SELF-SYNCHRONIZED CONVERTER FOR FAST

SYNCHRONIZATION BETWEEN LOW VOLTAGE

MICROGRID AND INVERTER CONNECTION

MD RUHUL AMIN

UNIVERSITI TUN HUSSEIN ONN MALAYSIA

SELF-SYNCHRONIZED CONVERTER FOR FAST SYNCHRONIZATION

BETWEEN LOW VOLTAGE MICROGRID AND INVERTER CONNECTION

MD RUHUL AMIN

A thesis submitted in

fulfillment of the requirement for the award of the

Degree of Master of Electrical Engineering

Faculty of Electrical and Electronic Engineering

Universiti Tun Hussein Onn Malaysia

April 2017

iii

To my beloved parents and wife

iv

ACKNOWLEDGMENTS

Alhamdulillah, all praise to Allah Subhanahu Wa Ta'ala, the Most Graceful and Most

Merciful, for giving me the utmost strength to have this thesis completed.

I would like to express my greatest and sincere gratitude to Dr. Shamsul Aizam

bin Zulkifli, my research supervisor for his insightful comments, guidance and support

given throughout the duration of my research. My sincere love and thank goes to my

wonderful parents and wife for their loves and support throughout my life and

education. My sincere love and thank goes to my beloved wife for her great support

during my period of studies.

My deepest appreciation and warmest thank goes to all my friends for

encouraging an extraordinary level of encouragement and assistance during my

research work. These unique and affectionate friends have added their valuable

contributions, offering feedback and criticism. May Allah bless them all.

Finally, I wish to thank all postgraduate colleagues, all staff in the Faculty of

Electrical and Electronic Engineering and Center for Graduate Studies for their

painstaking support, cooperation and contribution during the period of my research. I

will be happy to shower my exceptional acknowledgement to my prestigious

institution ‘University Tun Hussein Onn Malaysia (UTHM).’

v

ABSTRACT

In this research, a fast-synchronization between single-phase microgrid and inverter in

low voltage, which is based on virtual synchronous converter (VSCon) technique is

been developed. This technique does not require any phase locked loop (PLL) circuit

as an external control structure for the synchronization of the inverter. As known, PLL

is a common technique in order to synchronize the amplitude, phase-angle and

frequency between a microgrid and an inverter in distributed generation network.

Previous studies show that, the disadvantage of PLLs is where the non-linear

characteristic on the signal process will result the inverter control to be non-linear.

Therefore, it is difficult and lengthy process to tune the PLL gains to reach suitable

performance in order to synchronize the voltage, phase-angle and frequency between

microgrid and inverter. As a result, a VSCon is been developed in which it is a self-

synchronized inverter which is based on synchronous generator mathematical model.

This controller acts like as synchronous generator operating system in inverter control

loop in order to achieve fast voltage, phase and frequency synchronization between

inverter-microgrid connection. This technique has been modeled, simulated and tested

in Matlab/Simulink software. It is by using a single-phase AC source input system

connecting with several load variations during simulation period. This VSCon has

been placed in inverter control loop to generate a pulse width modulation (PWM)

signal that responds to the grid information for to synchronizing the inverter output

voltage with the grid voltage. A single source input at 120V, 50Hz frequency and

240V, 50Hz frequency are used in order to see the self-synchronization response

between a microgrid and an inverter. Furthermore, it also been tested when the grid

frequency has been changed from the rated frequency at 50Hz to 51Hz and the result

shows that, VSCon takes nearly 1-cycle to synchronize to this new frequency value.

The grid phase angle test also has been conducted. For this test, the voltage grid which

has 100 phase delay has been created in the input voltage source at microgrid side. As

a result, the VSCon is also able to achieve self-synchronization with this new source

vi

input phase delay in just within 1.5 cycles or 30ms after the inverter has been

connected to the microgrid. From all the results that have been collected, this technique

can be an improved model for inverter to have synchronization between inverter and

microgrid which is not require a PLL circuitry model in order to maximize the power

transfer from the inverter to the microgrid when it been applied to the distributed

generation network.

vii

ABSTRAK

Dalam kajian ini, penyegerakan cepat antara micro-grid dan fasa tunggal penyongsang

arus terus (AT) kepada arus ulang (AU) alik bagi voltan peringkat rendah, yang

berasaskan maya penukar segerak teknik (VSCon) telah dibangunkan. Teknik ini tidak

memerlukan mana-mana litar gelungan kunci fasa (PLL) sebagai struktur kawalan

penyongsang AT-AU sebagai fungsi penyegerakan antara micro-grid dan penyonsang

didalam penjanaan pembahagian litar elektrik. Seperti yang diketahui, PLL adalah

kaedah biasa untuk menyelaraskan amplitud, frekuensi dan fasa-sudut antara

microgrid voltan dan penyongsang. Kajian sebelum ni menunjukkan, kelemahan PLL

adalah di mana ciri-ciri bukan linear keatas proses isyarat yang akan menyebabkan

kawalan penyongsang sebagai tidak linear. Oleh itu, VSCon telah dibangunkan di

mana, pemodelan matematik penyegerakan cepat diasaskan berdasarkan struktur

penjana segerak. Kawalan yang direka adalah berdasarkan penjana segerak yang akan

digunakan dalam kawalan penyongsang AT-AU untuk menghasilkan penyegerakan

cepat dengan sambutan frekuensi dan voltan yang tepat pada sambungan

penyongsang-microgrid. Teknik ini telah dimodelkan disimulasikan dan diuji didalam

perisian Matlab / Simulink. Ia dengan menggunakan sistem satu fasa sumber input

kepada penyongsang AT-AU tunggal yang akan dihubungkan dengan beberapa beban

variasi didalam jaringan microgrid. VSCon yang direka akan dilaksanakan dalam

kawalan penyongsang untuk menjana modulasi lebar denyut (PWM) isyarat yang

mana maklumat-maklumat di grid bertindak balas untuk menyelaraskan voltan

keluaran penyongsang dengan voltan grid tanpa menggunakan PLL dengan cepat.

