I

Yarmouk University

Hijjawi Faculty for Engineering Technology

Detection of electrical treeing in polyester exposed to

electrical and mechanical stress

Done By:

Muna AL Zoubi 2011975138

Nisreen AL Naji 2011975129

Aseel Hamdan 2010975011

Advisor

Dr. M. H. AL Zoubi

March 2015

II

DEDICATION

We dedicate our work and effort to our families and

friends. A special feeling of gratitude to our loving

parents whose words of encouragement and push for

tenacity ring our ears.

III

Abstract

In this project, the impact of mechanical stress on the

treeing process is studied. The results achieved in this

project are based on experimental work executed in the

department's laboratory. All composite samples, used in

the experiments, were prepared by the authors in the

department of archaeology and anthropology. An optical

system is used to observe and monitor the tree growth.

Also a physical model is used to apply the mechanical

stress on the wanted specimens. All the specimens were

maintained under observation and pictures were taken

and saved in a computer connected to the optical system.

The pictures were studied and compared to conclude the

relationship between the mechanical stress and the

breakdown process. The tree growth in pure polyester

resin were made as a reference to compare other

samples with. Therefore the applied mechanical stress

significantly decrease the lifetime of the insulation in

other words it speeds up the treeing process.

IV

Table of Contents CHAPTER ONE INTRODUCTION.......................................................... 1

1.1 Background ........................................................................................ 2

1.2 General overview of insulation material. .......................................... 2

1.3 basic electrical properties of dielectric materials. ............................. 3

1.3.1 Dielectric breakdown strength (DBS). ........................................... 3

1.3.2 Breakdown voltage. ....................................................................... 4

1.3.3 Non-electrical properties of dielectric. .......................................... 4

1.4 Classes of insulation. ......................................................................... 4

1.5 Solid dielectric ................................................................................... 5

1.5.1 Types of solid insulation materials. ............................................... 6

CHAPTER TWO TREEING ...................................................................... 7

2.1 Electrical treeing ................................................................................ 8

2.1.1 Treeing stages................................................................................. 8

2.2 Insulation breakdown ...................................................................... 10

2.2.1 Short-time mechanism ................................................................. 11

2.2.2 Long-time mechanism .................................................................. 13

CHAPTER THREE EXPEREMINTS SETUP ........................................ 15

Sample preparation ................................................................................ 16

3.1: Materials ......................................................................................... 16

3.2: Method ............................................................................................ 17

CHAPTER FOUR RESULTS AND ANALYSIS ................................... 22

4.1 The function of the barrier ............................................................... 23

4.2 Microscopic of samples ................................................................... 23

4.2.1 Tree growth in sample 1 (without barrier and

withoutmechanical stress): ……………………………………..24

4.2.2 Tree growth in sample 2 (with barrier and without mechanical

stress): …………………………………...……………….………… 25

V

4.2.3: Tree growth in sample 3 (with barrier and with mechanical

stress): .................................................................................................. 27

4.3 Analyses of electrical tree in samples ............................................. 28

CHAPTER FIVE CONCLUSION ........................................................... 31

5.1: Conclusions .................................................................................... 32

References:................................................................................................ 33

1

CHAPTER ONE

INTRODUCTION

2

1.1 Background

Polymeric material is a commonly used material as insulation of high

voltage cables due to excellent electrical thermal and mechanical

properties.

One of the main reasons for the long term degradation and breakdown of

polymeric insulation when exposed to electrical stress is the electrical

treeing [3].

The researches on the properties of electrical trees have been conducted

since 1950s to determine more efficient insulation systems since

insulators are key elements in transmission and distribution systems [1],

but to evaluate the insulation material it is necessary to take in to account

the mechanical and environmental conditions of the application because

mechanical failure often leads to electrical failure [3].

Needle plane composite insulation were made from polyester are used for

initiation of electrical trees, both for electrical and mechanical stress.

1.2 General overview of insulation material.

The main difference between a conductor and a dielectric lies in the

availability of free electrons in the outermost atomic shells to conduct

current.

So insulation material minimize or prevent leakage current from high

voltage conductors to the grounded conductors. Although the charges in a

dielectric are not able to move about freely, they are bound by finite

forces and a displacement is expected when an external force is applied

[13].

3

This is why electrical breakdown of insulation happens when stressed

continuously with electric field.

