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IOSR Journal of Electrical and Electronics Engineering (IOSR-JEEE) e-ISSN: 2278-1676 Volume 5, Issue 1 (Mar. - Apr. 2013), PP 34-42 www.iosrjournals.org

Preventive Maintenance of Circuit Breaker 1

Mr. B.Babu, 2Mr.G.Ashok Kumar, 3Ms. B.R.Supraja, 4Ms.A.Gayathri 1

Head Of the Department, 2Asst.Prof., 3Student 4Student (E.E.E Dept., M.R.R.I.T.S/J.N.T.U.A/A.P/INDIA)

Abstract: Worldwide utilities and industries have seen various technological advancements in circuit breaker (CB) testing and analysing. Yet most of the owners underperform the maintenance activity when it comes to circuit breaker maintenance and many limit themselves to an operating time test or a static contact resistance test using inappropriate test equipment. The reasons cited for this were ‘complicated test procedures which involve longer outage time on the feeder’, ‘lack of availability of the feeder for maintenance’ and ‘lack of permission to remove the earth rod or earth switch while the breaker was in shutdown condition’. Also the decision making process and pin pointing the fault in a breaker after performing a basic circuit breaker test will often be a challenge. Though the basic tests like timing test on a circuit breaker provides preliminary information about the breaker condition, actual condition of a deteriorating breaker becomes obvious in a well matured state and often revealed only by a catastrophic failure while in service. Different utilities, switch gear manufacturers, researchers and test equipment manufacturers have stressed the importance of various advanced test methodologies to save time and money. Those tests include coil current analysis, dynamic resistance measurement, testing on dual ground conditions, vibration testing, first trip analysis, online testing and so on. These circuit breaker testing technologies often provide critical information of the circuit breaker and make the decision making process simple. Pinpointing the fault in a circuit breaker and attempting the maintenance activity in a very short time will be complicated if the directions guided by the test methodologies or equipments are not clear enough or easy enough. Also one particular method, tool or technology will never provide a complete solution on the circuit breaker analysis. In this paper we would like to present a report on how some of the latest technologies in the testing industry with an integrated approach of different test methods have maverted major catastrophes in substations and has made the CB testing and analysing processes simple, safe, effective and efficient. Keywords: Circuit Breaker, Dual ground, vibration, DRM, DCM.

I.

Introduction

The term „circuit breaker (CB) test‟ has evolved now to mean either „timing test‟ measuring the operating time or „contact resistance test‟ measuring the static contact resistance (SRM) value. The following were some of the valuable additional field CB tests available in general but were scarcely done by the maintenance teams. Motion recordings, Auxiliary contact lead lag time, Pre-insertion resistor (PIR) contact timing value, PIR contact resistor value, Coil current graph recording, Coil voltage graph recording, Coil resistance measurement, Dynamic resistance measurement (DRM) for CB contacts, Vibration analysis (both online and offline), Online first trip recordings, Pressure graph recording, Motor current graph recording, Minimum coil operating voltage or Force / energy test, Vacuum integration check for vacuum interrupters, Synchronised switching / controlled switching test, Tan delta measurement for the grading capacitors, Insulation resistance measurement and so on. “To develop “ideal” maintenance management, information about the system and about the individual device must bebrought together” [6]. The more information obtained by performing a reliability centred maintenance the higher the reliability. If underperformed, the reliability of the circuit breaker‟s operation and thereby the performance of the transmission system naturally goes under question. The process of making circuit breaker (CB) testing as effective and efficient depends on the quality of the results obtained and the time taken to perform such tests. The Dynamic Resistance Measurement test (DRM) which records and plots the variation in contact resistance in a time scale is one of the most effective methods of performing circuit breaker testing for the highly populated SF6 and Gas Insulated Substation (GIS) type breakers. The Dual ground method of testing the circuit breakers in a live substation with both sides of the breakers earthed is a unique solution for practical onsite issues in CB testing. As per Cigre reports [2] the highest percentage of failures occurs in the mechanical parts of the circuit breaker. Vibration test is directly recording the mechanical vibrations occurring in a circuit breaker during operation and performing Dynamic Time Warping (DTW) analysis. The biggest benefit of the vibration analysis is the option for performing both online (when the breaker is in service) or to perform offline (when the circuit breaker is under shut down condition). In this paper, the way these three innovative technologies have transformed and have adapted to the need of the industry will be explained along with some field proven case examples. www.iosrjournals.org

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Preventive Maintenance Of Circuit Breaker II.

