Title Circulating Current in Parallel Transformers Transformer Electrical Grid Electrical Impedance Series And Parallel Circuits Electric Current 132.5 KB 4
Circulating current in parallel transformers
Same Voltage Ratio
Same Percentage Impedance
Same Polarity
Same Phase Sequence

##### Document Text Contents
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Circulating current in parallel transformers

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

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.

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.

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- Whenever 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.

1 L. F. Blume & A. Boyajian, Transformer Connections, Chapter VII (Schnectady: General
Electric, 1940).

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Unequal
impedances—
Unequal ratios—
Different kVA
No Yes Yes Yes No

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Same Voltage Ratio

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