Network operators have to conduct power quality (PQ) measurements in medium voltage grids to verify compatibility levels of disturbances such as harmonics.

Network operators have to conduct power quality (PQ) measurements in medium voltage grids to verify compatibility levels of disturbances such as harmonics.  Instrument transformers, optimised for 50 Hz use, need to be characterised to find the ratio and phase errors at various frequencies across the desired measurement range.  In some cases, resonances at certain frequencies can introduced huge errors in ratio and phase.  Extensions for PQ measurement to frequencies up to 9 kHz further compound the need for traceability for these ITs.

This leads to a need to define accuracy and uncertainty limits of voltage and current Instrument Transformers (ITs) used for Power Quality (PQ) measurements (i.e. for the measurement of harmonics, total harmonic distortion, and other PQ disturbances) in distribution grids, considering the high number of installed VTs and the grid complexity, with rated voltage up to 36 kV and currents up to 2 kA. Voltage Waveforms with frequency spectrum up to 9 kHz are considered in the analysis.

To this end, suitable performance indices, which quantify the IT errors in the measurement of the single PQ parameter, need to be defined as well as an overall ‘aggregated’ performance index that can directly qualify the ITs performance in PQ measurements. Definition of different classes of limits for this synthetic index, based on a deep knowledge and classification of the errors introduced by the ITs when measuring the PQ parameters, is required.

The introduction of this synthetic index and its associated accuracy limits allows the extension of the concept of accuracy class, presently defined in the relevant IEC 61869-X standard family as the permissible percentage voltage and current errors for rated voltage, and the introduction of a new, PQ specific, accuracy class index.

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