Active And Reactive Power Control For Renewable Energy Generation

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02 Nov 2017

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by

SAHARUDDIN BIN OTHMAN

A thesis submitted in fulfilment of the

requirements for the award of the degree of

Doctor of Philosophy

September 2011

DECLARATION

I declare that this thesis entitled "Active and reactive power control for renewable energy generation’’ is the result of my own research except as cited in the references. The thesis has not been accepted for any degree and is not concurrently submitted in candidature of any other degree.

Signature : ....................................................

Name : ..Saharuddin Bin Othman.............

Date : .1st September 2011....................

DEDICATION

To my beloved mother and father,

Late Othman Ya’akub and Sabariah Mohd Amin,

and

My heartiest thanks are due to my wife

Rafidah Mohamed,

my sons and daughter

Ilmi Abdul Hakim, Ilmi Hazim, Ilmi Hariz, Ilmi Puteri Insyirah and Ilmi Hafiz

Thank you for giving me their fullest

cooperation, understanding, patience, sacrifices and encouragement

during the hard years in completing this project

ACKNOWLEDGEMENTS

Bismillahirrahmaanirrahim,

In the name of Allah, the most compassionate and the most merciful

Alhamdulillah, all praise is due to Allah SWT, the most beneficent and the most merciful, who has taught me what I knew not. For His guidance, blessings, for granting me patience and perseverance to accomplish this thesis successfully.

I would like to express my sincere gratitude to my supervisor, Professor Dr. Malcolm R. Irving, for his guidance, encouragement and continuous support throughout the course of this work. His extensive knowledge, advice and creative thinking have been an invaluable help to this research work.

I would also like to thank my co-supervisor, Dr. Gary A. Taylor, for his invaluable discussion and time.

I am also indebted to Universiti Kuala Lumpur – British Malaysian Institute and Majlis Amanah Rakyat (MARA) for giving me the chance undertakes this PhD study.

Recognition is extended to all staff at the School of Engineering and Design, Brunel University for a pleasant working atmosphere.

My fellow postgraduate students should also be recognised for their support. My sincere appreciation also extends to all my colleagues and others who have provided assistance at various occasions. Their views and tips are useful indeed. Unfortunately, it is not possible to list all of them in this limited space. I am grateful to all my family members.

ABSTRACT

Research has been conducted into the problem of active and reactive power production in renewable energy generation. The purpose of the research being to achieve the overall objective of minimum generation cost and acceptability of other system parameters, subject to constraints that ensure power system feasibility. The problem will involve simulation, decomposed into a series of discrete time steps. It is intended that the duration of each time step will be chosen so that points can occur at any critical points of the predicted demand curve within the time period. It is anticipated that the student will use existing hardware facilities at Brunel University including experimental photo-voltaic, D.C. power generators and a wind-turbine generator. System constraints and dynamic rate limits will be included. Within each time step, reactive power problem is represented by a static model, influenced by the linking constraints on generator rate of change of real power output.

It is intended that the proposed research will build on existing expertise, and software available, at Brunel Institute of Power Systems, to extend the solution of the problem in the case of various renewable energy paradigms.

COPYRIGHT

Attention is drawn to the fact that copyright of this thesis rests with its author. This copy of the thesis has been supplied on condition that anyone who read it is understood to recognise that its copyright rest with its author and no information derived from it may be published without the prior written consent of the author. This thesis may be made available for consultation within the university library and may be photocopied or lent to other libraries for the purposes of consultation.

Uxbridge, September 2011

TABLE OF CONTENTS

CHAPTER

TITLE

PAGE

DECLARATION

ii

DEDICATION

iii

ACKNOWLEDGEMENTS

iv

ABSTRACT

v

COPYRIGHT

vi

TABLE OF CONTENTS

vii

LIST OF TABLES

viii

LIST OF FIGURES

viii

LIST OF ABBREVIATIONS

LIST OF SYMBOLS

1

INTRODUCTION

1

1.1

Background

1.2

Objectives

1.3

Project methodology

1.4

Scope and contribution

1.5

Thesis outline

2

BACKGROUND AND LITERATURE REVIEW

2.1

Introduction

2.2

Power Load flow

3

METHODS EXPERIMENT

4

SIMULATION RESULTS AND DISCUSSION

5

CONCLUSION AND FUTURE WORK

5.1

Conclusion

5.2

Suggestion and future work

APPENDICES

GLOSSARY

LIST OF REFERENCES

BIBLIOGRAPHY

INDEX

CHAPTER 1

INTRODUCTION

The world today faces two major problems with its energy supply, which is highly dependent on fossil fuel. The first problem is related to the security of supply of fossil fuel and the second is the effect of its use on the environment. These two problems have forced the energy industry to face up to the prospect of renewable energy as a basis for mainstream power generation

