POLITECNICO DI MILANO Scuola di Ingegneria Industriale … · POLITECNICO DI MILANO Scuola di...

POLITECNICO DI MILANO

Scuola di Ingegneria Industriale e dell'Informazione

Corso di Laurea Magistrale in Ingegneria Elettrica

Comparative Investigation on the Properties of Transformer-used

High-temperature Resistant Oil and Paper Insulation Materials

Relatore: Prof. Giovanni Dotelli

Correlatore: Saverio Latorrata

Tesi di Laurea Magistrale di:

Shen Shuhang

Matr. 813546

Anno Accademico 2015-2016

Contents

1

Contents

CONTENTS .............................................................................................................................................. 1

ABSTRACT ............................................................................................................................................... 4

1 INTRODUCTION ............................................................................................................................ 8

1.1 RESEARCH BACKGROUND AND SIGNIFICANCE ..................................................................................... 8

1.2 STATE-OF-ART OF HIGH TEMPERATURE RESISTANT INSULATION PAPER ............................................ 9

1.2.1 The Development of High-Temperature Resistant Insulation Paper ......................................... 9

1.2.2 Main Components of High-Temperature Resistant Insulation Paper ...................................... 13

1.3 AGING MECHANISMS OF HIGH TEMPERATURE RESISTANT INSULATION PAPER ................................. 17

1.3.1 Aging Process of High Temperature Resistant Insulation Paper ............................................ 17

1.3.2 Thermal Aging Kinetics of Insulation Material ....................................................................... 26

1.3.3 Prediction of Transformer Lifespan ........................................................................................ 29

1.4 RESEARCH STATUS OF VEGETABLE OIL IMPREGNATED INSULATION SYSTEM ................................... 31

1.4.1 Characteristics of Vegetable Oil.............................................................................................. 31

1.4.2 Thermal Aging Researches on Vegetable Oil Impregnated Insulation Paper ......................... 33

1.5 RESEARCH CONTENT OF THIS THESIS ................................................................................................ 34

2 EXPERIMENTAL DESIGN OF HIGH-TEMPERATURE RESISTANT INSULATION

MATERIALS PROPERTY COMPARISON AND THERMAL AGING .............................................. 36

2.1 MATERIALS AND TEST PARAMETERS OF PROPERTY COMPARISON .................................................... 36

2.2 MATERIALS AND TEST PARAMETERS OF THERMAL AGING ................................................................ 37

2.3 PRECONDITIONING OF PAPER AND OIL MATERIAL ............................................................................. 39

2.3.1 Preconditioning of Paper ........................................................................................................ 39

2.3.2 Preconditioning of Oil ............................................................................................................. 40

2.4 CHAPTER CONCLUSION ...................................................................................................................... 42

3 PHYSICAL AND MECHANICAL PERFORMANCES OF HIGH TEMPERATURE

RESISTANT INSULATION PAPER ..................................................................................................... 43

3.1 BASIS WEIGHT AND DENSITY ............................................................................................................ 43

3.1.1 Test Method ............................................................................................................................. 43

3.1.2 Test Results .............................................................................................................................. 44

Contents

2

3.2 WATER CONTENT .............................................................................................................................. 45

3.2.1 Test Method ............................................................................................................................. 45

3.2.2 Test Results .............................................................................................................................. 47

3.3 TENSILE STRENGTH ........................................................................................................................... 48

3.3.1 Test Method ............................................................................................................................. 48

3.3.2 Test Results .............................................................................................................................. 49

3.4 OIL ABSORPTION ............................................................................................................................... 52

3.4.1 Test Method ............................................................................................................................. 52

3.4.2 Test Results .............................................................................................................................. 53

4 DIELECTRIC PERFORMANCES OF HIGH TEMPERATURE RESISTANT INSULATION

PAPER ..................................................................................................................................................... 58

4.1 PERMITTIVITY AND DISSIPATION FACTOR ......................................................................................... 58

4.1.1 Test Method ............................................................................................................................. 58

4.1.2 Test Results .............................................................................................................................. 60

4.2 BREAKDOWN ELECTRIC FIELD IN AIR ................................................................................................ 68

4.2.1 Test Method ............................................................................................................................. 69

4.2.2 Test Results .............................................................................................................................. 70

4.3 BREAKDOWN ELECTRIC FIELD IN OIL ................................................................................................ 71

4.3.1 Test Method ............................................................................................................................. 71

4.3.2 Test Results .............................................................................................................................. 71

5 PHYSIOCHEMICAL PERFORMANCES OF HIGH TEMPERATURE RESISTANT

INSULATION OIL ................................................................................................................................. 76

5.1 VISCOSITY ......................................................................................................................................... 76

5.1.1 Test Method ............................................................................................................................. 76

5.1.2 Test Results .............................................................................................................................. 77

5.2 TOTAL ACID NUMBER........................................................................................................................ 78

5.2.1 Test Method ............................................................................................................................. 78

5.2.2 Test Results .............................................................................................................................. 79

5.3 WATER CONTENT .............................................................................................................................. 80

5.3.1 Test Method ............................................................................................................................. 80

5.3.2 Test Results .............................................................................................................................. 81

6 RESULTS AND ANALYSIS OF ACCELERATED AGING EXPERIMENT OF HIGH

TEMPERATURE RESISTANT INSULATION SYSTEM .................................................................... 83

6.1 VARIATION IN INSULATION PAPER PERFORMANCE BEFORE AND AFTER AGING ................................ 83

6.1.1 Tensile Strength ....................................................................................................................... 84

Contents

3

6.1.2 Breakdown Voltage.................................................................................................................. 86

6.2 VARIATION IN INSULATION OIL PERFORMANCE BEFORE AND AFTER AGING .................................... 87

6.2.1 Viscosity ................................................................................................................................... 87

6.2.2 Total Acid Number................................................................................................................... 89

6.2.3 DGA ......................................................................................................................................... 90

7 DISCUSSION ON THE AGING PROCESSES AND MECHANISMS OF HIGH

TEMPERATURE RESISTANT INSULATION SYSTEM .................................................................... 98

7.1 ANALYSIS OF AGING MECHANISMS OF PAPER AND OIL MMATERIALS .............................................. 98

7.2 ANALYSIS OF LONGER REMAINING LIFESPAN OF T910 AND DMD PAPER ....................................... 100

8 CONCLUSIONS AND PROSPECTS .......................................................................................... 103

8.1 CONCLUSIONS OF THE THESIS .......................................................................................................... 103

8.2 PROSPECTS OF THE THESIS ............................................................................................................... 104

9 BIBLIOGRAPHY ........................................................................................................................ 105

APPENDIX ............................................................................................................................................ 108

WEIBULL DISTRIBUTION ................................................................................................................ 108

ACKNOWLEDGES .............................................................................................................................. 110

Abstract

4

Abstract

As the distribution transformers experience more severe seasonal overloading in recent

years, the reliability of electricity supply to urban and rural residences is jeopardized.

Therefore, it is necessary to improve transformers’ anti-overload ability. One economical

way is to substitute the insulation system with a higher temperature resistant system. There

are already several kinds of high temperature resistant paper candidates which are needed

to be investigated on their applicability to fluid-filled transformers. Meanwhile, compared

with traditional mineral oil, vegetable oil has much higher fire and flash points and is readily

biodegradable. Therefore, vegetable oil has higher fire safety and the advantage of

environmental friendliness and is increasingly applied in transformers. The combination of

high temperature resistant paper and vegetable oil would provide a potential to safely

increase the transformer anti-overload ability. It is necessary to investigate on the behavior

of high temperature resistant oil and paper insulation materials.

This thesis first selects NOMEX T910 paper and Thermally Upgraded Kraft(TUK) paper

and three kinds of vegetable oil, namely FR3,Vinsoil and Dupont EBF#2. The initial physical,

chemical, mechanical and dielectric properties of all the paper and oil material are studied

and the results are compared with conventional Kraft paper and mineral oil system. On this

basis, the thesis further sets up a thermal aging platform and then performs a 150℃

accelerated thermal aging tests on T910 paper and PET Composite(DMD) paper in FR3

impregnation in a sealed tube. The change on typical properties of paper and oil along aging

is analyzed.

The results show that T910 paper owns a better performance on water content and oil

absorption. But on the initial mechanical strength, TUK paper and Kraft paper have higher

values, and T910 paper still meet the requirement of the relevant standard. The dissipation

factor tanδ of TUK paper and Kraft paper are lower, while T910’s is greater. The dielectric

constants εr of all three kinds of paper increase as the density goes up. T910 paper shows

higher breakdown electrical field values in air and most oil impregnation conditions, and

Abstract

5

mineral oil is more sensitive to temperature rise. As for the oil, all the vegetable oils have

higher water content, total acid number and viscosity.

After 720h of the 150℃ aging, the breakdown voltage of paper shows little change. Kraft

paper has more obvious tensile strength decrease and drops to 60% of the initial value,

whereas T910 paper and DMD paper nearly keep contant and high level all the time, remain

95% and 90% respectively. The vegetable oil has a stable viscosity behavior but distinct

increase on total acid number. The DGA results indicate higher gaseous content of C2H6

and CO and are diagnosed as T1 low-temperature fault by IEC standard. The aging process

makes it clear that the paper and oil degrade as the hydyolysis process dominates. All the

paper and oil materials experience the molecular chain cleavage led by the breakdown of

glucosidic bonds or ester bonds and some by-products are formed simultaneously. The

supporting role of high thermal stability of NOMEX fiber and the interactive

transesterification effect between PET and triglyceride make the T910 paper and DMD

paper have great tensile stability in FR3 oil. T910 and DMD paper both show greater tensile

retention than Kraft paper. However, the limitation of aging time could not verify the

temperature resistivity of T910 and DMD paper in FR3 oil sufficiently. Therefore, the

possibility of cooperations of T910 paper and DMD paper with FR3 oil to improve

transformer’s anti-overload ability is still needed to be assessed.

Abstract

6

Abstract

Poiché i trasformatori di rete hanno evidenziato, negli ultimi anni, seri problemi dovuti ai

sovraccarichi stagionali, l’affidabilità dei sistemi di alimentazione urbana e non è messa a

repentaglio. Perciò, è necessario migliorare l’abilità dei trasformatori di gestire tali

sovraccarichi. Una via economica è rappresentata dalla possibilità di sostituire il sistema

isolante con uno maggiormente resistente alle alte temperature. Esistono già diversi tipi di

carta resistente alle alte temperature candidati a questo ruolo che meritano un’analisi di

applicabilità ai trasformatori. Allo stesso tempo, se paragonato agli oli minerali tradizionali,

l’olio vegetale ha un più alto flash point ed è facilmente biodegradabile. Perciò esso offre

una maggiore sicurezza e il vantaggio di essere meno impattante dal punto di vista

ambientale; per questi motivi il suo impiego nei trasformatori sta aumentando notevolmente.

La combinazione di un foglio resistente alle alte temperature e di olio vegetale

comporterebbe un potenziale miglioramento nell’abilità nella gestione dei sovraccarichi.

Questa tesi ha selezionato dapprima due fogli, NOMEX T910 e Thermally Upgraded Kraft

(TUK),e tre tipi di olio vegetale, FR3, Vinsoil e Dupont EBF#2. Le proprietà fisiche,

chimiche, meccaniche e dielettriche iniziali degli isolanti impiegati sono state studiate e i

risultati paragonati con le proprietà di un foglio Kraft e di un olio minerale convenzionali.

Su questa base, si è sviluppato un piano di test termici di invecchiamento e in seguito un test

di stress accelerato a 150 °C. La variazione delle proprietà tipiche dei materiali impiegati

nel corso dell’invecchiamento è stata analizzata.

I risultati hanno mostrato che il T910 è in grado di fornire una performance migliore in

termini di contenuto di acqua e assorbimento di olio. Tuttavia,il TUK e il Kraft hanno

mostrano valori iniziali di resistenza meccanica più elevati. Il fattore di dissipazione tanδ

del TUK e del Kraft è più basso. Le costanti dielettriche εr dei tre paper aumentano

all’aumentare della densità. Il T910 mostra i più alti valori di breakdown in aria e l’olio

minerale è maggiormente sensitivo a un aumento di temperatura. Gli oli vegetali hanno

maggiori contenuto di acqua, numero di acidità totale e viscosità.

Dopo 720 h di invecchiamento, la tensione di breakdown dei fogli mostra una leggera

variazione. Il Kraft mostra una ovvia riduzione della resistenza a trazione (fino al 60 % del

Abstract

7

valore iniziale), mentre T910 e DMD mantengono tale parametro pressoché costante per

tutto il tempo.

L’olio vegetale mostra un comportamento stabile della viscosità, ma un netto aumento del

numero di acidità totale.

I risultati DGA indicano un più elevato contenuto di C2H6 e CO.

Il processo di invecchiamento ha evidenziato che il foglio e l’olio degradano se il processo

di idrolisi risulta dominante. Tutti i fogli e gli oli minerali hanno evidenziato la frattura della

catena polimerica dovuta alla rottura dei legami glucosidici o esterei e alla formazione

simultanea di sottoprodotti.

I fogli T910 e DMD mostrano una migliore tenuta a trazione rispetto al Kraft. Tuttavia, il

limite imposto di tempo durante le prove di invecchiamento potrebbe aver inciso sui risultati

ottenuti. Pertanto, la possibilità di unione delle proprietà di T910 e DMD nell’olio FR3 per

migliorare la gestione dei sovraccarichi meriterebbe un’ulteriore valutazione.

Introduction

8

1 Introduction

1.1 Research Background and Significance

As the commonly used electrical apparatus in power system, the distribution transformers

play a critical role in electricity transmission, distribution and utilization. The safe operation

of distribution grid directly determines the stability and reliability of power supply. Currently,

most distribution transformers apply the oil-immersed type insulation, that is the windings

wrapped by insulation paper are immersed in insulation oil. Transformers in some areas may

experience seasonal overloading in particular periods, for example farming or festivals.

Under such conditions, the transformers temperature rise will increase, leading to the melting

of protecting fuse and then blackout. Meanwhile, the urban industries and household

electricity also call for a higher requirement of safe and stable electricity supplement.

Therefore, how to enhance the anti-overloading ability of distribution transformers

economically and efficiently has become a prominent problem in recent years.

The anti-overloading ability of distribution transformers can be improved by the following

methods: (1) increasing the transformer capacity. However, the daily residential electricity

consumption is limited, as the loading rates in normal operation just reach approximately

40%. The raising of transformers capacity will also increase the light-loading losses, causing

additional energy wasting. (2) supplement of draught fans or increasing the size of oil tank.

(3) choosing alternative high-temperature resistant oil-paper insulation systems. The third

method has the highest technique-economic performance. The appearance of high-

temperature class insulation paper provide a solid support. At the same time, the high fire

and flash points, the environment-friendly property and the characteristic that could prolong

insulation paper’s life of vegetable insulation oil make the economic increasing of

transformers anti-overloading ability possible.

The conventional transformer oil and paper insulation system consists of Kraft paper and

mineral oil. During the long-term operation, oil-paper insulation materials will deteriorate

under electrical, thermal, mechanical and environmental factors, among which the thermal

stress is the main degrading source. Insulation paper will age accelerated when facing the

Introduction

9

temperature rise caused by overloading. According to the Montsinger’s rule[1], the life of

insulation paper will be halved as the operating temperature increases by every 6 to 8℃.

Hence, in order to increase the transformers anti-overloading ability, we need to select highly

temperature resistant paper material. There are plenty choices in real applications.

NOMEX® paper, thermally upgraded Kraft (TUK) paper, DMD paper and NOMEX T910

are typical paper materials with high thermal class. Due to the different performances, the

suitability of such materials in anti-overloading transformers is still needed to be investigated.

Although the mineral oil is widely used as insulation oil, the low fire and flash points make

it has low fire security, unsuitable for areas where the population is dense or great fire safety

concerns are drawn. Mineral oil is hard to degrade naturally. Once it is leaked, the

environment will be contaminated. However, the high fire and flash points and readily

biodegradability allow the vegetable oil to be more safe and environment-friendly[2]. There

are also researches[3] showing that the impregnation by vegetable oil could postpone the

hydrolysis of cellulose and thus extend the paper and transformers life.

To sum up, investigating the differences of fundamental properties of different insulation

materials could provide a basis to increase the transformer anti-overloading ability. It will

also be helpful for engineers to design a suitable and stable insulation system.

1.2 State-of-Art of High Temperature Resistant

Insulation Paper

1.2.1 The Development of High-Temperature Resistant Insulation Paper

Before the 1920s, a variety of fibrous materials, both cellulosic and non-cellulosic, were

used for electrical insulation: cotton rag, silk, jute, asbestos, etc. Although varnished cambric

cloth and other textiles were used in cables, varnished or ‘boiled-in-oil’ pressboard made up

of cotton rags and paper clippings was used in transformers. In 1920, blends of kraft wood

fibers and manila-hemp fibers began to be used for telephone insulation. In capacitors, linen

was used until the late 1920s. The 1920s and 1930s were periods of much experimentation

on how to improve the dielectric performance of the paper-oil system. A better

understanding of fibers and impurities in the pulp resulted in better insulation. It seems that

by the late 1920s and early 1930s, kraft paper insulation began to be used in combination

Introduction

10

with insulating oil in transformers. This combination was needed to satisfy the increasing

insulation requirements as the voltage ratings escalated. In the 1940s, kraft paper in

combination with oil was the dielectric material of choice for HV use as evidenced by the

number of cellulose material studies done. Much more information on paper chemistry was

generated in the 1950s and later at the Institute of Paper Chemistry. But interest in synthetic

dielectric materials slowly developed in the late 1950s, and such materials began to replace

cellulosic insulation in power cables and capacitors[4]. Mixtures of cellulosic and synthetic

materials are now used in many transformer insulation applications.

It may be noted that transformer insulation had to be developed almost concurrently with

transformer development, but it took a few decades before the paper-oil combination became

reliable and well accepted. The transformer had been invented as far back as 1885 by a team

of Austrian engineers and further developed by other inventors, especially George

Westinghouse and his team. The one built by George Westinghouse in 1885 based on the

work of his team of experts in the U.S. was, in principle, similar to theirs, and was a dry-

type distribution transformer with 500-V primary and 100-V secondary. It used air as coolant.

Cellulose-oil insulation was critical for all transformers developed since the 1920s.

Transformer oil itself had been introduced for transformer use in 1892 by GE and underwent

improvement from paraffinic to naphthenic by 1925. Vacuum filling of oil was introduced

in 1932.

A. Kraft Paper and Board

It is difficult to pinpoint the time when electrical grade paper was introduced, but it is

known that such papers were used for capacitors and cables extensively before becoming

the primary solid insulation in transformers. The use of resin-impregnated paper for

transformer insulation was introduced at the turn of the 20th century. The introduction of

oil impregnation of paper led to the discontinuation of resin-impregnated paper.

Although resin-impregnated cylinders functioned remarkably well in the earlier days, they

were not desirable in high-stressed areas such as angles and corners (boundary areas) as the

voltage rating increased. By the late 1920s in Switzerland, Weidmann had developed

transformerboard (now called pressboard) from kraft pulp, which could be easily fabricated

into formed items, and these were ideal for high-stressed areas. The wet sheets, built up from

Introduction

11

a number of required plies, pass through compressing and drying cylinders and emerge as

dry sheets. The calendered pressboard is ideal also for washers and tubes used in power

transformers. Another European manufacturer of calendered board, based in Sweden, is

Figeholm, which started its operation in 1931. Other companies once in production have

been acquired by other companies or shut down. Figeholm itsef is now owned by ASEA in

Sweden[5].

B. Creped Paper Turn Insulation

Although plain kraft paper is widely used for conductor insulation in transformers in many

countries, creped kraft paper is used for such purposes in the U.S. Crepe paper for turn

insulation was introduced by Dennison Paper Company in Framingham, MA in the 1970s

with the blessing of the Westinghouse Large Power Transformer Plant in Muncie, IN, which

was interested in a tear-free paper for taping[5]. The tough hemp-kraft paper used for taping

at the time had very little stretch. The crepe paper has as much as 20% stretch (elongation).

The creping is done on the regular sheet of paper, as a drum of it unrolls and is picked up by

another drum revolving at a slower speed; the paper goes through an aqueous bath containing

a creping compound. The crepe paper described here should not be confused with the 100%

stretch lead tape used in transformers that was available earlier. The introduction of the crepe

paper was a few years after thermal upgrading agents were put into paper (see subsequent),

so the crepe paper could be thermally upgraded at the same time from a non-upgraded paper.

C. Thermal Upgrading of Paper

As the rating of transformers climbed in the 1950s and 1960s and as transformers were

occasionally overloaded, the concern for transformer life, or rather, paper insulation life, was

raised. Thermal upgrading of the paper insulation was considered one remedy and was

attempted by several research groups associated with transformer or paper manufacturing in

the late 1950s through the 1970s; upgraded paper began to be used in the U.S. since the mid

1960s. An EPRI Report on thermal upgrading agents released in 1987 gives both historical

and ongoing studies. The upgrading systems developed include Insuldur (Westinghouse),

Cyanoethylate (GE), Thermacel (McGraw Edison), Celloflex (Allis Chalmers), Mannitherm

(Manning Paper Co.), HAS (McGraw Edison), Hovotherm (Hollingsworth & Vose), and

Rigel 65°C Rise (Rigel Products)[6]. The superiority of the upgraded papers was

demonstrated by both short-term and long-term aging. The purpose of upgrading is to

increase the insulation life. Accelerated aging studies confirmed that cellulose degradation

Introduction

12

is considerably slowed by upgrading agents. Transformers rated at 55°C oil rise could be

upgraded to 65°C oil rise, which meant the insulation life was extended by at least three

times. Also, cellulose paper has a thermal class rating of only 105°C, whereas the TUK paper

usually has at least 15°C elevation, which means its thermal class could reach up to 120°C[7].

D. Synthetic Materials-NOMEX®

Special synthetic formulations such as Aramid (an aromatic polyamide) developed by

DuPont in 1960s under the trade name Nomex® are being used for making paper sheets and

pressboard for limited transformer use. Nomex® has a considerably higher thermal rating

(220°C vs. 105°C for cellulosic paper)[5]. Moisture absorption by Nomex® paper is

significantly lower than for cellulosic paper, e.g., for 0.075-mm (3-mil) thick papers,

saturation values at room temperature at 50% humidity are as follows: Nomex®, 4%;

cellulose paper, 6.5%. Hybrid insulation structures in distribution, mobile, and small power

transformers containing both Nomex® turn insulation and cellulosic structural parts are in

commercial use. The higher cost of Nomex® insulation prevents its widespread use in

medium and large power units.

E. Flexible Laminates-DMD

Flexible laminates have been used as basic insulation in the electrical industry for over 30

years, but they are still one of the least understood components in an electrical insulation

system. DMD laminates are three-ply constructions of nonwoven polyester fiber mat bonded

to both sides of polyester film originally known as Dacron® - Mylar® - Dacron® in the

industry, and subsequently shortened to DMD[8]. These composites could be considered to

be the first modern flexible laminates developed specifically for insulating electrical motors.