Insulation materials are influenced by several factors, such as filling

material, morphology, mechanical stress, insulation thickness,

environments, these factors have different effect based on the insulator

function where insulators perform several important functions such as

mechanical support and heat transfer [3].

1.3 basic electrical properties of dielectric materials.

There are four types of insulators based on the material states and they

are solid, liquid, gas, and composite.

These states have different values for its electrical properties, the material

state is chosen according to the application and availability, and every

insulation material state has better values than others in some properties,

the good designer can select the optimum material for the present case

[3]. The properties of a dielectric material are outlined here.

1.3.1 Dielectric breakdown strength (DBS).

Dielectric strength is defined as the maximum allowable stress that the

material with stand before breakdown.

The dielectric strength depends upon the applied voltage, distance

between electrodes, pressure, temperature, humidity, impurities, nature

and configuration of the electrodes.

4

1.3.2 Breakdown voltage.

Breakdown voltage of a dielectric is defined as the minimum voltage that

cause a portion of a dielectric to become electrically conductive.

Breakdown voltage is always given in peak values insulators are

characterized by atoms with tightly bound electrons. The atomic forces

holding these electrons in place exceeds most outside voltages that might

induce electrons to flow. This force is finite however, and can always

potentially be exceeded by an external voltage, which will cause electrons

to flow at some rate through the substance [11].

1.3.3 Non-electrical properties of dielectric.

Density, specific capacity, thermal conductivity, chemical and thermal

stability, mechanical properties , toxicity, all of these chemical, physical

and mechanical properties need to be considered when a choice among

different dielectric material is performed, because the dielectric materials

have other functions such as mechanical support, thermal cooling and arc

quenching in addition to their function as electrical insulation[3].

1.4 Classes of insulation.

Insulation materials cab be classified according to the state of material or

according to the restoring of its properties or they can be classified into

external and internal.

According to the state, they can be classified into five main categories,

namely gas, liquid, solid, vacuum and composite.

5

According to the restoring of its properties, they can be self-restoring and

non-self-restoring; insulation that completely recovers insulating

properties after a disruptive discharge caused by the application of a

voltage is called self-restoring insulation and the non-self-restoring

insulation is the insulation that loses insulating properties or does not

recover completely after a disruptive discharge caused by the application

of a voltage. Examples of self-restoring insulations are vacuum and oil.

Examples of non-self-restoring insulations are XLPE and polyester.

According to external insulation and internal insulation. Examples of

external insulations are porcelain shell of a bushing and bus support

insulators. Examples of internal insulation is transformer insulation.

Equipment may be a combination of internal and external insulation.

Examples are a bushing and a circuit breaker [3].

1.5 Solid dielectric

Solid insulation cab be classified into three groups: organic materials,

inorganic materials and synthetic polymers, and simple, bonded, and

impregnated materials. They differ in their electrical, physical, chemical

properties, which are used as a guide in selecting the material appropriate

for each application but in general they have extremely high electrical

resistivity and high dielectric strengths below certain temperatures. Solid

dielectrics have high breakdown strength as compared to liquids and

gases.

6

1.5.1 Types of solid insulation materials.

These material include paper, fibers, mica and its products, glass,

ceramics, rubber, plastics, polyethylene, nylon, PVC, polystyrenes, epoxy

resins and composites [11].

In this project, composite insulation is used.

1.5.1.1 Composite insulation

Excellent performance and reliability can be assured for the lifetime of

electrical equipment by introducing composite insulators into line service.

In many engineering applications, more than one type of insulation are

used together such as solid/liquid insulation, solid/ vacuum insulation and

solid/solid insulation.

Composite insulation are used in many applications such as High voltage

circuit breakers (live tank and dead tank), Surge arresters, Cable

terminations, Transformer bushings. Examples of composite insulations

are oil impregnated paper, oil impregnated metalized plastic film and

polyester resin reinforced by glass fiber.

The composite should be chemically stable and will not react with each

other under the application of combined thermal, mechanical and

electrical stresses over the expected life of the equipment.

They should also have nearly equal dielectric constant, because the

intensity of the electric field that determines the onset of breakdown and

the rate of increase of current before breakdown, it is very essential that

the electric stress should be properly estimated and distributed in a high

voltage apparatus using composites [3].

