Overcurrent Protection Relay

Relay protection against the high current was the earliest relay protection mechanism to develop. From this basic method, the graded overcurrent relay protection system, discriminative short circuit protection, has been formulated. This should not be mixed with ‘overload’ relay protection, which typically utilizes relays that function in a time-related to some degree to the thermal capacity of the equipment to be protected. On the contrary, overcurrent relay protection is completely directed to the clearance of short circuits, even though with the settings typically assumed some measure of overload relay protection may be obtained.

Types of Overcurrent Relay

• Instantaneous Overc1r11e1nt Relay
• Inverse Time Overcurrent Relay (IDMT Relay)
• Directional overcurrent relay
• 1Mixed IDMT and high set instantaneous

Instantaneously overcurrent relay operates when the current exceeds its Pickup value. The operation of this relay is based on the current magnitude and it is without any time delay.

STANDARD IDMT OVERCURRENT PROTECTION RELAYS

The current/time-tripping characteristics of IDMT protection relays may need to be changed according to the functioning time needed and the characteristics of other relay protection elements used in the electrical network0.10 For these needs, IEC 60255 determined the number of standard characteristics. These are:

• Standard Inverse characteristic (SI)
• Very Inverse characteristic (VI)
• Extremely Inverse characteristic (EI)
• Definite Time characteristic (DT)

MIXED IDMT AND HIGH SET INSTANTANEOUS OVERCURRENT PROTECTION RELAYS

A high-set instantaneous device can be utilized where the source impedance is small in comparison with the protected circuit impedance. This allows a decrease in the operating time at high short circuit levels possible. It also enhances the overall electrical system grading by allowing the ‘discriminating protection curves’ behind the high set instantaneous device to be reduced. One of the benefits of high-set instantaneous devices is to decrease the tripping time of the circuit protection. If the source impedance stays constant, it is then feasible to accomplish high-speed relay protection over a large part of the protected circuit. The quick short circuit clearance time helps to decrease damage at the short circuit location. Grading with the protection relay directly behind the protection relay, which has the instantaneous devices enabled, is accomplished at the current setting of the instantaneous devices and not at the maximum short circuit level.

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DIRECTIONAL OVERCURRENT RELAY

When a short circuit current can go in both directions through the protection relay location, it may be required to make the response of the protection relay directional by the initiation of a directional control device. The device is provided by the use of extra voltage inputs to the protection relay. There are many ways for an appropriate connection of voltage and current signals. The different connections depend on the phase angle, at a unity system power factor, by which the current and voltage used in the protection relay are displaced

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