Ujikaji pertama, ia diuji apabila sumber input tunggal pada 120V, dengan frekensi

50Hz dan sumber input 240V, 50Hz digunakan untuk melihat tindak balas pada

penyegerakan diri antara microgrid dan inverter. Selain itu pula, ia juga telah diuji

apabila perubahan kekerapan grid dari frekuensi semasa iaitu 50Hz kepada 51Hz yang

mana hasilnya menunjukkan bahawa, VSCon hanya mengambil masa hampir satu

kitaran lengkap untuk menyegerakkan kepada nilai kekerapan baru ini yang diperlukan

oleh grid. Untuk ujian sudut fasa grid juga dijalankan dimana grid voltan yang

mempunyai 10 sudut kelewatan fasa telah diwujudkan atas sumber voltan input di

sebelah microgrid. Hasilnya, VSCon dapat membuat penyegerakan pantas dengan

viii

kelewatan baru ini, dalam masa satu setengah kitaran atau dalam masa 30ms selepas

penyongsang AT-AU disambungkan kepada microgrid itu. Dari semua keputusan

yang telah dihasilkan, teknik kawalan ini boleh menjadi model yang lebih baik kepada

penyongsang AT-AU untuk dapat memberi penyegerakan pantas antara penyongsang

AT-AU tanpa model litar PLL untuk memaksimumkan pemindahan kuasa antara

penyongsang dan microgrid itu sekiranya perlu didalam talian pembahagian

penjanaan.

ix

CONTENTS

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vii

CONTENTS ix

LIST OF FIGURES xii

LIST OF TABLES xvi

LIST OF SYMBOLS AND ABBREVIATION xvii

CHAPTER 1 INTRODUCTION 1

1.1 Introduction 1

1.2 Problem Statement 3

1.3 Objectives 4

1.4 Scope of Project 4

1.5 Thesis Organization 5

CHAPTER 2 LITERATURE REVIEW 6

2.1 Distributed Generation and its Benefits 6

2.2 Structure of Microgrids 7

2.3 Methods of Synchronization and PLL 9

2.4 Synchronous Generator Model 17

2.4.1 Electrical Part 18

2.4.2 Mechanical Part 19

2.5 Synchronverter 20

2.6 Inverter and Controls 21

2.6.1 PCM Control 23

2.6.2 VCM Control 23

2.6.3 PI Control 23

2.7 Summary 24

x

CHAPTER 3 RESEARCH METHODOLOGY 27

3.1 Introduction 27

3.2 The overall project methodology and

phases

28

3.2.1 Phase I: Modeling of Voltage Phase-angle

Synchronization

29

3.2.2 Phase II: Modeling of Frequency

Synchronization

30

3.2.3 Mathematical Development Model on

VSCon

31

3.3 Concept of VSCon 31

3.4 Design of VSCon 34

3.5 Modeling of Phase-angle and Frequency

Tracking System on VSCon

36

3.5.1 Determination on Phase-angle Tracking

System

36

3.5.2 Determination on Grid-frequency Tracking

System

37

3.5.3 Design of Logic Check for Circuit Breaker

Operation When the Load Changing

39

3.6 Summary 42

CHAPTER 4 RESULTS AND DISCUSSIONS 43

4.1 Case I: 24V dc-linked Inverter 44

4.1.1 Connecting Microrid-inverter and Sharing

Load Current

46

4.1.2 Automatic Phase and Frequency

Synchronization with VSCon

49

4.1.3 Synchronization Effect during Load

Changes

53

4.2 Case II: 120V dc-linked Inverter 55

4.2.1 Connecting Microgrid-Inverter and

Inverter Sharing Load Current

56

xi

4.2.2 Automatic Frequency and Phase

Synchronization with VSCon

60

4.2.3 Synchronization Effect during Load

Changes

63

4.3 Case III: 240V dc-linked Inverter with

Linear and Non-linear Loads

65

4.3.1 Inverter and AC Source Voltage Response 65

4.3.2 Connecting Grid-Inverter and Sharing

Load Current

66

4.3.3 Automatic Frequency and Phase

Synchronization with VSCon

69

4.3.4 Synchronization during Load Changes 72

4.4 Non-linear Load and Synchronization 73

4.5 Comparison of Synchronization between

using Conventional PLL and VSCon

76

4.6 Summary 79

CHAPTER 5 CONCLUSION AND FUTURE RESEARCH 80

5.1 Conclusion 80

5.2 Future Research 81

References 82

Appendix I 88

Appendix II 90

Appendix III 91

Appendix IV 92

Appendix V 94

Appendix VI 95

Appendix VII 96

Appendix VIII 97

Appendix IX 98

List of Publications 99

VITA 100

xii

LIST OF FIGURES

1.1 Typical control structures for low voltage microgrid-

connected inverter

2

2.1 Typical structure of microgrid 8

2.2 Complete block diagram of PLL control scheme 10

2.3 Linearized small signal model of SRF-PLL 12

2.4 Rotating Coordinate 13

2.5 DSOGI-PLL block diagram 16

2.6 An ideal three-phase synchronous generator model 18

2.7 Typical voltage source inverter 22

2.8 Typical current source inverter 22

2.9 Block diagram of PI controller 24

3.1 Block diagram of proposed microgrid connected inverter

structure

27

3.2 Flow chart of the whole project 28

3.3 Flow chart of the VSCon in voltage phase-synchronization 29

3.4 Flow chart of the VSCon in frequency synchronization 30

3.5 Generator is connected to prime mover 31

3.6 Block diagram of VSCon inverter interfacing along with

microgrid

33

3.7 Circuit diagram of synchronous generator connected with

infinite bus

33

3.8 The VSCon controller connecting microgrid voltage 34

3.9 Determination of phase-angle of microgrid voltage 36

3.10 Simulink model of frequency tracking system 37

3.11 Obtained microgrid frequency 38

3.12 Output frequency of frequency tracking system 38

3.13 Complete Simulink model of VSCon 39

3.14 Simulink model of combinational logic diagram of circuit

breaker driver

40

xiii

3.15 Complete circuit diagram of inverter-connected with 240V

at 50Hz microgrid along with linear loads

41

4.1 Single-phase H-bridge inverter with microgrid 45

4.2 (a) Inverter output voltage (b) nominal microgrid voltage

24V in 50Hz

46

4.3 (a) Microgrid current while connecting loads, (b) inverter

current while connecting loads and (c) load current while

connecting loads

47

4.4 THD analysis of microgrid current at 50Hz 48

4.5 (a) Inverter sharing active power with loads and (b)

inverter sharing reactive power with loads

49

4.6 (a) The e is connected with PWM signal at 0.3s (b) the

microgrid voltage and inverter voltage is synchronized at

51 Hz

50

4.7 (a) The e is connected with PWM signal at 0.196s, (b) the

microgrid voltage and inverter voltage are synchronized at

0.196s while changing the microgrid voltage frequency 51

Hz

51

4.8 The microgrid voltage is shifted by 10 52

4.9 The e is shifts phase-angle by 10 at nearly 0.02s 52

4.10 The microgrid voltage phase and inverter voltage phase

are synchronized at 0.05s

53

4.11 Inverter voltage and grid voltage while connecting loads 54

4.12 (a) The nominal microgrid voltage of 120V in 50Hz

frequency (b) the inverter output voltage of 120V in 50Hz

frequency

55

4.13 (a) Inverter current contributes to loads, (b) microgrid

current contributes to loads and (c) overall load current

57

4.14 (a) Inverter sharing real power to microgrid and (b)

