pv diesel generator based power plant

Modelling, Simulation and Performance Analysis of PV-Diesel Generator Based Power Plant

For a rural community

Prepared by:

Faith Kanhukamwe KNHFAI001

Department of Electrical Engineering

University of Cape Town

Prepared for:

Dr. Sunetra Chowdry

Department of Electrical Engineering

University of Cape Town

October 2013

Submitted to the Department of Electrical Engineering at the University of Cape Town in partial fulfilment

of the academic requirements for a Bachelor of Science degree in Electrical Engineering

Key Words: photovoltaic, diesel, stand-alone, grid-connected, active power, demand

i

Declaration 1. I know that plagiarism is wrong. Plagiarism is to use another's work and pretend that it is one's own.

2. I have used the IEEE convention for citation and referencing. Each contribution to, and quotation in, this

final year project report from the work(s) of other people, has been attributed and has been cited and

referenced.

3. This final year project report is my own work.

4. I have not allowed, and will not allow, anyone to copy my work with the intention of passing it off as

their own work or part thereof

Name: Kanhukamwe, Faith

Signature: Date: 11 October 2013

ii

Terms of Reference The topic Modelling, Simulation and Performance analysis of a PV-Diesel Generator Based Power Plant,

was presented as an undergraduate Electrical Engineering research project. This was initiated in May 2013

by Dr. Sunetra Chowdry, Senior Lecturer at the University of Cape Town, Department of Electrical

Engineering. The need for the research arose as a result of the increasing use of Hybrid power systems (HPS)

as a cost effective and energy efficient method of rural electrification in many countries. This project related

to the modelling and simulation of a power plant which has solar photovoltaic (PV) and a diesel generator.

The application of the PV-diesel hybrid system would be such that the diesel generator would provide

backup power for the PV in order to ensure a continuous supply to a rural community.

The details and instructions of the project are outlined below:

1. Model a PV-diesel generator based power plant.

2. Determine whether the system will be tested with or without a battery.

3. Determine the dispatch strategy to be followed in designing the hybrid system.

4. Model a controller to regulate the schedule of operation of the PV array and diesel generator according

to resource availability.

5. Carry out simulations in MATLAB and/or DigSILENT.

6. Test the developed models for stand-alone and grid-connected operating modes with various loading and

sunlight conditions.

7. Submit a report and all the material used in accomplishing the projects objectives on the 15th of October

2013.

iii

Acknowledgements Firstly, I would like to thank my heavenly Father for the fullness of the Holy Spirit living in me, and

constantly reminding me that I am the helped of God. And for Jesus Christ, the word who became flesh, full

of grace and truth, through whom I am able to accomplish anything.

I would like to acknowledge my supervisor Dr. Sunetra Chowdhury for her supervision and guidance

through every step of my research project. And to the Electrical Engineering (EE) class of 2013 and BLW

UCT, thank you all for the contributions you made helping me reach this far.

To my loving, generous and awesome parents Mr. and Mrs. Kanhukamwe, thank you for your financial

support, sacrifices, prayers and most of all spiritual guidance in encouraging me to look to God for direction.

Finally to my siblings and family, thank you for your support and prayers. The time spent apart was working

for the greater good for all of us. I am truly grateful.

iv

Abstract In an attempt to provide low cost electrical energy for rural and remote areas and to encourage the use of

renewable technologies, the electrical industry has been de-regulated. Hybrid power systems are being

implemented in order to increase reliability distributed generation and minimise the carbon footprint from

electrical energy generation. As a solution to this problem, this project presents a model of the PV-Diesel

HPS with a battery. A controller is required to determine the dispatch of the generators. The analysis is done

in terms of the controller operation and operational cost in rural electrification or electrification of remote

areas. The system is designed to supply a rural community and was tested for both the stand-alone and the

grid-connected modes of operation.

Initially a thorough review of literature was carried out to identify the research that has already been carried

out and determine the course of action for this project. The Digital Simulation and Electrical Network

(DigSILENT) software was then chosen to be used for the simulations required to analyse the performance

of the system. Next, the complete hybrid system was described and the individual components were

modelled separately. The components modelled were the photovoltaic generator, the diesel generator, the

battery energy storage system and a secondary controller. The model for the PV array was verified in Matrix

Laboratory (MATLAB). The model of the external grid already available in the DigSILENT library was

used the simulations. Simulations were run for various case studies of varying loading and sunlight

conditions. Results from the simulations were then analysed and compared for the stand-alone and grid-

connected mode of operation. Finally conclusions were drawn from the work and recommendations were

made on how to improve the modelled controller.

The results showed that the hybrid system used the load following dispatch strategy. This implied that the

sum of the active power outputs from each source was equal to the demand. The photovoltaic (PV) generator

supplied a constant output depending on the solar radiation levels. Backup energy was provided by the diesel

generator according to primary and secondary control. Furthermore, the battery allowed excess energy to be

stored and the battery output was used as backup power to restore system balance. It was also seen in the

grid-connected mode that excess power supplied by the PV generator was supplied to the main grid. Finally,

the results showed that the main grid has a higher priority than the diesel generator in providing backup

energy, thus resulting in lower operation costs.

From the results the following conclusions were reached. The cost of supplying a rural community with

electrical energy depends on the available power sources. Although a battery would increase the capital costs

of the system, a battery energy storage system is required to improve the performance of the system.

However, there is a great need for a controller to determine the dispatch between the generators, particularly

in the stand-alone mode, hence increasing the overall efficiency of the system. The availability of many

power sources in the grid-connected mode results in faster reactions to power system imbalances. Thus, a

controller is required in the stand-alone mode to maintain balance in the power system. It was therefore

recommended to further improve the controller to also monitor and control voltage fluctuations.

v

Table of Contents Declaration ..................................................................................................................................................................... i

Terms of Reference .................................................................................................................................................... ii

Acknowledgements ................................................................................................................................................... iii

Abstract ......................................................................................................................................................................... iv

Table of Contents ......................................................................................................................................................... v

List of Figures ............................................................................................................................................................. vii

List of Abbreviations ................................................................................................................................................ ix

1. Introduction ......................................................................................................................................................... 1

1.1. BACKGROUND TO STUDY........................................................................................................................................ 1

1.2. OBJECTIVES OF THIS STUDY........................................................................................................................................ 1

1.3. SCOPE AND LIMITATIONS ............................................................................................................................................ 2

1.4. PLAN OF DEVELOPMENT .............................................................................................................................................. 2

2. Literature Review .............................................................................................................................................. 3

2.1. HYBRID POWER SYSTEMS (HPSs) ............................................................................................................................. 3

2.1.1. Cost of electricity generation ......................................................................................................................... 3

2.1.2. Feasibility of HPSs for rural electrification .............................................................................................. 4

2.2. CONTROL STRATEGY ...................................................................................................................................................... 5

2.3. OPTIMIZATION OF PV TECHNOLOGIES .................................................................................................................. 6

2.3.1. Components and operation ............................................................................................................................ 6

2.3.2. Financial considerations ................................................................................................................................. 7

2.3.3. Issues affecting PV performance .................................................................................................................. 8

2.4. OPTIMIZATION OF DIESEL GENERATOR ............................................................................................................... 8

2.4.1. The concept of diesel generators ................................................................................................................. 8

2.4.2. Environmental impact ...................................................................................................................................... 9

2.4.3. Financial considerations ................................................................................................................................. 9

2.5. PV-DIESEL GENERATOR ............................................................................................................................................. 10

2.5.1. Economic dispatch .......................................................................................................................................... 10

2.5.2. Benefits of PV-Diesel HPSs .......................................................................................................................... 10

2.5.3. Factors affecting optimal operation ........................................................................................................ 11

