Table Of ContentHolistic Design of Resonant DC Transformer
on Constant Voltage Conversion, Cascaded Stability
and High Efficiency
· · · ·
Xin Zhang Fanfan Lin Hao Ma Bin Zhao
Jingjing Huang
Holistic Design of Resonant
DC Transformer on Constant
Voltage Conversion,
Cascaded Stability and High
Efficiency
Xin Zhang Fanfan Lin
College of Electrical Engineering Nanyang Technological University
Zhejiang University Singapore, Singapore
Hangzhou, Zhejiang, China
Bin Zhao
Hao Ma Institute of Electronics
College of Electrical Engineering Chinese Academy of Sciences
Zhejiang University Beijing, China
Hangzhou, Zhejiang, China
Jingjing Huang
School of Electronic and Information
Engineering
Xi’an Jiaotong University
Xi’an, Shanxi, China
ISBN 978-981-19-9114-1 ISBN 978-981-19-9115-8 (eBook)
https://doi.org/10.1007/978-981-19-9115-8
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Preface
Nowadays, due to the rapid development of renewable energy generations and DC
loads, DC microgrid is becoming more and more popular. However, in the DC micro-
grid, it is necessary to convert the AC power generated by wind turbines into DC
power through AC/DC conversion technique, which requires multiple conversion
steps and introduces additional system power loss and DC microgrid complexity.
Therefore, the hybrid AC/DC microgrid has come up to promote the connection
of various renewable AC and DC power sources and loads. DC transformer (DCT)
plays a significant role in the hybrid AC/DC microgrid. It is generated by adding
a number of overdriven switchers to a high-frequency AC transformer. It not only
inherits the electrical isolation function of the traditional AC transformer, but also
has high power density and high efficiency as the power converter. There are many
popular DCTs used in the hybrid AC/DC microgrid, including CLLC-type DCT,
asymmetrical CLLC-type DCT and other types of DCTs.
This book comprehensively studies the design problem of the DCT and empha-
sizes the optimization of the DCT in the hybrid AC/DC microgrid. It not only
considers the influence of the actual fluctuating values of the inductors/capacitors, but
also considers multiple design objectives (i.e., voltage conversion gain (VCG), effi-
ciency, stability and so on). This book first reviews the existing DCTs and its design
challenges in the hybrid AC/DC microgrid and then develops a family of optimization
design methodologies for the DCTs, to ensure that the designed DCT enjoys good
power transmission and voltage regulation ability in the hybrid AC/DC microgrid
even when its actual inductors/capacitors’ values vary with the practical power and
temperature. The book also puts forward a family of multi-objective optimization
design approaches for the DCTs to ensure that it meets the requirements of VCG,
efficiency and system stability simultaneously. Besides, this book also discusses how
to control the DCT in an optimal and general way to make it work well in the hybrid
AC/DC microgrid.
This book comprehensively provides self-contained knowledge on the DCT
design from Prof. Zhang Xin’s research Group. The goals of this book are to provide
circuit and control design methodologies of the DCT in a systematical way to fix
the VCG, efficiency, stability and parameter drift issues occurred in the hybrid
v
vi Preface
AC/DC microgrid. The main audience includes senior undergraduate and postgrad-
uate students, educators, scientists, researchers, engineers and innovators. This book
is mainly for scholars and researchers from both academia and industry who are
working in the fields of power electronics and power system.
