Degradation of Cellulose Insulation in Liquid-Filled … 2005 San Antonio...Interpretation of...
Transcript of Degradation of Cellulose Insulation in Liquid-Filled … 2005 San Antonio...Interpretation of...
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Degradation of Cellulose Insulation in
Liquid-Filled Power Transformers
presented by:
Thomas A. PrevostEHV-Weidmann Industries, Inc.
W-ACTI2005 Fourth Annual Technical Conference
New Diagnostic Concepts for Better Asset Management November 15, 2005
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Title:
Degradation of Cellulose Insulation in Liquid-Filled Power Transformers
Thomas A. PrevostEHV Weidmann Industries, Inc.
Abstract: The life of a transformer is limited to the life of its solid insulation. Many diagnostic techniques are used to assess the condition of the solid insulation. This presentation will give a review of cellulose insulation, both paper and pressboard, used in liquid filled power transformers. The manufacture of paper and pressboard will be reviewed with an emphasis on those critical properties that determine functional life. The degradation process of paper and pressboard will be reviewed including those byproducts of aging that are used in diagnostic analysis. Techniques to prolong the life of the solid insulation will be presented as well.
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The life of a transformer is limited to the life of the solid insulation.
Much of the diagnostics performed on power transformers is an attempt to determine the health of the insulation system.
In order to understand the proper diagnostics to perform and interpret the results of these tests a fundamentalunderstanding of the solid insulation materials is essential.
Cellulose paper and pressboard is the most commonly used solid insulation in oil-filled power transformers.
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What is the Life of an Transformer?
IEEE C57.91-1995 “Guide for Loading Mineral-Oil-Immersed Transformers”
Definitions:
3.5 transformer insulation life: For a given temperature of the transformer insulation, the total time between the initial state for which the insulation is considered new and the final state for which dielectric stress, short circuit stress, or mechanical movement, which could occur in normal service, and would cause an electrical failure.
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Table 2—Normal insulation life of a well-dried, oxygen-free 65 °C average winding temperature rise insulation system at the reference temperature of 110 °CBasis Normal insulation life
Hours Years50% retained tensile strength of insulation(former IEEE Std C57.92-1981 criterion) 65 000 7.42
25% retained tensile strength of insulation 135 000 15.41
200 retained degree of polymerization ininsulation 150 000 17.12
Interpretation of distribution Transformerfunctional life test data(former IEEE Std C57.91-1981 criterion) 180 000 20.55
What is the Life of an Transformer?IEEE C57.91-1995 “Guide for Loading Mineral-Oil-Immersed Transformers”
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Materials Critical to Functional Life of a Transformer
•Conductor Insulation•Thermally Upgraded Paper
•Duct Spacers•High Density Pressboard
•Lead Insulation•Crepe Paper
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Critical Properties of Paper and Pressboard that Determine Functional Life
•Chemical Purity
•Mechanical Strength
•Dielectric Strength
•Thermal Stability
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Cellulose Basics:
Part I) Fiber Source
Boreal Forest•White Spruce•Black Spruce•Balsam Fir•Hemlock
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Chemistry of Wood
Wood contains four major substances:
Cellulose
Hemicellulose
Lignin
Extractives
For making paper and paper products, it is desirable to retain asmuch of the cellulose and hemicellulose as possible.
Lignin is the “chemical glue” that holds the fiber together.
Most extractives are removed during pulping.
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Kraft Pulp• Cellulose materials used for electrical papers and pressboard are usually manufactured from coniferous trees pulped by the Kraft process.
• Kraft Process
• “Cook” the wood chips using heat, pressure, and chemicals (pulping liquors)
• Wash the pulp to remove the pulping liquor
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•Conventional kraft cooking removes 92-96% of the lignin from softwoods. Softwood is generally cooked to a kappa number of 32 which corresponds to a lignin content of 4.8%
Kappa Number
The Kappa number measures the amount of lignin present in a pulp.
Kappa Number x 0.15 = % lignin in pulp
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Handbook for Pulp and Paper Technologists
Figure 13-8. Photomicrographs of kraft softwood pulp before and after refining (Courtesy of Institute of Paper Science and Technology).
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BM2 Wet End
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View from “Wet End” of 1.4 metre machine, BM2
This is a cylinder machine affording a multi-ply construction of the paper.
The machine also features a CLUPAK ® facility, twin head MEASUREX computer control, float drying, size press, and on-line calendering.
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Board Machine
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Pulp - Transformerboard Flow
CutterDryer
Cutting Table
Hot Press
Forming Roll
Sheet Forming
Water
Sulfate Pulp
Stock Chests
Deflakers
RefinersStorage Chests
MixingChests
WhiteWater
MachineChest
Fig. 23 (Machine diagram for production of Transformerboard precompressed.)
