D. Aguglia & R. Rebeschini windpower@trasfor.ch rebeschini@trasfor.ch

1. Power Transformer Role for Gearbox Mechanical Stress Mitigation during Voltage Dips Applied to Doubly-Fed Induction Generator Based Wind Turbines D. Aguglia & R. Rebeschini • windpower@trasfor.ch • rebeschini@trasfor.ch Trasfor S.A. , Customized Transformers & Inductors 6995-Molinazzo diMonteggio, Switzerland. www.trasfor.com

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) • Downtime Data from: J. Ribrant, « Reliability performance and maintenance – A survey of failures in wind power systems », M.Sc. Thesis. Failure frequency

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 Worst case at full load operation ! 3-phase to ground short-circuit on a 1.5MW DFIG using crowbar protection system.

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

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

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

9. Series transformer connection as a solutions for gearbox protection • Series connection to the grid Injection of series voltage during Grid voltage dips Voltage dip seen by DFIG Faulty DFIG currents and torque attenuated! Series connection Classic shunt 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. Turbine’s power is a cubic function of the wind speed • Steady-state analysis Active power balance imposed by DFIG!

13. Adapted specifications for a series transformer • Steady-state analysis DFIG stator current flowing through series transformer : Igrid = Is Transformer maximal voltage ~0.5 p.u. • 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!!!

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. Thank you very much! Trasfor S.A. , Customized Transformers & Inductors 6995-Molinazzo diMonteggio, Switzerland. www.trasfor.com Please refer to our paper for details and contact us: windpower@trasfor.ch rebeschini@trasfor.ch

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: