on the damageto metallized film capacitors …

Electrocomponent Science and Technology1975, Vol. 2, pp. 201-208

(C) Gordon and Breach Science Publishers Ltd.Printed in Great Britain

ON THE DAMAGE TO METALLIZED FILM CAPACITORSCAUSED BY ELECTRICAL DISCHARGES

J. W. BURGESS, R. T. BILSON and N. F. JACKSONAllen Clark Research Centre, The Plessey Company Limited, Caswell, Towcester, Northants, U.K.

(Received April 23, 19 75; in final form June 20, 19 75)

High voltage operation of metallized film capacitors leads to electrical spark breakdown in pockets of gas trappedwithin the sealed capacitor unit. A simple picture of gas discharge processes in the capacitor is used to identify therole played by electric discharges in the deterioration and ultimate failure of capacitors under accelerated life test.

1 INTRODUCTION

The development of metallized solid polymer filmcapacitors for use at high voltage requires consider-able care with the processing technology. In theabsence of liquid impregnants, the application ofpeak voltages above 300 V across the capacitor canresult in electrical spark breakdown being initiated inpockets of air trapped within the sealed unit, causinglocalized damage to the polymer film and themetallization, and a considerable generation of heat.In the present paper, the occurrence of breakdownresulting from gas discharges in wound capacitor unitsof the extended foil construction is discussed. Thepolymer t’tim is metallized on one side and wound ina staggered configuration allowing the whole lengthof metallized electrode to be short circuited to an endconnection in the form of a fine spray (Figure 1).Using simple physical considerations, a picture of thedamage caused by electrical breakdown and thesubsequent deterioration in capacitor performancemay be developed.

2 INITIATION OF SPARK BREAKDOWN

When the electric stress through a gas is increased toexceed some critical value, a spark breakdown willinitiate a momentary discharge in the gas. The criticalstress is known as the Discharge Inception Stress (Ei)and is dependent on gas composition, temperature(T) and pressure (P). The voltage associated with pathof length d’ over which a uniform critical stress ismaintained, is the Discharge Inception Voltage (Vi).Over a wide range of conditions, the Discharge

201

Inception Voltage is found to be a function of theproduct of gap (d’) and pressure (P), the relationshipbeing known as Paschen’s Law,

V f(P d’) (1)

Wound unit

(b) Section through wound unit

"-----End spray connection

Metallisaticn

Polymer

FIGURE 1 Extended foil configuration: (a)Wound Unit,(b) Section through wound unit.

202 J.W. BURGESS, R. T. BILSON AND N. F. JACKSON

FIGURE 2 Discharge inception voltage for air, N and SF at 20C versus pressure and gap.

In Figure 2 Paschen plots for air, nitrogen andsulphur hexafluoride at 20C are shown2 The rise in

Vi for small pressure and gap is a consequence ofinsufficient molecules for the usual avalanche ofionization to be produced. This relationship results ina minimum value of Inception Voltage which, forpressures near atmospheric, corresponds to a break-down path length of 10 pan. The nature of the solidinterfaces from which eletric stress is applied has alimited effect on the inception voltage, but can alterthe discharge characteristics. For example, in the caseofdischarges across a gaseous cavity within a dielectricsolid, the discharge magnitude is limited by theimpedance of the dielectric and the repetition fre-quency is determined by the time taken for chargeleakage to restore the electric stress across a pre-viously discharged path (typically 102 sec. in plasticmaterial). Breakdown will occur along paths ofminimum inception voltage, of which there may beseveral associated with a single cavity. Repeateddischarging to a local site on the boundary of a cavityeventually causes considerable damage to the dielec-tric. Under a.c. applied stress the frequency ofrepetition for discharges is equal to twice the signalfrequency 3, and can be some orders of magnitudehigher than that for the d.c. case, resulting in anacceleration of damage caused to the dielectric.

