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Auto Transformer Connection

(7) Auto Transformer Connection:

An Ordinary Transformer consists of two windings called primary winding andsecondary winding. These two windings are magnetically coupled andelectrically isolated. But the transformer in which a part of windings iscommon to both primary and secondary is called Auto Transformer.

n Auto Transformer two windings are not only magnetically coupled but alsoelectrically coupled. The input to the transformer is constant but the outputcan be !aried by !arying the tapings.

The autotransformer is both the most simple and the most fascinating of theconnections in!ol!ing two windings. t is used "uite e#tensi!ely in bul$ powertransmission systems because of its ability to multiply the effecti!e %&Acapacity of a transformer. Autotransformers are also used on radialdistribution feeder circuits as !oltage regulators. The connection is shown in

'igure

The primary and secondary windings of a two winding transformer ha!einduced emf in them due to a common mutual flu# and hence are in phase.The currents drawn by these two windings are out of phase by *+. Thisprompted the use of a part of the primary as secondary. This is e"ui!alent tocommon the secondary turns into primary turns.

The common section need to ha!e a cross sectional area of the conductor tocarry (,-) ampere.

Total number of turns between A and C are T. At point B a connection ista$en. ection AB has T, turns. As the !olts per turn/ which is proportional tothe flu# in the machine/ is the same for the whole winding/ & : &, 0 T : T,

1hen the secondary winding deli!ers a load current of , Ampere thedemagneti2ing ampere turns is ,T,. This will be countered by a current flowing from the source through the T turns such that/ T 0 ,T,

A current of ampere flows through the winding between B and C. Thecurrent in the winding between A and B is (, - ) ampere. The cross sectionof the wire to be selected for AB is proportional to this current assuming aconstant current density for the whole winding. Thus some amount ofmaterial sa!ing can be achie!ed compared to a two winding transformer. The
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magnetic circuit is assumed to be identical and hence there is no sa!ing in thesame. To "uantify the sa!ing the total "uantity of copper used in an autotransformer is e#pressed as a fraction of that used in a two windingtransformer As

copper in auto transformer 3 copper in two winding transformer 0((T -T,) 4 T,(, - ))3T 4 T,,

copper in auto transformer 3 copper in two winding transformer 0 5(,T, 3(T 4 T,,))

But T 0 T,, so The 6atio 0 5(,T, 3 ,T) 0 5(T,3T) This means that an auto transformer requires the use of lesser quantity

of copper given by the ratio of turns. This ratio therefore the savingsin copper.

As the space for the second winding need not be there/ the window space canbe less for an auto transformer/ gi!ing some sa!ing in the lamination weightalso. The larger the ratio of the !oltages/ smaller is the sa!ings. As T,approaches T the sa!ings become significant. Thus auto transformersbecome ideal choice for close ratio transformations.

The auto transformer shown in 'igure is connected as a boosting autotransformer because the series winding boosts the output !oltage. Care mustbe e#ercised when discussing primary88 and secondary88 !oltages inrelationship to windings in an auto transformer.

n two9winding transformers/ the primary !oltage is associated with theprimary winding/ the secondary !oltage is associated with the secondarywinding/ and the primary !oltage is normally considered to be greater thanthe secondary !oltage. n the case of a boosting autotransformer/ howe!er/the primary (or high) !oltage is associated with the series winding/ and thesecondary (or low) !oltage is associated with the common winding but the!oltage across the common winding is higher than across the series winding.

;imitation of the autotransformer

One of the limitations of the autotransformer connection is that not all typesof three9phase connections are possible. 'or e#ample/ the


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windings/ so the neutrals of the circuits connected to these windings cannotbe isolated.

A sing an autotransformer connection pro!ides an opportunity for achie!ing

lower series impedances and better regulation. ts efficiency is more whencompared with the con!entional one.

ts si2e is relati!ely !ery smaller. &oltage regulation of autotransformer is much better. ;ower cost ;ow re"uirements of e#citation current. ;ess copper is used in its design and construction. n con!entional transformer the !oltage step up or step down !alue is fi#ed

while in autotransformer/ we can !ary the output !oltage as per outre"uirements and can smoothly increase or decrease its !alue as per ourre"uirement.

?isad!antages of the autotransformer:

The autotransformer connection is not a!ailable with certain three9phaseconnections.

@igher (and possibly more damaging) short9circuit currents can result from alower series impedance.

hort circuits can impress !oltages significantly higher than operating!oltages across the windings of an autotransformer.

