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CIRCULATING CURRENTS TESTING AND ADJUSTING SUBJECT: Currents - Circulating in Paralleled Generators BUSINESS: Building Services, Marine, Material Handling PRODUCT/APPLICATION: Generator Sets When   two   or   more   generator   sets   are   operated   in parallel, a current may circulate between the generators. This    current    will    exist    when    the    internal    voltage generated by each generator is slightly different, but the terminal   or   bus   voltage   is   the   same.      In   the   most elementary  form,  current  will  flow  out  the  line  leads  of one  generator,  through  the  paralleling  bus  and  into  the second  generator.    It  does  not  flow  into  the  load.    This current,  called  "circulating  current,"  is  in  addition  to  the normal   line   current   supplied   to   the   connected   load. When  more  than  two  generators  are  in  parallel,  current could  flow  out  of  any  generator  and  into  one  or  more  of the   other   generators.      Circulating   currents   can   take many paths into and out of the several generators. We  are  concerned  with  these  "wattless  amperes"  only when  they  interfere  with  normal  generator  set  operation or    when    the    normal    on-line    kVA    capacity    of    the generators  must  be  reduced  because  of  excessive  line currents.  With no-load (zero kilowatts) on a generator in a   parallel   system,   Caterpillar   generators   can   readily tolerate  a  circulating  current  equal  to  20%-25%  of  the line  ampere  rating  shown  on  the  generator  name  plate. At load conditions (100% kW load), Caterpillar generators will tolerate a circulating current of up to 10% of the rated line amperes. Since   circulating   currents   pass   through   the   generator coils, these currents heat the coils the same as does the load   current.      Further,   since   circulating   currents   are superimposed  on  the  load  current  passing  through  the circuit  breaker,  circulating  currents  can  cause  a  breaker to   trip   as   the   breaker   could   "see"   an   actual   ampere overload.        More    complex    control    systems    include "reverse    current"    relays    which    sense    counter    flow currents.    Currents  in  excess  of  the  relay  setting  will actuate the circuit breaker trip mechanism. Observed  line  current  (as  indicated  by  panel  ammeters) in a parallel generator set system is a summation of two or three currents: 1. Load  current  --  that  current  which  is  supplied  to the  load.    It  may  be  in  phase  with  the  voltage (unity  power  factor)  or  somewhat  out  of  phase with the voltage (power factor less than unity). 2. Harmonic    current    --    usually    third    harmonic current  which  flows  through  the  entire  system when  "Y"  connected  paralleled  generators  have their  neutral  leads  connected,  either  directly  or through an earth or ground connection. 3. Circulating   current   --   that   current   which   flows between     generators     for     reasons     explained below. Each   of   the   above   currents   contribute   heat   to   the generator coils, the amount being equal to the square of the  sum  total  current  times  the  resistance  of  the  coils. Thus, if the current doubles, the heat loss increases by a factor    of    four.        Coil    heating    reflects    in    possible overheating    and    lowered    efficiency.        In    very    large generators, this is an important consideration. Significance     of     efficiency     decreases     with     smaller generators.    However,  coil  heat  is  always  a  factor,  as  it must  be  removed  by  ventilation  or  radiation  to  keep  coil temperatures to an acceptable maximum. The load alone determines the load current. Reactors or switches  can  be  placed  in  neutral  leads  to  reduce  or eliminate   third   harmonic   currents.      Proper   generator voltage    adjustment    can    bring    operating    circulating currents to a minimum. Circulating  currents  perform  an  important  function:  they account   for   misadjustment   of   the   generator   voltage control  system  as  well  as  slight  variations  in  the  control systems. 122

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