In Europe and elsewhere in the world, distribution network operators (DNOs) are transitioning to substations that use digital communication. The outputs of measuring instruments used in digital substations are digital signals that are generated in accordance with IEC 61850-9-2 and must be metrologically checked. At the same time, there is no solution for the calibration of measuring instruments used in digital substations, and the testing and development of intelligent electronic equipment is therefore left without metrological control. With Metrosert’s participation, the three-year EURAMET/EPM project “Metrology for Digital Substation Instrumentation” (DIGITAL-IT) was launched in 2022, which relates to the calibration of measuring instruments used in digital substations. The development activities will make it possible to extend the capability of the existing reference standard of electric power to the calibration of measuring instruments with digital signals. The plan is also to extend the measuring range of the standard to 100 A, using precision current shunts.

The traceability of the results of measurements performed with the reference standard of the electric power unit is ensured by the calibration of measuring instruments included into the  sampling wattmeter at the Estonian national measurement standard laboratory and at primary level metrology institutes. The measuring system consists of four main components: analogue-to-digital converters (A/D converters): two multimeters which are equipped with a sampling function and which are used as A/D converters. Voltage dividers: nine voltage dividers designed and produced at Metrosert with a small AC-DC difference and phase shift at the nominal voltage of up to 1000 V. Current shunts: nine coaxially shaped current shunts at the nominal voltage of up to 20 A. The structure and choice of components of the voltage dividers and current shunts make it possible to reduce the transducers’ AC-DC difference, the phase shift, temperature and power factors. The data acquisition and processing software developed in Metrosert is based on the multiharmonic sine fitting algorithm. The components of the measuring system have been calibrated to the respective standards, using a step-up calibration procedure and a sampling method. The measuring capability of the sampling wattmeter was estimated using the Monte-Carlo method. In order to check the measurement capability of the sampling wattmeter, Metrosert participated in the international comparison Euramet.EM-K5.2018, the final report of which will be published in 2024.

Andrei Pokatilov
Andrei.pokatilov@metrosert.ee