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Is it mandatory to have same X/R ratio of the transformers meant for parallel operation apart from same %impedance, voltage ratio and vector group?

Responding to Krishna’s 25-Aug (11:20) query…

No, it is not mandatory! For details, search the Control List Archives. There is a host of related topics!

Regards,
Phil Corso, PE

Does it mean that the transformer of different impedance angle can be operated parallely if the %impedance is same? Pls help me to know how.

Responding to Krishna’s 29-Aug (23:23)… Yes!

If their X to R ratios are equal, then of course their percent impedances are equal, making the equations simpler! If any or all of the 3-factors are different a circulating current will exist between the two transformers, even at no-load!

Regards, Phil Corso, PE (cepsicon@aol.com

Thanks sir,
So can I understand that even if the X/R is different the XTR can opetate parallel if the %impedance is same with the same vector group and turns ratio,preventing circulating current.

Regards

Responding to Krishna's 03-Sep (15:41)... your understanding is not correct:

1) A circulating current exists if they have equal pct impedances but different X/R ratios!

2) A circulating current exists if they have equal X/R ratios, but different pct impedances!

For either case you must calculate the level of circulating current to determine its impact on safe operation!

Regards, Phil Corso (cepsicon@aol.com)

What happens if the voltage ratio is different (but the secondary voltage is same)? Can we parallel two transformers for short time for motor starting?

Naga, I believe it’s best to answer your question by using examples. Hopefully, those who abhor formulas and equations won’t have a melt-down! The following should help:

a) Consider two transformers, A and B, having equal capacity, voltage-ratio, and percent-impedance! They will each carry 50% of the load, and circulating-current is virtually zero!

b) If A’s impedance is twice that of B, say 6% vs 3%, respectively, then A will carry 2/3 of the load, while B carries 1/3. In other words, if two 1,000-kVA transformers are supplying a 1,800-kVA load the one with the 6% impedance will carry 600-kVA and the other 1,200-kVA!

c) The case when the impedances are equal but X/R ratios are different doesn't yield a simple “word-description” that will adequately express the outcome!

d) Addressing your question about different voltage-ratios, regardless of impedances and/or X/R ratios differences: consider transformer A with a 400-V as its secondary no-load voltage, while B’s is 415-V; further, let Za and Zb represent the impedances (in ohms) of transformer A and B, respectively. Then the circulating current, equals,

Ic = [Vb (higher secondary voltage) – Va (the lower secondary voltage] / [Za + Zb], Amps

In conclusion, for the condition when voltage-ratios are equal, but the impedance and/or X/R ratios are different, then the load carried by one transformer is proportional to the other’s impedance divided by the sum of both impedances. Hence, the distribution is fairly easy to establish. On the other hand, circulating-current is much more difficult to determine! For the simple case when voltage-ratios are different, the formula in part d) can be applied! But, popularizing the famous song… “Don’ worry”… because manufacturers build to standards that try to limit resistance and reactance variations. Thus, it is unlikely (but, still possible) that circulating-currents will exist! So, when in doubt… run the numbers!

Regards, Phil Corso (cepsicon@aol.com)

Let me know how this circulating current effects power system. Especially transformer side. And how much circulating current can be permitted in parallel power system?

Regards,
Lee, Hyonwoo

Responding to Hyonwoo Lee's 12-Sep (01:38) questions, discussion requires the identification of currents, as follows:

There are three distinct currents: two are the transformer secondary load-currents, Ia and Ib, respectively; and the third, Ic, representing the circulating-current! The latter results from parameter asymmetries noted in earlier posts. And, because the subject becomes more difficult for three-phase transformers, I will discuss single-phase transformers. Also, please note, that when sums of components are involved, they must be added vectorially, and not arithmetically:

1) Supply current, Iï¿œa + Iï¿œb, (Ia + Ib, adjusted for voltage-ratio.)

2) Primary-winding currents, Iï¿œa and Iï¿œb, respectively (Ia + Ib, adjusted for voltage-ratio.)

3) Secondary-winding currents, Ia + Ic, and Ib - Ic, respectively.

4) Load current, Ia + Ib.

I realize that verbal description is difficult, so if you have a specific case in mind, contact me on or off List!

Regards, Phil Corso

How do I calculate the circulating current for the following problem:

I have 2 transformers in parallel. One 12/20 MVA 67/4.16kV, Z=7.29% and another 7.5/9.375MVA 70/4.360kV (because of how the taps are set) and Z=7.88%.

Thanks,

Mani

Responding to Mani's 27-Nov (00:33) problem... contact me for a Spread-Sheet solution.

Unfortunately, it not only contains Vector-Algebra, but Complex-Number notation as well, so the mathematically-impaired need not apply!

Regards,
Phil Corso (cepsicon@aol.com)

Naga, I want to point out an error in my last post; the two fractions 1/3 and 2/3 in item b) must be interchanged. That is, the 1/3-value belongs with the transformer whose impedance is 6%. Conversely, the 2/3-value belongs with the transformer whose impedance is 3%!

Krishna, turns-ratio and voltage-ratio are different parameters. You must use voltage-ratio, that is, the primary phase-to-phase voltage divided by the secondary phase-to-phase voltage!

Regards, Phil Corso (cepsicon@aol.com)

Krishna & Naga… if you would like an example calculation covering your case, please send specific data.

Regards, Phil Corso (Cepsicon@aol.com)

Thanks Phil very much for clearing concept. Now can you give me an idea as to why for delta-star (secy) transformer, (normally 11kV/0.433kV level), if a barrel fuse blown on primary side results one phase healthy, second phase dim, third phase no supply?

Krishna, since your question is not related to the original thread, please submit a new thread!

Phil Corso

Hey ive been reading this forum and i have this problem. There are two transformers that i want to make parallel. Both are Dyn11 transformers which is 630kVA and voltage ratios are 33kV/415kV. Both have the same tap positions and made by the same manufacturer. But one is made in year 2000 and its imp voltage is 4.91% and the other one is made in year 2006 and its imp voltage is 4.68%.

can i make this two transformers parallel?will there be significant circulating currents which will damage the transformers in long term? at the moment these two transformers supply separate loads which are unequal and the highest load so far is 580kVA from one transformer and 450kVA from the other transformer.. on average the total load is around 500 - 600kVA. Please help me phill.thanks for the above posts.

KDX, I apologize for not responding earlier. If you are still interested in an answer to your 14-Oct post, please contact me off-list!

Regards, Phil Corso (cepsicon@aol.com)

Responding to KDX's 14-Oct questions... based on the data provided:

1) The transformer with the lower impedance will carry 51% of any combined load, to 1,200-kVA!

2) The other will carry 49%!

3) Circulating current is nil!

My earlier offer to Mani applies to your situation as well.

Regards, Phil Corso (cepsicon@aol.com