Overcurrent Relay Classification
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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?

  • Inverse Time Relay
  • Definite Time Relay
  • Inverse Definite Minimum Time Relay
  • Instantaneous Relay (correct)

    • 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?

  • 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?

    <p>Definite Time relay</p> Signup and view all the answers

    What is the fault current at bus A?

    <p>480 A</p> Signup and view all the answers

    What is the main difference between an Inverse Time relay and an Inverse Definite Minimum Time relay?

    <p>The tripping time characteristics</p> Signup and view all the answers

    What is the operation time of relay R1 from the standard inverse curves?

    <p>0.439 sec</p> Signup and view all the answers

    What is the PSM of relay R2 from the standard inverse curves?

    <p>4.8</p> Signup and view all the answers

    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?

    <p>Transmission systems</p> Signup and view all the answers

    What is the purpose of the instantaneous element in an Inverse Time relay?

    <p>To detect high fault currents</p> Signup and view all the answers

    What is the operation time of relay R2 from the standard inverse curves?

    <p>0.38 sec</p> Signup and view all the answers

    What is the main advantage of using an Inverse Definite Minimum Time relay?

    <p>It provides a fixed tripping time</p> Signup and view all the answers

    What is the main difference between a Definite Time relay and an Inverse Definite Minimum Time relay?

    <p>The tripping time characteristics</p> Signup and view all the answers

    What is the significance of the '125%' value in setting the instantaneous unit in transmission systems?

    <p>It is the symmetrical r.m.s. current for the maximum fault level</p> Signup and view all the answers

    What is the primary characteristic of an Inverse Definite Minimum Time Relay?

    <p>It is inverse in the initial part and tends to approach a definite minimum operating time characteristics as the current becomes high</p> Signup and view all the answers

    What is the main difference between an Instantaneous Relay and an Inverse Definite Minimum Time Relay?

    <p>One has a pick-up setting and a time setting, the other only has a pick-up setting</p> Signup and view all the answers

    What type of relay is widely used and has both time and pick-up settings?

    <p>Inverse Definite Minimum Time Relay</p> Signup and view all the answers

    Why is an Instantaneous Relay not suitable for backup protection?

    <p>It has a pick-up setting but no time setting</p> Signup and view all the answers

    What is the primary purpose of Overcurrent Protection?

    <p>To detect faults in a system</p> Signup and view all the answers

    What type of relay is used for short length feeders?

    <p>Time Delay Definite Time Relay</p> Signup and view all the answers

    What happens to the operating time of an Inverse Definite Minimum Time Relay as the current increases?

    <p>It approaches a definite minimum operating time</p> Signup and view all the answers

    What is the main principle behind Overcurrent Protection?

    <p>When the current exceeds a predetermined value, it indicates the presence of a fault</p> Signup and view all the answers

    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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    Description

    Learn about the classification of overcurrent relays, including definite time relays and their applications in electrical protection systems.

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