Volume 3, Issue 10, October – 2018 International Journal of Innovative Science and Research Technology

ISSN No:-2456-2165

Enhancement of Dielectric and Tensile Strengths for

Tree Retardant Cross-Linked Polyethylene (TR-XLPE)
Loai Nasrat, Abd Allah Ibrahim Esraa Sayed
Electrical Power and Machines Eng. Dept. Kom Ombo Industrial Secondary School.
Aswan University, Aswan, Egypt Aswan, Egypt

Abstract- The use of the polymeric materials in high and at the semiconducting shield/insulation interfaces.
voltage applications is considered especially interesting in TR-XLPE insulation materials are expected to retard the
the electrical and industrial materials sectors. In this growth of water trees in a manner that will slow cable aging
context, the purpose of this paper is to improve the when operated in wet environments [2-4].
electrical properties of tree retardant cross-linked
polyethylene (TR-XLPE) with respect to mechanical Kaolin has particle size of 42 μm and density 2.23 g/cm3
characteristics by adding Kaolin filler with 15%, 30%, and melting point 1755°C.It is available in high purity and
40% and 50% concentration percentages. The dielectric large quantity at low cost [5].
strength of the composites was tested under several
conditions such as (dry, wet and salty wet at (20000µS/cm, Optimization is the act of obtaining the best result under
30000µS/cm and 50000µS/cm)). given circumstances. Optimization can be defined as the
Tensile strength test was applied to investigate the process of obtaining the conditions that give the maximum or
mechanical properties of TR-XLPE after adding Kaolin minimum value of a function. Thus without loss of generality,
filler with 15%, 30%, 40% and 50% concentration optimization can be taken to mean minimization since the
percentages. Soft program (Curve fitting) is used to select maximum of a function can be found by seeking the minimum
the most proper equation between dielectric strength of the negative of the same function [6].
values under each condition and different percentages of
Kaolin filler also between tensile strength and the same  Grey Wolf Optimizer (GWO)
percentages of Kaolin filler. Grey Wolf Optimizer (GWO) Mirjalili for the first time simulated the mathematical
was applied to find the best suitable percentages of Kaolin model of the behavior of the Grey Wolf Optimizer (GWO).
filler to obtain the best optimal values of dielectric strength GWO is a meta-heuristics natural inspired method belongs to
and tensile strength. swarm intelligence (SI) algorithms. SI is “The emergent
collective intelligence of groups of simple agents”. The
Keywords – TR-XLPE; Kaolin; Dielectric strength; Tensile inspirations of SI techniques originate mostly from natural
strength; GWO. colonies, flock, herds and schools. It imitates the grey wolves
behavior in attacking and hunting a prey. Grey wolves prefer
I. INTRODUCTION living in packs with a robust social dominant hierarchy.

Underground cables are an important part of any Power The first three fittest wolves are considered as alpha,
distribution system. These cables are laid in ducts or may be beta and delta who guide other remaining wolves (ω) toward
buried in the ground. Unlike in overhead lines, air does not promising areas of the search space. For an optimization
form part of the insulation and the conductor must be problem, the best solution is represented by alpha wolves, the
completely insulated. This means that the selection of cable second and third best solutions are beta and delta wolves while
must be based on the losses, cost and environmental factors omega wolves provide all the other solutions [7-8].
surrounding [1].
II. SAMPLES PREPARATION
Since the 1960s, polyethylene and later cross-linked
polyethylene (XLPE) have been used as insulation materials The preparation of TR-XLPE depends on different
for low, medium and high voltage power cables that operate in parameters such as the ratio of TR-XLPE, types and
both wet and dry environments on underground electric utility concentrations of filler that affect on properties of the base
systems. In the early 1980s, tree retardant cross-linked material and final product. Samples have been prepared in the
polyethylene (TR-XLPE) was introduced which claimed laboratory of the polymers and pigments department National
improved cable performance compared with XLPE. The Research Center (NRC). Polymer composite was prepared by
degradation of XLPE insulation due to moisture (water trees) mixing different ratios of Kaolin filler (0, 15, 30, 40 and 50
has been a trouble for many years and extensive studies have wt. %). The samples preparation were operated at the room
been made to improve the resistance of XLPE to water treeing. temperature (25°C ±1), until curing occurred. The samples
Water trees result from the combined effects of water, ionic codes were represented in the table (I) with different
impurities and electrical stress enhancements in the insulation concentration of Kaolin filler.

