transformer failure due to circuit breaker induced switching transients

TUTORIAL

Transformer Failure Due to Circuit Breaker Induced Switching Transients

2011 I&CPS CONFERENCE 2011

NEWPORT BEACH, CA

MAY 1 – 4, 2011

David D. Shipp, PE Fellow, IEEE

Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086

[email protected]

Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086

[email protected]

i

TABLE OF CONTENTS

1.0 INTRODUCTION.....................................................................................1-1

2.0 CASE STUDY OVERVIEW.....................................................................2-1

3.0 EMTP MODELING & CASE STUDIES...................................................3-1

4.0 MOTOR STARTING AUTOTRANSFORMER CASE STUDY.................4-1

5.0 PT FAILURES.........................................................................................5-1

6.0 SNUBBER PERFORMANCE MEASUREMENTS ..................................6-1

7.0 SUMMARY..............................................................................................7-1

8.0 REFERENCES........................................................................................8-1

1-1

1.0 INTRODUCTION Switching transients associated with circuit breakers have been observed for many years. Recently this phenomenon has been attributed to a significant number of transformer failures involving primary circuit breaker switching. These transformer failures had common contributing factors such as 1) primary vacuum or SF-6 breaker, 2) short cable or bus connection to transformer, and 3) application involving dry-type or cast coil transformers and some liquid filled. This tutorial will review these recent transformer failures due to primary circuit breaker switching transients to show the severity of damage caused by the voltage surge and discuss the common contributing factors. Next, switching transient simulations in the electromagnetic transients program (EMTP) will give case studies which illustrate how breaker characteristics of current chopping and re-strike combine with critical circuit characteristics to cause transformer failure. Design and installation considerations will be addressed, especially the challenges of retrofitting a snubber to an existing facility with limited space. Finally, several techniques and equipment that have proven to successfully mitigate the breaker switching transients will be presented including surge arresters, surge capacitors, snubbers and these in combination

2-1

2.0 CASE STUDY OVERVIEW

Transformer Failure Due to Circuit Breaker Induced Switching TransientsPart 1: Case Study Overview

David D. Shipp, PEFellow, IEEE

Eaton Electrical Group130 Commonwealth Dr.Warrendale, PA 15086

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

Case Study Overview:Data Center Transformer Failure

Attributed to Circuit Breaker Induced

Switching TransientsSystem Configuration, Failure Analysis

and RC Snubber Fix

R. McFadden – JB&B – NY, NYD. Shipp – Eaton Electrical – Pgh, PA

Presentation Purpose/Summary

• Purpose #1 Safety:– Alert Design & User Community– Drive Solutions Through Market Interest

• Unique Case History– Forensic Evidence– High Speed Power Quality Capture

• Presentation Summary– Simplified System Overview– Forensic Review– Power Quality Snapshots– Installed RC Snubber Fix

• Presentation Summary: David Shipp, Eaton– Phenomenon Detailed Explanation– Science of RC Snubber Design

Site Specifics

• Utility Service:– 26kV– Double Ended Loop Through

• Transformers– (6) Total– 26kV Primary, 480V Secondary– VPI– 3000kVA AA / 3990 kVA FA– 150 kVbil

• Switching Device– Vacuum Circuit Breaker

• Cable– 35kV, 133% EPR Insulation, 1/3 Concentric Ground

M

T-A3CRACsChillers

T-A2UPS

T-A1OTHER

40’

80’

100’+

26kv Utility “A”

To Normal SideOf 480vac ATS(Typical)

M

T-B3CRACsChillers

T-B2UPS

T-B1OTHER

40’

80’

100’+

26kv Utility “B”

Simplified System Configuration

Failure/Sequence of Events• All Transformers Fully Tested:

• Pre-functional: Turns Ratio, Insulation Resistance, etc• Functional:UPS Full Load Tests, UPS Transient Tests, Data Center

Room Validation Testing• Final Pull The Plug Test (PTP)

• 4 Electricians “simultaneously” open (4) 26KV vacuum breakers to simulate a general utility outage.

• All systems successfully transfer to standby generation but:• Loud Pop heard in Substation Room B• Relay for VCB feeding TB3 signaling trip

• Decision made to shutdown generator test and investigate issue in “B” substation room

• 2 Electricians “simultaneously” close (2) 26KV vacuum breakers to Substation Room “A”

• Transformer TA3 fails catastrophically.

M

T-A3CRACsChillers

T-A2UPS

T-A1OTHER

40’

80’

100’+


0.8MW

M

T-B3CRACsChillers

T-B2UPS

T-B1OTHER

40’

80’

100’+


Pre-PTP Condition

1.5MW

0.9MW

1.3MW

M

T-A3CRACsChillers

T-A2UPS

T-A1OTHER

40’

80’

100’+


M

T-B3CRACsChillers

T-B2UPS

T-B1OTHER

40’

80’

100’+


Failure #1: De-Energization During PTP

Audible Pop In Substation

Room

Relay Signaling

Trip

M

T-A3CRACsChillers

T-A2UPS

T-A1OTHER

40’

80’

100’+


M

T-B3CRACsChillers

T-B2UPS

T-B1OTHER

40’

80’

100’+


Failure #2: Energization During PTP

CatastrophicFailure

Relay Trips VCB

Coil to Coil Tap Burnt Off

Failure #2Transformer Failure On Energization

Coil to Coil ConductorBurnt Off

Suspected Area of Initial Flash





Upward TwistFrom Lower Blast


Flash/Burn Marks

Failure #1Transformer Failure On De-Energization

Close up of Flash/Burn Marks

Coil to Coil notWinding to Winding


This Transformer was Returned to the Factory and Passed all

Standard IEEE Tests!

No Coil to CoilCable Supports

Some Burn Marks Match

Point ofCable Contact

15KV CableIn 26KV

Transformer


Some Burn Marks Match

Vicinity of Cable

Cable is Undamaged


Factory InstalledCable Support

Unaffected Transformer

Current Transient: Failure #2Transformer Energization

Event Initiated A-B Phase 19,000 amp, 3 Phase

5 Cycle CB Clearing Time

Second TransformerInrush

AØ

BØ

CØ

AØ

BØ

CØ

Current Transient: Failure #1Transformer DeEnergization

8000amp A-B Phase

Approx ¼ Cycle

C Phase Shows2nd Xfmr Load

& Event Duration

Breaker Induced Switching Transients

Case 1Hydro Dam, MT 2005

• MV Vac Bkr Replacements Vendor “A”• 13.8 kV• 20 feet of cable• 50 kV BIL (W) ASL Dry Type Txmrs• Customer Energized before Vendor OK• Txmr Failed• No Surge Protection Applied

CASE 2Cleveland Hospital 3/06

• Vacuum Breakers – Vendor “A” and Vendor “C”

• 13.8 kV• 95 kV BIL• Dry Type Txmr• 27 feet of Cable• Bkr Manufacturer Paid to Repair Failed 2500

KVA Txmr• Surge Protection Added Afterwards

CASE 3RAILROAD SUBSTATION 11/06

• Vacuum Breaker – Vendor “A”• 26.4 kV• 150 kV BIL• Generic Liquid Filled Rectifier Txmr• 37 feet of Cable

CASE 4NJ DATA CENTER 12/06

• 26.4 kV – Vendor “B”• 4 Txmrs Switched Under Light Load• 2 Txmrs Failed-1 on Closing/1 on

Opening• 40 Feet of Cable• 2 other Txmrs Did Not Fail - 80 Feet of

Cable• Arresters Were Present

CASE 5OIL FIELD – AFRICA 6/07

• Vacuum Breaker – Vendor “D”• 33 kV • 7 Feet of Cable• Dry Type Txmr in 36 Pulse VSD• Arresters Were Applied• Txmr Failed Upon Energization

CASE 9Oil Drilling Ship – 6/2002

• Vendor “A” IEC Vac Bkrs in Vendor “D”Swgr

• 11kV, 60 HZ• Cast Coil Dry Type IEC VSD Propulsion

Txmr Failed (7500 kVA) – 75 kV BIL?• < 30 feet of Cable• Fed from Alternate Bus – Now 80 feet of

Cable• No further Failures Reported

CASE 11Hospital – Slide 1

• 13. 8 kV System• Vendor C Breakers/

Vendor F Txmrs• 2 Txmrs Failed During

Commissioning• Differential Relay

Tripped During motor Starting – under Highly Inductive Load



CASE 12Motor Starter Auto Txmr Failure

• 4160V• 5000 HP• Reduced Voltage

Auto Txmr Failed• Arresters on Wye

Point• Internal Resonance• Layer Wound• Failed Layer to Layer

CASE 12 Run Contactor Closes 3516 HZ

CASE 12 - Sweep Frequency Test 4500 HZ (Admittance)

CASE 12 - Run Contactor Closes – 844 HZ With Snubber

SWITCHING TRANSIENTS DUE TO VACUUM / SF6 BREAKERS

• Opening -- Current Chop• Closing -- Prestrike/Re-ignition/Voltage

Escalation• Vacuum Bkrs --Both Closing and

Opening• SF6 -- Opening• Air -- Generally Acceptable

Current Chop

Current ChopCurrent Chop in Vacuum is a Material Problem

Current Chop

• All Types of Interrupters Chop Current

• This is not a Unique Feature of Vacuum

Switching Inductive Circuits

• Closing• Opening• Voltage Escalation• Surge Suppression

Switching Inductive LoadsCLOSING

Switching Inductive LoadsOPENING

Switching Inductive LoadsVOLTAGE ESCALATION

SWITCHING TRANSIENT THEORY

• “Thou Shalt Not Change Current Instantaneously in an Inductor”

• Conservation of Energy –– You Cannot Create or destroy Energy –

You Can Only Change Its Form

ENERGY EQUATION

22

21

21

CVLI

OR

C

LIV

TOTAL VOLTAGE

• Vt = Venergy + Vdc + Vosc

• Venergy is from the Energy Equation• Vdc = DC Off-set that Sometimes is

Present• Vosc = the Oscillatory Ring Wave

TRANSFORMER LIMITS

• Magnitude – BIL Ratings• Dv/dt Limits• Both MUST Be Met• Dry Type Txmrs Particularly

Susceptable• Liquid Filled Not Immune.• Consider “Hammer Effect”

ANALYSIS

• When Open Bkr, Txmr is Left Ungrounded

• Ring Wave is a Function of its Natural Frequency

LCNF

21

CASE 3Waveforms Without Snubbers

CASE 3Current – Without Snubbers

Ichop

CASE 3Waveforms With Snubbers

CASE 4Data Center 12/06

• 26.4 kV• Vendor “B” Breakers• 4 Bkrs Switched at Once• 2 Dry Type Txmrs Failed (40 Ft of

Cable)• 2 Txmrs Did not Fail (80 Ft of Cable)• Unfaulted Txmr Winding Failed BIL

@162 kV ( Rated 150 kV)

CASE 4Data Center, NJ 12/06 - W/O Snubbers

08-Jan-07 14.53.47

0 5 10 15 20 25

-200

-150

-100

-50

0

50

100

150

200

v [kV]

t [ms](42) XFMA3C (48) XFMB3C

Plot of the Transformer A3 and B3 primary voltage Phase C

Open circuit breaker with chopped current of 9 Amperes

-207 kV

CASE 4Data Center, NJ 12/06-With Snubbers

09-Jan-07 10.50.25

0 5 10 15 20 25

-40

-30

-20

-10

0

10

20

30

40

v [kV]

t [ms](29) XFMA3B (35) XFMB3B

Plot of the Transformer A3 and B3 primary voltage Phase B

Open circuit breaker with chopped current of 9 Amperes

Application of the R-C snubber circuit

-39 kV

CASE 6Data Center 2, NJ 12/06

• 13.2 kV• Vendor “B” Breakers• 3 MVA Dry Type Txmrs• 60 ft Cable – Required Snubbers• 157 ft Cable – Required Snubbers• No Problem at Startup

CASE 613.2 kV SYSTEM – 119 kV @ 678 HZ

CASE 6WITH SNUBBERS – 33.6 kV @ 236 HZ

CASE 7Chemical Plant, NC 3/07

• 12.47 kV System• 20+ Year Old Oil Filled Txmrs• Vendor “A” Vacuum Bkrs retrofitted on

Primary• 10 Feet of Cable• No Problem at Startup

CASE 7 425 kV - 12. 47 kV - 23 kHZ Ring Wave

CASE 7Added Snubbers – 12.47 kV

CASE 8DATA CENTER – Indiana 6/07

• 12.47 kV System• Vendor “A” Breakers• 270 Feet of Cable• No Additional Surge Protection

Required

CASE 8-55 kV at 800 HZ

CASE 11 Hospital186.9 kV / 2000 HZ

CASE 10 – SnubberReduced to 33.8 kV / 368 HZ

MITIGATING TECHNIQUES

• Arresters• Surge Capacitors• Snubbers (RC)• Hybrid / Combinations• Liquid vs Dry Type• Electronic Zero Crossing Switching

Snubber - 3 Phase CapacitorGenerally Solidly Grounded System• Paper Mill - AL• 13.8 kV

Snubbers – 2nd paper mill - beta site

Snubber – Single Phase CapacitorsLow Resistance Grounded Systems

• Paper Mill• Vendor “A” Swgr• 13.8 kV

Snubber – Single Phase Capacitors

• Silicon “Chip” Plant• Montana• Very Specialized

Dry Type Txmrs• 13.8 kV• Cables < 100 feet• Primary Fused

Switch AF Solution• Vendor “A” Vac Bkrs

Capacitor

Resistor

Fuse

LightningArrestor

Blown FuseViewing Window

RC Snubber Installed – Case 4


Capacitor

Resistor


Fuse

Blown Fuse

Indicator

SWITCHING TRANSIENT STUDY

• Quantifies Problem• Predict Exposure / Risk• Select Best / Most Cost Effective

Solution• Do “What if” Cases• Verify Results

RECOMMENDATIONS

• Factor into Design Up-front• Do Study – Results Are Bkr Manufacturer

Specific• Use Protection Only When / Where Needed

(if not there, cannot fail)• Fused or Unfused Snubbers??• Loss of Fuse Detection??• Fear Not! - Mitigating Techniques Have Been

Proven.• Discrete Snubber Components??

Conclusion/Next Steps• This is a System Problem

– Transformer, Cable, Switching Device, Proximity– Statistical Event , Possible Undetected Failures

• Data Centers Fall into the Highest Risk Category– High Power Density– Close Proximities– Frequent Switching

• Draft IEEE C57.142– A Guide To Describe The Occurrence and Mitigation Of Switching Transients

Induced By Transformer And Switching Device Interaction– Does not accurately warn users of all areas of concern– This case did not meet the areas of concern noted in Draft C57.142– Need to push for formalization of the standard with new lessons learned

• RC Snubbers– Transformer Manufacturer appears best positioned to implement the solution– Limited Cataloged Product (One Manufacturer)– Not embraced by all transformer manufacturers– Design Parameters of RC Snubber not clearly defined

• Lives, Property and Uptime are all at risk

QUESTIONS ?

3-1

3.0 EMTP MODELING & CASE STUDIES

1


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

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


2 2

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

3 3

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

2

4 4

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

5 5

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

6 6

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

3

7 7

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

8 8

Complete System Model

TRANSFORMER

RLRG

LTRAN RTRANT1 T2

CH CL

N:1

CABLE13.8KV

C1 LCABLERCABLE C2

C/2C/2


XUTILRUTIL

VUTIL

UN

U UT

VCBBKR

9 9

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

4

10 10

Switching Transient Simulations

• Switching transient simulations• EMTP and model requirements

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

• Successful techniques/solutions

11 11

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

12 12

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

5

13 13

Ferry - Oneline

VacuumBreaker

Short Cable

Dry-Type Transformer

Forward Reverse

14 14

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

15 15

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

6

16 16

MeasurementsTransient Overvoltages at Primary of Rectifier Transformer – VCB Closes

17 17

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.

18 18

Case 1 – Model ValidationTransformer primary voltage

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

7

19 19

Case 2 – Valid Model with SnubberTransformer primary voltage

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

20 20

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

21 21

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

8

22 22

Case 5 – Current ChopTransformer primary voltage

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

23 23

Case 6 – Current Chop with SnubberTransformer primary voltage

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

24 24

Summary of Current Chop Cases

9

25 25

Summary of Re-Ignition Cases

26 26

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

27 27

Data Center

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

10

28 28

Data Center - Oneline

VacuumBreaker

Short Cable

Cast-Coil Transformer

29 29

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

30 30

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

11

31 31

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.

32 32

Case 13 – no surge protectionLoad current at 13.2kV VCB

Load current of 10A

Chopped current of 6A

33 33

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

12

34 34

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

Load current of 10A


35 35

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

36 36

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

Load current of 10A


13

37 37

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

38 38

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

39 39

Comparison of Results

No Snubber

0.25uF surge cap

SnubberR=30ohmC=0.25uF

14

40 40

Measured Transients with Snubber

41 41

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

42 42

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

15

43 43

LMF & EAF - Oneline

VacuumBreaker

Short Bus

Oil-Filled Transformer

Existing EAF New

LMF

44 44

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

45 45

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

16

46 46

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

47 47

Case 1 – Open VCBLMF Transformer primary voltage

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

48 48

Case 2 – Open VCB with SnubberLMF Transformer primary voltage

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

17

49 49

Case 3 – Open SF6 BreakerAuto-regulating Transformer primary voltage

V max of 23kV < 200kV BILNo oscillating frequency

50 50

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

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

51 51

Case 5 – VCB Re-ignitionTransient Recovery Voltage for VCB

18

52 52

Case 5 – Highly Inductive CurrentInrush current to LMF transformer

6000A Peak Inrush

VCB Opens

VCB Closes

53 53

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

54 54

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

19

55 55

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

14000A Peak Inrush

SF6 CB Opens

SF6 CB Closes

56 56

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

57 57

Summary of Current Chop Cases

20

58 58

Summary of Re-Ignition Cases

59 59

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

4-1

4.0 MOTOR STARTING AUTOTRANSFORMER CASE STUDY

1

Medium Voltage Reduced Voltage AutotransformerStarter Failures

– Explaining The Unexplained –

Larry Farr, Senior Member IEEE Principal Engineer

Eaton ElectricalAsheville NC

Technical Advisor TC-17A IECChair of CANENA TC-17A SC-3

Arthur J. Smith III, Member IEEEVice President

Waldemar S. Nelson and Co. Inc.New Orleans, LA

• Background



TODAY’S AGENDA

• Investigation• Testing 1988• Findings•Testing 2002-03• Findings

• Solution• Conclusions

• Background



2

• For the past century the Auto-transformer or “Korndorfer”Starter has been a standard in the electrical industry.

Background

• In the late 1970s, unexplained “High Voltage Stress” failures started occurring.

Background

3

Failures Were Occurring Worldwide• North Sea Platform 12kV starters failed multiple

times. Problems solved with single phase autotransformers.

Background

• 20,000 hp-15kV starter in British Columbia failed four times until surge arresters from the Zero tap to ground were installed.

• South America 2400 volt failed multiple times.

• Gulf of Mexico – Multiple 5kV starter failures

• IEC 60470 Clause 6.102.7 Change-over Ability Test

• Off the West Coast of Africa – Multiple 5kV starter failures

3 Inches !!

What Happened?Zero Tap Circuit to Ground

Background

High Voltage Flashover

Telescoping CoilsLayer to Layer

Failure ModesBackground

4

High Voltage FlashoverTap to Tap Over the Surface

Background

High Voltage FlashoverCoil to Ground

Background

High Voltage FlashoverLayer to Layer Failure

Background

5

High Voltage FlashoverZero-tap Circuit to Ground

Background

Telescoping Coils

Background

• S and R Conducting Simultaneously• Flashover of S with R Closed

Background

Flashover of S with R Closing

6



• Background

• Investigation • Testing 1988

Testing

Initial Testing

Testing

Initial Testing

7

Testing

Initial Testing

Testing



• Background

• Investigation • Testing 1988

• Findings

8

Design Modifications

• Changed Design to Three Coil Three Legged Autotransformer

Findings

• Changed Control Circuit to Include a Transition on Current Below 125% FLA

• HOWEVER, Failures Continued at a Rate of Two to Three a Year !

The High Voltage Stress FailuresOccurred When:

• Transformers were on the 80% tap

Findings

• Transitions Were Forced at or Near Locked Rotor Current

Remaining Failures Required a Closer Look and Additional Testing

Findings

9



• Background

• Investigation • Testing 1988• Findings

• Testing 2002-03

Additional Testing

• However, The Sample Rate Was only 50,000 Samples Per Seconds and I suspect some level of smoothing.

• Confirmed the Test Results From 1988 on Three Coil Three Legged Transformers

(February 2002 High Power Lab in Pennsylvania)

Dielectric WithstandTwo Layers of .005 Nomex 14kV rms for 60 Secs.

Additional Testing

10

Partial DischargeAdditional Testing

November, December and JanuaryBelmont, North Carolina

Additional Testing

A Closer LookAdditional Testing

11

FebruaryArden, North Carolina

Additional Testing



• Background

• Investigation• Testing 1988• Findings• Testing 2002-03

• Findings

Issue Found !

Findings

12

Current Flat During Voltage EscalationFindings

When Reignition OccursFindings

Each Burst 70,000 Volts/Micro Sec

Each Burst70,000 Volts/Micro Sec

Findings

13

• Background• Investigation

• Testing 1988• Findings• Testing 2002-03• Findings

• Solution



New Circuit for Autotransformer Starters

Solution

With Surge Arresters12-13,000 Volts at 12-18 amps for 800 Micro Sec

Solution

14

• Background• Investigation

• Testing 1988• Findings• Testing 2002-03• Findings

• Solution• Conclusions



Conclusions

• When controllers are configured for transition on current sensing below 125% FLA, high voltage potential is reduced but not eliminated.

•When autotransformer starters are forced to transition before they reach near full speed, high voltages are generated on the 0% taps

Conclusions

• Vacuum contactor autotransformer starters and starter retrofits require surge arresters even if no problems were encountered with air break contactors

• When 6 kV distribution surge arresters are installed on a 4,160 volt circuit from the 0% tap to ground, the voltage is clamped to 13 kV without the resultant high dv/dt across the transformer coil regardless of transition current

15

Conclusions

• Care must be taken so the Metal Oxide arrestors do not conduct near normal operating voltages.

• The 15kV Autotransformer Starter in British Columbia was set on the 55% tap. The starter used MV Circuit Breakers. Therefore all autotransformer starters need surge arrestors zero tap to ground or across the VI,s.

5-1

5.0 PT FAILURES

1


Part 4: Potential Transformer FailuresDavid D. Shipp, PE



2 2

Case Studies

• Switching Transients – Opening• Switching Transients – Closing• Ferro- Resonance – Closing• Ferro-Resonance – Opening (20 HZ

Saturation)• Internal Resonance

3 3

CASE 12Midwest Data Center

• 12.47 kV System / 120 MW Load• Bkr Pairs with Unloaded wye-wye PTs for Auto

Transfer Sensing at Load End of Cables• Multiple Open and Closed Operations were

Performed Preceding the Failure.• 1st failure – Smoke But fuses did not Blow –

Cleared Manually.

