Implementation of space charge measurement using the Pulsed Electro- Acoustic method during ageing of HVDC model cable
Résumé
Due to high dielectric performances, cross-linked polyethylene (XLPE) is one of the preferred materials for the insulation of HVDC cables. However, polymeric insulating materials tend to store electric charges, which disturb the electric field within the material and may eventually lead to a structural modification of the polymer thus promoting its ageing. This degradation of the insulation may induce a decrease in the life expectancy of the cables. Nowadays many studies are carried out to bring knowledge on the degradation mechanisms in polyethylene. Indeed, an improved understanding of the behaviour of charges makes it possible to better correlate the measurements with the ageing of the material. However, it is often not possible to measure the dielectric characteristics (current, space charge) during ageing, and most generally this has to be done a posteriori. The objective of this research is to measure the variations of the dielectric properties of the polyethylene insulation in a model cable, either during ageing conditions, i.e. when a thermo-electric stress is applied to the cable, or a posteriori. To do so, two approaches have been developed and put into practice. The first approach is based on the adaptation of the Pulsed Electro-Acoustic (PEA) measurement method to an already charged model cable having undergone thermo-electric ageing. In this case, the PEA measurement is not performed online, but on samples taken regularly from a long cable length under thermo-electric stress. Hence, for each measurement, a different piece of cable, already charged when received, is under study. The PEA method requires a calibration step, normally performed on a non-charged sample to obtain the response of the system. We have developed a calibration procedure on an already charged cable without removing the possible charges that are present in the insulation. In this way crucial information on the state of charge before any further electrical stressing can be analysed. The treatment of the PEA signal has been revised, and it is now possible to quantify the charge present on an already stressed cable sample prior to any measurement. The improvement of the PEA signal conditioning will be presented in the paper. Moreover, the results of space charge measurements at different ageing times will be discussed.
The second approach is focused on an online space charge measurement at laboratory scale, using the PEA method on model cables over long periods of time. One of the difficulties when measuring during a long time, all the more when the set-up is not at room temperature, is the possible drift of the system configuration, as the mechanical pressure inside the PEA cell for example may vary even slightly. It may lead to changes in the transfer
function of the whole system and possibly to some mistakes on the space charge profiles. The dedicated test bench has been developed with realizing periodic calibration using the procedure described below. It allows measuring space charge during long periods of time, and moreover during grounding and during polarity inversion cycles. Space charge profiles will be presented for long measurement times (several days) where the polarity is reversed
each 30 min, for a mean applied field of 60 kV/mm and under thermal gradient
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