Wind Energy Based Power System-Final

8/6/2019 Wind Energy Based Power System-Final

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Bengal Engineering and ScienceUniversity, Shibpur

The Modeling and Simulation

OfWind Energy Based Power System

Submitted ByAnup Narayan Kavimandan

Soumyadip SanyalSoumi Chaki

Abhishek Dey

Arnab Choudhury

Under The Guidance OfProf. Biswarup Basak

Dept. of Electrical EngineeringBengal Engineering and Science University

In partial fulfillment for the award of the degreeOf

Bachelor of EngineeringIn

Electrical Engineering

2009-2010

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DEPARTMENT OF ELECTRICAL ENGINEERING

BENGAL ENGINEERING AND SCIENCE UNIVERSITY,SHIBPURP.O.-BOTANICAL GARDEN

HOWRAH-711103

CERTIFICATE OF APPROVAL

I do hereby forward the final report on project entitled The modeling and simulation of

wind energy based power system, prepared by Anup Narayan Kavimandan, SoumyadipSanyal, Soumi Chaki, Abhishek Dey, Arnab Choudhury under my supervision ,as partial

fulfillment of requirements for the completion of Degree of Bachelor of Engineering

(Electrical) of Bengal Engineering and Science University.

Prof. Biswarup Basak

Dept. of Electrical

EngineeringBengal Engineering and Science University.

Countersigned by:

Prof .S.K. Mallik

Professor& Head

Department of Electrical Engineering

Bengal Engineering and Science University, ShibpurHowrah-711103.

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Table of Contents

1. Introduction 4

1.1 Electrical issues relevant to wind energy 5

2. Aim of the project 6

3. Review of previous work done on this project 11

4. The modeling aspects of a Wind Energy based

Power System

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4.1 Generator model 15

4.2 Wind Turbine model 19

4.3 Infinite Bus/Grid model 21

4.4 Load model 22

4.5 Rectifier model 28

4.6 Battery Charger model 36

4.7 Inverter model 48

5. Conclusion 58

6. Scope of future work 59

7. References/ Bibliography 61

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The modeling and simulation of a wind energy based powersystem

1. Introduction

One of the most significant developments of the late20th century was the re-emergence of the wind as a potentialsource of energy generation. An emerging awareness of thefiniteness of earths fossil fuel reserves as well as of theadverse effects of burning those fuels for energy had causedmany people to look for alternatives. The growing concernabout the emissions from fossil fuel generation andincreased government support has helped flourish wind

power installation in India and abroad. Provision of incentivesinstituted by the Ministry of New & Renewable Energy(MNRE) has made wind electricity competitive in India. Indiastands 5th in wind power installed capacity with a stunning9587 MW considering data till 2008.The need of energygeneration, the potential and the technological capacitywere the reasons to foster the emergence of wind energy.

Wind is air in motion and this energy is actually derivedfrom solar energy. About 2% of the total solar flux that

reaches the earths surface is transformed into wind energydue to uneven heating of atmosphere. This kinetic energy ofwind is used to gain the rotational motion of the wind turbinewhich is coupled with an electrical generator to supply powerover a region acting as stand alone or supplying power to agrid. In an actual WECS, the issues to be considered are asfollows:-

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1.1 Electrical Issues relevant To Wind EnergyPower Generation Generators

Power Electronic ConvertersStorage Batteries

RectifiersInverters

Interconnection & Distribution Power CablesSwitch GearCircuit BreakersTransformers

Control SensorsControllerYaw or Pitch MotorsSolenoids

Lightning Protection GroundingLightning RodsSafe paths

End Loads Lighting, Heating and MotorsSite Monitoring Data Measurement & Recording

Data Analysis

Of these only the most basic and essential parts of aWECS were included in the project work.Now, WECS can be used in two different ways:A) Isolated standalone systemB) Grid connected system.

Isolated standalone systems are employed to cater theneeds of small townships or small scale industries located atfar off places/ remote areas. Such systems are set up with arestricted objective to avoid transmission costs over longdistances.

Whereas, emphasis on grid connected systems isincreasing with the integration of renewable energy systemsinto the grid, which leads to increased energy efficiency,robustness of the system, voltage support , diversification ofenergy sources , reduced transmission and distribution

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losses and reliability of the system .As wind is one of themost promising distributed generation sources and theirpenetration level to the grid is also on the rise, this mode ofgeneration is thus our centre of focus.

2. Aim of the Project :This project is concerned with the modeling and

simulation of a grid connected wind-driven electricitygeneration system or WECS (an acronym for Wind EnergyConversion System). Our aim is to design and test a powersystem of 14.9 KVA capacity, operating at 440V, 20m/s basewind speed, induction generator based-wind energy systemvia. MATLAB simulation.

The different components of a wind energy systemnamely the wind turbine, generator, controller system,rectifier-inverter, battery , load and other equipmentsincluding transformers, grid etc. were decided as per thescope of this project following which the design steps andmodeling of the system in MATLAB simulation environmentwas undertaken.

A Block Diagram for the overall wind energy

conversion scheme:-

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To further clarify the aim of this project, brief layouts of thedifferent components are enlisted as follows:

2.2 Wind Turbine:A wind turbine is a rotating machine which converts

the kinetic energy of wind into mechanical energy. The windturbine is the prime mover which drives a generator.

