Vector Groups of Transformers

8/13/2019 Vector Groups of Transformers

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Vector Group of Transformer

Introduction:

Three phase transformer consists of three sets of primary windings, one for eachphase, and three sets of secondary windings wound on the same iron core. Separatesingle-phase transformers can be used and externally interconnected to yield thesame results as a 3-phase unit.

The primary windings are connected in one of several ways. The two most commonconfigurations are the delta, in which the polarity end of one winding is connected tothe non-polarity end of the next, and the star, in which all three non-polarities orpolarity! ends are connected together. The secondary windings are connectedsimilarly. This means that a 3-phase transformer can have its primary and secondarywindings connected the same delta-delta or star-star!, or differently delta-star orstar-delta!.

It"s important to remember that the secondary voltage waveforms are in phase with

the primary waveforms when the primary and secondary windings are connected thesame way. This condition is called #no phase shift.$ %ut when the primary andsecondary windings are connected differently, the secondary voltage waveforms willdiffer from the corresponding primary voltage waveforms by 3& electrical degrees.This is called a 3& degree phase shift. 'hen two transformers are connected inparallel, their phase shifts must be identical( if not, a short circuit will occur when thetransformers are energi)ed.$

Basic Idea of Winding:

• *n ac voltage applied to a coil will induce a voltage in a second coil where thetwo are lin+ed by a magnetic path. The phase relationship of the two voltages

depends upon which ways round the coils are connected. The voltages willeither be in-phase or displaced by & deg• 'hen 3 coils are used in a 3 phase transformer winding a number of options

exist. The coil voltages can be in phase or displaced as above with the coilsconnected in star or delta and, in the case of a star winding, have the starpoint neutral! brought out to an external terminal or not.

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Six Ways to wire Star Winding:

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Six Ways to wire Delta Winding:

olarity:

• *n ac voltage applied to a coil will induce a voltage in a second coil where thetwo are lin+ed by a magnetic path. The phase relationship of the twovoltages depends upon which way round the coils are connected. Thevoltages will either be in-phase or displaced by & deg.

• 'hen 3 coils are used in a 3 phase transformer winding a number of optionsexist. The coil voltages can be in phase or displaced as above with the coilsconnected in star or delta and, in the case of a star winding, have the starpoint neutral! brought out to an external terminal or not.

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• 'hen air of /oil of Transformer have same direction than voltage induced in

both coil are in same direction from one end to other end.• 'hen two coil have opposite winding direction than 0oltage induced in both

coil are in opposite direction.

'inding connection designations:

• First Symbol: for ig! Voltage : *lways capital letters.• 121elta, S2Star, 2Interconnected star, 424eutral• Second Symbol: for "ow #oltage : *lways Small letters.• d21elta, s2Star, )2Interconnected star, n24eutral.• T!ird Symbol: hase displacement expressed as the cloc+ hour number

,5, !• $xample % Dyn&&

Transformer has a delta connected primary winding D ! a star connectedsecondary y ! with the star point brought out n ! and a phase shift of 3& degleading &&!.

• The point of confusion is occurring in notation in a step-up transformer. *sthe I6/5&&75- standard has stated, the notation is 80-90 in se uence. ;orexample, a step-up transformer with a delta-connected primary, and star-connected secondary, is not written as , but . The indicatesthe 90 winding leads the 80 by 3& degrees.

•

Transformers built to *4SI standards usually do not have the vector groupshown on their nameplate and instead a vector diagram is given to show therelationship between the primary and other windings.

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0ector ?roup of Transformer:

• The three phase transformer windings can be connected several ways. %ased

on the windings" connection, the vector group of the transformer isdetermined.

• The transformer vector group is indicated on the 4ame late of transformerby the manufacturer.The vector group indicates the phase difference between the primary andsecondary sides, introduced due to that particular configuration oftransformer windings connection.

