Jean-Luc Bessede

Improvement of power transformers by using mixtures of mineral oil with synthetic esters

This paper is aimed at the improvement of power transformers through the improvement of the characteristics of mineral oil by mixing this later with other insulating liquids for transformers namely silicon and synthetic ester oils. A comparison of the main properties such as the heat transfer, breakdown voltage (BDV), ageing stability and electrostatic charging tendency (ECT) of different mixtures as well as liquids alone is presented. It is shown that the optimal mixture enabling to improve the dielectric properties and the ageing stability of mineral oil without degrading its viscosity and ECT is that of mineral oil with 20% of synthetic ester oils.

Dielectric breakdown of epoxy-based composites: relative influence of physical and chemical aging

The effect of aging on the dielectric strength of epoxy-inorganic particle composites used for insulators in the high voltage industry is reported. A differential scanning calorimetry analysis of an insulator aged twenty years in actual service conditions indicated both a chemical degradation and a structural recovery of the polymer network. This composite exhibited however a breakdown field comparable to that of a fresh sample with the same formulation. An accelerated physical aging was thus performed which lead to a large increase in the high voltage performance of the newly processes composite over time. This improvement was attributed to a densification of the thermoset resin, which impeded tree growth. It was also observed that the choice of the electrode geometry greatly alters the measurements under high electric field. In a quasi-homogeneous field configuration, the breakdown was mainly governed by the major flaws at the sample scale, namely the reinforcing particles. On the contrary, under a divergent field (with a point-plane electrode arrangement), the field was essentially localized at the point electrode tip, and the major flaws might not be reachable by the damage tree. It hence appeared that the measurements performed in a quasi-homogeneous field are not very sensitive to the variations within the polymeric matrix as are the measurements under a divergent field.