2

4 4


• 2nd Failure – Identical Switching Events• Open Transitioned Back to Source “A”• A few Minutes Later A Load “Pop” Was heard.• More Smoke + B Phase Fuse Blew• Measurements Were Taken – Snuck Up on

Problem without PT Loading – Risked Failure

5 5


6 6

CASE 12 Closing From Generator Source – 300 kW Load

3

7 7

CASE 12 Opening From Generator Source – 300 kW Load

8 8

CASE 12Primary Measurements - 300 kW Load Opening

9 9

CASE 12Primary Measurements – Snubbers + 300 KW Load

4

10 10

CASE 12 - Closing Primary Measurements - 300 kW Load

11 11

CASE 12 – Closing - SnubberPrimary Measurements - 300 kW Load

12 12

PT Failure – Example 13

• PT 1500VA, 14400/120V, open delta, in gen-tie swgr• System 13.8 kV wye-solidly grounded• Upstream VCB switching• Cable lengths of 250 to 3600 feet• Failures

• #1 – catastrophic failure of PT in gen-tie swgr (2700ft cable)• #2 – a month later, catastrophic failure of PT in gen-tie swgr• No switching at time of failure/100s prior to failure

• Possible failure modes• Ring frequency of transient overvoltage on closing• Ferroresonance (PT saturation) on opening

5

13 13

CASE 13Switching Transient 1st Failure

14 14

PT transient on VCB closing (EMTP simulated)

15 15

PT - test without snubber (measured)

• Transient overshoot to -17.5kVpeak• High frequency oscillation follows • Oscillation persists for more than ¼ cycle on phase-b

Transient followed by high frequency ring

Oscillation continues beyond ¼ cycle

6

16 16

PT - test without snubber (zoom view)

1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 3200-18000

-16000

-14000

-12000

-10000

-8000

-6000

-4000

-2000

0

2000

4000

6000

8000

10000

12000

Volts

Recorded Volts/Amps/Hz Zoomed Detail: 04/27/2011 19:21:07.855.770.000 - 04/27/2011 19:21:07.857.283.100

Time 04/27/2011 19:21:07.854000 + us

• Magnitude is not the only problem• High frequency as well as high magnitude• Overtime both will damage PT

Magnitude effects –17.5kV

High frequency effects – 50kHz

11.27kVpeak nominal

17 17

PT - test with snubber

• Overshoot to 12.25kVpeak• Lower frequency oscillation• Oscillation very well damped

Transient just above normal crest

Oscillation well damped

18 18

PT – test with snubber (zoom view)

• Both magnitude and frequency are acceptable• PT is well protected by the snubber

2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800

-12000

-10000

-8000

-6000

-4000

-2000

0

2000

4000

6000

8000

10000

12000

Volts

Recorded Volts/Amps/Hz Zoomed Detail: 04/27/2011 20:09:12.582.800.000 - 04/27/2011 20:09:12.584.990.800

Time 04/27/2011 20:09:12.580000 + us

Low frequency 2000Hz

11.27kVpeak nominalLow Magnitude

12.25kV

7

19 19

PT saturation on VCB opening

simulated

measured

20 20

PT saturation on VCB opening (EMTP)

21 21

PT with 100W loading resistors VCB opening

6-1

6.0 SNUBBER PERFORMANCE MEASUREMENTS

1


Part 5: Snubber Performance MeasurementsDavid D. Shipp, PE



2 2

Snubber Performance Measurements

• Data Center• Purpose of Measurements• Test procedures• Medium voltage measurements

• Selecting a meter• Using voltage dividers• Safety concerns

• Data Center challenges

3 3

Purpose of Performance Measurements

• Ensure the proper operation of the snubbers• Measure the transient overvoltage waveforms at the transformer

primary • produced during switching of the primary vacuum circuit breakers• verify they do not exhibit high frequency transients (magnitude, rate-

of-rise and frequency). • Specify the meters and voltage dividers needed for the transient

measurement• guide electrical contractor with their installation• electrical contractor to operate all equipment following the test

procedure• After completion of the site monitoring during the snubber tests,

analyze the measurement data and document the test results in an engineering report.

2

4 4

Data Center

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

5 5

Data Center – Measurement Locations

VacuumBreaker

Short Cable

Cast-Coil Transformer

6 6

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

3

7 7

RC Snubber at MVS - Typical

200 A Disconnect to transformerRC Snubber

13.8 kV 0.25 microFarad30 ohm

Cast Coil3750KVA

Transformer

8 8

RC Snubber at MVS - Typical

Snubber Specs13.8 kV0.25 microFarad30 ohm

9 9

Test Procedure

• Test Procedure required by contractor• Prepared 2 weeks in advance of testing• developed the instructions • Included safety briefing each day• Site specifics supplied by the contractor

• LO/TO instructions• Breaker operations

• All meter connections made de-energized & LO/TO• No one in transformer room during tests• Signature of “Responsible Engineer” before test began• detailed test procedure form

4

10 10

Measurement Equipment

• Measurement equipment included voltage dividers and a transient recording device

• Voltage Dividers• Capacitive and resistive elements• 10MHz frequency response

• Three-Phase Power Quality Recorder• transient voltage waveshape sampling• 200 nsec sample resolution• 5 Mhz sampling

11 11

Test Measurement Setup (Typical)

12 12

Voltage Divider Connections

• Highly stranded No. 8, 15 kV insulated hookup wire• Insulated wire was a precaution – could have used

bare conductor• maintained 8inch minimum separation for 15kV

between phases and ground• Routed wires with gradual curve – no 90 degree

bends • Connection at surge arrester at bus to transformer

primary windings

5

13 13


14 14


• Base of each divider grounded• Grounded to ground bus in primary disconnect• Used same highly stranded no. 8, 15kV insulated

hookup wire

• Connection to PQ Meter• Cables to match input impedance of meter to that

of voltage divider• Voltage divider was calibrated prior to shipping to

site

15 15

CT Connections

• Primary interest was voltage at transformer• Secondary interest was snubber current• Connected CTs at ground side of snubbers

6

16 16

Initial view of events

Energize(see zoom)

De-energize(see zoom)

• RMS trend of voltage• VCB energized and de-energized transformer

17 17

Zoom View of Energize

• Slight overshoot above normal crest• Ring well damped

18 18

Zoom View of De-Energize

• No transient on opening• Well damped

7

19 19

Other Tests – transformer natural frequencies

• Sweep Frequency Response Analysis (SFRA)

Impe

danc

eP

hase

Ang

le

Frequency

20 20

Data Center Challenges

• Contractor requested measurements• Advised contractor

• First hookup/measurement – 1 day• Remaining 5 transformers – 2nd day• Prepare two voltage divider carts in advance

• Contractor agreed to schedule but…• This was a construction site• Project manager had other objectives• Snubber testing viewed as an “inconvenience”• Voltage divider carts were not prepared

• Be prepared to negotiate

21 21

Summary

• Prepare test procedure – 2 weeks in advance• Request onelines and site photos to plan hookups• Ship all equipment in advance• For safety, two engineers perform measurements• Follow test procedure

• Make all connections de-energized, LO/TO• No one in transformer room during test• Radio contact with contractor performing switching

• Download data after each test• Analyze results before proceeding to next test• Begin building test report while on-site

7-1

7.0 SUMMARY

1


SummaryDavid D. Shipp, PE



2 2

The Problem – In a Nutshell

• Switching transients associated with circuit breakers observed for many years

• Breaking opening/closing interacts with the circuit elements producing a transient

• The severity of the transient is magnified by breaker characteristics• Current chopping on opening• Pre-strike or re-ignition on closing

• In limited instances, the transient overvoltage exceeds transformer BIL resulting in failure

• RC snubber in combination with surge arrester mitigates the transient

3 3

What to look for…

“Rules of Thumb” to screen applications:• Generally, short distance between circuit breaker and

transformer • about 200 feet or less

• Dry-type or cast coil transformer • oil filled not immune and low BIL

• Inductive load being switched • transformer, motor, etc. (light load or no load)

• Circuit breaker switching characteristics: • chop (vacuum or SF6) or restrike (vacuum)

2

4 4

Predicting Transients – EMTP Simulations

• For purposes of screening applications for damaging TOVs• Source, breaker, cable and transformer modeled• Breaker models for current chop and re-ignition

TRANSFORMER

RLRG

LTRAN RTRANT1 T2

CH CL

N:1

CABLE13.8KV

C1 LCABLERCABLE C2

C/2C/2


XUTILRUTIL

VUTIL

UN

U UT

VCBBKR

5 5

Designing the Snubber

• 15kV typical snubber & arrester• transformer protection

• non-inductive ceramic resistor• 25 ohms to 50 ohms

• surge capacitor• capacitor ratings

0.15 μF to 0.35 μF• 3-phase 13.8kV solidly ground• 1-phase 13.8kV LRG

R

C

SA

TX

Surge Arrester

Resistor

Surge Cap

6 6

Performance Measurement Equipment

• Test equipment includes voltage dividers and a transient recording device

• Voltage Dividers• Capacitive and resistive elements• 10MHz frequency response

• Three-Phase Power Quality Recorder• transient voltage waveshape sampling• 200 nsec sample resolution• 5 Mhz sampling

8-1

8.0 REFERENCES Switching Transients 1. Shipp, Dionise, Lorch and MacFarlane, “Transformer Failure Due to Circuit

Breaker Induced Switching Transients”, IEEE Transactions on Industry Applications, April/May 2011.

2. Shipp, Dionise, Lorch and MacFarlane, “Vacuum Circuit Breaker Transients During Switching of an LMF Transformer”, IEEE Industry Applications Society Annual Meeting 2010, October 2010.

3. ANSI/IEEE, A Guide to Describe the Occurrence and Mitigation of Switching Transients Induced By Transformer And Switching Device Interaction, C57.142-Draft.

4. D. Shipp, R. Hoerauf, "Characteristics and Applications of Various Arc Interrupting Methods," IEEE Transactions Industry Applications, vol 27, pp 849-861, Sep/Oct 1991.

5. ANSI/IEEE, Standard for AC High-Voltage Generator Circuit Breakers on a Symmetrical Current Basis, C37.013-1997.

6. ANSI/IEEE, Application Guide for Transient Recovery Voltage for AC High-Voltage Circuit Breakers, C37.011-2005.

7. D. Durocher, “Considerations in Unit Substation Design to Optimize Reliability and Electrical Workplace Safety”, ESW2010-3, 2010 IEEE IAS Electrical Safety Workshop, Memphis.

Ferroresonance 1. IEEE Standard Dictionary of Electrical and Electronics Terms, ANSI/IEEE Std

100-1984. 2. Hopkinson, R.H., “Ferroresonant Overvoltages Due to Open Conductors,”

General Electric, 1967, pp. 3 - 6. 3. Westinghouse Distribution Transformer Guide, Westinghouse Electric Corp.,

Distribution Transformer Division, Athens, GA, June 1979, revised April 1986, Chapter 4 Ferroresonance, pp. 36 - 40.

4. IEEE Guide for Application of Transformers, ANSI/IEEE C57.105-1978, Chapter 7 Ferroresonance, pp. 22 – 28.

5. Distribution Technical Guide, Ontario Hydro, Ontario, Canada, May 1999, original issue May 1978, pp. 72.1-1 – 72.1-10.

6. Greenwood, A., “ Electrical Transients in Power Systems”, Wiley & Sons, 1971, pp. 91-93.

7. Kojovic, L., Bonner, A., “Ferroresonance - Culprit and Scapegoat”, Cooper Power Systems, The Line, December 1998.

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 707

Transformer Failure Due to Circuit-Breaker-InducedSwitching Transients

David D. Shipp, Fellow, IEEE, Thomas J. Dionise, Senior Member, IEEE,Visuth Lorch, and Bill G. MacFarlane, Member, IEEE

Abstract—Switching transients associated with circuit breakershave been observed for many years. Recently, this phenomenonhas been attributed to a significant number of transformer failuresinvolving primary circuit-breaker switching. These transformerfailures had common contributing factors such as the following:1) primary vacuum or SF-6 breaker; 2) short cable or bus con-nection to transformer; and 3) application involving dry-type orcast-coil transformers and some liquid-filled ones. This paper willreview these recent transformer failures due to primary circuit-breaker switching transients to show the severity of damagecaused by the voltage surge and discuss the common contributingfactors. Next, switching transient simulations in the electromag-netic transients program will give case studies which illustratehow breaker characteristics of current chopping and restrikecombine with critical circuit characteristics to cause transformerfailure. Design and installation considerations will be addressed,particularly the challenges of retrofitting a snubber to an exist-ing facility with limited space. Finally, several techniques andequipment that have proven to successfully mitigate the breakerswitching transients will be presented, including surge arresters,surge capacitors, snubbers, and these in combination.

Index Terms—Electromagnetic Transients Program (EMTP)simulations, RC snubbers, SF-6 breakers, surge arresters, switch-ing transients, vacuum breakers.

