Effect of Size on Electrical Performance

This paper was presented at IEEE International Symposium on Electrical Insulation, June 2006. ©2006 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. Digital Object Identifier: 10.1109/ELINSL.2006.1665249The electrical breakdown performance, either unaged or after ageing (laboratory or service), is often used as the basis for qualification of a device, design or material. Many of the features that affect these performance levels have been discussed in other documents; contaminants, propensity for water treeing, insulating and semiconducting materials. However the size of cable tested is rarely discussed. This is somewhat surprising as it has been long recognized that electrical failure is an extreme value (the Weibull distribution is a member of this family) or weakest link process. In extreme value processes the performance of the whole device is determined by the single "weakest link". Thus when more "weak links" are present the chance of failure is consequently higher: the measured performance depends on weak link concentration or size of the device. Additionally at some dimensions the thickness of the dielectric can influence the breakdown mechanism itself; especially if the thermal influences are present. This paper will attempt to discuss a number of these size related issues for both AC & impulse conditions; these will include: 1) the effect of the dielectric volume actual mechanism of failure, 2) prediction of performance on service length cables from short length laboratory tests. This has practical relevance on the selection of appropriate qualification levels which will have direct relevance to service performance, 3) the requirements for cable quality when increasing the size (thickness or length) installed

The Role of Degassing in XLPE Power Cable Manufacture

This paper appears in: Electrical Insulation Magazine, IEEE. ©2006 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.This article focuses on a range of elements critical to this process; form the fundamental chemistry, through computational and measurement techniques to the solutions that are in use today. Degassing contributes greatly to the quality of power cables by improving the certainty in electrical testing and improving the dielectric properties. To ensure that the degassing process delivers the expected benefits, it is important to measure and model the process by which the crosslinking byproducts are desorbed from the cable polymers. Weight loss and HPLC have shown themselves to be the most effective and practical measurement techniques

Control of Water Tree Length and Density in Cable Insulation Polyethylenes

This paper was presented at the IEEE International Symposium on Electrical Insulation, June 2006. ©2006 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.Digital Object Identifier: 10.1109/ELINSL.2006.1665284The goal of this paper is to study the influence of the chemical crosslinking of polyethylene on the water tree initiation and propagation in polymer insulation. For this, the water tree resistances of crosslinked and thermoplastic low density polyethylene were compared. Three types of crosslinked polyethylene systems were evaluated: one containing only peroxide and the other two having, beside peroxide, a free retarding additive system. The results were compared with those obtained on their thermoplastic correspondents. The data show that there are differences in both the water tree length and density that can be ascribed to the polyethylene systems. However, only differences in the water tree density could be ascribed to the material form (thermoplastic or crosslinked). The observed results are consistent with differences, on microscopic level, in permittivities and local breakdown strengths.This work was partly supported by the Romanian Ministry of Education and Research, in the frame of Grant CNCSIS A-1461

Structural Aspects of Peroxide Crosslinking of Polyethylene

Despite major developments in the ways of polymerising ethylene (the processes as well as the catalysts) the use of polyethylene is still restricted in several applications due to its low melting point. This shortcoming can in some areas be overcome by the introduction of crosslinks between the polymer chains. This thesis discusses some recent insights in how crosslinks are introduced and utilised in the network structure. Vinyl groups, especially those of pendant type, have been found to have a tremendous effect on the amount of gel formed and on the network density reached. For these studies, two new ethylene copolymers, poly(ethylene-co-1,9-decadiene) and poly(ethylene-co-divinylsiloxane), were used. The vinyl groups were found to be rapidly consumed during the crosslinking reaction, most probably in a polymerisation reaction. Allylic hydrogens were found not to be needed for the reaction of vinyl groups. Reacted vinyl groups contribute to the network structure as chemical crosslinking points. Deeper studies of the network structure revealed that maximum one third of the existing network points present are of chemical nature (i.e. originating from macroradical combination or from a reacted vinyl group). The remaining two thirds consist of entanglements that become trapped to different degrees by the chemical crosslinks. The creation of intermolecular crosslinking points proved to govern the macroscopic network build-up. Thus, the topography of the polymer coil has a significant effect on the network formation. The reference LDPE used in this work was found to occupy only 15% of the volume of a linear polymer having the same average molar mass. This leads to that a large fraction of the network points, both chemical and physical, will be of intramolecular type thereby not contributing to the macroscopic network build-up in an efficient way. The here presented insights can aid in further development of polyethylene resins, especially those designed for use in crosslinked applications