Wind turbines can rotate about either a horizontal orvertical axis. We have considered a Horizontal Axis Wind

Turbine for our project. The wind turbine model used forsimulation purpose in this project is a standard MATLAB7.0.4 variable pitch wind turbine block. This block simulates

an actual wind turbine whose output mechanical torquevaries with the speed of the generator, the pitch of theblades and the wind velocity considered during operation.We have implemented pitch control in our system using a PIcontroller to control the mechanical torque output of theturbine and in turn, the total generation of the system.

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2.3 Generator:Generator is one of the most important components of

a power system. The efficiency of a power system isdependent to a large extent on the efficiency of the

generator operating in the system. It is concerned with thegeneration of electrical power in order to meet the loaddemand on a continuous basis. In wind turbine applicationsthey are used occasionally on large grid-connected turbines,or in conjunction with power electronic converters in variablespeed wind turbines.

Induction machines are now the most common type ofgenerator on wind turbines, and they are used for otherdistributed generation as well. Induction machines are

popular because:

1. They have a simple, rugged construction and require lessmaintenance,2. They are relatively inexpensive, and3. They may be connected and disconnected from the gridrelatively simply.

The generator used for this wind energy conversion

system is 14.9 KVA, 3 phase, 440V Asynchronous (Induction)Generator connected to the system.

Particular attention must be paid to synchronizing thegenerator with the network to which it is to be connected.Further details about the generator are provided in the latterhalf of this report.

2.4 Rectifier:Earlier AC to DC conversion was achieved using Motor-

Generator set, Mercury arc-rectifiers etc. With the advent ofhigh power thyristors presently thyristor converters are usedto convert AC to DC. The thyristors are phase-controlled toget variable direct voltage. They make use of LineCommutation i.e., natural reversal of AC source voltage forcommutating the thyristors conducting earlier.

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The AC/DC conversion module provides the maximumpower of a generator within a variable speed range withtorque control. The thyristors are triggered using the cosine

wave crossing technique.

The three phase fully-controlled bridge rectifierderives its input from the generator output terminals andprovides a controlled dc voltage at its output. With theadvent of high power thyristors and high voltage D.Ctransmission systems, A.C output of the 3 phase generator isrectified using a bridge rectifier and then converted back toA.C using line commuted inverters.

The rectifier also serves a very important purpose ofcharging the battery with dc at its output terminals. This is avery crucial role of a rectifier as the battery comes in veryhandy during shortages in generation.

2.5 Battery:The battery acts like a reservoir and stores energy

when the generation is over and above the load demand.This is later on utilized to feed energy to the grid when the

demand is exceeds the generation of the WECS. Thisconversion also facilitates the operation of a backup/reservebattery charger module which taps the DC power duringnormal operation only to supply the grid later duringunsatisfactory wind turbine generator operation i.e. whenthe generator fails to produce the required power.

As can be seen in the block diagram of the system, itis connected to the dc link between the Rectifier and theInverter. The battery being considered in this project is astandard BP100-12 12V battery manufactured by B.B BatteryCo. ltd. As per load current requirement up to 21 of thesebatteries can be connected in series and additional units

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may also be included, which is exactly what is done in realsystems.

2.6 Inverter:

Although, we have a battery in our system which actsas a source of DC power, but it still has to be converted intoAC for it to be of any use, as only then can it besynchronized with the bus and can successfully feed powerto the grid.

The inverter performs exactly this, very important,function of converting DC power into AC power at desiredoutput voltage and frequency. We have incorporated a threephase Voltage Source Inverter (VSI) connected in bridge

configuration. It would receive dc power from the Rectifier-battery side and convert it into 3-phase AC power beforesupplying it to the grid through a step-up transformer. Wehave used SPWM technique to control the output voltage ofthe inverter. For more information on the inverter regardingits working principles, you can refer to the section oninverter in the latter half of the report.

2.7 Step-up Transformer:

The battery used in our system has a nominal voltageof 12 Volts. As a result, the line-to-line voltage at the inverteroutput is around 10 Volts. So, obviously this voltage shouldbe stepped up to a level same as the grid before we canconnect the inverter output to the grid.

Here, we have used a three phase 2.5 KVA, 10/440V, 50 Hzdelta/delta transformer. In the process, we faced quiet anumber of interesting problems which we have discussed inthe inverter section and brought to light in the report.

2.8 Load:

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Every power system is designed keeping the load hat it has tocater, in consideration. The entire system has been modeled so asto be able to successfully supply power to the load on it. It is theprime consumer of electrical power and it also dictates the working

of the system in a number of ways. For e.g., whether the batterywill charge or discharge depends on the charging current as well ason the load conditions. We have implemented a three phase load inthe system which can be changed as per the requirement.

We have also made an attempt to perform load fluctuationson our system in the running condition using a combination of anIDEAL SWITCH and a MANUAL SWITCH. We have placed theswitches in such a manner so that even after cutting the load, itremains balanced. This process is known as LOAD MANAGEMENT.

