Application of PDC analysis to identify effect of overheating on dielectric response and conductivity of mineral insulating oil of in-services transformers

The diagnosis and maintenance of in-service power transformers is important for a safe and reliable of electrical power supply. Several testing and diagnostic measurement tools have been developed to detect ageing of insulation system due to thermal and electrical stresses as well as incipient faults which can increase the risk of transformer failure. The dielectric response measurement either in time and frequency domain were widely used presently to determine and assess the condition of insulation system. This paper presents the analysis result of dielectric response and conductivity of mineral insulating oil under overheating condition using time domain technique known as Polarization and Depolarization Current (PDC) measurement. The analysis results found that the insulating with overheating condition has higher polarization and depolarization current magnitude. In addition, the calculated conductivity demonstrated that overheating oil sample has higher conductivity level compared to normal oil sample.