stability improvement of a wind diesel system using an energy storage unit

8/8/2019 Stability Improvement of a Wind Diesel System Using an Energy Storage Unit

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Stability Improvement of a Wind/Diesel

System Using an Energy Storage Unit

Chung Der Chyuan (11012902)

25th October 2002

A Thesis submitted for partial fulfillment of the degree of

Bachelor of Engineering (Electrical)


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Stability Improvement of a Wind/Diesel System

Using an Energy Storage Unit

Chung

Der Chyuan

25th October 2002

Bachelor Of Engineering

Dr. W.W.L. Keerthipala

This report presents the modeling and simulation of an isolated

Wind/Diesel system. The simulation was done using the

PSCAD/EMTDC software package. Individual system components were

simulated and discuss in details. Stability aspect of the Wind/Diesel

system under various type of disturbances were analyzed and

improvements technique was introduced to the system. Finally,

recommendations were iven for future work and conclusion made.

Wind Turbine, Induction Generator, Diesel Generator, Energy Storage

Unit, Stability, Disturbances, PSCAD/EMTDC

Electrical


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Mr. Chung Der Chyuan

04 Werribee Crescent

Willetton

Perth, W.A. 6155

25th October 2002

Professor A. M. Zoubir

Head of school

School of Electrical and Computer Engineering

Curtin University of Technology

P.O. Box U1987

Perth, W.A. 6000

Dear Professor Zoubir,

Project Thesis for Engineering Honours Project 402

Please find the attached thesis on the project titled Stability Improvement of a

Wind/Diesel System using Energy Storage Unit, in the fulfillment of the

requirements of the Engineering Honours Project unit for the Award of Bachelor of

Engineering (Electrical)

Thank you & best regards,

Chung Der Chyuan

Student No: 11012902


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Abstract

The main objective of this final year project is to analyze and simulate the complete

scheme of wind/diesel system. The analysis of typical wind/diesel energy systems for

remote area applications is being performed using the PSCAD/EMTDC software.

Using software modeling of a wind/diesel system provides an in-depth understanding

of the system operation before building the actual system and also the testing and

experiments of system operation under disturbances is not possible on the actual

system.

The first stage of the project is to analyze each individual wind/diesel system

components using mathematical equations and to model them in software simulation.

The simulation was done in time domain analysis and the resulting waveforms are

shown in the report.

In the second phase of the project, each individual system components were

combined to form the overall wind/diesel system. Dynamic stability analysis on the

wind/diesel system was performed and stability improvement technique was being

used to stabilize the system performances.

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Acknowledgments

The author would like to thank the people who have contributed to the development

and progress of this final year project. Without their guidance and patience, the

project would not have come to this stage.

Many thanks must be extended to Dr. W.W.L. Keerthipala, who has been so

generous with his time in support, guidance, patience and advice throughout the

project. Much gratitude is due to the project co-supervisor, Mr. James Goh for his

interest and time spent in this project, as well as his assistance in providing

information and in discussing certain aspects of the wind/diesel system.

Many thanks to the School of Electrical Engineering for the computer facilities and

software during the working process of this project.

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Table of Contents

Abstract......................................................................................................................... i

Acknowledgments........................................................................................................ ii

Nomenclature.............................................................................................................. ix

CHAPTER ONE

1.0 INTRODUCTION ................................................................................................. 1

1.2 Background information ........................................................................................ 1

1.3 Wind Power ........................................................................................................... 4

1.4 Wind Component................................................................................................... 6

1.4.1 Wind Turbine Components............................................................................. 7

CHAPTER TWO

2.0 LITERATURE REVIEW ...................................................................................... 9

2.1 Introduction............................................................................................................ 9

2.2 Wind Turbine Generators..................................................................................... 11

2.2.1 Induction Generator Applications................................................................. 12

2.2.3 Analysis Of Induction Generator.................................................................. 13

2.2.4 Dynamic Performance Of Wind Turbine-Induction Generator .................... 14

2.2.5 Unbalanced-Voltage Problem In Wind Turbine Generation ........................ 15

2.2.6 Wind Turbine Generator Site Selection........................................................ 17

2.3 Wind/Diesel System............................................................................................. 18

2.3.1 Wind/Diesel System Fundamental................................................................ 19

2.3.2 Wind/Diesel System Strategies..................................................................... 22

2.4 Wind Hybrid System............................................................................................ 25

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2.5 Wind Farm ........................................................................................................... 26

2.5.4 Wind Farm Power Quality............................................................................ 27

2.5.1 Wind Farm Protection................................................................................... 29

2.6 Flickers................................................................................................................. 30

2.7 Noise .................................................................................................................... 31

CHAPTER THREE

3.0 Modeling of Wind/Diesel System........................................................................ 32

3.1 Introduction.......................................................................................................... 32

3.1 Induction Machine ............................................................................................... 33

3.2 Synchronous Machine Model .............................................................................. 36

3.3 Synchronous Machine Automatic Controller ...................................................... 39

3.3.1 Automatic Voltage Regulator (AVR) ........................................................... 40

3.3.2 Excitation system .......................................................................................... 42

3.4 Compensation Capacitor Bank Model................................................................. 44

3.5 Energy Storage Unit............................................................................................. 45

3.5.1 Battery Energy Storage System .................................................................... 46

3.5.2 Superconducting Magnetic Energy Storage (SMES) Unit ........................... 47

3.5.3 Flywheel Energy Storage.............................................................................. 50

CHAPTER FOUR

4.0 Computer Simulation ........................................................................................... 52

4.1 Introduction.......................................................................................................... 53

4.2 Wind Turbine Model............................................................................................ 55

4.3 Induction Generator Model.................................................................................. 59

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4.4 Diesel Generator Model....................................................................................... 62

4.5 Power-Conditioning Unit ..................................................................................... 64

4.5.1 Rectifier Model ............................................................................................. 65

4.5.2 Inverter Model .............................................................................................. 67

4.6 Overall System Operation.................................................................................... 70

4.6.1 Synchronization ............................................................................................ 70

CHAPTER FIVE

5.0 System Operation With Energy Storage Unit ...................................................... 73

5.1 Disconnection of Diesel Generator From System ............................................... 74

5.2 Fault Occurrence at Wind Generator Bus-bar...................................................... 76

5.3 Wind speed variation ........................................................................................... 80

CHAPTER SIX

6.0 Future Recommendations .................................................................................... 83

CHAPTER SEVEN

7.0 Conclusion ........................................................................................................... 85

CHAPTER EIGHT

8.0 References............................................................................................................ 86

Appendix A: Overall System Operation

Appendix B: Disconnection of Diesel Generator From System

Appendix C: Fault Occurrence in Wind Generator Bus-Bar

Appendix D: Wind Speed Variation

Appendix E: Load Profile of Albany

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List of Figures

Figure 1. 1: Growth of wind and PV capacity [15]...................................................... 3

Figure 1. 2: Wind Turbine Components [47]............................................................... 7

Figure 2. 1: Single line diagram for typical wind turbine generator configuration. .. 11

Figure 2. 2: Connection of wind energy system [18]................................................. 16

Figure 3. 1: d-q equivalent circuit of induction machine........................................... 35

Figure 3. 2: Illustration of the positions of d-q axis on a two-pole machine ............. 37

Figure 3. 3: Phasor diagram of synchronous machine in steady state ....................... 37

Figure 3. 4: IEEE Type 1 AVR model [38]. .............................................................. 42

