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CAPACITOR FAILURE ANALYSIS:ATROUBLESHOOTING CASE STUDY

Thomas M. Blooming, P.E.t.blooming@ieee.org

Eaton ElectricalAsheville, North Carolina

Abstract: Adding power factor correctioncapacitors provides well-known benefits toindustrial plants. These benefits include powerfactor correction, voltage support, and release ofsystem capacity. Engineering judgment must beused, however, when applying capacitors in powersystems with excessive harmonics and/ortransients. Capacitors might not survive long insuch environments if they are improperly applied.

Capacitors are also known to cause powerquality problems, such as oscillatory switching

transients, and can possibly amplify otherproblems, such as harmonics. The same piece ofequipment that solves some problems can causeother problems. Adjustable speed drives (ASDs)are another example of such equipment.

This paper investigates capacitor failures andfuse operations in an automatically switchedcapacitor bank in an industrial facility. The fusesthat cleared were protecting individual capacitorsteps in the bank. It was initially believed thatharmonics were the source of the problem. Theinvestigation determined that transients from anunlikely source were to blame.

Key Words: Capacitors, transients, harmonics,fuses, measurements, power quality.

I. Introduction

A steel processing plant was experiencingunexplained capacitor failures and fuse operationsin an automatically switched capacitor bank. Theplant rolls and galvanizes sheet steel for theautomotive industry. Any problem that interfereswith production schedules affects the bottom line.With increased productivity demands, the plantcannot afford to devote man-hours to recurringproblems. Plant personnel need to solve problems

as they occur rather than continue to replace failedequipment or restart shut-down processes.Maintaining an acceptable power factor is

important to the plant because the utility ratestructure includes a penalty for low power factor.The accounting department did notice a reductionin the electric bill when the capacitors were added,proving that they are definitely contributing to thebottom line.

Due to the variable loading on one of theplants 480 V buses that needed power factorcorrection, plant engineers chose an automaticallyswitched capacitor bank with four variable steps.When capacitors and fuses in the bank began tofail the electric bill increased and plant processeswere affected.

II. Electric Power System

A. System Description

A simplified one-line diagram showing theportions of the power system relevant to this paperis shown in Figure 1.

The steel processing plant is served at 13.2 kVat the end of a radial overhead distribution line.This line has a relatively low short-circuit MVA forthis voltage level. The short circuit MVA at 13.2 kVis 55 MVA with an X/R ratio of 2.99. From themetering, there are four transformers servingvarious parts of the plant. These transformersrange from 1000 to 3000 kVA.

One of the transformers, a 13.2 kV-480Y/277V, 1500 kVA delta-grounded wye with a 5.6%impedance, serves the 480 V bus where the

automatically switched capacitor bank is installed.It is this bank that has been experiencing capacitorfailures and fuse operations.

The capacitor bank contains two 50 kvar fixedsteps and four switched steps of 50 kvar each for atotal of 300 kvar. The capacitors which make upeach of the steps are 16.67 kvar, three-phasecells. All of the kvar ratings are at 480 V. None ofthe steps are configured as harmonic filters. Each50 kvar group is protected by its own set of currentlimiting fuses. The steps switch in and out ofservice automatically based on the power factorcorrection control algorithm in the bank.

The variable steps in the bank are switched by

means of electro-mechanical contactors. Thecontrol algorithm switches steps in and out in orderto maintain a target power factor. There is a timedelay when switching, either adding or removingcapacitors, to avoid hunting, the excessiveswitching in and out of a step.

The control algorithm also avoids switching ina step within one minute after it has beendisconnected. This allows trapped charge todissipate to less than 50 V before reconnecting

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From Utility: 13.2 kV, 55 MVA, 2.99 X/R

Transformer #21000 kVA, 5.75% Z13200-480Y/277 V

Transformer #31500 kVA, 5.6% Z13200-480Y/277 V

Transformer #13000 kVA, 7.4% Z13200-480Y/277 V

Transformer #41500 kVA, 6.7% Z13200-720 V

450 A 600 A 800 A 400 A400 A800 A

MCC Bus Duct

DC DrivesDC Drives

Bus Duct

Fixed50 kvar

Step 150 kvar

Step 250 kvar

Step 350 kvar

Step 450 kvar

Fixed50 kvar

13.2 kV

480 V

Figure 1. One-Line Diagram of the Power System

them. This is done so that capacitors are notswitched in when they have a trapped charge,which might lead to an excessive switchingtransient.

These two limitations allow short periods oftime when the power factor criterion is not met. Onbalance, however, the overall power factor from ademand point of view is maintained above the setlevel.

The load on this 480 V bus includes four DCdrives, served from two isolation transformers (twodrives per transformer). These drives operateintermittently as the process demands. Theaverage load on the main 1500 kVA transformerwas 550 A with a maximum of 990 A during themeasurements. The drives are the only significantharmonic sources on the bus. When the drivesdraw their maximum current, they can compriseabout 40% of the bus load. This does not happenvery often, however.

With the power factor correction capacitors,The steel processing plant benefits from voltagesupport in addition to cost savings due to reduction

in power factor penalties. Release of systemcapacity was not an issue on this particularservice. A multi-step, automatically-switched bankwas chosen because of the intermittent nature ofmany of the loads on this particular bus.

