Stefano Serra

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.

Theory of inception mechanism and growth of defect-induced damage in polyethylene cable insulation

We have investigated theoretically the inception mechanism and growth of the damage inside the insulation system of a polymeric cable under working conditions. We focused, in particular, our attention on damage originating from microscopic defects such as voids. In order to clarify the implications of these defects for cable failure, we have developed a theoretical model based on the theory of electrical avalanche solving numerically its basic equations. Calculations of the ionization rates of atmospheric gas filling the voids are done as a function of the applied electric stress and void dimensions. Estimates of the energy release and local damage in polyethylene produced by the resulting hot-electron discharge are given. The developed physical model of damage growth compares reasonably well with known experimental data.

Advanced modeling of electron avalanche process in polymeric dielectric voids: Simulations and experimental validation

This paper deals with aging phenomena in polymers under electric stress. In particular, we focus our efforts on the development of a novel theoretical method accounting for the discharge process (partial discharge) in well known defects present in polymers, which are essentially tiny air gaps embedded in a polymeric matrix. Such defects are believed to act as trigger points for the partial discharges and their induced aging process. The model accounts for the amplitude as well as the energy distribution of the electrons during their motion, particularly at the time in which they impact on the polymer surface. The knowledge of the number of generated electrons and of their energy distributions is fundamental to evaluate the amount of damage caused by an avalanche on the polymer-void interface and get novel insights of the basic phenomena underlying the relevant aging processes. The calculation of such quantities would require generally the combined solution of the Boltzmann equation in the energy and space...

Model of ageing inception and growth from microvoids in polyethylene-based materials under AC voltage

Several degradation mechanisms may affect polymeric insulation system reliability. Some authors postulate that, even in a perfect dielectric, nanoscale cavities can enlarge due to various mechanisms (from mechanical fatigue to lowering of the degradation reaction energy barrier) up to a point where highly energetic phenomena, which bring about breakdown, can be incepted. Other authors are more focused on the inherent limits of manufacturing processes, which leave cavities within the insulation system whose size is large enough to cause electron avalanches, thus a measurable partial discharge (PD) activity, from the time the system is put in service or as a function of external factors (e.g. mechanical damage, thermal shrinking, overvoltages). Given the time scale of polymeric system failures, this latter mechanism seems to be more plausible. It is therefore worthwhile to investigate in depth the degradation rates associated with PD in micrometric cavities in polymeric insulation systems subjected to AC voltage. The proposed model is based on damage accumulation on cavity surfaces caused by PD phenomena. The main degradation mechanism associated with PD is considered to be the hot-electron induced bond-breaking process. This process accumulates with time, leading to the creation of a damaged region of critical size and, ultimately, to breakdown. The proposed model describes the defect induction and growth phase until treeing phenomena start, which is, normally, the largely prevailing component of breakdown time.

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.

Theoretical study of inception mechanism and growth of defect-induced damages in XLPE cable

We have investigated theoretically the inception mechanism and growth of the damage inside a cable in working conditions. We focused, in particular, our attention to the inception points originated from microscopic defects like voids. In order to clarify the implication of these defects with the failure and breakdown of cables, we have developed a theoretical model based on the theory of electrical breakdown solving numerically its basic equations. Calculations of the ionization rates of atmospheric gas filling the voids have been done as a function of the applied electric stress and void dimensions. Estimates of the energy release and local damages produced by hot electrons are given. A model of growth of damage is also proposed.

Aging model of polyethylene-based materials for HV cables founded on damage inception and growth from air-filled voids

In this paper, the aging of polyethylene-based materials for HV cables is addressed to inception and growth of damage from microscopic defects like air-filled microvoids. Such process is considered as divided in three subsequent phases: accumulation of electrons at PE-void interface and relevant injection in the air-filled void, hot-electron avalanche production in air, damage accumulation in PE due to avalanches. The rate of production of hot electrons inside the void is derived by solving the electron diffusion equation in the energy-time domain, and the growth rate of damage in polyethylene due to hot-electron discharges at the void-polymer interface is evaluated estimating local bond-breaking produced by hot electrons. An application of the proposed aging model to a PE-based material in typical working conditions for HVDC cables, including life estimation as a function of void size and applied electric field, is also presented.

First electron availability and PD generation in insulation cavities

An investigation on the behavior of partial discharges occurring in insulation cavities as a function of environmental conditions is presented in this paper. In particular, focus is made on peculiar conditions where semi-transparent insulation is subjected to different visible light intensity, is irradiated by UV or is kept in the dark. Experiments are carried out on cells with an artificial defect, obtained producing a spherical void in epoxy resin. It is shown that not only PD inception voltage is sensitive to fight wavelength and intensity, but also PD average delay time changes considerably. It can be speculated that inception voltage can be related prevailingly to either background radiation or field-assisted electron detrapping from defect surfaces, depending on defect geometry and environmental conditions.

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.

Charge density stabilised local electron spin pair states in insulating polymers

A model is presented that addresses the energy stability of localized electron states in insulating polymers with respect to delocalized free electron-like states at variable charge densities. The model was derived using an effective Hamiltonian for the total energy of electrons trapped in large polarons and spin-paired bipolarons, which includes the electrostatic interaction between charges that occurs when the charge density exceeds the infinite dilution limit. The phase diagram of the various electronic states with respect to the charge density is derived using parameters determined from experimental data for polyethylene, and it is found that a phase transition from excess charge in the form of stable polarons to a stable state of bipolarons with charge = 2 and spin number S = 0 is predicted for a charge density between 0.2 C/m3 and ∼2 C/m3. This transition is consistent with a change from low mobility charge transport to charge transport in the form of pulses with a mobility orders of magnitude highe...