Protective Relay and its Type

What is Relay?

�A protective Relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the faulty element from rest of the healthy power system.�

Protection of electrical equipment plays a vital role in Power System not only from the point of view of economy behind costly equipment but also for Power System Stability.

Protection Scheme provides a mean to isolate the faulty part of the Power System from the rest healthy network. Also, it protects the costly equipment installed from damaging. The detection of fault and disconnection of faulty section or equipment is achieved using Relays in conjunction with Circuit Breaker.

A protective Relay should possess the following qualities to perform its intended function:

• Selectivity - Protective Relay shall only operate in case it senses a fault. It should selectively isolate the faulty section from the healthy network.

• Speed - The time of operation of relay shall be as low as possible. This is due to the following reasons:

1. Equipment may get damage if the carry fault current for a longer duration.
2. A fault on the system lead to reduction of voltage. If faulty section is not isolated from the healthy section as quick as possible them the network voltage may collapse. Due to this collapse of system voltage, power system will become unstable leading to complete blackout.
3. There always exists a change of development of more severe type of fault if it is not isolated quickly.

• Sensitivity - Sensitivity is the ability of relay to operate with low value of actuating quantity.
• Reliability - Protection system should be reliable. Reliability means that relay operates only when there is a fault. Unreliable protection system will lead to spurious tripping of circuit breaker.
• Simplicity � Simplicity increases the reliability. The simpler the system, the greater will be its reliability.
• Economy - A protection system should not cost more than 5% of total cost of system.

Types of Protective Relay

Protective relay can be classified as follows:

• Electromagnetic Relays
• Static Relays
• Digital / Numerical Relays

Electromagnetic Relays

Electromagnetic Relays are either of attracted armature or induction cup / induction disc type. These possess mechanical inertia and therefore take longer time to operate as compared to static Relays. Besides, the burden imposed by these relays on the Current Transformer and Potential Transformer are substantial.

Static Relays

Static Relays use solid state devices to process the input signals in analog form. The burdens imposed on CTs by these relays are very low compared to Electromagnetic relays. It is possible to obtain higher speed of operation with static relays.

Digital / Numerical Relays

Numerical Relays are programmable version of solid state relays based on digital signal processing by microprocessors. Its modular architecture allows the same unit to be programmed in to different types of relays.