Partial Discharge Characteristic of Low Density Polyethylene and Silica Nanocomposite

Abstract

Studies on mechanical properties of linear low density polyethylene (LLDPE) and natural rubber (NR) added by fillers such as carbon black, silica and calcium carbonate show an improvement in the mechanical properties. With the development of nanotechnology, the opportunity to increase the usage of LLDPE and NR composites are promising. The common method is by adding a small weight percentage of nano particles during the blending process. This study focuses on analyzing the partial discharge characteristic of different weight percentage (w%) of titania (TiO2) nano particle to LDPE and NR blend. The specimens were subjected to aging using CIGRE Method II system. Partial discharge pulses were counted for both polarities. Phase resolve pattern were used to characterize the aging process. A microscopic physical surface analysis using an optical microscope was also carried out. In contrast to the prediction, the results show that the PD pattern is negatively affected by nano titania particles, where no improvement was noted with a certain amount of w% of nano silica. The PD characteristic of virgin LDPE/NR is found to be better than the nano titania filled DLPE/NR. The physical investigation also shows a similar result, where a smaller damage on the surface of the specimens was observed for the case of virgin composite.

Full text

Partial Discharge Characteristic of Linear Low Density Polyethylene and
Silica Nanocomposite
Aulia1,2,a, Z. Abdul-Malek1,b, Y.Z. Arief1,c, M.A.M. Piah1,d, M. Jaafar3,e
1Institute of High Voltage and High Current, Faculty of Electrical Engineering
Universiti Teknologi Malaysia, Johor 81310, Malaysia
2 Jurusan Teknik Elektro, Fakultas Teknik, Universitas Andalas, Padang, Indonesia, 075186
3 School of Material & Mineral Resources Engineering, Engineering Campus
USM,14300 Nibong Tebal, Penang.
aaulia007@gmail.com, bzulk@fke.utm.my, cyanzarief@fke.utm.my, dfendi@fke.utm.my,
emariatti@eng.usm.my
Keywords: linear low density polyethylene, natural rubber, nano titania, partial discharge
Abstract – Studies on mechanical properties of linear low density polyethylene (LLDPE) and
natural rubber (NR) added by fillers such as carbon black, silica and calcium carbonate show an
improvement in the mechanical properties. With the development of nanotechnology, the
opportunity to increase the usage of LLDPE and NR composites are promising. The common
method is by adding a small weight percentage of nano particles during the blending process. This
study focuses on analyzing the partial discharge characteristic of different weight percentage (w%)
of titania (TiO2) nano particle to LDPE and NR blend. The specimens were subjected to aging
using CIGRE Method II system. Partial discharge pulses were counted for both polarities. Phase
resolve pattern were used to characterize the aging process. A microscopic physical surface analysis
using an optical microscope was also carried out. In contrast to the prediction, the results show that
the PD pattern is negatively affected by nano titania particles, where no improvement was noted
with a certain amount of w% of nano silica. The PD characteristic of virgin LDPE/NR is found to
be better than the nano titania filled DLPE/NR. The physical investigation also shows a similar
result, where a smaller damage on the surface of the specimens was observed for the case of virgin
composite.
Introduction
Nanodielectrics material which consists of polymeric matrix and nanofillers has gained a
significant attention due to their potential advantageous in electrical power industries [1-3]. Only
when the processing is done correctly these advantageous can be fully explored and utilized [4].
Adding a nanoparticles to a matrix and accompanied by coupling agent will introduce a stable and
large interfacial thickness that related to the interaction zone that possibly could help to improve the
PD resistance [5].
Partial discharge, one of the most important factors leading to the breakdown in electrical
insulation has drawn tremendous attention by researchers worldwide. Through the incorporation of
nanofillers in polymer matrices, it could enhance the partial discharge properties of the insulation
materials. A limited reference about partial discharge (PD) characteristic of low density
polyethylene (LDPE) and nanosilica composite and related works were explained in [6-12]. There is
evidence that low density polyethylene (LDPE) and nanosilica (SiO2) composite is stronger than
the pure LDPE under PD stress even if during a longer period of time. The smoother surface of this
nanocomposite after the PD test shows the indication PD endurance of the material [8].
The CIGRE Method II was used by many authors to investigate the aging mechanism of
insulation. The main advantage of this method is that the discharge taken place in a concentrated
area. LDPE nanocomposite containing 0, 2, 4, 6, and 8w% of nanosilica were previously
investigated. The aim of this work is to determine the effects of nanosilica filler content on the
partial discharge characteristics of the LDPE composite based on CIGRE Method II test setup.
Applied Mechanics and Materials Vol. 554 (2014) pp 133-136
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© (2014) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.554.133
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Sample Preparation and Experimental Set-up
Sample Preparation. Cosmothene® F410-1 type of low density polyethylene (LDPE) was used
as the base insulating material. The filler material is Aldrich S5130 silica fumed powder with a
mean size of 7 nm. Compounding of LDPE and silica fillers was performed in two roll mill heaters.
The LDPE compounds were prepared according to the various filler contents (0wt%, 2wt%, 4wt%,
6wt% and 8w%, namely sample A, B, C, D and E). The temperature for both roll mills was set at
150°C. The mixing of LDPE and silica fillers took 10 min, followed by compression moulded into
rectangular specimens having 200 x 150 x 1 mm3 size in an electrically heated hydraulic press at
150°C and a pressure of 1500 psi. Preheating at the compression moulding was carried out for 4
min followed by 3 min for pressure using electrically heated hydraulic press model GT-7104-A30C.
The subsequent cooling under hydraulic pressure of 100 kg/cm3 was conducted for 3 minutes. The
specimens were prepared in the form of 40-mm-diameter circular sheets with a thickness of 1 mm.
PD Test Set-up. An AC voltage of 6.5 kV rms at 50 Hz was applied to the test electrode while
the plane electrode was earthed. Within the given experimental conditions, it was believed that no
PD took place from areas other than the void. Fig. 1 shows the laboratory set-up consisting of an
AC high voltage supply and its measuring system, the CIGRE METHOD II (CM-II) electrode
system [3], and the data acquisition system (PC-connected Picoscope 6). To detect the PD signals, a
high pass filter with a range of frequency between 10 kHz up to 400 kHz was used. Also to get the
50 Hz sinusoidal wave reference power supply, a step down transformer was used and then
connected to the Picoscope. The PD pulses were sampled for 10 s corresponding to 1000 cycles at
every 10 minutes.
Fig. 1 Test set-up for the partial discharge test
Results and Analysis
Positive (+ve) and Negative (-ve) PD Trend. Fig. 2 shows the positive and negative PD trend
of all samples and nanocomposite. The PD number for sample A is linearly increasing with time
both for positive (+ve) and negative (-ve) PD. The -ve PD number is slightly higher than +ve PD
number (Fig. 2 (a)). The PD number of sample B also increases linearly where the +ve PD number
is a little bit higher than the -ve PD number (Fig. 2 (b)). In contrast, for sample C, the +ve PD
number is lower than the -ve PD number (Fig. 2 (c)). This observation is also true for samples D
and E (Fig. 2 (d) and (e)).
Average Positive (+ve) and Negative (-ve) PD Number. The average PD number every ten
minutes for all samples are shown in Fig. 3. From this figure, it can be observed that the
consecutive 10-minute averages of all samples do not follow one common trend. For the virgin
sample, the 10-minute average PD number increases with time for both polarities. Sample C (4wt%
nanosilica) follows a trend of more or less constant value for both polarities. Samples B, D, and E
seem to follow a similar trend in that the PD number is fluctuating at every 10 minute
measurement.
The Total PD Number of LDPE nanocomposites. Fig. 4 shows the total of PD number for the
LDPE nanocomposite. The total PD number increases for all samples as shown in Fig. 4 (a). The
PD number per interval decreases after about one hour PD test duration. The lowest PD number is
134 Mechanical and Materials Engineering

