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INVESTIGATION OF CIRCUIT BREAKER

SWITCHING TRANSIENTS FOR SHUNT

REACTORS AND SHUNT CAPACITORS

Mohd Shamir Ramli, B.Eng

Submitted in fulfilment of the requirements for the degree ofMaster of Engineering

School of Engineering Systems

Faculty of Built Environment and Engineering

Queensland University of Technology

2008


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Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors

M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page ii

Abstract

Switching of shunt reactors and capacitor banks is known to cause a very high rate of

rise of transient recovery voltage across the circuit breaker contacts. With improvements in

circuit breaker technology, modern SF6 puffer circuits have been designed with less interrupter

per pole than previous generations of SF6 circuit breakers. This has caused modern circuit

breakers to operate with higher voltage stress in the dielectric recovery region after current

interruption. Catastrophic failures of modern SF6 circuit breakers have been reported during

shunt reactor and capacitor bank de-energisation. In those cases, evidence of cumulative re-

strikes has been found to be the main cause of interrupter failure.

Monitoring of voltage waveforms during switching would provide information about the

magnitude and frequency of small re-ignitions and re-strikes. However, measuring waveforms at

a moderately high frequency require plant outages to connect equipment. In recent years, there

have been increasing interests in using RF measurements in condition monitoring of switchgear.

The RF measurement technique used for measuring circuit breaker inter-pole switching time

during capacitor bank closing is of particular interest.

In this thesis, research has been carried out to investigate switching transients produced

during circuit breaker switching capacitor banks and shunt reactors using a non-intrusive

measurement technique. The proposed technique measures the high frequency and lowfrequency voltage waveforms during switching operations without the need of an outage. The

principles of this measurement technique are discussed and field measurements were carried out

at shunt rector and capacitor bank installation in two 275 kV air insulated substations. Results of

the measurements are presented and discussed in this thesis.

The proposed technique shows that it is relatively easy to monitor circuit breaker

switching transients and useful information on switching instances can be extracted from the

measured waveforms. Further research works are discussed to realise the full potential of the

measuring technique.


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M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page iii

Table of Contents

Keywords ....................................................... ............................................................... ...........................i

Abstract ..................................................................... .................................................................... ......... ii

Table of Contents .............................................................. .................................................................... iii

List of Figures .................................................................. .................................................................. ....vi

List of Tables ..................................................................... ....................................................................ix

List of Abbreviations .............................................................. ................................................................x

Statement of Original Authorship ..................................................................... .....................................xi

Acknowledgments....................................................................... ......................................................... xii

CHAPTER 1: INTRODUCTION....... ..................................................................... .......................... 1

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

1.2 Research conducted ............................................................... ......................................................2

1.3 Thesis outline...............................................................................................................................3

CHAPTER 2: LITERATURE REVIEW...... ..................................................................... ............... 4

2.1 Review of current interruption in circuit breakers.......................................................................4

2.2 Reactive equipment switching ................................................................ .....................................6

2.3 Review on capacitor bank switching ................................................................... ........................82.3.1 Interrupting capacitor bank...............................................................................................82.3.2 Energising capacitor bank .............................................................. ................................13

2.4 Review of reactor bank switching .................................................................... .........................142.4.1 Interrupting shunt reactor bank.......................................................................................152.4.2 Current chopping............................................................................................................162.4.3 Reignition .................................................................. .....................................................202.4.4 Oscillation modes .............................................................. .............................................242.4.5 Interaction between phases.............................................................................................262.4.6 Energising transients.......................................................................................................28

2.5 Limitation of overvoltage transient during reactive switching ..................................................282.5.1 Over voltage limitation..................................................................................................282.5.2 Controlled switching.......................................................................................................29

2.6 Failure of circuit breaker due to restriking ............................................................... .................30

2.6.1 Importance of detecting restrike ................................................................ .....................362.7 Condition monitoring for circuit breakers ............................................................................ .....39

2.7.1 Detecting restrikes or interrupter experiencing prolong restrikes ..................................402.7.2 Alternative monitoring methods.............................................................. .......................40

CHAPTER 3: NEW METHODS FOR CONDITION MONITORING OF RESTRIKING EHV

CBS.................... ............................................................... .................................................................. . 42

3.1 Non-invasive circuit breaker monitoring using radiometric measurement................................42

3.2 Research methodology...............................................................................................................43

3.3 Developing measuring equipment ...................................................... .......................................44

3.4 Active broadband Antenna ....................................................... .................................................45

3.5 Capacitive Coupling antenna ................................................................... ..................................473.5.1 Construction of the Passive Antenna..............................................................................47


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M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page iv

3.5.2 Review on capacitive coupling.......................................................................................493.5.3 Single phase capacitive coupling model.........................................................................513.5.4 Three phase coupling inside substation ..........................................................................55

3.6 Recording instruments .............................................................. .................................................563.6.1 Digital oscilloscopes.......................................................................................................563.6.2 Coaxial cable ...................................................................... ............................................583.6.3 Measurement requirement in substation.........................................................................58

CHAPTER 4: EXPLORATORY MEASUREMENT ON SINGLE-PHASE REACTOR

SWITCHING AND CAPACITOR BANK SWITCHING ............................................................. 60

4.1 Field measurement at Ergon Laboratory....................................................................................604.1.1 Purpose of measurement.................................................................................................604.1.2 Restriking in Vacuum Circuit Breaker ............................................................. ..............604.1.3 Test and measurement set up..........................................................................................624.1.4 Reactor opening at 3kV with Passive Antenna located close to the supply transformer644.1.5 Conclusion......................................................................................................................67

4.2 Exploratory three phase capacitor bank switching measurement at Blackwall substation ........684.2.1 Purpose of site measurement ............................................................. .............................684.2.2 Site details and arrangement...........................................................................................684.2.3 Measurement set up........................................................................................................704.2.4 Summary of tests/measurement carried out....................................................................724.2.5 Opening operation ........................................................ ..................................................734.2.6 Closing operation............................................................................................................754.2.7 Discussion.......................................................................................................................77 4.2.8 Improvement to be taken ...................................................... ..........................................77

CHAPTER 5: MEASUREMENT OF CAPACITOR BANK SWITCHING............................... 79

5.1 Site details and arrangement ....................................................................... ...............................79

5.2 Test and measurement set up ..................................................................... ................................81

5.3 Summary of tests/measurement carried out ................................................................. ..............835.4 Background measurement..........................................................................................................84

5.5 Capacitor bank closing...............................................................................................................87

5.6 Imperfect capacitor bank closing...............................................................................................90

5.7 Capacitor bank opening ............................................................ .................................................94

5.8 Summary on capacitor bank switching tests..............................................................................995.8.1 Closing operation............................................................................................................995.8.2 Opening operation ........................................................ ................................................100

CHAPTER 6: MEASUREMENT OF SHUNT REACTOR BANK SWITCHING .................. 101

6.1 Site arrangement ............................................................. .........................................................1016.2 Test and measurement set up ..................................................................... ..............................103

6.3 Summary of tests/measurement carried out ................................................................. ............105

6.4 Background measurement........................................................................................................106

6.5 Shunt reactor bank closing.......................................................................................................108

