9. Tracking and Breakdown of Solids due to Discharges

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9. Tracking and Breakdown of Solids due to
Discharges

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? Introduction

• 9.1 Tracking
• 9.2 Low-voltage tests
• 9.3 Medium-voltage test
• 9.4 High-voltage test
• 9.5 Theory of tracking
• 9.6 Breakdown by discharges in solids
• 9.7 Discharge detection

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9.1 Tracking

• Definition
• Tracking is the formation of a permanent
  conducting path across a surface of the
  insulation.
• The conduction results from degradation of the
  insulation itself.
• Tracking is an untidy process.
• Incidence depends upon the insulation but its
  inception depends upon several other factors.
• Its necessary for organic insulation to be
  present if tracking is to occur.

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• Three essentials of the tracking phenomenon
• The presence of a conducting film across the
  surface of the insulation
• A mechanism whereby the leakage current through
  the conducting film is interrupted with the
  production of sparks
• Degradation of the insulation must be caused by
  the sparks

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? the process of tracking
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• The conducting film is usually moisture from the
  atmosphere absorbed by some form of contamination
  such as salt in coastal areas, carbonaceous dust
  from fuel or brush gear, industrial deposits, or
  cellulose fibres.
• Moisture is not essential, as a conducting path
  can arise from metal dust.
• Interruption of moisture film is caused by drying
  of the surface following the heating effect of
  the leakage current.
• Sparks are drawn between the separating moisture
  films, which act as extensions to the electrodes,
  and damage is done. ?

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• This represents a significant difference between
  tracking and discharge failure.
• For a discharge to occur there must be a voltage
  at least equal to the Paschen minimum for the
  particular state of the gas, 380V at s.t.p. in
  air, whereas tracking can occur at well below
  100V it does not depend on gaseous breakdown.
• In the case of conducting particles in oil the
  mechanism of interruption is one of bad contact
  between adjacent particles, which draws sparks
  across the surface of the supporting insulation.
• Degradation of the insulation is almost
  exclusively the result of heat from the sparks,
  and this heat either carbonizes or volatilizes
  the insulation if tracking is to occur.

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• This emphasizes the point that for all practical
  purposes tracking can only occur with organic
  insulation.
• Carbonization results in a permanent extension of
  the electrodes and usually takes form of a
  dendritic growth.
• Degradation by discharges is accelerated by the
  high stress at the end of a discharge channel.
• With tracking, it is the concentration of current
  at the end of the carbonized channel that causes
  drying of subsequent moisture films at this
  point, and further damage.

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• Erosion, the second form of damage, is not so
  disastrous as carbonization, since it does not
  immediately produce concentrations of leakage
  current.
• Erosion may lead to penetration of the insulation
  to electrode or may even cause mechanical
  failure.
• It causes a roughening of the surface, which aids
  contamination and way to carbonization.
• Perspex is an example of a material which will
  not carbonize although considerable erosion can
  occur.

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• Degradation may be accelerated by extraneous
  processes, such as physical weathering,
  ultra-violet radiation and chemical attack.
• Ozone and oxides of nitrogen generated by
  discharges may degrade the insulation and provide
  sources of contamination.

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? We know what tracking is two problems arise.

1. How do we stop tracking?
2. How do we assess the tracking liability of
   insulation?

Prevention of tracking
Design

• Help by limiting access of dirt
• Avoiding its accumulation areas between
  conductors.

clean
dry
Undamaged surfaces
Undamaged surfaces
Track resistant of the materials
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9.2 Low-voltage tests

• Some well-established insulating materials that
  are used in high-voltage engineering will track
  at 100V, so a low-voltage test method is needed.

I.E.C. test given in publication 112
Two chisel-edged electrodes, usually of brass,
are rested on the horizontal test piece 4mm apart
Drops of specified size of 0.1 ammonium chloride
solution fall between the electrodes at 30-s
intervals.
Voltage is applied to the electrodes
Each drop is boiled off by the current passing
through it.
Drying of each drop sparks appear on the surface,
may damage the insulation.
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• The number of drops required to cause failure is
  found for several voltages. ? curve of
  drops-to-failure against voltage constructed.
• As the voltage is decreased the number of drops
  increases and at a particular voltage the curve
  becomes asymptotic.
• For the majority of insulation the value of the
  voltage corresponding to 50 drops is a good
  approximation to the asymptote, and is taken as
  the C.T.I., but for some materials, tests must be
  made for 100-200 drops before an asymptote is
  apparent.
• C.T.I. (Comparative Tracking Index) used to
  measure the electrical breakdown (tracking)
  properties of an insulating material.

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• Kaufmann describes a modification in which the
  solution is sprayed onto a sloping test-piece so
  that unused liquid runs off taking with it
  electrode and insulation contamination.
• This method claims to be independent of the
  electrode material something that is certainly
  not true for the standard I.E.C. method.
• ? C.T.I. values on the I.E.C. or B.S test

Tracking index(V) materials
100-140 A phenolic bonded paper board
200-300 Mineral-filled alkyd resins
600 or more Perspex, silicone rubber, polythene
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9.3 Medium-voltage test

• Brief mention will be made to the dust-fog
  test(A.S.T.M.D.2132-62T)

A high-voltage electrode ½2 in is placed on the
insulation
- Test is started by cleaning a path around the
high-voltage electrode(applying 500-700V)
Two similar earthed electrodes are placed either
side of the high-voltage electrode and 1 inch away
- When sparking is established 1500V are applied
until failure occurs.
- Changes in conductivity of the fog sometimes
have to be made to maintain sparking.
A dust comprising 94 inorganic inert material,
3 sodium chloride, 3 cellulose, is put on the
insulation
- One test can take 100h or more.
Tap-water fog is created with a specified
deposition rate.
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9.4 High-voltage test

• The inclined-plane test is a similar test to the
  I.E.C. test, but can be used up to 6 or 7kV. It
  is being proposed as a standard test
  internationally.

