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 pro-
vide accessibility and protect transformers.
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 trans-
former 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 ser-
vice 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).
The distribution transformer in Fig. 15.2 is self-protected. It is equipped with a lightning
arrester, a weak-link or protective-link expulsion-type fuse (installed under oil in the trans-
former tank), a secondary circuit breaker, and a warning light. The transformer primary
bushing conductor is connected to one phase of the three-phase primary circuit through a
partial-range current-limiting fuse. The transformer tank is grounded and connected to the
FIGURE 15.2 Typical pole-type dis-
tribution 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 trans-
former if an internal fault develops.
primary and secondary common-neutral ground wir