Unit-IV - Voli. 2 Polyphase Transformers

8/11/2019 Unit-IV - Voli. 2 Polyphase Transformers

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UNIT - IV

POLYPHASE TRANSFORMERS


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Comparison between Single 3-PhaseTransformer

and

Bank of Three Single Phase Transformers

forThree Phase System


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http://en.wikipedia.org/wiki/File:Drehstromtransformater_im_Schnitt_Hochspannung.jpg


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The three phase system which has been adopted worldover to

Generate,

Transmit and

Distribute electrical power

Therefore to change the level of voltages in the system

three phase transformers should be used.

Three number of identical single phase transformers canbe suitably connected for use in a three phase system andsuch a three phase transformer is called a bank of threephase transformer.

Alternatively, a three phase transformer can be constructedas a single unit


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It is found that generation, transmission and

distribution of electrical power are more

economical in three phase system than singlephase system.

For three phase system three single phase

transformers are required. Three phase transformation can be done in two

ways, by using single three phase

transformer or by using a bank of three singlephase transformers.


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Both are having some advantages over other.

Single 3-phase transformer costs around 15%

less than bank of three single phasetransformers.

It occupies less space than later.

For very big transformer, it is impossible totransport large three phase transformer to thesite and it is easier to transport three singlephase transformers which is erected separately

to form a three phase unit. Another advantage of using bank of three

single phase transformers is that, if one unit ofthe bank becomes out of order, then the bankcan be run as open delta.


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POLYPHASE TRANSFORMERS

CONNECTIONSAND

PHASOR DIAGRAMS


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Marking or Labelling the Different

Terminals of Transformer

Terminals of each phase of HV side should be labeledas capital letters, A, B, C

And

Those of LV side should be labelled as small letters,a, b, c.

Terminal polaritiesare indicated by suffixes 1 & 2.

Suffix 1sindicate similar polarity ends and so do 2s.


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If more terminals are brought out from a

winding by way of taps there are numbered in

the increasing numbers in accordance to theirdistance from 1 (eg A1, A2, A3...).

If the induced emf at an instant is from A1 to

A2on the HV winding it will rise from a1 to a2on the LV winding.


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The individual transformers are connected in a variety of ways in apower system.

Due to the advantages of polyphase power during generation,transmission and utilization, polyphase power handling is veryimportant.

As an engineering application is driven by techno-economicconsiderations, no single connection or setup is satisfactory for all

applications.

Thus transformers are deployed in many forms and connections.

Star and mesh connections are very commonly used.

Apart from these, vee or open delta connections, zig zag connections,T connections, auto transformer connections, multi windingtransformers etc. are a few of the many possibilities.


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A few of the common connections and the technicaland economic considerations that govern their usageare discussed here.

Literature abounds in the description of many other. Apart from the characteristics and advantages of these,

one must also know their limitations and problems, tofacilitate proper selection of a configuration for anapplication.

Many polyphase connections can be formed usingsingle phase transformers.

In some cases it may be preferable to design, developand deploy a polyphase transformer itself.

In a balanced two phase system we encounter twovoltages that are equal in magnitude differing in phase

by 90


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Similarly, in a three phase system there are three

equal voltages differing in phase 120 electrical

degrees. Further there is an order in which they reach a

particular voltage magnitude.

This is called the phase sequence. In some applications like a.c. to d.c. conversion,

six phases or more may be encountered.

Transformers used in all these applications mustbe connected properly for proper functioning.


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Out of the different polyphase connections threephase connections are mostly encountered due tothe wide spread use of three phase systems forgeneration, transmission and utilization.

Three balanced 3-phase voltages can beconnected in star or mesh fashion to yield a

balanced 3-phase 3-wire system. The transformers that work on the 3-phase supply

have star, mesh or zig-zag connected windings oneither primary secondary or both.

In addition to giving different voltage ratios, theyintroduce phase shifts between input and outputsides.

.


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Connection Primary Winding Secondary Winding

Delta D d

Star Y y

Interconnected Z z

Transformer Winding Identification


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In a single phase transformer, we have only two

coils namely primary and secondary.

Primary is energized with single phase supplyand load is connected across the secondary.

However, in a 3-phase transformer there will be

3 numbers of primary coils and 3 numbers ofsecondary coils.

So these 3 primary coils and the three secondary

coils are to be properly connected so that thevoltage level of a balanced 3-phase supply may

be changed to another 3-phase balanced system

of different voltage level.