'or the same !oltage surge at the line terminals/ the impressed and induced!oltages are greater for an autotransformer than for a two9windingtransformer.

Autotransformer consists of a single winding around an iron core/ whichcreates a change in !oltage from one end to the other. n other words/ theself9inductance of the winding around the core changes the !oltage potential/but there is no isolation of the high and low !oltage ends of the winding. oany noise or other !oltage anomaly coming in on one side is passed through

to the other. 'or that reason/ Autotransformers are typically only used wherethere is already some sort of filtering or conditioning ahead of it/ as inelectronic applications/ or the downstream de!ice is unaffected by thoseanomalies/ such as an AC motor during starting

Applications:

>sed in both ynchronous motors and induction motors.


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>sed in electrical apparatus testing labs since the !oltage can be smoothlyand continuously !aried.

They find application as boosters in AC feeders to increase the !oltage le!els. Used in HV Substation due to following reasons.

. f we use normal transformer the si2e of transformer will be !ery high which

leads to hea!y weight/ more copper and high cost.,. The tertiary winding used in Auto transformer balances single phase

unbalanced loads connected to secondary and it does not pass on theseunbalanced currents to rimary side. @ence @armonics and !oltage unbalanceare eliminated.

. Tertiary winding in the Auto Transformer balances amp turns so that Autotransformer achie!es magnetic separation li$e two winding transformers.

!" Scott#T Connection of Transformer$

Transforming hase to , hase:

There are two main reasons for the need to transform from three phases totwo phases/

. To gi!e a supply to an e#isting two phase system from a three phase supply.,. To supply two phase furnace transformers from a three phase source.

Two9phase systems can ha!e 9wire/ 9wire/ or D9wire circuits. t is needed tobe considering that a two9phase system is not ,3 of a three9phase system.Balanced three9wire/ two9phase circuits ha!e two phase wires/ both carryingappro#imately the same amount of current/ with a neutral wire carrying .times the currents in the phase wires. The phase9to9neutral !oltages are E*F

out of phase with each other. Two phase 9wire circuits are essentially Gust two ungrounded single9phase

circuits that are electrically E*F out of phase with each other. Two phase D9wire circuits ha!e four phase wires plus a neutral the four phase wires areE*F out of phase with each other.
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The easiest way to transform three9phase !oltages into two9phase !oltages iswith two con!entional single9phase transformers. The first transformer isconnected phase9to9neutral on the primary (three9phase) side and the secondtransformer is connected between the other two phases on the primary side.

The secondary windings of the two transformers are then connected to thetwo9phase circuit. The phase9to9neutral primary !oltage is E*F out of phase

with the phase9to9phase primary !oltage/ producing a two9phase !oltageacross the secondary windings. This simple connection/ called the Tconnection/ is shown in 'igure

The main ad!antage of the T connection is that it uses transformers withstandard primary and secondary !oltages. The disad!antage of the Tconnection is that a balanced two9phase load still produces unbalanced three9phase currents i.e./ the phase currents in the three9phase system do notha!e e"ual magnitudes/ their phase angles are not ,*F apart/ and there is aconsiderable amount of neutral current that must be returned to the source.

The cott Connection of Transformer:

A cott9T transformer (also called a cott connection) is a type of circuit usedto deri!e two9phase power from a three9phase source or !ice9!ersa. Thecott connection e!enly distributes a balanced load between the phases of thesource.

cott T Transformers re"uire a three phase power input and pro!ide twoe"ual single phase outputs called Hain and Teaser. The HAI and Teaseroutputs are E* degrees out of phase. The HAI and the Teaser outputs mustnot be connected in parallel or in series as it creates a !ector currentimbalance on the primary side.

HAI and Teaser outputs are on separate cores. An e#ternal Gumper is alsore"uired to connect the primary side of the HAI and Teaser sections.

The schematic of a typical cott T Transformer is shown below:

cott T Transformer is built with two single phase transformers of e"ualpower rating. The HAI and Teaser sections can be enclosed in a floor mount

enclosure with HAI on the bottom and Teaser on top with a connectingGumper cable. They can also be placed side by side in separate enclosures. Assuming the desired !oltage is the same on the two and three phase sides/

the cott9T transformer connection consists of a center9tapped : ratio maintransformer/ T/ and an J.JK (*.DL) ratio teaser transformer/ T,. Thecenter9tapped side of T is connected between two of the phases on thethree9phase side. ts center tap then connects to one end of the lower turncount side of T,/ the other end connects to the remaining phase. The other


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side of the transformers then connects directly to the two pairs of a two9phase four9wire system.

