NETA Protective Relay Applications 2 — Questions and Answers
Question 1: What is the primary function of a distance relay (21) in transmission line protection?
- Measure line current magnitude only
- Measure impedance to the fault location (Correct answer)
- Monitor voltage at the sending end
- Detect zero-sequence current only
Correct answer: Measure impedance to the fault location
A distance relay measures the impedance from the relay location to the fault, using both voltage and current inputs to determine fault location.
Question 2: Which relay characteristic is best suited for protecting a transformer against internal turn-to-turn faults?
- Overcurrent relay (51)
- Differential relay (87) (Correct answer)
- Distance relay (21)
- Overvoltage relay (59)
Correct answer: Differential relay (87)
The differential relay (87) compares current entering and leaving the transformer; internal faults cause an imbalance that triggers tripping.
Question 3: In a Mho distance relay, the operating characteristic on an R-X diagram is shaped like a:
- Rectangle
- Lens
- Circle passing through the origin (Correct answer)
- Straight line
Correct answer: Circle passing through the origin
The Mho relay characteristic is a circle on the R-X diagram that passes through the origin, providing inherent directionality.
Question 4: What does the term 'reach' mean in the context of a distance relay?
- The maximum voltage the relay can measure
- The impedance setting that defines the protected zone boundary (Correct answer)
- The relay's operating time in cycles
- The current threshold for instantaneous tripping
Correct answer: The impedance setting that defines the protected zone boundary
Reach is the impedance value (in ohms) up to which the relay will operate, defining the boundary of the protection zone.
Question 5: A pilot protection scheme uses communications between line terminals primarily to:
- Increase the relay's current sensitivity
- Achieve high-speed tripping for faults anywhere on the protected line (Correct answer)
- Reduce the number of CTs required
- Provide backup protection for bus faults
Correct answer: Achieve high-speed tripping for faults anywhere on the protected line
Pilot schemes use communications to allow both terminals to trip simultaneously for internal faults, achieving high-speed protection for the full line length.
Question 6: Which type of bus protection scheme uses current from all feeders connected to the bus and compares them?
- Frame leakage protection
- Directional comparison protection
- Differential bus protection (87B) (Correct answer)
- Overcurrent bus protection
Correct answer: Differential bus protection (87B)
Differential bus protection sums all currents flowing into and out of the bus; any difference indicates an internal fault.
Question 7: What is the purpose of a check zone in high-impedance bus differential protection?
- To measure bus voltage during normal operation
- To provide a redundant differential element covering all bus sections (Correct answer)
- To detect CT saturation in individual feeder CTs
- To supervise the breaker failure timer
Correct answer: To provide a redundant differential element covering all bus sections
The check zone covers all bus sections and must operate simultaneously with a zone element, preventing false trips due to CT errors.
NETA Protective Relay Applications 2
What is the primary function of a distance relay (21) in transmission line protection?