3 Review of Previous Work done on this Project

We had a preliminary knowledge about the project topicfrom the Renewable Energy Classes we had in the 7th

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Semester. Since our task was to carry on this project fromwhere our seniors had left in the previous year, we gatheredsubstantial idea related to the scope of this project. Thoughthe idea we got from that project report was mostly

irrelevant to the project work carried out by us, the batterymodel designing procedure as mentioned previously by ourseniors, formed the basis of our battery design in MATLAB.Using the report as reference, the project was initiated byfirst and foremost modeling a DC generator setup for thewind turbine. On satisfactory simulation and testing, whichhelped us in familiarizing with MATLAB tools and techniques,we decided on introducing an Asynchronous Machine as windturbine generator. The first Induction Generator modeledworked well for its motoring operation but on transition to

generation mode, there were severe oscillations,disturbances and computational problems owing to themathematical framework of the model.Heres a brief insight into our first Induction Generatormodel:

Simulink Representation of the Induction Generator:

1. The input to the turbine is wind speed. The p.ugenerator speed is feedback from the speed

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computation blockto the turbine. Pitch angle isassumed constant till now.

2. As we want to make it work as a generator, so we passthe turbine output torque (pu) through a negative gainblock to get the actual generator torque.

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3. above mathematical model is used for the per phaseelectromagnetic torque computation and the per phasepower output. It is then computed for all the 3 phases withthe voltages of the individual phases equal in magnitude and

displaced 120 degrees apart, as is the case in an infinite bus-bar.

4. It is important to point out here that this model ofinduction machine is working satisfactorily in themotoring mode of operation. But as soon as it is tryingto enter the generator mode and the slip becomes one ,there are severe oscillations and disturbances in thesystem due to which it is unable to operate smoothly as

a generator , as is also evident in the performancecharacteristics of the generator.

5. We obtained the following performance characteristics from ourfirst Induction generator model on simulating it in MATLAB:

3-PHASE TORQUE:

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3-PHASE POWER:

P.U GENERATOR SPEED:

SLIP:

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4. The modeling aspects of a Wind Energy based PowerSystem

4.1 GENERATOR MODEL:

The Asynchronous Machine block used here is a built-in

model of MATLAB 7.0.4 present in the Simpower Systems of

Simulink Library. It operates in either generator or motor

mode. The mode of operation is dictated by the sign of the

mechanical torque: If Tm is positive, the machine acts as a

motor. If Tm is negative, the machine acts as a generator.

The electrical part of the machine is represented by a

fourth-order state-space model and the mechanical part by a

second-order system. All electrical variables and parameters

are referred to the stator. This is indicated by the prime

signs in the machine equations given below. All stator and

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rotor quantities are in the arbitrary two-axis reference frame

(dq frame).

Inputs and Outputs:

Tm

The Simulink input of the block is the mechanical

torque at the machine's shaft. When the input is a positive

Simulink signal, the asynchronous machine behaves as a

motor. When the input is a negative signal, the

asynchronous machine behaves as a generator.

The input signal is in p.u as we have used the p.u maskhere i.e., Asynchronous Machine p.u Units.

M

The Simulink output of the block is a vector containing

21 signals. We have demultiplexed these signals by using

the Bus Selector block provided in the Simulink library.

Depending on the type of mask we use, the units are in SI, or

in p.u. In our project we have used it in p.u mask.

GENERATOR SPECIFICATIONS:

ITEMS SPECIFICATIONS

TYPE OF GENERATOR SQUIRREL CAGE INDUCTION GENERATOR

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KVA RATING 14.9 KVA

RATED VOLTAGE 440 V

RATED SPEED 1000 sync R.P.M

RATED POWER 11 KW @ 0.86 p.f

NO. OF POLES 6GENERATOR INERTIA 25 kg-m2

RATED TORQUE 105 N-mSTATOR RESISTANCE 1.41 ohm i.e., 0.1085 p.u

STATOR REACTANCE 2.3 ohm i.e., 0.502 p.u

ROTOR RESISTANCE 135 ohm i.e., 0.2124 p.u referred to stator

ROTOR REACTANCE 2.3 ohm i.e., 0.502 p.u

MAGNETISING REACTANCE 98.4 ohm i.e., 7.573 p.u

WINDING CONNECTION DELTA

WORKING OF A 3-PHASE INDUCTION MACHINE

CONNECTED TO THE GRID

We have chosen a 3-phase Squirrel Cage InductionMachine coupled to a Wind Turbine with Delta connectedwinding as our Generator for the Wind energyconversion system. The generator is connected to aninfinite bus/Grid and is catering the load connected to it.

There are various aspects associated with thegeneration of wind energy which need to highlighted atthis juncture to make the report meaningful andunderstandable for the reader :

1. With a constant wind speed and without pitch control, if

there is an increase in load demand, it will be observed

that the bus current and the load current would

increase, even though the generator output current

remains constant. The reason is that as the load

demand increases, the load current also increases. Butsince the wind speed is constant (the input mechanical

torque to the generator also remains constant), so the

generator output current remains constant. As such,

the excess load current is taken from the grid/bus.

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2. There is no effect of increase in load demand on the

rotor speed or the electromagnetic torque developed

when the pitch angle became zero, as then the turbine

output torque cannot increase further unless the windspeed increases.

3. It should be understood at this point of time that if

there is no load then the generator would feed all the

power to the bus only. But that would be useless.

Hence we employ pitch control to control the input

mechanical torque from the turbine to the generatorand in turn the power generated by the generator.

4. Further, as long as the load demand is below the power

rating of the generator, we use pitch control so that the

generator develops as much power as is required by

the load. Otherwise, what will happen is that the excess

power will be fed into the bus, which is useless.

