Our aim is to understand the electrical ageing of insulating polymeric materials in the absence of any pre-existing gaseous cavities in the bulk of the dielectric. We used a needle electrode moulded into the insulant with the aim to produce local field enhancement. The transition between the discharge-free to micro-partial discharge state (early electrical tree propagation phase) is investigated with a sensitivity reaching 0.01 pC. Correlation is made between length of tree and amplitude of discharges before any branching of the initial tree channel. The physics of discharge in such small volume is discussed.
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