impact of utility switched capacitor on customer systems -part 2 - adjustable speed drive concerns

7/30/2019 Impact of Utility Switched Capacitor on Customer Systems -Part 2 - Adjustable Speed Drive Concerns

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1623EEE Tmwmions on Power Delivery, Vol. 6,NO. 4, October 1991IMPACT OF UTILITY SWITCHE D CAPACITORSON CUSTOMER SYSTEMS

PART 11 - ADJUSTABLE-SPEED DRIVE CONCERNSM. F. McGranaghan T. E. Grebe, PE G. Hensley T. Singh M. SamotyjSenior Member, IEEE

Electric Power Research InstituteMembers, IEEE Members, IEEE

Electrotek Concepts, Inc Pacific Gas and Electric CompanyKnoxville, Tennessee San Ramon, California Palo Alto, California

Abstract - This paper describes a concern for nuisancetripping of adjustable-speed drives caused by capacitorswitching on the utility distribution system. Thecharacteristics of the drive system, the customer electricalsystem, and the utility distribution system that affect thisphenomena are analyzed using sensitivity analysissimulations. Possible solutions to the nuisance trippingproblem are alsopresented.Keywords: power quality, capacitor switching,adjustable-speed drive, transient voltage magnification

I"Adjustable-speed drives (ASDs) are being applied in

increasing numbers due to the improved efficiencies andflexibility that can be achieved. Smaller drives (less than50 HP) using pulse width modulation (PWM) invertertechnology are becoming particularly popular because theycan be applied to virtually any induction motor and aresimple to apply. Despite the many advantages provided byASDs, there are still a number of concerns associated withtheir application. These concerns include harmonicdistortion levels, component failures due to transientvoltages, nuisance tripping, motor overheating, and audiblenoise.

91 WM 086-9 PWRDthe IEEE Transmission and Distribution Committee ofthe IEEE Power Engineering Society for presentationat the IEEE/PES 1991 Winter Meeting, New York, NewYork, February 3-7, 1991. Manuscript submittedSeptember 4, 1990; made available for printingJanuary 22, 1991.

A paper recommended and approved by

One concern that has become quite widespread isnuisance tripping during capacitor switching events on theutility distribution system. Capacitor energizing operationscause a transient oscillation on the primary distributionsystem [1,2]. This transient does not usually causeproblems on the distribution system unless there are specialconditions that result in local resonances [3]. However, acompanion paper [4] and previous publications [5 ]describes how the transient oscillation can be magnified atcustomer buses where low voltage power factor correctioncapacitors have been added. Even when customers do nothave low voltage capacitors, ASDs that use voltage sourceinverters (such as PWM inverters) can experience nuisancetripping because of the large dc capacitors used to maintaina constant voltage for the inverter.

The problem of nuisance tripping due to utilirycapacitor switching has been recognized [6] but there hasbeen no effort to completely characterize the phenomena.This paper describes the basic phenomena and then presentsthe results of sensitivity analysis simulations performed toexplore the various parameters which can affect theproblem. The important parameters include the sourcestrength at the switched capacitor, the switched capacitorbank size, the customer step down transformer size, thedrive characteristics, and system loading.

The simplest method to control the nuisance tripping isthe addition of inductance in series with the individualdrives. The effect of the series inductance on the dctransient voltage during capacitor switching is illustratedand conclusions related to the choke size requirements toavoid problems are presented. The measurements an dsimulations performed to characterize this phenomena limebeen performed under Electric Power Research Institute(EPRI) contractRP2935-91and RP 2935-13 [8].

CTERISTJCSAn AS D system consists of three basic componcnts and

a control system as illustrated in Figure 1. The rectifierconverts the three phase ac input to a dc voltage.Depending on the type of system, a reactor, a capacitor, or

0885-8977D1$01 ooO 991 IEEE


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a combination of these is used to smooth the dc signal. Theinverter circuit uses the dc signal to create a variablefrequency ac signal to control the speed of the ac motor. Adc motor drive differs from this ASD configuration in thatthe rectifier is used to control the motor directly.

