ABSTRACT Combining advantages of conventional high‐voltage direct‐current (HVDC) and flexible HVDC, the hybrid cascaded HVDC system adopts a structure where a line commutated converter (LCC) and a modular multilevel converter (MMC) are cascaded on the inverter side. The valve‐side single‐phase‐to‐ground (SPG) fault is one of the most critical issues affecting the safe operation of hybrid cascade HVDC systems. However, impacts of valve‐side SPG faults in hybrid cascade HVDC systems have not been thoroughly investigated. This paper first analyses the characteristics of a valve‐side SPG fault of the MMC converter transformer and the generation mechanism of the fault‐induced inrush current (FIIC). The impact of FIIC on the commutation within the LCC inverter is investigated. It is indicated that the harmonics in the FIIC cause voltage distortion on the LCC inverter AC bus, thereby leading to the subsequent commutation failure. Considering the insufficiency of the existing protection schemes in mitigating FIIC, the new suppression strategy based on the bypass circuit breaker of the starting resistor with auxiliary thyristor‐pairs is proposed to achieve the purpose of suppressing subsequent commutation failure. Finally, results obtained from the PSCAD/EMTDC platform demonstrate the validity of the theoretical analysis and the practicality of the proposed solution.
Zheng et al. (Thu,) studied this question.