Bushings

PRESENTATION  ON  BUSHINGS 

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Topics to be covered  

What

is a bushing



Types of bus hings



Salient design features



Testing of bus hings



Factors affecting bus hings



Causes for failure of bus hings



Do¶s and don¶ts on bus hings

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Topics to be covered  

What

is a bushing



Types of bus hings



Salient design features



Testing of bus hings



Factors affecting bus hings



Causes for failure of bus hings



Do¶s and don¶ts on bus hings

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Bushing 



Bushing is defined as a structure carrying one or several conductors t hrough a partition such as a wall or tank etc. and insulating it or t hem therefrom  A bus hing has to provide  Electrical

insulation insula tion to the conductor for the working voltage and for various over  voltages, which occur in service.

 Mechanical

support against various Mechanical forces. Jump to first page

TYPES OF BUSHINGS 1.1 Application i) Alternator bushing ii) Transformer bushing iii) Bushings for switch gear iv) Wall bushing or roof bushing v) Cable end and joint box bushing 1.2 Application: i) Indoor type ii) Outdoor type

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TYPES OF BUSHINGS    

  

 

2.1 Type of Bushing: i) Stem type ii) Pull-through type 2.2 Type of Bushings: i) Condenser core type ii) Non-condenser core type (Hollow Porcelain type) iii) Gas filled bushing iv) Solid Bushing Solid Ceramic Bushing  Epoxy Bushing 

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TYPES OF BUSHINGS 3. Type of insulation used in manufacturing Condenser Bushing: i) Synthetic Resin Bonded Paper (SRBP) Oil filled Compound filled ii) Oil Impregnated Paper (OIP) iii) Resin impregnated Paper (RIP)

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800 kV OIP Condenser  Bushing

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MAJOR PARTS OF BUSHING

Top Cap

Top Porcelain

Flange

Bottom Porcelain

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Major Parts of  Bushing

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C REEPAGE DISTAN C E  

   

The shortest distance along the contour  of the external insulating surface between the metal parts which normally have the operating voltage between them. Lightly polluted atmosphere Medium polluted atmosphere Heavily polluted atmosphere Very heavily polluted atmosphere Jump to first page

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Nominal Specific C reepage Distance Level I

Light

16 mm/kV

Level II

Medium

20 mm/kV

Level III

Heavy

25 mm/kV

Level IV

Very Heavy

31 mm/kV

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Salient Design features  Electrostatic

Field Controlled design of  Condenser Core by Computer 

 Suitable

for Operation under  heavily polluted atmosphere

 Reliable,

relatively easy erection and practically maintenance free.

 Better

Dielectric Strength and Partial Discharge free up to Working Voltage

 Space

for mounting of Ring Type Current Transformer  Jump to first page

Salient Design Features( Co  nt¶d)  Self

Earthing Test Tap for measurement of Tan Delta and Capacitance at site.

 Environmental

friendly.

 Bushings

for special applications can be designed and supplied to suit client¶s specifications.

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M anufacturing Requriements  Dust



Winding Machines

 Oil



free pressurized winding chamber 

Treatment Plants for oil processing

Process Plants for Vacuum Impregnation at very fine Vacuum (0.005-0.001mm of  Mercury)

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he ta ar pro u t ra ge of

Voltage (kV)

Current(Amps.)

ushi gs is as elo

CT Length (mm)

52

1250, 2000, 3150

100, 300

72.5

800, 1250

100, 300

145

800, 1250, 2000

100, 300, 600

245

800, 1600, 2000

300, 600

420

800, 1250

400 Jump to first page

Bushing under  Type Test

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Bushing under  Testing in UHV Lab. Jump to first page

Testing Facilities ( C ont¶d) 

Routine Tests on Bus hings are conducted in a separate electro magnetically s hielded Test Laboratory which is provided with  2.0

Million-Volt, 60 kJ Impulse Generator   Schering bridge  Standard capacitors  PD measuring equipment  Facility for conducting Cantilever Load Test

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ROUTINE TESTS ON BUSHINGS

Sn

Test details

IEC Cl. 52 kV 72.5 kV 145 kV 245 kV 420 kV o. 

1

Measurement of Dielectric loss and Capacitance

8.1

2

Dry lightning Impulse Voltage withstand test

8.2

A

3

Dry Power Fre uency Voltage withstand test kV

8.3

Measurement of Partial disc harge 5 6









A

A

A

1 25

5

1 0

2 5

60

630

8.











Test on test tap

8.5











Leakage test

8.











Tightness test at t h e Flange or ot h er fixing device

8.











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Bushing under  Routine Test

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Factors affecting performance of  Bushing  Atmospheric

condition like pollution, humidity, and ambient temperature.

 Temperature  Internal

Corona

 Thermal  Loss

rise of joints.

instability

of oil

 Altitude

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C auses of failure in Bushings  Insulation  Ingress

of moisture

 Leakage  Internal

failure

of oil

discharges

 Disconnection  Hair

of eart hing link of test tap

crack in porcelain

 Looseness

of joint in t he current carrying pat h

 Failure

due to high voltage such as lightning, switching etc. Jump to first page

Tan Delta M easurement  Tan Delta is the dielectric loss angle. Limiting Value for OIP Bus hing = 0.00 Max. For measuring tan delta at site Sc hering Bridge is re uired. 

