Insulator flasovers 1

[KillBill7]

Hello all..... we have 13.8kV distribution line which runs near the costal area and we recently experienced few flasover where the inusulator show arc marks on the top shed and the bottom shed only. Installed insulators are composite type with silicon rubber shed and 40mm/kV creepage distance. Flashover happened during the rain so my assessment is rain created high conductivity solution on the insulator surface that allowed leakage current to flow. The leakage current tends to burn carbon track on the insulator shed which resulted in power arc. What doesnt make sense to me is that there are no burn/arc marks on middle sheds but the arc marks at the bottom shed, so if tracking didn't happend how did the arc reach the bottom shed? I am thinking ,as it was raining, the surroundig air may have become the shortest path for the arc so instead of flowing through insulator surface (tracking) it arced directly from top shed to bottom shed? Any feedback is much appreciated.

 

[waross]

In coastal Central America, we had multiple flash-overs at the start of every rainy season.
During the dry season, onshore winds built up a thin layer of dry salt on the insulators.
When the rains started, the salt would become conductive and flash over.
In our small island utility we used 35 kV insulators on our 13.2 kV system.
That avoided flashovers.
An engineer for the National Grid on the mainland told me of their approach;
Lines parallel to and close to the sea coast were relocated further from the ocean and loads near the ocean were then fed by radial lines at right angles to the sea shore.
Secondary lines to loads near the sea were run for several spans to keep the transformers and high voltage lines further from the sea.
Over the years I have seen two instances of lightning strikes where a piece of wood was blown out with no burn marks.
One was a high straight tree.
A strip of wood, about 2 inches wide was blown out from top to bottom.
The exposed wood was white with no scorching.
The other was a power pole with a strip about 1 1/2 inches wide blown out from top to bottom with no scorching.
My theory is that the current followed a layer of sap or moisture and flashed it into steam, hence a cooler explosion.
As a WAG, the tracking may have been started pre-flashover by corona like action. The actual flashover may have been cooled by the moisture that encouraged the flashover.



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Ohm's law
Not just a good idea;
It's the LAW!

 

[Kiribanda]

It looks to me that your thinking is correct.