generator basic concept

ELECTRICAL MACHINES

ALTERNATING CURRENT MACHINESSYNCHRONOUS MACHINESo SYNCHRONOUS MACHINES CYLINDRICAL (Laminated / solid)M tl d l it G t d i b t / Mostly used as large capacity Generator driven by steam / gas turbines. Speed 1500 rpm/ 3000 rpm. (Solid rotor)Motors for high speed applications 3000 and above) are Motors for high speed applications 3000 and above) are with solid rotor and lower speed are with laminated rotor.

SALIENT POLE (Laminated / solid)Slow speed design used with hydro turbines.p g yLarge & medium capacity motor, having low speeds.

Basic Principal of Operation of Generator

• In case of turbogenerators, Rotor winding is supplied with DC current (through sliprings or brushless exciter) which produces constant magnetic field.

• 3 phase stator winding is laid in stator core. • When generator rotor is rotated (by a turbine) magnetic flux• When generator rotor is rotated (by a turbine) magnetic flux

produced by rotor winding also rotates. • Voltage is induced in stator winding (Faraday’s Law*)Voltage is induced in stator winding (Faraday s Law )• 3 phase stator winding also produces magnetic flux revolving

at synchronous speed (=120*f/2p). Rotor also rotates at synch speed. Both the magnetic fields are locked and rotate together.

*Faraday’s Law : E M F (Voltage) is induced in a closed path*Faraday s Law : E.M.F. (Voltage) is induced in a closed path due to change of flux linkages and is proportional to rate of change of flux linkages. The change in flux linkages can be caused by change in flux in a stationary coil or by motion of coil with constant flux or by both.


• Stator winding, which is stationary, experiences change in flux linkages and thus E M F (voltage) is generated in statorflux linkages and thus E.M.F. (voltage) is generated in stator winding


Induced EMF E N (dΦ/dt) 10-8 voltsInduced EMF E = N.(dΦ/dt).10-8 voltsΦ = Φm.Sin(.t)


Generator at No LoadGenerator at No Load


Armature Reaction

•MMF generated by stator winding current.•Armature Reaction interacts with Rotor MMF and a resultant Magnetic Field is created.

•Armature Reaction may be considered as mechanical reaction necessary to balance the energy equation.


P = (Et.Ef / Xd).sin( / )

Generator at Unity Power Factor Load


Generator at Unity PFGenerator at Unity PF


•Power can only be delivered by an d f b l i h advance of rotor by an angle in the

direction of rotation.R t l i f P t •Rotor angle is a measure of Power as at no load is zero.

P = (V.Ef / Xd).sin( )


Generator at Zero Power Factor (Lag) LoadGenerator at Zero Power Factor (Lag) Load


Direct axis Reactance Xd

Under short circuit condition terminal voltage is zero. It means that excitation required to circulate zero. It means that excitation required to circulate the stator current must be that just to overcome armature reaction.

The ratio of excitation current to circulate rated stator current to excitation required to generate stator current to excitation required to generate rated open circuit voltage (measured on air gap line) will give Unsaturated Synchronous reactance line) will give Unsaturated Synchronous reactance Xd.


Open Circuit and Short Circuit Curve

A BO


Generator at Lag LoadGenerator at Lag Load


Generator at Lagging LoadGenerator at Lagging Load


Generator Capability DiagramGenerator Capability Diagram


V CV-Curve

Generator Module Selection Parameters

Location/ Environment Conditions Ambient Temperature, Coolant Temperature Atmospheric Conditions

Dust, Chemicals, Explosive Gases Humidityy Altitude

Driving Equipments Type of Turbine (Driving System)Type of Turbine (Driving System) MW rating Speed Speed

Generator Module Selection Parameters

Performance Requirements Type of Generator (Cylindrical / Salient Pole Type)yp ( y yp ) Voltage (for large TGs voltage is arrived by its own design) Frequency (50 Hz or 60 Hz)Frequency (50 Hz or 60 Hz) Variation in V & f Power FactorPower Factor Short Circuit Ratio Negative Sequence CapabilityNegative Sequence Capability Asynchronous capability Cooling Method Cooling Method Class of Insulation & Winding Temperature limits

Li it ti Di i d i ht Limitation on Dimensions and weight

Sizing of Generator Module

Almost all the requirements / parameters influence Sizeand Design of the machine in one way or other.

It is difficult to consider all the parameters in one go. At first step, design is worked out considering those

parameters which directly influence basic Machine Sizep y In subsequent steps, the design is checked for other

parameters / requirementsparameters / requirements


Basic Equation for Sizing of Electrical Machines2P = K.As.B. D2L.n

or, D2L = P/ (K.As.B.n)

Here,P = MW outputAs = Electric Loading (Amp cond/cm)As Electric Loading (Amp.cond/cm)B = Magnetic loading (gauss)D = Stator bore diameter (cm)D = Stator bore diameter (cm)L = Stator core length (cm)n = Rated speed


“D2L” represents Volume of Rotor or Size of theMachine.The Output Equation assumes several key factors : Complete Stator Geometry Relative proportion of stator Outer & Inner Dia.p p Slot and Tooth Dimensions Flux densities in the Iron PartsFlux densities in the Iron Parts.

