Investigation of circuit breaker switching transients for shunt reactors and shunt capacitors

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(1)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 of Master of Engineering School of Engineering Systems Faculty of Built Environment and Engineering Queensland University of Technology 2008.

(2) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. Keywords Antenna, arcing time, capacitor bank, capacitive coupling, circuit breaker, condition monitoring, controlled switching, current interruption, high frequency, high voltage, non-intrusive, on-line monitoring, overvoltages, prestrike, restrike, reignition, shunt reactor, switching, timing, transients, voltage sensor.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page i.

(3) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 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 restrikes 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 low frequency 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.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page ii.

(4) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 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 ...........................................................................................8 2.3.1 Interrupting capacitor bank...............................................................................................8 2.3.2 Energising capacitor bank ..............................................................................................13. 2.4. Review of reactor bank switching .............................................................................................14 2.4.1 Interrupting shunt reactor bank.......................................................................................15 2.4.2 Current chopping ............................................................................................................16 2.4.3 Reignition .......................................................................................................................20 2.4.4 Oscillation modes ...........................................................................................................24 2.4.5 Interaction between phases.............................................................................................26 2.4.6 Energising transients.......................................................................................................28. 2.5. Limitation of overvoltage transient during reactive switching ..................................................28 2.5.1 Over voltage limitation ..................................................................................................28 2.5.2 Controlled switching.......................................................................................................29. 2.6. Failure of circuit breaker due to restriking ................................................................................30 2.6.1 Importance of detecting restrike .....................................................................................36. 2.7. Condition monitoring for circuit breakers .................................................................................39 2.7.1 Detecting restrikes or interrupter experiencing prolong restrikes ..................................40 2.7.2 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 .....................................................................................................47 3.5.1 Construction of the Passive Antenna..............................................................................47. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page iii.

(5) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 3.5.2 Review on capacitive coupling.......................................................................................49 3.5.3 Single phase capacitive coupling model.........................................................................51 3.5.4 Three phase coupling inside substation ..........................................................................55 3.6. Recording instruments ...............................................................................................................56 3.6.1 Digital oscilloscopes.......................................................................................................56 3.6.2 Coaxial cable ..................................................................................................................58 3.6.3 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....................................................................................60 4.1.1 Purpose of measurement.................................................................................................60 4.1.2 Restriking in Vacuum Circuit Breaker ...........................................................................60 4.1.3 Test and measurement set up..........................................................................................62 4.1.4 Reactor opening at 3kV with Passive Antenna located close to the supply transformer 64 4.1.5 Conclusion......................................................................................................................67. 4.2. Exploratory three phase capacitor bank switching measurement at Blackwall substation ........68 4.2.1 Purpose of site measurement ..........................................................................................68 4.2.2 Site details and arrangement...........................................................................................68 4.2.3 Measurement set up ........................................................................................................70 4.2.4 Summary of tests/measurement carried out....................................................................72 4.2.5 Opening operation ..........................................................................................................73 4.2.6 Closing operation............................................................................................................75 4.2.7 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 ...............................................................................83. 5.4. 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..............................................................................99 5.8.1 Closing operation............................................................................................................99 5.8.2 Opening operation ........................................................................................................100. CHAPTER 6: MEASUREMENT OF SHUNT REACTOR BANK SWITCHING .................. 101 6.1. Site arrangement ......................................................................................................................101. 6.2. 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......................................................................119 6.7.1 Background measurement ............................................................................................119 6.7.2 Closing operation..........................................................................................................119 6.7.3 Opening operation ........................................................................................................119. CHAPTER 7: ANALYSIS OF RESULTS ..................................................................................... 121. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page iv.

(6) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 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 .........................................................................................138 7.3.1 Analysing using Fast Fourier Transform (FFT) ...........................................................138 7.3.2 Analysing using Short Time Fast Fourier Transform (ST FFT) Analysis....................139. CHAPTER 8: CONCLUSION........................................................................................................ 144 REFERENCES................................................................................................................................. 144 APPENDICES .................................................................................................................................. 153 Appendix A: Site Measurement Procedure..............................................................................153 Appendix B : Sample of forms used for site measurement......................................................156 Appendix C : Matlab Program for Capacitive Divider Model (In Chapter 7) .........................158. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page v.

(7) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 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) and nozzle (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 ...............................................................................................................51 Figure 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. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page vi.