5. But once the load demand exceeds the generator

rating, pitch control is of no use as the turbine has to

develop the rated torque (i.e., the blade angle is set to

zero where the mechanical torque developed by the

turbine is maximum) and the load draws the excess

power from the bus.

6. Turbine can only supply active power. So, the load

will draw all the reactive power from the bus. Further,

Induction Generator draws reactive power (Magnetizing

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VAR) from the bus. It cannot supply reactive VAR to the

bus or the load.

7. If the machine is generating more than the requirementof the load, the excess active power will be fed into the

bus. So, there must be a perfect balance of power (both

active & reactive) between the bus, load and the

generator. Thus,

ACTIVE POWER generated by the machine= LOAD

ACTIVE POWER + BUS ACTIVE POWER (as the excessgeneration by the machine is fed into the bus).

REACTIVE POWER CONSUMPTION BY THE LOAD +

MAGNETISING CURRENT (VAR) OF THE GENERATOR =

REACTIVE POWER DELIVERED BY THE BUS.

A three phase V-I Measurement block has beenplaced at the output terminals of the generator to computethe line voltages and line currents coming out from thegenerator, which can also be viewed in the scope.Another measurement block present in the SimPowerSystem of the Simulink Library by the name ofDiscrete 3-phase Power has also been used. This measurement block

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takes the generator line voltages and currents as its inputsand enables us in computing the instantaneous power andmean power produced by the generator at its output.

4.2 WIND TURBINE MODEL:

This block implements a variable pitch wind turbine model.

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The first input is the generator speed in per unit of thebase generator speed i.e., we feedback the rotor p.u speedto the wind turbine model.

The second input is the blade pitch angle (beta) in degrees.

It is a controlled input fed through a pitch controller schemeimplemented using a PI controller. The PI controller willtrack the error signal as long as the generator outputpower is not equal to the reference power (whichcontrols the generation).

The third input is the wind speed in m/s.

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We have prepared the model of the grid/infinite bususing three balanced AC voltage sources connected in starand shifted in phase by 120 degrees. The phase to phasevoltage of the bus is taken as 440 volts so that the phase toneutral is 440/1.732. The frequency is set at 50 Hz and so itacts as an infinite bus.It has a three phase V-I Measurement block in series whichgives us the phase to neutral voltages and currents entering

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the bus. These currents and voltages are then fed to a 3-phase instantaneous Active & Reactive Power block whichgives us the Bus Active power and Reactive power flowing inthe bus.

The generator output terminals are connected to connect tothe grid.

4.4 LOAD MODEL:

The figure above shows the three phase load block. Weimplemented this block using series R-L branches where wehave the freedom to feed the values of resistance R and

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inductance L and vary the load. We have prepared 3 suchsets and connected them in series in each phase.

LOAD MANAGEMENT:

In physical/real systems there is always some fluctuation inthe load on the system. We have made a sincere attempthere to implement this feature in our system using acombination of IDEAL SWITCH and MANUAL SWITCH. Usingthis feature we can have load fluctuation in the system evenin the running condition. However, Care has been taken toensure that even after increasing/decreasing load, it remainsbalanced. This is because our project involves study ofbalanced currents.We have placed the switches to perform load fluctuations on

the system in the running condition in such a manner so thateven after cutting the load, it remains balanced. When themanual switch is at 0, the load is in the system. When themanual switch is at 1, that part of the load gets shorted in allthe 3 phases.A power computation block has been used which gives usthe load power i.e., 3-phase instantaneous active andreactive power.

OVERALL SYSTEM LAYOUT:The following figure describes our final system with all thecomponents present

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CASE 1:Load Demand is greater than the generator output.

Load parameters: R= 8 ohm/phase, L=0.02 H/phase.Load Demand (3-phase active power) = 14.97 KW

Generator Output = 13 KWSo, the excess load power is supplied by the bus which is1.968 KW.Load voltage = 440/1.732 V/phase.Mechanical Torque= 1.091 p.u ; Pitch angle = 1.361 degrees

Snap shot of the system after it has settled at theoperating point

CASE 2:

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Generator Output = 10.01 KWBus Power = 0.96 W (almost equal to zero, as expected, asthe entire load demand is taken care of by the generatoralone).

Load voltage = 440/1.732 V/phase.Mechanical Torque= 0.8055 p.u; Pitch angle = 14.06degrees.

Snap shot of the system after it has settled at theoperating point

4.5 RECTIFIER MODEL

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The Rectifier model has been realized using a 3-phaseUniversal Bridge Simulink block. The scheme of modelingis as follows:

The Universal Bridge block implements a universalthree-phase power converter that consists of up to six powerswitches connected in a bridge configuration. Thyristorshave been chosen as the power switches and a 3 armconverter configuration is adopted using the universal bridgeblock properties.

A 3-phase full controlled thyristorized bridge rectifierschematic

The Universal Bridge model as available in Matlab 7.04:-

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The gating signals for all six SCRs i.e. Thyristors have beenmodeled using the logic as mentioned below:

The Relational Operator block performs the specifiedcomparison of its two inputs. The inputs to this block are areference control voltage corresponding to the desired firingangle and a phase voltage (Vy a cosine wave) as per thethyristor meant for this gate pulse. The control voltage can

be set between the peaks of the phase voltage. Therectangular output of the relational operator i.e. comparatoris the fed to a monostable multivibrator. The monostableoutput is subsequently logically operated (ANDed) with atrain of pulses from a pulse generator model used in here.