DC Llnk

Figure 1 - ASD ComponentsThere are a wide variety of specific configurations for

the power electronics of an ASD. However, for most of thepower quality considerations,ASDs can be divided into twobasic types:

1. Voltage Source Inverter (VSI) Drives. Thesedrives use a large capacitor in the dc link to providea relatively constant voltage to the inverter. Theinverter then chops up this dc voltage to provide thevariable frequency ac voltage for the motor. Thesecan be off the shelf drives and most commonlyemploy pulse width modulation (PWM) techniquesto improve the quality of the output voltagewaveform. These are by far the most common typesof drives up to at least 100 HP.

2. Current Source Inverter (CSI) Drives. Thesedrives are typically used for larger HP pplicationswhere custom designs can be justified. The dc linkconsists of a large choke to keep the dc currentrelatively constant. The inverter then chops up thiscurrent waveform to provide the variable frequencyac signal for the motor.

An important characteristic of the ASDs is theirtransient voltage withstand capabilities. Many ASDs arebuilt around power semiconductor switches that have apeak inverse voltage (PIV) rating of only 1200 volts. On a480 Volt distribution system, this PIV rating equates to177% of normal system voltage. Most powersemiconductor switch assemblies are equipped with on-board metal oxide varistors (MOVs) for protection. Whilethe MOVs are effective for many low energy transients,they can be destroyed by magnified capacitor switchingtransients [4].

The sensitivity of the drives to transient disturbancescan also be related to the drive topology and the control

system characteristics. VSI type drives require smoothingof the dc link voltage with a large capacitor for properoperation. For protection of the dc capacitor and theinverter components, the dc bus voltage is monitored andthe drive is tripped when it exceeds a preset level. In manycases this level is around 760 Volts (for 480 Voltapplications), which is only 117% of the nominal dcvoltage.

The drive controls can also be sensitive to momentaryinterruptions or voltage sags on the input voltage. Thischaracteristic is very dependent on the specific controlsinvolved but it is not uncommon for voltage sags lastingonly a few cycles to cause drives to trip.

NT O V E R V O L U G E CON-Energizing a shunt capacitor from a predominantly

inductive source creates an oscillatory transient voltage thatcan approach two times the normal peak voltage. Thisenergizing transient (Figure 2) is important because it canexcite an LC circuit, resulting in magnified transientvoltages at remote locations. When customers apply lowvoltage capacitors for power factor correction, significantlyhigher transient voltage magnitudes can occur at the lowvoltage bus. This phenomena and the concern for botharrester duties and component ratings was described in acompanion paper [4].

CapacitorBusI I I

- 2 t """Figure 2 - Capacitor Energizing Transient

ASDs that have large capacitors in the dc link tosupply voltage source inverters are particularly sensitive tothese capacitor switching transients. There are two reasonsfor this sensitivity:

1. The dc capacitors form part of an LC circuit (withthe inductance between the drive and the switched

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capacitor) that can be excited by the capacitorswitching transient. The result is a significantcurrent surge into the dc capacitor, increasing thevoltage on the dc link.

2. The drive controls are very sensitive to overvoltageson the dc link. In order to protect the dc capacitorand the inverter components, the controls are usuallyset to trip whenever the dc link voltage exceedsapproximately 1.2 times the normal dc voltage.

The result is that small ASDs often trip when utilitiesswitch capacitorson the primary distribution system.

