EXCITATION LOSS DETECTION OF SYNCHRONOUS GENERATOR …

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Power Systems Engineering Thesis

2020-03-16

EXCITATION LOSS DETECTION OF

SYNCHRONOUS GENERATOR ON

POWER SYSTEM PERFORMANCES

Ygzaw, Alganesh

http://hdl.handle.net/123456789/10370

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i

EXCITATION LOSS DETECTION OF SYNCHRONOUS GENERATOR ON

POWER SYSTEM PERFORMANCES

Alganesh Ygzaw

A thesis submitted to school of Research and Graduate Studies of Bahir Dar Institute

of Technology, BDU in partial fulfilment of the requirements for the degree of Masters

of Science in Electrical Engineering with specialization in Power System Engineering

in Electrical and Computer Engineering Faculty.

Advisor Name: Dr.-Ing. Belachew Banteyirga (PhD)

Bahir Dar, Ethiopia

March, 2019

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DECLARATION

I, the undersigned, declare that the thesis comprises my own work. In compliance

with internationally accepted practices, I have acknowledged and refereed all

materials used in this work. I understand that non-adherence to the principles of

academic honesty and integrity, misrepresentation/ fabrication of any

idea/data/fact/source will constitute sufficient ground for disciplinary action by

the University and can also evoke penal action from the sources which have not

been properly cited or acknowledged.

Name of the student_______________________________ Signature _____________

Date of submission: ________________

Place: Bahir Dar

This thesis has been submitted for examination with my approval as a university

advisor.

Advisor Name: __________________________________

Advisor’s Signature: ______________________________

Date: __________________________

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© 2019

ALGANESH YGZAW TEFERI

ALL RIGHTS RESERVED

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v

To My Brother

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ACKNOWLEDGEMENT

First of all, glory to God for all blessings I have earned in my whole life. I am very

humbled and field with a great gratitude to acknowledge the people that had a great

help in this work. Without his kindness, patience and guidance, the complete of this

work will be impossible; I would like to thank my advisor Dr.-Ing. Belachew B. I need

to give a great appreciation to my family also for their endless support and love

throughout my life. Last but not least I want to sincerely thank and acknowledge all my

friends and individuals for sharing their idea and support in every way.

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ABSTRACT

Generating steadiness of synchronous generators is highly dependent on their exciter,

as the direct current from excitation system sustains generator stator and rotor windings

magnetically coupled. However, any excitation system failure grades generator loss of

excitation and suspends power transmission from generating unit to customers. It is

typically characterized by high active power flow out of the generator with large

reactive power flow into the generator. And this power imbalance increases rotor speed

of generator beyond synchronous speed which result in voltage and current instability

in the grid connected with the generator. At this state, excitation loss protection must

isolate the faulted generator from the remaining system to avoid any damage that can

possibly happened due to excitation loss. This thesis work studies generator excitation

loss relay detection ability on IEEE 9-bus test system and Tana Beles-I power plant on

various excitation loss events. The proposed schemes have simulated and evaluated

using MATLAB/SIMULINK software.

The simulation results show that the relay tripping duration is highly dependent on

initial loading condition of the generator, type of excitation loss and reactive power

support from interconnected systems. Comparatively excitation loss relay shows a good

performance in full loss of excitation than in partial excitation loss. The relay is able to

detect any full excitation loss in less than 6.4second after failure initiated. But it detect

partial excitation loss long after the failure for heavily loaded generators and not detect

at all for lightly loaded generators. On the other hand, the relay has mal-operated for

stable and unstable power swings which are failures outside the exciter.

To overcome mal-operation of excitation loss relay, a back-up protection scheme has

proposed based on study of field voltage, quadrature-axis voltage and generator reactive

power variation in excitation loss event. The proposed algorithm limits the reactive

power consumption of excitation loss generator considering system stability. The back-

up protection has improved the excitation loss detection length to twice less for heavily

loaded generators and 16% less for medium and lightly loaded generators. It has also

differentiate system failures and excitation loss events successfully.

Key words: Excitation Loss, Excitation Loss Protection, Synchronous Generator

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Table of Contents

DECLARATION .................................................................................................... ii

ACKNOWLEDGEMENT ..................................................................................... vi

ABSTRACT ......................................................................................................... vii

LIST OF ABBREVATIONS ................................................................................. xi

LIST OF SYMBOLS ............................................................................................ xii

LIST OF FIGURES ............................................................................................. xiv

LIST OF TABLES .............................................................................................. xvi

CHAPTER-1

INTRODUCTION .................................................................................................. 1

1.1 Background ................................................................................................... 1

1.2 Excitation Systems ........................................................................................ 2

1.2.1 Types of Excitation Systems .................................................................... 3

1.2.2 Control and Protective Function of Excitation System ............................. 5

1.2.2.1 AC and DC Regulators ....................................................................... 6

1.2.2.2 Excitation System Stabilising Circuits ................................................ 6

1.2.2.3 Power System Stabiliser (PSS) ........................................................... 7

1.2.2.3 Volts-per-Hertz Limiter and Protection (V/Hz) ................................... 8

1.3 Overview of Synchronous Generator Protection............................................. 8

1.4 Excitation Loss ............................................................................................ 11

1.5 Problem Statement ....................................................................................... 13

1.6 Objective of the Study ................................................................................. 14

General Objective .......................................................................................... 14

Specific Objectives......................................................................................... 14

1.7 Scope of the Study ....................................................................................... 14

1.8 Significance of the Study ............................................................................. 14

1.9 Document Organization ............................................................................... 15

ix

CHAPTER-2

LITERATURE REVIEW ..................................................................................... 16

CHAPTER 3

SYSTEM MODELLING AND MATHEMATICAL OVERVIEW OF EXCITATION

LOSS....................................................................................................................... 20

3.1 System under Study ..................................................................................... 20

3.2 Synchronous Generator Modelling ......................................................... 21

3.3 Characteristics of Synchronous Generator in Excitation Loss Event ............. 25

3.3.1 Initial Loading Effect............................................................................. 30

3.4 Excitation Loss Protection Relay ................................................................. 32

CHAPTER 4

SIMULATION RESULTS AND DISCUSSIONS ................................................ 35

4.1 Full Loss of Excitation ................................................................................. 36

4.1.1 Field Winding Short Circuit ................................................................... 36

4.1.2 Sudden Main Circuit Breaker Failure ..................................................... 39

4.1.3 Sudden Loss of AC Voltage to Excitation System.................................. 40

4.1.4 Field Winding Open Circuit ................................................................... 42

4.2 Partial Loss of Excitation ............................................................................. 44

4.2.1 30% Field Voltage Loss ......................................................................... 46



4.3 Effect of Excitation Loss on Parallel Connected Generators ......................... 50

4.4 Power Swings .............................................................................................. 52

4.3.1 Short Circuit Faults................................................................................ 53

4.3.2 Outages ................................................................................................. 55

4.5 Backup Protection for Excitation Loss Detection ......................................... 57

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CHAPTER 5

CONCLUSIONS AND RECOMMENDATION ................................................... 67

5.1 Conclusion .................................................................................................. 67

5.2 Recommendations for Future Work ............................................................. 69

REFERENCES ..................................................................................................... 70

APPENDIX A ...................................................................................................... 74

A.1 Full Loss of Excitation ................................................................................ 74

A.2 Partial Loss of Excitation ............................................................................ 74

A.3 IEEE ST1A Excitation System .................................................................... 75

A.4 Two-axis Model Initial Values .................................................................... 75

APPENDIX B ...................................................................................................... 76

B.6 Excitation Loss Relay Protection Zones ...................................................... 78

APPENDIX C ...................................................................................................... 81

Physical Representation of Excitation Loss Event .............................................. 81

xi

LIST OF ABBREVATIONS

AC Alternating current

ANSI American National Standards Institute

AVR Automatic Voltage Regulator

BiT Bahir Dar Institute of Technology

CB Circuit Breaker

CLOE Complete Loss of Excitation

DC Direct current

EPS Ethiopian Power Systems

FFL Field flux linkage

FFT Fast Fourier Transform

FW Field Winding

G-1 Generator-1

G-2 Generator-2

IEEE International Electrical Engineering and Electronics

LOE Loss of Excitation

MATLAB Matrix Laboratory

MVA Mega Volt Ampere

MVAR Mega Volt Ampere Reactive

MW Mega Watt

OEL Over Excitation Relay

OOS Out of step

PID Proportional integrator differentiator

PLOE Partial Loss of Excitation

rms Root Mean Square

R-X Resistance- Reactance

SPS Stable Power Swing

SVM Space Vector Machine

UEL Under Excitation Relay

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LIST OF SYMBOLS

Ed′ Direct axis voltage behind transient reactance

Eq′ Quadrature axis voltage behind transient reactance

Te Electrical torque output

Tm Mechanical torque

Xd D-axis steady state reactance

Xd′ D-axis transient reactance

Xq Q-axis steady state reactance

Xq′ Q-axis transient reactance

φd D-axis stator flux linkages

φq Q-axis stator flux linkages

ωs Synchronous speed (rad/s)

∆ω Speed deviation

D Damping constant

Efd Field voltage

H Inertia constant (MWs/MVA)

Id D-axis current

Iq Q-axis current

It Generator terminal current

Pm Mechanical power input

Pt Active power

Qt Reactive power

Ra Armature resistance

s Slip

T’’d0 D-axis sub transient open loop time constant(s)

T’’q0 Q-axis sub transient open loop time constant(s)

T’d0 D-axis transient open loop time constant(s)

T’q0 Q-axis transient open loop time constant(s)

Vd D-axis terminal voltage

Vq Q-axis terminal voltage

Vref Reference voltage

Vt Generator terminal voltage

X’d D-axis transient reactance (pu)

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X’q Q-axis transient reactance (pu)

Xd D-axis steady state reactance (pu)

Xq Q-axis steady state reactance (pu)

Z Terminal impedance

δ Generator’s rotor angle

xiv

LIST OF FIGURES

Figure 1: Excitation system of synchronous generator .............................................. 2

Figure 2: Excitation system control and protective circuits ....................................... 6

Figure 4: Coordination of UEL, LOE relay and stability limit ................................... 8

Figure 5: Synchronous generator protective relays .................................................. 10

Figure 6: Single line diagram of (a) IEEE-9 bus test system (b) Tana Beles-I power

plant ........................................................................................................................ 20

Figure 9: Mechanical block diagram of synchronous generator ............................... 23

Figure 10: Simplified model of ST1A excitation system ......................................... 23

Figure 11: Generator(a) Terminal and internal voltage (b) q-axis and d-axis voltage in

LOE event created at 1second .................................................................................. 26

Figure 12: Generator (a) Active and reactive power (b) d-axis and q-axis currents in

LOE event ............................................................................................................... 27

Figure 13: Generator (a) Rotor speed (b) load angle in LOE event at 1second ......... 28

Figure 14: Generator terminal (a) resistance and reactance (b) impedance in excitation

loss event ................................................................................................................. 29

Figure 15: Generator parameter variation in LOE event at various generator loading

conditions ................................................................................................................ 31

Figure 16: IEEE 9 bus system G-1parameters in G-2 LOE event ............................ 31

Figure 17: Impedance trajectory of LOE relay in different system conditions ......... 33

Figure 18: Flow chart of LOE relay ........................................................................ 34

Figure 19: Simulink model of IEEE 9-bus system with LOE event and LOE relay .. 35

Figure 20: (a) G-2 excitation current and voltage (b) rotor speed and reactive power in

field winding short circuit ........................................................................................ 36

Figure 21: Power flow direction of synchronous machines ..................................... 37

Figure 22: G-2 impedance trajectory in field winding short circuit .......................... 38

Figure 23: G-2 field voltage and current in main CB failure .................................... 39

Figure 24: G-2 impedance trajectory in main CB failure ......................................... 40

Figure 25: G-2 field voltage and terminal impedance in sudden loss of AC voltage to

exciter terminal ........................................................................................................ 41

Figure 26: G-2 impedance trajectory in sudden AC voltage loss to exciter .............. 41

Figure 27 : G-2 Field voltage and field current in field winding open circuit ........... 43

Figure 28: G-2 impedance trajectory in field winding open circuit .......................... 43

xv

Figure 29: Reactive power of G-2 in 70%Efd loss ................................................... 45

Figure 30: G-2 parameter variation in partial loss excitation ................................... 46

Figure 31: G-2 impedance trajectory in 30% field voltage loss................................ 47

Figure 32: G-2 impedance trajectory in 50% loss of excitation................................ 47

Figure 33: G-2 impedance trajectory in 70% loss of excitation................................ 48

Figure 34: Simulink model of Tana Beles-1 power plant ......................................... 51

Figure 35: Tana Beles-I (a) G-1 and (b) G-2 parameters in G-1 LOE event ............. 51

Figure 36: Tana Beles-I G-1 and G-2 impedance trajectories .................................. 52

Figure 37: Tana Beles-I G-1 and G-2 impedance trajectories in field winding (a) short

circuit with SPS (b) open with SPS .......................................................................... 52

Figure 38:G-2 impedance trajectory (a) three phase (b) two phase to ground (c) phase

to phase (d) three phase cleared after 250ms at G-2 terminal .................................... 54

Figure 39: G-2 impedance trajectory (a) three phase fault at G-2 terminal (b) L7-8

outage (c) load rejection (d) G-outage ...................................................................... 55

Figure 40: Flow chart of proposed back up protection ............................................. 57

Figure 41: Q-V curve of LOE relay and proposed back up protection ..................... 60

Figure 42: Terminal voltage reduction in LOE relay and back-up protection .......... 61

Figure 43: G-2 terminal voltage in Partial loss of excitation (a) medium load 20%Efd

loss (b) light load 30%Efd loss .................................................................................. 64

Figure 44A-1: 9-bus G-2 impedance trajectory in 90% and 70% loading ................ 74

Figure 45A-2: 9-bus G-2 impedance trajectory in (a) 90% and (b) 60% field voltage

loss .......................................................................................................................... 74

Figure 46A.3: Block diagram of IEEE ST1A excitation system .............................. 75

xvi

LIST OF TABLES

Table 1: Parameters of generators under study ........................................................ 21

Table 2: LOE relay detection ability in main causes of full excitation loss .............. 44

Table 3: LOE relay detection ability in partial loss of excitation in different loading

conditions ................................................................................................................ 49

Table 4: Performance of LOE relay in power swings .............................................. 56

Table 5: Comparison of actual and proposed excitation loss detection in field winding

short circuit ............................................................................................................. 62

Table 6: Comparison of actual and proposed excitation loss detection in field winding

open circuit .............................................................................................................. 62

Table 7: Comparison of actual and proposed excitation loss detection in partial field

voltage loss .............................................................................................................. 63

Table 8: Comparison of actual and proposed excitation loss detection in system

disturbances ............................................................................................................. 65

Table 9 B-1: IEEE 9-bus system required Machine Data ......................................... 76

Table 10 B-2: IEEE 9-bus system load data ............................................................ 76

Table 11 B-3: IEEE 9-bus Transmission line Data .................................................. 77

Table 12 B-4: IEEE 9-bus system excitation system data ........................................ 77

Table 13 B-5: Tana Beles-1 System data ................................................................. 78

1

CHAPTER-1

INTRODUCTION

1.1 Background

The versatility behaviour of electrical energy has grasp researchers attention in the

recent centuries to develop the efficiency of electric power delivery and sustain the

activities of life as easily as possible. Ever since discovered, electrical energy has

gradually improve human life to better than it was before in manufacturing, health and

generally transformation of life activities to easy and labour intensive system. In

consequence, the dependence of human life on electricity has increase gradually from

time to time and a lot of researchers have focused on improvement of reliability and

security of energy transmission from the source to customer. On the other hand

unprotected and unsecured electrical energy deliver can cause a serious damage to

properties even to life of living things. So, a modern power system concern must be

consistency and security on all part of power system (generation unit, transmission unit

and distribution unit) to make the system secure and economical since survival comes

before any gain.

