Criteria for the Application of Vacuum Interrupters in HVDC Circuit Breakers

It has become necessary to develop HVDC circuit breakers to protect against fault currents that may occur in HVDC grids. One possible HVDC circuit breaker design uses vacuum interrupters as the main DC current path and the DC current interrupter using a counter-pulse system to create an artificial current zero. As the counter-pulse has a relatively high -di/dt, the vacuum arc mode at the current zero is critical when withstanding the resulting Transient Interruption Voltages (TIVs). The fault current in the DC system can also increase rapidly to a very high value. In order to prevent serious damage to the DC grid, it is imperative that the vacuum interrupter open soon after the fault is detected and the counter pulse applied in order to limit its magnitude. This paper discusses the vacuum arc modes as the contacts open and the requirements to create a fully developed diffuse vacuum arc at the counter-pulse current zero. The vacuum interrupter also has to develop a large enough contact gap in order to withstand the expected recovery voltages (TIVs). To do this a high-speed opening mechanism is required. Thus, the vacuum interrupter will accelerate faster than it would have experienced in a HVAC vacuum circuit breaker. The paper presents the critical design parameters for the vacuum interrupter to operate in a HVDC vacuum circuit breaker and contrast them with vacuum interrupters used in HVAC circuit breakers.