Bushing power transformer

1
PRESENTATION ON BUSHINGS
2
Topics to be covered

• What is a bushing
• Types of bushings
• Salient design features
• Testing of bushings
• Factors affecting bushings
• Causes for failure of bushings
• Dos and donts on bushings

3
Bushing

• Bushing is defined as a structure carrying one or
  several conductors through a partition such as a
  wall or tank etc. and insulating it or them
  therefrom
• A bushing has to provide
• Electrical insulation to the conductor for the
  working voltage and for various over voltages,
  which occur in service.
• Mechanical support against various Mechanical
  forces.

4
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
5
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

6

• 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)

7
(No Transcript)
8
(No Transcript)
9
800 kV OIP Condenser Bushing
10
MAJOR PARTS OF BUSHING
Top Cap
Top Porcelain
Flange
Bottom Porcelain
11
Major Parts of Bushing
12
CREEPAGE DISTANCE

• 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

13

14
Nominal Specific Creepage 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
15
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

16
Salient Design Features(Contd)

• 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 clients specifications.

17
Manufacturing Requriements

• Dust free pressurized winding chamber
• Winding Machines
• Oil Treatment Plants for oil processing
• Process Plants for Vacuum Impregnation at very
  fine Vacuum (0.005-0.001mm of Mercury)

18
(No Transcript)
19
(No Transcript)
20
Bushing under Type Test
21
Bushing under Testing in UHV Lab.
22
Testing Facilities (Contd)

• Routine Tests on Bushings are conducted in a
  separate electro-magnetically shielded 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

23
(No Transcript)
24
Bushing under Routine Test
25
Factors affecting performance of Bushing

• Atmospheric condition like pollution, humidity,
  and ambient temperature.
• Temperature rise of joints.
• Internal Corona
• Thermal instability
• Loss of oil
• Altitude

26
Causes of failure in Bushings

• Insulation failure
• Ingress of moisture
• Leakage of oil
• Internal discharges
• Disconnection of earthing link of test tap
• Hair crack in porcelain
• Looseness of joint in the current carrying path
• Failure due to high voltage such as lightning,
  switching etc.

27
Tan Delta Measurement

• Tan Delta is the dielectric loss angle.
• Limiting Value for OIP Bushing 0.007(Max.)
• For measuring tan delta at site Schering Bridge
  is required.
• New Bushing yet to be installed
• Bushing already in service

28
Significance of Tan Delta Measurement

• It reveals increased dielectric losses (I2R) with
  other source of heating may eventually lead to
  thermal runaway
• High level of partial discharges are reflected
  in tan delta
• Source of heating include dielectric losses,
  ambient temperature and load fluctuations

29
Factors affecting Tan Delta Measurement

• Ambient conditions
• Temperature of transformer oil around the lower
  end of bushing
• Pollution level at site
• Induction effect of the neighboring lines

30
Guidelines for measurement of Tan Delta at site

• Position of bushing should be vertical
• If mounted on transformer, remove top terminal
  connections
• Clean the porcelain with a non-fluffy cloth
• Check for oil level and any leakage
• Short the bushings of other phases which are not
  under test
• Avoid measurement on a rainy season or when
  humidity is more than 70

31
Guidelines for measurement of Tan Delta at site

• Note temperature and correct it at 20ºC
• The bushing should be tested in Ungrounded
  Specimen Test (UST)mode.
• The bridge should be placed on a vibration free
  base.
• Test tap should be clean and dry
• Connecting leads should be shielded
• Make sure the bare connector on the HV lead
  extends away from the bushing porcelain

32
Analysis of Results

• 1) High Tan delta (between0.007 and 0.01)
• Ingress of moisture in bushing insulation
• 2)Very High Tan delta (Beyond 0.01)
• High humidity, Strong ageing of insulation
• 3) Low Tan delta
• Weak Potential connection
• 4) High Capacitance
• This indicates Partial Breakdown.

33
Analysis of Results

• Variation in Capacitance
• With in 5 - Bushing Acceptable
• With in 5 to 10 - Monitor Bushing closely
• More than 10 - Inform BHARAT HEAVY ELECTRICALS
  LIMITED or Replace Bushing

34
Modifications made in design

• Bushing sealing is improved by increasing the
  compression load
• Hydraulic tightening introduced
• Cork gaskets replaced by rubber gaskets
• Design of top dome of 72.5 kV improved to protect
  ingress of rain water and better dissipation of
  heat
• Self Earthing Test Tap

35
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.
36
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.
37
DOs AND DO NOTs FOR BUSHINGS DO 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.
38
DOs AND DO NOTs FOR BUSHINGS DO NOTs....... 5.
DO NOT fill oil in the bushing without specific
instructions from BHEL . 6. DO NOT climb the
porcelain to tighten the top terminal. 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.
39
Bushing

• BHEL started manufacturing of Bushings with
  Synthetic Resin Bonded Paper technology in 1962
• Oil Impregnated Paper(OIP) bushings were
  developed in BHEL in 1982.
• Commercial Production of OIP Bushings started in
  1984 up to 420 kV rating.
• 525 kV Bushing manufactured and commercialized in
  1997
• BHEL has already developed 800 kV Bushing
  (Prototype) .

40
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.

41
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

42
Testing Facilities (Contd)

• 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 137,
IS 2099, BS 223 and ANSI can be done within BHEL
43
Quality Aspects

• BHEL Bhopal is ISO 9001 and 14001 certified
  Company
• Testing system in accordance with National
  Accreditation Board for Labs. (NABL) 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 0.7 as specified in standards
• All castings are routine tested before assembly.

44
Thank You
45
(No Transcript)
46
(No Transcript)
47
Equipotential condensers