Electrical power transformer working principle

What is an electrical power transformer?

An electrical transformer is a stationary device where the AC supply voltage is increased or decreased concerning the current. The electrical power transformer is widely used in the power sector. Like generating station, an electrical substation, it means that Transformer is used where the voltage level is changed. On the other hand, a power transformer is a static electrical device or machine that allows power and frequency to change without any electrical connection.

The transformer is used to change voltage levels. It means the transformer can change voltage level with constant frequency, it depends on the number of turns of the transformer. Only two coils connected magnetically can transfer energy from one circuit to another by increasing or decreasing the voltage as required.

What is the turn ratio of the transformer?

The relationship between the number of patches or current in the Transformer’s primary and secondary coils is called the Transformer’s transformer ratio or turns ratio of the transformer. Basically, the turns ratio of the transformer is found by a simple calculation, such as the number of turns on the secondary winding divided by the number of turns on the primary winding. The transformation ratio is given below

Vp / Vs = Np / Ns = Is / Ip


Vp = Primary Voltage

Vs = Secondary voltage

Np = Primary number turns

Ns = Secondary number of turns

What is the reason for using Transformer?

The transformer is an electromechanical device rotating device. Transformer works on the principle of mutual induction or Faraday’s law of electromagnetic induction process. Transformers are commonly used for voltage up and down voltage. For example, if the substation voltage is 11KV, but the consumer voltage is 400V or 220 volts, then we use a transformer to step down and convert these 11 kilovolts to 400 by 220.

How does electrical power transformer work?

Basically, the transformer works on the principle of Faraday’s law of electromagnetic induction and mutual induction. Transformers work mainly based on mutual induction. Now you may be wondering what mutual induction is. This process is called mutual induction when an electromotive force is generated due to a change in current between one circuit and another. The two circuits are magnetically connected.

When an electrical supply is supplied, a magnetic field is generated around it, which collects the secondary coils resulting in a mutual induction between the primary and secondary coils, and electricity flows in the secondary. In other words, there is no moving part in the Transformer, which means it is a wholly fixed type of device. Its structure is straightforward, such as two or more copper wires wrapped in insulated steel or iron. We know that a transformer has two windings.

  • Primary winding
  • The other is the secondary winding

When voltage is supplied to the primary winding, a magnetic field is created, and the secondary winding goes through the magnetic flux, and a magnetic field is created there, resulting in voltage at the secondary coil. The amount of current flowing on the secondary side and the primary side will depend on the number of patches in the primary and secondary coils, which is called the transformation ratio. There are two types of transformers depending on voltage level.

  • Step up
  • Step down

Step-up transformer

When an alternating voltage (AC) is applied to the Transformer’s primary side, high voltage is found in the secondary winding. It is called a step-up transformer. Basically, a step-up transformer is the main component in the electrical power station. It is because electricity generation voltage is stepped up from low voltage (11KV) to high voltage (132KV, 230KV, 400KV, etc). We also use step-up transformers in UPS, inverters, etc

Step down transformer

We know that the transformer is used to change the voltage level but it depends on the number of turns of the transformer windings. When the voltage in the Transformer’s primary is given, a relatively low voltage is obtained in the secondary winding which is called a step-down transformer. The step-down transformer is largely used in the power grid substation and distribution substation. It means wherever you need to reduce voltage level or high voltage to low voltage, you can use a step-down transformer, and step-down transformers are also used in most household appliances. 

Transformer is based on which formula?

Transformers are based on Faraday’s formula, or the transformer principle is Faraday’s law of the electromagnetic induction process. Now the question may be that Faraday said that if a magnetic field is to be occupied by another coil, one of them must move, and the flux will not be settled. Yes, it was true, but it is right in the case of DC, and the Transformer runs on AC DC, so it no longer has to be moved separately because its waves tend to be transmitted automatically. So transformer is based on the faradays law of the electromagnetic induction process.

What is a Transformer Coil?

The electrical power transformer has two main windings, the primary winding, and the secondary winding. Each winding is twisted by the coil. The primary and secondary coils of a transformer have to be made of outstanding quality original copper. Besides, it creates additional resistance so that the voltage drop increases and the number of losses increases.

What is Leakage Flux?

By leaks flux, we know that current flows through the mutual inductance of the Transformer. Now flux refers to the voltage generated in the primary and secondary coils. If the amount of flux in the secondary coil is more than the flux amount in the primary coil, the output voltage will be less than expected. The lamination of transformers ranges typically from 0.4 mm to 0.5 mm, but the lamination or thickness of many transformers used in the power lines is much less, such as 0.2 mm.

What is the type of transformer test?

The transformer is the static device used to step up and step down the electrical power. Basically, the transformer transfers energy from the primary winding to the secondary winding. If the primary winding is high and the secondary winding is low, then the transformer is called a step-down transformer.

