SUMMARYA new 72-kV rated low-pressure dry-air insulated switchgear applying electromagnetic actuation and a function that supports CBM has been developed. First, the dielectric characteristics in dry air under lightning impulse application were investigated for bare and insulator-covered electrodes. The dependence of the breakdown electric field strength on the effective area was found in order to apply it to the configuration design of the insulation mold for the vacuum interrupter. In addition, the dependence of the moisture volume on the surface resistance was found in order to decide the moisture volume in the gas pressure tank. Next, a new vacuum circuit breaker (VCB) was designed. To keep the dimensions the same as in the previous 72-kV SF 6 gas insulated switchgear, the distance between contacts in the vacuum interrupter must be shorter than in the previous switchgear. The voltage withstand capability between electrodes practically designed for the vacuum interrupter was investigated under DC voltage application, in a small capacitive current breaking simulation. The gap configuration, including contacts and slits, was optimized and the distance was shortened by 11% from the previous switchgear. As a result, the new low-pressure dry-air insulated switchgear was designed to be comparable in external size to the previous SF 6 gas insulated switchgear. Using dry air as an insulation medium with low pressure makes it possible to reduce the environmental burden.

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A new electromagnetically actuated vacuum circuit breaker (VCB) has been developed for a 72 kV rated switchgear. Each phase of this VCB has a plurality of compact electromagnetic actuators linked mechanically providing the required driving energy. The mechanical linkage working as a lever magnifies an actuator stroke to the required stroke of a 72 kV rated vacuum interrupter. An electromagnetic analysis coupled with motion, which considers the mechanical linkage of the plural actuators, has been developed for designing the driving behavior of this VCB. Using this analytical method and a quality engineering method known as the Taguchi method, we have clarified effective parameters to reduce the time difference of the driving behavior for tolerance specifications. Moreover, analyzing the oscillatory behavior closing the contacts, a structure of this VCB has been designed to reduce the bounce duration. The developed new VCB has been confirmed that a time difference is short enough and bounce duration is reduced. This VCB is highly reliable against variations in manufacturing and environment.

Partial discharge (PD) monitoring system for cubicle type gas-insulated switchgear (C-GIS) filled with compressed dry air as an insulation gas was developed. The shield electrodes embedded in the solid insulator of bushings are used as PD sensors. For verification of the sensitivity of the sensors and propagation characteristics of PD pulse, artificial defects such as fixed particle on high voltage conductor and defect on creepage surface of insulator in C-GIS were used. It is clarified that the sensors had good sensitivity to PD pulse propagating along high voltage conductor in the range from VHF to UHF and that the amplitude of the sensor output had good correlation with apparent charge, pC, detected by conventional PD detector. The PD monitoring system is connected with our condition based maintenance (CBM) controller, which also monitors the abnormality of circuit breaker, disconnecting switch, gas pressure and so on, as a total diagnosis tool for our C-GIS.