Dynamic Resistance Measurement (DRM)

Dynamic resistance measurement is measuring the contact resistance by Voltage/current method continuously during breaker operation and plotting the resistance graph over time. A typical schematic arrangement is as shown in Fig-1. By performing DRM measurement the arcing contact length and the condition can be reliably estimated without dismantling the CB. Knowing the condition of the main and arcing contacts is vital as the contacts may get deteriorated during heavy fault interruptions.

Over years this technology has proven the effectiveness in identifying the faulty breakers and saving the breakers. Some maintenance teams cite the complicated test set up and thereby the time consumed to perform the test as a drawback. The figures 2a, 2b, and 3, show the evolution of this DRM technology.

As shown in Fig – 2a the DRM measurements were made by heavy batteries and heavy cables that can handle around 200A [5]. The major problem was the weight and also the time required to complete for 3 or 6 breaks in a 1 break per phase or 2 breaks per phase interrupter CBs. The unit has to be connected and disconnected for each break in the site to get the full result. As shown in Fig – 2b the design was drastically improved in reducing the weight, but the current output was still single channel. Then came the super capacitors or ultra capacitor based designs which reduced the weight around two Kilo gram per unit and can offer 2 channels of 250A. 6 channels of current output from the DRM units were done in a single shot recording and the analysers were capable of recording 18 channels of information for DRM 3 currents, 3 voltages, and 3 resistance plots for all 3 phases. Fig – 3 shows the schematic of a DRM recording on a 2 break per phase design based on super capacitors.

This latest solution answers the genuine issue of complicated test set up and the time consumption issue of the maintenance teams. www.iosrjournals.org

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Preventive Maintenance Of Circuit Breaker 2.1 Field cases – DRM In this chapter some of the field cases that have saved the High Voltage circuit breakers from replacement state to repair state by timely maintenance and thereby improving the reliability of the whole system will be discussed. The DRM recordings have become the key tool to identify the issues in these circuit breakers where the classical timing tests and contact resistance tests couldn‟t identify. The following were some of the recordings performed by Power Grid India: World's Leading Power Transmission Utility maintaining 82,354 Circuit km line and 135 EHVAC and HVDC substations [8]. CB Population: 2500 Nos. (Approximately) Causes of CB failures observed: Manufacturing defect, Design defect, and Lack of maintenance Common Failures: Mechanical 80% Electrical / Dielectric16%, Others 4%. Components of failures: Loose Helicoils, Loose PIR contact Actuator ring failure, Magnetic ventils Pitting and erosion in contacts Leakage of hydraulic oil, PTFE rod in PIR assembly breakage PIR pull rod bending, SF6 gas leakage, Main cylinder bolts loose, Tilted piston in main drive, Operating rod loose Casting failures of the operating assembly and so on Case – 1: 400kV SF6 Bus Reactor CB with 8 years service and 1400 operations: During routine maintenance differences were observed in DRM graph recordings between poles as shown in Fig – 4a and 4b.

Issues identified on dismantling the suspected CB were:“Moving arcing contact found deformed and expanded. One screw of moving main contact assembly found loose. PTFE Nozzle of moving contact assembly found cracked. Scratch marks on main moving Contact.” Fig 5 shows the condition of the CB in dismantled condition

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Preventive Maintenance Of Circuit Breaker Case – 2: 400kV, 50MVAR Bus reactor circuit breaker: This breaker was in frequent switching operation for controlling the bus voltage. The static resistance was beyond the limit during annual maintenance and the DRM recordings were performed. The recorded DRM graph was as shown in Fig –6a in the R- Phase Bus side interrupter and was similar in the Y phase Bus interrupter too. Hence on suspicion, the CB was dismantled. The nozzles in the moving contact assembly /Puffer Cylinders were found damaged in most of the interrupters. The severest one was as shown in the Fig – 7a. Fig – 6b shows the DRM recording done in the same pole after the Nozzle replacement. This breaker would have exploded for the next fault trip and would have cost multiples of money and time compared to the repairs cost and time spent.

2.2 DRM – A closer look Older designs use batteries as current source for SRM and DRM. To switch the current injection on/off they use the circuit breaker itself by always performing a „Close-Open‟operation sequence. This can be very dangerous if the circuit breaker fails to open. Batteries can become overheated and explode in worst case. These designs cannot perform the simplest and most accurate way of measuring the „Open‟ DRM sequence alone as shown in Fig - 8a and 8b. The DRM graph when superimposed on the motion trace will clearly provide the arcing contact length. Whereas performing a „close DRM‟ or „Close-Open DRM‟ cannot provide the finest details accurately for analysis. The problem will be the bounces during close which will oscillate the current and therefore the DRM graph. The Open DRM will be consistent and will be easy to analyse as no bounces will be experienced during the breaker opening.

