Impact of transformer inrush currents on sensitive protection functions How to configure adjacent relays to avoid nuisance tripping?

Abstract

Transformer inrush currents are known to cause problems for sensitive protection functions applied to a power transformer itself, or in a near vicinity of a transformer. This paper reviews impact of transformer inrush currents on four protection functions. For each category the problem is reviewed and quantified, and practical solutions are presented. First, an impact of inrush currents combined with CT saturation on restricted ground fault (RGF) protection is analyzed. Second, the impact of inrush currents of a step-up transformer on generator protection is presented. Third, impact of the combination of inrush currents and saturated CTs on sensitive ground overcurrent protection is addressed. Fourth, distance zones, if set too far-as compared with the amount of inrush current-could pickup spuriously when energizing a large power transformer is analysed. The paper delivers analysis of the above phenomena and provides practical application guidelines regarding settings and scheme logic.