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CONTROL STRATEGIES FOR DGS TO IMPROVE
OPERATION PERFORMANCE OF MICROGRID
by
Loc Nguyen Khanh
A Dissertation
Submitted to the Faculty of
INHA UNIVERSITY
In Partial Fulfillment of the Requirements
for the Degree of
DOCTOR OF PHILOSOPHY
Graduate School of Electrical Engineering
August 2011
CONTROL STRATEGIES FOR DGS TO IMPROVE
OPERATION PERFORMANCE OF MICROGRID
by
Loc Nguyen Khanh
A Dissertation
Submitted to the Faculty of
INHA UNIVERSITY
In Partial Fulfillment of the Requirements
for the Degree of
DOCTOR OF PHILOSOPHY
Graduate School of Electrical Engineering
August 2011
ACKNOWLEDGEMENT
I wish to express my sincere gratitude and appreciation to my Advisor
Professor Dr. Dong-Jun Won for his invaluable guidance, advice and
financial support throughout the course of this work. Without his help, this
work would not be possible and I am grateful to him for his supportive and
friendly attitude. I would also like to express my sincere thanks to Dr. Seon-
Ju Ahn for his valuable and outstanding advice. I would also like to thank the
members of my laboratory for their kind helps throughout the PhD course at
INHA University. I would also like to thank the members of my committee
for reviewing the dissertation and attending my defense: Prof. Dr. Kim
Yeong Seok, Prof. Dr. Lee Bok Hee, Prof. Dr. Song Seung Ho, and Prof. Dr.
Chung Il Yop. Their advice and patience are appreciated.
Lastly, I would like to thank my family for their continuous support and
encouragement, and especially my wife, Mrs. Tran Thi Hong Nga, for her
unconditional love, support, and patience.
ABSTRACT
This dissertation presents a control strategy for distributed generators
including controllable and uncontrollable sources to improve operation
performance of micgrogrids.
In order to achieve the control strategy for controllable DGs, the
frequency and active power responses according to the droop characteristic
have been investigated to show advantages and disadvantages of different
microgrid configurations and different control modes. The analysis is based
on three conditions of operations, i.e. grid-connected mode, islanded mode,
and transition mode, and four possible configurations of the microgrid
namely the FFC-series-, FFC-parallel-, UPC-series-, and UPC-parallel-
configuration.
According to the analysis, control strategy for controllable DGs is
proposed to take the advantages and overcome the disadvantages of each
configuration. Namely, the control strategy for controllable DGs is to operate
the microgrid as constant load from the utility viewpoint, minimize the
frequency change in the transition mode, and control the frequency
unchanged during the islanded mode.
Uncontrollable sources like PV and Wind turbine coupled with
controllable sources like Fuelcell in a hybrid system can become controllable.
The operation algorithm for hybrid sources, i.e. the PV-PEMFC hybrid
source, based on the unit-power-control mode and feeder-flow-control mode
ii
is proposed to operate in the microgrid with a high efficiency, maximize the
solar energy, and optimize the PEMFC operational efficiency.
Overall control strategy for microgrid with presence of controllable
DGs and the PV-PEMFC hybrid system is finally implemented in a multi-
feeder microgrid in EMTDC /PSCAD. The simulation results show
advantages and improvements of proposed method over conventional control
method.
KEYWORDS:
Microgrid, microgrid control, distributed generation, microsource,
energy management system, control strategy, hybrid system, droop
characteristic, unit power control, feeder flow control mode, inverter
control.
STUDENT NUMBER: 2207-2231
iii
CONTENTS
Abstracts .......................................................................................................... i
Contents ........................................................................................................ iii
Abbreviation ................................................................................................. vii
Index of Figures and Tables ........................................................................... ix
Chapter I: Introduction ............................................................................. 1
1.1 Motivations and Purposes ................................................................... 1
1.2 Highlights and Major Contributions ................................................... 4
1.3 Dissertation Structure ......................................................................... 6
Chapter II: Microgrid Concept and Control ............................................. 9
2.1 Overview ............................................................................................ 9
2.2 Microgrid Management and Control .................................................. 10
2.3 Microsource Controller ...................................................................... 14
2.3.1 The Two Power Control Modes ......................................................... 18
2.3.2 Droop Control through Active and Reactive Power ........................... 20
Chapter III: Analysis of P and f Responses According to Microgrid
Configurations ............................................................................................. 31
3.1 Overview ........................................................................................... 31
3.2 Active Power and Frequency Response in a Microgrid ..................... 33
3.2.1 Active Power and Frequency Response in Transition Mode .............. 33
iv
3.2.2 Active Power and Frequency Response in Grid Connected Mode ..... 34
3.2.3 Active Power and Frequency Response in Islanded Mode ................. 42
3.2.4 Comparison of the Different Microgrid Configurations ..................... 48
Chapter IV: Control Strategies for Controllable DGs in Microgrid ........ 53
4.1 Overview ........................................................................................... 53
4.2 Control strategy for Microgrid in the Grid-connected Mode .............. 54
4.2.1 Control Strategy for Single-Feeder Microgrid ................................... 54
4.2.2 Control Strategy for Multi-Feeder Microgrid ..................................... 63
4.3 Control Strategy for Microgrid in the Islanded Mode ........................ 65
4.4 Simulation Studies and Results .......................................................... 67
4.4.1 Test System and Simulation Scenarios .............................................. 67
4.4.2 Simulation Results in the Grid-Connected Mode ............................... 69
4.4.3 Simulation Results in the Transition Mode ........................................ 73
Chapter V: Control Strategies for a Hybrid Source Connected to
Microgrid ................................................................................................. 77
5.1 Overview ........................................................................................... 77
5.2 Hybrid System Description ................................................................ 78
5.2.1 Structure of Grid Connected Hybrid System ...................................... 78
5.2.2 PV Array Model ................................................................................ 79
5.2.3 PEMFC Model ................................................................................... 81
5.2.4 Maximum Power Point Tracking Control .......................................... 82
5.3 Control Algorithm of the Hybrid System ........................................... 85
v
5.3.1 Control Strategy for the Hybrid System in the UPC mode ................. 85
5.3.2 Overall Control Strategy for the Hybrid System .............................. 92
5.4 Simulation Studies and Results .......................................................... 97
5.4.1 Simulation Results in the Case without Hysteresis ............................ 97
5.4.2 Improving Operation Performance by Using Hysteresis .................... 99
5.4.3 Discussion ........................................................................................ 100
Chapter VI: Case Study .......................................................................... 105
6.1 Introduction to Case Study ............................................................... 105
6.1.1 System Configuration ...................................................................... 106
6.1.2 System Parameter ............................................................................ 106
6.1.3 Simulation Scenarios ....................................................................... 107
6.2 Results and Discussion .................................................................... 108
6.2.1 Simulation Results without Hybrid Source ...................................... 108
6.2.2 Simulation Results with the PV-FC Hybrid Source ......................... 117
6.2.3 Simulation Results with Multiple Feeder Microgrid ........................ 132
6.2.4 Proposed control method versus frequency restoration control scheme
in island mode .................................................................................. 137
Chapter VII: Conclusions and Future Extensions .................................. 141
7.1 Conclusions ..................................................................................... 141
7.2 Future Extensions ............................................................................ 143
Bibliography .............................................................................................. 145
Appendix A: Frequency and Active Power Responses in the Islanded
Mode ........................................................................................................... 155
vi
A.1 PSCAD Model of Three-DG Microgrid ........................................... 155
A.2 Simulation Results ........................................................................... 157
Appendix B: Case Study ........................................................................... 161
B.1 PSCAD Model of Whole System ..................................................... 161
B.2 PSCAD Model of Photovoltaic ........................................................ 162
B.3 PSCAD Model of MPPT Algorithm of PV ...................................... 163
B.4 PSCAD Model of PEMFC ............................................................... 165
B.5 PSCAD Model of Buck-Boost DC/DCs and Controllers ................. 169
B.6 PSCAD Model Inverter Controller .................................................. 170
vii
ABBREVIATION
BESS Battery energy storage system
CANMET Canada Centre for Mineral and Energy Technology
CERTS Consortium for Electric Reliability Technology Solutions
CHP Combined heat and power
CSI Current source inverter
CV Constant voltage
DER Distributed energy resources
DG Distributed generation
DGs Distributed generators
DMS Distribution management system
ED Economic dispatch
EMS Energy management system
EMTDC Electromagnetic transients including DC
FFC Feeder flow control
HB Hybrid
IC Internal combustion
INC Incremental conductance
LCs Load controllers
LD Load demand
MCs Microsource controllers
viii
MGCCs Microgrid central controllers
MGT Micro-gas turbine
MPPT Maximum power point tracking
NEDO New Energy and Industrial Technology Development
Organization
P&O Perturbation and observation
PCC Point of common coupling
PEMFC Proton exchange membrane fuel cell
PSCAD Power system computer aided design
PV Photovoltaic
PWM Pulse width modulation
RE Renewable energy
SC Super capacitor
SS Static switch
UC Unit commitment
UPC Unit power control
VSI Voltage source inverter
ix
INDEX OF FIGURES AND TABLES
FIGURES
Fig. 1.1 Dissertation structure ..................................................................... 7
Fig. 2.1 Microgrid architecture diagram ...................................................... 9
Fig. 2.2 Microgrid control architecture ....................................................... 12
Fig. 2.3 Microgrid energy management system .......................................... 13
Fig. 2.4 Inverter-based microsource diagram ............................................. 16
Fig. 2.5 Power-control mode of a DG ........................................................ 17
Fig. 2.6 Inverter-based system .................................................................... 19
Fig. 2.7 Voltage vs. Reactive power droop ................................................. 21
Fig. 2.8 Diagram of P vs. f droop control block ......................................... 25
Fig. 2.9 Power vs. Frequency droop ........................................................... 26
Fig. 2.10 Diagram of FL vs. f droop control block ....................................... 27
Fig. 2.11 Control block diagram to enforce the output limit ......................... 28
Fig. 2.12 Microgrid configuration with two Sources .................................... 29
Fig. 2.13 Feeder flow vs. Frequency ............................................................ 29
Fig. 2.14 Control block diagram to enforce the output limit ......................... 30
Fig. 3.1 Different configurations of microgrid ........................................... 32
Fig. 3.2 FL vs. f characteristic in transition mode for series–FFC
configuration ................................................................................. 34
Fig. 3.3 P vs. f characteristic in transition mode for series–UPC
configuration ................................................................................. 36
x
Fig. 3.4 FL vs. f characteristic in transition mode for parallel-FFC
configuration. ................................................................................ 37
Fig. 3.5 FL vs. f characteristic in transition mode for parallel-UPC
configuration ................................................................................. 39
Fig. 3.6 FL vs. f characteristic in islanded mode for series-FFC
configuration when load LDi increases ......................................... 42
Fig. 3.7 P vs. f characteristic in islanded mode for series-UPC
configuration and parallel-UPC configuration ............................... 45
Fig. 3.8 FL versus f characteristic in islanded mode for parallel-FFC
configuration ................................................................................. 46
Fig. 4.1 Grid-connected Microgrid with Multiple DGs .............................. 54
Fig. 4.2 Feeder Flow vs. Frequency Droop (DG1 & DG3: FFC mode) ...... 57
Fig. 4.3 Algorithm of the control-mode change of UPC-mode DGs ........... 63
Fig. 4.4 Microgrid with m feeders .............................................................. 64
Fig. 4.5 Power sharing for an islanded microgrid ....................................... 65
Fig. 4.6 Power sharing for a multi-feeder microgrid in islanded mode ....... 66
Fig. 4.7 The Grid-connected Microgrid with Three DGs ............................ 68
Fig. 4.8 Simulation results for the conventional method ............................ 70
Fig. 4.9 The simulation results for the proposed method ............................ 72
Fig. 4.10 The frequency change in the transition mode ................................ 74
Fig. 5.1 Grid Connected PV-FC Hybrid System ........................................ 79
Fig. 5.2 P&O MPPT Algorithm.................................................................. 83
Fig. 5.3 Buck-Boost converter and control ................................................. 84
Fig. 5.4 Operation Strategy of Hybrid Source in the UPC .......................... 86
Fig. 5.5 Control Algorithm Diagram in UPC mode .................................... 90
xi
Fig. 5.6 Hysteresis Control Scheme for PMSref Control ............................... 91
Fig. 5.7 Overall Operating Strategy for the Grid Connected Hybrid System93
Fig. 5.8 Simulation result without hysteresis .............................................. 99
Fig. 5.9 Improving operation performance by using hysteresis ................ 102
Fig. 6.1 System Configuration .................................................................. 105
Fig. 6.2 Active power response as LD1 changes ...................................... 108
Fig. 6.3 Voltage and frequency response in grid-connected mode ........... 109
Fig. 6.4 Frequency change due to islanding ............................................. 110
Fig. 6.5 Active power response as LD1 changes ...................................... 110
Fig. 6.6 Voltage and frequency response in grid-connected mode ........... 111
Fig. 6.7 Frequency change due to islanding ............................................. 112
Fig. 6.8 Active power response as LD1 changes during island mode ....... 113
Fig. 6.9 System voltage ............................................................................ 113
Fig. 6.10 System frequency ........................................................................ 114
Fig. 6.11 Active power response as LD1 changes during island mode ....... 115
Fig. 6.12 DG2 control mode ....................................................................... 115
Fig. 6.13 System Voltage and frequency .................................................... 116
Fig. 6.14 Load demand and feeder flow in front of HB source ................... 118
Fig. 6.15 Power response in case without control strategy ......................... 119
Fig. 6.16 System voltage and frequency ..................................................... 120
Fig. 6.17 Load demand and feeder flow in front of HB source ................... 121
Fig. 6.18 Power response in case with control strategy .............................. 121
Fig. 6.19 The control mode of DG2 and hybrid source .............................. 122
Fig. 6.20 System voltage and frequency ..................................................... 123
Fig. 6.21 Load demand and feeder flow in front of HB source ................... 125
xii
Fig. 6.22 Power response in case without control strategy ......................... 125
Fig. 6.23 The hybrid source control mode .................................................. 126
Fig. 6.24 System voltage and frequency ..................................................... 127
Fig. 6.25 Load demand and feeder flow in front of HB source ................... 128
Fig. 6.26 Power response in case with control strategy .............................. 129
Fig. 6.27 The control mode of DG2 and hybrid source .............................. 130
Fig. 6.28 System voltage and frequency ..................................................... 131
Fig. 6.29 Active power responses of feeder 1 ............................................. 133
Fig. 6.30 Active power responses of feeder 2 ............................................. 133
Fig. 6.31 Power from the main grid ............................................................ 134
Fig. 6.32 Active power responses of feeder 2 in island mode ..................... 135
Fig. 6.33 Active power responses of feeder 1 in island mode ..................... 135
Fig. 6.34 Feeder flow in front of DG2, Pf1_DG2 ....................................... 136
Fig. 6.35 Microgrid frequency .................................................................... 136
Fig. 6.36 Active power responses in case of frequency restoration method 138
Fig. 6.37 Microgrid frequency in case of frequency restoration method .... 138
Fig. 6.38 Active power responses in case of proposed method ................... 139
Fig. 6.39 Microgrid frequency in case of proposed method ....................... 139
Fig. A.1 Three-DG microgrid in series configuration ................................ 155
Fig. A.2 Three-DG microgrid in parallel configuration ............................. 156
Fig. A.3 Power sharing of three DGs in series-FFC configuration ............ 157
Fig. A.4 Frequency response in series-FFC configuration......................... 157
Fig. A.5 Power sharing of three DGs series-UPC configuration ............... 158
Fig. A.6 Frequency response in series-UPC configuration ........................ 158
Fig. A.7 Power sharing of three DGs in parallel-FFC configuration ......... 159
xiii
Fig. A.8 Frequency response in parallel-FFC configuration ...................... 159
Fig. A.9 Power sharing of three DGs in parallel-UPC configuration ......... 160
Fig. A.10 Frequency response in parallel-UPC configuration ..................... 160
xiv
TABLES
Table 2.1 Microsource Types and Typical Capability ................................ 14
Table 2.2 Typical Line Parameters ............................................................. 24
Table 3.1 Frequency and Active Power Change in Transition Mode .......... 40
Table 3.2 Frequency and Active Power Change in Islanded Mode ............. 48
Table 3.3 Comparison of Four Configurations of Microgrid ...................... 50
Table 3.4 Simplified Comparison of Four Configurations of Microgrid ..... 51
Table 4.1 The System Parameters ............................................................... 68
Table. 5.1 The Hybrid System Parameters ................................................... 97
Table. 6.1 Scenarios without Hybrid Source .............................................. 107
Table. 6.2 Scenarios with Hybrid Source ................................................... 107
Table. 6.3 Scenarios with Multiple Feeder Microgrid ................................ 107
Table. 6.4 Proposed Method and Frequency Restoration Control Scheme 108
Chapter I:
INTRODUCTION
1.1. Motivations and Purposes
Conventional power systems are currently facing the challenges of
handling increasing electricity demand levels together with environmental
consciousness and market deregulation. This has resulted in two main
trends developed in power system recently. The first is a wide utilization of
renewable energy (RE) sources along with combined heat and power (CHP)
systems. The second is decentralization of power generation, so called
distributed generation [1, 2]. The global warming causes many challenges
to human being and it has become a hot issue than ever. In addition, the
earth’s natural resources are becoming exhausted. Therefore, the use of
renewable energy sources in power system is exploiting rapidly. There is
universal agreement that by the end of this century the majority of our
electrical energy will be supplied from RE sources. Generators powered
from renewable energy sources (except large-scale hydro and large
offshore and onshore wind farms) are typically much smaller than the
fossil fuelled and nuclear powered generators. Small generators cannot be
connected to the transmission system due to the cost of high voltage
transformers and switchgear. Also, the transmission system is often a long
way away as the geographical location of the generator is constrained by
the geographical availability of the resource. Small generators must
therefore be connected to the distribution network. Such generators are
CHAPTER I
2
known as distributed generators or microsources. They are in range of
kilowatts to megawatts.
The electricity supply is more and more based on distributed
generators (DGs). Presently, they are only injecting the available active
power into the interconnected network. At higher penetration levels they
might jeopardize the stability of the network. In a more sophisticated
approach, they should participate in network operation to guarantee a
sustainable and secure electricity supply. Distributed generation also has
the potential to increase system reliability and power quality due to the
decentralization of supply. Increase in reliability levels can be obtained if
DG is allowed to operate autonomously in transient conditions, namely
when the distribution system operation is disturbed upstream in the grid.
The DGs are connected to the grid through an electronics device such
as DC-DC converter, DC-AC inverter etc. Power electronics interface has
many advantages such as easy and flexibility in operation due to digital
controls; faster dynamic response compared to the electro mechanical
converters; lower acoustic noise when compared to electromagnetic
controllers, relays and contactors; high efficiency due to low losses in the
thyristors; long life and reduced/minimal maintenance due to the absence
of mechanical wear; control equipments using thyristors are compact in
size.
