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.

No abstract

Instrument transformers (ITs) are vital components of power quality (PQ) measurement systems in electrical transmission and distribution grids. Whereas measurement methods and accuracy are well defined for PQ measurement instruments, accurate knowledge about IT performance in presence of grid disturbances is missing. Main research themes of a project recently funded within the EMPIR program are described in the paper. The project focuses on the set-up of a metrological framework to enable the traceable measurement of PQ parameters when ITs are included in the measurement chains. Attention is focused on the development of reference measurement systems and the definition of IT test procedures, as well as on the quantification of the performance of ITs under realistic disturbance conditions. Moreover, an extensive characterization of voltage and current ITs will provide the basis for the identification of different sets of limits for the IT accuracy performance in PQ application and the definition of PQ accuracy classes.

No abstract

A practical method with high accuracy in generation and application of error values for calibration of current transformer test sets is described. A PC-controlled three-phase power source with a standard wattmeter is used for generating the nominal and error test currents while an electronically compensated current comparator is used to provide summation and subtraction of them, precisely. With this method, any ratio error and phase displacement values could be generated automatically and nominal and test currents could be grounded on the test set safely. Because of its high accurate ratio and phase error generating capability, any type of test set regardless of its operating principles could be calibrated.