I. INTRODUCTION

TODAY, medium-voltage metal-clad and metal-enclosedswitchgears that use vacuum circuit breakers are applied

over a broad range of circuits. These are one of many typesof equipment in the total distribution system. Whenever aswitching device is opened or closed, certain interactions ofthe power system elements with the switching device can causehigh-frequency voltage transients in the system. The voltage-transient severity is exacerbated when the circuit breaker op-erates abnormally, i.e., current chopping upon opening andprestrike or reignition voltage escalation upon closing. Suchcomplex phenomena in combination with unique circuit char-acteristics can produce voltage transients involving energies

Manuscript received June 4, 2010; accepted July 9, 2010. Date of publicationDecember 23, 2010; date of current version March 18, 2011. Paper 2010-PPIC-188, presented at the 2010 IEEE Pulp and Paper Industry Conference,San Antonio, TX, June 21–23, and approved for publication in the IEEETRANSACTIONS ON INDUSTRY APPLICATIONS by the Pulp and Paper Indus-try Committee of the IEEE Industry Applications Society.

The authors are with Eaton Electrical Group, Warrendale, PA 15086 USA(e-mail: [email protected]; [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIA.2010.2101996

Fig. 1. Simplified electrical distribution system for data center.

which can fail distribution equipment such as transformers.Transformer failures due to circuit-breaker-induced switchingtransients are a major concern, which is receiving attention in adraft standard [1] and the focus of this paper.

A. Forensic Evidence for a Unique Case

Consider the case study of a new data center with a 26-kVdouble-ended loop-through feed to six dry-type transformerseach rated at 3000 kVA AA/3390 kVA FA and 26/0.48 kVand delta–wye solidly grounded, as shown in Fig. 1. The trans-former primary winding is of 150-kV basic impulse insulationlevel (BIL). A vacuum breaker was used to switch the trans-former. The 4/0 cable between the breaker and the transformerwas 33 kV and 133% ethylene propylene rubber. Primaryarresters were installed. The transformers were fully tested,including turns ratio, insulation resistance, etc. Functional testswere completed, including uninterruptible power supply (UPS)full load, UPS transient, data center room validation, etc. Inthe final phase of commissioning, a “pull-the-plug” test wasimplemented with the following results.

1) De-Energization Failure #1. Four electricians “simultane-ously” opened four 26-kV vacuum breakers to simulatea general utility outage. All systems successfully trans-ferred to standby generation but a “loud pop” was heardin Substation Room B and the relay for the vacuum circuitbreaker feeding transformer TB3 signaled a trip.

2) Energization Failure #2. Minutes later, two electri-cians “simultaneously” closed two 26-kV vacuum break-ers to substation Room A. Transformer TA3 failedcatastrophically.

0093-9994/$26.00 © 2010 IEEE

708 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011

Fig. 2. Transformer failure #2 during energization.

Fig. 3. Transformer failure #1 during de-energization.

Failure #2 is shown in Fig. 2. Examination of the primarywindings revealed that the coil-to-coil tap burnt off and thewinding terminal showed an upward twist. The burn marksfrom the initial Flash indicated the transient concentrated onthe first turns of the windings. Typically, closing the vacuumbreaker to energize the transformer is the worst condition.

Failure #1 is shown in Fig. 3. Examination of the primarywindings revealed Flash and burn marks on the B-phase wind-ing at the bottom and middle. Those at the top indicate a coil-to-coil failure, not a winding-to-winding failure, and indicate atransient voltage with high dv/dt. Those in the middle were aresult of the cable (used to make the delta connection) swingingfree. Supports were only lacking for this jumper (oversightduring manufacturing) which could not withstand the forces ofthe transient. This transformer passed the BIL test at 150-kVBIL but ultimately failed at 162-kV BIL.

All six transformers and cables were identical, but onlytwo failed during the vacuum-circuit-breaker switching. Thesignificant difference was that the two failed units had 40 ftof feeder cable while the others had 80 or 100 ft of feedercable. This short 40-ft cable, high-efficiency transformer, andvacuum circuit breaker proved to be the right combination toproduce a damaging voltage transient on both energization andde-energization.

B. History of Failures and Forensic Review

The previous example is not an isolated case. Instead, itis representative of a growing number of transformer failures

TABLE IHISTORY OF TRANSFORMER FAILURES RELATED TO

PRIMARY VACUUM BREAKER SWITCHING

due to primary switching of vacuum breakers. Table I details ahistory of transformers related to primary switching of vacuumbreakers occurring within the past three years.

In Case 1, in a hydro dam, the transformer was “valueengineered” with a 13.8-kV primary-winding BIL of 50 kV. TheBIL should have been 95 kV for the 13.8-kV class.

The 1955 switchgear was replaced with modern vacuumbreakers with only 20 ft of cable to the transformer. The userchose to energize the transformer before conducting a switchingtransient analysis and failed the transformer primary winding.The post mortem analysis revealed that no surge protection wasapplied.

In Case 2, in a hospital, the vacuum breaker wasclose-coupled through 27 ft of cable to a 2500-kVA dry-typetransformer with a 95-kV-BIL primary winding. The vacuumbreakers were supplied with no surge protection because theparticular vacuum breaker installed had a very low value of cur-rent chop. During vacuum breaker switching of the transformer,the transformer failed. The transformer was rewound, and surgeprotection/snubbers were installed.

In Case 3, in a railroad substation, vacuum breakersapplied at 26.4 kV were used to switch a liquid-filled rectifiertransformer with 150-kV-BIL primary winding. The switchingtransient overvoltage (TOV) failed the middle of the primarywinding. Forensic analysis determined a rectifier with dc linkcapacitors, and the transformer inductance formed an internalresonance that was excited by the switching. Such an LC seriesresonance typically fails the middle of the transformer primarywinding.

In Case 4, in a data center, vacuum breakers applied at26.4 kV were used to switch six dry-type transformers with150-kV-BIL primary windings under light load. Two transform-ers failed, one on breaker closing and the other on opening.The failed transformers were connected by 40 ft of cable to thevacuum breaker, while the other transformers had either 80 or100 ft of cable. Arresters were in place at the time of failure,but there were no snubbers.

In Case 5, in an oil field, a dry-type transformer for a variablespeed drive (VSD) had multiple windings to achieve a 36-pulseeffective “harmonic-free” VSD. A vacuum breaker at 33 kVwas separated from the transformer by only 7 ft of cable.

SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 709

TABLE IICURRENT CHOP VERSUS CONTACT MATERIAL

Arresters were applied on the primary winding. However, uponclosing the breaker, the transformer failed.

Finally, in Case 6, in an oil drilling ship, vacuum breakersdesigned to International Electrotechnical Commission (IEC)standards were applied at 11 kV and connected by 30 ft ofcable to a dry-type cast-coil propulsion transformer rated at7500 kVA. The transformer was also designed to IEC stan-dards, and the primary winding had a BIL of 75 kV. The IECtransformer BIL is much lower than the American NationalStandards Institute (ANSI) BIL for the same voltage classwinding. The transformer failed upon opening the breaker.

C. Common Parameters

The severity of the voltage surge, i.e., high magnitude andhigh frequency, and the damage caused by the voltage surgeare determined by the circuit characteristics. The following aresome “rules-of-thumb” to screen applications for potentiallydamaging switching transient voltages.

1) generally, short distance between circuit breaker andtransformer (about 200 ft or less);

2) dry-type transformer (oil filled and cast coil not immune)and low BIL;

3) inductive load being switched (transformer, motor, etc.);4) circuit-breaker switching characteristics: chop (vacuum

or SF-6) or restrike (vacuum).

D. Underlying Concepts

3) Current Chop. When a vacuum breaker opens, an arc burnsin the metal vapor from the contacts, which requires ahigh temperature at the arc roots [2]. Heat is supplied bythe current flow, and as the current approaches zero, themetal vapor production decreases. When the metal vaporcan no longer support the arc, the arc suddenly ceases or“chops out.” This “chop out” of the arc called “currentchop” stores energy in the system. If the breaker opensat a normal current zero at 180◦, then there is no storedenergy in the system. If the breaker opens, choppingcurrent at 170◦, then energy is stored in the system.

Current chop in vacuum circuit breakers is a ma-terial problem. Older vacuum interrupters (VIs) usedcopper–bismuth. Modern VIs use copper–chromium.Most copper–chromium VIs have a low current chop of

Fig. 4. Voltage escalation due to successive reignitions.

3–5 A, offering excellent interruption performance and amoderate weld strength. Table II shows the average andmaximum levels of current chop for copper–chromium,copper–bismuth, and other contact materials. It shouldbe noted that both vacuum and SF-6 interrupters currentchop. Current chop is not unique to vacuum breakers.

4) Reignition. Current chop, even though very small, coupledwith the system capacitance and transformer inductancecan impose a high-frequency transient recovery voltage(TRV) on the contacts. If this high-frequency TRV ex-ceeds the rated TRV of the breaker, reignition occurs.Repetitive reignitions can occur when the contacts partjust before a current zero and the breaker interruptsat high-frequency zeros, as shown in Fig. 4. On eachsuccessive reignition, the voltage escalates. The voltagemay build up and break down several times before inter-rupting. Although current-chop escalation with modernVIs is rare, a variation of this concept applies on closingcalled prestrike.

5) Switching inductive circuits. The transformer is a highlyinductive load with an iron core. The effect of switchingthis inductive load and core must be considered. Thecurrent cannot change instantaneously in an inductor.Energy cannot be created or destroyed; only the formof energy is changed. The energy in the inductor isdescribed by

1/2LI2 = 1/2CV 2 or V = I√

L/C. (1)

From the energy equation, it can be seen that, for shortcables, C is very small, which results in a very high surgeimpedance

√L/C. Energizing a cable produces a travel-

ing wave which reflects when it meets the discontinuity insurge impedance between the cable and the transformer. Thesurge impedance of a cable may be under 50 Ω, while the surgeimpedance of the transformer is 300–3000 Ω. In theory, thereflection can be as high as 2 per unit.


Fig. 5. Important circuit elements for EMTP modeling.

Vacuum circuit breakers are prone to current chopping andvoltage reignition while SF-6 circuit breakers are more proneto just current chopping. Air circuit breakers are not proneto either of any significant magnitude. Manufacturers designvacuum-circuit-breaker contacts to minimize the severity andoccurrence of abnormal switching leading to severe voltagesurges (the lowest current-chop characteristics are 3–5 A).Regardless of the circuit-breaker manufacturer, voltage surgesdo occur.

E. Characteristics of the Voltage Transient andTransformer Limits

The voltage transient that develops following the vacuum-circuit-breaker switching is influenced by three factors: storedenergy, dc offset, and the oscillatory ring wave. The voltagecomponent is due to the stored energy. The dc offset is de-termined by the X/R ratio of the cable and the transformer.The oscillatory ring wave is a result of the capacitance andinductance of the cable and the transformer. The magnitude ofthe voltage transient is compared with the transformer BIL. Ifthe magnitude is excessive, then the transformer winding willlikely fail line to ground. If the voltage transient has excessiverate of rise (dv/dt), then the transformer winding will likelyfail turn to turn (natural frequency of ring wave). For thetransformer to survive the transient, the insulation must be ableto withstand both the magnitude and the dv/dt. Dry-type trans-formers are particularly susceptible to vacuum or SF-6 breakerswitching transients. However, oil- or liquid-filled transformersare not immune. The oil has capacitance and acts like a surgecapacitor to slow the rate of rise of the voltage transient.The trend in modern-day power systems is to install trans-formers with high-efficiency design. As a result, these high-efficiency transformers have a very small resistance, whichoffers little or no damping to the voltage transient. Also, therepetitive effect, i.e., small indentations in the insulation, canoccur with each successive peak of the voltage transient.

II. PREDICTING PERFORMANCE WITH SIMULATIONS

A. Modeling the Circuit

When a statistical approach is taken for switching transients,complex modeling requires a frequency-dependent transformermodel and an arc model of the circuit breaker. For purposesof screening applications for potentially damaging switchingtransients, a simpler approach is suggested with the important

Fig. 6. Simplified electrical system for ship propulsion drive.

circuit elements modeled in electromagnetic transients program(EMTP) consisting of the source, breaker, cable, and trans-former, as shown in Fig. 5. The cable is represented by aPi model consisting of the series impedance and half of thecable charging at each end. In some cases, multiple Pi modelsare used to represent the cable. The vacuum or SF-6 breakeris represented by a switch with different models for opening(current chop), restrike (excessive magnitude of TRV), reig-nition (excessive frequency of TRV), and closing (prestrike).The three-phase transformer model consists of the leakage im-pedance, magnetizing branch, and winding capacitances fromhigh to ground and low to ground. For oil-filled transformers,the oil acts like a dielectric so the high-to-low capacitanceis modeled. In cases requiring more detail, the transformersaturation and hysteresis effects are modeled.

