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tap-changing transformers (LTCs), substation feeder or bus
voltage regulators (employed in Figure 7.3), line voltage
regulators, and fixed or switched shunt capacitors.

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Primary Distribution

The primary distribution system consists of the feeders
emanating from the substation and supplying power to 1 or more
secondary distribution systems. Such feeders are usually 3-phase
circuits.
Feeders are almost always radial from substation to loads (i.e.,
one way flow of power) in rural areas, usually radial in residential
neighborhoods, and they are often radial even in urban areas. In
densely populated urban areas, particularly commercial and
business districts where reliability is critical, feeders may be
looped. The prices to pay for such a reliability benefit are as
follows:
 Protection is more costly since a given fault on the loop will
require at least two protective devices to operate, and to obtain
the reliability benefits, multiple switching devices must be
installed along the feeder.
 The fault currents tend to be lower, closer to normal load
currents, and therefore there is less margin between breaker
trip current and normal load current.
 Voltage control is complex since there are 2 control points.
One way to obtain the reliability benefit of a looped
configuration while avoiding some of the above difficulties is to
operate a looped configuration in open-loop, i.e., employ a
normally open switch mid-way in the loop. Then when the loop is
faulted, the normally open switch can be closed while a switch just
downstream of the fault can be opened, and all of the de-energized
loop up to the downstream switch can be supplied. This is
illustrated in Figure 7.4.
As indicated previously, the standard primary distribution
voltage levels include 4.16kV, 7.2kV, 12.47kV, 13.2kV, 14.4kV,
23.9kV, and 34.5kV.
However, equipment is specified in terms of voltage class.
Equipment of one voltage class may be utilized in at any operating
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