multi-levelac-dc power electronic converter for applicationsin … · 2016-07-21 ·...
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Multi-Level AC-DC Power Electronic Converter
for Applications in PMG-Based WECSs
by
A B M Saadmaan Rahman
BSc.E, Military Institute of Science and Technology,
Dhaka, Bangladesh, 2012
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
Masters of Science in Engineering
In the Graduate Academic Unit of the Electrical and Computer Engineering
Supervisor: Saleh Saleh, PhD, Electrical and Computer EngineeringExamining Board: Yevgen Biletskiy, PhD, Electrical and Computer Engineering
Rodney Cooper, PhD, Computer Science
This thesis is accepted
Dean of Graduate Studies
THE UNIVERSITY OF NEW BRUNSWICK
June, 2016
c©A B M Saadmaan Rahman, 2016
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Abstract
Due to their structural and operational features, permanent magnet generators
(PMGs) have gained popularity in wind energy conversion systems (WECSs).
The general structure of a PMG-based WECS is composed from a voltage source
(VS) ac-dc power electronic converter (PEC)(generator-side PEC), which feeds
a VS dc-ac PEC (grid-side PEC) that delivers both active and reactive powers
to a grid and/or a load. Such a structure (usually called a back-to-back PEC
WECS) offers an independent control of both PECs to accommodate variable
wind speed operation. Despite the flexible control, back-to-back PEC PMG-based
WECSs suffer a disadvantage due to the current harmonics generated on inputs of
the generator-side PEC. These current harmonics create distortions in the stator
magnetic field of PMG, and cause pulsations in the electromagnetic torque. The
pulsations in electromagnetic torque of a PMG can lead to several undesired op-
erating conditions, including sustained mechanical vibrations in the wind turbine
tower, damages to the turbine shaft and rotor mechanical assembly, wear-outs of
mechanical fittings of the PMG and turbine couplings, and difficulties in realizing
the maximum power point tracking (MPPT) operation. This research aims to
investigate the possibility of reducing the pulsations in the electromagnetic torque
of a PMG, when used in back-to-back PEC WECSs. The proposed approach is
based on employing a 3φ, multi-level, VS, ac-dc PEC as the generator-side PEC.
The ability of multi-level ac-dc PECs to reduce current harmonics on their inputs
will be employed for achieving the objectives of this research.
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Acknowledgments
I would like to express my gratitude to my Supervisor, Dr Saleh for his guidance
and help during this journey of research. He has always encouraged me to
bring innovations into my work and enriched my knowledge to the world of
Power Electronics. I also want to take this opportunity to thank department of
Electrical and Computer Engineering, University of New Brunswick for providing
support in every phase of my graduate study.
I dedicate this work to my mother who has always been my strength during
any struggle. I also would like to thank my wife, Binti, for being the source of
inspiration in every possible way to encourage me completing this degree.
Last but on the least, I would like to thank my family and friends for their support
during this entire journey of graduate research.
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Contents
Abstract ii
Acknowledgements iii
Contents iv
List of Tables vii
List of Figures viii
1 Introduction 1
1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.4 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Structure of Permanent Magnet Generator Based Wind Energy
Conversion Systems 7
2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 The Structure of PMG-based WECSs . . . . . . . . . . . . . . . . . 8
2.3 Modeling of PMG . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
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2.4 Generator-side PEC . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.5 DC-Link . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.6 Grid-side PEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.7 Grid Connection Circuitries . . . . . . . . . . . . . . . . . . . . . . 17
2.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3 Multi-Level AC-DC Generator-Side PEC 20
3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2 Multi-level AC-DC PEC . . . . . . . . . . . . . . . . . . . . . . . . 21
3.3 Changing the Layout of the H-Bridge . . . . . . . . . . . . . . . . . 27
3.4 3φ Multi-Level AC-DC PEC . . . . . . . . . . . . . . . . . . . . . . 31
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4 Performance Testing of 3φ ac-dc 5-level PEC 40
4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.2 Square-Wave Switching . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.3 Multi-Pulse Switching of a 3φ Multi-level ac-dc PEC . . . . . . . . 41
4.3.1 PWM-Based Multiple Pulse Switching for Multi-Level PECs 43
4.3.2 Level-Shifted PWM . . . . . . . . . . . . . . . . . . . . . . . 43
4.4 Performance Results . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.4.1 The Ideal 3φ Supply . . . . . . . . . . . . . . . . . . . . . . 46
4.4.2 The PMG Test Case . . . . . . . . . . . . . . . . . . . . . . 50
4.5 Performance Comparison . . . . . . . . . . . . . . . . . . . . . . . . 54
4.5.1 Harmonic Distortion in Input Currents . . . . . . . . . . . . 54
4.5.2 Produced Electromagnetic Torque . . . . . . . . . . . . . . . 56
4.5.3 The Ripple in Electromagnetic Torque . . . . . . . . . . . . 57
4.5.4 The Output Power . . . . . . . . . . . . . . . . . . . . . . . 59
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4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5 Conclusion and Future Work 62
5.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.3 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.4 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
Bibliography 67
Curriculum Vitae
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List of Tables
3.1 Switching Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 Square-Wave Switching Pattern for a 5-Level AC-DC PEC shown
in Figure 3.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.3 Square-Wave Switching Pattern for a 3φ 5-Level AC-DC PEC. . . 37
4.1 Parameters of the PMG . . . . . . . . . . . . . . . . . . . . . . . . 50
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List of Figures
2.1 A schematic diagram for a PMG-based WECS. . . . . . . . . . . . 9
2.2 The equivalent circuits for a PMG using the d-q-axis components. . 11
2.3 Schematic diagram of (a) 3φ full-wave diode rectifier (b) 3φ VS,
6-pulse ac-dc PEC. . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4 Schematic diagram of (a) 3φ full-wave diode rectifier with capacitor
dc-link (b) 3φ, VS, 6-pulse ac-dc PEC with capacitor dc-link. . . . . 14
2.5 Schematic diagram of (a) 3φ full-wave diode rectifier with dc boost
PEC dc-link (b) 3φ, VS, 6-pulse ac-dc PEC with dc boost PEC
dc-link. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.6 Schematic diagram of 3φ VS, 6-pulse dc-ac PEC with grid synchro-
nization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1 Schematic diagram of a 2-level dc-ac PEC operated as an ac-dc PEC. 22
3.2 Schematic diagram of a 5-level dc-ac PEC operated with square-
wave switching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.3 Schematic diagram of a 5-level dc-ac PEC operated with square-
wave switching to realize an ac-dc PEC function. . . . . . . . . . . 24
3.4 Configuration of a conventional single ac supply 5-level dc-ac PEC
with square wave switching actions to realize an ac-dc PEC function. 26
3.5 Interval Combinations of the Switching actions. . . . . . . . . . . . 26
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3.6 The changes in the configuration of controlled switches in the H-
bridge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.7 Proposed layout for a 5-level ac-dc PEC (2 H-bridges). . . . . . . . 29
3.8 Simulated currents drawn by a 1φ 5-level ac-dc PEC and a conven-
tional 1φ ac-dc PEC. . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.9 Layout of a single H-bridge to construct a 3φ ac-dc PEC. . . . . . . 32
3.10 Layout of a 3φ 5-level cascaded H-bridge ac-dc PEC. . . . . . . . . 33
3.11 The conduction switches and flow of the 3φ currents for the 3φ
5-level ac-dc PEC over T1. . . . . . . . . . . . . . . . . . . . . . . . 34
3.12 The conduction switches and flow of the 3φ currents for the 3φ
5-level ac-dc PEC over T2. . . . . . . . . . . . . . . . . . . . . . . . 34
3.13 The conduction switches and flow of the 3φ currents for the 3φ
5-level ac-dc PEC over T3. . . . . . . . . . . . . . . . . . . . . . . . 35
3.14 The conduction switches and flow of the 3φ currents for the 3φ
5-level ac-dc PEC over T4. . . . . . . . . . . . . . . . . . . . . . . . 35
3.15 The conduction switches and flow of the 3φ currents for the 3φ
5-level ac-dc PEC over T5. . . . . . . . . . . . . . . . . . . . . . . . 36
3.16 The conduction switches and flow of the 3φ currents for the 3φ
5-level ac-dc PEC over T6. . . . . . . . . . . . . . . . . . . . . . . . 36
3.17 Simulated 3φ input currents drawn by a 3φ 5-level ac-dc PEC. . . . 38
4.1 Reference and carriers for a 5-level PEC. . . . . . . . . . . . . . . . 45
4.2 Generated LSPWM Switching Pulses for a 5-level PEC. . . . . . . . 45
4.3 Performance results of the developed 3φ, 5-level, ac-dc PEC, when
fed by an ideal 3φ supply: (a) the 3φ input line voltages and (b)
the output dc voltage across the dc-link. . . . . . . . . . . . . . . . 47
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4.4 Performance results for the developed 3φ, 5-level, ac-dc PEC, when
fed by an ideal 3φ supply: the 3φ input currents. . . . . . . . . . . 48
4.5 Performance results for the developed 3φ, 5-level, ac-dc PEC, when
fed by an ideal 3φ supply: the harmonic spectrum of IA. . . . . . . 49
4.6 Performance results for a 3φ, diode rectifier, when fed by an ideal
3φ supply: the harmonic spectrum of IA. . . . . . . . . . . . . . . . 49
4.7 Performance results of the developed 3φ, 5-level, ac-dc PEC, when
fed by a non-ideal 3φ supply: (a) the 3φ input line voltages (the
base value is 300 V) and (b) the output dc voltage across the dc-link
(the base voltage is 300 V). . . . . . . . . . . . . . . . . . . . . . . 51
4.8 Performance results for the developed 3φ, 5-level, ac-dc PEC, when
fed by a non-ideal 3φ supply: the 3φ input currents (the base current
value is 100 A). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.9 Performance results for the developed 3φ, 5-level, ac-dc PEC, when
fed by a non-ideal 3φ supply: the harmonic spectrum of IA. . . . . . 52
4.10 Performance results for a 3φ, ac-dc PEC, when fed by a non-ideal
3φ supply: the harmonic spectrum of IA. . . . . . . . . . . . . . . . 53
4.11 The harmonic distortion in input currents of the 5-level ac-dc PEC,
3φ, 6-pulse, PWM ac-dc PEC, and 3φ full-wave rectifier for input-
side frequencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.12 Electromagnetic torque produced by the PMG for using the 5-level
ac-dc PEC, 3φ, 6-pulse, PWM ac-dc PEC, and 3φ full-wave rectifier
as generator-side PECs. The base torque is 1000 N.m. . . . . . . . . 56
4.13 The ripples in electromagnetic torque produced by the PMG, when
using the 5-level ac-dc PEC, 3φ, 6-pulse, PWM ac-dc PEC, and 3φ
full wave rectifier as generator-side PECs. . . . . . . . . . . . . . . 58
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4.14 The output power of the 5-level ac-dc PEC, 3φ, 6-pulse, PWM ac-dc
PEC, and 3φ full wave rectifier when used as generator-side PECs
for different input-side frequencies. The base power is 50 kW. . . . 60
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Chapter 1
Introduction
1.1 General
During the past few years, power electronics has been playing an important role
in converting and controlling electric power generated from wind turbines with
the rapidly growing wind energy capacity around the world. Desired functions of
a variable speed wind energy conversion system (WECS) are to produce electric
power over a wide range of wind speeds, and to deliver electric power over a wide
range of power factors. Permanent magnet generator (PMG)-based WECSs have
demonstrated good abilities to perform such functions at various power ratings.
