Harmonic current control for LCL-filtered VSCs connected to ultra-weak grids

This paper addresses the harmonic current control for LCL-filtered Voltage-Source Converters (VSCs) connected to ultra-weak (high-impedance) grids. It is shown that the harmonic current controllers tend to be unstable as the Short-Circuit Ratio (SCR) of the system reduces. An active stabilizing control scheme is thus proposed by feeding back the filter capacitor voltage and the converter-side current. The method not only stabilizes the harmonic current control with a wide range of SCR values, but also mitigates harmonic distortions in the grid-side current of the VSC. The stabilizing mechanism of the approach is elaborated by the impedance-based analysis. The time domain simulations and experiments corroborate the theoretical analysis.