Such power grid trends make the future grid more environmentally
friendly, more reliable, more intelligent, higher power quality, as well as
enhanced the operational efficiencies. These trends also lead to new grid
concepts such as “SmartGrid”, “Intelligrid”, and “Microgrid”. As the
INTRODUCTION
3
penetration of distributed generation (DG) increases at the distribution
level, managing these systems effectively becomes increasingly
challenging. The microgrid is one effectively proposed way to manage
these systems. Microgrids are also seen as one of the cornerstones of the
future smartgrids [3].
The interconnection of small modular generation system (PV, fuel
cell, micro-turbines, small wind generator) and storage devices to Low
Voltage (LV) distribution grids will lead to a new energy system paradigm,
usually referred as the Microgrid [4-9]. Microgrid is generally defined as a
low to medium voltage distribution networks (e.g. a small urban area, a
shopping center, or an industrial park) comprising various controllable
DGs, such as micro-turbines, fuel cells, etc., uncontrollable DGs like
photovoltaic, together with storage devices, i.e. flywheels, energy
capacitors and batteries, and controllable loads that can operate either
interconnected to the main distribution network or in an islanded mode [9-
13]. The microgrid can be thought as a controllable cell of the power
system from the utility view point. For example, microgrid could be
controlled as a single dispatchable load which can respond in seconds to
meet the requirements of the transmission system. The microgrid can also
be designed to meet special needs of the customers such as local reliability
enhancement, feeder losses reduction, local voltage support, voltage sag
correction, increase of the efficiency through use of waste heat etc [4].
Microgrid is a new type of the power system. Many efforts have been
being put into it by different organizations as well as countries.
Considerable researches on microgrids currently carried out are 1) the
CHAPTER I
4
Consortium for Electric Reliability Technology Solutions (CERTS) by the
National Science Foundation Industry/University Cooperative Research
Center, the U.S., 2) the “MICROGRID” and the “MORE MICROGRIDS”
projects funded by the European Commission, 3) the three projects by the
New Energy and Industrial Technology Development Organization
(NEDO), Japan, and 4) the microgrid project by CANMET Energy
Technology Center, Varrenes, Canada. The technique is still not mature
nowadays. A lot of works have to be done until it can be put in the market.
It is still under the research and experimental stage [9].
In addition, the increase of distributed energy resources (DERs)
including intermittent renewable resources will pose many challenges, such
as protection, control, and operational cost [14], for the future power grid
operators. Especially for the distribution infrastructure and the system
operator, new flow patterns caused by DERs may require changes to the
protection and control strategies, voltage and Var management, and overall
enforcement of distribution grid infrastructure.
Under the circumstances, this dissertation will focus on the operation
and control of DGs including controllable and uncontrollable sources to
meet the various requirements of the Microgrid.
1.2. Highlights and Major Contributions:
The dissertation will focus on the active power and frequency responses in
the microgrid. The DGs output power and/or microgrid frequency can be
affected by three cases of operation: i) load variation during the grid-
connected mode, ii) load variation during islanded mode, and iii)
disconnection from the main grid.
INTRODUCTION
5
i) During the grid-connected operation mode, the local load variation can
be compensated by the DGs in the microgrid to keep the power from the
main grid constant and thus the microgrid becomes a controllable load
from the utility view point.
ii) During islanded mode, the frequency should be maintained constant
due to the load variation.
iii) During transition mode, the microgrid frequency and DGs’ outputs are
changed due to the loss of power from the utility grid. In this mode, the
local frequency and DGs’ outputs should be maintained within
predetermined limits.
In addition, different control mode of DGs and different
configuration of microgrid will lead to different changes in the microgrid
frequency and DGs’ outputs. Therefore, a proper configuration as well as
control strategies should be achieved to satisfy the above requirements.
The contributions in this dissertation will deal with such problems and can
be summarized as follows:
1) Investigate the active power and frequency response in microgrid
2) Suggestion of a proper configuration for microgrid in terms of the
structure and the DGs’ control mode.
3) Proposed control strategies for a microgrid, in grid connected and
islanded mode, to
– Improve power quality
– Reduce the loss of power
– Enhance operation efficiency
4) Proposed control strategies for hybrid source in a microgrid to
CHAPTER I
6
– Enhance generation performance
– Enhance microgrid stability
– Operate hybrid source more economically (PV-MPPT…)
5) Microgrid operation with presence of hybrid source
1.3. Dissertation Structure
While the main subject of this dissertation is analysis of droop control
scheme for different microgrid configurations and control strategies for
DGs in the microgrid, it will be necessary to look in some details at the
issues surrounding the microgrid too. To this end, Chapter II will be
devoted to the microgrid concept and control including unit power control
mode, feeder flow control mode, and droop characteristics to control the
voltage and frequency as well as power output. The droop characteristics
and power sharing among DGs in the microgrid and their advantages,
disadvantages are investigated in Chapter III. In Chapter IV, a control
strategy for controllable DGs in microgrid is proposed to take the
advantages and overcome disadvantages of different configurations and
control modes pointed out in Chapter III. Chapter V deals with operation
and control issues of hybrid system when it connects to the microgrid, a
control strategy is also proposed. The operation schemes mentioned in
Chapter IV and V are independent therefore, the two operation strategies
are integrated to a multiple feeder microgrid and the overall simulation
results as well as discussions are presented in Chapter VI. Finally, the
conclusions and future extensions are devoted in Chapter VII. The
structure of the dissertation can be summarized as in Fig. 1.1.
INTRODUCTION
7
Fig. 1.1 Dissertation structure
Chapter 5: Control Strategy for a Hybrid Source Connected to Microgrid
Chapter 2: Microgrid Concept and Control
Chapter 3: Analysis of P and f Responses in Microgrid
Chapter 1: Introduction
Chapter 4: Control Strategy for Controllable DGs in Microgrid
Chapter 7: Conclusions and Future Extensions
Chapter 6: Case Study
CHAPTER I
8
Chapter II:
MICROGRID CONCEPT AND CONTROL
2.1. Overview
Fig. 2.1 Microgrid Architecture Diagram [15]
In the context of increasing in distributed generation sources including
renewable and CHP systems, the microgrid concept is an advanced
approach for adding value to distributed energy resources by aggregating
them into autonomous grids that provide high levels of efficiency, security
DG
A
DG
B
DG
C
DG
DStatic Switch
Sensitive Loads
Non Sensitive
Utility Grid PCC
CHAPTER II
10
and controllability. The Microgrid concept is also enable integration of an
unlimited quantity of DGs into the electricity. The concepts of microgrid
were firstly proposed by Robert H. Lasseter in the CERTS microgrid
project. Basic microgrid architecture is shown in Fig. 2.1. This consists of
a group of radial feeders, which could be part of a distribution system or
building’s electrical system. The feeders are connected to the utility grid at
a single point called point of common coupling (PCC) [4]. Some feeders
(Feeder A-C) have sensitive loads, which require local generation. The
non-critical load feeders do not have any local generation. Feeders A-C can
autonomously island from the utility grid using the static switch.
The CERTS microgrid has two critical components, the static switch
and the microsource. The static switch is able to autonomously island the
microgrid from disturbances such as faults and power quality event. During
the islanded mode, the sensitive loads are supported by the local generation.
The power can be seamlessly balanced by microsource using a power
versus frequency droop controller. When the disturbances are eliminated,
the reconnection of the microgrid is achieved autonomously by the static
switch. The synchronization is achieved by using the frequency difference
between the microgrid and the main grid to match frequency and phase
angles at the connection point [16].
2.2. Microgrid Management and Control
Microgrid controllers have responsibilities to ensure that [15]:
(1) Microsources work properly at predefined operating point of slightly
different from predefined operating point but still satisfy the
operating limits;
MICROGRID CONCEPT AND CONTROL
11
(2) Active and reactive powers are transferred according to necessity of
the microgrids and/or the distribution system;
(3) Disconnection and reconnection processes are conducted seamlessly;
(4) Market participation is optimized by optimizing production of local
microsources and power exchanges with the utility;
(5) Heat utilization for local installation is optimized;
(6) Sensitive loads, such as medical equipment and computer servers are
supplied uninterruptedly;
(7) In case of general failure, the microgrid is able to operate through
black-start; and
(8) Energy storage systems can support the microgrid and increase the
system reliability and efficiency.
Based on the above responsibilities and the controller coordination,
the microgrid controls can be classified as local controls, centralized
controls, and decentralized controls.
Local controls are the basic category of microgrid control. The
measured data for local controllers are local voltages and currents [4, 15].
Local controllers are aimed to control operating point of the microsources
and their power-electronic interfaces without communication systems, to
ensure peer-to-peer and plug-and-play function of microsources, to
seamlessly connect to or disconnect from the distribution network when
needed [17].
Centralized controls, that will be used in this dissertation, base on
hierarchical controls as shown in Fig. 2.2, including three levels of control,
Distribution Management System (DMS), Microgrid Central Controllers
CHAPTER II
12
Fig. 2.2 Microgrid Control Architecture
(MGCCs), and local controllers consisting of Microsource Controllers
(MCs) and Load Controllers (LCs) [18-20]. The MCs, presented in details
below, in centralized controls have similar principle as the Local
Controllers. LCs are installed at the controllable load and commonly used
for demand side management. The MGCC has main responsibility of
optimizing the microgrid operation, MCs and LCs follow the orders of
MGCC during grid-connected mode and have autonomy to perform their
own control during islanded mode [18]. DMS has responsibility to manage
MICROGRID CONCEPT AND CONTROL
13
the operation of medium and low voltage areas in which more than one
microgrid may exist.
Decentralized controls have similar description to the centralized
control and can be explained based on Fig. 2.2. The main responsibility of
the decentralized controls is given to the MCs that compete to maximize
their production in order to satisfy the demand and probably provide the
maximum possible export to the grid taking into account current market
prices [18, 21-23].
Fig. 2.3 Microgrid Energy Management System
In order to maximize the benefit of microgrid and minimize the
global energy cost, an Energy Management System (EMS) supported by a
communication infrastructure has to be built. The EMS uses the
information on the local electrical and heat demands, weather, electricity
RES Conventional generation
Controllable loads
Uninterruptible loads
EMS
Generation Demand
Batteries, Super capacitors, Flywheel
Energy Storage
Scheduling Energy Storage
Load ForecastingLoad
Planning Generation Forecasting
Optimization of Generation
CHAPTER II
14
price, fuel cost, power quality requirements, demand side management
requests, congestion levels, etc. The information exchanged between the
EMS and the local devices are summarized in Fig. 2.3. The key functions
of the EMS are as follows [24-26]:
- to provide the individual power and voltage set point for each
microsource controller;
- to insure that heat and electrical loads are met;
- to insure that the microgrid satisfies operational contracts with the
bulk system;
- to minimize emissions and system losses;
- to maximize the operational efficiency of the microsources;
- to provide logic and control for islanding and reconnecting the
microgrid during events.
2.3. Microsource Control
TABLE 2.1: MICROSOURCE TYPES AND TYPICAL CAPABILITY
Microsource Type Capability Range
Internal combustion engines 10kW~10MW Mini- to small- size combustion
turbines 0.5~50MW
Micro turbines 20~50MW Fuel Cells 1kW~10MW
Photovoltaic systems 5W~5MW Wind turbines 30W~10MW
MICROGRID CONCEPT AND CONTROL
15
Microsource controls need to insure that new microsources can be added to
the system without modification of existing equipment, set-points can be
independently chosen, the microgrid can connect to or isolate itself from
the grid in a rapid and seamless fashion, reactive and active power can be
independently controlled, and can meet the dynamic needs of the loads.
Each microsource controller must autonomously respond effectively to
system changes without requiring data from the loads, the static switch or
other sources [4, 6].
Microsource comprises a wide range of prime mover technologies,
such as internal combustion (IC) engines, gas turbines, micro turbines,
photovoltaic, fuel cells, and wind power. The suitable generation
technologies for microgrid are shown in Table 2.1 [27]. Among those DGs,
the renewable sources like PV and Wind turbine are uncontrollable sources
if tracking maximum power point. They can be coupled with other
controllable source as a hybrid system to improve the operational
performance and make them become controllable [28-32]. In this
dissertation those controllable sources will be coordinated with each other.
The stand-alone uncontrollable sources will be considered as negative
loads.
Basically, there are two classes of microsources; one is a DC source,
such as fuel cells, and battery storage, the other is a high frequency AC
source such as the micro turbine, which needs to be rectified. In both cases,
the resulting DC voltage is converted to an acceptable AC source using a
voltage source inverter [4]. In addition, most of microsource technologies
that can be installed in a microgrid are not suitable for direct connection to
CHAPTER II
16
the electrical network due to the characteristics of the energy produced.
Therefore, power electronic interfaces (DC/AC or AC/DC/AC) are
required. Inverter control is thus the main concern in microgrid operation
[7].
Generally, inverters can be classified as line-commutated and self-
commutated inverters as shown in Fig. 2.4 [33]. The line-commutated
inverters are not used in DG interfacing because it needs an extensive filter
due to low order harmonics, and it is not capable to operate in islanded
mode since it relies on the distribution system waveform for
communication. Most inverter interfaces for DG, however, utilize self-
Inverters
Self-Commutated Inverters
Voltage Source Inverters
Current Source Inverters
Current-Control Scheme
Voltage-Control Scheme
Line-Commutated Inverters
Fig. 2.4 Classification of Inverters
MICROGRID CONCEPT AND CONTROL
17
commutated inverter because the high switching frequency can be easily
filtered, and unlike the line-commutated inverter, the total distortion of the
self-commutated inverters does not increase in proportionality to the
number of installed units due to phase cancellations. The self-commutated
inverter can be divided into two main types based on nature of the DG link:
current source inverter (CSI) and voltage source inverter (VSI). The CSI is
equipped with a large inductor on the DC side and act as a current source.
On the other hand, the VSI is normally equipped with a large capacitor on
the DC side to act as a voltage source. VSIs are typically used in DG
interfacing since DGs resemble voltage sources more than current sources.
According to the above expressions, in this study, the VSIs will be used to
interface DGs with grid. The voltage source inverter-based microsource
diagram is shown in Fig. 2.5.
Inverter Controller
To
n XF
CF
X
Gate Pulses
VDC vabc(t)
eabc, iinv,abc
iabc
E
Filter InductorVSI
DC Storage
Micro-source
From Grid
Loca
l Fee
der
Fig. 2.5 Inverter-based Microsource Diagram
CHAPTER II
18
Fig. 2.5 shows a block diagram of the power-electronic-interface
microsource that is coupled to a microgrid through a voltage-source
inverter. The inverter is connected to a DC voltage generated by the
microsource and converts the input DC voltage to a three-phase voltage
with a desired frequency, voltage magnitude and phase angle at the output
terminals. If the generation at the microsource site is retrofitted into the
facility, a common transformer arrangement is delta-wye (grounded) [34-
36]. This arrangement is typically chosen to provide isolation for the utility
from ground faults in a microsource system’s facility, and to supply a
ground source for that facility. I does not cause an over voltage for a
ground fault in the microgrid site.
2.3.1 Power Control Modes
Unit Power Control Mode:
In the UPC mode, the DG output power is regulated at a constant level
(PDGref). In order to control the DG output, the voltage (V) at the
interconnection point and the current (I) injected by the DG are measured
as shown in Fig. 2.6(a). The active power injected by the DG (PDG) is
calculated from the measured voltage and current, and then fed back to the
inverter controller.
When the microgrid is connected to the main grid, DG regulates its
output to a constant power regardless of the load variation. If the load
demand is changed anywhere in the microgrid, the extra power will be
compensated by the main grid. On the other hand, when the microgrid
disconnects from the main grid with respect to the islanded mode, DGs
MICROGRID CONCEPT AND CONTROL
19
must follow the load demand accurately.
In numerous studies, a power versus frequency (P–f) droop control, that will be discussed later on, has been adopted for DG power-sharing methods [9, 12, 37-39]. This control uses the frequency of the microgrid as a common signal among the DGs to balance the active power generation of the system [12]. P–f droop-based power controllers have proven to be robust and adaptive to variation in the power system
(a)
(b)
Load
V
PDG
PDGref
Grid
DG
Static Switch
Controller
I
V
PDG
FLLineref
Grid
DG
Static Switch
IFeeder
FLLine
LoadController
Fig. 2.6 Power-control mode of a DG: (a) Unit output power control (UPC), (b) Feeder flow control
CHAPTER II
20
operational conditions, such as frequency- and/or voltage-dependent loads and system losses [12, 38].
Feeder Flow Control Mode:
The objective of the FFC mode is to control the active power flow in the
feeder where the unit is installed at a desired value (FLLineref). In this mode,
the DGs regulate the voltage magnitude at the connection point and the
power flow in the feeder at connection point (FLLine). The feeder current
(IFeeder) and voltage at the connection point (V) are measured in order to
calculate the power as shown in Fig. 2.6(b).
During the grid-connected mode, extra load demands are picked up
by the DGs, and power supplied from the main grid remains unchanged
regardless of the load variation within the microgrid. Thus the microgrid
looks like a controllable load from the utility view point. On the other hand,
in the islanded mode when the microgrid is disconnected from the main
grid, the feeder flow versus frequency (FL–f) droop characteristic is used
to share the load demand [4, 16].
Derivation of droop characteristics used in the FFC mode and the
UPC mode will be discussed below.
2.3.2 Droop Control through Active and Reactive Power
The power injecting to the feeder by the microsource, as represented in Fig.
2.7, is described as follows [40, 41]:
MICROGRID CONCEPT AND CONTROL
21
Fig. 2.7 Inverter-based system (a) Simplified Interface of Inverter System
(b) Phasor diagram.
( ) ( )
**
2
2.1
j
j
jj
V EP jQ S VI VZ
V EeVZe
V VEe eZ Z
δ
θ
θ δθ
−
+
⎛ ⎞−+ = = = ⎜ ⎟
⎝ ⎠⎛ ⎞−
= ⎜ ⎟⎝ ⎠
= −
Thus, the active and reactive powers flowing into line are
( ) ( )
( ) ( )
2
2
cos cos 2.2
sin sin 2.3
V VEPZ Z
V VEQZ Z
θ θ δ
θ θ δ
= − +
= − +
(a)
Inverter
V∠0 E∠-δ
Z=R+jX
P, Q
I∠-Φ
V
E
-δ
-φ
I RI
jXI
(b)
CHAPTER II
22
With jZ Ze R jXθ= = + , (2.2) and (2.3) are rewritten as
( ) ( )
( ) ( )
2 2
2 2
cos sin 2.4
sin cos 2.5
VP R V E XER X
VQ RE X V ER X
δ δ
δ δ
= − +⎡ ⎤⎣ ⎦+
= − + −⎡ ⎤⎣ ⎦+
Or
( )
( )
sin 2.6
cos 2.7
PX QRVE
PR QXV EV
δ
δ
−=
+− =
For overhead line X R , this means that R may be neglected. Equations
(2.4) and (2.5) then become
( )
( ) ( )
sin 2.8
cos 2.9
VEPXVQ V EX
δ
δ
=
= −
E is the voltage magnitude at the point of common coupling.
If the power angle δ is also small (less than 10 degrees [42]), then
sin and cos 1δ δ δ . Equations (2.6) and (2.7) then become
MICROGRID CONCEPT AND CONTROL
23
( )
( )
2.10
2.11
PXVE
QXV EV
δ
− =
Equation (2.10) and equation (2.11) show that the power angle depends
pre-dominantly on P, whereas the voltage difference depends pre-
dominantly on Q. In other words, the angle δ can be controlled by
regulating P, whereas the inverter voltage V is controllable through Q.
