PT 6014 Genset Ups Compatibility En

8/8/2019 PT 6014 Genset Ups Compatibility En

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Generator set andUPS compatibility

Power topic #6014 | Technical inormation rom Cummins Power Generation

This document discusses problems that canbe encountered in operating UPS equipmenton generator set equipment, explains reasonsbehind some problems, and identies stepsthat can be taken to minimize compatibilityproblems between generator sets and staticUPS equipment.

Conventional wisdom,and associated problems

There is a conventional wisdom that a generator setprovided or use with a UPS should be sized so that thegenerator set is always two to ve times the capacity othe UPS.

One problem with that guideline is that there is no rmconsensus in the industry o what exact number to useand little technical basis or a recommendation basedon sizing alone. There have been observed cases whereSCR loads as small as 3 kW have disrupted operationo a 100kW generator set. Would anyone suggest thatgenerator sets be derated to cover that situation?

Moreover, even i one ollows that rule, there is noguarantee that the generator set will successully powerthe load. The UPS supplier generally cant oer aniron-clad guarantee that a specic generator set willwork in an application, and the generator set supplierwont guarantee that a UPS will operate successullyon a specic generator set.

With as little as a two times derating actor, eveni the load is successully operated, the over-sizingo the generator set can result in unnecessarily highinstallation and equipment costs, excess acility spacerequirements and design costs, and potential operating

and service problems with the generator set romunder-loading the engine on the generator set.

Over-sizing alone will not guarantee successul opera-tional perormance, and will result in unnecessarilyhigh installation and operation costs. So, what can bedone to improve the cost, perormance and reliability opower systems that include UPS and generator equip-ment? The answers lie in understanding the problemsthat can occur and what suppliers can do to preventthem rom happening.

Generator set compatibility problems are mostpronounced on small (under 100 kW) generator sets,and particularly on natural gas ueled generator sets.Problems most commonly occur when the UPS is theonly load on the generator set, or is the single largestload on the system. Simply designing around situationsthat t these situations could avoid many problems.

Generator set and UPS incompatibility is particularlymystiying when identical equipment at two separatesites may perorm dierently; and especially when theUPS and generator set will both perorm normally untilan attempt is made to run them together.

When a system ailure occurs, it is a natural reactionto assume that something must be wrong in theperormance o either the generator set or the UPS.That assumption is oten not true. So, it is important torst understand that there are several dierent ailuremodes in the UPS/generator set compatibility problem,and then apply the appropriate solution to the specicproblem encountered.

The rst action is to be sure that the generator set andUPS equipment are both operating correctly. This mayrequire testing o the genset under various loading

> White paper

By Gary Olson, Director, Power Systems Development


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www.cumminspower.com

2007 Cummins Power Generation

FIGURE 2A

FIGURE 2B

FIGURE 2C

80 KVA UPS/60KW Load Running on utility power

+150A

-150A

0A

75.0A/div vertical 3.3ms/div horizontal

+500V

-500V

0A

250V/div vertical 3.3ms/div horizontal

80 KVA UPS/60KW Load Running on 150KW Cummins Power Generation genset

+150A

-150A

0A

75.0A/div vertical 3.3ms/div horizontal

+500V

-500V

0A

250V/div vertical 3.3ms/div horizontal

amount o energy (the area under the excitation curve)willreachtheexciter.(FIGURE3a)NotethattheSCRis switched o by sel-commutation as the voltage

waveorm approaches its zero-crossing point.

Consequently, voltage level on the output drops; theAVRtriestoincreasevoltagelevelbyswitchingontheSCRs sooner, and overcompensates or the problem.The net result is that voltage level on the output othe alternator begins to oscillate. Since the actualkilowatt load on the engine is a direct unction o the

Voltage regulator sensitivity

Ageneratorsetusesanautomaticvoltageregulator(AVR)tomonitortheoutputvoltageofthegeneratorsetand control the eld strength o the machine to main-tain a constant voltage on the output o the generatorsetundervaryingsteadystateloadconditions.FIGURE

1 illustrates a typical generator set design.

The voltage regulator senses output voltage level o thegenerator set directly rom the output power connec-tions and, based on a set reerence point, changesoutput power to the generator exciter to maintainvoltage. Note that the power to operate the voltageregulator is derived rom the output o the generatorset. This is termed a shunt type excitation system.