The removal of extra excitation device and the adoption of the direct-driven form
make PMG a suitable option for variable speed WECSs. One of the most popu-
lar designs of PMG-based WECSs is the back-to-back power electronic converter
(PEC) PMG-based WECS, which is typically composed of
i) a 3φ voltage-source (VS) ac-dc PEC (called the generator side PEC);
ii) a dc link (could be a capacitor or a dc-dc PEC);
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iii) a 3φ or a 1φ VS dc-ac PEC (called the grid-side PEC)
iv) other auxiliary components, such as a coupling filter, a grid connection trans-
former and a grid synchronization unit.
The back to back PEC PMG-based WECS offers controlling the generator-side
PEC and grid-side PEC independently. The generator-side PEC is responsible for
producing regulated dc power and the grid-side PEC is used to control the active
and reactive power delivered to the grid [1]. In addition, this WECS can facilitate
a simple realization of the maximum power point tracking (MPPT) operation.
1.2 Motivation
A back-to-back PEC PMG-based WECS has a 3φ VS ac-dc PEC as the generator-
side PEC, which suffers a tendency to generate harmonic components in its input
ac currents. These harmonic components flow through the stator windings of a
PMG, and can create distortions in its 3φ rotating magnetic field. Such distor-
tions in the stator magnetic field are capable of triggering significant pulsations
in the electromagnetic torque, which can result in several operational and control
problems. The reduction of torque pulsations in back-to-back PEC PMG-based
WECSs can be achieved by employing multi-level ac-dc PECs. The ability of a
3φ multi-level ac-dc PEC to reduce harmonics in its input ac current motivates
its application in PMG-based WECS.
1.3 Literature Review
The technology for constructing PMG-based WECSs has been through several
stages. The earliest designs fo PMGs have been mostly used for co-generation
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systems that are operated with a direct connection to a host grid or a load. The
use of PMGs in WECSs has mandated power conditioning circuits to eliminate the
variations of voltage and/or frequency. These mandates have motivated the use
of the back-to-back PECs. These PECs are mainly designed to convert variable
frequency voltages and currents produced by a PMG to DC ones, before convert-
ing them into controllable currents and fixed voltages at a constant frequency.
The early designs of PMG-based WECSs have employed 3φ, full-wave, diode rec-
tifiers as generator-side PECs. These designs offered simple and efficient perfor-
mance in terms of the number of switching elements [1,2]. However, the main chal-
lenges for employing diode rectifiers as generator-side PECs include uncontrolled
outputs and significant harmonics in the input 3φ currents. Although integration
of different control methods with the 3φ diode rectifiers has demonstrated good
ability to reduce harmonic components from its input side, the uncontrolled diode
rectifiers degrade the performance of PMG-based WECSs eventually [3,4]. These
challenges for 3φ rectifiers (in PMG-based WECSs) have resulted in complications
for variable speed operation, along with pulsations in the electromagnetic torque
produced by PMGs.
The employment of switch mode rectifiers, as generator-side PECs, has offered
controlling the electromagnetic torque in order to facilitate the variable-speed op-
eration of PMG-based WECSs. However, the control of electromagnetic torque
becomes difficult in the presence of torque pulsations, which are produced by
distortions in the stator magnetic field. Such distortions are mainly caused by
the harmonics in the stator currents, that can result from switching actions of a
generator-side PEC. One of the popular methods developed to control the elec-
tromagnetic torque of PMGs is the switch-mode rectifier (SMR) [5-8]. An SMR
is typically composed of a 3φ full wave diode rectifier that feeds a boost dc-dc
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PEC. The main objective of an SMR is to control the output current of the rec-
tifier, hence controlling the stator current of the PMG. Several designs of SMRs
have been developed and employed in PMG-based WECSs. SMRs in PMG-based
WECSs have improved the performance of these WECSs in terms of controlling
the electromagnetic torque to facilitate generating the maximum power at differ-
ent wind speeds [5]. Despite their simplicity and efficiency, SMRs still use 3φ full
wave diode rectifiers that can create significant current harmonics on their input
side [1,6]. Such current harmonics can be a major cause for pulsations in the
electromagnetic torque of PMGs. Finally, the new PMG-based WECSs are being
designed for high power ratings (in the MW ranges) at higher operating voltages.
Such high ratings may result in some operational complications for the ac-dc PEC,
especially for the voltage and current transients
The limitations demonstrated by the 3φ rectifiers and SMRs in PMG-based
WECSs have motivated the employment of 3φ ac-dc PECs. These PECs are typ-
ically configured as VS, 6-pulse converters with a dc-link capacitor on the output
side [2,10]. The literature reports various switching strategies utilized to operate
3φ, VS, 6-pulse, ac-dc PECs for applications in PMG-based WECSs. Pulse width
modulation (PWM), and its various implementations, and space vector modu-
lation (SVM) have been widely used in operating 3φ ac-dc PECs employed in
PMG-based WECSs [2,9-12]. The 3φ, VS, ac-dc PECs have offered several advan-
tages including, reduced current harmonics on the input side (relative to the 3φ
rectifiers and SMRs), good ability to be integrated with various control methods
to establish MPPT operations, and improved ability to support independent con-
trol of the generator-side and grid-side PECs. Although 3φ, VS, ac-dc PECs have
shown the ability to improve their input current, the high switching frequency may
degrade the overall efficiency of PMG-based WECSs. In addition, the operation of
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these PECs for variable frequency ac inputs (as a result of variable wind speeds)
may impact their ability to maintain reduced current harmonics.
In general, for most ac-dc PECs (rectifiers, SMRs and 3φ ac-dc PECs), the har-
monics in the input currents, are usually reduced by filters on their input side.
Such filters are composed of a series inductance and a shunt capacitance [1,2,6].
However, for PMG-based WECSs, utilizing such filters may create partial res-
onance with the stator windings of the PMG (high equivalent inductance) and
aggravate torque pulsations and affect the control of the electromagnetic torque.
The 3φ ac-dc generator-side PEC is a critical component in PMG-based WECSs,
where its function is to convert a variable frequency ac power into a regulated dc
power. This function has been extended to controlling the electromagnetic torque
of PMGs in order to facilitate MPPT operation for variable wind speeds. Existing
topologies of 3φ ac-dc generator-side PECs have been mostly based on single-level
PECs to achieve the aforementioned functions [1,2,9]. The main challenge for ex-
isting generator-side PECs is due to their inherent tendencies to create harmonic
distortions in their ac currents, which flow through the stator windings of PMGs.
Such harmonic distortions are the major cause of torque pulsations that can ad-
versely affect the overall performance of PMG-based WECSs [12]. The reduction
of torque pulsations can be achieved by employing a generator-side PEC that is
capable of significantly reducing the harmonic distortions in its input currents.
The development and operation of such a 3φ ac-dc PEC is the main motivation
of this research work.
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1.4 Objectives
The main objective of this research is to design and test a PEC that can reduce
the pulsations in the electromagnetic torque of a PMG, when used in back-to-back
PEC WECSs. The desired PEC is designed as a 3φ, VS, multi-level, ac-dc PEC
to be used as the generator-side PEC. Furthermore, this thesis aims to investigate
the performance of the multi-level ac-dc PEC, when used in variable-speed PMG-
based WECSs.
1.5 Thesis Outline
This thesis will be split into following chapters:
i) Chapter 2 overviews the general structure of a PMG-based WECS, with the
focus on the variable speed ones that employ back-to-back PECs. This chap-
ter also highlights the challenges that can result from harmonic distortion
produced by the generator-side PECs.
ii) Chapter 3 describes the proposed 3φmulti-level ac-dc PEC, both the structure
and operation as generator-side PEC. Furthermore, chapter-3 discusses the
switching strategies that can be used to operate the proposed ac-dc PEC.
iii) Chapter 4 provides performance results for employing the developed 3φ multi-
level ac-dc PEC in a variable-speed PMG-based WECS. In addition, chapter-4
provides performance comparisons with other generator-side PECs (used in
variable-speed PMG-based WECSs).
iv) Chapter 5 provides a summery of the thesis, its contributions, and avenues
for possible future work.
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Chapter 2
Structure of Permanent Magnet
Generator Based Wind Energy
Conversion Systems
2.1 General
The steady increases in the demands for clean and sustainable electrical power
supplies have motivated the integration of various renewable energy sources.
Wind, solar, tidal, geo-thermal, and fuel-cells are the popular renewable en-
ergy sources, which have been interconnected to power systems. Wind En-
ergy Conversion Systems (WECSs) have gained remarkable interest over other
types of renewable energy sources due to their diverse designs, control, power
ratings, and ability to contribute to different functions in their host power
systems both in steady state and transient conditions [1,11,12].
There are several approaches to classify WECSs, among which are:
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i) Grid-interconnected; without power electronic converters (PECs) (partial
or full interface), or with PECs
ii) Speed; variable-speed, or fixed-speed
iii) Power ratings; small, medium, large
iv) Type of electric generator; induction, synchronous, or permanent magnet.
Permanent magnet generators (PMGs) have demonstrated several structural
and operational advantages over other generators used in WECSs. Among
these advantages are the robust and stable field structure, wide range of op-
erating speeds, simple control over different values of power factor [13-17]. In
addition, the recent advances in PMG technology have facilitated designing
these machines with high power-to-weight ratios; along with high efficiencies
[16-17].
2.2 The Structure of PMG-based WECSs
The general structure of PMG-based WECS is set to achieve three main
functions; power generation, power conditioning, and power delivery [17].
The realization of these functions is achieved by the following stages:
i) Power generation, which is composed of the wind turbine assembly and
PMG.
ii) Power conditioning, which is composed of an ac-dc PEC (generator-side
PEC), a dc-link, a dc-ac PEC (grid-side PEC), and grid-connecting cir-
cuitries (an ac filter and a transformer).
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iii) Power delivery, which may include a synchronizing unit (for grid-
connected operation) or load feeder (for stand-alone operation).
Figure 2.1: A schematic diagram for a PMG-based WECS.
These three stages comprising a grid-connected PMG-based WECS are illus-
trated in Figure 2.1.
The operation of the generator-side and grid-side PECs is set based on control
approaches, which depend on the mode of operation of the WECSs. The
control of the generator-side PEC is mainly designed to accomplish maximum-
power-point-tracking (MPPT). Such a control can be implemented using a dc-
dc PEC (in the dc-link), or by decoupled control of the ac-dc PEC [1,9,10,18].