Fig. 2 Cumulative PD number
for various samples of LDPE
nanocomposites over 60
minutes duration (a) Sample A
(virgin LDPE), (b) Sample B,
(c) Sample C, (d) Sample D,
and (e) Sample E.
Fig. 3 10-minutely average PD
number for various samples of
LDPE nanocomposites (a)
Sample A (virgin LDPE), (b)
Sample B, (c) Sample C, (d)
Sample D, and (e) Sample E.
Fig. 4 The weight percentage (w%) effect of nanosilica filler to LDPE PD number, (a) PD trend for 60
minutes, and (b) PD trend every 10 minutes (c) The total PD numbers
Applied Mechanics and Materials Vol. 554 135

achieved by the LDPE blended with 8w% of nanosilica. Adding only 4w% nanosilica to the LDPE
blend apparently increases the PD numbers, but then it significantly decreases after a higher amount
of nanosilica is added to the LDPE. This could be explained by inspecting the dispersion of the
composite and surface condition of the sample before the test.
Conclusion
PD characteristic of LDPE nanosilica was presented and discussed in this paper with the
following results; adding a certain w% to LDPE blend will not improve PD characteristic directly,
but should be proved by experiment before a final conclusion remarked. Adding w% of nanosilica
to LDPE blend up to 4 w% increased the PD number, but a bigger w% of the filler, such as 8 w%,
cause the PD number to decrease significantly.
Acknowledgment
The Authors wish to thank Ministry of Science, Technology and Innovation (MOSTI), Ministry
of Education (MOE), and Universiti Teknologi Malaysia (Research Vote Nos. 4S045, 03H59, and
4F291) for the financial aid.
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Mechanical and Materials Engineering
10.4028/www.scientific.net/AMM.554
Partial Discharge Characteristic of Low Density Polyethylene and Silica Nanocomposite
10.4028/www.scientific.net/AMM.554.133
DOI References
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term failure in nanodielectrics: a review, IEEE Trans. on Dielectrics and Electrical Insulation, 17 (2010)
1523-1535.
http://dx.doi.org/10.1109/TDEI.2010.5595554