7

CHAPTER TWO

TREEING

8

2.1 Electrical treeing

Electrical treeing in a degradation phenomenon developing in polymer

material exposed to high electric fields. It is named for its shape being

similar to the nature trees [1].

Electrical treeing affects the practical life time of various power

equipment, it is a damaging process due to partial discharge or inclusions

and progresses through the stressed dielectric insulation, in a path

resembling the branches of a tree [2].

2.1.1 Treeing stages

Electrical treeing is a pre breakdown mechanism and it is a complicated

electro-erosion phenomenon and a consequence of several process

including: collision ionization, oxidation decomposition, partial

discharge, electro mechanical stress, physics deformation, chemical

decomposition, etc. also the level of research relies on the advancement

of experiment instrument not only on the human's understanding the

tree[1], but in general there are three stages of treeing development to

cause the final breakdown of insulation.

The first stage is the initiation stage before discernible damage is found in

the form of a small tube or cavity at the high stress point, large enough to

support partial discharges. Under continual ac field application the

electrical tree will propagate across the insulation following initiation.

During propagation the electrical tree can adopt complex forms [3].

2.1.1.1 Initiation

The starting point for a tree growth is the injection of charges due to an

electrode geometry producing high divergent field or by gas discharge in

9

voids. The evidence for this is the detection of electro-luminescence that

appears before partial discharges (PD's). The first tubule of electrical

trees is a product of a chain of events. The injected charges are

accumulated and for a certain level it is enables excitation of molecules

from some kind of energy release. It might be due to; there kinetic

energy, recombination of charges of opposite polarity or trapping. The

excitation or ionization enables bond breaking of the polymer chain due

to chemical reactions which makes the final damage needed for the tree

initiation channel[4],[5].

Mechanical properties such as tensile strength, elastic modulus and

fracture toughness has been proven to influence both initiation and tree

growth in polyester. A higher tensile stress, will give rise to a faster tree

growth [6].

2.1.1.2 Propagation

This stage starts at tree initiation and it ends when the first branch has

reached the opposite electrode.

There are two main categories of electrical tree structures; branch and

bush. The names are given by their geometrical shapes, the bush tree

being denser than the branch tree. They are attributed with different

discharge rates. Branch trees propagate with a much higher rate than the

bush [7], [8].

There are also difference in how branch and bush are formed.

For branch-trees the channel becomes conducting and it will suppress

partial discharges to take place inside the channel [9]. At the same time

charges are injected into the solid building a net space charge resulting in

11

a high local field at the tip of the channel. As a consequence partial

discharges arise at the channel tip and with the thermal energy released

from the discharges there is energy enough to degenerate the insulation

further [7]. The branch tree will extend in this way by stepwise

breakdown of the dielectric material.

For the bush tree the first channel is started as that of a branch tree. Then

branching will continue as a result of partial discharges throughout the

tree, from the electrode tip to the channel end. This is enabled by the lack

of conducting particles on the walls, i.e. there is enough resistance for a

voltage drop inside the channel [9]. Due to more branches created from

channels close to the electrode the bush will have a dense structure.

2.1.1.3 Runaway

The acceleration stage occurs when a tree is close to crossing over the

material to the other electrode. It is still not clear how the acceleration is

triggered. One explanation is that when the propagation has ceased

discharges may still be present in the tree, making the channel wider.

With larger voids bigger discharges are possible and this together with

the second electrode being close might give a sufficiently high electric

field at the branch tip, giving rise to the accelerating growth [4].

2.2 Insulation breakdown

Electrical breakdown is often associated with the failure of solid or liquid

insulating materials used inside high voltage transformers or capacitors in

the electricity distribution grid, usually resulting in a short circuit or a

blown fuse. Electrical breakdown can also occur across the insulators that

11

suspend overhead power lines, within underground power cables, or lines

arcing to nearby branches of trees [10].

Breakdown of solids may be caused by four mechanisms, two of them are

short-time mechanisms:

1. Intrinsic breakdown.

2. Thermal breakdown.

And two of them are long-time mechanisms:

1. Breakdown caused by partial discharge (PD).

2. Breakdown caused by inclusions.

Here adscription in details for these mechanisms:

2.2.1 Short-time mechanism

2.2.1.1Intrinsic breakdown

Intrinsic breakdown appears at extremely high field strength under ideal

laboratory conditions where all interfering effects have been prevented.

