wind power transformer design - ieee-sa · 2013. 10. 23. · wind power transformer design by...
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Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 1
Wind Power Transformer at base of Tower Critical link to successful Wind Turbine connection to the grid Subject of Many complaints 1. Gassing 2. Winding Failures 3. Contact coking 4. Arc-Flash 5. Low liquid level
Doble Engineering Conference 021913
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
1/22/2013 2 Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 3
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
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 4
Typical Wind Turbine transformer concerns 1. High Hydrogen gas 2. High voltage winding failures 3. Safety of HV load-break switch 4. Carbonized HV Switches and Tap
changers 5. Low liquid levels in cold
temperatures
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 5
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.
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 6
High Hydrogen Gas always partial discharge related 1. But windings not involved 2. Leads not involved in most cases 3. Hot spots in windings OK 4. Some gassing improves with time 5. If not windings or leads then what
else?
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 7
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
Doble Engineering Conference 021913
Transformer Core Grounds in Wound Cores By Philip J Hopkinson, PE
1. The issue is gassing and Partial Discharge 2. Dielectric Breakdown in windings not the problem 3. Gassing and PD worst at 34.5 kV 4. Gassing not an issue at 5 kV 5. Gassing at 15 kV an annoyance only 6. Gassing coming from Core 7. Solution via grounding or shielding
1/22/2013 8 Doble Engineering Conference 021913
Transformer Core Grounds in Wound Cores By Philip J Hopkinson, PE
1/22/2013 9
Ground Lead
Vi Outer Core Vi Inner Core Vi Inner Core Vi Outer Core
C5 C2 C1 C3 C4 C1 C1 C4 C4 C1 C1 C4 C3 C1 C2 C3 C5
C5 C5 C5 C5
LV Grounded Clamp
HV
L
L
Doble Engineering Conference 021913
Transformer Core Grounds in Wound Cores By Philip J Hopkinson, PE
1/22/2013 10
Ground Lead
Vi Outer Core Vi Inner Core Vi Inner Core Vi Outer Core
C5 C2 C1 C3 C4 C1 C1 C4 C4 C1 C1 C4 C3 C1 C2 C3 C5
C5 C5 C5 C5
LV Grounded Clamp
HV
L
L
HV = 34500 LV = 600 Gr Y
kVA = 1850 Frequency = 60 Hz
C1-C5 calculated In EXCEL
Doble Engineering Conference 021913
Transformer Core Grounds in Wound Cores By Philip J Hopkinson, PE
1. Outside core grounds at 34.5 kV result in high core volts
2. Inside core grounds eliminate static charges and drop core volts to volt/turn levels
1/22/2013 11
Vi Outer Core Loop Inside Surface
804 Volts
LV Start is Grounded
0 VoltsC2 C1
L-G Volts
19919
Core Ground
C3 0 Volts
Case I, Core Ground Connected to Frame
Voltage drop in Outer Core stacks = 804 Volts
HV
Doble Engineering Conference 021913
Transformer Core Grounds in Wound Cores By Philip J Hopkinson, PE
1. Outside core grounds at 34.5 kV result in high core volts
2. Inside core grounds eliminate static charges and drop core volts to volt/turn levels
3. Core shields equally effective
1/22/2013 12
0.030" thick static shield
0.020" thick paper
Outside Core Leg
Core Ground
Grounded Electrostatic Shield
Doble Engineering Conference 021913
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
1/22/2013 13 Doble Engineering Conference 021913
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
1/22/2013 14 Doble Engineering Conference 021913
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
1/22/2013 15 Doble Engineering Conference 021913
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
1/22/2013 16 Doble Engineering Conference 021913
High Voltage Winding Failures from Switching By Philip J Hopkinson, PE
1/22/2013 17
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
Doble Engineering Conference 021913
High Voltage Winding Failures from Switching By Philip J Hopkinson, PE
1/22/2013 18
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
1/22/2013 19 Doble Engineering Conference 021913
High Voltage Winding Failures from Switching By Philip J Hopkinson, PE
1/22/2013 20
Vacuum breakers compact, clean, efficient, generally safe.. But Transformers often in trouble!
