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  • Resonant Boost Converter for Distributed Maximum Power

    Point Tracking in Grid-Connected Photovoltaic Systems

    by

    Gregor Simeonov

    A thesis submitted in conformity with the requirements for the degree of Masters of Applied Science

    Graduate Department of Electrical and Computer Engineering

    University of Toronto

    Copyright c© 2010 by Gregor Simeonov

  • Abstract

    Resonant Boost Converter for Distributed Maximum Power Point Tracking in

    Grid-Connected Photovoltaic Systems

    Gregor Simeonov

    Masters of Applied Science

    Graduate Department of Electrical and Computer Engineering

    University of Toronto

    2010

    This thesis introduces a new photovoltaic (PV) system architecture employing low volt-

    age parallel-connected PV panels interfaced to a high voltage regulated DC bus of a

    three-phase grid-tied inverter. The concept provides several improvements over existing

    technologies in terms of cost, safety, reliability, and modularity. A novel resonant mode

    DC-DC boost converter topology is proposed to enable the PV modules to deliver power

    to the fixed DC bus. The topology offers high step-up capabilities and a nearly constant

    efficiency over a wide operating range. A reduced sensor maximum power point tracking

    (MPPT) controller is developed for the converter to maximize energy harvesting of the

    PV panels. The reduced sensor algorithm can be generally applied to the class of con-

    verters employing pulse frequency modulation control. A ZigBee wireless communication

    system is implemented to provide advanced control, monitoring and protection features.

    A testbench for a low cost 500 W smart microconverter is designed and implemented,

    demonstrating the viability of the system architecture.

    ii

  • Acknowledgements

    First and foremost I would like to thank my loving parents Alex and Zdenka for their

    support, my brother Andrej for his long-lasting friendship, and my girlfriend Linh for

    sticking with me through thick and thin over the last six years.

    I would like to extend my gratitude to my supervisor Dr. Peter Lehn for his wis-

    dom, patience, and for giving me the opportunity to explore my practical and academic

    interests with this work.

    I wish to acknowledge and give thanks for the financial support from Hydro One

    in the form of the H.W. Price Research Fellowship, and Hatch Limited for the Hatch

    Sustainable Energy Research scholarship.

    Finally I would like to reflect on my years at the University of Toronto; a journey of

    learning, maturing, discovering, achieving, and building friendships and memories that

    will last a lifetime.

    iii

  • Contents

    1 Introduction 1

    1.1 Grid-Connected PV Systems . . . . . . . . . . . . . . . . . . . . . . . . . 1

    1.1.1 Module-Integrated Converters . . . . . . . . . . . . . . . . . . . . 2

    1.1.2 Micro-Inverters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    1.1.3 Low Voltage Inverter . . . . . . . . . . . . . . . . . . . . . . . . . 4

    1.2 Grid-connected System Description . . . . . . . . . . . . . . . . . . . . . 5

    1.2.1 Microconverter Requirements . . . . . . . . . . . . . . . . . . . . 7

    1.2.2 Parallel vs. Series Connected Panels . . . . . . . . . . . . . . . . 8

    1.3 Overview of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    2 Resonant Boost Converter Analysis 13

    2.1 Overview of Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    2.1.1 Hard Switched Converters . . . . . . . . . . . . . . . . . . . . . . 13

    2.1.2 Resonant Mode Converters . . . . . . . . . . . . . . . . . . . . . . 14

    2.2 Theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

    2.2.1 Power Stage Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 16

    2.2.2 Power Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    2.2.3 Peak Current and Voltage Analysis . . . . . . . . . . . . . . . . . 22