SincetheAVRdirectlysensestheoutputofthe

alternator, it must be designed to operate successullywhen the sensed voltage waveorm is distorted by thepowering o non-linear loads. The load current drawnby a UPS in normal operation does not ollow a sinusoi-dal pattern. Consequently, the voltage waveorm o thesource supplying power to the UPS is also distorted.The waveorm distortion eects, which can be asignicant detriment in the operation o utility powereddistribution systems due to heating eects, will haveeven more pronounced eects on generator sets.

This voltage waveorm distortion can cause misopera-tion o generator sets with some voltage regulator

types,especiallyAVRsthatutilizeSCRstoswitchexcitationpoweronandoff.TheseAVRsprovidegoodperormance when powering linear loads, but can ailto operate in a situation where the voltage waveormis disrupted by non-linear loads. Under normal (linearload)conditions,theAVRwillsensethevoltagelevelonthe output o the alternator and based on that voltagelevel will time the ring o the SCR so that a measured

Power topic #6014 | Page 3

FIGURE 2 (a,b,c) Comparison o current and voltage distortion

seenattheinputofan80kVAUPSrunningwith60kWloadwhile

operating on the utility and on a 150kW genset with lters operat-

ing, and not operating. Total harmonic distortion o voltage is about11.4% when running on the genset without lters.

FIGURE 1Inthisschematicdrawingofashunt-excitedalternator,note that the voltage regulator senses voltage level and draws exci-

tation power rom the output o the main eld o the alternator.

Sensing and Power Electrical Power Output

Output

AutomacticVoltageRegulator

ExiterRotor

and Stator

SHUNT-EXCITED ALTERNATOR

Rotating

Rectifiers

Main Rotor

Main Stator

Rotating

Mechanical

Power

Input


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voltage,theAVRvoltagevariationresultsinrealpower

pulsations to the engine o the generator set. Thesepower pulsations cause pulsating governor action,and oscillating requency, which makes the problemeven worse, since most generator sets incorporate avoltage roll o with a change in requency. The quicklychanging requency can also result in misoperation oUPS equipment. See Control Loop Compatibility inthis paper.

Many governors on many generator sets were changedbeore the core problem was diagnosed. With theproper diagnosis in hand, the rst attempts to correcttheproblemrevolvedaroundisolatingtheAVRfromthe

waveorm distortion by:applyinglterstotheAVRsensinginputtopreventvoltagedistortionfromreachingtheAVR;

applyingisolationtransformerstotheAVRsensinginput

These solutions, however, were not always withoutside eects.

The same input lter that prevented waveormdistortionfromdisruptingtheSCR-basedAVRalsoprevented the regulator rom quickly responding to realKVAdemands,inlargemotorstartingapplications,for

example. So, even though the lter helped the systemremain stable, it could prevent proper operation oother loads, unless the generator set was over-sized.Anotherproblemwithlteringwasthatittendedtobesuccessul only when waveorm distortion was not toosevere-such as when total SCR load on the generatorwas only 30-50% o the total generator capacity.

The isolation transormers applied would not alwaysbe successul in removing enough o the waveormdistortion to allow stable operation since specializedtransormer arrangements are needed to successullyremove signicant waveorm distortions (other than the

triplen harmonics).BecauseofthelimitationsoflteringAVRinputs,manymanuacturers designed high-speed voltage regulatorsthat provided pulse width modulated output to theexciterofthealternator.TheseAVRsaccuratelysensetrue RMS voltage value and provide excitation power inshort bursts, rather than depending on commutationoftheACvoltagewaveformtoswitchofftheexcitationpower.(FIGURE3c)

Consequently, the amount o excitation power deliveredto the exciter o the alternator is not aected by wave-formnotching,andtheAVRisimmunetooperation

problems related to waveorm distortion.

Useof3-phasesensingAVRsisalsohelpfulinreduc-ing the eect o waveorm distortion on an alternatorcontrolsystem.Itwillbeparticularlyeffectivewhen

FIGURE 3b Note that the single trigger turns on at the correct

time,butwaveformnotchingcausestheAVRtoswitchoffpowerto

the exciter too soon.

