Estimation method of degraded vacuum in vacuum interrupter based on partial discharge measurement

Loss of vacuum in vacuum interrupter (VI) is becoming an interesting theme for VI used in medium voltage range beyond its lifetime of typically 20-30 years. The early detection of abnormality in the vacuum insulated equipment is important not only for the operating vacuum circuit breakers (VCBs) in medium voltage but also for the development of higher voltage VCBs, such as cubicle type gas insulated switchgear (C-GIS) with compressed gas and/or insulation gas. The purpose of present study is to estimate the vacuum degree in VI by measuring partial discharges (PDs) occurring in medium vacuum range for more than 1 Pa. As a leakage gas, atmospheric air and SF
₆
gas were sealed into the VI with close-state, whose pressure was regulated through vacuum pump systems, which corresponds to the left region of its PD inception voltage (PDIV) curve. From PD current waveforms measured by detecting resistor connected to plane electrode outside the VI, PD charges, occurrence frequency and phase characteristics of PDs for each applied AC voltage and internal vacuum pressure were determined. It was concluded that, by statistical processing using PD parameters and histogram, a number of occurrence of PD and PD charges would be dominant parameter for determination of the internal pressure of VI with the leakage gas of air.