If the primary winding or number of turns (coil turns) becomes small and the secondary winding or the second number of turns (coil turns) becomes high such transformer is called a step-up transformer. It is noted that a step-up transformer is used for the generating station.

It means whereas we need to higher the voltage level (132KV, 232KV, etc.) step-up transformer is used there. Basically, the transformer is a highly efficient device. But the transformer needs some tests to determine transformer efficiency and voltage regulation. For this reason, the transformer needs two simple tests, such as

  • Open circuit test
  • Short circuit test

Open circuit test on transformer

To the open circuit test, the rated voltage is applied to the primary side of the transformer (usually low voltage winding) whereas the transformer’s secondary side is left open-circuited. The primary applied voltage V1 is measured by the voltmeter, no-load input power W0 is measured by the wattmeter, and no-load current I0 is measured by the ammeter.

If the normal voltage is applied to the primary side of the transformer, then the normal iron loss will occur in the transformer core. Wattmeter will record the iron loss and small copper loss on the primary side of the transformer. The average no-load current I0 ratio is very small (usually 2 to 10% of rated current).

But copper loss Cu is very small (inconsiderable) as compared to iron loss. It is noted that the wattmeter reading practically gives the iron loss, and Iron loss is measured by the wattmeter. Iron loss is the same at all loads. The open circuit or no load test transformer equation is given below:

Input power W= V1 I0 cos ɸ0

No load power factor cos ɸ= W0/ V1 I0

                Iw = I0 cos ɸ0

                Im = I0 sin ɸ0

The open circuit test gives PI0 cos ɸ0, Iw and Im

Whereas, wattmeter reading = W0

Ammeter reading = I0

Voltmeter reading = V1

It is noted that the OC test of a transformer or the open circuit test in a transformer is used to measure or determine the no-load current and iron losses of the transformer.

Short circuit test on transformer

In the short circuit or impedance test, the transformer’s secondary side (usually low voltage winding) is short-circuited by the thick conductor, and variable low voltage is applied to the primary side. The short circuit voltage is rapidly increased till at voltage Vsc full load current I1 flows in the primary side of the transformer. Under this condition, the copper loss is the same as the full load condition.

Whenever the transformer becomes short-circuited then, the transformer doesn’t show any output. In this case, iron loss in the transformer core is negligible and copper Cu loss is high. It is noted that the transformer short circuit test gives full load copper Cu loss R01 and X01. It is noted that a short circuit test on the transformer is conducted to determine full-load copper Cu loss. The insulation resistance test is also called a routine test transformer

What is the dyn11 vector group test?

The vector group test of transformer indicates the phase difference between the primary and secondary directions of the transformer. Vector grouping has been introduced due to the special configuration of the winding connections on both sides of the transformer. It is very important to determine the vector group of transformers before connecting two or more transformers in parallel.

Dyn11 is the transformer vector group notation. This means Low Voltage winding, which is star connected by High Voltage winding which is delta connected. Basically, we connect the + -30-degree connection of the transformer.

D = delta connection to the primary, y = star connection to the secondary side of the transformer, and n = neutral point to the secondary side. Dyn 11 means that the voltage of the secondary star winding increases the initial phase voltage by 30 degrees and corresponds to 11 o’clock.

Dyn11 transformers are largely used for high-voltage transmission purposes. During the conduction of electricity, the voltage is increased at some points, and at some point, the voltage goes down step by step. So to make the system more efficient, delta winding is used on L.V. Side, and star is used on H.V. Side.

Why Iron Core is used in Transformer?

The transformer is a crucial device in the electrical power system. The transformer is used to change the voltage level. We know that a high voltage electrical power transformer consists of two wildings primary and secondary winding. Both windings are separated from each other. Electrical energy passes from primary to secondary winding by the mutual flux or alternating flux.

It means alternating flux or mutual flux links between the two windings. This action will happen if we use an air core, but the iron core is much more efficient than the air core. It means if we use an iron core then alternating flux or mutual flux can easily link between two windings efficiently. On the other hand, the air core has much lower permeability than the iron core. For this reason, an iron core is used in transformers.

Why Core of the Transformer is Laminated?

Basically, the transformer core is the iron core. We know that transformer has a very high efficiency than some other electrical devices or electromechanical devices. Basically, transformer efficiency is usually 95% or more. But ideal transformer efficiency is usually 100%. But lamination is used in the transformer core due to reducing eddy current loss. Though transformer has comparatively low loss and most of the losses occur due to heat and eddy current loss is produced by heat. For this reason, the transformer core is laminated to reduce eddy current loss.

Does Iron Core Transformer Use in High-Frequency Application?

