CONTROL STRATEGIES FOR DISTRIBUTED GENERATION BASED MICRO-GRIDS

by

PARIMITA MOHANTY

Centre for Energy Studies

Submitted

in fulfillment of the requirements of the degree of Doctor of Philosophy

to the

INDIAN INSTITUTE OF TECHNOLOGY DELHI

MARCH 2013

i

CERTIFICATE

This is to certify that the thesis entitled “Control strategies for Distributed Generation based

microgrids”, being submitted by Ms. Parimita Mohanty for the award of the degree of Doctor of

Philosophy, is a record of bonafide research work carried out by her in the Centre for Energy Studies of

the Indian Institute of Technology Delhi.

Ms. Parimita Mohanty has worked under our supervision and guidance and has fulfilled the

requirements for submission of this thesis, which to our knowledge has reached the requisite standard.

The matter recorded in this thesis has not been submitted in part or full for the award of any other degree

or diploma.

(Prof. R. Balasubramanian) (Prof. G. Bhuvaneswari)

Centre for Energy Studies Department of Electrical Engineering

Indian Institute of Technology Delhi Indian Institute of Technology Delhi

HauzKhas, New Delhi – 110016, India. HauzKhas, New Delhi – 110016, India

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ACKNOWLEDGEMENT

It is my immense pleasure to express my heartiest gratitude to my supervisors

Prof. R. Balasubramanian, Centre for Energy Studies and Prof. G. Bhuvaneswari,

Department of Electrical Engineering, IIT Delhi for giving me the lifetime

opportunity to undertake my PhD work under their esteemed guidance and

supervision. It would not have been possible to reach this stage without their

continuous support and motivation.

I am also thankful to Prof. T. S. Bhatti, Dr. Nilanjan Sen Roy and all my SRC

members who have constantly encouraged me and provided very valuable suggestions

in different stages of my PhD work.

I am extremely obliged to Dr. Akanksha Chaurey, former Director, TERI,

Dr. R.K Pachauri, Director General TERI, Dr. Leena Srivastava, Executive Director,

TERI and Vice Chancellor, TERI University for prompting me to do PhD and

constantly encouraging me throughout my research activities. Without their support I

could not have accomplished this work.

I take immense pleasure in expressing my heart-felt gratitude to my fellow

research scholar Mr Hemant Ahuja for his cooperation, friendly behavior, assistance

and support right throughout. I also truly acknowledge the support I got from

Mr. Praveen Kumar and Ms. Navdeep Kaur during my research work. I would like to

express the deepest appreciation to all my near and dear who are involved directly or

indirectly for taking my research work to the completion stage

My thanks are due to my family members who have constantly encouraged me

to complete the research work. I have no word and sufficient enough for my parents,

my husband Mr. Abhay Kumar and my dearest son Sai Sankalp who had taken all the

pains and sacrifice and have given me all the comforts so that I can manage both the

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professional activities at TERI with extensive tours and my PhD activities

simultaneously. I salute them for their hard work and enthusiasm and dedicate this

work to my parents, my husband and my beloved son.

Parimita Mohanty

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ABSTRACT

Indian power sector has shown impressive growth in last few decades with the total

installed power generation capacity having grown to 202 GW by 2012. At present, the

gross electricity generation is 855 billion units. Despite such growth, spiraling power

demand, huge transmission and Distribution losses are the main challenges that has

been faced by India. Also, around 400 million people in India lack access to

electricity. In such a scenario, enhancing energy-security and energy-access to all her

citizens, is one of the major challenges that India has to deal with. Distributed

Generation based micro-grid has been proposed as one of the solutions to tackle these

challenges. Distributed Energy Resources (DER) can, not only deliver power to the

local areas (where it is installed and distributed) more efficiently and reliably, but it

can also feed excess power, if any, to the utility grid.

During recent years, Solar PV technology has emerged as one of the most

promising renewable energy candidates for micro-grid due to its improved efficiency

and maturity level with decreasing trend in cost and enhanced life with minimal

operation and maintenance cost. There exists a huge market potential of Solar

Photovoltaic (SPV) technology with worldwide average annual growth rate of around

40% per annum and prediction of similar annual market growth till 2020. Although

SPV systems are not technologically new, they have received increased attention

because of their ability to meet peak power demand, to provide backup power, to offer

improved power quality and reliability to micro-grids. The SPV systems require

specific power electronic converters to convert the power generated into useful power

that can be directly interconnected with the utility grid and/ or can be used for specific

consumer applications locally. The roles of these power converters become very

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critical, particularly when it is used in one of the most expensive energy generating

sources such as solar PV.

