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Research on Maximum Power Point Tracking Control System of Variable Speed Wind Turbine Fengting Li 1,a , Enrang Zheng 1,b 1 Laboratory of Wind Power Generation College of Electric and Communication Engineering, Shaanxi University of Science and Technology, Xi'an ,China a [email protected], b [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 3 1 (, ) 2 p P C Rv πρ λβ = (1) Where ρ is air density, β is propeller pitch angle, (, ) p C λβ is wind energy utilization coefficients, R is rotor blade diameter, v is wind speed, λ is tip speed ratio. That is R V ω λ = , ω is angular frequency ( / rad s ). Applied Mechanics and Materials Vol. 65 (2011) pp 389-393 Online available since 2011/Jun/22 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi: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)

Research on Maximum Power Point Tracking Control System of Variable Speed Wind Turbine

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Page 1: Research on Maximum Power Point Tracking Control System of Variable Speed Wind Turbine

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)

Page 2: Research on Maximum Power Point Tracking Control System of Variable Speed Wind Turbine

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

Page 3: Research on Maximum Power Point Tracking Control System of Variable Speed Wind Turbine

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

Page 4: Research on Maximum Power Point Tracking Control System of Variable Speed Wind Turbine

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

Page 5: Research on Maximum Power Point Tracking Control System of Variable Speed Wind Turbine

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.

References

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application,2007,34(2):1-4,15

[2] XUE Juan-ni, MA Peng. Analysis of the most high-power capture for the wind power

system[J].Equipment manufacturing technology,2008,(2):42-44

[3] ZHAO Pei-hong, LI Jian-lin, E Chun-liang. Research of maximum wind power capture control

for variable speed wind turbine[J].Electric Drive, 2009,39(6):3-6

[4] Katsumi Nishida, Tarek Ahmed, Mutsuo Nakaoka. A Cost Effective High Efficiency Power

Conditioner with Simple MPPT Control Algorithm for Wind Power Grid Integration [J]. IEEE,

PEDS2009: 476-481

[5] Joanne Hui, Alireza Bakhshai. A New Adaptive Control Algorithm for Maximum Power Point

Tracking for Wind Energy Conversion Systems [J]. IEEE, 2008: 4003-4007.

[6] Ahmad Nadhir, Takashi Hiyama. Maximum Power Point Tracking Based Optimal Control

Wind Energy Conversion System [J]. IEEE 2010 Second International Conference on

Advances in Computing, Control, and Telecommunication Technologies:41-44.

Applied Mechanics and Materials Vol. 65 393

Page 6: Research on Maximum Power Point Tracking Control System of Variable Speed Wind Turbine

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