The phenomenon of high-frequency distortion (HFD) in the electric grids, at both low-voltage (LV) and medium-voltage (MV) levels, is gaining increasing interest within the scientific and technical community due to its growing occurrence and the associated impact. These disturbances are mainly injected into the grid by new installed devices, essential for achieving decentralized generation based on renewable sources. In fact, these generation systems are connected to the grid through power converters, whose switching frequencies are significantly increasing, leading to a corresponding rise in the frequency of the injected disturbances. HFD represents a quite recent issue, but numerous scientific papers have been published in recent years on this topic. Furthermore, various international standards have also covered it, to provide guidance on instrumentation and related algorithms and indices for the measurement of these phenomena. When measuring HFD in MV grids, it is necessary to use instrument transformers (ITs) to scale voltages and currents to levels fitting with the input stages of power quality (PQ) instruments. In this respect, the recently released Edition 2 of the IEC 61869-1 standard extends the concept of the IT accuracy class up to 500 kHz; however, the IEC 61869 standard family provides guidelines on how to test ITs only at power frequency. This paper provides an extensive review of literature, standards, and the main outputs of European research projects focusing on HFD and ITs. This preliminary study of the state-of-the-art represents an essential starting point for defining significant waveforms to test ITs and, more generally, to achieve a comprehensive understanding of HFD. In this framework, this paper provides a summary of the most common ranges of amplitude and frequency variations of actual HFD found in real grids, the currently adopted measurement methods, and the normative open challenges to be addressed.

Measurements of harmonics and, in general, of power quality in distribution grids are becoming more and more an essential task, due to the proliferation of switching power devices. Instrument Transformers (ITs), that are necessary for these kind of measurements in distribution grids, in turns, need to be frequency characterized. Although international standards about ITs do not suggest procedures for their frequency characterization, there are some characterization procedures that are acknowledged in scientific literature. They are based on the application of realistic power system waveforms to the ITs, in order to test their behaviour in conditions very similar to the actual operating conditions. A drawback of these procedures is the quite long duration of the tests, in the order of ~ 1 - 10 hours. Therefore, this paper proposes a new waveform for the frequency characterization of inductive Voltage Transformers (VTs), composed by the superposition of a fundamental component and a periodic transient phenomenon, namely a damped sine wave. With its use, it is possible, with a single waveform, to evaluate the frequency behaviour of an inductive VT, thus with a test duration in the order of ~ 1 s. The waveform is theoretically analysed and experimentally applied to two commercial VTs for medium voltage distribution grids. Results are compared with those obtained with the IT frequency characterization methods most acknowledged in scientific literature.

This paper proposes a generation and measurement setup for the characterization of Voltage Transformers (VTs, both inductive as well as Low Power Voltage Transformers, LPVTs) at power frequency and from 9 kHz to 150 kHz with waveforms having amplitudes at MV level. It involves the independent generation of the fundamental tone and of the high-frequency components to produce the desired distorted test waveform. The generation of the fundamental frequency component, in MV amplitude range, is obtained by means of a step-up VT, whereas the high frequency components are generated through a series connected voltage amplifier. As regards the measuring stage, the fundamental tone is measured through a commercial divider. Instead, high frequency tones are measured through a reference device, operating in the range [9, 150] kHz, specifically designed, realized and characterized. As an application, the characterization of two commercial VTs, an inductive VT and a LPVT, are presented.

This paper presents the development and characterization of a reference high-voltage sensing chain for the calibration and conformity assessment of instrument transformers with Class 0.1-WB3, in the extended frequency range up to 150 kHz, according to IEC 61869. The sensing chain, composed of a high-voltage divider, precision attenuators and high-pass filters, has been specifically developed and characterized. The chain features two parallel measurement paths: the first path, comprising the high-voltage divider and attenuator, is optimized for measuring the fundamental frequency superimposed with high-amplitude harmonics; the second path, consisting of the high-voltage divider followed by a high-pass filter, is dedicated to measuring very-low-level superimposed harmonic components by enhancing the signal-to-noise ratio. These two paths are integrated with a digitizer to form a complete and modular measurement chain. The expanded uncertainty of measurement has been thoroughly evaluated and confirms the chain’s ability to support assessment of instrument transformers with Class 0.1-WB3 compliance. Additionally, the chain architecture enables a future extension up to 500 kHz, addressing the growing need to evaluate instrument transformers under high-frequency power quality disturbances and improving the sensing capability in this field.

The aim of the study here presented is to evaluate how temperature variations affect the metrological performance of inductive voltage transformers (VTs) in harmonic measurements. The analysis specifically targets the harmonics up to 1 kHz, where measurements using VTs can be challenging due to their non-linear behavior. In fact, recent scientific literature has evidenced that when a VT operates under distorted conditions, the harmonics at its secondary side are not simply a scaled version of those injected. Instead, they are a combination of the injected harmonics and the spurious ones generated by the iron core due to its B-H curve. This phenomenon has been investigated in laboratory under controlled conditions, and specific techniques have been proposed to compensate for it. However, no studies have analyzed the impact of temperature variation on this phenomenon and, consequently, on the introduced errors. This paper addresses this open issue through experimental activities on two different VTs using a high-accuracy reference setup. Results show that, for different temperatures, the measured VT harmonic errors can differ up to an order of magnitude, indicating that this influence quantity cannot be neglected.

Medium Voltage (MV) grids are experiencing the same Power Quality issues that Low Voltage (LV) grids begun to suffer from some tens of years. In particular, recent advance in semiconductor technology has enabled the development of power converters, directly connectable to MV grids, with higher and higher switching frequencies. Consequently, high-frequency components—specifically, harmonics related to the switching frequency—will be present within MV grids. As happened at LV level, also at MV level it is essential that such disturbances are accurately measured. At the same time, recent international standards set accuracy requirements for Instrument Transformers (ITs) up to 500 kHz. This scenario increased the performance requirements of ITs, particularly in terms of measurement accuracy over an extended frequency range. Unfortunately, no ready-to-use commercial solutions are available to assess the performance of MV ITs up to hundreds of kilohertz. Therefore, this paper presents a reference setup for the calibration of MV Voltage Transformers (VTs), up to 35 kV, at fundamental frequency and in a frequency range from 9 kHz up to 150 kHz. An innovative generation scheme, based on the parallel connection of two voltage sources, allows to generate waveforms composed by a fundamental tone at MV level (up to 35 kV) and by reduced-amplitude spectral components from 9 kHz up to 150 kHz, is proposed. Moreover, also an innovative measuring chain, based on a compensated resistive divider, with the possibility to be cascaded with an additional attenuator or a high-pass filter, is shown. The implementation of the setup is discussed along with the results of the characterization of a commercial MV Low Power VT.