Transformers are a key topic in electrical and instrumentation engineering interviews. This page presents a collection of the most important and frequently asked transformer basic interview questions, specially designed for freshers. Whether you're preparing for a job interview or a college viva, these questions cover essential topics such as transformer working principles, construction, parts, types, efficiency, and real-world applications. Get ready with clear and concise answers to boost your confidence.<
A transformer is an electrical device used to change the voltage level in an AC (Alternating Current) electrical circuit without changing the frequency. It works on the principle of electromagnetic induction. A transformer consists of primary and secondary windings wound over a magnetic core. It is commonly used to step up (increase) or step down (decrease) voltages in power distribution systems.
Transformers are used to efficiently transmit and distribute electrical energy. At power generation stations, the voltage is stepped up to reduce transmission losses, and then stepped down at distribution points to usable levels for residential, commercial, and industrial applications.
The modern transformer was developed by Lucien Gaulard and John Dixon Gibbs in the 1880s. Later, it was improved by William Stanley, who built the first practical transformer based on the concept of alternating current and electromagnetic induction.
Transformers are classified into various types:
The winding that receives electrical energy is called the primary winding. The winding that delivers the transformed voltage to the load is called the secondary winding. In a step-up transformer, the primary has fewer turns than the secondary. In a step-down transformer, the secondary has fewer turns.
The core is typically made of laminated silicon steel sheets. These laminations reduce eddy current losses and improve efficiency by directing the magnetic flux efficiently through the windings.
A transformer works on the principle of **Faraday's Law of Electromagnetic Induction**. When alternating current flows through the primary coil, it produces a varying magnetic field, which induces an electromotive force (EMF) in the secondary coil due to mutual induction.
No, a transformer cannot work with direct current (DC) because mutual induction requires a changing magnetic field, which only occurs with alternating current (AC). If DC is applied, it may damage the transformer due to continuous current and heating.
An ideal transformer is a theoretical model that assumes 100% efficiency, meaning no energy losses. It has perfect magnetic coupling, no winding resistance, and no leakage flux. It is used for simplified analysis.
Transformer efficiency is the ratio of output power to input power. It is usually very high (95–99%) because transformers have no moving parts and minimal losses.
Efficiency (η) = (Output Power / Input Power) × 100
The transformation ratio (also called the turns ratio) is the ratio of the number of turns in the secondary winding to the number of turns in the primary winding. It is denoted as:
K = N₂ / N₁ = V₂ / V₁
Voltage regulation is the difference between no-load and full-load secondary voltage, expressed as a percentage of full-load voltage. It indicates the ability of the transformer to maintain constant secondary voltage under varying load conditions.
Mutual induction is the phenomenon where a changing magnetic field in one coil induces a voltage in another nearby coil. This is the fundamental operating principle of a transformer.
Insulation in a transformer prevents short circuits between windings and ensures safe operation. It also provides electrical isolation between the core and windings and between layers of windings.
Transformers are designed for specific frequencies. In most countries, the operating frequency is either 50 Hz or 60 Hz. Transformers do not alter frequency; the input and output frequencies are the same.
No-load current is the small current drawn by the primary winding when the secondary circuit is open. It is required to magnetize the core and supply iron losses.
Instrument transformers are used for measurement and protection in high-voltage systems. They include:
An autotransformer uses a single winding that acts as both the primary and secondary. It is more compact and efficient but lacks electrical isolation between input and output.
Transformers are highly efficient because they have no moving parts and minimal losses. Their efficiency typically ranges from 95% to 99% depending on load and design.
If DC is applied to a transformer, there will be no changing magnetic field, hence no mutual induction. The primary winding will draw a high steady current, causing overheating and possible damage to the transformer.
The laminated core reduces eddy current losses. Laminations are insulated from each other and prevent the flow of circulating currents, thereby reducing heat and improving efficiency.
Transformers are rated in kVA (kilovolt-ampere), not in kW. Standard ratings include 25 kVA, 63 kVA, 100 kVA, 250 kVA, 500 kVA, 1000 kVA, etc. The rating depends on voltage, current, and application.
Isolation means electrical separation between primary and secondary circuits. It helps protect the user and equipment from electric shocks and interference. Isolation transformers are commonly used in sensitive electronic circuits.
Yes, small transformers like signal transformers, pulse transformers, and audio transformers are used in electronic circuits for isolation, impedance matching, and voltage transformation.
A distribution transformer is designed to deliver power at the end of a distribution network. It typically operates at low voltages (400V or 230V) and is installed near residential or commercial areas. It has high efficiency and low losses.
Transformers are rated in kVA because they handle both real power (kW) and reactive power (kVAR). The current flowing through the transformer depends on apparent power (kVA), not just active power, hence the kVA rating.
The basic working principle of a transformer is **Faraday’s Law of Electromagnetic Induction**. When alternating current (AC) flows through the primary winding, it produces a time-varying magnetic field in the core. This changing magnetic flux links to the secondary winding and induces an electromotive force (EMF). The voltage induced in the secondary depends on the number of turns and rate of change of magnetic flux.
Faraday’s Law states that **an EMF is induced in a coil when the magnetic flux linking that coil changes over time**. Mathematically:
EMF = -N × (dΦ/dt)
Where:
N = number of turns in the coil, Φ = magnetic flux, dΦ/dt = rate of change of flux.
The negative sign indicates Lenz's law — the induced EMF opposes the change in flux.
When AC is applied to the primary winding, it produces an alternating magnetic field in the transformer’s core. This field continuously changes direction and strength. The secondary winding, placed within the same magnetic circuit, is exposed to this changing flux. As a result, voltage is induced in the secondary coil. This is mutual induction, where magnetic coupling transfers energy without physical contact.
A transformer requires a **changing magnetic field** to induce voltage in the secondary winding. Alternating current naturally creates a continuously changing magnetic field due to its sinusoidal nature. DC current produces a constant magnetic field, which fails to induce EMF after the initial instant. Hence, transformers only work with AC.
