Title Page
Abstract
Contents
List of Abbreviations 15
I. Introduction 16
1.1. AC and DC Distribution Systems 16
1.2. Unipolar and Bipolar DC Distribution Systems 17
1.3. Power Converters for Constructing DC System 18
1.4. Introduction of Enhanced Dual-Active-Bridge Converters 20
II. Enhanced Full-Bridge Dual-Active-Bridge Converter 22
2.1. Topology 22
2.2. Operating Principles 23
2.2.1. DC Inductors for Voltage Balancing Capability 23
2.2.2. Additional Switching Modulation To Improve Voltage Balancing 27
2.3. Full-Load-Range ZVS Methodology 29
2.4. Experimental Results 33
2.4.1. Prototype Design 33
2.4.2. Voltage Balancing Operation 34
2.4.3. ZVS Capability 34
2.4.4. Comparison with Conventional Methods 36
III. Enhanced T-type Dual-Active-Bridge Converter 38
3.1. Motivation 38
3.2. Topology 38
3.3. Operating Principles 39
3.3.1. Enhanced Switching Modulation for Separate Power Distribution 39
3.3.2. Mathematical Derivation 47
3.4. Experimental Results 51
3.4.1. Prototype Design 51
3.4.2. Voltage Balancing Operation 51
3.4.3. Comparison with Conventional Methods 53
IV. Enhanced Three-port based Dual-Active-Bridge Converter 57
4.1. Motivation 57
4.2. Topology 58
4.3. Operating Principles 59
4.3.1. Independent Power Flow Capability 59
4.3.2. Voltage Balancing Under Short Circuit Fault 64
4.4. Full-Load-Range ZVS Methodology 65
4.5. Experimental Results 70
4.5.1. Prototype Design 70
4.5.2. Voltage Balancing Operation 71
4.5.3. ZVS Waveforms 73
4.5.4. Comparison of Power Conversion Efficiency 74
V. Enhanced Three-level Dual-Active-Bridge Converter 76
5.1. Motivation 76
5.2. Topology 77
5.3. Operating Principles 78
5.3.1. Switching Modulation For Separate Power Distribution 78
5.3.2. Mathematical Derivation 84
5.4. Experimental Results 87
5.4.1. Prototype Design 87
5.4.2. Voltage Balancing Operation 87
VI. Enhanced Four-port based Dual-Active-Bridge Converter 90
6.1. Motivation 90
6.2. Topology 91
6.3. Operating Principles 94
6.3.1. Power Flow Control Strategy 94
6.3.2. Three Independent Power Flows 95
6.4. Full-Load-Range ZVS Methodology 99
6.5. Experimental Results 103
6.5.1. Prototype Design 103
6.5.2. Voltage Balancing Operation 103
6.5.3. ZVS Waveforms 104
6.5.4. Comparison of Power Conversion Efficiency 106
VII. Conclusion 108
7.1. Conclusion 108
7.2. Future Plans to Optimize Enhanced DAB Converters 109
REFERENCE 110
TABLE I. CONVERTER SPECIFICATIONS 32
TABLE II. COMPARISON BETWEEN PROPOSED DAB AND CONVENTIONAL SYSTEM 37
TABLE III. CONVERTER PARAMETERS 51
TABLE IV. COMPARISON BETWEEN PROPOSED DAB CONVERTERS AND CONVENTIONAL CONVERTER 56
TABLE V. SPECIFICATIONS AND DESIGN PARAMETERS 70
TABLE VI. CONVERTER PARAMETERS 87
TABLE VII. PROTOTYPE SPECIFICATIONS 103
TABLE VIII. COMPARISON BETWEEN ENHANCED DAB CONVERTERS 108
Fig. 1.1.1. Structure of distribution systems: (a) ac, (b) dc. 16
Fig. 1.2.1. Types of dc distribution systems: (a) Unipolar, (b) Bipolar. 17
Fig. 1.3.1. Construction dc distribution systems using power converters: (a) Unipolar, (b) Bipolar. 18
Fig. 1.3.2. Simulation result under unbalanced load conditions: (a) Only DAB converter, (b) DAB... 19
Fig. 1.4. Concept of enhanced DAB converter for bipolar dc distribution system 20
Fig. 2.1.1. Circuit schematic of the enhanced full-bridge DAB converter. 22
Fig. 2.2.1. Key theoretical waveforms. 23
Fig. 2.2.2. Three cases for dc currents according to bipolar load conditions: (a) When the bipolar load is... 24
Fig. 2.2.3. Operating modes: (a) Mode 1 (θ0-θ1), (b) Mode 2 (θ₁-θ₂), (c) Mode 3 (θ₂-θ₃), (d) Mode 4 (θ₃-θ₄).[이미지참조] 26
Fig. 2.2.4. Volt-second balance theory for deriving balanced bipolar voltage equation. 27
