Lightning Protection System for Distribution Substation circuits

[NAZ55]

Could you all provide any input or guidance on what would be the best way to protect distribution substation low side (25kV and below) inside the substation from lightning. We use rolling sphere for High voltages such as 69,138,345kV etc, but rolling sphere would be very cost ineffective with low voltages, because of the small protection radius. For the distribution Utility I worked for in the past I don't recall using any shield wires inside the sub.

I will appreciate any help I can get.

Thanks in advance

 

[NAZ55]

Just wanted to add to my question. I am also aware of IEEE recommendation from 5.3.6 Std 998-1996 that for stations below 115kV it may be appropriate to use minimum stroke current of 2kA for shielding. I just wanted to figure out how others are going about protecting their low voltage stations against lightning strikes.

 

[jghrist]

You can design for a particular time between shielding failure, say 100 years, instead of designing for complete protection.

For a typically sized distribution substation, in most areas the probability of strokes are such that you can get away with using about 10kA and a strike distance of 100 ft. Using 2kA would be conservative.

 

[NAZ55]

jghrist,

Could you guide me how should I go about figuring out a 100 years failure rate?

Is there a method or procedure for this?

Thanks for your help
zazmat

 

[jghrist]

zazmat,
Annex D of the IEEE Guide for Direct Lightning Stroke Shielding of Substations gives a method for calculation of failure probability.