Different Kinds of Power Distribution Transformers
Although commercial electric companies use a large number of transformers throughout their power distribution system, that industry pretty much reserves the name "distribution transformer" for the final transformer in the system before the electricity enters a residence or a business. Various types of distribution transformers serve different needs according to amount of power needed as well as physical and environmental considerations.
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Step Up or Step Down
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A basic transformer consists of two coils of wire wrapped around metal cores. Voltage applied to the primary or input coil induces voltage into the secondary or output coil by a process called magnetic flux. The amount of voltage induced depends on the ratio of the number of turns of the wires around the core. With a "turns ratio" of 2:1, the transformer halves, or steps down, the voltage. With a ratio of 1:2, it steps it up. Distribution transformers usually step down voltage from power lines for residential or commercial use.
Three Phases, Wye of Delta
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A single generator in a power plant produces three phases of electricity on one armature, and all three phases are distributed along high tension lines. All three phases can be applied into a single distribution transformer, specially designed with the primary wound in a "wye" or "delta" pattern, depending on how the neutral connects in relation to the three phases. Many industrial applications use three-phase distribution transformers. Most residential applications use single phase distribution transformers.
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Tapped or Center Tapped
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Distribution transformers can have more than one secondary to provide outputs to more than one location. Also, a single secondary of a step up type of winding can have "taps" along it to pick off several outputs of equal or different voltages from anywhere along the winding, for distribution to multiple points. A transformer can be tapped in the center to split the output in half. Terminology for these types include "tapped" and "center tapped."
Pole Mounted or Stationary
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Distribution transformers come in a wide variety of sizes and shapes, from rectangle shapes with cooling fins to complex arrangements of mechanical shapes to insulate the electricity and contain the multitude of shapes of the coils inside. Three-phase transformers can look like three separate transformer connected together. They can be small enough to mount on utility poles or can be mounted on concrete pads protected by fences or locked cabinets, depending on their size.
Oil-Filled or Dry
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For larger applications, filling a distribution transformer with a highly refined mineral oil serves several purposes. The oil helps insulate the internal parts of the transformer that conduct high voltages to prevent arcing. The oil also helps cool the transformer or keep it at a constant temperature as well as reducing hum and vibration. These are known as oil-filled, liquid-filled or wet transformers. Those with no liquid filling are called dry transformers.
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