The relay may be defined as an electrical device interposed between the main circuit and the circuit breaker in such a manner that any abnormality in the circuit acts on the relay, which in turn if the fault is dangerous then it causes the circuit breaker to isolate and so to remove the faulty element. The relay ensures the safety of the circuit equipment from any damage which might be otherwise caused by the fault.
All the relay has three essential elements. These elements are measured element, comparing element and controlling element. Measured element measured the change in the actuating quantity, whereas comparing element compare the actuating quantity on the relay with a preselected relay setting. The controlling element manages a sudden change in the controlled quantity such as the closing of the current operative circuit.
Relays may be classified on the basis of the electrical quantity (actuated by the voltage, current, power, etc.); the mechanical quantity (actuated by pressure, the velocity of outflow of a liquid gas or gas, etc.,) and thermal quantity( actuated by heating effect) optical, acoustical, and other types of relays.
The electrical protective relay can be broad, classified into two categories (i) Electromagnetic Relay and (ii) Static Relay. According to the principle of operation and construction, the relay may be classified such as the electromagnetic attraction type, electromagnetic induction type, electrodynamic type, moving coil type, electro-thermal type, physics electric type, and static relays.
Electromagnetic attraction relays are of two types, namely attracted armature type and solenoid type. In an attracted armature type relay operation depends on the movement of an armature under the influence of attractive force due to magnetic field set up by the current flowing through the relay winding.
While in a solenoid type operation depends on the movement of an iron plunger, attracted armature hinged, and balanced beam types of relays fall under this category. Such relays are actuated by DC or AC quantities.
Electromagnetic Induction Relays
Its operation depends on the movement of a metallic disc or cylinder free to rotate by the interaction of the induced currents and the alternating magnetic fields producing them. Electromagnetic induction relays are most generally used relays. It involves only AC quantities for the protective relaying purpose.
These relays operate on the simple principle of split-phase induction motors. Actuating force is developed on a moving element, by the interaction of electromagnetic fluxes with eddy currents that are induced in the rotor by these fluxes.
Electrodynamic Type Relays
In an electrodynamic type relay moving member consists of a coil free to rotate in an electromagnetic field as in the case of a moving coil instrument.
Moving Coil Type Relays
In a moving coil type relay moving member consists of a coil free to rotate in the air gap of a permanent magnet. In this relay, the magnetic coil freely rotates in the magnetic field of a permanent magnet. The torque is developed by the interaction between the field of the permanent magnet and the coil field developed due to the flow of actuating current into the coil.
The time-current characteristic of such relay is an inverse time characteristic. The operating torque is proportional to the actuating current. Such a relay has uniform torque for different positions of the coil and therefore, can be accurately set.
In electro-thermal relay, the movement depends on the action of the heat produced by the current flowing through the element of the relay. These relays operates on the principle of thermal effect of electric current. It is most widely used for the protection of low-voltages squirrel cage induction motors or dc motors of lower output ratings.
A static relay refers to a relay in which there is no moving contacts and response is developed by thermionic valves, transistor or amplifiers. It is combination of both static and electromagnetic units. In static relays, the sensing is taken out by static circuits consisting of comparators, detectors, filters etc.
According to the connection of sensing elements, relays are classed as primary and secondary relays. Primary relays are those whose sensing elements are directly connected in the circuit or element they protect, while the secondary relays are those whose measuring elements are connected to the circuit they protect through instrument transformers.
Normally secondary relays are used in power system protection because of high values of line voltages and current. According to applications the relay may be classified as
Overvoltage, Overcurrent, and Overpower Relay – The relay operates when the voltage, current or power arises above a specified value.
Undervoltage, Undercurrent, and Underpower Relay – The relays operate when the voltage, current or power falls below a specified value.
Directional or Reverse Current Relay – The relay operates when the applied current assumes a specified phase shift on the supply voltage and the relay is compensated for the fall in voltage.
Directional or Reverse Power Relay – The relay operates when the applied voltage and current assumed specified space displacement and no compensation is allowed for fall in voltage.
Differential Relay – The relays operate when some specified phase or magnitude difference between two or more electrical quantities occurs.
Distance Relay – In this relay operation depends on the ratio of the voltage to the current.
According to timing characteristic, the relays can be divided into the following classes.
Instantaneous Relays – These relays employed after a small time duration from the incidence of the current or other quantity resulting in operation. The times require for the operation of such relays is less than 0.2 seconds.
Definite Time Lag Relays – In these relays, the time of operation is sensibly independent of the magnitude of the current or other quantity causing operation.
Inverse Time Lag Relays – In this relay, the magnitude of the current or other quantity causing operation is inversely equal to the time requires for the operation.
Inverse Definite Minimum Time Lag Relays – In these relays the time of operation is approximately equal to the lesser values of current or other quantity like voltage, frequency, etc., causing operation and tends to be a specific minimum time as the value rises without limit.