Advances in PD measuring system techniques: Parallel synchronous detection, time domain and frequency domain processing for improved sensitivity

The diagnostic examination of partial discharges (PD) for the condition assessment of insulating systems of high voltage (HV) equipment, formerly carried out mostly in test bays to control new products after the manufacturing process, has reached currently numerous on-site/on-line applications as well as being used for the permanent monitoring of HV equipment with e.g. strategic importance. Basic characteristics of modern PD measuring equipment should include the suppression of electromagnetic interferences - which could be external noises from the measuring surroundings or internal noises from the HV test circuit. These electromagnetic interferences have generally either sine-shaped (broadcast, television, telecommunication) or pulse-shaped behaviour. The pulse-shaped disturbances could be subdivided into periodic (e.g. switched inverters) and stochastic noise (e.g. external discharges, system switching). Regarding the relevant suppression techniques frequency selective filtering, software or hardware windowing and gating is used for the above mentioned disturbances. In the time domain modern digital filter design rely on finite and infinite impulse response filters, while in the frequency domain center frequency tuning gives frequency spectrum analyzer functionality and advantagous narrow-band measurements. Enhanced diagnostic approaches as clustering, rejection of common-mode disturbances in 3-phase arrangements are enabled through parallel and synchronous measurement at various channels which are interconnectable and synchronized in the nanoseconds range.