• The 1etermination of vector group of transformers is very important beforeconnecting two or more transformers in parallel. If two transformers ofdifferent vector groups are connected in parallel then phase difference existbetween the secondary of the transformers and large circulating current flowsbetween the two transformers which is very detrimental.

hase 1isplacement between 80 and 90 'indings:

• The vector for the high voltage winding is ta+en as the reference vector.1isplacement of the vectors of other windings from the reference vector, withanticloc+wise rotation, is represented by the use of cloc+ hour figure.

• IS: @&@5 art 0!- A77 gives @5 sets of connections star-star, star-delta,and star )ig)ag, delta-delta, delta star, delta-)ig)ag, )ig)ag star, )ig)ag-delta.1isplacement of the low voltage winding vector varies from )ero to -33&B insteps of -3&B, depending on the method of connections.

• 8ardly any power system adopts such a large variety of connections. Some ofthe commonly used connections with phase displacement of &, -3&&, - &Cand -33&B cloc+-hour setting &, , 5 and !.

• Symbol for the high voltage winding comes first, followed by the symbols ofwindings in diminishing se uence of voltage. ;or example a @@&D55D +0Transformer connected star, star and delta and vectors of 55 and +0windings having phase displacement of &B and -33&B with the reference @@&+0! vector will be represented *s 'y( % 'd&& .

• The digits &, , etc! relate to the phase displacement between the 80 and90 windings using a cloc+ face notation. The phasor representing the 80winding is ta+en as reference and set at @ o"cloc+. hase rotation is alwaysanti-cloc+wise. International adopted!.

• Ese the hour indicator as the indicating phase displacement angle. %ecausethere are @ hours on a cloc+, and a circle consists out of 35&B, each hourrepresents 3&B.Thus 2 3&B, @ 2 5&B, 3 2 A&B, 5 2 &B and @ 2 &B or35&B.

• The minute hand is set on @ o"cloc+ and replaces the line to neutral voltagesometimes imaginary! of the 80 winding. This position is always thereference point.

• 6xample:• 1igit & 2&B that the 90 phasor is in phase with the 80 phasor

1igit 23&B lagging 90 lags 80 with 3&B! because rotation is anti-cloc+wise.• 1igit 2 33&B lagging or 3&B leading 90 leads 80 with 3&B!


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• 1igit F 2 F&B lagging 90 lags 80 with F&B!• 1igit 5 2 &B lagging 90 lags 80 with &B!• 'hen transformers are operated in parallel it is important that any phase shift

is the same through each. aralleling typically occurs when transformers arelocated at one site and connected to a common bus bar ban+ed! or locatedat different sites with the secondary terminals connected via distribution ortransmission circuits consisting of cables and overhead lines.

)!ase S!ift*Deg+

,onnection

& =y& 1d& 1)&

3& lag =d 1y =)

5& lag 1d@ 1)@

@& lag 1dG 1)G

F& lag =dF 1yF =)F

& lag =y5 1d5 1)5

F& lead =d7 1y7 =)7

@& lead 1d 1)

5& lead 1d & 1) &

3& lead =d 1y =)

• The phase-bushings on a three phase transformer are mar+ed either *%/,

E0' or @3 80-side capital, 90-side small letters!. Two winding, three phasetransformers can be divided into four main categories

Group -.cloc/ T,?roup I & o"cloc+, &B deltaDdelta, starDstar?roup II 5 o"cloc+, &B deltaDdelta, starDstar?roup III o"cloc+, -3&B starDdelta, deltaDstar?roup I0 o"cloc+, H3&B starDdelta, deltaDstarinus indicates 90 lagging 80, plus indicates 90leading 80


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/loc+ 4otation: &

/loc+ 4otation :

/loc+ 4otation: @

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/loc+ 4otation: G

/loc+ 4otation: F

/loc+ 4otation: 5

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/loc+ 4otation: 7

/loc+ 4otation:

oints to be consider while Selecting of 0ector ?roup:

• 0ector ?roups are the I6/ method of categori)ing the primary and secondarywinding configurations of 3-phase transformers. 'indings can be connectedas delta, star, or interconnected-star )ig)ag!. 'inding polarity is alsoimportant, since reversing the connections across a set of windings affects thephase-shift between primary and secondary. 0ector groups identify thewinding connections and polarities of the primary and secondary. ;rom avector group one can determine the phase-shift between primary andsecondary.

