The interface between cable accessory insulation and cable insulation is always coated by silicone grease.However

the swelling effect of silicone grease will reduce the mechanical properties and insulation properties of the silicone rubber（SiR）. The research on how silicone grease swelling will influence the PD performance at interface is rarely available in the literature. Therefore

in this paper

an experimental interface with an air-gap defect was firstly designed and fabricated to test the partial discharge inception voltage（PDIV） of the XLPE-SiR interface. Secondly

the mass test

FTIR spectrometer test

FDS spectrum test and SEM observation of the SiR before and after swelling were carried out.Finally

a simulation analysis of the interfacial electric field distribution was carried out. The results show that the PDIV of the interface will increase significantly at the very beginning when silicone grease is coated

however

with the swelling effect continuing

the PDIV will drop rapidly

from 17.5 kV to 10.8 kV in only 36 h. The swelling of silicone grease enables the dielectric constant of SiR under power frequency to increase from 2.87 to 3.3

which is resulted from the increasing number of polarized molecules in unit volume of SiR. The increasing dielectric constant of SiR will cause the electric field distortion in the air gap between the interface

thereby reducing the PDIV at the interface. Thus

the dielectric constant change of SiR caused by silicon grease swelling is one of the important factors that affect the PD initiation of the XLPE-SiR interface.