Lightning Arrestor Help

[Ztrain1985]

For a job we are looking at installing a lighting suppression system, but my company has no information or previous experience with such jobs.

The system in question is an ungrounded 4160V, stepped down from a 2500 kVA Wye Primary, Delta Secondary Transformer, that feeds several buildings up to 2000 FT away.

What I have gathered so far is that we need an arrestor good for about 33kV, or about 10. p.u of the L-L voltage. It will be a Metal Oxide (MOV) type as well.

I have also read that the valid protective distances for arrestors are something to the effect of several miles, so we could in theory get away with a single set of arrestors per building feeder, however one at the transformer and 1 at the building (reflection point) would be best.

Is there anything else I need to know about this? And is my game plan at least correct in theory?

 

[davidbeach]

You need an arrester at each device to be protected. I've never seen anything that would suggest several miles of coverage for a single arrester. Normally a distribution line that provides arrester protection for service transformers will have several arresters per mile of line if not one at each transformer.

 

[dpc]

Agree. Basic rule of thumb is put arresters as close as possible to the device to be protected. Since transformers generally have the lowest BIL in a distribution system, arresters are often (normally)located at each transformer.

The other rule is to run generously sized ground wire from the arrester directly to earth (and transformer tank) with as few bends as possible.

 

[jghrist]

You need an arrester good enough for the highest voltage it will see - 4160V in you case, plus whatever overvoltage can occur. The arrester MCOV rating has to be as high as the maximum continuous overvoltge. You'll need a 5.1 kV MCOV, or 6 kV Duty Cycle Rating. Using a 33 kV arrester will not clamp the voltage low enough to protect the equipment.

Are the 4160V feeds to the buildings overhead or underground?

 

[Ztrain1985]

They are run underground in Concrete Ducts, but the transformer and part of the line is exposed above ground. I guess I didnt have as good of an understanding as I thought I did

So when you say an arrestor for each device, you mean 1 for the Trans and 1 for the building? (technically 3 for each as there are 3 phases right)?

So 5.1 kV? Is there any book or through online resource that discusses this? I have a feeling I need to see some equations to get a better handle on this.

 

[jghrist]

5.1 kV is the lowest standard MCOV above 4.16 kV. See

http://www.hubbellpowersystems.com/powertest/literature_library/pdfs4lib/OB/EU1136-H.pdf

METAL-OXIDE SURGE ARRESTER
PROTECTION OF DISTRIBUTION SYSTEMS, Ohio Brass.

 

[dpc]

Yes, one per phase. When you say 1 for the building - the arrester should be located at whatever piece of equipment that feeder goes to. Switchgear, fused switch, whatever. The arrester doesn't protect the building, it protect the equipment.

If you want lightning protection for the building, that's another story entirely.

 

[arvinolga]

Hi,

Just an inquiry, if your requirement is a building lightning protection, how do you install the downconductor (say 25 storey building). Is it encased in a pipe sleeve and embedded in the column? If so, how do you account for the splicing of the downconductor?

Thanks

 

[jghrist]

At 4 kV, you may be able to achieve adequate protective margins with arresters on the source end of the cable only. It depends on the characteristics of the arrester, the lead lengths, the circuit length, and the BIL of the equipment. See C62.22-1997 IEEE Guide for Application of Metal-Oxide Surge Arresters for Alternating-Current Systems.