Abstract
Most Inverter-Based Resources (IBRs) are grid-following and rely on Phase-Locked Loop (PLL) units to track the system frequency. During fault conditions, PLL errors may occur, causing IBRs to inject currents at frequencies different from the nominal system frequency. This off-nominal frequency injection can affect the operation of several protection functions.
For distance protection, it produces rotations of the apparent fault resistance, impacting the line reactance response under resistive faults. Directional units may also be affected due to the phase shift between voltage and current caused by the frequency deviation. In line differential protection and external fault detectors based on directional comparison units, rotations occur between the local and remote currents, potentially compromising correct operation. This paper analyzes these effects across different polarizations of the line reactance, various types of directional units and differential and directional comparison units. The analysis is done through RTDS simulations using full-converter generator models. Solutions are proposed for units prone to misoperation under these conditions, improving the reliability of protection schemes in networks with high penetration of grid-following IBRs.
In addition, in networks with high penetration of grid-following IBRs, very high Rates of Change of Frequency (ROCOFs) may occur, affecting frequency tracking. Periods without adaptation of the sampling frequency to the system frequency can cause maloperations in certain units, such as voltage memory-based directional units or units based on prefault current. The impact of high ROCOFs on different protection functions, including distance, line differential protection and directional comparison is analyzed using oscillography from real events. Solutions are proposed for units that may operate incorrectly under these conditions.