Electrical Breakdown Characteristics of Polypropylene for an Eco-Friendly Power Cable
(Dewimaruto Ratri)
1iD
(Bella Eliana)
1iD
(Seunghee O)
1iD
(Younghun Park)
2iD
(Hyoungku Kang)
†iD
-
(Master course, Dept. of Electrical Eng., Korea National University of Transportation)
-
(Undergraduate course, Dept. of Electrical Eng., Korea National University of Transportation)
Copyright © The Korean Institute of Illuminating and Electrical Engineers(KIIEE)
Key words
Dielectric characteristics, Eco-friendly, Electrical breakdown, Polypropylene, Recyclable, XLPE
1. Introduction
XLPE has been widely used as an insulation material to fabricate a power cable due
to its excellent electrical and mechanical characteristics (1). It is known that the lifetime of a power cable is about 40 years (2). When a power cable is disposed after the operation in a power grid, copper used
as a core of a power cable is usually recycled to various industrial applications.
However, the insulation material, XLPE could not be recyclable due to its thermosetting
characteristics (3). Recently, many research institutes are actively conducting research on the development
of eco-friendly materials as an electrical insulation material for a power cable (4). Polypropylene (PP) is considered as the most promising insulation material for a
power cable to replace XLPE due to its thermosetting characteristics and relatively
high melting point (5), (6).
In this paper, dielectric experiments on the electrical breakdown characteristics
of the six types of PP specimens and conventional XLPE specimen are conducted under
AC and lightning impulse voltage according to the standard of American society for
testing materials (ASTM) D-149 which deals with electrical breakdown experiment on
various solid materials (7).
2. Dielectric Experiments
2.1 Properties of Specimens
In this study, PP samples developed by Hu-innovation Co. Ltd with six different types
are measured and compared with the conventional XLPE through a experimental study.
The specifications of specimens are described in Table 1. The specimen, P represents the PP sample manufactured with conventional process
and the specimens, #1, #2, and #3 denote the samples fabricated with modified process
by using different lot compared with the conventional process. Specimen A means the
sample made by mixing metal powder and specimen B is the sample made by mixing dust
at the manufacturing process.
Table 1. Specifications of Specimens
|
Specimen
|
Process
|
Resistivity [Ω·m]
|
Size [mm]
|
|
P
|
original
|
0.5 × 1017
|
100 ×100
|
|
#1, #2, #3
|
modified by lot
|
> 1 × 1017
|
|
A
|
mixing metal powder
|
> 1 × 1017
|
|
B
|
mixing dust
|
> 1 × 1017
|
The six types of specimens are manufactured in order to verify the influence of manufacturing
process on the dielectric characteristics of PP samples for a power cable.
2.2 Experimental Set-Up
The schematic drawing of dielectric experiment is shown in Fig. 1. The electrical breakdown voltages of PP and conventional XLPE are measured and compared
by the dielectric experiments which conform to the standard of ASTM D-149. Dielectric
experiments are conducted by an AC power supply with a capacity of 100kV at 60Hz and
a lighting impulse power supply with a capacity of 600kV whose waveform of 1.2/50$\mu$s.
According to the ASTM D-149, a sphere to sphere electrode system made with stainless
steel with a diameter of 12.7mm is used to determine the dielectric characteristics
of PP and XLPE samples. The sphere to sphere electrode with a diameter 12.7mm is a
representative of an experimental condition suggested by the international standard,
ASTM D-149.
Fig. 1. Schematic drawing of dielectric experiment
Table 2. Specifications of electrode system
|
Material of electrode
|
stainless steel
|
|
Electrode system type
|
sphere to sphere
|
|
Diameter of sphere electrode [mm]
|
12.7
|
|
Insulating material
|
silicone oil
|
|
Applied voltage
|
AC, Imp.
|
Fig. 2. Structure of electrode system
Table 3. Specifications of specimens
|
Specimen
|
Thickness [mm]
|
Size [mm]
|
|
#1, #2, #3,
P, A, B
|
1.0 ± 0.1
|
120×120
|
Fig. 3. Comparison of electrical breakdown voltage between PP according to various manufacturing processes and XLPE.
The specifications of electrode system are shown in Table 2. Fig. 2 shows the electrode system made with MC Nylon. In order to avoid a creepage discharge
along the surface of specimens, the electrode system and specimen are immersed in
silicone oil at the temperature of 285K. Table 3 shows the specifications of PP samples.