(8) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 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 Powerlink’s 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 Powerlink’s 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 Powerlink’s 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 Powerlink’s portable recorder for Test 4....................................109 Figure 6.10 Waveforms captured by PA 1,2 and 3 during CB closing operation for Test 4 ..............110. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page vii.

(9) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 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 Powerlink’s 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..................................117 Figure 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 ................................130 Figure 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. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page viii.

(10) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 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 21st May 2007 ...............................................72 Table 5.1 Summary of tests conducted at Blackwall on 7th August 2007.............................................83 Table 5.2 Voltage measured by each Passive antenna during background measurement.....................86 Table 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 21st August 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. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page ix.

(11) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 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. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page x.

(12) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 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:. _________________________. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page xi.

(13) Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors. 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 really needed. ALHAMDULLILLAH.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page xii.

(14) Chapter 1: Introduction. Chapter 1: Introduction. 1. 1.1. BACKGROUND Modern SF6 puffer circuit breakers have been designed with far fewer interrupters per. pole than previous generations of SF6 circuit breakers. This has meant that modern circuit breakers have to contend with far higher voltage stress in the dielectric recovery region than previous types. The increased stress has caused dielectric re-ignition of some types of circuit breakers on capacitive switching duties [1]. In line with this, new standards [2] have been developed that require a large number of tests and provide a classification of circuit breakers based on their probability of restriking for capacitive switching. Switching of shunt reactors is recognised as a duty that causes a very high rate of rise of transient recovery voltage across the circuit breaker contacts [3]. Restrikes of modern SF6 circuit breakers have been observed during disconnection of shunt reactors but the high-frequency reignition current is interrupted at an early current zero and often there is no external evidence of any adverse effects on the circuit breaker interrupters. With the ongoing development in circuit breakers, re-strike free operation of circuit breaker is not guaranteed for the stressful capacitive and inductive switching duty. [2] states that a very low restrike probability as the best possible performance for circuit breakers on capacitor bank switching duty. [1] and [3] give guidance on the application of AC high voltage circuit breakers for capacitor bank and shunt reactor bank switching respectively. Precautions are also taken during the design stage by selecting suitable breaker duty, carrying out system studies and evaluating methods to reduce overvoltage transients. Controlled switching to reduce switching transients seems to be the preferred method chosen by utilities. Despite all these measures, failures of modern circuit breaker during capacitor bank and shunt reactor bank switching have been observed. There has been increasing evidence that recent failures of circuit breakers have been due to restriking during reactor switching [4] and capacitor switching. Generally, restrikes do not cause immediate failures but they gradually degrade the nozzle over time leading to catastrophic failure. Spencer [5] has suggested that the high-frequency re-ignition currents during interruption cause “parasitic arcing” in the circuit breaker nozzle and that this phenomena leads to gradual deterioration of the nozzle that may. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 1.

(15) Chapter 1: Introduction. 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 Moore’s 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 online 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.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 2.

(16) 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. It starts 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.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 3.

(17) Chapter 2: Literature Review. Chapter 2: Literature Review. 2. 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.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 4.

(18) Chapter 2: Literature Review. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. 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. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 5.

(19) Chapter 2: Literature Review. 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 a CB in a predominantly capacitive or inductive network may result in inrush currents. The highfrequency 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-frequency current 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 circuit breaker determines amplitude, frequency, shape of the current and voltage oscillations.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 6.

(20) Chapter 2: Literature Review. 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 minimal disturbance 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.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 7.

(21) Chapter 2: Literature Review. 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 deenergisation 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 the time 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.. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. 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, Cb represents the capacitance of the substation busbar and other equipment. The impedance of the source is represented in the circuit by R1 and L1.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 8.

(22) Chapter 2: Literature Review. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. 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. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 9.

(23) Chapter 2: Literature Review. 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.. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. 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 relatively weak 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. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 10.

(24) Chapter 2: Literature Review. cases the circuit breaker is capable of interrupting it at the first current zero. If this should occur soon after the contacts have parted, a voltage of twice the system voltage will 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:. fo =. ω 1 = 2π 2π ( LC )1 / 2. (2.1). where L is 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. ⎛L⎞ ir = 2Vp ⎜ ⎟ ⎝C ⎠. 1/ 2. sin ω 0 t. (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 4Vp across 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].. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 11.

(25) Chapter 2: Literature Review. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. 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.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 12.

