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PRINCIPLESOF AC GENERATION I K I Energy Technologyr -BRTraining Module: 01.04.01 Issue: A Date: September 2002 Page: 3 of 10

1 FARADAY S LAW OF ELECTROMAGNETICINDUCTION

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Figure 1: Electromagnetic Induction IFaraday sLaw Of Electromagnetic Induction, illustrated in Figure 1,states that, if a conductor is movedin a magnetic field, then an electromotive force (emf) or simply, a voltage is induced in thatconductor.It follows that, if the ends of the conductor are connected to an external load, then an electric current,driven bythat voltage, will flow from the conductor, through the load and back again.Faraday showed that if a wire moves in a magnetic field, an artificial charge, or voltage, will be createdin that wire. Faraday also showed that the magnitude of the voltage induced in the moving conductordepends on the strength of the magnetic field and the speed of movement, and on nothing else. Thesetwo laws form the whole basis of electricalpower generation, bothAC and DC.Fleming s Right Hand Rule for generators determines how this is achieved. Figure 2 illustrates therelationship between the magnetic field (North to South), direction of motion and direction of emf(voltage) induced in the conductor.

Field

Induced emf IFigure 2: Fleming s Right Hand Rule

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FKI Energy TechnologyIr APRINCIPLES OF AC GENERATIONRUSH BEM Ltd.Training Module: 01 04 01 I Issue:A Date: September2002 Page: 4 of 11Figure 3 shows a loop of stiff wire on a shaft which can be turned. Suppose each end of the wire isconnected to a slipring, insulatedfrom the shaft, upon which there are brushes that are connected to aload.

Figure 3: AC Generation Fixed FieldAs the shaft is turned, one bar passes the N-pole as the other passes the S-pole. Voltage is inducedone way in one bar and the oppositeway in the other. b Figure4 illustrates how an alternatingcurrentwaveform sinewave) is induced in the rotatingcoil as it passes the fixed magnetic field.

a) ELECTROMAGNETICNDUCTION

Figure4: Alternating Current b

01.04.01 A) PrinciplesOf AC Generation.doc O Brush Electrical Machines Ltd. 2 2


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KI Energy Technology IRINCIPLES OF AC GENERATION rRUSH M Ltd ITraining Module: 01 04 01 Issue: Date: September 2002 I Page: of 10Faraday s theory required only that the conductor should be moving through a magnetic field i.e. thatthere should be relative motion between conductor and field. It would work just as well if the magneticfield moved past the conductor. Inthe arrangement shown in Figure 5 this is ust what s happening.

Figure 5: Rotating Field (Permanent Magnet)Inthe above diagram, the stiff wire loop is fixed, and the permanent magnet is rotated past it and insideit. As a pole passes a fixed conductor a maximumvoltage is induced in it, opposite voltages on oppositesides, and they add up to give double voltage at the terminals or at the voltmeter.. In this arrangementno sliprings or brushes are needed which would be advantageous for a number of reasons, not leastthe reduced maintenancerequirement.

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01.04.01 A) PrinciplesOfAC Generation.doc 8 BrushElectricalMachinesLtd 2002


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FKI Energy Technology- . --~ .PRINCIPLESOF AC GENERATIONRUSH 6 EM Ltd.Training Module: 01.04.01 Issue: A Date: September 2002 I rage o U USo far only permanent magnets have been considered for producing the magnetic field. Far betterresults can be achieved by using an electromagnet, which can produce much stronger fields andtherefore much higher inducedvoltages (See Figure6).

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Figure 6: Rotating Field (Electromagnet)In this case however DC power must be provided to the coil which magnetises it. This can come from abattery or other DC source, but a pair of sliprings and brushes must be used to bring the batterycurrbntto the moving magnetising coil called the field coil . This coil is said to excite the field and the wholeprocess is called excitation1.Becausethe field magnet is not permanent but is an electromagnet, it is possible to vary the coil currentby a resistance and so vary the strength of the magnetic field itself. This in turn will vary the amount ofthe inducedvoltage. bUsing this principle, it is possible for an Operator to control the machine s voltage (remotely) byvaryingthe excitation. This is illustrated in the following diagram.

