This article follows-up on a story in the Geelong Advertiser (page 4, 19/01/2000). A rash of pole fires on the morning of Sunday, 16th January to the south-west of Geelong, left up to 22,000 households without electricity for varying periods. A combination of several factors may have triggered this "epidemic" of fires on multiple power poles. The accepted explanation for sporadic, isolated pole fires occurring in electricity distribution networks is well-known and well accepted:
New Theory on Pole Fires
- coastal areas are most prone to this phenomenon.
- it often occurs during extended dry spells.
- salt spray builds up on insulators, because rain has not rinsed the salt away.
- risky and costly maintenance (humans washing the insulators) may reduce the chance of problems. Scheduled replacement of all cross-arms with galvanised steel replacements can eliminate fires, but may not prevent insulator failure.
- morning mist or humidity can greatly increase the leakage current on the surface of the salty insulator, or cause frank breakdown with arcing.
- if the insulator is mounted on an old-fashioned wooden cross-arm, the leakage current can initiate arcing within the cross-arm, especially if there is some decay or rot in the wooden beam.
- once a glowing ember is formed, a breeze may facilitate the ignition of a flame.
- the wooden beam is then consumed by fire, the HV conductor falls to the ground, or across other wires, and the HV feeder supply will be interrupted by fault protection equipment.
But it seems odd that such a sequence of events could cause NINE pole fires to occur in a small region within the space of a few hours. One possible explanation for fires occurring in small "epidemics" within a very short time frame on interconnected feeders is as follows:
A feeder or feeders with bifurcations or multiple branchings, may have one branch isolated by fault-protection equipment. All the remaining feeders will most likely experience a sudden increase in voltage, because the fault-protection has the effect of shedding load from the originating zone substation. Thus, if there were any other cross-arms ready to ignite, they will suddenly have a more intense and energetic arc-fault eating away inside them. The generation of heat will therefore also be increased, and the likelihood of another fire within a short space of time on these remaining "live" feeders is also increased.Footnote: Electric sparking (arcing) in HV power lines produces a tell-tale sign on low-band VHF television pictures: Australia's television system has a picture rate of fifty half-images per second, and a mains frequency of 50 Hertz. It therefore follows that bands of "snow" on your TV screen, perhaps slowly moving up or down the screen are almost certainly Power Line Interference (PLI). This is seen most clearly on Channel 2 (the national public TV broadcaster). Check with all your neighbours, video the problem if severe, go for a walk and listen for buzzing power poles in your area. Then report the problem to your utility: Your action may prevent a pole fire in areas where wooden cross-arms are still in use.
If this scenario has any merit, the historical record should show that pole fires are more likely at "off-peak" times (i.e. times of low electricity usage - such as Sunday mornings!) when we can reasonably expect that voltages will be very high in the distribution network.
One way that distribution utilities may be able to reduce the frequency of pole fires, is to reduce the supply voltage to nominal whenever the customer power usage is low. The technology to do this is already present in all zone substations: it is called a Line Drop Compensator (LDC). Long rural feeders may benefit from the use of line drop compensators at each regulator along the feeder, but this would be expensive. However, one suspects that the present cost burden to rural customers of excessive voltage habitually around 254 volts is a much greater cost to the Australian economy, so the introduction of improved voltage control in both rural and urban settings will have microeconomic reform benefits to the national economy as a whole. We do not live in a third-world Banana Republic do we?
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