Wind turbines use power electronics like doubly fed induction generators (DFIGs) consisting of wound rotors, induction generators, and AC/DC/AC converters. DFIGs allow variable speed operation, reduce converter size/cost, and enable reactive power control. They work by converting the turbine's mechanical energy to electrical energy. The stator is directly connected to the grid while the rotor is fed at variable frequency by the converter. The converter controls the rotor speed and active/reactive power flow. DFIGs offer advantages like low losses, compact design, and speed regulation from ±20-25% but have disadvantages like slip ring maintenance and complex control.

1. Power Electronics in Wind Turbine Systems
Manasa.K
CWB0912002, FT-2012
M. Sc. (Engg.) in Electronic System Design
Engineering
Module leader: Mr. R Sreekrishna
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2. Contents
• Introduction
• Modern Power Electronics
• Wind Energy Conversion
• Doubly Fed Induction Generator
• Operating Principal of DFIG
• Advantages
• Disadvantages
• Conclusion
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3. Introduction
• Wind turbine technology is the most promising renewable energy technology. It
started in 1980’s with a few tens of kW production of power per unit. And today
multi-MW size wind turbines are being installed.
• Wind power production in the beginning, did not have any impact on the power
control system and was based on the induction generator where the pulsations in
the wind was directly transferred to the grid. There was no control on active and
reactive power which are the important control parameter to regulate frequency
and voltage.
• As the power range of the turbines increases those control parameters become
more important and it is necessary to introduce power electronics as an interface
between the wind turbine and the grid. The power electronics is changing the
basic characteristic of the wind turbine from being an energy source to be an
active power source.
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4. Modern Power Electronics
The interface of Wind power converter between generator and power grid should
satisfy the requirements on both the sides. It has to store the active power and boost up
the voltage from generator side to grid side.
• Generator side: * It should control stator current and adjust the rotating speed.
* Extract maximum power from turbine.
• Power grid side: * It should have the ability to control the inductive/capacitive
reactive power and perform fast active power response.
*Frequency and voltage should be fixed for normal operation
* Harmonic distortion should be maintained low.
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5. Wind Energy Conversion
Wind energy conversion systems convert wind energy into electrical energy, which is
then fed into electrical grid.
• The turbine rotor, gear box and generator are the main three components for energy
conversion.
• Rotor converts wind energy to mechanical energy.
• Gear box is used to adapt to the rotor speed to generator speed.
• Generator with the variable speed wind turbine along with electronic inverter
absorbs mechanical power and convert to electrical energy.
• The power converter can not only transfer the power from a wind generator, but
also improve the stability and safety of the system.
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6. Doubly Fed Induction Generator
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7. Doubly Fed Induction Generator
• Wind turbines use a doubly-fed induction generator (DFIG) consisting of a
wound rotor , induction generator and an AC/DC/AC IGBT-based PWM
converter.
• The stator winding is connected directly to the grid while the rotor is fed at
variable frequency through the AC/DC/AC converter.
• Vr is the rotor voltage and Vgc is grid side voltage.
• The AC/DC/AC converter is basically a PWM converter which uses sinusoidal
PWM technique to reduce the harmonics present in the wind turbine driven
DFIG system.
• Crotor is rotor side converter and Cgrid is grid side converter.
• To control the speed of wind turbine gear boxes or electronic control can be used
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8. Operating Principal of DFIG
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9. Operating Principal of DFIG
•
Below the synchronous speed in the motoring mode and above the synchronous speed in the
generating mode, rotor-side converter operates as a rectifier and stator-side converter as an
inverter and where slip power is returned to the stator.
•
Below the synchronous speed in the generating mode and above the synchronous speed in
the motoring mode, rotor-side converter operates as an inverter and stator side converter as
a rectifier, where slip power is supplied to the rotor.
•
For super synchronous speed operation, Pr is transmitted to DC bus capacitor and tends to
rise the DC voltage. For sub-synchronous speed operation, Pr is taken out of DC bus
capacitor and tends to decrease the DC voltage.
•
•
Cgrid is used to generate or absorb the power Pgc in order to keep the DC voltage constant.
In steady-state for a lossless AC/DC/AC converter Pgc is equal to Pr and the speed of the
wind turbine is determined by the power Pr absorbed or generated by Crotor.
•
The phase-sequence of the AC voltage generated by Crotor is positive for sub synchronous
speed and negative for super synchronous speed.
Crotor and Cgrid have the capability for generating or absorbing reactive power and could
be used to control the reactive power or the voltage at the grid terminals.
•
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10. Advantages of DFIG’s
• Advantages of the system include, low losses, which assures a
high overall efficiency, and an outstanding availability due to the
compact design with a minimal number of components
• Significantly reduced power rating and cost of the converter
• Possible speed regulation for optimal utilization of energy
(typically ±20-25%)
• Sub-synchronous and Super-synchronous operation is possible
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11. Disadvantages of DFIG’s
• Slip rings wear and tear, maintenance
• Complex control of the entire unit
• Direct connection to the grid is somewhat difficult
• The output voltage is dependent on the rotor speed and stator.
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12. Summary
• Wind energy conversion systems convert wind energy into electrical energy,
which is then fed into electrical grid.
• Power electronics as an interface between the wind turbine and the grid.
• Power converter is used to store the active power and boost up the voltage from
generator side to grid side and it also reduces harmonics.
• DFIG is a AC/DC/AC IGBT-based PWM converter.
• Crotor and Cgrid have the capability for generating or absorbing reactive power
and could be used to control the reactive power or the voltage at the grid
terminals
• Advantages and disadvantages.
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13. References
[1] F. Blaabjerg, Z. Chen Power electronics in Wind Turbine System
Aalborg University, Institute of Energy Technology Denmark.
[2] Prof. K. B. Mohanty Thesis on Study of wind turbine driven
DFIG using ac/dc/ac converter National Institute of Technology
Rourkela.
[3] E. Sheeba Percis Comparative Analysis of Variable Speed Wind
Energy Conversion Systems Dr. MGR University Second
International Conference on Sustainable Energy and Intelligent
System July 20-22, 2011 Chennai.
[4] Macro Liserre Power Converters and Controls of Renewable
Energy Systems CEMD Research Group Italy.
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14. Thank You
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