three phase transformer connections and basics

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Three Phase Transforme

Transformer Connections

Transformer Coil Winding

Three Phase Transformers

Three Phase Transformer Basics

Thus far we have looked at the construction and operation of the single-phase, two winding voltage

transformer which can be used increase or decrease its secondary voltage with respect to the primary supply

voltage. But voltage transformers can also be constructed for connection to not only one single phase, but for

two-phases, three-phases, six-phases and even elaborate combinations up to 24-phases for some DC

rectification transformers.

If we take three single-phase transformers and connect their primary windings to each other and their secondary

windings to each other in a fixed configuration, we can use the transformers on a three-phase supply.

Three-phase, also written as 3-phase or 3 supplies are used for electrical power generation, transmission, and

distribution, as well as for all industrial uses. Three-phase supplies have many electrical advantages over Single-phase

Power (http://amazon.in/s/?field-keywords=Electrical+and+Electronic+Principles+and+Technology) and when

considering three-phase transformers we have to deal with three alternating voltages and currents differing in phase-

time by 120 degrees as shown below.

Three Phase Voltages and Currents
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Where: V is the line-to-line voltage, and V is the phase-to-neutral voltage.

A transformer can not act as a phase changing device and change single-phase into three-phase or three-phase into

single phase. To make the transformer connections compatible with three-phase supplies we need to connect them

together in a particular way to form a Three Phase Transformer Configuration.

A three phase transformer or 3 transformer can be constructed either by connecting together three single-phase

transformers, thereby forming a so-called three phase transformer bank, or by using one pre-assembled and balanced

three phase transformer which consists of three pairs of single phase windings mounted onto one single laminated

core.

The advantages of building a single three phase transformer is that for the same kVA rating it will be smaller, cheaper

and lighter than three individual single phase transformers connected together because the copper and iron core are

used more effectively. The methods of connecting the primary and secondary windings are the same, whether using

just one Three Phase Transformeror three separate Single Phase Transformers. Consider the circuit below:

Three Phase Transformer Connections

L P


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The primary and secondary windings of a transformer can be connected in different configuration as shown to meet

practically any requirement. In the case of three phase transformerwindings, three forms of connection are possible:

star (wye), delta (mesh) and interconnected-star (zig-zag).

The combinations of the three windings may be with the primary delta-connected and the secondary star-connected, or

star-delta, star-star or delta-delta, depending on the transformers use. When transformers are used to provide three or

more phases they are generally referred to as a Polyphase Transformer.

Three Phase Transformer Star and Delta ConfigurationsBut what do we mean by star (also known as Wye) and delta (also known as Mesh) when dealing with three-phase

transformer connections. A three phase transformer has three sets of primary and secondary windings. Depending

upon how these sets of windings are interconnected, determines whether the connection is a star or delta

configuration.

The three available voltages, which themselves are each displaced from the other by 120 electrical degrees, not only

decided on the type of the electrical connections used on both the primary and secondary sides, but determine the flow

of the transformers currents.

With three single-phase transformers connected together, the magnetic fluxs in the three transformers differ in phase

by 120 time-degrees. With a single the three-phase transformer there are three magnetic fluxs in the core differing in

time-phase by 120 degrees.

The standard method for marking three phase transformer windings is to label the three primary windings with capital

(upper case) letters A, B and C, used to represent the three individual phases of RED, YELLOW and BLUE. The

secondary windings are labelled with small (lower case) letters a, b and c. Each winding has two ends normally labelled

1 and 2 so that, for example, the second winding of the primary has ends which will be labelled B1 and B2, while the

third winding of the secondary will be labelled c1 and c2 as shown.

Transformer Star and Delta Configurations

Symbols are generally used on a three phase transformer to indicate the type or types of connections used with upper

case Y for star connected, D for delta connected and Z for interconnected star primary windings, with lower case y, d

and z for their respective secondaries. Then, Star-Star would be labelled Yy, Delta-Delta would be labelled Dd and

interconnected star to interconnected star would be Zz for the same types of connected transformers.


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Transformer Winding Identification

Connection Primary Winding Secondary Winding

Delta D d

Star Y y

Interconnected Z z

We now know that there are four ways in which three single-phase transformers may be connected together between

primary and secondary three-phase circuits. The configurations are delta-delta, star-star, star-delta, and delta-star.

Transformers for high voltage operation with the star connections has the advantage of reducing the voltage on an

individual transformer, reducing the number of turns required and an increase in the size of the conductors, making the

coil windings easier and cheaper to insulate than delta transformers.

The delta-delta connection nevertheless has one big advantage over the star-delta configuration, in that if one

transformer of a group of three should become faulty or disabled, the two remaining ones will continue to deliver

three-phase power with a capacity equal to approximately two thirds of the original output from the transformer unit.

