Moisture Estimation in Transformer Insulation Peter Fong OMICRON electronics l t i C Corp. USA
Content 1. The Effects of Water in Transformer 2 H 2. How D Does W Water G Get IIn? ? 3 How to measure moisture? 3. 4. What can be Done about it?
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9 November 2009
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The Effects of Water in Transformer • Generates Bubbles • Triggers Ti Partial P ti l Di Discharge h
Premature e atu e Failure a ue of Good Transformers
• Lowers Breakdown Voltage of Oil • Accelerates the Aging of Insulation
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9 November 2009
Shorten Life Expectancy Of Transformer
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Effect: Bubbling inception temperature will decrease with higher moisture content
Temp perature [°C]
Effect 1: Bubbling 200 Kraft Paper New Oil TU Paper New Oil
180
Aged Kraft Aged Oil 160
140
120
Risks: • Inception p of Partial Discharge g 100 • Lower Dielectric Strength • Short-Circuit of Windings 80
1,0
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2,0
3,0
4,0 5,0 6,0 Moisture in paper [%] Page 4
Effect 2: Dielectric Strength B kd Breakdown V Voltage lt [kV]
Effect: • PD Inception Voltage decreases with higher moisture content • Dielectric strength of Oil decreases
Cw=3,5% 3,5%
Temperature p
Risks: • Failure of Transformer Insulation
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Effect 3: Aging
Risks: • Lower L th the expected t d lif life off transformer • Run transformer at lower rating
1000 Life expe ectance / a
Effect: High temperature and moisture content will dramatically lower the mechanical strength of paper insulation i l ti
Dr y 1%
100
10
2% 3%
1
4%
0,1 50
70
90
110 130 Temperature / °C
L. E. Lundgaard, “Aging of oil-impregnated paper in power transformers”, IEEE Transactions on Power Delivery, Jan. 2004 © OMICRON
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Paper Insulation • Paper insulation is made up of chains of ‘glucose rings’ • As the paper ages, these rings breaks up and forms water molecules • Water also accelerates the breaking up of these rings
Network transformers Generator stepup units
New: ~ 1200 DP End of Life: ~ 200 DP [Neumann, Micafil-Symposium, Stuttgart, 2004] Condensation
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-H 2O
+H 2O
Age / years
Hydrolysis
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The Aging Process • When a transformer is manufactured in the factory, the paper insulated windings are subjected to extended drying before they are oil impregnated • At this stage, the transformer has a moisture content g in p paper p and 6ppm pp in oil of < 0.5% byy weight • As the transformer ages, the moisture content will increase progressively • In a severely deteriorated system, the moisture content could reach > 4% • Insulation aging is directly related to moisture content © OMICRON
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Where Does Water Come From?
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I. Grundlagen: Feuchtigkeit in Öl-Papier
So rces of Water Sources
• Normal aging of paper produces water • Leaks L k could ld expose insulation to atmospheric moisture • Exposure to atmospheric moisture during maintenance • Failure to dry out the insulation during manufacturing
Water content in the paper/Pressboard: New: 0.4 to 0.8 % Aged: 3 to 5 %
Increase of water 0,1-0,2% per year [Cigré WG12.18 Life Management of Transformers, 1999]
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Wh Where is i the th Water? W t ? Most of the water is contained in the cellulose! ->>> >>> M More iimportant t t to t measure the th water t content in paper than oil Mass of the oil: 100000 kg
Mass of the solid insulation: 13000 kg
Water content at 60 °C: 40 ppm
Water content at 60 °C: 4%
Mass of the water, desolved in the oil:
Mass of the water contained in the paper:
4 kg
520 kg
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How to Measure Water in Paper Insulation?
