Behavior of instrument transformers under distorted conditions and corresponding compensation strategy

This thesis comes as the final report of a three-year Ph.D. program in Electrical Engineering carried out at Politecnico di Milano, Milan, Italy. The aim of my research activity described in this thesis is to contribute to the subject of characterizing the behaviour of instrument transformer under distorted input condition or real working condition, in particular, focusing on the aspect related to the harmonic measurement. The justification of this research is described following. As more and more nonlinear loads and renewable energy sources have been connected to the grid, the topic of monitoring harmonics in the power system attracts more attention; in other words, an accurate measurement of power system harmonics using the already installed instrument transformer (IT) is needed. However, most researches and standards in the past proposed to characterize an IT only considering its behaviour at the fundamental frequency or, in the advanced cases, treating the IT as a linear device and adopting the frequency sweep as the method for IT characterization over frequency. Nevertheless, if we consider the transformer behaviour, when harmonics are present in the input signal, the transformer study based only on the fundamental frequency is not sufficient to describe its behaviour at higher frequencies as well as, being the transformer a nonlinear device, its behaviour cannot be fully interpreted by the sinusoidal frequency response. In fact, the harmonic measurement is very useful, which can help to improve the power quality since its measurement result provides useful information for harmonic elimination approaches like the active power filter. In this respect and considering, in particular, the voltage measurement transformer (VT), the main goal of this work is the proposal of an accurate transformer model capable of describing the device in terms of its linear and nonlinearity behaviours. In the meantime, a measurement setup has been developed. Thanks to its software-based controller, this setup is able to test the transformer under the discussed condition, process the experiment data, and calculate the proposed model automatically. Finally, the proposed model has been validated on different transformers through experiments. In addition, we designed other experiments in order to extend the application of this model, which can demonstrate its function. This thesis is organized in the following structures. Chapter 1 covers a preliminary introduction of the voltage transformer in terms of working principle, parameters of interest, and the classical frequency response approach of characterizing a VT. Then, we introduce the transformer nonlinearity and its influences on the measurement of harmonics in chapter 2. In chapter 3, we start with a simple review of the commonly used transformer nonlinear models. After that, the Volterra series method is introduced. Based on the Volterra series, we propose a simplified model for the VT; in particular, the assumptions applied for the coefficient reduction have been carefully discussed in this section. Chapter 4 presents the experiment setup used in this thesis. This setup is flexible that can support experiments at both the medium and low voltage level. Refer to the setup designed for medium voltage; a simple compensation method is presented in order to reduce the harmonics amplitude introduced by the additional step-up transformer. Finally, in chapter 5, the proposed model is firstly validated by experiments and then applied to three different applications to prove its effectiveness and improvement with respect to the traditional frequency response method. Finally, some applications are deeply discussed. In particular, they are the measurement of harmonics of the primary side, the identification of an inner-turn fault, and the measurement of the DC-bias in the transformer.