POWER TRANSFORMER & its Types with Working Principle Explained

(Last Updated On: August 19, 2020)

Power Transformer

Power Transformer:-

Power Transformer and its Types- A Machine used to transfer electrical energy from one circuit to another circuit or multiple circuits without a change in the frequency. A transformer is provided with Primary and Secondary sides. A varying current which is the Alternating current in any one coil of the transformer produces a varying magnetic flux in the transformer core, which interns induces a varying EMF Electromotive force across any other coils wrapped around the same core. So, with the help of a transformer, electrical energy can be transferred from one coil to one or multiple coils without the physical connection i.e. the Primary and Secondary sides are not electrically connected.

A transformer is not used to generate electrical energy, but it is used to transfer the electrical energy from one circuit to another circuit to multiple circuits. The transformer coil side which is connected with the input Alternating current is called the Primary side, while the side of the transformer which is connected with the output load is called the secondary side of the transformer, while the core of a transformer is an electromagnetic device that increases for decreases the voltage flow according to the output requirements.

  • If there are a greater number of turns on the secondary coil than on the primary coil, the alternating current will have a higher voltage than Input voltage on the Primary side. This is known as a step-up transformer.
  • If there are fewer turns on the secondary coil than on the primary coil, the output alternating current will have a lower voltage than the input voltage on the Primary coil. This is known as a step-down transformer.

Power Transformers increase or decrease line voltages, and, if needed for integrated circuit or other specialized circuit operations, can aid with the transformation from AC voltage to DC voltage.

General description of the transformer

Basically a transformer is a four-terminal device that is used to transform an AC input voltage into a higher or lower AC output voltage. Regardless of the Voltage Levels, no matter the voltage is increased or the voltage is decreased, It transforms power from a particular circuit to another with no frequency changes. Normally a transformer consists of three major components: primary winding, which acts as an input, the secondary coil, secondary winding, which acts as the output, and the iron core, which serves to strengthen the magnetic field generated. If you open or disassemble a Transformer you will find that a transformer has no internal moving parts, and it transfers energy from one circuit to another by electromagnetic induction, the primary and secondary sides of the transformer remains completely isolated i.e. having now physical metallic connection.

For the low load transformers no external cooling is necessary, but transformers used in 1500 watt and higher needs the cooling, this is why if you open such stabilizers and Inverters you will find small fans, so in a nutshell, the high ampere or high load transformers are provided with external cooling systems which include Radiators, Oil Pumps, Fans, and heat exchangers, etc. You will find transformers in villages, towns, cities, industries, etc because a change in voltage is needed.. Power transformers are defined as transformers rated 500 kVA and larger (In figure 1 is shown typical power transformer).

Power Transformer

Power Transformers are used to move electrical energy between different circuits totally isolated from one another and this permits utilizing high voltages for transmission lines, bringing about a lower flow. Higher voltage and lower current decrease the necessary size and cost of transmission lines and diminish transmission misfortunes.

They don’t require, as much consideration as most different gadgets; in any case, the consideration and support, which they truly require, is totally fundamental. Due to their unwavering quality, upkeep is once in a while overlooked, which decreases administration life and some of the time altogether disappointment

Transformer Principle of operation

A Transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction.  A varying current in one winding of the transformer produces a magnetic field which in turn induces an Electromotive force or voltage in a second Winding.  Power can be transferred between the 2 Windings through the magnetic field, without a metallic connection between the two circuits. A changing current in a conductor sets up a changing magnetic field around the conductor. If secondary winding is placed within this changing magnetic field a voltage will be induced into that winding.

Power Transformer

Transformer Turns Ratio

The voltage induced into the secondary winding would have a magnitude that depends on the TURNS RATIO of the transformer. I.e. if the secondary winding has half the no of turns of the primary winding, then the secondary voltage will be half the voltage across the primary winding. If the secondary winding has twice the no of turns of the primary winding the secondary voltage will be Two times the primary voltage.