The choice of the integration time step will depend uponthe anticipated frequency of the voltage transient. If it is toolarge, the time steps will “miss” the frequency effects. If it istoo small, then this will lead to excessive simulation times. TheNyquist criteria call for a minimum sample rate of twice theanticipated frequency. In switching transients, the anticipatedfrequency is 3–25 kHz. When the circuit breaker opens, thetransformer primary winding is ungrounded. Also, the ringwave is a function of the natural frequency of the circuit

fnatural = 1/(2π√

LC). (2)

The iron core of the transformer dominates the inductanceof the circuit. The capacitance is very small for the dry-typetransformer and short cable. Consequently, the circuit’s naturalfrequency is 3–25 kHz with relatively short cables.


Fig. 7. Matching the simulation to field measurements.

B. Mitigating the Switching Transient

Various surge protection schemes exist to protect thetransformer primary winding from vacuum-breaker-switching-induced transients. A surge arrester provides basic overvolt-age protection (magnitude only). The arrester limits the peakvoltage of the transient voltage waveform. The surge arresterdoes not limit the rate of rise of the TOV. A surge capacitor incombination with the surge arrester slows down the rate of riseof the TOV in addition to limiting the peak voltage but doesnothing for the reflection or dc offset. The number of arresteroperations is greatly reduced because of the slower rate ofrise. There is a possibility of virtual current chopping. Finally,adding a resistor to the surge capacitor and surge arresterprovides damping, reduces the dc offset of the TOV waveform,and minimizes the potential for virtual current chopping. Theresistor and surge capacitor are considered an RC snubber.Selecting the values of resistance and capacitance are bestdetermined by a switching transient analysis study, simulatingthe circuit effects with and without the snubber.

C. Matching the Model to Measurements

The results obtained from simulation of switching transientsin EMTP are only as good as the choice of model and data used.When available, field measurements taken during the switchingtransients enable verification of the EMTP model. The EMTPmodel can be adjusted as needed to match the actual field-measured conditions. To illustrate this approach, consider theship propulsion electrical system in Fig. 6.

The system consists of 3 × 2865-kW generators, a 4160-Vthree-phase bus, two 1865-kW drives/motors for forward

Fig. 8. TRV leading to reignition during energization of drive transformerwith and without snubber.

TABLE IIICURRENT CHOP AND REIGNITION CASES FOR DRIVE PROPULSION

TRANSFORMER SWITCHING

propulsion and identical drives for reverse propulsion, eight1185-kVA dry-type transformers, and eight 630-A vacuumcircuit breakers. The critical parameters are the vacuum circuitbreaker, 50 ft of cable, and dry-type transformer of 30-kVBIL. Fig. 7 shows that the EMTP simulation results match thetransients captured in the field with a high-speed power-qualitymeter (closing). The simulation shows 4.96 kVpeak, which isless than 30-kV BIL; however, the oscillation frequency of20.2 kHz exceeds an acceptable limit of dv/dt. Having verifiedthe model, a series of current-chop cases and reignition caseswere run. Fig. 8 shows the TRV leading to reignition and theTRV with a snubber installed. Reignition occurs because theTRV peak, time to crest, and rate of rise of recovery voltageexceed IEEE ANSI C37.06 limits.


Fig. 9. Simplified electrical distribution system for Tier III data center.

The snubber reduces the TRV below the IEEE/ANSI limitsfor general-purpose vacuum breakers [3] and for generatorbreakers [4]. Table III summarizes the reignition cases andthe current-chop cases. In all cases, the snubber is effective inreducing the transient voltage.

D. Borderline Case

It is important to note that not all applications involvingprimary switching of transformers using vacuum breakers re-quire snubbers. The large majority of applications do not re-quire snubbers. Switching transient studies are conducted todetermine when snubbers are needed. In this paper, the caseswere selected to show different situations requiring snubbers.For the system shown in Fig. 9, the results were border-line; therefore, a snubber was still applied for reliability pur-poses. The Fig. 9 system is a Tier III data center with two24.9-kV incoming lines, two 12.5-MVA 25/13.2-kV transform-ers, a 13.2-kV ring bus, two 2250-KW generators, and six3750-kV cast-coil transformers.

Data centers fall into the highest risk categories because oftheir high load density, close proximities of circuit components,highly inductive transformers (high-efficiency designs), andfrequent switching. The critical parameters for the Fig. 9 systemare vacuum circuit breakers, 90-kV-BIL transformers, and cablelengths ranging from 109 to 249 ft. For the cable of 109 ft, theresults of opening the vacuum breaker with current chopping of8 A are shown in Fig. 10. The TOV is as high as 123 kVpeak

on phase A which exceeds the transformer BIL of 95 kV.The TOV exhibits a significant dc offset because there is verylittle resistance in the highly inductive circuit. The oscillationfrequency of 969 Hz is slightly less than the acceptable limit.

A snubber is required to reduce the peak below 95-kV BIL.The results of adding a snubber are shown in Fig. 10. Notethe significant reduction in the dc offset. The resistor in thesnubber provides the reduction in dc offset as well as damping.The peak is reduced to 28.6 kV and an oscillation of 215 Hz,

Fig. 10. TOV initiated by current chop during de-energization of cast-coiltransformer with and without snubber.

both within acceptable limits. Finally, field measurements weretaken after the snubber was installed to ensure that the snubbersperformed as designed. The field test setup for the snubberperformance measurements is discussed in Section IV. Thefield measurements showed that the snubber limited the TOVwithin acceptable limits.

E. Case of Switching a Highly Inductive Circuit

Now, consider the vacuum breaker switching of a highlyinductive circuit, such as the starting current of a large grinder


Fig. 11. Simplified electrical distribution system for LMF.

motor or an electric arc furnace. The vacuum-circuit-breakerswitching of an electric arc furnace and ladle melt furnacetransformers raises concern because of their high inductive cur-rents. High-frequency transients and overvoltages result whenthe vacuum breaker exhibits virtual current chop and multiplereignitions. As an example, the arc furnace circuit of Fig. 11consists of a 50-MVA power transformer, 2000-A SF-6 breaker,56-MVA autoregulating transformer, 1200-A vacuum breaker,and 50-MVA furnace transformer. The switching of the SF-6and vacuum breaker was studied. The vacuum breaker, becauseof the 28-ft bus to the furnace transformer, was the worstcase. The results opening the vacuum breaker with and withoutsnubbers are show in Fig. 12. The TOV of 386 kVpeak exceedsthe transformer BIL of 200 kV, and the oscillation of 1217 Hzexceeds the acceptable limit. Application of the snubber resultsin a TOV of 56.4 kVpeak that is below the transformer BIL,and the oscillation of 200 Hz is below the acceptable limit. Theresults for cases involving current chop and reignition are givenin Table IV.

F. Concerns for the Pulp and Paper Industry

The previous examples illustrate that circuit-breaker-inducedswitching transients can fail transformers for specific com-binations of circuit parameters and breaker characteristics.

Fig. 12. TOV during de-energization of LMF transformer with and withoutsnubber protection.

The examples show that the problem is not unique to oneindustry, application, vendor’s breaker, or transformer design.For the pulp and paper industry, there are many situationswhere circuit-breaker-induced switching transients are likelyto damage transformers. The following examples are some ofthe more common scenarios encountered in the pulp and paperindustry.

1) Vacuum breaker retrofit for primary load break switch ina unit substation. In the pulp and paper industry, thereare numerous unit substation installations with primaryload break fused switch and no secondary main breaker.This arrangement results in arc Flash issues on the low-voltage secondary. Limited space on the low-voltageside prevents installation of a secondary main breakerto mitigate the arc Flash issues. Retrofitting a vacuumcircuit breaker in the primary of the unit substation, inplace of the primary load break switch, and sensing onthe secondary is a solution that provides both primaryand secondary fault protection [5]. Unit substations mayhave oil-filled or dry-type transformers. The secondariesmay be solidly grounded or resistance grounded. With thevacuum breaker closely coupled to the transformer, surgearresters and snubbers are most likely needed.

2) Vacuum breaker and rectifier (or isolation) transformerinstallation. Rectifier transformers are installed to servedc drives such as those needed for feed water pumps tothe boilers. Also, isolation transformers are installed toserve a large VSD or groups of smaller drives. Primaryvoltages may be 13.8 or 2.4 kV, and secondary voltages


may be 600 or 480 V. In both situations, vacuum breakersare installed in the primary and closely coupled to thetransformer through a short run of bus or cable. Often,these transformers are inside and of dry-type design.

3) New unit substation with primary vacuum breaker.Recently, a paper mill installed a new metal-enclosedvacuum switchgear and a new 13.8/2.4-kV 7500-kVAtransformer for a bag house for the generator boilersto meet Environmental Protection Agency requirements.The vacuum breaker was connected to the transformerthrough 5 ft of bus. While doing the coordination and arcFlash studies, the switching transient issue was identified.The equipment was installed and was awaiting startupand commissioning when the studies raised the concern.Before energizing the transformer, snubbers were quicklysized, obtained, designed, and installed.

The screening criteria previously mentioned identify theaforementioned examples for potential damaging switchingtransient voltages due to vacuum breaker switching. Thevacuum breaker, short distance to transformer, and dry-typetransformer (or aged oil-filled transformer) are key variablesto consider. With such short distance between breaker andtransformer, most of these installations will require snubbers.One might conclude that standard snubbers could be applied.However, a switching transient study is still recommended todetermine the unique characteristics of the circuit and customdesign the snubber for the application. Given the limited spacein each of these examples, it is unlikely that off-the-shelfstandard snubbers would fit. A substantial part of the designeffort includes determining how to best fit the snubbers into thenew or existing unit substation or transformer enclosure.

III. DESIGNING THE SNUBBER

The preceding analysis has shown that, in some cases,switching transients can produce overvoltages that can resultin equipment insulation failure. If the results of the switchingtransient study indicate a risk of overvoltage greater than theBIL of the equipment and/or if the dv/dt limits are exceeded,a surge arrester and snubber should be applied. The switch-ing transient study may also indicate that multiple locationsrequire surge arresters and snubbers to protect the generator,transformer, or large motor. Additionally, the study specifies thenecessary protective components and determines how close theprotection must be placed to provide effective protection.

A. Design Requirements

At this point, custom engineering design determines how tobest provide the protection needed for the equipment. The fol-lowing questions must be answered to ensure that the snubberdesign meets all criteria and specifications.

1) Is the switching transient protection cost effective?2) What is the value of the equipment being protected?3) What is the cost of lost production if the equipment fails

from switching transients?4) Can the protection be installed within existing equipment

enclosures?

Fig. 13. Typical snubber and arrester arrangement for transformer protection.

Fig. 14. Example of standard snubber package for transformer.

5) Will equipment modification void equipment warranties?6) Are there standard equipment packages that can accom-

plish the switching transient protection?7) Is the application an indoor or outdoor application?8) Is there available space to mount the protective equipment

in an external enclosure?9) How can it be verified if the switching transient protection

components are working effectively?10) What alarms are necessary if the protective equipment

components fail?

B. Custom Engineering Design

Fig. 13 shows the typical snubber arrangement for trans-former protection. A noninductive ceramic resistor and asurge capacitor are the basic components of a snubber design.Resistance values typically range from 25 to 50 Ω. Standardcapacitor ratings that range from 0.15 to 0.35 μF are the basisof the design.

Fig. 14 shows a standard 15-kV surge protection package.The arresters are mounted on the top of the enclosure. A three-phase surge capacitor is mounted on the bottom. Insulators andbus are located in the center. The cables can enter from top orbottom. A ground bus is located on the center right. If spaceheaters are required for outdoor locations, they are located onthe lower left.

Fig. 15 shows one phase of a custom snubber circuit. Thecustom design was required because there was not enough roomin the transformer for the snubber components. The enclosure


TABLE IVCURRENT CHOP AND REIGNITION CASES FOR LMF TRANSFORMER SWITCHING

Fig. 15. 15-kV snubber mounted above the transformer.

had to be mounted above the transformer. The cable connec-tions from the transformer were field installed and land on thecopper bus. A 15-kV nonshielded jumper cable was used tomake the connection. Each phase passed through an insulationbushing to the transformer below. Bus work was required toprovide a solid support for the fragile resistors. Normally, onlyone resistor would be provided, but for this application, toachieve the delivery schedule, parallel resistors were designedto obtain the correct ohmic value (the correct single resistorvalue had long delivery).

