Overvoltage and Surge Protection in Variable Frequency Drives A Dissertation Presented by Dawood Talebi Khanmiri to The Department of Electrical and Computer Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the field of Electrical Engineering Northeastern University Boston, Massachusetts October 2020 Acknowledgment I would like to thank my advisor, Professor Brad Lehman, for his continuous support of my PhD research, for his patience and guidance, without whom this thesis could not be completed. Besides my professor, I would like to thank Mersen who partially funded this research and generously provided the lab for high power experiments, especially Roy Ball, Jerry Mosesian, and late Craig McKenzie, who helped me during my research. I would, also, like to thank my thesis committee members, for their time and their insightful comments in my proposal review. I would like to thank all of my labmates and fellow grad students, too, for all the good memories that we have had together. Last, but not least, I would like to thank my family who supported me in my studies, especially my wife, Dr. Nasibeh Nasiri, without whom I would not have been able to complete my PhD. i Abstract Overvoltage and surge protection of electric devices is an indispensable part of the system protection. Metal Oxide Varistors (MOVs) are the most common components used for surge protection. However, MOVs are known to degrade over time when they experience severe discharges. This degradation may, particularly, lessen protection capabilities for fast switching PWM applications, such as inverters and drives. For instance, Variable Frequency Drives (VFDs) are becoming more and more widespread for motor speed control and energy saving purposes. However, when connecting long cables between the inverter and the motor, high frequency overvoltages appear on motor terminals due to the voltage reflection phenomenon. These overvoltages can damage the motor and the cable insulation, and at the same time, cause installed MOVs to conduct high currents and fail. In this research, we investigate the MOV’s behavior and degradation in such applications. The VFD systems and characteristics of the overvoltages are first modeled. MOVs behavior is investigated through experimental tests and simulations. Finally, new methods of applying MOVs in PWM motor drives are proposed to mitigate the overvoltages. ii Table of Contents 1. Introduction .................................................................................................................................... 1 1.1. Motivation ................................................................................................................................. 1 1.2. Background ............................................................................................................................... 3 1.2.1. Surges and overvoltages ................................................................................................... 3 1.2.2. Standard Surge waveforms ............................................................................................... 7 1.2.3. Metal Oxide Varistors ....................................................................................................... 9 1.2.4. MOV models .................................................................................................................. 12 1.2.5. MOVs’ failure modes ..................................................................................................... 12 1.2.5.1. Thermal runaway ........................................................................................................ 13 1.2.5.2. Puncture ...................................................................................................................... 14 1.2.5.3. Cracking ..................................................................................................................... 17 1.2.5.4. Flashover .................................................................................................................... 17 1.2.6. Effective Factors in MOVs’ Failure Modes .................................................................... 18 1.3. Problem Statement................................................................................................................... 23 1.4. Dissertation Organization ........................................................................................................ 25 2. Degradation and Health Monitoring of Metal Oxide Varistors ................................................... 27 2.1. Energy Absorption Capability ................................................................................................. 27 2.2. Degradation of MOVs ............................................................................................................. 33 2.3. Degraded MOV’s V-I curve .................................................................................................... 39 2.4. Health Monitoring of MOVs ................................................................................................... 40 2.4.1. Effects of temperature and voltage on health monitoring and lifetime estimation ......... 44 2.4.2. Proposed Health Monitoring Algorithm ......................................................................... 46 2.5. Summary and Conclusions ...................................................................................................... 48 3. Overvoltages in VFDs ................................................................................................................. 49 3.1. Introduction ............................................................................................................................. 49 3.2. Effects of High Frequency Overvoltages................................................................................. 53 3.2.1. Effects on Motor ............................................................................................................. 53 3.2.2. Effects on Drive .............................................................................................................. 55 3.2.3. Other Effects ................................................................................................................... 57 3.3. Causes and Effective Factors ................................................................................................... 58 3.3.1. Voltage Rise Time .......................................................................................................... 60 3.3.2. Cable Length ................................................................................................................... 62 3.4. Voltage waveforms in a PWM VFD system ........................................................................... 63 3.5. Simulation of a VFD system for overvoltages ......................................................................... 70 3.5.1. Cable model .................................................................................................................... 70 iii 3.5.2. Measurement of cable parameters .................................................................................. 72 3.5.3. Motor Models ................................................................................................................. 75 3.5.4. Measurement of motor parameters ................................................................................. 77 3.5.5. Experimental setup and Measurements........................................................................... 79 3.5.5.1. Cable parameters ........................................................................................................ 81 3.5.5.2. Motor parameters ....................................................................................................... 84 3.5.6. Simulation results ........................................................................................................... 86 3.6. Summary and conclusion......................................................................................................... 92 4. Overvoltage Mitigation and Protection in VFDs ......................................................................... 93 4.1. Surge Protection in VFDs ........................................................................................................ 93 4.2. Overvoltage Mitigation in VFDs ............................................................................................. 97 4.2.1. Passive Filters ................................................................................................................. 98 a. Filters on Motor Terminals ...................................................................................................... 99 b. Filters on Inverter Output ...................................................................................................... 101 c. Filters on Both Motor Terminals and Inverter Output ........................................................... 104 4.2.2. Active Filters ................................................................................................................ 105 4.2.3. PWM Control ............................................................................................................... 108 4.2.4. DC Cable between Rectifier and Inverter ..................................................................... 112 4.2.5. Energy Varistor ............................................................................................................