Research on a Non-PLL Control Strategy for a Flexible Fault Current Limiter and Its Application in Improving the FRT Capability of Microgrids

Aiming to achieve fault ride-through (FRT) in a microgrid under the condition of an asymmetric power grid, this paper proposes a scheme that applies the cascaded H-bridge flexible fault current limiter to realize both voltage regulation and current limiting at the point of common coupling (PCC). In order to overcome the instability of phase-locked loop (PLL) and its other problems, in the double-synchronous rotating coordinate system, the dq-axis components of each sequence electric quantity can be extracted without PLL by setting the rotational angular frequency of the coordinate axis to a given value. To eliminate the deviation between the set frequency and the actual frequency of the power grid, the calculation formula for the frequency correction amount is deduced, and, based on this, an iterative correction method for the set frequency of the rotating coordinate system is proposed. In this way, the set frequency can adaptively follow the actual frequency of the power grid, which ensures the effectiveness of the non-PLL scheme. Based on this, a sequence decomposition and compensation control strategy for the CHB-FFCL is presented. Finally, through the MATLAB/Simulink platform simulation, the effectiveness of the proposed strategy is verified and thereby shown to improve the FRT capability of microgrids under complex grid conditions.