In 1999, the national AC power standard at PTB is based on a fully synchronised generation and sampling-based measurement core for power frequencies. It covers nominal voltages from 60 V to 240 V and nominal currents from 100 mA to 10 A, while the standard exhibits an expanded measurement uncertainty of 5 μW/VA at 120 V and 5 A. However, after nearly 25 years of use, a modernisation effort is necessary as some of the electronic components of the system are becoming obsolete. As part of this optimisation, new voltage and current sensors with extended ranges, along with an optimised sampling core, have been developed. This results in a reduction in overall attainable uncertainty.

This paper deals with aspects relevant to setting up a current-to-voltage converter, based on a compensated current transformer (CT) with a temperature-controlled, active measuring resistor for use in the alternating current (AC) power standard at PTB. The attained errors of the CT are in the range of 10−7. It allows the measurement uncertainty of the current path of the AC power standard to be reduced by a factor of around three.

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.

Power Quality (PQ) monitoring in electrical transmission and distribution grids relies on the accurate measurement of wideband high voltages and high currents. Ensuring traceability of the measurements carried out is generally only partially accomplished by calibration of the low voltage (LV) PQ measurement instrument, whereas the calibration of the instrument transformer (IT) is mostly limited to the power frequency or performed at LV. In the paper, objectives and main outputs of the research project “Measurement methods and test procedures for assessing accuracy of Instrument Transformers for Power Quality measurements” (IT4PQ), which is focused on medium voltage (MV) grids, are outlined. An overview of the metrological framework developed within IT4PQ is given, together with details of the characterization of different types of MV ITs, both under controlled laboratory conditions and in the presence of influence parameters. Laboratory calibration measurement uncertainty is within a few hundreds of ppm and microradian up to 9 kHz, for ratio and phase errors respectively.

In this paper, three different approaches to precision measurement of high AC voltage are described and compared. These approaches also include the metrological traceability of this measurand. These are traceability via the measured capacitance, current and voltage. As these different approaches all aim to preserve and pass on the measurand “high AC voltage”, they are compared in their results and complexity after the respective traceability. The comparison shows agreement below the PTB's measurement uncertainty of ±50 μV/V. This result leads to a confirmation of all three methods and is a foundation for future work towards a reduction of the best possible measurement uncertainties as well as a simplification and automation of the traceability.

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.

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 A/A and 0.1 mu rad.

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).

This paper introduces the reference measurement systems for calibrating instrument transformers under power quality phenomena and their associated novel uncertainties. The systems are lately developed at the national metrology institutes in European counties within the frame of the European project 19NRM05 “IT4PQ - Measurement methods and test procedures for assessing the accuracy of instrument transformers for power quality measurements”. It's worth noting that the setups, as presented in the paper, are an improved extension of the system already developed in terms of generated waveforms and system characterization level. Especially, the detailed system setups and methods for combined instrument transformer calibrations are debuted. Moreover, new characterization tests were performed by using all the presented reference measurement systems.

The accuracy of power quality measurements relies on the use of common measurement procedures and traceable measurement systems, which necessarily include instrument transformers. The paper provides an overview of the progress achieved within the EU project 19NRM05 IT4PQ in developing the needed metrological framework, in terms of performance indexes to qualify the instrument transformers for PQ measurements, simplified testing procedures and set-ups, and quantification of their behaviour under multiple influence factors.

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 four 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 $0.5\times 10^{-6}$ 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 $20~\mu \text{V}$ /V for ratio error and $20~\mu $ rad for phase displacement.

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 reference system for current sensor calibrations at power frequency and for wideband frequencies. The reference system is lately used for researches at PTB in the frame of two European projects: "IT4PQ - Measurement methods and test procedures for assessing the accuracy of instrument transformers for power quality measurements" and "WindEFCY - Traceable mechanical and electrical power measurement for efficiency determination of wind turbines". The relevant standards of IEC61000 were used for the power quality related tests. The proposed calibration system was validated for current sensor calibrations with the expanded uncertainties (k = 2) below 20 μA/A and 10 µrad at 50 Hz up to 5 kA and 0.01 % and 0.04 crad for the frequencies up to 9 kHz. Additionally, a centring unit for the Rogowski coil improved the measurement uncertainty 10-fold.

This is dataset for paper published: Crotti, Gabriella, Yeying Chen, Huseyin Çayci, Giovanni D’Avanzo, Carmine Landi, Palma Sara Letizia, Mario Luiso, Enrico Mohns, Fabio Muñoz, Renata Styblikova, and Helko van den Brom. 2022. "How Instrument Transformers Influence Power Quality Measurements: A Proposal of Accuracy Verification Tests" Sensors 22, no. 15: 5847. https://doi.org/10.3390/s22155847 Excel file provides data in the time domain for tests performed on the inductive VT

Extensive integration of renewables in the electrical energy distribution system is essential for the implementation of “green growth” strategies. However, it contributes to the degradation of the quality of the electrical power, as such power quality (PQ) monitoring holds greater and greater importance. Currently, there are no standards related to the characterisation of the instrument transformers (ITs) for PQ measurements, even if ITs can introduce significant errors in the PQ measurement chain. Therefore, a project has recently funded to investigate gaps in accuracy assessment and characterisation of ITs when used in PQ measurements and to work out specific performance indices, reference measurement systems and test procedures to evaluate the ITs accuracy and uncertainty contributions in PQ measurements. Guidelines and recommendations for the calibration of ITs used in PQ measurements will be provided to the Technical Committee IEC TC 38.

This paper describes the setup of a traceable calibration system for conventional or non-conventional current sensors even with digital output. The system is lately developed at PTB within the frame of the European project “FUTUREGRID II”. Details of the system components are presented. The absolute phase errors of the two-channel generator related to the pulse per second time reference of the global positioning system can be configured to almost zero. The accuracies of the reference current transformers are within ±10 μA/A and μrad at power frequency. The sampled value receiver box is firstly validated for the sample rate of 4 kHz according to the IEC standard 61869-9 [1].

Within the frame of the European project “Future Grid II – Metrology for the next-generation digital substation instrumentation”, several partners developed traceable calibration systems which allow the calibration of conventional or non-conventional instrument transformers (IT) even with a sampled value (digital) output according to IEC 61869-9. Different setups are prepared to allow the calibration with complex test waveforms to emulate steady state, dynamic or temporary events during the assessment of the ITs. The laboratory calibration setups for either current transformers or voltage transformers are briefly described. Several results obtained for different kind of voltage or current transformers are presented.