Control of the frequency dynamically controls the power angle and, thus,
the real power flow. As a result, by adjusting P and Q independently,
frequency and amplitude of the grid voltage are determined. In other words,
it is possible to independently control the real and reactive power. These
conclusions form the basis for well-known frequency and voltage droop
regulation through respectively active and reactive power:
( ) ( )( ) ( )
0 0
1 0 0
2.12
2.13P
q
f f k P P
E E k Q Q
− = − −
− = − −
f0 and E0 are rated frequency and grid voltage respectively, and P0 and Q0
are the (momentary) set points for active and reactive power of the inverter.
The droop characteristics shown in equation (2.12) and equation
(2.13) are for the conventional power system where the active power
relates with the frequency and reactive power relates with the voltage. On
the contrary, the medium voltage line has mixed parameters and the low
voltage line is even predominantly resistive as shown in Table 2.2 [43].
CHAPTER II
24
The characteristic of low voltage line causes reversed droop characteristics
where the active power is related with the voltage and the reactive power is
related with the frequency [43, 44]. However, it is also stated in [43] that
the droops used in the conventional grid can be effectively used in the low
voltage level due to their “indirect operation”. Therefore, the control
strategy of conventional grid can be down scaled to the low voltage level
without any restrictions.
TABLE 2.2:
TYPICAL LINE PARAMETERS
Type of line R Ω/km
X Ω/km
R X
Low voltage line 0.642 0.083 7.7
Medium voltage line 0.161 0.190 0.85
High voltage line 0.06 0.191 0.31
Reactive Power (Q) versus Voltage (V) Droop:
Integration of large number of microsources into a microgrid could
experience voltage and/or reactive power oscillations. Therefore, instead of
basic unity power factor controls, voltage regulation is necessary for local
reliability and stability [6, 42].
Voltage control must insure that there are no large circulating
reactive currents between sources. Unlike a large power system, the
impedance between microsources in a microgrid is small. With small errors
in voltage set points, the circulating current can exceed the ratings of the
MICROGRID CONCEPT AND CONTROL
25
microsources. This can be prevented by a voltage versus reactive power
droop control, Fig. 2.8, which is described in equation (2.13).
Fig. 2.8 Voltage vs. Reactive Power Droop
Fig. 2.8 illustrates the basic function of a controller that operates
based on the voltage versus reactive power droop. When the reactive power
generated by the microsource becomes more capacitive, the local voltage
set point is reduced. Conversely, as Q becomes more inductive, the voltage
set point is increased. The reactive power limit Qmax is a function of the
volt-ampere VA rating of the inverter and the real power of the prime
mover P as follows:
( ) ( )22 2max 2.14Q VA P= −
Unit Power (P) versus Frequency (f) Droop:
When the microgrid is connected to the utility grid, loads receive power
both from the grid and from local microsources, depending on the
Qmax -Qmax
Qcapacitive Qinductive
V
CHAPTER II
26
customer’s situation. With loss of the grid due to voltage drops, faults,
blackouts etc. the microgrid smoothly transfer to island operation.
When regulating the output power, corresponding to UPC mode, each
microsource has a constant negative slope droop on the P, f plane. With the
disconnection from the grid, the microsources will participate in sharing
the local loads according to P versus f droop characteristic as in equation
(2.12). The droop characteristics described in equation (2.12) can be
rewritten as follows:
( ) ( )0 0 2.15Uf f K P P− = − −
Where, KU is droop constant of the UPC-mode DG.
The control diagram of equation (2.15) is expressed in Fig. 2.9. The
error of active power is then sent to the PI controller to control the output
power to the predetermined value, P0.
Consider two microsources with the power set points P01, P02 for two
units, as shown in Fig. 2.10. This is amount of power injected by each
source when connected to the utility grid, at system frequency.
f0 -1/K
PI
f P0
P
+ _
_ +
+
Fig. 2.9 Diagram of P vs. f Droop control block
MICROGRID CONCEPT AND CONTROL
27
Fig. 2.10 Power vs. Frequency Droop
If the system transfers to island when importing power from the grid,
then the generation needs to increase power to balance power in the island.
The new operating point will be at a frequency that is lower than the
nominal value. In this case both sources have increased their power output
(P1’ and P2’). If the system transfer to island when exporting power to the
grid, then the new frequency will be higher, corresponding to a lower
power output from the sources (P1” and P2”).
The characteristics shown in Fig. 2.10 are steady state characteristics.
Each source has a fixed slope in the region where the unit is operating
within its power range. The slope becomes vertical as soon as any limit is
reached. The droop is the locus where the steady state points are
constrained to come to rest, but during dynamics the trajectory will deviate
from the characteristic.
f
P
fo
Exporting to Grid
Importing from Grid
P1’
f ’
f”
P2’P1” P2”
P01 P02
CHAPTER II
28
Feeder Flow (FL) versus Frequency (f) Droop:
When regulating the feeder flow FL, the FL vs. f can be derived from (2.12)
as follows:
( ) ( )0 0 2.16Ff f K FL FL− = −
Where KF is the FFC droop constant.
The control diagram of equation (2.16) is expressed in Fig. 2.11. The
error of feeder flow is then sent to the PI controller to regulate the feeder
flow to the predetermined value, FL0.
When regulating FL the relative location of loads and source is
important [6]. Fig. 2.12 shows two possible microgrid configurations,
series and parallel. Fig. 2.13 shows the set points FL01 and FL02 for the two
units when connected to the utility system.
f0 1/KF PI
f FL0
FL
+ _
_ +
+
Fig. 2.11 Diagram of FL vs. f Droop control block
MICROGRID CONCEPT AND CONTROL
29
Fig. 2.13 Feeder Flow vs. Frequency
f
fo
Exporting to Grid
Importing from Grid
f ’
f” FL01 FL02
FL
LD
FL1
Utility Grid
LD
FL2
LD
FL1
Utility Grid
LD
FL2
(a) Series Configuration
(b) Parallel Configuration
DG1 DG2
DG1
DG2
Fig. 2.12 Microgrid Configuration with Two Sources
CHAPTER II
30
When in series configuration, FL01 is the grid flow. The microgrid is
exporting power to the grid, since flow is negative. When the system
transfers to the island, the flow reaches zero and the frequency increases
(squares).
In the parallel configuration, the grid flow is the algebraic sum of the
two flows. Since |FL02|>|FL01| the microgrid is importing power from the
grid. Fig. 2.13 shows that in island mode FL01=-FL02 and the frequency is
reduced (triangles).
In both the unit power control mode and the feeder flow control
mode DG output is limited to its generation band by using the control
block shown in Fig. 2.14.
f0 -1/K
PI
f P0
P
+ _
_ +
+
Fig. 2.14 Control block diagram to enforce the output limit
Pmax +
_
Kis
0
Pmin+
Kis
0
P
_
+
+ +
errPmax
errPmin
Chapter III:
ANALYSIS OF ACTIVE POWER AND FREQUENCY
RESPONSE IN MICROGRID
3.1. Overview
This considering multiple-DG microgrid connects to the main grid at a
point of common coupling (PCC) via a static switch (SS). DGs in the
microgrid can connect to the feeder either in series or parallel. Each DG
can also operate in unit power control (UPC) mode or feeder flow control
(FFC) mode. In this chapter responses active power and frequency in the
microgrid with different configurations (parallel/series), different control
modes (UPC/FFC), and different operation modes (grid-
connected/islanded) is investigated. Four possible configurations, as shown
in Fig. 3.1, are considered. The analysis shows the power sharing among
DGs in the microgrid according to the droop characteristic as well as the
frequency changes according to the load change and mode change.
Additionally, advantages and disadvantages of each configuration are
achieved.
The DGs connect to the microgrid in series or parallel and form the
microgrid in different configurations. Four possible configurations that will
be investigated, as shown in Fig. 3.1, are:
- Configuration 1: Series-FFC, the DGs are connected in series and the
DGs’ control modes are FFC.
CHAPTER III
32
Fig. 3.1. Different configurations of microgrid: (a) Configuration 1: Series –
FFC, (b) Configuration 2: Series – UPC, (c) Configuration 3: Parallel – FFC, (d)
Configuration 4: Parallel – UPC
- Configuration 2: Series-UPC, the DGs are connected in series and
DGs’ control modes are UPC.
FL1
FFC
…
FLPCC
P1
(c)
1
Main Grid
LD1
FL2
FFC
P2
2 LD2
FLn
FFC
Pn
n LDn
FL1
UP
…
FLPCC
P1
(d)
1
Main Grid
LD1
FL2
UP
P2
2LD2
FLn
UP
Pn
nLDn
(a) (b)
1
LD1FLPCC
LD2
FFC
FL2 FLn
UPC UPC UPC
LDn
MainGrid
LD1FLPCC
LD2
FL2 FLn
LDn
2
FFC
n
FFC
1 2 n Main Grid
ANALYSIS OF ACTIVE POWER AND FREQUENCY RESPONSE IN MICROGRID
33
- Configuration 3: Parallel-FFC, the DGs are connected in parallel and
DGs’ control modes are FFC.
- Configuration 4: Parallel-UPC, the DGs are connected in parallel and
DGs’ control modes are UPC.
The two control modes, UPC and FFC, as well as droop
characteristics were basically presented in chapter II.
3.2. Active Power and Frequency Response in a Microgrid
Both UPC mode and FFC mode were investigated in [4, 9, 12, 16, 31, 32,
37-39]. The active power and frequency responses were also presented in
[4, 6, 42] where the two-DG microgrid was mentioned. In addition, the
change in frequency and active power was investigated in three different
conditions, condition of load change during the grid-connected mode,
disconnection from the main grid, and load change during the islanded
mode. A comparison was also achieved according to the analysis [45].
However, the references only consider the active power and frequency
responses in case of either one- or two-DG microgrid. In analysis of [45],
the droop coefficient of UPC mode (|KU|) and FFC mode (|KF|) are chosen
the same. Therefore, the conclusion was then applicable to particular cases.
In this section, an analysis of frequency and power changes for
multiple-DG microgrid is investigated with arbitrary droop coefficients
[46]. The four possible configuration of microgrid, Fig. 3.1, considered.
The analysis is based on three typical conditions: (A) Disconnection from
the main grid, (B) Load change during grid-connected mode, and (C) Load
change during the islanded mode.
CHAPTER III
34
3.2.1 Active power and frequency response during transition mode
In this section the change of active power and frequency when the
microgrid disconnects from the main grid is considered. Before the
disconnection, the system frequency is equal to nominal value, DGs’ output
is Pi0 and feeder flow is FLi
0, i=1÷n. When the microgird disconnects from
the utility grid, the loss of power from/to the main grid may cause the
change in the local frequency and DGs’ outputs. The change of frequency
and active power are changed according to the droop characteristic.
Configuration 1: Series–FFC
Fig. 3.2. FL vs. f characteristic in transition mode for series–FFC
configuration
In this configuration all DGs work in the FFC mode, Fig. 3.1(a). The
feeder flow vs. frequency droops of DGs are depicted in Fig. 3.2. The
change in feeder flow, frequency and thus the DGs’ output depends on the
droop characteristic and initial conditions. The DGs are connected in series
therefore the first feeder flow FL10 is the power from/to the main grid.
f 1
f 0
FL11=0 FL1
0 FL21
f
FLn1 FLn
0 FL20
DG1 DG2 DGn
f f
ANALYSIS OF ACTIVE POWER AND FREQUENCY RESPONSE IN MICROGRID
35
After the disconnection, FL1 become zero due to non-power from the main
grid then we have:
( )1 11 0 3.1PCCFL FL= =
From (2.16) and (3.1) we have:
( )
( )
0 1 01
22
* 3.2
,..., 3.3
PCC
nn
f f f KF FLf fFL FL
KF KF
Δ = − =
Δ ΔΔ = − Δ = −
And the changes in DGs’ output are as follows:
( )1 1
1 1 2
3.4...
n n
n n n
P FLP FL FL
P FL FL
− −
Δ = −Δ⎧⎪Δ = −Δ + Δ⎪⎨⎪⎪Δ = −Δ + Δ⎩
From (3.2)-(3.4) we have:
( )
01
1
*3.51 1
PCC
ii i
f KF FL
P fKF KF+
⎧Δ =⎪
⎛ ⎞⎨Δ = Δ −⎜ ⎟⎪
⎝ ⎠⎩
Equation (3.5) shows the frequency and DGs’ output change when the
microgrid disconnects from the main grid for configuration 1.
Configuration 2: Series–UPC
CHAPTER III
36
Fig. 3.3. P vs. f characteristic in transition mode for series–UPC
configuration
In this configuration all DGs work in the UPC mode, Fig. 3.1(b). In grid-
connected mode the load change is compensated by the main grid, the DGs
regulate the output power to the reference value. When microgrid is
isolated, all the DGs participate in sharing the load according to the active
power vs. frequency droop characteristic as shown in Fig. 3.3. After the
disconnection, the total power change of DGs equal to the loss of power
from the main grid, therefore:
( )0
1
3.6n
PCC ii
FL P=
= Δ∑
From (2.15) and (3.6) we also have:
f 1 f 0
P10 P1
1 P20
f f
P21
DG1 DG2 DGn Pn
0
f
Pn1
ANALYSIS OF ACTIVE POWER AND FREQUENCY RESPONSE IN MICROGRID
37
( )
0
1
13.7
PCCn
i i
Ui
i
FLf
KUfP
KU
=
⎧Δ = −⎪⎪⎪⎨⎪ Δ⎪Δ = −⎪⎩
∑
Configuration 3: Parallel–FFC
Fig. 3.4. FL vs. f characteristic in the transition mode for the parallel-FFC
configuration.
In this configuration, the DGs are connected in parallel, all DGs work in
FFC mode as depicted in Fig. 3.1(c). Before the disconnection,
( )0 0 0 01 2 ... 3.8n PCCFL FL FL FL+ + + =
Unlike configuration 1, after disconnection the FL11 is not equal to zero,
however, the sum of all feeder flow is equal to zero:
( )1 2 ... 0 3.9nFL FL FL+ + + =
f 2 f 1
FL10FL1
1 FL21
f f
FLn1
f
FLn0 FL2
0
DG1 DG2 DGn
CHAPTER III
38
From (2.16) we can also have:
( )
01 1
1
02 2
2
0
3.10...
n nn
fFL FLKF
fFL FLKF
fFL FLKF
Δ⎧ = −⎪⎪
Δ⎪= −⎪
⎨⎪⎪
Δ⎪ = −⎪⎩
From (3.8), (3.9) and (3.10) we have:
( )
0
1
11
3.11
PCC n
i i
ii
f FL
KFfP
KF
=
⎧Δ =⎪⎪⎪⎨⎪ ΔΔ =⎪⎪⎩
∑
Configuration 4: Parallel–UPC
In this parallel configuration, all DGs work in UPC mode. If the
disconnection from the main grid happens, the DGs output will change
according to the droop characteristic shown in Fig. 3.5 to meet the load
demands. From droop characteristic will also have:
ANALYSIS OF ACTIVE POWER AND FREQUENCY RESPONSE IN MICROGRID
39
Fig. 3.5. FL vs. f characteristic in transition mode for parallel-UPC
configuration
( )
01 1 1
1
02 2 2
2
0
1
13.12
...1
n n nn
P P P fKU
P P P fKU
P P P fKU
⎧Δ = − = − Δ⎪⎪⎪Δ = − = − Δ⎪⎨⎪⎪⎪Δ = − = − Δ⎪⎩
( )01 2 ... 3.13n PCCP P P FLΔ + Δ + + Δ =
From (3.12) and (3.13) we have:
( )
0
1
13.14
PCCn
i i
ii
FLf
KUfP
KU
=
⎧Δ = −⎪⎪⎪⎨⎪ Δ⎪Δ = −⎪⎩
∑
f 1
f 0
P10 P1
1 P20
f f
P21
DG1 DG2 DGn Pn
0
f
Pn1
CHAPTER III
40
The above four configurations have been analyzed in term of the frequency
and active power change due to the disconnection from the main grid.
From (3.5), (3.7), (3.11) and (3.14) the change of frequency and active
power can be summarized as shown in Table 3.1.
TABLE 3.1:
FREQUENCY AND ACTIVE POWER CHANGE IN TRANSITION MODE
Δf ΔP
Config. 1 01 * PCCf KU FLΔ = −
1
1 1i
i i
P fKU KU +
⎛ ⎞Δ = −Δ −⎜ ⎟
⎝ ⎠
Config. 2 &
Config. 4
0
1
1PCC
n
i i
FLf
KU=
Δ = −
∑ 1
ii
P fKU
Δ = −Δ
Config. 3
0
1
1PCC
n
i i
FLf
KU=
Δ =
∑ 1
ii
P fKU
Δ = −Δ
3.2.2 Active power and frequency response during grid-connected
operation mode
The frequency remains unchanged in all cases during grid-connected
operation mode.
Configuration 1: Series–FFC
If the load changes, the DG output will change to match the load and
regulate the feeder flow unchanged. The response of DG output starts from
ANALYSIS OF ACTIVE POWER AND FREQUENCY RESPONSE IN MICROGRID
41
the nearest DG to the furthest DG (DG1) from the load. For example, when
load demand LDi increases, DGi increase its output until its maximum,
then the next DG (DGi-1) increases until its limit, and so on. If DG1 output
reaches its maximum, the variation in load will be matched by the main
grid, hence the feeder flow will be changed and the microgrid is no longer
a constant load from the main grid view point.
Configuration 2: Series–UPC
All DGs are in the UPC control mode therefore their output powers are
regulated to be unchanged. As a result, the power comes from/to the main
grid will change according to the load variations and the microgrid is a
non-dispatchable entity from the utility point of view.
Configuration 3: Parallel–FFC
Every change in load demand is picked up by the DG in the same feeder.
Once the DG output reaches to its limit, the rest of load variation will be
matched by the main grid, thus the feeder flow at PCC is non-constant and
the microgrid is no longer a constant load from the utility view point. In
this configuration, the change in load of each feeder does not affect to
other feeders operation. In brief, the feeder flow at PCC remains
unchanged until one of the DGs reaches to their limit.
Configuration 4: Parallel–UPC
The power sharing in this configuration is same to the configuration 2, all
DGs’ output powers are regulated unchanged and the load variations are
matched by the main grid. Therefore, the feeder flow at PCC point is
CHAPTER III
42
always changed according to the load change and the microgrid becomes a
non-constant load from the main grid view point.
3.2.3 Active power and frequency response in islanded operation mode
In this section, the islanded operation mode is considered to analyze the
frequency and active power responses. The four configurations will also be
investigated as occurring load variations.
In islanded mode, there is no power from the main grid therefore all
DGs have to participate in sharing the load demand. If the load changes,
each of the different configurations will lead to the different changes in
frequency and DGs’ output.
