instead of one, and provides each feeder with enough capacity to carry all
the load. The extra investment (that is, providing for alternate primary
feeders and selector switches) is compensated by the quick restoration of
service in the event of primary feeder failure.
8.2.7
Secondary Selective-Radial Distribution System. The secondary
selective-radial distribution system (see Figure 11) carries duplicate
feeders up to the secondary bus of the load centers, thus compensating for
any fault or malfunction in primary feeders or transformers. A common
configuration involves pairs of unit substations connected through a
normally open secondary tie circuit breaker. If the primary feeder or a
transformer fails, the main secondary circuit breaker on the affected
transformer is opened and the tie cirucit breaker closed. Operation may be
manual or automatic. Normally the stations operate as radial systems. With
the loss of one primary circuit or transformer, the total substation load
may be supplied by one feeder and one transformer. To allow for this
condition, one (or a combination) of the following alternatives are
applicable:
a) Oversizing the primary feeders and both transformers so that
one transformer can carry the total load.
b) Oversizing the primary feeders and providing forced-air cooling
to the transformer in service for the emergency period.
c)
Shedding nonessential load for emergency period.
d) Oversizing the primary feeders and using the temporary overload
capacity in the transformer and accepting the loss of transformer life.
DM 4.2 reads: ". . . Each transformer and its associated equipment shall be
capable of carrying the essential loads of both sections. In sizing the
transformers, take into consideration allowable overloading. . . ."
8.2.8
Simple Network Distribution System. A simple network distribution
system (see Figure 12) must be used for high-density, heavy, concentrated
loads. The main advantages of network distribution systems are continuity of
service under any kind of fault in the system, automatic clearing of faults,
flexibility and adaptability for shifting loads, and good voltage
characteristics.
8.2.8.1
Network Protector. The network protector supplies forward power
into the network, avoids any feedback into the primary line, and clears any
fault automatically.
8.2.8.2
Limitation. The main objection to the use of network systems is
the initial cost involved. Also, when secondary ties extend considerable
distances, the probability of faults occurring is greater.
8.2.8.3
Secondary Loops. The purpose and advantages of the secondary loop
are the same as those for the banked secondary-radial distribution type
(para. 8.2.5). The number of individual cable sets connecting load centers
will determine the reliability of the system. Install limiter lugs of the
cable size used at both ends of any secondary ties. The size of ties shall
be to supply all the load of any load center from one secondary tie.
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