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International Journal of Electrical & Computer Sciences IJECS-IJENS Vol: 12 No: 03
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Space Charge Characteristics of LDPE/MMT
Nano-Composite Insulation Material under
different Fields and Temperatures
Shakeel Akram, Liao Ruijin, Muhammad Tariq Nazir, Lijun Yang, Baige
The State Key Laboratory of Power Transmission Equipment & System Security and New Technology
Chongqing University, Shapingba District, Chongqing 400044 CHINA
Abstract—Low-density polyethylene (LDPE) is one of the
types of solid polymers and has been used as insulating
material of electrical power cables. When a voltage is
applied on LDPE, space charges appear and accumulate.
However, the electric field distribution in the sample could
be distorted by the accumulation of space charge. The
uneven distribution of electric field due to space charge
usually results in the electric breakdown of LDPE power
cables. Therefore, it is important to understand the
mechanism and accumulation of space charges in LDPE
power cables and to explore the means of elimination. An
improvement in LDPE using Nano-fillers may be one of the
ways to overcome this problem. If the Nano-fillers could
suppress the accumulation of space charges or improve the
migration of space charges in LDPE, the electric
breakdown from space charges will be decreased to some
extent and due to its high resistivity under high dc stress
this material is expected to be used as insulation material in
HVDC underground power cables in future. For this
purpose a series of measurements with 1% content of MMT
and pure LDPE have been taken using PEA technique at
different temperatures and electric field. The results
showed that temperature, electric field and Nano-filler has
great influence on the performance of insulation material.
Index Terms—Space charge, LDPE/MMT Nano-Composite,
Pulsed electro-acoustic (PEA), Electric Field, Temperature
electrons, charge particles or ions; all charge carriers which can
exist within the dielectric material and can be trapped by
material or transport through the material under external electric
field can be called as space charges. These space charges can
distort electric field distribution and this uneven distribution of
electric field is one of the main reasons for electric breakdown
of crosslink power cables [7, 8]. Recently many efforts have
been made to improve the space charge behavior, breakdown
characteristics, conductivity and mechanical performances of
LDPE by adding MgO nanocomposite [9, 10]. However, the
majority of such research work was carried out at room
temperature while the operational temperature of DC cables is
not constant. The first time results have been published on space
charge measurements on HVDC cable paper insulation [11, 12].
However, the influence of temperature has not been discussed.
In this paper MMT is used as nano filler to improve the space
charge behavior of LDPE and based on the PEA technique, a
series of measurements were carried out when the LDPE/MMT
sample was subjected to different applied fields and
temperatures. The pulsed electroacoustic (PEA) technique was
first developed in 1980s and it has been widely used in space
charge measurement because of its low cost and ease of
implementation. The PEA method allow space charges to be
observed during poling, i.e. under electric field, and after
electric field removal, like during depolarization, thus providing
thorough information on space charge behavior. [13]
2. EXPERIMENTAL DETAILS
1. INTRODUCTION
The research works on the nano-fillers into the LDPE have
attracted the attention of researchers for suppressing space
charges in the past two decades [1-6]. Due to its higher
resistivity under high dc stress Low density Polyethylene
(LDPE) mixed with montmorillonite (MMT) nano filler is
expected to be used as insulation of HVDC underground power
cables transmission. To use the material as the dc power cable, it
is necessary to study the space charge characteristics, especially
at high temperature and different voltage levels because an
actual power cable is used at high temperature and different
level of fields.
The meaning of space charge accumulation in the dielectric
material is that the excessive electrical charge is being
distributed over a region of continuum of space rather than over
a distinct point. It means no matter whether these are holes,
2.1 Sample Details
Low density polyethylene (LDPE 2426H, density: 0.9227
g/cm3, melt index: 1.0 g/10min) was purchased by Lanzhou
Petrochemical Co., Ltd. (Gansu, China). A natural
montmorillonite (MMT) clay surface modified with
octadecylamine and silane coupling agent (Nanomer I.31PS,
Nanocer) was used as the reinforcement filler.
The LDPE/MMT nanocomposites were prepared in two steps.
Firstly, mixtures of LDPE containing 1% mass of organically
modified MMT were mixed in a high-speed two-roll mill for 10
min at 100°C. Secondly, the mixture of LDPE and the modified
MMTs were compounded in a counter-rotating twin-screw
extruder at 160°C. The screw rotation speeds and feeding rate is
120rpm. The pure LDPE (hereinafter “PE”) was then prepared
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for comparison with the properties of the PE/MMT composites.