    2.2.4 Voltage and Current Conversion Ratio . . . . . . . . . . . . . . . 24

    2.3 Switching Loss and Efficiency Considerations . . . . . . . . . . . . . . . . 24

    iv

  • 2.3.1 Control Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

    3 Photovoltaic System Design and Implementation 30

    3.1 Power Stage Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

    3.1.1 Resonant Frequency, Inductor and Capacitor . . . . . . . . . . . . 31

    3.1.2 Semiconductor Devices . . . . . . . . . . . . . . . . . . . . . . . . 33

    3.1.3 Input Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

    3.1.4 Output Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

    3.1.5 Lossless Snubber . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

    3.1.6 Digital Controller and Gate Drivers . . . . . . . . . . . . . . . . . 37

    3.1.7 PCB Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

    3.2 PV Emulator Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

    3.2.1 PV Emulator Implementation . . . . . . . . . . . . . . . . . . . . 40

    3.2.2 PV Cell Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

    3.3 Inverter Emulator Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

    4 Maximum Power Point Tracking Control System 45

    4.1 MPPT Control Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

    4.2 Controller Model and MPPT Algorithm . . . . . . . . . . . . . . . . . . 47

    4.3 Converter Control System . . . . . . . . . . . . . . . . . . . . . . . . . . 49

    4.4 Considerations on Control System Improvements . . . . . . . . . . . . . 52

    5 Communication System 54

    5.1 Communication System Requirements . . . . . . . . . . . . . . . . . . . . 54

    5.1.1 ZigBee Wireless Networks . . . . . . . . . . . . . . . . . . . . . . 56

    5.2 System Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

    5.3 PV Communication Module . . . . . . . . . . . . . . . . . . . . . . . . . 59

    5.3.1 Software Description . . . . . . . . . . . . . . . . . . . . . . . . . 61

    5.4 Server Graphical User Interface . . . . . . . . . . . . . . . . . . . . . . . 62

    v

  • 5.5 Future Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

    6 Experimental Results 67

    6.1 Converter Model Validation . . . . . . . . . . . . . . . . . . . . . . . . . 67

    6.1.1 Nominal Operating Conditions . . . . . . . . . . . . . . . . . . . 68

    6.1.2 Hold State and Soft Switching Operation . . . . . . . . . . . . . . 69

    6.1.3 Input Filter Validation . . . . . . . . . . . . . . . . . . . . . . . . 72

    6.2 Converter Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

    6.2.1 Weighted Efficiency Results . . . . . . . . . . . . . . . . . . . . . 74

    6.3 MPPT Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

    6.3.1 PV Emulator Parameters . . . . . . . . . . . . . . . . . . . . . . . 78

    6.3.2 MPPT Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

    7 Conclusion 81

    Bibliography 84

    A Converter PCB Schematics 87

    B Converter PCB Bill of Materials 91

    C Communication Module PCB Schematics 93

    D Communication Module PCB Bill of Materials 95

    E Converter Microcontroller Source Code 97

    F PV Emulator Microcontroller Source Code 103

    vi

  • List of Tables

    3.1 Resonant boost converter parameters. . . . . . . . . . . . . . . . . . . . . 39

    5.1 XBee API commands supported by the communication module. . . . . . 61

    6.1 Expected and measured parameters at the nominal operating point. . . . 68

    6.2 Recorded efficiency with hold state control mode. . . . . . . . . . . . . . 75

    6.3 Weighted efficiency results. . . . . . . . . . . . . . . . . . . . . . . . . . . 76

    6.4 Emulated PV panel parameters. . . . . . . . . . . . . . . . . . . . . . . . 78

    B.1 Converter PCB bill of materials. . . . . . . . . . . . . . . . . . . . . . . . 92

    D.1 Communication module PCB bill of materials. . . . . . . . . . . . . . . . 96

    vii

  • List of Figures

    1.1 Module integrated converters. . . . . . . . . . . . . . . . . . . . . . . . . 3

    1.2 Micro-inverter module technology. . . . . . . . . . . . . . . . . . . . . . . 4

    1.3 Low voltage inverter technology with parallel panel collection. . . . . . . 5

    1.4 Proposed system featuring distributed MPPT using DC-DC boost con-

    verters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

    1.5 I-V characteristic curves of two panels at different irradiance levels. . . . 8

    1.6 P-I curve of panel A and B in series. . . . . . . . . . . . . . . . . . . . . 9

    1.7 P-V curve of panel A and B in parallel. . . . . . . . . . . . . . . . . . . . 10

    2.1 Frequency response of a series resonant LC circuit. . . . . . . . . . . . . 15

    2.2 Proposed resonant boost converter topology. . . . . . . . . . . . . . . . . 16

    2.3 Operating states of the resonant boost converter. . . . . . . . . . . . . . 19

    2.4 Theoretical waveforms of the resonant boost converter. . . . . . . . . . . 20

    2.5 Input current waveform de

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