Scott Connection of Transformer

Scott Connection of Transformer

A Scott connection is the most common method to connect transformers to perform the three-phase to two-phase conversion and vice-versa.

In this method, two transformers are connected electrically. But both are divided magnetically. It requires two transformers. One transformer is named the main transformer, and the second is called an auxiliary transformer.

The connection diagram of Scott’s connection is as shown in the figure below.

Scott connection
Scott connection

The auxiliary transformer is also known as a teaser transformer.

The main transformer’s primary winding is centre-tapped at point D, And it is connected between two phases (B and C) of a three-phase supply. A secondary winding of the main transformer is connected between a1 and a2.

The primary winding of an auxiliary transformer is connected between point D and A-phase. And secondary winding is connected between b1 and b2.

For Scott connection, identical interchangeable transformers are used. In this type of transformer, there are many turns TP in primary winding provided with tapings of 0.289TP, 0.5TP, and 0.866TP.

Phasor Diagram of Scott Connection

The line voltage of a three-phase transformer is VAB, VBC, and VCA. In balance load conditions, these voltages are equal and 120˚ apart. These voltages can be placed in the form of a triangle, as shown in the figure below.

Scott connection phasor diagram
Scott connection phasor diagram

Here, we have taken VBC as reference. Hence,

    \[ V_{BC} = V_L \angle 0 \circ \]

    \[ V_{CA} = V_L \angle -120 \circ \]

    \[ V_{AB} = V_L \angle +120 \circ \]

The above figure shows the voltage on the primary winding of the main and auxiliary transformers. Here, the primary winding of main transformer BC divides into two equal parts at point D. A number of turns in portion BD are similar to the number of turns in portion DC.

Hence, the voltage VBD and VDC is equal and it is half of the line voltage.

    \[ V_{BD} = V_{DC} = \frac{1}{2} V_{BC} = \frac{1}{2} V_L \angle 0 \circ \]

    \[ V_{AD} = V_{AB} + V_{BD} \]

    \[ V_{AD} = V_L \angle +120 \circ + \frac{1}{2} V_L \angle 0 \circ \]

    \[ V_{AD} = V_L \left( cos(120) + j sin(120) \right) + \frac{1}{2} V_L \]

    \[ V_{AD} = V_L \left( \frac{-1}{2} + j\frac{\sqrt{3}}{2} \right) + \frac{1}{2} V_L \]

    \[ V_{AD} = j \frac{\sqrt{3}}{2} V_L \]

    \[ V_{AD} = 0.866 V_L \angle 90 \circ \]

Therefore, the primary voltage of auxiliary transformer VAD is 0.866 of the main transformers and at an angle of 90˚.

Position of Neutral Point N

If both transformers have four terminals to connect with three-phase supply, the tapping point N is provided in the auxiliary transformer in such a way that,

Voltage across AN = VAN = phase voltage

    \[ V_{AN} = \frac{V_L}{\sqrt{3}} \]

Voltage across AD is;

    \[ V_{AD} = \frac{\sqrt{3}}{2} V_L \]

Voltage across ND is;

    \[ V_{ND} = V_{AD} - V_{AN} \]

Hence,

    \[ V_{ND} =  \frac{\sqrt{3}}{2} V_L - \frac{V_L}{\sqrt{3}} \]

    \[ V_{ND} = \frac{V_L}{2 \sqrt{3}} \]

In order to keep same voltage per turns;

Turns in portion AN;

    \[ T_{AN} = \frac{T_P}{\sqrt{3}} = 0.577 T_P \]

Turns in portion ND;

    \[ T_{ND} = \frac{T_P}{2 \sqrt{3}} = 0.288 T_P \]

Turns in portion AD;

    \[ T_{AD} = \frac{\sqrt{3}}{2} T_P = 0.866 T_P \]

Application of Scott Connection

The application of Scott connection is as listed below.

  • This type of transformer connection links a three-phase system with a two-phase system with a flow of power in either direction.
  • It is used to supply single-phase load like electric trains, which are scheduled to keep the load on the three-phase system as nearly balanced as possible.
  • It is desired to operate two single-phase electrical furnaces together and run a balanced load from the three-phase supply.

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