P 06 Transformer Failures 30 Mar

7/30/2019 P 06 Transformer Failures 30 Mar

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Knowledge Is Power SM Apparatus Maintenance and Power Management

for Energy Delivery

Doble 2005 regional seminar

FAI LURE ANALYSI S FORPOWER TRANSFORMERS

Presentat ion P 6Doble 2005 Regional seminar s

2 Transformer failures Doble 2005 regional Seminar

Introducing Failures - Transformers Do Fail

3 Transformer failures Doble 2005 regional Seminar 4 Transformer failures Doble 2005 regional Seminar

There are three categories of Transformer Failure

1. Unexpected ones leading to Forced outage

2. Managed failures in a Scheduled outage

3. Removals at end of useful life- rare for transformers- more for switchgear

Introducing Failures


But there are several definitions and no consensusof Failure across plant groups

1. Failure to perform ie a trip, causing an outagebut only minor and easily re-instated- more typical for switchgear

2. End of useful life failures leading to removal,from plinth for repair at works or scrap




WE MUST LEARN How to avoid unexpected failures

By understanding failure history

By linking failures to some indicator

By using the indicators to detect potential problems =Condition assessment

By tracking a worsening condition= asset health review

Removing the Transformer just prior to failure = effectiveasset management


2/12


Cost of losses Maintenance cost index Third party costs at failure Cost of constraints Cost of clean-up Cost of secondary damage Cost of replacement Current net worth Bushings and Tap changers

Failure costs


This Presentation

1. Introduction to Failure 2. Asset Life Concepts- what causes physical

failure3. Defining Failure Modes4. Understand your design groups and their failure

modes5. Distribution and generation/ transmission units6. Leading to other presentations:

-Defining the indicators of those modes-Ranking of your fleet-Managing the risk


At specificati on stage Most utilities expect 35-40 years for staticplant, although generator transformers weregiven 25y in UK in 1960s.

Exper ience Now Some transformer design groups, in some

applications will last 50-80 years.

2 The technical Asset life


2 - Failure rate with age

Hazard Rate %


22 -- Technical Life

When the condition of any integral part of theunit is beyond both acceptable risk for furtherservice and economic refurbishment.


When the 400 kV system was built most transformerexperts thought their specs were wonderful and theirtransformers would fail at old age- when the windingpaper fell apart.

If a unit was held at rated maximum hot spottemperature IEC thought it would last 40y (test tube R&D) IEEE testing on models and distribution units

indicated 19y life- but thought it should last40y since rating not maintained.

2 - The Asset life assigned values


3/12


Exper ience Now

Most transformer failures are not old agemechanisms- except in some 3 rd W areas

Most failures are due to some limitation inspecification/ application/ design/manufacture/ care, ave age 16-19y.

But in some applications some design groupswill last 50-80 years.

2 - The Asset life Realities


Variety of modes and causes

Design weaknesses Abnormal system conditions - trigger

Aged condition / service loading

Pre-existing faults

Timescales

2 - Failures


Increasing AgeNew Old

Insulation Strength

IncidentsInsulationStress

InsulationSpareMargin

Reducing Strengthwith t ime and afterincidents

Failure

Failure Model


Thermal

Dielectric

Mechanical

3 - Failure Modes


1. The so called long term Intrinsic Ageing of winding paper

2. But also early life failures from accelerated localageing =Same process just different causes

3. Core bolt insulation/ leads/ connections

3 And Three types of Thermal Failure


End of Life of the win ding insulation-

is when the paper is too brittle to withstand

mechanical and electrical stresses and shortedturns occur.

Transformers are designed to operate underrated conditions of ambient temperatures andload for an acceptable life duration

3 Winding Thermal Failure


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The thermal rating is the steady state current thatproduces a temperature rise of 65 0 C under the

following conditions. It is a normal test duringfactory acceptance- although not always done forevery transformer.

A starting oil temperature of 30 0 C

A final top oil temperature of

(30 + 65) = 95 0 C

A presumed hottest spot conductortemperature of 95+ 15 = 110 0 C

3 - Rated temperatures IEEE C57.91-1995


Most assessments looking at time -at -temperature

support the view that lives 60+ years arereasonable.

