2005 prevost presentation

A Member of the Group

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


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.


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.


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.


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”


Materials Critical to Functional Life of a Transformer

•Conductor Insulation•Thermally Upgraded Paper

•Duct Spacers•High Density Pressboard

•Lead Insulation•Crepe Paper


Critical Properties of Paper and Pressboard that Determine Functional Life

•Chemical Purity

•Mechanical Strength

•Dielectric Strength

•Thermal Stability


Cellulose Basics:

Part I) Fiber Source

Boreal Forest•White Spruce•Black Spruce•Balsam Fir•Hemlock


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.


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


•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


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).


BM2 Wet End


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.


Board Machine


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.)


Transformerboard Mechanical Role

• Support Windings During Short Circuits

• Maintain Dielectric Clearances

• Support High Voltage Leads

• Support Auxiliary Equipment

- LTC, DETC, Bus Bar etc.


Transformer’s Forces

Radial Forces Axial Forces

Core

Inner WindingOuter Winding


F Clamping Pressure = f(moisture,temperature,age)

F

Frigid clamping distance

transformerwindingcoil

pressboardpresspapercopper


Schematic of 550 kV BIL core and coil layout.


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


Characteristics of Transformerboard

Physical and Mechanical

0

5

10

15

20

25

%

Oil Absorption Compression

Hi-ValT-IV


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


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


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


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.


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.


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.


Cellulose Molecule


Single Glucose Ring


CyanoethylationRef. General Electric Company


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.


Aging Curves

Thermally upgraded paperRegular Kraft paper

Source: Westinghouse/ABB Brochure on Insuldur®

Aging Curves

(Paper severely aged below this line)


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”


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!


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


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


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


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


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.


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


T.V. Oommen et al, Atlanta, 2001

Generation of gas bubbles at high temperature


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


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


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


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


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


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


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

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


CH2OH

O

H

HH

OH

OHO

H

H O

Glucose Unit

Cellulose Degradation


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


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.


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


Source:1999 data from S.D. Myers on 13 units [4]

Relationship between 2FAL concentration and DP


2- Furfural vs. DP Correlation Plots


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


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


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



•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



• 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.




•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



Thank you for your attention

Questions??

2005 prevost presentation

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