useful for the technical students

1. POWER ELECTRONIC CONVERTER
IN WIND TURBINE
Prepared By:-
SONU KUMAR BAIRWA

2. INTRODUCTION
• By controlling the active power of
the converter, it is possible to vary the
rotational speed of the generator, and hence
the speed of the rotor of the wind turbine. ...
The power electronic converters for
variable-speed generators have the ability to
control both the active and
reactive power delivered to the grid.

4. Soft starting of grid-connected WTs.
• switching to the grid of WTs equipped with
induction generators (soft starting) .
• Direct connection of the induction generator to
the grid results in high inrush currents, which
are undesirable particularly in the case of weak
grids and can also cause severe torque
pulsations and probably damage to the gearbox.
• For this reason AC voltage regulators are
frequently employed,
• As illustrated in figure 1, utilizing phase-
controlled anti parallel thyristors switches for
the regulation of the applied stator voltages.

5. • Usually, the turbine accelerates under pitch-
control to the synchronous speed through the
wind power alone and only then is it switched
onto the grid.
• AC controllers have been also reported for the
connection to the grid at zero speed and
subsequent fast acceleration to the operating
speed, which is particularly useful in stall
controlled WTs.

6. • After the synchronization of the turbine the
thyristor switches are normally by-passed. It is
possible, however, to utilize the AC controller in
the normal operation of the turbine, in order to
reduce its stator voltage and hence the
magnetizing losses, when operating under light
wind.

7. Variable speed Wind turbines
• The wind power captured by the turbine rotor
and converted to mechanical power is dependent
not only on the average wind speed over the
rotor surface, but on the rotational speed of the
rotor as well.
• Therefore, maximum wind energy capture can
be achieved only if the rotor speed is varied,
tracking the changes of the wind, for optimal
rotor aerodynamic power coefficient.

8. • Additional advantages associated with variable
speed operation are the reduction of the drive
train mechanical stresses, which permits the use
of lighter and cost-effective transmissions, the
improvement of the output power quality and
the reduction of the acoustical noise emitted
from the WT.
• If the electrical generator of the WT is directly
coupled to the grid, then its rotor speed is
practically fixed by the constant network
frequency.

9. • In order to achieve variable speed operation, a
frequency converter must in general be inserted
between the stator or the rotor of the generator
and the grid, in exactly the same way as for any
variable speed electrical drive.
• Hence, the variable speed WT technology is
directly transferred from the field of electric
drives and inevitably benefits from the great
advances made in it.
• In the following, three main types of variable
speed WT electrical drives are separately
examined and discussed:

10. • 1. WTs equipped with squirrel cage induction
generator, connected to the grid through a stator
converter cascade.
• 2. WTs equipped with wound rotor induction
generator, connected to the grid through a rotor
converter cascade or a cycloconverter.
• 3. WTs equipped with synchronous generator
and a stator DC-link cascade for network
connection.

11. WTs equipped with squirrel cage
induction generator
• Squirrel cage induction generators are the most common
choice for small and medium size WTs due to their
ruggedness, low cost and maintenance-free operation.
For VS operation they are connected to the network
through a variety of stator DC-link converter cascades.

12. • From a converter point of view the simplest possible
configuration utilizes a self-excited induction
generator through a capacitor bank connected at its
stator terminals, as illustrated in figure 2,
• An uncontrolled diode bridge rectifier or a naturally
commutated thyristor converter can be connected at
the machine terminals, since the capacitors can
provide the necessary reactive power for their
operation.
• The network-side inverter can also be a simple line-
commutated thyristor bridge.
• Such a scheme, however, is not favored in practice
due to the complexity introduced by the self
excitation of the induction generator and resonance
problems, particularly in the lower speed region,

13. • In figure 3(a) another scheme is shown, where a
thyristor force-commutated current source
inverter (CSI) is connected at the generator
terminals and a line-commutated thyristor
bridge converter at the grid side,

15. • The CSI has been the most common choice of
industrial induction motor drives of medium
power for many years and its technology is
mature and well-known.
• Its main advantages are its reliability and low-
cost and its inherent ability to operate in
motoring or regenerative mode by simply
reversing the DC-link voltage.
• Disadvantages include the commutation circuits,
which add complexity to the converter and the
large machine current harmonics which give rise
to torque pulsations.

16. • In general, force-commutated current-fed
thyristor inverters are becoming obsolete and
are gradually replaced by GTO current-fed PWM
converters,
• Besides, the 6- pulse line-commutated converter
is characterized by very poor power factor and
generates significant low order harmonics into
the network, which are both issues of increasing
concern as the penetration of the wind power
increases, particularly in weak grids.

17. • The control system structure for the variable
speed WT drive of figure 3(a) is shown in figure
3(b), . The measured wind speed determines the
turbine speed reference and subsequently the
induction generator stator frequency.
• A ordinary constant volts/Hz ratio control
scheme is adopted. Compared with high-
performance industrial drives, the performance
requirements for the electrical drives of WTs are
relatively reduced, and hence advanced control
methods, such as the field oriented control, only
recently have begun to find their way in WT
applications.

18. • There is a significant trend in industrial
induction motor drives towards PWM converters
of the voltage source type , which inevitably
affects the variable speed WT technology.
• The use of pulse width modulation techniques
results in almost sinusoidal machine voltages
and currents,

19. • and thus in enhanced dynamic behavior, better
efficiency and absence of torque pulsations and
consequent unwanted mechanical resonances in
the drive train.
• In figure 4 a dual PWM voltage-source converter
cascade is utilized, similar in principle to the
schemes of .
• In standard industrial drives of this type it is
customary to use at the grid side a simple line-
commutated bridge, for cost reduction reasons.
However, the phase controlled converter, apart
from the power factor and harmonics problems,

20. • does not permit both generator and motor
operation (e.g. for accelerating the WT during
starting up), unless a second anti-parallel bridge
is added at the output. Utilizing a second PWM
converter at the output, a solution which is
gaining momentum,
• bidirectional power flow is possible, while the
injected harmonics to the grid are minimized
and the output power factor can be regulated at
unity (or even leading, if the converter rating
permits it, providing thus voltage support to
weak grids).

22. • In figure 5 the control system structure for the
converters of figure 4 is shown, .
• The machine-side converter performs the speed
control of the WT, using the wind speed as input
and assuming a constant V/f ratio for the
induction generator.
• The grid side converter controls the power flow
to the bus and hence the DC capacitor voltage, as
well as the output power factor.