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The measurement of harmonics is one of the key tasks in the modern power systems, performed basically for power quality assessment, verification of planning levels compliance, immunity and compatibility purposes and so on. Especially at Medium Voltage (MV) and High Voltage (HV) levels, harmonics are always measured through Voltage and Current Transformers (VTs and CTs), which very often are inductive. Recent papers show that inductive VTs and CTs, due to the intrinsic iron core nonlinearity, can introduce errors up to some percent in harmonic measurements. The paper aims to investigate in-depth this phenomenon by analyzing how the accuracy of harmonic measurements performed through an inductive MV VT is affected by the presence subharmonics in its input voltage. Subharmonics are spectral components having frequencies lower than the power frequency, that is 50 Hz for European countries. Some theoretical considerations are carried out and the performance of two different inductive MV VTs are tested in various operating conditions. Experimental results show that subharmonics can worse the harmonic measurements done through inductive VTs up to some tens of percent.

The paper presents a method for the frequency characterization of voltage transformers (VTs) for medium voltage (MV) grids which involves only sine waves. It is called extended SINDICOMP, since it is an extended version of the previously developed technique SINDICOMP. It requires, in the first step, an evaluation and a compensation of the non-linearity introduced by the VT when it is supplied with a 50 Hz sinusoidal input at rated value. Then, the VT is characterized with a low voltage sinusoidal frequency sweep from the second harmonic frequency up to the first resonance frequency. Some rules to build the approximated frequency response, starting from these two sets of data, are given in the paper. The proposed approach is applied to three commercial MV VTs. Significant improvement of the VT performance is obtained, compared to the use of a frequency response obtained from the low voltage characterization.

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Within the framework of distribution and transmission grids, knowledge of Instrument Transformers (ITs) behavior in distorted conditions is a topics of great interest. Its relevance stems from the ITs wide use in metering, protection, monitoring and control applications, where their role is to reduce voltage and current to levels compatible with measuring instruments input. In force standards require that the performance of measuring instruments is assessed under realistic conditions. On the contrary, performance test of ITs are generally carried out only at rated conditions, so that their behavior under actual waveforms is not fully known. To cover this gap and starting from the hardware used for the Medium Voltage (MV) Voltage Transformer (VT) frequency characterization, a suitable setup is developed for the traceable test of VTs under a quite large set of static and time varying test waveforms. In the paper, after a short description of the setup, examples of its applications to the evaluation of the performances of ITs under different test voltage waveforms are given, including a realistic reproduction of a PQ event and focusing on the amplitude and phase modulation tests, as those suggested for the Phasor Measurement Units in IEEE Std. C37.118.1. Preliminary experimental results are shown.

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.

Measurements of Power Quality (PQ) are gaining more importance due to increasing presence of switching power converters that deform the waveform of the distributed voltage further and further away from a sine wave. Especially at medium and high voltage levels, PQ measurements are carried out by means of Instrument Transformers (ITs). A recently started European metrology project, EMPIR 19NRM05 IT4PQ, aims at establishing the methods and procedures for assessing the accuracy of ITs used for PQ measurements. This paper, that is written in the framework of the IT4PQ project, presents a thorough review of the current state-of-the-art of literature and international standards about ITs and PQ. The main results from several papers and the main information from IT and PQ related international standards are summarized.

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