e) The configuration of the high voltage bus will be selected for
reliability and economy. Alternative bus arrangements include main and transfer bus,
ring bus, and breaker and a half schemes. The main and transfer arrangement, shown in
Figure 27, is the lowest cost alternative but is subject to loss of all circuits due to
a bus fault. The ring bus arrangement, shown in Figure 28, costs only slightly more
than the main and transfer bus arrangement and eliminates the possibility of losing all
circuits from a bus fault, since each bus section is included in the protected area of
its circuit. Normally it will not be used with more than eight bus sections because of
the possibility of simultaneous outages resulting in the bus being split into two parts.
The breaker and a half arrangement, shown in Figure 29, is the highest cost alternative
and provides the highest reliability without limitation on the number of circuits.
a) Connections between the generator and switchgear bus where distribution
is at generator voltage, and between generator and step up transformer where
distribution is at 34.5 kV and higher, will be by means of cable or bus duct. In most
instances more than one cable per phase will be necessary to handle the current up to a
practical maximum of four conductors per phase. Generally, cable installations will be
provided for generator capacities up to 25 MVA. For larger units, bus ducts will be
evaluated as an alternative.
b) The power cables will be run in a cable tray, separate from the control
cable tray, in steel conduit suspended from ceiling or on wall hangers, or in ducts,
depending on the installation requirements.
c) Cable terminations will be made by means of potheads where lead covered
cable is applied, or by compression lugs where neoprene or similarly jacketed cables are
used. Stress cones will be used at 4.16 kV and above.
d) For most applications utilizing conduit, cross-linked polyethylene with
approved type filler or ethylene-propylene cables will be used. For applications where
cables will be suspended from hangers or placed in tray, armored cable will be used to
provide physical protection. If the cable current rating does not exceed 400 amperes,
the three phases will be triplexed; i.e., all run in one steel armored enclosure. In
the event that single-phase cables are required, the armor will be nonmagnetic.