Modular AC Nano-Grid with Four-Quadrant Micro-Inverters and High-Efficiency DC-DC Conversion ...

175 Pages·2016·5.25 MB·English

Modular AC Nano-Grid with Four-Quadrant
Micro-Inverters and High-Efficiency DC-DC Conversion
by
Shahab Poshtkouhi
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Graduate Department of Electrical and Computer Engineering
University of Toronto
Copyright ' 2016 by Shahab Poshtkouhi
Abstract
Modular AC Nano-Grid with Four-Quadrant Micro-Inverters and High-Efficiency
DC-DC Conversion
Shahab Poshtkouhi
Doctor of Philosophy
Graduate Department of Electrical and Computer Engineering
University of Toronto
2016
A signi(cid:12)cant portion of the population in developing countries live in remote communi-
ties, where the power infrastructure and the required capital investment to set up local
grids do not exist. This is due to the fuel shipment and utilization costs required for
fossil fuel based generators, which are traditionally used in these local grids, as well as
high upfront costs associated with the centralized Energy Storage Systems (ESS). This
dissertation targets modular AC nano-grids for these remote communities developed at
minimal capital cost, where the generators are replaced with multiple inverters, con-
nected to either Photovoltaic (PV) or battery modules, which can be gradually added
to the nano-grid. A distributed droop-based control architecture is presented for the PV
and battery Micro-Inverters (MIV) in order to achieve frequency and voltage stability, as
well as active and reactive power sharing. The nano-grid voltage is regulated collectively
in either one of four operational regions. Effective load sharing and transient handling
are demonstrated experimentally by forming a nano-grid which consists of two custom
500 W MIVs.
TheMIVsformingthenano-gridhavetomeetcertainrequirements. Atwo-stageMIV
architecture and control scheme with four-quadrant power-(cid:13)ow between the nano-grid,
the PV/battery and optional short-term storage is presented. The short-term storage
is realized using high energy-density Lithium-Ion Capacitor (LIC) technology. A real-
ii
time power smoothing algorithm utilizing LIC modules is developed and tested, while
the performance of the 100 W MIV is experimentally veri(cid:12)ed under closed-loop dynamic
conditions.
TwomainlimitationsoftheDABtopology,asthecoreoftheMIVarchitecture’sdc-dc
stage,areaddressed: 1)Thistopologydemonstratespoorefficiencyandlimitedregulation
accuracyatlowpower. Theseareimprovedbyintroducingamodi(cid:12)edtopologytooperate
the DAB in Flyback mode, achieving up to an 8% increase in converter efficiency. 2) The
DAB topology needs four digital isolators for driving the active switches on the other
side of the isolation boundary. Two Phase-Locked-Loop (PLL) based synchronization
schemes are introduced in order to reduce the number of required digital isolators, hence
increasing reliability and reducing the implementation costs. One of these schemes is
demonstrated on a discrete 150 W DAB prototype, while both of them are implemented
on-chip in a 0.18(cid:22)m 80V BCD process. In addition, the power-stage of the primary-side
of a 1 MHz, 50 W DAB converter is fully integrated on the same die. By using such a
high switching frequency, the size of passive elements in the DAB is reduced, resulting
in further cost reductions for the MIV.
The results of this dissertation pave the way for affordable nano-grids with minimal
capital cost, reliable performance and reduced complexity.
iii
Acknowledgements
\You are not a drop in the ocean, you are the entire ocean in a drop."
{ Rumi, 13th century Persian poet, jurist and scholar
First and foremost, I would like to thank my doctoral advisor, Professor Olivier
Trescases,forhisexceptionallevelofguidanceandsupportthroughoutmyentiregraduate
studies. Every time I left his office after a research meeting, I was more motivated
and much happier than when I was going in. His wisdom and vast technical knowledge
catalyzesuniquesynergieswithin\OTgrp", ourresearchgroupwhichIhavebeenaproud
member of since its early days of foundation. I have learned so much from Olivier, not
only in the technical aspect, but also in business, management, and personal networking,
for which I will always be grateful.
