• Energy Research

The aim of this paper is to show that the developed inductive current transformer may ensure the required wideband transformation accuracy and it may be applied, as the reference source, in the measuring system for the evaluation of the transformation accuracy of inductive current transformers for harmonics of distorted current. This device ensures 5 A and 1 A RMS secondary currents to provide the opportunity to use the differential measuring setup. Such solutions are characterized by the significantly reduced measurement uncertainty in relation to the comparative measurements made between two current/voltage channels. The problems required to ensure the high wideband transformation accuracy, including the self-generation phenomenon of the low order higher harmonics to the secondary current and a too-low frequency range of operation, were overcome in the design process. The values of its ratio error and the phase displacement of the developed reference wideband inductive current transformer did not exceed ±0.2%/° up to 1 kHz, ±0.4%/° from 1 kHz up to 1.5 kHz and ±0.5%/° from 1.5 kHz up to 3 kHz, as is required to perform the test procedure in accordance with the optional requirements for the inductive current transformers defined in the new edition of the standard IEC 61869-1.

Abstract The aim of this paper is to present results explaining the phenomenon of inductive current transformers (CT) additional secondary current distortion in condition of variation of their primary current rms value caused by dips and interruptions of the power line's voltage. To assess its accuracy of current transformation for such transients, harmonics spectrums’ of primary and secondary currents are compared. To ensure repeatability of measurement results with the usage of FFT algorithm, regardless of the beginning of the sampling process, the window of 16 fundamental periods consisting 4 dips/interruptions cycles is used. Additional CT's secondary current distortion caused by repeatable transients results in increased error of indirect measurement of rms values of power line's current higher harmonics and additional inaccuracy of power quality estimation.

This paper presents the performance of the 26 kVA inductive high-current transformer (HCT) during operation in the frequencies range of transformed harmonics from 50 Hz to 5 kHz. Performed research concerns the determination of the possibility of obtaining an order of the higher harmonic of a given RMS value in its distorted output current for the required RMS value of the main component and the maximum safe instantaneous value of the input voltage equal to 400 V. The results are presented for serial, serial-parallel and parallel configurations of primary and secondary windings (9 cases). Therefore, the most favourable configuration of the primary and secondary windings sections may be chosen. The tests are performed for the transformation of the distorted current containing a fundamental component and one higher harmonic of order from the 5th changed by the 5 up to the 100th. The constant 10% higher harmonic level in relation to the main component of the distorted secondary current is set. The measurements are performed for different resistances and inductances of the secondary winding’s load resulting from the length of the connected current track.

Voltage transformers (VTs) are an important element of the measuring system that allows measuring the energy flow in medium and high voltage networks. Additional problems with the accuracy of the measurement introduced by the appearance of sources and nonlinear receivers cause deformation of the voltage shape in the energy system. Due to the high metrological requirements, the design of voltage transformers requires high accuracy (for class 0.2 ΔU ≤ 0.2, phase displacement ≤10 min), which is not possible with the use of analytical methods using approximate models. Therefore, only the application of numerical modeling by the finite element method, taking into account real three-dimensional phenomena, allows achieving high modeling accuracy. The article concerns the phenomenon of the influence of voltage higher harmonics of supply voltage on the accuracy (up to the 100th harmonic) of the measuring inductive voltage transformer (IVT). The applied modeling method takes into account the phenomena in the transformer core and the circuit equations resulting from the winding arrangement, which allows for the study of the deformation voltage transformation. Experimental tests on a real model to evaluate the method used were necessary. The article presents simulations for a model transformer, and results have been confirmed by experimental tests.