Transformer Power ratio

The transformer is a passive component it cannot produce more power out from its secondary than is applied to its primary. Therefore if the secondary voltage is greater than the primary voltage by a particular amount, the secondary current will be smaller than the primary current by a similar amount.

Principle of working

A Transformer is a simple stationary “with no moving parts” Electromagnetic Passive electrical device that works on the principle of Faraday’s Law of induction by converting electrical energy from one value to another. Actually, mutual induction between two or more winding is responsible for transformation action in an electrical transformer. Faraday’s Laws of Electromagnetic Induction (second law) states that the magnitude of emf (E) induced in the coil is equal to the rate of change of flux that linkages with the coil. The flux linkage of the coil is the product of the number of turns in the coil and flux associated with the coil.

Power Transformer

As we said before, the transformer has three main parts, which are:

  • Primary Winding of transformer

  • Secondary Winding of transformer

  • Magnetic Core of transformer

The type of the transformer depends on the number of turns used on the Primary side and Secondary side. To recognize a Transformer whether it is a step-up or step-down is very simple. All you need is count the number of turns on the primary side and also on the secondary side. If the number of turns on the primary side are greater than the number of turns on the secondary Side this it is a Step-Down Transformer. While on the other hand, if the number of turns on the Primary side are less than the number of turns on the secondary side then it is a Step-up transformer.

For the Step-up transformer, Secondary side turns > Primary side Turns

For the Step-down Transformer Secondary side turn < Primary side Turns

Utilization of transformer

The most widely recognized and significant gadget in an electric force framework is a Power transformer, while transformers are typically utilized for the conveyance of the Power. That a transformers utilized in numerous utilizations of intensity framework, for example, across the board power dispersion over a force lattice, power dissemination and voltage coordinating for power circulation to structures, and giving low voltages to machine control.

Kinds of Transformers

Step-up Transformer and Stepdown Transformer

Step-up transformers convert the low voltage (LV) and high current from the primary side of the transformer to the high voltage (HV) and low current value on the secondary side of the transformer.

Step down transformers converts the high voltage (HV) and low current from the primary side of the transformer to the low voltage (LV) and high current value on the secondary side of the transformer.

Three Phase Transformer and Single Phase Transformer

A three-phase transformer generally has the 3 magnetic circuits that are interlaced to give a distribution of the dielectric flux between the higher and lower voltage coils. Single-phase power can be derived from a three-phase source. Transformers cannot convert a single-phase source to a three-phase source. The typical method to convert single-phase power to three-phase power is to utilize devices generally termed as rotary or static phase converters.

The single-phase transformer contains 2 windings, one on primary and the other on the secondary side. They are used in a single-phase electrical power system. The three-phase system application means using three single-phase units connected in the three-phase system.

Advantages of single-phase transformer:

  • Simple network.
  • Cost-effective.
  • Most efficient AC power supply for up to 1000 watts.

Limitations of the single-phase transformer:

  • Supply only 1-ph load.
  • Used to light loads and small electric motors.
  • Minimum power transfer capability.
  • Power failure occurs.

Advantages of three-phase transformer:

  • Large motors or heavy loads materials.
  • Transmission of power to long-distance through the magnetic field.
  • Maximum power transfer capability.
  • Power failures do not occur.

Limitations of the three-phase transformer:

  • Requires many cooling systems depending on the transformer rated power.
  • Complex network.

Indoor Transformer and Outdoor Transformer

Transformers that are intended for introducing at indoor will be indoor transformers and transformers intended for introducing at outside are open air transformers.

Indoor Advantages:

  • Low maintenance cost.
  • Safer option as compare to oil-filled transformer.

Indoor Limitations:

  • Higher operating loss.
  • Noise pollution.

Outdoor Advantages: 

  • Smaller and more efficient.
  • Lower operational costs.

