8. Evaluation of Discharges

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8. Evaluation of Discharges

• 2003. 10. 18

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8. Evaluation of Discharges

• Ultimate goal is to evaluate
• Must be ascertain
• - Observed discharges Harmful or not ?
• - Absence of discharges safe level ?
• (minimum
  detectable discharge)
• In connection with other test( tand, overvoltage
  test..)
• make clear that observed dielectric will be
  sufficiently safe
• Three section
• - Recognition Type of discharge
• - Mechanism deterioration of the insulation
• - Evaluation discharge levels

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8.2 Recognition

• 8.2 Recognition
• Oscillogram
• - discharge pattern -gt valuable indication
  to the type
• and origin
  of discharges
• - distribution of phase angle
• - ratio between positive and negative pulses
• 2. X-Y diagram
• - discharge magnitude (function of the test
  voltage)
• - recorded in pC(logarithmic scale)
• 3. Time effect
• - in some cases discharge magnitude and the
  extinction
• voltage change(when testing for sometime
  at high voltage)
• - X-Y diagram(good practice) rising and
  declining voltage
• - ignition effects and time effects can be
  shown

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8.2 Recognition

• 4. Ambience
• - Character or location of the discharge
  determined
• by making changes in or around the sample
• a. Immersion in oil or applying grease
  surface discharge
• b. changing pressure in GIS distinguish
  between
• discharges in the pressurized gas
• and isolated discharges
• c. increasing the temperature fissures
• - temperature cycles interstices
• - good tool combined with antecedents of the
  sample and
• with the
  results of other tests

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8.2 Recognition

• 8.2.1 Dielectric-bounded discharges
• - Occur between dielectric bounds(internal or
  superficial)
• fairly symmetric patterns(Fig8.1)
• - Occur in advance of the test voltage peaks
• - Impulse (same amplitude, number and location)
  appear at the positive and negative half cycles
• - There is some random variation (amplitude and
  phase angle)

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8.2 Recognition

• 8.2.1 Dielectric-bounded discharges
• 1. A squarely shaped diagram limited cavity(Fig
  8.2,3)
• - Further discharges not possible (after
  ignition of the cavity, the available space is
  occupied)

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8.2 Recognition

• 2. A triangular diagram(Fig8.4) somewhat larger
  cavity
• - discharges can expand first and remain
  constant (fill with discharge)

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8.2 Recognition

• 3. An ever increasing diagram(Fig8.5)
• - surface discharge(Fig8.6 a)
• - discharge in a large gap(Fig8.6 b)
• 100,000pC observed
• - discharge in an interface(Fig8.6 c)

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8.2 Recognition

• 4. Effect of time (1)
• - at its maximum value keep for some
  time(e.g. 30min)
• discharge magnitude may change
• - magnitude decrease and higher extinction
  voltage(Fig8.7)
• - longer period(e.g. 24hr) more

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8.2 Recognition

• - Elastomeric insulation fissures
• (direction of
  the electric field, Fig 8.8)
• - Round cavities in thermoplastics(Fig 8.9)
• gradual increase in discharge magnitude (a
  number of cavities of various sizes)
• higher extinction voltage (formation of
  electrically conducting layers at the surface of
  the cavity)
• rested for sometime(e.g. 24h) restored

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8.2 Recognition

• 5. Effect of time (2)
• - inception voltages are lowered instead of
  increased by testing the sample at high
  voltage(Fig 8.10,11)
• Fig 8.10 Voltage is kept at maximum value,
  discharge level is gradually increased and
  becomes stable(e.g.15min)
• - Extinction voltage is lower than the inception
  voltage
• epoxy resin(small amount of moisture during
  manufacture)

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8.2 Recognition

• 5. Effect of time (2)
• Fig 8.11 extreme case of deterioration by
  voltage
• - oil-impregnated paper(not sufficiently dry)
• - bubbles are generated and increase in size and
  number during testing.
• - rested for some time discharges will
  disappear

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8.2 Recognition

• 8.2.2 Electrode-bounded discharges
• pattern asymmetric(Fig 8.12)
• Between electrode and dielectric
  boundary
• - Accur in advance of the test voltage peaks
• One half cycle small number of large
  discharges
• The other cycle larger number of smaller
  discharges
• Amplitude differece 3 10 times

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8.2 Recognition

• 8.2.2 Electrode-bounded discharges
• - squarely shape X-Y diagram(Fig8.2) internal
  discharge in an
• 
  electrode-bounded cavity(Fig 8.13 a)
• - Ever-growing X-Y diagram(Fig8.14) surface
  discharge
• ( starting from
  one of the electrodes)

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8.2 Recognition

• 8.2.2 Electrode-bounded discharges
• - Time effect small
• Self-healing effect with surface discharge
  spurious
• discharges are seen and
  extinguish again
• (voltage is increased or kept constant
  for observation)
• Fig8.15
• internal discharge lower voltage
• surface discharge higher voltage

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8.2 Recognition

• 8.2.4 Floating objects
• Metallic parts peculiar discharge
  patterns(Fig8.16)
• - Equal amplitude, number and phase angle at
  both half cycles
• - More-or-less equally spaced, sometimes occur
  in pairs
• - Move around the time base, disappear(certain
  point) and
• restart(moment)
• - X-Y diagram square (Fig8.2)
• - Unaffected by time

