Pulse Withstand Capability of Self-healing Metalized Polypropylene Capacitors in Power Applications. An Experimental Investigation

Transcription

1 146 IEEE Transactions on Electrical Insulation Vol. 26 No. 1, February 1991 Pulse Withstand Capability of Self-healing Metalized Polypropylene Capacitors in Power Applications An Experimental Investigation C. A. Nucci, S. Pirani and M. Rinaldi Istituto di Elettrotecnica Industriale, Facolti di Ingegneria Universiti di Bologna, Bologna, Italy. ABSTRACT One of the most frequent reasons for failure of metalized polypropylene capacitors is the detaching of the sprayed ends from the electrode edges. This is typical in power electronic applications due to the action of voltage and current pulses which may result in electrical, thermal and mechanical stresses. A study is carried out starting from the hypothesis that, according to previous results obtained, one of these stresses is prevailing and a combined effect is not to be considered. Assuming that the thermal stress is the most significant one, several tests have been carried out in order to verify this hypothesis. This is performed by checking whether current pulses with different wave-shapes, but with the same value of the Joule Integral, lead to the same degradation level of the end-edge contact. The degradation level is ascertained by measuring the tan6 variation, since degradation of the contact results in an increase of the equivalent series resistance of the capacitor. Contrary to the starting assumption, the results obtained show only a partial dependence of the degradation level on thermal stress, revealing also a contribution from the electrical and mechanical stresses. This has to be taken into account both to improve capacitor design in view of their application, and to develop significant type- test standards. INTRODU CTlON ET*LIZED PolYProPYlene self-healing capacitors are M widely employed in many applications due to their low cost and small size. On the other hand, there are some applications where they do not exhibit the required reliability, SO that various manufacturers recommend components made according to other technologies. One of the most frequent reasons for failure of metalized polypropylene (MPP) capacitors is the detachment of the sprayed ends from the electrode edges. In ac steady-state conditions such an event rarely occurs, whereas in transient state conditions it happens frequently. This is the case of capacitors employed in automatic power factor correction, where the inrush current due to frequent switching operations may result in the detaching of the ends. Analogous effects occur in impulsive conditions 1991 IEEE

2 IEEE Transactions on Electrical Insulation Vol. 26 No. 1, February typical of power electronic applications; in this case the wave shape of the current pulses can differ noticeably according to the circuit functions, and the pulse frequency can be very high. In the first case, inductors are connected in series to the capacitive units to eliminate the damaging effects of the inrush currents, whereas in many applications of power electronics this solution cannot be used due to the low value of inductance required in the circuit. The reason why the ends detach is not yet completely understood. It is reasonable theoretically to ascribe it to electrical, thermal and mechanical stresses. The problem is to understand whether one of them prevails or whether a combined effect has to be considered. A better knowledge is therefore required both to improve capacitor design in view of their application, and to develop significant type-test standards, which have not yet been drafted. This paper deals with the study of the end-edge formed, and the experimental results are presented and discussed. STRESSES TO THE SPRAYED END-ELECTRODE EDGE CONTACT DUE TO RAPID VOLTAGE VARIATIONS T is well-known, that the sprayed ends of MPP capac- I itors are formed by spraying a melted material on the two ends of the element, which is generally cylindrically shaped. The melted material is sprayed on the ends of the wound roll from a certain angle and distance. It attaches to the polypropylene and to the thin stripe of the metallic edge seen by the metallic jet (the stripe is as long as the polypropylene rolled band but only a fraction of a mm in height). The layers of metalized polypropylene which must be electrically connected to the end are obviously alternated with layers of film whose edges are connected to the opposite end, as shown in Figure 1. The lead wires which lead to the external connections of the capacitors are then connected, by soldering, to the ends. In what follows we consider some of the stresses which may result from the application of voltage and current pulses to the contact. ELECTRICAL STRESSES 3) I 4) Figure 1. Schematic view of the sprayed ends-electrode edge contact. 1: Sprayed end-electrode edge contact. 2: Polypropylene film. 3: Sprayed end. 4: Electrode. contact degradation; in particular, according to the results of previous investigations which seem to show that the thermal effect is the pre-eminent one, an attempt is made to verify whether current pulses with different waveshapes, but characterized by the same value of the Joule Integral (JI) may result in the same degradation level of the contact. After some consideration of the possible causes of deterioration, when capacitors are stressed by voltage and current pulses typical in power electronic applications, a description is given of the test procedure per- A voltage pulse between the edges of capacitors may result in discharges in the dielectric. As is known, during the service of MPP capacitors a number of discharges occur which contribute to a decrease in the capacitance [l] and to the development of gas [2]. Some discharges may also be localized at the end-edge contact region, leading to a degradation of the contact itself. This has been observed experimentally by the authors in tests carried out in conditions more severe than the rated ones, in order to emphasize the phenomenon. But, as already mentioned in the Introduction, the hypothesis of the investigation carried out is that this cause of degradation is negligible compared to the thermal one. MECHANICAL STRESSES The combination of both the Coulomb force and the electrodynamic force between the electrodes may result in a *mechanical stress on the end-edge contact. The computation of these forces is complicated by the irregular distribution of the current along the various cylinders

