Electrical Transformers: Complete Guide (Working, Types, Components, Testing & Applications)
What is an electrical transformer?
Transformer is a device that transfers electrical energy from one electrical circuit to another electrical circuit through the medium of magnetic field and without a change in the frequency. The electric circuit which receives mains supply is called primary winding and the other circuit which delivers electrical energy to the load is called secondary winding.
In other words, Transformer is an electromagnetic energy conversion device, since it converts the energy received in the primary winding into magnetic energy which is then reconverted into electrical energy at the secondary side. The primary and secondary sides of the transformer are not connected electrically but are coupled magnetically. This coupling magnetic field allows the transfer of energy in either direction
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Why electrical transformers are essential in power systems?
Transformers are used extensively in ac power systems because they make possible power generation at the most desirable and economical level (10–20 kV), power transmission at an economical transmission voltage (as high as 400–1000 kV) and power utilization at most convenient distribution voltages (230/400 V) for industrial, commercial and domestic purposes. Additionally in industrial applications voltages may have to be as high as 3.3, 6.6 or 11 kV for large motors.
Where transformers are used today
We will understand transformer uses by understanding what are the features of the transformer which makes them useful in power transfer. In general, transformers are used under following circumstances:
(i) For decreasing or increasing voltage and current levels from one circuit to another circuit (or circuits when there are 2 or more output windings) in low and high current circuits. This ability of transformer makes them suitable for use in Generating stations, transmission lines, distribution stations, industries, mills and for supply to homes/residences etc.
(ii) For matching the impedance of a source and its load for maximum power transfer in electronic and control circuits.
This feature of transformer finds its use in audio systems to match the output impedance of an amplifier to the impedance of a speaker. This makes highest power transfer and thus a louder and clearer sound.
A similar use case is found in communication system to match impedances of transmitter output with antenna for maximum energy radiation.
One more such use case can be found in electronics multistage amplifier circuit to match impedances between different amplifier stages.
(iii) for isolating DC. while permitting the flow of AC between two circuits or for isolating one circuit from another.
This feature of transformer finds its application in medical devices, computers, lab equipment, UPS power supplies, industrial control systems which are sensitive to voltage spikes etc.
How a Transformer Works
The core principle behind working of transformer is Faradays law of electromagnetic induction. Faraday had proposed two laws of electromagnetic induction.
The first law states “Whenever the magnetic flux linked with a circuit changes, an electromotive force (emf) is induced in the circuit”.
Faradays second Law states “The magnitude of the induced emf is directly proportional to the rate of change of magnetic flux linkage”
Mathematically:
where:
- e = induced emf
- N = number of turns
- Φ = magnetic flux
- The negative sign is derived from Lenz’s law (the induced emf opposes the cause producing it).
For a sinusoidal AC input to transformer its EMF equation is derived with the help of Lenz’s Law and it comes out to be:
- Erms = induced emf
- N = number of turns
- Φmax = Peak value of magnetic flux
- f = frequency of sinusoidal input signal.
Step-up vs. step-down behaviour explained simply
Recall the EMF equation described above. You will notice that EMF produced in the coil placed in changing magnetic flux is directly related to number of turns in that coil.
So if we keep different number of turns in primary and secondary windings(coils) and keep them linked to the same changing magnetic flux, we will get different voltages in its primary and secondary windings (Coils).
Lets understand Step and Step Down transformer with illustrations:
EMF Equation for Primary Winding
EMF Equation for Secondary Winding
Divide the secondary emf by the primary emf:
Upon cancelling common terms, we get:
Under Ideal conditions i.e. no loss this ratio can be written as :
Case1: Step-Up Transformer:
We Get;
Which mean Secondary voltage is greater than primary voltage.
Case2: Step-Down Transformer:
We get;
Which means Secondary voltage is less than primary voltage.
Electrical diagram
Losses and efficiency
Core loss:
When a magnetic material undergoes cyclic magnetization, two kinds of power losses occur in it—hysteresis and eddy-current losses—which together are known as core-loss.
In transformer, Hysteresis loss happens as there is a loss of energy during magnetization and demagnetization of core which occurs cyclically at supply frequency.
The eddy current losses occurs when a magnetic core carries a time-varying flux, voltages are induced in all possible paths enclosing the flux. The result is the production of circulating currents in the core (all magnetic materials are conductors). These currents are known as eddy-currents and have power loss (i2R) associated with them called eddy-current loss.
Copper Loss:
Copper loss occurs in winding resistances when the transformer carries the load current; varies as the square of the loading expressed as a ratio of the full-load.