Configuration 1: Series-FFC
Fig. 3.6. FL vs. f characteristic in islanded mode for series-FFC
configuration when load LDi increases
f(1)
0
f(2)
FLi(1,2)
f f
FLn(1) FLn
(2)
f
DG1 DG2÷i DG(i+1)÷n
(1)
(2)
(1)
(2)
(1): LDi increases & DG1 does not reaches to its limit P1<P1max
(2): P1=P1max, LDi keep increasing ΔLDi
Δf
ANALYSIS OF ACTIVE POWER AND FREQUENCY RESPONSE IN MICROGRID
43
In this configuration, if load demand increases/decreases, the nearest
DG will increase/decrease its output to match the load change and regulate
the feeder flow unchanged as long as the nearest DG’s output reaches its
limit, then the next DG will compensate the load change. In other words,
the load change is matched by from the nearest DG to the furthest DG
(DG1). If the DG1 reaches its limit, the power will be shared by the
remained DGs, as a result, the frequency and feeder flow will change
according to the FL-f droop characteristic as shown in Fig. 3.6.
When the frequency starts to decrease from Fig. 3.6 we have:
( )1 2
11
... 03.15
,...,
i
i ni n
FL FL FLf fFL FL
KF KF++
Δ = Δ = = Δ =⎧⎪
Δ Δ⎨Δ = − Δ = −⎪⎩
( )
1 1
1 1 2
1
...3.16
0...
0
n n
n n n
i i i
i
P FLP FL FL
P FL FLP
P
− −
+ + +
Δ = −Δ⎧⎪Δ = −Δ + Δ⎪⎪⎪Δ = −Δ + Δ⎨⎪Δ =⎪⎪⎪Δ =⎩
Equations (3.15) and equation (3.16) deduce:
CHAPTER III
44
( )1 1
1 3.17n
kk i
P fKF= +
Δ =Δ∑
On the other hand,
( )1
3.18n
k ik
P LD=
Δ =Δ∑
From (3.17) and equation (3.18) we have:
( )1 3.19i if KF LD+Δ = Δ
( )
( )( )
1
0; 1...3.201 1 ; 1...
k
kk k
P k i
P f k i nKF KF +
Δ = =⎧⎪
⎛ ⎞⎨Δ = Δ − = +⎜ ⎟⎪
⎝ ⎠⎩
Configuration 2 & 4: Series-UPC & Parallel-UPC
The frequency and power responses in both configuration 2 (series-UPC)
and configuration 4 (parallel-UPC) are the same for islanding operation
mode. All DGs work in UPC mode therefore all DGs participate in sharing
the load demand according to P vs. f droop characteristic, Fig. 11.
ANALYSIS OF ACTIVE POWER AND FREQUENCY RESPONSE IN MICROGRID
45
Fig. 3.7. P vs. f characteristic in islanded mode for series-UPC
configuration and parallel-UPC configuration
The frequency and DGs’ output is changed whenever the load changes.
According to the droop characteristic shown in Fig. 3.7 we have:
( )1
3.21n
ii
LD P=
Δ = Δ∑
( )0 3.22Ui i i
i
fP P PKUΔ
Δ = − = −
Equations (3.21) and equation (3.22) deduce:
( )
1
3.231n
i i
LDf
KU=
ΔΔ = −
∑
f (2)
f (1)
P1(1) Pi
(1)
f f
Pi(2)
(DG1) (DGi) (DGn)
Pn(1)
f
Pn(2) P1
(2)
CHAPTER III
46
Configuration 3: Parallel-FFC
Fig. 3.8. FL versus f characteristic in islanded mode for parallel-FFC
configuration
In this configuration, all DGs play the same role to regulate its own feeder
flow unchanged as long as its output power does not reach the limit. If the
DG’s output reaches its limit, the remained DGs will participate in sharing
the load.
Fig. 3.8 shows the droop characteristic of all DGs when load change
in LD1. Any change in load demand LD1, the load variation is picked up
by DG1 first therefore FL1 and frequency remain constant, corresponding
to f(1). If DG1 output reaches its limit, the load will be shared by remained
DGs (DG2÷DGn) thus the frequency and feeder flow will be changed
according to FL vs. f droop characteristic, corresponding to f(2). As seen in
Fig. 3.8, the frequency is determined by the droop characteristics of other
DGs except DG1. Therefore we have:
f(1)
0
f(2)
FL21FL2
2
f f
FLn1 FLn
2
f
(DG1) (DG2) (DGn)
FL12FL1
1
ANALYSIS OF ACTIVE POWER AND FREQUENCY RESPONSE IN MICROGRID
47
( )
( )
1 1
1 2
3.24, 2...
... 0 3.25
ii
n
FL LDfFL i n
KF
FL FL FL
Δ = Δ⎧⎪
Δ⎨Δ = − =⎪⎩Δ + Δ + + Δ =
We also have:
( )3.26i ii
fP FLKFΔ
Δ = −Δ =
From equations (3.24), (3.25) and (3.26) we have:
( )
( )
1
1
3.271
3.28
n
i i
ii
LDf
KF
fPKF
=
ΔΔ =
ΔΔ =
∑
From the analysis of four configurations in islanded operation mode
we can also summarize the change of frequency and DGs’ output as shown
in Table 3.2.
It is noted that, in configuration 2 and configuration 4 any change in
load will lead to the change in frequency. However, in configuration 3, the
frequency only changes if one of the DGs output reaches its limit. Besides,
in configuration 1, the frequency will change if the outputs of all the DGs
(DG1÷DGi) in front of the load change (LDi) reach their limit. Therefore,
CHAPTER III
48
the reserve before the change in frequency in configuration 1 is higher than
other configurations.
3.2.4 Comparison of the Different Microgrid Configurations
TABLE 3.2:
FREQUENCY AND ACTIVE POWER CHANGE IN ISLANDED MODE
Δf ΔP
Config. 1 1i if KF LD+Δ = Δ
0; 1...kP k iΔ = =
1
1 1 ;
1...
kk k
P fKF KF
k i n+
⎛ ⎞Δ = Δ −⎜ ⎟
⎝ ⎠= +
Config.
2 & 4 1
1n
i i
LDf
KU=
ΔΔ = −
∑
Ui
i
fPKUΔ
Δ = −
Config. 3 1
1
1n
i i
LDf
KF=
ΔΔ =
∑
ii
fPKFΔ
Δ =
According to the above analysis the comparison of both cases are presented
in Table 3.3 and simplified in Table 3.4, where the notation “X” means less
advantage or negative and notation “√” means more advantage or positive.
Table 3.4 shows that configuration 1 is more advantage than other
configurations in grid-connected mode and islanded mode. However, in
ANALYSIS OF ACTIVE POWER AND FREQUENCY RESPONSE IN MICROGRID
49
transition mode this configuration has more drawbacks in terms of
frequency and DGs’ output changes. Besides, if the DGs operate in the
UPC mode with respect to configuration 2 and configuration 4, it shows
negative performance comparing to other configurations in both grid-
connected mode and islanded mode. Table 3.4 shows that the configuration
3 has more advantage over other configurations except the reserved power
in the islanded mode. The comparison also shows that the FFC mode
presents a better performance in both cases parallel and series
configuration.
From the analysis it is suggested that the first DG of each feeder
should work in FFC mode to regulate the microgrid as a constant load from
the utility viewpoint.
In this chapter, the microgrid with four possible configurations is
considered. The changes of the frequency and active power of each
configuration due to the different control mode of DG (UPC/FFC) and
different operation modes (grid-connected / islanded / transition) were
investigated. The study shows that: 1) in both parallel and series
configurations, the FFC mode has more advantages over the UPC mode in
terms of frequency change and active power reserve; 2) in islanded
operation mode, the configuration of series–FFC has more reserve to
regulate frequency unchanged and therefore it has more advantage over
other configurations. Otherwise, in grid-connected mode and transition
mode, the parallel–FFC configuration is better.
Simulation results show the active power and frequency responses in
islanded mode of a three-DG microgrid are shown in Appendix A.
CHAPTER III
50
ANALYSIS OF ACTIVE POWER AND FREQUENCY RESPONSE IN MICROGRID
51
TABLE 3.4:
SIMPLIFIED COMPARISON OF FOUR CONFIGURATIONS OF MICROGRID
Configuration Config. 1 Config. 2&4
Config. 3
Transition mode
Δf x √ √ ΔP x √ √
Grid-Connected
f √ √ √ FL √ x x
Islanded f √ x x
reserve √ x x
Each configuration has advantages and disadvantages as well.
According to these results, a control strategy is proposed in Chapter IV to
overcome the disadvantages and inherit the advantages as well.
CHAPTER III
52
Chapter IV:
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
4.1. Overview
As mentioned earlier, the microgrid can either connect to the main grid or
work autonomously with respect to the grid-connected operation mode or
the islanded operation mode, respectively. In the grid-connected operation
mode, the microgrid is connected to the main grid at the point of common
coupling (PCC) to deliver power to the load. During the transition mode
(from the grid-connected mode to the islanded mode or vice versa) the
frequency changes due to the loss of power from/to the main grid. Thereby,
in the grid-connected mode, the higher the power that comes from/to the
main grid, the larger the frequency changes that occurs due to
disconnection or reconnection. Additionally in some cases, e.g. the contract
between the utility operator and distributor, the microgrid needs to
minimize the change of power flow between the microgrid and the main
grid [7, 47]. To achieve this goal, a multiple-FFC configuration and related
power sharing method was proposed in [6, 45], in which the first DG that
is connected to PCC was always operated in the FFC mode to regulate the
power from/to the main grid to remain unchanged. However, this
conventional method has limitations in terms that, if the first FFC-mode
DG output reaches its limit, the variation in load will be matched by power
coming from the main grid and hence the feeder flow no longer remains
CHAPTER IV
54
unchanged.
In order to overcome the above limitations and the disadvantages
indicated in Chapter III, as well as to take over the advantages of each
configuration, a control strategy is proposed in this chapter to operate the
microgrid based on the two control modes of DG, the UPC mode, the FFC
mode, and droop characteristic. The mixed configuration with the first DGs
in the FFC mode is used hereafter to derive the control scheme.
4.2. Control strategy for Microgrid in the Grid-connected Mode
4.2.1. Control Strategy for Single-Feeder Microgrid
Fig. 4.1. Grid-connected Microgrid with Multiple DGs
This section presents a method to operate DGs in the microgrid as depicted
in Fig. 4.1. The studied microgrid includes small scale DGs which are in
UPC- or FFC-mode, and the control mode of the first DG (connecting to
MainGrid
LD1 LD4FLPCC
P2
LD2 LD3
FFC FFCUPC UPCFLPCC
ref P20 P4
0FL3ref
LD1max LD4
maxLD2max LD3
max
P1 P3 P4
1 2 3 4
P1max P3
maxP2max P4
max
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
55
the PPC) is the FFC. Before discussing the proposed power sharing method,
the key points of the conventional approach is stated.
a. Conventional Control Strategy
In general, the power generated by each DG is determined by its control
mode (UPC/FFC) and the droop characteristic [6, 16, 45]. In the grid-
connected operation mode, the features of the microgrid can be
summarized as follows:
- The frequency is equal to the main grid frequency (e.g. 60Hz).
- UPC mode DGs output remains unchanged.
- Any variation in load is matched by the FFC-mode DGs and by the
power that coming from the main grid.
- The first DG (DG1) regulates the feeder flow at the PCC point
(FLPCC) to a constant by changing its output to match the load
demand. FLPCC remains constant, and thus the microgrid can be a
constant load from the main grid view point until the DG1 output
reaches its limits. If the variation of loads exceeds the capacity of the
DG1, the microgrid becomes a non-constant load from the main grid
view point.
In the system depicted in Fig. 4.1, DG1 and DG3 are in the FFC
mode whereas DG2 and DG4 are in the UPC mode. If the load demand
LD3 and/or LD4 increases, DG3 output will increase in order to
compensate the extra power, whereas the UPC-mode DGs (DG2 and DG4)
output remains unchanged. If DG3 output reaches its maximum, DG1 will
increase its output to supply the load instead of DG3 and regulate FLPCC to
CHAPTER IV
56
a constant (i.e. FLPCC0). If DG1 output reaches its maximum, the extra
power will be provided by the main grid and hence the feeder flow at the
PCC point FLPCC will be increased. Therefore, the microgrid is no longer a
constant load from the utility view point.
If the main grid power is lost because of IEEE 1547 events, e.g.
voltage sags, faults, blackout, etc., the microgrid can autonomously
transfer to island operation [48]. In the islanded mode, the microgrid
frequency is controlled by the DGs based on the power versus frequency
(P-f) droop characteristic. If the system transfers to island when importing
power from the grid, then the DGs needs to increase the power output to
balance the power in the island. The new operating point will be at a
frequency that is lower than the nominal value. On the contrary, if the
system transfers to island when exporting power to the grid, then the new
frequency will be higher than the nominal value. The feeder flow vs.
frequency droop characteristic for FFC-mode DGs is shown in Fig. 4.2.
DG1 and DG3 are in the FFC mode and the feeder flow vs. frequency
droops for DG1 and DG3 are shown in Fig. 3a and Fig. 3b, respectively. In
the grid-connected operation mode, the microgrid frequency is equal to the
main grid frequency of (f0), the DG1 feeder flow is the power coming from
the main grid FLPCC0. If the operation mode changes to the islanded mode,
the frequency of the microgrid is changed to f ' due to the loss of power
from/to the main grid (FLPCC becomes zero). The frequency change of the
microgrid during the transition mode can be calculated as shown in
equation (2.16):
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
57
( )0 ' 01 * 4.1F
PCCf f f K FLΔ = − =
Where, K1F is the droop constant of DG1. Equation (4.1) shows that the
frequency deviation depends on grid flow (FLPCC0). Therefore, the change
of frequency in the transition mode can be out of its limits if |FLPCC0| is
high enough.
From the above expression it is seen that, in the conventional power
sharing method, the microgrid can be a non-constant load from the utility
view point and the frequency change during a transition mode can be out of
its limits.
Fig. 4.2. Feeder Flow vs. Frequency Droop (DG1 & DG3: FFC mode).
b. Proposed Control Strategy
From the above analysis, it is clear that the conventional power sharing
method has limitations, especially during heavy or light load conditions, in
f 0
FL10=FLPCC
0
f
FL30 FL3’
(a) DG1 droop (b) DG3FL1
’=FLPCC’=0
f
Grid-connected mode Islanded mode
f
0
CHAPTER IV
58
terms that the microgrid can be a non-constant load from the utility view
point and the frequency change during the transition mode can be out of its
limits. To overcome such disadvantages, a new power sharing method is
proposed in which a proper feeder flow reference is determined and the
algorithm of changing the DG control mode is presented.
In order for the microgrid to be a constant load from the main grid
view point regardless of the load conditions, the reference feeder flow of
the DG1 FLPCCref must be increased high enough. However, if the feeder
flow at the PCC point is very high, the microgrid frequency can be beyond
its limits during the transition mode. The higher the feeder flow at the PCC
(FLPCC0), the larger the change of frequency during transition mode.
Therefore, to minimize the change of frequency in the transition mode, the
reference value of DG1 feeder flow should be minimized. The proposed
power sharing method will deal with those two conflicting objectives, and
overcome the disadvantages of the conventional method. In a microgrid the UPC-mode DGs output power is regulated to a
constant, the FFC-mode DGs compensate for any change in load demand
and maintain the feeder flow to be unchanged and equal to a reference
power. The feeder flow at the PCC FLPCC is calculated as follows:
( )0 4.2PCC i ji FFC j UPC
FL LD P P∈ ∈
⎛ ⎞= − +⎜ ⎟
⎝ ⎠∑ ∑
Where:
1
n
ii
LD LD=
=∑ : Microgrid load demand
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
59
iP : The output of the ith DG which is in the FFC mode.
0jP : The reference output power of the jth DG which is in the
UPC mode.
The first DG is in the FFC mode, and the feeder flow will be equal to
the reference value (i.e., FLPCC = FLPCCref), if the FFC-mode DGs outputs
Pi are within their limits. Otherwise, the extra power is compensated by the
main grid, and hence the FLPCC becomes non-constant. In order for FLPCC
to remain unchanged, the feeder flow reference (FLPCCref) must be high
enough so that the outputs of FFC-mode DGs do not reach their limits at
the peak load. Therefore, from equation (4.2) we have:
( )max max 0 4.3refPCC i j
i FFC j UPCFL LD P P
∈ ∈
⎛ ⎞≥ − +⎜ ⎟
⎝ ⎠∑ ∑
Where:
max max
1
n
ii
LD LD=
=∑
Pimax : The maximum capacity of the ith DG
If FLPCCref satisfies the inequality (4.3) the feeder flow will not
change even if the load reaches its peak value.
Meanwhile, it is seen from equation (4.1) that the change of
frequency (Δf), during the transition mode, is directly proportional to
FLPCC. Thus, in order to reduce the frequency variation during the
transition mode, FLPCCref should be minimized while satisfying equation
CHAPTER IV
60
(4.3), as follows:
( )max max 0 4.4refPCC i j
i FFC j UPCFL LD P P
∈ ∈
⎛ ⎞= − +⎜ ⎟
⎝ ⎠∑ ∑
In equation (4.4), FLPCCref can be reduced further by increasing the power
output reference of UPC-mode DGs (i.e., Pj0). If the power references of
UPC-mode DGs are set to their maximum (Pjmax), FLPCC
ref can be
minimized:
( ) ( )max max maxmin 4.5refPCC i j
i FFC j UPC
FL LD P P∈ ∈
⎛ ⎞= − +⎜ ⎟
⎝ ⎠∑ ∑
Equation (4.5) means that the feeder flow can be minimized if all DGs,
including the UPC-mode DGs, increase the output powers to their
maximum when load demands reach the peak point. It is noted that the
condition set in equation (4.5) corresponds to the positive value of FLPCC
and thus the direction of power is from the main grid to the microgrid. In
case of the reversed direction the approach is similar. Even though the
power references of the UPC-mode DGs are not normally set at their
maximum, and the load demand is also usually not the peak value,
equation (4.5) suggests that the two aforementioned conflicting objectives
can be satisfied by changing the control mode of UPC-mode DGs if
necessary. In other words, when the load is heavy and other FFC-mode
DGs reach their maximum limits, we can maintain the FLPCC constant by
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
61
changing the control mode of UPC-mode DGs to FFC. The principle of
control mode change will be explained by using the sample microgrid
system shown in Fig. 4.1. The microgrid comprises four DGs; two DGs
(DG1 and DG3) are always in the FFC mode whereas the other two DGs
(DG2 and DG4) are in the UPC mode.
The outputs of DG2 and DG4 are regulated unchanged, 02P and 0
4P ,
respectively, while the change in the load is matched by the two FFC-mode
DGs (DG1 and DG3). Any variation in load demands LD1 and LD2 is
firstly compensated by DG1. However, if DG1 output reaches its limit, the
control mode of DG2 will be changed from the UPC to FFC, and hence the
output of DG2 can be increased more to match the load demand. As a
result, the feeder flow at PCC will not change when DG1 output reaches its
limit. The condition of DG2 mode change is that the DG1 output reaches
its maximum.
