Considerations When Paralleling Generating Sets White Paper by Robert Patrick, Lead Project & Systems Application Engineer

Power topic #5410679 | Technical information from Cummins Considerations when Paralleling Generating Sets White Paper By Robert Patrick, Lead Project & Systems Application Engineer

Applications where several generating Paralleling Considerations sets are paralleled together are quite This document looks at the various ways common today. Either to supply in which groups of similar or dissimilar electrical power to a facility in island generating sets can safely electrically mode or paralleled together with the synchronize together as a Power Plant. Utility in an infinite bus topology. There are many different forms in which Standby generators are frequently these generators could be electrically paralleled together to protect critical connected. For the design engineer the watch-out is how will their installation stand applications such as hospitals or data up to the test of time? centres in the event of a failure from the Utility. In other situations they are Over the next few pages various methods of used for periodic emergency support Synchronising, Paralleling and Load Sharing are discussed with a focus on equipment to directly back-up the national capabilities and how robust are the options. electricity network. Then there is the scenario where a Utility supply The advice provided here, a technical precis of Power Topics #9015, #9016, #9017 and is not even available and paralleled T-016 is based on European Norms aimed at groups provide the only source of questions raised when paralleling both Low reliable energy to a specific site. The Voltage and Medium Voltage generator sets. configurations are immense. If the reader’s specific interest is for medium Whatever the application, paralleling or high voltage paralleling applications, it is a fundamental concept in power is suggested additional advice should be generation and invariably introduces solicited regarding the system earthing. specific challenges that must be overcome. Contents

1 Generator Size Compatibility 2 7.2 Circulating Neutral Currents due to Alternator Pitch Differences 8 2 Generator Synchronising 3 7.3 In Summary - Compensating for 2.1 Slip Frequency Synchronising 3 Pitch Differences 10

2.2 Phase Lock Active Synchronising 3 8 Load Sharing Methods 10

2.3 Limits to Synchronising Zone Parameters 4 8.1 Droop Load Sharing 10

3 Picking the First Generator to Close 8.2 Isochronous kW and kVAr Load Sharing 12 to the Bus 4 8.3 Cross Current Compensation 3.1 Random Access paralleling system 4 (Droop Circuit CT) 12

3.2 Dead Bus paralleling system 5 8.4 Using Different Operating Modes for 3.3 Paralleling System Comparison 5 Load Sharing 13

4 Compatible Engines 6 9 General Recommendations 15

5 Load Sharing Factors 6 9.1 Key Considerations 15

6 Compatible Load Sharing Control Systems 7 9.2 Additional reference reading on this subject 15

7 Compatible Alternators 7

7.1 Mechanical Alternator Design Characteristics Driving Harmonics 7

1. Generator Size Compatibility no less than 30% of the capacity of the largest generator set in the system. Not all installations will have the same sized generating ■■ Generator sets must have a means to determine sets. Some dissimilar size generator considerations are: which generator set/s will close to the bus first in ■■ An emergency system with generator sets that a “black start” (first start) situation. have matching kW ratings can support a higher ■■ Some manufacturers are unable to provide first priority load than a system that has generator equipment that is certain to parallel within 10 sets with dissimilar kW ratings. For example, an seconds. Generally speaking older larger engines emergency system with two 1000 kW generator are slower to start than their smaller counterparts. sets will handle a first priority load as large as So in situations where emergency load is required 1000 kW. A system with one 1500 kW generator within 10 or 15 seconds, the system design must set and one 500 kW generator set, while having prevent the smaller machines from closing to the the same total power rating should be limited to bus first, or ensure that first priority loads can always a 500 kW first priority load. This is because if the be served by the smallest machine in the system. first generator set closing to the bus is the 500 kW machine, any load greater than 500 kW could ■■ Generator sets may be monitored by either a

cause it to be overloaded. site Building Management System (BMS), or by an external monitoring system, such as for ■■ From a load shed perspective with dissimilar- service contract facilitation or external Power sized machines, it is desirable to drop load in Management System (PMS). Control and status large enough steps to relieve the 500 kW set in information should be compatible for this duty. the event that the 1000 kW unit becomes the unit that is not available. In general, a manageable

Power Topic #5410679 Power Topic system is when the smallest generator set is 02 2. Generator Synchronising Figure 1 below illustrates phase angle difference between sources. Notice that when synchronised To parallel generators, they must first be synchronised. (indicated by the green OK TO CLOSE region), the Synchronising means that the output voltage waveform phase angle difference and therefore the voltage of one generator must match the output waveform across the synchronising circuit breaker is near zero. of another source in terms of voltage, frequency and phase angle. A phase angle difference between When the sources are out of synchronism, the phase the two waveforms creates a difference in potential angle difference is large and therefore the voltage across between the two sources. The potential difference the synchronising circuit breaker is large. If an attempt should be as small as possible within practical limits is made to close the circuit breaker in this condition, before closing the paralleling circuit breaker. the low impedances in the circuit means that very large and potentially damaging currents would flow. In a slip It is imperative that at the instant of closing the frequency application there will be alternating moments of paralleling Circuit Breaker, the transient current being in phase (synchronism) and being out of phase. surge experienced by the in-going generator does not exceed 50% of that generators rated current. 2.2 Phase Lock Active Synchronising Achieving this critical requirement will limit the forced levels of synchronising alignment torques experienced Figure 2 (page 4) illustrates the Phase Lock loop (Phase throughout the generating set. Match) method of synchronising. Notice that while there is initially a large phase angle difference between There are two forms of synchronising: waveforms, the difference is reduced and maintained.

