Partial discharge behaviour of transformer liquids and the influence of moisture content

This study investigated the partial discharge (PD) behaviour of a mineral oil Gemini X, a synthetic ester Midel 7131 and a Gas-To-Liquid (GTL) based synthetic hydrocarbon liquid Diala S4 ZX-I, and examined how their PD behaviour can be affected by moisture. Measurements of PD inception voltage (PDIV), PD magnitude represented by maximum apparent charge and PD number represented by pulse repetition rate of dry and moist liquids were compared. Results indicate that the PD magnitude of Midel 7131 is higher than the other two liquids at the same voltage. At voltages higher than PDIV, those liquids exhibit rather different pulse repetition rates. Midel 7131 has a higher pulse repetition rate than the other two liquids at the same voltage. As a result of increasing the relative moisture content of the liquid samples to 50%, it has been observed that the pulse repetition rates of both Gemini X and Diala S4 ZX-I increase, however, in contrast, that of Midel 7131 decreases.