To keep electrical stress of power systems from posing any kind of risk to people and property, electrical devices and systems are designed with protective relays. The machine’s insulation is often stressed by the rise in temperature caused by mechanical forces affecting different parts of the equipment.

Preventive measures are essential in power systems to address abnormalities and overloads so that the devices can function safely. Overheating can occur at any time but having a protective relay in place can stop the issue before it does extensive damage to the system. It also ensures that the system’s optimal performance is maintained for years to come.

What Is a Protective Relay?

Protective relaying is a feature of electrical system design that works to minimize disturbances to service and prevents damage to machines in the event of an electrical failure. It is a special kind of relay that monitors voltage, current, frequency, and other power measurements from a load or generating source that can trigger a circuit breaker to work if any abnormal condition is detected.

Circuit breakers in a protective relay are essentially electromechanical, much like in commercial and residential use where a bimetallic strip bends due to an increase in temperature, activating the circuit breaker.

A protective relay removes any element of the electrical system from service if overheating happens, or if it deviates from standard conditions that might affect the operation of the entire system. The relaying equipment, with the help of a circuit breaker, can disconnect the faulty element as the need arises.

Classifying Protective Relays

By nature, a relay is a combination of a single or several open or closed contacts, which can reverse based on their state as soon as actuating parameters are set. That means closed contacts can become open, and vice versa, which will activate the relay. There are many ways to classify protective relays, but they can be categorized under three types:

• Electromagnetic relay. The electromagnetic action of the solenoid (cylindrical coil wiring carrying current and acting as a magnet) prompts the opening or closing of contacts.
• Mechanical relay. Relay contacts open or close based on the mechanical displacement of gear levels.
• Static relay. A semiconductor takes control of the switches in a static relay.

Another way to group protective relays are their manner of application. A primary relay works as the first line of defense for power systems, while the backup relay operates only when the primary relay suffers a failure. Backups are generally slower than primary relays.

Protective Relays for Overheating

When a motor is overloaded, it draws more current from the power line, which causes it to heat up gradually. Sustained overloads pose a danger to the whole system and the operators, which is why a protective overlay is used to reduce the risk significantly.

Overheating can be a result of the stalled rotor, unbalanced stator currents or continuous balance overloading on a system element. Therefore, three-phase power systems need to have an overloading element in all phases.

A three-phase motor is designed so that the power transformer that feeds it will provide twice as much current to travel through one phase compared to the rest of the system, which means that the heavily loaded phase will be prone to overloading, which consequently causes overheating.

While it might be logical to apply an over current relay, this arrangement discriminates by time, which means that it will not be able to detect cooling faults in the system.

Working Principle

The most efficient method of detecting overheated elements is through installing temperature detector coils at several points in the system. These detectors could either be thermocouples, resistance temperature detectors or thermistors.

These thermal protection relays are equipped with three bimetal strips working together with a trip mechanism on an insulated housing. These strips are heated by the motor current, which then causes the strip to bend to activate the trip mechanism. The activation varies on the current-settings applied to the relay.

This mechanism enables an auxiliary switch position that indicates a tripping condition. The bimetal strips can either be directly heated when current flows through it or by heating the insulated winding around the bimetal piece. The indirect heating causes a delay, which increases the inertia of the protective relay. Both these methods of heating are used in combination.

As soon as the tripping condition happens, the motor control circuit disengages the operating coil from the power supply. It results in the stoppage of the operation of the affected element.

Safeguarding Power Systems

Protective relays work in conjunction with devices that detect and correct faults as they happen. These preemptive measures like protective relays are crucial in any application, particularly in large industries that rely on the optimal performance of equipment to keep the business going.

Present Group has over two decades of experience in providing specialist electrical testing and commissioning, operations and maintenance, including protective relaying. We can hep you with your electrical engineering needs. Ask us how today!