Introduced in the late 1950s, DMD laminates are composed entirely of synthetic organic

materials combined together to achieve a truly synergistic effect-the performance of the

laminate far exceeds the performance of the individual components. It should be noted that

early experiences with DMD laminates pointed out potential problems with the nonwoven

mat surface. The soft, fibrous mat surface exhibited poor abrasion resistance and tended to

snag and pull on rough surfaces. This would cause problems in automatic insertion

operations when the laminate would hang up in the slot or jam the insertion equipment. This

problem was overcome by saturating, or overcoating, the nonwoven mat surfaces with a hard

resin, usually polyester or epoxy, to tie down the surface fibers and create a smooth, abrasion

resistant surface.

Introduction

13

F. Cellulose-Synthetic Composite NOMEX T910®

Since 1980s, there are little modifications made on the insulation paper. In recent years,

DuPont developed a new and unique solid insulating material under the trademark NOMEX

T910®. This new paper is composed of high quality electrical grade cellulose pulp and web-

like binders made from high-temperature meta-aramid synthetic polymer without any

additional binders. NOMEX T910® has a thermal class of 130°C, filling the blank space in

thermal class between 120°C(TUK) and 220°C(NOMEX). This new solid insulation paper

has shown noticeably better thermal capability than the incumbent cellulose paper, with a

cost between the cellulose and the synthetic aramid papers.

1.2.2 Main Components of High-Temperature Resistant Insulation Paper

A. Kraft Paper

For the manufacturing of paper and pressboard for electrical insulation, mainly unbleached

softwood kraft pulp is used. The cellulose is refined from the tree by the so-called "sulphate"

or "kraft" process. Wood is a natural composite material that is made up of flexible tubes of

cellulose bound together by lignin, a brownish aromatic polymer that is mostly removed

during the pulping process. After processing the typical composition of unbleached kraft

pulp is 78-80% cellulose, 10-20 % hemicellulose and 2-6 % lignin[9].

Cellulose, the essential component of paper and pressboard, is a polymer of glucose units

linked to one another in a special manner as shown in Figure. 1-1[10]. It may be represented

simply as [C5H10O5]n, ignoring the extra atoms on the end groups, where n is the degree of

polymerization (DP). The repeating unit, however, is cellobiose, consisting of two glucose

units.

Figure 1-1. Structural formula of cellulose

The DP values for paper samples can be estimated by specified methods such as ASTM D-

4243. The DP of kraft pulps ranges from 1100 to 1200, but mixed pulp fibers can have much

higher DP, e.g., 1400 to 1600.

Introduction

14

When the cellulose molecule is fully extended it takes the form of a flat ribbon with highly

hydrophilic hydroxyl groups protruding laterally and capable of forming both inter- and

intramolecular hydrogen bonds. The surface of the ribbon consists mainly of hydrogen atoms

linked directly to carbon and is therefore hydrophobic. These two features of the molecular

structure of cellulose are responsible for its supramolecular structure and this in turn

determines many of its chemical and physical properties.

B. Thermally upgrading Kraft(TUK) Paper

There are basically two types of thermal upgrading processes that were developed[11].

1) Modification of the cellulose chains specifically at OH groups by cyanoethylation and

acetylation.

In cyanoethylation the cellulose is chemically modified with some of the less-stable water-

forming hydroxyl groups in the cellulose chain being replaced by more stable cyanethyl

groups. (see Figure 1-2). This process must be done in the pulping stage of paper

manufacturing. The replacement of some of the hydroxyl groups also reduces the number of

hydrogen bridges between the molecules. This reduces mechanical strength.

Figure 1-2. The Cyanoethylated Reaction of Cellulose.

2) Addition of chemicals to protect the cellulose from oxidation: this is primarily achieved

with nitrous compounds such as urea, melamine, dicyandiamide, and polyacrylamide.

In amine addition nitrous compounds such as dicyandiamide are added to the paper to act as

stabilizing agents. The addition of stabilizers suppresses the self-catalyzing character of the

aging process by a chemical reaction with the aging products during which the additives are

consumed. The stabilizing agents consume water by reacting chemically with it. They also

contain organic bases which partially neutralize the acids which are also a by-product of

aging. Figure 1-3 shows some typical stabilizing agents[10].

Introduction

15

Figure 1-3. Agents for thermally upgrading of paper

All processes result in an increased content of nitrogen in the solid insulation, but the

treatment is by no means standardized, and the concentration of additives may vary. The

nitrogen content of the various upgrading systems ranged from 0.3 to 2.7%.

C. DMD Paper

DMD laminates are three-ply constructions of nonwoven polyester fiber mat bonded to both

sides of polyester film. The polyester molecule structure, which is co-polymerized from

terephthalic acid and glycol through ester bonds, is shown in Figure 1-4[12]. Polyester film

is an excellent electrical insulation material exhibiting high dielectric strength, tensile

strength and resistance to tear initiation. However, if nicked or scratched by a sharp edge or

a burr, polyester film has very little resistance to tear propagation. Polyester film is usually

rated for use at service temperatures up to 130'C. On the other hand, nonwoven polyester

fiber mat, due to its random fiber structure, offers virtually no electrical insulation value, but

is extremely resistant to tear propagation. The high strength of the individual polyester fibers

offers resistance to cut-through and puncture. Nonwoven mats exhibit relatively low tensile

strength, especially in the cross-machine direction, and no inherent stiffness.

Figure 1-4. Structural formula of polyester

The importance of the third component in DMD, the other laminates-the resin system used

to bond the substrates together-should not be overlooked. In high temperature applications

the resin acts as a protective coating on the polyester film that retards oxidation and

hydrolytic attack. This permits polyester film to be used in laminates that qualify for use in

electrical insulation systems at service temperatures exceeding 130°C. As a result of this

Introduction

16

phenomenon, there are many insulation systems recognized by Underwriters Laboratories,

Inc., as Thermal Class 155, which utilize DMD laminates as the primary slot insulation.

A DMD laminate thus exhibits the best characteristics of its individual components:

excellent electrical insulation properties, high tensile strength, stiffness and formability from

the polyester film; resistance to tear, puncture and cut-through from the nonwoven mat; and

high temperature capabilities as a result of the resin system used.

D. NOMEX® Paper

The component of NOMEX fiber is meta-linked aramid. The term aramid is derived from a

composite of “aromatic polyamides” and describes a form of synthetic solid insulation

commonly used in high temperature applications[13]. The chemical structure of NOMEX

fiber is shown in figure 1-5.

Figure 1-5. Structural formula of NOMEX fiber(Aramid)

Aramid is far more robust that cellulose insulation, and provides high levels of electrical,

chemical and mechanical integrity. Chemically it is resistant to hydrolysis and oxidation and

does not produce the levels of gas and water by-products as does cellulose.

E. NOMEX® T910 Paper

Nomex® T910 has a unique three-ply construction. Figure 1-6 shows the structure of T910.

The two plies on the outside of the sheet are a combination of cellulose and synthetic aramid.

The center ply is composed of cellulose, providing bulk and mechanical support, but with

reduced cost. These three plies are made in the wet forming of the sheet, and when meshed

together, dried and densified using a commercial paper machine, the final sheet becomes a

single consolidated product. This multilayered structure of cellulose and aramid construction

is unique, unlike any previous combination of these two materials.

Introduction

17

Figure 1-6. Picture of Nomex® T910 Structure

The synthetic aramid ingredient in the outside layer is a fibrid, which is a non-granular,

fibrous, or film-like particle. These particles can be prepared by precipitation of a solution of

polymeric material, using a non-solvent under high shear. The integral and inherent part of

this high-temperature fibrid gives thermal resistance in the outside layer to the hot active part

of a transformer and also contributes excellent electrical properties, which is a direct result of

the filmy fibrid particles that have been interwoven in the structure.

1.3 Aging Mechanisms of High Temperature Resistant

Insulation Paper

During the operation, the performance of transformer oil-paper insulation system will

definitely de-escalate, namely aged, under the synergistic effects of thermal, electrical,

mechanical and environmental stresses. Once the oil degrades to a unacceptable grade, it can

be substituted by new oil, whereas paper is very hard to replace. Therefore, the key factor

determining the transformer lifespan is the insulation paper. Among the many aging factors,

thermal stress plays the most important role in the paper degrading process. Due to the

differences in the components of different types of paper, the aging processes and affecting

factors may have divergence. This part will mainly focus on different aging mechanisms of

different papers.

1.3.1 Aging Process of High Temperature Resistant Insulation Paper

1) Aging Process of Cellulose

Most researches of insulation paper aging are focused on cellulose, since it is the most widely

applied material. In cellulose, the amorphous region takes up around 30%, whereas the rest

70% volume is crystal. Cellulose molecules are arranged rather uniformly and compact.

Small molecules are hard to enter such zones, thus the molecules in crystal region are very

stable. However, the arrangement of cellulose molecules in amorphous region is disordered,

loose and with large intermediate space. Small molecules are easy to intrude, so chemical

Introduction

18

interactions are more likely to happen in amorphous region, where cellulose aging is also

initiated[10]. It is commonly accepted that the main factors influencing the degradation

paper insulation are temperature, water, oxygen and acids, and that the main aging routes

are hydrolysis, oxidation and pyrolysis[7]. Hydrolysis is commonly considered as the most

threatening factor to cellulose aging. Figure 1-7 briefly summarizes the various mechanisms

of paper degradation and its products.

Figure 1-7. Cellulose aging mechanisms

a) Hydrolysis

Presence of water will increase the rate of degradation. At the beginning of a transformer’s

life, the kraft insulation contains less than 0.5 % water, and the oil is also dried. The water

content levels within the transformer may increase up to 5 % during its lifetime (Fallou,

1970). Fallou (1970) showed that the rate of degradation of the paper at initial value of 4%

water content was 20 times greater than that at 0.5 % water content. So, in principle as the

transformer ages the rate at which the insulation deteriorates is expected to increase.

Recently, Lundgaard et al[14-16] suggests that the hydrolysis of cellulose is a catalytic

process where the reaction rate depends on dissociated acids or rather H+-ions that can get

into the amorphous zones of the cellulose. Low molecular weight water-soluble acids, that

are formed by the paper ageing and to some degree also by the oil ageing, are more efficient

than the larger hydrophobic acids, which mainly stems from the oil ageing. The fact that acid

catalyzed hydrolysis generates organic acids and at the same time is governed by their

presence makes the process auto-acceleratory. Under the attack of water molecule, the

glucose bond will be broken up, leading to hydroxyl groups attached on the adjacent C atoms

and thereby the molecular chain cleavage. The hydrolysis process is described as in Figure

Introduction

19

1-8[17]. This reaction produces free glucose molecules which decompose further to form

furans and water. The water then permits further hydrolysis.

Figure 1-8. Hydrolytic Degradation Reaction of Cellulose[17]

b) Oxidation

Oxygen will also accelerate the degradation reaction. If the oxygen level in the oil is held

below 2000 ppm the rate of degradation of the full insulation system is reported to be five

times lower than that of a free breathing transformer. Transformer oil can reach about 30

000 ppm when the oil if fully saturated, but in reality most free breathing transformers in

service (warm) only contain 20 000 ppm as a result of dissolution of gas from the air. Cox’

results suggest that by extracting the oxygen to below 300 ppm, using for example

semipermeable membranes the effect of oxygen can be reduced to a sixteenth of that of

normal operating conditions. These results contrast with experimental evidence that ageing

of paper in oil with access to oxygen is only about 2-3 times higher than ageing under

vacuum. Oxidation promotes accumulation of additional ageing accelerators as e.g.

acids[14].

It is suggested that the oxidative depolymerisation is catalyzed by hydroxy-radicals (HO•),

which are produced by decomposition of hydrogen peroxide, H2O2 and of organic

hydroperoxides (ROOH). Hydrogen peroxide can for example be formed from oxygen and

water by reactions catalyzed by transition metal cations (such as Cu+/Cu2+ or Fe2+/Fe3+).

Hydroxy-radicals are formed from H2O2 or ROOH in a reaction catalyzed by traces of Fe3+

Introduction

20

or other active metals, together with small amounts of autooxidizable compounds such as

phenols, aromatic amines or thiols. It is here also suggested that the oxidation is reduced in

an acidic environment, which would reduce the importance of these reactions with time[10].

c) Pyrolysis

By pyrolysis we mean a process that can take place without access to water and/or oxygen,

or any other agent to initiate the decomposition. At normal operating or overload

temperatures (i.e. <140℃) such processes are considered to be of little relevance. At high

temperatures, which may occur at defects such as poor soldering or magnetic induced local

failure currents, pyrolysis may well occur. Generation of CO and CO2 may follow. However

this is outside the scope of this report.

The researches of A.M Emsley group[18] showed that temperature and moisture are the main

affecting factors in the process of paper aging, while oxygen plays a secondary role.

Meanwhile, temperature and moisture have very high synergetic effect, much greater than

the effect between temperature and oxygen. They also discovered that, with low water

content in paper, water and oxygen have a antagonistic interaction behavior. Water has less

importance on aging as the oxygen concentration increases. Authors account this for the

potential reason for the fact that insulation could still remain intact in some scrapped

transformers.

In a real transformer all these processes – hydrolysis, oxidation and pyrolysis act

simultaneously, resulting in a non-linear Arrhenius plot[19] - which hampers the application

of one single activation energy - describing the full complexity of the degradation processes.

Which process will dominate depends on the temperature and the condition. Probably also

synergetic effects takes place between the different reactions; e.g. oxidation may activate

hydrolysis. However, we will for illustrative purposes assume independent processes. The

total degradation then being the sum of degradation from each process becomes:

𝜂𝑡𝑜𝑡 = (𝐴𝑂𝑥𝑖 ∙ 𝑒−𝐸𝑂𝑥𝑖𝑅∙𝑇 + 𝐴𝐻𝑦𝑑 ∙ 𝑒−

𝐸𝐻𝑦𝑑

𝑅∙𝑇 + 𝐴𝑃𝑦𝑟 ∙ 𝑒−𝐸𝑃𝑦𝑟

𝑅∙𝑇 ) ∙ 𝑡

where oxi, hyd and pyr are used as subscripts to identify the activation energy E and the

environment factor A for the singular processes mentioned above. Somewhat simplified one

can say the degradation rates from these reactions will depend on activation energy and the

Introduction

21

environmental for each process at the given temperature. The ageing rates dependence on

temperature will vary depending on which process that dominates in the specific region as

suggested in Figure 1-9[10].

Water is a dominant degradation product of cellulose paper and board. It is formed by

dehydration reactions following hydrolysis (which itself actually consumes water), but is

also an end product in the oxidation of oil as well as that of paper. The amount of water

formed may constitute several % by weight of the total mass of solid insulation.

Figure 1-9. Sketch of ageing rates due to different ageing mechanisms. The arrow shows the effect of

increased water content increasing the A-factor for hydrolysis.[10]

The hydrolysis of paper produces acids. The mechanisms are fairly well understood from

the study of model compounds. The initial hydrolysis reaction causes scission of the

cellulose chain. Dehydration reactions follow, where 5-hydroxymethyl-2-furfuraldehyde is

a major product. This substance readily decomposes into levulinic acid and formic acid. It

should be noted that these acids also undergo further reactions. Levulinic acid may form an

acidic polymeric compound (known as “caramel”) while formic acid may decompose into

carbon monoxide and water.

Oxidation also produces acids. There may be free acids formed (e.g. small carboxylic acids),

as well as acidic groups attached to the cellulose molecular chain. Some of the latter are

present in the cellulose already in new paper and board.

Introduction

22

Obviously the oxidation of paper (as well as oil) is associated with the presence of oxygen.

The higher the oxygen content the higher the rate of oxidation. In the absence of oxygen the

oxidation rate will be insignificant. Reducing the O2 content will reduce the oxidation rate

but not necessarily in direct proportion to the change of O2 content. However, as already

pointed out, acids can be formed by other routes than oxidation, especially hydrolysis.

Many of these acids will have a low molecular weight thus being volatile and having a low

boiling point. They will also have a high polarity, meaning that they will be hydrophilic,

tending to dissolve well in paper.

“Furanic compounds” refers to a whole family of compounds, all of which could be

described as furane derivatives. The most abundant is 2-furfural (2FAL), but 2-acetylfuran

(2ACF), 5-methyl-2-furfural (5MEF), 5-hydroxymethyl-2-furfural (5HMF), and 2-

furfurylalcohol (2FOL) have also been found in oil and paper[20]. These furanic compounds

are shown in figure 1-10. Their determination in insulating oil is described in IEC 61198.

Figure 1-10. Furanic Intermediate Product of Cellulose Aging

5HMF and 2FAL are formed by dehydration reactions following hydrolysis of the cellulose

and hemicellulose. But perhaps more important, all the furanic compounds mentioned above

are also formed by oxidative pyrolysis.

The presence of furanic compounds is not generally considered to influence the ageing of

oil or paper significantly. However, the measurement of furanic compound content,

especially 2FAL, has found some use in transformer diagnostics. There is believed to be

some correlation between the degree of polymerizaton of paper and 2FAL (or total furanic

compound) content of the oil[21].

CO and CO2 are ultimate degradation products of all the constituents of paper and board. In

lesser amounts also CO and CO2 are oxidation products of the oil. The oils content of these

gases is always measured in Dissolved Gas Analysis, and the production rates and relative

Introduction

23

amounts of CO2 and CO are used in the interpretation of the results. Very high CO2 contents

from paper degradation may influence the acidity of the oil, and may thus complicate the

assessment of oil condition.

2) Aging Process of Polyester

The three layers of DMD are all made up from polyester(PET) macromolecular compound.

The PET fibers on the outside layer has a non-woven loose and porous structure and are the

weakest link in DMD paper. Therefore, the long-term behavior of DMD is directly restrained

by the PET fiber condition. PET macromolecules would degrade in different ways under

different conditions, mainly including hydrolysis and pyrolysis. Normally, water exists

inevitably, thereby the ester bonds in PET could break up under the attack of water molecule.

Hydrolysis is also the main degrading form of PET[22]. The process is shown in figure 1-

11.

Figure 1-11. Hydrolysis Process of PET

When experiencing cleavage, a alcoholic hydroxyl group and a carboxyl group will be

formed on each end of the broken molecular chain. Ideally speaking, the end products when

all the ester bonds break up are terephthalic acids and glycols. When heated, the carboxyl

groups on the ends may be decarboxylated and CO2 will be released. Some researches also

show that the hydrolysis of PET is auto-catalyzed, whose reaction rate depends on the

concentration of carboxyl groups.

If the temperature is high enough, PET itself is also pyrolyzed. Researches show that, around

300℃, the main degradation product of pyrolysis is annular oligomers; when temperature

rises up to 400℃, the main product are acetaldehyde and anhydride-containing oligomer[23].

The pyrolysis routes are described in figure 1-12.

Introduction

24

Figure 1-12. Pyrolysis of PET Molecule

At 300℃, some C-O single bonds in ester bonds may break up temporarily, and then reunite

with an adjacent C=O double bond at the far-end fracture point. Thus, an annular oligomer

is formed. At 400℃ , glycol group on molecular chain could be eliminated, producing

formaldehyde and oligomer bonded by anhydride bond.

3) Aging Process of NOMEX Fiber

The class of synthetic materials known as aromatic polyamides have received considerable

attention in the last decade because of their thermal stability at temperatures as high as 500℃.

The inherent stability of aromatic polyamides has led to several studies of mechanism of

thermal degradation with the ultimate goal of engineering polymers with superior resistance

to heat.

Several studies of the vacuum pyrolysis of aromatic polyamides have appeared in the

literature in recent year, one of the first being by Krasnov et a1. [24] who studied the vacuum

pyrolysis of a polyamide (I) made from 1,3-phenylenediamine and isophthalic acid:

Introduction

25

Figure 1-13. NOMEX Molecule Structure

Carbon monoxide, carbon dioxide, and water were reported as the major volatile products

up to 450℃. Benzene, toluene, benzonitrile, and hydrogen cyanide were additional products

observed when the sample was heated to 530℃. On a basis of these products, Krasnov

suggested that polymer degradation proceeded by a pathway, as proposed by Kamerbeck:

that involved the cleavage of the bond between the aromatic ring and the -NH group to form

an aromatic amide[25]:

Figure 1-14. Cleavage of Polyamide(I)

The amide, in turn, lost H2O to form an aromatic nitrile:

Figure 1-15. Dehydration of Amide

The liberated water can react with another amide linkage to form a carboxyl end group and

an aromatic amine:

Figure 1-16. Hydrolysis of Polyamide(I)

The formation of CO2 is a product of the decarboxylation of the acid functional group:

Figure 1-17. Decarboxylation of Intermediate Product (III)

Krasnov et a1. also proposed an alternate mechanism for the formation of CO2 that involved

the formation of a carbodiimide structure by an isocyanate intermediate:

Introduction

26

Figure 1-18. Cleavage of Benzene-Carbonyl Bond

In another report Friedman et al.[26] semiquantitatively analyzed the effluent gases from

vacuum pyrolysis of the same polymer up to 1000℃. These authors found 11 degradation

products; the prominent compounds were CO, C02, H2, HCN, and NH3. Their results

disagreed with those reported by Krasnov et al. in that NH3 was a major product and H2O

was undetected in a significant quantity. Friedman et al. did not propose a mechanism to

account for their experimental findings.

In a more quantitative study Ehlers and co-workers[27] studied the vacuum pyrolysis

products from the same polymer at 20-550°C and monitored the formation of eight major

volatile product. They reported CO, CO2, H2, HCN, and CH4 as the most abundant

compounds formed in polymer degradation. Because H2O was not a major product, Ehlers

and his associates reasoned that the proposed mechanism in the formation of a carbodiimide

intermediate was the most plausible degradation pathway. Their results were in disagreement

with those of Friedman et al. in that NH3 was not observed as a degradation product.

1.3.2 Thermal Aging Kinetics of Insulation Material

1) Introduction of Thermal Aging Kinetics

Montsinger published a pioneering paper in 1930 focusing on the aging of transformer

internal insulation, which pointed out that the insulation material’s mechanical properties,

especially the tensile strength, could reflect the deterioration level of the material. He also

put forward that the lifespan will be halved as temperature increases every 5~10℃, which is

modified as “6~8℃ Law” later by some scholars. But one thing to note is that the

correspondent temperature rise value of life halving is different in different temperature

ranges.

Introduction

27

In 1948, Darkin[28] made further research on the aging rate. He attributed the thermal aging

of cellulose to chemical reactions. Therefore, the physical quantity that could be measured

is able to give index to the chemical reaction rate.

On this basis, Darkin further proposed the aging kinetics of insulation material and built up

a linear polymerized molecule aging kinetic model. According to the chemical reaction

kinetics, the reaction rate dc/dt has a relationship with the concentration c of reactant as

followed:

𝑑𝑐

𝑑𝑡= −𝑘𝑐𝑛 (1-1)

Where:

k------reaction rate;

n------index of reaction order.

