Secondary concentric neutral terminations

[duh]

Hey all,

Had a question regarding concentric grounding terminations. We have a 138 - 25 kV delta-Yg power transformer feeding 25 kV switchgear via 3 circuits of 25 kV - 750 kcmil triplexed cable with concentric neutrals on each phase. Our standard is to have the concentric neutral tied to the power transformer (at X0 which is tied to the ground grid), and leave it disconnected at the switchgear end. This would prevent circulating currents through the concentric neutral. However, I was thinking that any unbalanced current would instead flow through the ground grid to return to the transformer, which doesn't really make sense to me. I am curious to know what others are doing in this type of installation (tying the concentric neutral at the switchgear end or not). Why is one better than the other?

Thx,
Pete

 

[davidbeach]

If you are covered by the NEC there are prohibitions against using only earth as a ground return path. I believe there are similar in the NESC. You need to tie your concentric neutral to the neutral bus at the switchgear end. Why, oh why, would you spend the money for concentric neutrals and then not connect them? If you only need three phases, just use shielded cable and run a 600V insulated ground conductor, but you need a conductor as a path for ground fault return.

 

[apowerengr]

Assuming the concentric is rated for the fault current (I'm guessing you have concentril rated for at least 1/3rd neutral with the triplexed 750) you connect it at both ends. What happens if you have a ground fault in the switchgear? You want the concentric to carry the fault current back to the power transformer. The only time you would not connect both ends is if you have tape shield which is only for shielding and is not sized to handle the fault current. This installation is no different than any URD feeder with the same conductors.

 

[Marmite]

Another way to do it is connect the concentric screen wires to ground at one end only, but run a separate rated ground continuity conductor. This is the approach used by the company I work for. If you connect both ends you will get circulating currents in the screens. By preference I would connect the screens to ground at the switchgear end, as this is likely to be visited more by staff. Under fault conditions you will get a potential difference between the screens and the local ground at the disconnected end, and for this reason the screens should be shrouded with insulation. This applies whichever end you choose, but there is slightly more chance that staff will be at the switchgear under fault conditions than the transformer, as they may be fault switching for example.
Regards
Marmite

 

[RalphChristie]

I also prefer Marmite's method.

http://tinypic.com/view.php?pic=4vn0som

A picture taken of a damaged 630mm² singe-core cable due to circulating currents. The cable was installed and grounded at both ends, no-one considered circulating currents, and it was probably energized for two weeks before it blasted.
In the picture you can see the damaged cable with a Raychem cable-end (red) and a radiator-fin of the 20MVA transformer. The white thing in the right top corner is a current-limiting reactor with a piece of the 11kV busbar connected to it.

In this particular incident the expected circulating current was 280A at a load of 670A - leaving a few people with red faces!

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376

 

[slavag]

Hi.
It's not so simple. It's function of length of cable.
In short cables we grounded cable on one end ( supply end).
In case of long cables on both sides. One of the reasons is safety, on not grounded end possible HV potential because long cable capacitance.
I'll try found my old instruction about this issue and I'll send to you.
BTW, we also add grounding cable 95mm^2 between several
building and current circulate in this cable.
Regards.
Slava

 

[davidbeach]

Ralph's picture is a shielded cable, not a cable with a concentric neutral. Concentric screen and concentric neutral are two very different things.

http://www.okonite.com/Product_Catalog/section2/sheet38.html

shows a concentric neutral conductor and

http://www.okonite.com/Product_Catalog/section2/sheet4.html

shows a shielded cable.

The concentric neutral provides the equipotential plan provided by the shield, but is also intended as a current carrying conductor. Concentric neutrals are available in 33% rating and 100% rating. The 33% is used on three phase circuits, where three 33% in parallel provides full capacity. The 100% is used on single phase circuits, allowing a single phase drop to be run using a single cable.

If one is using concentric neutral cable for applications where shielded cable would work, one is spending way too much and has a cable that is much more difficult to work with than necessary. If the concentric neutral is actually appropriate for the installation at hand, then it is imperative that the concentric neutral be connected at both ends.

Shielded cable and a separate neutral conductor can be used in stead of a concentric neutral cable, or you can run a separate neutral in parallel with a concentric neutral and then treat the neutral as a really expensive shield, connecting it at only one end.