Stray Load Loss:
Stray Load Loss largely results from leakage fields inducing eddy-currents in the tank wall, and conductors.
Dielectric-loss:
This loss occurs in the insulating materials, particularly in oil and solid insulations.
Main Components of a Transformer
Core
As we know that a common magnetic flux links both primary and secondary winding of the transformer. For better flux linkages between the primary and secondary windings, magnetic cores are generally used.
CRGO: cold-rolled grain-oriented steel (CRGO) material is most commonly used as core in transformer as it gives high permeability to magnetic flux i.e CRGO steels can be magnetized easily without much losses.
Core types:
There are various types of transformers core such Shell type, Core type, Distributed core type, Toroidal Core type, Spiral or wound core and amorphous core. Of these, core type and shell types transformer widely used power system while toroidal core types are widely used in electronics circuits.
Windings
Copper and Alluminium are two most commonly used winding materials. These windings help in transferring electrical energy through electromagnetic induction. Windings are carefully wrapped around core to have maximum flux linkages.
Types of windings (primary, secondary, tertiary):
Primary winding is connected to input supply and it receives electrical energy from the source. The current which flows in the primary winding creates magnetic flux in the transformer core.
Secondary winding is generally connected to main load at the output side of the transformer. The magnetic flux which is created by current in the primary side links with the secondary side to generate voltage.
Tertiary winding is occasionally provided in the transformer for some auxiliary power supplies. These winding are similar to secondary windings but they used to supply auxiliary loads. For example in a generating station, generator transformer is provided with tertiary winding to supply power to various equipment which help in power generation.
Insulation system
There are two modes of providing insulation in the transformer. The first one is around windings to prevent shorting between coils, the other is provided to fill the empty space of the transformer tank such as oil.
The insulation applied to the windings (Paper, Varnish, Epoxy, fiberglass etc) are generally decided based on its insulation class. Insulation class is a method to segregate insulating materials based on their temperature withstand capacity. These insulating class is defined as A,B,F,H and C for their maximum temperature withstand capacity of 105, 130, 155, 180 and more than 180 degree Celsius respectively.
Transformer oil
Transformer oil is the oil which is filled in the body of main tank and conservator. The main purpose of transformer oil is providing cooling and insulation. Additionally it also help in arc quenching, moisture protection and preservation of insulation for long life.
Conservator tank
Conservator is placed above main tank and is connected with main tank though a pipe. It is air tight to prevent any moisture ingress. It acts as reservoir to hold extra oil. It provides space for oil expansion and contraction. Oil expansion and contraction occurs due to heating when transformer is in operation.
Conservator is often connected through silica gel breather. Silica gel absorbs the moisture content of the air present above the oil surface in the tank. Moisture degrades the dielectric strength of the insulating oil.
Radiators/cooling equipment
Radiator is a set of thin metal tubes/panels which are attached to main tank. Its purpose is to provide increased surface area for transformer oil to cool down. There are various types of cooling systems installed in the transformer such as ONAN (oil Natural Air Natural), ONAF (Oil Natural Air Forced), OFAF (Oil forced Air Forced) and OFWF (Oil Forced Water Forced)
Tap changer (off-load vs. on-load)
Tap Changers are provided with the transformer for to regulate output voltage to a certain value. Loads are often designed at a fixed rated voltage. Thus customers demands rated voltage at their end. But sometime due to system load fluctuation or during peak load hours, the output voltage differs from its rated value. To keep output voltage constant, tap changers are provided. Tap changer basically changes the number of turns of turns to generate the relevant voltage at the output.
There are two types of Tap changer, one is On-Load Tap changer and other is Off Load Tap Changer.
Bushings & terminals
Transformer bushings are provided to insulation between live parts transformer body while bring conductor out of main tank. It acts as safety layer between live part and grounded tank body without compromising electrical insulation and tank sealing.
Explosion vent/PRV
Explosion Vents are provided for to protect transformer from dangerous pressure buildups caused by severe internal faults. It provides safe passage to rapidly expanding gases to escape and prevent tank rupture.
Pressure relief valve is provided in modern day transformer to release excess internal pressure. Once the pressure normalizes, the valve closes again.
Types of Electrical Transformers (Detailed Classification)
A. Based on Function
1. Step-up Transformers
Step Up transformer increase voltage in its secondary with respects to applied voltage at primary side. It also reduces the current in the secondary side in the same ratio. It is used in power transmission for transmitting generated low voltage power at high voltage to prevent ohmic loss during transmission. Other uses of it is found in X-Ray Machines, Microwave ovens etc.
