A new control method for three phase inverters under unsymmetrical voltage sag conditions

Under grid fault conditions, especially the unbalanced grid faults, the PCC voltage of DG will suffer notably unbalanced voltage droops, which may cause the unnecessary disconnection of DGs according to the grid codes. Moreover, the overcurrent risk during voltage sag will also result in the disconnection of DGs, and even damage the inverter. In this paper, a new fault control strategy including three control objectives, was proposed to enhance the low-voltage ride-through (LVRT) capability for three-phase inverters. Firstly, the positive sequence (PS) voltage method is proposed to maximize the voltage support capability in any types of unbalanced voltage sags. As to ensure the safe operation of the inverter, a current limitation algorithm is designed based on different operation scenarios. Also, the active power delivery is considered as an ancillary service to fully use the capacity of the inverter. Then, a new control method towards the scenario classification and reference current selection is proposed to simultaneously achieve these control objectives. Finally, the simulation results based on MATLAB/Simulink are presented to verify the effectiveness of the proposed fault control strategy.