Overcurrent Relay

Overcurrent Relay

The relays are used in a power system network to sense abnormal conditions. An overcurrent relay gives an output when the fault current exceeds a preset value.

The current value at which it operates is called the pickup value of current. It is adjusted in terms of a relay’s Plug Setting Multiplier (PSM). And the operating time is adjusted in terms of Time Multiplier Setting (TMS).

PSM is defined as the below equation.

    \[ PSM = \frac{Fault \; Current \; in \; Relay \; Coil}{Rated \; CT secondary \; Current \times Current Setting \; of \; Relay} \]

In normal conditions, PSM is less than one, and fault current is less than the pickup value. In abnormal conditions, PSM must be greater than one (1) for the relay to pick up.

Classification of Overcurrent Relay

The overcurrent relays are classified according to operation time and the nature of time-current characteristics.

It is classified as listed below.

  • Instantaneous Overcurrent Relay
  • Inverse Definite Time Relay
  • Inverse Definite Minimum Time (IDMT) Relay
  • Very Inverse Time Relay
  • Extremely Inverse Time Relay

Instantaneous Overcurrent Relay

An instantaneous overcurrent relay operates without any intentional time delay; however, as the name suggests, this type of relay operates very fast (instantaneously).

The operating time of instantaneous relay is very low. The relay that operates within 20 to 40 msec is considered an instantaneous relay.

This type of relay has only a current setting and no time setting. The instantaneous overcurrent relay can be made by various construction types like; attracted armature type or induction type relay.

An instantaneous relay is effective when the impedance between the relay and source ZS is minimal compared to the impedance of protected section ZL.

The typical characteristics of instantaneous overcurrent relay are shown in the figure below.

characteristics of instantaneous overcurrent relay
characteristics of instantaneous overcurrent relay

Definite Minimum Time (DMT) Relay

A Definite Minimum Time (DMT) relay is also known as an inverse definite time relay. In this type of relay, two adjustments are to be done; Current setting and Time setting.

Therefore, a definite time relay operates after a substantial period of time once the current exceeds the pickup value.

The time setting is in the range of 0.1-1s, 1-10s, or 6-60s. And the current setting is in the range of 50-200% of relay rated current.

The following equation can achieve the relay characteristic of the electromechanical relay.

    \[ T_{OP} = \frac{A}{{MP}^B-1} \times TDS + C \]

Where,

TOP = Time of operation of relay

MP = Multiple of pickup current or Plug Setting Multiplier (PSM)

TDS = Time Dial Setting

A, B, and C = circuit constant that is decided by the characteristic of the relay.

Relay C equals zero for a definite minimum time and A, B are very small (near zero).

For the static relay, the following equation gives the characteristic of the relay.

    \[ T_{OP} = \frac{a}{MP^n - C} \times TDS + b \times TDS + K \]

Where

n = exponent

a, b, C, and K (preferably 0.01) are constant.

The typical characteristic of the Definite Minimum Time overcurrent relay is shown in the figure below.

characteristic of Definite Minimum Time overcurrent relay
characteristic of Definite Minimum Time overcurrent relay

Inverse Definite Minimum Time (IDMT) Relay

An Inverse Definite Minimum Time (IDMT) relay is a widely used relay as it follows two characteristics. Initially, the relay follows the characteristic of inverse law, and after that, the current increases, it follows a definite minimum time characteristic.

In other words, the inverse characteristic dominates near the pickup value of current and then becomes a definite time characteristic for higher current.

This operation can be achieved because of the constant operating torque due to flux saturation at a high value of current in the electromechanical relay.

The inverse characteristics are obtained for plug setting multiplier values less than ten, while for values more than ten, the characteristic is minimum definite time characteristics.

The constant’s values are given in the table at the end of this article. And the characteristic of the IDMT relay is given in the below figure curve-A.

Very Inverse Time Relay

A very inverse time relay gives more inverse characteristics compared to an IDMT relay. This characteristic is achieved as the core saturates at a further later stage than the IDMT relay.

The inverse nature continues for a more extended range and takes curve-B as shown in the below figure. This type of relay gives better selectivity than the IDMT relay.

The mathematical relation between Time Multiplier Setting (TMS) and Plug Setting Multiplier (PSM) is given by the below equation.

    \[ T = \frac{13.5 \times TMS}{PSM-1} \]

If the short circuit current drops rapidly with the distance from the source, this type of relays are suitable. And this relay is adequate for ground faults.

Extremely Inverse Time Relay

In the extremely inverse relay, the curve is more inverse than the very inverse relay as the core saturation occurs at a very later stage.

The mathematical equation of this relay is shown in the below equation.

    \[ T = \frac{80 \times TMS}{PSM^2 -1} \]

This type of relay is used to protect distribution feeders, alternators, transformers, expensive cables, etc.

The extremely inverse time relay curve is as shown in the figure below (curve-C).

Operating Time Characteristics of Overcurrent Relay
Operating Time Characteristics of Overcurrent Relay
Relay Characteristic Electromechanical Relay Static Relay
A B C a b c
Normal Inverse 0.092 0.02 0.149 5.4 0.18 2.00
Very Inverse 18.92 2.00 0.492 5.4 0.11 2.00
Extremely Inverse 28.08 2.00 0.13 5.4 0.03 2.00
IDMT 0.14 0.02 0.00 0.14 0.0 0.02

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