SHIBAURA INSTITUTE OF TECHNOLOGY Non-linear Analysis of Electrical and Thermal Stress Grading System in Multi-Level Inverter-Driven Medium Voltage Motors by Nguyen Nhat Nam A thesis submitted to Shibaura Institute of Technology in fulfilment of the requirements for the degree of Doctor Engineering Graduate School of Engineering and Science September 2014 Abstract SHIBAURA INSTITUTE OF TECHNOLOGY ABSTRACT Advanced Research Program on Environmental Energy Engineering Graduate School of Engineering and Science by Nguyen Nhat Nam Energy has been one of the most important problems in the world. Beside numerous efforts to explore and to apply renewable resources of energy, the efficient use of energy has become a good solution to face the depleted situation of fossil fuels. In practice, applications of adjustable speed drives are demonstrated being able to enhance the efficiency of using electric power. However, this trend becomes a big challenge for stress grading systems of stator end-winding insulations in AC motors because of fast and high voltage impulses in the output of the frequency-variable drives.
Hence, a comprehensive understanding about the behaviour of stress grading systems in the inverter source conditions is an inevitable demand. Originated from this desire, the aim of our research work is to analyze the electrical and the thermal stress grading mechanisms of a typical stress grading system in invert-fed medium voltage motors. Based on a finite element method software package named COMSOL, two models of electric field and heat transfer analyses are developed taking into account the nonlinearly electrical behaviour of the semiconductive tape in the stress grading systems. Besides, a mathematical model of surge travelling is ii Abstract modified and built in Matlab/Simulink to compute overshot voltages at the motor terminal caused by the cable-motor impedance mismatch.
In the inverter applied conditions, the electric field stress and the dissipated power in the conductive armour tape of the stress grading system are validated existing during the short rise time interval of the impulses. The dissipated power density is observed to be greatest in the area at the stator slot exits, hence the highest temperature rise locates in this region. Moreover, the effects of voltage overshooting and ringing due to the cable-motor impedance mismatch on the stress grading system behaviour are clarified in detail. This phenomenon can increase both the intensity and the lasting time of the electric and thermal stresses, exacerbate the ineffective situation of the stress grading system, especially in the cases of long connecting cables, high cable-motor surge impedance differences, and newer inverters with very fast electronic switches.
With the results achieved above, the application of our simulation models can be a promising way for the improvements and optimal designs of stress grading system compatible to inverter-fed motors. iii Acknowledgements I would like to express my highest gratitude to Professor Satoshi Matsumoto for his enthusiastic guidance, valuable suggestions and helps not only in the research work but also in my daily life. Financial supports from Japan International Cooperation Agency (JICA) for my study and living are highly appreciated. I gratefully acknowledge my examination committee members; Professor Hiroyuki Nishikawa, Professor Goro Fujita, Professor Kan Akatsu, and Professor Akiko Kumada for their valuable comments and kindly cooperation in reviewing my research work.
I would like to thank Emeritus Professor Yoshikazu Shibuya for his useful advices and encouragements during my research work. I am thankful to my dear friend, Mr. Le Dinh Khoa for allowing me to use his simulation model of Series Connected H-Bridge Voltage Source Converter, and for all his useful discussions. Special acknowledgement is sent to Mr.
Takahiro Nakamura from Tokyo University for his precious experiment information. The helps and supports from Ms. Junko Okura, Ms. Makiko Hagiwara and Mr.
Seiji Mizuno of JICA during my living in Japan are highly appreciated. I would like to thank all the members of Matsumoto Laboratory, and University staffs at Shibaura Institute of Technology, especially the Global Initiative Section and Graduate School Section for all their helps and supports. Finally, I would like to acknowledge the endless supports from my family including my grandparents, my parents, my beloved wife, my sister and my brother in law. iv I would like to devote this thesis to my grandfather… v Contents CONTENTS Declaration of Authorship i Abstract ii Acknowledgements iv List of Figures viii List of Tables xiii Abbreviations xiv Symbols xv 1 Introduction 1 1.2 Stress grading system structure .1 Conductive armour tape .3 Temperature and field dependence of materials in stress grading systems .5 Objective of the present study .1 Approaches applied for stress grading analysis in previous works .2 FEM based models of stress grading system .1 Electric field analysis model .2 Heat transfer analysis model .3 Series Connected H-Bridge voltage source converter model .4 Mathematical model for PWM surge transmission in ASD networks .1 Frequency response of the stress grading system .2 Operation analysis of the stress grading system under PWM voltage sources .1 Output voltages of the SCHB VSCs .2 Electric field analysis .3 Heat transfer analysis .3 Investigation of the effect of overshot voltage due to impedance mismatch between cable and motor .1 Electric field and thermal stresses in the SGS .2 Validation of the FEM based analysis models.
65 5 Conclusion and Future works 70 5. 71 A List of publication 74 References 77 vii List of Figures Fig. 1-1: A general configuration of inverter-driven motor. 1-2: A typical structure of type II form wound insulation system.
1-3: Microstructure view of semiconductive materials based on SiC and ZnO [8]. 1-4: Measuring samples of CAT or SCT in [111. 1-5: DC resistivity of CAT measured at 22 oC, 80 oC, 110 oC and 155 o C in [11]. 1-6: DC resistivity of SCT measured at 22 oC, 80 oC, 110 oC and 155 o C in [11].
1-7: Electric field dependent curves of the electrical conductivity of SCT measured at DC, 60 Hz, 3 kHz and 5 kHz in [12]. 1-8: A typical stress grading configuration for optimization problem in [13]. 1-9: Spark gap generator circuit proposed in [18]. 1-10: Sectionalized structure of stress grading system with an additional conductive tape in [19].
