Lecture 4 Energy Conversion in Wind Turbine

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2 Tuuli- ja aurinkovoimateknologia ja -liiketoiminta Wind and Solar Energy Technology and Business BL20A1200 Lecture 4 Energy Conversion in Wind Turbine LUT Energia, Olli Pyrhönen

3 Equations for energy conversion Newton s second law for rotation d dt d dt 2 2 T J - angular acceleration [rad/s 2 ] - angular speed [rad/s] - rotation angle [rad] T - torque [Nm] J - moment of inertia [kgm 2 ] Kinetic energy in turbine Turbine mechanical power 1 E J 2 2 P t T U p U pp I p - phase voltage [v] - mains voltage [V] - Phase current [A] cos - elect. power factor Generator electrical power P g 3 U p I p pp p cos 3 U I cos

4 Equations for energy conversion Effect of gearbox, gear ratio Mechanical power is same on the both sides P T 1 k n n n1 n2 1 T1 2 T n - rotational speed [rpm] J - moment of inertia [kgm 2 ] - angular speed [rad/s] K shaft stiffness [Nm/rad] K 1 n=n 1 J 1 K 1 J 2 n=n 2 In analysis parameters are reduced to the same speed K 1 k 2 K 2 J 1 k 2 J 2 n=n 1 J K tot tot J 1 K k 1 2 k J 2 2 K 2

5 WIND TURBINE DRIVE TRAIN Wind turbine drive train includes all the components converting turbine mechanical power to electrical power Main components are gearbox, generator and power converter Traditional wind turbine drive train setup 3-stage gearbox high speed induction generator (double fed induction generator) Power converter for frequency control Sometimes called as Danish concept PM generator technology and high gear box failure ratio in the past has brought alternative solutions to the market Modern turbines use also other drive train alternatives Direct Driven system without gearbox Low speed ratio gearbox Multigenerator solutions

6 WIND TURBINE DRIVE TRAIN BASE LINE Drive train setup with high speed generator

7 WIND TURBINE DRIVE TRAIN BASE LINE GEARBOX Typical gear topology consists of two planetary stages and one helical stage or one planetary and two helical stages Gear ratio about 1:100 or more needed traditionally With one planetary stage about 1:6 gear ratio possible Planetary stages are used in low speed end Helical stages used at high speed end Efficiency approximately 98 % Oil cooling oil to air heat exchanger Manufacturers Moventas, Hansen, Bosch Siemens, GE Transmission

8 WIND TURBINE DRIVE TRAIN BASE LINE GEARBOX Example of compound planetary helical gearbox structure

9 Electric Drive in Wind Turbine Example of WT speed and power [1] Figure shows 1 MW WT aerodynamic power curves at different wind speeds (5 m/s 14 m/s) Additionally two possible generator power curves have been presented If constant generator speed is applied, optimal tip speed ration can be achieved only with a certain wind speed (here between 6 and 7 m/s) With variable speed generator the optimal operational point can be achieved in the whole wind speed range In the past, fixed speed generators were applied, Today, all modern large scale (P > 1MW) wind turbines have some method to vary the generator speed WT power output at different wind speeds and two different contol schemes [1] 9

10 Drive Train Alternatives in WT Traditional Electrical Drive solutions Fixed speed asynchronous generator Two speed asynchronous generator Wounded rotor asynchronous generator with variable rotor resistance Modern Electrical Drive solutions Double fed induction generator (DFIG) Electrically excited synchronous generator (EESM) Permanent magnet synchronous generator (PMSM) 10

11 Drive Train Alternatives in WT Technology trends in WT Drive trains Superconducting generator Permanent Magnet Generator Doubly-fed induction Generator Fixed speed Generator Source: The Switch 11

12 Drive Train Alternatives in WT Fixed speed asynchronous generator Asynchronous generator torque production is based on the slip frequency Slip s is relative speed difference between synchronous speed and rotor mechanical speed n n n s 0 n 0 60 f p grid rpm Example: f grid = 50 Hz, number of pole pairs p = 2, nominal slip is -3% (negative slip in generator mode), then nominal speed n nom is n nom s n rpm Torque curve of asynchronous generator [1]. Speed range between n 0 And n nom 12