Basically, the iron core transformer is in high-frequency applications due to its limitations. The iron core transformer has high eddy current loss and hysteresis loss, the iron core is low effective than the magnetic core in high-frequency applications. To avoid such issues iron core transformer is not used in the high-frequency application.

What happens if Transformer Primary Winding is Connected to DC Supply?

There are two types of electrical energy, such as AC electrical energy and DC electrical energy. There are lots of differences between AC and DC power supply systems. Basically, the electrical transformer is designed for the alternating current. However, if you apply DC power to the primary winding of the transformer then the primary winding will produce constant flux due to the steady current.

In this case, transformer primary winding will draw more current due to the low resistance, because back e.m.f will never be generated. As a result, the transformer’s primary winding will be overheated and it can be burned out. For this reason, be careful never to connect the transformer’s primary winding with the DC power supply.

When can you apply DC supply to the transformer’s primary side?

You can apply DC supply to the primary side of a transformer but there is some limitation to it. If you apply DC supply to the primary winding then you have to connect high resistance with the primary winding in series. If you connect the high resistance in series this resistance will control the transformer’s primary current to safe a DC value, and it will prevent the transformer from burning out. It is noted that you should avoid applying DC supply to the transformer.

What is the transformer protection system?

The high-voltage electrical power transformer is an oil-immersed and highly efficient static device. The electrical power transformers are enclosed with the steel sheet and have a low chance of occurring fault. After also that fault occurs in the transformer. If we don’t take action to prevent fault it may cause a big explosion.

To avoid such unexpected explosions transformer protection system is used. Basically, an automatic protection system is used for the high-voltage transformer. On the other hand, an automation protection system is not required for all categories of the transformer. A small distribution transformer is not required automatic protection system.


A small distribution transformer is directly connected to the supply line through the fuse but the circuit breaker is not used for the small transformer protection purposes. Some common transformer faults occur for different reasons. These are some common transformer faults that also occur in the normal case, such as

  • Open circuit Fault
  • Short-circuit Fault/internal fault
  • Overheating                    

Open circuit Fault

Open circuit fault can occur in one phase of the three-phase transformer and it can create undesirable or overheating. In this case, no automatic protection is required or a relay protection system is not provided. Because open circuit fault is comparatively harmless. Whenever such faults occur then the transformer can manually be disconnected from the supply.

Transformer overheating

Overheating of the transformer is a common reason in the electrical power system. Basically, there are three main reasons to occur overheating in the transformer, such as

  • Sustainable overload on the transformer.
  • Failure of the cooling system of the transformer.
  • Short-circuit occurs in the transformer winding.

Such faults occur in midrange distribution transformers and high voltage transformers. If this fault occurs then automation relay protection is not necessary. In this case, thermal accessories (bank of fans) are used to control such faults.  

Winding short-circuit/internal fault

A winding short circuit fault is a severe fault in the transformer. Transformer short circuit faults can be of three types, such as earth fault, phase-to-phase fault, and inter-turn fault. Basically, a transformer winding short circuit fault occurs due to the transformer winding deterioration.

Transformer winding deterioration occurs due to mechanical injury and insulation failure of the winding. The transformer must be disconnected from the electrical supply system as soon as possible whenever a short circuit occurs because an arc can cause oil fire in the transformer. For this reason, an automatic protection system or relay protection system is mandatory.

What are the transformer protection devices?

 A transformer protection device is most necessary in the high voltage power system to be safe from surge voltage. There are different types of protection systems used to prevent and take action against transformer faults. Some protective device is given below.

  • Buchholz relay
  • Over-current relay
  • Earth fault relay
  • Differential relay

Buchholz relay

Buchholz relay is an important device to prevent all kinds of transformer faults. Basically, the Buchholz relay is the protective device and gas actuated. Those are the oil-immersed transformer Buchholz relay is installed there. All oil-immersed transformers don’t require a Buchholz relay. Those transformers rating more than 750 KVA such transformer require Buchholz relay to prevent any kind of incipient faults. Whenever a fault occurs the relay gives the alarm and disconnects from the supply system. If the transformer oil level is decreased then it also gives an alarm.  

Over current relay

Over-current relay is also a comprehensive protective device that is widely used for transformer protection purposes. Basically, over current relay provide protection against transformer phase-to-phase faults and overloading. This protection system is used for high-voltage transformers.

Earth fault relay

The earth fault relay provides protection against earth fault only. Earth’s fault is a dangerous fault. If an earth fault occurs then the transformer must be disconnected from the supply system. For this reason, a relay protection system is absolutely necessary to take action against such faults.

Differential relay

Differential relay plays an important role to provide protection for the phase fault and earth fault. Basically, a differential relay protection system is used for high-voltage transformer protection purposes. Differential relay is sometimes called the circulating current system.

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