i ANI CHUKWUNONSO CALEB PG/M.ENG/13/66409 AN INSULATION CO-ORDINATION PROCEDURE FOR POWER SYSTEM FACULTY OF ENGINEERING DEPARTMENT OF ELECTRICAL ENGINEERING Azuka Ijomah Digitally Signed by: Content manager’s Name DN : CN = Webmaster’s name O= University of Nigeria, Nsukka OU = Innovation Centre
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i
ANI CHUKWUNONSO CALEB
PG/M.ENG/13/66409
AN INSULATION CO-ORDINATION
PROCEDURE FOR POWER SYSTEM
EQUIPMENT
FACULTY OF ENGINEERING
DEPARTMENT OF ELECTRICAL ENGINEERING
Azuka Ijomah
Digitally Signed by: Content manager’s Name
DN : CN = Webmaster’s name
O= University of Nigeria, Nsukka
OU = Innovation Centre
ii
DEPARTMENT OF ELECTRICAL ENGINEERING
UNIVERSITY OF NIGERIA NSUKKA
A THESIS SUBMITTED IN PARTIAL FULFILMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
Master of Engineering
TOPIC
AN INSULATION CO-ORDINATION PROCEDURE FOR POWER
SYSTEM EQUIPMENT
BY
ANI CHUKWUNONSO CALEB
PG/M.ENG/13/66409
iii
SUPERVISOR: PROF. T.C. MADUEME
OCTOBER, 2015
iv
CERTIFICATION
This is to certify that this project work titled “AN INSULATION COORDINATION
PROCEDURE FOR POWER SYSTEM EQUIPMENT” was carried out by ANI
CHUKWUNONSO CALEB, with Reg. No.: PG/M.ENG/13/66409 in the department of
Electrical Engineering, University of Nigeria Nsukka and meets the regulations governing the
Award of Degree of Master of Engineering(M.ENG) of the University of Nigeria Nsukka
………………………………… ……………..
Engr. Prof. T.C. Madueme Date
(Project Supervisor)
……………………………….. …………….
Engr. Prof E.C Ejiogu Date
(Head of Department)
……………………………… …………….
Prof. A. O. Ibe Date
v
External Examiner
………………………………… ……………..
Engr. Prof. E. S. Obe Date
Faculty PG Rep.
vi
APPROVAL
The contentS of this report are true reflection of the project undertaken by Ani Chukwunonso
Caleb (PG/M.ENG/13/66409). It is hereby accepted by the Department of Electrical
Engineering, Faculty of Engineering, University of Nigeria, Nsukka in partial fulfillment of
the requirement of for the award of master of engineering in Electrical Engineering
(M.ENG.) of University of Nigeria, Nsukka.
………………………………… ……………..
Ani Chukwunonso Caleb Date
Student
………………………………… ……………..
Engr. Prof. T.C. Madueme Date
Project Supervisor
……………………………….. …………….
Engr. Prof E.C Ejiogu Date
Head of Department
vii
……………………………… …………….
Prof. A. O. Ibe Date
External Examiner
………………………………… ……………..
Engr. Prof. E. S. Obe Date
P.G. Faculty Rep.
viii
DEDICATION
This work is dedicated to the Almighty God and to my Parents; Mr and Mrs Chibuzo Ani.
ix
ACKNOWLEDGEMENTS
I would like to use this opportunity to express my profound gratitude to my supervisor Prof
T.C. Madueme, for his guidance, encouragement, and total support throughout the course of
this thesis work. It was an extremely useful learning experience for me to be one of his
students. From him I have gained not only extensive knowledge, but also a careful research
attitude.
To Prof E.C. Ejiogu; who taught me that hard work and persistence is an important
research instrument. I also admire the motivation you gave me during my research period.
I also want to appreciate Prof S. E. Obe who has been my guidance and a counselor
during this research period. Moreover, I thank Dr. C.U. Ogbuka, Dr. B.N Nnadi and Engr.
