Dealing with front-end white noise on differentiated measurements such as frequency and ROCOF in power systems

Andrew J. Roscoe, Steven M. Blair, William Dickerson, Gert Rietveld

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Abstract

This paper describes the way that white noise (including quantised input section sampling) imparts errors onto frequency and rate-of-change-of-frequency (ROCOF) measurements. The main paper focus concerns the use of filtered heterodyned (i.e. Fourier) analyses for single-phase and 3-phase systems, and the filtered Clarke transform for 3-phase systems. The rules and equations governing the effect of white noise on frequency and ROCOF are formulated for these techniques, explaining the subtle effects of aliasing, splitting signals and noise into their positive and negative frequency components, and the correlation or de-correlation of noise. It is shown that - as expected - for 3-phase AC measurements, averaging 3 single-phase Fourier measurements produces the same performance against noise as using a method based on Clarke’s transform, if identical filtering is used. Furthermore, by understanding the theory behind the frequency and ROCOF measurement processes, it is shown that to achieve the lowest RMS errors, in the presence of front-end white noise (alone, ignoring other dynamic signal and power quality aspects), a filter which provides ~40 dB/decade attenuation (i.e. a 2-boxcar cascade) is recommended for a frequency measurement, but a filter which rolls off at ~60 dB/decade (i.e. a 3-boxcar cascade) is recommended for a ROCOF measurement.
Original languageEnglish
Number of pages14
JournalIEEE Transactions on Instrumentation and Measurement
Early online date25 Apr 2018
DOIs
Publication statusE-pub ahead of print - 25 Apr 2018

Keywords

  • frequency measurement
  • frequency estimation
  • white noise
  • Gaussian noise
  • colored noise
  • signal to noise ratio
  • power system measurements
  • finite impulse response filters
  • Fourier transforms
  • phasor measurement units

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    Filter design masks for C37.118.1a-compliant frequency-tracking and fixed-filter M-class Phasor Measurement Units (PMUs)

    Roscoe, A. J., Dickerson, B. & Martin, K. E., Aug 2015, In : IEEE Transactions on Instrumentation and Measurement . 64, 8, p. 2096-2107 12 p.

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