A publcaton of CHMICAL NGINRING TRANSACTIONS VOL. 33, 213 Guest dtors: nrco Zo, Pero Barald Copyrght 213, AIDIC Servz S.r.l., ISBN 978-88-9568-24-2; ISSN 1974-9791 The Italan Assocaton of Chemcal ngneerng Onlne at: www.adc.t/cet DOI: 1.333/CT1333185 Health Montorng of DC lnk Capactors Maawad Makdess*, Al Sar, Pascal Venet Laboratore Ampère UMR 55 CNRS, Unversté Claude Bernard - Lyon 1, 69622 Vlleurbanne Cedex, France maawad.makdess@unv-lyon1.fr lectrolytc and metallzed polypropylene flm (MPPF) capactors are among the most popular capactors used n electronc equpments. The choce of capactors s of major mportance because one of the most frequent causes of the equpment breakdowns results from the falure of capactors, and wll therefore determne the overall lfetme of the system. lectrolytc capactors present hgher capacty per volume unt and lower cost than MPPF capactors; however, they appear as the most lfe-lmtng components exhbtng a hgh falure rate. MPPF are a good alternatve to electrolytc capactors thanks to ther hgh delectrc breakdown strengths, low dsspaton factors and good stablty over a wde range of frequences and temperatures. ven though MPPF capactors are very relable components due to ther unque selfhealng capablty, they are not free of falures and they release noxous gases n ths case. Therefore, sutable dagnostc technques are needed to prevent catastrophc falures. The typcal falure mode of electrolytc and polypropylene flm capactors lays on the ncrease of the capactor quvalent Seres Resstance (SR) and the decrease of ts capactance. Ths paper presents an approach for the montorng of the capactor s electrcal parameters n functon of the ageng tme when used as DC-lnk capactors. A comparson between the flm and electrolytc technologes wll be also made n ths paper. 1. Introducton Capactors are one of the key components n power electroncs equpments; ther use extends over wde domans of applcatons and reaches the most crtcal operatng systems. lectrolytc capactors (C) assume a specal poston among the varous types of capactors snce ther prncple of operaton reles, n part, on electrochemcal processes. Ths structure, despte beng senstve to frequency and temperature varatons, provdes electrolytc capactors hgher capactance values, hgher volumetrc effcency and an excellent prce over performance rato (Joubert et al., 27). Therefore, C becomes the usual and the best choce for low-frequency applcatons such as n power supply flters. Metallzed polypropylene flms (MPPF) capactors on the other hand, n vew of ther good delectrc breakdown strengths, low dsspaton factors and good stablty over a wde range of frequences and temperatures, becomes the preferred polymer materals for capactve energy storage-devces. They proved to be more relable components than C thanks to ther unque self-healng propretes and are favored for hgh voltage applcatons and applcatons where hgh capactances are not requred such as n hgh frequency flterng, snubbers and resonant crcuts. However, capactors, whether belongng to electrolytc or metallzed flms technologes, reman one of the most unrelable components snce they are responsble of more than 3 % of the equpments breakdowns (Venet et al., 1999). Health montorng of these passve components becomes of paramount mportance; a good acquantance of ther deteroraton over tme would enable us to perform a predctve mantenance on the component and thus to mprove the avalablty of the whole system. The object of the present paper s to focus on the montorng of both electrolytc and MPPF capactors when used as DC-lnk capactors. Ths operaton, through adequate measurements and technques must ensure analyss of the components degradaton, an essental step to predct the capactor performance and ts remanng lfetme; the fnal nterest would be to provde warnngs n advance of catastrophc falures. Please cte ths artcle as: Makdess M., Sar A., Venet P., 213, Health montorng of dc lnk capactors, Chemcal ngneerng Transactons, 33, 115-111 DOI: 1.333/CT1333185 115
