Three Phase Transformer

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Three phase transformer connections and phasor groups:

1. H.V. terminals are marked by capital letters such as A, B, C and L.V. terminals are by small letters such as a,b,C.

2. Tertiary winding terminals if provided are marked as 3A, 3B &3C. 3. Each winding has two ends designated by the subscript numbers 1,2 or if there are

sections of the same winding these are numbered then in the order of their separation from end 1.

4. Neutrals are provided at extreme left.

Three phase transformer terminals are connected in Y, ∆ or Z fashion in its H.V and L.V terminals. But H.V side lines to neutral voltages with respect to L.V side line to neutral voltages are different in magnitude as well as in angular displacement also. So to completely specify a three phase transformer along with H,V& L.V windings connections , angular phase displacement or simply phase angle between them will also be required.

The phasor group indicates the phase difference between the primary and secondary sides, introduced due to that particular configuration of transformer windings connection.

For example Dyn11 connection meaning that- Delta connected HV winding;star; star connected LV winding with neutral brought out, LV is leading HV with 30°

Explanation:-

Phase displacement Phase rotation is always anti-clockwise. (International adopted convention) Use the hour indicator as the indicating phase displacement angle. Because there are 12 hours on a clock, and a circle consists out of 360°, each hour represents 30°. Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°.The minute hand is set on 12 o'clock and replaces the line to neutral voltage (sometimes imaginary) of the HV winding. This position is always the reference point. Because rotation is anti-clockwise, 1 = 30° lagging (LV lags HV with 30°) and 11 = 330° lagging or 30° leading (LV leads HV with 30°)

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Phasor groups

Connections

Phase angle

Clock positions Remarks

I Yy0, Dd0, Dz0

0°

00∠

H.V Phasor is denoted by minute hand L.V phasor is denoted by hour hand. Rotation of L.V phasor is always in anti-clock wise direction.

II Yy6, Dd6, Dz6

180°

0180∠

III Dy1,

Yd1, Yz1

-30°

030∠−

IV Dy11,Yd11, Yz11

+30°

030∠+

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Phasor

groups

Winding connections diagrams Phasor diagrams

HV LV HV LV

Yy0

2c2C

Dd0

Dz0

Yy6

Dd6

Three Phase Transformer

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Dz6

Dy1

Yd1

Yz1

Dy11

Yd11

n

b2

c2

a2+30

0

Yz11

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Three phase transformer connections

A three-phase transformer consists of three transformers, either separate or combinedon one core. The primaries and secondaries of any three-phase transformercan be independently connected in either a wye (Y) or a delta (∆.)This gives a totalof four possible connections for a three-phase transformer bank. 1. Wye- wye (Y - Y) 2. Wye-delta (Y -∆) 3. Delta-wye (∆-Y) 4. Delta-delta (∆-∆)

1. Wye- wye (Y - Y) connection:

The Y-Y connection of 3 phase transformer is shown in above fig.2. in Y-Y connection, the

primary voltage on each Phase of the transformer is given by =3LV

. Now primary phase voltage

is related to secondary phase voltage by the turns ratio of the transformer.

So, phase voltage at the secondary =3LkV

. Hence secondary line voltage= kVL.

Therefore, in Y-Y connection -

primary line voltage

secondary line voltagek=

primary line current 1

secondary line current k=

3LV

3LkV

I

k

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Phasor diagram:-

From above phasor diagram it is seen that the voltages of the corresponding phases (and therefore the corresponding lines) are in phase. This is known as 00connections. If the windings terminals on secondary side are reversed, the 1800 connection is obtained.

Advantage of Y-Y Connection:

� No Phase Displacement: The primary and secondary circuits are in phase; i.e., there are no phase angle displacements introduced by the Y-Y connection.

� Required Few Turns for winding: Due to star connection, phase voltages is (1/√3) times the line voltage. Hence less number of turns is required. Also the stress on insulation is less. This makes the connection economical for small high voltage purposes.

� Required Less Insulation Level: If the neutral end of a Y-connected winding is grounded, then there is an opportunity to use reduced levels of insulation at the neutral end of the winding. A winding that is connected across the phases requires full insulation throughout the winding.

� Handle Heavy Load: Due to star connection, phase current is same as line current. Hence windings have to carry high currents. This makes cross section of the windings high. Thus the windings are mechanically strong and windings can bear heavy loads and short circuit current.

� Use for Three phases Four Wires System: As neutral is available, suitable for three phases four wire system.