But:But: Localised hot spots can lead to acceleratedageing

3 - Thermal Failure


Causes of Localised hot spots Poor design of conductor sizing/ transpositions

Joints defective

Leakage flux heating due to poor stress shield/

shunts

Blocked cooling ducts

3 - Thermal Failure- winding paper


Premature Ageing


Localised overheating


Causes of Localised hot spots Joints defective

Defective earth insulation leading to Circulating

currents- frame or core and tank

Eddy currents from Defective core bolt insulation

3 - Thermal Failure elsewhere


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Core bolt insulation


FrameFrame -- Earth FaultEarth Fault OverheatingOverheating


3 - Dielectric failures

A properly designed t ransformer should withstand normal andtransient voltages

Causes of dielectric failure Incorrect stress distribution calculation and turn

over-stresses with impulse ( a factory test) Inadequate phase to phase insulation,(winding or

bushing to tank, tap changer clearance) Moisture leading to tracking in phase-to-phase

barrier boards and flashover Bubble formation at overloading a wet unit Static electrification at high flow rates Debris from sludge and carbon due to degraded

oil, paper fibres and metal from pumps Turn faults following mechanical and thermal

ageing


Inter Phase Barrier Board Failure


3 - I n ter-winding insu lat ion


Note the insulation in the 2 previous slides

Int er phaseboards

Interwinding wrap


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Inter-turn failure


Sparking at clamping bolt

Not all arcing leads to a failure


A Properly specified and designed unit shouldwithstand system fault levels and frequency

Causes of mechanical failure1. Poor design tools used for older units2. Design not proven by test or service

3. Clamping pressure deterioration4. Fault levels higher and/ or frequency higher than

specified

3 - Mechanical Failure


3 Mechanical Failure Ageing Effects

As a transformer ages the insulation shrinks and

clamping pressure is lost.-reduces strength.

Any minor winding deformation usually results in

electromagnetic imbalance increases stresses

during subsequent faults.


3 - Mechanical Failure Service I ssues

The number and severity of short-circuit events

suffered are important.

Close-up short circuits are a common cause of

winding movement failures.

Tap-changer faults can cause tap winding

failures.

Faulty synchronisations can cause windingdamage and failures.


3 - Mechanical Failure Detection Problems

DGA will only indicate a problem when theinsulation has been damaged ( usually too lateto repair ).

Internal visual inspections often inconclusive

Winding failures can usually be diagnosed byvarious electrical tests ( but not always ).

But interim damage is much harder to detect.

Your transformer may be critically damagedwithout you being aware of it !


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3 - Mechanical Fai lure Modes

Hoop buckling of inner winding

Conductor tipping Conductor telescoping Coil clamping failure End insulation collapse Spiral tightening Lead displacement


Buckled w indings

Capacitance detectedmovement anddeformation of transformer windings.


Twisted Winding- not much seen externally


3 - Mechanical Failure Winding damage seen now


3 - Mechanical Failure -Core Frame


Part 4

Understanding your design families


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Your company data

Natio nal Associat ions, eg CEA, EA

Collaborati ve Groups, eg Doble

I nternational surveys, eg CI GRE



Two questions

1. What are the failure rates for your designgroups?

2. What aspects cause troubles and failuresfor each family

Understanding your design famil ies


4 - Hazard Rates

Where mathematical techniques have beenapplied to failure and population statisticsidentify future hazardrates and modes.

Examples are shown for two design families of transformer, A & B in the next two slides.


4- Company A- Two design famil ies


4 - Failure Probability density f unction


4 - Company A - Hazard Rates

Type B has a high random failure rate and avery ill defined end of life for this poor designfamily.

Type A has a very low failure rate until trueageing effects begin after 30+ years, soallowing effective asset life planning.


9/12


Company X had plenty of data of its own

Company Y didnt, and so they filed a question onthe Ask Doble forum about failures of one typeof design. They got 10 replies and a data setfrom someone who did have data- company X.



THE SUMM ARI ES OF ASK DOBLE


Substation Distribution Transformer Failure Rate

Distribution Transformer Failure Rates

0.13 = Industry Avg.