Dr. C.M. Nwosu for your persistent advice as regards my research work. I appreciate all staff
of electrical engineering department and my colleagues in the division of power electronics
group and all the post graduate students in general for their support.
x
ABSTRACT
Generally, for existing Insulation co-ordination studies the power system has been modeled
either by deterministic mathematical techniques or by statistical methods. The shortcoming of
the existing conventional mathematical technique of Insulation co-ordination analysis is that
it assumes that the power system dynamics is linear. This makes analysis of over voltage
response of the system under transients less optimal for determining over voltage withstand
of system elements. Thus, this work seeks to model a lightning induced over voltage transient
in a High voltage power system substation(132/33KV) used as a case study) using Hidden
Markov Model, to determine the maximum likelihood lightning surge signal. The station
data and configuration was modeled/simulated (in a MATLAB environment), which
implements the algorithms used in the work. The Hidden Markov algorithm(which makes use
of observable parameters to study what is happening at the hidden states), was used to
formulate the problem, while the Baum-welch and Viterbi algorithm were used to
find/identify the maximum likelihood lightning overvoltage waveform. These hidden states
are represented with different scenarios introduced in the work and the waveform identified,
is used to determine the Basic Insulation level(BIL), which is used to determine other
parameters accurately, which in turn helps to ensure an optimal/novel Insulation coordination
procedure for power system equipment in the station.
The results showed that the minimum required margin(15%) exceeded by a little value(i.e.
about 1.08) and the evaluation carried out to raise the protection margin to 18% meant the
relocation of the arrester to within 5.56m of the transformer.
xi
TABLE OF CONTENTS
Pages
Title Page i
Certification ii
Approval iii
Dedication iv
Acknowledgements vi
Abstract vii
Table Of Contents viii
List Of Figures xi
List Of Tables xii
List Of Symbols And Abbreviation xiii
Chapter One: Introduction
1.1 Background of the Study 1
1.2 Statement of the Problem
2
1.3 Objectives of the Study 3
1.4 Significance of the Study 4
xii
1.5 Scope of the Study 5
Chapter Two: Literature Review
2.1 Historical Trends 6
2.2 Definition of Terminology 8
2.3 Over Voltages 11
2.3.1 Power Frequency Overvoltages 13
2.3.2 Overvoltage Caused by an Insulation Fault 13
2.3.3 Overvoltage by Ferromagnetic Resonance 13
2.3.4 Switching Overvoltages 14
2.3.5 Normal Load Switching Overvoltage 14
2.4 Insulation Coordination Principle 14
2.4.1 Highest Power Frequency System Voltage(Continuous) 15
2.4.2 Temporary Power-Frequency Overvoltages 15
2.4.3 Transient Overvoltage Surges 15
2.4.4 Withstand Levels of the Equipment 16
2.5 Line Insulation Coordination 18
2.6 Station Insulation Coordination 20
2.7 Strategy of Insulation Co-Ordination 23
2.7.1 Conventional Method of Insulation Co-Ordination 24
xiii
2.7.2 Statistical Approach to Insulation Coordination 26
2.8 Hidden Markov Model 30
2.8.1 Brief History of Markov Process and Markov Chain 31
2.8.2 Brief History of Algorithms Need to Develop Hidden Markov Models 32
2.8.3 The Expectation-Maximization (E-m) Algorithm 33
2.8.4 The Baum-Welch Algorithm 34
2.8.5 The Viterbi Algorithm 34
2.9 Mathematical Basics of Hidden Markov Models 35
2.9.1 Definition of Hidden Markov Models 35
2.10 Summary of Related Literatures 36
Chapter Three: Research Methodology
3.0 Model Design 37
3.1 The Model Design Strategy 37