When reaction model is deemed to be a first-order reaction, as shown in the following

equation:

A → B + C (1-2)

For this case, n is equal to 1. And the equation aforementioned could be modified as:

r = −𝑑𝑐

𝑑𝑡= k(T)c (1-3)

where:

c-----concentration of reactant;

k(T)----reaction rate factor, related to temperature.

The reaction rate is often expressed in Arrhenius equation:

k(T) = A ∙ 𝑒−𝐸𝑎𝑅𝑇 (1-4)

where,

R------molar gas constant(8.314J/mole/K);

T------Kelvin absolute temperature;

Ea-----activation energy, unit in kJ/mole;

A------preposition factor, depending on the chemical environment.

By taking logarithms to both sides of this equation, we get another form of Arrhenius

formula:

Introduction

28

ln(k) = ln(A) −𝐸𝑎

𝑅∙

1

𝑇 (1-5)

Simplified as:

ln(k) = 𝑎 −𝑏

𝑅 (1-6)

2) Aging Kinetics of Insulation Paper

As ageing proceeds the molecular weight and DP of the cellulose is reduced due to molecular

cellulose chains being cut. The relation between the chain scissions (η) and measured DP

is[29]:

η =𝐷𝑃0

𝐷𝑃𝑡− 1 (1-7)

DP0 is the initial degree of polymerisation and DPt is value after an ageing period t. Most

analyses of degradation have been based on the work of Kuhn and co-workers in 1930, which

was extended by Ekenstam in 1936 to relate rates of degradation to DP. Ekenstam considered

random, first order chain scission and showed a direct relationship of reciprocal DP with

time and that this relation combined with the Arrhenius equation to include temperature

dependence can be mathematically expressed as:

1

𝐷𝑃𝑡−

1

𝐷𝑃0= A ∙ 𝑒−

𝐸𝑎𝑅𝑇 ∙ 𝑡 (1-8)

1

𝐷𝑃0(

𝐷𝑃0

𝐷𝑃𝑡− 1) = 𝐴 ∙ 𝑒−

𝐸𝑎𝑅𝑇 ∙ 𝑡 (1-9)

So from several reasons it is convenient to focus on changes in the DP value or the rate at

which chain scissions occur. The equation above can be explained and supported by the

following procedure and considerations[10] : If we instead of plotting DP vs. time as shown

in Figure 1-19(a), plots 1/DP as shown in Figure 1-19(b) we get fairly straight lines, showing

that the rate of change (k) is quite linear over time up to a certain value of 1/DP. This is in

accordance with a model saying that Δη/Δt = k, which is a first order reaction rate model.

Plotting the natural logarithm of k vs. 1/T gives straight lines as would be the case for a

thermally activated process described in an Arrhenius plot. In equation 8 it is the value of Ea

describes the slope of the curve in Figure 1-19(c); the higher this value is the more

temperature dependent will the reaction rate be (steeper curve). Table 1 shows how the

activation energy is related to the temperature increase giving a 50% life reduction: Chemists

prefer to relate the energy to joule per mole, while physicists use eV per molecule. In

principle the A-values determine the intercept of the curves in 14c with a virtual Y-axis from

Introduction

29

0; the higher the value the higher the location of the curve above the abscissa and the faster

the ageing.

Figure 1-19. Ageing of kraft paper with a high initial water content versus time for four different

temperatures. a: DP-value versus time, b: 1/DP versus time, c: Reaction rate versus inverse absolute

temperature[10]

Knowing the end-of-life (EOL) criterion, we can reorganize equation 6 to express life

expectancy as a function of temperature T, and the parameters E and A:

𝐸𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝐿𝑖𝑓𝑒 =

1

𝐷𝑃𝑡−

1

𝐷𝑃0

𝐴×24×365∙ 𝑒

𝐸

𝑅𝑇[𝑦𝑒𝑎𝑟] (1-10)

Which is equal to what was is suggested in the standards, except from the EOL-criterion

being based on DP value instead of mechanical rigidity of the paper as Montsinger did.

Table 1-1. Correlation between activation energy and temperature rise for halving of life.

Activation

Energy kJ/mol 70 90 110 130 150

ΔT(50%) ℃ 11.7 9.1 7.4 6.2 5.4

The average activation energy of oil-paper insulation material calculated by Emsley et.al.

ranges in 105~117kJ/mol. Lundgaard et.al. found that the activation energy for hydrolysis

reaction is around 90 kJ/mol, while the activation energy for oxidation reaction is much

lower, approximately 70 kJ/mol. When temperature exceeds 140℃, in a water and oxygen

free environment, the activation energy derived by Fung et.al. is about 150 kJ/mol.[10]

1.3.3 Prediction of Transformer Lifespan

Due to the replaceability of insulation oil and the inconvenience of substitution of aged

insulation paper, transformer has a lifespan restrained by paper’s condition. The lifespan

prediction of transformer is actually the forecast of insulation paper’s life. The most

indicative factors to indicate paper’s life are its mechanical properties.

Introduction

30

From the aforementioned paper aging kinetics, we can observe that the direct parameter that

represents the polymerization strength is DP value. As paper ages, the chain scissions will

lead to the decrease of DP value. The initial DP value of cellulose in pulp ranges in

1300~1400, which afterwards drops to around 1200 through the kraft process. Generally

speaking, when DP of cellulose declines to 200, cellulose insulation paper is considered to

reach the end of its life[30]. The measurement of DP value is based on four kinds of

molecular weight definitions: number-averaged DP(DPn), weight-averaged DP(DPw), Z-

averaged DP(DPz) and viscosity-averaged DP(DPv) respectively[31]. Strictly speaking, any

DP value should be denoted with the correspondent molecular weight estimation type.

Considering the measurement speed and economy, the most widespread determination

method is viscosity measurement, that is measuring DPv. Polymer materials(like cellulose)

are dissolved by some particular solvents, and the viscosity of that solution is proportional

to the volume(length) and the concentration of macromolecular chain. The DP value of

polymer material could be converted by the measurement of viscosity. The solvent often

used to dissolve polymer materials is (CUEN), which is totally able to dissolve cellulose.

However, thanks to the chemical robustness and stability of NOMEX fibers, NOMEX paper

could hardly be dissolved by most solvents. Therefore, the determination of DP value of

NOMEX is not easy to fulfil.

DP value is the parameter to reflect the degrading condition of insulation paper from a

microscopic structural point view, while the parameter which most directly represents the

mechanical strength macroscopically is tensile strength and other mechanical properties.

Hence, tensile strength is also usually taken to indicate the level of paper deterioration. A

50% retained tensile strength is generally accepted as corresponded to the DP value of 200.

Paper will become quite brittle and unsuitable for further operation when its tensile strength

drops below 50% of the initial value. This retention is always utilized as the paper end-life

criteria[32].

The simultaneous sampling of insulation paper when transformer is running is very hard to

fulfil. And some indirect quantitative ways to interpret paper operation condition based on

the analysis of paper aging intermediate products are proposed. Among the different methods,

the most deeply carried-out methods are furan compounds and DGA analysis.

Introduction

31

Furan compounds could be formed as cellulose degrades. A high furan compound content is

an indicator of transformer over-temperature fault. Some researchers have established

relationships between furan compounds content and DP value, and expressed in formulas.

The formulas raised up by Chendong and De Pablo et.al. are most representative. Researches

on the comparisons between the furan compounds generated by Kraft paper and that by TUK

paper showed that TUK paper has lower furan compounds yield.

DGA analysis working as an effective transformer condition interpretation method has been

implemented for several years. The interpretation is achieved by measuring the contents of

hydrocarbon and carbon oxide gases in oil and by determining their relevant relationship.

Abundant experimental results and on-site experiences manifest that the degradation of

insulation paper will lead to a sharp increase in carbon oxide gases in oil[33]. Therefore, the

carbon oxide gases are treated as prime indicator on the level of paper degradation. Due to

the potential possibility of external intrusion of CO2 from the atmosphere, although it could

also be generated by paper aging, the most indicative gas for determination of paper

condition is CO. In transformer, the normal ration of CO2 over CO is lower than 7. Once this

ratio increases sharply, the insulation paper would be experiencing severe aging. Large

portion of CO content(for example, more than 30% of total carbon oxide gases) would

definitely be a sign of insulation paper over-temperature fault.

1.4 Research Status of Vegetable Oil Impregnated

Insulation System

1.4.1 Characteristics of Vegetable Oil

The main component of vegetable oils, also called as natural esters, is triglycerides extracted

from natural seeds, comprising more than 95% portion of vegetable oil. There are also

diglycerides, monoglycerides, glycerin and some fatty acids existing in the vegetable oil[34].

Triglycerides have a common chemical structure as showed in figure 1-20.

Introduction

32

Figure 1-20. Chemical Formula of Triglycerides

In the formula, R, R’ and R’’ stand for fatty acid groups. There are various fatty acid groups

in vegetable oil, with C atom numbers ranging from 12 to 22 and with single bonds to triple

bonds. Vegetable oils are differentiated by different fatty acid groups. Currently, the most

widely applied vegetable oils are FR3 produced by Cargill and Biotemp produced by ABB.

Due to the differences in the basic ingredients, vegetable oil has some distinctions in

performance compared with mineral oil. The abundant componential fatty acids make

vegetable oil has far greater total acid number than mineral oil. The main element,

triglyceride, has longer molecular chain and larger average molecular weight. Thereby,

vegetable oil has bigger viscosity value, which is detrimental for transformers heat

dissipation. Hydrocarbon compounds are the major ingredients in mineral oil and they have

lower polarity, while the fatty acids in vegetable oil own a higher polarity behavior. Given

that the polarities are distinct, vegetable oil is much more hydrophilic than mineral oil,

resulting in a higher water saturation value. Normally, the moisture saturation value for

mineral oil could just be around 50ppm, while for vegetable oil, this value could be as high

as 1000ppm[35]. On the dielectric performances, the dielectric constants of vegetable oils

are always higher than that of mineral oil, as well as the dissipation factor and electrical

conductivity. But both vegetable oils and mineral oils have very high electrical strength.

Beyond this, a remarkable disadvantage of vegetable oil is the readily oxidation, which is

caused by the unsaturated bonds in its chemical structure. The more double bonds in

vegetable oil, the easier for vegetable oil to be oxidized. On another side, higher double

bonds composition could reduce the viscosity of vegetable oil and this is contributed to

improve the heat dissipation of transformers. Therefore, vegetable oils are only

recommended for applications of sealed-type transformers.

Introduction

33

The high viscosity and poor anti-oxidation performance could be compensated by

optimization design of transformer insulation system, and brilliant high fire and flash points

and readily biodegradable performances have also propelled vegetable oil filled transformers

to be widely installed in distribution power grid since 2000. According to some reports, until

2009, more than 45,000 transformers have adopted vegetable oil as liquid insulation

material[36].

1.4.2 Thermal Aging Researches on Vegetable Oil Impregnated Insulation Paper

As new alternative insulation liquids, assessments and evaluations of long-term condition of

vegetable oils must be performed before formal operation. Long time working lifespan is

usually guaranteed under normal working temperature. In order to shorten the time duration,

thermally accelerated experiments are usually performed on new insulation materials. To be

more specific, commonly used accelerated aging methods are: Sealed Tube Aging(IEC

62332-2[37]), Functional Life Aging, namely Lockie Method(IEEE C57.100[38]) and Dual

Temperature Model(IEC 62332-1[39]).

McShane[40] simulated the aging of vegetable oil in modern sealing type transformer by

adopting sealing tubes. DP value and tensile strength of insulation paper were measured and

recorded to indicate the deterioration of insulation paper, which is shown in figure 1-21.

Basically, the aging rates of insulation paper in vegetable oil are lower than in mineral oil.

Figure 1-21. Aging of Kraft Paper in Vegetable Oil and Mineral Oil Under 150℃ and 170℃

Sealed tube aging[41] and functional life aging[42,43] (aimed at 25kVA single-phase

transformer) experiments carried by ABB ETI and other aging experiments by ChongQing

Introduction

34

University[44], Stuttgart University[45] and DuPont[46] all exhibit that vegetable oil could

retard the aging speed of insulation paper, while the transformer hot spot temperature using

vegetable oil could be 15℃ higher than that of mineral oil.

Some explanations are given by researchers to explain this phenomenon:

ABB experts ascribe the paper life prolonging effect to the dynamic moisture equilibrium

between oil and paper. Due to the higher hydrophilia of vegetable oil, the moisture balance

will move more to oil rather than paper, lowering the water content in paper and thereby the

aging speed of paper is slowed down. Some researchers[47] consider this as the result of so-

called transesterification process(as shown in figure 1-22): by X-ray photoelectron

spectroscopy, there are differences in the bond energies of different C structures in different

locations on cellulose molecule before and after aging. After aging, there are new peak C5

in cellulose aged in vegetable oil, which is correspondent to the bonding energy of ester bond

–COOR. A new peak is also discovered at 1746cm-1 in the measurement of infrared

spectroscopy on cellulose which is absent in the cellulose aged in mineral oil. This peak also

climbs up as aging proceeds. They believe that the extension of paper’s life is attributed to

the particular hydrolysis process of vegetable oil. Glyceride and fatty acids are formed after

the vegetable oil is hydrolyzed, and the fatty acids could further react with cellulose molecule

by the esterification reaction. Long side-chains will be formed on the surface of cellulose

and thus the structure of cellulose is modified and intensified. These long side chains are

beneficial to prevent the water intrusion to some extent.

1.5 Research Content of This Thesis The research contents of the thesis could be divided into two parts:

(1) T910 paper and TUK paper are chosen as the research objects and their fundamental

physiochemical, mechanical and dielectric properties are investigated and compared

with traditional Kraft paper. For paper properties requiring insulation oil cooperation,

the impact of different oils is also studied. Three typical vegetable oils and one

conventional mineral oil are selected and their initial key properties are also investigated.

By the experiments aforementioned, the relative merits and performance differences of

several oil and paper materials could be analyzed, providing a reference

Introduction

35

Figure 1-22. Transesterification of Cellulose Molecule in Vegetable Oil

basis for transformer insulation system design.

(2) Fast accelerated aging tests are performed on T910 paper, DMD paper and Kraft paper

under the impregnation of FR3 vegetable oil. The changes of the mechanical and

dielectric strength of paper and the physiochemical properties of oil along aging are

investigated. The aging tests are conducted by using stainless steel vessel under single

temperature aging mode. The thesis tries to discuss on the aging mechanisms of T910

and DMD paper aged in FR3 oil and analyze the performance change of paper and oil

materials.

Experimental Design of High-Temperature Resistant Insulation Materials Property Comparison and Thermal Aging

36

2 Experimental Design of High-Temperature Resistant

Insulation Materials Property Comparison and

Thermal Aging

2.1 Materials and Test Parameters of Property

Comparison

In this part, the thesis chooses several typical high-temperature resistant oil and paper

materials as research objects. Kraft paper, TUK paper and T910 paper are included as paper

material candidates. Kraft paper is produced by Sanmu Manufacturer and is mechanically

strengthened for power transformer use; T910 paper is produced by DuPont; TUK paper is

produced by Ruitai Insulation Material Manufacturer and is a type of DDP(diamond dot)

pattern. For Kraft and T910 paper, they have three thickness specifications: 0.08mm,

0.13mm and 0.18mm, while TUK paper only has 0.08mm thickness.

Oil materials included in the experiments are Karamay #45 produced by China CNPC, FR3

produced by US Cargill, Vinsoil produced by China NARI and EBF#2 produced by US

DuPont. Karamay #45 is a kind of common-used pour point improved mineral oil, whose

pour point could be as low as -45℃. FR3, Vinsoil and EBF#2 are three vegetable oils based

on different vegetable seeds. The general descriptions on typical properties of Karamay #45

mineral oil and FR3 and Vinsoil vegetable oils are shown in table 2-1.

Table 2-1 Comparisons on Typical Properties of Karamay, FR3 and Vinsoil Transformer Oil

Typical Property Karamay#45 Mineral Oil FR3 NARI Vinsoil

color transparent light green light yellow

chemical type hydrocarbon ester ester

gravity at 25℃ 0.89 0.92 0.92

viscosity at 40℃ (cSt) 9.7 36 32.4

pour point（℃） -42 -21 -16

interfacial tension

（dynes/cm） 45 24 30

flash point(closed)（℃） 142 319 331

water content(ppm) 20 56.5 142

tan δ(at 90℃) 0.0009 0.03 0.0201

breakdown voltage(kV) 55 54.4 57

total acid number (mg 0.009 0.03 0.01


37

KOH/g)

The thesis measures the typical physical, chemical, mechanical and dielectric properties of

insulation papers under both dry and oil-immersed condition. All the experiments refer to

relevant ASTM standards. The detailed test items and relevant standards are shown in table

2-2.

Table 2-2 Test Properties of Insulation Paper

Test Item Reference Standard

Basis Weight ASTM D646

Density（g/cm3） ASTM D646

Water Content（%） ASTM D644、ASTM D3277

Tensile Strength（N/cm） ASTM D828

Dielectric Constant ASTM D202

Dissipation Factor ASTM D202

Breakdown Strength，in air ASTM D149

Breakdown Strength，in oil ASTM D149

Besides of paper material, the thesis also investigates the initial differences of different oil

properties and mainly focuses on the physiochemical performances, including total acid

number, viscosity and water content. The experiments also refer to ASTM standards.

Table 2-3 Test Properties of Insulation Oil

Test Item Reference Standard

total acid number（mg KOH/g） ASTM D664

viscosity（cst） ASTM D445

water content（ppm） ASTM D1533

2.2 Materials and Test Parameters of Thermal Aging

In order to more closely simulate transformers in real operation, except oil and paper

insulation materials, copper wire and silicon steel are also needed to be incorporated to

simulate windings and core steels.

The paper material research objects included in the thermal aging part are T910 and DMD

paper, and Kraft paper is also investigated as a basis comparison material. The thickness

specification of three kinds of paper is all 0.13mm. The papers are all aged in FR3 vegetable

oil. The anaerobic brass wire is chosen as the simulation of copper wire in the experiment,

while for core steel simulation 0.2mm thick oriented cold-rolled silicon steel is used. Before


38

aging, the brass wire and silicon steel are all polished by 2000 mesh sandpapers to eradicate

the surface oxidized layer.

The determination of four aging components’ ratio should be under the reference to real

portions in transformers. Here, the recommended material ratio in IEC 62332-2 aging

standard is referred. Concerning about the volume of aging vessel and the thermal expansion

of insulation oil, the four components’ ratio is set as volume of oil : weight of paper : surface

area of copper wire : surface of silicon steel=2L: 95.4g: 8cm2:42.6cm2.

The aging vessels are made up by stainless steel, which is a cylinder with a volume of 2.375L.

The cylinder is sealed by fluorine apron which is resistant to 190℃.The vessel has three

valves on the top, functioning as vacuuming, nitrogen injection and oil injection/sampling

respectively. On account of the fact that the vessel and valves have to be operated under high

temperature for a long term, the sealability and temperature resistance are highly demanded.

In the experiments, American company Swagelok’s SS-6BW type bellows seal valves are

used, which has very high temperature, oil and corrosion resistances and high sealability.

In order to exclude the impacts of oxygen and water in the environment on the aging process,

a nitrogen protective blanket is inserted into the head space of the vessel. The material

assembly and condition controlling process is as following: solid materials are placed inside

the vessel and the vessel is sealed first, then open the vacuum valve to vacuum the vessel

and inject oil through oil valve under vacuum condition. Close the vacuum and oil valves

and open the nitrogen valve to insert a nitrogen blanket. After the materials are aged to

required time durations, the internal nitrogen is compressed and thus the pressure increases.

Therefore, at such sampling nodes, oil sampling valve is open first and some oil sample

would be extruded out. After the inner and outer pressures are balanced, nitrogen is filled in

to extrude more oil samples. After oil is sampled, the vessel is opened and paper materials

are taken out.

The temperature in the aging experiments has a decisive impact on the aging process.

According to IEC 60126.4[48], the temperature fluctuation of aging ovens in the range of

80°C to 180°C should not exceed 5°C. Ovens used in the thesis meet the requirement,

confirmed by infrared thermometry measurement.


39

The deterioration rates of oil and paper materials have a close relationship with temperature.

Aging experiments are usually accelerated by elevating temperature. Due to the time

limitation of the thesis, the aging temperature is set 150℃ and the durations are determined

as 6h, 12h, 24h, 48h, 96h, 192h and 720h.

Along the aging, both paper and oil key parameters are investigated. The detailed test

parameters are shown in table 2-4. All the tests are performed under the guidance of ASTM

standards.

Table 2-4 Test Parameters Along Aging Experiment

Material Parameter Reference Standard

Paper Breakdown Voltage ASTM D149

Tensile Strength ASTM D202

Oil

Water Content ASTM D1533

Viscosity ASTM D445

DGA ASTM D3612

2.3 Preconditioning of Paper and Oil Material

2.3.1 Preconditioning of Paper

Based on the different experimental requirements, the paper preconditioning processes can

be divided as the following three kinds:

1) Comparison experiments on physiochemical and mechanical performance

According to reference standards, paper samples should be maintained in a standard

environment of 23.0±1.0℃ and 50.0±2.0% relative humidity for 24h before the experiments.

The thesis utilizes a temperature-and-moisture variant heat chamber whose temperature

range is -10℃~150℃ and moisture range is 20%~100%.

2) Comparison experiments on dielectric performance

For such tests, the impact of moisture has to be excluded as much as possible. Thus, paper

samples should be dried. The specific procedure is to dry the paper samples in an air-

circulating oven under 105℃ for 12h. The determination of the water content of the paper

after dry shows that the moisture level is lower than 0.5%.

3) Experiments of paper under oil impregnation and aging tests


40

Normally, in order to exclude the effects of water and oxygen and ensure the adequate

impregnation, the impregnation process should be conducted under vacuum and the paper

should also be dried to maintain a low initial water content level. Therefore, before the

experiments which require insulation oil cooperation, paper samples should be dried and

vacuum impregnated. The specific process is as following. For oil impregnated breakdown

experiment, due to the limitation of the vessel amount, glass bottles are used as impregnation

container. Paper samples are placed inside the bottle and dried under 105℃ for 12h in air

circulating oven. Afterwards, insulation liquids are filled in and the bottle is transferred to a

vacuum oven and paper samples are vacuum impregnated under 90℃ for at least 48h before

experiments. For thermal aging test, the paper samples are first dried under 105℃ for 12h in

air circulating oven and then transferred into a vacuum container(aging vessel) and are

further vacuum dried under 90℃ for 2h. Insulation oil is then filled in with vacuum degree

maintained below 266Pa and the paper is vacuum impregnated under 90℃ for more than 6h.

2.3.2 Preconditioning of Oil

Insulation oil has a potential to be contaminated by the intrusion of water, oxygen and

particles during the storage, transportation and sampling process. These substances will to

some extent affect the performance and aging rates of paper and oil materials. Before the

experiments, all the oil samples should be filtered, degassed and dehydrated. Therefore, the

thesis establishes the oil preconditioning platform.

