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Introduction and Life Cycle Management ofTransformers
Prof. Dr.- Ing. habil. H. Borsi
Institute of Electric Power Systems, Division of High Voltage
Engineering
- Schering-Inst itut -
Leibniz Universitt Hannover
Germany
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INDEX
Life Cycle in HV Equipment
Quality Assurance
in
High Voltage Equipments
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SOCIALENVIROMENT
Social Influece
Quality
Enviroment
Legislation
TECHNOLOGICALENVIROMENT
Technological evolution
Kind of network
UTILITY FITTINGS
Asset Management Philosophy: Condition based, Risk based,..
AVAILABILITYRELIABILITY
COSTS
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Life Cycle ManagementSTARTS
during manufacturing
PLANIFICATION TECHNICAL SPEC. PROJECT
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Planification, Technical Spec., Project
Design, Manufacturing and
Acceptance Testing
Site Installation
Service Operation
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Design, Manufacturing and
Acceptance Testing
Site Installation
Service Operation
Planif ication, Technical Spec., Project
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Site InstallationQuality Assurance:
- Design Review
- Manufacturing Inspections
- Factory Testing----------------------------
- On site Testing
Assembly and Commisioning:
- Site preparation- Transportation and assembly
- Commissioning
Service Operation
Design, Manufacturing and
Acceptance Testing
Planification, Technical Spec.,Project
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- Strict and controled planning of manufacturing and acceptance testing of HV
Equipment.
- Complete Quality Assurance proccess of HV Equipment on a basis of
homogeneity and rigour. Each part of the proccess must maximize global
reliability.
- Integration and coordination of Quality Assurance proccess in the whole life
cycle of HV equipment.
Main Goals
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Design
Review
Manufacturing
InspectionsAcceptance
Testing
Supplier Qualification
Factory
Tests
Special
Tests
Quality Assurance
Comissioning
Tests
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Supplier Qualification
Quality Management:- Quality policy (real degree of implementation)
- Quality control plan (incoming materials and in- production)
- Traceability
Global Assessment of the factory and manufacturing
processes:
- General data, Incoming materials, sub-suppliers- Manufacturing processes, facilities condition, tools
- HV Testing Laboratory
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Design Review
- Internal design analysis(materials, dielectrics,
electrodynamics, thermal)
- Identification of weak points (design and
manufacturing) and design margins.
-Acceptance tests plan(rutine, type, specials)
- Fitting with functional requeriments
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Manufacturing Inspections
Detailed assessment of manufacturing proccesses:
- Check of incoming controls and material traceability.
- Warehouse management, facilities condition
- Manufacturing proccesses and technology assessment
- Check of in-production quality controls.
- Sub-suppliers management and control
- Maintenance and calibration of machinery, tools and sensors.
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Acceptance testing
Factory tests:
- Checks and tests to validate a correct design and
manufacturing
- Compliance with tests plan and standards
- Correct functionality and calibration of instruments
- Laboratory personnel qualification
On-site tests:
- Checks and tests to validate a correct transportation and
assembly
- Comparison with factory testing
- Initial fingerprint for predictive maintenance
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Transformers arekey equipmentfor power transmission and distribution
Power Transformers belong to the
most expensiveequipment of power networks
Power Transformers are
custom madeand not available as stock equipment
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FACTORY TESTS
-Turn ratio
- Windings resistance
- Insulation resistance
- Capacitance and tg delta
- Load losses and Zcc.
