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 paper describes the design and setup of different measuring systems for the calibration of both voltage and current transducers which are recently developed at VSL, PTB, and TUBITAK. The output levels of non-conventional transducers are adapted to traditional levels using step-up and step-down converters so that they can be measured using traditional measurement bridges. Verification and performance of a recently developed commercial transformer test set is investigated. The reference measuring systems are developed within the frame of the European project “FUTURE GRID - Non-conventional voltage and current sensors for future power grids”.

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).