Figure 3. 5:Synchronous excitation control system................................................... 44

Figure 3. 6: Battery Energy Storage System interface to power system ................... 47

Figure 3. 7: SMES basic operation ............................................................................ 48

Figure 3. 8: Schematic Configuration Of A Superconducting Magnetic Energy

Storage Unit. [38]..................................................................................... 50

Figure 4. 1: Wind/Diesel system block diagram........................................................ 54

Figure 4. 2: Power curve of manufacturer vs. polynomial fit curve for 225kW turbine

.................................................................................................................. 56

Figure 4. 3: PSCAD circuit diagram of 225kW wind turbine ................................... 57

Figure 4. 4: Power curve of 225kW wind turbine...................................................... 58

Figure 4. 5: Induction Generator model..................................................................... 59

Figure 4. 6: Power generated by Induction generator................................................ 60

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Figure 4. 7: Line and phase voltages generated......................................................... 61

Figure 4. 8: Diesel Generator model in PSCAD........................................................ 62

Figure 4. 9: Diesel Generator output power............................................................... 63

Figure 4. 10: AC to DC conversion rectifier model................................................... 65

Figure 4. 11: Rectifier output voltage........................................................................ 66

Figure 4. 12: Rectifier input phase voltages .............................................................. 66

Figure 4. 13: Inverter circuit model ........................................................................... 67

Figure 4. 14: Gate signals and PWM circuit.............................................................. 68

Figure 4. 15: PWM switching technique ................................................................... 69

Figure 4. 16: Load phase voltages due to synchronization........................................ 71

Figure 4. 17: Load power due to synchronization ..................................................... 72

Figure 4. 18: Overall System circuit diagram in PSCAD.......................................... 72

Figure 5. 1: Battery equivalent circuit model ............................................................ 73

Figure 5. 2: Generated power without energy storage unit........................................ 74

Figure 5. 3: Generated power with energy storage unit............................................. 75

Figure 5. 4: Generated power and voltage during fault ............................................. 76

Figure 5. 5: Over-current circuit representation......................................................... 78

Figure 5. 6: Over-current Relay control logic............................................................ 78

Figure 5. 7: Generated power during fault with battery storage unit......................... 79

Figure 5. 8: Wind speed variation.............................................................................. 80

Figure 5. 9: Generated wind power increase.............................................................. 81

Figure 5. 10: Diesel generator power during increase of wind speed........................ 81

Figure 5. 11: Diesel generator power with battery bank during increase in wind speed

.......................................................................................................................................82

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List of Tables

Table 1: Data of power vs. wind speed curve............................................................ 55

Table 2: Result of fitting manufacturer curve data to polynomial curve of several

orders........................................................................................................... 56

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Nomenclature

Induction Generator

ids, iqs Peak stator d and q axes currents

idr, iqr Peak rotor d and q axes currents

is Rms stator current

vds,vqs Peak stator d and q axes voltages

vg Peak magnitude of air-gap voltage

im Peak magnetising current

Lm Magnetising inductance

Lr, Ls Rotor and stator self inductances

Llr, Lls Rotor and stator leakage inductances

rr, rs Rotor and stator resistances

wr Shaft speed (rad/sec)

we Electrical frequency (rad/sec)

ds,qs Peak stator d and q axes flux linkages

ds, qs Peak rotor d and q axes flux linkages

B, J Net friction and inertia of the rotating parts of the system

Tm Mechanical torque

N Number of poles

X open circuit reactance

X open circuit transient reactance

To rotor open circuit transient time constant (sec)

Vt a.c machine terminal (bus-bar) voltages (p.u)

Vd, Vq Voltage coordinates: Vt = Vd + jVq (p.u)

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Ed, Eq Voltages behind transient reactance X (p.u)

IA Current magnitude (p.u)

Id, Iq Current coordinates (p.u)

Synchronous Generator

H Inertia time constant

Xd, Xq Direct and quadrature axis reactance

Xd, Xq Direct and quadrature axis transient reactance

Xd, Xq Direct and quadrature axis sub-transient reactance

Ed Voltage behind the transient reactance Xd (p.u)

Ed, Eq Voltages behind sub-transient reactance Xd and Xq (p.u)

Tqo Open circuit sub-transient time constant (sec)

Tdo, Tdo Open circuit transient and sub-transient time constants of

direct axis (sec)

Automatic Voltage Regulator

R, A, F, E Subsystems indices: regulator input filter (R), amplifier (A),

stabilizer (S), exciter (E)

K Gain constant of subsystem indices

T Time constant of subsystem indices

Vref Reference voltage (p.u)

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CHAPTER ONE

1.0 INTRODUCTION

1.2 Background information

The total annual energy consumption in 1997 was 1015 BTUs and almost half of the

total primary energy used is for generating electricity. To meet this demand,

electrical generating capacity installed must be increased. [41] Today, in many parts

of the world, the decisions for new capacity installation become complicated due to

the fact that finding new sites for generation and transmission facilities of any kind

are difficult. Particularly rural areas in the developing world where most of the

population is located, most people lack the essential energy services to satisfy most

of their basic needs. The cost of grid connection in these rural areas is very high due

to a low density of population; therefore various organizations have turn to explore

alternative solutions. [21]

One of the most economical and reliable alternatives is to use diesel power

generation, but diesel power generation is very inefficient when the load is a small

percentage of the rated power of the engine. The fact that every time, there is a need

for power the engine has to operate makes it very inconvenient and reduces the

efficiency and lifetime of the power generation system. As a result, wind energy

system has been suggested to provide a good solution to supply energy loads in these

rural areas. Wind energy system has been proven to be more profitable than other

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electrification utilities for rural areas and also it can provide an uninterrupted supply

of electricity, ease of installation, low maintenance and high reliability. [21]

Farmers have used wind energy for centuries in many different applications, but due

to todays business conditions, it is difficult to decide which applications will be

economic to use. [6] Comparing windmills in the early years, todays wind turbines

uses innovative technology that have substantially reduced the cost of electricity

generated from wind power. In the 1920s and 1930s, farm families throughout the

world used wind to generate enough electricity to power their lights and electric

motors. The use of wind power declined with the government subsidized

construction of utility lines and fossil fuel power plants [41]. However, due to the

energy crisis in the 1970s and growing concern for the environment have gained an

interest in alternative, environmental friendly energy resources. Today, homeowners

in rural and remote locations again examine renewable generated energy to provide

electricity for domestic needs [26].

Wind is a natural resource created by heating of the earths surface. Using wind

turbines, wind can be utilized and convert into electrical energy by connecting

mechanically rotating blades to a generator.

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With the technical advances in wind turbine technology, the efficiency of wind

turbines increases and the cost per kilowatt-hour (kWh) of wind generated electricity

decreases. The wind industry also achieved in areas such as noise reduction, power

quality and also the power output capacity which is the amount of electricity a wind

turbine can produce at specific wind speed increasing. As a result, world wind energy

capacity is currently doubling every three years and two months; this is shown in

figure 1.1. From figure1.1, it can be seen that as a comparison, photovoltaic is

growing at a slower rate than wind. [15]

Figure 1. 1: Growth of wind and PV capacity [15]

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1.3 Wind Power

Winds are only an intermittent source of energy. From past investigations and

studies, the intermittency of wind energy is no barrier to large-scale usage. The most

basic and important application of wind is to generate electricity, with the wind

turbines operating with utility grid systems or in parallel with diesel engines in

remote locations. Utilities have the flexibility to accept a contribution of about 20%

or more from wind energy systems and more than 50% fuel savings from wind-diesel

systems. [32]

The kinetic energy in the air mass m moving with speed V is given by [41]:

Kinetic Energy =2

1. m . V2 joules 1.1

The power in moving air is the flow rate of kinetic energy per second.

