CIRCULATING CURRENTSTESTING AND ADJUSTINGSUBJECT:Currents - Circulating in Paralleled GeneratorsBUSINESS:Building Services, Marine, Material HandlingPRODUCT/APPLICATION:Generator SetsWhen two or more generator sets are operated inparallel, a current may circulate between the generators.This current will exist when the internal voltagegenerated by each generator is slightly different, but theterminal or bus voltage is the same. In the mostelementary form, current will flow out the line leads ofone generator, through the paralleling bus and into thesecond generator. It does not flow into the load. Thiscurrent, called "circulating current," is in addition to thenormal line current supplied to the connected load.When more than two generators are in parallel, currentcould flow out of any generator and into one or more ofthe other generators. Circulating currents can takemany paths into and out of the several generators.We are concerned with these "wattless amperes" onlywhen they interfere with normal generator set operationor when the normal on-line kVA capacity of thegenerators must be reduced because of excessive linecurrents. With no-load (zero kilowatts) on a generator ina parallel system, Caterpillar generators can readilytolerate a circulating current equal to 20%-25% of theline ampere rating shown on the generator name plate.At load conditions (100% kW load), Caterpillargenerators will tolerate a circulating current of up to 10%of the rated line amperes.Since circulating currents pass through the generatorcoils, these currents heat the coils the same as does theload current. Further, since circulating currents aresuperimposed on the load current passing through thecircuit breaker, circulating currents can cause a breakerto trip as the breaker could "see" an actual ampereoverload. More complex control systems include"reverse current" relays which sense counter flowcurrents. Currents in excess of the relay setting willactuate the circuit breaker trip mechanism.Observed line current (as indicated by panel ammeters)in a parallel generator set system is a summation of twoor three currents:1.Load current -- that current which is supplied tothe load. It may be in phase with the voltage(unity power factor) or somewhat out of phasewith the voltage (power factor less than unity).2.Harmonic current -- usually third harmoniccurrent which flows through the entire systemwhen "Y" connected paralleled generators havetheir neutral leads connected, either directly orthrough an earth or ground connection.3.Circulating current -- that current which flowsbetween generators for reasons explainedbelow.Each of the above currents contribute heat to thegenerator coils, the amount being equal to the square ofthe sum total current times the resistance of the coils.Thus, if the current doubles, the heat loss increases by afactor of four. Coil heating reflects in possibleoverheating and lowered efficiency. In very largegenerators, this is an important consideration.Significance of efficiency decreases with smallergenerators. However, coil heat is always a factor, as itmust be removed by ventilation or radiation to keep coiltemperatures to an acceptable maximum.The load alone determines the load current. Reactors orswitches can be placed in neutral leads to reduce oreliminate third harmonic currents. Proper generatorvoltage adjustment can bring operating circulatingcurrents to a minimum.Circulating currents perform an important function: theyaccount for misadjustment of the generator voltagecontrol system as well as slight variations in the controlsystems.122
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