The electrical strength of a set of LDPE micro and nanocomposites with silica, laminar silicates and fibrous silicates has been studied. A significant 60% increase of the breakdown electrical field is shown by the microsized microdispersed LDPE/montmorillonite composites while only a 20% increase is obtained with a nanosized laminar silicate. Nanosized nanodispersed composites of the spherical silica and fibrous silicate with LDPE show a lower increase of the breakdown electrical field. The data point scatter in repeated tests is very high in the case of the microcomposites, indicating strong morphological heterogeneity, while it is very low in the nanocomposites. This implies that these nanocomposites are remarkably defect-free as regards electrically ''weak'' sites. An analysis of the crystalline structure, semicrystalline morphology and inorganic particle size and distribution evidences the importance of the role played not only by the inorganic particles but also by the semicrystalline morphology in the final dielectric performance of the composite.

No abstract

Partial Discharge (PD) measurements output,\ud when non-sinusoidal voltages are applied to insulating\ud systems, can have different meanings, depending on the\ud voltage waveform frequency content and on the PD\ud probe circuit. A PD detection circuit is designed for a\ud low noise-to-signal ratio, thus for suppression of the\ud residual high frequency (HF) voltage. Such a\ud suppression varies with the applied voltage rise time: for\ud fast rise rate voltage pulses the PD measurements\ud deserve a specific study. When multiple PD sites are\ud active in the same insulation system and when they give\ud rise to PD inception during the voltage rise/fall, effects\ud due to the signal superimposition should be considered.\ud During quasi-square wave PD tests on twisted pairs the\ud voltage rise rates, the voltage test levels and the high PD\ud repetition rate during voltage "pulses" have been\ud suitably inter-played, taking in account analogue and\ud digital PD acquisition systems, in order to evidence the\ud PD measurements actual information content. The\ud obtained results evidenced the limits of high and low\ud frequency PD measurements; in certain instances they\ud did negate both the validity of the relation and the possibility of\ud resolving each single PD pulse

An electric strength (ES) test campaign in alternated current and space charge (SC) measurements via the pulsed electro-acoustic (PEA) method has been carried out on films of low-density polyethylene (LDPE) nanocomposites. The nanocomposite materials were obtained by the dispersion by melt mixing of organically modified layered silicates in an LDPE matrix. Before the electrical tests, the materials were characterized by means of x-ray diffraction (XRD) and thermogravimetric analysis (TGA). The LDPE-silicate nanocomposites evidenced some smart behaviour in AC.

Nowadays electrical motors used in industry applications are usually coupled with inverter drives so that the stator motor windings are subjected to severe voltage waveforms such as PWM. When electrical motors having stator windings realized by means of type I random wire wound are subjected to these kinds of voltage waveforms, reliability problems due to degradation phenomena increase. In this research work different insulating nanocomposite enamels have been analyzed: suitable twisted pair specimens insulated by the studied enamels have been prepared and tested. Breakdown voltage tests and electrical aging tests have been performed: during the electrical aging tests the specimens have been subjected to pulsed voltage waveforms having amplitude higher than the partial discharges inception voltage level in order to operate in presence of partial discharges. Enamel breakdown condition has been chosen as end-point criterion and both the relevant voltage breakdown and life time values have been collected and compared. Analyzing the obtained results it has been possible to point out the behavior of the different insulating organic enamels when subjected to electrical degradation phenomena.