B. Description of the ProblemThe steel processing plant was experiencing

problems with the automatically switched capacitorbank for some time before they investigated theproblem. The problem was not discoveredimmediately because the bank is not checkedregularly. The problem was first noticed in the

electric bill. Permanent on-site monitoring mayhave detected the problem sooner.

The natural first action was to simply replacethe blown fuses that were found. It was laternoticed that some capacitor cells had also failed.These were also replaced. When the problemspersisted a detailed examination was undertaken.

At the time of the measurements, some fuseswere blown and some capacitor cells had failed.The fuses in variable Steps 1 and 4 were blown

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and one of the three 16.7 kvar (three-phase) cellsin Step 3 had failed so Step 3 was supplying only33.3 kvar rather than its nominal 50 kvar.

No obvious cause was observed during themeasurements that were performed. Either theproblem was due to cumulative effects over time orit was an intermittent problem that did not occur

during the measurements.The fact that failures did not occur during the

measurements made further analysis necessary todetermine the cause of the problem. If failures hadoccurred during the measurements, themeasurement data at the time of the failures couldhave been analyzed and the cause may have beendetermined much sooner.

III. Power System Measurements

A. Harmonic Measurement ResultsPossible causes for the capacitor failures and

fuse operations included excessive harmonics and

transients (overvoltages). Measurements wereperformed to quantify the harmonic voltages andcurrents in the capacitors in order to study whetherharmonics were the cause of the failures. Thepower monitor used for these measurementswould also catch transients if they were to occur.Measurements were also performed on other partsof the power system, including the DC drives whichare known to cause harmonics, as part of a largerstudy effort.

The average values for the voltage totalharmonic distortion (THD) and the rms,fundamental, and harmonic voltages at thecapacitor bank with different kvar step

configurations are presented in Table 1. Allconfigurations also include the 100 kvar fixed step.

All values given are three-phase averages. Allharmonics are given in percent of fundamental.

Higher order even harmonics such as the 8th,

10th, 12

th, 14

th, et cetera are not normally reported,

but they were in this case. This was done toinvestigate a possible harmonic resonancecondition near those frequencies.

The average values for the current THD andthe rms, fundamental, and harmonic currentsflowing in the capacitor bank with different kvarstep configurations are presented in Table 2. Allconfigurations also include the 100 kvar fixed step.

All values given are three-phase averages. Allharmonics are given in percent of fundamental.

The average values for the current THD andthe rms, fundamental, and harmonic currentsflowing in several other important locations arepresented in Table 3. All values given are three-phase averages. All harmonics are given inpercent of fundamental. For the DC drives, all

data is presented during periods of significant load.Time when the drives were not operating is notincluded in the drive data.

Table 2. Capacitor Current Measurement Summary

CASE 1 CASE 2

QUANTITY STEPS 2&3 ZERO STEPS STEP 3 STEP 2 STEPS 2&3

ITHD 27.10% 33.42% 23.02% 34.71% 35.20%

IRMS (A) 219.4 120.4 158.0 182.4 225.0

I1 (A) 211.2 114.2 153.8 171.8 211.4

I2 0.12% 0.11% 0.09% 0.12% 0.14%

I3 0.52% 0.52% 0.53% 0.58% 0.58%

I4 0.34% 0.44% 0.38% 0.45% 0.48%

I5 5.66% 5.79% 7.83% 5.95% 7.26%

I6 0.49% 0.61% 0.54% 0.60% 0.72%

I7 4.54% 3.01% 2.00% 2.69% 3.96%

I8 0.80% 0.71% 0.60% 0.82% 1.19%

I9 1.39% 0.90% 1.01% 1.51% 2.14%

I10 0.87% 0.45% 0.55% 0.73% 1.09%

I11 19.95% 11.56% 6.18% 23.84% 31.36%

I12 1.05% 0.54% 1.07% 1.34% 1.06%

I13 12.00% 17.96% 15.46% 19.05% 8.91%

I14 0.50% 1.11% 1.54% 0.83% 0.53%

I15 0.86% 2.39% 2.20% 1.26% 1.03%

I17 5.52% 17.66% 8.83% 10.46% 7.01%

I19 2.24% 10.05% 4.76% 3.12% 2.38%

I21 0.35% 0.98% 0.45% 0.47% 0.39%

I23 1.98% 5.10% 2.25% 3.24% 2.10%

I25 1.38% 3.55% 2.40% 1.60% 1.63%

Table 1. Capacitor Voltage Measurement Summary

CASE 1 CASE 2

QUANTITY STEPS 2&3 ZERO STEPS STEP 3 STEP 2 STEPS 2&3

VTHD 2.62% 2.59% 2.25% 3.10% 3.40%

VRMS(VLL)