In a similar manner, the change of load demands LD3 and LD4 are
firstly matched by DG3. However, once DG3 output reaches its maximum,
DG1 will then match the load demand. In the other words, the change of
load demand is compensated for by the FFC-mode DGs, in order from the
nearest DG to the furthest DG (DG1). When the DG1 output reaches its
maximum, the control modes of the UPC-DGs are changed to FFC. For
example, when DG1 output is maximized, the control mode of DG2 is
changed to FFC mode, and DG2 participates in sharing the load together
with DG1. Similarly, if the load keeps increasing and the outputs of DG1
and DG2 reach their limits, the control mode of DG4 will be changed to
FFC mode. It can be seen that, the condition of changing the control mode
CHAPTER IV
62
of DG2 and DG4 is that the DG1 output reaches its maximum. However,
DG4 only changes its control mode when DG2 is in the FFC mode. In
other words, the UPC-mode DG will change its control mode to FFC if
DG1 output reaches its maximum P1max and the UPC-mode DG in front of
DG4 is in FFC mode, e.g. the UPC-mode DG in front of DG4 is DG2. DG2
has no UPC-mode DG in front of itself so the condition that causes it to
change its mode is only the status of DG1 output.
Changing the control mode of UPC-mode DGs to FFC allows the
DGs to operate at maximum capacity, and hence the load can be shared
more by the UPC-mode DGs. It can be seen from equation (4.5) that,
changing the control mode of UPC-mode DGs allows the feeder flow
reference at PCC ( refPCCFL ) to be minimized, and it is set as follows:
( )max max max 4.6refPCC i j
i FFC j UPC
FL LD P P∈ ∈
⎛ ⎞= − +⎜ ⎟
⎝ ⎠∑ ∑
Equation (4.6) means that, the feeder flow at PCC can be minimized
and always remained unchanged although the load reaches maximum, if
the control modes of UPC-mode DGs are changed to FFC.
The algorithm of the control-mode change, as depicted in Fig. 4.3,
shows that the DG will not change its control mode to FFC if either the
front UPC-mode DG is in the UPC mode or DG1 output does not reach its
maximum. In addition, the UPC-mode DG will return to UPC mode if its
output get back to the reference power and the front UPC-mode DG is in
the FFC mode.
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
63
Fig. 4.3. Algorithm of the control-mode change of UPC-mode DGs
4.2.2. Control Strategy for Multi-Feeder Microgrid
A microgrid with m feeders is depicted in Fig. 4.4. As mentioned earlier in
this chapter, the first DG of each feeder is in the FFC mode. The feeder
flow references of the feeders and control strategy of single feeder case
presented in 4.2.1 can be applied to the multi-feeder case because, in grid-
connected mode with the first DG in the FFC mode, the feeders operate
independently with each other. Therefore, the operation of each feeder is
same to a single feeder microgrid. The feeder flow reference of each feeder,
P1 ≥ P1max
Yes
Mode = UPC
Front UPC-DGis in FFC?
Front UPC-DG is in FFC?
Mode = FFC Mode = UPC
No
Yes Yes
PDG ≤ PDG0
Yes
No
Yes
CHAPTER IV
64
FLiref, is determined by equations (4.6) and thus the feeder flow at PCC is
equal to the sum of all feeders’ power flow. Each feeder flow is regulated
unchanged therefore the power from the utility side does not change and
equal to1
m
ii
FL=∑ ; m: number of parallel feeders. Each feeder flow is
minimized and hence FLPCC is minimized. During transition mode, the
change of frequency is determined as in equation (3.11) and depends on
FLPCC0. Therefore frequency change in transition mode is also minimized.
DG11
LD11 LDn1
DGn1DG21
LD21 LD31
…
MainGrid
FLPCC
…
FL1
DG12
LD12 LDn2
DGn2DG22
LD22 LD32
…
FL2
DG1m
LD1m LDnm
DGnmDG2m
LD2m LD3m
…
FLm
Fig. 4.4 Microgrid with m feeders
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
65
4.3. Control Strategy for Microgrid in the Islanded Mode
In the conventional control strategy, the loads are shared by the DGs
according to the droop characteristic as following:
‐ The change in load ith will be first compensated by the FFC-mode
DGs between the load and the PCC, DGj, j = 1...i-1, and frequency
does not change.
‐ When all DGj (j< i) reach to their maximum, the load change will be
picked up by all remaining DGs, and the frequency will be changed.
The frequency change is depicted in Fig. 4.5.
Using the control strategy presented in the grid-connected mode for
islanded mode, the UPC-mode DGs will change the control mode
according to the control algorithm shown in Fig. 4.3. By means of the
proposed control algorithm, the microgrid’s frequency does not change as
long as the DGs’ output reach maximum.
For multi-feeder microgrids, the operation of DGs in each feeder is
same to single-feeder case. However, when all DGs in a feeder increase to
Fig. 4.5 Power sharing for an islanded microgrid
f 1 f 0
FL211
f
P221P22
0
f
FL210 P22
2 FL231 FL23
0 FL232
FL212
f 2
f
CHAPTER IV
66
limits the load will be shared by DGs of the remaining feeders according to
droop characteristic and thus the frequency changes. The power sharing
during load variation of multi-feeder microgrid in the islanded mode can be
summarized as follows:
f 1 f 0
FL111
f
P121P12
0
f
FL110 P12
2 FL1n1 FL1n
0 FL1n2FL11
2
f 2
f 1 f 0
FL211
f
P221P22
0
f
FL210 P22
2 FL231 FL23
0 FL232FL21
2
f 2
f
f
f 1 f 0
FLm11
f
Pm21Pm2
0
f
FLm10 Pm2
2 FLmn1 FLmn
0 FLmn2FLm1
2
f 2
f
Fig. 4.6 Power sharing for a multi-feeder microgrid in islanded mode
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
67
‐ When a variation in load, the extra power is compensated by other
FFC-mode DGs in the same feeder, from the nearest to the furthest
DG (the first DG - DGi1). The frequency remains unchanged until
DGi1 output reaches to its limit.
‐ When DGi1 output is the limit, other UPC-mode DGs in the same
feeder will change the control mode to FFC. The manner to change
the control mode of UPC-mode DGs is same to the single-feeder case
(presented in the above).
‐ When all UPC-mode DGs in front of the load change, e.g. LDk, the
load variation is compensated by all other DGs in the microgrid,
including other feeders’ DGs. Therefore, the frequency of the
microgrid is changed. Due to the change of frequency, the load
sharing in other feeder is also changed.
‐ The power sharing is depicted in Fig. 4.6.
4.4. Simulation Studies and Results
4.4.1. Test System and Simulation Scenarios
In order to verify viability of the proposed power sharing method, a grid-
connected microgrid with three DGs and three-phase loads was simulated
by using PSCAD. The system configuration is shown in Fig. 4.7, and the
system parameters are listed in Table 4.1.
The control modes of DG1 through DG3 were FFC, UPC, and UPC,
respectively. The feeder flow reference of DG1 was determined to 5 kW,
based on equation (4.6) and the parameters shown in Table 4.1, and this
value was used in all simulation cases, including conventional method and
CHAPTER IV
68
proposed method. LD1 and LD2 were not changed during the simulation,
while the LD3 value was varied as follows. LD3 was initially 10 kW and
changed to 20 kW, 27 kW, 20 kW, and 10 kW at 2 s, 4 s, 6 s, and 8 s,
respectively (see Fig. 4.8 (b)).
Fig. 4.7. The Grid-connected Microgrid with Three DGs
TABLE 4.1 THE SYSTEM PARAMETERS
Parameter Value Unit PDG1
max 17 kW PDG2
max 15 kW PDG3
max 9 kW PDG2
0 5 kW PDG3
0 5 kW FLPCC
ref 5 kW LD1
max 5 kW LD2
max 11 kW LD3
max 30 kW
Main Grid
LD1 FLPCC0
PDG
LD2 LD3
FFC UPC UPC
P1max P3
max P2max
FLPCCref P2
0
LD1max LD2
max LD3
max
P30
DG1 DG2 DG3
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
69
4.4.2. Simulation Results in the Grid-Connected Operation Mode
This section describes the simulation results during the grid-connected
operation mode according to different power sharing method (conventional
and proposed). Fig. 4.8(a) shows the simulation results for the
conventional power sharing method. During the low load (from 1 s to 2 s),
the power from the main grid was regulated to the reference value (5 kW).
At 2 s, DG1 increased its output to compensate for the increase of LD3.
However, since DG1 reached its maximum (17 kW), the remainder load
was picked up by the main grid, and thus the FLPCC was changed to 8 kW.
Since the power output of DG2 and DG3 were fixed (control mode was not
changed), between 2s and 8 s, the microgrid was no longer a constant load
from the main grid viewpoint, as was discussed in Section III-A.
Fig. 4.9 shows the simulation results for the proposed power sharing
method. The DGs power and/or flow references and the load demand were
identical to those of the above case.
However, in the proposed method, the control mode of the UPC-
mode DGs were controlled according to the algorithm shown in Fig. 4.3. It
can be seen from Fig. 4.9 (a) that, by using the proposed method, the
feeder flow at PCC (FLPCC) remained unchanged, i.e. although the load
demand LD3 increaseed between 2 s and 8 s, FLPCC was regulated to
reference value (5kW). To accomplish this, the control modes of DG2 and
DG3 were changed from UPC to FFC at 2 s and 4 s, respectively, as shown
in Figs. 4.9 (b) and 4.9 (c).
CHAPTER IV
70
(a)
(b)
Fig. 4.8. Simulation results for the conventional method (Control mode of
DGs are not changed). (a) Active power generation (FLPCC changes during
heavy load conditions). (b) Load demand (LD1,LD2 remain unchanged,
LD3 changes).
LD3
LD1+LD2
FLPCC
FLPCC > FLPCCref
PDG1 PDG2+PDG3
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
71
(a)
(b)
FFC mode
UPC mode
PDG2 PDG1
FLPCC = FLPCCref
PDG3
CHAPTER IV
72
(c)
Fig. 4.9. The simulation results for the proposed method. (a) Active power
generation (FLPCC remains unchanged). (b) DG2 control-mode change. (c)
DG3 control-mode change.
When LD3 increased to 19 kW at 2 s, DG1 firstly increased its output,
but the variation could not be compensated by DG1 within its limit.
Therefore, DG2 changed its control mode to FFC, and increased output
until the load demands were matched. When LD3 increased further at 4 s,
DG2 also reached its maximum limit (15 kW). Accordingly, DG3 started to
increase its output by changing the control mode to FFC. When LD3
decreased at 6 s, DG3, which is the nearest DG upstream from LD3, firstly
decreased its output. Since the DG3 output returned to its initial power
reference (5 kW), the control mode was changed to UPC, and DG2
decreased its output. As LD3 decreased further at 8 s, the control mode of
FFC mode
UPC mode
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
73
DG2 was also returned to UPC, and the DG1 output was decreased.
4.4.3. Simulation Results in the Transition Mode
(a)
(b)
Δf=0.5Hz
Δf=1.05Hz
CHAPTER IV
74
(c)
Fig. 4.10. The frequency change in the transition mode. (a) Conventional
method, disconnection at 5s. (b) Conventional method, disconnection at 7s.
(c) Proposed method, disconnection at 7s.
In this section, we investigated the advantage of the proposed method
during the transition mode. To accomplish this, we simulated the
intentional islanding, and observed the microgrid frequency during the
transition mode. In order to investigate the effect of load level on the
microgrid frequency, we simulated the islanding at 5 s and 7 s, respectively,
for each power sharing method.
Fig. 4.10 summarizes the simulation results. It can be seen that the
change of frequency in the proposed method was always the same since the
feeder flow at PCC point was unchanged as depicted in Fig. 4.10 (c). On
the other hand, in the conventional method, FLPCC depended on the load
Δf=0.3Hz
CONTROL STRATEGY FOR CONTROLLABLE DGS IN MICROGRID
75
condition and it was no longer a constant if the first DG output reached its
maximum as shown in Fig. 4.8 (a). Therefore, the change of frequency due
to a disconnection from the main grid was not constant, but depended on
the feeder flow power at which the disconnection occurred. Figs. 4.10 (a)
and 4.10 (b) show the changes of frequency during the transition mode at
5s and 7s, respectively. The frequency change in case of disconnection at
5s (1.05Hz) was larger than that in case of disconnection at 7s (0.5Hz),
since the FLPCC at 5s was larger than FLPCC at 7s.
Additionally, if the transition mode occurred between 2s and 8s,
FLPCC was larger than the feeder flow reference (5kW), and hence the
change of frequency in the conventional method was always larger than the
change in the proposed method. As depicted in Figs. 4.10 (a) and 4.10 (b),
the changes of frequency in conventional method were 1.05Hz and 0.5Hz
respectively, whereas that was 0.3Hz for the proposed method (see Fig.
4.10 (c)).
From the simulation results it is seen that with proposed method the
change of frequency during the transition mode (grid-connected
mode/islanded mode) is minimized, the feeder flow at PCC is remained
unchanged during grid-connected mode, and frequency of islanding
operation mode can be regulated unchanged as long as the load is shared
by its own feeder. According to the proposed method, the DG output is
mobilized during heavy load condition thus reduce the burden to the utility
grid in the peak time and the microgrid operates more stably. Otherwise,
the UPC-mode DGs do not change their control mode, and thus they can
operate at high efficiency (e.g. economical condition, high performance
CHAPTER IV
76
band…). The control strategy is also applied to the multiple feeder
microgrid and simulation results will be presented in Chapter VI.
Chapter V:
CONTROL STRATEGIES FOR HYBRID SOURCE IN
MICROGRID
5.1. Overview
Renewable energy is currently widely used. One such resource is solar
energy. The PV array normally uses maximum power point tracking
(MPPT) techniques to continuously deliver the highest power to the load
when there are variations in irradiation and temperature. The disadvantage
of PV energy is that the PV output power depends on weather conditions
and cell-temperature making it become an uncontrollable source [49].
Furthermore, it is not available during night time. In order to overcome
these inherent drawbacks, alternative source, such as the proton exchange
membrane fuel cell (PEMFC) that will likely play a major role in
distributed generation and microgrids in near future [27, 50], should be
installed in the hybrid system. By changing FC output power, the hybrid
source output becomes controllable. However, PEMFC, in its turn, works
only at a high efficiency within a specific power range ( low upFC FCP P÷ ) [51, 52].
The hybrid source like PV-FC is connected to feeders of microgrid
and work as a distributed generator. Therefore the hybrid source as a DG
also has two control mode, UPC mode and FFC mode. In the UPC mode,
variations of load demand are compensated by the main grid because the
hybrid source output is regulated to a reference power. Therefore, the
CHAPTER V
78
reference value of the hybrid source output PMSref must be determined. In
the FFC mode, the feeder flow is regulated to a constant, the extra load
demand is picked up by the hybrid source and hence the feeder reference
power Pfeederref must be known.
The purposes of hybrid source operation are to operate PV in MPPT
mode to mobilize the free of charge energy, and to operate FC in a high
efficiency band. Therefore, the control mode of hybrid source must be
determined so that it can meet the above assumptions.
The proposed operating strategy is to coordinate the two control
modes and determine the reference values of the UPC mode and FFC mode
so that all constraints are satisfied, the operation of such a complicated
system should be simplified, and the performance of microgrid operation is
improved.
5.2. Hybrid System Description
5.2.1 Structure of Grid Connected Hybrid System
The system consists of a PV-FC hybrid source with the main grid
connecting to loads at the PCC as shown in Fig. 5.1. The Photovoltaic [53,
54] and the PEMFC [55, 56] are modeled as nonlinear voltage sources.
These sources are connected to DC/DC converters which are coupled at the
DC side of a DC/AC inverter. The DC/DC connected to the PV array
works as an MPPT controller. Many MPPT algorithms have been proposed
in the literature such as Incremental Conductance (INC), Constant Voltage
(CV), and Perturbation and Observation (P&O). The P&O method has
been widely used because of its simple feedback structure and fewer
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
79
measured parameters. The P&O algorithm with power feedback control
[57-59] is shown in Fig. 5.2. As PV voltage and current are determined, the
power is calculated. At the maximum power point, the derivative (dP/dV)
is equal to zero. The maximum power point can be achieved by changing
the reference voltage by the amount of ΔVref.
Fig. 5.1. Grid Connected PV-FC Hybrid System
5.2.2 PV Array Model
The mathematical model [53, 54, 60-62] can be expressed as below
AC DC
DC DC
MPPT
PV
PWM1
Vref
D1
V I
DC DC PWM2
VDCref
D2
PCC
FC
DC Bus
PFeeder
PMS
PLoad
Load
VDC
CHAPTER V
80
( ) ( )5.1ph sat sqI I I exp V IR 1
AKT⎧ ⎫⎡ ⎤= − + −⎨ ⎬⎢ ⎥⎣ ⎦⎩ ⎭
Where:
:Electronic charge:A dimensionless factor:Boltzmann constant
qAk
Equation (1) shows that the output characteristic of a solar cell is non-
linear and vitally affected by solar radiation, temperature and load
condition.
Photo-current Iph is directly proportional to solar radiation Ga
( ) ( )5.2aph a sc
as
GI G IG
=
The short-circuit current of solar cell Isc depends linearly on cell
temperature:
( ) ( ) ( )5.3sc scs sc sI T I 1 ΔI T T⎡ ⎤= + −⎣ ⎦
Thus, Iph depends on solar irradiance and cell-temperature:
( ) ( ) ( )5.4aph a scs sc s
as
GI G ,T I 1 ΔI T TG
⎡ ⎤= + −⎣ ⎦
The saturation current Isat also depends on solar irradiation and cell-
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
81
temperature and can be mathematically expressed as follows:
( ) ( )( )( )
( )5.5oc
t
ph asat a V T
V T
I G ,TI G ,T
e 1⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠
=
−
Where
Gas: Standard irradiation (1000 W/m ).
Isc: Short-circuit current.
Iscs: Short-circuit current at standard condition (Gas and Ts)
Ts: Standard temperature (298 K).
Voc: Open-circuit voltage
Vt: Thermal voltage
5.2.3 PEMFC Model
The PEMFC steady-state feature of a PEMFC source is assessed by means
of a polarization curve, which shows the non-linear relationship between
the voltage and current density. The PEMFC output voltage is as follows
[52, 56, 63-65]:
( )5.6out Nerst act ohm concV E V V V= − − −
Where, ENerst is the “thermodynamic potential” of Nerst, which represents
the reversible (or open-circuit) voltage of the fuel-cell. Activation voltage
drop Vact is given in Tafel equation as below:
CHAPTER V
82
( ) ( )5.7actV T a bln I⎡ ⎤= +⎣ ⎦
a, b: Constant terms in Tafel equation (in volts per Kelvin)
The overall ohmic voltage drop Vohm can be expressed as:
( )5.8ohm ohmV IR=
The ohmic resistance Rohm of PEMFC consists of the resistance of the
polymer membrane and electrodes, and the resistances of the electrodes.
Concentration voltage drop Vconc is expressed as:
( )5.9conclimit
RT IV ln 1zF I
⎛ ⎞= − −⎜ ⎟
⎝ ⎠
5.2.4 Maximum Power Point Tracking Control
Many MPPT algorithms have been proposed in the literature such as INC
[66], CV [67], and P&O [57, 59]. The two algorithms often used to achieve
maximum power point tracking are the P&O and INC methods. The INC
method offers good performance under rapidly changing atmospheric
conditions. However, four sensors are required to perform the
computations. If the sensors require more conversion time, then the MPPT
process will take a longer time to track the maximum power point. During
tracking time, PV output is less than its maximum power. This means that
the longer the conversion time the larger amount of power loss. On the
contrary, if the execution speed of the P&O method increases, then the
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
83
system loss will decrease. Moreover, this method only requires two sensors,
which results in a reduction of hardware requirements and cost [68].
