A new type medium-voltage STATCOM based on main-auxiliary IGCT inverter

This paper aims at reactive power compensation of three phases balancing loads for 6 KV medium-voltage source, and proposes a new type of main-auxiliary structure STATCOM that is directly connected to power source. This structure combines the characteristics of both diode-clamped multilevel inverter and cascaded multilevel inverter: the main inverter takes the form of three-level diode-clamped inverter, while the auxiliary inverter adopts H bridge (full bridge) inverter. The three auxiliary H-bridge inverters are connected to the corresponding phase of main inverter in series, so that both of them constitute a five-level hybrid inverter. In this paper, the auxiliary H-bridge inverter is designed to produce a square wave voltage whose frequency equals to the AC power supply, and the square wave acts as the basis of expected output phase voltage. The main inverter has two functions: one is to generate the compensatory part of the expected output phase voltage; the other is to compensate the harmonic components of output voltage. In the end, the superiority of the proposed STATCOM structure is validated by simulation.