A NEW FUZZY LOGIC BASED ApPROACH FOR A VARIABLE SPEED, V ARlABLE PITCH WIND TURBINE
M. Perales, J. perez, F. Barrera, J. L. Mora, E. Galvan, J. M. Carrasco, L.G. Franquelo. D. de la Cruz*, L. Femandez* , A. Zazo* Grupo de Tecnologfa Electr6nica, Departamento de Ingeneirfa Electr6nica.ESI de la Universidad de Sevilla. Avda. Camino de los Descubrimentos SIN, 41092 SEVILLA. Tlf: 4487368, Fax: 4463153. e-mail:
[email protected] (*) Desarrollos E6licos S. A. Avda. de la Buhaira SIN. SEVILLA Tlf: 4937000. e-mail: lmfernandez@d_eolicos.abengoa.com
Abstract In this paper is a system used to evaluate a \'a riable speed and pitch angle wind turbine by fuz.~Y logic techniques is described. A fllzzy logic control has been proposed based on the speed wind estimation in order to get maxim lim power and stability of the system. A II kW prototype system using two inverter connected by a DC-Link capacitor has been lIsed: the first one works as a controlled rectifier implementing vector control and the second one handles the power inyected into the utility g rid. Experimental resliit of a II KW generator \I'ill be sho\\'lI.
1
Introduction
Many hori zon tal axes. grid-connected, med ium to large scale wind turbines are regulated by pitch control and most of wind turbines so far built have practically constant speed . since they use an AC generator. directly connected to the distribution grid. which determines its speed of rotation. In the last years. variable speed control has been added to pitch-angle con trolled design in order to Impro ve the performance of the sys tem ([ I 1-14]). Variahle speed operati on of a wind turhine has
important advantages versus constant speed ones. The reduction of electric power tluctuations by changes in kinetic energy of the rotor, the potential reduction of stress loads on the blades and the mechanical ' transmissions and the possibility to tune the turbine to ; local conditions by adjusting the control parameters are main advantages of variable speed wind turbine. On the other hand, variable speed control is normally used with fix pitch angle and very few works using both controls have been reported. ObJectivf' for variable speed control system are sumarized by the following general goals: to regulate and smooth the power generated, to maximize the energy captu, e, to alleviate the transient loads throughout the wind turbine . unity power factor in the line side with 110 harmonics current injection. to reduce the machine f('tor !lux at light load reducing core losses. Objectlve'; for the pitch--angle control are similar to the variable speed ones but only can be match a maximum in energy capture. which will be minor than variable speeci If pitch angle control is used together with variable \·elocit y. better performances are obtained. For example. 10 permit the starling hlades angle to dilTer from the operati on blade pitch angle. hence allowing eas ier starting and optimum running. power and speed can he limited through rot or pitch regulation.
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power control.
2
Control Strategies 2.1 Induction Generator Control Block.
In order to control the induction generator, two inverter connected by a DC--Link capacitor have been used. The first one works as a controlled rectifier and the second one handles the power injected into the utility grid (figure I). The control of the induction generator can be divided in three parts: induction generator control, power injected into the utility grid control and fuzzy
This control is based on the well known indirect vector method. The rotor speed is added to slip speed to obtain the synchronous speed on the reference frames.
Public Grid
380V.llkW
380V
Generator Control Board
Utility Grid control board
Figure 1. Variable speed Power circuit. can
In that reference frame, the torque and flux control be performed controlling the stator current
components, I q and I d' respectively, in those axes. A
,,
Space Vector technique is used to control the induction generator current. This current control has been implemented in a static reference frame. The control block diagram is represented in
~~~~~~~--~--- -
Figure 3. Public Grid current injection Block Diagram. Control of the Power Injected to the Public Grid.
Figure 2. Indirect Vector Control Method Block Diagram.
The DC-link capacitor voltage must be controlled to maintain a constant reference voltage. This is performed, injecting in the public grid the active power delivered by the induction generator. The control block diagram is represented in Figure 3. A space vector technique is also used to control the current injected into the utility grid. In this case, the current control has been implemented in a stationary reference frame.
