AEP Troubleshooting & Diagnostics 3 — Questions and Answers
Question 1: A substation bus voltage drops suddenly but feeder current remains normal. What is the most probable cause?
- Load increase on feeders
- Voltage regulator malfunction or tap changer failure (Correct answer)
- Damaged feeder cable
- CT saturation
Correct answer: Voltage regulator malfunction or tap changer failure
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:
- Correct CT operation
- Shorted secondary turns (Correct answer)
- Open secondary winding
- Incorrect burden connected
Correct answer: Shorted secondary turns
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?
- Three-phase bolted fault
- High-impedance ground fault (Correct answer)
- Phase-to-phase fault
- Overload condition
Correct answer: High-impedance ground fault
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?
- Normal balanced load
- Phase transposition error or unbalanced fault (Correct answer)
- Capacitive reactive power
- Harmonic distortion only
Correct answer: Phase transposition error or unbalanced fault
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?
- Reset the recloser and re-energize immediately
- Patrol and inspect the feeder for a permanent fault before re-energizing (Correct answer)
- Increase recloser pickup setting
- Bypass the recloser and energize from the substation bus
Correct answer: Patrol and inspect the feeder for a permanent fault before re-energizing
Lockout after three operations indicates a permanent fault; a physical inspection must locate and clear the fault before re-energizing.
Question 6: Thermal imaging of a cable termination shows a hot spot at the lug. What should you check first?
- Cable insulation resistance
- Lug torque and connection integrity (Correct answer)
- Phase-to-phase voltage
- Grounding of cable shield
Correct answer: Lug torque and connection integrity
Hot spots at lugs are most commonly caused by loose or improperly torqued connections that increase contact resistance.
Question 7: A power quality meter records high total harmonic distortion (THD) on a bus supplying a VFD load. The best mitigation is:
- Increase system voltage
- Install a line reactor or harmonic filter (Correct answer)
- Reduce VFD speed
- Replace the VFD with a soft starter
Correct answer: Install a line reactor or harmonic filter
Line reactors and harmonic filters are standard mitigation devices that reduce harmonic currents injected by VFDs into the power system.
AEP Troubleshooting & Diagnostics 3
A substation bus voltage drops suddenly but feeder current remains normal.
What is the most probable cause?