Essentially
all of the world’s electric power is generated by synchronous machines. The
synchronous generator has proven to be a reliable and efficientdevice for
converting mechanical power to electric power.
Since the typical power system uses
alternating current (60 Hz in the United States), the chief requirement of such
a device is that it produces power at a controllable voltage at a constant
frequency.
A typical
synchronous machine consists of a rotor with a field winding and a stator with
a three-phase ac winding. The rotor has a dc power supply and the stator is
connected to the power system through a generator step-up transformer. The
turbine rotates the field at a constant speed, often as high as 3,600rpm.
If the
stator windings are connected to a load, current flows through the windings and
the load. As the electrical load increases, the prime mover (turbine) must
expend more mechanical energy to keep the rotor turning at a constant speed.
Thus
mechanical energy input by the turbine is being transformed into electrical
energy. Generators in hydro plants are also synchronous machines, but rotate at
lower speeds than steam unit or gas turbines.
The
electrical power produced by a synchronous generator is almost equal to the mechanical
power input, the efficiency being in the range of 98%. The division of electric
load among a number of generators is determined by a number of factors,
including economics.
At a given
operating point each turbine generator has an incremental cost, which is the
cost per kWh to generate an additional small amount of power. Maximum system
economy results when all generators are operating at the same incremental
production cost.
The control
of the real power and regulation of the speed (which must be held constant to
provide a constant frequency) is done with the speed governor and automatic
generator controls (AGC) and interaction with the system control center.
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