Comparison of the charge trapping tendency between ester impregnated cellulose sheets and mineral oil ones

Natural esters are nowadays attracting a lot of utilities which are thinking of installing (or have already installed) the first large power transformers with positive expectations in terms of thermal and long term performance. Instrument transformers manufactures are anyway still resilient to experiment with such fluids, because of the lower advantage-disadvantage ratio related to their application (higher costs not immediately translated into outstanding performance). In a previous investigation, we discovered an interesting so far unknown failure mechanism, which seemed not to be related to manufacturing errors, but to a possible charge trapping related phenomenon. Pulsed Electro-Acoustic measurements have been carried out on simple impregnated sheets samples to compare the different behavior between mineral oil and natural esters. Preliminary obtained results highlight a very low voltage injection thresholds in both cases, and a comparable injected charge, but the depolarization trends seems to be lower in the case of natural impregnated paper even if the investigation is still far to be considered concluded.