University of Calgary PRISM: University of Calgary's Digital Repository Graduate Studies The Vault: Electronic Theses and Dissertations 2015-06-29 Effect of STATCOM Location on Distance Protection Relay Operation Sun, Peng Sun, P. (2015). Effect of STATCOM Location on Distance Protection Relay Operation (Unpublished master's thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/25810 http://hdl.handle.net/11023/2324 master thesis University of Calgary graduate students retain copyright ownership and moral rights for their thesis. You may use this material in any way that is permitted by the Copyright Act or through licensing that has been assigned to the document. For uses that are not allowable under copyright legislation or licensing, you are required to seek permission. Downloaded from PRISM: https://prism.ucalgary.ca
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University of Calgary
PRISM: University of Calgary's Digital Repository
Graduate Studies The Vault: Electronic Theses and Dissertations
2015-06-29
Effect of STATCOM Location on Distance Protection
Relay Operation
Sun, Peng
Sun, P. (2015). Effect of STATCOM Location on Distance Protection Relay Operation (Unpublished
master's thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/25810
http://hdl.handle.net/11023/2324
master thesis
University of Calgary graduate students retain copyright ownership and moral rights for their
thesis. You may use this material in any way that is permitted by the Copyright Act or through
licensing that has been assigned to the document. For uses that are not allowable under
copyright legislation or licensing, you are required to seek permission.
Downloaded from PRISM: https://prism.ucalgary.ca
UNIVERSITY OF CALGARY
Effect of STATCOM Location on Distance Protection Relay Operation
Flexible AC Transmission System (FACTS) devices are playing an increasingly important role
in electrical power systems to satisfy the function of achieving better power transferability and
enhancing power system controllability. The presence of FACTS devices in power systems has
brought up some challenges to the protection schemes in the grid. Distance protection, as a major
transmission line protective scheme, is facing such a challenge to meet the basic requirements for
its accuracy, selectivity, reliability and security. This dissertation reviews FACTS concepts, and
studies the shunt connected STATCOM and its modeling. Based on the dynamic behaviour of a
shunt connected STATCOM in a two-machine system, where a distance protection scheme is
applied to protect the transmission line connecting the two machines, performance of the two
zone distance protection scheme has been evaluated in EMTDC/PSCAD simulation environment
for various contingent conditions. This includes different STATCOM installation locations,
various STATCOM voltage settings, various fault locations & types. To overcome the mis-
operation of the distance relay and make the distance scheme operational and reliable when the
transmission line is shunt compensated with STATCOM, studies on some communication-aided
protection schemes, including PUTT, POTT and DCB, are conducted. These pilot protection
schemes have proven to be effective for fast clearance of the faults on the transmission line and
meet the requirements for protections, regardless of STATCOM installation locations.
iii
Acknowledgements
The dissertation is with great support and patience from my family in the past 4 years during
which we suffered the deep sorrow of losing an important family member and overcame the
unpredictable challenges of life together. The spiritual motivation from family is the power
encouraging me to move towards the completion of this project.
I wish to solemnly express my sincere gratitude and deep appreciation to my supervisors Dr. Ed
P. Nowicki and Dr. O. P. Malik at this time for their constant guidance, encouragement and
support throughout the whole program. A new window is open for me in Electrical Engineering
with their direction, from which I greatly broadened my horizon in the application of power
electronics and hence my professional career has benefited tremendously from the exciting
learning procedure. My foremost thanks go to them and I also wish to extend my appreciation to
other professors and support staff in the Department of Electrical and Computer Engineering in
University of Calgary for their help during my study here.
I also wish to thank some of my friends for their continuous support and constructive suggestions
that inspired and motivated me to complete the part time study, without whom I would be unable
to finish my project successfully.
iv
Table of Contents
Abstract ............................................................................................................................... ii Acknowledgements ............................................................................................................ iii
Table of Contents ............................................................................................................... iv List of Tables ..................................................................................................................... vi List of Figures and Illustrations ........................................................................................ vii List of Symbols, Abbreviations and Nomenclature .............................................................x
Step Distance Schemes .........................................................................................5 1.1.2 Pilot Schemes ....................................................................................................6
1.2 Introduction to FACTS ..............................................................................................6
1.3 Type of converters ...................................................................................................11 1.4 Summary ..................................................................................................................13
CHAPTER TWO: STATCOM PRINCIPLE AND LITERATURE REVIEW .................16 2.1 Introduction to FACTS ............................................................................................16
2.2 STATCOM ..............................................................................................................17 2.2.1 Introduction to STATCOM .............................................................................17
2.2.2 Basic Principle of a STATCOM ......................................................................19 2.2.3 STATCOM Control .........................................................................................23
Introduction to STATCOM Topologies ..............................................................23 Basic Control Approaches of a STATCOM .......................................................25
Indirect Control .................................................................................................26 Direct Control ....................................................................................................28
2.2.4 Steady State and Transient Characteristics of a STATCOM ..........................29
2.2.5 Harmonic profile of STATCOM .....................................................................33 2.2.6 Detailed Mathematical Model of STATCOM .................................................36
Static Module of STATCOM ..............................................................................37 Dynamic Module of STATCOM .........................................................................38
CHAPTER THREE: MODELING OF DISTANCE PROTECTION IMPEDANCE .......46 3.1 STATCOM installed at mid-point of the transmission line .....................................46
3.1.1 Single phase fault after the STATCOM ..........................................................48 3.1.2 Single phase fault before the STATCOM .......................................................51