Mutual induction is the process where a change in current in one coil induces a voltage in a second nearby coil. In transformers, the **primary coil's magnetic field induces voltage in the secondary coil** via the iron core. It is the core mechanism of energy transfer from input to output without any direct electrical connection.
The magnetic core of the transformer acts as a low-reluctance path that guides the alternating magnetic field produced by the primary coil. This flux passes through the core and links with the secondary winding. The better the magnetic coupling and core design, the higher the flux linkage and efficiency.
Leakage flux is the portion of magnetic flux generated by the primary winding that does not link with the secondary winding. This flux escapes through the surrounding air or nearby space instead of the core. It reduces efficiency and voltage transfer. Designers reduce leakage flux by tight coupling and core design.
The **induced EMF** is the voltage developed across the secondary winding due to the changing magnetic flux from the primary winding. According to Faraday’s Law, it is directly proportional to the number of turns and the rate of change of magnetic flux:
EMF = N × (dΦ/dt)
For sinusoidal AC, the RMS value of the EMF induced in the coil is:
E = 4.44 × f × N × Φmax
Where:
E = induced voltage (V), f = frequency (Hz), N = number of turns, Φmax = maximum magnetic flux (Wb).
This formula is used to design windings and determine voltage levels.
The transformer core is typically made of laminated silicon steel and acts as the path for magnetic flux. It serves the following functions:
Laminating the core reduces **eddy current losses**, which are circular currents induced in the core material due to the alternating magnetic field. These currents cause heating and power loss. By using thin, insulated steel sheets, eddy current paths are interrupted, greatly improving efficiency.
The voltage ratio between primary and secondary windings is equal to the ratio of their number of turns:
V2 / V1 = N2 / N1
Where:
In a **core-type transformer**, the magnetic flux travels through the core limbs where windings are placed. In a **shell-type transformer**, the flux is confined within a central limb and two side limbs, which surround the windings. Shell-type designs offer better flux containment and reduced leakage.
Yes, frequency has a direct impact on transformer operation. At higher frequencies, core losses (especially eddy currents and hysteresis) increase. Transformers are designed for specific frequencies (e.g., 50 Hz or 60 Hz), and using a different frequency may cause overheating or saturation of the core.
When a load is connected to the secondary winding, current flows. This load current induces an opposing magnetic field, which causes the primary to draw more current to maintain the magnetic balance. Thus, the **primary current increases proportionally to the secondary load**.
In an ideal transformer, **input power equals output power**:
Pin = Pout
Or: V1 × I1 = V2 × I2
In real transformers, a small amount of power is lost due to core losses, copper losses, and stray losses. But overall, transformers maintain high power conservation.
Magnetic saturation occurs when the core material is magnetized to its maximum capacity. Any further increase in current does not produce proportional increase in flux, resulting in distortion, overheating, and reduced efficiency. Transformers are designed to operate below the saturation point.
Induced EMF is the voltage generated inside the winding due to magnetic flux. Terminal voltage is the voltage measured across the winding terminals. Due to internal impedance and voltage drops, terminal voltage is slightly less than induced EMF under load conditions.
Reluctance is the opposition offered to magnetic flux in a magnetic circuit (similar to resistance in electrical circuits). Lower reluctance in the core ensures better flux linkage and higher efficiency. Magnetic materials like silicon steel are chosen for low reluctance.
Transformer polarity indicates the direction of the induced voltage relative to the input. It is essential for proper parallel operation. Polarity can be checked using a simple test setup and observing voltage polarity marks (dot convention). Incorrect polarity can cause circulating currents or damage.
Magnetizing current is the small no-load current required to establish magnetic flux in the core. It lags the applied voltage by nearly 90 degrees and is typically 2%–5% of rated current. It helps maintain the flux needed for induction.
Core flux (Φ) is the magnetic flux in the core due to magnetizing current. It can be calculated from the applied voltage:
Φ = E / (4.44 × f × N)
Where E = EMF, f = frequency, N = number of turns.
Energy is transferred via the changing magnetic field. The AC in the primary coil generates alternating flux in the core, which induces EMF in the secondary coil. This magnetic coupling allows energy transfer without any electrical contact between windings.
Leakage reactance is caused by leakage flux that doesn't link both windings. It causes voltage drops and phase shifts under load, affecting regulation and short-circuit performance. Lower leakage reactance improves voltage stability and short-circuit withstand capacity.
Yes, when there’s no load connected to the secondary, the transformer still draws a small magnetizing current to establish flux in the core. This is called the no-load condition, and the input power is used mainly to overcome core losses.
Inrush current is a large, short-duration current that flows when the transformer is first energized. It is caused by the sudden magnetization of the core and can be 5–10 times the rated current. Protection systems must account for it to avoid false tripping.
Lenz’s Law states that the direction of induced EMF is such that it opposes the cause producing it. In transformers, the induced EMF in the secondary creates current that generates a magnetic field opposing the original flux from the primary, maintaining energy conservation.
If the transformer core saturates, it leads to distortion in flux waveform, high magnetizing current, noise, overheating, and core losses. It degrades performance and may damage insulation. Proper core design and voltage levels prevent saturation.
Flux density (B) is the amount of magnetic flux per unit area of the core. It is expressed in Tesla (T) or Wb/m². High flux density can increase efficiency but must be kept below the saturation limit of the core material.
When load is suddenly removed, the secondary current drops to zero, reducing the opposing flux. The magnetizing current slightly increases to maintain core flux. However, the overall input power decreases, and transformer operates in no-load condition, drawing minimal current.
A transformer typically consists of the following major components:
The core serves as the magnetic circuit of the transformer. It provides a low reluctance path for magnetic flux and helps in efficient transfer of energy from the primary to the secondary winding through electromagnetic induction. The core is made of laminated silicon steel to reduce eddy current losses.
Core laminations reduce **eddy current losses**, which are circulating currents induced in solid metal when exposed to changing magnetic fields. Thin laminations separated by insulation restrict the path of eddy currents, minimizing heat generation and improving efficiency.
Transformer cores are typically made of **cold-rolled grain-oriented (CRGO) silicon steel**. This material has high magnetic permeability, low hysteresis loss, and excellent flux-handling capacity. For small transformers or special applications, ferrite cores may be used.