This allows for a sustained period of synchronism. 2.1 Slip Frequency Synchronising This particular design of synchroniser by controlling In a slip frequency application, a synchroniser voltage, frequency and phase angle, makes this mode normally is used to match the voltage of the incoming of operation possible. The synchroniser analyses the generator set to that of the busbar and the frequency generator output voltage and makes corrections to the of the incoming generator is set to a fixed difference engine speed (via the governor) and controls the AVR to that of the busbar. The different frequencies permit to adjust voltage amplitude and phase angle to achieve a moment of minimal phase angle and therefore sustained synchronism. potential difference between sources.

Genset voltage synchronizing to bus voltage

Bus voltage Genset voltage

180

Ok to close 0 Phase angle di erence between genset & bus

Figure 1 - Phase Angle difference when Slip Frequency synchronising. Power Topic #5410679 Power Topic 03 The synchroniser becomes active when the paralleling 1. Frequency must match within 0.1 Hz. bus is energised and the generator is running. Power 2. Rate of change of frequency 0.1 Hz/sec. Command 3.3 controllers have a synchroniser enable 3. Voltages must match within < 1% setting that must be true (on) as well. Most paralleling 4. Maximum phase angle within < 10% systems today will use Phase Lock (active) synchronising. When slip synchronising, the engine speed should always be faster than the utility frequency so power 2.3 Limits to Synchronising Zone will flow from the generator to Utility when paralleled Parameters together. In all cases the incoming generating set With either system outlined above, there are limits to voltage should be set equal to or higher than the the ‘OK to Close’ zone parameters. If this window is busbar voltage to ensure an export of kVAr (lagging incorrectly set then the generator set will be exposed to power factor) from the generator. A generator voltage “Rough Synchronising” – a synchronisation event where that is lower than busbar voltage will reduce the there is excessive voltage across the synchronising magnetic field strength of the field and may result in circuit breaker contacts, which is damaging to both the the generator becoming unstable when paralleled. alternator and engine. To avoid long term progressive damage to the equipment it is suggested as a worst 3. Picking the First Generator case, transient current is limited to 50% of the alternator to Close to the Bus Full Load Current (FLC). There are two distinct systems for First Start generator To achieve a transient current level of less than 50% systems. at the moment of closure of the synchronising circuit breaker, it is necessary to reduce the level of voltage 3.1 Random Access Paralleling System miss-match and ensure the slip frequency is below With Random Access paralleling systems, all 0.1Hz/s and that the closing of the circuit breaker occurs generator sets receive a start command at the within acceptable closing phase angle alignment limits. same time and independently build up their voltage It is suggested that the following parameters should be and speed to rated values at which point they are ideally set...