The pulse generator has been configured so as to producepulses over the period equal to the width of the monostableoutput. The outputs of the logic block are the requiredtriggering pulses for the thyristor. It is important to mention

that the triggering of thyristors is based on the CosineWave Triggering Technique.

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Proper armvoltage say

-Vy for the

Q1 gate

Pulses

Gating

Pulse

Signal to

the

Thyristor

Q1 gate.


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The control voltage was set at 115 V corresponding to 60degree firing angle. A more resolved view shows the pulsesclearly.

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Heres the proposed rectifier model design:-

A resistive load has been placed across the Rectifier Outputto account for current even during no load condition forsimulating this model.

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The rectifier output and the gate pulses for all six thyristorsof the bridge rectifier as observed using a scope across theterminals:-

EXPECTED RECTIFIER BATTERY MODEL

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The figure above outlines the model of the rectifier and thebatterywhich we had set out to achieve. It tries toimplement the firing angle control scheme using whichwe could have been able to control the firing angle to beused in the rectifier by sensing the charging current.

But due to some computational problems in the battery thiscould not be achieved and we could not connect this schemein our system.

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4.6 THE BATTERY CHARGER MODEL

The Wind Energy Conversion System scheme requires abattery charger system for a convenient and consistentpower supply to the load which stores energy during normaloperation of the wind generator while supplying to the loaddirectly and feeding the stored energy to the load through ainverter/rectifier circuit when generator is not operatingsatisfactorily or it develops a technical snag.

The development of the battery charger model startedfrom the observation of terminal voltage vs. state of charge,discharge characteristics of a 12 V DC standard battery. The

coordinates are extracted from the given plot to obtain stateof charge values and their corresponding terminal voltagevalues. The values were then entered in a tabular form onMS Excel and subjected to the non-linear least squarecurve fitting method using Excels inbuilt solver add-in.

The constants A, C, D were modified during the execution ofthe solver to the curve fitted values for the values of X(state of charge) and E (terminal voltage) obtained fromthe standard characteristics of the 12V battery. The A,C,D

values were then used in Kinetic Battery Model where it isassumed that terminal voltage E varies with state ofcharge(SOC) as follows,

E=E0 + A*X + C*X/ (D-X)Where,

E0 = Extrapolated voltage at zero current of a fullycharged battery.

A= Initial linear variation of internal batteryvoltage with SOC

C, D= Parameters reflecting the sharpness of end ofdischarge voltage drop (commonly called the knee ofcurve). A smaller value of C will give a sharper knee of curve.

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D indicates the maximum discharge capacity at a particulardischarging current.

X= Normalized capacity removed from the batteryat a given discharge current:

X= (qout / qmax (I)) * qmax

qmax (I) = Capacity of the battery at each discharge current I.

qmax = Maximum ampere-hour capacity, it corresponds tothe charge that may be obtained when a battery isdischarged at a very slow rate i.e. I 0 A

qout = Amount of charge that has been removed by a certain

point (in discharging from a full battery.)

GIVEN PLOT OF TERMINAL VOLTAGE VS. SOC FORDIFFERENT DISCHARGING CURRENTS:-

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Terminal voltage vs. State of Charge data table:

For 9.5 amps discharging current: For 17.5 ampsdischarging current:

For 9.5 A the constants areFor 17.5 A the constants are

A= -0.01179 A= -0.01325C= -0.5195 C= -0.20515D= -46.7329 D= -3.32264

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X E(volts)

0 12

2.5 11.98

5 11.94

7.5 11.92

10 11.9

12.5 11.89

20 11.88

22.5 11.87

25 11.86

30 11.85

32.5 11.83

37.5 11.82

40 11.8

42.5 11.78

45 11.77

47.5 11.75

50 11.74

52.5 11.73

55 11.72

57.5 11.68

60 11.67

62.5 11.61

65 11.55

67.5 11.48

70 11.42

72.5 11.36

75 11.26

X E(volts)

0 12

2.5 11.99

5 11.98

10 11.92

15 11.86

20 11.8

25 11.74

30 11.72

35 11.7

37.5 11.69

40 11.67

45 11.64

47.5 11.62

50 11.61

52.5 11.6

55 11.59

57.5 11.56

60 11.54

62.5 11.48

65 11.42

67.5 11.35

70 11.29

72.5 11.23

75 11.19

77.5 11.16

80 11.1

82.5 11.07

85 10.97

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For 25 Amps discharging current: For 60 Amps discharging current:

For 25 A constants are For 60 A constants are

A= -0.0156 A= -0.06748C= -0.33595 C= -3.52479

D= -8.08126 D= -41.9415

Next we wrote a small MATLAB program to plot the verydischarge characteristics (E vs. X) using the formula. The

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X E(volts)

0 12

2.5 11.99

5 11.98

10 11.92

15 11.86

20 11.8

25 11.74

30 11.72

35 11.7

37.5 11.69

40 11.67

45 11.64

47.5 11.62

50 11.61

52.5 11.6

55 11.59

57.5 11.56

60 11.54

62.5 11.48

65 11.42

67.5 11.35

70 11.29

72.5 11.26

75 11.2

77.5 10.97

80 10.83

82.5 10.58

X E(volts)

