LOW VOLTAGE RIDE-THROUGH METHODS BASED ON GRID

Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 5(8): 221-226 of Emerging in Engineering and(ISSN: Applied Sciences (JETEAS) 5(8): 221-226 (ISSN: 2141-7016) ©Journal Scholarlink ResearchTrends Institute Journals, 2014 2141-7016) jeteas.scholarlinkresearch.com

LOW VOLTAGE RIDE-THROUGH METHODS BASED ON GRID CODES FOR DOUBLY FED INDUCTION GENERATOR DRIVEN WIND TURBINE Kenneth E. Okedu and Roland Uhunwangho

Department of Electrical Engineering University of Port Harcourt, Nigeria. Corresponding Author: Kenneth E. Okedu -----------------------------------------------------------------------------------------------------------------------Abstract Rapid wind power development has led to a shift from small generators to large generators and from single distributed units to large centralized clusters of generators commonly known as wind farms. Amongst the modern variable speed wind turbines, the doubly fed induction generator (DFIG) has recently been very attractive to wind turbine manufacturers due to its ability to decouple both active and reactive powers via its high frequency power converters. The suitability of wind turbines driven by DFIG operation is analyzed based on the recent grid codes for low voltage ride through (LVRT) of wind turbines in this paper. Different low voltage ride through methods or alternatives were discussed for the quick recovery of the DFIG wind turbine system during low voltage or fault in the grid. Two methods were proposed to protect the rotor current of the DFIG driven wind turbine during LVRT. Simulations were carried out in the standard laboratory software package using power system computer aided design and electromagnetic transient including DC (PSCAD/EMTDC). Keywords: wind energy, doubly fed induction generator, induction generator, low voltage ride through, wind turbines, and grid codes INTRODUCTION Wind power has become one of the preferred alternatives for new energy production in the world, and wind power projects for grid connection are growing both in size and numbers. The introduction of larger wind farms with power plants, have a great influence on the power system stability, due to the stochastic nature of wind energy, thus changed the way transmission system operators (TSO’s) and grid owners look at wind disconnection issues during low voltage in the grid system. The low voltage ride through (LVRT) performance is of concern for the wind power owners and also the sub transmission network owners to ensure that turbine equipment survived voltage deviations during a voltage dip in the grid system (Coughlan Y., et al, 2007, Erlich I., Shewarega F., 2007, and Wang-Hansen M., 2008).

This paper presents the various methods to overcome the LVRT of the DFIG driven wind turbine considering grid requirements. The methods presented are the Crowbar switch, DC-link chopper braking resistor, the STATCOM (Static synchronous compensators), static series compensators (SSC) or dynamic voltage restorer (DVR) and the HVDC. Two methods were proposed in this study; using the control of the rotor side converter current during low voltage and the insertion of series dynamic resistor into the grid side of the DFIG wind turbine system. Effect of Low Voltage on DFIG Driven Wind Turbines The use of the partial-scale frequency converter in the generator’s rotor makes DFIG attractive as a wind generator from an economical point of view. However, on the other hand, this converter arrangement requires advanced protection system, as it is very sensitive to disturbances on the grid (Hasan A.D., et al, 2007). If there is no protection system, the DFIG can suffer from large transient currents in the stator during a grid fault since its stator circuit is directly connected to the grid. Because of the magnetic coupling between the stator and the rotor, the stator transient is transmitted to the rotor, resulting in both large rotor currents and voltages during the grid faults. Furthermore, the surge following the fault includes a rush of power from the rotor terminals towards the converter. Since the stator-to-rotor voltage ratio of the DFIG is designed according to the desired variable speed range, in case of grid faults it might not be possible to achieve the desired rotor voltage in order to control the large rotor currents. This means that the frequency converter reaches fast its limits and as a consequence it loses the independent control during the grid fault. As the grid voltage drops in the fault moment, the grid side

New grid codes require the ability of wind turbines to maintain its connection to the network during and after grid failures (Morren J., Haan S. W. H., 2005, Sun T., Chen Z., and Blaabjerg F., 2005). In 2006, some Transmission System Operators adopted even more restrictive code requiring also the ability to provide reactive currents to the network (Ullah N.R., Thiringer T., Karlsson D., 2007 and Requisitos de Respuesta, 2006). This ability would lead to a better stability performance of the network increasing residual terminal voltages during grid faults. Wind turbines based on DFIG is the most widely used in the wind power generation market because of its low cost and ability to control active and reactive powers during LVRT, but have the stator windings connected directly to the grid, making the rotor winding susceptible to high currents induced during grid faults (Lopez J., et al, 2008, Erlich I., et al, 2007, and J. M. Rodriguez J. M., et al, 2002).

