This document provides an overview of relays presented by five students. It defines a relay as an automatic device that senses abnormal electrical circuit conditions and closes contacts to complete a circuit breaker trip coil circuit to disconnect the faulty portion of an electrical circuit. The document discusses key relay functions and requirements including selectivity, reliability, sensitivity, and speed. It describes common relay types such as reed, electromagnetic, and static relays, and explains their basic workings.

1. 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

2. 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

3. SELECTIVITY
The 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
RELAY
RELIABILITY
The 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.

4.  Sensitivity
The relaying equipment must be sufficiently sensitive so that it can be
operated reliably when level of fault condition just crosses the predefined
limit.
 Speed
The 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.

5. 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.

7. Reed Relay
Electromagnetic Relay
Solid State Relay
 Types of Relays
Types 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.

8.  Reed Relay :-
A reed relay, which uses an electromagnet to control one or several reed switches, has
contacts
which are mounted on magnetic reeds that are scaled into a long narrow glass tube. An
actuating
coil is wound on an auxiliary ferritecore
structure or around the glass tube in order to provide the
magnetic field that is required for the relay to operate.
 Working
It uses an electromagnet to
control one or more reed switches. The contacts are of
magnetic material and the electromagnet acts directly on them
without 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 low
resistivity but is prone to corrosion when exposed, rather than
Corrosion resistant but more resistive gold as used in the
exposed contacts of high quality relays.

10. • 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.

11. 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 Relay
The 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.

12. 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 Relay
The 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 rectifier
Rectifier rectifies the relaying quantity i.e., the output from a CT or PT or a
Transducer

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