Influence of instrument transformers on harmonic distortion assessment

Determination of harmonic emission levels is becoming a very important task in power distribution networks. Accurate measurements of harmonic voltages and currents are extremely important for correct evaluation of harmonic emissions. Voltages and currents in MV and HV networks are usually measured by the instrument transformers but the accuracy of these transformers for frequencies above their rated frequency is usually not known. The paper presents the behavior of the voltage and current instrument transformers at higher frequencies (above 50 Hz or 60 Hz). The harmonic magnitude and phase angle measurements errors are evaluated. At the end the impact of the measurements errors on the harmonic emission levels calculation is discussed.