Page 266 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Fig 5

Fig 6

Star delta (Y D) connection (Fig 7)
This connection is used at the end of the transmission linewhere the voltage is to be stepped down. The primary
winding is Y-connected with grounded neutral. The ratio between the secondary and primary line voltage is 1/3
times the transformation ratio of each transformer. There is a 30° shift between the primary and secondary line
voltages .A Y D transformer bank cannot be paralleled with either a Y Y bank. Also, third harmonic currents flows
in the D to provide a sinusoidal flux.
Delta star (D Y) connection (Fig 8)

At the beginning of high transmission system.
The neutral of the secondary is grounded for providing 3-phase 4-wire service.
Can be used to serve both the 3-phase power equipment and single-phase lighting circuits.
Primary and secondary line voltages and line currents are out of phase with each other by 30°.
Impossible to parallel such a bank with DD or Y Y bank of transformers even though the voltage ratios are correctly
adjusted.

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CITS : Power - Electrician & Wireman - Lesson 41-49 CITS : Power - Electrician & Wireman - Lesson 41-49