SWGRM

April 17, 2023 PMI Revision 00 1

SWITCHGEAR

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Presentation outline

Basics of switchgear

Components and Classification

Basic design aspects

Breakers, relays and fuses

Typical parameters


What is a Switchgear ? “The apparatus used for Switching, Controlling and Protecting

the Electrical Circuits and equipment”.

Need of Switchgear : * Switching during normal operating conditions for the

purpose of Operation and Maintenance.

* Switching during Faults and Abnormal conditions and

interrupting the fault currents.


SWITCHGEAR

Historical background :

• up to 1875 : knife switches

• 1885 : Bulk Oil circuit breaker

• 1900 : Arc extinction devices, MOCB

• 1930’s : ABCB

• 1950’s : SF6

• 1960’s : Vacuum CB


PARTS OF SWITCHGEAR

Switching device

Power circuit

Control circuit

Measurement and display

Protection


Switching devices :

Circuit breakers / contactors

Isolators

Earthing switch

Control Circuit :

service / test /isolated position selectors

Tripping and closing circuit

Spring charging, anti pumping arrangement

Supply monitoring , space heaters , indications

Measurement :

Ammeter, voltmeter, energy meter

Protection :

Relays, CT, PT,


SWITCHGEAR Various symbols :

Isolator / Disconnecting switch

Circuit Breaker

Earthing switch

Lightening Arrestor

CT PT

Ammeter Voltmeter

VA

April 17, 2023 PMI Revision 00 8SWGR : CIRCUIT BREAKERS


CLASSIFICATION OF SWITCHGEARS :

Method of arc quenching :

Bulk oil, Min. oil, Air Break, Air Blast, SF6 , Vacuum

Working voltage :

440 v, 6.6 kV, 11 kV, 400 kV etc.

Indoor / out door

SOME INTERLOCKS :

Check synchronisation for closing

Master relay contacts for trip and close

HV & LV Breaker interlocks

Main / Reserve supply change over


Swgr: Basic Design Aspects

• The Auxiliary power system in a power plant must form a RELIABLE source of power to all unit and Station auxiliaries. The basic function of Switchgear is to control supply of electric power and to protect the equipment in the event of abnormal conditions.Hence the Switchgears have to be RELIABLE , SAFE, and ADEQUATE .

• Defining the reliability, safety aspects and adequacy aspects in terms of Quantitative parameters forms the essential part in “SPECIFICATIONS”


Swgr: Basic Design Aspects

33 KV, 11 KV, 6.6 KV and 3.3 KV Switchgears

• Indoor, metal clad single front and fully Compartmentalized , with degree of protection IP42 and IP52 for metering compartments. For 33 KV the switchgears can be metal enclosed either.

• Circuit Breakers of either SF6 or Vacuum type. They shall comprise of three separate identical single pole interrupting units operated through a common shaft by a sturdy mechanism.


Basic Design Aspects …• Breakers are suitable for Switching transformers and

motors at any load and also for starting 3.3 KV - Above 200 KW to 1500 KW , 11 KV- above 1500 KW for 500 MW units and 6.6 KV- above 200 KW for 210 MW units.

• Surge arresters are provided for all motor feeders to limit the over voltages

• For Coal handling plant Motors where frequent start/stop of motors is called for HRC fuse backed contactors are provided.

• Suitable Interlocks are provided to ensure that Breaker is off before opening the rear doors/covers.


Basic design features: Control and Safety

• VSTPP -II onwards all Circuit Breakers/contactors are being controlled normally from remote through DDCMIS/PLC. The control Switch located on the Switchgear is normally used only for testing.

• All the logic for incomers, buscouplers , ties, transformer feeders and motor feeders is being generated in DDCMIS only . The reverse blocking schemes are still incorporated in Swgr(hardwired)

• In earlier projects these logics were generated in Swgr / remote panel (HARDWIRED).

• Such a type of control has facilitated Flexibility ,simpler wiring and helped in standardizing interface requirements . A typical Control scheme being implemented in Talcher-II is as displayed.

• Isolation Transformers for Lighting distribution to limit the fault of lighting Boards to 9 KA.

