Current Transformers Effects on the Measurement of Harmonic Active Power in LV and MV Networks

The harmonic active power is used to determine the location of polluting loads, the direction of harmonic power flow and to estimate how consequential a certain current harmonic source is. The highest uncertainties, when measuring harmonic active power, are introduced by the current and voltage transducers. This paper presents experimental results which show the impact of current transformers (CTs) on the accuracy of the harmonic power measurements. An expression for the evaluation of the power error in distorted conditions was developed which can be easily utilized in the case of linear current transducers. It is shown how CTs cannot be considered linear transducers and how the higher the absolute value of the CT phase angle error is, the higher becomes the harmonic power error. The harmonic power error reaches maximum value when the phase shift between the voltage and current harmonic nears +-90°. Even high accuracy class CTs may not be adequate enough to assist on harmonic power measurement, while linear current transducers will perform satisfactorily.