Analysis on outside transformer fault causing CT saturation and its influence on transformer protection

In recent years, transformer instantaneous differential protection misoperated several times on the site. As a secondary protection, transformer instantaneous differential protection should act as fast as possible without any blocking logic when serious internal faults occur in the transformer and the differential current value is bigger than the trip value. Through on-site analysis of several accidents, CT saturation during external fault is the direct reason that causes instantaneous differential protection misoperation. Through analyzing the reason why P level CT is easily saturated, this paper improves the instantaneous differential protection's action logic for different fault types, which is very easy to implement in computer protection. The results show that the improved scheme can significantly improve the protection's capacity against CT saturation in external fault without reducing the acting sensitivity in internal fault.