Maximizing the DG output power using multilevel transformerless inverters with un-equal DC rail voltages

Inverter based small scale power sources are widely used, which usually use a coupling transformer to connect to the network. In the transformerless inverters, this transformer replaces with a proper impedance. A new algorithm for controlling the multilevel cascaded transformerless inverters with non-equal DC rail voltages is proposed in this paper, called the Voltage Look-up Table method, to maximize the active power injection from the DG to the network, while keeping the THD factor in an acceptable range. Simplicity of implementation and a very low switching frequency would be the major advantages of this method. Both simulative results by MatLab/Simulink and experimental results on a 1 kW prototype of the circuit show that this method can build up a very accurate current reference with a very acceptable THD.