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Ageing in Epoxy Resin As a Precursor to Electrical Treeing

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Ageing in Epoxy Resin As a Precursor to Electrical Treeing Ageing in Epoxy Resin as a Precursor to Electrical Treeing A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering 2020 Harry McDonald School of Engineering Department of Electrical and Electronic Engineering Table of Contents Abstract ................................................................................................................................................. 11 Acknowledgements ............................................................................................................................... 13 1. Introduction .................................................................................................................................. 14 1.1. Cables .................................................................................................................................... 14 1.1.1. Cable Background ......................................................................................................... 14 1.1.2. XLPE Cables ................................................................................................................... 16 1.1.3. Electrical Tree Formation .............................................................................................. 17 1.2. Electrical Treeing Background ............................................................................................... 18 1.2.1. Stages of Electrical Treeing ........................................................................................... 18 1.2.2. Standard Experimental Design ...................................................................................... 19 1.2.3. Concerns regarding the needle-plane methodology .................................................... 20 1.2.4. Electrical Treeing In Different Materials ....................................................................... 21 1.3. Tree Initiation ........................................................................................................................ 22 1.3.1. What is Tree Initiation? ................................................................................................. 22 1.3.2. Incubation Period .......................................................................................................... 22 1.3.3. Tree Initiation Mechanism ............................................................................................ 29 1.3.4. Impulse Treeing ............................................................................................................. 30 1.4. Tree Growth .......................................................................................................................... 31 1.4.1. Tree Shape .................................................................................................................... 32 1.4.2. Bifurcation (Branching) ................................................................................................. 32 1.4.3. Stages of Tree Growth .................................................................................................. 33 1.4.4. Growth Mechanisms ..................................................................................................... 34 1.4.5. Electromechanical Fracturing ....................................................................................... 35 1.5. Aims and Objectives .............................................................................................................. 37 1.6. Thesis Summary .................................................................................................................... 38 2. Plane-Plane Sample Testing .......................................................................................................... 39 2.1. Fine Tree Initiation Testing ................................................................................................... 39 2.1.1. Introduction .................................................................................................................. 39 2.1.2. Background ................................................................................................................... 40 2.1.3. Methodology ................................................................................................................. 41 2.1.4. Fine Tree Testing Results .............................................................................................. 57 2.2. Interfacial Tracking Tests ...................................................................................................... 61 2 2.2.1. Introduction .................................................................................................................. 61 2.2.2. Background ................................................................................................................... 63 2.2.3. Methodology ................................................................................................................. 64 2.2.4. Interfacial Tracking Test Results ................................................................................... 65 2.3. Water Absorption Testing ..................................................................................................... 68 2.3.1. Introduction .................................................................................................................. 68 2.3.2. Background ................................................................................................................... 68 2.3.3. Methodology ................................................................................................................. 70 2.3.4. Water Absorption Result............................................................................................... 76 2.4. Conclusion ............................................................................................................................. 78 3. Atomic Force Microscopy with Infrared Spectroscopy (AFM-IR) - Technique .............................. 80 3.1. Introduction .......................................................................................................................... 80 3.1.1. Background information on chemistry in electrical treeing ......................................... 80 3.1.2. Atomic Force Microscopy with Infrared Spectroscopy – How it works ........................ 88 3.2. AFM-IR Chemical Analysis Testing Methodology .................................................................. 91 3.2.1. Samples ......................................................................................................................... 92 3.2.2. Preparation for AFM-IR ................................................................................................. 96 3.2.3. AFM-IR Operation and Data Analysis .......................................................................... 100 3.2.4. Conclusion ................................................................................................................... 104 4. Atomic Force Microscopy with Infrared Spectroscopy (AFM-IR) - Testing and Results ............. 105 4.1. AFM-IR Tree Initiation Testing Results ................................................................................ 105 4.1.1. Pre-initiation Tests ...................................................................................................... 105 4.1.2. Post-initiation Tests .................................................................................................... 118 4.2. AFM-IR Tree Growth Testing Results .................................................................................. 132 4.2.1. AFM(1)-A1(2) – Earliest Stages of Tree Growth .......................................................... 132 4.2.2. AFM(1)-G1 – 100 µm from the needle tip .................................................................. 135 4.2.3. AFM(1)-G2 (Channel 1 – 100 µm from the needle tip) ............................................... 138 4.2.4. AFM(1)-G2 (Channel 2 – 500 µm from the needle tip) ............................................... 142 4.3. Conclusion ........................................................................................................................... 148 5. Discussion .................................................................................................................................... 150 5.1. Plane-Plane Sample Configurations .................................................................................... 150 5.1.1. Sample Design Issues .................................................................................................. 150 5.1.2. Water Saturated Samples ........................................................................................... 150 5.1.3. Tracking Samples......................................................................................................... 151 3 5.1.4. Reverse Tree Samples ................................................................................................. 151 5.1.5. Improved Sample Analysis Needed ............................................................................ 152 5.1.6. Application of AFM-IR to plane-plane electrode configurations ................................ 153 5.2. Discussion on Tree Initiation Findings ................................................................................ 154 5.2.1. Degradation processes occurring prior to initiation ..................................................
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