1. 7. Breakdown of commercial Liquid and Liquid-Silid Dielectrics

2. 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

3. 7.1 Breakdown of commercial liquids
Initial stages of breakdown
electrode surfaces free from oxide films
insulating liquid
= homogeneous dielectric
highly degassed
Pure insulating oil
impurity
carefully filtered
breakdown strength : over 1MV/cm
Tests done on highly purified transformer oil
Breakdown strength has a small but definite dependence on electrode material
Breakdown strength decreases with increasing electrode spacing
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.

4. 7.1 Breakdown of commercial liquids
transformer oil
절연파괴전압 및 기타의 전기적 특성이 양호할 것
냉각성능이 양호할 것
물리적 또는 화학적으로 안정할 것
절연물
절연내력 철 납 구리
알루미늄 금 아연 은
Benzin
400
435
455
450 -
490
Hexan
355
380
440
430
475
480
Xylol
465
470
485
515
535

5. 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

6. 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 ε1
(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
bubble
gas formation
discharge
breakdown

7. 7.1.1 Breakdown due to gaseous inclusions

8. 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) ε2 ε1
pressure due to surface tension(σ) at (z,x) :
(3)
E2 : in the distorted bubble is uniform
pressure at any point x on the globule-medium interface :
(4)

9. 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, x=b
(6)
Subtracting (6) from (5) and substituting form (4)
(7)
κ12 > 0 (except for the trivial case ε1= ε2),
Cx > Cb (except at the equator)
globle elongates in the direction of the field, whether ε1> or < ε2

10. 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

11. 7.1.3 Breakdown due to solid particles
ε1≠ ε2 in an electric field E
fiber
(14)
solid particle ε2
r = radius of the particle
ε2 = permittivity of the particle
ε1 = permittivity of the liquid ε1
insulating liquid
ε2→∞
(15)

12. 7.2 breakdown of liquid-solid dielectrics
Drawback
great affinity for water
mineral oil or
chlorinated diphenyl
the form of thin sheet
inexpensiveness
consistency
ease of manipulation
Paper
Availability
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

13. 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

14. 7.2.1 Deterioration due to internal discharges
Cause of discharges produced gradual deterioration in organic liquid-solid dielectrics
disintegration of the solid dielectric under bombardment by electrons and ions generated by the discharges;
chemical action on the dielectric of the products of ionization of the gas;
high temperature in the region of discharges.
Breakdown
through
the cracks
Discharge in gas inclusion
Local
heating
mechanical
stress
Formation
of crack
Inorganic dielectric
Discharge of incompletely impregnated solid dielectric
Voids can be removed by careful impregnation
Rated
stress
Increase in the discharge inception stress

15. 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 paper containing of moisture
Gas from thoroughly dried paper
10 V/um
100 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

16. Sheet cellophane immersed in mineral oil
7.2.1 Deterioration due to internal discharges
Needle electrode
Measure of moisture content
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

17. 7.2.1 Deterioration due to internal discharges
ideal case of a completely dry paper
Gas formation
regions of highest stress
decompose the molecules of the oil
gas bubble formation
Oil paper dielectric
Ei > rated stress
rapid growth of the bubble
Discharge inception stress
enough for the gas to dissolve in the oil
restore the initial high discharge inception stress
permanent damage by the discharges

18. expected breakdown time
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.

19. 7.2.2 Electrochemicla deterioration
Garton`s theory give the following expression for the decrease of tanδ with stress: where, The total loss angle of on film of the impregnant is given by where, tan δ0 is the loss angle in the absence of ions.

20. 7.2.2 Electrochemical deterioration
One of the main causes of failure of liquid-impregnated paper dielectrics
Dependent on concentration of ions -
Garton`s theory
decrease of tanδ with stress:
(16)
The total loss angle of on film of the impregnant :
(17)
tan δ0 : the loss angle in the absence of ions.