In the past, most national grid codes and standards did not require wind turbines to support the power system during a disturbance. For example during a grid fault or sudden drop in frequency wind turbines were tripped off the system. However, as the wind power penetration continues to increase, the interaction between the wind turbines and the power system has become more important. This is because, when all wind turbines would be disconnected in case of a grid failure, these renewable generators will, unlike conventional power plants, not be able to support the voltage and the frequency of the grid during and immediately following the grid failure. This would cause major problems for the systems stability. Therefore, wind farms will have to continue to operate during system disturbances and support the network voltage and frequency. Network design codes are now being revised to reflect this new requirement. A special focus in this requirement is drawn to both the fault ride-through capability and the grid support capability. Fault ride-through capability addresses mainly the design of the wind turbine controller in such a way that the wind turbine is able to remain connected to the network during grid faults (e.g. short circuit faults). While grid support capability represents the wind turbine capability to assist the power system by supplying ancillary services, i.e. such as supplying reactive power, in order to help the grid voltage recovery during and just after the clearance of grid faults. Due to the partial-scale power converter, wind turbines based on the DFIG are very sensitive to grid disturbances, especially to voltage dips during grid faults. Faults in the power system, even far away from the location of the turbine, can cause a voltage dip at the connection point of the wind turbine. The abrupt drop of the grid voltage will cause over-current in the rotor windings and over- voltage in the DC bus of the power converters. Without any protection, this will certainly lead to the destruction of the converters. In addition, it will also cause over-speeding of the wind turbine, which will threaten the safe operation of the turbine. Thus a lot of research works have been carried out on the LVRT ability of DFIG wind turbines under the grid fault. These LVRT strategies can be divided into two main types: the active method by improving control strategies, the passive scheme with additional hardware protective devices.

1. IMPROVED REACTIVE POWER CAPABILITY WITH GRID
CONNECTED DOUBLU FED INDUCTION GENERATOR

2. CONTENT
 INTRODUCTION
DOUBLY FED INDUCTION GENERATOR
OPERATION
REACTIVE CHARACTERISTICS OF DFIG
MODELLING OF DFIG
CAPABILITY CURVE OF DFIG
CONCLUSION
REFERENCE

3. INTRODUCTION
 The reactive power capability (RPC) curve of DFIG considering three
limitations( rotor voltage, rotor current, stator current) for reactive
power/consumption.
 It is established that the total reactive power generation is limited by
rotor voltage at low speeds and by rotor currents at higher speed.
Selection of partial rated converter rating is crucial in terms of cost,
efficiency, operating speed range and reactive power capability.
The effect of converters rating on the enhancement of RPC of DFIG
IS analyzed.
 Complete capability curves of DFIG for different stator voltages are
developing considering grid side converter(GSC) contribution towards
RPC

4. DOUBlY FED INDUCTION GENERATOR
DFIG equipped with self-commutated insulated gate bipolar transistor
(IGBT) voltage source converter (VSC) is one of the most popular
topologies used in wind power systems.
The reactive power capability is subject to several limitations which
change with the operating point.
Around synchronous operating point, a special attention is needed
since the limitation of maximum junction temperature of the IGBTs
cause a reduction on maximum permissible output current at the rotor
side.
Simulation results show that appropriate selection of PWM type is
necessary at around synchronous speed to increase the maximum
permissible rotor current as well as reactive power capability.

5. OPERATION OF DFIG

6. The DFIG consists of a 3 phase wound rotor and a 3 phase wound stator.
As the rotor rotates the magnetic field produced due to the ac current also
rotates at a speed proportional to the frequency of the ac signal applied to
the rotor windings.
The speed of rotation of the stator magnetic field depends on the rotor
speed as well as the frequency of the ac current fed to the rotor windings.
The whole system consists of two back to back converters – a machine side
converter is used to control the active and reactive powers by controlling
the d-q components of the rotor ,torque and speed of the machine.
Grid side converter is used to maintain a constant dc link voltage and
ensures the unity power factor operation

7. REACTIVE CHARACTERISTICS DFIG
It facilitates flow of active power from generation sources
to load centers and maintains bus voltages within
prescribed limits.
Stable operation of power systems requires the availability
of sufficient reactive generation.
The presence of power electronics control in DFIG makes
them a fast acting dynamic reactive resource as
compared to direct grid connected synchronous
generators.
Reactive power capability for a wind plant is a significant
additional cost compared to conventional units which
possess inherent reactive capability.

8. MODELLING OF DFIG
 The doubly fed induction generator has been used for years for
variable speed drives.
 Using vector control techniques, the bidirectional converter
assures energy generation at nominal grid frequency and nominal
grid voltage independently of the rotor speed.
 To compensate for the difference between the speed of the rotor
and the synchronous speed with the slip control
The main characteristics may be summarized as follows:
Limited operating speed range (-30% to + 20%)
Small scale power electronic converter (reduced power losses and
price)
Complete control of active power and reactive power exchanged
with the grid
Need for slip-rings
Need for gearbox (normally a three-stage one)

9. For a DFIG associated with a back-to-back converter on the rotor
side and with the stator directly connected to the grid
 An SFOC (stator flux oriented control) system is used in order to
control separately the active and reactive power on the stator side

10. CAPABILITYCURVE OF DFIG
The power output of a generator is usually limited to value within the
MVA rating by the capability of its prime mover.
When real power and terminal voltage is fixed, its armature and field
winding heating limits restricts the reactive power generation from the
generator.
The armature heating limit is a circle with radius, centered on the origin

11.  From the figure that at 100% plant output, the use of the capability
curve does not give much additional reactive support compared to the
0.95 leading operation.
Wind parks will very seldom operate continuously at 100% output in
the periods of operation below 100%, there is significant additional
reactive power available that could aid in improved system
performance.

12. CONCLUSION
The operation of DFIG wind farm implementing a
capability curve paves the way for regulatory changes.
In general guidelines for interconnecting wind farm are
used a restricted power factor.
 When DFIG work with capability curve, fully utilizing the
potential of DFIG wind farm may be obtain at no extra
cost to the wind farm owner.
 As the levels of wind penetration continues to increase
the reactive power the certain point it should be in limit.
 At the 100% penetration the limit of reactive power in
both CC and RPF are almost same.

13. REFERENCE
S.Chandrasekaran, C.Rossi, D.Casadei, A.Tani, “Improved Control
Strategy of Wind Turbine with DFIG for Low Voltage Ride Through Capability”
International Symposium on Power Electronics, Electrical Drives, Automation
and Motion, 2012, Sorrento, Italy.
Jiabing Hu, Hailiang Xu, Yikang He: “Coordinated Control of DFIG’s RSC
and GSC Under Generalized Unbalanced and Distorted Grid Voltage
Conditions”, IEEE transactions on industrial electronics, vol. 60, no. 7,
July 2013.
T. Sun, Z. Chen, and Frede Blaabjerg, “Voltage Recovery of Grid-
Connected Wind Turbines with DFIG After a Short-circuit Fault,” 2004 35th
Annual lEEE Power Electronics Specialists Conference.
J. I. Jang, Y. S. Kim, and D. C. Lee, "Active and reactive power control of
DFIG for wind energy conversion under unbalanced grid voltage," IPEMC
Shanghai, vol. 3, pp. 1487-1491, Aug. 2006.

14. THANK YOU