NETA Protective Relay Applications 3 — Questions and Answers
Question 1: When testing a directional overcurrent relay (67), which reference quantity is typically used for directional discrimination?
- Zero-sequence voltage (3V0)
- Positive-sequence current
- Polarizing voltage or current (Correct answer)
- Negative-sequence impedance
Correct answer: Polarizing voltage or current
Directional relays use a polarizing quantity (voltage or current) as a reference to determine whether the fault is in the forward or reverse direction.
Question 2: A generator protection relay setting for loss-of-field (40) is based on the measurement of:
- Positive-sequence voltage only
- Reactive power absorption (leading current) (Correct answer)
- Negative-sequence current
- Over-frequency condition
Correct answer: Reactive power absorption (leading current)
Loss of field causes the generator to absorb reactive power (operate in an under-excited leading power factor condition), which the impedance-based relay detects.
Question 3: What is 'CT saturation' and how does it affect differential relay performance?
- CT output exceeds rated burden, causing relay over-voltage
- CT core saturates under high current, producing distorted secondary current that can cause false differential trips (Correct answer)
- CT ratio changes permanently under fault conditions
- CT saturation improves relay sensitivity during external faults
Correct answer: CT core saturates under high current, producing distorted secondary current that can cause false differential trips
CT saturation during external faults produces distorted secondary currents that appear as differential current, potentially causing false relay operation.
Question 4: In transformer differential protection, what is the purpose of the harmonic restraint feature?
- To prevent operation during lightning transients
- To block tripping during magnetizing inrush current which contains significant 2nd harmonic (Correct answer)
- To detect turn-to-turn faults from harmonic distortion
- To compensate for CT ratio mismatch
Correct answer: To block tripping during magnetizing inrush current which contains significant 2nd harmonic
Inrush current during transformer energization contains high 2nd harmonic content, and harmonic restraint uses this to distinguish inrush from internal fault current.
Question 5: Which ANSI device number is assigned to a breaker failure relay?
- 50BF (Correct answer)
- 51BF
- 50
- 62
Correct answer: 50BF
ANSI device number 50BF designates the breaker failure protection function, which initiates backup tripping if the primary breaker fails to open.
Question 6: A breaker failure relay initiates after the primary relay trips. What starts the breaker failure timer?
- Fault inception detected by the relay
- The trip signal issued by the primary relay (Correct answer)
- Manual operator intervention
- Loss of DC control power
Correct answer: The trip signal issued by the primary relay
The breaker failure timer typically starts when the primary relay issues a trip signal, timing out if current still flows after the expected breaker opening time.
Question 7: What does the pickup current setting of an overcurrent relay represent?
- The maximum continuous current the relay can carry
- The minimum secondary current at which the relay element begins to operate (Correct answer)
- The instantaneous trip threshold for zone 1
- The rated current of the protected equipment
Correct answer: The minimum secondary current at which the relay element begins to operate
The pickup current is the minimum secondary current value at which the relay element energizes and begins timing (or trips instantaneously).
NETA Protective Relay Applications 3
When testing a directional overcurrent relay (67), which reference quantity is typically used for directional discrimination?