Physics of electrical treeing in silicone gel

Mancinelli Paolo - Ciclo XXIX - 2017

Abstract - Silicone gel is commonly employed in electrical insulation, especially in power module encapsulation. In the recent years, the increase of the operation voltage of this application led to a higher electrical stress on the silicone gel, thus, electrical treeing has become a serious issue to the reliability of power modules. In this research, the degradation produced by the electrical treeing has been evaluated dividing it into two main parts: the tree inception and the tree growth, which have been assessed for different waveforms of the applied voltage. The tree inception, the preliminary stage of this phenomenon, has been tested and an innovative model has been proposed explaining this stage. The tree growth has been evaluated in function of the waveform voltage obtaining useful comparisons between the possible electrical stresses. This thesis highlights the peculiar behavior of silicone gel under high electric field and furnishes useful guidelines for designing and testing the electrical insulation made by silicone gel.

Fundamental Study and modeling of nanofluids

Negri Fabrizio - Ciclo XXIX - 2017

Abstract - In spite of all the recent studies, the insulating materials employed for high voltage (HV) transformer manufacturing are still those in use since several decades. For the fluid insulation, mineral oil (MO) still dominates thanks to its excellent dielectric and cooling performances. Anyway, since it is a carcinogenic agent, the electric industry searched alternatives for applications where environmental concerns are of greater concern (e.g., offshore equipment, trains). In this context, many efforts have been done to study green fluids, mostly natural and synthetic esters, to replace MO. However, green alternatives have a low resistance to electrical discharge propagation. Some researchers have started to experiment with MO-based nanofluids (i.e., colloidal solutions of nanoparticles in a base fluid), with the aim of improving the MO heat exchange capabilities. They have started to study nanofluids because of their outstanding thermal properties, but these fluids should be excellent also regarding all the electrical properties, ranging from the power losses to the discharge propagation resistance. Target of this thesis is to perform a fundamental study aimed at understanding if nanofluids are a real possible alternative for mineral oil.

Electrospun materials for energy applications: from Lithium-ion batteries to electrets

Zaccaria Marco - Ciclo XXVIII - 2016

Abstract - The aim of this work is focused on the application of electrospinning technology, an innovative manufacturing technique to design nanostructured polymers, for energy storage, energy harvesting and sensor applications. These nanostructures are suitable to increase specific performances of components: from the reduction of internal resistance of Lithium-ion batteries to the increase of specific electrical response of materials for energy harvesting. The increase of specific power of Lithium-ion batteries represents one of the key factors for the development of competitive storage systems for automotive and power grid. Indeed, electrochemical performances at high currents significantly decay, strongly limiting the competitiveness on the market beyond portable applications. Furthermore, recovery of dissipated energy will be one of the most promising challenges for the establishment of green technologies and for the reduction in power consumption. Finally, health monitoring and/or impact sensors could represent suitable technologies for the growth of smart materials. This Ph.D. thesis covers the study of PVdF-based polymers to develop enhanced materials for Lithium-ion batteries and energy harvesters.

PARTIAL DISCHARGE PHENOMENA IN CONVERTER AND TRACTION TRANSFORMERS: IDENTIFICATION AND RELIABILITY

Carlos Gustavo Azcárraga Ramos - Ciclo XXVI - 2014

Abstract - After the development of power electronics converters, the number of transformers subjected to non-sinusoidal stresses (including DC) has increased in applications such as HVDC links and traction (electric train power cars). The effects of non-sinusoidal voltages on transformer insulation have been investigated by many researchers, but still now, there are some issues that must be understood. Some of those issues are tackled in this Thesis, studying PD phenomena behavior in Kraft paper, pressboard and mineral oil at different voltage conditions like AC, DC, AC+DC, notched AC and square waveforms. From the point of view of converter transformers, it was found that the combined effect of AC and DC voltages produces higher stresses in the pressboard that those that are present under pure DC voltages. The electrical conductivity of the dielectric systems in DC and AC+DC conditions has demonstrated to be a critical parameter, so, its measurement and analysis was also taken into account during all the experiments. Regarding notched voltages, the RMS reduction caused by notches (depending on firing and overlap angles) seems to increase the PDIV. However, the experimental results show that once PD activity has incepted, the notches increase PD repetition rate and magnitude, producing a higher degradation rate of paper. On the other hand, the reduction of mineral oil stocks, their relatively low flash point as well as environmental issues, are factors that are pushing towards the use of esters as transformer insulating fluids. This PhD Thesis also covers the study of two different esters with the scope to validate their use in traction transformers. Mineral oil was used as benchmark. The complete set of dielectric tests performed in the three fluids, show that esters behave better than mineral oil in practically all the investigated conditions, so, their application in traction transformers is possible and encouraged.

