1. D. Aguglia & R. Rebeschini
Power Transformer Role for Gearbox Mechanical Stress Mitigation during Voltage Dips Applied to Doubly-Fed Induction Generator Based Wind Turbines
D. Aguglia & R. Rebeschini
Trasfor S.A. , Customized Transformers & Inductors
6995-Molinazzo di Monteggio, Switzerland.

2. Outline Motivations - Gearbox failures statistics
Crowbar protection system for DFIG
Worst fault conditions & gearbox mechanical stress
Series transformer connection as a solutions for gearbox protection
Adapted specifications for a series transformer
Conclusion

3. Motivations - Gearbox failures statistics
Gears failures issues (data: 2000–2004, Sweden)
Failure frequency
Downtime
Data from: J. Ribrant, « Reliability performance and maintenance – A survey of failures in wind power systems », M.Sc. Thesis.

4. Crowbar protection system for DFIG
Main purpose : Power converter protection during grid faults
Crowbar designed to protect rotor side power converter.
Specifications impose to remain connected to grid during faults/voltage dips.
This protection system impose an over dimensioning of the gearbox!

5. Worst fault conditions & gearbox mechanical stress
3-phase to ground short-circuit on a 1.5MW DFIG using crowbar protection system.
Worst case at full load operation !

6. Worst fault conditions & gearbox mechanical stress
Phase to ground short-circuit on a 1.5MW DFIG using crowbar protection system.
Worst case at full load operation !

7. Worst fault conditions & gearbox mechanical stress
Phase to phase to ground short-circuit (to 0V) on a 1.5MW DFIG using crowbar protection system.
Worst fault!
Worst case at full load operation !

8. Worst fault conditions & gearbox mechanical stress
Summary for crowbar protection system efficiency
Designed to protect power converter.
Not suitable for gearbox/gears protection.
Implies gearbox over-dimensioning.
Faulty peak torque must be reduced!

9. Series transformer connection as a solutions for gearbox protection
Series connection to the grid
Classic shunt connection
Injection of series voltage during Grid voltage dips
Voltage dip seen by DFIG
Faulty DFIG currents and torque attenuated!
Series connection

10. Series transformer connection as a solutions for gearbox protection
Series connection to the grid
…through H Bridges
Advantages: possibility to compensate unbalances during “normal operation”.
Drawback: more switches!

11. Series transformer connection as a solutions for gearbox protection
Series connection to the grid
…through conventional 2-level power converter
Advantages: classic topology with less switches.
Drawback: difficult to compensate asymmetrical faults.

12. Adapted specifications for a series transformer
Design specifications of series transformer
The series connection system should be designed taking into account the steady-state and fault operation.
Steady-state analysis
Active power balance imposed by DFIG!
Turbine’s power is a cubic function of the wind speed

13. Adapted specifications for a series transformer
Steady-state analysis
DFIG stator current flowing through series transformer : Igrid = Is
In hypo-synchronous operation DFIG stator voltage increases! grid Grid voltage selection issues!
In hyper-synchronous operation DFIG stator voltage decreases! Series voltage already with the right phase angle. Ready to react against faults!!!
Transformer maximal voltage ~0.5 p.u.

14. Adapted specifications for a series transformer
Steady-state analysis
- Transformer maximal current 1 p.u.!!!
- Max voltage 0.5 p.u.
- Transformer apparent power increased!!!
Transformer & converter ratings
Different currents seen by the GSC – losses are not the same!

15. Adapted specifications for a series transformer
Fault operation analysis
Worst DFIG operation range for faults behavior
Series voltage already in the right polarity & ready for protection during faults.
If series transformer rated at 0.5 p.u., possibility to provide 50% of voltage.
That means the worst voltage dip seen by DFIG is 50% only…enough to protect gearbox!

16. Conclusion
Crowbar protection system contributes to gearbox mechanical stress – need for over-dimensioning.
Series transformer topology very interesting for fault ride-through capabilities & decrease of gearbox dimensions.
Steady-state specifications analysis is enough to guarantee a good series transformer design for gearbox mechanical stress reduction.
Series transformer will have a slight apparent power increase compared to shunt topology.

17. Please refer to our paper for details and contact us:
Thank you very much!
Trasfor S.A. , Customized Transformers & Inductors
6995-Molinazzo di Monteggio, Switzerland.
Please refer to our paper for details and contact us:

18. Worst fault conditions & gearbox mechanical stress
Peak torque & peak rotor voltages depends on fault time occurrence.

19. Power transformer role & Solutions for new protection systems
As WT size increases, generator’s voltages will be higher and higher.
A power transformer for DFIG rotor connection to the grid will be needed!
OR: