Internal overvoltage is called switching overvoltage or resonance overvoltage

and it threatens the safe operation of electrical equipment and the personal security of operating personnel.Through analyzing the mechanism of internal overvoltage coming into being and the mechanism of preventive measures in several typical switching operations（stopping main transformer in neutral point directly earthed system

substituting the circuit of substation with special bypass bus;putting electromagnetic potential transformer into operation in the system with non-directly earthed neutral;changing busbar to hot standby in neutral point directly earthed system）

it is illustrated that overvoltage will come into being during the process of switching operation

so different preventive measures should be adopted aiming at difference power system and electrical equipment to prevent overvoltage from coming into being completely and to ensure the security of electrical equipment and operating personnel effectively.