Wind is air in motion. Since the earth’s surface is made of various land and water formations, it absorbs the sun’s radiation unevenly. Wind is produced by the uneven heating of the earth’s surface by the sun. Wind is one of the renewable energy from the sea. It’s a fact that 71% of earth surface is sea or ocean and only 10% of it has been fully utilized. When the land area is fully utilized by population, people will start to find another place to stay. That means, some people will have to live on sea like what’s happening in Brunei. When this happens, power generation in the middle of the sea may be considered as one of the solutions to provide electricity for on-water-cities. In the middle of the sea winds tend to flow at higher speeds for constant generation. Malaysia is a country which consists of a peninsular and an island, where 60% of its area is covered by sea. Having such technology can help utilize such resources.

A hybrid system between wind, solar, batteries and diesel is the best way to provide full utilization of the sea area for energy generation and then connected to the grid system of the country.

However, before the construction of the project, a simulation should be done first to evaluate the effects of the impact of renewable energy to the grid system. Using MATLAB software, we can test the system generation, and load is.

The primary aim of the project is to

The objectives of the study are;

The methodology, which is suitable for this research is to

Background

Active and Reactive Power Control for Renewable Energy Generation. Research will be conducted into the problem of active and reactive power production in renewable energy generation. The purpose of the research being to achieve the overall objective of minimum generation cost and acceptability of other system parameters, subject to constraints that ensure power system feasibility. The problem will involve simulation, decomposed into a series of discrete time steps. It is intended that the duration of each time step will be chosen so that points can occur at any critical points of the predicted demand curve within the time period. It is anticipated that the student will use existing hardware facilities at Brunel University including experimental photo-voltaic, D.C. power generators and a new wind-turbine generator that we intend to install in the near future. System constraints and dynamic rate limits will be included. Within each time step, reactive power problem is represented by a static model, influenced by the linking constraints on generator rate of change of real power output.

It is intended that the proposed research will build on existing expertise, and software available, at Brunel Institute of Power Systems, to extend the solution of the problem in the case of various renewable energy paradigms.

Objectives

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Project methodology

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Scope and contribution

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Thesis outline

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CHAPTER 2

BACKGROUND AND LITERATURE REVIEW

Introduction

70-100 journal review of the research area dated up to 2011

Ideally, all power utilities provider should provide their customers with a quality power supply which has constant magnitude and frequency of sinusoidal voltage. Unfortunately is a hard task to maintain this quality power supply for constant magnitude and frequency of sinusoidal voltage. In reality the supply waveforms are always got distorted. [Bachry, A., et.al., 2000, Power Notes-Power System Harmonics, 2003]. Then solar, wind and other renewable energy adapt by power utilities provider to reduce the generation cost due to the constant fossil fuel price rising, this Intermittent energy power generation is tends to be unsteady because they are influenced by natural and meteorological conditions [Wang, X. Y., et.al., 2008] or more intermittent than conventional power sources in normal operational conditions. Intermittency is a problem related to dispatch ability, or the ability to match the generated supply of electricity to actual demand makes the task harder to the power utilities provider company.

Most of the world's present demand met by fossil and nuclear power plants. A small proportion met by renewable energy technologies, such as wind, solar, biomass, geothermal and ocean. Among sources of renewable power, wind and solar has experienced very rapid growth in the past 10 years. Both are pollution-free sources of abundant power. In addition, they generate power close to load centers, thus eliminating the long-term needs of the high-voltage transmission lines through rural and urban landscapes.

Why should we use renewable energy?