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Transformerboard Mechanical Role
• Support Windings During Short Circuits
• Maintain Dielectric Clearances
• Support High Voltage Leads
• Support Auxiliary Equipment
- LTC, DETC, Bus Bar etc.
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Transformer’s Forces
Radial Forces Axial Forces
Core
Inner WindingOuter Winding
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F Clamping Pressure = f(moisture,temperature,age)
F
Frigid clamping distance
transformerwindingcoil
pressboardpresspapercopper
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Schematic of 550 kV BIL core and coil layout.
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Types of Transformerboard
* Difference is due to type of final drying• Calendered - Low Density Formable
- Dried Unrestrained
• Precompressed - High Density
- Dried Under Pressure and Restrained
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Characteristics of Transformerboard
Physical and Mechanical
0
5
10
15
20
25
%
Oil Absorption Compression
Hi-ValT-IV
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Compression of Radial SpacersEffect of Screen Pattern
Material = .059 Inch Thick T-IV
Note: Tested in accordance with ASTM D-3394 Bedding Pressure 150 PSI, Compacting 3000 PSI
5.57
2 .05
4 .31
1
0
1
2
3
4
5
6
C o m p re s s io n C o m p re s s io n S e t
W ithS creenP atternW ith o u tS creenP attern
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Effect of aging on the thickness of a stack of Transformerboard.
Aging of Pressboard Under Compression
88
90
92
94
96
98
100
102
0 50 100 150 200 250 300
Aging Time (Days)
Spac
er S
tack
Hei
ght (
mm
)
135 Deg. C
150 Deg. C
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Shrinkage after 250 Days of AgingAged at 135 C Aged at 150 C
4.8% 11.0%
Degree of Polymerization after 250 Days of AgingInititial Values Aged at 135 C Aged at 150 C
1190 164 152
•Large difference in shrinkage versus Aging Temp.•Slight difference in DP versus Aging Temp.•While DP appears to have leveled off at a DP value
that would indicate end of life, the thickness ofthe spacer material continues to decline.
Shrinkage versus DP
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Thermal Upgrading of Insulation
In the late 1950’s transformer manufacturers developed Thermally Upgraded Papers (TUK).
In 1962 NEMA officially recognized TUK in standard TR-1-1962 by establishing another temperature rise limit of 65 °C for oil-immersed transformers using TUK.
Today 65 °C rise transformers are the norm in N. America.
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The thermal limit of transformer windings is the insulation on the conductor at the winding hot spot. The average winding rise is calculated as follows:
55 C Rise 65 C RiseAmbient 30 30 Average Wndg Rise 55 65 Hot Spot Differential 10 15 Hot Spot Temperature 95 110 *
* Only attainable with thermally upgraded insulation.
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Two types of Thermal Upgrading processes:
Modification of the cellulose chains specifically at OH groups by cyanoethylation and acetylation.
Addition of chemicals to protect the cellulose from oxidation: this is primarily achieved with nitrous compounds such as urea, melamine, dicyandiamide, and polyacrylamide.
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Cellulose Molecule
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Single Glucose Ring
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CyanoethylationRef. General Electric Company
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Amine Addition - Dicyandiamide
•Chemical Additive to paper.•Consumes water as it is produced.•Neutralizes acids as they are produced.
•(ref Lundgaard)
•Suppresses the self-catalyzing character of aging process by chemical reaction.
•During this process the stabilizing agent is consumed.
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Aging Curves
Thermally upgraded paperRegular Kraft paper
Source: Westinghouse/ABB Brochure on Insuldur®
Aging Curves
(Paper severely aged below this line)
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Nitrogen•All of the various thermal upgrading processes contain nitrogen.•Nitrogen is not found in cellulose
Nitrogen quantity is used to determine the amount of thermal upgrading agent added to paper.
Different thermal upgrade processes will have different nitrogen content levels to assure sufficient upgrading.ASTM D-982/ TAPPI T-418 “ Organic Nitrogen in Paper and Paperboard”
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Verification of 65 °C Rise Insulation
•Presently there is no clause in the standards which state that the transformer manufacturer must verify that Thermally Upgraded Paper is used.
•Presently no acceptance test will indicate if thermally upgraded paper is not used.
•Currently being considered for IEEE C57.12.00
•The transformer purchaser needs to specify!