Fig. 16(a) and (b) shows a snubber assembly mounted inmedium voltage switchgear. The photo on the left shows thesingle-phase surge capacitors mounted vertically. The blackcylinders are ceramic resistors. A variety of options are avail-able to detect if the snubbers are functional. They range fromnothing (oversized but treated like a lightning arrester) to verysophisticated loss of circuit detection. Glow tube indicators areshown at the top of Fig. 16(a), and a close-up is shown inFig. 17(a). These glow tubes are visible through a window inthe switchgear door and provide a visual indication of snubber

Fig. 16. 15-kV snubber mounted in switchgear with glow tubes and currentsensors.

continuity. The purpose of the blue current sensors at the bottomof Fig. 16(a) is to monitor the continuity of the resistor and fuse(optional) and alarm on loss of continuity. A close-up of thecurrent sensor is shown in Fig. 17(b). Some industries mandatefused protection. If there should be a broken resistor or a blownfuse, an alarm signal can be sent to the plant distributed controlsystem or supervisory control and data acquisition system toalert the operating personnel that these snubber componentshave failed. Fig. 16(b) shows a continuation of the same snub-ber assembly. Three fuses are attached to the tops of the resistor.The fuses will isolate any fault that may occur in the snubberassembly and prevent loss of the breaker circuit.

C. Special Design Considerations

The nature of high-frequency switching transients requiresspecial design considerations. The snubber designer should


Fig. 17. 15-kV snubber visual indication (glow tubes) and continuity verifi-cation (current sensors).

consider the location of the switching transient source when de-veloping the custom design layout of the protective equipment.Abrupt changes in the electrical path should be avoided. A lowinductive reactance ground path should be designed, using non-inductive ceramic resistors and flat tin braided copper groundconductors. The minimum clearances of live parts must meet orexceed the phase-to-phase and phase-to-ground clearances ofNEC Table 490.24. The enclosure should be designed to meetthe requirements of IEEE Standard C37.20.2 1999. When theenclosure is mounted greater than 10 ft from the equipment tobe protected, NEC tap rules may apply to the cable size requiredand additional circuit protective devices may be required.

D. Custom Designs for the Pulp and Paper Industry

As mentioned previously, given the limited space in eachof the examples related to the pulp and paper industry, it isunlikely that off-the-shelf standard snubbers would fit. Instead,a substantial part of the design effort includes determining howto best fit the snubbers into the new or existing unit substationor transformer enclosure. Following are three examples of thecustom design effort needed for snubber installation.

1) 13.8-kV Solidly Grounded System in Paper Mill. A vac-uum breaker was retrofitted into the enclosure for theprimary load break switch of the unit substation witha dry-type transformer. The space for the snubber wasextremely limited, as shown in Fig. 18 (left). Because thesystem was solidly grounded, the voltage on the surgecapacitor was limited to 8 kV line to ground; therefore,it was possible to use a three-phase surge capacitor. Thetight clearance required the use of glastic to insulate thecomponents at line potential from ground.

2) 13.8-kV Resistance Grounded System in Paper Mill. An-other example of retrofitting a vacuum breaker for aprimary load break switch with a 30-year-old oil-filledtransformer. Because this snubber was needed immedi-ately, the only available resistors had to be paralleled toobtain the desired resistance, as shown in Fig. 18 (right).Again, glastic was used for insulation and to support theresistors.

3) 13.8-kV Low Resistance Grounded System. Snubberswere provided for a new metal-enclosed vacuumswitchgear and a new 13.8/2.4-kV 7500-kVA transformer

Fig. 18. Snubber with (left) three-phase and (right) single-phase surge capac-itors for 13.8-kV paper mill application.

Fig. 19. Snubber for metal-enclosed 13.8-kV vacuum breaker.

for a bag house. Single-phase surge capacitors were used.Single resistors of the right ohmic value were available.Adequate clearance did not require the use of glasticas shown in Fig. 19.

IV. MEASUREMENTS TO VERIFY

SNUBBER PERFORMANCE

Following the installation of the snubbers, power qualitymeasurements may be taken to ensure the proper operation ofthe snubbers. A high-speed scope or power quality disturbanceanalyzer should be used to measure the TOV waveforms at thetransformer primary produced during switching of the primaryvacuum circuit breaker. The measurements are used to verifythat the waveforms do not exhibit excessive high-frequencytransients (magnitude, rate of rise, and frequency).

The test measurement setup generally consists of voltagedividers and a transient recording device. The voltage dividersshould be made of capacitive and resistive components witha bandwidth of 10 MHz. The scope or power quality metershould be capable of transient voltage wave shape sampling,


Fig. 20. Snubber performance measurement setup for 15-kV transformerusing high bandwidth voltage dividers.

Fig. 21. Voltage divider connections at 18-kV surge arrester on transformerprimary bushing.

8000-Vpeak full scale, and 200-ns sample resolution (5-MHzsampling).

An outage and lockout/tagout are needed to install the volt-age dividers, make connections to each of the three phases attransformer primary bushing, and make secondary connectionsto the transient recorder. Fig. 20 shows the test measurementsetup for a typical cast-coil transformer. Voltage divider con-nections to the transformer primary were made, as shown inFig. 21.

Additionally, for tests requiring load at a transformer, aportable load bank may be connected to the transformer sec-ondary. A resistive load bank configurable to different loadlevels (300 or 100 kW) prevents destructive testing.

V. CONCLUSION

This paper has reviewed recent transformer failures due toprimary circuit-breaker switching transients to show the sever-ity of damage caused by the voltage surge and discuss commoncontributing factors. Next, switching transient simulations inEMTP were presented to illustrate how breaker characteristicsof current chopping and restrike combine with critical circuitcharacteristics to cause transformer failure in unique situations.In these limited instances, mitigation of the transients is accom-plished with snubbers custom designed to match the specificcircuit characteristics. Design and installation considerationswere addressed, particularly the challenges of retrofitting a

snubber to an existing facility with limited space. Finally, theperformance of the snubbers is verified with field measurementsat the medium-voltage primary winding of the transformer.

REFERENCES

[1] ANSI/IEEE, A Guide to Describe the Occurrence and Mitigation of Switch-ing Transients Induced by Transformer and Switching Device Interaction.C57.142-Draft.

[2] D. Shipp and R. Hoerauf, “Characteristics and applications of various arcinterrupting methods,” IEEE Trans. Ind. Appl., vol. 27, no. 5, pp. 849–861,Sep./Oct. 1991.

[3] Standard for AC High-Voltage Generator Circuit Breakers on a Symmetri-cal Current Basis, C37.013-1997, 1997.

[4] Application Guide for Transient Recovery Voltage for AC High-VoltageCircuit Breakers, C37.011-2005, 2005.

[5] D. Durocher, “Considerations in unit substation design to optimize reliabil-ity and electrical workplace safety,” presented at the IEEE IAS ElectricalSafety Workshop, Memphis, TN, 2010, Paper ESW2010-3.

David D. Shipp (S’72–M’72–SM’92–F’02) re-ceived the B.S.E.E. degree from Oregon State Uni-versity, Corvallis, in 1972.

He is a Principal Engineer with the ElectricalServices and Systems Division, Eaton Corporation,Warrendale, PA. He is a distinguished scholar inpower system analysis and has worked in a widevariety of industries. He has spent many years per-forming the engineering work associated with hispresent-day responsibilities, which include a widerange of services covering consulting, design, power

quality, arc flash, and power systems analysis topics. Over the last few years,he has pioneered the design and application of arc-flash solutions, modifyingpower systems to greatly reduce incident energy exposure. He has written over80 technical papers on power systems analysis topics. More than 12 technicalpapers have been published in IEEE Industry Applications Society (IAS)national publications and two in EC&M. He spent ten years as a professionalinstructor, teaching full time. He occasionally serves as a legal expert witness.

Mr. Shipp is currently the Chair for the IEEE Industrial and CommercialPower Systems-sponsored Working Group on generator grounding. He hasreceived an IEEE IAS Prize Paper Award for one of his papers and conferenceprize paper awards for six others. He is very active in IEEE at the national leveland helps write the IEEE Color Book series standards.

Thomas J. Dionise (S’79–M’82–SM’87) receivedthe B.S.E.E. degree from The Pennsylvania StateUniversity, University Park, in 1978, and theM.S.E.E. degree with the Power Option fromCarnegie Mellon University, Pittsburgh, PA, in 1984.

He is currently a Senior Power Systems Engi-neer with the Power System Engineering Depart-ment, Eaton Corporation, Warrendale, PA. He hasover 27 years of power system experience involvinganalytical studies and power quality investigationsof industrial and commercial power systems. In the

metal industry, he has specialized in power quality investigations, harmonicanalysis and harmonic filter design for electric arc furnaces, rectifiers, andvariable-frequency drive applications.

Mr. Dionise is the Chair of the Metal Industry Committee and a memberof the Generator Grounding Working Group. He has served in local IEEEpositions and had an active role in the committee that planned the IndustryApplications Society 2002 Annual Meeting in Pittsburgh, PA. He is a LicensedProfessional Engineer in Pennsylvania.


Visuth Lorch received the B.S.E.E. degree fromChulalongkorn University, Bangkok, Thailand, in1973, and the M.S. degree in electric power engi-neering from Oregon State University, Corvallis, in1976, where he was also a Ph.D. candidate and wasinducted as a member of the Phi Kappa Phi HonorSociety. During his Ph.D. studies, he developed theShort Circuit, Load Flow, and Two-Machine Tran-sient Stability programs. The Load Flow programhas been used in the undergraduate power systemanalysis class. He also prepared a Ph.D. thesis on the

Load Flow program using the third-order Taylor’s series iterative method.In 1981, he joined Westinghouse Electric Corporation, Pittsburgh, PA. He

was responsible for conduction power system studies, including short circuit,protective device coordination, load flow, motor starting, harmonic analysis,switching transient, and transient stability studies. He also developed the Pro-tective Device Evaluation program on the main frame Control Data Corporationsupercomputer. In 1984, he joined the Bangkok Oil Refinery, Thailand, wherehis primary responsibility was to design the plant electrical distribution systemas well as the protection scheme for the steam turbine cogeneration facility. Hewas also responsible for designing the plant automation, including the digitalcontrol system for the plant control room. In 1986, he rejoined WestinghouseElectric Corporation. He performed power system studies and developed theShort Circuit and Protective Device Evaluation programs for the personalcomputer. In 1998, he joined the Electrical Services and Systems Division,Eaton Corporation, Warrendale, PA, where he is currently a Senior PowerSystems Engineer in the Power Systems Engineering Department. He performsa variety of power system studies, including switching transient studies usingthe electromagnetic transients program for vacuum breaker/snubber circuitapplications. He continues to develop Excel spreadsheets for quick calculationfor short circuit, harmonic analysis, soft starting of motors, capacitor switchingtransients, dc fault calculation, etc.

Bill G. MacFarlane (S’70–M’72) received theB.S.E.E. degree from The Pennsylvania State Uni-versity, University Park, in 1972.

He began his career in 1972 with Dravo Corpo-ration, Pittsburgh, PA, as a Power System DesignEngineer supporting this engineering/constructioncompany with expertise in the design and construc-tion of material handling systems, chemical plants,pulp and paper mills, steel facilities, ore benefaction,mine design, and computer automation systems. Heprovided full electrical design services in heavy in-

dustrial applications, primarily in the steelmaking and coal-preparation-relatedsectors. His responsibilities included client and vendor liaison and supervisionof engineers, designers, and drafters. From 1978 to 2003, he was a principalProcess Controls Systems Engineer with Bayer Corporation, Pittsburgh. Heprovided design and specification of instrumentation and control systems forchemical processes. He configured programmable logic controller and distrib-uted control systems. He was a Corporate Technical Consultant for powerdistribution systems. He had been the principal Electrical Engineer on manymajor projects with Bayer Corporation. He joined the Electrical Services andSystems Division, Eaton Corporation, Warrendale, PA, in 2004. At Eaton,he has been involved in a 230-kV substation design, ground grid design forsubstations, as well as in short circuit, protective device coordination, load flow,and arc-flash hazard analysis studies. He has also done power factor correctionand harmonics studies to resolve power quality issues.