FIGURE 3c Multiple triggers and pulsed output provide proper

levels o excitation regardless o level o waveorm distortion.

Input AC Waveform

Triggers

Excitation

SCR-CONTROLLED EXCITATION SYSTEM

WITH DISTORTED INPUT VOLTAGE WAVEFORM

FIGURE 3a Note that a single trigger input turns on the excita-

tionpower,andtheACvoltagecrossingturnsitoff.Whenthatregu -

lator is applied with non-linear loads, the waveorm notching causestheSCRintheAVRtoswitchoffattheincorrecttime,sotheexciter

does not get the proper level o energy to maintain generator output

voltagelevel(FIGURE3b).

Input AC Waveform

Triggers

Excitation

SCR-CONTROLLED EXCITATION SYSTEM

Input AC Waveform

Triggers

Excitation

PULSE WIDTH MODULATED (PWM) EXCITATION SYSTEM


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single-phase SCR loads are causing the system disrup-tion. Three phase RMS sensing circuits are also helpulin maintaining reliable voltage regulation or generatorsets serving non-linear loads, since they can accuratelysense voltage level regardless o the magnitude o thedistortion on the alternator output voltage waveorm.

Anotheralternatorsystemenhancementthathasbeenapplied is the use o permanent magnet generator(PMG)supportedexcitationsystems.InaPMG-typegeneratorset,theAVRdrawspowerfromapermanentmagnet generator mounted on the alternator shat,ratherthanfromthealternatoroutput.(FIGURE4)

Since the PMG will provide constant power output aslong as the alternator shat is turning, and since it iscompletely isolated rom load-induced voltage wave-orm distortion, the PMG will provide reliable isolatedpowersupplyfortheAVRinsituationswheresignicantvoltage waveorm distortion is present.

Heating effects

Generator sets, like other building electrical compo-nents, are aected by the heating caused by non-linearloads. These heating eects include abnormalhysteresis, eddy current, and skin eect losses. Simplyput, the alternator operates at a higher than normalinternal temperature when serving SCR loads. Failureto properly size or the eects o non-linear loads

can result in premature ailure o the alternator due toabnormal heating in the machine.

Generator sets, however, are generally less aectedthan many loads and distribution system components.The primary issues in generator sets concern propersizing and allowance or neutral conductors oappropriate sizes to be connected to the generator set.

Alternatorderatingfornon-linearloadswillbeneces-sary only i the alternator standby rating and generatorset rating is equal.

For example, it a 500kW alternator with class F insula-tion is provided on a 500kW generator set then with thegenerator set operating at rated load and power actor

and carrying non-linear loads, the alternator wouldbe operating at a temperature in excess o its ratedtemperature. This would lead to alternator overheating,and premature alternator ailure.

However, i the same alternator were provided withclass H insulation rather than class F, there would bea 20 degree centigrade margin o thermal protectionadded; so the alternator insulation temperaturerating probably would not be exceeded even with thenon-linearloadapplied.Inapracticalsense,virtuallyall generator sets are provided initially with oversizedalternators in order to provide needed motor starting

capability and to provide required voltage dip peror-mance with the maximum load to be applied on thesystem (even though the load diversity is such that it ishighly unlikely that the entire load will be applied to thegenerator set simultaneously).

Consequently, when non-linear loads are used in aacility and operated by a generator set there generallyis excess capacity in the alternator design, so alternatoroverheatingduetonon-linearloadsisnotamajorconcern. However, i the total amount o nonlinear loadsin the system is close to the steady state rating o thegenerator set, use o derated alternators can provide

added capacity needed to operate the machine withnon-linear loads applied.

Since alternator capacity is relatively inexpensive, thisis not a signicant burden to the rst cost o the acility.Class H insulation provides additional high temperatureresistance over Class F insulation systems.

The system designer should veriy that an appropriatelysizedjunctionboxandlugterminationareaisprovidedor those applications where an oversized neutral isspecied.

Output voltage distortion

Facility designers generally are well aware o thepotential impact o non-linear loads on the voltagewaveorm quality in their applications. The degradationin voltage waveorm quality can result in heating eectsand shortened lie in motors, misoperation o someloads, and even bypass not available alarms in UPSequipment.

