Space charge and associated electroluminescence processes in XLPE cable peelings

Abstract

The intent of this paper is to correlate the information obtained by space charge profile analysis and electroluminescence detection in cross-linked polyethylene samples under DC\fields, with the objective to make a link between space charge phenomenon and energy release as revealed by detection of visible photons. Space charge measurements carried out at different electrical fields by the pulsed electro-acoustic method. show the presence of a low-field threshold, close to 15-20 kV/mm, above which space charges begins to accumulate considerably in the insulation. Charges are seen to cross the insulation thickness through a packet-like behavior at higher fields, starting at about 60-70 kV/mm. Electroluminescence measurements show the existence of two distinct thresholds, one related to the excitation of electroluminescence under voltage, the other upon specimen short-circuit. The former occurs at values of fields corresponding to space charge packet formation, and the latter to space charge accumulation. The two techniques give therefore consistent information on space charge phenomenology and associated energy release in the optical EM spectrum. Introduction Measurements of electrical properties useful for insulation characterization and for aging diagnosis were carried out by different research groups in the framework of the European project "ARTEMIS" (Aging and Reliability Testing and Monitoring of Power Cables : Diagnosis for Insulation Systems), with the aim to investigate the degradation mechanisms and to evaluate reliability of high voltage cross-linked polyethylene (XLPE) cables. Among these properties, space charge, chargingdischarging current, polarization loss and electroluminescence (EL) measurements are associated with the mechanism of charge injection and transport. There is a general agreement among researchers that electrical aging of synthetic insulation used in high voltage cables could be started by space charge accumulation. However, there is no clear picture of the nature of the degradation reaction that would be involved at a microscopic level. Electroluminescence emission enables a link to be made between space charge phenomenology and the release of energy due, for example, to radiative charge recombination. It is therefore of importance to be able to define levels of fields and associated space charge profiles that are liable to give rise to luminescence excitation since EL is the evidence of the existence of highly reactive excited states in the insulation that could promote accelerated aging [1]. This paper presents and discusses the results obtained from space charge and EL measurements, focusing on the limits of electrical field (thresholds) which promote space charge formation and luminescence excitation. Experimental proceduresMeasurements 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...

Space Charge Profile of TR-XLPE Cables Aged Under Multiple Stresses

“…In general, it is accepted that if the polymer is aged the local concentration of space charge is enhanced and this produces the local modifications on the structure of the polymer towards critical levels in which voids, with sufficient size to develop partial discharges, and electric trees are created [7,8,9]. Presence of chemical defects and crosslinked by-products in the polymer can give the start point for the formation of degrading areas because they contribute with the increasing of the space charge accumulation [10,11].…”

Morphology Effects on Space Charge Characteristics of Low Density Polyethylene

“…Several papers gave very different threshold field values of space charge packet initiation such as 50, 70, or 120 kV/mm in low density polyethylene (LDPE), etc. [10][11][12][13][14][15] Measures also have been taken to test and diminish the effects of space charge on electrical properties. 16,17) Among these measures, annealing which can influence electrical properties via its modification effects on morphology 6,[18][19][20][21] is an important process in manufacturing of polymeric insulating materials.…”

Electric field requirements for charge packet generation and movement in XLPE

“…In these circumstances space charge is observed to cross the insulation in the form of a packet rather than through the steady advance of a charge front. Such packets have been observed both in cable insulation [5,6], and in thin films (see for example [7,8,9]), and are now a widely accepted phenomenon in polymers. Since they occur at high fields before an electric failure occurs they are of interest as a possible pre-breakdown phenomenon.…”

“…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.…”

“…This would argue for an extraction/neutralisation rate dependent upon the hetero-charge concentration as well as the local field. There is in fact evidence for positive charge injection through the electro-luminescence observed when the applied field exceeds 70kV/mm [9], which has been assigned to charge recombination. In our experiments it appears that positive injection just exceeds the negative charge density produced by dissociation, and consequently the anode field on packet exit decreases slightly with each succeeding packet.…”