Same Phase Cable Dearting in Trefoil

[coppabox]

Has anyone come across a simple formula or rule of thumb for calculating the derating of cables when the same phases are grouped together? For example, I am running 3 x 400mm2 single core cables per phase from switchboard to generator. To avoid derating the cables they can be run in RED-YELLOW-BLUE trefoil. But for ease of termination, it is preferable to have RED-RED-RED in trefoil, YELLOW-YELLOW-YELLOW in trefoil and BLUE-BLUE-BLUE in trefoil (hence there will be no cross-overs when terminating). Is there a simple way to calculate the derating caused by this arrangment? Any help much would be much appreciated.

 

[electricuwe]

Running cables with a cross section that large in parallel without aranging them red-yellow-blue will require a severe derating. The exact factor depends on the the distances and the calculation has to consider electromagnetic issues as well as thermal issues.

You will already have to derate the cable if you arrange them in trefoils red-yellow-blue. There are even some special arrangements (changing the position of each phase in the different trefoils) to keep this derating factor low.

I will try to find some literature on that topic.

 

[coppabox]

Thanks electricuwe.

I already have a recommended layout (from AS3008) which alternates the R-Y-B arrangment (it keeps Red on top, and alternates the Yelllow and Blue for each trefoil group). This is what I intend on doing, but naturally they will cross at termination. The entire run is relatively short - about 20m. But at each end where they cross over, around 4m will probably have to be run in R-R-R, Y-Y-Y and B-B-B arrangement. Hence 40% or the run could be in this unfavourable formation (and may be exposed to metal cable ladder etc..).

 

[dpc]

You are probably aware of this, but running individual phases in steel conduit or tray can cause serious induction heating of the metal raceway.

I don't see why de-rating due strictly to thermal effects would be any different if conductors are grouped by phase or not. But maybe I'm missing something...

 

[DanDel]

Cables should be run in, as you say, red-yellow-blue combinations to prevent high levels of impedance due to inductive reactance. This is standard practice(and required in the USA).
As dpc says, any ferrous metal between cable phases or phase groups will cause heating of the metal due to this inductive effect.
I suppose you would have to consider the heat created as the ambient temperature and apply that to cable derating. It seems a huge waste of energy and material just to keep the cables from crossing over.

 

[jghrist]

If the cables are shielded, running the phases together will also increase shield losses resulting in heating and derating.

 

[coppabox]

Thanks guys,

I would just like to make note that the cables are direct buired and are not shielded.

Regarding the final comment from DanDel, I am not trying to avoid a crossover (that would be impossible). The problem (the way i see it) is that a large portion of the cable run is in 'same phase' configuration. Hence I feel that some further derating might be appropriate.

 

[busbar]

If you had a higher-compliance breaker test set, an impedance test into a short-circuited cable run may prove revealing.

 

[jbartos]

Suggestion: It is easy to consider the cable bundled by subconductors into three phase cable rather than to have more three phase cables without any subconductor per cable since some busways are run such a way that several buses are run together per phase. The busways configured this way do not seem to experience major problems.
Visit

http://www.sea.siemens.com/busway/product/Bus-Spec-Instal.pdf

for busway configurations
The proper way to do it is to calculate bundled cable impedances or simulate the bundled cable run.