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Preventive Maintenance Of Circuit Breaker Another draw back in using the circuit breaker as a switch for the current injection is that it is impossible to make a measurement with two sides grounded as the current will pass through the grounding even when the circuit breaker is open. The designs that can control, the switching of test current, only during measurement, can perform „Open DRM‟ and also can perform on dualground condition. The need to perform the DRM measurement with dual ground condition is vital as the rise in current from 0 to 200 or 220A though it‟s DC, the ramp may trigger the bus bar protection circuits. Even if the CB under test is under tripped condition and physically isolated, feeder CT secondary leads will still be connected to the bus bar protection differential relays in most designs. Hence due care must be taken during DRM testing on selecting the test lead connection points in the feeder and the test procedure being used.

III.

“Dual Ground” Testing

The most common CB test as referred earlier the „timing test‟ was performed by most of the utilities in an unsafe manner.The reason behind was the lack of technical feasibility. One side of the CB will be earthed and the test current will be flowing through the breaker for performing the operating timemeasurement. But the induction current from the neighbouring live feeders also will be flowing through the test kit as shown in Fig – 9a. The test will be continued relying on the instrument‟s earth and the shield earth of the test cables. In the event of a heavy fault in the neighbouring feeder, high voltage will be induced in the test circuit which may become lethal to the testing personnel. To overcome this issue a patented technology was introduced in 2007 named as Dual-Ground technology where the CBs can be tested when the they are earthed on both sides during shut down as shown in Fig – 9b. The technology used to achieve this is the dynamic capacitance measurement. The circuit breaker can be modelled as a variable capacitor and the test is performed using high frequency ac signals instead of using the conventional DC voltage current methods [1]. When high frequency AC signals were involved the earth leads or earth switches on both side of the breaker were seen as two impedance paths and not as resistance paths.The frequency was varied by the test system and tunes the whole CB circuit with both sides earthed to identify the resonant frequency based on the capacitance of the circuit breaker. When the CB opens from the close position the capacitance varies and hence the resonant frequency varies. The test equipment can sense this difference and can provide accurate results on both sides grounded condition too.�The ground loop can even have lower resistance than the main contact/arcing contact path and it still works. This is particularly crucial when testing GIS breakers and Generator breakers but also for Air Insulated Substations (AIS) breakers having very efficient grounding systems.

3.1 Field case – Dual Ground In this chapter one interesting field case that have proven dual ground testing as a unique solution other than offering safety will be discussed. The single line diagram shown in Fig – 10is a typical feeder arrangement in the Beauly 132 KV substation, Inverness maintained by Scottish and Southern energy, UK. The permit to perform the feeder maintenance work was issued to all three substations interconnected by the feeders because of the T junction arrangement. All the feeders will have to be earthed as shown in the Fig - 10 as per the permit and will not be allowed to remove during the maintenance work. In this condition to perform CB test with the classical one side earthed condition was not possible at all. The “Dual ground” was the solution to conduct the CB tests safely with out removing the earth rods.

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Preventive Maintenance Of Circuit Breaker

IV.

Vibration Testing – online and offline

“One of the major contributing factors in failure of hydraulic switch gears is occurrence of mechanical faults, which the conventional method of monitoring failed to detect and locate- TNB Malaysia [4]” “Out of the Power grid, India CB failures, mechanical defects contribute 80%” “Mechanical Malfunction of the operating mechanism is by CIGRE WG 1.06 reported to be an important failure mode for high voltage circuit breakers [2]” Vibration testing is a well associated procedure with rotating machinery, but getting very popular in recent years for Circuit breaker analysis. Though the technology has been commercially introduced for field testing of circuit breakers in 1997, the advantages of online first trip testing using vibration has increased its efficiency and is creating new interests. The technology behind is the direct measurement and recording the physical vibrations that will occur in a circuit breaker during the breaker operation [7]. As the technology involved is a direct measurement of mechanical events, detecting mechanical issues in circuit breaker is straight forward. Small accelerometers that can measure the acoustic vibration signals over a wide frequency range are rigidly mounted on the circuit breaker at one or more points [2]. Those accelerometers will be connected to the CB analysers through a signal conditioning amplifier. Once the breaker is operated for performing the vibration analysis, either on-line or off-line, the graph recordings will be captured in the analyser. A typical vibration graph with 2 pick ups (accelerometer) one connected to theinterrupter and another to the operating mechanism is as shown in the Fig -11. This graph can provide fine details of the breakers „sequence of events‟ from the moment of command initiation to the full operation execution in a time base scale. This vibration graph recording will become the reference graph or finger print for this breaker for future reference.