• Transformer vector group depends upon1. 0emo#ing !armonics: 1y connection J y winding nullifies 3rd

harmonics, preventing it to be reflected on delta side.

2. )arallel operations: *ll the transformers should have same vectorgroup K polarity of the winding.

3. $art! fault 0elay: * 1d transformer does not have neutral. to restrictthe earth faults in such systems, we may use )ig )ag woundtransformer to create a neutral along with the earth fault relay..

G. Type of 4on 9iner 9oad: systems having different types of harmonics Knon linear Types of loads e.g. furnace heaters ,0;1S etc for that wemay use 1yn , 1yn@ , 1yn3 configuration, wherein, 3& deg. shiftsof voltages nullifies the 3rd harmonics to )ero in the supply system.

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5. Type of Transformer 1pplication: ?enerally for ower exporttransformer i.e. generator side is connected in delta and load side isconnected in star. ;or ower export import transformers i.e. inTransmission urpose Transformer star star connection may bepreferred by some since this avoids a grounding transformer ongenerator side and perhaps save on neutral insulation. ost of

systems are running in this configuration. ay be less harmful thanoperating delta system incorrectly. =d or 1y connection is standard forall unit connected generators.

5. There are a number of factors associated with transformer connectionsand may be useful in designing a system, and the application of thefactors therefore determines the best selection of transformers. ;orexample:

For selecting Star ,onnection:

• * star connection presents a neutral. If the transformer also includes a deltawinding, that neutral will be stable and can be grounded to become a

reference for the system. * transformer with a star winding that does 4LTinclude a delta does not present a stable neutral.• Star-star transformers are used if there is a re uirement to avoid a 3&deg

phase shift, if there is a desire to construct the three-phase transformer ban+from single-phase transformers, or if the transformer is going to be switchedon a single-pole basis ie, one phase at a time!, perhaps using manualswitches.

• Star-star transformers are typically found in distribution applications, or inlarge si)es interconnecting high-voltage transmission systems. Some star-startransformers are e uipped with a third winding connected in delta to stabili)ethe neutral.

For selecting Delta ,onnection:• * delta connection introduces a 3& electrical degree phase shift.• * delta connection


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• rovision of a neutral earth point or points, where the neutral is referred toearth either directly or through impedance. Transformers are used to give theneutral point in the maMority of systems. The star or interconnected star !winding configurations give a neutral location. If for various reasons, onlydelta windings are used at a particular voltage level on a particular system, aneutral point can still be provided by a purpose-made transformer called a


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• Star point facilitates mixed loading of three phase and single phase consumerconnections.

• The delta winding carry third harmonics and stabili)es star point potential.• * delta-Star connection is used for step-up generating stations. If 80 winding

is star connected there will be saving in cost of insulation.• %ut delta connected 80 winding is common in distribution networ+, for

feeding motors and lighting loads from 90 side.

@! Star-Star =y& or =y5!

• ainly used for large system tie-up Transformer.• ost economical connection in 80 power system to interconnect between two

delta systems and to provide neutral for grounding both of them.• Tertiary winding stabili)es the neutral conditions. In star connected

transformers, load can be connected between line and neutral, only ifa! the source side transformers is delta connected orb! the source side is star connected with neutral connected bac+ to thesource neutral.

• In This Transformers. Insulation cost is highly reduced. 4eutral wire canpermit mixed loading.

• Triple harmonics are absent in the lines. These triple harmonic currentscannot flow, unless there is a neutral wire. This connection producesoscillating neutral.

• Three phase shell type units have large triple harmonic phase voltage.8owever three phase core type transformers wor+ satisfactorily.