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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 5(8): 221-226 (ISSN: 2141-7016) inverter is not able to transfer the power to the grid that obtains from the rotor side converter and therefore, the excess energy in the DC-link capacitor rises the DC-bus voltage rapidly. It is therefore necessary to protect the frequency converter against overcurrents, the rotor of the generator against overvoltages and the DC-link against overvoltages (Okedu K.E., et al, 2010, and Okedu K.E., et al, 2011). The protection system monitors usually several signals such as rotor current and the DC-link voltage. When at least one of the control input signals exceeds its respective relay setting, the protection is activated. Grid Requirements for Low Voltage Ride Through of Wind Turbines A summary of the grid requirements for the low voltage ride through of wind turbines are shown in the following Figs.1-5 based on (E. ON NETZ GmbH, 2006).

Fig. 4: Requirements placed on the output power of a generating plant to the grid for certain periods as function of grid frequency and grid voltage as set by E.ON Netz The magnitude of the voltage is controlled by the reactive power exchange, since in most networks; the reactive power is directly proportional to the voltage. Fig. 1 displays the typical requirement for low voltage ride through grid code. The wind farm must remain connected if the voltage drops, defined by the retained voltage r.m.s value, and the duration of the fault are also shown in the curve (E.ON NETZ GmbH, 2006). Figure 2 shows the required reactive current support from the generating plants during voltage dip, while in the event of frequency drops above the thick line in Figs. 3 and 4, the active power output must not be reduced even if the generating plant is being operated at rated power. However, in Fig. 5, if there is a sudden change in generation or load, the system frequency is allowed to deviate by up to ± 0.5 Hz.

Fig.1 Fault ride through requirement for wind farm

Figure 3 Frequency envelope for frequency drops in which there may be no limitation of the active power output as set by E.ON Netz

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Fig. 5 Requirements placed on the reactive power provision of a generating plant at frequencies between 49.5 and 50.5 Hz and without limiting the active power as set by E.ON Netz

Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 5(8): 221-226 (ISSN: 2141-7016) Low Voltage Ride Through Methods for DFIG Driven Wind Turbine −

+

V

FACTS (STATCOM) method

Vg>0.9:1(turn on) Normal operation Proposed method 1

Wind Turbine

SDBR Control method

Gearbox

d csta t

Vg < 0.9 : 0(turn off) Fault condition

Infinite

Switch

0.1+j0.6

50MW 0.0005+j0.005 DFIG

T1

Vgrid

Resistor

Rext

Pitch Control

j0.2

0.1+j0.6 Fault

System base (66kV, 100MVA) DC Chopper method Self capacity base (6.6kV)

T6

control

Variable Voltage Frequency

RSC Control

Proposed

Fixed Voltage and Frequency

Edc

DC-link DC-

K(Idr,Iqr)

Vg

Transformer

GSC

RSC

GSC Control Pulse

Edc Edcmax

Fault condition look up table controller method

+

Comparator

Rext IGBT

Edc >Edcmax :1(turn on) Edc < Edcmax : 0(turn off)

