Case Study of the Failure of two 13.8kV Control & Metering Transformers that caused significant Equipment Damage

Transcription

1 LLNL-CONF Case Study of the Failure of two 13.8kV Control & Metering Transformers that caused significant Equipment Damage G. R. Dreifuerst, D. B. Chew, H. L. Mangonon, P. W. Swyers September 1, 2011 IEEE Electrical Safety Workshop Daytona Beach, FL, United States January 30, 2012 through February 3, 2012

2 Disclaimer This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.

3 CASE STUDY OF THE FAILURE OF TWO 13.8KV CONTROL & METERING TRANSFORMERS THAT CAUSED SIGNIFICANT EQUIPMENT DAMAGE Copyright Material IEEE Paper No. ESW-12 (do not insert number) Gary Dreifuerst PhDEE Dennis Chew, P.E. Harry Mangonon, P.E. Paul Swyers, P.E. Member, IEEE Member, IEEE LLNL M/S L-604 Member, IEEE LLNL M/S L-449 LLNL M/S L-601 Livermore, CA LLNL M/S L-604 Livermore, CA Livermore, CA USA Livermore, CA USA USA USA Abstract - The degradation and failure of cast-coil epoxy windings within 13.8kV control power transformers and metering potential transformers has been shown to be dangerous to both equipment and personnel, even though best industrial design practices were followed. Accident scenes will be examined for two events at a U.S. Department of Energy laboratory. Failure modes will be explained and current design practices discussed with changes suggested to prevent a recurrence and to minimize future risk. New maintenance philosophies utilizing partial discharge testing of the transformers as a prediction of end-of-life will be examined. Index Terms 13.8kV, arc flash, arc blast, CPT, MPT, designing for safety Auspices This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA I. INTRODUCTION This Department of Energy lab was founded in 1952 with the mission to ensure the safety and security of the nation through applied science and technology. This lab houses world-class high performance computing facilities, the world s most powerful laser and numerous other high technology facilities on its one square mile location. The electric utility supply for these requires high availability and quality of service. The base line power supply is about 180MVA. This is distributed on-site through double-ended 15kV-class metal-clad switchgear lineups. The line-up circuit breakers are switchable through a Supervisory Control And Data Acquisition (SCADA) system. Each line-up has two sets of Control Power Transformers (CPT) and four sets of Metering Potential Transformers (MPT) to drive local relaying protection networks and automatic transfer schemes. These are all 15kV-class transformers using cast-epoxy solid dielectrics. A. Background II. CPT FAILURE AUGUST 29, 2007 [1] The first failure of a cast-epoxy 15kVA 13.8kV primary transformer in our recent history was destructive to the transformer and to the 13.8kV line-up. Substantial debris was circulated through the medium-voltage switchgear with very little mechanical damage to the surrounding structure. A risk would have existed for any worker present during this incident. The CPT, which feeds one side of the line-up, failed its primary (13.8kV winding) side insulation. It experienced turn-turn and layer-layer shorting (i.e., the transformer shorted-out and failed) during the failure event. The failure was not sufficient to cause the E type primary fuse (7E) to clear the fault. This device is constructed using a cast-epoxy, copper primary winding. The failure of this transformer caused the loss of AC power to various transducers. This event was logged by the SCADA system. Protective actions resulted in shutting down power to 107 buildings. B. Technical Review The metal-clad switchgear line-up was manufactured in the 1990s. There are six line-ups on the lab s 13.8kV system. Four of the line-ups have identical layouts and CPTs. The rest of the line-ups have slightly different CPTs. This was the first failure of a CPT. It is not possible to perform an accident reconstruction because of the magnitude of the damage to the CPT. The specific initiator is not known. It is known that no power system disturbances were observed by the SCADA system until the CPT started to fail at 07:14:40. The 13.8kV primary power was still applied until 07:25:21 when the bus differential relays tripped. Over that time, the CPT failure progressed until the epoxy cast primary was sufficiently overheated, which boiled off resin creating substantial smoke which prompted the call to the Fire Department. The CPT failure may have started with a simple shorted-turn in the winding due to interwinding (turn-turn or layer-layer) insulation failure. That would have increased the primary current, in order to apply more power to the failed winding area. As the temperature increases, the remaining insulation starts to fail, causing additional damage sites to develop and to deposit even more heat. This dynamic would not require a substantial amount of current to destroy the transformer. Figure 1 shows the result that the extreme heat caused by the arcing, progressively destroying the insulation and welding the wire fragments together. The arc debris damaged the two vertical compartments which were exposed to the CPT. Just above the CPT is the compartment for the Main Breaker for the main bus. After disassembly for cleaning and rewiring, arc spots to the frame were observed in the rear of the CPT compartment. The presence of these arcs is the likely reason that the bus differential relay opened the bus breakers at [Type text]

