Pitch Control of CFVWS Turbine Using PID Controlled Internal Pitch Loop

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International Journal of Electrical, Electronics and Computer Systems (IJEECS)

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Pitch Control of CFVWS Turbine Using PID Controlled Internal Pitch Loop

Umesh Kumar Soni

Department of Electrical Engineering, MNNIT, Allahabad, U.P., India E-mail: [email protected]

Abstract- Constant-Frequency Variable Wind Speed (CFVWS) generating system using variable-pitch turbine is mostly used in wind power, hydro power, and aerospace.

In this work the maximum wind energy capture is realized in a PMSG and the active and reactive power outputs regulated through proportional integral and derivative (PID) control of pitch angle of rotor blades with a internal pitch loop has been investigated for different reference value of rpm and wind speeds ranging from 6-12 m/s. It is recognized that the most important advantage of the variable pitch angle turbine over the conventional constant pitch angle turbines is the improved dynamic characteristics, resulting in the reduction of the drive train mechanical stresses and output power fluctuations. It accomplishes challenge to maximize available energy in the low rated wind speed areas .The internal pitch loop improves the fastness of the response to changes in wind speed. The system parameters set at the operating point gives the desired performance at nearly all ranges of the reference rpm and pitch angles. In all cases we have obtained the rpm variation within the allowable ranges of frequency and rpm. The model has been simulated using SIMULINK/MATLAB and satisfies the theoretical analysis.

Index terms: Proportional Derivative, pitch angle control, wind generator, constant frequency variable wind speed.

I. INTRODUCTION

„Wind power” describe the process by which the wind is used to generate mechanical power for getting electricity through electric generators. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding gain or pumping water) or a generator can convert this mechanical power into electricity Wind has served mankind as a source of power. The constant speed constant frequency (CSCF) and variable speed constant frequency (VSCF) types of wind turbines are widely used in wind power, [1], hydro power, aerospace Wind electrical power system are recently getting lot of attention, because they are cost competitive, environmentally clean and safe renewable power sources, as compared to fossil fuel and nuclear power generation. High speed and high efficiency of turbines are the most desirable requirement for successful electricity generation [2].At the end of 2009,

more than 82 countries have utilized the wind turbine for the electricity purpose with a capacity of 159.21 GW [3]. It is a challenge to extract power from varying wind, especially in the low rated wind speed areas. To maximize available energy pitch control system proves to be the most useful technique [4-11]. The pitch angle control strategy is useful to optimize the output power of the wind turbine for low rated wind speed, to reduce torque fluctuations and output power variation for high variation in wind speed [6].The method of the pitch angle control for variable –speed constant –frequency wind turbine [7] There are some strategies of pitch control have been proposed [5-11].Conventional pitch control strategies such as proportional derivative (PD) controller need the knowledge of system dynamics [5].

Advanced control strategies such as a fuzzy logic controller can be used when the system is not well known or contains non-linearities [8, 9, 10,]. In this paper, a pitch angle control of wind turbine has been modeled and the result has been investigated for various wind speeds and various desired frequencies.

II. WIND TURBINE SYSTEM

In a general wind turbine system, the transformation to mechanical torque Tr (see Figure 1) is done by aero dynamical forces acting on the wind turbine rotor blades connected to the actuator disc. Usually there is a gearbox also called the drive train coupling the slowly rotating turbine shaft and the more rapidly rotating generator shaft.The wind turbine shaft through gears then transports the power to the electrical generator connected to the electrical grid. Electromagnetic torque condition is feedback to control the drive train speed thereby control of pitch angle of rotor blades.the figure.2 represents the energy storage and utilization mechanism where the AC to DC converts the AC into DC form PMSG and then the power is fed for storage battery units while at the same time the power needs are also fulfilled by DC-AC converter(inverters ) synchronized at the system frequency and voltage level

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Figure 1 : Power flow and feedback for a Wind Turbine

Figure 2: Schematic of Power conversion from wind generator to Grid.

III. CONSTRUCTION OF WIND TURBINE SYSTEM

A wind turbine is made up of the following components:

shown in Figure 3.

1. Tower 2. Foundation 3. Nacelle 4. Rotor blade 5. Hub

6. Transformer (this is not a part of the Wind Turbine).

Figure3. The structure of tubular tower wind turbine

IV. THEORETICAL ANALYSIS OF WIND TURBINE

Where

Pm = Mechanical output power of the turbine Cp = Performance coefficient of the turbine ρ = Air density (Kg/m³)

A = Turbine swept area (m²) v³ = Wind speed (m/s)

β = Blade pitch angle (degree) λ = Tip speed Ratio of the rotor blade λ= ^R_v^ω ………. ……….Equation (2) ω = turbine rotor speed and

R = the radius of the wind turbine blade.

The power coefficient is a function of pitch angle and tip speed ratio „λ‟. The following model has been used to approximate the relation of Cp with Tip Speed Ratio and pitch angle.

Fig. 4 the variation of performance coefficient of the turbine Cp with respect to different tip speed ratios.

Figure 4 shows the graph where performance coefficient of the turbine Cp for wind turbine varies with varying tip speed ratio „λ‟ with different fixed values of pitch angle „β‟. C1 =0 .5176, C2 = 116, C3 = 0.4, C4=5, C5 = 21, and C6 = 0.0068. The maximum value of Cp = 0.48 is achieved for β = 0 degree and for λ = 8.1.

