The basic features of the ultra-high voltage（UHV）transmission system and the composition of the UHV 110 kV low voltage side are briefly described.The reactive-load compensation equipment of 110 kV UHV station utilizes the load switch，which will switch normal load current but will not interrupt short circuit current.Therefore，the concept of high current blocking function is proposed，and its main criterion is discussed.Starting from protection devices，such as bus protection，capacitor protection and reactor protection，the driving method of tripping outlet contacts for protection action is put forward，when the criterion for the high current blocking function is satisfied.In addition，the trip modes are analyzed when the bus and reactive-load compensation equipment are broken down，respectively.Measures of reserve protection segregating the breakdown progressively are discussed when the blocking action cannot shut down close by due to the high current.