Showing posts with label Relays. Show all posts
Showing posts with label Relays. Show all posts

Current Differential Relay Working || Types of Differential Relays

Current Differential Relay Working || Types of Differential Relays

What is differential relay?

In the over-current relays, a current is sensed but such relays are not very sensitive as these relays cannot distinguish between heavy loads and minor fault conditions. in such castes. differential relays can be need.

A differential relay in defined an the relay that operates when the phasor drill-me of two or more similar electrical quantities exceeds a predetermined value,

Thus a current differential relay operates on the result of comparison between the phase angle and magnitudes of the currents entering and leaving the system to be protected. Under normal conditions. the two currents are equal in phase and magnitude hence relay is inoperative. But under fault conditions. this condition no longer exist.
The relay is connected in such a manner that the difference between current entering and current leaving flows through the operating coil.If this differential current exceeds a preset value then the relay operates and opens the circuit breaker.

Almost any type of relay when connected in a certain way can be made to operate as a differential relay.

Types of differential relays:

1. Current differential relay.
2. Biased beam relay or percentage differential relay.
3. Voltage balance differential relay.

Current differential relay working:

Most of the differential relays are at current differential type. Consider an over current relay connected in the circuit so as to operate as the current differential relay. This it shown in the figure.
Two current transformers are used having same ratio are connected on the either  side of the section to be protected.The secondaries of current transformers are  connected in series, so they carry induced currents in the same direction. Let current I is flowing through the primary of current  transformers towards the external fault. As the current transformers are identical, the secondaries of current transformers will carry equal currents. Due to the connection of relay, no current will flow through the operating coil for the relay, Hence relay will remain Inoperative. So relay cannot operate if there is an external fault.

The current flows through the fault from both sides. The two secondary currents through C.Ts are not equal. The current flowing through the relay coil  is now i1+i2. This high current causes the relay to operate.
It should be noted that the fault current need not always flow to the fault from both sides. A flow on one side only or even some current flowing out of one side while a large current entering  the other side can cause differential relay to operate. Thus the amount of current flowing through a relay coil depends upon the way the fault is being fed‘

Disadvantages of current differential relay:

1. The current transformers are connected through cables called pilot cables. The impedance of such pilot cables generally causes a slight difference between the currents at the ends of the section to be protected, A sensitive relay can operate to a very small difference in the two currents, though there is no fault existing.
2. The relay is likely to operate inaccurately with heavy through current flows.This is because the assumed identical current transformers may not have identical secondary currents due to the constructional errors and pilot cable impedance.
3. Under severe through fault conditions,. the current transformers may saturate and cause unequal secondary currents. The difference between the currents may approach the pick value to cause the inaccurate operation for the relay.
4. Under heavy current flows. pilot cable capacitance may cause inaccurate operation of the relay.
All these disadvantages are overcome in biased beam relay.

Instantaneous Relay Operation

Instantaneous Relay Working Operation 

What Is Instantaneous Relay ?

An instantaneous relay is one in which there is no time delay provided intentionally. More specifically ideally there is no time required to operate the relay. Although there is some time delay which can not be avoided.

Instantaneous relay. An instantaneous relay is one in which no intentional time delay is provided. In this case, the relay contacts are closed immediately after current in the relay coil exceeds the minimum calibrated value. Figure shows an instantaneous solenoid type of relay. Although there Will be a short time interval between the instant of pickup and the closing of relay contacts, no intentional time delay has been added.

Instantaneous Relay Working Operation

The instantaneous relays have operating time less than 0-1 second. The instantaneous relay is effective only Where the impedance between the relay and source is small compared to the protected section impedance. The operating time of instantaneous relay is sometimes expressed in cycles based on the power-system frequency
e.g. one-cycle would be If 50 second in a 50-cycle system.


  • instantaneous relay wikipedia
  • instantaneous relay definition
  • instantaneous relay operating time
  • difference between idmt and instantaneous relay


Induction Disc Relay-Construction,Working

Induction Cup Relay Construction 

Induction Cup Relay is also a type of  induction disc relays.The basic principle on which induction cup relay works is same as that of induction motor. In construction also it is identical to induction motor.
Note: If you know the working of induction motor,then this article will be very easy to understand read working of induction motor here
Structure of induction cup relay is Shown in figure.