1) Filtering

To filter the insulation oil, an upper suction outlet flask of 5L in volume is used. Filtering is

fulfilled under the cooperation of funnel, filter paper and vacuum pump. The filtering

platform is shown in figure 2-1. The platform adopts ceramic Buchner funnel of 125mm in

diameter, above which Whatman No.5 qualitative filter paper(125mm in diameter, pore size

of 2.5µm) is placed. The funnel is placed onto the bottle and the bottle port is connected with

a vacuum pump. The oil sample is filtered through the filter paper under the help of internal

vacuum condition.


41

Figure 2-1 Sketch of Oil Filtering Platform

2) Degasing and dehydration

The degasing and dehydration platform uses a upper and lower outlet flask of 10L in volume.

In addition, vacuum pump, magnetic thermal stirrer, nitrogen bag and three-way valve are

used. The platform is shown in figure 2-2.

Figure 2-2 Sketch of Oil Degassing and Dehydration Platform

The upper and lower outlet bottle is placed on the magnetic stirrer. The lower outlet is

connected with a valve to control the oil outflow and the upper outlet is connected with a

three-way valve to vacuum and fill in nitrogen into the bottle. A stir bar is put inside the

bottle.

The degassing and dehydration process is described as following: first, the oil outflow outlet

is switched off and the oil after being filtered is transferred into the flask. Switch on the

magnetic stirrer and set the temperature at 90℃(corresponding to the internal oil temperature

70℃). Adjust the stirring rate to agitate the oil as fast as possible and stay focused in the

center. Turn the three-way valve to the vacuum side and switch on the vacuum pump to

vacuum the flask inner space. Under vacuum condition, it could be observed that lots of

bubbles emerge inside the oil. Maintain such state for at least 12h to degas and dehydrate


42

the oil. Afterwards, close the vacuum pump and turn the three-way valve to nitrogen side

and open the nitrogen bag. When the pressures inside and outside the flask are balanced, the

oil outflow outlet is open and oil sample is flowed into aging vessels or oil-impregnation

containers.

The results of the water content measurements of the insulation oil after dehydration and

degassing show that the mineral oil moisture level could be reduced to 10ppm, while

vegetable oil has a water content around 50ppm. This water content levels meet the

requirements for the insulation oil before transformer operation(<20ppm for mineral oil and

<200ppm for vegetable oil).

2.4 Chapter Conclusion

This chapter mainly introduces the test materials and parameters involved in this thesis. The

preconditioning methods of solid and liquid materials depending on different requirements

of experiments are also described. The content in this chapter provides an overview of

experimental preparation and a basis for the work hereinafter.

Physical and Mechanical Performances of High Temperature Resistant Insulation Paper

43

3 Physical and Mechanical Performances of High

Temperature Resistant Insulation Paper

This chapter mainly introduces the experimental results on physical and mechanical

performance of three paper materials, which are T910 paper, TUK paper and Kraft paper.

The experiment methods are also described. The preconditioning process of the materials

are mention in chapter 2. In order to avoid contamination of water and particles from hand,

the experimenter wears rubber gloves during the whole session of experiments.

3.1 Basis Weight and Density

3.1.1 Test Method

Due to the error existed during manufacture, the basic physical index of paper sample could

have bias from the rated values. The thesis measures the real thickness, basis weight and

density according to ASTM D202 Standard Test Methods for Sampling and Testing

Untreated Paper Used for Electrical Insulation[49].

.

The paper samples to be tested are cut into 10 sheets with size of 200±5×200±5mm.

Micrometer screw with the division value of 0.01mm is used to measure the thickness of one

paper sample sheet at any 5 locations. The separation of each location and the edge should

be larger than 6mm. The average value of these 5 thickness measurements is taken as the

measured thickness of the sample. And the average of 10 paper samples thickness is taken

as the measured real thickness of the paper.

An analytical Balance with accuracy of 0.0001g is used to measure each paper sample sheet

weight M. The length H and width W are determined by a steel ruler with division value of

0.5mm. The basis weight is then calculated by formula (3-1), and the average of 10 paper

samples’ basis weight is taken as the measured value. Meanwhile, the density is further

determined based on the thickness measurement.

WH

M

A

MBW

(3-1)

TWH

M

V

MD

(3-2)

https://www.astm.org/Standards/D202.htm



44

Where：

BW——Basis weight of paper sample，g/m2

M——Weight of paper sample，g

A——Area of paper sample，m2

H——Length of paper sample，m

W——Width of paper sample，m

T——Thickness of paper sample，m

3.1.2 Test Results

The results of the real thickness, basis weight and density measurements are shown in Table

3-1.

Table 3-1 Real Thickness, Basis Weight and Density of Kraft Paper, T910 Paper and TUK Paper

Kraft T910 TUK

Rated Thickness

（mm) 0.080 0.130 0.180 0.080 0.130 0.180 0.080

Measured Thickness

（mm) 0.080 0.130 0.177 0.090 0.139 0.179 0.096

Basis Weight (oz/yd2) 1.90 4.19 5.75 2.14 3.78 4.74 2.68

Density (g/cm3) 0.80 1.09 1.10 0.80 0.92 0.90 0.96

From the results, we can first observe that, on the real measured thickness, Kraft paper has

the best consistence with the rated values, only existing little bias on 0.18mm specification.

T910 and TUK papers has larger biases, especially at lower thickness specifications.

Secondly, the basis weight increases as the paper thickness goes up. This is easy to

understand since the basis weight is defined as the paper weight per unit area, regardless of

thickness. There exists bigger steps on basis weight from 0.08mm to 0.13mm rather than

0.13mm to 0.18mm. For example, the basis weight of Kraft paper of 0.13mm is greater than

twice of the value of 0.08mm. This outcome may attributes to the lower density values of

thin paper samples. Thirdly, except the close density values(both are 0.8 g/cm3) on 0.08mm

specification, for thicker samples T910 always shows smaller densities than Kraft paper.

This is mainly due to the 30% component of NOMEX fibers in T910 paper, which originally

has lower density. Compared with T910 and Kraft paper, TUK paper has the largest basis

weight and density.


45

Due to the porous structure of insulation paper which is a composite system of air and fibers,

the density could directly reflect the fiber content in paper materials. The mass of air could

be neglected and thus higher density value means higher fiber content in the paper and the

more dense the paper is. For same kind of material, density values could be used to analyze

the paper densification degree. Therefore, we can draw a conclusion that the Kraft papers of

0.13mm and 0.18mm in thickness are more compressed than of 0.08mm. The compression

degree has a direct relationship with some key performances of insulation paper, e.g. oil

absorption and dielectric constant etc.

3.2 Water Content

3.2.1 Test Method

Since the water exists in paper could affect paper performance and accelerate its aging, it is

necessary to determine the initial water contents of different kinds of paper. ASTM D644

Standard Test Method for Moisture Content of Paper and Paperboard by Oven Drying[50]

and ASTM D3277 Standard Test Methods for Moisture Content of Oil-Impregnated

Cellulosic Insulation[51] are both referred as the test accordance. Oven drying and Karl

Fisher titration methods are adopted to measure the water content in insulation paper.

The basic rule of oven drying method is to use oven to dry out the water that exists inside

paper samples and measure the differences of the paper weight before and after drying. Then

the water content could be deduced. During the experiments, glass bottles of 100mL in

volume are selected. The bottle has good airtightness with 145℃ resistant PP screw cap and

fluorine rubber gasket is placed inside the cap.

The experiment process is described as following. Firstly, the glass bottle is open and placed

into a heating oven whose temperature is set as 105℃. After heating 1h, the oven is open

and the cap is screwed up onto the bottle as soon as possible and then the bottle is cooled

down to room temperature in a desiccator for 1h. Afterwards, the glass bottle is weighed by

analytical balance. Secondly, around 2g paper samples are put inside the bottle by a tweezers

and the bottle is screwed up and weighed again. The difference of two successive weight of

the bottle is taken as the initial paper sample wet weight W1. Thirdly, heat the paper sample

along with the bottle open inside the oven under 105℃ for 2h. Then, open the oven and


46

screw up the bottle immediately and put it into the desiccator. After the bottle is cooled down

to room temperature, weigh the bottle with paper sample inside and then take out the paper

sample and only weigh the empty bottle and the screw cap. The difference of these two

successive weight of the bottle is taken as the paper sample dry weight W2. Repeat the

aforementioned weighing process, until the difference of two successive weight in no larger

than 0.002g. The water content of paper sample is then calculated by formula (3-3). For each

thickness specification, three paper samples are performed the test and their average value

is calculated as the water content for such paper.

%1001

21

W

WWWC (3-3)

Where,

WC——Water content of paper sample，%

W1——Weight of paper sample before drying，g

W2——Weight of paper sample after drying，g

The basic rule of Karl Fisher titration is the reduction reaction of iodine by SO2 under the

existence of water. The I2 involved in is electrolyzed on electrode. By measuring the total

amount of electric charges transferred, the amount of water involves could be determined.

Karl Fisher titration experiment is carried out by using Metrohm 831 Karl Fisher titrator,

under the cooperation of Metrohm 860 thermoprep. When testing the water content of solid

materials, thermoprep is first used to evaporate the water exists inside the material. Then the

vaporized water is carried into the titrator by air dried through molecular sieve. Inside the

titrator, there is commercial-used KF reagent, including iodide, SO2, imidazole and methanol.

Reagent oxidizes on the anode and yields iodine. The key point is that the required iodine

amount is derived by electrolysis process, and thus the equipment could directly calculate

the iodine amount by recording the electric charges. Finally, the water amount enters the

titrator is determined.

Before the formal titration, three blank samples are first titrated and their average value is

taken as the background water content. Small paper stripe sample weighing around 0.1g is

placed inside the sample bottle. The sample bottle is then put into the center of thermoprep


47

to be heated under 220℃. Open the gas pump to carry the water vaporized into the titrator.

The residual water amount subtracted by background value is considered as the water content

in the paper sample. Two measurements are conducted and the average value is recorded.

3.2.2 Test Results

The results of water content measurements of paper samples in standard environment(23℃，

50% relative humidity) by two methods are shown in table 3-2

Table 3-2 Water Content of T910, Kraft and TUK paper

T910 Kraft TUK

Rated Thickness（mm) 0.08 0.13 0.18 0.08 0.13 0.18 0.08

Oven Drying（%） 4.02 3.86 4.03 5.85 5.93 5.96 6.95

Karl Fisher（%） 3.77 4.30 4.88 5.47 5.64 5.68 6.55

From the test results, certain biases between two measurement technologies are observed.

Nevertheless, T910 paper owns the lowest water content level, from 3.77% to 4.88%. TUK

paper’s water content is the highest(6.55%~6.95%), while Kraft paper has intermediate

values, from 5.47% to 5.96%. The Karl Fisher tests show that the water content increases as

the thickness goes up, and this rule is only suitable for Kraft paper under oven drying method.

The biases between two methods of T910 results are bigger, up to 0.8%; while the

differences for Kraft and TUK paper are small, just around 0.3%.

From the components of three kinds of paper, we know that 30% of T910 paper are NOMEX

fibers, with the rest being cellulose and TUK paper has some nitrogenous additives. It is the

difference on the component and structure that determines the particular water content level.

To be more specific, NOMEX fiber has a lower polarity of approximately 2.5 and has no

highly polar groups on molecular chain. However, on cellulose molecular chain, there are 3

hydroxyl groups on each glucose unit, and this greatly enhances its polarity. Commonly, the

reported polarity of cellulose is around 6.0. Therefore, Kraft and cellulose-based paper have

naturally greater hydrophilicity and NOMEX fiber could reduce the overall water content of

T910 composite system. This explains why the water content of T910 is 20% lower than

Kraft paper. For TUK paper, except the cellulose basis, some nitrogenous additives, e.g.

dicyandiamide and urea, including amino(-NH2) are added. These additives could form

hydrogen bond with water molecule and also owns high hydrophilicity. For instance, the

dicyandiamide has a water solubility of 32g/L, while cellulose will only swell in water.


48

Consequently, small nitrogenous additive addition will increase the water content of TUK

paper.

3.3 Tensile Strength

3.3.1 Test Method

Tensile strength is defined as the maximum tension that paper per unit width could maintain

under standard test condition. Referring to ASTM D202 Standard Test Methods for

Sampling and Testing Untreated Paper Used for Electrical Insulation[49], tensile strength

of paper sample is measured by constant elongation rate method. Electronic universal

mechanical testing machine is used.

Cut the insulation paper into paper strips with standard size of 200mm×25mm. The paper

strip is fixed onto the machine clampers. Select a tensile rate that can break up paper strips

in 10~15s. Usually the tensile rate on paper machine direction is set as 20mm/min, while on

cross-machines direction is 50 mm/min. Start the machine and the tensile experiment will

be automatically carried by the machine. After paper strips are ruptured, record the

maximum fracture force, time and position. Measure the width W of the fracture position.

Tensile strength TS of paper sample is calculated by formula (3-4) and the 10 paper samples

are performed the test to derive the average value.

W

FTS max (3-4)

Where,

TS——Tensile strength of paper sample

Fmax——Maximum force when fracture

W——Width of paper sample

One thing to note is that, depending on the type of insulation paper and its direction, elastic

deformation or elastoplastic deformation may exist during the stretching process. When

calculating, Fmax should choose the maximum stress that causes the paper fibers start to

deform.




49

3.3.2 Test Results

In this thesis, both paper samples on direction and cross-direction are measured and their

tensile stress-strain curve examples are shown in figure 3-1. Since all three kinds of paper

share the same tensile rules, here the thesis only exhibits the stress-strain curve of 0.18mm

Kraft paper to analyze the fracture process of paper sample. In the figures, x-axis is the strain

distance and y-axis corresponds to tensile stress.

(a) Machine direction (b) Cross-machine direction

Figure 3-1 Stress-strain curve of 0.18mm Kraft paper

From the curves, a linear relationship between stress and strain on paper machine direction

is clearly observed. When the displacement increases to a certain level, the stress reaches the

climax and the paper is ruptured. But on cross-machine direction, the stress and strain keeps

linear before 2mm displacement. Between 2mm and 3mm, the curve bends a little and the

slope reduces. After 3mm, the change of stress along the displacement gets mitigated. And

finally the stress reaches the maximum maintained value and break paper strip up.

Kraft paper is cellulose based material. From the composition of T910 and TUK paper, we

know that their main components are cellulose as well. Therefore, the tensile strength

mechanism is quite related to the cellulose fracture mechanism.

Tensile strength mechanism of cellulose paper has been studied for a long time. Page[52]

proposed the famous theory in 1950s and tried to establish a relationship between the


50

physical characteristic of paper to essential features of fiber. He ascribed the paper’s tensile

strength to the strength of single fiber and bonding strength of inter-fibers. The fracture of

paper could be caused by the overloading of paper fiber and also by the transcendence of

low bonding strength. It is commonly acknowledged that tensile strength of paper is affected

by fiber strength itself, inter-fiber bonding strength and the arrangement of fibers.

Except the effect of fiber raw material, the effect of paper manufacture on the inter-fiber

bonding strength could not be neglected. During the dehydration and wet compression

process, micro fibers contact each other and form conjunction. During the drying process, as

the water content in paper decreases, hydrogen bonds are formed between micro fibers and

small fibers. The bridging effect brought about by hydrogen bond is the main contribution

to paper strength on cross-machine direction.

Herein, the thesis tries to analyze the paper fracture process.

During the tensile experiment on paper machine direction, paper sample is extended along

the fiber own stretching direction and elastic deformation happens. The tensile behavior

conforms to Hooke's law as elastomers. When the elongation distance reaches a certain level,

the chemical bonding inside fiber macromolecules is broken up and thus fibers are pulled

off, leading to the fracture of paper strips.

When paper samples are stretched along the cross-machine direction, the stretching direction

is perpendicular to fiber direction. The hydrogen bonds provide the dominant bonding

strength. At the beginning, strains are small and stress remains within the inter-fiber elastic

limits, and the deformation belongs to elastic category. Once the strain keeps increasing,

hydrogen bonds exists inter-fiber are elongated and partially broken. Fibers partially slide

over and this process behaves like a elastoplastic deformation. When the tensile elongation

increases more, hydrogen bonds are totally broken up and excessive creep deformation

happens. The paper strength on cross-machine direction will be maintained only by fiber

physical entanglement. This is the reason why the slope of curve at the second stage is much

lower than at the first elastic stage, since hydrogen bonds are much stronger than physical

entanglement.


51

The calculated tensile strength on both machine and cross machine direction are shown in

table 3-3.

To better and more conveniently compare the data, results are drawn in bar plots as shown

in figure 3-2.

Table 3-3 Tensile Strength of Kraft, T910 and TUK Paper

Kraft T910 TUK

Rated Thickness

（mm) 0.08 0.13 0.18 0.08 0.13 0.18 0.08

Machine Direction

（N/cm） 101.07 277.24 327.40 74.22 130.79 147.86 107.80

Cross machine

direction（N/cm） 14.69 52.91 96.53 14.94 38.49 43.94 35.38

(a)Machine Direction (b) Cross-machine Direction

Figure 3-2 Tensile Strength of Kraft, T910 and TUK paper

From the plots, a apparent conclusion could be drawn that the tensile strength on machine

direction is far greater than on cross-machine direction. This is mainly caused by the

different bonding approaches on two directions. The strength on machine direction is mainly

provided by chemical bonds, while on cross-machine direction the hydrogen bonds are the

prior source. Chemical bonds are strong interaction, whose bonding energy usually ranges

in 125~840kJ/mol. Hydrogen bonds are weaker interaction, whose bonding energy is usually

below 40kJ/mol. Therefore, the energy needs to destroy chemical bonds is much greater than

the energy to break hydrogen bonds. This is the reason why paper owns higher tensile

strength on machine direction.


52

Besides, it could be clearly seen that, compared with the other two papers, T910 paper always

shows lower tensile strength no matter what direction and thickness specifications are. TUK

is the most robust material where all three materials could be compared. Only under 0.08mm

thickness specification, Kraft paper has similar tensile strength value with T910 paper on

cross-machine direction. On other conditions, Kraft paper has much higher tensile strength

level. And the leading of Kraft paper over T910 paper becomes more significant as thickness

increases. For instance, on machine direction, the tensile strength of 0.08mm Kraft paper

exceeds 36% over that of T910 paper. For 0.13mm specification, the percentage of Kraft

paper tensile strength surpass T910 paper is 113%, while it reaches up to 122% under

0.18mm specification. This outcome is resulted from the fact that Kraft paper in experiment

is specially mechanically strengthened for the aim of power transformer application and

TUK paper in experiment belongs to a DDP strengthened type.

T910 paper owns lower tensile strength on machine and cross machine directions due to its

particular structure. At the entanglement position of NOMEX and cellulose fibers, due to

the intrinsic differences of macromolecular structures, their hydrogen bonding effect is weak.

NOMEX fiber does not have polar groups such as hydroxyls, so it is not easy to form

hydrogen bonds with other polar groups. Therefore, at the intersection of two fibers, the

bonding strength is weaker and such areas are the weak link of T910 paper. Although T910

paper has much lower tensile strength, according to ASTM D1305 Standard Specification

for Electrical Insulating Paper and Paperboard-Sulfate (Kraft) Layer Type[53], its tensile

strength still meets the requirement and so as other two paper materials. The threshold values

of different thicknesses on machine direction are shown in figure 3-3(a) by the dotted line.

3.4 Oil Absorption

3.4.1 Test Method

The real operation of insulation paper is achieved by the impregnation in insulation oil. The

impregnation level and the oil content of insulation paper are reflected by oil absorption

value. Because insulation papers are porous materials, oil will fill in the pores when

impregnating. Oil absorption is the key parameter to indicate the oil absorption ability of

insulation paper. It is normally required for insulation paper to have a higher oil absorption

value. Thus the composite oil impregnated paper system will have a lower composite


53

permittivity, approaching closer to the lower permittivity of insulation oil. Therefore, the

electric field is more balanced between paper and oil and the higher electric stress sustained

by oil is alleviated.

The oil absorption measurement refers to ASTM D3394[54]. Three paper samples with size

of 80mm×80mm of each kind of paper are measured the oil absorption value and the average

value is calculated. The detailed measurement process is as following. Dry the paper samples

inside an air-circulating oven under 105℃ for 12h. Then place the dried paper samples into

a desiccator and cool them down to room temperature. Weigh this paper as the paper weight

before oil impregnation W1. Afterwards, transfer the paper samples into a vacuum

atmosphere, and further dry the paper for 2h under 90℃. Fill in the insulation oil to cover

the paper sample slowly, ensuring the vacuum degree below 266Pa during the vacuum

impregnation. Once the paper samples are fully covered, immediately stop the vacuum

condition and interrupt the heating. Take out paper samples after the vessel is cooled down

in room temperature for at least 6h. Wipe out the surface excess oil of paper samples by

paper tissue. By weighing the difference in the weights of paper samples before and after oil

impregnation, the oil absorption could be determined by formula (3-5).

%100 1

12

W

WWAbsoprtionOil (3-5)

Where,

Oil Absorption——Oil absorption Value of Paper Sample，%;

W1——Weight of paper sample before oil impregnation，g;

W2——Weight of paper sample after oil impregnation，g.

3.4.2 Test Results

All three paper materials are impregnated in four kinds of mineral and vegetable oils. The

test results are shown in table 3-4.

Table 3-4. Oil Absorptions of T910, Kraft and TUK Paper Under Mineral and Vegetable Oils

Oil Absorption/% T910 Kraft TUK

0.08mm 0.13mm 0.18mm 0.08mm 0.13mm 0.18mm 0.08mm

Karamay #45 42.5 33.0 39.1 37.5 21.4 19.4 19.5


54

FR3 42.1 37.1 37.9 36.7 21.5 21.4 16.9

NARI Vinsoil 42.3 36.5 39.6 38.4 22.9 21.3 21.4

DuPont EBF#2 43.9 38.3 40.7 45.5 23.9 22.1 21.2

Firstly, the differences among three kinds of paper under each kind of impregnation oil are

analyzed and the following figures 3-3(a)-(d) could be drawn. The four bar plots refer to the

impregnations of Karamay #45 mineral oil, FR3 vegetable oil, Vinsoil vegetable oil and

EBF#2 vegetable oil respectively.

(a) Oil Absorption of Karamay #45 Impregnation (b) Oil Absorption of FR3 Impregnation

(c) Oil Absorption of Vinsoil Impregnation (d) Oil Absorption of EBF#2 Impregnation

Figure 3-3 . Oil Absorptions of T910, Kraft and TUK Paper Under Four Kinds of Oil

From the bar plots, it can be observed that T910 paper always shows higher oil absorption

level no matter in mineral oil impregnation or vegetable oil impregnation. And the prevailing

of T910 paper over the other two papers becomes more significant under higher thickness

specifications. For 0.08mm specification, the oil absorption values of T910 paper under four


55

oil impregnations are all above 40%. The values of Kraft paper are around 37% for mineral

oil and FR3 and Vinsoil vegetable oil impregnations. Only in EBF#2 vegetable oil

impregnation does the oil absorption value of Kraft paper surpasses the T910 paper oil

absorption of just 2%. TUK paper samples have a much lower oil absorption level, which is

just around half of the value of T910 paper. Under 0.13mm and 0.18mm specifications, the

T910 paper samples have obviously larger values on oil absorption over Kraft paper, at least

15% higher than Kraft paper.

