TESTING OF ESB s 20kV FAULTED PHASE EARTHING SYSTEM

Transcription

1 TESTING OF ESB s 20kV FAULTED PHASE EARTHING SYSTEM N. McDonagh, MIEI, MIET, ESBI, Ireland, neil.mcdonagh@esbi.ie, W. Phang, MIEI, MIET, MIEEE, ESBI, Ireland W. Bridgeman, ESB Networks, Ireland Keywords: Faulted Phase Earthing, Earth Fault Controller, Continuity of Supply, High Impedance Fault 1 Introduction 1.1 Background In order to reduce customer hours lost (CHL) and customer interruptions (CI), proposed changes in the method of operation of the Irish 20kV distribution system have been reported in [3]. Two of the methods described in [3] are currently on trial. The first involves earthing 20kV transformer neutrals via an automatically tuned Peterson coil. The second is the use of Faulted Phase Earthing (FPE) through the use of a custom built Earth Fault Controller (EFC) [4], [5]. The proposed system will operate as an interchangeable high resistance earthed/isolated system in conjunction with FPE, by the use of a custom designed EFC. The proposed system uses a 300Ω Neutral Earth Resistor (NER), which is switched out during sustained faults. This NER is required to restrict switching over-voltages. During a single line to ground fault, the faulted phase will be earthed and transformer neutral will be isolated, as shown in Figures 1 and 2. As the phase to phase voltage is unchanged with the FPE applied, and load is connected via delta windings the supply voltage to customers is maintained. The concept for this controller was initially developed and was by the ESB (Electricity Supply Board, Ireland) in 1997 [4] and as described in [5]. Figure 1. System under normal operation Figure 2. Operation under single phase fault 1.2 Existing Methods of Operation At present the operations policy states that 20kV transformer neutrals be earthed via a 20Ω NER. Sensitive Earth Fault (SEF) protection on the earthed neutrals is installed to detect and trip for high resistance earth faults. This existing system has the ability to detect single line to ground faults with a resistance of not more than 3kΩ. The 20kV network includes extensive two phase sections resulting in a high level of capacitive unbalance on the 20kV system. Even though there is no fault on the system, single pole switching may in some cases lead to mal-operation of protection. This is due to significant levels of neutral current as a result of capacitive imbalance. 1.3 Controller Development Initial attempts at deployment of the EFC have failed due to issues surrounding the algorithms used by the EFC. Also a lack of priority around the project until CHL/CI penalties and incentives were introduced meant that the project remained dormant. In late 2007 ESBI (Ireland), was commissioned to reassess the function of the controller, and significant changes have been reported in [1]. This paper presents the development of algorithms associated with the EFC, and the testing of these

2 algorithms. The controller provides a robust method of detecting earth faults of up to 12kΩ, and can distinguish between high resistance faults and single pole switching events. The benefit of this method of system operation is to maintain supply during a single line to ground fault. 1.4 Controller Operation One of the main operational challenges for the EFC is to discriminate between single line to ground faults and single pole switching events. This challenge is overcome by new algorithms. The EFC computes the change in currents and voltages before, during and after the FPE cycle, as opposed to only analysing the data before and after the initial event as set forth in [2]. During this analysis the controller specifically looks changes in zero sequence current to determine if there is an earth fault on the system. Using this method in coupled with equations outlined in [1-2] the controller can distinguish between the single line to ground faults and single pole switching events. The algorithms used and the operational aspects of the new system are presented in [1], and therefore will not be discussed in detail in this publication. 2 Simulation A software model of the system was created in order to assist in algorithm development, in order to provide current and voltage inputs to test algorithms. Due to the high level of unbalance and asymmetry on this network, it would not be acceptable to use conventional program utilising sequence components [6], for this reason an Electromagnetic Transient Program (EMTP) is preferred, and Alternative Transient Program (ATP) was selected for this purpose. The simulated network model consists of transformers, three phase feeders, two phase feeders, three phase loads, two phase loads, NER and FPE switches. A simplified illustration of the ATP model is shown in Figure 3. The actual model has three feeders, but for this illustration only one feeder is shown in Figure 7. Phasor values were assessed upon the application of a fault; 2 seconds after fault application, during the FPE auto reclose cycle and after the FPE reclose cycle. A large number of single line to ground faults with different properties (line, phase, fault resistance), and switching events were simulated and the phasor data was used to test the controller. 3 Testing Figure 3. ATP Model Used for Simulation Testing has been carried out in a number of ways: firstly by injecting of simulated currents and voltages by primary injection, and secondly by applying earth faults to the system, and carrying out single pole switching operations. The controller has preformed well using both methods. 3.1 Simulated Tests During the commissioning process, simulated voltages and currents were injected at the primaries of measurement voltage and current transformers associated with the EFC. Over one hundred tests were carried out and the EFC successfully identified earth faults and switching events.

