Analysis of the Hybrid-MMC-HVDC Short-Circuit Current under MMC AC System Symmetrical Fault

The characteristics of short-circuit current provided by modular multilevel converter (MMC) is analyzed after three-phase short-circuit fault occurs at PCC (Point of Common Coupling) of AC system, based on the operation control principle and short-circuit grounding modes of the converter in this paper. First of all, this paper introduced the structure and control principle of MMC. Furthermore, the calculation method of dq component of short-circuit current is proposed, and then the short-circuit current characteristics provided by MMC were studied. When there is a short-circuit near the flexible DC converter bus and the converter bus maintains a certain residual voltage, the contribution of MMC circulating current to short-circuit current mainly depends on the reactive short-circuit current component of MMC during short-circuit fault. Finally, the influence of MMC control modes, short-circuit grounding modes and fault time on the transient peak value of reactive short-circuit current is analyzed and the model is built on PSCAD for simulation verification. This research provides a reference for the control parameter setting of MMC, short-circuit current evaluation and selection of AC circuit breaker in future flexible DC project.