Oil absorption level could reflect the porosity of paper materials to some extent and has

direct relationship with the paper density and its polarity. The more pores existing inside

paper, the more space left for oil to occupy and the higher the oil absorption level. At the

meantime, the more porous the paper, the smaller volume fraction of the fiber and the lower

density of the paper. By comparing the densities of three paper materials of each thickness,

it could be found out that T910 paper has highest density under 0.13mm specification, which

is 0.922g/cm3 and has lowest density under 0.08mm specification, which is 0.804g/cm3.

Therefore, the oil absorption level of T910 paper under 0.08mm specification corresponds

to the highest value and T910 paper has a lowest oil absorption level under 0.13mm

specification. For Kraft paper, it has very close density of around 0.8g/cm3 with T910 paper

under 0.08mm specification. But for thicker paper samples, the densities of Kraft paper are

around 1.1g/cm3, greater than the density of T910 paper, around 0.9 g/cm3, of same thickness

specifications. Therefore, the oil absorption levels of T910 paper are much higher than that

of Kraft paper under 0.13mm and 0.18mm thickness specifications.

As for the abnormal situation of the higher absorption level of Kraft paper over T910 paper

under 0.08mm thickness specification, the cause may attribute to the differences in polarity

of different oil materials. The oil absorption level is mainly related to the density of paper

materials but also affected by the polarity relationship between oil and paper. Closer

polarities could enhance the adsorption effect and thus increase the amount of oil absorbed.

Since the EBF#2 vegetable oil is artificially added high molecular acids to increase its anti-

oxidation ability, the polarity of EBF#2 vegetable oil should be higher and closer to the high

polarity of paper. Therefore, it is possible for paper impregnated in EBF#2 vegetable oil to

own a slightly higher oil absorption level.


56

If we consider the impacts of different impregnation oils on the oil absorption of insulation

papers, the following figures comparing the oil absorption results under different oil

impregnations of each paper material could be drawn, as shown in figure 3-4(a)-(c).

(a) Oil Absorption of T910 Paper under Different Oil Impregnation

(b) Oil Absorption of Kraft Paper under Different Oil Impregnation


57

(c) Oil Absorption of T910 Paper under Different Oil Impregnation

Figure 3-4 . Impacts of Different Impregnation Oils on T910, Kraft and TUK Paper

It could be directly seen that vegetable oil incurs an overall comparable or even larger oil

absorption level, compared with mineral oil. For T910 paper, DuPont EBF#2 vegetable oil

impregnation always owns slightly higher oil absorption levels than other oil impregnations.

For 0.13mm T910 paper that with higher density, vegetable oil impregnation generates

apparently higher oil absorption levels than mineral oil impregnation. However, for 0.08mm

and 0.18mm T910 papers that with lower densities, the oil absorption levels of paper

impregnated in mineral oil and vegetable oils are similar to each other. For Kraft paper,

EBF#2 vegetable oil impregnation also incurs higher oil absorption levels at all three

thickness specifications, especially for 0.08mm specification, whose oil absorption level is

approximately 7% higher than other three oil impregnations. The higher oil absorption level

of paper in vegetable oil is related to the higher polarity of vegetable oil as well. The polarity

of common vegetable oil is around 3.2, which is higher than the mineral oil polarity(2.2).

The polarity of vegetable oil is closer to fiber polarity. Therefore, the oil absorption level of

paper in vegetable oil impregnation is usually greater than that in mineral oil impregnation.

The differences of the oil absorption levels in different vegetable oils are probably caused

by their different polarities. Higher polarity means a higher adsorption effect between paper

fiber and oil and thus a higher oil absorption value. This is the potential reason to extrapolate

the higher oil absorption level of EBF#2 vegetable oil over the other two vegetable oils,

since the EBF#2 has a larger polarity.

Dielectric Performances of High Temperature Resistant Insulation Paper

58

4 Dielectric Performances of High Temperature

Resistant Insulation Paper

For good insulating materials, dielectric properties are key indicators on their insulating

performance. Usually low dielectric constant and dissipation factor and robust breakdown

behavior are required. In this chapter, the experimental results of dielectric constant,

dissipation factor and breakdown electric field in air and oil are analyzed. All dielectric

properties are investigated under low and high temperatures to study the temperature effect

on its performance.

4.1 Permittivity and Dissipation Factor

4.1.1 Test Method

The dielectrics will be polarized under electric field, along with the heating led by energy

dissipation. From a macroscopic point of view, the polarization and heating effect are

indicated by dielectric constant εr and dissipation factor tanδ. Insulating materials are usually

equalized to RC paralleled circuit, as shown in 4-1(a). Because of the existence of small

amount of free charges and polarized molecules, under high electric field, not only capacitive

current flows throughout dielectrics, but also conductive current and polarization current.

The ratio of active power dissipated on conductance over reactive power dissipated on

capacitance is called the loss tangent tanδ. The loss factor phase angle is shown in figure 4-

1(b). Since the real electric displacement vector has a different phase with electric field,

except conductive loss, there also exists polarization loss, as shown in figure 4-1(c).

(a) Equivalent Circuit (b) Phase Diagram Neglecting Relaxation Polarization


59

(c) Phase Diagram Including Relaxation Polarization

Figure 4-1 Loss Factor Equivalent Circuit and Phas Diagram

tan𝛿 = (𝛾 + 𝜔휀0휀‘’)(𝜔휀0휀‘)−1 (4-1)

where：

γ — conductivity；

ε’ — Real part of complex permittivity；

ε’’ —Imaginary part part of complex permittivity；

ε0 —Permittivity of vacuum ε0。

The reference standard is ASTM D202 Standard Test Methods for Sampling and Testing

Untreated Paper Used for Electrical Insulation[49]. The apparatus used is Switzerland

Tettex 2821 high and low voltage Schering bridge. The bridge has auxiliary three-electrode

systems under room and high temperatures. Under room temperature, the measuring

electrode has an area of 20cm2 and the electrodes are placed in a glass shield. Pressure can

be applied onto the paper samples. In experiments, 3N/cm2 pressure is applied onto the paper

surface to ensure the sufficient contact between sample and electrode. Under higher

temperatures, the electrode systems are placed inside a oven and the measuring electrode is

38mm in diameter. Connect the bridge HV cable and measuring cable to the correspondent

ports and then experiments can begin. The experiment source is 50Hz commercial electricity

and the highest voltage level is 2000V.

In experiment, the voltage level should be chosen as high as possible under the premise of

stable indicating. After voltage application, adjust the capacitance and resistance of the

measuring arm alternatively and let the zero-point indicator between two bridge arms

reaches zero. Increase the sensitivity level of the indicator progressively and balance the

bridge until level 5. Record the readings of capacitance and resistance and the εr and tanδ

could be calculated. Five paper samples with size of 100mm×100mm of each thickness


60

specification are carried out the experiment. Their average result is taken as the measurement

result. One thing to keep in mind is that water has a significant effect on the test results.

Therefore, drying process must be implemented on paper samples. Dried paper samples

should be stored in desiccator. The εr experiments are carried out under different

temperatures, including 23℃, 40℃, 60℃, 75℃ and 90℃. The tanδ experiments are carried

out under 40℃, 50℃, 60℃, 70℃, 80℃ and 90℃. And only 0.13mm T910 and Kraft paper

and 0.08mm TUK paper are carried the tanδ experiments.

4.1.2 Test Results

The measured εr changing under different temperatures of three paper materials are shown

in figure 4-2. The x-axis corresponds to temperature with unit in ℃.

Figure 4-2 εr of Kraft, T910 and TUK Paper

From the curves, we can observe that on εr all paper materials show fluctuated downward

trend. On 0.08mm specification, the magnitude relationship among three paper materials is

not very obvious. While on 0.13mm and 0.18mm specifications, it is clearly seen that T910

paper has a lower dielectric constant than Kraft paper.

To interpret this result, we have to start with the polarization process and the physical

meaning of loss factor. Under the electric field, the dipoles inside materials will occur a

displacement of its positive and negative charge centers or a orientation polarization process.

According to the type of material and its structure, the polarization process can be classified

as electronic polarization, ionic polarization, interfacial polarization and orientation

polarization etc. Electronic polarization is the most fundamental polarization form. For high


61

molecular polymers, due to the existence of polar groups on the molecular chain, the chain

could rotate to some extent under the alternative changing of electric field, leading to the

orientation polarization. Therefore, for polar polymers such as insulating paper, their

polarization process has the characteristics of electronic and orientation polarization.

Macroscopically speaking, polarization is scaled by relative permittivity.

Temperature

ε r

Figure 4-3 Schematic Sketch of εr Changing with Temperature

Generally speaking, εr will first increase and then decrease as temperature goes up, as shown

in figure 4-3. In the low temperature region, the mobility of molecules is weak and the

molecular thermal motion gets stronger as temperature increases. The dipoles are built up

more easily and thus leads to an ascend on εr. In high temperature region, as temperature

increases, excessively violent thermal motion will begin to hinder the polarization process

and thus εr will decrease. In real temperature variation process, several peak εr values may

appear along temperature increase and there may exist some turning points depending on

material type and polarization methods.

Based on the aforementioned polarization knowledge, the changing rules of εr along

temperature are analyzed. It could be seen that all paper materials experience early decrease

as temperature increases from room temperature. Thereafter, T910 paper has a εr trough at

around 50℃ and reaches another peak value at 75℃, while Kraft paper reaches the trough

at around 45℃ and the peak value at 60℃ . This is probably caused by the different

sensibility of orientation polarization of different papers.

When temperature increases from room temperature, the initial temperature rise has not

reached the glass transition temperature. Macromolecules are still sticking together and the

ability of its polar groups to rotate along with electric field is limited. Under this condition,

electronic displacement polarization is the main form and usually is called elastic dipole


62

polarization. The thermal motion of molecules are already intensified and hinders the

electronic polarization. Thereby, the macroscopic εr shows a descending trend. As the

temperature keeps increasing, the rotating mobility of polar groups is strengthened. When

temperature exceeds above glass transition temperature, paper fibers are in highly elastic

state and their polar groups and chain links could rotate obviously as electric field alternates.

The orientation polarization is thus intensified and contributes more to the overall

polarization in a certain temperature range. The orientation polarization is stronger and its

intensity is larger than electronic polarization. Thus, εr again begins to increase. When

temperature further increases, the more intensified thermal motion will impede the

orientation of polar groups and chain links. The macromolecules again become disordered

and thus εr will experience a final decrease. During the changes, there is a peak value of εr

and based on the change of εr the range of paper fibers glass transition temperatures could

be qualitatively analyzed. The uprising interval of Kraft paper εr is 45~65℃, while for T910

paper and TUK paper the uprising intervals are 50~75℃ and 60~80℃ respectively. The

glass transition temperatures of each material are considered to lie in these temperature

ranges. T910 paper has two polymer components and TUK paper has surface epoxy film,

both of which are two phase system. Additional interfacial polarization may also exists and

contributes to excess polarization intensification. This is the potential reason for the delay of

the εr trough and peak values of T910 and TUK paper over Kraft paper.

The complete dielectric constant is represented by complex dielectric constant, and could be

divided into real part ε’ and imaginary part ε’’. During the dielectric polarization, due to the

orientation of dipoles or the existence of macroscopic polarization current, some energies

are dissipated as heat radiation. The loss caused by dielectric polarization is often called as

relaxation loss. The relaxation loss factor is defined as the ratio of imaginary part ε’’ over

real part ε’, as shown in formula (4-2).

''

tan'

(4-2)

Where,

ε’ —real part of complex permittivity；

ε’’ —imaginary part of complex permittivity。

ε’ and ε’’ could be represented as:


63

2 2

'1

s

(4-3)

2 2

( )''

1

s

(4-4)

Where,

ε∞ — relative permittivity when the frequency of electric field tends to infinity.

Light frequency is high enough and so ε∞ is often called relative permittivity under light

frequency.

εs —relative permittivity under stable condition；

τ — relaxation time factor, referring to the time it needs to establish the electric field to

1-1/e or reduce the electric field to 1/e.

The measured tanδ changing under different temperatures of three paper materials are shown

in figure 4-4. Only 0.13mm Kraft and T910 paper and 0.08mm TUK paper are selected as

the representative paper samples and are performed the tanδ measurement.

Figure 4-4 tanδ of Kraft, T910 and TUK Paper

From the results, we can observe that T910 paper has an apparently larger loss factor tanδ

than other two papers. And TUK paper has a very close but slightly greater tanδ than Kraft

paper. All three kinds of paper keep a rather constant tanδ value before 60℃.After that, all

the tanδ begin to increases. Nevertheless, compared with the slight increase in Kraft and

TUK papers, the tanδ of T910 experiences a dramatic increase. The tanδ of T910 at 100℃

is nearly 5 times the value at 40℃. But for TUK and Kraft paper, the tanδ still keep a low

level.


64

Except the energy dissipated by dielectric polarization, conductance of the material can also

cause energy loss, which is often quoted as conductive loss. The complete physical meaning

of loss factor is described by formula (4-5). The energy loss factor is determined by the sum

of conductivity and equivalent conductivity of relaxation polarization divided by the product

of ε’ and angular frequency. Therefore, the increase on conductivity usually leads a an

obvious increase on loss factor.

tan'

g

(4-5)

''g (4-6)

Where,

γ——conductivity of the material；

g——equivalent conductivity of the relaxation loss。

On the basis of understanding the two sources of loss, the relationship between loss factor

and temperature can be further analyzed. The usual changing rule of tanδ along with

temperature is indicated in figure 4-5. The overall changing pattern is a up-down-up trend.

Temperature

tanδ

Figure 4-5. Schematic Sketch of tanδ Changing with Temperature

The qualitative analysis of typical changing rule of dielectric dissipation factor with

temperature is as following. When temperature increases in the low temperature range, the

thermal motion is strengthened and thus polarization is intensified. At the meantime,

dissipation factor goes up as well. When temperature increases to some certain levels, the

more intensified thermal motion will hinder the establishment of polarization. But the

conductive loss increase at this time is still little and not remarkable. Thus the overall

dissipation factor will go down for a while. As temperature keeps increasing, although the


65

relaxation loss are further reduced, an exponential increasing of conductive loss incurred by

high temperature will happen. Finally, the overall dissipation factor will obviously increase.

Considering 40℃ is already above room temperature, the molecular thermal motion has

already got intensified to some extent. The change in tanδ may lie in the plain region before

the final rapid high-temperature boost. In this temperature region, the decrease in relaxation

loss indicated by a reduction in εr is somewhat compensated by the increase on conductive

loss. Therefore, the overall loss factor could maintain a close level for small temperature

rises. Afterwards, tanδ of T910 increases dramatically after 70℃ but Kraft paper and TUK

paper have a slight increase in tanδ. This is probably due to the differences on the change of

conductive loss of different papers. Conductive loss is directly related to material

conductivity, which usually decreases as temperature goes up. The more conductivity

decreases, the more the resistivity increases and the more conductive loss increases.

Therefore, an inference could be made that, as temperature goes up, the decrease in

conductivity of T910 paper is much larger than the decrease of Kraft and TUK paper. And

thus the conductive loss and then the overall tanδ of T910 show a rapid ascending trend.

Another thing to note is that the εr of three kinds of dried paper changes with thickness.

Theoretically speaking, as the same kind of material, the εr is one inherent characteristic and

should not change with thickness and other size parameters. This is due to the fact that dried

paper samples are not pure paper fibers but composite systems of fiber and air. The εr of a

composite dielectric is dependent on the relative content of each component. Papers with

different thickness specifications have different densities. This means that the portions of

fiber are different, leading to discrepancy in εr. Generally speaking, the higher the paper

density, the greater the εr.

The current theoretical researches mainly focus on double component composite

dielectrics[55-58]. The models built up are either paralleled or in series, as shown in figure

4-6.


66

Figure 4-6 Structural Diagram of Double Component Dielectric

(a) in-series model (b) paralleled model

To interpret the relationship between composite system εr and the portion of each component,

many researchers have raised up different empirical formulas. Each formula is derived from

the modelling of different composite structure and different conditions. Table 4-1

summarizes typical formulas and their applications.

Table 4-1 Conversion Methods of Double Component Dielectric εr

Name of Formula Formula Application

Rayleigh’s a

f a f

f a f

V

Cylinder particles(air) with

small volume fraction

immersed in medium base

Maxwell-Garnett 2 2

a

f a f

f a f

V

Spherical particles with small

volume fraction but big

separation, randomly

embedded in medium base

Wiener’s

1 f a

f a

V V

， series

f f a aV V ， paralleled

Layered composites

Lichtenecker and

Rother

ln ln lnf f a aV V

f f a aV V ( Refractive index)

Powder and granular material

Bruggemann 02 2

a

f af

f a

V V

Spherical particles with large

volume fraction but small

separation, randomly

embedded in medium base

Goldschmidt [ ( )( )]

( )

aa a f a f

a f a

V fV

fV

Fiber materials；suitable for

paper when f=1/2


67

For insulation paper, its main body is a composite of air and paper fiber. Taking Kraft paper

for instance, it is the cellulose and air constituent the paper. T910 and TUK paper have a

more complicated structure which is a three components system. For the aim of convenience,

the thesis will only analyze the Kraft paper case to interpret the relationship of density and

εr.

εr of air is 1, while of cellulose is around 6. The εr of pure cellulose could be calculated from

experiment data and be compared with the reported values. The thesis takes the εr of air as

known condition and then converts the measured dried paper εr to pure cellulose εr by

aforementioned 6 formulas. The volume fractions of air and cellulose are determined by oil

absorption experimental results. The absorbed oil is considered to occupy the volume of air

and densities of oil, air and cellulose are taken as known conditions. The volume fraction

calculation method is described as following.

(a)oil oil oilV m (4-7)

( )f f a a f a paperV V V V (4-8)

Where,

(a)oil aV V ——the volume of oil and the air，under test ；

fV ——volume of cellulose, to be determined；

f ——density of cellulose，1.53g/cm3；

a ——density of air，approximately of 0 g/cm3；

oil ——density of oil，FR3 oil’s density is 0.92 g/cm3；

paper ——density of dried insulation paper，measured density is used, knwon；

oilm ——absorbed oil mass，knwon；

From the above formulas, the volume fractions of air and cellulose could be calculated and

normalized. The conversion results of pure cellulose εr is listed in table 4-2. Meanwhile, the

calculation results of paper composite εr based on reported cellulose εr value are also

compared with the measured values, as shown in table 4-3.


68

Table 4-2 Calculation Results of Pure Cellulose εr by Different Methods

Thickness

/mm Ray’s Max-Garn Wie seires

Wie

parallel

Lich and

Roth Refraction Brug Golds Reported

0.08 2.75 2.60 6.01 2.41 2.92 2.60 2.69 3.2

6 0.13 3.81 3.60 11.92 3.35 4.11 3.62 3.70 4.87

0.18 4.29 4.03 25.94 3.74 4.71 4.07 4.15 5.92

Table 4-3 Comparison Between Calculation Results of Kraft paper εr and Measured Values

Thickness

/mm Ray’s Max-Garn Wie seires

Wie

parallel

Lich and

Roth Refraction Brug Golds Measured

0.08 2.79 3.05 1.69 3.45 3.35 2.93 2.76 2.08 1.69

0.13 3.69 3.95 2.25 4.33 3.30 3.86 3.79 2.81 2.57

0.18 3.74 3.99 2.28 4.37 2.41 3.91 3.84 2.86 2.85

From the tables, we can observe that, under most conditions, the calculated pure cellulose εr

has great discrepancy with the reported value by using all calculation methods. And the

differences between different methods are significant as well. It is normal to happen this

kind of result. This is because that all the modelling and calculation is under the simulation

of materials with ideally symmetrical or continuously and homogenously distributed

structures. But real insulation paper has a very discontinuous structural system. Considering

the unavoidable additional errors caused by water intrusion, it is therefore very hard to

accurately calculate cellulose εr by using ideal models. However, it could be found that,

whether in calculation of pure cellulose εr or in calculation of paper εr based on reported

value, the results of Wiener in-series method and Goldschmidt method have very good

consistence with measured results of 0.08mm Kraft paper and 0.18mm Kraft paper

respectively.

Although the theoretical models could not give us an accurate prediction, we can still

qualitatively interpret the relationship between paper density and εr. From the results, all the

results show an ascending rule in εr as thickness goes up. The results of Kraft paper density

measurements tell us that the density of Kraft paper increases monotonously as thickness

increases. Concerning that larger density refers to greater volume fraction of cellulose, it

could be drawn a conclusion from a qualitative point of view that the higher the paper density,

the greater the paper εr.

4.2 Breakdown Electric Field in Air


69

4.2.1 Test Method

Insulation materials must have high enough dielectric strength to ensure the stable and

healthy operation. The dielectric strength is commonly determined by breakdown

experiments. The experiment applies increasing voltage on the test sample until its electrical

breakdown and the electric breakdown strength is derived by the division of breakdown

voltage over sample thickness. The breakdown strength is defined as:

𝐸𝐵 =𝑈

𝑇 (4-9)

Where,

EB——Breakdown electric field；

U——Breakdown voltage；

t ——Thickness.

The thesis first measures the paper samples’ breakdown electric field in air, according to

ASTM D149 Standard Test Method for Dielectric Breakdown Voltage and Dielectric

Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies[59].

The breakdown experiment system is composed of a voltage booster, electrode system and

grounding wire. For breakdown experiment in air, the cylinder electrodes with 51mm

diameter specification are adopted. Due to the lower dielectric strength in air, the voltage

increasing rate has to be chosen a lower value. In experiments, voltage increasing rates

within 150V/s to 200V/s are selected according to different paper thickness. The basic rule

is to control the breakdown time lies within 10s to 20s. For paper samples of each thickness

specification, a total number of 20 breakdown experiments are carried and the breakdown

voltages are divided by paper average thickness result. The breakdown experiments of paper

in air are only performed under room temperature.

To interpret the derived breakdown electric field results, Weibull statistic method is used.

And the breakdown electric field corresponding to the breakdown probability of 63.2% is

taken as the characteristic value. The Weibull statistic method is introduced in Appendix

more in detail.


70

4.2.2 Test Results

The Weibull statistic data of the results of electric breakdown experiments of paper in air

are shown in table 4-4. To better illustrate the results, the characteristic breakdown electric

fields are plotted in bars, as shown in figure 4-7.

