Distribution transformers are pole-type transformers that supply relatively small amounts of power to residences. They are used at the end of an electrical utility’s delivery system. Both single-phase and three-phase products are available. Performance specifications for distribution transformers include primary frequency and maximum primary voltage rating. The primary frequency is measured in Hertz (Hz) and indicates the frequency of incoming voltage signals to primaries. Choices include 50 Hz, 60 Hz, 50 and 60 Hz, and 400 Hz. Typically, 400 Hz distribution transformers are designed for aerospace and military applications. Products with other frequency options may also be available. The maximum primary voltage rating is measured in volts (V) and represents a maximum value for devices that use several nominal voltages.
For distribution transformers with a single-phase primary configuration, size and cost increases with the number of leads. Devices with only one winding in the primary side can accept one nominal voltage. Conversely, devices with two primary-side windings can accept two nominal voltages. Single-phase distribution transformers that use the quad (2+2) configuration have dual windings in the primary side where each winding has a tap that can accept two nominal voltages. Devices with 5-lead voltage selectors can accept five possible nominal voltages. Distribution transformers with a ladder configuration consist of windings in a cascade, and a series of inductances connected between adjacent windings. As a rule, a ladder is the least economical primary configuration.
Three-phase distribution transformers are connected in delta or wye configurations. A wye - delta distribution transformer has a primary winding connected in a wye (Y) and a secondary winding connected in a delta. A delta - wye distribution transformer has a primary winding connected in a delta and a secondary winding connected in a wye. Choices for three-phase distribution transformers include delta - delta, delta - wye, wye - wye, wye - delta, wye - single phase, delta - single phase, and international. International transformers are fully compliant with both U.S. and international standards from organization such as the International Electrotechnical Commission (IEC) and the National Electrical Manufacturer’s Association (NEMA). International distribution transformers can be used in the U.S or in any country that supports their voltage and frequency requirements.