Analysis of inverse time zero-sequence current relay and its selectivity

This paper analyzes the distribution of zerosequence current to ground faults.It discusses that in some operating conditions,zero-sequence current of the fault line maybe less than or equal to the next line.So the inverse time zero-sequence relay of next line will trip before the fault line.Even if the breakers of the the next line trip successively,the selectivity of inverse time zero-sequence current relay can not be provided.As an alternative of simplifying zero-sequence protection,inverse time zero-sequence current protection with a lagged timer can be used as an auxiliary to the grounding distance protection.It will provide sensitivity to high impedance ground faults.