March 2017 IEEE NCS Power and Instrument Transformer ...sites.ieee.org/northern-canada-pesias/documents/2017/04/march-2017... · • Dry-type or Low BIL more susceptable, ... or more

Power and Instrument Transformer Failures Root Causes and Modern Methods Used to Mitigate Risk

Tamer Abdelazim, PhD, PEngSenior Member, IEEE

Eaton Corporation2611 Hopewell Pl NECalgary, AB T1Y 3Z8

Scott Basinger, P.Eng.Senior Member, IEEE

Eaton Corporation12465 153 Street NW

Edmonton, AB T5V 1E4

IEEE-NCS,IAS/PES (March 2017)

2

Outline

• Introduction

• Power Transformer & VT Failure

• Underlying Concepts

• Baseline Testing

• Solution Design

• Solution Testing

• Conclusions

3

Introduction

• Over the past few years there have been a number of high profile failures of Power Transformers and Voltage Transformers in medium voltage (5-38kV) applications.

• These failures have become more frequent because of changes in equipment selection and design in associated power systems.

• For MV Power Transformers, failures have generally been associated with circuit breaker transient switching:• Modern MV Breakers chop current on opening, and pre-strike

current on closing. • High di/dt interacts with the circuit system characteristic

producing a transient voltage of high frequency and high voltage, sometimes exceeding the TRV / RRRV withstand of the breaker, and the BIL rating of the transformer.

• Dry-type or Low BIL more susceptable, but oil-filled not immune.

• Short cable or bus connection to transformer.

4

Introduction

• Most applications do not require mitigation (RC snubbers), but some design choices will make the issue more likely:• higher “DOE” energy-efficient transformers, • shorter distance between MV breaker and transformer,• dry-type MV transformers with lower BIL designs,• MV breaker Vacuum Interrupters designed with higher (>5A)

current chop.

• Switching Transient Studies can help identify the risk before it becomes a problem.

5

Introduction

• For PTs/VTs (Potential Transformers / Voltage Transformers), switching transients can lead to failure as well, although based on our experience, this is the least common mode of failure.

• The mode of failure we`ve started to see as a more common mode of failure in the field is due to ferroresonance.

• IEEE Standard 100-1984 defines ferroresonance as “A phenomenon usually characterized by overvoltages and very irregular wave shapes, and associated with the excitation of one or more saturable inductors in series with capacitance.

• In a ferroresonant circuit, the capacitance Xc can be the capacitance of the cable, overhead lines, or stray capacitance of transformer windings or bushings. Under normal circumstances Xc is smaller than XL.

• In a switching event, voltage can increase, driving the transformer core into saturation and XL is lowered. This `jolts` the transformer XL into a lower saturated value where XL = Xc and ferroresonance starts.

6

Introduction

• After the upstream breaker opens, there will be a DC trapped voltage on the open line. This trapped voltage can be high enough to saturate the PT and `ring` with a square waveform.

• This phenomenon is made worse on an under-loaded PT.• Modern protective relays have very low burdens compared to

electromechanical.• The combination of the two design characteristics can lead to

some fairly costly failures.• Study is required to determine when, but failures observed in the

field are typically associated with longer lines and low burdens on switchgear PTs.

• Can mitigate by several methods to burden the PTs such as loading resistors, saturable reactor (new approach), or a frequency relay tied to a resistor and a timer.

7

Transient Overvoltages: 3 Problems

• Transient Overvoltage (TOV and/or dv/dt) Condition (magnitude and frequency) on Power Transformers leading to Failures.

• Transient Recovery Voltage (TRV), Rate of Rise of Recovery Voltage (RRRV) important to proper Vacuum Circuit Breaker interruption.

• PT Ferroresonance due to saturation in VCB switching operations.

