Impact of DC side fault protection on performance and operation of multi-terminal DC (MTDC) systems

With the development of modular structured Voltage Source Converters (VSC), Multi-Terminal DC (MTDC) transmission systems have now became a feasible solution to transmit power at high voltage levels which greatly improves the electric power transmission system. The MTDC grid has lower capital costs and lower losses than an equivalent AC transmission system. Thus for long distance power transmission, MTDC grid becomes a very attractive solution. Since the MTDC network is now built based on VSCs, it automatically offers better quality of transmitted power along with more flexibility in power transmission over the conventional current source converters. However, VSC based MTDC transmission systems are vulnerable to DC side fault and often expensive DC circuit breakers are required to protect them against DC fault. In this paper, we demonstrate the effect of DC Circuit Breaker (DCCB) performance on the rating, control and operation of modular multi-level converters (MMC) inside a MTDC system. Furthermore, it is established that depending on the type of DCCBs the converter component ratings can be reduced which contributes to the overall reduction in system cost. Performance analysis has been done to investigate the fault current limiting capabilities of each of the types of DCCBs. PSCAD and real-time control hardware-in-the loop (C-HIL) simulations are used to prove the relevance of the analysis.