474.6 470.6 474.3 472.2 474.2

V1 (VLL) 474.4 470.4 474.2 472.0 473.9

V2 0.06% 0.06% 0.05% 0.07% 0.07%

V3 0.19% 0.20% 0.20% 0.21% 0.21%

V4 0.09% 0.11% 0.10% 0.11% 0.12%

V5 1.13% 1.16% 1.57% 1.19% 1.45%

V6 0.08% 0.10% 0.09% 0.10% 0.12%

V7 0.64% 0.43% 0.28% 0.38% 0.56%

V8 0.10% 0.09% 0.08% 0.10% 0.15%

V9 0.15% 0.10% 0.10% 0.16% 0.22%

V10 0.09% 0.04% 0.05% 0.07% 0.10%

V11 1.79% 1.04% 0.55% 2.13% 2.81%

V12 0.08% 0.04% 0.09% 0.11% 0.09%

V13 0.90% 1.36% 1.66% 1.43% 0.67%

V14 0.03% 0.08% 0.10% 0.06% 0.04%

V15 0.05% 0.15% 0.13% 0.08% 0.06%

V17 0.32% 1.01% 0.51% 0.60% 0.40%

V19 0.11% 0.51% 0.24% 0.16% 0.12%

V23 0.08% 0.21% 0.09% 0.13% 0.08%

V25 0.05% 0.13% 0.09% 0.06% 0.06%

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The measurements show relatively high, butnot unusual, levels of harmonics being produced

by the pulse width modulated (PWM) drives. Bycomparison, the harmonics in the capacitor bankand in the transformer have much higher thanexpected levels of 11

thand 13

thharmonics relative

to the harmonics injected into the system by thedrives. This suggests a harmonic resonancecondition. This phenomenon is explored further inSection IV, Harmonic Analysis.

B. Transient Measurement ResultsDuring the course of the measurements there

were only a few significant transients measured,none of which would be expected to causeproblems. The highest voltage transient was 1.74per unit. None of the transients with significantlyhigh voltage lasted for more than 50 sec.

The only voltage transients that hadcorresponding increases in current were somecapacitor switching transients. Recall that theobjective is to find the cause of the fuse operationsas well as the capacitor failures. Therefore currentis also of interest, not just voltage. One of thetransients recorded is shown in Figure 7 and isdiscussed in Section VI.

IV. Harmonic Analysis

IEEE Std 519-1992 [2] discusses the possibleeffects of harmonics on capacitors. Portions of

Section 6.5 of this document are presented below:

A major concern arising from the useof capacitors in a power system is thepossibility of system resonance. Thiseffect imposes voltages and currents thatare considerably higher than would bethe case without resonance.

The reactance of a capacitor bankdecreases with frequency, and the bank,

therefore, acts as a sink for higherharmonic currents. This effect increasesthe heating and dielectric stresses.

Table 3. Load Current Measurement Summary

QUANTITY FLATTENER DRIVE RECOILER DRIVE TRANSFORMER

I 76.15% 48.14% 8.90%THDThe result of the increased heating

and voltage stress brought about byharmonics is a shortened capacitor life.

I (A) 23.23 72.4 545.3RMS

I (A) 19.51 68.3 543.11

I 9.49% 6.75% 0.92%2I 4.88% 1.93% 2.04%3

Adding capacitors will cause the power systemto be tuned to a certain harmonic. This is knownas parallel resonance between the capacitors andthe source (including the transformer) inductance.

A parallel resonance presents a high impedance toinjected harmonics at or near the resonantfrequency. This should not be confused withseries resonance, which is utilized in harmonicfilters to present a low impedance to a certainfrequency to remove that frequency from thesystem.

I 4.44% 3.05% 0.53%4

I 56.31% 40.79% 3.72%5

I 3.23% 1.18% 0.36%6

I 33.49% 14.50% 1.09%7

I 6.73% 8.08% 5.09%11

I 5.02% 3.85% 3.57%13

I 4.40% 1.20% 1.27%17

I 3.07% 1.99% 0.66%19

I 1.75% 2.51% 0.45%23

I 1.77% 1.45% 0.32%25

If the parallel resonant frequency is close to

injected harmonic frequencies within the plant,voltages and currents at these frequencies will be

amplified. This is more likely when the capacitorbank is a switched bank with multiple steps sincethere are several possible resonant frequencies.Resonance can result in increased harmonicproblems and can lead to capacitor failures.

Calculations were performed to estimate theresonant frequencies of the power system withdifferent levels of capacitance on-line. Theresonant frequency of a system, at a transformersecondary, can be estimated with the followingformula. h is the tuned harmonic of the system,XCis the capacitive impedance of all capacitorsconnected to the secondary bus of the transformer,and XL is the inductive impedance of thetransformer (plus primary source inductiveimpedance, if available).

L

C

X

Xh =

The information for transformer #3 is asfollows: 1500 kVA, Z=5.6%, 13.2 kV-480Y/277 V.The short circuit MVA at the 13.2 kV level (primaryof the transformer) is 55 MVA with an X/R ratio of2.99.

The resonant frequency calculations yieldedthe results shown in Table 4.

Harmonic impedance scans are shown in

Figure 2. These scans show the impedance at arange of frequencies for three systemconfigurations. The first configuration is withoutany capacitors or filters connected to thetransformer secondary. The second configurationis with 150 kvar on-line, as was often the caseduring the measurements. The third configurationis with a 150 kvar capacitor bank replaced with a4.7th harmonic filter.

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The impedance scans are performed withoutplant loads connected to the system for a worstcase analysis. Connected loads tend to damp,and slightly alter, a systems impedance scans byrounding off (lowering), and possibly slightlymoving, the peaks in the plot. The purpose of theimpedance scans is to identify possible systemresonant frequencies. To allow these frequenciesto stand out more clearly, the analysis is performedwithout connecting the plant loads to the system.

