Maximum Power Point Tracking of Wind Turbine Conversion Chain Variable Speed Based on DFIG

Received Oct 8, 2017 Revised Dec 10, 2017 Accepted Jan 23, 2018 Wind energy has many advantages, it does not pollute and it is an inexhaustible source. However, the cost of this energy is still too high to compete with traditional fossil sources. The yield of a wind turbine depends on three parameters: the power of the wind, the turbine power curve and the ability of the generator to respond to fluctuations in the wind. This article presented the MPPT of a wind turbine system equipped with an asynchronous generator has dual power under Matlab Simulink program, in the first time we simulated all the conversion chain with complete model of DFIG and vector control in second step then applied the extracted maximum power MPPT strategists, this command is effective and has several advantages it offered to kept the maximum power delivered to network despite all the parameter is change. Keyword:


INTRODUCTION
Electrical energy is crucial to any socio-economic development [1]. The demand for electricity is very important to this dilemma, it is necessary to appeal to new energy sources that will be without consequence for man and environment. Wind energy represents a sizeable potential for bearing damping demand increasingly rampant, after centuries of evolution and further research in recent decades and some wind power projects developed by major central wind turbines provide electricity in parts of the world at a competitive price than the energy produced by conventional plants [2].
Today, the development and proliferation of wind turbines have led researchers in Electrical Engineering to conduct investigations in order to improve the efficiency of electromechanical conversion and quality of the energy supplied [3]. It is in this context that this paper describes that we present a study on the use of asynchronous machines has dual power in a wind system. Initially modeling and conversion system simulation (turbine and asynchronous generator dual feed) Then the vector control in stator active and reactive powers are proposed in a last step And improved the system study and extract maximum power presents MPPT strategists and the simulation results. The objective of this paper is to develop a novel combined MPPT-pitch angle control system of a variable speed wind turbine.

. Model of the wind turbine
The wind turbine collects the kinetic energy of the wind and converts it into a torque, which turns the blades of the rotor [3]. In our study, this system used a wind turbine based on DFIG, which supplies to generate the torque required to the load in Figure 1.

Wind modeling
In our case, the wind speed will be modeled as a sum of several harmonics [5]: (1)

Aaerodynamic power
Which is defined by: The coefficient of power Cp represents the aerodynamic efficiency of the wind turbine. We will use an approximate expression of the power coefficient given by [6]: (4) The simulation of the power coefficient is shown in Figure 2:

529
For the mechanical part can thus lead to a more simple mechanical model [7 and 14] (Figure 3). The dynamic behavior of the generator can be represented by the following equation: Where Jt is the rotational moment of inertia of the rotor and the generator kg.m², ω is the angular velocity of the rotor in red / s, T e is the mechanical torque applied to the alternator shaft in Nm, T em is the electromagnetic torque developed by the generator in Nm and f m is the viscous friction coefficient in Nm.

DFIG Modeling
We modeled the DFIG with the implementation of the transformation of the park following Repository related rotating field This repository is called system of axes (X, Y), it rotates with the speed of the electromagnetic field is obtained [9]: The stator: The rotor: The equations of flow and after the simplification are: Also for the rotor and in the same way are: Where: is the dispersion coefficient.
The expression of the electromagnetic torque of the DFIG in reference Park [10]. The general form of electromagnetic torque is:

Power Modeling DFIG 2.3.1. Modeling of phase rectifier diodes
If we neglect the effect of encroachment, the rectifier output voltage will be defined as following:

Modeling of DC bus
To reduce the ripple of the voltage source adding a low pass filter LC Their operation is governed by the following equations:

VDC Association to DFIG
We will have the following system: In our case, the control of the switches of the inverter is performed by use of the command modulation or PWM pulse width.

MAXIMUM POWER EXTRACTION TECHNIQUE
The goal of the (MPPT) strategy is to pick up the maximum power from the wind; it involves the following of the power curve shown in Figure 4, given by in equation (15) An erroneous speed measurement therefore inevitably leads to degradation of the power captured by the first extraction technique. This is why most wind turbines are controlled without control of the speed. This second control structure based on the assumption that the wind speed varies very little steady state.

APPLICATION VECTOR CONTROL WITH THE ORIENTATION FLOW STATOR
We consider the DFIG works in hypersynchronous mode, the principle is to direct the stator flux along the axis of the rotating frame [9].
So we have: and we have: Relations between current stator and rotor currents: The technical guidance of stator flux is applying on the couple become: Relations between rotor and rotor currents tensions: One could express the rotor voltages depending on rotor currents, we can write: In steady state, the terms involving derivatives disappear, we can write: Vector control in stator wind power system based on a PI controller [13,14]:

Direct Vector Control
This control mode ensures proper decoupling between flux and torque but the problem this disadvantage by the sensor. We neglect the terms of coupling between the two axes of control because of the low value of the slip. We are getting as shown in Figure 5.

Vector control indirectly
The flow is controlled in open loop. It is not measured or estimated. The quantities (voltage or current) ensuring the flow direction and are evaluated `decoupling from the equations of the machine transient.
The values of the rotor voltages depending on power and found we calculate: (26) The Figure 6 shown the isolation carried at the outputs of regulators in rotor currents without any return to the system. The Figure 7 shown the method of control with power. In this method, the isolation is carried out at the outputs of the current regulators rotor with a return of the system.

RESULTS
To evaluate and test the indirect control technique with power loop of active and reactive power by the PI controllers, a simulation was performed under the MATLAB / Simulink. In this case, the gains of the PI controllers are based to method of design, which is based on the compensation of the time constant of the regulator with that of the process of the quantity to be regulated, and were refined after simulation: a

DISCUSSION
We obtained good results in dynamic handling and response to regulations imposed and reactive power. The fluctuations in the power due to the PWM inverter and the dependence of these powers slip. The figure 7 and 8 shown the results of the stator current and rotor current that have the same paces with the trend of the wind and the power. Are sinusoidal, implying a clean energy without harmonics supplied or absorbed by the DFIG.
The results obtained in Figure 12, 13 and 14, we can conclude that this type of control is more efficient than direct control in terms of a variable speed operation, since it is able to provide a decoupled control of active power and responsive regardless of the drive speed.

CONCLUSION
This paper present the modelling of the various components of a wind system for distributed generation of electricity and the study of different double-fed asynchronous machine control systems (DEIG) representing generator for production this energy. In the second part, we have begun the maximum power extraction technique in the operation of the wind; this method proves and gives good results for the maximum generated power to the grid. Subsequently, we developed vector of control reactive power in the stator level, the proper follow instructions for the two powers by the real powers debited by the stator of the machine showed the effectiveness of the applied control.