- No load losses -> before dielectric tests
- FQ and DGA oil analysis -> before dielectric tests
- OLTC tests
- Switching Impulse for 220kV trafos
- Lighting Impulse (HV, LV, Tertiary and Neutrals)
- Separate Source withstand voltage
- Long Duration Induced voltage with PD meassurement
- Short Duration Induced voltage with PD when Switching Impulse do not apply-No load losses -> after dielectric tests
- FQ and DGA oil analysis -> after dielectric tests
- FRA and FDS test
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- Insulation resistance
- Capacitance and tg delta (trafo and bushings)
- Excitation test
- Turn ratio
- Leakage reactance
- Windings resistance
- Conmutation dynamic resistance
- DGA oil analysis- FRA test
- FDS test
COMISSIONNING TESTS
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Definition of life time
the life time of a transformer is definedas the life time of its rigid insulation
system
The end of life time is defined by the reduction of the
mechanical strength by more than 50%
In general this is the case at DP-values < 200 ( - 75%)acc to IEEE C57.91-1995
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Failure Curve of Technical Equipment
TimeNew End of
Design Life
FailureRa
te Bathtub Curve
Transformers can fail any time in life cycle Most likely early in life or near the end of life
Due to high age and increase in load the risk for failure oftransformers in service is HIGH
The average expected life time for transformers isbetween 25 and 35 years
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Failure rates on Power
Transformers
windings
21%
tank
15%
core
2%
others
8%
tap changer
39%
bushings
15%
Most failures occur suddenly
The outage time often is very long
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Aging of transformer insulation
Thermal
Degradation
Electrical and
Dielectric
Degradation
Mechanical
Degradation
ChemicalDegradation
Resistive and magnetic losses, capacity
of the cooling system
Alterations of the insulating material due
to the electrical field
Vibrations, deposits out of pumps, fans,
gaskets,...
Oxygen out of the atmosphere (forbreathing type transformers) together
with catalysts leads to acidification, water
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Electrical causes
Local overstressing in the insulationcauses aging of the cellulose and the
insulation oil Acid generation and contamination in the oil
Water content increasing
Gas and sludge generation
Acceleration of the cellulose depolymerisation
Partial discharge or breakdown causes strong transient
overstressing
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Electromagnetic causes
High Current (e.g. due to short circuit in
network) generate high forces
Winding deformation
Paper insulation breakage in
particular in the aged places
Be careful for partial discharge and breakdown
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Dielectric causes
Aging of the cellulose causes breaks ofthe glucose chains (Depolymerisation)
Generation of: Water
Gas (CO, CO2)
Aldehyd groups (Alkaline)
Carboxyl groups (Organic acids)
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Thermal causes
Eddy current losses in the core
Ohm's losses in the coil
Variations in load lead to heating up and
cooling
Increasing the water content of insulationdue to breathing
Hot spot temperature (IEC 60354: Loading
Guide) has crucial influence on the li fe Span
of the transformer insulation
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Ch i l
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Chemical causes
Due to aging generates organic acids They attack in particular the paper
insulation
Metals such as copper, iron, aluminum and
zinc act additionally as catalysts Accelerated aging
Microstructure of
Paper with
NZ [mg/kg]0.05 (left)
0.1; 0.2;
0.3 (right)
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Aging of Insulation
Thermal aging
Electrical and dielectrical aging
Mechanical aging
Chemical Aging
Generation
of Water
Additional water is coming from outside (free breathing
transformers)
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Due to ageing the glucose ring chains of the
cellulose break (Depolymerisation) The result
Water
Gases (CO, CO2)
Aldehyd Groups (Alkaline)
Carboxyl Groups (organic acids)
Ageing of Paper
C O
C C
CH CH
O
H2COOH
H
OH
H
H
OH
CH CH
C O
H2
COOH
H
C C
OH
H
H
OH
H2O
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Aging Process
Heat and water accelerate the aging process
1
10
100
1000
0 1 2 3 4RelativeDepolymer
isationsgeschwindigkeit
Wassergehalt im Papier
80 C
100 C
120 C
[%]
Water Content in the Paper
RelativeVelocityofDepaolarization
