XIII.  Parallel  Operation 

25.  Any  alternator  can  be  operated  in  parallel,  or  synchronized 
with  any  other  alternator.  A  single-phase  machine  can  be  syn- 
chronized with  one  phase  of  a  polyphase  machine,  or  a  quarter- 
phase  machine  operated  in  parallel  with  a  three-phase  machine  by 
synchronizing  one  phase  of  the  former  with  one  phase  of  the 
latter.  Since  alternators  in  parallel  must  be  in  step  with  each 
other  and  have  the  same  terminal  voltage,  the  condition  of  satis- 
factory parallel  operation  is  that  the  frequency  of  the  machines 
is  identically  the  same,  and  the  field  excitation  such  as  would 
give  the  same  terminal  voltage.  If  this  is  not  the  case,  there  will 
be  cross  currents  between  the  alternators  in  a  local  circuit;  that 
is,  the  alternators  are  not  without  current  at  no  load,  and  their 
currents  under  load  are  not  of  the  same  phase  and  proportional 
to  their  respective  capacities.  The  cross  currents  between 
alternators  when  operated  in  parallel  can  be  wattless  currents  or 
power  currents. 

If  the  frequencies  of  two  alternators  are  identically  the  same, 
but  the  field  excitation  not  such  as  would  give  equal  terminal 
voltage  when  operated  in  parallel,  there  is  a  local  current  between 
the  two  machines  which  is  wattless  and  leading  or  magnetizing 
in  the  machine  of  lower  field  excitation,  lagging  or  demagnetiz- 
ing in  the  machine  of  higher  field  excitation.  At  load  this  watt- 
less current  is  superimposed  upon  the  currents  from  the  machines 
into  the  external  circuit.  In  consequence  thereof  the  current  in 
the  machine  of  higher  field  excitation  is  lagging  behind  the  cur- 
rent in  the  external  circuit,  the  current  in  the  machine  of  lower 
field  excitation  leads  the  current  in  the  external  circuit.  The 
currents  in  the  two  machines  are  thus  out  of  phase  with  each 
other,  and  their  sum  larger  than  the  joint  current,  or  current  in 
the  external  circuit.  Since  it  is  the  armature  reaction  of  leading 
or  lagging  current  which  makes  up  the  difference  between  the 
impressed  field  excitation  and  the  field  excitation  required  to  give 
equal  terminal  voltage,  it  follows  that  the  lower  the  armature 
reaction,  that  is,  the  closer  the  regulation  of  the  machines,  the 
more  sensitive  they  are  for  inequalities  or  variations  of  field 
excitation.  Thus,  too  low  armature  reaction  is  undesirable  for 
parallel  operation. 

With  identical  machines  the  changes  in  field  excitation  re- 
quired for  changes  of  load  must  be  the  same.  With  machines 


154     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

of  different  compounding  curves  the  changes  of  field  excitation 
for  varying  load  must  be  different,  and  such  as  correspond  to 
their  respective  compounding  curves,  if  wattless  currents  shall 
be  avoided.  With  machines  of  reasonable  armature  reaction 
the  wattless  cross  currents  are  small  even  with  relatively  great 
inequality  of  field  excitation.  Machines  of  high  armature  re- 
action have  been  operated  in  parallel  under  circumstances  where 
one  machine  was  entirely  without  field  excitation,  while  the  other 
carried  twice  its  normal  field  excitation,  with  wattless  currents, 
however,  of  the  same  magnitude  as  full-load  current.