The efficacy of electrical energy transmission in all part of power system is highly

dependent on the reliability of synchronous generating machines at any situation so

that a truthfulness of power transfer from the generating unit to customer is maintained

continually. Generally synchronous generators have two inputs; mechanical input from

turbine and field voltage from excitation system. And at normal condition they are able

to produce and deliver active power due to the mechanical input and reactive power

due to the field voltage. Indeed a secure and well protected excitation system should

be one of the important concerns in modern power plants.

However, any failure in excitation system grades excitation loss in the generator and

the generator will suddenly start to consume reactive power from the grid connected

with it. In this condition the generator must be isolated from the remaining system. In

actual power system industries generator terminal impedance variation is used to detect

excitation loss event. However, generator terminal impedance variation with system

disturbances threatens the accuracy of excitation loss relay to mal-operate for un-

necessary power swings that created due to failures outside the generator. In addition

this excitation loss detection method has an apprehended detection ability in partial

excitation loss and the mal-operation of the relay for system disturbances results in

2

unwanted generator tripping. And this further jeopardize stability of the gird due to

unnecessary generator outage.

1.2 Excitation Systems

One of the most significant elements of electric power system is synchronous generator

that changes the mechanical energy from turbine into electrical energy [1]. Energy

transformation is possible only if generator have excitation system which defines the

generator reactive power output values. This means that generator excitation regulation

is actually regulation of generator output energy and also impacts the stability of entire

electric power system. Excitation system is part of generating units in which it produces

flux by passing current in the field winding to supply its output to synchronous

generating machines through either brushes or slip rings to run or excite synchronous

machines [1]. Its power makes up generally 0.2-0.8% of the generator power to

maintain the terminal voltage of the generator within the accepted voltage range by

responding quickly to system component variation [2].

Limitation and

Protection unit

Measuring

elements

Controller Exciter Generator

Power system

stabilizer

Reference value

System

Figure 1: Excitation system of synchronous generator

Generally excitation system consists of two relatively independent components,

excitation regulator (AVR) and the exciter itself with requirements to keep the

generator in a condition when it is possible to transmit the power close to line power

limit, ensure sufficient dynamic stability reserve, damping power swings of generator

after any failure, maintain stability during change of properties of system and to ensure

high operating reliability of the system [2] [3]. Thus, it is able to control voltage and

3

reactive power flow by ensuring if the machine does not exceed the capability limits.

Generally, an excitation system includes the following elements.

The components of excitation system works inter-correlate to provide Direct Current

(DC) to the generator field winding [1].

Controller- It processes and amplifies input control signals to a level and form that is

appropriate to control the exciter.

Measuring elements- this includes the terminal voltage transducer which sense, rectify

and filtered the generator terminal voltage to a DC quantity and a load compensator to

compare the terminal voltage with a reference voltage.

Power system stabilizer- provides additional input signal to the regulator to damp

power system oscillations.

Limiters and protective circuits- limit the capability limit of exciters and generators.

Co-operating the above excitation system components, the main properties of excitation

system regulation should include these three characteristics: speed of system operation,

autonomy of excitation system and maximal drive security. Speed of operation is

important to maintain stability of electric power system in the meaning of reactive

power transmitting and receiving, fast de-excitation in case of internal failure and

overvoltage limitation in case of sudden unloading.

Autonomy of excitation systems means that excitation system supply must be ensured

in every condition of a drive. And drive security is function of reliabilities of all

incorporated components [4] [5].

1.2.1 Types of Excitation Systems

Excitation systems of synchronous generators can be classified in the meaning of

construction as static or rotating and according to excitation energy source as separate

excitation systems and self-excited systems [5]. In static excitation systems energy

needed for excitation is brought to generator field winding via slip-rings with carbon

brushes. To perceived use of brushes when supplying high field current to large

synchronous machines, use of brushes in static excitation systems have been eliminated

in rotating excitation systems [1]. They are brushless excitation systems but direct

measurement of generator field current and voltage is impossible in this type of exciters.

Brushless systems are used for excitation of larger generators (power over 600MVA)

and in flammable and explosive environments. Brushless system consists of Alternating

4

Current (AC) exciter, rotating Diode Bridge and auxiliary AC generator realized with

permanent magnet excitation.

Separate excitation systems are independent of disruptions and faults that occur in

electric power system, and have possibility to force excitation [4] [5]. On the other

hand, self-excited excitation systems are connected to the grid and utilizes part of

generator power. Generally there are three major groups of generator excitation

systems, with nineteen different excitation system models altogether: Direct Current

Commutator Exciters (DC), Alternator Supplied Rectifier Excitation Systems (AC) and

Static Excitation Systems (ST) [6].

DC excitation systems

These type of exciters uses direct current generators as sources of excitation power and

provided current to the rotor of the synchronous generator through slip rings. The

exciter may be placed on the same shaft with power generator or separately driven by

a motor. Nowadays, DC type exciters are mainly suppressed by the other two types and

a few new synchronous machines are being equipped with these. This group consists

of four models as described in [6].

DC1A model is used for self-excited shunt fields with voltage regulator operating in a

buck-boost mode. It represents field-controlled DC commutator exciters with

continuously acting voltage regulators that have generator output voltage as main input.

And it have improved to DC2A by adding voltage regulator output limits. On the other

case DC3A model is used in DC commutators with non-continuously acting regulators.

DC4B is newly added model that differs from DC1A in implemented controls and

contains PID controller.

AC excitation systems

In this types of excitation system AC machines are used as sources of the main

generator excitation power and rectification of AC voltage is carried out through

controlled or non-controlled rectifiers to provide DC to the generator field winding. But

these systems do not allow negative field current except AC4A model. This is the main

disadvantage of this type of systems because it does not allow de-excitation of

generator. AC1A model is used for field-controlled alternator-rectifier excitation

systems, with non-controlled rectifier in case of separate excitation. AC2A differs from

AC1A in additional compensation of exciter time and exciter field current limiting

elements. AC3A and AC4A models are used for self-excitation systems and for systems

with full thyristor bridge in the exciter output circuit respectively [7]. AC5A is

5

simplified model for brushless excitation systems with separate excitation whereas

AC6A represents field-controlled alternator-rectifier excitation systems with system

supplied electronic voltage regulators. AC7B and AC8B model incorporates newer

controls and PID controller. Here, proportional, integral and differential gains are

defined with separate constants [5] [6].

Static excitation systems

In static excitation systems all the elements are stationary. Such systems directly

provide synchronous generator field winding with excitation current by means of slip

rings and the rectifiers gain power from generator through auxiliary windings or a step-

down transformer. In such systems generator itself is power source or the generator is

self-excited. This type of excitation system consist of seven models and the possibility

to produce negative excitation current is their significant advantage. Thus, it provides

quick de-excitation which may be needed in case of generator internal fault. ST1A

model represents systems in which excitation power is supplied from generator

terminals or separate bus. Having this advantage, in this thesis work ST1A exciter

model will be used in all scenarios of the study. ST2A is model for systems that utilize

both current and voltage generator terminal quantities to comprise power source [6].

Model ST3A uses a field voltage control loop to linearize control characteristic of the

exciter and ST4B only varies from ST3A model due to usage of PI instead of lag-lead

controller. ST5B is variation of ST1A with alternative over excitation and under

excitation inputs and additional limits. Voltage regulator of ST6B model consists of a

Proportional Integrator (PI) voltage regulator with an inner loop of field voltage

regulation and pre-control. ST7B model represents static potential-source excitation

systems, with PI controller which may be turned into PID controller if phase lead-leg

filter used in series, which is typical case for brushless excitation systems.

Today, most excitation systems are AC or static types because of the fast response

ability [8].

1.2.2 Control and Protective Function of Excitation System

Capability limit of exciters and generators are limited through the limiters and

protective circuits of the exciter. This function includes set limits of field current,

terminal voltage limit, volts-per-Hertz limit, maximum and under-excitation limits.

The type of limiters and their output signals location have given in Fig. 2. For secure

6

and reliable generator operation, most of these limiting circuits are included as part of

excitation system [5].

1.2.2.1 AC and DC Regulators

The main function of AC and DC regulators is to maintain stator voltage and to hold

the field voltage at constant respectively. DC regulator is also used as back-up of AC

regulator to test and start-up and to outfit to situations when AC regulator is at fault. In

this condition the field voltage is regulated and a manual adjusting of set point is

required thus DC regulator is called as Manual control.

Voltage sensing and load

compensation

PSS

Voltage sensing

DC regulator

AC regulator

Exciter Field

shortingGenerator

Exc. Sys.

stabilizing

circuits

Overexc. limiter

Under exc.

limiter

V/Hz limiter and

protection

AC voltage adjust

DC voltage adjust

Figure 2: Excitation system control and protective circuits

1.2.2.2 Excitation System Stabilising Circuits

Stabilising circuits are used to improve the dynamic performance of the excitation

system for both AC and DC exciters by minimization of the phase shift caused by

element time constants before synchronization or after load rejection. Depend on type

excitation system, the level of stabilizing system may differ according to time constant

7

effects. The negligible inherent time delays of time constants for static excitation

systems have avoid the requirement of excitation control-system stabilization. The

general derivative feedback of excitation control can be summarized in Fig.3.

1.2.2.3 Power System Stabiliser (PSS)

Power system stabiliser is used for further improvement of power system dynamic

performance of synchronous machines. It stabilize either shaft speed, terminal

frequency or terminal power to output stabilized voltage by damping system

oscillations.

Exciter and AVR

sKF/1+sTF

fdE To generator field

Compensation

Ve

Figure 3: Excitation control system stabilization

1.2.2.4 Under Excitation Limiter (UEL)

UEL prevents generator excitation from reduction of stability limit or stator core end

region heating limit. The main control signal of UEL can be derived either from the

combination of voltage and current or active and reactive power of the generator. When

the UEL set limit is achieved, the limiter controls the excitation system until the signal

reaches below set limit. Since the limit point setting is based on the instability or stator

core heating, the limiter should coordinated with excitation loss relay and small signal

stability limit as shown in Fig.4 [5].

1.2.2.5 Over Excitation Limiter (OEL)

OEL protects generator from overheating from prolonged field overcurrent. It detects

the high field current condition and then after a time delay act through the AC regulator

to ramp down the excitation to about 110% of rated field current; if unsuccessful, trips

the AC regulator and transfers to DC regulator to reposition the set point corresponding

to rated value [5]. If this also does not reduce the excitation to a safe margin, OEL will

initiate an exciter field breaker trip and so a unit trip will be created.

8

1.2.2.3 Volts-per-Hertz Limiter and Protection (V/Hz)

These protection schemes are used to protect generator core and step-up transformer

from damage due excess overheating resulted from extreme magnetic flux of low

frequency and over voltage system condition. V/Hz limiter controls field voltage so as

to limit the generator voltage when the ratio exceed the pre-set limit. Thus the limiter

can also be used as over voltage relay for frequency greater than 60/50Hertiz.

Figure 4: Coordination of UEL, LOE relay and stability limit

1.3 Overview of Synchronous Generator Protection

[Synchronous generators supply almost all the electric power we consume today and

always there is a constant need for reduction of operational and maintenance costs of

large sized synchronous generators. The most efficient way of reducing these costs

would be continuous monitoring of the condition of these generators. This allows for

early detection of the degeneration of the generator’s health, facilitating a proactive

response, minimizing downtime and maximizing productivity [5].

Despite the efficient design and protection of synchronous generators, faults occurring

within the machine cannot be avoided completely. However, the generator protection

relays make sure the faults are eliminated within a short period of time. Since

eliminating a generator from a system may be costly and create instability in the whole

system, faults from outside should be cleared as efficiently as possible before creating

permanent damage in the generator [9]. To achieve the selectivity and sensitivity of the

9

protection relays, a proper coordination should be taken by considering abnormal

operating conditions and type of faults occurring within the machine. Abnormal

conditions of a generator can arise due to stator or rotor field failure or system

disturbances [3] [10] and protective relays for each failures using variety signals are

meant to monitor and provide proper signals to alarm or remove the generator from the

system under faulty conditions [9].

The general protective devices of synchronous generators are given in Fig.5 which

protect the generator uniquely with their own element characteristics and should work

coordinated but without overlapping. The numbering are given according to American

National Standards Institute (ANSI) standard.

Distance relay (Device 21) - it is an impedance relay which uses voltage and current

phases to measure the impedance in front of the generator. Basically this device protects

the generator from an external fault. If impedance falls into the relay characteristic,

relay will trip the generator [11].

Over excitation relay (Device 24)-when the ratio of the voltage to frequency

(volts/Hz) exceeds 1.05 pu for a generator, severe overheating can occur due to

saturation of the magnetic core of the generator and the subsequent inducement of stray

flux in components not designed to carry flux. Such over excitation most often occurs

during start-up or shutdown while the unit is operating at reduced frequencies, or during

a complete load rejection which leaves transmission lines connected to the generating

station or during excitation system failures. Over excitation relay detects this

phenomenon [12] [13].

Power direction relay (Device 32) – it is a reverse power relay which monitors the

direction of generator power to prevent any reverse flow of active power (motoring

mode of operation). Motoring is an abnormal condition that can cause serious

mechanical damage to prime mover. In some applications this relay could be used for

load shedding [14].

Excitation loss relay (Device 40) – it uses the impedance variation of generator

terminal to protect excitation loss event. This relay will be discussed in detail in this

thesis work.

Current unbalance relay (Device 46) - current unbalance relay monitors the negative

sequence component of the current and if this current exceeds from the relay setting,

relay will operate. The most common causes of unbalance current are system

asymmetries, unbalance loads, unbalance fault and open phase [12].

10

These system conditions produce negative-phase-sequence components of current

which induce a double-frequency current in the surface of the rotor. These rotor currents

may cause high and possibly dangerous temperatures in a very short time [13].

CB

64F

61

49

Field Ground

Generator Inte-

rturn

Stator Temp.

87G

Gen.Diff

60 Voltage Balance

78

40

32

21

51V

5981O/

U

24

46

Aux VTs

Sys. BackupNeg. Seq. Current

Over/Under Freq.

V/Hz

59N

51N

Gen. Neutral

Overvoltage

Loss of Feild

Loss of Sync

Over-Voltage

Reverse power

MV Line

Figure 5: Synchronous generator protective relays

Over temperature (Device 49) - this relay senses the temperature at different spots of

the generator and provides a thermal protection. Usually this relay is not used for

primary protection [6].

Time delay over current (Device 51) - monitors currents flowing through generator

windings and provide a time delay over load protection for the particular part. The relay

has an inverse time characteristic and provides a time delay which is inversely

proportional to the over load current magnitude [15]. Device 51V is the voltage

restrained time delay over current relay which provides better protection when under

voltage condition exists [16].

11

Over voltage relay (Device 59) -generator overvoltage may occur during a load

rejection or excitation control failure. In case of hydro generators, upon load rejection

the generator may speed up and the voltage can reach high levels without necessarily

exceeding the generator’s V/Hz limit. Over voltage relay is for monitoring the voltage

and if the voltage exceeds from the relay pre-set level, it will trip.

Voltage Balance Relay (Device 60) - it compares two voltages from two different set

of Voltage Transformers (VTs) and trip if these two voltages are not balanced. Most

common use of this relay is to detect VT fuse failure.

