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College of technology and engineering, udaipur

A seminar reportSubmitted to Ms.kusumlata mamDepartment of electrical engg.Submitted byVinay kumar maliB.Tech final year

IMPROVED REACTIVE POWER CAPABILITY WITH GRID CONNECTED DOUBLY FED INDUCTION GENERATORContent of this presentationIntroductionDoubly fed induction generatorWind turbineElectricity generation using DFIGImproved control strategies implementation of DFIGRequirements and control methodEnhancement of reactive power capabilityConclusionReferences

In the past, most national grid codes did not require wind turbines to support the power system during a disturbance.

sudden drop in frequency wind turbines were tripped off the system.

these renewable generators will, not be able to support the voltage and the frequency of the grid during

This would cause major problems for the systems stability. (tripped off the system) so design should be such that the wind turbine able to remain connected to the network during grid faults.

the wind turbine should be support the power system by supplying ancillary services, i.e. such as supplying reactive power, in order to help the grid voltage.

DFIG are very sensitive to grid disturbances, especially to voltage dips during grid faults. The abrupt drop of the grid voltage will cause over-current in the rotor windings Without any protection, this will certainly lead to the destruction of the converters

introduction

DOUBLY FED INDUCTION GENERATORIt is a 3 phase induction generator where both the rotor and stator .windings are fed with 3 phase AC signal. multi phase windings placed on both the rotor and stator bodies multiphase slip ring assembly to transfer power to the rotorGenerally the wind turbine generators work in a range of wind speed between the cut in speedand cut off speed As rotor rotates the magnetic field produced due to the ac current also rotates at a speed proportional to the freq. of the ac signal applied to the rotor windingsAs a result a constantly rotating magnetic flux passes through the stator windings which cause induction of ac current in the stator winding, depends on rotor speed and frequency of current fed to rotorSTATOR AND ROTOR MEGNETIC FIELD IN SAME DIRECTION STATOR AND ROTOR MEGNETIC FIELD IIN OPPOSITE DIRECTION

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Inner part of DFIG

transformerDFIGCONVERTERMV GRID

Wind turbine

PERFORMANCE OF THE DFIG DURING VOLTAGE DIPS

The modified vector control strategy can provide adequate control of the DFIG during grid voltage dips, its ride-through capability is limited by the relative small rating of the rotor side converter compared to the generator rating If the depth of the dip is small and the required voltage does not exceed the maximum voltage that the rotor side converter can generate, the current remains controlled. But for larger dips, an increased rotor voltage will be needed to control the rotor currents.When voltage exceeds the voltage limit of the converter, it is not possible any longer to control the current as desired. It is confirmed that if the stator voltage decreases to zero.Therefore, an additional protection device is always needed in the case of large voltage dips. Protection devices such as:-crowbar circuits, energy storage system, stator switches been used to protect DFIG during grid faults.

CONTROL METHODS

CONTROL METHODS DFIG wind turbine system with DVR( dynamic voltage regulator) shown:- The basic functions of a controller in a DVR are the detection of voltage sag/swell events in the system computation of the correcting voltage, correction of any anomalies in the series voltage injection and termination of the trigger pulses when the event has passed.

The control is based on the comparison of a voltage reference and the measured terminal voltage (Va,Vb,Vc).The voltage sags is detected when the supply drops below 90%.

Rotor side converterLine side converterDynamic voltage restorergridgearshaftRotor bladeSchematic diagram of DFIG wind turbine system with DVR

IMPROVED CONTROL STRATEGIES IMPLEMENTATION

Traditional vector control based on1 stator flux orientation or 2 stator voltage orientation has been widely usedWith this kind of control strategy, the PI controller is usually used in order to regulate independently the active and reactive powerBut when there is a sharply voltage dip on the grid side, the PI controller will get saturation easily, and it is hard to get back to the effective regulate state The command ability of the DFIG is then lost.

The researchers around the world have proposed many improved strategies to achieve LVRTThe advantage of this method is that it can be applied to all types of symmetric and asymmetric grid failures. This new method proposed to control the rotor-side converter so that the rotor current contains components in order to oppose the undesired components in the stator-flux linkage.

MODELING OF THE ACTIVE CROWBAR

The crowbar protection circuit is composed of three phase bidirectional switches and bypass resistors

The behavior of such systems during grid faults is greatly affected by the resistor value of crowbar

Low crowbar resistance leads to a higher electrical torque, over currents and low rotor voltages. And vice-versa

Therefore; the crowbar resistors should be sufficiently low to avoid large voltages on the converter terminals. On the other hand, they should be high enough to limit the rotor current.

DFIG based wind turbine with crowbar protection

DFIGWINDTURBINECrowbar protectionconverterInductor and resistorsTo grid

LVRT STRATEGIES WITH HARDWARE IMPLEMENTATION

In doubly fed wind power generation systems, the capacity of rotor side converter is small compared with the rated capacity of generator, the rotor side converter can provide partial control of the generator. Therefore when power system faults occur and a deep drop of generator terminal voltage occurs, the rotor side converter will lose the control of rotor currents. That is why an additional hardware protection circuit is necessary

Hence a path for the rotor over current is provided, so that the rotor side converter can be well protected

new active crowbars, using active switches such as IGBT and GTO, due to low operation process of thyristor the power system can be more flexible, taking less time to return to a normal operating mode

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PMSGRectifierInverterLVRT moduleField busgridTHEDIAGRAMOFLVRTPRINCIPLE

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ENHANCEMENT OF REACTIVE POWER CAPABILITY

According to short term interruption (STI) is allowed under specific circumstances. It requires resynchronization within 2 s and a power increase rate of at least 10% of the nominal power per second.In area 2 the interruption time allowd is much less, just a few hundred milliseconds.wind turbines have to supply at least 1.0 p.u. reactive current already when the voltage falls below 50%

conclusionWhen 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, which not only facilities reduced system losses but also improves the post fault voltage recovery following a disturbanceThe real and reactive power capability of the DFIG model is analyzed for various firing angles .The reactive power production is improved.

ReferencesInternational Journal of Engineering Innovation & Research Volume 1, Issue 5, ISSN : 2277 5668IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 26, NO. 3, SEPTEMBER 2011 A Fault Ride-Through Technique of DFIG Wind Turbine Systems Using Dynamic Voltage Restorers Fault- Ride Through of a DFIG Wind Turbine using a Dynamic Voltage Restorer during Symmetrical and Asymmetrical Grid Faults ISSN (PRINT) : 2320 8945, Volume -1, Issue -4, 201Thank you