6.6 Shunt reactor bank opening ................................................................ .....................................114

6.7 Summary on shunt reactor bank switching tests......................................................................1196.7.1 Background measurement ....................................................... .....................................1196.7.2 Closing operation..........................................................................................................1196.7.3 Opening operation ........................................................ ................................................119

CHAPTER 7: ANALYSIS OF RESULTS............................................................................ ......... 121


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M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page v

7.1 Three phase capacitive coupling model...................................................................................121

7.2 Observations of arcing signals in shunt reactor opening .........................................................129

7.3 Analysis in frequency-time domain ................................................................. ........................1387.3.1 Analysing using Fast Fourier Transform (FFT) ...........................................................1387.3.2 Analysing using Short Time Fast Fourier Transform (ST FFT) Analysis....................139

CHAPTER 8: CONCLUSION............................................. ........................................................... 144

REFERENCES....................................... ..................................................................... ..................... 144

APPENDICES................................................................... .............................................................. . 153Appendix A: Site Measurement Procedure..............................................................................153Appendix B : Sample of forms used for site measurement......................................................156Appendix C : Matlab Program for Capacitive Divider Model (In Chapter 7).............. ...........158


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M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page vi

List of Figures

Figure 2.1 Typical Circuit Interruption (from [7])..................................................................................5

Figure 2.2 Single Phase Capacitor Bank circuit (from [10])...................................................................8

Figure 2.3 Capacitance Switching (a) System voltage and current. (b) Capacitor voltage (c) Voltage

across CB contact. (from [9]) ..................................................................... ....................................9

Figure 2.4 Capacitance switching showing the effect of source regulation (from [9])..................... ....10

Figure 2.5 Capacitance switching with a restrike at peak voltage. (from [9]) ......................................12

Figure 2.6. Capacitance switching with multiple restrikes. (from [9]) .................................................13

Figure 2.7. Single phase equivalent circuit [11]...................................................................................16

Figure 2.8 Current chopping phenomena (from [3]).............................................................................17

Figure 2.9 Chopping Phenomena in single phase (from [11]) ......................................................... .....19

Figure 2.10 Reignition Windows (from[3]).................... ............................................................... .......21

Figure 2.11 Reignition at recovery voltage peak for a circuit with low supply side capacitance (from

[3])................................................................................................................................................23

Figure 2.12 Maximum re-ignition overvoltages (from [3]) ........................................................... .......24

Figure 2.13 Oscillation Mode in the reactor circuit ............................................................... ...............25

Figure 2.14 Load side oscillation with circuit breaker located close to shunt reactor (from[3]) .........27

Figure 2.15 Load side oscillation with circuit breaker located remote from shunt reactor (from [3]).27

Figure 2.16 (a) Typical schematic of SF6 CB showing main contacts (1), arcing contacts (2) andnozzle (3). (b) Voltage distribution in interrupter chamber. [21].................................................33

Figure 2.17 Analysis of voltage breakdown for main and arcing contacts along the contact gap. (from

[21])..............................................................................................................................................34

Figure 3.1 Photo of Broadband Active Antenna...................................................................................46

Figure 3.2 Gain vs Frequency for RF amplifier [62] ........................................................................... .46

Figure 3.3 Passive Antenna Drawing....................................................................................................47

Figure 3.4 Photo of Passive Antenna......... ..................................................................... ......................48

Figure 3.5 Electrostatic coupling between a HV conductor and secondary circuit...............................49

Figure 3.6 Capacitive Divider...............................................................................................................51Figure 3.7 Passive Antenna Equivalent Circuit .................................................................... ................52

Figure 3.8 Bode Diagram......................................................................................................................54

Figure 3.9 Capacitive coupling between three phase conductors and three passive antennas. .............55

Figure 3.10 Recording instrument arrangement....................................................................................56

Figure 4.1 Restriking Process During CB Opening (from [58]) .................................................... .......61

Figure 4.2 Measured Voltage at reactor terminal (from [58])...............................................................61

Figure 4.3 Experimental Circuit arrangement.......................................................................................62

Figure 4.4 Photograph showing Laboratory arrangement.....................................................................63

Figure 4.5 Photograph showing Laboratory arrangement.....................................................................63


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M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page vii

Figure 4.6 (a) Waveforms during opening of vacuum circuit breaker at 3 kV (b) Area A Restrikes

on Reactor Voltage (c) Area A Restrikes detected by passive antenna.....................................65

Figure 4.7 HF restriking pulses detected on Active Antenna ...................................................... .........66

Figure 4.8 Magnification on one HF pulse ........................................................ ...................................67

Figure 4.9. Blackwall Substation Interconnection .......................................................................... ......69

Figure 4.10 Blackwall Capacitor Bank Layout............................................................... .....................70

Figure 4.11 Measuring equipment layout ................................................................ .............................71

Figure 4.12 Antenna waveform on CB opened at point A....................................................................73

Figure 4.13 HF pulses during opening..................................................................................................74

Figure 4.14 Three typical HF Pulses During Opening..........................................................................74

Figure 4.15 Passive Antenna waveform on closing .............................................................. ................75

Figure 4.16 HF markers during closing .......................................................... ......................................75

Figure 4.17 (a) to (f) typical HF Pulses during closing.........................................................................76

Figure 5.1 Three-phase voltage waveforms and controlled closing points for a Capacitor Bank.........80

Figure 5.2 Three-phase current waveforms and controlled opening points for a Capacitor Bank.......80

Figure 5.3 Measuring equipment layout for tests at Blackwall.............................................................81

Figure 5.4 Capacitor Bank Installation .................................................................. ...............................82

Figure 5.5 Recording Instrumentation ................................................................... ...............................82

Figure 5.6 Plan view of antenna positions at Capacitor Bank installation during background

measurement.................................................................................................................................85

Figure 5.7 Waveform from Background measurement.........................................................................86

Figure 5.8 Plan view of the antenna positions for Test 5......................................................................87

Figure 5.9 Waveforms captured during CB close operation for Test 5 ................................................88

Figure 5.10 Waveforms captured on Powerlinks portable recorder for Test 5....................................89

Figure 5.11 Plan view of the antenna positions for Test 3....................................................................91

Figure 5.12 Waveforms captured on Powerlinks portable recorder for Test 3....................................92

Figure 5.13 Waveforms captured during CB close operation for Test 3 ..............................................93

Figure 5.14 Plan view of the antenna positions for Test 8....................................................................94

Figure 5.15 Waveforms captured on Powerlinks portable recorder for Test 8-Open..........................96

Figure 5.16 Waveforms captured during CB open operation for Test 8...............................................98

Figure 6.1 Three-phase voltage waveforms and controlled closing points for a Shunt Reactor Bank102

Figure 6.2 Three-phase current waveforms and controlled opening points for Shunt Reactor Bank .102

Figure 6.3 Measuring equipment layout at Braemar substation..........................................................103

Figure 6.4 Shunt Reactor Installation..................................................................................................104

Figure 6.5 PASS MO Circuit Breaker.................................................................................................104

Figure 6.6 Passive and active antennas location ........................................................................ .........105

Figure 6.7 Plan view of the antenna positions at shunt reactor installation........................................107

