Robust Active Power Filter Controller Design for Microgrid and Electric Vehicle Applications Part-1

PART-1
Robust Active Power Filter Controller Design
for Microgrid and Electric Vehicle Application
BUDDHADEVA SAHOO
Registration No. 1781001006
Department of Electrical Engineering
Institute of Technical Education & Research
Siksha ‘O’ Anusandhan
(Deemed to be University)
Bhubaneswar-751030, Odisha, India
2021

Robust Active Power Filter Controller Design
for Microgrid and Electric Vehicle Application
Thesis submitted in partial fulfilment of the requirements
for the degree of
Doctor of Philosophy in Engineering
by
Buddhadeva Sahoo
Registration No. 1781001006
CSIR ACK No: 143460/2K19/1
Supervisor
Prof. (Dr.) Sangram Keshari Routray
Associate Professor
Electrical and Electronics Engineering Department,
Siksha ‘O’ Anusandhan (Deemed to be) University, Bhubaneswar, Odisha, India
Co-supervisor
Prof. (Dr.) Pravat Kumar Rout
Professor
Siksha ‘O’ Anusandhan (Deemed to be) University, Bhubaneswar, Odisha, India
Department of Electrical Engineering
Institute of Technical Education and Research (ITER)
SIKSHA ‘O’ ANUSANDHAN (Deemed to be University)
Bhubaneswar, Odisha, India
2021

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MR BUDDHADEVA - SAHOO
DR SANGRAM KESHARI ROUTRAY
ELECTRICAL AND ELECTRONICS DEPARTMENT
SIKSHA O ANUSANDHAN DEEMED TO BE UNIVERSITY,KHORDHA ORISSA - 751030
Date Of Examination : 01/12/2020
Roll Number : 143460/2K19/1
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CERTIFICATE
This is to certify that the thesis entitled "Robust Active Power Filter Controller
Design for Microgrid and Electric Vehicle Application" submitted by Mr.
Buddhadeva Sahoo, Registration No:1781001006, for the award of Doctor of
Philosophy from Siksha *0' Anusandhan (Deemed to be University) is a record of
an independent research work done by him under our supervision and guidance. This
work is original. This has not been submitted elsewhere to any other University or
Institution for the award of any degree or diploma. In our opinion, the thesis has
fulfilled the requirements according to the regulation and has reached the standard
necessary for submission.
To the best of our knowledge, Mr. Buddhadeva Sahoo bears a good moral
character and descent behavior.
Prof. (Dr.) Sargram Keshari Routray
Associate Professor
Dept of Etectrical &Electronics Engg.
ITER, SOA Deemed to be University
Bhubaneswar, india-751030
Supervisor
Prof. (Dr.) Sangram Keshari Routray
Associate Projessor,
Siksha O Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India
Prof.(Dr.) Pravat Kumar Rout
EEE Department
Siksha '0' Anusandhan
(Deemed to be University)
Co-supervisor
Prof. (Dr.) Pravat Kumar Rout
Professor,
Siksha O Anusandhan (Deemed to be University), Bhubaneswar, Odisha, india

APPROVAL SHEET
Title of Dissertation: Robust Active Power Filter Controller Design for
Microgrid and Electric Vehicle Application
We the bellow signed, after checking the dissertation mentioned above and the official
record book(s) of the student, hereby state our approval of the dissertation submitted in
partial fulillment of the requirement of the degree of Doctor of Philosophy in
Engineering at Electrical Department under Siksha "0' Anusandhan (Deemed to be
University), Bhubaneswar. We are satisfied with the volume, quality, correctness, and
originality of the work.
Examiners
DS.Smtasa KeoD
NITwaampal-so6&oy
Supervisor(s)
Prof. (Dr.) Sangram Keshar Routray-
Ascociate Piofcrsor HOR
Dept. of Ejectrica &Electronics ncg-
TER, SOA Dcen ed to be Uiiversity
8huhaneswai, india-751030
Pqt avatKumar Rout
EE Department
Siksha 'O' Anusandhan
T®eemed to be University)
Ph.D.Chairman
Dr. Renu'
Shama
Professor &FHea
Denariment oi Etectrica! Engineedng
HEA SADgemgdiohoiloiversity
Bnubanes, 751030
Date:19»)202|
Place:
ii

DECLARATION
1, Mr. Buddhadeva Sahoo do hereby declare that the thesis entitled "Robust Active
Power Filter Controller Design for Microgrid and Electric Vehicle
Application" being submitted to the Siksha 0' Anusandhan (Deemed to be
University) for the partial fulfillment of the requirements for the degree of Doctor of
Philosophy in Electrical Engineering represents my ideas in my own words and
where others' ideas or words have been included, I have cited and referenced the
original source files. I also declare that I have adhered to all principles of academic
honesty and integrity and have not misrepresented or fabricated or falsified any
idea/data/fact/source in my submission. I understand that any violation of the above
will cause disciplinary action by the Institute and can also evoke penal action from
the sources which have thus not been properly cited or from those whose proper
permission has not been taken when needed.
veldhacown laho
Buddhadeva Sahoo
Registration No: 1781001006,
Department ofElectrical Engineering
Siksha O Anusandhan (Deemed to be University),
Date: 12 202
Place: hnbaniiD

ACKNOWLEDGEMENTS
Firstly. I thank CGod (Gopal Bhai) for letting me through all the difficulties and standing
with me every time. I have experienced His guidance and support day by day.
I want to thank my supervisor Prof. (Dr.) Sangram Keshari Routray, ITER, for his
valuable guidance and support. I appreciate him for their valuable contribution of time and
ideas to make my Ph.D. experience and stimulating. The joy and enthusiasm for his research
were contagious and motivational for me even during the tough times in the Ph.D. pursuit.
At the same time, with much pride and delight, I express my heartfelt sense of gratitude
and am indecbted to my co-supervisor Prof. (Dr.) Pravat Kumar Rout, ITER, for his
valuabie guidance, supervision, and cncouragement throughout the tenure. I am privileged
to have him as my co-supervisor. He has spared much of his valuable time for discussion
pertaining not only to this study but also for most of his empirical findings of the inverter
design and control application in the microgrid problem.
I would like to thank the Council of Scientific and Industrial Research, Govt of India,
and SOA Deemed to be University for providing me the fellowship (SRF-Direct, File no.
09/969(11117/2021-EMR-I) during the Ph.D. Journey.
I would also like to thank my committee members Dr. Manohar Mishra, Dr. Manoj
Debnath, and Dr. S.N Bhunya for serving as my DAC member. I am thankful to Prof. (Dr.)
J.K Nath.Dean Research for his valuable suggestion and providing official support during
my work. I am also thankful to the Head of Electrical department, Prof (Dr.) Renu Sharma
for her support during the work of the thesis.
I am deeply indebted to my father Mr. Basudeva Sahoo and Smt. Jyotsna Sahoo for
their blessing, prayer, and mental support, which enabled me to carry out this research. I
would like to extend my heartiest thank to Smt. Prativa Mohanty for insisting on me to do
a Ph.D., blessing, and mental support to carry out this research. I would like to thank my
brother Jayadeva Sahoo and sister Swapna Mohanty for their emotional deprivation during
the entire period of work.
Lastly, but not least,
support from my friends Soumya Mohanty, S. Priyadarshini, Sairam Mishra, and Dr.
Shetal Chandak. I thank them for their emotional and mental support during the entire
period of research.
could not complete this work without the love, affection, and
ndhaclrala lahr
BuddhadevaSahoo
Registration No: 1781001006,
Department ofElectrical Engineering
Siksha O Anusandhan (Deemed to be Universiry),
iV

CHAPTER-1
INTRODUCTION
Robust Active Power Filter Controller Design for Microgrid and
Electric Vehicle Application
Background of the study, Literature survey regarding the
active filter control scheme, Microgrid application,
Merits and demerits, Objective, Contribution
Title of Dissertation
Introduction
(Chapter-1)
Robust Controller
(Chapter-4)
Summary
(Chapter-7)
Development and Design Stage
Implementation Stage
Conclusion Stage
Future Scope
C
O
M
P
L
E
T
S
T
U
D
Y
Reduced Switch
Multi-level Inverter (RSMLI)
Enhanced Instantaneous
Power Theory (EIPT)
(Chapter-2) (Chapter-3)
Hybrid Microgrid Application Electric Vehicle Application

Chapter-1
INTRODUCTION
1.1 Prologue
The detailed explanation such as motivation, challenges, and possible solutions related to
the smart microgrid application is discussed. In addition to that, the different
configurations and classifications of SMG are detailed in this section.
1.1.1 Motivation
Specifically, low/medium voltage-based autonomous MGs are distributed in nature and
mainly depends upon renewable energy systems like solar and wind plant, storage devices,
and hybrid vehicles [1-2]. The increased integration of distributed renewable energy (DRE)
resources in the power distribution system not only fulfills the excess energy demand but
also reduced the investment cost compared to the traditional power plant. The smart MGs
are developed by interlinking different MGs such as AC-MG, DC-MG, and hybrid MG,
and provide autonomous operation [3-4]. To avail of autonomous grid operation, the SMG
can be operated in a grid-connected or islanded mode of operation. During the grid
disturbance condition, the SMG has detached the interlinked MG from the grid and made
the system islanded and fed only to the local load, not to the utility grid [5-6]. In both
autonomous modes of operation, the power quality (PQ) and power reliability (PR) issues
are considered as major challenges during real-time applications. The novel design of MG
necessitates further development and amendment of planning, operation, and power
management in the electrical power distribution system, suburban, and industrial
applications [7-8]. The related development includes design, modeling, and control
solutions such as renewable-based system control, optimal size, and novel maximum power
algorithm for MG operation [9-10]. According to the Electric Power Research Institute
(EPRI) [11-12] and Smart Electric Power Alliance (SEPA) study [13], the advanced power
electronic switch-based converter is used as a reduced cost device for facilitating successful
grid-integration by reducing the challenges related to the grid. Due to the excess application
of power electronic devices, storage systems, sensors, sensitive loads, and hybrid electric
vehicle applications, new challenges related to the power system control are increased to
provide better stability, PQ, and PR operation [14]. In this regard, the important control
constraints for MG operation are voltage, frequency, stability, real and apparent power.

