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Craig L. Stiegemeier; ABB TRES Transformer Remanufacturing & Engineering Services; August 20, 2013

ABB Red TIE Series - PomonaTransformer failure modes

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Slide 2

Transformer Failure ModesAgenda

Definition of a transformer

Primary Causes of Transformer Failure

Balancing the three leg stool Thermal degradation

Dielectric withstand

Mechanical performance

Causes of insulation system degradation

Identification of failure vulnerabilities including keytransformer components

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Slide 3

Transformer Failure ModesDefinition of a transformer

IEC 60076-1

A Static piece of apparatus with two or more windings

which, by electromagnetic induction, transformers asystem of alternating voltage and current into another

system of voltage and current usually of different values

and at the same frequency for the purpose of

transmitting electrical power.

IEEE C57.12.80

A static device consisting of a winding, or two or more

coupled windings with or without a magnetic core forintroducing mutual coupling between electrical circuits.

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Slide 4

Transformer Failure ModesFundamental laws of a transformer

Maxwell 2nd Law (No 2 - induction law )

Ui = - N d / dt or converted to

Ui = 4.44 f N B AFe or U1 / U2 = N1 / N2

where:

Ui r.m.s value of the induced voltage [ v ]

f frequency [ Hz ]

N number of turns

B peak value of the magnetic induction [ T ]

AFe section of the iron core [ m2 ]

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Slide 5

Transformer Failure ModesGeneral fundamental of a transformer

HV Winding

LV Winding

Electrical Voltage applied to

the HV winding

Magnetizes the Core

And the voltage is inducedinto the LV winding

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Slide 6

Transformer Failure ModesMagnetic Coupling between coils and secondary EMF

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Transformer Failure ModesCore Form Transformer

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Slide 7

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Slide 8

Transformer Failure ModesStresses Acting on Power Transformers

Mechanical Stresses

Between conductors, leads and windings due to

overcurrents or fault currents caused by short circuits andinrush currents

Thermal Stresses

Due to local overheating, overload currents and leakagefluxes when loading above nameplate ratings; malfunction

of cooling equipment

Dielectric Stresses

Due to system overvoltages, transient impulse conditions

or internal resonance of windings

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Slide 9

The fault current is

governed by:

Open-circuit voltage

Source impedance

Instant of fault onset

Displacement of current

Transformer Failure ModesMechanical Stresses in Power Transformers

In the case of externalshort-circuits, the firstpeak of the fault currentthrough the transformer

will increase to amultiple of the ratedcurrent

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Transformer Failure ModesMagnetic field lines

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Slide 10

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Slide 11

Transformer Failure ModesMechanical Stresses in Power Transformers

A short circuit gives rise to:

Mechanical forces

Temperature rise The transformer must be designed so

that permanent damage does not takeplace

Electromagnetic forces tend to increase

the volume of high flux Inner winding to reduced radius

Outer winding towards increasedradius

Winding height reduction

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Slide 12

Inner

winding

Outer

winding

Radial forces inwards

compressive stress

Radial forces outwards

tensile stress

Fmean

Transformer Failure ModesMechanical Stresses in Power TransformersEffect of the radial forces on windings

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Slide 13

Inner

winding

Outer

winding

Transformer Failure ModesMechanical Stresses in Power Transformers

Radial forces result in:

Buckling for inner windings

Increased radius for outer windings Spiraling of end turns in helical winding

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Slide 14

Axial short circuit forces accumulate towards winding mid-height

The radial

component of

the leakage

flux createsforces in axial

direction

Transformer Failure ModesMechanical Stresses in Power TransformersEffect of the axial forces on windings

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Slide 15

B B Fax Fax

B B Fax Fax

Axial imbalance

will create extraaxial forces

The forces tend

to increase the

imbalance

Transformer Failure ModesMechanical Stresses in Power Transformers Axial

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Slide 16

Failure mode Spiraling:

Characteristic failure mode for

inner and outer winding

Failure mode Buckling:

Characteristic failure

mode for inner winding

Transformer Failure ModesMechanical Stresses in Power Transformers - Radial

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Slide 17

Transformer Failure ModesMechanical Stresses in Power Transformers

Two examples showing buckling of inner windings

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Transformer Failure ModesMechanical Stresses in Power Transformers

Another example of buckling of the inner windings

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Slide 18

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Slide 19

Axial force failure modes:

Collapse of winding end support

Tilting of winding conductors

Telescoping of windings

Bending of cables between spacers Damage of conductor insulation

Transformer Failure ModesMechanical Stresses in Power Transformers

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Slide 20

Failure mode

Conductor tilting

Failure mode

Bending of cables

Failure mode

Collapse of end support

Transformer Failure ModesMechanical Stresses in Power Transformers

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Slide 21

Transformer Failure ModesMechanical Stresses in Power Transformers

Axial forces cause:

Mechanical withstand of insulation material

Risk for tilting

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Transformer Failure ModesMechanical Stresses in Power Transformers

Example for axial forces

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Slide 22

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Slide 23

Transformer Failure ModesShort-Circuit Failure

Unit Auxiliary Test Transformer FailureInternal High Speed Film Camera Footage

ABB Inc.

