Yoshinao Murata

Superiority of dielectric properties of LDPE/MgO nanocomposites over microcomposites

Dielectric properties were compared between two kinds of low-density polyethylene (LDPE) composites prepared by adding near spherical MgO fillers with different diameters of around several ten nm and several ¿m. In the whole range of temperature from 0°C to 90°C, the conductivity is decreased and the permittivity is increased by the addition of fillers, irrespective of their sizes. However, the decrement in the conductivity is more significant and permittivity increment is suppressed more in the case of the nm fillers. In addition, a drastic increase in dielectric loss factor, observed at low frequencies, is suppressed more significantly by the addition of the nm fillers. While the formation of packet-like charge, observed in LDPE, is suppressed by both fillers, the effect is far more significant in the nanocomposites. This means that the nano-fillers suppress the conductivity enhancement by the voltage application more effectively.

Investigation of electrical phenomena of inorganic-filler/LDPE nanocomposite material

A nanocomposite material, composed of nano-size inorganic-filler added to a low-density polyethylene (LDPE), was subjected to electrical property investigations. The volume resistibility of LDPE, under 80 kV/mm DC field at 90 C, increases by the power of 10 due to the addition of only a few percent of nano-filler. On the other hand, lightning impulse breakdown strength was estimated by using a sphere-plate electrode system not dependent on MgO content in the area of 5 phr or less. From these results, it can be considered that the addition of nano-size inorganic-filler drastically raises the volume resistibility of LDPE without reducing the LDPEs original impulse breakdown strength.

Effects of nano-sized MgO-filler on electrical phenomena under DC voltage application in LDPE

A nanocomposite material, composed of nano-sized MgO-filler added to a low-density polyethylene (LDPE), was subjected to electrical property investigations under DC voltage application. The volume resistivity of LDPE, under 40-80 kV/mm DC field at 90 /spl deg/C, increases by the power of 10 due to the addition of only a few percent of nano-filler. The amount of space charge in LDPE under high electric field is reduced and the ratio of stress enhancement by space charge becomes less due to the addition of nano-sized MgO-filler. The DC breakdown strength of LDPE is increased by adding nano-sized MgO-filler. Based on these results, the mechanism where by the nano-sized MgO filler had improved DC characteristics of LDPE was discussed.

PD Resistance Evaluation of LDPE/MgO Nanocomposite by a Rod-to-Plane Electrode System

Low density polyethylene (LDPE) was melt-compounded with MgO nano-fillers to produce LDPE/MgO nanocomposite. Its partial discharge (PD) resistance was evaluated by using a rod-to-plane electrode system. Erosion depth was measured as an index of PD resistance. Permittivity and tan delta were also measured to investigate interfacial morphological change due to nanostructuration. It was found that LDPE exhibits the increase in its partial PD resistance, if it is filled with the nanofillers by 1 to 10 phr (parts per hundred parts of resins). It should be noted that marked improvement appears in their content even as small as 1 phr. This tendency also appears for the permittivity of the same nanocomposite, i.e. the permittivity seems to show a minimum point, when nanofillers are added by 1 phr. PD resistance and dielectric properties of LDPE/MgO nanocomposite are discussed in terms of a multi-core model

Development History of HVDC Extruded Cable with Nanocomposite Material

In this paper, the outline of the development history of HVDC cable with nanocomposite material is introduced. Nano-sized MgO-filler added into XLPE, which had been developed for HVDC cable insulation, was not investigated from the view point of nanocomposite material until 2001. Since then, nano-scale observation of the dispersion state of nano-sized MgO-filler in HVDC cable insulation has been carried out. It has been found that the dc material, which had been developed for 500 kV dc cable, is XLPE/MgO nanocomposite material in which nano-sized MgO homogeneously dispersed in nano-scale

Space charge behavior in multi-layered dielectrics with LDPE and LDPE/MgO nanocomposites

Space charge distributions were investigated in multi-layered dielectric films consisting of combinations of low-density polyethylene (LDPE) and its nanocomposite (NC) with MgO nano-fillers during application of a dc electric field of 100 kV/mm at room temperature and 50 °C. At 100 kV/mm, a packet-like charge appears in LDPE, but not in NC. Similar behavior is also observed in multi-layered samples. These phenomena are explained by assuming that the nano-fillers effectively suppress the increase in conductivity induced by the high electric field, which agrees well with related work done previously by the authors.

