1. Introduction

There is now general acceptance that the burning of fossil fuels is having a significant influence on the global climate. Effective mitigation of climate change will require deep reductions in greenhouse gas emissions, with UK estimates of a 60–80% cut being necessary by 2050 [1], Still purer with the nuclear power, this last leaves behind dangerous wastes for thousands of years and risks contamination of land, air, and water; the catastrophe of Japan is not far[2], to avoid the problems of the pollution, the energy policy decision states that the objective is to facilitate a change to an ecologically sustainable energy production system such as wind power [3], but the major problem is how associate the wind power stations to the grid  with assure the linking conditions[4]. In addition, now a day’s power transmission and distribution systems face increasing demands for more power, better quality and higher reliability at lower cost, as well as low environmental effect. Under these conditions, transmission networks are called upon to operate at high transmission levels, and thus power engineers have had to confront some major operating problems such as transient stability, damping of oscillations and voltage regulation etc [5], in this work we interest to the transient stability, this last indicates the capability of the power system to maintain synchronism when subjected to a

severe transient disturbances such as fault on heavily loaded lines, loss of a large load etc [6].Generator excitation controller with only excitation control can improve transient stability for minor faults but it is not sufficient to maintain stability of system for large faults occur near to generator terminals [6]. Researchers worked on other solution and found that flexible AC transmission systems (FACTS) are one of the most prominent solution [7], [8].

The objective principal to use FACTS technology for the operators of the electric power is to have an opportunity for the control of the power flow and by increasing the capacities usable of these lines under the normal conditions. The parameter which controls the operation of transmission of energy in a line such as the impedances series and shunts, running, tension and phase angle is controlled by utilizing FACTS controllers. FACTS devices increases power handling capacity of the line and improve transient stability as well as damping performance of the power system [7], [8].

According to the specialized literature we find several types of FACTS [6-11], in our work we are limited to the study a great disturbance, so the FACTS element used for reactive power compensation both assuring the low cost and high efficiency is STATCOM.

The static synchronous compensators (STATCOM) consist of shunt connected voltage source converter through coupling transformer with the transmission line. STATCOM can control voltage magnitude and, to a small extent, the phase angle in a very short time and therefore, has ability to improve the system [7], [8].

2. Wind Turbine Model

2.1. Squirrel Cage Induction Generator

The fixed speed wind generator systems have been used with a multiple-stage gearbox and a SCIG directly connected to the grid through a transformer [11].

The well-known advantages of SCIG are it is robust, easy and relatively cheap for mass production [11], electrically fairly simple devices consisting of an aerodynamic rotor driving a low-speed shaft, a gearbox, a high-speed shaft and an induction generator [12].

The gearbox is needed, because the optimal rotor and generator speed ranges are different, we find also a polechangeable SCIG has been used in some commercial wind turbines; it does not provide continuous speed variations [11]. The generator is directly grid coupled. Therefore, rotor speed variations are very small, because the only speed variations that can occur are changes in the rotor slip[13], because the operating slip variation is generally less than 1%, this type of wind generation is normally referred to as fixed speed [12].

A SCIG consumes reactive power. Therefore, in case of large wind turbines and/or weak grids, often capacitors are added to generate the induction generator magnetizing current, thus improving the power factor of the system as a whole [13].

The power extracted from the wind needs to be limited, because otherwise the generator could be overloaded or the pullout torque could be exceeded, leading to rotor speed instability. In this concept, this is often done by using the stall effect. This means that the rotor geometry is designed in such a way that its aerodynamic properties make the rotor efficiency decrease in high wind speeds, thus limiting the power extracted from the wind and preventing the generator from being damaged and the rotor speed from becoming unstable [13], so the operating condition of a squirrelcage induction generator, used in fixed-speed turbines, is dictated by the mechanical input power and the voltage at the generator terminals. This type of generator cannot control bus bar voltages by itself controlling the reactive power exchange with the network. Additional reactive power compensation equipment, often fixed shunt-connected capacitors, is normally fitted [12]; this system concept is also known as the 'Danish concept' and is depicted in Fig 1 [13].

The slip is generally considered positive in the motor operation mode and negative in the generator mode. In both operation modes, higher rotor slips result in higher current in the rotor and higher electromechanical power conversion. If the machine is operated at slips greater than unity by turning it backwards, it absorbs power without delivering anything out i.e. it works as a brake. The power in this case is converted into I heat loss in the rotor conductor that needs to be dissipated [14].

Fig. 1 shows the torque-slip characteristic of the induction machine in the generating mode. If the generator is loaded at constant load torque  only 1 is stable. The loading limit of the generator i.e. the maximum torque it can support is called the breakdown torque and represented in the Fig.1 as  If the generator is loaded under a constant torque above , it will become unstable and stall, draw excessive current and destroy itself thermally if not properly protected [14].

