This paper addresses the challenges posed by high Grid-Following (GFL) Photovoltaic (PV) penetration on the dynamic performance of memory-polarized mho relays, crucial for close-in fault protection in power systems. Traditional memory-polarized mho relays, designed for synchronous generator-dominated systems, utilize a scalar weight to dynamically expand their mho characteristics based on memory voltage, enhancing resistive reach. However, the unique transient behavior of Inverter-Based Resources (IBRs) like GFL PV during faults can disrupt this mechanism, compromising relay reliability. To overcome this limitation, this research introduces a novel algorithm that employs a complex weight parameter in the memory polarization process, replacing the conventional scalar approach. This complex weight allows for more precise and adaptable control of the mho characteristic’s dynamic expansion, enabling the relay to better respond to the complex voltage and current transients introduced by GFL PV. The study investigates the dynamic expansion of the mho element’s maximum diameter (dmax) and memory vector angle (θm) under various fault scenarios (three-phase, single-line-to-ground, and line-to-line) to evaluate the algorithm’s effectiveness. The proposed complex weight algorithm is validated across diverse fault types, varying complex weight factors, and different fault resistances, considering GFL PV generators with reactive power priority and IEEE Standard 2800–2022 compliant Low/High Voltage Ride-Through capabilities. The results demonstrate significantly enhanced reliability and stability of memory-polarized mho relays in systems with high GFL PV penetration, showcasing the superior performance of the complex weight approach.

This paper investigates the influence of dynamic voltage support on Positive Sequence Memory-Polarized (PSMP) mho relays in the presence of Grid Following Photovoltaic (PV) generators. The widespread integration of Inverter-Based Resources (IBRs) has significantly altered power system dynamics. Traditionally, memory polarization in mho elements has ensured reliable operation during close-in faults. A dynamic expansion of mho characteristics results from the use of memory voltage for polarizing the mho elements, enhancing the resistive reach in systems dominated by Synchronous Generators (SGs). However, the fault current characteristics of IBRs differ from SGs, potentially compromising the dynamic mho expansion behavior. This work explores how dynamic voltage support employed by large-scale PV generators affects the expansion of PSMP mho characteristics. The PV generators used in this study have reactive power priority and Low Voltage Ride-Through /High Voltage Ride-Through capabilities and are compliant with the IEEE Standard 2800-2022.

The restructuring of the power system by replacing Synchronous Generators (SGs) with Inverter-Based Resources (IBRs) has resulted in a drastic change in power system dynamics. Positive Sequence Memory Polarized (PSMP) mho elements have been used to ensure the reliable operation of mho relays during close-in bolted faults. The use of memory voltage for polarization of the mho element causes dynamic expansion of the mho characteristics. In an SG-only system, the memory polarization-induced dynamic expansion of these mho characteristics helps to increase the resistive reach. However, as the integration of IBRs increases, the dynamic mho expansion will be affected due to the different fault characteristics of IBRs. Most IBRs currently integrated into the power system are Grid-Following (GFOL) type, which uses Phase-Locked Loops (PLL) for synchronization. The variations in control parameters and bandwidths of the PLLs used in the GFOL IBRs may also affect the dynamic mho expansion. The studies presented in this paper demonstrate the impact of different types of PLLs and their control parameters on the dynamic expansion of PSMP mho relays. For this analysis, a modified IEEE-39 bus system with a GFOL Photovoltaic (PV) generator is used. The PV s have Voltage Support (VS), Low Voltage Ride-Through (LVRT)/ High Voltage Ride-Through (HVRT) features and comply with IEEE 2800–2022 standard.