Capacitors Age And Capacitors Have An End Of Life
A White Paper from the Expertsin Business-Critical ContinuityTMCapacitors Age andCapacitors Have an End of Life
Executive SummaryThis white paper discusses large DC aluminum electrolytic and AC polymeric film capacitorsfor use in a UPS application: specifically field aging, failure modes, expected service life andpreventative maintenance.UPS systems use large capacitor banks. These capacitor banks are made up of both DCelectrolytic and AC polymeric film capacitors. AC polymeric film and DC electrolytic capacitorsboth degrade under field operating conditions. The field aging of the capacitor is a slowprocess which takes place over years but eventually the field aging leads to a capacitor failureunless the capacitors are periodically replaced.1
Introductioncomponents in UPS systems which field ageand a corresponding list of recommendedreplacement times. For more informationon capacitor preventative maintenancesee the white papers The Effect of RegularSkilled Preventative Maintenance on CriticalPower System Reliability and Longevity of KeyComponents in Uninterruptible Power Systems,available at www.liebert.com.High quality capacitor manufacturers allaround the world provide a capacitor servicelife rating. The service life rating is, at best,a guideline. The number lacks sufficientaccuracy to be used as a predictor of whenthe first capacitor in a large population willfail. Capacitor failure models do exist andwill generate a failure time for a specificfailure rate but the number contains alarge variance and has a low confidencelevel. Replacing capacitors periodicallyis the only way to insure a very high MTBFfor capacitors. This white paper discusses thereasons capacitors fail, the dispersion in timeover which a group of capacitors fail, failuremodeling for capacitors and the cost effectivesolution of a capacitor replacement program.Why UPS systems use largepower capacitorsOn line UPS systems contain five main parts:as shown in Figure 1.1. An AC filter at the input line2. A rectifier which converts the filteredAC to DC3. A DC bus, containing both a largebattery bank and a DC capacitor bankfor bus hold up and DC filtering4. A power inverter, which convertsDC to AC5. An AC filter at the output lineCapacitors are not the only component inUPS systems which experience field agingand can cause UPS systems to transfer offlineor to a bypass source. Liebert has a list ofStaticBypassSwitchInput LineStaticBypassSwitch2Rectifier1Input AC FilterCouplingTransformerOutput Line43DC Buss Hold Up and FilterInverter5Output AC FilterAuto TransformerBatteryCabinetNeutralNeutralFigure 1. UPS circuit showing filtering banks.2
DC electrolytic capacitors are used to filter theDC signal and AC polymeric film capacitorsare used for filtering the AC signal. The DCelectrolytic capacitors are also used to holdthe DC bus voltage at a near constant level.Without the capacitor filters the UPS systemwould supply poor quality power, which isnot an adequate AC source for highly reliablesystems like a large electronic database.The benefits of two differentcapacitor technologiesCapacitors are selected for electricalcircuits based on a complete set ofelectrical, mechanical and environmentalparameters. After narrowing the searchbased on the complete specification, thenquality, size (energy density) and mountingconfigurations determine the final selection.DC electrolytic capacitors are smaller in size(larger energy density) than AC polymericfilm capacitors. Unfortunately, DC electrolyticcapacitors can only be used on DC circuits.AC polymeric film capacitors can be used onboth circuits but using AC film capacitors forDC circuits would increase the size and costof a UPS system. As a result, DC electrolyticcapacitors are used in the DC circuit andAC polymeric film capacitors are used inthe AC circuits.are only packaged in large round aluminummetal cases. Pictures of a typical large ACpolymeric film and DC aluminum electrolyticcapacitor are shown in Figure 2. As you cansee, these are large components typically 60mm to 100 mm in diameter and can vary inheight from 120 mm to 220 mm.The polypropylene dielectric film used inAC polymeric film capacitors is extrudedfrom dielectric grade resin (very purepolypropylene resin which contains aminimum of contaminates but it is not zero)into a thin film which is then stretched toachieve a very thin film (3 µm to 20 µm).The average film thickness can vary by 3to 4% over the web width. In addition, thefilm contains thickness variances on a smallarea scale (smaller than the cross section ahuman hair). The contaminants have both anaverage concentration on a large surface areascale and variances on a small area scale. Thenon uniformities in materials and processingdrive variances in the capacitor performanceThis is a very simplistic model but it servesto introduce the concept that performancevariances play a major role in determining theservice life and the reliability of a capacitor.The difference between service life andreliability will be discussed in section 7.Typical