3 148 Nucci et al.: Pulse Withstand Capability of Polypropylene Capacitors Figure 2. Expressions of the parameter WO for some voltage pulses wave-shapes typical in power electronic applications. which constitute the capacitive roll: the different current paths are in fact characterized by uneven values of the resistance due to manufacturing technology. At the end-edge contact, the resultant of these forces theoretically may result in strains which might cause a progressive detaching of the film from the external bond represented by the sprayed ends, via micro-rupture. In the capacitor body, this strain is not expected to produce any degradation due to the absence of external bonds. However, monitoring of this phenomenon is difficult. Other mechanical stresses may originate from gas development due to discharges already mentioned. For this case also, the working hypothesis has been that the effect of such a stress can be neglected. THERMAL STRESSES If one assumes that detaching of the ends is essentially due to thermal effects, then the main cause of deterioration of the end-edge contact is assumed to be the decreasing effectiveness of the PP film s support due to a temperature increase. In fact, a temperature increase Figure 3. MPP capacitors used for the tests. of - 30 C may produce a shrinkage of the PP film (the shrinkage becomes evident above 100 C).,.

4 IEEE!lkansactions on Electrical Insulation Vol. 28 No. 1, February , Rockwell AIM 65 - Datalab - DL 905 CRT Figure 4. Wave shapes of the applied pulses. (a) damped oscillation, (b) aperiodic pulse. Local heating of the end-edge contact region may be caused, as assumed also by other researches [4], by the current pulses produced by rapid voltage variations that are typical in power electronic applications: these pulses have a short duration in comparison with the thermal time constants of the surrounding region, and thus the temperature increase produced by the Joule effect on the end-edge contact resistance may be considerable. The effects of such a dissipation can still be present when a subsequent pulse is applied, depending on the time interval. An attempt has been made by some authors [4-61 to define a parameter independent of the single capacitor and suitable for assessing these thermal effects. On the basis of this work a series equivalent circuit of MPP capacitors can be put forward, with R being the contribution of the end-edge contact to the total capacitor series resistance. The energy J dissipated in R as a consequence of a voltage variation in time T which produces a current i(t) is T J = R J i2(t)dt (1) 0 where the integral in the right-hand side of (1) is the Joule Rc> >Rd Figure 5. Block scheme of the test facility. integral (JI). The current i(t) in (1) can be expressed by where V is the voltage, C = cs/d is the capacitance, E is the permittivity and d is the thickness of the film; S = 2hl is the active area of the electrodes where h is the active height of the electrodes and 1 the length of the wound roll of polypropylene [7]. It is then possible to express the energy loss per unit length of the end-edge contact by introducing a parameter which does not depend directly on the capacitance and (4) 4r,~ k=- d2 (5) where rp characterizes the resistance of the end-electrode contact. Note that the total resistance of the end-electrode contact R, can be obtained by dividing rp by the total length 1 of the wound roll.

