Problems of Long-Term Reliability for UHV Transformer Insulation

Through long-term (3 years) partial discharge (PD) tests on some component models for transformer insulation, the authors have revealed that transformer insulation has a threshold voltage, Vol below which no partial discharge will occur regardless of how long the voltage may last. The value of VO is more than 62.5 percent of a 50-percent probability PD inception voltage at one-minute ac test.