Ground fault (Device 64) -the function of this device is to detect ground fault in the

stator or rotor field winding. It is a common practice to ground all types of generators

through some form of external impedance. The purpose of this grounding is to limit the

mechanical stresses and fault damage in the generator, to limit transient voltages during

faults and to provide a means for detecting ground faults within the generator. The

magnitude of stator ground-fault current decreases almost linearly as the fault location

moves from the stator terminals ground fault near the neutral of a wye-ground fault

current becomes small toward the neutral of the generator [17].

Out-of-step relay (Device 78) - out of step relay detects generator loss-of-synchronism

condition. It contains two blinder elements supervised by a mho relay to prevent

nuisance tripping for stable swings. It detects unstable condition of generator through

prolonged system disturbances.

Over/Under frequency (Device 81) - the operation of generators at abnormal

frequencies (either over-frequency or under-frequency) generally results from full or

partial load rejection or from overloading of the generator. Full or partial load rejection

may be caused by clearing of system major system disturbance. Load rejection will

cause the generator to over-speed and operate at some frequency above normal value

[12]. Over/under frequency relay detect this conditions.

Differential Relay (Device 87) - this relay looks into a zone defined by location of

current transformers and if the input current does not match with output current in that

zone, it rapidly trips the generator [9].

1.4 Excitation Loss

Any failure in excitation system directly interrupt the generating capability of the

synchronous machine and transmission of power to the system. The phenomenon where

the generators lose its excitation is called excitation loss. In excitation loss event, the

12

excitation system fails to deliver DC current and the generator seek a way to stay

excited which causes the faulty generator to absorb a large amount of reactive power

from the system connected with it. And consequently reduces reactive power delivery

from generator to system and lead to power system voltage and current instability and

if it continues to blackout of the whole system [18] [19]. If the reduction in reactive

power continues until a pre-determined under excitation limit, the generator will lose

synchronism and result in generator rotor speed up. This causes stator overloading as a

result of reactive power decrease on the grid, heating up of rotor winding due to induced

currents, asynchronous operation and active power swings which may decrease

generator’s life time [19].

Generator instability after excitation loss may lead to complete or partial excitation loss

of the synchronous generator. Complete loss of excitation (CLOE) can occur when field

winding open or short circuit or sudden opening of the field supply breaker happen

whereas partial loss of excitation (PLOE) can occur when suddenly field voltage drop

or short-circuiting in a section of the field winding is happen.

No matter how it caused, loss of excitation (LOE) represents huge damage on the

generator and on the whole system if an early protection is apprehended.

Damage to the generator:

When a synchronous generator lost excitation; its excitation current gradually decreases

which result in reduction of internal electromotive force and the electromagnetic

relation of stator winding and rotor windings. As the interaction of stator and rotor part

of the generator decreases, the reactive power of the generator terminal starts to reduce

in value as it is dependent on electromotive force. The less interaction of rotor and

stator windings creates slip which causes rotor overheating. As the machine operates

as an induction machine after loss of excitation, large amount of reactive power

supplied by stator current is required and the stator may suffer over heating because of

this large current. Under heavy load condition, the generator may suffer from severe

mechanical stress because of the power reduction which may damage both generator

and system [2] [20].

Damage to the system:

The asynchronous behaviour of synchronous machines after excitation loss results

decline of system voltage. For some weak system, the system voltage may collapse due

to the loss of excitation of an important generator and increases the reactive power

output of other generators in the system. This may cause the overloading in some

13

transmission lines or transformers. Thus the power swing and voltage drop caused by

loss of excitation may affect the normal operating generators and lead to loss of

synchronism of some normal operating generators in the system.

As a result a well-designed and an accurate protection against LOE is needed to detect

any failure in excitation system of synchronous generators to maintain the system stable

and safe at normal and abnormal (system swings) condition of the system [19].

As studies show a mal operation of excitation loss protection devices have been the

main causes of black out in many power grids throughout the world [18] [21]. This

arises due to weak setting of LOE relays and variable behaviour of the system depend

on load condition [18] [9]. An imperfection of the protection devices also lead to sense

some external system failures which is totally different from loss of excitation but that

have similar effect on the impedance variation of the generating unit. So, it is desirable

to install an excitation system with a highest possible operating reliability since outages

or failures in excitation systems can have very unfavourable operating consequences in

the whole system including the adjacent generators.

1.5 Problem Statement

Excitation system by its behaviour not only run the generator but also receive the

generator terminal voltage back through the transducer and compares it with a stability

margin reference value to keep the system in stable condition. And the terminal voltage

of the generator mostly affected by the grid working performances, which have the

probability to affect the exciter in addition with different failures of excitation system.

The actual excitation loss relay in power system industries mal-operates in prolonged

faults. Furthermore, excitation loss detection period of the relay varies on type of

excitation loss and severe of excitation loss (complete or partial excitation loss). An

apprehended detection of partial excitation loss leads to system instabilities and

blackouts in many systems. This is due to the dependence of relay characteristic design

on generator terminal parameters. Considering the above reasons, in this work

detection ability of excitation loss relay will be studied on various types of excitation

loss and system conditions.

14

1.6 Objective of the Study

General Objective

The main objective of this thesis is to study the accuracy and performances of excitation

loss detection method of synchronous generating machines on different excitation loss

causes and power grid performances.

Specific Objectives

The main specific objectives of the work includes:

Study synchronous generator and its excitation system characteristics

Investigate loss of excitation and its causes

Examine LOE detection methods on different power system structures and

performances

Improve the conventional methods of LOE detectors to overcome mal-operation

of the relay

Modelling IEEE 9-bus system and Tana Beles-1 power plant to study excitation

loss phenomenon

1.7 Scope of the Study

Excitation loss detection performance of IEEE 9-bus test system will be studied and

simulated on different system condition and different causes of excitation loss using

MATLAB/SIMULINK programming language.

1.8 Significance of the Study

The main significances of this work are:

Identifying the drawback of actual excitation loss relay in power system

industries

Give an appropriate setting for various causes of excitation loss to

decrease the mal operation of excitation loss relay

Avoiding the threshold robustness of the LOE detecting methods which

need a tedious simulation settings of threshold set,

Differentiate system failure and loss of excitation easily without the

dependence of the grid parameters.

Improve excitation loss relay detecting duration in all excitation loss

events

15

1.9 Document Organization

This report contains five chapters and their general organization can be summarized as

following:

Chapter 1: Is the general overview part which gives a detail background to the work

including the general highlight, problem statement, objective, significance, scope and

outline of the study.

Chapter 2: Present the previous works regarding with the excitation loss detection

method improvement through different parameters of synchronous generators.

Chapter 3: Deals with the modelling and control of synchronous generator components

with and without excitation loss event.

Chapter 4: The proposed algorithms simulation results will be discussed on different

excitation loss causes and system conditions.

Chapter 5: Presents the conclusion and possible recommendations for future work.

16

CHAPTER-2

LITERATURE REVIEW

For about six centuries, various LOE detecting methods has been suggested depend on

different parameters of the generator that are assumed really sensitive in case of

excitation system failures. But still the actually excitation loss protection in power

system industries is so called impedance type protection proposed almost four decades

ago. Since the protective relays should be designed with requirements of sensitivity to

sense all possible failures of excitation systems for all types of generators and accuracy

and reliability as should be easy and less complex to set the threshold of the protective

devices after a possible failure. Even if there are generator operations where this type

of protection mal-operate, the methods that have been suggested since then no matter

how sensitive in advance they are, the vast simulation process requirement of their

algorithms makes them unpractical. Thus the protection relays on the generating unit

are expected not only to ensure the reliability of the system but also to accurately

operate in face of faulty conditions where a precise setting and a practical coordination

of protective relays is important to minimize unwanted disconnection of components

and inactivating trip when it is important to protect the system which is the power

system reliability concern above the economic issue [2] [3].

So, it is desirable to install an excitation system with a highest possible operating

reliability and a well-designed and an accurate protection or detection against LOE and

is needed to detect any failure in excitation system since outages or failures in

excitation systems can have very unfavourable operating consequences in the whole

system.

The generator terminal voltage and terminal current measurement is used to protect

against excitation loss event. In 1949, Mason [22] suggests a negative off-set mho-type

distance relay to sense the impedance variation of generator terminal point due to

excitation loss through the variation of terminal voltage and terminal current of the

generator in excitation loss event. When the impedance falls under predefined

protective zone in Resistance-Reactance (R-X) plane for a pre-set time delay

determined using the longest oscillation of swing angle, the relay detects loss of

excitation and send a trip signal to the field breaker. It is the basis for most of the

methods that have been created since, but it has high relay operation time and

difficulties in differentiating system failures like stable power swing which are failures

17

outside the excitation system. Shortly afterwards, in 1975, Berdy [20] presented a

method based on the addition of another mho unit to this protection scheme proposed

in [22]. This type of protection is the most common method of LOE protection which

detects the generator terminal parameters variation at any cause of excitation failures

and it is the actual technique used in most power system industries until now.

The above methods have a mho function characteristics that uses the current and

voltage measured at the relay point of the generator terminal to determine if the

apparent impedance plots within the mho characteristic. The relay characteristic is an

offset circle which has an angle of maximum torque that falls on the (-X) ordinate. As

viewed from the machine terminals the relay will operate for any impedance phasor

that terminates inside the circular characteristic. When the relay was introduced in

1949, it was recommended the offset be set equal to one-half of the direct axis transient

reactance (X’d/2) and the diameter of the circle set equal to the direct axis synchronous

reactance (Xd). It was shown that with the machine reactance that existed at that time,

these settings would detect a loss of excitation from any machine loading and that there

would be optimum selectivity against operation during stable power swings. Machine

direct axis synchronous reactance was in the range of 1.1 to 1.2 per unit. But, with the

improvement of synchronous machines synchronous reactance the method finds

difficulties in detecting LOE event with Xd greater than 1.6pu and lightly loaded

generators. Thus, addition of a second mho-unit relieved this problem.

Some years later, the above methods have been modified using modern computational

methods, such as neural networks [23] [24], decision tree [25] and fuzzy [26]

algorithms in protection against loss of excitation. These methods may present good

results, however require a considerable amount of training and depend on the

characteristics of the system. In 2005, S. R. Tambay and Y. G. Paithankar [27]

proposed to use a relay with quadrangular characteristic and the use of rate of change

of the reactance seen in the terminals of the machine with the help of digital relays.

Again with the advent of digital relays, another method on the basis of Space Vector

Machine (SVM) technique to discriminate between LOF and stable power swing SPS

is presented in [28]. However, both of the above mentioned schemes need a significant

amount of data for training and are dependent on the system characteristics.

In 2016, Behnam M. and Jian Guo Zhu, [29] present a setting free approach that the

rate of resistance variations at the generator terminal is introduced as an excitation loss

detector, which it will become and remain negative a short period after the event

18

occurred. Since the measured resistance has an oscillatory nature due to the speed

variation associated with slip frequency, the proposed algorithm may reset for loss of

excitation events with the high slip frequency.

On the same year, M. Abedini et al. [10], proposes a method using the rate decay of the

generator internal voltage with the field flux linkage variation. An adaptive and

threshold loss of excitation index is introduced depend on terminal voltage to

discriminate system disturbance from excitation failure such if the generator achieve

greater excitation loss index for a given samples, then loss of excitation will be

detected. This method have accurate sensing results since it uses the capability curve

of the generator, however the set points identification is a difficult task and may involve

extensive simulation processes which makes it unpractical. Those authors modify the

mentioned criterion in 2017 [21], which uses Fast Fourier Transform (FFT) coefficient

of three-phase active power to prevent the mentioned algorithm from mal-operation in

the face of SPS.

A combined index based on generator terminal voltage, reactive power and power

angle variations is presented in [18], where power angle is estimated by measuring the

rotor speed. Although this technique can be implemented by considering a special case

of the network operation, regarding the network combination is inevitable.

Excitation loss detection through generator internal parameters can be evaluated also

using flux interaction of the stator and rotor windings, internal voltage or internal

current measurements. A flux based method is presented in [30], which it uses the

installed search coils in stator slots to measure the air-gap flux. This scheme however

should normally be implemented by the generator manufacturer.

Despite the fact that the methods presented by [22] and [20] have been introduced more

than 40 years ago, they are still available in the most commercial relays for generator

protection due to the fact that the most recent methods require a considerable amount

simulation in order to obtain thresholds, especially the methods present in [10], [23]

and [25]. So, the real life implementation of recent proposed methods is still a difficult

task. The second reason is because it is really challenging to identify the best methods

among several techniques tested under different systems, load conditions, frequency

sample rate, etc. In other words, there is no paper that compares the methods under the

same test conditions. So, there is no knowledge and practical understanding about this

particular condition of LOE in the recent presented methods [2] [3].

19

In [31], different structures of a simple power system network was presented to study

the behaviour of the parallel generators in loss of excitation situation; as a result the

excitation failure in one generator have an effect on the performance of nearby

generators where the close ones were highly affected than the further generators.

20

CHAPTER 3

SYSTEM MODELLING AND MATHEMATICAL OVERVIEW

OF EXCITATION LOSS

3.1 System under Study

For this thesis work, the IEEE 9-bus test system and Tana Belese-I from Ethiopia Power

Systems (EPS) have been modelled in MATLAB/SIMULINK simulation tool to study

LOE scenarios in different system conditions. IEEE 9-bus test system includes three

generators, six transmission lines and three loads connected through nine possible

buses. Also, Tana Beles-I power plant from Ethiopian Power Systems has been used in

this thesis work to study effect of excitation loss on interconnected generators. This

generation unit consists of four identical generators connected on the same bus and the

rest of the system connected within this unit will be considered as infinite bus system.

In addition the IEEE standard models IEEE ST1A type excitation system and hydro

turbine governor from the Simulink Sim-power library have been used to study

excitation loss phenomenon [8]. The single line diagrams of IEEE 9-bus system and

Tana Beles-I have given in Fig.6 and generator under study data in table1.

Figure 6: Single line diagram of (a) IEEE-9 bus test system (b) Tana Beles-I power

plant

G-1

G-2

G-3

B-1

B-2

B-3

B-4

B-5B-6B-7

B-9

B-8

T-1

T-2

T-3L5-8

L8-9

L4-6

L5-7

L4-5

L6-9

Where

B-Bus

G-Generator

T-Transformer

L-Line

(a)

G-1

G-2

G-4

Rest of EPS

Gird G-3

Common BUs

(b)

21

Table 1: Parameters of generators under study

Generator MVA kV Xd X’d T’do

9-Bus G-2 192 18 1.72 0.23 8

Beles G-1 133 15 1.03 0.25 9.2

3.2 Synchronous Generator Modelling

A synchronous generator generally has two inputs, a torque input from a turbine

coupled to its rotor and an excitation current coupled to field winding of rotor. The

mechanical torque from turbine run the rotor and generate rotating magnet field in the

air gap which cuts the stationary coils in the stator and induce a voltage whereas the

excitation current from the excitation system produces magnetic poles in rotor [32].

Thus, the mechanical torque supplied by a turbine is converted to electrical torque

through the flux linkage and transmitted to grid as voltage and current. As

electromagnetic induction principle states, “when a coil of copper wire is rotated in a

magnetic field in such a way as to cut across the lines of magnetic force, an electric

charge is created or induced in the wires”; the synchronous generator also follows this

principle as when the magnetic circuit of the generator rotor rotates, the generator field

winding excites the rotating magnetic circuits to establish field flux linkage (FFL) and

consequently an internal voltage which have a proportion amplitude to the magnitude

of FFL is generated on the rotor field winding. This generated voltage will be delivered

to the whole system connected with the generator unit [33].

