Inrush currents and their effect on protective relays

Abstract

This paper explains in detail the phenomena of inrush in single-phase transformers during three conditions: transformer energization, external fault clearing and sympathetic inrush. It then focuses on the inrush in three-phase transformers, explaining the phenomena that occurs when there is a delta winding and analyzing the types of transformer configurations that allows the flow of zero-sequence current. The paper describes the influence of the inrush current on different types of protection functions as transformer differential, overcurrent, distance, busbar and line differential. It focuses mainly on the transformer differential describing the most common methods used for maintain the security during the inrush condition: harmonic restraint and harmonic blocking. Their differences are explained. It then explains the different crossed logics used and selects the most appropriate one. It finally describes a logic to inhibit the harmonic restraint / blocking based on an external fault detector. This logic reduces the tripping time of the differential unit mainly during internal faults with CT saturation. Different cases are considered based on RTDS simulations and real events.