Interactions between a half‐bridge MMC and a hybrid DC breaker during fault current interruption compared to a full‐bridge MMC

Abstract

The large-scale integration of remote renewable energy resources into existing AC grids is one of the main drivers for the construction of multi-terminal direct current (MTDC) systems. Today, a few MTDC systems are already in execution stage or even in operation. However, with a growing number of extended MTDC systems, the probability of high-voltage DC (HVDC) insulation faults is likely to increase and therefore, reliable solutions for handling of such disturbances are crucial in order to maintain high availability. Since most HVDC converters are realised by modular multilevel converters (MMC) converters are based on half-bridge (HB) modules, they lack an inherent capability to control DC fault currents. Therefore, additional equipment like HVDC breakers needs to be installed as well. Among the various breaker types, the so-called ‘hybrid’ DC circuit breaker (DCCB) is considered most popular. This contribution provides a more holistic view on the overall system behaviour, represented AC grid, MMC HVDC converter, DC reactor and DCCB altogether. It discusses the physical interactions of these main components in MTDC systems. In particular, the influence of the electrical parameters of the hybrid DCCB and its required DC reactors on the MMC converter's internal current and voltage conditions is outlined.