Effects of Distorted Voltages on the Performance of Renewable Energy Plant Transformers

Abstract

The significant global growth in renewable energy production has led to increasing concerns about the problems associated with electrical equipment in power plants connected with this type of energy. The crucial electrical components of renewable energy generation are step-up transformers, with respect to which, gassing problems and premature insulation failures have been extensively reported in recent years. One of the factors related to the reported problems is the presence of high-frequency high-dV/dt voltages that are created by switching operations in wind energy plants. Addressing this challenge necessitates the investigation of transformer insulation systems under high-dV/dt pulse voltages. This thesis presents research undertaken in order to examine the performance of wind turbine step-up transformers under distorted voltages, with consideration of internal resonance phenomena and high-frequency dielectric effects. To evaluate the effect of distorted voltage on the acceleration of the ageing process in transformers, model transformers have been aged under distorted power converter voltage as well as under pure sinusoidal power grid voltage. Relevant parameters are monitored as indicators of the condition of the transformer insulation, and the results are compared throughout the ageing period. In addition, to study the transformer behavior under high-frequency high-dV/dt voltages, a detailed high-frequency model is necessary. High frequency modeling of large power transformers is complex and time-consuming due to their sizes. Therefore, as an alternative approach, this work proposes a modeling method that considers the high-frequency behavior of a scale down model transformer, and then relates it to the behavior of the actual size transformer. To verify the proposed modelling method, an experimental study investigates the correlations between the frequency responses of the two model transformers of different power ratings and sizes. Comparison of the frequency responses of the scaled and original transformers validates the proposed approach of scaling transformers for high-frequency study. High-frequency measurements are performed on an actual wind turbine transformer to represent a linear wideband black-box model of wind turbine step-up transformer in an electromagnetic transient study. Using the simulation results for switching impulsive waveforms imposed on wind turbine transformers due to the adjacent breakers operations, this work evaluates the effects of impulsive voltage parameters such as rate of rise and repetition frequency on inception voltage and intensity of partial discharge, generally assumed to be the main long-term cause of insulation deterioration. Partial discharge parameter measurement under high-dV/dt voltages is challenging due to interferences from fast oscillations, and difficulties of PD energy measurements. To avoid such issues, which are related to electromagnetic detection methods under pulse energization, this work uses a chemical approach to compare PD, based on the rate of hydrogen generation in a controlled test chamber with oil/paper samples. Gas monitoring of the oil containing impregnated paper samples show good linear correlation between the amount of hydrogen detected and PD energy level. Transformers installed in renewable energy plants require specific design considerations in order to protect them from the adverse effects of abovementioned voltage distortions. A number of manufacturers practice the implementation of electrostatic shields in transformer windings to filter the transferred voltages. Although this method has shown some improvements, effectiveness of the electrostatic shielding for a broad frequency range requires further studies. The effectiveness of electrostatic shielding in alleviating the transfer of high-frequency distortions from LV winding to HV winding and vice versa is evaluated with an experimental study. To compare the internal field enhancement at different frequencies in the presence and absence of an electrostatic shield, the frequency response of the voltage distribution inside the transformer’s winding is also measured and analyzed. Internal short circuit is one of the far-reaching incidents that has been recently reported for many wind-farm transformers. Detecting the location of an internal short circuit in a transformer winding is beneficial in improving future designs by defining the critical spots for target oriented insulation reinforcement. In an effort to identify trends with inter-turn fault locations and frequency responses, this research investigates the effect of the location of deliberately initiated internal faults on parameters such as transfer voltages and input impedance by means of frequency response analysis. Finally, alternative approaches are suggested for wind-farm design based on a method for recognizing the transformer compatibility with its surrounding devices such as power converters, breakers and cables.