3.2 Scenario Description 39
3.2.1 Surge Event Scenario A 39
3.2.2 Surge Event Scenario B 39
3.2.3 Surge Event Scenario C 39
3.3 The Overvoltage Transient Assessment Based on the Hmm 39
3.4 The Overvoltage Training Disturbance Classification 40
3.4.1 The Processing Block 43
xiv
3.5 Computing for the Insulation Coordination 50
3.6 Modeling the Power System 53
3.6.1 Transmission Line Conductors Model 53
3.6.2 Transmission Line Towers Model 54
3.6.3 Surge Arresters Model 54
3.6.4 Transformer Model 54
3.6.5 Lightning Surge Model 54
3.7 Assumption for Lightning Surge 55
Chapter Four: Simulation and Result Evaluation
4.0 Simulation and Result Evaluation 56
4.1 Simulation of the Three Lightning Overvoltage Transient Scenarios 56
4.1.1 Surge Event Scenario A 60
4.1.2 Surge Event Scenario B 61
4.1.3 Surge Event Scenario C 61
4.2 Waveform at the Strike Point 63
Chapter Five: Recommendation and Conclusion
5.1 Summary 70
xv
5.2 Conclusion 71
5.3 Recommendation 71
5.4 Suggestion for Further Studies 72
References 73
Appendix 76
LIST OF FIGURES
Figure
Pages
2.1 Statistical Impulse Withstand Voltage 10
2.2 Stastical Impulse Voltage 11
2.3 Allegheny Power System's 500-kV Tower. 19
2.4: 330kv Nigeria Power Transmission Tower 20
xvi
2.5 The Strike Distances and Insulation Lengths in a Substation. 21
2.6 Margin of Protection and Insulation Withstand Level 24
2.7 Coordination Using Gaps 25
2.8 Coordination of Bils and Protection Levels Classical Approach) 26
2.9 Method of Describing the Risk of Failure. 28
2.10 Reference Probabilities for Overvoltage and for Insulation Withstand Strength 29
2.11 The Statistical Safety Factor and its Relation to the Risk of Failure 30
3.1: Flow Chart of Proposed Logarithm for Insulation Coordination 42
3.2: Steps for the Signal Processing and Observation Evaluation Problem 43
3.3: Flow Chart of the Hmm Training Process for the Observation Evaluation Problem 46
4.1 Model of the 330kV/132kV Power Station
At New Haven Enugu Nigeria 57
4.2 The illustration of incoming transmission surge wave of scenario A 60
4.3. Resultant Waveform for Surge Event Scenario A 61
4.4. The illustration of incoming transmission surge wave of scenario B1 60
4.5. Resultant Waveform for Surge Event Scenario B 62
4.6 The illustration of incoming transmission surge wave of scenario C 63
4.7. Resultant Waveform for Surge Event Scenario C 64
4.8 Resultant Waveform Of Three Surge Event Scenarios (The Combined Plot) 65
4.9 Resultant Waveform At The Strike Point 66
4.10 Location Of Arrester 4 (On Phase A) To The 132/33kv Transformer Supplying The
Kingsway Line I 68
xvii
LIST OF TABLES
2.1: Characteristics of the various Overvoltages types 12
2.2: Basic Impulse Insulation Levels 16
3.1: Observed electrical feature for HMM classification of the lighting overvoltage transients
of the power system. 44
4.1: Station Parameters/Data supplied by PHCN 58
4.2: Transmission Line data 59
4.3: Corona damping constant 𝐾𝑐𝑜 65
xviii
LIST OF SYMBOLS AND ABBREVIATIONS
HMM: Hidden Markov Model
BIL: Basic Insulation Level
BSL: Basic Switching Level
MV: Mega Volts
PF: Power Frequency
LOV: Lightning Over Voltage:
SOV: Switching Over Voltages
E.H.V: Extra High Voltages
U.H.V: Ultra High Voltages
𝑉𝑆: Statistical Overvoltage
BW: Baum Welch
xix
P(𝑂|λ): Maximum Likelihood Probability
F(𝐼𝑚): Lightning Current Probability
𝐾𝑐𝑜: Corona damping Constant. µs/(KV.m)
IEC: International Electro-technical Commission
IG: Impulse Generator
𝑋𝑝: Limit Overhead line distance within which lightning event occurs, m
T: Longest Travel time of Surge Current, (µs)
𝑈𝑝𝑙 is the lightning impulse protective level of the arrester, KV
U: Overvoltage Amplitude, KV.