Transformer Delta and Delta Connections

In a delta connected ( Dd ) group of transformers, the line voltage, V is equal to the supply voltage, V = V . But the

current in each phase winding is given as: 1/3 I of the line current, where I is the line current.

One disadvantage of delta connected three phase transformers is that each transformer must be wound for the full-line

voltage, (in our example above 100V) and for 57.7 per cent, line current. The greater number of turns in the winding,

together with the insulation between turns, necessitate a larger and more expensive coil than the star connection.

Another disadvantage with delta connected three phase transformers is that there is no neutral or common

connection.

In the star-star arrangement ( Yy ), (wye-wye), each transformer has one terminal connected to a common junction, or

neutral point with the three remaining ends of the primary windings connected to the three-phase mains supply. The

number of turns in a transformer winding for star connection is 57.7 per cent, of that required for delta connection.

L L S

L L


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The star connection requires the use of three transformers, and if any one transformer becomes fault or disabled, the

whole group might become disabled. Nevertheless, the star connected three phase transformer is especially convenient

and economical in electrical power distributing systems, in that a fourth wire may be connected as a neutral point, ( n )

of the three star connected secondaries as shown.

Transformer Star and Star Connections

The voltage between any line of the three-phase transformer is called the line voltage, V , while the voltage between

any line and the neutral point of a star connected transformer is called the phase voltage, V . This phase voltage

between the neutral point and any one of the line connections is 1/3 V of the line voltage. Then above, the primaryside phase voltage, V is given as.

The secondary current in each phase of a star-connected group of transformers is the same as that for the line current

of the supply, then I = I .

Then the relationship between line and phase voltages and currents in a three-phase system can be summarised as:

Three-phase Voltage and Current

Connection Phase Voltage Line Voltage Phase Current Line Current

Star V = V 3 V = 3 V I = I I = I

Delta V = V V = V I = I 3 I = 3 I

Where again, V is the line-to-line voltage, and V is the phase-to-neutral voltage on either the primary or the secondary

side.

Other possible connections for three phase transformers are star-delta Yd, where the primary winding is star-connectedand the secondary is delta-connected or delta-star Dy with a delta-connected primary and a star-connected secondary.

L

P

LP

L S

P L L P P L L P

P L L P P L L P

L P


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Delta-star connected transformers are widely used in low power distribution with the

primary windings providing a three-wire balanced load to the utility company while the

secondary windings provide the required 4th-wire neutral or earth connection.

When the primary and secondary have different types of winding connections, star or

delta, the overall turns ratio of the transformer becomes more complicated. If a three-

phase transformer is connected as delta-delta ( Dd ) or star-star ( Yy ) then the

transformer could potentially have a 1:1 turns ratio. That is the input and output voltages

for the windings are the same.

However, if the 3-phase transformer is connected in stardelta, ( Yd ) each star-connected

primary winding will receive the phase voltage, V of the supply, which is equal to 1/

3 V .

Then each corresponding secondary winding will then have this same voltage induced in

it, and since these windings are delta-connected, the voltage 1/3 V will become thesecondary line voltage. Then with a 1:1 turns ratio, a stardelta connected transformer will

provide a 3:1 step-down line-voltage ratio.

Then for a stardelta ( Yd ) connected transformer the turns ratio becomes:

Star-Delta Turns Ratio

Likewise, for a deltastar ( Dy ) connected transformer, with a 1:1 turns ratio, the transformer will provide a 1:3 step-upline-voltage ratio. Then for a delta-star connected transformer the turns ratio becomes:

Delta-Star Turns Ratio

Then for the four basic configurations of a three-phase transformer, we can list the transformers secondary voltages

and currents with respect to the primary line voltage, V and its primary line current I as shown in the following table.

Three-phase Transformer Line Voltage and Current

Primary-Secondary

Configuration

Line Voltage

Primary or Secondary

Line Current

Primary or Secondary

Delta Delta

Delta Star

P

L

L

L L
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Star Delta

Star Star

Where: n equals the transformers turns ratio (T.R.) of the number of secondary windings N , divided by the number of

primary windings N . ( N /N ) and V is the line-to-line voltage with V being the phase-to-neutral voltage.

Three Phase Transformer Example

The primary winding of a delta-star ( Dy ) connected 50VA transformer is supplied with a 100 volt, 50Hz three-phase

supply. If the transformer has 500 turns on the primary and 100 turns on the secondary winding, calculate thesecondary side voltages and currents.

Given Data: transformer rating, 50VA, supply voltage, 100v, primary turns 500, secondary turns, 100.

Then the secondary side of the transformer supplies a line voltage, V of about 35v giving a phase voltage, V of 20v at

0.834 amperes.

