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Degradation of Cellulose Insulation
in
Liquid-Filled Power Transformerspresented 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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The life of a transformer is limited to the life of the solid
insulation.
Much of the diagnostics performed on powertransformers 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 fundamental
understanding 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 2Normal insulation li fe of a well-dried, oxygen-free 65 C average windingtemperature rise insulation system at the reference temperature of 110 C
Basis Normal insulation life
Hours Years
50% 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 in
insulation 150 000 17.12
Interpretation of distribution Transformer
functional 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 FirHemlock
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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 as
much 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 fromconiferous 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
Cutter
Dryer
Cutting Table
Hot Press
Forming Roll
Sheet Forming
Water
Sulfate Pulp
Stock Chests
Deflakers
Refiners
Storage Chests
Mixing
Chests
White
Water
Machine
Chest
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 Auxil iary Equipment
- LTC, DETC, Bus Bar etc.
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Transformers Forces
Radial Forces Axial Forces
Core
Inner WindingOuter Winding
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F Clamping Pressure = f(moisture,temperature,age)
F
F
rigid clamping distance
transformer
windingcoil
pressboard
presspapercopper
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6.>3
>(,'0$11
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-Val
T-IV
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Compression of Radial Spacers
Effect 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 r e s s io n C o m p r e s s io n S e t
W i t hScreenPat tern
W i t h o u tScreenPat tern
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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 Tim e (Days )
SpacerS
tackHeight(mm
)
135 Deg. C
150 Deg. C
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Shrinkage after 250 Days of Aging
Aged at 135 C Aged at 150 C
4.8% 11.0%
Degree of Polymerization after 250 Days of Aging
Inititial Values Aged at 135 C Aged at 150 C1190 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 of
the spacer material continues to decline.
Shrinkage versus DP
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Thermal Upgrading of Insulation
In the late 1950s 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 theconductor at the winding hot spot. The average winding rise is
calculated as follows:
55 C Rise 65 C RiseAmbient 30 30Average Wndg Rise 55 65Hot Spot Differential 10 15Hot 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 withnitrous 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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Cyanoethylation
Ref. 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 agingprocess by chemical reaction.
During this process the stabilizing agent is
consumed.
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Aging Curves
Thermally upgraded paper
Regular Kraft paper
Source: Westinghouse/ABB Brochure on Insuldur
Aging Curves
(Paper severely aged below this line)
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NitrogenAll 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 verifythat Thermally Upgraded Paper is used.
Presently no acceptance test will indicate if thermallyupgraded 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 Insulation
Causes:Moisture
Oxygen
Temperature
Effects:Breakdown of the Cellulose Polymer
Reduced Mechanical Strength
Shrinkage (Under compression)
Byproducts:
Moisture
GasCarbon 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 Liters
3,000 13.2 453.6 0.15 22.7 23.1
10,000 115 1,605.7 0.16 80.3 81.8
16,000 115 1837 0.11 91.6 93.1
20,000 132 2612.7 0.13 130.6 132.9
30,000 154 3637.8 0.12 181.9 185.1
40,000 230 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 12
Moisture content in paper (% W/W)
Ageingacce
lerationfactor
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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 )
VoltageU(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 )
Powerfactortan(%)
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 tobe 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 beenidentified :
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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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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0
20
40
60
80
100
120
140
1 10 100 1000 10000
Temperature(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)
VonGuggenberg (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 Oil
CO 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 unitsper molecular chain.
DP of Insulation Components prior to processing ~1200DP 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
Aging process : Cellulose depolymerization
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Aging process : Cellulose depolymerization
CH2OHO
OH
OH
O
CH2OH
OH
OH
OO
CH2OH
OH
OHO
CH2OH
O
OH
OHO H
CH2OH
OH
OHOO
CH2OH
OH
OHO
OH
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CH2OH
O
H
H
H
OH
OH
O
H
H O
Glucose Unit
Cellulose Degradation
Degradation of Cellulose
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HOH
CO HOH
HOH
CH2OH
OH
O
O H
HH
HO
C
OHH
O CHO
H
H
H
WATER
WATER
WATER
FURAN
CARBON
MONOXIDE
Degradation of Cellulose
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Furans
Most labs determine the concentration of five furanic
compounds:
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 furaniccompounds are less stable. They tend to form and then degrade
to 2FAL over a time period of months.
F
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Furans
Causes of Specific Furan Compounds:
Compound Cause2-furaldehyde (2FAL) General overheating, Normal ageing5-methyl-2-furaldehyde (5M2F) High temperatures
5-hydroxylmethyl-2-furaldehyde (5H2F) Oxidation
2-acetyl furan (2ACF) Rare, Causes not fully defined
2-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-FURALDEHYD
E(ppb,microg/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 T3
ALK 1-2B
ALK 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
T h i t Miti t th A i P
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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
T h i t Miti t th A i P
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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
Degradation of Cellulose Insulation in Liquid-Filled Power
Summary and Conclusion
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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.
Degradation of Cellulose Insulation in Liquid Filled Power
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 DioxideAcids
Furans
These by-products are also catalysts for the ageing process.
Removal of these by-products will slow down the ageing processMeasurement of these by products can also be used to assess
insulation life.
Degradation of Cellulose Insulation in Liquid-Filled Power
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
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Thank you for your attention
Questions??
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