Parallel Operation of Governors If Unit A had been operating alone carrying full load, the system frequency would be 60 cycles as shown by Line "A"  at  60  cycles  and  100%  load.    Now,  if  the  frequency of  Unit  B  is  adjusted  by  the  engine  speed  control  to  be equal to that of Unit A and the circuit breaker of Unit B is closed,    the    system    would    be    operating    under    the following conditions: 1.  Unit A is at 60 cycles and 100% load. 2.        Unit    B    is    at    60    cycles    and    0%    load.        The characteristic  of  the  governor  of  Unit  B  at  this  time  is shown by the dotted line B. In   order   for   Unit   B   to   carry   load,   it   is   necessary   to advance   the   speed   setting   of   the   governor.      If   it   is advanced    to    the    full    load    position,    the    governor characteristic  of  B  will  coincide  with  the  characteristic  of A.  Since the load on the system was 100% of one unit, no  change  was  made  in  total  load,  and  the  available capacity  is  now  200%,  the  system  will  operate  at  50% load on each unit, and 60.9 cycles for two units of equal capacity  (Point  E).    For  units  of  unequal  capacity,  the load   will   be   divided   in   proportion   to   the   ratio   of   the capacity   of   each   to   the   total   capacity.      The   system frequency   will   be   determined   by   the   points   on   the governor  characteristics  corresponding  to  these  loads. The   frequency   will   be   the   same   for   both   units   since paralleled alternators must operate at the same speed. If  Unit  A  had  been  a  60  KW  unit  fully  loaded  and  a  100 KW   Unit   B   was   paralleled   with   it   and   the   governor adjusted  to  the  full  load  position,  the  final  load  division and frequency would be determined as follows: a.  System load - 60 KW b.  System capacity - 160 KW System Load KW rating for unit c.  The 60 KW unit will carry 60 x 60 KW = 22.5 KW 160 System capacity KW load for unit d.  100 KW unit will carry 60 x 100 KW = 37.5 KW 160 e.      The   system   frequency   can   be   determined   readily from step c or step d.  The load carried by each unit is 22.5 or 37.5 which figures out to be 60 100 37.% of the capacity of either unit Again, using Figure 1 for the governor characteristic of the 100 KW unit and reading up from the value of load (37.5% to point F), we find the system frequency to be 61.125 cycles. Figure 2-Paralleling two units, one with a 3% governor and the other with a hydraulic governor set for isochronous operation. Example 2-One 3% Governor and one 0% (Isochronous) Governor. The characteristics of the 3% governor (Unit A) is shown by   line   A   of   Figure   2   and   the   characteristics   of   the isochronous governor (Unit B) is shown by line B.  Only at  full  load,  60  cycles,  do  the  frequencies  of  the  units have   the   same   value.      It   is   customary   to   operate   a system  of  this  type  with  a  system  load  greater  than  the capacity of Unit A.  In this way Unit A carries its full load at 60 cycles and the additional load and load swings are handled  by  Unit  B,  also  at  60  cycles.    The  system  can maintain constant frequency by this method of operation. The   system   described   in   example   1   cannot   maintain constant  frequency  with  load  changes  because  of  the speed droop characteristics of the governors. In  the  system  described  in  this  example,  if  the  load  is less   than   the   capacity   of   Unit   A   (which   has   the   3% governor)  and  can  be  carried  by  Unit  B,  the  governor setting   of   Unit   A   can   be   reduced   to   give   a   governor characteristic  such  as  A,  so  Unit  A  will  still  carry  the steady  part  of  the  load  and  Unit  B  will  carry  the  load swings.  If the system load is reduced to the point where Unit   A   is   not   operating   at   the   60   cycle   point   of   its governor  characteristic,  Unit  A  will  try  to  motor  Unit  B and   the   system   frequency   may   be   greater   than   60 cycles.  The reason for using an isochronous governor in a power system is to maintain constant frequency above approximately 40% load. 18