𝛽: Reflection Coefficient
𝑆: Steepness of Surge Voltage, Kv/µs.
𝐸𝑡: Surge Voltage at the Transformer Terminal.
𝑙𝑡: Separation between Transformer and the Arrester.
𝐸𝑎: Arrester BIL.
MOP: Margin of Protection.
CFOV: Critical Flashover Voltage.
xx
CIGRE: Conseil International Des Grands Reseaux Electriques(International Council on
Large Electric Systems).
NEMA: National Electrical Manufacturers Association
NELA: Nigeria Electric Light Association
AIEE: Advanced International Electronic Equipment.
EEI: Edison Electric Institute
CDMA: Code Division Multiple Access
GSM: Global System for Mobile Communication
LAN: Local Area Network
CVT: Capacitor Voltage Transformer.
PHCN: Power Holding Company of Nigeria.
CFO: Critical Flash Over
1
CHAPTER ONE
INTRODUCTION
1.0 Background of the Study
The demand for the generation and transmission of large amounts of electric power today,
necessitates its transmission at extra-high voltages. In modern times, high voltages are used
for a wide variety of applications covering the power systems, Industry and research
Laboratories. Such applications have become essential to sustain modern civilization[1].
The diverse conditions under which a high voltage apparatus is used necessitate careful
design of its insulation and the electrostatic field profiles[2]. This entails the analysis of the
electrical power system to determine the probability of post insulation flashovers. For
instance, analysis must be carried out to determine that the insulation contained within power
system components like transformers has the acceptable margin of protection. Since the
internal insulation is not self-restoring, a failure is completely unacceptable. An insulation co-
ordination study of a substation will present all the probabilities and margins for all transients
entering the station.
Over voltages are phenomena which occur in power system networks either externally or
internally. The selection of certain level of over voltages which are based on equipment
strength for operation is known as Insulation co-ordination[3]. It is essential for electrical
power engineers to reduce the number of outages and preserve the continuity of service and
electric supply. In another perspective, Insulation co-ordination is a discipline aiming at
achieving the best possible techno-economic compromise for protection of persons and
equipment against over voltages, whether caused by the network or lightning, occurring on
2
electrical installations. The purpose of Insulation co-ordination is to determine the necessary
and sufficient insulation characteristics of the various network components in order to obtain
uniform withstand to normal voltages and to over voltages of various origins.
However over voltages are extremely hard to calculate. They cannot generally be
predetermined, since they involve incalculable elements which vary from site to site. Hence
effective Insulation co-ordination requires accurate modeling of the power system. Modeling
transmission lines and substations help engineers understand how protection systems behave
during disturbances and faults.
Though a number of techniques have been developed for modeling transient disturbances in
power systems, the problem of doing optimal Insulation co-ordination is still limited by
accurate model of the power system. Generally, for existing Insulation co-ordination studies
the power system has been modeled either by deterministic mathematical techniques or by
statistical methods. The shortcoming of the existing conventional mathematical technique of
Insulation co-ordination analysis is that it assumes that the power system dynamics is linear.
This makes analysis of over voltage response of the system under transients less optimal for
determining over voltage withstand of system elements. While the statistical technique,
though more accurate[4][5][6], is known that the statistical evaluation of the risk cannot be
assessed if the breakdown behavior of the insulation is unknown or if it is referred only to the
basic Impulse level(BIL) of the power system component.
Hence a novel Insulation Co-ordination procedure for power system equipment is proposed in
this work.