A high impedance at a given frequency meansthat any harmonic currents injected into the systemat that frequency will cause greater voltagedistortion than injected currents of the samemagnitude at different frequencies. Harmonicresonance problems occur when harmoniccurrents are injected at frequencies with highimpedances.

Figure 3 shows the possible magnification ofharmonic frequencies due to the presence of a capacitor

bank or a filter bank relative to having neither. Theimpedances of the system with the capacitor bank andwith the filter were divided by the impedance of thesystem with neither. Again, without the presence ofresistive loads to provide damping, this is a worst caseanalysis.

The presence of the capacitor bank clearlyamplifies a range of harmonics. Characteristic

harmonics of six-pulse drives include the 5th th, 7 ,

11Table 4. Resonant Frequency Calculations

th th th th, 13 , 17 , 19 , et cetera, in decreasing

amounts. But during the on-site measurementsthe capacitors and the main 1500 kVA transformerwere carrying significantly more 11

CONNECTED CAPACITANCE(KVAR)

RESONANT FREQUENCY(HARMONIC NUMBER)

100 th and 13th

harmonic current than 5

13.5th

and 7th133 * 11.7 . This occurred

despite much higher 5th th

and 7 harmonic current

injections. This can be explained by the tuning ofthe system with the capacitor bank on-line. Thereis clearly some degree of harmonic resonance inthis system.

150 11.1

183 * 10.0

200 9.6

250 8.6

300 Except for a small range of frequencies (due tothe parallel resonance of the filter) the filter wouldtend to reduce the harmonic impedance relative tothe system without any capacitors. The filter wastuned below the lowest characteristic harmonicfrequency produced by the six-pulse drives toavoid amplifying any harmonic currents producedby the drives.

7.8

* Possible with one 16.7 kvar cell in Step 3 out of service

Figure 4 shows line-to-line voltage and totalcurrent into the capacitor bank with 150 kvar on-

line, recorded during the measurements. Thesewaveforms show what current and voltagewaveforms can look like in a resonant condition.Note that there are additional frequencies riding onthe 60 Hz waveforms, especially the currentwaveform.

With 150 kvar on-line, the calculationsestimate a resonance at approximately the 11.1

st

harmonic. Frequencies near this harmonic mayalso be amplified. The current waveform shows astrong 11

th thand 13 harmonic components

superimposed on the 60 Hz. The resonance canbe identified in the waveform by counting thenumber of peaks due to the resonant frequencythat occur within one 60 Hz cycle. This issomewhat less clear in this case because thereare both 11

th thand 13 harmonics, but one can

count 11 dominant peaks in one 60 Hz cycle.Figure 5 shows the harmonic spectrum

calculated for the current waveform in Figure 4. Itclearly shows the dominant 11

th thand 13

0.001

0.01

0.1

1

10

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Harmonic

Impedance(ohm

s)

With Filter

Without Capacitors

With 150 kvar Capacitors

0.01

0.1

1

10

100

1000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Harmonic

Magnification

With 150 kvar Capacitors

With Filter

Figure 2.Impedance Versus Harmonic Frequency Figure 3.Magnification Versus Harmonic Frequency

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V. Examination of Failed Equipment

harmonics despite the fact that the harmonic-

producing load is generating more 5th and 7thharmonic current.

A detailed harmonic analysis studying how aharmonic filter could reduce harmonic levels anddesigning such a filter was not performed due tosubsequent discoveries.

Although harmonics were not found to be thecause of the problems in the capacitor bank, thecapacitors were causing a harmonic resonancesituation. For this reason, or if harmonics becomemore of a problem in the future, it wasrecommended that if power factor correction wasneeded elsewhere in the plant where there werefewer harmonic-producing loads, it would be a

good idea to move this capacitor bank to that area.It should then be replaced by a bank configured asa harmonic filter.

Another possibility, not investigated in thisstudy, would be to de-tune the capacitor bank.This would not tune the bank to filter harmonics,but would tune it to avoid causing harmonicresonance. The addition of the de-tuning reactorswould also reduce the transient overvoltagesduring capacitor switching.

-700

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

Time (seconds)

Voltage(V

)andCurrent(A)

Voltage

Current

A. IntroductionIn cases like this, an analysis of the failed

equipment often yields valuable clues and thiscase was no exception. Fuses which had clearedwere x-rayed to determine the cause of their

operation. This x-ray was sent to the fusemanufacturer for examination. A failed capacitorcell was examined by the manufacturer.

B. Capacitor ExaminationThe capacitor manufacturer found that the

dielectric fluid in the failed capacitor was almostblack from carbon deposits. Carbon deposits arecaused by arcing which burns or breaks down thedielectric material.

Figure 4. Waveforms Showing Harmonic Resonance

The internal discharge (or bleed-off) resistors(required by the National Electric Code [3] todischarge capacitors rated 600 V and lower to 50V or less within one minute) were found to have

burned and disconnected connection tabs. It is notclear whether this was a cause or effect of thefailure.

0

25

50

75

100

125

150

175

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Harmonic

Current(A)

To check the discharge resistors in capacitorswhich had not failed, several of the goodcapacitors were disconnected from the systemafter they had been on-line. The voltages werethen monitored to see whether the capacitorsdischarged properly. In every case, the capacitorsdischarged properly indicating that the dischargeresistors were still connected and doing their job.

