System Model

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(c) Increase distribution line capacity.
(d) Add more distribution feeders from the existing substation.
(e) Analyze the system at different distribution voltage levels, thereby, adding fewer
substations at higher voltage and more at lower voltages.
To achieve orderly growth over the next twenty years, and after establishing a long range load
design, the next feasible step is to develop the load levels for the intermediate periods (i.e., load
levels between present and long range load levels). Perform the steps of long range planning for
each transition load level as described above.
Substation requirements for each service area are analyzed for the various voltage levels (e.g.,
24/4.16 kV, 7.2/12.5 kV, and 14.4/24.9 kV). A system model, discussed in subparagraph 8.4.3.3,
is useful in determining load density, the construction cost for the substation, and the number of
feeders from each substation. The system model will also help to establish the number of
substations and number of feeders required for the system to accommodate the loads of the
present service area.
8.4.3.3 System Model. A system model is represented in Figure 8-1. The figure
illustrates a substation with four primary feeders, eight laterals, and a rectangular service area.
The general model can represent any number of primary feeders and any polygon shaped service
area. For this particular study, the number of primary feeders is varied from three to six. It is
believed that this is a representative number of the possible distribution feeders for this study.
The number of laterals in the model is dependent upon the allowable spacing between the
laterals, i.e., the area served by each lateral. Lateral spacing of four miles would mean that no
customer would be more than two miles away from a three-phase lateral.
The factors that can be varied as part of the model are:
o
System voltage.
o
Number of primary feeders.
o
Primary feeder conductor size.
o
Lateral feeder conductor size.
o
Area served by lateral feeders.
o
Load density in kW per square mile.
o
Cost per mile of primary and lateral feeders.
o
Cost of distribution substation facilities.
o
Cost of energy losses.
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