Characterization of Voltage Instrument Transformers Under Nonsinusoidal Conditions Based on the Best Linear Approximation

Voltage instrument transformers are usually tested at the rated frequency. In order to assess their performance in measuring harmonic components, typically, the frequency response function (FRF) is evaluated. Therefore, this conventional characterization does not consider nonlinear effects that may have a nonnegligible impact on the accuracy, especially when the transducer under test is represented by an inductive voltage transformer (VT). In this paper, a simple procedure for the characterization of voltage instrument transformers is presented. The method is based on the concept of best linear approximation of a nonlinear system. It requires applying a class of excitation signals that resembles the typical voltage waveforms found in power systems. Results consist of the FRF that permits the best linear compensation of the transducer response, and sample variances that allow quantifying the impact of noise and nonlinearities on the accuracy. The method is presented and explained by means of numerical simulations. After that, it has been applied to the characterization of a conventional inductive VT. Experimental results show how the accuracy of the transducer under test is heavily degraded by nonlinear phenomena when low-order voltage harmonics are considered.