3.2 Phase to phase fault ..................................................................................................54 3.2.1 Phase to phase fault after the STATCOM .......................................................54 3.2.2 Phase to phase fault before the STATCOM ....................................................56
4.1 System Simulation ...................................................................................................59 4.1.1 Transmission System Module .........................................................................59
System configuration ..........................................................................................59
Transmission line ...............................................................................................60 Generator and load ............................................................................................61
4.1.2 STATCOM modelling and its Control Circuit ................................................63 STATCOM model ...............................................................................................63 Voltage Control Loop ........................................................................................65
PWM Control Module ........................................................................................65 4.1.3 Distance Protection Module ............................................................................68
Voltage & Current Signal Processing ...............................................................70 Distance Mho Characteristic .............................................................................71
Fault resistance is 50 Ω .....................................................................................78 4.2.2 Near-end bus connected STATCOM simulation ............................................82 4.2.3 Far-end bus connected STATCOM simulation ...............................................84
4.2.4 Effect of Voltage Setting of STATCOM .........................................................87 4.3 Concluding Remarks ................................................................................................90
Active power control, damping oscillations, transient and dynamic stability, voltage stability
Unified Power Flow Controller (UPFC)
Active and reactive power control, voltage control, VAR compensation, damping oscillations, transient and dynamic stability, voltage stability, fault current limiting
Thyristor-Controlled Voltage Limiter (TCVL)
Transient and dynamic voltage limit
Thyristor-Controlled Voltage Regulator (TCVR)
Reactive power control, voltage control, damping oscillations, transient and dynamic stability, voltage stability
Interline Power Flow Controller (IPFC)
Reactive power control, voltage control, damping oscillations, transient and dynamic stability, voltage stability
11
1.3 Type of converters
In general, FACTS Controllers are based on an assembly of AC/DC or DC/AC converters or
high power AC switches [2]. A converter is an assembly of valves in which each valve is an
assembly of solid state power devices comprising of turn-on/turn-off gate drive circuits with
snubber circuits for damping purpose. Similarly, each AC switch is an assembly of back-to-back
connected solid state power devices along with their snubber circuits and turn-on/turn-off gate
drive circuits.
Compared to the self-commutating converter, the line-commutating converter must have an AC
source connected and will consume reactive power and suffer from occasional commutation
failures in the inverter mode of operation. Hence converters applicable to FACTS Controllers
often employ the self-commutating type [2]. There are two basic categories of self-commutating
converters:
Current-sourced converter
In Current-sourced converter (CSC), direct current always has one polarity, and the power
reversal takes place through reversal of DC voltage polarity.
Voltage-sourced converter
In Voltage-sourced converter (VSC), the direct voltage always has one polarity, and the power
reversal takes place through reversal of DC current polarity.
For the reasons of economy and performance, voltage-source converter is often preferred for
FACTS applications and it will be presented in the following.
12
Figure 1.3 Valve for a voltage-sourced converter
A voltage-sourced converter valve that is made up of an asymmetric turn-off device such as a
GTO, with a parallel diode connected in reverse is shown in Figure 1.3.
Figure 1.4 Voltage-Sourced Converter
The basic function of a voltage-sourced converter is shown in Figure 1.4. In this figure the
converter valve is schematically represented by a box that has a valve and a diode inside it. On
the DC side, the voltage is supported by a capacitor that is large enough to handle a sustained
charge/discharge current that accompanies the switching sequence of the converter valves. The
capacitor is also able to satisfy the current shifts in the phase angle of the switching valve
13
without significant changes in the DC voltage. The DC current can flow in either direction
hence it can exchange power with the connected DC system in either direction. On the AC side,
the generated AC voltage from the converter, Ua, is connected to the AC system via an
inductor. To the AC system, the converter output is a voltage source with low internal
impedance. Therefore, an inductive interface between the converter and the AC system is
important to ensure that the DC capacitor will not discharge rapidly into a capacitive AC load,
such as a transmission line, when there is a short circuit. In application, an interface transformer
can be utilized to achieve multi-functions including inductive interface, voltage regulation and
harmonic cancellation.
1.4 Summary
A brief description of the major protection on power transmission lines, distance relay, and other
protection schemes based on it is given above. Further, four basic types of FACTS controllers
are introduced and different attributes of the controllers are briefly discussed. Self-commuting
converters, the basic power electronic unit in FACTS Controllers, are also introduced. The
operation of voltage-sourced converter that is most applicable to FACTS Controllers is discussed
in the last section. The topic of how the traditional protection schemes are affected by the new
emerging FACTS devices is raised.
1.5 Thesis Outline
The thesis is organized as follows:
In Chapter 2, description of shunt connected STATCOM with its operating principles is
introduced first. Then different topologies of STATCOM based on GTOs are discussed along
14
with control methods of STATCOM. Both external and internal control approaches are presented
by providing different control logistics. Afterwards, discussion of the stable and transient
characteristic of STATCOM is given. The Equal criterion method is applied to analyze the
improvement of system stability with a STATCOM installed. Harmonics in a 6 pulse voltage
sourced converter are analyzed as well. Mathematical models for static and dynamic behaviour
of a STATCOM are presented, from which the current and voltage of a STATCOM can be
obtained with equations provided. Introduction to some worldwide STATCOM applications is
given in the last section.
In Chapter 3, mathematical model of distance protection impedance is built up so as to have a
clear analysis of the measured impedance of a distance relay when a STATCOM is installed at
mid-point of a transmission line. In the discussion for single phase to ground fault and phase to
phase fault, the method of symmetrical components is utilized to obtain the equations for the
measured impedance of the distance relay under different conditions. Conclusion of typical mis-
operations of distance relays can be made based on the new impedance equations.
