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.
A new electromagnetically actuated vacuum circuit breaker (VCB) has been designed and developed on the basis of the transient electromagnetic analysis coupled with motion. The VCB has three advanced bistable electromagnetic actuators, which control each phase independently. The VCB serves as a synchronous circuit breaker as well as a standard circuit breaker. In this work, the flux delay due to the eddy current is analytically formulated using the delay time constant of the actuator coil current, thereby leading to accurate driving behavior. With this analytical method, the electromagnetic mechanism for a 24 kV rated VCB has been optimized; and as a result, the driving energy is reduced to one fifth of that of a conventional VCB employing spring mechanism, and the number of parts is significantly decreased. Therefore, the developed VCB becomes compact, highly reliable and highly durable.
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