MW Rating :Size of machine (D2L) is directly proportional to its outputSize of machine (D2L) is directly proportional to its output(MW)


Speed :2Size of machine (D2L) is inversely proportional to its

SpeedSynch. Speed (RPM) = 120*F / (2*P)

F=Freq., P= Pole PairqFor 50Hz supply, Speeds are (2*P = 2 – 16) :3000 1500 1000 750 600 500 428 375 rpm3000, 1500, 1000, 750, 600, 500, 428, 375 rpm .

Frequency : Frequency :Size of machine (D2L) is inversely proportional to itsFrequency.


Magnetic loading B (Air-gap magnetic flux density)Machine Size is inversely proportional to B Large B : High Magnetising Current (field current),

High Iron Losses & Core Temperature, Low Efficiency,High Overload Capability.

V l f B i i d d i i i Value of B is restricted due to magnetic saturation inany parts of Magnetic Circuit (Core/ Teeth).

Ch i f B l d d th d f t i Choice of B also depends on the grade of stampings(ETS) used.

C ld R ll d N G i O i t d (CRNGO) Sili (0 3 Cold Rolled Non-Grain Oriented (CRNGO), Silicon (0.3-4.5%) ETS are used for Generators : Low Loss,Eli i t A i L bilitEliminates Aging, Low permeability.

Sizing of Generator ModuleConsideration for Electrical Loading ‘As’

From Output Equation, Machine Size is inverselyproportional to ‘As’proportional to As

Electrical Loading ‘As’ is indicative of Winding Losses. Hi h th l ll d O t t P Higher the losses are allowed, more Output Power can

be obtained from Machine.Winding Temperature increases with increase in Losses.

Sizing of Generator ModuleConsideration for Electrical Loading As

One of the most important aspect of deciding theMaximum output which can be obtained from a givenMaximum output which can be obtained from a givenframe size is limitation on winding temperature.

Wi di t t i li it d b th Cl fWinding temperature is limited by the Class ofInsulation being used in the machine.

World-wide, for High Voltage AC Machines, Epoxybased Class-F insulation (140 °C ) is used.

Winding Temperature is limited to 120 °C (Class-B).

Insulating Material, Class & TemperatureClass Y (Temperature : 90 C)

Cotton, Silk, Paper (unimpregnated)Class A (Temperature : 105 C)Class A (Temperature : 105 C)

Cotton, Silk, Paper (impregnated in natural resins, oil)Class E (Temperature : 120 C)Class E (Temperature : 120 C)

Synthetic Resin enamels, Cotton & Paper laminates withformaldehyde bondingClass B (Temperature : 130 C)

Mica, Glass fibre, asbestos with suitable bonding (bitumen)Class F (Temperature : 155 C)

Class B material with bonding material of higher thermal stability.Class H (Temperature : 180 C)Class H (Temperature : 180 C)

Glass fibre and Asbestos, and buildup mica, with Silicon Resin.Class C (Temperature : > 180 C)Class C (Temperature : 180 C)

Mica, Ceramic, Glass, Quartz with Silicon Resin of higher thermalstability.

Sizing of Generator ModuleFactors effecting winding temperature Losses Losses Cooling method and its Heat carrying capacity Coolant (Cooling Media) temperature Coolant (Cooling Media) temperature Ambient temperature

Thermal Steady State Condition Losses produced in machine

= Losses dissipated from Machine Losses dissipated (Tw – Tc) {Temp. Rise} Tw depends on Insulation Class = 120 °C (Class-B)p ( )

Tw = Winding Temp. Tc = Coolant Temp.


Cooling Methods for very Large Size Machines :Machine Size is a critical and important aspect of design ofp p g

very Large Capacity Machines from handling,transportation point of view. (turbo-gen)p p ( g )

Size can only be limited with very high ‘As’. This isachieved by more efficient cooling methods such as. Stator : Indirect air cooling, Rotor : direct air cooling Stator : Indirect H2 cooling, Rotor : direct H2 cooling Stator : Direct H2 cooling, Rotor : direct H2 cooling (axial) Stator : Direct Water cooling, Rotor : direct H2 cooling (axial)

St t Di t W t li R t di t W t H2 li Stator : Direct Water cooling, Rotor : direct Water H2 cooling

Large Capacity Generator modules are classified w r t coolingLarge Capacity Generator modules are classified w.r.t coolingmethod and arrangement.