• * tertiary mesh connected winding may be re uired to stabili)e the oscillatingneutral due to third harmonics in three phase ban+s.

3! 1elta J 1elta 1d & or 1d 5!

• This is an economical connection for large low voltage transformers.• 9arge unbalance of load can be met without difficulty.• 1elta permits a circulating path for triple harmonics thus attenuates the

same.• It is possible to operate with one transformer removed in open delta or$ 0$

connection meeting F percent of the balanced load.• Three phase units cannot have this facility. ixed single phase loading is not

possible due to the absence of neutral.

G! Star- ig-)ag or 1elta- ig-)ag =) or 1)!

• These connections are employed where delta connections are wea+.Interconnection of phases in )ig)ag winding effects a reduction of thirdharmonic voltages and at the same time permits unbalanced loading.

• This connection may be used with either delta connected or star connectedwinding either for step-up or step-down transformers. In either case, the)ig)ag winding produces the same angular displacement as a delta winding,and at the same time provides a neutral for earthing purposes.

• The amount of copper re uired from a )ig)ag winding in FN more than acorresponding star or delta winding. This is extensively used for earthingtransformer.


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• The conventional method is to connect the red phase on *Da, =ellow phase on%Db, and the %lue phase on /Dc.

• Lther phase displacements are possible with unconventional connections forinstance red on b, yellow on c and blue on a! %y doing some unconventionalconnections externally on one side of the Transformer, an internal connected1d& transformer can be changed either to a 1dG - @&B! or 1d H @&B!

connection. The same is true for internal connected 1dG or 1d transformers.

*5+ Group II: 6xample: 1d5 &B displacement between 80 and 90!.

• %y doing some unconventional connections externally on one side of theTransformer, an internal connected 1d5 transformer can be changed either toa 1d@ -5&B! or 1d & H5&B! connection.

*6+ Group III: 6xample: 1yn -3&B displacement between 80 and 90!.

• %y doing some unconventional connections externally on one side of theTransformer, an internal connected 1yn transformer can be changed eitherto a 1ynF - F&B! or 1ynA HA&B! connection.

*7+ Group IV: 6xample: 1yn H3&B displacement between 80 and 90!.

• %y doing some unconventional connections externally on one side of theTransformer, an internal connected 1yn transformer can be changed eitherto a 1yn7 H F&B! or 1yn3 -A&B! connection.

)oint to be remembered:

• For Group2III 8 Group2IV: %y doing some unconventional connectionsexternally on both sides of the Transformer, an internal connected ?roup-IIIor ?roup-I0 transformer can be changed to any of these two groups.

• Thus by doing external changes on both sides of the Transformer an internalconnected 1yn transformer can be changed to either a: 1yn3, 1ynF, 1yn7,1ynA or 1yn transformer, This is Must true for starDdelta or deltaDstarconnections.

• For Group2I 8 Group2II: /hanges for deltaDdelta or starDstar transformersbetween ?roup-I and ?roup-III can Must be done internally.

'hy 3&Bphase shift occur in star-delta transformer between primary and secondaryO

• The phase shift is a natural conse uence of the delta connection. The currentsentering or leaving the star winding of the transformer are in phase with thecurrents in the star windings. Therefore, the currents in the delta windingsare also in phase with the currents in the star windings and obviously, thethree currents are @& electrical degrees apart.

• %ut the currents entering or leaving the transformer on the delta side areformed at the point where two of the windings comprising the delta cometogether J each of those currents is the phasor sum of the currents in theadMacent windings.


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• 'hen you add together two currents that are @& electrical degrees apart,the sum is inevitably shifted by 3& degrees.

• The ain reason for this phenomenon is that the phase voltage lags linecurrent by 3&degrees.consider a deltaDstar transformer. The phase voltages inthree phases of both primary and secondary. you will find that in primary thephase voltage and line voltages are same, let it be 0P= ta+e one phase!.but,the corresponding secondary will have the phase voltage only in its phasewinding as it is star connected. the line voltage of star connected secondaryand delta connected primary won"t have any phase differences between them.so this can be summari)ed that #the phase shift is associated with the waveforms of the three phase windings.

