Compensation of DC link voltage variation of a multilevel series-connected H-bridge inverter

The phase voltages of a series-connected H-bridge multilevel inverter are formed from galvanically isolated secondaries of complicated transformer. The reactive energy of a reactive load flowing back to the DC links is not transferred to other phases the same way as is in two-level inverters. 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 proposed. The method is shown to be applicable to both space vector modulation and duty-cycle modulation schemes. The algorithm also includes circulation of the modules to ensure equal loading of different DC links. The proposed method was simulated using the parameters of a 3 kV three-phase seven-level inverter. Laboratory measurements with a 100 kVA inverter also verify the operation. The simulations and test results show the algorithm to fix unideal voltages to correspond the demanded reference. Also DC link voltages remain very well in balance, because of the use of module circulation.