The zero-sequence model of single phase earth fault is made on the basis of distributed parameter lines.

But when the old direct server can not recovery in time, the newdirect server will become a latent single fault spot, this will reduce the system utility.

Taking the Chenjiaqiao Changshou electromagnetically coupled power loop as an example, which is the main channel for sending Sichuan power eastward, the necessity of contingency analysis of the tripping of line without fault and single line permanent ground (SLG) fault on UHV line prescribed in the Guideline on Security and Stability for Power System is analyzed.