Wind Power Transformer Design
By Philip J Hopkinson, PE
Wind Power
Transformer at base of
Tower
Critical link to
successful Wind
Turbine connection to
the grid
Subject of Many
complaints
1.
2.
3.
4.
5.
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Doble Engineering Conference 021913
Gassing
Winding Failures
Contact coking
Arc-Flash
Low liquid level
1
Wind Power Transformer Design
By Philip J Hopkinson, PE
1. Consideration focused on Generator step-up
Transformers from 600 volt to 34,500 volt
2. Inputs from field experience
3. Standards work within IEC TC 14 and IEEE
4. Arc-Flash Safety considerations from NFPA 70E 2009
5. Work with Manufacturers of Transformers
6. Analytical studies
7. Attempt to develop meaningful universal standard
requirements for transformers rated up to 10 MVA.
8. IEEE Transformer Working Group P60076-16
9. IEC Document 60076-16
10. Currently 100 members on IEEE Working Group
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Typical Wind Turbine
transformer
1. 3-Phase Pad mounted
2. 600 volt low voltage Gr Y
3. 34,500 volt high voltage
4. Commonly Wye high voltage
5. Sometimes Delta high voltage
6. Most with 5-leg wound cores
7. Some with 3-leg stacked cores
8. Nearly 100% liquid filled
9. Nearly all with sheet low voltage
10. All with wire high voltages
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Typical Wind Turbine transformer
concerns
1.
2.
3.
4.
High Hydrogen gas
High voltage winding failures
Safety of HV load-break switch
Carbonized HV Switches and Tap
changers
5. Low liquid levels in cold
temperatures
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Wind Power Transformer Design
By Philip J Hopkinson, PE
High Hydrogen Gas
1. Present in Up to 50% of
transformer populations
2. Hydrogen as high as 20,000 ppm
3. Small amounts Ethane and
Ethylene and Methane
4. Windings apparently OK
5. Most transformers returned to
factories pass Routine Tests,
including Impulse.
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Wind Power Transformer Design
By Philip J Hopkinson, PE
High Hydrogen Gas always partial
discharge related
1.
2.
3.
4.
5.
1/22/2013
But windings not involved
Leads not involved in most cases
Hot spots in windings OK
Some gassing improves with time
If not windings or leads then what
else?
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Wind Power Transformer Design
By Philip J Hopkinson, PE
What about the iron core?
1. Magnetic energy reservoir
2. Voltage generator from changing
flux
3. Many laminations of thin steel
4. Very thin inter-laminar insulation
5. High inter-laminar capacitance
6. Susceptible to static charge
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Transformer Core Grounds in Wound Cores
By Philip J Hopkinson, PE
1.
2.
3.
4.
5.
6.
7.
1/22/2013
The issue is gassing and Partial Discharge
Dielectric Breakdown in windings not the problem
Gassing and PD worst at 34.5 kV
Gassing not an issue at 5 kV
Gassing at 15 kV an annoyance only
Gassing coming from Core
Solution via grounding or shielding
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Transformer Core Grounds in Wound Cores
By Philip J Hopkinson, PE
Ground Lead
Vi Outer Core
Vi Inner Core
Vi Inner Core
Vi Outer Core
L
C5
C2
C1
C5
LV
L
C3
C4
C1
C1
C4
C5
C4
C1
C1
C5
C4
C3
C1
C2
C3
C5
C5
Grounded Clamp
HV
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Transformer Core Grounds in Wound Cores
By Philip J Hopkinson, PE
Ground Lead
Vi Outer Core
Vi Inner Core
Vi Inner Core
Vi Outer Core
L
C5
C2
C1
C5
LV
L
C3
C4
C1
C1
C4
C4
C5
1/22/2013
C1
C4
C3
C5
C1
C2
C3
C5
C5
Grounded Clamp
HV
HV = 34500
LV = 600 Gr Y
C1
kVA = 1850
Frequency = 60 Hz
C1-C5 calculated
In EXCEL
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Transformer Core Grounds in Wound Cores
By Philip J Hopkinson, PE
Vi Outer Core Loop Inside Surface
804 Volts
LV Start is Grounded
0 Volts
HV
C2
C1
L-G Volts
19919
C3
Core Ground
0 Volts
Case I, Core Ground Connected to Frame
Voltage drop in Outer Core stacks =
804 Volts
1.
2.
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Outside core grounds at 34.5 kV result in high core volts
Inside core grounds eliminate static charges and drop core
volts to volt/turn levels
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Transformer Core Grounds in Wound Cores
By Philip J Hopkinson, PE
0.030" thick static shield
0.020" thick paper
Outside Core Leg
Core Ground
Grounded Electrostatic Shield
1.
2.
3.