Several good capacitors were also removedfrom service in order to check their capacitance. Inall cases the capacitance was very close to the

expected value.

Figure 5. Harmonic Current Spectrum from Figure 4

The manufacturer suggested two possiblecauses for the failures: excessive harmonic currentdraw and overvoltage conditions due to anintermittent connection. Excessive harmoniccurrent could be due to motor drives or a resonantcondition. An intermittent connection can leave atrapped charge on the capacitor which can resultin more severe switching transients (higherovervoltages) when voltage is re-applied. This iswhy one should be careful when manuallyswitching capacitor banks. When a step ismanually switched off it should be left off for atleast one minute for it to discharge to 50 V or less.

This is discussed further in Section VII, CapacitorSwitching Transients.

C. Fuse BackgroundThe capacitor fuses in this case are current

limiting fuses. Using current limiting fuses toprotect capacitors is common at low voltages but isgenerally not done with medium or high voltagecapacitors (4160 V and higher) due to the cost.

Current limiting fuses can clear in two ways:overload and short circuit, in the words of fuse

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manufacturers. Consulting power engineers alsocall these two events overcurrent and impulseenergy(I

2t).

The National Electric Code [3] defines anoverload as follows:

Operation of equipment in excess of

normal, full-load rating, or of a conductorin excess of rated ampacity that, when itpersists for a sufficient length of time,would cause damage or dangerousoverheating. A fault, such as a shortcircuit or ground fault, is not an overload.

An overload is a current that is typicallybetween one and six times the normal currentlevel. [4] A fuse will operate, or clear, if theoverload is present for a certain length of timebased on its time-current characteristic (TCC). Ifthe overload is very short in duration, fuses aregenerally designed to ignore it. For example,

motor inrush and transformer energization arenormal system events which cause high currentsfor a brief time and should not cause a fuse tooperate.

A short circuit is an overcurrent whichexceeds the normal full-load current of a circuit bya factor many times (tens, hundreds, or thousands)greater. [4] Unlike an overload, a short circuit isoften caused by a fault.

The National Electric Code [3] defines acurrent-limiting overcurrent protective device asfollows:

a device that, when interruptingcurrents in its current-limiting range, willreduce the current flowing in the faultedcircuit to a magnitude substantially lessthan that obtainable in the same circuit ifthe device were replaced with a solidconductor having comparableimpedance.

Current limiting fuses are designed to limitpeak fault current magnitude and reduce fault timeduration for better equipment protection. [5] Theycan interrupt a short circuit current in less thanone-half cycle, before the current would havereached a natural current zero.

Current limiting fuse characteristics, when thecurrent is high enough for them to operate in acurrent limiting mode, are described by their I

2t

values. I2t is a value that is proportional to energy

(which would be I2Rt). Since the resistance, R, is

constant within the fuse, the performance of thefuse is expressed in terms of the I(current) and t

(time) variables. Often I2t is used interchangeably

with energy, as will be done in the rest of thispaper.

There are two types of energy values minimum melt I

2t and let-through I

2t. Minimum melt

I2t is an indication of the amount of energy

necessary to melt a fuses element. Let-through I2t

is an indication of the amount of energy a fuse willlet through to a fault before operating and clearinga current. [5]

The type of fuse used to protect the capacitorbank is a full range current limiting fuse. Thismeans that it has a TCC that allows it to operatefor overloads as well as operate in a currentlimiting mode for high short circuit currents. It hasseparate elements to perform each of thesefunctions.

Within the fuse there is an M spot which ismade of an alloy that is designed to melt and clearfor overloads but will not operate for short circuits.There are also several weak spots or weak

links that are designed melt and clear for shortcircuits but not for overloads.

If there is a problem with excessive harmonicscausing additional steady-state current, this wouldbe expected to cause the M spot to melt and clear.If there is a problem with short circuits the weakspots would be expected to melt and clear.

D. Fuse ExaminationAs mentioned earlier, fuses which had cleared

were x-rayed to determine the cause of theiroperation. This x-ray was sent to the fusemanufacturer for examination.

Figure 6 shows an x-ray of six of the fuses

which cleared. In none of the six fuses did the Mspot clear indicating that an overload was not toblame. In all of the six fuses one, two, or threeweak spots cleared. If there had been a shortcircuit or a fault in the capacitor bank, all four weakspots would have cleared.

The engineer with the fuse manufacturer whoanalyzed the x-rays stated:

Note how the M spots on the linksare not melted. This suggests that thecurrent was over 500% of the fusesrating. Now, not all of the weak spotsare opened. This suggests an overload,

not a short. Put the two together & youget something in the magnitude of 600%- 800%. The harmonics should only addto the heating effects, not be the mainconcern.

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Figure 6.X-Rays of Cleared Fuses

According to the manufacturer, the 100 Acurrent limiting fuses used to protect the capacitorbank had a minimum melt I

2t of 5,000 A

2sec and a

peak let-through I2t of 11,000 A

2sec. This means

that for a short circuit that had an I2t of 5,000

A2sec, the weak spots in the fuse would start to

melt and clear. All of the weak spots would not beexpected to clear, however. For a very high shortcircuit, all of the weak spots would be expected toclear.