• Safety Measures for Switchgear include provision of insulated mats in front.


Typical Control Scheme


STN Supply

6.6 KV

415 V

DC Chargers

Unit Supply One Line Diagram

STNSupply

400KV Bus BarGT

20.5 KV 6.6KV Unit Bus

6.6KV

415 V

6.6 KV Unit Bus-2

6.6 KV

415V

Motors

MotorFeeders


Relays


Relay : “A device that detects the fault and initiates the operation

of the Circuit breaker to isolate the defective element

from the rest of the system”.

The relay detects the abnormal conditions in the electrical

circuits by constantly measuring the electrical quantities

which are different under normal and faulty conditions.

RELAY

CT CBLoadSource

PT


Requirements of Protecting relaying :

Selectivity - Ability to select the faulty part and isolate that

part without disturbing the rest of the system. Speed - Ability to disconnect the faulty part at the

earliest possible time. Sensitivity - Ability of the relay to operate with low value of actuating quantity. Reliability - Ability of the system to operate under pre-

determined conditions


Simplicity

- Should be simple so that it can be easily maintained.

- The simpler the protection scheme, the greater is the

reliability

Economy

- Availability at lower cost.

- Generally, the protective gear should not cost more than 5% of the total cost. However, when the apparatus to be protected is of utmost importance (e.g. Generator, GT

etc) economic conditions are subordinated to reliability.


Classification of Relays based on Design :

RELAYS

Electromagnetic&

Electro thermalStatic

Microprocessor based Numerical

* Attracted Armature* Induction disc* Printed disc dynamometer* Permanent magnet* Moving coil* Polarised moving Iron

* Bimetallic Strip

* Relay consists of Electronic circuitry such as Transistors, ICs, Diodes etc

* Uses VLSI technology* Can be Programmed


Basic classification of Relays based on Function :

* Current based, with and without directional feature.

* Voltage based

* Impedance based

* Frequency based

* Power based, with and without directional feature


Circuit Breakers


Main Parts of a Circuit Breaker : * Fixed Contact

* Movable Contact

* Operating Mechanism and control circuit

* Arc extinguishing medium


Basic trip circuit :

CT

CB

PT

RELAY

DC supply

Bus

Line

Trip Coil


Few definitions :

Breaking Capacity: Max fault current at which a CB is capable of breaking.

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 across the breaker contacts after the arc is finally extinguished and transient oscillations die out.


Fault clearing process :

During any Fault….. * Fault impedance will be low, so fault current will increase 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 :

- 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 the contacts. 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.


Events/Timings during fault clearing process

Fault clearing Time

Relay time Circuit breaker Time

InstantOf

Fault

Closure ofTrip

Circuit

Final arcExtinction

Circuit breaker Time

Closure ofTrip

Circuit

= +

=

= to

toRelay time


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 :


Air Blast CB :

April 17, 2023 32

ABCB- Principle of arc quenching


Bulk Oil CB :


Minimum Oil CB :


SF6 CB :

1. Op mechanism

2. Interrupter

3. Support

4. Op rod

5. Linkage

6. Terminals

7. Filters

8. Puffer cylinder

9. Nozzle

10. Fixed position

11. Fixed contact

12. Moving contact

13. Gas inlet


* 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 a 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 common compressor.

* The decomposition products of arcing are not explosive hence no chance of fire.

Disadvantages

* SF6 gas condensates at low temperature & high pressure

Advantage of SF6


Vacuum CB :


* Used up to 66 KV. * Vacuum is of the range of 10ˉ6 to 10ˉ8 torr. * Vacuum is highly dielectric, so arc can’t persists. * Separation of contacts causes the release of metal vapour

from the contacts, the density of vapour depends on the fault current.

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

* No emission to atmosphere, hence pollution free. * Non- explosive and silent operation. * Compact size.

Advantage of vacuum CB


* 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.

Disadvantage of vacuum CB



Few definitions : Rated current : - Current which can be carried without fusing Minimum fusing current : - Min value of the rms current at which the fuse melts. Fusing factor : - FF = Min fusing current / Rated current Prospective current : - Current which would have flown if the fuse is absent. Cut-off current : - Maximum value of fault current actually reached before the fuse elements melts.


