Transformer paralleling

[gsjhand]

Can I operate two transformers; 3MVA 440 Volts, 50 Hz, 6% impedance and 4MVA 440volts, 50Hz, 7% impedance; in parallel?
If yes what will be the percentage load sharing by each of them?

 

[gsjhand]

Forgot to mention that vector group is same for both of them.

 

[slavag]

Hi.
Usually, recommended 10% difference between Uk%.

possible check oveload and undeload situation.
according:
Uk% for both=(3+4)/( 3/6+ 4/7)=6.54%

S1=3x6.54/6=3.28
S2=4x6.54/7=3.72

 

[waross]

Normalize the impedances. The 4 MVA will derate to 3.43 MVA at 6% impedance. Total load may be 6.43 MVA and the transformers will share the load in proportion to their normalized MVA ratings (3 MVA and 3.43 MVA).
If the X/R ratios are not equal the total allowable load will be less than 6.43 MVA.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

I did an "exact" calculation and the result is not far from the results in above posts.
The load will be shared depending- but only a bit- on load pf. If 440 V is the rated transformer voltage the Load voltage will be less due to the drop voltage on transformers.
For instance if the rated Load Sload= 7 MVA and pf=0.8 then S1=3.125 and S2=3.57 MVA. But if pf=0.9 then S1=3.154 MVA and S2=3.604 MVA.