Outdoor Limitations:

  • High Maintenance cost.
  • Require periodic sampling of the oil.

Oil Cooled and Dry Type Transformer

In oil cooled transformer the cooling medium is transformer oil while in dry sort transformers air is utilized as the cooling medium rather than oil.


Generators, as a rule, produce voltages in the range 11–25kV, which is expanded by transformers to the principle transmission voltage. At substations, the associations between the different parts of the framework, for example, lines and transformers, are made and the exchanging of these segments is done. A lot of intensity are communicated from the producing stations to the heap place substations, for instance at 400kV and 275kV in Britain, and at 765, 500, and 345kV in the USA.

Appropriation Systems:

Appropriation systems vary from transmission systems in a few different ways, very separated from their voltage levels. The quantity of branches and sources is a lot higher in dispersion systems and the overall structure or geography is extraordinary. A run of the mill framework comprises of a stage down (for example 132/11kV) on-load tap-changing transformer at a mass flexibly point taking care of various circuits which can shift long from two or three hundred meters to a few kilometers. A progression of venture down three-stage transformers, for instance, 11kV/433V in Britain or 4.16kV/220V in the USA, are divided along the course and from these are provided the customer three-stage, four-wire systems which give 240V, or, in the USA, 110V, single-stage supplies to houses and comparative burdens.

Phase-Shifting Transformer-

A phase moving transformer is a gadget for controlling the force move through explicit lines in a mind-boggling power transmission organize.

Reasons for Phase-Shifting transformers:

  1. a) To control the force stream between two enormous autonomous force frameworks.
  2. b) To change the viable phase removal between the info voltage and the yield voltage of a transmission line, hence controlling the measure of dynamic force that can stream in the line.

Transformers in Power Systems (Transmission and Distribution frameworks)

Transmission alludes to the mass exchange of intensity by high-voltage joins between focal age and burden focuses. Circulation, then again, depicts the movement of this capacity to buyers by methods for lower voltage systems.

Present-day transformers utilized in transmission and appropriation frameworks have exceptionally high efficiencies up to 90%-99%. This implies they can send up to 90%-99% of the electrical vitality contribution to them when venturing up or venturing down the voltage.


Generators, as a rule, produce voltages in the range 11–25kV, which is expanded by transformers to the principle transmission voltage. At substations, the associations between the different parts of the framework, for example, lines and transformers, are made and the exchanging of these segments is done. A lot of intensity are communicated from the producing stations to the heap place substations, for instance at 400kV and 275kV in Britain, and at 765, 500, and 345kV in the USA.

Appropriation Systems:

Appropriation systems vary from transmission systems in a few different ways, very separated from their voltage levels. The quantity of branches and sources is a lot higher in dispersion systems and the overall structure or geography is extraordinary. A run of the mill framework comprises of a  step down (for example 132/11kV) on-load tap-changing transformer at a mass flexibly point taking care of various circuits which can shift long from two or three hundred meters to a few kilometers. A progression of venture down three-phase transformers, for instance, 11kV/433V in Britain or 4.16kV/220V in the USA, are divided along the course and from these are provided the customer three-phase, four-wire systems which give 240V, or, in the USA, 110V, single-phsae supplies to houses and comparative burdens.

Power Transformer

Countless transformers of various classes and sizes are required in the transmission and appropriation arrange, with a wide scope of working voltages. The last change step into the purchaser mains voltage (in Europe 400/230V) is finished by the conveyance transformer. Conveyance transformers worked and possessed by power dissemination organizations are liable for providing about 70% of low voltage power to definite clients. Voltage levels are named:

  • Extra high voltage: transmission grid(>150kV) regularly 220–400kV(ultra high>400kV)
  • High voltage >70 kV up to 150 kV
  • Medium voltage >1 kV up to 70 kV (regularly up to 36 kV)
  • Low voltage < 1kV (for example 110V, 240V, 690 V).