Originally taken by The General Electric Company atPittsfield, Massachusetts

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ABB Inc. 2013 - Slide 24

Movies should be screened in thegrey area as featured here, sizeproportion 4:3. No titles should beused.

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Slide 25

Transformer Failure ModesRisk: Short Circuit Forces & Stresses

Through faults are often the cause oftransformer failures

Many older designs have insufficient

margin for todays fault currents Loose coils due to aging can cause

failures

Normal aging can cause brittle

insulation and increased failures Even brief overloading may cause

significant aging

Oxygen in the oil can double theaging rate

Moisture in the insulation increasesaging rate 2-5 times depending onthe amount of moisture

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Slide 26

Transformer Failure ModesMechanical Risk: Short Circuit Forces & Stresses

Figure 3. Results of the Short-Circuit Strength Design Analysis used in a Life Assessment Study

HV Radial

(Hoop)

HV Axial

(tipping or

crushing)

LV Radial

(Buckling)

LV Axial

(tipping or

crushing)

LTC

Winding

Radial

(Buckling)

LTC

Winding

Axial

(tipping)

Design #1

Design #2

Design #3

Design #4

Little Risk of Failure

Slight Risk of Failure

High Risk of FailureDesi

gnMargin

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Slide 27

Transformer Failure ModesThermal Stresses in Power Transformers

Loading is primarily limited by highest permissible temperatures inthe transformer, especially within the windings

Temperature limits are based on:

Expected lifetime The risk for oil vaporization

Permissible temperatures are generally expressed as temperaturerises above ambient

Ambient temperature is in turn defined by current standards 24 hour ambient temperature average 30 C

Maximum ambient 40 C

In accordance to Standards:

Winding temperature rise 65 K Top oil temperature rise 65 K

Hot spot temperature rise 80 K

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Slide 28

Winding hot spot

Top oil rise

hot spot factor

Winding average rise

Copper over winding oil gradient

AmbientWinding

Temperature

Bottom oil

Copper over tank oil gradient

Transformer Failure ModesWinding Temperature Rise and HS Calculation

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Slide 29

Transformer Failure ModesThermal Risk: Intensive aging

T f F il M d

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Transformer Failure ModesThermal Risk: Intensive aging

T f F il M d

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Slide 31

Transformer Failure ModesDegree of Polymerization - DP

Degree of polymerization is a measure of the number ofintact chains in a cellulose fiber. It provides an indication ofthe ability of the transformer insulation to withstand

mechanical force (due to through-faults, etc).Cellulose Fiber Chain

T f F il M d

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Slide 32

Transformer Failure ModesFactors affecting DP and Measurement Method

The DP of the insulation is affected by the followingconditions:

Moisture content

Acidity of the oil

Oxygen content

Temperature

The DP is measured by viscosity measurements according

an ASTM method after dissolving the paper samples incupriethylene diamine solvent.

Paper samples must be taken from enough differentareas in a transformer in order to get a profile ofdeterioration of the cellulose

When combined with detailed design knowledge,measurements in one area of the transformer can giveinformation on the condition of paper in inaccessibleareas of the windings.

T f F il M d

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Slide 33

0.1

1.0

10.0

100.0

1000.0

10000.0

50 60 70 80 90 100 110 120 130 140 150

Temperature [oC]

L

ife

E

xp

ect

an

cy

(years)

Dry & Clean (Insuldur)

Acidic Oil (Insuldur)

1% Water Content (Insuldur)

3-4% Water Content (Insuldur)

Transformer Failure ModesLife Expectancy Based on DP and Other Factors

It is assumed that the DP of transformer insulation is approx. 1,000 at the start of life and approx.200 at the end of life. This graph shows the expected life of thermally upgraded insulation(Insuldur) under various conditions:

For long insulation life expectancy, it is important to keep the insulation dry, keep acidity

and oxygen concentration of oil low and provide good cooling for insulation

Transformer Failure Modes

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Slide 34

Transformer Failure ModesThermal Stresses in Power Transformers

Life Expectancy Based on DP and Other Factors

Transformer Failure Modes

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Slide 35

Transformer Failure ModesDielectric Stresses in Power Transformers

Overvoltage integrity

Overvoltages can be divided into two classes:

Continuous

Transitory

Continuous overvoltage is related to the core and itsmagnetization (normal 50Hz or 60 Hz stresses)

Transitory overvoltage refers to intermittent stressesplaced on the insulation system, usually at much higherlevels than the power frequency stresses

Transformer Failure Modes

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Slide 36

Lightning and switching impulse

surges are called Transients

because their duration is short. The frequencies are much higher

than the power frequency (60 Hz

here) operation frequency.