Vacuum Circuit Breaker Transients During Switching of an LMF Transformer

David D. Shipp, PE Eaton Electrical Group

Power Systems Engineering

Warrendale, PA Fellow, IEEE

Thomas J. Dionise, PE Eaton Electrical Group


Warrendale, PA Senior Member, IEEE

Visuth Lorch Eaton Electrical Group


Warrendale, PA

William MacFarlane, PE Eaton Electrical Group


Warrendale, PA Senior Member, IEEE

Abstract— Switching transients associated with circuit breakers have been observed for many years. With the wide-spread application of vacuum breakers for transformer switching, recently this phenomenon has been attributed to a significant number of transformer failures. Vacuum circuit breaker switching of electric arc furnace and ladle melt furnace transformers raises concern because of their inductive currents. High frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple re-ignitions. This paper will present a detailed case study of vacuum breaker switching of a new ladle melt furnace transformer involving current chopping and re-strike simulations using the electromagnetic transients program. A technique that involves a combination of surge arresters and snubbers will be applied to the ladle melt furnace to show the switching transients can be successfully mitigated. Additionally, some practical aspects of the physical design and installation of the snubber will be discussed.

Keywords- Switching Transients, vacuum breakers, SF-6 breakers, LMF transformer, EMTP simulations, surge arresters, RC snubbers.

I. INTRODUCTION Electric Arc Furnaces (EAF) are used widely in the steel

industry in the production of carbon steel and specialty steels. The Ladle Melt Furnace (LMF) maintains the temperature of liquid steel after tapping the EAF and facilitates changes in the alloy composition through additives. In both cases, the furnace transformer is a critical component of the furnace circuit that is exposed to severe duty. The demands of the melt cycle may result in extensive damage to the furnace transformer due to electrical failures in the transformer. With advances in technology and metallurgy, the operation of arc furnaces today is significantly different. Heats of 4 to 5 hours with periods of moderate loading have been reduced to 3 to 4 hours with consistently high loading. Accompanying the shorter heats of sustained loading are many more switching operations. Combined, these factors impose thermal and electrical stresses on the transformer.

Frequent switching operations have been enabled by the development of the vacuum switch. The vacuum switch has been designed for hundreds of operations in a day, for long life and low maintenance. With the advantages of the vacuum switch, also come the disadvantages of switching transient overvoltages. Depending on the characteristics of the vacuum switch and the power system parameters, these switching transient overvoltages can be of significant magnitude and frequency to cause transformer failure. High frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple re-ignitions. According to statistics compiled by one insurance company [1], the application of vacuum switches has resulted in numerous failures of arc furnace transformers. These failures rates have been reduced by the application of surge arresters, surge capacitors and damping resistors [2]. The transients produced by the vacuum circuit breaker switching of an LMF transformer and their mitigation are the focus of this paper.

A. The LMF Circuit of Interest Consider the new LMF circuit of Fig. 1 that consists of a 50

MVA, 135/26.4 kV power transformer, a 2000 A SF-6 breaker, a 56 MVA, 27/10 kV auto-regulating transformer, a 1200 A vacuum breaker and a 50 MVA, 25/0.53 kV furnace transformer. The SF-6 circuit breaker is separated by 53 feet from the auto-regulating transformer. The vacuum circuit breaker is separated by 28 feet from the LMF transformer. The normal configuration (1) consists of the new LMF and the existing 4EAF operating in parallel. One alternate configuration (2) of the LMF circuit consists of 4OCB and 4EAF out-of-service with 4LTC in standby-service. A second alternate configuration (3) of the LMF circuit consists of LMF LTC out-of-service with 4LTC switched on-line to source the LMF. Each of these three possible configurations of the LMF circuit was considered. Of the three, the normal configuration results in the shortest bus length between the vacuum breaker and the LMF transformer. The normal configuration also results in the shortest bus length between the SF-6 breaker and the LMF transformer.

4AUTO LTC56MVA

27-10KV3.3%Z

1700A

EAF XFMR50/56MVA25/.53KV

2.5%Z4EAF38MW

(EXISTS)

NC NO

4OCB2000A

138KV

UTILITY4713MVA 3PH SC9.26 X/R

50/66/83MVA135.3/26.4KV

7.5%Z

1600A

13OHMNC

NC NO

2

2

3

3

AUTO LTC56MVA

27-10KV3.3%Z

VACUUM BREAKER1200A

LMF XFMR50/56MVA25/.53KV

2.5%ZLMF

20MW(NEW)

HEAVY DUTYCOPPER PIPE

28FEET

NC

SF-6 BREAKER2000A

27KVNC

ALUMINUMIPS BUS53FEET

NO NC

2

1

XFER BUS

XFER BUS

Figure 1. Simplified Electrical Distribution System for New LMF

B. Critical Characteristics of the Furnace Ciruit The severity of the switching transient voltage; i.e. high

magnitude and high frequency, and the damage caused by the transient overvoltage (TOV) are determined by critical characteristics of the LMF power supply circuit:

• Short distance between circuit breaker and transformer • BIL of the transformer • Inductive load being switched (transformer) • Circuit breaker switching characteristics: chop (vacuum or

SF-6) or re-strike or re-ignition (vacuum)

In the case of the furnace circuit, the vacuum or SF-6 breaker induced switching transients can be amplified by the short bus or cable length between the breaker and transformer. This amplification is due to the vacuum or SF-6 breaker chopped current and the system stray capacitance, especially that of the short bus or cable. In modeling the system for such switching transient analysis, it is important to accurately represent the vacuum or SF-6 breaker chopped current, stray capacitance of the short bus or cable and inductance of the transformer being switched.

The study approach was to evaluate the normal configuration (shortest bus lengths) shown in Fig. 1 which produces the worst case TOV during vacuum and SF-6 breaker switching and size the RC snubber for this worst case. The performance of the RC snubber was proven for this worst case of the normal configuration. The RC snubber designed for the worst case will therefore reduce the less severe TOVs produced during vacuum and SF-6 breaker switching for the two alternate configurations. For breaker opening cases, the transient recovery voltage (TRV) of the breaker was evaluated.

II. SWITCHING TRANSIENTS SIMULATIONS Switching transients simulations were conducted in the

electromagnetic transients program (EMTP) to investigate the possible failure of the new LMF transformer due to transient overvoltages during the circuit switching of the new vacuum and SF-6 circuit breakers. The LMF circuit model developed in EMTP consisted of the source, breaker, cable and transformer. The cable was represented by a Pi model consisting of the series impedance and half of the cable charging at each end. In some cases, multiple Pi models are used to represent the cable. The vacuum or SF-6 breaker was represented by a switch with different models for opening (current chop), re-strike (excessive magnitude of TRV), re-ignition (excessive frequency of TRV) and closing (pre-strike). The three phase transformer model consisted of the leakage impedance, magnetizing branch, winding capacitances from high-to-ground and low-to-ground. For oil filled transformers, the oil acts like a dielectric so the high-to-low capacitance was modeled. Two worst-case switching scenarios involving the 2000 A SF-6 breaker and the 1200 A vacuum breaker were simulated: 1) current chop by the breaker on de-energization of the LMF transformer and 2) re-ignition following opening of the breaker during energization of the LMF transformer. Also, the surge arrester was not modeled to show worst case.

A. Modelling Current Chop for Vacuum and SF-6 Breakers When a vacuum breaker opens, an arc burns in the metal

vapor from the contacts which requires a high temperature at the arc roots [3]. Heat is supplied by the current flow and as the current approaches zero, the metal vapor production decreases. When the metal vapor can no longer support the arc, the arc suddenly ceases or “chops” out”. This “chop out” of the arc called “current chop” stores energy in the system. If the breaker opens at a normal current zero at 180 degrees, then there is no stored energy in the system. If the breaker opens chopping current at 170 degrees, then energy is stored in the system. For modern breakers, current chop can range from 3 – 21 A depending on the contact material and design. Both vacuum and SF-6 interrupters current chop. Current chop is not unique to vacuum breakers. Fig. 2 illustrates the LMF transformer load current at time of the vacuum circuit breaker opening is 10 Amperes. The Phase-B pole opens first at 2.7891 msec, followed by Phase-A at 6.1875 msec and Phase-C at 6.4377 msec. The vacuum circuit breaker was modeled with 6 A chopped current. The SF-6 breaker was modeled similarly.

Figure 2. Current Chop for the Vacuum Breaker

B. De-Energize the LMF Transformer at Light Load by Opening the 1200A Vacuum Breaker In Case 1, the 1200 A vacuum breaker feeding the 56

MVA LMF transformer was opened to interrupt light load. The vacuum breaker was modeled as previously described with 6 A chopped current. This value provides a small safety margin for the vacuum breaker with an actual value of current chopping of 3 to 5 A for a vacuum breaker of this design. The results opening the vacuum breaker with and without snubbers are shown in Fig. 3. The TOV of 386 kVpeak exceeds the transformer BIL of 200 kV and the oscillation of 1217 Hz exceeds the acceptable limit. This TOV is unacceptable and indicates the need for an RC snubber in addition to a surge arrester.

An RC snubber was designed to protect the LMF transformer as explained in Sec. III. Sec. III provides the specifications for the RC snubber. In Case 2, the RC snubber was modeled with the same switching conditions of Case 1. In Case 2, application of the snubber results in a TOV of 56.4 kVpeak which is well below the transformer BIL and the oscillation of 200 Hz is below the acceptable limit as shown in Fig. 3. This TOV is acceptable and shows the RC snubber effectively controls the TOV. In Fig. 3, notice the resistor in the snubber reduces the DC offset of the transient voltage waveform. The resistor also provides damping of the transient voltage waveform. The vacuum breaker, because of the short distance of 28 feet of bus to the furnace transformer, produced the worst case TOV.

Figure 3. TOV During De-Energization of LMF Tranformer by the Vacuum

Breaker With and Without Snubber Protection

C. De-Energize the Auto-Regulating Transformer at Both No Load and Light Load by Opening the 2000A SF-6 Breaker In Case 3, the 2000 A SF-6 breaker feeding the 56 MVA

auto-regulating transformer was opened to interrupt no load. At the time, the 1200 A vacuum breaker feeding the LMF transformer was open. The SF-6 breaker was modeled with 6 A chopped current similar to that of the vacuum breaker. As with the vacuum breaker, this provides a safety margin for the manufacturer’s stated actual value of current chopping of 3 to 5 A. In Case 4, the switching conditions are the same as for Case 3, except the vacuum breaker is closed and the RC snubber circuit is applied at the primary side of the 56 MVA auto-regulating transformer. Fig. 4 compares the results for Cases 3 and 4 and shows the TOV magnitude at the primary side of the LMF transformer was negligible in both cases. A snubber is not required.

The results of the switching transient study of the LMF operation during current chop conditions are summarized in Table I. For each case the magnitude and frequency of the TOV are given. Acceptable and unacceptable levels of transient overvoltage are noted.

Figure 4. TOV During De-Energization of LMF Tranformer With and

Without Snubber Protection

TABLE I. SUMMARY OF CURRENT CHOP CASES FOR LMF TRANSFORMER SWITCHING

CurrentChop1 TOV Freq Transf

(A) (kV) (Hz) BIL1 Open IC 6 N 386.5 1,217 200 U2 Open IC 6 Y 56.4 200 200 A3 IO Open 6 N N/A N/A 200 A4 IC Open 6 Y 54.7 197 200 A

Notes: U = unnacceptable. A = acceptable. 1 = current chop on breaker opening. IO = initially open. IC = initially closed.

NoteCaseVacuum Breaker

SF6 Breaker RC

Case 2 - Voltage Waveform With Snubber

Case 1 - Voltage Waveform Without Snubber

Case 4 - Voltage Waveform With Snubber (light load)

Case 3 - Voltage Waveform Without Snubber (no load)

D. Modelling Re-ignition Current chop, even though very small, coupled with the

system capacitance and transformer inductance can impose a high frequency transient recovery voltage (TRV) on the contacts. If this high frequency TRV exceeds the rated TRV of the breaker, re-ignition occurs. Repetitive re-ignitions can occur when the contacts part just before a current zero and the breaker interrupts at high frequency zeros [4]. On each successive re-ignition, the voltage escalates. The voltage may build up and break down several times before interrupting.