Transformer EMF Equation

Let’s derive the equation for voltage, turns, and flux of the transformer.

The emf induced in each winding of the transformer can be calculated from its emf equation.

Power Transformer

The linking of the flux is represented by the Faraday law of electromagnetic induction. It is expressed as,

Power Transformer

The above equation may be written as,

Power Transformer


Power Transformer

For a sine wave, the r.m.s value of e.m.f is given by

Power Transformer

The emf induced in their primary and secondary winding is expressed as,

Power Transformer

The secondary RMS voltage is

Power Transformer

Where φm is the maximum value of flux in Weber (Wb), f is the frequency in hertz (Hz) and E1 and E2 in volts.

If, Bm = maximum flux density in the magnetic circuit in Tesla (T)

A = area of cross-section of the core in square meter (m2)

Power Transformer

Voltage Ratio and Turns Ratio

The ratio of E/T is called volts per turn. The primary and secondary volts per turns is given by the formula

Power Transformer

The equation (1) and (2) shows that the voltage per turn in both the winding is same, i.e.

Power Transformer

The ratio T1/T2 is called the turn ratio. The turn ratio is expressed as

Power Transformer

The ratio of primary to secondary turn which equals to primary to secondary induced voltage indicates how much the primary voltage lowered or raised. The turn ratio or induced voltage ratio is called the transformation ratio, and it is denoted by the symbol a. Thus,

Power Transformer

Any desired voltage ratio can be obtained by shifting the number of turns.


  • Appropriate for high voltage applications (more prominent than 33KV).
  • High protection level.
  • Limit the force misfortune.
  • Practical


  • Stacked for 24 hours at the transmission station, in this way, the center, and copper misfortune will happen for the entire day.
  • Huge in size.

Extraordinary sorts of transformers

Instrument transformers

Instrument transformer is an electrical gadget used to change flow just as a voltage level. Now and again, they are additionally called as confinement transformers. Instrument transformers are normally used to securely disengage the optional winding when the essential has high current flexibly and high voltage with the goal that the estimating instrument transfers or vitality meters, which are associated with the auxiliary side of the transformer won’t get harmed. The instrument transformer is separated into two sorts: Current Transformer (CT) and Potential Transformer (PT)

Power Transformer

Advantages of Instrument Transformers

  1. The large voltage and current of the AC Power system can be measured by using a small rating measuring instrument i.e. 5 A, 110 – 120 V.
  2. By using the instrument transformers, measuring instruments can be standardized. Which results in a reduction of the cost of measuring instruments. More ever the damaged measuring instruments can be replaced easily with healthy standardized measuring instruments.
  3. Instrument transformers provide electrical isolation between high voltage power circuit and measuring instruments. This reduces the electrical insulation requirement for measuring instruments and protective circuits and also assures the safety of operators.
  4. Several measuring instruments can be connected through a single transformer to a power system.
  5. Due to low voltage and current levels in measuring and protective circuits, there is low power consumption in measuring and protective circuits.

Current Transformer (CT)

A current transformer is an instrument transformer, used along with measuring or protective devices, in which the secondary current is proportional to the primary current (under normal conditions of operation) and differs from it by an angle that is approximately zero.

Current transformers perform the following functions:

  • Current transformers supply the protective relays with currents of magnitude proportional to those of power circuits but sufficiently reduced in magnitude.
  • The measuring devices cannot be directly connected to the high magnitude supplies. Hence current transformers are used to supply those devices with currents of magnitude proportional to those of power.
  • A current transformer also isolates the measuring instruments from high voltage circuits.
  • The basic principle of the current transformer is the same as that of the power transformer. Like the power transformer, the current transformer also contains a primary and a secondary winding. Whenever an alternating current flows through the primary winding, alternating magnetic flux is produced, which then induces an alternating current in the secondary winding. In the case of current transformers, the load impedance or “burden” is very small. Therefore the current transformer operates under short circuit conditions. Also, the current in the secondary winding does not depend on load impedance but instead depends on the current flowing in the primary winding.