1/22/2013
Outside core grounds at 34.5 kV result in high core volts
Inside core grounds eliminate static charges and drop core
volts to volt/turn levels
Core shields equally effective
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Transformer Core Grounds in Wound Cores
By Philip J Hopkinson, PE
With Outside Core Ground and no core shield:
1. Hydrogen at 15 kV typically 100-300 ppm
2. Hydrogen at 34.5 kV typically 3,000-10,000 ppm
3. Hydrogen accompanied by small amounts of
a. Ethane
b. Ethylene
c. Methane
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Transformer Core Grounds in Wound Cores
By Philip J Hopkinson, PE
With Outside Core Ground and no core shield:
1. Partial discharge best detector
2. PD should be conducted per Class II transformers
a. Start at 50% of rated volts
b. Go to 100% of rated volts and record
c. Go to 110% of rated volts and record
d. Go to 150% of rated volts and hold for 1 hour
e. Drop back to 110% of rated volts and hold for at
least 10 minutes and record.
f. Drop back to 100% of rated volts and record.
g. Drop back until pd extinguishes below 100 pc
3. Transformer fails test if extinguish Pd (< 100 pc)
occurs at less than 110% of rated volts
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Transformer Core Grounds in Wound Cores
By Philip J Hopkinson, PE
Conclusions:
1. Wound Cores most responsible for High
Hydrogen with Outside Core Grounds
2. Absolute Inside Core Grounds OK
3. Shielded Cores OK
4. 3-Leg Stacked Cores generally immune
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Transformer Core Grounds
By Philip J Hopkinson, PE
Recommendations:
Specify Core Precautions in Standard
1. For 5 Leg Wound Cores
a. Absolute inside core grounds or
b. Shielding
2. 3 Leg Stacked Cores
3. 4 and 5 leg stacked cores May need some
shielding
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High Voltage Winding Failures from Switching
By Philip J Hopkinson, PE
Load-break Switching often
fails transformers
1. Switching often not done at
transformer
2. Groups of ~15 transformers
switched by vacuum breakers
3. First and / or Last transformer
in Group most vulnerable
4. Current Chopping and
Reignition Transients are
failure-initiators
5. IEEE C57.142 Addresses Issues
6. Resistor-Capacitor Snubbers
needed
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High Voltage Winding Failures from Switching
By Philip J Hopkinson, PE
Common Denominators for
switching-induced problems
1. Switching done at Vacuum
breaker
2. Transformers connected to
vacuum breakers by shielded
cables
3. No arresters at transformers..
but it wouldn’t help
4. Switched currents < 6 amps
5. Current chopping and
reignition transients present
Winding failures in tap section
or at line ends
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High Voltage Winding Failures from Switching
By Philip J Hopkinson, PE
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High Voltage Winding Failures from Switching
By Philip J Hopkinson, PE
Vacuum breakers compact, clean, efficient, generally safe..
But Transformers often in trouble!
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High Voltage Winding Failures from Switching
By Philip J Hopkinson, PE
Shielded Cables integral part
of damaging voltage
transients
1. Act like Transmission
lines
2. Wave velocity half the
speed of light
3. Surge impedance
independent of length
4. Low energy dissipation
5. EPR generally superior
to XLPE due to higher
dissipation
Shielded cables amplify resonances with reflected waves
and voltage doubling
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High Voltage Winding Failures from Switching
By Philip J Hopkinson, PE
Ping test #5 at Tap 4-5 & 9.5 amps Feb. 1. 06
Island Park Substation Unit #2
Line 1 to Gr. Voltage
With 20:1 Attenuators & Arc Gap@ 5.5"
Load-break Switching
often fails transformers
3. Current decays and is chopped out
of conduction and voltage oscillates to
zero.
60
40
kV
20
0
-20 0
-200
200
400
600
800
1000
Current chopping
unavoidable at light
currents < 6 amps
-40
-60
Reignition transients likely
at low power factor
-80
-100
-120
Microseconds
1. Circuit breaker
contacts open and
transient recovery
voltage (TRV) rises
by Ldi/dt to (-)100
kV in~90 µsec.
2. Contacts reignite back into
conduction, current rises to
peak and decays to chopping
level, inducing oscillatory
transient. Voltage rises by
+145 kV in <1 µsec., then
oscillates to zero.
5. Contacts
4. Second TRV
voltage rises to (-)80 open
sufficiently to
kV, then breaker
prevent
reignites, raising
reignition
voltage by +120 kV current and
in <1 µsec., etc.
interruption is
completed
Circuit damping key to
solving problems
The higher the system energy efficiency the more likely are winding failures
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High Voltage Winding Failures from Switching
By Philip J Hopkinson, PE
Resistor-Capacitor Snubbers
provide needed damping
1. Can be mounted in
transformer
2. Can also be mounted in
switchgear
3. Current chopping will still
happen
4. Reignition transients will
totally disappear
5. Transformer survives!
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High Voltage Winding Failures from Switching
By Philip J Hopkinson, PE
Conclusion
R-C Snubbers in Vacuum breaker cabinet can prevent switching failures
Recommendation
Include brief tutorial and recommendation in the new Wind Power
Document P60076-16
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Thermal Stability of
Electrical Contacts in
switches and tap changers
1. Desired stable life for
30+years
2. Many electrical contacts
susceptible to oxidation
3. Key Variables are:
a. Contact materials
b. Contact pressures
c. Type of fluid
d. Current amplitude and
variability
e. Temperature
f. Worsened by high
current harmonics
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Thermal Stability of Electrical
Contacts in switches and tap
changers
1. Functional life test being
documented in IEEE PC57.157
2. 30 day test with acceleration
of 1000 times
3. 2 XN Current daily for 8 hrs.
in 130 C bath followed by 16
hours cool down deenergized.
