An improved phase locking scheme for grid-connected-inverter under non-ideal grid conditions

A precise frequency and phase angle observation of the utility grid voltage is essential to the grid-connected-inverter control. In order to improve performance of the conventional phase locked loop algorithm under non-ideal grid conditions, a positive sequence filtering network based phase locking scheme is proposed. Owing to the positive sequence filtering network, only positive sequence component can be passed on to the phase detection stage. Furthermore, a feed-forward loop of angular frequency observation is introduced to make the phase locking scheme utility grid frequency adaptive. Simulation results show that the proposed scheme can operate in an even heavily polluted grid environment and continue to generate a precise utility grid frequency and phase angle information.