Charge Transport and Breakdown Physics in Liquid/Solid Insulation Systems by Jouya Jadidian B.Sc., University of Tehran (2006) M.Sc., University of Tehran (2008) Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2013 © Massachusetts Institute of Technology, MMXIII. All rights reserved. Author__________________________________________________________________ Department of Electrical Engineering and Computer Science May 20, 2013 Certified by______________________________________________________________ Markus Zahn Thomas and Gerd Perkins Professor of Electrical Engineering Thesis Supervisor Accepted by_____________________________________________________________ Leslie A. Kolodziejski Chairman, Department Committee on Graduate Students Charge Transport and Breakdown Physics in Liquid/Solid Insulation Systems by Jouya Jadidian Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degree of! Doctor of Philosophy Liquid dielectrics provide superior electrical breakdown strength and heat transfer capability, especially when used in combination with liquid-immersed solid dielectrics. Over the past half- century, there has been extensive research characterizing “streamers” in order to prevent them, as they are the main origins of electrical breakdown in liquid dielectrics. Streamers are conductive structures that form in regions of liquid dielectrics that are over-stressed by intense electric fields. Streamers can transform to surface flashovers when they reach any liquid-immersed solid insulation. Surface flashovers usually propagate faster and further than streamers in similar electric field intensity. Charge generation and transport is crucially important in liquid dielectric breakdown, since without the presence of the electric charge and its ability to migrate in the liquid dielectric volume and on the interface of liquid/solid dielectrics, streamers and surface flashovers are unable to develop. In this thesis, we develop a finite element method transport model in one, two and three- dimensional geometries to help understand the complicated dynamics of electric charge transport and streamer breakdown in liquid dielectrics. This electrohydrodynamic model clarifies many of the mechanisms behind streamer/surface flashover formation, propagation and branching in typical liquid/solid dielectric composite systems. Several key mechanisms have been identified and added to the transport model of streamers, such as effects of electric field intensity on the ionization potential of liquid dielectric molecules and electron velocity saturation, which make the modeling results more realistic. In addition to improving the understanding of electrical breakdown physics in liquid-based insulation systems, a significant effort is made throughout this thesis research to enhance the stability, convergence, speed and accuracy of the model, making it a convenient and reliable tool for designing high voltage components that contain pure liquid dielectrics, nanofluids and liquid immersed insulation systems. This model, for the first time, is able to treat any given electrode shape and gap distance as well as any applied voltage waveform with accurate results, which provides a convenient preliminary way to verify the performance of an insulation system in terms of breakdown voltage, time to breakdown, electric field intensity distribution and ionization level. The model precision is validated through experimental records, analytical solutions and alternative modeling approaches wherever available. Specifically, we verify our one-dimensional numerical results with exact analytical solutions, and our two and three-dimensional modeling results with experimental data found in the literature or provided by ABB Corporate Research, Sweden. The streamer initiation voltages, number of streamer branches, breakdown voltages and currents are in excellent agreement with the experimental data compared to the prior theoretical research on liquid breakdown physics. Identical results obtained using a finite volume method also confirm the correctness of the finite element approach used in this thesis. The presented model can be employed to search for novel configurations of liquid immersed insulation systems including nanofluids and liquid/solid composite systems. Thesis Supervisor: Markus Zahn Title: Thomas and Gerd Perkins Professor of Electrical Engineering ! - 3 - ACKNOWLEDGEMENT I have really enjoyed my MIT life. I have had a unique advisor, Professor Markus Zahn, who has treated me like his own son. I am profoundly grateful to him for sharing his knowledge and experience with me, for his encouragement and friendship during the course of my