Power Systems Analysis & Troubleshooting Flashcards

When performing a power factor test on a transformer bushing, a low power factor reading typically indicates:

Answer: Moisture or contamination in the insulation

Low power factor on a bushing test indicates moisture ingress or contamination degrading the dielectric insulation.

A cable insulation resistance test shows dramatically different readings between phases. The MOST likely cause is:

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.

In a protective relay coordination study, the primary goal is to ensure that:

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.

What does a tan delta (dissipation factor) trending upward over successive test intervals on a cable system indicate?

Answer: Progressive insulation degradation requiring attention

An upward trend in tan delta values over time indicates progressive deterioration of the cable insulation system.

The ANSI device number for an overcurrent relay is:

Answer: 50/51

ANSI device number 50 designates an instantaneous overcurrent relay and 51 designates a time-overcurrent relay.

During a transformer turns ratio (TTR) test, a ratio error exceeding acceptable limits on one winding most likely indicates:

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.

When conducting a circuit breaker contact resistance (micro-ohm) test, a high reading compared to the manufacturer's specification indicates:

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.