2
Chapter-1 INTRODUCTION
Therefore, the above factors motivate to develop robust controllers for successful
monitoring the following conditions [15,16,17]:
✓ During both modes of operation, improve voltage and frequency stability.
✓ During both modes of operation, by providing real and reactive power support, the MGs
avail better power-sharing operation.
✓ Offer seamless transition from grid-following mode to grid forming mode of operation.
✓ Generate optimal power sources and participate in the energy market.
✓ Provide uninterrupted power supply to critical loads like the hospital, school, and
traction drive, etc.
✓ Capability to facilitate black start during a grid failure condition.
✓ Enhancing the monitoring cost of energy production and power transfer capability of the
microgrid.
✓ Reduced the harmonic/ non-linear component.
✓ Facilitate better power quality and power reliable operation.
✓ Capable to provide better energy management by appropriately tracking the grid and
load demand.
1.1.2 Challenges
However, during the controller design and implementation stage, the researchers face a lot
of problems and a few of the prominent problems are discussed below [18-24].
✓ In AC-MG, due to the presence of electromagnetic and mechanical parameters, the
computation of output power fluctuation in the distribution generators is quite difficult
[18].
✓ DRE resources-based MGs depend upon the environmental condition, temperature, and
wind speed variation. Therefore, the output power of the MG is greatly affected and
unable to maintain the desired power level [19].
✓ Excess integration of DRE based MGs lead to voltage and frequency variations and
affects the stability and PQ of the system [19].
✓ Increasing power electronic switches, inverters, battery, synchronous and asynchronous
machine, increase the system complexity, losses, heat, and affects the stability [20].
✓ To improve stability, traditional AC-MG systems require additional devices like
coupling switches, tap changing transformer, and capacitor banks [21].
✓ Bidirectional power flow is prevented due to the electromagnetic device present in the
AC-MG. Due to the excess integration of DRE resources, advanced power electronic
devices, and cooperative loads, the bidirectional power flow is possible. However, the
conventional relay protection is no longer valid [22] and most of the AC transformer
also does not facilitate bidirectional power flow.
✓ In AC-MG and DC-MG topological structures and less capacity for interconnection, are
unable to facilitate a high proportion to the DRE [23].
✓ Balancing power is hardly possible due to the uncertainty of power generation [21].

3
✓ Flexible networking is desired for achieving an inclusive range of power at mutual aid
[22].
✓ Conventional AC distribution sectors are adopted region operation for HV networks,
ring-method with the open-loop operation for medium voltage network, and radial-
method with the week-feeder operation for LV network. Due to the above structures,
the MG systems lack flexible power transfer capability in networking [23].
✓ Excess AC-DC/DC-AC conversion results in more conversion losses. In many cases, to
improve the voltage and frequency stability, an AC-DC-AC converter is needed.
However, the conversions require an excess number of inverters, converter, and
transformers, which leads the MG results to increased cost and losses, and reduces the
overall operation and efficacy [24].
1.1.3 Possible Solutions and Related Challenges
Looking at the above possible challenges, the power engineers are suggesting different
control and design solutions for the enhancement of the MG performance. A few of the
prominent techniques are discussed below [25-35].
✓ Looking at the above problems, for a real-time application point of view, the hybrid-MG
system is the most suitable, flexible, efficient, and cost-effective solution. The enhanced
networking, innovative design, and reduced switch power electronic-based hybrid-MG
structures offer improved PQ and PR of the MG. In addition to that, it reduces the
fluctuation of DRE resources, limiting the range and design complexity of synchronous
devices, apprehending distributed control, and decreased losses [25-26].
✓ In every situation, the selection of hybrid-MG systems cannot be a wise decision.
Therefore, it is essential to develop appropriate control and MG structures for better and
flexible operation [27].
✓ To avail reliable and stable MG operation with multiple DGs, better power management
technique (PMT) is becoming a robust technique for both modes of operation. The
enhanced PMT sets the active and apparent power limits for the respective generation
units, to facilitate synchronized and momentary energy transfer operation balanced the
generation and load demand ratio, and quick settlement of system frequency during any
fluctuations [28-29].
✓ The PMT also offers seamless transition operation from grid forming mode to grid
following mode of operation [31].
✓ In [31-32], a novel controller is proposed for the parallel operation of multiple
uninterruptable inverters. Due to the proposed approach, each inverter output has equal
voltage and current magnitude, frequency, and phase for facilitating equal load sharing
operation. However, the physical imbalance of converter and misalliance among line
impedance affects the load sharing problems.
✓ In [33], a solution-based droop control method is suggested to facilitate better
performance by considering the local measurement signals.

4
✓ To consider the local measurement signals, MG control is designed by using a unified
control hierarchy (UCH). The UCH embraces three parts and presented as follows [34].
1. Require distribution and market network for low/medium voltage operation.
2. Require a unified main controller to operate the local controller.
3. Require a local controller for collecting each essential data.
✓ The networking hierarchy also operates in a similar manner, where the unified main
controller tracks the regulator set conditions from the distribution and market network
and is directed to the local controller for optimal performances.
✓ For a successful MG operation, it is necessary to follow a few interlinked standards as
presented in [35] for both grid following and grid forming operation.
1.1.4 Configuration and Classification of SMG
Fuel Cell
Flywheel Super Cap. Solar Cell
ElectricBulb
Mobile
Char.
Battery
Sensitive Load
Flywheel bank to charge EV
Implemented in Israel
Microgrid
Solar Cell
Building Wind Generator
1 2 3
4 5 6
Solar Farm Solar Farm
Wind
Turbine
1 2 3
4 5 6
Wind Farm Wind Farm
DC-GRID
AC-GRID MAIN-GRID
Figure 1. 1 Overall SMG architecture
Figure 1. 1 illustrates the overall architecture of the smart microgrid (SMG) system. As
illustrated in Figure 1. 1, the SMG is designed by combining the ac-grid, dc-grid, and main
grid. To improve the performance of the SMG, it is co-ordinately operated and facilitates

5
different load integration. In addition to that, the SMG also having the capability to
separate from each other through a circuit breaker during any fault and transient condition.
The SMG contains both ac and dc-grid, to facilitate both ac and dc-load integration by
minimizing the power electronic component, cost, and size. The common joining point of
ac, dc, and the main grid is termed as a point of common coupling. The SMG also having
the capability to increase the size according to the requirement. All the used
inverter/converter operation is depended upon the design control strategy of the SMG
system. Therefore, to make the MG smart, it is necessary to develop robust and coordinated
control strategies for improving the power quality and power reliability of the system. The
related robust control techniques for the respective MG system is discussed in the next
section. Looking at the excess demand for protection, and reliability aspects, recently
hybrid MG systems are gaining interest. SMG has operation is divided into different types
according to the requirement and few of them are discussed below.
1) Concerning the power form (dc and ac) [36], the SMG is classified into two categories
as dc-MG [37] and ac-MG operated at the high-frequency range. The dc-MG and ac-
MG are used to facilitate better power quality (PQ) and power reliability (PR) by
reducing the variability and uncertainty that occurred due to the integration of renewable
energy and the use of a lot of energy conversion devices. In this regard, high-frequency
ac-MG [38] and Hybrid MG based SMG systems are gaining interest [39].
2) Concerning the application point of view [40], the SMG is divided into three sections as
utility MG (for example a district of a country is operated as an MG),
industries/commercial plant-based MG, and remote MG system.
3) Concerning the structure [36,41-44], the SMG is divided into two categories as single-
stage MG [43] and two-stage MG [44]. The stages are formulated according to the
power stage conversion process. Generally, two-stage MG is widely used to achieve
better power reliability operation. For example, as illustrated in Fig.1, the solar farm
used two back to back converter for the grid integration. One converter is used to obtain
maximum power from the solar plant and another converter is used to convert the dc-ac
power for grid integration. However, recently single-stage MGs are also gaining interest
due to the development of multi-level inverter applications. The single-stage MG
requires a lesser number of components, reduced size, and cost as compare to two-stage
MG.
4) Concerning supervise control, the SMG is divided into two sections as unified control
and decentralized control. In a centralized controller (CC), the master control passes the
desired values to the local controller through a two-way communication channel.
However, the CC technique is less reliable and redundant [45]. The decentralized
controller (DC) is a multiagent system and the communication between the local
controllers occurred through a communication network [46]. DC technique facilitates
flexible and reliable operation as compared to the CC technique.
5) Concerning about renewable energy integration [15,36], the SMG is classified into two
categories as converter-based MG and conventional DG based MG. MG may also be
categorized as a single-phase/three-phase MG and low/medium voltage MG system.

6
6) Concerning the mode of operation, the SMG is classified into two categories as grid
forming and grid following mode of operation. Each mode of operation control strategy
has its advantages and disadvantages according to the requirement [47].
As per the above classifications and related literature survey [15-47], it is necessary to
focus on single-stage/ two-stage operation, hybrid MG, supervised control strategies, three-
phase MG, and interlink converter-based MG for future SMG application. In this regard,
the associated control strategies are a greater impact on availing a Smart MG system. The
advanced controllers for ac, dc, and hybrid microgrid system is explained in the following
section of the thesis.
1.2 Background
Looking at the excess energy demand and population, renewable energy such as solar and
wind-based distribution generation (DG) integrations are gaining interest. However, during
the real-time implementation and excess non-linear load integration, the design of modern
DGs are facing a lot of challenges. The major challenges are presented as follows.
• Synchronization
• Excess energy sector integration
• Excess electricity demand
• Better energy management
• Required to maintain the rated voltage profile
• Required to reduce the power losses
• Decrease in power factor
• Harmonic distortion
• Design complexity of the controller
• Reduction of global warming/ Pollution
• Grid-connected/ Islanded mode of operation
• Reactive power support
• Frequency mismatch
• Non-linear load application
• Power quality
• Power reliability
• Customer satisfaction
• Stability
• Suitable Active filter design
• AC/DC hybrid microgrid operation
• Electric vehicle application
• Electric vehicle design

7
1.3 Literature Review
As per the title, the whole thesis is based upon active power filter (APF) and related control
design for a smart microgrid system. In this section, the detailed literature survey regarding
APF operation and involved control strategy is discussed. From the structure point of view,
APFs are divided into two types such as shunt active filter (SHAF) and series active filter
(SEAF). In the planned thesis, due to the superiority like lesser component, lower switching
frequency, lesser component, lower switching frequency, light-weighted, independent upon
system impedance and load shading condition, harmonic mitigation, avoidance of
resonance problem, avoidance of capacitor aging, necessitates active switching
components, and excellent power factor correction, the shunt active filters are selected for
microgrid operation [48]. Firstly, the detailed working principle of SHAF design and
secondly, the related control strategy is explained for different microgrid operation.
Specifically, the SMG system is divided into three sections like AC-MG, DC-MG, and
hybrid MG. To operate the respective MGs through an active filter, few specific controllers
are discussed for optimum performances of the MG operation. In the following sections,
the related controllers comprised of primary, secondary, and tertiary control levels are
discussed for each of the individual MG operations.
1.3.1 Overall Design and Working Principle of SHAF
a
b
c
n
A
B
C
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Grid
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A
B
C
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a
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+
-
+ + +
-
-
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+
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L
f
L
SHAF
dc
C
SHAF
P
g
P l
P
l
T 3
T 5
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4
T 6
T 2
T 2
n
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1
n
T
abc
,
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V abc
,
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I abc
,
l
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,
l
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dc
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Measurement Measurement
Nonlinear
Load
SCT based
Controller
NET based
Controller
CRT based
Controller
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l
I
*
abc
,
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I
in
abc
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CRT based
Controller
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DC-VCT based
Controller
dc
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*
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de
I

d
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dc
I

+
-
Pulses
Figure 1. 2 Overall SHAF design with important controller applications

