MIL-HDBK-1003/7
20.3.3.2 Incremental Load Reduction. In designing a manual load shedding plan, the
total amount of load to be shed manually should be sufficient to relieve the anticipated
plant overload.
Manual load shedding can be performed in one or several steps to take
account of varying system conditions, such as the difference between summer and winter
peaks or seasonal changes in plant capability because of scheduled maintenance.
Point of Application. In order to determine the point of application, local
20.3.4
or remote, the designer must take into account the location of the specific loads to be
shed and the electric distribution system configuration. This determination will also
be influenced by the load shedding method; underfrequency, transfer trip, or manual.
20.3.4.1 Local Application. When load shedding is initiated locally, that is, at the
power plant, individual outgoing feeders are tripped. Generally, each feeder will be
tripped sequentially in a predetermined order of preference until the overload has been
corrected.
Local load shedding is the system that is the most common and easiest to
implement and can be utilized with underfrequency, transfer trip, or manual systems.
A disadvantage is that, if a feeder is tripped, then all loads served from
that feeder are dropped, regardless of their criticality. In some systems, all non-
essential loads can be grouped on a single feeder, and this feeder is tripped first. In
other systems, this approach may not be practical.
20.3.4.2 Remote Application. With remote application, individual loads are tripped
rather than entire feeders. With this approach, each load can be given a priority and
tripped sequentially in accordance with that priority.
Remote tripping can be readily implemented with underfrequency relays
installed at the remote circuit breaker. With manual or transfer trip load shedding,
the trip signal must be transmitted to the remote circuit breaker.
Electrical Usage and Critcality. In general, the least critical
20.4
loads should be tripped first, and the load shedding should proceed in stages with
progressively more critical loads being shed at each stage. The following provides
guidelines for the determination of the relative criticality of loads.
258
Previous Page