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Suppose you take three identical transformerseach of rating 10 kVA, 200 V / 100 V, 50 Hz andto distinguish them call them as A, B and C.

For transformer-A, primary terminals aremarked as A1A2

and the secondary terminals are

marked as a1a2.

The markings are done in such a way that A1 and

a1 represent the dot () terminals.

Similarly terminals for B and C transformers are

marked and shown in f igure below.


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Star-star connection


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Star-Star Transformer


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N l t j i th t i l A B d C


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Now let us join the terminals A2, B2and C2

of the 3 primary coils of the transformers

and no inter connections are made betweenthe secondary coils of the transformers.

Now to the free terminals A1, B1 and C1a

balanced 3-phase supply with phasesequence A-B-C is connected as shown in

figure below.

Primary is said to be connected in star.


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It may be noted that individually eachtransformer will work following the rules ofsingle phase transformer i.e,

Induced voltage in a1a2 will be in phase withapplied voltage across A1A2

and the ratio ofmagnitude of voltages and currents will be as

usual decided by K where K = N2/N1, the turnsratio.

This will be true for transformer-B andtransformer-C as well i.e., induced voltage in

b1b2 will be in phase with applied voltageacross B1 B2

and induced voltage in c1c2will be

in phase with applied voltage across C1C2.


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Since the secondary coils are not interconnected, thesecondary voltage phasors too have been shownindependent without any interconnections between

them. In contrast, the terminals A2, B2and C2are physical lyjoined forcing them to be equipotential which has been

ref lected in the primary coil voltage phasors as well

where phasor points A2, B2 and C2

are also shownjoined.

Coming back to secondary, i f a voltmeter is connected

across any coil i.e., between a1 and a2 or between b1 and

b2

or between c1

and c2

i t wi l l read 100 V.

However, voltmeter wil l not read anything i f connected

between a1and b1

or between b1and c1 or between c1

and

a1 as open circui t exist in the paths due to no physical

connections between the coils.


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Imagine now the secondary coil terminals

a2, b2and c2

are joined together physical ly as

shown in f igure below. So the secondary coil phasors should not be

shown isolated as a2 b2 and c2

become

equipotential due to shorting of theseterminals.

Thus, the secondary coil voltage phasors

should not only be parallel to the respectiveprimary coil voltages but also a2, b2

and c2

should be equipotential.


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Therefore, shif t and place the phasors

in such a way that they remain parallel to the

respective primary coil voltages and thepoints a2, b2and c2 are superposed.


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Here obviously, if a voltmeter is connectedbetween a1

and b1or between b1

and c1or

between c1and a1

it will read corresponding

phasor lengths a1b1 or b1c1

or c1a1which are all

equal to 1003 V.

are of same magnitude and displaced mutually

by 120 to form a balanced 3-phase voltage

system.Primary 3-phase line to line voltage of 2003Vis therefore stepped down to 3-phase, 1003Vline to l ine voltage at the secondary.


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The junction of A2, B2and C2can be used as

primary neutral and may be denoted by N.

Similar ly the junction of a2, b2 and c2may be

denoted by n for secondary neutral.


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A wrong star-star connection

In continuation with the discussion of the lastsection, we show here a deliberate wrongconnection to highlight the importance ofproper terminal markings of the coils with dots

(). Let us start from the first figure (26.2) where

the secondary coils are yet to be connected.

To implement star connection on the secondary

side, let us assume that someone joins theterminals a2, b1 and c2

together as shown insecond figure (26.4).


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A wrong star-star connection

To implement star connection on the

secondary side, let us assume that someone

joins the terminals a2, b1 and c2together as

shown in second figure (26.4).


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The question is: is it a valid star connection? If


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The question is: is it a valid star connection? If

not why?

To answer this we have to interconnect the

secondary voltage phasors in accordance with

the physical connections of the coils.

In other words, shift and place the secondary

voltage phasors so that a2, b1 and c2

overlap

each other to make them equipotential.


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The lengths of phasors

are no doubt, same and equal to 100 V but they

do not maintain 120 mutual phase displacementbetween them as clear from second figure (26.4).

The magnitude of the line to line voltages too will

not be equal.

From simple geometry, it can easily be shown

that


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Thus both the phase as well as line voltagesare not balanced 3-phase voltage.

Hence the above connection is useless so faras transforming a balanced 3-phase voltageinto another level of balanced 3-phase

voltage is concerned. Appropriate polarity markings with letters

along with dots () are essential in order tomake various successful 3-phase

transformer connections in practice orlaboratory.


