EVALUATION OF DIELECTRIC STRENGTH OF EPDM ELASTOMER LOADED WITH ATH FILLERTamer Sheta 1
|
Blend symbol |
ATH% |
EPDM% |
A |
0 |
100% |
B |
10% |
90% |
C |
20% |
80% |
D |
30% |
70% |
E |
40% |
60% |
(A) (B) (C) (D) (E)
Figure 1: Different types of
composite samples.
The test circuit contained a variable power
transformer (100kV-5kVA). A resistor of 5MΩ should be connected in series
with the secondary coil of the transformer to limit the current. The sample was
placed between two electrodes which connected directly to the transformer. The
high voltage circuit should be placed in an earthed cage. The output of the
transformer is gradually increased until breakdown occurs, all results had been
recorded, repeated and the average were taken. The dielectric strength for the specimen
was taken according to ASTM D149. Specimens have been immersed in tap water for
24 h, and then were tested using ac voltages. To investigate the impact of
adding some contaminant other specimens have been sunken in 5% NaCl solution
for the same period (24 h). Dust over the samples should be removed by cleaning
them well by using distilled water prior to the test [6].
The tensile strength is a kind of the
mechanical test to evaluate the ability of the samples to withstand the
mechanical force. The mechanical features of the samples were observed
according to ASTM D412. Three samples are used for each composition. The
dimension of the sample is 1mm thickness and 5cm in length has a dumbbell shape
as in Figure (2).From the two ends the sample is clamped then the machine is
running with cross speed of 50mm/min until break, then the tensile strength was
measured in (MPa) [7].
Figure 2: The structure of samples
used for Mechanical test.
Figure (3) represents the results of
the breakdown test for the five composites of EPDM with different ATH filler
percentages. At 25°C, the recorded value is 30kV/mm while at 130°C, the
recorded value is 26kV/mm for blend composite. By increasing the temperature
over the EPDM composites as shown in Figure (3), one can notice that the break
down voltage decreases. By adding ATH
filler the break down voltage
improved until it reaches to its maximum at 30% ATH percentage where It
recorded 32.5kV/mm at 130°C, while it has recorded 37kV/mm at 25°C. Increasing
the ATH percentage over 30% BDV decreases. This can be ascribed to the
increased composite absorption to water because of the ATH properties. Thus,
the breakdown voltage for EPDM composite insulators increases by increasing ATH
filler percentages up to 30% then decreases.
Figure 3: The effect of temperature
on the breakdown voltage for different
EPDM/ATH composites.
Further,
the dielectric strength results of EPDM composites with different ATH filler percentage
under wet and salt conditions are shown in Fig (4). At blend composites the BDV
recorded 27kV/mm and 24kV/mm respectively at wet and salt conditions, while it
recorded 30kV/mm at the dry condition. This illustrates the main effect of
contaminated condition in decreasing the BDV. By adding ATH the BDV will
improve until it reaches to its maximum at 30% of ATH added to EPDM recording
34kV/mm and 32.5kV/mm at wet and salt conditions respectively. This means that
the BDV is enhanced by almost 26% for wet condition and by 35% for salt
conditions. Therefore, one can conclude that the best percentage of ATH filler
is 30% to give high breakdown voltages for EPDM/ATH composite, i.e. the
breakdown voltage for EPDM increases gradually by increasing the filler percentage
up to 30% at different conditions which are simulated by salt and wet.
Figure 4: The effect of wet and salt
conditions on the breakdown voltage for different EPDM/ATH composite.
Thus,
the above results show that the presence of water droplets over the surface of
the insulator improves the electric field intensity distribution and this
causes the electrical breakdown to increase. Water drops on the surface of the
insulators were found to vibrate in many ways and were moved away from the high
electric stress zones to low electric stress zones. Drops of water on the
surface of the polymer grow larger by absorbing droplets of water from the fog
or by coalescing together [8].
EPDM composite under salinity
conditions are more sensitive than wet and dry conditions. The breakdown
voltage (BDV) of composite insulator is decreasing due to the absorption of
composite material to salt solution. Immersion in NaCl solution resulted in
further drop in the insulation properties, probably due to hydrolysis of NaCl
resulting in the formation of NaOH and evolution of HCl gas which further dissolves in water.
