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Transformer Failure Due to Circuit Breaker Induced Switching Transients

Part 2: EMTP Modeling and Case StudiesDavid D. Shipp, PE

Fellow, IEEEEaton Electrical Group130 Commonwealth Dr.Warrendale, PA 15086

Thomas J. Dionise, PESenior Member, IEEEEaton Electrical Group130 Commonwealth Dr.Warrendale, PA 15086

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Transient Analysis Tools

• Electromagnetic Transients Program (EMTP)• Developed by Hermann Dommel brought to BPA• Major contributors: Scott Meyer & Dr. Liu at BPA

• Alternative Transients Program (ATP)• Alternative to commercialized EMTP• Free to all if agree not to commercialize it

• EMTP-RV• PSCAD EMT/DC

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ATP

• ATP Draw interface• 3-phase modeling• Solution method - Trapezoidal integration

• Robust for wide variety of modeling needs

• Extensive library of models• Many options and features

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Source Model

• “3-phase wye cosine”• Resistance in ohms• Inductance in mili-Henries• Source Impedance

• Z1 and X/R• Z0 and X/R

SYSTEM SOURCEAT 13.8 KV

XUTILRUTIL

VUTIL

UN

U UT

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Cable Model

• “Pi Model”• Resistance in ohms• Inductance in mili-Henries• Cable charging in micro-Farads

• ½ charging on sending end• ½ charning on receiving end

• Multiple pi models in some cases

CABLE13.8KV

C1 LCABLERCABLE C2

C/2C/2

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Breaker Model

• “Switch”• Time dependent switch

• Open or close at a specific time• Opens at current zero unless specify otherwise

• Current dependent switch• Needed for current chopping• Define current at which to open switch

• Independent Poles (A, B and C independent)• Data request

• Vacuum or SF6 breaker nameplate• Current chop (ask manufacturer)• Transient Recovery Voltage Ratings – T2, E2 and RRV

VCBBKR

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Transformer Model

• “Three Phase Model”• Resistance in ohms• Inductance in mili-Henries• Magnetizing current• Winding capacitance

• CH and CL for dry-types• CHL for oil-filled

• More detailed modeling• Saturation• Hysteresis

• Data Request• %Z and X/R, MVA• BIL TRANSFORMER

RLRG

LTRAN RTRANT1 T2

CH CL

N:1

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Complete System Model

TRANSFORMER

RLRG

LTRAN RTRANT1 T2

CH CL

N:1

CABLE13.8KV

C1 LCABLERCABLE C2

C/2C/2

SYSTEM SOURCEAT 13.8 KV

XUTILRUTIL

VUTIL

UN

U UT

VCBBKR

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Time Step

• Time Step – Integration Time Step• Choice depends on frequency of expected transient• Too large – miss high frequency effects• Too small – excessive simulation times

• Nyquist Criteria• Minimum sample rate = 2 x frequency• Example 10-20KHz ring for VCB transient

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Switching Transient Simulations

• Switching transient simulations• EMTP and model requirements

• Case studies• Data Center• Ferry Propulsion• Laddle Melt Furnace

• Successful techniques/solutions

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Ferry Propulsion

Highlights• 60K passengers per day• 20M passengers per year• 4160V distribution• 3 x 2865kW diesel gens• 2 x 1865KW forward propulsion motors• 2 x 1865KW reverse propulsion

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Problem

• Forward propulsion drives ferry• Upon reaching dock, reverse propulsion stops ferry• July 1, 2009, reverse propulsion was lost entering dock

• lost power and hit a pier at full speed• one serious injury and nine minor injuries• 750 to 800 passengers were aboard • impact did not send any passengers overboard

• 1185KVA rectifier transformer failed on reverse propulsion motor

• Evidence of insulation failure on first few turns of primary winding

• Investigation into source of such failure VCB switching

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Ferry - Oneline

VacuumBreaker

Short Cable

Dry-Type Transformer

Forward Reverse

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Ferry – Electrical Highlights

• 3 x 2865kW diesel generators• 4160V, 3-bus system

• Vacuum circuit breakers – 630A mains, ties, feeders

• 2 x 1865kW motors (forward propulsion)• 2 x 1865kW motors (reverse propulsion)• 8 x 1185KVA dry-type transformers, 30kV BIL

• 3-winding rectifier transformers• 12-pulse effective (6-pulse VFD per winding)• Feeder cable lengths of 50 feet each

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Worst Case Scenarios

• Feeder cable lengths of 50 feet• Each 1185KVA transformer has one 4160V feeder• Need to examine both feeders for each transformer• “Worst Case” Switching Transient Simulations

• Close 4160V VCB to transf. with 50ft. cable (model validation)• Close 4160V VCB to transf. with 50ft. cable (re-ignition)• Open 4160V VCB to transf. with 50ft. cable (current chop)