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Preventive Maintenance Of Circuit Breaker In the next vibration recording, on this breaker, during maintenance, the new recording should be identical with the previous and follow the same pattern within the maximum allowable limits. If the breaker is having a mechanical defect the vibration graphs will deviate in either x-axis or y-axis. The x-axis variation is the time variation either leading or lagging. The y-axis variation will be amplitude variation which will be level of vibration recorded either higher or lower but usually higher. Comparing the two graphs and figuring out the deviations will not be possible by naked eye and hence the signals will the processed with Fast Fourier Transformation (FFT) logic, compared using an acoustic analysis technique called “Dynamic Time Warping” (DTW) and a resultant graph will be provided. This analysis software will search for the best possible match between the two recordings and provide the time and amplitude deviation graphs. The resultant deviation graphs will assist the maintenance team to exactly pin point the location of fault in the operating mechanism. A recording of a circuit breaker with and without time lag is shown in Fig 12. Multiple techniques like, wavelet theory, envelope analysis, pattern recognition, empirical mode decomposition (EMD) energy entropy, and Multiclass support vector machine (MSVM) and so on for analysing the vibration graphs were continuously evolved [3].

4.1 Field case In this chapter one interesting field case that have proven vibration testing as a unique solution other than diagnosing mechanical defects will be discussed. A 275kV, 6 breaks per phase design Oil CB at Eakring, UK has to be tested for timing measurements along with the PIR (Pre-Insertion Resistor) contact timing. Each phase has 6 serial connected main contacts with parallel PIR contacts. PIR resistor value is 1.2kOhm, or 7.2kOhm per phase. The high ohmic value andthe sliding oil-film on the moving contact made normal timing impossible with safe measuring voltages. It was decided to use vibration testing and was started with sampling and filtering at 10 kHz. The PIR contact touch started to appear but the vibration trace was not showing signature of the main contacts as shown in Fig – 13a. When the sampling frequency was increased to 40 kHz in the CB analyser with corresponding filtering, both PIR and main contact touch were clearly visible as shown in Fig – 13b. In this case vibration testing became a unique solution for measuring the PIR contact timing that couldn‟t be achieved with a conventional timing test unit.

4.2 Vibration analysis in online condition “First Trip analysis” / online testing are the latest trends for testing the most critical breakers. Online testing for high voltage circuit breakers are done to ensure the circuit breaker‟s healthiness while the circuit breaker is in service. Two options are available to do this online testing i ) By providing a „FIRST TRIP‟ command to capture the first trip test values before removing the CB from service for maintenance works. ii) By providing a „RECLOSE‟command for recording the tripping and reclosing measurements of circuit breaker. Based on the results recorded the need to remove the breaker from service for maintenance or to continue in www.iosrjournals.org

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Preventive Maintenance Of Circuit Breaker service can be considered. The characteristics and hence the test results have been proven todiffer to the first trip of a breaker that occurs after a considerable period of continuous CB service and the test that has been done after the breaker is manually tripped and tested again. Hence First trip recording of the vibration measurements along with the other possible first tip measurements will be an excellent way of identifying the exact cause of the CB issues. Fig – 14 shows the online field set up of 3 channel vibration measurements along with 3 phase motion recording, 3 phase CT secondary current recording, auxiliary contact timing, coil currents and coil voltage recording and remote trigger control for performing first trip analysis.

V.

Conclusion

Circuit breaker testing is not „Timing test‟ and „Contact resistance test‟ alone. The latest advancements in the testing and analyzing capabilities of the CB analyzers have proven they can provide even the finest details in the shortest time and simplest ways. Citing the reasons like „complicated test procedures‟ „longer outage time on the feeder‟, „consumes more time‟ „lack of availability of the feeder for maintenance‟ and „lack of permission to remove the circuit main earths during shutdown‟ can no longer be valid for underperforming the breaker maintenance. Any outage caused in any part of the power system component will collapse the reliability of the whole system. To improve the reliability, performing effective and efficient CB testing with proper analysing tools is a must. In this paper some of the latest trends in CB analysing like the DRM, Dual ground, Vibration testing with online first trip analysis were discussed. The application of these testing in some field cases which has averted major failures and also has made the testing process simple has been discussed. What ever the technology offers and however the analysing process simplifies, it‟s the practical knowledge and field experience of the testing engineer, which will identify the crucial differences and can make the decision. These tools help them to make that decision making process effective and efficient.

