Question

For which fault types will there be a zero sequence current that is non-zero? (L-L line-line; SLG single lin to-ground; DLG double line-to-ground; L-L line-to-line) a. 3-phase, L-L, SLG, DLG b. L-L, SLG, DLG c. SLG, DLG d. 3-phase, L-L 5. (10 points) The impedance of a 10MVA,22/220kV transformer is 0.2 per unit. What is this impedance in per unit for a power base of 10MVA and a voltage base of 11kV on the low voltage side of the transformer? a. 0.05 p.u. b. 0.10 p.u. c. 0.20 p.u. d. 0.40 p.u. I e. 0.80 p.u.

Answer 1

For the first question, the fault types that will have a non-zero zero sequence current are SLG (single line to ground) and DLG (double line to ground). Therefore, the correct answer is option c. SLG, DLG.

For the second question, to find the impedance in per unit on the low voltage side of the transformer, we need to calculate the per unit impedance at the low voltage base. Since the power base is 10 MVA and the voltage base is 11 kV, we can use the formula:

Zpu = Zbase / (Vbase)^2

Zpu = 0.2 / (11kV)^2 = 0.2 / (0.011MV)^2 = 0.2 / 0.000121 = 1.65 p.u.

Therefore, the impedance in per unit for a power base of 10 MVA and a voltage base of 11 kV on the low voltage side of the transformer is approximately 1.65 p.u.