1-11: Illustration of capacitive stress grading systems using embedded foils. 2-1: Equivalent circuit of a stress grading system [26,30-31]. 2-2: Schwarz-Christoffel conformal transformation mapping plane (z) onto plane () [13, 32]. 2-3: Detailed size of the studied stress grading system in 2D axially symmetric coordinate (r, z).
2-4: Electrical conductivity of the materials versus electric field strength. 2-5: Boundary conditions used in the electric field analysis models. 2-6: A typical cooling system for high power motors [37]. 2-7: Boundary conditions used in the heat transfer analysis model.
2-8: The mesh profile of the SGS used in the two analysis models. 2-9: Typical topology of a (2M+1)-level SCHB VSC [1, 42]. 2-10: Computation algorithm of VM for Matlab/Simulink used in [45]. 2-11: Improved computation algorithm of VM for Matlab/Simulink in this work.
3-1: Electric potential and tangential electric field stress on the surfaces of the SGS in case of 50 Hz sinusoidal voltage source. 3-2: Maximum value along z-axis of average dissipated power density in the SGS and maximum temperature on the surfaces of the SGS in case of 50 Hz sinusoidal voltage source. 3-3: Maximum tangential electric field stress on the surfaces of the CAT and the SCT in case of sinusoidal voltage sources with the frequency from 50 Hz to 5 MHz. 3-4: Maximum tangential electric field stress on the surfaces of the SGS in the four cases 50 Hz, 26 kHz, 123 kHz and 5 MHz sinusoidal voltages.
3-5: Maximum temperature on the surfaces of the CAT and the SCT in case of sinusoidal voltage sources with the frequency from 50 Hz to 20 kHz. 3-6: Electric potential and tangential electric field stress on the surfaces of the SGS in case of 5 MHz sinusoidal voltage source. 3-7: Output phase-to-ground voltages of the 5L SCHB and 11L SCHB VSCs. 3-8: Harmonic spectra in the phase-to-ground voltages of the 5L and the11L SCHB VSCs.
3-9: Maximum tangential electric field stress on the surfaces of the SGS in the three cases of 5L, 11L SCHB VSCs and 9 kV RMS (phase-to- phase) sinusoidal voltage source. 3-10: Tangential electric field on the surfaces of the SGS at the two points (z = 0 mm and z = 56.9 mm) in the two cases of 5L and 11L SCHB VSCs during the interval from 10 ms to 20 ms. 3-11: Tangential electric field on the surfaces of the SGS at the two points (z = 0 mm and z = 56.9 mm) in the case of 5L SCHB VSC during the interval from 12. 3-12: Tangential electric field on the surfaces of the SGS at the two points (z = 0 mm and z = 56.9 mm) in the case of 11L SCHB VSC during the interval from 12.
3-13: Distribution of electric potential and tangential electric field on the surfaces of the SGS during the interval from 12.18 ms in the case of 5L SCHB VSC. 3-14: Distribution of electric potential and tangential electric field on the surfaces of the SGS during the interval from 12.475 ms in the case of 11L SCHB VSC. 3-15: Maximum value along z-axis of average dissipated power density in the SGS in the cases of 5L and 11L SCHB VSCs. 3-16: Temperature distribution on the surfaces of the SGS in the three cases of 11L, 5L SCHB VSCs and sinusoidal voltage source after 30 hours.
3-17: Comparison of maximum tangential electric field stress on the surfaces of the SGS under the 11L SCHB voltages between the two cases without and with voltage overshooting. 3-18: Tangential electric field on the surfaces of the SGS at the two points (z = 0 mm and z =56.9 mm) in the cases of 11L SCHB voltages during the interval from 10. 3-19: Tangential electric field on the surfaces of the SGS at the two points (z = 0 mm and z = 56.9 mm) in the cases of 11L SCHB voltages during the interval from 12. 3-20: Maximum value along z-axis of average dissipated power density in the SGS in the cases of 11L SCHB voltages without and with voltage overshooting.
3-21: Temperature distribution on the surfaces of the SGS in the cases of 11L SCHB voltages without and with voltage overshooting after 30 hours. 3-22: Simplified waveform of the 11L SCHB VSC used in the ns- scaled electric field analysis. 3-23: Tangential electric field stress at z = 0 mm on the surface of the CAT under the simplified and the 11L SCHB voltages. 3-24: Voltage overshot factor versus the cable length for the two motor impedance values of 100 and 1000 .
3-25: Maximum tangential electric field on the CAT at z =0 versus the rise time of the inverter voltages. 4-1: RC low pass filter circuit. 4-2: Frequency response of a low pass filter output. 4-3: Maximum values along z-axis of dissipated power density in the SGS.
4-5: Hypothesis explained for PD on the middle surface of CAT in [29]. 69 xii List of Tables Table 2-1: Electrical properties of the materials .24 Table 2-2: Thermal properties of the materials. 26 Table 2-3: Basic data of the SCHB VSCs. 31 Table 3-1: Fundamental harmonic and THD of the phase voltage provided by the 5L and the 11L SCHB VSCs.
42 Table 3-2: Amplitude, appearance time and location of maximum tangential electric field stress on the surface of the SGS in the three cases of 5L, 11L SCHB VSCs and 9 kVRMS (phase-to-phase) sinusoidal voltage source. 44 Table 3-3: Important differences of tangential electric field stress at the two special positions on the SGS in the cases of 5L and the 11L SCHB VSCs. 46 Table 3-4: Important data of temperature distribution in the SGS from the heat transfer analyses. 49 Table 3-5: Surge characteristics of the inverter, the cable and the motor in the fundamental case.