The control of the grid-side PEC is mostly developed as a decoupled current
controller for grid-connected operation. However, for a stand-alone operation,
the control is designed as two nested loops, the inner one is for current control
while the outer one is for voltage control. The use of two PECs in the power
conditioning stage is commonly called back-to-back structure, and its main
advantage is the ability to operate and control the generator-side and the
grid-side PECs independently [14,15,17].
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2.3 Modeling of PMG
The deployment of modern design methods in permanent magnet machines
(e.g. finite element methods) has offered manufacturing PMGs with large
number of poles, as well as high power ratings. Such PMGs have become
popular in fixed and variable speed WECSs [15,17].
In order to provide an insight of PMGs, the popular approach is to consider
their model, developed in terms of d-q-axis components as:
Vds =Rsid + Ld
diddt
− ΩrLqiq (2.1)
Vqs =Rsiq + Lq
diqdt
+ ΩrLdid + Ωrψm (2.2)
where Vds and Vqs are the direct-axis and quadrature-axis stator voltages
respectively, Id and Iq are the direct-axis and quadrature-axis stator currents
respectively, Rs is the stator resistance, Ld and Lq are the direct-axis and
quadrature-axis inductances respectively, ψm is the magnetic flux, and Ωr is
the rotor speed.
The model, in equations (2.1) and (2.2), can be further illustrated by con-
structing the equivalent circuits in the d-axis and q-axis, as shown in Figure
2.2 [15,17].
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Figure 2.2: The equivalent circuits for a PMG using the d-q-axis components.
For the sake of presenting a complete model of a PMG, the developed elec-
tromagnetic torque, Te, can be expressed as [15];
Te =3P
4((Ld − Lq)iqid − ψmiq) (2.3)
where P is the number of magnetic poles.
2.4 Generator-side PEC
The general theory of the 3φ synchronous machine, including the permanent
magnet machine, relates the induced voltage in the stator windings with the
rotor speed and the flux density of the rotor magnetic field. However, for
permanent magnet machines (motors or generators), the flux density of the
rotor magnetic field is constant due to the use of permanent magnets. As
a result, the magnitude and frequency of the voltage induced in the stator
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windings of a PMG are dependent on the rotor speed. When used in WECSs,
the voltage on the terminals of a PMG will have variable magnitude and
frequency depending on the wind speed [17]. These features of the induced
voltage do not allow the delivery of the power generated by PMG driven by a
wind turbine directly to loads or a host power system. In order to utilize the
power generated by a variable-speed PMG, the frequency has to be maintained
constant, while the voltage has to be close-to-sinusoidal wave form (to match
the nominal voltage).
Back-to-back PEC designs for WECSs are used to interface PMGs with loads
or a host power system for meeting such a requirement. The first PEC (com-
monly called the generator-side PEC) in the back-to-back PECs design is a
power converter that can eliminate the variations in the voltage magnitude
and frequency. Such a PEC can meet this objective by converting ac voltages
with variable magnitude and frequency into dc ones. This function of the
ac-dc PEC can create undesired frequency components in its input currents.
Since the input currents of the generator-side PEC flow in the stator windings
of the PMG, torque pulsations and mechanical vibrations may be created by
the current distortion.
Existing designs of the PMG-based WECSs utilize two main topologies for
the generator-side PEC. These topologies are:
i) 3φ full-wave diode rectifiers
ii) 3φ, VS, 6-pulse, ac-dc PECs
Existing designs for the generator-side PEC employed in PMG-based WECS
are illustrated in Figure-2.3.
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Figure 2.3: Schematic diagram of (a) 3φ full-wave diode rectifier (b) 3φ VS, 6-pulseac-dc PEC.
The performance of these generator-side PECs can be evaluated based on the
structure, operational requirements, efficiency, quality of input ac currents,
and controllability [3,5,14]. Nevertheless, the new designs of high power rated
PMGs for WECSs demand the use of generator-side PECs that can reduce
the harmonic distortion in their ac input currents.
2.5 DC-Link
The dc voltage produced by the generator-side ac-dc PEC contains harmonic
components, which can adversely impact the operation of the grid-side dc-ac
PEC. In order to avoid such impacts on the grid-side dc-ac PEC, a dc-link
is used as a mid-stage between the generator-side and the grid-side PECs.
The main functions of the dc-link are to stabilize the input dc voltage to the
grid-side PEC, to facilitate its operation, and simplify its control. In general,
the dc-link, for WECSs, can be designed as:
i) a capacitor;
ii) a dc-dc PEC, which can be designed as buck, boost, or buck-boost dc-dc
PEC
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Capacitor dc-links can offer a simple structure and a good ability to prevent
harmonic components from flowing through the grid-side PECs. These dc-
links are widely used for fixed speed WECSs, as well as integral horse-power
frequency motor drives (high power 3φ motor drives that are fed by back-
to-back PECs). However, for variable speed WECSs, capacitor dc-links have
shown limited abilities to maintain stable voltage at the input of the grid-
side PECs. For variable speed WECSs, several dc-links are designed with dc
PECs, which can offer stabilizing the voltage at the input side of the grid-side
PECs.
Figure 2.4: Schematic diagram of (a) 3φ full-wave diode rectifier with capacitor
dc-link (b) 3φ, VS, 6-pulse ac-dc PEC with capacitor dc-link.
The general configurations of dc-links are demonstrated in Figure 2.4 and
Figure 2.5.
Figure 2.5: Schematic diagram of (a) 3φ full-wave diode rectifier with dc boost
PEC dc-link (b) 3φ, VS, 6-pulse ac-dc PEC with dc boost PEC dc-link.
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In addition, the control of the dc PEC can be employed to regulate the input
currents to the generator-side PEC. This feature of dc PEC-based dc-links has
offered adjusting the torque of the generator. The control of torque developed
by the generator has facilitated the implementation of the MPPT. The dc
PEC-based dc-links have been widely used with 3φ, full wave, diode rectifiers
for generator-side PEC. These types of dc-links have been also used with 3φ,
VS, ac-dc PECs, when used as generator-side PECs. One of the operational
requirements for the dc PEC-based dc-links is switching frequency, which has
to be set higher than switching frequencies of the generator-side and grid-side
PECs [9,19,20,21]. The recommendation for the switching frequencies are
generally set as:
(fs)dcPEC ≥ 10(fs)dc−acPEC (2.4)
(fs)dcPEC ≥ 15(fs)ac−dcPEC (2.5)
where (fs)dcPEC is the switching frequency of the dc PEC, (fs)dc−acPEC is the
is the switching frequency of the grid-side dc-ac PEC and (fs)ac−dcPEC is the
switching frequency of the generator-side ac-dc PEC.
The conditions of the switching frequency of the dc PEC are set to ensure
stable dc voltage on the input of the grid-side PEC. These settings for the
switching frequency of the dc PEC also ensure fast adjustments of the dc
voltage fed to the grid-side PEC. However, the high switching frequency tends
to increase the power losses, thus decreasing the efficiency of the PMG-based
WECS.
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Figure 2.6: Schematic diagram of 3φ VS, 6-pulse dc-ac PEC with grid synchro-nization.
2.6 Grid-side PEC
In order to deliver the electric power generated by a PMG-based WECS, to a
host power system, the power generated by the PMG has to be converted to ac
power at a frequency and a voltage that meet those of the host power system.
Such a goal is achieved by employing a voltage source dc-ac PEC, which
is commonly called the grid-side PEC. The functions of the grid-side PEC
include controlling the active and reactive powers delivered to the host power
system, while maintaining an output voltage that meets the requirements of
the host power system. The critical role of the grid-side PEC in variable
speed WECSs, has motivated the development of several PEC topologies,
along with different controllers. In general, the majority of the controllers
developed for the gird-side PEC, are designed as current controllers. However,
some controllers for the grid-side PEC are designed with current and voltage
controllers.
The grid-side PEC can be configured as a 1φ, mostly used for low power rated
WECSs, or a 3φ PEC. Finally, the grid-connection of the grid side PEC is
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usually realized by a grid-coupling filter, a grid-connection transformer, and
a grid-synchronizing unit. Figure 2.6 shows a schematic diagram for a 3φ,
6-pulse grid-side PEC and its grid-connection circuitries.
2.7 Grid Connection Circuitries
The grid connection circuitries are composed of three major parts, which are:
a) The AC filter: This part is responsible for removing the harmonic com-
ponents produced by the grid-side dc-ac PEC, as well as rejecting distur-
bances originated from the host power system. The grid-coupling filter
can be designed as L, LC, and LCL filters. The LCL filter has advantages
over other filters, including the attenuation of voltage and current ripples
created at the output of the grid-side PEC, good abilities to reject distur-
bances from the grid side, and a realization with small components. The
LCL grid-coupling filter is usually designed with the following constraints
[20,23]:
i) The value of the capacitor CF is limited by an acceptable change in
the power factor (∆PF ≤ 5%), at the point of common coupling
(PCC) for delivering the maximum power through PCC. Moreover,
the stability of the dc voltage (on the input of the grid-side PEC) is
maintained by limiting the maximum value of LI and LG as:
(LI + LG) ≤ 0.1H (2.6)
ii) The resonance frequency, fr of the LCL grid-coupling filter has to be
selected higher than the grid frequency, fg and less than the switching
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frequency of the grid-side PEC, (fs)dc−ac. The design constraint is
commonly realized by setting fr as:
10fg ≤ fr ≤(fs)dc−ac
2(2.7)
This selection is made to avoid any possible frequency resonances
between the elements of the LCL filter. It should be noted that the
switching frequency of the grid-side PEC is usually selected as:
15fg ≤ (fs)dc−ac ≤ 25fg (2.8)
iii) The resistance, RD is selected as a tradeoff between the required
damping and power losses.
b) The grid-connection transformer: This transformer is responsible for iso-
lating the PMG-basedWECS from its host power system, especially during
ground faults on either side of PCC. In addition, this transformer can be
configured to allow a grounding path for the PMG-based WECS, thus sim-
plifying the ground fault protection and improving the voltage stability at
PCC.
c) The synchronizing unit: This part is responsible for establishing the grid
connection mode of operating the PMG-based WECS, as it maintains the
grid connection as long as the voltage and frequency conditions are met.
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2.8 Summary
A PMG-based WECS is structured to convert the power captured from the
wind to an ac power, with variable frequency, on the output of the PMG. This
ac power is then fed into the generator-side PEC to convert it to a dc power,
which then flows through the dc-link. The dc-link maintains a constant dc
voltage, and delivers the dc power to the grid-side PEC. The dc power is
converted back to an ac one at a frequency and voltage that meet those of
the host power system (or the ratings of an isolated load). Finally, the ac
power is delivered to a host power system, or a load, via grid-connecting
circuitries. These stages of the power conversion introduce undesired levels of
distortion. The most critical distortion is the one created by the generator-
side PEC, which can adversely affect the PMG operation. Existing solutions
represent trade-offs between distortion levels, efficiency, and controllability.
