Enhancement of the direct power control applied to DFIG-WECS

Received Oct 29, 2018 Revised Jul 20, 2019 Accepted Aug 29, 2019 This work is dedicated to the study of an improved direct control of powers of the doubly fed induction generator (DFIG) incorporated in a wind energy conversion system 'WECS'. The control method adopts direct power control 'DPC' because of its various advantages like the ease of implementation which allows decoupled regulation for active and reactive powers, as well as a good performance at transient and steady state without PI regulators and rotating coordinate transformations. To do this, the modeling of the turbine and generator is performed. Therefore, the Maximum Power Point Tracking (MPPT) technology is implemented to extract optimal power at variable wind speed conditions. Subsequently, an explanation of the said command is spread out as well as the principle of adjusting the active and reactive power according to the desired speed. Then, the estimation method of these two control variables will be presented as well as the adopted switching table of the hysteresis controller model used based on the model of the multilevel inverters. Finally, the robustness of the developed system will be analyzed with validation in Matlab/Simulink environment to illustrate the performance of this command.


INTRODUCTION
The production of electrical energy in the world generates various pollutions. Thus, thermal power stations (coal, oil) are responsible for atmospheric emissions linked to the combustion of fossil fuels. In contrast, nuclear power plants, whose development will increase following the oil crisis, have no adverse influence on air quality although they produce radioactive waste that causes storage problems, treatment or transport.
Today, the fear of being limited to ephemeral energies, the awareness of the negative impact of these on the environment, the craze for renewable energies and the opening of the market of the production of energy towards other alternatives are factors that give an important place to these energies (hydraulic, wind, solar, biomass, ...) in the production of electricity [1][2][3].
Among the most coveted renewable energies, we find the wind energy that interests more and more countries as it produces a clean and sustainable energy. We also notice that a large part of wind turbines installed today is equipped with a doubly fed induction generator (DFIG). The latter allows the production of electricity under variable speed, this makes it possible to better exploit turbine resources. These turbines are also equipped with variable blade pitch angle in order to be adapted to different wind conditions. The turbine is controlled so as to permanently maximize the power produced independently of the variation of the wind profile [4].

MAXIMUM
Wind turb xploiting the e mechanical or e r Point Track possible to cha ontrol of the g s itself to ea systems also ind becomes t In this co (Tip Speed R ol because of aintain λ at an

DOUBLY FE
In the lite marized in four

Flux equation
The stator

Electromagnetic torque
The expression of the electromagnetic torque as a function of the stator flux and rotor current is given by

Mechanical equation
The evolution of the mechanical speed from the total mechanical torque (T ) is determined by the fundamental equation of dynamics:

DIRECT POWER CONTROL APPLIED TO THE DFIG 5.1. Principle of the direct power of control
The basic principle of direct power control (DPC) was proposed by Noguchi [15], it is based initially on the direct control of torque (DTC), intended for the control of the electric motors [16][17][18][19][20]. In the case of DPC, active and reactive powers replace torque electromagnetic and the amplitude of the stator flux of the DTC. This non-linear control strategy is defined as a technique of direct control because it chooses the appropriate voltage vector of the converter without any modulation technique. The basic concept is to select the appropriate switching states from a switch table based on errors, which are limited by a band hysteresis, present in the active and reactive powers.
Instant active and reactive powers are calculated from the expressions below: P ω |ψ ||ψ |sinδ (9) The reference active power is calculated from the output of the DC bus voltage regulator U DC [21]. The reference of the reactive power is maintained at zero in order to ensure a unit power factor. Then, the powers are compared and the errors obtained are applied to regulators of hysteresis.

Hysteresis controller
The main idea of direct power control is to maintain the instant active and reactive powers in a desired band. This control is based on two hysteresis comparators which use as input the error signals between the reference values and estimates of the active and reactive powers. These two controllers are responsible for deciding how much a new switch and/or output voltage vector of the inverter is applied. If the error of the power (ePs or eQs) is increasing and reaches the higher level, the hysteresis controller changes its output to '1'.

Vector selection
The influence of each output vector on the active and reactive powers is very dependent on the actual position of the vector of the source voltage Thus, in addition to the signals of the two hysteresis controllers, the switching table operates according to the position of the vector of the source voltage, which turns to the pulsation ( m ), in the complex plan. However, instead of introducing to the switching table the exact position of the vector of the voltage, the sector selection block informs us in which domain the current vector of the source voltage is located [22,23]. Therefore, we propose to use a modified DPC which, unlike the conventional DPC, can produce twenty seven voltage vectors instead of the eight vectors. In other words, we will decompose twelve sectors instead of six in order to increase the accuracy and also to avoid the problems encountered in boundaries of each control vector. With this in mind, we used a five-stage hysteresis corrector for the reactive power and a three-level corrector for the active power.

Switching tab
The switch r of the inver based on the p is work, we a mal minimizati

Setpoint trac
The syste synchronous, tion generato mulation are s Eng Enhancem ble hing table is t rter in order to position of the adopted a mo on in error po

Robustness te
The param erature increas e parameter v ollowing condi esistance R m ductances L a

CONCLUSION
This work proposes an improvement of the classical DPC control applied to the doubly fed induction generator (DFIG) integrated in a wind energy conversion system 'WECS'. The whole system is modeled and simulated in the environment Matlab/Simulink. Also, a technique (TSR) to reach the maximum power point (MPP) is presented in order to capture the maximum of power. The results (setpoint tracking and robustness test) in steady and transient regimes show a complete correlation. They both prove the robustness and efficiency of the method developed. In general, the simulation's results obtained during the application of the control under variable speed show an excellent dynamic performance and tracking ability of the powers generated at the corresponding reference values with the preservation of sinusoidal shapes for both currents and voltages (stator and rotor).