The Use of Voltage Transformers for the Measurement of Power System Subharmonics in Compliance With International Standards

The measurement of subharmonics in distribution systems requires instrument transformers to reduce voltage and current to levels fitting with the low-voltage input of the power quality (PQ) instruments. The in-force international standards establish algorithms and methods for detecting, measuring, and reporting subharmonics. In particular, the IEC 61000-4-7 suggests performing the discrete Fourier transform over basic time frames of ten cycles (12 cycles) for the 50-Hz (60 Hz) power frequency. Considering the case of 50-Hz constant power frequency, the spectral analysis is performed with a fixed spectral resolution of 5 Hz; thus, subharmonics with a frequency not integer multiple of 5 Hz could introduce inaccuracies in the measurements because of the spectral leakage. In this framework, this article investigates the additional error contributions that can be introduced by voltage transformers (VTs) used, at the input of PQ instruments, to measure subharmonics in compliance with international standards. The analysis is conducted through numerical simulations and experimental tests on two commercial VTs based on different operating principles. Results show that the use of a VT to measure subharmonics, in compliance with international standards, could introduce higher additional errors compared to the ratio errors of the same device evaluated at subharmonic frequencies.