G. Chen

Electrodes and charge injection in low-density polyethylene using the pulsed electroacoustic technique

The effects of electrode materials on space charge formation in low-density polyethylene (LDPE) have been investigated experimentally using the pulsed electroacoustic (PEA) technique. Common electrode materials used in either the laboratory or power cable industry were selected, i.e. aluminum, gold and carbon loaded crosslinked polyethylene (XLPE), and space charge accumulation after the application of high electric stress was monitored. Experimental results demonstrated that charge injection processes take place in all cases once the applied stress has exceeded a threshold. However the amount of charge, and the polarity of the dominant injected charges showed a significant dependence on the electrode materials under the same applied electric stress.

The dimensional effect of breakdown field in ZnO varistors

The relationship between the breakdown electric field EB (electric field corresponding to the current density 1 mA/cm2) and thickness d for three types of ZnO varistors were investigated. The geometric dimensional effect, observed in these varistors, is referred to as the responsible parameter for the change in EB with the variation in the thickness of the varistor samples. The variation in the diameter of the ZnO grains and the corresponding aspect ratio due to the irregularity of each ZnO grain shape are used to characterize the microstructural heterogeneity of the resulting ZnO grain size distribution. The distribution of the ZnO grain size is statistically analyzed, and thereby a model of the microstructure is proposed. The thickness dependence of the breakdown field, EB, obtained via computer simulation shows a similar dimensional effect to the one observed experimentally. It was found that the critical thickness, dc, increases linearly with the dispersive ratio of the ZnO grain length, and the corresponding slope (b2) in the large thickness domain is directly proportional to the aspect ratio of the ZnO grains.

Influence of radiation environments on space charge formation in /spl gamma/-irradiated LDPE

In the present paper the formation of space charge in irradiated low-density polyethylene (LDPE) was investigated using the pulsed electroacoustic (PEA) technique. Specimens of LDPE, /spl sim/100 /spl mu/m thick, were irradiated to various doses at room temperature in a /sup 60/Co /spl gamma/ source in one of three different environments: ambient room air, oxygen-free nitrogen and vacuum. The results obtained indicate that the space charge distribution is dependent on the irradiation environment. There is a large concentration of positive charge evident in the sample irradiated in air compared to very little for material irradiated in either vacuum or nitrogen under similar conditions. This indicates that the presence of oxygen during irradiation, and thereby oxidation itself, has a major influence on the mechanism by which space charge evolves. Thus the monitoring of space charge may in itself be an indicator of the degree of oxidation and of aging in a material. The evolution of space charge and its decay are discussed.

A modified PEA space charge measuring system for power cables

A modified cable pulsed electro-acoustic (PEA) system using the plate outer electrode, which enables the technique to be applied to different cable sizes, is introduced to measure space charge distribution in extruded polymeric power cables. Some preliminary measurement results from cable samples are presented.

Space charge measurements in cables using the PEA method:-signal data processing considerations

Acoustic characteristics in the new cable PEA system have been analysed and it has been confirmed theoretically that the new system can provide the same sensitivity and resolution as the curved electrode system. The new system offers several advantages such as easy assembling sample, intimate contacts at the interfaces and its suitability for different sized cables. Several factors that influence the quantitative charge distribution in cable geometry such as divergence (both in pulsed electric stress and acoustic pressure) and attenuation/dispersion of the acoustic wave in thick insulation are discussed. A data processing technique has been developed which considers all of the above factors in order to improve the measurement accuracy both in sensitivity and spatial resolution. The results from the two XPLE power cables with different thermal treatments have demonstrated that the new system, together with data processing algorithms is capable of measuring space charge distribution and its change with the treatments.

