Overview

Extensive integration of renewables in the electrical energy distribution system is essential for the implementation of  the EU objective of net-zero greenhouse gas emission by 2050. However, this integration 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, this project aims to develop specific performance indices, reference measurement systems and test procedures to evaluate the ITs accuracy and uncertainty contributions in PQ measurements (PQM). Guidelines and recommendations for the calibration of ITs used in PQM will be provided to the Technical Committee 38 of the International Electrotechnical Commission (IEC TC 38 Instrument transformers).

Need

Proliferation of disturbances on the electrical transmission and distribution grids implies degradation of the quality of the delivered power. It is estimated that poor PQ costs; both industry and commerce in the EU; about €150 billion annually. Henceforth, there is a need to address the unavoidable task of monitoring PQ.
Effective PQ monitoring requires accurate measurement instrumentation. When PQ measurements are performed in transmission and medium voltage distribution grids, ITs have to be included in the measurement chain to reduce the grid high voltages and currents to low levels, compatible with the PQ measurement instrument inputs. At present, international standards deal only with limits for public distribution system disturbances, measurement methods for PQ phenomena, and accuracy requirements and test methods for measurement instruments, without considering ITs. On the other hand, standards for ITs are mainly focused on accuracy verification in the absence of PQ disturbances, i.e at 50/60 Hz, or only provide accuracy limits for harmonic components. At present, no National Metrology Institute (NMI) presently declares calibration capabilities of ITs for PQ.
Furthermore, there is no shared and common method that can be adopted by the interested community (grid operators, instrument manufacturers, test and calibration laboratories) to characterise ITs and estimate their uncertainty contribution in PQ measurements. There is a need for undertaking pre-normative research to fill these gaps, as highlighted by IEC TC 38 “Instrument Transformers”.

Objectives

The project aims at developing of the metrological framework for the traceable calibration of ITs employed for PQ measurements in electricity grids, that which can be used as a basis for relevant future standardisation.The specific objectives of the project are:

• To define accuracy and uncertainty limits of ITs in PQ measurements, by introducing performance indices for the single PQ parameter and by defining a new “PQ Accuracy Class” for ITs, as an extension of the concept of classical “accuracy class”.
• To establish suitable reference measuring systems for ITs and methods for the evaluation of the relevant uncertainty contribution of ITs to PQ indices.
• To establish traceable test procedures for reference setups to calibrate ITs used for PQ measurements in electricity grids by covering limits for PQ disturbances in the available standards.
• To evaluate performance of ITs in PQ measurements in presence of multiple influence factors (e.g. temperature and temperature gradients, adjacent phases, proximity effects, vibrations).
• To contribute to a revision or issues of  new reports and standards of the IEC 61869 family product (e.g. 61869-1, 61869-6, etc.) by providing the IEC TC 38 Instrument transformers data, methods, guidelines and recommendations, necessary for the calibration of ITs used in PQ measurements.