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rive -- c MainGmeramr

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dx.or attay ControlPanelFlgure7: Voltage Control b ka01.04.01 A) PrinciplesOfAC Generation.doc (8 Brush ElectricalMachines Ltd. 2002


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KI nergy Technology


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/ PRINCIPLESOFAC GENERATION r KI Energy Technologym rnI un e LLU.ra ig Module: 01 04 01 Issue: A Date: September2002 I je: 8 of 10

3 GENERATOR EXCITATION CONTROL SYSTEMSFigure 7 showed how it would be possible (for an Operator) to control the main machine s voltage byadjustmentof the resistancewhich in turn varies the excitation i.e. If the Operator knows the voltage hewants to see on a voltmeter connected to the generator output, he can adjust the resistance until therequired value is achieved. This is called excitation control .To make the process automatic, an electronic device called an Automatic Voltage Regulator (AVR) orExcitation Controller is used to sense the output voltage and compare it with a datum wbich haspreviously been set by hand. The AVR decides whether the output voltage is correct, too high or toolow.There commonly usedtypes of excitation control systems for acgeneratorsoutput control are:3 1 Conventional Excitation Svstem IDC GeneratorCommutator Exciter)

Figure 10: Excitation System ConventionalInthis system, a dc control signal is fed from the excitation conjrol to the stationaryfield of thedc exciter. The rotating element of the exciter then supplies a direct current to the fieldwinding of the main ac generator. The rotating armature of the dc exciter is either driven fromthe same shafl as the rotating main field of the generator, or can be on a separate motordriven shaft. In both cases, a dc commutator is required on the exciter, and brushes andsliprings (collector rings) are required on the rotating generator field to cam/ the maingeneratorfield current. This system issometimes usedonsmaller or older machines. bStatic Excitation Svstem

Figure 11: Static Excltatlon System

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PRINCIPLES OFAC GENERATIONS HE M Ltd.rraining Module: 01.04.01 I Issue:A Date: September2002 Page: 9 of 10 I

01.04.01 A).... .

I EnergyTechnolog

Static excitation systems obtain power from the electrical output of the generator or from theconnected system to feed rectifiers in the regulating system, which in turn supply directcurrent to the main field winding of the generator through brushesand sliprings.Brushless Excitatlon SystemBrushgenerators are nowalmostexclusively fitted with brushless excitationsystems in whichthe exciter shares a common shaft thus doing away with the need for sliprings and brushes.Since a DC generator used as an exciter would require the brushgear to rotate, the mainexciter is another, but smaller, AC generator with stationary field and rotating armature. TheAC output from this armature is taken converted to DC through rectifiers rotating with theshaft, and then fed to the rotatingfield winding of the main generator.

Figure 12: Brushless ExcitationSystemIn this system the ac armature of the exciter, the rotating three phase diode bridge rectifier,and the main field of the ac generator are all mounted on the same rotating shaft system. Allelectricalconnectionsare made along or throughthe centre of the shaft.It is commonto add a small second, or pilot exciter (or permanent magnet generator PMGto excite the main exciter.

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PilotciterRectifierMainPI

Figure13: BrushlessExcitationSystemWith Pilot Exciter

Principles OfAC Generation.doc. Sam-


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POWER GENERATION SYSTEMSI Ltd.Tralnlng Module: 04.01.01 Issue: B Date: April 2003 Page: 3 of 14KI tnergy lecnnology