E. Interrupt Inrush Current to LMF Transformer Followed by Re-Ignition of the Vacuum Breaker In Case 5, after the LMF transformer was energized for

about 3 cycles, the 1200 A vacuum breaker tripped open as shown in Fig. 5. On this initial opening, the TRV has an E2 of 62.95 kVpeak, T2 of 19 μsec and RRRV of 3.3133 kV/μsec. T2 and RRRV exceed the limits of [5] and [6] of 63 μsec and 1.1270 kV/μsec as shown in Table II. As a result, re-ignition occurs and the breaker opens at the next current zero. This second opening of the vacuum breaker produces a TRV with E2 of 217.67 kVpeak, T2 of 16 μsec and RRRV of 16.6044 kV/μsec, all exceed the limits of [5] and [6] of 71 kV, 63 μsec and 1.1270 kV/μsec respectively as shown in Table II. The voltage escalation due to the successive re-ignitions is shown in Fig. 6. The initial TRV and subsequent TRVs due to re-ignition are unacceptable and indicate the need for an RC snubber. The conditions of Case 6 are the same as Case 5, except an RC snubber is applied at the primary side of the 56 MVA LMF transformer. In Case 6, the vacuum breaker interrupting the inductive current of the LMF furnace transformer produces a TRV with E2 of 49.90 kVpeak, T2 of 130 μsec and RRRV of 0.3839 kV/μsec that are well below the limits of [5] and [6], and re-ignition is avoided.

Figure 5. TRV Across Vacuum Breaker During Interuption of LMF

Transfomer Inrush Current With and Without Snubber Protection

Figure 6. Voltage Escalation Due to Sucessive Re-ignitions

TABLE II. STD C37.06 EVALUATION OF TRV FOR VACUUM BREAKER FOR CASES 5 & 6

CASE 5 -

First Opening CASE 5

Reiginition CASE 6 with

Snubber IEEE ANSI

C37.06 Limit E2 (kV) 62.952 217.670 49.901 71 T2 (μsec) 19 16 130 63 RRRV (kV/μsec) 3.3133 13.6044 0.3839 1.1270

F. Interrupt Inrush Current to Auto-Regulating Transformer

by the SF-6 Breaker SF-6 breakers do not experience re-ignition. For

illustration purposes only, the conditions leading to re-ignition of the vacuum breaker are duplicated for the SF-6 breaker. In Case 7, after the auto-regulating transformer was energized for about 3 cycles, the 2000 A SF-6 breaker tripped open. At this time, the 1200 A vacuum breaker to the LMF transformer was open. The conditions of Case 8 were the same as Case 7, except with the application of the RC snubber circuit at the load side of the 2000 A SF-6 circuit breaker. Fig. 7 compares the results for Cases 7 and 8. For both Cases 7 and 8, the TRV across the SF-6 breaker was within the limits of [5] and [6] as shown in Table III. The TRV was not sufficient to cause re-ignition. The snubber is not required for SF-6 switching.

Figure 7. TRV Across SF-6 Breaker During Interuption of Auto-Regulating


Case 6 - TRV With Snubber

Case 5 - TRV Without Snubber (one phase shown)

Case 5 – Voltage Without Snubber(one phase shown)

Case 7 - TRV Without Snubber

Case 8 - TRV With Snubber

However, the snubber provides an additional benefit during closing of the SF-6 breaker to energize the auto-regulating transformer. In Case 8, with the application of the snubber circuit, the period of the transient voltage following closing of the SF-6 breaker to energize the auto-regulating transformer was reduced from 1,100 μsec to 230 μsec as shown below as shown in Fig. 8. This advantage alone does not justify the installation of an RC snubber at the primary of the auto-regulating transformer. A surge arrester at the primary of the auto-regulating transformer is adequate.

This results of the switching transient study of the LMF operation during re-ignition conditions are summarized in Table IV. The re-ignition was simulated for either the 1200 A vacuum breaker or 2000 A SF-5 breaker. The condition of the other circuit components for each case is described. For each case the magnitude of the transient recovery voltage (E2), the time to crest of the recovery voltage (T2) and the rate-of-rise of the recovery voltage (RRRV) are given. Acceptable and unacceptable levels of transient overvoltage are noted.

TABLE III. STD C37.06 EVALUATION OF TRV FOR SF-6 BREAKER FOR CASES 7 & 8

CASE 7

Reiginition CASE 8 with

Snubber IEEE ANSI

C37.06 Limit E2 (kV) 29.4 29.4 136 T2 (μsec) 75 75 106 RRRV (kV/μsec) .3923 .3923 1.1270

Figure 8. Transient Period of During Energizaton of Auto-Regulating


TABLE IV. SUMMARY OF RE-IGNITION SWITCHING CONDITIONS AND SWITCHING TRANSIENT RESULTS

Current IEEEChop1 ANSI

(A)C37.06 Limit

E2 (kV) 217.7 71T2 (μsec) 16 63

RRRV (kV/μsec) 13.6 1.127

E2 32.7 71T2 99.4 63

RRRV 0.3293 1.127E2 29.4 136T2 75 106

RRRV 0.3923 1.127E2 29.4 136T2 75 106

RRRV 0.3923 1.127Notes: U = unnacceptable. A = acceptable. 1 = current chop on breaker opening. IO = Initially open. IC = initially closed.

Transient Recovery Voltage Note

Y A

N

N U

8 IOOpen, No Reignition 6

Y A

7 A

6Open, No Reignition IC 6

IOOpen, No Reignition 6

5Open,

Reignition IC 6

CaseVacuum Breaker

SF6 Breaker RC

III. MITIGATING THE SWITCHING TRANSIENT Various surge protection schemes exist to protect the

transformer primary winding from vacuum breaker switching induced transients. A surge arrester provides basic overvoltage protection (magnitude only). The arrester limits the peak voltage of the transient voltage waveform. The surge arrester does not limit the rate-of-rise of the transient overvoltage. A surge capacitor in combination with the surge arrester slows down the rate-of-rise of the transient overvoltage in addition to limiting the peak voltage but does nothing for the reflection or DC offset. The number of arrester operations is greatly reduced because of the slower rate-of-rise. There is a possibility of virtual current chopping. Finally, adding a resistor to the surge capacitor and surge arrester provides damping, reduces the DC offset of the transient overvoltage waveform and minimizes the potential for virtual current chopping. The resistor and surge capacitor are considered an RC snubber. Selecting the values of resistance and capacitance are best determined by a switching transient analysis study, simulating the circuit effects with and without the snubber [7].

A. RC Snubber Ratings The specifications for the RC snubber circuit are given in

Fig. 9. The resistor average power rating at 40 °C is 1000 W and the peak energy rating is 17,500 joules. This RC snubber circuit, which has the same ratings as the one presently installed at the 3EAF, provides adequate mitigation of the voltage transients produced by either the vacuum breaker or SF-6 breaker and simplifies the inventory of spare parts, i.e. only one type of resistor and capacitor must be stocked.

R100OHM

C0.15uF

SA36kV Rating29kV MCOV

LMF TX50/56MVA

Figure 9. Snubber Specifications and Surge Arrester Arrangement for the

LMF Transformer Protection

Case 7 – Voltage waveform Without Snubber

Case 8 – Voltage waveform With Snubber

B. Locating the Snubber to Maximize Effectiveness To maximize effectiveness, apply the RC snubber circuit to

the primary-side of the 56 MVA LMF furnace transformer. Cases 1 and 2 have shown the RC snubber circuit reduces the magnitude of the transient overvoltage during switching of the vacuum breaker to be within the BIL of the LMF furnace transformer and reduces the oscillating frequency of the transient overvoltage to be less than 1,000 Hz. Cases 5 and 6 have also shown the RC snubber reduces the TRV of the breaker, when interrupting the inductive current of the LMF transformer, to be well below limits of [5] and [6].

Applying the RC snubber at the load-side of the SF-6 breaker is optional. In Cases 3 and 4 the transient overvoltage was negligible. In Cases 7 and 8 the transient recovery voltage is not sufficient to cause re-ignition. The only reason for applying the RC snubber circuit at the load-side of the SF-6 breaker is to reduce the transient period during energization of the LMF transformer. This advantage alone does not justify the installation of an RC snubber at the primary of the auto-regulating transformer. A surge arrester at the primary of the auto-regulating transformer is adequate.

C. Custom Designing the Snubber Given the limited space in the transformer vault, it was

unlikely off-the-shelf standard snubbers would fit. Instead, a substantial part of the design effort determined how best to fit the snubbers into the new transformer vault. Fig. 10 shows one phase of the custom RC snubber circuit . The custom design was required because of the 36 kV rating and the need to locate the snubber in close proximity to the LMF transformer terminals in a highly congested transformer vault.

Figure 10. 36 kV RC Snubber for LMF Transformer Protection

Figure 11. Plan View of LMF Transformer Vault Showing RC Snubber

The RC snubber is mounted 16 feet above the floor. The surge capacitor is mounted horizontally on an insulated stand-off bolted to the transformer vault wall. An insulator string was required to provide a solid support for the surge capacitor to counter the torque-arm of the 39.5 inch, 150 lb. component . The 39.5 inch, 20 lb. resistor was also mounted horizontally and the base was bolted to the transformer wall but did not require any additional support.

The nature of high frequency switching transients require special design considerations. The snubber designer should consider the location of the switching transient source when developing the custom design layout of the protective equipment. Abrupt changes in the electrical path should be avoided. A low inductive reactance ground path should be designed, using non-inductive ceramic resistors and flat tin braided copper ground conductors. The minimum clearances of live parts must meet or exceed the phase to phase and phase to ground clearances of NEC Table 490.24.

The cable connections from the snubber capacitor land on the 25 kV, 1.5 inch copper bus supplying the LMF transformer as shown in Fig. 11. 35 kV non-shielded jumper cable was used to make the connection between snubber and bus. The elevation of the bus is 18 feet and that of the surge capacitor is 16 feet with the difference allowing for a gradual curve of the jumper cable. Because of the high frequency transient overvoltages, gradual arcs are preferred over 90 degrees bends. Such an arrangement allows the snubbers to remain in place during the removal of the transformer, should maintenance or service be required. Also, the height of the snubbers provides adequate floor space for maintenance personnel.

IV. CONCLUSIONS This paper presented the findings of a detailed case study of

the vacuum breaker and SF-6 switching of an LMF furnace transformer. Through EMTP switching transient simulations, it was shown high frequency transient overvoltages result when the vacuum breaker exhibits virtual current chop and multiple re-ignitions. The simulations showed the SF-6 breaker switching transients were negligible primarily due to the longer distance of bus between the breaker and transformer. It was shown a technique that involves a combination of surge arresters and RC snubbers applied to the LMF transformer primary effectively mitigates the switching transients due to the vacuum breaker switching. Additionally, the practical aspects of the physical design and installation were discussed including the nature of high frequency switching transients which require the avoidance of abrupt changes in the electrical path, and the use of a low inductive reactance ground path.

REFERENCES

[1] Factory Mutual Insurance Company, “Arc Furnance Transformer Installations”, Property Loss Prevention Data Sheets, pgs. 1-13, January 2002.

[2] ANSI/IEEE, A Guide to Describe the Occurrence and Mitigation of Switching Transients Induced By Transformer And Switching Device Interaction, C57.142-Draft.

[3] D. Shipp, R. Hoerauf, "Characteristics and Applications of Various Arc Interrupting Methods", IEEE Transactions Industry Applications, vol 27, pp 849-861, Sep/Oct 1991.

[4] A. Morre, T. Blalock, “Extensive Field Measurements Support New Approach to Protection of Arc Furnace Transformers Against Switching Transients”, IEEE Transactions on Power Apparatus and Systems, vol. PAS-94, no. 2, March/April 1975, pgs. 473-481.

[5] ANSI/IEEE, Standard for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis, C37.06-1997.

[6] ANSI/IEEE, Application Guide for Transient Recovery Voltage for AC High-Voltage Circuit Breakers, C37.011-2005.

[7] D. Shipp, T. Dionise, V. Lorch, B. MacFarlane, “Transformer Failure Due to Circuit Breaker Induced Switching Transients”, 2010 IEEE Pulp and Paper Industry Conference Record, San Antonio.