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Transformer rating with single phase loads
3

Transformer rating with single phase loads

Transformer rating with single phase loads

(OP)
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

 

RE: Transformer rating with single phase loads

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

RE: Transformer rating with single phase loads

edison is correct.  Should be 69 kVA.

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

RE: Transformer rating with single phase loads

(OP)
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?

RE: Transformer rating with single phase loads

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

RE: Transformer rating with single phase loads

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

RE: Transformer rating with single phase loads

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

RE: Transformer rating with single phase loads

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

RE: Transformer rating with single phase loads

69 kVA. Odlanor is incorrect.

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

RE: Transformer rating with single phase loads

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

RE: Transformer rating with single phase loads

(OP)
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.

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