AC polymeric film capacitance ratingsfor large UPS applications are 50 µF to 200 µF(microfarads). This is a single capacitor rating.These capacitors typically have a dielectricBasic design of an AC Polymer Film andDC Aluminum Electrolytic capacitor.Capacitors in general are constructed withtwo main components: a very thin dielectricmaterial with a large surface area and twothin conducting plates, referred to as currentcollectors. For both capacitor technologies,the dielectric material and the two currentcollectors are rolled into a section whichlooks very similar to a roll of paper towels.The AC polymeric film capacitors arepackaged in a large round metal or a largeplastic case. The DC electrolytic capacitorsFigure 2. The shiny metal case is the ACpolymeric film capacitor and the blue insulatedmetal case is the DC electrolytic capacitor. Thegrid in the background is 1 cm by 1 cm squares.3
and halogens for electrolytic capacitors arejust a few examples) lead to deterioration ofthe dielectric materials (polymeric film andaluminum oxide layer) which reduces thevoltage withstand capability of small localizedareas. These chemical reactions lead to anincrease in the probability that the dielectricmaterial will not withstand theapplied voltage.surface area in the range from 10 m2 to 40 m2(this is a general number and varies with thevoltage and service life of the capacitor). Thescale of these dimensions can be visualizedas a sandwich of two large garage doorswith a layer of shrink-wrap plastic betweenthem. This sandwich contains material andgeometry variances and the weakest pointin the dielectric sandwich becomes the weaklink for the capacitor performance.The second mechanism has to do with theleakage current. Dielectric materials areoften thought of as insulators which do notconduct current when voltage is appliedacross the insulation. In fact, all insulatingmaterials conduct a very small currentwhen voltage is applied. This current isa conduction (electrons and ions movephysically through the material and not by anelectronic mechanism) current and is usuallycalled a leakage current. The magnitude ofthis current is very small (typically a millionthof a millionth of an Ampere per cm2). Eventhough the leakage current is very small itflows in a very small channel and gives rise tolocalized heating and materialelectron interactions.DC aluminum electrolytic capacitors usean aluminum oxide layer as the dielectricmaterial and a dielectric grade aluminumfoil as the current collector. The aluminumoxide layer is grown on a dielectric gradealuminum foil. The aluminum foil currentcollector is first etched to achieve a largesurface area (composed of many smalldiameter tunnels which do not penetratethrough the aluminum foil) before thealuminum oxide is grown on the aluminumfoil. Both the materials and the processinghave non uniformities on a small scale. TypicalDC electrolytic capacitor ratings for large UPSapplications are in the 1,500 µF to 16,000µF (these values vary with the voltage andservice life of the capacitor and are commonratings for capacitors in the 350 to 400 Vdclevel). DC Aluminum electrolytic capacitorswith these capacitance ratings typically havea surface area in the range of 30 m2 to over300 m2. Like polymeric film capacitors, thisis a very large surface area, and materialplus processing variances (in addition toother issues) lead to weak points within thecapacitor. Once again, the weakest points inthe large surface area become the weak linkfor the capacitor performance.The aging mechanism presented in this whitepaper is very superficial but it introducesthe concept of capacitor aging. Capacitorsare not static electrical components whichjust sit and operate in a circuit. Over time,the internal reactions and the leakagecurrent lead to a reduction in the dielectricvoltage withstand capability (increase inthe probability for a capacitor to fail). As thereactions progress, the capacitance valueslowly decreases and the resistance valueslowly increases. (All capacitors have anequivalent series resistance).Capacitor field agingTwo basic mechanisms lead to capacitorfield aging. The first mechanism is chemicalreactions. Combinations of heat andchemical contaminates (oxygen, moisture,Capacitors have an end of lifeThe aging process in the capacitor can bevisualized by considering a water dam with4
