Abstract: Summation impedance protection is a crucial means of solving the sensitivity problem of single-phase high resistance grounding fault in the high voltage transmission line. Through analyzing the criterion, the paper shows that the operation sensitivity of such protection is affected by the capacitance compensation method and its operation security is threatened by the system oscillation. In order to solve these problems, a new protection criterion was proposed, trying to eliminate the influence of long line’s capacitance and system oscillation fundamentally. Theoretical analysis is made of the proposed protection criterion, including phase selection, transition resistance tolerance and the performance dealing with fault during the system oscillation. A suitable phase selection criterion was designed, which was combined with the main criterion of summation impedance relay to realize the fault phase selection tripping. The effectiveness of the proposed protection criteria was verified by simulation.