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1.2 ELECTRIC POWER GENERATION. A generator is a machine that transforms mechanical
energy into electric power. Prime movers such as engines and turbines convert thermal or
hydraulic energy into mechanical power. Thermal energy is derived from the fission of nuclear
fuel or the burning of common fuels such as oil, gas, or coal. The alternating current generating
units of electric power utilities generally consist of steam turbine generators, gas combustion
turbine generators, hydro (water) generators, and internal-combustion engine generators.
1.2.1 Prime Movers. The prime movers used for utility power generation are predominantly
steam turbines and internal-combustion machines. High-pressure/high-temperature and
high-speed (1800 to 3600 rotational speed (rpm)) steam turbines are used primarily in large
industrial and utility power generating stations. Internal-combustion machines are normally of
the reciprocating-engine type. The diesel engine is the most commonly used internal-combustion
machine, although some gasoline engines are also used.
1.2.2 Generators. Generator Capacity. Turbine units can be built for almost any desired capacity.
The capacity of steam turbine driven generators in utility plants range from 5 MW to 1000 MW.
Most of the installed steam turbine generators are rated less than 500 MW. Gas turbine
generators for electric power generation generally have capacities ranging from 100 kW to 20
MW (but are used in multiple installations). The applications of gas turbine generators include
both continuous and peak load service. Diesel engine generator sets have capacities ranging
from 500 kW to 6500 kW. These units are widely used in auxiliary or standby service in
portable or stationary installations, but they may be used as the primary power source in some
locations. Smaller units (steam turbine, gasoline, or diesel engine) are also available for special
applications or industrial plants. See NAVFAC MO-322 for testing procedures. Generator Voltage. Large generators used by commercial utilities are usually
designed with output voltages rated between 11 and 18 kV. Industrial plant generators are
normally rated 2.4 kV to 13.8 kV, coinciding with standard distribution voltages. The generated
voltage is stepped up to higher levels for long distance power transmission. Generator Frequency. Power generation in the United States is standardized at 60
Hz. The standard frequency is 50 Hz in most foreign countries. Generators operating at higher
frequencies are available for special applications.
1.2.3 Voltage and Frequency Controls. Voltage Control. The terminal voltage of a generator operating in isolation is a
function of the excitation on the rotor field winding. The generator output terminal voltage is
normally maintained at the correct level by an automatic voltage regulator that adjusts the field


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