M s e Lower Raise LowerFigure Main ComponentsOf A GeneratingPackage

1.2 Prime Mover GovernorThe prime mover is mechanically linked, or coupled, to the generator either directly or by agearbox. It would typically be a turbine (gas, steam, water or wind) or a diesel engine.function is to rotate the generator. As the generator is usually a synchronous machine, throtationalspeed is requiredto be keptfairly constantand this is the function of the governor.Modern governors are normally electronic, providing a fast, closed loop control but the outpufmay take many forms to suit the prime mover being controlled.The governor output can be afuel, water or gas valve; being opened to increasespeed or closed to reduce it. Some form ospeed signal is fed to the governor and compared with an adjustable reference. Thedifference, the emr, is usedto controlthe output.The speed to which the governorcontrols, the speed datum, is adjustable over a small range;the adjustment usually being made by means of a 'speeder motor' in the case of mechanicalgovernors or by an upldown counter in electronic units. The raisenower signals might omefrom a control switch, an automaticsynchroniseror an automaticcontrol system. b

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KI Energy TechnologvOWERGENERATIONSYSTEMS-BRUSH B EM Ltd.TrainingModule: 04.01.O1 I Issue: Date: April 20031.3 Generator AVR

The Generator converts rotational mechanical energy produced by the prime mover intoelectrical energy.Figure 2 illustrates how the various elements are connected to a brushless generator AVRsystem.

PMG EXCITER0 D l

PRIMEMOVER ROTORI STATOR

SENSINGVT AW

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iLOADFigure2: GeneratorlAVR BlockDiagrama The purpose of the pilot exciter is to provide a source of excitation power whenever themachine is running. The pilot exciter is a single phase permanent magnet generator PMG),with the magnets mounted on the shaft, and the AC output being generated in the stator.

The main exciter is of the brushlesstype and comprisesa fixed part called the main exciterstator, and a rotating part called the main exciter armature. The main exciter stator iscomprises laminatedsteel field polesaround which are the field colls.The three phase AC output from the main exciter armature is connected to the rotatingrectifier assembly, which converts the AC output to the DC input required in the generatorrotor winding See Figure3) The rotating rectifier assembly is a three phase full wave bridgeconfiguration, with fuses in series with each rectifier diode. On larger machines more thanone fuselrectifier diode may be fMed to each arm of the bridge. Electrical connectionsbetween the rectified output and the generator rotor winding are carried in the central borethrough the machine shaft.

04.01 O1 (B) PowerGenerationSystems.doc BrushElectricalMachinesLtd. 2003

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ROTATlNGRECTlFlER-


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FKI EnergyTechnology 1POWER GENERATIONSYSTEMSEM L t d 9Tralnlng Module: 04 01 01 n Issue: B Date: April 2003 I Page: of 14 -In single and parallel operation it is important to realise that power is determined by the fwersupply to the prime mover, and that excitation determines voltage when single running, npower factor when parallel running.b

3 AUTOMATIC VOLTAGE CONTROL

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Figure6: PrlnclpalComponents Of A GeneratorAnd AVRThe above diagram shows the principalcomponentsof the generator and itsAVR.The voltage transformer W) provides a signal proportional to line voltage to the VR where it iscomparedto a stable referencevoltage.The difference (error) signal is amplifiedand then used to control the output of a thyristor rectifierwhichsupplies a portion of the PMGoutput to the exciter field.If the load on the generatorsuddenly increasesthe reduction in output voltage produces an error signalwhich, when amplified, causes an increase in exciter field current resulting in a correspondingincreasein rotor current and generator output voltage.Conversely, load reductionwill cause the generator voltage to suddenly increase, and in this case theamplifiederror signal will cause a reduction in exciter field current resulting in a correspondingreductionin rotor current and generator output voltage.Because of the high inductanceof the generator field windings, it is difficult to make rapid changes infield current. This introduces a considerable lag in the control system which makes it necessary toinclude a stabilising circuit in the AVR to prevent instability and optimisethe generator voltage responseto load changes. Without a stabilising circuit, the regulator would keep increasing and reducingexcitationand the line voltage would continuouslyfluctuate above and belowthe requiredvalue.Modemvoltage regulatorsare designed to maintainthe generator line voltage within better than*l fnominalfor wide variations of machine load. b

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