circulating current in parallel transformers

7/24/2019 Circulating Current in Parallel Transformers

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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. 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, !ut now it will !e superimposed on the load current in each transformer, i.e. for a transformer

producing circulating current, this will !e added to its load current, and for a transformer receiving

circulating current, this will !e su!tracted from its load current.

Thus voltage control of parallel transformers with the circulating current method aims to minimi"e the

circulating current while #eeping the voltage at the target value.

$n case of a parallel operation of transformers, the electric current carried !y these transformers are

inversely proportional to their internal impedance. Thin# of it as two parallel impedances in a simple

circuit !ehind a voltage source, you will have equal currents through each impedance only if you have

two identical impedances, in some cases as stated a!ove, tapping could !e a pro!lem, the other one

is the actual manufacturing tolerances which could diverge !y almost %-&', if the transformers are

manufactured !y different suppliers or not within the same !atch. o, the difference in current!etween the currents through these two impedances is !asically the circulating current as it is not

seen outside these parallel impedances.

The currents that are produces due to magnetic flu* circulation in the core are called eddy currents

and these eddy currents are responsi!le for core losses in transformer.

While the circulating currents are the "ero sequence currents that may !e produces due to following

causes.

&- when there is three phase transformer the (+rd, %th, th....) harmonic currents which are called "ero

sequence currents from all the three winding of three phase transformer add up and !ecome

considera!le even in loaded conditions these currents have no path in connection of transformer

so a tertiary winding is provided co conduct these currents !ut in d or /y connection these currents

circulate in delta winding.

0- Whenever there is un!alanced loading in transformer. $n which with positive sequence, negative

sequence and "ero sequence currents are also produced which cause circulating currents.

+- When the transformer !an#s are used and the transformers have phase !etween them then

circulating currents are produced !etween them, than transformers in the !an# get loaded without

!eing shearing the power to the load.

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

Electric, 1940).


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Table 1: Overall Connection SummaryTransformerParallelConnection

Types EqualLoadingUnequalLoadingOverloadingConcernsCirculatingCurrentsecommended

ConnectionEqual impedances

Equal ratios

Same kVA

Yes No No No Yes

Equal impedances

Equal ratios

Different kVA

No Yes No No Yes

Unequal

impedances

Equal ratios

Same kVA

No Yes Yes No No

Unequal

impedancesEqual ratios

Different kVA

No Yes Yes No No

Unequal

impedances

Unequal ratios

Same kVA

Yes No Yes Yes No


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Unequal

impedances

Unequal ratios

Different kVANo Yes Yes Yes No

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If two transformers of different voltageratio are connected in parallel with sameprimary supply voltage, there will be a difference in secondary voltages. Now say

the secondary of these transformers are connected to same bus, there will be acirculating current between secondaries and therefore between primaries also. As

the internal impedance of transformer is small, a small voltage difference maycause sufficiently high circulating current causing unnecessary extra I2R loss.

Same Percentage Impedance

The current shared by two transformers running in parallel should be proportional

to their !A ratings. Again, currentcarried by these transformers are inverselyproportional to their internal impedance. "rom these two statements it can be said

that, impedance of transformersrunning in parallel are inversely proportional to

their !A ratings. In other words, percentage impedance or per unit values ofimpedance should be identical for all the transformers that run in parallel.

Same Polarity

#olarity of all transformers that run in parallel, should be the same otherwise huge

circulating current that flows in the transformer but no load will be fed from thesetransformers. #olarity of transformer means the instantaneous direction of induced

emf in secondary. If the instantaneous directions of induced secondary emf in two

transformers are opposite to each other when same input power is fed to both of thetransformers, the transformers are said to be in opposite polarity. If theinstantaneous directions of induced secondary emf in two transformers are same

when same input power is fed to the both of the transformers, the transformers aresaid to be in same polarity.

Same Phase Sequence

The phase se$uence or the order in which the phases reach their maximum positive

voltage, must be identical for two parallel transformers. %therwise, during thecycle, each pair of phases will be short circuited.

The above said conditions must be strictly followed for parallel operation of

transformersbut totally identical percentage impedance of two different

transformers is difficult to achieve practically, that is why the transformers run in

parallel may not have exactly same percentage impedance but the values would be

as nearer as possible
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circulating current in parallel transformers

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