Pole Mounted Transformer Calculation (Overload)

[KDBJ]

Hi All,

If I have the following information, how do I determine if the transformer is overloaded

R-RED
W-WHITE/YELLOW
N-NEUTRAL

Phase to neutral Voltages:

R-N 123V
W-N 122V
R-W 246V

CURRENTS

NEUTRAL 115 A
RED PHASE 257 A
WHITE 444 A


pOEL MOUNTED TRANSFORMER SIZE 75KVA


Also, if I had the same information (plus line voltages and curents) for a three phase bank with 3 50 KVA pole mounted transformers (stra/star configuration) how do i tell if a transformer is overloaded?

Thanks in advance for any reponse

 

[gar]

130902-0959 EDT

As a rough approximation I would do the following:

1. Consider each half of the secondary as a transformer alone. You are concerned with internal power dissipation. Thus, assume you have two 75/2 = 37.5 kVA transformers.

2. The current rating of the transformer secondary is 75,000 / 240 = 312 A.

3. Yes, one side of the secondary is way overloaded. Even if you equalized your load so you had 350 A on each side, then the transformer is overloaded. Roughly speaking your total load is 84,000 kVA.

4. In your 3 phase question treat each transformer by itself.

.

 

[suemarkp]

Something doesn't add up. The 240V loads, or balanced 120V loads, will suck the same current from each end of the transformer (R and W). Since R is 257A, the W component that is purely 240V should be the same. The higher current on W could be from an imbalance, so if you add all the neutral current of 115A to the lower phase (R), that should match what the meter says for W. But 257 + 115 = 372A and you measured 444A. Is there a fault on one phase or a leak to ground? Where is that missing 70+ amps?

 

[petersonra]

Something doesn't add up. The 240V loads, or balanced 120V loads, will suck the same current from each end of the transformer (R and W). Since R is 257A, the W component that is purely 240V should be the same. The higher current on W could be from an imbalance, so if you add all the neutral current of 115A to the lower phase (R), that should match what the meter says for W. But 257 + 115 = 372A and you measured 444A. Is there a fault on one phase or a leak to ground? Where is that missing 70+ amps?

a good question.

something here literally does not add up.

in any case, if this is a utility xfmr they routinely allow them to be run over their nominal ratings. it is just cheaper to do so than to replace it with a larger one until it fails.