Impact of Inverter-Based Resources on Memory-Polarized Distance and Directional Protective Relay Elements

Distance and directional protective relay elements use memory polarization to ensure proper operation in case of close-in faults where the short-circuit voltage may be too small to be accurately measured. A side advantage of memory polarization is an increase in the resistive reach of distance mho characteristics. The proper operation of memory polarization is based on the following assumptions: (i) the amplitude of source impedance behind the relay is predictable and consistent, allowing the additional resistive reach to be accurately calculated and (ii) voltage phase angle does not significantly change during a fault, allowing the phase angle of the short-circuit voltage to be estimated by that of the memory voltage (i.e., the pre-fault voltage). While these assumptions are valid in a traditional Synchronous Generator (SG)-dominated power system, they may no longer be valid when Inverter-Based Resources (IBRs) displace a large amount of SGs, leading to potential misoperation of the memory-polarized elements. The paper studies these misoperation problems. Specifically, conducted simulations on a multi-Wind Park (WP) transmission test system show a case where WPs cause a variable expansion of a memory-polarized distance mho circle, thus leading to unintentional operation of the element. In another case, WPs cause a significant shift in the phase angle of short-circuit voltage, leading to an incorrect directionality decision. The objective is to identify such potential protection challenges and the cause thereof as a first step towards developing future solutions to ensure effective protection under IBRs.