Therefore, the P&O method is used to control MPPT process.
Fig. 5.2. P&O MPPT Algorithm.
In order to achieve maximum power, two different applied control
methods often chosen are voltage-feedback control and power-feedback
control [58, 59]. Voltage-feedback control uses the solar array terminal
voltage to control and keep the array operating near its maximum power
point by regulating the array’s voltage and matching the voltage of the
array to a desired voltage. The drawback of the voltage-feedback control is
CHAPTER V
84
its neglect of the effect of irradiation and cell-temperature. Therefore, the
power-feedback control is used to achieve maximum power. The P&O
MPPT algorithm with a power-feedback control [57, 59] is shown in Fig.
5.2. As PV voltage and current are determined, the power is calculated. At
the maximum power point, the derivative (dP/dV) is equal to zero. The
maximum power point can be achieved by changing the reference voltage
by the amount of ΔVref.
In order to implement the MPPT algorithm, a Buck-Boost DC/DC
converter is used as depicted in Fig. 5.3.
a)
b)
Fig. 5.3. Buck-Boost converter and control: a) Buck-Boost topology,
b) PWM circuit
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
85
The Buck-Boost converter consists of one switching device (GTO)
that enables it to turn on and off depending on the applied gate signal D.
The gate signal for the GTO can be obtained by comparing the saw-tooth
waveform with the control voltage [58]. The change of the reference
voltage ΔVref obtained by MPPT algorithm shown in Fig. 5.2 becomes the
input of the Pulse-Width-Modulation (PWM) circuit depicted in Fig. 5.3b.
The PWM circuit generates a gate signal to control the Buck-Boost
converter and thus maximum power is tracked and delivered to the AC side
via a DC/AC inverter.
5.3. Control Algorithm of the Hybrid System
As mentioned above, the purpose of the operating algorithm is to
determine the control mode of the hybrid source and the reference value
for each control mode so that the PV is able to work at maximum output
power and the constraints are fulfilled. Once the constraints (PFClow, PFC
up,
and PFmax) are known, the control mode of the hybrid source (UPC mode
and FFC mode) depends on load variations and PV output. The algorithm
to choose control mode of hybrid source will be presented in section 5.3.2.
In the UPC mode, the reference output power of the hybrid source PMSref
depends on PV output and the constraints of FC output. The algorithm
determining PMSref is presented in 5.3.1 and is depicted in Fig. 5.4.
5.3.1 Control Strategy for the Hybrid System in the UPC mode
In this subsection, the presented algorithm determines the hybrid source
works in the UPC mode. Such an algorithm allows the PV to work at its
maximum power point, and the FC to work within its high efficiency band.
CHAPTER V
86
In the UPC mode, the hybrid source regulates the output to the reference
value. Then
refPV FC MSP P P+ = (5.11)
Equation (5.11) shows that the variations of PV output will be
compensated for by the FC power and thus the total power will be
Fig. 5.4 Operation Strategy of Hybrid Source in the UPC Mode.
PMSref PPV
PMS3ref = PMS2
ref + ΔPMS
PMSiref = PMSi-1
ref + ΔPMS
: : PPV3
PPV2
PPV1
PPVi
Area 2
Area 1
PMSnref = PMSn-1
ref + ΔPMS PPVn
: :
ΔPMS
ΔPMS PMS2ref = PMS1
ref + ΔPMS
PMS1ref = PFC
max
PFClow
0 0
PPV0 = 0
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
87
regulated to the reference value. However, FC output must satisfy its
constraints and hence PMSref must set at an appropriate value. Fig. 5.4
shows the operation strategy of the hybrid source in UPC mode to
determine PMSref . The algorithm includes two areas, Area 1 and Area 2.
In Area 1, PPV is less than PPV1, and then the reference power PMS1ref
is set at PFCup. Where:
( )1 – 5.12up lowPV FC FCP P P=
( )1 5.13ref upMS FCP P=
If PV output is zero, then equation (5.11) deduces PFC to be equal to PFCup.
If PV output increases to PPV1, then, from equations (5.11) and (5.12), we
get PFC equal to PFClow. In other words, when PV output varies from zero to
PPV1, the FC output will change from PFCup to PFC
low. As a result, the
constraints for FC output are always reached in Area 1. It is noted that the
reference power of the hybrid source during UPC mode is fixed at a
constant PFCup.
Area 2 is for the case in which PV output power is greater than PPV1.
As examined earlier, when PV output increases to PPV1, the FC output will
decrease to its lower limit, PFClow. If PV output keeps increasing, the FC
output will decrease below its limit, PFClow. In this case, to operate the PV
at its maximum power point and the FC within its limit, the reference
power must be increased. As depicted in Fig. 5.4, if PV output is larger
than PPV1, the reference power will be increased by the amount of ΔPMS,
CHAPTER V
88
and we get
( )2 1 5.14ref refMS MS MSP P P= + Δ
Similarly if PPV is greater than PPV2, the FC output becomes less than
its lower limit and the reference power will be thus increased by the
amount of ΔPMS. In other words, the reference power remains unchanged
and equal to PMS2ref if PPV is less than PPV2 and greater than PPV1. Where
( )2 1 5.15PV PV MSP P P= + Δ
It is noted that ΔPMS is limited so that with the new reference power, the
FC output must be less than its upper limit, PFCup. Then we have
( )– 5.16up lowMS FC FCP P PΔ ≤
In general, if PV output is between PPVi and PPVi-1 (i=2, 3, 4…), then we
have
( )1 5.17ref refMSi MSi MSP P P−= + Δ
( )1 5.18PVi PVi MSP P P−= + Δ
Equations (5.17) and (5.18) show the method of finding the reference
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
89
power when PV output is in Area 2. The relationship between PMSiref and
PPVi is obtained by using equations (5.12), (5.13), and (5.18) in (5.17), and
then
( ), 2, 3, 4 5.19ref lowMSi PVi FCP P P i= + = …
It can be generalized for the determination of PMSref in both Area 1 and
Area 2 by starting the index i from 1. Therefore, if PV output is
1 , 1, 2, 3PVi PV PViP P P i− ≤ ≤ = …
then we have
( )min , 1, 2, 3 5.20refMSi PVi FCP P P i= + = …
( )1 , 2, 3, 4 5.21PVi PVi MSP P P i−= +Δ = …
It is noted that, when i=1, PPV1 is given in equation (5.12), and
( )1 0 0 5.22PVi PVP P− = =
In brief, the reference power of the hybrid source is determined
according to PV output power. If PV output is in Area 1, the reference
power will always be constant and set at PFCup. Otherwise, the reference
value will be changed by the amount of ΔPMS, according to the change of
PV power. The reference power of the hybrid source PMSref in both Area 1
and Area 2 is determined by equations (5.20) and (5.21). PPV0, PPV1, and
CHAPTER V
90
ΔPMS are shown in equations (5.22), (5.12), and (5.16) respectively.
Fig. 5.5 shows the control algorithm diagram determining the
reference power automatically. The constant C must satisfy equation (5.16).
If C increases, the number of change of PMSref will decrease and thus the
performance of system operation will be improved. However, C should be
small enough so that the frequency does not change over its limits (±5%).
PMSref= PFC
max, ∆PMS = 0
PMSref(new) = PMS
ref(old) + ∆PMS
PPV > PPV1
PFC ≤ PFCmin
PFC ≥ PFCmax
∆PMS = –C ∆PMS = +C ∆PMS = 0
No
Yes
No
Yes
Yes
No
Fig. 5.5. Control Algorithm Diagram in UPC mode (PMSref
Automatically Changing)
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
91
In order to improve the performance of the algorithm, a hysteresis is
included in the simulation model. The hysteresis is used to prevent
oscillation of the setting value of hybrid system reference power PMSref. At
the boundary of change in PMSref, the reference value will be changed
continuously due to the oscillations in PV maximum power tracking. To
avoid the oscillations around the boundary, a hysteresis is included and its
control scheme to control PMSref is depicted in Fig.5.6.
C
PMSref = PMS
ref –C
PPV
PPV2
PPV
PPVn
PMSref = PMS
ref
+C
PMSref = PMS
ref –C
PMSref = PMS
ref –C
PMSref = PMS
ref
+C
PMSref = PMS
ref
+C
I
II
: :
+ C– C0
Fig. 5.6. Hysteresis Control Scheme for PMSref Control
CHAPTER V
92
5.3.2 Overall Control Strategy for the Hybrid System
It is well known that, in the microgrid, each DG as well as the hybrid
source has two control modes, the UPC mode and FFC mode. In the above
subsection, a method to determine PMSref in the UPC mode is proposed. In
this subsection, an operating strategy is presented to coordinate the two
control modes. The purpose of the algorithm is to decide when each control
mode is applied and to determine the reference value of the feeder flow
when the FFC mode is used. Such an operating strategy must enable the
PV to work at its maximum power point, FC output and feeder flow to
satisfy their constraints.
If the hybrid source works in the UPC mode, the hybrid output is
regulated to a reference value and the variations in load are matched by
feeder power. With the reference power PMSref proposed in subsection A,
the constraints of FC and PV are always satisfied. Therefore, only the
constraint of feeder flow is considered. On the other hand, when the hybrid
works in the FFC mode, the feeder flow is controlled to a reference value
Pfeederref and thus the hybrid source will compensate for the load variations.
In this case, all constraints must be considered in the operating algorithm.
Based on those analyses, the operating strategy of the system is proposed
as demonstrated in Fig. 5.7.
The operation algorithm in Fig. 5.7 involves two areas (Area-I and
Area-II) and the control mode depends on the load power. If load is in
Area-I, the UPC mode is selected. Otherwise, the FFC mode is applied
with respect to Area-II.
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
93
In the UPC area, the hybrid source output is PMS
ref. If load is lower
than PMSref, the redundant power will be transmitted to the main grid.
Otherwise, the main grid will send power to the load side to match load
demand. When load increases, the feeder flow will increase
correspondingly. If feeder flow increases to its maximum Pfeedermax, then
the feeder flow cannot meet load demand if the load keeps increasing. In
order to compensate for the load demand, the control mode must be
MS
refP
2loadP
P FC
1loadP
P Fee
derm
ax
FC
PV
Feeder
FC
Load Shedding
P FC
up–
P FC
Area-I(UPC)
Area-II(FFC)
P FC
max
– P F
C
PLoad maxloadP
P PV
0
Fig. 5.7. Overall Operating Strategy for the Grid Connected Hybrid System
CHAPTER V
94
changed to FFC with respect to Area-II. Thus the boundary between Area-I
and Area-II Pload1 is
( )max1 5.23ref
Load Feeder MSP P P= +
When the mode changes to FFC the feeder flow reference must be
determined. In order for the system operation to be seamless, the feeder
flow should be unchanged during control mode transition. Accordingly,
when the feeder flow reference is set at Pfeedermax then we have
max ref
Feeder FeederP P= (5.24)
In the FFC area, the variation in load is matched by the hybrid source.
In other words, the changes in load and PV output are compensated for by
PEMFC power. If FC output increases to its upper limit and load is higher
than total generating power, then load shedding will occur. The limit that
load shedding will be reached is
( )max2 5.25up
Load FC Feeder PVP P P P= + +
Equation (5.25) shows that Pload2 is minimal when PV output is at 0 kW.
Then
( )min max2 5.26up
Load FC FeederP P P= +
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
95
Equation (5.26) means that, if load demand is less than Pload2min, load
shedding will never happen.
From the beginning, FC has always worked in high efficiency band
and FC output has been less than PFCup. If load is less than Pload2
min, load
shedding is ensured not to occur. However, in severe conditions, FC should
mobilize its availability, PFCmax, to supply the load. Thus, the load can be
higher and the largest load is
( )max max max 5.27Load FC FeederP P P= +
If FC power and load demand satisfy equation (5.27), load shedding will
never happen. Accordingly, based on load forecast, the installed power of
FC can be determined following equation (5.27) to avoid load shedding.
Corresponding to the FC installed power the width of Area-II is calculated
as follows:
( )max – 5.28upArea II FC FCP P P− =
In order for the system to work more stably, the number of mode
changes should be decreased. As seen in Fig. 5.7, the limit changing the
mode from UPC to FFC is PLoad1, which is calculated in equation (5.23).
Equation (5.23) shows that PLoad1 depends on PFeedermax and PMS
ref.
PFeedermax is a constant, thus PLoad1 depends on PMS
ref. Fig. 5.4 shows that in
Area 2 PMSref depends on ΔPMS. Therefore, to decrease the number of mode
changes, PMSref changes must be reduced. Thus ΔPMS must be increased.
CHAPTER V
96
However, ΔPMS must satisfy condition (5.16) and thus the minimized
number of mode change is reached when ΔPMS is maximized
( )max – 5.29up lowMS FC FCP P PΔ =
In summary, in a light load condition the hybrid source works in UPC
mode, the hybrid source regulates output power to the reference value
PMSref, and the main grid compensates for load variations. PMS
ref is
determined by the algorithm shown in Fig. 5.4, and thus the PV always
works at its maximum power point and the PEMFC always works within
the high efficiency band (PFClow...PFC
up). In heavy load conditions, the
control mode changes to FFC, and the variation of load will be matched by
the hybrid source. In this mode, PV still works with the MPPT control, and
PEMFC operates within its efficiency band until load increases to a very
high point. Hence FC only works outside the high efficiency band
(PFCup...PFC
max) in severe conditions. With an installed power of FC and
load demand satisfying equation (5.27), load shedding will not occur.
Besides, to reduce the number of mode changes, ΔPMS must be increased
and hence the number of mode changes is minimized when ΔPMS is
maximized, as shown in equation (5.29). Additionally, in order for system
operation to be seamless, the reference value of feeder flow must be set at
PFeedermax.
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
97
5.4. Simulation Studies and Result
5.4.1 Simulation Results in the Case without Hysteresis
A simulation was carried out using the system model shown in Fig. 5.2 to
verify the operating strategies. The system parameters are shown in Table
5.1.
In order to verify the operating strategy, the load demand and PV
output were time varied in terms of step. According to the load demand
and the change of PV output, PFC, PMSref, PFeeder
ref, and operating mode are
determined by the proposed operating algorithm. Fig. 5.8 shows the
simulation results of the system operating strategy. The changes of PPV and
PLoad are shown in Fig. 5.8-a (Δ line) and Fig. 5.8-b (ο line) respectively.
TABLE 5.1 THE HYBRID SYSTEM PARAMETERS
Parameter Value Unit PFC
low 0.01 MW PFC
up 0.07 MWPFeeder
max 0.01 MW ∆PMS 0.03 MW
Based on PPV and the constraints of PFC shown in Table 5.1, the
reference value of the hybrid source output PMSref is determined as dipicted
in Fig. 5.8-a (ο line). From 0s to 10s, PV operates at standard test
conditions to generate a constant power, and thus PMSref is constant. From
10s to 20s, PPV changes step by step and thus PMSref is defined as the
algorithm shown in Fig. 5.4 or Fig. 5.5. PEMFC ouput PFC, as shown in
Fig. 5.8-a (• line), changes according to the change of PPV and PMS. Figure
CHAPTER V
98
5.8(c) shows the system operating mode. The UPC mode and FFC mode
correspond to values 0 and 1, respectively. From 4s to 6s, the system works
in FFC mode and thus PFeedermax becomes the feeder reference value.
During FFC mode, the hybrid source output power changes with respect to
the change of load demand, as in Fig. 5.8(b). On the contrary, in UPC
mode, PMS changes following PMSref, as shown in Fig. 5.8(a).
It can be seen from Fig. 5.8 that, the system only works in FFC mode
when the load is heavy. The UPC mode is the major operating mode of the
system, and hence the system works more stably.
It can also be seen from Fig. 5.8(a) that, at 12s and 17s, PMSref
changes continuously. This is caused by variations of PPV in the maximum
power point tracking process. As a result, PMS and PFC oscillate and are
unstable. In order to overcome these drawbacks, a hysteresis was used to
control the change of PMSref, as shown in Fig. 5.6. The simulation results of
the system, including the hysteresis, are depicted in Fig. 5.9.
Fig. 5.8. (a)
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
99
Fig. 5.8. (b)
Fig. 5.8. (c)
Fig. 5.8. Simulation result without hysteresis: (a) The operating strategy of
the hybrid source; (b) The operating strategy of the whole system; (c) The
change of operating modes.
5.4.2 Improving Operation Performance by Using Hysteresis
Fig. 5.9 shows the simulation results when a hysteresis was included with
the control scheme shown in Fig. 5.6. From 12s to 13s and from 17s to 18s,
the variations of PMSref (Fig. 5.9a, ο line), FC output (Fig. 5.9a, • line), and
feeder flow (Fig. 5.9b, Δ line) are eliminated, and thus the system works
Feeder control
Unit power control mode
CHAPTER V
100
more stably compared to a case without hysteresis (Fig. 5.8). Fig. 5.9d
shows the frequency variations when load changes or when the hybrid
source reference power PMSref changes (at 12s and 18s). The parameter C
was chosen at 0.03MW, and thus the frequency variations did not reach
over its limit (±5%*60 = ±0.3Hz).
5.4.3 Discussion
It can be seen from Fig. 5.9(b) that, during the UPC mode, the feeder flow
(Δ line) changes due to the change of load ( line) and hybrid source
output (• line). This is because in the UPC mode the feeder flow must
change to match the load demand.
However, in a real-world situation, the microgrid should be a
constant load from the utility view point [9]. In reality, the microgrid
includes some DGs connected in parallel to the feeder [16, 47]. Therefore,
in the UPC mode, the changes of load will be compensated for by other
FFC mode DGs and the power from the main grid will be controlled to
remain constant.
In the case in which there is only one hybrid source connected to the
feeder, the hybrid source must work in the FFC mode to maintain the
feeder flow at constant. Based on the proposed method, this can be done by
setting the maximum value of the feeder flow (PFeedermax) to a very low
value, and thus the hybrid source is forced to work in the FFC mode.
Accordingly, the FC output power must be high enough to meet the load
demand when load is heavy and/or at night time without solar power.
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
101
Fig. 5.9(a)
Fig. 5.9(b)
Fig. 5.9(c)
Feeder control mode
Unit power control mode
C=0.03MW
CHAPTER V
102
Fig. 5.9(d)
Fig. 5.9. Improving operation performance by using hysteresis: (a) The
operating strategy of the hybrid source; (b) The operating strategy of the
whole system; (c) The change of operating modes; (d) Frequency
variations occur in the system.
From the above discussions, it can be said that the proposed
operating strategy is more applicable and meaningful to a real-world
microgrid with multi DGs.
In summary, the main operating strategy shown in Fig. 5.7 is to
specify the control mode and feeder flow reference; the algorithm shown in
Fig. 5.4 is to determine PMSref in the UPC mode. With the operating
algorithm, PV always operates at maximum output power, PEMFC
operates within the high efficiency range (PFClow ÷ PFC
up), and feeder
power flow is always limited to its maximum value (PFeedermax). The change
of the operating mode depends on the current load demand, the PV output
and the constraints of PEMFC and feeder power.
