Grid Current Feed-forward based Transfer Scheme for Grid-tied Inverters with Anti-islanding Schemes

Grid-connected distributed resources operate in current or power control mode when in grid-tied conditions. However, in islanded conditions, the inverters operate in grid forming mode to feed local loads. This paper proposes a method for the inverter to transfer from grid-tied to grid-forming mode after an islanding event considering the operating point tolerances when detecting an unintentional island. This transfer is undertaken using a control structure comprising of an instant switch from current control to voltage control mode utilizing an islanding detection signal and a grid current feed-forward term. Islanding detection plays a significant role in the transfer of control scheme by providing the initial stimulus for transfer. Hence, all transfer schemes act after an island is detected and the system has deviated from its steady-state conditions. The magnitude of deviation depends on the generation-load imbalance and the time taken for detection. Active anti-islanding schemes tend to force a larger deviation. The proposed method thus returns the deviated system to its nominal operating range within one power cycle by providing a grid current feed-forward signal to offset the effect of generation-load imbalance. This is achieved with minimal operating point disturbance as seen by the local load.