Individual Phase Current Control With the Capability to Avoid Overvoltage in Grid-Connected Photovoltaic Power Plants Under Unbalanced Voltage Sags

Independent current control of each phase of a three-phase voltage source inverter under unbalanced voltage sags is proposed to effectively meet grid code requirements for grid-connected photovoltaic power plants (GCPPPs). Under current grid codes, GCPPPs should support grid voltages by injecting reactive currents during voltage sags. Such injection must not allow the grid voltages of the nonfaulty phases to exceed 110% of their nominal value. However, grid overvoltages can occur in the nonfaulty phases, especially if the currents injected into the grid by the GCPPP are balanced. Based on a new requirement of the European network of transmission system operators published in 2012, a transmission system operator is allowed to introduce a requirement for unbalanced current injection. In this letter, this grid code is addressed by controlling individual phases and injecting unbalanced currents into the grid during voltage sags. Experimental results from a 2.8-kV.A inverter are presented, confirming the effectiveness of the proposed control method.