Three phase Over current relays; Over current relay characteristics

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Overcurrent Relays and Their Characteristics

Overcurrent protection is one of the most widely used protection schemes in power systems. It protects equipment such as transmission lines, feeders, transformers, and generators against excessive current caused by short circuits or overloads.



Depending upon the time of operation, overcurrent relays are categorized into the following types:

  1. Instantaneous Overcurrent Relay
  2. Inverse Time Overcurrent Relay
  3. Inverse Definite Minimum Time (IDMT) Overcurrent Relay
  4. Very Inverse Overcurrent Relay
  5. Extremely Inverse Overcurrent Relay

Let us study each one in detail.

1. Instantaneous Overcurrent Relay

  • Principle: Operates instantly (about 0.1 sec) when the current exceeds the preset value.
  • Working: Achieved using hinged armature relays or solid-state relays without any intentional delay.
  • Characteristic: A vertical line on the time-current curve.
  • Application: Used for short line protection and backup protection, where selectivity is not critical.

2. Inverse Time Overcurrent Relay

  • Principle: Operating time decreases as fault current increases.
  • Working: The relay becomes more inverse near the pickup value, and less inverse as current rises further. Achieved with induction-type relays having a non-saturating core.
  • Characteristic: Curve (a) – sharply inverse near pickup, flattens at high currents.
  • Application: Commonly used in distribution protection where time grading with downstream relays is required.

3. Inverse Definite Minimum Time (IDMT) Overcurrent Relay

  • Principle: Time of operation is inversely proportional to fault current near pickup, but after a certain value, the operating time becomes nearly constant (minimum time).
  • Working: Achieved by using an electromagnetic core that saturates just above pickup current.
  • Characteristic: Curve (b) – inverse initially, then horizontal (constant time).
  • Application: Widely used in overhead lines and feeder protection due to its balance between speed and selectivity.

4. Very Inverse Overcurrent Relay

  • Principle: Relay saturation occurs at a later stage compared to IDMT.
  • Working: Time decreases steeply with increase in current, but eventually tends to definite time after saturation.
  • Characteristic: Curve (c) – much steeper inverse than IDMT.
  • Application: Suitable for protection of distribution transformers and long lines where high fault currents need faster clearing.

5. Extremely Inverse Overcurrent Relay

  • Principle: Core saturation occurs at a very late stage, making the relay highly sensitive to overloads.
  • Equation: K=I2×tK = I^2 \times t → i.e., operating time is inversely proportional to the square of current.
  • Characteristic: Curve (d) – very steep inverse, then flattens at definite time.
  • Application: Common in transformer differential protection and motor protection, where overload protection is critical.

Comparison of Overcurrent Relays

Relay Type

Operating Time Characteristic

Best Suited For

Instantaneous

No delay, ~0.1 sec

Short lines, backup

Inverse Time

Inverse with current

Distribution feeders

IDMT

Inverse + definite minimum time

Transmission & feeders

Very Inverse

Strong inverse

Transformers, long feeders

Extremely Inverse

I2tI^2 t type curve

Transformer & motor overloads

✅ Key Takeaway:

  • Instantaneous = fastest but less selective.
  • Inverse time relays = better selectivity.
  • IDMT, very inverse, extremely inverse = provide flexibility for different system conditions.

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