implementation of first order sliding mode control of...

2487

www.ijifr.com Copyright © IJIFR 2015

Research Paper Paper

International Journal of Informative & Futuristic Research ISSN (Online): 2347-1697

Volume 2 Issue 8 April 2015

Abstract

In this paper, Direct Power Control (DPC) strategy for Doubly Fed Induction Generator (DFIG) based Variable-Speed Wind Energy Conversion System (VS-WECS), is presented. The stator of the generator is directly connected to the grid while the rotor is connected through a back-to-back converter. Rotor Side Converter (RSC) usually does the active power control and maximum power tracking from the turbine while Grid Side Converter (GSC) keeps the voltage of the DC-link constant so as to control reactive power and thus the power factor. For this, a Variable Structure Controller (VSC), called Sliding Mode Controller (SMC), which has been proved to be the most robust controller is adopted along with Maximum Power Point Tracking (MPPT) algorithm, to track the DFIG torque, so that maximum power is extracted from wind. A First Order Sliding Mode Controller (FOSMC) is designed here, for direct active and reactive power control of the Rotor side and Grid side through MATLAB SIMULINK and EMBEDDED MATLAB. It operates well for various wind velocities and gives quick dynamic response. Steady state stability analysis is carried out through Lyapunov Theorem and the results shows convergence at 0.3 sec.

1. Introduction

Wind power is the most reliable and developed renewable energy source. Due to the advancement in

the field of power electronics, wind energy systems have been effectively connected to the grid. A

Controller is needed to ensure the power system operation in terms of reliability and stability. This is

because most of the wind turbines are located at remote places. [1]. In this Paper, VS-WECS are

employed using DFIG due to the fact that it can able to track the changes in wind speed by changing

Implementation of First Order Sliding

Mode Control of Active and Reactive

Power for DFIG based Wind Turbine Paper ID IJIFR/ V2/ E8/ 022 Page No. 2487-2497 Research Area

Electrical &

Electronics Engg.

Key Words Doubly Fed Induction Generator (DFIG), Maximum Power Point Tracking

(MPPT), Direct Power Control (DPC), First Order Sliding Mode Control

(FOSMC)

B. Kiruthiga Assistant Professor, Department Of Electrical & Electronics Engineering Velammal College of Engineering and Technology, Madurai

2488

ISSN (Online): 2347-1697 International Journal of Informative & Futuristic Research (IJIFR)

Volume - 2, Issue - 8, March 2015 20th Edition, Page No: 2487-2497

B. Kiruthiga:: Implementation of First Order Sliding Mode Control of Active and Reactive Power for DFIG based Wind Turbine

the shaft speed, and it generates optimal power. It is thus employed because of its dynamic

behaviour, reduction in the mechanical stress, an increased energy- capture and improved power

quality. Typically, the system use aerodynamic controls to regulate speed and power [2].

The function of Rotor-Side Converter (RSC) in VS-WECS is to control the active power and it

extracts the optimum power from the turbine and the function of Grid-Side Converter (GSC), is to

keep the DC-link voltage constant and hence the power factor retains to unity. There are so many

linear controllers available for the control of RSC and GSC. But they have less efficiency and

instability due to parameter variations. A robust control is needed to improve the efficiency to

extract the maximum power from the wind and hence VSC is designed to deal with uncertainty

parameters [6].Regulation of the power produced by the generator is the prime objective and to fully

extract the maximum power from wind, Maximum Power Point Tracking (MPPT) control scheme is

employed. In this context, this paper proposes an advanced First Order Sliding Mode Controller

(FOSMC) to control the wind turbine according to reference given by MPPT[3].MATLAB

simulation is used to verify the system accuracy and effectiveness of the control strategy proposed

in the paper.

2. Modelling Of Turbine

The schematic diagram of the DFIG based WT along with first order SMC is shown in Fig.1.Wind

turbines extracts the kinetic energy present in the wind and converted into mechanical energy for the

DFIG connected system.

The power input from the wind is given by

P = ½(air mass per unit time) (wind velocity)

(1)

The output power from the wind turbine is given by

Where

(2)

isthe air density in kg/m3 , A is the area of the turbine blades in m

2 and is the velocity of

wind in m/sec.

Figure 2.1: Schematic diagram of the DFIG based WT using first order SMC

2489




The power coefficient gives only the fraction (59%) of the kinetic energy that is converted

into mechanical energy by the wind turbine. It is a function of the tip speed ratio and the blade

pitch angle () for pitch- controlled turbines [1].

(3)

Where, R is the radius of the wind turbine rotor (m). The output power of the turbine will be

maximum for a particular speed, called Thetip speed ratio corresponds to this speed is

called optimum tip speed ratio Corresponds to this ratio, is the maximum power coefficient.

Variable speed turbines can efficiently be operated to capture this maximum energy in the wind.

