Quantcast Paralleling AC Generators

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AC GENERATOR OPERATION AC Generators Paralleling AC Generators Most electrical power grids and distribution systems have more than one AC generator operating at one time.   Normally, two or more generators are operated in parallel in order to increase the available  power.    Three  conditions  must  be  met  prior  to  paralleling  (or  synchronizing)  AC generators. Their terminal voltages must be equal.   If the voltages of the two AC generators are not equal, one of the AC generators could be picked up as a reactive load to the other AC generator.  This causes high currents to be exchanged between the two machines, possibly causing generator or distribution system damage. Their frequencies must be equal.   A mismatch in frequencies of the two AC generators will cause the generator with the lower frequency to be picked up as a load on the other generator  (a  condition  referred  to  as  "motoring").    This  can  cause  an  overload  in  the generators and the distribution system. Their  output  voltages  must  be  in  phase.    A  mismatch  in  the  phases  will  cause  large opposing  voltages  to  be  developed.    The  worst  case  mismatch  would  be  180°  out  of phase, resulting in an opposing voltage between the two generators of twice the output voltage.   This high voltage can cause damage to the generators and distribution system due to high currents. During  paralleling  operations,  voltages  of  the  two  generators  that  are  to  be  paralleled  are indicated through the use of voltmeters.   Frequency matching is accomplished through the use of output frequency meters.  Phase matching is accomplished through the use of a synchroscope, a  device  that  senses  the  two  frequencies  and  gives  an  indication  of  phase  differences  and  a relative comparison of frequency differences. Types of AC Generators Figure 5    Stationary Field, Rotating Armature AC Generator As  previously  discussed,  there  are  two  types  of AC   generators:   the   stationary   field,   rotating armature;    and    the    rotating    field,    stationary armature. Small  AC  generators  usually  have  a  stationary field  and  a  rotating  armature  (Figure  5).    One important disadvantage to this arrangement is that the slip-ring and brush assembly is in series with the  load  circuits  and,  because  of  worn  or  dirty components, may interrupt the flow of current. ES-10 Page 10 Rev. 0


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