0 12

2.5 12

5 11.93

7.5 11.87

10 11.82

12.5 11.78

15 11.74

17.5 11.71

20 11.66

22.5 11.61

25 11.55

27.5 11.5

30 11.44

32.5 11.4

35 11.36

37.5 11.32

40 11.29

42.5 11.22

45 11.14

47.5 11.08

50 10.95

52.5 10.75

55 10.49

57.5 10.23

60 10

62.5 9.61

65 9.35

67.5 8.83


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program for plotting discharge characteristics for 60 Amps islike,

X=0:5:80;

For I= 1:17,V (I) = 12-0.06748* X (I) +3.52479*X (I)/ (41.9415-X (I));End;Plot(X, V)Comparison of given plot & plot obtained from manipulatingthe equation:For 9.5 Ampere Discharging Current

Red- given plot

For 17.5 Ampere Discharging Current

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Green- Given plot

For 25 Ampere Discharging Current

Indigo- Given plot

For 60 Ampere Discharging Current

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Pink- Given plot

Modeling of the Battery Model:-

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45

Low batterycut-off

Over charging cut-off


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E=E0 + A*X + C*X/ (D-X)

This equation forms the basis of our battery model. TheBattery subsystem model has two voltage terminals (+ve

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terminal and -ve terminal) and four output terminals viz. (i)Display of State of Charge

(ii) Low voltage Signal(iii)Display of voltage

(iv)High voltage SignalSince the above equation is based on the State of Charge(x),so we are using a Current Measurement block to get thecurrent flowing through the circuit. By convention, State ofCharge is always taken to be positive. So we use an absolutevalue block to get the absolute value of the current and thenwe use an integrator to obtain the flow of charge (qout). Thisqout is multiplied by maximum ampere-hour capacity (qmax)which is taken to be 120 for this standard Kinetic BatteryModel. Now to get the State of Charge(x) we divide the

product by capacity of battery at a particular dischargecurrent (qmax (I) ) which is fed from a Look up Table withpreviously obtained values from a standard characteristicscurve of the battery we are working with.

X= (qout / qmax (I)) * qmaxSo from the obtained State of Charge one branch is sent fordisplay to output terminal and the other is sent for

computation towards A,C and D. The Lookup Table blockcomputes an approximation to some function y= f(x) givendata vectors x and y. The values of the constants A, C and Dare dependent upon values of current only. So look up tablesare used to feed values of A, C and D depending on thevalues of current. After that necessary mathematicalcalculation blocks are applied wherever necessary to realizethe above mathematical equation.

E=E0 + A*X + C*X/ (D-X);The equation implemented so far is actually true fordischarging condition. For charging condition the sign of theconstants A and C needs to be changed. So sign blocks arefed to the multiplier block which sensing the sign of current(which depends on charging or discharging) changes thesign of the product of A*X and C*X .Then E is obtainedby summing up all three components of the basic equation.

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Now to derive two signals (one for high voltage and anotherfor low voltage signal) two relational operator blocks areused. The voltagesignal now available is of simulink level, therefore, to use it

on simpower blocks, an arrangement of voltage controlledsource and current measurement block is employed. Fordifferent values of current drawn, the minimum cutoffvoltage as permitted by the battery specifications can bederived using an appropriate look-up table. The output ofthis look-up table is then fed to a relational operator to becompared with the value of output voltage for the battery. Incase the battery output voltage goes below the minimumvoltage as furnished by the look-up table (corresponding tothe output current), the relational block output triggers an

ideal switch to break the circuit. Likewise, it is necessary tobreak the circuit once the output voltage crosses-over thepermitted charge capacity (which is 110% of the ratedvoltage for the battery) in order to avoid over-charging. So,another relational operator is used to compare the outputvoltage with the maximum voltage capacity i.e. 13.2V in thiscase. When the output voltage exceeds this value, anotherideal switch is triggered to break the circuit. A DC sourcemodel along with a resistance branch is connected across

the battery terminals to simulate and test the operation ofthis battery model. On varying the voltage magnitude of theDC voltage source (above and below the 12V standardcapacity voltage), the battery is simulated for charging anddischarging conditions. The results obtained from thissimulation were as expected.As observed during simulation, the battery model workedsatisfactorily when simulated as an isolated system. But, itshowed computational errors once this model was connectedto the rectifier model.

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The following plots are taken from the working BatteryModel:-

Terminal Voltage vs. State of Charge plot depicts thecharging characteristics of the battery model; Charging

current = 16.7 Amps*

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Terminal Voltage vs. State of Charge plot depicts thedischarge characteristics of the battery model; Dischargingcurrent= 10.21 Amps*

*The battery considered for this model, as stated before, has astandard output voltage of12 V.4.7 INVERTER:

Inverters are used to convert dc power into ac power atdesired output voltage and frequency. Some of theimportant applications of inverters are in variable frequencyac drives, standby aircraft and uninterruptible powersupplies (UPS), high voltage dc transmission and many more.

Presently, our main purpose of using the inverters here

is to convert the dc power from the battery into 3 phase acpower. This power is used to meet the load demands underemergency conditions when there is either a shortage ofgeneration or that of the wind. Since the voltage level of thisac power is around 10 V so we have also used a step-uptransformer to boost up this voltage before connecting it tothe grid.

