G.C. Montanari

The incorporation of space charge degradation in the life model for electrical insulating materials

The time function of the Eyring reaction rate theory of insulation life is modified to demonstrate a physical origin for temperature threshold. Various mechanisms by which trapped charges may be involved in degrading the polymer are examined, and incorporated into the life model through an alteration to the free energies of the reacting system. The corresponding life functions are shown to possess the field-dependent threshold form previously obtained phenomenologically for insulating materials. The physical interpretation of the model parameters is discussed.

Space charge phenomenology in polymeric insulating materials

Topics related to space charge phenomenology, particularly the origin of space charge and its interaction with the host insulating polymer, have been widely discussed in recent years. Due to the development of reliable techniques for direct observation of space charge it is now possible to obtain, besides obvious indications on space charge and electric field magnitude and location, quantities that provide additional information on space charge accumulation and depletion rate, apparent mobility and trap depth distribution. Therefore, commonly debated topics such as the nature of charge carriers, the conduction mechanism, the charge dynamics at interfaces and the relation with nanostructure can be re-examined on the basis of space charge observation and this is, indeed, the purpose of this paper.

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.

First electron availability and partial discharge generation in insulation cavities: effect of light irradiation

An experimental investigation of the behavior of partial discharges occurring in insulation cavities as a function of light irradiation conditions is presented in this paper. Partial discharge inception voltage (PDIV) measurements were performed for different irradiation conditions of the test objects (visible light, of variable intensity, ultra violet (UV) light or absence of irradiation), defect geometry and conductivity of insulating material. It is shown that changes of such factors involve considerable alteration of partial discharge inception features, in terms of both inception voltage and delay time. Depending on applied field, defect geometry, insulating material characteristics and irradiation condition, in fact, partial discharge inception voltage may be influenced prevailingly by background radiation, as generally stated in literature, or by field-assisted electron detrapping from material-cavity interface.

Conduction current measurements on XLPE and EPR insulation

Electrical conduction of crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR) is studied by means of DC charging current measurements. The threshold for space charge accumulation and the DC conductivity of XLPE and EPR have been investigated at different electric fields and at different temperatures. Current-voltage characteristics show a transition from ohmic behavior to high field behavior, above a threshold value of the applied electric field. An Arrhenius-type dependence of both XLPE and EPR conductivity with temperature was found. Measurements were performed also on two-layer XLPE/EPR combinations. Based on conductivity values obtained by measurements on a single layer sample, the electric field distribution of the two-layer combination is discussed.

A new algorithm for the identification of defects generating partial discharges in rotating machines

This paper describes a new algorithm for the automatic identification of the type of partial discharge sources in insulation systems of rotating machine stator windings. The proposed algorithm is based on fuzzy rules that associate each partial discharge phenomenon to a fuzzy space class trough a tree path selection. The output is a simple frame where the defect type and a membership are evidenced. Applications of the algorithm to analyze data collected both on field and in laboratory are reported and discussed to show its validity and robustness.

The effect of space charge on phenomenology of partial discharges in insulation cavities

The effect of space charge accumulated in insulation cavities on partial discharge activity is dealt with in the paper. Partial discharges were measured on non-polarized and DC-polarized XLPE specimens. In order to characterize space charge distribution, pulsed-electroacoustic (PEA) space charge measurements were also performed on polarized and PD-aged specimens. The results show that space charge can affect, although in some complex way, partial discharge phenomenology. Moreover, space charge measurements on stressed specimens can help in the inference of degradation mechanisms (e.g., trap formation at the cavity surface) associated to partial discharge activity.

Diagnostic evaluations of electrical apparatus by a novel partial discharge measurement system

The application of a novel partial discharge measurement system to the identification of defects in insulation systems of electrical apparatus is presented in this paper. It records the whole waveshape of a large number of partial discharge pulses and, then, splits the measured dataset into subgroups on the basis of the pulse waveshape. When different waveshapes can be attributed to different partial discharge sources or, possibly, noise, artificial intelligence techniques enable the system to reject noise and apply identification/risk-assessment routines to homogeneous patterns, that is, to phase-resolved partial discharge patterns relevant to a single partial discharge source.

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.

Simulation of bipolar charge transport in polyethylene featuring trapping and hopping conduction through an exponential distribution of traps

We present an asymmetric bipolar model of conduction intended to describe temporal and spatial evolution of space charge density and external current in cross-linked polyethylene under a uniform applied DC field. Carriers of both polarities are provided by injection at the electrodes according to the Schottky law. Charge transport is described by a hopping mechanism in which carriers move from site to site by getting over a potential barrier. An exponential distribution of trap levels with a maximum limit in trap depth is adopted for the model. Net charge density profiles and external current are obtained by solving the continuity equation for electrons and holes and the Poisson equation for electric field coupled to transport equation. The aim of this work is to use the simulations to evaluate the influence of temperature and applied field on the net charge density profiles.