8
The complete SHAF based system modeling with its important four control strategies are
illustrated in Figure 1. 2. In the complete system modeling, the non-linear/sensitive load is
directly connected to the grid and the SHAF is connected to the point of common coupling
(PCC) in between the grid and non-linear load. The complete working principle of SHAF is
majorly dependent upon two factors such as voltage/current source inverter/converter and
control strategy. Specifically, the important four control techniques are known as non-
linear extraction technique (NET), dc-voltage control technique (DC-VCT), current
regulation technique (CRT), and synchronizer control technique respectively. Each of the
control operations is discussed below.
1.3.1.1 NET Based Controller:
In this control technique, by considering the non-linear load current signal ( l
I ) from the
high-frequency load, the NET-based control design is started. After gathering sufficient
knowledge about the harmonic percentage of current, it is passed through the linear current
controllers for isolating the high-frequency component and extracting the fundamental
current component. Lastly, by using the fundamental current component, the reference
current ( *
g
I ) for the SHAF operation is developed. Meanwhile, the main aim of the NET-
based controller is to develop the reference current generation and otherwise known as the
reference current extraction technique [49].
1.3.1.2 DC-VCT Based Controller:
In this control technique, the actual dc voltage ( dc
V ) of the SHAF is compared with the
reference dc voltage ( *
dc
V ). The compared result ( de
I ) is passed through a linear controller,
to compute the appropriate active current ( d
I ) component for charging the SHAF. The
computed current is the amount of dc-current required to be haggard by the SHAF for
facilitating the switching operation by which the system able to maintain its dc-link voltage
of the capacitor at its desired value [50].
1.3.1.3 CRT Based Controller:
In this control technique, the output responses of the NET and DC-VCT based controller
are considered to extract appropriate switching pulses ‘ P ’ for the inverter operation, by
which the inverter behaves like a SHAF. The CRT based controller is designed by
considering a space vector pulse width modulation (SVPWM) technique for appropriate
pulse generation and a current regulation loop is required to guarantee that the generated
injected current ( in
I ) is properly synchronized with the reference current ( *
g
I ) [51].

9
1.3.1.4 SCT Based Controller:
The SCT based control approach is designed based upon the phase-locked loop (PLL)
approach. In this control technique, the controller takes the grid voltage as an input
parameter and extracts a synchronization angle ( s
 ), so that the injected current generated
by the SHAF is easily synchronized with the grid voltage. It also ensures that there is not a
necessity of explicit SCT for the SHAF controller operation [52].
Other related important factors for the SHAF operation are discussed below.
1.3.1.5 Voltage Source Converter (VSC):
As illustrated in Figure 1. 2, this is a power electronic component-based device, which is
used to generate an appropriate injection current for reducing the power system non-
linearity. The dc capacitor-based energy storage device is used to reduce the active power
fluctuations that occurred during the dynamic study of SHAF operation. The VSC
modeling also incorporates a filter inductor by which it mitigates the higher ripples present
in the injection current. Recently, multi-level voltage inverters are also gaining interest due
to their significant contribution such as improved voltage levels, better power quality,
reduced harmonic, lesser switching component, and reduced size [53].
1.3.1.6 Non-linear Load:
This type of load injects harmonic to the linear/stable power system through PCC. The
application of these types of the load is gradually increased day by day and few of them are
illustrated as switched power supply, industrial application, furnace, speed driver,
converters, battery charger, etc. These types of practical loads generate higher harmonics
and an increase in reactive power components. However, during the Simulink model
design, an uncontrolled RL, RC, and R based bridge controller is used as it generates
excess harmonics [54,55,56].
1.3.2 SHAF Design
As illustrated in Figure 1. 2, mathematical SHAF modeling is presented [57-60]. At first
assume that the SHAF is not connected to the system model, the undertaken system current
flow equation is mathematically represented as.
n
fu
l
g I
I
I
I +
=
= (1.1)
where g
I is the grid current, l
I is the load current, fu
I is the fundamental current, and n
I is
the non-linear current component generated by the non-linear loads. Due to the absence of
SHAF, the grid current is equal to the load current, which indicates that the grid current is
distorted and changes its phase. However, by connecting the SHAF to the PCC of the
undertaken system as illustrated in Figure 1. 2, two supplementary currents such as SHAF
injection current ( in
I ) and dc-link current ( dc
I ) are flowing in the system. in
I is used to

10
mitigate the nonlinear current generated by the sensitive load and dc
I is used to
compensate for the switching losses of the SHAF and to regulate the dc-link voltage of the
inverter. Therefore, after using the SHAF in the design system, the new current flow
equation is mathematically represented as.
( ) dc
in
n
fu
g I
I
I
I
I +
−
+
= (1.2)
From Eq.1.2, it is visualized that the main role of SHAF is to eliminate the nonlinear
current by injecting appropriate injection current and make the grid current sinusoidal
current. In this way, the SHAF can regain the sinusoidal characteristics of the grid and in-
phase with the grid voltage. After eliminating the non-linear current, Eq.1.2 is simplified
as.
dc
fu
g I
I
I +
= (1.3)
After computing an appropriate current flow equation, the related power flow equations of
the system are computed as follows. The instantaneous grid voltage ( )
t
(
Vg ) of the
undertaken system is presented as.
t
sin
V
)
t
(
V a
g 
= (1.4)
The related instantaneous non-linear current ( )
t
(
Il ) equation is presented in terms of the
fundamental and non-linear components as.
( ) ( ) ( )


 


 


 

 

linear
non
2
k
k
2
l
Fundamenta
1
1
1
k
k
k
l t
k
sin
I
t
sin
I
t
k
sin
I
)
t
(
I
−

=

=

 +
+
+
=
+
= 




 (1.5)
By using Eq.1.4 and Eq.1.5, the instantaneous non-linear load power ( )
t
(
Pl ) can be
computed as.
( )




 




 




 



 



 


 

(t)
powerP
linear
non
2
k
k
k
a
)
t
(
P
power
reactive
1
1
a
(t)
powerP
ctive
a
2
l
a
l
g
l
n
r
a
t
k
sin
I
t
sin
V
sin
t
cos
t
sin
I
V
cos
t
sin
I
V
)
t
(
I
)
t
(
V
)
t
(
P
−

=
 +
+


+

=
+
=








(1.6)
From the active power component as illustrated in Eq.1.6, the respective three-phase
reference grid current components ( )
t
(
I*
ga , )
t
(
I*
gb , and )
t
(
I*
gc ) are computed as.
t
sin
I
t
sin
cos
I
)
t
(
V
)
t
(
P
)
t
(
I m
1
1
g
a
*
ga 

 =
=
= (1.7)
)
120
t
sin(
I
)
t
(
I m
*
gb

−
=  (1.8)
)
120
t
sin(
I
)
t
(
I m
*
gc

+
=  (1.9)

11
The maximum current component ( m
I ) is regulated by controlling the dc-link voltage of
the SHAF through a PI or other linear controllers.
1.3.3 AC-MG Control Operation
The AC-MG facilitates several potential advantages and sets a novel paradigm for future
power system applications as enumerated below [61-63].
1. During the occurrence of uncertainty/transient condition at the grid side, the ability of
smooth isolation from the grid facilitate less distortion to the loads within the MG
operation.
2. The performance of the normal power grid is optimized.
3. During the peak load demand, it protects the grid failure by regulating the load demand.
4. Significant environmental condition improvement is possible by using low/zero-
emission power generators.
5. The system improves the overall efficiency of the system by facilitating plenty of energy
sources and reduced heat conditions.
6. The production and availability cost of the electricity is decreased for the users.
7. Facilitating enhanced power quality and reliability during sensitive load-based MG
application.
However, few of the demerits of the AC-MG system operation are enumerated below [64-
65].
1. Major drawbacks during an increased number of renewable energy source integration
are presented in the following section:
• Increased cost and net metering for MG integration.
• Requires expert power engineers and well-equipped engineering techniques.
• Necessary to follow/develop interconnection standards for maintaining
consistency.
2. The control and protection aspect are considered as major problems for availing grid
forming and grid following mode operation.
3. Resynchronization/restoration of the AC-MG is also considered as a major challenge
due to the following reasons.
• As per the stability aspects, the synchronization after island operation is difficult.
• Voltage angle and phase mismatch occurred during the resynchronization
process from grid-forming to grid-following mode of operation.
4. The error arises in voltage setpoint, which increases the circulating current between the
MG and the main grid. The increase in circulating current also increases the active and
reactive power oscillations.
5. In the islanded mode of operation, to track the load frequency change, the MG is
necessary to regulate the operating power point. The regulation of power also creates a

12
problem for frequency error generation. These affect the voltage, phases, and PQ of the
system.
6. The related impedance like line and DG also affects reactive power control and sharing
during both grid-connected and islanded mode respectively.
For example, the overall control structure comprised of primary, secondary, and tertiary
control levels of an AC-MG system is illustrated in Figure 1. 3. As illustrated in Figure 1.
1, the primary loop is used to regulate impedance, voltage, and current parameters of the
MG system. Similarly, the secondary loop is used to regulate the voltage and frequency.
The tertiary control is used to regulate the active and reactive power of the system for
facilitating optimum power exchange with the grid [66-67].
d
V q
V d
I q
I
+
- dc
V
dq
abc
dq
abc
gd
V gq
V gd
I gq
I
PCC
gd
P gd
Q d
P d
Q
GRID
INVERTER SIDE
GRID SIDE
INVERTER
+
-
+
-
d
P
d
Q
*
d
P
*
d
Q
abc abc
dq dq
p
K
q
K
-
-
+
+
+
-
+
-
gd
P
gd
Q
*
gd
P
*
gd
Q
p
K
q
K
-
-
+
+




Inv
V
v
K

K
V



V



+
+
+
+
*
gd
E
*
g
E
*

*

*
e
E
*
e

Three
phase
voltage
source
inverter
)
t
sin(
E
V
*
e
*
e
*

=
Virtual
Impedance
+
+
-
-
Inv
I
*
d
V
*
q
V
*
de
V
*
qe
V
+-
+-
d
V
q
V
PI
PI
*
a
V
*
r
V
*
d
I
*
q
I
+-
+-
q
I
d
I
PI
PI
*
de
I
*
qe
I
+- d
U
d
V
+
-
q
V
q
U
dq
abc
Pulses
Pulses
Tertiary Control Secondary Control
Voltage regulation
Frequency regulation
Programmable output
Impedance Voltage control droop Current control droop
Primary Control
Droop control and Sine Generator
Primary Control
1. DG unit control loop
2. Local Measurement
3.Virtual Impedance
Secondary Control
1. PCC voltage Regulation
2. Frequency Regulation
3.Power quality issues
Tertiary Control
1. Power exchange with
grid
Figure 1. 3 Overall control diagram of AC-MG system
1.3.3.1 Primary Approach (PA) for AC-MG:
This controller is achieved through the local controllers available for the regulation of the
utility and load integrating converter. As indicated in Figure 1, the main aim of the primary
control is to provide appropriate real and reactive power support between the DGs by