Figure 6.8 Waveform on Background measurement ....................................................... ...................108

Figure 6.9 Waveforms captured on Powerlinks portable recorder for Test 4....................................109Figure 6.10 Waveforms captured by PA 1,2 and 3 during CB closing operation for Test 4 ..............110


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M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page viii

Figure 6.11 Waveforms captured during CB close operation in Test 4 by each antenna ...................111

Figure 6.12 Comparison of voltage magnitude of closing pulses at each closing event.....................112

Figure 6.13 Waveforms captured on Powerlinks portable recorder for Test 7..................................114

Figure 6.14 Waveforms captured during CB open operation in Test 7 ..............................................115

Figure 6.15 Waveforms captured during CB open operation by each antenna..................................117Figure 7.1 Capacitances between passive antennas and three phase conductors with symmetrical

spacings ....................................................... .................................................................... ...........122

Figure 7.2 Equivalent circuit for passive antenna at location 1 measuring three phase voltages. ......123

Figure 7.3 Output waveforms for Case 1............................................................................................125

Figure 7.4 Capacitances between passive antennas and three phase conductors with unsymmetrical

distances ................................................................ .................................................................. ...127



Figure 7.7 Waveforms recorded by Active antenna on capacitor bank opening ................................130Figure 7.8 Waveforms recorded by Active antenna on shunt reactor opening during Test 7.............131

Figure. 7.9 Test 7 - AA signals without noise and load oscillation ....................................................131

Figure. 7.10 Test 7 - Cumulative energy against time ..................................................................... ...132

Figure 7.11 Test 7 - Density of Pulses with time................................................................................133

Figure 7.12 Test 7 - Cumulative Pulses against time.............. ............................................................133

Figure 7.13 Test 5 - AA signals without noise and load oscillation .................................................134

Figure 7.14 Test5 - Cumulative energy against time ................................................................. .........135

Figure 7.15 Test 5 - Density of Pulses with time................................................................................136

Figure 7.16 Test 5 - Cummulative pulses against time .......................................................................136

Figure 7.17 RF Measurement showing arc signal UD, switch voltage Us and current Is [50]............137

Figure 7.18 Reactor Opening Test 7 (a)Time domain plot of PA1 waveform for opening from 20 ms

to 50 ms (b) Frequency content of waveform in (a)................................................................. .139

Figure 7.19 (a) Voltage-Time domain plot of waveforms from PA2 (b) ST FFT contour plot of PA2

waveforms for opening- from 23 ms to 28 ms .................................................. .........................141

Figure 7.20 (a) Voltage-Time domain plot of AA (b) ST FFT contour plot of AA (from 0-2MHz) (c)

ST FFT contour plot of AA (from 0-10MHz) for opening from 23 ms to 28 ms ......................142


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M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page ix

List of Tables

Table 2.1 Results of the overhaul of the circuit breakers (from [4]).....................................................31

Table 2.2 Results of tests made to examine effects of parasitic arcing.(from [25])..............................36

Table 2.3 Statistics cause of failure of circuit breaker (from [28]) ......................................................38

Table 3.1 Characteristics of Agilent Digital Oscilloscope....................................................................56

Table 3.2 Characteristics of Yokogawa Digital Oscilloscope...............................................................57

Table 4.1 Calibration between test voltage, supply voltage and Passive antenna.................................64

Table 4.2 Summary of tests conducted at Blackwall on 21stMay 2007 ...............................................72

Table 5.1 Summary of tests conducted at Blackwall on 7thAugust 2007.............................................83

Table 5.2 Voltage measured by each Passive antenna during background measurement.....................86Table 5.3 Summary of CB timing and pole sequence for capacitor bank closing Test 5......................90

Table 5.4 Summary of CB timing and pole sequence for capacitor bank closing Test 3......................94

Table 5.5 Summary of CB timing and pole sequence for capacitor bank Test 8 ..................................97

Table 6.1 Summary of tests conducted at Braemar substation on 21stAugust 2007 ..........................106

Table 6.2 Voltage measured by each Passive antenna during background measurement...................108

Table 6.3 Summary of CB timing and pole sequence for shunt reactor bank closing Test 4 .............113

Table 6.4 Summary of CB timing and pole sequence for shunt reactor bank opening Test 7 ............118

Table 7.1 Calculated results and measured values from Braemar substation .....................................126

Table 7.2 Differences between original waveform and reconstructed waveforms with 10% error on

capacitances................................................................................................................................129


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M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page x

List of Abbreviations

(Sort in alphabetical order.)

AA - Broadband active antenna

AIS - Air Insulated Switchgear

CIGRE - International Conference on High Voltage Systems, Paris

CB - Circuit Breaker

EHV - Extra High Voltage

HF - High frequency

PA - Passive antenna (capacitive coupling antenna)

PASS - Plug And Switch System

SF6 - Sulphur hexafluoride

VCB - Vacuum circuit breaker


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M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page xi

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To the best of my

knowledge and belief, the thesis contains no material previously published or written by another

person except where due reference is made.

Signature: _________________________

Date: _________________________


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M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page xii

Acknowledgments

First and foremost, my most sincere thanks must go to my supervisors, Associate

Professor David Birtwhistle and Dr. Tee Tang, for their advice, guidance and most of all their

patience and understanding throughout this research.

Special acknowledgement is made towards Powerlink Queensland for the financial

support given to this research project and also to Dr Jose Lopez Roldan of Powerlink for his

assistance and contribution in this research. The assistance of staff of Powerlink Queensland in

carrying out field measurements is greatly acknowledged.

I would also like to thank the Head of the School of Engineering Systems, Queensland

University of Technology, for the use of the facilities. Special thanks go to the technical staff at

Level 9, S Block, for their assistance with laboratory works, giving jokes that brighten up the

day and technical advices on equipment. I also thank my colleagues for their help throughout the

research work.

I gratefully acknowledge the management of Tenaga Nasional Berhad (TNB), Malaysia

for awarding a scholarship to me to further my studies and to embark on a very good research

that will benefit the organisation directly or indirectly.

Finally, my love and thanks go to my wife who has been giving support to me while

completing this research. Also to my children for cheering me up at times when it is reallyneeded.

ALHAMDULLILLAH.


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Chapter 1: Introduction

M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 2

eventually puncture the nozzle material and result in the failure of the interrupter. Failures due to

re-strike/re-ignition are becoming more of a concern as it is difficult to detect re-strike

occurrence. Failures can be catastrophic and they can affect the availability, reliability, safety

and cost of the system which can greatly affect the utilities.

Condition monitoring of circuit breakers is thus important in order to ensure the safe

operation and reliability of circuit breakers. To date, no specific technique has been developed to

detect re-strike. Currently, shut downs are required to physically connect monitoring equipment

to measure switching transients and re-strike. An on-line non-intrusive technique would be an

advantage in monitoring circuit breaker re-strike occurrence during switching of reactive

equipment.

Moore [6] has demonstrated the practicality of measuring time between pole-closing in

circuit breakers during capacitor switching duty from measurement of emitted radio waves. In

this thesis research is conducted to determine whether it is possible to extend Moores

methodology to investigate switching transients produced during capacitor bank and shunt

reactor bank switching. Techniques for monitoring the magnitude and number of re-strikes

occurring during reactor switching using this or similar methods have also been explored..

