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Questions and Answers
What type of relay is used for providing protection against earth fault at the secondary side of the 2.5MVA, 11/3.3kV transformer?
What type of relay is used for providing protection against earth fault at the secondary side of the 2.5MVA, 11/3.3kV transformer?
- Inverse Time Relay
- Definite Time Relay
- Inverse Definite Minimum Time Relay
- Instantaneous Relay (correct)
What is the main characteristic of a Definite Time relay?
What is the main characteristic of a Definite Time relay?
- Its pickup setting is dependent on the fault location
- Its tripping time decreases as the current increases
- Its tripping time is fixed and independent of the pickup setting (correct)
- It is suitable for backup protection
What is the percentage of the zero sequence setting in relay R1?
What is the percentage of the zero sequence setting in relay R1?
- 30%
- 40%
- 10%
- 20% (correct)
What type of relay is used for short length feeders where the fault current does not change significantly with the location of the fault?
What type of relay is used for short length feeders where the fault current does not change significantly with the location of the fault?
What is the fault current at bus A?
What is the fault current at bus A?
What is the main difference between an Inverse Time relay and an Inverse Definite Minimum Time relay?
What is the main difference between an Inverse Time relay and an Inverse Definite Minimum Time relay?
What is the operation time of relay R1 from the standard inverse curves?
What is the operation time of relay R1 from the standard inverse curves?
What is the PSM of relay R2 from the standard inverse curves?
What is the PSM of relay R2 from the standard inverse curves?
In what type of system is the setting of the instantaneous unit carried out by taking at least 125% of the symmetrical r.m.s. current for the maximum fault level?
In what type of system is the setting of the instantaneous unit carried out by taking at least 125% of the symmetrical r.m.s. current for the maximum fault level?
What is the purpose of the instantaneous element in an Inverse Time relay?
What is the purpose of the instantaneous element in an Inverse Time relay?
What is the operation time of relay R2 from the standard inverse curves?
What is the operation time of relay R2 from the standard inverse curves?
What is the main advantage of using an Inverse Definite Minimum Time relay?
What is the main advantage of using an Inverse Definite Minimum Time relay?
What is the main difference between a Definite Time relay and an Inverse Definite Minimum Time relay?
What is the main difference between a Definite Time relay and an Inverse Definite Minimum Time relay?
What is the significance of the '125%' value in setting the instantaneous unit in transmission systems?
What is the significance of the '125%' value in setting the instantaneous unit in transmission systems?
What is the primary characteristic of an Inverse Definite Minimum Time Relay?
What is the primary characteristic of an Inverse Definite Minimum Time Relay?
What is the main difference between an Instantaneous Relay and an Inverse Definite Minimum Time Relay?
What is the main difference between an Instantaneous Relay and an Inverse Definite Minimum Time Relay?
What type of relay is widely used and has both time and pick-up settings?
What type of relay is widely used and has both time and pick-up settings?
Why is an Instantaneous Relay not suitable for backup protection?
Why is an Instantaneous Relay not suitable for backup protection?
What is the primary purpose of Overcurrent Protection?
What is the primary purpose of Overcurrent Protection?
What type of relay is used for short length feeders?
What type of relay is used for short length feeders?
What happens to the operating time of an Inverse Definite Minimum Time Relay as the current increases?
What happens to the operating time of an Inverse Definite Minimum Time Relay as the current increases?
What is the main principle behind Overcurrent Protection?
What is the main principle behind Overcurrent Protection?
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Study Notes
Classification of Overcurrent Relays
- Definite Time relay: can be adjusted to issue a trip output after a specified delay when the relay picks up, with a fixed delay independent of pickup setting, used for short length feeders.
- Inverse Definite Minimum Time (IDMT) relay: inverse in the initial part, approaches a definite minimum operating time at high currents, widely used.
Instantaneous Relay
- Fast action, only pickup setting, no time setting, not suitable for backup protection.
- Used for transmission systems, setting based on symmetrical r.m.s. current for maximum fault level at the next substation.
Inverse Time Relay
- Operating time decreases as current increases.
Principle of Overcurrent Protection
- Relay decision based on current magnitude, used in radial distribution systems, principle used in overcurrent relaying and fuse protection.
Example Computation
- Instantaneous unit setting: 50% of maximum short-circuit current at CT connection point or 125% of symmetrical r.m.s. current for maximum fault level at the next substation.
- Relay R1: instantaneous and standard inverse units, zero sequence setting 20%, SLG fault current at bus A 480 A, at bus B 650 A.
- Relay R2: time dial setting 0.138, operation time 0.38 sec.
Lecture 6: Overcurrent Protection
- Overcurrent protection principle, classification of overcurrent relays, and relay computations.
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