Simulation studies for a transmission line with a source at each end and with STATCOM
installed are given in Chapter 4. Various control modules including transmission line module,
VSC-based STATCOM module, voltage control loop module, PWM (Pulse Width Modulation)
control module, distance relay voltage processing module, distance relay Mho module and
Distance relay output module are described. Simulations are run for midpoint connected
STATCOM, near-end bus connected STATCOM and far-end bus connected STATCOM with
different fault conditions. Comparison of the performance with different simulation studies is
15
presented along with an analysis of the behaviour of the distance relay. The effect of output
voltage setting of STATCOM is also considered. Results of all simulation studies should be
consistent with the conclusions made in Chapter 3.
Possible solutions to overcome the mis-operation of the distance scheme when a STATCOM is
installed on a transmission line are given in Chapter 5. Some communication-aided schemes,
including Permissive Overreach Transfer Trip, Permissive Underreach Transfer Trip, Directional
Comparison Blocking and Line Differential scheme, are analyzed and tested in the simulation
system. As a conclusion, Permissive Overreach Transfer Trip is found to be the most suitable
scheme to improve the performance of a traditional distance relay when STATCOM is installed
on the transmission line.
Summary of this work is given in Chapter 6. Further discussion of the distance protection with
installed STATCOM is provided. Also, future work is considered and possible research
approaches, such as pilot schemes and adaptive setting, are discussed for better improvement of
distance relay in similar applications.
Results of research on the distance protection of a transmission line with the shunt compensation
device, STATCOM, are reported in this thesis. By conducting mathematical modeling for
distance protection and by building power system simulation model for STATCOM and step
distance scheme, this work provides a solid solution to overcome the mis-operation of a distance
relay protection, i.e. Underreach and Overreach, on a transmission line where STATCOM is
installed.
16
Chapter Two: STATCOM Principle and Literature Review
2.1 Introduction to FACTS
From power system equation, real power and reactive power transferred between two power
sources are [10]:
(2.1)
(2.2)
where:
U1 is the RMS voltage at power source 1
U2 is the RMS voltage at power source 2
θ1 is the power angle at power source 1
θ2 is the power angle at power source 2
XL is the transmission line reactance connecting the two sources
From equations 2.1 and 2.2, the power flow can be controlled in either direction in theory by
adjusting the variables of the equations on the right side, such as transmission line reactance XL,
system voltages U1 & U2 and system power angles θ1 & θ2. In practical applications, various
FACTS controllers can be used to achieve the different functions of adjusting specific system
parameters in the system they connect. The shunt connected SVC or STATCOM can provide the
supporting voltage to the compensated system. Other FACTS controllers can change the phase
angles between the two systems, such as TCPST. TCSC can be series connected in a long
transmission line to change the line reactance [2]. All the FACTS controllers mentioned above
17
can rapidly change the power flow within one cycle and even increase the power transfer limit at
normal operating conditions. When the power system is in abnormal or faulty conditions,
FACTS controllers can enhance the system stability with the inherent capability to change the
system parameters continuously. Especially in a ring connected power system, by applying SC
and TCPST, it is possible to meet the requirement of satisfying power demand, reducing
transmission line loss and increasing power transmission capacity [2].
The dynamic control of FACTS devices is based on the real time adjustment of power electronic
switching devices (turn on/off is within one millisecond). Therefore, a FACTS controller can
respond more quickly than a traditional circuit breaker when the FACTS controller is functioning
as an interrupting device (the fastest interrupting time of a circuit breaker is 2 cycles [11]).
Moreover, it is impossible for the mechanical apparatus to conduct the same functions that a
FACTS controller has. As common sense, mechanical device such as circuit breakers and
disconnect switches, cannot be operated so continuously at such high operating speeds without
any safety concerns and any power losses due to their inherent attributes. A circuit breaker can
be used to connect a fixed valued capacitor bank into the system; however, continuous
adjustment of compensation current from the capacitor bank is not possible.
2.2 STATCOM
2.2.1 Introduction to STATCOM
The IEEE defines the STATCOM as [8]:
18
“Static Synchronous Compensator (STATCOM): A Static synchronous generator
operated as a shunt-connected static VAR compensator whose capacitive or inductive output
current can be controlled independent of the AC system voltage.”
From this definition, a STATCOM is a shunt-connected reactive power compensation device that
is capable of independently generating/absorbing reactive power at its output terminals. In
addition, the compensating reactive power of a STATCOM device can be varied to control the
specific parameters of the electric power system to which it is connected [12].
In summary, a STATCOM can improve power system performance in the following areas:
1) Independent dynamic voltage control of transmission and distribution systems
2) Power-oscillation damping in power transmission systems
3) System transient stability enhancement
4) Voltage flicker control
5) Control of both reactive and active power on the connected line with an energy storage
source.
Furthermore, in practical engineering a STATCOM has some other application benefits due to its
small physical size and modular constructive characteristic compared to other shunt connected
FACTS devices such as SVC. This makes STATCOM have a minimum environmental impact
and more economic efficiency [12]. However, as new FACTS based technology, the
STATCOM is less commonly employed than the SVC in the conservative market. Nevertheless,
19
more projects with STATCOM applications have been commissioned worldwide recently. Some
examples of STATCOM projects are introduced later in this chapter.
2.2.2 Basic Principle of a STATCOM
A STATCOM is analogous to an ideal synchronous machine [12] that generates a balanced set of
sinusoidal voltages at the fundamental frequency with controllable amplitude and phase angle,
and also generates either capacitive or inductive VARs for the system.