4 07:25:21. The action of the differential relay properly opened the main, tie and all feeder breakers, but could not open the source side breaker (remote from this line-up) which powers the main breaker. The extent of the debris from the CPT failure bridged the bus insulators to ground. The source feeder breaker tripped on overcurrent. 124 buildings for at least 2 seconds and 173 buildings for approximately 4 hours. An internal failure in one of the two single-phase MPTs located in the lower compartment of the vertical section containing the tie breaker caused a short-circuit and ground fault at the MPT s input contacts, melting and vaporizing the contacts. The resulting damage is shown in Figure 2. This created an arc across the 13.8kV input contacts of the MPT, resulting in an arc flash and an arc blast (an arc flash of significant power and energy sufficient to create a pressure wave able to mechanically destroy facility equipment) that blew the lower compartment front door off the enclosure and damaged the rear panels in that compartment. Figure 1: Damaged 15kVA CPT protected by 7E fuses C. Manufacturer s Installation Instructions for Replacement CPTs reveals likely fault scenario If a transformer develops a winding fault, in many instances the primary fuses do not clear the fault until the transformer has split open from internal pressure and vented ionized gases. These gases can travel to the top of the vertical section and can cause a phase-to-phase or phase-to-ground fault on an uninsulated bus, which can then travel to adjacent sections in a line-up. Good practice would indicate that the sections with a transformer should be designed to prevent ionized gas traveling into adjacent sections. A. Background III. MPT Failure December 17, 2010 [2] Figure 2: MPT failure in bottom drawer of feeder breaker cabinet. The tie breaker, located in the upper compartment above the MPTs, began to open in order to clear the fault condition. However, the tie breaker experienced a failure at its input connections due to the arc blast originating in the lower compartment, which created a second arc flash and resulting arc blast. The section s front panel and the upper compartment door were blown off. See Figure 3. The building showed minor evidence of overpressurization, with some sheet metal screws popped out of the metal siding and a double-hung window tipped outward after shearing off two of the four framing screws. The extent of damage resulted in the Incident Analysis Report [2] concluding that death may have resulted for someone in front of the line-up s faulted vertical section, and organ or hearing damage for anyone in the room. The second failure of a cast-epoxy 13.8kV primary transformer was destructive to the transformer and to the 13.8kV switchgear line-up. A major risk of harm would have existed for any worker present during this incident. The failure was not sufficient to cause the E type primary fuse (0.5E) to clear the fault. Substantial debris was circulated through the medium-voltage switchgear with significant mechanical damage to the surrounding structure and observable over-pressurization effects for the building. B. Technical Review At approximately 11:37, an electrical outage occurred due to the unintentional opening of a tie breaker. This outage affected Figure 3: Line-up with damaged feeder breaker cabinet. Revision 2,

