Three Phase Transformer Connections

Three Phase Transformer Connections

AU: May-04,08,10,16, Dec.-04

• The primary and secondary windings of three phase transformers as three phase windings can be connected in different ways such as in star or in delta. With suitable connection the voltage can be raised or lowered. In this section some commonly used connections for three phase transformers are discussed. The most useful connections are,

i)    Star-Star (λ- λ) connection

ii)  Delta-Delta (∆ - ∆) connection

iii) Star-Delta (λ - ∆) connection

iv) Delta-Star (∆ - λ) connection

v) Open Delta or V connection

vi) Scott connection or T-T connection.

 

1. Star-Star Connection

• In this type of connection, both the primary and secondary windings are connected in star as shown in the Fig. 7.4.1 and as phase voltage is 1/√3 times that of line voltage, the number of turns per phase and the quantity of insulation required is minimum. The ratio of line voltages on the primary and secondary sides is the same as the transformation ratio of each transformer. It can be noted that there is a phase shift of 30° between the phase voltages and line voltages on both primary and secondary side. The line voltages on both sides and the primary voltages are in phase with each other.

• The voltages on primary and secondary sides can be represented on the phasor diagram shown in the Fig. 7.4.1.

• The main difficulty with this type of connection is that it can work satisfactorily if the load is balanced. But the neutral point shifts which makes phase voltages unequal if load is unbalanced.

• This point can be understood properly if we consider a single phase load placed between phase r and neutral on the secondary side. The phase R on primary side will have to supply the power to the load. But the phase R will not supply the necessary power as it is in series with phases Y and B whose secondaries are open.

• The primary coil 1 carries small current from the line as phases Y and B act as high impedances. Thus secondary r is unable to provide necessary power. If we consider that very small resistance is connected then very small current will flow. Due to neutral shift the voltage V_m will reduce. But there is increase in voltage V_yn and V_bn without affecting line voltages on primary side. The primary phase Voltage V_RN will reduced to zero. The voltages V_YN and V_BN is increased to full primary voltage.

• This difficulty can be overcome by connecting the primary neutral to the generator. This will make on phase R on primary side to take its power from its line and neutral. The same effect can be observed with the connection of single phase load between any two lines.

• The connection of primary neutral to the neutral of generator has an added advantage that it eliminates distortion in the secondary phase voltages. If the flux in the core has sinusoidal waveform then it will give sinusoidal waveform for the voltage. But due to characteristics of iron, a sinusoidal waveform of flux requires a third harmonic component in the exciting current. As the frequency of this component is thrice the frequency of circuit at any given instant, it will try to flow either towards or away from the neutral point in the transformer windings. With isolated neutral, the triple frequency current cannot flow so the flux in the core will not be a 2 sine wave and the voltages are distorted.

• If primary neutral is connected to generator neutral the triple frequency currents get the path to solve the difficulty. The alternative way of overcoming with this difficulty is the use of tirtiary winding of low kVA rating. These windings are s are connected in delta and provides a circuit in which triple frequency currents can flow. Thus sinusoidal voltage on primary will give sinusoidal voltage on ve sinusoidal voltage on secondary side.

• If VL1 is the line voltage is the line voltage on the primary side then phase voltage on primary side is given

• If K is the turns ratio or transformation ratio then phase voltage on secondary side is given as,

Note: Suffix 1 indicates primary side whereas suffix 2 indicates secondary side

The line voltage V_L2 on secondary side is given as,

Key Point: The line voltage on secondary side is K times line voltage on primary side.

• Thus in summary we can state the advantages and disadvantages of star-star connection as follows.

a. Advantages art egribni

1. Due to star connection, phase voltage is 1 / √3 times line voltage. Hence less number of turns are required. Also the stress on insulation is less. This makes the connection economical for small high voltage purposes.

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

3. There is no phase shift between the primary and secondary voltages.

4. As neutral is available, it is suitable for three phase, four wire system.

b. Disadvantages

1. If the load on the secondary side is unbalanced then the performance of this connection is not satisfactory then the shifting of neutral point is possible. To prevent this, star point of the primary is required to be connected to the star point of the generator.

2. Eventhough the star or neutral point of the primary is earthed, the third harmonic present in the alternator voltage may appear on the secondary side. This causes distortion in the secondary phase voltages.