Hangzhou, China Xin Zhang
Contents
1 Introduction ................................................... 1
1.1 Introduction to Hybrid AC/DC Microgrid ...................... 1
1.1.1 Concepts of Hybrid AC/DC Microgrid .................. 1
1.1.2 Challenges of Hybrid AC/DC Microgrid ................. 1
1.2 Background of DC Transformer .............................. 3
1.2.1 Functions of DC Transformer .......................... 3
1.2.2 Types of DC Transformer ............................. 3
1.3 Control Strategy for DC Transformer .......................... 6
1.3.1 Open-Loop Control for DC Transformer ................. 6
1.3.2 Closed Loop Control for DC Transformer ............... 7
1.4 Challenges in DC Transformer Design ......................... 8
1.4.1 Challenges in Voltage Conversion Gain ................. 8
1.4.2 Challenges in Cascaded System Stability ................ 8
1.4.3 Challenges in Efficiency Improvement .................. 9
References ..................................................... 9
2 The Proposed Robust Circuit Parameters Design
for the CLLC-Type DC Transformer in the Hybrid AC/DC
Microgrid ..................................................... 11
2.1 Introduction ............................................... 12
2.2 Review: Power Flow Scenarios of the Hybrid AC/DC
Microgrid and the CLLC-DCT Control Scheme ................. 15
2.2.1 Power Flow Scenarios of Hybrid AC/DC Microgrid ....... 15
2.2.2 Cooperation Between the DCT and the BIC .............. 16
2.2.3 Control Scheme of the CLLC-DCT in the Hybrid
AC/DC Microgrid .................................... 17
2.3 Robust Circuit Parameters Design of the CLLC-DCT
in the Hybrid AC/DC Microgrid .............................. 18
2.3.1 Target Designed Parameters ........................... 19
2.3.2 Design Requirements ................................. 20
2.3.3 Design Challenges and Criteria ........................ 23
vii
viii Contents
2.3.4 Robust Circuit Design Method for the CLLC-DCT
According to the Design Criteria ....................... 27
2.3.5 Design Example ..................................... 32
2.4 Experimental Verification .................................... 34
2.4.1 Experimental Waveforms of CLLC-DCT
with Bidirectional Power Flow ......................... 35
2.4.2 Power Transmission Ability Verification
of CLLC-DCT ....................................... 37
2.4.3 Voltage Conversion Gain (M) Verification of DCT ........ 38
2.4.4 Characteristics of the CLLC-DCT Under
the High-Power Condition ............................. 38
2.5 Conclusion ................................................ 39
References ..................................................... 41
3 The Proposed Simplified Resonant Parameters Design
of the Asymmetrical CLLC-Type DC Transformer
in the Renewable Energy System via Semi-Artificial Intelligent
Optimal Scheme ............................................... 45
3.1 Introduction ............................................... 46
3.2 Design Challenges of the ACLLC-Type DCT ................... 49
3.2.1 Parameters Need to be Designed ....................... 49
3.2.2 Design Challenges ................................... 50
3.2.3 Objective ........................................... 51
3.3 PT and VCG Oriented Design Criteria ......................... 51
3.3.1 Design Criterion-I to Ensure PT ........................ 52
3.3.2 Design Criterion-II to Ensure VCG ..................... 54
3.3.3 Further Simplification of the Above Criteria ............. 55
3.4 Proposed Semi-AI Circuit Parameter Design ................... 57
3.4.1 Preliminary of the Proposed Semi-AI Design Method ..... 58
3.4.2 Proposed Semi-AI Design Method ...................... 59
3.4.3 Determine the Parameters L ,L ,L ,L ,
r1R r2R m1R m2R
C ,C ........................................... 61
r1R r2R
3.5 A Design Example Based on the Proposed Semi-AI Approach .... 62
3.5.1 Step-By-Step Parameter Determination .................. 63
3.5.2 Transformer Design Based on the Required
Parameters in Step 5 .................................. 63
3.6 Experimental Verification .................................... 66
3.6.1 Bidirectional Power Flow Characteristics ................ 67
3.6.2 PT and VCG Verification .............................. 69
3.6.3 Other Characteristics of the Proposed DCT .............. 71
3.6.4 Comparison Between the Approach in [25]
and the Proposed Approach ............................ 77
3.7 Conclusion ................................................ 79
References ..................................................... 79
Contents ix
4 The Proposed Two-Stage Parameter Design Methodology
of a Generalized Resonant DC Transformer in Hybrid AC/DC
Microgrid with Optimum Active Power Transmission ............. 83
4.1 Introduction ............................................... 84
4.2 Problems Description ....................................... 87
4.2.1 Problems in Resonant DCT ............................ 87
