The current status of 20 kV line protection is introduced first. Then focusing on neutral low resistance grounding power system the advantages and disadvantages as well as the applicable scope of zero-sequence over current protection and zero-sequence power directional protection are analyzed. With the results of theoretical study and laboratory fault tests， the problem of single-phase high-impedance grounding faults causing line protective relay miss-operation is further discussed. At last， based on simulation analysis， technical measures are provided to improve the protective relay operation performance for single-phase high-impedance grounding faults.