Determination of the influence of ageing parameters on the streamer characteristics of natural and synthetic esters under lightning impulse voltage

Summary

Ester liquids as alternative to conventional mineral oil as insulation fluid become increasingly accepted in the transformer market. It must be ensured that the esters remain chemically and electrically stable even under thermal stress. This paper presents experimental studies on accelerated ageing of natural and synthetic esters with respect to the chemical and electrical modifications of the insulating fluids. First the esters were aged in a furnace at 150 °C for 1010 h, with varying exposure to oxygen. Samples were taken each week to analyse breakdown voltage, streamer velocity, total acid number, water content and viscosity of the esters to monitor the degradation of the chemical and electrical characteristics. For the breakdown voltage and streamer velocity analyses, positive standard lightning impulse tests were carried out with a point sphere geometry at 4 mm. A decrease in breakdown voltage was found for the first three weeks, remaining at a nearly constant level afterwards. An increase in breakdown voltage could be observed for two of the esters in week 5 and 6. The velocity of the streamers observed was found to be highest in week 1 for the synthetic esters, whereas the velocity for the natural ester stayed nearly unchanged. For the chemical development of the aged liquids it was found that oxidation has the highest impact on the water content and total acid number rather than heat and metal catalysts. The kinematic viscosity of the synthetic esters was not influenced by the ageing procedure. For the natural ester viscosity solely increased under the presence of oxygen.

Additional informations

Keywords

ester, insulation, transformer, aging, streamer