Section 8:
SELECTION OF DISTRIBUTION SYSTEMS
8.1
Selection Factors. In selecting the distribution system, the
designer must consider load requirements, permissible voltage regulation,
reliability, flexibility, and life cycle cost.
8.2
Systems Available. The various systems recommended for use are for
medium-voltage distribution systems, unless noted otherwise, and are
described in paras. 8.2.1 through 8.2.11.
8.2.1
Conventional Simple-Radial Distribution System. The conventional
simple-radial distribution system (see Figure 5) requires minimum transformer
capacity, because it takes advantage of the total diversity among loads and
requires less space. The main disadvantages are low flexibility, poor
voltage regulation, and not much reliability because a simple fault in the
main bus may shut off service completely. Cable cost and energy losses are
high. The system should ordinarily be limited to l,000 kVA of maximum
demand and to a low-voltage distribution system.
8.2.2
Modern Simple-Radial Distribution System. The modern simple-radial
distribution system (see Figure 6) can be used for capacities in excess of
l,000 kVA. The length of low-voltage feeders is kept to a minimum, reducing
cable cost and energy losses. Size each transformer to handle the peak load
of the area served. The main inconvenience of this system is that a fault in
the primary circuit can shut off service to all transformers.
8.2.3
Modified Modern Simple-Radial Distribution System. The modified
modern simple-radial distribution system (see Figure 7) provides all the
advantages of the modern simple-radial type, but includes important
improvements in reliability. The initial cost, however, is higher.
8.2.4
Loop Primary-Radial Distribution Type. The loop primary-radial
distribution system (see Figure 8) has the same characteristics as the modern
simple-radial distribution type, but in addition provides a quicker
restoration of service in the event of primary feeder or transformer failure.
Advantages are lower installation costs and energy losses than those of any
of the distribution systems previously discussed.
8.2.5
Banked Secondary-Radial Distribution System. The banked
secondary-radial distribution system (see Figure 9) provides quick
restoration of service for the whole system after primary cable fault or
transformer failure. The secondary loop provides an emergency supply to
affected areas, equalizes loads on all transformers, permits taking
advantage of diversity among loads gaining transformers capacity, and allows
large motors to start across the line. In general, the secondary loop
provides greater flexibility, high efficiency, and good voltage regulation.
Short circuit contribution from all transformers connected in parallel within
the system should be taken into consideration.
8.2.6
Primary Selective-Radial Distribution System. The primary
selective-radial distribution system (see Figure 10) differs from the modern
simple-radial distribution type in that it uses two or more primary feeders
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