5 150 Nucci et al.: Pulse Withstand Capability of Polypropylene Capacitors If one makes the assumption that the thermal stresses are the most significant with respect to the end-edge contact, as was the case in a number of preliminary results [8], it follows that, for capacitors made according the same technology, the application of pulses characterized by different wave-shapes but the same WO at a given frequency must result in comparable degradation levels. This kind of comparison cannot be limited only a few specimens: data can be considered reliable only if referred to a great number of tests on different specimens. It is also important to adopt test pulses characterized by significantly different wave-shapes, in order to have very different values of voltage and current but with the same value of WO ~ IO I i 8 I I I I I 4 It is worth observing that WO is proportional to the JI value through C and thus, for capacitors with the same value of capacitance, J I values instead of WO can be adopted. LABORATORY TESTS SPECIMENS 01 I I I b ) o 50 IC0 150 M) 253 m m SOTJ Figure 6. NUMBER OF PULSES Average tan6 value (referred to the initial value) vs. the number of applied pulses for the elements tested. (a): aperiodic pulse, (b): damped oscillation. Confidence limits: 90%. As mentioned above, the parameter J/l does not depend directly on the capacitance, but on the construction technology, the electrode height and the applied voltage by means of the term WO; thus it can be significant even for capacitors with different capacitance made with the same technology. For a given capacitor, the J/1 values which lead to the ends detaching after a certain number of applied pulses at a certain frequency can be determined, even though they are affected by the typical dispersion of this type of experimental data. Of course, different values of J/l can be obtained according to the different technologies adopted by the various manufacturers. The value of parameter WO (usually expressed in V2/ps) depends on the wave-shape of the pulses: in Figure 2 some expressions of WO as a function of the different wave-shapes are given (e.g. [5,6]). HE tests have been carried out on over two hundred T specimens of the same type from the same production run, made by a well-known Italian manufacturer (Figure 3). These are 400 V rated voltage, 16 pf rated capacitance elements made with 8 pm layers of polypropylene wound into a roll as shown in Figure 1, with aluminum electrodes obtained by evaporation under vacuum. The sprayed ends are made with pure zinc. DESCRIPTION AND AIM OF THE TESTS The elements have been divided into two groups which have been stressed by the same thermal stress obtained from pulses with two different wave-shapes. This has been done in order to verify whether current pulses with the same repetition rate, JI, but different wave-shapes, result in comparable degradation levels when applied to capacitors of the same type. The first current pulse (Figure 4a) is a damped oscillation obtained by short-circuit discharging the element under test; the wave-shape is that typical of the discharge of a capacitance C in an RLC series circuit with a resistance lower than the critical value 2(LC) /. The second one (Figure 4b) has been obtained by discharging the capacitor under test in an RLC series circuit with a resistance greater than the critical one.. I

6 IEEE Transactions on Electrical Insulation Vol. 26 No. 1, February Figure 7. Enlarged views of the detached sprayed end for two elements stressed by damped oscillations Figure 8. Enlarged views of the detached sprayed end for two elements stressed by aperiodic pulses. The tests have been carried out by making use of a bench built for the purpose, including two contactors, two wide-band Pearson current transducers, a multiplexer, a Datalab DL905 transient recorder and a Rockwell AIM65 microprocessor-based controller card. The block diagram is shown in Figure 5. The AIM65 controls all the test operations. With this facility two batteries, each of 4 capacitors, can be tested. For each battery, the capacitors are connected in parallel and in series during the charging and discharging phase respectively. The discharge resistors have been selected with different values in order to produce the two different pulses discussed earlier. This way of proceeding made it possible to carry out life tests on more than one capacitor at a time. During the charge-discharge cycles, the capacitance and tan6 values have been periodically measured. The latter has been assumed to be an index of the degradation process which causes the detaching of the film from the ends: in fact, this detaching results in an increase in the equivalent series resistance of the capacitor. The capacitance remained practically constant throughout the tests. These measurements have been performed at 1 khz frequency by using a GenRad 1656 measuring bridge suit-

