As these factors are varied, the model will determine:
(a) The maximum area that can be served without exceeding a 7 percent voltage drop at
any point in the area.
(b) The total cost for constructing all substation facilities and primary and lateral feeder
circuits; the costs are represented on a per sq mile basis and assume an 8 percent annual cost.
(c) The total losses in the system and their associated costs. Losses are based on the
planning load levels.
(d) The total cost for the plan, including the annual cost on a per sq mile basis for the
substation and distribution feeders, as well as the cost of losses.
The model represents all main and lateral circuits as three-phase lines. Voltage drop factors for
the lines and the resistance data for loss calculation are taken from the Electrical Distribution
Handbook. The model assumes uniform load densities for representation of system loading
conditions. For long range planning purposes, this allows a satisfactory representation of system
loading. If, for a particular system, an unusually large concentrated load did exist, then this
would be considered separately. Extensive studies of a variety of distribution feeder circuit
patterns have indicated that the assumption of uniform load density, as represented in the model,
allows a meaningful analysis of the system.
The model is intended to represent actual system conditions, including voltage drop and system
losses. The service area is initially set at one square mile and the voltage drop is computed. The
service area is then expanded and the voltage drop is recomputed for each increase in service
area. When a maximum voltage drop of 7 percent is detected at the end of the last lateral, the
area expansion is stopped. The model is then used to determine the total number of primary and
lateral feeder circuit miles required for the expanded service area. This is possible because as the
service area is expanded, the model automatically adds the required primary feeder circuits and
lateral feeder circuits necessary to serve the increased area. The model is then used to determine
the total energy losses on the system. Finally, the model provides computation of the substation
size required, given the maximum possible service area and the load density within that area.
Using cost data for constructing primary and lateral circuits as well as substation costs and
evaluation of energy losses, the model is used to determine the total cost on a square mile basis
for the particular planning alternative. The model then provides computation of the number of
substations that will be required to provide service for the area being considered.
Using this approach, the model represents an actual operating system. The flexibility of the
model, however, allows a number of different system configurations to be investigated and
evaluated. In this way, many alternatives can be evaluated before deciding on the final system
configuration. A report is then compiled for various alternatives and cost data is assembled
indicating the economic approach for system development.