13
regulating the inverter voltage and frequency through voltage and current controller.
Therefore, it facilitates the internal voltage and current control of inverter and avail power
exchange operation by using both unified and decentralized control methods. Due to the
fast control action, the PA is also used to detect the grid-forming condition, power
exchange, output voltage, and current control, and facilitate to change the modes of the
controller [68]. The primary current control approach is especially divided into two types as
(1) linear current controller and (2) non-linear current controller. Examples of the linear
current controller are synchronous and stationary reference frame-based PI and PR
regulator, feedback controller, adaptive, predictive, and dead-beat regulator [70], etc. For
dc-input, the PI regulator is preferred due to zero steady-state error, and for ac-input, the
PR regulator is selected due to the faster action [70]. Similarly, a few examples of the non-
linear regulator are hysteresis, SMC, wavelet, signal processing, Fuzzy techniques, and
ANN techniques [70]. The primary current control approach is used to regulate the dc-link
voltage and active power of the MG by considering the active and reactive current
component. A few of the important techniques are discussed below.
(a) V-P/F-Q DA:
To overcome the shortcomings of the TDA approach, in [71], V-P/F-Q based ADA is
proposed for LV and high resistive distributed line application. In this advanced approach,
the output voltage magnitude is decreased with an improved in real power and an increase
in frequency with a boost in apparent power output. The relation between V-P /F-Q is
illustrated in Eq.1.10 and Eq.1.11 respectively.
Z
V
E
V
P
2
inv
inv −
= (1.10)
z
E
V
Q inv 

−
 (1.11)
E

E 
max
P
0
E


max
Q
min
Q

*

0
Figure 1. 4 Decrease/increase characteristic: (a) V-P droop approach, (b) F-Q
increase approach
where ‘Z’ is the reactive impedance of the line. P-V decrease and F-Q increase
characteristics are illustrated in Eq.1.12, Eq.1.13, Eq.1.14, and Eq.1.15 respectively.
P
D
V
V t
r
n −
= (1.12)

14
Q
It
r
n +
= 
 (1.13)
m
t
P
E
D

= (1.14)
m
t
Q
2
I


= (1.15)
The related characteristic diagram is illustrated in Figure 1. 4. Here r
 and r
V are known as
the rated angular frequency and RMS voltage of the inverter respectively, and t
D and
t
I are known as decrease and increase coefficient for V-P and F-Q characteristic,
respectively. This can be played a vital role in the LV resistive distributed network, but it
lags the performance during sensitive load application.
(b) Q-dV DA:
P&Q
Calculation
LPF
Pf
droop
Q-dV
droop
t
sin
E *
*
 +-
Voltage &
Current
Controller
PWM
Controller
+- -
+ -+ -+
inst
,
x
P
inst
,
x
Q
x
P
x
Q
*

*
E
*
x
V
x
V
x
V x
I
ref
,
x
V
x
V
x
I
x
0
dV
x
dV
x
dV

Rx
resQ
K
s
1 x
0
Q
x
Q
x
Q

x
n
x
0
dV
x
dV
s
1 x
V

0
V
*
x
V
Magnified Figure
Figure 1. 5 Q-dV DA control diagram
By increasing the reactive power support to the MG application, the Q-dV DA is designed
by considering the reactive power (Q) and the rate of change of voltage (dV) [72-73]. The
proposed Q-dV DA avoids the coupling dependency of the system. The rate of change of
voltage is changed continuously until the system achieves its desired Q value and it is also
independent upon the line impedance value. The overall control diagram of the system is
illustrated in Figure 1. 5 [72-73]. The related equation of Q-dV DA is presented in the
following section.
)
Q
Q
(
n
dV
dV x
x
0
x
x
0
x −
−
= (1.16)

15

+
=


d
dV
V
V x
x
0
*
x (1.17)
where x
n and x
0
dV are the decrease constant and fundamental x
dV (during initial condition
equal to zero) respectively. x
0
Q is the fundamental x
Q at the fundamental x
dV related to the
required reactive power of MG. x
0
V and *
x
V is the fundamental magnitude voltage and the
reference voltage of the MG. During steady-state conditions, the *
x
V is set to zero to protect
the system from varying output conditions. Therefore, the related equation becomes:
)
dV
dV
(
Q
K
Q
dt
d
x
x
0
Rx
res
x
0 −
= (1.18)
The proposed approach depends upon the initial parameter condition and may create
stability problems during small disturbance conditions.
(c) Phase Angle Droop Approach (PADA):
dc
V
Q
Q
nom
Q
dc
V
nom
,
dc
V
nom
,
g
V
g
V
nom
f
f
dc
P
nom
,
dc
P
g
V
nom
,
g
V
Voltage
Controller
dc
P
VSI
g
V
f
controller
droop
V
V dc
g − controller
droop
V
P dc
−
controller
droop
f
Q −
dc
V
Figure 1. 6 PADA control diagram
In [74-75], a novel PADA is proposed to regulate the phase angle of the DG voltage
sources as compared to a common time reference. Due to that, the desired power can be
fulfilled between the DGs, like to TDA by decreasing the magnitude and angle of the
voltage. The PADA design diagram is illustrated in Figure 1. 6. By using the PADA
strategy the load sharing accuracy of the MG system is improved significantly without
affecting the steady-state frequency. The related PADA equations are illustrated as follows.
)
P
P
(
M nr
n
p
r
n −
−
= 
 (1.19)
)
Q
Q
(
N
E
E nr
n
q
r
n −
−
= (1.20)

16
where r
 and r
E is the set corresponding slant and magnitude of the voltage. n
P and n
Q are
the real and reactive power outcomes of the inverter respectively. n
 and n
E are the
related voltage angle and magnitude of the system respectively. nr
P and nr
Q is the set real
and reactive power value of the respective inverter respectively. p
M and q
N are associated
with decrease real and reactive coefficient respectively.
(d) Virtual Power Transformation Droop Approach (VPDA):
In [76-77], VPDA is used as a linear quadrature transfer matrix to compute the real and
reactive power transfer equation in a novel reference condition without considering the line
impedances. The related real and reactive power matrix is illustrated as.












−
−
=






=








Q
P
sin
cos
cos
sin
Q
P
T
Q
P
PQ




(1.21)
In this method, the exact value of R/X is not recognized. However, an exact computation of
R/X may be enough to operate the method. Similarly, the frequency and amplitude of
inverter output voltage are changed regarding the virtual frame, and the related parameters
like  and E are used to compute the voltage magnitude and frequency references to the
inverter voltage control loop. The detailed diagram of VPDA is illustrated in Figure 1. 7.
The related matrix equation is presented in Eq.1.22.












−
−
=






=








E
sin
cos
cos
sin
E
T
E







 (1.22)
min
E
max
E
Ê
min
 max

̂
min
E
min

E



a
b
c
d
Ê

ˆ


E

Figure 1. 7 Detailed VPDA diagram

17
1.3.3.2 Secondary Approach (SA) for AC-MG:
The complete control structure of SA for the AC-MG system is illustrated in Figure 1. 3.
This control method is used to regulate the energy management system of the MG. SA is
used to improve the power quality (PQ) by retuning the voltage and frequency of the MG,
as previously affected by the primary approach. In addition to that, this proposed approach
also facilitated resynchronization operation among the utility and DGs [34]. By utilizing
the frequency and voltage error signal, the SA is used to generate the reference working
signal through Eq.1.23 and Eq.1.24.
 −
+
−
= dt
)
(
G
)
(
G MG
*
MG
I
MG
*
MG
P 




 
 (1.23)
 −
+
−
= dt
)
V
V
(
G
)
V
V
(
G
V MG
*
MG
IV
MG
*
MG
PV
 (1.24)
where 
P
G , 
I
G , PV
G , and IV
G are the closed-loop transfer function regulator, *
MG

and *
MG
V are the reference frequency and voltage unit, MG
V and MG
 are the actual frequency
and voltage unit of the MG respectively. The V
 and 
 are the corrected voltage and
frequency magnitude at the MG terminal respectively. The distributed and unified control
techniques are discussed in the following sections.
(a) Model Predictive Approach (MPA):
MPA is used to solve the optimization problem by appropriate forecasting the generation
and load demand [78-79]. By using the feedback control and regulating the power system
constraints, the proposed MPA is applied to resolve the multivariable evolution problem
[79-80]. To regulate the voltage instability, a voltage predictive approach (VPA) is
proposed by appropriately injecting reactive power in the MG system [80]. To overcome
the above problem and improve the robustness of the system, a two-layer MPA approach
is proposed in [80] for solar-diesel-energy storage-based MG applications. In [81], a two-
way predictive approach is suggested to regulate the connection and disconnection period
of the diesel generator and solve the boundary difficulties by considering the reference
value from the first layer of the controller. The related control diagram is illustrated in
Figure 1. 8. The BVP is used to take care of the forecast error and an increase in solar
power oscillations. Similarly, the second layer includes an optimization technique for
transferring the connection and disconnection of the diesel generator through boundary
difficulties. The computation of the connection and disconnection period is evaluated by
sensing the actual system states, forecasting generation, and load demand. The key
function of the second layer evolutionary technique is to set the predefined SOC path
( *
SOC
X ) and real SOC ( SOC
X ) of the energy system. The boundary difficulties are
computed through energy storage device dynamics and presented as follows.
)
t
(
X
)
t
(
X s
*
SOC
s
SOC = (1.25)
)
t
(
X
)
t
(
X f
*
SOC
f
SOC = (1.26)

18
Determine optimum power dispatch
1) Update the weight by analyzing the error
{1,2,3}
l
with
1 

2) Compute forecast P(load), P(pv)
3) Compute reference SOC of battery and Power of diesel
generator by
min J (terminal stage cost and transition cost)
Subject to:
• Shepherd equation with additional normalized
capacity coefﬁcient
• Diesel generator Off time
• State of each generator
• Parameter constraints
• Physical and operational level of battery and DG
Adjustment of diesel generator on/off
1) Compute forecast and prediction of power at each time
step P(load), P(pv)
2) Compute shift of diesel generator turn on/off time by
solving a boundary value problem
Energy System(ES)
1) Apply the diesel generator shift power to ES
2) Feedback system state diesel power trajectory, battery
SOC trajectory and diesel generator turnoff trajectory
Model Predictive Approach
First Layer
Second Layer
*
SOC
X
*
i
,
dg
P
k
,
SOC
X
k
,
dgi
P
k
,
toffi

k
,
SOC
X
Every 2 min
Every 10 min
i
,
dgshift
P
Figure 1. 8 Two-layer MPA approach
(b) Consensus Theorem Approach (CTA):
+
-
+
-
+
+
+
+
z
1
z
1
1
i
,
i
a −
1
i
,
i
a +
)
n
(
1
i
,
i −

)
n
(
1
i
,
i +

)
1
n
(
1
i
,
i +
+

)
1
n
(
1
i
,
i +
−

  ++
z
1
)
n
(
X 1
i +
)
n
(
X 1
i −
Neighboring
States
)
0
(
Xi
)
1
n
(
Xi +
)
n
(
Xi
Consensus algorithm
Figure 1. 9 CTA design
In [55], CTA based distributed control is used to solve a distributed optimization problem
to obtain a congregated result for all DG units. In [81], a load restoration technique is
suggested where the agents chose their verdicts by using the local data from the

19
neighbouring agent and global data by using the average consensus theorem. In [82], a
dynamic CTA is suggested to regulate the negative sequence current component with
distorted voltage mitigation. The proposed dynamic CTA design is illustrated in Figure 1.
9. The design dynamics of the proposed CTA are presented in the following equations.


