PD evolution and their effect in natural and synthetic ester liquids

The employment of natural and synthetic ester liquid as dielectrics in electrical equipment is increasing in the latest decades. They have higher fire and environmental performances if compared with the most used traditional mineral oils. Natural and synthetic esters are designed to provide an alternative to mineral oils, silicone liquids and dry-type transformers. Furthermore, natural and synthetic ester liquids have higher water saturation points than mineral oils and, for this reason, their dielectric performances are also greater. This paper presents and discuss experimental results obtained in testing commercial mineral oils and natural and synthetic ester liquids, used in high voltage electrical apparatus (mainly transformers), in terms of partial discharge inception voltage (PDIV). It is also shown as a continuous Partial Discharge (PD) activities may be at the origin of high viscosity by-products and wax formation. Different PDIV test method determinations are also compared, including the experimental method defined by the actual IEC TR 61294 'Determination of the partial discharge inception voltage (PDIV) - Test procedure'.