4. Test passed if:
a. Stable
b. Resistance change < 25%
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Thermal Stability of Electrical
Contacts in switches and tap
changers
CuCu Average Resistance
700.0E-6
Resistance in Micro-Ohms
600.0E-6
500.0E-6
400.0E-6
300.0E-6
200.0E-6
100.0E-6
000.0E+0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Days
1/22/2013
CuCu (C147) oil (1-9-96/2-15-96)
CuCu (C147) sil (1-9-96/2-15-96)
CuCu (C147) sil (3-28-96/6-1-96)
CuCu (C110) sil (10-23-97/1-21-98)
CuCu FR3 (12-15-04/2-21-05)
CuCu FR3 (4-18-05/6-27-05)
1. Functional life test being
documented in IEEE PC57.157
2. 30 day test with acceleration
of 1000 times
3. 2 XN Current daily for 8 hrs. in
130 C bath followed by 16
hours cool down deenergized.
4. Test passed if:
a. Stable
b. Resistance change < 25%
Doble Engineering Conference 021913
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Thermal Stability of Electrical
Contacts in switches and tap
changers
AgCu Average Voltage Drop
120.0E-3
Voltage Drop in Volts
100.0E-3
80.0E-3
60.0E-3
40.0E-3
20.0E-3
000.0E+0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Days
AgCu sil (10-7-98/12-16-98)
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AgCu fr3 (8-6-04/10-13-04)
AgCu oil (12-15-04/2-21-05)
1. Functional life test being
documented in IEEE PC57.157
2. 30 day test with acceleration
of 1000 times
3. 2 XN Current daily for 8 hrs. in
130 C bath followed by 16
hours cool down deenergized.
4. Test passed if:
a. Stable
b. Resistance change < 25%
AgCu oil (4-18-05/6-27-05)
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Thermal Stability of Electrical
Contacts in switches and tap
changers
AgAg Average Resistance
300.0E-6
Resistance in Micro-Ohms
250.0E-6
200.0E-6
150.0E-6
100.0E-6
50.0E-6
000.0E+0
1
3
5
7
9
11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49
Days
AgAg (Soft) sil (3-28-96/6-1-96)
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AgAg (Hard) sil (6-4-96/11-5-96)
1. Functional life test being
documented in IEEE PC57.157
2. 30 day test with acceleration
of 1000 times
3. 2 XN Current daily for 8 hrs.
in 130 C bath followed by 16
hours cool down deenergized.
4. Test passed if:
a. Stable
b. Resistance change < 25%
AgAg (Line 1) fr3 (8-6-04/10-13-04)
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Thermal Stability of
Electrical Contacts in
switches and tap changers
1. Test quite meaningful
2. 40+ years proven to find
stable contacts
3. IEEE PC 57.12.157 Guide
should be helpful to the
industry
4. Tests should be performed
by each manufacturer for
each switch type
5. These conclusions to be
included in Wind Power
Transformer Standard
P60076-16
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Arc-Flash Concerns
1. NFPA 70E reference for
2009
2. Both HV and LV Cabinets
have concerns with LV
worst
3. Suit-up needed to just
check gauges
4. These conclusions to be
included in Wind Power
Transformer Standard
P60076-16
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Arc-Flash Concerns
Desire third cabinet
1. Gauges
2. Load-break switch
Handle
3. Schrader valve
4. Sampling port
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Arc-Flash Concerns
Desire HV Cabinet
Door to open first
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Low Liquid Level
Concerns
1.
2.
3.
4.
5.
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Meter shows level
below minimum
No leak evidence
Cold temperatures
Transformer may
be de-energized
Critical switchgear
and other
accessories may
not be immersed
in the liquid
34
Wind Power Transformer Design
By Philip J Hopkinson, PE
Low Liquid Level
Concerns
1.
Volumetric change
of mineral oil with
temperature
Delta V =0.0007*(C-1)
2.
3.
4.
5.
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Assume tank filled
at +70 C
Assume coldest
temperature (-) 40 C
Temperature change
is (-) 110 C
Volume shrink = 7.6%
35
Wind Power Transformer Design
By Philip J Hopkinson, PE
Low Liquid Level
Concerns
Suppose original liquid
height at 50”
A 7.6% volume
reduction is (-) 4”
Recommendation
Specify added liquid
level to fully cover
switches and accessories
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Wind Power Transformer Design
By Philip J Hopkinson, PE
Summary
IEEE P60076-16 should
be an important
document for Wind
Power Transformers
1. Normal C57.12.00
considerations
2. Core Grounds and
shielding
3. Resistor –Capacitor
Snubbers
4. Stable electrical
contacts
5. Arc-Flash issues
6. Added liquid level
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