Most of the relays in commission on facility these days treat the principle of magnetic force attraction or magnetic force induction. notwithstanding the principle concerned, relays square measure typically classified in line with the operate they’re known as upon to perform within the protection of power circuits. as an example, a relay that acknowledges overcurrent in an exceeding circuit (i.e. current bigger than that which might be tolerated) associate degreed initiates corrective measures would be termed as an overcurrent relay regardless of the relay style. Similarly, associate degree overvoltage relay is one that acknowledges overvoltage in an exceedingly circuit and initiates the corrective measures. though there square measure many varieties of special functions relays, solely the subsequent vital varieties are mentioned during this chapter.

  1. Induction type overcurrent relays
  2. Induction type reverse power relays
  3. Distance relays
  4. Differential relays
  5. Translate scheme

Induction type overcurrent relays:


This type of relay works on the induction principle and initiates corrective measures once current within the circuit exceeds the planned worth. The activating supply may be a current within the circuit provided to the relay from a current electrical device. These relays area unit used on a.c. circuits solely and may operate for fault current flow in either direction.

Constructional details. The necessary constructional details of a typical nondirectional induction kind overcurrent relay. It consists of a gold (aluminum) disc that is liberal to rotate between the poles of 2 electromagnets. The higher magnet incorporates a primary and a secondary coil. the first is connected to the secondary of a C.T. within the line to be protected and is abroach at intervals. The tappings area unit connected to a plug-setting bridge by that the amount of active activates the relay operational coil are often varied, thereby giving the specified current setting.

The secondary coil is energized by induction from primary and is connected asynchronous with the winding on the lower magnet. The dominant force is provided by a spring. The spindle of the disc carries a moving contact that bridges 2 fastened contacts (connected to the trip circuit) once the disc rotates through a pre-set angle.

Operation. The driving torsion on the metal disc is ready up thanks to the induction principle as mentioned in Art. 21.5. This torsion is opposed by the restraining torsion provided by the spring. underneath traditional in operation conditions, restraining torsion is bigger than the driving torsion created by the relay coil current. Therefore, the metal disc remains stationary. However, if the present within the protected circuit exceeds the pre-set worth, the driving torsion becomes bigger than the restraining torsion. Consequently, the disc rotates and also the moving contact bridges the mounted contacts once the disc has revolved through a pre-set angle.

Induction type reverse power relays:

This type of relay operates once power within the circuit flows in a very specific direction. not like a nondirectional overcurrent relay, a directional power relay is therefore designed that it obtains its operative torsion by the interaction of magnetic fields derived from each voltage and current supply of the circuit it protects.


Constructional details. the essential elements of a typical induction sort directional power relay. It consists of AN metal disc that is liberal to rotate between the poles of 2 electromagnets. The higher magnet carries a winding (called potential coil) on the central limb that is connected through a possible electrical device (P.T.) to the circuit voltage supply. The lower magnet encompasses a separate winding (called current coil) connected to the secondary of C.T. within the line to be protected.

Operation. The flux φ1 because of current within the potential coil is going to be nearly 90o insulation behind the applied voltage V. The flux φ2 because of current coil are going to be nearly in part with the operative current I once the facility within the circuit flows within the traditional direction, the driving torsion and also the restraining torsion (due to spring) facilitate one another to show away the moving contact from the fastened contacts.

Consequently, the relay remains down. However, the reversal of current within the circuit reverses the direction of driving torsion on the disc. once the reversed driving torsion is giant enough, the disc rotates within the reverse direction and also the moving contact closes the trip circuit. This causes the operation of the breaker that disconnects the faulty section.

Distance relays:

The operation of the relays discussed to date depending upon the magnitude of current or power within the protected circuit. However, there’s another cluster of relays during which the operation is ruled by the magnitude relation of the applied voltage to current within the protected circuit. Such relays are known as distance or resistivity relays. In Associate in Nursing resistivity relay, the torsion created by a current part is opposed by the torsion created by a voltage part. The relay can operate once the magnitude relation V/I is a smaller amount than a preset worth.


A distance or resistivity relay is basically Associate in Nursing meter and operates whenever the resistivity of the protected zone falls below a pre-determined worth. There are 2 styles of distance relays in use for the protection of power provide, namely ;


  • Definite-distance relay that operates in a flash for fault up to a predetermined distance from the relay.
    Operation. underneath traditional operative conditions, the pull thanks to the voltage part is bigger than that of the present part. Therefore, the relay contacts stay open. However, once a fault happens within the protected zone, the applied voltage to the relay decreases whereas the present will increase. The magnitude relation of voltage to current (i.e. impedance) falls below the present worth.
  • Time-distance relay during which the time of operation is proportional to the space of fault from the relay purpose.
    Operation. underneath traditional load conditions, the pull of the coil is quite that of the induction part and thence the trip circuit contacts stay open. However, on the incidence of a short-circuit, the disc of the induction current part starts to rotate at a speed relying upon the operative current. because the rotation of the disc issue, the spring coupling is tense until the strain of the spring is enough to drag the coil aloof from the pole face of the voltage-excited magnet. right away this happens, the spindle carrying the coil and bridging piece moves chop-chop in response to the strain of the spring and trip contacts are closed.

