Comparison of control schemes for multilevel inverter with faulty cells

Cascaded H-bridge multilevel inverter has been receiving wide attention mainly due to its capability of high voltage operation without switching devices connected in series. In real applications, one or more cell may fail and the output voltage is unbalanced, causing damaging results to load. In this paper, three control schemes including NS 1 and NS 2 based on neutral shift (NS) with delta injection and space vector (SV) PWM way, are compared in terms of the output line-to-line voltages THDs and their difference ratio. The reason of the difference is given for the first time, which has not been fully understood before. Based on this, a modified SV method is proposed. The theoretical results are also given, which shows that the NS has better performance than SV in theory. However, NS 1 and NS 2 are only approximately way to implement NS and they cannot achieve the best performance of NS, while modified SV has much better performance in terms of voltage difference ratio, which plays the most important role in a system with faulty cells. Simulation and experimental results are given to verify the algorithm.