13 Drive Train Alternatives in WT Two speed asynchronous generator As shown in previous example, asynchronous generator has a very narrow speed range, ( ) rpm When generator speed can not be controlled, the blades must be designed to work properly on wider tip speed range If the generator has two sets of stator windings representing two different pole pair numbers, a two speed generator is obtained Then generator has two operational modes: low and high wind speed mode (this is referred as Danish concept in [2]) If generator has pole pair numbers p=2 and p=3, (s= , f = 50 Hz), two nominal speeds for low and high wind speeds are achieved n rpm low_ wind n high _ wind rpm 13

14 Drive Train Alternatives in WT Wounded rotor induction generator This slip frequency can be adjusted by changing the rotor resistance If rotor resistance is to be changed, a wounded rotor and slip rings to access rotor winding system are needed If addditional resistance is connected to rotor winding via slip rings, a torque characteristics of the machine can be changed Fig. shows the torque curve with different rotor resistance values Rotor resistance adjustment can be used to extend the torque control of asynchronous generator Torque curves of asynchronous generator with different rotor resistances [1]. This feature has been used e.g. in Vestas Optislip turbine control method 14

15 Drive Train Alternatives in WT Double fed induction generator In double fed induction generator stator is directly grid connected and wounded rotor is connected to the grid through a frequency converter This way the electrical frequency of the rotor can be tuned independently on the mechanical speed Example: 50 Hz DFIG, pole pair number p=2, should rotate 1350 rpm. What electrical frequency should be supplied to the rotor? Stator has grid frequency 50 Hz and p=2 -> stator magnetic field has synchronous rotating speed n 0 = 1500 rpm The electrical frequency in stator and rotor must be the same f r,s = f s =50 Hz in stator coordination system Rotor rotates 1350 rpm, which is 90% of synchronous speed. Then electrical rotor frequency must add 10% to the mechanical speed (=5 Hz), to have the same electrical frequency on both stator and rotor. Thus rotor inverter supplies + 5 Hz frequency to the rotor Slip in the double fed drive is defined as = = = 0.1 In double fed system both stator and rotor windings participate to power generation Typically rotor winding creates maximum 1/3 of the whole generator power If rotor rotates slower then stator magnetic field, rotor consumes power If rotor rotates faster than stator magnetic field, rotor produces power 15

16 Drive Train Alternatives in WT Layout of DFIG Stator directly connected to the grid Rotor connected to the grid through power converter Additionally system includes crowbar to protect converter in grid voltage drop Generator torque controllable independent ly from mechanical speed Double fed induction generator layout, source The Switch 16

17 Drive Train Alternatives in WT Features of DFIG DFIG system design realizes typically 1:2 speed range 2 3 n n sync 4 3 Speed range limited by the rotor power Winding system Slip rings Converter At low rotational speed n < n sync, rotor is consuming power (P 2 > 0) At high rotational speed, n>n sync, rotor is producing power P 2 < 0) The total generator power (P el ) is a sum of rotor and stator power, as shown in figure. The rotor converter can also control reactive power of the system Reactive control capability is limited due to limited current ratings of the rotor converter In grid voltage transients the rotor circuit must be shortcircuited to protect rotor converter (crowbar) DFIG system has a limited capability to handle grid voltage transients Additional equipments might be ineccessary to fulfil new grid codes Stator (P1) and rotor power (P2) of DFIG generator as a function of speed [1] 17

18 Drive Train Alternatives in WT Synchronous generators In synchronous generator rotor rotates in synchronism with the magnetic field Traditionally electrically excited synchronous generators (EESM) have been used in power production with direct grid connection Since the WT requires some torque and speed control, synchronous generator requires always frequency conversion, when applied to WT-application Slow speed EESM s have been adapted also to wind power applications Permanent magnet synchronous machines (PMSM) have been developing rapidly during the last decade offering an advantageous solution also for WTs Compared to EESM, PMSM is more simple due to lacking rotor windings and slip rings New turbine designs have more and more PM drives PMSM technology is available in different speed ranges high speed ( > 500 rpm) Medium speed ( rpm) Direct driven (< 30 rpm) 18