a small leak. Over time, the small water leakgrows. The movement of the water throughthe dam causes deterioration within thedam structure. In spite of the growth inthe leak rate, the leak rate is still small andthe dam still functions as a dam. As watercontinues to leak, the structure of the damis compromised. When sufficient damageoccurs, the probability for a near term failurebecomes very high and the dam needs to betaken out of service.industry experience, the end of life numbersstill cover a range and are only guidelines.Failure mode for a capacitoras it approaches its end of lifeDuring the capacitor aging process theelectron leakage current and the chemicalreactions both cause a decrease in thecapacitance value and an increase in theresistance value. Both of these changes(decrease in capacitance and increase inthe resistance) are tied to damage takingplace inside of the capacitor. Once sufficientdamage to the capacitor has been sustained,the probability for the capacitor to failincreases and when this probabilitybecomes high, the capacitor shouldbe taken out of service.Capacitor industry guidelines exist whichdefine end of capacitor service life. Theseare based on a decrease in the capacitanceand/or an increase in the series resistance.Typical values are shown in Table I. Thecapacitor still operates at end of life, just asthe dam still holds water, but the capacitorhas a high probability for a short circuit failureand the capacitor should be taken out ofservice. Note that after years of capacitorBoth capacitor technologies age by losingcapacitance and developing higher internalresistance. If this process is allowed tocontinue, a point will be reached where thecapacitor will fail into a short circuit andwill lose the ability to withstand the ratedvoltage. The capacitor aging process alsogenerates gas which increases the internalgas pressure in both capacitor constructions.The polymeric film capacitor has a pressureinterrupter built into the case and whenthe pressure reaches a preset value, theinterrupter opens inside of the capacitorcase and disconnects the capacitor from thecircuitry. The pressure interrupter design forlarge polymeric film capacitors are based ona notched wire which breaks open as shownin Figure 3B. When the pressure interrupteroperates, the capacitor case expands upwardand is very visible indicating the capacitorhas reached end of life and is no longer inthe circuit. Other pressure interrupter designsare used around the world but they all openunder pressure and disconnect the capacitorfrom the circuitry. For DC aluminumelectrolytic capacitors, the build up inpressure pushes up a rubber bung at a preset pressure. The rubber bung is shownin Figure 3A. This does not disconnect thecapacitor from the circuit but it providesElementsFilm TechnologyAluminum ElectrolyticTechnologyMax loss of capacitance-5% to -10%-15% to -20%Max increase in seriesresistance 100% to 150% 200% to 300%Table 1. End of Life Parameters.5
capacitors which again will impact powerquality. In the extreme case where the lossof capacitance can lead to a problem with theUPS, the UPS will go to by-pass mode whichmaintains the load but the UPS is no longer inthe circuit. In both cases (when the pressureinterrupter opens or the rubber bung popsup), the capacitor is at its end of life andshould be replaced.a visible sign when the capacitor is reachingend of life. (Simple voltage interrupterswitches do not exist for DC circuits andas a result DC electrolytic capacitors donot contain pressure interrupters.The UPS system monitors the quality of theAC power being supplied by the UPS systemand can detect reduction in capacitance forboth the AC and the DC capacitor banks.The UPS actually monitors power qualitynot capacitance or resistance but the twoare related. When the UPS detects that apreset value of capacitance reduction hastaken place, a signal and or alarm is sent toan appropriate location indicating servicingof the unit is required. As capacitancedecreases increase stress on the remainingcapacitance can develop. In the event apressure interrupter does operate, additionalstress can also be placed on the remainingThe aging AC polymeric film capacitorcontains a large reactive power flow inaddition to an internal gas pressure (relativeto atmospheric pressure). The DC aluminumelectrolytic capacitor contains a large electricpotential energy in addition to the internalgas pressure. If the capacitor develops a shortcircuit, there is the potential for a large energyflow (capacitors assembled into large bankscan discharge all their energy into the onefailed capacitor) in a very short period of timeand it is not always possible to disconnect thecapacitor before damage has been doneto the UPS system.The difference between expectedservice life and reliabilityExpected service life and reliability are similarconcepts but they are not the same. Expectedservice life is a general classification for theservice life. It is a general classification, likeone year, five years, ten years, etc. It doesnot mean that all the capacitors fail after fiveyears if it is a five year design. Capacitors canand do fail inside the expected service liferating. The expected service life ratingcan only be used as a general guideline.Reliability is a probabilistic statement of thecumulative failure probability for a specificset of operating conditions and a specificoperating time.Figure 3A. Rubber bung is shown for the DCelectrolytic capacitor.Figure 3B. Notched wire is shown for the ACpolymer film capacitor.6
