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Research on Maximum Power Point Tracking Control System of Variable Speed Wind Turbine
Fengting Li1,a, Enrang Zheng1,b
1Laboratory of Wind Power Generation College of Electric and Communication
Engineering, Shaanxi University of Science and Technology, Xi'an ,China
[email protected],[email protected]
Key words: Variable speed constant frequency, Variable speed wind turbine, Wind power captured, Maximum power point tracking
Abstract: This paper analyzes the operating characteristics of wind turbine and introduces the
principle of maximum power point tracking control system of variable speed wind turbine. A
improved maximum power tracking control strategy is proposed for large inertia wind power
systems in order to achieve maximum wind power capture and increased utilization of wind energy
when wind turbines is below the rated wind speed. A variable speed wind power generation system
is modeled and simulated in the Simulink environment of the Matlab .The simulation results proves
the correctness and feasibility of the tracking control strategy suggested in this paper.
Introduction
At present, to reduce the production cost and improve the economic benefit of wind power,
comparing with traditional constant speed constant frequency wind power generation technology,
variable constant frequency technology which has developed in the 20th century can achieve
maximum wind power of capture, improve the output power, and reduce the mechanical stress and
torque[1]. The advantage of variable speed constant frequency wind turbine generator is can
maximize the wind energy capture, thus generating capacity is the more than constant speed
constant frequency generator with equivalent capacity. It has more wide speed range in order to
adapt to requirement of wind machine speed change. Adopting proper control strategy can flexibly
adjust reactive or active power of the system, and can restrain harmonics, reduce loss, improve
power.
The key problem of variable speed constant frequency wind power technology is in the low wind
speed how to effectively control the rotational speed according to the change of wind speed to make
wind turbine captures the biggest wind power. This kind of control mode of variable speed running
wind turbine generators rotational speed relate to the overall efficiency of the wind power converter
system[2,3]. Thus maximum wind power tracking control method is a key factor to enhance the
efficiency of wind power generation system.
Operation Characteristic of the Wind Machine
Operation characteristic of the wind machine mainly includes the following three parts:
(1) Tip speed ratio and wind power utilization coefficient
According to the Bates theory, the power which wind turbines capture from the wind is [4]:
2 31( , )
2pP C R vπρ λ β= (1)
Where ρ is air density, β is propeller pitch angle, ( , )pC λ β is wind energy utilization
coefficients, R is rotor blade diameter, v is wind speed, λ is tip speed ratio. That is R
V
ωλ = , ω is
angular frequency ( /rad s ).
Applied Mechanics and Materials Vol. 65 (2011) pp 389-393Online available since 2011/Jun/22 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.65.389
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.118.88.48, Penn State University, University Park, United States of America-30/09/13,13:24:19)
The relationship between wind energy utilization coefficient and tip speed ratio is shown in
figure 1.
Fig 1 Relation curves of wind energy utilization
coefficient and tip speed ratio
Fig 2 Wind turbines with the maximum
power tracing process curve
optλ λ= , max( , )p pC Cλ β = is the maximum of the wind energy utilization coefficients. And,
when R
V
ωλ = , correspondingly adjusting the rotor speed of wind wheel, λ can be maintained in
optλ λ= , now wind energy utilization coefficient is maximum maxpC . Wind turbines have captured
the largest wind power, namely running in the maximum power point.
(2) Power characteristic curve
By formula (1)
2 3 31( )
2 30m p
RP R C n
πρπ
λ= (2)
For a given wind speed, the output power of wind machine change along with the changing of
rotational speed. The rotational speed has an optimal value. In this value, a wind turbine output max
machinery power. wind speed is the relationship between optimal speckling ratio relations; In under
different wind speed. To connect this Maximum power point can get a maximum output mechanical
power curve. Any point on the curve the relation between rotational speed with wind speed is
optimal tip speed ratio Therefore in the different wind speed, controlling wind turbines in order to
make it achieve optimal rotational speed can realize most power tracing control.
On the basis of formula (2), power adjusting of wind turbine entirely depends on the
aerodynamic characteristics of blades and speed regulation device. The wind energy and wind
turbine speed has been restricted. Further considering the fluctuation of wind speed and direction,
the output power of wind turbine theoretically is [5]:
21
2
0 ,
p in e
m e e out
in out
R C V V V
P P V V V
V V V V
ρπ ≤ <
= ≤ < < ≥
(3)
Where, inV , eV , outV respectively denote cut-in wind speed of wind turbines, rated wind speed and
cut-out wind speed. eP is denote rated power.
(3) Torque - speed characteristics
Due to the system itself restriction of mechanical properties and electric property, torque, power and
rotation speed impossibly get to infinity. After reaching limits, operating protection control must be
carried out to make the system safety operation. Wind turbines have six operation stages [6]: the
setup phase, shifting operation stage, constant speed operation stage, constant power operation
stage, constant torque phase. Wind machine system would close down when it achieve cut-out wind
speed.