. 18

. 8 8

. 2 0. 14

. 7 1

. 18. 12

. 7 5

. 0 19

. 9 0

. 18. 2 4

. 0 2 2

. 2 4

. 0 2 3

. 8 4

. 16. 2 2

. 0 2 0 . 0 2 2

0

0.2

0.4

0.6

0.8

1

3ph w/ o FP E FP E only 1ph

1998

1999

20002001

2002

F a

i l u r e

R a

t e

1 9 9 8

1 9 9 9

2 0 0 0

2 0 0 1

2 0 0 2

1 9 9 8

1 9 9 9

2 0 0 0

2 0 0 1

2 0 0 2

1 9 9 8

1 9 9 9

2 0 0 0

2 0 0 1

2 0 0 2

1 9 9 8

1 9 9 9

2 0 0 0

2 0 0 1

2 0 0 2

X X


The best international survey is that byCigre done in the 1970s and published in1983

But that was a long time ago



Features

Web-Based Access

Who has access to the reports

Simplified Data-entry Form

Pick Lists for Standardized Info Monthly Reports Containing Anonymous Details

Reports are clearly understandable

Partnering With Other Organizations

= more involvement, more entries = more knowledge for you!

Dobles Trouble and Failure ReportingSystem


Dobles Trouble and Failure ReportingSystem


10/12


CIGRE DOBLE DOBLESubstation ALL ONE

DESIGN

WINDINGS 19% 43% 47%CORE 3 4 7

TANK& OIL 13 8 -TAP CHANGER 41 16 12BUSHINGS 12 19 8OTHER 12 10 8Period 1968-78 1993-8 1980-2000Number of failures 1000 1500 100

Fault & Defects for Transform ers


Visual I nspection at tear down


Sim ple stat s can conf use



1993-98 analysis of 300 failures Opinions of tw oexperts

CONTACTS 44% 8 0% 2 5%MECHANI SM 18% 20% 75%I NSULATI ON 12%LEADS 7%REACTOR 4%OTHER 15%

Fault and defects - LTCs


Distribution of Power Transformer Failures Per Year of Manufacture

0

1

2

3

4

5

6

7

8

9

P e r c e n t a g e

% of Failures

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010


YEAR



Failures Versus Age, Adjusted for Quantities in theDatabase Manufactured in the Year.

0

1

2

3

4

5

6

0 2 4 6 8 1 0

1 2

1 4

1 6

1 8

2 0

2 2

2 4

2 6

2 8

3 0

3 2

3 4

3 6

3 8

4 0

P e r c e n t a g e

All

Major

AGE


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69k V and Below

5 What about Dist r ibut ion uni t s?


Ave age oldest Comment

30y 47y

29y 53y replace on failure

36y 77y ave age at fail= 14y

42y 60-70y (25% >50y) Evaluation prog

30y 65-70y (5 units) replace on failure




35y 34% >40y Health index

38y 77y

30y 45-50y replace on failure

What is the age of Distri but ion Units in USA?


CIGRE DOBLE DOBLE

Substation ALL 69 kV

WINDINGS 19% 43% 47%

CORE 3 4 6

TANK& OIL 13 8 3

LTC 41 16 11

BUSHINGS 12 19 3

OTHER 12 10 4

UNKNOWN 28

Period 1968-78 1993-8 1955-85

Number of failures 1000 1500 955

Where is the site of t he fault?


Email forum and Doble data indicates-

There are a lot of old units out there workingwell at 40+ years

Failures tend to be at 15-20y and maininitiating causes are lightning andthrough faults

But are failure rates higher?

Distr ibution Transformers


Causes of thru-faults-


Thermal Overload


12/12


2003 Doble conference paper- Prout & Wilson

Have more but lower intensity through faults

More tolerant to poorer dielectric

Have poorer preservation systems

Flush bushing flanges can allow water ingress

Failure of fans/ gauges more consequential

Fewer LTCs below 69 kV/ NLTCs reliable

Old fleet well designed

>1970s units more unreliable

Distri bution Transformers


Looking at Failures is all very int eresting butwe need to move forward

1. Assess the Risk

2. Create a fa i lure t ree

3. Link an assessment program to t ree

6 Moving forw ard

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P 06 Transformer Failures 30 Mar

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