� Eliminate Distortion in Secondary Phase Voltage: The connection of primary neutral to the neutral of generator eliminates distortion in the secondary phase voltages by giving path to triple frequency currents toward to generator.

Disadvantage of Y-Y Connection:

� The Third harmonic issue: The voltages in any phase of a Y-Y transformer are 1200 apart from the voltages in any other phase. However, the third-harmonic components of

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each phase will be in phase with each other. Nonlinearities in the transformer core always lead to generation of third harmonic. These components will add up resulting in large (can be even larger than the fundamental component) third harmonic component. The result is a very large third-harmonic component of voltage on top of the 50-Hz fundamental voltage. This third-harmonic voltage can be larger than thanfundamental voltage itself.

� If loads on the transformercircuit are unbalanced, then the voltages on the phases of the

transformer can become severely unbalanced.

Both the unbalance problem and the third-harmonic problem can be solvedusing one of

two techniques:

1) Solidly ground the neutrals ofthe transformers, especially the primary winding's a. neutral. This connection permits the additive third-hannonic components b. to cause a current flow in the neutral instead of building up large voltages. The c. neutralalso provides a return path for any current imbalances in the load.

2) Add a third (tertiary) winding connected in ∆to the transformer bank. If a third

a. ∆-connected winding is added to the transformer. then the third-harmoniccomponents of voltage in the ∆ will add up, causing a circulating current flowwithin the winding. This suppresses the third-hannonic components of voltagein the same manner as grounding the transformer neutrals. The∆-connected tertiary windings need not even be brought out of thetransformer case, but they often are used to supply lights and auxiliary powerwithinthe substation where it is located. The tertiary windings must be largeenough to handle the circulating currents, so they are usually made aboutone-third the power rating of the two main windings. Oneor the other of these correction techniques must be used any time a Y – Ytransformer is installed. In practice, very few Y -Y transformers are used, since thesame jobs can be done by one of the other types of three-phase transformers

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2. Wye-delta (Y -∆)

3LV

Ik

3LkV

LkV

3I

k

Fig.4 shows the basic Y-∆ connection diagram of 3 phase transformer. At primary star connection is formed by connecting together with the suffix 1 with suffix 2 terminals and at secondary side by connecting c1a2 , a1b2 and b1c2.

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The connection diagram and phasor diagram are shown in fig.5 and fig.6 for -300 and +300 connection.it is seen from phasor diagram that on the delta side sum of voltages around delta is zero.

The Y-∆ connection has no problem with 3rd harmonics compounds since are consumed in a circulating current on the delta side. So for unbalanced load this connection is more stable.

But there is +300 or -300 phase shift between secondary phase voltages to primary phase voltage. This phase shift can cause problem in paralleling the secondary of two transformer banks together.

In Y-∆ connection, the primary voltage on each Phase of the transformer is given by =3LV

. Now

primary phase voltage is related to secondary phase voltage by theturns ratio of the transformer

So, phase voltage at the secondary =3LkV

. Hence secondary line voltage= 3LkV

Therefore, in Y-∆ connection -

Advantages

� The primary side is star connected. Hence fewer numbers of turns are required. This makes the connection economical for large high voltage step down power transformers.

� The neutral available on the primary can be earthed to avoid distortion. � Large unbalanced loads can be handled satisfactory. � The Y-∆ connection has no problem with third harmonic components due to circulating

currents in ∆. It is also more stable to unbalanced loads since the ∆ partially redistributes any imbalance that occurs.

� As Grounding Transformer: In Power System Mostly grounded Y- ∆ transformer is used for no other purpose than to provide a good ground source in ungrounded Delta system.

Disadvantages

� In this type of connection, the secondary voltage is not in phase with the primary. Hence it is not possible to operate this connection in parallel with star-star or delta-delta connected transformer.

� One problem associated with this connection is that the secondary voltage is shifted by 300 with respect to the primary voltage. This can cause problems when paralleling 3-

primary line voltage

secondary line voltage 3

k=

primary line current 3

secondary line current k=

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phase transformers since transformers secondary voltages must be in-phase to be paralleled. Therefore, we must pay attention to these shifts.

Application:

� It is commonly employed for power supply transformers. � This type of connection is commonly employed at the substation end of the transmission

line. The main use with this connection is to step down the voltage. The neutral available on the primary side is grounded. It can be seen that there is phase difference of 30° between primary and secondary line voltages.

3. Delta-wye (∆-Y)

This connection is simply the interchange of primary and secondary roles in the star/delta connection also have the same advantages and the same phase shift as the Y -∆transformer.