2. Step-down Transformers
Step Down Transformer decreases voltage in secondary side with respect with the applied voltage on the primary side. It also increases current in the secondary side in the same ratio. These transformers have wide variety of applications such as distribution transformer, transformers provided in household appliances, electronic circuits, welding machines etc.
3. Isolation Transformers
Isolation transformer is used for isolating circuits on the primary side with circuits on the secondary side. It has 1:1 voltage ratio. This type of transformer prevents electrical shocks, noise, surges to transfer to secondary side. It also helps in elimination of ground loops.
4. Auto-Transformers
When the primary and secondary windings are electrically connected so that a part of the winding is common to both primary and secondary, the transformer is known as an autotransformer.
B. Based on Construction
1. Core-type transformer
In core-type construction the windings are wound around the two legs of a rectangular magnetic core. In core type transformer half of the LV and HV windings are wounded on the two legs of the core to prevent flux leakage.
The core-type construction has a longer mean length of core and a shorter mean length of coil turn. This type is better suited for EHV (extra high voltage) requirement since there is better scope for insulation
2. Shell-type transformer
In shell-type construction, the windings are wound on the central leg of a three-legged core.
The shell-type construction has better mechanical support and good provision for bracing the windings.
3. Toroidal transformer
Toroidal Transformer uses toroid shaped (doughnut shaped) core made of silicon steel or ferrite. Windings are wound uniformly around the core. This shape of core provides good magnetic coupling and low leakage flux. These type of transformers finds it use in electronics circuits.
C. Based on Insulating Method
1. Oil-Immersed (ONAN, ONAF, OFWF, OFAF)
In oil type Transformer, active parts (windings and core) are immersed in Oil to provide insulation and cooling. Usually, mineral oil or ester roil is used in these transformers. These are most common type of transformers used in electrical power system.
These transformers are further classified into ONAN, ONAF, OFWF, OFAF based on the arrangements made for cooling of this oil.
2. Dry-Type (VPI, Cast Resin)
Dry type of transformer uses air, resin or solid insulation to cool and protect windings. Cooling in these transformer by natural or forced air. Insulation in these transformers are provided with materials such as Epoxy Resin, fiberglass, polyester film, cast resin etc. These type of transformers are generally used in High rise buildings, Hospitals and Schools, Wind Turbines and in Marine applications.
D. Based on Application
1. Power Transformers
Power transformer as the name suggests is used for power transmission networks to transmit large power efficiently at high voltages. These transformers are designed to operate at nearly full load, thus these transformers are designed to operate at high efficiency at full load. Typical operation voltage is 33kV and above.
2. Distribution Transformers
Distribution transformers are used electrical distribution network. Most of these transformer are step down transformer to transfer the power from grid to distribution network. Power in our homes is supplied from distribution transformer.
3. Instrument Transformers (CT, PT)
Instrumentation transformers are used for measuring current and voltages in electrical power system. Current transformers are used to measure currents usually found in both high and low voltage systems. Protentional transformers are used to measure voltages in electrical system. PT is generally found in medium to high voltage system.
4. Earthing Transformers
Earthing Transformer also called as neutral grounding transformer is used to provide neutral point for grounding purposes in power system where neutral is not available in the system. In a three phase delta connected system, provision of neutral is not available. Path for ground fault currents is not available in such system and hence their protection is difficult. Earthing transformers provides artificial neutral to solve such issue.
5. Pulse Transformers
Pulse transformer are used in electronics circuit to transfer electrical pulses from one circuit to another circuit without distorting signal. It is used in high frequency operations, to provide accurate pulse by keeping electrical isolation.
6. Furnace Transformers
Furnace transformer is used to supply very high current at low voltage to electrical furnances used in steel plants, foundries and other metal melting industries. These transformers experiences extreme electrical, mechanical and thermal stress due to high fluctuating currents.
7. Traction Transformers
A traction transformer as the name suggests typically used in railway electrical systems to supply power to electric trains. These are step down transformers to lower traction voltages to desired voltage level of traction motor.
8. Welding Transformers
Arc Welding requires high current and low voltage. These transformers are generally used for SMAW arc welding. Typical output voltages are 20-50v and current are typically in the range of 100A to 600A.
Transformer Protection & Safety Devices
1. Buchholz relay
The Buchholz relay is a small device installed with the connecting pipe which connects main tank and conservator. It is used in transformers for protection against all kinds of faults. It is a gas-actuated relay and installed in oil-immersed transformers. It will give an alarm in case of incipient faults in the transformer. This relay also disconnects the transformer in case of severe internal faults. It uses mercury switches for alarm and trip signals.