7 152 Nucci et al. : Pulse Withstand Capability of Polypropylene Capacitors Figure 9. Enlarged views of the region of the sprayed end under the tin drop, for two of the elements tested. Figure 10. Enlarged views of a region of the sprayed end far from the tin drop, for the same capacitors of Figure 9. ably modified to estimate tan6 variation with a resolution of about - Further, the JI value has been monitored in order to keep it constant throughout the test, by changing the voltage of the charging Computation of the JI value has been performed by,.he AIMG5 with the processing of data stored in the Datalab Transient Recorder to the detaching of the ends in a reasonable number of pulses has been fixed at 15 A s (corresponding to about V /ps) on the basis of some previous tests [8]. The test of each capacitor was stopped when the tan6 value reached a value lox the initial one. we consider that at this value the contact end-edge has deteriorated which sampled the current pulse values at a frequency of 5 MHz. RESULTS AND COMMENTS The test pulses have been applied at a frequency of four pulses every minute; the value of the JI able to lead HE results obtained have been presented in two graphs T(Figure 6). tan6 values relative to the initial value are

8 IEEE Transactions on Electrical Insulation Vol. 26 No. 1, February deteriorations of the ends. This process is more rapid when caused by aperiodic pulses. Since the imposed thermal stress is the same for the two cases, one can deduce that this stress is not the only reason for deterioration. Furthermore, since the pulse feature which differs more markedly is the peak value of the current (and the voltage), it seems reasonable to conclude that significant electrical and mechanical stresses play a not negligible role in the degradation mechanism of the sprayed end-electrode edge contact. Figure 11. Views of the end of an element pulled out from its case prior to testing. Figure 12. View of an element removed from its case during the application of a series of pulses. plotted versus the number of applied pulses. To allow for easier analysis, only the average curves for the two cases have been plotted. The confidence limits for 90% refer to a normal probability distribution. The curves show a certain dispersion of the increase in tan6 with the applied pulses. This expected result is, however, more evident for the specimens stressed with damped pulses. Analysis of the curves reveals that the application of pulses having different wave-shapes but the same JI values on the same type of capacitors gives rise to different The last observation seems to be confirmed also from an analysis performed by making use of a scanning electron microscope on the sprayed ends of a number of capacitors which had been completely detached from the roll after the application of a cycle of pulses. In Figures 7 and 8 the enlarged views of some areas of the ends are shown. Figure 7 refers to two elements tested with the damped oscillation, whereas Figure 8 refers to two others stressed by the aperiodic pulse. In both Figures the detaching appears neat, unlikely to be caused by thermal shrinkage and softening of the polypropylene following a local temperature increase; in fact, if this were the case, a more diffused fringing of the polypropylene attached to the end would be expected. In Figures 9 and 10 the enlarged views of different areas of the same end for two different elements (a and b) are shown. The first (Figure 9) refers to areas under the tin drop which provides the electrical connection of the lead wire to the end. In this area, where the prevalence of the thermal effect is particularly likely because of the local temperature increase which may take place due to both the heat caused by the lead wire soldering process and the high value of current density, the morphology differs from that of Figures 7 and 8. The fringing of the polypropylene in the detached end is evident: it seems reasonable to ascribe it to a local temperature increase. Figure 10 shows other parts of the same ends of Figure 9, far from the previous ones. In this case the morphology is markedly different and looks like that of Figures 7 and 8: the detachment seems cleaner. In order to investigate the causes of these phenomena further, other tests have been carried out on elements which had been pulled out of their cases. In Figure 11 the view of the end of a capacitor is shown as it looks prior to testing. Metalized polypropylene capacitor cases are often filled with insulating liquid which may slightly penetrate into the wound unit through the ends. It has been observed that, if an element is removed from its case, a spill of liquid is evident under the influence of the applied pulses. Figure 12 shows an instance of this phenomenon.