Structural Aspects of Peroxide Crosslinking of Polyethylene

Despite major developments in the ways of polymerising ethylene (the processes as well as the catalysts) the use of polyethylene is still restricted in several applications due to its low melting point. This shortcoming can in some areas be overcome by the introduction of crosslinks between the polymer chains. This thesis discusses some recent insights in how crosslinks are introduced and utilised in the network structure. Vinyl groups, especially those of pendant type, have been found to have a tremendous effect on the amount of gel formed and on the network density reached. For these studies, two new ethylene copolymers, poly(ethylene-co-1,9-decadiene) and poly(ethylene-co-divinylsiloxane), were used. The vinyl groups were found to be rapidly consumed during the crosslinking reaction, most probably in a polymerisation reaction. Allylic hydrogens were found not to be needed for the reaction of vinyl groups. Reacted vinyl groups contribute to the network structure as chemical crosslinking points. Deeper studies of the network structure revealed that maximum one third of the existing network points present are of chemical nature (i.e. originating from macroradical combination or from a reacted vinyl group). The remaining two thirds consist of entanglements that become trapped to different degrees by the chemical crosslinks. The creation of intermolecular crosslinking points proved to govern the macroscopic network build-up. Thus, the topography of the polymer coil has a significant effect on the network formation. The reference LDPE used in this work was found to occupy only 15% of the volume of a linear polymer having the same average molar mass. This leads to that a large fraction of the network points, both chemical and physical, will be of intramolecular type thereby not contributing to the macroscopic network build-up in an efficient way. The here presented insights can aid in further development of polyethylene resins, especially those designed for use in crosslinked applications

The Effect of Crosslinking on the Electrical Properties of LDPE

Two low density polyethylene (LDPE) grades with varying vinyl content were crosslinked with partly different mechanisms. The electrical tree inception voltage, using a double needle setup, was measured on the two materials when crosslinked with increasing amounts of peroxide. The morphology changed from fully developed spherulites to more or less randomly distributed lamella stacks with increasing density of the crosslinked network. A gradual decrease in crystallinity was also noted. It was found that the increasing network density with the subsequent changes in morphology and crystallinity had a major effect on the tree inception voltage. In the densely crosslinked networks sometimes no trees were observed after visual inspection, although the test equipment indicated a tree. There were no significant differences between the two materials

The Effect of Crosslinking on the Electrical Properties of LDPE

Two low density polyethylene (LDPE) grades with varying vinyl content were crosslinked with partly different mechanisms. The electrical tree inception voltage, using a double needle setup, was measured on the two materials when crosslinked with increasing amounts of peroxide. The morphology changed from fully developed spherulites to more or less randomly distributed lamella stacks with increasing density of the crosslinked network. A gradual decrease in crystallinity was also noted. It was found that the increasing network density with the subsequent changes in morphology and crystallinity had a major effect on the tree inception voltage. In the densely crosslinked networks sometimes no trees were observed after visual inspection, although the test equipment indicated a tree. There were no significant differences between the two materials

The effect of different type of crosslinks on electrical properties in crosslinked polyethylene

In this study LDPE has been crosslinked below and above Tm with the use of different techniques. Two of the materials were crosslinked with dicumyl peroxide (DCP), and one material was crosslinked with silane crosslinking, where silane groups are converted to silanol that form crosslinks via a condensation reaction in the presence of a catalyst. A major difference between the crosslinking methods is that peroxide crosslinking takes place in the melt, whereas silane crosslinking take place in the solid state. It has been found that degree of crosslinking and morphology is of importance for a number of electrical degradation properties, i.e. water treeing and electrical treeing