Following periods of extended service interruption, the advantages
provided by load diversity are often lost. The term cold load pickup refers to
the energization of the loads associated with a circuit or substation following
an extended interruption during which much of the diversity normally
encountered in power systems is lost.
For example, if a feeder suffers an outage, interrupting all
customers on the feeder during a particularly cold day, the homes and
businesses will cool to levels below the individual thermostat settings.
This situation eliminates the diversity normally
experienced, where only a fraction of the heating will be required to operate
at any given time.
Once power is restored, the heating at all customer
locations served by the feeder will attempt to operate to bring the building
temperatures back to levels near the thermostat settings.
The load experienced by the feeder following reenergization
can be far in excess of the design loading due to lack of load diversity.
Cold load pickup can result in a number of adverse power
system reactions. Individual service transformers can become overloaded under
cold load pickup conditions, resulting in loss of life and possible failure due
to overheating.
Feeder load levels can exceed protective device
ratings=settings, resulting in customer interruptions following initial service
restoration.
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