When earth faults occur at the end of long power lines in strong source networks

traditional zero sequence (negative sequence) directional element may not operate due to very low zero sequence (negative sequence) voltage which is lower than the threshold of the directional element

although zero sequence current reaches the set value of protection relay. Two kinds of solution to this problem are proposed: One is that using positive sequence voltage instead

when zero sequence (negative sequence) voltage is lower than the threshold of the directional element

the other is that using nondirectional zero sequence current protection with inverse time characteristic as a backup protection and applying new principle of current transformer circuit supervision.