Improvement of Grid Current Quality for Droop-Controlled Grid-Connected Inverters under Distorted Grid Conditions

Grid-connected inverters based on droop control have been widely used in distributed generation (DG) systems, as they can ensure smooth transition between grid-connected mode and islanded mode. However, the traditional active power-frequency (P-omega) and reactive power-amplitude (Q-E) droop control method is vulnerable to the grid voltage distortion, because of the following two reasons. Firstly, the inverter under droop control behaves like a controlled voltage source connected in parallel with utility grid, and the grid current is not directly controlled. Secondly, the line impedance interlinking the inverter and the grid is usually very small, so a slight distortion in grid voltage can lead to serious distortion in grid current. To suppress grid current distortion, this paper proposes to employ harmonic virtual impedance (HVI), which is effective only at harmonic frequency, to increase the equivalent impedance between inverter and grid. Compared with the existing control methods, the proposed HVI control scheme does not need to detect grid voltage and it is easy to be implemented. Comprehensive simulation and experimental results are provided to validate the effectiveness of the proposed method.