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The authors have recently developed a number of instruments for measuring harmonic composition of power grid signals. These instruments have a simple, predominantly digital architecture and they are based on an assumption that the frequency of the measured signals equals its nominal value (50 Hz, 60 Hz or 400 Hz). This approach has enabled the measurement of a high number of harmonics within a single period. However, the internal oscillator in the instrument generates the nominal frequency and cannot adapt to frequency changes in the input signal. This paper presents a method for the improvement of the operation of the developed instruments in cases when the fundamental frequency drifts from its nominal value as is the case with real power grid signals. Based on this method, modified versions of the harmonic measurement instruments have been developed. A comparison of the measurement error with and without the application of the proposed method is presented in the paper.

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In this paper, it is shown that the use of random uniform dither in harmonics measurement can significantly shorten the word of applied A/D converter and shorten the word of applied base function (sine and/or cosine), stored in memory, without accuracy loosing. This fact enables design of simple instrument for harmonics measurement. It is proven theoretically, by simulation and by experiment that 6-bit dithered A/D converter word, 8-bit dithered base function word, and 6 x 8 (14-bits) multiplier word assure measurement of 16 harmonics (16 sine and 16 cosine components) with 13-bit accuracy. Putting n such simple devices in parallel it is possible to measure n x 16 harmonics simultaneously. The measurement of 1 x 16 harmonics is realized in the small PLD chip.

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