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 and function of the station and its relation to the system. Frequency control is sometimes
assigned to one of the largest generating stations. Very large systems are sometimes divided into
load districts, each with its own load dispatcher, often with a central load-dispatching agency for
general supervision over the districts.
(c) Automatic load-frequency control is necessary for maintenance of good overall
system operations, proper sharing of load between generating stations, suitable regulation of
tie-feeder loading between systems, and maintenance of proper frequency and time control. The
problem of control resolves itself into:
o
The measurement of a quantity.
o
Interpretation of the measurement in terms of deviation from a control point.
o
The application of corrections to restore the measured quantity to its normal
value. In some cases more than one measurement is required for proper
operation of the control equipment.
Generator, station, and system loads are measured through the summation of various
thermal-converter millivolt outputs. Frequency is measured by a frequency-bridge type
instrument. As this data is fed into a master controller, it is able to detect the need for more or
less generation and to send impulses to the different stations calling for load increase or
reduction. By the use of area requirement (proportional load control), the equipment is able to
call for changes at the several generating stations. In effect, they supply the load of their
respective areas, thereby causing a minimum of power flow over tie feeders from one station to
another.
6.2.3.2 Reactive Power and Voltage Control. A generator basically controls two of the
key parameters in a power system: the amount of power being generated to meet frequency
specifications and the amount of reactive power being supplied to meet voltage specifications.
The amount of field current supplied to the generator's rotor winding controls the amount of
reactive power generated. Normally, a monitoring system senses the voltage and automatically
adjusts the field current to the generator to maintain that voltage at some prescribed value. If the
supply for a generator's field current fails, or if a short circuit develops within the field winding,
the generator's ability to control the voltage within a power system is lost. Its ability to continue
supplying electric power, therefore, is greatly reduced. When a generator in this condition is
allowed to remain connected to the system, it will cause a severe local voltage depression. This
forces the system to supply a flow of reactive power to the generator from other sources in order
to maintain generator excitation. In many cases, however, the generator will continue to supply
electric power, even after it loses its excitation supply, by operating as an induction machine. It
is then functioning at a slightly higher speed than that corresponding to system frequency.
Consequently, the iron in its rotor's solid forging may overheat. A loss-of-field or
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