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Volume 3, Issue 10, October – 2018 International Journal of Innovative Science and Research Technology
ISSN No:-2456-2165
Table I
By using curve fitting methods, we can create access and
adjusted curve fitting objects. That allowed to like plot and
Kaolin filler integrate, to perform operations that uniformly process the
Samples code TR-XLPE ratio
percentage (%) entirety of information encapsulated in a curve fitting object.
Dielectric properties were inspected at five conditions; dry,
Blank 100 0 wet and salty wet at (20000 µS/cm, 30000µS/cm and
50000µS/cm).
Kaolin15 85 15 The average of dielectric strength (kV/mm) for TR-XLPE
with different percentages of Kaolin filler under several
Kaolin30 70 30 conditions, i.e.: dry, wet and salty wet at 20000µS/cm,
30000µS/cm and 50000µS/cm shown in table (II) and Fig (2).
Table II
Kaolin40 60 40
The average of dielectric strength (kV/mm)
for five samples under several conditions
Kaolin50 50 50 Kaolin filler
percentages Salty Salty Salty
(%) wet wet wet
Dielectric strength is measured through the thickness of Dry Wet
(20000 (30000 (50000
specimen (which is equal to 1 mm) and it is expressed in µS/cm) µS/cm) µS/cm)
(kV/mm). Mechanical tests such as: tensile strength test is
performed to illustrate the ability of composite samples to 0 31.03 26.52 22.12 20.94 17.58
withstand the mechanical forces. The dimensions of the
sample are 5 cm length and 1 mm thickness for tensile
strength. 15 33.83 29.79 26.24 24.22 21.57

III. RESULTS AND DISCUSSIONS

30 36.11 32.09 29.56 27.68 25.23
 Dielectric break down strength test
The dielectric strength of a material is a measure of the
electrical strength of an insulator. It is defined as the 40 31.09 26.14 23.04 20.59 17.86
maximum voltage required to produce a dielectric breakdown
through the material and is expressed in terms of Volts per
unit thickness. The higher the dielectric strength of a material 50 23.29 20.24 19.51 17.12 15.76
the better an electrical insulator it makes.

The failure is characterized by an excessive flow of From table (II), It can be observed that the dielectric
current (arc) and by partial destruction of the material. strength of TR-XLPE was improved by adding Kaolin filler
Dielectric strength is measured through the thickness of the until definite value then the dielectric strength decreased, the
specimen (which is equal to 1mm) and is expressed in volts dielectric strength decreased in wet condition as compared
per unit of thickness. Samples are in the form of disc with with those in dry condition and the exposure to salt water
diameter 5 cm and thickness 1 mm. For each test, the average solution drastically affected the dielectric strength where the
result of five samples has been taken to minimize the error. dielectric strength is inversely proportional with the amount of
Fig (1) shows the circuit used for dielectric break down salinity.
strength test.
Applying Curve fitting (regression analysis) using
Matlab program, the best curve fitting for the obtained results
from the test are shown in Fig (2) for dry condition.
Representing the data by a 3rd degree polynomial equation to
minimize the error as possible we get.

The equations of curve fitting results for the dielectric

strength of blends under dry, wet and salty wet at
(20000µS/Cm, 30000µS/Cm and50000µS/Cm) are as follows:

 1-Under dry condition is:

Y = -0.00029*X 3 + 0.0082*X 2 + 0.16*X + 31 (1)
Fig 1:- Schematic diagram for the dielectric strength testing
circuit.  2-Under wet condition is:

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Volume 3, Issue 10, October – 2018 International Journal of Innovative Science and Research Technology
ISSN No:-2456-2165
3 2
Y = -0.00022*X + 0.0021*X + 0.28*X + 26 (2)

 3- Under salty wet at 20000µS/cm condition is:

Y = -5.6e-0.005X^3 - 0.0079*X 2 + 0.48*X + 22 (3)

 4-Under salty wet at 30000µS/cm condition is:

Y = -7.9e-0.005X^3 - 0.0058*X 2 + 0.4*X + 21 (4)

 5- Under salty wet at 50000µS/cm condition is:

Y = 9.9e-0.006X^3 - 0.012*X 2 + 0.54*X + 17 (5)

Where Y parameter is the dielectric strength value

(KV/mm) at all different conditions, X is the percentage of
concentration of Kaolin filler in the samples.

GWO was applied to find the best percentage of Kaolin

filler by inserting the negative of the equations (1-5) in the
code of the program to obtain the best optimal value of
dielectric strength in each condition.
Fig 3:- The optimal value of dielectric strength under dry
condition.

At salty wet condition the best solution obtained by

GWO is 30% of kaolin filler to obtain the best optimal value
of dielectric strength for 20000 µS/cm and 30000 µS/cm while
22.51% of Kaolin filler to obtain the best optimal value of
dielectric strength for 50000µS/cm.

Note: The optimal value of dielectric strength is negative in

Fig (3) because of inserting a negative sign to equation (1).

 Mechanical analysis
Tensile strength tests (TS) are carried out in order to
illustrate the ability of TR-XLPE filled with different
percentages of Kaolin filler to withstand the mechanical
forces.

