Impact of the condition of oil on the polarisation based diagnostics for assessing the condition of transformers insulation

The oil/paper insulation system in a transformer degrades under various chemical, thermal and electromechanical stresses. Among the newer dielectric response measurements, polarisation and depolarisation current (PDC) measurement has the strength to monitor the quality of oil and papers conditions separately on the overall diagnostic results. In the current research project, the ageing condition and the moisture content of both paper and oil samples have been varied over a controlled range and their effects on the dielectric response measurements have been investigated. Particular emphasis has been given to use different oil samples from field transformers. Experimental results are described in this paper to demonstrate the effects of both ageing and moisture content of oil on PDC results. A mathematical tool has also been used to model the equivalent circuit of polarisation process to monitor the impact of oil on diagnostics.