Self-Synchronising Stationary Frame Current Regulation for Grid-Connected LCL Converters Under Unbalanced Grid Voltage Conditions

This paper proposes a new self-synchronising current control strategy for an LCL grid-connected converter that is straightforward, and very robust to unbalanced grid voltages. The strategy estimates the grid voltage sequence components from the internal variables of a resonant current regulator, so that the phase-locked-loop can synchronise to this estimate. The same variables are also used to compensate for the LCL filter capacitor current. This allows a current regulator acting on the converter side to maintain accurate grid current control even under extreme unbalanced grid voltages. The effectiveness of the approach has been verified using detailed simulation and experimental investigations.