industrial power considerations for vsd.pdf

7/27/2019 INDUSTRIAL POWER CONSIDERATIONS FOR VSD.pdf

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INDUSTRIAL POWERCONSIDERATIONS FOR VARIABLESPEED DRIVES (VSD s)

ABSTRACTJames M. oelscher

Electronic variable speed drives (VSD's)are the preferred choice for industrial ACmotor speed control. Offering manyadvantages over their mechanicalcounterparts, the VSD's can provideimproved efficiency and flexibility,enhanced product quality, a higher degreeof accuracy and computer compatibility.However, they are more sensitive to powerline disturbances such as transients,harmonics, voltage variations andmomentary interruptions. These concernsmust be addressed at the time a VSDinstallation is designed and installed inorder to avoid future problems with boththe VSD and other plant electronicequipment.I. INTRODUCTIONIn the U.S. t is customary for most largeindustrial facilities to be supplied with athree phase source of electrical power at afrequency of 60 hz. The most commonuser voltage is 460VAC and matches thenameplates of most AC induction motors.AC induction motors being the mostcommon type of load. Normally, plantdistribution systems are either the fourwire wye with ground or the groundedand ungrounded delta .Each of these types of power systems aresusceptible to line disturbances which maybe introduced via the utility's transmissionlines or generated internally within theuser's own facility. Most power linedisturbances are voltage rather thanfrequency related. Several of these voltagedisturbances are explained here.Undervoltage and overvoltage - RMSvoltage fluctuations that exceed allowablelimits for more than 2.5 seconds.

Undervoltages are also commonly referredto as brownouts and most are intentionalby the utility to extend system capacityduring heavy load conditions.Overvoltages are generally caused by poorline regulation during light loads.Surges and sags - These are special shortterm cases of undervoltage and overvoltageconditions generally exceeding allowablelimits for some significant portion of acycle. These voltage fluctuations areusually of larger amplitude and of ashorter duration than undervoltages andovervoltages.Transient impulses - These are deviationsfrom the ideal A C sine wave of very shortduration and typically last from a fractionof a microsecond to a few milliseconds.The amplitude of the transients is such thatthey greatly increase or decrease theinstantaneous voltage. Increases are alsoreferred to as voltage spikes anddecreases are referred to as notches .Dropouts and line interruptions - Voltagedecreases down to zero volts are calleddropouts and usually last only a portionof a cycle. These are generally caused byutility breakers opening and closing in thepresence of a fault condition. When thepower system is removed for a moresignificant period of time, these lineinterruptions usually represent a seriousutiliiy problem.Frequency variations - Defined asdeviations of or .5 Hz or less of inputline frequency (This is seldom a problemin the U.S.)Presently acceptable limits vary from stateto state and no national standards arepresently set. Consult your utilitycompany for data and an analysis of your

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power system.11. LINE TRANSIENTSTransients are variations from the normalAC sine wave having very short durationas compared with one cycle typicallyfrom nanoseconds to milliseconds. Thetransients range from single pulses withsharp rise times and progressive decay tooscillatory disturbances lasting for five toten cycles within a gradually decayingwindow. They may either increase ordecrease the instantaneous magnitude ofthe AC sine wave and are defined as aspike or notch , respectively.Line transients can range in magnitudefrom a fraction of peak voltage up tohundreds or thousands of volts and by farcause the largest voltage swings of anytypes of line disturbances. Note fromFigure 1 that transient magnitude ismeasured from the point it occurs on thesine wave and not from zero voltage.Because of this, the proper way to examinea transient disturbance is to block out thesine wave so that the transient appears byitself. It is also important to know thesource and direction of a transient. Was itgenerated in the VSD or arrive via thepower line? Either is possible, and, ofcourse, the solutions to the two problemsare very different.There are two types of transients: commonmode transients where the voltages toground of the AC power line phases riseand fall together, and normal-modetransients where the phase-to-groundvoltages vary from phase to phase.Common mode transients are caused bysuch things as lightning or utility breakertripping and closure and usually result insingle spikes or notches on the AC sinewave. In contrast, normal mode transients

are typically the result of connecting ordisconnecting heavy loads or power factorcorrection capacitors to the line. Thesetransients exhibit decaying oscillatorycharacteristics at frequencies up to andabove 5 KHZ. Both common mode andnormal mode transients are shown inFigure 1. It is important to know the typeof transient since the methods ofsuppressing them are different.

