Generators for wind power conversion B. G. Fernandes Department of Electrical Engineering Indian Institute of Technology, Bombay Email : bgf@ee.iitb.ac.in
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 2/53
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 3/53
Wind Resources in India 4/53
Wind classes and power density 5/53
Fixed Speed η Types of Wind Turbine is more at a particular speed that will result in optimum tip speed ratio Suitable for high wind speed IG is used Variable Speed Optimum tip speed ratio in a certain range of wind speed Change rotor speed with wind speed Higher Energy Capture Suitable for Low wind speed Requires PE interface Either IG or SM 6/53
Wind Turbine Model Two dimensional characteristics C p = f ( β, λ) =>Max. value of = 0.593 =>Varies with ω β = Pitch angle λ = Tip speed ratio Power in Wind A = Swept Area v = Wind velocity ρ = Air Density Pw = 1 ρ Av 2 3 7/53
Contd.. Turbine Output Power P = P w C p λ = ω m R v ω m R = Speed of( rotation )Wind Turbine = Radius of rotating Wind Turbine P max = = 1 2 Kω ρc 3 m P (max) A Rω λ K => function of ρ and turbine parameter opt m Sensing rotor speed or frequency of output voltage it is possible to operate at C P(max) 3 8/53
C p versus wind speed 9/53
Output power versus wind speed 10/53
Variation C p with wind speed 11/53
Wind Energy Generators 12/53
Types of wind energy generators Constant speed with gear box Field wound Synchronous generator Induction generators Variable speed with gear box Field wound Synchronous generator Permanent magnet synchronous generator Doubly fed induction generator Variable speed without gear box ( direct driven) PMSM Axial flux machines 13/53
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 14/53
Contd 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 15/53
PM Machines New Freedom Magnet strength proportional to depth MMF due to winding is Proportional to its area To reduce size and increase in efficiency, replace coil with PM 16/53
PM Machines New Freedom Flexibility in rotor geometry. 17/53
PM Machines Concept is not new 1 st 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) 18/53
B-H characteristics of PM 19/53
Permanent Magnet materials B-H Characteristics 20/53
Permanent Magnet Materials PM material development 21/53
Magnet volume and (BH)max Minimum volume => Nd FeB magnet Minimum volume => Miniaturization => less weight => Reduced cost Maximum operating Temp. Corrosion 22/53
Surface mount PMSM Magnets are mounted on the rotor Large air gap => Armature reaction is negligible. Reluctance is high X d =X q T = 3 Ψ m I s sinσ / 2ω Torque - load angle characteristics similar to cylindrical rotor synchronous motor 23/53
Interior PMSM Magnets are buried inside the rotor Rotor is more robust High speed operation Uniform air gap X d less than X q Small air gap => Armature reaction T = {3 Ψ m I s sinσ + (X d -X q )(I s )2 sin(2σ)}/ 2ω Net torque is negative for low values of load angle T max when σ >90 24/53
Magnetic flux paths in IPMSM d axis flux path Q axis flux path Stator q - Axis Stator q - Axis S S N N S d - Axis S d - Axis Rotor N S Rotor N S Magnet 25/53
Torque-load angle characteristic-ipmsm 5 4 Te1+Te2 3 Te1 Torque 2 1 Te2 0-1 N o lo a d a n g le, -2 0 20 40 60 80 100 120 140 160 180 Load Angle Where Te 1 and Te 2 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. 26/53
Doubly Fed Induction Generator (DFIG) Sub synchronous Generation Speed below synchronous speed Power fed to rotor Total power output = P stator - P rotor Super synchronous Generation Speed above synchronous speed Rotor feeds power to grid Total power output = P stator +P rotor Ps P r Ps Pr Pm Pm 27/53
DFIG operation with Back to Back PWM converter Two stage power conversion Independent control of converter Converter of reduced power rating lower voltage rating of the devices Lower dc bus voltage -reduces the capacitor bank rating DFIG Wind turbine Gears grid ac dc dc ac Back to back PWM converter 28/53
DFIG operation with Back to Back PWM converter-control Rotor side converter will control the active and reactive power flow in either direction Limited slip range - lower dc bus voltage hence reduces the capacitor bank rating By 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 29/53
Direct driven wind generatorwithout gear box Advantages It eliminates expensive gear box and its losses. Variable speed operation allows the turbine to capture higher energy. Disadvantages Direct driven generators are large in diameter. low speed generator have higher losses. 30/53
