power system

1. Switchgear
A switchgear is a devices associated with control,
metering and regulating of electrical power systems.
In other words systems used for switching, controlling
and protecting the electrical power circuits and different
types of electrical equipment are known as switchgear.
It is used to protect, isolate, and protect electrical
equipment from fault currents.

2. • Switchgear includes fuses, switches, relays, isolators,
circuit breaker, potential and current transformer,
indicating device, lightning arresters, etc. that protects
electrical hardware from faulty conditions.
• Automated protective switchgear consists of a relay
and circuit breaker.
• The relay acts whenever there is a fault. The relay
closes the faulty circuit and disconnects the disrupted
line.
• Relays provide that the faulty part remains isolated
while the healthy part continues to function as usual.

3. Functions of Switchgear
• The main functions of this equipment include the following.
• It protects the equipment from short-circuits & fault currents.
• This device gives isolation to the circuits from power supplies.
• It increases the availability of the system by allowing more than one
source to feed a load.
• It can open & close the electrical circuits under the conditions of
normal & abnormal.
• In normal conditions, it can operate manually so it ensures the
safety of the operator & also proper electrical energy utilization.
• In abnormal conditions, it operates mechanically. Once a fault
happens this device detects the fault & detaches the damaged part
in the power system. So it protects the power system from damage.
• Switchgear Types

5. Types of Switchgear
• switchgear are three categories :
• High Voltage (H.V.) Switchgear
• Medium Voltage (MV) Switchgear
• Low Voltage (LV) Switchgear
• LV Switchgear
• This switchgear range up to 1 kV.
• It used at low voltage circuit breakers, offload electrical
isolators, earth leakage circuit breakers, switches,
miniature circuit breakers (MCB), moulded case circuit
breakers (MCCB), and H.R.C. fuses.
• LV switchgear is present in the LV distribution board. It
consists of incomers, sub-incomer, and feeders.

6. MV switchgear
• An MV switchgear can operate between 3kv to
36 kV.
• MV switchgear operates tasks like interrupting
short circuit current, switching capacitive
winds and inductive currents, performing the
usual On/Off switching function, etc.

7. • Faults are of two type
• 1. Short circuit fault- current
• 2 Open circuit fault
• Classification of short circuited Faults
• Three phase faults (with or without earth connection)
• Two phase faults (with or without earth connection)
• Single phase to earth faults Classification of Open
Circuit Faults
• Single Phase open Circuit
• Two phase open circuit
• Three phase open circuit

8. isolator
• An isolator is a switch used to isolate a
section of a circuit from any energised
conductors, by presenting a visible break in
the circuit.

9. air break switch
• An “air break switch' is a switchgear device that
uses air as the dielectric. Air Break Switches
(ABS) are widely used as both is isolation or
switching points
• The switch whose contacts open in the air and
quenching of an arc achieves
by compressed air, such
type of switch is called an
air break switch.
. The maximum voltage for
the switches is up to 35kV .

10. SF6

11. • SF6 (sulphur hexa fluoride) gas has excellent dielectric, arc
quenching, chemical and other physical properties which have
proved its superiority over other arc quenching mediums such as oil
or air.
• Working Principle of SF6 Circuit Breaker
• In the normal operating conditions, the contacts of the breaker are
closed.
• When the fault occurs in the system, the contacts are pulled apart,
and an arc is struck between them. The displacement of the moving
contacts is synchronised with the valve which enters the high-
pressure SF6 gas in the arc interrupting chamber at a pressure of
about 16kg/cm^2.
• The SF6 gas absorbs the free electrons in the arc path and forms
ions which do not act as a charge carrier.
• These ions increase the dielectric strength of the gas and hence the
arc is extinguished.
• This process reduces the pressure of the SF6 gas up to 3kg/cm^2
thus; it is stored in the low-pressure reservoir.
• This low-pressure gas is pulled back to the high-pressure reservoir
for re-use.

12. Properties of Sulphur hexafluoride
Circuit Breaker
• It possesses very good insulating and arc quenching properties. These properties
are
•
It is colourless, odourless, non-toxic, and non-inflammable gas.
• SF6 gas is extremely stable and inert, and its density is five times that of air.
• It has high thermal conductivity better than that of air and assists in better cooling
current carrying parts.
• SF6 gas is strongly electronegative, which means the free electrons are easily
removed from discharge by the formation of negative ions.
• It has a unique property of fast recombination after the source energising spark is
removed. It is 100 times more effective as compared to arc quenching medium.
• Its dielectric strength is 2.5 times than that of air and 30% less than that of the
dielectric oil. At high pressure the dielectric strength of the gas increases.
• Moisture is very harmful to SF6 circuit breaker. Due to a combination of humidity
and SF6 gas, hydrogen fluoride is formed (when the arc is interrupted) which can
attack the parts of the circuit breakers.