-

Fig. 6 Proposed DFIG model system and some LVRT methods The DFIG is equipped with a pitch-change mechanism to adjust the blade tip speed as shown in Fig. 6 above, so they have a better power coefficient profile in a wind farm. In the case of direct-connect wind turbine, the electrical generator speed is fixed by the grid. In turn, the rotor speed is also fixed since it is directly connected to the generator via a gearbox. This means that the blade tip speed is practically unchanged. Therefore, as the wind speed increases, the Cp (power coefficient) of a direct-connect fixed speed wind turbine in a wind farm will increase at first, and then decrease after the rated power is reached. The DFIG is doubly fed by means that the voltage on the stator is applied from the grid and the voltage on the rotor is induced by the power converter. The converter consists of two conventional voltage source converter (Rotor Side Converter RSC, and the Grid Side Converter GSC) and a common DC-link bus voltage as shown in Fig. 6. The methods for low voltage ride through for the DFIG driven wind turbine is discussed in the following sub sections. Crowbar Control Method For converter protection reasons, as part of any LVRT system, a crowbar is connected between the rotor and the rotor-side converter as shown in Fig.6 (Salles M.B.C., et al, 2010, and G. Pannell G., et al, 2010). The crowbar system used in modern wind turbines is based on a threephase series resistance controlled by power electronics. The crowbar system is activated during over-current on the rotor windings or an over-voltage on the DC link, which appears most of the time after short circuit close to the wind farm. The pulse signal to trigger the crowbar is when the DC-link voltage Edc exceeds Edc-max. The steps involved during the activation and deactivation of the crowbar system are; disconnection of the rotor windings from the rotor side converter (RSC), insertion of the three-phase resistance in series to the rotor windings, disconnection of the crowbar system from the rotor windings, and reconnection of the RSC to the rotor 223

windings. These actions will help to prevent the high rotor currents and excessive DC-link voltage. The resulting voltage amplitude in the rotor circuit is determined by the crowbar resistors. The crowbar resistor also acts as an active power sink, burning off active power to mitigate rotor over-speeding. During the time the crowbar is activated, the generator works as a conventional induction machine with high rotor resistance. Several chains of events can follow a crowbar action, and those are effectively the different LVRT strategies. One possibility is to overrate the IGBT modules in the converter to allow for an extended, voltage tolerance of the DC-link, another is the possibility to disconnect the rotor side converter but not the grid side converter and a third is to disconnect the stator and continue the active operation of both converters and the DC-link. The main goal of the LVRT system regardless of principal system solution is instantaneous resumed active operation for the wind turbine after grid fault clearance. DC-link Braking Resistor Method A chopper or braking resistor (dumped load) could be added on the DC-link (Fig. 6) with a similar function to that of the rotor side crowbar, reducing the DC-link voltage (Takahashi R., et al., 2006, and Okedu K. E., et al, 2010). The chopper facilitates a voltage-raising action from the converter terminals during the LVRT and thereby enabling a faster regain of the control of the DClink voltage. The protective device in this scheme is a simple chopper circuit. The pulse signal to trigger the IGBT is activated when Edc exceeds Edc-max, and thus, the chopper is turned on and the energy is dissipated by the internal resistance. Crowbar and STATCOM Method FACTS (flexible ac transmission network) device like the STATCOM (static synchronous compensator) can effectively enhance the performance of the DFIG, when it is disconnected by the crowbar switch during grid fault

Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 5(8): 221-226 (ISSN: 2141-7016) by providing additional reactive power to the system, thus improving the voltage instability performance of the DFIG and the wind farm as well (Wei Q., et al., 2009). The crowbar switch is triggered if the Edc-max value in the excitation circuit is exceeded. The FACTS device which is the STATCOM is connected to the point of common coupling (PCC) as shown in Fig.6. STATCOM can enhance the transient stability and significantly minimize the blade-shaft torsional oscillation of wind turbine

generators (Suul J. A., et al., 2010 and Muyeen S. M., et al., 2008). The reactive power of a STATCOM is provided by voltage source converter (VSC). The VSC converts the DC voltage into a three-phase output ac voltage with desired amplitude, frequency and phase. The control scheme and switching strategy for a STATCOM used in this study is shown in Fig. 7, where the d axis current controls the DC voltage and the q axis current controls the reactive of the STATCOM respectively. l

θ

*

V

d ,q

PLL

V

* a ,b ,c

a

V

V

b

Generated Switching Reference

E

ref

−

( S ta t )

P I K

+

Q + −

T

*

P I T

p i

= 1 0 .0

= 0 .0 1

+

d stat

+

-

V

ST A T

p i

T

−

= 2 .0

= 0 .1 0

I

0 .0 2 (

1 + 0 .0 0 4 s ) 1 + 0 .0 0 1 s

p i

I

c

K T

p i

= 1 .8

= 0 .0 0 2

q stat −

P I K

b ,

P I

+

sta t

−

K

I

( S ta t )

