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SWITCH A switch is simply a means of

opening or closing the current path

in an electric circuit. There is no special opening

condition for short circuit or other

fault. It is operated manually.

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FUSE A fuse is used for over current

and short circuit protection.

Fuse has the major drawback ofreplacement after every use.

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CIRCUIT BREAKERThe circuit breaker is a piece of

equipment which can do any one of thefollowing tasks:

Makes or breaks a circuit either

manually or by remote control undernormal conditions

Breaks a circuit automatically under

fault conditions

Makes a circuit either manually or byremote control under fault conditions

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CIRCUIT BREAKER

A circuit breaker is a switching i.e.

current interrupting or makingdevice in switchgear.

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SWITCHGEAR

It is the generic term including the

entire range of switching devices andtheir combination with associated

control, measuring, protecting, and

regulating equipment.

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FUNCTIONS OF SWITCHGEAR Carrying the normal load current

Making or breaking the normal load

current Clearing the fault current (for which

sensing devices like CT, PT, and

various relays are employed)

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COMPONENTS OF SWITCHGEAR Circuit Breaker

Current Transformer (CT)

Potential Transformer (PT)

Protective Relays

Measuring Instruments

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COMPONENTS OF SWITCHGEAR Switches

Fuses

Surge Arresters

Isolators

Miniature Circuit Breakers (MCB)

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SWITCHGEAR REQUIREMENTSThere are several voltage levels andfault situations between the generatingstations to the final load points. Henceswitchgear requirements may varydepending upon following factors:

Location

Ratings Local Needs

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LOW VOLTAGE SWITCHGEARS

Switchgears for low voltage

applications are rated up t01500 V.

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LV Switchgears

Oil Circuit Breakers (OCB)

Air-Circuit Breakers (ACB)

Switch-Fuse Units (SFU)

Off-Load Isolators

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LV Switchgears HRC Fuses

Earth Leakage Circuit

Breakers(ELCB) Miniature Circuit Breakers (MCB)

Moulded Case Circuit Breakers

(MCCB)

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MEDIUM VOLTAGE SWITCHGEARS

Switchgears for medium

voltage applications are rated inbetween 3.3 kV to 33 kV.

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MEDIUM VOLTAGE SWITCHGEARS

This type of switchgears is mainly

used for the distribution of electrical

energy, connected to variouselectrical networks.

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HIGH VOLTAGE SWITCHGEARS

The high voltage switchgears

range from 66 kV to 800 kV.

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SWITCHGEARS CONSIDERATIONS

While using switchgears in a power

system certain considerations mustbe kept in mind.

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SWITCHGEARS CONSIDERATIONS

1.Location of the equipment

Indoor Outdoor

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2. System Parameters

System Earthing

Frequency Insulation Level

Ambient Conditions

Ratings (both normal and shortcircuit ratings)

SWITCHGEARS

CONSIDERATIONS

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ERECTION OF SWITCHGEARSIt includes:

Placing the equipment on foundation

Assembly of loose supplied parts

Mechanical and electricalinterconnections

Connections of control and powercircuits

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SWITCHGEAR COMMISSIONING Commissioning is done to carry out pre-

defined checks and tests.

Testing is necessary to ensure that the

installation work meets the specifiedrequirements and safety norms.

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Switchgear classification

By the current rating.

By interrupting rating (maximum short circuit current that the device can safelyinterrupt) Circuit breakers can open and close on fault currents Load-break/Load-make switches can switch normal system load currents Isolators may only be operated while the circuit is dead, or the load current is very

small.

By voltage class:

Low voltage (less than 1,000 volts AC) Medium voltage (1,00033,000 volts AC) High voltage (more than 66,000 volts AC)

By insulating medium: Air Gas (SF6 or mixtures) Oil Vacuum

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Switchgear classification

By construction type: Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure

type) Outdoor Industrial Utility Marine Draw-out elements (removable without many tools) Fixed elements (bolted fasteners) Live-front Dead-front Open Metal-enclosed

Metal-clad Metal enclose & Metal clad Arc-resistant

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Switchgear classification

By operating method: Manually-operated

Motor-operated

Solenoid/stored energy operated

By type of current: Alternating current

Direct current

By application:

Transmission system Distribution.

