Incremental Load Reduction. The amount of load to be shed at each critical
frequency should be related to the expected percentage overload. In general, this
determination can only be made in an approximate manner, because system loads vary and
cannot be determined accurately in advance.
It is important to note that with a multi-stepped underfrequency load
shedding system, quite often the overload will be eliminated and system frequency will
start to recover after only one or two critical frequencies are reached. Herein lies
another advantage of underfrequency load shedding; with a well designed load-shedding
plan, only the minimum amount of load necessary for system recovery is shed.
Transfer Trip Load Shedding. With transfer trip load shedding, protective
relays and circuit breakers are interlocked so that the event that causes a loss of
power supply also causes certain loads to be dropped, thereby preserving a balance
between load and generation. For instance, with an interconnected plant, auxiliary
contacts on the interconnection circuit breaker could be used to trip certain loads
whenever the interconnection failed. Similarly, for an isolated plant, the loss of one
generating unit could be sensed and used to initiate the tripping of a block of load
equal to the generating capacity lost.
Although transfer trip load shedding has the advantage of simplicity, the
use of this method will almost always result in greater loss of load than with
underfrequency load shedding. For this reason, the transfer trip method is generally
Manual Load Shedding. In general, manual load shedding cannot be used to
drops too rapidly for an operator to respond and the plant can be lost within seconds.
Manual load shedding is appropriate to relieve mild overloads; that is,
overloads that are small enough to be accommodated by governor action (thereby
preventing underspeed operation), but that are still large enough to cause excessive
equipment stress (overheating, accelerated wear, etc.) if uncorrected.
It is appropriate to combine manual load shedding (to relieve mild
overloads) with underfrequency load shedding (to relieve severe overloads).
Load Shedding Plan. With manual load shedding, it is appropriate to have a
load shedding plan. This plan can be quite simple; it need consist of no more than a
list of what loads (or feeders) should be tripped and how. It is important that the
plan be prearranged so that the plant operator can implement it quickly under emergency