In a power transmission system

there may appear partially coupled lines at different voltage levels because of the difference in construction time. When faults occur with this kind of lines

the zero-sequence mutual inductance will make it difficult for fault analysis. To solve this problem

as is conventionally done

the partially coupled lines are first decoupled into completely decoupled positive and negative sequence components and the zero sequence component with coupling. Then

the zero sequence is divided into the same direction component and the circumfluent component according to the six sequence fault component theory for further decoupling zero sequence mutual inductance. Finally

by use of the relation between the zero sequence current and voltage

the parameters of the uncoupled part will be modified. Based on the boundary conditions of the faults

the characteristics of sequence components are studied and the related sequence networks of single line faults are presented for their role in solving the complicated problem of fault calculation of partially coupled lines at different voltage levels. PSCAD simulation results show that this method is appropriate for fault analysis on partially coupled lines at different voltage levels

and the short circuit analysis and calculation prove to be correct

practical and valid. This work is supported by National Nature Science Foundation of China (No. 51177066

No. 51377104).