POWERPOINT PRESENTATION
ELECTRICAL ENGINEERING
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INTRODUCTION
In the power system, whenever any
abnormal condition occurs, protective devices are employed to isolate the
abnormal condition instantaneously or in some cases, after the predetermined
time delay. The faulty section of the power system
must be isolated from the healthy section automatically before it could
diameter costly equipment or apparatus. The most common protective devices
employed in the power system to protect the system against heavy currents are
fuses and relays. Moreover the protection is also provided for the persons
operating the power system which achieved by earthing all the metallic bodies
of all equipment and apparatus.
Sample slide:-
Main features of a good
protective devices
A good protective gear must
have following main features:
a)Selectivity: The
ability of protective gear to select correctly the faulty section of the power
system and disconnect the same without disturbing the rest of the system is
called selectivity is called selectivity.
b)Sensitivity: The ability of the protective gear to
operate with low value of actuating quantity is called its sensitivity.
c)Reliability: The ability of protecting gear to
operate under predetermined condition without fail is called
its reliability.
d)Quickness or speed: How fast the protective gear
respond to the foult, this ability of protective gear is called its speed or quickness. The
quickness of a protective gear improve quality of service, increase safety of
life and equipment and stability of operations.
e)Economy: While selecting a protective gear,
a due consideration is given to its cost. However, when out most important
apparatus like alternator and transformer is to be protected, the economic
consideration are often subordinated to reliability.
sample slide:-
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Type of protective
devices
1.FUSE
2.Types of Fuse
3.Relay
4.Types of Relays
5.Earthing
FUSE a fuse is a short piece of a metal
inserted in series with the circuit which melts when excessive current flows through it and does breaks the circuit.
The material used for fuse element should possess the
following properties:
•Low melting point
•High conductivity
•Free from oxidation
The common material used for Fuse
element are copper, tin-lead alloy (tin 63% lead 37%), silver, aluminum, etc. A fuse is connected in series with
the circuit to protected and carries the load current without overheating under
normal conditions. However when abnormal condition occurs an excessive current flow through it(more or
equal to predetermined value for which the future design). This raises the
temperature which metal the fuse/melt element and open the circuit. This
protects the machine or apparatus from damage which can be caused by the excessive currents.
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TYPES
OF FUSES
There are two main
types of fuses:
1.Low voltage fuses
, 2. High voltage fuses
1. Low
Voltage fuse:
(a)Semi-closed rewire able fuse and (b)High rupturing capacity
cartridge fuse.
2. High Voltage fuse: Two types of high voltage fuse namely
(a)Cartridge type
(b) Liquid type.
Low Voltage fuse: The essential parts of typical HRC cartridge fuse, it consists of heat
resistant ceramic cylindrical body having low coefficient of thermal expansion
and has the ability to withstand internal pressure. The body has the brass end
caps to which is welded with silver current carrying element. The space within
the body surrounding the element is completely packed with a felling powder
usually quartz, which is excellent arc
extinguishing property however chalk, plaster of paris or marble dust may also be used as
filler powder. Under normal conditions, fuse element carries the normal full
load current without overheating when a fault occur the current increases and
the fuse element melts before the fault current reaches its for peak. The heat
produced in process vaporises the melted silver element, a chemical reaction take place between the
Silver vapor and the filler powder which forms high resistive substance, which helps
in quenching the arc. these fuse may be built with a breaking capacity of 16500
to 30000 amperes at 440 voltage.
High Voltage fuse: It is essentially consists of a
glass tube filled with carbon tetrachloride solution and sealed at both ends
with brass caps. Inside the tube at one end high resistance fuse wire is welded
and the other end of the tube fuse wire is weld by strong Phosphorus bronze
spiral spring fixed to the other end of the glass tube. Sometimes a flexible
copper wire is also connected in parallel with resistance fuse element. Under
normal condition, it carries the full load current without overheating. The
corona effect is eliminated by carbon tetrachloride filled in the tube. When
fault occur the heavy current flow which melt a high resistance fuse element
and spring pull back the flexible copper wire. The arc the produce is
extinguished by carbon Tetrachloride vapour.
ADVANTAGES
1.It is cheapest form of protection.
2.It requires no maintenance.
3.It interrupt heavy current without noise or smoke.
4.The smaller size of fuse element impose
a current limiting effect under short circuit.
5.They have high speed of operation.
6.They are quite reliable and can be
selected for proper discrimination.
7.They are capable of clearing high as
well as low current.
8.They have inverse time current
characteristics.