Genset voltage synchronizing to bus voltage

Bus voltage Genset voltage

180 Phase angle di erence between bus & generator Ok to close 0

Figure 2 - Phase Angle difference with Phase Lock synchronising. Power Topic #5410679 Power Topic 04 ready to close to the paralleling bus. The generator 3.2 Dead Bus Paralleling System sets will not be in synchronism with each other so With the Dead Bus paralleling system, all generator the generator set controls must have some kind of sets start simultaneously with their paralleling breakers arbitration scheme allowing only one generator set closed to the bus and excitation circuits disabled. to close to the dead bus. A dead bus sensor on the This allows generator sets to be connected in parallel common bus prevents out-of-phase paralleling when without being in sync because no voltage is being one generator wins the arbitration. generated. Cummins Power Command Controls (PCC) use As engines reach a pre-set speed, the generator set a scheme called First Start (Sensor) to determine controls turn on and ramp up excitation levels. This whether or not to close the generator paralleling causes the voltage on the bus to build up and forces breaker on to a dead bus in paralleling applications. the generator sets to come into sync with each other. First start is based on the “first start arbitration”. The PCC joins first start arbitration when first start There is a variation of this method known as Dead conditions are met. The winner of the arbitration Field paralleling in which the generator sets start with gets permission to close to the dead bus. All other the paralleling breakers open and then close them as generators in the system must then wait for the bus to the engine starter disengages. “Exciter paralleling”, become energised and then synchronise to it. “run up synchronization” and “close before excitation” are other terms that describe this same basic The PCC has a number of conditions that must be paralleling algorithm. met before first start becomes enabled: However, a stationary alternator connected to the bus ■■ The paralleling bus must be de-energised. is effectively a short circuit potentially compromising ■■ The PCC must be in Automatic mode. the entire system. A means of detection is required to ■■ The control must not have any shutdown faults disconnect the alternator. This method of paralleling present. is not considered by Cummins to be as robust and ■■ The unit must be in a Ready to Load (90% reliable as Random Access paralleling and therefore voltage and frequency) state. not supported by PowerCommand controls. ■■ There must not be Inhibit signals present. The standard Cummins PowerCommand controls If these conditions are met, generators will arbitrate currently require the addition of a third party module through the first start communication lines. Each to support this method of paralleling. generator tries to send a series of random bursts (1 - 8ms each) through this connection for 800ms. 3.3 Paralleling System Comparison If a generator control detects a burst from another generator during that time, the burst stops, waits There are Pros and Cons with each system. for 2.5 seconds and starts over. Otherwise, after As Dead Bus or Dead Field Paralleling have no need 800ms, the control turns on the first start connection for arbitration or synchronising of multiple generator (a breaker inhibit signal), to prevent other generators sets, dead bus paralleling can bring a paralleling bus from getting permission to close. The control then to rated speed and voltage relatively quickly. closes the breaker to the dead bus. At this point the Additionally, it provides the capability of magnetising other generating sets recognise the bus is now live a system that has a number of transformers with big and they synchronise and close to the bus according inrush currents and reduces great stresses on the to the type of synchronising method chosen. alternator from large transformers during start-up. Power Topic #5410679 Power Topic 05 However, this system is considered a less robust ■■ Inertia in the rotating components method of paralleling as each generator set represents ■■ How fast the governing and air intake systems a single point of failure. A control or excitation system can increase the fuel rate into the engine. fault on any generator set can compromise the Active load sharing during transient conditions is entire system without sensing device to detect if a dependent upon the dynamic response of the engines set is “lazy” in starting or has failed and remove it by that are connected in parallel. Smaller engines declaring it “failed” and opening its breaker. tend to respond more quickly due to their lighter The overwhelming majority of paralleled generating components – particularly turbochargers – and in the sets in operation today use Random Access event of a large step load being applied to a system Paralleling to synchronise and connect to a paralleling with unequally sized generating sets, a degree of bus. The single most important point in favour of unbalance may occur between sets. This should not random access being system reliability and ability be detrimental and will be rapidly equalised by the to consistently provide emergency power. Even if governing and load sharing system. Sets cannot be a single generator set fails or is slow to come up seriously overloaded, since the maximum load that to speed, the rest of the generator sets are not in can be contributed by any generating set is the fuel any way affected and the electrical system is not stop power. compromised in anyway. Keep in mind that at lower load level changes, voltage For detailed correspondence on Random Access vs and frequency transients are lower and recovery Dead Bus Paralleling please refer to Cummins White times will be shorter so transients may be very similar Paper #GLPT-6174-EN between the machines. This way dissimilar transient performance of the machines can be dealt with by 4. Compatible Engines adding and shedding loads in smaller steps.

Real power (kW) provided by a generator set In general terms, compatible engines are those with operating in parallel with others is a direct function of similar load sharing capabilities and similar transient engine real power output. performances, sufficient to meet the load demands. Care should be taken when considering paralleling ■■ Compatible engines share load nearly equally generating sets with dissimilar control and governing at all load levels whilst operating at steady systems as similarity assumptions may not be valid. state load levels and during transient loading conditions. ■■ If incompatible engines are paralleled, load 5. Load Sharing Factors sharing problems may occur, particularly on When a generator set is operating in a paralleled application or rejection of large load steps. arrangement, the voltage and frequency outputs of the generator sets are forced to exactly the same As loads are added to a generator set, particularly values when they are connected to the same bus. in large increments, frequency will momentarily drop Consequently, generator set control systems cannot until the engine governor can drive more fuel into simply monitor bus voltage and frequency as a the engine to recover back to its nominal speed reference for maintaining equal output levels, as they (frequency). The amount of speed drop and recovery do when operated in isolation from one another. time are a function of:

Power Topic #5410679 Power Topic 06 If for example, one set operates at a higher excitation Several types of load sharing control are available: level than the other sets, the reactive load will not ■■ Droop governing and voltage regulation (reactive be shared equally – this will be demonstrated by the droop compensation) generators operating at different power factors and ■■ Isochronous kW load sharing load currents. Similarly, if a generator set is regulated ■■ Cross current compensation for VAr load sharing to a different speed settings than the others, even ■■ Isochronous Voltage VAr load sharing. though its frequency will be the same as the others, it will not share kW load properly with other generator In general, these communication tasks are handled by sets in the system. Each generator set in the system commonly available communication practices such as has two active control systems always in operation: analogue signals or by a digital communication load share line. ■■ Excitation control system regulating the voltage ■■ Fuel control system regulating the engine speed (kW). 7. Compatible Alternators Real power sharing (expressed as kW or unity power Paralleled alternators are compatible if they can factor load) depends on speed setting and fuel operate in parallel without having damaging or rate control between the generator sets based on disruptive neutral currents flowing between them. percentage of kW load. Reactive power (expressed The magnitude of neutral current flow related to the as kVAr or zero power factor load) is primarily dissimilarity between paralleled sets depends upon dependent upon voltage setting and excitation the shape of their voltage waveforms. system control that is dependent on the percentage Depending on the alternators temperature rise of kVAr load between the generator sets. characteristics, age and insulating ratings, some 6. Compatible Load Sharing neutral current flow between generator sets may not Control Systems necessarily be damaging. However, keep in mind that a high neutral current could cause disruption in Generators can be sharing kW load and may have protective relay operation, particularly for ground fault problems sharing their reactive power load (expressed sensing and may cause electromagnetic interference. as kVAr). With only a small increase of excitation on one set in a paralleled system, reactive load (kVAr) will 7.1 Mechanical Alternator Design increase and may not be equally shared with the other Characteristics Driving Harmonics generator sets. Alternator designers can control the magnitude and Reactive power is primarily dependent upon voltage orders of harmonics produced in an alternator by matching and excitation system control between the manipulation of several design factors, the most generator sets and the means of VAr load sharing important of which is alternator pitch. (reactive load sharing), is discussed below. For paralleling applications, it is highly desirable to Although it is sometimes possible to integrate load utilise 2/3 pitch designs because no third-order share methods of different manufacturers, generator harmonics are created by the machine. Paralleling set governors and load sharing controls should ideally compatibility with utility (mains) sources or other 2/3 be of the same manufacturer to avoid conflicts in pitch machines is assured because there aren’t any responsibility for proper system operation. neutral currents related to third-order harmonics.

The higher-order harmonics see relatively greater impedances due to higher frequencies and so are much less of a problem in terms of neutral current flow. Power Topic #5410679 Power Topic 07 08 Power Topic #5410679 and canresult inlargeneutralcurrent flowsbetween problematical becausetheydirectly addintheneutral Third-order harmonics(andtheirmultiples)are effects inthepowersupplyanddistributionsystem. operation ofloadsandonextraneousheating because theyhavethegreatest impactonthe etc.) are ofgreatest tosystemdesigners concern In general,odd-order harmonics(3rd, 5th,7th,9th higher pitchmachines. the useofmore copperforthesamekWoutputthan statorlesseffectivelyuses thealternator andrequires the generatorcost.Ashorterpitch(lowerratio) optimise thegeneratorwaveformshapeandminimise generator isadesignparameterthatcanbeusedto that machinewouldbe2/3pitch.Thepitchofa winding thatspans8slots,sowith12slotsperpole, the righthalfofillustrationweseeanalternator slot-to-coilratiois10/12,or“5/6pitch”.In alternator slots perpole,andsincethecoilsspan10slots, a 4polemachinewith48totalslots,there willbe12 generator pole.InFigure 3 (lefthalf),whichshows statortothenumberofwindingslotsper alternator number ofslotsenclosedbyeachcoilinthe characteristic ofagenerator. Itistheratioof Pitch isatermusedtodefinemechanicaldesign Figure design:5/6and2/3. 3-Pitchinalternator MAIN ROTOR (4 POLE)

SHAFT S S

N 5/6 OF12SLOTS=10 MAIN STATORWINDING (COIL SPANS1-11) FOR 48SLOTS (48 SLOT) FULL PITCH MAIN ROTOR magnitude through alowimpedancecoupling. the twomachinesare lockedtotheexactsame When generatorsare paralleled,thevoltagesof generator sets. Section applicabletoLowVoltage, 3phase,4wire, PitchDifferencesAlternator 7.2. Circulating NeutralCurrents dueto than 3.0%inanysinglephase. measured linetoandneutral,notmore Distortion atanyloadbetweennoandfullload, should havenotmore than5%Total Harmonic size from roughly 100kWto4MWisthatthemachine A goodstandard to achieve formachinesrangingin higher pitchmachines. can bereduced tomagnitudes ofalevelsimilarto pitch machine,thefifthandseventh-order harmonics load devices.However, withcareful designofa2/3 will causesomelevelofabnormalheatinginrotating because theyare “negative sequence”currents, and (and theirmultiples)are considered tobeaconcern some transformertypes.Fifth-order harmonics because theycanmigratethrough thesystemacross paralleled machines.Theyare alsomore problematic