The general circuit illustrated in Figure 3was used toillustrate this concern and to evaluate the variousparameters which affect the dc overvoltage that occursduring capacitor switching. The base conditions for thisanalysis are as follows:

System Source Strength at the Substation=200 MVASwitched Capacitor Bank Size=3 MVArTotal Feeder Load =5 M WCustomer Transformer Size=1500kVA (6%Customer Power Factor Correction=0 kVArCustomer Resistive Load =200 kWdc Capacitor Size=400ASD Choke Size=0.5 mH

Impedance)

Distribution Feeder

T Capacitor ~ 4 n kT& Capacitor

~ ~ ~ ~ ~

Figure 3 - One Line Diagram for Example System

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This is because the transient current charges up the dc bus(Figure 4c) and the diodes in the rectifier front end cannotconduct again until the dc voltage decays.

acLineCumnt10050

n2 5 . 0

3 looF-l 0

0.02 a04 o m o.oe aio 0.12nme (s)Figure 4a - ASD Input Current

dc Bus Current260

150n

3 %5 0

0.02 0.04 0.08 0.08 ai0 0.12Time (s)Figure 4b - ASD dc Output Current

dc BusVQttqpI

0.02 0.04 0.08 0.08 a10 0.12I Time (s)The effect of varying these parameters on the resulting

transient voltages and currents are analyzed using theElectro-Magnetic Transients Program (EMTP). The circuitin Figure 3 is the basis for the EMTP model. Figure 4provides typical waveforms for the base case conditionswhich illustrate the concern for the transient overvoltage onthe dc link. Figure 4a illustrates the transient current on theac side of the drive and Figure 4b shows the correspondingcurrent on the dc side. Note that the transient lasts for onlyone half cycle of the oscillation frequency (300-800 Hz).

Figure 4c - ASD dc Link Voltage-he waveforms in Figure 4 illustrate the concern fortransient overvoltage on the dc link. Typical drives appliedon 480 Volt systems would have a dc overvoltage trip

setting of approximately 760 Volts. The dc voltage in


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Figure 4 reaches approximately 900 Volts, which wouldcause the drive to trip. This can be a very serious problemfor many customers where the drives are applied in criticalprocesses. The following sections illustrate the effect ofthe most important system parameters on this transientvoltage.

The waveforms in Figure 4 were obtained by closingall three phases of the capacitor switching devicesimultaneously at the peak of the phase A voltage. Thisresults in a maximum voltage on Phase A of thedistribution system but is not necessarily the worst case forthe transient that occurs at the customer bus. Figure 5illustrates a distribution of transient overvoltages on the dclink for randomly varied closing instants of the capacitorswitching device.

Distribution of dc Tmnsient omvdtpges

Figure 5 - Distribution of dc Transient OvervoltagesThe distribution in Figure 5 illustrates that the three

phase simultaneous closing case results in close to theworst case transient on the dc link. Therefore, this baseclosing sequence will be used to analyze the otherimportant parameters. The other important observationfrom Figure 5 is that approximately 65% of the capacitorclosing cases (assuming random switch characteristics,which may not be valid for some switching devices) resultin a transient on the dc link which could cause tripping ofthe drive.

The switched capacitor size on the distribution systemdetermines the frequency of oscillation (usually in therange 300-800 Hz) and also the energy available to exciteLC circuits that include the low voltage buses. Figure 6illustrates the effect of the switched capacitor size on theresulting dc link transient voltage magnitude. For theseconditions, any capacitor 1200 kVAr or larger is likely tocause tripping of the drive,

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Figure 6 - Effect ofSwitched Capacitor Size on deOvervoltage

dc CThe size of the capacitor used in the drive's dc link is

also very important. As the size of the dc link capacitor isincreased, the energy required to charge it up increasesaccordingly. The result is shorter conduction times duringnormal operations and lower transient voltages during thecapacitor switching events being studied here.

Figure 7 illustrates the effect of the dc link capacitorsize for two different switched capacitor sizes. For the 3MVAr capacitor used in the base case analysis, there is aresonance associated with a dc capacitor size o lapproximately 100@ (much smaller than actual capacitorsizes for most drives). With the 3 M V A r switchedcapacitor, drive tripping can be expected for dc capacitorsizes less than approximately 1000 pF. With the smaller1200 kVAr switched capacitor, the drive is only likely totrip for capacitor sizes less than approximately300 pF.