Table 4-4 Weibull Distribution of Breakdown Electric Field of Kraft, T910 and TUK Paper in Air

Parameter 0.08mm 0.13mm 0.18mm

T910 Kraft TUK T910 Kraft T910 Kraft

Size

(kV/mm) 12.32 9.17 11.51 13.90 11.72 12.76 11.68

Shape 17.25 25.92 28.61 22.35 22.83 26.00 27.74

Figure 4-7 Characteristic Breakdown Electric Fiels of Kraft, T910 and TUK Paper in Air

From the bar plots, we can clearly see that among three kinds of paper T910 paper always

has the highest breakdown electric field in air at any thickness specification. The breakdown

electric fields of T910 paper are 12.32kV/mm at 0.08mm, 13.90kV/mm at 0.13mm and

12.76kV/mm at 0.18mm. Kraft paper has the lowest breakdown performance in air. The

breakdown electric fields of Kraft paper are 9.17kV/mm at 0.08mm, 11.72kV/mm at

0.13mm and 11.68kV/mm at 0.18mm, which are 3.15kV/mm、2.18kV/mm and 1.08kV/mm

respectively lower than that of T910 paper. As for TUK paper, its breakdown electric field,

which is 11.51kV/mm, is slightly lower than T910 paper at 0.08mm. All three paper

materials reach their own maximum values in breakdown electric field at 0.13mm.


71

According to the requirement for breakdown electric field in air regulated in ASTM

standard[53], as shown in dashed lines in figure 4-4, all three paper materials have electric

fields above the threshold values.

4.3 Breakdown Electric Field in Oil

4.3.1 Test Method

For oil-immersed breakdown electric field experiments, the composition of testing system

is same as the experiments in air. The main difference is the electrode system. In oil-

immersed experiments, an oil test cup with Φ25mm cylinder electrode placed inside. The

paper sample is clamped in between the electrodes and approximately 400mL insulation oil

is poured into the test cup to cover the insulation paper. The voltage increasing adopts the

continuous method and the rate is set as 1kV/s in order to let the breakdown happens within

10 to 20s. The size of paper sample in oil immersed breakdown is 80mm×65mm. 5

breakdowns are performed on different location of one paper sample. And 5 paper samples

of each thickness specification are carried out the experiment, that is 25 total breakdown

points are recorded. The experiments are performed under both room temperature and high

temperature of 90℃. The results are also interpreted by Weibull distribution.

In order to investigate the impacts of different impregnation oils on paper breakdown

performance, four kinds of insulation oil are used in the experiments. The liquid

impregnation materials include Karamay #45 mineral oil, FR3 vegetable oil, NARI Vinsoil

vegetable oil and DuPont EBF #2 vegetable oil. All three kinds of paper sample are measured

the breakdown electric field under all four kinds of oils.

4.3.2 Test Results

The characteristic breakdown electric fields processed by Weibull distribution of three kinds

of paper under four kinds of oil impregnation are shown in figure 4-8(a)-(d).


72

(a)Breakdown Electric Field in Karamay #45 (b) Breakdown Electric Field in FR3

(c) Breakdown Electric Field in Vinsoil (d) Breakdown Electric Field in EBF#2

Figure 4-8 . Breakdown Electric Field of T910, Kraft and TUK papers under 23℃ and 90℃ in four

kinds of insulating oils

From the shape of bar plots, we can first observe that the changes of breakdown electric field

along temperature and thickness in mineral oil and in vegetable oil belong to different

patterns. Under room temperature, when three kinds of paper are immersed in Karamay #45

mineral oil, T910 paper always has the highest value no matter what the thickness is. Kraft

paper has a lower breakdown strength than T910 paper, while the breakdown electric field

of TUK paper is the lowest, which is just 82kV/mm at 0.08mm thickness. But if the

impregnation oil alternates to vegetable oils, even if T910 paper still precedes over the other

two papers, the breakdown performance of TUK paper prevails over Kraft paper

alternatively. For thicker paper samples, T910 paper has very close breakdown electric field

values with Kraft paper. The differences are just within 2kV/mm.


73

When temperature is elevated to 90℃, under the mineral oil impregnation, the relationship

of three papers is just reversed. For 0.08mm specification, TUK becomes the most robust

material in breakdown strength, which is 97kV/mm. And T910 paper shows the lowest

breakdown electric field, which is just 92kV/mm. For thicker paper samples, Kraft paper

still prevails T910 paper of around 5kV/mm. However, in vegetable oil impregnation, Kraft

paper is still the weakest material but TUK paper replaces T910 paper to be the material

owning highest breakdown electric field. Furthermore, for thicker paper samples, the

differences between Kraft paper and T910 paper are not significant.

If we consider the temperature effect on paper breakdown electric field, we can find out that

for paper under mineral oil impregnation, except the 0.08mm T910 paper, the temperature

rise could always enhance the breakdown electric fields of three kinds of paper. But for paper

impregnated in vegetable oils, temperature rise has different impacts on papers with different

thickness specifications. By the temperature elevation up to 90℃, T910 paper shows a

descending trend in breakdown strength, but the breakdown electric field of TUK paper gets

magnified. And temperature rise has little impact on Kraft paper. For thicker paper samples,

the change in breakdown electric field caused by temperature increase is not significant.

Thereby, the breakdown electric field of insulation paper in mineral oil impregnation is more

sensitive to temperature rise.

When considering the impacts of different insulating oils on paper breakdown strength under

two temperatures, bar plots shown in figure 4-9(a)-(e) could be drawn.

(a) Oil effects on T910 paper under 23℃ (b) Oil effects on T910 paper under 90℃


74

(c) Oil effects on Kraft paper under 23℃ (d) Oil effects on Kraft paper under 90℃

(e) Oil effects on TUK paper under 23℃ (f) Oil effects on TUK paper under 90℃

Figure 4-9. Effects of different insulation oils on T910, Kraft and TUK paper

From the plots, we can observe that for 0.08mm specification, no matter under room

temperature or 90℃, three kinds of paper all have higher breakdown strength in mineral oil

impregnation rather than in vegetable oil impregnations.

Under 0.13mm and 0.18mm thickness specifications, the breakdown strength of three kinds

of paper under room temperature differ little in two kinds oil impregnations, and breakdown

strength in vegetable oil impregnation even has higher values than in mineral oil

impregnation. Furthermore, there are some differences in the effects of different vegetable

oils on paper breakdown performance. For T910 paper, its breakdown electric field reaches

maximum under FR3 vegetable oil impregnation. But for Kraft and TUK papers, Vinsoil

seems to be the vegetable oil that allows their breakdown strength reach maximum.


75

However, when temperature is elevated to 90℃, mineral oil exhibits its advantage in higher

incurred electric field, especially for Kraft paper. The advantage of 0.13mm Kraft paper’s

breakdown strength in mineral oil over that in vegetable oil is around 10kV/mm. And for

0.18mm Kraft paper, the breakdown strength in mineral oil is approximately 15kV/mm over

that in vegetable oil. The advantages of T910 paper in mineral oil over in vegetable oil are

more moderate. The differences are only within 5kV/mm.

Physiochemical Performances of High Temperature Resistant Insulation Oil

76

5 Physiochemical Performances of High Temperature

Resistant Insulation Oil

The performance of insulation oil also has great influence on transformer healthy and safe

operation. Transformer insulation system usually consists of solid and liquid materials and

its insulation condition is determined by both materials. Insulation paper is immersed in

insulation oil and oil condition will affect the aging rate and operation condition of paper to

some extent. In this chapter, three key parameters indicating insulation oil behavior of all

four kinds of oil materials are investigated, including viscosity, total acid number and water

content. All the experiments are carried out towards oil samples unused and preconditioned

by degassing and dehydration. The differences among four oils are analyzed.

5.1 Viscosity

5.1.1 Test Method

When liquids are flowing, the nature of the inter-molecule friction is known as the viscidity

of liquid, whose magnitude is scaled by viscosity. The notion of viscosity of insulation oil is

the same as normal liquids, which is the internal friction. Under the external force, the

insulation oil will have a laminar flow phenomenon and there will exist internal friction

among macromolecules. The greater the internal friction of insulation oil, the bigger the

viscosity and the harder the oil could flow. This means that the insulation oil has a bad

performance on heat dissipation. Viscosity is a key thermal parameter of transformer oil,

indicating its cooling ability. At the meantime, viscosity is a mirror to reflect the aging

condition of insulation oil. After insulation oil is oxidized, its viscosity usually experiences

an increase. There are two commonly used viscosity types, kinematic viscosity and dynamic

viscosity.

According to Newton fluid law, during the flowing of fluid, the shear stress τ is proportional

to the velocity gradient of the fluid. The scaling factor is called kinematic viscosity.

dX

dV (5-1)

Where,

τ——shear stress，force applied per unit area，MPa, Pa, mPa；


77

dX

dV——shear rate，velocity gradient formed in flowing layer，1/s；

η——kinematic viscosity，cp, 1cp=1mPa·s.

The dynamic viscosity is the ratio of the fluid kinematic viscosity and its density ρ under

same temperature. The unit of dynamic viscosity is m2/s. The calculation formula is as

following.

(5-2)

The thesis utilizes Brookfield DVII+Pro rotational viscometer to measure the kinematic

viscosity of oil samples according to ASTM D445 Standard Test Method for Kinematic

Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)[60].

The rotational viscometer adopts the viscosity rotational measurement principle. The

viscosity is determined by measuring the torque generated by the continuous rotation of a

rotor placed inside the oil sample under test. The torque is proportional to the resistance

formed by viscous dilatory effect of the rotor immersed in the samples and then proportional

to viscosity. The kinematic viscosity of each oil sample is measured under both 40℃ and

90℃. Two oil samples of each oil type are measured and the average result is calculated

according to the standard.

5.1.2 Test Results

The results of viscosity measurement under 40℃ and 90℃ of four kinds of oil are listed in

table 5-1.

Table 5-1 Comparison of Kinematic Viscosity Between Mineral Oil and Vegetable Oils

Temperature FR3 EBF #2 Vinsoil Karamay #45

40℃/cP 29.70 34.65 29.05 8.4

90℃/cP 8.43 9.00 8.16 2.51

From the table, we can find out that the viscosities of vegetable oils are much greater than

mineral oil on both two temperatures. The viscosity of vegetable oil is nearly four times of

that of mineral oil under 40℃, while under 90℃ the ratio becomes to 3.5. As the temperature

rises, both mineral oil and vegetable oil experience a decline in viscosity and the decreasing

levels of mineral oil and vegetable oil are close. As temperature rises from 40℃ to 90℃, the


78

viscosities of vegetable oils drop around 72%, from 29~35cP to 8~9cP, while the viscosity

of mineral oil drops 70%, from 8.4cP to 2.51cP. Among the three vegetable oils, the viscosity

of DuPont EBF#2 is the greatest, while the viscosity of NARI vinsoil is the lowest.

The higher viscosity of vegetable oil is determined by its structural characteristic. As its

main component, the triglycerides have longer molecular chains and bigger average

molecular weight. This will hinder the flow of vegetable oil molecule and thus its viscosity

is greater than mineral oil. The greater viscosity is sort of disadvantage of vegetable oils. But

it could be partially compensated by reasonable design of transformer oil-flow structure and

strengthening of the cooling methods. Therefore, the conventional requirements on viscosity

aimed at mineral oil are not completely suitable for vegetable oils. Some international

standard organizations, e.g. ASTM, IEEE and IEC[61-63], have proposed new technical

specifications on vegetable oils. The novel requirements on viscosity of vegetable oils is

below 50cP under 40℃. All the test results meet that requirement.

5.2 Total Acid Number

5.2.1 Test Method

The total acid number is a key indicator on the insulation oil condition. As aging goes on,

the acids are accumulated in the oil and thus the total acid number of oil will increase.

Therefore, the total acid number could reflect the aging condition of insulation oil. Besides,

the low molecular acids are considered to threaten the healthy condition of insulation paper.

So the initial total acid number of oil should maintain at a low level. For traditional mineral

oil, the requirement on the initial total acid number of IEC and other international standards

is below 0.01mgKOH/g.

The total acid number is defined as the amount of KOH in mg to neutralize 1g sample under

test, and its unit is mgKOH/g. The measurement of total acid number of oil samples refers

to ASTM D664 Standard Test Method for Acid Number of Petroleum Products by

Potentiometric Titration[64]. The titration apparatus used is Metrohm 848 Titrino plus

titrator, whose buret accuracy is 1/10000. The test electrode is Metrohm 6.0229.100.


79

The total acid number is determined by automatic potentiometric titration in the experiments.

During the titration, the ionic concentration of the solution is constantly changing. According

to Nernst equation, the potential of the indicator electrode that measures the potential of

solution is also constantly changing. When it comes to the end of titration, there is a sudden

change in the potential and this is an indicator of the total neutralization of acids in the test

solution.

The test procedures are described as following.

(a) Weigh a proper amount of KOH and transfer it into a Erlenmeyer flask. Add isopropanol

into the flask and ensure the concentration of KOH to be around 0.1mol/L. Heat up the

KOH contained isopropanol by water bath until boiling. During the boiling, add small

amount of Ba(OH)2 in order to remove the carbonic acids dissolved in the solution.

Switch off the water bath after half an hour. Cool the flask down and wait 72 hours to

allow the thorough participation.

(b) Transfer the KOH solution into the brown glass bottle of the test apparatus. Standardize

the KOH solution by using standard potassium hydrogen phthalate and acquire the real

concentration of KOH solution.

(c) Titrate the mixed solution of isopropanol and toluene, which is the background solution

dissolving the insulation oil, by the KOH solution with known concentration. The total

acid number of background solution is acquired.

(d) Finally, dissolve the oil sample in a mixed solution of isopropanol and toluene. Stir the

mixed solution to ensure sufficient mixing. Titrate the solution and the total acid number

of the oil sample is the value measured subtracting the background acid number.

For each kind of oil, two measurements are carried and their average result is taken as the

total acid number of that kind of oil.

5.2.2 Test Results

The test results of the initial total acid number of four kinds of oil are listed in table 5-2.

Table 5-2 Comparison of Initial Total Acid Number Between Mineral Oil and Vegetable Oils

FR3 EBF #2 Vinsoil Karamay #45

Total Acid Number

/mgKOH/g 0.0479 0.1937 0.0831 0.0077


80

From the above table, we can see that the total acid number of mineral oil meets the

requirement of below 0.01mgKOH/g, while the results of vegetable oils are much greater

than that threshold. And there are big discrepancies among three vegetable oils. Due to the

particular structure, vegetable oils have intrinsically higher total acid number level. Except

the natural acidic groups on its triglyceride backbone, vegetable oils contain free fatty acids.

Therefore, vegetable oils always contain more carboxyl groups, leading to higher total acid

number level. For mineral oil, the main components are paraffins or naphthenics and no

acidic groups are contained. Only under oxidation would the mineral oil generates acidic

products and leads to total acid number increase. Therefore, it is not suitable to apply the

assessment criteria of mineral oil on vegetable oil.

Considering the characteristic of high total acid number, IEC international standard

organization first raised the limit of the total acid number for vegetable oils to 0.06mgKOH/g.

According to this specification, herein only FR3 meets the requirement. But it does not mean

that the other two vegetable oils are harmful for safe operation. According to the introduction

of paper aging mechanism in chapter 1, it is the low molecular acid that plays a harmful role

in cellulose hydrolysis. In vegetable oils, the acid groups usually are high molecular acids.

Therefore, the high total acid number of vegetable oils has a limited threats on transformer

normal operation. Due to the complex sources of vegetable oils, the content of acidic

substances differs a lot with the type of seed and the difference in manufacturing crafts. The

DuPunt EBF#2 vegetable oil is added high molecular acids intentionally to increase its anti-

oxidation ability and the Vinsoil may originates from seeds containing more acids. These

are the potential reasons to interpret why these two oils have much higher total acid numbers.

Therefore, it is natural and reasonable for vegetable oils to have higher total acid number.

But the current 0.06mgKOH/g requirement may be too strict, considering the various sources

of the vegetable oils. It is hard to set a universal requirement that is suitbale for all kinds of

vegetable oils.

5.3 Water Content

5.3.1 Test Method

The water contained in oil-paper insulation system will directly affect the aging process of

oil and paper material. Water behaves as raw material for paper and oil hydrolysis process.

It is impossible to fully eliminate the water existence inside the materials, since the insulation


81

materials have certain polarity levels. However, the water content of oil and paper materials

could be reduced by some drying processes.

According to ASTM D1533 Standard Test Method for Water in Insulating Liquids by

Coulometric Karl Fischer Titration[65], the water contents of oil samples are determined by

Metrohm 831 Karl Fisher titrator. The titration principle is the same with the determination

of water content in paper, as introduced in section 3.2.1. The only difference is that oil

samples are directly injected into the titrator, while the water in paper samples are evaporated

and carried into the titrator.

In the experiment, 5mL Disposable syringes are used to extract proper amount of oil sample.

Then 1~2mL oil sample is injected into the titrator through a 8cm syringe needle passing

through the silicone gasket on the top of the titrator. Then the titration program starts and

the water content is automatically determined by entering the oil sample weight data into the

apparatus. For each oil sample, two measurements are performed and the average value is

taken.

5.3.2 Test Results

The test results of water content measurement of four kinds of insulation oil are listed in

table 5-3.

Table 5-3 Comparison of Initial Water Content Between Mineral Oil and Vegetable Oils

FR3 EBF #2 Vinsoil Karamay #45

water content/ppm 57.9 60.7 44.9 12.0

From the results, under the same preconditioning conditions, a much greater water content

levels of vegetable oils over mineral oil are observed. The water contents of vegetable oils

lie within 45~60ppm, while the water content of mineral oil is just 12ppm.

The higher water content in vegetable oil is mainly caused by its higher polarity. On

vegetable oil molecular chains, there exists unsaturated bonds and hydroxyl groups in fatty

acid groups. Therefore, vegetable oil molecules usually have higher polarity. However, the

compositions of mineral oil are mainly hydrocarbons, which have low polarity. The higher

polarity means more significant hydrophilicity. Thereby, the vegetable oil has intrinsic

higher water content level.


82

Main international standard organizations have established the water content restrictions for

mineral oil and vegetable oil separately. The upper limit of water content for mineral oil

before operation is 20ppm and the upper limit for vegetable oil is 200ppm. The test results

of all four oil samples after preconditioning meet the requirements of relevant standard.

Results and Analysis of Accelerated Aging Experiment of High Temperature Resistant Insulation System

83

6 Results and Analysis of Accelerated Aging Experiment

of High Temperature Resistant Insulation System

Good insulation materials should not only have excellent initial insulating properties, but

also maintain a stable and healthy state during the long-term operation. The comparison of

initial performances of insulating materials is not sufficient enough to reveal the advantages

and disadvantages thoroughly. The changes of key parameters during the long-term run

should also be investigated. Due to the time limitation, the aging experiment usually elevates

the temperature in order to accelerate the aging and thus acquire the anticipated outcome in

a short time. For a traditional 55℃ temperature-rise Kraft paper insulation system, assuming

the room temperature to be 30℃, the designed hot spot temperature inside transformer is

95℃. Under this temperature, Kraft paper could run steadily and healthily for many years,

as long as 30 years according to CIGRE D1.323 report. Therefore, the thesis adopts 150℃

as the aging temperature. According to Montsinger 6~8℃ rule, the aging rate is sharply

increased and the life of material is substantially shortened, and thus an obvious change in

properties could be observed. TUK paper has been applied for more than 50 years and it has

been commonly acknowledged that it has a 10~15℃ leading in thermal class over Kraft

paper.

In this chapter, the thesis more focuses on the novel high-temperature resistant materials.

Therefore, T910 and DMD papers are chosen as the solid aging objectives, and Kraft paper

is also aged as a base comparison material. All paper materials are aged in FR3 vegetable

oil impregnation. This chapter mainly introduces the test results from a point of view of the

changing in the properties of paper and oil materials. The test methods are consistent with

the experiments of initial paper and oil properties and have been described in former chapters.

Thus the detailed test methods are omitted in this chapter.

6.1 Variation in Insulation Paper Performance Before

and After Aging


84

6.1.1 Tensile Strength

Tensile strength, as the most indicative parameter of paper degradation level, has been used

to reflect the aging condition of transformer insulation system for a long time. In most cases,

50% residual tensile strength is considered as the end-of-life criteria. The changes in tensile

strength of three paper materials are measured along the aging process and are plotted in

figure 6-1. The y-axis refers to the residual tensile strength and 50% threshold is

demonstrated by the dashed line. The x-axis is the aging time.

Figure 6-1. Changes of Tensile Strength of T910, Kraft and DMD Paper During 150℃ Aging

From the curves, we can observe that,

(1) Compared with T910 and DMD papers, Kraft paper shows the fastest descending trend

in tensile strength. Till the end of 720h aging experiment, the residual tensile strength of

Kraft paper has dropped to around 60%. And the decreasing trend is also apparent and

monotonous.

(2) After the initial decrease, DMD paper has a stable residual tensile strength before 96h,

which is around 95%. Then its residual tensile strength experiences a sudden drop to 88%

at 192h. Afterwards, DMD paper maintains this level until the 720h aging end.

(3) T910 paper exhibits the highest residual tensile strength level along all the aging period.

Around 95% residual tensile strength is remained till the end of aging. T910 paper shows

a slight increase at the beginning of aging. During 6h to 24h, its residual tensile strength

reaches a peak value of 105%. After 48h, T910 paper has a very stable residual tensile


85

strength.

Therefore, to sum up, T910 paper and DMD paper both maintain a high level residual tensile

strength during the aging and T910 paper is the most stable paper material that has the

highest residual level. However, Kraft paper shows a significant reduction and has the lowest

residual tensile strength.

The aforementioned summary is drawn upon the changing on tensile strength compared with

the initial values. A slower decreasing rate is what we desire. But the absolute tensile strength

behavior also draws our concerns. The initial tensile strength on machine and cross-machine

directions are measured and listed in table 6-1.

Table 6-1 Comparison on Initial Tensile Strength of T910, DMD and Kraft Paper

T910 Kraft DMD

Machine（N/cm） 155.4 119.0 129.0

Cross-machine（N/cm） 27.0 47.8 90.9

From the table, it could be seen that on machine direction T910 paper still owns the largest

initial tensile strength, which is 155.4N/cm, and Kraft paper is the weakest material, whose

tensile strength is just 119.0N/cm. However, on cross-machine direction, T910 paper

becomes to the material owning the lowest tensile strength of just 27.0N/cm.

This is mainly caused by the uniqueness of T910 paper structure. NOMEX fiber itself has

powerful mechanical strength and that is reflected in the high bonding strength along the

molecular chain direction. Thus T910 paper has a excellent behavior in machine direction

tensile strength. However, the cross-machine direction strength is provided by hydrogen

bonds mainly. Since NOMEX fibers do not have polar groups, when in hybrid weaving with

cellulose, it is hard to form hydrogen bonds between NOMEX and cellulose fibers, unlike

the pure cellulose molecules. Therefore, the intersection part of NOMEX and cellulose fibers

is the weak link of the material, due to the weak hydrogen bonding effect. When mechanical

stress is applied on cross-machine direction, the paper fibers in this region will first be

stretched out and thus T910 paper shows a low value of tensile strength on cross-machine

direction. Nevertheless, the insulation paper on transformer windings is wrapped along its


86

machine direction. The tensile strength on machine direction is usually more crucial than

that on cross-machine direction.