As our inverter is fed by a small dc source withnegligible internal impedance, so it is known as a Voltage

Source Inverter (VSI).The inverter that we have used here is an IGBT-DIODE

based inverter connected in the bridge configuration. It hasthree bridge arms and hence the name Bridge Inverter. Itsconfiguration is as shown below:

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To implement this bridge inverter, we have used the

Universal Bridge model present in the Simpower Systems ofthe Simulink Library in Matlab 7.0.4 .

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One of the most promising means of controlling theinverter output voltage is to incorporate time ratio controlwithin the inverter using a suitable modulation scheme i.e.,by suitably controlling the ON-OFF time of the switches wecontrol the output voltage. These inverters are called Pulse

width modulated inverters.

The advantages of this method of control are:1. We can control the output voltage without significantly

adding to the total number of components of theinverter.

2. It is also possible to substantially eliminate the lowerorder harmonic frequencies. The higher orderharmonics can be filtered by the load inductanceitself.So, without using a filter we are able to reduce theharmonics in the load current to a large extent.

Pulse width modulation techniques can be of the followingtypes:

52

The Inverter Simulink Model


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a) Single Pulse Modulation (SPM)b) Symmetrical multiple pulse modulation (SMM)c) Multiple Pulse Modulation with selective reduction of

harmonics (MMSR)

d) Sinusoidal Pulse-Width Modulation (SPWM).

Out of these 4 methods, the one we have employed is thelast one i.e., SPWM technique. So we would discuss thismethod of inverter output voltage control in detail.

SPWM TECHNIQUE:The gate pulse requirement of the inverter bridge is taken care of using a PWM

Generator

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The frequency ofcarrier triangular signal= 2 KHzThe fundamental frequency of the output voltage of thecontrolled bridge= 50HzModulation index is used to control the amplitude of thefundamental component of the output voltage and is equalto 0.5.

As the modulating signal is generated internally, itmeans the reference modulating signal is a three

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phase sinusoidal wave. The triangular carrier signal iscompared with the sinusoidal modulating signal. When themodulating signal is greater than the carrier pulse 1 is high(1) and pulse 2 is low

The gating pulses generated by the PWM generatorof our system.

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0 0.005 0.01 0.015 0.02 0.025 0.030

0.5

1

1.5

0 0.005 0.01 0.015 0.02 0.025 0.030

0.5

1

1.5

0 0.005 0.01 0.015 0.02 0.025 0.030

0.5

1

1.5

0 0.005 0.01 0.015 0.02 0.025 0.030

0.5

1

1.5

0 0.005 0.01 0.015 0.02 0.025 0.030

0.5

1

1.5

0 0.005 0.01 0.015 0.02 0.025 0.030

0.5

11.5

Six pulses are generated for a three-arm bridge. Pulses1, 3, and 5 fire the upper devices of the first, second, andthird arms. Pulses 2, 4, and 6 fire the lower devices.

INVERTER OUTPUT VOLTAGE AND CURRENT

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Substantial amount of harmonic content is introducedin the output voltage, particularly at low values of the outputvoltage. Some important points for the evaluation ofharmonic content (Assuming that there are m triangular

pulses per cycle of sinusoidal voltage):

a) mth harmonic and its integral multiples are notpresent.

b) The predominant harmonics are (2m-1) and (2m+1).The other lower amplitude harmonics are (m-2),(m+2),(3m-1),(3m+1),(4m-1) and (4m+1).c) Increasing m will shift the dominant harmonic to a

higher frequencyd) Harmonic content is higher at lower values of

modulation index.

Before connecting it to the grid:

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

-10

0

10

tim e , t--------------->

Va

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

-10

0

10

tim e , t--------------->

Vb

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

-10

0

10

time ,t--------------->

Vc

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

-0.05

0

0.05

tim e , t--------------->

Ia

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

-0.05

0

0.05

tim e , t--------------->

Ib

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

-0.05

0

0.05

tim e , t--------------->

Ic

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As can be seen, the inverter output voltages are twostepped square wave. The triplen harmonics can beconsidered to be absent and will be zero. The inverter outputcurrent wave is four stepped square wave and it alsocontains several harmonics.

After connecting it to the grid through a step uptransformer

The output voltage waveforms present at the primary

and secondary of the step up transformer have been shownin the figure below.After connecting the inverter output voltage to the grid, ittries to synchronize with the bus voltage. Moreover, it mustalso be noted that the carrier frequency has been taken as2KHz.So high frequency components are also introduced inthe inverter output voltage. It is due to this reason that thevoltages and currents appearing at the output of the inverterhave ripples (due to high frequency components) over-riding

their sinusoidal fundamental components.

So before connecting the inverter to the grid, we mustensure that the inverter output voltages are free from thesehigh frequency components and are synchronized with thegrid, otherwise circulating currents would flow.

So, we have connected chokes (inductive coils) in serieswith the step up transformer and the grid. These chokesbypass the high frequency components.

