Low cost procedure for frequency characterization of voltage instrument transformers

The paper introduces a simplified method for the frequency characterization of instrument transformers for transmission and distribution grids. The proposed method makes extremely easy and affordable the MV instrument transformers frequency characterization. It involves, in the first step, an evaluation and a compensation of the non-linearity introduced by the voltage transformer when it is supplied with a 50 Hz sinusoidal input at rated value. Then, the voltage transformer is characterized with a low voltage sinusoidal signal from the second harmonic frequency up to the first resonance frequency. In this way, its approximated frequency behavior is obtained by compensating the non-linearity errors at the lower harmonics and using a piecewise curve at the higher frequencies. The proposed approach is applied to the evaluation of the ratio errors of a commercial voltage transformer up to the first resonance. Significant improvement of the VT performance is obtained, compared to the use of a frequency dependent ratio obtained from the low voltage characterization.