1.1 Statement of Problem
3
With reference to the limitation of the deterministic mathematical and statistical approach of
power system insulation co-ordination, as highlighted in the background of this study. Thus
given a high voltage(HV) power station and its associated transmission line, the problem to
be tackled by this work is to model overvoltage transient disturbance from lightning using
Hidden Markov Model(HMM), to determine the maximum likelihood lightning surge
waveform. This is to enable the determination of voltage stresses within the station during
surge event and to determine voltage withstand of systems insulation elements; i.e. the Basic
Impulse Insulation(BII) in order to determine protection margin based on equipment data and
to make optimal placements of protection devices within the system.
1.2 Objectives of the Study
The major objective of this work is to develop a model that enables the investigation of over
voltages due to lightning voltages in order to effectively carry out insulation Co-ordination of
a high voltage substation power system. Hence, this work realizes the following specific
objectives:
1. To model a lightning induced over voltage transient in a High voltage power system
substation using Hidden Markov Model, to determine the maximum likelihood
lightning surge signal.
2. To carry out simulation of the response of the power system to lightning over voltages
and determining over voltages induced at specific junctions of the substation.
3. To carry out evaluation of the results of the simulation to determine voltage withstand
capabilities(Basic Impulse insulation; BIL), evaluating protection margin based on the
systems equipment data and optimal placement of protection devices throughout the
substation.
4
4. To carryout validation of the findings and make recommendation for both actual
implementation of the proposed insulation co-ordination technique and
recommendations for further improvement of the technique.
1.3 Significance of the Study
The power system constitutes a huge factor in the national and global economy. When
power system equipment is not properly protected during over voltage, this equipment
gets damaged necessitating repairs. Hence improper equipment protection against
over voltages increases causes of repairs and cost of power system maintenance. This
means substantial impact on the economy. Hence the realization of the objectives of
this study to develop a novel model to enhance the reliability of insulation co-
ordination of power systems is significant to the reduction of system downtime,
reduction of power system repair and maintenance cost. This means the success of
this work helps to enhance the economy, since all modern services (including
banking, telecommunication, agriculture, manufacturing, health care etc.) that depend
on reliable electric energy benefits from interruptible supply of power.
High voltage insulation failure poses danger to persons and equipment. Hence the
significance of a research that seeks to enhance protection technique for persons and
equipment is in no doubt. Therefore, the proposed study presents much promise for
the enhancement of human and equipment safety from over voltages within electric
power systems.
One of the things that hamper effective administration of electric power generation,
transmission and distribution is planning and control. These activities are in turn
5
hampered by inaccurate evaluation and prediction of equipment and systems failure
rates and lack of reliable probability estimate of post insulation flashovers. This
problem is substantially caused by lack of accurate model of power systems. With
accurate modeling of power system for insulation co-ordination activities, it would be
possible to estimate proper withstand capabilities of power system limits, estimate of
probabilities of failures and proper equipment protection margin. Thus, proper
planning and control of power systems can be done, ensuring effective administration
of electric power systems.
Also, with reference to the modeling of the power system proposed in this study, this
would help increase the understanding of power system engineers about the behavior
of power system components under lightning induced disturbances.
This work makes a contribution regarding the use of Hidden markov model
(HMM), in determining the probabilistic maximum likelihood of surge wave signal,
based on the digital model of a power system. Therefore, this contribution would
benefit further research on both power systems modeling and insulation co-ordination
studies of high voltage power systems.
1.4 Scope of the Study
This work covers modeling of lightning induced overvoltage transients in HV power
substation and its associated transmission lines. It covers insulation coordination, involving
lightning arresters, their placement relative to substation transformers and the evaluation of
protection margin. However, insulation coordination for switching overvoltage and substation
shielding are not considered. PHCN 330/132/33KV Transmission station switch yard New
Haven Enugu, was used as a case study.
