Sizing wire and Transformer

[reyweesor]

Please help me verify if i got it right for the size of my wire for main generators and transformer. since i find it complicated.

Please see attach file for my questions and scenario.

Stay safe, Thank you!

 

[tom baker]

Do you use the NEC? If not we can't help you.

 

[Carultch]

You are correct in simply using the reciprocal of the voltage ratio, to determine the current on both sides. Transformers are analogous to a gearbox. Just like a gearbox trades torque for rotation speed, a transformer trades current for voltage. BIG V*little i = little v *BIG I, assuming the limiting case that it is 100% efficient.

However, you now have medium voltage on the 13.2kV side, which changes a lot about how you size your cables. You'll need to get cables that have significantly thicker insulation, and are rated for the 15kV class of circuits. The sizing algorithm is not the same as it is for sub-1000V circuits. The procedure is detailed in Chapter 3 of the NEC, in a section dedicated to medium voltage circuits.

 

[reyweesor]

You are correct in simply using the reciprocal of the voltage ratio, to determine the current on both sides. Transformers are analogous to a gearbox. Just like a gearbox trades torque for rotation speed, a transformer trades current for voltage. BIG V*little i = little v *BIG I, assuming the limiting case that it is 100% efficient.

However, you now have medium voltage on the 13.2kV side, which changes a lot about how you size your cables. You'll need to get cables that have significantly thicker insulation, and are rated for the 15kV class of circuits. The sizing algorithm is not the same as it is for sub-1000V circuits. The procedure is detailed in Chapter 3 of the NEC, in a section dedicated to medium voltage circuits.

Okay sir! Thank you so much! Yeah, here in Philippines, we do use over head lines aluminum conductor for the 13.2kV.

 

[Julius Right]

If the maximum load is 1035 A at 3*230 V 60 Hz the required VA=√3.1035*230=412314.7 VA=412.3 kVA
A transformer of 500 kVA presents a copper and core losses of 6 kW [approx]. 2 transformers 2*6=12 kW.
On 13.2 kV side the rated current will be 500/√3/13.2=21.9 A
An overhead line of ACSR [aluminum steel reinforced] of #6 [sun+wind] 85 A ampacity at 40oC air.
The resistance [ac 75oC] 4.26 ohm/mile and reactance 0.63 ohm/mile
So the voltage drop at 21.9 A 0.85 p.f on 13.2 kV DV=[4.26*0.85+0.63*√(1-0.85^2)]*21.9*√3=150 Vpro mile [1.14%]
At 480V side Irated=500/√3/0.48=601.4 A
According to NEC 2017 250 MCM 75oC copper 255 A ampacity.
3*255=765 A. If all 9 loaded wires are in the same conduit a 0.7 factor is required. so you need 601.4/0.7=860 A. 3*300 mcm will be the minimum.