Further, in this respect, the authors
believe that the composite insulation with a high percentage of ATH filler of
30% can form a surface resistance stack keeping a uniform voltage distribution
or quasi-uniform field under the voltage stress, thus increasing the breakdown
voltage. The surface wet due to water or humidity can have the same effect.
With further increase of the filler parentage and/or the surface wet or
pollution, a more conductivity path can form decreasing the breakdown voltage.
Figure
(5) illustrates the effect of increasing ATH loading on the tensile strength of
EPDM/ATH composites. Table (2) shows the tensile strength at different filler
percentages. It recorded 1.66 MPa for blend sample while it recorded 2.93 MPa
for 20% of ATH, this means that the mechanical stress improved by 76.5%. By
increasing the filler, the mechanical stress will decrease recording 2.43 MPa
at 30% of ATH and 1.76MPa at 40% of ATH.
Table 2: Average results for tensile
strength.
SAMPLES |
A |
B |
C |
D |
E |
|
EPDM % |
100 |
90 |
80 |
70 |
60 |
|
ATH % |
0 |
10 |
20 |
30 |
40 |
|
1 |
Tensile Strength (MPa) |
1.8 |
1.9 |
2. 7 |
2. 4 |
2. 0 |
2 |
1.7 |
2. 2 |
3.0 |
2. 3 |
1.6 |
|
3 |
1.5 |
2. 3 |
3. 1 |
2. 6 |
1.7 |
|
Average Tensile Strength |
1.66 |
2.13 |
2.93 |
2.43 |
1.76 |
Figure 5: The elongation force of
sample against different percentages of filler.
The above obtained results of the
effect of adding ATH filler to the EPDM on the mechanical properties of the
composite insulation can be explained in the light of the resulting increased
rigidity of the composite insulation. This may be further investigated through
more investigation on the microscopic structure using X-ray diffraction (XRD)
and Transmission Electron Microscope (TEM) investigations.
1) The changes of electrical
and mechanical properties of EPDM by adding different percentage of ATH filler
are investigated under various temperature and atmospheric conditions.
2) The dielectric strength of
EPDM reached to 37 kV/mm by adding 30% of ATH under dry condition which means
that the dielectric strength is improved by almost 23%.
3) The tensile strength of EPDM
reached to 2.93 MPa with adding 20% of ATH filler which means that tensile
strength is improved by almost 76%.
4) All properties such as
physical, mechanical and electrical have decreased at higher salinities and
moisture.
5) ATH filler plays a vital
role for the electrical properties of
EPDM insulating material; breakdown voltage increases with the increase of the
ATH filler up to 30%. Therefore, it is recommended to add ATH filler at this
percentage to obtain the optimum values of breakdown voltage.
None.
None.
The authors wish to thank the staff of the High Voltage Laboratory, the Electrical Engineering Dept, Ain Shams University and the National Research Center, Polymers and Pigments Dept. where large portion of the samples preparation and experimental work were completed.
[1] Jeffry Mackevich, et al, " Polymer Outdoor Insulating Materials Part I: Comparison of Porcelain and Polymer Electrical Insulation", IEEE Electrical Insulation Magazine Vol. 13, No. 3, 1997.
[3] Y. Kurata, et al, “Evaluation of EPDM Rubber for High Voltage Insulator", IEEE, proceedings of 1995 conference on electrical insulation and dielectric Phenomena.1995.
[5] Isaías Ramírez, et al," Silicone Rubber and EPDM Micro Composites Filled with Silica and ATH”, Annual Report Conference on Electrical Insulation and Dielectric Phenomena, IEEE, Cancun, Mexico 2011.
[8] A. Krivda, et al.” Breakdown between Water Drops on Wet Polymer Surfaces”, Annual Report Conference on Electrical Insulation and Dielectric Phenomena (Cat. No.01CH37225), IEEE, Kitchener, Ontario, Canada, 2001.
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