Low speed direct driven PM generator Speed range=30-50 rpm Buried magnet Large no. of poles Flux concentration Low cost ferrites magnets Less noise as gear box is eliminated 31/53
Electrical Machines Axial flux Radial flux 32/53
Axial Flux Machine High power density. Less winding overhang, Higher flux density Winding with overhang Winding without overhang 33/53
AFM basic construction AFM with single stator and rotor Rotor Stator Shaft Disadvantage Strong axial magnetic attraction force between stator and rotor 34/53
AFM basic construction A) Single stator & double rotor B) Single rotor & double stator ` Rotor Stator Shaft AFM with multiple stators and multiple rotors 35/53
AFM-stator and rotor construction N-N type rotor with toroidal shaped winding B S N Rotor i F B Stator core F i S N Windings arrangement Requires back iron in stator to provide return path for the working flux Reduction in end winding, hence less copper losses 36/53
AFM-stator and rotor construction (contd.) N-S type rotor with trapezoidal shaped winding B S N Rotor i F F B i N S Stator core Winding arrangement No need of stator back iron, hence better possibilities of cooling Large end winding connections exits, causing extra I 2 R losses 37/53
Flux lines in two rotor configuration N S N S N A) N-N type rotor N S N S N S N S N S B) N-S type rotor N S N S N 38/53
AFM- Torque Peak value of current density at radius r is A m m (r) = 2N πr 1 I a ----- (1) R o r I a B Force on the current element Idr is df x = I a g (dr B ) = A(r)(dS B g ) R i ----- (2) Torque at the element Idr is dt = 2 1 Td = α 4 d πα i i k mn w1 t A(r)B K w1 B m m r D 2 2 o dr (1 λ 2 ) I ----- (3) a ----- (4) 39/53
Contd D out Selection of the outer diameter = 3 π 2 λk w1 60. εp n s B m out Aηcosφ D α out 3 n s (Taking other parameters to be constant) 1 So less speed requirement Much more increase in outer diameter More slots can be made to achieve higher pole pairs (lower speed) on large diameter) Optimization goal K w1, by the proper selection of winding configuration ε = E f /V 1,depend on winding inductance B m by proper selection of magnet shape and span 40/53
Contd Selection of number of turns per phase E f = π 4 2n s N k 1 w1 B m D 2 out (1 λ Where E f is the rms value of the generated emf per phase Where E f is the rms value of the generated emf per phase Optimization goal N 1 = f(e f, k w1,b m,λ), Keeping other quantities to be fixed E f is limited by the rated output/input voltage of machine K w1 and B m are the quantities which can be optimized 2 ) 41/53
Design aspects Ratio of inner to outer diameter = λ It is industrial practice to choose λ = 1/ 3 for getting maximum torque output Torque in PU λ 42/53
Comparison of AFM with RFM High torque to weight ratio R T = ratio of torque densities of AFM to RFPM R T R T = p π 1+ 4p π 2 p 43/53
Torous type axial flux machine low speed wind generator Features Large no.of poles No cogging torque Less resistance High efficiency 44/53
Wind Turbine Generator (16 pole configuration) 1 kw, 375 rpm, 110 V, 50 Hz For direct driven wind generator. 45/53
Doubly salient PM Machines- Three Phase Flux Reversal Machine Magnets and winding are stationary 6/8 pole configuration n r = n s (n pp +1/3) f =(n n r )/60 Equivalent to 16 pole PMSM PM required are 12 Flux pattern of 2 poles 46/53
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 n r )/60 Flux pattern of 2 poles 47/53
Doubly Salient PM Machine-6/14 pole FRM Sinusoidal induced voltage Stator poles = 6, Rotor poles =14 No. of Flux pattern poles = 2 Machine with fictitious electrical gear (Gear ratio = 14) Advantages of PMSM and SRM Suitable for low speed direct driven Wind Power application 48/53
Fictitious Electrical Gear FRM 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 49/53
FRM Gear ratios S.No Machine Configuration No. of Magnets Gear ratio Speed at 50 Hz (rpm) 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 Flux pattern poles 50/53
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 same number of turns and machine size) 51/53
Conclusion Direct driven variable speed wind energy conversion systems are more efficient and are possible using PM machines. For low speed operation axial flux machines are preferred FRM topology is suitable for low speed low power application 52/53
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