FIGURE 4 Schematic drawing o an alternator with permanentmagnetgenerator(PMG)forexcitationsupport.Voltageregulatorsenses output voltage level rom output o alternator, but excitation

power is derived rom the PMG.

ALTERNATOR WITH A PERMANENT MAGNET GENERATOR (PMG)

Sensing Electrical Power Output

Power

Output

Rotating

Mechanical

Power

Input

AutomacticVoltageRegulator

ExiterRotor

and Stator

PMGRotor

and Stator Rotating

Rectifiers

Main Rotor

Main Stator


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powering 3-phase loads will be in the 5th and 7thorders). This is an advantage to distribution circuitsbecause 3rd order harmonics add directly in the neutralo the system, and can cause signicant heating indelta/wye transormers, and neutral bus/cable systems.

The disadvantage o 2/3 pitch machines is that i 3rd

order harmonics are induced in a distribution systempowered by a 2/3-pitch alternator, they will tend to havegreater heating impact on the alternator than otherharmonic orders. However, since most loads seenby an alternator are 6-pulse, and since 6-pulse loadsproduce no 3rd order harmonics, this is not normallya signicant issue. (2-pulse loads are typically singlephase devices, and are separated rom the generatorset by a transormer, which normally isolates the gen-erator set rom 3rd order harmonics and their multiples(triplen harmonics). 12-pulse loads produce no triplenharmonics, and most o the distortion produced is inthe 11th and 13th orders.

Alternatorswithallotherpitchesmayhavewaveformdistortion present at all harmonic orders, typically withthe highest distortion in the 3rd order. Since all theinjected3rdorderharmonicswilladdintheneutralofthe system, and potentially cause heating eects thatare additive to the heating caused by non-linear loads,Cummins Power Generation recommends that only2/3 pitch alternators be used, to avoid this potentialproblem.

Since speciying alternator pitch may restrict thenumber o suppliers that can provide hardware or a

specicproject,manydesignerswillelecttospecifymaximum permissible distortion on an alternator(poweringlinearloads.)Atypicalspecicationistoallow not more than a total harmonic distortion o 5%produced by the alternator, with no single harmoniclevel being more than 3%, with the alternator operatingat rated load level with linear loads.

Generator set metering errors

ACoutputmeteringforgeneratorsetsisoftenaveragereading analog type, which will not indicate true RMSvalues when serving non-linear loads. The primarydisadvantage o this is that an operator unamiliar with

thesystemmaymisadjustthegeneratorsetbasedonerroneous meter readings.

Digital true RMS metering sets are available and canbe mounted on generator sets, but this metering isnot particularly useul in monitoring and servicing thegenerator set because a fashing digital display doesntas clearly indicate rate o change or maximum change

o metered parameters under transient load conditions,and doesnt as clearly indicate stability o a meteredunction.

Since the primary unction o the metering mounted onthe generator set is to enhance the serviceability o thegenerator set, analog metering should be provided on

the generator set in spite o the potential inaccuracyo its indicated values under some system operatingconditions. Supplementary digital metering setsreading load conditions in switchboards or switchgearwill accurately provide data necessary or operatingpersonnel to monitor and manage system loads.

Control loop compatibility

When an SCR-controlled device operates, it dependson accurate timing o the ring o the SCR to accuratelycontrol output power o the rectier. This ring time iscritical because the SCR will be turned on at a specic

time, but the o command is derived rom a zero-crossingevent.Inautilitypowersystemthisisnotamajorproblembecausethepowersourcefrequencyisconstant,sothemajorproblemsinvolvedinthetiming o the ring revolve around proper detection othe zero-crossing which may be somewhat masked byvoltage waveorm distortion.

On a generator set the requency o the output voltageisnotconstant.Itvariesevenwithsteadystateloadson the machine and under transient load conditions itcan vary considerably. The term slew rate is used todescribe the rate o change o requency.

You can begin to better understand the magnitude opotential compatibility problems between generatorsets and SCR-based loads, when you realize that manyload devices require that the slew rate will not exceed0.5 hertz/second. When a load is applied to a generatorset, slew rates o 10-15 hertz/second are not uncom-mon or short time periods.

