Influence of thermally aged barrier on corona discharge activity in transformer oil under AC voltages

Corona Inception voltage (CIV) / breakdown voltage increases when the barrier is placed anywhere from needle tip to the 20% of the needle-plane electrode gap above which a marginal reduction in breakdown voltage is observed but its value is much higher than the breakdown strength of the electrode gap without barrier. Thermally aged oil impregnated pressboard insulation, as barrier, have high impact on CIV and breakdown voltage. The injected current due to corona activity, its rise time, measured using HFCT, in the negative and positive half cycle of the AC voltage measured to be 600 ps and 500 ps respectively. The shape of the current pulse is the same with the aged pressboard insulation barrier in the electrode gap. The corona inception voltage was measured by using UHF sensor and the signal has frequency contents between 0.5 -2.2 GHz. The surface charge measurements indicates that the amount of charge accumulated is high with thermally aged oil impregnated pressboard (OIP) than the unused material. Polarity of surface charge acquired by OIP due to corona charging depends on the surface condition of the material. To understand the mechanical behavior of the aged specimen, tensile test and flexural test were carried out and could conclude that ageing of material reduces the fundamental mechanical properties of the material. SEM analysis indicates that scission of fibers occurs due to thermal ageing at high temperatures. Partial Discharge studies on corona activity indicate, on corona inception, discharges occur near peak of the applied AC voltage, irrespective of position of barrier. The phase width of the PD generated due to corona discharge activity is slightly high with thermally aged specimen compared with unused specimen.