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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
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Introducing Failures - Transformers Do Fail
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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
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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
Introducing Failures
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Introducing Failures
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
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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
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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
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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
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2 - Failure rate with age
Hazard Rate %
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22 -- Technical Life
When the condition of any integral part of theunit is beyond both acceptable risk for furtherservice and economic refurbishment.
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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
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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
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Variety of modes and causes
Design weaknesses Abnormal system conditions - trigger
Aged condition / service loading
Pre-existing faults
Timescales
2 - Failures
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Increasing AgeNew Old
Insulation Strength
IncidentsInsulationStress
InsulationSpareMargin
Reducing Strengthwith t ime and afterincidents
Failure
Failure Model
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Thermal
Dielectric
Mechanical
3 - Failure Modes
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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
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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
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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
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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
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Premature Ageing
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Localised overheating
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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
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FrameFrame -- Earth FaultEarth Fault OverheatingOverheating
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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
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Inter Phase Barrier Board Failure
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3 - I n ter-winding insu lat ion
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Note the insulation in the 2 previous slides
Int er phaseboards
Interwinding wrap
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Inter-turn failure
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Sparking at clamping bolt
Not all arcing leads to a failure
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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
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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.
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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.
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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
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Buckled w indings
Capacitance detectedmovement anddeformation of transformer windings.
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Twisted Winding- not much seen externally
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3 - Mechanical Failure Winding damage seen now
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3 - Mechanical Failure -Core Frame
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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
Understanding your design families
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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
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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.
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4- Company A- Two design famil ies
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4 - Failure Probability density f unction
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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.
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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.
Understanding your design families
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THE SUMM ARI ES OF ASK DOBLE
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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
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The best international survey is that byCigre done in the 1970s and published in1983
But that was a long time ago
Understanding your design famil ies
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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
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Dobles Trouble and Failure ReportingSystem
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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
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Visual I nspection at tear down
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Sim ple stat s can conf use
Understanding your design families
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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
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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
Understanding your design families
YEAR
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Understanding your design famil ies
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?
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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
23y 51y replace on failure
36y 77y replace on failure
32y 62y 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?
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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?
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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
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Causes of thru-faults-
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Thermal Overload
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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
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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