Transformer rating with single phase loads 3

[Murlinor]

Hi.

Assume that the single phase load is connected as follows (balanced):

L1-L2 = 23 kVA
L2-L3 = 23 kVA
L3-L1 = 23 kVA

All loads are resistive.

Based on the above load, what will be the kVA rating for a 3 phase supply transformer? The network is 230 V i.e. 3 wire system with isolated neutral. (Phase-earth voltage is 230/sqrt(3))

Best Regards
Murtaza

 

[edison123]

69 KVA

Muthu

http://www.edison.co.in

 

[odlanor]

Nmono= 23;
Ntr =Nmono * sqrt(3) => 39.84 kVA

 

[magoo2]

edison is correct. Should be 69 kVA.

If it were line currents that you start with, then something like what odlanor did would apply.

 

[Murlinor]

Thanks!

But the current in L1-L2 is 100 A, similiary in the other loads. Shouldnt the the rating be: S = sqrt(3)*230*100 = 39.8 kVA? That is 13.3 kVA in each phase?

 

[cranky108]

Try vector addition of coil currents to find the actual line currents.

 

[odlanor]

murlinor,

I calculated 0.1A.
If is 100A so loads are 23MVA.
So, with a 3-phase transformer of 39.84 MVA you get in Y connection:
Iline-line=Iline-neutral = 39.84 *1000/(230*1.732) = 100 A

 

[rbulsara]

Murlinor:

Current between L1-L2 will be the phase current in the load, connected in delta. The current in the formula you used is the line current, which will be 1.732*100=173A in a balanced phase system.

L1 is involved in feeding both L1-L2 and L1-L3 loads and so on.



Rafiq Bulsara

http://www.srengineersct.com

 

[itsmoked]

Murlinor; As Rafiq sez, the 100A you are stating has to be measured directly in line with the individual 23kVA loads. Not the current coming from the transformer.

When you state that each load is 23kVA conservation of energy is going to demand the transformer feeding the three loads to transform the same total kVA. (23kVA x 3).

That would even go for single phase.
Three single phase 23kVA loads would still demand the same (23kVA x 3) transformer capacity.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[stevenal]

69 kVA. Odlanor is incorrect.

If you use LN voltage with line currents you will get the same result.

 

[odlanor]

itsmoked,
Yes, I agree. With Lavoisier I do not discuss!
ronaldo

 

[Murlinor]

Thanks people. I drew a three phase diagram and saw that the load will be connected in delta, so the 100 A will be the current through each load. I had calculated 173 A, but got confused.

Thanks everybody. Appreciate it.