Chapter 3 presents another approach to reduce the distortions on the inputs
of the generator-side PEC. The intended approach is based on employing a
multi-level ac-dc PEC, which can offer good abilities to reduce the energy in
harmonic components.
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Chapter 3
Multi-Level AC-DC
Generator-Side PEC
3.1 General
The distortions in the stator current of a PMG are among the most crit-
ical concerns for the operation, control, efficiency, and stability of PMG-
based WECS. The source of these distortions is the switching actions of the
generator-side ac-dc PEC. Existing approaches for reducing such distortions
are mostly based on controlling the stator currents of the PMG. These ap-
proaches can perform efficiently for low levels of distortion. As a result, re-
sponses of existing approaches are contingent upon the inherent capabilities
of the conventional generator-side ac-dc PECs. In this work, an alternative
approach to minimize the distortion in stator currents of a PMG is proposed
based on the employment of the multi-level ac-dc PEC. The rationale for
proposing a multi-level ac-dc PEC is due to its ability to reduce the energy
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in the harmonic components in its input ac currents without the need for
increasing the switching frequency. This chapter presents the topology of a
multi-level ac-dc PEC as generator-side PEC in PMG-based WECSs.
3.2 Multi-level AC-DC PEC
The increasing demands for high power-rated PECs have prompted develop-
ing new technologies for PECs. Among the new developed PEC topologies;
are the multi-level PECs, which have been developed to achieve high input-to-
output power transfer, high voltages and currents, and reduced levels of input
and output harmonic distortions. These new PECs have been designed as dc-
ac PECs for applications in electric transportation systems, medium-voltage
motor devices and high voltage DC (HVDC) systems. The encouraging per-
formance of multi-level dc-ac PECs has prompted the development of different
topologies for these PECs, including:
i) Diode-Clamped
ii) Flying-Capacitor
iii) Cascaded H-Bridge
Nowadays, multi-level dc-ac PECs are manufactured with power ratings that
exceed a hundred kilowatt at voltages that can reach 10kV. The success in
multi-level dc-ac PECs has attracted interest for their possible employment as
ac-dc PECs. Multi-level dc-ac PEC can be controlled and operated to perform
ac-dc conversion. This function of multi-level PECs has been successfully
adapted in electric transportation systems and motor drives [24-27]. However,
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such applications represent a limited deployment of multi-level PECs as ac-
dc PECs. Furthermore, the operation of multi-level PEC as an ac-dc PEC
produces different dc output voltages as illustrated in Figure 3.1.
Figure 3.1: Schematic diagram of a 2-level dc-ac PEC operated as an ac-dc PEC.
The employment of multi-level PECs as ac-dc PECs has been based on operat-
ing a dc-ac PEC as a four-quadrant PEC. In such an operation, input voltages
and currents can be either positive or negative. In order to achieve an ac-dc
PEC operation using multi-level PECs, the control has to be set to realize
a four-quadrant operation of a multi-level dc-ac PEC. If the application in
PMG-based WECS is considered, a four-quadrant operated multi-level dc-ac
PEC may not be able to reduce the distortion on its input-side. In addition,
the use of capacitors in some of the multi-level PECs may raise concerns re-
garding the possibility of partial resonance with stator windings of the PMG.
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The previous discussion suggests using a multi-level PEC that does not have
capacitors, while being able to be operated as an ac-dc PEC without the
need for a four-quadrant control. The H-bridge multi-level PEC can meet
such requirements as it operates without capacitors, and its topology is flex-
ible to accommodate ac-dc PEC function. The desired ac-dc function, using
an H-bridge multi-level PEC can be achieved by changing the layout of the
switching elements in switching cells. Each switching cell is typically com-
posed of 4 switching elements, which can be operated to produce a dc output.
The required modifications of each switching cell, for ac-dc function, can be
achieved by the following changes:
i) Configurations of switching elements
ii) Configuring connecting points of the switching cells to structure multi-
level function
In general, the dc-ac function of an H-bridge multi-level PEC can be achieved
by switching the diagonal switching element in each switching cell. The out-
put of each switching cell is connected in series with other switching cells in
order to create desired switching levels in the overall output voltage. Figure
3.2 shows the output voltage for a 5-level H-bridge multi-level PEC, when
operated as dc-ac PEC.
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Figure 3.2: Schematic diagram of a 5-level dc-ac PEC operated with square-wave
switching.
If the H-bridge multi-level dc-ac PEC is to be operated as an ac-dc PEC
(four-quadrant control), then using the principle of duality, each switching
cell will have its own ac supply. This case is illustrated in Figure 3.3.
Figure 3.3: Schematic diagram of a 5-level dc-ac PEC operated with square-wave
switching to realize an ac-dc PEC function.
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The need for multiple ac supplies to feed the multi-level dc-ac PEC, for op-
erating as an ac-dc PEC, may not be realistic for applications, where one ac
supply is available. One of such cases is the PMG-based WECSs. In addi-
tion, the operation of the H-bridge multi-level dc-ac PEC as an ac-dc PEC
may create high conduction and switching losses, hence reducing the overall
efficiency.
In order to overcome the need for multiple ac supplies, the configuration of
the switching elements can be changed. The changes in the configuration of
switching elements, in each H-bridge, will require changing the connection of
H-bridges to construct the multi-level topology. In general, the number of
required H-bridges in a multi-level PEC can be related to the number of the
levels in the output as [24,25];
d =m− 1
2(3.1)
where d is the number of H-bridges and m is the number of levels in the
output.
As could be seen from Figure 3.3, the employment of a multi-level dc-ac PEC
to function as an ac-dc one requires using multiple ac-supplies. In order to
use one ac supply, a path of the same current has to be created between
the cascaded H-bridges. Such a current path requires activating switching
elements in all H-bridges. This operation of the multi-level PEC can still be
achieved without changes in the original layout of each H-bridge, as illustrated
in Figure 3.4. The operation of multi-level dc-ac PEC as multi-level ac-dc
PEC, as shown in Figure 3.4, can be realized by the switching actions listed
in Table 3.1. It should be noted that the time intervals (T1, T2, T3, T4, T5, T6)
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in Table 3.1 are set based on one cycle of the ac supply, as shown in Figure
3.5.
Figure 3.4: Configuration of a conventional single ac supply 5-level dc-ac PEC
with square wave switching actions to realize an ac-dc PEC function.
0
Time
T1 T2 T3
T4 T5 T6
Figure 3.5: Interval Combinations of the Switching actions.
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Table 3.1: Switching Pattern
Interval OFF Switching Elements ON Switching Elements
T1 Q11,Q12,Q21,Q23 Q13,Q14,Q22,Q24
T2 Q11,Q12,Q21,Q22 Q13,Q14,Q23,Q24
T3 Q11,Q12,Q21,Q23 Q13,Q14,Q22,Q24
T4 Q23,Q24,Q13,Q11 Q12,Q14,Q22,Q21
T5 Q23,Q24,Q13,Q14 Q11,Q12,Q21,Q22
T6 Q23,Q24,Q13,Q11 Q12,Q14,Q22,Q21
The switching actions in Table 3.1 show that during each interval, there are
two active elements in each H-bridge. Since each ON action, by each switch-
ing element, is achieved by deactivating the controlled switch which allows
the anti-parallel diode to conduct. Similarly, each OFF action is created by
activating the controlled switch in order to stop the anti-parallel diode from
conducting. The switching actions by controlled switches are listed in Table
3.1 for each interval over one cycle of the ac supply. The aforementioned
description of operating a multi-level dc-ac PEC as a multi-level ac-dc PEC
fed from one ac supply, indicates high switching losses, due to active switch-
ing elements during both ON and OFF actions. The next section provides
a solution for reducing the needed switching elements for each ON and OFF
action in a multi-level ac-dc PEC fed from one ac supply.
3.3 Changing the Layout of the H-Bridge
The operation of a multi-level dc-ac PEC in four quadrant mode (ac-dc PEC),
implies that anti-parallel diodes provide a path for the current to flow from
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the ac supply. In order to create paths for the current through the controlled
switches, the diodes have to be removed, and the configuration of each con-
trolled switch in a H-bridge cell has to be reversed. These changes are shown
in Figure 3.6.
Figure 3.6: The changes in the configuration of controlled switches in the H-bridge.
The changes of the H-bridge layout, shown in Figure 3.6, allows constructing
a multi-level ac-dc PEC that can be employed in a PMG-based WECS.
Since the desired multi-level ac-dc PEC is to be constructed from more than
one H-bridge, the points-of-connection between the H-bridges have to be
formed with the following constraints:
i) The same current flows through all H-bridges;
ii) Each H-bridge has to be able to perform ac-dc PEC function on its own;
iii) No short circuits are created across any H-bridges.
Considering the previous constraints, a layout for a multi-level ac-dc PEC
(5-level) can be designed as shown in Figure 3.7.
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Figure 3.7: Proposed layout for a 5-level ac-dc PEC (2 H-bridges).
The operation of the multi-level ac-dc PEC in Figure 3.7 offers drawing a
current from the ac supply to flow through all H-bridges. Such a current will
have different magnitudes depending on the number of H-bridges (levels). A
multi-level current can have lower harmonic distortion than that drawn by a
single level ac-dc PEC. This feature of multi-level ac-dc PECs indicates that
increasing the number of levels ensures lower harmonic distortion in the input
ac current. However, increasing the number of levels causes higher switching
losses, as well as complex structures of the multi-level ac-dc PEC.
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Table 3.2: Square-Wave Switching Pattern for a 5-Level AC-DC PEC shown in
Figure 3.7
Interval ON Switching Elements OFF Switching Elements
T1 Q11,Q12,Q22 Q13,Q14,Q21,Q23,Q24
T2 Q11,Q12,Q23 Q13,Q14,Q21,Q22,Q24
T3 Q11,Q12,Q22 Q13,Q14,Q21,Q23,Q24
T4 Q24,Q13,Q14 Q11,Q12,Q21,Q22,Q23
T5 Q21,Q13,Q14 Q11,Q12,Q22,Q23,Q24
T6 Q24,Q13,Q14 Q11,Q12,Q21,Q22,Q23
The desired current can be drawn from ac supply by creating adequate switch-
ing actions of all the controlled switches in the multi-level ac-dc PEC in Fig-
ure 3.7. If the square-wave switching is considered (for simplicity), then the
switching pattern can be stated as in Table 3.2. It should be noted that Ta-
ble 3.2 lists the switching patterns over one cycle of the ac supply. Using the
switching pattern in Table 3.2, the simulated current drawn by a 1φ 5-level
ac-dc PEC, together with the current drawn by a conventional 1φ ac-dc PEC
are shown in Figure 3.8.
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−2
−1
0
1
2
i S[p
.u]
−2
−1
0
1
2
Time
i S[p
.u]
Conventional ac-dc PEC
5-Level ac-dc PEC
Figure 3.8: Simulated currents drawn by a 1φ 5-level ac-dc PEC and a conventional
1φ ac-dc PEC.
