Improvement of sensitivity in online PD-measurements on transformers by digital filtering

Sensitive PD measurements can be carried out in electromagnetically shielded laboratories. The PD measuring technique is also used for the online monitoring of apparatus in substations. In substations on-site, a sufficient measuring sensitivity can only be achieved by specific filtering methods. Different digital filtering methods had been developed to improve the signal processing in order to suppress efficiently narrowband noise as the most frequent disturbance source and corona discharges which usually cannot be distinguished from the internal discharges of the PD specimen. Examples for a successful application of filtering principles to difficult on site conditions are discussed as well as combinations of the newly developed methods. The limits in the sensitivity of the algorithm are reported.