Interest in renewable energy sources is growing because fossil fuels are running out and because people are worried about the way burning fossil fuels damages the environment e.g. the greenhouse effect and acid rain

The greenhouse effect is caused by an increase in the concentration of gases such as carbon dioxide (CO2) in the earth’s atmosphere. This prevents heat from being radiated out into space and causes the Earth’s surface, oceans and atmosphere to heat up

Carbon dioxide (CO2 ) is the major gas which causes the greenhouse effect.

Carbon dioxide (CO2) is released when fossil fuels are burnt

A temperature rise of just one or two degrees Celsius can cause flooding, drought, crop failures and stormy weather

Gases such as sulphur dioxide (SO2) and nitrous oxides (NOx) are released when fossil fuels are burned. They react with rain drops to form acid rain.

Acid rain can damage crops and forests, it can make lakes and rivers acidic which can harm fish and aquatic life. It can also damage buildings

Chimneys at power stations can be designed to stop harmful gases from being released to the atmosphere

There are grants available to help meet the costs of using renewable energy

If we can generate some of our own energy using renewable resources then we do not have to rely on other countries for our energy

Did you know that scientists estimate that global temperatures are increasing at a rate of 0.3°C every ten years?

Did you know that the UK government has signed an international agreement called the Kyoto Protocol to reduce UK carbon dioxide CO2 emissions by 12.5% by the year 2008?

Did you know the energy required for heating, lighting, and powering equipment in an ordinary school classroom releases about 4 tonnes of carbon dioxide gas (CO2) every year?

Power or Load flow Analysis

Power flow analysis is to describe or monitor the situation as a whole power system operation including network generators, transmission lines, and loads can represent municipal area as small or as large as several states. With known parameters quantify certain parameters, the amount of power produced and used in a different location, flow analysis allows to determine the quantity of other quantities. The most important quantity is the voltage at locations across the transmission system, which, for alternating current (ac), consists of both the magnitude and the time element or phase angle. Once known voltage, the current flowing through each link transmission can be easily calculated.

In order to analyze any such circuit, a reference point is identified which has electricity distinct and there are a number of differences between their impedance, which can accommodate potential differences. This reference point is called a node. When representing the power system on a large scale, the node is called a bus, because they represent the actual physical busbar where different components of the system meet.

Based on the assumption that to the best approximation, everything that happens in a Phase also occur in every other phase, with that the three Phase can condense to model drawings into a single line, making the so-called one line diagram.

Bus admittance matrix is generally use as a solution methods developed suitable for computer solution calculation for power systems network and load flow studies.

In the transmission line as the length of line increase for example 250 km and longer the exact effect of the distribution parameters must be considered. Expressions for voltage and current at any point on the line are derived. then, based on these equations an equivalent π model is obtained for long line.

Picture6.png

Nominal π Method

picture9.png

busbarphd.bmp

3bus.png

shunt.png

shunt2.png

From Hadi Saadat (1999) review that power flow is the analysis to determine the steady-state complex voltages at all buses of the network and also the real and reactive power flows in every transmission line. These is the routine power network calculations, which can used in power system planning, operational planning, and operation or control. Types of methods to analysis power flow for examples impedance matrix methods, Newton-Raphson methods, and Decoupled Newton power flow methods [4].

Power flow analysis is a basic tool and very important for the analysis of any power system as it is used in the planning and design stages as well as during the operational stages. Some applications need repeated fast power flow solutions especially in the fields of optimizations of power system and distribution automation. It is imperative in these applications that the power flow analysis is solved as efficiently as possible. Power flow solutions are needed for the system under the following conditions.

With certain equipment out aged

Addition of new generators

Addition of new transmission lines or cables

Interconnection with other systems

Load growth studies

Loss of line evaluation

With the widespread and invention use of digital computers in 1950s, many methods for solving the power flow problem have been developed such as indirect Gauss-Seidel Load Flow (bus admittance matrix), direct Gauss-Seidel Load Flow (bus impedance matrix), Newton-Raphson Load Flow and its Fast Decoupled Load Flow versions. Voltage solution in all these methods is initially assumed and the improved upon using some iterative process until convergence is reached. The first method Gauss-Seidel Load Flow is a simple method to program but the voltage solution is updated only node by node and hence the convergence rate is poor. Newton-Raphson Load Flow and Fat Decoupled Load Flow methods update the voltage solution of all the buses simultaneously in each of iteration and hence have faster convergence rate [1].

