Static electrification in a 700 MVA 23.7/345 kV generator step up transformer
- Commonwealth Edison Co., Chicago, IL (United States)
On April 5, 1994, Generator Step-Up Transformer 2E at the Braidwood nuclear station of Commonwealth Edison was tripped off line by protective relays due to a B O high voltage winding failure. At the time of the trip, there were no system disturbances and the unit was at 98% load. Transformer 2E had been in service at Braidwood Station since 1984. Two identical 625/700 MVA, 3 O, 23.7-345 KV transformers (2E and 2W) were in services as the GSU transformers for Braidwood Unit 2. Both are shell form designs furnished to Commonwealth Edison between 1975 and 1980. A teardown inspection of the failed transformer was performed, and this inspection revealed that a turn to turn failure had occurred within the first section of the B O 345 KV winding. The failure appears to have originated strand to strand within conductors of the same turn and later progressed into the turn to turn fault detected on April 5, 1994. During this inspection, evidence of discharges resulting from static electrification were found. These were rather light surface discharges which had not progressed to the dark or worm hole appearing state. It was concluded that the static electrification discharges did not contribute to the failure. Prior to August 1992, the operating policy regarding cooler operation on the Braidwood transformers was to activate all pumps upon closing of the generator field breaker. This operating mode provided conditions conducive to static electrification. Since August 1992, the cooler controls were divided into a group 1 and group 2 operating sequence with group 1 being initiated upon closing of the generator field breaker, and group 2 by top oil temperatures of 60 C. It is assumed that the discharges observed had occurred prior to the group 1 and group 2 cooler control modification. It is also believed that a more serious problem could have resulted from these discharges if the transformer would have continued to be operated without the use of the cooler temperature controls.
- Research Organization:
- Electric Power Research Inst. (EPRI), Palo Alto, CA (United States)
- OSTI ID:
- 96469
- Report Number(s):
- EPRI-TR-105019; CONF-9409355-; TRN: 95:018955
- Resource Relation:
- Conference: Static electrification in power transformers workshop, Milwaukee, WI (United States), 28-30 Sep 1994; Other Information: PBD: May 1995; Related Information: Is Part Of Static electrification in power transformers: Proceedings; PB: 369 p.
- Country of Publication:
- United States
- Language:
- English
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