3 3.2 Live Tests Live testing has been carried out during three periods September 2008, March 2009 and October Tests carried out in September 2008 did not involve the operation of the FPE switches, and were simply to assess the abilities of the EFC to acquire, assess and record data. Tests carried out in March and October 2009 involved the operation of FPE and NER switches as per Figure 2. Due to the voltage stress applied to the network as a result of the FPE method of operation, there were a number of insulation failures during preliminary testing. This was not unexpected as this network had never previously seen the full line to line voltage as a phase to ground voltage. Insulation failures occurred at lightning arrestors and cable sealing ends. For this reason a voltage soak was carried out by applying FPE to each phase individually for a number of hours. ESB networks crews were on standby to replace any damaged equipment. During this voltage soak a number of failures did occur, damaged items of plant were replaced and testing continued. During testing Power Quality meters were connected at the test site in Kilmacthomas, Co. Waterford, Ireland. Waveform data from these meters was extracted, some of which are presented for analysis in Sections and As testing was coming to a close during October 2009 an extraordinary event occurred where the NER was damaged and a fuse in the house transformer cubicle blew. It was not known what caused this event, therefore the EMTP model was used to try and replicate measured waveforms Test Results for Switching Events Single pole switching may cause conventional protection to detect an SEF and malopearte. Therefore it is essential that the EFC can differentiate between these events and earth faults successfully. The algorithms of the EFC are designed to discriminate single pole switching events from single line to ground faults. In order to prove the EFC s ability a number of single pole switching events were carried out. These were correctly identified as switching events Test Results for Line to Ground Faults Figures 4 displays the different switching operations involved in carrying out the live earth fault testing, steps include; fault application, FPE application, isolation of neutral, switching in neutral and isolation of faulted phase. Figures 5-8 display the voltage waveforms measured at the substation during live earth fault testing in October The waveforms correspond to the following actions of the FPE system: Closing fault, FPE open, NER closed Closing of FPE, NER closed, Fault on Removal of FPE, NER closed, Fault on Removal of FPE, NER closed, Fault off Of particular interest are the voltage waveforms in Figure 8 where upon the removal of FPE voltages return to healthy levels almost immediately. This may be contrasted with measurements taken on ESB 10kV system where FPE is operated on an isolated system. Figure 9 shows the voltages measured on the 10kV system upon the removal of the FPE switch. It may be seen that in that line to ground voltages rise above the line to line voltage level as a result of ferroresonance. This topic is discussed in more detail in [7]. The observation may be made that during the operation of the new FPE system voltages do not rise above the line to line voltage level. Photos at of the test site setup are shown in Figures 10 and 11, which shows the switch used to apply the earth fault and the array of earth rods to connect the fault to ground. Figure 4. Test System Figure 5. Fault Applied

4 Figure 6. Closing of FPE, NER closed, Fault on Figure 7. Opening of FPE, NER closed, Fault on 2 0 [k V ] Figure 8. Opening of FPE, NER closed, Fault cleared Figure 9.FPE removal at 10kV Figure 10. Earth Fault Test Switch Figure 11. Earth Fault Test Site Analysis of NER Fault During testing an unexplained event occurred which damaged the NER and caused a fuse to blow. Voltage waveforms measured during this event are shown in Figures 12 and 13. In order to try and explain this event an EMTP model was developed as per Figure 14, and a number of simulations were carried out to ascertain the causes of the fault. The occurrence of events in the order listed adjacent to Figure 14 appears to provide an adequate explanation for the events. Corresponding waveforms of modelled voltage at the fault site and the substation are shown in Figures 15 and 16. From analysis of the results it is apparent that the fault was applied correctly. The EFC then behaved in the correct way and earthed the faulted phase after two seconds. Within two cycles of this an arc developed in the NER. This created a low impedance loop between the transformer neutral and the FPE switch which enabled a large current to flow between the neutral and FPE switch. This caused the fuse connected in series to the FPE switch to blow. The blowing of the fuse had the effect of clearing the arc on the neutral and leaving the earth fault untreated on the system. Due to the nature and relative speed of occurrence of these events on the day in question it was difficult to ascertain the cause of the initial problem. Results suggest that the initial event was an arc in the NER caused by the application of the FPE switch. A replacement NER have has been ordered with a higher voltage rating, the damaged NER was rated for 12kV. The model suggests that approximately 1000A of fault current was present. The data sheet of the fuse in question suggests that