SHAFT S S

N (SPAN 1to9) MAIN STATORWINDING 2/3 PITCH 8 SLOTS Figure System. 5-FourWire only andcanbereduced oreliminatedbyadjusting differences involtagesetting)appearaslinecurrent machines duetolinevoltagedifferences (e.g. In general,circulating currents flowingbetween the neutralofsysteminwhichcurrent canflow. current andisapparent whenthere isapaththrough paralleled. Thisisreferred toascirculating neutral current willthen flowbetweenthemachinesonce difference betweenthemachinespriortoparalleling, So, atanypointinthecyclewhere there isavoltage generated isathird-order harmoniccurrent. times ineachhalfcycle,thecurrent magnitude blue andred voltagelinescross eachother three to more clearlyillustratethephenomenon.As The magnitudeofthecurrent shownisexaggerated represented bythegreen line. in current flowbetweenthemachines,whichis on acommonbus,thedifferences involtage result versa. Whenthemachinesare connectedtogether time, thebluevoltageishigherthanred, andvice RMS voltagemagnitude,butatdifferent pointsin these voltagewaveformsmaybeexactlythesame lines) are superimposeduponeachother. Notethat In Figure 4,twovoltagewaveforms(thered andblue phenomenon. instantaneous voltage.Figure 4illustratesthis instantaneous voltagetothemachine(s)withlower large current flowfrom themachinewith higher Small differences involtage,willresult inrelatively NEUTRAL CURRENTPATH LV 4-WIREPARALLELED GENSETS G N C B A Figure 4-Voltages before andafterparalleling. no loadonthesystem. by displayingcurrent flowingfrom eachgeneratorwith current metering.Thesystemwillbemostapparent devices butisoftenvisiblewithconventionalAC This willbeclearlyseenwithproper measuring conducted. with balancedlinearload(orevennoload)canbe current flowbetween themachineswhenoperating load sharing,aharmonicanalysistestoftheneutral If thiscurrent flowcannot beadjustedoutbykVAr consult themachinemanufacturer foradvice. on thesemachines,whichwillvarywithapplication– considering theconnectionofneutralconductors often 5/6pitched.Care musttherefore betakenwhen windings are uncommon onthesemachines,whichare distributed neutralconductorsand2/3pitched Higher voltagesystemsdonotnormallyhave connected neutralconductorsasinfigure 5 below. with 2/3pitchedwindingsthatwillallowdirectly is commonpracticetoprovide lowvoltagealternators system toallowsinglephaseloadconnectionandit the neutralconductorisdistributedthroughout the In mostlowvoltageelectricalsystems(below1000V), voltage settings. conductor isunlikelytobeduedifferences in the voltagesettings.Current appearingintheneutral OF TWODISSIMILARMACHINES VOLTAGES PRIORTOCONNECTION PARALLELING (MAGNIFIED) RESULTANT CURRENTFLOWAFTER

09 Power Topic #5410679 10 Power Topic #5410679 3. 2. 1. alternators. possibility thesegeneratorsetmayhaveincompatible When parallelingdissimilargeneratorsets,there is Differences 7.3 InSummary-CompensatingforPitch incompatibility. system, thenthisisalmostcertainlyduetoalternator system) withnoloadoralinearappliedtothe 150Hz ina50Hzsystemand180Hz60Hz current isflowingathigherthan60Hz(particularly application ofsinglephaseloads.However, ifneutral flow isa result ofinaccuratekVAr loadsharingorthe same asthesystemoperatingfrequency, thecurrent If thefundamentalfrequency ofthecurrent isthe ■ ■ ■ Depending onthemagnitude,thismaycause: hasbeenpurposelyoversized. the alternator if thegeneratorisoperatingatlowload,or especially not bedamagingtothealternators Keep inmindthiscirculating current mayor susceptible tofailure. willbehighestandmost of thealternator temperaturethat isthepointatwhichinternal magnitude ofcurrent flowatratedload,because However, willbethe typicallythemajorconcern apparent atsomeload levelsthanatothers. operating withdissimilargeneratorsmaybemore varies withtheload,negativeeffects of As harmoniccontentofageneratorwaveform by thedifferences pitch. inalternator then itcanbeassumedtoaconditioncaused compensation (droop circuit kitorotherdevices), by manipulationofvoltagesettingorcrosscurrent circulating linecurrent andcannotbeeliminated neutral path,ifthisisnotaccompaniedby Should acirculating current befoundin the ■ ■ ■ harmonic currents. Disturbance toequipmentthatissensitive in thesystem Nuisance trippingtootherprotective devices earthing resistors (iffitted) Heating intheneutralconductors,or Figure 6-Droop governing. called VAr droop compensation. regulation system.VAr loadsharingviadroop isoften The samepracticescanalsobeappliedtothevoltage generator sets. drops result inequalloadsharingbetweenparalleled as theloadincreases. Identicalspeedandvoltage decline byapredetermined percentage (typically3%) engine speed(measured voltageto in Hz)oralternator Droop orvoltageregulation governing allowsthe to withinstablefrequency andvoltagevalues. Importantly thegeneratorscanthenalsobecontrolled identical droop frequency andvoltage atthesamerate. isolated busaslongthegeneratorscanbesetupto two ormore compatiblegeneratorsoperatingonan many years.Thisallowsloadsharingbetweenany been historicallyusedforisolatedbusparalleling Droop andvoltageregulation governing systemshave 8.1 Droop LoadSharing 8. LoadSharingMethods 4. PERCENT HZ OR VOLTS 1 5 measures tolimitneutralcurrent flow. expansion flexibility, or require othersystem machines), buttheirusemaylimitfuture system used (andinconjunctionwith2/3pitch operation ofgenerators.Otherpitchesmaybe 2/3 pitchisnotrequired forsuccessfulparallel DROOP PERCENT LOAD ISOCHRONOUS 1 95 Generator 2. different droop rates.Generator1alwayscarriesmore loadthan Figure 8-Generatorsstartatsamenoloadfrequency buthave supervisory systemisfitted. vary considerablyovertimeunlessanadditional busbar frequency, especiallywhere theloadcan this systemwillrequire attendedcontrol tomaintain of someloads.Themajordisadvantagebeingthat with droop control canbedisruptivetotheoperation engines mayvary, thefrequency variationsthatoccur torque curvesatdifferent loadlevelsondissimilar significant inisolatedbussystems,butbecausethe occur duetodroop operation atthislevelare not from noloadto fullload.Thevoltagevariationsthat can bedifferent, typicallyintherangeof3%to5% Common droop settings forfrequency andvoltage no loadGenerator2experiencesreverse kVAR. voltages. Generator1alwayscarriesmore loadthanGenerator2.At Figure 7-Generatorsdroop atthesameratebutstartdissimilar 1 PERCENT Hz PERCENT VOLTS 1 GENERATOR 2 GENERATOR 2 GENERATOR 1 GENERATOR 1 PERCENT LOAD(kVAR) PERCENT LOAD(kW) 1 1 95 95 significant, especiallyinemergency/standbysystems variations thatoccurduetodroop operationcanbe significant inisolatedbussystems,butthefrequency variations thatoccurduetodroop operationare not fortheirloadsharinginterface.Thevoltage concern it alloweddissimilarmachinestobeparalleledwithout The majoradvantageinusingdroop parallelingisthat often termed“reactive droop compensation”. the othercanbedroop. VAR loadsharing viadroop is same typeofsystem.Onecanbeisochronous and load sharing,buttheydonotbothneedtobethe Note thatsystemsalwaysrequire bothkWandkVAr Figure 8showstheimpactofdissimilardroop settings. reverse kVAr atnoload. load thanGenerator2.2experiences voltage settings.Generator1alwayscarriesmore Figure 7illustratestheimpactofincorrect noload FL =fullload NL =noload Voltage (V)droop: (100)[(V Frequency (Hz)droop: (100)[(Hz Droop canbecalculatedasfollows: droop donotneedtobethesamepercentage. It isworthnotingthatfrequency droop andvoltage ■ ■ ■ conditions needtoexist: For adroop systemtofunctioncorrectly, thefollowing ■ ■ ■ same ratefrom noloadtofullload. Each enginemustbesettodrop frequency at the the sameratefrom noloadtofullload. Each generatormustbesettodrop voltageat disconnected from thebus. load frequency andvoltagewhentheyare The generatorsmusthavethesameno NL -V FL ) /V NL -Hz FL ] FL ) /Hz FL ]

