Analisi delle scariche parziali per la diagnostica delle macchine elettriche

The main aim of the present work has been the investigation of the characteristics of partial discharge current pulses as a diagnostic tool for electrical machines: in particular the possibilities offered by the Ultra Wide Band (UWB) partial discharge (PD) current acquisition technique have been thoroughly examined and implemented. This method allows to obtain information not only from the charge associated to the current pulse itself but also from the pulse waveform and therefore it can be used to discriminate among different types of PD sources and to provide hints for their localisation inside the insulation system. UWB techniques have been applied to the investigation of partial discharges within the windings of cast resin transformers even if observations on other types of MV equipment were carried out as well. A complete measuring system has been built around a fast digitizer thus obtaining an "open source" product which allows the analysis of several practical problems likely to influence the results, such as noise level and memory management techniques. Several analysis algorithms were implemented and examined; the Time-Frequency analysis method has been chosen as a preferential tool as it proved were effective in the process of discrimination between different clusters of PD pulses. A further improvement has been obtained with the introduction of a new parameter derived from the properties of analytic functions. The geometrical properties of measuring circuits and the related stray parameters have been investigated to obtain the bandwidth limits of typical MV testing layouts. MV cast resin transformers design techniques were studied and the various types of possible defects were discussed taking advantage also from the data provided by the manufacturer. Within the frame of the work several aspects dealing with partial discharge metrology and integration error in relation to measurements carried out on MV cast resin transformers were also discussed. An extensive experimental study has been performed to validate the above mentioned theoretical considerations.