2. Prncples and Technologes 2.1 Alumnum electrolytc capactors Alumnum electrolytc capactors are composed of two alumnum fols, an electrolyte support (also called separator) and an alumnum oxde (AL 2 O 3 ) layer consttutng the delectrc (Martnez-Vega, 21). The composton of such a capactor s represented n Fgure 1 (a). Real electrolyte cathode Anode Alumnum fol Apparent cathode Alumnum fol Metallzaton + - Delectrc flms Alumnum oxde delectrc (Al 2 O 3 ) lectrolyte support: Paper mpregnated wth electrolyte Al 2 O 3 Mandrel Schoopage (a) (b) Fgure 1: Composton of alumnum electrolytc capactors (a), and MPPF capactors (b) As mentoned prevously, due to the thn oxde layer and the engravng structure of the alumnum fol consttutng the anode, capactance values for ths type of capactors can be very hgh. The alumnum oxde, consttutng the delectrc, s formed by electrolyss on the anode durng manufacturng processes. The electrolyte mbbng the paper has two man functons, t wll, n the frst hand, ensure electrc conducton between the alumnum fol (brought to the negatve potental) and the alumnum oxde, and on the other hand, t wll regenerate the ntegrty of alumnum oxde n case of default. 2.2 Metallzed polypropylene flms capactors Capactors usng a plastc flm as delectrc are wdespread and ther characterstcs depend on the materal that has been used. Plastc flms are themselves coated wth znc or alumnum of a few nanometers thckness to consttute the electrode (see Fgure 1 (b)). Metallzed flms are then wrapped together on a cylndrcal nsulated base called mandrel. To assure connectons wth an external crcut, a sprayed metal technque known as Schoopage s used on both sdes of the wndng. Metallzed flms capactors have been used snce the 195 s (Torta et al., 24) and were coveted for ther abltes of self-healng; defects such as pnholes, embedded foregn partcles or even mcro-flaws n the delectrc materal can lead to a localzed breakdown of the flm. Such a breakdown event results from a sudden and localzed dscharge of a porton of the stored charge under the nfluence of temperature and pressure. Durng ths ntense dscharge, a puncture s developed n the delectrc materal and the thn metallzaton layer near the defected ste wll be rapdly vaporzed and blown away and the ste becomes electrcally solated. Thus, metallzed flm capactors can undergo a large number of breakdowns wth as only vsble mpact a slght drft of ts electrcal parameters. Unlke electrolytc capactors, plastc flms capactors do not requre a gven polarzaton and behave well under hgh current and voltage condtons (l-hussen et al., 23). Metallzed flms capactors are a very nterestng alternatve to electrolytc capactors (Buatt et al., 21) and present hgher relablty, hgher RMS currents and small capactance change regardless of the appled voltage. 2.3 quvalent electrcal crcut All real components ncludng capactors have parastc parameters not taken nto account n deal models. These factors can have a major mpact on electrcal behavour wthn a crcut. A model whch s smple and takes nto account parastc parameters nvolved s showed n Fgure 2. 116
