space charge characteristics of ldpe/mmt nano-composite...

International Journal of Electrical & Computer Sciences IJECS-IJENS Vol: 12 No: 03 50

126803-5757 IJECS-IJENS © June 2012 IJENS I J E N S

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

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,

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

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

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



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 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

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

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.

(a) 30kV/mm at 24°C

(b) 30kV/mm at 50°C

(c) 30kV/mm at 70°C

(d) 50kV/mm at 24°C



(e) 50kV/mm at 50°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







Fig. 3 Space charge dynamics of M1sample at different fields and temperatures

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



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

3.3 Space Charge Decay in Pure LDPE and M1 Samples after Volt Off

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.







Fig. 4 Space charge decay of pure LDPE sample after DC electric stressing









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

0

( ) ( , )d

Q t x t Sdxρ= ∫

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

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



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

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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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.

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

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.

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.

space charge characteristics of ldpe/mmt nano-composite...

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