MIL-HDBK-1003/7
Section 6.
GENERATOR AND ELECTRICAL FACILITIES DESIGN
6.1
Typical Voltage Ratings and Systems
6.1.1
Voltages
General. Refer to ANSI Standard C84.1, Electric Power Systems and Equipment -
6.1.1.1
Voltage Rating, for voltage ratings for 60 Hz electric power systems and equipment. In
addition, the standard lists applicable motor and motor control nameplate voltage ranges
up to nominal system voltages of 13.8 kV.
Generators. Terminal voltage ratings for power plant generators depend on the
6.1.1.2
size of the generators and their application. Generally, the larger the generator, the
higher the voltage. Generators for a power plant serving an installation will be in the
range from 4160 volts to 13.8 kV to suit the size of the unit and primary distribution
system voltage. Generators in this size range will be offered by the manufacturer in
accordance with its design, and it would be difficult and expensive to get a different
voltage rating. Insofar as possible, the generator voltage should match the
distribution voltage to avoid the installation of a transformer between the generator
and the distribution system.
6.1.1.3
Power Plant Station Service Power Systems
a) Voltages for station service power supply within steam electric
generating stations are related to motor size and, to a lesser extent, distances of
cable runs. Motor sizes for draft fans and boiler feed pumps usually control the
selection of the highest station service power voltage level. Rules for selecting motor
voltage are not rigid but are based on relative costs. For instance, if there is only
one motor larger than 200 hp and it is, say, only 300 hp, it might be a good choice to
select this one larger motor for 460 volts so that the entire auxiliary power system can
be designed at the lower voltage.
b) Station service power requirements for combustion turbine and
internal combustion engine generating plants are such that 208 or 480 volts will be
used.
Distribution System. The primary distribution system with central in-house
6.1.1.4
generation should be selected in accordance with MIL-HDBK-1004/1.
6.1.2
Station Service Power Systems
General. Two types of station service power systems are generally in use in
6.1.2.1
steam electric plants and are discussed herein. They are designated as a common bus
system and a unit system. The distinction is based on the relationship between the
generating unit and the auxiliary transformer supplying power for its auxiliary
equipment.
a) In the common bus system the auxiliary transformer will be connected
through a circuit breaker to a bus supplied by a number of units and other sources so
that the supply has no relationship to the generating unit whose auxiliary equipment is
being served. In the unit system the auxiliary transformer will be connected solidly to
the generator leads and is switched with the generator. In either case, the auxiliary
equipment for each generating unit usually will be supplied by a separate transformer
with appropriate interconnections between the secondary side of the transformers.
b) The unit type system has the disadvantage that its station service power
requirements must be supplied by a startup transformer until the generating unit is
synchronized with the system. This startup transformer also serves as the backup supply
in case of transformer failure. This arrangement requires that the station service
power supply be transferred from the startup source to the unit source with the
auxiliary equipment in operation as a part of the procedure of starting the unit.
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