Ifthefrequencyofthepowersourcechangesfasterthan the load device is designed to deal with the SCRring will occur at the wrong time, resulting in either toomuch or too little energy transmitted to the load device.On the next timing cycle the load control logic may

then start to make corrections or the error condition.Atthesametimethatthisishappeningthegeneratorsetcontrols(theAVRandforengineswithelectronicgovernors, the electronic governor control) will start tomake corrections to maintain engine speed and volt-age. The generator set control system corrections canbegin to resonate with the load system corrections.


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OSCILLOGRAPHIC RECORDING OF A GENERATOR SET WITH UPS RECTIFIER RAMPING ON

Phase A Input

Phase B Input

Input Current

Phase A Out

When that happens, a control loop incompatibilityexists. This incompatibility may be dicult to correctlydiagnose, because its symptoms are very similar toother generator set control problems. The symptomsmay occur immediately when the generator set tries topowerupaspecicload.InFIGURE6youcanseean

oscillographic tracing o a 200kW generator set outputwhileattemptingtopowera125KVAUPS.ThetoptracingisA-phasevoltage,thesecondlineisA-phasecurrent, then B-phase voltage, B-phase current, andgenerator set excitation system current.

Prior to picking up this load, the generator set was stableunder all steady state and transient conditions. TheUPS input locked on to the generator set, and begin toramp up load on the generator set. However, as loadincreased, the system became unstable, and the genera-tor set governing system began to oscillate wildly.

Itistemptingatthispointtostartmakinggovernor

stabilityandgainadjustments,wheninfactthegenera-tor set is simply responding to real load being appliedto its output. This real load is pulsating due to errorsin the ring o the SCRs in the load due to requencychanges in the generator set.

The control incompatibility is also dicult to diagnosebecause it can occur at any time and at any load level.IntheexampleshowninFIGURE6a,thegeneratorset

was running smoothly, and carrying the UPS load withno problem. Suddenly, or no apparent reason, thesystem began to oscillate as shown in this gure.

AnotherpotentialproblemisthatarectierinaUPSisa constant load device, so voltage reduction unctionsin generator set voltage regulators that are intended to

reduce load on the engine to allow actor recovery romtransient load conditions, will not unction as expected.Reduction in generator voltage output does not resultin reduced load unless voltage drops to such a lowlevelthattherectierswitchesoff.Infact,thevoltagereductioneffortsbythegeneratorAVRmaymaketheperormance o the system worse, especially whenmostoftheloadisUPSequipment.Inthosecasesthevolts/hertzroll-offfunctionmayneedtobeadjustedtooccur at a signicantly lower requency.

This situation maybe dicult to diagnose since thestability and response characteristic o a generator

set vary with ambient temperature, uel type andcondition,andmanyotherfactors.Itispossiblethatthesystem can be stable one day and unstable the next.Obviously, one cant physically inspect any particulargovernor control, voltage regulator, or UPS circuitboard, and deduce whether or not they will exhibitcontrol loop compatibility problems. Similarly, under-standing o the logic and control o one component in

FIGURE 6aThetoprecordingisanA-phasevoltage.ThesecondrecordingisanA-phaseoutputcurrentfromUPS.Thethirdrecordingis

a B-phase input voltage to UPS, and ourth line is an input amperage to UPS rectier.


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Power topic #6014|Page9

OSCILLOGRAPHIC RECORDING OF A GENERATOR SET OPERATING UPS SHOWING SYSTEM OSCILLATION

Phase A Input

Phase B Input

Current

Input Current

Phase A Out

the system will not provide enough inormation to solvetheproblem.Acooperativeeffortfromallequipmentsuppliers will be needed to resolve these issues.

Ifcompatibilityisapotentialissueonasite,itcanbeproven only by test and experience. The solutions tocontrol loop compatibility problems may involve modi-

cations to the SCR ring control logic, sizing o thegeneratorset,orproperadjustmentofthegeneratorsetcontrols.Thesegeneratorsetcontroladjustmentswill be more successul i the generator set has a broadadjustmentrangeinboththeenginespeedgoverningand voltage regulation controls.

