Download 9. Tracking and Breakdown of Solids due to Discharges

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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
 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.
 Three essentials of the tracking phenomenon
(1)The presence of a conducting film across the
surface of the insulation
(2)A mechanism whereby the leakage current
through the conducting film is interrupted with
the production of sparks
(3)Degradation of the insulation must be caused by
the sparks
♦ the process of tracking
Surface pollution
Leakage current
Evaporation by joule heat
Formation of dry band
Generation of scintillation
Surface erosion
Generation of conductive track
Progressing of track
Breakdown by tracking
- 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. ☞
- 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.
- 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.
- 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.
- 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.
☺ We know what tracking is two problems arise.
(1)
(2)
How do we stop tracking?
How do we assess the tracking liability of insulation?
Prevention of
tracking
clean
Undamage
d
Undamage
dsurfaces
surfaces
Design
dry
Track
resistant of
the
materials
- Help by limiting access of
dirt
- Avoiding its accumulation
areas between conductors.
 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.
- The number of drops required to cause failure is
found for several voltages. ☞ curve of drops-tofailure 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.
- 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
Mineral-filled alkyd
resins
200-300
600 or more
Perspex, silicone
rubber, polythene
 Brief mention will be made to the dust-fog
test(A.S.T.M.D.2132-62T)
A high-voltage electrode
½*2 in is placed on the
insulation
Two similar earthed
electrodes are placed
either side of the highvoltage electrode and 1
inch away
A dust comprising 94%
inorganic inert material,
3% sodium chloride, 3%
cellulose, is put on the
insulation
Tap-water fog is created
with a specified
deposition rate.
- Test is started by cleaning a path
around the high-voltage
electrode(applying 500-700V)
- When sparking is established 1500V
are applied until failure occurs.
- Changes in conductivity of the fog
sometimes have to be made to
maintain sparking.
- One test can take 100h or more.
 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 undersurface to the lower electrode.
In steps of 0.25kV, which will cause
uniform sparking over the wetted surface
is applied.
- 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.
1. A test is complete in 4 to 6h
2. 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.
-
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
√ The lower the fraction ∆Hc/ ∆HcPD the less likely is
the material to track.
√ ex) 분수의 값이 0.4이상이면 트랙킹 파괴형이고,
0.4이하 0.2까지는 방전에 의한 침식형이고 이 침식형의 고분자 재료
는 내트래킹성이 양호하다고 말 할 수있다.
☞ this theory is a very simplified one and cannot take into account
the effect, for example, of fillers in resins, or of thermal conductivity.
Fraction plotted against time to failure for several
materials tested in the dust-fog test
- 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.
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
 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.)
 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.
- 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 1.5~7 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.
- 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.
- 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 6pF~250pF with sensitivities of 0.005 pC~15pC.
- 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.

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.
♦ 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.