Parallelling transformers with different turn ratio and rating?

I got stuck in a problem while solving the previous exam question for my course...





Two transformers with unequal turn ratio and unequal ratings are connected in parallel and have the same secondary voltage. How will the load be distributed between them? Hence find the circulating current.





I know how to find the circulating current when the secondary voltages are different. But how can there be a circulating current if the secondary voltage is same?


And I know about the load distribution being inversely proportional to the line impedance of the transformers, but what will be the change taking circulating current into account?|||Think of it this way - the two transformers are connected in parallel on the primary. They have different turns ratios. As a consequence, they want to produce different open circuit secondary voltages. But the fact that they are also connected in parallel on the secondary means that the secondary voltages must be the same.



So what happens is that the DIFFERENCE between the open circuit secondary voltages is the voltage that drives the circulating current between the transformers.



You didn't mention the impedances of the transformers. If you have transformers with equal nameplate impedances but different ratings in parallel, load will split between them in proportion to their respective ratings. Except for losses, the circulating current is reactive and not 'load'.|||If I understand this correctly, then you have this situation,


let T1 be the first transformer and T2 the second one





The input voltage is connected to the primary of T1


The secondary of T1 is connected to the primary of T2


the secondary of T2 is connected to the load





If the secondaries are at the same voltage then


T1 secondary voltage = T1 primary voltage = T2 secondary voltage.





This can only happen if the turns ratio of T2 is 1:1, i.e it has the same number of turns on the primary and the secondary.


Does that help?|||To me it is a typical circuit analysis problem.





From an application engineers point of view , to analyse the problem, the following needs to be checked first





1) Open circuit Voltage of both the TRXs


2) Internal impedences of TRXs


3) Total Load to be catered to ( converted to in terms of Z)


4) Ratings of the TRXs ( Rather allowable current that can be drawn from each TRXs)








Having known first three quantities , we can make the parallel circuts and calculate the current in each branch ( through TRX1, TRX2, and the load) andtheir directions





The fourth quantity will tell us if loading is within limit or not.