Transformer-Arrester Connection on Pole-Mount Transformers 4

[joozu6]

I work for a electrical distribution utility. This past season we have had over a dozen transformers that were struck by lightning, resulting in the transformer spliting from the top of the transforer close to the primary bushing and down to the tap changer (we use dual voltage 7.2kV/14.4kV transformers). Our current method of connection is from the overhead line, through the cutout and then to the primary bushing. The arrester is connected by connecting a jumper from the bushing wire to the top of the arrester. The arrester lead is grounded to the tank ground.

It seems to me that this connection will incourage a ligtning surge to go through the bushing, resulting in our tank-splitting problem. Does this seem logical?

 

[joozu6]

Update: I asked around at some of our surrounding utilities that use the same transformers we use. We seem to be the only utility having multiple xfmr failure due to lightning. The only difference I could determine was our connection. All the utilities I called make the connection that Cooky depicted above. We connect from the cutout to the bushing, but tap a jumper from the bushing line to the top of the arrester. The arrester lead is connected to the tank ground. I found out we also had some fail the same way where the arrester was connected up with the cutout on an L-bracket, where the ground lead went directly to the pole ground.

I also found out that the transformers are splitting from the tap changer up to the bushing (determined by the fact that some of our xfmrs only split from the tap changer but not all the way to the top). Too bad we cant attach pics to this.

Based on the responses I'm thinking of changing to Cooky's connection, but running the arrester ground directly to the pole ground. I just can't see putting a lightning surge through the tank. I can't see how the L di/dt will make much of a difference with such a small transformer. Increasing the ground wire size may be beneficial as well.

 

[mc5w]

Some areas have more than their fair share of lightning. Pittsburgh, for instance, has a lot of lightning due to the mountain uplift making thunderstorms more severe.

Lightning protection is only as good as the ground. If your soil has very few rocks you can use a Borit or Borzit ( 2 competitive companies ) to sink a 1.25 inch copper of steel water pipe way deep into the ground. Figure your budget on the basis that you will have to leave the drill bit at the bottom of the hole.

This is a cheap hand held directional drilling machine to which you hook up a water hose and an air drill. Their directions say that you can use an electric drill of the type that electricians use to drill holes in woodwork but an air compressor is cheap insurance.

Do not forget to use Burndy Penetrox A or A13 or Ideal Noalox on galvanized pipe threads. For copper or bronze pipe use Penetrox E.


Mike Cole, mc5w@earthlink.net

 

[tinfoil]

One difficulty with connecting the arrestor in parallel with the transformer bushing, downstream of the cutout fuse, is that the arrestor is then 'removed from service' if the fuse blows or the switch is manually opened.

Often, xmfr arrestors are the ONLY ones installed in the system for miles around. The lightning surge would then have to find another path to earth, likely damaging one or more insulators (which can be a bitch to find in the middle of the night during an electrical storm!).

 

[mc5w]

And cause a poletop fire.

 

[GOTWW]

Some clarification on this post is still needed, I believe. This is that this type of connection with a single ground rod is only allowable per NESC C2-1997, allowed per rule 97B, and 97C. The 97C requires at least 4-grounds minimum per mile on the MGN. Otherwise 2-separate ground rods are required, and separated by 20 foot, and a spark-gap.

 

[tinfoil]

Many utilties within the USA do not adhere perfectly to NESC standards.

Most utilities outside of the USA have not even heard of the NESC standarda!

 

[steelerfan28655]

Here in the mountains of North Carolina two ground rods can not always be seperated by more than a few feet due to the rocky terrain and the other existing utilities. We use a single 17' ground rod (a 10' section coupled with a 7' section) along with #2 stranded copper conductor up the pole. Since we began this practice six years ago, we have seen a significant reduction in the amount of lighting damage to our own and our customer's equipment.

Happiness is a way of travel, not a destination.