The capacity
of the distribution system is determined in most cases by the thermal ratings
of the equipment. In more rural areas with low load density it may be
determined by voltage limits.
The
distribution substation capacity depends on the size of transformers and the
provision of an additional spare transformer. If a substation has two
transformers, all load must be supplied by the remaining one if one fails.
In this
case, the substation capacity will depend on the capability of the remaining
transformer to carry the load for the time required to replace the failed
transformer, with the capacity being lower if the replacement time is longer.
For
substations with a single transformer, load is limited to what can be transferred
to other substations at remote feeder tie points.
The
allowable primary feeder loading can be limited by the size of conductors used
and the characteristics of the load supplied. If the load varies, higher maximum
loads can be carried by the feeder than steady loads, since the rating of the
feeder depends on the heating effect of the current over time.
Feeder
loading may also be limited by the voltage drop that occurs at the end of
longer feeders.
The
distribution transformer capacity is determined by the size of the transformer and
the characteristics of the load. In some cases, the distribution transformers
are single phase. When a three phase supply is needed, three single phase transformers
can be used, each connected to a different phase of the three-phase primary
supply.
Alternatively,
a three-phase transformer may be used in which the three phases are in a single
tank. The capacity of the secondaries is determined by the size of wire used,
their length, and the characteristics of the load they supply.
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