Similar breakdown characteristics are observed fordischarges in a gas space between the surface ofa dielectric material and an external conductor. In thiscase the local discharges are still repeated every half-cycle of the a.c. signal, but the discharge magnitudeincreases rapidly as the voltage is increased above Vi.A significant volume of gas is contained in the

wound capacitor unit principally near the staggerednon-contact edge of the polymer film (Figure 1), butalso trapped within the windings. Some gas remainsafter the conventional heat treatment and further gaspockets are contained within the polymer film,usually associated with inclusions formed during thecasting of the film, and known as voids (Figure 3).When the capacitor is operated at high voltage,trapped gas pockets are susceptible to breakdownwith consequent damage to the polymer film andaluminium metallization. The initiation of dischargesin the capacitor is thought to be governed by thegeneral theory of gas breakdown, the discharge gapsbeing typically <10/am. Some deviations from Pas-chen’s Law have been observed with small gaps(<5/am), the inception voltage always being lowerthan the Paschen value (4), but not below thecharacteristic Paschen minimum. Typical values ofuniform electric stress developed in working capaci-tors do not exceed thresholds for field emission of

METALLIZED FILM CAPACITORS 203

electrons from the metallization, and consequentlysevere departures from Paschen’s Law should notoccur frequently. The a.c. operation serves to maskpolarity effects, which are not discussed.

(a) Cavity Model

..:,..... ’.-.:

::.’....i..

(b) Cavity Discharge Inception Voltage Versus Gapfor Various Pressures

FIGURE 3 Void in the film as viewed with reflected light.Typical void densities are 25 to 100 cm -2.1 Cavity Discharge

In Figure 4(a), two types of gaseous cavity whichmight lie within the wound unit are shown in section.The sketch illustrates voids within the polymer filmand localized gas pockets trapped within the wind-ings. The standard winding gap is assumed to be<0.5/am, with breakdown being localized at trappedgas pockets. In each case, the cavity is roughlydisc-shaped and minimally disturbs the uniform elec-tric field pattern between the aluminium electodes.The concentration of electric stress at regions oflower dielectric constant results in the DischargeInception Voltage for cavity breakdown (3) being

where Vg is the gas breakdown voltage, d is thedistance between adjacent electrodes, d’ is the dis-charge gap (Figure4(a)), and e is the dielectricconstant of the polymer. Values ofV as a function ofgas path length are shown in Figure 4(b) for variouspressures within the cavity for 8/am thick polymerfilm in air at 20C. The gas breakdown voltage Vg istaken as the Paschen value appropriate to d’ and thepolymer dielectric constant is taken as 2.3.

5 Atmos.I.

O. Atmos.

oAtmos.

0.4O 4 b 8 IO 12

Discharge Gap (lm)

FIGURE 4 Cavity discharges: (a) Cavity model, (b) Cavitydischarge inception voltage versus gap for various pressures.

2.2 ConductorEdge Discharge

The extended foil configuration utilizes staggeredalternate polymer films, for which the non-contactaluminium electrode is recessed. The enhancement ofelectric field near the edge of the aluminium elec.trode causes this region to be highly vulnerable todischarges. Assuming a uniform field across the gaspath d’ shown in Figure 5(a), the Discharge InceptionVoltage Vi for conductor edge breakdown (3) may bedetermined as

v,:v 1where d is the thickness of the polymer film. InFigure 5(b), values of Vi are shown as a function ofgas path length d’ for various pressures, again wth8/am thick polymer film in air at 20C.

In a typical capacitor unit, the value of Vi for theconductor edge only loosely corresponds to thatcalculated from Eq. 3, because of the strong non-


(a) Conductor Edge Model It is seen from Figure 6 that E > Eo for distances/m from the conductor edge. Corona streamersare readily initiated within the high field region andwill propagate into regions of low stress, some ofthem reaching the polymer surface and causingcomplete breakdown of the gas. The propagation ofstreamers once formed has been observed to continuein regions of quite low field strength6. Overalldischarge characteristics are quite complex, beingcritically dependent on the polarity of the conductoredge, the composition of gas and ambient conditions.