There are several kinds of synchronous generator models with different complexity and

sophistications. The mathematical model of synchronous machine can be described by

a set of differential equations representing the dynamics of the machines, exciters and

other controls and algebraic equations representing the network relation. However, at

first to model a synchronous machine mathematically all the windings that should be

included have to be identified first.

In this work, the synchronous generator behaviour and the generator parameters

calculation is analysed using a fourth order (two-axis) generator model for simplicity

of calculations as presented in [32] and Fig.7. In this generator model the transient

effects are accounted for, while the sub transient effects are neglected.

22

DC

DC

)2

('''' ])([

j

qqdqd ejEIXXE

)2

(

)(

j

qd ejII

'

djXsR eR epjX

vsj

seV

vsj

qd ejVV )(

Figure 7: Dynamic model of two-axis synchronous generator model

Where X’d is generator transient reactance, Rs generator internal resistance, Re and Xeq

are equivalent resistance and reactance of system connected with the generator

respectively.

The transient effects are dominated by the rotor circuits, which are the field circuit in

d-axis and an equivalent circuit in the q-axis formed by the solid rotor. Thus the time

constantsT’’d0 and T’’q0 are equal to zero [34]. The assumption is based on the fact that

the effect of damper windings on the transient is small enough to be negligible. The

general model of synchronous generator can be simplified as the following blocks

which work simultaneously to each other [35].

Excitation

system Rotor Electrical

Block

Torque-Angle

Loop

Terminal Voltage

(vt)

Mechanical

Torque(Tm)

Output current (It)Angular position

Field voltage

(Efd)Speed

Figure 8: Synchronous machine model block diagram

The torque angle loop in figure above represents turbine and generator mechanical

system. Inputs to this block are mechanical and electrical torques, and outputs are

rotating speed and rotor position as can represented in Fig.9 and equations (2.1-2.3).

dqqdqqdde iiXXiEiET )( '''' (2.1)

s

. (2.2)

)]([2

1.

semm DPPH

(2.3)

23

Where s is the synchronous speed, speed deviation, generator speed, mP is

mechanical power and eP is electrical power. Here the real power from the internal

source is exactly equal to the electrical torque across the air gap for this model. Then if

mechanical torque and electrical torque are balanced; the rotor will rotate at a constant

speed called synchronous speed.

D

s

0

Hs2

1

eT

mT

Figure 9: Mechanical block diagram of synchronous generator

A change in either of the torques will cause the speed variation. Another output of the

block is the rotor position. Consider any fixed point in the rotor, it will circulate in a

circle and hence the rotor position will change from 0 to 360 degree [32].

The rotor electrical block represents flux dynamics in the machine windings with

generator terminal voltage and current outputs and excitation system block compares

terminal voltage magnitude with a reference voltage and outputs field voltage. The

terminal mathematical representation for these two blocks is given as the following

equations (2.4-2.10) for both the input and output variables [36].

A

A

sT

K

1RsT1

1tV 1V

refV

fdE

Figure 10: Simplified model of ST1A excitation system

Generally, in all the thesis work the excitation voltage is considered IEEE ST1A

excitation system field voltage as can be given in equation (2.6) and Fig.10. The d and

24

q-axis voltages and field voltage of synchronous generator are also given as the

following.

]))([(1 ''

'

0

'

fddddq

d

q EiXXET

Edt

d (2.4)

)])([(1 ''

'

0

'

qqqd

q

d iXXET

Edt

d (2.5)

)][(1

fdtA

A

fd EVKT

Edt

d (2.6)

The terminal active and reactive power of generator identifies the capability of the

machine to feed the gird connected with it. Since any synchronous machine have direct

and quadrature axis, the terminal parameters of generator are also given in terms both

axis components. The terminal current, voltage and power are given as equation (2.7-

2.11).

qdt jVVV (2.7)

qdt jiiI (2.8)

Where the d-axis and q-axis currents are given as:

'

'

d

qq

dX

VEi

And

'

'

q

ddq

X

EVi

(2.9)

qqddt iViVP (2.10)

qddqt iViVQ (2.11)

The overall dynamic representation of the two-axis model showed in Fig.7 can be

summarized in the following complex circuit in terms of the internal voltage of the

generator and system voltage assuming a single machine is connected to infinite bus

[32].

vsjjjj

s

e

qdepe

e

qdds

e

qqdqd VjiijXRjiijXRjEiXXE

)

2()

2()

2(

))(())((])([ '''''

Where Vs stands for system voltage.

Practically, the science of generating power by synchronous generators and power

consumption in customer side in any power system is defined by set of inter-related

non-linear differential equations which are dependent in time. Before t=0, the system

will be assumed an equilibrium state and calculation of these states would be important

25

for power system stability analysis. So, the initial values of two-axis model have given

in Appendix A.5.

3.3 Characteristics of Synchronous Generator in Excitation Loss Event

The main electrical and mechanical quantities of the generator including voltage,

current and rotation speed will deviate from the related steady-state values during LOE

event. In loss of excitation, the synchronous apparent power of a generator falls off to

zero within a short time [5] [2] which causes a mismatch between the mechanical power

input and the electrical power output. Generator speed increases exponentially and

eventually reaches asynchronous condition. At this point, the mechanical power

produced by the turbine equates with the asynchronously developed electrical power

[25]. The mechanical power produced is given by the reference power-setting and the

static drop of the turbine-governor. The electrical power is determined by the

parameters of the equivalent circuit and the slip. In terms of operational impedance of

an alternator, the average asynchronous torque in LOE event is given as:

))(

1()

)(

1Re(

2 00

2

jsjXjjX

VT

qd

tas (2.12)

Where Vt is terminal voltage, Re indicates real part of the equation, s 0 is

synchronous speed. Thus, an LOE occurrence can end with a decrease in terminal

voltage and output reactive power, oscillate output current and active power, and

increase the generator speed and power angle (i.e. loss of synchronism) [18].

At normal condition a synchronous generator generates active power due to the shaft

or steam and reactive power due to excitation or field current as shown in Fig.8. At this

state, the synchronous machine delivers reactive power to the system connected with

it. However, if the excitation system fail by any means; the generator losses its

excitation and the Mega Volt Ampere Reactive (MVAR) delivering to the system will

suddenly stop. Since the generator continue generating Mega-Watt (MW) power due to

the shaft, its speed increases rapidly and it only remains in synchronism until the speed

of the generator exceeds the synchronous speed [2]. However, when the synchronous

speed becomes much less than the generator speed, a slip current starts to induce in the

rotor surface which establish magnetic field and the generator lose synchronism and

finally black out may attain if it is not protected early. Because of reactive power drawn

by the LOE generator, the system voltage is immediately reduced while the armature

26

current of the generator is increased [37]. Under such circumstances, disturbance in the

power system would take place.

When synchronous generators are subjected to excitation loss event, the accurate

calculation of the machine transient performance depends on the load, open-circuit

resistance and the saturation condition of their main flux paths. From equation (2.1-

2.11), we can re-write the internal voltage, terminal voltage, power and speed of the

generator in terms of excitation voltage and current as the following equations.

)11

(1

'

'

0

'

0

''

'

0

'

ddqa

d

fd

d

d

ddqddaq

q

qd

qdt iXiRsT

Ei

sT

XXjiXiRi

sT

XXjVVV

Whereqddaq

q

qd

d iXiRisT

XXV '

'

0

'

1

and

ddqa

d

fd

d

d

ddq iXiR

sT

Ei

sT

XXV '

'

0

'

0

'

11

(2.13)

And the internal voltage of the generator in terms of field voltage, d-axis and q-axis

voltages can be stated in equation (2.14).

)11

(1 '

0

'

0

'

'

0

'

''

d

fd

d

d

ddq

q

qd

qdisT

Ei

sT

XXji

sT

XXjEEe

(2.14)

(a)

(b)

Figure 11: Generator (a) Terminal and internal voltage (b) q-axis and d-axis voltage

in LOE event created at 1second

Thus, as can be seen from equation (2.13 and 2.14) and Fig.11a, the terminal and

internal voltage of the synchronous generator varies with generator field voltage

reduction. And in case of full loss of excitation since the field voltage is null which

indeed results in reducing of terminal and internal voltage of the synchronous machine.

Comparing with the terminal voltage of a generator, the internal voltage parameters of

0 1 2 3 4 5 6 70

0.5

1

1.5

Time

internal voltage(pu)

Terminal Voltage

Internal Voltage

0 1 2 3 4 5 6 7-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Time

q-axis voltage(pu)

d-axis voltage

q-axis voltage

27

the machine responses faster especial the q-axis voltage of the generator since the q-

axis voltage is the voltage which is proportional to field flux linkage as it have been

stated in equation(2.4) and Fig.11b. This indicates that the terminal parameters of the

generator are composted by the condition of the gird connected with the synchronous

machine since terminal voltage is the voltage delivered to the whole system.

As can be observed from Fig.12, the d-axis current increase and q-axis current remains

constant till the generator instability happen in LOE event. Thus, the active power and

reactive power can be rewritten as equations (2.15 and 2.17) using the d-axis and q-axis

currents. From equation (2.15-2.17), the reactive power will indeed reduce to negative

so does the reality in LOE event generator consume reactive power from the system.

And the active power almost remains constant till loss of synchronism. A change in the

real power demand affects essentially the frequency whereas a change in the reactive

power affects mainly the voltage magnitude and so mainly excitation loss affects

voltage parameters. If the faulted generator did not isolated from system, a loss of

synchronism will happen when the system weaken to feed the generator. The

asynchronous value of active and reactive power can be given as equation (2.16) and

(2.18) in a workable expression by avoiding sub-transient time constants for two-axis

model.

(a) (b)

Figure 12: Generator (a) Active and reactive power (b) d-axis and q-axis currents in

LOE event

2

'

0

'

'

0

'

0

'

)1

()1

)1

(( q

q

qd

d

d

fd

d

d

ddt i

sT

XXi

sT

Ei

sT

XXQ

(2.15)

0 1 2 3 4 5 6 7-1

-0.5

0

0.5

1

1.5

Time

<Output active power Peo (pu)>

Reactive Power

Active Power

0 1 2 3 4 5 6 7-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time

<Stator current iq (pu)>

d-axis current

q-axis current

28

])(1

)()

11[(

2 2'

2'

'

2

d

d

dd

tsT

sT

XX

VQ

(2.16)

dq

q

qd

q

d

fd

d

d

dddq

q

qd

t iisT

XXi

sT

Ei

sT

XXii

sT

XXP )

1()

11(

1 '

0

'

'

0

'

0

'

'

0

'

(2.17)

])(1

)11

[(2

)1(2'

'

'

2

d

d

dd

tsT

T

XX

VssP

(2.18)

Fig.12a illustrates the active and reactive power variation in LOE event. If the system

is able to feed the excitation system a reactive power, the generator’s parameter

remains in synchronism. However, the moment the system reaches the maximum limit

of reactive power delivering, the whole system lose synchronism. In this simulation as

can be shown in Fig.12, LOE event is created in 1second and the generator loses

synchronism after 4.23second, until this limit the active power of the generator is not

almost affected since it is the power generated due to the shaft of the machine.

qqd

dda

q

dda

iXE

ViRViR''

(2.19)

Where the d-axis and q-axis induced stator flux linkages are given as:

qqdq iXE '' And ddqd iXE '' (2.20)

(a)

(b)

Figure 13: Generator (a) Rotor speed (b) load angle in LOE event at 1second

The speed of the generator rises under loss of excitation event as shown in Fig.13a.

When the speed of the generator increases above the synchronous speed, the machine

will act like an induction generator which induces rotor surface slip currents. This is

because the speed of a rotating magnetic field is proportional to the frequency of

excitation current. Since the shaft of the generator keeps generating MW power, the

load angle 𝜃 will increase as well. The maximum mechanical power can increase until

0 1 2 3 4 5 6 70.95

1

1.05

1.1

1.15

1.2

1.25

Time

<Rotor speed wm (pu)>

Rotor speed

0 1 2 3 4 5 6 7-200

-150

-100

-50

0

50

100

150

200

Time

<Load angle delta (deg)>

Load angle

29

the load angle reaches 𝜃=90◦, after that the mechanical power will be greater than the

electrical power and the generator will loss equilibrium point between mechanical

power input and electrical power output. For the same reason, when the mechanical

power input is fixed and maximum electrical power output increase, the load angle will

increase as the intersection of mechanical power and electrical power moves up to the

peak. Generally, high increase in load angle leads to loss of stability and pole slipping

and turbine would suddenly go into over speed with AC current flowing in the rotor.

Fig.13b. shows variation of load angle with excitation loss. In LOE event, load angle

increases and 4second after the failure the load angle raises further to more than 90◦and

the generator becomes unstable.

Voltage in excitation loss depresses to such extent under voltage relays may sense it.

This reduces the terminal impedance of a generator which can be expressed in terms

of terminal voltage and current as equation (2.21).

22

2

22

2

tt

t

tt

t

QP

QVj

QP

PVjXR

I

VZ

(2.21)

Where 22

2

tt

t

QP

PVR

and

22

2

tt

t

QP

QVX

Thus, when LOE occurred, resistance of the generator declines to zero gradually

proportional to terminal voltage decay since the active power is almost unaffected by

excitation loss until the generator loss synchronism. On the other hand the terminal

reactance decreases to negative value proportional as the reactive power decline as can

be shown in Fig.14. This results in terminal impedance reduction in excitation loss

event as given in Fig.14b.

(a) (b)

Figure 14: Generator terminal (a) resistance and reactance (b) impedance in LOE

event

0 1 2 3 4 5 6 7-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Time

Terminal Resistance

Terminal Reactance

0 1 2 3 4 5 6 70

0.2

0.4

0.6

0.8

1

1.2

1.4

Time

Terminal Impedance(pu)

Terminal Impedance

30

3.3.1 Initial Loading Effect

At normal state synchronous generator is expected to run at constant value of voltage,

limited between 95% and 100% of rated voltage. However, the variation of loading

condition results generator voltage to vary. During periods of light loads, the voltage

drop is minimal through system parts such as transformers and lines. When loading

condition increases, voltage drop increases. Initial load of synchronous generator is the

ability of the generator to produce active power to the system in normal operation.

Thus, to operate in stability the loading effect of the generator is also inter-related with

the initial field voltage. The variation in active power production ability of the

generator directly affects the initial field voltage and the initial mechanical power of

the generator. The initial mechanical power declines proportionally with the initial load

reduction as the mechanical power is equal with initial electrical power as can be shown

in equation (2.22). Thus, the influence of excitation loss decreases as the initial load

decrease. When the generator operates in light load, the duration of generator instability

during loss of excitation is much longer than in full load as the amount of reactive

power needed to excite the generator is less in value compare to full load. Thus, the

reactive power consumption in light load is so slow.

0000000 )()( qqaqddadem iiRViiRVPp (2.22)

0

''

00 )( dddqfd iXXEE (2.23)

Fig.15 shows the variation of generator parameters in full, medium and light load

conditions in LOE event created at 1second. The generator parameters vary in

magnitude relative to initial load variation. However, the reactive power reduction in

excitation loss event in light load (30%Peo) is much slower than in heavy load as can

be observed in Fig.15(c).

On the same generator and at similar excitation loss event, the duration taken to reach

reactive power consumption margin from the system differs when the loading effect

changed. In full load condition, the generator was able to consume reactive power from

the system for about 4.23second but for medium and light load conditions it takes

14second and 30.5second to loss generator stability after loss of excitation.

From the above simulations, we can see that excitation loss does not only affect the

faulted generator itself, but also affect the entire system. Since the LOE generator tries

to absorb large reactive power, the rest of the system has to produce heavy MVAR to

make up for the additional reactive power demand. If the rest of the system cannot

31

provide the desired amount of reactive power, the excitation loss condition can then

degenerate into a voltage collapse.