Hence:

Power =2

1. (mass flow rate per second). V2 1.2

Letting P = mechanical power in moving air

= air density, kg/m3

A = area swept by the rotor blades, m2

V = velocity of the air, m/s

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then, the volumetric flow rate is A.V, the mass flow rate of the air in kilograms per

second is *A*V, and the power is given by [41]:

P =21 (AV). V2 =

21 AV3 watts 1.3

The kinetic energy available in the wind is not possible to be harnessed total. This is

because to utilize all the kinetic energy of the wind and convert it to some other form

of energy, this would mean that the velocity of the air particles after the collector

device would be zero. Provided that one has available an infinite vacuum space to

collect the zero velocity air particles, this would not be possible. Therefore under

most ideal conditions, only 16/27 of wind energy can be utilized and converted to

other form of energy. This factor is known as the Betz Coefficient.

Therefore, the wind power from wind turbine is given by:

P =2

1CpAV

3 watts 1.4

Where, Cp is the power coefficient.

From the above equation, the power output from a wind turbine is proportional to the

cube of the wind speed; therefore site selection is an important factor to consider.

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1.4 Wind Component

The wind power system is consist of one or more units, operating electrically in

parallel, comprising of the following components:

Wind tower

Two or three blades wind turbine

Yaw mechanism

Mechanical gear

Electrical generator

Speed sensors and control

In addition to modern wind power system the following components are often

included:

Power electronics

Control electronics

Battery storage for improving the load availability in stand-alone

configuration

Transmission link for grid connection

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1.4.1 Wind Turbine Components

Figure 1. 2: Wind Turbine Components [47]

Disc Brake: The disc brake is used in case of failure of the aerodynamic brake,

or when the turbine is being serviced.

Gearbox: The gearbox has the low speed shaft to the left. It makes the high-

speed shaft to the right turn approximately 50 times faster than

the low speed shaft.

Generator: The electrical generator is usually an induction generator or

asynchronous generator.

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Hub: The hub of the rotor is attached to the low speed shaft of the wind

turbine.

Hydraulics System: The hydraulics system is used to reset the aerodynamic brakes of

the wind turbine.

Body Frame: The body frame is to support the generator and contains the key

components of the wind turbine, including the gearbox, and the

electrical generator.

Main Shaft: The rotor main shaft of the wind turbine connects the rotor hub to

the gearbox.

Radiator: The radiator contains an electric fan that is used to cool the

electrical generator. In addition, it contains a hydraulics system

which is used to cool the oil in the gearbox.

Yaw Motor: The yaw motor uses electrical motors to turn the nacelle with the

rotor against the wind. The yaw drive is operated by the

electronic controller that senses the wind direction using the

wind vane.

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CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 Introduction

The generation of wind power is increasingly becoming popular in the past few

years. In the United States, many applications of wind power are related to large-

scale, utility-size wind farms where thousands of wind turbines are interconnected to

generate large-scale electricity and in other parts of the world, smaller scales wind

turbines are installed to provide electricity that is sufficient to demands. [18]

In the past, wind turbines are used as a direct shaft power. The rotor axle was directly

connected to the millstone at the old windmills. Today the output power from the

wind turbines is utilized in two ways. One is by direct use of the mechanical shaft

power, the other is letting the wind turbine power an electrical generator and then

utilize the power as electrical power. The problem of using mechanical shaft power

is of course that the wind turbine has to be close to the place of the machine used. By

letting the wind turbine drive an electrical generator, one can transfer the power over

a large distance to the final utilization. [47]

The energy produced by wind is clean and safe to use and has low external and social

costs. Liabilities related to decommissioning of obsolete power plants will not occur

on wind energy conversion systems. As comparing to fossil fuelled electricity

production, wind turbines do not produce green house gases. [47] Numerous research

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and studies were conducted to investigate the environmental impact of wind energy

on the environment. These studies include the noise and visual effects on the

surrounding environment in the early year. In the recent years, studies in the United

States, Germany, the Netherlands, Denmark and the United Kingdom had concluded

that wind turbines do not pose any substantial threat to birds. [47]

Wind power is an ideal technology for electrification of rapidly industrializing

countries as well as industrialized areas and countries. Wind energy application can

include all types of systems such as: grid connected wind farms, hybrid energy

systems, and stand-alone systems. Wind energy is a reliable technology for both fuel,

and small remote grids and special applications such as desalination as well as large

grids. Since wind power is modular, more power can be added quickly as the demand

increased and it is a cost effective technology in many developing areas and

countries. [47]

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2.2 Wind Turbine Generators

Most of the wind turbine generators installed, except for the case of stand-alone

systems, are directly connected to the grid. This configuration of connection is

widely used in most of the countries in the world due to its cost-effectiveness and

robust solution for the wind turbine owners.

However the main draw back of this configuration is that the wind turbine generators

consume reactive power for the excitation of the rotors. In order to compensate the

reactive power consumption, the wind turbine uses capacitor bank as shown in

Figure 3.1. Other wind turbines have installed additional capacitors to compensate

for the reactive power consumption in the transformer. The capacitor banks are

typically designed only to compensate for the generator no-load consumption of

reactive power. The capacitor bank for no-load compensation is connected to the rid

in steps, immediately after the connection of the generator to the grid. Also the

capacitor bank is divided into steps to limit the transients during switching.

GearBox

Generator

CapacitorBank

TransformerGridWindTurbine

Figure 2. 1: Single line diagram for typical wind turbine generator configuration.

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2.2.1 Induction Generator Applications

In most wind energy system applications, the use of self-excited induction generator

(SEIG) is well known. [5, 55] Induction generators are capable of generating power

from variable speed as well as constant speed prime movers, however fixed speed

may not generate as much energy yield as a fully variable speed system. Induction

generators configuration is simple and the maintenance cost is low. [5] The

operating concepts of self-excited induction machines nowadays is increasingly

studied and well documented. Several methods of estimation of the generator

performance during steady state and during transient have been developed. [62]

Several domestic applications of induction machines driven from wind turbine have

been studied. The concept of using a self-excited induction generator driven from a

wind turbine for water storage heating was discussed in [55]. The primary objective

of this paper is to operate the wind turbine at its optimum tip-speed ratio using an

appropriate designed controller. [55] Using induction machine for wind energy

conversion as well as for water pumping is proposed in [36]. Since induction

generator has the disadvantage that it is a sink of reactive power and cannot provide

the reactive power requirements of consumers, a static VAR compensator is used for

providing the magnetizing currents of the induction machines in the system. [36]

From the simulated and experimental results presented in [36], through the adopting

of an indirect induction generator stator flux control strategy, the system steady state

and dynamic operation is able to maintain at a nearly constant flux operation,

meaning that the system can operate better and over wider range of speed range. [36]

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2.2.3 Analysis Of Induction Generator

For a wind energy conversion system that uses induction generator, a dc link

converter is essential for power conversion. The induction generator produces current

at variable frequency. This current is rectified onto the dc link using a converter with

six active switches. To convert the dc to a fixed frequency of the utility, a second

converter with six switches is needed. This results in many switches needed for wind

energy conversion system. Hence a new method that uses a six-switch current

regulated pulse width modulated inverter and a zero sequence filter is proposed to

eliminate some of the switches used and still retaining the original functionality of

the system. [17]

The study of induction generator steady state analysis and performance

characteristics is important due to the speed fluctuations of unregulated wind

turbines, the terminal voltage may increase to dangerously high levels which have

been reported to cause capacitor failure at wind farms. Over-voltages are the major

cause of excitation capacitor failure. Using a saturable transformer connected to the

terminals of the induction generator will improve voltage regulation and also

protection against over-voltages. [62]

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2.2.4 Dynamic Performance Of Wind Turbine-Induction

Generator

The problem of using wind as an input source of power generation is that wind varies

from time to time due to wind gusts, and is further disturbed by the effect of

supporting tower shadow. However with the advances in power electronics, the use

of static VAR compensator to regulate voltage produced from wind generator system

became an alternative solution to overcome the problem of input variation. To

achieved stability of the system, a state and output PI controller is proposed to

control the static VAR controller and the mechanical input power to the generator.

