Mitsuru Tsukima

Synchronous controlled switching by vacuum circuit breaker (VCB) with electromagnetic operation mechanism

Synchronously controlled switching to suppress transient overvoltage and overcurrent resulting from when the circuit breakers on medium voltage systems are closed is described. Firstly, by simulation it is found that if the closing time is synchronously controlled so that the contacts of the circuit breaker close completely at the instant when the voltage across contacts of the breaker at each of the three individual phases are zero, the resulting overvoltage and overcurrent is significantly suppressed when compared to conventional three phase simultaneous closing. Next, an algorithm for determining the closing timing based on a forecasted voltage zero waveform, obtained from voltage sampling data, is presented. Finally, a synchronous closing operation experiment utilizing a controller to implement the algorithm and a VCB with an electromagnetic operation mechanism is presented. The VCB was successfully closed at the zero point within a timing error of 200 microseconds.

Vacuum Switch Chopping Current in an Inverter Circuit

When small currents are interrupted by vacuum switch, it is well known that chopping current and the associated overvoltage are observed. Various investigations on the current-chopping phenomena in AC circuits have been reported, but there are no data available on inverter circuits in spite of their recent wide application to synchronous motor systems. In this paper, chopping-current levels and the overvoltages in an inverter circuit under a simple three level pulse width modulated (PWM) control are investigated. As a result, the mean chopping-current level is 3–4 times higher than that in an AC circuit. In particular, high chopping currents occur only when the timing of the current interruption by the vacuum switch synchronizes with that of the inverters switching, which we call ‘double chopping’. High chopping current due to double chopping might be harmful since the high overvoltage proportional to the chopping-current level could damage the insulation of electric equipment. However, we experimentally confirmed that a typical surge suppressor could reduce overvoltage.