lightning arresters and cutouts on 25kV lines

[k2panman]

Looking for anyone from an electric utility with experience on the use of lightning arresters on distribution lines, particularly using a lightning arrester in front of a cutout in an attempt to save the fuse from blowing when a lightning strike occurs. If you have actual experience before and after installing LAs I would certainly appreciate hearing from you.


K2ofKeyLargo

 

[matwong]

I haven't encountered the cutout need LA to protect but I rather perceive the issue was on the positioning of LA's lead wire whether is on the equipment itself (e.g. Trx) or on the cutout.
Articles from Hubbellpowersystems may be of interesting:

http://www.hubbellpowersystems.com/powertest/literature_library/pdflib_lit_OhioBrass.html(EU1509WB)

 

[NewfieSparky]

I am familiar with your topic, however I do not have a great deal of experience. From what I seen, you have two choices when installing LA's. First is to put the LA on the primary side of the cutout - If lightening strikes, excess voltage will be dissipated from your line, thus protecting the entire line. Second is on secondary of cutout - LA will only dissipate voltage until the fuse blows in the cutout. Basically, I look at it this way: Are we protecting the line or a xfrm, or similar device, connected to the secondary of the cutout. There are many factors when installing and choosing LA's. Do you have a specific question?

Hope this helps.

 

[cuky2000]

Try to obtain the EPRI report # EL-1140 "Study of Lightning Current Magnitude Through Distribution Arresters."

The enclose site discuss this issue briefly.

http://www.sandc.ca/webzine/050602_1.asp

 

[ccamts]

Hi I found this topic interesting, but a little confusing also, when we have a cut out and below that pothead in parallel with lightning arrester, So just wondering, is this true according to this arrangement the fuse will blow first for a lightening and a switching surge ?.
cheers!

 

[mbalea]

In this arrangement, the Surge Arrester is installed between phase and ground. The cut-out fuse is in series, and usually protects a transformer. Now, if the transformer is grounded, than a portion of the current shall go thru. Normally, the transformer insulation level (BIL) is at least as high as the line insulation, such that the lightning surge should first jump an insulator and go to ground. To have a lightning strike on a distribution line means that the line is a rural one, otherwise there are other striking points. In order to offer a better protection to the line, there are the following solutions:
1. the line should be bult with a protectiove conductor above
2. If the line is an existing one, can be improved by installing from place to place a set of surge arresters - for examople, each 1-2miles; this will have the effect of stopping the surge on line, before reaching critical equipment. How many you install depend on the isokeraunic indices of your area.

Still, more than likely is a problem of insualtion coordination (chek the BILs)

 

[jghrist]

First, I assume that the lightning arrester in question is to protect equipment, like a transformer, that is protected by the fused cutout. Otherwise, you would not consider putting the arrester after the cutout.

If the arrester is placed ahead of the cutout, then the protective level of the equipment is reduced because the arrester lead length between the arrester ground and the equipment ground will be longer. This is because the surge current through the arrester will have to flow through the arrester ground lead and there will be an inductive (L·di/dt) voltage buildup in the lead. The inductive voltage will add to the arrester voltage so that a higher voltage will be impressed on the equipment. You want to keep lead lengths that carry the surge in parallel with the equipment as short as possible.

If the arrester is placed after the cutout, then the lightning surge current will flow through the fuse and you might get nuisance fuse blowing. You can get high surge fuse links to reduce this. The article referenced by cuky2000 discusses this and provides some probability curves of the fuse link blowing.

 

[NorthSlope]

Surge arresters respond to voltage and rate-of-rise of voltage. Fuses respond to current. Up here we do have arresters in front of fuses. How many it has saved over the years I can't say. With a lot of calculation you could take the fuse clearing curve, the anticipated rate-of-rise of the voltage wave and a guestimate of the "Z" of the transformer lead and winding and figure out who'd go first.
Admittedly, surge arresters cost money - but so do fuses - AND re-fusing. Unless you're in a high isokeraunic area I think I'd just fuse and let nature take her course.

 

[jghrist]

It isn't the cost of surge arresters that drives putting arresters after the fuse. It's the cost of transformers. Protective level is better with the arrester close to the transformer, because of the L·di/dt voltage in the arrester lead.