User-defined constant switching frequency current control strategy for a four-leg inverter

A novel current control strategy called user-defined constant switching frequency method for a four-leg voltage source inverter is presented. This scheme allows the user to directly set the switching frequency of all the legs. Unlike the hysteresis current control where the switching frequency is variable and the fourth leg operates at a very high frequency, the proposed method ensures a constant and reduced switching frequency for all the switches of the inverter. This is achieved by exercising an indirect control on the occurrence and duration of the uncontrollable (zero vector) states. The three legs of the inverter are operated in hysteresis current tracking mode and the switches of the fourth leg are controlled with a square pulse of fixed user-defined frequency. The above advantages supplemented with the simplicity, fast response and robustness make this an attractive scheme. The feasibility of the proposed strategy is verified through extensive simulation and experimental studies on a distribution static compensator to compensate for unbalanced and non-linear loads in a distribution power network.