The presented work is used in the projects 19NRM05 (IT4PQ) and 19ENG08 (WindEFCY) which have received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme.
The integration of renewable energy sources on a large scale in the electrical energy distribution systems, as well as the widespread of non-linear loads, has led to a significant increase in power quality (PQ) disturbances. For this reason, PQ monitoring is also becoming a key task in medium voltage (MV) grids. The measurement of PQ at MV levels can only be performed using instrument transformers (ITs) to scale down the level of voltage and current to levels suitable for the input stage of PQ instruments. However, no international standards currently require the verification of the errors introduced by ITs in the measurement of PQ phenomena. Moreover, this issue is only partially addressed in the scientific literature, where papers dealing with specific and limited aspects of the problem can be found. For this reason, this paper aims to comprehensively assess the issue, proposing IT accuracy verification tests for different PQ parameters. First, a set of PQ phenomena relevant for IT testing is chosen, as well as the associated ranges of variation, based on a review of the enforced standards and the scientific literature. For each selected PQ phenomenon, possible performance indices and test waveforms are proposed. Finally, the proposed procedure is validated by applying it to the characterization of two different types of commercial voltage transformers.
This paper describes the setup and characterization of a set of wideband Current-to-Voltage (C-to-V) transformers equipped with symmetrical primary windings. The objective is to use the C-to-V transformer set to calibrate digital instrument transformer (IT) accurately at power frequency and within a wider frequency range to cover power quality aspects. The errors of the 50 A and the 200 A reference current transformer (CT) were within ±10 ppm and μrad at 50 Hz. The frequency responses of the two CTs up to 12 kHz were below 0.1 % and 0.2 crad.
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