Measuring the intrinsic dielectric strength requires using pure material,

perfect electrodes, testing of small volume, and short test period.

These conditions lead to format a single avalanche, a free electron gets

enough energy above a certain field and electrons liberate from the outer

shell of the adjacent atoms by collisions. The energy of the electrons lost

during collisions, but when the energy gained by an electron exceeds the

ionization potential, more electrons will be liberated due to collision of

the first electron. The formation of an electron avalanche results due to

the repeated process, when the avalanche exceeds a certain critical size

breakdown will occur.

12

In practice, breakdown occurs by the formation of many avalanches

which are extending step by step through the entire thickness of the

material [11].

2.2.1.2 Thermal breakdown

When an electric field is applied to a dielectric, conduction current flows

through the material. Current heats up the specimen and the temperature

rises.

Heat generated is transferred to the surrounding medium by conduction

and radiation.

Excessive dielectric heating, either by dielectric losses in the case of AC

or by conductive losses in the case of D.C.

Equilibrium is reached when the heat generated (W.dc or W.ac) is equal

to the rate of cooling (heat dissipated) (Wt).

If the rate of heating exceeds the rate of cooling the temperature rises.

The losses increase with increasing temperature, so that more heat is

generated and instability occurs. The temperature rises fast and a narrow

channel is formed where the material burns out.

Thermal breakdown is more serious at high frequencies since the heat

generated is proportional to the frequency.

Thermal breakdown stresses (MV/cm) are lower under A.C condition

than under D.C. [12].

13

2.2.2 Long-time mechanism

2.2.2.1 Breakdown caused by partial discharge (PD)

Partial discharge are the main cause of breakdown in case of A.C. they

occur in gas-filled cavities in a dielectric and cause a slow erosion of the

material.

Solid insulation materials contain voids or cavities within the medium or

the boundaries between the dielectric and the electrodes. These voids are

generally filled with a medium of lower dielectric strength, and the

dielectric constant of the medium in the voids is lower than that of the

insulation.

The electric field strength in the voids is higher than that across the

dielectric. Therefore, even under normal working voltages the field in the

voids may exceed their breakdown value, and the breakdown may occur.

Partial discharges have the same effect as treeing on the insulation.

Effect of partial discharge may break chemical bonds and cause erosion

of the material and consequent reduction in the thickness of insulation.

Life of insulation with partial discharges depends upon the applied

voltage and the number of discharges [11].

2.2.2.2 Breakdown caused by inclusions.

Inclusions are dust, fibers, slivers, metal particles, etc. are found in

insulation materials and there is no dielectric free from these inclusions

ever.

14

Many industrial materials have to be made according to standards, where

the maximum allowable size is X and the maximum allowable size is Y

are allowed in Z cm^3 material.

However the modern manufacturing techniques guarantee a high degree

of freedom of inclusions, many particles of 1 to 100 micro occur in the

raw materials, which results in thousand to millions of foreign particles in

a finished product.

These inclusions may cause breakdown and it can be classified as:

Inclusions of low dielectric strength. For example: cavities and

globules of heavily oxidized or even burnt polyester and inclusions

of dust.

Inclusions (insulating or conductive) which are not well-embedded

in the insulation.

Sharp conductive inclusions. Field concentrations at a sharp edge

may initiate breakdown.

In all these cases, electrical trees are formed in the same way as in the

case of cavities, and the time to initiate a tree is extremely long compared

to the time of formation.

At high field strengths of 20 to 30 KV/mm the process can cause

breakdown in hours, even minutes [11].

