Lightning does strike twice

[steelerfan28655]

I wanted to get your opinion of this situation.

We have an electric substation that was built in the late 1950's early 1960's. It is a steel structure with an exposed copper IPS bus on top. The voltage is 12,470 wye/7200.

Two years ago we had a lightning strike at the station. It damaged one of the vacuum breakers inside the steel structure. The shell of the breaker was penetrated in two spots with pencil size holes. We checked the grounding and bonding when we installed a replacement breaker.

Two nights ago, the same breaker position was destroyed again. The shell of the breaker has multiple holes in it.
The voltage regulators for this circuit are located outside of the structure, and they have no damage. The relay inside the breaker was not damaged. It seems like the steel shell of the breaker got all of the damage.

We will begin resistance testing on the station ground grid tommorrow.

Any ideas about what could be causing this? How can the lightning bypass the grounded steel structure and damage the same breaker in the same position? Could this be ground up lightning?

Happiness is a way of travel, not a destination.

 

[davidbeach]

How well is the case of the breaker grounded? If it isn't well grounded, you could be getting flash from ground or grounded structures to this ungrounded/poorly grounded breaker.

 

[rhatcher]

Do you have any pictures??

I am seeing this a little differently. I think that the holes in the circuit breaker frame indicates that the frame is solidly grounded. The arc occurred from the breaker bus to the grounded frame.

If this is true then there are burns on the breaker bus that correspond to the frame burns but there are no burns in the structure that correspond to an arc path from the breaker frame to the breaker mounting structure.

If the frame was not grounded and somehow became energized, there would be corresponding burn marks (holes) in the structure where the arc struck from the breaker to the structure and no corresponding burn marks on the breaker bus. Since you are not mentioning damage to the structure, this scenario seems to be a possibility.

I am thinking that the problem may be a lightning induced surge coming from the load side and finding the metal enclosed vacuum circuit breaker as the first weak link in the air gap/insulation chain.

The phase to ground arc I am describing will be from the circuit breaker bus to the part of the metallic breaker frame that is closest. The arc would immediately start to burn a hole in the grounded metallic breaker frame. Of course, the air will become very hot, very ionized, and very conductive. This will allow the arc to continue to burn even as the hole gets larger and theoretically the arc length gets longer. The observation that I have made is that this often results in a surprisingly round hole.

At some point either a protective device interrupts the arc or the air gap distance becomes so great due to burning that the arc is naturally extenquished at the zero crossing point.

If this idea is correct then the installation of a three-phase surge suppressor on the load side of the breaker will correct it.

Do you have any pictures???

 

[steelerfan28655]

Here are the pictures we have so far.

This picture shows the top of the breaker. You can see some of the flash damage to the line side and load side connections.

http://files.engineering.com/getfil...4a89-a0b7-446de3b29ce2&file=04-12-11_0752.JPG

This picture shows destruction of the porcelain insulators and melting of line side connections.

http://files.engineering.com/getfil...432a-a50c-b38c67b1c207&file=04-12-11_0753.JPG

This picture shows destruction of the porcelain insulators and the load side connections.

http://files.engineering.com/getfil...4ece-a361-4d2d5729b3ef&file=04-12-11_0754.JPG

This picture show the blade of the line side disconnect switch that was destroyed.

http://files.engineering.com/getfil...46cf-8e1f-8d07a46e337e&file=04-12-11_0805.JPG

I am leaning towards a strike to the exposed copper line side bus. We will inspect the bus this morning. The breaker has been replaced and I will investigate the destroyed breaker today.

Happiness is a way of travel, not a destination.

 

[jghrist]

It appears to me that the strike was to the exposed copper bus and the breaker terminals flashed over, damaging the bushings and the breaker enclosure. I'd recommend analyzing the direct stroke protection, using the rolling sphere EGM method explained in IEEE Std 998, IEEE Guide for Direct Lightning Stroke Shielding of Substations. You could probably add shielding masts to the structure to protect the bus.