6
7
CHAPTER TWO
LITERATURE REVIEW
2.1 Historical Trends
Coordination of insulation was not given serious consideration until after the First World
War, mainly because of lack of information on the nature of lightning surges and the surge
strength of apparatus insulation. Since concrete data were lacking on the actual surge strength
of insulation or the discharge characteristics of protective equipment, early attempts at
coordination were rule-of-thumb methods based on experience and individual ideas. The
result was that some parts of the station were over-insulated while others were under-
insulated. Also, the gradual increasing of line insulation in an attempt to prevent line
flashovers were eliminated at the expense of apparatus failures. Growth of power systems
demands for improved power service, and more economical system operation focused more
and more attention on the problems of surge voltages, adequate insulation, and its protection.
Thus, during the period from about 1918 to 1930 considerable work was done by individual
investigators and laboratories in collecting data on natural lightning and in developing
insulation testing methods and technique. Although progress was seemingly slow, it resulted
in a fair knowledge of the nature of lightning surges and the establishment of universal surge
producing and measuring devices. Very little correlation between laboratories was attempted
during that period[7].
In 1930, the NEMA-NELA Joint Committee on Insulation Coordination was formed to
consider laboratory testing technique and data, to determine the insulation levels in common
use, to establish the insulation strength of all classes of equipment, and to establish insulation
levels for various voltage classifications. After ten years of study and collection of data this
8
schedule was fairly well completed. Numerous articles in trade magazines show the results.
In a report dated January 1941, the committee, now known as the joint AIEE-EEI-NEMA
Committee on Insulation Coordination, rounded out the program by specifying basic impulse
insulation levels for the different voltage classifications.
Test specifications for apparatus are prepared on the basis of demonstrating that the
insulation strength of the equipment will be equal to or greater than the selected basic
insulation level and the protective equipment for the station should be chosen to give the
insulation meeting these levels as good protection as economically justified[7].
The following are the basic definition of insulation coordination in its most fundamental and
simple form:
(a). Insulation coordination is the selection of the insulation strength.
If desired, a reliability criterion and something about the stress placed on the insulation could
be added to the definition. In this case the definition would become
(b). Insulation coordination is the "selection of the insulation strength consistent with the
expected overvoltages to obtain an acceptable risk of failure"[6].
In some cases, engineers prefer to add something concerning surge arresters, thus, the
definition is expanded to
(c). Insulation coordination is the "process of bringing the insulation strengths of electrical
equipment into the proper relationship with expected overvoltages and with the
characteristics of surge protective devices"[8].
The definition could be expanded further to
9
(d). Insulation coordination is the "selection of the dielectric strength of equipment in relation
to the voltages which can appear on the system for which equipment is intended and taking
into account the service environment and the characteristics of the available protective
devices" [9].
(e). "Insulation coordination comprises the selection of the electric strength of equipment and
its application, in relation to the voltages which can appear on the system for which the
equipment is intended and taking into account the characteristics of available protective
devices, so as to reduce to an economically and operationally acceptable level the probability
that the resulting voltage stresses imposed on the equipment will cause damage to equipment
insulation or affect continuity of service" [10].
2.2 Definition of Terminology used in Insulation Coordination
(i). Nominal System Voltage: It is the r.m.s. phase-to-phase voltage by which a system is
designated. Also it is the phase to phase voltage of the system for which the system is
normally designed. S as 11KV,. S as 11KV,33KV, 132KV, 220KV, 400KV systems[11].
(ii). Maximum System Voltage: It is the maximum rise of the r.m.s. phase-to-phase system
voltage.
(iii). Factor of Earthing: This is the ratio of the highest r.m.s. phase-to-earth power
frequency voltage on a sound phase during an earth fault to the r.m.s. phase-to-phase power
frequency voltage which would be obtained at the selected location without the fault. This
ratio characterizes, in general terms, the earthing conditions of a system as viewed from the
selected fault location.
(iv). Effectively Earthed System
10
A system is said to be effectively earthed if the factor of earthing does not exceed 80%.
Factor of earthing is 100% for an isolated neutral system, while it is 57.7% (1/√3 = 0.577) for
solidly earthed system.
