An electrical protection relay is an intermediate device that bridges the function of a current transformer or a similar fault-detecting device to one or more circuit breakers. In other words, the relay connects the fault-sensing device with the fault-clearing device. Essentially, a relay has a simple magnetic coil that receives current from the current transformer. Additionally, in its simplest form, it has a plunger attached to a normally open contact. That contact is in series with the DC circuit of the circuit breaker trip coil.
Here, the current transformer serves as the fault-sensing device. The circuit breaker functions as the fault-clearing device. Whenever a fault occurs in the system, the current through the primary increases. Accordingly, the secondary current of the current transformer increases, which further increases the electromagnetic force of the coil. Therefore, the increased magnetic force moves the plunger attached to the coil. As a result, the normally open contact closes, and the DC voltage reaches the tripping coil of the circuit breaker. Therefore, the circuit breaker trips to isolate the fault.
Working Principle of an Electrical Protection Relay
This is the simplest form of a relay.
Therefore, in an electrical system, a relay is a switch-like device. The relay uses very low power to control a very high-powered circuit. The relay coil consumes only a few tens of VA from the CT secondary, even under fault conditions. In contrast, the circuit breaker interrupts fault power in the range of hundreds or thousands of MVA during fault clearing.
In the most basic model of a relay, there will be a coil connected across the abnormality-sensing device, which is the CT here. It will also have a contact that operates during a fault in the system. The contact of the relay then actuates the operation of the associated fault-clearing device, the circuit breaker.
Types of Electrical Protection Relay
There are two types of relays. The first is the electromechanical type, as discussed earlier. Obviously, it has an electromagnetic coil, which mechanically moves the contact of the relay. As soon as the contact changes position—from closed to open or open to closed—the circuit breaker initiates its operation and clears the fault. On the other hand, the second is the solid-state relay. Instead of using an electromagnetic coil, these relays use semiconductor devices to switch the circuit electronically without the mechanical movement of any contact.
Uses of Protection Relay
As the name suggests, electrical protection relays send signals of faulty conditions to fault-clearing devices. In this way, they protect power systems from permanent damage due to any fault.