Because, in all the fuses x-rayed, only one tothree of the four weak spots cleared, the I

2t of the

event which caused the fuses to operate wasexpected to be between 5,000 and 11,000 A

2sec.

Based on this information it was now clear thatit was transients which were causing the fuses toclear and, most likely, the capacitors to fail.Section VII, Capacitor Switching Transients,examines the cause of the transients and theunique situation that caused unexpectedly severe

transients to occur.

VI. Failure Analysis

A. Fuse AnalysisThe measurements showed that the rms

current in each of the fuses did not approach their100 A ratings. Recall that each set of 100 A fusesprotects a 50 kvar group of capacitors. The fullload current of each 50 kvar group is 60 A. The

fuse rating is 166% of the nominal full load current.When faster classes of fuses are used, they areoften sized even higher.

The fuse rating is selected to allow forcapacitor inrush currents (which can be muchhigher than full load) when each step is switched

in. This prevents the fuse from operating duringsuch normal system events.If harmonics were causing excessive heating

in the fuse the M spot should have clearedindicating a steady-state overload. This did notoccur. Although the capacitors are sinking a verysignificant amount of harmonics, the harmonicswere not the cause of the fuse operations.

If there were a fault within the capacitorcabinet, the current should be high enough to clearall of the weak spots in the fuse link. The availablethree-phase short circuit current at the 480 V busis 21.9 kA and the available line-to-ground shortcircuit current is 24.6 kA, both considering only

source and transformer impedance. Since all ofthe weak spots did not clear, a fault is not the likelycause of the fuse operations.

The approximate current which caused thefuse to operate was 600-800 A (600-800% of a100 A fuse) according to the manufacturer. Thiscurrent could be developed from a transient suchas a capacitor energization.

The problem is that the measurement dataalso did not contain any transient events which

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would be expected to cause the fuses to operate.In fact, during the measurements there were nofailures.

The transient waveform shown in Figure 7 is acapacitor energization when the 50 kvar Step 2was energized with the base 100 kvar already inservice. The steady-state currents before and

after the energization were approximately 124 Aand 180 A, respectively (60 A per 50 kvar group).The peak current in this event was -1480 A. Thiswas the largest peak current recorded during themeasurements.

The I2t associated with the 1480 A peak was

793 A2sec. Including the following positive peak

increases the I2t to 1058 A

2sec. These are both

well below the 5,000 A2sec fuse rating for the weak

spots to start to melt.This type of event is analyzed in greater depth

in Section VII of the paper, Capacitor SwitchingTransients. In Figure 7 it is also worth noting theresonance in the current waveform similar to that

in Figure 4.In summary, the measurement data did not

reveal why the fuses had cleared.

B. Capacitor AnalysisCapacitors must be built to tolerate voltages

and currents in excess of their ratings according tostandards. The applicable standard for powercapacitors is IEEE Std 18-1992, IEEE Standard forShunt Power Capacitors. [6] Additional informationis given in IEEE Std 1036-1992, IEEE Guide for

Application of Shunt Power Capacitors. [7]IEEE Std 18-1992 gives the following

allowable contingency continuous overload limits.

110% of rated rms voltage120% of rated peak voltage180% of rated rms current (nominal current

based on rated kvar and voltage)135% of rated reactive power

It should be noted that capacitors are oftenfused below 180% of rated rms current so the

180% limit is not usually approached.Short time overload voltages are specified in

IEEE Std 18-1992 and IEEE Std 1036-1992 andare given below. These standards state that acapacitor may be expected to see 300 suchovervoltages in its service life.

2.20 per unit rms voltage for 0.1 seconds(6 cycles of rms fundamental frequency)

2.00 per unit rms voltage for 0.25 seconds(15 cycles of rms fundamental frequency)

1.70 per unit rms voltage for 1 second1.40 per unit rms voltage for 15 seconds1.30 per unit rms voltage for 1 minute1.25 per unit rms voltage for 30 minutes

An older standard, IEEE Std 18-1980 alsoincluded the following permissible overvoltages.

3.00 per unit rms voltage for 0.0083seconds( cycle of rms fundamental frequency)

2.70 per unit rms voltage for 0.0167seconds(1 cycle of rms fundamental frequency)

None of these tolerances were exceededduring the measurements.

VII. Capacitor Switching Transients

A. OverviewA capacitor switching transient is a normal

system event that can occur whenever a capacitoris energized. This transient occurs because of thedifference between the system voltage and the

voltage on the capacitor. A basic characteristic ofcapacitors is that the voltage across them cannotchange instantaneously. If a capacitor is at zerovoltage and system voltage is applied to it, thesystem voltage will be pulled down to nearly zeromomentarily.

There will then be a capacitor inrush current asthe capacitor charges. The voltage on thecapacitor will then recover and overshoot thesystem voltage, and then oscillate around thesystem voltage. It is possible for this overvoltageto reach 2.0 per unit (twice the peak systemvoltage) if the capacitor is initially uncharged.System damping (resistance) usually keeps this

overvoltage below the theoretical peak.

