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!
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.
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 others 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't 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!
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!
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?
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?
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:
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.
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