what does mean by circulating current in a transformer? why its produced? plz explain

.Refer to any standard text on transformers for discussion of transformer internal impedances, magnetic characteristics of the transformer core (reluctance, permeability, etc.) The circulating currents are created by the mutual fluxes between the primary and secondary windings of the transformer.

Hi Nageen,
From my understanding and experience, the circulating current in a transformer is a current flowing internally which is there when the transformer energized. It will be generated by unbalanced three phase voltage or the phase angle between two phases is not 120 degree. When this things happened, the total voltage (three phase voltages plus together) will not be zero, then the current occurs, expeciall in dalta connection.
Another situation is when two transformers (with different impedances or ratio differences) operate in parallel through busbar, then it could cause the circulating current between these two transformers as well.

There are magnetic mutual fluxes that circulate within the core of the transformer. The electrical flux in the primary and secondary coils induces the respective electric currents that can be measured on the transformer primary or secondary terminals. The magnetic circuits within the core are one of the primary parameters that contribute to the heat gain within the core. Depending on how the sections of the core are laminated and the design of the transformer (including its materials), the amount of flux and heat gain can be varied in the design. The magnetic flux is analogous to the electric current.

For instrument transformers used for current measurement input to protective devices, the unused CTs must always be provided with shorting blocks, to prevent an open voltage on the secondary side of the CT.
Refer to a textbook on the basics of transformers and electrical machinery. It will go into much greater detail, including the differences in DC motors, synchronous AC motors and induction motors (and the different types of motor construction.

Dear ;

First, I agreed with what Mr. George said.

Second, there's a circulating current, bigger than that's generated by mutual fluxes, flows in transformer's primary if the following conditions are all realized :
- The transformer's connection is " Dy... " ( Delta for primary & Star for secondary )
- The " Multiple Third Harmonics " ( 3, 9, 15, ... ) are existed in LV side
As these Harmonics are " Zero sequence " the sum of them at primary side is " simple summing ", and they circulate in the primary's coils

If there is a percentage impedance mismatching in the system, the circulating currents are highly in chance to develop. By mismatching I mean that of equal values attached together. These circulating currents originate from the high impedance to the lower one & may cause heating & excessive losses in the system.

Quite often the term "Circulating current" between parallel transformers is simply a way to describe the difference in load that will be supplied by transformers that are not identical.
For some reason we do not use the same term to describe the difference in current between parallel resistors of different resistance. If we have a 2Ω resistor in parallel wit a 1Ω resistor, and 2V across them, the one would carry 2A and the other 1A, so we could just as well claim that there is a 1/3A "circulating current" between them. This could apply even better if we have impedances that are not purely resistive? I wonder if we should use the term "circulating voltage", if we have different impedances connected in series?

I assume that you are talking about circulating current between two (or more) transformers operating in parallel. Here comes short explanation:

When two transformers are in a parallel group, a transformer with a higher tap position will typically have a higher (LV side) no-­load voltage than the other one with a lower tap position. These unequal no­-load voltages (unequal tap positions) will cause a circulating current to flow through the parallel connected transformers. A transformer with higher no-­load voltage (typically higher tap position) will produce circulating current, while a transformer with lower no-­load voltage (typically lower tap position) will receive circulating current.
When load is connected on these two parallel transformers, the circulating current will remain the same, but now it will be superimposed on the load current in each transformer, i.e. for a transformer producing circulating current, this will be added to its load current, and for a transformer receiving circulating current, this will be subtracted from its load current.
Thus voltage control of parallel transformers with the circulating current method aims to minimize the circulating current while keeping the voltage at the target value.

Following up to Zoran's explanation above, in case of a parallel operation of transformers, the electric current carried by these transformers are inversely proportional to their internal impedance. Think of it as two parallel impedances in a simple circuit behind a voltage source, you will have equal currents through each impedance only if you have two identical impedances, in some cases as stated above, tapping could be a problem, the other one is the actual manufacturing tolerances which could diverge by almost 5-10%, if the transformers are manufactured by different suppliers or not within the same batch. So, the difference in current between the currents through these two impedances is basically the circulating current as it is not seen outside these parallel impedances.

Current can only circulate in closed circuit... So, current can only circulate in cases if ends of secondary winding are short connected or if they are connected with consumer...
Also, for initializing of circulation of current, you need to have difference of potential, that is voltage, in mentioned circuit. So, you need to have inducted voltage between ends of secondary winding which is a consequence of electromagnetic connection between primary and secondary winding. Of course, you need to connect ends of primary winding under voltage due of circulation of current in circuit of primary winding, which produces electromagnetic field between primary and secondary winding using electromagnetic induction.

The initial question was about circulating currents within a single transformer. There are magnetic currents that circulate between the primary and secondary of the primary due to the mutual flux. Refer to my preceding comment. Most textbooks on magnetics , transformers and machinery will explain in detail the theory and the mathematics of electric and magnetic fluxes and corresponding currents that are created.. Thy also discuss the BH curve that reflects the magnetic characteristics of the respective transformer

multiple core to ground connection also produces circulating current for power transformers, one possible reason to consider

The currents that are produces due to magnetic flux circulation in the core are called eddy currents and these eddy currents are responsible for core losses in transformer.
while the circulating currents are the zero sequence currents that may be produces due to following causes.
1- when there is three phase transformer the (3rd,5th,7th....) harmonic currents which are called zero sequence currents from all the three winding of three phase transformer add up and become considerable even in loaded conditions these currents have no path in Y/Y connection of transformer so a tertiary winding is provided co conduct these currents but in Y/d or D/y connection these currents circulate in delta winding.
2- when ever there is unbalanced loading in transformer. In which with positive sequence, negative sequence and zero sequence currents are also produced which cause circulating currents.
3- when the transformer banks are used and the transformers have phase between them then circulating currents are produced between them, than transformers in the bank get loaded without being shearing the power to the load.

Dear Mr. Shoaib ;

I'm very sorry, but you mentioned in paragraph no. 1 that : " 3rd, 5th, 7th " harmonics currents are called zero sequence, but please note :
- Only the Harmonics' orders " 3rd, 9th, 15th, ... " are " Zero Sequence "
- The " 5th " Harmonic is " Negative Sequence "
- The " 7th " Harmonic is " Positive Sequence "

yes dear I am sorry you are right 5th and 7th harmonics are not zero sequence

Just to elaborate one thing
Its not necessary to have the parallel conditioning of transformers for circulating currents.
If you have a three phase Transformer bank consisting of Single phase Transformers, even then circulating currents flow if 
there is a mismatch of Turn ratios on all three phases.