This paper presents experimental data and calculations concerning the electrical properties of positive streamers in mineral oil at large gaps and HV. The experiments concern the measurement of charge, electric field, and the determination of the potential drop along streamers, either in the liquid alone, or for streamers guided within insulating tubes. Calculations of charge and field distribution around streamers are carried out by charge simulation. To do this, streamers are represented by objects with simple shapes (spheres or cylinders) equivalent to their macroscopic aspect. These models lead to a correct agreement with measured streamer charge and field on the plane electrode. Qualitative correlations are established between calculated field distributions and streamer behavior such as velocity, transitions between propagation modes. It is also concluded that the potential drop in streamers and branching both act as regulating mechanisms that help to keep the streamer tip field, and hence the velocity, constant over a wide voltage range.

Positive streamer propagation in large oil gaps: electrical properties of streamers

The choice of the materials used as the dielectric media in large transformers continues to be a combination of pressboard and mineral oil. However, a number of failures in the field have been attributed to the surface discharge or creeping discharge over the oil/pressboard interface. This paper reports on an experiment to study surface discharge over oil/pressboard. The TJ (triple-junction) model was used to study the characteristic of partial discharge (PD) from inception to flashover. The max PD charge, pattern, frequency and current pulse were measured during the flashover process. Four oil/pressboard specimens with different aging degrees were used and their influence on the flashover process over oil/paper interface was studied in this experiment. The results indicate that the max PD charge and PD frequency increase with the increase of applied voltage. The oil aging has great influence while the pressboard aging has little influence on PD characteristic over oil/pressboard. The PD pattern and current pulse shape show great difference for various applied voltages. The results therefore suggest that the PD detection on the oil/pressboard can be used effectively to implement the early monitor of surface discharge thereby to avoid the formation of surface flashover.

Partial discharge characteristics over differently aged oil/pressboard interfaces

The properties of streamers traveling over the surface of oil-immersed solid dielectrics were experimentally studied under lightning impulse conditions. Streamer polarity and the position of a grounded side electrode significantly affected the relationship between the streamer extension length and the applied voltage. Solid surface charging also had a large effect on the streamer propagation. However, the streamer propagation properties showed a consistent dependence on the potential at the solid-liquid interfaces. In addition, the potential drop inside the streamer channel was measured as a function of normalized streamer length. The curve revealed that the potential drop increased drastically within the region of /spl sim/20% from the streamer tip. The streamer appeared to progress with a constant mean velocity.

Creepage discharge characteristics over solid-liquid interfaces with grounded side electrode

A needle immersed in transformer oil is located just above a sheet of glass of which the opposite surface is conducting. By applying high-voltage (HV) steps, streamers which propagate radially with numerous branches are generated. The characteristics of these discharges (mean propagation velocity, final extension, and electric current and charge) are determined, and the measured charge is compared with the predictions of two simple models. A technique for guiding the discharges gives single quasi-rectilinear streamers, which allows a more accurate investigation. The mean electric current and the final charge indicate that the streamer channels are, to a first approximation, at the potential of the needle. The simple picture of a finite potential gradient along the channels is discussed with reference to the intermittent character of the discharge phenomena. >

Streamer propagation over a liquid-solid interface

The electrical characteristics of creeping discharges and single creeping streamers in transformer oil first are compared with those of streamers developing in the liquid bulk. The distribution of electric potential along the channel of a single negative creeping streamer is determined using a capacitive probe technique. Then the distribution of the space charge associated with each streamer channel is discussed and the electric field around the channels is estimated. A strong correlation between the mean potential gradient and the capacitance of the streamer channels is found. The different results and considerations tend to support the hypothesis of the same basic physical mechanism for both creeping discharges and streamers developing in the bulk.

Properties of streamers in transformer oil

The growth characteristics of positive impulse surface discharges using a point-plane electrode geometry in oil have been studied using schlieren optics and a special probe was used for the measurement of the voltage drop across the streamer. The experiments are made in growth lengths up to, approximately 4 cm. The growth characteristics are discussed in conjunction with the probe measurements. We examine mechanisms for the continuous reduction of the potential gradient in the streamer channel as the tip grows, and then propose a new model for streamer growth, adding some findings on light emission in the channel.

S. Ohgaki

Papers

A Study of the Positive Streamer Growth under Surface Discharge Configuration in Liquid Paraffin