M. Di Lorenzo del Casale

Microscopic and nanoscopic EVA composite investigation: Electrical properties and effect of purification treatment

This work presents preliminary results of an investigation aiming at characterization of nanocomposite insulating materials. Focus is made on size and technological process effects, i.e. dependence of electrical properties on filler size, on polymer-nanofiller interaction (e.g. intercalation or exfoliation) and, in particular, on chemical purification procedures. The materials under investigation are (EVA) copolymer filled by layered micro and nanosized silicates consisting of montmorillonite and fluorohectorite, that is, natural and synthetic clays. Microstructural characterization, using transmission electron microscopy, TEM, and wide-angle X-ray spectroscopy, WAXS, and electrical measurement results, particularly space charge, are presented and discussed, showing that nanostructuration of EVA-synthetic clay, especially in the exfoliated morphology, can give rise to interesting modification of electrical properties, only if, however, chemical purification of nanofillers is carried out effectively.

A study about electrical treeing in different EVA-layered silicate nanostructured compounds

Electrical treeing in polymeric insulation is potentially a nano-structure dependent phenomenon. The present work is part of a preliminary study on electrical treeing in EVA-layered silicate nanocomposites having different morphology. Two parameters have been mainly considered: the treeing inception voltage (TIV) and the breakdown voltage (BDV). These parameters, especially the TIV, have shown a significant increase in the intercalated nanocomposite materials. The outcomes strengthen the concept that interface interactions due to the nanostructure can improve the treeing resistance of polymeric compounds

Partial Discharge Tests Using CIGRE Method II Upon Nanocomposite Epoxy Resins

Nanocomposite polymers are becoming a new family of dielectric materials with promising insulating properties. In order to apply this technology to insulation systems, a study about the material longterm endurance (life) is needed. In this paper results from comparative life tests performed upon base and nanostructured epoxy resin samples are presented. The nanofiller is a organophillic-modified ultra-clean montmorillonite. Specimens were subjected to ageing under surface discharge phenomena using a standard electrode configuration, that is the CIGRE method II. Life data and partial discharge phase resolved patterns, acquired during time under stress, are presented and commented, in order to characterize the ageing processes that leads to the specimen breakdown and to explain the longer life of the nanocomposite materials. A further insight on the breakdown mechanism is obtained through microscopic observations (using optical and SEM techniques) of the surfaces of the samples before and after ageing and breakdown. A different morphology of the discharge channel walls is observable for base and nanostructured resins.

Direct interaction between partial discharge and temperature on epoxies: phenomenological life models

In this paper the final results of basic research regarding the ageing process of insulations due to partial discharge (PD) activity under different thermal conditions are reported. According to the IEC publication 792-1 (IEC 1985 IEC Report Publication 792-1), the effects of a direct interaction between PD and temperature on material performance has been studied. We used, a simple electrode system with a flat cavity and an epoxy resin widely employed in high-voltage insulation apparatus to perform lifetime tests. On the basis of a previously performed analysis of the ageing mechanisms taking place in different materials, simple phenomenological life models are proposed in this work, giving a new approach to the problem that appears to be a useful tool for future application on electrical components. For this purpose, first the interaction between the temperature and PD was modelled, then a suitable PD quantity, depending on the energy wasted in the specimen during the ageing, was chosen to be assumed as a stress function in the inverse power law.

Electrical treeing in EVA-layered silicate nanocomposites

The present work presents a preliminary research work about electrical treeing inception and growth in polymer-layered silicate nanocomposites. Different silicates were dispersed in an ethilene-vynil acetate (EVA) co-polymer matrix: unmodified and organically modified synthetic fluorohectorite, as well as organically modified natural montmorillonite, were considered. A microcomposite and two nanocomposite compounds were thus obtained and their behaviour under diverging electric field is here investigated and compared to base EVA. In an early stage two parameters were taken into account: treeing inception voltage (TIV) and breakdown voltage (BDV). Then, some short term duration tests have been performed, too. The experimental outcome of this investigation (morphology studies and electrical treeing tests) can offer a basis to further understand how the presence of a layered inorganic nanofiller can influence the short term electric performances of nanostructured polymeric materials.

Voltage distortion effects on insulation systems behaviour in ASD motors

Adopting PWM-like voltage waves, the insulation systems of stator windings of medium voltage motors have been characterized in presence and in absence of partial discharges. In addition, the dielectric losses increase for similar test samples, stressed both by a quasi square voltage and by a sinusoidal voltage, have been evaluated and reported.

Dielectric spectroscopy analysis of EVA-silicate nanocomposite insulating materials

Nanocomposite materials show promising results for applications as electrical insulation. Among several materials presently under investigation, previous works indicate that polypropylene and ethylene-vinilacetate filled by nanosilicates present lower content of space charge and higher electric strength. However, efforts are needed to explain nanocomposite behaviour and characterize their electrical, thermal and mechanical properties. In this paper, the results of broad-band dielectric spectroscopy performed on ethylene-vinylacetate copolymer filled by layered micro and nanosized silicates are reported. The nanofillers consist of montmorillonite and fluorohectorite, that is, natural and synthetic clays. Isochronal and isothermal curves of complex permittivity, as well as activation energies of the relaxation processes are presented and discussed. It is shown that nanostructuration (verified by XRD and TEM analysis) gives rise to substantial changes in the polarisation and dielectric loss behaviour. While the relaxation process of EVA, associated to glass transition of the material amorphous phase, results unchanged, it has been observed the rise of a new process at higher temperatures, having different activation energy in intercalated and exfoliated samples, as well as a change of the distribution of relaxation times.

PD patterns in CIGRE II cells subjected to PWM voltages

This work reports the results of a basic research regarding the study of Partial Discharge (PD) behaviour in a flat cavity when the insulation is subjected to a square voltage waveform and to two different PWM-like voltage waveforms. Tests were performed on CIGRE II specimens realised using polyimide film samples.

Quality requirements of fibre reinforced polymer materials for HV composite longrod insulators

This paper reports on quality requirements of fibre reinforced polymer (FRP) rod materials used for high quality composite longrod insulators. A description of brittle fracture laboratory tests and additional diagnostic test methods regarding the materials PD behaviour and resistance will be given. Results of tests performed with rod samples prepared in the development laboratory and those taken from the current production line made of electrical grade corrosion resistant (ECR) glass fibre reinforced epoxy will be presented and discussed.

PD performance of dielectric insulations in presence of low frequency conducted disturbances: a life model approach

The problem of how 50 Hz power supply voltage distortions affect PD activity and the lifetime of epoxy resin insulation systems, is addressed. In this aim, harmonics up to the 5/sup th/ were summed up to the voltage fundamental component changing the waveform from the sinusoidal into its peaked or flattened resulting shape. In a previous paper, life tests were performed using a sphere-plane electrode configuration; now the experimental research has been carried out, and an analysis of the PD aging phenomena was performed in both sinusoidal and non-sinusoidal life tests by a digital PD system. A good agreement has been:found between the physical phenomena involved during the aging and the obtained new related lifetimes. Finally, a life model is proposed to establish a functional relationship between the shape characteristics of the applied voltage and the failure times, thus providing a useful tool to evaluate the epoxy life changes due to the presence of voltage harmonics with respect to the sinusoidal supply.