Date of Award

6-2023

Document Type

Dissertation

Degree Name

Ph.D.

Organizational Unit

Daniel Felix Ritchie School of Engineering and Computer Science, Mechanical and Materials Engineering

First Advisor

Maciej Kumosa

Second Advisor

Joseph Hoffman

Third Advisor

Paul Predecki

Fourth Advisor

Yun-Bo Yi

Fifth Advisor

David Gao

Keywords

Composites, Large power transformers, Materials, Reliability, Resilience

Abstract

Due to the current demands placed on the power grid in terms of climate change, increasing urbanization, and terrorist attacks, the U.S. government in response to these demands, mandated that all the grid components be modernized in order to increase their reliability. As a critical component of the grid, Large Power Transformers (LPTs) play a key role in ensuring sustainable power generation and distribution. A literature search performed in this work and the analysis of data retrieved from the search showed that the tanks of these LPTs are critical to their durability, longevity, and reliability. Therefore, the reliability of LPTs can be improved by the modernization of the LPT tanks by the replacement of the current LPT tank modern designs using other materials. This research established a material selection framework for selecting a suitable replacement material for Low Carbon Steel (LCS) which is presently been used in the current LPT tanks. The selection process favored two types of advanced Polymer Matrix Composites (PMCs): Glass Fiber Reinforced Polymer), and CFRP (Carbon Fiber Reinforced Polymer).

Numerical models were built and independently verified to examine the performance of the selected PMCs in ballistic simulations to compare their impact behavior against LCS. The results showed that a sandwich structure consisting of cross-ply GFRP and CFRP compares favorably or even be better than LCS in terms of its impact performance if the sandwich is strengthened with an elastomer such as polyurea (PU). To further improve the impact property of the sandwich structure developed in the ballistic study, a woven CFRP layer with graded crimping through the laminate thickness was proposed as a substitute for the CFRP cross-ply plate. This substitution was proposed because an additional numerical analysis using graded crimping of a plain weave composite made with CFRP yarns demonstrated noticeably improved properties compared to the same geometry and the number of plies of the unidirectional (UD) CFRP plate.

In furtherance of the study of the reliability of PMCs as a replacement material for LCS, the electrical behavior of PMCs in relation to hotspot generation in LPT tanks was studied. A comprehensive literature review was also performed to determine the trend of research outcomes on stray losses which are responsible for hotspot generation in LPT tanks. Subsequently, it was presented in this thesis that PMCs could have the capacity to mitigate or significantly reduce the volumetric loss identified as the heat generated in the material due to the alternating line current source imposed on it. GFRPs are classified as dielectrics therefore, they do not induce eddy currents and hotspots. On the other hand, CFRPs are conductive and could reduce the currents and hotspots in the tanks in comparison with LCS.

Numerical simulations of the current and potential new LPT tank geometries were also performed to evaluate the response of the LCS and PMC tanks to internal over-pressure to improve their mechanical reliability. The outcome showed that when the traditional rectangular LCS LPT tank was replaced with the combination of a capsular geometry based on PMCs, a significant increase in the safety factor was seen.

Finally, a cost analysis and decision-making study were carried out for three decision alternatives, which were LCS, CFRP, and GFRP, respectively. Five decision-making theories under uncertainty were used to find the best alternative under three conditions governed by the states of nature such as costs, demand, and the impact of the environment. The analysis recommended a decision that favored LCS as the most efficient material followed by GFRP.

Copyright Date

6-2023

Copyright Statement / License for Reuse

All Rights Reserved
All Rights Reserved.

Publication Statement

Copyright is held by the author. User is responsible for all copyright compliance.

Rights Holder

Babajide O. Williams

Provenance

Received from ProQuest

File Format

application/pdf

Language

English (eng)

Extent

239 pgs

File Size

4.5 MB

Discipline

Mechanical engineering

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