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The control action, I q2 is the output of the speed control and it can be obtained by a PI controller: Indirect Vector
I q2 = K p . (Wret - W,) + K I
.
f(W,et - W, )dt
In this control it is very important to use an anti window reset (the integral term is reseted during the transitions to other control zone).
Controller Wr
This action is only taken into account in a region below 1500 rpm. In this region the output is calculated by the fOllowing expression:
2.2 Fuzzy Power Control Block: The power control block implements different Figure 4. Fuzzy and vector Control.
= KI
I ,/3
control policies depending on rotor speed and generates a current torque control action I q ' ~roportional to the
W,
Over 1500 rpm, the flux component, I"
,
will be
changed in a similar form.
machine torque. In this case, several low level regulators are used. Such control loops are:
Controller
13 : Constant power control.
Controller
14 : Maximum speed control.
Actually this controller is a limiter. The action,
1;: Zero power control.
1'/4
~
force to the speed to decrease. I q4 is calculate in this form:
The control action is : I ql = 0 .
Controller 12 : Maximum power control.
Iq4
=-K 2 · W,
This is a more complex controller. We calculate the optimal velocity for maximum power capture. The
Each control loop listed before is responsible for maintaining its set point, and the output of each controller,
commanded velocity, W,et
I ql ' I q2 ' I q3' I q4 will be different for each other.
'
is obtained using a model of '
the wind turbine in order to estimate the wind speed. It is ; To combine all control policies, a fuzzy logic based control, similar to the controller proposed by Sugeno and well known that rotor speed must be proportional to the Takagi [7], has been presented in this paper. The fuzzy wind speed in a maximum of the power captured. The system acts as a supervisor which combines all outputs ratio for both speeds is defined like
Rw
A = --' , where R Vw
is the turbine radius, and
VII' is
w, is the turbine rotor speed
actions,
the wind speed. For the wind turbine used in the
experiments the optimal
value for
_
Aop,
Where
on
the
rotor
speed,
w,.
The
Membership functions for Velocity
A is Aop , = 4.7 .
Finally, the commanded generator speed can be obtained by the following expression:
W ret - P R
depending
membership functions for rotor speed are shown in Figure 5.
w,=0.8
V I\'
p is the ratio of generator speed and turbine
rotor speed and the wind speed , v w ' is obtained from a
Wr=I780
Figure 5. Membership function for rotor Velocity.
static inverse model of the wind turbine which uses the torque and the rotor speed as input.
Rule based inference process and defuzzyfication are listed in Figure 6.
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IfWris SMALL then I , is
I, I~
IfWr is MEDIUM. then I , is If Wr is LARGE. then I , is
I,
I, ~
~ I
I~
If Wr is VERY LARGE. then I , is
I,
q
1/
Figure 6. Inference and defuzzyfication process.
3
Experimental results
Figure 8. Photography of the test rig.
Figure 7 shows power and control systems of the test-rig used to evaluate the proposed control. In that figure, two subsystems can be found: the DC motor and the AC generator control. The DC ~otor is used to emulate the wind turbine and is controlled by a commercial full bridge rectifier implemented by thyristors, with a control system based on a microcomputer.
The DC motor is torque controlled in order to emulate the wind turbine including the pitch control, large inertia term and mechanics oscillations. Figure 9 shows oscillograms of the injected current in the public grid in nominal condition and the generator current. ~:g/ --- ---'--'------
. _ - - - --
- ..
16: '. :49
l1..
1it'lt
If
I
\.
\.
'II'"
IIU
iI'I
I" I
I.l
f,
;
""
~\
1\.
' !~
"..
,
/.
1. '1
l\
A"' \
/ '\
j
\.
fI /
!
I -\
, \
}
! \
I
I
\
Ln."O CEll TOR Ql1: SIGSAL
Chm2 O.J mV ! : D.OO ms
I
L._ Figure 7. Power control system implementation in the test rig.
