Scott-T Transformer Connection

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The Scott-T connection is a method of connecting two 1-phase transformers to perform the 3- phase to 2-phase conversion and vice-versa. In the Scott connection, the two 1-phase transformers are connected electrically (not magnetically), where one transformer is known as main transformer and the other is called as auxiliary or teaser transformer.

The connection diagram of the Scott connection is shown in the figure. The main transformer is the centre tapped transformer at point D and is connected across the lines Y and B of the 3- phase side. Thus, the primary of the main transformer is YB and the secondary is a₁a₂. The teaser transformer is connected between the line terminal R and the centre tapping point D. Thus, the teaser transformer has primary winding RD and the secondary winding b₁b₂.

Phasor Diagram of Scott-T Connection

The phasor diagram of the Scott connection is shown in the figure.

Here, the line voltages of the three-phase balanced supply systb are −

$$\mathrm{V_{RY} \: = \: V_{YB} \: = \: V_{BR} \: = \: V_{L}}$$

Let V_YB be taken as the reference phasor, then

$$\mathrm{V_{YB} \: = \: V_{L} \: \angle 0^{\circ}}$$

$$\mathrm{V_{RY} \: = \: V_{L} \: \angle + 120^{\circ}}$$

$$\mathrm{V_{BR} \: = \: V_{L} \: \angle - 120^{\circ}}$$

The figure also shows the voltages on the primary windings of the main and teaser transformers. Since, the centre tapping point D divides the primary winding YB of the main transformer into two equal halves. Therefore,

$$\mathrm{\text{Number of turns in portion } \: YD \: = \: \text{ Number of turns in portion } \: DB \: = \: \frac{N_{P}}{2}}$$

Hence, the voltages V_YD and V_DB are equal and in phase with the voltage V_YB i.e.

$$\mathrm{V_{YD} \: = \: V_{DB} \: = \: \frac{VBC}{2} \: = \: \frac{V_{L}}{2} \: \angle 0^{\circ} \:\: \dotso \:(1)}$$

Now, the voltage between terminals R and D is

$$\mathrm{V_{RD} \: = \: V_{RY} \: + \: V_{YD} \: = \: V_{L} \: \angle 120^{\circ} \: + \: \frac{V_{L}}{2} \: \angle 0^{\circ}}$$

$$\mathrm{\Rightarrow \: V_{RD} \: = \: \left[V_{L}\left(-\frac{1}{2} \: + \: j\frac{\surd 3}{2}\right)\right] \: + \: \left[\frac{V_{L}}{2}(1 \: + \: j0)\right]}$$

$$\mathrm{\Rightarrow \: V_{RD} \: = \: j\frac{\surd 3}{2}V_{L} \: = \: 0.866 \: V_{L} \: \angle 90^{\circ} \:\: \dotso \: (2)}$$

Thus, the primary voltage V_RD of the teaser transformer is 0.866 times of that of the main transformer and is 90° displaced from it in time.

The voltage V_RD is applied to the primary winding of the teaser transformer. Thus, the secondary terminal voltage V_2t of the teaser transformer will lead the secondary terminal voltage V_2m of the main transformer. For the same flux in each transformer, the voltage per turn should be the same. To make the voltage per turn same in the primary windings of the main and teaser transformers, the number of turns in the primary of the teaser transformer should be

$$\mathrm{\text{Number of turns in winding RD, } \: N_{RD} \: = \: \frac{\surd 3}{2}N_{P}}$$

Then,

$$\mathrm{\frac{V_{2t}}{N_{S}} \: = \: \frac{V_{RD}}{N_{RD}}}$$

$$\mathrm{\Rightarrow \: V_{2t} \: = \: \frac{N_{S}}{N_{RD}}V_{RD} \: = \: \left(\frac{N_{S}}{\frac{\surd 3}{2}N_{P}}\right)\left(\frac{\surd 3}{2}V_{L}\right)}$$

$$\mathrm{\Rightarrow \: V_{2t} \: = \: \frac{N_{S}}{N_{P}}V_{L} \: = \: V_{2m} \:\: \dotso \: (3)}$$

Therefore, the secondary windings of both the main and teaser transformers have equal voltage in magnitude, but they are displaced by 90° in time so that they results in a balanced two-phase systb.