The discussed changes in the layout of the H-bridge are focused for a 1φ multi-
level ac-dc PEC. As the objective of this work is to develop a multi-level ac-dc
PEC for a PMG-based WECS, the changes in the 1φ multi-level ac-dc PEC
are to be extended to the 3φ ones. The layout of a 3φ multi-level ac-dc PEC
is discussed in the next section.
3.4 3φ Multi-Level AC-DC PEC
The desired changes in the layout of an H-bridge, for constructing a 3φ multi-
level ac-dc PEC, have to consider the conduction modes of a 3φ ac-dc PEC.
Such modes are based on the fact that at any given time, the 3φ voltage will
have different magnitudes (due to the 120 phase shift). As a result, a change
in the conducting switches will take place each 1/6 of the the supply voltage
period. Such changes result in creating return paths of the currents through
one or two phases. This nature of 3φ ac-dc PECs suggests that each H-bridge
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has to be able to conduct the current from and back to the 3φ supply. In
addition, each H-bridge has to be able to conduct the current from and to
other levels (H-bridges in series). Such constraints can be met by constructing
each H-bridge from two half bridge PECs (forward half bridge and backward
half bridge) as shown in Figure 3.9.
Figure 3.9: Layout of a single H-bridge to construct a 3φ ac-dc PEC.
The proposed H-bridge, composed of two half bridges, can be used to con-
struct a 3φ multi-level ac-dc PEC, as shown in Figure 3.10, (for 5-levels).
For the sake of describing the operation of the proposed 3φ multi-level ac-
dc PEC, the square wave switching is considered. Similar to conventional
(single-level) controlled ac-dc PECs, the changes in the conducting switching
element can be selected based on the instantaneous 3φ supply voltages.
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Figure 3.10: Layout of a 3φ 5-level cascaded H-bridge ac-dc PEC.
As the change in the conducting switches occurs each 1/6 of the voltage
period, the operation of the proposed 3φ multi-level ac-dc PEC is described
over one period of the supply voltage; using 5-levels. For simplicity, the period
of the supply voltage is divided in 6 intervals, T1, T2, T3, T4, T5, T6. Figure
3.11 to Figure 3.16 show the conducting switches and the current flow for
each phase through each H-bridge over the intervals.
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Figure 3.11: The conduction switches and flow of the 3φ currents for the 3φ 5-level
ac-dc PEC over T1.
Figure 3.12: The conduction switches and flow of the 3φ currents for the 3φ 5-level
ac-dc PEC over T2.
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Figure 3.13: The conduction switches and flow of the 3φ currents for the 3φ 5-level
ac-dc PEC over T3.
Figure 3.14: The conduction switches and flow of the 3φ currents for the 3φ 5-level
ac-dc PEC over T4.
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Figure 3.15: The conduction switches and flow of the 3φ currents for the 3φ 5-level
ac-dc PEC over T5.
Figure 3.16: The conduction switches and flow of the 3φ currents for the 3φ 5-level
ac-dc PEC over T6.
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Table 3.3: Square-Wave Switching Pattern for a 3φ 5-
Level AC-DC PEC.
Switching Actions
Interval ON Switching Elements OFF Switching Elements
T1 QA11,QB14,QC11
QA12,QA13,QA14,QB12,QB13,
QB14,QC12,QC13,QC14,QA21,
QA22,QA23,QA24,QB21,QB22,
QB23,QB24,QC21,QC22,QC23,
QC24
T2QA11,QA13,QA21,QB12,QB14,
QB24,QC12,QC14,QC24
QA12,QA14,QA22,QA23,QA24,
QB11,QB13,QB21,QB22,QB23,
QC11,QC13,QC21,QC22,QC23,
T3 QA11,QB11,QC14
QA12,QA13,QA14,QB12,QB13,
QB14,QC12,QC13,QC14,QA21,
QA22,QA23,QA24,QB21,QB22,
QB23,QB24,QC21,QC22,QC23,
QC24
T4QA14,QA12,QA24,QB11,QB13,
QB21,QC12,QC14,QC24,
QB12,QB14,QC11,QC13,QA11,
QA13,QA21,QA22,QA23,QB22,
QB23,QB24,QC21,QC22,QC23
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Continuation of Table 3.3
Interval ON Switching Elements OFF Switching Elements
T5 QA14,QB11,QC14
QA11,QA12,QA13,QB12,QB13,
QB14,QC11,QC12,QC13,QA21,
QA22,QA23,QA24,QB21,QB22,
QB23,QB24,QC21,QC22,QC23,
QC24
T6QB11,QB13,QB21,QC12,QC14,
QC24,QA12,QA14,QA24
QB12,QB14,QB22,QB23,QB24,
QC11,QC13,QC21,QC22,QC23,
QA11,QA13,QA21,QA22,QA23
The switching actions listed on Table 3.3 provide a base case for operating
the developed 5-level ac-dc PEC. If these switching actions are set to create
a square-wave switching, the 3φ currents drawn from a 3φ supply can be as
shown in Figure 3.17. It should be noted that the 3φ input currents have
been simulated with a supply frequency of 60 Hz and phase voltage of 100 V.
2
0
2
IA[p.u]
−2
0
2
IB[p.u]
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
2
0
2
Time [msec]
IC[p.u]
Figure 3.17: Simulated 3φ input currents drawn by a 3φ 5-level ac-dc PEC.
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Figure 3.17 shows the 3φ currents, drawn by the developed 3φ 5-level ac-dc
PEC, have 5 levels, which meet the set constraints for the proposed layout of
the H-bridges.
3.5 Summary
This chapter presented the stages required to develop a multi-level ac-dc PEC.
These stages included the changes in the layout of each level so that the
current drawn from the ac supply flows in all levels. This constraint is critical
to ensure the ability of the developed multi-level ac-dc PEC to reduce the
harmonic distortion on its input-side. The developed 3φ multi-level ac-dc
PEC has been able to create levels in its input 3φ currents, thus meeting the
desired feature. The next chapter presents the performance of the developed
multi-level PEC, when employed as the generator-side PEC in a PMG-based
WECS.
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Chapter 4
Performance Testing of 3φ
ac-dc 5-level PEC
4.1 General
The previous chapter has presented a layout for an H-bridge that can be
employed in constructing a 3φ multi-level ac-dc PEC. The key feature of this
H-bridge is the two half-bridge PECs, which can facilitate the current flow
to and from one level to another. The preliminary tests of a 3φ 5-level ac-dc
PEC have shown good capabilities to reduce the distortion in the input ac
currents. This chapter presents performance evaluation of the 3φ 5-level ac-dc
PEC when used as generator-side PEC in a PMG-based WECS. In addition, a
switching strategy based on the level-shifted pulse width modulation (PWM)
will be used to generate the switching pulses for the tested PEC. Finally,
Chapter 4 discusses performance comparisons between the 3φ 5-level ac-dc
PEC, 3φ full-wave diode rectifiers, and 3φ, 6-pulse ac-dc PECs under similar
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operating conditions.
4.2 Square-Wave Switching
The square-wave switching or single pulse switching, is a simple method to
operate single-level and multi-level PECs. In this switching technique, each
switching element, of the operated PEC, has one ON action and one OFF
action over its interval of conduction. The square-wave switching has been
widely used to operate multi-level PECs during their early stages of devel-
opment. The main advantages of this switching technique include the simple
production of switching signals, reduced switching losses, and simple synchro-
nization with the modulating (reference) signal. However, the square-wave
switching suffers several disadvantages that are mainly due to the high con-
duction losses, and difficulty for integration within closed loop controllers.
These disadvantages make the square-wave switching less applicable for op-
erating PECs, which are employed in renewable energy systems.
4.3 Multi-Pulse Switching of a 3φ Multi-level
ac-dc PEC
The selection of the switching strategy for any PEC is typically specified by
the switching frequency and duty cycle. On the other hand, the switching
frequency is set as a scaled version of the fundamental frequency component,
either on the input (ac-dc PECs), or the output (dc-ac PECs) sides. The
scaling factor for setting the switching frequency is commonly called the fre-
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quency modulation index (mf ). On the other hand, the duty-cycle relates the
duration of ON to that for OFF over each switching period. The duty cycle
can be translated as the ratio between the peak value of a carrier signal and
a reference. Such a ratio is commonly called the modulation index (ma).
In general, the selection of mf is made to meet the constraints of switching
losses and harmonic distortion. A large value of mf increases the switch-
ing losses, while reducing the harmonic distortion. Similarly, a low value of
mf reduces the switching losses, while aggravating the harmonic distortion.
The recommended values for mf can be set to meet the switching losses and
harmonic distortion. Recommended values for mf can be selected as [28,29]:
5 ≤ mf ≤ 25 (4.1)
For the modulation index ma, it can be selected to meet the constraints of
misfiring and over-modulation. Low values for ma may lead to to misfiring
where switching elements may not be forward biased during their ON state.
Such a condition is viewed as, switching elements fail to respond to ON switch-
ing signals. However, low values of ma can reduce conduction losses, as the
ON durations become short. Large values of ma can eliminate the misfiring
conditions, but can result in high conduction losses. These constraints can be
met by selecting ma as [28,29]:
ma =Am
(m− 1)Ac
; with 0.4 ≤ ma ≤ 1 (4.2)
where Am is reference signal amplitude, and Ac is the carrier signal amplitude.
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4.3.1 PWM-Based Multiple Pulse Switching for Multi-
Level PECs
Several switching strategies have been developed to realize the multiple pulse
switching technique for multi-level PECs.These strategies include:
a) Selected harmonic elimination
b) Sinusoidal PWM
c) Trapezoidal PWM
d) Harmonic Injection PWM
As PWM strategy is widely used in single-level PECs, it has been adapted
for operating multi-level PECs. Among the popular PWM strategies used for
operating multi-level PECs are:
a) Level-shifted PWM (LSPWM)
b) Phase-shifted PWM (PSPWM)
c) Space Vector Modulation (SVM)
The phase-shifted PWM and SVM have been mainly developed for multi-
level PECs that are not constructed from H-bridges (diode-clamp and flying
capacitor PECs). As a result, these two strategies will not be considered in
this work.
4.3.2 Level-Shifted PWM
In general, the generation of PWM switching signals for any single-level PEC
is based on modulating a reference signal, Sm(t) (low frequency) by a carrier
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signal, Sc(t) (high frequency). This modulation is set to create ON and OFF
pulses as:
IF Sm(t) ≥ 0
IF Sm(t) ≥ Sc(t) ⇒ ON
IF Sm(t) < Sc(t) ⇒ OFF
(4.3)
IF Sm(t) < 0
IF Sm(t) ≤ Sc(t) ⇒ ON
IF Sm(t) > Sc(t) ⇒ OFF
(4.4)
Triangular and sawtooth carrier signals are popular in generating PWM pulses
for different PECs.
In order to adapt the PWM strategy to operate multi-level PECs, several car-
rier signals are needed. These carrier signals can be set to exhibit shifts either
in phase or in magnitude. These shifts are mandated to ensure generating
switching pulses for all levels. The level-shifted PWM is structured to have
K carrier signals that are shifted in magnitude by 1. The number of carrier
signal, K is related to he number of levels N by:
K = N − 1 (4.5)
The shift-by-1 in the magnitude of each carrier signal requires adjusting the
amplitude of the reference Sm(t), where the modulation index, ma is set as in
equation (4.2).