inverter provides reactive power to microgrid

58

4.15 THD analysis of microgrid current at 50Hz frequency 59

xiv

4.16 The VSCon produced e signal is connected with PWM at

0.2s

60

4.17 The microgrid voltage and inverter voltage is

synchronized at 0.2s

60

4.18 The microgrid voltage phase is shifted by 10 at 51Hz 61

4.19 The e signal is connected with PWM at 0.21s 61

4.20 The microgrid voltage and inverter voltage is

synchronized at 0.21s

62

4.21 Inverter and microgrid voltage synchronization is zoomed

at 0.15s to 0.30s

62

4.22 Load current at all load connected condition 63

4.23 Grid-inverter voltage when load are connected 64

4.24 (a) Nominal microgrid voltage of 240V in 50Hz frequency

(b) inverter output voltage

65

4.25 (a) Inverter current supplied to loads, (b) microgrid current

supplied to loads and (c) overall current consumes loads

67

4.26 (a) Inverter provides real power to loads and (b) inverter

provides reactive power to loads

68

4.27 THD of microgrid currents in loads connected condition 69

4.28 The VSCon produced e signal is connected with PWM at

0.1s

69

4.29 The microgrid voltage and inverter voltage is

synchronized at 0.1s

70

4.30 Time interval of 0.08s to 0.20s is zoomed of Figure 6.6 70

4.31 The microgrid voltage phase is shifted by 15 at 51Hz

frequency

71

4.32 The microgrid voltage and inverter voltage is

synchronized at 0.1s

71

4.33 Load current at all loads connected condition 72

4.34 Grid-inverter voltage when loads are connected 72

4.35 Rectifier as a non-linear load 73

4.36 PWM switching pulse for switches 73

4.37 Non-linear load current 74

xv

4.38 Voltage drop across the non-linear load 74

4.39 Grid-inverter voltage while connecting non-linear load 75

4.40 (a) The inverter voltage and microgrid voltage is

synchronized at 0.21s using PLL. (b) Inverter and

microgrid voltage synchronization is zoomed at 0.15s to

0.30s using PLL (c) Inverter and microgrid voltage

synchronization is zoomed at 0.15s to 0.30s using VSCon.

77

4.41 The inverter is connected with microgrid of 240V at 51Hz

at 0.1s and it is synchronized in nearly (a) 0.18s using PLL

and (b) 0.12s using VSCon

78

xvi

LIST OF TABLES

2.1 Benefits of distributed generation

2.2 A brief summary of synchronization techniques and characteristics

3.1 Logic table of circuit breaker operation

4.1 System parameters and components

4.2 Loads and connection timing of microgrid

4.3 Loads and power rating for 120V in 50 Hz microgrid

4.4 Loads and power rating for 240V in 50Hz microgrid

xvii

LIST OF SYMBOLS AND ABBREVIATIONS

D Friction Coefficient

fi Field current

J Virtual rotor inertia

LI Load current

CI Inverter current

M Inertia

fM Mutual field inductance

P Real Power

inP Input mechanical power

outP Output electrical power

Q Reactive Power

s Second

eT Electrical torque

mT Mechanical torque

gV Grid voltage

sX Synchronous reactance

coupling Voltage angle at coupling point

Virtual angular speed of rotor

Virtual angular acceleration of rotor

g Grid voltage phase

AC Alternate Current

xviii

CORDIC Coordinate Rotation Digital Computer

CSI Current Source Inverter

DC Direct Current

DER Distributed Energy Resource

DG Distributed Generation

EPLL Enhanced Phase Locked Loop

FLL Frequency Locked Loop

GI Generalize Integrator

GSC Grid Side Converter

MHDC Multi-harmonic Decoupling Cell

MW Megawatt

PCC Point of Common Coupling

PCM Power Control Mode

PI Proportional Integral

PID Proportional Integral Differential

PLL Phase Locked Loop

PMS Power Management System

PWM Pulse Width Modulation

RES Renewable Energy System

RMS Root Mean Square

SOGI-PLL Second Order Generalized Integral PLL

SRF-PLL Synchronous Reference Frame PLL

THD Total Harmonic Distortion

VCM Voltage Control Mode

VCO Voltage Controlled Oscillator

VSI Voltage Source Inverter

VSCon Virtual Synchronous Converter

CHAPTER 1

INTRODUCTION

1.1 Introduction

Distributed generations (DGs) have attracted the researchers around the world due to

several advantages such as reduce the emission of greenhouse gases, increasing the

reliability of the system and alleviating the pressure on power transmission system.

According to [1], the distributed energy resources based on microgrid system are a

good solution to transfer the electric power from DGs to the existing electrical grid

and at the meantime to supply the electric power to the local loads in order to ensure

reliable power supply if the primary power supply fails to deliver the power to the

load. Most of the distributed sources need power converter mechanisms in order to

control the power flows to the microgrid. When the DGs share the power, the power

system becomes noteworthy for preserving and it improve the stability of the power

system [2]-[3]. The important task during power sharing is to maintain an accurate

synchronization condition between both sources.

Commonly, two parts of power processing strategies are required in order to

send the power from DGs to the microgrid. First, the harvested energies from the

distributed sources are converted into electrical power by using dc power converters.

For example, the non-conventional energy sources such as PV offers direct

current/voltage to be stored into rechargeable battery bank, but it needs a dc converter

to maintain the voltage level at the dc-link battery. Then this battery connects to the

existing ac low voltage microgrid network through an efficient power processing

devices [4] such as the inverter. Due to these advantages, DG-microgrid is catching

more room to be researched in order to export more power to the grid from the DG. In

this point of view, smart and intelligent coupling between microgrid and DGs become

significant issue for transferring the power with good and fast synchronization

2

mechanism. As usual, current-source inverter (CSI) or voltage source inverter (VSI) is

used as a power processing converter to supply the power from the DG to the

microgrid. As compared to CSI, VSI is an inverter that has ability to control the

inverter output voltage and it does need external reference to keep the inverter voltage

to synchronize with the grid voltage [5]. After it has been synchronized, other control

systems such as frequency droop and voltage droop can be applied to perform

autonomous power flow to the microgrid [6]. Before the power can be transferred, both

inverter and microgrid require synchronization stage in terms of voltage, frequency

and phase angle to be the same in both side. Then the inverter can feed the power to

the grid; even the microgrid voltage changes it frequency, phase, and amplitude [7].

Figure 1.1 shows the typical synchronization and power control structure for

microgrid-connected inverter.

