Research on Out-of-Step Characteristics and Protection of Grid-Connected Converter

The grid-connected converter synchronizes with the power grid through the controller, so its out-of-step mechanism and out-of-step characteristics are different from synchronous generators. The out-of-step characteristics and out-of-step protection of grid-connected converter are urgent research topics at present. In this paper, the out-of-step mechanism of grid-connected converter under the low voltage ride through (LVRT) control strategy is analyzed by combining the phase plane method and the equal area criterion. The characteristics of current, voltage and measured impedance angle at the point of interconnection (PoI) are analyzed after the grid-connected converter is out-of-step, and compared with those of the traditional power system. On the basis of these, a novel out-of-step protection method for the grid-connected converter is proposed based on the measured impedance angle at the PoI. It judges the out-of-step by the characteristic that the measured impedance angle oscillates continuously and the difference of its extreme values increases gradually. The faster the out-of-step oscillation develops, the faster the protection acts, so it is adaptive. Finally, a MATLAB/Simulink simulation model is built to verify the effectiveness of the proposed protection method.