Harmonic resonance analysis and stability improvement for grid-connected inverters

The traditional dual-control-loop strategy is widely used in grid-connected inverters. However, due to uncertain grid conditions, a resonance phenomenon may arise in systems and grid current can be badly distorted. In addition, the systems themselves may be unstable. In this paper, an equivalent impedance model of a grid-connected inverter is established. Based on this, two concepts of circuit resonance and harmonic resonance are proposed. By using the Nyquist stability criterion, it is revealed that harmonic resonance is induced when system stability is decreased, and the harmonic amplification in the system is enhanced. Due to the fact that the dual-control-loop strategy has poor robustness against grid impedance variations, a linear quadratic regulator (LQR) based on full-state feedback is proposed in this paper. The design procedure for selecting the full-state feedback gain is presented. Finally, both simulation and experimental results are presented to verify the effectiveness of the proposed LQR controller.