Windings are coils of conductive wire, usually copper or aluminum. The **primary winding** receives input AC voltage, while the **secondary winding** delivers the transformed output voltage. The interaction of these windings via magnetic flux enables energy transfer.
Copper is preferred due to its:
Based on size and application, common winding types include:
Insulation materials electrically isolate the different conducting parts of a transformer and prevent breakdown due to voltage stress. Common materials include:
The tank is the outer steel enclosure that houses the core, windings, and insulating oil. It:
Bushings are insulated terminals that allow electrical connection between the transformer windings and external power circuits while passing through the tank wall. They prevent flashover and are usually made of porcelain or polymer.
The conservator is a small tank mounted above the main tank in oil-filled transformers. It accommodates the expansion and contraction of transformer oil due to temperature changes. It keeps the main tank completely filled and minimizes contact with external air.
Transformer oil serves dual purposes:
A Buchholz relay is a gas-actuated protection device used in oil-filled transformers. It detects internal faults such as insulation failure, short-circuits, or arcing. It is placed between the main tank and conservator and operates based on gas accumulation or oil flow.
The breather is a silica gel-filled device attached to the conservator. It filters and dehumidifies the air entering the conservator during oil expansion and contraction. This prevents moisture from contaminating the oil and reducing insulation quality.
Cooling methods include:
Radiators are extended surface heat exchangers attached to the main tank. They increase the surface area for cooling oil to dissipate heat to the surrounding air. Some radiators have fans to increase cooling efficiency (forced air cooling).
Tie rods and yoke bolts are mechanical fasteners that hold the transformer core and windings in place. They:
Core clamps hold the laminated core securely and maintain the magnetic alignment of the structure. They also support the assembly and reduce vibration and noise during operation.
The earthing terminal is connected to the metallic body of the transformer tank. It ensures that leakage currents or fault currents do not pose a shock hazard to humans or connected equipment. It is a key safety requirement.
Oil level indicators are mounted on the conservator tank and show the level of insulating oil. Maintaining proper oil level is critical to ensure insulation and avoid exposure of windings to air, which can cause oxidation and reduced dielectric strength.
A magnetic shunt is placed inside the core structure to control and guide the magnetic path and minimize stray flux. It reduces the magnetic leakage that could otherwise induce eddy currents in nearby structural parts.
Enclosures can be:
A tap changer is a device connected to the winding that allows adjustment of voltage ratios. It can be:
Placing the secondary winding near the core reduces leakage reactance and improves coupling efficiency. It also ensures better mechanical balance and simplifies heat transfer in large transformers.
During short-circuit conditions, currents produce electromagnetic forces:
A dry-type transformer uses air or solid insulation (like epoxy resin) instead of oil. It is:
Spacers are used to:
Assembly involves:
To prevent corrosion:
Proper construction:
Transformers can be classified based on various criteria. The main types include:
A power transformer is a high-voltage, high-rating transformer used in transmission networks to transfer bulk electrical energy over long distances. They operate at or near full load and are optimized for maximum efficiency at high loads.
Distribution transformers are used in low-voltage distribution networks to step down the voltage for end-user consumption. They are designed to operate with variable loads and typically have ratings below 500 kVA.
Power transformers are used in transmission with ratings above 500 kVA, designed for constant full load. Distribution transformers are used in local networks with variable loads, and typically rated below 500 kVA. Power transformers are more efficient at high loads, while distribution transformers must maintain efficiency across a range of loads.
An autotransformer uses a single winding with a common section for both primary and secondary. It is lighter, cheaper, and more efficient for small voltage differences but lacks isolation between input and output.
An isolation transformer has a 1:1 turns ratio, providing no voltage transformation but electrical isolation between input and output. It's used in sensitive medical and electronic equipment to eliminate ground loops and ensure safety.
Instrument transformers are used for measurement and protection in high-voltage systems. They include:
A CT steps down high current to a safer, measurable value proportional to the primary current. Commonly used in power system monitoring and protection relays. Example: A 1000:5 CT converts 1000 A to 5 A.
A PT, or Voltage Transformer, reduces high voltage to a lower standard level (like 110V) for safe metering. It maintains proportionality and phase relationship for accuracy.
A step-up transformer increases voltage from primary to secondary. It has more turns in the secondary winding than the primary. Used at generating stations to raise voltage for transmission.
A step-down transformer reduces voltage from primary to secondary. It has fewer turns in the secondary winding. Used in distribution to deliver usable voltages to consumers.
A single-phase transformer works on a single alternating voltage and current system. It's typically used in residential and light commercial applications with lower power needs.
A three-phase transformer is designed to handle three-phase power, either as a single 3-phase unit or by connecting three single-phase transformers in a bank. It’s widely used in industries and transmission networks.
Yes. This is called a transformer bank. While three single-phase units are flexible and easier to replace, a single three-phase transformer is more compact and efficient.
Core Type: Windings surround the core limbs.
Shell Type: Core surrounds the windings. More robust and used for high-voltage applications.
Dry type transformers use air for cooling instead of oil. They are safer for indoor and fire-prone environments like malls, hospitals, and high-rise buildings. Require less maintenance but are costlier.
Oil-filled transformers use insulating oil for cooling and insulation. The oil absorbs heat from the core and windings and dissipates it via radiators. Most commonly used outdoors.
A furnace transformer is a special type used to power electric arc furnaces (EAF) and induction furnaces. They must handle high short-circuit currents and rapid load fluctuations.
Used in railway systems to supply power to electric locomotives. They handle variable loads and are often mounted on trains. Operate under demanding mechanical and thermal conditions.
A zig-zag transformer is used to create a neutral point in ungrounded systems or to filter harmonics. Its winding configuration cancels out triplen harmonics.
These transformers control the power flow in transmission networks by adjusting the phase angle between input and output voltages. Useful in load balancing and parallel line control.
Grounding transformers provide a path to ground for ungrounded systems. They help stabilize system voltage during unbalanced load or fault conditions.
