Chapter 3 HV Insulating Materials: Gases

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Chapter 3HV Insulating Materials Gases

• Air is most commonly used insulating material
• Gases (incl. air) normally a good electrical
  insulating material
• Under high E-field conditions, gases become
  ionized, leading to corona, sparks and flashover
• Why??

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Discharges on an insulatorWhy?
How are these discharges formed?
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Ionisation processes Photo-ionization

• Bohr model of an atom electrons in fixed orbits
• Photo-ionisation Planck
• W hf (Joules)
• Energy gained from light raises electrons to
  higher energy level (orbit) or quantum band
• Energy is absorbed when moving to higher orbit
• Energy is emitted when falling back
• If energy gained exceeds the ionisation energy
  of the gas the electron leaves the atom.

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Ionisation processes Orbits and Energy Levels
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Ionisation by Collision

• Ionisation by collision Free initiating
  electrons always present (cosmic rays)
• - Initiating electrons accelerated by Lorentz
  force due to the E-field
• Electron gains kinetic energy
• Collide against gas atoms - kinetic energy
  converted to potential energy
• Ionisation occurs if this energy exceeds the
  ionisation energy of the atom, sets free more
  electrons and leaves positive charge behind.
• Other processes (in vacuum tubes)
• - Thermal ionisation due to external heat
• - Field emission

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Ionisation by Collision

• Townsends primary ionisation coefficient ?
• ? No. of ionising collisions for 1 mm length
  movement by one electron
• Exponential growth avalanche formation
• n n0exp(? x) number of electrons liberated
  at point x
• Electrons are more mobile than (relatively heavy)
  positive ions.
• Not a self-sustaining process (depends on
  initiating electron)
• Typical application - Geiger counter

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Ionisation by CollisionTownsends secondary
process

• An avalanche is not self-sustaining process
  stops if initiating electrons not available.
  Positive feedback thus required
• - Pos. ions move back to cathode (-) and collide
  against cathode, releasing more initiating
  electrons
• - ? new electrons gained at cathode by () ion
  impact
• - New avalanches form, plasma column formed -
  higher current leads to breakdown
• - Thus a self-sustaining process.

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Electronegative gases SF6

• Some gases are electronegative, have electron
  affinity electrons attach to the molecules
• Heavy SF6-molecules are formed thus lower
  mobility and collision ionization probability
• If ?gt?, then ionization stops

The following attachment processes occur in
SF6 SF6e ? SF5F2e SF6e ? SF6
SF6e ? SF5 F

? attachment coefficient
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SF6

• Sulphurhexafluoride (SF6) is an electronegative
  gas electron affinity
• Townsends first ionization coefficient (? ) is
  effectively lowered to (? - ? ) with ? the
  attachment coefficient
• Free electrons attach to the heavy molecules with
  lower mobility compared to electrons
• The electron attachment reduces the ionisation
  probability and raises the flashover voltage
  (roughly 4 times higher than air)

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SF6 Substations (GIS)

• Colourless, odourless, non-toxic, chemically
  inactive.
• 5 times heavier than air
• Also arc quenching medium in circuit breakers

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Paschens Law

• Sustained Townsend discharge leads to spark then
  arc (flashover). Formulated mathematically by
  Paschen, (see p 52)
• The flashover voltage is a function of the
  product of the gas pressure and the gap length
  for an uniform field
• Implications in practice
• - Altitude effect
• - Compressed gases
• - Vacuum contactors
• Approximation for curve

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Paschens Law
Low gas density - more kinetic energy gained but
less collisions High gas density more
collisions but less energy gained
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GASES PASCHENS LAW FLASHOVER Ud vs pd GRAPH
UNIFORM GAPS