The cott9T transformer connection may be also used in a bac$ to bac$ T to T

arrangement for a three9phase to phase connection. This is a cost sa!ing inthe smaller $&A transformers due to the , coil T connected to a secondary ,coil T in9lieu of the traditional three9coil primary to three9coil secondarytransformer. n this arrangement the Ieutral tap is part way up on thesecondary teaser transformer . The !oltage stability of this T to Tarrangement as compared to the traditional coil primary to three9coilsecondary transformer is "uestioned

%ey oint:

f the main transformer has a turn8s ratio of : / then the teaser transformerre"uires a turn8s ratio of *.JJ: for balanced operation. The principle ofoperation of the cott connection can be most easily seen by first applying acurrent to the teaser secondary windings/ and then applying a current to the

main secondary winding/ calculating the primary currents separately andsuperimposing the results.

%oad connected between phase&' and phase &( of the secondary$

econdary current from the teaser winding into phase M 0.* NE*F econdary current from the teaser winding into phase M, 09.*N E*F rimary current from @ phase into the teaser winding0 .D7N E*F
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rimary current from @, phase into the main winding0 *.D77 NE*F rimary current from @ phase into the main winding0 9*.D77N E*F The reason that the primary current from @ phase into the teaser winding is

.D7 due to *.JJ: turn8s ratio of the teaser/ transforming 3*.JJ0.D7 times the secondary current. This current must split in half at thecenter tap of the main primary winding because both hal!es of the main

primary winding are wound on the same core and the total ampere9turns ofthe main winding must e"ual 2ero.

%oad connected between phase )( and phase )' of the secondary$

econdary current from the main winding into phase M, 0.*N *F econdary current from the main winding into phase M0 9.* N*F rimary current from @, phase into the main winding 0.* N*F rimary current from @ phase into the main winding09 .* N*F rimary current from @ phase into the teaser winding0 * uperimpose the two sets of primary currents: @0 .D7 NE*F 4*0 .D7 NE*F @, 0*.D77 NE*F 4.*N *F0 .D7 N *F @ 0*.D77 NE*F4 .* N*F0.D7 N,*F Iotice that the primary three9phase currents are balanced i.e./ the phase

currents ha!e the same magnitude and their phase angles are ,*F apart.The apparent power supplied by the main transformer is greater than theapparent power supplied by the teaser transformer. This is easily !erified byobser!ing that the primary currents in both transformers ha!e the samemagnitude howe!er/ the primary !oltage of the teaser transformer is onlyJ.JK as great as the primary !oltage of the main transformer. Therefore*the teaser transforms only +!.!, of the apparent power transformedby the main.

1e also obser!e that while the total real power deli!ered to the two phase

load is e"ual to the total real power supplied from the three9phase system/the total apparent power transformed by both transformers is greater thanthe total apparent power deli!ered to the two9phase load.

The apparent power transformed by the teaser is *.JJ M @0 .* and theapparent power transformed by the main is .*M @, 0.D7 for a total of,.D7 of apparent power transformed.

The additional *.D7 per unit of apparent power is due to parasitic reacti!epower owing between the two hal!es of the primary winding in the maintransformer.

ingle9phase transformers used in the cott connection are specialty itemsthat are !irtually impossible to buy off the shelf 88 nowadays. n anemergency/ standard distribution transformers can be used

Ad!antages of the cott T Connection:

f desired/ a three phase/ two phase/ or single phase load may be suppliedsimultaneously

The neutral points can be a!ailable for grounding or loading purposes

?isad!antages when used for hase ;oading


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This type of asymmetrical connection ( phases/ , coils)/ reconstructs threephases from , windings. This can cause une"ual !oltage drops in thewindings/ resulting in potentially unbalanced !oltages to be applied to theload.

The transformation ratio of the coils and the !oltage obtained may be slightlyunbalanced due to manufacturing !ariances of the interconnected coils.

This design8s neutral has to be solidly grounded. f it is not grounded solidly/the secondary !oltages could become unstable.

ince this design will ha!e a low impedance/ special care will ha!e to be ta$enon the primary protection fault current capacity. This could be an issue if thesystem was designed for a ?elta9tar connection.

The inherent single phase construction and characteristics of this connectionproduces a comparati!ely bul$y and hea!ier transformer when compared witha normal three phase transformer of the same rating.