A pad-mounted transformer is a ground-mounted enclosed unit used in urban areas for underground distribution systems. It is tamper-proof, safe for public spaces, and often oil-cooled.
Mobile transformers are skid- or trailer-mounted and used as emergency power solutions during outages or maintenance. They’re compact, portable, and designed for temporary deployment.
Also called a shunt reactor, it absorbs reactive power and helps control voltage levels in transmission systems. Typically used in long high-voltage lines to reduce overvoltage during light load.
High Voltage (HV) transformers handle voltages above 33 kV. Low Voltage (LV) transformers operate below 1.1 kV. They are used based on application voltage requirements.
Custom transformers are designed for specific applications, such as testing equipment, aerospace systems, or medical devices. Their ratings and features vary based on client needs.
This is a new concept in nanotechnology and biomedical engineering where tiny transformers are designed at the molecular or nanoscale level for energy control in circuits or devices.
The right transformer is selected based on:
Transformer efficiency is defined as the ratio of output power to input power, usually expressed as a percentage. It indicates how effectively a transformer converts input power into output power without losses.
Efficiency (%) = (Output Power / Input Power) × 100
Since transformers have very low losses, efficiency is usually above 95%, especially in power transformers.
The two main types of losses in a transformer are:
Hysteresis loss occurs due to the continuous reversal of magnetization in the transformer core during each AC cycle. It depends on the material and frequency and is calculated using Steinmetz’s formula:
Wh = η × f × B1.6 × V
Where η = Steinmetz coefficient, f = frequency, B = flux density, V = volume of core.
Eddy current loss is caused by circulating currents induced in the core due to changing magnetic flux. It leads to heat generation and energy loss. Laminated cores are used to reduce eddy currents by increasing resistance to circulating paths.
Copper loss occurs in the transformer windings due to resistance when current flows. It is proportional to the square of the load current:
Pc = I² × R
It increases with load and is significant at higher loads.
Transformer efficiency improves with increasing load up to a certain point. At very low load, core losses dominate. At very high load, copper losses increase. Maximum efficiency occurs when copper loss equals iron loss.
Maximum efficiency occurs when:
Copper Loss = Iron Loss
This condition can be used to design the transformer for best performance at typical load conditions.
All-day efficiency considers energy over a full 24-hour period. It is more useful for distribution transformers where load varies throughout the day.
All-Day Efficiency = (Total Output Energy / Total Input Energy) × 100
Core losses can be reduced by:
Copper losses can be reduced by:
Transformer losses impact energy efficiency and cost. In large-scale power systems, even small losses translate to major energy and monetary losses. Minimizing losses is key to achieving higher performance and lower operating costs.
No-load loss is the power consumed by the transformer when it is energized but not supplying any load. It consists primarily of core losses (hysteresis and eddy current). These losses are nearly constant irrespective of load.
Load loss is the power loss that occurs in the windings when the transformer is supplying a load. It includes copper loss and leakage reactance effects. Load loss increases with the square of the load current.
Stray losses are caused by leakage fluxes that induce eddy currents in non-core metallic parts such as the tank, clamps, and structural supports. Although small, they contribute to the total losses and heating of the transformer.
Dielectric loss occurs in the insulating material when subjected to high voltage AC. These are usually very small but can be significant in high-voltage transformers.
Power transformers are more efficient due to better design and less load variation.
Efficiency is tested using open-circuit and short-circuit tests. These tests help calculate iron and copper losses, which are then used to estimate efficiency at different loads and power factors.
Increased temperature raises resistance of windings, increasing copper losses. Proper cooling is essential to maintain efficiency and prevent insulation damage.
Voltage regulation indicates the change in secondary voltage from no-load to full-load condition.
% Regulation = ((No-load V - Full-load V) / Full-load V) × 100
Lower regulation means better voltage stability under varying loads.
High-efficiency transformers reduce energy loss, improve power delivery, lower operational costs, and support environmental goals by conserving energy. They are essential for reliable and sustainable electrical infrastructure.
Iron losses, also known as core losses, occur in the transformer's core due to alternating magnetic flux. They consist of two components:
Iron losses remain constant regardless of the transformer load because they are dependent on voltage and frequency.
Copper losses occur due to the resistance of the transformer windings when current flows through them. The power loss is given by:
Pcu = I²R
Where I is the load current and R is the resistance of the winding. Copper losses vary with the square of the load current and are zero when the transformer is idle (no load).
Hysteresis loss is caused by the reversal of magnetization in the transformer's core during every AC cycle. The molecular structure of the core material resists this change, resulting in energy loss as heat. It is given by:
Ph = η × B1.6 × f × V
Where:
η = Steinmetz coefficient (depends on material)
B = maximum flux density
f = frequency
V = volume of core
Eddy currents are loops of induced current that circulate in the core material due to changing magnetic flux. These currents generate heat, leading to energy loss. To reduce this loss, the transformer core is made of thin laminated sheets insulated from each other.
Iron losses can be reduced by:
Copper losses can be reduced by:
Efficiency improves with load up to a certain point. At light loads, iron loss dominates. As the load increases, copper loss increases (I²R). The maximum efficiency occurs when:
Iron Loss = Copper Loss
Beyond this point, increasing load causes more copper losses, reducing efficiency.
Transformer efficiency can be determined through testing methods such as:
In practical applications, transformer efficiency typically ranges from 95% to 99.5%, depending on the rating and design. Large power transformers used in substations may exceed 99% efficiency under full load conditions.
If transformer losses increase, several negative effects occur:
Proper maintenance and design optimization help in keeping losses minimal.
Transformers generate heat due to iron and copper losses during operation. Excessive heat can damage insulation, reduce transformer efficiency, and shorten lifespan. Therefore, a proper cooling system is required to maintain the temperature within safe limits and ensure reliable performance.
Transformer cooling methods are classified into:
ONAN stands for Oil Natural Air Natural. It is a passive cooling system where:
It is suitable for smaller and medium-rated transformers.
ONAF stands for Oil Natural Air Forced. In this method:
Used in transformers with higher ratings for better cooling.
