Consideration on Switching Frequency and Multiple Connection at a Bulk Power Self-Commutated Converter System

Abstract

This paper describes which is better, a single bridge or multi bridges, what switching frequency is best, at a bulk power converter system for static var compensation from the viewpoint of power loss, utilization rate of power devices, distortion factor of current and voltage waveforms, control response and reliability. Our study shows that multiple connection of single-phase bridges with one pulse PWM and that of three-phase bridges with three pulse PWM are superior to a three-phase single bridge with multiple pulse PWM. These results are quite different from general thoughts in UPS or general purpose motor drive systems, where higher switching frequency has resulted in less harmonics, better response and smaller size.
At a bulk power converter system many power devices are required, thus multiple connection can be easily applied without the sacrifice of economy. In addition, it is very important, our newly adopted control strategy provides very quick transient response at the lowest (fundamental) switching frequency.