 Tensile strength test

It can be defined as the maximum tensile sustained by the
material being tested to its breaking point.

Fig 2:- Dielectric strength of TR-XLPE/Kaolin under dry Tensile strength = F/A.
condition.
It is measured in units of force (F) divided by units of
After applying GWO under dry condition by inserting area (A). The tensile strength of TR-XLPE filled with 0%,
the negative of equation (1), the best solution obtained by 15%, 30%, 40% and 50% of Kaolin filler has been tested.
GWO is 25.94% of Kaolin filler to obtain the best optimal Fig (4) shows the tensile strength of TR-XLPE with kaolin
value of dielectric strength as evident in Fig (3). filler.

At wet condition by inserting the negative of equation From Fig (4) it's obviously seen that the tensile strength
(2), the best solution of Kaolin filler percentage obtained by increased gradually with increasing the percentage of Kaolin
GWO is 24.02% to obtain the best optimal value of dielectric filler until 40% then the tensile strength decreased.
strength.

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Volume 3, Issue 10, October – 2018 International Journal of Innovative Science and Research Technology
ISSN No:-2456-2165

Fig 4:- Tensile strength (MPa) of TR-XLPE/Kaolin Fig 6:- The optimal value of tensile strength.

Applying Curve fitting (regression analysis) using Note: The optimal value of tensile strength is negative because
Matlab program, the best curve fitting for the obtained results of inserting a negative sign to equation (6).
from the test are shown in Fig (5).
From this study both dielectric and tensile strengths
The equation of curve fitting results for the tensile strength of increased with increasing the Kaolin filler percentage until
blends is: definite value then they decreased. This is due to the saturation
Y = -0.00011*X 3 + 0.007*X 2 - 0.061*X + 2.8 (6) that happened in the lattice of TR-XLPE and the more
addition of Kaolin filler to TR-XLPE made the composites
Where Y parameter is the tensile strength (MPa), X is brittle so both dielectric and tensile strength decreased, also
the percentage of concentration of Kaolin in the samples. the values of dielectric strength in wet and salty wet
GWO was applied to find the best percentage of Kaolin filler conditions are less than dry condition. This is due to
by inserting the negative of the equation (6) in the code of the increasing flow of leakage current that increases the
program to obtain the best optimal value of tensile strength. opportunity of break down to occur.

IV. CONCLUSION

The addition of Kaolin filler had resulted some

improvement in the electrical and mechanical properties of
TR-XLPE. GWO gives definite indications of filler percentage
to obtain the optimal values of dielectric strength and tensile
strength.

It can be concluded that, the suitable percentages of

Kaolin filler can be added to TR-XLPE which determined by
applying GWO are:

 In electrical properties

 Under dry condition

The best solution obtained by GWO is 25.94% of Kaolin
filler to obtain the best optimal value of dielectric strength.
Fig 5:- Tensile strength (MPa) of TR-XLPE/Kaolin
 Under wet condition
After applying GWO by inserting the negative of The best solution obtained by GWO is 24.02% of kaolin
equation (6), the best solution obtained by GWO is 37.49% of filler to obtain the best optimal value of dielectric strength.
Kaolin filler to obtain the best optimal value of tensile strength
as evident in Fig (6).  Under salty wet condition
The best solution obtained by GWO is 30% of kaolin
filler to obtain the best optimal value of dielectric strength for
20000 µS/cm and 30000 µS/cm while 22.51% of Kaolin filler

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Volume 3, Issue 10, October – 2018 International Journal of Innovative Science and Research Technology
ISSN No:-2456-2165
to obtain the best optimal value of dielectric strength for [3]. P. J. Caronia, P. J. Brigandi, T. J. Person and S. Cree, "A
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Linked Polyethylene Insulation for Long Life Power
 In mechanical properties Cables", IEEE Conferences, 2016 IEEE/PES
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[4]. V. Vahedy, "Polymer Insulated High Voltage Cables",
It may be recommended to use the average between IEEE Journals & Magazines, IEEE Electrical Insulation
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industrial application. [5]. Z. Y. Shnean, " Effect of Grain and Calcinations Kaolin
ACKNOWLEDGEMENTS Additives on Some Mechanical and Physical properties
on Low Density Polyethylene Composites", Al-
The authors would like to thank the staff of High Khwarizmi Engineering Journal, Volume: 4, No. 4, PP.
Voltage Laboratory, Electrical Engineering Dept. Aswan 37-44, 2008.
University and National Research Center, Polymers and [6]. S. S. Rao, "Engineering Optimization Theory and
pigments Dept.; where most of the samples preparation and Practice 4th Edition", Book, Canada, 2009.
experimental work were carried out. [7]. S. Mirjalili, S. M. Mirjalili, and A. Lewis, "Grey Wolf
Optimizer", Advances in Engineering Software, Volume:
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