OFAF stands for Oil Forced Air Forced:
This active cooling method enhances capacity and is used in large power transformers.
OFAF stands for Oil Forced Water Forced. In this system:
This is used in very large, indoor or underground transformers where air cooling is not sufficient.
Radiators increase the surface area for heat dissipation. Hot oil flows from the transformer tank to the radiator, where it cools and returns to the tank, thus maintaining oil temperature.
The conservator tank is a small tank mounted above the transformer main tank. It accommodates expansion and contraction of oil due to temperature changes. It helps prevent oil spillage and oxidation by maintaining an oil seal.
A breather is a silica gel chamber connected to the conservator tank. It prevents moisture from entering the transformer oil when air enters the conservator during cooling. The silica gel absorbs moisture from the incoming air, protecting the insulation system.
A Buchholz relay is a gas-actuated protective relay installed between the main tank and conservator. It detects gas accumulation or sudden oil flow due to internal faults and triggers alarms or tripping signals.
Transformers use two main types of insulation:
These insulations provide electrical separation and thermal stability.
Transformer oil serves two purposes:
It must be clean, dry, and free of contaminants to work effectively.
Good transformer oil must have:
Dry-type transformers are air-cooled and do not use oil. Cooling is achieved using:
They are suitable for indoor or fire-sensitive areas.
Insulation isolates different voltage levels and prevents electrical short circuits inside the transformer. It also withstands thermal and mechanical stresses during operation.
Common causes include:
Oil filtration is the process of removing moisture, gases, and impurities from transformer oil. It helps restore dielectric strength and prolongs transformer life. Regular filtration is recommended during maintenance or after faults.
It is the increase in temperature of transformer components above the ambient temperature due to load losses. Standards define permissible temperature rise to ensure safe operation (e.g., 55°C for oil, 65°C for winding).
ONAN: Natural oil and air circulation, passive system.
ONAF: Same oil circulation, but air is forced using fans, increasing cooling capacity and allowing higher load handling.
Low oil level reduces insulation and cooling. Windings may overheat, leading to insulation failure or complete transformer damage. It also allows air and moisture to enter, reducing oil effectiveness.
Oil level is checked using the level indicator on the conservator tank or oil gauge glass. Oil samples may also be taken for dielectric testing and dissolved gas analysis (DGA).
In sealed transformers, nitrogen gas is filled above the oil to prevent contact with moisture or oxygen. It also maintains pressure and prevents oil oxidation or fire hazards.
Flash Point: ~140°C — the temperature at which oil gives off enough vapors to ignite.
Fire Point: ~160°C — the temperature at which oil vapors burn continuously. High values are preferred for safety.
Cooling tubes or fins are external extensions of the tank that help dissipate heat from the oil to surrounding air. These increase the surface area for heat transfer and are essential in ONAN systems.
Fans blow air over radiators or the body of the transformer, accelerating heat dissipation. They are used in ONAF and OFAF systems, often controlled by temperature sensors.
Thermometers are used to monitor winding and oil temperatures. These help in controlling fans, alarms, and trip signals to prevent overheating and failures.
Temperature relays detect excessive temperature in windings or oil and activate alarms or cooling systems. Some are equipped with digital sensors and programmable trip points.
Overheating must be addressed immediately to prevent insulation breakdown or fire.
Insulation class defines the temperature range the insulation material can withstand continuously. Common classes include:
Routine inspection depends on size and criticality, but generally includes:
Transformers can be categorized based on their construction into:
In a core-type transformer, the windings are placed around two limbs of a rectangular magnetic core. Each limb carries half of the primary and half of the secondary windings, often arranged concentrically.
In a shell-type transformer, the core surrounds the windings. It typically has three limbs, with both windings placed on the central limb, improving magnetic coupling and reducing leakage flux. It's more compact and better for handling higher power.
| Core-Type | Shell-Type |
|---|---|
| Windings surround core | Core surrounds windings |
| High leakage flux | Low leakage flux |
| Easier to insulate and cool | Compact and better for high voltages |
Power transformers are used in transmission networks for stepping up or stepping down voltages at high power levels (typically above 200 MVA). They operate near full load most of the time and are designed for high efficiency and insulation.
Distribution transformers are used to supply power to residential or commercial loads. They typically handle lower power ratings (<200 kVA), operate at partial loads, and prioritize voltage regulation over efficiency.
An autotransformer uses a single winding that acts as both the primary and secondary. Part of the winding is common to both. It is more efficient and economical for small voltage changes but lacks isolation between primary and secondary.
A step-up transformer increases voltage from primary to secondary. It has more turns in the secondary winding than the primary. Commonly used in power generation stations to transmit power at high voltages.
A step-down transformer reduces voltage from primary to secondary. The secondary has fewer turns than the primary. It is commonly used in distribution systems to deliver usable voltage to homes and businesses.
Instrument transformers are used for measurement and protection. They scale down high voltages or currents to manageable levels for meters and relays. Types include Current Transformers (CTs) and Potential Transformers (PTs).
CTs are used to measure high currents. They produce a reduced current accurately proportional to the current in the circuit, allowing measurement and protection devices to operate safely.
PTs reduce high voltages to lower values for metering and protection. They provide an accurate, proportional voltage output suitable for instruments and relays.
A three-phase transformer is designed to work with three-phase power systems. It can be built either as a single unit with three windings or as three separate single-phase transformers connected together (banked).
A single-phase transformer has only one primary and one secondary winding. It's used in applications where only single-phase power is required, such as residential loads or small appliances.
Winding arrangement affects leakage reactance, voltage regulation, and cooling. Common arrangements include concentric and sandwich types. Proper arrangement enhances performance and efficiency.
In concentric windings, one winding (typically low-voltage) is wound directly over the other (high-voltage). This arrangement is common in core-type transformers and helps in reducing leakage flux.
Sandwich windings alternate layers of high and low voltage windings. This technique is commonly used in shell-type transformers to minimize leakage inductance and improve voltage regulation.
Transformer windings are typically made from:
The tank encloses the core and windings. It contains insulating oil for cooling and dielectric strength. The tank is usually made of steel and may have cooling fins or radiators attached to dissipate heat.
