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Phasing arrangement of 15kV paralleled power conductors in cable trays
2

Phasing arrangement of 15kV paralleled power conductors in cable trays

Phasing arrangement of 15kV paralleled power conductors in cable trays

(OP)
We are installing a large oil filled transformer whose primary will be connected to some overhead bus in a substation area. We are going to use 2-1/C 750 Aluminum shielded cable per phase.

As you can tell from the attached sketch the easiest way to make these connections is to run them AA BB CC in the cable tray. Is this ok to do this this way, or do you have to for electrical reasons triplex these cables A-B-C A-B-C or run them A-B-C-A-B-C in single file?

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

I think the flat layout (either A-B-C-A-B-C or A-A-B-B-C-C) is going to be better from a thermal point of view. It will be better if you can maintain some spacing between all of the conductors (even the A-A B-B C-C pairs).

I don't think it will make much difference from an impedance point of view (unless the run is long), but A-B-C-A-B-C might have a bit lower inductance. All things considered, keep the cable runs neat and the conductors the same length.

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

Option 2, with ABC bundles, will reduce circulating current in the shield, and all other things being equal, will increase the capacity.  It also will have lower impedance.
 

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

If you go with a flat configuration, then A B C C B A minimizes the mutual heating and results in a better current balance.

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

(OP)
Seems like the majority view is the A B C C B A configuration is the best. Thanks. Although from a design point of view and aesthetically it's not as appealing – but that's the purpose of the question.

In that proposal all the cables will be crossing each other at both ends...is this not a concern? I sketched it out on the attached. The cable is oversized, we need about 950A ampacity and this cable has a rated ampacity of 1080A. And in actuality it's only about 35 feet in length.

Would it really be that big a deal to put it in as originally proposed? AA BB CC?



 

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

First of all according to NEC 392.17(B.2)b and Table 310.60(C)70 ampacity of 1*750 MCM AL is only 700 A.
Even for copper cable  Table 310.60(C)69 it is only 885
The air temperature is 40dgr.C
I think the unbalance of currents in AA BB CC formation could be
up to 15%[let's say one would be 950/1.15 and another 950*1.15]
 

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

(OP)
7anoter4,

I'm talking of the total ampacity of (2) per phase per the manufacturer's literature: 540A x 2 = 1080A, which is based on NEC table 310.60(C)74. I just took that number from the chart they provided. And really in this situation we could use 310.60C.72 and it would be even higher: 1200A.

I don't understand why the currents would be unbalanced. You are saying the currents would be unbalanced in the A-A phase, B-B phase etc. between the individual paralleled cables? Why is that?

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

The unbalanced currents are due to the mutual impedances between each pair of conductors.  Each conductor will have a self impedance, determined by conductor size and type, and a mutual impedance, determined by the spacing between conductors and the amount of current flow.

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

2
From Introduction of the IEC 60287-1-3 standard:
"When single-core cables are installed in parallel the load current may not share equally
between the parallel cables. The circulating currents in the sheaths of the parallel cables will
also differ. This is because a significant proportion of the impedance of large conductors is
due to self reactance and mutual reactance. Hence the spacing and relative location of each
cable will have an effect on the current sharing and the circulating currents. The currents are
also affected by phase rotation. The method described in this standard can be used to
calculate the current sharing between conductors as well as the circulating current losses.
There is no simple rule by which the circulating current losses of parallel cables can be
estimated. Calculation for each cable configuration is necessary. "
http://www.google.co.il/url?sa=t&rct=j&q=parallel%20single%20core%20cables&source=web&cd=6&ved=0CHoQFjAF&url=http%3A%2F%2Fwebstore.iec.ch%2Fpreview%2Finfo_iec60287-1-3%257Bed1.0%257Den_d.pdf&ei=T26qT_OgC66Q4gS5qMGhCQ&usg=AFQjCNF3HBjBVXoWBmGnRoHzaL9xBYJauw
See also:
http://www.google.co.il/url?sa=t&rct=j&q=parallel%20single%20core%20cables&source=web&cd=5&ved=0CHUQFjAE&url=http%3A%2F%2Fjmst.ntou.edu.tw%2Fmarine%2F18-2%2F290-297.pdf&ei=T26qT_OgC66Q4gS5qMGhCQ&usg=AFQjCNEO9c8xtuL5SrAoblBF8_rq8XomrA
 

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

7anoter4 - thanks for the references, particularly the second one by Lee.  It looks like a pretty decent paper on the subject.

 

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

The Lee paper does not consider shield circulating current and losses.  There will be significant differences in shield losses with different phase configurations.  Shield losses significantly affect ampacity.
 

RE: Phasing arrangement of 15kV paralleled power conductors in cable trays

Hey jghrist,
  Would you have another reference to suggest on the subject?  I found some material on this subject in an EPRI Underground reference book which treats the subject at a fairly high level so you don't come away with an appreciation of how to model it.

I did find a paper by K. A. Petty titled Calculation of Current Division in Parallel Single-Conductor Power Cables for Generating Station Applications, IEEE Transactions on Power Delivery Vol. 6, No. 2, April 1991.  I'm trying to get a copy of it.

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