An Improved Voltage Support Scheme for the DG with Independent Active Power Control under Asymmetrical Grid Faults

Ever-increasing shares of renewable energy sources (RES) have made the grid operation challenging under faulty conditions. The grid codes in many countries for distributed generation (DG) powered by RES have incorporated voltage support capability during grid faults. The voltage support scheme (VSS) under asymmetrical faults tries to bring the point of common coupling (PCC) voltage within the specified limits. The voltage support to the medium voltage grid requires both reactive and active powers. The available power at the dc bus may be less or more for the voltage support; hence the independent control of the active power without impacting the voltage support is essential. Existing VSSs for asymmetrical faults, while supporting the voltage, either do not independently control the active power or control the power iteratively, leading to distortion in the current and active power. The proposed VSS tracks an active power reference independently without impacting the voltage support at the PCC. During asymmetrical faults, the proposed VSS restores the PCC voltage within limits while independently controlling the active power supplied to the grid. The analytical expressions are derived for the independent control under asymmetrical faults and are validated by simulation results.