CONTROL STRATEGY FOR HYBRID SOURCE IN MICROGRID
103
With the proposed operating algorithm, the system works flexibly,
exploiting maximum solar energy; PEMFC works within a high efficiency
band and hence improves the performance of the system’s operation. The
system can maximize the generated power when load is heavy and
minimize the load shedding area. When load is light, the UPC mode is
selected and thus the hybrid source works more stably.
The changes of operating mode only happen when the load demand is
at the boundary of mode change (PLoad1); otherwise, the operating mode is
either UPC mode or FFC mode. Besides, the variation of hybrid source
reference power PMSref is overcome by means of hysteresis. Additionally,
the number of mode changes is reduced. As a consequence, the system
works more stably due to the minimization of mode changes and reference
value variation.
CHAPTER V
104
Chapter VI:
CASE STUDY
Previous chapters have presented the control strategies for a microgrid in
general cases and the operation algorithm for a hybrid source connected to
the microgrid. The simulation was implemented to verify the algorithms
separately. In this chapter, overall control algorithms are together applied
to a tested microgrid with controllable DGs and PV-FC hybrid source.
6.1. Introduction to Case Study
Fig. 6.1 System Configuration
D
FL1 1
FFC LD
DUPC F
Main Grid
P
FL2 1 FL1 2 FL2 2 FL1 HB FL2 HB
D
FL1 3
FFC LD
FL2 3
PHB P2P1
SS
CB2
CB1
CHAPTER VI
106
6.1.1. System Configuration
The tested system configuration shown in Fig. 6.1 has two feeders. The
first feeder has two controllable DGs and a PV-FC hybrid source supplying
to load demand LD1. The second feeder has one controllable DG supplying
power to load LD2. The two feeders connect to the utility grid at PCC
through a static switch (SS). The circuit breakers CB1 and CB2 are to
isolate the feeder separately if necessary. The system is simulated in
PSCAD/EMTDC as shown in Appendix A.
6.1.2. System Parameters
General parameters:
‐ Microgrid nominal voltage: 380V
‐ Main grid nominal voltage: 22.9kV
‐ Nominal frequency: 60Hz
‐ Number of feeders: 2
DG1:
‐ P1max = 50kW,
‐ FFC mode: FLref = 0kW
DG2:
‐ P2max = 50kW
‐ UPC mode: P2ref = 20kW
Hybrid source:
‐ FC: Pfcmax = 50kW, Pfcmin = 10kW
‐ Pmsref = 40kW ( = Ppv1 = Pfcmax – Pfcmin)
‐ FL31max = 100kW ( = P1max + P2max + FL0)
CASE STUDY
107
‐ HB source change mode when: LD1 ≥ FL31max + Pmsref =
140kW.
6.1.3. Simulation Scenarios
TABLE 6.1
CASES WITHOUT HYBRID SOURCE
Operation mode
Control method
Grid connected
mode
Islanded mode
Without proposed control algorithm Case 1-1 Case 2-1
With proposed control algorithm Case 1-2 Case 2-2
TABLE 6.2 CASES WITH HYBRID SOURCE
Operation mode
Control method
Grid connected
mode
Islanded mode
Without proposed control algorithm Case 3-1 Case 4-1
With proposed control algorithm Case 3-2 Case 4-2
TABLE 6.3 CASES WITH MULTI-FEEDER MICROGRID
Operation mode Cases Grid Connected mode Case 5-1
Islanded mode Case 5-2
CHAPTER VI
108
TABLE 6.4 FREQUENCY RESTORATION CONTROL METHOD VS. PROPOSED METHOD
Operation mode Control method
Islanded mode
Frequency restoration control scheme Case 6-1 Proposed control scheme Case 6-2
6.2. Results and Discussion
6.2.1. Simulation Results in case without Hybrid Source
Case 1-1: Grid-connected mode, without control strategy
Fig. 6.2 Active power response as LD1 changes
CASE STUDY
109
(a)
(b)
Fig. 6.3 Voltage and frequency response in grid-connected mode
CHAPTER VI
110
Fig. 6.4 Frequency change due to islanding
Islanding at 5 seconds, Δf = 1.5Hz
Case 1-2: Grid-connected mode, with control strategy
Fig. 6.5 Active power response as LD1 changes
CASE STUDY
111
(a)
(b)
Fig. 6.6 Voltage and frequency response in grid-connected mode
Frequency remains unchanged in grid-connected mode and equals to
nominal value, 60Hz.
CHAPTER VI
112
Fig. 6.2 shows that, without control strategy the feeder flow is
change d when load is heavy, between 2s and 8s, and feeder flow increases
over the reference value FLref, 0kW. In other words, the microgrid
becomes uncontrollable load from the utility point of view. With control
strategy, Fig. 6.5, DG2 changes its mode according to the control
algorithm and thus the feeder flow remains unchanged all the time.
Frequency remains unchanged in grid-connected mode and equals to
nominal value 60Hz as depicted in Fig. 6.3(b) and Fig. 6.6(b).
Fig. 6.7 Frequency change due to islanding
Frequency does not change as islanding occurs.
The simulation results of case1-1 and case 1-2 show that without
control strategy, as load LD1 increases the feeder flow increases whereas it
remains unchanged in case with the control strategy as shown in Fig. 6.2
and Fig. 6.5, respectively.
CASE STUDY
113
In the transition mode the frequency drop in conventional method is
larger than in case with proposed control scheme as shown in Fig. 6.4 and
Fig. 6.7, respectively.
Case 2-1: Islanded mode, without control algorithm.
Fig. 6.8 Active power response as LD1 changes during island mode
Fig. 6.9 System voltage
CHAPTER VI
114
Fig. 6.10 System frequency
Frequency decreases when DG2 output increases
Fig. 6.8 shows simulation results of conventional method in islanded
mode. The DG2 control mode does not change all the time and it is in the
UPC mode. As DG1 reaches maximum, DG2 increases output according to
P-f droop characteristic to meet the load demand, 50kW. The frequency,
therefore, decreases as DG2 output increases as shown in Fig. 6.10.
CASE STUDY
115
Case 2-2: Islanded mode, DG2 does change its mode according to the
control algorithm.
Fig. 6.11 Active power response as LD1 changes during island mode
Fig. 6.12 DG2 control mode
CHAPTER VI
116
(a)
(b)
Fig. 6.13 System Voltage and frequency
Using the control algorithm, f is regulated to nominal value, 60Hz, all the
time
CASE STUDY
117
Fig. 6.11 shows that as DG1 reaches maximum 50kW, from 3s to 9s,
DG2 increases output to meet the load demand. However, with the
proposed control strategy the DG2’s control changes according to the
algorithm as shown in Fig. 6.12, and the frequency, therefore, remains
unchanged as shown in Fig. 6.13(b).
The simulation results of case 2-1 and case 2-2 show that with the
proposed control strategy the frequency remains unchanged all the time
whereas it changes according to P-f droop characteristic in the
conventional control method.
6.2.2. Simulation Results in case with the PV-FC Hybrid Source
The PV-FC hybrid source is controlled according to the algorithm
presented in Chapter V.
Case 3-1: Grid-connected mode, without control strategy
Fig. 6.14(a) shows that the feeder flow in front of HB source,
Pf1_HB, is limited to Pf1_HBmax, 100kW. Between the period of 5s and
7s Pf1_HB reaches maximum, the hybrid source’s control mode changes to
the FFC mode as shown in Fig. 6.14(b).
From Fig. 6.15 it is seen that when hybrid source’s control mode
changes to the FFC mode, from 5s to 7s, its output increases to track the
load demand and the feeder flow Pf1_HB is regulated unchanged.
Therefore, the presence of the hybrid source can limit the power shared by
other DGs and it actively participates in sharing the load with other DGs.
Otherwise, the hybrid source works in UPC mode and controls its output to
the reference power, 40kW.
CHAPTER VI
118
(a)
(b)
Fig. 6.14 (a) Load demand and feeder flow in front of HB source,
(b) Control mode of the hybrid source
CASE STUDY
119
Fig. 6.15 Power response in case without control strategy
Without proposed control method the feeder flow at PCC is changed as
P_DG1 reaches maximum (50kW).
The frequency and voltage are regulated to the nominal values and
do not change during the grid-connected mode as shown in Fig. 6.16.
CHAPTER VI
120
(a)
(b)
Fig. 6.16 System voltage and frequency
CASE STUDY
121
Case 3-2: Grid-connected mode, with control strategy
Fig. 6.17 Load demand and feeder flow in front of HB source
These parameters are same to Case 3-1.
Fig. 6.18 Power response in case with control strategy
CHAPTER VI
122
(a)
(b)
Fig. 6.19 The control mode of DG2 and hybrid source: (a) DG2 control
mode, (b) HB source control mode
CASE STUDY
123
(a)
(b)
Fig. 6.20 System voltage and frequency
CHAPTER VI
124
The operation of the hybrid source does not change the operation of
other DGs in proposed control method, the feeder flow at PCC (Pgrid)
remains unchanged as DG2 changes it control mode to increase the output
(P_DG2) as depicted in Fig. 6.18 by changing the DG2 control mode to
FFC mode between 3s and 9s as load is heavy, Fig. 6.19(a). The frequency
and voltage in proposed method are also regulated to the nominal values
and do not change during the grid-connected mode.
The simulation results of case 3-1 and case 3-2 show that, in grid-
connected mode, the participation of the hybrid source with its control
strategy does not change the microgrid operation performance. The feeder
flow is regulated unchanged with the proposed control strategy. In addition,
the operation of hybrid source allows limiting the feeder flow in front of its
connection point, Pf1_HB, therefore the operation of DGs between hybrid
source and PCC are easier and reduce the burden to theses DGs. The
simulation results show that the control strategy of the hybrid source
actively coordinate with the control strategy for controllable DGs
presented in Chapter IV.
Again the feeder flow does not change in case with proposed control
method, otherwise it changes according to the load condition and hybrid
source capacity.
CASE STUDY
125
Case 4-1: Islanded mode, without control strategy.
Fig. 6.21 Load demand and feeder flow in front of HB source
Feeder flow Pf1_HB is limited to 100kW
Fig. 6.22 Power response in case without control strategy
CHAPTER VI
126
Fig. 6.23 The hybrid source control mode
CASE STUDY
127
(a)
(b)
Fig. 6.24 System voltage and frequency
The frequency decreases according to the P-f droop characteristic of DG2
CHAPTER VI
128
Similar to grid-connected mode, the feeder flow at hybrid source is
limited to 100kW. The hybrid source’s control mode changes to the FFC,
from 5s to 7s, as its feeder flow reaches 100kW as depicted in Fig. 6.23
and, therefore, its output increases to meet the load demand, Fig. 6.22.
The voltage is regulated constant. However the frequency is changed
according to the droop characteristic as shown in Fig. 6.24 as the DG2’s
output increases between the duration of 3s to 9s.
Case 4-2: With HB source, islanded mode, DG2 changes its control mode.
Fig. 6.25 Load demand and feeder flow in front of HB source
CASE STUDY
129
Fig. 6.26 Power response in case with control strategy
CHAPTER VI
130
(a)
(b)
Fig. 6.27 The control mode of DG2 and hybrid source: (a) DG2 control
mode, (b) HB source control mode
CASE STUDY
131
(a)
(b)
Fig. 6.28 System voltage and frequency
The microgrid frequency remains unchanged as DG2 output increases
according to the FL-f droop characteristic.
CHAPTER VI
132
The parameters used in this case are same to those in case 4-1. When
DG1 output reaches maximum, DG2 increases its output to match the load
demand, as shown in Fig. 6.26. The hybrid source control mode changes to
the FFC when its feeder flow reaches limit, Fig. 6.27(a). The changes in
power output of DG1, DG2 and hybrid source are same to those in case 4.1.
However, in this case the DG2’s control mode changes to the FFC
whenever its output power increases as shown in Fig. 6.27(b) and,
therefore, the frequency does not change as depicted in Fig. 6.28(b).
Simulation results of case 4-1 and case 4-2 show that in the islanded
mdoe the hybrid source coordinates well with other DGs in the microgrid
and similarly to grid-connected mode. When the hybrid source increases its
output the control mode changes to the FFC, from 5s to 7s - Fig. 6.27b,
therefore the system frequency does not change. Additionally, with the
proposed control strategy the frequency does not change when the DGs
increase the outputs, Fig. 6.28, whereas it changes adequately according to
load in case without control strategy, Fig. 6.24.
6.2.3. Simulation Results with Multiple-Feeder Microgrid
In the grid-connected mode each feeder in the microgrid operates
independently and the operation of each feeder is similar to the single
feeder microgrid considered earlier in this section.
CASE STUDY
133
Case 5-1: Grid-connected mode, multi-feeder
Fig. 6.29 Active power responses of feeder 1
Fig. 6.30 Active power responses of feeder 2
CHAPTER VI
134
Fig. 6.31 Power from the main grid
Grid power increases to meet the load demand in feeder 2, LD2.
DGs in feeder 1 have sufficient capacities to meet the local load and
the feeder flow Pf1_DG1 is unchanged in grid-connected mode as shown
in Fig. 6.29. DG3 increases output to meet load demand LD2 as long as its
output is less than the maximum (50kW) and the feeder flow increases due
to the short of generation, Fig. 6.30.
The simulation results show that the control strategy applied to multi-
feeder microgrid in grid-connected mode is same to the single feeder case.
CASE STUDY
135
Case 5-2: Multi-feeder, islanded mode
Fig. 6.32 Active power responses of feeder 2 in island mode
Fig. 6.33 Active power responses of feeder 1 in island mode
CHAPTER VI
136
Fig. 6.34 Feeder flow in front of DG2, Pf1_DG2
Pf1_DG2 decreases due to the reversed power flow.
Fig. 6.35 Microgrid frequency
Frequency change according to droop characteristic
CASE STUDY
137
Fig. 6.32 shows that from 2s to 8s, the DG1 reaches its maximum
(50kW) thus the load LD2 is compensated by DGs on feeder-1 side and
feeder flow FL2 increases. Fig. 6.33 shows that both DG1 and DG2 in the
feeder 1 increase the outputs to compensate the load LD2 in the feeder 2.
When the load in feeder 2 is shared by DGs in the other feeder, feeder 1,
the frequency therefore changes according to the droop characteristic as in
Fig. 6.35.
From the simulation results it can be seen that the load shedding can
be achieved easily according to the change of frequency.
If DGs of each feeder can supply the load in its own feeder, the
power sharing among DGs in each feeder is same to the single-feeder case.
Therefore, the frequency does not change if the proposed control strategy
is applied as presented the earlier cases. Otherwise, the load in this feeder,
e.g. feeder 2, is compensated by the other feeder, e.g. feeder 1. The feeder
flow vs. frequency droop is applied and the frequency will be decreased in
case of high load. In such a manner load is shed according to the deviation
of frequency from the nominal value.
6.2.4. Proposed control method versus frequency restoration control
scheme in island mode:
Without control scheme, the frequency can be changed according to the
droop characteristic as microgrid is in the islanding mode due to the load
variations. With the proposed control scheme, the frequency can be kept
unchanged by changing the DGs’ operation mode. However, the frequency
can also be regulated to nominal value by means of frequency restoration
controller [69, 70].
CHAPTER VI
138
The following simulation results of single feeder microgrid show the
differences between the two control methods.
Case 6-1: Using frequency restoration control scheme
Fig. 6.36 Active power responses in case of frequency restoration method
Fig. 6.37 Microgrid frequency in case of frequency restoration method
CASE STUDY
139
Case 6-2: Proposed control scheme
Fig. 6.38 Active power responses in case of proposed method
Fig. 6.39 Microgrid frequency in case of proposed method
CHAPTER VI
140
The change in output power of DGs in case 6-1 and case 6-2 are
same as shown in Fig. 6.36 and Fig. 6.38.
From Fig. 6.37 and Fig. 6.39 it is seen that the frequency is
controlled unchanged in both two method, frequency restoration method
and proposed method. However, the frequency deviation due to the load
change in proposed method is much smaller than the one in frequency
restoration method. The frequency transient in conventional method
depends on the changes of load. The large load changes the higher
frequency deviates as shown in Fig. 6.37 because in the load is primarily
shared according to the droop characteristic. On the contrary, in the
proposed method the control mode of DG changes to the FFC therefore the
frequency does not change according to the droop characteristic. The
frequency deviation is caused by the mode change and thus it is much
smaller as shown in Fig. 6.39. As the result, the power quality of the
microgrid is improved in island mode. In addition, this characteristic
facilitates the load shedding as the generation is insufficient based on the
change of frequency from the nominal value.
Chapter VII:
CONCLUSIONS AND FUTURE EXTENSIONS
7.1. Conclusions
This dissertation has presented a control algorithm for the DGs and the
operation algorithm for the PV-FC hybrid source connected in a microgrid.
In order to achieve the control strategy, the frequency and active power
responses according to the droop characteristic have been investigated. The
analysis based on the three conditions of operations and four possible
configurations of the microgrid. According to the analysis, the proposed
control algorithm is to take the advantages and overcome the drawbacks of
each configuration. In addition, the operation of hybrid source is a
particular case and a control algorithm has proposed to operate in the
microgrid with a high efficiency and maximizing the solar energy.
Analysis of frequency and active power responses:
The analysis shows that: 1) in both parallel and series configurations, the
FFC mode has more advantages over the UPC mode in terms of frequency
change and reserved active power; 2) in islanded operation mode, the
configuration of series–FFC has more reserve to regulate frequency
unchanged and therefore it has more advantage over other configurations;
3) Otherwise, in grid-connected mode and transition mode, the parallel–
FFC configuration is more advantageous.
CHAPTER VII
142
Control strategies for controllable DGs in microgrid
With proposed method the change of frequency during the transition mode
(grid-connected mode/islanded mode) is minimized, the feeder flow at
PCC is remained unchanged during grid-connected mode and therefore the
microgrid becomes controllable load from the utility point of view. The
frequency of islanding operation mode can be regulated unchanged as long
as the load is shared by DGs in its own feeder. According to the proposed
method, the DG output is mobilized during heavy load condition thus
reduce the burden to the utility grid in the peak time and the microgrid
operates more stably. Otherwise, the UPC-mode DGs do not change their
control mode, and thus they can operate at high efficiency such as
economic condition, high performance band etc.
Control strategies for hybrid source
With the proposed operating algorithm, the system works flexibly,
exploiting maximum solar energy; PEMFC works within a high efficiency
band and hence improves the performance of the system operation. The
system can maximize the generated power when load is heavy and
minimize the load shedding area. When load is light, the UPC mode is
selected and thus the hybrid source works more stably.
The changes of operating mode only happen when the load demand is
at the boundary of mode change (PLoad1); otherwise, the operating mode is
either UPC mode or FFC mode. Besides, the variation of hybrid source
reference power PMSref is overcome by means of hysteresis. Additionally,
the number of mode changes is reduced. As a consequence, the system
CONCLUSIONS AND FUTURE EXTENSIONS
143
works more stably due to the minimization of mode changes and reference
value variation.
Microgrid operation with presence of hybrid source
The presence of the hybrid source together with other DGs in the microgrid
does not change the microgrid operation performance in both grid-
connected and islanded mode. It can also reduce the burden and facilitate
the operation of other DGs by limiting the feeder flow at its connection
point.