3I

3I

k

3kVL

3I

k

3

Ik

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In ∆-Y connection, the primary voltage on each Phase of the transformer is given by VL..Now primary phase voltage is related to secondary phase voltage by the turns ratio of the transformer

So, phase voltage at the secondary =kVL. Hence secondary line voltage= √3kVL

Therefore, in ∆-Y connection -

primary line voltage3

secondary line voltagek=

primary line current 1

secondary line current 3k=

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Advantages

Used at Three phase four wire System: On secondary side, neutral is available, due to which it can be used for 3-phase, 4 wire supply system.

� No distortion of Secondary Voltage: No distortion due to third harmonic components. � Handled large unbalanced Load: Large unbalanced loads can be handled without any

difficulty. � it provides a local source of ground current at the secondary that is isolated from the

primary circuit. Disadvantages

� In this type of connection, the secondary voltage is not in phase with the primary. Hence it is not possible to operate this connection in parallel with star-star or delta-delta connected transformer.

� One problem associated with this connection is that the secondary voltage is shifted by 300 with respect to the primary voltage. This can cause problems when paralleling 3-phase transformers since transformers secondary voltages must be in-phase to be paralleled. Therefore, we must pay attention to these shifts.

Applications:

� Commonly used in a step-up transformer: In recent years, this arrangement has become very popular for distribution system as it provides 3- Ø, 4-wire system.

� Commonly used in commercial, industrial, and high-density residential locations: To supply three-phase distribution systems

4. Delta-delta (∆-∆)

Below fig.10 shows the ∆-∆ connection and its phasor and connection diagrams. It is easily seen from the phasor diagram that the primary and the secondary line voltages are in phases. So it is 00connection. But if secondary leads a, b, c are taken out from the delta nodes a1b2, b1c2 c1a2, then the secondary voltages are in phase opposition to the primary voltages. This is 1800 connection.

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3I

3I

k

3I

kLkV

LkV

I

k

Advantage

� Sinusoidal Voltage at Secondary: In order to get secondary voltage as sinusoidal, the magnetizing current of transformer must contain a third harmonic component. The delta connection provides a closed path for circulation of third harmonic component of current. The flux remains sinusoidal which results in sinusoidal voltages.

� Suitable for Unbalanced Load: Even if the load is unbalanced the three phase voltages remains constant. Thus it suitable for unbalanced loading also.

� Carry 58% Load if One Transfer is Faulty in Transformer Bank: If there is bank of single phase transformers connected in delta-delta fashion and if one of the transformers is disabled then the supply can be continued with remaining tow transformers of course with reduced efficiency.

� No Distortion in Secondary Voltage: there is no any phase displacement between primary and secondary voltages. There is no distortion of flux as the third harmonic component of magnetizing current can flow in the delta connected primary windings without flowing in the line wires .there is no distortion in the secondary voltages.

� Economical for Low Voltage: Due to delta connection, phase voltage is same as line voltage hence winding have more number of turns. But phase current is (1/√3) times the line current. Hence the cross-section of the windings is very less. This makes the connection economical for low voltages transformers.

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� Reduce Cross section of Conductor: The conductor is required of smaller Cross section as the phase current is 1/√3 times of the line current. It increases number of turns per phase and reduces the necessary cross sectional area of conductors thus insulation problem is not present.

� Absent of Third Harmonic Voltage: Due to closed delta, third harmonic voltages are absent.

� The absence of star or neutral point proves to be advantageous in some cases.

Disadvantage

Due to the absence of neutral point it is not suitable for three phase four wire system.

� More insulation is required and the voltage appearing between windings and core will be equal to full line voltage in case of earth fault on one phase.

Application:

� Suitable for large, low voltage transformers.

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Merits –demerits among different connections:-

Connecti

ons

Line Voltage Line Current Remarks

Prima

ry

Secon

dary

Pri

mar

y

Secon

dary

Star-Star (Y-Y)

VL kVL. I Ik Less number of turns is required. Also

voltage stress on insulation is less. So this connection is economical for small high voltage purposes. Gauge of the wire is thicker and core cross section is less as compare to ∆-∆.

Star-Delta (Y-∆)

VL 3

LkV

I 3Ik

Delta-Star (∆-Y)

VL 3 LkV I 3

Ik

Delta-Delta (∆-∆)

VL kVL. I Ik More insulation is required as the2ndary

windings will have to withstand k time the line voltage. For same power and voltage rating core cross section is higher than Y-Y. And the gauge of the wire is thinner.