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1.95 1.955 1 .96 1.965 1.97 1 .975 1.98 1.985 1.99

-2 0

0

20

INVE RTER OUTPUT VOLTAGE

1.95 1 .96 1.97 1 .98 1 .99 2 2.01

-1

0

1

x 104

INVERTER OUTPUT CURRENT

1.95 1 .955 1.96 1.965 1 .97 1.975 1.98 1.985 1.99

-1000

0

1000

INVERTER OUTPUT VOLTAGE (AFTER STEPPING UP)

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5. CONCLUSION:We faced a number of problems while modeling thegenerator and the other components in our system in theSimulinkwindow of the MATLAB. It was a learning process

for us. So, we felt the need to discuss those issues and sumup our critical observations:

1. Phase sequence of the supply is important. It should be

kept such that the stator magnetic field rotates in the

same direction as the turbine. Otherwise, the generator

will not rotate and no torque will be produced.

2. As per the convention, if the mechanical torque from

the turbine is positive, the machine works as a Motor

otherwise as a Generator.

3. Computation of bus power, load power and the

generator output power is very important. It reveals

whether the model is working properly, as expected or

not.

4. It must also be noted that, if the load is purely resistive,

then phase current (= line current) will be in phase with

the phase voltages. But the phase voltages itself lagthe line voltages by 30 degrees i.e., the phase/line

currents will lag the line voltages by 30 degrees. Hence

if we try to compute the power consumption by the load

using line currents and line voltages as the inputs, we

found that even for a resistive load, there was

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significant reactive power consumption. Hence we

should feed the phase values of current &

voltages into the power computation blocks to

get the correct result.

5. When the machine settles at an operating point, there

will be a perfect balance of power, both active and

reactive, between the generator, bus and the load.

6. We took a single monoshot pulse having a width of 500micro-secs. The pulse train had a pulse width of80% i.e., 8 micro-secs for a period of 10 micro-secs.

This was done to ensure that all the thyristors caneasily latch. But still we found in the rectifier outputwaveform that a few thyristors missed their firingpulses. To overcome this problem, we increased theinductance of the load. Ad we could see from the outputvoltage waveform that all the thyristors were gettingtriggered.

6. SCOPE OF FUTURE WORK:-

1. The battery model, we have made here, is workingproperly in isolated condition. But when it is connected in the

whole system it was given some computational error. Thiscan possibly be attributed to the fact that it is completely amathematical model built by implementing thismathematical equation:-

E=E0+A*X+C*X/ (D-X).

As the battery model is mathematical, so when it isconnected to the Rectifier, which is a component model, it

possibly lacks the intelligence to judge the signals comingfrom the rectifier, work on it, and then decide whether tocharge or discharge. It showed erroneous values of voltages.So, ultimately we could not link the battery in our finalsystem.So this is one area which needs to be looked upon andrectified in future.

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2. Since the battery could not be connected in the ultimatemodel, so there is also the scope to connect it making use ofthe firing angle control scheme described in the Rectifier-

battery section. That would enable us to control the firingangle to be used in the rectifier by sensing the chargingcurrent.

3. It must be noted that, although the frequency of theinverter output voltage is kept at 50 Hz (same as the grid),but that alone does not ensure that the zero crossings of thereference sinusoidal signals used in SPWM are same as thezero crossings of the sine waves of the grid. So there may bea chance of a mismatch between the two voltages which

would later on create problems in synchronizing them. Toavoid this problem, what we can do is to take the sine wavesfrom the grid and use it for SPWM. This is exactly what isdone in real/physical systems nowadays and can beimplemented in future in this system.

4. We have connected chokes (inductive coils) in series withthe step up transformer and the grid. These chokes bypassthe high frequency components coming from the inverter.

Even though they largely reduce the ripples but it could notbe completely eliminated and the inverter output showssigns of still carrying them with it.

This results in a poor quality of power being obtained fromthe inverter. Moreover, there are Wide fluctuations in thebus power as well. So, this is the problem that needs to bedealt with as far as the working of the inverter is concerned.

5. Though the inverter has been connected to the systemand is almost working satisfactorily but there is still a lot ofscope in properly synchronizing the inverter output voltageswith the grid and locking them in phase with each other.

That would not only improve the stability of the system butalso go a long way improving the quality of the bus power aswell.

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6. We have not incorporated Reactive Power CompensationScheme in ourProject. There is enough scope to work on it in future,whereby

Compensation capacitor can be used under two differentschemes:i)NO-LOAD COMPENSATION : It refers to the condition whenthe capacitor, or rather capacitor bank, fully compensates the reactivepowerabsorption of the Induction Generator under no-loadcondition, andii)FULL-LOAD COMPENSATION : It refers to the conditionwhen the

capacitor fully compensates the reactive power absorption ofthegenerator under full-load condition.

7.For a grid connected transmission/distribution network, itmustcomply with certain GRID CODES i.e., fault ride throughcapability,reactive power range, voltage control, frequency range and

frequencycontrol etc. So, this aspect of our system can also be lookedinto inthe future.

8. During network fault, there is a risk that the inductiongeneratorWill lose stability during the fault. So the TRANSIENTINSTABILITYaspect of a power system , which has not been analyzedhere, can alsobe studied in the future.

7. References

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1) Wind Energy Explained-Theory, Design & Application - ByJ.F. Manwell, J.G. McGown, A.L. Rogers.

2) Wikipedia and Google Search were helpful for looking

up trivial issues.3) A Course in Electrical Machine Design - By AKSawhney.

4) The official website of B.B.Battery Company Ltd. USAfor BP-100 12V battery specifications.

5) Thyristorised Power Controllers - By GK Dubey, SRDoradla, A Joshi, RMK Sinha.

THANK YOU

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Wind Energy Based Power System-Final