2.5.4. IEEE standards for generator protection .............................................................................................. 11

2.6. SELECTION OF BATTERY MODEL ........................................................................................................................... 11

2.7. OPERATION AND PERFOMANCE IN DIFFERENT NETWORKS ................................................................... 12

2.7.1. Operation in stand-alone mode ................................................................................................................. 12

2.7.2. Operation in grid connected mode ........................................................................................................... 14

2.8. PURPOSE OF RESEARCH ............................................................................................................................................. 15

3. Model development and preliminary design ......................................................................................... 16

3.1. METHODOLOGY .............................................................................................................................................................. 16

3.1.1. Problem formulation ...................................................................................................................................... 17

3.1.2. System overview .............................................................................................................................................. 18

3.1.3. Software selection ........................................................................................................................................... 19

3.2. MODEL OF SYSTEM LOAD .......................................................................................................................................... 19

3.3. MODEL OF PHOTOVOLTAIC SYSTEM .................................................................................................................... 20

3.3.1. PV array model ................................................................................................................................................. 20

3.3.2. PV controller ...................................................................................................................................................... 21

3.4. MODEL OF DIESEL GENERATOR ............................................................................................................................. 22

3.4.1. Synchronous generator ................................................................................................................................. 22

vi

3.4.2. Diesel governor ................................................................................................................................................ 23

3.4.3. Voltage controller .............................................................................................................................................. 24

3.5. MODEL VALIDATION WITHOUT BATTERY ........................................................................................................ 25

3.6. BATTERY MODEL........................................................................................................................................................... 26

3.6.1. PWM converter................................................................................................................................................. 27

3.6.2. Charge controller ............................................................................................................................................. 28

3.7. MODEL OF SYSTEM CONTROLLER ......................................................................................................................... 29

3.8. SIMULATION CASES ...................................................................................................................................................... 31

4. Simulation Results ........................................................................................................................................... 33

4.1. MODEL VALIDATION .................................................................................................................................................... 33

4.2. Case study 1: Sunrise (6am to 7am), morning peak load .............................................................................. 36

4.3. Case study 2: Sunset (6pm to 7pm), evening peak load................................................................................. 38

4.4. Case study 3: Cloudy day (6am to 8am) ............................................................................................................... 41

4.5. Case study 4: PV out of service ................................................................................................................................. 43

4.6. Case study 5: Night time, maximum load ............................................................................................................. 46

4.7. Comparison of performance for stand-alone and grid-connected ............................................................ 48

5. Conclusions ........................................................................................................................................................ 49

6. Recommendations ........................................................................................................................................... 52

7. List of References ............................................................................................................................................. 53

8. Appendices ......................................................................................................................................................... 56

APPENDIX A: LOAD DATA .................................................................................................................................................. 56

APPENDIX B: DIESEL GENERATOR DATASHEET ..................................................................................................... 57

APPENDIX C: SOLAR PANEL DATASHEET ................................................................................................................... 59

APPENDIX D: BATTERY DATASHEET ............................................................................................................................ 60

APPENDIX E: GRAPHICS FOR CONTROLLER MODELLING .................................................................................. 61

APPENDIX F: MATLAB CODE FOR MODEL VALIDATION ...................................................................................... 62

APPENDIX G: SIMULATION DETAILS............................................................................................................................. 64

APPENDIX F: SIMULATION FILES ................................................................................................................................... 64

9. EBE Faculty: Assessment of Ethics in Research Projects ................................................................... 65

vii

List of Figures List of Illustrations Fig2.1.Schematic of a typical PV-diesel-battery hybrid power supply system [19] 5

Fig 2.2.Diesel consumption against normalized area [18] 6

Fig2.3. Control system model 6

Fig2.4. Model of a standalone PV system 7

Fig 2.5.Cost comparison of diesel generator and PV [11] 7

Fig2.6. Electricity consumption profile [21] 8

Fig2.7. Block diagram for diesel generator control [20] 9

Fig2.8. PV-Diesel hybrid system configuration [5] 10

Fig2.9. Battery capacity versus battery life for load-following strategy [18] 11

Fig2.10. Flow chart of how a PV-diesel hybrid works 12

Fig 2.11.Frequency compensation by ESS [9] 13

Fig 2.12.Power sources in Grid-connected mode 14

Fig 3.1.Iterative framework followed in overall research 16

Fig 3.2.Components in PV-diesel plant 18

Fig 3.3.Rural community daily load profile 20

Fig 3.4.Electrical equivalent of a solar cell [28] 20

Fig 3.5.Per phase equivalent of a synchronous generator [34] 22

Fig 3.6.Block diagram of speed governor model [31] 23

Fig 3.7.Block diagram of the AVR model 24

Fig 3.8.Load flow results for system without battery 25

Fig 3.9.Simplified battery model [26] 26

Fig3.10.Schematic of a PWM converter [35] 27

Fig 3.11.Battery energy storage control system [32} 28

Fig 3.12.Block diagram of system control 29

Fig 3.13.PT1 controller electrical circuit 29

Fig 3.14.Flow chart of secondary controller operation 30

Fig 3.15.Percentage power output over time for PV arrays [33] 31

Fig 4.1. Power-voltage curve of PV array 33

Fig 4.2. Current-voltage curve of PV array 34

Fig 4.3. Characteristics of diesel generator 35

Fig 4.4.Load following using secondary controller 36

Fig 4.5.Case study 1 results for stand-alone mode 36

Fig 4.6.Case study 1 results for grid-connected mode 37





Fig 4.11.Case study 4 results for stand-alone mode (demand low) 43

Fig 4.12.Case study 4 results for stand-alone mode (demand high) 44




viii

Fig 6.1.Voltage and frequency fluctuations with changes in demand 52

Fig E1. DigSILENT Governor model modified for secondary control [23] 61

Fig E2. DigSILENT Battery power-voltage controller modified for secondary control 61

Fig G1. Power source output without a secondary controller 64

List of Tables

Table2.1. Diesel generator consumption .......................................................................................................................... 8

Table 3.1. Appliance usage per household ..................................................................................................................... 19

Table3.2. Calculation values for PV array sizing ......................................................................................................... 21

Table 3.3. Diesel generator parameters (Ambient temperature 40) ................................................................. 23

Table 3.4. Governor Parameter values ............................................................................................................................. 24

Table 3.5. Voltage controller parameter values ........................................................................................................... 25

Table 3.6. Calculation values for battery sizing ............................................................................................................ 27

Table 3.7. Parameter of the charge controller .............................................................................................................. 28

Table 3.8. System controller parameters ........................................................................................................................ 31

Table 4.1. Summary for Case study 1 ................................................................................................................................ 38





Table A1. Rural demand data ................................................................................................................................................. 56

ix

List of Abbreviations

A, amps -amperes

AC -alternating current

Ah -ampere hour

AVR -automatic voltage regulator

BESS -battery energy storage system

C -capacitor

CaSos - Complex Adaptive System of Systems

DC -direct current

DGs -distributed generation

DigSILENT - Digital Simulation and Electrical Network

DSL - DigSILENT Simulation Language

emf -electromotive force

ESS -energy storage systems

FFC - feeder-flow control

H -constant of inertia

HPS -hybrid power system

K0 -modulation factor for sinusoidal PWM

kWh -kilo watt hours

MATLAB -Matrix Laboratory

MC -micro controller

Mi - imaginary part of PWM modulation factor

Mr - real part of PWM modulation factor

MPPT -maximum power point tracker

MW -megawatts

Opamp -operational amplifier

PBATTERY, PESS -active power output from battery

PCC -point of common coupling

PDIESEL -active power output from diesel

PGRID -active power output from main grid

PPV -active power output from photovoltaic generator

PSS -power system stabilizer

pu -per unit

PWM -pulse width modulation

Ra -armature resistance

SOC -state of charge

T -torque

UPC - unit-power control

VAC -alternating current voltage

-angular velocity

xal -leakage reactance

xar -magnetizing reactance

1

1. Introduction 1.1. BACKGROUND TO STUDY

The energy industry today is aiming at providing electrical energy for everyone. The need to generate and

distribute this energy as cheaply as possible has resulted in de-regulation of the electricity industry. More

importantly, the global Green- initiative aims at transforming the conventional energy sources to

renewable. However, most renewable technologies are highly subjective to availability of resources (wind

generation, hydro-generation) and unpredictable weather condition (solar PV, wind generation).

