Contents I. Introduction
PD are electrical discharges inside an insulating system, occurring in a limited portion of the dielectric material, therefore only partially short-circuiting the electrodes. The insulating system can be made of solid, liquid or gaseous material, or any combination. Those discharges occur in bubbles, voids or weak parts of the liquid, solid or gaseous dielectric. Every discharge event deteriorates the material, for the energy of the ions impacts, causing many sorts of chemical reactions [1]. It is still not possible to establish a direct relationship between amount of PD and time to failure, but, in general, failure occurs when an acceleration of the process takes place [2]. PD detection is based in the energy exchanges during the discharge process. Examples of those exchanges are impulsive electrical currents, dielectric losses, radiation, sound, increase of gas pressure, chemical reactions [3]. Most successful methods are the electrical ones, in which methods for the separation of the electrical impulses from any other phenomena are applied. In this case, PD activity needs be detected in the external terminals of the equipment, once a direct measurement inside the insulation material is not possible, in order of not do damage the material and its properties. In this research, PD detection is made through measurement of the high frequency current caused by the PD activity, using a HFCT installed in equipment's grounding conductor. Assuming that HFCT can be modelled using resistance, inductance and capacitance, as shown in Fig. 1, as it can act as a resonant RLC filter. PD pulse can be modelled as an impulsive current, entering RLC filter.
Equivalent circuit for HFCT, modelled as an RLC filter, where
