Propagation and structure of streamers in liquid dielectrics

Our purpose is to present a critical review of the current understanding of streamer propagation in dielectric liquids in order to help define the direction of future research. We show that the molecular structure has a significant effect on streamer propagation. The main parameter affecting propagation is the electronic affinity of the liquid molecules     

PD characteristics and mechanisms in artificial air-filled voids atroom and liquid nitrogen temperatures [comment and reply]

For original paper see Tsuru et al, ibid., vol.6, p.43 (1999). Tsuru et al. studied discharge mechanisms in cavities in some specific test samples at room and liquid nitrogen temperatures. They claimed that the sensitivity of their partial discharge (PD) measurements was 3 pC, while that of the phase-resolved measurement was 10 pC. Danikas thinks that such sensitivities are rather low if one wants to investigate in detail the mechanisms of PD in air-filled cavities. Tsuru et al reply to the comments     

Simulation of tree propagation in polyethylene including air void by using cellular automata: The effect of space charges

In the present paper tree propagation in polyethylene is simulated with the aid of cellular automata. The case of small air voids is studied for various space charge values. The influence of positive space charge is investigated and the simulation shows that tree propagation is strongly affected by the space charge density. The role of homocharges in the propagation of tree structures is studied and the results of the simulation are in qualitative agreement with published experimental data.     

Properties of electrical insulating materials at cryogenictemperatures: A literature review

Many insulating materials and systems for superconducting electrical equipment have been developed in response to the severe demands of safe usage, such as in cryogenic temperature, high mechanical stresses, high radiation, etc. They have been designed to fulfill very extreme service conditions, either at liquid helium or liquid nitrogen temperatures. Whatever the future industrial development, there is already abundant knowledge about insulating materials at cryogenic temperatures, and the feasibility has been proven. Many laboratories in the world continue research to increase this knowledge and to fill in the remaining gaps     

Influence of homocharges and nanoparticles in electrical tree propagation under DC voltage application

The influence of nanoparticles and homocharges on the propagation of electrical treeing in polymer insulation is examined for a needle-plane electrode arrangement. A simulation is carried out using a model based on Cellular Automata (CA). A DC voltage application on the needle electrode is assumed. Nanoparticles are introduced in the polymer matrix in the vicinity of the needle electrode, and simulations with different homocharge densities are performed. It is confirmed that the propagation of electrical trees is hindered by the presence of nanoparticles and homocharges. A larger quantity of homocharges forms a barrier to the injection of charge carriers in the nanocomposite sample. Electrical trees seem to go around and/or stop at nanoparticles and thus, their propagation becomes more difficult. In other words, the proposed simulations show that electrical trees follow a tortuous path, avoiding the nanoparticles.     

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

Contents Partial 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 (Pedersen's model) and its relation to the modem Pulse Height Analysis and Phase Resolved Measurement techniques is investigated. Furthermore, some relations based on Pedersen's 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.

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Teilentladungen in Hohlräumen in Epoxydharz und die Möglichkeiten und des Pedersen-Modells

Übersicht Teilentladungen 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.

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The experimental results were obtained at ABB, Baden/Switzerland. One of the authors is indebted to Dr. L. Niemeyer and Dr. B. Fruth for fruiful discussions.     

Study of samples of composite insulating system under electricaland thermal stresses

Factors affecting the performance of small composite samples are investigated, and the capabilities of the uniaxially oriented polyethylene (UOPE) tape/dodecylbenzene (DDB) oil insulation system are evaluated. The apparatus used to study the influence of the combination of electrical and thermal stresses on the performance of the samples is described. Experimental results are reported for both uniform and nonuniform electrode arrangements. The discharge energy/material damage tests showed that a satisfactory correlation between the total surface area damage of the tapes and the energy supplied by the discharge exists. This suggests that the measurement of total discharge energy is a very useful technique in evaluating electrical insulation and may prove useful as a diagnostic tool for predicting the remaining life of insulation     

Analysis of Surface Degradation of Silicone Rubber Insulation Due to Tracking Under Different Voltage Profiles

Tracking is basically a carbonaceous process. In the present study, the tracking phenomenon in silicone rubber material has been studied under AC and DC voltage. The influence of applied voltage magnitude, conductivity, and flow rate of the contaminant on tracking, were analyzed. It is observed that the tracking is more severe under DC voltages. The tracking time is less under negative DC voltage compared to the positive DC voltage. The tracking mechanism is explained in detail. The leakage current during the tracking studies was measured and its trends were analyzed by using the moving average technique. The leakage current magnitude is high with thermally aged specimens when compared to the virgin specimen, irrespective of the type of applied voltage. It is realized that the tracking time and the leakage current magnitude shows an inverse relationship. In addition, the surface condition of the insulation material was analyzed with the water aged specimen and the diffusion coefficient of the material was calculated. The reduction in contact angle of the specimen has direct impact in the reduction of its tracking time. The tensile test and DMA test results indicate that thermal aging of the specimen alter the mechanical properties of the material. The activation energy of the material was calculated through DMA studies, indicating that materials with high activation energy show an increase in tracking time of the specimen.     

Technical report-particles in transformer oil

Attention is drawn to a parameter that plays a crucial role in determining the breakdown strength of transformer oil, yet which tends to be ignored. This parameter is the settling time, which can mean either the time interval between two consecutive measurements or the time for which an oil sample is stored before measurements begin. The term is used here in the former sense. Research carried out with Bruce profile electrodes and for gap spacings of 1 and 4 mm showed that settling times between measurements of 1 and 5 mm gave widely differing breakdown strength results. The breakdown strength with 1-min settling time was inferior to that for a settling time of 5 min. A particle counting analysis performed on two oil samples supports the hypothesis that short settling times do not allow either the bubbles from the previous breakdown to be dissolved or the decomposition products to settle under the influence of gravity, thereby increasing the probability of breakdown     

Streamer propagation over a liquid/solid interface

Danikas discusses and comments on a paper by Atten and Saker (see ibid., vol. 28, no. 2, pp. 230-42, 1993). He points out that, using a similar point/plane electrode arrangement and different insulating oils (dodecylbenzene oil and polybutene oil), he reached mostly the same conclusions regarding creepage discharge figures. He also discusses inception electric stress and streamer discharges. In reply, Atten and Saker thank Danikas for his comments and acknowledge the points he raises