The dried LDPE specimens and LDPE/MMT nanocomposites
specimens were put into a mold of 0.1mm thickness and then
compressed this mold into plates by a plate vulcanizing machine
at 170°C for 15 min. The pressure of compression molding was
15 MPa. The LDPE mixed with 1% of organic modified MMT
named as M1.
2.2 Measurement of Space Charge using PEA
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Different polarity charge carriers have different injection
characteristics to those of LDPE/MMT film. In this work
negative DC field is applied on top copper electrode of PEA
system and positive is connected with aluminum electrode
known as anode. It is very clear from the space charge behavior
as shown in figure that anode peak is sharp and more evident as
compare to the cathode peak that is flat and wider, this is
because anode terminal is near to the sensor while at cathode
terminal the acoustic wave scatter and attenuated.
The space charge accumulation and effects of temperatures in
pure LDPE and M1 specimens were measured by using pulsed
electro-acoustic method. The pulse width of pulse generator is 5
ns; the range of pulse voltage is from 0 kV to 0.2 kV. The
thickness of the piezoelectric sensor film of polyvinylidene
fluoride is 25 µm. Ethylene-Vinyl Acetate Copolymer with
graphite and Aluminum is deposited on two sides of specimens
as electrodes. Top electrode where negative DC field is applied
is made of copper and lower ground electrode that is near to
PVDF sensor is made of aluminum. Silicone oil is used as an
acoustic coupling agent in order to make a good acoustic
contact between the specimen’s electrode and the measuring
electrode. The range of DC source voltages is from 0 kV to 20
kV. The principle of this method of measurement and
experimental details are described in [14, 15].
Before PEA tests, several pre-treatment steps were carried
out upon samples. Firstly, the pure samples of LDPE and
LDPE/MMT were cut into square shapes with a length of ~ 4 cm
and measure the thickness by using digital thickness gauge and
then clean it with ethanol. Then the samples were preheated for
10 minutes before measuring PEA. In the experiment, a
30kV/mm and 50 kV/mm of negative DC field was applied to
these samples according to thickness range between 0.14mm to
0.2mm for 0 to 60 minutes with different intervals, These
measurements were taken on three temperature levels (24°C,
50°C and 70°C) and temperature was controlled by using a
temperature controller STC-400 and heating load. Then these
specimens were short-circuited for 30 min to analyze the decay
rate with Voltage off. The space charge accumulation, charge
trapping and decaying in these specimens were observed and
measured through a digital oscilloscope by using Lab view
software. The measuring setup of PEA is as shown in figure 1
(a) 30kV/mm at 24°C
(b) 30kV/mm at 50°C
(c) 30kV/mm at 70°C
Fig. 1 Space Charge Measuring Setup
3. PEA TEST RESULTS AND DISCUSSION
3.1 PEA Test Results of Virgin LDPE at different
Fields and Temperatures
(d) 50kV/mm at 24°C
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(e) 50kV/mm at 50°C
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(c) 30kV/mm at 70°C
(d) 50kV/mm at 24°C
(f) 50kV/mm at 70°C
Fig. 2 Space charge dynamics of pure LDPE at different
temperatures and fields
3.2 PEA Results of M1 Sample at Different Fields
and Temperatures
(e) 50kV/mm at 50°C
(a) 30kV/mm at 24°C
(f) 50kV/mm at 70°C
Fig. 3 Space charge dynamics of M1sample at different fields
and temperatures
(b) 30kV/mm at 50°C
From figures 2 and 3 it is clear that mostly at room
temperature the peak value at electrodes decreases with the
increase in time of electric stress from 0 to 60 min, while in case
of temperatures at 50°C and 70°C the peak value increases with
the increase in time from 0 to 60 min. This is because at high
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temperature the mobility of negative electrons increases and it
try to move inside the sample while more and more positive
electrons accumulate at anode. Due to each applied electric
field, homocharges inject at both electrodes, in case of room
temperature the quantities of electric charges on both electrodes
decreases as the stressing time increases.