Key Point : Due to the disadvantages, this connection is rare in practice and used only for small high voltage transformers.

 

2. Delta-Delta Connection

• In this type of connection, both the three phase primary and secondary windings are connected in delta as shown in the Fig. 7.4.4.

• The voltages on primary and secondary sides can be shown on the phasor diagram as shown in the Fig. 7.4.5

• The another way of representing this type of connection is shown in the Fig. 7.4.6.

• This connection proves to be economical for large low voltage transformers as it increases number of turns per phase.

Key Point

It can be seen that there is no phase shift between primary and secondary voltages.

Let, V_L1 = Line voltage on primary side,

V_L2 = Line voltage on secondary side

V_ph1 = Phase voltage on primary side

V_ph2 = Phase voltage on secondary side,

K = Transformation ratio

For, delta connection,.

But again since secondary is connected in delta,

V_L2 = V_ph2 = K V_L1

• The advantages and disadvantages of this type of connection can be summerized as follows:

a. Advantages

1. 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.

2. Even if the load is unbalanced the three phase voltages remain constant. Thus it allows unbalanced loading also.

3. The important advantage with this type of connection is that 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 two transformers of course with reduced efficiency.

4. There is no distortion in the secondary voltages.

5. Due to delta connection, phase voltage is same as line voltage hence windings have more number of turns. But phase current is √3 times the current. Hence the cross section of the windings is very less. This makes the connection economical for low voltage transformers.

b. Disadvantage

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

Key Point: The connection is commonly used for large low voltage transformers.

 

3. Star-Delta Connection

• In this type of connection, the primary is connected in star fashion while the secondary is connected in delta fashion as shown in the Fig. 7.4.7.

• The voltages on primary and secondary sides can be represented on the phasor diagram as shown in the Fig. 7.4.8.

• The same type of connection can be represented in another way as shown in the Fig. 7.4.9.

• 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.

Key Point: The delta connection on secondary side allows third harmonic current to flow which

provides a sinusoidal flux.

Let, V_L1 = Line voltage on primary side,

V_L2 = Line voltage on secondary side

V_ph1 = Phase voltage on primary side

V_ph2 = Phase voltage on secondary side,

K = Transformation ratio

• The connection suffers no problems due to unbalanced load as secondaries are connected in delta. This type of transformers are commonly employed at receiving end.

• The advantages and disadvantages of this type of More connection can be stated as follows.

a. Advantages

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

2. The neutral available on the primary can be earthed to avoid distortion.

3. Large unbalanced loads can be handled satisfactorily.

b. Disadvantage

• 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.

4. Delta-Star Connection

• In this type of connection, the primary is connected in delta fashion while the secondary is connected in star fashion as shown in the Fig. 7.4.10.

• The voltages on primary and secondary side can be represented on the phasor diagram as shown in the Fig. 7.4.11.

• The another way of representing the same type of connection is shown in the Fig. 7.4.12.

• The main use of this connection is to step up the voltage i.e. at the begining of high tension ed-ms transmission system. It can be noted that there is a phase shift of 30° between primary line voltage and secondary line voltage as leading.

Key Point: As secondary side is star connected, use of three phase, four wire system is possible.

• Thus single phase and three phase loads can be supplied with this type of connection.

So Let, V_L1 = Line voltage on primary side,

V_L2 = Line voltage on secondary side

V_ph1 = Phase voltage on primary side

V_ph2 = Phase voltage on secondary side,

K = Transformation ratio

• The advantages and disadvantages with this type of connection are given below.

a. Advantages

1. On primary side due to delta connection winding cross-section required is less.

2. On secondary side, neutral is available, due to which it can be used for 3 phase,  4 wire supply system.

3. There is no distortion due to third harmonic components.

4. The windings connected in star makes it economical due to saving in cost of insulation.

5. Large unbalanced loads can be handled without any difficulty.

b. Disadvantage

• Due to phase shift between primary and secondary voltages, the limitation of star-delta connection continues for this type of connection as well.

Review Question

1. With proper connection and phasor diagrams describe the different ways of connecting three phase transformers. AU May-04, 08, 10, 16, Dec.-04, Marks 8