In order to increase reliability and minimise these effects, hybrid technologies are being designed and

implemented. These integrate operation of different types of renewable and low carbon sources. Energy

storage devices are incorporated for cost-effective electrification. This then justifies the lifecycle costs of

renewable technologies.

This research project investigates the performance PV-Diesel hybrid power system (HPS) with a dispatch

controller. The analysis is done in terms of the controller operation and operational cost in rural

electrification or electrification of remote areas. The HPS will be tested for both the standalone and grid

connected modes of operation. This report will involve modelling, simulation and analysis of performance

for a PV- diesel hybrid power system with an energy storage device using the load-following dispatch

strategy.

1.2. OBJECTIVES OF THIS STUDY The aim of this research project is model, simulate and analyse the performance of a PV-Diesel generator

based power plant in different modes of operation. Problems to be investigated include:

Conduct a literature review based on the PV-diesel HPS

Synthesis all theoretical viewpoints which justify the research problem (i.e. the analysis of the performance

of a PV-diesel HPS in standalone and grid connected modes using load-following).

Derive a load profile typical for a rural area

Model realistic rural loads and develop a suitable load profile.

Model the PV-Diesel generator system

The model should ensure minimum operating cost, continuous supply and efficient rural electrification. The

system comprises an array of photovoltaic (PV) cells, a diesel generator and a battery energy storage system

(BESS).

Model a controller to track the demand for load-following The controller monitors the hourly load demand. It also carries out the energy dispatch strategy such that the

diesel generator is only switched on if the PVs rated hourly output is less than the hourly load.

Implement the effective model in MATLAB and/or DigSILENT The mathematical models must then be simulated to obtain an energy efficient system.

Thus, the purpose of this study is to:

Analyse the performance of the PV-Diesel HPS

2

Performance analysis can be carried out for various loading and weather conditions. As the hourly load

demand varies, the power flows can be analysed after inputting parameters like solar radiation, storage

capacity and diesel content.

Explain reliability of system for grid-connected and standalone modes

Determine the most reliable and energy efficient mode of operation. The description of the controller

operation in either mode can be derived from the analysis.

Draw conclusions and recommendations for future work

Draw conclusions on the extent to which this model is optimal for rural electrification and recommend

strategies and fields for further research in the modelling of the controller.

1.3. SCOPE AND LIMITATIONS

This report investigates the performance of a PV-diesel hybrid system with battery energy storage system

(BESS). It was required to model and simulate the PV-diesel generator based power plant and controller. The

system is developed and tested for stand-alone and grid-connected operating modes with various loading and

sunlight conditions.

The research conducted will aim at answering the following questions:

What is the cost of supplying a rural community with electrical energy?

What is the most optimized model of a PV-diesel hybrid power system?

How do different load levels and weather patterns affect the performance of this model?

How would implementing controller model influence the energy efficiency of the system?

How is the stability of the system affected when connected to different networks?

Although the temperature of the system would affect performance, its effects have been neglected. For the

purpose of this research, rapidly changing solar irradiations and temperatures were not considered. The

battery storage system is assumed constant temperature.

1.4. PLAN OF DEVELOPMENT

The research begins with a critical review of the existing research necessary for this project. The literature

will consist mainly of material on optimal rural electrification, dispatch strategies for hybrid power systems

and concepts of the component models of the hybrid system.

Chapter 3 outlines the design process of the system. Initially the methodology of this research is described

and the suitable software for simulation is chosen. Mathematical and electrical models are then provided for

the various components of the PV-diesel hybrid system and the controller. Finally, case studies are defined

for the simulation

The design is implemented in Chapter 4. Modeled components are implemented in MATLAB and

DigSILENT. Simulations are then carried out to understand the capabilities of the system in stand-alone and

grid connected modes. Attention is then paid to diagrams and descriptions of the simulations. Simulation

results are then analyzed and discussed.

Conclusions on the performance of the system in different modes of operation are then drawn based on the

results obtained in Chapter 5. Finally, in Chapter 6 recommendations for future work on the controller

development are made based on the discussion in the conclusions.

3

2. Literature Review

2.1. HYBRID POWER SYSTEMS (HPSs)

2.1.1. Cost of electricity generation

All electricity generators deliver electrical energy. However, the price of this energy to the customer and the

rate of depletion of resources are determined by the type of generator. Owing to over-dependence on fossil-

fuels and poor energy efficiency, many countries are facing depletion of natural resources and environmental

pollution.

The main factor contributing to cost of generation is the cost of fuel. For efficient generation, high cost fuels,

for example diesel and gas are usually run as peak loads. Fuel costs also include transport costs and ash

disposal costs in the case of coal fired power plants. Another factor contributing to cost of generation is the

capital costs. These costs are determined by electrical, mechanical and civil works at a power plant.

Assuming a small generating power plant, these costs are usually very high in the cases of environmentally

friendly technologies such as photovoltaic panels. For large base load power stations, shut down costs are

very high and assumed to be equal to start up costs.

Maintenance costs are lower for new power plants and higher for old power plants. Labor costs also

contribute a small fraction to the overall cost of generating electricity. In the attempt to reduce greenhouse

gas emissions, some countries have included emissions cost into the cost of generating electricity. Emissions

costs have a significant impact on the cost of electricity. [2]

Despite the high capital costs of most renewable power technologies, their reduced costs of operation and the

fact that they are green technologies justifies their use in distributed generation (DGs). It should be noted that

some DGs for example diesel generators are neither environmentally friendly technologies nor inexpensive

to operate. As a result, depending on economic viability, technical features and environmental issues, it is

often necessary to combine two or more technologies to supply a load. These are known as Hybrid Power

Systems (HPS). HPS have the following advantages:

Portability: Similar to any stand-alone power generation system, HPS overcome reliability or

economic limitations caused by extension of the main grid to remote areas.

Design flexibility: There is a design trade-off between the two systems, for example, decreasing costs

of fuel, increases dependence on weather

Increased reliability: The use of more than one power source increases the overall reliability of the

system by exploiting the benefits of each source.

Controlled cost: Hybrid power systems can be designed to maximize the use of renewable energy.

According to [22] access to reliable electricity is a precondition boosting living standards and sustainable

development in any community. In order to achieve development, rural areas are in need of reliable and

affordable electricity. Most recent literature has shown that off-grid renewable energy is the most suitable for

rural electrification because it overcomes the need for long distribution infrastructures. However, there is

need to supply rural communities with steady, grid quality electrical energy. Hybrid power systems

capture the best features of each energy source. Consequently, they provide efficient and reliable electricity.

These systems can also be used as a backup solution for the main grid [22].

4

2.1.2. Feasibility of HPSs for rural electrification

Financial feasibility studies of stand-alone hybrid systems are aimed at assessing the ability of a proposed

venture to justify itself from an economic point of view. Literature describes several hybrid systems, for

example PV-Wind, PV-Battery-Wind, PV-Diesel, PV-Battery-Diesel and PV-Wind-Fuel cell [22].

