Reduced Switching Frequency Operation of Parallel Connected Two-level Inverters with Isolated DC-links for STATCOM Application

Parallel operation of two-level inverters with isolated dc-links are of profound importance for STATCOM operation in high voltage applications where the importance of cost, current sharing, modularity, effective multi-level voltage generation and consequently minimised ripple content of the STATCOM injected current are prevalent. Conventional approach using phase shifted carriers satisfies the criterion of effective multilevel voltage generation but suffers from the achievable current controller bandwidth with low switching frequency operation of the inverter. The proposed control method enables switching of individual inverters at a very low frequency while the total STATCOM current injected in to the grid is controlled at reasonably high bandwidth. Hence, the current ripple is minimised considerably. Also, due to isolated dc-links, the flow of zero sequence currents among the inverters is absent thus further reducing current ratings of individual inverters. The proposed control method is verified in simulation and experiment in a laboratory prototype and the results are presented.