1.2 RESEARCH CONDUCTED

This research investigates switching transients produced during the switching of three

phase capacitor banks and shunt reactors. The research includes the development of a monitoring

system which possesses most of the features required for the non-invasive, on-line monitoring of

EHV circuit breakers in AIS substations. The monitoring system may be used to carry out on-

line measurements at substations and the measured data are stored and analysed to give valuable

information on the switching transients.

Measurements may be correlated with the actual switching event using recorded

waveforms. Important information such as evidence of restrikes would be looked into from the

data gathered.


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Chapter 1: Introduction


1.3 THESIS OUTLINE

This thesis includes a description of the development of the measuring system, results of

measurements made in EHV substations, analysis of results, comparisons with other available

techniques and suggestions for further works to be pursued.

Chapter 2 includes a literature review on topics related to the research. It reviews the

principle of current interruption, capacitor bank and shunt reactor bank switching and failures of

circuit breakers switching shunt reactor and capacitor banks. The importance of preventing

failures is highlighted with the need to develop a suitable condition monitoring method to detect

potential failures.

Chapter 3 describes the proposed new technique for monitoring CB during switching. Itstarts with a review of the radiometric method used previously and describes the methodology

used in carrying out the research and development of the new monitoring system. The

measurement principles are described followed by details of the design and construction of the

high voltage transducers. . Chapters 4, 5 and 6 cover HV laboratory measurements, capacitor

bank measurements and shunt reactor bank site measurements respectively. Switching transients

are recorded and discussed. Results are analysed in time domain and important findings are

highlighted.

Chapter 7 deals with analysis of the recorded waveforms. Analysis in frequency domain

is shown to give more information and to correlate with the results from time domain analysis.

Chapter 8 reviews work done in the research highlighting important information obtained from

the measurements, the advantages of the measuring system and further work that can be done to

develop the measuring system.


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Chapter 2: Literature Review


2Chapter 2: Literature Review

This chapter contains a literature review of materials pertinent to this research. A basic

review of current interruption in circuit breakers is carried out followed by a review of reactive

switching, including capacitor bank and shunt reactor bank switching. The energisation and

deenergisation phenomenon for capacitor and shunt reactor bank is described. Failures of circuit

breakers due to restriking are considered and the case for detecting restrikes is established. This

is followed by a review of the current available condition monitoring methods and their

suitability for detecting restrikes. Finally, new monitoring methods are proposed.

2.1 REVIEW OF CURRENT INTERRUPTION IN CIRCUIT BREAKERS

The primary purpose of an interrupting device such as circuit breaker is to disconnect

the circuit at the point at which it is placed. When closed, the circuit breaker must carry

continuous rated current. The insulation to ground is stressed by the power frequency voltage

and any transient overvoltages. When open, the dielectric between the contacts is stressed by the

voltage developed across the open contacts.

During the transition period from closed to open and vice versa, a range of dynamic

condition arise. For instance, during a transition from closed to open, the current must be

interrupted to achieve electrical isolation. Interruption of current normally occurs at a current

zero of the sinusoidal waveform and a voltage known as the transient recovery voltage appears

across the open contacts of the circuit breaker. The ability of a circuit breaker to interrupt the

current depends on external circuit parameters, dielectric recovery, contact separation at the time

of current zero, interrupter design and the interrupting conditions e.g. normal load, reactive

switching or fault current. The rate of rise and the peak value of the transient recovery voltage

have a significant impact on circuit breaker performance. Waveforms of a typical circuit

interruption [7] sequence are given in Figure 2.1 for a fault on the load-side terminals of a circuit

breaker.


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Figure 2.1 Typical Circuit Interruption (from [7])

The circuit breaker contacts separate at Point A causing an arc to be drawn between the

contacts. This arc has a resistance that creates a small voltage drop, Va. The arc continues until

the current, I, drops to a level too small to maintain it. This occurs as the current passes through

zero, at which point the arc extinguishes and the transient recovery voltage appears across the

circuit breaker contacts. Successful interruption is achieved if the dielectric strength between the

contacts as they separate increases at a greater rate than that of the transient recovery voltage. In

addition, the breakdown strength of the gap between the contacts must exceed the peak value of

the transient recovery voltage. If not, the arc will re-establish and current interruption may occur

at a subsequent zero.

When the current ceases, the voltage between the contacts changes from virtually zero

(the arc voltage) to the instantaneously value of the power frequency voltage. This change

cannot take place instantaneously and a resultant overshoot occurs. The voltage approaches its

steady state value by a transient oscillation with a frequency that is determined by the values of

the circuit inductances and capacitances. The amplitude of the transient recovery voltage may

reach two times the steady state voltage change for the first pole to clear. However, in practice,

its value is usually less due primarily to system damping.

In addition, the instantaneous value of the recovery voltage at the instant of current

interruption is dependent on the power factor of the circuit. The amplitude of the voltage change

that occurs will depend on whether load, charging current or fault current is being interrupted.

Under fault conditions, power systems are primarily inductive. Therefore the power factor of the

circuit as seen from the circuit breaker is effectively zero lagging and the power frequency

This figure is not available online.Please consult the hardcopy thesisavailable from the QUT Library


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component of the transient recovery voltage has its maximum value at the instant of current

interruption as shown in Figure 2.1.

The capability of the circuit breaker [8]to successfully interrupt the current will depend

on the phenomenon of current extinction at current zero. After current interruption, the still-hot

gas between the breaker contacts is stressed by a steep rate of rise of the recovery voltage and in

the resulting electric field the present charged particles start to drift and cause a hardly

measurable so-called post arc current. The post arc-current, together with the transient recovery

voltage, results in energy input in the still-hot gas channel. When the energy input is such that

the individual gas molecules dissociate into free electrons and heavier positive ions, the plasma

state is created again and current interruption has failed. This is called a thermal breakdown.

Thermal breakdown normally occur within microsecond in a region known as thermal recovery

phase. When the current interruption is successful, the hot-gas channel cools down and the post

arc current disappears. However, if the dielectric strength of the gap between the breaker

contacts is not sufficient to withstand the transient recovery voltage, a dielectric failure can

occur. Dielectric failure normally occurs within milliseconds in a region known as dielectric

recovery phase.

2.2 REACTIVE EQUIPMENT SWITCHING

Switching of reactive equipment such as capacitor banks and shunt reactors is known to

produce overvoltage transients that may cause insulation breakdown and lead to power system

failure [8]. The reactive equipment is connected to the power system via circuit breakers, this

similar for equipment like overhead lines, transformers and generators. When circuit breakers

operate, parts of the power system are either separated from or connected to each other. This can

be either a closing or opening operation of the circuit breakers.

After a closing operation, transient currents will flow through the system. Closing of aCB in a predominantly capacitive or inductive network may result in inrush currents. The high-

frequency inrush current can cause problems by: production of severe mechanical stresses on

equipment; production of over voltages due to the system response to the inrush current; and

induction of undesirable transients into neighbouring circuits with low power relay and control

circuits being particularly vulnerable.After an opening operation, when a power-frequencycurrent is interrupted, a transient recovery voltage or TRV will appear across the terminals of the

interrupting device.The configuration of the network as seen from the terminals the circuitbreaker determines amplitude, frequency, shape of the current and voltage oscillations.