'hy when ?enerating Transformer is =d than 1istribution Transformer is 1y :

• This is the 80 Side or the Switchyard side of the ?enerator Transformer isconnected in 1elta and the 90 Side or the generator side of the ?T isconnected in Star, with the Star side neutral brought out.

• The 90 side voltage will #lag$ the 80 side voltage by 3& degrees.• Thus, in a generating station we create a 3& degrees lagging voltage for

transmission, with respect to the generator voltage.• *s we have created a 3& degrees lagging connection in the generating

station, it is advisable to create a 3& degrees leading connection indistribution so that the user voltage is #in phase$ with the generated voltage.*nd, as the transmission side is 1elta and the user might need three phase,four-wire in the 90 side for his single phase loads, the distribution transformeris chosen as 1yn .

• There is magnetic coupling between 8T and 9T. 'hen the load side 9T!suffers some dip the 9T current try to go out of phase with 8T current, so 3&degree phase shift in 1yn- +eeps the two currents in phase when there isdip.

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• So the vector group at the generating station is important while selectingdistribution Transformer.

0ector ?roup in ?enerating-Transmission-1istribution System:

• ?enerating T/ is =d transmitted power at G&&Q0, for G&&Q0 to @@&Q0 =y isused and by using 'd between e.g. @@& and 55 +0, then Dy from 55 to +0so that their phase shifts can be cancelled out. *nd for 90 G&&D@3&0!supplies at F& 8) are usually 3 phase, earthed neutral, so a #1yn$ 90 windingis needed. 8ere ?T side -3&lag =d ! can be nullify H3& by using distributionTransformer of 1y .

• * reason for using 'd between e.g. @@& and 55 +0, then Dy from 55 to +0is that their phase shifts can cancel out and It is then also possible to parallela @@&D +0 == transformer, at +0, with the 55D +0 a == transformeroften has a third, delta, winding to reduce harmonics!. If one went 1y J1y from @@& to +0, there would be a 5& degree shift, which is notpossible in one transformer. The #standard$ transformer groups in distributionavoid that +ind of limitation, as a result of thought and experience leading to

lowest cost over many years.

?enerator T/ is =d , /an we use 1istribution T/ 1yF instead of 1y .

• 'ith regards to theory, there are no special advantages of 1yn over 1ynF.• In Isolation 1pplication: In isolated applications there is no advantage or

disadvantage by using 1yF or 1y . If however we wish to interconnect thesecondary sides of different 1ny transformers, we must have compatibletransformers, and that can be achieved if you have a 1yn among a groupof 1ynF>s and vice versa.

• In )arallel ,onnection: ractically, the relative places of the phases remainsame in 1yn compared to 1ynF.

•

If we use =d Transformer on ?enerating Side and 1istribution side 1ytransformer than -3& lag of generating side =d ! is nullify by H3& 9ead atPeceiving side 1y ! so no phase difference respect to generating Side and ifwe are on the 80 side of the Transformer, and if we denote the phases as P-=-% from left to right, the same phases on the 90 side will be P- = -%, butfrom left to Pight.

• This will ma+e the Transmission lines have same color for identification!whether it is input to or output from the Transformer.

• If we use =d Transformer on ?enerating Side and 1istribution side 1yFtransformer than -3& lag of generating side =d ! is more lag by - F& 9ag atPeceiving side 1yF! so Total phase difference respect to generating Side is& deg -3&H- F&2- &! and if we are on the 80 side of the Transformer,

and if we denote the phases as P- =-% from left to right, the same phases onthe 90 side will be P- = -%, but from Pight to 9eft.

• This will ma+e the Transmission lines have 4o same color for identification!whether it is input to or output from the Transformer.

• The difference in output between the 1yn and 1nyF and is therefore &degrees.

Documents

Vector Groups of Transformers