Paralleling of transformers

You're no doubt going to get a vast amount of neat formulas to explain exactly how this works. Electrical calculations are pretty much straight mathematics, meaning, what you draw on paper you can generally count on to actually measure when built and functioning.

Here's a boiled down common sense characteristic. If the transformers were EXACTLY the same, then the current would divide evenly or each one could contribute it's full name-plate capacity... ( 2 ) 10 kva transformers would indeed provide 20 kva.

In reality, the impedance of LIKE transformers but from different manufacturers, would be different and ( 2 ) 10 kva transformers would NOT be "worth" 20 kva, rather, all paralleled transformers will be de-rated to the actual capacity of the weakest transformer. Example, the weakest transformer in a bank of paralled 8 kva, 9 kva and 10 kva would NOT deliver 27 kva or even 24 kva, it'll be less. The weakest link ( highest impedance ), gets the highest voltage drop therefor has an "unfair burden" of heat dissipation. The DC resistance of the core copper, the number of actual transformer turns, the number and thickness of the lamanations and the physical stature or ability to shed heat, ALL figure in a transformer's actual capacity.

Same concept as buying matching belts for pulleys or matching power transistors for an audio amplifier.

The different Z of Transformers will be loaded unevenly if it is paralled.Due to this uneven current, heat devoloped on the winding is more and the class of insulation to be with stand.Also the harmonics will be more at one transformer and it will be less at another transformer.After parallaling properly, this will be normalised subject to the above.It is not danger if you follow the condition for paralleling of Transformers.Practically many distribution transformer were paralled by electricity board and functioning normally with differant impedance.

I agree with your "diifferent Z" that leads to different loading, my point exactly. Different harmonics ? I'd think the harmonics are the same unless we're talking about audio transformers. And...you said the power companies do this all the time, yes, I know that for sure, I worked for an electric utility for 11 years. They can safely parallel transformers because they use MATCHED transformers. Once again, I assure you, can NOT deliver the sum total kva of unmatched transformers in parallel because the weakest transformer will overheat.

Hi there,

I don't know which one of you to address with this issue because a few you confusing me.

My Q is:

What role harmonics has here?

Why would anyone try to paralell up unidentical transformers on the secondary side?

As I can see it thre's no problem on the primary side, as long as it is connected to the grid, but it is definately a concern on the secondary side because of the one with higher dc impedance (weaker trf) would work into the one with the lower dc impedance(stronger trf) due to a phase shift difference between them, however small it maybe.

Does this make sense?

Power companies do it frequently in power sub stations but the primaries are (generally) fed from different sources. The main problem with these applications is voltage mismatch (and other phase angle displacements) that cause circulating currents to flow and tend to heat the transformers. Generally they are paralleled only for short durations (Via Bus tie breakers) in order to maintain continuous service to the customers during the transfer of loads.

Friends,

With all said and done is it impossible to overcome these obstacles of impedence mis-match? If I have three sources of power and make some kind of redundant power arrangement this idea/trick will be very useful. What is the way out? Other than using UPS/Inverters.

R. GANAPATHI RAO

INDIA.

See following Posts: 1.} Combining portable generator output <http://globalspec.ip09.com/rd/9z1z4o4akgbccedogn3ca55c4ivu29tpnsnh7vgo1dg> (Electrical Engineering) 2.) Haajee Power-User Join Date: Feb 2006 Location: Pakistan: 33.2N; 72.16 E Posts: 163 #2 [New Comment] Re: 05/10/2007 6:39 AM In parallel you should have exactly balanced O/P Voltage. If a slight difference is there sharing will be accordingly in: Imbalanced Load for a Double-Wound Transformer <http://globalspec.ip09.com/rd/9z1z2tkf2d9e2c2u1se3iu91si9bqn08lsrbmk4pue8> (Electrical Engineering) 3.) Davegur Active Contributor Join Date: Mar 2007 Posts: 12 #1 Re: Can I connect two 12v rails on a PC power supply? 04/27/2007 1:16 PM Yes, provided the 12volt rails have a over-crrent protection charecteristic, otherwise one may supply the majority of the output and overheat. and more ... in: http://cr4.globalspec.com/thread/7368/Can-I-connect-two-12v-rails-on-a-PC-power-supply Can I connect two 12v rails on a PC power supply? <http://cr4.globalspec.com/thread/7368/Can-I-connect-two-12v-rails-on-a-PC-power-supply> and more ...