Figure 4.1 shows the carrier signals and the reference signal for a 5-level PEC.
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0 0.005 0.01 0.015 0.02 0.025 0.03
−2
−1
1
2
Time
Am
plitude
Sc1(t)
Sc2(t)
Sc3(t)
Sc4(t)
Sm(t)
Figure 4.1: Reference and carriers for a 5-level PEC.
It can be seen from Figure 4.2 that the generation of PWM pulses is still valid,
where level n gets switching pulses after level n− 1 becomes over-modulated.
The generation of level-shifted PWM pulses for a 5-level PEC is shown in
Figure 4.2.
0
1
Level2+
0
1
Level1−
0 0.005 0.01 0.015 0.02 0.025 0.030
1
Time
Level2−
0
1
Level1+
Figure 4.2: Generated LSPWM Switching Pulses for a 5-level PEC.
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The switching pulses generated by LSPWM strategy are well suited to operate
H-bridge multi-level PECs. This switching pulse generation strategy will be
used to operate the developed multi-level ac-dc PEC.
4.4 Performance Results
The selection of the LSPWM switching pulse generation to operate the devel-
oped multi-level ac-dc PEC, allows testing the performance of the developed
multi-level PEC. The performance of the multi-level ac-dc PEC is tested for
5-levels, when supplied by:
a) an ideal 3φ supply
b) a 3φ PMG
The performance of the developed multi-level PEC will be evaluated for:
i) The harmonic contents in its input ac currents
ii) The output power
The performance testing of the 5-level ac-dc PEC is conducted using MAT-
LAB/Simulink software.
4.4.1 The Ideal 3φ Supply
The ideal 3φ supply test case was selected to provide a base line performance
for the developed multi-level ac-dc PEC. Such a supply is typically assumed
to deliver 3φ power at a fixed frequency, a pure sinusoidal voltage, and with a
zero internal impedance. In this test case, the ideal 3φ supply was selected to
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have 70.7 V (Vrms line-to-line voltage) at 60Hz. Furthermore, the developed
multi-level ac-dc PEC was constructed as a 5-level PEC and was operated by
LSPWM switching signals. The LSPWM switching signals were generated
at a switching frequency (the frequency of the 4 triangular carrier signals) of
fc = 1.26KHz, and a modulation index of ma = 0.98. Figure 4.3 shows the
3φ input voltages and the output dc voltages across the dc-link. Figure 4.4
shows the 3φ input currents, while Figure 4.5 shows the harmonic spectrum
for the current in phase A.
−100
−50
0
50
100
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
100
200
300
Time [msec]
(b)
(a)
Vdc [V]
VAN [V ] VCN [V ]VBN [V ]
Figure 4.3: Performance results of the developed 3φ, 5-level, ac-dc PEC, when fed
by an ideal 3φ supply: (a) the 3φ input line voltages and (b) the output dc voltage
across the dc-link.
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−20
0
20
IA
[A]
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
−20
0
20
Time [msec]
IC
[A]
−20
0
IB
[A]
Figure 4.4: Performance results for the developed 3φ, 5-level, ac-dc PEC, when
fed by an ideal 3φ supply: the 3φ input currents.
It can be seen from Figure 4.3 that the output dc voltage across the dc-
link had two levels, which was created by the actions of the 5-levels in the
developed ac-dc PEC. These actions also created 5 levels in the input currents,
as could be seen from Figure 4.4. The 5 levels in the input currents resulted in
reducing the harmonic currents, when compared to the case of a 3φ full-wave
rectifier (see Figures 4.5 and 4.6).
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0.02 0.03 0.04 0.05 0.06 0.07 0.08
−20
0
20
Time (msec)
IA
[A]
0 100 200 300 400 500 600 700 800 900 10000
5
10
15
20
Frequency (Hz)
|IA(60)| = 22.64, THDi = 16.96%
|IA(f)|
Figure 4.5: Performance results for the developed 3φ, 5-level, ac-dc PEC, when
fed by an ideal 3φ supply: the harmonic spectrum of IA.
0.02 0.03 0.04 0.05 0.06 0.07 0.08−20
−10
0
10
20
Time (msec)
IA
[A]
0 100 200 300 400 500 600 700 800 900 10000
5
10
15
20
Frequency (Hz)
|IA(60)| = 18.51, THDi = 30.46%
|IA(f)|
Figure 4.6: Performance results for a 3φ, diode rectifier, when fed by an ideal 3φ
supply: the harmonic spectrum of IA.
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4.4.2 The PMG Test Case
The test aims to demonstrate the performance of the developed multi-level
ac-dc PEC, when fed by a non-ideal 3φ supply. Such a supply can have a
non-zero internal impedance and/or possible non-sinusoidal voltages. The
non-ideal 3φ supply in this test is selected as a 3φ PMG that is structured to
feed the multi-level ac-dc PEC without an input side filter. The parameters
of the 3φ PMG used in this test are listed in Table 4.1.
Table 4.1: Parameters of the PMG
Parameter Value
Rated Power 50 [kW]
Rated terminal voltage 300 [V]
Number of poles 36
Torque constant 18.9 [N.m/A]
Voltage-flux constant 0.7 [V.s]
Stator per-phase resistance 0.18 [Ω]
Stator per-phase leakage inductance 14.45 [mH]
In this test, the developed multi-level ac-dc PEC was structured as a 5-level
PEC, which was operated by LSPWM switching signals. The LSPWM switch-
ing signals were generated with fc = 1.26KHz and ma = 0.98. The PMG
was running at a rotor speed of 200 rpm to produce a line-to-line terminal
voltage of 300 V. The 3φ terminal voltages and output dc voltage across the
dc-link are shown in Figure 4.7.
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0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4 0.41
.25
0.5
0.75
1
Time [msec]
Vdc[p
.u]
0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4 0.41−2
−1
0
1
2
Time [msec]
VA [p.u] VB [p.u] VC [p.u] (a)
(b)
Figure 4.7: Performance results of the developed 3φ, 5-level, ac-dc PEC, when fed
by a non-ideal 3φ supply: (a) the 3φ input line voltages (the base value is 300 V)
and (b) the output dc voltage across the dc-link (the base voltage is 300 V).
Figure 4.8 shows the 3φ currents drawn by the developed 5-level ac-dc PEC,
when fed by the PMG. In addition, the harmonic spectrum for the phase A
current is shown in Figure 4.9.
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−0.5
0
0.5
IA[p
.u]
−0.5
0
0.5
IB
[p.u
]
0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4 0.41
−0.5
0
0.5
Time [msec]
IC
[p.u
]
Figure 4.8: Performance results for the developed 3φ, 5-level, ac-dc PEC, when
fed by a non-ideal 3φ supply: the 3φ input currents (the base current value is 100
A).
0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39
−50
0
50
Time (msec)
IA
[A]
0 100 200 300 400 500 600 700 800 900 10000
20
40
60
Frequency (Hz)
|IA(60)| = 71.48, THDi = 6.99%
|IA(f)|
Figure 4.9: Performance results for the developed 3φ, 5-level, ac-dc PEC, when
fed by a non-ideal 3φ supply: the harmonic spectrum of IA.
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0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39
−50
0
50
Time (msec)
IA
[A]
0 100 200 300 400 500 600 700 800 900 10000
20
40
Frequency (Hz)
|IA(60)| = 50.21, THDi = 47.76%
|IA(f)|
Figure 4.10: Performance results for a 3φ, ac-dc PEC, when fed by a non-ideal 3φ
supply: the harmonic spectrum of IA.
The results obtained from the non-ideal 3φ supply test showed consistent abil-
ity of the developed multi-level ac-dc PEC to reduce harmonic distortion in
its input currents. This feature could be seen from the values of THDi for the
developed ac-dc PEC and that of the 3φ, 6-pulse, PWM ac-dc PEC (see figure
4.10). The reduction of the harmonic distortion in the input current resulted
in improved fundamental components of the currents. Such an improvement
could facilitate increasing the power transfer between the input and output
sides of the developed multi-level ac-dc PEC. This feature can significantly
increase the overall efficiency of the multi-level ac-dc PEC.
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4.5 Performance Comparison
The performance of the developed multi-level ac-dc PEC has demonstrated
good abilities to reduce the harmonic distortion in the input currents, thus
improving the power transfer between the input and the output sides. More-
over, the reduction of the harmonic distortion in input currents has been
maintained for ideal and non-ideal 3φ supplies.
To further demonstrate the advantages of the developed multi-level ac-dc
PEC, its performance is compared with the conventional 3φ full-wave rec-
tifier, and the 3φ, 6-pulse, PWM ac-dc PEC. The performance comparison
is conducted, when the tested PECs are fed by the same PMG. Finally, the
performance comparison of the 3 PECs is carried out for:
i) Input current total harmonic distortion (THDi)
ii) Average torque produced by the PMG
iii) Torque ripple produced by the PMG
iv) Output power of the PECs.
4.5.1 Harmonic Distortion in Input Currents
One of the critical requirements for ac-dc PECs is their ability to reduce the
harmonic distortion in their input currents. In order to demonstrate, such
a feature of the developed multi-level ac-dc PEC, the harmonic distortion in
the input currents was evaluated for different input-side frequencies, when fed
by a PMG. In these tests, the same PMG was used, and it was operated at
different speeds to generate power at different frequencies.
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At each frequency the harmonic distortion in phase A current was determined.
In addition, the same PMG was used to feed a 3φ full-wave rectifier and a
3φ, 6-pulse, PWM ac-dc PEC. The results of these tests are shown in Figure
4.11, where the total harmonic distortion factors for phase A currents of each
PEC are evaluated at each frequency.
10 20 30 40 50 60 70 80
5
10
15
20
25
30
Frequency (Hz)
TH
Di%
3φ Diode Rectifier
3 φ AC-DC PEC
5-Level AC-DC PEC
Figure 4.11: The harmonic distortion in input currents of the 5-level ac-dc PEC,
3φ, 6-pulse, PWM ac-dc PEC, and 3φ full-wave rectifier for input-side frequencies.
The results in Figure 4.11 confirm the ability of the developed multi-level ac-dc
PEC to reduce the harmonic distortion in its input currents. The determined
THDi at all input-side frequencies showed that the multi-level ac-dc PEC was
able to have the lowest THDi.
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4.5.2 Produced Electromagnetic Torque
Another desired feature for any generator-side PEC (in a PMG-based WECS),
is its ability to facilitate increasing the electromagnetic torque production.
Such a feature indicates that the stator currents (input currents to the
generator-side PEC) have large fundamental components, while having low
harmonic distortions.