SR SL C Fgure 2: Smplfed electrcal model of C and MPPF capactors SR s the quvalent Seral Resstance takng nto account all losses n the component; SL represents the equvalent seral nductance due to the capactors wndngs and electrodes, whle C s the nomnal capactance of the capactor. 3. Mode and falure rates of capactors 3.1 Falure mode Falures are classfed nto two man parts, parametrc and catastrophc modes: - Parametrc falures, e.g. devces degradaton are defned as the change of the component characterstcs whch drfts out of ts specfc tolerances. The most current degradatons of capactors are the ncrease of the equvalent seres resstance SR or of the loss factor tanδ and the decrease n the capactance C. - Catalectc or catastrophc falures represent sudden falures correspondng to the dsappearance of the component functon. They are most often charactersed by a short or open crcut of the capactor. 3.2 Falure rate The falure rate λ(t) s the condtonal falure probablty of a component by unt of tme; t gves an ndcaton on the rsks for a devce to breakdown durng a tme nterval ]t, t+δt] when Δt tends to zero and by consderng that ths devce lasted untl tme t. The falure rate λ(t) s expressed n 1-6 /hour or 1-9 /hour (mpled: falure/1 6 h or falure/1 9 h) or n FIT (Falure In Tme wth 1 FIT = 1 falure/1 9 h). The falure rate λ(t) follows for many components the bathtub curve represented n Fgure 3. λ(t) t a t b t Fgure 2: Falure rate λ(t) as a functon of tme As t can be seen, the curve can be dvded nto three parts: The early falure perod (t t a ), whch s due to the youth defects of capactors, durng ths phase the falure rate λ(t) decreases; The ntermedary perod (t ]t a, t b [), where the falure rate s approxmately constant, t corresponds to the normal lfetme perod; The wear out perod (t t b ) where the falure rate λ(t) ncreases drastcally. In ths perod, the falures take a systematc character. Generally, they correspond to the falures by degradaton. 4. Accelerated Ageng tests Accelerated ageng tests methods are often used as a way to assess the effects of the degradaton process through tme. Montorng and analyzng the degradaton of these test components provdes nformaton on ther behavour under specfc stresses and allows the dentfcaton of dfferent falure mechansms. These nformaton can also be used as a way to retroact on the concepton of the component and thus to mprove ts relablty (l-hussen et al., 23). Very often, for energy storage devces ncludng capactors, t s well recognzed that falure process s equvalent to a chemcal reacton. Its rate constant, accordng to Arrhenus law, s functon of the absolute temperature T (Lahyan et al., 1998). The typcal falure mode of capactors lays on the gradual ncrease of the capactor quvalent Seres Resstance (SR) and a decrease of ts man capactance C; a capactor s consdered at ts end of lfe when SR 117
becomes at least two tmes hgher than ts ntal value and/or when ts capactance C decreases of 2 % from ts orgnal value. Montorng the evoluton of these parameters through tme s of a major mportance and wll serve us as database to track the capactors ageng. 4.1 Ageng of alumnum electrolytc capactors As mentoned prevously, accelerated ageng tests allow studyng C and SR evoluton as functon of tme. For a 1 μf, 4 V alumnum electrolytc capactor, floatng ageng test at 85 C and 4 V was realzed. It conssts on applyng constant voltages and temperatures across the component termnals. Fgure 4 represents the nterpolated and extrapolated data wth the use of expermental measurements for SR and C parameters (Abdennadher et al., 21). xpermental measurements Interpolaton and extrapolaton xpermental measurements Interpolaton and extrapolaton SR ( ) C (Farad) Tme (hours) Tme (hours) Fgure 3: voluton of SR and C as functon of ageng tme for an electrolytc capactor (1μF / 4 V) These evolutons are manly due to the ageng of the electrolyte. In fact, lke all components based on lqud electrolyte, alumnum electrolytc capactor present a wear out perod, when t t b (see Fgure 3), falure occurs snce t becomes nevtable. As t can be seen from Fgure 4, the equvalent seres resstance SR s the parameter whch s the most affected by the component ageng. It manly depends on the resstance of the electrolyte mbbng the papers; when the electrolyte evaporates, the equvalent surface of ths latter decreases, leadng to an ncrease of SR and a decrease of C. It s of nterest to note that SR and C varaton as a functon of tme depends on the electrolyte type, on the component package and more partcularly on ts watertghtness. Several models descrbe SR and C evolutons as a functon of ageng tme. For example, for C, a lnear model can descrbe ts decrease versus ageng tme. Whle for SR more precse models descrbe these varatons accordng to exponental laws (Persse, 23). 