Two stainless-steel electrodes are clamped to the
specimen, with 50mm spacing.
Specimen is set up with its longer axis at 45 to
the vertical
Electrodes attached to the underside.
A solution of 0.1 ammonium chloride with a
little wetting agent is fed into a filter-paper
pad clamped under the top electrode and flows
down the under-surface to the lower electrode.
In steps of 0.25kV, which will cause uniform
sparking over the wetted surface is applied.
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• At the end of each hour thereafter the voltage is
  increased 0.25kV until failure or flashover
  occurs.
• In this way materials are graded in terms of the
  voltage at which they fail.
• Two advantages of this test.
• A test is complete in 4 to 6h
• The tracking path is very similar to that
  obtained over several years of exposure to normal
  weather conditions.
• ? This method appears to be suitable for rapid
  evaluation of materials that may be track
  resistant under hazardous exposure conditions.

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9.5 Theory of tracking

• A theory of tracking in terms of chemical bond
  energies has been suggested. by Parr
  Scarisbrick
• The tendency to track, depends on the proportion
  of the bonds which produce free carbon on
  pyrolysis.

?Hc The energy of all the bonds which on
breaking produce free carbon
?HcPD The total bond energy of the molecule
v The lower the fraction ?Hc/ ?HcPD the less
likely is the material to track.
v ex) ??? ?? 0.4???? ??? ?????, 0.4?? 0.2??? ???
?? ????? ? ???? ??? ??? ?????? ????? ? ? ???.
? this theory is a very simplified one and cannot
take into account the effect, for example, of
fillers in resins, or of thermal conductivity.
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9.5 Theory of tracking(2)
Fraction plotted against time to failure for
several materials tested in the dust-fog test
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9.6 Breakdown by discharges in solids

• Discharges on the surface, or in cavities in
  insulation, will occur whenever the stress in the
  gas exceeds its breakdown value.
• (If the discharge can choose its own path, as is
  the case for most surface discharges, then the
  critical stress will correspond to the Paschen
  minimum for the gas, which for air at s.t.p. is
  about 380kV/cm.
• If the path is pre-determined, as when it is
  across a cavity or between layers of insulation,
  then the critical stress will depend on the
  dimension of the gap parallel to the filed.
• Although in this case the stress may be below
  380kV/cm, the voltage across the gap will exceed
  380V.

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Cause of surface discharges Cause of internal discharges
inadequate stress relief At high voltages where a limited life is acceptable Designed stress relief may be invalidated by leakage due to contamination. Cavities in solid insulation Poor design manufacture Eventual effect failure of the insulation.
Erosion Erosion
initially the damage caused by discharges is erosion over a comparatively large area. Roughens the surface Slowly penetrates the insulation Not the cause of rapid failure. Gives way to channel propagation and dendritic growth through the remaining insulation. By virtue of the high stresses created at the tips of the channels they propagate rapidly and are the cause of failure. The channels are conducting either because their walls are carbonized or because the gases therein are highly ionized. In the presence of moisture, discharges can be suppressed by deliquescent discharge products cavities can cease to discharge (remain dormant until these products have diffused into the body of the material.)
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• The evils of over-voltage testing will be
  discussed.

a.c. voltage test d.c. voltage test
Several times the working voltage Apply to an equipment for 1min or more to satisfy a specification. Dangerous procedure. Object to detect gross faults in the insulation Its result may well be to initiate discharge channels and lower the discharge-inception voltage of the system below the working voltage. In 1min at 50c/s there may be 6000 discharges more than may be caused by surges or over-voltages in the whole of its normal service. ? the test should be replaced by a d.c. over-voltage test. This will still find the gross fault Few discharges since the rate of discharge now depends on the R.C. of the circuit. The use of an instrument which will detect small discharges will show the incidence and in many cases the cause of discharges in the region of their inception voltage.
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• Evaluation of materials in terms of discharge
  resistance is best done with the rod-plane
  system.
• A 6mm stainless-steel rod is cut off square and
  placed on the insulation on a stainless-steel
  plate.
• The system is put in a ventilated dry-air
  enclosure and voltage applied between the
  electrodes. (the discharge inception voltage
  1.57 times)
• ? result..
• - failure is not caused by thermal instability.
• - the frequency of the test voltage can be
  increased to accelerate this test.
• - Life is proportional to the number of
  discharges.

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9.7 Discharge detection

• Basic circuit.
• the impedances of the detector and of the
  high-voltage supply are assumed to be infinite to
  the step wave produced by the discharge, and the
  internal impedance of the step-wave generator is
  taken to be negligible.

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• matching units are tuned to give a resonant
  circuit in the range 12 to 50 kc/s
• matching units permit measurements to be made
  with specimen capacitances of from 6pF250pF with
  sensitivities of 0.005 pC15pC.
• Calibration is by the injection of a known
  step-wave voltage into the system.
• this gives direct calibration of discharge
  amplitude
• takes into account the response of the amplifier.

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• Experience enables an operator to distinguish
  between several types of discharge from the
  nature of the output of the amplifier which is
  displayed on a C.R.O. having an elliptical time
  base. This time base is produced from a
  phase-shifting R.C. network.

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• ? Essential requirements for this type of
  measurement
• discharge-free voltage supplies, mains filters.
• At low energies of discharge, screened rooms are
  essential.
• Any transformer is discharge-free up to half its
  rated voltage.
• ? Failures by tracking and discharges are not an
  essential hazard in electrical life.
• Proper design, manufacture, correct choice of
  materials will prevent this.