A conservator tank accommodates the expansion and contraction of transformer oil due to temperature changes. It is connected to the main tank via a pipe and partially filled with oil and air, sometimes separated by a bladder.
The breather prevents moisture from entering the transformer when it breathes. It contains silica gel that absorbs moisture from the air entering the conservator. This keeps the transformer oil dry and maintains insulation properties.
Bushings are insulating structures that allow electrical connections through the tank wall. They isolate the high-voltage terminals from the grounded transformer tank and prevent leakage currents.
Cooling fins or radiators increase the surface area of the transformer tank to dissipate heat generated inside. They are especially used in oil-immersed transformers to enhance natural or forced air/oil cooling.
A Buchholz relay is a gas-actuated protective relay placed between the transformer tank and conservator. It detects gas formation due to internal faults (e.g., winding faults or oil breakdown) and triggers alarms or circuit breakers.
Tap changers allow the adjustment of transformer turns ratio to regulate output voltage. They can be:
A tertiary winding is a third winding added in some three-phase transformers for:
A zigzag transformer is used to create a neutral point in ungrounded systems and to filter harmonics. It has interconnected windings to provide zero sequence current paths and help in grounding applications.
Key design factors include:
Transformer protection ensures safe and reliable operation by preventing damage from faults like overcurrent, short circuits, over-temperature, and internal insulation failures. It helps avoid power interruptions, costly repairs, and safety hazards.
Common faults in transformers include:
The Buchholz relay is a gas-actuated protective device used in oil-filled transformers (usually above 500 kVA). It detects internal faults like inter-turn short circuits or core insulation failure by sensing gas accumulation and oil movement between the main tank and conservator.
When a minor fault occurs, it generates gases due to oil decomposition. These gases accumulate in the relay chamber, activating a float switch for an alarm. In severe faults, rapid oil flow triggers a second float switch to trip the circuit breaker, disconnecting the transformer.
A differential relay protects transformers from internal phase and winding faults by comparing current entering and leaving the transformer. If the difference (differential current) exceeds a threshold, it indicates an internal fault and initiates tripping.
Percentage differential protection uses a restraint system to prevent false tripping during external faults or heavy inrush currents. It calculates a percentage bias based on the average of incoming and outgoing current to ensure stability under normal and external fault conditions.
Winding faults may result from:
Temperature relays monitor winding and oil temperatures using sensors (usually PT100 or thermocouples). If the temperature exceeds safe limits, the relay can activate alarms or trip the transformer to prevent thermal damage.
Overload protection is usually provided by:
Surge arresters protect transformers from high-voltage transients like lightning or switching surges. They divert excess voltage to ground, preventing insulation damage and dielectric breakdown of transformer windings.
Common indicators include:
Restricted Earth Fault (REF) protection detects earth faults in a specific zone (typically transformer winding area) by measuring zero-sequence current and voltage. It provides sensitive and fast fault detection within the winding zone.
Overfluxing protection monitors the V/f (voltage-to-frequency) ratio. If it exceeds the design limit, it can cause core saturation and excessive heating. Overfluxing relays protect the core from magnetic saturation and overheating.
Oil level sensors monitor the oil level inside the transformer tank. If the level falls below safe limits (due to leakage or evaporation), an alarm or trip signal is generated to prevent overheating and insulation failure.
Typical protection devices include:
Circuit breakers interrupt fault current when signaled by protection relays. They isolate the faulty transformer quickly, preventing damage to the transformer and connected systems.
Moisture in oil reduces dielectric strength. Detection methods include:
Major causes include:
Pressure relief devices release excess internal pressure in oil-filled transformers during fault conditions, preventing tank rupture or explosion. It may also activate an alarm or trip signal.
Protection methods include:
Insulation resistance (IR) testing helps assess the health of winding and core insulation. A low IR value may indicate moisture ingress, aging insulation, or contamination, which can lead to faults.
The most common tests are:
Modern digital/numerical relays record event logs, disturbance records, and fault waveforms. These can be analyzed later to identify the type and location of the fault.
Inrush current is a high magnetizing current occurring when the transformer is energized. Differential relays with inrush restraint features use harmonic analysis to differentiate inrush from faults and avoid false tripping.
Harmonics may cause incorrect operation of protection relays, especially differential and overcurrent relays. Harmonic restraint features and filtering are used to avoid misoperation during inrush or unbalanced loading.
Internal Faults: Occur within the transformer (e.g., winding faults, insulation failure).
External Faults: Occur in connected systems (e.g., line faults), leading to overcurrent conditions.
CTs step down high primary current to a measurable value for protection relays. Accurate CTs are critical for proper functioning of differential, overcurrent, and REF protection schemes.
Dissolved Gas Analysis (DGA) helps detect incipient faults like overheating, arcing, or insulation breakdown by analyzing gases like H₂, CH₄, C₂H₂, and CO in transformer oil.
Protective relays and systems should be tested at least once a year or after any major fault. Testing includes relay calibration, CT/PT verification, trip checks, and logic validation.
Signs include:
Transformers are critical components in power systems and are expensive to repair or replace. Protection systems are essential to detect faults, prevent damage, and ensure uninterrupted and safe operation. They also help isolate faulty transformers from the network.
Common transformer faults include:
Key protective devices include:
A Buchholz relay is a gas-actuated protection device used in oil-filled transformers. It detects slow-developing faults by sensing gas accumulation and fast faults by detecting oil movement. It’s mounted between the main tank and conservator and can trip the transformer or give an alarm.
Differential protection compares the current entering and leaving the transformer windings. If the difference exceeds a set threshold (indicating internal fault), the relay trips. It offers high sensitivity to internal phase-to-phase or turn-to-turn faults.
Overcurrent protection uses current relays to detect excessive current caused by external faults or overloading. It may trip the breaker if current exceeds preset limits. It is usually backed up by thermal protection.
Restricted Earth Fault (REF) protection detects internal earth faults near the neutral side of transformer windings. It offers fast, selective fault clearance within a zone, using a set of CTs and differential principle.