4.2 Ageng of metallzed polypropylene flms capactors Snce most of the research done n the feld of ageng laws of capactors deals wth the electrolytc type, we wll focus n ths secton on the dentfcaton of MPPF capactors ageng law. It s based on the analyse of the degradaton of the capactors parameters over the ageng tme; wth respect to Fgure 3, we wll be n ths case n the thrd zone of the bathtub. Although Arrhenus model gves a good approxmaton of the operatng lfe of a component, t s lmted to a temperature effect. In fact, durng ther operaton, capactors are not only subjected to temperature stresses but also to crtcal operatng condtons such as voltages and currents that may affect ther lfetme. In order to mprove the knowledge of the ageng law of MPPF capactors, a more sophstcated one must be establshed. yrng theory generalzes Arrhenus law to many factors besdes the temperature (ndcott et al., 1965). quaton (1) descrbes yrng law when two supplementary stresses are consdered: α C a τ ( T,S,S ) = A T.exp( + ( B + ).S + ( D + ).S ) (1) f _ yrng 1 2 1 2 kt T T Where, τ f_yrng s the useful lfe of the component (s), T the temperature stress n Kelvn (K), a the actvaton energy (ev), k Boltzmann constant (8.617.1-5 ev.k -1 ), S 1 and S 2 are the supplementary consdered stresses, A, B, C, D, and are dfferent constants. By consderng the DC voltage as the only supplementary stress for our tests, q (1) can be smplfed as shown n equaton (2): 118
τ yrng ( θ,u θ U ) = τ.exp( θ U ) (2) Where, τ yrng represent the useful lfe of a component at a gven temperature (ºC) and a gven voltage U ; and U are respectvely the acceleratng coeffcents of the temperature and voltage, whle τ s the theoretcal useful lfe of the component at ºC and V. In order to determne τ, and U, three floatng ageng tests, have been acheved respectvely on a set of ten metallzed polypropylene flms capactors (1μF-63V). The realzed accelerated tests are lsted below: - Floatng ageng 1: at 85 ºC 692 V - Floatng ageng 2: at 1 ºC 692 V - Floatng ageng 3: at 85 ºC 82 V Ths method s used to quantfy both temperature and voltage effects on the ageng of capactors. From the floatng ageng tests 1 and 3 we would be able to determne U, whereas from the floatng ageng tests 1 and 2, can be dentfed. Fgure 5 represent the evolutons of SR and C of MPPF capactors as functon of ageng tme when subjected to dfferent stresses. ΔSR/SR(%) 45 4 35 3 25 2 15 1 5 Floatng ageng 85ºC 693V Floatng ageng 1ºC 693V Floatng ageng 85ºC 82V Lnearzaton and extrapolaton of the measurements ΔC/C(%).2.1 -.1 -.2 -.3 -.4 -.5 -.6 -.7 Floatng ageng 85ºC 693V Floatng ageng 1ºC 693V Floatng ageng 85ºC 82V Lnearzaton and extrapolaton of the measurements 1 2 3 4 5 6 7 Tme (hours) -.8 1 2 3 4 5 6 7 Tme (hours) Fgure 4: SR and C evoluton as functon of ageng tme for MPPF capactors These evolutons are manly due to the self-healng capabltes of MPPF capactors; when the metallzaton evaporates, the equvalent surface area of ths latter decreases, whch leads to an ncrease of SR and a slght decrease of C. Wth respect to the expermental plots shown n Fgure 5, τ, and U can be dentfed by extrapolaton of the measurements. By consderng an ncrease of SR of 1 % we can note 5 % reducton of the lfetme o MPPF capactors for an ncrease of 1 ºC or an ncrease of 17 V on the appled voltage (U = 245 V; = 14.4 ºC). Assumng ths trend fathful, we can establsh an expectaton lfetme of MPPF capactors n functon of voltage and temperature as shown n Fgure 6. xpectng lfetme (days) 1 7 1 6 1 5 1 4 1 3 T= C T=2 C T=4 C T=6 C T=8 C T=1 C 1 2 2 4 6 8 1 Voltage (V) Fgure 5: xpectng useful lfe of MPPF capactors as functon of temperature and DC voltage stresses 119
