No abstract

No abstract

No abstract

No abstract

No abstract

The presence of slow space charge packets crossing the insulation thickness from one electrode to the other and causing significant electrical field distortion has been reported already in several papers. They are activated in general by very high DC fields or, in highly polluted materials, by relatively low fields and constitute an important ageing factor, concerning DC electrical stress. It has been observed, in fact, that such packets can cause accelerated breakdown of insulation.The development of fast systems for space charge measurements has allowed the presence of almost instant heterocharge to be observed close to electrodes in certain field and temperature conditions, especially in cable models. This has been explained often by the separation of ionic charge populations, even though such heterocharge appears also in materials, such as Polyethylene or cross-linked Polyethylene that represent the best extra-clean technologies. The measurements reported here use a high speed technique to investigate the build up of heterocharge in model cables that have been treated to remove volatile chemical species. They show that in fact the heterocharge is built up by many very small and very fast charge packets (i.e. charge packets having a high mobility), which are injected from both electrodes and cross the insulation in less than one second. Because the packet charge is unable to exit the counter-electrode at the same rate at which it arrives, hetero-charge is built up within just a few seconds from the beginning of the polarization. The mobility of these charges, depending significantly on temperature, is estimated through observation of charge packets as a function of time, and compared with that of the already-known slow packets, generally occurring at higher fields with respect to fast packets. The basis for the interpretation and modelling of such phenomena is discussed.

Space charge observation is becoming thc most widely used techniquc to evaluate polymcric materials for dc insulation applications, particularly HV cablcs. However, quantities are still lacking that can help to summarizc and interpret thc huge amount of data resulting from space charge measurements, and that also are associated with the electrical performance of the insulation. The purpose of this payer is to propose test methods and parameters, based mi the pulsed elcctroacoustic tcchiiique, which can be used for the evaluation of the electrical performance of polymeric insulation under dc ficlds. These parameters are related to the space charge magnitudc and mobility, as well as to the internal elcctric field amplification, the rate of charge accumulation, and the threshold field above which charge is stored in the insulation. In ordcr to investigate the meaning of the parameters proposed for thc asscssment of iiisulation performance, the corrclation between these parainctcrs associated with space charge and insulation lifc is discussed. Examples arc reported with rcfcrence to five different caiididatc matcrials for de cable insulation.

This paper investigates the changes in electrical and physico-chemical properties of lowvoltage power cables for nuclear application when subjected to the combined effects of gamma radiation and temperature. Electrical response is evaluated by means of the dielectric spectroscopy, while the physicochemical changes are analyzed at different structural scales through five complementary techniques (OIT measurements, FTIR spectroscopy, swelling measurements, DSC analysis and micro-indentation). The dielectric spectroscopy and the first two chemical techniques are shown to be appropriate for evaluating the development of radio-thermal ageing in low-voltage cables. Hence, the results reported in this article suggest the effectiveness of dielectric spectroscopy as a non-destructive technique for on-site cable diagnosis.