Root-Mean-Square Profiles under Varying Power Frequency: Computation and Applications

The operating power frequency fluctuates around its nominal value under normal conditions based on the dynamic balance between load demand and generation levels. This frequency variation creates a challenge in computing and analyzing voltage and current root-mean-square values over time, since sliding windows may not cover an integer number of cycles of the sampled voltage and/or current data. The overall objective of this paper is to present an adaptive resampling technique for the voltage and current waveforms, such that each sliding window used for computation of power quality parameters always cover an exact integer number of cycles. This technique is illustrated for a capacitor de-energizing operation, which is detected through a step change in the rms voltage profile. Once this de-energizing operation has been detected, the voltage waveform is resampled and the rms voltage profile with a high time resolution is obtained through a fast and recursive approach. Finally, the exact capacitor de-energizing instant is estimated. The proposed approach is shown to be simple, yet robust and accurate.