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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