Application:

'or ndustrial 'urnace Transformer.

-or Traction urpose$The power is obtained from the ,,* $& or , $& or* $& or JJ $&/ three9phase/ effecti!ely earthed transmission networ$ of thetate lectricity Board/ through single9phase transformers or cott connectedtransformer installed at the Traction ubstation. The primary winding of thesingle9phase transformer is connected to two phases of the transmissionnetwor$ or 1here cott9connected transformer is used/ the primary windingsare connected to the three phases of the transmission networ$.

The single9phase transformers at a Traction ubstation are connected to thesame two phases of the transmission networ$ (referred as single9phaseconnection)/ or alternati!ely to different pairs of phases9 the three singlephase transformers forming a delta9connection on the primary side. Out ofthree single9phase transformers/ one transformer feeds the o!erhead

e"uipment (O@) on one side of the Traction ubstation/ another feeds theO@ on the other side of the Traction ubstation/ and the third remains asstandby. Thus the two single9phase transformers which feed the O@constitute an open9delta connection (alternati!ely/ referred as &9connection)on the three9phase transformers networ$. The cott9connected transformerand &9connected single9phase transformers are effecti!e in reducing !oltageimbalance on the transmission networ$. The spacing between adGacentsubstations is normally between 7* and ** $m.

/" The 0ig1ag Connection$

The 2ig2ag connection is also called the interconnected star connection. Thisconnection has some of the features of the = and the < connections/combining the ad!antages of both.

The 2ig2ag transformer contains si# coils on three cores. The first coil on eachcore is connected contrariwise to the second coil on the ne#t core. The secondcoils are then all tied together to form the neutral and the phases areconnected to the primary coils. ach phase/ therefore/ couples with eachother phase and the !oltages cancel out. As such/ there would be negligiblecurrent through the neutral pole and it can be connected to ground


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One coil is the outer coil and the other is the inner coil. ach coil has thesame number of windings turns (Turns ratio0:) but they are wound inopposite directions. The coils are connected as follows:

The outer coil of phase a9a is connected to the inner coil of phase c,9I. The outer coil of phase b9b is connected to the inner coil of phase a,9I. The outer coil of phase c9c is connected to the inner coil of phase b,9I. The inner coils are connected together to form the neutral and our tied to

ground The outer coils are connected to phases a/b/c of the e#isting delta system.

f three currents/ e"ual in magnitude and phase/ are applied to the threeterminals/ the ampere9turns of the a,9I winding cancel the ampere9turns ofthe b9bwinding, the ampere-turns of the b2-N winding cancel the ampereturns of the c9c winding/ and the ampere9turns of the c,9I winding cancelthe ampere turns of the a9a winding. Therefore/ the transformer allows thethree in9phase currents to easily flow to neutral.

f three currents/ e"ual in magnitude but ,*F out of phase with each other/are applied to the three terminals/ the ampere9turns in the windings cannotcancel and the transformer restricts the current flow to the negligible le!el ofmagneti2ing current. Therefore/ the 2ig2ag winding pro!ides an easy path forin9phase currents but does not allow the flow of currents that are ,*Fout ofphase with each other.

>nder normal system operation the outer and inner coil winding8s magneticflu# will cancel each other and only negligible current will flow in the in theneutral of the 2ig 52ag transformer.

?uring a phase to ground fault the 2ig92ag transformer8s coils magnetic flu#are no longer e"ual in the faulted line. This allows 2ero se"uence.

f one phase/ or more/ faults to earth/ the !oltage applied to each phase ofthe transformer is no longer in balance flu#es in the windings no longer

oppose. (>sing symmetrical components/ this is a*0 b*0 c*.) Pero se"uence(earth fault) current e#ists between the transformers8 neutral to the faultingphase. @ence/ the purpose of a 2ig2ag transformer is to pro!ide a return pathfor earth faults on delta connected systems. 1ith negligible current in theneutral under normal conditions/ engineers typically elect to under si2e thetransformer a short time rating is applied. nsure the impedance is not toolow for the desired fault limiting. mpedance can be added after thesecondary8s are summed (the opath)


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The neutral formed by the 2ig2ag connection is !ery stable. Therefore/ thistype of transformer/ or in some cases an auto transformer/ lends itself !erywell for establishing a neutral for an ungrounded phase system.