Thermal protection uses winding and oil temperature indicators (WTI & OTI). When the temperature exceeds a preset limit, an alarm or trip signal is initiated. Some systems use RTDs or thermocouples embedded in the winding for accuracy.
Surge arresters are connected at transformer terminals to protect against high-voltage transients caused by lightning or switching. They absorb the surge energy and divert it safely to the ground.
Fuses protect small transformers by disconnecting them from the source during faults or short circuits. They are cheap, simple, and fast-acting, ideal for small distribution transformers and control transformers.
Pressure Relief Devices (PRDs) or explosion vents open automatically to release internal pressure buildup due to a severe internal fault. This prevents the transformer tank from rupturing or exploding.
OTI: Oil Temperature Indicator – measures the temperature of transformer oil.
WTI: Winding Temperature Indicator – gives approximate temperature of windings using a simulated heating element (typically in oil-filled transformers).
The conservator tank allows expansion and contraction of transformer oil due to temperature variations. It helps maintain oil level and prevents spillage or vacuum formation. It often integrates protection like Buchholz relay and breather system.
The breather contains silica gel, which absorbs moisture from the air entering the conservator tank during transformer breathing (cooling). This prevents moisture from contaminating the oil and degrading insulation properties.
Oil leakage is identified by visual inspection, oil level gauges, or through sensors. Protection schemes may include alarms for low oil level or low pressure in sealed-type transformers.
Key safety measures include:
Explosions occur due to internal arc faults that rapidly heat and vaporize the oil, increasing internal pressure. If not relieved, this leads to tank rupture. Overheating, oil degradation, and poor protection also contribute to explosions.
Fire prevention measures include:
Earthing provides a path for fault currents and stabilizes the system voltage. Transformer neutral and body are usually earthed to prevent dangerous potential build-up and ensure personnel safety.
NGR limits fault current during a ground fault in the transformer neutral. This allows detection of faults without excessive damage while maintaining system stability.
Correct setting ensures the relay is sensitive to internal faults but stable during through-faults or CT saturation. If improperly set, it may cause nuisance tripping or fail to trip during genuine faults.
CTs step down high current to measurable values for relays. They enable safe monitoring and accurate detection of fault conditions by protective relays.
Protection testing includes:
Alarms provide early warning of abnormal conditions like high temperature, low oil level, or Buchholz operation. Operators can take preventive action before full trip occurs, avoiding outages or damage.
Yes, large transformers use advanced digital relays with multiple zones of protection. Small transformers may only have overcurrent or fuse protection. Protection is designed based on capacity, criticality, and application.
Modern transformers use IoT-based sensors to monitor temperature, oil pressure, gas, and current. Data is sent to SCADA or cloud systems for analysis and alerts, enabling predictive maintenance.
Arc fault detection identifies high-energy faults within transformer enclosures by detecting sudden rise in light, sound, or current patterns. Fast-acting systems can trip supply within milliseconds to prevent fire.
Typical trip settings:
DGPT (Detection of Gas, Pressure, Temperature) relay is used in sealed transformers. It combines protection against gas generation, pressure rise, and overheating in a single compact unit.
Regular inspections help detect early signs of oil leakage, overheating, corroded terminals, or protective device malfunction. It reduces the risk of sudden failure and improves transformer lifespan and reliability.
Transformer testing is essential to ensure its safety, reliability, and performance before commissioning and during periodic maintenance. Testing helps detect manufacturing defects, insulation issues, winding faults, and other conditions that could lead to failure if undetected.
The IR test checks the condition of transformer insulation between windings and earth using a megohmmeter (megger). A high IR value indicates good insulation; a low value may indicate moisture ingress, dirt, or insulation breakdown.
The TTR test verifies the ratio of primary to secondary turns, ensuring the voltage transformation is as designed. A deviation from the rated ratio may indicate winding damage or incorrect connections.
In a short-circuit test, the secondary winding is shorted, and a reduced voltage is applied to the primary until rated current flows. This test determines copper losses and impedance. It is usually done at low voltage.
In an open-circuit test, the secondary winding is left open, and rated voltage is applied to the primary. It measures the core (iron) losses and no-load current. This helps calculate transformer efficiency.
Transformer oil serves as both an insulator and coolant. Oil testing (DGA, BDV, moisture content) ensures the oil is free from contaminants and breakdown products. Poor oil quality can cause insulation failure and overheating.
BDV test measures the voltage at which the insulating oil breaks down (sparks). It assesses the oil’s dielectric strength. A healthy transformer oil typically has a BDV > 30 kV. Low BDV indicates contamination or moisture.
DGA analyzes gases dissolved in transformer oil. Certain gases like H₂, CH₄, C₂H₂, and CO₂ are formed during insulation breakdown or arcing. Their type and concentration help detect internal faults before visible failure occurs.
Testing frequency depends on transformer size, criticality, and operating conditions. Routine tests like IR and BDV may be done annually, while full diagnostic tests like DGA are often done every 2–3 years or after a fault.
Winding resistance test detects issues like open windings, poor connections, or shorted turns. Abnormal resistance values can indicate winding damage, loose joints, or corrosion inside the transformer.
The vector group test verifies phase displacement and proper connection of windings (like Dyn11 or Yy0). It's critical for parallel operation and ensuring proper phase relationships during installation or repairs.
Polarity test determines the direction of instantaneous induced voltage. It's important in applications like parallel operation or CT testing to ensure proper connection of secondary windings.
Impulse testing checks the transformer's ability to withstand high-voltage surges like lightning. It’s typically done in factories using simulated impulse voltages and measures insulation robustness.
The heat run test evaluates the transformer's ability to operate under full-load conditions without overheating. It checks thermal stability, temperature rise, and cooling system performance.
PD testing detects tiny electrical discharges in insulation. These discharges degrade insulation over time and may lead to failure. PD detection helps in early diagnosis of insulation weakness.
Visual inspection includes checking oil level, leaks, bushing cracks, rust, paint condition, fan operation, breather silica gel color, terminal tightness, and external signs of overheating or arcing.