11 Power Topic #5410679 12 Power Topic #5410679 of copperwires (onepairkWandonekVAr) this loadshare lineisusuallyasimpletwistedpair to provide thisinterface.Asindicatedpreviously Several approaches are availableinthemarketplace load sharingcommunicationline. the system.Eachcontroller isthenconnectedwitha value asthepercentage ofreal andreactive loadon of kWandVAr loadonthegeneratortosame the fuelandexcitationtodrivesamepercentage Individual generatingsetcontrollers managesboth the system. value tothepercentage ofreal andreactive loadon on aspecificgeneratorsetandthencompare this calculating thepercentage ofreal andreactive load Isochronous loadsharing controls are activesystems using thebackupDroop mode. line, thesystemcanstillcontinueinanemergency sharing ifthere were tobedamagetheloadshare Whilst thesitelosesaccuracyofisochronous load share linesbetween synchronised generatorsiscut. Droop Backup incasethe“daisychain”ofload droop), are protected withabuilt-inpre-configured Today mostIsochronous systems(operatingwithout 8.2 Isochronous kW and kVAr Load Sharing same valuesonceconnectedtotheutility. of connectedgeneratorsetsare thenlockedtothe infinity source) asthevoltageandfrequency outputs generators are paralleledtoautilitysource (or other VAr/power factorcontrollers shouldbeusedwhen as theloadonsystemchanges. voltage levelatanypointinautilitydistributionsystem effective forutilityparallelingduetothegreatly varying usually veryconstant.However, reactive droop isnot paralleling systems,becausetheutilityfrequency is loading control insinglegeneratorset-to-utility Droop isgenerallyusedforgenerator governing from noloadtofullload. can bedifferent andare typicallyintherangeof3–5% Common droop selectionsforfrequency andvoltage where theloadcanvaryconsiderablyovertime. intentional droop in voltage from no load to full load on Using cross current compensationresults inno using cross current compensationare allidentical. practice istoensure allvoltageregulators inasystem regulators worktogetherinthismode,sobestplanning all ofthesamemanufacturer andmodel.Notallvoltage The systemworksbestwhenthevoltageregulators are an identicalvoltagebiastoeachAVR inthesystem. generator setCT’s are theninterconnected toprovide on “B”phaseofeachgenerator. Theseindividual of adroop current transformer(Qty1CT),usually intentional voltagedroop. Thisisachievedbyinsertion operation ofparalleledgeneratorsetswithout sets. Cross current compensationdescribesthe sets causedbydissimilarexcitationlevelsinthose Cross current istheflowofcurrent betweengenerator Circuit CT) 8.3 Cross Current Compensation(Droop performance isnotacceptable. dramatic systemchangesiftheloadsharinggateway gateway isnotfullyfunctionalorbeprepared formore for performanceofthesystemifproposed designer willneedtoclearlyidentifytheresponsibility digital communicationloadsharingequipment.A interface module(gateway)are availableforusewith nearly anygeneratorset.Insomecasesloadsharing This nowmakesthecontrols easiertointerfacewith controlbias togovernor andvoltagebiastotheAVR). which hasacommonloadsharinginterface(speed communication/control-based loadsharingequipment a singlecard), almostalluseproprietary digital (those thatprovide alltheparallelingfunctionson the current buildofintegratedparallelingcontrols equipment toanexistingsystem.However, in protocols makingitdifficulttoadddissimilar Historically everysupplierhaddifferent communication operating onanisolatedbus. generator controls interactwitheachotherwhen generator setcontrollers andistheonlypointwhere connected ina‘daisychain’toalltheother neutral notconnectedongeneratorG3. one dissimilarduetopitchdifference. The illustration showsthe Figure 9-Systemwithtwoidenticalgeneratorsets (G1&G2),and combinations ofgeneratingsetsare viable. of potentialfailure modes andtoensure thatall when consideringsuchasystem,totakeaccount in asystem,howevergreat care mustbetaken viable whenthere are nosuddenlargeloadchanges Programmable LogicController (PLC).Thissystemis the base-loadmachineswillbemanipulatedbya the availableload.Occasionallytotalloadon load sharingmodewill“float”withthebalanceof output, whilethegeneratorsoperatinginisochronous The baseloadmachinesoperateataconstant others operateinaloadshare state. sets inthesystemoperateatabaseloadlevel,and to configure thesystemsothatsomeofgenerator equipment from different suppliers,itisalsopossible When tryingtointerfacedissimilarloadsharing control upgradescanbeaccomplished. be addedtoexistingsystems,andhowparalleling range ofpossiblevariationsinhowgeneratorsetscan for upgradeofexistingequipmenthasledtoawhole The availabilityofsingleboard parallelingcontrollers Load Sharing 8.4 UsingDifferent OperatingModesfor performance perspective. to areactive droop compensation system,from a the system.Therefore itisconsidered tobesuperior GENSET 1 ENG 52 GENSET 2 ENG 52 GENSET 3 ENG 52 4. 3. 2. 1. Points regarding thisillustration: the othertwo. together inasystem,withonemachinedissimilarto three 1000kWgeneratorssetsare connected Figure 9illustratesasituationwhere forexample, this canbeusefulinlimitingearthfaultlevels. that G3willalsomakenocontributiontoearthfaults– basis ofG1andG2,insteadallthree. Notealso sequence limitwouldneedtobecalculatedonthe be significantimbalanceinthesystem,negative contribution tosingle-phaseloads.Ifthere were to to Line-Lineconnectedloads–itwouldmakeno also notethatG3wouldonlyprovide contribution the costofneutralgrounding switchgear. You should 2/3 pitchedandG3is5/6pitched.Thiswouldsave in athree generatorsetinstallationwhere G1& G2are Typically thisarrangementcouldbeusedforexample and protection scheme. be madetodetectthisanomalyinthecontrol should anearthfaultoccursandprovision should to closecouldcausedangerous systemvoltages protection. Failure ofthedesignatedcontactor contactors havetobetreated withback-up be usedinthisscenario(onLV generators),the Likewise ifNeutralEarthContactorswere to disconnect andshutdown. grounded generator, bothsetsmustimmediately of ashutdownfaultoccurred ontheonly with onlyoneothergeneratorset,intheevent If generatorG3iseveroperatinginparallel there wouldnotbea definedearth(Ground) path. Generator G3mustneveroperateonitsownas plant orsomeGridCapacitysupportgeneratorset. illustration wouldonlyapplytoanisolatedpower Code compliance.Itislikelythatthisproposed networks. Checksshouldbemadefirstforlocal permitted onpublicallydistributedgenerator regions, disconnected Neutralsare notalways It shouldbenotedthatinEurope andsomeother