I

a ,

Grid Side

PWM VSC

Pulse Generation System

vpu

I c

Coupling Transformer

Pulse Generation

Switching Pattern

E

q sta t

abc/dq

dq/abc

V

l

d sta t

Triangular Carrier Signal

Controller

+

= 2 .0

0 .0 1 (

1 + 0 .0 0 3 s ) 1 + 0 .0 0 1 s

P I K T

= 0 .2

p i

V

* q s ta t

= 0 .0 3

= 0 .0 0 2

V

* d sta t

Fig. 7 Control block and switching strategy of STATCOM Series Anti-parallel Thyristor and DC-chopper Method A new LVRT system is proposed in (Petersson A., et al., 2005), where the power ratings of the IGBTs in the converter connected to the rotor are dimensioned for higher current ratings, and anti-parallel thyristors are placed as in Fig. 8. When the grid voltage recovers, high transient currents appear in the stator. In order to avoid these high current when the voltage returns, anti-parallel thyristors can be connected in series with the stator in order to achieve a quick disconnection of the stator circuit. By interrupting the stator circuit, the flux oscillation will also be interrupted. As soon as the flux is interrupted it is possible to re-magnetize the DFIG quickly through the rotor converter and connect the stator circuit to the grid again. The converter needs to be disconnected from the rotor circuit since the valves of the converter are over dimensioned. In other to remove the excess power that is fed into the DC-link, a DC-link chopper is used to dissipate the excess power. The problem with this configuration is that the efficiency decreases due to the conduction losses in the thyristors since they are kept on during normal operation. This could be avoided by bypassing the thyristor with commutators, but the switching time of the commutators may be too slow and the system may not respond fast enough to a voltage dip. Also, the higher ratings of the IGBTs will increase the cost of the power converters. A dynamic voltage restorer (DVR) or a static series compensator (SSC) can be used as solution to isolate the

224

Dynamic Voltage Restorer/Static Series Capacitor Method Switch

DFIG

RSC

GSC

Fig. 8 Series anti-parallel thyristor for DFIG LVRT DFIG wind turbine during a voltage dip (Awad H., et al, 2004, Ibrahim A.O., et al, 2009). The SSC is a voltage source converter connected in series on the connection between the grid and the DFIG wind turbine, whose voltage adds to the grid voltage to obtain the desired load (DFIG) voltage VL (t) as shown in Fig. 9. The SSC may also control voltage unbalance, voltage regulation and annul low-order harmonics based on the control strategy employed. Different SSC/DVR topologies for the power converter like the multimode SSC (Okayama H., et al., 2003) where the transformer are substituted by series connection of power cells has been proposed. In (Alegria E., et al., 1998), a static voltage regulator with a variation of the load tap changer without storage element was proposed. The disadvantage of these methods is that the additional cost of the added power converters may override the advantage of the lower price of the DFIG wind turbine.

Rotor Current of DFIG [pu]

Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 5(8): 221-226 (ISSN: 2141-7016) Thyristor LgRg

VL (t)

Grid

VSC

LfRf

DFIG

TRF

+ Vi (t)

Vdc

Cf

-

Fig. 9 Static series compensator for DFIG LVRT

4

2 1 0 -1 -2 -3 0 .0

HVDC LVRT Method The use of HVDC in offshore DFIG wind farms may be helpful for LVRT. HVDC could also be of benefit to onshore wind farms if the cost of HVDC stations would drop. Some of the benefits HVDC schemes provides in comparison to the conventional AC connection for LVRT are; fully defined and controllable power flow, sending end and receiving end frequencies are independent, offshore network is isolated from mainland disturbance which provides for a ride through capability for the wind turbines during AC system faults at the receiving end, low cable power losses compared to AC, and fewer cables provide both environmental benefits and reduced civil works. Proposed LVRT Methods for DFIG (Main Contribution to Knowledge) Two methods as shown in Fig. 6 were proposed for the DFIG LVRT in this study; • The control of the rotor current of the DFIG during LVRT in combination with the DC-link protection, limiting the magnitude of the rotor current using a variable k. The insertion process of multiplying K to the measured d and q rotor currents during low voltage helps to limit the magnitude of the rotor current of the DFIG within its nominal value, and hence the use of expensive crowbar switch to disconnect the RSC from the DFIG during the low voltage can be avoided. • The insertion of a small value series dynamic braking resistor (SDBR) connected to the stator side of the DFIG (Fig. 6), instead of the rotor side of the DFIG together with the DC-link protection scheme to help improve further the overall performance of the DFIG during LVRT. Simulation Simulation was run in PSCAD/EMTDC (PSCAD/EMTDC Manual, 1994) software for the two proposed methods to protect the rotor current of the DFIG driven wind turbine during LVRT for a duration of 1.0 sec, with a simulation timing step of 0.00001sec. The result is shown in Fig. 10 below. The simulation result above shows that the two proposed methods were able to limit the rotor current of the DFIG within twice of its nominal value during LVRT, but the SDBR control gives a better performance than the control of the RSC Id and Iq rotor currents during voltage dip of 100ms duration, which occurs at 0.1sec.