19 Drive Train Alternatives in WT Electrically exited synchronous generator ENERCON applies DD EESM in the range of 300 kw 7500 kw Diode rectifier is used to connect generator stator to the DC-link Torque control is implemented by controlling both excitation current and DC-link voltage Stator current results from the voltage difference between diode bridge output voltage and dc-link voltage Inverter controls both active and reactive power of the wind turbine Output current must be filtered to fulfill grid harmonic requirements IEC defines harmonics limits ENERCON drive train, [ Figure 9. Example of PWM waveforms [2] 19

20 Drive Train Alternatives in WT Layout of PM drive Generator controlled by generator inverter Grid current controlled by grid inverter Good control dynamics on both generator and grid side Well controllable also during the grid transients Double fed induction generator layout, source The Switch 20

21 Drive Train Alternatives in WT Permanent magnet synchronous generator Modern concept for wind power drive train is PM generator and full power converter Both generator and grid currents are fully controlled PM machine can be design with good efficiency both for high and low speeds Direct drive machines have higher losses due to more complicated winding structure, but on the other hand gearbox losses are elimiated Full power conversion allows speed range from zero to overspeed range PM-concepts enables thus wider design freedom to aerodymics of turbine Examples of low, medium and high speed PM wind power generators Source: The Switch 21

22 Drive Train Alternatives in WT Comparison of typical efficiencies Drive train efficiency Double-fed induction DFIG generator Permanent magnet high PMHS speed Permanent magnet PMMS medium speed Permanent magnet direct PMDD drive Wind speed (% of rated) Comparison of different generator type efficiencies, source The Switch 22

23 DRIVE TRAIN EXAMPLES BASE LINE EXAMPLE I, VESTAS V80 DRIVE TRAIN Rated power Wind class Rotor speed Generator Gearbox Rotor diameter Rated wind speed 2000 kw IEC1A rpm 4 pole asynchonous 3-stage planetary/helical 80 m 16 m/s Tip speed ratio?? (rated point) Gear ratio approximation - Generator max. speed rpm - Rotor maximum speed 19.1 rpm : V80 k

24 DRIVE TRAIN EXAMPLES BASE LINE Alstom ECO 100 Platform Double fed generator Stator voltage 1000 V Three stage gearbox Tip speed ratio at nominal point

25 DRIVE TRAIN EXAMPLES BASE LINE EXAMPLE III, SIEMENS SWT

26 DRIVE TRAIN EXAMPLES DIRECT DRIVEN Due to some failure cases related to gearboxes interest towards direct driven systems has increased Specially in offshore technology gearless systems are preferred In direct driven technology low speed synchronous generator is used Induction generator design not feasible for low speed due to complexity and heavy losses Electrically excited direct driven generator solution by Enercon has been on the market more than a decade Permanent magnet direct driven generators were introduced during the last decade Scanwind introduced 3.7 MW DDPM turbine 2005 Goldwind has used PMDD outor rotor generator almost 10-years Siemens has introduced new PMDD 3.0 MW product recently

27 DRIVE TRAIN EXAMPLES DIRECT DRIVEN: CASE ENERCON Enercon has been a pioneer in direct driven technology Technology basis is electrically excited direct driven synchronous generator Relatively expensive machine structure due to wounded rotor Very good controllability-wide speed control area with high efficiency Own generator technology, not outsourced Wide range of turbines from 330 to 7500 kw

28 DRIVE TRAIN EXAMPLES DIRECT DRIVEN: CASE GE ENERGY Scanwind / GE Energy has a direct driven PM concept for offshre Generator technology has been developed by The Switch in co-operation with LUT Rotor and generator as counterweights Three parallel power threads

29 DRIVE TRAIN EXAMPLES DIRECT DRIVEN: CASE SIEMENS Siemens has introduced new DDPM product SWT Outer rotor PMDD generator Nacell weight has been reduced

30 DRIVE TRAIN EXAMPLES DIRECT DRIVEN: CASE GOLDWIND One of the main products is GW 1.5 MW PMDD Thousands of installed turbines in China Biggest PMDD market share in the world (OP s estimate) Important customer to The Switch

