Comparison of PD Characteristics Induced by Metal Particles and Bubbles in Flowing Transformer Oil

Metal particles and bubbles are two typical insulation defects in transformer oil, which can cause partial discharges (PD) and damage the reliable operation of power transformers. These insulation defects often move with transformer oil because of the forced oil convection or thermal convection due to temperature difference during the actual operation of power transformers. In order to obtain the PD characteristics induced by metal particles and suspended bubbles under flow state and AC condition, a large number of PD experiments were performed on an experimental platform of flowing transformer oil. PD signals were measured by RLC impedance detector, ultra-high frequency (UHF) antenna and non-inductive resistance. phi-q and phi-n patterns were constructed. The results show that there are significant differences in PD characteristics of the two kinds of defect. The rising time of current pulses of metal particles is shorter than that of bubbles. Metal particles PD can excite UHF signals, while bubbles PD can not. The PD inception voltage (PDIV) of metal particles is much lower than bubbles, and the PD frequency of bubbles is far lower than metal particles. In brief, the PD of metal particles is more likely to occur than bubbles PD.