Aging of nuclear power plant cables: in search of non-destructive diagnostic quantities

Luca Verardi - Ciclo XXVI - 2014

Abstract - The safety systems of nuclear power plants rely on low-voltage power, instrumentation and control cables. Inside the containment area, cables operate in harsh environments, characterized by relatively high temperature and gamma-irradiation. As these cables are related to fundamental safety systems, they must be able to withstand unexpected accident conditions and, therefore, their condition assessment is of utmost importance as plants age and lifetime extensions are required. Nowadays, the integrity and functionality of these cables are monitored mainly through destructive test which requires specific laboratory. The investigation of electrical aging markers which can provide information about the state of the cable by non-destructive testing methods would improve significantly the present diagnostic techniques. This work has been made within the framework of the ADVANCE (Aging Diagnostic and Prognostics of Low-Voltage I\&C Cables) project, a FP7 European program. This Ph.D. thesis aims at studying the impact of aging on cable electrical parameters, in order to understand the evolution of the electrical properties associated with cable degradation. The identification of suitable aging markers requires the comparison of the electrical property variation with the physical/chemical degradation mechanisms of polymers for different insulating materials and compositions. The feasibility of non-destructive electrical condition monitoring techniques as potential substitutes for destructive methods will be finally discussed studying the correlation between electrical and mechanical properties. In this work, the electrical properties of cable insulators are monitored and characterized mainly by dielectric spectroscopy, polarization/depolarization current analysis and space charge distribution. Among these techniques, dielectric spectroscopy showed the most promising results; by means of dielectric spectroscopy it is possible to identify the frequency range where the properties are more sensitive to aging. In particular, the imaginary part of permittivity at high frequency, which is related to oxidation, has been identified as the most suitable aging marker based on electrical quantities.

Accelerated degradation of ac-motor winding insulation due to voltage waveforms generated by adjustable speed drivers

Davide Fabiani - ciclo XIV - 2002

Abstract - This Ph. D. Thesis is the result of a three-year work carried out at Department of Electrical Engineering of University of Bologna. The causes of reliability reduction of motor winding insulation supplied by voltage waveforms generated by adjustable speed drives (ASD) are here thoroughly investigated. An innovative technique based on life tests, space charge and partial discharge measurements is proposed and validated experimentally aimed at understanding insulating material behaviour under such voltage waveforms. The provided experimental framework enables to compare different insulating materials candidate to be used in motor windings supplied by ASD through electrical property characterisation and electrical endurance evaluation.

Nuove metodologie per la caratterizzazione di materiali polimerici per applicazioni HVDC

Ghinello Ilich - ciclo XI - 1998

Abstract - Sebbene molti dei primi sistemi elettrici fossero basati sulla corrente continua (DC), storicamente i sistemi di grande potenza e quindi anche i sistemi di trasmissione dell’energia si sono sviluppati in corrente alternata (AC). Questa preferenza è senza dubbio da attribuirsi agli innumerevoli vantaggi che la corrente alternata presenta rispetto alla corrente continua, non ultimo la grande facilità di trasformazione, permessa dall’introduzione dei trasformatori, dei parametri tensione-corrente tra le varie fasi di generazione, trasporto e distribuzione dell’energia. I sistemi DC sono così stati per lungo tempo relegati ad applicazioni particolari; la più interessante è sicuramente quella dei cavi per il collegamento sottomarino dove gli effetti capacitivi dovuti alle lunghe pezzature di cavo impediscono l’utilizzo della AC. Oggigiorno, però, la situazione sta cambiando. 

Development of artificial intelligence systems for electrical insulation defect identification through partial discharge measurements  

Physical model of PD behavior and relevant damage growth from micro-cavities in polyethylene-based material under AC voltage