AEP Troubleshooting & Diagnostics 3

Question 1

A substation bus voltage drops suddenly but feeder current remains normal. What is the most probable cause?

Accepted Answer

Voltage regulator malfunction or tap changer failure

Answer

A sudden voltage drop without a corresponding load change typically indicates a voltage regulator or tap changer fault.

Question 2

When testing a protection CT with a turns-ratio test, you find the measured ratio is 10% higher than nameplate. This most likely indicates:

Accepted Answer

Shorted secondary turns

Answer

Shorted secondary turns reduce the effective secondary turns, causing the measured ratio to appear higher than the nameplate value.

Question 3

A ground fault indicator on a distribution feeder activates, but the feeder does not trip. What type of fault is most likely?

Accepted Answer

High-impedance ground fault

Answer

High-impedance ground faults produce limited fault current that may activate a ground indicator but not exceed overcurrent relay pickup.

Question 4

You measure negative sequence current in a balanced three-phase system. What does this indicate?

Accepted Answer

Phase transposition error or unbalanced fault

Answer

Negative sequence current appears when there is phase imbalance caused by an unbalanced fault or wiring error such as a phase transposition.

Question 5

An automatic recloser operates three times and locks out. The feeder remains de-energized. What is the correct next step?

Accepted Answer

Patrol and inspect the feeder for a permanent fault before re-energizing

Answer

Lockout after three operations indicates a permanent fault; a physical inspection must locate and clear the fault before re-energizing.

(AEP) American Electric Power Practice Test