According to [22], the costs of renewable technologies have been brought down by a combination of

improved technology and economies of scale. Hybrid systems based on renewable energy sources (RES)

have no significant operating costs. Moreover, even if these systems may include fuel generator as a backup,

the renewable energy is designed to supply at least 60% of the demand. Therefore, HPS have proven to

provide high quality, least cost energy for remote areas.

This research project will focus on a hybrid power plant consisting of photovoltaic (PV) panels, a diesel

generator, batteries and a DC-AC converter. According to recent studies [5, 19, 23], the PV-diesel HPS

requires mostly sunlight and minimizes the fuel used in the operation of the diesel generator in order to

generate electrical energy.

In general, grid extension to remote areas is not feasible. Access to reliable and affordable electrical energy

can play an important role in improving living standards and reducing deforestation in remote areas. Due to

the cost of transporting diesel for diesel generators, cost of electricity in remote areas becomes higher than

the national grid, yet it is also very expensive to extend the national grid to these areas. [7] The use of HPS

would make electricity available to the remote area. Moreover, because the fuel required is minimized for the

diesel generator through its use in a hybrid power system with a renewable energy technology, electricity can

be delivered to these rural areas cheaply.

The power demand for low income communities (rural, informal settlement and townships) is relatively less.

These communities require cheap energy and are not very critical of the quality of supply. This is mainly

because of the following reasons:

They cannot afford many electrical gadgets

Due to limited education levels, most people may not be aware of the latest technology appliances

available

These communities are not very safe, so customers avoid buying appliances which might be stolen in

the end

Use of security lighting, alarms, swimming pool pumps, electric blankets is not a common practice

Rural resident activities are less energy intensive than urban residents. However as a result of technology

advancements and rural electrification, township energy consumption could be expected to increase. Where

the main use of electricity is expected to be for lights and small appliances, there is no reason to apply the

design standards used for much more heavily loaded urban systems. However, it is still necessary to offer

customers a continuous supply of electricity.

A standalone hybrid power system is more cost effective for rural communities. If there is insufficient

renewable energy to supply peak load, the system should be designed with backup battery storage charged

with renewable energy storage. In addition, to ensure a continuous supply, diesel generators can be used as

backup during peak loads. The PV-diesel HPS can be operated for various loading and sunlight conditions.

[7] Moreover, relative to most Distributed Generation (DG) technologies, the diesel generator and PV are

cheap.

In the case where the HPS is grid-connected, it serves as an effective backup solution to the main grid [22].

This is particularly beneficial in case of blackouts or weak grids, and for professional load centers, for

example telecommunications and hospitals.

5

It should be noted that the diesel generator has a low efficiency at light loads. Thus, the PV-Diesel Hybrid

Power System incorporates batteries for energy storage which can be discharged at light loads. During peak

loads, both the battery and the diesel generator are used to supply the demand. Figure 2.1 shows the general

layout of the PV-diesel HPS.

Fig2.1.Schematic of a typical PV-diesel-battery hybrid power supply system [19]

2.2. CONTROL STRATEGY

In general, domestic loads levels vary with time. If generation is not adjusted properly to follow the demand,

fluctuations in the interaction between the demand and generated power could create imbalances in the

power system. This means that strategic control is required to monitor the demand and generation and hence

match both systems. Two different dispatch strategies are often used.

a. Night dispatch

In this strategy, the diesel generator is only switched on at night or on cloudy days, when there is no solar

radiation.

b. Load following

In the load following strategy, the diesel generator is switched on when the load equals or exceeds the rated

output of the solar-PV arrays [19].

Although the night dispatch strategy is relatively less complicated to control and offers a sophisticated

system, considering optimization of resources, studies in [18] show that the load-following dispatch strategy

is more superior. The study shows that the effective working hours for the diesel generator are less for the

load following strategy than for the night strategy. As a result the maintenance and fuel costs are higher for

the night strategy. Therefore, this research project will use the load following strategy because it is more

environmentally friendly in terms of cost and emissions into the environment. Figure 2.2 below shows that

diesel consumption decreases as PV array area increases.

KEY

~ AC power

= DC power

= INVERTER

~

AC LOAD

DIESEL

GENERATOR

~ BATTERY

CHARGER = ~

BATTERY

BANK

=

SOLAR

CONTROLLER

PV

ARRAY =

6

Fig 2.2.Diesel consumption against normalized area [18]

A controller is thus required to monitor the interaction between the hybrid system and the load. In order to

simplify this study, the control system is assumed to consist of real power proportional integral controls (Fig

2.3).

Fig2.3. Control system model

The purpose of the controller here is to ensure that the load demand is supplied at all times. The controller

will track the difference between the PV power output and the demand level. If the demand level is more

than or equal to the PV output threshold, the controller outputs a signal to switch on the diesel generator.

Similarly if demand falls below the PV threshold, the controller disconnects the diesel generator. At demand

levels above both the diesel generator output and the PV output, the controller prompts the battery to

discharge. However, if the system experiences loss of load, the controller disconnects the load from the

battery, thereby avoiding severe discharge. [19]

2.3. OPTIMIZATION OF PV TECHNOLOGIES

Capital costs of photovoltaic panels are very high. The optimization problem of PV technologies depends on

balancing between size and the price of the module. This results in the best selection of the PV panel to

harness the maximum solar energy while minimizing capital costs.

2.3.1. Components and operation

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Normalized area

Die

sel c

on

sum

pti

on

(lit

res/

ye

ar)

Load following

Night strategy

0 2 4 6 8 10

PV MODULE

CONTROLLER

BATTERY LOAD

MPPT

INVERTER

KP

Ki/s

P

Pref

Pin + +

+

-

7

Fig2.4. Model of a standalone PV system

The PV module comprises several PV cells joined together. Figure 2.4 above shows the general layout of a

stand-alone photovoltaic system. This module absorbs the sunlight energy (photons) and converts it to

voltage. The unidirectional flow of negative charge forms direct current (DC). To ensure a continuous supply

of electrical energy, batteries have a large storage capacity in order to be used when solar radiation is

minimal. The battery is connected directly to the DC bus of the grid connected inverter. The inverter

converts DC to alternating current (AC). It should have a high efficiency close to 100%. The maximum

power point tracker (MPPT) technique is used to extract maximum power from solar panels and

continuously deliver the highest power when load variations in irradiations and temperature occur. Although

the MPPT will ensure smooth charge and discharge of the battery, this alone is insufficient to increase the

battery life.

2.3.2. Financial considerations

Capital costs of PV systems comprise of costs such as labor, installation, mechanical and civil works and

also PV array costs which vary depending on the nature of the technology. Fig2.5. below shows that the

capital costs of PV systems is 6.5 times higher than that of a diesel generator. However, it should be noted

that little maintenance is needed for PV systems. Thus maintenance costs are negligible.

Fig 2.5.Cost comparison of diesel generator and PV [11]

Rural households on average consume less than 100kWh of electricity per month (Fig 2.6). The amount of

energy used by each household is insufficient to justify the high capital cost of installing a PV module of that

capacity. Since cost is a critical factor for rural electrification, the PV array model size needs to be optimal.

0

20000

40000

60000

80000

100000

120000

140000

160000

Year

Photovoltaic

Diesel generator(inflation 6%)

0 2 4 6 8 10 12 14 16 18 20

8

Fig2.6. Electricity consumption profile [21]

2.3.3. Issues affecting PV performance

Solar energy does not produce greenhouse gas. However, several reasons have contributed to it not being a

common electrical energy source. Its dependence on weather conditions results in unpredictable operation.

The dependence of PV on irradiation and temperature makes it an uncontrollable source. Furthermore, solar

energy is unavailable at night. The standalone power system is most effective in areas with high solar

radiation.

