Forensic Services Newsletter
Arcing horns on rural HT distribution networks are considered to be a good first line of defense against lightning strikes for both the lines and associated equipment and the downstream transformers to consumers’ premises. Their widespread use in the general overhead distribution networks is justified by both their low cost and their low maintenance factor. For a better level of protection at overhead system entry points into switchyards, the more sophisticated MOV arrestor is used because of its superior ability to divert the lightning surge away from the equipment it is protecting. But because they are considerably much more expensive than pairs of arcing horns they tend to be used only where the equipment being protected is much higher in value and more easily damaged by transients.
Although considered to be old technology and widespread throughout first world rural areas for basic, first-line protection for electrical HT distribution networks against damage from lightning strikes, the omission of the installation of arcing horns, leading to loss of HT equipment in a local consumer’s substation, has recently been encountered. Not only did the consumer suffer sizeable damage to his incoming substation installation, but there was also a considerable loss encountered through business interruption. The simple action of placing arcing horns at the nearest ‘upstream’ and ‘downstream’ support structures from the consumer’s point of connection to the HT supply would have considerably reduced the risk of exposure to damage from a lightning strike entering the distribution system anywhere close by.
The elevated plateau topography of the area where the distribution network to the consumer’s premises was located made it more than reasonable to consider that it would be prone to lightning strike damage. The facility affected was rurally located and remote from any other buildings or structures that could have presented a favourable alternative ‘target’ for a lightning strike. Considering these aspects, it was a profile that placed this installation in a category where there was a high likelihood of lightning strikes to be expected.
Arcing horns are basically a means of providing an air-gap interposed between the live side of the equipment and earth potential. They present an immediate path to earth accessible across an air-gap ‘tuned’ to the voltage of the circuit they are protecting. The air-gap is set so that dielectric strength of the air between the tip of the earthed electrode and the tip of the electrode connected to the live side of the installation being protected is sufficient to withstand breakdown at the ‘line voltage’ of the installation. When a voltage transient enters the HT distribution system such as that generated by HT switching, or more commonly a lightning strike, the voltage difference across the air-gap exceeds the breakdown voltage across the air-gap. At this point a spark forms which ionizes the air between the electrodes and considerable current flows, maintaining the arc until the potential difference on the line side of the installation has been discharged. This protects the ‘downstream’ equipment from the damaging current flows associated with lightning damage.
Extra high tension transmission lines often have their suspension insulator strings protected with arcing horns in order to divert the lightning discharge to the air gap rather than have the discharges finding their way to earth across the creepage paths on the insulator sheds, thus causing them to shatter with the heating effect from the passage of high levels of current.
Rurally located outdoor transformers are generally protected in this way with arcing horns set up across the HT bushings for the transformer so that any lightning strikes on the HT supply line to the transformer are discharged across the air-gap and are thus prevented from entering the transformer.
The transformer in our case mentioned at the beginning of this article had sustained a partially abated lightning strike within the substation. Some of the energy from the strike had been diverted through the rupturing of the insulation of the HT cable to the substation. Further energy from the strike had been diverted when the interposing HT switchgear had been compromised by flashovers. The walls of the transformer tank had comprised cooling fins and these had all enlarged. The HT windings of the transformer had been wound with resin reinforced fiberglass tapes and these had melted with the heat pulse generated by the lightning strike. Remarkably the transformer windings had remained intact and still exhibited good levels of insulation resistance.
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