Direct power control of voltage source inverters with unbalanced grid voltages

A new approach of direct power control (DPC) of a voltage source inverter for achieving symmetrical grid currents under unbalanced grid voltage conditions is presented. In these situations, negative sequence components appear on the grid voltage vector, leading to distorted currents if the power is maintained constant. Nevertheless, sinusoidal balanced grid currents can be achieved through control. Several current controllers have already been modified to overcome voltage imbalance with good results. Here, sinusoidal grid currents are obtained by directly controlling power, which becomes a novel approach and keeps DPC philosophy. A phase locked loop is necessary to obtain positive sequence voltage angle. A very simple positive and negative sequence calculator is also used so that power references can be modified in order to obtain balanced grid currents. Simulated and experimental results are presented to confirm the validity of the method.