8

Introduction

• Forensic evidence and history of failures

• Underlying concepts

• Predicting performance with simulations

• Mitigating the transients with snubbers

• Custom designing the snubber

• Snubber performance measurements

9

Unique Case – Forensic Evidence

• Four electricians “simultaneously” opened four 26kV VCBs• simulate utility outage• systems transferred to

standby generation• “loud pop” in Sub Rm B • the relay for VCB feeding

transformer TB3 signaled trip• Minutes later, two electricians

“simultaneously” closed two 26kV VCBs• breakers to Sub Rm A• transformer TA3 failed

catastrophically

10

Transformer Failure #1 – De-energization

• Examination of primary windings• Flash and burn marks on b-phase at bottom & middle

• Bottom - Indicate a coil-to-coil flashover (high dv/dt)

• Middle – cable used to make delta swung free (lack of support)

• Transformer passed BIL test at 150kV but failed at 162kV

Both failed units:

• 40 feet of cable

• High efficiency design

• VCB switching

11

Transformer Failure #2 - Energization

• Examination of primary windings• Coil-to-coil tap burn off

• Winding showed an upward twist

• Burn marks from the initial blast

• Transient on first turns of windings

12

History of Failures – Forensic Review

Case Facility VoltageCable Feet Bil Type Arrester

Failure Mode Vendor Switching

1* Hydro Dam 13.80 20 50 Dry No 1st turn A Close

2 Hospital 13.80 27 95 Dry No 1st turn A Close

3 Railroad 26.40 37 150 Liquid N/A middle A Open

4 Data Center 26.40 40 150 Cast coil Yes 1st turn B Close/Open80 150 Cast coil Yes None B Close

5 Oil Field 33.00 7 Dry No 1st turn C Close

6** Oil Drill Ship 11.00 <30 75 Cast coil Yes 1st turn C Close

Notes: * = 40-50yrs. old with new breaker. ** = 2 yrs. old. All others new.*** = All transformers unloaded or lightly loaded when switched.

Circuit Vacuum BreakerTransformer***

13

Common Parameters

“Rules of Thumb” to screen applications:

• Generally, short distance between circuit breaker and transformer • about 200 feet or less

• Dry-type transformer • oil filled and cast coil 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)

14

Underlying Concepts - Current Chop

• VCB opens, arc burns metal vapor

• Heat supplied by current

• As current goes to zero, metal vapor ceases

• Arc ceases or “chops”

• All breaker chop current

• low end 3 – 5A

• high end 21A

Contact Material Average (A) Maximum (A)CU 15 21Ag 4 7Cr 7 16W 14 50Cr-Cu (75 wt %) 3 5Cr-Cu-Bi (5 wt %) 1 3Cr-Cu-Sb (9 wt %) 4 11Cu-Bi (0.15 wt %) 6 21WC-Ag (50 wt %) 1.5 2.5W-Cu (30 wt %) 5 10Co-AG-SE 0.4 0.8Cu-Bi-Pb 1 9

6A current chop

Modern VI

Older VI

15

Reignition and Voltage Escalation

• Current chop plus system C and L imposes high frequency TRV on VCB contacts

• If TRV exceeds breaker rated TRV, then reignition occurs

• VCB closes and then opens high frequency current

• Multiple reignitions lead to voltage escalation

16

Switching inductive circuits

• Current cannot change instantaneously in an inductor (conservation of energy)

• Energy Equation ½ LI2 = ½ CV2 or V = I L/C

• Vtransient = Venergy + Vdc + Vosc

• Venergy is from the Energy Equation

• Vdc = DC Off-set due to system X/R

• Vosc = the Oscillatory Ring Wave

17

Predicting Performance – 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

SYSTEM SOURCEAT 13.8 KV

XUTILRUTIL

VUTIL

UN

U UT

VCBBKR

18

Matching model to measurements

VacuumBreaker

Short Cable

30KV BIL

V max of 4.96kV < 30kV BIL

Oscillation of 20.2kHz

3 x 2865KW Gens

1865KW motors

1185KVA

630A

19

Transient Mitigation

• Surge Arrester• Overvoltage protection (magnitude only)

