Using the information given in Table C and Fig. 31, calculate the approximate real values of the impedances of a 250 kVA, 4160 V/480 V, 60 Hz distribution transformer.

Question

TABLE C TYPICAL PER-UNIT VALUES OF TRANSFORMERS
Circuit element	3 kVA to 250 kVA	1 MVA to 100 MVA
R₁ or R₂	0.009-0.005	0.005-0.002
X_f1 or X_f2	0.008-0.025	0.03-0.06
X_m	20-30	50-200
R_m	20-50	100-500
I_o	0.05-0.03	0.02-0.005

Accepted Answer

We first determine the base impedances on the primary and secondary side. From the results of Example 8, we have

[latex]\begin{aligned}&Z_{ np }=69 \Omega \&Z_{ ns }=0.92 \Omega\end{aligned}[/latex]

We now calculate the real impedances by multiplying [latex]Z_{ np } ext { and } Z_{ ns }[/latex] by the per-unit values given in Table C. This yields the following results:

[latex]\begin{aligned}R_{1} &=0.005 imes 69 \Omega=0.35 \Omega \R_{2} &=0.005 imes 0.92 \Omega=4.6 m \Omega \X_{ f 1} &=0.025 imes 69 \Omega=1.7 \Omega \X_{ f 2} &=0.025 imes 0.92 \Omega=23 m \Omega \X_{ m } &=30 imes 69 \Omega=2070 \Omega=2 k \Omega \R_{ m } &=50 imes 69 \Omega=3450 \Omega=3.5 k \Omega\end{aligned}[/latex]

This example shows the usefulness of the perunit method of estimating impedances. The equivalent circuit of the 250 kVA transformer is shown in Fig. 32. The true values may be 20 to 50 percent higher or lower than those shown in the figure. The reason is that the per-unit values given in Table C are broad estimates covering a wide range of transformers.

Question 10.9: Using the information given in Table C and Fig. 31, calcula...

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