This paper presents the time varying characteristics of partial discharge (PD) activity in a cylindrical cavity within polymeric insulation. The samples are produced using a multiple layer technique with polyethylene terephthalate (PET) selected as the solid dielectric. The results highlight the degradation process of the cavity surface through the exposure to a HV AC electric field and the impact of this on PD activity. The variation in sample conditions are linked to PD analysis in each case with one sample having exhibited electrical tree growth. The morphological surface and chemical composites of the test samples are analysed using Raman spectroscopy and an optical microscope.