In summary, although T910 paper has lower tensile strength on cross-machine direction, it

shows the most excellent tensile behavior on the main direction, namely the machine

direction. As the aging goes on, T910 paper remains the highest residual level at the end of

aging. Kraft paper shows the fastest degrading rate and the lowest final residual tensile

strength. And DMD paper has the intermediate behavior and has good initial tensile strength

performance and slow aging speed.

6.1.2 Breakdown Voltage

The insulation materials should maintain a high breakdown strength for a long time to keep

the intact insulation. The aging experiment performs 20 breakdown measurements for all

three papers at each aging node and Weibull distribution is used to process the test results.

The obtained changes of breakdown voltage along aging of three kinds of paper are

illustrated in figure 6-2.

Figure 6-2. The Changes in Breakdown Voltage of T910, Kraft and DMD Paper During 150℃

Aging

Firstly, the magnitude relationship between three kinds of paper could be determined. DMD

paper has the highest breakdown voltage level, of around 12.7kV. T910 paper has a slightly

lower value of around 11.6kV. While Kraft paper is weakest material in breakdown

performance, its breakdown voltage is just around 10kV. It could be therefore inferred that


87

by weaving some NOMEX fibers into cellulose fibers, the breakdown strength of the

composite system could be enhanced. And the DMD PET fibers also own a very high

breakdown strength.

If we compare the changing trends of breakdown voltage along aging of three paper

materials, it could be found out that within the total aging period the changes in breakdown

voltage are not significant. It only shows small fluctuations at early stage and keeps an

overall stable level. By comparing the breakdown voltage performance with tensile strength,

we can observe that the reduction in tensile strength will not lead to a same decrease in

breakdown voltage, as apparently shown in Kraft paper case. Therefore, for insulation papers,

breakdown voltage is not suitable to reflect their aging conditions in a short duration, but

tensile strength could reflect more clearly.

6.2 Variation in Insulation Oil Performance Before and

After Aging

6.2.1 Viscosity

For insulation oil, viscosity is a key parameter to determine whether it undergoes oxidation.

When oxidized, insulation oil usually shows an increase in viscosity. Viscosity could directly

reflect the average molecular weight of a liquid, and its relationship is described by Mark-

Houwink formula[66].

MaK (6-1)

Where,

η — Kinematic viscosity；

K — Constant for certain liquid；

a — Constant for certain liquid；

M — Average molecular weight.

The kinematic viscosities of FR3 vegetable oil during the aging process are measured and

recorded. The changes in viscosity under 40℃ and 90℃ are shown in figure 6-3.


88

Figure 6-3 The Changes in Viscosity of FR3 Vegetable Oils During 150℃ Aging

From the Mark-Houwink formula, we can draw a conclusion that when polymerization

reactions exist among liquid molecules, the average molecular weight would increase and

so as the liquid viscosity. On the other side, if there exists depolymerization reactions in

liquid, the average molecular weight and viscosity will decrease. For vegetable oils, there

are two main degrading forms, namely hydrolysis and oxidation. The oxidation of vegetable

oil[67] will first generate some primary oxidation products along with free radicals R·.

Polymerization reactions will occur mutually among these produced free radicals R·. Since

the free radicals themselves usually own longer molecular chains and larger molecular

weight, their polymerized products would show obviously lengthened molecular chain.

Therefore, the oxidation of vegetable oil will cause an apparent increase in its viscosity. The

hydrolysis of vegetable oil[68] is a process which mainly undergoes a fracture process of

ester bonds in triglycerides. The hydrolysis of ester bonds is reversible and the produced

diglycerides or monoglycerides products and triglycerides will not polymerize among each

other. So there is no large-scale polymerization and depolymerization phenomenon and the

viscosity will not change significantly during the hydrolysis process.

From the curves, we can see that all the FR3 oils keep a stable viscosity level before 192h,

no matter under 40℃ or 90℃. Only small fluctuations happen during this aging period. The

viscosities of FR3 oils under 40℃ are around 30cP, while under 90℃ the viscosities are

around 8cP. From 192h on, the viscosities of FR3 oils increase a little. The viscosities rise


89

up to around 36cP under 40℃ and the increases under 90℃ are not apparent. Therefore, it

could be considered that the vegetable oils are not significantly oxidized before 192h, but

after 192h the vegetable oils are slightly oxidized. The aging vessels are filled in nitrogen to

exclude air and are kept airtight all the time. The effect of oxygen on vegetable oil aging

should be limited. The slight increase on viscosity after 192h may attribute to the air

intrusion during the sampling process. Considering the increases are not significant, the

influence of oxidation on vegetable oil is still limited. The aging process of vegetable oil is

mainly dominated by hydrolysis.

6.2.2 Total Acid Number

The total acid number is a key parameter to indicate whether the traditional mineral oil is

oxidized or not. Mineral oil molecules have no acidic groups or ester bonds that could be

hydrolyzed. But the oxidation of mineral oil will create some hydroxyls attached onto the

molecular chain. Thus a obvious total acid number of mineral oil is usually considered as

caused by oxidation. However, the situation is quite different for vegetable oil. The total acid

numbers of vegetable oils during aging process are measured and the test results are shown

in figure 6-4

Figure 6-4 The Changes in Total Acid Number of FR3 Vegetable Oils During 150℃ Aging

From the curves, we can observe that the total acid numbers of FR3 vegetable oils

cooperating three kinds of paper all increase sharply as aging going on. The total acid

numbers of impregnation FR3 oils of T910, Kraft and DMD paper reach 2.75mgKOH/g,


90

2.25mgKOH/g and 2.0mgKOH/g respectively. The ascending trends of total acid numbers

of three FR3 oils are all monotonous.

From the results of the aforementioned viscosity measurements, a conclusion could be drawn

that vegetable oils mainly endure the hydrolysis as the aging form, instead of oxidation. The

increase in total acid number of vegetable oil is mainly caused by its hydrolysis and this is a

big difference with mineral oil. As the hydrolysis of vegetable oil goes on, the ester bonds

on triglycerides backbone are broken up. And triglycerides will degrade to diglycerides,

monoglycerides and finally glycerin step by step. A long chain fatty acid could be formed

by each fracture of ester bond. Therefore, the complete hydrolysis of each triglycerides could

generate three long chain fatty acids. Thus the total acid number is increased obviously by

the hydrolysis of vegetable oil. This corresponds to what is observed in the experiments. The

long chain fatty acids created by the hydrolysis of vegetable oil are usually water-insoluble

high molecular acids, which have limited threats on insulation paper condition.

Except hydrolysis, the oxidation of vegetable oil will also increase its total acid number level.

Therefore, in real applications, the total acid number could reflect vegetable oil aging

condition, but it could not be used to discuss the effect of oxidation or hydrolysis individually.

6.2.3 DGA

The dissolved gas analysis(DGA) is a common evaluation method to determine the

transformer operation condition. A large quantity of on-site experiences have been

accumulated for many years. Due to the short application time of vegetable oil immersed

transformer, the demand for DGA of vegetable oil is higher and most DGA results of

vegetable oil are coming from laboratory. The gases generated by vegetable oil under fault

conditions share the same types with the gases generated by conventional mineral oil, but

the gas contents are different. The commonly gases measured by DGA are CH4, C2H6, C2H4,

C2H2, H2, CO and CO2. Compared with mineral oil, vegetable oil usually has higher gas

content in CH4, C2H6 and C2H4. The overheating of vegetable oil would incur a substantial

generation of CO and CO2. The production of hydrocarbons is related to the fracture of C-H

and C-C bonds on the backbones of mineral and vegetable oils. The active H and CH groups,

namely free radicals, produced by the fractures would combine each other and then form

hydrocarbons like H2, CH4, C2H6 and so on.


91

During the aging process, oil samples are taken by particular glass syringes and are sealed

hermetically. Then the oil samples are transferred into the DGA apparatus to measure the

dissolved gas contents. The DGA experiments refer to ASTM D3612 Standard Test Method

for Analysis of Gases Dissolved in Electrical Insulating Oil by Gas Chromatography[69].

Oil samples for each impregnated paper material are taken at 6h, 24h, 96h, 192h and 720h.

The gas contents of CH4, C2H6, C2H4, C2H2, H2, CO and CO2 are recorded. The test results

are shown in table 6-2.

Table 6-2. Results of DGA Measurements of Three Paper Impregnation FR3 Oils

Gas

Content

（uL/L）

T910 Kraft DMD

24h 96h 192h 720h 24h 96h 192h 720h 24h 96h 192h 720h

H2 43.7 10.6 30.7 494.8 39.8 7.7 14.7 584.2 26.6 3.6 28.9 284.4

CH4 38.1 16.1 29.2 51.1 44.7 19.3 23.4 46.9 50.5 24.7 83.3 31.2

C2H6 277.8 394.1 366.4 256.8 379.7 213.4 179.1 311.6 383.4 242.9 782.8 205.6

C2H4 11.6 4.2 5.2 18.4 8.4 4.6 4.2 16.4 7.1 4.9 17.4 11.7

C2H2 0 0 0 0 0 0 0 0 0 0 0 0

CO 595.7 286.5 372.2 2335 658.7 409.9 468.5 2820.0 485.2 209.4 765.6 321.9

CO2 18596 18089 12833 88203 16861 11867 10394 71611 4114 5010 12314 33782

TDCG 327.5 416.4 206.6 326.4 432.8 237.3 206.6 374.9 441.1 272.6 883.5 248.5

The changes of the content of each type of gas along aging time are plotted in figure 6-5.

(a) FR3 Oil Impregnating T910 Paper (b) FR3 Oil Impregnating Kraft Paper


92

(c) FR3 Oil Impregnating DMD Paper

Figure 6-5. Changing Trends of Dissolved Gases in FR3 Oils of Three Kinds of Paper

From the DGA results, we can observe that three FR3 oils all have apparently high values in

C2H6 and CO content along the entire aging periods. In most cases, the content level of CO

is higher than or close with the content level of C2H6. Only at the end of aging, apparent H2

contents are detected in three FR3 oils. The contents of CH4 and C2H4 are slight and all

below 50 uL/L. And C2H2 is entirely absent in all three FR3 oils all the time.

The changing trends of the gases dissolved in three FR3 oils also differ a little. Kraft paper

impregnation FR3 oil has similar performance in DGA with that of T910 paper impregnation

FR3 oil. Except C2H6, the rest gases all reach their peak values at the end of aging. Among

them, the magnitude of the changing of CO is the most dramatic. By the end of aging, the

CO contents of Kraft and T910 paper impregnation FR3 oils reach 2335 uL/L and 2880 uL/L

respectively. As for C2H6, its content levels first increase and then decline, and reach

maximum values at 192h. However, the DGA behavior of DMD paper impregnation FR3

oil is quite different. Except H2, all other gases reach the peak gas content levels at 192h and

the second peak values at 24h. The changing trend of C2H6 has very high similarity with CO.

If we compare the differences of the gas content levels among three FR3 oils at each aging

time node, the following figure 6-6 could be derived.


93

(a) 6h (b) 24h

(c) 96h (d) 192h

(e)720h

Figure 6-6 Comparisons Among the DGA Results of Three Paper Impregnation FR3 Oils at Different

Aging Nodes

It could be seen from the plots that there are little differences on the gas distribution of three

paper impregnation FR3 oils. But from 96h on, these differences are more significant. At

96h, the FR3 oil impregnating T910 paper shows higher C2H6 content and that of Kraft paper

shows higher CO content. At 192h, the FR3 oil impregnating DMD paper alternatively

shows the highest content of C2H6 and CO. By the end of aging, the contents of H2 in three


94

oils are more remarkable and the C2H6 and CO contents in FR3 oils impregnating T910 and

Kraft paper are higher.

DGA is an important approach to interpret the insulation oil condition and could be used to

accurately determine the fault type of a transformer. It has been successfully applied widely

in mineral oil applications. There are several diagnose methods raised up and approved by

international standards, including gas condition, Rogers ratio, Doerenburg ratio, CO2/CO

ratio and key gas methods given by IEEE[70], and Duval triangle and basic ratios methods

given by IEC[71]. However, for novel vegetable oils, these methods are not all appropriate

for vegetable oil and may require some modifications. Duval took the lead to modify his

triangle aiming at vegetable oils, specifically for FR3 oil. The manufacturer of FR3, Cooper

Power System[72], also gives suggestions with consideration of on-site and laboratory

experiences, which recommends the Duval triangle method most and considers IEC basic

ratio method as same suitable diagnose approach. Therefore, the Duval triangle and IEC

basic ratio methods are adopted to diagnose the experimental DGA results.

The Duval triangle method[73] determines the fault type according to the relative

percentages among CH4, C2H4 and C2H2 themselves. The three sides of the triangle refer to

the proportions of CH4, C2H4 and C2H2 occupying their own total content amount

respectively, ranging from 0% to 100%. Depending on the differences of the relative

contents of three gases, the triangle is divided into several regions, corresponding to different

fault zones. The fault types are PD(partial discharge), D1(low energy charging), D2(high

energy charging), T1(thermal fault <300 ℃ ), T2(thermal fault 300 ℃ ~700 ℃ ) and

T3(thermal fault>700℃). There are #1~#5 five Duval triangles depending on the type of

electrical apparatus and oil type. For non-mineral oils, the triangle is collectively called as

#3 Duval triangle. There are several triangles with small differences in this group to be in

line with different kinds of vegetable oil or synthetic esters. The specific Duval triangle for

FR3 oil is shown in figure 6-7. The main differences with other triangles are the boundaries

among different types of fault.


95

Figure 6-7 Duval Triangle for FR3 Vegetable Oil

Applying the Duval triangle method to diagnose the DGA experimental results, the obtained

triangle is shown in figure 6-8.

T910 6h

T910 24h

T910 96h

T910 192h

Kraft 6h

Kraft 24h

Kraft 96h

Kraft 192h

DMD 6h

DMD 24h

DMD 96h

DMD 192h

DMD 720hKraft 720hT910 720h

Figure 6-8 Diagnose Result of DGA Experiment by Duval Triangle

Due to the fact that in all cases FR3 oils do not contain any C2H2, all data are extrapolated

lying on the side of C2H4%. And for all situations, the CH4 percentages range within

77%~87%. Therefore, as indicated by the dots plotted on the triangle, all data lie in T1 region,

that is the thermal fault with temperature <300℃. The diagnose result conforms to the real

experiment condition. Thereby, the conclusion that Duval #3 triangle is suitable for

diagnosing DGA experimental results of FR3 oil could be drawn.


96

The IEC basic ratio methods is to compare the relationships among three gas content ratios,

which are C2H2/ C2H4, CH4/H2 and C2H4/C2H6. The fault type is determined by different

ratio categories. The fault types are differentiated as the same with Duval triangle. The fault

types and criteria are shown in table 6-3.

Table 6-3 Criteria of IEC Basic Ratio Diagnose Method

Fault Type C2H2/ C2H4 CH4/H2 C2H4/C2H6

PD Not significant <0.1 <0.2

D1 >1 0.1~0.5 >1

D2 0.6~2.5 0.1~1 >2

T1 Not significant >1, but not significant <1

T2 <0.1 >1 1~4

T3 <0.2 >1 >4

The differences of the fault criteria mainly lie in the differences of the ratio ranges of C2H2/

C2H4 and C2H4/C2H6. By calculating and organizing the obtained gas content results, the

diagnose results are shown in table 6-4.

Table 6-4 Diagnose Results of FR3 Oils by IEC Basic Ratio Method

C2H2/ C2H4 CH4/H2 C2H4/C2H6 Fault type

T910

6h 0 2.130 0.009 T1

24h 0 0.872 0.042 T1

96h 0 1.510 0.011 T1

192h 0 0.952 0.014 T1

720h 0 0.103 0.072 T1/PD

Kraft

6h 0 1.456 0.007 T1

24h 0 1.123 0.022 T1

96h 0 2.522 0.021 T1

192h 0 1.584 0.024 T1

720h 0 0.080 0.053 T1/PD

DMD

6h 0 1.107 0.005 T1

24h 0 1.904 0.019 T1

96h 0 6.869 0.020 T1

192h 0 2.877 0.022 T1

720h 0 0.110 0.057 T1/PD


97

From the results, it could be observed that, under the diagnose of this method, all the data

except 720h case are extrapolated as T1 thermal fault, while the data of 720h is interpreted

as whether T1 thermal fault or PD fault. Therefore, IEC basic ratio method is suitable for

most experimental results in the thesis except for the small inconsistency at 720h data with

Duval triangle diagnose result.

For traditional mineral oil, the ratio of CO2/CO is commonly used to reflect the aging

condition of insulation paper. Under normal conditions, this ratio should be above 7. Once

the cellulose starts to degrade, to be more specific, which refers mainly to oxidization and

pyrolysis, the amount of CO will increase dramatically leading to a drop in this ratio. IEC

and IEEE both take the limitation of below 3 of the CO2/CO ratio as the signal of an apparent

occurrence of cellulose deterioration. However, the vegetable oil itself would generate a

large amount of carbon oxides, which could shows several orders of magnitude higher than

that generated by mineral oil. The level of carbon oxides contents generated by vegetable oil

may prevail the level of gas generated by cellulose. From the results, we can calculate the

CO2/CO ratios of FR3 oils impregnating T910 and Kraft papers, which are both above 7 and

most of which are higher than 20. Therefore, according to the standards, it could not be

considered that the cellulose undergoes an apparent aging. However, this is obviously

inconsistent with the apparent drop in tensile strength of Kraft paper. This is probably due

to that the main degrading form of the cellulose fibers is hydrolysis and will not generate a

large amount of CO. Besides, the determination of CO2 may be interfered by the gases in the

testing environment. Errors could be induced by the air intrusion during the sample

transferring and testing. Overall, the levels calculated from the DGA test results could not

accurately reveal the real deterioration condition of insulation paper.

Discussion on the Aging Processes and Mechanisms of High Temperature Resistant Insulation System

98

7 Discussion on the Aging Processes and Mechanisms of

High Temperature Resistant Insulation System

The aging of oil and paper materials is a complex process and should be analyzed integrally

in combination with several parameters. Based on the aging experimental results, this

chapter mainly aims at analyzing and extropolating the mechanisms of the aging of paper

and oil materials.

7.1 Analysis of Aging Mechanisms of Paper and Oil

Mmaterials

From the changes in vegetable oils viscosity, it could be determined that oxygen has limited

impact on paper and oil aging. The aging of paper and oil is dominated by hydrolysis

thereafter. Both paper and oil undergo hydrolysis.

For cellulose based paper, the hydrolysis of paper will incur fractures on its molecular chain

and hydroxyl groups are attached to the ends of fracture. Thus the hydrophicility of paper is

increased. The hydrolysis process of cellulose is catalyzed by low molecular water-solutable

acids. After hydrolysis, a dehydration process will follow and happen to cellulose itself.

Furanic compounds intermediate products are produced, along with more water molecules

production. Theoretically speaking, the hydrolysis of cellulose is initiated by one water

molecule and followed by three water molecules production in the dehydration stage. Thus,

there is a two water molecules net production and they behave as reaction source for further

hydrolysis[14]. Some unstable furanic compounds, e.g. 2FAL, will deform into low

molecular acids and then in turn the cellulose hydrolysis rate is auto-accelerated. The

hydrolysis of cellulose and PET are described in figure 1-8 and figure 1-11 in chapter 1.

When molecular chain fracture numbers reach to some certain extents, the long molecular

chain is seperated into several short molecular chains. The former strong chemical bonding

effect is transformed into weaker hydrogen bonding effect. The hydrolysis positions on

molecular chains become to weak links. Therefore, a drop in tensile strength could be

observed macroscopically, especially for Kraft paper.


99

Although the fiber molecular chain is depolymerized into short fibers, the arrangement of

these short fibers are still dense enough. The paper sample is a product of superimposition

of several layers of fiber. A weak and penetrating conductive channel is still hard to form.

The paper fibers still show a high resistant ability for eletron movements. Therefore, at the

end of aging, all paper samples still maintain a high level breakdown voltage.

The hydrolysis of vegetable oil will open up the ester bonds on triglycerides and diglycerides,

monoglycerides and glycerin are formed step by step. One high molecular fatty acid would

be generated at each deformation step. The hydrolysis process of vegetable oil is shown in

figure 7-1.

Figure 7-1 Schematic Diagram of Hydrolysis of Vegetable Oil

As aging carries on, the total acid number of oil would increase continuously and

significantly. Most of these acids are high molecular acids that will do little harm on paper

aging. Thus, the threats from vegetable oil on insulation paper health are limited. The fatty

acids produced by hydrolysis will further induce fractures on C-C or C-H bonds under high

temperature. Some free radicals, such as H· and CH·, are formed and combined with each

other to form several kinds of dissolved gas.

The overall aging routes of paper and oil materials in experiments are indicated by the block

diagram, as shown in figure 7-2.


100

Figure 7-2 Block Diagram of Aging Routes of Paper and Oil Materials

7.2 Analysis of Longer Remaining Lifespan of T910 and

DMD Paper

As one of the most effective parameters to indicate the insulation paper aging condition in

real applications, the changes in tensile strength are helpful to determine the insulation

operation condition and residual lifespan of transformers. In the experiment of this thesis,

there exists big differences in the changes of tensile strength during the aging of three kinds

of paper. Kraft paper shows a continuous and most obvious decline, but T910 paper that also

contains 70% cellulose and DMD paper whose components are PET macromolecules both

remain higher residual tensile strength. By the end of aging, the retained tensile strength of

Kraft paper is just around 60%, but T910 and DMD paper remain a level as high as 90%.

From the hydrolysis of DMD paper, as shown in figure 1-11, we know that hydroxyl and

carbonxyl groups are formed at both ends of the fracture position of PET macromolecular

chain, and thus DMD paper’s hydrophicility is enhanced. As for T910 paper, since 70%

components are cellulose, the cellulose part will also go through apparent hydrolysis. Chain

scissions caused by hydrolysis will lead to the obvious drop in tensile strength of Kraft paper,

but have little impacts on T910 and DMD paper. This is probably due to the NOMEX fiber

component of T910 paper and the particular structures of DMD paper and FR3 oil.


101

For T910 paper, its external layers are mixed woven by NOMEX fibers and cellulose fibers.

From the characteristics of NOMEX fiber and its aging process, we know NOMEX fibers

have extremly high machanical and dielectric strength and the chain scissions of NOMEX

fibers happen only under high temperature(>250℃). It could be considered that under the

experimental condition of this thesis, no apparent declines in mechanical strength will occur

to NOMEX fibers. Although the cellulose fibers in T910 paper undergoes severe chain

scissions, the external NOMEX fibers would function as protective and bearing effect due

to its own high tensile strength and thermal resistance, as the role of steel reinforcement in

bridge. When nearby cellulose fibers are broken up, NOMEX fibers could steel maintain its

molecular chain integrity. The robust and stable chemical bonding of NOMEX fiber under

150℃ would provide enduring bracing of T910 paper. Therefore, T910 paper could own a

high level of retained tensile strength along the entire aging period.

