PhD defence - Mahdi Khanali

Candidate

Mahdi Khanali

Title

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

Supervisor

Sheshakamal Jayaram

Abstract

Gassing problems and premature insulation failures of wind turbine transformers have been documented lately, indicating concerns with insulation reliability of such power electronic converter-fed devices. Due to relative young age of the majority of type-IV wind turbine generators, there is yet little published about the associated issues. This research investigates the effects of distorted voltages on wind turbine step-up transformer with an insight to internal resonance and high-frequency dielectric effects. Model transformers are aged under converter PWM and power grid sinusoidal voltage to evaluate the effect of distorted voltage on transformer. In this regard, parameters are monitored as indications of transformer insulation condition and compared through the ageing period. To study transformer behavior under high-frequency high-dV/dt voltages, a datailed high-frequency model is required. This work proposes a modeling approach that considers the high frequency behavior of a scaled-down model transformer, and then relates it to the behavior of full size transformer. To verify the proposed modeling method, an experimental study investigates the correlations between the frequency responses of the two model transformers with different power ratings and sizes. To analyze the switching transients imposed on wind turbine transformers due to the operation of the adjacent vacuum circuit breakers, a linear wideband black-box model of an actual wind turbine transformer is prepared to represent wind turbine step-up transformer for a system study in PSCAD. Based on the simulation results for impulsive voltage oscillations at transformer terminals, an experimental study investigates effects of impulse voltage characteristics such as rate of rise and repetition frequency on PD activities in oil impregnated paper samples. To avoid interferences from fast oscillations and other electromagnetic related issues, this work uses a chemical approach to compare PD activities, based on the rate of hydrogen generation in a controlled test chamber containing oil/paper samples. Finally, installation of electrostatic shield for improvent of transformer high-frequency performance is analyzed; and alternative approaches are suggested for target-oriented insulation reinforcement based on fault location study and transformer compatibility.