Space charge accumulation and breakdown in LDPE and LDPE/MgO nano-composite under high dc stress at various temperatures

The relationship between space charge accumulation and breakdown at various temperatures is investigated by observing the space charge formation in low density polyethylene (LDPE) and LDPE/MgO nanocomposite material under dc high stress. We have reported, earlier, that, enhancement of electric field in LDPE by packet-like charge under dc stress might be a cause of its electrical breakdown. Furthermore, it has been found that such enhancement does not take place in LDPE/MgO nanocomposite under the same conditions. Therefore, it is believed that the addition of MgO to LDPE results in suppression of charge injection. To understand the effect of temperature, we tried to observe the charge accumulation under high dc stress at various higher temperatures. This is reported in this paper. It is seen that the packet like charge, in LDPE, plays an important role on the electric breakdown at higher temperature. On the other hand, the suppression effect of charge injection by MgO nano filler is still working well even at higher temperatures.

Impulse breakdown superposed on ac voltage in XLPE cable insulation

This paper discusses tree inception and breakdown voltage characteristics of XLPE cable insulation subjected to impulse voltages superimposed on ac voltage. The tree initiation tests were performed on laboratory-molded specimens equipped with needle electrodes, whereas the breakdown tests were conducted on a full-sized cable. The impulse tree initiation stress was found to be dependent on the magnitude of the pre-applied ac stress and the relative polarities of the impulse and the ac peak at the instant of their superposition. Although the impulse polarity has an effect on the tree inception, the general behavior is that the tree inception stress always decreases with an increase of the pre-applied ac stress. This phenomenon is discussed in terms of the space charge effect and the influence of the impulse voltage application itself. The impulse strength of a full-sized cable insulation was found to be independent of the pre-applied ac stress as long as that stress did not exceed the operating stress of a 500 kV cable insulation. However, subjecting cable insulation to higher ac stresses before impulse application caused a reduction of its breakdown strength as compared with the insulation without ac prestressing.

Space charge suppression effect of nano-size fillers added to polymeric materials

Space charge suppression mechanism in nano-composite polymer material is studied using experimental results and numerical simulation. Recently, many kinds of nano-composite polymeric materials have been reported to have improved their characteristics under high electric field. For example, LDPE/MgO nano-composite, which is made up of low density polyethylene (LDPE) and nano size filler of magnesium oxide (MgO), exhibits high volume resistivity and high dielectric strength under dc electric field. Authors have investigated the space charge behaviour in LDPE/MgO nano-composite under high electric field using pulsed electro-acoustic (PEA) method. It has been found that, compared to LDPE, the space charge formation is also suppressed in the nano-composite material. As a reason for the suppression, we have suggested that the induced dipole polarization around MgO filler formed by dc stress application might play a role of carrier trap sites. From the numerical calculation, distortion of electric potential around MgO is seen to be much larger than that around naturally included dipole. It means that the MgO acts as a deep trap site as different from some defect or ions included in LDPE. Using the numerical calculation based on such electric potential distortion, we have tried to simulate the space charge distribution in LDPE/MgO under high dc electric field. The simulation results are in good agreement with the experimental results.

Investigation of space charge distribution and volume resistivity of XLPE/MgO nanocomposite material under DC voltage application

It has been proved earlier that electric properties of polymer insulation can be improved by addition of nano-sized inorganic filler. These materials have attracted a lot of attention. With the addition of nano sized MgO filler in Low Density Polyethylene (LDPE), several improvements in material properties were observed and were already reported. In this paper, investigations on the volume resistivity and space charge distribution in XLPE/MgO nanocomposite (MgO nano-filler mixed with Cross Linked Polyethylene) material are reported. Investigations on packet like charge, observed earlier in LDPE, are also made both for XLPE and XLPE/MgO nanocomposite materials. The results indicate the suitability of XLPE/MgO nanocomposite material for HVDC insulation.