Induction Disc Relay-Construction,Working
Induction cup relay consist of following parts, 
(a) Stationary iron core (stator)
(b) Hollow cylindrical cup (rotor)
(c) Electromagnet and
(d) Coil.

1.Stationary iron core acts as stator which consists of two, four, or more poles. Now-a-days it is constructed for 4 or more poles.

2.Hollow cylindrical cup acts as rotor similar to a disc in other two relays. It is free to rotate in air gap between electromagnet and stationary iron core.

3.Electromagnet creates magnetic field.

4.Induction cup relay consist of two pairs of coil, when these coils are energized, flux is produced in the coils. These fluxes will have same frequency but will have phase delay (0 to
  ). These coils will then energize iron core thus producing a rotating flux in the air gap.

Induction Disc Relay Working

Due to these fluxes, eddy currents are made to circulate in rotor. Thus it will give rise to a force which acts on rotor. So, the hollow cup, rotates. in the direction of magnetic field between magnets and stationary iron core.

There is a control spring and back stop attached to spindle of cup to avoid rotation of rotor continuously.

Advantages of Induction Cup Relays

1. In induction cup relays the inertia of cup is much lesser than that of disc in induction disc relays.

2. Induction cup structures have very high torque to weight ratio and are therefore very sensitive .

3. They are very fast in operation due to light rotor (the operating time is less than 0.10 seconds) and are therefore suitable for high speed operation.

4. The leakage in magnetic circuit is minimum in induction cup relays due to more efficient magnetic systems. This also reduces the resistance to the induced current path in rotor.

5. Because of efficient magnetic system and light weighted rotor Volt-Ampere (VA) burden is greatly reduced in induction cup relays. The torque per VA is about three times that of induction disc relays.

6. Induction cup relays can be made to have linear characteristics by proper design. They can be made to operate accurately over a wide range with high ratio of reset to pick—up value.

7. They produce a steady non—vibrating torques.

8. The parasitic torque due to current or voltage alone is small in induction cup relay.

9. They are inherently less sensitive to D.C transients.

10. Induction cup relays are best suited for protection where the normal and abnormal conditions differ marginally.

Electromagnetic Attraction Relays Working,Construction

Most of the relays used in the power system operate by virtue of the current and/or voltage supplied by current and voltage transformers connected in various combinations to the system element that is to be protected.

Electromagnetic attraction relays operate by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid. Such relays may be actuated by d.c. or a.c. quantities.

Types of electromagnetic attraction relays are :

(i) Attracted armature type relay & working:

Fig.1 shows the schematic arrangement of an attracted armature type relay. It consists of a laminated electromagnet M carrying a coil C and a pivoted laminated armature. The armature is balanced by a counterweight and carries a pair of spring contact fingers at its free end. Under normal operating conditions, the current through the relay coil C is such that counterweight holds the armature in the position shown. However, when a short-circuit occurs, the current through the relay coil increases sufficiently and the relay armature is attracted upwards. The contacts on the relay armature bridge a pair of stationary contacts attached to the relay frame. This completes the trip circuit which results in the opening of the circuit breaker and, therefore, in the disconnection of the faulty circuit.
Attracted armature type relay & working
The minimum current at which the relay armature is attracted to close the trip circuit is called pickup current. It is a usual practice to provide a number of tappings on the relay coil so that the number of turns in use and hence the setting value at which the relay operates can be varied.

(ii) Solenoid type relay & working:

Fig. 2 shows the schematic arrangement of a solenoid type relay. It consists of a solenoid and movable iron plunger arranged as shown. Under normal operating conditions,the current through the relay coil C is such that it holds the plunger by gravity or spring in the position shown. 
Solenoid type relay & working:
Fig. 2
However, on the occurrence of a fault, the current through the relay coil becomes more than the pickup value, causing the plunger to be attracted to the solenoid. The upward movement of the plunger closes the trip circuit, thus opening the circuit breaker and disconnecting the faulty circuit.

(iii) Balanced beam type relay & working: 

Fig.3 shows the schematic arrangement of a balanced beam type relay. It consists of an iron armature fastened to a balance beam. Under normal operating conditions, the current through the relay coil is such that the beam is held in the horizontal position by the spring. 

Solenoid type relay & working:
Fig. 3
However, when a fault occurs, the current through the relay coil becomes greater than the pickup value and the beam is attracted to close the trip circuit. This causes the opening of the circuit breaker to isolate the faulty circuit.

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