confidence level deals with the variance in thesample test data (see the next paragraph).A reliability number has the following format:“Maximum cumulative failure of 5%when operated for 50,000 hours at maxrated conditions with a confidence levelof 90 %” ( 95% is also very common).The issue associated with sample testing iseasily understood by looking at Figure 5.The top box in Figure 5 shows a small groupof 24 capacitors which are being tested tofailure. The small round capacitors plottedalong the test time axis, show the firstcapacitor to fail, then the second capacitorto fail (this is a longer time than the timeit takes for the first capacitor to fail), thethird capacitor to fail and finally the fourthcapacitor. In a perfect world, the test shouldbe continued until all the capacitors fail, butthat takes too long and is rarely done.A simple representation of capacitor agingin Figure 4 shows what is going on.In the ideal world each capacitormanufactured with the same modelnumber and operated in the identical ratedconditions, would all reach the expectedservice life at the same time and fail slightlyover the expected service life as shownin Figure 4.In the real world, a group of capacitors allmanufactured with the same model numberand operated under the same max ratedconditions would all fail at different times asshow in Figure 4. A few of these capacitorswould actually fail inside of the expectedservice life.The second box shows the measured databeing curved fit to a distribution curve(for capacitors this curve is usually a 2parameter Weibull function). An analyticalrelationship is required if a failure time fora small cumulative failure rate like 0.01%is the number being sought. Measuringa cumulative failure time of 0.01% wouldrequire testing a group of over 10,000capacitors. This is just not practical.The challenge is to calculate the operatingtime for a small cumulative failure rate, like0.01% with a high confidence level. The highFigure 4. A simple figure representing capacitor aging. The right figure shows ideal aging, and theleft figure shows aging in the real world.7
from the total capacitor population. Eachtime the experiment is repeated, themeasured results would be different. If theexperiment is repeated N times each witha different set of capacitors, there would beN measured/calculated times. ConfidenceThe bottom box shows what would happenif another set of 24 capacitors were pulledMeasure time to failure for the first four capacitorswhich fail out of a group of 24 capacitors testedunder accelerated life conditions.Cumulative failure rateThe third box shows the cumulative failurerate for 0.01% being added to themeasured data.Test TimeCumulative failure rateCurve fit the cumulative failure rate testdata to a distribution (Weibull)Test TimeCumulative failure rateUse the mathematical curve fit to predicta service life for the required cumulativefailure rateFailure time forthe requiredcumulativefailure rateTest TimeSpecific failure rateRepeat this complete process N times. Eachindividual experiment of 24 capacitors willgenerate a different service life. Plot all theindividual different calculated service liveseach for the same cumulative failure rateIndividual calculatedservice life rates ofeach of N tested groupsTest TimeFigure 5. Chart showing the testing and analytical process for calculating reliability.8
level mathematics must be applied to theset of N measured/calculated times to geta final number.The capacitor manufacturer producesa large quantity of capacitors with a singlepart number. The quantity of capacitors witha single part number purchased by a largeUPS company can easily exceed one million.The number of capacitors within this groupthat are tested by the capacitor manufacturerwould typically be less than 150 units, madeup of 6 to 8 sample sets. The sample sizeproviding the tested data used to predictreliability is very small. Reliability numberscan be generated and are generated bythe industry but it is clear from the processthat achieving a high degree of accuracy isvery difficult. This leaves the industry withpreventative maintenance including fieldcapacitor replacement, based on years ofoperating experience, as the most accurateapproach to insure capacitors in UPS systemslast well beyond the service life of theUPS itself.Statistical modeling is requiredto predict failure probabilityfor capacitors.the right is much larger than the tail whichstretches to the left) and the distributionis very wide. Of the three models whichhave been used, the exponential curve isby far the most prevalent because it hasthe easiest math. The exponential model isthe basis for the “bathtub curve” which isstill actively referenced in the industry. TheWeibull model is the second most popularand the log normal curve is still used but hasbeen largely replaced by the Weibull modelover the past 20 years. An example of failuremodeling for AC polymeric film capacitorsis shown in Figur
AC polymeric film capacitors can be used on both circuits but using AC film capacitors for DC circuits would increase the size and cost of a UPS system. As a result, DC electrolytic capacitors are used in the DC circuit and The difference between service life and AC polymeric film capacitors are used in reliability will be discussed in section 7.
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