Figure 2.1 VSC-based STATCOM interface diagram in a power system
A voltage-sourced converter based STATCOM interface diagram in a power system is shown in
Figure 2.1. The shunt connected compensation system, STATCOM, consists of three major
components, a capacitor, converter and a coupling transformer. The capacitor C, functions as a
DC input voltage source. As output voltages of the STATCOM, the three phase voltages
20
produced by the converters are connected to the AC system through the coupling transformer.
The leakage impedance Xg of the coupling transformer normally is rated at 0.1 p.u to 0.15 p.u.
[2]. Hence it can also functions as a tie inductance between the STATCOM and the AC system.
Then the reactive power exchange can be controlled in a manner similar to that of the
synchronous machine by adjusting the amplitude of the converter output voltages.
Figure 2.2 STATCOM and associated phasor diagrams (capacitive) for Rg=0 and Rg≠0
21
The basic schematic connection of a VSC-based STATCOM for reactive power generation is
shown in Figure 2.2 with phasor diagrams for the cases of Rg= 0 and Rg≠ 0, where Rg represents
the total resistance of the STATCOM. The phasor diagrams are for the cases where the
STATCOM provides capacitive VARs. The Rg = 0 case is the ideal case where power loss in the
circuit is neglected and the STATCOM output voltages are in phase with system voltages.
Referring to Figure 2.2, the equations for the voltages are given below:
Us=Ug+ j IgXg+ IgRg (2.3)
Where
Us is the AC system voltage
Ug is the Converter output voltage
Xg is the reactance summation of the transformer leakage
Rg is the total resistance summation in STATCOM
For the Rg=0 case, the STATCOM current and reactive power exchanged is given by:
Ig=
(2.4)
Q=
(2.5)
For the sake of better understanding, the operation of a STATCOM is sometimes considered
analogous with the operation of a synchronous machine. Both equations 2.4 and 2.5 also apply
for a synchronous machine as well. For a synchronous machine, reactive power flow can be
controlled by adjusting the excitation of the machine, which in turn adjusts the magnitude of the
output voltage |Ug|. When the machine is over-excited, then it is |Ug| > |Us|. This will result in a
22
leading current, as shown in Figure 2.2. In this case the machine is sending VARs to the system;
consequently the machine can be seen by the system as a capacitor. Likewise, the machine can
function as a reactor in the under-excitation condition with |Ug| < |Us| (not shown in Figure 2.2).
A STATCOM functions in a similar way. This means if the amplitude of the converter output
voltage |Ug| is greater than system voltage |Us|, |Ug| > |Us|,the converter provides capacitive
reactive power to the system, i.e., the STATCOM behaves like a capacitor. On the other hand,
reactive power is absorbed from the system by controlling the converter output voltage to be
smaller than the system voltage, that is |Ug| < |Us|. In this case, the STATCOM behaves like an
inductor.
The resistance Rg in the circuit represents the total power loss of the STATCOM if the power
loss of the switching devices and coupling transformer are considered. In normal operation,
when the STATCOM is used for reactive power generation, the converter can keep the DC
capacitor charged at a desired voltage by making the output voltage of the converter Ug lag
behind the AC system voltages Us by a small angle, which is usually set between 0.1° and 0.2°
[2]. In this way, a small amount of real power from the AC system will be absorbed by the
converter to compensate for its internal real power loss and to meet the capacitor voltage
requirement. This approach can be applied to increase or decrease the capacitor voltage. Hence
VAR generation or absorption of the STATCOM can be controlled.
STATCOM control approaches are now discussed, to be followed by a discussion of
STATCOM’s applications and effects on distance protection.
23
2.2.3 STATCOM Control
Introduction to STATCOM Topologies
The topology of a STATCOM is related to the VAR capacity and to the harmonics profiles of the
STATCOM. Regardless the number of pulses, the voltage-sourced converter, is composed of
several high power switching devices such as GTO or IGBT devices, with a parallel diode
connected in reverse for each device [2].
A six-pulse STATCOM topology is shown in Figure 2.3. If a higher VAR capacity is needed,
then the 12-pulse topology of Figure 2.4 may be used. Other topologies exist, for example a 48-
pulse converter may be constructed using the multi-level converter approaches [2].
Figure 2.3 Topology of a three-phase, two-level, six-pulse voltage-sourced converter
24
Figure 2.4 Topology of a three-phase, three-level, twelve-pulse voltage-sourced converter
Referring to Figures 2.3 and 2.4, a switching device usually is comprised of a number of
(normally 3 to 10) series connected GTOs or IGBTs to increase the overall voltage peak
capability. Each of the three legs of the converter is controlled to produce a quasi-square wave
output voltage, or sometimes a pulse width modulated (PWM) output voltage waveform. The leg
waveforms are 120° phase shifted from each other in a three phase system.
A coupling transformer connection to the AC system is used to produce a stepped approximation
of a sine wave current waveform, in which a significant number of low order harmonics are
eliminated [2].
25
Basic Control Approaches of a STATCOM
A block diagram of the basic control functions of a STATCOM is shown in Figure 2.5.
Figure 2.5 Block Diagram of the basic control structure of a STATCOM
The control [2] of a STATCOM includes two main parts, external control and internal control.
External control provides the reference signals to determine the functional operation of the
STATCOM. The internal control provides the gating signals for the semiconductor power
switches of the voltage-sourced converter. Some reference signals for external control are
normally from operator instructions or system variables, such as system voltage fluctuation ΔUs
and reactive current IQref. With the support of the STATCOM, the system voltage at the
compensation point can be kept at a preset level. In applications, ΔUs is the voltage difference
between system voltage Us and reference voltage Uref and it has to be kept within a limit for
26
internal power loss. The STATCOM is able to increase the adjustment range with a fixed MVAR
capacity and to provide the flexible compensation to the system by following its V-I
characteristic slope, as discussed in section 2.2.4.
