ONSET OF NEGATIVE AND POSITIVE CORONA
3.4 FIRE AS A CATALYST FOR CORONA AND BREAKDOWN
3.4.1 Flame Temperatures
Fire has the fundamental component of high temperatures. The temperatures of the sugar cane fires in the proximity of the conductors has been estimated to be in the region of 110°C to 250°C. Flame temperatures have been measured up to 800°C and 1200°C, but it appears that no accurate measurements have been made to date. In limited measurements made in this project with advanced thermal detecting visual equipment, temperatures greater than 162 C were recorded at the conductors in a genuine sugar cane plantation fire. Further temperature recordings should be made to capture more information about the actual temperatures under which corona is being generated at the conductors. It is however, not expected that the temperatures would be high enough for thermal ionisation to play any major role in the corona process. As explained previously (section 3.2.3), work done byK.T. Compton [25] showed that temperatures of over eight hundred degrees are required to reduce the breakdown strength of air by even a small amount with the assistance of thermal ionisation.
3.4.2 Flame Lengths
The heat generated by the fire is a continuous "plasma" frequently releasing large pockets of heat from the main body of the flame. Depending on the environmental elements, flames of a sugar cane fire may reach the conductors from many meters away. Sugar cane fires have now been visually recorded with flames reaching heights of several meters above the earth wires on a triangular line configuration. With no wind present, the vertical heights obtainable from a continuous flame should therefore not be underestimated. Recordings have captured flames at heights of 25 to 30 meters. Inthe environment, humidity will also influence the eruptive nature
with the thermal vertical ascent being very much less limited. Likewise in windy conditions, the horizontal distances across which the flames could be distributed is at times extremely far (unexpectedly far for the inexperienced since IT. Bird refers to distances of up to 25 meters).
3.4.3 Air Density / Air Pressure
The fundamental activity occurring due to the presence of higher temperatures, is the decrease in air density. From equations (1) and (2), substantial increases in temperature (T) and decreases in pressure (P) will substantially increase the mean free path experienced by electrons under the influence of an external electric field. Hence the energy gained by those electrons will also increase substantially. The probability of ionisation, Pi, will therefore be higher and the rate of electron avalanche will increase. The nett result then follows that for fire-induced corona the voltages at which the different modes of corona start to occur should be lower than is the case for rain-induced corona. However, unlike rain, the flames of the fire are not continually in the same physical position in the column below the conductor (figure 3.3) but rather will shift and move around in that column touching and covering the conductor surfaces sporadically. Hence large streamers can be expected at lower voltages but due to the varying position of the flames at the conductors, these large streamers will not be continuous, as can be expected in the case of rain- induced corona.
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In some instances, flames will also pass by the conductors in pockets dislocated from the main body of the flames.
3.4.4 Particles
Figure 3. 5: Anexample of pockets of flame.
The presence of carbon-based particles in sugar cane fires is another factor which will severely influence the electric field about the conductors. The distortions caused by conducting particles to electric fields are well understood [30]. Sadurski postulated, "the presence of particles in bush fire flames are responsible for the flashover of transmission lines" [5]. In the proximity of the conductors the particles become dipoles (figure 3.4), ionised by the electric field and becoming part of the field lines emanating from the high voltage conductors.
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The increased local field due to the particle will start the process of ionisation by electron impact, and provided the electric field is high enough, the gap between conductor and particle will be bridged with streamers then emanating from the particle.
Figure 3.7: Anexample of particles in and above flames during a night-time fire.
Should there be sufficient particles in the gap between electrodes, the bridging affect from conductor to particle to particle will finally result in the bridging of the entire gap.
The final bridge to flashover may not necessarily be a particle bridge but rather the increased electric field stresses across the reduced gap between the last particles (front of the leader) and the other electrode. More research is being carried out in this field to determine the process by which particles bridge the gap and the requirements for such a flashover to occur due to the sugar cane particles. Also the role which particles play in the breakdown of the air insulation in the event of a sugar cane plantation fire under power lines [q].
3.4.5 Chemical Reactions
The research work performed here does not consider the influence of different flammable materials and as such does not compare the chemical reactions resulting from different combustion processes. For any given flammable material, combustion of the material will result in the release of electrons and ions according to the chemical reactions. The number of free electrons available for ionisation by collision will therefore vary from flammable material to flammable material. Any material producing more free electrons through the combustion process may induce higher corona intensities. The conductor and bundle dimensions and the applied electric field stresses are critical components which will influence the degree to which that corona intensity will exist further from the conductor surfaces.