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When interrupting a mainly capacitive load (e.g. capacitor bank for voltage regulation)

under normal load conditions, the current and voltage are approximately 90 degrees out of phase

and the current is leading the voltage. When interrupting a mainly inductive load (e.g. large

transformer or shunt reactor) under normal load conditions, current and voltage are also

approximately 90 degrees out of phase with the current lagging the voltage.

In interrupting capacitive or inductive current, if the current is interrupted at current

zero, the interruption is normal and the transient recovery voltages are within the specified

values [8]. However when premature interruption occurs due to current chopping, interruption is

abnormal and this may cause high-frequency re-ignitions and over voltages. If the interrupter

chops high current in a reactor a high-magnitude oscillatory voltage surge may be produced.. If

this process is repeated several times due to high-frequency re-ignitions, voltage doubling may

ensue with rapid escalation of voltage. If these overvoltages exceed the specified dielectric

strength for the circuit breaker, the interrupter and other parts of the circuit breaker may be

damaged.

Re-ignition [8,9] is a phenomenon where a dielectric breakdown of the arc channel

occurs within 5ms after current interruption. It is considered not detrimental to circuit breaker

though no evidence has been found in the literature to substantiate this. Re-ignitions during

recovery voltage are expected to cause 50Hz current to be re-established with minimaldisturbance and the final interruption of current is delayed about 10ms until the next natural

current zero for some types of circuit breakers. Puffer circuit breakers in which opening times are

very critical may be more seriously affected, though there is no published research on this topic.

Re-striking [8,9] is dielectric breakdown of the arc channel after 5ms of interruption,

when the recovery voltage is close to peak. The circuit breaker gap flashes over as the recovery

voltage is greater than the dielectric strength of the gap. Re-strikes can cause high over voltages

and high magnitude HF re-ignition currents that impose sever stresses on the circuit breaker and

adjacent equipment. Numerous re-strikes and interruptions of re-ignition current may will lead to

voltage escalation.

Voltage escalation is a phenomenon where voltage across the circuit breaker is increased

by one or more interruptions of re-ignition current followed by further re-strikes. Generally

interruption at the first or third re-ignition current zero or any odd zero leads to voltage

escalation.


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2.3 REVIEW ON CAPACITOR BANK SWITCHING

Shunt capacitor banks are extensively used to improve loading of the transmission lines

as well as to support system voltages. As these capacitor banks are frequently switched in and

out of duty, energisation and de-energisation transients are produced and raise important

concerns. The concerns on energisation are overvoltages and inrush current whilst for de-

energisation is restriking.

2.3.1 Interrupting capacitor bank

Capacitor switching presents circuit breakers with a difficult switching condition. While

interrupting capacitive current, the recovering circuit breaker can be severely stressed during thetime when it is prone to dielectric failure. The problem of dielectric failure arises because the

normal point of current interruption (current zero) occurs when the current leads the voltage by

around 90 degrees. At current zero, a maximum voltage occurs, resulting in a fully charged

capacitance upon disconnection from the source. The voltage due to this trapped charge creates

high stresses during the first half cycle after interruption.

To consider the phenomena associated with capacitor de-energisation, the basic single

phase circuit parameters are given in Figure 2.2.

Figure 2.2 Single Phase Capacitor Bank circuit (from [10])

A capacitor bank can be represented by a lumped capacitance, C, connected to busbar

via circuit breaker. A small capacitance, Cbrepresents the capacitance of the substation busbar

and other equipment. The impedance of the source is represented in the circuit by R1and L1.



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Figure 2.3 Capacitance Switching (a) System voltage and current. (b) Capacitor voltage (c) Voltage across

CB contact. (from [9])

Figure 2.3 shows events occurring before and after a capacitor bank disconnection,

which in this case was performed successfully. At point A, the most favourable condition for arc

interruption is present and arc extinction occurs at the first current zero after contact separation.

Because of the relative phase current and voltage (current leads the voltage by approximately 900

), the capacitor is fully charged to maximum voltage when the switch interrupts. The magnitude

of the trapped voltage is equal to the peak value of the supply voltage, V (as shown in b).

The voltage on the supply side of the circuit breaker continues to vary at the source

power frequency (as in (a)) so that the voltage across the circuit breaker builds up sinusoidally

immediately after current interruption (as in (c)). One half cycle after current interruption, the

voltage across the circuit breaker reaches a value equal to twice the source voltage, which is

potentially dangerous. Thus, for successful interruption to be maintained, the gap between the

This figure is not available online.Please consult the hardcopy thesis

available from the QUT Library


23/175



contacts must withstand twice the peak value of the source voltage, approximately 10ms after arc

extinction [9].

Figure 2.3 tends to oversimplify conditions to some extent in that when a capacitor is

connected to a system, the leading current that it draws, flowing through the inductance of the

system, causes the capacitor voltage to be somewhat higher than the open-circuit system voltage,

a negative regulation sometimes referred to as the Ferranti Rise. When the capacitor is

disconnected, the potential of the source side of the circuit breaker will return to this lower value,

but will do so by way of an oscillation involving the source inductance and the stray capacitance

adjacent to the breaker on the source side. A more accurate representation of the disconnecting

event is shown in Fig 2.4.

Figure 2.4 Capacitance switching showing the effect of source regulation (from [9])

Here V is the aforementioned negative regulation. It is important to recognise this

phenomenon exists as it can be important when interrupting capacitive current on relativelyweak systems [9]. A relatively weak system condition can be described where a lower voltage

system is being supplied by a higher voltage system through a step down transformer with cable

on the higher voltage side and the lower voltage breaker is called upon to interrupt the charging

current of the cable.

Some circuit breakers, when called upon to interrupt a load of fault current, do not do so

at the first current zero, but instead wait until sufficient gap has been established between their

contacts for their various arc-extinguishing effects to have a better chance of operating

successfully. The current involved in capacitance switching is frequently small, so that in most



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cases the circuit breaker is capable of interrupting it at the first current zero. If this should occur

soon after the contacts have parted, avoltage of twice the system voltagewill appear across the

contacts while their separation, so there is an increased likelihood of the device reigniting [9]. If

a restrike takes place precisely when the voltage reaches its peak, which in equivalent to

reclosing a perfect switch at that instant. There is in this case a series LC circuit so closing

inrush current would be expected to respond to this sudden disturbance by being a sinusoidal

oscillation with a natural frequency,fo, which is given by:

2/1)(2

1

2 LCfo

== (2.1)

where Lis the inductance of the supply and C the capacitance of the bank.. A reignition or

restrike can also be viewed as an inadvertent re-energisation with a trapped charge of 1 pu on the

capacitor. The restrike current will be the instantaneous voltage across the switch at restrike

divided by the circuit surge impedance, or

tC

LVpir 0

2/1

sin2

= (2.2)

Neglecting damping, the voltage will swing as far above the instantaneous system

voltage as it started below [9]. This is indicated in Figure 2.5, which shows the initial 50 Hz

clearing, the trapping of charge on the capacitor, and the subsequent restrike. The transient

voltage excursion to 3Vp is an abnormal overvoltage and is the consequence of the energy stored

in the capacitor bank at the time of the restrike.

