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Experiment6

Three phase transformer

Objectives:

- To study three phase transformer and apply circuits in laboratory and LVVL program.

- To be familiar with operating characteristics of three phase transformer.

- To study how can be connect transformer windings in Y and ∆.

Theory:

When it is necessary to raise or to lower the voltage of a three-phase power supply, by means of

three phase transformer. We can use three exactly alike single-phase transformers to form a

single three-phase transformer, however, a three-phase transformer is designed to have all six

windings on a common magnetic core as shown in figure.

Figure 1 three-phase transformer.

The three phase transformer consists three transformers either separate or combined with one

core. The primary and secondary of the transformer can be independently connected either in star

or delta. There are four possible connections for a 3-phase transformer bank.

1. Δ – Δ (Delta – Delta) Connection.

2. Υ – Υ (Star – Star) Connection.

3. Δ – Υ (Delta – Star) Connection.

4. Υ – Δ (Star – Delta ) Connection.

The choice of connection of three phase transformer depends on the various factors likes the

availability of a neutral connection for grounding protection or load connections, insulation to

ground and voltage stress, availability of a path for the flow of third harmonics, etc. The various

types of connections are explained below in details.

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Part1: ∆/∆ Connection

A ∆/∆ connection for the primary and secondary windings of a three-phase transformer is shown

in Figure2. The line-to-line voltage on each side of the three phase transformer is equal the

nominal voltage of the single-phase transformer.

Figure 2 ∆/∆ -connected three-phase transformer

The advantage of this connection is that even under unbalanced loads the three-phase load

voltages remain substantially equal. The disadvantage of the A/A connection is the absence of a

neutral terminal on either side.

Part2: Y/Y Connection

A Y/Y connection for the primary and secondary windings of a three-phase transformer is shown

in Figure 3. The line-to-line voltage on each side of the three phase transformer is √ times the

nominal voltage of the single-phase transformer.

Figure 3 Y /Y-connected three-phase transformer

The main advantage of a Y/Y connection is that we have access to the neutral terminal on each

side and it can be grounded if desired.

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Part3: ∆/ Y Connection

A Y/∆ connection for the primary and secondary windings of a three-phase transformer is shown

in Figure 4.

Figure 4 ∆/Y -connected three-phase transformer

On the primary side the voltages are from line to line because the primary side ∆ connection,

whereas the voltages are from line to neutral on the secondary side because the winding in

secondary side Y connection. Therefore, the voltage and the current in the primary are out of

phase by 30 degree with the voltage and the current in the secondary.

Part4: Y/∆ Connection

A Y/∆ connection for the primary and secondary windings of a three-phase transformer is shown

in Figure 5.

Figure 5 Y/∆ -connected three-phase transformer

On the primary side the voltages are from line to neutral because the primary side Y connection,

whereas the voltages are from line to line on the secondary side because the winding in

secondary side ∆ connection. Therefore, the voltage and the current in the primary are out of

phase by 30 degree with the voltage and the current in the secondary.

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Experimental Procedures:

Part1: Y/Y Three phase Transformer Connection

1- Connect the three phase transformer circuit as shown figure.

Figure 6 Measure phase and line voltage in Y/Y Three phase transformer

2- Set the value of variable voltage source by knop 50%.

3- Find the value of E1-6 and E1-2, then check that the equation √ is true:

Figure 7 Measurements reading for three phase

4- By using phasor analyzer, Check line to line voltage lead phase voltage by 30 degree.

Figure 8 Phasor diagram between line and phase voltage in three phase Y/Y connection

E1-6 = VL-L = 199.1V

E1-2 = Vph = 115V

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5- Find the value of E1-6 and E3-8 then find the turns ration of three phase transformer.

Figure 9 Measure primary and secondary voltage in Y/Y Three phase transformer

6- Find the value of E1-6 and E1-2, then find turns ratio:

Figure 10 Measurements reading for three phase

7- Check the voltage between primary and secondary side in phase by phasor analyzer.

Figure 11 Phasor diagram between primary and secondary voltage in Y/ Y Three phase transformer

E1-6 = VL-L = 199.1V

E3-8 = Vph = 197.1V

N = E1 6

E3 8

199.1

197.1 ≈ 1

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Part2: Y/∆ Three phase Transformer Connection

1- Connect the three phase transformer circuit as shown figure.

Figure 12Measure phase and line voltage in Y/∆ Three phase transformer

2- Set the value of variable voltage source by knop 50%.

3- Find the value of E1-6 and E1-2, then check that the equation √ is true:

Figure 13 Measurements reading for three phase

4- By using phasor analyzer, Check the line to line voltage lead the phase voltage by 30

degree.

Figure 14 Phasor diagram between line and phase voltage in three phase Y/∆ connection

E1-6 = VL-L = 199.1V

E1-2 = Vph = 115V

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5- Find the value of E1-6 and E3-8 then find the turns ration of three phase transformer.

Figure 15 Measure primary and secondary voltage in Y/∆ Three phase transformer

6- Find the value of E1-6 and E1-2, then find turns ratio:

Figure1 6 Measurements reading for three phase

7- Check the voltage between primary and secondary side out of phase 30 degree by phasor

analyzer.

Figure 17 Phasor diagram between primary and secondary voltage in Y/∆ Three phase transformer

E1-6 = VL-L = 199.1V

E3-8 = Vph = 114.6V

N = E1 6

E3 8

199.1

114.6 ≈ √

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Exercise: 1- Repeat the above procedure for ∆/∆ and ∆/Y Connection.