13 Power Topic #5410679 14 Power Topic #5410679 4. 3. 2. 1. load sharingcouldbeaccomplishedasfollows: generators share reactive loadviadroop. ThekW arrangement, it’s assumedthethree 1000kW In thisexamplethesystemisanisolatedbus Figure 10-G1&G2LoadShare withG3Pre-Set BaseLoadof500kW. kW machines. when inparallelwitheitherorbothoftheother to operatewithsaya500kWbaseloadoutput slightly lowerspeedthatissufficienttocauseit frequency, generatorG3issettooperateata theothertwomachinesoperatingatrated With It operatesindroop forkWloadsharing generators isonthebus It isnotusedunlessatleastoneoftheother first tostart Generator G3issetupsothatitcannotbethe CLOSES TOBUS GENSET 1OR2 ATS OPERATES CLOSES TOBUS GENSET 2OR1 OTHER ATS OPERATE GENSET 1AND2WHILE 3 ISOPERATING TOTAL LOADON TOTAL GENSET3CONTRIBUTION ATS ALLRETRANSFER machines operatinginloadshare mode. statetopreventgovern over- orunder-loading ofthe be usedtovarytheloadlevelonmachine(s)in system variessignificantly, aPLCorotherdevicemay In caseswhere thisisusedandtheloadlevelon operates isochronously andshares loadproportionally. or kVAr loadlevel,whilethebalanceofsystem machines inasystemtooperateatfixedkWand/ controls cansometimesbeusedtocausesomeofthe (grid-parallel)loading In asimilarfashion,loadgovern until itisdisconnectedfrom thebus. then operatesatafixed500kWbaseloadoutput Generator G3thenassumesitspre-set loadleveland synchronised andclosedtothebus. already runningandcarryingsystemload,G3isthen Therefore asillustratedinFigure 10,withG1andG2 TIME ■ ■ ■ ■ ■ ■ discussed inthispaper: should takeintoaccountsomekeyconsiderationsas When parallelingdissimilargeneratorsets,adesigner 9.1 KeyConsiderations 9. GeneralRecommendations ■ ■ ■ ■ ■ ■ can beautomaticallyelevatedtotheoff-load that theon-loadgeneratorsetbusbarvoltage voltage matchingmustbeemployedtoensure for utilityparallelingapplications,asystemof bus kVAr loadsharing.Ifdroop control isused power applications,utilityparallelingandisolated Droop control maybesuitableforsomeprime manufacturer andmodel. be ofthesametypeandpreferably ofthesame sharing controls (both kWandkVAr) should all For emergency/standbyapplications,load circulating currents. controls are usedtoprevent 3rd harmonic that ifneutralsare intended tobeinterconnected, areUnless 2/3pitchedalternators used,ensure cannotbeavoided. steps ifdissimilaralternators machines toavoidproblems. Take mitigating Specify either2/3pitchor5/6onall Verify isthesame. ifthepitchofallalternators power tripping. generator setswillnotresult innuisancereverse load steps(particularlyrejection) ofallthe generator setsinthesystemandverifythat Review thetransientperformanceofall under loaded. system loadswithoutbeingeitheroverloadedor means toshare activeandreactive applied Generators operatinginparallelmusthavea term issues. current atthemomentofcouplingtoavoidlong fullload limited tolessthan50%ofalternator Transient current whilstsynchronising shouldbe ■ ■ ■ ■ ■ ■ ■ ■ ■ Cummins WhitePapersandTechnical Manuals: Subject 9.2 AdditionalReference Readingonthis ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Applications Technical Manual#T-016 –Paralleling Pitch–ChrisWhitworth and 5/6Winding White Paper#WP105–ACGeneratorswith2/3 factor onsynchronous –GaryOlson alternators White Paper#6001–Impactofleadingpower 10 startingrequirements –LaLiberte&Kaderbhai White Paper#NAPT-5675-EN –NFPA110 Type vs DeadBusParalleling–RichScroggins White Paper#GLPT-6174-EN –RandomAccess simple Parallelingapplications–RichScroggins White Paper#5590–Reliabilityconsiderationsin Generators Part3–GaryOlson White Paper#9017–ParallelingDissimilar Generators Part2–GaryOlson White Paper#9016–ParallelingDissimilar Generators Part1–GaryOlson White Paper#9015–ParallelingDissimilar generator setisnotmore than10–15%. the voltagewaveformwithloadsrunningon sooveralltotalharmonicdistortionof alternators for thisdistortionistoprovide relatively large industrial load.Theonlywaytocompensate very highlevelsofcurrent distortioninan waveform quality. Itisnotuncommontohave Load canalsoaffect generatorsystemvoltage be impossible. utility voltageasotherwise,no-break transfermay

