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 1.5.2.6 Secondary Network Distribution System. In a secondary network distribution
system, two or more distribution transformers are each supplied from a separate primary
distribution feeder (Figure 1-10).
The secondaries of the transformers are connected in parallel through a special type of circuit
breaker, called a network protector, to a secondary bus. Radial secondary feeders are tapped
from the secondary bus to supply loads. A more complex network is a system in which the
low-voltage circuits are interconnected in the form of a grid or mesh.
(a) If a primary feeder fails, or a fault occurs on a primary feeder or distribution
transformer, the other transformers start to feed back through the network protector on the faulted
circuit. This reverse power causes the network protector to open and disconnect the faulty supply
circuit from the secondary bus. The network protector operates so fast that there is minimal
exposure of secondary equipment to the associated voltage drop.
(b) The secondary network is the most reliable for large loads. A power interruption
can only occur when there is a simultaneous failure of all primary feeders or when a fault occurs
on the secondary bus. There are no momentary interruptions as with transfer switches on
primary selective, secondary selective, or loop systems. Voltage dips which could be caused by
faults on the system, or large transient loads, are materially reduced.
(c) Networks are expensive because of the extra cost of the network protector and
excess transformer capacity. In addition, each transformer connected in parallel increases the
available short-circuit current and may increase the duty rating requirement of secondary
equipment.
1.5.2.7 Secondary Banking. The term banking means to parallel, on the secondary side, a
number of transformers. All of the transformers are connected to the same primary feeder.
Banking is usually applied to the secondaries of single-phase transformers, and the entire bank
must be supplied from the same phase of the primary circuit. All transformers in a bank are
usually of the same size and should have the same nominal impedance.
(a) The advantages of banking include: reduction in lamp flicker caused by starting
motors, less transformer capacity required because of greater load diversity, and better average
voltage along the secondary.
(b) Solid banking, where the secondary conductors are connected without overcurrent
protection, is usually not practiced because of the obvious risks. Three methods of protecting
banked transformers are shown in Figure 1-11. In each arrangement the transformers are
connected to the primary feeder through high-voltage protective links or fuses. Each method
has different degrees of protection, depending on the location of the protective devices in the
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