The document discusses issues related to doubly fed induction generators (DFIGs) used in wind turbines remaining connected to the power grid during faults. It notes that grid codes now require renewable generators to provide ancillary services like reactive power during faults to support voltage and frequency. While DFIGs are sensitive to voltage dips, various protection methods like crowbar circuits, energy storage, and dynamic voltage restorers can help DFIGs ride through faults by limiting current surges. The document examines different control strategies and protection devices that allow DFIG wind turbines to meet grid code low voltage ride-through requirements.

1. A seminar report
Submitted to
Ms.kusumlata mam
Department of electrical engg.
Submitted by
Vinay kumar mali
B.Tech final year

2. Content of this presentation

3. • 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

4. DOUBLY FED INDUCTION GENERATOR
• It 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 rotor
• Generally 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 windings
• As 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 rotor
STATOR AND ROTOR MEGNETIC FIELD IN SAME DIRECTION STATOR AND ROTOR MEGNETIC FIELD IIN OPPOSITE DIRECTION
N
N
s s
N
N
s s
Inner part of DFIG

6. transformer
DFIG
CONVERTER
MV
GRID
Wind
turbine

7.  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.

8.  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%.

9. Rotor side
converter
Line side
converter
Dynamic voltage restorer
grid
gear shaft
Rotor blade

10. • Traditional vector control based on
1 stator flux orientation or
2 stator voltage orientation has been widely used
• With this kind of control strategy, the PI controller is usually used in order to
regulate independently the active and reactive power
• But 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 LVRT
• The 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.

11.  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.

12. DFIG based wind turbine with crowbar protection
DFIGWIND
TURBINE
Crowbar protection
converter
Inductor and
resistors
To grid

13.  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

14. mastervf
PMSG
Rectifier
Inverter
LVRT module
Field bus
grid
N
N
N
N
S
S
S
S

15. • 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%

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

17. • International Journal of Engineering Innovation &
Research Volume 1, Issue 5, ISSN : 2277 – 5668
• IEEE 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, 201
Thank you