Protection & Relay Systems Flashcards
7 cards from real AEP practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Protection & Relay Systems flashcards as text
What does a distance relay measure to determine whether a fault is within its protected zone?
Answer: The impedance (Z) between the relay location and the fault point
Distance relays calculate the impedance seen from the relay to the fault; if that impedance falls within the set reach, the relay operates.
What is 'directional overcurrent protection' and why is it used?
Answer: Protection that only operates for currents flowing in one direction, used on looped or multi-source networks to prevent incorrect tripping
Directional overcurrent relays (ANSI 67) sense both magnitude and direction of current, ensuring they only trip for faults in the protected direction on networks with multiple sources.
What is the coordination time interval (CTI) in relay coordination?
Answer: The minimum time margin between a downstream relay tripping and an upstream relay tripping for the same fault
CTI (typically 0.2–0.4 seconds) ensures the downstream relay clears a fault before the upstream backup relay operates, maintaining selective fault isolation.
What is the purpose of automatic reclosing on a transmission or distribution line?
Answer: To automatically re-energize a line after a temporary fault clears, restoring service without manual intervention
Most faults on overhead lines are temporary (lightning, tree contact); auto-reclosers restore service in milliseconds after the arc self-extinguishes.
What is bus differential protection designed to do?
Answer: Detect faults on the substation bus by comparing all currents flowing into and out of the bus
Bus differential protection sums all currents entering and leaving the bus; a nonzero sum indicates a bus fault, causing all connected breakers to trip.
What is the 'reach' of a distance relay?
Answer: The maximum impedance (in ohms) the relay will respond to, defining how far along the line it is protected
Reach is the impedance threshold setting; faults with calculated impedance less than the reach value are within the protected zone and cause the relay to operate.
In a three-zone distance relay scheme, Zone 1 is typically set to protect what percentage of the line?
Answer: 80–85% of the line with instantaneous operation
Zone 1 is set to 80–85% of the line and trips instantaneously, with the underreach margin preventing unwanted trips for faults just beyond the remote bus.