Overcurrent Suppression Method for Hybrid Cascaded UHVDC System Based on Fuzzy Clustering and Identification

The hybrid cascaded ultra-high voltage DC (HC-UHVDC) system based on the line commutated converter (LCC) and the modular multilevel converter (MMC) has received extensive attention from engineering and academic circles. This paper establishes a model for the HC-UHVDC system in which the rectifier adopts a dual 12-pluse LCC and the inverter is composed of one LCC and three paralleled MMCs in series. And the generation mechanism of the DC overcurrent due to the commutation failure of LCC caused by the AC fault at the inverter station is analyzed. Then this paper proposes an overcurrent suppression method for HC-UHVDC system based on fuzzy clustering and identification. In this suppression method, the fuzzy clustering of multiple electric parameters at the rectifier station is carried out through the simulation in the case of AC fault on the inverter side. According to the characteristics of different transient stages at the inverter station identified by the clustering result, the staged firing angle instruction value is designd in advance. When an AC fault occurs in the system, the DC voltage of the rectifier station is adjusted in time based on the local information of the rectifier station, to quickly suppress the DC overcurrent. The results of the detailed electromagnetic transient simulation in PSCAD/EMTDC show that, under the three-phase and single-phase metallic short-circuit fault conditions on the inverter side, the proposed approach can suppress the DC overcurrent and overvoltage after the commutation failure of LCC at the inverter station to some extent, and can significantly improve the dynamic characteristics of the HC-UHVDC system. © 2021 Automation of Electric Power Systems Press.