5 IV. CONCLUSIONS A. Transformer Issues The first failure led us to believe that the temperature rise in the epoxy core and resulting insulation degradation was the likely initiator, so we implemented a pro-active replacement of all CPTs, regardless of their age, in our metal-clad switchgear line-ups. The second failure changed our thinking about temperature rise, because the MPTs are not a power-handling transformer. MPTs have no appreciable temperature rise in service. In reviewing the switchgear design, we confirmed that locating MPTs in switchgear is not a disallowed practice by API [3] and by IEEE [4]. Overcurrent HV-side fusing is set up to protect the buses and not the PTs. The insulation failure in both manufactured transformer types is now believed to be due to partial discharge (PD) activity. A regular measurement and trending of PD levels for the 13.8kV transformers is being planned to look for degradation tendencies, with replacement as part of the Preventative Maintenance (PM) program. This PM effort is being used to decrease not just the equipment failure rate, but also to reduce the risk exposure to the engineers and electricians that work on and around the lineups. The second incident had the good fortune to be monitored by a computer-based relay. The waveform data gathered by this relay provided a clear picture of exactly how fast the failure mechanism occurred. What showed up as an intermittent SCADA undervoltage relay error message overnight, became a fully involved 30kA three phase fault in less than 7 cycles (0.12 seconds) B. Safe Work Practices The standard work practices for the High Voltage Electricians included reviewing the status of panel mounted instruments and verifying connections or settings of components that are housed in the control compartments of the energized metal clad switchgear line-ups are operating correctly. Following the first incident detailed in section II above, there was no modification to that work practice. The engineering action of replacing all the CPTs was thought to have significantly decreased the probability of exposure to a repeat of that failure and therefore was viewed as an appropriate action that mitigated the risk. Both engineering and electrician personnel recognized that a risk still exists due to the destruction brought about from the second incident detailed in section III. Any work where a compartment must be opened while the line-up is energized is no longer allowed. The time that a worker must be in the vicinity of an energized line-up is also minimized. As PM data is gathered, further work modifications may be incorporated. V. ACKNOWLEDGEMENTS An outside consultant was hired to help analyze the second fault [5]. He agreed that the transformer location complied with industry practice and that the primary fusing for the transformer would not protect against the destruction of the transformer. VI. REFERENCES [1] Lawrence Livermore National Laboratory, Investigation of the LLNL Power Outage due to a Transformer Failure in LGS-15 on August 29, 2007, Incident Analysis Report , March 3, [2] Lawrence Livermore National Laboratory, Building U424 Electrical Utility Incident, December 17, 2010, Incident Analysis Report , March 7, 2011, LLNL-AR [3] Electrical Installations in Petroleum Processing Plants, API Recommended Practice 540, Fourth Edition, April [4] IEEE Std C Standard for Metal-Clad Switchgear. [5] Hank Lee, LLNL-Bldg 424, T200 15kV Switchgear West Bus Fault Investigation and Analysis, Electrolink Engineering Services, Inc., report, January 25, VII. VITA Gary R. Dreifuerst (M 1970) is an electronics engineer at Lawrence Livermore National Laboratory (LLNL) since June 1981 working in the Inertial Confinement Fusion Laser programs after receiving his electrical engineering PhD degree from the University of Wisconsin-Madison. Gary is the Electrical Safety Officer for the National Ignition Facility and the chair of the LLNL Electrical Safety Committee. Dennis B. Chew (M 2001) received his BSEE degree in 2002 from California Polytechnic State University, San Luis Obispo and is a registered Professional Electrical Engineer in California. Dennis has worked at Lawrence Livermore National Laboratory (LLNL) since 2002 as an Electrical Engineer. In 2008, Dennis began working for the LLNL Electric Utility Group and is currently the Electric Utilities Services Supervisor as well as a member of the LLNL Electrical Safety Committee. Harry L. Mangonon received his BSEE degree in 1971 from the University of the Philippines. He has been a Utility Engineer at Lawrence Livermore National Laboratory since He is a registered Professional Electrical Engineer in California and Wyoming. Harry has over 40 years experience as an engineer that included stints at Bechtel and Aker Kvaerner in the petrochemical, non-ferrous metals and power industries; and as a Field Service Engineer for Westinghouse Electric Corporation. Paul W. Swyers (M 1983) received his BSEE degree in 1984 from California Polytechnic State University, San Luis Obispo and his MSEE degree in 1994 from Santa Clara University. Paul worked at Pacific Gas & Electric Company from 1984 to 1989 as an electric utility distribution engineer. Since 1989, Paul has worked at Lawrence Livermore National Laboratory (LLNL) in the Electric Utility Group. Paul is currently the LLNL Electric Utility Group Leader and a member of the LLNL Electrical Safety Committee. Paul is a registered Professional Electrical Engineer in California. Revision 2,