Although system level studies on the overall design and planning of micro-

grid is critical, it cannot be effective without the most optimum design of power

converters and controls for each of the distributed energy resources. Hence, it is

imperative to do research on power conversion and control at each energy resource

level. Considering the abundant availability of solar energy and multiple advantages

of SPV system, SPV system has been extensively researched in this work. Despite

several advantages of solar PV system with different power conversion units, there are

several hurdles, which need to be addressed in terms of its maximum power capture,

power quality, reliability, efficiency and cost. Different DC-DC and DC-AC

conversion topologies and control schemes for solar PV based power generating

systems are reported; but, a comparison of different power conversion topologies (DC-

DC and DC-AC conversion) in terms of their responses under various operating

conditions, conversion efficiency, reliability, ease of implementation is yet to be

explored.

This work focuses on modeling and simulation of the medium scale (5-10

kWp) solar PV system. It involves modeling, simulation and analysis of SPV

module and array for carrying out extensive simulation studies in order to assess the

system performance at different insolation, temperature and other operating

conditions such as partial shadowing etc. The results obtained from the simulation

are compared with the experimental results to validate the model developed. Further,

modeling, simulation and analysis of widely used DC-DC converters (i.e push pull

and full bridge converters), for solar PV system in MATLAB/SIMULINK platform

are also performed and their comparative performances in terms of settling time,

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ripple in the output, device stress levels, and capability to harness maximum power

are assessed under different operating conditions.

The performance of MPPT techniques, namely, Perturb and Observe

(P&O), Variable step-size P&O, Incremental Conductance, Fractional open circuit

voltage, are compared on the basis of maximum power capture capability, response

time, oscillations around MPP etc under uniform insolation as well as under rapidly

changing atmospheric conditions. Further, this work proposes a new, modified P&O

technique which is developed and assessed and found to perform better than its

predecessors. It harnesses a reasonably good amount of energy with a short response

time.

In addition, modeling, simulation and analysis of different inverter

topologies namely full bridge and newly used H5 topology for SPV system are also

carried out.

Despite the multi-fold advantage of distributed generation based micro-

grids, there are several research challenges in terms of optimal planning, designing

and load scheduling in micro-grids in order to provide reliable quality power at an

affordable price. There is a diverse range of applications which need different

configurations of micro-grids, depending upon its priority in load dispatch, resource

availability and other operational constraints. There is a need of systematic research

on the performance of micro-grids under different operating scenarios.

This work, is therefore, also targeting assessment of the performance of

distributed generation based micro-grid. In this work, solar PV, small wind electric

generator, biomass gasifier system and diesel generator are considered as the

distributed generation options for creating the micro-grid. Two to three different

operating scenarios are created in HOMER based on the load profiles, load

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categorizations, resource availability, load dispatch strategies etc and different micro-

grid options are created and simulated for those operating scenarios. The performance

of these micro-grid options are simulated and analyzed and the most optimum

configuration for different scenarios are recommended based on their cost of

delivered energy, life, fraction of un-served energy, renewable energy contribution,

load dispatch strategies etc.

An experimental setup consisting of 13 kWp solar Photovoltaic (SPV)

system, 3.3 kWe wind electric generating system, 10-100 kWe biomass gasifier

system, 48V, 600 Ah storage battery and 100 kVA diesel generator is considered

here. Various experiments have been carried out on this setup to validate the

simulation results. The simulated as well as experimental results of the micro-grid

system show how the contributions of different energy resources change at different

time of the day with varying load scenarios and how this contribution varies with

setting/changing of different control parameters and with different control strategies.

The simulation results also show the techno-economic performance of the micro-

grid when operated under different operating scenarios.

The key contributions in this research work include a)Development of SPV

module and array models which can be used to assess the dynamic performance of the

SPV array under different operating scenarios b) Comparative assessment of Push-

Pull and Full-Bridge DC-DC converters under different operating scenarios when it is

connected to a medium scale (5-10kWp) SPV array so that a judicious choice can be

made based on the operating conditions c) Comparative assessment MPPT

techniques, widely used in SPV system under various operating conditions so that

appropriate technique can be adopted d) Development of a new modified P&O

MPPT technique e)Development of optimal configurations of distributed generation

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based micro-grids for various applications, depending upon their load profile and

priorities, resource availability other operational constraints etc f) Experimental setup

of solar/wind/biomass gasifier/diesel generator/battery based micro-grid has been

developed to validate various simulation studies.

