7. Breakdown of commercial Liquid and Liquid-Silid Dielectrics

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

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.

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

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

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

7.1.1 Breakdown due to gaseous inclusions

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)

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

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

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)

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

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

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

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

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

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

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.

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.

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.