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8.2 Recognition

• 8.2.4 Floating objects
• Peculiar behavior is caused
• - bad contact(floating parts or floating part
  and electrodes,
• loose connection to a screen or
  unwanted metal particle)
• - serious defect in insulation
• Disturbances in a discharge test
• - metallic objects laying on the floor
• - Clean floor 30kV or more

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8.2 Recognition

• 8.2.5 Corona discharges in SF6 gas and air
• Corona discharges Typical patterns(Fig 8.17)
• - Negative half cycle sharp edge is at high
  voltage
• - Positive half cycle
  at the earth side

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8.2 Recognition

• 8.2.5 Corona discharges in SF6 gas and air
• X-Y diagram(Fig8.18) at higher voltage large
  corona discharge
• - opposite half cycle and second step is added
• Time effect discharge pattern is little affected

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8.2 Recognition

• 8.2.5 Corona discharges in SF6 gas and air
• Corona discharge cause
• - sharp edges in a construction
• - disturbances(sharp edges in the high-voltage
  test circuit)
• - Free of sharp edges or points high-voltage
  leads and
• 
  earth side(30 100kV)
• 8.2.6 Corona in oil
• Occur Both half cycles, Symmetric
• about the voltage peaks (Fig 8.19)
• - equally spaced in time
• greater magnitude and spacing than the
  other half cycle

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8.2 Recognition

• 8.2.6 Corona in oil
• - smaller discharges equal magnitude
• - larger discharges equal magnitude or
  vary
• - In contrast with corona in air larger
  discharges start first
• - X-Y diagram similar Fig8.18
• Time effect little affected, take a short time
  to stabilize
• (first apply the voltage)
• - Larger discharge on positive half cycle the
  point is at high voltage

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8.2 Recognition

• 8.2.7 Contact noise
• - Coarse and irregular noise about zero points
• Make clear distinguished from normal
  discharges
• - Occur sample under test or itself from test
  circuit
• - Frequently in capacitors connection to the
  foils
• ? short-circuiting the capacitor after
  charging
• 8.2.8 Interference
• - external sources(Fig8.20)

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8.2 Recognition

• 8.2.9 Expert systems
• - Can provide a possible diagnosis
• After the discharge characteristics have
  been answered
• Further automation Pulse height analyser is
  coupled to the
• discharge
  detector
• - Analyser builds pulse height histograms
• - Algorithm programmed using the language of an
  expert system
• Cannot give better answer than are allowed by
  the physical
• correlation between pulse patterns and
  discharges
• 8.2.10 Recording
• - make of both the discharge pattern and the X-Y
  diagram

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8.3 Mechanisms of deterioration

• 8.3 Mechanisms of deterioration
• Internal and surface discharges
• cause deterioration to dielectrics
• 1. Heating the dielectric boundary
• 2. Charges trapped in the surface
• 3. Attack by ultraviolet rays and soft X-rays
• 4. Formation of chemicals such as nitric acid
  and ozone
• Cause depolymerization, stress cracking, gassing
• leading to erosion of dielectric surface

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8.3 Mechanisms of deterioration

• 8.3.1 Internal discharges in polymers
• Three stages of deterioration
• 1. A uniform surface erosion
• thermal degradation, ultraviolet radiation
• - ions laid down by consecutive discharges
  cannot neutralize electrons(be trapped below the
  surface)
• - Close proximity of charges high field
  strengths in the dielectric -gt reach breakdown
  strength -gt surface erosion

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8.3 Mechanisms of deterioration

• 8.3.1 Internal discharges in polymers
• 2. Discharges become concentrated
• because stress cracking cause micro-crack
• -gt deep pits are formed -gt discharges further
• concentrate, carbonization occur
• - First two stages major part of the time to
  breakdown
• high stress(at 10 to 20kV/mm) a few hours
• lower stress(at 3 to 5kV/mm) many years

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8.3 Mechanisms of deterioration

• 8.3.1 Internal discharges in polymers
• 3. Field concentration around sharp tip
• -gt approach dielectric strength over a
  distance of some microns -gt dielectric breakdown
  
• -gt field concentration moves new tip
• and narrow channels propagate
• - found in polyethylene and rubber(Fig8.21)
• - Third stage breakdown take place in a few
  voltage cycles

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8.3 Mechanisms of deterioration

• 8.3.2 Internal discharges in paper insulation
• - Discharges in voids adjacent to the conductor
• attack the insulation, penetrate the first
  paper layer
• -gt surface discharges occur along the layer
• -gt trees or carbonized tracks are formed
• -gt tracks follow the weakest points in the
  insulation(butt gap) Fig8.22

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8.3 Mechanisms of deterioration

• 8.3.2 Internal discharges in paper insulation
• - At the foot of the tree local overheating
• -gt thermal breakdown -gt trees and corbonize
  track attain enormous lengths

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8.3 Mechanisms of deterioration

• 8.3.3 Surface discharges
• - Deterioration by surface discharges
• same patterns internal discharges
• - Discharge resistance of different materials
• - Time to breakdown taken as a measure.
  (Fig8.23)

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8.3 Mechanisms of deterioration

• 8.3.4 Corona discharges
• - Occur around bare conductors
• - Indirect action by ozone formed by corona
• deteriorate neighbouring dielectrics
• - Corona discharge In SF6 gas not acceptable at
  all
• create aggressive by-products (very
  detrimental to dielectric surfaces)