Tecnologie Elettriche



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. 

Ghinello_PhD_tesi_part1.pdf   Ghinello_PhD_tesi_part2.pdf
Ghinello_PhD_tesi_part3_1.pdf   Ghinello_PhD_tesi_part3_2.pdf
Ghinello_PhD_tesi_part4.pdf


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

Conti Marco - Ciclo XVI - 2004

 

Abstract - The aim of the present study can be summarized in three issues, correspondent to the three sections of the thesis: 1. Introduction to those basic concepts which constitute the pre-requisite for the effectiveness of PD interpretation. These concepts are pertinent to two main aspects: discrimination of different PD sources (and noise rejection) and general description of the discharge mechanisms; 2. Description of the techniques used to process automatically the acquired PD data in order to derive significant and robust diagnostic markers. Description of the identification strategy and of the subsequent database construction. Discussion of the principles and assumptions which constitute the basis for PD interpretation. Testing and examples; 3. Description of the mathematical methods which were developed in order to transfer human experience to the machine, as well as to derive rules and concepts directly from a training database, by means of artificial intelligence techniques.


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

Wang Le - Ciclo XXIII - 2011

 

Abstract - In the framework of developing defect-based life models, in which breakdown is explicitly associated with partial discharge (PD)-induced damage growth from a defect, ageing tests and PD measurements were carried out in the lab on polyethylene (PE) layered specimens containing artificial cavities. PD activity was monitored continuously during aging. A quasi-deterministic series of stages can be observed in the behavior of the main PD parameters (i.e. discharge repetition rate and amplitude). Phase-resolved PD patterns at various ageing stages were reproduced by numerical simulation which is based on a physical discharge model devoid of adaptive parameters. The evolution of the simulation parameters provides insight into the physical-chemical changes taking place at the dielectric/cavity interface during the aging process. PD activity shows similar time behavior under constant cavity gas volume and constant cavity gas pressure conditions, suggesting that the variation of PD parameters may not be attributed to the variation of the gas pressure. Brownish PD byproducts, consisting of oxygen containing moieties, and degradation pits were found at the dielectric/cavity interface. It is speculated that the change of PD activity is related to the composition of the cavity gas, as well as to the properties of dielectric/cavity interface.