Date of Award

6-2021

Degree Name

Doctor of Philosophy

Department

Electrical and Computer Engineering

First Advisor

Dr. Pablo Gomez

Second Advisor

Dr. Richard Meyer

Third Advisor

Dr. Damon Miller

Keywords

Optimal design, transformer design, fast electromagnetic transients, insulation

Abstract

Power transformers are essential electrical power system devices. Using the principle of electromagnetic induction, they can transfer electric energy between two otherwise isolated circuits, as well as reduce or increase voltage levels without modifying the frequency. This allows reduction of power losses when electric energy is transferred between power plants and consumers. Because of their importance and widespread use, transformers failures can have a severe economic impact due to the temporary interruption of electric service.

The insulation design of power transformers is performed by considering the maximum values of voltage or current to which the device will be exposed during operation. Traditionally, this is done by analyzing the transformer behavior when a fast front voltage excitation, emulating the characteristics of lightning or switching, is applied across one of its terminals and ground. Due to the increase of electrical energy produced by renewable energy sources to reduce the environmental impact of traditional fossil fuels, transformers are also exposed to fast and repetitive voltage pulses created by power electronic converters used in solar plants, wind farms, and other types of distributed energy resources (DERs). The continuous exposure to these pulses can result in partial discharges, high electrical stresses, heating, and premature aging of the transformer insulation system. For this reason, it becomes necessary to develop new design techniques that consider these conditions to improve the lifespan of electrical equipment and to assess its normal operation once connected to the system.

In this dissertation, a procedure to find an optimized dielectric design of the insulation system of a transformer subjected to fast front voltage pulses created by power electronic converters is presented. The optimization procedure is based on the interaction between three main elements: the Final Element Method, a frequency-domain distributed-parameter winding model, and a single-objective optimization algorithm. To validate results, values of dielectric stress before and after optimization for three simplified cast-resin dry-type transformer winding configurations were compared: two-layer winding (Case 1), three-layer winding (Case 2) and fourteen-layer winding (Case 3). The results show an overall decrease of 3.32% and 12.73% of dielectric distances for cases 1 and 2, respectively; and an overall increase of 29.14% for Case 3; nevertheless, in all cases the optimization process ensured that the values of dielectric strength for the insulation materials were never surpassed, which substantially lessens the possibility of premature damage or failure of the device due to dielectric breakdown when compared to non-optimized designs.

Access Setting

Dissertation-Open Access

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