Figure 1.1: Typical control structures for low voltage microgrid-connected inverter

The synchronization unit, which is the digital PLL block shown in Figure 1.1 needs to

provide frequency, amplitude, and phase information from the microgrid voltage in

order to generate a reference phase signal for the inverter control [8] for

synchronization and power delivery. Many power control approaches focuses with the

aim of transferring power into the microgrids from DGs such as [9][10][11][12] with

voltage source control, power angle control, torque angle control and current control.

Before the power can be transferred, PLL is commonly used technique for

synchronization. Conversely, to adopt the synchronization using PLL, all parameters

that are the phase angle and frequency need to be included for inverter outer and inner

loop controllers before it can be synchronized. This slow process on microgrid

parameters will affect the overall control performances and increase the time of

synchronization.

VDC

Controller PLL

V

I

Q

Microgrid

Bus 240 V,

50 Hz

AC Ic

Ls Lg Cf

Rd

=

3

As known, the synchronous generator does not required PLL to transfer the

power to the electrical network. It is when, the synchronous generator has reached a

mature technology for electrical power transfer which is not required PLL and this had

triggered a concept of self-synchronization for DGs inverter application [13]. It is

where the output voltage profile depends only on the generator physical construction

and input mechanical power. In such cases, frequency and amplitude of voltage are

depended on angular speed of prime mover and field excitation of rotor coil of

generator. For replacing the PLL operation, several mathematical models and

algorithms that can behave as a generator inside the inverter control can be

implemented. This technique is called synchronverter that has been proposed in

[13][14]. Based on this concept, several improvements have been made especially on

the frequency and phase tracking of the inverter-microgrid with low voltage

connection. Therefore, this improved version of synchronverter can work in grid-

connected or stand-alone mode since it has ability to control the voltage and the

frequency of the inverter output. Moreover, by maintaining the proper control on the

voltage and the frequency on inverter, it will able to share real power and reactive

power into the grid at the same time.

In this research, synchronous generator model has been used to develop a

virtual synchronous converter (VSCon) with an improved version of synchronverter

in order to achieve the capability of self-synchronizing with a low voltage microgrid

system without the aid of a dedicated PLL synchronization unit. The VSCon

synchronization between inverter and low voltage microgrid has been attained in 20ms

in case when 10 phase shift happens to the input voltage source. However, it has

widened the frequency bandwidth of the system and it is able to reduce the time needed

for synchronization, and improves the accuracy of synchronization. It also improves

the performance of the system but also reduces the complexity of the overall

synchronization system mechanism.

1.2 Problem statement

The phase locked loops (PLLs) have a slow synchronization unit, where it could

inherently affect control performance and degrade system stability [14]. In addition, a

complex synchronization unit is often computationally intensive, which adds

4

significant burden to the controller. Therefore, the synchronization needs to be done

quickly and accurately in order to maintain the synchronization [14]. It makes the

design of the controller and the synchronization units is very challenging because the

synchronization unit is often not fast enough with acceptable accuracy. Moreover, in

distributed generation system, PLL circuitry increase computational burden for

controller unit for synchronization as well as normal PLL does not effect on phase

changes due to line impedance affect. In case of frequency, changing in the grid source

given in paper [14], which used 20V and 50Hz system has been studied and changing

in 0.1Hz frequency in the microgrid, it takes approximately 0.45s, which is nearly 22

cycles to stabilize the rated frequency.

1.3 Objectives

The objectives of this research are as follows:

1. To review various topologies of the synchronization between the inverter and

low voltage microgrid in DG connection system.

2. To develop the synchronous generator mathematical model for inverter control

strategy for fast synchronization between the inverter and the low voltage

microgrid without using PLL structure.

3. To simulate and test the proposed synchronization technique using

Matlab/Simulink in several load conditions.

1.4 Scope of project

The scope of this research project are given as below:

1. The proposed synchronized controller model is limited to accept a voltage level

between 120V and 240V at 50 Hz frequency in single-phase connection.

2. To limit the testing, the Matlab/Simulink simulation models to 24V, 120V and

240V inverter-grid voltage at different load conditions.

3. The VSCon inverter model also has been limited with full-bridge rectifier as a

non-linear load system condition in single-phase inverter.

5

4. This self-synchronization model will also been simulated in different phase-

angle and frequency changes along the simulation time.

1.5 Thesis Organization

This research work structured into five chapters. Details and specific explanation to

every section will be discussed below:

Chapter 1: This is the Introduction of the research project. It consists of background

of research, the research problem, research objective and scope of project.

Chapter 2: Literature review will look at the previous researches within the scope

especially on the PLL applications. It looks at factors contributing to synchronization

between inverter and microgrid connection with several structures of PLL in the grid

application. At the end of this chapter it will be organized and tabulated from all the

paper review for summaried based on parameters affecting performance of a

synchronization unit.

Chapter 3: This chapter will discuss the methods that will be adopted in self-

synchronization between low voltage microgrid and inverter. The flow charts of phase

synchronization and frequency synchronization have been discussed including

complete model of VSCon.

Chapter 4: In chapter 4, the VSCon model will be simulated in Matlab/Simulink in

various load conditions at different voltage levels of 24V, 120V and 240V at 50 Hz

frequency. The different significant changes in simulation results will be discussed

while changing grid voltage phase-angle and frequency also will be discussed. The

comparison between low voltage microgrid and inverter synchronization using PLL

and using VSCon also will be illustrated in this chapter.

Chapter 5: The last chapter in the research summaries the entire research work

conducted and the conclusion. The future research were given for possible actions to

be taken.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction to distributed generation

Nowadays, distributed energy resources (DERs) [15][16] have attracted to extra

consideration in various technological advances since petroleum based energy

production contaminates large scale of greenhouse gases emission as well as high

maintenance cost [17][18]. This capacity is expected to reach 2,080 MW by 2020,

contributing to 7.8% of total installed capacity in Peninsular Malaysia and Sabah [19].

Distributed generation (DG) is not a new concept by itself. A number of patrons

utilize electricity from home-side electricity generation plant by their own facilities.

Nowadays, researchers emphasize more efforts to small resources of electricity

spreading all over the world. With the advantages of new technology, it becomes one

of the most competitive positions in the business of energy. It is developed with driving

contribution not only on green energy but also on utilizing fewer resources. Many

countries minimize the transmission and distribution cost by introducing locally

producing energy and locally consuming energy rules strategy of distributed

generation [20].

Moreover, distributed generators are not only limited to synchronous

generators, induction generators, reciprocating engines, micro turbines, combustion

turbines or gas turbines but also fuel cells, solar photovoltaic and wind turbine. DG is

meant to shift the structure of the utility system from a centralized, radial system to

energy source connected on the distribution level.

The benefits of applying DG in existing distribution system network have both

of economic and technical point of positive although benefits ultimately counted in

7

terms of money over the technical advantages. The benefits in two groups of technical

and economic [21][22][23] as shown in Table 2.1.

