Y. F. F. Ho

Temperature effect on space charge dynamics in XLPE insulation

This paper reports on space charge evolution in crosslinked polyethylene (XLPE) planar samples approximately 1.20 mm thick subjected to electric stress level of 30 kVdc/mm at four temperatures 25, 50, 70 and 90 degC for 24 h. Space charge profiles in both as-received and degassed samples were measured using the laser induced pressure pulse (LIPP) technique. The DC threshold stresses at which space charge initiates are greatly affected by testing temperatures. The results suggest that testing temperature has numerous effects on space charge dynamics such as enhancement of ionic dissociation of polar crosslinked by-products, charge injection, charge mobility and electrical conductivity. Space charge distributions of very different nature were seen at lower temperatures when comparing the results of as-received samples with degassed samples. However at higher temperature, the space charge distribution took the same form, although of lower concentration in degassed samples. Space charge distributions are dominated by positive charge when tested at high temperatures regardless of sample treatment and positive charge propagation enhances as testing temperature increases. This can be a major cause of concern as positive charge propagation has been reported to be related to insulation breakdown

Heat treatment of cross-linked polyethylene and its effect on morphology and space charge evolution

It is believed that space charge build-up in cross-linked polyethylene (XLPE) insulation is the main cause of premature failure of underground power cables. The space charge activities in XLPE depend on many factors such as additives, material treatment, ambient temperature, insulator/electrode interface, etc. Degassing is the process of subjecting the XLPE to a moderately high temperature, thereby removing volatile residual by-products and additives that are commonly employed in cable manufacturing to improve insulation performance. While there have been reports comparing space charge activities (as-received) un-degassed and degassed samples, little attention has been placed on degassing conditions effect on space charge. In this paper, investigations of morphology and space charge distribution of planar XLPE samples was carried out as parameters such as degassing temperature, degassing duration and sample thickness were adjusted. Changes in the morphological structure of the material were probed with the differential scanning calorimeter and the transmission electron microscopy, whereas space charge distribution of the samples subjected to DC electric stress was monitored for 24 h using the laser induced pressure pulsed technique. It was revealed that degassing temperature, duration and the thickness of the degassed sample has a direct effect on the morphological structure and space charge distributions of the samples. It is believed that the presence and amount of by-products takes precedence over the morphology of the material as far as space charge evolution is concerned.

Temperature effect on the space charge characteristics in as-received and degassed XLPE insulation under DC stressing condition

In this paper, the DC space charge measurement results obtained from 24 hours ageing of 1.2mm XLPE planar samples at 35 kV (/spl sim/30 kV/mm/sub dc/) with four different temperature conditions (i.e. 25/spl deg/C, 50/spl deg/C, 70/spl deg/C and 90/spl deg/C) are reported. Emphasis has been placed on comparing the space charge characteristics of the XLPE plaque at different temperatures under DC ageing conditions. The effects of sample degassing on space charge dynamics at different temperatures are also be presented.

Do semicons affect space charge

This paper reports on space charge measurements on /spl sim/1.5 mm thick XLPE and polyolefinic plaques with two different treatments and semicon electrodes, namely, the same XLPE and polyolefin loaded with carbon. Samples were subjected to both DC and 50 Hz AC electric stresses in the region of 25 kV/mm (at room temperature). Measurements of charge profiles and stress distributions were made using the modified laser induced pressure propagation system. Emphasis has been placed on comparing the space charge characteristics of the two insulation systems with the different sernicon electrodes. The effects of sample treatment (i.e. degassing) on the space charge dynamics are also presented.

The effect of degassing on morphology and space charge

It is believed that space charge buildup in cross-linked polyethylene (XLPE) insulation is one of the main cause for premature failure of underground power cables. The space charge activities in XLPE depend on many factors such as additives, material treatment, ambient temperature, insulator/electrode interface, etc. Degassing is one of the material treatment processes commonly employed in cable manufacturing to improve insulation performance. In this paper, investigation on the effect of degassing period on the morphology and space charge was carried out. Planar XLPE samples of the same composite were subjected to different degassing time. It is discovered that apart from removing volatile by-products, degassing also anneals XLPE material; changing the morphology as a result.

Optimization of the aluminium target thickness on space charge measurement using the LIPP technique

In this paper, the influence of the Aluminium (Al) disc thickness on the acoustic reflection, spatial resolution of space charge measurements in 1.2mm and 2mm XLPE plaques using the LIPP system has been studied. Experimental results indicate that there exists a minimum thickness of Al for each sample, which requires eliminating the signal reflection. It is also revealed that the spatial resolution determinate with the thickness of Al. Using a optimal thickness of Al for the target, preliminary results obtained from 2mm as-received and degassed planar XLPE samples subjected to a dc electric stress of 30 kV/mm (at room temperature) for 24 hours are reported.