In distribution system studies, particularly where growth and expansion
are contemplated, several plans are usually taken under consider-
ation. For purposes of comparison, design voltage-drop limits are determined
beforehand, and these same limits are used in all of the studies. Comparable
layouts are made to provide adequate voltages and suitable equipment
loadings with the design loads. A light-load voltage profile is assumed
to fall within permissible limits, although regular changes at the substation
may be relied upon to maintain maximum permissible voltage at
the first customer.
The utilization voltage spread of 15 V, the estimated service and
customer wiring voltage drops, and contact-making-voltmeter (voltageregulating
relay) bandwidth setting are derived from industry standards
accepted for the design of utilization devices.
Distribution circuit is a rather long and tedious process. A method for
making calculations easily and providing a record of basic data employs
a system of “unit voltage drops”; a self-explanatory data form
also indicates the procedure for making the computations.
For convenience, the unit selected for the voltage-drop calculations
may be 10,000 kVA-ft, that is, 10 kVA at a distance of 1000 ft
or 100 kVA at a distance of 100 ft, etc.
Circuit loads are assumed distributed in proportion to the connected transformers except where
relatively large individual loads are known, in which case equivalent
transformer capacity is estimated on the basis of known demands.
Included with the calculations is a sketch of the existing and
proposed circuit with station numbers for each branch, transformer or
group of transformers, or wire size change; these correspond to numbers
on the calculation sheet. Factors for each wire size to convert from
percent voltage drop to percent kilowatt loss in some smaller
branches are neglected to simplify calculations,
but results are sufficiently accurate for purposes of comparison.
In making comparisons, evaluation of losses should include both
a demand component reflecting system investment and an energy
component. Also, costs of conversion reflecting both additional capital
investment and annual operating and maintenance expenses should be
taken into account. All of these values are different for each utility.