+

+
=
+
i
N
j
ij
i
i )
1
n
(
)
0
(
X
)
1
n
(
X  (1.27)
))
n
(
X
)
n
(
X
(
C
)
n
(
)
1
n
( i
j
ij
ij
ij −
+
=
+ 
 (1.28)
where )
n
(
Xi denoted as the agent ‘i’ statistics information at nth
iteration, ij
C is the
connection link among the point ‘i’ and ‘j’. i
N denoted as the set of files respect to the
agent ‘i’.
(c) Multi-Agent Approach (MAA):
DG agent
Battery agent
Load agent
SCADA agent
System operator
agent Static switch
agent
GCC agent
EF agent
EDC agent
System Control Level
MG CentralControl Level
Local Control Level
Client server
Agent server
Agent server
Figure 1. 10 Client and agent server-based MAA
An agent that is physically/virtually present in the atmosphere is required to design the
proposed MAA. The agent is self-sufficient to react any disturbances or changes in
atmospheric conditions [83-86]. In MAA, two/more agent's capability is estimated by its
reaction to any change in atmospheric condition, pro-activeness, and communication
among agents [87].
In [83], a multiagent based hybrid energy managements for MG application is suggested.
The client and agent server-based MAA is illustrated in Figure 1. 10. The above figure
shows that to design an MAA, there are three control levels are required as local control

20
level, MG central control level, and system control level. A detailed analysis of the MAA
design is presented in [83]. The proposed MAA is based upon a contract with the internet
protocol, multi-agent commutation method, market race, and better coordination. A
detailed explanation of MAA regarding power application is presented in [87]. Recently,
MAA based cooperative approach method is suggested for offering better synchronization
of the independent MG [88].
1.3.3.3 Tertiary Approach (TA) for AC-MG:
TA is used to regulate the power flow among the MG and main grid, for optimizing the
performance of the system. TA facilitates better coordination of interlinking multiple MG
and supplies the desired voltage and frequency to the main grid [89]. From the data
management system, the TA receives the reference power component and regulates the
error among the real and set parameters. The related voltage and frequency equations are
presented as follows.
 −
+
−
= dt
)
P
P
(
P
K
)
P
P
(
P
K g
ref
,
g
i
g
ref
,
g
p
ref
 (1.29)
 −
+
−
= dt
)
Q
Q
(
Q
K
)
Q
Q
(
Q
K
E g
ref
,
g
i
g
ref
,
g
p
ref (1.30)
where ref
,
g
P and ref
,
g
Q are the reference active and reactive power
component, g
P and g
Q are the actual active and reactive power component, ref
 and ref
E
are the reference frequency and voltage component used for a secondary approach to adjust
the frequency and voltage of the interlinking converter respectively [16]. p
K and i
K are
the proportional and integral constants of the PI regulator respectively.
The TA control process is slower as compared to other approaches. The regulators are
applied to regulate the disturbances among the real and apparent power and supplied to the
main grid concerning the set point.
1.3.4 DC-MG Control Operation
The DC-MG facilitates several potential advantages and sets a novel paradigm for future
power system applications that are enumerated below [90].
• No necessity to regulate the reactive power and frequency of the system.
• No necessity to worry about the synchronization of the MG.
• Inrush current is avoided due to the avoidance of the transformer.
• AC-DC/ DC-AC conversion losses are neglected.
• Better fault ride-through capability is provided by the system.
• Facilitate direct integration of dc-load.
However, few of the demerits of the DC-MG system are enumerated below [90].
• A private dc-distribution line is required for improving the power flow.

21
• Recently, the protection of the DC-MG becomes more challenging due to the absence of
the zero-sequence current.
• The stability of voltage depends upon the active power alone. However, in the AC-MG
system without affecting the real power flow, the voltage stability is maintained at its
desired value by regulating the reactive power of the system.
A few power transfer methods and regulation approach for numerous interlinking converter
application are suggested for parallelly linked dc-dc load allocation technique such as
unified approach [78], master-slave [79], average load transfer approach, and spherical
chain control [91]. In addition to that, for better inverter operation droop control approach
was also selected for MG operation [34]. Specifically, in the literature, two control methods
such as unified and decentralized methods are proposed for DC-MG operation. The unified
control approach depends upon the communication parameters and the main controller, to
generate stable voltage and energy outcomes [34]. Still, in the decentralized method, to
regulate the DG units’ outcomes, the auxiliary controller requires lesser communication
parameters and the sovereign of the main controller.
1.3.4.1 Decentralized Method Based Primary Approach (PA):
(a) Traditional Droop Approach (TDA):
The voltage DA used for generating an appropriate inverter current result is illustrated in
Figure 1. 11 (a). The related voltage droop method is presented as follows.
o
ref
,
dc
o kI
V
V −
= (1.31)
where o
V is the system output voltage, ref
,
dc
V is the dc-reference bus voltage,‘ k ’ is the
droop coefficient, and o
I is the system output voltage. k is computed as:
o
V
max
V
min
V
ref
V
k
min
I max
I
o
I
2
o
V
max
V
ref
,
dc
V

+ -
min
V
k
vsc
I
+ -
2
dc
V
e
V
s
K
s
K vi
vp +
-1
ref
P
max
V
ref
,
dc
V
+ -
min
V
k
bat
I
+ -
dc
V
e
V
s
K
s
K vbi
vbp +
ref
V
ref
V
o
V
ref
,
bat
I
1
M
2
M
ref
,
bat
I
1) VSC droop approach
2)Battery droop approach
Droop approach
(a) (b)
Figure 1. 11 (a) Overall voltage DA for inverter output current result, (b)VSC and
battery-based droop approach

22
)
V
(
P
2
V
k dc
m
ref

= (1.32)
where dc
V
 is the maximum voltage variation and m
P is the maximum power of the
converter during this droop approach. To operate a total power system, the same droop
approach is applied for both the converter and battery control system. By using the TDA
approach, the appropriate power-transfer among the main grid, DG, and battery system is
attained. To offer a better power-transfer operation, the voltage droop control approach is
suggested and illustrated in Figure 1. 11 (b-1). The original grid voltage reference is
attained by comparing the voltage reference and converter input current with the droop
constant. Similar to VSC control, the battery energy controller is illustrated in Figure 1. 11
(b-2). In Figure 1. 11 (b-2), the switch 1
M is ON to support the main grid and 2
M is ON to
activate the charging condition of the battery. In [92], it states that the droop approach-
based MG system produces a steady-state error on the DC-grid. The paper also highlighted
that the variation of dc-link voltage is an essential part of the MG behaviour. To overcome
the variation related issue in real-time systems, for operating the dc-load generally, power
electronic-based converters are selected for maintaining the stability and providing better
synchronization operation. The TDA is easy to design, consistent, and no communication
linkage is needed.
(b) Virtual Resistance Droop Approach (VRDA):
+-
)
s
(
GMG +-
Voltage
Control
Current
Control
SVPWM
DC
Source
MG
V
*
MG
V 0
V
0
I
v
R
0
V

ref
V
*
0
V
Secondary Control
Primary approach
Inner-Loop
VRDA
Figure 1. 12 VRDA control design
In [34], by considering the line resistance, a novel droop approach-based control technique
is proposed, to offer better power transfer operation. In this proposed approach, a virtual
line resistance ( v
R ) is used in the feedback path of the droop approach, to eliminate the
effects of the line resistance. The related output voltage equation can be represented as.
o
v
o
ref
*
o I
R
V
V
V −
+
=  (1.33)
where *
o
V and ref
V are the output and the reference voltage of the MG at no-load
condition, o
I is the output current of the controller, and o
V
 is the regulator output voltage,

23
used to restore the MG voltage as illustrated in Figure 1. 12. The related equations are
presented as follows.
 −
+
−
= dt
)
V
V
(
K
)
V
V
(
K
V MG
*
MG
i
MG
*
MG
p
o
 (1.34)
where *
MG
V and MG
V are denoted as the reference terminal voltage and the actual terminal
voltage of the MG respectively.
1.3.4.2 Distributed Secondary Approach (DSA):
(a) Typical Current Control Approach (TCCA):
In [93], a distributed control technique is proposed for better voltage control with optimum
load sharing conditions. In [93], by using TCCA as illustrated in Eq.1.35, the traditional
droop equation is modified by adding 0
n
V
 as presented in Eq.1.36. The droop
characteristic equation is coordinated with the voltage axis by summing 0
n
V
 with the
traditional droop equation and presented as follows.
t
t
0
t
0
t
*
t I
d
V
V
V −
+
=  (1.35)
SECONDARY
CONTROL
+
- +
IV Loop
Converter
droop
tth
0
t
V

n
I
I
n
1
i
pu
i
ob
t

=
=
rated
t
I
ob
t
I
t
G
t
d
rated
t
I
1
0
t
V
MG
DC −
InnerLoop
Measured source current
Digital average current sharing
Figure 1. 13 Distributed control of DC-MG with TCCA
The change in 0
t
V
 rise/fall depends on the simultaneous rise/fall of the total system load.
Due to this, the reference voltage ( *
t
V ) is closer to the nominal voltage ( 0
t
V ). In this
condition, t
d is denoted as the droop constant. The related diagram of the proposed
technique is illustrated in Figure 1. 13. The obtained current ( ob
t
I ) inserted by all the
generation sources are presented as.

