Impact of CT saturation on overcurrent relays

An overcurrent relay analyses and processes the secondary currents from a set of current transformers (CTs) and if the currents exceed the operating threshold initiates the tripping of a circuit breaker after an appropriate time. An important issue for a protection relay is how it responds when the steel core of the CT is saturated. The exact behaviour of a CT depends on various factors such as core size, CT ratio, primary current magnitude, DC offset, secondary burden resistance and remnant flux. The aim of this study is to investigate the impact of CT saturation on overcurrent relays using both a physical relay test bench that includes actual protection CTs, and a modern relay test set configured to simulate the test bench and its CTs. The relay operating times, obtained under similar fault conditions using both the test bench and the test set, will then be compared. The results obtained will then be used to determine how different levels of CT saturation affect different generations of an inverse time overcurrent relays. The final section of this study will explain how CT saturation affects an overcurrent grading study involving a mixture of electromechanical, static and numerical relays.