A. Motori

Space-charge trapping and conduction in LDPE, HDPE and XLPE

The mechanisms of charge injection, transport and trapping in low-density, high-density and cross-linked polyethylene (LDPE, HDPE and XLPE) are investigated in this paper through charging-discharging current measurements and space-charge observations. The conductivity of LDPE is much larger than that of XLPE and HDPE. The threshold for space-charge accumulation and that for a space-charge-limited current mechanism, coinciding for the same material, are almost identical for LDPE and HDPE, while the threshold of XLPE is higher. However, HDPE accumulates more charge than the other two materials. The depolarization space-charge curves and the conduction current versus field characteristics indicate that the mobility of LDPE is larger than that of XLPE and HDPE, which supports the significant difference in conductivity. The lower mobility, as well as the nature, depth and density of trap sites, can explain the difference in space-charge accumulation and thresholds.

A contribution to the study of aging of XLPE insulated cables

The electrical, thermal, and multiple-stress (thermal and electrical) aging of cross-linked polyethylene (XLPE) cable models has been investigated in order to get information on aging effects and mechanisms. After endurance tests, XLPE cable models have been subjected to chemical, physical, electrical, and microstructural characterization. Thermal aging results reveal that bulk degradation occurs in the cables at temperatures higher than the melting point. Multiple-stress aging data emphasize a synergistic effect of electric field and temperature. Significant microstructural changes detected in the cables aged under multiple stress are evidence of this effect and can partly explain the time behavior of the electric strength. >

Electrical properties of nickel hydroxide for alkaline cell systems

Abstract The electrical behaviour of nickel hydroxide/oxyhydroxide electrodes for advanced alkaline cell systems is investigated as a funciton of oxidation degree which usually changes during practical operation of these systems. The d.c. electrical conductivity, relative dielectric constant and loss factor as a function of oxidation degree, temperature and frequency (a.c. properties) are investigated and discussed. Moreover, the effect of water absorption on the electrical conductivity is investigated. An insulating behaviour and ionic mechanism of conduction is pointed out for the uncycled and reduced electrodes, while the oxidized electrode behaves essentially as a semiconductor. Water absorption causes a strong increase in the electrical conductivity of the uncycled and reduced electrodes. Cycling enhances the dielectric losses of the active material.

An insight into thermal life testing and characterization of EPR-insulated cables

Conventional and analytical procedures for thermal endurance characterization of insulating materials, described in IEC Standards, are applied to EPR-insulated cable models. Close results are obtained by the two procedures, provided that the diagnostic properties, the end points and the test temperatures are carefully chosen. Moreover, the thermal endurance indices do not vary using either the Arrhenius model, assumed by IEC 216, or the Eyring model. However, modifications of the life models are required in order to take into account the compensation effect, which has been observed for the diagnostic properties and the end points considered for thermal endurance characterization. The comparison of the values of the thermal endurance indices obtained for EPR-insulated cable models with those derived in the past for XLPE cable models shows that the tested EPR has slightly better thermal endurance characteristics.

Charging properties and time-temperature stability of innovative polymeric cellular ferroelectrets

After appropriate mechanical and electrical treatments, some cellular polymers become able to retain space charge for a long time, i.e. they acquire electret behavior. The electrical treatment consists of charging under high levels of DC electric field. The mechanical treatment, based on the application of stretching forces to cellular polymer slabs that were before expanded under pressurized gas, affects the cavity size and shape, and therefore also the effectiveness of the charging process itself. An investigation of charging mechanisms, as well as of mechanical treatment, is therefore fundamental for optimizing the ferro- and piezo-electret properties. The aim of this paper is to discuss the effect of the physical dimension of the cavities on the charging behavior of cellular ferroelectrets and to focus on the time-temperature stability for two families of polymeric cellular ferroelectrets based on polypropylene (PP) and on a cyclo-olefin copolymer (COC). Emphasis will be given to the stretching process and in particular to the expansion rate applied during the manufacturing process (which affects the radial dimension and the height of the cavities, respectively). Space-charge and partial-discharge measurements as a function of time and temperature are the main tools to infer the influence of the cavity size on charging and stability characteristics

Improving thermal endurance properties of polypropylene by nanostructuration

Isothermal thermogravimetric analysis is used as analytical technique for short-term thermal endurance characterization of nanostructured isotactic polypropylene. The nanofiller consists of organophilic-modified fluorohectorite, that is, a layered silicate. It is shown that the temperature index improves considerably with respect to the base polypropylene, with only 6 wt.% of nanofiller, thus enabling the potential use of the thermoplastic nanostructured material at significantly higher operating temperatures.

Searching for short-term techniques for the inference of electrical threshold of PET. DC conductivity measurements

A possible way for the detection of electrical threshold in dielectric polymers is illustrated in this paper. On the basis of the consideration that the lowest threshold level for electrical aging is related to build up of steady space charge, high-field current measurements performed at different voltage and temperature values, seeking space charge limited current conditions, can provide estimations of electric field at threshold. This approach is applied to PET films under dc voltage. The values of electrical threshold thus obtained are compared with those derived from conventional accelerated life testing, showing that there is good correspondence of values at the lowest temperature of the test range, while deviation between estimates increases with temperature.

Application of oxidation induction time and compensation effect to the diagnosis of HV polymeric cable insulation

Oxidative stability tests were performed on field and laboratory-aged crosslinked polyethylene (XLPE), ethylene-propylene rubber (EPR) and polypropylene (PP) insulations using differential scanning calorimetry. Flat films and miniature cables aged in the laboratory were subjected to a wide range of aging conditions that included thermal, electrical and a combination of thermal-electrical aging, in dry and wet environments. The results were analyzed using the Eyring rate theory. It is shown that for a given material the oxidative stability data can be described by a single linear relationship between the activation entropy /spl Delta/S and the activation enthalpy /spl Delta/H of the oxidative process, and thus are governed by the so called compensation effect. It is argued that the position of a data point representing a certain operating condition of an insulation on the /spl Delta/S vs. /spl Delta/H compensation plot is a measure of the degree of degradation and can be used as a diagnostic indicator of the operating conditions of the insulation.

Degradation and electrical behavior of aged XLPE cable models

Results of DC electrical conductivity measurements performed at different aging times on cross-linked polyethylene (XLPE) cable models subjected to thermal and combined thermal-electrical stresses are discussed. It is shown that the variations of electrical conductivity with aging time are related to those of other properties (density, crystallinity, electric strength, and microstructure) and hence to the aging processes. Electrical conductivity is shown to be a bulk property well able to indicate the degradation of an insulating material due to electrical and thermal stresses, even if the failure due to electrical stress is primarily a localized phenomenon.<<ETX>>

Comparison of electrical aging tests on EPR-insulated minicables and ribbons from full-sized EPR cable

The results of electrical endurance tests performed on ribbons cut from full-sized EPR cables are compared with those obtained by tests performed on cable models, having the same insulation as the full-sized cable. It is shown that the voltage endurance coefficient estimated on the basis of the short-term life test data has approximately the same value for cable models and ribbons, according to both inverse-power and exponential models. However, the surface roughness resulting from cutting the specimens can affect the result at high stresses.