Electromagnetic relays used for power system .pptx

In this relay, the armature is attracted to the pole of a magnet. The electromagnetic force exerted on the moving element is proportional to the square of the current flow through the coil. This relay responds to both the alternating and direct current.

For AC quantity the electromagnetic force developed is given as

equation-1The above equation shows that the electromagnetic relay consists two components, one constant independent of time and another dependent upon time and pulsating at double supply frequency. This double supply frequency produces noise and hence damage the relay contacts.

The difficulty of a double frequency supply is overcome by splitting the flux developing in the electromagnetic relay. These fluxes were acting simultaneously but differ in time phase. Thus the resulting deflecting force is always positive and constant. The splitting of fluxes is achieved by using the electromagnet having a phase shifting networks or by putting shading rings on the poles of an electromagnet.

The electromagnetic attraction relay is the simplest type of relay which includes a plunger (or solenoid), hinged armature, rotating armature (or balanced) and moving iron polarised relay.

1. UNIT -I Power System Protection & Switchgear Topic 2 : Electromagnetic relays & Static relays

2. Relays Relays are

3. Relays: Some Important Terms Relay: A relay is an automatic device which senses an abnormal condition in an electric circuit and closes its contacts. These contacts in turn close the circuit breaker trip coil circuit, thereby it opens the circuit breaker and the faulty part of the electric circuit is disconnected from the rest of the healthy circuit. Pick up Level: The value of the actuating quantity (current or voltage) which is on the threshold (border) above which the relay operates. Reset Level: The value of current or voltage below which a relay opens its contacts and comes to original position. Operating Time: The time which elapses between the instant when the actuating quantity exceeds the pick-up value to the instant when the relay contacts close. Reset Time: The time which elapses between the instant when the actuating quantity becomes less than the reset value to the instant when the relay contact returns to its normal position.

4. Relays: Some Important Terms Contd . Primary Relays: The relays which are connected directly in the circuit to be protected. Secondary Relays : The relays which are connected in the circuit to be protected through current and potential transformers. Auxiliary Relays: Relays which operate in response to the opening or closing of its operating circuit to assist another relay in the performance of its function. This relay may be instantaneous or may have a time delay. Reach: A distance relay operates whenever the impedance seen by the relay is less than a prespecified value. This impedance or the corresponding distance is known as the reach of the relay. Underreach: The tendency of the relay to restrain at the set value or the impedance or impedance lower than the set value is known as underreach. Overreach: The tendency of the relay to operate at impedances larger than its setting is known as overreach.

5. Functional Characteristics of Protective Relays A protective relay is required to satisfy four basic functional characteristics: reliability, selectivity, speed, and sensitivity. Reliability: The relay should be reliable is a basic requirement. It must operate when it is required. There are various components which go into operation before a relay operates. Therefore, every component and circuit which is involved in the operation of the relay plays an important role; for example, lack of suitable current and voltage transformers may result in unreliable operation.

6. Functional Characteristics of Protective Relays Inherent reliability is a matter of design based on long experience. This can be achieved partly by: simplicity and robustness in construction, high contact pressure, dust free enclosures, good contact material, good workmanship, and careful maintenance. (b) Selectivity: It is the basic requirement of the relay in which it should be possible to select which part of the system is faulty and which is not and should isolate the faulty part of the system from the healthy one. Selectivity is achieved in two ways: ( i ) unit system of protection, and ( ii ) non-unit system of protection.

7. Functional Characteristics of Protective Relays Speed: A protective relay must operate at the required speed. It should neither be too slow which may result in damage to the equipment, nor should it be too fast which may result in undesired operation during transient faults. Sensitivity: A relay should be sufficiently sensitive so that it operates reliably when required under the actual conditions in the system which produce the least tendency for operation.

8. Relay Connection The proper operation of the power system requires an efficient, reliable and fast acting protection scheme A protective relay, acting as a brain behind the whole system. It detects abnormal conditions on a power system by constantly monitoring the electrical quantities of the system. The basic electrical quantities which are L i kel y to c h an g e during abnor m al co ndi tio n s are cur r ent, v ol tage , p h ase angle (direction) and frequency. Relay connection

9. Relay Connection Contd. Protective relays are broadly classified into the following three categories depending on the technologies they use for their construction and operation. Electromechanical relays Static relays Numerical relays

10. ELECTROMECHANICAL RELAYS Operate by mechanical forces generated on moving parts due to electromagnetic or electrothermic forces created by the input quantities. Types of electromechanical relays: Based on the principle of operation Electromagnetic relays Attracted armature relays, and Induction relays 2. Thermal relays

11. Attracted Armature Relays Attracted armature relays are the simplest type which respond to ac as well as dc. These relays operate through an armature which is attracted to an electromagnet or through a plunger which is drawn into a solenoid. The ele c tro m ag n et i c force e xe r te d on th e m ov i n g ele m ent, i . e . , th e a r m at u re or plu n g e r , is proportional to the square of the flux in the air gap or the square of the current. In dc relays this force is constant . In case of ac relays, the total electromagnetic force pulsates at double the frequency. The motion of the moving element is controlled by an opposing force generally due to gravity or a spring.

