The influence of pulse voltage on the accuracy of charge density distribution in the pulsed electroacoustic technique (PEA) is discussed. It is shown that significant error can be introduced if a low dc voltage and high pulse voltage are used to calibrate charge density. However, our main focus in the present paper is to deal with one of the practical situations where space charge exists in the material prior to any measurements. The conventional calibration method can no longer be used to calibrate the charge density due to the interference by the charge on the electrode induced by space charge. A method has been proposed which is based on two measurements. Firstly, the sample containing the charge is measured without any applied voltage. The second measurement is carried out with a small external applied voltage. The applied voltage should be small enough so there is no disturbance of the existing charge in the sample. The difference of the two measurements can be used for calibration. An additional advantage of the proposed method avoids the influence of the pulse voltage on calibration and therefore gives a more accurate representation of space charge. The proposed method has been validated.

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, insulatorr r r r relectrode 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.

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 kV dc /mm at four temperatures 25, 50, 70 and 90 o C for 24 h. Space charge profiles in both asreceived 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.

This paper reports on the result of space charge evolution in cross-linked polyethylene (XLPE) planar samples approximately 220 µm thick. The space charge measurement technique used in this study is the pulsed electroacoustic method.There are two phases to this experiment. In the first phase, the samples were subjected to dc 30 kV dc mm −1 and ac (sinusoidal) electric stress levels of 30 kV pk mm −1 at frequencies of 1, 10 and 50 Hz ac. In addition, ac space charge under 30 kV rms mm −1 and 60 kV pk mm −1 electric stress at 50 Hz was also investigated. The volts-off results showed that the amount of charge trapped in XLPE sample under dc electric stress is significantly bigger than samples under ac stress even when the applied ac stresses are substantially higher.The second phase of the experiment involves studying the dc space charge evolution in samples that were tested under ac stress during the first phase of the experiment. Ac ageing causes positive charge to become more dominant over negative charge. It was also discovered that ac ageing creates deeper traps, particularly for negative charge.This paper also gives a brief overview of the data processing methods used to analyse space charge under ac electric stress.

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.

In this paper, the DC space charge measurement results obtained from 24 hours ageing of 1.2"XLPE planar samples at 35 kV (-30 k V / a ) with four different temperature conditions (i.e. 25'C. 50°C, 70°C and 90°C) are reported. Emphasis has heen 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.

This paper reports on space charge dynamics in XLPE samples, which have been pre-stressed under ac electric stress, when subjected to 30kV dc /mm dc electric stress for 24 hours. The samples used for this study were planar XLPE samples of ~ 220 µm thick aged under 30 kV pk /mm, 30 kV rms /mm and 60 kV pk /mm at 50 Hz ac condition for 24 hours. In addition, the experiments were also extended to samples that were pre-aged under 1 Hz and 10 Hz ac 30 kV pk /mm for 24 hours.

Abstraet. In this paper, the influence of the Aluminium LIpP principle (Al) disc thickness on the acoustic refection, spatial resolution of space charge measurements in 1.2mm and 2"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 asreceived and degassed planar XLPE samples subjected to a dc elecmc stress of 30 kV/mm (at room temperature) for 24 hours are reported.