In order to evaluate the electromagnetic torque production for the three tested
PECs, the same PMG was used to supply at different frequencies. Figure 4.12
shows the produced electromagnetic torque at each frequency, when using the
developed multi-level ac-dc PEC, 3φ, 6-pulse, PWM ac-dc PEC, and 3φ full-
wave rectifiers as generator-side PECs.
10 20 30 40 50 60 70 80
−1
−0.8
−0.6
−0.4
−0.2
0
Frequency (Hz)
Ele
ctrom
agnetic
Torque
[p.u
]
5-Level AC-DC PEC
3φ Diode Rectifier
3 φ AC-DC PEC
Figure 4.12: Electromagnetic torque produced by the PMG for using the 5-level
ac-dc PEC, 3φ, 6-pulse, PWM ac-dc PEC, and 3φ full-wave rectifier as generator-
side PECs. The base torque is 1000 N.m.
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The results in Figure 4.12 show that the developed multi-level ac-dc PEC,
when used as a generator-side PEC, was able to facilitate the production of
the largest electromagnetic torque. Such a capability was consistent with
the reduced harmonic distortion in the stator current. In addition, the large
production of electromagnetic torque confirmed the ability of the developed
ac-dc PEC to have high power transfer between its input and output side,
thus improving the power production of the PMG-based WECS.
4.5.3 The Ripple in Electromagnetic Torque
The mechanical and electrical stabilities of a PMG in a WECS are highly
dependent on the quality of the produced electromagnetic torque on one hand,
poor quality of the electromagnetic torque implies high ripple, which can
create mechanical oscillations capable of inflecting physical damages. On
the other hand, torque ripple can cause excessive harmonic distortion in the
terminal voltages of the PMG, where possible full or partial resonance can
take place. These are several factors that contribute to the torque ripple
in a PMG. These factors include the type permanent magnets in the rotor,
distribution of the stator windings, harmonic distortion of the stator currents
etc.
The harmonic distortions in the stator currents can create high frequency flux
components that interact with rotor flux. Such an interaction yields torque
ripples, which have magnitudes related to the harmonic components in the
stator currents. Since the harmonic distortion in the stator currents depend
on the generator-side PEC, it is critical to select the generator-side PEC with
a good ability to reduce harmonic distortion.
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In order to investigate the impacts of the developed multi-level PEC on the
torque ripple, the PMG was used to feed the developed PEC at different
input-side frequencies. In addition, the same PMG was used to feed a 3φ,
6-pulse, PWM ac-dc PEC, and a 3φ full-wave rectifier at different input-side
frequencies. At each input-side frequency, the torque ripple was determined
for each tested PEC. The determined torque ripples for the developed multi-
level ac-dc PEC, 3φ, 6-pulse, PWM ac-dc PEC, and a 3φ full-wave rectifier
at different input-side frequencies are shown in Figure 4.13.
10 20 30 40 50 60 70 80−0.08
−0.07
−0.06
−0.05
−0.04
−0.03
−0.02
−0.01
Frequency (Hz)
Torque
Rip
ple
%
5-Level AC-DC PEC
3φ Diode Rectifier
3 φ AC-DC PEC
Figure 4.13: The ripples in electromagnetic torque produced by the PMG, when
using the 5-level ac-dc PEC, 3φ, 6-pulse, PWM ac-dc PEC, and 3φ full wave
rectifier as generator-side PECs.
One can see from Figure 4.13 that the developed multi-level ac-dc PEC was
able to ensure the lowest torque ripple in the PMG. This feature of the devel-
oped ac-dc PEC was in-line with its ability to reduce the harmonic distortion
in the stator currents, and to help in increasing the production of electromag-
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netic torque.
4.5.4 The Output Power
Another desired feature of generator-side PECs, in a PMG-based WECS,
is their ability to transfer high power from the input to the output. This
feature is mandated to ensure efficient function of a PMG-based WECS. In
addition, the ability to have high output power can be viewed as an indication
of reduced harmonic distortions on the input and output sides of a generator-
side PEC.
The output power of the developed multi-level PEC was evaluated as the
PMG was operated to produce different input-side frequencies. Moreover, the
PMG was operated for different input-side frequencies, when a 3φ, 6-pulse,
PWM ac-dc PEC, and a 3φ full wave rectifier were used as generator-side
PECs. Figure 4.14 shows the output powers produced by the tested PECs for
different input-side frequencies.
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10 20 30 40 50 60 70 80−0.2
0
0.2
0.4
0.6
0.8
1
Frequency (Hz)
Pow
er
(p.u
.)5-Level AC-DC PEC
3φ Diode Rectifier
3 φ AC-DC PEC
Figure 4.14: The output power of the 5-level ac-dc PEC, 3φ, 6-pulse, PWM ac-dc
PEC, and 3φ full wave rectifier when used as generator-side PECs for different
input-side frequencies. The base power is 50 kW.
The results in Figure 4.14 show that the developed multi-level ac-dc PEC
was able to produce higher output powers than those produced by other
PECs. This observation was consistent with the abilities of the developed
PEC to reduce harmonic distortion in its input currents. Finally, the results
of this test provided support for the possible improvements in the operation
of a PMG-based WECS, when using the developed multi-level ac-dc PEC as
generator-side PEC.
4.6 Summary
Chapter 4 has presented and discussed the performance testing of the devel-
oped multi-level ac-dc PEC. Test results for the ideal and no-ideal 3φ supplies
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have shown the ability of the developed ac-dc PEC to reduce the harmonic
distortion in its input currents, facilitate a PMG to produce large and high
quality electromagnetic torque, and transfer high power between its input and
output sides. These features have been further demonstrated through perfor-
mance comparison with the 3φ, 6-pulse, PWM ac-dc PEC, and 3φ full wave
rectifier, when used as generator-side PECs in the same PMG-based WECS.
The results of these comparisons have shown that the developed multi-level
ac-dc PEC is able to outperform the other PECs in terms of high quality input
currents, high quality PMG torque, and high output power. Test and compar-
ison results support the applicability of the developed multi-level ac-dc PEC
as generator-side PEC to improve the function and stability of PMG based
WECSs. The next chapter, Chapter 5, provides a summary of contributions
of this research, along with some recommendations for future work.
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Chapter 5
Conclusion and Future Work
5.1 Summary
Permanent magnet generator (PMG)-based wind energy conversion systems
(WECSs) have become widely used in grid-connected WECSs. These WECSs
have gained such popularity due to their ability to deliver power over a wide
range of wind speeds. The dominating designs of PMG-based WECSs are
based on the back-to-back power electronic converters (PECs), where the
PMG feeds an ac-dc PEC (generator-side PEC), which supplies a dc-ac PEC
(grid-side PEC) through a dc-link. The back-to-back PMG-based WECSs
have shown encouraging performance that is supported by independent con-
trol of the generator-side and grid-side PECs. Due to its operation as the
first-stage in a PMG based WECS, the generator-side PEC is considered to
have a vital impact on the function and efficiency of a PMG-based WECS.
This consideration is set due to the impacts of the generator-side PEC on the
mechanical and electrical stabilities of the PMG, as well as its influence on
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the quality and quantity of the power produced by the PMG.
Existing PMG-based WECSs employ 3φ, 6-pulse, PWM ac-dc PECs, and 3φ
full wave rectifiers as generator-side PECs. The major concerns for employing
these PECs are due to their limited abilities to reduce the harmonic distortions
in their input currents. Such harmonic distortions can reduce the quality of
the produced power of the PMG, and create ripples in the electromagnetic
torque. In addition, the harmonic distortion in the input currents of the
generator-side PEC may complicate the design and function of maximum-
power-point-tracking controllers which are common in PMG-based WECSs.
This research work has employed the concept of multi-level PECs in order
to develop a multi-level ac-dc PEC, which can be used as a generator-side
PEC in a PMG-based WECS. The developed multi-level ac-dc PEC has been
designed using H-bridges, which have been modified to facilitate the current
flow between different levels of the PEC. The modification of H-bridges are
structured to create a forward half-bridge and a backward half-bridge. The
developed multi-level ac-dc PEC have been operated by square-wave switch-
ing, and level-shifted PWM, when fed by ideal and non-ideal 3φ supplies. The
capabilities of the developed multi-level ac-dc PEC have been demonstrated
for its operation as generator-side PEC in a PMG-based WECS. Performance
results have shown good abilities to reduce the harmonic distortion in the in-
put currents, large and high quality torque production by the PMG, and high
output power. These features have been further highlighted through perfor-
mance comparison with a 3φ, 6-pulse, PWM ac-dc PEC, and a 3φ full wave
rectifier under similar operating conditions. Comparison results have pro-
vided an additional support to the employment of the developed multi-level
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ac-dc PEC as a generator-side PEC in PMG-based WECSs.
5.2 Conclusions
The work presented in this thesis has been dedicated to the development of
an ac-dc PEC that can be employed in PMG-based WECSs. The desired
ac-dc PEC has been developed using the H-bridge multi-level PEC, where
each H-bridge has been modified to facilitate the function of an ac-dc PEC.
Test results of the developed multi-level ac-dc PEC have shown good abili-
ties for a generator-side PEC in a PMG-based WECS. The development and
performance of a multi-level ac-dc PEC, as generator-side PEC, can lead to
the following conclusions:
• The development of a multi-level ac-dc PEC that can naturally operate
as a 4-quadrant PEC.
• A combination of a forward and a backward half-bridge PEC can be used
to modify the conventional H-bridge in order to construct a multi-level
ac-dc PEC.
• The constructed multi-level ac-dc PEC can be operated to allow cur-
rent flow between different levels. Such a feature can help reduce the
harmonic distortion in input currents.
• The inherent abilities of the developed multi-level ac-dc PEC, to re-
duce the input-side harmonic distortion, can improve the electromag-
netic torque produced by the PMG.
• The high quality electromagnetic torque production can improve the
overall power transfer from the PMG and dc-link through the generator-
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side PEC.
• The level-shifted PWM and square-wave switching can be used to oper-
ate the developed multi-level ac-dc PEC.
5.3 Contributions
The work presented in this thesis has achieved several contributions that can
be summarized as:
• The modification of the conventional structure of the H-bridge in order
to allow current flow between levels to operate a multi-level PEC as an
ac-dc one.
• The use of the modified H-bridge in developing a multi-level ac-dc PEC
that exhibits the features of multi-level PECs.
• The performance evaluation of the developed multi-level ac-dc PEC as
a generator-side PEC in a PMG-based WECS.
• The demonstration of improvements in the operation of a PMG-
based WECS, when using the developed multi-level ac-dc PEC as the
generator-side PEC.
5.4 Future Work
The employment of the developed 3φ 5-level ac-dc PEC in the generator-
side of PMG-based WECS can provide grounds for future research work.
The following are recommendations for additional works that go toward the
experimental testing of the developed multi-level ac-dc PEC:
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• Developing a prototype of the developed multi-level ac-dc PEC for ex-
perimental performance evaluation.
• Testing the performance of the developed multi-level ac-dc PEC for 7
and 9 levels.
• Testing the space vector modulation switching strategy for operat-
ing the developed multi-level ac-dc PEC. Micro-controllers or field-
programmable gate array (FPGA) platform could be used for imple-
mentation of such switching strategies.
