1.What is transformer?

transformer is a static device that transfers electrical energy between two or more circuits through electromagnetic induction. A varying current in one coil of the transformer produces a varying magnetic field, which in turn induces a voltage in a second coil. It transforms power from one circuit to another without changing its frequency but may be in different voltage level.
A transformer: –

  • Does not modify the power between primary and secondary.
  • Does not modify the frequency of AC current or voltage.
  • Only modify the voltage and current.

2. List of international standards of transformer

  • IEC 60076-8 Power transformers –Application guide. IEC (International Electro Technical Commission).
  • IEC 60050(421):1990, International Electro Technical Vocabulary (IEV) – Chapter 421: Power transformers and reactors
  • IEC 60354:1991, Loading guide for oil-immersed power transformers
  • IEC 60722:1982, Guide to the lightning impulse and switching impulse testing of power transformers and reactors.
  • IEC 60905:1987, loading guide for dry-type power transformers.
  • IEC 61378-1: 1997, Convertor transformers – Part 1: Transformers for industrial applications

3. Types of transformer

  • Step up and Step down Transformer

This type of transformer is categorized on the basis of a number of turns in the primary and secondary windings and the induced emf.

Step up transformer transforms a low voltage, high current AC into a high voltage, low current AC system in this type of transformer the number of turns in the secondary winding is greater than the number of turns in the primary winding. If (V2 > V1) the voltage is raised on the output side and is known as Step up transformer

Step down transformer converts a high primary voltage associated with the low current into a low voltage, high current. With this type of transformer, the number of turns in the primary winding is greater than the number of turns in the secondary winding. If (V2 < V1) the voltage level is lowered on the output side and is known as Step down transformer.

  • Power Transformer

The power transformers are used in the transmission networks of higher voltages. The ratings of the power transformer are as follows 400 KV, 200 KV, 110 KV, 66 KV, 33 KV. They are mainly rated above 200 MVA. Mainly installed at the generating stations and transmission substations. They are designed for maximum efficiency of 100%. They are larger in size as compared to distribution transformer.

At a very high voltage, the power cannot be distributed to the consumer directly, so the power is stepped down to the desired level with the help of step-down power transformer. The transformer is not loaded fully hence the core loss takes place for the whole day, but the copper loss is based on the load cycle of the distribution network. If the power transformer is connected in the transmission network, the load fluctuation will be very less as they are not connected at the consumer end directly, but if connected to the distribution network there will be fluctuations in the load.

The transformer is loaded for 24 hours at transmission station, thus, the core and copper loss will occur for the whole day. The power transformer is cost effective when the power is generated at low voltage levels. If the level of voltage is raised, then the current of the power transformer is reduced, resulting in I2R losses and the voltage regulation is also increased.

  • Distribution Transformer

This type of transformer has lower ratings like 11 KV, 6.6 KV, 3.3 KV, 440 V and 230 V. They are rated less than 200 MVA and used in the distribution network to provide voltage transformation in the power system by stepping down the voltage level where the electrical energy is distributed and utilized at the consumer end. The primary coil of the distribution transformer is wound by enamel coated copper or aluminum wire. A thick ribbon of aluminum and copper is used to make secondary of the transformer which is high current, low voltage winding. Resin impregnated paper and oil is used for the insulation purpose.

The oil in the transformer is used for

  1. Cooling
  2. Insulating the windings
  3. Protecting from the moisture

The distribution transformer less than 33 KV is used in industries and 440, 220 V is used for the domestic purpose. It is smaller in size, easy to install and has low magnetic losses and is not always loaded fully. As it does not work for constant load throughout 24 hours as in the daytime its load is at its peak, and during the night hours it is very lightly loaded thus the efficiency depends on load cycle and is calculated as All Day Efficiency. The distribution transformers are designed for maximum efficiency of 60 to 70%.

  • Instrument Transformer

They are generally known as an isolation transformer. Instrument transformer is an electrical device used to transform current as well as voltage level. The most common use of instrument transformer is to safely isolate the secondary winding when the primary has high voltage and high current supply so that the measuring instrument, energy meters or relays which are connected to the secondary side of the transformer will not get damaged.The instrument transformer is further divided into two types

  1. Current Transformer (CT)
  2. Potential Transformer (PT)

The current and potential transformer is explained below in detail:

Current Transformer

The current transformer is used for measuring and also for the protection. When the current in the circuit is high to apply directly to the measuring instrument, the current transformer is used to transform the high current into the desired value of the current required in the circuit.

