CIRCULATING CURRENTS TESTING AND ADJUSTING The kilowatt (or horsepower) load on parallel alternators is  entirely  a  function  of  the  driving  source.    Thus,  to increase  the  load  demand  on  one  generator  set  in  a parallel   system,   the   governor   speed   setting   of   that generator set must be increased. Changing the voltage setting on one generator   does not change  the  kilowatt  load  division  between  generators. This   fact   is   often   confusing,   as   observation   of   line ammeters after a voltage level adjustment will indicate a current increase, leading to the belief that one generator has   picked   up   "load."   It   has   not.      Instead,   currents circulating   between   generators   have   changed.      The panel ammeters indicate this change. Paralleled alternators must operate at the same terminal voltage  since  they  are  physically  connected  through  the paralleling  bus.    If  internally  generated  voltages  are  not exactly equal, one alternator will automatically supply an exciting  or  magnetizing  current  to  the  other  alternator  to raise its internally generated voltage.  At the same time, the  second  alternator  will  supply  a  current  to  the  first, which  will  lower  the  generated  voltage  of  this  unit.    The net   result   of   circulating   or   "cross"   current   is   equal generated voltages. This  action  is  inherent  and  automatic.    The  amount  of circulating   current   flow   is   entirely   a   function   of   the internal    voltage    generated    by    each    of    the    several alternators in the parallel system. The   amount   and   type   of   connected   load   also   affects internally    generated    voltage.        Induction    motors,    for example,    will    tend    to    lower    the    generated    voltage because    the    motors    require    magnetizing    current    in addition   to   power   producing   current.      The   generator which  is  trying  to  produce  the  higher  generated  voltage will    supply    a    proportionately    greater    share    of    the magnetizing  current  not  only  to  the  motors  but  to  other generators on the bus. When  generators  are  run  in  parallel,  a  current  sensing system  must  be  added  to  each  voltage  regulator.    The current    sensing    system    samples    the    generator    line current not only in quantity but also in its phase (angular) relation  to  the  voltage.    The  current  sensing  or  droop system  produces  a  voltage  that  adds  to,  or  subtracts from,    the    voltage    sensed    by    the    voltage    regulating system.      (This   accounts   for   the   name   often   used: Voltage  Droop  System.)  The  resultant  regulating  voltage level  (plus  or  minus  droop  voltage)  causes  the  regulator to   adjust   the   alternator   exciting   current   downward   for lowered   generated   voltage,   or   upward   for   increased generated  voltage.    Within  limits,  the  complete  regulator keeps   individual   generated   voltages   nearly   equal   and amperes balanced. In any alternator power system -- single or multiple -- the system   voltage   level   is   established   by   the   level   of generator  excitation.    When  the  system  is  supplying  a purely    resistive    load    (unity    power    factor),    generate excitation    is    normally    expected    to    come    from    the individual   generator   exciters   (static   or   rotary).      If   one generator   exciter   in   a   parallel   system   is   somewhat deficient,   the   additional   excitation   will   be   supplied   by circulating currents from other generators on the bus. When the system is supplying induction motors, a higher exciting  or  magnetizing  current  is  needed  to  provide  the magnetic  forces  in  the  motors.    This  motor  excitation subtracts  from  the  total  generator  excitation  driving  the generated    voltage    downward.        All    of    the    voltage regulators    in    the    system    sense    this    decrease    and individually  raise  the  excitation  level  and  the  generated voltage of their respective generators. If   the   voltage   regulator   action   and   resultant   generator performance    are    precisely    uniform,    each    generator would supply its exact proportion of additional magnetizing   current.      In   practice   this   does   not   occur. Very small differences result in relatively large differences   of   current   supplied.      The   voltage   droop system  senses  these  currents  (in  amount  and  in  phase or   power   factor)   and   causes   the   voltage   regulator   to react   in   the   correct   direction,   raising   or   lowering   the individual    excitation    level.        The    result    is    controlled division of total line current. Droop  systems  will  function  correctly  only  if  the  current sensing transformers of the several generators are all in the  same  phase  or  line  lead.    (T-2  in  Caterpillar  SRCR Generators, T-8 in SR 4 Generators.) Droop   systems   are   proportionate.      This   means   that droop  system  reaction  is  proportionate  to  the  ampere load  on  an  individual  generator  set.    Example:  the  total kW  load  on  the  system  is  150  kW  at  0.8  P.F.    One generator  is  supplying  50  kW,  and  the  other  100  kW. The  total  kVA  (187.5)  should  be  proportioned  with  62.5 kVA on the 50 kW unit, and 125 kVA on the 100 kW unit. Indicated     individual     line     amperes     would     also     be proportionate,  with  one  third  of  the  total  current  coming from  the  50  kW  unit  and  two  thirds  of  the  load  current coming from the 100 kW unit. 123