MIL-HDBK-1003/7
e) In
no event should the current carrying capacity of the power cables
emanating from the
generator be a limiting factor on turbine generator output. As a
rule of thumb, the
cable current carrying capacity will be at least 1.25 times the
current associated
with kVA capacity of the generator (not the kW rating of the
turbine).
6.3.2.2
Segregated Phase Bus
a) For gas turbine generator installations the connections from the
generator to the side wall or roof of the gas turbine generator enclosure will have been
made by the manufacturer in segregated phase bus configuration. The three-phase
conductors will be flat copper bus, either in single or multiple conductor per phase
pattern. External connection to switchgear or transformer will be by means of
segregated phase bus or cable. In the segregated phase bus, the three bare bus-phases
will be physically separated by nonmagnetic barriers with a single enclosure around the
three buses.
b) For applications involving an outdoor gas turbine generator for which a
relatively small lineup of outdoor metal clad switchgear is required to handle the
distribution system, segregated phase bus will be used. For multiple gas turbine
generator installations, the switchgear will be of indoor construction and installed in
a control/switchgear building. For these installations, the several generators will be
connected to the switchgear via cables.
c) Segregated bus current ratings may follow the rule of thumb set forth
above for generator cables, but final selection will be based on expected field
conditions.
6.3.2.3
Isolated Phase Bus
a) For steam turbine generator ratings of 25 MVA and above, the use of
isolated phase bus for connection from generator to step up transformer will be used.
At such generator ratings, distribution seldom is made at generator voltage. An
isolated phase bus system, utilizing individual phase copper or aluminum, hollow square
or round bus on insulators in individual nonmagnetic bus enclosures, provides maximum
reliability by minimizing the possibility of phase-to-ground or phase-to-phase faults.
b) Isolated phase bus current ratings should follow the rule of thumb set
forth above for generator cables.
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