Enrico Mohns

For the setup of wideband power calibration systems at PTB and at INMETRO, two wideband multi-range voltage dividers that integrate (remotely) selectable voltage ranges from 60 V to 480 V, have been build up. The design is based on a phase-compensated, buffered, resistive voltage divider, in which the effect of stray capacitances within the high voltage arm has been minimised. The basic errors at 50 Hz show that all ranges are within ±60 ppm and ±5 µrad. The frequency-dependent amplitude errors in all four voltage ranges up to 100 kHz are practically identical and below 500 ppm. The phase errors up to 100 kHz follow a simple and almost linear phase characteristic with frequency, described by time constants with values within ±5 ns for each range. It could be shown that the frequency responses do not change with the applied input voltage. Self-heating effects, observable in the ratio error have been minimised by a fan to be smaller than 10 ppm at 480 V.

This article presents a step-up method for calibrating a set of error compensated current comparators (CCs) for currents from 4 A to 60 kA from the Physikalisch-Technische Bundesanstalt (PTB)’s instrument transformer laboratory for the use as reference standards for calibrating current transformer (CT). The method is based on some hypotheses and previous knowledge about the error characteristics of a current comparator of the set. Extensive measurements between the CCs confirm the hypotheses in the order of 10−7 and show for the CCs that their ratios and phase errors and corresponding uncertainties are also in the range of $0.1 ~\mu \text{A}$ /A and $0.1 ~\mu $ rad.

Instrument transformers (ITs) play a key role in electrical power systems, facilitating the accurate monitoring and measurement of electrical quantities. They are essential for measurement, protection, and metering in transmission and distribution grids and accurately reducing the grid voltage and current for low-voltage input instrumentation. With the increase in renewable energy sources, electronic converters, and electric vehicles connected to power grids, ITs now face challenging distorted conditions that differ from the nominal ones. The study presented in this paper is a collaborative work between national metrology institutes and universities that analyzes IT performance in measuring distorted voltages and currents in medium-voltage grids under realistic conditions. Both current and voltage measuring transformers are examined, considering influence quantities like the temperature, mechanical vibration, burden, adjacent phases, and proximity effects. The study provides detailed insights into measurement setups and procedures, and it quantifies potential errors arising from IT behavior in measuring distorted signals in the presence of the various considered influence quantities and their combinations. The main findings reveal that the temperature has the most evident impact on the inductive voltage transformer performance, as well as the burden, causing significant changes in ratio error and phase displacement at the lower temperatures. As for low-power ITs, establishing a priori the effects of adjacent phases and proximity on the frequency responses of low-power ITs is a complex matter, because of their different characteristics and construction solutions.

A two-stage voltage transformer (VT) with several rated transformer ratios and with a rated maximum voltage of 40 kV and a rated minimum voltage of 5 kV has been designed. To guarantee high accuracy over the full medium voltage range, the primary winding has been split into 4 primary windings, which can be configured in three different series and parallel connections. With five windings of the secondary, 15 different voltage ratios can be accomplished. This connection scheme, which is already used in older commercial standard VTs of the error class 0.02, achieves a very low effective output resistance, which in turn results in very small additional errors of less than 0.5·10 ^-6 due to the loading by the input of the transformer bridge. According to the measuring results, for all the 15 different ratios, the error is less than 20 μV/V for ratio error and 20 μrad for phase displacement.

Main text: A comparison of high voltage transformer measuring systems of TÜBİTAK UME (Türkiye) and PTB (Germany) was conducted between August 2021 and August 2022. The comparison was registered as EURAMET Project No: EM 1492 and EURAMET.EM - S43 in the BIPM key comparison database. Ratio error and phase displacement of the voltage transformers were measured at voltages from 3 kV to 110/√3 kV, at 50.2 Hz and 60 Hz. The main targets of the comparison were to underpin and strengthen the capabilities of the National Metrology Institutes. The results of the participants were found to be equivalent, comparable and in agreement with the comparison reference values within stated uncertainties of measurements. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/. The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

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.

Sampling-based calibration systems for calibrating “Sampled Value” (SV)-based instruments for substation automation require synchronised and time-aligned sampling processes. As the signal frequency of the power grid is always asynchronous to the standardised sampling frequencies according to IEC 61869-9, the sampled waveforms of the calibration system and of the SV-based device under test can be resampled to be synchronised and to allow better accuracy in the following measurements based on the Discrete Fourier Transform (DFT) of the resampled waveforms. The paper presents simulations and results for different resampling algorithms. A modified sinc interpolation method with a finite impulse response (FIR) is presented. The deviation of the results for the root mean square (RMS) and phase angle is in the order of 10⁻⁸V/V (or rad) for normalised frequencies of up to 20% of the sampling frequency. No practical degradation in the presence of noise and harmonics could be observed. In addition, laboratory experiments demonstrate the realization of the proposed resampling process in the future SV-based calibration systems for SV-based instrumentation.

This paper presents the new fundamental step-up method for standard voltage transformers (VTs) at the Physikalisch-Technische Bundesanstalt, as well as the setup of the required components. A main component is an active low-voltage capacitor, designed to realize capacitive voltage dividers by means of compressed gas, high-voltage capacitors for rated primary voltages between 5 kV and 800 kV/√ 3. This divider has a negligible voltage dependence and low phase errors at 50/60 Hz. Using the other components, mainly a two-stage VT with integrated inductive voltage divider and a sampling-based voltage comparator, a transfer uncertainty for one build-up step of below 1· 10⁻⁶ (k = 2) has been reached.

State-approved test centres in Germany supplying accuracy tests for instrument transformers must, in the future, provide measurement uncertainty budgets for their quality management systems. In this work, the cause of ratio error and phase displacement of instrument transformers are therefore discussed. The traceability to the national standards of PTB, the attainable uncertainty, and the permitted error limits of test equipment for testing instrument transformers are presented. Finally, an example of an uncertainty budget for a current transformer of the class 0,2 S is given.