Transient calculations are used to

find the time dependent distribution

of transient voltages, applied on the

line terminals, over the windings.

Transformer Failure ModesDielectric Stresses in Power Transformers

Transient Voltages

Transformer Failure Modes

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Slide 37

Winding

Win-

dinglength

Voltage

Winding oscillation

Transformer Failure ModesDielectric Stresses in Power Transformers

Transformer Failure Modes

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Slide 38

2 D field plotscan be used to

check thedesign of themain insulation

2 D Field Plot

Transformer Failure ModesDielectric stresses - Main insulation design

Transformer Failure Modes

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Transformer Failure ModesDielectric stresses failure - Main insulation design

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Slide 39

Transformer Failure Modes

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Slide 40

Field distribution over the barriers andHV-LV windings

CAD-model

FLC evaluation

Transformer Failure ModesAnalysis of Bushing Failure

525 kV unit assumed bushing failure

Simulation showed electric stress was greatest on the paperinsulation around the shield ring

Used simulation to redesign insulation barriers

Transformer Failure Modes

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Slide 41

Transformer Failure ModesWhat we know

Top transformer failures (78%) (from Doble):

43% winding insulation

19% bushings

16% tap changers

Other areas of concern:

Pollution, dust & debris affecting bushings &cooling systems

Cooling System inefficiency

COPS Tank elevation

Specific issues:

Streaming Electrification

Nitrogen Gas Bubble Evolution

Blocking / GE Mark II Clamping

Shell Form Rewedging

GE Type U Bushings

Transformer Failure Modes

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Transformer Failure ModesDe-energized tap changer

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Slide 42

Transformer Failure Modes, grounding of the active part

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Transformer Failure Modes, grounding of the active partHot metals gassing

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Slide 43

Core Clamp grounding point

Core clamp grounding totank

Transformer Failure Modes - Thermal Scan Value

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Loose Bushing Terminal Connection

When there is a loose connection at the terminal from thebushing to the bus work, it will lead to overheating of thebushing top terminal when under load.

The thermograph will show the bushing terminal as hot, whilethe body of the porcelain will show normal temperatures.

ABB Inc. 2013

Slide 44

Transformer Failure Modes - Thermal Scan Value

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Blocked Oil Flow in Radiators

In case of a malfunction that stops or restricts the flow of oilthrough a radiator, this will show up on an infrared scan.

The image will reveal dim areas where the oil flow is restricted

and brighter areas where normal oil flow is taking place

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Slide 45

Transformer Failure Modes / Diagnostic Techniques

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Slide 46

g qHighly Effective On-line Actions are Best

PROBLEMS DIAGNOSTIC TECHNIQUESSERVICE CONDITIONS

OF THE EQUIPMENT[1]

PROVEN

EFFECTIVENESS[2]

MECHANICAL

1. Excitation Current2. Low-voltage impulse3. Frequency response analysis4. Leakage inductance measurement5. Capacitance

OFF-SOFF-SOFF-SOFF-SOFF-S

MLH

M/HH

THERMAL

GAS-IN-OIL ANALYSIS6. Gas chromatography7. Equivalent Hydrogen method

ONON

HM

OIL-PAPER DETERIORATION8. Liquid chromatography-DP method9. Furan Analysis

ONON

M/HM/H

HOTSPOT DETECTION10. Invasive sensors11. Infrared thermography

ONON

LH

DIELECTRIC

OIL ANALYSIS12. Moisture, electric strength, resistivity, etc.

ON M

13. Turns ratio OFF-S L

PD MEASUREMENT14. Ultrasonic method15. Electr ical method

ONON

M/HM/H

16. Power Factor and Capacitance17. Dielectric Frequency Response

OFF-SOFF-S

HH

ABB Service Handbook for Transformers, Table 3-1, Page 72

[1] OFF-S = equipment out of service at site, OFF-L = equipment out of service in laboratory, ON = equipment in service[2] H=High, M=Medium, L=Low

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