F. Palmieri

Modification of electrical properties and performance of EVA and PP insulation through nanostructure by organophilic silicates

Nanostructured materials are attracting increased interest and applications. Exciting perspectives may be offered by electrical insulation. Polymeric nanofilled materials may find new and/or upgraded applications in the electrical and electronic industry, replacing conventional insulation to provide improved performances in electrical apparatus, as regards, e.g., reliability, environmental compatibility and power rating. This paper shows that electrical properties of nanocomposite insulating materials for DC applications, specifically space charge, conductivity and breakdown voltage, can improve significantly with respect to the basis, unfilled materials. Reference is made to two polymeric materials, i.e. poly(ethylene-covinylacetate) (EVA) and polypropylene (PP), that are widely used as electrical insulation, e.g. for cables and capacitors. The nanofiller consists of an organophilic layered silicate, specifically an extra-pure synthetic fluorohectorite modified by means of interlayer exchange of sodium cations for protonated octadecylamine NH/sub 3//sup +/ (ODA), in a weight concentration of maximum 6%. In both materials the space charge accumulation rate as a function of applied electric field is significantly reduced, while the electrical conductivity is raised. The breakdown voltage can increase for the nanofilled materials.

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.

Quantities extracted from space-charge measurements as markers for insulation aging

The relation of space charge-derived quantities with aging stresses and time is discussed in this paper. Polyethylene-based materials, aged for different times under electrical and/or thermal stress, are subjected to space charge measurements. Quantities are derived, that is, the threshold for space charge accumulation and the apparent, trap-controlled mobility, obtained by the threshold and the depolarisation characteristics, respectively, which show time variations depending on the level of applied stress and aging time, i.e. on the extent of degradation. This indicates that such quantities can be considered as aging markers useful for the diagnosis of bulk degradation in insulation systems aged under electrical field, dc or ac, and temperature.

Mobility estimation in polymeric insulation through space charge profiles derived by PEA measurements

Inference of apparent mobility of charge carriers in polymeric materials by means of space charge measurements performed during depolarization is presented in this paper. New expressions are proposed here, which are alternative to those already developed in a previous work, both applicable to space charge quantities measured during depolarization. These expressions are applied to the results of space charge measurements performed on unaged and aged XLPE insulation for HVDC cables. It is shown that the two approaches provide close apparent mobility estimations (as a function of depolarization time), which also agree with some literature data. Hence, it can be speculated that the results thus obtained support the validity of both approaches.

Investigation of charge transport and trapping in LDPE and HDPE through space charge and conduction current measurements

Space charge observation and conduction current measurements have proved to be fundamental tools for the characterisation of the behaviour under electrical field of insulating materials. They provide complementary information on the processes of charge injection and transport, from the threshold for space charge formation to the conduction mechanism, which can help in understanding the relation between chemical-physical-microstructural properties and the electrical properties. Even apparently simple and widely-used materials such as low-density polyethylene (LDPE) and high-density polyethylene (HDPE) have not been exhaustively investigated as regards injection and transport mechanisms. The purpose of this paper is to provide a contribution to the investigation of charge trapping and transport mechanisms under DC electrical field, using both space charge and charging current measurements. The information obtained by these two techniques allows the correlation between charge accumulation and transport to be discussed. In particular, the threshold characteristics, which provide the behaviour of trapped charge and conductivity as a function of electrical field, are determined.

Dependence of space-charge trapping threshold on temperature in polymeric DC cables

This paper shows that the charge detrapping rate follows an Arrhenius-type law when the poling electrical field is large enough to cause copious injection of electrons, so that the transport and trapping process is dominated by electronic carriers. At fields close to the charge accumulation threshold, the tested materials, LDPE and XLPE, suggest that the conduction mechanism takes into account both ionic and electronic charge carriers. The threshold field for space charge accumulation significantly decreases as the temperature increases, at least in the range 20 to 60 /spl deg/C, even if it seems that the temperature dependence becomes smoother above 50 to 60 /spl deg/C. The increase of mobility with temperature and, thus, of charge depletion rate, after each polarisation voltage reversal, also follows an Arrhenius-type law, with smaller activation energy values for XLPE. These results must be considered when designing dc polymeric insulation systems, such as HVDC cables, since charge accumulation is a degradation acceleration factor and the temperature distribution in the cable can change significantly with loading conditions.

The effect of temperature on space charge behavior of epoxy resins containing both micro and nano sized fillers

This paper presents results of space charge and conduction current measurements performed at three different temperatures on epoxy resin samples containing micro and nano fillers. The base material is modified by silica with different filler content (ranging from 0 to 65% in weight for the micro filler and 0 to 5% for the nano filler). The results show that space charge accumulation is largely temperature dependent for specimens containing both nano and micro fillers, i.e. space charge decreases considerably with temperature. Specimens with the nanofiller alone, on the contrary, exhibit smaller temperature dependence of space charge accumulation and conduction current (lower activation energy).

The effect of humidity on space charge accumulation in XLPE

This paper investigates the effect of different humidity concentration in cross-linked polyethylene (XLPE) material, cut from HV cables, on space charge accumulation, through space charge measurements performed by the pulsed electroacoustic technique. Space charge measurements were carried out in a climatic chamber, at room temperature and humidity concentration from 5 to 95%. Tests were also performed in laboratory environment, with relative humidity close to 40%. Variation of the threshold for space charge accumulation and, in particular, for charge packet formation was detected as a consequence of relative humidity modification. In particular, a reduction of space charge amount and disappearance of charge packets increasing relative humidity concentration was observed at high fields. The results of conduction current measurements support the space charge results, showing significant variation of quasi steady-state charging current.

Techniques for the estimation of ac space charge accumulation threshold for insulating materials

A new method for the detection of the ac electrical threshold for space charge injection in polymeric materials is presented in this paper. The ac charge injection-extraction dynamic in discussed, and experimental procedures are proposed to establish the electrical field for charge injection. These procedures are based on the observation of the space charge profiles obtained by the Pulsed Electroacoustic technique. The ac threshold data provided by the two tested materials i.e. LDPE and XLPE, are compared with dc threshold results, showing that the r.m.s. ac injection threshold values are very close to, but slightly smaller than, the dc ones.

Space charge behavior of chemically cross-linked and radiation cross-linked polyethylene

This paper presents results of an investigation having the aim to compare electrical and oxidative performances of chemically and radiation cross-linked polyethylene. Gamma irradiated and peroxide crosslinked specimens are characterized through dc space charge observations, as well as ac electric strength and oxidation induction time measurements. Similar results, in terms of space charge accumulation and electric strength, are found for specimens having the same gel fraction and obtained by chemical and radiation cross-linking, while longer oxidation times are derived for the latter. This confirms the interest in investigating deeply the possibilities related to gamma radiation crosslinking of polymeric energy cables.