(26) Chapter 2: Literature Review. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. Figure 2.6. Capacitance switching with multiple restrikes. (from [9]). 2.3.2. Energising capacitor bank Energisation of capacitor banks are usually associated with transient voltages and. currents. Transients produced during energising capacitor bank are inrush currents causing a voltage dip and overvoltages resulting from the system response to the voltage dip. It is common nowadays to have more than one capacitor bank connected to the same bus. This has no influence on the conditions at interruption. The inrush current during closing is affected to a high degree [1]. Two different situations may occur: •. The capacitor bank is energised from a bus that does not have other capacitor banks energised. This situation is called isolated capacitor bank switching.. •. The capacitor bank is energised from a bus that has other capacitor banks energised. This situation is called back-to-back capacitor bank switching. It is worth noting that even energised capacitor banks in nearby substations may contribute to the inrush current representing a back-to-back situation.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 13.

(27) Chapter 2: Literature Review. 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 assumption is 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. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 14.

(28) Chapter 2: Literature Review. 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.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 15.

(29) Chapter 2: Literature Review. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. 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:. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 16.

(30) Chapter 2: Literature Review. •. the characteristic “chopping number” of one interrupting unit of the switching device;. •. the effective parallel capacitance;. •. the number of breaks in series.. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. 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. ich = λ C t. (2.3). where ich. = current level at the instant of chopping (A). Ct. = total capacitance in parallel with the breaker (F). λ. = chopping number for a single interrupter (AF −0.5). M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 17.

(31) Chapter 2: Literature Review. The chopping number, λ, 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. Ct = C p +. CsCL Cs + CL. (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-tophase 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 Ct is given by. Ct = C p + C L. (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:. I ch = λ NC t. (2.6). The level of current chopping may be dependent on arcing time. This tends to be the case for SF6 puffer 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. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 18.

(32) Chapter 2: Literature Review. circuit, i.e. the reactor itself and all equipment connected between the circuit-breaker and the reactor.. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. Figure 2.9 Chopping Phenomena in single phase (from [11]). M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 19.

(33) Chapter 2: Literature Review. 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 :. ⎛i ⎞ L k a = 1 + ⎜⎜ ch ⎟⎟ * ⎝ uo ⎠ C L. (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>>CL and Cp is negligible, the overvoltage is dependent on ich only. Equation 2.4.2.5 can then be rewritten as. ka = 1 +. 3 Nλ 2 2ωQ. (2.8). where Q. = three-phase reactor rating (V· A). λ. = the chopping number (AF-0.5) for a single interrupter. ω. = 2Πf = 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-. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 20.

(34) Chapter 2: Literature Review. 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.. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. 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. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 21.

(35) Chapter 2: Literature Review. 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.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 22.

(36) Chapter 2: Literature Review. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. 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).. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 23.

(37) Chapter 2: Literature Review. This figure is not available online. Please consult the hardcopy thesis available from the QUT Library. 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. fL =. 1 2π LC L. (2.9). and is in the range 1 to 5 kHz. This oscillation may be modulated due to phase interaction as described later.. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 24.

(38) Chapter 2: Literature Review. First Parallel Oscillation. Second Parallel Oscillation Main Circuit Oscillation. Load Circuit Oscillation. Figure 2.13 Oscillation Mode in the reactor circuit. Reignition oscillation Reignition phenomena are described in detail in [13] but is described briefly to give appreciation on oscillations involved during reignition. Three different oscillation circuits are involved in reignitions. A “first parallel” oscillation occurs when CP discharges through the circuit breaker; the frequency of this oscillation is. f p1 =. 1 2π LP C P. (2.10). and is in the order of 1 to 10 MHz. The circuit breaker will not interrupt the current associated with the “first parallel” oscillation. A “second parallel” oscillation (reignition overvoltage oscillation) will follow, as a result of which, the voltages across Cs, and CL are equalized, i.e., the voltage across the circuit breaker is reduced to zero for an instant. The frequency of the “second parallel” oscillation is given by. f p2 =. 1 2π. CL + Cs Lb C L C s. (2.11). and is in the range 50 to 1000 kHz. The circuit breaker may interrupt the current associated with the “second parallel” oscillation. If it does not, then a “main circuit” oscillation develops. This oscillation involves the total circuit and generally leads to a new loop of current. M.Shamir Ramli, M.Eng Thesis, QUT, 2008. Page 25.

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