AC GENERATOR OPERATION
AC Generators
Summary
The important information covered in this chapter is summarized below.
AC Generator Operation Summary
Power (kW) ratings of an AC generator are based on the ability of the prime
mover to overcome generation losses and the ability of the machine to dissipate
the heat generated internally. The current rating of an AC generator is based on
the insulation rating of the machine.
There are three requirements that must be met to parallel AC generators:
1)
Their terminal voltages must be equal. A mismatch may cause high
currents and generator or distribution system damage.
2)
Their frequencies must be equal. A mismatch in frequencies can cause
one generator to "motor," causing an overload in the generators and the
distribution system.
3)
Their output voltages must be in phase. A mismatch in the phases will
cause large opposing voltages to be developed, resulting in damage to the
generators and distribution system due to high currents.
The disadvantage of a stationary field, rotating armature 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.
A stationary armature, rotating field generator has several advantages: (1) a load
can be connected to the armature without moving contacts in the circuit; (2) it
is much easier to insulate stator fields than rotating fields; and (3) much higher
voltages and currents can be generated.
The advantage of the delta-connected AC generator is that if one phase becomes
damaged or open, the remaining two phases can still deliver three-phase power
at a reduced capacity of 57.7%.
The advantage of a wye-connected AC generator is that each phase only has to
carry 57.7% of line voltage and, therefore, can be used for high voltage
generation.
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