D. Aguglia & R. Rebeschini [email protected] [email protected]
description
Transcript of D. Aguglia & R. Rebeschini [email protected] [email protected]
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Power Transformer Role for Gearbox Mechanical Stress Mitigation during Voltage Dips Applied to
Doubly-Fed Induction Generator Based Wind Turbines
D. Aguglia & R. [email protected]@trasfor.ch
Trasfor S.A. , Customized Transformers & Inductors6995-Molinazzo di Monteggio, Switzerland.
www.trasfor.com
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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
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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)
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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!
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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 c
urre
nts
i s [p.u
.]
0 0.05 0.1
-5
0
5
Time [s]
Rot
or c
urre
nts
i r [p.u
.]
0 0.05 0.10
0.5
1
1.5
2
2.5
Time [s]
Torq
ue T
e [p.u
.]
0 0.05 0.1-500
0
500
Time [s]
Rot
or V
olta
ge v
r' [V]
(a) (b)
(d)(c)
Worst case at full load operation !
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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 c
urre
nts
i s [p.u
.]
0 0.05 0.1
-4
-2
0
2
4
Time [s]
Rot
or c
urre
nts
i r [p.u
.]
0 0.05 0.1-1
0
1
2
Time [s]
Torq
ue T
e [p.u
.]
0 0.05 0.1-400
-200
0
200
Time [s]
Rot
or v
olta
ges
v r' [V]
(a) (b)
(c) (d)
Worst case at full load operation !
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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 c
urre
nts
[p.u
.]
0 0.05 0.1
-5
0
5
Time [s]
Rot
or c
urre
nts
[p.u
.]
0 0.05 0.1
0
1
2
Time [s]
Torq
ue [p
.u.]
0 0.05 0.1
-500
0
500
Time [s]
Rot
or v
olta
ges
V r' [V]
(a) (b)
(c) (d)
Worst fault! Worst case at full load operation !
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Worst fault conditions & gearbox mechanical stress
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• 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!
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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!
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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!
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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.
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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.
sP
sPP
ssPP
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 [-]
Pow
er [p
.u.]
Pturb.
Ps
Pr
Turbine’s power is a cubic function of the wind speed
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Adapted specifications for a series transformer
13
• Steady-state analysis
gridsturbr
s
gridseriesturbr
IVs
PsPP
IVssPP
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 [-]
Vol
tage
[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.
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-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!
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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 [-]
Vol
tage
[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!
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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.
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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:[email protected]@trasfor.ch
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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
roto
r vol
tage
Vr [
V]
• Peak torque & peak rotor voltages depends on fault time occurrence.
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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!