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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
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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
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 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
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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
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
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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
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)
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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 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
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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
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.)
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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
Figure 7.7: Rotor And Grid Side Converter Pulses
Figure 7.8: ACTIVE POWER (P.U.)
Figure 7.9: REACTIVE POWER (P.U.)
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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
Figure 7.10: RSC CONVERGENCE
Figure 7.11: GSC CONVERGENCE
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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
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.
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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
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