I have had the pleasure of collaborating with many forward-looking companies as
well as a great number of talented individuals. I want to thank Solantro Semiconductor
for their visionary outlook and keen interest in niche humanitarian applications, which
prompted the nano-grid project. In particular, I have had the honor of working closely
with Ray Orr and Ben Bacque, whose creative approach to solving technical difficulties
has had a lasting in(cid:13)uence on me. I also want to thank Mihai Varlan, Tony Reinberger,
Chris Gerolami, Nikolay Radimov, Edward Keyes, Edward MacRobbie, and Christian
Cojocaru from Solantro Semiconductor for their technical support and long fruitful dis-
cussions during my internship at Solantro. I have learned a lot from all of them. I have
had the pleasure of collaborating with Magnachip Semiconductor for the IC design and
fabrication presented in Chapter 5. I want to thank Michael Sun and Ikjoon Choi for
securing the silicon space on one of their highly popular shuttle runs as well as providing
much needed technical support throughout the design stage. I would like to express my
sincere gratitude to David King Li for his continuous effort on IDP (Chapter 2), Hus-
sam Hussein and Aliakbar Eski, for their signi(cid:12)cant contributions to the development
iv
and testing of many DAB prototypes (Chapters 3 and 4), and Miad Fard, for his valued
cooperation in the design and testing of the SPM IC (Chapter 5). This thesis would not
have got so far without their important contributions.
I want to thank all the friends I have made and worked with in my graduate studies.
I will never forget my shared memories with Victor (Yue) Wen, Mazhar and Andishe
Moshirvaziri, Vishal Palaniappan, Shuze Zhao, Shawkat Zaman, Steven Chung, Masa-
fumiOtsuka,andAhmadDiabMarzouk. Ithankthemforthediscussion,encouragement,
and friendship.
I received generous (cid:12)nancial support from Ontario Centres of Excellence (OCE),
Solantro Semiconductor, Hatch, NSERC, OGS, and Rogers Graduate Scholarship. With-
out this funding, my Ph.D. would not have been as smooth as it was. I acknowledge the
support from all these sources.
I know that I can never be thankful enough to my mother, whose hard work and
ambitions have always been the main source of inspiration to me. I know that I can
never fully embrace all the dedications and sacri(cid:12)ces my father has done for me. If I were
to choose, I could not have possibly asked for better parents, whose unconditional love
and support have always been there for me.
I de(cid:12)nitely cannot thank Tania enough. Now that I look back at it, I know that
I found her just in time, at a time when I was overwhelmed with stress and endless
uncertainties. Her patience and understanding boosted me through this rough stage of
my life. Had it not been for her, my Ph.D. track would have been longer, harder, and
de(cid:12)nitely lonelier. I admit that when I developed the \smoothing" algorithm in Chapter
3, I did not believe it could be done any better any smoother. Well, she never stops
proving me wrong, and I love it.
v
Contents
1 Introduction 1
1.1 Modular AC Nano-Grids . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Bidirectional Topologies . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 Thesis Outline and Objectives . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Nano-Grid Architecture and Control 18
2.1 Proposed Droop Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.1 Nano-Grid Voltage Partitioning . . . . . . . . . . . . . . . . . . . 24
2.1.2 PV and Battery Inverter Control . . . . . . . . . . . . . . . . . . 24
2.2 Nano-Grid Test Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3 Nano-Grid Experimental Results . . . . . . . . . . . . . . . . . . . . . . 35
2.4 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3 MIV Architecture and Control 43
3.1 DC-DC Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.2 DC-AC Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3 Short-Term Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.4 Power Smoothing Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.5 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.5.1 MIV Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.5.2 Smoothing Algorithm Performance Evaluation . . . . . . . . . . . 57
vi
3.6 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4 DAB Converter: Drawbacks and Enhancements 64
4.1 Low-Power Efficiency Enhancement . . . . . . . . . . . . . . . . . . . . . 65
4.1.1 Modi(cid:12)ed DAB Architecture and Principle of Operation . . . . . . 65
DAB Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Flyback Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Dual Mode Control . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.1.2 Regulation Accuracy at Low Power . . . . . . . . . . . . . . . . . 70
