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Wind Energy 4P720 Assignment 3: Drive train and electrical conversion system Editors: Thieo Thevarayan Gillis Hommen 12 December 2006 0472316 0551901 Contents 3.1 DOWEC/V120 drive train and generator layout ..................................... 3 3.a Turbine Configuration ...................................................................... 3 3.b Mechanical components .................................................................. 4 3.c Multibrid design ............................................................................... 5 3.2 Electrical conversion system.................................................................. 5 3.d/e Nominal speeds and frequencies ................................................. 7 3.f Voltages without converter ............................................................... 7 3.g Rotor power with DFIG .................................................................... 9 3.h Grid current ................................................................................... 10 3.i Frequencies and Power .................................................................. 11 3.1 DOWEC/V120 drive train and generator layout 3.a Turbine Configuration The DOWEC 6 MW wind turbine has the same configuration as the Vestas V120. The layout of the V120 is given in figure 3.1. Figure 3.1 Nacelle layout taken from Vestas V120 pdf file. The V120 illustrated in figure 3.1 has a similar concept to the concept which is very common for onshore turbines of the MW+ class: 3-bladed, variable speed, pitch regulated. The hub height is 100 m and the rotor diameter is 120 m, with flap wise pre-bending, resulting in an upwind tip position of 2.0 m out of plane in unloaded situation. The V120 uses a two stage planetary gearset combined with a final parallel stage. The gearbox has a transmission ratio of 138. The variable speed operation is obtained with a doubly fed generator with a 30% power inverter. The nominal power rating is 6 MW. 3.b Mechanical components The mechanical components that transmit torque are the rotor, the main shaft, couplings, brakes and the generator. The function of these components will be briefly described. The rotor consists of three blades and has a diameter of 120 metres. The rotors are designed for the purpose of extracting significant power from the wind and converting it to rotary motion. The rotor is connected to the main shaft (low-speed or rotor shaft), which is the principal rotating element, thereby providing for the transfer of torque from the rotor to the rest of the drive train. The main shaft supports the weight of the rotor. The main shaft is directly connected (without the use of a coupling) to the gearbox. The gearbox consists of two planetary stages and parallel stage. The two planetary gearsets are needed to reduce the enormous torques produced by the 120 diameter rotor. A composite disc coupling is used to connect the gearbox between the gearbox output shaft and the generator. The primary function of this coupling is to transmit torque between the gearbox output shaft and the generator. The function of the generator is to convert the mechanical power from the rotor into electrical power. The DOWEC uses a mechanical disc brake to prevent the rotor from turning when the turbine is not operating. When the power of a wind turbine is doubled the needed area of the rotor also doubles. This means that the diameter of the rotor has to increase with a factor of 2 . When a constant tip speed ratio is maintained, the rotational speed of the rotor will be a factor 2 lower. The rotor torque is given by: T P (3.1) It shows that if the power is doubled and the rotational speed decreases with a factor 2 , the transmitted torque must be a factor 2 2 greater. The volume of the generator is proportional to its rated torque. Therefore the volume of the generator also has to increase with a factor 2 2 . The volume is equal to frontal area times generator length: 1 V lD 2 , 4 (3.2) with l the length and D the diameter of the generator. A sufficiently high volume can be established by giving the generator a larger diameter rather than a larger length. 3.c Multibrid design The task is to modify the nacelle and drive train configuration of the DOWEC 6 MW wind turbine into a Multibrid system. Only the power is adapted to the Multibrid system and all the other parameters are re-dimensioned. The data for the Multibrid M5000 is collected from the Multibrid website and are presented in the first column of table A In the second column, the data of the DOWEC 6 MW are presented, which is partly given in the matlab file. Some of the data for the DOWEC are not present and therefore the data of the VESTAS V120 4,5 MW was used. The 180.000 kg for the nacelle of the DOWEC is obtained by subtracting 220.000 kg (the weight of the tower of the VESTAS V120 4,5 MW) from the weight given in the matlabfile (weight of the nacelle and the tower of the DOWEC). M5000 DOWEC Modified General Radius Rated power Rated speed [m] [kW] [rpm] 58,00 5.000 14,80 60,00 6.000 12,40 54,77 5.000 13,58 Gearbox Rated power Rated torque [kW] [kNm] 5.540 3.575 6.649 5.120 5.541 3.895 Generator Rated power [kW] 5.315 6.383 5.319 Masses Blade Hub Nacelle [kg] [kg] [kg] 16.500 6.100 199.300 20.000 8.500 180.000 15.780 6.466 136.930 Table A: Re-dimensioning of the DOWEC into a multibrid. In the third column the modified DOWEC is presented. The re-dimensioning is done by using scaling rules. The rated power of the DOWEC is multiplied with 5/6 to get the same power as the M5000 and the other parameters can be compared. The following scalings are used: Radius: R P 1 2 1 Rotational velocity: P Torque: Blade mass: T P 2 M P 1.3 3 2 Hub mass: Nacelle mass: M P 3 M P 3 2 2 In table A can be seen that by re-dimensioning the masses of the DOWEC to a 5 MW turbine, the masses of the blade and the nacelle will become lower than the masses of the Multibrid system. Only the mass of the hub will be somewhat higher than in the case of the Multibrid system. The mass of the blade decreases, because also the radius of the blade decreases. And a smaller radius is found for the 5 MW DOWEC than for the Multibrid system is given. The rotational speed increases with decreasing power and is still smaller than for the Multibrid system. The torque in the gearbox will be somewhat higher than for the Multibrid system. 