“…The modified Debye model can be used as a potential tool to evaluate the ageing condition of insulating fluids such as mineral oil, ester oil etc. Since the degradation of the insulation due to ageing correlated with the physical polarisation process, the modified circuit with a reasonable number of parallel R–C branches provides more accurate diagnostic information to quantify the degradation [21, 22]. Depending on the ageing condition, the relaxation time‐constant of each branch follows a certain time lag in their physical polarisation and depolarisation process [23, 24].…”