The purpose of a distribution transformer is to reduce the primary voltage of the electric distribution system to the utilization voltage serving the customer. A distribution transformer is a static device constructed with two or more windings used to transfer alternating-current electric power by electromagnetic induction from one circuit to another at the same frequency but with different values of voltage and current.

Figure 15.1 shows distribution transformers in stock at an electric utility company service building. The distribution transformers available for use for various applications, as shown, include pole-type (Figs. 15.2 and 15.3), pad-mounted (Fig. 15.4), vault or network type (Fig. 15.5), and submersible (Fig. 15.6).

Figure 15.1: Electric utility distribution storage yard. Forklift trucks are used to load transformers on line trucks. Storage area is covered with concrete to provide accessibility and protect transformers

Figure 15.2: Typical pole-type distribution transformer installation with the transformer bolted directly to the pole. The unit is equipped with a surge arrester, a low-voltage circuit breaker, and an overload warning light. A partial-range current-limiting fuse is mounted on the primary bushing, connecting it in series with the high-voltage winding to prevent a violent failure of the transformer if an internal fault develops

Figure 15.3: Two view construction standard detail of a conventional transformer to be installed on the distribution system. Note the fused cutout and lighting arrestor combination external to the transformer. (Courtesy MidAmerican Energy Co.)

Figure 15.4: Three-phase pad-mounted transformer installed in shopping center parking lot. Transformer reduces voltage from 13,200Y/7620 to...