From software simulation results, the proposed controller shows good damping

performance for the wind generation system under severe wind gust and large

electrical system disturbances. [19] Another method of damping oscillations in terms

of induction generator mechanical point of view is to use a sliding mode control. The

sliding mode control provides suitable compromise between maximizing conversion

efficiency and damping torque oscillations under disturbances. [26, 27]

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2.2.5 Unbalanced-Voltage Problem In Wind Turbine Generation

In a utility system, an unbalanced voltage occurs when single-phase loads are not

uniformly applied to all three-phases. Induction machines operating in unbalanced

conditions will results in heating problems and decreasing of operating efficiency.

Unbalanced voltage usually occurs in rural electric power systems having long

distribution lines that are fed by induction generators driven by wind turbines. Other

causes of unbalanced voltage are form unsymmetrical transformer windings or

transmission impedance, unbalanced loads in transmission lines and many others.

[18]

In a wind generation system as shown in Figure 3.2, unbalanced loading at the point

of common coupling (PCC) will result in unbalanced voltage at PCC. This

unbalanced voltage will cause large negative sequence currents due to low negative

sequence impedance of induction generator. Eventually these large currents will

cause unbalanced heating (hot spot) in the machine windings that can eventually lead

to machine failure. Also the unbalanced voltage operation will create a pulsating

torque which produces speed pulsation, mechanical vibration and acoustic noise. [18]

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InductionGenerator

Unbalanced Load

Point Of CommonCoupling (PCC)

InfiniteBus

Transmission Line

LR

WindTurbine

Figure 2. 2: Connection of wind energy system [18]

Using steady state and dynamic analysis and simulation on the impact of unbalanced

voltages on the three-phase induction generator, solutions to improve the unbalanced

conditions are deduced which are to increase the power capability of transmission

lines, redistribute load periodically to equalize any unbalanced load and to use power

converter with wind turbine on the utility side. [18]

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2.2.6 Wind Turbine Generator Site Selection

To design and operate a wind energy system efficiently it is important to match the

wind turbine to the potential site. If the matching requirements are not fulfil, the

energy generated from the wind energy system will not be optimal. Factors to

consider are:

variation of wind speed distribution,

cut-in velocity of wind turbine,

rated velocity of wind turbine and,

cut-out velocity of wind turbine.

To achieve the matching requirements, methods like the Weibull statistical model

and the power curves normalization method are used. Through case studies

developed in [64-66], these methods being implemented can serve as useful tool to

make judicious choice of potential site and wind turbine generator system from the

available potential sites and wind turbine generator.

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2.3 Wind/Diesel System

Some years ago, wind-diesel system was configured as putting a wind turbine into

a small diesel network. These early and unsophisticated systems saved little if any

diesel fuel. As technologies advances and research being carried out, the necessary

requirement of wind-diesel system is to be able to stop the diesel engines as partially

loaded diesels run very inefficiently. [44] Grid connected wind energy conversion

system influences the system to certain extent due to varying power output from the

wind energy conversion system. On a large interconnected grid system, this is seen

mainly in localized voltage variations and fluctuations on a variety of timescales,

from voltage waveform harmonics through flicker to long-term voltage variations.

As a result a wind/diesel schemes strategy will be required, such as energy storage or

load control, so that sudden drops in wind power can be buffered in one way or

another. [54]

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2.3.1 Wind/Diesel System Fundamental

Wind/Diesel system can be classified according to different levels of wind

penetration. Wind penetration is defined as: [72]

Instantaneous penetration =Wind Power Output (kW)

Primary Electrical Load (kW) 2.1

Average Penetration =Wind Energy Produced (kWh)Primary Energy Demand(kWh) 2.2

In low wind penetration, the diesel generator will run at full time with the wind

power reducing the net load on the diesel generator. All the wind energy generated

will be supplying the primary load. Low wind penetration system is the easiest to

integrate with existing diesel system and modification to the diesel plant is not

necessary; therefore it has the lowest capital cost. Due to the simplicity of this

configuration, fuel savings is only up to ~20%. [72]

In medium wind penetration, the diesel generator will operate at full time. During

high wind power levels, the secondary loads will be dispatched to ensure sufficient

diesel loading and alternatively, wind turbines are curtailed during high winds and

low loads. To achieve this dispatching of loads, simple control system is required. As

compared to low wind penetration, medium wind penetration required some diesel

controls modification, automated diesel operation and integration of secondary

loads. It also has higher capital cost and greater fuel savings of ~40%. [72]

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In high wind penetration, the diesel generator can be shut down during high wind

availability and auxiliary components are required so as to regulate voltage and

frequency. Conventional diesel control will be modified with new diesel control and

sophisticated supervisory control system required to monitor the system operation.

This configuration provide as high as ~70% of fuel savings and higher capital cost.

[42]

When the wind generator output is sufficient to supply the load demand, it is

impractical to keep the diesel generator on-line as spinning reserve to cover short-

term deficits in wind generator output. Hence frequent start-stop cycling of diesel

generator is impractically high. This frequent start-stop diesel engine cycling is a

fuel-inefficient mode that also may have a detrimental impact on engine and engine

starter life. Also, there are certain types of diesel engines that have warm up

requirements, which made it impossible for rapid start-stop operation. Therefore,

under these typical circumstances it is favorable for short-term energy storage system

to be integrated into the wind-diesel system operation during prolonged windless

periods or when the wind generator output is insufficient to supply the load demand.

[1]

Seal lead-acid battery is used as a form of energy storage for the wind/diesel system.

Past objections of lead-acid batteries usage due to the requirement watering and the

hydrogen gas explosion hazard have been eliminated by the improved sealed design.

