C Synchronous machines - ggn.dronacharya.info

1 Synchronous machines Generator MotorPelec Pmech toPtoPhMotorSynchronous PelectoPmech machinesmachinesInduction machinesSynchronous MachinesSynchronous machines Synchronous generatorsor alternatorsare used to convert ygmechanical power derived from steam, gas, or hydraulic-turbine to ac electric powerShh iflil Synchronous generators are the primary source of electrical energy we consume today Large ac power networks rely almost exclusively on Synchronous Large ac power networks rely almost exclusively on Synchronous generators Synchronous motorsare built in large units compare to induction ygpmotors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drivesConstruction Bit fht Basic parts of a Synchronous generator: Rotor - dc excited winding St t3hidiihihth fitd Stator -3-phase winding in which the ac emf is generated The manner in which the active parts of a Synchronous pymachine are cooled determines its overall physical size and structureTypes of Synchronous MachineTypes of Synchronous MachineAccording to the arrangement of the fieldandarmaturewindingssynchronousandar maturewindings,synchronousmachines may be classified Type Thearmaturewindingisontherotorandthefiel dThearmaturewindingisontherotorandthefie ldsystem is on the stator.

2 The generated current isbrought out to the load via three (or four) slip-rings. Insulation problems, and the difficultyinvolved in transmitting large currents via thebhli itthittdbrushes,limitthemaximum poweroutputandthe generated electromagnetic field (emf). Thistypeisonlyusedinsmallunitsanditsmain typeisonlyusedinsmallunits,anditsmainapp lication is as the main exciter in largealternators with brushless excitation field typeRotating field typeThearmaturewindingisonthestatorandTh earmaturewindingisonthestatorandthe field system is on the rotor. Fieldcurrent is supplied from the excitervia twoppslip-rings, while the armature current isdirectly supplied to the load. This type isldillihi hemployeduniversallysince veryhighpower can be delivered. Unless otherwisestatedthesubsequentdiscussionre fersstated,thesubsequentdiscussionrefers specificallytorotating-fieldtypesynchron ousmachinessynchronousmachinesab c NabcNIfb a c bcSPelec cbca P=2 Pmech Armature windings are located on the VfIfstator Field windings are located on the rotorlocated on the rotorFig.

3 1 Advantages of rotating field and istationary armature Ease of constructionEase of construction Number of slip rings requiredBtt i lti tt Better insulation to armature Reduced Rotor weight and Rotor Inertia Improved ventilation arrangementVarious Types Salient-pole Synchronous machine Clidi ldhhi Cylindrical or round-rotor Synchronous machineSalient-Pole Synchronous Generator1. Most hydraulic turbines have to turn at low speeds (between 50 and 300 r/min) 2A large number of poles are required on the large number of poles are required on the rotorNon-uniformNd-axisD 10 mNonuniform air-gapNSSqaxisTurbineHd ( t )SSNq-axisHydro (water)HydrogeneratorSynchronous machine The stator is similar in construction that of a induction motor The rotor can be Salient or Non-Salient Field excitation is provided on the rotor by either permanent or electromagnets with number of poles equal to the poles of theRMF caused by statorRMF caused by stator Non-excited rotors are also possible as in case of reluctance motorsSynchronous machine (2)

4 The rotor gets locked to the RMF and rotates unlike induction motorat Synchronous speedunder all load condition All conventional power plants use Synchronous generators forAll conventional power plants use Synchronous generators for converting power to electrical form They operate at a better power factor and higher efficiency thanequivalent induction machinesSynchronous machine ConstructionSalient-Pole Synchronous GeneratorStatorCylindrical-Rotor Synchronous GeneratorD 1mTu r b i n eL 10 mSteamUniform airgapStator windingN High speedd-axisStatorUniform air-gapRotor winding 3600 r/min 2-pole 1800 r/min 4-pole Directconductor cooling (usingq-axisRotorS Direct-conductor cooling (using hydrogen or water as coolant) Rating up to 2000 MVAT urbogeneratorSynchronous Generator: Rotor Salient-pole rotors Used for low speed applications (<300rpm) which ilb f lhiidrequire large number of poles to achieve required frequencies ( hydro turbines) Cylindrical rotors Cylindrical rotors Used for high-speed applications (steam/gas turbines).)

5 Minimum number of poles is 2, so for 50Hz the p,maximum speedis 3000rpm. High speed of rotation produces strong centrifugal forces, which impose upper limit on the rotor diameterCylindrical-Rotor Synchronous GeneratorStatorCylindrical rotorOperation PrincipleppThe rotor of the generator is driven by a prime-movergypA dc current is flowing in the rotor winding which ggproduces a rotating magnetic field within the machineThe rotating magnetic field induces a three-phase voltage in the stator winding of the generatorPrinciple of machine Operation: If a three-phase set of currents, each with equal magnitude and differing in phase by 120 degrees, flows in an armature winding, then it will produce a rotating magnetic field of constant flux wave will travel in the air gap at the speed of nfPsynce=120where feis the frequency of the three Peqyphase aVb c DMagnetic axis FrequencyElectrical frequency produced is locked or synchronized to the mechanical speed of rotation of a synchronousthe mechanical speed of rotation of a Synchronous generator:nP120menPf=wherefe = electrical frequency in HzP = number of polesnm= mechanical speed of the rotor, in r/minVTIfRaEaRjXs+TVlililine - line VEVffVjXsittreactance synch.

