Dynamic of partial discharges in cavities in a MV AC cable under DC conditions

In order to ensure the energy transmission from remote production stations to load centres, it is well known that High Voltage Direct Current (HVDC) presents some advantages compared to High Voltage Alternative Current (HVAC). The growth of HVDC network and systems in recent decades raises several questions. Even with the constant improvement of the production line, cable suppliers are aware that it is not possible to avoid the formation of microvoids inside the insulation of crosslinked polyethylene.

Cavities in the micrometre range have sizes below critical values leading to partial discharges (PD). Thus, they cannot be detected by conventional PD tests. The possible impact of these small cavities on the cable lifetime and behaviour is unknown.

A question arises whether these microscale cavities may increase in size inside the polymer matrix, due to the various constraints applied, and reach a critical size leading then to critical discharges. More broadly, may the shape and dimensions of the cavities evolve under the combined effects of electric field, including locally enhanced field due to space charges, harmonics due to the converters, and the temperature ?

In a first step, this work is devoted to discharging cavities. The theoretical critical dimensions of the bubbles, estimated thanks to Paschen criteria in the conditions of electric field and temperature submitted to these cables, will be recalled. A cable was produced with cavities inside the dielectric medium. Parts of this cable have been subjected to different temperature levels and/or DC electric field during various ageing durations. Partial discharges activity was followed during the ageing. The dynamic of these PD will be described and assumptions regarding the behaviour of the larger discharging cavities will be proposed.

P. DANIEL1,2,3, P. RAIN2,3, O. GALLOT-LAVALLEE2,3
M. DARQUES1, W. FRELIN1, A. TUNDO1

1 SuperGrid Institute, Villeurbanne, France
2 Univ. Grenoble Alpes, G2Elab, Grenoble, France
3 CNRS, G2Elab, Grenoble, France