However, lead-acid batteries are ill suited to short-term storage application, which is

characterized primarily by the rate, rather than by the amount of energy transfer. As

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such that a battery system of suitable power rating will be considerably oversized

with respect to its energy storage rating and thus not cost effective. [1]

Another short-term energy storage is the conventional steel flywheels, which is

directly coupled to a synchronous generator driven by diesel engine during wind

deficits. In the system the synchronous generator can be decoupled from the diesel

by clutch means but it is still connected to the load bus. The advantage of using

flywheel is that it is simple to construct. But the specific energy capacity of steel

flywheel is relatively low and especially so for rotor disks which are pierced to

receive shaft. [1] Power electronic drive capable of bi-directional power flow is

required between the diesel generator and synchronous machine so that energy from

a flywheel can be store and retrieve at continuous changing speed. In addition,

accurate control of the power in and out of the flywheel requires fast and accurate

control of the torque on the shaft of the synchronous machine. With the combination

of flywheel energy buffer and fast-response power electronic integrated into the

wind/diesel system, voltage disturbances at the point of coupling and in the

distribution system due to frequent wind power fluctuations will be avoided, also

frequent start/stop cycling of diesel generator in autonomous power systems

prevented. [29]

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2.3.2 Wind/Diesel System Strategies

Many investigations have been conducted to model wind/diesel system. In [9], a

computer simulation program has been developed to investigate the dynamics of an

isolated power system supplied from diesel and wind under 100% wind penetration

in wind/diesel power supplied isolated network. A wind/diesel system model was

developed using mathematical equations and is presented in the paper. A control

policy was developed to minimize frequency and voltage disturbances due to wind

turbulence and gusts and load demand fluctuations. An eigenvalue analysis followed

by the participation-matrix technique is applied to identify weak points in the

stability of the system. Through simulation results, the implementation of the control

policy into wind diesel system under 100% wind penetration has achieved acceptable

performance.

The difficulties of integrating wind energy and various approaches in solving these

problems have been discussed in [13]. Also the role of energy storage is examined,

both to deal with operational problems and to improved wind energy utilization. A

summary of actual installations of wind/diesel system is presented to illustrate the

various approaches to problems covered in different locations. Results from

monitoring exercises are quoted, where possible, to indicate the performance

expected from the different system designs in practice. It was emphasize that it is

misleading to suggest that there exists one preferred design for wind/diesel systems.

The optimum design will depend on the particular needs of the consumer. [13]

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System sizing is an important factor to consider when implementing a stand-alone

wind/diesel system. Through correct sizing of the system, the required power

supplied to consumer is enhance and energy produced will not be wasted [51]. A

reliable/cost evaluation model described in [51] has been applied to obtain optimal

utilization of wind energy sources in small isolated power system capacity

expansion. A useful concept regarding appropriate dates, types of energy sources and

their penetration levels in the formulation of potentially beneficial capacity schemes

for composite small isolated power system. In [22], a sizing methodology was

developed based on the power curve of wind turbine, physical (charge current

limitation) and cost considerations. The optimized energy system for various levels

of satisfaction (LLP), i.e. both able to satisfy the load with a certain percentage of the

load not satisfied and to offer the lowest kWh cost was determined and studied. An

existing isolated system is being use to justify the methodology developed and

various findings has been concluded. [22]

Besides using battery or flywheel as a form of short-term energy storage, the use of

Superconducting Magnetic Energy Storage unit (SMES) can be implemented into

wind/diesel system. Due to its fast acting characteristics, SMES can effectively damp

electromechanical oscillations in the system when there is a sudden change in power

requirement. It also provides energy storage capacity in addition to kinetic energy of

the generator rotor. Through simulation results, it was shown in [57] that the system

dynamic and transient stability with SMES increases as compare to the system

without SMES. Further improvements in the control of SMES on wind/diesel system

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are shown in [35]. A complete model of the wind/diesel system with SMES was

developed and was used for eigenvalue analysis and in the design of controllers.

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2.4 Wind Hybrid System

In a wind hybrid system, the certainty of meeting load demands at all time is greatly

enhanced using more than one power source. Besides using diesel generator with

wind turbine for electrical generation, batteries are used to meet the daily load

fluctuation in short terms and diesel generator are used to meet long term load

fluctuation. [39]

In some hybrid systems the wind turbine is combined with PV panels to generate

electricity for stand-alone applications. Such systems usually consist of wind turbine

generators, PV panels, storage batteries and backup generators. The hybrid wind/PV

power system greatly enhance the generation of electricity and being emission free,

the energy coming from the wind and sunrays is available at no cost. [12,39]

When there is more than one source of generation in a system, a smart controller is

needed so as to protect the equipments in the system and to ensure sufficient power

flow to the load. Most controllers are electronic controller, which based on

parameters such as load demand, generator status and battery sate of charge to

perform the necessary tasks required. [12] Due to the high complexity in designing

the controller, most controllers are custom designed. Besides controllers, an inverter

or a power-conditioning unit, PCU, is necessary to drive ac loads from dc source and

also a rectifier is required to charge the battery from engine generator. [20]

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2.5 Wind Farm

Wind farms are clusters of wind turbines that generate electricity. Wind farms are

usually located in areas with reliably favourable wind speeds. [50] Most of the wind

farms developed by private companies are in the United States. These private

companies uses their own land from farmers and ranches or in some cases from the

government to developed wind farm and sell the electricity produced to power

marketers, electric utilities, etc. [73]

The largest wind farms, in terms of the number of turbines in a single area, are in

California. The reasons for the California wind farms being the largest are because

the California wind farms are in places where very favourable wind occur and also

they are near to electric power transmission lines and large cities. Hence, peak winds

in these areas occur approximately at the same times as the peak electricity demand

in the cities. [73]

The three largest wind farms in the world presently are at Altamont Pass, California,

just east of San Francisco, in the Tehachapi Mountains in Kern Country, and at San

Gorgonio Pass, just north of Palm Springs, where there are thousands of wind

turbines at each of these wind farms. [73]

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2.5.4 Wind Farm Power Quality

In remote/rural regions where electric power supplies are not economically favored,

the use of renewable energy resources is of great importance. Wind energy being the

most commercially interesting applications are widely used in these areas. However,

the problem with wind power installations is the power quality impact that wind

turbines have on weak or rural grids. Variations in the wind speed and also variations

in output power from wind turbines create power pulsations. This has led to high

costs for connecting the wind turbines to the grid, up to 20% of the total cost of a

wind turbine installation. [68] From the technical point of view the integration of the

power produced by standard wind turbines in a weak grid should be dealt with care

and the requested design tools must be developed and used in order to maintain the

grid power quality. [7]

On weak networks in rural areas, the majority of customer loads are single-phase. If

these are not correctly shared out between the phases, voltage unbalance will result.

Although induction machines connected to such networks will act to reduce the

unbalance, in the operating process induction machines will be subject to

overheating. It is known that in some cases voltage unbalance has been above the

specified levels, and significant wind turbine downtime has occurred. [4]

One way to improve power quality is to use turbines of various kinds in order to

optimize the energy output per invested amount of money. Or, to use converters only

at lower powers where there is an energy gain and use directly connected induction

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generators at higher wind speeds. Also the converter at this configuration can, at high

wind speeds, be used as reactive power sources. [68]

The real and reactive power problems due to the wind turbines in wind farm will

affect voltages on the network as it flows through the network impedances. For

weak/rural networks, the connections of wind farms can results in significant impact

on voltages, load demands and power losses. [63]

To minimize the impact of voltage variations on the network, static compensators,

STATCOMs, can be used to improve both the steady state and dynamic impact of a

wind farm on the network. To further improve the use of STATCOMs, control

strategy is being adopted to prevent over-voltages that occur under islanding

conditions. [77] Another method to minimize voltage variations is the use of reactive

power regulation ability of an advanced power electronic interface, thus voltage

fluctuation can be minimized to acceptable levels. Also the system harmonics

requirements can be met by using higher frequency pulse width modulation, PWM,

switching technique together with a relatively low cost harmonic filter. [76]

Power utilities have also specified limits on the maximum instantaneous step change

in voltage that a customer can cause. Wind turbines can cause voltage step changes

when starting, or when changing between generators. For very weak points on a

network, this issue may be the limiting factor on the number and size of wind

turbines that may be connected. [7]