In Figure 7 is shown the power control system implementation. There are two subsystems: The DC motor subsystem and the AC generator subsystem. The overall system is governed by a personal computer, which includes the control board based on a DSP and the softy/are used to emulate the wind turbine. A photograph of the test rig used in the experiments is shown in.
lIt.:.
00
: -2
"----
48 nV
OC B' _
CH l ,1 'J CH2 SOrV = ildlY 10 -5
Figure 9. Current injected into the public grid (channel I) and the generator current (channel 2) in nominal conditions (1500 rpm 380 V). Channel I IONdiv, channel 2 5Ndiv and 10ms/div.
In order to evaluate the overall system, a wind data file between 6 and 14m1s has been used, which is shown in Figure 10. Using this waveform, we can get easily the transitions between zones.
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smoth torque depending on the wind speed have been implemented by a new fuzzy controller which combines all these factors and calculates the current torque reference. An inverse model of the wind turbine has been used to estimate the wind speed and to obtain the optimal speed reference. Experimental results have been shown confirming the validity of the proposed control method.
,. 16
5
oL-~
o
__
~
__
10
~
15
__
~
20
__
~~~~
25
30
__- L_ _
35
40
~~
45
50
Tiampa (5)
Figure 10. Wind wavefonn used in the experiments. In Figure llis shown the evolution of the generator during the experiments. The upper graph shows the generator speed and below it can be observed the tip speed ratio ( A ). 1500 ,----~--~~~~--~--~--~_=~--~--_,
1000
I 500 r
oL I __- L_ _~. _ _~_ _~_ _~_ _~_ _~_ _~_ _~~ o 10 15 20 25 30 35 40 45 50 Tiempo (5)
°0~--~~1~ 0--~1~5--~2O~~25~~30~~35~~ 40~-4~5~~50 Tiempo (s)
Figure 11. Generator Rotor Speed (upper graph) and tip speed ratio measured (continuous line) tip speed ratio estimated (dashed line). The optimal value A is 4.7 and the wind turbine is force to work with that value during the test. It can be observed a good estimation.
4
ACKNOWLEDGMENT
This work has been supported by the First User Program (FUSE) included in the ESPRIT project, entitled "DSP-Based control for variable speed wind turbine" experiment number 2063, CICYT (T AP-96037I ) and Desarrollos E6licos S. A.
6 Bibliography [I] R. David Richardson and Gerald M. McNerney, "Wind Energy Systems", Proceeding of IEEE, VOL. 81, NO.3, March 1993. [2]. M. Godoy Simoes, Bimal K. Bose, "Fuzzy Logic Based Intelligent control of a Variable Speed Cage Machine Wind Generation System". IEEE, 1995. [3]. Torbjonrn Thiringer and Jan Linders, "Control by Rotor Speed of a Foxed--Pitch Wind Turbine Operating in a Wide ' Speed Range". IEEEJPES Summer Meeting , Seattle, 1992. ,. [4]. A. D. Simmons, L.L. Freris and J. A. M. Bleij s, , "Comparison of Energy Capture and Structural Implementations of Various Policies of Controlling Wind Turbines", Wind (Amsterdam Energy: Technology and Implementation EWEC'91). [5]. Donald S. Zinger, Eduard Muljadi . "Annualized Wind Energy Improvement Using Variable Speeds". IEEE Transactions on Industry Applications, VOL. 33 , NO.6, NovDec 1997, pp. 1444-1447. [6]. F. Barrero, J.L. Mora, M.Perales, A. Marchante, E.Galvan, J.M Carrasco, A. Torralba and L.G. Franquelo. "A Test-Rig to evaluate a Wind Turbine Generation Control System based on DSP". EPE'97. Trondheim, Norway, pp 2.642-2.645. [7J Sugeno. M. "An Introductory Survey of Fuzzy Control ". Information Scien ces, 36(1985), pp. 59-83.
Conclusions
A control system based on a DSP has been used In this paper in order to evaluate some control policies for wind -energy conversion systems. The generator control has been implemented using a vector control method. The different control policies, maximum power capture and
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