Answer Posted / tarun aggarwal ymca

what u have to first take into mind what type of cable u want to purchase....then its size matter

usually we use XLPE insulated cable for Power cable and Aluminum or copper its depend upon the cost and loses

mean if u need large length cable you have to prefer aluminum in shorter length copper cable is preferred as its resistivity is less

for example

VLL = 11kV (line volatge)
L = 1.3km
Maximum allowable Vdrop = 2.5% of VLL i.e.Vdrop = 275V

SkVA = 3000kVA (Total Power we have to transfer of to be used by the connected load)
Full load Current If= SkVA/root3 VLL i.e If = 157.5A

Demand factor is what we actually demand the current (it not always full load current what we demand) Let D.F = 80% of If

Id =D.F x If (Demand Current) where D.F = Demand Factor i.e Id = 126.0A

The cable current carrying capacity is always kept larger than the demand current let S.F for Cable in % = 30% Extra Id

Ic= (1+S.F/100) Id i.e Ic = 163.8A

This current is what will be safer current that will be carried out by cable

Now derating factor K is calculated by the following equation

K = Kground x Ktherm.res x Kdepth x Kgroup

Vdrop = 3IdL root(Rac cos􀄳+ Xsin􀄳)

Now cable data is refered

Type 3 Core, Copper Conductor, XLPE insulated, Copper tape Screened, PVC bedded, Steel Wire Armoured, PVC, 8750/15000V.


Parameter -------- 50 mm2---- 70 mm2 ------ 95 mm2------- 120 mm2 ------- 150 mm2 ----- 185 mm2
Irated (A) --------- 187----------- 227- ------------ 267--------------303 -----------------338------------------ 383
Kground----------------0.86
Ktherm.res-- ----------- 1
Kdepth-------------------0.98
Kgroup ------------------0.7
K-------------------------- 0.590
Icor (A) -----------110.32 --------- 133.92----------157.52--------- 178.76-------------199.41------------225…
Rac (ohm/km) --- 0.493 --------0.342------------ 0.247------------0.196---------------0.1… ------------ 0.128
X (ohm/km) --------0.118 --------0.112 ------------0.106------------0.102------…
P.F = Cos--------0.85
(In radian)-------- 0.5548------ 0.5548----------0.5548-----------0.5548-…
Vdrop (V)-----------196.76-------167.73-------…

Considering the values of Ic, Icorrected and Vdrop the best suited cable is 120 mm2
cable for connecting switchyard to substation

Sorry extra long answer but i want to explain in detail

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