4.1.3 Transformer Design . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.1.4 Efficiency Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Conduction Losses . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Switching Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Core Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Loss Comparison in Two Modes . . . . . . . . . . . . . . . . . . . 78
4.1.5 Simulation and Experimental Results . . . . . . . . . . . . . . . . 79
4.2 Reliability and Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.2.1 PTS Scheme for the DAB Topology . . . . . . . . . . . . . . . . . 87
Digital PLL Implementation Scheme and Analog Interface . . . . 90
Data Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4.2.2 Effect of Transformer Leakage Inductance . . . . . . . . . . . . . 92
4.2.3 Extension to Other Isolated Bidirectional Topologies . . . . . . . 96
4.2.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.3 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
5 On-Chip Synchronization and Integrated DAB Converter 110
5.1 PLL Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
5.2 DIS Scheme Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . 113
vii
5.3 PTS Scheme Synchronization . . . . . . . . . . . . . . . . . . . . . . . . 116
5.4 Integrated Power Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5.5 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.6 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6 Conclusions 134
6.1 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
A Intelligent Distribution Panel 140
A.1 IDP Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
A.2 Measurements and Processing . . . . . . . . . . . . . . . . . . . . . . . . 143
A.3 IDP Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . 145
A.4 Appendix Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
B Household Load Modeling and Emulation 150
B.1 Load Pro(cid:12)le Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
B.2 Load Pro(cid:12)le Emulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
B.3 Appendix Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
viii
List of Tables
1.1 Comparison of AC and DC Micro and Nano-Grids . . . . . . . . . . . . . 6
2.1 Operational Regions of V and Interpretations for PV/Battery MIVs and
g
Local Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2 Parameters of the Simulated Nano-Grid . . . . . . . . . . . . . . . . . . . 31
3.1 MIV Prototype Speci(cid:12)cations . . . . . . . . . . . . . . . . . . . . . . . . 54
3.2 LIC and PV Speci(cid:12)cations . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.1 Dual Mode DAB Converter Prototype Speci(cid:12)cations . . . . . . . . . . . . 81
4.2 DAB Converter with PTS Scheme Prototype Speci(cid:12)cations . . . . . . . . 100
5.1 DAB Converter Prototype Speci(cid:12)cations with External Power Stage . . . 123
5.2 DAB Converter Prototype Speci(cid:12)cations with Internal Power Stage as
Primary-Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
5.3 Comparison of Driving Schemes in Isolated Converters . . . . . . . . . . 131
A.1 Comparison of IDP MSOGI-FLL Outputs and Readings from E-Load for
Four Test Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
ix
List of Figures
1.1 Projected world energy consumption between 2010 and 2040 [1]. . . . . . 2
1.2 Visualizationoftheremoteplacesonearth, createdbytheEuropeanCom-
missions Joint Research Centre and the World Bank [6]. . . . . . . . . . 2
1.3 Price history of silicon PV cells ($/W ) [8]. . . . . . . . . . . . . . . . . . 3
p
1.4 Architecture of (a) a nano-grid with central ESS and generator, and (b)
the proposed modular nano-grid. . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Two-stage MIV architecture with integrated storage targeted in this work. 10
1.6 The full system implementation of the nano-grid. . . . . . . . . . . . . . 11
2.1 Equivalent Thevenin model of a single MIV connected to the nano-grid. . 20
2.2 P-V droop characteristic for (a) battery, and (b) PV MIVs. . . . . . . . 22
2.3 Q-f droop characteristic for battery and PV MIVs. . . . . . . . . . . . . 23
2.4 Control approaches for the PV and battery inverters based on regulation
of virtual resistance and the synthetic internal voltage. . . . . . . . . . . 25
2.5 (a) P, and (b) Q for the two parallel battery inverters with different SOC.
SOC = 0.25, and SOC = 0.1. The inverters are cold started at t = 0 s. 28
n1 n2
2.6 (a) P, and (b) Q for the two parallel battery inverters with different series
resistances. R = 0.1 Ω, and R = 0.3 Ω. . . . . . . . . . . . . . . . . . 29
L1 L2
2.7 Block diagram of the inverter control implementation. . . . . . . . . . . . 30
2.8 The RMS nano-grid voltage, V , throughout the simulated test scenario. . 31
g
2.9 The real power of MIV1-5, P , throughout the simulated test scenario. 32
1(cid:0)5
x