6 Line-line stRXVE aresistancearmature aRcEEbRaRbajXsjXsRabcSynchronous motorsynchronous motor SynchronousmachineisprovidedwithSynchron ousmachineisprovidedwithdamper windings Damperwindingisashortcircuited Damperwindingisashort-circuitedwindingsi milartothesquirrel-cagewindingofinductio nmotorwindingofinductionmotorInduced Emf in a Synchronous himachine E(average)=PZN /60AE(average)=PZN /60A E(rms)/E(average)= () 2 22P NT/60 (Z 2T) E(rms)= NT/60 (Z=2T) N=120f/P 2f=PN/60 E=2 22 T* T 2f E= fTGenerated Voltage(OCC)The generated voltage of a Synchronous generator is given byecfKE =where = flux in the machine (function of If)f=electrical frequencyecf fe electrical frequencyKc= Synchronous machine constantEIfSaturation characteristic of a Synchronous machine Equivalent CircuitSynchronous ReactanceSynchronous Reactance Equivalent circuit of a Synchronous generator:Ehh hiREach phase has resistance Rand inductance LSynchronous reactance: Synchronous reactance:Xs = 2 fLR is typically << Xs, therefores typ ca ys, t e e o eneglected unless interested inefficiency or heating effectsVoltage RegulationA convenient way to compare the voltage behaviour of twogenerators is by theirvoltage regulation(VR).

7 TheVRof asnchronosgeneratoratagi enloadpo erfactorandatratedsynchronousgeneratorat agivenload,powerfactor,andatratedspeed is defined asVE%VVEVR flflnl100 =WhereVflis the full-load terminal voltage, andEnl(equal toEf)is the no-load terminal voltage (internal voltage) at rated lagging power factor (PF),VRis fairly positive, for unityPF,VRis small positive and for leadingPF,VRis Circuit_1oThe internal voltageEfproduced in a machine is not usually thevoltage that appears at the terminals of the same as the output voltageofaphase iswhen there is no armature current flowing in the factors that cause the differencebetweenEfandVt: The distortion of the air-gap magnetic field by the current flowingin the stator, called the armature reaction The self-inductance of the armature coils. The resistance of the armature coils. The effect of salient-pole rotor Circuit_2motorIajXjXlRageneratorIajXjla+ ++gEfEresVt+Equivalent circuit of a cylindrical-rotor Synchronous machinePhasor DiagramPhasor diagram of a cylindrical-rotor Synchronous generatorPhasor diagram of a cylindrical-rotor Synchronous generator, for the case of lagging power factorLagging PF: |Vt|<|Ef| for overexcited conditionLeading PF: |Vt|>|Ef| for underexcited conditionThree-phase equivalent circuit of a cylindrical-rotor Synchronous machinesynchronous machineThe voltages and currents of the three phases are 120oapart in angle, but otherwise the three phases are identical.

8 +VL-LVtIa1Ef1jXsRa+VL-L=3 VtDetermination of the parameters of the equivalent circuit from test datacircuit from test data The equivalent circuit of a Synchronous generator that has been derived contains three quantities that must be determined in order de ved co a see quaesaus be de eedo deto completely describe the behaviour of a real Synchronous generator: The saturation characteristic: relationship between Ifand (and therefore between Ifand Ef) The Synchronous reactance, Xs The armature resistance, Ra The above three quantities could be determined by performing the fll i th t tfollowing three tests: Open-circuit testShortcircuit test Short-circuit test DC testOpen-circuit test The generator is turned at the rated speed The terminals are disconnected from all loads, and the field current is set to set to zero. Then the field current is gradually increased in steps, and the terminal voltage is measured at each step along the way.

9 It i thibl tbt iiith titif It is thus possible to obtain an open-circuit characteristic of a generator (Efor Vtversus If) from this informationI+VdcIfVdcVtShort-circuit test Adjust the field current to zero and short-circuit the terminals of the generator through a set of ammetersthe generator through a set of ammeters. Record the armature current Iscas the field current is increased. Such a plot is called short-circuit characteristic. A+IfAVdcIscDC Test The purpose of the DC test is to determine Ra. A variable DC voltage source is connected between two stator terminals. The DC source is adjusted to provide approximately rated stator current The DC source is adjusted to provide approximately rated stator current, and the resistance between the two stator leads is determined from the voltmeter and ammeter readings thenDCDCDCVRI= If the stator is Y-connected, the per phase stator resistance is2 DCaRR= If the stator is delta-connected, the per phase stator resistance is2a3 DCaRR23=Determination of Xs For a particular field current IfA, the internal voltage Ef(=VA) could be found from the occ and the short-circuit current flow Isc,Acould be found from the scc.

10 Then the Synchronous reactance Xscould be obtained using ()fAEV=22 Efor Vt(V)Air-gap lineOCCI(A)()scAfAunsat,saunsat,sIXRZ=+= 2222 RZX =OCCIsc(A)SCCV ratedaunsat,sunsat,sRZX: Rais known from the DC (A)VAIsc,BIsc, Aoc,tfVEX Since Xs,unsat>>Ra,IfAIf(A)IfBscAscAunsat,sIIX = Xsunder saturated condition()EVAt V=Vrated,Efor Vt(V)Air-gap lineOCCIsc(A)SCCV ratedV()scBfratedsat,sasat,sIEVXRZ==+=22 IfAIf(A)VAIsc,BIsc, AIfB22asat,ssat,sRZX =: Rais known from the DC circuit and phasor diagram under conditionIjXsRa+EfV=0 Equivalent circuit and phasor diagram under conditionIaEfVt=0+EfVt0jIaXsIRIaIaRaShor t-circuit RatioAnother parameter used to describe Synchronous generators is the short-circuit ratio (SCR). The SCR of a generator defined as the ratio of thefield current required for the rated voltage at openratio of the field current required for the rated voltage at open circuitto the field current required for the rated armature current at short circuit.