Partial Discharge Repetition Rate Research Articles

Electrical measurements for the characterization of electret properties of cellular polymers

Cellular polymeric materials may show interesting quasi-piezoelectric properties, upon charging under specific conditions of electrical field, temperature, pressure and environment. Electrical charging is the trigger of electret properties. It has been proved that the occurrence of electrical avalanches in the cavities which are present in a material is associated with the appearance of electret behavior, but the precise nature of the phenomena involved in this transformation is still under investigation. This paper provides a contribution to the study of these materials focusing on the relation between the appearance of electret properties and the presence of charge injection and partial discharge phenomena. For this purpose, space charge, partial discharges and low-frequency polarization measurements were carried out on polypropylene (PP) cellular materials, at different fields and temperatures. The existence of a threshold field for the start up of electret behavior that is larger than the threshold for space charge injection and partial discharge inception was observed. Steady charge is accumulated in the material, thus generating electret behavior, only if partial discharge repetition rate becomes sufficiently high and space charge is injected from electrodes. Space charge measurements were performed through the pulsed electroacoustic technique, in spite of significant attenuation of acoustic waves due to the cellular nature of the materials. Partial discharge measurements were carried out by an advanced system recording each discharge pulse, thus allowing observation of single and multiple pulses, besides achieving statistics of discharge pulse amplitude and phase. By such approach, space charge and partial discharge measurements proved to constitute a valuable tool to investigate the phenomena leading to piezoelectric behavior and, also, to infer electret time stability and optimize charging methodologies.

Partial discharges measured with an automated system in epoxy resin and polyethylene

The partial-discharge (PD) characteristics of a filled epoxy casting resin were compared in long-term tests with those of polyethylene (PE). For this purpose, a newly developed automatic measuring system was used to detect and process PD parameters such as the apparent amount of charge, the PD repetition rate, the apparent PD energy and the polarity of the impulses, together with the position of the latter with respect to the phase of the applied 50-Hz AC voltage. The measurements were made on needle-plate electrode model specimens. The results recorded over long stress periods at constant voltage show that the PD impulses in epoxy resins arise discontinuously and with constantly changing amplitude. So called impulse pauses lasting several hours were recorded in these materials. PE was found to differ fundamentally from epoxy resins in their PD characteristics. In the PE material, the PD impulses occurred fairly regularly and were of almost constant amplitude. As a result of the very high impulse frequency in PE over the whole period of measurement, breakdown development was relatively rapid. Interpretation of the results is based on the concept of the origin of PD channels in the extremely inhomogeneous AC field, and, in particular, use is made of the recorded phase histograms and of the changes observed in the apparent PD energy with time.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the influence of measuring circuit on a DC partial-discharge repetition rate

A simple model of DC partial discharges running in cable and capacitor dielectrics is described in order to investigate the influence of the measuring circuit on the DC partial-discharge repetition rate. This model shows that the measuring circuit can only influence the discharge repetition rate temporarily. On reaching the stationary state the DC partial discharges are governed by the electric qualities of the dielectric used.