Figure 2.2: CP–λ Characteristics Curve

Fig.2.2 shows that there is one specific at which the turbine is most efficient. Normally, a

variable speed WT follows the Cpmax to capture the maximum power up to the rated speed by varying

the rotor speed to keep the system at ƛopt. The rotor power (aerodynamic power) isalso given in

terms of aerodynamic torque as

(4)

Where, ωmr is the rotor speed & Ta is the aerodynamic torque [1].

3. Modelling Of Generator

The DFIG-based WT finds increasing application, particularly in the megawatt range, in VS-WECS.

Theoretically, the power handling capacity will be twice in generating mode than in motoring mode.

The stator of the DFIG is directly connected to the grid and the rotor is connected to the grid through

the back-to-back converter, hence the converter costs and the power loss are considerably (typically

25%) reduced. Also, it offers variable speed operation (+ 33% around synchronous speed) along

with four-quadrant power capabilities. It requires little maintenance [24]. Decoupled control of tive

2490




and reactive power of DFIG is carried out in Synchronous reference frame oriented along d-axis.The

equations’ describing the DFIG is given

}

(5)

Where,

4. Direct Power Control

This is achieved from the generator model for reasons of simplification. In d-q reference frame, flux

is assumed to align, related to the stator spinning field pattern and thus here, stator flux is assumed to

be aligned on the d-axis. Moreover, the stator resistance can be neglected. In an asynchronous

generator stator, the Active Power (Ps) and Reactive Power (Qs) are given as:

(6)

(7)

Rotor side voltages are given as

( -

)

- ( -

)ω (8)

( -

)

( -

)ω

(9)

Thus DPC involves control of q-component of current for active power and d-component of current

for reactive power.

5. Sliding Mode control

A sliding mode control (SMC) as a variable control structure is basically an effective non-linear

controller. It gives robust performance for parameter variation and uncertainties. The design of SMC

includes the selection of sliding surface for the desired closed loop performance as a reference and

the next step is to find out the trajectories and the control is designed in such a way that the state

trajectories are forced towards the sliding surface. It stays remain on the surface. A variable structure

controller is first calculated to evaluate performances of the system under varying wind speed

conditions. The different steps of the controller synthesis and original ways allowing improving the

behaviour for power references tracking and DC bus voltage variations are then analyzed [15].

Figure 5.1: System Behavior around the Sliding Surface

2491




Let us consider the non-linear system below:

(10)

Where, x is the system error and u the delivered signal of the controller. The sliding surface is given

by:

(11)

As the control is applied, the system will take in one of the two forms presented below:

- ( -)

}(12)

6. Stability and Convergence

In order to guarantee the system stability and convergence condition, consider the following

Lyapunov equation:

(13)

Where, S1 is the error. Differentiating it with respect to time:

(14)

Forany , it always guarantees that So, if , according to Lyapunov theory, the

system will be stable. Lyapunov method makes the surface attractive and invariant.

Control Algorithm:

The control algorithm is defined by the relation

(15)

Where, U - control signal, Ueq

- equivalent control signal, Un

- switching control term, K is the

controller gain.

(a) Active Power Control:

To control the active power, for the sliding order n=1, the expression of the active power control

surface becomes:

S =(

) (16)

Taking its derivative, we get and during the sliding mode and in steady state, we have

(17)

The equivalent control signal is found as:

(18)

During the convergence mode,

S is verified,

And consequently, the switching term is given by

( ) (19)

To verify the system stability condition, the parameter must be positive.

(b) Reactive Power Control:

The same procedure as for active power is followed for reactive power, replacing P by Q we get the

expression:

S =(

) (20)

Taking its derivative, we get and after computation, the equivalent control signal is found as:

(21)

2492




The switching term is given by

( )(22)

To verify the system stability condition, the parameter must be positive.

Overall block diagram of the DFIG based WT along with the sliding mode controller is shown in

Figure 6.1

Figure 6.1: Detailed Block Diagram Of The Proposed System

7. Simulation Results

A complete electrical model consisting of Wind turbine, DFIG, Back-to-Back converter, Sliding

Mode controller and Grid, is designed in MATLAB - Simulink and Embedded MATLAB codes.

Results plotted in the following figures shows the power generated when reference signals are

applied.

Figure 7.1: GRID VOLTAGES (P.U.)AT 525V BUS BAR

Figure 7.2: GRID CURRENTS (P.U.)AT 525V BUS BAR

2493




Figure 7.3: GRID VOLTAGES (P.U.)AT 25KV BUS BAR

Figure 7.4: GRID CURRENTS (P.U.)AT 25KV BUS BAR

Figure 7.5: VOLTAGE (P.U.) AT DC LINK CAPACITOR

Figure 7.6: ROTOR REFERENCE SPEED (P.U.)

2494




Figure 7.7: Rotor And Grid Side Converter Pulses

Figure 7.8: ACTIVE POWER (P.U.)

Figure 7.9: REACTIVE POWER (P.U.)