Although ground mounted PV systems are cheaper than roof-mounted PV systems, the existence of tall

structures (buildings and trees) will shade the PV system. Thus, large scale solar systems will require large

areas of land to be cleared. Temperature has an effect on the output of a PV module. By increasing the

temperature at high voltages the voltage may be susceptible to drop.

2.4. OPTIMIZATION OF DIESEL GENERATOR

2.4.1. The concept of diesel generators

Diesel generators are sized to be able to carry the load even during peak power demand. For safety reasons,

they are being sized to leave room for upgrades, and they never run at full load (always around 30% to 60%)

in order to avoid undue mechanical stress [15].

Table 2.1 below shows that in supplying rural loads, diesel generators are not used efficiently due to low

loading levels.

Table2.1. Diesel generator consumption

Generator Loading

Fuel consumption (L/h)

Fuel consumption (L/kWh)

0.75 1.98 0.26

0.50 1.56 0.31

0.30 1.04 0.52

94%

6%

0

20

40

60

80

100

120 Proportion of households in each household whose monthly consumption falls in each slab (2010, Rural Households)

Poorest quantile

2

3

4

Richest quantile

ALL

0-100kwh 100-400kwh >400kWh 0-100kwh 100-400kwh >400kWh

0%

9

Figure 2.7 below shows the block diagram of a diesel generator. The frequency is kept constant by

maintaining the rotor speed via its governor. Output voltage is controlled by controlling excitation current.

Fig2.7. Block diagram for diesel generator control [20]

2.4.2. Environmental impact

One major issue associated with the use of diesel fuel is the emission of green house gases. This increases

the amount of carbon dioxide released into the atmosphere. Electricity generation using diesel driven

generators is one of the major sources of air pollutants which have a negative effect on public health. Studies

have shown this to cause premature mortality due to respiratory and cardiovascular symptoms. Due to the

intangible aspect of health impact, the economic value of health cost cannot be directly estimated.

Although pollutants would be more concentrated at the generating plant, dispersion of air borne pollutants is

affected by meteorological conditions and emission control technologies. Implementation of the latter has a

significant impact on the total cost of the diesel generator system. Secondary pollutants which are formed

due to chemical reactions in the atmosphere could be dispersed over long distances relative to the primary

pollutants. However, due to the highly spacious arrangement of rural dwellings, the impact of pollution is

lower than it would be in densely populated areas.

Another environmental problem associated with diesel generator is noise pollution. This particularly has an

effect of interfering with wild life in remote areas.

2.4.3. Financial considerations

The overall cost of operating a diesel generator is very high. These include cost of fuel, cost of transportation

of the fuel and maintenance costs of generator.

The operation of diesel generators is highly dependent on the cost of diesel. Also capital costs are relatively

higher than that of PV modules. These vary depending on the size and the brand of the diesel generator. The

emission penalties are fixed at $2.25/l as per international standard [7] .The pricing considerations of a

generator are combined result of factors such as motor speed, noise level and load factor.

Due to high maintenance and operating costs of diesel generators, after about seven years, the investment

costs equal that of PV (Fig 2.2.2). The operating cost of diesel generators is very high due to fluctuating

demand. Cost of diesel generator is very expensive when starting up or shutting down.

Diesel Engine

Governor

Exciter Synchronous

generator

V

+

-

f

-

+

To the rest of the power system

10

2.5. PV-DIESEL GENERATOR

2.5.1. Economic dispatch

Optimization of PV-diesel hybrid systems involves optimal selection of PV modules and diesel generator

capacity. The procedure involves modeling of the diesel generator and then selecting the PV and battery

sizes by determining the minimum number of storage days and the minimum PV array area based on a well-

defined weather profile. In addition an energy dispatch strategy is determined to obtain set points for the

starting and stopping of the diesel generator in order to minimize the overall system costs. [10]

Although installation costs form part of the total cost, Economic Dispatch aims at minimizing the total

operating costs. This considers how the real power output of the diesel generator unit is controlled to meet a

given load for optimal operation.

Depending on the level of demand and the state of charge of the battery, the generated energy is used is one

of the following ways:

Consumed by load

Supplied to the battery

Wasted (dumped energy)

Assuming the load-following strategy is followed in stand-alone mode of operation, at any hourly-instant,

the diesel generator is either off or on (at rated output). If PLOAD < PDIESEL + PPV, where PLOAD is the

hourly-average demand and PDIESEL is the rated supply of the diesel generator, the diesel generator

supplies the load and the excess goes into charging the battery. Any excess energy to this supply then goes to

waste. If PLOAD > PDIESEL + PPV, the load is then supplied by the combined supply of the diesel

generator, solar-PV array and the battery.

It should be noted that the system design makes it impossible for the hourly average demand to exceed the

total output of the PV array, diesel generator and battery. The system experiences loss of load under these

circumstances. [19]

2.5.2. Benefits of PV-Diesel HPSs

Fig2.8. PV-Diesel hybrid system configuration [5]

Fig 2.8 above shows the concept of a PV-Diesel hybrid simulated in this project. The diesel generator

provides backup to loads in the case of unavailability of PV module output. Thus, a continuous and more

reliable power system is achieved. Moreover, the cost of diesel fuel used is minimized by maximizing use of

solar energy. Relative to a solar-PV only generator system, a hybrid power generation system reduces the

initial cost of the system and benefits from low PV maintenance. Due to diversity of power sources, the

system has reduced noise pollution and emissions.

Bidirectional

inverter

PV

module

Battery

MPPT ~

= Diesel

Generator

LOAD

11

2.5.3. Factors affecting optimal operation

To ensure a more efficient and economically viable system, the following must be considered:

Minimize diesel consumption of generator

Utilization rate of the photovoltaic panel must be maximized

Minimize battery capacity storage

The net present cost is a direct result of both the amount of solar irradiation experienced and the size of the

solar panels. Thus, the amount of diesel used can only be minimized if solar energy is supplied for a longer

period of time.

2.5.4. IEEE standards for generator protection

Multiple isolated generator systems are normally operated manually, but load-sensing controls and automatic

synchronizing relays can be used. The rated voltage is usually at the utilization voltage or the highest

distribution voltage level, or both, such as 4.16 kV or 13.8 kV. Rural loads are mostly domestic loads.

Therefore the rated voltage of the hybrid system should be 0.4kV.

If the system is effectively grounded, the generator neutral is grounded with a neutral inductive reactance. If

the system is not effectively grounded, the generator neutral is grounded through a small resistor [3]. For the

purpose of this research, the protection system of the generator will not be considered.

2.6. SELECTION OF BATTERY MODEL

Even though addition of storage elements increases the initial cost of the system, it reduces the overall

operating cost of energy produced using a hybrid system significantly. Battery energy storage systems

(BESS) are run as back-up when there is low solar irradiation or when diesel generator runs out of fuel. In

some cases, the battery storage system runs as cover-up when the generator is shut down for maintenance.

Figure 2.9 below shows that battery life increases with capacity. Although an increase in capacity means an

increase in the initial cost, a long life means reduced replacement costs

Fig2.9. Battery capacity versus battery life for load-following strategy [18]

0

2

4

6

8

10

12

0.5 0.6 0.8 1 1.3 1.4 1.6 1.8

Bat

tery

life

(ye

ars)

Battery capacity/ daily load

Varied normalized area

2

3

4.5

12

The battery storage system can also be used to store electrical energy while the diesel generator is running.

This improves the running efficiency of the diesel generator by pushing the diesel generator to run at full

load instead of values in Table 2.3.1 above. The flow chart below shows the operating cycle for diesel

generator-battery system.