Power grids are a combined system where the electrical energy produced by the power plants is transmitted to consumers. This forms a specific interdependence where the recipients have a significant impact on the power quality. Therefore, the nonlinear loads connected by households and industrial customers cause current and voltage distortion in the power networks. This creates the need for accurate measurement of nonsinusoidal voltage and current composed not only from the fundamental component but also containing higher harmonics, interharmonics, and subharmonics. In order to ensure high transformation accuracy of distorted current and voltage, the inductive instrument transformers have to be tested in these conditions. Many papers describe their behavior during the transformation of sinusoidal current or voltage. Nowadays, the scientific field in this scope is focused on the evaluation of their exploitation properties for distorted signals. The common problem of inductive instrument transformers is the self-generation of low-order higher harmonics to the secondary current or voltage. In the case of the inductive VTs, an additional problem results from the resonance caused by the parasitic capacitance of the primary winding. The proposed solutions to compensate for the values of current or voltage errors and phase displacement of inductive instrument transformers are also analyzed.

In this paper, we present an investigation into the influence of the load power factor of secondary winding on the metrological performance of inductive CTs with frequencies from 50 Hz to 5 kHz of the harmonic of a transformed distorted current. The results clearly indicated that the inductive load caused a deterioration in the transformation accuracy of the inductive CT. To ensure the most advantageous conditions of their operation, a resistive load should be used. The inductive CTs for the frequencies of the transformed harmonic of a distorted primary current from 50 Hz to 5 kHz may ensure the accuracy class designated for the transformation of the sinusoidal current of a frequency of 50 Hz with the same limiting values of errors. Moreover, an analysis of the generated low-order harmonics by a 300 A/5 A CT determined for the power factor of 0.8 inductive and 1.0 of the secondary winding was investigated. These results for the transformed distorted currents of 3rd, 5th and 7th higher harmonics were evaluated for a rated load and 25% of this value.

Current transducers/transformers have a significant impact on the overall accuracy of the measuring system used for electrical power metering and current measurement, especially in non-sinusoidal conditions. This research concerns an evaluation of the impact of the Rogowski coil and the combined transducer on the distorted current measuring accuracy of the digital power meter. It covers a characterization of their influence in conditions similar, at present, to the high RMS value of the distorted current in order to ensure the required test scenario for the equipment designed for the power network metering purposes. It has been established that the active component—the integration circuit (voltage-to-voltage converter or voltage-to current-converter)—has the most significant effect on the overall performance of the combined transducer. However, its impact is different depending on the selected equipment. Therefore, the presented analysis and results are essential for a conscious choice of the proper current transducer for the substation equipment.

This paper presents the evaluation of tested inductive CTs’ accuracy for distorted current harmonics in accordance with the optional accuracy class WB1 introduced by the new edition of the standard IEC 61869-1 published in the year 2023. The tests were performed in compliance with the interpretation sheet IEC 61869-2:2012/ISH1:2022. Therefore, the resistive and the resistive–inductive loads of the secondary winding of tested inductive CTs were used, as this was required for the given test conditions. The results indicate that the units designed for the transformation of a sinusoidal current of a frequency of 50 Hz ensure the high wideband transformation accuracy of the distorted current harmonics, as demanded by the power quality monitoring and distorted electrical power and energy requirements. The key to this is proper design using modern magnetic material(s) for the magnetic core and its oversizing in relation to the requirements for a given accuracy class defined for the transformation of sinusoidal currents with a rated frequency. Both tested inductive CTs with a rated primary current RMS value equal to 300 A, class 0.2 and 0.5, ensured compliance with the requirements of the WB1 wideband accuracy class.

In the case of the inductive voltage transformer (VT), the resonance phenomenon may be the main reason for its poor transformation accuracy of the non-sinusoidal voltage. This problem mainly results from the leakage inductance and the parasitic capacitance of its primary winding. The application of the sinusoidal voltage with a frequency from 20 Hz to 20 kHz presented in this study ensures proper identification of the resonance frequencies of the medium-voltage (MV) inductive VTs. The results are consistent with the values obtained in the reference condition at their nominal primary voltage. Therefore, it is proven that the proposed solution is effective in all cases. The influence of the main frequency variation of the non-sinusoidal primary voltage on the resonance properties of the inductive VT is also studied. Moreover, the tests indicate that the capacitance of the load of the secondary winding may cause a decrease in their resonance frequency.