1200Wk Ritfhedb n k Typic01 TripVow

Figure 7- Effect of dc Capacitor Size on dc OvervoltugeUnfortunately, the dc capacitor size is generally not

published in the manufacturers literature for ASDs. Thesize can be determined by physically inspecting the driveand noting the sizes of the large capacitors used in the dc

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link. Typical sizes for small drives (less than 25 I-IP) are inthe range400- lo00 pF.

The discussion so far has been for the case without lowvoltage power factor correction capacitors. A companionpaper [4] describes the transient voltage magnification thatcan occur when a customer has low voltage capacitors.Since the ASD dc link capacitor is effectively connectedphase-to-phaseon the low voltage bus at any given momentin time (through the diode bridge), the transientmagnification experienced on the low voltage bus will alsobe experienced on the dc link. A case with a 300 kVArcapacitor bank on the 480 Volt bus was used to illustratethis concern.

Table I compares the transient voltages with andwithout the low voltage power factor correction. Note theincrease in the 48 0 Volt transient and the dc link transientwhen the power factor correction is added. The effect oflow voltage capacitors must be considered whendeveloping a solution to the drive tripping problem. Themost common solution involves the addition of a choke orisolation transformer in series with the drive (see nextsection). Table I shows that a choke size that could solvethe problem for the base case conditions could beineffective if low voltage power factor correctioncapacitors are added.

TableIEffectof Low Voltage Power Factor Correction Capacitorson the dc Overvoltage48 0 Volt Choke 480Volt dcBusCompensation Size Bus (pu) (Volts)

None 0.5 mH 1.55 907None 1.42 mH 1.52 780

300 kVAr 0.5 mH 2.28 1116300 kVAr 1.42 mH 2.27 897

300 kVAr filter(5th) 1.42m H 1.40 73 2

200 kVAr filter(5th)+100 kVAr 1.42 mH 2.00 806

One possible solution to avoid the magnificationproblem involves applying the low voltage power factorcorrection capacitors as harmonic filters (capacitors inseries with tuning reactors) rather than just capacitors [4].

This detunes the circuit and prevents the magnification.Table I shows the effectiveness of a 300 kVAr filter ascompared to just capacitors. It is important to note that thissolution is generally not effective unless all of thccompensation is applied as harmonic filters. If part of thecompensation includes capacitors without tuning reactors,the magnification problem can still exist. This is illustratedby the last case in Table I.

TIONS TO T W R I V E TBUJPINGSolutions to the drive tripping problem can be applied

at one of three different levels:1. The problem can be solved by controlling the

capacitor switching transienton the utilitydistribution system. The parametric analysisillustrates that this can be accomplished by limitingthe capacitor sizes on the distribution system. Thecapacitor switching transient can also be controlledusing a synchronous closing control on a vacuumbreaker [7] or with closing resistors in the switchingdevice[4].Synchronous closing control has beenproven efficient for large substation banks andtransmission system capacitors. These solutionshave not typically been employed for feedercapacitors.

2. The problem can be solved by adding isolatinginductance in series with the individual drives. Thisinductance can be in the form of a simple ac chokeor an isolation transformer. The sizes are usuallyspecified in 96 on a kVA or HP base. Figurc 8illustrates the effect of choke size on the dc linktransient for two different switched capacitor sizes.The base case included a choke of approximately0.8%on a 10 HP base (0.5 mH). Without thischoke, the dc link transients could be even higherthan the values given. As the choke size isincreased, the resulting dc link transient is reducedsubstantially because the dc capacitor becomeseffectively isolated from the ac bus at the transientfrequencies.A choke size of 3% on the drive EIPbase is usually sufficient to avoid tripping problems.The only cases where a larger size choke would berequired would be for very large switched capacitorsor if there is a magnification problem associatedwith low voltage power factor correction capacitors.


