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Protective Relays:

A protecting relay may be a device that detects the fault and initiates the operation of the breaker to isolate the defective part from the remainder of the system. The relays observe the abnormal conditions within the electrical circuits by perpetual activity the electrical quantities that square measure totally different beneath traditional and fault conditions. The electrical quantities which can amendment beneath fault conditions square measure voltage, current, frequency, and point. Through the changes in one or additional of those quantities, the faults signal their presence, type, and site to the protecting relays.


  1. Initial half is that the primary of a current electrical device (C.T.) that is connected asynchronous with the road to be protected.
  2. The second half consists of the secondary coil of C.T. and therefore the relay operational coil.
  3. The third half is that the tripping circuit which can be either a.c. or d.c. It consists of a supply of provisions, the trip coil of the breaker and therefore the relay stationary contacts.

When a brief circuit happens at purpose F on the cable, the present flowing within the line will increase to a colossal price. This leads to an important current flow through the relay coil, inflicting the relay to work by closing its contacts. This, in turn, closes the trip circuit of the breaker, creating the breaker open and analytic the faulty section from the remainder of the system.

Basic Relays:

Most of the relays employed in the ability system operate by virtue of this and/or voltage equipped by current and voltage transformers connected in varied combos to the system part that’s to be protected. Through the individual or relative changes in these 2 quantities, faults signal their presence, type, and placement to the protecting relays. Having detected the fault, the relay operates the trip circuit which ends within the gap of the breaker and thence within the disconnection of the faulty circuit.

  • Electromagnetic Attraction Relays
  • Electromagnetic induction Relays

Electromagnetic Attraction Relays:

Electromagnetic attraction relays operate by virtue of AN coil being drawn to the poles of AN magnet or a plunger being drawn into a coil. Such relays could also be motivated by d.c. or a.c. quantities. The vital varieties of magnetic force attraction relay area unit :


  1. Attracted coil kind relay. the schematic arrangement of AN attracted coil kind relay. It consists of a laminated magnet M carrying a coil C and a pivoted laminated coil. The coil is balanced by a counterweight and carries a combination of spring contact fingers at its free finish. below traditional operational conditions, the present through the relay coil C is such counterweight holds the coil within the position shown.
  2. Coil kind relay. It consists of a coil and movable iron plunger organized as shown. below traditional operational conditions, the present through the relay coil C is such it holds the plunger by gravity or spring within the position shown. However, on the prevalence of a fault, the present through the relay coil becomes quite the pickup worth, inflicting the plunger to be drawn to the coil. The upward movement of the plunger closes the trip circuit, therefore gap the electrical fuse and disconnecting the faulty circuit.
  3. Balanced beam kind relay. the schematic arrangement of a balanced beam kind relay. It consists of AN iron coil fixed to a gymnastic apparatus. below traditional operational conditions, the present through the relay coil is such the beam is controlled within the horizontal position by the spring. However, once a fault happens, the present through the relay coil becomes bigger than the pickup worth and also the beam is drawn to shut the trip circuit.

Electromagnetic induction Relays:

Electromagnetic induction relays operate the principle of induction motor and area unit wide used for protecting relaying functions involving a.c. quantities. they’re not used with d.c. quantities because of the principle of operation. associate induction relay basically consists of a pivoted metal disc placed in 2 alternating magnetic fields of constant frequency however displaced in time and area. The torsion is created within the disc by the interaction of 1 of the magnetic fields with the currents iatrogenic within the disc by the opposite.


  1. Shaded-pole structure. the final arrangement of shaded-pole structure It consists of a pivoted metal disc unengaged to rotate within the air-gap of the associated magnet. One 1/2 every pole of the magnet is encircled by a copper band referred to as shading ring. The alternating flux φs within the shaded portion of the poles are going to be because of the reaction of this iatrogenic within the ring, lag behind the flux φu within the unshaded portion by associate angle α. This two a.c. fluxes differing in the section can manufacture the required torsion to rotate the disc.
  2. Watthour-meter structure. This structure gets its name from the actual fact that it’s employed in watthour meters. the final arrangement of this sort of relay is It consists of a pivoted aluminum disc organized to rotate freely between the poles of 2 electromagnets. The higher magnet carries 2 windings; the first and therefore the secondary. the first winding carries the relay current I1 whereas the coil is connected to the winding of the lower magnet. the first current induces e.m.f. within the secondary and then circulates a current I2 in it. The flux φ2 iatrogenic within the lower magnet by this within the coil of the higher magnet.
  3. Induction cup structure. the final arrangement of associate induction cup structure. It most closely resembles associate induction motor, except that the rotor iron is stationary, solely the rotor conductor portion being unengaged to rotate. The moving part could be a hollow cylindrical rotor that activates its axis. The rotating field is created by 2 pairs of coils wound on four poles as shown. The rotating field induces currents within the cup to supply the required driving torsion.

