Paralleling Transformers

[govender1]

What are the effects of operating two power transformers of different impedances in parallel?

 

[nag400]

If you operate two transformers with different impedance in parallel, (assuming that the transformers are of the same rating) the transformer which has low impedance path will try to carry most of the loads (because current always follows the least impedance path) and the other will carry slightly lesser load. This may be seen as in power system a case of unbalance and chances of circulating current flowing through the transformer is high. The chances of circulating current are high, if the taps of both the transformers are kept in different positions while paralleling.

-nag400

 

[cuky2000]

This is a good response by Nag400.

Paralleling transformer seen to be a hot topic in this forum.

Please see the enclose threads that address similar question: thread238-7093 thread238-16427 thread238-30275

A related topic could be the phase shifting transformer use to increase the power flow in a paralleling situation. The following thread has good reference thread238-27103

 

[busbar]

A couple of added comments... If the transformers are not in close proximity, the system’s line-impedance characteristics will affect circulating current also.

It may be obvious, but circulating currents will present a highly reactive load to the source, and will exist regardless of loading.

 

[acc]

hi govender1,
please put your values (i.e. uk,Sn,S)in to
basic formulas given here under and calculate the loads shared by transformers.Then you will see the effects.

uk = S /((Sn1/uk1)+(Sn2/uk2)) [total imp.of parallel tr's.]
S1 = S*(Sn1/Sn)*(uk/uk1)
S2 = S*(Sn2/Sn)*(uk/uk1)

where;
Sn1;uk1 = rated power;imp.of TR1
Sn2;uk2 = rated power;imp.of TR2
S1,S2 = load shared by TR1,TR2
S = existing load fed by parallel transformers.
Sn = Sn1+Sn2

 

[govender1]

Thanks guys for the valuable information. Note that the transformers are approximately 8m apart will this still create circulating currents. The impedance of the transformers are 7.69% and 8% respectively. Is there an impedance tolerance where paralleling the transformers will not have any negative effects or rather must the impedances be identical?