For DMD paper, since PET and FR3 molecules both own ester bonds on their backbones,

there would exist more obvious and strong transesterification effect when they are

hydrolyzed simultaneously. The transesterification effect was first proposed to explain the

prolonged lifespan of Kraft paper in vegetable oils and has been introduced in section 1.4 in

chapter 1. The long chain fatty acids hydrolyzed from triglycerides are esterified with the

hydroxyl groups on cellulose. Thus long side chains are attached onto the cellulose chain

and prevent cellulose from the intrusion of water, and the life of cellulose based paper is

extended. The hydrolysis of PET and triglycerides will both create hydroxyl and carbonxyl

groups on each ends. The three hydroxyls on glycerin produced by hydrolysis of

triglycerides could react and esterify with carbonxyls on several ends produced by hydrolysis

of PET. The backbone of glycerin will reconnect the broken-up PET molecules, which

functions as bridging and crosslinking. The broken PET molecules are strengthened in this

way. The transesterification process is described in figure 7-3. Unlike the transesterification

between FR3 oil and Kraft paper, the molecular chain of PET could be strengthened as

bridged or crosslinked during the transesterification. The strengthening effect is more robust

than that in Kraft paper. Therefore, DMD paper shows a more stable tensile strength

performance.


102

CH2

CH2

CH2

OH

OH

OH

R1-COOH

R2-COOH

CH2

CH2

CH2

OOCR1

OOCR2

OOCR3R3-COOH

3H2O

Figure 7-3 Transesterification Process Between DMD Paper and FR3 Oil

By the end of 720h aging under 150℃, T910 and DMD paper show obviously higher

retained tensile strength over Kraft paper in FR3 vegetable oil impregnation. As the tensile

strength is a key parameter to indicate insulation paper lifespan, a qualitative conclusion

could be drawn that T910 and DMD paper have better temperature resistant behavior and

longer anticipated lifespan. Nevertheless, some researches[74] have reported that in mineral

oil impregnation, under 150℃ or higher temperatures, DMD paper may occurs severe

damages and lose entire mechanical strength in the same aging time span. In the FR3

vegetable oil impregnation, after 864h aging under 180℃, DMD paper shows delamination

phenonmenon. This is different from what is observed in the experiment of this thesis and

this difference is probably caused by the limitation of aging time in the thesis. The thorough

aging behavior of aging materials is not revealed sufficiently. Therefore, the aging

experimental results are not sufficient to reflect the comprehensive differences on the high-

temperature tolerability of T910, DMD and conventional Kraft paper. It needs further

investigations in the future to confirm the suitability of these materials on the enhancement

of transformer anti-overloading ability.

Conclusions and Prospects

103

8 Conclusions and Prospects

8.1 Conclusions of the Thesis

The thesis analyzes the fundamental properies of T910, TUK and Kraft paper and performs

acceleareted aging test under 150℃ on T910, DMD and Kraft paper impregnated in FR3

vegetable oil. The following conclusions could be drawn.

(1) The fundamental properties comparison test results show that T910 paper has lower

density and water content and higher oil absorption value. On initial tensile strength,

TUK and Kraft paper show higher level, but T910 still meets the requirement of relevant

standard. Compared with T910 paper, TUK and Kraft paper have lower dissipation factor.

And relative dielectric constants of three paper increase as the thickness goes up.

Moreover, T910 paper owns higher breakdown electric field in air and most oil

impregnation cases. And the impact of temperature rising on paper breakdown strength

in mineral oil impregnation is more significant. For insulation oil initial physiochemical

properties, all vegetable oils show higher water content, viscosity and total acid number.

(2) The thesis analyzes and compares the accelerated aging performances of T910, DMD

and Kraft papers in FR3 vegetable oil impregnation under 150℃ for up to 720h. As aging

going on, there is no significant change in paper breakdown voltage, whose fluctuation

magnitude during aging is no larger than 10%. The Kraft paper shows obvious and

monotonous decline in tensile strength, retaining 60% of initial tensile strength at the

end of aging. T910 and DMD papers both maintain residual tensile strengths higher than

90% along the entire aging duration. The total acid numbers of FR3 oils all keep

increasing and the viscosities exhibit rather stable levels. The results of DGA

measurements show that all three FR3 oils impregnating three kinds of paper dissolve

C2H6 and CO in large quantities during the aging process, and the results are diagnosed

as T1 thermal fault by IEC standard.

(3) The NOMEX fibers contained in T910 paper provide a supportive effect as the fibers

possess high mechanical strength and high temperature resistance. There exists

Conclusions and Prospects

104

transesterification process during the hydrolysis aging of DMD paper in FR3 oil, the

broken PET molecules are bridged or crosslinked and thus its structural strength is

enhanced. Therefore, by the end of 720h aging, the tensile strengths of T910 and DMD

papers remain high levels. Although the retained tensile strengths of T910 and DMD

papers are higher than Kraft paper, due to the limitation of aging duration, the thorough

aging performances of different paper materials are not revealed sufficiently. It needs

further investigations to confirm their high temperature resistance.

8.2 Prospects of the Thesis

(1) The thesis only performs the aging experiment under one single temperature and has not

observed a decline of tensile strength below 50% retained tensile strength. Thus a longer

aging duration is anticipated in order to investigate the changes of the properties of oil

and paper materials more sufficiently and comprehensively.

(2) The single temperature aging experiment could not derive the thermal class and the

thermal index of the aged materials directly. According to relevant standards, at least

three temperature levels should be implemented with at least four aging time nodes

under each temperature level, in order to calculate the thermal index. Therefore, in future

works, more temperature levels and longer aging time could be supplemented to obtain

the thermal classes of the materials and to directly compare and verify the temperature

resistances.

(3) Calculate the temperature rise conditions at different positions on transformer winding

and in insulation oil. Investigate the impacts of winding temperature rising on the

lifespan of insulation paper. Thus, more proofs could be provided to verify the

improvement of transformer anti-overloading ability by applying high temperature

resistant insulation system.

Bibliography

105

9 Bibliography

[1]. Montsinger VM. Loading Transformers By Temperature[J]. American Institute of Electrical

Engineers, Transactions of the 1930;49(2):776-790.

[2]. McShane CP, Gauger GA, Luksich J. Fire resistant natural ester dielectric fluid and novel insulation

system for its use[C]. IEEE Transmission and Distribution Conference, Vols 1 & 2 1999:890-894.

[3]. Frimpong GK, Oommen TV, Asano R. A Survey of Aging Characteristics of Cellulose Insulation in

Natural Ester and Mineral Oil[J]. IEEE Electrical Insulation Magazine 2011;27(5):36-48.

[4]. Schaible M. Electrical Insulating Papers - an Overview[J]. Electrical Insulation Magazine, IEEE

1987;3(1):8-12.

[5]. Prevost TA, Oommen TV. Cellulose insulation in oil-filled power transformers: Part I - history and

development[J]. Electrical Insulation Magazine, IEEE 2006;22(1):28-35.

[6]. Ford JG, Leonard MG, Gainer GC, et al. Insuldur-Another Milestone in Transformer Insulation

Development[J]. Power Apparatus and Systems, Part III Transactions of the American Institute of

Electrical Engineers 1958;77(3):804-808.

[7]. Oommen TV, Prevost TA. Cellulose insulation in oil-filled power transformers: Part II - Maintaining

insulation integrity and life[J]. IEEE Electrical Insulation Magazine 2006;22(2):5-14.

[8]. Simpson RW. Flexible Laminates: Key Insulation for Motors and Generators[J]. Electrical Insulation

Magazine, IEEE 1986;2(1):9-13.

[9]. Emsley AM, Stevens GC. Review of Chemical Indicators of Degradation of Cellulosic Electrical

Paper Insulation in Oil-Filled Transformers[J]. IEEE P-Sci Meas Tech 1994;141(5):324-334.

[10]. Cigre D1.01.10 Ageing of Cellulose in Mineral-oil insulated Transformers[R]. 2007.

[11]. Prevost TA. Thermally upgraded insulation in transformers[C]. Proceedings: Electrical Insulation

Conference and Electrical Manufacturing Conference 2005:120-125.

[12]. Kanuga, K. Degradation of polyester film exposed to accelerated indoor damp heat aging[J].

Photovoltaic Specialists Conference (PVSC), 37th IEEE; 2011 19-24.

[13]. Cigre A2-35.Experiences in Service with New Insulating Liquids[R].2010.

[14]. Lundgaard LE, Hansen W, Linhjell D, et al. Aging of oil-impregnated paper in power transformers[J].

IEEE Transactions on Power Deliver 2004;19(1):230-239.

[15]. Lundgaard L E HW, Ingebrugtsn S.et al. Aging of Kraft Paper by Acid Catalyed Hydrolysis[C]. IEEE

International Conference on Dielectric Liguids, 2005:281-384.

[16]. Lundgaard LE, Hansen W, Ingebrigtsen S. Ageing of mineral oil impregnated cellulose by acid

catalysis[J]. IEEE Transactions on Dielect Electric Insulation 2008;15(2):540-546.

[17]. Lelekakis N, Martin D, Wijaya J. Ageing Rate of Paper Insulation used in Power Transformers Part

1: Oil/paper System with Low Oxygen Concentration[J]. IEEE Transactions on Dielect Electric

Insulation 2012;19(6):1999-2008.

[18]. Emsley AM, Xiao X, Heywood RJ, et al. Degradation of cellulosic insulation in power transformers.

Part 3: effects of oxygen and water on ageing in oil[C]. Science, Measurement and Technology, IEE

Proceedings - 2000;147(3):115-119.

[19]. Emsley AM. The kinetics and mechanism of degradation of cellulosic insulation in power

transformers[J]. Polymers Degradation and Stability 1994;44:343-349.

[20]. Furanic Compounds in Dielectric Liquid Samples: Review and Update of Diagnostic Interpretation

and Estimation of hulation Ageing[C]. 7th lntemational Conference on Properties and Applications

of Dielectric Materials; 2003.

[21]. Emsley AM, Xiao X, Heywood RJ, et al. Degradation of cellulosic insulation in power transformers.

Part 2: Formation of furan products in insulating oil[J]. IEEE P-Sci Meas Tech 2000;147(3):110-114.

[22]. Villain F, Coudane J, Vert M. Thermal-Degradation of Polyethylene Terephthalate - Study of

Polymer Stabilization[J]. Polymer Degradation and Stability 1995;49(3):393-397.

[23]. Samperi F, Puglisi C, Alicata R, et al. Thermal degradation of poly(ethylene terephthalate) at the

processing temperature[J]. Polymer Degradation and Stability 2004;83(1):3-10.

[24]. D. A. Chatfield, R.W.Mickelson,and J.H.Futrell. Analysis of the Products of Thermal Decomposition

of an Aromatic Polyamide Fabric[J]. Polymer Science 1979;17:1367-1381.

[25]. Y.P.K. and E.M.P, et al. Thermal Degradation of Some Aromatic Polyamides and Their Model

Diamides[J]. Polymer Science 1981;19:2817-2834.

Bibliography

106

[26]. Villar-Rodil S, Paredes JI, Martinez-Alonso A, et al. Atomic force microscopy and infrared

spectroscopy studies of the thermal degradation of Nomex aramid fibers[J]. Chemical Material

2001;13(11):4297-4304.

[27]. Power J. and James.R.B.Thermal Degradation of Aramids:Part I--Pyrolysis/Gas

Chromatography/Mass Spectrometry of Poly(1,3-Phenylene Isophthalamide) and Pply(1,4-

Phenylene Terephthalamide)[J]. Polymer Degradation and Stability 1982;4:139-392.

[28]. Dakin TW. Electrical Insulation Deterioration Treated as a Chemical Rate Phenomenon[J]. American

Institute of Electrical Engineers, Transactions of the 1948;67(1):113-122.

[29]. Gasser, H. P. et al. Determining the ageing parameters of cellulosic insulation in a transformer[C].

High Voltage Engineering, 1999 Eleventh International Symposium on ; .

[30]. Heywood RJ, Emsley AM, Ali M. Degradation of cellulosic insulation in power transformers. Part

1: Factors affecting the measurement of the average viscometric degree of polymerisation of new

and aged electrical papers[J]. IEEE P-Sci Meas Tech 2000;147(2):86-90.

[31]. Krause C, Dreier L, Fehlmann A, et al. The Degree of Polymerization of Cellulosic Insulation:

Review of Measuring Technologies and its Significance on Equipment[C]. 2014 Electrical Insulation

Conference 2014:267-271.

[32]. Emsley AM, Heywood RJ, Ali M, et al. Degradation of cellulosic insulation in power transformers.

Part 4: Effects of ageing on the tensile strength of paper[J]. IEEE P-Sci Meas Tech 2000;147(6):285-

290.

[33]. Khan IU, Wang ZD, Cotton I, et al. Dissolved gas analysis of alternative fluids for power

transformers[J]. IEEE Electrical Insulation Magzine. 2007;23(5):5-14.

[34]. Oommen TV. Vegetable Oils for Liquid-Filled Transformers[J]. IEEE Electrical Insulation Magzine.

2002;18(1): 6-11.

[35]. Fofana I. 50 years in the development of insulating liquids[J]. Electrical Insulation Magazine, IEEE

2013;29(5):13-25.

[36]. Stockton DP, Bland JR, McClanahan T, et al. Seed-oil-based coolants for transformers[J]. Industry

Applications Magazine, IEEE 2009;15(1):68-74.

[37]. IEC 62332-2. ELECTRICAL INSULATION SYSTEMS (EIS) – THERMAL EVALUATION OF

COMBINED LIQUID AND SOLID COMPONENTS -Part 2: Simplified Test[S]. 2013.

[38]. IEEE Standard Test Procedure for Thermal Evaluation of Insulation Systems for Liquid-Immersed

Distribution and Power Transformers[S]. C57.100. 2011.

[39]. IEC 62332-1. ELECTRICAL INSULATION SYSTEMS (EIS) – THERMAL EVALUATION OF

COMBINED LIQUID AND SOLID COMPONENTS -Part 2: General requirements[S]. 2005.

[40]. McShane, C. P. et al. Aging of paper insulation in natural ester dielectric fluid[C]. Transmission and

Distribution Conference and Exposition, 2001 IEEE/PES; 2001 2001.

[41]. Oommen TV, Claiborne C, Le H. BIOTEMP paper aging study[R]. Unpublished ABB ETI Report

Feb. 2001.

[42]. Nattrass D. Functional life test on transformers filled with a biodegradable electrical insulating fluid

(Biotemp)[R]. Unpublished ABB ETI Report Nov. 1998.

[43]. Lanni A. Functional life tests on revised BIOTEMP™ formulation[R]. Unpublished ABB ETI

Report Mar. 1999.

[44]. Liao RJ, Liang SW, Yang LJ, et al. The Improvement of Resisting Thermal Aging Performance for

Ester-immersed Paper Insulation and Study on Its Reason[C], 2008.

[45]. Tenbohlen S, Koch M. Aging Performance and Moisture Solubility of Vegetable Oils for Power

Transformers[J]. IEEE T Power Deliver 2010;25(2):825-830.

[46]. Jr. RA, Cheim L, Cherry D, et al. Thermal evaluation of cellulosic board in natural ester fluid for

hybrid insulation systems[C]. 78th Annual International Conference of Doble Clients, Boston, MA

2011.

[47]. Liao RJ, Liang SW, Yang LJ, et al. The Improvement of Resisting Thermal Aging Performance for

Ester-immersed Paper Insulation and Study on Its Reason[C]. 2008 IEEE Conference on Electrical

Insulation and Dielectric Phenomena 2008:633-636.

[48]. IEC 60126-4. Electrical insulating materials - Thermal endurance properties - Part 4-1: Ageing ovens

- Single-chamber ovens[S]. 2006.

[49]. ASTM D202 Standard Test Methods for Sampling and Testing Untreated Paper Used for Electrical

Insulation[S]. 2008. [50]. ASTM D644. Standard Test Method for Moisture Content of Paper and Paperboard by Oven

Bibliography

107

Drying[S]. 2007.

[51]. ASTM D3277. Standard Test Methods for Moisture Content of Oil-Impregnated Cellulosic

Insulation[S]. 2001.

[52]. PAGE DH. [J]TAPPI 1969;52:674.

[53]. ASTM D1305. Standard Specification for Electrical Insulating Paper and Paperboard—Sulfate

(Kraft) Layer Type[S]. 2009.

[54]. ASTM D3394. Standard Test Methods for Sampling and Testing Electrical Insulating Board[S]. 2009.

[55]. Kurtinec M, Hamar R, Pihera J. Complex Permittivity Models of Composite Dielectrics[J]. 2007

IEEE International Conference on Solid Dielectrics, Vols 1 and 2 2007:39-42.

[56]. Niklasson GA, Granqvist CG, Hunderi O. Effective medium models for the optical properties of

inhomogeneous materials[J]. Applied optics 1981;20(1):26-30.

[57]. Morgan VT. Effects of frequency, temperature, compression, and air pressure on the dielectric

properties of a multilayer stack of dry kraft paper[J]. IEEE Transactions on Dielectric Electrical

Insulation 1998;5(1):125-131.

[58]. Stuart 0. Nelson.Permittivity and density relationships for granular and powdered materials[C].

Antennas and Propagation Society International Symposium, 2004 IEEE; 2004 20-25 June 2004.

[59]. ASTM D149. Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of

Solid Electrical Insulating Materials at Commercial Power Frequencies[S]. 2013.

[60]. ASTM D445. Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids

(and Calculation of Dynamic Viscosity) [S]. 2014.

[61]. ASTM D6871. Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical

Apparatus[S]. 2008.

[62]. IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers[S]. IEEE Std

C57147-2008 2008:1-31.

[63]. IEC 62770.Fluids for electrotechnical applications - Unused natural esters for transformers and

similar electrical equipment[S]. 2013

[64]. ASTM D664. Standard Test Method for Acid Number of Petroleum Products by Potentiometric

Titration[S]. 2011.

[65]. ASTM D1533. Standard Test Method for Water in Insulating Liquids by Coulometric Karl Fischer

Titration[S]. 2012.

[66]. R J Young, PJ Lovell. Introduction to Polymers[M], Nelson Thornes: Cheltenham, 2000.

[67]. E Souza, G Belinato, R Otero, et al. Thermal Oxidative Stability of Vegetable Oils as Metal Heat

Treatment Quenchants[J]. Journal of ASTM International, 2011, 9(1):1-30.

[68]. Rooney D, Weatherley LR. The effect of reaction conditions upon lipase catalysed hydrolysis of high

oleate sunflower oil in a stirred liquid-liquid reactor[J]. Process Biochemistry 2001;36(10):947-953.

[69]. ASTM D3612. Standard Test Method for Analysis of Gases Dissolved in Electrical Insulating Oil by

Gas Chromatography[S]. 2009.

[70]. IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers[S]. IEEE Std

C57104-2008:1-36.

[71]. IEC 60599. Mineral oil-impregnated electrical equipment in service — Guide to the interpretation

of dissolved and free gases analysis[S]. 1999.

[72]. Dissolved Gas Guide of Envirotemp FR3 Fluid[R]. Cooper Power System. 2006.

[73]. Duval M. The Duval Triangle for Load Tap Changers, Non-Mineral Oils and Low Temperature

Faults in Transformers[J]. IEEE Electrc Insulation Magzine 2008;24(6):22-29.

[74]. Xiaojing Zhang, et.al. High-temperatrue Insulation System Thermal Evaluation for Fluid Filled

Transformers[C]. IEEE Electrical Insulation Conference. unpublished paper. 2016

Appendix

108

Appendix

Weibull Distribution

The failures of solid insulation materials could be interpreted by extreme statistics, such as

Weibull distribution and Gumbel distribution. Logarithmic function is also used once.

Weibull distribution, as most frequently utilized, has extensive applicability and is a kind of

extreme value distribution when a system breaks up at its weakest link.

Weibull distribution is a statistical theory to describe phenomena of chain-shaped

vulnerability damages, that is the whole system fails when its weakest link is broken up.

The cumulative probability function of double parameter Weibull distribution is:

, , 1 exp{ ( ) }t mF t m

(A1-1)

Where,

t ——the variable under test；

F(t) ——failure probability corresponding to t。

ŋ ——size parameter；

m ——shape parameter.

The size parameter ŋ refers to the failure time or specific quantity, where the failure

probability is 0.632. ŋ could be voltage, electric field and time et.al. The shape parameter m

is a reflection of the range of failure time or voltage. The greater the m, the more

concentrated the time or voltage data.

The double parameter Weibull distribution is a particular form of more common three

parameter Weibull distribution.

1 exp{ ( ) }t mF t

t (A1-2)

Appendix

109

The new parameter γ is called position parameter. When t<γ, F(t)=0 and it means the failure

probability is zero. When voltage or time is smaller than a particular value, the sample will

not be broken up.

Theoretical reseaches and experimental results both prove that the reliability analysis is

influenced by some factors, e.g the sample amount of the electric breakdown experiment and

the estimation method of the Weibull distribution parameters. The most common method to

estimate the parameters is the ridge regression. One best fitting straight line is found by least

squares method, so that the sum of the variances is minimum. The ridge regression is

considered as the standard estimation approach, due to the high accuracy of most suitable

datasets.

By taking twice natural logarithmic transformations to formula (A1-1), a linear equation is

derived:

1

ln ln ln ln1 ( )

Y m t mF t

(A1-3)

Assuming the sample size is n, F(i,n) is ranked in a ascending manner and i is the sequence

number. There are four main ridge regression estimation methods of the failure probability.

0.5

( , ) 100%i

F i nn

(A1-4a)

( , ) 100%1

iF i n

n

(A1-4b)

0.3

( , ) 100%0.4

iF i n

n

(A1-4c)

0.44

( , ) 100%0.25

iF i n

n

(A1-4d)

From (A1-3), the slope of the straight line is m and the intercept of y axis is mlnη. Y and lnt

has a linear relationship and the unknown parameters could be estimated by the linear least

square method. The failure probability estimator 0.44

( , ) 100%0.25

iF i n

n

is found to have

the highest accuracy and convenience, and therefore is considered as the best method in

engineering designs.

Acknowledge

110

Acknowledges

My deepest gratitude goes first and foremost to my supervisor, Prof. Giovanni

Dotelli, for his instructive advices, corrections and useful suggestions on my

thesis. I am deeply grateful of his help in the completion of this thesis. Without

his continuous and illuminating instruction, this thesis could not have reached

its present form.

High tribute shall be paid to Mr. Saverio Latorrata. The many times discussions

with him about the thesis enlighten my ideas very much and his kind helps and

suggestions have made my thesis completed more smoothly and effectively.

Special thanks shall go to my friends who have put considerable time and efforts

into their comments on the draft.

Finally, I am indebted to my belove parents and girlfriend for their consistent

supports and encouragements.

POLITECNICO DI MILANO Scuola di Ingegneria Industriale … · POLITECNICO DI MILANO Scuola di...

Documents

Transcript of POLITECNICO DI MILANO Scuola di Ingegneria Industriale … · POLITECNICO DI MILANO Scuola di...