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Tsinghua Science and Technology, December 2001, 6(5), pp. 497-499 Air Breakdown During Fires* WANG Liming Department of Electrical Engineering, Tsinghua University, Beijing 100084,
China; Received: 2000-09-24; revised: 2000-11-30 *Supported by EPSRC from United Kingdom Code Number: ts01104 Abstract: This paper analyzes the different discharge processes occurring at normal temperatures and high temperatures. The theoretical results show that the net charges in the streamer channel at normal temperatures are zero, but they are positive at high temperatures so that the advancing field is reinforced more than that at normal temperatures. Therefore, the field required for streamer propagation is reduced at high temperatures. The sparkover voltage is largely reduced with increased temperature, which is influenced by the solid materials in the flame. Key words: higher temperature; streamer; sparkover Introduction It has long been accepted that the insulation strength of air depends upon the air density. For most operating conditions, changes of the air density are due to the changes of the air pressure. However, the high temperatures are occasionally encountered, for instance, forest fires occur close to power lines[1]. Although much work has been carried out at low temperatures, where the temperature changes are small[2-4], there is a relative lack of information for high temperatures, where the pressure remains more or less constant. The recent fire in the channel tunnel has focused attention on this point, since it is thought temperatures approaching 1000 K were experienced. Therefore, it is important to acquire some data to estimate effectiveness of the insulation of overhead lines at temperatures in a range up to this limitation. The present paper seeks to summarize the theoretical knowledge already available and presents some test results. 1 Breakdown Process under Reduced Air Density At normal temperatures and pressures, the ionization of air is brought about by the impact of electrons on neutral molecules, so creating positive ions. However, due to the electronegative nature of the 2(H2O)n complexes (where n is usually between 0 and 5), the electrons produced in ionizing collisions attach themselves to these molecules and the resulting "mixture" contains roughly equal numbers of ions of each polarity, with few free electrons. Most breakdowns in electrical equipment occur in highly non-uniform electrical fields between metallic electrodes having a variety of profiles. A successful model envisages a single electron multiplying by successive ionization until a significant concentration of the positive ions is created ( Fig. 1 (a) ). This "space charge" sets up a field reinforcing the applied field, so that further ionization occurs around it and the space charge concentration is replicated in the direction of the applied field. However, the electrons, after leaving the positive space charge region, slow down and quickly attach to the O2(H2O) n complexes to form negative ions which, on the time-scale of electron velocities, can be regarded as stationary. On account of the low mobility of both positive and negative ions which extend back towards the positive electrode, the advancing space charge in front and the trail of ions left behind are known as a "streamer". A streamer is endowed with certain precise properties and characteristics, one property is worthy of note. A streamer can progress in an electrical field of about 5 kV/cm at normal temperature and this determines the average field at which the final breakdown occurs after the streamer has crossed the inter-electrode gap. An important change takes place as temperature increases. The electrons attached to the 2(H2O)n complex is only loosely attached, requiring the acquisition of energy >0.1 eV to detach it. This is easily achieved as the random kinetic energy of molecules increases with temperature, so at several hundred degrees it can be assumed that negative ions have such a short lifetime that their concentration is negligible and all negative charges are free electrons[5]. The model adopted for streamer development at normal temperature is thus no longer valid, since electrons will remain free and will be drawn quickly up the channel of Fig. 1 (a) to the positive electrode, so that a purely positively charged channel will remain as in Fig. 1(b). It has also been demonstrated that branching and multiplication of streamers increase as the temperature increases in experiments carried out up to ~200 K[3]. Thus the picture changes from one region in which narrow filamentary streamers having no significant net charge to a broader region of positive charge advancing towards the cathode. At the same time, the field required for streamer propagation reduces with increased temperature[3]. 2 Alternating Voltage Breakdown Behavior under Conditions of Fire The effect of a flame is to introduce ionization to the heated air; if the flame results from combustion of solid materials, conducting particles will also be introduced. A direct comparison is made between the experiments reported in Fig. 2, where high temperatures were produced in a furnace and results obtained when a "clean" butane flame spanned the inter electrode space and when ash particles were introduced into the same space in the presence of the flame. The results are re-plotted as the mean peak stress in Fig.3 . The comparison with Fig.2 shows that the two experiments were carried out with "small" and "larger" gaps. The breakdown stresses were reduced by the presence of a flame at an estimated 1173 K to less than 200 kV/m compared with over 1000 kV/m for heated air without flame in the very small 5 mm gap. The mean stress increased with increasing gap for lengths up to 30 mm but decreased with increasing gap for longer gaps in the range 200 - 1000 mm. The stress appeared not to depend upon the electrode shape in both sets of experiments. The introduction of solid particulate matter (aluminum powder, sawdust or fine ash) to the same flame caused a very large reduction in breakdown stress to the order of 7 kV/m. Here, the stress remained constant over the range of gaps used from 200 to 1000 mm. Therefore, it is reasonable to conclude that it represented a characteristics dielectric strength of the air under these conditions. Sadurski also noted the effect of adding solid particles to the air in the test apparatus at room temperature. In this case, the sparkover voltage was reduced by 25% - 30% - much smaller than that in the presence of the flame. In a separate experiment, Fonseca[5] measured the breakdown voltage in conductor-conductor and conductor-plane gaps subjected to a fire of sugar cane leaves. In each case the inter-electrode gap was 1 m and sparkover voltage was35 kV . Based on these test results, Fonseca proposed that the minimum clearance between conductors up to a maximum potential difference of 245 kV, should be given by the formula, D=U/35 (m). West and MacMullam[6] have measured the sparkover stress in field experiments using two phase test lines supported by vertical towers and wood fires were started beneath the line. The following sparkover stresses were: Conductor-conductor, 65 kV/m; It is clear from the forgoing that the presence of solid particles in a flame greatly reduces the dielectric strength of an air gap. Conditions in field experiments are less controllable than those in the laboratory, in which the strength reduction was very great. But the available evidence suggests that the strength in a fire originating from solid combustible materials will depend on the concentration of solid particles as well as on factors such as temperature, turbulence and so on. 3 Conclusions (1) The sparkover voltage is greatly reduced at high temperatures. The main reason is that the net positive charges in the streamer channel at high temperatures greatly increase the advancing field, so the field required for streamer propagation is reduced. (2) The presence of solid particles can introduce additional charges to the discharge channel, so that sparkover voltage (minimum average field 7 kV/cm ) is much smaller in the presence of the solid particles than for a clean flame. (3) For high temperatures, the sparkover voltage against the air density does not follow the law driven from normal temperatures and pressures, so more work is needed to clarify the high temperature effects. References
Copyright 2001 - Tsinghua Science and Technology The following images related to this document are available:Photo images[ts01104f3.jpg] [ts01104f1.jpg] [ts01104f2.jpg] |
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,
GUAN Zhicheng 
,
HU Qihao 
,
LIANG Xidong 
,
SHENG Xinfu 
,
N. L. Allen†
