The mixed-voltage multi-circuit transmission lines system cross-voltage fault prone

which makes higher requirements on relay protection. Earth-fault distance protection is one of the main protections of high voltage transmission lines. The earthed cross-voltage fault in mixed-voltage transmission lines is influenced by two factors

one is zero-sequence mutual inductance of adjacent lines

the other is the fault current of another fault circuit. By taking both factors into consideration

a method of calculating the cross-voltage fault in mixed-voltage transmission lines is proposed. Then the impedance measuring expression considering two factors is calculated. Finally

the relationship between impedance measuring and electromotive force phase difference of the two systems are analyzed. The ranges of the phase in which the earth-fault distance protection can operate and mal-operate are given

respectively. Corresponding measures are recommended. A 500 kV/220 kV four-circuit model is built in PSCAD to verify the correctness of the analysis. This work is supported by National Basic Research Program of China (973 Program) (No. 2012CB215206) and the Fundamental Research Funds for the Central Universities (No. 2015XS12).