GOVT. COLLEGE OF ENGG. CHANDRAPUR

With Grid Connected Doubly Fed Induction GeneratorImproved Reactive Power CapabilityGandhi Institute of Engg. & Technology Gunupur

A Brief Overview on Reactive PowerActive power is the energy supplied to run an Electric Appliances but reactive power is what that provides the important function of regulating voltage.

If voltage on the system is not high enough, active power cannot be supplied.

Reactive power is used to provide the voltage levels necessary for active power to do useful work.

Reactive power is essential to move active power through the transmission and distribution system to the customer.

DragThrustLiftGravityReactive PowerActive PowerLift does not get you any closer to your destination but without it you are not flyingINTRODUCTIONThe reactive power capability of DFIG depends on three parameter (rotor voltage, rotor current, stator current) for reactive power consumption.

From the principle of DFIG, the total reactive power generation is limited by rotor voltage at low speeds and by rotor currents at higher speed.

Reactive Power Capability also depends on the factor of selecting Power Converters.

However Complete Reactive Power Capability also depends on the Grid Side Converters.DOUBlY FED INDUCTION GENERATORDoubly Fed Electric Machinesareelectric motorsorelectric generatorsthat have windings on both stationary and rotating parts, where both windings transfer significant active power between shaft and electrical system.

Usually the stator winding is directly connected to the three-phase grid and the three-phase rotor winding is fed from the grid through a rotating or static frequency converter.

Doubly fed machines are typically used in applications that require varying speed of the machine's shaft in a limited range around the synchronous speed.

Today doubly fed drives are the most common variable speed wind turbine concept.

DOUBLY FED INDUCTION GENERATORThe 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 HOW IT OPERATES??HOW IT OPERATES??

REACTIVE CHARACTERISTICS DFIGIt 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.CONCLUSIONThe 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.THANK YOU ALL !!!