• Repeat each case with Snubber

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MeasurementsTransient Overvoltages at Primary of Rectifier Transformer – VCB Closes

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Study Cases

• Case 1 – Closing the 4.16 kV feeder breaker feeding the three winding rectifier transformer with the distance of 50 feet. Simulated to match the Siemens measurements to ensure that the computer model is accurate. (Model Validation)

• Case 2 – Same as Case 1, except with the RC snubber.• Case 3 - Closing the 4.16 kV feeder breaker feeding the three

winding rectifier transformer, then the 4.16 kV feeder breaker opens during the inrush current. Shows the possibility of the vacuum breaker re-ignition. (Re-ignition)

• Case 4 - Same as Case 3, except with the RC snubber.• Case 5 - Open the 4.16 kV feeder breaker feeding the three

winding rectifier transformer under the light load condition. (Current Chop)

• Case 6 – Same as Case 5, except with the RC snubber.

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Case 1 – Model ValidationTransformer primary voltage

V max of 4.96kV < 30kV BILOscillation of 20,203Hz > 1000Hz

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Case 2 – Valid Model with SnubberTransformer primary voltage

V max of 3.982kV < 30kV BILOscillation of < 1000Hz

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Case 3 – Re-ignitionTransient Recovery Voltage (TRV) at VCB

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Case 4 – Re-ignition with SnubberTransient Recovery Voltage (TRV) at VCB

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Case 5 – Current ChopTransformer primary voltage

V max of 31.9kV > 30kV BILOscillation of 958Hz ~ 1000Hz

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Case 6 – Current Chop with SnubberTransformer primary voltage

V max of 8.9kV < 30kV BILOscillation of 299Hz < 1000Hz

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Summary of Current Chop Cases

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Summary of Re-Ignition Cases

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Recommendations

• Install snubber at primary of each 1185KVA rectifier transformer

• 40ohm resistor• Non-inductive• Peak voltage – 6 kVpeak• Peak energy – 2100 Joules• Average power – 190 Watt

• 0.5uF capacitor• Rated voltage - 4.16kV

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Data Center

Highlights• Tier III• LEEDS Certified• 12.5MVA Capacity• 13.2KV Ring Bus

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Data Center - Oneline

VacuumBreaker

Short Cable

Cast-Coil Transformer

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Data Center – Electrical Highlights

• 2 x 24.9 kV lines from Factory Shoals and Buzzard Roast

• 2 x 12.5 MVA transformers • 13.2 kV “ring-bus” configuration

• MSA and MSB and generator bus GPS• Vacuum circuit breakers – 600A mains & ties, 200A feeders

• 3 x 2250 KW generators• 6 x 3750KVA cast coil transformers, 90kV BIL

• 3 x DE Subs CSA, CSB and CSC• 3 x SE Subs MDSA, MDSB and MLB• Feeder cable lengths vary from 109 to 249 Feet

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Worst Case Scenario

• Feeder cable lengths vary from 109 to 249 Feet• Each 3750KVA transformer has two 13.2kV feeders• Need to examine both feeders for each transformer• Shortest of all 13.2kV cable runs to 3750KVA

Transformers• MSB to Transformer CSC – 109 feet• MSA to Transformer CSC – 111 feet

• “Worst Case” Switching Transient Simulation• Open 13.2 kV VCB at MSB to CSC with 109ft. cable• Open 13.2 kV VCB at MSA to CSC with 111ft. cable

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Worst Case – Study Case 13

• Study Case 13 - Open the 13.2 kV feeder breaker at Bus MSB feeding the 3,750 kVA dry type transformer CSC with the shorter distance of 109 feet. The result for this case will represent the “worst-case” condition, the other feeder from Bus MSA has a longer feeder distance of 111 feet.

• Study Case 14 – Same as Case 13, except with the application of the 0.25 μF surge capacitor.

• Study Case 15 – Same as Case 13, except with the application of the snubber circuit with 30ohm and 0.25 μF surge capacitor.

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Case 13 – no surge protectionLoad current at 13.2kV VCB

Load current of 10A

Chopped current of 6A

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Case 13 – no surge protectionTransformer CSC primary voltage

VC max of 65.3kV > 95kV BILVB max of 116kV > 95kV BILVA max of 123kV > 95kV BILOscillation of 969Hz > 1000Hz

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Case 14 – 0.25uF surge capLoad current at 13.2kV VCB

Load current of 10A

Chopped current of 6A

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Case 14 – 0.25uF surge capTransformer CSC primary voltage

VA max of 29.4kV < 95kV BILVB max of 19.1kV < 95kV BIL VC max of 15.7kV < 95kV BIL Oscillation of 215Hz < 1000Hz

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Case 15 – snubber 30ohm and 0.25uFLoad current at 13.2kV VCB

Load current of 10A

Chopped current of 6A

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Case 15 – snubber 30ohm and 0.25uFTransformer CSC primary voltage