Fig2.10. Flow chart of how a PV-diesel hybrid works

The batterys primary use is to provide the load with power in periods of no irradiation, such as that of night time. Thus, the battery only needs to be able to provide the load for a few hours. Since the batteries charging

and discharging cycles will be very short, its lifespan will reduce greatly. According to [17], the following

are some tradeoffs:

High quality, deep cycle batteries are required.

Lifespan reduction is dependent on factors such as battery type, depth of cycle (how long the battery supplies the load), rate of recharging (how often the battery is recharged)and ambient temperature

The energy lost during recharging It is vital that the battery be fully charged after every cycle, otherwise the capacity will staircase down each cycle. Eventually the storage system will stop working.

According to [19], the battery can only discharge when its depth of discharge is less than the maximum

allowed.

For computer simulations, several battery models have been proposed based on the electrical parameters

exhibited by the battery. These include Kinetic battery model (KiBaM), Universal lithium-ion model,

Dynamic lithium-ion model, Impedance based lithium-ion model and Capacity based lead-acid model. [8]

From the BESS modeling carried out in [8], the lithium-ion chemistry exhibits the best characteristics for the

photovoltaic application. This is followed by the nickel-cadmium and finally lead acid battery technology.

2.7. OPERATION AND PERFOMANCE IN DIFFERENT NETWORKS

2.7.1. Operation in stand-alone mode

An important factor to consider is the economic aspects associated with reliability of operation in isolated

mode. The main disadvantage operation in standalone mode is the high installation costs associated with

microgrids. The technical issues in rural areas are the lack of proper communication infrastructure. This is

because telecommunication protocols required for real-time and off-line control and energy management are

relatively expensive.

Battery storage

fully charged.

Diesel generator

turned off again

STEP 4

Hybrid system

initiates with

batteries fully

charged.

STEP 1

Diesel generator

turned off. Load

supplied by

battery only.

STEP 2

Battery storage

discharged to

preset level.

Diesel generator

turned on

STEP 3

13

Renewable power sources such as PV modules with unpredictable behavior under natural conditions are

incapable of supplying power demand exactly. This becomes a major issue of concern for microgrids with

small capacity in isolated mode. Hence stable sources like batteries and diesel generators are incorporated to

supply demand balance. Reliability evaluation uses two approaches which are deterministic and

probabilistic. However, random behavior in microgrid systems cannot be determined using deterministic

approach. The use of battery storage systems can increase the reliability of the stand-alone mode. It is also

necessary to include the use of a storage system in the reliability evaluation. [1]

Frequency oscillations and power fluctuations of renewable sources such as solar-PV are major causes of

instability and efficiency drop for hybrid systems operating in isolation. A compensation control strategy is

often required to improve power quality.

Fig 2.11 below shows the concept of conventional frequency compensation by ESS (Energy storage system)

in a standalone micro-grid.

Fig 2.11.Frequency compensation by ESS [9]

ESS generates a compensation power (PESS) from a variation of system frequency and voltage. A simple

load sharing and frequency control method whose characteristic for ESS is shown in Fig 2.11 is droop. This

is effective for damping control of frequency oscillation but not for improving efficiency.

On the other hand, literature in [13] states that most distributed resources together with the energy storage

devices are DC based. In order to maintain system stability, some AC generators require inverter-based

interfaces for connection to an AC grid. Current applications require that the generated AC power be

converted to DC before connecting to the AC grid. This is an issue of concern since very few projects for DC

power-grid have been implemented. Thus the converter will always be necessary and a control will be

required for stabilizing DC and AC voltages. The DC bus is interconnected to the AC bus (feeds the

conventional AC loads) via a bi-directional converter.

Most challenges experienced in stand-alone mode involve maintaining a balance between supply and

demand. The controller is necessary in order to maintain a constant DC bus voltage. However, literature

explains the importance of DGs in voltage dip mitigation within a microgrid. DGs can be oversized in a

microgrid to produce reactive power locally. In realistically sized systems, automatic voltage regulators are

used to maintain voltages to values between 0.9pu and 1.1pu of the nominal voltage value

~ Grid

PCS

Storage

ESS

PESS

ESS Output Generator

Pg f V P

PESS = g(fV)

fV

14

2.7.2. Operation in grid connected mode

By integrating DG, the distribution network becomes active. A microgrid is coupled to the main utility grid

through a PCC (Point of common coupling) switch. By selecting a mode of operation, the PCC is controlled

to connect or disconnect the microgrid from the main grid. Reclosers are used to ensure that only the faulted

section of the feeder or branch would be disconnected and isolated in order for normal operation to continue

and keep customer interruption to a minimum.

It is vital that the DG penetration be kept within the acceptable limits to ensure stability. The access point of

the microgrid affects the level of penetration. [12].For the purpose of this research, it shall be assumed that

the microgrid is connected correctly to the main grid.

The power delivered from the main grid as well as from the hybrid system must be properly coordinated to

meet the demand. When a fault occurs in the main grid, the micro-grid operates in standalone mode. For grid

electrification to be financially viable, customer density has to be above an identifiable minimum. Extension

of national power systems are limited by Environmental and Economical policies.

According to [14] the hybrid source has two control modes: 1) unit-power control (UPC) mode and feeder-

flow control (FFC) mode. The UPC mode involves compensation of load demand variations by the main grid

because the hybrid source output is regulated to reference power. Therefore, the reference value of the hybrid

source output must be determined. The FFC mode involves regulation of the feeder flow to a constant; the

extra load demand is picked up by the hybrid source. Thus, it is vital that the feeder reference be known.

Coordination of the two operating modes and determining set point values for each mode will minimize the

number of operating mode changes, improve performance of the system operation, and enhance stability.

[14]

Considering instantaneous levels of power generated and demand, four sub-modes are possible. Fig 2.12

summarizes the grid-connected sub-modes of operation.

Fig 2.12.Power sources in Grid-connected mode

Literature in [15] describes the following as the power flow in the case of a PV-diesel hybrid in grid

connected mode:

PPV + PBATTERY + PDIESEL = PGRID + PLOAD

Diesel

Generator

Photovoltaic

Battery

Load

~ Grid

15

Grid-connected sub-mode 1:

In this instance, the flow of power is such that PPV + PBATTERY + PDIESEL > PLOAD. Thus, the grid

absorbs the extra generated power.


This occurs when PPV + PBATTERY + PDIESEL = PLOAD. In this case, the grid neither absorbs nor

supplies power to the load.


This mode is established when PPV + PBATTERY + PDIESEL < PLOAD. The grid provides the excess

power required to meet the demand.


At an instant where PPV + PBATTERY + PDIESEL = 0, the grid provide the entire demand. There is no

power flow from the hybrid system.

2.8. PURPOSE OF RESEARCH

A growing empirical literature has shown the benefits of supplying rural communities with electrical energy

from renewable energy sources. In this review an attempt has been made to impose a solution for efficient,

reliable and cost effective rural electrification using HPS. However, this solution could be based on

misinterpretation of evidence because if the hybrid system is resized to supply a whole village, it might not

be optimal to run the system in both standalone mode and grid-connected modes.

The main objective of this research is to model and simulate the optimum configuration of a PV-diesel

hybrid generator system and controller to supply a rural community. This is to be tested using the load-

following dispatch strategy for two modes of operation. The load following strategy ensures continuous

supply for different power levels.

Mode A: Stand-alone mode

The HPS will supply demand within its capacity. If demand exceeds the maximum output of the HPS, the

system shuts down.

Mode B: Grid connected mode

If demand levels exceed HPS output, power can be supplied main grid to meet demand. Within the main

grid, if demand exceeds generation, the HPS supplies the main grid.

The results from the various modes will then be analyzed to determine the most economical and reliable PV-

diesel hybrid generator system and mode of operation. Conclusions will then be drawn based on the results

and recommendations will be made on future models of the controller.

