Sizing of MV (13.8 kV) cable for a 200kVA, 500 kVA, 1000kVA and 1500 kVA Transformers

[MFather]

Dear colleagues,
Kindly help sizing the subject cables considering they are for link between the transformers and the RMU only and they should be single core cables

 

[ninjutsu]

Dear colleagues,
Kindly help sizing the subject cables considering they are for link between the transformers and the RMU only and they should be single core cables

Use normal formula of power to estimate max current.Please also include the other factors such as transformer starting current, temperature. Calculate the short circuit let through energy of the circuit breaker to determine the correct cable size

 

[Julius Right]

The transformer rated power is not important. You may check the cable in steady state load, but:
For instance: 3 cables laid underground at 30 dgr.C and 1.5 K.m/W, trefoil configuration,
50 sqr.mm Aluminum conductor cable 12/20 KV, xlpe insulation: maximum permissible current: 140 A [3400 KVA].
As ninjutsu said, you have to know:
-the short-circuit current [at least at RMU level].You may reduce it if you recalculate the short-circuit current at the supplied transformer terminal.
-fault clearance time [it will be better at the RMU main breaker]
-insulation [XLPE, EPR, PVC or else]
-conductor material [CU, AL]
-frequency [50 or 60 Hz]
For instance: 25 KA 1sec XLPE /CU 50 Hz the minimum conductor cross section will be 150 sqr.mm [300 MCM]. [X/R=10].

 

[MFather]

Thanks Julius Right and ninjutsu for your participation, but a little bit of clarity is required that's why I am adding the requested details here-below considering the worst scenario (i.e. 90% of the transformers' rated power ) - although not required - and assuming the following:
1. Short circuit current @ the RMU is 21 kA.
2. Fault clearance time @ the RMU breaker: 85 ms.
3. Breaker rating: 630 A.
4. Insulation type is XLPE (non-armored)
5. Conductor material: Copper (Cu).
6. Frequency: 60 Hz.
7. Installation conditions:
7.1. Route length: 7 meters (~ 21') each.
7.2. Burial depth: 1.5 m (~ 4.5').
7.3. Max. ambient temp.: 60 Deg. C (140 Deg. F).
7.4. Ground temp.: 45 Deg. C (113 Deg. F).
7.5. Soil thermal resistivity: 1.5 K.m/W
8. Configuration: Single cores, flat, buried inside conduits, 0.25 m (~ 0.82') spaced.

Please advise if any other parameters are required to help precisely size the subject cables... Thanks in advance

 

[Julius Right]

First of all I think you need to take into consideration the possibility the cable breaker could fail and the RMU general breaker will work instead. So I think the fault clearing time could be 0.2 sec [200 msec]. In this case the minimum copper cross section would be 70 sqr.mm.
You can employ this formula:
Asqrmm=IkA*10^3/SQRT (TCAP*10^2/ror/alpha/ts*LN ((Ko+Tm)/ (Ko+Ta)))
TCAP=3.42,ror=1.72;alpha=0.00393;Tm=250;Ta=90;Ko=234.5;ts=0.2 sec
For underground cable laid in ducts you can calculate the required conductor cross section or take it from IEC 60364-5-52. According to IEC 60364-5-52:
Reference method D, xlpe ins.,3cables per conduit 70sqr.mm Cu I=178 A[one circuit,20 dgr.C earth 2.5 k*m/W].
For 4 circuits 0.25 m distance k1=0.75. For 1.5 k*m/W K2=1.1. For 45 dgr.C Earth k3=0.80.
Total I=178*0.75*1.1*0.8=117.48 A maximum permitted for 70 sqrmm Cu.
So 70 sqr.mm has to be the cross section [from short-circuit condition]