Download - Electrical treeing
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
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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.
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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.
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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].
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CHAPTER TWO
TREEING
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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
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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
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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
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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.
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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].
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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.
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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
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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.
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Figure2: Needle-plane arrangement of specimen.
Figure3: Physical model used to apply mechanical stress on the specimens.
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Figure4: Polyester resin, hardener and cobalt.
Figure5: Optical system used to observe the specimens
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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.
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CHAPTER FOUR
RESULTS AND
ANALYSIS
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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.
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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.
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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
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34
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