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WATER IN THE TRANSFORMER
A temperature increase of 6 - 8C is doubling the
depolymerisation speed
A moisture increase of 1% is also doubling the
depolymerisation speed
4% moisture at 50C leads to a moisture content in the oil
of 50 ppm. Is the oil quickly cooled down (power failureduring winter), is it possible to have free water already at
20 C
With a too high moisture content, there is the risk of
bubble formation in the insulation at much lower hot spottemperatures as 140C as with dry insulation
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Possible Measurements
Oil Analysis DGA
Oil parameter
Furan analysis
DP Electrical Failures
Resistance test
Insulation resistance test
Ratio test
FRA (Frequency Response Analysis)
PD (Partial Discharge)
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Oil parameter(IEC 60422, VDE 0370-2) Color
Insignificant
General condition
Particle
Water content
Important parameter
Breakdown voltage Important parameter
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Oil parameterTan
Increasing with thermal stress
Interfacial tension
Aging of oil, degradation product Acidity (total acid number)
Acidity decreases the strength of paper
Inhibitor content
Consumption is a measure for aging
Density, Flash Point, Viscosity
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DGA (Dissolved Gas Analysis) Nitrogen N2
Oxygen O2
Carbon Monoxide CO
Carbon Dioxide CO2 Hydrogen H2
Methane CH4
Ethan C2H6
Ethen (Ethylene) C2H4 Ethin (Acetylene) C2H2
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/DGA
DGA
NORMAL PD PD LOW PD HIGH T< 300C
300C
< T>700C T> 700C
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IEC 60599 Method
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Furan analysis 5-Hydroxylmethyl-2-Furfurol (5HMF) 2-Furfurylalkohol (2FOL) 2-Furfurol (2FAL) 2-Acetylfuran (2ACF) 5-Methyl-2-Furfurol (5MEF)
1200
900
2002FAL
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Resistance Measurement
Measuring of DC resistance for all coils at different tap changerposition (typically: US about 20 m, OS about 1 )
Contacts condi tion (Tap changer, bushing) as well as windingshorts and disconnection in the current path
Insulation Resistance Measurement
Insulation evaluation due to measuring the insulation between coiland ground (typical a few 100 M). Determination of Kr-Factors=R60/R15
With Kr>3 moisture in insulation
Ratio Measurement
Ratio measurement in each position of tap changer Maximum deviation 0,5% Determination the short circui t or disconnection in windings
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PD-measurement Sensitive measurement for insulation
condition evaluation
Chemical (DGA)
Optical (e.g. UV camera for corona)
Acoustic (sensors on the tank)
Electric (narrowband or wide-band) UHF (antenna in transformer)
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P ibl
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Possible measurements
Dielectric measurementsRVM (Recovery Voltage Measurement)
PDC (Polarisation Depolarisation Current)
FDS (Frequency Domain Spectroscopy)
The procedures use similar models.
The comparison between model response and
measurement is used to determine the watercontent in pressboard
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Recovery Voltage Measurement (RVM)
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Recovery Voltage Measurement (RVM)
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PDC
SampleElectrometer
Possible measurements
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FDS
Frequency (Hz)
Moisture 4%
Moisture 2,5%
Moisture 1%
Moisture 0,2%
Possible measurements
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Vacuum, heat
Vacuum, heat
Technologies for transformer drying
Drying potentially endangers the solid insulation asthe winding coil usually is not re-fastened after drying
(>>> stability in case of shorts?)
Cellulose fibers
Water molecules
Insulating liquid
Vacuum, heat
Vacuum, heat
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C ti t f i l ti d i
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Continuous transformer insulation drying
Vacuum and heat
Hygroscopic materials
(molecular sieves,
Zeolites)
Use of the water
equilibrium at different
temperatures
Applicable technologies:
Interfering the
Dissolved GasAnalysis
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R li ti
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Realization
Gentle, continuousdesiccation without
influencing the DGA
Upgraded insulating liquid
Cooling circuit
Cooler
Warm, wet oil
Cellulose filter cartridge
Pump
Transformervessel