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Main functions of switchgear

Electrical protection

Electrical isolation of sections of an

installation Local or remote switching

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Electricalprotection against

Isolation Control

Overload currents

Short-circuitcurrents

Insulation failure

Isolation clearly

indicated by anauthorized fail-proof mechanicalindicator

A gap orinterposedinsulating barrierbetween theopen contacts,

clearly visible

Functional

switchingEmergencyswitching

Emergency

stoppingSwitching off formechanicalmaintenance

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Electrical protection The protection of circuits

Against overload;

Against short-circuit currents

The protection of persons Against insulation failures.

The protection of electric motors

Against overheating

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Isolation To separate a circuit or apparatus from the

remainder of a system which is energized. An isolating device must fulfil the following

requirements:

All poles of a circuit, including the neutral must be provided with a locking system inopen position

must comply with a recognized national or

international standard

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Isolation To separate a circuit or apparatus from the

remainder of a system which is energized. An isolating device must fulfil the following

requirements:

All poles of a circuit, including the neutral must be provided with a locking system inopen position

must comply with a recognized national or

international standard

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Switchgear controlFunctional control

This control relates to all switchingoperations in normal service conditions forenergizing or de-energizing a part of a

system or installation, or an individualpiece of equipment, item of plant etc.

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Switchgear controlEmergency switching

An emergency switching is intended to de-energize a live circuit which is, or couldbecome, dangerous (electric shock or fire).

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Switchgear controlMaintenance operations on the power

system

This operation assures the stopping of a

machine and its impossibility to beinadvertently restarted while mechanicalmaintenance work is being carried out on

the driven machinery.

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Supply fromsource

Equipment

RELAY

BatteryTrip Coil

ofBreaker

CB

CT

Basic relay circuit scheme

Main Parts of a Circuit Breaker :

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Main Parts of a Circuit Breaker :

* Fixed Contact

* Movable Contact

* Operating Mechanism

* Arc extinguishing medium

Fault clearing process :

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Fault clearing process :

During any Fault..

* Fault impedance will be low, so fault current willincrease and relay senses this increase in current.

* Relay contacts closes and sends trip signal to circuit

breaker and the trip coil of the circuit breaker will get

energized.* Operating mechanism of the circuit breaker will

operate and separate the contacts.

* Arc will be initiated between the contacts and it is

extinguished by suitable methods.

Arcing phenomenon :

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Arcing phenomenon :

- When a fault occurs, heavy current flows through the contacts

of the circuit breaker before they are opened by the protective system.

- At the instant when the contacts begin to separate, the contact

area decreases rapidly and current density (I/A) increases and hence

rise in temperature.

-The heat produced is sufficient to ionise the medium between thecontacts. This ionised medium acts as conductor and an arc is struck

between the contacts.

- The potential difference between the contacts is very small and

is sufficient to maintain the arc.

- The current flow depends upon the Arc resistance.

Few definitions :

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Few definitions :

Breaking Capacity Max fault current at which a CB is capable

of breaking a circuit.

Making Capacity Max current a CB can withstand if it closing on

existing Short circuit.

Restriking Voltage After the arc has been extinguished, the

voltage across the breaker terminals does not normalize

instantaneously but it oscillates The transient voltage which appears

across the breaker contacts at the instant of arc being extinguished.

Recovery Voltage Power frequency voltage which appears acrossthe breaker contacts after the arc is finally extinguished and transient

oscillations die out.

Events/Timings during fault clearing process :

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Events/Timings during fault clearing process :

Fault clearingTime

Relay time Circuit breakerTime

InstantOf

Fault

Closure ofTrip

Circuit

Final arc

ExtinctionCircuit breaker

Time

Closure ofTrip

Circuit

= +

=

= to

toRelay time

Various types of CBs :

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Various types of CBs :

(i) Miniature CB

(ii) Air Break CB

(iii) Air Blast CB

(iv) Oil CB

(v) SF6 CB

(vi) Vacuum CB

Bulk Oil CB

Minimum Oil CB

Air Break CB :

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Air Break CB :

Air Blast CB :

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Air Blast CB :

ABCB Principle of arc quenching

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ABCB- Principle of arc quenching

Bulk Oil CB :

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Bulk Oil CB :

Minimum Oil CB :

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Minimum Oil CB :

http://electricalandelectronics.org/wp-content/uploads/2008/10/low-oil-circuit-breaker.gif

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SF6 CB :

http://electricalandelectronics.org/wp-content/uploads/2008/10/low-oil-circuit-breaker.gif

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SF6 CB :

1. Op mechanism

2. Interrupter

3. Support

4. Op rod5. Linkage

6. Terminals

7. Filters

8. Puffer cylinder

9. Nozzle

10. Fixed position

11. Fixed contact

12. Moving contact

13. Gas inlet

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400kV Circuit Breaker

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Advantage of SF6

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* Inert gas with high dielectric strength.