DISADVANTAGES
1.Considerable time is lost in rewiring or replacing fuse after every
operation.
2.One short-circuits, determination
between fuses in series can only be obtained if there is considerable
difference in relative sizes of the fuse concerned.
3.They have to be replaced after each operation.
4.Interlocking between fuses is not possible.
5.The lack relays in complete
discrimination.
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RELAY a
relay is a device that detects the fault mostly in high voltage circuit and
initiated the operation of circuit breaker to isolate the defective section
from the rest of the circuit.
Fundamental requirements of a relay
The main function of protective relay
is to disconnect the faulty section of the power system through circuit breaker
abruptly before damaging The costly equipment. In order to perform this
function satisfactory, it should have the following important features:
1.Selectivity
2.2. Sensitivity
3.3. Reliability
4.4. Quickness
5.5. Simplicity
6.6. Economy
TYPES
OF RELAY
According to construction and
principle of operation
1.Thermal relay 2.Electromagnetic attraction relay 3.Induction relay
According to application
1. Overcurrent, overvoltage or over power relay 2. Under current, under
voltage or under power relay 3. Directional or reverse current relay
4.Directional or reverse power relay 5. Distance relay 6. Differential relay
According to the time of
operation:
1. Instantaneous relay 2. Definite time lag relay 3. Inverse time lag relay
4. Inverse definite minimum time
lag relay(IDMT)
THERMAL RELAY
The systematic diagram of an
indirect heated thermal relay is shown. It has bimetallic strip which is heated
by heating element. The heating element get supply from the current
transformer, an insulated contact arm caring moving contact is pivoted by and
is held by a spring. The other contact of trip circuit is fix contact. The
spring tension can be varied by changing the position of contact arm with the help of sector
plate. under normal condition the current flowing through heating element is
proportional to normal full load current of the circuit. The heat produced by
the heating element under this condition is not sufficient to band the
bimetallic strip, however when the fault occur current flow through the heating
element increased which produce heat and sufficient to band the bimetallic
strip. This release the contact arm because of spring tension the relay contact
are closed, which closes the strip coil circuit. once the trip coil circuit is
closed, it operated the circuit breaker which open the circuit. These over
currents trip relays are used mostly for motor control and their heating
element is designed to with stand short time over load upto 7 times the normal full load.
ELECTROMAGNETIC
ATTRACTION RELAY
Under normal condition the current
flowing through the relay coil at such that spring tension is more than the
attractive force of the electromagnetic relay. Therefore armature is held in
open position, however when the fault occur current flowing through the relay
coil this increased the attractive force
of electromagnet at the instant. When force of electromagnet is greater than
force of attraction. The armature still downward and moving contact bridge
become the fixed contact this close the trip coil circuit and open the circuit
breaker. The current setting can we adjust by the changing the number of
tipping. The time setting can be adjust by changing the tension of the string.
INDUCTION
RELAY
Induction type overcurrent relay
it consists of metallic disc which is free to rotate placed in between the two
Electromagnet. The upper magnet has three limbs where as the lower magnet has
two limbs. The tapped primary winding is wound on the central limb of upper
magnet which is connected to C.T of the line to be protected. The tapping is connected a plug setting bridge
by which the number of active turns of a primary winding can be varied, there
by the desired current setting is obtain. The secondary is closed winding and
wound on the central limb of the upper magnet and the lower magnet. this
winding is energized by the primary winding.. The controlling torque is
provided by connecting spiral spring on the spindle of a disc. The spindle of
the disc also carries a moving contact when the disc rotates through a preset
angle the moving contact bridges the two fix contact of the trip coil circuit.
The preset angle can be adjusted to any value between 0 to 360 degree. By
adjusting the angle the travel of moving contact can be adjusted and the relay
can be set for any desired time setting. When primary winding carry current
voltage is induced in the secondary winding by induction. Since secondary is
closed a current flow through it. The flux are produced by the current flowing
through these winding. This flux are separated by phace and space and this produce driven
torque on the disc.this torque oppose by restrain torque
provided by the spring. Under normal condition restrianing torque is more than the Driven
torque. Therefore the disc remain stationary. When the fault occur current flow
through the primary exceed the preset value. Drive the torque become more than
the stationary torque ,consquently the disc rotates and moving
contact bridges the fixed contact when the disk rotated through a preset angle.
QUESTIONS
1.Name any three
main feature of a good “Protective device”?
2.Name any two
type of protecting device?
3.What are the
advantages and disadvantages of fuse?
4. Type of relay?
5.What is relay?
6.What is fuse?
7.You can add by your self……..