5. Conclusions

The increasing penetration of renewable energy sources in the grid, high demands, caused destabilized the electrical network, so the researchers must be finding and master a new techniques for produced more power, better quality and higher reliability at lower cost. In first section a global description of system was presented, for each its component a brief presentation are given, modeled and simulated. 

In the second section, the dynamics of the gridconnected wind farm is compared with and without the presence of STATCOM under fault, our test network contain three wind farm each wind farm has two equal wind turbine, according to the simulation results, it clearly illustrates the need of STATCOM improvement when the wind farm recovers its operation after the fault and takes its stability and do not leave the wind farm disconnect in the insufficient of the excitation condenser case. In the last section, a several successive simulation are executed for understand the relation between the STATCOM dimension and the CCT.

References

1. AL. Olimpo, J. Nick, E. Janaka, C. Phill and H Mike, “Wind Energy Generation Modelling and Control,” John Wiley & Sons, Ltd 2009.
2. H. Mellah and K. E. Hemsas, “Design and Analysis of an External-Rotor Internal-Stator Doubly Fed Induction Generator for Small Wind Turbine Application by Fem,” International Journal of Renewable and Sustainable Energy. vol. 2, no. 1, pp. 1-11, 2013.
3. A. Petersson, "Analysis Modeling and Control of DoublyFed Induction Generators for Wind Turbines,” PhD thesis, Chalmers university of technology, GOteborg, Sweden 2005.
4. B .BOUHADOUZA, “Amélioration de la Stabilité Transitoire des Fermes Eoliennes par l’utilisation des STATCOM,” magister theses of Setif university, Algeria, 2011.
5. A. Pahade and N. Saxena, “Transient stability improvement by using shunt FACT device (STATCOM) with Reference Voltage Compensation (RVC) control scheme,” International Journal of Electrical, Electronics and Computer Engineering, vol. 2, pp.7-12, 2013.
6. S. Chauhan, V. Chopra, S. Singh, “Transient Stability Improvement of Two Machine System using Fuzzy Controlled STATCOM,” International Journal of Innovative Technology and Exploring Engineering, vol. 2,pp. 52-56, March 2013. 
7. V. K. Chandrakar, A. G. Kothari, “Comparison of RBFN based STATCOM, SSSC and UPFC Controllers for Transient Stability improvement,” Power Systems Conference and Exposition, PSCE '06, pp. 784 – 791, IEEE PES 2006.
8. M. Mohammad Hussaini, R. Anita, "The Study of Dynamic Performance of Wind Farms with the Application Trends in Engineering,” International Journal of Recent Trends in Engineering, vol. 2, pp. 158-160, 2009
9. P. Kumkratug, “The Effect of STATCOM on Inter-Area Power System Stability Improvement,” IEEE computer society conference, Second UKSIM European Symposium on Computer Modeling and Simulation, pp. 359 – 363, 2008.
10. X. Zhang, C. Rehtanz and B. Pal, “Flexible AC Transmission Systems: Modelling and Control,” Publisher: Springer, 2006.
11. H. Li, Z. Chen, "Overview of different wind generator systems and their comparisons,” IET, Renewable Power Generation, vol. 2, pp. 123–138, 2008.
12. A. Olimpo, J. Nick, E. Janaka, C. Phill and H Mike, “Wind Energy Generation Modelling and Control, John Wiley & Sons, Ltd 2009.
13. J.G. Slootweg, S. de Haan, H. Polinder, W. L. Kling, “Modeling wind turbines in power system dynamics simulations,” Power Engineering Society Summer Meeting, Vol. 1, pp. 22 – 26, IEEE, 2001.
14. M. Huong Nguyen, T. K. Saha, “Dynamic simulation for wind farm in a large power system,” Power Engineering Conference, AUPEC '08. Australasian Universities, pp. 1 – 6, IEEE, 2008.
15. L. Shenghu , L. Zhengkai , H. Xinjie , J. Shusen, “Dynamic equivalence to induction generators and wind turbines for power system stability analysis,” 2nd IEEE International Symposium on Power Electronics for Distributed Generation Systems (PEDG),  pp. 887 – 892, IEEE 2010.
16. H. Mellah, K. E. Hemsas, “Simulations Analysis with Comparative Study of a PMSG Performances for Small WT Application by FEM,” International Journal of Energy Engineering, vol.3, no 2, pp. 55-64, 2013.
17. B. wu, Y. Lang, N. Zargari, S. Kouro, “power conversion and control of wind energy systems,” IEEE press, wiley, Canada 2010.
18. E.Acha, V.G.Agelidis, O.Anaya-lara, T.J.E.Miller, “power Electronic Control in Electrical Systems", Newnes. A division of reed educational and professional publishing ltd, 2002.
19. M. A. Kamarposhti, M. Alinezhad, “Comparison of SVC and STATCOM in Static Voltage Stability Margin Enhancement", International Journal of Electrical Power and Energy Systems Engineering, 2010.