Secondary concentric neutral terminations
Secondary concentric neutral terminations
(OP)
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
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






RE: Secondary concentric neutral terminations
RE: Secondary concentric neutral terminations
RE: Secondary concentric neutral terminations
Regards
Marmite
RE: Secondary concentric neutral terminations
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
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RE: Secondary concentric neutral terminations
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
RE: Secondary concentric neutral terminations
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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.