Newton-Raphson

Newton-Raphson method begins with initial guesses of all unknown variables

(voltage magnitude and angles at Load Buses and voltage angles at Generator

Buses). Next, a Taylor Series is written, with the higher order terms ignored, for each

of the power balance equations included in the system of equations.

Project methodology

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Scope and contribution

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Thesis outline

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Abstract

The increase of oil prices forces a developing country such as Malaysia to start investing in using its own natural resources for its energy generation. Perhentian Islands in the north east Malaysia peninsular is a pilot project which combines wind turbine installations with solar panels. In the day, when there is less wind, the solar panels will cover the extra load. At night, the wind turbines will be generating more power. If the solar panels and wind turbines do not create enough power, a diesel generator will automatically run to compensate for the deficiency. Since the back-up generators are fuelled by diesel and turn on only when needed, this can actually save significantly on fuel cost but the inconsistent of wind and solar energy will definitely produce frequency deviation in the system. The effect of reactance’s frequency dependent in ZIP loads and Exponential loads will be assess and evaluate. Such a situation can be modelled and simulated using a Stepwise Power Flow method created in the MatPower package and executed with MATLAB. From the experiment it is shown that the renewable energy very good in saving cost but the effect toward frequency of the systems could also damage the system.

KEYWORDS: Wind power; Solar power; Simulation; Power Flow; Intermittent generation

Introduction

Regular power flow calculations assume a balance between scheduled generation and actual load, but this is formally correct only once, or a few times, during the hour in a real power system; where the load is always changing, as discussed by Bakken et al.[2]. The Perhentian island system as figure 1 below is the real system that we want to simulate but since the real data of bus and transmission line parameters not yet obtain, the simulation has to be done using a nearly similar system artificial data namely as the Ward Hale system. It is belief that the result of simulation will be similar between real and artificial system.

Perhentian.bmp

Figure 1: Perhentian island System

The Stepwise Power Flow method is simulated in 5 minute steps for 24 hours data using a modified Ward Hale system model as shown in figure 2.

modified ward hale.bmp

Figure 2: Modified Ward Hale System

The Ward-Hale model consists of 3 loads which will be varied in 5 minute steps for 24 hours following the island power demand. Exact network data for the north east Malaysia peninsular pilot project is not yet available, so the Ward Hale model has been adapted and is used as an example.

There are 3 generation systems attached to the system.

Generator fuelled by diesel, in ‘swing bus’ mode.

Wind turbine generator

Photovoltaic (PV) Solar panel

Simulation Data

The 24 hour wind speed data were obtained from [6] is adjusted by increasing the data 65% to get a reasonable wind speed curve as in figure 2 for generation because the wind turbine will only function with the wind speed between 5 m/s and 15 m/s. according to the specification of the wind turbine. If the wind speed is less than 5 m/s the blades will not move and the blades will be automatically stopped if the wind speed is more than 15 m/s.

Figure 2: Wind speed in hourly intervals.

The data recorded is shown on the remote monitoring system at the generator set station. It is noted the wind turbine is produce 60 KW.

A set of 24 hour measured illuminance data at 5-min intervals was obtained from [7] then smoothed, as in figure 3, to get a reasonable illuminance and irradiance curve for solar generation.

Figure 3: Measured irradiance and illuminance.

The recorded data, shown in figure 3, is from the remote monitoring system at the generator station. It is noted that the solar PV produces 65 KW.

It was also noted that the site load requirement on that day is 120 kW then a reasonable set of 24 hour estimated loads is design for the purpose of simulation using the stepwise power flow.

Figure 4: Estimate daily load profile

Simulation

The data collected is then converted into the power generation and 3 loads to be used in the simulation of the stepwise power flow.

Figure 5: Renewable Power Generation

The main idea is to use a sequence of stationary power flow analyses to capture slow system dynamics in the minutes range. For each time step, loads are updated and generation is distributed among all units according to schedule. A sequence of modified power flow analyses are run in 288 steps of power flow cases. The system load is assumed to change every 5 minutes following the demand graph throughout the 24 hour period, while scheduled diesel generation and the renewable generation is changed only at the change of hour, see Figure 6.