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Degradation of Cellulose InsulationCauses:
•Moisture•Oxygen•Temperature
Effects:•Breakdown of the Cellulose Polymer
•Reduced Mechanical Strength•Shrinkage (Under compression)
Byproducts:•Moisture •Gas
•Carbon Monoxide/ Carbon Dioxide•Acids•Furans
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High Moisture Content in Insulation Can Cause:
• Accelerated Aging of the Cellulose
• Significant Reduction in Dielectric Strength
• Bubble Formation and Dielectric Failure
• Partial discharges in the Insulation
Dry = Cellulose < 0.5% by weight
& Oil < 10 ppm H O2
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Paper and Water in TransformersKVA Weight of 5% Initial Moisture
Rating KV Paper (kg) kg/KVA Kilograms Liters3,000 13.2 453.6 0.15 22.7 23.1
10,000 115 1,605.7 0.16 80.3 81.816,000 115 1837 0.11 91.6 93.120,000 132 2612.7 0.13 130.6 132.930,000 154 3637.8 0.12 181.9 185.140,000 230 4808.1 0.12 240.4 244.5
Ref. S.D. Meyers
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Moisture Accelerates Ageing Process
0
5
10
15
20
25
0 2 4 6 8 10 12Moisture content in paper (% W/W)
Age
ing
acce
lera
tion
fact
or
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Effect of moisture on Dielectric strength of Insulation
0
10
20
30
40
50
60
30 40 50 60 70 80 90 100
Temperature (°C )
Volta
ge U
(kV) x = 1%
x = 4%x = 6%x = 8%x = 10%
0
5
10
15
20
25
30
30 40 50 60 70 80 90 100 110
Temperature (°C )
Pow
er fa
ctor
tan
(%)
x = 1%x = 4%x = 6%x = 8%x = 10%
High-voltage insulation systems of Transformerboard must be properly dried and impregnated with oil. The insulation has to be dried because moisture increases the dielectric power factor and increases the risk of thermal breakdown.
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Moisture Promotes Bubble Evolution
• Residual moisture in winding insulation can lead to generation of gas bubbles at high temperature
• This is the dominant concern in the selection of a limiting hot spot temperature for safe operation
• Determinant factors for bubble generation have been identified :– Moisture content in insulation– Hydrostatic pressure– Duration of the high temperature
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T.V. Oommen et al, Atlanta, 2001
Generation of gas bubbles at high temperature
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Critical temperature for bubble evolution
50
70
90
110
130
150
170
190
0 2 4 6 8 10
WCP % w/w
Tem
pera
ture
Kobayashi rapid heatingKobayashi slow heating
Davydov
Oommen gas free
Oommen gas saturated
Ref. Sparling, Brian; GE Energy, Tutorial Transformer Insulation Condition Monitoring RVP-AI Mexico, 2005
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Diagnostics techniques for assessing the condition of insulation
•Moisture of Oil
•Dissolved Gas Analysis (DGA)
•Degree of Polymerization (DP)
•Furans
•Power Factor
•Polarization Index
•Return Voltage
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Equilibrium ConditionsWater in Oil & Paper
0
1
2
3
4
5
6
7
0 10 20 30 40 50 60 70 80 90 100
Water in Oil (ppm)
Wat
er in
Pap
er (%
)
20°C 30°C 40°C 50°C
60°C
70°C
80°C
90°C
100°C
Ref. Norris
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0
20
40
60
80
100
120
140
1 10 100 1000 10000
Tem
pera
ture
(°C
)
Diffusion time constant (hours)
Davydov et al. (winding model)
Davydov et al. (pressboard)
Griffin (insulated conductor)
Sokolov andVanin (full size transformer)
Oommen (distribution transformer)
Du et al. (theoretical)
Von Guggenberg (theoretical)
Sokolov et al. (theoretical)
FARADAY™ Model approximation
Diffusion Time Constant on Insulation Material
Ref. Sparling, Brian; GE Energy, Tutorial Transformer Insulation Condition Monitoring RVP-AI Mexico, 2005
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Dissolved Gas Analysis
The causes of fault gasses are classified into three categories:
1. Partial discharge
2. Thermal Heating
3. Arcing
When the insulation system is thermally overstressed, gasses are produced and they will dissolve in the oil.
Hydrogen from the OilCO and CO2 from the insulation
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Degree of Polymerization
Measurement of intrinsic viscosity after dissolving the cellulose in a specific solvent.
Gives an average measurement of the number of glucose units per molecular chain.
•DP of Insulation Components prior to processing ~1200
•DP of Insulation Components following processing ~1000
•DP level considered as “over-processed” ~800
•DP level considered end of life ~200
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Effects of aging:- darkening of color- loss of electrical and mechanical strength; trans. failure- shortening of cellulose chains – DP lowered- paper becomes wetter, and acidic- by-products contaminate the oil Source ABB Power Technologies, Inc.