Field voltage

Reactive power

Terminal voltage

Rotor speed

Figure 15: Generator parameter variation in LOE event at various generator loading

conditions

Figure 16: IEEE 9 bus system G-1parameters in G-2 LOE event

0 2 4 6 8 10 12 14 160

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Time sec

Fie

ld V

oltage P

u

Vf in Full load

Vf in Medium load

Vf in Light load

0 5 10 15 20 25 30 35-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

Time sec

Genera

tor

term

inal R

eactive p

ow

er

Pu

<Output reactive power Qeo (pu)>

Qt in Full load

in Medium load

in Light load

0 5 10 15 20 25 30 350.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

Time sec

Term

inal voltage P

u

vt in Full load

in Medium load

in Light load

0 5 10 15 20 25 30 35 400.95

1

1.05

1.1

1.15

1.2

1.25

Time sec

Roto

r speed P

u


Speed in Full load

in Medium load

in Light load

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5

3

3.5

time sec

<Output active power Peo (pu)>

G-1 Field voltage

Reactive power

Active power

32

When the reactive power of G-2 of IEEE 9-bus test system lessen because of excitation

loss, the reactive power and field voltage of other generators on the system starts to

increases in value to compensate the loss and to keep the system in stability. But the

active power reduce in value so does bus voltages. If faulted generator was not

disconnected from the grid it also causes swings on the remaining machines as can be

showed in Fig.16.

3.4 Excitation Loss Protection Relay

In this work, the actual excitation loss detection method in power system industries

which is the impedance protection relay has used to study excitation failures in different

power system performances. This method of excitation loss protection is based on the

calculation of the impedance at generator terminals. As stated above, there are different

methods that use generator terminal impedance for loss of excitation protections; a one

circle mho relay proposed by Mason, two circle off-set mho relay proposed by Berdy

and two circle off-set mho relay with directional element and others. In modern power

system commercial industries, Berdy method is the most popular and in this thesis work

this method will be investigated in different power system conditions. The main

indicator in capturing the probability of excitation failure is the significant flow of

reactive power into the generator. This should be captured by the relay. The two circle

off-set mho relays have two protection circle zones plotted on the R-X plane of the

terminal impedance as can be observed in Fig.17. The protection zones are positioned

in the negative reactance coordinate of the R-X plane with offset value X’d/2 and with

circle zones of 1pu and Xd for zone-1 and zone-2 respectively.

As can be seen from equation (2.21) and Fig.17, in normal operating condition, the

generator generate and deliver active and reactive power to the system which means

both R and X are positive and the terminal impedance is located in the first quadrant in

R-X plane. When LOE event happened, the generator starts to draw reactive power

from the system and reactance becomes negative from the relay point of view. As a

result, the terminal impedance trajectory in R-X plane moves to the forth quadrant and

the endpoint of terminal impedance ranges between the sub transient reactance and

synchronous d-axis reactance. The endpoints depend on the initial load and type of

excitation loss cause as shown in figure below. The amount of time delay used with the

large setting should be the minimum time required to ride over transient conditions. A

time delay of 0.5 or 0.6second appears to be sufficient to ride over stable transient

33

wings. While there is no data available on the transient performance of voltage

regulators, it would appear that 1second external time delay should prevent undesired

operation due to voltage regulator undershoot [20]. In either case, when selecting a time

delay the user should determine the effect of the time delay on possible generator

damage and on the overall operation of the system.

Figure 17: Impedance trajectory of LOE relay in different system conditions

It should be noted that even in the case of a lightly loaded generator, a loss of excitation

can cause a considerable VAR drain from the system. A prolonged VAR drain may

cause the tripping of transmission lines and general system instability. As a smaller

mod zone one reach, the trajectory of the impedance entry time for the altered zone one

is higher than entry time for the initial zone two reach. These lead the usage of a typical

original zone two time delay of 1second may be insufficient to trip the generator before

the synchronous loss is taking place. A power swing conditions are selected to control

and enhance the performance of the protection scheme in steady power swing [20].

Thus, the relay is set with a time delay to distinguish between a recoverable swings and

an LOE condition for zone-2 0.5 and 1second delay have been suggested to send alarm

and trip signals consequently. But for zone-1no delay for trip signal have been

suggested.

34

Measure

Generator

terminal

parameters

Fast Fourier

Transform

R-X loci r enter

zone-1

No

Calculate R

and X

R-X loci

enter zone-

2&wait

for

1sec

LOE event trip G

No

Yes

Yes

SPS or normal

state

Figure 18: Flow chart of LOE relay

35

CHAPTER 4

SIMULATION RESULTS AND DISCUSSIONS

In this chapter, the excitation loss detection relay will be scrutinised on different causes

of excitation loss and power system disturbances. Excitation loss of generator can be

initiated through field winding short or open circuit, poor brush contact, AVR control

failure, AC voltage loss to exciter, main circuit breaker failure or slip ring flash over

[29]. Despite the cause of excitation loss, either the field voltage or field current reduce

in value or to zero depend on the type of excitation loss cause or sever of the failure.

Some of the causes of LOE have similar characteristics such as AVR failure have

similar characteristics with field winding short circuit which introduce reduction of

field voltage to zero. Accuracy of excitation loss relay to detect these causes of

excitation loss will be study on different initial loading conditions of the generator.

Figure 19: Simulink model of IEEE 9-bus system with LOE event and LOE relay

In the remaining sub-topics, excitation loss events will be study on generator two (G-

2) of IEEE 9-bus test system shown in Fig.19 except for effect of excitation loss on

parallel connected generators which will study on generator one (G-1) of Tana Beles-

I. Parameters of the generators under study is given in table1. And the required machine

data, transmission line and load data are given in Appendix B. Also, in all the

simulation scenarios of the remaining sub topics, the LOE event and system

disturbances are assumed initiating at time t=1second. The general lay out of this

chapter can summarized as the following four sub-topics.

36

Study main causes of full excitation loss

Partial loss of field voltage

LOE relay reaction on power swings and

Proposed back up protection

4.1 Full Loss of Excitation

Generator loses its excitation completely when the field voltage or field current

supplied to the synchronous generator from the excitation system is totally lost and the

excitation system fails to excite the synchronous generator completely. In this

condition the synchronous generator is able to produce active power due to the

mechanical input but it completely stops producing reactive power. Full loss of

excitation is initiated either due to field winding failure, main circuit breaker between

excitation system and generator failure or sudden AC voltage loss to exciter.

4.1.1 Field Winding Short Circuit

A short circuit on the exciter field winding is the most common type of excitation

system fault. In field winding short circuit the field voltage literally decline to zero as

shown in Fig.20a. But the field current remains high and it is able to swing when the

generator lose synchronism due to self-exciting excitation system is used and field

current is dependent on parameters of generator as given in equations (2.6), (2.23) and

(2.24).

Figure 20: (a) G-2 excitation current and voltage (b) rotor speed and reactive power

in field winding short circuit

Fig.20b shows response of generator parameters when field winding short circuit

happens at 1second. Region-(a) is the first stage of excitation loss event and the

synchronous generator starts to consume reactive power from the system. In this stage,

the system is capable of feeding reactive power to the generator and the generator

(a)

(b)

0 1 2 3 4 5 6 7-5

-4

-3

-2

-1

0

1

2

3

4

5

Time sec

Fie

ld V

olt

age a

nd c

urr

ent

Pu

<Field current ifd (pu)>

Field voltag

Field current

0 1 2 3 4 5 6 7 8-1

-0.5

0

0.5

1

1.5

Time


Rotor speed

Reactive power

LOE

a

b

37

remains in synchronism. The reactive power amplitude consumed by the generator

could be as large as 1.56 times rated power of the generator which indicates the

machine is trying to run the faulted generator and to keep the system stable. On the

second stage shown point (b), the system reaches the maximum limit of reactive power

delivering and the generator speed rises rapidly beyond synchronous speed due to

power imbalance (excessive mechanical power of turbine). In this stage, the relation

between the stator and rotor is already very week due to under-excitation limit is

achieved. And over speed of the generator consequences the generator to loss

synchronism and bring a huge damage to the system.

The power flow direction of synchronous machine can be summarized in the four

quadrature as given in Fig.21. If the synchronous machine deliver both active and

reactive power to the system, it acts as synchronous generator and the impedance

trajectory remains in quadrant-1. However, if the machine starts to receive reactive

power from the system while delivering active power, it is acting as induction generator

and the impedance trajectory R-X loci lays on the fourth quadrant. The same for

synchronous motor principles as stated in the figure. So, in normal condition of

synchronous generator the tip of the impedance trajectory looking from the generator

terminals should remain within the first quadrant and both resistance and reactance

should be positive.

+jX+MVAR

+jR

+MW

-MVAR

-MW

-jX

-R

P

Q

Machine acts as

synchronous

Generator

Q-I

Machine acts as

an Induction

Generator

PQ

Q-IVMachine acts as

an Induction

Motor

Q

Q

P

P

Machine acts as

Synchronous

Motor

Q-III

Q-II

Figure 21: Power flow direction of synchronous machines

But on occurrence of excitation loss, the reactance becomes negative and resistance

decrease in value. So, the impedance trajectory starts moving toward to the fourth

38

quadrant and is expected to enter the protection zone of excitation loss relay to protect

the whole system from a possibly damage that can outcome due to the faulted

generator. At this stage the relay will sense the fault and will provide the proper signal

to trip or alarm after a pre-set time delay.

Fig.22 shows the impedance trajectory of G-2 for field winding short circuit in different

loading effects; heavy load, medium load and light load conditions. In this thesis work

the normal rated power is considered as heavy load, 50% and 30% of rated power as

medium load and light loading conditions correspondingly. In heavy load condition the

fault detected at 4.16second through zone-1 since the first zone of impedance protection

is modelled to protect heavy loaded generators. On the other hand, when the loading

effect starts to decrease, zone-2 is capable of detecting the LOE event with a reasonable

time delay.

Heavy load

Medium load

Light load

Tripping signal

Figure 22: G-2 impedance trajectory in field winding short circuit

For 80% initial load, the impedance trajectory of G-2 enters zone-2 and zone-1 at

4.165sand 6.415second respectively. Zone-2 sends an alarm signal after 0.5 second.

Though, the tripping signal initiates through zone 2 after 1second at 5.165second.

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

Impedance Trajectory in heavy load

in 80% load

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

Impedance Trajectory in 60% load

in 50%load

-1 0 1 2 3 4

-3

-2

-1

0

1

2

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 30%load

0 1 2 3 4 5 6 7 8 90

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Time sec

Tripin

g S

ignal


Triping signal in Heavy load

Triping signal in medium load

Triping signal in light load

39

As the loading effect of the generator decrease, detection duration increases. In medium

load the event is detected after 5.804second through zone-2. On the same manner, in

light load condition, the generator delivers 30 % of the rating power, and field winding

short circuit is detected after 6.286second. But, the impedance trajectory enters zone-1

after 13.43second which is much slower than in heavy load case.

It can be understood that the detection duration of LOE relay in field winding short

circuit increases as the initial load of the generator decrease but still zone-2 plays a

great role in detection of light loaded generators in short period of time compare to

zone-1. And LOE relay detection ability have shown a great response in field winding

short circuit for different loading conditions. The remaining studies of field winding

short circuit have summarized in table3 and Appendix A1 with required time reaching

of both protection zones.

4.1.2 Sudden Main Circuit Breaker Failure

When the main circuit breaker between the exciter and generator opened, the exciter

and the generator totally isolated and the generator loses its field voltage totally. In this

case, the exciter is at normal state but due to the main circuit breaker between exciter

and synchronous generator fail the field voltage entering to the generator is reduced to

zero as shown in Fig. 23. And this results in complete excitation loss in generator. The

reduction of generator field voltage to null let this cause of excitation loss to have

similar characteristics to field winding short circuit as can be observed in Fig.20a and

Fig.23.Similarly to field winding short circuit, field current is also able to swing after

generator lose synchronism since it is dependent on generator parameters.

Figure 23: G-2 field voltage and current in main CB failure

Fig.24 shows the LOE relay detection ability when the main circuit breaker between

the exciter and the generator fail at 1second. Before fault happen the generator was

0 1 2 3 4 5 6 7-5

-4

-3

-2

-1

0

1

2

3

4

5

Time sec

Fie

ld V

olt

age a

nd c

urr

ent

Pu


Field voltag

Field current

40

delivering 0.884pu active power and 0.63pu reactive power in heavy load condition.

Again the heavy load condition is detected through zone-1 after 4.16second the same

to field winding short circuit. For 60% and 40% loading effect the failure have been

detected after 5.575second and 6.286second after the event happen through zone-2

respectively. To avoid unnecessary repetitive actions, the excitation loss causes that

have similarity with field winding short circuit have been eliminated.

Figure 24: G-2 impedance trajectory in main CB failure

4.1.3 Sudden Loss of AC Voltage to Excitation System

If the AC voltage that run the exciter interrupted by any means, the excitation system

and also the generator connected with the exciter will be faulted. In this work self-

excited static exciter is used, so sudden loss of AC voltage to the exciter is studied

when the generator terminal voltage interning to ST1A excitation system and the

exciter are isolated. Thus, AC voltage loss to the exciter also results in full loss of

excitation in generator as can be shown in Fig.25 below. Complementary to other

causes of excitation loss, excitation system got support from the other components of

Heavy load

Medium load

Light load

Tripping signal

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 80%load

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 50%load

-1 0 1 2 3 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 30%load

0 1 2 3 4 5 6 7 8 90

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Time sec

Tripin

g S

ignal





41

the exciter in sudden AC voltage loss to exciter since the field voltage is dependent on

the initial value of field voltage and the terminal voltage of the generator as have been

stated in equation (2.6).

Figure 25: G-2 field voltage and terminal impedance in sudden loss of AC voltage to

exciter terminal

Heavy load

Medium load

Light load

Tripping signal

Figure 26: G-2 impedance trajectory in sudden AC voltage loss to exciter

0 1 2 3 4 5 6 7 8-0.5

0

0.5

1

1.5

2

2.5

Time


Field voltage

Terminal impedance

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 80%load-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 50%load

-1 0 1 2 3 4

-3

-2

-1

0

1

2

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 30%load

0 1 2 3 4 5 6 7 8 90

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Time sec

Tripin

g S

ignal





42

For simulation, let the excitation system of G-2 suddenly loss AC voltage at 1second,

and the characteristic of LOE relay on G-2 have been observed in Fig.26 below. The

relay detects the event at 4.223second through zone-1 in heavy load condition which

is 0.63second longer than the above causes.

For medium load conditions, the failure is detected after 5.878second for 50% loading

and 5.664second for 60% loading effect through zone-2. Similarly to the above cases,

in light load condition, the event is detected slowly at 6.348second after fault happened.

Comparing to the above causes of excitation loss, sudden AC voltage loss to exciter is

detected slowly. The remaining simulation results have been summarized in table2 in

different initial loading conditions.

4.1.4 Field Winding Open Circuit

In field winding open circuit, field current is terminated to null. It is associated with

inserting of an infinite discharge resistance which tends to reduce the field current to

almost null from the relationship of equation (2.24) below [2] [12]. This is the worst

cause of excitation loss event that generator lose synchronism in fraction of

microseconds in any loading conditions. As can be observed from Fig.28, comparing

the courses of generator values during loss of excitation in field winding open circuit,

system reactive power feeding limit is reached at 0.06second after fault which is almost

4seconds before in other causes of excitation loss.

fd

fd

fd

d

adufd

fd

R

EI

T

XXR

'

0 (2.24)

Despite the faster response of generator parameters in field winding open circuit, by

comparing Fig.17 and 28, it can be noticed that the peak values of active and reactive

power are smaller in the open-circuit case. From Fig.28, it can be seen that after the

fault is initiated the seen impedance by the relay moves from quadrant-one toward the

fourth-quadrant of the R_X plane and then moves back to the third-quadrant. And this

movement increases when the initial loading of the generator decreases.