3 DETERIORATION OF CAPACITORPERFORMANCE

Conductor Edge Discharge Inception Voltage VersusGap for Various Pressures

1"2

0.6-

0"4-

O 4 b 8 IO 12

Discharge Gap (lm)

P 5 Atmos.

Atmos.

P= Atmos.

a Atmos.

FIGURE 5 Conductor edge discharges: (a) Conductor edgemodel, (b) Conductor edge discharge inception voltage versusgap for various pressures.

uniformity of electric field in the gas. This electricfield patern dose to the electrode edge has beencalculated for the geometry shown in Figure 6 bystandard mapping techniques The average windinggap is taken to be 0.5/,trn (this value is not critical tothe electric field behaviour) and the electrode thick-ness to be 300 A. A contour representation of theelectric field is shown, where the field magnitude isnormalized to the uniform field Eo across the polymer(e 2.3, d 8/an)

VEo - (4)

The simple models developed here are consideredrepresentative of the types of gas breakdownoccurring in a metallized film capacitor. A similarrepresentative approach can be used to assess the im-pact of gas discharges on the performance of a typicalcapacitor. In the cases shown in Figures 4(a) and 5(a)surface damage arising from localized discharges maybe expected on both internal and external surfaces ofthe polymer and on the aluminium electrodes. Forhigh applied voltage (>500 V) the discharge magni-tude and consequent damage increases more for thealuminiumpolymer discharges, than for thoseoccurring between two polymer surfaces7

/

/

!

Thickness(lm)

-3

-2

POLYMER

/2 Eo x Eo

\.Eo 12Eo 4E "l-"""..lOEo ,4E AIR

0

Width (pm)

FIGURE 6 Electric field distribution near conductor edgerelative to uniform field, E0.


The damage to polymer and electrode surfacesmay be chemical, via active gases formed within adischarge, or physical due to bombardment by chargecarders. A discharge may be summarily described interms of electrons accelerated under the applied field,colliding with and ionizing neutral gas moleculeswhen sufficient energy has been gained. A typicalenergy for charge carders arriving at a solid surface isthen ~8 eV (half of the ionization potential of thegas). With polymeric bond energies being "-’4eV,considerable chemical modifications in the polymerwill occur near a discharge site. Observed dischargemagnitudes are "1 to 1000 pC, and the associatedenergy can produce very high local surface temper-atures. A single discharge of 10 pC produces 10 -11

calories at the electrode surface (assuming 8 eVenergy on arrival) which is sufficient to sublime0.25/am square area of the aluminium electrode. Theerosion of internal polymer surfaces by discharges hasbeen observed7 and amounts of displaced polymerare similar to those estimated for the aluminiumelectrode.

The effects of repeated discharging thus causessome chemical modification of the polymer surface,erosion of the polymer and eventual formation ofbreakdown channels by treeing8, and also erosion ofthe aluminium electrode. Evidence for chemicaldamage in the present work was obtained by infra-redspectroscopy which indicated the presence ofcarbonyl groups. Local damage to the polymer filmhas little effect on capacitor performance until acomplete breakdown channel through the f’flm isdeveloped. Complete sublimation of the aluminium atdischarge sites leads to a loss of electrode area andproportional loss of capacitance. The removal ofaluminium, together with modifications to capacitorgeometry caused by heat generated in the dischargesare major contributors to the loss of capacitance andfinal catastrophic breakdown of the devices.

From the representative models considered, twocontributions to the capacitance loss under life testmay be identified.

1) Repeated discharges across gas pockets trappedwithin the windings lead directly to removal of thealuminium electrode. Once a small area of electrodehas been removed, further discharges will occur fromeach point on the periphery of the surroundingaluminium electrode via the conductor edge mechan-ism, leading to the growth of large circular clearanceareas in the alummium.