This test ensures the transformer core is properly grounded at a single point. Multiple grounds or floating core can cause circulating currents, overheating, or increased core losses.
Oil filtration removes moisture, gases, and solid particles using a filtration machine under vacuum and heat. It's done when oil quality degrades, typically indicated by low BDV or high moisture levels.
The breather (usually silica gel-based) prevents moisture from entering the transformer during air exchange. It must be regularly checked and replaced when the silica gel changes from blue to pink (moisture-saturated).
Proper grounding prevents high voltage buildup, protects against lightning, and ensures safety. Ground connections of the tank, core, and neutral must be checked for continuity and corrosion.
A tap changer adjusts the transformer's turns ratio to regulate output voltage. Testing includes checking contact resistance, operation timing, insulation, and oil quality (in case of OLTC – On Load Tap Changer).
Capacitance and power factor (tan delta) tests are done on bushings to detect insulation deterioration. Visual inspection and thermal imaging can also detect heating or loose connections.
This test (also called power factor test) measures the energy lost as heat in insulation. A high power factor indicates poor insulation. It's a key diagnostic test for aged transformers.
Thermal imaging cameras detect hotspots in transformer components like terminals, bushings, or radiators. It’s a non-contact method that helps locate abnormal heating due to loose connections or overloads.
Signs include oil leaks, low insulation resistance, high operating temperature, strange noises, tripping of protection relays, or unusual gas generation. These warrant detailed inspection and testing.
Dehydration removes moisture from transformer insulation and oil. It is needed when oil moisture exceeds safe limits (usually >25 ppm). Vacuum drying or hot oil circulation are common methods.
The insulation resistance should typically be >1000 MΩ for new transformers. In service transformers may have lower values depending on voltage level, humidity, and age. Values must also be compared across phases.
Routine maintenance includes:
Noise is usually due to magnetostriction in the core or loose parts. Sudden changes in sound may indicate core lamination loosening, winding movement, or fault conditions. Acoustics analysis helps detect issues early.
Pre-commissioning tests verify that the transformer is installed properly, has no internal damage from transportation, and meets safety standards. They include IR test, winding resistance, ratio, vector group, and oil BDV. It ensures safe and efficient operation from day one.
Transformers are widely used in electrical power systems for voltage transformation, isolation, and distribution. Applications include substations, power generation plants, residential areas, industries, and electronic devices.
In power stations, step-up transformers increase the voltage generated by alternators (e.g., from 11 kV to 220 kV or more) to reduce transmission losses over long distances.
Transformers in substations step down the high transmission voltage (e.g., 220 kV or 132 kV) to medium or low voltage levels suitable for distribution to consumers.
Small power or electronic transformers are used in household appliances like TVs, refrigerators, microwaves, chargers, and doorbells to step down voltage for internal use.
Distribution transformers operate at lower voltages and are located near consumers to supply final step-down voltage. Power transformers are used in transmission networks to handle higher voltages and power levels.
Isolation transformers electrically isolate the primary and secondary circuits for safety. They are used in sensitive devices, hospitals, control panels, and testing labs to prevent shock and interference.
CTs (Current Transformers) and PTs (Potential Transformers) are used in power systems for measurement and protection. They scale down high voltages and currents to measurable levels for relays and meters.
By stepping up voltage at the generation point, transformers reduce the current for the same power level, thereby minimizing I²R losses in transmission lines.
Transformers in solar/wind farms step up DC (via inverters) or AC output to grid-compatible voltage levels for efficient transmission and integration into the utility grid.
Electronic circuits use ferrite core transformers or pulse transformers for high-frequency switching, isolation, and voltage adaptation in devices like SMPS and inverters.
Electric arc or induction furnaces use special furnace transformers designed to supply high currents at low voltages suitable for melting metal.
In electric railways, traction transformers step down the overhead high-voltage AC supply (typically 25 kV) to required levels for traction motors and control systems.
Flameproof or dry-type transformers with specialized enclosures are used in oil and gas environments to prevent ignition in hazardous atmospheres.
Transformers in UPS systems provide isolation and voltage conversion between batteries, inverter output, and the load. Isolation ensures safety and clean power delivery.
Yes, transformers with corrosion-resistant enclosures and compact dry-type construction are used in offshore platforms, ships, and marine installations.
Control transformers step down incoming AC supply (e.g., from 415V to 230V or 24V) to operate relays, timers, and PLC systems safely in industrial control panels.
In smart grids, transformers are integrated with sensors, communication modules, and monitoring systems to enable load management, fault detection, and energy efficiency.
Mobile transformers are mounted on trailers or skids for emergency backup, construction sites, or temporary power in field installations.
Transformers may be used in harmonic filtering circuits or to match the voltage level of capacitor banks with the main distribution system.
Specialized transformers deliver low voltage, high current output for electroplating baths and welding machines, providing controlled and safe operation.
A rectifier transformer supplies AC voltage to a rectifier circuit that converts it to DC. It is used in DC furnaces, electrolysis, and industrial applications.
Elevators use control transformers to power circuits, indicators, and logic systems, while isolation transformers protect the electronics from surges and faults.
Small signal and pulse transformers are used for impedance matching, signal isolation, and surge protection in telecommunication devices and modems.
Audio transformers are used in sound systems for impedance matching, balanced signal transmission, and noise isolation in microphones, amplifiers, and mixers.
Laboratory-grade isolation transformers provide clean, shock-free power for sensitive instruments, ensuring precision measurements and operator safety.
CTs and PTs provide proportional voltage/current to energy meters from high-voltage circuits for accurate billing in substations and industries.
Flameproof or explosion-proof transformers are used in hazardous areas like mines and chemical plants to prevent ignition of flammable gases or vapors.
Transformers provide multiple isolated low-voltage supplies for PLCs, sensors, HMIs, and control units in automated manufacturing environments.
Yes, compact and lightweight transformers are used in aircraft power systems to step down voltage and isolate control electronics under stringent conditions.
Yes, transformers can be customized with specific enclosures, cooling methods, voltage ratios, and protective features to suit mining, marine, medical, or high-altitude applications.