Development of a large capacity static var compensator capable of high efficiency and quick response

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In recent years, attention has been paid to advanced static var compensators (ASVC) of 50-300MVA consisting of voltage-source inverters using GTO thyristors not only for improving power factor but also for stabilizing transmission systems. The ASVCs can adjust the amplitude of output voltage of the inverters by means of PWM or non-PWM operation, thus producing either leading or lagging reactive power. In practical applications of large capacity ASVCs to power systems, high efficiency is the first priority as well as high reliability be.In this research, a dc voltage-controlled ASVC of 10kVA,consisting of quad-series voltage-source non-PWM inverters, was developed and tested by the authors. The following items are clarified by analysis and experiment.1.It was verified by experiment that quick control of reactive power with a time constant of 5ms, which had been considered to be impossible, was achieved by adding a feedback control loop. The experimental result agreed well with the analytical one.2.It was verified by experiment that a lossless snubbing circuit proposed by the authors works well, resulting in a reduction of switching losses and electromagnetic noises.3.It was pointed out by analysis and experiment that series or parallel resonance may occur between the dc capacitor and line inductance so that the capacity of the dc capacitor has to be designed to avoid the resonance.