24
n
I
I
n
1
i
pu
i
ob
t

=
= (1.36)
where pu
i
I is the current from the generating unit ‘i’ (pu.) and n is the total generating
sources required for availing power transfer operation.
rated
t
ob
t
t
0
t I
I
G
V =
 (1.37)
where t
G is the power transfer gain and rated
t
I is the rated current of the involved generation
sector ‘t’.
(b) Hierarchical Approach:
To fully solve the dc-voltage variation issue, the hierarchical control approach is suggested
to regulate the VSC and battery control strategy [16]. The droop approaches are used for
the primary communication line. The secondary approach is used as a communication line
to identify and correct the variations. However, the secondary communication process is
slower as compared to primary communication and easier to implement. By adding the
corrected voltage signals with the AVDA, the proposed control equation becomes:
V
I
)
SOC
(
k
V
V 0
ref
,
dc
o 
+
−
= (1.38)
2
o
V
max
V
ref
,
dc
V

+ -
min
V
k
vsc
I
+ -
2
dc
V
e
V
s
K
s
K vi
vp +
-1
ref
P
ref
V
2) VSC droop approach
Hierarchical approach
+
+
V

max
V
ref
,
dc
V
+ -
min
V
bat
I
+ -
dc
V
e
V
s
K
s
K vbi
vbp +
ref
V
o
V
ref
,
bat
I
1
M
2
M
ref
,
bat
I
3)Battery droop approach
)
SOC
(
k
+
+
V

o
V
o
V
ref
,
dc
V
+ -
dc
eV
dc
V
s
K
s
K is
ps + max
V

min
V

V

1) Secondary approach
Figure 1. 14 Control diagram of the hierarchical approach

25
where o
V and o
I is the control output voltage and current signals, ref
,
dc
V denoted as the
required reference voltage, k is the droop constant which depends upon the energy storage
device SOC, and V
 denoted as the corrected voltage of the system.
The enhancement of voltage can be attained through a sluggish PI regulator and presented
as follows.
 −
+
−
= )
V
V
(
K
)
V
V
(
K
V ref
dc
vi
ref
dc
vp
 dt (1.39)
The complete control structure of the hierarchical approach is illustrated in Figure 1. 14.
The change in voltage by the droop approach is regulated by the secondary approach. In the
absence of a secondary approach, the primary approach guarantees the system stability and
makes the system reliable.
1.3.5 Hybrid AC-DC MG Control Operation
A typical hybrid microgrid system planning is illustrated in Figure 1. 15. The hybrid-MG
facilitates several potential advantages and sets a novel paradigm for future power system
applications. The merits of hybrid MG is the combination of both AC and DC MG. It is
more flexible, reliable, eliminates unnecessary converter complexity, environment friendly,
improves power quality, and cost-efficient [69].
However, a few of the demerits and integration challenges of the hybrid-MG system are
enumerated below.
• Complex structure and controls [94].
• During grid forming mode of operation, power-sharing among the AC and DC MG is
not possible by traditional P/F and Q/V DA.
• For sharing an accurate power demand, an energy management system requires a
specific droop approach.
• Due to excess implementation of sensitive load, the regulation of harmonic power is also
a point of concern.
• There is a trade-off between reactive power support and voltage control.
• The droop approach is independent upon the line impedance between the converter and
AC/DC bus for appropriate regulation of energy and voltage demand.
• To improve the performance, there is a necessity to design a robust control for hybrid-
MG operation.
Looking at the above challenges, possible control solutions are illustrated below.
1.3.5.1 Unified Primary Approach (UPA):
UPA based control techniques applied for the hybrid-MG system is presented in [106-107].
Similarly, a hybrid standalone hybrid MG system with renewable energy sources, diesel
generator, and intelligent control method is suggested in [95]. In [95], to regulate the pitch
angle of the wind power generation system, a novel Elman neural network-based maximum
power algorithm is proposed for MPPT operation. In addition to that for solar PV MPPT
operation, a radial basis based neural network is proposed. However, the proposed system
does not consider the application of a non-linear/sensitive load. In [96], the main controller

26
is used to regulate the hybrid-MG system consisting of a wind power station, fuel energy
generation, flywheel, and an ultracapacitor.
Integration Structure,
Load exchange
Role of Converter
Hybrid AC-DC System
Type
Location
Ratings and Port number
Control
Bus-loop structure
Bus/loop structure
Tie-line structure
Connection/ no connection (binary matrix)
AC/DC (binary matrix)
AC-AC/AC-DC/DC-AC/DC-DC
CSI/VSI, DG-inv, DC transformer, electronic
transformer
Generation/Load/Tie-line/Group/Unified
Number of used buses, Load demand and
power transfer margin
AC-AC/AC-DC/DC-AC/DC-DC
AC-DC network
design
DG, EMS,EV charging
system
Transformation
Topological Structure
DG operation
Design structure
Constrints
Ac-bus dc transformation, ac/dc feeding, multi
connection, unified seamless
Radial/double/ring/star
DC/AC, Wind turbine, BES, Photothermal
generator
Location (Continuous/discrete),Capacity, grid
connected and islanded mode
DC/AC access, unified/decentralised
Reactive power support,
harmonic mitigation
Generation-load-BES
partition
Clustering feature
Source-load ratio
Types of load (AC/DC), location, reliability, power
quality
DC load ratio, Renewable energy integration
Type of the device
Constraints
Capacitor, SVC, STATCOM,SHAF, UPFC/DVR
Location (connected bus no. ) and total capacity
Types of voltage
AC-voltage rating
Form of dc-bus
HV and LV bus
connection
Single/double bus, unipolar/bipolar device
Power frequency conversion, high frequency
conversion
AC-DC fault control
regulator
RELATED TERMS
Grounding DC
equipment
Constraints
Power electronic switching/ super conducting
resistance
Ungrounded, high resistance grounding, low
resistance grounding, IT grounding mode
Installation short circuit branch, open circuit
impedance, series reactance, line impedance
DC-voltage rating
kV
10
,
kV
6
,
kV
3
,
V
400
,
V
380
,
V
220
)
kV
20
,
kV
6
,
kV
3
,
V
750
,
V
375
,
V
48
(

ASPECTS
VARIABLE
Type of the device
Figure 1. 15 Typical hybrid-MG system planning

27
The main controller based on a hierarchical approach with an energy balancing unit is
proposed for generating the reference voltage and frequency for an auxiliary controller. In
[96], the energy balance technique consists of a grid-connected technique (dc-bus voltage is
regulated through line current loop method), and the source-following technique (line
current loop regulates the active power of the system and wind generating stations regulates
the dc-bus of the MG). The experimental test results illustrate the superiority of the source-
following techniques over grid-connected techniques. However, the proposed approach
lags the performance due to the absence of grid-forming mode and sensitive load operation.
In [97], to overcome the above demerits, a coordinated control technique is suggested for
islanded hybrid MG with MLI operation. In this approach, the superiority of the proposed
controller is tested by using a sensitive load application. Due to the application of sensitive
load, in this proposed manuscript proportional resonant control technique is used to
regulate the generation parameter.
1.3.5.2 Decentralized Primary Approach (DPA):
An I-V based DA is suggested for proportionate energy transfer between the interlinking
AC/DC converter in a hybrid-MG system. In this proposed approach, the current regulators
are PR regulator and voltage regulators are PI regulators. The I-V DA equation is
represented as.
vd
dc
ref
,
dc
dc R
I
V
V −
= (1.40)
where dc
V , ref
,
dc
V , and dc
I are represented as dc-link voltage, reference dc-link voltage,
and dc-link current respectively. The acceptable range of the virtual resistance vd
R range is
computed as.
  )
fl
(
dc
mrg
,
dc
mn
,
dc
mrg
,
dc
mx
,
dc
vd I
)
V
V
(
)
V
V
(
R −
−
−
 (1.41)
pu
V
max
V
min
V
,max
dc
P
Inter-link
Converter
pu
F
max
F
min
F
max
,
ac
P
2
sh
V 2
dc
V
2
V


sh

(a) (b)
Figure 1. 16 (a) Power-transfer in hybrid-MG, (b) Hybrid-MG with dead zone
where mx
,
dc
V mrg
,
dc
V and mn
,
dc
V are known as an optimum, predefined boundary, and
lesser voltage boundary respectively. )
fl
(
dc
I is denoted as the full load dc-current.

28
However, in [25], the paper is not considered an ac-side droop approach for hybrid MG
operation. As a solution, a bidirectional droop regulation loop based on standardized and
mutual per unit range for proportionate energy transfer is proposed in [98]. The related
diagram is shown in Figure 1. 16 (a). The frequency of the AC-grid and voltage of the DC-
grid is set as per the Eq.1.42 and Eq.1.43 respectively.
)
F
F
(
5
.
0
)
F
F
(
5
.
0
F
F
mn
mx
mn
mx
pu
−

+

−
= (1.42)
)
V
V
(
5
.
0
)
V
V
(
5
.
0
V
V
mn
mx
mn
mx
pu
−

+

−
= (1.43)
where pu
F and pu
V are the standardized frequency and dc-bus voltage unit respectively.
mx
F and mn
F are the maximum and minimum frequency value on the ac side, and mx
V and
mn
V are the extreme and least voltage rating of the hybrid-MG system respectively.
A bidirectional droop approach is proposed for a hybrid-MG system and the performance is
tested at different operating conditions with a primary objective of IC loading condition
[99]. In [99], a hybrid AC-DC droop approach is proposed as illustrated in Figure 1. 16 (b).
In this approach, the output of the proposed droop is applied in P
Vdc − and P
−
 droops
of the IC.  and V
 are the dead zones that represent the allowable frequency and voltage
variations respectively.
1.3.5.3 Unified Secondary Approach (UPA):
(a) Evolutionary Technique-Based USA (ET-USA):
An ET-USA is proposed for hybrid-MG system applications in [100] with an objective to
increase the use of RES, reduced the use of the conventional power generations, improved
the energy storage device life span, and bound the implementation of the IC among the AC
and DC grid by considering the forecasting error and intermittence issues. Here, the energy
storage device charging and discharging conditions are set according to the robust optimal
unified control approach through the auxiliary PI regulator.
(b) Synchronized Control of the Hybrid-MG System:
A synchronized control scheme for the hybrid-MG system during both grid-following and
grid-forming conditions are suggested in [94]. In [94], the energy management scheme
guarantees the MPP conditions of both wind and solar PV during the grid-following mode
of operation. In grid forming mode of operation, the ON/OFF MPPT for solar and wind
power generation depends upon the rule-based energy management system. A unified
control approach for the wind-battery based hybrid-MG is proposed in [101]. Due to the
above approaches, the proposed system is more reliable and efficient for real-time
applications.