2. Differential protection
Differential protection is a scheme to protect transformer from internal faults. It is most sensitive and reliable protection methods. This protection scheme is based on the principle that current entering a protected zone should be equal to the current leaving it. CTs are installed in both terminals to make comparison feasible. Thus accuracy of CTs are important for preventing false alarm in this protection scheme.
3. Overcurrent & Earth Fault protection
Over current and earth fault protections are most important and basic protection schemes. In this scheme, when current exceeds a preset limit due to overload or short circuit conditions. For overcurrent protection a time delay is provided based on trip characteristics of relay. For short circuit, transformer should trip instantaneously to prevent damage.
4. Reduced Earth Fault (REF protection)
Restricted Earth fault protection is used to protect transformer winding. It provides fast and accurate detection of interturn earth faults within restricted zone. Standard earth fault protection relays are not sensitive to low current earth faults inside transformer winding. REF is used to protect transformer winding from inter turn faults, phase to earth faults, neutral side winding faults and low level faults below differential pickup.
5. Temperature detectors
Winding temperature detectors and oil temperature indicators are provided to protect transformer from temperature rise. As the name suggest winding temperature detector measures winding temperature and gives alarm and trip signal if the temperature crosses preset values.
Similarly Oil Temperature indicator comes with dial to show oil temperature and gives alarm and trip signal when temperature reaches above preset value.
6. Pressure relief devices
Pressure relief valves are provided with the modern transformers to release pressure generated due to gas formation in the transformer tank body. When pressure crosses certain preset limit, this pressure release valve opens to release gases and oil vapours. Once pressure releases, this valve closes automatically.
7. Surge arresters
Surge arresters are provided to protect transformers from high voltage surges. These voltage surges may arise due to lightning strikes or switching surges or line faults etc. These high voltage surges may damage winding insulation of the transformer. The surge arrestor provides high voltage surges a low resistive path to ground them.
Transformer Testing (Factory & Field Tests)
A. Factory Tests
These tests are performed by the manufacturer at their premises before its installation at site.
1. Routine tests :
These tests are performed on every transformer produced by the manufacturer before delivering it to customer. These tests determine the characteristic parameter of any particular transformer.
Tests include Winding Resistance test, Turn Ratio Test, polarity test, no load test, short circuit test, insulation resistance test, Hi-pot test, Induced voltage test and oil tests (BDV, moisture, acidity etc).
2. Type tests
Type tests are done on one transformer of a particular design or type. These tests demonstrates that the transformer meets the design standards.
Tests include Temperature Rise Test, Lightning impulse test, Short circuit withstand test and switching impulse tests etc.
3. Special tests
Special tests are done customer requirements such as Harmonics test, Sound level tests, Vibration test, Zero sequence test, Tan Delta Test etc.
B. Field Tests
Field tests are performed after transformer installation. Some of the type tests are again repeated in the field. These tests include Insulation resistance test, Tan delta test, SFRA (Sweep Frequency Response Analysis), Oil BDV test, Partial discharge test, Load & no-load tests, Thermography
FAQs
Why are transformers rated in kVA?
Transformers are rated in kVA because their heating and losses depend only on voltage (V) and current (I), not on the power factor (pf) of the load. The copper loss depends on current and the iron loss depends on voltage, both independent of power factor. Since the transformer cannot control the load’s power factor, it is rated by the apparent power (kVA) which ensures safe operation for any type of load.
Can transformers work on DC?
No, transformers cannot work on DC.
How long does a transformer last?
A transformer typically lasts 25 to 35 years, but with proper maintenance, good cooling, and healthy insulation conditions, it can operate for 40 years or more.
What causes transformer noise?
Transformer noise is mainly caused by magnetostriction — a property of the core material. When AC voltage is applied, the iron core expands and contracts twice every cycle (50 Hz → 100 vibrations per second). This rapid mechanical vibration produces the familiar “humming” sound.
Conclusion
Transformers play a vital role in the generation, transmission, and distribution of electrical energy, making them one of the most essential components in modern power systems. Their ability to efficiently step up or step down voltage levels ensures safe, reliable, and economical power delivery across vast distances. With diverse types such as power, distribution, isolation, and special-purpose transformers, they serve a wide range of industrial, commercial, and domestic applications. As technology advances, improvements in insulation, cooling, and protective systems continue to enhance their performance and lifespan. Ultimately, transformers remain indispensable in sustaining the world’s electrical infrastructure and enabling the smooth functioning of everyday life.
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