15

CHAPTER THREE

EXPEREMINTS SETUP

16

Sample preparation

3.1: Materials

Specimens: The experiments in this project were done using the needle-

plane arrangement (figure1) and a physical model (figure3). The

specimens used are composed of epoxy (resin+hardener) and cobalt

(figure4). The hardener used was methyl ethyl ketone hydroperoxide. A

rubber mould was used to prepare the samples. The specimens have a

composite insulation of epoxy and barrier (layer of fiber). The ratio of

resin to hardener was 2:100 ml and the ratio of resin to cobalt was

0.2:100 ml. The cobalt is then added to the epoxy and mixed thoroughly

before casting the mixture in a mould at 80° C for 3 and half hrs. Before

inserting the samples in to the oven the needle is inserted inside each

sample to remove any mechanical pressure would remain in the

specimens. The distance between the tip of the needle and the barrier

(layer of fiber) is almost 1 mm, and the distance between the barrier and

the earth barrier is also almost 1mm. Before inserting the needles in the

oven, now all the samples are degassed for 15 minutes to remove the

trapped air bubbles. This type of resin was chosen because of its ease of

handling, rapid curing, good physical and electrical properties,

dimensional stability and optical quality [3]. The fiber layer is called a

barrier and it is used to increase the tree growth resistance of the polymer.

All specimens were inspected by a high resolution microscope to see the

electrical trees clearly.

Optical system: High resolution microscope with 1000 to 40100

magnification factors was used to observe the samples. A digital camera

17

is inserted in the eyepiece tube of the microscope to capture the electrical

trees propagation figure5. .

Test equipment: The high voltage equipment consist of a setup

transformer 220 V \ 20 KV 50 Hz supplied from a control unit and all

system is grounded.

3.2: Method

Firstly the mechanical stress was applied to the specimens using the

model. Secondly samples were tested under AC voltage of 8 KV RMS for

a specific periods of time. The experiment were done under room

temperature and pressure. The electrical trees propagation were observed

and captured using the microscope and the digital camera. See figures 8-9

Figure1: Schematic diagram of needle-plane arrangement of specimen.

18

Figure2: Needle-plane arrangement of specimen.

Figure3: Physical model used to apply mechanical stress on the specimens.

19

Figure4: Polyester resin, hardener and cobalt.

Figure5: Optical system used to observe the specimens

21

Figure6: Step up transformer with control unit.

Figure7: Earth electrode.

21

Figure8: Applying mechanical stress on the specimens.

Figure9: Applying electrical stress on the specimens.

22

CHAPTER FOUR

RESULTS AND

ANALYSIS

23

4.1 The function of the barrier

Composite insulation systems are widely used in high voltage equipment.

Typical composites are glass fiber reinforced materials. During their service the

composites may be exposed to partial discharges and electrical treeing for long

periods. The main function for the barriers is to delay the breakdown of

insulators [14].

4.2 Microscopic of samples

The samples exposed to a high voltage of different voltages for 3 months. The

response of each samples, voltage, and the time to fail was different. Some of

these samples were made from pure resin (without a barrier) and others had

single barrier. Also some samples exposed to a high voltage only but others

exposed to a high voltage and mechanical stress.

- This table shows the different samples with number of hours needed for each

sample until reaching the barrier.

24

Table 4.1: Tested samples with test duration time

Test duration hours Material type Sample No.

8 Polyester resin only Sample 1

46 Polyester resin reinforced by glass fiber Sample 2

46 Polyester resin reinforced by glass fiber

With mechanical stress

Sample 3

The microscopic images of electrical tree were captured by the digital

camera and recorded by the computer.

Now we will show the images for each sample and see how much voltage

and time needed until reaching the barrier.

4.2.1 Tree growth in sample 1 (without barrier and without

mechanical stress):

Figure 1 below shows an image for the electrical tree of sample 1 cast

from pure polyester resin without a barrier. With the continuity of voltage

application on this tested sample and gradual growth in tree and without

barrier, this will accelerates the sample breakdown.The time for initiation

was 8 hours.

25

Figure 10: Tree of sample 1 at inception.

4.2.2 Tree growth in sample 2 (with barrier and without mechanical

stress):

Figure 2 below presents a series of images for electrical tree grown in

sample 2 cast with a barrier inserted midway between the pin tip and

earth electrode. On each image a time tag was fixed to relate the growth

with time 46 hours. The time for initiation was 10 hours.

8h

26

Figure 11: Tree growth of sample 2 until reaching the barrier.

The second stage of tree development was the growth through the barrier.

More than 46 hours were needed for the tree to penetrate the barrier after

reaching it. After long time the tree succeeded in passing through the

barrier.

With the continuity of voltage application on the tested samples and

gradual growth in tree, the distance between the tree and the earth

electrode is reduced.