HS Processing Pickle Line Capacitors, Phase C, Sep 25 1998 15:01:41.327

-1500

-1000

-500

0

500

1000

0.035 0.04 0.045 0.05 0.055

Time (sec)

Voltage(V)andCurrent

(A)

Current Voltage

The capacitor voltage will continue to oscillatearound the 60 Hz fundamental waveform, with theoscillation gradually getting damped out, usuallywithin a cycle. The magnitude of the transient andits characteristic oscillation frequency will dependon the characteristics of the electric power systemin question.

Figure 7. Measured Capacitor Energization Transient

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The magnitude of the transient will vary basedon two variables at the time of the switching.These variables are the initial voltage on thecapacitor (trapped charge, usually close to zero ifthe capacitor has been allowed to discharge) andthe instantaneous system voltage at the time of theswitching. The greater the difference between

these two voltages, the greater the magnitude ofthe transient. The worst case transient will occurwhen the system voltage is at peak voltage andthere is a trapped charge on the capacitor of peaksystem voltage at the opposite polarity.

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

0 45 90 135 180 225 270 315 360

60 Hz Degrees

Voltage(perunit)

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

C

urrent(A)

Voltage

Current

Recall that the National Electric Code requiresresistors to discharge capacitors rated 600 V andlower to 50 V or less within one minute. Thecontrol algorithm in the capacitor bank avoidsswitching in a step within one minute after it hasbeen disconnected. So in normal operation thereshould be very little trapped charge on thecapacitors when switching.

Figure 8. Capacitor Energization Transient (Sim.)No Prior Charge on Capacitor, I

2t=1,857 A

2sec

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

0 45 90 135 180 225 270 315 360

60 Hz Degrees

Voltage(perunit)

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

Current(A)

Voltage

Current

If the transient voltage is high enough the

capacitor could fail immediately. If not, thecumulative effects of the transient voltages(greater than peak system voltage) may stress thedielectric to the point of failure over time.

The transient currents will cause high I2t levels

which may cause nuisance fuse blowing if they arehigh enough.

B. Capacitor Energization SimulationsCapacitor energization simulations were

performed for two reasons. The event that causedthe capacitor failures and fuse operations did notoccur during the measurements and theexamination of the fuses indicated that transients

were the likely cause. The information from thesteel processing plant power system was used tosimulate some capacitor switching events underdifferent conditions.

Figure 9. Capacitor Energization Transient (Sim.)Prior Charge on Capacitor (-300 V), I

2t=5,661 A

2sec

C. Back-to-Back Capacitor SwitchingAnother type of capacitor switching transient is

called back-to-back switching. This is when asecond capacitor is switched on in close proximityto a previously energized capacitor. In this case afast transient occurs as the two capacitors sharetheir charge with each other and come to the samevoltage. Then there is another transient as the pairof capacitors cause the voltage to oscillate aroundthe 60 Hz fundamental voltage, as describedabove, as if they were a single capacitor bank.

Figure 8 shows the energization of a 50 kvarcapacitor step with no trapped charge and with noother capacitor steps in service. The energizationoccurred at peak system voltage. This transienthad an I

2 2t of 1,857 A sec.

Without any charge on the capacitors beingswitched into the circuit, the I

2t values are below

5,000 A2sec, the minimum melt I

2t value of the

fuses used to protect the capacitors. This is, ofcourse, an expected result. If this were not the

case, the fuses would operate regularly forcommon system events.

Figure 10 shows the energization of a 50 kvar

capacitor step with trapped charge and with 150kvar of other capacitor steps in service. Theenergization occurred at peak system voltage.This transient had an I

Figure 9 shows the energization of a 50 kvarcapacitor step with trapped charge and with noother capacitor steps in service. The energizationoccurred at peak system voltage. This transienthad an I

2t of 5,178 A

2sec. The time

scale for Figure 10 is greatly zoomed in from thatin Figures 8 and 9. This was done to better showthe higher frequency initial transient.2 2t of 5,661 A sec.

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D. Trapped ChargeIn both the simple capacitor energization and

the back-to-back switching, when some trappedcharge on the capacitors was assumed in themodel, I

2t values rose above the 5,000 A

2sec

which would cause the fuses to operate. In bothcases, the I

2t values did not exceed 11,000 A

2sec

which would be expected to cause all of the weakspots in the fuses to open. This was true even inthe worst case scenario with the system voltage atits peak and a trapped charge on the capacitor ofpeak system voltage of opposite polarity.

It was known that the fuses operated due to I2t

values between 5,000 and 11,000 A2sec based on

how many weak spots in the fuses had cleared.The analysis showed that capacitor switching

transients, with trapped charge on the capacitors,could cause I2t values in this range. The trappedcharge could have occurred in three ways:1) The discharge resistors failed or became

disconnected.2) The capacitors were switched manually before

they were allowed to discharge.3) The capacitor control unit switched the

contactors too quickly, before the capacitorshad adequate time to discharge their trappedcharge, leaving a trapped charge on thecapacitors at the time of reconnection.

E. Correlation with ObservationsAfter the various steps in the analysis, it was

believed that the failures that were occurring weredue to capacitor energization transients, mostlikely due to switching a bank with trapped charge.This had not yet been confirmed, however.

Plant personnel had reported that thecontactor for some of the 50 kvar steps in thecapacitor bank had been chattering occasionally,opening and closing very rapidly. This chatteringdid not occur at any time during the measurementsso it was not able to be detected at that time. The

plant electricians stated that the chattering wasmuch more common during periods of hightemperature, which was not the case during themeasurements.