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Bank of three phase transformer

In the background of the points discussed inprevious section, we are now in a position tostudy different connections of 3-phase

transformer.

Let the discussion be continued with the

same three single phase identicaltransformers, each of rating 10kVA, 200V /100V, 50Hz.


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Star star connection


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Star-star connection

We have discussed in length in the last section, the

implementation of star-star connection of a 3-phasetransformer.

The connection diagram along with the phasor diagramare shown in figure below.

As discussed earlier, we need to apply to the primaryterminals (A1B1C1) a line to line voltage of 200 3 V sothat rated voltage (200 V) is impressed across each ofthe primary coils of the individual transformer.

This ensures 100 V to be induced across each of the

secondary coil and the line to line voltage in thesecondary will be 100 3 V.

Thus a 3-phase line to line voltage of 200V is steppeddown to a 3-phase line to line voltage of 100 3


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Now we have to calculate how much load current or kVA can be

supplied by this bank of three phase transformers without over loading

any of the single phase transformers. From the individual rating of

each transformer, we know maximum allowable currents of HV and

LV windings are respectively

IHV

= 10000/200 = 50A and ILV

= 10000/100 = 100A.

Since secondary side is connected in star, line current and the windingcurrents are same.

Therefore total kVA that can be supplied to a balanced 3-phase load is


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Note: While solving problems, it is not

necessary to show all the terminal markings

in detail and a simple and popular way ofshowing the same star-star connection

Star Star Transformer is formed in a 3 phase


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transformer by connecting one terminal of each phaseof individual side, together.

The common terminal is indicated by suffix 1 in the

figure above.

If terminal with suffix 1 in both primary and secondaryare used as common terminal, voltages of primary andsecondary are in same phase.

That is why this connection is calledzero degree connection or 0o- connection.

If the terminals with suffix 1 is connected together inHV side as common point and the terminals with suffix

2 in LV side are connected together as common point,the voltages in primary and secondary will be inopposite phase.

Hence, Star Star Transformer connection is called

180o- Connection, of three phase transformer.

Star Connection Key points


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Star Connection Key points

As Primary in Star connected Line voltage on Primaryside = 3 X Phase voltage on Primary side.

So Phase voltage on Primary side = Line voltage onPrimary side / 3

Now Transformation Ration (K) = Secondary Phase

Voltage / Primary Phase Voltage

Secondary Phase Voltage = K X Primary Phase Voltage.

The neutralavailable on the primary can be earthed toavoid distortion.

The neutral point allows both types of loads (singlephase or three phases) to be met.
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Harmonic Suppression:

The magnetizing current must contain odd harmonics for the

induced voltages to be sinusoidal and the third harmonic is the

dominant harmonic component.

In a three-phase system the third harmonic currents of all three

phases are in phase with each other because they are zero-

sequence currents.

In the Y-Y transformer connection, the only path for third

harmonic current is through the neutral.

Dd connection
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Dd connection Dd connection in three phase banks with mesh connection on

both primary side and secondary side provides a closed path

for the triplen harmonics to circulate currents.

Thus the supply current is nearly sinusoidal for the non-triplen harmonic currents.

The triplen harmonic currents inside the closed mesh windingcorrect the flux density wave to be nearly sinusoidal. Thesecondary voltages will be nearly sinusoidal.

Third harmonics currents flow both in the primary and thesecondary and hence the magnitudes of these currents, so alsothe drops due to them will be lower.


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delta-delta transformer


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delta delta transformer

In delta-delta transformer, 1 suffixed terminals

of each phase primary winding will beconnected with 2 suffixed terminal of next phaseprimary winding.

If primary is HV side, then A1will be connectedto B2, B1will be connected to C2and C1will beconnected to A2.

Similarly in LV side 1 suffixed terminals of each

phase winding will be connected with 2 suffixedterminals of next phase winding.

That means, a1will be connected to b2, b1will beconnected to c

2

and c1

will be connected to a2

.

That means, a1 will be connected to b2, b1 will be


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That means, a1 will be connected to b2, b1 will beconnected to c2and c1will be connected to a2.

If transformer leads are taken out from primaryand secondary 2 suffixed terminals of the winding,then there will be no phase difference betweensimilar line voltages in primary and secondary.

This delta-delta transformer connection is zerodegree connection or 0o- Connection.

