Group Derating Factor for Trefoil Formation 1

[alanElec]

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

 

[7anoter4]

The reduction factor for groups of more than one circuit only, I'm afraid is still far from 1.According to IEC 60364-5-52 Standard the factor will be 0.7.If the distance is 1500 mm k=0.94 [calculated].
However, other factors depend on earth temperature, on earth thermal resistance[RHO=0.7-2.5 usually]and depth.

 

[bacon4life]

The attachment appears to have the distance as 170 mm rather than 170 cm. Would it be one trench 3.2 meters wide or separate trenches for each set? If it is 170mm, the reduction factor will be much less than 1 rather than slightly less than 1.

 

[healyx]

Hi Alan,

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

 

[alanElec]

I am having a total of five 530 KW chillers..By my sizing I need 9 Runs(3 Trefoils) for one chiller..Im planning to take 18 runs(6 trefoils spaced 170mm apart) in one trench...My soil resistivity is 2.4 n ambient temp 35 C..

 

[7anoter4]

According to ABB Switchgear Manual ed.11 [DIN/VDE-0276-603] I[initial] =663 A ,but for one 3 single-core cables in trefoil formation 0.6/1 kV copper conductor xlpe[90 dgr.C] insulated ,earth th.res. 1 K.m/w, 20 dgr.C,load factor=0.7.
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 .

 

[alanElec]

Thank You All for your kind responses..I've checked myself with the Cable Manufacturer and they sent me derating factors for trefoil formation from ERA report Table 70.Based on that,I've sized the cable as 3 Runs of 1Cx630 sq.mm per phase.

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..

 

[ScottyUK]

630mm²? Wow, your installers are going to love you.

A few additional things to consider:
[ul]
[li]Does the chiller and switchgear have provision for landing multiple cables of this size?[/li]
[li]You will need to provide cleats to restrain the cables during a fault.[/li]
[li]You will need a non-ferrous glandplate to control induced currents.[/li]
[li]You may have problems with further thermal de-rating due to image currents if you bond the armour at both ends, or have a non-zero potential on the armour at the un-bonded end.[/li]
[li]If you don't bond the armour, make sure you include adequate additional earths.[/li]
[/ul]

 

[alanElec]

They may hate me..But I couldn't think of a better option...If i were to use 400 sq.mm I would have required at least 5 runs..thats costs more and delivery might also take longer..

Regarding Earth Cables, I have considered 3 runs of 240 sq.mm earth cable..Do I need to add more?

 

[7anoter4]

According to NEC Art.250.24 (C)(1)-(2) the grounding conductor cross section has to be not less than 12.5% of all parallel live conductor[per phase].So, since there are 3*630 sqr.mm per phase the minimum will be 240 sqr.mm per group of 3 trefoil.
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].

 

[1capybara]

alansam, the installers are going to hate you because
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

 

[alanElec]

Im a newbie and from what I've asked and learned from Senior Engineers,630 sq.mm is a more standard size than 400 sq.mm cables here..Yes,it is true installation is difficult..But the cable is directly buried and not through ducts or laid in concrete trenches..distance is also quite short...