LOE relay detect field winding open circuit in 0.0835second for heavy load,

0.0467second for medium and 1.131second for light initial load conditions. As shown

in table2 the impedance trajectory in field winding open-circuit fault enters the

protection zones much faster than other excitation causes in any loading condition

43

which create overvoltage in the machine. A crowbar system is proposed to change

disconnection of field winding to field winding short circuit [18]. Every time field

disconnection happen the crowbar will be activated to avoid system damage due to

overvoltage. But, without crowbar, LOE relay detects the failure in less than 1.2second

in any event which is about 5second faster than other causes of LOE event.

Figure 27 : G-2 Field voltage and field current in field winding open circuit [

Figure 28: G-2 impedance trajectory in field winding open circuit

Table 2 summarizes the detection ability of LOE relay in all main causes of full

excitation loss in different initial loading conditions. From all the causes, field winding

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5-0.5

0

0.5

1

1.5

2

Time


Field voltage

Field current

Heavy load

Medium load

Light load

MVAR and speed

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 80%load

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 50%load

-1 0 1 2 3 4 5-4

-3

-2

-1

0

1

2

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 30%load

0 1 2 3 4 5 6 7-1

-0.5

0

0.5

1

1.5

Time sec

Q,w

m p

u


G-2 Reactive power

Rotor Speed

44

open circuit is the worst cause of excitation loss that the relay detects the fault in every

loading effect in short period of time comparing to others. And the remaining cause of

full LOE event; field winding short circuits, sudden loss of AC voltage to the exciter

and main circuit breaker failure have the same characteristics that the field voltage is

null and LOE relay detection ability is almost similar with micro and millisecond

differences. And it can also observe that, impedance protection of excitation loss is

highly dependent on initial generation ability of the generator. But, impedance

protection of excitation loss (ANSI 40 relay) have detect full loss of excitation in less

than 6.5second in all loading conditions.

Table 2: LOE relay detection ability in main causes of full excitation loss

4.2 Partial Loss of Excitation

Partial loss of excitation is happen when field winding voltage of the generator decrease

in value by any reason. In light load case as shown in table 3, the effect of PLOE may

not be visible by the relay and in some cases the system still remain in stability

especially until reduction of 30% of the field voltage. But in heavy loaded generators

it may cause severe damages as much as full loss of excitation. In addition to the PLOE

causes introduced above, AVR action in system disturbances is also the most common

Initial Loading(in

pu)

Type of LOE and Generator tripping

time(second)

FW

short

circuit

AC

voltage

loss

Main

CB

failed

FW open

circuit

0.8485+j0.06307 4.160 4.223 4.160 0.0835

0.7604+j0.04781 4.961 5.027 4.961 0.06485

0.6792+j0.03679 5.165 5.236 5.165 0.05115

0.5943+j0.02962 5.355 5.426 5.355 0.04555

0.5094+j0.0268 5.575 5.664 5.575 0.0481

0.4245+j0.03389 5.804 5.878 5.804 0.0467

0.3396+j0.03424 5.773 5.853 5.773 0.0341

0.2547+j0.3 6.286 6.348 6.286 1.131

45

reason of partial excitation loss. In PLOE, the filed voltage does not subject to null, so

there will be some reactive power generation but not enough to feed the system so the

generator still consumes reactive power from the system even if that is slower than full

loss of excitation. From equations (2.5-2.10) the internal voltage, terminal voltage,

generator impedance and reactive power reduced proportional to the field voltage

reduction as can be observed in Fig.30. The detection ability of LOE relay will be

studied for different percentage losses of field voltage starting from the worst partial

loss which is decrease Efd by 1.6497pu till the least loss 0.1833pu Efd reduction as can

be scrutinized in table 3.

Fig.30 summarizes characteristics of synchronous generator on 30%, 50% and 70%

loss of field voltage in heavy load condition. In all cases, the partial loss of field voltage

is happened at 1second and the normal field voltage value of G-2 right before partial

loss is 1.833pu. When the field voltage reduced to 30% of its normal value, the system

is able to feed reactive power to the synchronous generator for about 15second after

fault happen. On the other hand, when the generator lost 50% and 70% of the field

voltage, the synchronous generator was able to consume reactive power from the

system for about 9second and 7second respectively without loss of synchronism.

During partial loss of excitation, the generator can operate for longer time with

synchronism compared to the complete loss of excitation. However, both full and

partial loss of excitation on the same initial condition reaches the required value

decrement as can be compared Fig.17 and Fig.30. For both cases in heavy load, the

generator reactive power diminishes until -0.413pu before loss of synchronism. Thus,

the field voltage reduction only affects the reactive power consumption duration and

variable changing rate of the machine and the initial condition determines the generator

output during loss of excitation.

Figure 29: Reactive power of G-2 in 70%Efd loss

0 5 10 15 20 25-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

time sec

MV

AR

pu


Reactive power in medium load

in light load

46

In case of medium and light load conditions, the system will remain in stability without

losing synchronism for about 18 and 24.6second respectively as can be shown in Fig.29

for 70% of field voltage. Generally, effect of partial excitation loss in variation rate of

generator terminal parameters highly depends on initial loading of the generator. Thus,

the slow terminal impedance decrement leads slow operation of excitation loss relay in

generator partial field voltage loss.

Field voltage

Reactive power

Terminal impedance

Rotor speed

Figure 30: G-2 parameter variation in partial loss excitation

4.2.1 30% Field Voltage Loss

In this case, the field voltage is forced to reduce 30% of its initial value. This may threat

the stability of system but except for heavy loaded generators the reduction is not

visible by LOE relay. Fig.31 shows the impedance trajectory of G-2 in 30% field

voltage loss. The field voltage reduction in pu for all the three conditions have given

in table3 below. The LOE relay successfully detects the 30% diminishing of field

voltage for heavy load after 15.210second.

0 1 2 3 4 5 6 7 8 9 100.5

1

1.5

2

Time

Fie

ld v

olta

ge

30% field voltage loss

50%field voltage loss


0 2 4 6 8 10 12 14 16 18-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

Time

Re

active

po

we

r





0 2 4 6 8 10 12 14 16 180

0.2

0.4

0.6

0.8

1

1.2

1.4

Time

Te

rmin

al im

pe

da

nce





0 2 4 6 8 10 12 14 16 180.95

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

Time

Ro

tor

sp

ee

d





47

Figure 31: G-2 impedance trajectory in 30% field voltage loss


In 50% field voltage loss, the field voltage reduces to 0.9856pu in heavy load, 0.4545pu

in medium load and 0.2232pu in light load cases.

Heavy load

Medium load

Light load

Tripping signal(Heavy and

50%loading)

Figure 32: G-2 impedance trajectory in 50% loss of excitation

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Re

acta

nce

(X

) P

u

Zone-1

Zone-2

Impedance Trajectory in Full load

in Medium load

in Light load

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 80%load

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 50%load

-1 0 1 2 3 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 30%load

0 2 4 6 8 10 12 14 16 180

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Time sec

Tripin

g S

ignal




48

Again the remaining percentage reduction values are given in table3. Fig.32 shows the

impedance trajectory of G-2 in different initial percentage load for 50% field voltage

loss. If the excitation loss generator did not isolated from the system the interconnected

generator keeps generating more reactive power and a voltage instability in the system

may lead to voltage collapse and to more severe instabilities. Thus excitation loss event

should be detect at any condition in short period of time.

Reduction half of field voltage remains un-detected for 24.15second in medium load

and not detected at all in light condition. This threat system stability and may lead to

blackout of the whole system. But in heavy load the field voltage reduction is detected

after 8.563second.


In 70% field voltage loss, as observed in Fig.33, the field voltage reduction is detected

5.913, 9.242 and 13.41second for heavy, medium and light load conditions

respectively.

Heavy load

Medium load

Light load Tripping signal

Figure 33: G-2 impedance trajectory in 70% loss of excitation

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Re

acta

nce

(X

) P

u

Zone-1

Zone-2


in 80%load

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 50%load

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


in 30%load

0 5 10 15 20 250

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Time sec

Tripin

g S

ignal





49

Thus, the relay detect the case in less than 13second in all loading conditions. However,

the stability condition of the system is threaten highly in this duration. The system

voltage is reduced to about 0.58pu in 13second which may lead to system instability.

Even if the relay detect partial loss of excitation, instability of the system should be

also the main concern to avoid unwanted blackouts and component damages.

The field voltage reduction tests showed that in some cases the impedance trajectory

does not enter the protecting zones of the relay since the remaining field voltage allows

the defective machine and the system to find a new operating point and we can see that

under the same partial field voltage loss, the system response is much different in

various initial loading effects. From above simulations, we can see that decreasing the

initial load of the synchronous generator will give the LOE relay more time before the

system collapses.

Table 3: LOE relay detection ability in partial loss of excitation in different

loading conditions

It can be conclude that the relay have a good performance in detecting field voltage

loss greater than half of the rated field voltage as have been shown in table3 and

Appendix A2. On the other hand, for partial excitation loss less than half of the rated

field voltage of the synchronous generator, it fails detecting field voltage diminishing

except for heavily loaded generators.

Initial Loading(in

pu)

Partial Loss of Excitation(% Efd loss) and G-tripping duration

[Y(sec)/N]

30% 40% 50% 60% 70% 80% 90%

0.8485+j0.06307 15.210 10.940 8.563 6.99 5.913 5.153 4.592

0.7604+j0.04781 18.210 12.51 9.569 7.878 6.816 6.077 5.493

0.6792+j0.03679 22.29 14.100 10.48 8.426 7.161 6.296 5.659

0.5943+j0.02962 N 16.64 11.81 11.81 7.681 6.657 5.918

0.5094+j0.0268 N 21.33 13.74 10.31 8.341 7.094 6.223

0.4245+j0.03389 N N 17.19 11.95 9.242 7.632 6.569

0.3396+j0.03424 N N 24.15 13.88 10.04 7.925 6.644

0.2547+j0.3 N N N 23.57 13.41 9.354 7.35

50

From the above simulations, we can see that partial or full excitation loss have much

different system responses in different initial loading conditions. The detection duration

is highly dependent on the type of excitation loss and scheduled MW output of the

generator. The duration varies inversely with the output active power. The less MW

output, there is more time before system collapse and detected by LOE relay and vice

versa. And comparing full and partial excitation loss, partial field voltage loss is

detected much slower than full excitation loss. LOE relay detect all the scenarios of

FLOE but fails to detect field voltage reduction less than half of the rated field voltage

in lightly loaded synchronous generators as have summarised in table3.

4.3 Effect of Excitation Loss on Parallel Connected Generators

Generators are rarely used in isolated situations, more commonly they are used in

parallel. In parallel connection of generators, a number of identical machines share

their power to one bus. But, to connect a number of generators in parallel, rms line

voltages and phase sequence, phase angle of corresponding phases and frequency of

the generators must be the same. In this section, Tana Beles-I power plant from

Ethiopian Power System is used to study LOE event on parallel connected generators.

This power plant contains four identical generators connected at one common bus as

shown in single line diagram in Fig.3 and Simulink model in Fig.34. As was presented

above in the discussions, if generator lost excitation it changes into significant

consumer of reactive power. If this generator is not removed, the adjacent generators

start increasing the production of reactive power up to the limit when their limiters of

rotor and stator currents act [18] [28] as can be shown in Fig.35b. This results in voltage

drop in the remaining generator terminals, but only the generator which loss excitation

G-1 absorbs reactive power. In field winding open and short circuit conditions, the

impedance trajectory of G-1 and G-2 have given in Fig.36. The trajectory of G-1 for

both failures enter the protection zones of the relay at 4.659second and 0.0731second

respectively.

On the other hand, despite the voltage drop of the other three generators the impedance

trajectories of remaining three generators for both scenarios remain outside of the

operation zone of the relay. In strong system condition of parallel connected generators

excitation loss only affects the impedance variation of the faulted generator. And the

strong help of MVAR of the remaining generators help their impedance trajectories to

stay out of the protection zones.

51

Figure 34: Simulink model of Tana Beles-1 power plant

(a)

(b)

Figure 35: Tana Beles-I (a) G-1 and (b) G-2 parameters in G-1 LOE event

The worst condition on interconnected generators have been found when LOE event

and SPS event created on the same time. In this case for both field winding short and

open circuit with SPS event have simulated at 1second. SPS event is created by three-

phase short circuit fault at the common bus cleared after 150ms by opening the circuit

breakers from the receiving and sending end. From the simulation result, it have shown

that field winding short circuit and SPS event on G-1 still keeps the impedance

trajectories of the other three generators outside the protection zones of the relay. This

is due to slow generator parameter reduction in field winding short circuit than in SPS

event so the system is able re-stabilize the system through protective devices.

0 1 2 3 4 5 6 7-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

Time


G-1 Reactive power

Active power

Terminal voltage

Field current

0 1 2 3 4 5 6 7-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Time


G-2 Reactive power

Active power

Terminal voltage

Field current

52

Figure 36: Tana Beles-I G-1 and G-2 impedance trajectories

However, in field winding open circuit and SPS event created at the same time both

failures jeopardize the stability of the system in very short period of time, the nearby

generator G-2 impedance trajectory was able to enter the protection zones of the relay

in 1.285second even if the generator excitation system remains at normal state as

shown in Fig.37b. Thus, in parallel connected generator, field winding open circuit

with SPS condition have affected the impedance trajectories of the remaining

generators.

Figure 37: Tana Beles-I G-1 and G-2 impedance trajectories in field winding (a)

short circuit with SPS (b) open with SPS

4.4 Power Swings

Power swing is the oscillation of machine rotor angle due to power system disturbances

like a fault, generator or line outages and load propagation that alters the mechanical

equilibrium of one or more machines. Power swings can be either stable or unstable

Field winding short circuit

Field winding open circuit

(a)

(b)

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

Impedance Trajectory of G-1

Impedance T. of G-2

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


Impedance T. of G-2

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


Impedance T. of G-2

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


Impedance T. of G-2

53

depend on severity of disturbances and the action of protective devices. It is stable

when the rotation speed of the machine returns back to synchronous speed and it is

unstable when the machine or part of the system loses synchronism after a major

disturbance. Mostly unstable swings are caused due to prolonged disturbances. System

failures directly affect the terminal voltage of the generator and as a result all the

behaviour of the system. Thus, the excitation system compromises the terminal voltage

to keep the system in stable through voltage transducer and load compensator which is

the part of excitation system that sense and compare the generator terminal voltage

with a pre-defined reference point. Large power swings can cause unwanted relay

operations that can further aggravate the power system disturbance and results outage

in the system [38]. In spite of the excellent performance of LOE relay in most system

disturbances, the concern that the relay might operate incorrectly during stable swings

has persisted. In view of this concern, an investigation was made to determine the

proximity of stable swings to the relay characteristic. During system disturbance, the

generator impedance trajectory may enter the LOE protection zones and stays for a

short time. If the maximum waiting time exceeds tripping time delay of LOE

protection, the relay will mal-operate. Faults cleared in less time than the critical

clearing time are stable and prolonged faults are unstable. In some cases of short-circuit

faults, the impedance at generator terminal does reach the protected area, but should be

studied and coordinated to the loss of excitation tripping time.