31 DRIVE TRAIN EXAMPLES MEDIUM SPEED DRIVE TRAIN Geared wind power drive train is lighter than direct driven When higher nacelle elevation is a target, nacelle weight should be reduced By reducing gear stages weight can be reduced and reliability increased Medium speed drive train system has been developed as a compromise Multibrid is a patented technology, where one stage gearbox has been integrated with generator Benefits from DD and geared system are combined WinWind uses multibrid technology in 1 MW and 3 MW turbines Source: WWD-3 datasheet

32 DRIVE TRAIN EXAMPLES MEDIUM SPEED DRIVE TRAIN: CASE WinWind Note: Electric power is typically NOT Electric power to the grid!! Source: WWD-3 datasheet

33 DRIVE TRAIN EXAMPLES SPECIAL TOPOLOGIES: CASE CLIPPER Clipper Liberty uses multiple generators

34 TURBINE CONTROL BASICS CONTROL MODES At low wind speeds control goal is constant tip speed ratio Pitch angle is not controlled Torque is adjusted according to generator speed Close to nominal wind speed control goal is constant rotational speed Torque is adjusted by a speed controller Above nominal wind speed control goal is constant power and speed Generator torque and speed are kept constant Turbine torque is controlled by adjusting the pitch angle

35 TURBINE CONTROL BASICS CONTROL MODES A Constant tip speed ratio, torque adjusted by generator speed B Constant speed torque adjusted by speed controller C Constant power turbine power adjusted by pitch control P [kw] T [knm] A C B P [kw] T [knm] n [rpm*10] 500 A B C U [m/s] Rotational speed vs. torque and power* Wind speed vs. torque and power *)Calculated from Enercon E82 power curve with nominal speed assumption 17 rpm (uncertain)

36 TURBINE CONTROL BASICS A.D. Wright and L.J. Fingersh, Advanced Control Design for Wind Turbines Part I: Control Design, Implementation, and Initial Tests, Technical Report, NREL/TP , March 2008

37 TURBINE CONTROL BASICS PITCH CONTROL Above nominal wind speed turbine power is kept constant by reducing the power coefficient In pitch controlled turbines this is done by turning the blades In commercial turbines pitch control uses same reference for all the blades The rate of change is typically less than 5 deg/sec in large turbines Both hydraulic and electrical pitch actuators are used Lucy Y. Pao, Kathryn E. Johnson, A tutorial of dynamics and control of wind turbines and wind farms, American control confrerence 2009

38 TURBINE CONTROL BASICS YAW CONTROL Turbine should be directed to wind with good accuracy Yaw control is using wind direction measurement as feedback Yaw rates are typically less than 1 deg/s to avoid dangerous gyroscopic forces Simple PI-controller with a very slow bandwidht can be applied Additional logic is needed to avoid cable twisting in tower (max angle 360 deg) Measured wind direction at Puumala, Finland. Masurements by LUT Energy & FMI Note the wide fluctuation of wind direction at low hights (measurement points 90, 60 and 30 m)

39 WIND TURBINE STANDARDS Source: Antikainen et al., Tuulivoimalan aerodynamiikka, kuormitukset ja standardointi, Tuulivoima tutuksi 2010

40 WIND TURBINE STANDARDS Source: Antikainen et al., Tuulivoimalan aerodynamiikka, kuormitukset ja standardointi, Tuulivoima tutuksi 2010

41 LITERATURE Interesting Web pages Turbine manufacturer home pages Vestas, Siemens, GE Energy, Gamesa, GoldWind, Sinovel, WinWind Component manufacturers The Switch, Moventas, ABB, Hansen, Bosch

42 LITERATURE References [1] Martin O.L. Hansen, Aerodynamics of Wind Turbines, 2nd edition, 2008 [2] Manfred Stiebler, Wind Energy Systems for Electric Power Generation, 2008 [3] National Renewable Energy Laboratory, WindPACT Drive Train Alternative Design Study Report, 2005 USA [4] Connection Code for Connection of Wind Power Plants to Finnish Power System, Fingrid