16

3. Model development and preliminary design This chapter describes the process followed in developing models for the PV-diesel hybrid system.

3.1. METHODOLOGY

This research methodology follows an iterative framework which is nearly identical to the Complex

Adaptive System of Systems (CASoS) Engineering framework [29]. Fig 3.1 below shows a flow chart of the

iterative process to be followed.

Fig 3.1.Iterative framework followed in overall research

(RE)DESIGNING AND

SIMULATIONS

INTERPRET RESULTS

AND MAKE

RECOMMENDATIONS

FOR FUTURE WORK

(RE)DEFINITION

Definition of

objectives of

report

Problem

identification and

formulation of PV-

diesel HPS

Theoretically

valid

Testing

objectives

Definition of conceptual

models (consult

published literature)

Choice of

software and

collection of

real system

data (use

datasheets)

Establishment of

experimental

conditions for

simulations

Implementation of

solutions in

DigSILENT and

MATLAB

Comparison

of solutions

under worst

case case

study

System

succeeds

No

Yes

No

Yes

Error!

17

The first step designing the system was to define the purpose and objectives of the research. A rural load

demand analysis was then carried out. This involved obtaining appliance information on the hours of usage

and the power ratings. An estimate daily rural load profile was plotted to determine the behaviour of active

power with appliance usage. Next the PV-Diesel hybrid system was designed. The design identified the

individual components of the system and their connection points. Particular attention was paid to

components required in the control systems in order to meet the test objective, which was a hybrid system

with load following dispatch. These were the diesel generator governor, AVR, PV controller and battery

charge controller.

By considering the complexity of the system and the limited time available to complete the research project,

DigSILENT was chosen as the most suitable software to simulate the system performance. MATLAB was

used to plot the curves for the load profile and PV characteristics. The following step was to define the

electrical models for each of the system components. Using information from datasheets, the parameters to

be used for the simulations were outlined.

The study cases to be simulated were defined for varying sunlight and loading conditions. Simulations were

then carried out and the active power outputs from the power sources were plotted. Detailed explanations of

the graphs were given. A discussion on the comparative behaviour of the system in between the stand-alone

and grid-connected modes was then conducted. Conclusions were drawn from the information extracted in

the literature review, the simulation results and the subsequent discussions. Finally recommendations were

then made to improve the controller to also monitor reactive power flow.

3.1.1. Problem formulation

This research project investigates the performance of the PV-diesel hybrid system under two modes of

operation: grid connected and standalone mode. Since the main objective of this system is to use a controller

to supply a continuous, low cost electrical supply to a rural area, the first step will be to establish the dispatch

strategy to be used. From the literature review conducted in Chapter 2, the ideal strategy for this design will

be the load following strategy. The PV system will provide the base supply.

Standalone Mode: A micro-grid is basically a combination of DGs at distribution voltage level. In general,

micro source controllers (MC) independently control power flow of the power source in response to any load

changes. The power sources are provided with power electronic interfaces which implement the following

functions in both standalone and grid connected modes: control, metering and protection. For this research,

secondary control will be required to coordinate the given zone of network. A power frequency controller

model will determine the load sharing between the power supplies. The system output shall be set to

400/230Vac.

Grid-connected Mode: The micro grid is coupled with the distribution level voltage of the main grid

through a point of common coupling (PCC). In both modes of operation secondary control will determine the

dispatch of the diesel generator and the battery system.

18

3.1.2. System overview

The PV-diesel hybrid system supplies a domestic rural load using the load following dispatch strategy.

A secondary controller will be used to enable the load following functionality. The system operates in

two modes: the stand-alone mode and the grid connected mode. The system will consist of a PV system,

diesel generator, battery system, AC load and a simulated concept of the main electrical grid. The load,

power sources and storage devices are connected to a common generator bus. The main grid will be

disconnected from the rest of the system in the stand-alone mode. However in the grid-connected mode the

main grid is connected to the system through the common bus. Fig 3.2 below summarizes the components of

the main test system which will be investigated.

Fig 3.2.Components in PV-diesel plant

An understanding of the individual components of the PV-Diesel hybrid system is necessary in order to

model the system. The components are thus modelled separately before investigating the performance of the

complete system.

DC to AC converter

Energy storage controller

DC voltage source

Power-voltage controller

AC load Photovoltaic controller

Static generator

Diesel governor

Synchronous generator

Main Grid

DIESEL GENERATOR

PHOTOVOLTAIC

SYSTEM

BATTERY SYSTEM

19

3.1.3. Software selection Due to the time constraints associated with this research project, the system models presented in this report

will be simulated using Digital Simulation and Electrical Network (DigSILENT) Power Factory version

14.1.6. DigSILENT modelling capabilities replace a lot of other software systems thereby minimizing

project execution time and costs. Modelling in DigSILENT can be carried out in two ways.

1. Built in models for standard power systems, which can be obtained from the Power Factory library 2. User defined entities which are modelled using DSL (DigSILENT Simulation Language).

However, the license version used in this system does not allow modelling through DSL. Thus, all the

electrical components to be used for this work have in-built models in DigSILENT.

Since the license version of DigSILENT does not allow writing code. In order to verify the model of the PV

array, Matrix Laboratory (MATLAB) was used to write the relevant code and plot graphs.

3.2. MODEL OF SYSTEM LOAD

A realistic load profile is necessary for the load following dispatch strategy. Using household appliance

usage data obtained from personal experience of appliance usage in low income households, an estimate load

profile was derived. In deriving the system load, the following assumptions have been made:

The load is purely domestic. The system will supply 30 households whose inhabitants have similar

behavioural patterns.

Apart from the refrigerator whose load levels fluctuate due to ambient temperature and thermostat

settings, the electrical appliances are deterministic loads with known and pre-defined usage.

There is no hot water cylinder mounted. The electric kettle is used for heating bathing water.

Light bulbs are CFL lamps.

The load consumes active power only.

Table 3.1.Below summarizes the appliance usage per household

Table 3.1.Appliance usage per household

APPLIANCE POWER (W) TIME OF DAY

3 14W light bulbs 42 4am-6am, 5pm-11pm

Radio 60 6am-8am

19 colour TV 300 5pm-11pm

DVD player 35 9pm-11pm

Satellite TV decoder 15 5pm-9pm

2 cell phone chargers 14 5pm 4am

1.2l kettle 2000 4am-8am, 6pm-9pm

Iron 1000 5am -6am, 6pm-9pm

50l refrigerator 75 All day

Hotplate 1200 6am-8am, 5pm-9pm

The individual household peak hourly demands have been added to determine the total demand for the rural

area (Appendix A). The sizes of the diesel generator and the PV cells shall be selected based on this power

demand. Fig 3.3 below shows the estimated load profile for the community.

20

Fig 3.3.Rural community daily load profile

3.3. MODEL OF PHOTOVOLTAIC SYSTEM

3.3.1. PV array model

The PV array is simply the generator of the system. Since there is no rotating part, it is essentially a static

generator. An inverter transforms the DC to AC current. Fig 3.4 shows the ideally equivalent circuit or a

solar cell. A more accurate model includes a series resistance to account for any resistance in the path

through the conductor.

Fig 3.4.Electrical equivalent of a solar cell [28]

I = Iph I0 (

- 1) (equation 3.1)

U = UTln (

) (equation 3.2)

I0 = Ik

(equation 3.3)

Iph = Ik (equation 3.4)

Where: Ik = short circuit current

I0 = reverse bias current

Uoc = open circuit voltage

UT = thermal voltage

R ID,

UD

U

Iph

I

21

The following methods can be used to model a PV array in DigSILENT [28]:

1. DC Current source A DC Current Source is connected to a DC terminal. The main disadvantage of this model is that the direct

current must then be converted to alternating current.

