7. Breakdown of commercial Liquid and Liquid-Silid Dielectrics

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7. Breakdown of commercial Liquid and
Liquid-Silid Dielectrics
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7.1 Breakdown of commercial liquids
The passage of a spark through a liquid involves
i) the flow of a relatively large quantity of
electricity determined by the characteristics of
the circuit
ii) a bright luminous path from electrode to
electrode
iii) the evolution generally of bubbles of gas
and the formation of solid products of
decomposition (if the liquid is of the requisite
chemical nature)
iv) formation of small pits on the electrodes
v) an impulsive pressure through the liquid with
an accompanying explosive sound
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7.1 Breakdown of commercial liquids
Initial stages of breakdown
electrode surfaces free from oxide films
Pure insulating oil
highly degassed
carefully filtered
breakdown strength over 1MV/cm
Tests done on highly purified transformer oil

1. Breakdown strength has a small but definite
   dependence on electrode material
2. Breakdown strength decreases with increasing
   electrode spacing
3. Breakdown strength is independent of hydrostatic
   pressure(0-800 mm Hg) for degassed oil but
   increases with pressure if oil contains gases
   like N2 or O2 in solution.

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7.1 Breakdown of commercial liquids
transformer oil

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Benzin 400 435 455 450 - 490 -
Hexan 355 380 435 440 430 475 480
Xylol 430 465 470 480 485 515 535
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7.1 Breakdown of commercial liquids
Impurities which lead to breakdown of commercial
liquids
Impurities which have a breakdown strength lower
than that of the liquid itself
bubbles of gas
Impurities which are unstable in an electric
field
globules of water
Impurities which result in local enhancement of
electric field in liquid
conducting particles
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7.1.1 Breakdown due to gaseous inclusions
changes of temperature
insulating liquid impurity
Bubbles of gas
changes of pressure
agitation
Electric field
The electric field in a gas bubble which is
immersed in a liquid of permittivity e1
(1)
E0 the field in the liquid in the absence of
the bubble
when Eb the limiting field for gaseous
ionization
liquid
decomposition of oil molecules
electrode
electrode
gas formation
bubble
discharge
breakdown
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7.1.1 Breakdown due to gaseous inclusions
breakdown
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7.1.2 Breakdown due to liquid globules
Electric field
plane y 0 R1, R2 principal radii of curvature
at (z,x)
z
the total curvature at (z,x)
(z,x)
electrode
electrode
(2)
e2
e1
pressure due to surface tension(s) at (z,x)
(3)
E2 in the distorted bubble is uniform
pressure at any point x on the globule-medium
interface
(4)
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7.1.2 Breakdown due to liquid globules
For equilibrium at the point (z,x)
(5)
Pb pressure inside the globule P external
pressure.
At the equator of the globule, xb
(6)
Subtracting (6) from (5) and substituting form (4)
(7)
?12 gt 0 (except for the trivial case e1 e2),
Cx gt Cb (except at the equator)
globle elongates in the direction of the field,
whether e1gt or lt e2
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7.1.2 Breakdown due to liquid globules
For the solution of (7), using (2)
(7a)
The solution of (7a)
(8)
I0 and I1 modified Bessel functions of the
first kind
Electric field
Globule of bubble
Globule of bubble
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7.1.3 Breakdown due to solid particles
e1? e2 in an electric field E
fiber
(14)
e2
solid particle
r radius of the particle e2 permittivity of
the particle e1 permittivity of the liquid
e1
insulating liquid
e2?8
(15)
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7.2 breakdown of liquid-solid dielectrics
liquid-solid dielectric
Drawback
great affinity for water
the form of thin sheet inexpensiveness consistency
ease of manipulation
mineral oil or chlorinated diphenyl
Availability
Paper
breakdown
energy loss
on the type of stress applied (ac or dc)
liquid-solid dielectric
gradual deterioration
on operating temperature
on the homogeneity of the dielectric
Breakdown
Electric field
Type of breakdown of liquid-solid dielectrics
deterioration due to internal discharges
electrochemical deterioration
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7.2.1 Deterioration due to internal discharges
Different breakdown strength of cinstituent parts
of a composite dielectric
Increasing stress
Breakdown of the weaker dielectric
breakdown
Partial breakdown as a discharge
Breakdown of the gas phase in solid dielectrics
containing gas inclusions
discharge
discharge
discharge
uniform field
divergent field
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7.2.1 Deterioration due to internal discharges
Cause of discharges produced gradual
deterioration in organic liquid-solid dielectrics

1. disintegration of the solid dielectric under
   bombardment by electrons and ions generated by
   the discharges
2. chemical action on the dielectric of the products
   of ionization of the gas
3. high temperature in the region of discharges.

Breakdown through the cracks
Discharge in gas inclusion
Formation of crack
Local heating
mechanical stress
Inorganic dielectric
Voids can be removed by careful impregnation
Discharge of incompletely impregnated solid
dielectric
Rated stress
Increase in the discharge inception stress
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7.2.1 Deterioration due to internal discharges

• Ei depends on electrical processes which lead
  to gas formation and in oil impregnated paper
  these are
• decomposition of moisture present in paper
• local electrical breakdown of the oil

Gas from thoroughly dried paper
Gas from paper containing of moisture
100 V/um
10 V/um
discharge inception stress
discharge inception stress
The gas first formed arises from electrochemical
decomposition of water held in the paper.
Dielectric of high water absorption
Dielectric of low water absorption
Gas evolution
No gassing
stress
Breakdown stress
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7.2.1 Deterioration due to internal discharges
Measure of moisture content
Needle electrode
The gas bubbles moved rapidly away from the
needle and, in de-gassed oil, dissolved in a few
minutes.
Sheet cellophane immersed in mineral oil
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7.2.1 Deterioration due to internal discharges
ideal case of a completely dry paper
Gas formation
regions of highest stress
gas bubble formation
decompose the molecules of the oil
Oil paper dielectric
rapid growth of the bubble
Ei gt rated stress
Discharge inception stress
enough for the gas to dissolve in the oil
restore the initial high discharge inception
stress
permanent damage by the discharges
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7.2.1 Deterioration due to internal discharges
life test
expected breakdown time
oil-impregnated paper capacitor
operating voltage
On opening up a broken-down unit, widespread
carbonization of the paper is observed and, with
mineral oil-impregnated paper capacitors,
extensive fluorescence is seen under u.v. light.
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7.2.2 Electrochemicla deterioration

• Gartons theory give the following expression for
  the decrease of tand with stress
• where,
• The total loss angle of on film of the impregnant
  is given by
• where, tan d0 is the loss angle in the absence of
  ions.

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7.2.2 Electrochemical deterioration
Electrochemical deterioration
One of the main causes of failure of
liquid-impregnated paper dielectrics
Dependent on concentration of ions
Gartons theory
decrease of tand with stress
(16)
The total loss angle of on film of the impregnant

(17)
tan d0 the loss angle in the absence of ions.