Cooling Methods and value of As :Normally factor AsxJs is taken as the basis for design.y gJs = current density assumed in stator winding

Indirect Air cooling = 1600-2000

Indirect H2 cooling = 2000-3000g

Stator indirectly & Rotor directly cooled (H2) = 3000-3600

Stator water cooled, rotor H2 cooled = 6500-12000

Generator Cooling Methods

Generator Cooling Methods

Development of Cooling Methods for Large Size M/cs :


M h i l St th L th f C li S t ti f T Ri i

D L B A

P D2 . L . B . As . n

Mechanical Strength of Material

Length of Cooling Paths

Saturation of Magnetic Paths

Temp. Rise in Armature Winding

Alternating Expansion during

Dynamic Stability Stator Core Dimension

Temp. Rise in Field Winding p g

Start-Ups & Shut Downs

Conductor & Insul. Stresses during

g

Sensitivity to Bowing

Vibration of Stator Core

Temp. Rise in End Zone of Stator

Start-Ups & Shut Downs

Transportation Sizes & Weights)

Core

Alternating Bending Stresses

Temp. Rise in End-Zones of Core

Bar Forces

Manufacturing Facility (Sizes & Weights)

Core

Torsional Stresses in Shaft & Coupling

Protection against Overvoltages

Short Circuit Forces in End Winding

Requirement for Requirement for improved Balancing Techniques

Flux Density in Rotor

Excitation & Short-Circuit Losses (Eff)

Transient Stability (Reactances)

No increase of utilization Increased utilizationSame or higher mfg. Costs Lower mfg. costs possible

(Reactances)


Separation of D and L

= Pole Pitch = .D/2P (= .D/2 for turbogenerator)L/ factor 2*L / D2*L / .D

1= L1/D1 = 2 to 6 higher value > smaller weight1 L1/D1 2 to 6, higher value > smaller weight

Da ≈ 2.1 D1

L1 ≈ L2

1 ≈ 5, minimum stator copper weight1 also has role stator winding leakage which decides1 also has role stator winding leakage which decides sub-transient reactance


Separation of D and L

2 = L2/D2 ≈ 2.7, minimum rotor copper weight

2 effects critical speed of rotor.p

Air gap is determined from SCR requirement

Selection of Voltage and Stator SlotsVoltage E=4.44*f*W1**fw1Voltage E 4.44 f W1 fw1

E/W1 = 4.44*f**fw1 = E/W1 = 4.44*f*(B*.D.L) *fw1

E/W1 D.L

Voltage has no role in sizing : E=4.44*f*W1**fw1g g


SLOT SIZE : Current density & No. of Slots

Voltage and Frequency variations

Zone A

Rated Point

Practically a machine will sometimes be required to operate outside the zone A

Zone B

operate outside the zone-A. Such excursion should be limited in value, duration and f f frequency of occurrence.


Short Circuit Ratio

Short circuit ratio may be defined as stator current (PU) under 3 phase short circuit condition when field excitationunder 3 phase short circuit condition when field excitation corresponding to rated open circuit voltage is applied. It is just inverse of direct axis reactance Xd. It is often used as rough and ready basis for calculating excitation for various loading.

SCR is also indicative of generator stability level.

SCR reduces as generator capacity increases


Negative Sequence Capability

In practice, generators are asymmetrically loaded, i.e. each phase draw different load current This leads toeach phase draw different load current. This leads to asymmetrical distribution of the current between stator phase windings.Thus asymmetrical current in winding phases may consist of all three symmetrical components – Positive I1, Negative I2 and Zero Io sequence currentsNegative I2 and Zero Io sequence currents.

Negative sequence current I2 produces counter-Negative sequence current I2 produces countersynchronous MMF which induces e.m.f. in rotor winding, rotor body and damper windings.

Basic Principal of Operation of GeneratorNegative Sequence Capability

Presence of closed paths allows large currents to

flow in these components, resulting losses and temperature rise.

Limits :Limits : Neg. seq. current I2Product I22.t


Asynchronous Operation

In case of load angle reaches 90°, and no corrective action is taken, rotor angle will continue to increase untill synchronism is lost There are two types of asynchronoussynchronism is lost. There are two types of asynchronous operation.

1. Pole slipping : Field exc. not strong enough to hold

h isynchronism.2. True Asynch. Operation:

Complete loss of excitationComplete loss of excitation. Generator runs as induction generator and draws magnetising current from grid.


Asynchronous Operation

3. Motoring operation of Generator : Connected to grid but turbine is not charged

It causes :

but turbine is not charged.

It causes :•Stator heating : Severe end core heating. Low power factor increases stator current and thus stator copper loss.pp•Rotor Heating : Rotor losses (rotor body, winding & damper I2R losses) are proportional to slip. Temperature

frise shall be rapid if slip is high.•Generator transformer heating•Generator voltage reduction during asynchronous•Generator voltage reduction during asynchronous running occurs at full load

Design ConsiderationsAltitude At high altitude, air density reduces thus effecting At high altitude, air density reduces thus effecting

cooling of motors Operation under Voltage andFrequency VariationFrequency Variation

UT

PUT

Typical Curves

OTO

R O

UTypical Curves• Altitude : 1000 M

• Altitude : 1500 M

% M

O



COOLING AIR TEMPERATURE


generator basic concept

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