ω ω3 ω2 ω1
P1
P2
P3
v3
v2 v1
P
A
E
C
D
F
B
B 10 15 λ 0
0.1
0.2
0.3
0.4
0.5 Cpmax
λopt
Cp
390 Mechatronic Systems and Automation Systems
Principle of Variable Speed Wind Generator Maximum Power Tracing
Figure 2 shows the tracking process of variable speed wind turbines vs. maximum power point
while wind speed changes. While wind speed at 1v , wind turbine run on point A. To track the
maximum power point B, rotation speed ω should be increased.
When 1ω ω= , wind machine running in B, the change process of A→B namely is the tracking
process for maximum power point B while wind turbine at 1v . While wind speed increase from 1v to
3v , the running state of wind turbine would jump from point B to C. For tracking the maximum
power of the wind turbine at this wind speed, ω should be increased. When 3ω ω= , wind turbine
run on D. The change process of C→D namely is the tracking process for maximum power D while
variable speed wind turbine at wind speed 3v . Similarly, while wind speed decrease from 3v to 2v ,
the running state of wind turbine jump from point D to E. Point F is maximum power while wind
speed at 2v . Point E locates at right of F. So rotation speed ω should be decreased until 2ω ω= .
Now, speed turbine run on max power point F. The change process of E→F is tracking process of
max power point while wind speed at 2v . Above-mentioned content is the tracking process of max
power point of variable speed wind turbine while wind speed change.
Control Method and Realization
For large inertia of large wind machine system includes:
* *m load f
dp p T J
dt
ωω ω− − = (4)
Where mp is wind machine mechanical power, loadp is the system output power, fT is friction
torque, ω is wind turbines, J is wind turbine speed of inertia.
By formula (4), we can conclude that when this wind-driven inertia J is bigger, there are likely
to cause the system output power greatly increases or decreases, even speed oscillation is minor.
Because in revolving speed variations, large inertia wind system will release or storage large
amounts of energy, leading to the maximum power point tracking control algorithm can effectively
control wind turbines. This also is climbing search algorithm cannot successfully applied in large
inertia system. However by t formula (4):
* *m load f
dp J p T
dt
ωω ω= + + (5)
Where, * load
dJ p
dt
ωω + is the mechanical power at wind turbines, *fT ω relatively small can be
ignored. Due to this wind-driven mechanical power cannot direct measurement, so will wind
turbines output power loadp and system energy storage variation *d
Jdt
ωω as a whole into
consideration, * load
dJ p
dt
ωω + as wind machine mechanical power approximation, used in the wind
turbines with the maximum power point tracking control. This can greatly reduce wind power
system in search of maximum power point process inertial negative effects; make big inertia wind
turbines for maximum power point has good tracking control effect.
Based on the above analysis, figure 3 is the design of maximum power point tracking control of
the control mode principle diagram. In the figure, a wind turbine speed ω after differentiator get the
rate of change d
dt
ω, and rotate speed ω ,multiply rotate speed ω and rotational inertia J gets wind
Applied Mechanics and Materials Vol. 65 391
machine system energy storage variation *d
Jdt
ωω . Then addition wind turbine output power loadp
get approximation * load
dJ p
dt
ωω + of mechanical power mp (because the relatively small, so
neglected).
*load
dJ p
dt
ωω +
*d
Jdt
ωω
d
dt
ω
loadp
Fig 3 Maximum power point tracking control system diagram of wind generators
Approximate value * load
dJ p
dt
ωω + of wind turbine mechanical power mp and rotate speed ω
through time-lapse comparison get their variation p∆ , ω∆ ; Logic is according to the p∆ , ω∆
positive and negative relationship judgment the actual wind machine with maximum power point
nod the position relations.
System Modeling and Simulation
Fig 4 System simulation model
Maximum power point tracking control system is constructed by applying Simulink of the
simulation toolbox in Matlab. Simulation model chart is as figure 4:
Simulation parameters are as follows: Nominal mechanical output power (W):30361;Base power
of electrical generator(VA):27567.6;
The simulation results as shown in figure 5, figure 6, respectively is under low speed the wind
turbines output power and generator output power.
392 Mechatronic Systems and Automation Systems
Simulation results show that when the low speed the output power of wind turbines is relatively
small. And the wind power system’s response time is short. Wind turbines, motor’s output have
certain fluctuation, but they are in a small range. This shows that double of the variation of the
controller system has high response speed, thus reducing the transformation time between the
operation state points to make wind machine allows fast to track maximum power point.
0 0.5 1 1.5 20
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
Pwind(MW)
t(s)
Fig 5 Wind turbine output power curve
0 0.5 1 1.5 2-0.01
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07P(MW)
t(s)
Fig 6 Generator output power curve
Conclusion
An advanced maximum power point tracking control strategy for variable speed wind power
systems have excellent control effect to control wind turbines. Not only with wind power system
used the converter topology related, also concerned with the own control modes. This paper
presents a method of variable speed windmills for maximum power tracking control system.
Through modeling in the Matlab/Simulink simulation, verifies the feasibility and correctness of this
method.
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[3] ZHAO Pei-hong, LI Jian-lin, E Chun-liang. Research of maximum wind power capture control
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Applied Mechanics and Materials Vol. 65 393
Mechatronic Systems and Automation Systems 10.4028/www.scientific.net/AMM.65 Research on Maximum Power Point Tracking Control System of Variable Speed Wind Turbine 10.4028/www.scientific.net/AMM.65.389