15 Power Topic #5410679 About the author Robert (Bob) Patrick is a Lead Project & responsible for the installation and handover of several Systems Application Engineer in the Sales large medium voltage turn-key projects, specifically in Application Engineering Team that supports Saudi Arabia and Nigeria. Distributors across Europe and Russia. His first contact with the Power Generator Throughout the whole of his career Bob has industry was during his Electrical Technician gained extensive experience in both LV and apprenticeship. This also included a period working MV Systems, Diesel Gensets, Synchronising in the development laboratory plus on-site test and Switchgear. commissioning work of generators. Prior to joining Cummins in 2001, he spent Bob is a certified 6 Sigma Green Belt and today the previous 17 years in the Middle East this lifetime experience of the product is channelled and Africa working with both switchgear into providing training and technical support to his and generators. During this period Bob’s colleagues across Europe. role was project management, primarily installing new power plant and was directly

Acknowledgements:

My thanks are extended to colleagues here in the Sales Application Engineering European Team, headed by José F Rodriguez who provided me with both support and data for each section throughout the preparation of this document. However, my special thanks are also extended to two people who contributed at the final stages with detailed support during the review stages to ensure documentary accuracy:

■■ Richard Meadows IEng FIET who kindly provided both technical advice and guidance during the editing of this document. ■■ Andrew D Frazer who also provided further detailed advice and guidance during the process of final editing of this document.