Overcoming Frequency Response Measurements of Voltage Transformers: An Approach Based on Quasi-Sinusoidal Volterra Models

Abstract

The most recent IEC standards about voltage transformers warn about nonlinearity, which may have significantly impact on the harmonic measurement performance. However, there is a lack of scientific literature about this topic: usually their characterization consists of frequency response measurements, which are clearly not able to capture the nonlinear behavior.In this paper, an innovative approach based on simplified frequency domain polynomial models developed by the authors is proposed. The method is applied to two different medium voltage inductive transformers. Models are identified and validated with a large set of realistic primary waveforms injected by a proper setup. Experimental results confirm the remarkable accuracy of the proposed models especially for low-order harmonics, which are the most affected by nonlinearity.

A Simple Method for Compensating Harmonic Distortion in Current Transformers: Experimental Validation

“…Similar to the case of CTs, also the metrological performance of conventional voltage transformers (VTs) employed for harmonic measurement is strictly related to their dynamic and nonlinear behavior [ 23 ]. In this respect, the authors of the present paper have shown that nonlinear behavioral models based on a simplified Volterra representation [ 24 , 25 ] can be used to accurately predict the secondary voltage spectrum from the primary side [ 26 ]. This enables a thorough characterization of VTs: linear and nonlinear effects can be separated, nonlinearities can be classified, and their individual impact on the overall metrological performance can be studied.…”

Extended SINDICOMP: Characterizing MV Voltage Transformers with Sine Waves

“…The metrological performances of these sensors in presence of harmonic distortion significantly depend on their operating principles. In the particular case of the frequently used inductive instrument transformers (ITs), not negligible ratio and phase errors can be introduced both at low and high frequency harmonics [4][5][6][7][8][9][10][11].…”

“…In [5,6,8,9] the ITs characterizations performed under distorted conditions have highlighted errors up to some percent for the first 10-15 harmonics. In [10], some of the authors have shown that the current transformers (CTs) ratio and phase errors at power frequency can increase up to exceed their accuracy class limits in presence of modulated current signals.…”

“…In many cases, the ratio and phase errors of the instrument transformers are evaluated by carrying out a frequency sweep at low voltage under sinusoidal supply [12][13][14][15][16], not considering the non-linearity of the device, which can lead to additional errors of the order of the percent. Alternative approaches have been proposed, reproducing more realistic situations; for example, in [4,[7][8][9]17] the ITs are characterized by applying a bi-tone signal composed of a fundamental tone at rated voltage and a superimposed harmonic tone, whereas in [5,6,8,9] the ITs are tested under signals composed by a fundamental at rated voltage plus a certain number of superimposed harmonics.…”

“…Some papers in literature face the issue of compensation of nonlinearity of VTs and CTs. For example, in [6] the proposed compensation method is based on the simplified Volterra method, in [8] the CTs nonlinearities are compensated by analyzing the CT secondary-side current waveform whereas the technique presented in [9] requires the use of the Frequency Coupling Matrices. Some of the authors have already presented a simple technique for characterization and compensation of VT non-linearity called SINDICOMP (SINusoidal characterization for DIstortion COMPensation) [7].…”