New simulation models to dynamically test impedance relays

Modern protective relays are becoming more sophisticated and faster as time progresses. Relay test systems also become more sophisticated and flexible. Yet some protective relay manufacturers claim that there is no need to test numerical relays, such as impedance relays, as the operating characteristics are accurate and do not change with time. Why then test numerical relays in the same way as it has always been done? This paper takes a critical look at the conventional test techniques and the associated simulation models. More realistic test models are discussed and a test system that combines all the necessary test models is introduced