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N o C o n to l Id ,Iq R S C C o n t r o l+ D C C h o p p e r S D B R C o n t r o l+ D C C h o p p e r

3

0 .2

0 .4 0 .6 T im e [s e c ]

0 .8

1 .0

Fig. 10 Rotor current for DFIG LVRT for two proposed methods CONCLUSIONS Low voltage ride through (LVRT) is necessary for doubly fed induction generator (DFIG) driven wind turbine wind power generation system. In this paper, the crowbar, DClink chopper or braking resistor, crowbar and static synchronous compensator (STATCOM) combination, series anti-parallel and DC- chopper combination, dynamic voltage restorer (DVR) or static series compensator (SSC), and HVDC LVRT methods were discussed. Also, two methods using the control of the rotor current and the insertion of a series dynamic braking resistor during LVRT were proposed. The two proposed methods were able to control the rotor current of the DFIG driven wind turbine during voltage dip, hence, may be a good solution due to low cost and reduced circuitry, for LVRT of DFIG driven wind turbines system during voltage dip in the grid. REFERENCES Alegria E., Khan A., Rajda, J., Dewan S., “Static Voltage Regulator (SVR)-Ride through Support for Semiconductor Facilities,” Power Quality Conference, Power Systems World’ 98, Santa Clara, California, November, 1998. Awad H., Svensson J., and Bollen M., “Mitigation of Unbalanced Voltage Dips using Static Series Compensator,” IEEE Transaction on Power Electronics, vol. 19, pp. 837-846, May, 2004. Coughlan Y., Smith P., Mullane A., and Mark O’Malley, “Wind Turbine Modeling for Power System Stability Analysis - A System Perspective,” IEEE Transaction on Power Systems, vol. 22, no. 3, August, 2007. E.ON NETZ GmbH, Grid Connection Regulation for High and Extra High voltage, 2006. Erlich I.,and Shewarega F., “Insert Impact of large-Scale Wind Power Generation on the Dynamic Behaviour of Interconnected Systems,” iREP Symposium-Bulk Power System Dynamics and Control – VII, Revitalizing Operational Reliability, August 19-24, 2007, Charleston, SC, USA. Erlich I., Kretschmann J., Fortmann J., MuellerEngelhardt S., and Wrede H., “Modeling of Wind Turbines Based on Doubly-fed Induction Generators for Power System Stability Studies,” IEEE Transaction on Power System, vol. 22, no. 3, pp. 909-919, Aug. 2007.

Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 5(8): 221-226 (ISSN: 2141-7016) Hasan A.D., and Michalke G., “Fault Ride-Through Capability of DFIG Wind Turbines,” Renewable Energy, Vol. 32, pp.1594-1610, 2007. Ibrahim A. O, Nguyen T.H, Lee D., and Kim S., “Ride Through Strategy for DFIG Wind Turbine Systems using Dynamic Voltage Restorers,” Proceedings IEEE-ECCE (Energy Conversion Congress and Exposition), California, USA, 2009. Lopez J., Gubia E., Sanchis P., Roboam X., and Marroyo L., “Wind Turbines Based on Doubly Fed Induction Generator under Asymmetrical Voltage Dips,” IEEE Transaction on Energy Conversion, vol. 23, no. 1, pp. 321-330, March, 2008. Morren J., de Haan S.W.H., “Ridethrough of Wind Turbines with Doubly Fed Induction Generator during a Voltage Dip,” IEEE Transaction Energy Conversion vol. 20, pp. 435-441, 2005. Muyeen S.M, Mannan M.A., Ali M.H, Murata T., and Tamura J., “Stabilization of Wind Turbine Generator System by STATCOM,” IEEJ Transaction on Power and Energy, vol. 126B, no. 10, pp. 1073-1082, 2008 Okayama H., Fujii T., Tamai S., Jochi S., Takeda M, Hellested R., and Reed G., “Application and Development Concepts for a New Transformer-less FACTS device-the Multimode Static Series Compensator,” Proceedings of the IEEE PES, Dallas USA, September, 2003. Okedu K.E, Muyeen S.M , Takahashi R., and Tamura J., “Comparative Study between Two Protection Schemes for DFIG-based Wind Generator,” International Conference on Electrical Machines and Systems (ICEMS), Seoul, South Korea, 2010. Okedu K.E, Muyeen S.M.,Takahashi R., and Tamura, J., “Stabilization of Wind Farms by DFIG-based Variable Speed Wind Generators,” International Conference on Electrical Machines and Systems (ICEMS), Seoul, South Korea, 2010 Okedu K.E., Muyeen S.M., Takahashi R., and Tamura J., “Participation of FACTS in Stabilizing DFIG with Crowbar during Grid Fault Based on Grid Codes,” IEEEGCC Conference and Exhibition, Dubai, UAE, Feb. 2011 Pannell G, Atkinson D.J, and Zahawi B., “Minimumthreshold Crowbar for a Fault Ride Through Grid Code Compliant DFIG Wind Turbine,” IEEE Transaction on Energy Conversion, vol. 25, no. 3, pp. 750-759, September, 2010. Petersson A., Lundberg S., and Thiringer T., “A DFIG Wind Turbine Ride Through System Influence on Energy Production,” Wind Energy Journal, vol. 8, pp. 251-263, 2005.

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Petersson A., “Analysis, Modeling and Control of Doubly-Fed Induction Generators for Wind Turbines,” PhD Thesis, Division of Electric Power Engineering, Department of Energy and Environment, Chalmers University of Technology, Sweden, 2005. “PSCAD/EMTDC Manual”, Manitoba HVDC research center, 199 Requisitos de Respuesta frente a Huecos de Tension de las Instalaciones Eolicas, BOE no. 254, 2006, pp. 3701737019, (in Spanish). Available on line: http://www.ree.es/operacion/pdf/po/PO_resol_12.3_Resp uesta_huecos_eolica.pdf.. Rodriguez J. M, Fernandez J. L, Beato D, Iturbe R, Usaola J., Ledesma P., and Wilhelmi J. R., “Incidence on Power System Dynamics of High Penetration of Fixed Speed and Doubly Fed Wind Energy Systems: Study of the Spanish Case,” IEEE Transaction on Power Systems, vol. 17, no. 4, pp. 1089-1095, November, 2002. Salles M.B.C, Hameyer K., Cardoso J.R, Grilo A. P., and Rahmann C., “Crowbar System in Doubly Fed Induction Wind Generators,” Energies Article Journal, ISSN 19961073, vol. 3, pp. 738-753, 2010. Sun T., Chen Z., and Blaabjerg F., “Transient Stability of DFIG Wind Turbines at an External Short Circuit Fault”, Wind Energy Journal, vol.8, pp.345-360, 2005. Suul J. A, Molinas,M., and Undeland T., “STATCOMbased Indirect Torque Control of Induction Machines during Voltage Recovery after Grid Faults,” IEEE Transaction on Power Electronics, vol. 25, no. 5, pp. 1240-1250, 2010. Takahashi R., Tamura J, Futami M., Kimura M., and Idle K., “A new Control Method for Wind Energy Conversion System Using Doubly Fed Synchronous Generator,” IEEJ Trans. Power and Energy, Vol. 126, no. 2, pp. 225-235, 2006. Ullah N. R., Thiringer T., Karlsson D., “Voltage and Transient Stability Support by Wind Farms Complying with E. On Netz Grid Code,” IEEE Transaction Power System, vol. 22, pp. 1647-1656, 2007. Wang-Hansen M., “Wind Power Dynamic Behavior-Real Case Study on Linderodsasen Wind Farm,” Dissertation, Department of Energy/Envir. Division of Elect. Power Engineering, Chalmers University, Sweden, 2008. Wei Q., Venayagamorthy G. K., and Harley R.G., “Realtime Implementation of a STATCOM on a Wind Farm Equipped with Doubly Fed Induction Generators, “IEEE Transaction on Industrial Application, vol. 45, no.1, pp. 98-107, January, 2009.