M. Goldman

In situ mass spectrometric analysis of the deterioration of polymer surfaces by low pressure AC discharges

Mass spectra of positive ions extracted from low-pressure (300-500 Pa) AC discharges to polymer surfaces show that polyethylene and polypropylene emit significant amounts of water when exposed to dry air of air-SF/sub 6/ coronas, and that this is due to the action of oxygen. This suggests that water trees in polymers may grow without externally supplied water if microdischarges and oxygen are present. The discharge conditions used, pd approximately=2.5 Pa-m, are near the Paschen minimum and correspond to a void of 25- mu m thickness at atmospheric pressure. This is the width at which a partial discharge will initiate at the lowest voltage. The corona ion source mass spectrometer has proved to be a very sensitive device for in situ measurement of the interaction between a gas discharge and a surface.<<ETX>>

Chemical aspects of the ageing of solid insulating materials submitted to partial discharges

The authors attempt to correlate the characteristics of the electrical signals due to partial discharges (PD) with the chemical aspects of the ageing of the insulating material subjected to these discharges. The samples are 19- mu m-thick foils of polyethylene terephthalate, low-density polyethylene, and polypropylene. The PD development both in time and in space is related to the electrical pulse shape and to the chemical composition of the surface subjected to PD. The pulse shape can be related to the ageing of the insulating material. Several ageing steps appear. The surface wettability increases at the very beginning of exposure to PD. Changes of the surface morphology are studied using electron scanning microscopy. The by-products of chemical reactions due to PD have been analyzed by infrared microspectroscopy. The production of carboxylic aid by PD and the effect of this product and water on the ageing processes are also discussed.<<ETX>>

On the role of water in the ageing of polymers in air-insulated electrical systems

Experimental results are presented on the direct interaction between corona and insulating surfaces, including the generation of water by corona oxidation of the polymer. Measurements of indirect corona-polymer interaction by corona exposure of distilled water with submerged polymer samples are also reported. It is shown that corona discharges in ambient air will form a water layer on any exposed polymer surface, both by bringing water to the surface made hydrophilic by the corona, and by producing water from the polymer itself, by oxidation. Water exposed to air corona becomes acid and oxidizing, and causes marked aging effects on polymer surfaces. Thus, any model of the action of air corona discharges on polymer surfaces must include the presence and effects of an intermediary aqueous electrolyte surface layer.<<ETX>>

Charge transfer by contact between metallic samples and insulating coatings

The paper deals with experiments carried out in ambient air to study charge transfer in a quasi equilibrium state from metallic samples into insulating layers covering their surface. Experimental results concern the surface potential variations undergone when the thickness of these layers is increased. They show that below a critical thickness xcrit depending on the layers nature, the electrical behaviour of the layers is governed by a surface charge, and that beyond this thickness value, it becomes space charge dominated. A model fitting with these results is proposed.

In-situ measurements of the electrical stresses induced by corona streamers on polymer foils under AC conditions

This work deals with air corona gaps submitted to AC voltages with an insulating coating on the low field electrode. Such gaps with a point-to-plane geometry are suitable for inducing accelerated streamer ageing phenomena on the surface and in the bulk of insulating materials. They are thus as useful for dielectric testing of materials, as they are for studies on partial discharges effects or on other failure phenomena which can cause malfunctions in high voltage equipment. The aim of the present paper is to report on in-situ oscillographic measurements which can give a direct access to the electrical properties of dielectric samples submitted to severe electrical stresses (in this case, polypropylene films 15 /spl mu/m thick). An AC point-to-plane corona deposits charges of alternating sign on the sample surface. The resulting potential across the polymer foil and the current through it are recorded. As it has been shown that this current determines the lifetime of the polymer foil, it follows that the present method will be of interest for the testing and the qualification of similar insulating materials.

Evolution of the surface layer appearing with the corona ageing of epoxy samples

The impact of ions, photons and neutral activated particles produced by a corona can be an important parameter in the mechanism of polymeric indoor HV insulation ageing. Corona treatment is known to induce a wettability increase on polymer surfaces, and can thus be one of the first steps towards the emergence of a wet conductive layer on the surface. Therefore, this chemical influence of the discharge on the surface greatly influences the flashover voltage. We present here optical and electrical measurements on epoxy samples submitted to the activated species produced by an alternating corona discharge. Corona treatment on polymers by the means of an AC corona discharge is of common industrial use nowadays to increase the wettability and adhesivity of a polymer surface. However, the efficiency of corona treatment is limited by the development of a surface layer, composed of oxidation products of the polymer, and weakly bound to the substrate, since it can be removed by washing.

Ageing kinetics of solid insulating materials submitted to partial discharges

An attempt is made to correlate the characteristics of the electrical signals due to partial discharges (PDs) with the ageing kinetics of the insulating material subjected to these discharges. The light emitted by a single PD and the corresponding electrical pulses have been recorded using a high-speed camera and a computerized signal acquisition system. The discharge development both in time and space is related to the electrical pulse shape, i.e., the amplitude of each frequency component of the electrical signal, and can be related to the ageing of the insulating material. Several steps of insulation ageing appear. The surface wettability and thus surface conductivity increase at the very beginning of exposure to PD.<<ETX>>

Drilling effects of corona streamers on insulating films

The interactions of streamers with polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) films were studied experimentally. Streamers impinging on polymers subjected to corona discharges were found to add localized disruptive effects to the dispersed chemical effects produced by the corona-activated gas. The time elapsing before disruption of a polymer film has been used to analyze the phenomenon as function of the foil thickness and the discharge current. It is shown that there are at least two different possible mechanisms of disruption, ion sputtering which has been observed on PE films, and another mechanism, observed on PP and PET films, which produces blistering effects like tears.<<ETX>>

Current imbalance and charge build-up on insulators in AC air coronas

By replacing an AC-corona-stressed insulator surface by a metal electrode subjected to superposed AC and DC corona, the authors studied the processes responsible for the DC charging of insulators. A main result is that the charging is critically dependent on corona processes at the high-field electrode. A corona electrode that gives positive streamers will give substantially less net charge than an electrode burning in the positive glow (Hermstein corona) mode. However, the streamers may do other damage to insulator surfaces. The balance between the synchronous and impulse current components is found to be the decisive factor in determining the charging of corona-exposed dielectric surfaces.<<ETX>>