4.2.2 Objective ........................................... 91
4.3 Preliminary: APTR and VCG Oriented Design Requirements ..... 92
4.3.1 General Topology .................................... 92
4.3.2 Design Requirement Based on APTR ................... 94
4.3.3 Design Requirement Based on VCG .................... 95
4.4 Challenges and Design Criteria in the Presence of Parameter
Variations ................................................. 96
4.4.1 Challenges .......................................... 96
4.4.2 Proposed Design Criteria .............................. 97
4.5 Proposed Two-Stage Parameter Design ........................ 99
4.5.1 Preliminary Designs .................................. 99
4.5.2 Proposed Two-Stage Parameter Design Method
to Achieve the Optimum k and g ....................... 100
4.5.3 Determination of the Parameters Lr , L , L ,
1R r2R m1R
L , C , C ..................................... 102
m2R r1R r2R
4.5.4 An Illustrative Design Example ........................ 102
4.6 Experimental Validation ..................................... 104
4.6.1 APTR Ability Comparison ............................ 104
4.6.2 VCG Ability Comparison ............................. 106
4.6.3 Loss Breakdown Comparison .......................... 108
4.6.4 Bidirectional Power Flow Characteristic
of the Proposed DCT ................................. 110
4.7 Conclusion ................................................ 110
References ..................................................... 112
5 Design of Symmetrical CLLC Resonant DC Transformer
Considering Voltage Transfer Ratio and Cascaded System
Stability ....................................................... 115
5.1 Introduction ............................................... 115
5.2 Preliminaries Regarding Parameter Design for the CLLC
Resonant DCT ............................................. 118
5.2.1 Basic Design for Circuit Parameters of CLLC
Resonant DCT ....................................... 119
5.2.2 Analysis on the VTR Design Objective for the CLLC
Resonant DCT ....................................... 120
5.2.3 Analysis on the System Stability Design Objective
for the CLLC Resonant DCT .......................... 122
5.3 Problems in Parameter Design for the CLLC Resonant DCT
Considering Parameter Fluctuations ........................... 127
x Contents
5.3.1 Fluctuations of Values of Inductance and Capacitance ..... 127
5.3.2 Problem Regarding VTR Design Objective Resulting
from Parameter Fluctuations ........................... 128
5.3.3 Problem Regarding System Stability Design Objective
Resulting from Parameter Fluctuations .................. 129
5.4 The Proposed Parameter Design Approach for CLLC
Resonant DCT with Assistance of PSO ........................ 130
5.4.1 The Proposed Parameter Design for CLLC Resonant
DCT with Assistance of PSO .......................... 131
5.4.2 RODDPSO Adopted to Consider Parameter
Fluctuations ......................................... 134
5.4.3 Design Case of the Proposed Design Approach ........... 136
5.5 Experimental Results ....................................... 138
5.5.1 Experiments of Operation at Rated Conditions ........... 139
5.5.2 Experiments of VTR Verification ....................... 139
5.5.3 Experiments of System Stability Verification ............. 139
5.6 Conclusion ................................................ 141
References ..................................................... 142
6 Parameter Design for Symmetrical CLLC-Type DC
Transformer Considering Cascaded System Stability
and Power Efficiency ........................................... 145
6.1 Introduction ............................................... 146
6.2 Impacts of Parameter Design for the CLLC-Type DCT
on Cascaded System Stability and Power Efficiency ............. 149
6.2.1 Preliminaries: Basic Parameter Design
for the CLLC-Type DCT .............................. 149
6.2.2 Impacts of Parameter Design for the CLLC-Type
DCT on Cascaded System Stability ..................... 150
6.2.3 Impacts of Parameter Design for the CLLC-Type
DCT on Power Efficiency ............................. 155
6.3 Challenges of Parameter Design for CLLC-Type DCT
Considering Cascaded System Stability and Power Efficiency ..... 157
6.3.1 Fluctuating Values of Inductors and Capacitors ........... 157
6.3.2 Challenges in Parameter Design for the CLLC-Type
DCT Regarding Cascaded System Stability and Power
Efficiency ........................................... 158
6.4 The Proposed Four-Stage PSO-Aided Parameter Design
Approach for the CLLC-Type DCT Considering Cascaded
System Stability and Power Efficiency ......................... 160
6.4.1 The Proposed Four-Stage PSO-Aided Parameter
Design Approach for the CLLC-Type DCT .............. 160
6.4.2 RODD-PSO [23] Adopted in the Parameter Design
for the CLLC-Type DCT .............................. 163
6.4.3 Design Example ..................................... 165
