Partial Discharge Behaviours of Different Aged Insulating Liquids

The origin of PDs is always due to the imperfections or voids presence somewhere in the overall insulation. In traditional electrical oil-paper insulated components, PD activities usually take place in terms of robust charge pulses for the presence of well-formed and stable cavities due to poor or decay of impregnation. In the case of liquid dielectrics, the situation becomes much more complex because of such voids (bubbles) should not be present and surely not stable in dimension and location; they form, grow, develop and collapse randomly under the action of the electric field and in accordance with Paschen law. In consequence of this dynamic situation, PDs in pure insulating liquids appear as fast events, which appear as bursts of rapid PD pulses of ascending magnitude.This paper presents and discuss experimental results obtained in testing new and aged samples of insulating liquids used in high voltage electrical apparatus (mainly transformers), in terms of electrical, chemical and physical analysis. For these liquids the periodical chemical and physical analysis have been compared with electrical tests, in terms of partial discharge activities, dissipation factor (tan6) and breakdown voltage, evaluating also their evolutions. Different PDIV test method determinations are also compared, including the experimental method defined by the actual IEC TR 61294.