By computing the magnitude and phase angle of the STATCOM current Ig from external control
and the pre-set reference voltage, the internal control of the STATCOM can be achieved to
generate a set of coordinated timing waveforms, that can operate the converter power switches to
produce output voltage waveforms Ug, and provide the real/reactive power exchange requested
for the compensation. These timing waveforms have a gating pattern that determines the Turn-
ON and Turn-OFF periods of each individual switch of the converter. The pre-defined phase
relationship between the waveforms is determined by different factors, such as the converter
pulse number, the method used for constructing the output voltage waveforms and the required
angular phase relationship between outputs in each phase (normally 120 degree).
There are two methods to achieve the function of internal control: Indirect Control and Direct
Control.
Indirect Control
A simple block diagram of the indirect control of a STATCOM for pure reactive compensation is
shown in Figure 2.6.
27
Figure 2.6 Indirect control diagram of a STATCOM
In this approach, magnitude of the output voltage from the converter is proportional to DC
capacitor voltage [2]. By varying the DC capacitor voltage through the temporary phase shift δ
between the STATCOM output voltage Ug and the AC system voltage Us, reactive current from
the converter can be controlled indirectly. The inputs from external control to the indirect control
are AC system bus voltage Us, converter output current Ig and the reactive current IQref. Voltage
Us operates a Phase Lock Loop circuit that provides the basic synchronizing signal angle θ.
Current IgQ is the reactive component of the converter output current Ig. It is compared with the
reference current IQref. The resulting error obtained provides an angle Δδ after suitable
amplification. The angle Δδ defines the necessary phase shift between converter output voltage
and the AC system voltage. Accordingly, Δδ is added to θ to provide Δδ+θ, which represents the
desired synchronizing signal for the converter and is processed by the Gate Pattern Logic circuit.
28
The Gate Pattern Logic circuit generates the gate drive signals for individual power switches.
When the control procedure is complete, there should be only reactive power exchange between
the STATCOM and the system, and the final δ is zero (if Rg=0).
Direct Control
A simple block diagram of the direct control approach of a STATCOM is shown in Figure 2.7.
Figure 2.7 Direct control diagram of a STATCOM
In this approach [2] the reactive output current can be controlled directly by the internal voltage
control mechanism of the converter while the DC voltage of capacitor is kept constant. To make
this possible real power exchange is needed and Pulse Width Modulation (PWM) is applied to
29
control the output real power and output voltage. Inputs from the external control circuit to the
indirect control are AC system bus voltage Us, converter output current Ig and the reactive
current IQref, plus the DC voltage reference Udcref. The DC reference voltage determines the real
power that the converter absorbs from the AC system in order to compensate its internal power
loss. As illustrated in Figure 2.7, the reactive component of the STATCOM output current is
compared with reference current IQref from external control. The real part is compared with IPref
from DC voltage regulation loop. After suitable amplification, the real and reactive current
error signals are processed to calculate the magnitude and phase angle Δδ. As in the case of
indirect internal control, Δδ is added to the basic synchronizing signal angle θ that is from the
Phase Locked Loop. As a result, the angle summation (Δδ + θ) together with the desired
converter output voltage, Ug, operates the Gate Pattern Logic circuit to provide the individual
gate drive logic signals to the switches. The internal control scheme operates the converters
with a DC power supply, the internal real current reference, IPref, can be summed to an
externally provided real current reference. This current, IgP, can indicate the desired real power
exchange with the AC system.
2.2.4 Steady State and Transient Characteristics of a STATCOM
V-I characteristic
The V-I characteristic of a STATCOM [13] is shown in Figure 2.8.
30
Figure 2.8 V-I characteristic of a STATCOM
On the Y axis in Figure 2.8, Vt is the per unit system voltage. The intersection of a given
characteristic sloped line with Y-axis provides the STATCOM operating voltage, i.e. the Y
intercept is the STATCOM voltage. It can be observed from the figure that the STATCOM can
be operated as either a capacitive or an inductive compensator. It is also depicted in Figure 2.8
that the STATCOM is able to control its output current. As shown in the figure, the STATCOM
can provide full rated steady-state reactive current even in the case that the system voltage is as
low as 0.15 p.u rated. This outstanding capability, compared to other shunt connected FACTS
devices, is particularly useful for the situations in which the STATCOM is needed to support
the system voltage during or after fault conditions.
31
Transient Stability
To examine the concept of transient stability, consider Figure 2.9, that shows a two-machine,
two-line power system with a STATCOM installed in the middle of one line.
Figure 2.9 Two-machine, two-line power system with a STATCOM
In Figure 2.9, Ui is the generator terminal voltage; Uj is the voltage at the receiving end. Ub1 and
Ub2 are voltages at the sending bus and the receiving bus while U1 represents the system voltage
at the STATCOM connection point. ZS1 is the impedance between the generator and the
STATCOM; Zr1 is the impedance between STATCOM and the receiving generator. P, Q and I
represent, respectively, real power, reactive power and current at various locations of the system.
The effectiveness of a STATCOM on transmission line stability improvement can be
conveniently explained with the equal area criterion [10] for the system in Figure 2.9.