Doble Engineering Conference 021913
High Voltage Winding Failures from Switching By Philip J Hopkinson, PE
1/22/2013 21
Shielded cables amplify resonances with reflected waves and voltage doubling
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
Doble Engineering Conference 021913
High Voltage Winding Failures from Switching By Philip J Hopkinson, PE
1/22/2013 22
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"
-120
-100
-80
-60
-40
-20
0
20
40
60
-200 0 200 400 600 800 1000
Microseconds
kV
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.
3. Current decays and is chopped out
of conduction and voltage oscillates to
zero.
5. Contacts
open
sufficiently to
prevent
reignition
current and
interruption is
completed
4. Second TRV
voltage rises to (-)80
kV, then breaker
reignites, raising
voltage by +120 kV
in <1 µsec., etc.
Load-break Switching often fails transformers Current chopping unavoidable at light currents < 6 amps Reignition transients likely at low power factor Circuit damping key to solving problems
The higher the system energy efficiency the more likely are winding failures
Doble Engineering Conference 021913
High Voltage Winding Failures from Switching By Philip J Hopkinson, PE
1/22/2013 23
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!
Doble Engineering Conference 021913
High Voltage Winding Failures from Switching By Philip J Hopkinson, PE
1/22/2013 24
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
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 25
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
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 26
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 de-energized.
4. Test passed if: a. Stable b. Resistance change < 25%
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 27
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 de-energized.
4. Test passed if: a. Stable b. Resistance change < 25%
CuCu Average Resistance
000.0E+0
100.0E-6
200.0E-6
300.0E-6
400.0E-6
500.0E-6
600.0E-6
700.0E-6
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
Resis
tan
ce in
Mic
ro
-O
hm
s
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)
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 28
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 de-energized.
4. Test passed if: a. Stable b. Resistance change < 25%
AgCu Average Voltage Drop
000.0E+0
20.0E-3
40.0E-3
60.0E-3
80.0E-3
100.0E-3
120.0E-3
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
Vo
ltag
e D
ro
p in
Vo
lts
AgCu sil (10-7-98/12-16-98) AgCu fr3 (8-6-04/10-13-04) AgCu oil (12-15-04/2-21-05) AgCu oil (4-18-05/6-27-05)
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 29
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 de-energized.
4. Test passed if: a. Stable b. Resistance change < 25%
AgAg Average Resistance
000.0E+0
50.0E-6
100.0E-6
150.0E-6
200.0E-6
250.0E-6
300.0E-6
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
Resis
tan
ce in
Mic
ro
-O
hm
s
AgAg (Soft) sil (3-28-96/6-1-96) AgAg (Hard) sil (6-4-96/11-5-96) AgAg (Line 1) fr3 (8-6-04/10-13-04)
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 30
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
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 31
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
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 32
Arc-Flash Concerns
Desire third cabinet 1. Gauges 2. Load-break switch
Handle 3. Schrader valve 4. Sampling port
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 33
Arc-Flash Concerns
Desire HV Cabinet Door to open first
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 34
Low Liquid Level Concerns
1. Meter shows level below minimum
2. No leak evidence 3. Cold temperatures 4. Transformer may
be de-energized 5. Critical switchgear
and other accessories may not be immersed in the liquid
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 35
Low Liquid Level Concerns
1. Volumetric change of mineral oil with temperature
Delta V =0.0007*(C-1) 2. Assume tank filled
at +70 C 3. Assume coldest
temperature (-) 40 C 4. Temperature change
is (-) 110 C 5. Volume shrink = -
7.6%
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 36
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
Doble Engineering Conference 021913
Wind Power Transformer Design By Philip J Hopkinson, PE
1/22/2013 37
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
Doble Engineering Conference 021913
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