Distribution Transformer

Distribution Transformer

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 demonstrate as early as 1882.

Generally, these transformers are available in different sizes with efficiencies along with insulating oil. These transformers are available in various sizes and efficiencies. The selection of this transformer mainly lies in the requirement and budget of the user.

Moreover, There are four types of distribution transformer connections available like star-star, delta-delta, star-delta, delta-star and Zig Zag/delta zigzag.

Construction of Distribution Transformer

The designing of a distribution transformer can be done similarly to small size transformers. The main parts of this transformer mainly include Oil Tank, Conservator, Buchholz Relay, Breather Unit, Oil Indicator, Temperature Detector, Pressure Relief Device, Thermal Relay, Radiator, and Bushing.

  • The oil tank is use to soak the windings by placing it in.
  • A conservator is arrange above the oil tank at the outside of the transformer frame. It is connect to the main tank with the help of a metallic tube. The oil within the tank can be easily contact & enlarge throughout loading so that the temperature of the oil can be increased & decrease.
  • Buchholz relay is use when a conservator tank is use. Because it indicates errors like loss of oil once it goes low, improper flow of oil between the tank & transformer.
  • Breather Unit includes silica gel that absorbs moisture in the oil. It changes its color from blue color to pink color it is not capable to absorb moisture in the oil.
  • The oil indicator indicates the level of the oil within the conservatory unit.
  • The temperature detector monitors the temperature of the oil. If the temperature of the oil increases to a certain level then the transformer will be disconnected from the service.
  • Pressure relief device decreases the pressure within the transformer to avoid an explosion of the transformer.
  • Thermal relay is use as an indicator for the temperature of the winding
  • The radiator is use to increase the transformer’s cooling efficiency.
  • The bushing is use to connect the internal windings of the transformer with the help of an exterior electrical network.

Types of Distribution Transformer

Based on the application or requirement, these transformers are categorize into different types like single phase, three phases, underground, pad-mounted, pole-mounted transformers.

Single Phase

These transformers are specially is use for networks wherever a three-phase supply is not require. Therefore, these are use for repairing overhead distribution loads in residential. These are also applicable in industrial lighting, light commercial loads & power applications.


Three Phase

This kind of transformer is use to hold electrical energy from the main distribution circuit to a minor distribution circuit. This type of transformer transmits the current to a secondary distribution circuit and also reduces the voltage of the primary distribution circuit. These transformers reduce the voltage supply for the primary circuit based on the consumer requirement.


This voltage always changes & can be dissimilar for the users of commercial, residential & light industry. Meanwhile, These transformers work on different levels of voltage & frequency based on the standards existing in different countries. These transformers are available in single-phase & three phases. Single-phase is use in residential applications whereas 3-phase with a pad is use in underground primary circuits.


This type of transformer includes a locked steel cup board that is arrange on a concrete pad. Moreover, This type of transformer is install in places where they do not have space for a fenced enclosure. Meanwhile, This transformer is use with electric power distribution lines at an overhead electrical line for reducing the primary voltage to supply for the customers. A single transformer of this type can serve many homes/a large building. The power rating of this transformer ranges from 75 kVA to 5000 kVA and includes fixed switches & fuses.


Pole Mounted

These transformers are mount on an electrical service pole at the height of the overhead cables. These are use for changing high distribution voltage to low like 120/240 volt power. These types of transformers are used in a wide rural area, ranges from 16 kVA to 100kVA. These are available in small size and easy to fit on single-pole structures. These transformers are reliable to harsh climates when they used in remote areas.


Tanks of the transformer can be shaped and protected from the corrosive materials & accumulation of water. In coastal areas, tanks can be protected from zinc sprays whereas, in highly corrosive regions, stainless tanks are used.


The uses of the distribution transformer include the following.

  • This transformer changes from high voltage electricity to low voltage electricity, used in homes & businesses.
  • The main function of this is to step down the voltage to provide isolation between two windings like primary & secondary
  • This transformer distributes the power to remote areas which are generated from the power plants
  • Generally, this transformer distributes the electrical energy to industries with less voltage under 33KV and 440volts to 220volts for domestic purposes.

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