32
In normal practice voltage amplitudes on both ends of the transmission lines are equal. From
equation 2.1, it means:
|Ub1| =|Ub2| = U
Defining:
δ = θ1- θ2 (2.6)
equation 2.1 can be re-written as:
(2.7)
With the STATCOM installed at the mid-point of the transmission line system, the real power
transferred through the line is:
(2.8)
Based on equations 2.7 and 2.8, the curves in Figure 2.10 show the power transmitted in the
system without STATCOM and with STATCOM installed, respectively. The system is
represented by the P versus δ curve ‘a’ and it is operating at angle δ1 to transmit power when a
fatal fault occurs on line 2 [14]. During the fault, the system is characterized by the P versus δ
curve ‘b’. During the fault transient, the transmitted power drops significantly but at the same
time the mechanical input power to the sending generator remains substantially constant
corresponding to P1. As a result, the generator accelerates and the system angle increases from δ1
to δ2, at which time the protective breakers disconnect the fault line 2 and the generator still
accelerates. The additional energy absorbed by the generator during this transient corresponds to
the area ‘A1’. After the fault is cleared, the system without line 2 is represented by P versus δ
curve ‘c’. At angle δ2 on curve ‘c’ the transmitted power exceeds the mechanical input power P1
and the generator starts to decelerate. However the angle keeps increasing up to δ3 due to the
33
kinetic energy stored in the machine. δ3 is the maximum angle where the decelerating energy
(area A1) is equal to accelerating energy (area A2). The limit of transient stability is reached at
δ4, beyond which the decelerating energy would not balance the accelerating energy and system
synchronism would be lost. The area ‘Amargin’ between δ3 and δ4 represents the transient stability
margin of the system. From both curves it can be observed that the Amargin in the case with a
STATCOM installed is significantly bigger than that in the case without the STATCOM. The
above illustrates that the system stability has been improved by the STATCOM installation.
Without STATCOM With STATCOM
Figure 2.10 Illustration of equal area criterion for transient Stability
2.2.5 Harmonic profile of STATCOM
As mentioned before, converters in STATCOM always have an inductive impedance interface
with the AC system (usually through a coupling transformer). The function of the inductance in
the circuit is to ensure that the DC capacitor does not discharge rapidly into a capacitive load
such as a transmission line [2]. The inductance also reduces the resultant harmonic current flow.
It is preferable if the STATCOM converter generates lower amplitude harmonics. Following is
an analysis of a simple six-pulse VSC-based STATCOM to illustrate harmonics generation.
34
As shown in Figure 2.3, the elementary 6-pulse VSC-based STATCOM consists of six self-
communicating semiconductor high power switches, such as IGBT or GTO, with anti-parallel
diodes. The converter can produce a balanced set of three quasi-square voltage waveforms at a
given frequency. The output voltage of the STATCOM is a staircase type synthesized waveform.
It has substantial harmonics in addition to the fundamental. The following analysis is for a 180°
conduction sequence, a sequence where three switches in different legs conduct for equal time
intervals and conduct at a time [12].
Using Fourier-series equation, the STATCOM output voltage may be expressed as
(2.9)
where coefficients a0, an and bn can be determined by considering one fundamental period of Vab.
If Vab has no dc component, then a0=0. With odd wave symmetry, an=0. The coefficient bn can be
determined as:
(2.10)
Then
=
(2.11)
Therefore
35
(2.12)
For a 180° conduction sequence, α = 30°, where α is half of a step interval. The triplen
harmonics are zero in the output line voltage as per equation 2.13, because when n=3k, if
k=1,3,5,…, then cos (nα) =0 and if k=2,4,6… then cos(nα) = . It also can be noted that when
n=5k, cos(nα)= . Hence the STATCOM output voltage only includes the harmonic
components of (6k ) f0 in its output voltage, where f0 is the fundamental output frequency and
k=1,2,3…
The magnitudes of various harmonics in the converted voltage from the 6 pulse STATCOM are
shown in Figure 2.11.
Figure 2.11Typical Harmonics in 6-pulse STATCOM voltage output
36
To reduce the harmonic generation in the system, various converter configurations and converter
switching techniques are utilized in practice. This could involve transformer configurations,
different topologies of the STATCOM with multiple-level, multiple-pulse converter controls,
etc. [12].
2.2.6 Detailed Mathematical Model of STATCOM
Since a STATCOM produces a synchronous voltage with the AC system, it can be considered as
a synchronous voltage source. The real power and the reactive power can be solved with the
Park’s Transformation.
The equivalent circuit of a 6-pulse VSC-based STATCOM including a coupling transformer is
shown in Figure 2.12.
Figure 2.12 STATCOM Equivalent Circuit
37
The inductance L is primarily from the reactance of the coupling transformer; Rs represents the
total loss from the converters and transformer; Udc is the DC voltage on the capacitor and idc is
the DC current. For analysis, the following assumptions are made:
1) All the switches are ideal.
2) Only three phase sinusoidal voltages with 120 degrees phase displacement are generated
from STATCOM, and the AC system voltage is symmetrical.
3) All harmonics are neglected.
Static Module of STATCOM
As discussed before, a STATCOM can be taken as a synchronous voltage source with
controllable output voltage magnitude and phase angle. Refer to phasor diagram in Figure 2.2 for
the following discussion.
The reactive current Ig of the STATCOM and the corresponding reactive power Q exchanged is
determined by equations 2.4 and 2.5, repeated here for convenience:
Ig=
(2.4)
Q=
(2.5)
It can be shown that:
Ug =
(2.13)
Therefore, the real power and active power from the STATCOM to the AC system are:
P =
(2.14)
38
Q =
(2.15)
Recall the Ratio of Modulation M defined as:
M =
(2.6)
Then:
Udc =
=
(2.16)
It can be observed from the above equations that when the real power loss of the STATCOM,
which is represented by Rg in the phasor diagram and equations, is included, the phase angle δ
can be used to determine,
1) The STATCOM output voltage
2) DC voltage of capacitor bank
3) The magnitude and direction of real power and reactive power.
