RELAY

PRESENTED BY :-* M NAEEM 2K15 ELE 70* M MASHAB 2K15 ELE 66* M WAQAS ASHRAF 2K15 ELE 54* RUKAN ABDULLAH 2K15 ELE 59* M.DAUD ARSLAN 2515 ELE 56

An electrical device, typically incorporating an electromagnet, which is activated by a current or signal in one circuit to open or close another circuit.

What is relay ?A relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.

or

SELECTIVITYThe relay must be operated in only those conditions for which relays are commissioned in the electrical power system. There may be some typical condition during fault for which some relays should not be operated or operated after some definite time delay hence protection relay must be sufficiently capable to select appropriate condition for which it would be operated.

FUNCTIONAL REQUIREMENTS OF PROTECTION RELAYRELIABILITYThe most important requisite of protective relay is reliability. They remain inoperative for a long time before a fault occurs; but if a fault occurs, the relays must respond instantly and correctly.

SensitivityThe relaying equipment must be sufficiently sensitive so that it can be operated reliably when level of fault condition just crosses the predefined limit.

SpeedThe protective relays must operate at the required speed. There must be a correct coordination provided in various power system protection relays in such a way that for fault at one portion of the system should not disturb other healthy portion. Fault current may flow through a part of healthy portion since they are electrically connected but relays associated with that healthy portion should not be operated faster than the relays of faulty portion otherwise undesired interruption of healthy system may occur. Again if relay associated with faulty portion is not operated in proper time due to any defect in it or other reason, then only the next relay associated with the healthy portion of the system must be operated to isolate the fault. Hence 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.

When a power is supplied to the coil, it generates a magnetic force that actuates the switch mechanism. The magnetic force is, in effect, relaying the action from one circuit to another. The first circuit is called the control circuit; the second is called the load circuit.

Basic working principle

All relays contain a sensing unit, the electric coil, which is powered by AC or DC current. When the applied current or voltage exceeds a threshold value, the coil activates the armature, which operates either to close the open contacts or to open the closed contacts. When a power is supplied to the coil, it generates a magnetic force that actuates the switch mechanism. The magnetic force is, in effect, relaying the action from one circuit to another. The first circuit is called the control circuit; the second is called the load circuit.

Reed Relay

Electromagnetic Relay Solid State Relay

Types of RelaysTypes of protection relays are mainly based on their characteristic, logic, on actuating parameter and operation mechanism. Based on operation mechanism protection relay can be categorized EMR , SSR , Reed Relay.

Reed Relay :-A reed relay, which uses an electromagnet to control one or several reed switches, has contactswhich are mounted on magnetic reeds that are scaled into a long narrow glass tube. An actuatingcoil is wound on an auxiliary ferritecorestructure or around the glass tube in order to provide themagnetic field that is required for the relay to operate.

WorkingIt uses an electromagnet tocontrol one or more reed switches. The contacts are ofmagnetic material and the electromagnet acts directly on themwithout requiring an armature to move them. Sealed in a long,narrow glass tube, the contacts are protected from corrosion,and are usually plated with silver, which has very lowresistivity but is prone to corrosion when exposed, rather thanCorrosion resistant but more resistive gold as used in theexposed contacts of high quality relays.

• Electromagnetic Relay Working Practically all the relaying device are based on either one or more of the following types of electromagnetic relays. 1- Magnitude measurement,2- Comparison,3- Ratio measurement.

Electromagnetic Relay Electromagnetic relays are those relays which are operated by electromagnetic action. Modern electrical protection relays are mainly micro processor based, but still electromagnetic relay holds its place. It will take much longer time to be replaced the all electromagnetic relays by micro processor based static relays. So before going through detail of protection relay system we should review the various types of electromagnetic relays.

Static relays are superior to electro-magnetic relays in the following aspects:The moving parts and the contacts are largely eliminated. The only moving element in a static relay is the final tripping contact.C.T.s and P.T.s employed are of lesser VA rating as static relays require a very little volt-ampere for their operation.More precise and high speed operation

Static RelayThe conventional relay type of electromagnet relays can be replaced by static relays which essentially consist of electronic circuitry to develop all those characteristics which are achieved by moving parts in an electro-magnetic relay.For example, in an induction type relay, the operating time can be adjusted by adjusting the distance travelled by the disc whereas in a static relay it can be achieved by adjusting the value of the resistance in the R-C time delay circuit. In other words static relays are capable of performing the same functions with the use of electronic circuit control as an electro-magnetic relay performs with the use of moving parts or elements.

The rectified output is supplied to a measuring unit comprising of comparators, level detectors, filters, logic circuits.The output is actuated when the dynamic input (i.e., the relaying quantity) attains the threshold value. This output of the measuring unit is amplified by amplifier and fed to the output unit device, which is usually an electromagnetic one

Operation of RelayThe essential components of static relays are shown in figure below. The output of CT and PT are not suitable for static components so they are brought down to suitable level by auxiliary CT and PT. Then auxiliary CT output is given to rectifierRectifier rectifies the relaying quantity i.e., the output from a CT or PT or a Transducer

THANK YOU ALL