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Figure 1. Transient Disturbance111. TRANSIENT OVERVOLTAGECONCERNS ASSOCIATED

W I T H C A P A C I T O RSWITCHINGTransient overvoltages 8 are always aconcern when power factor correctioncapacitor switching is involved. Each timea capacitor is energized, a transientoscillation occurs between the capacitorand the system inductance. The result,(see Figure 2), is a transient overvoltagewhich can be as high as 2.0 per unit (ofthe normal voltage) at the capacitorlocation. The magnitude is usually lessthan 2.0 per unit due to damping providedby system loads and losses. The transientover voltage caused by energizing

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capacitors is generally not a concern todrive manufacturers because they areusually below the level at which surgeprotective devices operate (1.5 2.0 perunit). A much more severe situationoccurs when there are larger capacitorsswitched on the power line and lowervoltage capacitors have been added withinthe customers facility. The simplifiedcircuit of concern is shown in Figure 3.4

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CUSTOMERTRANSFORMER

PSUBSTATIONE C l T O RCAPACITORS LOAD(V5o.r)

Figure 3. AC circuit for problems witht r a n s i e n t v o l t a g emagnification.

Figure 2. Capacitor bank energizingtransient.When the frequency of a transientovervoltage matches the series-resonantfrequency of the customers transformercoupled with the customers capacitor(s), alow impedance, high current conditionresults. As this high current passesthrough the transformer, it induces a largevoltage potential that crosses through zerovoltage to create a large voltage ofopposite sign at the resonant frequency.The VSD and the customers paralleledcapacitor(s) (and their surge protectiondevices) then see this magnified voltage(compared to distribution feeder voltage).When the resonant frequency currentcompletes its path to ground through thecapacitor, the voltage experiences aboost in respect to the ground referencevoltage.

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The magnification of capacitor switchingtransients is most severe when thefollowing conditions exist:The capacitor switched on the highervoltage system is much larger (KVAR)than the capacitor at low voltage bus.

- The frequency of oscillation fl.)which occurs when the high voltagecapacitor that is energized is close tothe resonant frequency a)ormed bythe step-down transformer in serieswith the low voltage capacitor fl =f2

There is little damping provided byloads on the low voltage system, as isusually the case for industrial plants.The transients that occur at the low voltagecapacitors are normally in the range of 2.0to 4.0 per unit. These transients are likelyto cause the failure of protective devicesand electronic components (SCRs, powerdiodes, power darlington transistors).VSDs are specifically vulnerable to thesetransients because of the relatively low



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peak inverse voltage (PIV) ratings of thesemiconductor devices and the low-energyof the metal-oxide varistors (MOVs) usedto protect the VSD power devices.These magnified transient overvoltages canbe controlled in a number of ways:

The capacitor switching transient canbe controlled by using vacuumswitches with synchronous closingcontrol to energize the capacitor bank.Provide high energy MOVprotection on the 480V buses. Theenergy capability of these arrestersshould be at least several K-Joules.

- Use tuned filters for power factorcorrection instead of using only shuntcapacitor banks. The tuned filterschange the response of the circuit andusually prevent magnification frombeing a problem.Something that cannot be overlooked is thevariable speed drives (VSDs) capability towithstand voltage transients. Thesemiconductor devices used on the powerside of the VSD are normally only ratedfor a PIV of 1200 to 1400 volts. On atypical power distribution system this PIVrating figures to approximately 170 to250 of normal system voltage. Toprotect against line transients, most drivemanufacturers will incorporate on-boardMOVs for protection. MOVs aretypically more effective for manylow-energy transients, but when faced witha magnified voltage transient (i.e. powerfactor capacitor switching) they can becompletely destroyed. While the drivesusceptibility or sensitivity to transientdisturbances is a function of thesemiconductor device rating, it may alsobe related to the drive topology. Drives,

in the 1 to lo00 HP ranges wt eithervoltage source inverter VSl) or pulsewidth modulated (PWM) outputs typicallyrequire a smoothing of tbc DC busvoltage, with a large capacitor bank forproper operation Sec Figme 4).

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Figure 4 . Simplified diagram of therectifier circuit showing theDC smoothing capaator stage.For protection of the VSD components heDC bus voltage is monitored and the driveis tripped when it exceeds a preset level.In most cases, the level is approximately158 to 172% of a 480 volt input linevoltage. Since the capacitor bank isessentially connected across each of thethree phases on the input line, drives ofthis type can be extremely sensitive toovervoltages on the A C power input side.A current transient, see Figure 5),resulting from the overvoltage is conductedthrough the rectifier to the DC bus,sharply raising the DC voltage SeeFigure6) above the operational level and trippingthe VSD. This type of nuisance tripping isnormally associated with power factorcapacitor switching, since it is a verycommon source of overvoltages on thepower distribution system.

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Figure 5 . Current surge in VSD as aresult of capacitor energizing.

0.04 0 05 0.06 0.07 0.08 0.09 0.10Time (Seconds)

Figure 6. DC bus voltage duringcapacitor energizing.IV. RE DUC ING NUIS ANCET R I P P I N G D U E T OCAPACITOR SWITCHINGTRANSIENTSThe best or most efficient way to eradicatenuisance tripping of VSDs is to isolatethem from the power line with series linereactors or line isolation transformers.

