Compensation of unequal voltages in the DC links of a multilevel series-connected H-bridge inverter

In series-connected H-bridge multilevel inverters, the voltage level of each phase is supplied independently by a complicated transformer. The energy flowing back via the free-wheeling diodes may, hence, not be transferred to another phase. Therefore, the DC link voltages in multilevel inverters fluctuate under load, and cannot be considered constant. Voltage fluctuation causes several problems including erroneous voltage vector production and undesired harmonics to voltage and current. A simple compensation method for such a DC link voltage fluctuation in series-connected multilevel inverters is introduced. The method is shown to be applicable to both space vector modulation and duty-cycle modulation schemes. The proposed method was simulated using the parameters of a 3 kV three-phase 7-level inverter. The simulations and test results showed the algorithm to be effective. Laboratory measurements with a 100 kVA inverter also verify the operation.