During the last two decades, partial discharges (PDs) modelling methods have been used as a complement of insulation diagnosis systems of electrical assets. Finite-element-analysis models for simulating PDs in cavities within solid dielectric materials are reviewed and a novel model is presented, which combines the main advantages of electrostatic and electric current models. The theoretical background is presented and some limitations and restrictions are discussed. A case of study was implemented for three different ageing conditions and simulation results exhibit good agreement with reported values by other authors in the literature. An analysis of variations of PD behaviour with ageing as a function of temperature and pressure is briefly presented. It is concluded that more research is needed to include physical and chemical interactions on the void surface and that the cavity surface conductivity plays a fundamental role in the PD simulations at advanced ageing conditions.