37h

hhh

hh

46h

29h 20h

10h 0h

27

4.2.3: Tree growth in sample 3 (with barrier and with mechanical

stress):

Figure 3 below presents a series of images for electrical tree with mechanical stress

grown in sample 3 cast with a barrier inserted midway between the pin tip and earth

electrode. On each image a time tag was fixed to relate the growth with time 46 hours.

The time for initiation was 10 hours.

Figure 12: Tree growth of sample 3 until reaching the barrier.

0h 10h

20h 29h

37h 46h

28

We can see that More than 29 hours were needed for the tree to penetrate

the barrier after reaching it, and this shows how the mechanical stress

caused to breakdown very fast.

4.3 Analyses of electrical tree in samples

It is worth to make the development in tree images into several stages.

1. The first stage is the tree inception:

The time for this stage starts when the voltage is applied and ends

with the first initiation of the tree from the pin tip. The presence of

barrier reduces the effect of electric field and leads to a delay in the

inception time for the tree. The total stress of electric field is reduced and

the time to breakdown becomes longer. The inception times for different

classes of examined specimens are shown in table 4.2.

For pure polyester resin without barrier the inception time is lower than

that (only 8 hours). This is clear evidence about the role of barrier in

hindering the tree propagation within composite dielectrics.

29

Table 4.2: Inception time for different samples.

Inception time in

hours

Material type Sample No.

8 Polyester resin only Sample 1

10 Polyester resin reinforced by glass fiber Sample 2

10 Polyester resin reinforced by glass fiber

With mechanical stress

Sample 3

2. The second stage of tree growth is the propagation phase:

the time spent in this stage is effected by the presence of barrier and

mechanical stress. The barrier caused a delay for the propagation time.

But the mechanical stress decreases it.

31

Table 4.3: The propagation time for different samples.

Propagation time in

hours

Material type Sample No.

29 Polyester resin reinforced by glass fiber Sample 2

20 Polyester resin reinforced by glass fiber

With mechanical stress

Sample 3

3. The third phase is the breakdown stage:

This is one of the most critical phases of insulation life. This phase

depends on the previous stages and it comes as a result of the tree

development during these stages.

The attempt to delay the breakdown is a goal of all insulation designers.

It is worth to know that the closeness of tree tip to the earth electrode

accelerates the breakdown. The total time to breakdown is a good

indicator of insulation resistivity to breakdown.

31

CHAPTER FIVE

CONCLUSION

32

5.1: Conclusions

After analysis of the experimental results and discussion presented, the

following conclusions may be drawn:

1_ Electrical treeing is a complex and random process because it depends

on many factors and get affected by several variables.

2_ There are three stages of electrical treeing inception, growth or

propagation and breakdown.

3_ Electrical trees can come in different shapes depending on the applied

voltage and the magnitude of the electric field. These shapes are branch,

bush, pine-branch, bine-branch and mixed configurations.

4_ It is found that adding barrier (fiber layer) will improve the tree

growth resistance of the polymer. The specimens with barrier have shown

the highest delay, whereas, the specimens without barrier have

experienced fast breakdown, at the barrier boundaries refraction law of

electric field is applied. Because of the barrier higher permittivity

(compared to polyester resin) it prevents the electric field to concentrate

in it this causes a delay of tree propagation at the barrier.

5_ It was observed that applying mechanical stress on the specimens will

weaken the tree growth resistance of the polymer and this will speed up

the breakdown process.

33

References:

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XLPE Cable insulation by investigating a double electrical tree

structure", IEEE Transactions on dielectrics and electrical insulation

Vol.15, No.3; June 2008

[2]http://en.wikipedia.org/wiki/Electrical_treeing

[3] Gasem K.AL-Ruwaili," The reduction of propagation speed of the

electrical tree by adding barriers in composite insulation materials using

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Transactions on dielectrics and electrical insulation Vol.EI-15, PP. 33-42,

1980.

34

[8] J.C. Fothergill, L.A. Dissado and P.J.J. Sweeney, "A Discharge-

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http://en.wikipedia.org/wiki/Electrical_breakdown#Failure_of_electrical_

insulation

[11] F.H. Kreuger "Industrial high voltage", Delft University press,

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[12] V. Kamaraju, M. S. Naidu, "High voltage engineering"

5e,McGraw Hill Education India, 2013

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New York, Oxford university press, 2007

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propagation along barrier-interfaces in epoxy resin