It is entirely possible that the circuit breaker will interrupt the restrike current, perhaps at

point A in Figure 2.5. If this happens, the high voltage is left trapped on the capacitor. The

source voltage, on the other hand, would continue on its way, so that after another half cycle

there would be approximately 4Vpacross the interrupter. This can be shown by the sequence

drawn in Fig. 2.6 where the Rs represent sequential restrikes and the Cs subsequent clearings [9].

If a second breakdown occurs, a second oscillatory discharge would be initiated. However, since

there is now twice the voltage across the switch, the current would be twice as high, and the

voltage excursion would be from +3Vp to -5Vp(the voltage excursion, neglecting damping, is

always twice the voltage across the switch). It is technically possible for the voltage to escalate

still further by the same mechanism until an external flashover occurs or the capacitor fails [9].


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Figure 2.5 Capacitance switching with a restrike at peak voltage. (from [9])

The sequence is idealized and to some extent oversimplified. Restrikes will not always

occur precisely at the voltage peak, so that the voltage, if it escalates, does so more slowly.

Again, the circuit is more complicated. Some capacitance will exist on the source side of the

breaker, which will introduce higher frequency disturbances, as was pointed out in Fig 2.5.

When the switch recovers after point A, the potential at the switch is quite high. But the

source would have it be at its potential. The source side of the switch, therefore, goes through a

high-frequency transient involving an oscillation of the aforementioned capacitance and the

inductance of the source. In fact, at this time, it is possible for a voltage of 4 pu to be developed

across the switch, a point which is often overlooked. A reignition may occur at this time rather

than half a cycle later, which will probably result in the switch conducting current for another

half cycle.



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The back-to-back switching normally gives rise to an inrush current of very high

magnitude and frequency which is higher than isolated capacitor bank switching. This inrush

current needs to be limited in order not to be harmful to the circuit breaker, capacitor banks

and/or the network. The magnitude and frequency of the inrush current is a function of the

following [1]:

Applied voltage during closing i.e. point on the voltage wave at closing.

Capacitance of the circuit

Inductance in the circuit (amount and location)

Any charge on the capacitor bank at the instant of closing.

Any damping of the circuit due to closing resistors or other resistances in the

circuit.

It is assumed that the capacitor bank is discharged prior to energization. This assumptionis reasonable, as capacitor units are fitted with discharging resistors that will discharge the

capacitor bank. Typical discharge times are in the order of 5 min.

The transient inrush current to an isolated bank is less than the available short-circuit

current at the capacitor bank terminals. It rarely exceeds 20 times the rated current of the

capacitor bank at a frequency that approaches 1 kHz [1]. Because a circuit breaker must meet the

making current requirements of the system, transient inrush current is not a limiting factor in

isolated capacitor bank applications.

When capacitor banks are switched back-to-back (i.e., when one bank is switched while

another bank is connected to the same bus), transient currents of prospective high magnitude and

with a high natural frequency may flow between the banks on closing of the circuit breaker. The

effects are similar to that of a restrike on opening. This oscillatory current is limited only by the

impedance of the capacitor bank and the circuit between the energized bank or banks and the

switched bank. This transient current usually decays to zero in a fraction of a cycle of the system

frequency. In the case of back-to-back switching, the component supplied by the source is at a

lower frequency; therefore, small it may be neglected.

2.4 REVIEW OF REACTOR BANK SWITCHING

Shunt reactors are mainly used in transmission networks. Their function is to consume

the excess reactive power generated by overhead lines under low-load conditions, thus stabilize

the system voltage. They are switched in and out almost on a daily basis, following the load


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situation in the system. Shunt reactors are normally connected to substation busbars, but also

quite often directly to overhead lines. They may also be connected to tertiary windings of power

transformers. The reactors may have grounded, ungrounded, or reactor grounded neutral.

2.4.1 Interrupting shunt reactor bank

Shunt reactor switching imposes a unique and severe duty on the connected system and

circuit breaker [3,11]. At high voltages, the current to be interrupted is generally less than 300A,

yet successful interruption is a complex interaction between the circuit breaker and the circuit.

Shunt reactor load currents are referred to generically as small inductive currents. The

capability of circuit breakers to interrupt small inductive currents is generally not a concern. The

circuit breaker will typically interrupt the current at the first current zero after contact parting, but

may not be immediately capable of withstanding the high magnitude recovery voltages that can

then appear across the contacts. This can result in a reignition followed by an additional loop of

rated frequency current and successful interruption.

The switching of directly grounded reactors can be analysed using the equivalent single

phase circuit shown in Figure 2.7. Basically, circuit breakers have no difficulty interrupting

shunt reactor current; in fact, the current is forced prematurely to zero, a phenomenon referred to

as current chopping. However, the chopping of the current and subsequent possible reignitions

can result in significant transient overvoltages.

The following two types of overvoltages are generated:

Chopping overvoltages with frequencies up to 5 kHz

Reignition overvoltages with frequencies up to several hundred kilohertz (kHz)

The switching process may be significantly influenced by two other circuit-breaker

characteristics:

Rise of the dielectric withstand of the contact gap after interruption which

influences the probability of re-ignitions occurring;

Capability to interrupt high-frequency currents after re-ignitions which influences

the risk of multiple re-ignitions and voltage escalation.


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Ls = supply side (short-circuit) inductance

Cs = supply side capacitance

CB = circuit breaker

Lp,Cp = stray inductance and capacitance across circuit-breaker CB

Also known as first parallel circuit inductance and capacitance

Lb = inductance of re-ignition circuit

Also known as connection series inductance

CL = capacitance parallel to the reactor (load side capacitance)

L = inductance of shunt reactor

Figure 2.7. Single phase equivalent circuit (from [11])

2.4.2 Current chopping

Current chopping is caused by arc instability, which exhibits itself in the form of a

negatively damped current oscillation superimposed on the load current [3,11]. The oscillation

amplitude increases rapidly, creating a current zero at which the circuit breaker usually interrupts

as shown in Figure 2.8. The frequency of the oscillation determined by Cs, CL and Lb (Figure

2.7) and is usually several hundred kHz and therefore current chopping can reasonably be

assumed to be instantaneous for purposes of calculating load transients.

The chopping level is determined by:



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the characteristic chopping number of one interrupting unit of the switching

device;

the effective parallel capacitance;

the number of breaks in series.

a) Current through circuit breaker b) Voltage across shunt reactor

Figure 2.8 Current chopping phenomena (from [3])

For a single interrupter circuit breaker, the chopping current level is given by the

equation

tch Ci = (2.3)

where

ich =current level at the instant of chopping (A)

Ct =total capacitance in parallel with the breaker (F)

= chopping number for a single interrupter (AF 0.5)



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The chopping number, , is a characteristic of the circuit breaker and typically given by

the manufacturer of the circuit breaker. [3] gives a typical range of chopping number for SF6

circuit breaker of 4-17 x 104.

Referring to Figure 2.7, Ct is given by the following equation

Ls

Lspt

CC

CCCC

++= (2.4)

Where

CP =circuit breaker parallel capacitance (F)

Cs =

source side capacitance to ground (F)CL =effective load side capacitance to ground (F).