V. MODEL SIMULATION AND RESULT

We have implemented the controller design in MATLAB 7.10.0(R2010a). The simulink model here

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pitch angle ß=45 degree . Here the pitch angle error variation rate 5 degrees and input wind speed variation of maximum 12 m/s is assumed. The average wind speed assumed as 10m/s. Generator reference rpm 500 to 1800 rpm. Simulation time taken is 1 sec. The PID parameters have been fixed at kp=2,kd=0.001,Ki=0.05 after a trial and procedure for the operating point 1700 rpm at 45 degree pitch angle. The gear ratio has been assumed =1/10 between turbine and generator. Effect of varying the PID parameters Ki, Kd and Kp are on the peaks on control, oscillations in control and level of obtained rpm as per the given rpm respectively. At low pitch angles the obtained speed is more than desired while at higher value of pitch angle the obtained rpm is less than the given rpm reference. The simulink model here includes inside the subsystems for actuator disc and Cp calculation. Following are the PMSG parameters:

Torque rating : 67.27 Nm -70.2Nm Output voltage: 560 Vdc

Rated Rpm for rated voltage: 1700 rpm Connection: wye (Y) with a neutral point Back emf waveform: sinusoidal

Stator phase resistances:0.085 ohm Ld= 0.00095H Lq = 0.00095H Flux linkage: 0.192 v. s

Voltage constant: 139.2998 Vpeakl-l/krpm Torque constant: 1.152 Nm/ampere peak Pole numbers=4

Inertia factor =0.008 Friction factor=0.001147 Load active power P=40KW,

Load inductive reactive power Ql=500VAR, Load capacive reactive powerQc=50kvar

The fast pitch control has been obtained by the following system called the pitch loop.

Fig. 5 Internal pitch loop with pitch angle (ß) control.

Fig.6 simulation model of the system in SIMULINK.

UNCONTROLLED CONDITION

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[1] beta _zero =0 degree

[2] beta zero= 45 degree

[3] beta_zero= 90 degree

CONTROLLED CONDITION [4] RPM reference =500, beta zero=0 degree

[5] Reference rpm =500, Beta zero=45 degree

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[6] reference rpm =500 , beta_zero=90.

[7] Reference RPM =1700 beta zero=0 degree

[8] Reference rpm=1700 ,Beta_Zero=45 degree

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[11] Reference rpm=3000 ,Beta_Zero= 45 degree

[12] Reference rpm=3000 ,Beta_Zero= 90 degree

VI. RESULT ANALYSIS AND CONCLUSION.

From results in cases [1], [2] and [3] it is clear that the system is uncontrolled and as the wind speed changes ,also the torque varies . The frequency also varies. The magnitude of the voltage sinusoid also varies as per speed of wind. The maximum power capture reduces as we change the pitch angle from 0 to 90 degree.

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rpm respectively. We got the variation of 539- 544, 1700-1735 and 2940-3080 rpm respectively or variation of 1%, 2.05% and 4.6% of reference rpm respectively which is considerable. The results in cases [5],[8] and [11] for the initially set value of the pitch angle equal to 45 degree, show that the rpm output vary in the range of 517-522, 1680-1715 and 2950-3030 respectively or vary as 1%, 2.05% and 2.66% of reference rpm respectively which is considerable. The results in cases [6],[9] and [12] for the initially set value of the pitch angle equal to 90 degree show the variation of 496.5-500.5, 1655-1695 and 2930-3020 respectively or variation within the range of 1%, 2.05% and 1.66% respectively which is considerable. Other important information which comes out in this study is that the 45 degree pitch angle is more controllable pitch angle because the free range of 45 degree is available both side. Also higher we decide the reference rpm to capture, higher the initial pitch angle gives more satisfying result. The thick lines in the voltage sinusoid in case [3] actually show the presence of high distortion in waveform in low frequency or low rpm which can only be seen when we zoom the waveform.

Fig. 7 harmonic distortion in voltage waveform at low wind turbine rpm

By here we conclude that the internal pitch loop with zero crossing detector together with PID control action perform excellently.

REFERENCES

[1] Fei Ye, Xueliang Huang, Chaoming Wang, Gan Zhou, Ping Luo “The Impact and Simulation on

Large Wind Farm Connected to Power System”, DRPT2008 6-9 April 2008 Nanjing China.

[2] Yifan Tang Member IEEE and Longya Xu Senior Member IEEE, “A Flexible Active and Reactive Power Control Strategy for a Variable Speed Constant Frequency Generating System” , IEEE Transaction on Power Electronics, Vol 10. No.4, July 1995.

[3] [email protected] Energy Report Charles-de-Gaulle-Str.5 (53113) Bonn Germany, 2009.

[5] Y. El-Tous, “Pitch Angle Control of Variable speed Wind Turbine”, American of Engineering and Applied Sciences I(2):118-120,2008.

[6] J. H. Laks, L. Y. Pao, and A. D. Wright, “Control of Wind Turbine: Past, Present, and Future”, US National Science Foundation (NSF Grant CMMI- 0700877) 2009.

[7] E. Muljadi, “Pitch-Controlled Variable-speed Wind Turbine Generator”, IEEE Transactions on Industry Applications Vol.37,N0.1 January- February 2001.

[8] J. Zhang, M. Cheng, Z. Chen, X. Fu, “Pitch Angle Control for Variable speed Wind Turbine”, Nanjing China, 6-9 April 2008.

[9] A. Z. Mohamed, M. N. Eskader, F. A. Ghali,

“Fuzzy logic control based maximum power tracking of a wind energy system”, Renewable energy 23(2001).

[10] C. Xiao, L. Zhang, J. Yan, “Fuzzy PID Controller for Wind Turbine”, Second International Conference on Intelligent Networks and Intelligent System. DOI 10.1 109/ICINIS. 2009.

[11] F. M. McNeil and E. Thro, “Fuzzy logic a practical approach”, Academic Press Ltd., London, 1994.

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