Originally Posted by

**Erased**
Now, I got a slightly different answer, so someone correct me if I'm mistaken.

First calculate HV currents.

Primary:

50000000/sqrt(3)*138000 = 209.18

Secondary:

50000000/sqrt(3)*69000 = 418.37

Now calculate CT currents. An electromechanical scheme on a Delta-Wye will have Wye-Delta CT's to account for the phase shift. Delta CT current must be multiplied by sqrt(3).

Primary - Wye CT:

209.18/50 = 4.18

Secondary - Delta CT:

418.37/100 = 4.18

4.18*sqrt(3) = 7.24

Calculate the percent mismatch as follows: On two-winding transformers,

determine the ratio of the two relay currents and the tap values selected. The

difference between these ratios, divided by the smaller ratio, is the percent

mismatch.

((4.18/5)-(7.24/9)) / (7.24/9) = 0.0392

3.92% with rounding errors.

Looks like your answer came from incorrectly dividing by the larger ratio.

(4.18/5)-(7.24/9) / (4.18/5) = 0.0377

3.77% with rounding errors.