power transformer role for gearbox mechanical stress mitigation during voltage dips applied to...
TRANSCRIPT
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
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:[email protected]@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!