An improved pulsed electroacoutic system for space charge measurement under AC conditions

In this paper, an improved space charge measurement system based on the pulsed electro acoustic technique (PEA) is presented. The new system gives an essential way to examine the role of space charge in electrical aging process under AC conditions. The system setup for AC measurement is presented and detailed in this paper with comparison to the old system. There are two features with improved PEA system. A pulse generator with a 3 kHz repetition rate is utilized to reduce the measurement time. The Eclipse data acquisition system is used to achieve the high data acquisition rate. The results which were taken from both old and new PEA system show that heterocharge can be formed in the region close to the lower electrode under AC electric field. Apparently the results captured from the new system have better phase resolution than the old system. The space charge decay profile measured by the new system can reflect vividly on the charge dynamically changing. The utmost space charge information was saved as the measurement time was dramatically shortened by the improved system.

Surface potential decay measurements on fluorinated polymeric insulation for high voltage DC applications

Space charge accumulation in polymeric insulation is an increasingly important area in high voltage DC transmission as its presence distorts the local electric field, leading to premature failure. By using fluorination process, the surface of polymeric insulation is chemically treated and consequently modifies charge transport characteristics of the material. In doing so, excellent surface properties similar to fluoropolymers can be obtained without compromising the bulk characteristics of the polymeric insulation. The modifications in chemical components at the surface of polymeric insulation should lead to corresponding modifications in electrical properties as well. Different surface fluorinating conditions were investigated and the fluorinated samples were electrically characterised and tested, so an optimal processing condition can be achieved to meet practical requirements for high voltage DC insulating material. This paper will focus on the surface potential decay from negative corona discharge and PEA measurements on fluorinated epoxy samples of different fluorination times in a controlled environment. It has been found that there is a significant change in surface potential decay characteristics with the introduction of surface fluorinated layer. The effects of fluorination process on surface potential decay mechanisms responsible for the observed phenomena were thoroughly discussed.

Influence of fluorination time on surface flashover of polymeric insulation

Charge build up under high voltage DC is a big concern in transmission system as its presence distorts the local electric field. By chemically treat polymeric insulation via fluorination process, the charge transport characteristics of the material can be modified. In doing so, excellent surface properties of fluoropolymers can be attained without compromising the bulk properties of the original polymeric insulation. The change in chemical components at the surface of polymeric insulation should lead to corresponding change in electrical properties at the surface and consequently suppress charge build up. The purpose of this paper is to review recent research into the fluorination of epoxy resins in order to suppress charge accumulation and improve surface DC flashover strength. Surface DC flashover test using a pair of finger electrodes had been conducted. Modelling and simulation of electric field and current density distribution had also been carried out for the fluorinated polymeric insulator. It has been found that the introduction of fluorinated surface layer on epoxy resins plays an important role in improving the surface dielectric properties as evident from experimental and simulation results. The influences of fluorination process on surface flashover mechanisms were thoroughly discussed.

Space charge measurement in XLPE cable with temperature gradient through the insulation

Space charge accumulation in XLPE power cable insulation under dc electric stress at room temperature and gradient temperature across the insulation was experimentally studied. It has been found that space charge is of a constant accumulation rate at room temperature and quite stable as soon as it is generated. In contrast, elevated temperature has not only reduced the charge density but also accelerated space charge accumulation and decay speed.

Polyethylene/silica nanocomposites: absorption current and the interpretation of SCLC

The topic of nanodielectrics continues to receive significant attention from todays dielectrics community, due to the property enhancements that can stem from the unique interfacial features within such material systems. Nevertheless, understanding the interfacial phenomena that occur in nanodielectrics, which determine their electrical behaviour, is challenging. In this paper, we report on an investigation into the absorption current behaviour of two nanocomposite systems, one containing an untreated nanosilica and the other containing the same nanofiller chemically modified using trimethoxy(propyl)silane. The results indicate that the absorption current behaviour of all the nanocomposites is very different from that of the reference, unfilled polymer; while the current flowing through the unfilled polyethylene decreased monotonically with time in a conventional manner, all the nanocomposites revealed an initial decrease followed by a period in which the current increased with increasing time of electric field application. Possible mechanisms leading to the observed absorption current behaviour in the nanocomposites are discussed with the aid of space charge measurements. The presence of space charge limited conduction (SCLC) and its trap-filled limit is proposed.