Abstract: To implement online detection of fault component and improving accuracy and tolerance, the protection entropy is proposed using seven types of wide-area protection actions, as well as the relative protection entropy. Meanwhile, the acquisition of threshold for relative protection entropy is discussed. Its sensitivity to some situations of protection action errors is studied, especially those of quite a few protection rejections on fault line and protection misoperations on adjacent lines. In most situations, the fault line can be accurately detected by protection entropy with simulation test. The method of nominal transition resistance based on limited phasor measuring unit（ PMU） is studied, which is used as a supplement to situations in which the protection entropy of suspected line is near to the threshold. The simulation results on IEEE 14-bus system show that, by combining relative protection entropy with nominal transition resistance, the proposed algorithm has higher accuracy and tolerance.