Single Length Copper Tube for Sub Station

Copper tubing is pretty heavy (3" electrical copper tubing is 8.75 lb/ft = 13kg/m) so it's not surprising vendors don't stick it, because 12m lengths will weigh over 150kg. That's very hard to handle in the field and unwieldy to ship.

Have you started with a Google search on "copper electrical tubing":

http://www.google.com/search?q=copper+electrical+tubing&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7GGHP_en

That could net you some sources, but they might be continents away from you.

It's pretty rare (in our part of the world) to have the main(?) busbars made out of copper tubing instead of aluminium (which is FAR, far cheaper and lighter).

I don't think you will be able to find a single 12m length unless you have it specially manufactured and even then transport is going to be a serious problem if you cannot fit the copper tube and its supporting packaging in a 40 foot shipping container. You may very well need shorter lengths and flexible expansion joints.

If it were me I would call up local general suppliers of copper tube and ask them, as they may know where you could get such a long continuous length.

You will also need to pay special care to ensuring the substation security is good enough as hundreds of kgs of copper is a VERY tempting theft target.

....client insisting us to provide 3" (75 MM )Copper tubes in 12meter length,...

does that mean that you bid on and won this job on the basis that you could get away with joining the copper tubes and now the client has the gall to demand that you actually follow the specs?

or

you are bidding on this job and you have tech-queried the need for continuous length and suggested an alternative that is easier to source but the client has politely instructed you to bid according to the published spec.

if it is the first then too bad mate.

if it is the latter then start calling copper tube manufacturers for a special order and include the obviously higher marked up cost in your proposal. Don't forget to charge for the additional work scope that this demands.

We encountered a similar concern in bus bars that routinely carry 250,000 amps It was resolved by brazing the copper bus bars together using 1145 silver brazing alloy and a large oxyacetylene torch. Now over 10 years in the field with no problems.

It can also be TIG welded, with enough heat. I'd want to confirm the electrical characteristics, but I've been told it is done on transformer bushings.

I assume OP plans to butt join two copper pipes to increase length. A welded butt joint can approach the mechanical and electrical properties of a continuous length of pipe, but can not equal or exceed them, especially if the pipe is work hardened. The weld will always be the weak link in the chain. Joining pipes with a sleeve using a high conductivity silver based braze alloy will result in a joint that is stronger and have better electrical conductivity than the native pipe. I recommend brazing unless there is a reason why this method can't be used. Even if one decides to weld, there are certain grades of copper that are very difficult to arc/TIG weld, so tread lightly.