22 Questions
What type of relay is used for providing protection against earth fault at the secondary side of the 2.5MVA, 11/3.3kV transformer?
Instantaneous Relay
What is the main characteristic of a Definite Time relay?
Its tripping time is fixed and independent of the pickup setting
What is the percentage of the zero sequence setting in relay R1?
20%
What type of relay is used for short length feeders where the fault current does not change significantly with the location of the fault?
Definite Time relay
What is the fault current at bus A?
480 A
What is the main difference between an Inverse Time relay and an Inverse Definite Minimum Time relay?
The tripping time characteristics
What is the operation time of relay R1 from the standard inverse curves?
0.439 sec
What is the PSM of relay R2 from the standard inverse curves?
4.8
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?
Transmission systems
What is the purpose of the instantaneous element in an Inverse Time relay?
To detect high fault currents
What is the operation time of relay R2 from the standard inverse curves?
0.38 sec
What is the main advantage of using an Inverse Definite Minimum Time relay?
It provides a fixed tripping time
What is the main difference between a Definite Time relay and an Inverse Definite Minimum Time relay?
The tripping time characteristics
What is the significance of the '125%' value in setting the instantaneous unit in transmission systems?
It is the symmetrical r.m.s. current for the maximum fault level
What is the primary characteristic of an Inverse Definite Minimum Time Relay?
It is inverse in the initial part and tends to approach a definite minimum operating time characteristics as the current becomes high
What is the main difference between an Instantaneous Relay and an Inverse Definite Minimum Time Relay?
One has a pick-up setting and a time setting, the other only has a pick-up setting
What type of relay is widely used and has both time and pick-up settings?
Inverse Definite Minimum Time Relay
Why is an Instantaneous Relay not suitable for backup protection?
It has a pick-up setting but no time setting
What is the primary purpose of Overcurrent Protection?
To detect faults in a system
What type of relay is used for short length feeders?
Time Delay Definite Time Relay
What happens to the operating time of an Inverse Definite Minimum Time Relay as the current increases?
It approaches a definite minimum operating time
What is the main principle behind Overcurrent Protection?
When the current exceeds a predetermined value, it indicates the presence of a fault
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.
Learn about the classification of overcurrent relays, including definite time relays and their applications in electrical protection systems.
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