• Surge Arrester + Surge Capacitor• Overvoltage protection

• Slows down rate-of-rise

• Surge Arrester + RC Snubber• Overvoltage protection

• Slows down rate-of-rise

• Reduces DC offset and provides damping

20

Breaker opening followed by reignition

R = 40ohm

C = 0.5uF

TRV exceeds limit

TRV within limit

C37.011

C37.011

21

A Borderline Case

• Tier III Data Center

• 2x 24.8kV lines

• 2 x 12.5MVA service

• 13.2kV ring bus

• 2 x 2250KW generators

• 6 x 3750KVA cast-coil transformers 90kV BIL

• VCBs on primary side

• 109 – 249 ft. cables

123kV 969Hz

38.6kV 215Hz

R = 30ohm

C = 0.25uF

22

Switching a Highly Inductive Circuit

386kV 1217Hz

56.4kV 200Hz

R = 100ohm

C = 0.25uF

138KV

UTILITY4713MVA 3PH SC9.26 X/R

50/66/83MVA135.3/26.4KV

7.5%Z

SF-6 BREAKER2000A

1600A

27KV

13OHM

AUTO LTC56MVA

27-10KV3.3%Z

ALUMINUMIPS BUS53FEET

VACUUM BREAKER1200A

LMF XFMR50/56MVA25/.53KV

2.5%Z

HEAVY DUTYCOPPER PIPE

28FEET

LMF20MW

VacuumBreaker

Short Bus

200KV BIL

23

Concerns for the Industry

• VCB retrofit for primary load break switch (LBS)• Units subs with LBS and no secondary main• Arc flash issues on sec main (no room to install secondary

main breaker)• Retrofit VCB in LBS box solves AF issue

• VCB for rectifier (or isolation) transformer• DC drives for feed water pumps• VCB on primary • Short run of cable to transformer (often dry type)

• New unit sub with primary VCB• Metal enclosed vacuum switchgear • 7500KVA transformer for gen boilers to meet EPA requirement• 5 feet of bus

24

Test without snubber & with snubber

Energize without snubber

Transient followed by high frequency ring

Oscillation continues beyond ¼ cycle

Transient near normal crest

Oscillation well damped

Energize with snubber

25

VT failures – Circuit configuration

• A site with a total of 25 SWGRs at 24.9kV

• A simplified one line diagram

• Each SWGR has two line-end VTs, one bus VT, and one bus CPT.

• The UTS breakers are VCBs

• The UPM devices are manually operated fused padmounted switches

• Failed VTs during commissioning

• Performed a comprehensive transient study

• analyzing transient overvoltage and ferroresonance

• coupled with test measurements

26

Line-end VT failures

• Two line-end VTs at 24.9 kV in the switchgear failed on Nov. 18, 2013 which initiated our investigation

27

Line-end VT failures

• When the dielectric rating of the insulation is exceeded, adjacent windings short together, heat and pressure build-up, and the VT housing ruptures.

• Could take days to weeks before the external evidence occurs.

28

Bus VT failures

• During the investigation, the bus VT for the same gear failed on Dec. 28, 2013

29

VT Failures – Ferroresonance and transient overvoltage

• Switching transients associated with circuit breakers observed for many years

• Breaker opening/closing interacts with the circuit elements producing ferroresonance or a transient overvoltage

• VTs and CPTs applied in system designs not seen 10 years ago

• We have investigated nearly 150 VT and CPT failures in past 3 years alone

• Failures may take hours, days or weeks to be detected

• Across all voltages – 5kV, 15kV, 25kV and 35kV

• Failures not unique to any one manufacturer

• Mitigating the ferroresonance • with damping resistors (conventional solution)• with saturable reactors (specialized, custom solution)

• Mitigating the transient overvoltage• RC snubber in combination with surge arrester

30

Underlying Concepts - Ferroresonance phenomenon and the VT

• When the source of current is interrupted, the trapped dccharge on the cables (capacitance) discharges into the VT.

+

-

+

-

+

-

+

- Dc trapped charge

VTNonlinear

inductance

31

Underlying Concepts - Simplified diagram

• The primary system circuit can be further simplified by lumping the system capacitance into a single representative capacitor and representing the inductance of the transformer as multiple smaller inductors in series.

• The over voltage across the primary winding is not equally distributed, resulting in high stress voltage differences across some of the windings.

• If the voltage difference between winding is in excess of their dielectric/insulation limits.

Dc trapped charge

VTNonlinear

inductance

High stress

32

Underlying Concepts – VT excitation curve

• Referring to the excitation diagram, one can see that it takes very little overvoltage to drive significantly higher excitation current through the thin primary conductors.