9 154 Nucci et al.: Pulse Withstand Capability of Polypropylene Capacitors Figure 13. View of the ends of some elemer its after the application of a number of pulses. It is worth mentioning that some tests performed, using a transparent cell made for the purpose and filled with the insulating liquid, have confirmed the occurrence of discharges at the end-edge contact region, and development of gas. In Figure 13, the ends of some elements after the application of a number of pulses are shown. The analysis of the polypropylene film from the capacitor after the tests has shown the absence of any damage (partial and breakdown discharges, self-healing processes) in the inner part of the element, confirming that the main causes of this kind of capacitor failure must take place in the sprayed end-electrode edge contact region. This research activity is still continuing with the aim of further verifying the results obtained. CONCLUSIONS NE of the critical points of self-healing metalized poly- 0 propylene capacitors is the contact between the electrodes and the sprayed ends. When these capacitors are employed in power electronic apparatus they are affected by rapid voltage variations which can stress the contact electrically, mechanically and thermally. Assuming that the thermal stress is the heaviest, as was the case in a number of preliminary results, tests have been carried out in order to verify this hypothesis on a large group of specimens by applying current pulses with different waveshape, but the same value of the JI. The experimental results do not seem to agree with the assumption above. The specimens stressed with the current pulses characterized by higher current-peak and voltage values have shown faster degradation compared with the others stressed by pulses with the same JI but lower current peak values. These results lead to the conclusion that electrical and mechanical stresses can play a non-negligible role in the degradation process of the end-edge contact. The analysis performed by means of a scanning electron microscope on some sprayed ends which had detached from the capacitive rolls after the tests seem to agree with this conclusion, which is thus to be taken into consideration both in improving the design of MPP capacitors and in developing suitable type-test standards. ACKNOWLEDGMENTS HE Authors wish to express their thanks to ITAL- T FARAD Company of Bologna for their collaboration in the manufacture of the specimens tested. Also thanks to Prof. Din0 Zanobetti for helpful discussions during the development of this work. REFERENCES [l] Y. Yoshida, K. Yamanaka, K. Matsunaga, K. Sato, J. Kano, T. Umemura, Y. Hayashi, Status Quo of Self- Healing Film Power Capacitors in Japan, CIGRE, paper 15-04, Paris, [2] S. Theoleyre, M. Nicholas, Condensateurs secs autocicatrisants: durce de vie et comportement en fin de vie, Rev. Gen. Electr., Nr. 6, June, 1988.

10 IEEE Transactions on Electrical Insulation Vol. 26 No. 1, February [3] J. Samat, J. J. Courtet, G. Bernard, P. Jay, P. Bernard, The Development of Dielectric All-Film Capacitors and the Evaluation of Their Endurance, CIGRE, paper 15-06, Paris, [4] H. Gottlob, H. Kessler, Charge impulsionelle admissible de condensateurs a feuilles plastiques mctallisies, Les Composants Electroniques, Nr. 5, pp , [5] G. Aglietti, M. Rinaldi, Report on the utilizz+:m of metalized polypropylene capacitors for SCR commutation in choppers for electric traction applications, IEE International Conference on Power Electronics- Power Semiconductors and their Applications, London, [6] Applicazioni impulsive dei condensatori in film plastico, Ducati Elettrotecnica Company: Technical Manual, Bologna, [7] F. Fritze, Metalized film capacitors, high pulse handling capability through stacked design, Siemens components, Nr. 2, [8] C. A. Nucci, S. Pirani, M. Rinaldi, Tests on the Electrode contact Degradation Caused by Current Pulses in Self-Healing Low Voltage Metalized Polypropylene Capacitors for Power Electronics, CIGRE, paper 15-05/86-25, Paris, This manuscript is based on a paper given at the 2nd International Conference on Properties and Applications of Dielectric Materials, Beijing, China, September Manuscript was received on 12 Jun 1989, in final form 10 Sep 1990,

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