Operation of Relay | Fundamental Requirements of Relay

 What is Protective Relay?

A device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element/section from the rest of the system. Relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and faulty conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. Through the changes in one or more of these quantities, the protective relay operates.

Operation of Relay to Open Circuit Breaker

The operation of a relay to open a circuit breaker is a critical process in the protection of electrical power systems.

When a fault occurs in an electrical circuit, such as an overload or short circuit, the current flowing through the system increases beyond safe levels. This is where the relay comes into play. The relay is an intelligent device that monitors the electrical currents and voltages to detect any abnormalities.

The core component of a relay is an electromagnet. When the relay detects a fault current, it energizes this electromagnet. The magnetic field generated by the electromagnet then exerts a force on a movable armature, which is mechanically linked to a set of contacts¹.

In its resting state, the relay’s contacts are either normally open (NO) or normally closed (NC). For a relay that operates to open a circuit breaker, the contacts would typically be in the NO position. Upon detecting a fault, the energized electromagnet pulls the armature, causing the contacts to move from the NO to the closed position. This action completes a secondary circuit that is connected to the tripping mechanism of the circuit breaker².

The tripping mechanism is usually a spring-loaded mechanism that, once activated, physically separates the contacts within the circuit breaker. This separation of contacts interrupts the flow of current, effectively isolating the faulty section of the circuit from the rest of the electrical system³.

The speed of this entire process is crucial. It must happen within milliseconds to prevent damage to the system and ensure safety. After the fault is cleared and the system is deemed safe, the relay is reset to its original state, ready to protect the circuit once again².

This protective action of the relay and circuit breaker is analogous to a home’s smoke detector system. The smoke detector (relay) senses the presence of smoke (fault) and triggers the home’s alarm system (circuit breaker) to alert the occupants and prevent further hazards.

By operating in such a manner, relays ensure the protection and longevity of electrical systems, preventing potential hazards and maintaining operational stability.

Learn more about Protective Relay

Fundamental Requirements 

1.      Selectivity: The cornerstone of a protective relay’s functionality is its ability to discern and isolate only the malfunctioning segment of an electrical network. This precision prevents unnecessary interruptions in the rest of the system. For instance, if a fault transpires on a transmission line, the relay should exclusively trip the breaker for that specific line. This is akin to a surgeon removing only the affected tissue and leaving the healthy parts untouched.

2.      Speed: Time is of the essence when it comes to protective relaying. The relay must actuate the circuit breaker with alacrity to curtail the duration of the fault, thereby safeguarding equipment from damage and averting potential instability within the power system. Imagine a fire alarm system in a building; the faster it detects and alerts the occupants, the quicker the evacuation and response.

3.      Sensitivity: A protective relay should be finely tuned to detect minute anomalies that could presage larger issues. It’s similar to a smoke detector’s sensitivity to smoke particles, which allows it to alert occupants to a fire before it becomes unmanageable. In electrical terms, this could mean responding to slight overcurrents that exceed normal operational levels.

4.      Reliability: Unfailing performance is non-negotiable for protective relays. They must operate correctly under all circumstances, akin to a parachute deploying reliably during a skydive. This ensures that whenever a fault occurs, the relay will unfailingly initiate the breaker to disconnect the fault.

5.      Simplicity: The design and operation of protective relays should be straightforward, minimizing complexity to facilitate ease of maintenance and reduce the risk of malfunction. This principle is similar to the design of emergency exits in buildings, which are made to be intuitively used even in high-stress situations.

6.      Economy: While incorporating all the aforementioned attributes, the protective relay system should also be cost-effective. It should deliver optimal protection without imposing exorbitant costs for installation and upkeep. This is akin to constructing a bridge that is not only strong and durable but also economical to build and maintain.

To illustrate, consider a current transformer (CT) based relay system. When a fault occurs, such as a short circuit, the CT senses the surge in current and signals the relay. The relay, acting swiftly and selectively, trips the circuit breaker connected to the faulty line, isolating it from the rest of the network while allowing the rest of the system to continue functioning normally.

By adhering to these fundamental requirements, protective relays play a pivotal role in maintaining the safety and reliability of electrical power systems, ensuring that any faults are addressed promptly and effectively.