Abstract: Current increment protection is the main protection of the traction power network in response to high impedance grounding faults. The current increment protection may fail to operate for the too high setting value. The protection may refuse to operate under some special working conditions, based on some improvement program in lowering the current increment setting value. By recording and analyzing the load current of the feeder line at the Suning North Substation of the Shuohuang Railway in China, and collecting data of high impedance grounding faults, the characteristics and causes of the load current and high impedance grounding fault current of electric locomotives are obtained. The calculation method of total harmonic content is defined. An improved scheme for blocking current increment protection based on total-harmonic content is proposed. It is recommended to lower the current increment protection setting value according to the actual operation condition of electric locomotives on the power supply arm. Based on the analysis and verification of on-site measured feeder current, it is shown that the total-harmonic content blocking element proposed can effectively block the potential misoperation of current increment protection caused by large current increments caused by electric locomotive load, improve the sensitivity and reliability of current increment protection.