A fault nearer to the relay can operate it ahead of a fault farther aloof from the relay. it should be more here that space relays are created by modifying either of 2 styles of basic relays; the exerciser or the induction disc.

Differential relays:

Most of the relays mentioned up to now relied on the surplus of current for his or her operation. Such relays square measure was less sensitive as a result of they can’t create the right distinction between significant load conditions and minor fault conditions. so as to beat this issue, differential relays square measure used.

A differential relay is one that operates once the phasor distinction of 2 or additional similar electrical quantities exceeds a preset worth. In alternative words, it’s not such a lot the relay construction because the method the relay is connected in an exceeding circuit that produces it a differential relay. There square measure 2 basic systems of differential or balanced protection viz.


  1. Current balance protection
    A combine of identical current transformers square measure fitted on either finish of the section to be protected (alternator winding during this case). The secondaries of CT’s square measure connected asynchronous in such the way that they carry the induced currents within the same direction. The operative coil of the overcurrent relay is connected across the CT secondary circuit. This differential relay compares this at the 2 ends of the generator winding.
  2. Voltage balance protection
    In this theme of protection, 2 similar current transformers square measure connected at either finish of the component to be protected (e.g. Associate in Nursing generator winding) by suggests that of pilot wires. The secondaries of current transformers square measure connected asynchronous with a relay in such the way that below traditional conditions, their induced e.m.f.s’ square measure con.


Under healthy conditions, equal currents (I1 = I2) flow in each primary windings. Therefore, the secondary voltages of the 2 transformers square measure balanced against one another and no current can flow through the relay operative coil. once a fault happens within the protected zone, the currents within the 2 primaries can dissent from each other (i.e. I1 ≠ I2) and their secondary voltages cannot be in balance. This voltage †difference can cause a current to flow through the operative coil of the relay that closes the trip circuit.

Translate scheme:

This system is that the changing variety of voltage-balance system. though the principle of balanced (opposed) voltages is preserved, it differs from the higher than the voltage-balance system in this the balance or opposition is between voltages evoked within the secondary coils wound on the relay magnets and not between the secondary voltages of the road current transformers.


Constructional details. The simplified diagram illustrating the principle of Translay theme. It consists of 2 identical double winding induction kind relays fitted at either finish of the feeder to be protected. the first circuits (11, 11a) of those relays square measure provided through a try of current transformers. The secondary windings (12, 13 and 12a, 13a) of the 2 relays square measure connected asynchronous by pilot wires in such some way that voltages evoked within the former opposes the opposite.

Operation. below healthy conditions, current at the 2 ends of the protected feeder is that the same and also the primary windings (11, 11a) of the relays carry identical current. The windings eleven and 11a induce equal e.m.f.s within the secondary windings twelve, 12a and 13, 13a. As these windings square measure thus connected that their evoked voltages square measure con, no current can flow through the pilots or operative coils and therefore no torsion is going to be exerted on the disc of either relay. within the event of a fault on the protected feeder, current deed the feeder can dissent from this getting into the feeder.

Consequently, unequal voltages are going to be evoked within the secondary windings of the relays and current can flow into between the 2 windings, inflicting the torsion to be exerted on the disc of every relay. because the direction of the secondary current is going to be opposite within the 2 relays, therefore, the torsion in one relay can tend to shut the trip circuit whereas, within the alternative relay, the torsion can hold the movement within the traditional unoperated position.

Types of Protection:

When a fault happens on any a part of the electrical power grid, it should be cleared quickly so as to avoid injury and/or interference with the remainder of the system. it’s a usual apply to divide the protection theme into 2 categories viz. primary protection and back-up protection.

  1. Primary Protection. it’s the protection theme that is meant to guard the element elements of the ability system. therefore concerning, every line has AN overcurrent relay that protects the road. If a fault happens on any line, it’ll be cleared by its relay AND circuit breaker. This forms the first or main protection and is the primary line of defense. The service record of primary relaying is incredibly high with a brim over ninety p.c of all operations being correct.
  2. Back-up protection. it’s the second line of defense just in case of failure of the first protection. it’s designed to work with sufficient time delay so primary relaying are going to be given enough time to operate if it’s ready to. therefore concerning A provides back-up protection for every one of the four lines. If a line fault isn’t cleared by its relay and breaker, the relay A on the cluster breaker can operate once an exact time delay and clear the complete cluster of lines. it’s evident that once back-up relaying functions, a bigger half is disconnected than once primary relaying functions properly.