When a 3-phase line reactor is used inconjunction (in series with the AC inputline) with an AC VSD, the magnitude ofthe input current surge is limited, and thecapacitor bank charges more slowly. Ifthe line reactor impedance is properlyselected, the capacitor voltage will notreach trip levels within the surge timelength.The required line reactor size is a functionof surge magnitude and duration,distribution system impedance, drive triplevel, and specific drive design.Experience indicates that line reactors of1.5% or 3 reactance will eliminate mostovervoltage trips. Note,however, that linereactors are much less effective on ACdrives that do not contain large capacitorson the DC bus.The application of line reactors issuggested under the following conditions:

The VSD periodically trips with anovervoltage fault indication and thenormal measured line voltage is withinthe drive rating. (Most AC drives willindicate the cause for shutdown.)

V.

VSD shutdowns occur at about thesame time of day, usually earlymorning or late afternoon. (These arethe times when capacitors on thepower line are usually switched inresponse to changing load conditions.)The VSD has periodically faultedwith damage to the input diodes.

THE BENEFITS OF LINEIMPEDANCEWhen many VSDs are powered off of thesame AC lines, a certain degree of crosstalk is injected by the VSDs themselves.

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This reduces the probability of trouble freeoperation. This interference is in the formof spikes', notches , or harmonicssuperimposed on the power line waveform.When DC VSD s are involved, transientsproduced by the switching of the powerdevices are usually absorbed by a snubbercircuit designed into the VSD. In mostDC VSD s, this consists of a capacitor andresistor connected in series across theswitching device. To successfully suppressthese transients, each snubber circuit relieson a certain amount of inductance offeredby the power wiring, providing a tunedL-R-C filter. It is the inductance thatchiefly limits the rate of rise of voltagethat is impressed across the device.With many DC VSD s on the same ACpower lines, insufficient inductance isoffered by the power wiring to limit therate of change of voltage, dv/dt. If thisdvldt value is high enough to exceed thespecification's of the SCR, the result is themisfiring of the device and a resultantnuisance trip, if not the loss of fuses, ordamage to the SCR s. This calls foradditional inductance to be inserted into thelines. This could take the form of eitheran isolation transformer if isolation ispreferred, or a line reactor, if isolation isnot necessary. The value of the requiredadditional inductance of this transformer orline reactor can be calculated (contactWarner Electric/SECO for appropriatespecifications), depending upon the linevoltage, the snubber circuit componentsand the specifications of the switchingdevices.The clear conclusion is that the applicationof power factor correction capacitors, in anelectronic VSD environment, must beassessed very carefully. Regardless of the

significant benefits in using power factorcorrection capacitors, there arc a numberof important concerns which should beevaluated when the capacitors arc applied.These concerns require particularevaluation to avoid possiile damage to theVSD s located on the samc power lines.VI. HARMONIC PROBLEMSHarmonics generated by the nonlinearloading of the power system can distortboth the current and voltage wave forms.Harmonically distorted load currentsproduce some voltage distortion as theyflow through the source impedance. Thisvoltage distortion is g e d y ow whenthe source impedance is I d l ercentageof the load impedance.A major source of harmonics U found inboth AC and DC drives due to therequirement of rectifying tht AC power.While this is also true of many other loadssuch as computer power supplies Thesepower requirements arc much lower thanthat required by the motar controller.The modern day DC drive controller(employing six pulse phut controlledrectification and connected to a motor ofthe correct power design code)produces avery well defined harmonic currentdistortion on the power source. (SeeFigure 7). However this harmonic currentdistortion is displaced fiom the systemvoltage by a power facta that variesclosely with speed. DC drives shouldalways be geared to run at tbe base speedor the field-weakened speed of the motorfor the best possible power factor.Today's AC drive controller ranges from1 through 250HP and k normally a PWMtype AC output fed from an AC rectifiedvoltage source. The input to this type of



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drive appears capacitive and would have avery low impedance to the power systemwhich would result in very large amplitudecurrent pulses appearing near the peak ofthe line voltage wave form. Likewise, avery large current distortion would bepresent. To prevent this, an impedance inthe form of an AC line reactor or DC buschoke is used to smooth the rectified ACcurrent and thereby significantly reducingthe harmonic current distortion. This typeof drive achieves an excellent power factorusually greater than 0.9 which varies littlewith speed. It is possible to achieveharmonic current levels approaching thoseas shown in Figure 7for the DC convertorthrough proper design.

