Protective Relay Applications Flashcards
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Read the first 7 Protective Relay Applications flashcards as text
What is the purpose of the 'security' aspect of a protective relay system?
Answer: To prevent the relay from operating for conditions outside its intended protection zone
Security refers to a relay's ability to refrain from tripping for out-of-zone faults or non-fault conditions, preventing unnecessary outages.
For ground fault protection on a resistance-grounded system, which relay type is most appropriate?
Answer: Ground overcurrent relay with sensitive pickup (51G)
Resistance grounding limits ground fault current, so a sensitive ground overcurrent relay (51G) with a low pickup threshold is required for detection.
What is meant by 'zone 2' reach setting in a stepped distance protection scheme?
Answer: The impedance zone covering 100–120% of the protected line, clearing with a time delay
Zone 2 is set to reach beyond the remote bus (typically 120% of line length) and operates with a time delay to coordinate with remote terminal Zone 1.
Which protection philosophy uses two completely independent relay systems with separate CTs, VTs, DC supplies, and trip coils?
Answer: Primary and secondary protection (dual main)
Dual main (primary and secondary) protection provides two fully independent relay systems so that failure of any single component does not leave equipment unprotected.
What does the negative-sequence overcurrent relay (46) detect in a rotating machine?
Answer: Unbalanced currents caused by phase asymmetry or unbalanced loading
Negative-sequence currents from phase unbalance induce double-frequency currents in the rotor, causing rapid heating that relay 46 protects against.
In a permissive overreaching transfer trip (POTT) pilot scheme, the relay at each terminal sends a permissive signal when:
Answer: The relay detects an internal fault in the forward direction
In POTT, each terminal sends a permissive signal if it detects a forward fault; tripping occurs only when both the local relay sees a forward fault AND receives the permissive signal from the remote end.
What is the function of a synchronism check relay (25) in a substation?
Answer: To confirm that voltage, frequency, and phase angle across an open breaker are within acceptable limits before closing
The synchrocheck relay (25) supervises breaker closing by verifying voltage magnitude difference, frequency difference, and phase angle are within set limits to prevent damaging power surges.