What is a Distribution Transformer and Where is it Used?

Transformers are an essential part of the electric power supply system and they are of many kinds. A distribution transformer or a service transformer provides the final voltage before it is distributed to the customer. This transformer is generally a step-down transformer that reduces the high voltage necessary for long distance transmission to the voltage that the customers require.

Distribution transformers in India, like in every other country, must adhere to a set of guidelines that detail their specifications necessary for use.

Applications of Distribution Transformers
Generally, distribution transformers are used to reduce the primary distribution voltage of the electric power (between 2kV and 35 kV) to the utilizable voltage near households or other settlements. The utilizable voltage for households in India is 220 volts at 50 Hertz frequency.

Railway Lines:
Distribution transformers are part of the overhead electric lines near railway lines. Here, single phase distribution transformers are used.

Power Supply:
Distribution transformers are part of power supply grids outside of settlements, farmyards, isolated houses, or pumping stations that require electricity at voltages below 30 kilovolts. 

Wind Farms:
Here, several distribution transformers may be found connected to the turbines of windmills, to step up the power generated by the turbines before it is transmitted to a substation. 

Padmount Transformers:
In urban areas that have underground power supply lines, the distribution transformers are found in the form of pad mount transformers. They are transformers that are mounted on a concrete pad and enclosed and locked in a steel box. Since all electrical supply lines are securely hidden, this kind of transformer is safe to be installed in areas that cannot be fenced.

The number of distribution transformers in an area depends on the number of households that are dependent on it for their power supply. Sometimes, several homes can be fed off of one distribution transformer; while in other cases; one transformer may be able to power a single household.

How Does a Distribution Transformer Work?
Distribution transformer generally consists of a core made from laminated sheets of steel, held together using resin. The primary and secondary coils are wound around this core, and they do not touch each other at all. Since distribution transformers are generally step-down transformers, the primary coils, where electricity enters the transformer, have more windings than the secondary coils through which electricity exits the transformer.

Through the process of mutual induction, the primary coils induce an electric current in the secondary coils through the magnetic force produced by the primary coil. Since the number of windings in the secondary coil is less, the electric current induced in the secondary coil will automatically have a lower voltage.

The number of windings of the two coils is entirely dependent on the voltage of the electric power that is transmitted to the transformer and other specifications of its use.

Classification of Distribution Transformers
Distribution transformers are classified in the following ways:

  • According to where it is mounted, they can be classified as pole mounted transformers, pad mounted transformers, and underground vault transformers.
  • Depending on the type of insulation used, they can be either liquid-immersed transformers or dry-type transformers
  • According to the number of phases it has, they can be either single-phase or three-phase transformers.
  • They can be further classified on the basis of voltage class and basic impulse insulation level.

Distribution transformers are energized throughout the day but they rarely work at full load. Due to this, they are designed in a way that gives them maximum efficiency at lower loads. To ensure efficiency and keep the voltage regulation at a minimum, these distribution transformers should be built with a small leakage reactance.

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