12. Attracted Armature Relays The following are the different types of construction of attracted armature relays. Hinged armature type Plunger type Balanced beam type Moving-coil type Polarised moving-iron type Reed type

13. Attracted Armature Relays: Hinged armature type Hinged armature-type relay The coil is energised by an operating quantity proportional to the system current or voltage. The operating quantity produces a magnetic flux which in turn produces an electromagnetic force. The electromagnetic force is proportional to the square of the flux in the air gap or the square of the current. The attractive force increases as the armature approaches the pole of the electromagnet. https://www.youtube.com/watch?v=cunddFiQzrk&t=18s

14. Attracted Armature Relays: Hinged armature type In the case of ac relays, sinusoidal current flows through the coil and hence the force of attraction is given by The restraining force is provided by a spring. The reset to pick-up ratio for attracted armature type relays is 0.5 to 0.9. The VA burden is low, which is 0.08 W at pick-up for the relay with one contact, 0.2 W for the relay with four contacts. The relay is an instantaneous relay. For a modern relay, the operation time is about 5 ms.

15. Attracted Armature Relays: Plunger-Type Relays Plunger-type relay In this type of a relay, there is a solenoid and an iron plunger which moves in and out of the solenoid to make and break the contact. The movement of the plunger is controlled by a spring .

16. Attracted Armature Relays: Balanced Beam Relays I t co nsi s t s of a bea m carr y i ng two electromagnets at its ends. One gives the electromagnetic torque , while the other re t rai n ing torque . The beam is su p p orted a t the middle and it remains horizontal co ndi tio n s under normal condition. When the operating torque exceeds the restraining torque, an armature fitted at one end of the beam is pulled and its contacts are closed. Balanced beam relay .

17. Attracted Armature Relay: Moving Coil Relays Polarised dc moving coil relay. It can b e u sed with a c actuati n g q u a n tit i es in conjunction with rectifiers. Modern relays have a sensitivity of 0.1 mW . Costlier than induction cup or moving iron type relays. These are used as slave relays with rectifier bridge comparators. Two types: Rotary moving coil, and Axially moving coil Rotating moving coil relays

18. Attracted Armature Relay: Moving Coil Relays 1. Rotary moving coil Similar to a moving coil indicating instrument. Components: a permanent magnet, a coil wound on a non-magnetic former, an iron core, a phosphor bronze spiral spring to provide resetting torque, jeweled bearing, spindle, etc. The operating torque is produced owing to the interaction between the field of the permanent magnet and that of the coil. The operating torque is proportional to the current carried by the coil. Rotating moving coil relays

19. Attracted Armature Relay: Moving Coil Relays 2. Axially moving coil It is more sensitive than the rotary moving coil relay. It is faster than the rotary moving coil relay because of light parts. Its coils are wound on a cylindrical former which is suspended horizontally. The relay has an inverse operating time/current characteristic. The axially moving coil relay is a delicate relay and since the contact gap is small, it has to be handled carefully. Axial moving coil relay Operating time is 30msec. Sensitivities as low as 0.1 mW .

20. Attracted Armature Relay: Polarised Moving Iron Relays The per m ane n t m ag n et pr o duces f lux in addition to t he main flux. U s ing tra n s i s t or a m pl i fi e rs, a relay ’ s s e nsi t i v ity c an be increased to 1 microwatts for pick-up. As its current carrying coil is stationary, it is more robust than the moving coil type dc polarised relay. Its operating time is 2 msec to 15 msec depending upon the type of construction. An ordinary attracted armature type relay is not sensitive to the polarity of the actuating quantity whereas a dc polarised relay will only operate when the input is of the correct polarity. Polarised moving iron relay

21. Attracted Armature Relay: Reed Relays The coil surrounds the nickel –iron strips ( reed contact). When the coil is energised , a magnetic field is produced which causes the reeds to come together and close the contact. Used as protective relays, slave relays, bounce free and are more suitable for normally-closed applications. Their input requirement is 1 W to 3 W and they have speed of 1 or 2 msec. Heavy duty reed relays can close contacts carrying 2 kW at 30 A maximum current or at a maximum of 300 V dc supply. The voltage withstand capacity for the insulation between the coil and contacts is about 2 kV. The open contacts can withstand 500 V to 1 kV. Reed relay Closed Glass Capsule

22. Induction Relays Induction relays use electromagnetic induction principle for their operation. Two types of construction: Induction disc induction cup In both types of relays, the moving element (disc or cup) is equivalent to the rotor of the induction motor. There is one contrast from the induction motor, i.e., the iron associated with the rotor in the relay is stationary. The moving element acts as a carrier of rotor currents, whereas the magnetic circuit is completed through stationary magnetic elements.

