Parallel Operation of Distribution Transformers 2

[sunilji]

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 ?

 

[jnims]

There can be several advantages to paralleling transformers. First, having two transformers provides some redundancy and would tend to reduce restoration time if one unit were to fail. Second, using parallel units can allow you to invest in transformer capacity only as it is needed. A third reason could be related to the transformers in the warehouse and your company's spare tranformer practices.

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.

 

[electricpete]

I vote jnims a star for a good response. I would like to add one advantage of running transformers in parallel vs split.... the load sharing between transformers will be determined by the system designer, not the load. Specifically, if you select two transformers (not necessarily the same size) with the same impedance when expressed on their respective MVA base, then they will divide the load so that they are both loaded to the same percent of their full rating. (ie if one is at 75% of its full capacity, then the other is also at 75% of its capacity). This gives optimal bang for the buck in terms of serving the load with minimum MVA of transfomrers without overloading any transformer. But as you mention, circulating currents due to unequal voltage ratio's creates additional load in the form of circulating current which must be factored into transformer loading.

 

[jbartos]

Suggestions: Transformer paralleling is usually used for some justifiable reasons, e.g.
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.

 

[sunilji]

Thanks to jnims, electricpete and jbartos for their informed and valued response ( At least 3 transformer manufacturers I talked to could not give a more definitive reply ! )
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 ?

 

[electricpete]

Now that you have explained that you are upgrading an existing installation....it seems more releveant that you heed the cautions by jnims and jbartos... and make sure your existing circuit breakers can handle the increased fault current.

 

[SooryaShrestha]

sunilji some additional points in addition to the points given by jnims, jbartos and electricpete and an explanation of "In some instances (after a proper economic analysis) it just may or may not be worth it to parallel transformers" by jbartos.
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.

 

[jbartos]

Suggestion (due): Reference:
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.&quot

 

[sunilji]

(A Diversion )
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 !

 

[BJC]

I had the same book in Machinery class.
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.

 

[jbartos]

Suggestions: Visit

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