(v). Insulation Level: Every electrical equipment has to undergo different abnormal transient
over voltage situation in different times during its total service life period. The equipment
may have to withstand lightning impulses, switching impulses and/or short duration power
frequency over voltages. Depending upon the maximum level of impulse voltages and short
duration power frequency over voltages that one power system component can withstand, the
insulation level of high voltage power system is determined.
(vi). Lightning Impulse Voltage: The system disturbances occur due to natural lightning can
be represented by three different basic wave shapes. If a lightning impulse voltage travels
some distance along the transmission line before it reaches to an insulator, its wave shaped
approaches to full wave, and this wave is referred as 1.2/50 wave. If during travelling, the
lightning disturbance wave causes flash over across an insulator the shape of the wave
becomes chopped wave. If a lightning stroke hits directly on the insulator then the lightning
impulse voltage may rise steep until it is relieved by flash over, causing sudden, very steep
collapse in voltage. These three waves are quite different in duration and in shapes[11].
(vii). Switching Impulse: During switching operation a uni-polar voltage appears in the
system. The waveform of which may be periodically damped or oscillating. Switching
impulse wave form has steep front and long damped oscillating tail.
(viii). Short duration power frequency withstand voltage: This is the prescribed rms value
of sinusoidal power frequency voltage that the electrical equipment shall withstand for a
specific period of time normally 60 seconds.
11
(ix). Protective Level of Protective Device: These are the highest peak voltage value which
should not be exceeded at the terminals of a protective device when switching impulses and
lightning impulses of standard shape and rate values are applied under specific conditions.
(x). Conventional Impulse Withstand Voltages: This is the peak value of the switching or
lightning impulse test voltage at which an insulation shall not show any disruptive discharge
when subjected to a specified number of applications of this impulse under specified
conditions.
(xi). Conventional Maximum Impulse Voltage: This is the peak value of the switching or
lightning overvoltage which is adopted as the maximum overvoltage in the conventional
procedure of insulation co-ordination.
(xii). Statistical Impulse Withstand Voltage: This is the peak value of a switching or
lightning impulse test voltage at which insulation exhibits, under the specified conditions, a
90% probability of withstand. In practice, there is no 100% probability of withstand voltage.
Thus the value chosen is that which has a 10% probability of breakdown[12].
12
Figure 2.1: Statistical impulse withstand voltage
(xiii). Statistical Impulse Voltage: This is the switching or lightning overvoltage applied to
equipment as a result of an event of one specific type on the system (line energizing,
reclosing, fault occurrence, lightning discharge, etc.), the peak value of which has a 2%
probability of being exceeded[12].
Figure 2.2: Statistical Impulse voltage
Insulation coordination is a discipline aiming at achieving the best possible technico-
economic compromise for protection of persons and equipment against over voltages,
whether caused by the network or lightning, occurring on electrical installations. It helps
ensure a high degree of availability of electrical power. Its value is doubled by the fact that it
concerns high voltage networks. To control insulation coordination:
the level of the possible over voltages occurring on the network must be known;
the right protective devices must be used when necessary;
the correct overvoltage withstand level must be chosen for the various network
components from among the insulating voltages satisfying the particular
constraints[13].
13
2.3 OVER VOLTAGES
An overvoltage is an abnormal voltage between two points of a system that is greater than the
highest value appearing between the same two points under normal service conditions.
• Overvoltages are the primary “metric” for “measuring” and “quantifying” power system
transients and thus insulation stress.
Also, these are disturbances superimposed on circuit rated voltage. They may occur:
• between different phases or circuits and are said to be differential mode;
• between live conductors and the frame or earth and are said to be common mode.
Their varied and random nature makes them hard to characterize, allowing only a statistical
approach to their duration, amplitudes and effects. Table 2.1 presents the main characteristics
of these disturbances.
In point of fact, the main risks are malfunctions, destruction of the equipment and,
consequently, lack of continuity of service. These effects may occur on the installations of
both energy distributors and users.
