DC circuit breaker requirements in multi-terminal HVDC systems

The DC circuit breaker (DCCB) is an important component in future super DC grids to isolate a DC fault. Its requirements, during pole-to-ground and pole-to-pole fault conditions, are addressed in this paper, based on a three-terminal HVDC link. In the event of a pole-to-ground fault, DC cables and transformers are exposed to significant DC stresses. However, the converter arm currents and DC current can be suppressed to zero by blocking the converters following fault detection, thus a pole-to-ground DC fault poses less a challenge than a pole-to-pole DC fault. To isolate a pole-to-pole DC fault and protect the stations, the DCCB response time must be in the range of 1ms while the current breaking capability needs to be many kA. Due to the characteristics of the metal-oxide surge arrestor, the DCCB voltage rating will be 50% higher than the system voltage. Currently, there is no commercial solution available for DC circuit breakers to meet these requirements. This problem might be circumvented by adding DC-link inductance in series with the DCCB, where the fault current peak and energy absorbed by the metal-oxide surge arrestor are reduced by around 50%.