A three-phase overhead distribution transformer is shown in the figure below. Where pole mounted overhead distribution is used to supply three-phase power, three-phase transformers occupy less space than a bank of transformers, and they weigh less.
Moreover, the cost of installation and maintenance is lower for a three-phase overhead transformer than for a bank of three single-phase units.
Three-phase overhead transformers are made with ratings from 30 to 300 kVA. Primary voltages range from 4.16 to 34.5 kV, and secondary voltages range from 120 to 480 V.
The basic impulse level (BIL) ratings are 45 to 150 kV. They are available with wye, delta, or T–T connections. These transformers have four output connections, X0, X1, X2, and X3, and their cases are filled with electrical-grade mineral oil.
These transformers are manufactured in ratings from 45 to 7500 kVA with high-voltage ratings from 2.4 to 46 kV. The standard connections are delta–wye, grounded wye–wye, delta–delta, wye–wye, and wye delta.
The transformers are housed in steel cabinets with front-opening, three-point latching steel doors. As in the overhead transformers, the cases of pad-mounted transformers are filled with electrical-grade mineral oil.
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