Switching transient effects on high voltage current transformers. Final report
- BDM Federal, Inc., Albuquerque, NM (United States)
Very high frequency, transient stresses in free-standing high voltage current transformers (HVCTs), caused by disconnect switching in transmission substations, are investigated to determine if they cause or promote early deterioration or failure of the insulation. Attention has focused on free-standing HVCTs because a small number have failed explosively while in service. Industry groups investigating HVCT failures report a number of possible causes, including designs, materials, workmanship, and field service conditions. Free-standing HVCT designs insulate curved high voltage conductors and are more complex than those CT designs which placed the secondary cores on straight bushings on deadtank oil circuit breakers. In the free-standing hairpin or eyebolt HVCT designs, oil/paper tape, wrapped around the conductors to form the insulation, introduces small oil spaces at paper edges where the layers overlap, and results in a weaker design than found in bushings. At least one HVCT failure was found to occur at the site of a much larger oil space, formed by several stacked sheets of crepe paper. Disconnect switching and an increase in partial discharge activity occurred shortly before the failure. Some failed HVCT designs also employed high resistance conducting layers for grading the voltage across the dielectric. Disconnect switching transients were investigated because they were suspected of being more severe than normal power frequency stresses, and HVCTs are exposed to their full strength without protection. Furthermore, transient stresses inside of oil voids in the otherwise solid oil/paper systems result in even higher stresses. This project has confirmed that very fast switching transients with 100% voltage polarity reversals may overstress some CTs. However, the tests did not confirm the hypothesis for the CT failure mode. Resistor-type disconnect switches were found effective in reducing bus transients, and dielectric stresses caused by them, by up to 80%.
- Research Organization:
- Electric Power Research Inst. (EPRI), Palo Alto, CA (United States); BDM Federal, Inc., Albuquerque, NM (United States)
- Sponsoring Organization:
- Electric Power Research Inst., Palo Alto, CA (United States)
- OSTI ID:
- 61138
- Report Number(s):
- EPRI-TR-104961; TRN: AHC29516%%74
- Resource Relation:
- Other Information: PBD: Mar 1995
- Country of Publication:
- United States
- Language:
- English
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