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2.5.1 Wind Farm Protection

In a wind farm the protection of equipments against fault is necessary because when

a fault occur, the wind farm reliability is maintained and also lower the cost of

replacing breakdown equipments. Normally the wind turbine induction generator in a

wind farm is individually connected to a transformer and in turn connected to one or

more medium voltage power collection circuits. In a wind farm the main protection

of the generator transformer is through a medium voltage fuse that is house in a ring

main unit. In case of a low/high current fault occurrence, the fuse is able to clear the

fault in the low voltage winding as well as in the medium winding. [61]

In [61], an alternative relay is use to replace the fuse currently used to reduce the

wind farm construction cost. This proposed relay have the ability of protecting the

power collection circuit and the medium and low voltage windings of each generator

against faults and thus eliminating the use of medium voltage fuse and the ring main

unit that houses them. However the elimination of low voltage fuses is not necessary

since the low voltage fuse are inexpensive. Several monitoring studies have been

carried out based on practical feasibility of proposed relay in wind farm and

simulation to confirm the operations of relay during fault occurrence.

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2.6 Flickers

Flicker is the technical expression for root mean square, RMS, voltage variation in

the frequency range above 1Hz which occurs in the electric power systems and

causes light bulbs to flicker. The cause of flicker has commonly been attributed to

wind speed variations. Variable-speed wind turbines generally produce significantly

lower flicker than fixed-speed machines. Flicker can be an important issue for weak

networks. The limits on the flicker produced by an installation vary between utilities.

Some have a fairly simple process, whereas the international standards describes a

complex methodology designed to share out equitably the flicker capacity of the

network amongst all network users. [70]

In paper [78], two models have been developed to use as an analysis tool for

prediction of flicker induced by large wind turbine. A simple model was developed

and used to identify important busbars where flicker levels are critical. The complex

model was used to predict accurately flicker severity. In this model, all types of

existing static loads are accommodated and input to the model to investigate the

effects of flickers. A dynamic load representation was also implemented which

enabled the effects of spinning loads and their dynamics on flicker to be predicted.

[78]

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2.7 Noise

One of the obstacles currently standing in the way of wind farms development is the

fear that noise from the wind turbine operation will adversely affect the living

environment near to wind farm. This fear of possible noise nuisance action against

the wind farm s makes the risk of development too high, and consequently

potentially valuable energy resources remain unclaimed. Therefore the calculation of

wind farm noise levels is essential in the development phase of wind farm design.

[49]

Calculating the far field noise levels requires two major steps. First, the sound power

output of the wind turbines must be known across the entire range of operational

wind speeds. Second, the manner in which sound attenuates as it travels from the

wind turbines to the receiver locations must be known. [49]

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CHAPTER THREE

3.0 Modeling of Wind/Diesel System

3.1 Introduction

In the wind energy conversion system studies, there are many elements affecting the

stability of the system. These factors are required to be addressed first before

proceeding to the design and simulation of the system.

Take for example; when the stability analysis involves simulation times longer than

about one second, any effects due to machine controllers such as automatic voltage

regulators (AVR) and speed governor must be incorporated. The AVR has a

substantial effect on the transient stability when varying the field voltage to try to

maintain the terminal voltage constant. Also that the stability factor contributed from

the wind turbine in the overall system should not be discarded as the variation of

mechanical power occurs as a function of time.

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3.1 Induction Machine

The operation of induction machine will be determined from the sign of the

electromagnetic torque and the slip, that is negative torque and slip correspond to

generator operation whereas positive torque and slip correspond to motor operation.

In order to model the induction machine, the d-q equivalent circuit of induction

machine, as shown in Figure 3.1, is required for the formulating of necessary

equations.

The direct-axis is assumed to align with the stator terminal voltage phasor, therefore

all the rotor variables are referred to the stator side. Hence, the current equations that

describe the dynamic behavior of the induction machine is given by the following

electromechanical equations [30]:

( ) drrmqrrdsrmeqsspeakqs iwLKirKiwLKwirKi 1221)( ++= 3.1

( ) dsdrrqrrmdssqsrmepeakds VKirKiwLKirKiwLKwi 12112)( +++= 3.2

( )( drersqr

r

rmrdsrsqsspeakqr iwwKLi

L

rKLriwKLirKi +

++= 1

222)( ) 3.3

( )( )

dsqr

r

rmrqrersdssqsrspeakdr VKi

L

rKLriwwKLirKiwKLi 2

2122)( +

++= 3.4

( ) mqrdsdrqsmrpeakr TJ

Niiii

J

LNw

J

Bw

+

+

=28

3 2

)( 3.5

where( )21 mrs

r

LLL

LK

= and

( )22 mrsm

LLL

LK

=

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The induction machine equations are derived from Park equations after some

simplifications [30]. The most important is that the stator electrical transients are

neglected as much faster compared to the rotor ones. The equations are in per-unit

(p.u.) system with respect to the synchronous reference frame [38].

The algebraic stator equations are:

qdsdd IXIrEV '' += 3.6

dqsqq IXIrEV '' ++= 3.7

The differential equations describing the dynamics of the rotor windings are:

( )[ ] qqdo

d EsIXXETdt

dE'''

'

1'++= 3.8

( )[ ] ddqo

qEsIXXE

Tdt

dE'''

'

1'= 3.9

The electromagnetic torque equation is:

qqddAe IEIET '' += 3.10

The output active and reactive power, voltage and current under steady state

operation are given as:

{ }Ate IVP *Re= 3.11

{ }Ate IVQ *Im= 3.12

22qdt VVV += 3.13

22qdA III += 3.14

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For a wind farm consisting of more than one induction machines, the number of

differential equations becomes very large. Therefore, to reduce the number of

equations, it is appropriate to assume that the number of induction machines operate

under the same wind conditions. Or the wind farm can be partitioned into groups of

n-machines which operate identically and each group can than be replaced by

equivalent machine [23].

Iqm

qr(we-wr)qswe

Iqr

iqsrr

Lm

LlrLls

Vqs

Rs

d-axis

idm

dr(we-wr)dswe

idr

ids

rr

Lm

LlrLls

Vds

Rs

q-axis

Figure 3. 1: d-q equivalent circuit of induction machine

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3.2 Synchronous Machine Model

A two-dimensional reference frame commonly defined the

electrical characteristic equation describing a three-

phase synchronous machine. This involves in the use of

Parks transformations [30] to convert currents and flux

linkages into two fictitious windings located 90 apart.

A typical synchronous machine consists of three stator

windings mounted on the stator and one field winding

mounted on the rotor. These axes are fixed with respect

to the rotor (d-axis) and the other lies along the

magnetic neutral axis (q-axis), which model the short-

circuited paths of the damper windings. Electrical

quantities can then be expressed in terms of d and q-axis

parameters. Figure 3.2 presents the diagram of d-q axis

in the machine and the phasor diagram of the synchronous

machine operating in steady state is as shown in Figure

3.3.

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Figure 3. 2: Illustration of the positions of d-q axis on a two-pole machine

Figure 3. 3: Phasor diagram of synchronous machine in steady state

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These equations do not take into account the stator transients as much faster

compared to the rotor ones.