• With a turns ratio of 208/1, for only a 50% increase in voltage (180 V), the primary excitation current alone is 0.06 A (i.e. 13.346 / 208) – or the full load rating of the primary winding.

Amperes volts

0.036727 10

0.050097 20

0.071112 30

0.09186 40

0.11584 50

0.13756 60

0.16037 70

0.18175 80

0.20233 90

0.22254 100

0.24381 110

0.28236 120

0.38503 130

0.61993 140

1.3408 150

3.2221 160

6.9868 170

13.346 180

475%

4700%

100%

33

Baseline Testing - Instrumentation

• Test equipment includes voltage dividers and a transient recording device

• Voltage Dividers (Capacitive and resistive elements, 10MHz frequency response, SF6 insulated)

• Three-Phase Power Quality Recorder (Transient voltage waveshape sampling)

• 200 nsec sample resolution

• 5 Mhz sampling

34

Baseline Testing - UTS breaker opening

17:50:35.640 17:50:35.660 17:50:35.680 17:50:35.700 17:50:35.720 17:50:35.740

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

Volts

Recorded Volts/Amps/Hz Zoomed Detail: 02/25/2014 17:50:36 - 02/25/2014 17:50:36

HH:MM:SS.mmmV Waveform Avg AG V Waveform Avg BG V Waveform Avg CG

(f ile C a s e 3 . p l4 ; x -v a r t ) v : S 1 C 0 3 A v : S 1 C 0 3 B v : S 1 C 0 3 C 0 .0 0 0 .0 5 0 .1 0 0 .1 5 0 .2 0 0 .2 5 0 .3 0[s ]

- 2 5 .0 0

- 1 8 .7 5

- 1 2 .5 0

- 6 .2 5

0 .0 0

6 .2 5

1 2 .5 0

1 8 .7 5

2 5 .0 0

[kV ]

Measurement

Simulation

Squarewaves with 17Hz

Squarewaves with 17-25Hz

35

Baseline Testing - UTS breaker opening

17:30:05.050 17:30:05.100 17:30:05.150-100

-80

-60

-40

-20

0

20

40

60

80

100

Volts


HH:MM:SS.mmmV Waveform Avg AN V Waveform Avg BN V Waveform Avg CN

Measurement



36

Baseline Testing - UPM switch closing

11:39:32.885 11:39:32.890 11:39:32.895 11:39:32.900 11:39:32.905

-200000

-150000

-100000

-50000

0

50000

100000

150000

200000

250000

300000

Volts



11:26:17.685 11:26:17.690 11:26:17.695 11:26:17.700 11:26:17.705 11:26:17.710

-300000

-250000

-200000

-150000

-100000

-50000

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

Volts



Measurement

Measurement

300kV

430kV

37

RC Snubber + Surge Arrester (2)

Saturable Reactor (1)

Solution Design – Saturable Reactor + RC Snubbers + Surge Arresters

100 Ohm Noninductive22 kVpeak17 . 5 KJoules0 . 25 μ F at 27 kV

SA

VT 1 1

2 2

1

38

Solution Design - Saturable Reactor Circuit• The reactor is sized such that as Vs increases, the reactor saturates just before the VT.

It is designed to absorb the high excitation current and act as a switch to insert its internal resistance.

• In addition to matching the magnetic characteristics to the specific VT, the reactor’s internal resistance is “tuned” to the system parameters for critical damping.

• Since the reactor saturates before the VT and inserts its resistance into the secondary circuit, the VT never goes into saturation.

• Our modeling optimized the saturation characteristic and the resistance value.

• The high secondary current is reflected back into the primary, but it is such a short duration that it does not overheat the primary windings.

Xm Rm

Xs

Ie

IPIp

VP

RpXp

Vp

Rs

Vs

Is

Is

R

39

Solution Design – Saturable Reactor

• Desired Solution - only apply increased loading on the VT when it experiences ferroresonance.

• The saturable reactor on the secondary side of the VT only conducts current when the voltage begins to increase above its saturation voltage – but below that of the VT’s saturation voltage.

• An “off the shelf” reactor was initial used for testing, but produced worse results since it saturated below the secondary operating voltage of the VT – premature saturation.

• Since the reactor saturates before the VT and inserts its resistance into the secondary circuit, the VT never goes into saturation.

40

Solution Design – RC Snubber

• 27kV typical snubber & arrester• voltage transformer protection

• non-inductive ceramic resistor• 100 ohms – Typically 25 to 100

ohms

• surge capacitor• 0.25 μF - Typically 0.15 μF to

0.35 μF

• 3-phase 24.9kV solidly ground

Surge Arrester

Resistor

Surge Cap

100Ohm Noninductive22kVpeak17.5KJoules0.25μF at 27kV

SA

VT

41

Solution Design – Damping Resistor

• A conventional solution was developed using damping resistors of 250W per phase• Easily available

• Provided damping in 1200ms

• Can affect VT accuracy

• Will consume high power during steady state

• Requires switching circuit to be active only during ferroresonance

(file CaseRp.pl4; x-var t) v:S1C03A v:S1C03B v:S1C03C

0.0 0.3 0.6 0.9 1.2 1.5-25.00

-18.75

-12.50

-6.25

0.00

6.25

12.50

18.75

25.00*103

(file CaseRp.pl4; x-var t) v:R -

0.0 0.3 0.6 0.9 1.2 1.50

100

200

300

400

500

VT Voltage

VT Power

42

Solution Design – Saturable Reactor

• The special design using saturable reactors• Custom design

• Requires extensive simulations

• Provided damping in 250ms

• No affect on VT accuracy

• Will consume very small power during steady state

• No additional switching circuits required to make it work

(file CaseLp.pl4; x-var t) v:S1C03A v:S1C03B v:S1C03C

0.0 0.3 0.6 0.9 1.2 1.5-25.00

-18.75

-12.50

-6.25

0.00

6.25

12.50

18.75

25.00*103

(file CaseLp.pl4; x-var t) v:RL -

0.0 0.3 0.6 0.9 1.2 1.50

1000

2000

3000

4000

5000

6000

7000

8000

VT Voltage

VT Power

43

Solution Testing - UTS breaker opening

13:05:54.550 13:05:54.600

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

Volts



(file Case_H_DesignE-Final1.pl4; x-var t) v:S1C03B v:S1C03A v:S1C03C

0.00 0.05 0.10 0.15 0.20 0.25[s]-25.00

-18.75

-12.50

-6.25

0.00

6.25

12.50

18.75

25.00[kV]

Only 250ms

1 1

2 2

1

Measurement

Simulation

1 = saturable reactor 2 = snubber & arrester

44

Solution Testing - UTS breaker opening

1

14:05:55.300 14:05:55.350 14:05:55.400

-100

-80

-60

-40

-20

0

20

40

60

80

100

Volts



(file Cell3_inductor_H_Design.pl4; x-var t) v:TX0001- v:TX0003- v:TX0002-

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35[s]-100

-75

-50

-25

0

25

50

75

100[V]

Only 250ms

1 1

2 2

1

Measurement

Simulation


45

Solution Testing - UPM switch closing

F

F

14:42:15.290 14:42:15.295 14:42:15.300

-150

-100

-50

0

50

100

150

200

Vo

lts



(file NM-1-Close-PD.pl4; x-var t) v:TX0001- v:TX0003- v:TX0002-

0 5 10 15 20 25 30[ms]-100

-50

0

50

100

150

[V]

Magnitude and

Frequency are well damped

1 1

2 2

1

Measurement

Simulation


220V

125V

46

Conclusions

• Failures due to MV breaker switching transients is on the increase due to higher efficiency transformers, installations with shorter cable lengths (~200ft or less), or VI’s with high current chops (>5A).

• These failures can be mitigated by `hardened` transient resistant transformer designs, or RC snubbers – a study is needed to identify the risk.

• System characteristics and nature of the switching can cause VT ferroresonance

• Damping resistors or saturable reactors both are able to mitigate ferroresonance

Documents

March 2017 IEEE NCS Power and Instrument Transformer ...sites.ieee.org/northern-canada-pesias/documents/2017/04/march-2017... · • Dry-type or Low BIL more susceptable, ... or more