• Testing the performance of the developed multi-level ac-dc PEC for other
applications such as motor drives and storage systems.
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Bibliography
[1] Z. Chen, J.Guerrero, and F. Blaabjerg, “A Review of the State of the Art
of Power Electronics for Wind Turbines,” IEEE Transactions on Power
Electronics, Vol. 24, No. 8, pp. 1859–1875, 2009.
[2] B. Sing, B. Sing, A. Chandra, K. Haddad, A. Pandey and D.P. Kothari,
“A Review of Three-Phase Improved Power Quality AC-DC Converters,”
IEEE Transactions on Industrial Electronics, Vol. 51, No. 1, pp. 641–660,
2004.
[3] A. Venkataraman, A. Maswood, N. Sarangan, O. Gabriel, “An Efficient
UPF Rectifier for a Stand-Alone Wind Energy Conversion System,” IEEE
Trans. on Industry Applications, Vol. 50, No. 2, pp. 1421–1431, 2014.
[4] V. Yaramasu, B. Wu, S. Alepuz and S. Kouro, “Predictive Control for
Low-Voltage Ride-Through Enhancement of Three-Level-Boost and NPC-
Converter-Based PMSG Wind Turbine,” IEEE Trans. on Industry Appli-
cations, Vol. 61, No. 12, pp. 6832–6843, 2014.
[5] M. E. Haque, M. Negnevitsky and K. M. Muttaqi, “A Novel Control
Strategy for a Variable-Speed Wind Turbine with a Permanent-Magnet
Synchronous Generator,” IEEE Trans. on Industry Applications, Vol. 46,
No. 1, pp. 331–339, 2010.
67
![Page 79: Multi-LevelAC-DC Power Electronic Converter for Applicationsin … · 2016-07-21 · Multi-LevelAC-DC Power Electronic Converter for Applicationsin PMG-Based WECSs by A B M Saadmaan](https://reader034.vdocuments.site/reader034/viewer/2022042411/5f2987aec321c1106347ceb7/html5/thumbnails/79.jpg)
[6] D. J. Perreault and V. Caliskan, “Automotive Power Generation and Con-
trol” IEEE Transactions on Power Electronics, Vol. 19, No. 3, pp. 618–
630, 2004.
[7] W. L. Soong and N. Ertugrul, “Inverterless High-Power Interior
Permanent-Magnet Automotive Alternator,” IEEE Trans. on Industry
Applications, Vol. 40, No. 4, pp. 1083–1091, 2004.
[8] C. Xia, Z. Wang, T. Shi and Z. Song, “A Novel Cascaded Boost Chopper
for the Wind Energy Conversion System Based on the Permanent Mag-
net Synchronous Generator” IEEE Transactions on Energy Conversion,
Vol. 28, No. 3, pp. 512–522, 2013.
[9] D. Oliveira, M. Reis, C.Silva, L. Barreto, F. Antunes and B. Soares, “A
Three-Phase High-Frequency Semicontrolled Rectifier for PM WECS,”
IEEE Transactions on Power Electronics, Vol. 25, No. 3, pp. 677–685,
2010.
[10] G. Kim and T. A. Lipo, “VSI-PWM Rectified/Inverter System with a
Reduced Switch Count,” IEEE Transactions on Industrial Applications,
Vol. 32, No. 6, pp. 1331–1337, 1996.
[11] J. M. Alonso, J. Cardesin, E. L. Corominas, M. R. Secades, and J. Garcia,
“Overview of Multi-MW Wind Turbines and Wind Parks,” IEEE Trans-
actions on Industrial Electronics, Vol. 58, No. 4, pp. 1081–1095, 2011.
[12] G. Buicchi, E. Lorenzani, F. Immovilli, and C. Bianchini, “Active Recti-
fier With Integrated System Control for Microwind Power Systems,” IEEE
Transactions on Sustainable Energy, Vol. 6, No. 1, pp. 60–69, 2015.
[13] X. Yuan, J. Chai, and Y. Li, “A Transformer-Less High-Power Converter
for Large Permanent Magnet Wind Generator Systems,” IEEE Transac-
68
![Page 80: Multi-LevelAC-DC Power Electronic Converter for Applicationsin … · 2016-07-21 · Multi-LevelAC-DC Power Electronic Converter for Applicationsin PMG-Based WECSs by A B M Saadmaan](https://reader034.vdocuments.site/reader034/viewer/2022042411/5f2987aec321c1106347ceb7/html5/thumbnails/80.jpg)
tions on Sustainable Energy, Vol. 3, No. 3, pp. 318–329, 2012.
[14] Y. Zhao, C. Wei, Z. Zhang, and W. Qiao, “A Review on Position/Speed
Sensorless Control for Permanent-Magnet Synchronous Machine-Based
Wind Energy Conversion Systems,” IEEE Journal of Emerging and Se-
lected Topics in Power Electronics, Vol. 1, No. 4, pp. 203–216, 2013.
[15] M. Sing, V. Khadkikar, and A. Chadra, “Grid synchronization with har-
monics and reactive power compensation capability of a permanent mag-
net synchronous generator-based variable speed wind energy conversion
system” IET Power Electronics, Vol. 4, No. 1, pp. 122–130, 2011.
[16] M. Sing, V. Khadkikar, and A. Chadra, “PM Wind Generator Topolo-
gies” IEEE Transactions on Industry Applications, Vol. 41, No. 6,
pp. 1619–1626, 2005.
[17] S.A. Saleh, M. Khan and M.A. Rahman, “Steady-state performance anal-
ysis and modeling of directly driven interior permanent magnet wind gen-
erators” IET Renewable Power Generation, Vol. 5, No. 2, pp. 137–147,
2011.
[18] F. Blaabjerg, M. Liserre and K. Ma, “Power Electronics Converters
for Wind Turbine Systems” IEEE Transactions on Industry Applications,
Vol. 48, No. 2, pp. 708–719, 2012.
[19] V. Yaramasu, B. Wu, P. Sen, S. Kouro and M. Narimani, “High-Power
Wind Energy Conversion Systems: State-of-the-Art and Emerging Tech-
nologies” Proceedings of the IEEE, Vol. 103, No. 5, pp. 740–788, 2015.
[20] A. Uehara, A. pratap, T. Goya, T. Senjyuu, A. Yona, N. Urasaki and
T. Funabashi, “A Coordinated Control Method to Smooth Wind Power
69
![Page 81: Multi-LevelAC-DC Power Electronic Converter for Applicationsin … · 2016-07-21 · Multi-LevelAC-DC Power Electronic Converter for Applicationsin PMG-Based WECSs by A B M Saadmaan](https://reader034.vdocuments.site/reader034/viewer/2022042411/5f2987aec321c1106347ceb7/html5/thumbnails/81.jpg)
Fluctuations of a PMSG-Based WECS” IEEE Transactions on Energy
Conversion, Vol. 26, No. 2, pp. 550–558, 2011.
[21] J. Chen, J. Chen and C. Gong, “On Optimizing the Transient Load of
Variable-Speed Wind Energy Conversion System During the MPP Track-
ing Process” IEEE Transactions on Industrial Electronics, Vol. 61, No. 9,
pp. 4698–4706, 2014.
[22] S. Saleh and R. Ahsan, “Resolution-Level-Controlled WM Inverter for
PMG-Based Wind Energy Conversion System” IEEE Transactions on In-
dustry Applications, Vol. 48, No. 2, pp. 750–763, 2012.
[23] H. Geng, G. Yang, D. Xu and B. Wu, “Unified Power Control for PMSG-
Based WECS Operating Under Different Grid Conditions” IEEE Trans-
actions on Energy Conversion,Vol. 26, No. 3, pp. 822–830, 2011.
[24] C. Cecati, A. Dell’Aquila, M. Liserre and V. Monopoli, “Design of H-
Bridge Multilevel Active Rectifier for Traction Systems” IEEE Transac-
tions on Industry Applications,Vol. 39, No. 5, pp. 1541–1540, 2003.
[25] L. Tarisciotti, P. Zanchetta, A. Watson, S.Bifaretti, J. Clare and P.
Wheeler, “Active DC Voltage Balancing PWM Technique for High-Power
Cascaded Multilevel Converters” IEEE Transactions on Industrial Elec-
tronics,Vol. 61, No. 11, pp. 6157–6167, 2014.
[26] A. Aquilla, M. Liserre, G. Monopoli and P. Rotondo, “An Energy-Based
Control for an n-H-Bridges Multilevel Active Rectifier” IEEE Transac-
tions on Industrial Electronics,Vol. 52, No. 3, pp. 670–678, 2005.
[27] K. Tsang, and W. Chan, “Multi-level multi-output single-phase active
rectifier using cascaded H-bridge converter” IET Power Electronics,Vol. 7,
No. 4, pp. 784–794, 2013.
70
![Page 82: Multi-LevelAC-DC Power Electronic Converter for Applicationsin … · 2016-07-21 · Multi-LevelAC-DC Power Electronic Converter for Applicationsin PMG-Based WECSs by A B M Saadmaan](https://reader034.vdocuments.site/reader034/viewer/2022042411/5f2987aec321c1106347ceb7/html5/thumbnails/82.jpg)
[28] A. Tsunoda, Y. Hinago and H. Koizumi, “Level- and Phase-Shifted
PWM for Seven-Level Switched-Capacitor Inverter Using Series/Parallel
Conversion” IEEE Transactions on Industrial Electronics,Vol. 61, No. 8,
pp. 4011–4021, 2014.
[29] D. Sreenivasarao, P. Agarwal and B. Das, “Performance evaluation of car-
rier rotation strategy in level-shifted pulse-width modulation technique”
IET Power Electronics,Vol. 7, No. 3, pp. 667–680, 2014.
71
![Page 83: Multi-LevelAC-DC Power Electronic Converter for Applicationsin … · 2016-07-21 · Multi-LevelAC-DC Power Electronic Converter for Applicationsin PMG-Based WECSs by A B M Saadmaan](https://reader034.vdocuments.site/reader034/viewer/2022042411/5f2987aec321c1106347ceb7/html5/thumbnails/83.jpg)
Curriculum Vitae
Candidate’s full name: ABM Saadmaan Rahman
Universities attended: BSc. in Electrical and Electronic Engineering
Military Institute of Science and Technology
Dhaka, Bangladesh, Feb 2008 to Jan 2012
Major: Electrical and Electronic Engineering
Publications: ABM Saadmaan Rahman, “ The PerformanceAnalysis of 3φ DC-AC Power ElectronicConverters With Different SwitchingSchemes”Submitted to International conference
on Smart Energy Grid Engineering, Oshawa,Canada, August 2016.
ABM Saadmaan Rahman, S. Ismail and A.Siddique, “A Technique to Sense Current forDigitally Controlling a Power Factor Correc-tion Boost Rectifier” Published in Interna-
tional Conference Computational on Intelli-
gence, Modelling and Simulation, Kuantan,Malaysia, 2012.