Fig. 2.2.5. Equivalent circuit analysis considering parasitic components: (a) When S₁ is on, (b) When S₂... 28
Fig. 2.2.6. Advanced switching modulation. 28
Fig. 2.2.7. Closed-loop control system with advanced switching modulation. 28
Fig. 2.3.1. Theoretical operating waveforms at no load condition. 31
Fig. 2.3.2. Theoretical analysis of transformer current: (a) Equivalent circuit of transformer, (b)... 31
Fig. 2.3.3. Extended ZVS range of the enhanced DAB converter. 32
Fig. 2.4.1. Experimental waveforms under various bipolar load conditions: (a) Pout1=1.45 kW and Pout2...[이미지참조] 35
Fig. 2.4.2. Experimental waveforms using advanced switching modulation under Pout1=2.9 kW and...[이미지참조] 35
Fig. 2.4.3. Experimental waveforms of ZVS at no load condition: (a) ZVS of S₁, (b) ZVS of S₄, (c) ZVS... 36
Fig. 2.4.4. Comparison of power conversion efficiency. 37
Fig. 3.1.1. Size comparison between core and power switch (TO-247) at 15kW design. 38
Fig. 3.2.1. Circuit schematic of the enhanced T-type DAB converter. 39
Fig. 3.3.1. Power flow of conventional and proposed modulation: (a) Conventional modulation, (b)... 39
Fig. 3.3.2. Operating modes according to load conditions: (a) D=0 with balanced load, (b) 0 〈D 〈π... 40
Fig. 3.3.3. Detailed switching modulation of proposed method. 42
Fig. 3.3.4. Operating stages of Mode A shown in Fig. 3.3.2: (a) Stage 1 (θ0-θ1), (b) Stage 2 (θ₁-θ₂), (c)...[이미지참조] 43
Fig. 3.3.5. Operating stages of Mode B shown in Fig. 3.3.2: (a) Stage 1 (θ0-θ1), (b) Stage 2 (θ₁-θ₂), (c)...[이미지참조] 44
Fig. 3.3.6. Operating stages of Mode A under D=π shown in Fig. 3.3.2: (a) Stage 1, (b) Stage 2, (c)... 45
Fig. 3.3.7. Operating stages of Mode B under D=π shown in Fig. 3.3.2: (a) Stage 1, (b) Stage 2, (c)... 46
Fig. 3.3.8. Power graphs in 3-D coordinate according to φ and D: (a) PoutA, (b) PoutB.[이미지참조] 49
Fig. 3.3.9. Power graphs in 3-D coordinate according to φ and D: (a) PoutA+PoutB, (b) PoutB-PoutA.[이미지참조] 50
Fig. 3.3.10. Closed-loop control system of the proposed converter. 50
Fig. 3.4.1. Steady-state operating waveforms according to various bipolar load conditions: (a) PoutA=1...[이미지참조] 52
Fig. 3.4.2. Steady-state operating waveforms under PoutA=1 kW and PoutB=700 W.[이미지참조] 53
Fig. 3.4.3. Power efficiency comparison with enhanced full-bridge DAB and conventional converter. 54
Fig. 3.4.4. Power density comparison between enhanced DAB converters and conventional converter:... 55
Fig. 4.1.1. High system reliability of bipolar dc system. 57
Fig. 4.1.2. Excessive current of voltage balancer under short circuit in one of dc poles. 57
Fig. 4.2.1. Circuit schematic of the enhanced three-port based DAB converter. 58
Fig. 4.3.1. Key operating waveforms of the proposed switching modulation 59
Fig. 4.3.2. Equivalent circuit models: (a) Conventional TP-DAB, (b) Proposed DAB, (c) Independent... 60
Fig. 4.3.3. Operating modes: (a) Mode 1 (θ0-θ1), (b) Mode 2 (θ₁-θ₂), (c) Mode 3 (θ₂-θ₃), (d) Mode 4 (θ₃-θ₄).[이미지참조] 62
Fig. 4.3.4. Control block diagram of the proposed converter. 62
Fig. 4.3.5. Equivalent circuit when port C has a short circuit fault. 63
Fig. 4.3.6. Operating waveforms when port B is under full-load and port C has a short circuit fault. 63
Fig. 4.4.1. Theoretical voltage and current waveforms of the proposed modulation at no-load condition. 65
Fig. 4.4.2. Operating regions according to φb: (a) 0 ≤ φb ≤ D/2, (b) D/2 〈φb.[이미지참조] 66
Fig. 4.4.3. Detailed operating regions according to φb: (a) φb=0, (b) 0 〈φb 〈D/2, (c) φb=D/2, (d) D/2...[이미지참조] 67
Fig. 4.5.1. Steady-state operating waveforms under unbalanced load conditions: (a) PB=0 W and PC=...[이미지참조] 71
