Research on Startup Control Strategy of Inverter Hybrid Cascade LCC-MMC UHVDC Transmission System

A novel inverter hybrid cascaded LCC-MMC UHVDC system is studied in this paper. The rectifier of the system is consisted of LCC; the inverter is consisted of LCC and parallel MMCs. Based on the analysis of the coordinated control strategy of LCCs and MMCs, a detailed startup control strategy for the inverter hybrid cascaded LCC-MMC UHVDC system is proposed. The startup control strategy is divided into 4 periods: MMC uncontrolled charging period, MMC unlocking period, DC loop closing period and power boosting period. In order to verify the performance of the proposed startup strategy, a model of the inverter hybrid cascaded LCC-MMC UHVDC system based on detailed engineering parameters was established in the PSCAD/EMTDC environment. The simulation results show that the system has a good dynamic performance except that MMCs may exchange power during power boosting period. However, after adopting the optimized startup strategy, the power exchange between MMCs has been significantly suppressed.