Degradation of Cellulose Insulation

7/30/2019 Degradation of Cellulose Insulation

1/70

A Member of the Group

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


2/70


3/70


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.


4/70


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.


5/70


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


6/70


Materials Critical to Functional Life of a Transformer

Conductor Insulation

Thermally Upgraded Paper

Duct Spacers

High Density Pressboard

Lead Insulation

Crepe Paper


7/70


Critical Properties of Paper and Pressboard that

Determine Functional Life

Chemical Purity

Mechanical Strength

Dielectric Strength

Thermal Stability


8/70



9/70


Cellulose Basics:

Part I) Fiber Source

Boreal Forest

White Spruce

Black Spruce

Balsam FirHemlock


10/70



11/70


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.


12/70



13/70


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


14/70


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


15/70



16/70



17/70


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).


18/70


BM2 Wet End


19/70


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.


20/70


Board Machine


21/70


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.)


22/70


Transformerboard Mechanical Role

Support Windings During Short Circuits

Maintain Dielectric Clearances

Support High Voltage Leads

Support Auxil iary Equipment

- LTC, DETC, Bus Bar etc.


23/70



24/70



25/70


Transformers Forces

Radial Forces Axial Forces

Core

Inner WindingOuter Winding


26/70


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

F

F

rigid clamping distance

transformer

windingcoil

pressboard

presspapercopper


27/70


6.>3

>(,'0$11

Schematic of 550 kV BIL core and coil layout.


28/70


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


29/70


Characteristics of Transformerboard

Physical and Mechanical

0

5

10

15

20

25

%

Oil Absorption Compression

Hi-Val

T-IV


30/70


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


31/70


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


32/70


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


33/70


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.


34/70


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.


35/70


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.


36/70


Cellulose Molecule


37/70


Single Glucose Ring


38/70


Cyanoethylation

Ref. General Electric Company


39/70


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.


40/70


Aging Curves

Thermally upgraded paper

Regular Kraft paper

Source: Westinghouse/ABB Brochure on Insuldur

Aging Curves

(Paper severely aged below this line)


41/70


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


42/70


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!


43/70


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


44/70


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


45/70


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


46/70


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


47/70


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.


48/70


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


49/70


T.V. Oommen et al, Atlanta, 2001

Generation of gas bubbles at high temperature


50/70


51/70


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


52/70


53/70


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


54/70


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


55/70


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


56/70


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


57/70


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


58/70


CH2OH

O

H

H

H

OH

OH

O

H

H O

Glucose Unit

Cellulose Degradation

Degradation of Cellulose


59/70


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


60/70


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


61/70


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


62/70


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

Relationship between 2FAL concentration and DP


63/70


2- Furfural vs. DP Correlation Plots


64/70


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


65/70


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


66/70


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


67/70


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



68/70



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.




69/70



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


70/70


Thank you for your attention

Questions??

Degradation of Cellulose Insulation

Documents

Transcript of Degradation of Cellulose Insulation