Frequency response and harmonic distortion testing of inductive voltage transformer used for power quality measurements

International standards related to power quality measurements define methods and accuracies for the measuring instruments, but do not specify the accuracy of instrument transformers. Therefore, it is not possible to specify an overall accuracy for such measurements. Inductive voltage transformers (IVTs) which are used for power quality measurements should be tested to determine the ratio correction factors (RCFs) and phase angle errors at higher frequencies. In this paper, harmonic distortion and frequency response tests are performed to determine the level of harmonics generated by IVT and its ability to transform harmonics from high voltage (HV) to low voltage (LV) side. Complex voltages consisting of fundamental voltage and certain amount of superimposed harmonic are used to check the frequency response of IVT. RCFs and phase angle errors of the IVT are determined and can be applied to power quality monitors for compensation of errors that occur at high harmonic frequencies. Different test circuits are proposed for generation of HV consisting of fundamental voltage and harmonic voltages with amplitudes in range 5-15 % of the applied fundamental voltage. In order to improve the testing capabilities at higher voltage levels (for equipment with 123 kV≤Um≤420 kV), a compensation for both fundamental and harmonic voltages is proposed with a special connection through blocking and pass filters.