Modulation and Voltage Balancing of a Five-Level Series-Connected Multilevel Inverter With Reduced Isolated Direct Current Sources

The series connection of low-power modules is an alternative solution to realize a multilevel inverter. Unlike a cascade connection, the series connection approach significantly reduces the number of isolated dc sources, thereby the design complexity, cabling of the transformer, and overall system cost become low. In this article, the operation of the series-connected multilevel inverter is presented for a five-level operation, and it is realized by using two three-level half-bridge diode clamp converter modules per phase. Hence, each phase of the series-connected multilevel inverter requires a single isolated dc source. In the series-connected multilevel inverter, the net dc-bus voltage will be equally distributed between four dc-bus capacitors. Therefore, the converter generates a multilevel voltage waveform with uniform steps and ensures equal voltage stress on the semiconductor devices. To achieve these objectives, a space vector pulsewidth modulation scheme with an additional voltage balancing approach is employed. The feasibility of series-connected multilevel inverter is validated through simulations and a scaled-down laboratory prototype under steady-state and transient conditions.