VB max of 28.6kV < 95kV BILVA max of 19.4kV < 95kV BIL VC max of 15.9kV < 95kV BIL Oscillation of 215Hz < 1000Hz

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Case 15 – snubber 30ohm and 0.25uFSnubber Current – important for duty on resistor and capacitor

IC peak of 7.7AIB peak of 8.0A IA peak of 6.8A

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Comparison of Results

No Snubber

0.25uF surge cap

SnubberR=30ohmC=0.25uF

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Measured Transients with Snubber

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Recommendations

• Install snubber at primary of each 3750KVA cast coil transformer 30ohm and 0.25uF

• Install surge caps 0.25uF at each emergency generator

• Install surge arresters at the following locations:• both incoming power transformers• every distribution dry type transformer• every generator• line side of both main circuit breakers• line side of the three generator circuit breakers• load side of every feeder breaker and every tie circuit breaker

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Laddle Melt FurnaceHighlights• Retiring 3 x 38MW EAFs• Adding 1 x 155MVA EAF• Adding 2 x 138kV lines to new EAF• Adding SVC at 34.5kV• Adding 1 x 20MW LMF

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LMF & EAF - Oneline

VacuumBreaker

Short Bus

Oil-Filled Transformer

Existing EAF New

LMF

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138KV

UTILITY4713MVA 3PH SC9.26 X/R

50/66/83MVA135.3/26.4KV

7.5%Z

SF-6 BREAKER2000A

1600A

27KV13OHM

AUTO LTC56MVA

27-10KV3.3%Z

ALUMINUMIPS BUS53FEET

VACUUM BREAKER1200A

LMF XFMR50/56MVA25/.53KV

2.5%Z

HEAVY DUTYCOPPER PIPE

28FEET

LMF20MW

LMF

VacuumBreaker

Short Bus

Oil-Filled Furnace

TransformerNewLMF

SF6Breaker

Short Bus

Oil-Filled Auto-Regulating

Transformer

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LMF Circuit – Electrical Highlights

• 50MVA Power Transformer 135/26.4kV• 27kV system

• SF6 circuit breaker – 2000A• Bus length of 53 feet• 56MVA autoregulating transformer, 200kV BIL• Vacuum circuit breaker – 1200A• Bus length of 28 feet• 50MVA oil-filled LMF transformer, 200kV BIL

• 20MW LMF

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Worst Case Scenarios

• Examine switching transients at both transformers• “Worst Case” Switching Transients for Auto-Reg Transf

• Open SF6 breaker to transf. with 53ft. bus (6A current chop)• 3 cycles after energizing Auto-Reg Tran, open SF6 bkr to transf.

with 53ft. bus (re-ignition) highly inductive current

• “Worst Case” Switching Transients for LMF Transf• Open VCB to transf. with 28ft. bus (6A current chop)• 3 cycles after energizing LMF Tran, open VCB to transf. with

28ft. bus (re-ignition) highly inductive current

• Repeat each case with Snubber

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Case 1 – Open VCBLMF Transformer primary voltage

V max of 386kV > 200kV BILOscillation of 1217Hz > 1000Hz

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Case 2 – Open VCB with SnubberLMF Transformer primary voltage

V max of 56.4kV < 200kV BILOscillation of 200Hz < 1000Hz

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Case 3 – Open SF6 BreakerAuto-regulating Transformer primary voltage

V max of 23kV < 200kV BILNo oscillating frequency

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Case 4 – Open SF6 Breaker with SnubberAuto-regulating Transformer primary voltage

VB max of 54.7kV < 200kV BILOscillation of 197Hz < 1000Hz

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Case 5 – VCB Re-ignitionTransient Recovery Voltage for VCB

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Case 5 – Highly Inductive CurrentInrush current to LMF transformer

6000A Peak Inrush

VCB Opens

VCB Closes

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Case 6 – VCB Re-ignition & SnubberTransient Recovery Voltage for VCB

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Case 7 – SF6 Breaker Re-ignitionTransient Recovery Voltage for Siemens SF6

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Case 7 – Highly Inductive CurrentInrush current to Auto-Regulating transformer

14000A Peak Inrush

SF6 CB Opens

SF6 CB Closes

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Case 8 – SF6 Breaker Re-ignition & SnubberTransient Recovery Voltage for SF6 Breaker

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Summary of Current Chop Cases

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Summary of Re-Ignition Cases

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Recommendations

• Install snubber (100ohm and 0.15uF) at primary of each transformer

• 100ohm resistor• Non-inductive• Peak voltage – 38 kVpeak• Peak energy – 17,500 Joules• Average power – 1000 Watt

• 0.15uF, 34.5kV surge capacitor (not available)• 1-pole, 24kV, 0.13uF• 2-pole, 14.4kV, 0.5uF• Series combination gives 0.103uF