Download 3.2 Electrical conversion system

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