29
(c) Intelligent Coordinate Approach:
A unified rule-base control approach is proposed for a standalone hybrid-MG with the wind
power plant, diesel plant, and capacitor bank for AC-grid, and PV-energy storage device
for dc-grid. In this, an FLC is used to set the charging and discharging condition of the
battery. Here, a total of 15 operating modes is illustrated, in which 4 operating modes are
used to no-power transfer among the power grid, and 11 operating modes are used to
facilitate power-transfer among the MG. In [102], a novel energy management technique
(EMT) based hybrid-MG system is proposed and experimentally tested at the MG research
lab (Aalborg University). The EMT is designed by using an optimization technique and
used to minimize the operating costs by considering 2 stages charging phenomenon
technique.
1.3.6 Major Findings
In this section, a few of the important control strategies of the respective MG operations are
highlighted. In addition to that, comparative tables are presented by showing the merits and
demerits of the primary and secondary control strategies. The major control findings of the
respective microgrid are presented in the following section.
1.3.6.1 Findings of AC-MG Control:
The respective control merits and demerits of the AC-MG is presented in Table.1. 1. Some
of the important strategies are highlighted for future SMG operation.
• Looking at the system parameter and operating condition, an appropriate current and
voltage controller is required to select for AC-MG operation.
• For highly resistive and inductive transmission line, FVDA based control techniques are
chosen without affecting the voltage and frequency of the system [103].
• A modified voltage and current control are suggested for better performance of the AC-
MG operation without considering the system constraints [104,105].
• To facilitate better load sharing operation, an appropriate hierarchical control approach
is selected for improving the voltage and frequency response of the system [34].
• Looking at the unbalanced/distorted MG, and nonlinear/sensitive load applications,
different adaptive control methods as high-frequency signal insertion, harmonic current
separation, and dc component extraction are selected for renewable energy-based AC-
MG operation.
• Due to the absence of communication-based control techniques, the droop approaches
are not worried about the DG and load position. However, due to the absence of the
information regarding the DG position make the system less reliable and suitable for
SMG operation.
• In the supervise control method, the decentralized method is gaining interest because of
the decreased risk of a single failure concerning the centralized technique. The

30
decentralized technique also facilitates other intelligent techniques like multi-agent,
Fuzzy, and optimization-based techniques.
1.3.6.2 Findings of DC-MG Control:
The respective control merits and demerits of the DC-MG is presented in Table.1. 2. Some
of the important strategies are highlighted for future SMG operation.
• Similar to AC-MG approaches, the hierarchical control technique is selected with a
slight change by considering the DG output voltage as an input. In DC-MG, the grid is
isolated from the system.
• Distributed control strategies such as model predictive control and multiagent based
methods are used to regulate the dc-link voltage of the respective inverters [106-107].
• As compared to both AC and hybrid MG operation, the DC-MG does not necessitate
additional power and frequency-based droop controller in the primary control for
isolated MG operation.
• By considering the merits of both decentralized and distributed control method, a novel
dc-bus voltage signal based secondary control is suggested for better performance of the
DC-MG system.
• Looking at the excess integration of dc load such as hybrid vehicles, and traction
devices, DC-MG control is gaining interest for reliable and easy control operation.
Table.1. 1 Merits and Demerits of AC-MG control Strategies
NO. Strategies Merits Demerits References
01. Traditional
DA
• Easy to integrate
• Limits the SG operation
• Affected by system constraints
• Voltage regulation is not guaranteed
[109]
02. TDA with
extra inductor
• Increase decoupling among power o/p
• Compromise power flow accuracy
• No nonlinearity regulation
[108]
03. Converse DA • Easy to integrate • Oscillations in Voltage and frequency
• Absence of direct integration
[31]
04. Bulky TDA
gain
• Decreases the apparent power transfer
• Oscillations in Voltage and frequency
[113]
05. V-P/F-Q DA • Easy to integrate
• Change in the bandwidth not affect the
frequency and voltage
• Affected by system constraints [71]
06. Q-dV DA • Independent of line resistance • Affected by system initial conditions
• Stability not guaranteed
[72-73]
07. Virtual
impedance
DA
• Not affected by variation in system
constraints
• Change in the bandwidth affect the
frequency and voltage
[109]
08. RL-VIMDA • Easy to integrate
• Improves fundamental current
transfer
• Reactive power transfer cannot be
neglected
• Increases voltage harmonic distortion
[110]
09. RC-VIMDA • Easy to integrate
• Improves fundamental and harmonics
current transfer
• Reactive power transfer cannot be
neglected
• Increases voltage harmonic distortion
[111]
10. Virtual power
DA
• Decoupled real and reactive power
controller
• Used for a well-known microgrid
impedance
• Difficult in each transformer angle
regulation
[76-77]
11. Virtual inertia
DA
• Improvement in stability and damping
ratio
• Protect the system during the
malfunction of relays
• Affected by system constraints
• Functions as TDA, during the lesser
frequency than threshold
[139]
12. Average • Improves the voltage regulation and • Change in the bandwidth of real and [112]

31
voltage DA transient response
• Not affected by system constraints
reactive power affect the frequency
and voltage
13. Multiagent
approach
• Active, intelligent and Social adaptive • Because of the trust issue,
communication is unsecured
[82-85]
14. Model
predictive app
• Robust and adaptive during transients • Frequency response cannot be
visualized
[78-80]
15. Consensus
Theorem
• During unbalanced load, improved
negative current transfer
• Transmission delay [68-81]
16. nth
-Nonlinear
DA
• Facilitates selective harmonic
elimination
• Attenuates both voltage and current
harmonics
• Complex control design, reactive
power transfer cannot be eliminated
and voltage -frequency fluctuations
[112]
17. NVH-Z • Regulates fundamental and harmonic
transfer
• Decreases reactive power transfer
among DGs
• Oscillations in voltage-frequency are
not eliminated
impedance
[105]
18. ESDA
for 1st
and 3rd
generation
• No energy storage is required
• Eliminates voltage and frequency
fluctuations
• No regulation of voltage and current
• Synchronization between each
generation is more complex
[99,104]
19. ESDA for 2nd
generation
• Eliminates voltage and frequency
fluctuations
• Require storage element,
synchronization is complex
[65]
20. PR regulator • Facilitates selective harmonic
eliminations
• Oscillations in voltage-frequency are
not eliminated
[30]
Table.1. 2 Merits and Demerits of DC-MG control Strategies
01. Traditional
DA
• Decreases current transfer exactness
[92]
02. Virtual
resistance DA
• Not affected by line resistance
• Increase decoupling among power o/p
• Decrease voltage regulation capacity
[34]
03. Adaptive
voltage DA
• Reduces the circulating current
• Decrease current flow among the VSC
• Known VSI resistance
• Known dc-bus voltage of the VSC
[114-115]
04. Intelligent DA • FLC-virtual resistance computation
• Decreases the voltage fluctuation
• Variation in dc-bus voltage [116]
05. Mode
Adaptive DA
• Avoid overloading condition among
DGs and energy storage device
• Sensors not detect minute variations
• Problems in proper voltage selections
[117]
06. Typical
current CA
• Avail better voltage regulation and
better current transfer
• Transmission delay [93]
07. Decentralised
SA
• Robust, adaptive
• Flexible, Extensive
• Transmission delay
• Security and protection issue
[92]
08. FLC based
USA
• Easy design
• Improved voltage regulation and load
sharing
• Heat and trail method adopt for
membership function
• Computational burden
[116]
09. Evolutionary
technique
• Increase the current transfer accuracy • Unsuitable for excess load, and excess
time is taken
[118]
10. Improved DA • Easy to integrate
• Better voltage management and load
sharing
impedance
• Unsuitable for excess load
[71]
11. Droop
coefficient
improvement
approach
• Improvement in stability and damping
ratio
• Reinstate dc-bus voltage and enhance
accuracy in current
• Improvement of the coefficient is
difficult to get
• Complex structure, the
computational burden
[119]
12. Impedance
Identification
approach
• Offers adequate voltage regulation
• Enhances the system performance
• Complex design and excess time are
taken
• Unsuitable for excess load application
[120]
13. Average
voltage CA
• Better voltage management • Unsuitable for complex load [121]
14. Dc-bus
regulation
• Improve transient response • The problem in power coupling [92]
15. Dynamic gain
DA
• Better voltage management • Unsuitable for larger SSMG
approach
[122]

32
16. Flexible
control
approach
• Electrical isolation
• Increase reliability
• Implementation cost is not
considered
[123]
17. Modular
power CA
• Multi-directional power conversation • Multifaceted line impedance
condition is not checked
[124]
18. EMS power
approach
• Eliminates voltage fluctuations and
better power regulation
• Lack of operation and control
applications
[125]
19. Power
regulation CA
• Improved robustness
• Plug and play
• Complex DG integrated system is not
considered
[126]
20. Plug and play
approach
• Improved robustness • Complex DG integrated system is not
considered
[127]
Table.1. 3 Merits and Demerits of hybrid-MG control Strategies
01. Bidirectional
DA
• Offers appropriate power-sharing
• Bad reactive power transfer
[98]
02. Hybrid droop
with dead zone
• Facilitates load sharing operation
• Decrease voltage regulation capacity
• Load dependent
[99]
03. I-V based DA • Reduces the circulating current
• Hierarchical approach
• Constraints affected performance
[25]
04. UPA • Appropriate load and power-sharing
• Better voltage regulation
• dc-bus voltage regulation required
• Non-linear load application
[95]
05. Model
predictive appr.
• Robust controller
• linearizing Power transfer
• Unsuitable for excess load
• Absence of Frequency response
[100]
06. Evolutionary
based appr.
• Robust, efficient
• Optimal
• Global optimum is difficult to achieve
[101]
07. Decentralised
SA
• Proactive
• Autonomous microgrid
• Traditional DA
• Decreased power-sharing operation
[128]
08. Distributed SA • Easy design
• voltage and frequency regulation
• Computational and security issue
[129]
09. Droop
Approach
• low cost, efficient,
• more reliable
• Coupling between power flow
• Steady-state error increase
[34]
10. Communication-
based Approach
• Improved efficiency, reduce losses
• Lesser time delay
• Increase cost and complexity
• Compromise the stability
[130]
11. Standardized
DA
• Design with one PI controller
• Facilitates better real power control
• Increase in losses
• computed variable dependency
[34]
12. Improved DA • Offers adequate voltage regulation
• Reduce transition losses
• Increased cost and complexity
• Computed variable dependency
[71]
13. FLC-approach • Optimum power generation • Coordination problem [131]
14. Sugeno Fuzzy
approach
• Avail better charging and
discharging condition
• Adaptation is inadequate during
peak shaving
[132]
15. Fuzzy with 2 i/p • Better voltage management • Unsuitable for increasing settle time [133]
16. Addition of FLC
and Crisp logic
• Battery Fuzzy agent
• Better control
• Power fluctuation is not considered
• HES optimum performance is not
achieved
[134]
17. Fuzzy-PID
control
• Multi-directional power conversation • Multifaceted line impedance
condition is not checked
[135]
18. Fuzzy-PI
approach
• Better voltage control, robust control • Perturbation of load is not
considered
[136]
19. Fuzzy with 6 i/p • Location of the storage system
• Better ESS operation
• Stability prob. among grid-connected
and islanded operation
[137]
20. Adaptive Fuzzy • Improved stability • Slower mitigation
• Increase the number of the learning
algorithm
[138]
21. Fuzzy Logic • TCR operation
• SVC operation
• larger SSMG operation
• Complex system
[139]
22. Neuro-Fuzzy
approach
• Damping power oscillation • Two-machine test system [140]
23. Fuzzy PID with
ZN and PSO
• Decrease the communication
problem between VSC operation
• Green energy is not separately
considered
[141]