13. Advantage of SF6 circuit breaker
• SF6 gas has excellent insulating, arc extinguishing compare to others.
• The gas is non-inflammable and chemically stable. Their decomposition
products are non-explosive and hence there is no risk of fire or explosion.
• Electric clearance is very much reduced because of the high dielectric
strength of SF6.
• Its performance is not affected due to variations in atmospheric condition.
• It gives noiseless operation, and there is no over voltage problem because
the arc is extinguished at natural current zero.
• There is no reduction in dielectric strength because no carbon particles are
formed during arcing.
• It requires less maintenance and no costly compressed air system is required.
• SF6 performs various duties like clearing short-line faults, switching, opening
unloaded transmission lines, and transformer reactor, etc. without any
problem.

14. Disadvantages of SF6 circuit breakers
• SF6 gas is suffocating to some extent. In the case of
leakage in the breaker tank, the SF6 gas being heavier
than air and hence SF6 are settled in the surroundings
and lead to the suffocation of the operating personnel.
• The entrance of moisture in the SF6 breaker tank is very
harmful to the breaker, and it causes several failures.
• The internal parts need cleaning during periodic
maintenance under clean and dry environment.
• The special facility requires for transportation and
maintenance of quality of gas.

15. AIR BLAST CIRCUIT BREAKER
• Air Circuit Breaker is an automatically operated electrical switch
that uses air to protect an electrical circuit from damage caused
by excess current from an overload or short circuit.
• Its primary function is to interrupt current flow after a fault is
detected.
• Gasses such as carbon dioxide, nitrogen, freon or hydrogen are
used as the arc interrupting medium, compressed air is the
accepted circuit breaking medium for gas blast circuit breakers.
• it uses a high-pressure air as the arc quenching medium.
• In this type of circuit breaker when the contacts are separated,
high-pressure air is forced on the arc through a nozzle.

16. Types of ABCB
• Air blast circuit breakers are classified on the
basis the direction of air blast to the arc. They
are classified into :
• Axial Blast Type – air blast is directed along
the arc path.
• Cross Blast Type – air blast is directed at right
angles to the arc path.
• Radial Blast Type – air blast is directed radially

17. • circuit breaker compressed air is stored in a tank
and released through a nozzle to produce a high-
velocity jet; this is used to extinguish the arc. Air
blast circuit breakers are used for indoor services
in the medium high voltage field and medium
rupturing capacity. Generally up to voltages of 15
KV. The air blast circuit breaker is now employed
in high voltage up to 220 KV lines.
Radial Blast

18. • the flow of air is longitudinal along the arc.
• Air blast circuit breaker are single blast or
double blast(radial blast circuit ).
• Breaking employing double blast breakers as
the air flows radially into the nozzle or space
between the contacts.
• The fixed and moving contacts are kept in a
closed position by spring pressure under
normal operating conditions. The air reservoir
tank is connected to the arc chamber through
an air valve, which is opened by a triple
impulse.

19. Axial Blast
•
• Under Normal Condition
• The fixed and moving contacts are held in a closed position with the help of spring pressure. There
is an air reservoir connected to the arcing chamber through an air valve.
• The air valve controls the flow of air into the arcing chamber. The valve is closed under normal
conditions.
• Under Faulty Condition
• When a fault occurs a tripping impulse is produced which causes the opening of the air valve.
• Since the air valve connects the air reservoir and the arcing chamber, a high-pressure air
enters the arcing chamber. This air pushes away the moving contact against the spring
pressure.
• The moving contact is separated and an arc is struck. At the same time, high-pressure air
blast flows along the arc and takes away the ionized gases along with it. Consequently, the
arc is extinguished and the current flow is interrupted.

20. • The contacts are separated, and an arc is developed between them.
The air flowing at a great speed axially along the arc cause removal
of heat from the edge of the arc and the diameter of the arc
reduced to a very small value at current zero.
• The flow of fresh air removes the hot gasses between the contact
space and rapidly build up the dielectric strength between them.

21. Cross Blast Air Circuit Breaker
• an arc blast is directed at right angles to the arc.
• A moving contact arm is operated in close spaces to
draw an arc which is forced by a transverse blast of air
into the splitter plates, thereby lightening it to the
point when it cannot restrike after zero current.

22. Advantages of Air Blast Circuit
Breaker:
• It eliminates the risk of fire
• Operating speed is high
• Requires less maintenance
• Suitable for frequent operation
• Short and consistent arcing time
• Cheap if using air as medium
https://youtu.be/yQ1BhEZQA9o

23. Drawback of Air Blast Circuit Breaker
• it is necessary to maintain correct pressure all
the times.
• In largest installation of a plant with two or
more compressors required.
• Air compressor needs to be maintained.
• it costly for low voltage as compared to oil or
air break circuit breaker.
• It produces a noise when the air is discharged

24. Reactor
• A reactor is a coil wired in series between two points in a power system
To minimize inrush current, voltage notching effects, and voltage spikes.
Reactors may be tapped so that the voltage across them can be changed to
compensate for a change in the load that the motor is starting.
1 dry-type are used
low-voltage applications.
2 oil-immersed are used
high-voltage applications.
Reactors may be used as line or
load reactors.