In the islanded mode, the frequency variation of proposed control
strategy is smaller than the one of the method with frequency restoration
scheme. Therefore, the power quality in islanded mode is improved. In
addition, the control strategy also facilitates the load shedding scheme in
the islanded operation mode by the frequency deviation.
7.2. Future Extensions
Control strategy for microgrid
The control strategy for microgrid did not mention the characteristic of
DGs. In a real microgrid various types of DGs are used such as PV, Wind,
FC, battery energy storage system (BESS), super capacitor (SC), micro-gas
turbine (MGT) etc. Each type of DGs has different operation characteristic,
for instance, MGT response time is slow, SC is a fast dynamic storage
(from seconds to minutes), BESS is long-term storage (from minutes to
hours) [71-75], etc. Among the available energy storage technologies,
batteries, flywheel and super-capacitors are more applicable for microgrid
CHAPTER VII
144
[76-78]. Therefore, the characteristic of DGs, especially the one of energy
storage technologies, should be taken into account to build up a control
strategy for whole system. For further extension, the microgrid control
strategy taken the DGs’ characteristics will be studied.
On the other hand, the control strategy has been proposed under the
technical view point. In order to, however, minimize the global energy cost
in the microgrid the control strategy should be put together with the
Economic Dispatch and the Unit Commitment problems. In other word, the
proposed control algorithm could be considered under circumstance of
energy management system.
Control strategy for hybrid source
The Fuelcell responses to the change of power slowly therefore the use of
fast response DGs such as BESS or SC can improve the operating
efficiency. The operating algorithm taking operation of BESS and SC into
account to enhance operation performance of the system will be considered.
BIBLIOGRAPHY
[1] P. Biczel, et al., "Power Electronic Devices in Modern Power
Systems," in EUROCON, 2007. The International Conference on
Computer as a Tool, 2007, pp. 1586-1586.
[2] L. Fresis and D. Infield, Renewable Energy in Power Systems: A
John Wiley & Sons, 2008.
[3] "European Technology Smartgrid Platform, "Smartgrids: Vision and
strategy for European electricity networks of the future"," 2006.
[4] R. H. Lasseter, "MicroGrids," in Power Engineering Society Winter
Meeting, 2002. IEEE, 2002, pp. 305-308 vol.1.
[5] R. Lasseter and P. Piagi, "Providing premium power through
distributed resources," in System Sciences, 2000. Proceedings of the
33rd Annual Hawaii International Conference on, 2000, p. 9 pp.
[6] P. Piagi and R. H. Lasseter, "Autonomous control of microgrids," in
Power Engineering Society General Meeting, 2006. IEEE, 2006, p. 8
pp.
[7] J. A. P. Lopes, et al., "Defining control strategies for MicroGrids
islanded operation," Power Systems, IEEE Transactions on, vol. 21,
pp. 916-924, 2006.
[8] J. A. P. Lopes, et al., "Management of Microgrids," JIEEC2003,
Bilbao, 2003.
[9] H. Jiayi, et al., "A review on distributed energy resources and
MicroGrid," Renewable and Sustainable Energy Reviews, vol. 12, pp.
2472-2483, 2008.
BIBLIOGRAPHY
146
[10] A. G. Tsikalakis and N. D. Hatziargyriou, "Centralized Control for
Optimizing Microgrids Operation," Energy Conversion, IEEE
Transactions on, vol. 23, pp. 241-248, 2008.
[11] F. Katiraei, et al., "Micro-grid autonomous operation during and
subsequent to islanding process," Power Delivery, IEEE
Transactions on, vol. 20, pp. 248-257, 2005.
[12] F. Katiraei and M. R. Iravani, "Power Management Strategies for a
Microgrid With Multiple Distributed Generation Units," Power
Systems, IEEE Transactions on, vol. 21, pp. 1821-1831, 2006.
[13] N. L. Soultanis, et al., "A Stability Algorithm for the Dynamic
Analysis of Inverter Dominated Unbalanced LV Microgrids," Power
Systems, IEEE Transactions on, vol. 22, pp. 294-304, 2007.
[14] A. P. S. Meliopoulos, "Challenges in simulation and design of
μGrids," in Power Engineering Society Winter Meeting, 2002.
IEEE, 2002, pp. 309-314 vol.1.
[15] A. A. Lasseter R, Marnay C, Stephens J, Dagle J, Guttromson R, et
al., "CERTS microgrid concept. White Paper on Integration of
distributed energy resources, prepared for Transmission Reliability
Program," Office of Power Technologies, U.S. Department of
EnergyApril 2002.
[16] R. H. Lasseter and P. Piagi, "Control and Design of Microgrid
Components," Jan. 2006.
[17] I. A. Hiskens and E. M. Fleming, "Control of inverter-connected
sources in autonomous microgrids," in American Control
Conference, 2008, 2008, pp. 586-590.
BIBLIOGRAPHY
147
[18] H. N, et al., "Centralized and decentralized control of microgrids,"
International Journal of Distributed Energy Resource vol. 1, pp.
197–212, 2005.
[19] P. a. L. JA, et al., "Control strategies for microgrids black start and
islanded operation," Int J Distr Energy Resour, vol. 2, pp. 211–31,
2006.
[20] H. N., "Active distribution network. The Effect of Distributed and
Renewable Generation on Power Systems Security," Available:
http://www.microgrids.eu/micro2000/presentations/21.pdf., 2005.
[21] M. Barnes, et al., "Microgrid laboratory facilities," in Future Power
Systems, 2005 International Conference on, 2005, pp. 6 pp.-6.
[22] T. Logenthiran, et al., "Multi-agent coordination for DER in
MicroGrid," in Sustainable Energy Technologies, 2008. ICSET 2008.
IEEE International Conference on, 2008, pp. 77-82.
[23] K. De Brabandere, et al., "Control of Microgrids," in Power
Engineering Society General Meeting, 2007. IEEE, 2007, pp. 1-7.
[24] G. Celli, et al., "Optimal participation of a microgrid to the energy
market with an intelligent EMS," in Power Engineering Conference,
2005. IPEC 2005. The 7th International, 2005, pp. 663-668 Vol. 2.
[25] J. D. Kueck, et al., "Microgrid Energy Management System,"
CERTSJanuary 29, 2003.
[26] L. K. Siow, et al., "Wi-Fi based server in microgrid energy
management system," in TENCON 2009 - 2009 IEEE Region 10
Conference, 2009, pp. 1-5.
[27] "Program on technology innovation: The Galvin Path to Perfect
BIBLIOGRAPHY
148
Power-A Technical Assessment," project report, EPRI, Mar. 2007.
[28] C. Gyu-Yeong, et al., "Comparative study of power sharing
algorithm for fuel cell and photovoltaic hybrid generation system," in
Power Electronics Conference (IPEC), 2010 International, 2010, pp.
2615-2620.
[29] A. M. Sharaf and A. A. A. El-Gammal, "A novel PSO-based hybrid
PV-FC-Diesel-Battery electric PID-controller drive system for
electric vehicle traction," in Electric Power and Energy Conference
(EPEC), 2010 IEEE, 2010, pp. 1-6.
[30] W. Caisheng and M. H. Nehrir, "Power Management of a Stand-
Alone Wind/Photovoltaic/Fuel Cell Energy System," Energy
Conversion, IEEE Transactions on, vol. 23, pp. 957-967, 2008.
[31] K. Loc Nguyen, et al., "Improvement of a PV-FC hybrid source
operation in a microgrid," in Power Electronics Conference (IPEC),
2010 International, 2010, pp. 717-720.
[32] K. Loc Nguyen, et al., "Power-Management Strategies for a Grid-
Connected PV-FC Hybrid System," Power Delivery, IEEE
Transactions on, vol. 25, pp. 1874-1882, 2010.
[33] T. K. Abdel-Galil, et al., "Project report, "Protection Coordination
Planning with Distributed Generation", Clean Energy Technologies
Techniques, CANMET Energy Technology Centre, June 2007."
[34] J. Balakrishnan, "Distributed generator interconnection protection,"
in Industrial and Information Systems, 2007. ICIIS 2007.
International Conference on, 2007, pp. 367-372.
[35] IEEE, "IEEE P-1547 Update on the Current Status of DG
BIBLIOGRAPHY
149
Interconnection Protection," ed.
[36] W. Hartmann and B. Electric. (Nov. 2002) Winding Arrangements
for Distributed Generation. On-Peak Performance.
[37] J. M. Guerrero, et al., "Decentralized Control for Parallel Operation
of Distributed Generation Inverters Using Resistive Output
Impedance," Industrial Electronics, IEEE Transactions on, vol. 54,
pp. 994-1004, 2007.
[38] N. Pogaku, et al., "Modeling, Analysis and Testing of Autonomous
Operation of an Inverter-Based Microgrid," Power Electronics, IEEE
Transactions on, vol. 22, pp. 613-625, 2007.
[39] C. L. Moreira, et al., "Using Low Voltage MicroGrids for Service
Restoration," Power Systems, IEEE Transactions on, vol. 22, pp.
395-403, 2007.
[40] K. De Brabandere, et al., "A voltage and frequency droop control
method for parallel inverters," in Power Electronics Specialists
Conference, 2004. PESC 04. 2004 IEEE 35th Annual, 2004, pp.
2501-2507 Vol.4.
[41] B. M. Weedy and B. J. Cory, Electric Power Systems, 4 ed.: New
York: Wiley, 1998.
[42] H. Nikkhajoei and R. H. Lasseter, "Distributed Generation Interface
to the CERTS Microgrid," Power Delivery, IEEE Transactions on,
vol. 24, pp. 1598-1608, 2009.
[43] A. Engler and N. Soultanis, "Droop control in LV-grids," in Future
Power Systems, 2005 International Conference on, 2005, pp. 6 pp.-6.
[44] H. Laaksonen, et al., "Voltage and frequency control of inverter
BIBLIOGRAPHY
150
based weak LV network microgrid," in Future Power Systems, 2005
International Conference on, 2005, pp. 6 pp.-6.
[45] A. Seon-Ju, et al., "Power-Sharing Method of Multiple Distributed
Generators Considering Control Modes and Configurations of a
Microgrid," Power Delivery, IEEE Transactions on, vol. 25, pp.
2007-2016, 2010.
[46] N. K. Loc, et al., "Analysis of Active Power and Frequency Response
in Microgrid," in IEEE Trondheim PowerTech 2011, to be published.
[47] M. Barnes, et al., "Real-World MicroGrids-An Overview," in System
of Systems Engineering, 2007. SoSE '07. IEEE International
Conference on, 2007, pp. 1-8.
[48] I. S. P. D. 10, "Draft Application Guide for IEEE Standard 1547,
Interconnecting Distributed Resources with Electric Power Systems,"
ed, Mar. 2008.
[49] N. K. Loc, et al., "Analysis of the Effects of Irradiation and Cell-
Temperature on the Dynamic Responses of PV System with MPPT,"
in KIEE Sumer Conference, July 2008.
[50] "Large scale integration of micro-generation to low voltage grids,"
project report, 2005, available online at http://www.microgrids.eu.
[51] T. Bocklisch, et al., "Predictive and Optimizing Energy Management
of Photovoltaic Fuel Cell Hybrid Systems with Short-time Energy
Storage," in 4th European Conference PV-Hybrid and Mini-Grid,
2008, pp. 8-15.
[52] J. Larmine and A. Dicks, Fuel Cell Systems Explained: New York,
Wiley, 2003.
BIBLIOGRAPHY
151
[53] X. Weidong, et al., "A novel modeling method for photovoltaic
cells," in Power Electronics Specialists Conference, 2004. PESC 04.
2004 IEEE 35th Annual, 2004, pp. 1950-1956 Vol.3.
[54] D. Sera, et al., "PV panel model based on datasheet values," in
Industrial Electronics, 2007. ISIE 2007. IEEE International
Symposium on, 2007, pp. 2392-2396.
[55] H. Chihchiang and S. Chihming, "Comparative study of peak power
tracking techniques for solar storage system," in Applied Power
Electronics Conference and Exposition, 1998. APEC '98. Conference
Proceedings 1998., Thirteenth Annual, 1998, pp. 679-685 vol.2.
[56] W. Caisheng, et al., "Dynamic models and model validation for PEM
fuel cells using electrical circuits," Energy Conversion, IEEE
Transactions on, vol. 20, pp. 442-451, 2005.
[57] E. Koutroulis, et al., "Development of a microcontroller-based,
photovoltaic maximum power point tracking control system," Power
Electronics, IEEE Transactions on, vol. 16, pp. 46-54, 2001.
[58] H. Chihchiang and L. Jong Rong, "DSP-based controller application
in battery storage of photovoltaic system," in Industrial Electronics,
Control, and Instrumentation, 1996., Proceedings of the 1996 IEEE
IECON 22nd International Conference on, 1996, pp. 1705-1710
vol.3.
[59] H. Chihchiang, et al., "Implementation of a DSP-controlled
photovoltaic system with peak power tracking," Industrial
Electronics, IEEE Transactions on, vol. 45, pp. 99-107, 1998.
[60] S. Weixiang, et al., "Mathematical model of a solar module for
BIBLIOGRAPHY
152
energy yield simulation in photovoltaic systems," in Power
Electronics and Drive Systems, 2009. PEDS 2009. International
Conference on, 2009, pp. 336-341.
[61] J. Leuchter, et al., "Mathematical modeling of photovoltaic systems,"
in Power Electronics and Motion Control Conference (EPE/PEMC),
2010 14th International, 2010, pp. S4-1-S4-4.
[62] R. F. Coelho, et al., "A proposed photovoltaic module and array
mathematical modeling destined to simulation," in Industrial
Electronics, 2009. ISIE 2009. IEEE International Symposium on,
2009, pp. 1624-1629.
[63] C. Wang, et al., "Control of PEM fuel cell distributed generation
systems," Energy Conversion, IEEE Transactions on, vol. 21, pp.
586-595, 2006.
[64] M. Tanrioven and M. S. Alam, "Modeling, Control, and Power
Quality Evaluation of a PEM Fuel Cell-Based Power Supply System
for Residential Use," Industry Applications, IEEE Transactions on,
vol. 42, pp. 1582-1589, 2006.
[65] Y. Zhu and K. Tomsovic, "Development of models for analyzing the
load-following performance of microturbines and fuel cells," Electric
Power Systems Research, vol. 62, pp. 1-11, 2002.
[66] L. Fangrui, et al., "A Variable Step Size INC MPPT Method for PV
Systems," Industrial Electronics, IEEE Transactions on, vol. 55, pp.
2622-2628, 2008.
[67] Z. Ye and X. Wu, "Compensation Loop Design of a Photovoltaic
System Based on Constant Voltage MPPT," in Power and Energy
BIBLIOGRAPHY
153
Engineering Conference, 2009. APPEEC 2009. Asia-Pacific, 2009,
pp. 1-4.
[68] T. Esram and P. L. Chapman, "Comparison of Photovoltaic Array
Maximum Power Point Tracking Techniques," Energy Conversion,
IEEE Transactions on, vol. 22, pp. 439-449, 2007.
[69] J. A. P. Lopes, et al., "Control strategies for microgrids emergency
operation," in Future Power Systems, 2005 International Conference
on, 2005, pp. 6 pp.-6.
[70] A. Madureira, et al., "Secondary Load-Frequency Control for
MicroGrids in Islanded Operation," in International Conference of
Renewable Energy Power Quality, Spain, 2005.
[71] L. Di, et al., "Design of a power management system for an active
PV station including various storage technologies," in Power
Electronics and Motion Control Conference, 2008. EPE-PEMC
2008. 13th, 2008, pp. 2142-2149.
[72] J. Li Jun, "Modeling and Simulation of Micro Gas turbine Generation
System for Grid Connected Operation," in Power and Energy
Engineering Conference (APPEEC), 2010 Asia-Pacific, 2010, pp. 1-
4.
[73] L. Xiao, et al., "Modeling and control of aggregated Super-Capacitor
Energy Storage system for wind power generation," in Industrial
Electronics, 2008. IECON 2008. 34th Annual Conference of IEEE,
2008, pp. 3370-3375.
[74] W. Juanhua, et al., "Study on a Super Capacitor Energy Storage
system for improving the operating stability of Distributed
BIBLIOGRAPHY
154
Generation system," in Electric Utility Deregulation and
Restructuring and Power Technologies, 2008. DRPT 2008. Third
International Conference on, 2008, pp. 2702-2706.
[75] B. Wu, et al., "Super-capacitors energy storage system applied in the
microgrid," in Industrial Electronics and Applications (ICIEA), 2010
the 5th IEEE Conference on, 2010, pp. 1002-1005.
[76] T. Sels, et al., "Electrical Energy Storage Systems: Existing Systems
versus Newest Systems-an Overview," in Power Generation and
Sustainable Development International Conference, 2001, pp. 215-
220.
[77] "Review of Electrical Energy Storage Technologies and Systems and
of their Potential for the UK," EA Technology, 2004.
[78] N. W. A. Lidula and A. D. Rajapakse, "Microgrids research: A
review of experimental microgrids and test systems," Renewable and
Sustainable Energy Reviews, vol. 15, pp. 186-202, 2011.
Appendix A:
FREQUENCY AND ACTIVE POWER RESPONSES IN THE ISLANDED MODE
A.1 PSCAD Model of Three-DG Microgrid
Fig. A.1 Three-DG microgrid, series configuration
APPENDIX A
156
Fig. A.2 Three-DG microgrid, parallel configuration
APPENDIX A
157
A.2 Simulation results
Configuration 1: Series-FFC
Fig. A.3 Power sharing of three DGs in series-FFC configuration
Fig. A.4 Frequency response in series-FFC configuration
APPENDIX A
158
Configuration 2: Series UPC
Fig. A.5 Power sharing of three DGs series-UPC configuration
Fig. A.6 Frequency response in series-UPC configuration
APPENDIX A
159
Configuration 3: Parallel-FFC
Fig. A.7 Power sharing of three DGs in parallel-FFC configuration
Fig. A.8 Frequency response in parallel-FFC configuration
APPENDIX A
160
Configuration 4: Parallel-UPC
Fig. A.9 Power sharing of three DGs in parallel-UPC configuration
Fig. A10 Frequency response in parallel-UPC configuration
Appendix B:
CASE STUDY
B.1 PSCAD Model of Whole System
APPENDIX B
162
B.2 PSCAD Model of Photovoltaic
Short circuit current of solar cell:
Photon current:
The saturation current:
APPENDIX B
163
PV voltage and current:
V-I characteristic:
B.3 PSCAD Model of MPPT Algorithm of PV
APPENDIX B
164
APPENDIX B
165
B.4 PSCAD Model of PEMFC
Enerst:
APPENDIX B
166
Activation loss, Vact:
APPENDIX B
167
Concentration loss, Vconc:
APPENDIX B
168
Ohmic loss, Vohm:
FC output voltage and current:
APPENDIX B
169
B.5 PSCAD Model of Buck-Boost DC/DCs and Controllers
PV’s DC/DC controller (MPPT controller):
FC’s DC/DC controller (DC voltage controller):
APPENDIX B
170
B.6 PSCAD Model Inverter Controller
APPENDIX B
171