Power Transformer

Potential Transformers (PT)

The potential transformer is additionally called as the voltage transformer. The primary capacity of the Potential transformer (PT) is to step down the voltage level to a sheltered breaking point or worth. They are utilized with voltmeters, wattmeter’s, watt-hour meters, power-factor meters, recurrence meters, synchronizing device, defensive and managing transfers, and under-voltage and overvoltage trip coils of circuit breakers. One potential transformer might be utilized for various instruments if the all-out current required by the instruments associated with the auxiliary winding doesn’t surpass the transformer rating.

Power Transformer


An Autotransformer is a transformer with only one winding wound on a laminated core. Autotransformers are less costly and are smaller for small voltage changes than standard transformers. Autotransformers transfer much of the power directly through a wire connection. Moreover, less current flows through the shunt winding, whereas most of the current pass through the lower voltage series winding at the top.

When comes to distribution systems, these type of transformers has two main applications.

  • Voltage regulators:

The regulator is an autotransformer with adjustable taps, which, as a rule, is able to regulate the voltage by ± 10%.

  • Step banks:

Normally the autotransformers are used instead of using the traditional transformers on the step banks and even substation transformers, where is the relative voltage change is moderate.

Autotransformer with an equivalent circuit is shown in figures a, b below

Power Transformer

Figure a. Autotransformer with an equivalent circuit

𝐼2 = 𝑛1 /𝑛1 + 𝑛2 *𝐼1 = 𝐼1/ 𝑏

𝑉2 = 𝑉1 + 𝑛2 /𝑛1 *𝑉1 = 𝑏𝑉1,

Where, b- Voltage change ratio, in per unit and it is equal to

𝑏 = 𝑛1 + 𝑛2 /𝑛1,

𝑛2 /𝑛1 = 𝑏 − 1 ,

Power Transformer

The required rating of the autotransformer depends on the voltage change between the primary and secondary windings. The rating of each winding as a percentage of the load is defined as:

𝑆 = 𝑏−1/ 𝑏

To obtain a 10% voltage change (b = 1.1), an autotransformer only has to be rated at 9% of the load kVA. For a 2:1 voltage change (b= 2), an autotransformer has to be rated at 50% of the load kVA. By comparison, a standard transformer must have a kVA rating equal to the load kVA.

The series impedance of autotransformers is less than an equivalent standard transformer. And the equivalent series impedance of the autotransformer is defined as:

𝑍𝑎𝑢𝑡𝑜 = (−1/ 𝑏) 2𝑍

Where, Z – impedance across the entire winding

Technical aspects of transformers

Transformer cooling

Usually, the efficiency of power transformers is more than 99% and because of this, the input and output powers are almost the same. Because of the small amount of inefficiency, losses occur inside the transformer. These losses are losses such as losses in conductors, losses in electrical steel due to the changing flux, which is carried, and losses in metallic tank walls and other metallic structures cause by the stray time-varying flux. These losses lead to temperatures increases, which must be controlled by cooling. The primary cooling media for transformers are oil and air.

In oil-cooled transformers, the windings and core are immersed in an oil-filled tank. The oil is then circulated through radiators or other types of heat exchangers so that the ultimate cooling medium is the surrounding air or possibly water for some types of heat exchangers. In small distribution transformers, the tank surface in contact with the air provides enough cooling surface so that radiators are not needed. Some time in these units, the tank surface area is augmented by means of fins or corrugations.

The cooling medium in contact with the WIndings and core must provide adequate dielectric strength to prevent electrical breakdown or discharge between components at different voltage levels. For this reason, oil immersion is common in higher voltage transformers since oil has a higher breakdown than air. Often one can rely on the natural convection of oil though the windings, driven by buoyancy effects, to provide adequate cooling so that pumping is not necessary. Air is a more efficient cooling medium when it is blown by means of fans through the windings for air-cooled units.

