Power Systems: Transmission, Faults, Stability and Protection — Practice

Question 1

A 30 MVA generator has X = 0.2 pu on its own base. Find X on a 100 MVA base.

Accepted Answer

0.2 x 100/30 = 0.667 pu.

Question 2

Name the three bus types in load flow and what is specified at each.

Accepted Answer

Slack: voltage and angle. PV: real power and voltage magnitude. PQ: real and reactive power.

Question 3

A bus has a Thevenin reactance of 0.667 pu on a 100 MVA base. Find the fault level.

Accepted Answer

Fault MVA = 100/0.667 = 150 MVA.

Question 4

Z1 = Z2 = j0.2 and Z0 = j0.1 pu, E = 1 pu. Compare three-phase and single line-to-ground fault currents.

Accepted Answer

Three-phase: 1/0.2 = 5 pu. Single line-to-ground: 3/(0.2 + 0.2 + 0.1) = 6 pu, which is larger because Z0 is less than Z1.

Question 5

Incremental costs are 0.2 P1 + 10 and 0.4 P2 + 8. Dispatch 100 MW economically.

Accepted Answer

0.2 P1 + 10 = 0.4(100 - P1) + 8 gives P1 = 63.3 MW, P2 = 36.7 MW and lambda = 22.7.

Question 6

State the equal-area criterion and how fast clearing helps.

Accepted Answer

A machine on an infinite bus is stable if the accelerating area equals the decelerating area before the angle passes its critical value. Faster clearing reduces the accelerating area, so stability improves.

Question 7

What does zone 1 of a distance relay cover and why is it not set to 100 percent of the line?

Accepted Answer

About 80 percent of the line, because errors in measurement and the line impedance could cause it to over-reach into the next line and trip wrongly.

Question 8

When does the Ferranti effect occur?

Accepted Answer

On a lightly loaded or open-circuited long line, where charging current raises the receiving-end voltage.

Practice — Power Systems: Transmission, Faults, Stability and Protection

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