In order to reveal the mechanism of how the re-ignition of a vacuum circuit breaker in an offshore wind farm causes the damage of the insulation of the generators with step-up transformers,the high-frequency modeling and the analysis of the vacuum circuit breaker are performed in this paper.First,a simplified simulation model of the medium-voltage cable collector system PSCAD/EMTDC for the offshore wind farms is constructed by using a self-defined vacuum circuit breaker model.Then,the effects of the re-ignition characteristics of the circuit breaker,the initial phase angle of switching and the cable length on the transient overvoltage on the high voltage (HV) side of the generators with the step-up transformers and the re-ignition number are investigated.Simulation results show that the dielectric recovery strength of the three characteristic parameters of the circuit breaker has the greatest in- fluence on the transient overvoltage and the re-ignition number,and that,when the initial phase angle of switching is 45° and the re-ignition phenomenon caused by the interruption of an inductive load circuit breaker occurs,the overvoltage reaches the maximum and its amplitude and steepness are up to 3. 26p. u.and 12. 46p. u.respective- ly.Finally,the scheme of installing the series of choking coils,the parallel small capacitor and the overvoltage pro- tection device of zinc oxide arrester on the HV side of the generators with the step-up transformers at the same time is proposed.Simulation results show that，after the overvoltage protection device is installed,the overvoltage ampli- tude and steepness as well as the re-ignition number are effectively suppressed.