HVDC circuit breakers for HVDC grid applications

High Voltage Direct Current (HVDC) transmission has been expanding due to rapid development of power electronics technology and by the need for connection of offshore/remote wind farms and large hydro power generators. An HVDC grid will be required to operate the healthy lines continuously, even if a voltage collapse occurs at the remote end. Rapid fault clearing is essential for DC Circuit Breaker (DCCB) even though the requirement varies depending on DC transmission system configurations, Voltage Source Converter (VSC) design, transmission capacity, and DC reactor connected in series with the line/cable, etc. In this paper, the requirements for DCCB were analytically evaluated using a four-terminal radial HVDC network model. The results show that DC fault interruption current and fault clearing time are achievable by using a mechanical DCCB with the forced current zero formation scheme. Furthermore, interruption performance of the mechanical DCCB composed of HV vacuum interrupter was evaluated. This DC circuit breaker successfully interrupted a current equivalent of up to 16 kA DC in the laboratory. The prototype adopts forced current zero formation scheme and comprises of a high-voltage AC vacuum circuit breaker at transmission voltages connected to an external capacitor equipped with a triggering gap. A series of interruption tests performed on this breaker verified the clearance of short circuit currents as high as 16 kA DC within a few ms after an opening command.