Four research activities are mainly carried out by Prof. Fabiani.
The first topic, started in 1998, regards the study of accelerated aging of insulation systems produced by non-sinusoidal voltages. First the effect low-frequency harmonics (up to 1 kHz) has been analyzed on cables and capacitors, showing that the increase of peak-to-peak voltage and not of current (and, thus temperature) is the cause of insulation accelerated aging. Secondly, the effect on motor insulation of high-frequency harmonics, such as those generated by power electronic converters, which produce impulsive waveforms (e.g. PWM inverters), have been studied. The research conducted shows that these motors are subject to premature failure due to inception of partial discharges (PD), normally absent under sinusoidal conditions. Prof. Fabiani has then developed an innovative system for PD measurement, which was specifically designed for inverter-fed motors, whose high switching interferences make difficult the measurements by means of traditional systems. For this purpose, therefore, an innovative probe, sensor (antenna) - filter, capable of reducing the switching noise of solid-state devices, without significantly affecting the PD signal. Prof. Fabiani has studied the effect of PD generated by impulse voltages on machine insulation systems. Using a specially developed experimental protocol, the writer has studied and proposed some diagnostic indicators, obtained by PD measurements, which could be correlated with the aging level of the insulation system. Moreover, the location of the discharge source by a phase-resolved PD pattern properly designed for PWM supply has been developed, leading to an international patent application. Finally, Prof. Fabiani has contributed to the development of a new method for on-line diagnostics of electric motors controlled by PWM. This method allows to realize a two-dimensional PD pattern by means of the Park transformation, from which the diagnostic indicators for discharge source identification can be directly extracted, even during the normal motor operation.
The second topic, started in 1999, deals with the study of aging and diagnostics of materials, components and insulation systems for high-voltage AC and DC. The application and development of specific statistical methods, using the Weibull probability distribution and Design of Experiments (DOE) technique has been carried out, as well as new technique design for the assessment of the state of aging and the diagnosis of the most common insulation systems present in the high-voltage components (cables, transformers, generators, motors, GIS, etc.. ), through PD measurements.
The third topic, developed since 1999, concerns the study of conduction mechanisms and space charge accumulation in polymeric cable insulation under HVDC, which still is very often realized by paper-oil, due to space charge that accumulates inside polymeric materials under DC fields. This research regards in particular:
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the impact of interfaces (semicon/insulation and insulation/insulation) in cables and cable accessories (joints and terminations) on space charge accumulation and electric field distortion inside the insulation;
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the effect of temperature gradient, applied to the cable insulation in DC, on the accumulation of space charge;
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the study and modeling of the space charge accumulation in HV cable models;
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the study of conduction mechanisms at high fields in polymers. In particular, an important discovery has been the observation of small “charge packets” (positive and negative) which are injected from the electrodes and crosses the insulation with surprisingly high mobility, accumulating close to the opposite electrode. This discovery, made possible thanks to the sensitive space charge acquisition system developed in the laboratory led to the development of an original theory on the formation and transport of electric charge in insulators at high fields through charge soliton and the application of an international patent.
The last research topic, developed from two national projects, MISTRAL (2004-2005) and CRINE (2007-2008) and many other international collaborations in the year after, in which the writer took an active part, concerns the development of new polymeric nanocomposites for applications in electrical and electronic equipment. Some inorganic nanofillers (clays, SiO2, TiO2, etc.) were evaluated and mixed to the polymeric material matrix in order to improve the electrical characteristics of the base material. In particular, the writer has observed that the use of clays as nano-additive can lead sometimes to a deterioration of the insulation electrical characteristics if the nanofiller, particularly with high aspect-ratio, is not completely dried during the mixing phase with the polymer. Considerable improvements in the electrical characteristics of most of the nanostructured materials analyzed were observed, however, in terms of reduced space charge accumulation, high charge mobility and better dielectric strength and temperature resistance, as well as greater resistance to surface erosion caused by partial discharges. Studies in the field are advancing considerably thanks to the collaboration with the University of Freiburg, Genoa, Polytechnic of Turin, Université Paul Sabatier of Toulouse, University of Southampton, Ecole de Technologie Superieure and HydroQuebec of Montreal (Canada). As a result of the research activities in the field of dielectric nanotechnology, the writer was invited to write a book chapter for Springer in 2010 on the implementation of dielectric nanocomposites (as co-author), and then another for Research Signpost on the electrical properties of nanocomposites in 2012 (as single author).
In 2006, the writer has founded the Research Group on Electrospinning (RGE) along with other researchers from the University of Bologna focused on electrospinning as a methodology to manufacture nanostructured materials for various applications (from biomedical materials to fuel cell and lithium battery components), through the application of appropriate high electric fields. In 2011 this group joined the Industrial Research Centre on Advanced Mechanics and Materials. In this context, Prof. Fabiani deals with the design of electrostatic fields to optimize the production of nanofibers and the development of lithium batteries with electrospun components (electrodes and separator), as well as composite multifunctional materials containing electrospun piezoelectric nanofibers. This absolutely innovative topic is the subject of a PRIN (Smart Composite Laminates) and a European Project H2020 (MyLeg). He also studied the use of plasma to improve the dispersion of nanoadditives (subject of a patent in 2013) and the electrospinnability of polymer solutions.
Over the past years, in collaboration with some researchers of Chemistry and an Italian company, Prof. Fabiani has participated in the development of insulating materials with nanostructured surface coating, particularly containing graphene. These materials show better resistance to partial discharges, reduced space charge accumulation and lower thermo-oxidation, thanks to the oxygen barrier created by the coating. It should be noted that the nano-structured coating can also be applied on the insulating component after manufacturing, e.g. by spray-coating or dip-coating. These studies have recently led to the filing of a patent application.
