UHV AC grid makes higher requests to stability control devices. An electromagnetic transient model is set up for UHV transmission line, and the adaptability of the traditional stability control devices fault tripping criteria is analyzed. When a fault occurred at the opposite side of a transmission line, the problems which traditional criteria may encounter are presented. Based on the unique fault characteristics of UHV transmission lines, an improvement single phase to ground fault tripping method considering zero sequence current is proposed, and a new phase to phase fault tripping method considering is also suggested. Real-time digital simulation (RTDS) tests are carried out to validate the new methods, the results show that the improved methods are correct, reliable and effective.