Total harmonic distortion calculation by filtering for power quality monitoring

Measuring and monitoring quality parameters of AC power systems requires several calculations, such the total harmonic distortion (THD) of voltage and current. This calculation is performed with samples of the monitored waveforms, at sample frequency equal to a power of two multiple of the frequency of waves. The samples are converted to digital values by analog-to-digital converters, with a finite number of bits. Numeric algorithms applied to these digital values insert some errors in the final results, due to the number of bits used in calculations. The most used algorithm to obtain the THD is FFT. Conventional 16-bit DSP FFT algorithms use only 16-bit calculations, despite the DSP 40-bit hardware accumulator. The results are errors in amplitude of each harmonic order. Using these amplitudes to calculate the THD, these errors are summed, and the final error is very greater. The purpose of this article is to present an algorithm to calculate the THD using the DSP 40-bit hardware accumulator and resulting more accurate results.