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I C O J : : : : : : : : : : i0 1 2 3 4 5 t 7 8 9 1D 11

Ch&a U. b nIO VAU

Figure 8 - Effect of Choke Size on the dc Overvoltage

3. The problem can be solved in the design of thedrive. This can be accomplished by rating thecomponents to withstand the dc link transientvoltages. The capacitor and the inverter componentswould have to be selected accordingly and the dcovervoltage used to trip the drive would have to beraised substantially. The problem can also beavoided by better design of the dc link. Largercapacitors are less likely to have the trippingproblem. Alternatively, the design can include asignificant choke in the dc link which wouldeliminate the need for a choke on the ac side. Sincethese switching transients are considered normal onthe distribution system, the best long term solution isto design the drives to handle these withoutinterruption.

4. It is worth noting that MOV arresters are oftenemployed to solve transient problemswith sensitiveelectronic loads. However, MOV protective levelsare in the range 1.6-2.0pu. As a result, they cannotprevent nuisance tripping due to small increases inthe dc voltage (i.e. 117%).

CONCLUSIONSCapacitor energizing operations on the utility

distribution system can cause nuisance tripping of ASDswithin customer facilities. Small drives using voltagesource inverters are susceptible to this tripping problembecause of the capacitors used in the dc link between therectifier and inverter.

The potential for nuisance tripping is dependentprimarily on the switched capacitor size, the dc linkcapacitor size, and the inductance between the twocapacitors. For normal conditions, the nuisance Yippingcan be prevented by including a choke or isolationtransformer in series with the individual drives. A size of

3% on the drive HP base will generally be sufficient toavoid problems.

The tripping problem can be made worse by theaddition of low voltage power factor correction capacitors.These capacitors can cause magnified transients on the lowvoltage bus which can result in component failures withinthe drive and higher dc link transient voltages. To avoidthese problems, the low voltage power factor correction callbe applied as harmonic filters instead of just shuntcapacitors.

REFERENCES[11 H. M. Pflanz and G. N. Lester, "Control ofOvervoltages on Energizing CapacitorBanks,'' EE ETransactions PAS , Vol. 92 , No. 3, pp 907-915,

May/June, 1973.[2] E. W. Boehne and S. S . Low, "Shunt CapacitorEnergization with Vacuum Interrupters - A Possible

Source of Overvoltages," EEE Transactions PAS ,Vol. 88, No. 9 , pp 1424-1443, September, 1969.

[3] C. G Troedsson,E. F. Gramlich,R. F. Gustin, and M.F. McGranaghan, "Magnification of Switching Surgesas a result of Capacitor Switchingon a 34.5kVDistribution System,"Proceedings of the AmericanPower Conference, 1983.

[4] G. Hensley,T. Singh, M. Samotyj, M. McGranaghan,andR. Zavadil, "Impactof Utility SwitchedCapacitors on Customer Systems, Part I -Magnification at Low Voltage Capacitors," Submittedfor the 1991 IEEE-PES Winter Power Meeting.

[5] A. J. Schultz, I.B. Johnson, and N. R. Schultz,"Magnification of Switching Surges," EE ETransactions on Power Apparatus and Systems, Vol77,February 1959, pp 1418-1425

[6] V. E. Wagner, J. P. Staniak, andT. L. Orloff, "UtilityCapacitor Switching and AdjustableSpeedDrives,"Proceedings of the Industrial and Commercial PowerSystems Conference, Detroit, MI, 1990.[7] R. W. Alexander, "Synchronous Closing Control forShunt Capacitors,''IEEE Transactions PAS, Vol. 104,No. 9, pp 2619-2626,September, 1985.[SI "Power Quality Considerations for ASD Applications",

EPRI CU.3036,1991

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impact of utility switched capacitor on customer systems -part 2 - adjustable speed drive concerns

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