Transmission line tripping during lightening..

[RAgrawal]

I am having about 60 KM long 220 kV double circuit Transmission line, which passes through the forest and hilly area. During rainy season, heavy lightening takes place and causes the frequent tripping of line and on checking, we often find flash marks on the disc. Quite often, we found the flash marks in 'Y' phase (middle phase) disc. My question:

1. Why 'Y' phase insulator disc is getting flashed. since it is i the middle, common sense says that it should be well protected from lightening.
2. How can we reduce the line tripping during lightening?
3. We are facing problem to improve the tower footing resistance in rocky area. How can we improve the same?

Regards,

Rajesh

 

[magoo2]

Does the line have a static wire or is it unshielded?

 

[RAgrawal]

It has static wire in the form of OPGW.

 

[Marmite]

When lightning strikes the tower or the shield wires transient voltages are generated and travel along the tower and the shield wires increasing the voltage on the top of the tower.
At same time induced voltages are generated on the phase conductors due to the coupling factors between the shield wire and the phase conductor. This can lead to back flashover of the insulator string. Transmission line lightning performance is dramatically affected by the transient behaviour of the tower earthing system.
Options include increasing the dry arc distance of the insulator string, increasing the height of the shield wire, and reducing the tower earthing impedance. All of these solutions are expensive, and the best answer may be to install line arresters.
Line arresters can often be used to good effect in regions with high thunderstorm activity and poor earthing. The arresters are installed in parallel with the insulator string.
There are two types available:- Externally Gapped Lightning Arresters: EGLA, Non-gapped lightning arresters – NGLA

Regards
Marmite

 

[RAgrawal]

Thanks Marmite,

But in hilly area, where improving the earth resistance is challenge, how the energy of LA will dissipate, if we install LA across the insulator string?

 

[Elecband]

You can try use software like CDEGS to model the line, tower footing resistance etc. to see how the existing performance and also the possible improvements.