Parallel Operation of Distribution Transformers
Parallel Operation of Distribution Transformers
(OP)
I would like to know whether there are any advantages of running Distribution Transformers in parallel ( secondary side ). I have heard of the hazards ( circulating currents ) and precautions to be taken ( matching parameters ) but are there any tangible benefits to be had by paralleling ?






RE: Parallel Operation of Distribution Transformers
However, there are also disadvantages (in addition to the ones you mentioned). Two or more transformers will probably have higher no-load losses than a single unit sized for the initial load plus capacity for expansion. Multiple transformers will also require more space and increase maintenance costs. Finally, when paralleling transformers, make sure to review the change in available fault current versus breaker interupt ratings and switchgear bracing.
RE: Parallel Operation of Distribution Transformers
RE: Parallel Operation of Distribution Transformers
1. Power distribution expansion or rebuilt with some expansion (caution: short circuit current level will increase)
2. Significant savings on using smaller standard transformers off-the-shelf.
3. Specialty applications, e.g. to obtain high currents
4. Etc.
When paralleling transformers, the following supposed to be kept in mind for satisfactory performance of the parallel transformers:
a) Essential features of the same polarity
b) The same phase-sequence
c) The same zero relative phase-displacement angle
d) Near identity of voltage ratio
e) A limited disparity in per-unit impedance
f) About the same K-rating for harmonics, if K-rated
g) Etc.
In some instances (after a proper economic analysis) it just may or may not be worth it to parallel transformers.
RE: Parallel Operation of Distribution Transformers
The case in hand is this : I have
a) 1000KVA, 22 kV/433V 3 phase, Delta/Star and
b) 500KVA, 22 kV/433V 3 phase, Delta/Star transformers feeding isloated sections of the load in the same factory premise. An increase in the total power requirement necessitates either installing a new transformer of 500 KVA or upgrading the 500 KVA to 1000 KVA. I am in favour of replacing the 500 KVA TX with a 1000 KVA TX and parallel it with the existing 1000 KVA. This would allow all additional and existing loads to be fed from the common bus without worrying about overloading any transformer. Both transformers would be loaded equally and their load losses would be minimum.
Jbartos, can I depend on you to refer any literature on this subject to me ?
RE: Parallel Operation of Distribution Transformers
RE: Parallel Operation of Distribution Transformers
If you have two transformers, which can be run in parallel, the losses will not be minimum when you run the two transformers in parallel at all the loads. Since you have two transformers, one 500 kVA and the other 1000 kVA or two 1000 kVA, they will have different noload and load losses. The total losses of a transformer at any load Ki is:
Pnlx + (Ki/K)^2 * Pllx, where Pnlx and Pllx are noload and full load losses of unit x and Ki and K are any rating and full load ratings respectively. Draw graphically the total losses vs the rating Ki. for each transformer. Again draw the two parallel operation scheme losses of Pnl1 + Pnl2 + (Ki/(K1+K2))^2 * (Pll1+Pll2) vs Ki.
Plot all the three curves on the same graph paper or in any program like mathcad. The above equations need certain changes if you have noload and load implications different like cost of losses then the losses can be multiplied by the capitalized cost of losses.
There will be clear three segments in the graph where running the tranformer first one, second one or both will make least losses.
RE: Parallel Operation of Distribution Transformers
1. Say M. G., "Alternating Current Machines," John Wiley & Sons, New York, 1978
Reference 1 page 182 section 5.15 "Parallel Operation" (Under Chapter 5 Transformers: Theory and Performance.")
RE: Parallel Operation of Distribution Transformers
jbartos, you may be surprised to know that "A.C. Machines " by M.G.Say was the prescribed text book for the subject 'Electrical Machine Design' when I took my degree 22 years ago ! And I have still preserved it, finding it an invaluable reference time and again. Just goes to show real classics are always timeless !
RE: Parallel Operation of Distribution Transformers
We also used a small paper back book published by General Electric titled " Transformer Connections". It was as of 3 or 4 years ago still in print. The parallel connection of two transfromers is covered in great detail including voltage/ phasor diagrams.
Whe I took the PE exam there was a problem on paralleling transformers, I had taken the GE booklet to the exam and looked up paralleling to help me with the problem. The problem in the book and the problem on the exam were exactly the same--easy ten points.
RE: Parallel Operation of Distribution Transformers
http://www.neiengineering.com/sub/sub.htm
where the transformer paralleling is under "Protective Relaying, Control, Metering and Monitoring
http://www.beckwithelectric.com/calendar.htm
for the technical paper "Advanced Transformer Paralleling"
http://scholar.lib.vt.edu/theses/available/etd-173510281975580/
open apend.pdf for a few equations good for a leisure time