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The composite insulation consisting of PTFE film layers with interstitial silicone gel, emerging as a new alternative type of insulation structure has been very successfully used for over 15 years in dry type capacitance-graded core insulation of high voltage equipments, such as wall bushings, current transformers and cable terminals. This composite insulation structure is found in particular to have excellent partial discharge performance with the unique characteristic of self-attenuating when in operation, sometimes down to non-detectable levels. Numerous observations are described from early field-testing, laboratory testing and dissection analysis. The consistency of the phenomenon, even after 15 years of operation is confirmed. Simulation experiments were able to reproduce the characteristic and demonstrate that the level of voltage applied, the structure design and the presence of silicone gel govern the self-attenuation. This paper then proposes a mechanism showing that the initial partial discharge helps to raise the breakdown voltage of the capacitor elements, thus reducing the onset of further discharges.