PhD research. I have learned so many invaluable things from him beyond the scope of graduate studies. I also appreciate insightful comments of my thesis committee members, Professors Jeffrey Lang and Luca Daniel, at the defense and at our committee and individual meetings, which helped me take one step back and look at the big picture of my thesis research. I am truly indebted to members of the ABB Corporate Research team in Västerås, Sweden, with whom I worked closely on this project: Dr. Nils Lavesson, Dr. Ola Widlund and Dr. Karl Borg, who provided technical advice, constructive criticism and encouragement. I would like to acknowledge the ABB Corporation for the financial and technical support of this research. I am thankful to my friends and colleagues at the Laboratory for Electromagnetic and Electronic Systems for many discussions and their company: Dr. Shahriar Khushrushahi, Dr. George Hwang, Mohammad Araghchini, Uzoma Orji, Samantha Gunter, Samuel Chang, Wei Li, Richard Zhang, Juan Antonio, Jiankang Wang, Wardah Inam and Seungbum Lim. My warm appreciation is due to Janet Fischer and Dimonika Bray and all other MIT administrative staff, who always had time for helping me out, no matter how busy they were. The work that resulted in this thesis began long before I arrived at MIT. The preparation for my PhD studies has to be traced back to my several years at the High Voltage Laboratory of the University of Tehran. I particularly would like to thank my former advisors, Professors Hossein Mohseni, Kaveh Niayesh and Amir Abbas Shayegani and all my colleagues at this lab, many of whom are now spread around the world. I especially would like to pay tribute to a wonderful friend and an outstanding scholar, Jamal Shakeri, who regrettably succumbed to cancer. This thesis is dedicated to my parents, Marzieh and Abbas, who have always been concerned about my education. They gave me a rare name, a pure Persian word, which means a person who is ravenous to search out the causes of things: a researcher. As far back as I can remember, I have marveled at their selection: my name is the best gift I have ever received. They enthusiastically started teaching their four-year-old son reading and basic arithmetic, and now I sincerely believe whatever I have achieved is their accomplishment too. I also dedicate this thesis to my sister, Darya, who will soon be graduating with her undergraduate degree. Although it has been extremely difficult for all of us to be apart, they have embraced the separation with patience, only for the sake of my well-being and success. I would like to express my sincere gratitude to my best friend and my wife, Sepideh, for her unconditional love and unwavering faith throughout the years of college and graduate school. She has been astonishingly strong and a steady source of confidence in difficulties we both have faced. I hope I will be able to return the favor, her technical comments and endless support, during the remainder of her PhD research at Tufts University. ! - 4 - Contents' ! ! ! 1. Introduction 29 1.1. Charge Generation and Transport in Liquid Dielectrics Leading to Streamer Formation and Electrical Breakdown 30 1.2. Analysis and Modeling of Streamer Development in Liquid-Solid Insulation Systems 31 1.2.1. One-Dimensional Analysts of Charge Transport in Liquid- Solid Composite Dielectric Systems 33 1.2.2. Two-Dimensional Axisymmetric Modeling of Single Column Streamer 33 1.2.3. Two-Dimensional Modeling of Breakdown Phenomena 34 1.2.4. Modeling of Surface Flashover on Liquid Immersed Dielectrics 34 1.2.5. Fully Three-Dimensional Modeling of Streamer Branching 35 1.3. Main Contributions of Thesis 35 1.4. Thesis Outline 41 2. Pre-breakdown Mechanisms in Transformer Oil-Based Insulation Systems 43 2.1. Streamer Initiation, Propagation and Branching in Transformer Oil 43 2.2. Interactions of Streamers Initiated in Transformer Oil with Adjacent Solid and Gaseous Dielectrics 46 2.3. Effects of Additives on Breakdown in Transformer Oil 48 2.3.1. Different Hydrocarbon Molecules in Transformer oil 48 2.3.2. Nanoparticle Additives and Microparticle Contaminations 50 ! - 5 - 3. Numerical Simulation Problem Posing/Approach 53 3.1. Introduction to COMSOL Multiphysics Modeling of Streamer/Surface Flashover 53 3.2. Problem Posing Elements 53 3.2.1. Simulation Geometry 54 3.2.2. Governing Equations 54 3.2.3. Boundary Conditions 55 3.2.4. Stabilization Techniques 55 3.2.5. Meshing Policies and Mesh Element Definitions 57 3.2.6. Numerical Solvers 59 4. One-dimensional Analysis and Modeling of Unipolar Charge Transport in Liquid-Solid Composite Dielectric Systems 63 4.1. Migration of Unipolar Charge