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Characterising Instrument transformers to develop new standards for more reliable power quality measurements

Renewable electricity can cause grid-injected disturbances, damaging power quality. Reducing network currents and voltages down to a level compatible with power quality measurement instruments requires the use of Instrument Transformers. However current international standards fail to fully account for instrument transformer performance in the presence of these disturbances, since they do not detail specific characterisation tests to quantify their uncertainty contribution, which are needed to perform accurate power quality measurements in electricity grids with a known level of confidence.

Completed EMPIR project Measurement methods and test procedures for assessing accuracy of instrument transformers for power quality measurements (19NRM05, IT4PQ) has defined accuracy and uncertainty limits for instrument transformer in power quality measurements, developed reference measuring systems and traceable test procedures and assessed instrument transformer performance with multiple influence factors. At the end of the project in 2023, recommendations were given to IEC TC38 to incorporate into new standards. Increased confidence in power quality measurements will allow electricity utilities and industries to take appropriate actions to help prevent or solve any power quality related issues.

First exploitations of the new wideband calibration techniques developed in the project are taking place.

Specific achievements from the project include:

• Frequency characterisation of both traditional inductive voltage and current transformers and low power instrument transformers have been carried out for instrument transformer manufacturers. The characterised instrument transformers will be used for wideband measurement in distribution grids.
• The traceable characterisation of an industrial voltage transformer under real harmonic distorted primary waveforms, reproducing specific laboratory test conditions, was studied. A simplified circuit was used in the premises of a high voltage instrument transformer manufacturer, partially employing the existing laboratory measurement systems and using reference sensors calibrated by comparison with those, developed within the project.
• Ten datasets relevant to publications, one dataset of measurement data vs test conditions relevant to transformer performance characterised in the presence of separate and combined influence factors were produced and made available on the project Community Zenodo repository. Indications and good practice guides were produced and made available to IEC TC 38 Instrument Transformers and in particular to its Working Group 47 which is focused on the evolution of transformers requirements for the modern market.
• Exploitation of simplified calibration setups for combined instrument transformers was demonstrated on the premises of two manufacturers laboratories by re-establishment of their existing individual current and voltage transformer test setups for combined use.
• An overall picture of the achievements of this EMPIR project was presented at the 27th CIRED 2023 the International Conference on Distribution Grids.

The project outputs in terms of reference systems and simplified test procedures, as well as measured data made available on instrument transformers, are supporting manufactures of transformers and test systems, as well as test and calibration laboratories in extending their products and provided services to power quality measurement. Traceability and adoption of common and standardised instrument transformer test procedures will ensure both grid operators and transformer manufacturers to operate in fairer market conditions.

Project coordinator Gabriella Crotti from INRIM said

‘The joint effort of project partners from National Metrology laboratories, Academia and a Research centre, with complementary expertise, and the fruitful interaction with the relevant IEC TC 38 working groups were the basis for the success of IT4PQ. By the extensive calibration campaign of off-the-shelf sensors, carried out by the developed reference systems, we got a picture of the accuracy of the different sensors under PQ phenomena. This, on one side increases operators’ confidence in the use of the more recent wideband current and voltage sensors, and on the other enables a more informed utilisation of the grid widely installed inductive instrument transformers. Their specific limit performances as a function of the frequency can support the adoption of more specific criteria for accuracy classification of ITs for PQ measurement in distribution grids. This will benefit, besides instrument transformer manufactures and test laboratories, also distribution grid operators and industry users in terms of a more faithful evaluation of grid disturbances, and timely identification of critical situations.’

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States