Same Phase Cable Dearting in Trefoil
Same Phase Cable Dearting in Trefoil
(OP)
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.






RE: Same Phase Cable Dearting in Trefoil
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.
RE: Same Phase Cable Dearting in Trefoil
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..).
RE: Same Phase Cable Dearting in Trefoil
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...
RE: Same Phase Cable Dearting in Trefoil
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.
RE: Same Phase Cable Dearting in Trefoil
RE: Same Phase Cable Dearting in Trefoil
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.
RE: Same Phase Cable Dearting in Trefoil
If you had a higher-compliance breaker test set, an impedance test into a short-circuited cable run may prove revealing.
RE: Same Phase Cable Dearting in Trefoil
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.