5. Health montorng of capactors Snce the drfts of the dfferent parameters are known and can be expressed as equatons, predctve mantenance systems can be elaborated (Abdennadher et al., 21). Some authors propose an ndvdual supervson of the capactors: by measurng ther mpedance near the resonance frequency, the equvalent seres resstance SR can be determned whlst functonng and compared to that of a healthy component. An example of such smart devce s gven on Fgure 7 (Venet et al., 22).The actual value of the SR s computed from the current flowng n the capactor and the voltage rpple. It s compared to the value of SR for a healthy capactor at the measured case temperature T c of the component. If the dfference s too hgh, a sgnal ndcates that the capactor s near ts end-of-lfe and should be replaced. Fgure 6: Smart electrolytc capactor 6. Concluson Ths paper presents a montorng approach for the capactor s electrcal parameters n functon of the ageng tme for both electrolytc and MPPF capactors. Furthermore, MPPF capactors ageng laws were dentfed usng yrng law as functon of the appled DC voltage and temperature. Ths latter can be mplemented n an algorthm allowng us to perform a predctve mantenance on the test capactors. However, snce DC-lnk capactors are subjected to gradents of temperature due to the electrcal stresses encountered durng ther normal operaton of the component, mprovements n yrng law should be made n order to take nto account the mpact of RMS current on the capactor ageng. References Abdennadher K., Venet P., Rojat G., Retf J.M., Rosset C., 21, A Real-Tme Predctve-Mantenance System of Alumnum lectrolytc Capactors Used n Unnterrupted Power Supples. Industry Applcatons, I Transactons on, 46, 1644-1652. Buatt G.M., Martn-Ramos J. A., Garca C.H.R., Amaral A.M.R., Cardoso A.J.M., 21, An Onlne and Nonnvasve Technque for the Condton Montorng of Capactors n Boost Converters. Instrumentaton and Measurement, I Transactons on, 59, 2134-2143. l-hussen M.H., Venet P., Al-Majd A., Fathallah M., Rojat G., Ferrera J.A., 23, Manufacturng technology effect on current pulse handlng performance of metallzed polypropylene flm capactors. Journal of Physcs D: Appled Physcs, 36, 2295. ndcott H., Hatch B., Sohmer R., 1965, Applcaton of the yrng Model to Capactor Agng Data. Component Parts, I Transactons on, 12, 34-41. Joubert C., Rojat G., Venet P., 27, Capactors: revstng a classcal technology to face new challenges. 2nd edton of the nternatonal conference Automotve Power lectroncs, Pars. Lahyan A., Venet P., Grellet G., Vverge P.J., 1998, Falure predcton of electrolytc capactors durng operaton of a swtchmode power supply. Power lectroncs, I Transactons on, 13, 1199-127. Martnez-Vega J., 21, Delectrc Materals for lectrcal ngneerng. John Wley & Sons Inc, Chapter 18 lectrolytc capactors, P. Venet, 43-434. Persse F., 23, tude et analyse des modes de défallances des condensateurs électrolytques à l alumnum et des thyrstors, applquées au système de protecton du LHC (Large Hardon Collder). Thess Unversty of Lyon. Torta J.H., Bonfac N., Denat A., 24, Dagnostc of the self-healng of metallzed polypropylene flm by modelng of the broadenng emsson lnes of alumnum emtted by plasma dscharge. Journal of Appled Physcs. Venet P., Lahyan A., Grellet G., Ah-Jaco A., 1999, Influence of agng on electrolytc capactors functon n statc converters: Fault predcton method. The uropean Physcal Journal - Appled Physcs, 5, 71-83. Venet P., Persse F., l-hussen M.H., Rojat G., 22, Realzaton of a smart electrolytc capactor crcut. Industry Applcatons Magazne, I, 8, 16-2. 111