Itshouldbenotedthatemergency/standbygeneratorsets are typically designed and set up to provide astestpossible service to acility loads, and astest possibletransient perormance response when a load is applied.Generator set manuacturers sometimes point outtransient perormance results with considerable pride

in their work. This perormance is achieved by pushingthe generator set to respond so ast that it is unstable,

andthenbackingofftheadjustmentssothatitcanbeclose to instability when running sensitive loads.

When powering UPS loads, ast response is not aprime concern. UPS equipment is very tolerant ovoltage and requency excursions, but very intoleranto ast changes in these parameters. So, setting up the

generator set in a typical emergency/standby ashionmay not result in best overall system perormance,especiallywhentheUPSisthemajorloadontheemergency/standby system.

Atthesametimeifthegeneratorsetisdetunedforbest perormance with the UPS, it must be remem-bered that this will impact on the ability o the generatorset to best service other loads in the system, particu-larly in the starting o large motors. Use o a motorstarting/sizing procedure that incorporates actualgenerator set data along with an understanding o therequirements or successul motor starting can help to

achieveinthebestsystemperformanceadjustments.

FIGURE 6bThetoprecordingisanA-phasevoltage,secondisanA-phasecurrent,thirdisaB-phasevoltage,fourthisaB-phasecurrent,

and the bottom recording is the voltage regulator output.


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OSCILLOGRAPHIC RECORDING OF A GENERATOR SET RUNNING NORMALLY WITH A MINOR LOAD CHANGE

Phase A Input

Phase B Input

Current

Input Current

Phase A Out

Whenreadjustmentofthegeneratorsetperformanceisnecessary, it is desirable to begin by damping the gainontheAVR.TheAVRrespondsmuchmorequicklythanthe engine governing system, so is more likely to bethe source o a problem than the governor, even whenrequency changes appear to be the problem.

Anothertacticthatcanbeusedtolessentheprob-ability o control loop incompatibility is designing thesystem so that the generator set will not be loaded insuch a ashion as to see large requency transients,especially while the generator set is powering up theUPS ater a power ailure. This means that load stepsthat are applied to the system are as small as possiblerelative to the generator set size, and that the generatorset is carrying other acility loads beore the UPS rampson.Iffrequencycanbecontrolledtoaplusorminus0.5 hertz operating band by careul sizing and applica-tion o loads, control loop incompatibility will be less oan issue.

Some acilities have incorporated resistive loadracks or generator sets that exclusively serve UPS

equipment, so that some linear load is present onthe generator set when the UPS begins the ramp onprocess.Ingeneral,aloadbanksizedforapproxi-mately 10% o the generator set standby rating shouldbe sucient to stabilize the system.

Finally, the solution to the compatibility between the

system components may be most easily addressed inthe UPS SCR-ring control logic. Many UPS systemsnow include control logic that compensates moreeectively or rapid changes in input power requency.

Generator set poweredUPS system limitations

Ageneratorsetisnotautilitypowersourceanditcantduplicate the perormance o the utility in powering aUPSandtheUPSloads.Inparticular,considerationmust be given to the act that generator requency willnot be as constant as on the utility, and this will impact

the perormance o the overall system.The best commercially available steady state per-ormance rom a reciprocating diesel engine driven

FIGURE 7 Generator set running normally, and apparently stable, until a minor load change or requency excursion initiates an oscillating

condition.ThetopwaveformisthegeneratorphaseAoutputvoltage,secondistheUPSoutputvoltage,thirdisthegeneratorphaseBout -putvoltage,andfourthisthecurrentinputtotheUPS.NotethattheAVRoutputismovingwiththevoltageandcurrentvariations.


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generator set is 60 hertz plus or minus 0.25%. (Naturalgas ueled generator sets may not be able to meet thislevel o perormance.) On application o a load, voltageand requency may dip up to 20-30%. The magnitudeo the dip can be controlled by proper sizing o thegenerator set.

Generator sets may be provided with either droop orisochronousgovernors.Inadroop-governedgeneratorset, the requency o the output is not a constant 50or 60 hertz, but varies as a direct unction o the loadon the system. With droop-governed generator sets,the steady state requency will vary rom 3-5% romno load to ull load, and generally, the steady stateperormance will not be as good as with an electronicisochronous governor.

