Troubleshooting & Diagnostics Flashcards

A breaker fails to close when commanded from the control room but closes manually at the panel. The most likely cause is:

Answer: Failed remote close relay or control wiring issue

If local manual close works but remote command fails, the issue lies in the remote control circuit, such as a relay, wiring, or communication link.

Elevated hydrogen gas alone in a DGA sample from a transformer most likely indicates:

Answer: Corona or partial discharge activity

Hydrogen as the dominant gas with little methane or acetylene is characteristic of low-energy partial discharge (corona) in transformer insulation.

During a load flow analysis, a feeder shows voltage at the end closer to the substation is higher than near the load. This is abnormal because:

Answer: Voltage naturally drops along a feeder toward the load end

Under normal loaded conditions, voltage drops progressively away from the source due to line impedance; a voltage rise toward the load indicates a capacitive source or reverse power flow.

A circuit breaker trips on overcurrent during cold-load pickup after a prolonged outage. The best corrective action is:

Answer: Temporarily raise the overcurrent relay pickup or use time-delay settings during restoration

Cold-load pickup causes high initial inrush current; temporarily adjusting relay settings or using time-delay elements prevents nuisance tripping during restoration.

A power transformer's winding resistance test shows one winding is 15% higher than the factory baseline. This suggests:

Answer: Partial open circuit or deteriorated connection in that winding

A 15% increase in winding resistance compared to baseline indicates a developing open circuit, poor connection, or deteriorated winding in that tap.

A voltage regulator on a distribution feeder is hunting (continuously adjusting) rather than settling. The most common cause is:

Answer: Incorrect bandwidth or time-delay settings on the regulator control

Hunting occurs when the bandwidth is set too narrow or the time delay too short, causing the regulator to continually chase small voltage variations.

A technician finds that a feeder's ground overcurrent relay (51G) operates for every external fault on adjacent feeders. This indicates:

Answer: CT polarity or wiring is incorrect, causing false zero-sequence current

Incorrect CT polarity or wiring errors in a residual ground scheme produce spurious zero-sequence current during external faults, causing false relay operation.