A complete detecting impedance analysis model of transmission lines with mutual inductance is developed from the angle of setting. In the model

the influence mechanism of various factors on the measured impendence of grounding distance protection is qualitatively analyzed and quantitatively validated. With these results

the conditions for the simplified model which can calculate the extreme values of the measured impendence are proposed. On this basis and by referring to typical extreme value cases in the power system

the sideline current addition influence of the detecting impendence of transmission lines with mutual inductance is further analyzed. Finally

the optimal relay setting strategy for the grounding distance protection based on the analysis results is proposed. The strategy is implemented in Guangdong power grid. Actual application results show that the optimal strategy is able to improve the adaptability and performance of grounding distance protection.