* Colour less and odour less.* Non-toxic and non- inflammable.

* Sf6 is blown axially to the arc, hence it removes the heat

by axial convection and radial dissipation. As result the

arc dia reduces and comes to zero at current zero.

* Gas pressure in the chamber is at 5 ksc.

* SF6 is filled at a pressure of 12 ksc in the tank and

maintained by means of an individual or a commoncompressor.

* The decomposition products of arcing are not explosive

hence no chance of fire.

Advantage of SF6

Vacuum CB :

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http://electricalandelectronics.org/wp-content/uploads/2008/10/vacuum-circuit-breaker1.jpg

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http://electricalandelectronics.org/wp-content/uploads/2008/10/vcb.gif

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Advantage of vacuum CB

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* Used up to 66 KV.

* Vacuum is of the range of 106 to 108 torr.

* Vacuum is highly dielectric, so arc cant persists.

* Separation of contacts causes the release of metal vapour fromthe contacts, the density of vapour depends on the fault current.

* At current zero the vapour emission will tends to zero and thedensity will becomes zero and dielectric strength will build up andrestriking will be prevented.

* No emission to atmosphere, hence pollution free.

* Non- explosive and silent operation.* Compact size.

Advantage of vacuum CB

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Disadvantages

* High initial cost.

* Surge suppressors (R or RC combination) are to be connected

at load side for limiting switching over-voltage while

switching low pf loads.

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MOVING PORTION

(Mechanism cover

removed)

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MECHANISM AND ITS OPERATION :

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GEAR BOX

MOTOR

CLOSINGSPRING

CLOSINGCOIL

TRIPPINGCOIL

MECHANISMSHAFT

LIMITSWITCH

TRIPPINGSPRING

SAFETY INTERLOCKS:

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The various interlocks provided on the breakerensure that-

(A) VCB Truck cannot be racked in inside the panelunless:

i) Door is closed .ii) Breaker is in tripped condition.iii) Plug & socket are engaged properly.

(B) VCB Truck can not be racked out from panelunless:

i) Door is closed.ii) Breaker is tripped .

Door cannot be opened unless the breaker is intripped condition .

Breaker can be closed only in SERVICE or in TEST position.

AUTO RECLOSURE

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AUTO RECLOSURE

Over 80% of the faults are transient in

nature. Designed to carry out one or two fast closing

operations followed by automatically locked

open.

AUTO RECLOSURE

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AUTO RECLOSURE

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FEATURES OF ISOLATORS OPERATES UNDER NO LOAD CONDITION

INTERLOCKED WITH BREAKERS ANDEARTHSWITCHES

MOTOR DRIVEN AND HAND DRIVEN

LOCAL AS WELL AS REMOTE OPERATIONPOSSIBLE

ISOLATES SECTIONS FOR MAINTENANCE USED TO SELECT BUS BARS

SELECT CT FOR BUS BAR PROTECTION

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400kV Isolator

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Isolator

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FEATURES OF EARTH SWITCHES USED TO GROUND SECTIONS REQUIRED FOR

MAINTENANCE

GROUND INDUCTION VOLTAGES

INTERLOCKED WITH BREAKERS AND ISOLATORS

CAN OPERATE FROM LOCAL ONLY

MOTOR DRIVEN AS WELL AS HAND DRIVEN SAFETY DEVICE

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CB

Earth S/W

IsolatorIsolator Load

Source

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Lightning Arrestors

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Purpose :

- To discharge the high voltage surges in the power

system due to lightning to the ground.

Apparatus to be protected :

* Overhead linesEarth/Ground wires

* HV equipmentLAs

* Substation...Lightning Masts, Earth wires

Types :

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Rod gap LA :

Insulator

Equipment

body

Rod gap

* Gap length is such that

the break-down occurs at

80% of the spark voltage

* After the surge, the arc

in the gap is maintained

by the normal supply

voltage.

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400kV Surge Arrester

Metal Oxide surge arrestor

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g

The gapless arresters are

based on metal oxide (MO)resistors, which have anextremely non-linear V/I

characteristic and a highenergy-absorptioncapability.

They are known as metal

oxide surge arresters, MOarresters for short.

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THANK

YOU