Cut-off current

Fault occur

Pre-arcing time

Arcing time

Total operating time

Prospective current

t

I Characteristics of Fuse :


Characteristics of Fuse element : - Low melting point……. (Tin, Lead)

- High conductivity……… (Silver, Copper)

- Free from deterioration due to Oxidation…… (Silver)

- Low cost……………… (Lead, Tin, Copper)

Outstanding feature of a Fuse element is that it isolates the circuit well before the fault current reaches its first peak current. This gives the fuse a great advantage over a Circuit breaker since the most severe thermal and electromagnetic effects of Short-circuit currents (which occurs at the peak value of the Prospective current) are not experienced with fuses.


Types of Fuses : LV fuses

Semi-enclosed rewirable fuse :

- consists of porcelain base and a fuse carrier. - used where low currents are to be interrupted. - used in domestic and lighting applications HRC cartridge fuse : - consists of heat resistant ceramic body and the cartridge is filled with filler material such as chalk, plaster of Paris, quartz or marble dust which acts as arc quenching and cooling medium. - when fuse element (silver) melts, high resistance substance is formed due to the chemical reaction between the silver vapor and the filling powder. Thus Arc is quenched.


HV fuses (i) Expulsion type :

- consists of hollow tube made of synthetic-resin

bonded paper in which fuse wire is placed

- when fuse element melts, it causes decomposition of

the inner coating of the tube resulting in formation

of gases which extinguishes the arc.

- used in the level of 11 KV, 250 MVA and generally

used for protection of distribution transformers.

(ii) Drop-out fuse :

- Expulsion type fuse.

- when fuse melts, the fuse element carrying tube drops

down due to gravity, so that, can be spotted easily.


Coordination between Fuses and a O/C

protection devices:

Capacity of Circuit breaker

Char of Fuse

Char of O/C prot devices

I

t


STAGE-I

RATING MAKE QUENCHING MEDIUM

6.6 KV ASEA MOCB

415V NGEF AIR

STAGE-II

RATING MAKE QUENCHING MEDIUM

6.6 KV SIEMENS VACUUM

11KV SIEMENS VACUUM

415V EE AIR

Details of various CBs in RSTPS


OS I

LOCATION RATING MAKE MEDIUM

SLURRY P/H-1 415 V NGEF AIR

RAW WATER P/H 6.6 KVASEA/

ABB

MOCB/

VACCUM

COMPR: - 1 6.6 KV ASEA MOCB

COLONY S/S 11 KV GEC MOCB

CW P/H – 1 6.6 KVASEA/

ABB

MOCB/SF6/ VACUUM


LOCATION RATING MAKE MEDIUM

SLURRY P/H - 2415V&

6.6 KV

EE/

SIEMENS

AIR/

VACUUM

CW P/H - 2 415V EE AIR

DM PLANT MCC 415V EE AIR

33 KV S/S

1 & 233 KV MG SF6

OS II & 33KV


Make Voltage Rating

(KV)Operating Mechanism

Arc Ext Medium

AEG 400 CENTERALISED AIR COMPRESSOR SYSTEM

SF6

NGEF 220 & 400 INDIVIDUAL AIR COMPRESSOR SYSTEM

SF6

BHEL 220 & 400 HYDRAULIC SF6

ABB 220 & 400 INDIVIDUAL AIR COMPRESSOR SYSTEM

SF6

BHEL 132 MOTORISED OIL

S/Y


Make Rated Pr Low Pr AlarmFunctional

Lock out

AEG 8.0 7.0 6.5

NGEF 8.0 7.0 6.5

BHEL 7.5 6.7 6.5

ABB 7.0 6.2 6.0

SF6 gas pressures of CBs : (bar)


MakeAEG

(Air)

NGEF

(Air)

BHEL

(Hydr)

ABB

(Air)

Operating Pr: 37 30.6 303 31.5

A/R

Lock out 34 28 30 30

Operation Lock out 30 27.3 253 23

Safety Valve Open 42 42 NA 33

Operating medium parameters of CBs : (bar)


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