The primary winding of the current transformer is connected in series to the main supply and the various measuring instruments like ammeter, voltmeter, wattmeter or protective relay coil. They have accurate, current ratio and phase relation to enable the meter accurately on the secondary side. The term ratio has a great significance in CT.

For example, if its ratio is 2000:5, it means a CT has an output of 5 Ampere when the input current is 2000 amp on the primary side. The accuracy of the Current Transformer depends upon many factors like Burden, load, temperature, phase change, rating, saturation, etc.In the current transformer, the total primary current is the vector sum of the excitation current and the current equal to the reversal of secondary current multiplied by turn ratio.
Ip – primary current
Is – secondary or reversal current
I– excitation current
KT – turn ratio

Potential Transformer

The potential transformer is also called as the voltage transformer. The primary winding is connected across the High voltage line whose voltage is to be measured, and all the measuring instruments and meters are connected to the secondary side of the transformer. The main function of the Potential transformer is to step down the voltage level to a safe limit or value. The primary winding of the potential transformer is earthed or grounded as a safety point.

For example, the voltage ratio primary to secondary is given as 500:120, it means the output voltage is of 120 V when the 500 V is applied to the primary.

Single Phase Transformer

A single phase Transformer is a static device, works on the principle of Faraday’s law of mutual Induction. At a constant level of frequency and variation of voltage level, the transformer transfers AC power from one circuit to the other circuit. There are two types of windings in the transformer. The winding to which AC supply is given is termed as Primary winding and in the secondary winding, the load is connected.

Three Phase Transformer

If the three single phase transformer is taken and connected together with their all the three primary winding connected to each other as one and all the three secondary windings to each other, forming as one secondary winding, the transformer is said to behave as three phase transformer, that means a bank of three single phase transformer connected together which acts as a three-phase transformer.

Three phase supply is mainly used for electric power generation, transmission and distribution for industrial purpose. It is less costly to assemble three single phase transformer to form three-phase transformer than to purchase one single three-phase transformer. The three-phase transformer connection can be done by Star (Wye) and Delta (Mesh) type.

4. Construction

Basically a transformer consists of two inductive windings and a laminated steel core. The coils are insulated from each other as well as from the steel core. A transformer may also consist of a container for winding and core assembly (called as tank), suitable bushings to take out the terminals, oil conservator to provide oil in the transformer tank for cooling purposes etc.

In all types of transformers, core is constructed by assembling (stacking) laminated sheets of steel, with minimum air-gap between them (to achieve continuous magnetic path). The steel used is having high silicon content and sometimes heat treated, to provide high permeability and low hysteresis loss.

Laminated sheets of steel are used to reduce eddy current loss. The sheets are cut in the shape as E, I and L. To avoid high reluctance at joints, laminations are stacked by alternating the sides of joint. That is, if joints of first sheet assembly are at front face, the joints of following assemble are kept at back face.

5. Transformer Working Principle

Transformer works on the principle of mutual induction of two coils or Faraday Laws Of Electromagnetic induction. When current in the primary coil is changed the flux linked to the secondary coil also changes. Consequently, an EMF is induced in the secondary coil due to Faraday laws of electromagnetic induction.


The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism)and second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

6. Materials of Iron Core of Transformer

  • Silicon steel for low cost, low core loss, and high permeability at high flux densities (1.0 to 1.5 T).
  • Compressed powdered ferromagnetic alloys known as ferrites- used in the core of small transformer used in communication circuits at high frequencies and low energy levels.

7. Applications

  • It can rise or lower the level of level of Voltage or Current (when voltage increases, current decreases and vice versa because P =V x I, and Power is same) in an AC Circuit.
  • It can increase or decrease the value of capacitor, an inductor or resistance in an AC circuit. It can thus act as an impedance transferring device.
  • It can be used to prevent DC from passing from one circuit to the other.
  • It can isolate two circuits electrically.


  • “Electrical Power System” by S. Rao
  • “Electrical technology “by B.L. THERAJA

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