3.2 Electrical conversion system 3.d/e Nominal speeds and frequencies The known parameters are as follows Gearbox ratio 138 Nominal wind speed 12m/s, tip speed ratio 7.4 From this the rotor and generator rotational velocities are calculated Rotor velocity 14.13rpm Generator velocity 1950rpm Considering 130% synchronous operation (slip factor s=-0.3) at rated speed and with 2 pole pairs the resulting grid frequency will be gr sh p rel 1950 1950 2 0.3 2 50 Hz 60 60 1.3 (3.1) Which equals to the grid frequency. In this case ωrel is -15Hz. This is derived using the slip factor equation: Prel rel Ps s Ps . gr (3.2) Where Prel is the power passing through the convertor, Ps the power through the stator and Psh the power from the rotor. Using a slip factor of -0.3 returns a converter power Prel = 0.23Psh, which is less than 30% of rated power. The reason for this is that rated power is actually the power at 130% synchronous operation. The actual percentage of power through the converter is at this point 100(30/130) % . 3.f Voltages without converter Assuming constant tip speed ratio the relation between wind speed and rotor speed is linear from the cut-in speed and constant from the rated speed up. Assuming a cut-in windspeed of 3.5m/s, rated windspeed of 12m/s and rotor speeds between 4.1rpm and 14.1rpm this relation can be plotted. This is done in figure 3.2 Rotor speed Vs. Wind speed 15 rotor speed [rpm] 10 5 0 0 5 10 15 Wind speed [m/s] 20 25 Figure 3.2 Rotor speed for operational wind speed range. Generator speed Vs. Windspeed 250 Generator-rotor speed [rad/s] 200 150 100 50 0 0 5 10 15 Wind speed [m/s] 20 25 Figure 3.3 Generator speed for operational wind speed range. The resulting induced voltage can now be derived using (3.1) and taking into account generator speed is 138 times rotor velocity, as seen in figure 3.3. Ef = p g M 2 If The results are shown in figure 3.4. (3.3) Induced voltage without DFIG 1200 Induced stator voltage [V] 1000 800 600 400 200 0 0 5 10 15 Wind speed [m/s 20 25 Figure 3.4 Stator induced voltage for operational wind speed range. 3.g Rotor power with DFIG Rotor power is calculated using the slip equation and the power balance. Considering that rel gr sh p rel P , Ps PSH Prel and s s it can be concluded that for Prel the following relation holds Prel Prel P gr 1 p sh gr (3.4) Where Prel is the rotor power and P is turbine power. The result is plotted in figure 3.4. It should be noted that negative power indicates the direction of power flow is inverted. 5 4 Convertor power Vs Windspeed x 10 2 Converter Power [W] 0 -2 -4 -6 -8 -10 -12 -14 0 5 10 15 Wind Speed [m/s[ 20 25 Figure 3.5 Converter power directed to rotor windings for operational wind speeds. 3.h Grid current Using the power curve from previous assignments grid current can be calculated according to P 3VLN I L 3VLL I L . (3.5) Where VLL is given to be 3000V. Figure 3.6 shows the result. Grid current Vs. Wind speed 1200 1000 current [A] 800 600 400 200 0 0 5 10 15 Wind speed [m/s] Figure 3.6 Current induced to grid. 20 25 3.i Frequencies and Power The magnitude and direction of power flows in the doubly fed induction generator vary greatly with wind speed. The basic constraints are generator speed (frequency), determined by wind speed (figure 3.3) and grid frequency which is 50Hz at all times. The electric power converter is needed to bridge the difference in frequency of the grid and the generator. This is realized by imposing a controlled voltage and current on the rotor to effectively synchronize its induction frequency with the grid. The rotor voltage and current represent a certain power flow. The direction and magnitude of this flow depends on the sign of the slip factor. The slip factor characterizes the frequency difference between grid and rotor according (3.2). Figure 3.7 illustrates the power flows as a function of s. Psh=(1-s)Ps Gen. Ps Pgr s=rel/s AC-DC-AC Converter Prel=sPs Pc Figure 3.7 Power flow diagram of the doubly fed induction generator for s>0 The converter frequency ωrel is given by (3.1) and is plotted in figure 3.8. Converter frequency Vs. Wind speed 200 Converter frequency [rad/sec] 150 100 50 0 -50 -100 0 5 10 15 Wind speed [m/s] Figure 3.8 Converter power frequency 20 25 At low wind speeds this frequency is positive, meaning that the generator frequency has to be increased to reach grid frequency. In this case power flows from the grid to the rotor, as in figure 3.7. At synchronous speed converter frequency is zero and so is power. In this case the wind turbine is spinning at 10.87rpm and the generator shaft is inducing a 50hz voltage on the stator. At higher wind speeds generator shaft velocity increases and has to be effectively decreased by the power converter. This can be seen as a negative converter frequency, as in figure 3.8 for wind speeds above 10.87rpm. Power flow is now also negative, indicating that power is extracted from the rotor instead of fed to it. In this case wind power flows partially through the stator and partially through the rotor to reach the grid. Electric power flow through the rotor, and thus the power converter, is given in figure 3.5, where positive power indicates power is fed from the converter to the rotor and negative power indicates power is extracted from the rotor. With both converter and generator shaft power known, stator power is given by Ps PSH Prel (3.5) A graph of stator power Ps is given in figure 3.9. At rated windspeed and above the stator only extracts about 4.6MW, the rotor connected to the power converter extracts the other 1.4MW. This is also the rated power that the converter must have. Generator stator power Vs Windspeed 5 4.5 4 Power [MW] 3.5 3 2.5 2 1.5 1 0.5 0 0 5 10 15 Wind speed [m/s] 20 25 Figure 3.9 Generator stator power for operational windspeeds