Distribution Transformer: Construction | Type | Rating

Definition of Distribution Transformer :


A Distribution transformer is a step-down transformer that is used to transform electrical supply from a higher voltage to a suitable lower voltage, which is used in domestic and commercial appliances. In the case of a single-phase transformer output of the transformer is as minimum as 240 v. and in the case of a three-phase transformer, the voltage can be as low as 440v. these are small in size.and the rating of distribution transformers are of 10kva, 25 kva,63 kva, 100 kva,250,400 kva,630 kva.

In the distribution network, the pole-mounted transformer is used if the electric line passes above ground through the pole. And Pad-mounted distribution transformers are used in an urban area where is no space is available for overhead lines due to which line is drawn through underground means. In the case of the pad-mounted distribution transformers, there is a requirement for a plinth to place that transformer on it.

Distribution Transformer
three phase distribution transformer

Construction of Distribution Transformer

There are three main parts in distribution transformer –

  1. core
  2. Primary winding
  3. Secondary winding

Core consist of the magnetic sheet which is made of silicon steel and these steel sheets are attached /glued with using resin or banded. The core is usually designed in such a way that it reduces core losses and minimizes magnetic energy losses to reduce heating in the core. Reducing core loss is also economically important as if the loss will be high then distribution loss will increase which increase the rate of per-unit cost of electrical energy. Core losses are occurred due to two effects hysteresis loss and eddy current loss. The reason for using silicon steel is to minimize hysteresis loss and the laminated plate core construction is used to reduce eddy current losses. The size of the core in the distribution transformer is usually made in C and E shape. On each pillar of the core first, the primary winding is fitted, and then after secondary winding is fitted. And connection is made in delta and star method of winding. 

Construction of Distribution Transformer
construction of distribution transformer

Primary winding high voltage sides are made from enamel coated copper or aluminum wire due to low current flows through it. While low voltage secondary side winding is made of a thick ribbon of aluminum or copper. The whole winding of primary and secondary laminate from resin impregnated paper for insulation. Core assembly will be placed in a steel tank. then after Steel tank is used to filled with transformer oil. transformer oil is made from such material so that it would be non-conductive and inert to electric supply and its BDV( Break Down Voltage) value must be above 35 kv. the main function of transformer oil is to provide cooling and insulation, also prevent moisture to stop breakdown.

Types of Distribution Transformer

  1. Pole mounted distribution transformer
  2. Pad-mounted distribution transformer


Pole mounted transformer is used in an area where overhead lines network is in use. These pole-mounted transformers are in singe phase-type. The rating of this transformer is from 10 kva, 16 kva, 25 kva. These transformers are used only in household utility. Output voltages of these transformers range from 240 or 440 volts. single-phase distribution transformers are installed on single pole or two-pole structures. and 3 phase pole mounted transformers are installed on two-pole or 4 pole structures. In the case of single-phase transformers up to 25KVA capacity, these transformers can normally be installed on the single-pole through an angle as a base.

pole mounted sub station ot pole monted transformer.
pole mounted sub station

Pad Mounted Distribution Transformer

Pad-mounted distribution transformers are used in those place where there is no space for is available for fenced compound and electric lines are laid through underground cables. These types of transformer consist of inbuilt fuses and switches and h t side power cable are used to connect with the lug connector. the outgoing lower voltage is used to supply to domestic or utility consumers. Pole mounted transformers rating ranges from 10 kva to 1000 kva. Its place a plinth made from bricks. It is also made in single-phase and 3 phase system both.

Pad Mounted Distribution Transformer
Pad Mounted Distribution Transformer

Uses of Distribution Transformer

A few of the applications of distribution transformer to be discussed are as follows:

  • Distribution transformers are extensively used for household applications.
  • Distribution transformers are used for domestic and small commercial utility purposes. In villages and urban areas power supply to the household is given through distribution transformer.
  • It is used in railway systems powered up with AC Current.
  • It is Used in industries where motor based system operation used in.
  • It is also used in power collector networks.

Distribution Transformer Connections

In the distribution transformer, the winding connection is made in delta-star type connection, which means primary in delta and star type in secondary. The reason for the delta star connection is due to the requirement of neutral on the secondary side. This neutral plays a major role in providing the minimum required voltage up to 230 volts. other reasons for the requirement of neutral in the distribution network are as follow-

Connection and Working Principle of Transformer
Connection and Working Principle of Transformer

The Benefit of Neutral of Distribution Transformer :

 (1) In the case of line-to-ground faults occur, which is a very common fault in the distribution system, as we know fault current used to flows to the earth which is sensed by the relay or fuses. Therefore neutral earthing gives a great option to the detection of ground faults on the distribution lines, and thus the faulty line is isolated easily.

 (2) It provides safety to worker or staff against electrical shocks and also safety to electrical equipment against damages due to heavy fault currents if occur. due to neutral availability, earth the metallic body (frame) of electrical equipment locally for safety. 

 (3) Due to neutrality, grounding provides a path for fault currents, third harmonic currents, and zero sequence currents. This, in turn, helps in preventing the system from overvoltages, waveform distortions, and electrical equipment damages.

Turn ratio of transformer- 

Turn ratio is decided by voltages in primary and secondary. 

The formula for turn ratio-

Calculation of no. Turn for 100 kva Transformer-

In 100 KVA distribution transformer if high voltage primary side, the line voltage is 11 kv and secondary side, the line voltage is 400 v then no of turn will be calculated as follow-

transformer turn ratio

TR= 1.732 x 11000/ 415 v

= 46 

Usually, no turn in the secondary low voltage side is used to set 74

Then no of turn in primary high low voltage side will be-

3443 no.

So in 100 kva transformer, we usually set primary side no, of turn 75 and secondary side total turn is used to set 3443.

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