The probable best slew rate (rate o change orequency) that can be achieved by a generator set witha stable load is 1 hertz per second. When loads are

applied to the generator set, the slew rate will usuallyexceed 3 Hz/second or a short period o time.

This impacts the system in two areas. First, when theUPS ramps on to the generator set ater a normalpower ailure, the generator requency will drop as loadisapplied.Iftherateofloadapplicationishighenoughthe requency o the generator set can drop to belowthe input requirements o the rectier, causing it to beswitched o the generator set, returning the system tobattery power. The generator set requency will thenquickly correct, and the rectier will ramp on again.This can lead to the appearance o instability in the

generator set governing, when in act multiple real loadadditions cause the requency changes.

Ifthisoccurs,adjustmentoftherectierrampingtime and ramping rate will minimize the requencydisturbance and allow the rectier to stay on line withthe generator set. Generator set requency changesduring load addition rom other loads while the genera-tor set is running the UPS can oten cause bypass notavailable alarms rom the UPS. When the UPS seesstable voltage and requency on the input to its recti-er, it will synchronize the output o its inverter to theinput, so that i the inverter ailed, a static switch caninstantly connect the load directly to the power source.When the source power supply requency and voltagechange, the UPS will attempt to ollow those changesand remain synchronized with the source, unless thevoltage and requency change more than is acceptable,ortheslewrateistoohigh.Atthatpoint,theUPSwillinterpret the input power as unacceptable and disableits automatic bypass ability (through the static switch)and issue a bypass not available alarm.

IfaloadisappliedtothegeneratorsetservingaUPSthe requency and voltage change or slew rate maybe unacceptably high, and a momentary bypassdisabled alarm may result in the UPS. The voltagerequency, and slew rate acceptance ranges or bypassenableintheUPSareadjustable,butitislikelythat

normal operating perormance o a generator set willnot be suitable to maintain bypass enable capabilityunder all load adding and shedding conditions.

The generator set normally will not be able to duplicatethat voltage waveorm perormance o the utility, socare must be taken to provide enough generatorset capacity so that waveorm distortion level willbe acceptable. When speciying systems that utilizeexisting equipment, care should be taken to be surethat the generator set or UPS could perorm properlygiven the quality o power that can be provided by thegenerator set used.

Transfer switch andpower transfer issues

Anotheremergency/standbypowersystemcomponentthat can be misapplied in systems that power UPSequipment is the automatic transer switch. Transerswitches can cause problems in the ollowing areas:

Oversizedneutralrequirements

GroundfaultsensingandUPSinput sensing problems

NuisanceUPSinputbreakertripping

Facility designers should be aware that oversizedneutral capacity in a transer switch is not currentlycommercially available, so transer switches must bederated or non-linear loads i a neutral o signicantlygreater capacity than the phase conductors is needed.This is less o an issue in 3-pole transer switches,since there is a solid neutral connection between thesources.

UPS systems are independent power productionsources that can be designed as either separatelyderived, or non-separately derived systems. That is, theneutraltogroundbondingjumpercanbeeitheratthe

service entrance o the acility (not separately derived),or close to the UPS (separately derived).

InaUPSsystemthatispoweredonlybytheutilityservice, the primary considerations in this area arebalancing potential added costs o a separately derivedsystem with the benets o system isolation oered bythat design.


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When the system operating voltage and acility capac-ity requires the use o ground ault protection, generallyit will be necessary to design the acility to incorporatea separately derived UPS and generator system, whichuses power transerred by a switched neutral (4-pole)transer switch. When a single transormer serves a

UPS system (either internally or externally), it will benecessary to switch the UPS load rom generatorsource to utility source using a transer switch that hasarelativelyslowoperatingspeed.Ifthetransferswitchopens the contacts on the generator set source, andthen closes on the utility source beore the residualvoltage generated by the transormer decays, nuisancetripping and potentially even equipment damage canoccur in breakers upstream rom the transormer. Toeliminate this problem, the open time on the transerswitch should be approximately 0.5 seconds. Mosttransferswitcheswiththecapabilityofadjustingoperating speed will allow customizing o this operating

time to be as ast as possible without causing nuisancetripping in the system. The duration o the open time onswitching o the transer switch is not important to theUPS (as long as its not too ast) because once powerails, it normally will wait or a short time to re-energizethe system rom its connected source.

