Evaluating Methods for Measuring Grid Frequency in Low-Inertia Power Systems

Accurate measurement of grid frequency is a critical component of reliable grid control. Traditionally, inverters and phasor measurement units (PMUs) have used methods such as phase locked loops (PLLs) and discrete Fourier transforms (DFTs) to measure frequency. However, as inverter-based resources (IBRs) such as solar and wind have increased, these conventional frequency measurement methods have yielded incorrect frequency measurements leading to unreliable control in some cases. One challenge is measuring frequency during transient events. During these events, measured frequency may contain significant spikes due to the disrupted waveform, much more rapid and significant than any expected physical frequency dynamics. New methods must balance between suppressing spikes in frequency during faults, and providing fast, accurate, measurements in all other grid operation conditions, especially during events with a high rate-of-change-of frequency (ROCOF), which are more prevalent in high-IBR power systems. This paper first surveys frequency measurement methods that have been proposed to reduce measurement errors during transient events in low-inertia grids. Then, both conventional and more novel frequency measurement methods are tested against an IEEE standard and industry recommendations, and their performance is evaluated for events simulated in PSCAD. Results quantify trade-offs in performance during different grid conditions and lead to suggestions for the most appropriate frequency and ROCOF measurement methods for low-inertia grids.