The algebraic stator equations in p.u. are [38]:

qqdsdd IXIrEV '''' += 3.15

ddqsqq IXIrEV '''' = 3.16

The differential equations corresponding to the rotor winding dynamics in p.u. are:

( )[ ]qqqdqo

d IXXETdt

dE '''''''1'' = 3.17

([ ]dddqqdo

qIXXEE

Tdt

dE''''''

''

1''+= ) 3.18

+

= q

dd

ddq

dd

ddfd

do

qE

XX

XXE

XX

XXE

Tdt

dE''

'''

''

'''

''

'

1'3.19

The electromagnetic torque equation is:

qdqdqqddDe IIXXIEIET ''''''' += 3.20

The equations for synchronous machine current, output power and output voltage are

the same as for the induction machine.

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3.3 Synchronous Machine Automatic Controller

In dynamic power system simulations of more than one second, it is recommended to

include the effects of the machine controllers, at least for the machine most affected

by the disturbance. Also the use of controller representation is becoming a must for

first swing stability with systems being operated at their limits with near critical fault

clearing times [30].

Two main controllers used for a turbine generator set are the automatic voltage

regulator (AVR) and the speed governor. In the AVR model, there consists of a

voltage sensing equipment, comparators and amplifiers controlling a synchronous

machine that can be generating or motoring. In the speed governor there consists of

similar equipment used in the AVR but in addition it requires taking the turbine into

account [30].

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3.3.1 Automatic Voltage Regulator (AVR)

Many different AVR models have been developed for different types of power

systems. The importance of AVR is to provide the synchronous machine the proper

field voltage, hence maintaining the desired voltage and reactive power that the

synchronous machine generates. The main advantage of using AVR is that it can

respond immediately to voltage deviations during both normal and emergency

operation [38].

The IEEE had defined several AVR types and the main and most commonly used

two are the Type1 and Type2 model [30]. The differential equations of the IEEE

Type 1 AVR model (Fig 3.3.1) can be written in a matrix form for convenience:

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+

=

0

0

01

0

10

01

000

1

.

.

.

.

ref

A

A

t

R

R

r

F

A

R

E

E

E

EF

EF

FEF

F

A

A

AA

A

R

R

r

F

A

R

VT

K

VT

K

V

V

V

V

T

K

T

TT

KK

TTT

K

T

K

TT

K

T

K

V

VV

V

3.21

A

ST

K

+1 E STK +1

SE

F

ST

SK

+1

R

ST

K

+1

-

+

Vmin

Vmax

VA

|Vt|

VF

Input Filter

Derivative Feedback

Exciter

Regulating amplifier

--

+Vref

VR

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Figure 3. 4: IEEE Type 1 AVR model [38].

3.3.2 Excitation system

The issue of power system stability is becoming more

crucial. The excitation and governing controls of the

generator play an important role in improving the dynamic

and transient stability of the power system. Typically

the excitation control and governing control are designed

independently. Changes in the values of these controls

affect the transient response of the machine. Different

types of governors and AVRs would then have different

output characteristics that must be considered in this

thesis in order to simulate the response with a set of

accurate time constants of the synchronous machine.

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Typically the excitation system is a fast response system where the time constant is

small. Its basic function is to provide a direct current to the field winding.

Furthermore, the excitation system performs control and

protective functions essential to secure operation of the

system by controlling the field voltage. Hence the field

current is within acceptable levels under a range of

different operating conditions. The protective functions

of the excitation system ensure that the limits of the

synchronous machine, excitation system and other

controlling equipments are not exceeded. Its control

functions include the monitoring of voltage and reactive

power flow. These contribute as an important factor in

power system stability. Figure 3.5 illustrates typical

excitation systems within a control system. This diagram,

which is originated from IEEE Trans., vol. PAS-88, Aug

1969, illustrates a typical excitation control system for

a synchronous generator. It clearly defines the elements

of the various subsystems.

The regular governing control is a traditional PID

control, which is similar to IEEE type 1 model. The

excitation control in this thesis will assume a linear

optimal control.

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Figure 3. 5:Synchronous excitation control system

3.4 Compensation Capacitor Bank Model

Induction generator absorbs reactive power for its excitation and therefore causing an

increased in the reactive load in the power system. The capacitor banks are

connected to the wind/diesel system to maintain the reactive power to acceptable

level and also it contributes a positive effect in voltage regulation.

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The capacitor banks are connected in steps to the wind/diesel system. In the

simulations they are interpret as injected currents in the network.

The injected current is given as [38]:

cap

tcap

X

VjI

~~

= 3.22

The required reactive power for the induction generator is:

)(

3lineavelineline

IVQ

= 3.23

3.5 Energy Storage Unit

When the wind/diesel system experienced disturbances, the generating units are not

always able to respond rapidly enough to keep the system. However, if high-speed

real or reactive power control is available, load shedding or generator dropping may

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be avoided during disturbances. One of the high-speed reactive power controls

available is through the use of flexible ac transmission systems (FACTS) devices.

However, a better alternative would be to have the ability to rapidly vary real power

without impacting the system through power circulation. Hence, energy storage units

are important in the role of maintaining system reliability and power quality. Ideally

for energy storage units is to be able to damp oscillations, respond to sudden changes

in load, supply load during transmission or distribution interruptions, correct load

voltage profiles with rapid reactive power control, and still allowing the generators to

balance with the system load at normal speed.

The following sections will present a few of the energy storage units available and

some operation details of each of the storage units.

3.5.1 Battery Energy Storage System

Batteries are one of the most cost-effective energy storage technologies available,

with energy stored electrochemically. A battery system is made up of a set of low-

voltage/power battery modules connected in parallel and series to achieve a desired

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electrical characteristic. Batteries are charged up when they undergo an internal

chemical reaction under a potential applied to the terminals. They delivered the

absorbed energy when they reverse the chemical reaction. Some of the advantages of

using battery energy storage system are: high energy density, high energy capability,

round trip efficiency and cycling capability [41].

Battery stored dc charge, so power conversion is required to interface a battery with

an ac system. The used of power electronics converters which can provide bi-

directional current flow and bi-directional voltage polarity with rapid response

improves the battery technology. A simple block diagram, Figure 3.6, summaries the

battery energy storage system interface to the system.

BatterySystem

ControlSystem

Powersystem

Discharging

Charging

AC/DCPowerConditioningsystem

Figure 3. 6: Battery Energy Storage System interface to power system

3.5.2 Superconducting Magnetic Energy Storage (SMES) Unit

In superconducting magnetic energy storage (SMES), electric energy is stored by

circulating a current in a superconducting coil, or inductor. Because no conversion of

energy to other forms is involved (e.g., mechanical or chemical), round-trip

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efficiency can be very high. SMES can respond very rapidly to dump or absorb

power from the grid, limited only by the switching time of the solid-state

components doing the DC/AC conversion and connecting the coil to the grid [45].

As an energy storage device, SMES is a relatively simple concept. It stores electric

energy in the magnetic field generated by DC current flowing through a coiled wire

(Fig 3.7). If the coil were wound using a conventional wire such as copper, the

magnetic energy would be dissipated as heat due to the wire's resistance to the flow

of current. However, if the wire is superconducting (no resistance), then energy can

be stored in a "persistent" mode, virtually indefinitely, until required [35].

Current I

Figure 3. 7: SMES basic operation

The SMES unit being considered here consists of a superconducting coil, a forced-

commutated converter and a controller. The forced-commutated converter uses gate

turn-off, GTO, thyristors so that the SMES system is able to generate and absorb

active and reactive powers in four-quadrants. An SMES system with the above

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Figure 3. 8: Schematic Configuration Of A Superconducting Magnetic Energy Storage Unit.

