Control strategy for L-type grid-connected inverters under ultra-weak grid conditions

Low power grid-connected inverters using L-type filters have the advantages of simple structures. However, due to the weak suppression of higher harmonics and the fact that the voltage of point of common coupling (PCC) is no longer clamped by the grid voltage under the ultra-weak grid, if the PCC voltage is directly fed forward, the quality of the grid current is affected. To improve the grid current quality under an ultra-weak grid, a virtual capacitor is introduced. Then repetitive control prediction is adopted to compensate for the lag phase of the feedforward PCC voltage caused by the virtual capacitor. Under an ultra-weak grid, the phase angle margin of the inverter decreases drastically, and an easy-to-implement strategy is proposed in this paper. In addition, in the situation of a large grid impedance and severe PCC voltage distortion, a method to eliminate the influence of the phase-locked loop (PLL) on the system stability is discussed. Finally, an experimental platform of the L-type inverter with an adjustable short circuit ratio (SCR) is built to verify the correctness of the analysis and effectiveness of the proposed strategy.