25. • A line reactor (an electrical reactor or a choke) is a
variable frequency drive (VFD) accessory that consists
of a coil of wire that forms a magnetic field as current
flows through it.
• This magnetic field limits the rate of rising of the
current, thus reducing harmonics and protecting the
drive from power system surges and transients.
• The reactors are normally classified according to their
modes of application.
• Shunt Reactor
• Current Limiting and Neutral Earthing Reactor
• Damping Reactor,
• Tuning Reactor
• Earthing Transformer
• Arc Suppression Reactor and Smoothing Reactor.

26. Inrush Current
• Inrush Current
• Many electrical devices
draw high currents at startup
or have very low impedance
• to the flow of current.
• This inrush current can cause voltage sags that trip out other
equipment.
• Many full-voltage motor starters use reactors to increase the
impedance and limit the inrush current.
• Large capacitor banks used to correct for low power factor have very
low impedance when the capacitor bank is first switched ON, and the
capacitors begin charging.
• Low impedance means that the flow of current is very high.
• A reactor can be added in series to increase the reactance. The
increased reactance increases the impedance and reduces the inrush
current

27. Reduced Notching
• When impedance in the form of a reactor is added in series
with an SCR controller, the notch voltage is distributed
across the new impedance and the impedance already
existing in the feeder lines.
• The added impedance reduces the notch depth and widens
the notch width.
• the reactor eliminate the extra zero crossovers that cause
problems.
• Higher impedance may cause problems with sensitive
equipment because the wider notch may be seen as a loss
of voltage.
• Lower impedance may not reduce the notch depth enough
to eliminate the problems.

28. Reactors
• Reactors are installed in a circuit to introduce
inductance for motor starting, combined with a
capacitor to make a filter, controlling the current,
and paralleling transformers.
• Current-limiting reactors are installed to limit the
amount of current that can flow in a circuit when
a short circuit occurs.
• two Types: iron cores and those with no magnetic
materials in the windings. air cooled or oil
immersed

29. Current Limiting Reactor
• A current limiting reactor is used when the prospective short-circuit
current in a distribution or transmission system is calculated to exceed
the interrupting rating of the associated switchgear.
• Current Limiting Reactors are connected in series with the power system
essentially to damp the short circuit fault current. During normal
operation, a continuous current flows through the reactor.
• CLR reduce the available short circuit current by providing additional the
impedance in the fault circuit.

31. Current Limiting Reactors
• The main motive of using current limiting reactors is to reduce
short-circuit currents.
• They can also be used to protect other system components from
high current levels.
• To limit the inrush current when starting a large motor.
• A line reactor is an inductor wired between a power source and a
load.
• In addition to the current limiting function, the device serves to
filter out spikes of current and may also reduce injection of
harmonic currents into the power supply.
• The most common type is designed for three-phase electric power,
in which three isolated inductors are each wired in series with one
of the three line phases.
• Line reactors are generally installed in motor driven equipment to
limit starting current, and may be used to protect Variable-
frequency drives and motors

32. Transformer secondary reactor
Feeder reactors
Bus tie reactors
Duplex reactor

33. Classification of Reactors
• There are three types of reactors:
(1) generator reactors: The reactors are located
in series with each of the generators. current
flowing into a fault
F from the generator
is limited.

34. Disadvantages:
• In the event of a fault occurring on a feeder, the voltage
at the remaining healthy feeders also may lose
synchronism requiring resynchronization later.
• A constant voltage drop in the reactors and also power
loss, even during normal operation.

35. Feeder reactors
• Feeder reactors: In this method of protection, each feeder is
equipped with a series reactor.
• In the event of a fault on any feeder the fault current drawn is
restricted by the reactor.
Disadvantages: 1 Voltage drop and power loss still occurs in the
reactor for a feeder fault.
the voltage drop occurs only in that particular
feeder reactor.
2 Feeder reactors do not offer any protection for bus
bar faults.
3 series reactors inherently
create voltage drop, system
voltage regulation will be impaired
(weaker). Used in special case such
as for short feeders of large cross-section.

36. Bus bar reactors
• In both the above methods, the reactors carry
full load current under normal operation.
• The c disadvantage are constant voltage drops
and power loss can be avoided by dividing the
bus bars into sections and interconnect the
sections through protective reactors.
• There are two ways of doing this: Ring system
and Tie bar system.

37. • Ring system: In this method, each feeder is fed by one
generator. Very little power flows across the reactors during
normal operation. Hence the voltage drop and power loss are
negligible. If a fault occurs on any feeder, only the generator
to which the feeder is connected will feed the fault and other
generators are required to feed the fault through the reactor

38. • Tie bar system: This is an improvement over the ring
system. Current fed into a fault has to pass through
two reactors in series between sections.
• advantage is that additional generation may be
connected to the system without requiring changes
in the existing reactors.
• disadvantage is that this system requires an
additional bus bar system, the tie bar.