IMPROVED REACTIVE POWER CAPABILITY WITH GRID CONNECTED DOUBLU FED INDUCTION GENERATOR

CONTENT

INTRODUCTION

DOUBLY FED INDUCTION GENERATOR

OPERATION

REACTIVE CHARACTERISTICS OF DFIG

MODELLING OF DFIG

CAPABILITY CURVE OF DFIG

CONCLUSION

REFERENCE

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

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.

OPERATION OF DFIG

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

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.

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)

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

CAPABILITY CURVE 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

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.

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.

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.

THANK YOU