Recommendations

The generator set provided to power UPS applicationsshould include an automatic voltage regulator that isimmune to misoperation due to waveorm distortion.Three phase sensing regulators and separately excitedalternators are recommended but not necessarilyrequired or proper system perormance.

Where available, class H generator set insulationprovides an added protection against generator over-heating over class F insulation. The lowest temperaturerise alternator, that is practical or an application,should be provided in order to minimize voltagewaveorm distortion. Typically, this will be an alternatorrated or 105C rise over a 40C ambient. The costpenalty or this enhanced rating will vary with generatorsize and manuacturer, but generally will be a 1% to4% premium over units rated or maximum insulationtemperature rise at their kW rating. Where available, the

generator set should have a subtransient reactance o15% or less. Remember that lower reactance results inbetter perormance.

Speciy UPS operating parameters that are compatiblewith generator set perormance capability. Keep inmind that this will change with the size and type ogenerator set provided. Speciy UPS with input lters

(especially with 0-pulse UPS) to minimize voltagewaveformdistortion.Aseparateisolationtransformerto serve the UPS (making the UPS a separately derivedsystem) will be useul in avoiding operational problemswith the UPS, providing better quality power to theload, and preventing ground ault sensing problems.

Speciy program transition (open time = 0.5 seconds)or transer switches eeding UPS loads - Generatorsets usually will need electronic isochronous governorsto provide 50 or 150 hertz output at all load levelsand enable the UPS output to be synchronized to thegenerator power. (Governing systems that have droopoperating characteristics will generally power up theUPS rectier, but may not orce operation close enoughto proper operating requency to allow sychronizingthroughtheUPSstatIcswitch.)

Generator sets should be sized based on total loadappliedbytheUPS.Itshouldconsider:

UPSoperatingatfullratedload,eveniftotalloadonthe UPS is planned to do signicantly less than 100%o the UPS rating. This is necessary because atera power ailure, the system will current limit to 100-125% o the steady state rating o the UPS, in orderto recharge the system batteries.

Batteryrechargerateafterpowerrestorationtothe UPS.

UPSoperatingpowerfactorandefciencyatull load.

Thesizingshouldconsiderwaveformdistortion

goals.Apracticalgoalistoachieveasystemwithnotmore than 10% THD when operating on generatorset power. Better perormance in this area can beachieved by careul consideration o alternator sizeand capabilities.

The generator set supplier should help the systemdesigner establish the proper size o the generator set,based on the perormance requirements set by thesystem designer.

Anemergency/standbypowersystemdesignthatincorporates a UPS system will have a signicantlyhigher probability o success i the UPS is not the onlyloadonthegeneratorset.Additionofotherloadsofvarious types will reduce the relative transient loadapplied to the generator set by any one load, andimprove the stability o the generator set, especiallywhen the UPS it is powering is lightly loaded.

Generator sets and UPS equipment are not by natureincompatible (Owners and system designers trying to


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wade through incompatibility issues between suppliersmay nd this hard to believe, but its true.)

Finally, other rectier-based loads can have problemssimilar to UPS equipment when running on a generatorset. The solutions described in this paper will be helpulin resolving operational problems when running recti-er-based equipment on a generator set.

For additional technical support, please contact yourlocal Cummins Power Generation distributor. To locateyour distributor, visit www.cumminspower.com.

About the author

GaryOlsongraduatedfromIowaStateUniversity with a BS in MechanicalEngineeringin1977,andgraduatedrom the College o St. Thomas with anMBAin1982.


He has been employed by Cummins Power Generationor more than 30 years in various engineering andmanagement roles. His current responsibilities includeresearch relating to on-site power applications, technicalproduct support or on-site power system equipment,and contributing to codes and standards groups.He also manages an engineering group dedicated todevelopment o next generation power system designs.


2008CumminsPowerGenerationInc.Allrightsreserved.

Cummins Power Generation and Cummins are registered

trademarksofCumminsInc.Ourenergyworkingforyou.

is a trademark o Cummins Power Generation.

PT-6014(09/08)Formerly900-0280

Documents

PT 6014 Genset Ups Compatibility En