Enhanced multiloop control scheme for an LCL-filtered grid-connected inverter under abnormal grid voltage conditions

To facilitate the transition between the standalone mode and grid-connected mode of a grid-connected inverter with LCL filters, multiloop and indirect current control schemes have been widely used. By considering the capacitor voltages as control variables, these control schemes can provide seamless transfer of operation modes to the inverter even at the event of unintentional islanding. However, since these schemes are mainly based on the proportional integral controller which is generally unable to cope with sinusoidal disturbances, the injected currents are adversely influenced by distorted grid voltages. To address such a problem, this paper proposes an enhanced multiloop control scheme for an LCL-filtered grid-connected inverter under abnormal grid voltage conditions. Since resonant controllers are incorporated into grid-side current control loop, the proposed scheme can guarantee the high quality of injected currents to grid regardless of adverse grid voltage conditions. Comparative simulation results are given to confirm the effectiveness of the proposed enhanced multiloop control scheme.