Also, because of the angle δ, the current of STATCOM is not completely orthogonal to the AC
system voltage.
Dynamic Module of STATCOM
For the AC voltage, if ω is the system frequency:
(2.17)
39
where Usa, Usb and Usc are the system voltages.
The voltage generated by the STATCOM is a three phase symmetrical voltage that has a phase
angle, δ, with the AC system. Then:
(2.18)
From the circuit of Figure 2.13 and using the principle of conservation of energy:
(2.19)
Using the Park’s Transformation, neglecting zero sequence components:
= Pk
(2.20)
= Pk ·
(2.21)
in which Pk is the a-b-c to d-q-0 transformation operator:
40
Pk
(2.22)
Then the mathematical model in the d-q-0 frame of reference is:
(2.23)
Pk
(2.24)
When the system is in asymmetrical operation there are still no zero sequence components
because of the delta connection of the STATCOM converters. The system voltage then can be
decomposed into positive and negative sequence components according to the symmetrical
component method. Taking phase A as the reference, the angle for positive voltage is zero and
that for negative voltage is , then in the time domain:
=
(2.25)
41
Using Park’s transformation, the following can be obtained:
(2.26)
Then from equations 2.21 and 2.26 the current and voltage of a STATCOM can be resolved in
the case of asymmetrical conditions.
2.2.7 STATCOM applications
Over the past few decades Voltage Sourced Converter based technology has been successfully
applied in a number of FACTS projects. In 1980, Kansai Electric Power Co. Inc. (KEPCO) and
Mitsubishi Motors developed the first STATCOM in the world, a 20 MVAR STATCOM using
forced-commuted thyristor inverters [15]. Recent STATCOM projects in North America have
demonstrated the advantages of the application of the FACTS in power systems. In 1994
Tennessee Valley Authority (TVA), USA, developed a ±100 MVAR static condenser at the
Sullivan substation for voltage control of transmission systems [16]. This installation was the
first demonstration of a STATCOM under the EPRI flexible AC transmission systems program,
and at that time was the largest installation of its type in the world with the availability of high
power GTO thyristors for the development of controllable reactive power in transmission
systems. In 1997, American Electric Power (AEP) installed the world's first Unified Power Flow
Controller (UPFC) at the Inez substation in eastern Kentucky. In phase I of the project two ±160
MVAR voltage-sourced GTO-thyristor-based STATCOM were installed. This was the first
practical demonstration of the UPFC concept with the highest power GTO-based STATCOM
equipment ever installed [17]. On May 1st, 2001, the Vermont Electric Power Company, Inc.
42
(VELCO) placed a +133/-41 MVAR, 115 kV STATCOM system on line at the Essex Substation
located near Burlington, VT, USA. The STATCOM was installed to provide dynamic voltage
support and reactive compensation on the VELCO transmission system [18] [19]. In October
2002, San Diego Gas & Electric (SDG&E) initiated the installation of a 138 kV STATCOM-
based dynamic reactive compensation system with capacity rating of 100MVAR in a major
transmission system enhancement project involving a key 230/138 kV substation [20]. In a
Northeast Utility project, a 150MVAR rated STATCOM at Glenbrook 115kV substation located
in Hartford Connecticut is split into two halves, each rated at 75MVAR. The STATCOM is to
provide fast acting dynamic reactive compensation for voltage support during contingency
events [21]. In November of 2002, BC Hydro installed a small STATCOM, an 8 MVA D-VAR
device, in their system at the Fort St. James substation to prevent voltage collapse in the 66 kV
long radial system and as a means to defer costly transmission reinforcement. It has shown that
utilizing small size STATCOMs distributed in multiple locations in a power grid is quite
effective in addressing issues such as: voltage support in contingencies; power transfer
limitations on interconnected systems; and integration of wind farms to grids [22].
As illustrated by the above projects, when a STATCOM is shunt connected in the system with
the FACTS using power semiconductor switching technology, several benefits may result:
dynamic voltage support; system stabilization; system transfer capacity increase and enhanced
power quality for both transmission and distribution systems. In normal practice, when a
STATCOM is used for voltage support, improving system stability or improving HVDC link
performance, the device is often installed at the end of a transmission line or on a bus in a power
43
substation [18] [20] [21]. For controlling power flow or increasing the power transfer limit of a
transmission line, the mid-point of the line is the best location for a STATCOM [2].
With the presence of a STATCOM in a system, there are concerns to be considered such as
harmonics caused by switching converters and potential effects on various protective schemes. In
this thesis, the focus is on the distance relay performance when a STATCOM is installed in a
transmission system. The following are some references focusing on this topic.
A general survey of the FACTS devices and a review of the effect of a STATCOM connected at
the midpoint of a transmission line on the performance of distance protection relays are
presented in Ref. [23].
The effect of the STATCOM installation locations on the measured impedance is considered in
Ref. [24]. Three locations were investigated, i.e. at the relaying point, mid-point and the remote
end of the transmission line.
Analytical and simulation results based on steady operation for modelling the STATCOM are
presented and the effect of STATCOM on a distance relay in both normal and faulty conditions
under different load levels were studied in Ref.[25].
The effect of the balanced fault in distribution system with STATCOM was analyzed and
simulated in Ref. [26]. The operating behaviour of the instantaneous over-current protection,
time-delayed instantaneous over-current protection, and definite time over-current were also
studied.