2. Pulse Width Modulator(PWM) Converter A DC current source is used to model the PV array and the PWM converter models a PV inverter. The array

is connected to the converter to allow a sinusoidal output onto the grid. Although this method offers simple

control, the hard switching increases system losses.

3. Static generator Fig 3.2.1 below shows the Static generator model of a PV array. This model has no rotating parts and

includes not only the PV array but also the DC terminal and the PV inverter.

The real part of the current defines the active power of the static generator. Thus, the value of the real part of

the reference current is defined by the PV controller. [23]

The inverters of the PV field convert the DC current into AC current at the frequency of the grid. The

inverters can also control the output power by changing the operation point of the PV field. However, it is

not possible to increase the output power above the maximum power point at a particular irradiation.

For the simulations in this research project, the basic PV system developed by the static generator shall be

used. The model of a PV array can be achieved by connecting the above PV model in a series parallel combination giving the output current of the PV array.

MPPT Technique:

DigSILENT does not have a MPPT system to monitor the amount of energy absorbed by the PV array. Thus,

the power generated by the PV array will be changed manually for different case studies.

Sizing Photovoltaic array:

A suitable solar panel for building this design is attached in Appendix C. The system should supply load

during the day and charge battery for the peak load. Thus load supply directly from the PV module is

between 6am and 6pm. The calculations for the array size are shown in table 3.2 below.

Table3.2. Calculation values for PV array sizing

STEP CALCULATION VALUE

1 Total energy usage per day* 5698080Wh/day

2 Divide by hours of sunlight(7hours) 801154.3W

3 Divide by panel size(280W) 2861.26

4 Total number of panels 2862

For the simulations, the PV Generator template available in the DigSILENT global library will be used. The

parameter values available in the datasheet (Appendix C) will be used for the array model. The

characteristics of the PV array are verified in MATLAB (Appendix F).

3.3.2. PV controller

The purpose of this controller is not to provide energy dispatch since the PV module is the primary energy

source. Thus the controller is required to respond to frequency disturbance in the system, which is a result of

power imbalance. When the frequency of the grid exceeds the nominal value, the inverters in the PV

controller start regulating to reduce output power. However, according to [23], if the frequency increases or

decreases beyond the predefined threshold, the photovoltaic module should be disconnected from the grid.

22

3.4. MODEL OF DIESEL GENERATOR

A diesel generator provides real power at its rated value by converting the chemical energy of the fuel into

mechanical torque on the shaft. The generator output must be continuously regulated to match active power.

An automatic voltage regular (AVR) will prevent voltages from going beyond prescribed limits. Power

system stabilizers (PSSs) add damping to generator oscillations. However, to use a PSS, at least a third order

generator should be considered. For the purpose of this report, the representation is insufficient to model the

system.

3.4.1. Synchronous generator

Fig 3.5.Per phase equivalent of a synchronous generator [34]

Where:

Ra = stator winding resistance

xar = magnetizing reactance

xal = leakage reactance Ef = excitation voltage

Vf = terminal voltage

Figure 3.5 above shows a schematic of a synchronous generator. The synchronous generator converts

mechanical energy from a diesel engine into electrical energy. The diesel engine has a standardized value

which is approximately equal to the synchronous generators nominal power. The synchronous generator

produces active power received from the diesel engine through the electromagnetic torque. [30]

According to [38], the mechanical system of a synchronous generator is described by the following equations

(t) =

(equation 3.5)

Mechanical =

(equation 3.6)

Where:

= change in angular speed

H = constant of inertia

Kd = damping factor

Tm = mechanical torque

Te electromagnetic torque

xar xal

Ef Ea

Ra

Vt

Ia

~ Es

23

For this report, the diesel generator shall be modelled using the DigSILENT Power Factory (ElmSym) built

in synchronous generator model. The datasheet for the modelled generator for this system is attached in

Appendix B. Table 3.3 shows the parameters of the simulation model.

Table 3.3 Diesel generator parameters (Ambient temperature 40)

DIESEL GENERATOR

Parameter Description Value

Nominal power 180kW

Rated MVA 180kVA

Rated speed 1500RPM

Reactance, Xd 0.312pu



Torque, Tdo 0.105s

Torque, Tqo 0.010s

3.4.2. Diesel governor

A governor is required to control the shaft speed and the power output. In general engine speed control is

achieved governors are analogue proportional integral derivative (PID) or mechanical droop governors. The

governor can be modelled and analyzed for small changes in load-frequency. The control performance of the

governor is shown by stabilisation of diesel engine speed and fast response to changes in load. A simplified

block diagram of the governor system is shown in Fig 3.6 below. The feed-back loop provides droop

compensation.

Fig 3.6.Block diagram of speed governor model [31]

For the governor, this project will use the IEEE based DigSILENT built-in model named gov_DEGOV1 [23] found in the Standard Models folder. Table 3.4 below shows the model parameters which determine the

droop characteristics of the governor. The known values are the frequency deviation and rated power. The

other values shall be obtained by trial and error method. The characteristics of the governor shall be

determined by the parameters of the common model of the diesel governor.

Temporary droop

Integrator

Permanent speed droop

To turbine

control

actuator

Set point

Unit speed

+

+

-

-

-

24

Table 3.4 Governor Parameter values

GOVERNOR

Parameter description Value

Actuator gain, K 0.44pu/pu

Actuator derivative time constant, T4 0.35s

Actuator first time constant, T5 0.002s

Actuator second time constant, T6 0.015s

Combustion delay, TD 0.024s

Frequency deviation, Droop 0.025pu/pu

Time constant power feedback, TE 0s

Electric control box first time constant, T1 0.018s

Electric control box second time constant, T2 0.0001s

Electric control box derivative time constant, T3 0.38s

Droop control(0=Throttle feedback, 1= Electrical power

feedback)

1

Prime mover rated power, PN 0.18MW

Minimum torque at minimum position of throttle, Tmin 0pu

Maximum torque, Tmax 1.2pu

3.4.3. Voltage controller In a synchronous generator system whose terminal voltage varies with load an automatic voltage regulator

(AVR) is used to adjust the field current to maintain a constant terminal voltage. The AVR plays an

important role in transient stability and primary voltage control at the generator. Fig3.7 below shows a

simplified block diagram of the AVR system. It should be noted that due to the delay in the time constants of

the AVR, transient emf will not change instantly. The voltage controller is particularly important for voltage

control in the grid connected mode. This research project uses the DigSILENT in-built IEEE model:

vco_IEEET1 whose parameter values are shown in table 3.5.

Fig 3.7.Block diagram of the AVR model

Amplifier Exciter Generator

Feedback

elements

Vref +

-

Vr Vt

25

Table 3.5 Voltage controller parameter values

VOLTAGE CONTROLLER

Parameter Description Value

Measurement delay, Tr 0.02s

Controller gain, Ka 150pu

Controller time constant, Ta 0.02s

Exciter constant, Ke 1pu

Exciter time constant, Te 0s

Stabilization path gain, Kf 0.01pu

Stabilization path time constant, Tf 0.1s

Saturation factor 1, E1 4.5pu

Saturation factor 2, Se1 1.5pu

Saturation factor 3, E2 6pu

Saturation factor 4, Se2 2.46pu

Minimum controller output, Vrmin 0pu

Maximum controller output, Vrmax 6pu

3.5. MODEL VALIDATION WITHOUT BATTERY

In an attempt to minimize capital costs, the system models were simulated without a battery as shown in Fig

3.8 below. The test was carried out to show the initial conditions for the worst case design: PV system

pv diesel generator based power plant

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