Power Angle-Frequency Droop Control to Enhance Transient Stability of Grid-Forming Inverters Under Voltage Dips

Due to the replacement of synchronous generators (SGs), grid operators are currently demanding to control grid connected inverters in grid-forming mode to make them participate in the maintenance of the grid. To carry this out, the traditional droop controls based on the active and reactive powers are usually adopted, achieving a satisfactory performance in normal operation. Nevertheless, the power-frequency (P - ?) droop may become transiently unstable under voltage dips. This is because of the modification of the active power response caused by the inverter current limitation together with the voltage reduction. To enhance this, the power angle-frequency (d(inv)-?) droop is proposed, consisting in employing an estimation of the inverter power angle as input to obtain the inverter frequency. The proposed method provides the inverter with the same performance as the P - ? droop in normal operation while enhancing considerably the transient stability margins under current limitation. This is due to the higher variation of the inverter power angle with the phase difference between the inverter and the grid. Simulation results show the transient stability problems of the P - ? droop as well as the superior performance of the proposed d(inv) - ? droop control, which has also been verified by means of HIL results.