The impact of STATCOM employed in a transmission system on the performance of distance
relay was analyzed in Ref. [27]. The simulation cases include different fault conditions, influence
44
of location of STATCOM, settings of STATCOM control parameters, and the operation mode of
STATCOM.
The effect of mid-point STATCOM compensation on the performance of an impedance distance
relay under normal load and fault conditions was investigated in Ref. [28]. The adaptive distance
relaying scheme for transmission line protection was proposed and implemented in a DSP
system. In Ref. [29], detail study on a quadrilateral characteristic distance relay in presence of
STATCOM in a transmission line was given; adaptive distance relay protection was proposed
based on the control parameters from SCADA information.
The effect of mid-point FACTS compensation on the distance relay was studied in Ref. [30]. In
this study, the errors introduced in the relay due to the presence of FACTS devices were
analyzed first. Then various situations with different fault conditions and system conditions were
simulated in EMTDC. Finally the results were confirmed by testing a commercial relay through
RTDS. Mitigation methods to improve the performance of distance relays, when transmission
lines are midpoint compensated by shunt-FACTS devices, are proposed in Ref. [31].
Some references in this chapter analyzed the impact of a STATCOM on the performance of
distance relays. All studies have shown that when a STATCOM is installed in fault loops in a
transmission system, the apparent impedance seen by a conventional distance relay is different
from the one in a system without STATCOM due to the VAR injection of STATCOM and the
steady and transient component changes in the fault. In order to give an overall analysis this
work is supposed to consider the following issues in detail with different system variables and
contingencies.
Normal conditions and fault conditions;
45
STATCOM installation positions, mid-point and end receiving side;
Setting voltage of STATCOM, 1.1pu, 1.0pu, and 0.90pu;
Fault types, signal phase to ground, phase to phase, phase to phase to ground, three-phase
to ground;
Fault locations, from sending terminal to receiving terminal;
Faulty resistances, from small to relatively large;
Comparison with the situations without STATCOM.
46
Chapter Three: Modeling of Distance Protection Impedance
As discussed in Chapter 2, the best location for the installation of a STATCOM to improve
system stability in a two-power source transmission system is the mid-point of the transmission
line. In this chapter, the impedance measured by a distance relay is analysed when a STATCOM
is installed in this way. The scenarios discussed in this chapter are investigated further by
simulations in Chapter four.
Fault impedance calculation by a distance relay relies on the voltage and current of each phase
measured at the relay location. How the transmission line impedance seen by a distance relay on
the incidence of a fault is modified, when a STATCOM is installed at the middle of the line, is
discussed in this chapter. Combination of the single phase to ground fault and phase to phase
fault schemes can cover all types of faults in the forward direction of the transmission line.
3.1 STATCOM installed at mid-point of the transmission line
The system shown in Figure 3.1 is utilized to perform an analysis of the distance relay protecting
a transmission line with a STATCOM installed at the mid-point. In the circuit, two generators,
G1 and G2, are connected with a transmission line. The distance relay is installed next to Bus 1
to protect the transmission line on which a STATCOM is installed at the mid-point (n=0.5 in
Figure 3.1). In this case, only the distance relay close to Bus 1 is analysed. Another distance
relay installed at the Bus 2 end to protect the transmission line should behave in a similar manner
when the same types of faults occur on the transmission line.
47
Figure 3.1 Transmission Line with a STATCOM at mid-point
In order to analyze the operation of the distance relay when a STATCOM is installed at the mid-
point of the line, a sequence network for a single phase fault is utilized. The apparent impedance
seen by the distance relay can be calculated with the symmetrical components of the voltage and
the current measured at the relay location.
The basic equation to calculate the apparent impedance seen by a distance relay for a single
phase to ground is [4]:
Z =
(3.1)
where:
VR, IR are the phase voltage and current at relay point
IR0 is zero sequence phase current
Z0, Z1 are zero and positive sequence impedance, respectively, of the
transmission line
48
For a distance relay on this transmission line, there are two possible fault locations to consider
relative to the STATCOM in the circuit: before and after the STATCOM point of installation.
3.1.1 Single phase fault after the STATCOM
A transmission line with a STATCOM installed at the mid-point and a single phase to ground
fault in the second half of the transmission line, i.e. after the STATCOM, is shown in Figure 3.2.
In the circuit, the distance relay is installed next to the sending Bus 1 and protects the
transmission line. The parameter ‘n’ is defined as the per unit distance from the fault location to
the relay location. Iline is the current in the transmission line after the STATCOM installation
point, Vs and Is are the voltage and current at bus 1, respectively, If is the ground fault current, Ist
is the shunt current injected from the STATCOM, Z is the combined impedance of the whole
transmission line.
Figure 3.2 Circuit with a fault after the STATCOM
49
Figure 3.3 Sequence Circuit with a single phase to ground fault after mid-point STATCOM
The sequence circuit for the case of a single phase to ground fault (A-G) in the transmission line
when the STATCOM is included in the fault loop is shown in Figure 3.3.
From Figure 3.3 it can be written that:
V1s= V1f + 0.5Z1I1s + (n-0.5)Z1(I1s + I1st) (3.2)
V2s= V2f + 0.5Z2I2s + (n-0.5)Z2(I2s + I2st) (3.3)
50
V0s= V0f + 0.5Z0I0s + (n-0.5)Z0(I0s + I0st) (3.4)
As:
Z1=Z2 (for a transmission line) (3.5)
Vs= V1s + V2s + V0s, (3.6)
V1f + V2f + V0f = 0 (for a direct short-circuit to ground) (3.7)