Group Derating Factor for Trefoil Formation
Group Derating Factor for Trefoil Formation
(OP)
Dear All,
I've been terribly confused over sizing cables for a 530 KW LV CHILLER.My current sizing is to go with 3 Run SIngle core 400 sq.mm cables per phase(total of 9).My cable arrangement for direct buried cables will be as attached.Can I take group derating factor to be 1 since trefoil centre to cente distance is 170 cm(Which is several times their dia).Please advise on this..Will this grouping have any adverse effect or cause huge derating to my cables..?
Kind Regards,
Alan
I've been terribly confused over sizing cables for a 530 KW LV CHILLER.My current sizing is to go with 3 Run SIngle core 400 sq.mm cables per phase(total of 9).My cable arrangement for direct buried cables will be as attached.Can I take group derating factor to be 1 since trefoil centre to cente distance is 170 cm(Which is several times their dia).Please advise on this..Will this grouping have any adverse effect or cause huge derating to my cables..?
Kind Regards,
Alan






RE: Group Derating Factor for Trefoil Formation
However, other factors depend on earth temperature, on earth thermal resistance[RHO=0.7-2.5 usually]and depth.
RE: Group Derating Factor for Trefoil Formation
RE: Group Derating Factor for Trefoil Formation
Can you tell us where you are so we know what standard applies? If you were in Australia (our standards are derived from IEC equivalents) I note the following.
3x400mm2 XLPE cables spaced at 170mm centres, buried direct (25 degrees c) would have a derating of 0.73, giving an aggregate cable rating of 1462A (after derating applied).
So, provided (as stated in your other thread) that the chiller is not just one big 530kW motor started direct online (assume multiple stages, time delayed), this cable would likely be fine, provided the run length is short (neglecting voltage drop).
I would hasten to add that the cables should be correctly arranged to reduce current imbalance caused by unequal inductance in the sets and should also be transposed (swap positions in the set). Neglecting to do this can result in surprisingly large current imbalances between the sets, even to the point where they couldn't carry the 1050A. Even doing this perfectly (arranged in a line) will result in at least another 0.95 derating for 3 or more sets.
You might also like to consider using 4x185mm2 XLPE cables, it is a more optimal solution and 185 is easier to work with than 400. Gets you 1228A after group derating.
Or perhaps 4x150mm2 XLPE multicore cables, which don't seem to be exposed to imbalances due to inbuilt transposition. Gets you 1139A after group derating.
Cheers,
Healyx
RE: Group Derating Factor for Trefoil Formation
RE: Group Derating Factor for Trefoil Formation
For 2.5 K.m/w ,load factor 0.5-1.0, 35 dgr.C f1=0.72 .
For 6 systems f2=0.5
Then permissible current [ampacity]=663*0.72*.5=238.7 A/each cable
If the cables would be laid-out in free open-air, if the clearance between groups it is one group diameter no ampacity reducing factor would be necessary .
RE: Group Derating Factor for Trefoil Formation
The manufacturer recommended six trefoils in a single trench each trefoil spaced 150mm apart.Also recommended adjacent tranches to be atleast 3 m away to make them thermally independent of each other.
Thanks Guys..Any Clarifications,im happy to explain..
RE: Group Derating Factor for Trefoil Formation
A few additional things to consider:
RE: Group Derating Factor for Trefoil Formation
Regarding Earth Cables, I have considered 3 runs of 240 sq.mm earth cable..Do I need to add more?
RE: Group Derating Factor for Trefoil Formation
IEC 60364 and BS7430 calculate the minimum cross section according to short-circuit current [rms] for 1 or 3 seconds fault clearing.
If we take the maximum standard transformer apparent power for secondary rated voltage of 400 V as Sn= 3000 kVA and 8% short-circuit impedance, the short-circuit impedance will be [considering the medium voltage short-circuit power as infinite Zsys=0]:
Ztrf=uk%*U^2/Sn=8/100*0.4^2/3=0.00426667 ohm.
I"k=400/sqrt(3)/0.00427=54 kA
According to BS7430 ch.14 permissible current density:
k=K*sqrt(ln((T2+b)/(T1+b))
If K=226 and b=254 [for copper conductor];T2=250 dgr.C.[XLPE insulation] and T1=35 dgr.[Earth temperature] k=168.54
I=k*scu/sqrt(time) or scu=I"k/k*sqrt(time)
For 54 kA and 1 sec you need 320 sqr.mm-one conductor XLPE insulated or 3*185 sqr.mm for 3 sec for each 3*3*1/c[3 trefoils].
RE: Group Derating Factor for Trefoil Formation
1. 630mm^2 may be a non-standard size or one you can't actually find very easily
2. single conductor cables larger than 400mm^2 are huge, heavy and hence difficult to handle and install.
for both the reasons above, consider never exceeding 400mm^2 for single conductor, 240mm^2 for 3 or 4 conductor
RE: Group Derating Factor for Trefoil Formation