switchgear cb
TRANSCRIPT
-
8/13/2019 Switchgear CB
1/175
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
-
8/13/2019 Switchgear CB
2/175
-
8/13/2019 Switchgear CB
3/175
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.
-
8/13/2019 Switchgear CB
4/175
Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors
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
-
8/13/2019 Switchgear CB
5/175
Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors
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
-
8/13/2019 Switchgear CB
6/175
Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors
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
-
8/13/2019 Switchgear CB
7/175
Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors
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
-
8/13/2019 Switchgear CB
8/175
Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors
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
-
8/13/2019 Switchgear CB
9/175
Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors
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 7.5 Output waveforms for Case 2............................................................................................127
Figure 7.6 Output waveforms for Case 3............................................................................................128
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
-
8/13/2019 Switchgear CB
10/175
Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors
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
-
8/13/2019 Switchgear CB
11/175
Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors
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
-
8/13/2019 Switchgear CB
12/175
Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors
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: _________________________
-
8/13/2019 Switchgear CB
13/175
Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors
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.
-
8/13/2019 Switchgear CB
14/175
-
8/13/2019 Switchgear CB
15/175
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.
-
8/13/2019 Switchgear CB
16/175
Chapter 1: Introduction
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 3
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.
-
8/13/2019 Switchgear CB
17/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 4
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.
-
8/13/2019 Switchgear CB
18/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 5
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
-
8/13/2019 Switchgear CB
19/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 6
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.
-
8/13/2019 Switchgear CB
20/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 7
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.
-
8/13/2019 Switchgear CB
21/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 8
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.
This figure is not available online.Please consult the hardcopy thesisavailable from the QUT Library
-
8/13/2019 Switchgear CB
22/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 9
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
-
8/13/2019 Switchgear CB
23/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 10
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
This figure is not available online.Please consult the hardcopy thesisavailable from the QUT Library
-
8/13/2019 Switchgear CB
24/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 11
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].
-
8/13/2019 Switchgear CB
25/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 12
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.
This figure is not available online.Please consult the hardcopy thesisavailable from the QUT Library
-
8/13/2019 Switchgear CB
26/175
-
8/13/2019 Switchgear CB
27/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 14
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
-
8/13/2019 Switchgear CB
28/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 15
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.
-
8/13/2019 Switchgear CB
29/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 16
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:
This figure is not available online.Please consult the hardcopy thesisavailable from the QUT Library
-
8/13/2019 Switchgear CB
30/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 17
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)
This figure is not available online.Please consult the hardcopy thesisavailable from the QUT Library
-
8/13/2019 Switchgear CB
31/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 18
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
-
8/13/2019 Switchgear CB
32/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 19
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])
This figure is not available online.Please consult the hardcopy thesisavailable from the QUT Library
-
8/13/2019 Switchgear CB
33/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 20
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-
-
8/13/2019 Switchgear CB
34/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 21
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
This figure is not available online.Please consult the hardcopy thesisavailable from the QUT Library
-
8/13/2019 Switchgear CB
35/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 22
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.
-
8/13/2019 Switchgear CB
36/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 23
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
-
8/13/2019 Switchgear CB
37/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 24
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.
This figure is not available online.Please consult the hardcopy thesisavailable from the QUT Library
-
8/13/2019 Switchgear CB
38/175
-
8/13/2019 Switchgear CB
39/175
-
8/13/2019 Switchgear CB
40/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 27
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
This figure is not available online.
Please consult the hardcopy thesisavailable from the QUT Library
-
8/13/2019 Switchgear CB
41/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 28
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
-
8/13/2019 Switchgear CB
42/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 29
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.
-
8/13/2019 Switchgear CB
43/175
-
8/13/2019 Switchgear CB
44/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 31
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
-
8/13/2019 Switchgear CB
45/175
Chapter 2: Literature Review
M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 32
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