A three-phase transformer has 2000 turns on the primary side and 400 turns on the secondary side. For a supply voltage of 33kV, calculate secondary line voltage when the windings are connected in the following:

Question

a) delta-star
b) star-delta

Answer 1

a) When the windings are connected delta-star:

Since the primary side is delta-connected, the phase voltage on the primary side is equal to the line voltage. Therefore, the phase voltage on the primary side is 33kV.

For a delta-star connection, the line voltage on the secondary side is √3 times the phase voltage on the secondary side. Therefore, the line voltage on the secondary side is:

Vline_secondary = √3 * Vphase_secondary
Vline_secondary = √3 * (33kV * (400/2000))
Vline_secondary = √3 * 6.6kV
Vline_secondary = 11.429kV

Therefore, the secondary line voltage when the windings are connected delta-star is 11.429kV.

b) When the windings are connected star-delta:

For a star-delta connection, the line voltage on the secondary side is equal to the phase voltage on the secondary side. Therefore, the line voltage on the secondary side is:

Vline_secondary = Vphase_secondary
Vline_secondary = 33kV * (400/2000)
Vline_secondary = 33kV * 0.2
Vline_secondary = 6.6kV

Therefore, the secondary line voltage when the windings are connected star-delta is 6.6kV.