Table 2.1: Benefits of distributed generation

Technical side Economic side

Reduces line losses Defers investment for facilities upgrades

Voltage profile improvement Reduces operation and maintenance cost

Reduces emissions of pollutants Enhances productivity

Increases overall energy efficiency Reduces health care cost by improving environment

Enhances system reliability and security Increases overall efficiency by reducing fuel costs

Improves power quality Reduces reserve requirements and the associated costs

Relieves transmission and distribution

congestion

Lowers operating costs due to peak shaving

The major drawback of renewable energy systems (RES) is the primary investment,

which is comparatively larger than conventional system. For example, a gas turbine

system may be built for USD500.00 per kilowatt, while for a wind turbine the

investment is quite high as much as USD 1000.00 per kilowatt. On the other hand,

RESs have special particulars of site and the unpredictability of the power generated.

The availability of renewable energy like sun, wind or water mostly determines the

feasibility of a RES and this may raise environmental issues. The unpredictability of

RES also means of higher cost for balancing the electricity microgrid and maintain

reserve capacity for example in the event that the fluctuation of wind speed and

direction. In this case of abnormalities and system control technology, most of the

DERs need power electronics interfaces to be connected to the microgrid [4].

2.2 Structure of microgrids

The consortium for electric reliability technology solutions (CERTS) first proposed

the definition of microgrids: CERTS microgrid concept believes an aggregation of

loads and micro-sources operating as a single system providing both power and heat.

The majority of the micro-sources must be power electronic based to provide the

required flexibility to guarantee operation as a single collective system. This control

flexibility permits the CERTS microgrid to present itself to the bulk power system as

8

a single controlled unit that meets local requirements for reliability and security [22].

The definitions of microgrid is generally recognized that a microgrid, which is made

of multiple DERs together with DGs and distributed storage systems (DSSs),

autonomous load centers, control and protection systems. It can drive both in grid-

connected and islanded modes, and also seamlessly switch between the two modes,

enhancing the robustness of distribution system, facilitating greater utility of

renewable energy, increasing energy efficiency and the level of local dependability

required by customers’ loads [23].

In general, remote microgrid in distant areas or islands used to heavily rely on

local power generation based on diesel or natural gas, because long distances and thus

exclusive transportation make the electrification of off-grid remote communities

uneconomic and slowly developed [24], while some distant areas are abundant of

renewable energy such as; wind energy, solar energy and biomass energy.

The remote microgrid is a probable solution for electrification of off-grid

remote communities by coupling optimally energy storage divisions and DGs in

combination with an optimal dispatching strategy. The basic structure of microgrid is

shown in Figure 2.1. The basic DGs and distributed storage can be selected and

accustomed according to local natural conditions and power requirements [22].

combined heat and power (CHP) systems can also be linked to remote microgrids to

provide both electricity and heat for achieving maximum benefits for local energy

need.

Figure 2.1: Typical structure of microgrid [16].

Wind Turbine

+ Inverter

Microturbine

+ Inverter

AC storage (Capacity

and Flywheel) +

Inverter + Controller

Main Inverter + Main Controller

DC BUS

Photovoltaic’s +

Converter

Fuel Cell + Converter DC Battery

AC BUS

9

To build a reliable and economic electrification of remote areas, the key is to construct

a remote microgrid, including power management, real-time control and protection

system. The main purposes of the power management strategy (PMS) of microgrids

for the DG units are to, 1) effectively share real and reactive power supplies of loads

among the DG units, 2) rapidly respond to disturbances and fluctuations DSSs, 3) pair

DSSs with DGs and find out the charge and discharge of DSSs, and 4) resolve the final

power generation set-points of the DG units to equilibrium power and restore

frequency of the system [25].

The optimal transmitting strategy based on mathematical model and

optimization algorithm can reduce the microgrid lifetime cost and emission, which has

been demonstrated by a remote community example located in the northern part of the

Canadian Province of Ontario [26]. Another power management strategy contains of

three (3) parts are [27]:

1. Pre-scheme is based on past date of periodic loading profiles and optimization

strategy;

2. Daily plan is based on weather forecast and probable load consumption;

3. Hourly observation is based on monitor device and simulation.

A three-step methodology is proposed, containing of local scopes, microgrids and

virtual power plants considering different optimization objectives [28].

2.3 Methods of synchronization and PLL

Microgrids enhance reliability through their ability to island or operate autonomously.

A microgrid is generally connected to the utility microgrid through a single connection

point and it can be easily islanded by opening the circuit breaker at that point. After

islanding, the microgrid should maintain to serve its loads without interruption. The

microgrid must also be able to resynchronize with the utility microgrid when the

condition that initiated islanding has been corrected [16]. To achieve this objective,

the ideas of active-power/frequency and reactive-power/voltage droop controls have

been considered [2].

Moreover, the concept of “plug and play” distributed micro-sources is the key

to microgrids. To accomplish this goal, the ideas of active-power as well as frequency

and reactive-power as well as voltage droop control is used. It allows micro-sources to

10

share power and maintain stability without the need for fast communications [29].

However, it has been a norm to adopt a synchronization unit, such as PLL to make

sure that the inverter is synchronized with the grid. This practically adds an outer-loop

controller (the synchronization unit) to the inverter controller [30]. Since, PLLs are

characteristically nonlinear, difficult and time-consuming to tune the PLL parameters

to reach satisfactory performance while controlling the inverter in power system.

The control scheme for the DER inverter is based on PLL for frequency and

phase detection together with conventional microgrid active-power/frequency and

reactive-power/voltage droops [2][31]. The block diagram of the complete control

strategy is shown in Figure 2.2.

Figure 2.2: Complete block diagram of PLL control scheme [30].

The opening action in the control algorithm is to convert phase voltages and

currents into stationary reference frame (α and β) quantities. The α and β voltage

components are used by the PLL to guesstimate the frequency and launch the phase

reference for the inverter. These quantities are endowed to the phase regulator that

calculates the required output phase of the inverter. The voltage regulator computes

and regulates the desired voltage magnitude of the inverter. Lastly, the PWM generator

takes the desired voltage magnitude and phase and creates the PWM output signals to

the thyristors gate to establish synchronism with microgrid by providing appropriate

phase-angle as well as frequency and voltage amplitude.

