R. Schifani

On PD mechanisms at high temperature in voids included in an epoxy resin

In this paper the effects of temperature on partial discharge (PD) activity taking place inside a spherical void in epoxy resin system are studied. Indeed, some experimental tests previously performed on specimens, having different void shapes, under multi-stress condition of temperature and voltage, have shown very different PD amplitude distributions at temperatures higher than ambient. However, this phenomenon cannot be explained only by taking into account the different thermobaric conditions of the enclosed gas. In consequence of the general physical inaccessibility of such voids, a study is here performed using a numerical model based on an evolutionary optimization algorithm. This is used to evaluate the range values for the physical parameters of the insulating system influencing the observed changes in PD activity. Finally, comments are presented about the adopted criteria by which the comparison between the experimental data and the simulated ones is performed, and about the interpretation of the dependence on temperature of the experimental PD.

Partial discharge tests using CIGRE method II

In this paper, the results of an experimental project on insulating material aging, performed in both Denmark and Italy, are reported. This study was concerned with partial discharge (PD) behavior at temperatures between 30 and 80/spl deg/C using CIGRE method II. The material tested was a commercial polymethylmethacrylate (PMMA) which was chosen not for its good dielectric properties but rather because much of its discharge resistance data at ambient temperature is already well documented. A description is given of the theoretical and experimental methodology followed in this work. Mixed Weibull analysis techniques in terms of the PD amplitude and phase distribution characteristics were employed to distinguish the presence of different aging mechanisms. Such a difference was observed at 30 and at 80/spl deg/C. At 30/spl deg/C the analysis inferred a single discharge aging process acting until breakdown, while at 80/spl deg/C the results suggested the predominance of a single PD aging mechanism for the first half of the insulation lifetime; however, for the remainder of the insulation life time, an additional degradation mechanism was evinced.

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 novel HV system for multi specimens aging tests under partial discharges and temperature

In this paper a novel high voltage (HV) system for testing five specimens in parallel under different combinations of voltage and temperature is presented. When partial discharges (PD) are present, the system allows under a computer supervision the cyclic acquisition of PD signals coming out from each specimen by mean of a new partial discharge phase resolved analyser (PDPRA) built in our laboratory. Each specimen is provided by a suitable circuit breaker, ad hoc designed, that disconnects the high voltage (HV) side of the broken-down specimen, without the central HV supply is interested. The specimen temperature regulation (up to 100/spl deg/C) is performed by a pt100 thermo regulator in a silicon oil bath. Finally, some tests have been performed to check the absence of any mutual PD interference between the specimens.

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.

Effect of water adsorption on the dielectric properties of polymer nanocomposites

In this paper, the effect of the water absorbed by inorganic nanofillers (e.g. during the manufacturing process prior to the extrusion) on the electrical properties of a thermoplastic polymer based nanocomposite is studied. Particularly, it is shown how the absorption of water by the nanofiller can affect significantly the nanocomposite electrical properties. Two nanofillers, i.e. fluorohectorite and bohemite, with two different levels of water content, were used to study the effects on electrical properties, in particular space charge accumulation, electric strength and conduction current.

Searching for PD-based indexes able to infer the location of internal defects in insulation

The aim of this paper is to derive PD-based indexes and criteria that are able to indicate whether an internal defect, a cavity, is located close to the HV electrode, to the ground electrode, or in the bulk of the insulation. For this purpose, several measurement sessions were carried out on laboratory samples whose geometry was suitably designed (CIGRE type I and type II cell). In addition, measurements were performed on a capacitive insulator, having known location defects. The measurements results and their evaluation are described in the paper. It is shown that indexes useful for PD source location can be extracted from PD phase distributions.

Optimisation of a numerical model for analysis of partial discharge phenomena in a flat cavity

The failure probability of the insulation in HV components can be studied through partial discharge (PD) measurements to detect defects. A numerical model, previously developed by the authors for the simulation of PD activity in a spherical void embedded in epoxy resin, is implemented for a flat cavity under a divergent electric field. This simulates very well the real working conditions encountered in the insulation of electrical machines. The model is developed taking into account all the geometric and electrical parameters, and a fine examination on physical parameters playing a fundamental role on the PD phenomenon is carried out. In particular, a more accurate formulation of the work function and a finite elements analysis (FEA) of the electric field inside the cavity has been applied. A new approach is investigated to reproduce the uncertainty of the discharge phenomenon. A probability function distribution, obtained by Weibull analysis, gains the best results. In order to obtain the best fitting of the model output to the experimental PD data, a strong procedure based on an heuristic algorithm is performed. This algorithm detects the minimum of a quality function that evaluates the distance between the PD patterns of the numerical model and of the experimental data. After a description of the numerical model adopted, a comparison between the experimental and the simulated data are presented together with comments and remarks.

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.