33
1.3.6.3 Findings of Hybrid-MG Control:
The respective control merits and demerits of the hybrid-MG are presented in Table.1. 3.
The hybrid MG system is designed by combining both AC and DC MG systems. Some of
the important strategies are highlighted for future SMG operation.
• For a coordinated and reliable operation of the hybrid-MG system, a complex control
approach is needed.
• The role of interlinking VSC operation and the related control operation is more
important to balance the power flow among the AC and DC MG for both grid following
and grid forming conditions.
• In recent power applications, the absenteeism of a universal term among both the AC
and DC MG set a novel task for hybrid-MG controller design. As a solution, recent
researches suggest a novel hybrid AC-DC droop, and frequency and voltage-based
control method for both of the sub-grid operation.
1.4 Objective of the Thesis
General objectives:
The main objective of this research work is to compute and regulate the effects of different
units such as solar, wind, and battery energy system integration under various penetration
levels and operating scenarios on distribution grid applications. To offer better power
quality, reliability and stability, in this thesis, specifically the reduced switch multi-level
inverter and controller design of a test microgrid system is focused. To circumvent the
above factors, the following specific aspects are needed to be considered.
Specific objectives:
The specific objectives of this study are:
• Looking at the real-time and IEEE standards, the accurate modeling of the non-
linear load and renewable energy-based microgrid system design is the first
objective of the thesis.
• To compensate the non-linear effects, the accurate modeling of the shunt active
filter is the second objective of the thesis. To offer optimum performance,
Multilevel inverters (MLIs) are used as a shunt active power filter. Therefore,
appropriate modeling of the reduced switch MLIs is much more important.
• In addition to the SHAF design and enhance the inverter performance, the
development of the controller is the third objective of the thesis. In this presented
thesis, different advanced control strategies like enhanced instantaneous power

34
theory (EIPT), robust control strategy, and morphological approach is developed
and used for different microgrid application.
• After developing the appropriate inverter and control model, the power loss, energy
management, battery charging and discharging condition, reactive power support,
single-stage operation, power quality and reliability of the renewable energy-based
microgrid system study is the fourth objective of the thesis.
• Recently, an appropriate electric vehicle modeling and control is considered as a
major problem. After developing an appropriate inverter and control model idea, the
electric vehicle and hybrid microgrid application is considered as the fifth objective
of the thesis. By properly designing the electric vehicle and hybrid microgrid
model, the improved power quality and stability of the system is evaluated and
tested.
• All the proposed models are developed and simulated through MATLAB/Simulink
software by considering the IEEE microgrid standards. In addition to that the
improved power quality of the test microgrid system is evaluated to look at IEEE-
1541,1459- 519 and MIL-STD-704E standards.
1.5 Thesis Major Contribution
Problem Formulation:
Looking at the excess energy requirement and increasing integration of renewable energy-
based microgrid systems, different power system problems such as harmonics, power
quality, power reliability, and stability are observed. Due to the above problems, renewable
energy-based DG performances significantly decreases. Therefore, there is an essential
requirement to circumvent the above problems for the efficient performance of the systems.
Looking at the above problems, the main focus of the thesis consists of the appropriate
design of controller and inverter modeling, avail maximum power from the generation
system, avail better energy management, better power quality and reliability operation,
excellent reactive power control, improve the stability, solve frequency and voltage
mismatch, generate an increased voltage level, and reduced switching losses, etc. Before
achieving the above factors, different renewable and non-linear based DGs are simulated
through MATLAB/Simulink software. The simulated model is operated and tested for both
grid-connected and islanded modes of operation. The simulated systems with appropriate
control and inverter modeling offer better performances by handling different power
system challenges. To handle the above problems, the following important factors are
considered to construct the proposed thesis. The details of the contribution are presented
below.

35
• Shunt Active Filter Design:
Looking at the excess use of non-linear load, after developing appropriate modeling, the
role of shunt active filter design is the first important contribution of the presented
thesis. For harmonic elimination and provide reactive power support to the system,
shunt active filter-based systems are offering excellent solutions. However, the
traditional inverters lag to show optimum performances due to the larger size, excess
component required, and cost. To overcome the above demerits, reduced switch multi-
level inverter applications are designed and implemented in the renewable energy-
based DG systems.
• Enhanced Instantaneous Power Theory:
From the literature, it is concluded that only shunt active filters are not providing an
appropriate solution. Therefore, to obtain an efficient solution, the controller design is
the second important contribution of the thesis. Traditionally, PQ and IPT based
current control theories offer the most attractive solutions to the DGs. However, during
dynamic and transient state conditions, the performance of the traditional controller is
decreased and affects the system performance. Therefore, to overcome the above
problems, appropriate switching pulse generation, and reduce the complexity, the
traditional control systems are enhanced and named as enhanced instantaneous
power theory (EIPT)based controller.
• Robust Controller Design:
Looking at the increased energy demand, multiple energy generation, and storage
integration, the traditional control methods are not providing an appropriate switching
solution for better SHAF operation. Undoubtedly, the EIPT based control solutions offer
an optimum solution with reduced cost and simpler design. However, to improve power
quality, power reliability, and stability, it is necessary to know the actual information
regarding the system model. Therefore, in this thesis presentation, the robust
controller design is the third important contribution. In the presented thesis, the
robust controller is designed by requiring reduced voltage and current sensors, filters,
linear controller, complexity, and proper mathematical analysis. The appropriate
mathematical modeling about the system gives actual information about the steady-state
and dynamic state condition of the system. In addition to that, in this section, a novel
dc-link voltage regulator is developed to improve the stability of the system.
• Hybrid Microgrid Design and Application:

36
After properly designing the inverter and controller, the designed model is implemented
in the hybrid microgrid operation. Undoubtedly, the developed model efficiently works
under single ac-grid performance. However, during a complex system application, the
actual testing of the above-developed models is justified. Therefore, for showing better
power quality, power reliability, and stability of the system, the developed controller and
RSMLIs are implemented and tested during both ac and dc grid-based microgrid
conditions. In this section, the synchronization between all the model parameters are
tested. This is the fourth important contribution of the thesis.
• Electrical Vehicle Design and Application:
Recently, due to excess vehicle demand and application, electric vehicle designs are
gaining interest. As the vehicle requires better coordination and operation between the
inverter and control model for better performances, there is a necessity to develop an
appropriate solution for availing better synchronization, improved stability, and better
power quality. Therefore, looking at the challenges, the developed inverter, and control
model performances are combining implemented to design an appropriate vehicle
model. This is the fifth and last important contribution of the thesis.
1.6 Thesis Organization
The proposed thesis is organized into seven chapters as illustrated in Figure 1. 17. The brief
discussion about the formulated chapters is described below.
Chapter-1 (Introduction)
This chapter provides a concise overview of the problem allied with the power distribution
system, specifically in the Micro-grid system. This chapter presents the introduction of
providing background information with the problem statement. The current status of
existing methods and the limitations are reviewed. In this section, the background of the
research problem is observed to clearly define the goal of the report. The major objectives
and findings of the thesis are also highlighted. The scope, limitations, and development
plan of the recent researches are stated to ensure a confined and fruitful research.
Chapter-2 (Reduced Switch Multilevel Inverter (RSMLI))
This chapter presents a detailed modeling and comparative study about the different shunt
active filter performances in real-time microgrid applications. In this section, the detailed
modeling of reduced switch multi-level inverter (RSMLI) and the related switching
operations are discussed. To show the advantages of the proposed RSMLI application, the
literature review regarding the traditional inverter and its limitations are discussed. Due to
the advancement of MLI application and to reduce the cost, complexity, and size, this
section is more focussed on single-stage grid integrated microgrid operation. In this section,
to circumvent the power quality and power reliability problems of the microgrid system,

37
the performance of RSMLI applications are tested through different testing scenarios such
as steady-state and dynamic conditions. Looking at the necessity of shunt active filter
requirement, this chapter is divided into three parts and each part contains different three-
phase RSMLI applications like 7-level and 32-level with different renewable energy-based
generation stations. The different studies are presented as follows.
Robust Active Power Filter Controller Design for Microgrid and
Electric Vehicle Application
Background of the study, Literature survey regarding the
active filter control scheme, Microgrid application,
Merits and demerits, Objective, Contribution
Introduction
(Chapter-1)
Robust Controller
(Chapter-4)
Major Findings, Summary
(Chapter-7)
Development and Design Stage
Implementation Stage
Conclusion Stage
Future Scope
C
O
M
P
L
E
T
S
T
U
D
Y
Reduced Switch
Multi-level Inverter (RSMLI)
Enhanced Instantaneous
Power Theory (EIPT)
Hybrid Microgrid Application Electric Vehicle Application
Title of Dissertation
Figure 1. 17 Detailed flow chart of the Thesis

38
Study-1:
In this study, to fulfil the main objective of the chapter, a reduced switch multi-level
inverter (RSMLI) based grid integrated photovoltaic (PV) system is studied at
different state conditions. A new topology of cascaded Seven-Level inverter by a
reduced number of switches (CSIR) is proposed to control active and reactive power
with enhanced power quality standard for a PV-battery based microgrid. To generate
appropriate switching signals for CSIR, a repetitive controller is used in the controller
design. In addition to that, an Incremental Conductance method based maximum
power point tracking method is used to extract maximum power from the PV system.
To justify the practical applicability of the proposed approach and to satisfy
IEEE1547 power quality constraints, an LCL filter is used to minimize the harmonics
in the grid side voltage levels. The enhanced performance of the proposed technique
is justified by presenting comparative simulation results with respect to Neutral
clamped inverter (NPC) and proportional-integral controller. By using MATLAB
software, the validity of the paper is studied by simulation at different battery
charging and discharging conditions.
Study-2:
Similar to study-1, to fulfil the main objective of the chapter-1, a RSMLI based grid
integrated wind energy system is studied at different state conditions. A new
topology of the cascaded 31-Level inverter by a reduced number of switches is
proposed to control both active and reactive power with enhanced power quality
standard for a grid-connected doubly fed induction generator (DFIG) for wind energy
conversion system (WECS). The repetitive control approach is considered for the
inverter operation due to its better controllability and accuracy under periodic
disturbance conditions. Further enhancing the system performance by supplying the
desired reactive power to DFIG and harmonic reduction, a 31-level cascaded inverter
topology with a reduced number of unidirectional switch operations is proposed and
implemented in the rotor side converter (RSC). This leads to the benefit of the highest
power extraction and offers the requisite reactive power to DFIG. In addition to that
grid side converter (GSC) an LC filter is added, to work as a hybrid active filter for
harmonic cancellation produced by the nonlinear load and so behaves like a static
compensator (STATCOM) even under shutdown condition of the wind turbine.
Indirect current control and flux-oriented reference frame control are implemented
for grid and rotor side converter respectively. The proposed approach is validated
with simulated test results under both steady-state and dynamic conditions.
Study-3:
Similar to study-1, to fulfil the main objective of the chapter-1, an RSMLI based grid
integrated photovoltaic (PV) system is studied at different state conditions with non-
linear load applications. This paper presents a Fuzzy Logic control (FLC) based

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