2.3 Experimental Results
The electrical breakdown voltage of PP is measured under AC and lighting impulse voltage
with the electrode system in a chamber filled with silicon oil like shown in Fig. 1. All experiments are repeated five times and analyzed by using the Weibull distribution
program, Minitab. In Weibull distribution, the probability value of 63.2% represents
the point where the shape and scale parameters are equal. The breakdown voltage values
with a 63.2% possibility are used to estimate the distribution's reliability and parameters.
Fig. 3 shows the dielectric experimental results under AC and lighting impulse breakdown
voltage for each specimen with the same condition. As shown in Fig. 3, The electrical breakdown voltage with the probability of 63.2%, VBD,63.2% of PP
is superior to that of XLPE. It is found that the VBD,63.2% of a sample, P is approximately
58% superior to that of XLPE. However, it is observed that the electrical breakdown
voltages of PP samples are not deeply dependent on manufacturing process. Also, it
is verified that the dielectric characteristics of PP regardless of manufacturing
process are always greater than those of XLPE.
3. Analysis
3.1 FEM Analysis
Analysis on the electric field distribution of a electrode system is conducted by
using a finite element method (FEM) through COMSOL in order to analyze the effect
of electric field intensity to electrical breakdown. Fig. 4 shows the FEM simulation result of electric field intensity on the P sample. The
mean value of electric field intensity at sparksover (EBD,MEAN) is calculated by multiplying
(VBD,63.2%) by the analytic mean value of electric field intensity (E1kV,MEAN) which
is deduced by an FEM analysis by placing 1kV into a upper electrode. The correlation
of EBD,MEAN, VBD,63.2%, and E1kV,MEAN can be represented as follows:
Fig. 4. Electric field distribution calculated by FEM analysis\
In (1), VBD does not have unit based on the Poisson’s equation which implies the voltage
of an electrode system is proportional to volume charge density.
3.2 Electric Field Intensity at Sparkover
In this paper, the dielectric experiments on PP specimens according to various manufacturing
processes are conducted and the experimental results are compared with those of XLPE.
Actually, PP considered as an eco-friendly material would be manufactured by the facilities
that manufacture XLPE. It means that the equipment used to manufacture XLPE can not
be completely cleaned. Therefore, the influence of impurities on the dielectric characteristics
of PP is investigated with different manufacturing conditions. The electrical breakdown
voltage of samples in Fig. 3 are converted to the electric field intensity at sparkover in Fig. 5 using an FEM analysis. As shown in Fig. 5, it is found that the electric field intensity at sparkover of PP is excellent compared
with that of XLPE regardless of impurities. Also, it is verified that the dielectric
characteristics of PP are not influenced by impurities such as dust and metal powder.
In the case of sample P, it is found that the electric field intensity at sparkover
is 58% better for AC and 84% better for lightning impulse compared with XLPE.
Fig. 5. Comparison of electric field intensity at sparkover between PP according to various manufacturing processes and XLPE.
4. Conclusions
In this paper, six types of PP specimens considering the impurities in the facilities
are developed as an alternative for the conventional insulation material, XLPE. Electrical
breakdown experiments on PP and XLPE specimens under AC and lightning impulse voltage
are conducted and the results are analyzed. As results, the dielectric characteristics
of PP are found to be 58% better for AC and 84% better for lightning impulse voltage
than conventional XLPE. Also, it is verified that the electric field intensity at
sparkover is not largely dependent on impurities.
Acknowledgements
This research was supported by “Human Resources Program in Energy Technology” of the
Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial
resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20184030202270).
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Biography
She received bachelor's degree in electrical engineering from University of Indonesia,
Indonesia in 2019.
Currently, master course in Dept. of electrical engineering, Korea national university
of transportation.
Her research interests are high voltage engineering, power asset management, and applied
superconductivity.
She received bachelor's degree in physics engineering from Telkom university, Indonesia
in 2018.
Currently, master course in Dept. of electrical engineering, Korea national university
of transportation.
Her research interests are high voltage engineering, power asset management, and applied
superconductivity.
She received bachelor's degree in electrical engineering from Korea national university
of transportation in 2020.
Currently, master course in Dept. of electrical engineering, Korea national university
of transportation.
Her research interests are high voltage engineering, power asset management, and applied
superconductivity.
He is currently a undergraduate course in electrical engineering of Korea national
university of transportation.
His research interests are high voltage engineering, power asset management, and applied
superconductivity.
He received doctor's degree in electrical engineering from Yonsei university in 2005.
Currently, professor in Dept. of electrical engineering, Korea national university
of transportation.
His research interests are high voltage engineering, power asset management, and applied
superconductivity.