In determining the design of distribution systems, three broad classifications
of choices need to be considered:
1. The type of electric system: dc or ac, and if ac, single-phase or
polyphase.
2. The type of delivery system: radial, loop, or network. Radial systems
include duplicate and throwover systems.
3. The type of construction: overhead or underground.
DESIRED FEATURES
Electrical energy may be distributed over two or more wires. The
principal features desired are safety; smooth and even flow of power, as
far as is practical; and economy.
Safety
The safety factor usually requires a voltage low enough to be safe
when the electric energy is utilized by the ordinary consumer.
Smooth and Even Flow of Power
A steady, uniform, nonfluctuating flow of power is highly desirable,
both for lighting and for the operation of motors for power purposes.
Although a direct current system fills these requirements admirably, it
is limited in the distance over which it can economically supply power
at utilization voltage.
Alternating current systems deliver power in a fluctuating manner
following the cyclic variations of the voltage generated. Such fluctuations
of power are not objectionable for heating, lighting, and small motors,
but are not entirely satisfactory for the operation of some devices
such as large motors, which must deliver mechanical power steadily and
therefore require a steady input of electric power.
This may be done by supplying electricity to the motors by two or three circuits, each supplying
a portion of the power, whose fluctuations are purposely made not
to occur at the same time, thereby decreasing or damping out the effect
of the fluctuations. These two or three separate alternating current circuits
(each often referred to as a single-phase circuit) are combined into
one polyphase (two- or three-phase) circuit. The voltages for polyphase
circuits or systems are supplied from polyphase generators.
Economy
The third factor requires the minimum use of conductors for delivery
of electric energy. This usually calls for the use of higher voltages
where conditions permit and the elimination of some conductors by
providing a common return path for two or more circuits.