According to the test results of PEA as mention in above
figures it is clear that temperature and MMT filler has
significant effect on space charge accumulation. By comparing
the peak values of charge density on anode terminal in all cases
of above tests it is concluded that charge density and electric
field increases with the increase in temperature. It indicates that
the combination of high temperature and electrical stress will
bring fatal impacts on the performance of LDPE/MMT
insulation of high power cables. It is also believed that the
injection from the cathode terminal is enhanced due to the
presence of positive charge. As the mobility of negative charges
is faster especially at higher temperature, so the injected
negative charges tend to move towards the anode and some of
them may be able to move across the sample without being
neutralized by the positive charge and that is why a small
amount of negative charge is held adjacent to the anode. PEA
technique only shows the net charge, the observed reduction in
positive charge across the sample indicates the existence of
negative charge. In Pure LDPE sample the amount of these
negative charges are more adjacent to anode terminal than the
M1 sample because the presence of MMT composite particle
provides hindrance for the movement of charges and neutralize
the holes and electrons
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(b) 30kV/mm at 50°C
(c) 30kV/mm at 70°C
3.3 Space Charge Decay in Pure LDPE and M1
Samples after Volt Off
(d) 50kV/mm at 24°C
After 60 min of DC stress, the space charge dissipation after
the removal of the applied voltage are shown in figures 4 and 5.
Compared to volts-on tests where charges are more easily
injected into the system, the charge movement under short
circuit condition is relatively slow. The charge decay speed
becomes much slower with the time increase. After 30 min,
about a quarter of space charges remains in the sample. Further
tests indicate that about 80% charges disappear after 1h. The
slow decay may be related to the positive charge at the
interfaces, the interfaces and filler act as a barrier to trap
positive charge and limiting the movement of positive charge.
The presence of positive charge affects the negative charge
decay.
(e) 50kV/mm at 50°C
(f) 50kV/mm at 70°C
(a) 30kV/mm at 24°C
Fig. 4 Space charge decay of pure LDPE sample after DC
electric stressing
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(a) 30kV/mm at 24°C
(b) 30kV/mm at 50°C
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(f) 50kV/mm at 70°C
Fig. 5 Space charge decay of M1 sample after DC electric
stressing
After the removal of the applied DC voltage the space charge
behaviors are shown in above section figures. At 50°C, more
than 80% charges decay after removing the applied voltage for
30 min, which is much faster than that at 24°C. In most cases the
space charge decay rate corresponds to its accumulation rate, i.e.
the faster the space charge accumulates, the faster it decays, or
vice versa [15]. Typical results which support this conclusion
are presented in figure 6. Charge decays much faster than that at
24°C, and almost all the space charge injected in the
LDPE/MMT sample diminishes in 10 minutes after removing
the applied voltage, only a little bit charges caused by pulse is
observed. The total space charge that related to the electric
performance as well as the physical, chemical and microcosmic
characteristics represents the property of space charge transport
inside the material. The total absolute amount of charge
accumulated in the samples can be calculated based on the
charge-density distribution by
d
Q (t ) = ∫ ρ ( x, t ) Sdx
0
(c) 30kV/mm at 70°C
(d) 50kV/mm at 24°C
Where ρ(x, t) is the charge density; S is the electrode area; d is
the thickness of the sample.
Fig. 6 Total charge decay rate at 50kV/mm under two
temperature levels
4. EFFECT OF TEMPERATURE AND
FIELD DURING VOLTS-ON
(e) 50kV/mm at 50°C
Considering the peak value of charge density at the anode on
the volts-on 60 min (figures 7, 8), both the applied field and
testing temperature have great effect on charge density at the
anode, the same may happen at the cathode as well, though not
so obvious due to acoustic scattering and attenuation in
LDPE/MMT insulation material. In general, at low temperature
(24°C or room), the applied field has a greater influence on the
charge density. However, with temperature increasing, the
influence becomes weak gradually. But when it comes to 70°C
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and the applied field is 50kV/mm, the charge density in pure
LDPE sample gets the maximum value. It indicates that the
presence of nanocomposite can reduce the accumulation of
space charge and combination of high temperature and
electrical stress will bring important impacts on the
performance of LDPE/MMT insulation material
Fig. 7 Comparison of Pure LDPE at different temperatures and
Fields
.
Fig. 8 Comparison of M1 sample at different temperatures and
Fields
There are two main sources of space charge in LDPE, on the
one hand the electrodes result in homo-charges injection, and on
the other hand hetro-charges are from defect which is located
the amorphous region and spherulites boundaries. In other
words, injected carriers (electrons or holes), during charge
transport process, will be captured by these defects under the
electric field and become space charge. MMT particles are
inorganic. A certain quantity of MMT particles can be used as
nucleating agent to increase the crystallinity degrees of LDPE
specimens. With the increasing of crystallization degree, the
defects located amorphous region and spherulites boundary
decrease in LDPE specimens, and the deep trap which captured
charge reduces. These indicate that increase of crystallization
degree is functional to suppress the accumulation of space
charges and improve the transfer rate of space charges.
