Primary voltage in the “13 kV class” is predominate among United States utilities. The 4-kV class primary systems are older and are gradually being replaced. In some cases 34 kV is used in new, high-density-load areas.

The three-phase, four-wire primary system is the most widely used. Under balanced operating conditions, the voltages of each phase are equal in magnitude and 120° out of phase with each of the other two phases. The fourth wire in these Y-connected systems is used as a neutral for the primaries, or as a common neutral when both primaries and secondaries are present.

The common neutral is also grounded at frequent intervals along the primary feeder, at distribution transformers, and at customers’ service entrances. Rural and suburban areas are usually served by overhead primary lines, with distribution transformers, fuses, switches, and other equipment mounted on poles.

Urban areas with high-density loads are served by underground cable systems, with distribution transformers and switchgear installed in underground vaults or in ground-level cabinets. There is also an increasing trend toward underground single-phase primaries serving residential areas.

Underground cable systems are highly reliable and unaffected by weather, but can have longer repair times. The costs of underground distribution are significantly higher than overhead. Primary distribution includes three basic types:

(1) radial, (2) loop, (3) and primary network systems.

Radial Systems
The radial system is a widely used, economical system often found in low-load density areas. To reduce the duration of interruptions, overhead feeders can be protected by automatic reclosing devices located at the substation or at various locations on the feeder.

These devices reenergize the feeder if the fault is temporary.To further reduce the duration and extent of customer interruptions, sectionalizing fuses are installed on branches of radial feeders allowing unaffected portions of a feeder to remain in service.

Loop Systems
The loop system is used where a higher level of service reliability is desired. Two feeders form a closed loop, open at one point, so that load can be transferred from one feeder to another in the event of an outage of one circuit by closing the open point and opening at another location.

One or more additional feeders along separate routes may be provided for critical loads, such as hospitals that cannot tolerate long interruptions. Switching from the normal feeder to an alternate feeder can be done manually or automatically with circuit breakers and electrical interlocks to prevent the connection of a good
feeder to a faulted feeder.

Primary Network Systems
The primary network system consists of a grid of interconnected primary feeders supplied from a number of substations. It provides higher service reliability and quality than a radial or loop system. Only a few primary networks are in operation today. They are typically found in downtown areas of large cities with high load densities.

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