Qestions:

a)  Draw the approximate equivalent circuit for a single phase power transformer. Identify all circuit elements and briefly explain their physical relevance.

b)  Short Circuit and Open Circuit tests have been conducted on a single-phase 19.2 kVA 480V/120V 50 Hz transformer, and the following values have been calculated from the results of these tests for the primary referred approximate equivalent circuit for this transformer:

 R_eq = 0.4 ω          X_eq = 0.88 ω          R_o = 512 ω                   X_o = 360

[i]  Calculate the voltage, current and power readings that would be expected from the open-circuit test on this transformer if the primary side was energised at rated voltage.

[ii] Calculate the voltage, current and power readings that would be expected from the short-circuit test on this transformer if the primary side was energised at rated current.

c)  The transformer of part (b) is supplied at rated primary voltage, and supplies a load current of 128A (measured on the secondary output side) with a lagging power factor of 0.866. Determine:

[i]   the secondary output voltage.

[ii]  The transformer efficiency.

d)  The tertiary, delta-connected winding in a Yy-connected three-phase transformer has a dual role.

Explain briefly the two reasons for its existence.