Table 2.1: Characteristics of the various Overvoltages types
Overvoltage Type (Cause) MV-HV
overvoltage
coefficient
Time Steepness of
Frequency
front
Damping
At Power frequency (Insulation
Fault
≤ √3
Long > 1s Power
frequency
low
14
Switching (short-circuit
disconnection)
2 to 4 Short 1ms Medium 1 to
200KHZ
medium
Atmospheric (direct lightning
stroke)
> 4 Very short
1 to 10µs
Very high
1,000KV/µs
high
Source:[13]
Disturbances may result in:
- Short disconnections (automatic reclosing on MV public distribution networks by overhead
lines);
- long disconnections (intervention for changing damaged insulators or even replacement of
equipment).
Protective devices limit these risks. Their use calls for careful drawing up of consistent
insulation and protection levels. For this, prior understanding of the various types of over
voltages is vital[13].
2.3.1 Power frequency overvoltages
This term includes all over voltages with frequencies under 500 Hz. The most common
network frequencies are: 50, 60 and 400 Hz. In normal operating conditions, network voltage
may present short duration power frequency overvoltages (a fraction of a second to a few
hours: depending on network protection and operating mode). Voltage withstand checked by
the standard one-minute dielectric tests is normally sufficient. Determination of this category
of characteristics is simple, and the various insulators are easy to compare.
15
2.3.2 Overvoltage caused by an insulation fault
An overvoltage due to an insulation fault occurs on a three-phase network when the neutral is
unearthed or impedance-earthed. In actual fact, when an insulation fault occurs between a
phase and the frame or earth (a damaged underground cable, earthing of an overhead
conductor by branches, equipment fault, ...), the phase in question is placed at earth potential
and the remaining two phases are then subjected, with respect to earth, to the phase-to-phase
voltage
U = V √3. (2.1)
Where U is the Line Voltage and V is the phase voltage.
More precisely, when an insulation fault occurs on phase A, an earth fault factor, 𝑆𝑑, is
defined by the ratio of the voltage of phases B and C with respect to earth, to network phase
to neutral voltage.
The following equation is used to calculate𝑆𝑑:
𝑆𝑑 =√3 (𝐾2 + 𝐾 + 1)
𝐾 + 2 (2.2)
2.3.3 Overvoltage by ferromagnetic resonance
In this case the overvoltage is the result of a special resonance which occurs when a circuit
contains both a capacitor (voluntary or stray) and an inductance with saturable magnetic
circuit (e.g. a transformer). This resonance occurs particularly when an operation (circuit
opening or closing) is performed on the network with a device having poles either separate or
with non-simultaneous operation.
2.3.4 Switching overvoltages
Sudden changes in electrical network structure give rise to transient phenomena frequently
resulting in the creation of an overvoltage or of a high frequency wave train of a periodic or
oscillating type with rapid damping[13].
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2.3.5 Normal load switching overvoltage
A normal load is mainly resistive, i.e. its power factor is greater than 0.7. In this case,
breaking or making of load currents does not present a major problem. The overvoltage factor
(transient voltage amplitude/operating voltage ratio) varies between 1.2 and 1.5.
2.4 Insulation coordination Principle
Study of insulation coordination of an electrical installation is thus the definition, based on
the possible voltage and overvoltage levels on this installation, of one or more overvoltage
protection levels. Installation equipment and protective devices are thus chosen accordingly.
Protection level is determined by the following conditions:
• Installation
• Environment
• Equipment use.
Study of these conditions determines the overvoltage level to which the equipment could be
subjected during use. Choice of the right insulation level will ensure that, at least as far as
power frequency and switching impulses are concerned, this level will never be overshot.
As regards lightning, a compromise must generally be found between Insulation level,
protection level of arresters, if any, and acceptable failure risk. Proper control of the
protection levels provided by surge limiters requires thorough knowledge of their
characteristics and behavior.
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The Insulation requirements are determined by considering the following:
2.4.1 Highest Power Frequency System Voltage(Continuous):
AC network has different nominal power-frequency voltage level(e.g. 400V, 3.3kV, 6.6kV,