Multi-terminal VSC-HVDC Pole-to-pole Fault Analysis and Fault Recovery Study

Voltage-Sourced Converter (VSC) HVDC is attracting significant attention as the UK tries to meet its renewable energy targets. However, compared with the traditional Current-Sourced Converter (CSC) HVDC, VSC-HVDC is more vulnerable to DC faults due to the lower DC side impedances and more sensitive power electronics used in its construction. This paper investigates the equivalent two-level converter system response under a pole-to-pole fault. A three-terminal VSC-HVDC system is developed in PSCAD/EMTDC to evaluate the system fault response and to verify theoretical analysis. Based on the results, it is found that the AC grid will initially contribute relatively little to the DC fault current seen by the DC side circuit breakers during a pole-to-pole fault. The converter fault recovery strategy has also been studied in this paper. It is found that after successful operation of the hybrid DC circuit breaker, with the aid of bump-less transfer control, the DC system has a good voltage recovery response. The contribution of this paper is that it gives an improved understanding of the system response and offers some guidance for the design of DC protection systems.