Comparative analysis of air-gap PD characteristics: Vegetable oil/pressboard and mineral oil/pressboard

Partial Discharge (PD) is considered to be one of the main reasons for aging and degradation of the oil/pressboard insulation system in power transformers. Vegetable oils which own excellent dielectric performance are introduced as potential insulation liquids substituting traditional mineral oil. In this paper, an air-gap PD model was adopted to investigate the PD characteristics of refined rapeseed oil and Karamay 25# mineral oil that are both currently employed in liquid filled power transformers. The PD current pulse waveform analysis (PD-CPWA) method was used to investigate PD mechanisms of two different insulation systems: mineral oil/pressboard and vegetable oil/pressboard. For both insulation systems, phase resolved partial discharge (PRPD) patterns throughout the accelerated deterioration experiments were compared. The extracted φ-q-n plots with respect to various PD times were analyzed. It is found that, the air-gap PD stage characteristics of vegetable oil/pressboard are more notable. It has fewer double-peak pulses, smaller inception phase angle, lower charge amplitude, higher repetition rate, and more remarkable `rabbit-ear' patterns than mineral oil/pressboard. The PD development process of both insulation systems can be characterized by four stages: initial discharge stage, weak developing stage, discharge burst stage and pre-breakdown stage.