CL is summation of the load side equipment capacitances to ground and the phase-to-

phase capacitance of the shunt reactor and associated connections. For many applications, the

latter may not be significant compared to former and can be ignored.

The maximum value of Ct and the worst-case condition for overvoltage generation

occurs when Cs>> CL, in which case Ctis given by

Lpt CCC += (2.5)

Equation (2.3) applies as noted only to circuit breakers with a single interrupter. For

circuit breakers with N interrupting units per pole, the following equation applies:

tch NCI = (2.6)

The level of current chopping may be dependent on arcing time. This tends to be the

case for SF6puffer type circuit breakers. Current chopping phenomena are discussed in detail in

[12,13].

Current Chopping Overvoltages

Fig 2.9 shows chopping phenomena in a single-phase circuit. When a premature current

interruption occurs at 6, the interruption is abnormal and causes an overvoltage. The energy

trapped in the load inductance and capacitance at the instant of chopping will oscillate between

this inductance and capacitance. The frequency of the oscillation is of the order of 1 kHz to 5

kHz in the HV and EHV range. It is determined by the natural frequency of the reactor load


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circuit, i.e. the reactor itself and all equipment connected between the circuit-breaker and the

reactor.

Figure 2.9 Chopping Phenomena in single phase (from [11])



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The first peak of the oscillation has the same polarity as the system voltage at the time of

interruption. This overvoltage is referred to as the suppression peak overvoltage. The maximum

chopping overvoltage to earth is usually the suppression peak voltage for directly earthed

reactors. Due to energy transfer between phases, the load side oscillation may in some cases

exhibit slightly higher peak values after one or two cycles of the oscillation. The highest

overvoltage to earth appears at the recovery peak for the unearthed and neutral reactor earthed

cases [11].

The magnitude of the suppression peak overvoltage, ka is given by the expression :

Lo

cha

C

L

u

ik *1

+= (2.7)

where

ich = chopped current

uo = peak system voltage to earth

L = reactor inductance

CL = load side capacitance.

For a given application (fixed uo, L, and CL), when Cs>>CLand Cp is negligible, the

overvoltage is dependent on ichonly. Equation 2.4.2.5 can then be rewritten as

Q

Nka

2

31

2

+= (2.8)

where

Q = three-phase reactor rating (V A)

= the chopping number (AF-0.5) for a single interrupter

= 2f = angular rated power system frequency

N = number of interrupting units in series per pole

The chopping overvoltage is thus only dependent on the chopping number and the

reactive power of the reactor [3,11].

2.4.3 Reignition

The circuit-breaker, after current interruption, is stressed by the difference between the

supply side voltage and the slowly decaying load side oscillating voltage. Circuit breakers with

very high chopping levels may exhibit reignitions before or at the suppression peak. Reignitions

if they occur have mainly the effect of reducing the chopping overvoltages. Most circuit-


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breakers, such as SF6 puffer type, which have low chopping levels and seldom reignite during

the suppression voltage loop.

At the recovery voltage peak the circuit-breaker is stressed by a voltage that may

approach the chopping overvoltage plus the peak of the supply side voltage. If the circuit-breaker

does not re-ignite before, or at this point, then the interruption is successful. If, however, the

instant of contact parting is such that the contact gap does not yet have sufficient dielectric

strength, then a re-ignition will occur as shown in Figure 2.10.

Figure 2.10 Reignition Windows (from[3])

Reference [3] states that all circuit-breakers will re-ignite when the interruption occurs

with a small contact gap. The re-ignition window may be narrow or wide depending on the

rate of rise of withstand capability of the increasing contact gap as illustrated in Figure 2.10. The

width depends on the design of the circuit-breaker i.e. interrupting medium, contact velocity,

electrode design, etc. Re-ignition-free interruption can practically be achieved by applying

auxiliary equipment to circuit breaker to limit overvoltages such as opening resistors, metal

oxide surge arresters and synchronous opening control devices (control switching). The latter



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device opens the contacts at sufficient time before the chopping to ensure that the dielectric

strength of the gap is always greater than the chopping overvoltage.

Re-ignitions occur only for relatively short arcing times in circuit-breakers with fast

dielectric recovery, and occur therefore generally only on the first phase of attempted

interruption. A further loop of power frequency current usually follows the re-ignition as in

Figure 2.9.

Reignition Overvoltages

Figure 2.11 [3] illustrates a case where a reignition occurs, the load side voltage rapidly

tends toward the source side voltage, but overshoots producing a reignition overvoltage. The

voltage breakdown at a reignition creates a steep voltage transient that is imposed on the reactor.The front time varies from less than one microsecond to several microseconds. Since the voltage

breakdown in the circuit breaker is practically instantaneous, the steepness is solely determined

by the frequency of the second parallel oscillation circuit, which in turn is dependent on the

system/station layout [3]. This steep transient may be unevenly distributed across the reactor

winding, stressing the entrance turns in particular with high interturn overvoltages.


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Figure 2.11 Reignition at recovery voltage peak for a circuit with low supply side capacitance (from [3]).

Figure 2.12 shows the maximum attainable overvoltages without damping for a

reignition at the recovery voltage peak. It can be seen that interruption with high current

chopping produces higher overvoltages than interruption with negligible current chopping. The

high theoretical overshoot assumes that the supply side capacitance dominates over the load side

capacitance (Cs>>CL).

This figure is not available online.Please consult the hardcopy thesis

available from the QUT Library


37/175



Figure 2.12 Maximum re-ignition overvoltages (from [3])

2.4.4 Oscillation modes

Four different oscillation modes occur during the interruption and reignition process for

a directly-ground reactor. Those oscillations are described below with reference made to Figure

2.13 which clearly shows the oscillations involved.

Load side oscillation

A successful interruption results in the slowly decaying load side oscillation with the

trapped energy oscillating between the inductance and capacitance of the load side circuit. The

frequency of the oscillation is given by

L

LLC

f2

1= (2.9)

and is in the range 1 to 5 kHz. This oscillation may be modulated due to phase

interaction as described later.



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three-phase (in one tank with a common core), the phase-to-phase coupling is significant and

results in beating such that the maximum recovery voltage peak can occur late in the oscillation.

[3] mentions that the interaction between phases is not a concern because the interaction

does not influence the recovery voltage in the region between current interruption and the

occurrence of the recovery voltage peak. If the circuit breaker successfully withstands the

recovery voltage peak, then no reignition will occur later even if subsequent peaks exceed the

chopping overvoltage peak value due to beating. The probability of high overvoltages occurring

due to superposition of transients from adjacent phases is considered to be remote.

Figure 2.14 Load side oscillation with circuit breaker located close to shunt reactor (from[3])

Figure 2.15 Load side oscillation with circuit breaker located remote from shunt reactor (from [3])


This figure is not available online.

Please consult the hardcopy thesisavailable from the QUT Library


41/175



2.4.6 Energising transients

Energizing (switching-in) a shunt reactor is a situation similar to the occurrence of a

reignition. However, the breakdown voltage across the circuit breaker will not exceed 1 pu for

directly grounded reactors, and the peak value of the energizing transient will be 1.5 pu or less.