Fundamental Elements of Protective Relays:

The principal operate of protecting relaying is to cause the prompt removal from service of the associated component of the facility system once it starts to control in an abnormal manner or interfere with the effective operation of the remainder of the system. so as that protecting relay system might perform this operate satisfactorily, it ought to have the subsequent qualities :


  1. Property. it’s the flexibility of the protection system to pick properly that a part of the system in bother and disconnect the faulty part while not worrisome the remainder of the system. A neat and economical relay system ought to be selective i.e. it ought to be ready to sight the purpose at that the fault happens and cause the gap of the circuit breakers nighest to the fault with minimum or no injury to the system.
  2. Speed. The relay system ought to disconnect the faulty section as quick as doable for the subsequent reasons : (a) Electrical equipment is also broken if they’re created to hold the fault currents for an extended time. (b) A failure on the system results in a good reduction within the system voltage. If the faulty section isn’t disconnected quickly, then the low voltage created by the fault might finish off consumers’ motors and therefore the generators on the system might become unstable. (c) The high-speed relay system decreases the likelihood of development of 1 sort of fault into the opposite a lot of severe kinds.
  3. Sensitivity. it’s the flexibility of the relay system to control with a coffee worth of causative amount. The sensitivity of a relay could be a operate of the volt-amperes input to the coil of the relay necessary to cause its operation. The smaller the var input needed to cause relay operation, a lot of sense is that the relay. Thus, a one VA relay is a lot of sensitive than a three VA relay. it’s fascinating that the relay system ought to be sensitive so it operates with low values of var input.
  4. Dependability. it’s the flexibility of the relay system to control below pre-determined conditions. while not dependability, the protection would be rendered for the most part ineffective and will even become a liability.
  5. Simplicity. The relaying system ought to be easy so it will be simply maintained. dependability is closely associated with simplicity. The less complicated the protection theme, the larger are going to be its dependability.
  6. Economy. the foremost necessary consider the selection of a selected protection theme is that the economic side. generally, it’s economically unreasonable to use a perfect theme of protection and a compromise technique must be adopted. As a rule, the protecting gear mustn’t price quite five-hitter of the full price.

Relay Timing:

By ‘the time of operation’ is supposed the length of the time from the moment once the activating component is energized to the moment once the relay contacts square measure closed. typically it’s fascinating and necessary to regulate the operational time of a relay. For this purpose, mechanical accessories square measure used with relays.


  1. Instant replay. a direct relay is one during which no intentional time delay is provided. during this case, the relay contacts square measure closed instantly once current within the relay coil exceeds the minimum mark worth. instant magnet style of the relay. though there’ll be a brief quantity between the moment of pickup and therefore the closing of relay contacts, no intentional time delay has been side. The instant relays have operational time but 0·1 second. The instant replay is effective solely wherever the electric resistance between the relay and supply is tiny compared to the protected section electric resistance.
  2. Inverse-time relay. Associate in Nursing inverse-time relay is one during which the operational time is close to reciprocally proportional to the magnitude of the activating amount. the time-current characteristics of Associate in the Nursing inverse current relay. At values of presently but a pickup, the relay ne’er operates. (a) In Associate in Nursing induction relay, the inverse-time delay will be achieved by positioning a static magnet (known as a tangle magnet) in much the way that relay disc cuts the flux between the poles of the magnet. once the disc moves, currents got wind of in it to turn out a tangle on the disc that slows its motion. (b) In different forms of relays, the inverse time delay will be introduced by oil dashpot or a time-limit fuse. inverse time magnet relay victimization oil dashpot. The piston within the oil dashpot connected to the moving plunger slows its upward motion.
  3. Definite break relay. during this style of the relay, there’s a precise time slip by between the moment of pickup and therefore the closing of relay contacts. This explicit time setting is freelance of the quantity of current through the relay coil; being constant for all worths of current in way over the pickup value. could|it’s going to|it should} be worthy to say here that much all inverse-time relays are supplied with definite minimum time feature so as that the relay may ne’er become instant in its action for terribly long overloads.

Time/P.S.M. Curve:


The curve between the time of operation and plug setting number of a typical relay. The horizontal scale is marked in terms of plug-setting number and represents the number of times the relay current is in more than this setting. The vertical scale is marked in terms of the time needed for relay operation. If the P.S.M. is 10, then the time of operation (from the curve) is three seconds. the particular time of operation is obtained by multiplying this point by the time-setting number. it’s evident from that for lower values of overcurrent, time of operation varies reciprocally with this however because the current approaches twenty times the full-load price, the operational time of relay tends to become constant. This feature is critical so as to confirm discrimination on terribly significant fault currents flowing through sound feeders.

Type of Relays:

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 : [ read more ]