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TABLE OF CONTENTS

Page No

Certificate i

Acknowledgements iii

Abstract v

Table of Contents xi

List of Figures xxi

List of Tables xxix

List of Abbreviations xxxi

List of Symbols xxxv

Chapter 1 Introduction 1

1.1 General 1

1.2 State of the Art 5

1.3 Scope of the Work 10

1.4 Organization of the thesis 13

Chapter 2 Literature Survey 17

2.1 General 17

2.1.1 Solar cell modeling 17

2.1.2 DC-DC converter for Solar PV system 19

2.1.3 Maximum power point tracking methods for solar PV 21

2.1.4 Inverter for solar PV system 23

2.1.5 Coordinated control for renewable energy based micro-grids 25

2.1.5.1 Series hybrid system 26

2.1.5.2 Parallel hybrid system 26

2.2 Research gaps 29

xii

2.2.1 Identified research areas 31

2.2.1.1 Development of solar cell model 31

2.2.1.2 Performance assessment of different DC-DC 31

converters under varying operating conditions

2.2.1.3 Performance assessment of different MPPT 32

techniques under different operating conditions

2.2.1.4 Performance assessment of inverter topologies 32

under different operating condition

2.2.1.5 Performance assessment of Distributed 32

Generation based micro-grids

2.3 Conclusion 33

Chapter 3 Micro-grids-Basic system description and experimental setup 35

3.1 General 35

3.1.1 Types and typical capacity of DG based micro-grids 36

3.1.2 Different applications of micro-grids 37

3.2 Experimental setup 38

3.2.1 General 38

3.2.2 Solar PV configurations 41

3.2.2.1 Configuration with Experimental setup 41

of 7.2kWp solar PV system

3.2.2.2 Configuration of 1kWp thinfilm solar PV system 43

3.2.2.3 Configuration of 5.3kWp solar PV system 44

3.2.3 Configuration of storage battery 45

3.2.4 Configuration of wind electric generator 46

3.2.5 Specifications of wind turbine 47

xiii

3.2.6 Specifications of inverter 47

3.2.6.1 Specificationsof 5kW Sunny Island 5048 inverter 47

3.2.6.2 Specificationsof 8kW Sunny min central inverter 48

3.2.6.3 Specifications of 1.2 kW Sunny Boy inverter 48

3.2.7 Biomass gasifier Generator 48

3.2.8 Data acquisition system 50

3.3 Conclusions 51

Chapter 4 Control strategies for solar PV system 53

4.1 General 53

4.2 Control strategies for DC-DC converter 54

4.2.1 Push-Pull converter 55

Full-Bridge converter 57

4.3 Control strategies for MPPT 58

4.3.1 MPPT control with constant voltage 62

4.3.2 MPPT control with perturb and observe method 63

4.3.3 MPPT control with variable step perturb and observe method 64

4.3.4 MPPT control with incremental conductance 65

4.3.5 MPPT control with fractional short circuit current 67

4.4 Control strategies for inverter 68

4.4.1 Control strategies with full bridge inverter 68

4.4.2 Control strategies for H5 inverter 71

4.5 Conclusions 74

Chapter 5 Modeling and simulation of solar PV system 75

5.1 General 75

5.2 Modeling and simulation of solar PV array 76

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5.2.1 Development of solar cell model 76

5.2.1.1 One diode solar cell model 76

5.2.1.2 Two diode model of solar PV cell 78

5.2.1.3 Development of solar cell model in 79

simscape environment

5.2.2 Validation of simulated results with experimental data 80

5.2.2.1 Simulated results of 75 Wp solar module 81

5.2.2.2 Experimental characteristics of the solar module 82

5.2.2.3 Development of complete solar PV array model 84

5.3 Modeling and simulation of DC-DC converter 85

5.3.1 Push-Pull DC-DC converter 85

5.3.1.1 Design principle 85

5.3.2 Full Bridge DC-DC converter 88

5.3.2.1 Design principle 88

5.3.3 Buck-Boost operation of DC-DC converter 89

5.3.4 Modeling and simulation 92

5.3.4.1 Modeling and simulation of Full Bridge converter 92

5.3.4.2 Modeling and simulation of Push-Pull converter 95

5.4 Modeling and simulation of inverter 96

5.4.1 Principle of operation of sinusoidal PWM bridge inverter 96

5.4.2 Principle of operation of H5inverter 97

5.4.3 Modeling and simulation of sinusoidal PWM 99

bridge inverter

5.4.4 Modeling and simulation of H5 inverter 100

5.5 Modeling and simulation of MPPT techniques 102

5.6 Modeling and simulation of the entire solar PV system 102

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5.7 Modeling and simulation of 7.2 kWp solar PV system with 104