Va Vαβ

Iαβ

P-f droop

Phase

Regulator

Positive

Sequenc

e filter

P-Calculation

Q-Calculation

PLL

V-magnitude

Q-v droop

And V

Regulator

PWM

Vb

Vc

11

The single-phase system usually takes up traditional zero-crossing detection

phase-locked method, which is simple and straightforward to execute, but zero-

crossing detection point would be exposed to external interference causing phase-

locked error or failure when the microgrid voltage is distorted. Reference [32]

proposed an enhanced phase-locked loop (EPLL) structure, which did not contain

synchronous reference frame (SRF). Its major improvement over the conventional

PLL lies in the phase delay mechanism, which allows more flexibility and provides

more information such as amplitude and phase-angle. Moreover, synchronization

method which not only demonstrates a superior performance as compared with the

existing methods with respect to corrupting factors of the signal, but it also provides

frequency adaptively and tolerance to unbalanced system circumstances. The main

building block of the synchronization method is an EPLL system, which operates as a

nonlinear dynamical system. However, the system dynamic performance became poor

due to the two closed-loop structure.

Designing inverter synchronization unit is based PLL, which consists of digital

processing unit, having many trigonometric functions. Frequent changes in microgrid

parameters make perturbation and computational burden for processing unit. The

second order generalized integrator FLL (SOGI FLL) could not allow derivative

trigonometric function rather than digital implementation of voltage controlled

oscillator (VCO) [33]. In case of disturbances in grid, VCO could response fast and

accurate information in terms of frequency, amplitude and phase, variables required to

meet the standards set by the IEEE 1547 standard while connecting and disconnecting

of systems power generation [9][10].

However, synchronous reference frame PLL (SRF-PLL) does not provide

individual values of the amplitude, phase and frequency rather than average

information. Therefore, this scheme cannot be applied to single-phase systems in

straightforward manner [34]. These techniques are implemented to detect the

magnitude of voltage and frequency at coupling point of grid-inverter. The interfacing

of these modification can be changed with application such as tracking harmonics,

variable frequency etc. The dq transformation system is usually used to determine the

phase difference and this would deal with self-synchronization. A number of

techniques can estimate this phase error. The computational methods for instances are

algebraic iteration, geometric arc tan and coordinate rotation digital computer

(CORDIC) algorithm [35] then PI controller [11]. In addition, it affords a beneficial

12

structure for single-phase PLLs as long as the 90 shifted orthogonal component of

the single phase input signal is created [12]. The linearized SRF-PLL is shown in

Figure 2.3 having two loops for grid-synchronization loop and self-synchronization

loop. So that, when the voltage is V , which is generated by the “self-synchronization

loop,” is greater than the maximum voltage V that can be generated by the “grid-

synchronization loop,” there is no equilibrium point that will produce zero q-axis

current.

Figure 2.3: Linearized small-signal model of SRF-PLL [36].

The loop is then unstable, as the negative feedback cannot overcome the offset of the

“Self-synchronization Loop” V . This provides to the stability criterion.

sin( ).

g

c L

c i g

VI I

Z

(2.1)

where inverter current cI is less than the line current LI and this would be inversely

proportional to microgrid impedance as well as would affect the real and reactive

current dynamically. Therefore, this condition could make the stability issue while

large amount current could be injected into microgrid [12]. The modified SRF-PLL is

proposed in [36] where the voltage is accounted due to reactive current injection to

estimate the grid . This difference is calculated in common coupling point to compute

the actual voltage phase and phase difference at coupling point independently. There

are a number of techniques are employed but using rotating synchronous reference

frame gives following expression (2.2).

13

1

2 2cos d

coupling

d q

V

V V

(2.2)

where the coupling is known as inaccuracy between voltage phase at coupling point and

phase-angle of grid. In case of smaller value affect the stability, it gives better precision

of angle even positive sequence having low-pass filter due to presence of noise in

microgrid voltage.

Thus, CORDIC algorithm is another method of phase estimation, which reduce

the computational burden and take less time to process the information in

microcontroller as well as other field programmable gate array (FPGA) platform [35].

This method also facilitates trigonometric as well as logarithmic functions evaluation

with iteration where optimum computational resources would be used by means of a

rotational vector in structure frame [9]. Figure 2.4 shows CORDIC method is applied

on conditional as well as shifting based operation. Any position is located by 0X and

0Y at Cartesian plane is rotating by angle can be defined by using (2.3) and (2.5).

The distance between position of Cartesian plan and the origin can be derived by using

(2.5).

0 [ cos ( ) sin ( )]i iX K X Y (2.3)

0 [ sin ( ) cos ( )]i iY K X Y (2.4)

2 2i iR X Y (2.5)

Figure 2.4: Rotating coordinate

14

The angular displacement between iX and iY can be calculated using (2.6).

1tan ii

i

X

Y

(2.6)

CORDIC has two mode of operation are rotation mode and vectoring mode

based on the application. In this application rotation mode is chosen to reduce the

resulting angle in each iteration by applying pseudo-rotation method, which is

described in [37] as given below.

0[ ( )]id sign a i (2.7)

Hence, the sign function can be expressed in real and imaginary axes form (2.5) and

(2.6). The iterative rotation can be expressed for both of components oX and oY for

new position estimation are expressed in (2.8), (2.9) and (2.10), (2.11) respectively.

Here K is scale factor is accumulated by 1cos tan 2 i , where 2iis defined as shift

operator and 0a is the output angle in (2.12).

0 0 0( 1) ( ) 2 ( )i

iX i X i d Y i (2.8)

0 0X KX (2.9)

0 0 0( 1) ( ) 2 ( )i

iY i Y i d X i (2.10)

0 0Y KY (2.11)

0 0( 1) ( ) i ia i a i d (2.12)

When i is fundamental angle makes iteration number can be written as

1tan (2 )i

i

(2.13)

This shifting angle compensates the delay produced by filters which is also

suited for unbalanced conditions. However, it has still complex feedback process and

tuning procedure to operate under abnormal condition. It reduces the time to estimate

phase because digital computer has fewer parameters to process in processor.

Another method is microgrid side converter (GSC) [32] is based on the PQ

controller which produces the reference currents, and the current controller which

holds the concern for a suitable fitting of current to be injected [9][11][38][39]. The

PQ controller can be implemented in the stationary or SRF as a closed-loop or an open-

loop controller. Thus, a proportional-resonant controller in the stationary reference

frame or a PI controller in the SRF can be adopted as the current controller [40].

15

Subsequently, the introduced current has to be synchronized with the microgrid

voltage the reaction of both controllers will be affected by the performance of the

synchronization method.

The quadrature SG (QSG) generates the quadrature voltage vector (Vαβ) [38]

along with higher order harmonics elimination filters of the voltage. The Multi-

harmonic Decoupling Cell (MHDC) [9] segment accomplishes the fast and accurate

decoupling of the fundamental voltage vector from the oscillations initiated by the

low-order harmonics. Finally, the nearly harmonic-less voltage signal is used by the

dq-PLL procedure to extract the voltage phase. The weaknesses of this method is the

small oscillation on the synchronization signals under non-nominal frequency due to

fractional outcome, which can be minimize using a radical fractional-order delay

method, and the increased implementation and computational complexity. Therefore,

the dynamic response and the invulnerability against voltage harmonic distortion of

the MHDC method can improve the performance of grid-inverters network as well as

improve the power quality.