[38]

Using equation 3.24 and 3.25, the firing angles of the converters are calculated at

each sampling instant [38].

3.5.3 Flywheel Energy Storage

Flywheels can be used to stored energy for power systems when the flywheel is

coupled to an electric machine. In most of the cases, a power converter is used to

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drive the electric machine to provide a wider operating range. Stored energy depends

on the moment of inertia of the rotor and the square of the rotational velocity of the

flywheel, as shown in 3.26. The moment of inertia, I, depends on the radius, mass,

and height or length of the rotor, as shown in 3.27. Energy is transferred to the

flywheel when the machine operates as a motor, charging the energy storage device.

The flywheel is discharged when the electric machine regenerates through the drive.

2

2

1IE= 3.26

2

2mhrI= 3.27

The energy storage capability of the flywheel can be improved either by increasing

the moment of inertia of the flywheel or by turning it at higher rotational velocities,

or both. Flywheel energy storage can be implemented in several power system

applications. If a flywheel energy storage system is included with a FACTS with a dc

bus, an inverter is added to couple the flywheel motor/generator to the dc bus.

Flywheel energy storage has been considered for several power system applications,

including power quality applications as well as peak shaving and stability

enhancement [41].

CHAPTER FOUR

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4.0 Computer Simulation

In order to model and simulate the wind/diesel system, a computer simulation

software package is a much useful tool. The computer simulation software package

used in this project is PSCAD/EMTDC. Engineers, researchers and students from

utilities, manufacturers, research and academic institutes are using this software in

planning, design, developing new concepts, testing ideas, understanding what

happened when equipment failed, commissioning, preparation of specification and

tender documents, teaching and research.

EMTDC is the software that performs the electromagnetic transient analysis on the

user-designed system. Ideas, concepts and models of portions of planned and existing

wind/diesel systems can be evaluated quantitatively using the EMTDC program. The

development of PSCAD, which is a graphical user interface for EMTDC, has greatly

simplified the tasks required to setup, run, and analyze the results of a simulation.

EMTDC does all the necessary calculations and the results can be plotted out in the

graphs and printed out.

PSCAD was designed to support multiple simulator architectures. One of these is the

EMTDC. PSCAD consists of a group of software modules. There are six modules

that make up PSCAD: the File Manager, Draft, T-Line/Cable, RunTime, MultiPlot

and UniPlot.

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4.1 Introduction

The wind/diesel system of interest to be modelled is as shown in Figure 4.1 in block

diagram. This wind/diesel system is used in the computer simulation studies and the

system configuration is similar to Western Australia Ten Mile Lagoon wind farm.

From the Albany load profile, Appendix, provided by Western Power Corp, the

system parameters were determined based on this load profile with some

assumptions made.

The wind/diesel system consist of:

Eight wind turbines

Eight induction generators

Two diesel engines

Two synchronous generators including excitation and governor controls

Capacitors banks

Transmission lines

Resistive load

A battery unit is to be included into the system to improve the stability performance

of the system.

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Figure 4. 1: Wind/Diesel system block diagram

Due to confidential reasons, the load demand obtained is only given for a particular

summer day on 3rd December 2001. The wind speed data of Ten Mile Lagoon could

not be obtained and hence theoretical wind speed is used for the simulation.

Certain data for the synchronous generators were not given in the specification sheet

from any manufacturer available, hence data for the synchronous generators were

taken from [57] to obtain a realistic modeling.

The PSCAD/EMTDC software is used for the simulation of the wind/diesel system.

Before the overall system is simulated, the individual system components are to be

constructed first.

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4.2 Wind Turbine Model

To represent the wind turbine, the power curve for a current model, 225kW rated

wind turbine manufactured by Vestas Danish Wind Technology A/S was used. The

manufacturers turbine specification sheet included the wind speed vs. power output

curve. From the power curve, a 23-point tabulation was formulated as shown in table

4.1. This data was truncated at the 18m/sec inflection point in the curve and a

polynomial fit was developed. Polynomial of second, third, fourth and fifth order fits

were than calculated with the results shown in table 4.2. The fifth order was chosen

since the residual variance will give a more accurate polynomial fit for the power

curve. The power curve when fit with the fifth order polynomial along with the

original power curve provided by the manufacturer is as shown in Figure 4.2.

Wind Speed (m/sec) Wind Speed (mile/hrs) Power output (kW)Polynomial power

output (kW)

3 6.7 0 3.3

4 8.9 6.8 2.3

5 11.2 19.5 14.3

6 13.4 31.2 35.1

7 15.7 57.1 61.2

8 17.9 87.6 89.7

9 20.1 118.7 118.3

10 22.4 149.6 145.3

11 24.6 171.4 169.3

12 26.8 189.3 189.6

13 29.1 206 205.7

14 31.3 216 217.4

15 33.6 224 224.8

16 35.8 227 228.5

17 38 229 228.9

18 40.3 230 226.8

19 42.5 225 223.0

20 44.7 215 218.5

21 47 212 214.0

22 49.2 210 210.4

23 51.5 210 208.5

24 53.7 210 208.8

25 55.9 210 211.7

Table 1: Data of power vs. wind speed curve

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Using PSCAD/EMTDC software, equation 4.1 is being modeled as a wind turbine as

shown in Figure 4.3.from the constructed circuit, the output power with respect to

wind speed is as shown in Figure 4.4 with a cut-in wind speed of 3m/sec and cut-out

wind speed of 25m/sec and power output of 225kW at 25m/sec.

Figure 4. 3: PSCAD circuit diagram of 225kW wind turbine

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Cut-in (3m/sec) Cut-out (25m/sec)

Figure 4. 4: Power curve of 225kW wind turbine.

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4.3 Induction Generator Model

In Figure 4.5, the SQ100 machine model was used to model as an induction

generator that is connected to an infinite bus. The generator is started at constant

speed of 1.01 that is higher than the rated speed. Hence with this configuration the

induction machine is generating power instead of absorbing power. However, this

configuration is used for initialization purposes only. At 0.5 second, the operation is

being switched to constant torque and run in steady state. The capacitor banks are

used to supply most of the reactive power needed by the induction generator.

Figure 4. 5: Induction Generator model

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From Figure 4.6, the generator active power from the induction generator is 0.35 p.u,

per unit, and absorbs 5.5 p.u of reactive power. The capacitor bank supplies most of

the reactive power and a small portion is being supplied by the source. The generated

phase-to-ground voltage is 7.9 kVRMS and the line-to-line voltage is 13.8 kVRMS as

shown in Figure 4.7.

Figure 4. 6: Power generated by Induction generator

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Figure 4. 7: Line and phase voltages generated

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4.4 Diesel Generator Model

The modeling of diesel generator in the PSCAD environment (Fig 4.8) is totally

based on the connection of an exciter to the synchronous machine.

The synchronous machine has option to model two damper windings in the Q axis

and hence it can be used as a round rotor machine or a salient pole machine. The

speed of the machine may be controlled directly by inputting a positive value into the

w input of the machines, or a mechanical torque may be applied to the Tm input of

the machine.

For general use, unknown parameters are left with the default values without

changing the expected performance of the machine. The general method of

initialization and start-up which is suggested be normally used is based on firstly

entering the terminal voltage magnitude and phase.

Figure 4. 8: Diesel Generator model in PSCAD

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stability improvement of a wind diesel system using an energy storage unit

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