But in LV side of transformer, if, a2 is connected tob1, b2is connected to c1and c2is connected to a1.

The secondary leads of transformer are taken outfrom 2 suffixed terminals of LV windings, and thensimilar line voltages in primary and secondary willbe in phase opposition.

This connection is called 180o- Connection, of threephase transformer.

This connection proves to be economical for large low


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voltage transformers as it increases number of turns per phase.

Key point: It can be seen that there is no phase shift betweenprimary and secondary voltages.

VL1= Line voltage on primary side. VL2= Line voltage on secondary side.

Vph1= phase voltage on primary side.

Vph2= Phase voltage on secondary side.

K = Transformer ratio.

For delta connection, VL1= Vph1

Now since Vph2/Vph1= K

... Vph2= K Vph1

But again since secondary is connected in delta

VL2= Vph2= K VL1

The advantages and disadvantages of this type ofconnection can be summarized as follows.

Advantages


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In order to get secondary voltage as sinusoidal, themagnetizing current of transformer must contain a thirdharmonic component.

The delta connection provides a closed path forcirculation of third harmonic component of current.

The flux remains sinusoidal which results in sinusoidalvoltages.

Even if the load is unbalanced the three phase voltagesremains constant.

Thus it allows unbalanced loading also.

The important advantage with this type of connection is

that if there is bank of single phasetransformers connected in delta-delta fashion and if oneof the transformers is disabled then the supply can becontinued with remaining tow transformers of coursewith reduced efficiency.


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Dy and Yd connection (without


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Dy and Yd connection (without

neutral connection)

Behaviour of the bank with mesh

connection on one side is similar to the one

discussed under Dd connection.

The harmonic currents and drops and the

departure of the flux density from

sinusoidal are larger in the present casecompared to Dd banks.

Yy connection without neutral wires


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y

With both primary and secondary connected in starno closed path exists.

As the triplen harmonics are always in phase, byvirtue of the Y connection they get cancelled in theline voltages.

Non-triplen harmonics like fundamental, become 3times phase value and appear in the line voltages.

Line currents remain sinusoidal except for non-triplen harmonic currents.

Flux wave in each transformer will be flat toppedand the phase voltages remain peaked. The potentialof the neutral is no longer steady.

The star point oscillates due to the third harmonicvoltages. This is termed as oscillatingneutral.


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Yy connection with neutral wires

When a neutral wire is provided the triplen harmoniccurrent can flow and the condition is similar to thesingle phase case (with a star connected 4 wire sourceor with the system earth).

The neutral wire carries three times the triplenharmonic current of one transformer as thesecurrents are co-phasal. Unloaded secondary neutralwill not be operative.

Other polyphase connections not discussed aboveexplicitly will fall under one type or the other of thecases discussed.

In a Yy connection, to obtain third harmonic


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In a Yy connection, to obtain third harmonicsuppression one may provide a third windingwhich is connected in mesh, which can be an

unloaded winding. It is called a tertiary. This winding improves the single phase to earth

fault detection also.

Further, this winding can be used to feed some

permanent station loads also. Suchtransformers are designated as Yydtransformers.

If the neutral wires are provided and also mesh

connected winding is present, then triplenharmonics are shared between themdepending upon their impedances.


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Three phase transformers units

As against a bank of three single phase transformersconnected to three phase mains, a three phasetransformer generally has the three magnetic circuitsthat are interacting.

The exception to this rule is a 3-phase shell typetransformer.

In the shell type of construction, even though thethree cores are together, they are non-interacting.

Three limb core type 3-phase transformer is the onein which the phases are magnetically also linked. Fluxof each limb uses the other two limbs for its returnpath.

This is true for fundamental and non


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This is true for fundamental and non-

triplen harmonics.

The triplen harmonics being co-phasalcannot use other limbs for the return path

(this holds good for zero sequence,

unbalanced fundamental mmf also). The flux path is completed through the air.

So substantially large value of the mmf

produces a low value of third harmonic flux

as the path of the flux is through the air and

has a very high reluctance.

Thus the flux in the core remains nearly


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Thus the flux in the core remains nearly

sinusoidal, so also the induced emf.

This happens irrespective of the type ofconnection used.

The triplen order flux, sometimes links the

tank and produces loss in the same. Other harmonics can be suppressed by

connecting tuned filters at the terminals.

Harmonic current compensation usingspecial magnetic circuit design is considered

to be outside the scope here.


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Documents

Unit-IV - Voli. 2 Polyphase Transformers