Distribution Transformers

Wooden utility poles are classified by the length, circumference at the top of the pole, and the circumference measured 6 feet from the bottom of the pole. Pole sizes begin at 25 feet and are increased in 5-foot increments up to 90 feet in length. The pole top circumference increases 2 inches for every class of pole. There are 10 classes of wooden poles numbered from 1 to 10. Class 1 is the smallest and class 10 is the biggest. The American National Standards Institute’s publication entitled Specifications and Dimensions for Wood Poles (ANSI 05.1) provides technical data for wood utility poles. Distribution Transformers For long-distance transmission, a voltage higher than normally generated is required. A step-up transformer is used to produce the high voltage. Most electrical equipment in the Navy uses 120/208 volts. The primary voltage distributed on Navy shore installations, however, is usually 2,400/4,160 and 13,800 volts. A distribution transformer (step-down) is required to reduce the high-primary voltage to the utilization voltage of 120/208 volts. The various types of transformer installations are discussed later in this chapter. Regardless of the type of installation or arrangement, transformers must be protected by fused cutouts or circuit breakers; and lightning arresters should be installed between the high-voltage line and the fused cutouts. Three general types of single-phase distribution transformers are in use today. The conventional type requires a lightning arrester and fuse cutout on the primary-phase conductor feeding the transformer. The self-protected (SP) type has a built-in lightning protector; the completely self-protected (CSP) type has the lightning arrester and current-overload devices connected to the transformer and requires no separate protective devices. You should review Module 2, Navy Electricity and Electronics Training Series (NAVEDTRA 172-02-00-91) for more information on transformer theory. In primary and secondary windings construction, the change in voltage in a transformer depends on the number of turns of wire in the coils. The high-voltage winding is composed of many turns of relatively small wire, insulated to withstand the voltage applied to the winding. The secondary winding is composed of a few turns of heavy copper wire, large enough to carry high current at a low voltage. Figure 4-5 shows a single- phase transformer with secondary windings connected in series and parallel. Figure 4-5.—Single-phase transformer with secondary windings connected in series and parallel. In a distribution transformer, a secondary coil is wound on each leg of the laminated iron core, and the primary coil is wound over the secondary coils. The primary leads pass through a steel tank and are insulated from the tank by porcelain bushings. The secondary leads are connected to studs on a terminal block. Copper straps on the secondary terminal block permit connecting the two secondary coils in series or in parallel. From the terminal block, three secondary leads pass through porcelain bushings to the outside of the tank. An oil-level line inside the tank marks the level to which the tank is filled with transformer oil. Several methods of cooling transformers are in use today, such as self-air cooling, air-blast cooling, liquid-immersed self cooling, and liquid-immersed water cooling. Self-air cooling types of transformers are simply cooled by surrounding air at atmospheric pressure; the heat is removed by natural convection (normal dissipation of heat by cooling). The self-air cooling transformer is called the dry type of transformer. The air-blast cooling transformer has the core and windings encased in a metal enclosure through which air is circulated by a blower. This type is used for large power transformers with ratings from 12,000 to 15,000 kVA. The liquid-immersed self-cooling transformer has its coils and core completely immersed in transformer 4-5