Three Phase Transformer Construction

S

P S P L P

L P


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We have said previously that the three-phase transformer is effectively three interconnected single phase transformers

on a single laminated core and considerable savings in cost, size and weight can be achieved by combining the three

windings onto a single magnetic circuit as shown.

A three-phase transformergenerally has the three magnetic circuits that are interlaced to give a uniform distribution

of the dielectric flux between the high and low voltage windings. The exception to this rule is a three-phase shell type

transformer. In the shell type of construction, even though the three cores are together, they are non-interlaced.

Three Phase Transformer Construction

The three-limb core-type three-phase transformer is the most common method of three-phase transformer

construction allowing the phases to be magnetically linked. Flux of each limb uses the other two limbs for its return

path with the three magnetic fluxs in the core generated by the line voltages differing in time-phase by 120 degrees.

Thus the flux in the core remains nearly sinusoidal, producing a sinusoidal secondary supply voltage.

The shell-type five-limb type three-phase transformer construction is heavier and more expensive to build than the

core-type. Five-limb cores are generally used for very large power transformers as they can be made with reduced

height. A shell-type transformers core materials, electrical windings, steel enclosure and cooling are much the same as

for the larger single-phase types.

The Current Transformer (http://www.electronics-tutorials.ws/transformer/current-transformer.html) | Bipolar

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Power TransformerPower & Distribution Transformer Power & Distribution Transformer
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Transformer Construction (http://www.electronics-tutorials.ws/transformer/transformer-construction.html)

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61 Responses to Three Phase Transformers

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page-3#comments)

Tags: AC Circuits (http://www.electronics-tutorials.ws/tag/ac-circuits)

Electromagnetism (http://www.electronics-tutorials.ws/tag/electromagnetism)

Power Supply (http://www.electronics-tutorials.ws/tag/power-supply) Transformer (http://www.electronics-tutorials.ws/tag/transformers)

deepak yadav

full detail

August 25th, 2015 (http://www.electronics-tutorials.ws/transformer/three-phase-transformer.html/comment-page-4#comment-7924)Reply (http://www.electronics-tutorials.ws/transformer/three-phase-transformer.html?replytocom=7924#respond)

LucianThere is an error in the example the current in primary is 0.289A, it is a three phase transformer so S=3x Vline xIline

August 23rd, 2015 (http://www.electronics-tutorials.ws/transformer/three-phase-transformer.html/comment-page-4#comment-7899)Reply (http://www.electronics-tutorials.ws/transformer/three-phase-transformer.html?replytocom=7899#respond)

Wayne Storr (http://www.electronics-tutorials.ws)

Thanks Lucian.


elango

super

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10/14



justyna

Hi, very nice and clearly written information. I would like to create a model of a circuit with a transformer, and I

see a lot of technical details how does something work but none answers to why they are configured in certain

way. Therefore I have a question, why are some configurations better than others? Is it that for high voltage one

type of winding is better, and for other a different one? Or does it depend on demands from load? Money saving

issue? Or backward compatibility?

I would appreciate any explanation about this.



This is very vague, I assume configurations means star or delta connected. 3-wire for transmission and

balanced loads, 4-wire for single phase and unbalanced loads.


subbu lakshmi.B

your answers are helpful to me.i have one doubt we can change the three phase winding transformer to two

phase winding transformer?is it possible?


MAHADEO D BHOSALE

BEST DATA FOR ELECTRICAL PERSON


swapnil a jadhavBEST INFORMATION OF ELECTRICAL
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July 22nd, 2015 (http://www.electronics-tutorials.ws/transformer/three-phase-transformer.html/comment-page-4#comment-7442)ep y ttp: www.e ectron cs-tutor a s.ws trans ormer t ree-p ase-trans ormer. tm rep ytocom= respon

Honeygood

July 16th, 2015 (http://www.electronics-tutorials.ws/transformer/three-phase-transformer.html/comment-page-4#comment-7389)Reply (http://www.electronics-tutorials.ws/transformer/three-phase-transformer.html?replytocom=7389#respond)

Meaidi

First thank for the great information then, my Q is why in open delta transformer we cant ground the twotransformer (in sec. side) in the middle bushing and if we do immediately the fuses/protection will activate



You can not ground one connection of a Delta (Mesh) connected transformer to create a neutral line, that is

crazy. A three-phase, three-wire delta configuration creates line voltages at different phase angles to one

another (120 degrees) at each connection so you are effectively shorting out one-phase which is why the

primary protection activates. If you want a secondary neutral connection for an unbalanced load you need a

Star (Wye) connected three-phase, four-wire system. Please, use some common sense.

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12/14



What's the Answer *

9 + = thirteen

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134

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13/14



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