In some applications, the choice of oil or air is dictated by safety considerations such as the possibility of fires. For units inside buildings, air-cooling is common because of the reduced fire hazard. While transformer oil is combustible, there is usually a tittle danger of fire since the transformer tank is often sealed from the outside air or the oil surface is blanketed with an inert gas such as nitrogen. Although the flashpoint of oils is quite high, if excessive heating or sparking occurs inside an oil-filled tank, combustible gasses could be released.

The environment also plays a big role in the choice of coolants. Mineral oil used in transformers is known to be detrimental to the environment if there is an accident. For transformers such as those used on planes or trains or units designed to be transportable for emergency use, air-cooling is preferred. For units that are not so restricted, oil is the preferred cooling medium, in general, oil-cooled transformers are used in everyday units, from the large generator or substation units to distribution units on telephone poles.

There are other cooling media, which find limited use in a certain application, such as sulfur hexafluoride gas, which is usually pressurized. This is a relatively inert gas and it has a higher breakdown strength than air, it is generally used in high-voltage units where oil cannot be used and where air does not provide enough dielectric strength. Usually, the standard transformer oil is used in oil-cooled transformers. Nevertheless, there are other types of oil are also used for specialized usage. For example, silicon oil. It can be used at a higher temperature than the standard transformer oil and at a reduced fired hazard.

Transformer Cooling Methods

Dry-type transformers

This method can be divided into two types:

Air Natural (AN)

Air natural or self-air cooled transformer is generally used for small rating transformers up to 3 MVA. Basically, this method uses the natural airflow surrounding the transformer as a cooling medium.

Air forced (AF)

The natural air-cooling method is adequate to use for transformers rated more than 3 MVA. Therefore, blowers or fans are required to force the air towards the core and windings so, hot air is gained cooled due to the outside natural conventional air. However, the air forced must be filtered to prevent the accumulation of dust particles in ventilation ducts. This method can be used for transformers up to 15 MVA.

Oil-immersed transformers generally, the transformer winding and core are immersed in the mineral oil, which has good electrical insulating properties to block the current flow through the oil and high thermal conductivity.

This method can be divided into four types:

Oil Natural Air Natural (ONAN)

This cooling method may be used for transformers up to about 30MVA. In this method, the heat generated in the core and winding is transferred to the oil. The heated oil moves in the upward direction and flows from the upper portion of the transformer tank according to the principle of convection. The heat from the oil will dissipate in the atmosphere due to the natural airflow around the transformer. In this case, the oil in the transformer will keep circulating because of natural convection and will dissipate heat in the atmosphere due to natural conduction.

Oil Natural Air Forced (ONAF)

Generally, this method of transformer cooling is useful for large transformers up to about 60 MVA. The heat dissipation can be improved by applying forced air on the dissipating surface. The heat dissipation rate is faster and more in the ONAF transformer cooling method than the ONAN cooling system. In this manner, fans are mounted near to the radiator and can be provided with an automatic starting arrangement, which turns on when the temperature increases beyond a certain value.

Oil Forced Air Forced (OFAF)

Oil Forced Air Forced (OFAF) cooling method is provided for higher rating transformers at substations or power stations. In this method, oil is circulated with the help of a pump, and then compressed air is forced to pass on the heat exchanger with the help of high-speed fans. Furthermore, the heat exchangers can be mounted separately from the transformer tank and connected through pipes at top and bottom as shown in the figure below.

Oil Forced Water Forced (OFWF)

We know that the ambient temperature of the water is much less than the atmospheric air in the same weather condition. Thus, water may be used as a better heat exchanger medium than air. The oil is forced to flow through the heat exchanger with the help of a pump, where the heat is dissipated in the water, which is also forced to flow. The heated water is taken away to cool in separate coolers. Generally, this type of cooling is used for very large transformers with a very high power rating above 500 MVA.

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My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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