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From LDPE to XLPE: investigating the change of electrical properties. Part I. space charge, conduction and lifetime

This paper discusses the results of space charge, conduction current and electrical lifetime measurements performed on widely-used materials for electrical insulation, that is, low density polyethylene (LDPE) and cross-linked polyethylene (XLPE). Space charge accumulation profiles were compared in LDPE, low density polyethylene plus antioxidant (LDPE+AO) and XLPE, with consideration of thermal treatment effects in LDPE and XLPE. Significant variation (decrease) of accumulated space charge and apparent mobility, as well as slight decrease of conduction current, can be seen going from LDPE to LDPE+AO and XLPE, which may be associated with formation of deeper trap levels (or an increase of their density). On the contrary, electrical life under AC voltage does not show significant differences from LDPE to XLPE. This latter result underlines that life under high AC electrical stress is mostly determined by defects (weak points) rather than material characteristics associated with charge injection and transport.

From LDPE to XLPE: Investigating the change of electrical properties. Part II. luminescence

Luminescence experiments have been used to probe the nature of trapping sites and the dynamic behavior of charge carriers in low density polyethylene (LDPE), antioxidant containing LDPE (LDPE+AO) and cross-linked polyethylene (XLPE). It is shown that deep traps of different natures are present in these materials. For example carbonyl functions provide deep traps in LDPE. New traps are seen in LDPE+AO and XLPE, associated with the anti oxidant itself or with its reaction produces, and to the by-products of the cross-linking reaction. Among those, acetophenone, cumyl alcohol and /spl alpha/-methylstyrene cannot account for the observed spectra. It is shown, in addition, that two XLPE compounds based on two different base resins exhibit the same behaviour demonstrating that the chemical and/or structural modifications introduced by the cross-linking process itself control the charge storage in XLPE. The electrical properties of the same materials are discussed in the light of the work reported in pt.I see ibid., vol.12 no.3 p.438-46, (2005).

Electrical performance of crosslinked polyethylene under uniform and divergent fields

This paper presents test techniques which enable electrical endurance characterization of HV cable insulating materials in relatively short times. Three crosslinked compounds are considered, i.e., two kinds of homopolymers, slightly differing in morphology, and a copolymer blend based on PE (polyethylene). The technique relies upon electric strength and life tests performed on plaques, subjected to uniform electrical field, and tree-inception tests, realized by needle-plane electrodes. The entire test cycle lasts /spl sim/500 h, but it proves to be able to distinguish among material performance not only in short times, but, probably even in the long run. In particular, it is shown that the copolymer blend behaves better, from the point of view of electrical endurance, than the homopolymers. It is thus emphasized that a test procedure as here proposed allows prompt feedback, which is fundamental for the technological development of insulating materials.

Chemical crosslinking of polyethylene and its effect on water tree initiation and propagation

The water tree resistance of chemically crosslinked polyethylene and of low density polyethylene were compared in order to elucidate whether the crosslinking itself influences or not the water tree characteristics of polymeric cable insulation materials. For this purpose, water trees were grown in compression moulded disks, obtained from pellets of either thermoplastic or chemically crosslinked polyethylene. Three types of crosslinked polyethylene were evaluated: one containing only peroxide (XLA) and two others having, besides peroxide, a tree retarding additive system (XLB and XLC), and the results were compared with those obtained on their thermoplastic correspondents (A, B and C). Results were determined for both Water Tree Propagation and Water Tree Initiation. The results obtained for Water Tree Propagation indicate that there is generally a large difference between the types (A, B & C) yet only small difference between thermoplastic and crosslinked samples. However for two cases (A & B) these small differences between crosslinking are statistically significant. The complementary Water Tree Initiation results show both large and significant differences between material types and the crosslinking.

Optimizing trading capabilities and reliability of power transmission networks

The European Commissions Energy Green Paper 2006 identifies the need to develop a flexible single European grid as one of the most important priorities of the European Union energy policy for the next few years. This fits a background of increased renewables generation and an increasing mix of generation and transmission technologies. In this paper we will present a scheme that addresses an approach of increasing the trading capacity, controllability, stability and reliability of such a future power transmission network. Further, the scheme seeks to achieve this with reduced life cycle costs and enhanced environmental benefits.

A study on the origin of space charge accumulation in polymeric HVDC cables

The paper covers results from a study aimed to elucidate the underlying mechanisms behind accumulation of space charge in polymeric high voltage direct current (HVDC) cables. It was observed that the charges in cable samples were mainly located in the outer part of the insulation while space charge profiles obtained on plaque specimens showed almost symmetrical shapes. In both cables and plaques hetero-charge accumulation was the dominant process. The performed experiments include space charge measurements using a pulsed electro-acoustic (PEA) system on cables with and without outer semi-conductive layer. The results suggest that the migration of polar species from the semi-conductive layers is not a prerequisite for space charge accumulation.

Analysis of Photoluminescence in Thermo-Electrically Aged Cross-Linked Polyethylene Cables

Photoluminescence measurements have been carried out along the radius of the insulation of un-aged and thermo-electrically aged HVAC cables. A specific emission, with maximum at 400nm has been isolated in the cross-linked polyethylene insulation in the vicinity of the semi-conducting screens. Complementary measurements on lab-compounded samples were carried in order to unravel the nature of the emitting species. It is shown that the antioxidant of the semi-conducting screens is at the origin of this emission and exhibits significant diffusion within the bulk of the insulation. Specific signatures have also been observed on cables having undergone significant yellowing.

Control of water tree length and density in cable insulation polyethylenes

The 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