4.3.1 Short Circuit Faults

In this sub-section the reaction of LOE relay on three-phase, phase to phase and phase

to ground faults on line 5-7 will be studied on different initial loading conditions. The

results have been summarised on table4 and discussion will be given here for rated MW

output. The pre-fault active and reactive powers of G2 in all the next scenarios are

0.8485and 0.6307 respectively for heavy load condition. In Fig.35a, a three-phase to

ground short-circuit was created on L5-7 at 1second and cleared after 150ms.

Impedance trajectory of G-2 never enter the protection zones of the relay so no action

is taken. Phase to ground (AB-G) and phase-to-phase (A-B) faults are not as severe as

three phase fault but during the faults large amount of negative sequence component

occurs in phase current. In this two cases the faults are cleared after 150ms and as given

in Fig.36b and c, the impedance trajectories of G-2 remains still out of the protection

zones. For SPS event a three phase to ground short circuit fault is created in L5-7 and

54

cleared after 150ms by opening the circuit breakers from the sending and receiving end

so that the generator experience a swing. LOE relay have not detect this scenario but

when LOE event is created after the fault cleared at 1second, the impedance trajectory

enter in the relay zones at 1.15second.

(a)

(b)

(c)

(d)

Figure 38:G-2 impedance trajectory (a) three phase (b) two phase to ground (c) phase

to phase (d) three phase cleared after 250ms at G-2 terminal

Contrary, for three-phase to ground short-circuit cleared after 250ms, the system

experiences unstable swing and the generator impedance trajectory as given in Fig.38d

swings in zone-1 for about 1.66second and in zone-2 for 0.325sec after fault happen.

The machine should be isolated from the system as soon as possible but it should not

be through LOE relay since it is not excitation loss event. In this condition, only the out

of step relay should operate and open the generator circuit breaker before LOE relay

mal-operation since the tripping time of ANSI 78 is faster than LOE relay. But if this

fails the LOE relay will mal-operate for out of step situation [18] [29].

For faults near to G-2 terminal, all short circuit fault scenarios have been depicted on

bus-7 near the low voltage output of G-2 transformer. And the impedance trajectory

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

G-2 Impedance Trajectory-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

G-2 Impedance Trajectory

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

G-2 Impedance Trajectory-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


55

reaches the protection zones of the relay unlike the above cases. For three-phase to

ground fault at bus-5, impedance trajectory enters zone-1 and zone-2 at 0.15 second

and 0.0157second as shown in Fig.39a. Thus, the relay mal-operate at 0.15second after

fault happen. Otherwise, for the remaining scenarios LOE relay shows a good

performance on isolating short circuit faults far from the generator.

4.3.2 Outages

In this sub-section critical line, generator and load outages will be studied to observe

their effect on detection ability of excitation loss relay.

(a)

(b)

(c)

(d)

Figure 39: G-2 impedance trajectory (a) SPS (b) L7-8 outage (c) load rejection (d) G-

outage

Outages of either of these system components results in over or under loaded condition

of synchronous generator and system frequency and voltage instabilities

consequentially. This variation also affects terminal impedance of the generator. The

critical transmission line outage is studied through outage of L8-9 which can be isolated

due to faults or Circuit breaker failures practically and either of them for simulation

purpose. And G-3 outage and 125MW load rejection from bus-5 have simulated to

study the scenarios. As have given in Fig.37b-d, it can be seen that however in this

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

Impedance Trajectory in SPS

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X) P

u

Zone-1

Zone-2

Impedance Trajectory

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

Impedance Trajectory

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2


56

cases the terminal impedance do not enter the protection zones, but the trajectory is quit

near the operation zone.

Table 4: Performance of LOE relay in power swings

In order to ensure the reliable operation of LOE relay there is a short time delay between

entering time to the operating zone and the initiation of a tripping signal. Comparing to

critical transmission line outage, main load rejection worsens the impedance trajectory

closeness to the relay characteristics as can be observed in Fig. 36c.

Table4 shows LOE relay performance on various system performances including SPS,

OOS, generating unit outages, line outage and load propagation. As can be observed

from the results, LOE relay had actually detected a loss of synchronism which was

caused by prolonged fault clearing times even in short period of time than LOE event.

Thus, the worst swings were happened when the system impedance was low and the

System Disturbances

Fault

clearing

time(ms)

Initial Loading(in pu) and

Tripping Status Y(sec)/N

Heavy

load

medium

load

Light

load

SPS

100 N N N

150 N N N

Short-circuit and LOE at1sec 150 Y(1.15s) Y(1.25) Y(1.053)

OOS

250 Y(0.856) Y(0.601) Y(0.638)

350 Y(0.420) Y(0.401) Y(0.401)

G3-outage &100MW load

addition in bus-2

- N N N

G2-outage - N N N

G2&G3 outage - N N N

L5-7 Outage - N N N

Load Rejection at bus-6 - N N N

Load Rejection at bus-5 and

short circuit fault at L8-9

150 N N N

57

fault clearing times were greater than to the critical clearing time. And short circuit

fault at the terminal of the generator is the sever fault detected in short period in

addition to out of step condition.

4.5 Backup Protection for Excitation Loss Detection

The main factors that affect the operation range of synchronous generator are armature

current, terminal voltage, limit of stability, field current, initial loading capacity and

minimum possible excitation. Thus, any variations of these parameters jeopardize

stability of the machine and the system as whole. Thanks to the study of LOE relay in

different power system performances, the relay have been found mal-operating in some

PLOE and some power system disturbances.

Efd<Efdi

& Vi=Vlimit

Efd status

Trip G

No

Yes

Efd<Efdi &

Q<=Qlimit

No

Where ith

values are

the previous

values

Yes

Calculate

E’q threshold

and Qlimit

Delay

Power

swing

Figure 40: Flow chart of proposed back up protection

In partial loss of excitation the relay detect the event long after the fault happen. In light

load case, even in full loss of excitation detection time was long after generator loses

stability. On the other hand, in power system disturbances the relay send trip signal for

58

un-wanted system disturbances as shown in table4. In this section considering the mal-

operation of LOE relay in partial loss of excitation and power swings, a backup

protection will be proposed considering the system stability in loss of excitation and

generator parameter variation in power swing conditions.

In any LOE event, the field voltage noticeably decreases from the initial value and so

the reactive power. Here, the reactive power keeps reducing to negative value till the

generator loses synchronism if any action is not taken. In some conditions of system

disturbances the reactive power also reduce to negative but the field voltage raise in

value to pay off the terminal voltage reduction. In this section, the stability of system

in loss of excitation will be studied to calculate the reactive power margin of generator

at specified field voltage using the q-axis voltage decay in excitation loss event. The

general scheme of the proposed algorithm can be sum up as Fig.40 flow chart.

From equation (2.2) and (2.13) we can see that the q-axis voltage is highly dependent

on field components and it is reasonable that it response really fast for field failures

than other parameters of the generator. The minimum e’q reduction that lead to voltage

collapse of the system will be calculated from the terminal voltage of the generator.

This will be the threshold and minimum q-axis voltage that keep the system in stability

at any moment.

)sin()1

(

1

'

0'

'

0

'

''' t

d

fd

qaq

d

d

dd

dqddq V

sT

EiRV

XsT

XX

XvXiE

(4.1)

At specified margin of terminal voltage, the q-axis voltage can be calculated from

equation (4.1) above where the q-axis voltage is given in terms of terminal parameters

of the synchronous generator and field voltage. The threshold q-axis voltage is also

used to identify the minimum possible value of reactive power the system can feed the

faulted generator without system collapse. Limiting the reactive power consumption of

the generator will be the main concern of this proposed algorithm since the actual

excitation loss detection mal-operation is due to the algorithm fails to limit how much

reactive power should the system feed the generator so as the system does not have to

collapse before the relay detect the failure. In some literatures this concept have been

used to detect excitation loss [39] but the unpredictable behaviour of power system in

light load condition, partial loss of field and system outages have threaten them. So, in

this algorithm limiting the reactive power consumption with the internal field

component will increase the sensitivity of the method for excitation loss event than

59

system disturbances. In the remaining parts of this thesis work, the generator terminal

voltage will be limited to 0.86pu so that q-axis threshold voltage will be calculated at

given field voltage. This voltage is limited by considering the transient and sub-

transient effects of the generator model through the specified time delay of the

algorithm. Thus, studying the possible swing of generator parameters in normal state

is important to compare the field voltage of the system.

After calculating eq threshold from equation (4.1), the remaining step is to identify the

amount of reactive power consumed by the generator in the threshold quadrature

voltage and at a given active power. Synchronous generator reactive power

consumption speed in a given field voltage is highly dependent on the initial MW

output of the machine. To verify the algorithm in different loading conditions active

power status is also one factor to identify the reactive power consumed by the

generator. Thus, the reactive power of synchronous generator in equation (2.11) can be

re-formulated in terms of q-axis voltage and active power. As have discussed in

previous sections, parameters of generator in excitation loss event varies till the fault

cleared. Except the limit voltage Vlimit, all the parameters required to calculate Qlimit are

used from the online status of the generator to avoid instability of the proposed

algorithm [40]. In full loss of excitation generator lose synchronism short after wards

after loss of stability that is why LOE relay response fast in FLOE. But in partial loss

of excitation, generator loses synchronism takes place long after system loses of

stability which results in system collapse and finally blackout [4].

q

d

qqt

dddqthret ii

iVPiXiEQ )(2''

(4.2)

Where e’qthre is the threshold q-axis voltage and Pt the output active power of the

synchronous generator. This reactive power identifies the ability of the system to

recover the reactive power loss due to excitation loss generator at the same time it is

the amount of reactive power consumed by the faulted generator without voltage

collapse. If the synchronous generator model consider sub-transient components, the

generator parameters swing in normal state should be counted through a reasonable

delay for tripping the generator to avoid un-necessary relay operation. But, in this thesis

work the two-axis generator model have used so the transient characteristics of the

generator have considered with time delay of 0.081second.

60

FLOE in heavy load

50%Efd loss in heavy load

FLOE in medium load

50%Efd loss in medium load

Figure 41: Q-V curve of LOE relay and proposed back up protection

To compare the detection ability of the proposed algorithm and LOE relay, the

scenarios used above in G-2 of IEEE 9-bus test system will be repeated in three

different loading conditions; heavy load, medium load and light load conditions. Fig.41

expresses the reactive power-voltage (Q-V) curve of LOE relay and the proposed

algorithm in full and partial loss of excitation in two different loading conditions. As

can be compared from the results, heavily loaded generators consumes more reactive

power in short period of time compare to other loading conditions which result also

system instability in less time. In full loss of excitation (field winding short circuit), the

generator consumes reactive power until the terminal voltage of the generator diminish

to 0.289pu before excitation loss relay detect the event. However, in the back-up

protection the generator was able to consume reactive power until the terminal voltage

reduce to 0.86pu. Comparatively, reactive consumption amount of generator before

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Reactive Power Q Pu

Term

inal V

oltage V

t P

u

MVAR limit of LOE relay

MVAR limit of Bacu up protection

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Reactive power pu

Voltage p

u


MVAR limit of back-up protection

-0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.050.82

0.84

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

Reactive Power Q Pu

Term

inal V

oltage V

t P

u


MVAR limit of Bacu up protection

-0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.050.84

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

1.04

Reactive power pu

Voltage p

u


MVAR limit of back-up protection

61

detected through LOE relay decreases with initial loading decrease but still it puts the

system condition at risk as can be shown in Fig.41b.

In partial loss of excitation, the back-up protection have response faster than LOE relay

as given in Fig.41c-d. At the same loading condition, generator reactive power

consumption is the same despite the type of excitation loss. On the other hand on the

same type of excitation loss type, power consumption differs for various initial loading

condition as shown in Fig.41a-d. Thus, for 50%field voltage loss, the generator was

able to consume reactive power until 0.289pu terminal voltage reduction before

detected through LOE relay in heavy load condition which is similar to full loss of

excitation. Also, for the remaining loading conditions the proposed algorithm have

shown a good performance in limiting reactive power consumption before system

collapse.

Full loss of excitation

50% Efd loss

Figure 42: Terminal voltage reduction in LOE relay and back-up protection

The main role of reactive power consumption limiting in excitation loss event is to

maintain the system from voltage collapse and system loss of stability to avoid un-

necessary other protective relays operation (under-excitation relay, loss of stability

relay) and blackout. From Fig.42, it can be understand that the terminal voltage of the

generator is kept to 0.86pu in the proposed back-up protection for full and partial loss

of excitation.

Table5 and 6 shows the comparison of excitation loss relay and the proposed back-up

protection in full loss of excitation. The back-up protection have improved the time

elapsed to detect excitation loss and reactive power consumption limit of the generator.

In field winding short circuit case, the proposed algorithm detect full loss of excitation

0 2 4 6 8 10 12

x 104

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

time sec

Term

inal V

oltage p

u

Vt reduction of LOE relay in heavy load

Vt reduction of LOE relay in medium load

Vt reduction of back-up protection for both cases

0 2 4 6 8 10 12 14 16 18

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Time

Term

inal V

oltage p

u

Vt reduction of LOE relay in heavy load

Vt reduction of LOE relay in medium load

Vt reduction of back-up protection for both cases

62

1.857second after fault happen which is 2.303second before LOE relay. Similarly, for

medium and light loaded generators the detection length have improved to about 16%

of excitation loss relay.

Table 5: Comparison of actual and proposed excitation loss detection in field

winding short circuit

Initial loading (pu)

Tripping Duration (sec)

Possible MVAR consumed by G-2

before fault detected (pu)

LOE relay Proposed LOE relay Proposed

Heavy load 4.16 1.857 -0.431 -0.194

Medium load 5.804 4.537 -0.321 -0.2688

Light load 6.286 4.104 -0.231 -0.2055

On the other hand, in field winding open circuit the proposed algorithm detects the

event in long duration than LOE relay except for light load condition and vice versa

for generator possible reactive power consumption before fault detected. Thus, the

detection duration of field winding open circuit have improved in medium load

condition and a little bit in heavy loaded generators. Thus, it can conclude field winding

open circuit highly affects the terminal impedance of the generator.

Table 6: Comparison of actual and proposed excitation loss detection in field

winding open circuit

Initial loading (pu)

Tripping Duration (sec)



LOE relay Proposed LOE relay Proposed

Heavy load 0.0835 0.0845 -0.3175 -0.3209

Medium load 0.0467 0.2851 -0.314 -0.363

Light load 1.131 0.409 -0.2964 -0.2089

So far, the back-up LOE protection seems working perfectly for FLOE in different

loading conditions. To verify the reliability of the scheme in different LOE events, the

method have been tested in all possible partial loss of field voltage in three different

loading conditions as shown in table7. Similarly to full loss of excitation, the proposed

algorithm detect partial loss of excitation in heavy loaded generators twice less time

than LOE relay in all possible field voltage reduction. LOE relay is not able to detect

field voltage reduction until half of the rated value in medium and light loaded

generators, and the proposed method have improved this.

