help for the clearance between the shield wire and PHASE conductor in substation

[jyj982001]

In a substation design, we choose shield wire for protection against lightning.
How to determine the clearance for the shield wire and the phase conductor in case of lightning couter attack?
Are there any IEC standards regarding to this issue?

P.S. if the distance is too short ,there may be an lightning counter-attack over-voltage on the phase conductor.But how about the distance?

 

[melspuds]

I do not know about IEC standards. But, IEEE Std 998 "Guide for Direct Lightning Stroke Shielding of Substations" is a good starting point.
Good luck,
Dave

 

[jyj982001]

Dear Melspuds,Thanks a lot for your kind reply! Unfortunately,I can't find any words relating to this topic in Std 998.
I want to check the air clearance between the shield wire and the phase conductor!Thanks!

 

[tinfoil]

The rule of thumb we have always used is that, since the shield wire is bonded to the ground grid by design, then apply similar minimum clearances from the phase wires 'UP' to the shield wire, that I would use 'down' to the ground.

But I have never found that to be the design-constraining issue.
Typically, I have found that desired height of my shield wires or 'rods' to provide effective lightning coverage provides considerably more clearance than that 'minimum' I mentioned previously.

 

[jyj982001]

Thank you Tinfoil.the reason is that if there is a thunder or lightning strike on the shield wire, and the air clearance is too short between the shield wire and the phase conductor,there would be overvoltage counter strike on the phase conductor. Thus we have to determine a distance for the wire and conductor .

 

[melspuds]

jyj982001
In my 45+ years of working in, designing or installing substations, never have I ran across this problem. The shield wire is always keep at least the minimum clearance for highest voltage phase wire to ground. Most of the time I like the clearance higher, due to maintenance issues using manlifts, cranes, ect. Sometimes you do not have that luxury of going up higher.
Hope this helps,
Dave

 

[jyj982001]

Thanks a lot for your kind reply,Dave. But I do think the air clearance between the shield wire and the phase conductor in the substation couldn't be that of the phase conductor to the ground.Cos Direct Lightning Stroke would result in very very high voltage which could break down the air in case that the Air-insulated Gap is too short.if that happens,the shield wire loses its function.

 

[jyj982001]

The question is that how much is the required separation distance in air of the shield wire and the phase conductor.the distance maybe relating to the system voltage to some extent. for a 500kV system or 10kV system it can be changed.

 

[tinfoil]

I was aware that lightning has a very high voltage!

Lightning is not 'hunting' for your phase wire.
It is seeking a discharge path between the air and ground.

If a given stroke of lightning hits a well-grounded shield wire, in my 25 years of experience it will not jump ANOTHER large air gap to damage your equipment.
Your shield wire should represent a fraction of an Ohm resistance to earth, compared to the megaOhms or gigaOhms of the several feet/meters of the air gap.

The separation I was quoting was to provide your energized equipment isolation from GROUND, including that of the grounded shield wire.