Multi-LevelAC-DC Power Electronic Converter for Applicationsin … · 2016-07-21 ·...

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

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.

ii

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.

iii

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

iv

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

v

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

vi

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

vii

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

viii

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











3.17 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

ix

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


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


fed by a non-ideal 3φ supply: the 3φ input currents (the base current

value is 100 A). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52


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

x

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

xi

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);

1

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

2

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

3

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

4

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.

5

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.

6

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:

7

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).

8

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].

9

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].

10

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

11

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.

12

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

13

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.

14

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.

15

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

16

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

17

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.

18

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.

19

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

20

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,

21

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.

22

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.

23

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.

24

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)

25

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.

26

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

27

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.

28

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.

29

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.

30

−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

31

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.

32

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.

33

Figure 3.11: The conduction switches and flow of the 3φ currents for the 3φ 5-level

ac-dc PEC over T1.


ac-dc PEC over T2.

34


ac-dc PEC over T3.


ac-dc PEC over T4.

35


ac-dc PEC over T5.


ac-dc PEC over T6.

36

Table 3.3: Square-Wave Switching Pattern for a 3φ 5-

Level AC-DC PEC.

Switching Actions


T1 QA11,QB14,QC11

QA12,QA13,QA14,QB12,QB13,

QB14,QC12,QC13,QC14,QA21,


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





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

37

Continuation of Table 3.3


T5 QA14,QB11,QC14





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.

38

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.

39

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

40

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-

41

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.

42

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

43

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.

44

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.

45

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

46

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.

47

−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).

48

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)|


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.

49

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.

50

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.

51

−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

]


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)|


fed by a non-ideal 3φ supply: the harmonic spectrum of IA.

52

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.

53

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.

54

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.

55

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.

56

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.

57

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-

58

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.

59

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

60

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.

61

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

62

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

63

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-

64

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:

65

• 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.

66

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71

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.

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