Push-pull DC-DC converter and bridge inverter

5.7.1 Modeling and simulation of 7.2 kWp solar PV 104

system with full-bridge dc-dc converter

and bridge inverter

5.7.2 Modeling and simulation of 1.6kWp solar PV array 105

With various MPPT techniques for battery charging

5.8 Results and Discussion 105

5.8.1 Simulated results of 7kWp PV system with 106

push-pull and full bridge converter with bridge inverter

5.8.1.1 Performance of DC-DC converter with 106

varying insolation

5.8.1.2 Performance of DC-DC converter with 113

varying insolation and temperature

5.8.2 Performance assessment of various MPPT techniques 114

under various operating conditions

5.8.2.1 Variation of power profile of 1.6kWpPV 114

system with different MPPT techniques at

fixed G-1000W/m2 and T-300K

5.8.2.2 Comparative assessment of different MPPT 118

Techniques under uniform G

5.8.2.3 Variation of power profile of 1.6kWpPV 120

system with different MPPT techniques at

varying G-1000W/m2 and T-300K

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5.8.2.4 Comparative assessment of different MPPT 123

Techniques under varying G

5.8.3 Development of a novel MPPT technique 124

5.8.3.1 Comparison of new two model modified P&O 126

method with other MPPT techniques

5.8.4 Experimental results of 7.2 kWp solar PV system with 127

DC-DC converter and inverter connected to dedicated

load

5.8.4.1 Solar insolation vs solar PV output power 127

5.8.4.2 Energy yield of the solar pv system 128

5.8.4.3 Voltage waveforms at different insolation 129

levels and load conditions

5.8.4.4 Voltage and current waveforms at the 131

inverter output

5.8.4.5 Efficiency of the power converter at 132

different load conditions

5.8.4.6 Operating frequency ranges of the power converter 134

5.9 Conclusion 135

Chapter 6 Modeling and simulation of micro-grids 139

6.1 General 139

6.2 Development of simulation models of micro-grids in HOMER 139

6.3 Development of micro-grid model for a typical village 141

6.3.1 Identifying the electrical load and developing 143

the load profile

6.3.1.1 Critical load 143

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6.3.1.2 Deferrable load 144

6.3.2 Development of energy resource profile 145

6.3.2.1 Solar energy resource 145

6.3.2.2 windenergy resource 149

6.3.3 Development of solar PV model 151

6.3.4 Development of wind model 152

6.3.5 Development of storage battery model 154

6.3.6 Development of inverter 156

6.3.7 Development of biomass gasifier model 157

6.3.8 Development of DG set model 159

6.3.9 Development of system controls and dispatch strategies 160

6.3.10 System economics and other details 162

6.3.11 Simulation and optimization for micro-grid for 162

a typical village

6.3.12 Results and discussions on micro-grids for typical village 163

6.3.12.1 Optimal production profile 163

6.3.12.2 Effect of maximum annual capacity shortage 164

6.3.12.3 Effect of distance from grid and optimal 165

breakeven point

6.3.12.4 Effect of different dispatch strategies 166

6.4 Development of microgrid model for peri-urban residential 167

complex - TERIRETREAT

6.4.1 Identifying electrical load and developing 168

the load profile

6.4.1.1 Critical load 169

xviii

6.4.1.2 Deferrable load 169

6.4.2 Development of energy resource profile 170

6.4.2.1 Solar energy resource 170

6.4.2.2 Wind energy resource 171

6.4.2.3 Development of solar PV model 172

6.4.2.4 Development of wind model 173

6.4.2.5 Development of storage battery model 174

6.4.2.6 Development of inverter 175

6.4.2.7 Development of biomass gasifier model 176

6.4.2.8 Development of DG set model 177

6.4.2.9 Development of system controls and dispatch 177

strategies

6.5 Simulated results and discussions 178

6.5.1 Optimum configurations of the micro-grids 178

6.5.2 Configuration of different micro-grid options with 179

no annual capacity shortage

6.5.3 Assessing the feasibility of installing 182

a wind electric generator

6.5.4 Effect of dispatch strategies 183

6.5.4.1 Forced on/off vs optimized operation of biomass 183

gasifier

6.6 Experimental results of micro-grids at TERI RETREAT 187

6.7 Simulated vs experimental results 189

6.8 Conclusion 189

Chapter 7 Conclusions and suggestion for further work 191

xix

7.1 General 191

7.2 Main Conclusions 194

7.3 Scope of Further Work 197

References 199

Annexures 207

A 3.1 Specification of battery 207

A 3.2 Specification of wind electric generator 207

A 3.3 Specification of inverter- 5 kW Sunny Island 5048 208

A 3.4 Specification of inverter- 8 kW Sunny Minni Central 8000TL 209

A 3.5 Specification of inverter- 1.2 kW Sunny Boy inverter 210

A 3.6 List of different electrical and physical parameter 211

List of Publications 213

Bio-data 215