1. Beijing Zhongheng JoinBright Digital Electrical Technology Company Limited, Beijing 100085, China;2. National Electric Power Dispatching and Control Center, Beijing 100031, China; 3. School of Electrical & Electronic Engineering, North China Electric Power University, Beijing 102206, China

Based on the phase component branch impedance equation, the decoupling method of the mixed-voltage four-circuit transmission line model on the same tower is deduced by using the three-sequence component method. It is proved that the mutual inductance between lines only affects the zero-sequence network, and the method for calculating the per unit value of the zero-sequence mutual inductance in the cross-voltage mode is deduced. The method maintains the symmetry of the node impedance matrix with unit values in the basic data model of the network before fault. Moreover, the universal fault boundary model for cross-line fault is studied. In order to adapt to the problem of mixed voltage, the actual admittance matrix of the fault boundary node is first established, and the calculation of its unit value in the cross-voltage mode is proposed. Then the fault current is calculated based on the compensation method. The example analysis shows that the proposed method is correct. The proposed method has been applied to the protection setting and online checking software.