Investigation on overvoltage caused by vacuum circuit breaker switching off shunt reactor in offshore wind farms

The multiple reignitions of vacuum circuit braker(VCB)while switching off the shunt reactor in an offshore wind farm(OWF)would produce a high overvoltage.The post-arc process of ion movement in the vacuum interrupter is analysed.Considering the sheath growth and ion concentration variation,the expression for dielectric recovery strength inside the VCB interrupter is derived.A novel VCB model is implemented in the environment of electromagnetic transient programme based on the new reignition criterion.Then,the switching overvoltage is computed and analysed in detail.For the OWF with high rising rate of transient recovery voltage,the sheath growth must be considered in the VCB model;otherwise,the overvoltage may be underestimated.Using resistor–capacitor(RC)snubber,a new installation scheme is proposed to suppress the overvoltage in the OWF.The results indicate that the proposed scheme can not only reduce the peak and steepness of overvoltage but also damp the high-frequency oscillation.It would not affect the steady-state operation of OWF.Besides,the capacitor of RC snubber normally does not bear the electric stress,and its life expectancy would be greatly enhanced.