3 Phase Transformer Input Line Current (Δ-Y)

  • Thread starter jegues
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Homework Statement



A 25 MVA, 69kV/660V 60Hz, Δ-Y connected three-phase power transformer has an equivalent series impedance of 2.28 + j17.15Ω, a croe loss resistance of 15235.2Ω and a magnetizing reactance of 5078.4Ω all refered to the HV side. The transformer delivers 20MVA to a three-phase load with 0.866 lagging power factor at 660V.

b)Determine the magnitude of the input line-to-line voltage.

c)Determine the input line current.

d)Calculate the real and reactive power input to the transfromer

Homework Equations





The Attempt at a Solution



Please see the figures attached for my attempt at the solution.

My strategy was to conver the 3 phase circuit into its Y-Y equivalent, and solve the single phase circuit on the HV side.

I am getting confused with how to find the input line voltage and current.

I was able to transfer everything to the HV side of the single phase circuit and solve for every voltage and current.

Since the HV side was turned into a Y equivalent circuit it leads me to believe that the input voltage is simply √(3)*input phase voltage I solved(i.e. Vshv) and the input line current should be the same as the phase current I solved.(i.e. Ishv)

Well it turns out I am correct for the input line voltage, but incorrect for the input line current.

It just so happens that the correct value of the input line current is (1/3) of my claimed line current. (i.e. Ishv)

Why is this? What am I discounting?
 

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