Since synchronous reference frame phase-locked-loop (SRF-PLL) is broadly

used in three-phase grid. When microgrid voltage is unbalanced, the sequence

components refer double frequency in phase information. This problem can be

improved by adding another SRF-PLL for getting positive and negative components,

frequency and phase angle in the network but still it is complex in terms of algorithmic

computation [11]. Instantaneous symmetrical components tracking technique is most

useful to determine the positive and negative sequence component where back to back

PLLs are connected known as enhanced-PLL (EPLL) [39].

It is mentioned that SOGI settling time is twice times greater or equal to FLL

settling time [41]. In addition, input frequency is determined by FLL and the phase is

taken form tangential component of q component and d component [42]. This

technique allows fast and accurate response of frequency tracing, voltage amplitude

and phase detection in faulty conditions as well as healthy condition of grid. Another

method is introduced to investigate automatic phase detection and frequency tracking

digitally is linear interpolation [43] method. Normally, input data is processed in

digital computer having well equipped with digital filter transfer function to process

data with 16-bit DSP processor. Sometimes, it is to increase the precision by

concerning the floating data and mathematical function. The delta operator based

16

digital filter operation is also introduced in [44] to minimize the lengthy sampling

effects of filter.

In addition, it can be extended into the single-phase system applications with a

few modifications successfully. Similar, double second order generalized integrator

PLL (DSOGI-PLL) extracts the fundamental positive sequence to the SF-PLL whereas

it operates a double second order generalized integrator to implement the quadrature-

signals generator [45]. Therefore, the precise sequence component extraction here

requires 90 phase shift for V

and V

. There are two solutions for this purpose,

the transport delay buffer method and the all pass filter method. Proper mixing band

pass filter (BPF) and low pass filter (LPF) remove the harmonics and 90 phase shift

by recognizing the angular frequency.

Figure. 2.5: DSOGI-PLL block diagram [45].

Figure 2.5 shows the DSOGI-PLL block diagram. On the other hand, enhanced

PLL (EPLL) is a frequency-adaptive nonlinear synchronization approach [46]. The

main progress over the conventional PLL lies in the PD mechanism is it permits more

flexibility and provides more facts such as amplitude and phase angle. EPLL also

provides higher degree of resistance and insensitivity to noise, harmonics and

unbalance of the input signal. Whereas, Three phase multi-PLL (3MPLL) adaptively

tracks and estimates the magnitude, phase angle and frequency of the input signal [30].

The operation principle of quadrature PLL (QPLL) is stated in [47] where this method

is based on estimating in-phase and quadrature-phase amplitudes of the fundamental

component of the input signal. In advantage, it is applicable for both in distributed

17

generation and communication system applications. In case of wide-range of

synchronization, the predictive PLL (PPLL) comprises of a predictor, oscillator, and

phase-shifter and data acquisition blocks [48]. It has phase locking mechanism similar

to the conventional PLL while PPLL locks to the phase of input signals, however, it

works the systematic expressions for computing the frequency, amplitude and phase

difference in a predicted manner [49].

2.4 Synchronous generator model

In this section, modelling of synchronous generator[50][51] is discussed in steady

state, balance sinusoidal voltage or current condition for simplification of analysis.

Most of the cases, assumptions are made to dynamic analysis of synchronous generator

for example for system analysis and controller design, damping-windings-less round

rotor machine is chosen having p pairs of poles per phase without effect of magnetic

saturation of iron core. Since, synchronous generator is driven by mechanical power

from prime-mover, this section is divided into two sections of electrical part and

mechanical part.

Figure 2.6: An ideal three-phase synchronous generator model [52].

18

2.4.1 Electrical part

The basics part of synchronous machines are ferromagnetic structures are hollow

cylindrical part is called stator or armature which is slotted for armature windings and

the rotor rotates inside the hollow cylinder [52]. The winding on the rotor, called the

field windings. The dc current is supplied to the field winding by an exciter to produce

high magnetomotive force (mmf). Figure 2.6 shows three such coils a , b and c which

represent the three armature windings on the stator round-rotor machine and

concentrated coil f , which represent the distributed field winding on the rotor. The

three stationary armature coils are identical in every respect and each has one of its

two terminals connected to a common point o. The other three terminal are a , b and

c . The axis of coil a is chosen at 0d and counter clockwise around the air gap

are the axes of the b coil and c coil are at 120d and 240d respectively.

Assume that, each of the concentrated coils a , b and c has self-inductance sL which

is equal to the self-inductance of distributed armature windings. In addition, the mutual

inductance between each adjacent pair of concentrated coils are negative constants

sM .The mutual inductance between the field coil f and each of the stator coils

varies with the rotor position d as a consinusoidal function with maximum value fM

are shown in (2.14), (2.15) and (2.16).

cosaf f dL M (2.14)

cos 120bf f dL M (2.15)

cos 240cf f dL M (2.16)

Flux linkage with each of the coils a , b , c and f are due to its own current

and the currents in the three other coils. Flux-linkages equations are therefore written

for all four coils as follows in (2.17) and (2.18)

Armature:

a s a s b c af f

b s b s a c bf f

c s c s a b cf f

L i M i i L i

L i M i i L i

L i M i i L i

(2.17)

19

Field: f af a bf b cf c ff fL i L i L i L i (2.18)

For considering the steady state condition, it is assumed that the current fi is dc with

a constant fI and that the field rotates at constant angular speed , when the machine

constructs with two-pole.

0;dd d

dt

dt

(2.19)

Where angle 0d denotes the initial position of field winding in (2.19). Hence (2.20)

can be rewrite considering the initial position of field winding.

Armature:

0

0

0

cos

cos 120

cos 240

a s s a f f d

b s s b f f d

c s s c f f d

L M i M I t

L M i M I t

L M i M I t

(2.20)

The (2.20) shows that a has two flux-linkage components, one due to field current

fI and the onther due to armature current ai , which is flowing out of the machine for

generator action. When coil a has resistance R then the voltage drop av across the

coil from terminal a to terminal o .

0sina aa a a s s f f d

d div Ri Ri L M M I t

dt dt

(2.21)

The last term of (2.21) denotes an internal emf or generated emf or synchronous

internal voltage is defined ae can be written as below.

0sina f f de M I t (2.22)

Hence, it shows that the generated emf magnitude and frequency is directly

proportional to the field excitation and angular velocity of field coil as well as prime

mover of machine.

2.4.2 Mechanical part

The mechanical part of the synchronous generator is governed by

20

m e pJ T T D (2.23)

Where J is the momentum of inertia of all parts rotating with the rotor, mT is the

mechanical torque, eT is the electromagnetic torque and pD is a damping factor. eT can

be found from the total energy E stored in the machine, which is the sum of the

magnetic energy stored in the stator and rotor magnetic fields and the kinetic energy

stored in the rotating parts as follow.

