c) The advantages of the unit system are that it reduces the number of
breakers required and that its source of energy is the rotating generating unit so that,
in case of system trouble, the generating unit and its auxiliaries can easily be
isolated from the rest of the system. The advantage of switching the generator and its
auxiliary transformer as a unit is not very important, so the common bus system will
normally be used.
Common Bus System. In this system, generators will be connected to a common
bus and the auxiliary transformers for all generating units will be fed from that common
bus. This bus may have one or more other power sources to serve for station startup.
a) Figure 22 is a typical one-line diagram for such a system. This type
system will be used for steam turbine or diesel generating plants with all station
service supplied by two station service transformers with no isolation between
auxiliaries for different generating units. It also will be used for gas turbine
generating plants. For steam turbine generating plants the auxiliary loads for each
unit in the plant will be isolated on a separate bus fed by a separate transformer. A
standby transformer is included, and it serves the loads common to all units such as
b) The buses supplying the auxiliaries for the several units shall be
operated isolated to minimize fault current and permit use of lower interrupting rating
on the feeder breakers. Provision shall be made for the standby transformer to supply
any auxiliary bus.
Unit Type System. The unit type station service power system will beused for
a steam electric or combustion turbine generating station serving