Types of Overcurrent Relay - Definite Time, Inverse Time & IDMT Relays

A protective relay that operates when the current flowing in the circuit reaches a predetermined value is called Overcurrent Relay. The predetermined value of the current at which the relay starts operating by initiating a trip signal is known as its pick-up value. It is essential to protect each and every electrical equipment from overcurrents, hence overcurrent relay is the most widely used protective relay in a power system.


The applications of overcurrent relay include protection of distribution lines, industrial systems, large motors, power equipment, etc. Most of the overcurrent relays used for overcurrent protection are of electromagnetic type. But due to the rapid evolution of technology, numerical overcurrent relays based on microprocessors or microcontrollers are used nowadays for overcurrent protection.


Types of Overcurrent Relays :

Based on the relay operating time-characteristics, overcurrent relay can be classified into the following types,
  • Definite time overcurrent relay
  • Instantaneous overcurrent relay
  • Inverse time overcurrent relay
  • Inverse definite minimum time (IDMT) overcurrent relay
  • Very inverse time overcurrent relay
  • Extremely inverse time overcurrent relay.

Definite Time Overcurrent Relays :

These relays operate after a predetermined time when the current exceeds its pick-up value. Here, the operating time of the relay does not depend on the magnitude of the current above the pick-up value. A time delay mechanism is provided to get desired operating time. They are used in applications where impedance ZS between the source and relay is small compared with the impedance ZL of the section to be protected. They are also used to serve as a check against short-time asymmetrical currents.


Instantaneous Overcurrent Relays :

They operate immediately when the current exceeds its pick-up value without intentional time delay. It is given as,
I2 t = k (constant)
Where I is the current in the relay coil and t is Relay time.

  • They are used for fast relaying operations having an operating time of less than 0.1 seconds.
  • Used for differential protection of transformers inrush currents.
  • Back-up protection for differential and distance types of relays.
  • They are used when the impedance between source and relay is less with respect to the impedance of the system to be protected.

Inverse Time Overcurrent Relays :

In these, the operating time of the relay reduces as the fault quantity increases in magnitude. Normal practice is such that they are more inverse when nearer to pick-up value and become less inverse as it increases. Inverse time relays are used where the source impedance is much smaller than the line impedance.

Inverse Definite Minimum Time (IDMT) Overcurrent Relays :

These relays have the combined characteristics of definite time and inverse time relays. The IDMT relays operate as inverse time relay i.e., there exists an inverse relationship between time and current for lower values of fault current. While for higher values of fault current the relay acts as the definite time relay where the operating time is independent of fault current.


The relay exhibits an inverse relation between operating time and fault current near pick-up value and becomes almost constant just above the pick-up value. IDMT relays are widely used for the protection of distribution lines or distribution feeders.


Very Inverse Time Overcurrent Relays :

These relays exhibit more inverse characteristics between time and current than that of an inverse time or IDMT relay. This type of relay saturation occurs at the later stages. The time-current characteristic lies between IDMT characteristic and extremely inverse characteristic. These relays give better selectivity than IDMT. They are used in cases where source impedance is much smaller than line impedance. They are more suitable for earth fault protection because of their steep characteristics.


Extremely Inverse Time Overcurrent Relays :

The time-current characteristics for these relays are steeper than that of very inverse overcurrent characteristics. They are required for fuse coordination and thermal protection of transformers, induction motors, alternators, expensive cables, etc. These relays are used for reclosing distribution circuits after a long outage. Also used for load restoration purposes.


Time-Current Characteristics :

The time-current characteristics of different types of overcurrent relays are shown in the below figure.

Overcurrent Relay

The general expression for time-current characteristic is given by,
t = K / (In - 1)
The following are the time-current characteristic of various overcurrent relays,
  • For IDMT relays,
    t = 0.14 / (I0.02 - 1)
  • For very inverse time overcurrent relays,
    t = 13.5 / (I - 1) [n = 1]
  • For extremely inverse time overcurrent relay,
    t = 80 / (I2 - 1) [n = 2]

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