generators for wind power nov 2008 - IIT Bombaynpsc2008/NPSC_CD/Data/Tutorial 2... · 14/53...

Generators for wind power conversion

Email : [email protected]

B. G. FernandesDepartment of Electrical EngineeringIndian Institute of Technology, Bombay

Nov 2008 Generators for wind power conversion 2/53

Outline of The Talk

Introduction

Constant speed wind turbine generator

=> Types and control

Variable speed wind turbine generator

=> Types and control

Axial flux machine for direct driven wind generator

Conclusion


Available wind Power

The amount of energy the wind turbine can produce is dependent on the wind regime where it is located and efficiency at which it captures energy

Wind regime is defined by three characteristics

=> Average wind velocity

=> The Weibull distribution of wind velocity

=> The shear of wind at the turbine location


Wind Resources in India


Wind classes and power density


Types of Wind TurbineFixed Speed

is more at a particular speed that will result in optimum tip speed ratio Suitable for high wind speedIG is used

Variable SpeedOptimum tip speed ratio in a certain range of wind speedChange rotor speed with wind speed Higher Energy CaptureSuitable for Low wind speedRequires PE interfaceEither IG or SM

η


Wind Turbine Model

Power in Wind = Swept Area= Wind velocity= Air Density

3

21 AvPw ρ=

vρ

),( λβfCp =

Two dimensional characteristicsTwo dimensional characteristics

= Pitch angle= Pitch angleββλλ = Tip speed ratio= Tip speed ratio

=>Max. value of = 0.593=>Max. value of = 0.593=>Varies with=>Varies with ωω

AA


Contd..

3

3

(max)max

21

m

opt

mP

K

RACP

ω

λωρ

=

⎟⎟⎠

⎞⎜⎜⎝

⎛=∴

K => function of and turbine parameterρSensing rotor speed or frequency of output voltageit is possible to operate at CP(max)

∴

Turbine Output Power

vRmωλ =

mωR

pwCPP =

= Speed of( rotation )Wind Turbine

= Radius of rotating Wind Turbine


CCp p versus wind speedversus wind speed


Output power versus wind speedOutput power versus wind speed


Variation CVariation Cpp with wind speedwith wind speed


Wind Energy Generators


Types of wind energy generators

Constant speed with gear boxField wound Synchronous generatorInduction generators

Variable speed with gear boxField wound Synchronous generatorPermanent magnet synchronous generator Doubly fed induction generator

Variable speed without gear box ( direct driven)PMSMAxial flux machines


Development Path of Electrical Machines

Improvement in (efficiency x Power factor)

Increase in torque/weight or power/weight

Compact size

Better dynamic performance of motor and converter unit

High MTBF (Mean Time Between Failures)

All benefits at minimum Pay back period


Higher Efficiency

Loss reduction

Better quality of electrical steel for magnetic circuit (i. e low core losses)

Doubly excited:1) Permanent Magnet 2) Current flowing in a coil

Contd…


PM Machines – New FreedomMagnet strength proportional to depthMMF due to winding is Proportional to its areaTo reduce size and increase in efficiency, replace coil with PM


PM Machines –New FreedomFlexibility in rotor geometry.


Concept is not new1st by J. Henry (1831) => poor quality hard material

(1932) => ALNICO

What is NEW?

Why we were told that “rating of synchronous M/C in MW?”

Reality : Being used in ceiling fan? (≈ 40 W)

Strong contender for irrigation pumps (2-5kW)

PM Machines


B-H characteristics of PM


Permanent Magnet materials

B-H Characteristics


Permanent Magnet Materials

PM material development


Magnet volume and (BH)max

Minimum volume => Nd FeBmagnet

Minimum volume => Miniaturization

=> less weight=> Reduced cost

Maximum operating Temp.

Corrosion


Magnets are mounted on the rotor Large air gap => Armature reaction is negligible.Reluctance is highXd =Xq

T = 3 Ψm Is sinσ / 2ω

Surface mount PMSM

Torque - load angle characteristics similar to cylindrical rotor synchronous motor


Interior PMSMMagnets are buried inside the rotorRotor is more robustHigh speed operation Uniform air gapXd less than Xq

T = {3 Ψm Is sinσ + (Xd-Xq)(Is)2 sin(2σ)}/ 2ω

Net torque is negative for low values of load angle

T max when σ >90

Small air gap => Armature reaction


Magnetic flux paths in IPMSM

d axis flux path

S

N

q - A

xis

N

S

S d - Axis

Rotor

Stator

Magnet

S

N

q - A

xis

N

S

S d - Axis

Rotor

Stator

Q axis flux path


Torque-load angle characteristic-IPMSM

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0-2

-1

0

1

2

3

4

5

L o a d A n g le

To

rqu

e

T e 1 + T e 2

T e 1

T e 2

N o lo a d a n g le ,

Where Te1 and Te2 are magnet and reluctance torque components.The developed torque is negative=0 and δo. The pull out torque of this type of machine occurs at more than 90º.


Doubly Fed Induction Generator (DFIG)Sub synchronous GenerationSpeed below synchronous speedPower fed to rotorTotal power output =

Pstator - Protor

Super synchronous GenerationSpeed above synchronous speedRotor feeds power to gridTotal power output = Pstator+Protor

Ps Pr

Pm

Ps Pr

Pm


DFIG operation with Back to Back PWM converter

Two stage power conversionIndependent control of converterConverter of reduced power rating – lower voltage rating of the devicesLower dc bus voltage -reduces the capacitor bank rating

DFIG

GearsWind turbine

dcac dc

Back to back PWM converter

ac

gridgrid


DFIG operation with Back to Back PWM converter-control

Rotor side converter will control the active and reactive power flow in either directionLimited slip range - lower dc bus voltage hence reduces the capacitor bank ratingBy controlling the phase and amplitude of the grid side converter voltage, flow of active and reactive power can be controlled.Unity power factor operation at the grid terminals possible due to control of reactive power


Direct driven wind generator-without gear box

Advantages It eliminates expensive gear box and its losses.Variable speed operation allows the turbine to capturehigher energy.

DisadvantagesDirect driven generators are large in diameter.low speed generator have higher losses.


Speed range=30-50 rpmBuried magnet Large no. of polesFlux concentrationLow cost ferrites magnetsLess noise as gear

box is eliminated

Low speed direct driven PM generator


Electrical Machines

Axial flux Radial flux


Axial Flux Machine

High power density.Less winding overhang, Higher flux density

Winding with overhang Winding without overhang


Shaft

Rotor Stator

Strong axial magnetic attraction force between stator

and rotor

Disadvantage

AFM – basic construction

AFM with single stator and rotor


`Rotor

Stator

Shaft

AFM – basic constructionA) Single stator & double rotor B) Single rotor & double stator

AFM with multiple stators and multiple rotors


B

Fi

F

B

i

Windings arrangement

Requires back iron in stator to provide return path for the working flux

Reduction in end winding, hence less copper losses

S

S

N

NRotor

Stator core

AFM-stator and rotor constructionN-N type rotor with toroidal shaped winding


N

N

S

SB

Fi

F

B

i Winding arrangement

No need of stator back iron, hence better possibilities of cooling

Large end winding connections exits, causing extra I2R losses

Rotor

Stator core

AFM-stator and rotor construction (contd.)N-S type rotor with trapezoidal shaped winding


Flux lines in two rotor configuration

A) N-N type rotor

B) N-S type rotor

N

N

S S

S SN N

NN

N N N

NNS S S

SS


Peak value of current density at radius r is

rIN2m

)r(A a1m π

=

)BdS)(r(A)Bdr(IdF ggax→→→→

×=×=

Force on the current element Idr is

a22

om1wtid I)1(DBKmN41T λ−α=

drrB)r(Ak2dT 2m1wid πα=

Torque at the element Idr is

----- (1)

----- (2)

----- (3)

----- (4)

B

Iar

Ro

Ri

AFM- Torque


Contd…Selection of the outer diameter

3ms1w

2out

out cosABnkP.60D

φηλπε

= 3s

out n1 D α

(Taking other parameters to be constant)

So less speed requirement Much more increase in outer diameterMore slots can be made to achieve higher pole pairs (lower speed) on large diameter)

Optimization goalKw1, by the proper selection of winding configurationε = Ef/V1,depend on winding inductanceBm by proper selection of magnet shape and span


)1(DBkNn24

E 22outm1w1sf λ−

π=

Where Ef is the rms value of the generated emf per phase

Where Ef is the rms value of the generated emf per phase

N1= f(Ef, kw1,Bm,λ), Keeping other quantities to be fixed Ef is limited by the rated output/input voltage of machineKw1 and Bm are the quantities which can be optimized

Contd…Selection of number of turns per phase

Optimization goal


Design aspectsRatio of inner to outer diameter = λ

It is industrial practice to choose λ = 1/√3 for getting maximum torque output

λ

Torque in PU


Comparison of AFM with RFMHigh torque to weight ratio

RT = ratio of torque densitiesof AFM to RFPM

p

RT

π

⎟⎟⎠

⎞⎜⎜⎝

⎛ π+=

p41

2

p

TR


Torous type axial flux machine –low speed wind generator

FeaturesLarge no.of polesNo cogging torqueLess resistanceHigh efficiency


Wind Turbine Generator (16 pole configuration)

1 kW, 375 rpm, 110 V, 50 Hz

For direct driven wind generator.


Doubly salient PM Machines- Three Phase Flux Reversal Machine

Magnets and winding are stationary6/8 pole configurationnr = ns (npp+1/3)f =(n × nr)/60Equivalent to 16 pole PMSM PM required are 12Flux pattern of 2 poles


Flux plot of 6/8 pole FRM

Observations

Number of rotor poles = 8

Machine is equivalent to 16 Pole PMSM from speed frequency relationship.

f =(n × nr)/60

Flux pattern of 2 poles


Doubly Salient PM Machine-6/14 pole FRM

Sinusoidal induced voltage

Stator poles = 6, Rotor poles =14

No. of Flux pattern poles = 2Machine with fictitious electricalgear (Gear ratio = 14)

Advantages of PMSM and SRM

Suitable for low speed directdriven Wind Power application


Fictitious Electrical GearFRM can be analyzed as PMSM with gear

Flux pattern speed and rotor speed is different

Gear ratio is defined as ratio of flux pattern speed to rotor speed


S.No MachineConfiguration

No. of Magnets

Gear ratio

Speed at 50 Hz(rpm)

Flux pattern poles

1 6/8 pole 12 8 375 2

2 12/16 pole 24 8 187.5 4

3 6/14 pole 24 14 214.28 2

4 12/28 pole 48 14 107.14 4

5 12/40 pole 60 20 75 4

FRM –Gear ratios


Full pitch winding FRM (FPFRM)

Electrical angle /slot= 60 deg.

Pitch factor of winding = 0.5 (CSPFRM)= 1 (FPFRM)

Voltage induced in FPFRM= twice of CSPFRM (for samenumber of turns and machine size)


Conclusion

Direct driven variable speed wind energyconversion systems are more efficient and are possible using PM machines.

For low speed operation axial flux machinesare preferred

FRM topology is suitable for low speed lowpower application

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