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

75 90 105 120 135 150 165

60 Hz Degrees

Voltage(perunit)

-6000

-4000

-2000

0

2000

4000

6000

8000

10000

C

urrent(A)

Voltage

Capacitor #2 Current

The chattering contactors would be a source oftrapped charge on the capacitors. This wouldaccount for the transient overvoltages which

damaged the capacitors and the transientovercurrents which caused the fuses to operate.

Once it was determined that the energizationtransients were most likely due to the chatteringcontactors, the contactors were replaced. Theproblems persisted, leading to further examinationby plant electricians.

They reported that when variable Step 2 wasbrought on-line with variable Step 1 already on-line, the contactor for Step 1 would drop out andpick up approximately six to eight times within oneminute. This would expose the capacitor to manyswitching transients. These would occur beforethe Step 1 capacitors would have had a chance to

discharge. Some the re-energizations would bebound to occur when there was a large differencebetween the capacitor voltage (due to trappedcharge) and the system voltage. This would leadto transient voltages and currents similar to thoseshown in Figure 10.

Figure 10. Back-to-Back Capacitor Switching (Sim.)Prior Charge on Capacitor (-350 V), I

2t=5,178 A

2sec

The next step was to replace the control boardin the capacitor bank which monitored the powerfactor and determined which steps to bring on-line.Since a new board was ordered and installed therehave been no capacitor failures or fuse operationsin the bank confirming that the control board wasthe problem.

VIII. Summary

A steel processing plant was experiencingcapacitor failures and fuse operations in anautomatically switched, multiple step power factorcorrection capacitor bank. Initial impressions werethat the problems were due to harmonics. Thiswould not be unexpected in a system whereharmonic sources, such as adjustable speeddrives, are electrically close to power factorcorrection capacitors.

A preliminary assessment of harmonicresonant frequencies, in addition to the measureddata, indicated that there was a resonant condition.

The measured values were not high enough,however, to be expected to cause the fuseoperations or the capacitor failures.

Examination of the fuses which had clearedindicated that low level transients, not harmonics,had caused them to operate. The measurementsdid not reveal any transients which would havecaused equipment problems, but no problemsoccurred during the measurements so there werelikely no significant transients to measure.

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Simulations were performed to determinewhether capacitor switching transients would havebeen able to cause the failures. The results of thesimulations indicated that the capacitor switchingtransients could generate high enough I

2t levels to

cause the fuses to operate. This was only true ifthere were high levels of trapped charge on the

capacitor step being switched in and the systemvoltage was near its peak at the time of theswitching.

With high levels of trapped charge duringswitching, the capacitor voltages can also reachwell over 2.0 per unit. These levels might notcause capacitors to fail immediately but couldcause cumulative degradation of the capacitordielectric, eventually leading to failure.

Even with worst case conditions, these I2t

levels would not reach the peak let-through I2t of

the fuses. The results of the simulations aretherefore consistent with the fact that not all theweak spots in any of the fuses had cleared.

With this information, the plant electriciansreplaced the contactors that were believed tochatter occasionally. When the problemspersisted, the electricians observed the operationof the capacitors and eventually replaced thecontrol board in the capacitor bank. Since thattime there have been no capacitor failures or fuseoperations in the bank.

References

[1] Electrical Transients in Power Systems,Second Edition, Allan Greenwood, JohnWiley & Sons, Inc. 1991.

[2] IEEE Std 519-1992, IEEE RecommendedPractices and Requirements for HarmonicControl in Electric Power Systems, Instituteof Electrical and Electronics Engineers, Inc.1993.

[3] NFPA 70, National Electric Code, 1999Edition, National Fire Protection Association,Inc. 1998.

[4] SPD Electrical Protection Handbook Selecting Protective Devices Based On TheNational Electric Code, Bussmann, CooperIndustries 1992

[5] Distribution System OverCurrent ProtectionWorkshop Course Notes, Cooper Power

Systems, Inc. 1996.[6] IEEE Std 18-1992, IEEE Standard for Shunt

Power Capacitors, Institute of Electrical andElectronics Engineers, Inc. 1993.

[7] IEEE Std 1036-1992, IEEE Guide forApplication of Shunt Power Capacitors, Institute of Electrical and ElectronicsEngineers, Inc. 1993.

Authors Biography

Thomas M. Blooming, P.E. is a Senior ProductApplication Engineer for the Power Quality Divisionof Eaton Electrical. Tom received a B.S. inElectrical Engineering from Marquette University,an M.Eng. in Electric Power Engineering from

Rensselaer Polytechnic Institute, and an M.B.A.from Keller Graduate School of Management.Tom works in the Power Factor Correction Groupof Eaton Electrical (Power Quality Division). Hehandles application issues related to power factorcorrection capacitor banks, harmonic filters, static-switched capacitor banks, and active harmonicfilters, as well as many power quality-relatedquestions. Tom formerly worked in the Cutler-Hammer Engineering Services & Systems(CHESS) group and provided clients with electricpower engineering expertise, focusing in the areasof power quality and reliability. Tom hasperformed numerous measurements and studies.

In addition, he has published technical papers andtaught engineering workshops and trainingseminars on power quality issues.