Figure 6: Schedule Generation

For each time step in SPF, the imbalance between generation and load is shared between all synchronized units and is measured and recorded.

During the simulation, power flow data for start (n = 1) and end (n = 288) cases are read, for 5 minute steps and 12 steps in an hour. The demand is increased or decreased by inputting the load demand from matrices, for each node i through the whole period. The renewable energy generation also will be loaded from the matrices for every step during the simulation. The diesel generator as a "swing bus" will compensate for the change in the network losses or over-generation. The frequency deviation in the system is calculate and recorded as shown in figure 7.

Figure 7: Frequency

Wind and solar energy is not a constant value and the generated power of a hybrid generation system is fluctuates. If hybrid system is installed substantially for a small power system, the output power fluctuations may cause frequency and voltage deviations.

The stepwise power flow goal is to obtained complete voltage angle and magnitude information for each bus in the power system for the load and generator real power and voltage condition. The result for voltage magnitude for all the bus is as below.

From the data of frequency deviation obtained, the program can then calculate the changes in inductive reactance (XL) and capacitive reactance (XC) values of the transmission line. The value of R is fixed and is independent of frequency. By using fixed values of reactance (XL) and (XC) for comparison the system is tested again to examine the effect of frequency dependent reactances. The value of capacitor and inductor as in the table below

Bus

Inductor (H)

Capacitor (f)

1

0.00329769

0.006144978

2

0.002355493

0.00860297

3

0.002591042

0.007820882

4

0.001909859

0.01061033

5

0.004074367

0.004973592

6

0.006684508

0.003031523

7

0.000846704

0.023933074

The result of stepwise power flow for each bus in the power system for the voltage magnitude is as below

The changes in value of reactances also change the frequency of the diesel generator as figure below.

When we program the system to operate with ZIP models, where PL0, QL0 and VL0 are the data retrieve from power flow values. The coefficient a, b and c represent the proportion of constant impedance (Z), constant current (I) and constant power (P).

where

But in the static load voltage characteristic described by exponentials

For a composite load, α is usually between 0.5 and 1.8, b between 1.5 and 6, Then set value of α and β is used and then stepwise power flow is run again.

α

β

0.5

1.5

1.15

3.75

1.8

6

Then the result is compared.

Conclusion

One of advantages of static models is their simplicity. But when voltage varies over a wide range, the model will lose its accuracy. A common method to cope with this problem is to use different model parameters over different voltage ranges. For voltage stability studies, it is believed that a constant power model is suitable for worst case studies.

In general, systems with load that has a leading PF have higher critical voltage and large load ability. Loads at low lagging PF cause excessive voltage drops in the system and are uneconomic to supply. Consumers are therefore encourage to reduce their reactive requirements by the application of tariffs in which a charge is made for reactive as well as active power.

LIST OF REFERENCES

[1] R.D. Zimmermann, C. E. Murillo-Sanchez: "MATPOWER: A MATLAB Power System Simulation Package, Version 3.2",PSERC, December 1997-2007.

[2] B. H. Bakken, A. Petterteig, E. Haugan, B. Walther: "Stepwise Power Flow-a new tool to analyse capacity shortage and reserve requirement", 15th Power Systems Computational Conference, Liege, Belgium, August 2005.

[3] I. Norheim, D. Pudjianto:"Method for Assessing Impact of Large-scale Wind Power Integration on Reseves" Wind Energy Research Article Wiley Interscience, John Wiley & Sons, Ltd, October 2007

[4] Bakken, B.H, Petterteig, A: "Alternatives to reduce reserve requirements and reserve costs in the Nordel system", SINTEF TR A6034, February 2005

[5] Wood A. J. Wollenberg B. F."Power generation, operational and control" 2nd ed. Wiley-Interscience Publication 1996.

[6] J. D. Glover, M. S. Sarma, and T. J. Overbye, Power System Analysis and Design, 4th ed. London: Thompson Learning, 2008.

[7] E.F. Fuchs & M.A.S. Masoum, Power quality in power systems and electrical machines (USA: Academic Press, 2008).

[8] Gilat A."Matlab An Introduction with application" 2nd ed. John Wiley and Sons, Inc. 2005



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