Development of Oil - Filled Microcellular Foam for Transformer Insulation by Julie

Microcellular plastics are investigated for a basis for application as insulation in large electrical transformers. These cellular foams contain billions of cells per cubic centimeter with diameters smaller than 10 microns. The insulation material concept involves saturating microcellular plastics with mineral oil. The electrical strength of the insulation is dependent on completely filling all voids with oil. An understanding of the mechanism by which the oil penetrates the matrix is necessary to determine this degree of saturation. Toward this end, microcellular foamed low-density polyethylene samples have been produced and subjected to a pressurized oil environment. An experimental procedure for oil saturation is discussed and presented. A theoretical model based on porous flow and molecular diffusion is compared to the experimental data. The effects of various parameters such as pressure, void fraction, and cell morphology are interpreted. Thesis Advisor: Nam P. Suh Title: Ralph E. and Eloise F. Cross Professor of Manufacturing Head of the Department ACKNOWLEDGMENTS I joined the Microcellular Plastics Lab because I though I could get my hands dirty and learn some new things. I've had very few regrets. Thank you, Professor Nam Suh. Your wisdom and depth of knowledge is truly inspiring to me. I wish to thank Mr. and Mrs. Lee and Jerry Martin for their generous financial assistance during my first year here. Your fellowship allowed me the freedom to test the waters and find my place. The National Science Foundation also has my appreciation for allowing me the luxury to look forward in time without financial worry. Thank you, Sung Cha, for teaching me how to make foam. Thank you, Dan Baldwin, for stimulating my brain into thinking more like the engineer I should be. Many thanks to all my friends, especially Benjamin Linder and Matthew Wall. I am so fortunate to know your friendship and love. You have truly held me up when I would have fallen. Thank you, Mom and Dad. Your unwavering love and support has carried me through. FOREWORD The Microcellular Plastics Research Laboratory in the Laboratory for Manufacturing and Productivity at MIT has been engaged in a program of research on microcellular plastics since the early 1980's. The goals of the research program have progressed from demonstrating the feasibility of foaming various polymers with cell sizes less than ten microns, through characterizing the properties of microcellular plastics, to designing and manufacturing an extrusion system which would produce microcellular plastics continuously. Commensurate with these goals, the research group is seeking industrial applications in which microcellular plastics can be used. The research group, in collaboration with Asea Brown Boveri, Co., is now involved with the design and development of a microcellular plastic insulation for use in high voltage transformers. The work performed for this thesis is motivated and supported by this project. This support is gratefully acknowledged. CONTENTS A BSTR A C T ............................................................................................................ 2 ACKN O W LEDGM EN TS ............................................................................................ 3 FO REW O RD .................................................... ................................................ 4 C O N TEN TS .. ........ ................................................................................................... 5 LIST O F FIG U RES ................................................................................................ 7 LIST O F TA BLES ................................................................................................... 9 1 IN TRO D U CTIO N ................................................................. ........................... 10 1.1 Thesis Objectives ....................................................... 10 1.2 Thesis Overview ........................................................ 11 2 BA C KG RO U N D .................................................................. ............................. 12 2.1 Microcellular Plastics Technology .......................................................... 12 2.2 Transformer Technology .......................................... ............... 14 3 INSULATION CONCEPT GENERATION ...................................... ..... 19 3.1 Open cell structures ..................................................... 21 3.2 Closed cell filled with SF 6 gas ................................................................ 21 3.3 Oil Saturation Methods ..................................... ...... ............... 24 3.3.1 Spinodal Decomposition ....................................... .......... 25 3.3.2 Mixing Molten Polymer and Oil ..................................... .. 28 3.3.3 High Pressure Saturation ...................................... ......... 28 4 EXPERIMENTAL METHODS ..................................... ..... ............... 30 4.1 Design of Experimental Apparatus ..................................... ....... 30 4.2 Sample Production Sequence ..................................... ... ........... 37 4.3 Material Evaluation ........................................................................... 40 4.3.1 Determining Void Fraction ....................................... ........ 41 4.3.2 Determining Cell Density ...................................... ......... 41 4.3.3 Optical Methods ..................................... 42 4.3.4 Point Count Procedure ....................................... ........... 45 5 THEORETICAL METHODS ...................................................................... 47 5.