Michael G. Danikas

Nanocomposites-a review of electrical treeing and breakdown

In this short review, we discussed some aspects of nanocomposites regarding electrical tree growth and breakdown. It appears that nanoparticles, properly mixed and dispersed in the polymer matrix, increase the breakdown strength and hinder the growth of electrical trees in the nanocomposite. Nanoparticles act as barriers obstructing electrical tree growth and delaying dielectric breakdown. This article also put forward a tentative proposal for the mechanisms of treeing and breakdown in nanocomposites.

Breakdown of transformer oil

Some of the factors affecting the dielectric strength of transformer oil are investigated. These factors include the stabilization phenomenon, oil and electrode pretreatment, the effect of oil velocity, the effect of a capacitance parallel to the test cell, and the effects of electrode area and gap spacing. The experimental apparatus and procedure are described, and the results are presented for each of the above factors. Suggestions for future research are offered.<<ETX>>

Simulation of electrical tree growth in solid dielectrics containing voids of arbitrary shape

In this paper the electrical tree growth in solid insulating materials containing voids of arbitrary shape is modelled using von Neumanns cellular automata (CA). The model is based upon the assumption that the electric stress at the end of a conducting tip quite often approaches the dielectric strength of the material and that progressive breakdown may occur by electrical tree formation. Because of tree advancement, the potential distribution within the insulating material changes with time, and it is calculated at each time step. An algorithm, based on this model, for the simulation of electrical tree growth in solid dielectrics containing voids of arbitrary shape has been developed. Simulation results show that voids constitute a danger for solid insulation, because they act as sources of secondary electrical trees. An interaction mechanism between voids within a dielectric is proposed and discussed.

The definitions used for partial discharge phenomena

A variety of definitions related to partial discharge (PD) phenomena are discussed. There is no general agreement at this time of writing as to the definitions of the various regimes of PD. A certain confusion exists which may be due to the multitude of PD manifestations. An attempt is made to elucidate some aspects of the PD definitions. Related problems needing more work are noted. >

Small partial discharges and their role in insulation deterioration

It is a well known fact that PD (partial discharge) affects the lifetime and the deterioration rate of an insulation. Research work has shown that a mixture of magnitudes and frequencies of PD may appear, depending on the type of the applied voltage, its frequency, the electrode arrangement, the type of the insulation under test, and a number of other parameters, such as temperature, humidity and pressure. Various types of discharges also have been recorded, such as streamer like discharges, Townsend discharges, glow and pseudoglow discharges. A type of discharges not yet well understood is the so called SPMD (swarming partial microdischarges), which seem to appear just before breakdown. In this paper, only aspects of the latter type of PD are investigated. Some suggestions for further research regarding the SPMD are made.

Partial discharges in epoxy resin voids and the interpretational possibilities and limitations of Pedersen's model

ContentsPartial discharges take place, among others, inside voids enclosed in insulations. The partial discharge behaviour of epoxy resin samples is studied with the aid of a recently developed streamer partial discharge model (Pedersens model) and its relation to the modem Pulse Height Analysis and Phase Resolved Measurement techniques is investigated. Furthermore, some relations based on Pedersens model are deduced. The advantages and disadvantages of the aforementioned model are analysed. Scanning electron microscope photographs show the nature of the deterioration of the void walls due to the partial discharges. The question whether initiation of the treeing phenomenon leading to breakdown from an enclosed spherical void is possible, is also investigated.ÜbersichtTeilentladungen verursachen eine Alterung der Isolierstoffsysteme. Sie können in Hohlräumen der Isolierstoffe stattfinden. Das Teilentladungsverhalten von Hohlräumen in Epoxydharz wird mit der Hilfe von Pulse Height und Phase Resolve Analysis studiert und mit der Hilfe des Pedersen-Modells analysiert. Einige mathematische Verhältnisse diese Modells werden vorgeschlagen. Die Vorteile und Grenzen dieses Modells werden analysiert. Rastermikroskop-Aufnahmen zeigen die Art der Schädigung der Hohlraumwände. Frage, ob das Phänomen der Bäumchen von einem Hohlraum zu einem Durchschlag der Isolation führen kann, wird nachgegangen.

Simulation of electrical tree growth in solid insulating materials

ContentsIn this paper the electrical tree growth in solid insulating materials is modeled using von Neumanns Cellular Automata (CA). The model is based upon the assumption that the electric stress at the end of a conducting tip quite often approaches the dielectric strength of the material and that progressive breakdown can occur by electrical tree formation. Because of tree advancement, the potential distribution into the insulating material changes with time and is calculated at each time step. An algorithm for the simulation of electrical tree growth in solid dielectrics based on this model has been developed. The algorithm is also used to simulate breakdown in solid dielectrics containing square or spherical voids.ÜbersichtIn diesem Artikel wird das Wachstum der elektrischen Bäume in Festisolierstoffen mittels von Neumannscher zellularer Automaten modelliert. Dieses Modell geht davon aus, daß das elektrische Feld an der Spitze der Elektrode oft in der Nähe der dielektrischen Festigkeit vom Material liegt und das ein fortschreitender Durch-schlag durch elektrische Bäume möglich ist. Durch das Wachsen der elektrischen Bäume ändert sich die Spannungsverteilung innerhalb des Festisolierstoffes mit der Zeit und wird in jedem Zeitschritt berechnet. Ein Algorithmus für die Simulation des Wachstums eines elektrischen Baums in Festisolierstoffen ist entwickelt worden. Der Algorithmus kann auch den Durchschlag in Festisolierstoffen mit zylindrischen oder kugelförmigen Hohlräumen simulieren.

Evidence and presumption in PD diagnostics

A general outline of the development of PD diagnostics is given with emphasis on problems of the validity of underlying models and the interpretation of results, because PD diagnostics is hardly ever direct and instead rests on a chain of inferences. Some major classical approaches are reviewed in this context, with a discussion of certain problems that also are significant for modern approaches. After a discussion of primary PD events and resulting degradation, some basic matters regarding measurement and testing are described and their problems are discussed. Particular attention is then paid to various approaches to PD pulse spectroscopy and its application to product diagnostics, particularly after algorithmic processing of the recorded data. The paper concludes with a discussion of conclusions that may or may not be drawn validly from PD diagnostics. A number of warnings and some suggestions of further development are given. >

Discharge studies in solid insulation voids

Partial discharge currents are studied with the aid of a digitizer for voids of various depths and constant diameter in solid insulation. Voids of 0.16, 0.32, 0.48, and 0.64 mm depth were tested. With the present arrangement, risetimes below 1 ns are measured. The results indicate a streamer mechanism taking place in the voids enclosed in solid insulation. Pedersens theory for ellipsoidal voids seems to agree somewhat better with experimental values than the classical capacitive model.<<ETX>>

A model for electrical tree growth in solid insulating materials using cellular automata

Models proposed to explain the breakdown mechanisms of the solid insulating materials are based, among others, on electromagnetic theory, avalanche theory and fractals. In this paper the breakdown of insulating materials is simulated using von Neumanns Cellular Automata (CAs). An algorithm for solid dielectric breakdown simulation based on CAs is presented with a point/plane electrode arrangement. The algorithm is also used to simulate breakdown in a solid dielectric having a spherical void.