Power Transformer Role for Gearbox Mechanical Stress Mitigation during Voltage Dips Applied to

Doubly-Fed Induction Generator Based Wind Turbines

D. Aguglia & R. Rebeschiniwindpower@trasfor.chrebeschini@trasfor.ch

Trasfor S.A. , Customized Transformers & Inductors6995-Molinazzo di Monteggio, Switzerland.

www.trasfor.com

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

2

Motivations - Gearbox failures statistics

3

Failure frequency Downtime

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

6,7 %13,4 %

5,5 %

17,5 %

5,6 %13,3 %

1,2 %9,8 %

14,1 %

12,9 %

Electrical system

GeneratorBlades /

pitch

Yaw System

Hydraulics

Mechanical Brakes

Gears

Sensors

Control system

Rest

13,3 %

9,4 % 8,9 %

14,3 %

5,3 %

4,4 %1,2 %

19,4 %

5,4 %

18,3 %

Electrical system

GeneratorBlades /

pitch

Yaw system

Hydraulics

Mechanical Brakes

Gears

Sensors

Control system

Rest

• Gears failures issues (data: 2000–2004, Sweden)

Crowbar protection system for DFIG

4

• Main purpose : Power converter protection during grid faults

Specifications impose to remain connected to grid during faults/voltage dips.

Crowbar designed to protect rotor side power converter.This protection system impose an over dimensioning of the gearbox!

Worst fault conditions & gearbox mechanical stress

5

3-phase to ground short-circuit on a 1.5MW DFIG using crowbar protection system.

0 0.05 0.1-5

0

5

Time [s]

Sta

tor

curr

en

ts i s [p

.u.]

0 0.05 0.1

-5

0

5

Time [s]

Ro

tor

curr

en

ts i r [p

.u.]

0 0.05 0.10

0.5

1

1.5

2

2.5

Time [s]

To

rqu

e T

e [p.u

.]

0 0.05 0.1

-500

0

500

Time [s]

Ro

tor

Vo

ltag

e v

r' [V]

(a) (b)

(d)(c)

Worst case at full load operation !

Worst fault conditions & gearbox mechanical stress

6

Phase to ground short-circuit on a 1.5MW DFIG using crowbar protection system.

0 0.05 0.1-4

-2

0

2

4

Time [s]

Sta

tor

curr

en

ts i s [p

.u.]

0 0.05 0.1

-4

-2

0

2

4

Time [s]

Ro

tor

curr

en

ts i r [p

.u.]

0 0.05 0.1-1

0

1

2

Time [s]

To

rqu

e T

e [p.u

.]

0 0.05 0.1-400

-200

0

200

Time [s]

Ro

tor

volta

ge

s v r' [V

]

(a) (b)

(c) (d)

Worst case at full load operation !

Worst fault conditions & gearbox mechanical stress

7

Phase to phase to ground short-circuit (to 0V) on a 1.5MW DFIG using crowbar protection system.

0 0.05 0.1

-5

0

5

Time [s]

Sta

tor

curr

en

ts [p

.u.]

0 0.05 0.1

-5

0

5

Time [s]

Ro

tor

curr

en

ts [p

.u.]

0 0.05 0.1

0

1

2

Time [s]

To

rqu

e [p

.u.]

0 0.05 0.1

-500

0

500

Time [s]

Ro

tor

volta

ge

s V

r' [V]

(a) (b)

(c) (d)

Worst fault! Worst case at full load operation !

Worst fault conditions & gearbox mechanical stress

8

• 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!

Series connection

Series transformer connection as a solutions for gearbox protection

9

Classic shunt connection• 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 transformer connection as a solutions for gearbox protection

10

…through H Bridges• Series connection to the grid

Advantages: possibility to compensate unbalances during “normal operation”.

Drawback: more switches!

Series transformer connection as a solutions for gearbox protection

11

• Series connection to the grid…through conventional 2-level power converterAdvantages: classic topology with less switches.

Drawback: difficult to compensate asymmetrical faults.

Adapted specifications for a series transformer

12

• Design specifications of series transformerThe series connection system should be designed taking into account the steady-state and fault operation.

s

P

s

PP

s

sPP

turbrs

turbr

1

1

.

.

• Steady-state analysis

Active power balance imposed

by DFIG! -0.200.20.40.60.8-0.2

0

0.2

0.4

0.6

0.8

Slip [-]

Po

we

r [p

.u.]

Pturb.

Ps

Pr

Turbine’s power is a cubic function of the wind speed

Adapted specifications for a series transformer

13

• Steady-state analysis

gridsturbr

s

gridseriesturbr

IVs

P

s

PP

IVs

sPP

31

31

.

.

DFIG stator current flowing through series transformer : Igrid = Is

-0.2-0.100.10.20.3

-0.5

0

0.5

1

1.5

Slip [-]

Vo

ltag

e [p

.u.]

Vs

Vseries

Vgrid• 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.

-0.2-0.100.10.20.30

0.2

0.4

0.6

0.8

1

Slip [-]

Cur

rent

[p.

u.]

Igrid

-0.2-0.100.10.20.3

0

0.2

0.4

0.6

0.8

1

Slip [-]

Cur

rent

[p.

u.]

Is

Ir

Igrid

Adapted specifications for a series transformer

14

• Steady-state analysisTransformer & converter ratings

- Transformer maximal current 1 p.u.!!! - Max voltage 0.5 p.u.- Transformer apparent power increased!!!

Different currents seen by the GSC – losses are not the same!

Adapted specifications for a series transformer

15

• Fault operation analysis

-0.2-0.100.10.20.3

-0.5

0

0.5

1

1.5

Slip [-]

Vo

ltag

e [p

.u.]

Vs

Vseries

Vgrid

Series voltage already in the right polarity & ready for protection during faults.

Worst DFIG operation range for faults behavior

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!

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.

Thank you very much!

17

Trasfor S.A. , Customized Transformers & Inductors6995-Molinazzo di Monteggio, Switzerland.

www.trasfor.com

Please refer to our paper for details and contact us:windpower@trasfor.chrebeschini@trasfor.ch

Worst fault conditions & gearbox mechanical stress

18

0 0.005 0.01 0.015 0.021

1.5

2

2.5

3

Short-circuit instant [s]

peak

torq

ue [p

.u.]

0 0.005 0.01 0.015 0.02300

350

400

450

500

550

600

650

Short-circuit instant [s]

peak

rot

or v

olta

ge V

r [V

]

• Peak torque & peak rotor voltages depends on fault time occurrence.

Power transformer role & Solutions for new protection systems

19

• As WT size increases, generator’s voltages will be higher and higher.

OR:A power transformer for DFIG rotor connection to the grid will be needed!