Live 400KV Insulator washing 2

[bdf5526]

Hello Guys,

On our site 400KV trafo bushing and insulator was cleaned with Online Wet washing last week. The supplier for the washing system was onsite for cleaning. Only for the first time.

We had a good water resistivity 5 micro siemens/cm. And at 25bar from the pump cleaning was done with min distance 8-10 meters (nozzle to 400kv line). Cleaning was executed from bottom to top as usual with alot of flushing.

There is something I do not understand. When the pressured water reaching almost 90% of the insulator from bottom of the we started to see arc/discharge from the 400KV conductor tracking down to insulator (like 20cm long) . Instead of stopping, the supplier hit the conductor and top 10% of the insulator with the water and the arc was quenched. I would expect if the arc already forming (maybe caused by dirt/conductive particle dissolve with water and since we are closer to sea and half desert and further more reaching closer to 400KV line) I would expect to stop shooting at the top 10% of the insulator. I would concentrate at 60% to 80% of the insulator from bottom with pressurized water to block the current path to the ground or stop shooting. Am I wrong? What could be the reason?

Thanks for the feedback.

 

[cranky108]

Just thinking, But pure water is not very conductive. But water with salts is. So wetting the insulator from the top would only be washing salty water down the insulator. Where starting from the bottom would clean the lower part of salt and reduce the salinity of the wash water, which would reduce the flashouer potential.

At 90% if you were seeing arcing from the top to the spray point, then the current must have been dividing at that point between the spray and the insulator bottom. By spraying the top you add 10% to the effective insulator length, and reduce the chance of a flashover.

I have not seen this process of studied it, but this is just my thinking of what might be happening.

 

[Compositepro]

Water is a good insulator, but not as good as air. The wet vs. dry insulator will form a voltage divider and a higher voltage will be seen by the dry insulator. This can lead to corona discharge on this dry part. However, the current will still be limited by the resistance of the wet insulator. On the other hand air is a very good insulator until it becomes ionized. Wetting the dry insulator that has a corona discharge on it, will stop this discharge. There will be a small current flow through the water, and I imagine that this will dry the insulator pretty quickly.

Corona discharge occurs when the electric field strength (volts/meter) is great enough to accelerate any free electrons to a high enough energy to knock-off another electron from the next molecule it hits. In gasses, the molecules are relatively far apart so the electrons have more distance to accelerate and gain energy. The ionization potential for air molecules is about 12 electron-volts. The free electrons, supposedly, are created by natural ionizing radiation and comic rays.

 

[bdf5526]

Thanks for the feedback guys,

Looks like voltage divider and length of the insulator definitely can be related to that scenario.