NETA - InterNational Electrical Testing Association Certified Technicians Protective Relay Applications Questions and Answers — Questions and Answers
Question 1: A technician is testing a bus differential (87B) protection scheme. The fundamental principle of operation for this relay is based on:
- The impedance of the bus changing during a fault.
- A time-delayed response to overcurrent conditions.
- Kirchhoff's Current Law, where the vector sum of currents entering and leaving the bus should be zero. (Correct answer)
- The detection of negative-sequence voltage during unbalanced faults.
Correct answer: Kirchhoff's Current Law, where the vector sum of currents entering and leaving the bus should be zero.
Bus differential protection (87B) operates on the principle of Kirchhoff's Current Law. It compares the sum of currents from all circuits connected to the bus. Under normal conditions or for an external fault, the currents entering the bus equal the currents leaving it, resulting in a near-zero differential current. For an internal fault on the bus, this balance is disrupted, causing a significant differential current that trips the relay.
Question 2: During the commissioning of a large industrial motor, a technician must verify the settings for the thermal overload protection (ANSI Device 49). Which of the following conditions is this relay primarily designed to protect against?
- Instantaneous short circuits within the motor windings.
- Sustained overcurrents that cause excessive heating. (Correct answer)
- Phase reversal that would cause the motor to run backwards.
- Ground faults on the motor frame.
Correct answer: Sustained overcurrents that cause excessive heating.
The ANSI Device 49, or thermal overload relay, is specifically designed to protect a motor from damage caused by overheating due to sustained overcurrent conditions. It models the thermal characteristics of the motor and will trip after a time delay if the current exceeds the motor's full-load ampere rating for too long, preventing insulation breakdown. It does not provide instantaneous protection for short circuits (Device 50) or protection against phase reversal (Device 46).
Question 3: A distance relay (ANSI Device 21) is most commonly applied for the protection of which of the following power system elements?
- Generators
- Transformers
- Capacitor banks
- Transmission lines (Correct answer)
Correct answer: Transmission lines
Distance relays (ANSI Device 21) are the primary protection for medium and long transmission lines. They calculate the impedance to a fault based on measured voltage and current. Since impedance is proportional to the length of the line, the relay can determine if a fault is within its designated zone of protection. This makes it highly selective and suitable for transmission line applications.
Question 4: A technician is performing maintenance on a generator's protection panel and encounters a relay designated as 51V. What is the function of this protective device?
- Instantaneous overvoltage protection.
- Inverse time overcurrent protection with a voltage restraint feature. (Correct answer)
- A synchronism check relay that verifies voltage match.
- A directional power relay based on voltage.
Correct answer: Inverse time overcurrent protection with a voltage restraint feature.
A 51V relay is a time-overcurrent relay (51) that includes a voltage restraint or voltage control feature (V). This is commonly used for generator backup protection. The voltage restraint feature allows the relay to be more sensitive to faults close to the generator, where the voltage drops significantly, while remaining secure against tripping on high load currents when the voltage is normal.
Question 5: When testing a differential protection scheme for a power transformer (87T), which of the following is a critical check to ensure the relay remains stable for external faults (through-faults)?
- Verifying the instantaneous trip setting is below the maximum fault current.
- Ensuring the time-dial setting allows for coordination with downstream devices.
- Confirming correct CT polarity and ratio mismatch compensation. (Correct answer)
- Testing the undervoltage blocking function.
Correct answer: Confirming correct CT polarity and ratio mismatch compensation.
For a transformer differential relay (87T), correct current transformer (CT) polarity is essential. The relay operates on the vector difference between the currents on the primary and secondary sides. If the CT polarities are reversed, the relay will see the sum of the currents during an external fault, causing an incorrect trip. Ratio mismatch compensation settings are also crucial to account for the transformer's voltage ratio and different CT ratios.
Question 6: Which of the following protective functions is NOT typically applied directly to a medium-voltage induction motor?
- Locked Rotor / Excessive Starting Time (51LR / 14)
- Thermal Overload (49)
- Differential Protection (87)
- Distance Protection (21) (Correct answer)
Correct answer: Distance Protection (21)
Distance protection (21) is used to protect transmission and distribution lines by measuring impedance to a fault. It is not an appropriate protection scheme for an induction motor. Motors are typically protected by thermal overload (49), locked rotor/stall protection (51LR/14), and for larger, more critical motors, differential protection (87M) to detect internal winding faults.
NETA - InterNational Electrical Testing Association Certified Technicians Protective Relay Applications Questions and Answers
A technician is testing a bus differential (87B) protection scheme.
The fundamental principle of operation for this relay is based on: