VOLTAGE SELECTION

The design of electric supply and distribution systems can proceed only after a distribution

voltage level has been determined. The electrical impact of the installation or facility as well as its location will influence the selection. A new service may be necessary or extension of an existing service may be acceptable. Before discussing selection of the system voltage, system voltage terminology and preferred voltage ratings need to be defined. Refer to the glossary for definitions of standard voltage terms.

System Voltage Classifications.

Voltage systems are classified either by the system use or the voltage range. More specific methods include using the voltage rating of equipment, the nominal voltage class, or the nominal system voltage.

a.     System use.

The requirement for electric power transfer will cause certain voltage levels to be more economical than others. A transmission system transfers energy in bulk between the source of supply (the utility) and the center for local distribution (the main electric supply station). A primary distribution system delivers energy from a main electric supply station to utilization transformers. A secondary distribution system delivers energy from a utilization transformer to points of utilization.

b.     Voltage ranges.

Voltage ranges are classified as low-voltage (1 kV or less); medium-voltage (above 1 kV to 99.9 kV); and high-voltage (above 99.9 kV).

c.      Voltage rating of equipment.

Voltage rating of equipment is based on nominal voltage classes which, in conjunction with the maximum voltage rating for that class, provides a simple method for rating equipment.

d.     Nominal system voltage.

The nominal system voltage is the nominal value assigned to designate a system of a given voltage class. Nominal system voltages are classified by IEEE Std 141 as standard and nonstandard voltages.

Selection of Primary Distribution Voltage for New installation.

A preferred nominal system voltage such as 12 kV, 12.5 kV, 13.2 kV or 13.8 kV, will be selected for the primary distribution system. On sizable installations where distances to loads are considerable or loads are large, the use of 34.5 kV or 24.9 kV primary distribution systems may be more  economical. Primary distribution voltages of the nominal 7.5 kV class and under will not be used, unless an off-site supply of a higher voltage is not available. Seldom is the lower voltage advantageous. For such cases, the size of the installation and the distances involved must make the use of voltages below 7.5 kV more economical in order to justify the selection.

 Selection of Primary Distribution Voltage  Existing installation.

When small facilities are added to an installation, the primary distribution system voltage within the addition will match the existing system. However, if the addition is substantial and large voltage drops or line losses can occur when existing voltages are retained, or if the main electric supply station is inadequate, then the economics of a higher voltage for the primary distribution system must be taken into account. The electrical master plan should have already provided for such deficiencies.

When a master plan indicates a contemplated voltage increase, transformers for use in ongoing construction will be specified to have dual primary voltages, when economic and transformer delivery time considerations permit such a requirement. When a dual voltage, high-voltage transformer is specified, taps are not normally available for the lower voltage. For existing voltage drop problems, not having transformer taps available may create an untenable situation, requiring a facility boost transformer, or other means to serve the facility until the distribution system is upgraded. If the facility to be added is not included in the master plan, an engineering study will be necessary to determine the most feasible method of providing service. Acquisition or preparation of maps of transmission and distribution systems with distances between principle points and single line diagrams of the systems will be required. Then a determination of the extent to which the existing system voltage can satisfy installation requirements, or the economics of a higher voltage level and benefits of such a system will be evaluated.