2) At the staggered non-contact edge of thealuminium electrode, discharges occurring via the

FIGURE 7 (a) Erosion of metallisation, illustrating overlapof edge erosion and circular clearance, (b) Erosion of polymerfilm.

conductor edge mechanism will cause the electrode torecede from its initial position, at a rate uniformthroughout the length of the film.

Both of these characteristic modifications areobserved in practical life tested capacitors (Figure 7).The circular clearance areas are found to lie atrandom positions, while the erosion of the non-contact electrode edge is uniform along the length offilm. Only an occasional correlation is found betweenthe electrode clearance areas and the sites of voidswithin the polymer itself. A study of life-testedcapacitors has shown that small areas of the alu-minium electrode can be sublimed from the polymerfilm, especially in the initial stages of life testing. Asthe life test proceeds, further electrode removal iseffected by the oxidation of aluminium. The rate ofelectrode removal is not constant with time, owing tochanges in the ambient conditions for discharging.Heat dissipation within the unit is fairly efficient, buta general temperature rise in the body of thecapacitor is observed, and this will lead to a reductionin the Paschen voltage of the gas. The gas pressureand composition are also continually changing, asealed air-filled capacitor often showing firstly a slightdrop in pressure associated with the using up ofoxygen, and later a sharp increase in pressure causedby copious generation of hydrogen gas.

Quantitative predictions of deterioration in a thinfilm capacitor unit are difficult, owing to uncertain-ties about the ambient conditions at a local dischargesite and the changes which occur after discharges havecommenced. However, the simple models used hereidentify the basic processes involved in capacitordeterioration, and may be used to provide a semi-quantitative assessment of the performance to beanticipated from particular devices. Within thecapacitor unit are many possible discharge paths,


0.8

0.7

-5

0.4

0"

0.:20 2 3 4 .5 6

Pressure Atmospheres

FIGURE 8pressure.

12.5 pro. Film

8pro. Film

Minimum discharge inception voltage versus

varying from 0.1/am to --5/am in length, andextending considerably further when the conductoredge mechanism (Figure 5(a)) is invoked. If a con-tinuous distribution of path lengths is available, thenthe characteristic Discharge Inception Voltage for thetwo representative models will be given simply by theminima of the curves plotted in Figures4(b) and5(b). The resulting variation of Discharge InceptionVoltage with pressure for 8/am and 12/am film isshown in Figure 8, taking the conductor edge modelas an example. Thus an increase in pressure will,under the present assumptions, raise the DischargeInception Voltage and, for a given test voltage, reducethe number of discharges. Reductions in the numbersof discharges may also be obtained by varying the gascomposition within the capacitor, in particular usingelectro-negative gases such as SF6. Figure 2 shows themeasure of increase in Paschen voltage (and thereforeof Discharge Inception Voltage) obtained with theuse of pure SF6. A nitrogen atmosphere is sometimespreferred in capacitor design, since although there islittle increase in Paschen voltage over that of air, theoxygen required for oxidation of the aluminium islimited to that available in residual moisture, and therate of capacitance loss can be slowed down.

To illustrate the use of discharge properties in thedevelopment of capacitor design, a set of life tests onseveral different types of capacitor has been analysed.The capacitance of the units was recorded at various

times within the duration of the tests, and theanalysis was made using a procedure analogous tothat employed for the assessment of bulk insulatingmaterials9 The minimum discharge inception voltageis calculated from a knowledge of capacitor con-struction and the ambient conditions, and the ratio oftest voltage to inception voltage (V/VI)is plottedagainst the number of continuous a.c. cyclesproducing a capacitance drop of 3% (Figure 9).Having taken the varying ambient conditions intoaccount in the calculation of inception voltage, theresults fall into two groups, according to the types ofencapsulation used. In this way, the techniquesdeveloped here can be used to identify and isolate theeffects of particular design variations on capacitorperformance.