References [1] [2] [3] [4] [5] [6] [7] [8]

L.Claesson, Z.Stanisic, H.Wernli, and K.Pettersson. “Safe Circuit Breaker Timing with New Technology”,m nternational Conference on Condition Monitoring and Diagnosis, Changwon, Korea, April 4, 2006. H.K.Høidalen and M.Runde “Continuous Monitoring of Circuit Breakers Using Vibration Analysis” IEEE transactions on power delivery, vol. 20, no. 4, October 2005 J.Huang, X.Hu, and X.Geng “An intelligent fault diagnosis method of high voltage circuit breaker based on improved EMD energy entropy and multi-class support vector machine” Electric Power Systems Research 81 (2011) 400–407. L.Kanth “Preventive Maintenance of Spring Switchgear Applying Vibration Diagnostic Tool- A Malaysian Experience” IEEE MELECON 2004, May 12-15, 2004, Dubrovnik, Croatia M.Ohlen, B.Dueck, and H.Wernli “Dynamic Resistance Measurements - A Tool for Circuit Breaker Diagnostics” 1995 Stockholm Power Tech International Symposium on Electric Power Engineering, Vol. 6, p. 108-113, Sweden V. Pitz and TH. Weber “Forecasting of Circuit-Breaker Behaviour in High-Voltage Electrical Power Systems: Necessity for Future Maintenance Management” Journal of Intelligent and Robotic Systems 31: 223–228, 2001. M. Runde, B. Skyberg, and M. Ohlen “Vibration analysis for periodic diagnostic testing of circuitbreakers” High Voltage Engineering Symposium, 22-27 August 1999, Conference publication No. 467. http://www.powergridindia.com/PGCIL_NEW/home.aspx accessed on 01/09/2011.

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Preventive Maintenance Of Circuit Breaker B.BABU was born in A.P, INDIA., in 1983 and received the B.Tech.,(hons) degree in electrical and electronics engineering and the M.Tech degree from S.V.UNIVERSITY, TIRUPATI, INDIA., in 2005 and 2011, respectively. His research interests include direct ac–ac power conversion, variable-speed ac motor drives using different circuit topologies, and moreelectric/electric automobiles. From 2006 to 2008, Lecturer at the CREC, involved in teaching and research in power electronic systems. Since 2010, he has been with the Power Electronics, Machines and Control Group, School of Electrical and Electronic Engineering, MRRITS , udayagiri ,INDIA., where he is currently a Senior Lecturer in Power Electronics. His research interests are power electronic converters and modulation strategies, variable-speed drive systems, and electromagnetic compatibility..

G.ASHOK KUMAR was born in A.P, INDIA., in 1986 and received the B.Tech.,(hons) degree in electrical and electronics engineering and the M.Tech degree from JNTU ANANTAPUR, , INDIA., in 2007 and 2012, respectively. His research interests include CONTINGENCY ANALYSIS BASED ON FUZZY APPROACH. From 2008 to till date working as Lecturer at the MeRITS, involved in teaching and research in power systems. Since 2008, he has been with the Power Electronics, Machines and Control roup, School of Electrical and Electronic Engineering, MRRITS , udayagiri ,INDIA., where he is currently a Senior Lecturer in Power Electronics. His research interests are Electrical power Systems and Protection B R SUPRAJA is persuing her graduation in the department of electrical and electronics engineering in MRRITS,udayagiri,A.P,INDIA.She did her project on testing of electrical equipments in substation at POWER GRID CO-ORPORATION OF INDIA LIMITED, MANUBOLU NELLORE (Dist). She also reviewed seminars on testing and also protection of electrical equipments at substation.

A.GAYATHRI is doing her graduation in electrical and electronics engineering in MRRITS,udayagiri,A.P,INDIA.she also completed her project ON testing of electrical equipments in substation at POWER GRID CO-ORPORATION OF INDIA LIMITED, MANUBOLU NELLORE (Dist).She reviewed seminars on different strategies which include testing of other electrical equipments.

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