21 1 12 2 2a a f fE i i J (2.24)

Since the mechanical rotor position m satisfies mp ,

sine f f a

m

E ET p pM I i

(2.25)

2.5 Synchronverter

Similarly, synchronverters are grid-friendly inverters that mimic synchronous

generators (SG) is discussed in [13]. Mathematically, a synchronverter is a model

which behaves as in same of synchronous machine to deliver a voltage supply. The

controlling principle of a power controller with integrated competency of voltage and

frequency parameter so it is able to achieve real power control, reactive power control,

frequency regulation, and voltage regulation. It is assumed that there are no damper

windings in the rotor, that there is one pair of poles per phase without having any

magnetic-saturation effects in the iron core as well as no eddy currents. The phase

voltage v of terminal can be obtained below. Where is flux per pole, sL is self-

inductance and sR is stator coil resistance.

S S S

d div R i R i L e

dt dt

(2.26)

sin cosf

f f f

die M i M

dt (2.27)

1( )m e pT T D

J (2.28)

,sine f fT M i i (2.29)

sinf fe M i (2.30)

21

,sinf fQ M i i (2.31)

Where it is assumed that three phase SG stator current i , can be written as below.

a

b

c

i

i i

i

,

sin

2sin sin

3

2sin

3

,

cos

2cos cos

3

2cos

3

Where mT , eT , e , , and Q are the mechanical torque applied to the rotor, the

electromagnetic torque, the three-phase generated voltage, the rotor angle, rotor

angular velocity and the reactive power respectively. Similarly, to the control of a

synchronous generator, the controller of a synchronverter has two channels: one for

the real power and the other for the reactive power. The real power is controlled by a

frequency droop control loop, using the (imaginary) mechanical friction coefficient

pD as the feedback gain. This loop regulates the (imaginary) speed of the

synchronous machine and creates the phase angle for the control signal e . The

reactive power is controlled by a voltage droop control loop, using the voltage droop

coefficient qD . This loop regulates the field excitation

f fM i , which is proportional to

the amplitude of the voltage generated. Hence, the frequency control, voltage control,

real power control, and reactive power control are all integrated in one compact

controller with only four parameters.

2.6 Inverters and controls

Power inverter is an electrical power converter that alters the dc into ac. The altered

ac form of electrical power can be transmitted to microgrid and consumed by

appliances in suitable form of voltage and frequency with the use of proper

transformers, switching and control circuits. Modern solid-state switch controlled

inverters do not require any moving parts and have wide range of applicability from

high frequency switch in computers application to large current carrying capability at

ultra-high-voltage for transmission bulk power to grid.

22

Most commonly used inverter type is voltage source inverter (VSI) where ac

power supplies at the output end as a function of voltage source and an independent

dc battery source called dc-link is used as input in sending end. This structure of

interfacing are mostly used in industrial arenas as a voltage source for using variable

speeds drivers and so on [53]. The single-phase VSI and three-phase VSI are used in

low-range power and high-range power application respectively where the voltage

amplitude, frequency and phase can be controlled. Figure 2.7 shows that a typical

voltage source inverter (VSI).

Figure 2.7: Typical voltage source inverter

On the contrary, current source inverter (CSI) supplies current at constant mode where

voltage is changed with the load requirements. In case of protection filter is normally

an inductor in series with dc source. The major advantage of the CSI is that it increases

the voltage towards the mains itself, so an additional dc/dc converter may not be

necessary [54]. Figure 2.8 shows the typical current source inverters.

Figure 2.8 Typical current source inverter

However, in case of interfacing distributed energy resources with the

conventional grid, a single inverter can characteristically work on two modes; one is

the grid-connected mode and another is the isolated mode. Equivalently, following two

modes, the controller has two individual modes of operation such as power-control

mode (PCM) and voltage-control mode (VCM).

_

+

_

+

Load Voltage DC C

+

_ _

+

ILoad

Load Current DC

L

23

2.6.1 PCM control

The voltage at grid-connected mode of operation does not actively regulate the power

management system (PMS). As a result, the logical choice of control of current

regulation is used by means of power flow control. The control calculates current

references for the system by the measured voltage and desired power levels, where the

current references are produced by the outer power control loops. This control mode

of operation control the real power and reactive power injecting to the grid. PCM may

deteriorate the power factor of ac source [55].

2.6.2 VCM control

When the inverter is disconnected from the utility grid, it has no longer to maintain the

microgrid voltage rather than the local load. This mode is called isolated mode or

standalone mode. In this case, inverter is operated in VSI mode where current

references are introduced by inner voltage control loop. The advantages of this off-

grid inverters are; the stored battery bank can be connected with inverter while

microgrid fails. In addition, remote areas or village uses this mode of inverter control

for electrification locally [55].

2.6.3 PI control

Proportional integral (PI) is proposed to improve the performance of the soft-switched

inverter [56]. PI controller controls the duty ratio of the inverter. The PI controller is

also improved to control the duty ratio of the inverter to provide optimal performance

at all operating conditions of the system.

PI control is a tradition linear control technique used in industrial applications.

The linear PI controllers are usually designed for dc-ac inverter and dc-dc converters

using standard frequency response techniques and based join the small signal model

of the converter [57]. A bode plot technique is used in the design to obtain the desired

loop gain, crossover frequency and phase margin. The stability of the system is

guaranteed by an adequate phase margin. However, the proportional integral derivative

(PID) and PI controllers can only be designed for one nominal operating pint. A boost

24

converter’s small signal model changes when the operating point varies. The poles and

a right-half pane zero as well as the magnitude of the frequency response are all

dependent on the duty cycle. Therefore, it is difficult for the PID controller to respond

well while changes in operating point. The PI controller is designed for the boost

converter for operation during a start-up transient and steady state respectively. Figure

2.9 shows the typical block diagram of PI controller.

Figure 2.9 Typical block diagram of the PI controller [58].

2.7 Summary

This chapter summarizes, the operation and structure of several PLLs that can be

applied in the inverter grid connection. These PLLs will give the information of the

phase angle of the grid but it totally depends to the time response of the PLL. A

synchronverter is compact control structure for a grid-connected inverter, where the

functions of synchronization, voltage regulation, and frequency regulation are

integrated into controller of synchronization unit. The synchronverter removes the

major part of non-linear element for example PLL that affects the synchronization

accuracy and reduce complexity of design. It also widens the bandwidth of the system

to increase the performance of the overall system. The brief summary of

synchronization techniques is given in Table 2.2.

-

+ + +

Proportional (P)

Plant

Integral (I)

82

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