NETA Power Systems Analysis & Troubleshooting 2 — Questions and Answers
Question 1: When performing a power factor test on a transformer bushing, a low power factor reading typically indicates:
- Moisture or contamination in the insulation (Correct answer)
- Excessive load current
- High contact resistance at the terminal
- Incorrect tap changer position
Correct answer: Moisture or contamination in the insulation
Low power factor on a bushing test indicates moisture ingress or contamination degrading the dielectric insulation.
Question 2: A cable insulation resistance test shows dramatically different readings between phases. The MOST likely cause is:
- Phase imbalance in the load
- Localized insulation damage or moisture on one phase (Correct answer)
- Incorrect meter calibration
- Overvoltage on the healthy phases
Correct answer: Localized insulation damage or moisture on one phase
Significantly different IR readings between phases point to localized insulation damage or moisture penetration on the affected phase.
Question 3: In a protective relay coordination study, the primary goal is to ensure that:
- All relays operate simultaneously during a fault
- The relay closest to the fault operates first (Correct answer)
- Upstream relays operate before downstream relays
- Instantaneous elements are disabled on all feeders
Correct answer: The relay closest to the fault operates first
Relay coordination ensures the device nearest the fault operates first, isolating only the faulted section while upstream devices serve as backup.
Question 4: What does a tan delta (dissipation factor) trending upward over successive test intervals on a cable system indicate?
- Normal aging within acceptable limits
- Progressive insulation degradation requiring attention (Correct answer)
- Improved insulation condition after drying
- Load growth on the cable circuit
Correct answer: Progressive insulation degradation requiring attention
An upward trend in tan delta values over time indicates progressive deterioration of the cable insulation system.
Question 5: The ANSI device number for an overcurrent relay is:
- 21
- 27
- 50/51 (Correct answer)
- 87
Correct answer: 50/51
ANSI device number 50 designates an instantaneous overcurrent relay and 51 designates a time-overcurrent relay.
Question 6: During a transformer turns ratio (TTR) test, a ratio error exceeding acceptable limits on one winding most likely indicates:
- Oil contamination in the conservator tank
- Shorted turns within that winding (Correct answer)
- Incorrect applied test voltage
- Phase reversal at the terminals
Correct answer: Shorted turns within that winding
A ratio error on a specific winding indicates shorted turns, which reduce the effective number of turns and alter the ratio.
Question 7: When conducting a circuit breaker contact resistance (micro-ohm) test, a high reading compared to the manufacturer's specification indicates:
- Excessive spring tension on the mechanism
- Worn, pitted, or oxidized contact surfaces (Correct answer)
- Correct breaker operation at rated voltage
- Dielectric strength below rated value
Correct answer: Worn, pitted, or oxidized contact surfaces
High contact resistance is caused by worn, pitted, or oxidized contacts that increase the resistive path through the breaker.
NETA Power Systems Analysis & Troubleshooting 2
When performing a power factor test on a transformer bushing, a low power factor reading typically indicates: