Power system restoration in a rapid and reliable way after blackout is important measure to reduce loss, extended black-start strategy is a novel way for power system restoration, but the network to be restore are more complex than that of black-start strategy in which one generator is started,the over-voltage problem of restoration path when the units to be start are charging is one of the important factors that can affect units start successfully or not. In order to obtain the reasonable model of charging,in this paper,the mechanism and characteris of power frequency overvoltage and switching overvoltage were analysed,then extended black-start scheme of southern power system of Hebei province was taked for example,different simulation models corresponding to the three charging modes were builded respectively to study,the results show that voltage rising from zero approach has obvious advantages,the conclusion can provide useful reference for subsequent industrial trails and studies.