IEEE Transformer Committee Panel Session – October 25, 2005
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Aging process : Cellulose depolymerization
CH2OH
O
OH
OHO
CH2OH
OH
OHO
O
CH2OH
OH
OH O
CH2OH
O
OH
OHO H
CH2OH
OH
OHO
O
CH2OH
OH
OH O
OH
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CH2OH
O
H
HH
OH
OHO
H
H O
Glucose Unit
Cellulose Degradation
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HOH
CO HOH
HOH
CH2OH
OHO
O H
H HH
OC
OHH
O CHO
H
H
H
WATER
WATER
WATER
FURAN
CARBONMONOXIDE
Degradation of Cellulose
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Furans
Most labs determine the concentration of five furaniccompounds:
1. 2-furaldehyde (2FAL)2. 5-methyl-2-furaldehyde (5M2F)3. 5-hydroxylmethyl-2-furaldehyde (5H2F)4. 2-acetyl furan (2ACF)5. 2-furfuryl alcohol (2FOL)
Note: 2FAL is stable for years while all other furanic compounds are less stable. They tend to form and then degrade to 2FAL over a time period of months.
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Furans
Causes of Specific Furan Compounds:
Compound Cause2-furaldehyde (2FAL) General overheating, Normal ageing5-methyl-2-furaldehyde (5M2F) High temperatures5-hydroxylmethyl-2-furaldehyde (5H2F) Oxidation2-acetyl furan (2ACF) Rare, Causes not fully defined2-furfuryl alcohol (2FOL) High Moisture
Ref: Stebbins, R.D., Myers, D.S., Shkolnik, A.B., “Furanic Compounds in Dielectric Liquid Samples: Review and Update of Diagnostic Interpretation and Estimation of Insulation Ageing”, Proceedings of the 7th International Conference on Properties and Applications of Dielectric Materials, 2003. Volume 3, 1-5 June 2003
Page(s):921 - 926 vol.3
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Source:1999 data from S.D. Myers on 13 units [4]
Relationship between 2FAL concentration and DP
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2- Furfural vs. DP Correlation Plots
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CORRELATION BETWEEN 2-FAL and DPV
DEGREE OF POLYMERISATION
2-FU
RA
LDEH
YDE
(ppb
, m
icro
g/L)
200 300 400 500 600 700 800 900 1000 1100 1200
10000
1000
100
10
0% 25% 50% 75% 100% Residual Life
VIT ST2
PAL T3ALK 1-2BALK 7-8A
ALK 5-6B
KLY 2RX2
KLY SP5RXPAL T2
ALK 3-4B
ASH T-1
RYL SPT1
RLY SPT3
MCA TX
Ref. GE Energy RVP-AI 2005
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Techniques to Mitigate the Ageing Process•It is not possible (today) to reverse the ageing of the cellulose insulation
•Control (slow down) the ageing process
•Remove the catalysts
•Moisture
•Acids
•Oxygen
•Process the oil
•Removes moisture, acids, particles, gasses
•Resets the Furan levels
•Dry the transformer
•Removes moisture from solid insulation
•Reduces the clamping pressure on windings
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Techniques to Mitigate the Ageing Process
•Control (slow down) the ageing process
•Reduce oxygen
•Maintain/Upgrade the Oil preservation system
•Membrane in oil conservator
•Reduce the temperature
•Increase cooling
•Control load
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Degradation of Cellulose Insulation in Liquid-Filled Power Transformers
•Selection of proper raw materials will prolong insulation life
•Pure/Clean cellulose processed with the Kraft process.
•Measured by Kappa number= low lignin content
•High mechanical strength
•High Density Pressboard Spacers with Surfaces Milled
•Improved compression characteristics= Short Circuit Withstand
•Thermally Upgraded Paper
•Determined by level of Nitrogen.
Summary and Conclusion
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Degradation of Cellulose Insulation in Liquid-Filled Power Transformers
• The rate of Insulation degradation is related to the presence of moisture, oxygen and temperature.
•The byproducts of insulation ageing are:•Moisture •Gas
•Carbon Monoxide/ Carbon Dioxide•Acids•Furans
•These by-products are also catalysts for the ageing process.•Removal of these by-products will slow down the ageing process•Measurement of these by products can also be used to assess insulation life.
Summary and Conclusion
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Degradation of Cellulose Insulation in Liquid-Filled Power Transformers
•Future Work
•Further development of moisture models.
•Diffusion
•Equilibrium
•Continue to verify Furan vs DP
•Need to measure retired/failed insulation.
•Include TUK vs Non-TUK
Summary and Conclusion
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Thank you for your attention
Questions??