23. Induction Disc Relay Types of construction of induction disc relays: Shaded pole type Watt hour meter type 1. Shaded pole type In the shaded pole type construction , a C-shaped electromagnet is used. One half of each pole of the electromagnet is surrounded by a copper band know as the shading ring. The shaded portion of the pole produces a flux which is displaced in space and time with respect to the flux produced by the unshaded portion of the pole. Torques are produced by the interaction of each flux with the eddy current produced by the other flux. The resultant torque causes the disc to rotate. (a) Simple construction (b) Construction in practice

24. Induction Disc Relay 2. Watt hour meter type In wattmeter type of construction , two electromagnets are used: upper and lower one. Each magnet produces an alternating flux which cuts the disc. To obtain a phase displacement between two fluxes produced by upper and lower electromagnets, their coils may be energised by two different sources. If they are energised by the same source, the resistances and reactances of the two circuits are made different so that there will be sufficient phase difference between the two fluxes. Fig. Wattmetric type induction-disc relay

25. Induction Disc Relay: Wattmeter It is robust and reliable. It is used for overcurrent protection. It gives an inverse time current characteristic and are slow compared to the induction cup and attracted armature type relays. It is used for slow-speed relays. Its operating time is adjustable and is employed where a time-delay is required. Its reset/pick-up ratio is high, above 95% because its operation does not involve any change in the air gap. The VA burden depends on its application, and is generally of the order of 2.5 VA. The torque is proportional to the square of the actuating current if single actuating quantity is used.

26. Induction Disc Relay: Wattmeter Current Setting In disc type units, there are a number of tapping provided on coil to select the desired pick-up value of the current. Time Setting The distance which the disc travels before it closes the relay contact can be adjusted by adjusting the position of the backstop.

27. Induction Disc Relay: Printed Disc Relay Printed Disc Relay Its operating principle is the same as tha t of a dyna m om e te r ty p e instrument. There is a per m a ne n t m ag n et to produce a magnetic field. The current from the CT is fed to the printed disc through a rectifier. Fig. Printed disc inverse time relay

28. Induction Disc Relay: Printed Disc Relay The construction of a printed disc extremely inverse time relay ( sqr (I)*t = K relay). To obtain sqr (I)*t = K characteristic, an electromagnet and a printed disc are used. The electromagnet is energised from the CT through a rectifier. Printed disc relays give a much more accurate time characteristic. They are also very efficient. Fig. Printed disc extremely inverse time relay

29. Induction Cup Relay The basis construction of this relay is just like four poles or eight pole induction motor. The number of poles in the protective relay depends upon the number of winding to be accommodated. The figure shows a four pole induction cup relay. In four pole unit, the eddy current produced in the cup due to one pair of poles, directly appears under other pair of poles. If magnetic saturation of the poles can be avoided by designing, the operating characteristics of the relay can be made linear and accurate for a wide range of operation. Fig. Induction cup relay

30. Induction Cup Relay: Working Principle As per working principle of induction motor, the cup starts rotating in the direction of rotating magnetic field, with a speed slightly less than the speed of rotating magnetic field. The aluminum cup is attached with a hair spring : In normal condition the restoring torque of the spring is higher than deflecting torque of the cup. But during faulty condition of system, the current through the coil is quite high, hence, deflecting torque produced in the cup is much higher than restoring torque of spring, hence the cup start rotating as rotor of induction motor. The contacts attached to the moving of the cup to specific angle of rotation.

31. Induction Cup Relay: Construction The magnetic system of the relay is constructed by attaching numbers of circular cut steel sheets. The field coils are wound on these laminated poles. The field coil of two opposite facing poles are connected in series. The aluminum cup or drum, fitted on a laminated iron core is carried by a spindle whose ends fit in jeweled cups or bearings. The laminated magnetic field is provided on inside the cup or drum to strengthen the magnetic field cutting the cup.

32. Induction Cup Relay: Theory of Induction Relay Torque Fig u re shows ho w force is produce d in a rotor which is c ut by ph i 1 and ph i 2 . These flu x es ar e alternating quantities and can be expressed as follows. where, theta is the phase difference between phi 1 and phi . The flux phi 2 leads phi 1 by theta. Voltages induced in the rotor are: Fig. Torque produced in an induction relay

33. Induction Cup Relay: Theory of Induction Relay Torque As the path of eddy currents in the rotor has negligible self-inductance, with negligible error it may be assumed that the induced eddy currents in the rotor are in phase with their voltages. The current produced by the flux interacts with other flux and vice versa. The forces produced are: As these forces are in opposition, the resultant force is

34. Induction Cup Relay: Theory of Induction Relay Torque The suffix m i s us u al l y drop p ed and th e exp r e s s i on is writ t en in th e form of F = K*p h i 1 *phi 2 * s i n theta . I n thi s exp r e s s i on, ph i 1 and ph i 2 are r m s v alue s . If the same current produces phi 1 and phi 2 the force produced is given by where, theta is the angle between phi 1 and phi 2 . If two actuating currents M and N produce phi 1 and phi 2 , the force produced is