Cross-Coupled Multiple Rotational Loop-Based Flexible Harmonic Control for GFM Inverters under Complex and Distorted Grid

The non-ideal power grid possesses the characteristics of varying grid impedance and harmonic distortion, which have a significant impact on the performance of grid-forming (GFM) Inverters, such as operating stability and power quality. The traditional multiple rotational loop control for harmonic current suppression might deteriorate the harmonic stability and excessively degrade the output voltage quality under a weak grid. To address the problem, this paper proposes a cross-coupled multiple rotational loop-based flexible harmonic control scheme for GFM Inverters under distorted grid with varying grid impedance. The proposed method introduces harmonic current error cross-coupling control within the harmonic current loop control, with adaptive PI controllers dynamically tuned in real time. The cross-coupling control is beneficial for phase margin improvement in harmonic current loop analysis. The adaptive PI controllers can optimize the harmonic voltage to a lower level, as well as sacrifice the minimum voltage quality to suppress harmonic current. With the proposed method, the GFM inverter is equipped with robust and flexible harmonic control capability across a wide range of grid strengths. The experiments are conducted on a prototype and the results validate the effectiveness of the proposed method.