2495




Figure 7.10: RSC CONVERGENCE

Figure 7.11: GSC CONVERGENCE

2496




8. Conclusion

The control of a wind energy conversion system based on a DFIG is proposed in this paper. First, a

model of the generator is proposed; then, a MPPT strategy to track maximum power and a sliding

mode control allowing the independent control of the power is also proposed. First order sliding

mode control for active and reactive power of rotor side and grid side are separately simulated and

tested through MATLAB SIMULINK and EMBEDDED MATLAB. It operates well for various

wind velocities and gives quick dynamic response. The decoupling, the stability and the

convergence towards the equilibrium are assured. Steady state stability analysis is carried out

through Lyapunov Theorem and the results show convergence at 0.3 sec. Furthermore, this strategy

proves to be a very simple robust control, which has the advantage to be easily implantable in a

computer control.

References

[1] ABB Sace, A division of ABB, Technical Application Papers-13, Wind Power Plants.

[2] Brice Beltran, Mohamed ElHachemiBenbouzid, and Senior Member “Second-order Sliding Mode

Control of a Doubly Fed Induction Generator Driven Wind Turbine” IEEE Transactions On Energy

Conversion, Vol. 27 NO.2.June 2012

[3] Brice Beltran, Tarek Ahmed-Ali, and Mohamed El HachemiBenbouzid, “Sliding Mode Power

Control of Variable-Speed Wind Energy Conversion Systems IEEE Transactions On Energy

Conversion, Vol. 23 NO.2, June 2008.

[4] Brice Beltran, Tarek Ahmed-Ali, and Mohamed El HachemiBenbouzid, “A Combined High Gain

Observer and High-Order Sliding Mode Controller for a DFIG-Based Wind Turbine” 2010 IEEE

International Energy Conference.

[5] Brice Beltran, Tarek Ahmed-Ali, and Mohamed El HachemiBenbouzid, “ High-Order Sliding Mode

Control of a DFIG-Based Wind Turbine for Power Maximization and Grid Fault Tolerance” IEEE

2009.

[6] .Mohamed Machmoum, Frederic Poitiers,” Sliding Mode Control of a Variable Speed Wind Energy

Conversion System with DFIG”, March 2009, MC2D & MITI.

[7] YoucefBekakra, Djilani and Ben attous, “ Active and Reactive Power Control of a DFIG MPPT for

Variable Speed Wind Energy Conversion using Sliding Mode Control”, World Academy of Science,

Engineering and Technology -60, 2011.

[8] Alvaro Luna, Daniel Aguilar and Raul-S-Aguilar Felipe Corcoles Pedro Rodriguez, “ Control

Proposals for the operation of Power Converters in Wind Power Systems”, Proceedings of the 2011

International Conference on Power Engineering, Energy and Electrical Drives.

[9] XuemeiZheng, Lin Li, DianguoXu and Jim Platts, “ Sliding Mode MPPT Control of Variable Speed

Wind Power Systems”, 978-1-4244-2487-0/09 -2009IEEE

[10] Oscar Barambones, PatxiAlkorta and Manuel De La Sen ,”Wind Turbine Output Power

Maximization based on Sliding Mode Control Strategy”, IEEE 2010

[11] F. Poitiers, M.Machmoum, R. Le Doeuff and M.E. Zaim, “ Control of Doubly-Fed Induction

Generator for Wind Energy Conversion Systems”,026-Poitiers-AUPEC01.

[12] Lingling Fan, Haiping Yin, and Zhixin Miao, “ On Active/Reactive Power Modulation of DFIG –

Based Wind Generation for Interarea Oscillation Damping

[13] Peng Zhou, Yikang He, and Dan Sun, “Improved Direct Power Control of Doubly-Fed-Induction-

Generator-Based Wind Turbines During Network Unbalance

[14] HengNian, Yipeng Song, Peng Zhou, and YikangHe,” Improved Direct Power Control of Wind

Turbine Driven Doubly-Fed-Induction-Generator During Transient GridVoltage Unbalance”, 0885-

8969 copyrights 2011 IEEE

[15] Mohamed Adjoudi, Mohamed Abid, AbdelghaniAissaouiYoucef, Ramdani, HouriaBounoua, “ SMC

of a DFIG for WT, Rev. Roum. Science Technology -Electrotechn.Et Energy, Bucarest, 2

[16] Hernan De Battista, Ricardo J- Mantz and Carlos F.Christiansen, ” Dynamical Sliding Mode Power

Control Driven Induction Generator”, May 1999. S-0885-8969(00)11006-X

2497




[17] Oscar Barambones, Jose Maria Gonzalez De Durana, PatxiAlkorta Jose Antonio Ramos and Manuel

De La Sen, “Adaptive Variable Structure Control Law for a Variable Speed Wind Turbine, Recent

Researches in Automatic Control. ISBN: 978-1-61804-004-6.

[18] Wind Energy Conversion System using DFIG Controlled by Backstepping and Sliding Mode

Strategies,”NihelKhemiri*, Adel Khedher**, Mohamed FaouziMimouni*** “,*Research unit ESIER,

National Engineering School of Monastir ENIM, Tunisia

implementation of first order sliding mode control of...

Documents