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Moisture Estimation • There is no practical direct way of measuring moisture levels in transformer insulation, therefore a wet condition may go unnoticed for a long time • This has led to indirect method of moisture estimation
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Direct Method • Take paper sample from transformer and test for moisture content using KFT – Limited use since possible only during repair or tear-down
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Indirect Methods • Measure properties of the insulation which can be related to moisture content 1. Moisture-in-Oil Measurements (Chemical Method) 2. Moisture Saturation (Relative Humidity) 3. Dielectric Spectroscopy (Electrical Method)
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Moisture-In-Oil Method • Karl Fischer Titration on oil sample • Use Equilibrium q Curves to correlate moisture in oil to moisture content in paper • Easy to Perform • Need to wait for equilibrium (days ~ weeks) • Uncertainty in Estimates • Errors introduced during handling (oil samples) • Not accurate for aged oil over estimate • Tend to over-estimate © OMICRON
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Equilibrium Curves
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Moisture iin oil [ppm]
340,5 pp ppm
180
60
50
40
Deviation from D m average [% %]
Round Robin Test at Oil Samples Uni Stuttgart B C D E F G
54,8
160 140
44,3 39,8 35,3 32 8 32,8
39,7
120
100
30 80 19,8
20 11,2 10
60
16,2 15,2 6,7 47 4,7
12,2 8,9
12,1 9,5
3,5
48 4,8
5,8
40 7,5 20 0
0 Sample A
Sample B
Sample C
US B
C
D
ÆComparison of oil sample done at 7 different laboratories December 2007
E
F
G
Moisture Saturation Method • Uses Sensor to measure moisture saturation level instead of ppm as in the lab • Moisture probes inside the transformer measures changes in capacitance of water molecules • Moisture diffused into the probe and changes its capacitance from this the moisture saturation is capacitance, determined • More accurate than oil sampling p g method since no handling is involved • Instrusive • Requires R i E Equilibrium ilib i © OMICRON
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Moistu ure in paperr [%]
Equilibrium Diagrams B Based d on Water W t Saturation S t ti 6 5 4 3 21°C 2
40°C 60°C
1
80°C 0 0
10
20
30
40
Moisture relative to saturation [%] Onsite and online application pp possible p M. Koch, “Advanced Online Moisture Measurements in Power Transformers” CMD 2006
Dielectric Spectroscopy • Electrical Method • Method of Choice due to: – – – – – –
Non-Destructive N D t ti (< 200V) Non-Instrusive High repeatability No need to wait for equilibrium No inaccuracies due to sampling / handling of oil Can be done as part of electrical tests on transformers during a maintenance outage
• Drawbacks – Time Ti consuming i (3 (3-9 9h hours)) © OMICRON
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Dielectric Spectroscopy • Time Domain Method – Polarization Spectrum
• Frequency F Domain D i M Method th d – Tangent Delta (Power Factor) vs. Frequency
• Both methods looks at the electrical response of the dielectric and compares to laboratory data and models • Accuracy of the estimation depends on the accuracy of the model (e.g. takes into consideration geometry of insulation,, conductivityy of oil,, etc.)) © OMICRON
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Frequency Domain Method • Tangent Delta (Power Factor) measurement over wide frequency range (0.1mHz to 2kHz) • Accurate A t models d l available il bl ffor ttransformers f • Widely used, reliable method • Automatic curve fitting and comparison to laboratory curves • Automatic estimation of water content and assessment based on international standards
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FDS Frequency Domain Spectrospy Sufficient data are gathered Hump
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Tan Delta vs. Water Content
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Curve fitting with model curves for estimating water content
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Example (On-Site Drying)
Before Drying ((5.4% w.c.))
After Drying (3 1% w.c.)) (3.1%
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FDS Disadvantages • Takes too long for one measurement – 1 to 3 hours for aged/wet transformers – 3 to 9 hours for new/dry transformers
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FDS Instrument
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Time Domain Method • Polarization-Depolarization Current (PDC) • Step DC voltage is applied to a fully discharged t transformer f • Polarizing current (pA) is measured until stable • Short circuit the voltage and measure the depolarizing current until stable • Compare the curve against laboratory models
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PDC Test T t Method M th d Polarization-Depolarization-Current
Principle
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Current
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PDC in Cellulose Dependency on the Water Content
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PDC Disadvantages • Very small current is measured (prone to power system interference) • Applied voltage must be pure DC, free of any ripple • Does not contain any information at higher frequencies
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PDC Instrument
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Combined PDC+FDS Method • Measure in Frequency Domain from 2kHz down to a 0.1Hz • Measure in Time Domain from 0.1Hz down to 0.1mHz • Transform Time Domain Data to Frequency Domain Curve • Perform comparison in Frequency Domain • Advantage: Significantly reduces testing time © OMICRON
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PDC up tto 0 0.1Hz 1H and d FDS above b 0 0.1Hz 1H
Switch-over Frequency
FDS
PDC
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Combined FDS and PDC FDS ⇒ 9 h measuring time
PDC ⇒ measurement only up to 0.1 Hz
0.1 - 2000 Hz 0.1 mHz – 0.1 Hz FDS+PDC ⇒ 0.1 0 1 mHz - 2000 Hz in less than 3h measuring time
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What Can Be Done? • Oven Drying • Hot H t Oil S Spray • On-line Oil Drying (not very useful since most of water is in paper)
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Oven Drying IV. Trocknung
Application •
Manufacture of new transformers
•
Refurbishment of aged units
Evaluation
December 2007
•
Very effective
•
Expensive disassembly and transport to workshop
Onsite-Drying
Low Frequency Heating •
Drying using LF heating
•
Removal / storage g of oil
IV. Trocknung
Vapour Phase / Hot Oil Spray •
Removal / storage of oil
•
Sprayer required
•
Effective drying
•
Transformer has to be switched off
•
Insulation may be damaged
Online-Drying y g
IV. Trocknung
Application as •
Molecular sieves
•
Oil drying plant
•
g „„fuller's earth“ Oil regeneration
Properties
OTP online
•
Fast recovery of oil insulation strength
•
Long g drying y g time for solid insulation (months – years)
Summary • Moisture in Paper Insulation can reduce the in-service in service life of a transformer significantly • Practical and reliable methods are available for moisture estimation • Keeping the transformer dry can extend the life of yyour most expensive p assets
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Thank you for your attention!
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9 November 2009
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