Abstract: Small hydro-generator of the over-current protection with compound voltage start as backup protection, the protection operation time limit setting need to cooperate with the biggest action time of the next component backup protection, adding a time limit, so the setting time is longer.if need to start the backup protection action, prolonged accidents are detrimental to unit operation, therefore, renovating and upgrading the electrical equipment and generallayout, on the premise of ensuring the selectivity of relay protection action of the power grid, it should be flexible and reasonable to configure relay protection devices of each circuit, so that each electrical component can be within the scope of the primary protection to ensure the safety and stability of the overall operation of the hydro-station.