Fig. 4.5.2. Step-load response when PC changes from 0.5 kW to 1 kW and PB is 1 kW.[이미지참조] 72
Fig. 4.5.3. Experimental waveforms when the output of port C becomes short circuit and port B... 72
Fig. 4.5.4. Proposed ZVS modulation at the worst ZVS conditions: (a) φb & φc=0, (b) φb & φc=D/2+ωd/2.[이미지참조] 73
Fig. 4.5.5. Gate-source and drain-source voltage waveforms at the worst ZVS conditions: (a) φb & φc=0,...[이미지참조] 73
Fig. 4.5.6. Proposed ZVS modulation at unbalanced load case of φb=0 and φc=D/2+ωd/2: (a) Steady-...[이미지참조] 73
Fig. 4.5.7. Power conversion efficiency curves. 74
Fig. 4.5.8. Power conversion efficiency in 3-D coordinate. 75
Fig. 5.1. DC bus voltage levels of LVDC distribution system. 76
Fig. 5.2. Entire voltage stress of dc-link voltage. 77
Fig. 5.3. Half voltage stress of three-level structure. 77
Fig. 5.2.1. Circuit schematic of the enhanced three-level DAB converter. 78
Fig. 5.3.1. Key waveforms of the enhanced switching modulation. 79
Fig. 5.3.2. Detail switching modulation of enhanced method: (a) Switching pattern α, (b) Switching... 80
Fig. 5.3.3. Operating modes of switching pattern α: (a) Mode 1, (b) Mode 2, (c) Mode 3, (d) Mode 4, (e)... 81
Fig. 5.3.4. Operating modes of switching pattern β: (a) Mode 2, (b) Mode 3, (c) Mode 6, (d) Mode 7. 83
Fig. 5.3.5. Operating modes of switching pattern α with no load condition at Vout2 side: (a) Mode 2, (b)...[이미지참조] 83
Fig. 5.3.6. Operating cases according to φ₁ and φ₂: (a) φ₁〉 φ₂, (b) φ₁ 〈φ₂ 84
Fig. 5.3.7. Power curves in 3-D coordinate according to φ₁ and φ₂: (a) Pout1, (b) Pout2.[이미지참조] 86
Fig. 5.3.8. Closed-loop control system of the proposed converter. 86
Fig. 5.4.1. Steady state operating waveforms under forward power flow: (a) Pout1=500 W and Pout2=...[이미지참조] 88
Fig. 5.4.2. Steady state operating waveforms under backward power flow when Pout1=-1.5 kW and...[이미지참조] 89
Fig. 5.4.3. Experimental waveforms of step load change in Pout1 from 500 W to 1300 W. 89
Fig. 5.4.4. Power conversion efficiency. 89
Fig. 6.1.1. Bipolar dc system using two bipolar dc buses: (a) System schematic, (b) Conventional three... 90
Fig. 6.1.2. Proposed bipolar dc system using enhanced four-port based DAB converter: (a) System... 91
Fig. 6.2.1. Conventional four-port based DAB converters: (a) Converter topology, (b) Equivalent circuit diagram. 92
Fig. 6.2.2. Proposed DAB converter: (a) Schematic of converter topology, (b) Equivalent circuit diagram. 93
Fig. 6.3.1. Power flow of the proposed power flow control strategy: (a) Conventional power distribution,... 94
Fig. 6.3.2. Theoretical operating waveforms and switching modulation. 96
Fig. 6.3.3. Implementation of power flow control strategies in proposed DAB converter: (a)... 96
Fig. 6.3.4. Operating modes: (a) Mode 1 (θ0-θ1), (b) Mode 2 (θ₁-θ₂), (c) Mode 3 (θ₂-θ₃), (d) Mode 4 (θ₃-θ₄).[이미지참조] 98
Fig. 6.3.5. Simplified block diagram of the closed-loop control system. 98
Fig. 6.4.1. Theoretical voltage and current waveforms to show ZVS strategy using magnetizing... 102
Fig. 6.4.2. Comparison of Q between the conventional method and proposed strategy. 102
Fig. 6.5.1. Balanced Bipolar voltage levels under asymmetrical load condition of Pport1=1.3 kW,...[이미지참조] 104
Fig. 6.5.2. Steady-state operating waveforms according to various asymmetrical load conditions: (a)... 105
Fig. 6.5.3. ZVS waveforms at no-load condition of every port: (a) Steady-state waveforms, (b) Gate to... 106
Fig. 6.5.4. Power efficiency comparison. PH is the compensated power by the converter which is the...[이미지참조] 107
Fig. 6.5.5. Conventional three-stage converter. 107
Fig. 7.2.1. Future plans to improve the performance of the enhanced DAB converters. 109