Research on compensation and protection of voltage in metal shield of 110kV power cable under three segments unsymmetrical state

110 kV power cable usually takes the form of single core, it is a general phenomenon that there is induced voltage in metal shield. For long cable, three segments transposition is often used to decrease the induced voltage in metal shield of high voltage power cable, but since city construction and other reasons, power cable is rebuilt more and more frequently, the length of three segments will be unequal while the power cable rebuilt, it will make the induced voltage in metal shield unbalanced, and produce the circulating current in metal shield. Through theoretical analysis of the induced voltage, the paper presents the mathematic model of induced voltage in metal shield under arbitrary arrangement ways of power cable; according to the model, simulating calculation is carried out, at the end of cable, the compensating device is installed to balance( counteract) the induced voltage and restrain circulating current. The paper presents that making use of standard ball gap to protect cable metal shield and its compensating unit, so as to prevent metal shield and compensating unit from severe over-voltage caused by short or thunder. The compensating device developed in the paper has been taken a great deal of spot tests in power company, the result has indicated that the method can reduce the induced voltage in metal shield effectively, thus the circulating current and cable losses can be reduced. At present, the device has been installed on spot for pilot running.