Page 290 - Introduction to Marine Engineering
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264  Electrical equipment

                 Alternating  current  supply

                 Three-phase  alternators  arranged  for  parallel  operation  require  a
                 considerable  amount  of  instrumentation.  This  will  include  ammeters,
                wattmeter, voltmeter,  frequency meter and a synchronising device. Most
                 of  these  instruments  will  use  transformers  to  reduce  the  actual  values
                 taken  to  the  instrument.  This  also  enables  switching,  for  instance,
                 between  phases  or  an  incoming machine  and  the  bus-bars,  so that  one
                 instrument  can  display  one  of  a  number  of  values.  The  wattmeter
                 measures the power being used in a circuit, which, because of the  power
                 factor  aspect of alternating current  load, will be less than the  product  of
                 the  volts and  amps. Reverse power  protection  is provided  to alternators
                 since  reverse  current  protection  cannot  be  used.  Alternatively various
                 trips may be provided  in the event of prime mover failure to ensure that
                 the  alternator  does  not  act as a  motor.
                   The  operation  of  paralleling  two alternators  requires  the  voltages to
                 be  equal  and  also  in  phase.  The  alternating  current  output  of  any
                 machine  is always  changing,  so  for  two  machines  to  operate  together
                 their  voltages must be changing at  the  same  rate  or  frequency  and  be
                 reaching  their  maximum (or any other  value) together. They  are then
                 said  to  be  'in  phase'.  Use  is nowadays made  of  a  synchroscope  when
                 paralleling two a.c. machines. The  synchroscope has two windings which
                 are connected one to each side of the paralleling switch. A pointer is free
                 to rotate  and  is moved by the  magnetic effect  of the two windings. When
                 the  two voltage supplies  are  in phase  the  pointer  is stationary in  the  12
                 o'clock  position.  If  the  pointer  is rotating  then  a  frequency  difference
                 exists  and  the  dial  is  marked  for  clockwise  rotation  FAST  and
                 anti-clockwise  rotation  SLOW,  the  reference  being  to  the  incoming
                 machine  frequency.
                   To  parallel an incoming machine to a running machine therefore  it is
                 necessary  to ensure  firstly  that both  voltages are equal  Voltmeters are
                 provided  for  this  purpose.  Secondly the  frequencies  must be  brought
                 into phase. In practice the synchroscope usually moves slowly in the  FAST
                 direction  and  the  paralleling  switch is closed  as the  pointer  reaches  the
                 11  o'clock  position.  This  results  in  the  incoming machine immediately
                 accepting  a small amount  of  load.
                   A  set  of  three  lamps  may also  be  provided  to  enable  synchronising.
                 The  sequence  method  of  lamp  connection  has  a  key lamp  connected
                 across  one  phase  with  the  two other  lamps  cross  connected  over  the
                 other  two phases.  If  the  frequencies  of the  machines are  different  the
                 lamps  will  brighten  and  darken  in  rotation,  depending  upon  the
                 incoming  frequency  being  FAST  or  SLOW.  The  correct  moment  for
                 synchronising is when the key lamp is dark and the other  two are equally
                 bright.
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