Stability Analysis and Robust Control Method for LCL-Type Three-Phase Four-Wire Split Capacitor Inverter Considering Zero-Sequence Loop

In distributed generation systems, the inverter is the main power interface and its stability directly determines the reliable operation of the grid-connected system. As a typical topology for a three-phase four-wire inverter, the LCL-type three-phase four-wire split capacitor inverter (LCL-TFSCI) is taken as the research subject of this paper. Compared with the three-phase three-wire inverter, there is an additional zero-sequence path in the LCL-TFSCI. Therefore, it is not only necessary to consider the stability of the positive and negative sequence system, but there is also the need to consider the stability of the zero-sequence system when performing stability analysis for the LCL-TFSCI. In this paper, a small-signal impedance model considering the zero-sequence loop of LCL-TFSCI is firstly established. Subsequently, the instability risk is revealed when LCL-TFSCI is connected to the grid with parallel compensation capacitors. Through instability analysis, an impedance-reshaping method based on the complex filter and combined differential elements is proposed, which can reshape the impedance characteristic of LCL-TFSCI within the wide frequency range and expand the stability domain of the grid-connected system. Finally, the proposed method is verified by simulation and experiment.