MIL-HDBK-1003/7
a utility transmission network. It will not be, as a rule, used for a diesel generating
station of any kind, since the station service power requirements are minimal.
The distinguishing feature of a unit type station power system is that the
generator and unit auxiliary transformer are permanently connected together at generator
voltage and the station service power requirements for that generating unit, including
boiler and turbine requirements, are normally supplied by the auxiliary transformer
connected to the generator leads. This is shown in Figure 23. If the unit is to be
connected to a system voltage that is higher than the generator voltage, the unit
concept can be extended to include the step-up transformer by tying its low side solidly
to the generator leads and using the high side breaker for synchronizing the generator
to the system. This arrangement is shown in Figure 24.
Station Service Switchgear. A station service switchgear lineup will be
6.1.2.4
connected to the low side of the auxiliary transformer; air circuit breakers will be
used for control of large auxiliary motors such as boiler feed pumps, fans, and
circulating water pumps which use the highest station service voltage, and for
distribution of power to various unit substations and motor control centers to serve the
remaining station service requirements. Figure 25 is a typical one-line diagram of this
arrangement. If the highest level of auxiliary voltage required is more than 480 volts,
say 4.16 kV, the auxiliary switchgear air circuit breakers will only serve motors 250 hp
and larger, and feeders to unit substations. Each unit sub station will include a
transformer to reduce voltage from the highest auxiliary power level to 480 volts
together with air circuit breakers in a lineup for starting of motors 100 to 200 hp and
for serving 480-volt motor control centers. The motor control centers will include
combination starters and feeders breakers to serve motors less than 100 hp and other
small auxiliary circuits such as power panels.
Startup Auxiliary Transformer. In addition to the above items, the unit
6.1.2.5
auxiliary type system will incorporate a "common" or "startup" arrangement which will
consist of a startup and standby auxiliary transformer connected to the switchyard bus
or other reliable source, plus a low voltage switchgear and motor control center
arrangement similar to that described above for the unit auxiliary system. The common
bus system may have a similar arrangement for the standby transformer.
a)
This common system has three principal functions:
(1) To provide a source of normal power for power plant equipment and
services which are common to all units; e.g., water treating system, coal and ash
handling equipment, air compressors, lighting, shops, and similar items.
(2) To provide backup to each auxiliary power system segment if the
transformer supplying that segment fails or is being maintained.
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