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11th 13th

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25thORDER OF HARMONIC

Figure 7. Theoretical and typical valuesof harmonic current for asix-pulse converter.Any solution to a harmonic problem mustbe prefaced by a thorough study todetermine the frequency and magnitude ofthe harmonics, their source and their effecton the power system, and whether thepower system is resonant at or near aharmonic frequency. Note, that the

application of power factor correctingcapacitors is especially critical to powersystems which have significant harmonicdistortion. This practice may causesevere damage to equipment on the powersystem during periods of switching orresonance with system components. Thebest solution is to reduce any harmonics atthe source if possible.VII. LINE NOTCHINGLine notching is a well-knownphenomenon associated with AC to DCconversion equipment operating in thecontinuous conduction mode. In the caseof a three phase full converter, thethyristors operate in pairs to convert AC oDC. The load current is switched betweenthe various thyristor pairs six times perAC line cycle. During this switchingprocess, which is known as commutation,a brief short circuit occurs which producesa notch in the line-to-line voltage waveform. As illustrated in Figure 8, twoprimary line notches and four secondaryones are produced in each line cycle. Thesecondary notches are of lower amplitudeand are the result of notch reflections fromother legs of the bridge. The timerequired for the commutation to occur is afunction of source impedance and outputcurrent magnitude; this time determines thewidth of the notch. Notch depth dependson the location of the measurement in thepower system. Figure 9 illustrates thedistribution of AC line impedance in atypical plant power system. A notchmeasurement made at the converterterminals would indicate a notch depth ofapproximately zero volts. If L1 = L2 =L3, the notch depth at Point B would be66 of the maximum depth due to thevoltage division of the three impedances.

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INE-TO-LINE INPUT VOLTAGEt o390 LEE

Figure 8. AC voltage wave form for fullconverter.Line notching can interfere with thecontrol and/or power circuits of VSD's. Ifthe notch width exceeds the duration of thegate trigger pulse of a thyristor, the devicecould fail to turn on. This problem can besolved by using wider gate pulses or burstfiring, a technique which applies a train ofpulses to each thyristor gate.Harmonic voltages and currents associatedwith line notches can adversely affect theoperation of sensitive electronic equipmentsuch as computers and communicationsequipment. AC line conditioners areavailable to prevent line voltageirregularities from reaching the sensitiveequipment.

1 lRIVEaFEEDERLINE TRANSFORMERon REACTOR

A+ TO D CCONVERTER

Figure9. A C l i n e i m p e d a n c edistribution.

As the line voltage xe uv n at the end ofa notch, it overshootsand then oscillates orrings at the resorrant frtqutncy of thepower system. Su- v i a s such asRC snubbers can be used to dampen hesetransients, but the suppr sors cansometimes be ovaworked by the repetitiveline notch oscillatiorrr.Often the simplest sdution to line notchingproblems is the addition of chokes or anisolation transformer 011 tbc input of theVSD causing the notching. Thc additionalinductance actually makes the notchingworse at the drive input, since the notcharea increases with incrtased inputinductance. However, the notching on thesource side of the inductance will be lesssevere because most of the notch voltagetransient will appear across the addedinductance. Therefore, the effect of theline notching on othef equipment will bereduced.SUMMARYIn conclusion, applying VSD s in anindustrial environment can be simplified ifthe guidelines outlined in this paper arecarefully practiced. Tksc techniques canresult in improved utility and customeroperations and thus a vast improvement inthe entire manufacturing processREFERENCES:1.

2.

SCR Drives Power Consideration forASD's GET-6468B.Power Line Considerations forVariable Frequency DrivesIEEE transactions on industryapplications.Jarc and ShiemanVOL 1A-21 NO. Sept./Oct. 1985

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3. Power Quality Considerations forASDs.Electric Power Research Institute4. ASDs in the Process IndustryGreg Porter, PSI EnergyPower Quality MayNune 19915. Why Power Factor CorrectionCapacitors May Upset AdjustableSpeed DrivesThomas Grebe, Electro ConceptsPower Quality Magazine MayNune19916. Reducing Nuisance Outages With LineReactorsTCI Tech Tips, TCIIssue 1, October, 19917. Understanding Power LineDisturbancesDranetz Technologies Publication

James M. oelscherSystems Engineering Manager,Warner Electric/SECOBSEET (1988) from the University ofNorth Carolina at Charlotte.Member of IEEEGrateful acknowledgement is given to thefollowing people for their assistance n textand technical advice.James Bums- Senior Project Engineer, WarnerElectric/SECOBSEE (1969) from the MilwaukeeSchool of Engineering.Member of IEEEWilliam H. tokesSenior Project Engineer, WarnerElectric/SECO.BSEE (1971) from North CarolinaState University.Member of IEEE8. IEEE Guide forHarmonic Control andReactive Compensation of StaticPower Converters.IEEE Standard 519, 1981.

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industrial power considerations for vsd.pdf

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