5 the fuse would blow after approximately 0.2s with 1000A of current, thus further confirming the hypothesis proposed. Photographs of the NER and the damage due to arcing are shown in Figures 17 and 18 receptively. Figure 12. Measured Fault Site Voltages Figure 13. Measured Line to Ground Voltages 1. Fault applied on S phase on 20kV feeder at 0 seconds 2. S phase FPE opened, 2s after fault 3. Arc forms in NER at within two cycles 4. NER switch attempts to open 200ms after FPE 5. Fuse blows on S phase 6. Arc extinguished Figure 14. Simplified ATP model of test System [kv] ,6 40 [kv] , Figure 15 Modelled Fault Site Voltages Figure 16 Modelled Line to Ground Voltages Figure 17. Neutral Earth Resistor Figure Figure 18. Damaged Neutral Earth Resistor

6 4 Conclusions The present analysis has shown the benefits of the new FPE system which is displayed through test results. The main benefits are: reduced fault site voltages and, reduced overvoltages. Fault site voltage is reduced substantially upon the application of FPE. Overvoltages as a result of the application and removal of the FPE switches has not exceeded the line to line voltages, see Figures 5-8. This is as a result of the NER always being in circuit for switching operations. These overvoltages may be contrasted to those from the isolated 10kV system which uses FPE, observed overvoltages on the 10kV system as shown in Figure 9. Faulted phase earthing using a custom built earth fault controller has been successfully implemented. The new system has the capability to both detect high impedance faults and maintain supply during single line to ground faults. Numerous benefits of this system have been identified such as: improved sensitivity, selectivity, and fault location. There are also a number of benefits associated with the FPE method of operation but not specifically associated with this work, such as: supply continuity, fault site safety. These benefits are discussed further in [1] and [2]. Future work involving the development of the EFC will entail further live testing and the creation of algorithms to detect remote side breaks on the network. There are drawbacks associated with increasing the voltage stresses applicable to any system. In this investigation there were a number of insulation failures at cable sealing ends and lightning arresters. These failures were expected and crews were in place to repair sections of network. EMTP is a valuable tool that can be used for testing algorithms of any device used on the network in order to minimise live testing, and is also useful in trying to explain extraordinary events that may not understood at the time of occurrence, such as the fault that occurred on the NER during testing. Acknowledgements The authors would like to show their gratitude to all the asset management team in ESB Networks with special thanks to Martin Hand and Hugh Borland, Kieran Rourke, Mick Murphy, Alan Staunton, Jim Walsh, Brendan Kennedy, Walter Keane, Michael Power, and all in Gracedieu to for all their hard work. Thanks all in the Power System Studies Group at ESBI, and to James Dooley for proof reading of this test. Biography Neil McDonagh, neil.mcdonagh@esbi.ie, Phone: Received a Bachelor of Engineering from University College Dublin in Mechanical Engineering, and MSc, with distinction, form the University of Bath, in Electrical Power Systems. Mr McDonagh has worked in the area of power systems studies for over 6 years with ESB International, specialising in: system modelling, earthing of high and medium voltage substations, lightning protection, and insulation coordination. References [1] N. McDonagh W. Phang Use of Faulted Phase Earthing Using a Custom Built Earth fault Controller, 10 th International Conference on Development in Power System Protection, IET, March 2010 [2] N. McDonagh Faulted Phase Earthing Using a Custom Built Controller Dissertation as part of MSc, University of Bath, 2009 [3] H. Borland, Influence of fault handling techniques on supply security IEE Colloquium on Improving Supply Security on 11 kv Overhead Networks, May 1990 [4] Electricity Supply Board, of Ireland, 1997, Fault Detection Apparatus and Method of Detecting Faults in An Electrical Distribution Network, Patent Application Number S [5] N. Tobin, B. Brady, Managing Earth Faults on Distribution Networks, Paper Presented to Institution of Engineers of Ireland. (1997) [6] D. Tziouvaras, EMTP Applications for Power System Protection, Protection, Automation & Control World, Spring [7] R. Willhem, M. Watters, Neutral Grounding in High Voltage Transmission Elsevier, 1956, pgs

7 Abstract To reduce customer hours lost (CHL) and customer interruptions (CI), the use of Faulted Phase Earthing (FPE) is being considered on the Irish 20kV distribution system. The operation of this particular FPE system is enabled by the use of a custom built Earth Fault Controller (EFC) that has the ability to detect high impedance faults of up to 12kΩ. The EFC can also successfully identify single pole switching events, which have at times caused the mal-operation of existing protection. FPE involves the earthing of a faulted phase during a single line to ground fault. This ensures that the fault site is made safer and that no customers are interrupted during the fault. This paper details the testing associated with ESB s 20kV FPE system controlled by the EFC. Both live and simulated tests have been carried out to outline the EFC s functionality. This testing is discussed, with specific focus given to live earth fault testing. This paper is a continuation of earlier work [1], which details the structure of the EFC and algorithms used.