Therefore, the dissipation rate of space charge in M1 sample
was much faster than that in pure LDPE specimens and the
accumulation of space charge are more difficult in the M1
sample of LDPE/MMT nanocomposites
5. CONCLUSION
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In this paper, the PEA measurements on LDPE/MMT
nanocomposite insulation material were presented. Space
charge dynamics under volts-on and decay conditions are
analyzed at different fields and temperatures. The conclusions
are summarized as follows:
(1) The formation and dynamics of space charge can affect the
performance of insulation material under different operating
conditions such as temperature and electric field.
(2) Space charge in Low Density Polyethylene mixed with 1%
of Nano-composite such as MMT has been investigated using
the pulsed electroacoustic (PEA) technique. Charge behavior in
pure LDPE and LDPE mixed with MMT has been analyzed and
the influence of temperature on charge dynamics was discussed.
The results show that homo-charge injection takes place under
all the test conditions, the applied DC field mainly effect the
amount of space charge, while the temperature has greater
influence on the distribution and mobility of space charge inside
LDPE/MMT samples. Organic nano-MMT improved the
breakdown strength and space charge suppression of LDPE
materials to some extent,
(3) As the Dc negative field applied on the surface of
LDPE/MMT insulation material the positive charges near anode
terminal start to accumulate on the surface and it increase as the
stressing time or temperature increase. This show that insulation
material layer surface act as barrier for positive charge, while
some negative charges also accumulate in the vicinity of anode.
This show that the mobility of negative charges is higher as
compare to negative charges so that some negative charges
cross the barrier of insulation material. This will affect the
distribution of electrical field and deteriorate the electrical
behavior of LDPE/MMT insulation and on higher temperature
this effect is more critical. In case of 1% content of MMT the
effect of temperature is lower as compare to pure LDPE sample.
(4) Experiments and analysis results showed that the amount
of injected charge, charge mobility and charge dissipation in
LDPE/MMT insulation are affected by the applied electric field
and temperature. Also the addition of MMT Nano-particles
restrained the accumulation of space charge of LDPE under DC
voltage, improved the rate of dissipation of space charge of
LDPE after the removal of the applied voltage, and reduced the
influence of space charge of cable for DC high voltage
transmission and high temperature.
Acknowledgment
Author wishes to thank the Chinese Government Scholarship
Council (CSC). This Project is supported by National Natural
Science Foundation of China (50807054)
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1999 he is a professor as well as head of Electrical Engineering
College at Chongqing University, China. His research activities
lie in the field of on-line monitoring of insulation condition and
fault diagnosis for high voltage apparatus, as well as aging
mechanism and diagnosis for power transformer. He is author/
coauthor of one book and over 80 journal and international
conferences.
Muhammad Tariq Nazir was born in
Punjab province, PAKISTAN, on
September 3rd, 1986. He received the
B.Sc. degree from University of
Engineering & Technology TAXILA,
PAKISTAN 2009. He is now working
towards M.Sc. degree in College of
Electrical Engineering, Chongqing
University. His main research interests
include high voltage technology, external insulation and
transmission line’s icing.
LijunYang was born in Sichuan,
China in 1980. She received her M.S.
and Ph. D. degrees in electrical
engineering
from
Chongqing
University, China respectively in
2004 and 2009. She was a visiting
researcher at the High Voltage
Engineering Division of Chalmers
University of Technology, Sweden, in
2011. Her major research interests
include online detection of insulation condition of electrical
devices, partial discharges and insulation fault diagnosis in high
voltage electrical equipment. She is author more than 20 journal
as well as conference papers.
Shakeel Akram was born in Punjab
province, Pakistan on August 08,
1987. He received his B.Sc. electrical
engineering
from
Bahauddin
Zakariya University Multan Pakistan
in 2010. He is now working towards
M.Sc.
degree
in
Electrical
Engineering
from
Chongqing
University China. His major research
field in high voltage technology is
space charge measurement of insulating materials used in
insulation of power cables and transformers.
Bai Ge was born in Sichuan, China in
1986. He received his B.Sc. degree in
2008 from college of physics
Chongqing
University
China.
Currently he is pursuing his MS.
leading to Ph.D. degree from college
of Electrical Engineering Chongqing
University China. His major research
area is Space charge measurement of
insulation paper.
Rui-jin Liao was born in Sichuan,
China in 1963. He received the M.S.
and Ph.D. degrees in electrical
engineering from Xi’an Jiaotong
University, China and Chongqing
University, China, respectively. Since
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