The results for each group of capacitors follow thecharacteristic form for lifetime of plastic materials,

N=K(Vi)n

V (s)

where N is the number of cycles, K is a constant andthe value of the index n is found from Figure 9 to ben 8. This value agrees with results previouslyobtained for a number of plastic bulk materials9

subjected to internal gas discharges.

4 CAPACITOR BREAKDOWN

As the life test proceeds, the deterioration of thedevice continues with further electrode removal andgeneration of heat until a complete failure eventuallyoccurs. Three possible failure mechanisms resultingfrom the occurrence of gas discharges are:

1) Breakdown of the polymer film by thermalrunaway or electromechanical effects.

2) The completion of a breakdown channelthrough the polymer film.

3) Damage to the sprayed end-connections leadingto an open circuited device.

It is thought that for typical polymer films,electromechanical breakdown will occur before athermal runaway condition develops3. The strongelectric field applied across the polymer is accom-panied by a compressive stress, causing a distortion ofthe film. In regions close to local discharge sites, thetemperature of the polymer is considerably increased,and the accompanying decrease of elastic stiffnessleads to a mechanical collapse of the film. Theerosion of polymer caused by repeated discharges will


0"

0"8

07.

xx

81m. Film

A Encopsulant A

Encopsulant B

1_

Number or" Ccles to cjive ’/’,FIGURE 9 Ratio of test voltage to discharge inception voltage versus number of a.c. test cycles to give a 3%capacitance loss.

form discharge channels which eventually propagatethrough the film, precipitating failure by mechanism(2). The distortion of the film during life test inducesstress at the sprayed end-connection, and damage tothe contact with the aluminium electrode is intensifiedwith the disruption of the dissipative flow of heat outof the capacitor. Failure by mechanism (3) can be acommon problem with the extended foil con-figuration.

aging of bulk plastic materials by discharges in gasfilled cavities within the plastic. Observed rates ofdeterioration for life-tested capacitors under variousambient conditions conform with predicted rates, soestablishing structural damage from discharges as aprimary contributor to capacitor deterioration andfailure.

ACKNOWLEDGEMENTS

5 CONCLUSIONS

A quantitative picture has been given of someprocesses of deterioration in metallised film capac-itors under life test at high voltage. Much of thestructural damage to the polymer and electrodes canbe associated with gas discharges within the woundunit. The deterioration in capacitor performance hasbeen analysed in terms of discharge characteristics fortwo types of gaseous pockets occurring in theextended foil configuration for a capacitor. Thepredicted behaviour has similar characteristics to the

The authors are grateful to Drs. J. C. Bass, P. J. Harrop andA. R. Morley for helpful suggestions and to the Directors ofthe Plessey Company for permission to publish this work.

REFERENCES

1. T. Robertson, "The properties of polypropylene and theirinfluence on the behaviour of polypropylene capacitors",Elect. Comp., October 1972, p. 948.

2. M.J. Schbnhuber, "Verlauf der zindspannung von edel-und molekilgasen in iner ebenen elektrodenanordnungim grob-, fein- und hochvakuumbersich", Arch. Electro-tech., 52, 28 (1968).

3. J.H. Mason, "Dielectric breakdown in solid insulation",Prog. in Dielect., 1, (1959).


4. T.W. Dakin and D. Berg, "Theory of gas breakdown",Prog. in Dielect., 4, 151 (1962).

5. P.M. Morse and H. Feshbach, "Methods of TheoreticalPhysics", (McGraw-Hill 1953).

6. E. Nasser, H. Heiszler and M. Abou-Seada, "Field criterionfor sustained streamer propagation". Z Appl. Phys., 39,3707 (1968).

7. J.H. Mason, "Breakdown of insulation by discharges",Proc. LE.E., 100, 149 (1953).

8. B. Bolton, R. Cooper and K. G. Gupta, "Impulse break-down of perspex by treeing", Proc. LE.E., 112, 1215(1965).

9. M.E. Baird, Electcal Properties ofPolymeric Materials,Plastics Institute (1973).

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