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					   HV BUSHINGS



TESTING & MAINTENANCE
      K.C.SEKHAR
        SS[EM]
         FUNCTION

• To provide an insulated entrance for
  an energized conductor into an
  apparatus tank or chamber
• To serve as a support for the
  energized parts
               TYPES
    CONDENSER
•   Oil-impregnated paper insulation
•   Resin-bonded paper insulation
    NON CONDENSER
•   Solid core
•   Gas filled
         ESSENTIAL PARTS
•   Center conductor
•   Level gauge/Sight glass
•   Insulating weathershed
•   Tap electrode with insulation & cover
•   Mounting flange
•   Ground sleeve
•   Lower/bottom insulator
          CONSTRUCTION

• Primary insulation is by oil-impregnated
  paper    insulation   with   interspersed
  conducting layers
• Primary insulation is contained in a
  weather proof housing, usually Porcelain
• Space between Primary insulation &
  Weather shed is filled with insulating
  medium [Oil/Compound]
           CONSTRUCTION
• A condenser bushing is essentially a
  series of capacitors between the center
  conductor & ground sleeve
• Equal     capacitances    produce   equal
  distribution    of   voltage   from   the
  energized center conductor to the
  grounded flange
• A conducting layer near the ground
  sleeve is tapped & brought out to a
  terminal outside known as ‘Test tap’
        INSULATION SYSTEM

  Main/Core insulation - C1
• Insulation between Center conductor and
  Tap electrode

  Tap insulation - C2
• Insulation between Tap electrode and
  grounded flange
             BUSHING TAP
                 Test tap
  Power factor tap
• Connected to one of the conducting
  layers of a capacitance graded bushing
• Facilitates to conduct a test on the main
  insulation without the need of isolation
• for measurement of Capacitance, Power
  factor, Partial discharge etc.
          BUSHING TAP

   Potential/Capacitance tap
• Connected to one of the conducting
  layers of a capacitance graded bushing
• Provides a capacitance voltage divider
  and additional equipment can be
  connected to this tap
• for measurement of Capacitance, Power
  factor, Partial discharge etc.
             FEATURES
• Simple device with no moving parts
• Hermetically sealed to maximise the life
  expentancy
• Considered to be maintenance free
• Have small volumes of oil
• Not possible to inspect the primary
  [internal] components at site
• 80 % of failures occur between 12-20
  years
              FEATURES
• 30 % of Transformer failures are due to
  bushing failures
• 90 % of failures are due to moisture entry
• Bushing failures are usually explosive in
  nature with huge fires
• Some times results in extensive damage
  to adjacent equipment & detrimental to
  personnel
               FEATURES
• Oil quality is very critical due to small oil
  to paper ratio
• Contamination is easy due to small
  volume of oil
• Philosophy is not to disturb the seal
• Periodic external inspection is essential
• Periodic external cleaning to remove dirt
  & contaminants to prevent flashovers
        BUSHING TROUBLES

• Cracked porcelain
• Gasket leaks
• Loose/Broken      connection   between
  ground sleeve & flange
• No oil/deteriorated oil
• Over heating of connections
• Partial discharges due to      internal
  deficiences
        BUSHING TROUBLES
• Porcelain contamination due to dirt, salts,
  semi-conducting sediments etc.
• Inadequate impregnation of paper by oil
• Short-circuited condenser layers
• Design problems
• Moisture ingress
• Improper storage of spare bushings
            TEST METHODS

    OFF - LINE
•   DC Insulation Resistance
•   Tap Insulation Resistance
•   Dissipation factor & Capacitance
•   Thermal Imaging [Infrared]
•   Partial discharge
•   Oil testing
          TEST METHODS

  ON - LINE
• Monitoring the change in sum current
  vector
• Monitoring the change in Tan delta
    INSULATION RESISTANCE
• Applicable to spare bushings
• Between Center conductor & Flange at
  5 KV
• Between Tap electrode & Flange at 500 V

				
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posted:9/1/2012
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