The unbalance of the three-phase voltage in the power system is mainly caused by the unbalanced load, the asymmetry of the three-phase impedance of the system and the improper tuning of the arc suppression coil. The background voltage imbalance caused by the system impedance asymmetry rarely exceeds 0.5%. The imbalance or unbalanced voltage of the general overhead power grid does not exceed the range of 0.5%～1.5%, in which more than 1% of the situation is often a segmented overhead power grid, and the transposition is realized on the bus of the substation. The asymmetry of the cable line is equal to zero, because whether it is a three-core cable or a single-core cable, each phase core cable is in a balanced position for the metal skin of the grounding cable.