63

Table 7: Comparison of actual and proposed excitation loss detection in

partial field voltage loss

%Efd

loss

Initial

Loading(in

pu)

Tripping Status

Y(sec)/N



LOE

relay

Proposed LOE relay Proposed

20% H 26.46 12.86 -0.4897 -0.204

M N N -0.2720(in30sec) -0.116(in30sec)

L N N -0.1906(in30sec) -0.05(in 30sec)

30% H 15.21 7.24 -0.4882 -0.2021

M N 29.017 -0.2735(in30sec) -0.19

L N N -0.1909(in30sec) -0.101(in30sec)

40% H 10.94 5.075 -0.4883 -0.198

M N 28.72 -0.2805(in30sec) -0.264

L N 29.803 -0.1909(in30sec) -0.1802

50% H 8.563 3.935 -0.4887 -0.1957

M 17.19 14.33 -0.2837 -0.2645

L N 15.37 -0.191 -0.1907

60% H 6.99 3.22 -0.4858 -0.1945

M 11.96 9.948 -0.2885 -0.266

L 9.673 9.402 -0.198 -0.1965

70% H 5.913 2.724 -0.482 -0.1938

M 9.242 7.648 -0.2919 -0.2669

L 7.332 7.11 -0.2076 -0.2045

80% H 5.153 2.359 -0.4792 -0.1929

M 7.632 6.216 -0.296 -0.265

L 6.143 5.697 -0.216 -0.206

90% H 4.592 2.079 -0.4768 -0.1939

M 6.569 5.241 -0.3669 -0.268

L 5.383 4.773 -0.2236 -0.2056

N.B. Where H is heavy load condition (0.8485+j0.06307pu) and M is medium load

condition (0.4245+j0.03389pu) and L is light loading condition (0.2547+j0.3pu). And

64

the negative sign in MVAR indicates the consumption of reactive power by the

generator.

(a) (b)

Figure 43: G-2 terminal voltage in Partial loss of excitation (a) medium load 20%Efd

loss (b) light load 30%Efd loss

LOE relay and also the proposed back-up protection have found not detecting 20%

field voltage for medium and light load conditions and 30%Efd loss for light load

condition. However, from the parameter variation of the generator and system as shown

in Fig.43 the terminal voltage of the generator remains in range of 0.93pu which is

stable and precise voltage range. So, the generator should not be tripped for stable case

since un-necessary eliminating of synchronous generators will further jeopardize

system stability despite its economic issue.

LOE relay have mal-operate for severe power swings as have shown in table4. To

investigate the proposed back-up protection performance during such disturbances, the

cases studied in power swing conditions have repeated with the same system conditions

and the results are compared with results of LOE relay. SPS and out of step conditions

are simulated by three-phase to ground fault at G-2 terminal with pre-fault initial

condition of 0.8485+j0.06307pu. In OOS condition the generator becomes unstable

and should be isolated from the remaining system but LOE protection should not give

any response for this condition. From the simulation results summarized in table7, all

the mal-operation of LOE relay in system disturbance have overcome through the

proposed aback-up protection. In LOE and SPS event created at 1second, LOE relay

send a trip signal after 0.15second SPS happened which is before the LOE event

detected through the relay but the back-up protection sends a trip signal after

1.857second which is the duration LOE event should be detected.

0 5 10 15 20 25 300.93

0.94

0.95

0.96

0.97

0.98

0.99

1

1.01

time sec

Vt pu

G-2 terminal voltage

0 5 10 15 20 25 300.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1

1.01

1.02

time sec

Vt pu

G-2 terminal voltage

65

Table 8: Comparison of actual and proposed excitation loss detection in

system disturbances

Improvement of LOE relay have been the main concern of researchers for almost six

decades. However, the complexity of threshold setting in every failure, several

simulation scenarios and only testing on one common type of excitation loss were the

main disadvantages to hold them from installing in actual industries of power systems

[41]. Some recent methods have proposed setting free algorithms [29] which greatly

differentiate system disturbances and LOE event in heavily loaded generators but the

longed parameter swing in PLOE and in power swing when initial loading of the

generator decrease have not considered. In this proposed back-up protection,

considering the drawbacks of the above methods excitation loss event detection

directly from measuring of generator parameter variation have tested in all possible

scenarios of LOE event and system disturbances.

Generally from the simulation results, the proposed back up protection detects all

stages of excitation loss event much faster than excitation loss relay, have differentiated

any excitation loss event from any system disturbance such no trip signal is issued

System Disturbances

Fault clearing

time(ms)

Tripping Status

Y(sec)/N

LOE relay Proposed

SPS

100 N N

150 N N

200 Y(1.15s) N

OOS

250 Y(0.856) N

350 Y(0.420) N

G3 outage and 100MW

load addition in bus-2

- N N

G2-outage - N N

G2&G3 outage - N N

L5-7 Outage - N N

Load Rejection at bus-6 - N N

Load Rejection at bus-5 and

short circuit fault at L8-9

100 N N

SPS and LOE at 1second 200 Y(1.15s) Y(1.857)

66

through the method for any power swings. And while detecting the excitation failure,

system stability is held by limiting the reactive power consumption of the generator to

ability of the system to feed the faulted machine without system collapse. This avoids

any power system component damage and unwanted gird protective devices

functioning like under voltage relay and under excitation relay due to excitation system

failure.

67

CHAPTER 5

CONCLUSIONS AND RECOMMENDATION

5.1 Conclusion

From the excitation loss event study in synchronous generators, it has shown that

excitation loss not only imperil the faulted generator but also the whole system’s

stability due to electrical and mechanical power imbalance on the generator. This thesis

investigates detection ability of excitation loss relay in various failures of excitation

system and different power system disturbances. The relay can detect any type of full

excitation loss in less than 6.4second after fault created but for partial excitation loss

the remaining field voltage allows the defective machine and the system to find a new

operating point as a result the scenario is detected long after it happen or may be not

detected at all for lightly loaded generators. In addition to the slow operation in partial

excitation loss, the relay also response for un-wanted system disturbances especially

for stable and un-stale power swings which further threaten system stability in addition

to economic issue of un-wanted generator tripping.

A back-up protection based on the q-axis voltage and reactive power flow to the

generator have proposed to overcome the mal-operation of excitation loss relay in

power swings and slow operation of the relay in partial loss of excitation. In heavily

loaded generators, the system voltage collapse happen at 2.85second after excitation

loss initiated but excitation loss relay detects the failure at 4.16second which is

1.31second later of voltage collapse. This have been the main reasons for system

blackouts due to excitation loss event. In this proposed algorithm reactive power

consumption of the generator have been limited to the ability of the interconnected

system to feed the faulted generator without system collapse. The proposed back up

protection detects field winding short and open circuit at 1.857 and 0.153second

respectively which is 44% improvement of actual excitation loss protection. The

improvements of the back-up protection can be summarized as:

Detect excitation loss before system collapse

Improve the detection duration of impedance protection relay to twice less for

heavy loaded generators and almost 16% less for lightly loaded generators

Differentiate system failure and excitation loss

Detect possible partial loss of excitation that can lead to system instability

68

Generally it can be conclude that in both excitation loss detection methods the time

available to trip an excitation loss generator depends on initial loading of the generator,

reactive power support from interconnected systems and type of excitation loss. More

MW output of generator and full excitation loss implies less time before detected

through excitation loss protection and vice versa for weak MVAR support from the

system and partial excitation loss.

69

5.2 Recommendations for Future Work

The excitation loss detection using the generator terminal impedance is the actual

method in power system industries until now. Even if a number of improvements using

other generator parameter variation in excitation loss event have been recommended,

the set point identification is a difficult task and may involve extensive simulation

processes which make them unpractical. For future work, it is recommended to modify

the actual excitation loss protection by co-ordinating reactive power consumption limit

of synchronous generator without system loss of stability. And I recommend further

development and testing of the proposed back-up protection scheme for general power

systems.

70

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74

APPENDIX A

A.1 Full Loss of Excitation

Field winding short circuit Field winding open circuit

Figure 44A-1: 9-bus G-2 impedance trajectory in 90% and 70% loading

A.2 Partial Loss of Excitation

(a)

(b)

Figure 45A-2: 9-bus G-2 impedance trajectory in (a) 90% and (b) 60% field voltage

loss

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

G-1Impedance T. in 90% loading:

70%loading

t=4.9605st=4.092s

t=4.3352s

t=6.2556s

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

G-1Impedance T. in 90% loading:

70%loading

t=0.0106s

t=0.0105s

t=0.065s

t=0.3239s

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Resistance (R) Pu

Reacta

nce (

X)

Pu

Zone-1

Zone-2

G-2ImpedanceT Heavy L.

in Medium L.:

in Light L.

t=9.672s

t=11.954st=7.988s

t=14.728s

t=18.139s

75

A.3 IEEE ST1A Excitation System

Figure 46A.3: Block diagram of IEEE ST1A excitation system

A.4 Two-axis Model Initial Values

000 )( aqatq IjXRVE

00 qE

00 aI

00 tV

)sin( 0000 ad II

)cos( 0000 aq II

)sin( 0000 td VV

)cos( 0000 tq VV

0

'

00

'

0 ddqaqq IXIRVE

0

'

00

'

0 qqdadd IXIRVE

0

''

0

'

0 )( dddqfd IXXEE

0000000 )()( qqaqddadem iiRViiRVPp

s 0

76

APPENDIX B

Table 9 B-1: IEEE 9-bus system required Machine Data

Parameters

Unit

Synchronous

Gen. 1

Synchronous

Gen. 2

Synchronous

Gen. 3

Bus-1 Bus-2 Bus-3

Rated Power MVA 247.5 192 128

Rated voltage KV 16.5 18 13.8

H S 9.55 3.33 2.35

D

2 2 2

Xd pu 0.361 1.72 1.68

Xq pu 0.2398 1.65 1.61

X’d pu 0.1504 0.23 0.232

X’q pu 0.159 0.23 0.232

X’’d pu 0.099 0.1728 0.19

X’’q pu 0.099 0.1728 0.19

T’d0 pu 8.96 4.8 8.9

T’q0 pu 0.48 0.07 0.5

T’d pu 1.01 1.302 1.302

T’’q0 pu .001 0.0007 0.070

Table 10 B-2: IEEE 9-bus system load data

Bus P[pu] Q[pu]

5 1.25 0.50

6 0.90 0.30

8 1.00 0.35

77

Table 11 B-3: IEEE 9-bus Transmission line Data

Line R[pu/m] X[pu/m] B

[pu/m]

Total

reactance

[ohm]

Estimated length of the line

based on line reactance

values [Km] From

bus

To

bus

4 5 0.01 0.068 0.176 2645 5290

4 6 0.017 0.092 0.158 3174 6348

5 7 0.032 0.161 0.3060 3703 7406

6 9 0.039 0.1738 0.358 4761 9522

7 8 0.0085 0.0576 0.1490 4232 8464

8 9 0.0119 0.1008 0.2090 4761 9522

Table 12 B-4: IEEE 9-bus system excitation system data

Parameters

Unit

Synchronous

Gen. 1

Synchronous

Gen. 2

Synchronous

Gen. 3

Tr S 0.000 0.000 0.060

Ka pu 200 30 25

Ta S 0.395 0.400 0.200

VRmax Pu 3.84 4.590 1.00

VRmin pu -3.84 -4.590 -1.000

Ke pu 1.000 -0.020 -0.0601

Te S .0000 0.560 0.6758

Kf pu 0.0635 0.050 0.108

Tf S 1.0000 1.300 0.350

E1 pu 2.880 2.5875 2.4975

SE

0.000 0.7298 0.0949

E2 pu 3.840 3.450 3.33

SE

0.000 1.3496 0.37026

78

Table 13 B-5: Tana Beles-1 System data

Parameters

Unit

Four Synchronous Generators with

identical ratings and system data

Active Power MW 46

Reactive Power VA -27.3

Rated voltage KV 15

H S 3.14

D

0

Xd pu 1.03

Xq pu 0.7

X’d pu 0.31

X’q pu 0.159

X’’d pu 0.25

X’’q pu 0.25

T’d0 pu 0.13

T’d pu 1.01

T’’q0 pu 0.1

B.6 Excitation Loss Relay Protection Zones

%%input Parameters of a generator f=60; theta =transpose( 0:.01:(2*pi)); S=192e6; V=18e3; %rated voltage H=2; %inertia constant Xd=1.72; %pu Xq=1.65; %pu Xd1=0.23; %pu Xq1=0.23; %pu Xd2=0.1728; %pu Xq2=0.1728; %pu Ra=0.0075; %pu Td01=1.302; % second Tq01=1.00; %second Td02= 0.023; %second Tq02=0.07; Rng=15; %pu

% Transmission Line Data L=0.24; %Henry, Rl=60; %

79

%Transformer data St=100e6; % VA VHv= 230e3; % V VLv=18e3; % V XTG=0.05; %pu cooperloss= 0.001; %pu

%% Mho characteristics Modelling

% Base impedance with respect to synchronous generator Zbase= (V^2)/S;

%MHo Zone-1 Settings %radius is 1 pu

radius1= Zbase/2; %Coordinate of centre % x-coordinate is set to zero % Y-coordinate of zone-1 is given by first calculating base value of

reactance with respect to synchronous generator %actual value of X'd Xd1a= Xd1*Zbase;

YC1=-((Zbase/2)+(Xd1a/2)); % MHO ZONE 2 settings % mho circle radius for ZONE 2 is xd/2 Xda= Xd*Zbase; radius2=Xda/2; %Y-coordinate of zone-2 YC2=-(Xd1a/2+Xda/2); %Relay Settings Forward and Reverse impedances ZFR=2*Xd1a; secZFR=ZFR*Ratio; ZbaseLV=VLv^2/St; XTGa=XTG*ZbaseLV; ZRV=1.5*XTGa; secZRV=ZRV*Ratio; % X and Y-axis values of the two protection zones xs=0+radius1*cos(theta); ys=YC1+radius1*sin(theta); x2=0+radius2*cos(theta); y2=YC2+radius2*sin(theta);

%Plots % zone1=[xs,ys]; % zone2=[x2,y2]; set(gcf,'visible','on')

%simulate the Simulink model

simout=sim('ninebus100');

plot(xs,ys,'r',x2,y2,'b'); axis([-1.5 4 -3.5 1]); hold on drawnow expose ;

plot(R.signals.values,X.signals.values,'r'); xlabel('Resistance (R) Pu'); ylabel('Reactance (X) Pu');

80

legend('Zone-1','Zone-2','Impedance Trajectory'); grid on set(gca,'Xlim',[0 ntimes],... 'Ylim',[2.5 4]) set(h,'XDataSource','R.signals.values') set(h,'YDataSource', 'X.signals.values')

hold off

81

APPENDIX C

Physical Representation of Excitation Loss Event

Excitation loss phenomenon can be easily understand through human body to have a

physical view of the event. So, generally human body can be represented as

synchronous generator which have an excitation system, grid connected to it and a

power (including the voltage and current) that will be delivered to the system. Thus,

the calories taken in terms of foods or drinks and stored in stomach will be an excitation

system, the stomach itself as generator, the energy that burn from the calories and

distributed to the whole muscles to keep the body functional as power of the generator

and the whole body as the gird connected to the synchronous generator.

When the stomach emptied, the human digestion system needs another way to deliver

energy to the whole muscles of the human body. This emptiness of the calories in

stomach is called excitation loss. So when our body losses excitation, the digestion

system starts to burn the accumulated energy as fat from the body and deliver it back

to the body’s muscles so as the body will remain functional until an additional calorie

added. And our body at this condition will feel hunger, thirsty or tired. Thus

synchronous generators consume the reactive power of the system when it loses its

excitation. However, if this continues for long time till the maximum limit of re-

energizing of the body through body’s energy, the delivering of energy will suddenly

stopped and the body will be so weak that may loss stability of handling each other.

May God avoid this from all human beings but if still not avoided the body will not be

able to resist the condition. When the system reach at maximum limit of reactive power

feeding to the generator, the synchronous generator loss synchronism and if it does not

isolated the whole system will be shut down.

Feeling of tired, hunger, thirst and unbalance are the response of our body to loss of

calorie in the stomach. So, if we are able to response to our feelings we can prevent it

before it loses its balance. The parameters including current, speed, terminal and

internal voltage and impedance of the generator starts to vary when a generator loses

its excitation. Thus by sensing the parameter variation we can prevent excitation loss

which is called excitation loss detection methods.

EXCITATION LOSS DETECTION OF SYNCHRONOUS GENERATOR …

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