Distribution transfomer

Construction

Oil-cooled three-phase distribution transformer, similar to one in above photo, with housing off, showing construction.

Distribution transformers are made using a core made from laminations of sheet steelstacked and either glued together with resin or banded together with steel straps. Where large numbers of transformers are made to standard designs, a wound C-shaped core is economic to manufacture. A steel strip is wrapped around a former, pressed into shape and then cut into two C-shaped halves, which are re-assembled on the copper windings. 

The primary coils are wound from enamel coated copper or aluminum wire and the high current, low voltage secondaries are wound using a thick ribbon of aluminum or copper. The windings are insulated with resin-impregnated paper. The entire assembly is baked to cure the resin and then submerged in a powder coated steel tank which is then filled withtransformer oil (or other insulating liquid), which is inert and non-conductive. The transformer oil cools and insulates the windings, and protects the transformer winding from moisture, which will float on the surface of the oil. The tank is temporarily depressurized to remove any remaining moisture that would cause arcing and is sealed against the weather with a gasket at the top.

Formerly, distribution transformers for indoor use would be filled with a polychlorinated biphenyl (PCB) liquid. Because these liquids persist in the environment and have adverse effects on animals, they have been banned. Other fire-resistant liquids such as silicones are used where a liquid-filled transformer must be used indoors. Certain vegetable oils have been applied as transformer oil; these have the advantage of a high fire point and are completely biodegradable in the environment. 

Pole-mounted transformers often include accessories such as surge arresters or protective fuse links. A self-protected transformer includes an internal fuse and surge arrester; other transformers have these components mounted separately outside the tank.  Pole-mounted transformers may have lugs allowing direct mounting to a pole, or may be mounted on crossarms bolted to the pole. Aerial transformers, larger than around 75 kVA, may be mounted on a platform supported by one or more poles. A three-phase service may use three identical transformers, one per phase.

Transformers designed for below-grade installation can be designed for periodic submersion in water. ^[

Distribution transformers may include an off-load tap changer to allow slight adjustment of the ratio between primary and secondary voltage, to bring the customer voltage within the desired range on long or heavily loaded lines.

Pad-mounted transformers have secure locked and bolted grounded metal enclosures to discourage unauthorized access to live internal parts. The enclosure may also include fuses, isolating switches, load-break bushings, and other accessories as described in technical standards. Pad-mounted transformers for distribution systems typically range from around 100 to 2000 kVA, although some larger units are also used.

Distribution transformer

A distribution transformer or service transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer. The invention of a practical efficient transformer made AC power distribution feasible; a system using distribution transformers was demonstrated as early as 1882. 

Classification

Distribution transformers are classified into different categories based on certain factors such as:

• Mounting location – pole, pad, underground vault
• Type of insulation – liquid-immersed or dry-type
• Number of Phases – single-phase or three-phase
• Voltage class
• Basic impulse insulation level (BIL).

  Use

  Distribution transformers are also found in the power collector networks of wind farms, where they step up power from each wind turbine to connect to a substation that may be several miles (kilometres) distant. 

• The number of customers fed by a single distribution transformer varies depending on the number of customers in an area. Several homes may be fed off a single transformer in urban areas; rural distribution may require one transformer per customer. A large commercial or industrial complex will have multiple distribution transformers. Padmount transformers are used in urban areas and neighborhoods where the primary distribution lines run underground. Many large buildings have electric service provided at primary distribution voltage. These buildings have customer-owned transformers in the basement for step-down purposes. In a secondary network system as used in urban areas, many distribution transformers may be connected in parallel, each equipped with its own network protector circuit breaker to isolate it from the secondary network in case of a fault.

• Distribution transformers are normally located at a service drop, where wires run from a utility pole or underground power lines to a customer's premises. They are often used for the power supply of facilities outside settlements, such as isolated houses, farmyards or pumping stations at voltages below 30 kV. Another application is the power supply of the overhead wire of railways electrified with AC. In this case single phase distribution transformers are used.