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During normal operation, power transformer insulation always experiences a temperature gradient along radial direction with respect to core axis. Over time, this gradient creates non‐uniform ageing within cellulosic insulation. Recent development such as Modified Debye Model (MDM) is found to be capable of modelling such non‐uniformity. Owing to non‐uniform ageing, measured tan δ value only provides the average of dielectric loss associated with various regions within the insulation. Researchers have observed tan δ value to be quite high (due to conductive ageing by‐products) in heavily degraded areas such as regions closer to winding. Also, the rate of deterioration of various regions within the insulation cannot be assumed to be constant. In fact, rate of degradation for regions with comparatively high tan δ values are expected to be much higher. tan δ measurement fails to assess the individual condition of different regions. A technique, based on MDM, is introduced in the present work that can identify the change in ageing severity of various regions within a non‐uniformly aged insulation between two consecutive testing. The proposed method is first successfully tested on multiple series connected laboratory samples. Thereafter, the methodology is applied to data recorded from real life power transformer. [ABSTRACT FROM AUTHOR]