Reactor surge arrester operation may occur if the switching device has a slow closing

speed resulting in multiple restriking and possible voltage escalation.

2.5 LIMITATION OF OVERVOLTAGE TRANSIENT DURINGREACTIVE SWITCHING

Switching of capacitor bank and shunt reactor produces overvoltage that can be harmful

to the system. The transient overvoltages may cause any of the following:-

Insulation degradation and possible failure of substation equipment

Operation of surge arrestors

Interference in the control wiring of substations

Increase in step potentials

Undesired tripping or damage to sensitive electronic equipment.

2.5.1 Over voltage limitation

Reference [1,3,14] state that several means are available to reduce the overvoltages

generated by the switching of capacitor bank. and shunt reactor.

Current-limiting reactors are normally used to reduce the current transients

associated with back-to-back switching of capacitor banks. They do not limit the

remote overvoltages.

Pre-insertion resistors limit the inrush current and remote overvoltages. It is a

basic solution widely used on transmission circuit breakers. They are usually fitted

on circuit breakers and as such add to the complexity of the equipment. Depending

on the design, the added complexity may or may not result in a reduced availability

of the equipment.

Surge Arrestersare the primary means of protection against fast transients and are

usually installed very close to the protected equipment in this case capacitor bank

and shunt reactor.

Controlled switching of circuit breaker meaning the opening and/or closing of

the circuit contacts at certain points on the waveform (such as at current or voltage

zeros, which is why it is often referred to as point-on-wave control) has long been

recognised as a way of reducing stress on circuit breaker contacts and systemcomponents during switching. A great deal of engineering is required for good


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precision of this kind of control. Controlled switching seems to be the chosen

method by utilities.

2.5.2 Controlled switching

Controlled switching is a method for eliminating harmful transients via time controlled

switching operations.Closing or opening commands to the circuit breaker are delayed in such a

way that making or contact separation will occur at the optimum time instant related to the

voltage, current and phase angle. By means of controllers, both energizing and de-energizing

operations can be controlled with regard to the point-on-wave position, and no harmful transients

will be generated.

Strategies are identified for energizing all types of shunt capacitor banks and harmonic

filter banks. The strategies involve energizing the load close to voltage zero across the circuit

breaker contacts thereby avoiding energizing transients. The strategy assumes that the banks are

discharged prior to energizing. For controlled opening, the strategy is to avoid short arcing times

resulting in the highest risk for reignitions or restrikes. The need for controlled opening will

depend on circuit breaker performance, load conditions and system frequency. All types of shunt

reactors, independent of magnetic and electric circuit, can be switched in a controlled manner.

The strategy for controlled opening is to select arcing times long enough to avoid re-ignitions at

de-energizing. The strategy may vary depending on the size of the shunt reactor. The strategy for

controlled closing is to energize at instants resulting in flux symmetry (current symmetry)

thereby minimizing the risk for nuisance tripping and rotor vibrations in nearby generators due to

zero sequence current.

Reference [15] suggested that controlled switching application requires CB with stable

operating times and have high and stable dynamic electric withstand capability between contacts,

both upon making and breaking conditions. In circuit breakers with independent mechanisms foreach pole the opening and closing operations can both be controlled. However if poles are

ganged and operated by a single mechanism, there are difficulties with control switching and

generally it is only possible to control either the opening or closing operation. It is mentioned in

[15] that it is difficult to quantify the effects of the control switching on reducing the probability

of restrikes because of the wide range of variability possible in the dielectric recovery

characteristics of circuit breakers.


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Sawir et al [18] reported that Tenaga Nasional Berhad (Malaysia) experienced incorrect

remote feeder tripping due to inaccurate operation of point-on-wave (POW) switching during

energisation of capacitor bank. [18] stated that although point-on-wave switching of capacitor

banks is an effective method in controlling system voltages, inaccurate operation of the POW

during switching in of the capacitor banks resulted in high voltage transients and caused

nuisance trippings. [18]suggested that the point-on-wave switching operation to be monitored

periodically.

Catastrophic failures of modern SF6 circuit breakers have been reported [4] when

disconnecting 420 kV shunt reactors. Detailed investigations were carried out and field

experience with HV SF6 circuit breakers switching reactors have been documented. [4] have

made few observations on the circuit breaker interrupters used for shunt reactor during the

investigation. Table 2.2 shows observations made on circuit breakers interrupter switching

shunt reactor during the investigation.

[4] described the external arcs as existences of signs of arcing between the main contacts

on the exterior of the nozzles, the perforations of the nozzles with a diameter of around 1mm in

the proximity of the moving contact with the farthest end of the moving contact and removal of

material on the internal face of the nozzle. It is further described the commutation arcs as

existences of signs of this arc between the main contacts and dirtiness and/or burr due to greaseand metal particles on the internal face of the breaker porcelain housing.

Table 2.1 Results of the overhaul of the circuit breakers (from [4]).

Peelo et al [19] reported that British Columbia Hydro and Power Authority experienced

a number of failures when switching out 500kV 3 x 45 MVAR shunt reactor banks. Series of

This table is not available online.Please consult the hardcopy thesisavailable from the QUT Library


45/175



reactor switching tests were performed over a three year period, to determine the causes of the

failures and to acquire knowledge of the switching duty in order to ensure adequate specification

of breakers for applications. Results of those field tests have provided valuable and important

information on interruption of small inductive currents. [19] concluded that high voltage shunt

reactor switching is a severe and unique duty, surge arresters play a significant role in reactor

switching application and reignitions during reactor switching can result in significant transients

in control circuits.

Khodabakchian et al [20] studied 420 kV circuit breaker failures during the opening of a

100MVAR shunt reactor in a 400kV one-and-a-half breaker transmission substation in central

part of Iran. The study shows that opposite-polarity high frequency arc-instability-dependant

oscillations caused mainly by current transformers on each side of the circuit breaker were

responsible for its thermal failures and thus the non-interruption of the low 50Hz reactor current

by the 50 kA circuit breaker. [20] mentioned the advantage in using simulation capabilities of

EMTP-RV to simulate large transients incorporating circuit parameters frequency dependency

and dynamic arc modelling which could contribute to improved reactor installation.

Lopez-Roldan et al [21] reported that Powerlink Queensland in recent years has

experienced several failures of modern SF6 circuit breakers used in Shunt Reactor switching

operations in the 275 kV network. An example of a failure was the breakdown of the CBswitching a line reactor with a neutral earthing reactor (NER) at the 275 kV substation. The CB

had been in service for over four years and had been operated almost daily. During a routine

opening operation, the dead-tank circuit breaker failed to clear on phase A and subsequently

faulted internally to ground. During the fault investigation and breaker disassembly, clear marks

of severe arcing puncture in the nozzle of the interrupter were found. The nozzle damage has

occurred prior to failure most likely due to re-striking during opening operations. Evidence of

severe nozzle puncture was also found in phase C.

The hypothesis [21] of the interrupter failure is that during the final opening, a re-strike

punctured right through the nozzle between the moving main contact and the fixed arcing

contact of the interrupter. The current within the nozzle was extinguished but ionized gases

forced though th

switchgear cb

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