Characterizing HV XLPE cables by electrical, chemical and microstructural measurements on cable peeling: effects of surface roughness, thermal treatment and peeling location

Abstract

This is the unspecified version of the paper.This version of the publication may differ from the final published version. Permanent repository link AbstractCharacterization of the electrical, chemical, and microstructural properties of high voltage cables was the first step of the European project "ARTEMIS", which has the aim of investigating degradation processes and constructing aging models for the diagnosis of cross-linked polyethylene (XLPE) cables. Cables produced by two different manufacturers were subjected to a large number of electrical, micro-structural, and chemical characterizations, using cable peelings, instead of lengths of whole cables, as specimens for the measurements. Here the effect of surface deformation and roughness due to peeling and the relevance and significance of thermal pre-treatment prior to electrical and other measurements is discussed. Special emphasis is put on space charge, conduction current and luminescence measurements. We also consider the dependence of these properties on the spatial position of the specimen within the cable. It is shown that even though the two faces of the cable peel specimens have different roughness, the low-field electrical properties seem quite insensitive to surface roughness, while significant differences are detectable at high fields. Thermal pre-treatment is required to stabilize the insulating material to enable us to obtain reproducible results and reliable inter-comparisons throughout the whole project. The spatial position of the specimens along the cable radius can also have a non-negligible influence on the measured properties, due to differential microstructure and chemical composition. IntroductionThe two HV cables investigated were insulated by XLPE produced using extra-clean materials and processes. The insulation thickness was 14 mm. The project started with the initial characterization of cable insulation, in order to constitute the reference for electrical, micro-structural, and chemical properties needed to single out aging markers on the basis of the property evolution with aging time. The first problems that were faced were the choice of the specimens to be used and the kind of pre-treatment necessary in order to have reproducible results. The choice made for the specimens posed a very delicate problem. Most of the measurements could not be carried out (or would have been much more expensive) on full-size cables. The use of small-size model cables does not seem to solve the problem, since they could not be realized with the same manufacturing conditions as full-size cables, and thus could be unrepresentative of the real cable behavior. For the same reason, use of insulation plaques by compres-

Space charge and associated electroluminescence processes in XLPE cable peelings

“…Measurements were performed on peels cut from unaged cross-linked polyethylene (XLPE) cables produced specifically for the project [2], width 80 mm and mean thickness 150 m. Before testing, specimens were pretreated in an oven for 48 h at 50°C in order to expel cross-linking by-products. Space charge observations were made by the pulsed electroacoustic technique (PEA) at various constant DC voltage levels from 1 -150 kV/mm.…”

Conductivity measurement of plaque samples obtained from the insulation of high voltage extruded cables

“…In majority of existing publications, the studies were conducted on very thin 100-200 um thick films obtained from cable insulation by means of peeling [4][10][11] [12]. The advantage of this method is that using a short piece of cable, it is possible to obtain samples from different locations of the insulation; besides, by voltage levels of few kV, electric fields representative to HVDC cables can be achieved.…”

“…Space charge measurement using PEA technique can provide further insight into the distribution of conductivity in the insulation but it can be hard to achieve reliable PEA measurements in cables with thick insulation especially when temperature gradients exist throughout the insulation. Considering the challenges above, since long, researchers have been interesting in testing samples obtained from full-scale cables [4] [10] [11] [12]. In this way, while enjoying the advantage of small scale lab condition tests, one obtains representative data from the actual cable insulation.…”

Electric field requirements for charge packet generation and movement in XLPE

“…The measurements were performed on tapes cut from unaged cross-linked polyethylene (XLPE) cables produced specifically for the project [10], by turning them on a lathe equipped with a specially designed cutting tool. The tapes had a width of 80mm and mean thickness 150 m. Before testing, specimens were cut from tapes from insulation located 3 1mm from the inner screen.…”

“…In one case [6] the packet is formed by injected charges and moves across the sample as a packet of excess charge carriers. Usually these packets are small in magnitude (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) ) and spread out as the packet moves so that the packet loses its identity as it crosses the sample. This type of packet can only be re-created a few times.…”