Resistive reach guidelines for impedance relay applications

Impedance relays must accommodate all types of faults. In order to enable this, it is necessary that the correct voltage and current quantities be applied to the measuring elements. The impedance characteristics are obtained from these three-phase voltage and current measurements. The main aim is to detect the faulted phases and react appropriately based on the applied relay settings, scheme design and philosophies. The difficulty arises in the distinction between the faulted and healthy phase impedance measurements. Continuous research and development of impedance relays are in progress to improve its operating performance. Unfortunately, the impact and limitations of such improvements on the power system applications are not studied with the same vigour. As such, it is necessary to investigate the influence of network topology, power system loading, fault resistance etc. on the reach capability of impedance relays when applied to power systems. In This work, the reach capability of impedance relays is investigated. A set of resistive and reactive reach guidelines for impedance relay applications are presented. The results from the research also show that the network topology and faulted phase selection algorithms play a major role in power system healthy phase impedance measurements. New impedance ratios namely SWIR and SNIR are introduced, and the well-known system impedance ratio (SIR) is enhanced for ease of illustrating network topology and associated impedance measurement.