New Bushing yet to be installed



Bushing already in service

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Significance of Tan Delta M easurement   It

reveals increased dielectric losses I2R with other source of  heating may eventually lead to thermal runaway



High level of partial disc harges are reflected in tan delta

 Source

of heating include dielectric losses, ambient temperature and load fluctuations

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Factors affecting Tan Delta M easurement   Ambient

conditions

 Temperature

of transformer oil around t he lower  end of bus hing

 Pollution  Induction

level at site effect of t he neighboring lines

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Guidelines for measurement of  Tan Delta at site  Position

of bus hing should be vertical

 If

mounted on transformer, remove top terminal connections

 Clean the  Check

porcelain with a non fluffy cloth

for oil level and any leakage

 Short the

bushings of other phases which are not under test

 Avoid

measurement on a rainy season or w hen humidity is more than 0% Jump to first page

Guidelines for measurement of  Tan Delta at site  Note

temperature and correct it at 20ºC

 The

bushing should be tested in Specimen Test ST mode.

ngrounded

 The

bridge should be placed on a vibration free base.

 Test

tap should be clean and dry

 Connecting

leads s hould be shielded

 Make

sure the bare connector on t he HV lead extends away from t he bushing porcelain Jump to first page

 Analysis of Results 1 High Tan delta between0.00 and 0.01 Ingress of moisture in bus hing insulation 2 Very High Tan delta Beyond 0.01 High humidity, Strong ageing of insulation 3 Low Tan delta Weak

Potential connection

High Capacitance This indicates Partial Breakdown. Jump to first page

 Analysis of Results Variation in Capacitance  With

in 5% Bushing Acceptable

 With

in 5% to 10% Monitor Bus hing closely

 More than

10% Inform BHARAT HEAV ELECTRICALS LIMITED or Replace Bus hing

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M odifications made in design  Bushing

sealing is improved by increasing t he compression load 

Hydraulic tightening introduced



Cork gaskets replaced by rubber gaskets

 Design

of top dome of 2.5 kV improved to protect ingress of rain water and better dissipation of heat

 Self

Earthing Test Tap Jump to first page

DOs AND DO

NOTs FOR BUSHINGS

DOs

1.

Check the packing externally for possible damage before un packing.

2.

Unpack with care to avoid any direct blow on the bushing or porcelain insulator.

3.

Store the bushing in a shed or covered with tarpoline to protect it from moisture and rains. If removed from the crate keep it indoors with lower end protective intact.

4.

Handle the bushing with manila rope slings without any undue force on porcelain insulators. Jump to first page

DOs AND DO

NOTs FOR BUSHINGS

DOs

5.

Clean the porcelain insulator thoroughly before taking any measurement or mounting the bushing on the transformer. Check for any foreign body adhered to the bottom porcelain.

6.

Check the oil level by making the bushing vertical

7.

Check for leakage of oil from any of the gasket joints. Each bushing is tested with the oil immersed in oil tank and so some traces of oil can be found which is actually not leakage.

8.

Check tan delta and capacitance ( if possible on mounted bushing without making any connections ).

9.

Maintain the log book of records of periodical checks. Jump to first page

DOs AND DO DO

NOTs FOR BUSHINGS

NOTs

1.

DO

NOT unpack the bushing from crate unless required to be mounted on the transformer.

2.

DO

NOT use metal slings on porcelain and avoid un due jerks while handling.

3.

DO

NOT store the bushing outdoors with out any protective covering

4.

DO

NOT measure IR value and tan delta with out thoroughly cleaning the porcelain and oil end portion. Jump to first page

DOs AND DO DO

5. 6.

NOTs FOR BUSHINGS

NOTs....... DO

NOT fill oil in the bushing without specific instructions from BHEL . DO

NOT climb the porcelain to tighten the top termina

Use elevators or seperate ladder for this purpose. 7.

DO

NOT dismantle or attempt to repair the bushing in

case the defect or mechanism is not clear. Jump to first page

Bushing  BHEL

started manufacturing of Bushings with Synthetic Resin Bonded Paper technology in 1 62

 Oil

Impregnated Paper OIP bushings were developed in BHEL in 1 82.

 Commercial

Production of OIP Bushings started in 1 8 up to 20 kV rating.

 525

kV Bushing manufactured and commercialized in 1

 BHEL has

already developed 800 kV Bushing Prototype .

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Bushing  More than

25,000 Bushings manufactured by BHEL are in service in India and abroad for  applications like Transformer, Traction, Switchgear, Cable box, Substations and Laboratories.

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Testing Facilities 

Type Tests  Type

tests are done in Ultra High Voltage (UHV) Laboratory which is one of the largest electro-magnetically shielded laboratories in the world.

 The

Laboratory offers very low background level during PD and RIV measurements.

 It 

is equipped with following facilities 4 Million Volt Impulse Generator 



1.5 Million Volt Cascade Test Transformer 



1.2 Million Volts DC Generator  Jump to first page

Testing Facilities ( C ont¶d) 

40 MVA 3-phase Motor-Generator set



Standard capacitors



Calibration and Recording Equipment

A

pollution test chamber is also attached to this for conducting Wet and Pollution Tests

With these

facilities all Type Tests to IEC 13 , IS 20 , BS 223 and ANSI can be done within BHEL Jump to first page

Quality Aspects  BHEL

Bhopal is ISO 001 and 1 001 certified Company

 Testing

system in accordance with National  Accreditation Board for Labs. N ABL recognized by  APLAC.

 Bushings

are manufactured in line with agreed Quality Plan with the purchaser or BHEL standard Quality Plan

 Raw

materials are accepted only after sample testing at BHEL Lab and comparing the properties with Plant Specifications.

 Process

is controlled to achieve tan delta less than 0.5% against 0. % as specified in standards

 All

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Thank ou

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