Relationship between Input and Output Currents

Let, I_R, I_Y and I_B are the line currents on the 3-phase input side.

I_1m = Primary Current of Main Transformer

I_2m = Secondary Current of Main Transformer

I_1t = Primary Current of Teaser Transformer

I_2t = Secondary Current of Teaser Transformer

Now, from the connection diagram,

$$\mathrm{I_{1t} \: = \: I_{R}}$$

As, the secondary windings of both the transformers are identical, thus,

$$\mathrm{|I_{2m}| \: = \: |I_{2t}| \: = \: I_{2}\:(say)}$$

If the magnetising current of the transformers is neglected, then the mmf balance equation of the teaser transformer is given by,

$$\mathrm{I_{1t}N_{RD} \: = \: I_{2t}N_{S}}$$

$$\mathrm{\Rightarrow \: I_{R}\left(\frac{\surd 3}{2}N_{p}\right)} \: = \: I_{2t}N_{S}$$

Therefore,

$$\mathrm{I_{R} \: = \: I_{1t} \: = \: \frac{2}{\surd 3}\left(\frac{N_{S}}{N_{P}}\right)I_{2t} \: = \: 1.15\:K\:I_{2t}\:\: \dotso \: (4)}$$

Where, K = $mathrm{(\frac{N_{S}}{N_{P}})}$ is the transformation ratio of the transformer. Hence,

$$\mathrm{I_{2t} \: = \: \frac{2}{\surd 3}\left(\frac{I_{R}}{K}\right)} \:\: \dotso \: (5)$$

Now, the MMF balance equation of the main transformer is,

$$\mathrm{I_{1m}N_{YD} \: - \: I_{1m}N_{BD} \: = \: I_{2m}N_{S}}$$

$$\mathrm{\Rightarrow \: I_{Y}\left(\frac{N_{P}}{2}\right) \: - \: I_{B}\left(\frac{N_{P}}{2}\right) \:= \: I_{2m}N_{S}}$$

$$\mathrm{\Rightarrow \: I_{Y} \: - \: I_{B} \: = \: 2\left(\frac{N_{S}}{N_{P}}\right)I_{2m}\:=\: 2\:K\:I_{2m}\:\:\dotso \: (6)}$$

Therefore,

$$\mathrm{I_{2m} \: = \: \frac{1}{2}\left({\frac{I_{Y} \: - \: I_{B}}{K}}\right)\:\: \dotso \: (7)}$$

Now, for a balanced 3-phase systb,

$$\mathrm{I_{R} \: + \: I_{Y} \: + \: I_{B} \: = \: 0}$$

$$\mathrm{\therefore \: I_{B} \: = \: - \: I_{R} \: - \: I_{Y} \:\: \dotso \: (8)}$$

Substituting the value of IB in the equation (6), we get,

$$\mathrm{I_{Y} \: - \: (-I_{R} \: - \: I_{Y}) \: = \: 2\:K\:I_{2m}}$$

$$\mathrm{\Rightarrow\: 2I_{Y} \: + \: I_{R} \: = \: 2\:K \:I_{2m}}$$

$$\mathrm{\Rightarrow\:I_{Y} \: = \: -\frac{I_{R}}{2} \: + \: K\: I_{2m} \:\: \dotso \: (9)}$$

Again, substituting the value of current IY in eq. (8), we have,

$$\mathrm{I_{B} \: = \: -I_{R} \: - \: \left(-\frac{I_{R}}{2} \: + \: K \:I_{2m}\right)}$$

$$\mathrm{\Rightarrow\:I_{B} \: = \: - \frac{I_{R}}{2} \: - \: K \:I_{2m} \:\: \dotso \: (10)}$$

These current equations are valid for balanced as well as unbalanced loads.

Applications of Scott-T Connection

The Scott connection of transformer is used in following applications −

• The Scott connection is used to link a 3-phase systb with a 2-phase systb with the flow of power in either direction.
• It is used to operate two 1-phase electric furnaces together when the power is drawn from the 3-phase supply.
• It is also used to supply 1-phase loads such as electric trains from the balanced three phase supply.