Hany times this type of transformer or auto transformer will carry a fairlylarge rating/ yet physically be relati!ely small. This particularly applies inconnection with grounding applications. The reason for this small si2e in

relation to the nameplate %&A rating is due to the fact that many types ofgrounding auto transformers are rated for , seconds. This is based on thetime to operate an o!er current protection de!ice such as a brea$er. Pig2agtransformers used to be employed to enable si2e reductions in dri!e motorsystems due to the stable wa!e form they present. Other means are nowmore common/ such as J phase star.

Ad!antages of Pig9Pag Transformer:

The < 92ig2ag connection pro!ides the same ad!antages as the


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connected transformers. But 2ig2ag connections are useful in specialapplications where con!entional transformer connections aren8t effecti!e.

? or = 3 Pig92ag are used in unbalanced low !oltage system 5 mostly withsingle phase appliances

7" Star#4elta Connection$

n this type of connection/ then primary is connected in star fashion while thesecondary is connected in delta fashion as shown in the 'ig.

The !oltages on primary and secondary sides can be represented on thephasor diagram as shown in the 'ig.

%ey point$

As rimary in tar connected
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;ine !oltage on rimary side 0 L M hase !oltage on rimary side. o hase !oltage on rimary side 0 ;ine !oltage on rimary side 3 L Iow Transformation 6ation (%) 0 econdary hase &oltage 3 rimary hase

&oltage econdary hase &oltage 0 % M rimary hase &oltage. As econdary in delta connected: ;ine !oltage on econdary side 0 hase !oltage on econdary side. econdary hase &oltage 0 % M rimary hase &oltage. 0% M (;ine !oltage on

rimary side 3 L) Secondary Phase Voltage = (K/3 ) X Line voltage on Primary side. There is s 89: 4egree or #9: 4egree hase Shift between Secondary

hase Voltage to rimary hase Voltage

Ad!antages of tar ?elta Connection:

The primary side is star connected. @ence fewer numbers of turns arere"uired. This ma$es the connection economical for large high !oltage stepdown power transformers.

The neutral a!ailable on the primary can be earthed to a!oid distortion. The neutral point allows both types of loads (single phase or three phases) to

be met. ;arge unbalanced loads can be handled satisfactory. The =9? connection has no problem with third harmonic components due to

circulating currents in?. t is also more stable to unbalanced loads since the ?partially redistributes any imbalance that occurs.

The delta connected winding carries third harmonic current due to whichpotential of neutral point is stabili2ed. ome sa!ing in cost of insulation isachie!ed if @& side is star connected. But in practice the @& side is normallyconnected in delta so that the three phase loads li$e motors and single phaseloads li$e lighting loads can be supplied by ;& side using three phase four

wire system. As 2rounding Transformer$n ower ystem Hostly grounded =9 s$1hen starting up pumps and fansfor e#ample/ the load tor"ue is low at the beginning of the start and increaseswith the s"uare of the speed. 1hen reaching appro#. *9D K of the motorrated speed the load tor"ue is e"ual to the motor tor"ue and the accelerationceases. To reach the rated speed/ a switch o!er to delta position is necessary/and this will !ery often result in high transmission and current pea$s. n somecases the current pea$ can reach a !alue that is e!en bigger than for a ?.O.;start.

Applications with a load tor"ue higher than D* K of the motor rated tor"uewill not be able to start using the start9delta starter.

%ow Starting Torque$The star9delta (wye9delta) starting method controlswhether the lead connections from the motor are configured in a star or deltaelectrical connection. The initial connection should be in the star pattern thatresults in a reduction of the line !oltage by a factor of 3L (D7.7K) to themotor and the current is reduced to 3 of the current at full !oltage/ but thestarting tor"ue is also reduced 3 to 3D of the ?O; starting tor"ue .

The transition from star to delta transition usually occurs once nominal speedis reached/ but is sometimes performed as low as D*K of nominal speedwhich ma$e transient par$s.

'eatures of star9delta starting

'or low9 to high9power three9phase motors. 6educed starting current i# connection cables 6educed starting tor"ue Current pea$ on changeo!er from star to delta Hechanical load on changeo!er from star to delta


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Application of tar9?elta tarter:

The star9delta method is usually only applied tolow to medium voltageand light starting Torque motors.

The recei!ed starting current is about 9: ,of the starting current duringdirect on line start and the starting tor"ue is reduced to about (/ ,of the

tor"ue a!ailable at a ?.O.; start. This starting method only wor$s when theapplication is light loaded during the start. f the motor is too hea!ily loaded/there will not be enough tor"ue to accelerate the motor up to speed beforeswitching o!er to the delta position.

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