Transformer parts

Transformer parts

Transformers consist of various essential parts that work together to convert electrical energy from one voltage level to another. Different parts of a transformer have distinct functions, and their structures can vary depending on the type of transformer. In this article, we will explore the essential components common to all transformers, while also mentioning those unique to liquid-filled transformers.


The transformer core is a critical component in electrical transformers, serving as the central element responsible for the efficient transfer of electrical energy. Typically constructed from high-grade laminated steel or other magnetic materials, the core forms a closed magnetic circuit. Its primary purpose is to provide a low-reluctance path for the magnetic flux generated by the transformer’s primary winding, thereby facilitating electromagnetic induction. By doing so, the core enables the conversion of electrical voltage and current from one level to another while minimizing energy losses due to hysteresis and eddy currents. The core’s design and construction are pivotal in determining the transformer’s efficiency, with factors like core material quality, thickness, and shape playing essential roles in its performance.


The winding of the transformer is a fundamental component serving as the conductive pathway through which electrical energy is transferred and transformed. In transformers, there are typically two main types of windings: primary and secondary winding. These windings play distinctive roles in the transformation of voltage and current levels, enabling the efficient distribution of electrical power across different parts of the electrical grid. To understand their significance, it is essential to explore each winding’s characteristics, functions, and importance within the context of transformer operation.

Primary Winding

The primary winding is responsible for receiving and carrying the incoming electrical power. It consists of a coil or coils of insulated wire wound around the transformer’s core. This winding is connected to the high-voltage side of the power system, where electrical energy is initially introduced into the transformer. As alternating current (AC) flows through the primary winding, it generates a changing magnetic field in the transformer’s core. This magnetic field is essential for the process of electromagnetic induction, which allows the transformer to transfer electrical energy efficiently from the primary winding to the secondary winding, albeit at a different voltage level.

Secondary Winding

The secondary winding plays a complementary role in the transformer’s operation. Like the primary winding, it consists of a coil or coils of wire wound around the same core, but with a different number of turns. This winding is connected to the low-voltage side of the power system or the load that requires the transformed electrical power. As the changing magnetic field induced by the primary winding passes through the core, it, in turn, induces a voltage in the secondary winding. The number of turns in the secondary winding compared to the primary winding determines the voltage transformation ratio, allowing the transformer to step up or step down the voltage as needed to meet specific electrical distribution requirements.


Transformers, whether dry-type or liquid-filled, rely on distinct insulation methods tailored to their unique designs and operational requirements. Let’s delve into the specifics of insulation for both dry-type and liquid-filled transformers to understand their significance.

Liquid-filled transformers

Liquid-filled transformers, like oil-immersed transformers, rely on insulating oil to provide both electrical insulation and cooling. Mineral oil or other specialized insulating fluids are used to immerse the windings and core. The oil acts as an excellent dielectric, preventing electrical discharge, and also helps dissipate heat generated during operation. It’s crucial to monitor the condition of the insulating oil regularly and filter or replace it when necessary to maintain the transformer’s insulating properties and overall performance.

Dry-type transformers

 In dry-type transformers, insulation typically consists of high-quality insulating materials such as epoxy resin or cast resin, which are poured or cast around the windings and core. This solid insulation not only provides electrical insulation but also effectively dissipates heat, preventing the risk of electrical breakdown. Dry-type transformers are known for their high resistance to environmental factors and contaminants due to their solid insulation.

Tap Changer

Tap changers are crucial components within transformers, allowing for the adjustment of the turns ratio and, consequently, the output voltage to meet varying load and voltage requirements. Two main types of tap changers are commonly used in transformers: De-energized Tap Changer (DETC) and On-Load Tap Changer (OLTC). Each type serves specific purposes and operates under different conditions to ensure the efficient and reliable performance of transformers in diverse electrical applications.

De-energized Tap Changer (DETC)

De-energized tap changers are designed for adjusting the transformer’s turns ratio when it is completely de-energized, meaning the transformer is not connected to the power supply. DETCs are commonly used in smaller distribution transformers and some medium-sized power transformers. During the tap-changing process, power must be temporarily interrupted to make adjustments, making it suitable for applications where brief power interruptions can be tolerated, such as in distribution networks or industrial settings. DETCs are known for their simplicity and reliability, as they do not have to operate under load, but their adjustments require a momentary loss of power.

On-Load Tap Changer (OLTC)

On-Load Tap Changers are designed to adjust the transformer’s turns ratio while it is under load, meaning it remains connected to the power supply during adjustments. OLTCs are typically used in larger power transformers, particularly in high-voltage transmission and substation applications. They offer the advantage of continuous voltage regulation without interrupting the power supply, making them indispensable in maintaining voltage stability in the grid. OLTCs are more complex than DETCs and require sophisticated control systems to ensure smooth and precise tap changes while the transformer remains operational. Their ability to adapt to changing load conditions is vital for efficient energy transmission and distribution.


Bushings serve as the interface between the high-voltage and low-voltage sides of the transformer. These insulating devices provide a safe and efficient means of connecting external wires or cables to the transformer’s windings while preventing electrical discharge or leakage. High-voltage bushings are typically located on the high-voltage side of the transformer, often extending through the transformer tank, while low-voltage bushings are positioned on the low-voltage side. Bushings are designed to withstand the electrical stresses and environmental conditions they encounter, ensuring the insulation and integrity of the transformer as it operates within power distribution and transmission systems. Their proper functioning is crucial for maintaining electrical safety and the efficient flow of electrical energy through the transformer.

Cooling System

Cooling systems have a critical role in regulating the operating temperature of transformers. These devices generate heat during the process of energy conversion, and efficient cooling is essential to prevent overheating and maintain safe and reliable operation. Two primary types of transformers—liquid-filled transformers and dry-type transformers – employ distinct cooling mechanisms tailored to their respective designs and applications. 

Liquid-filled transformers

In liquid-filled transformers, such as oil-immersed transformers, insulating oil not only serves as an electrical insulator but also as a coolant to dissipate the heat generated during the transformer’s operation. The cooling system typically includes various components, such as radiators, fans, cooling tubes, and sometimes oil pumps, designed to regulate the temperature of the insulating oil. Radiators, often equipped with cooling fins, facilitate heat exchange between the oil and the surrounding air. Cooling fans are employed to enhance airflow over the radiator surfaces, expediting heat dissipation.

Dry-type transformers

In dry-type transformers, the cooling system plays a vital role in dissipating the heat generated during operation without the use of insulating oil. These transformers primarily rely on natural convection and, in some cases, forced-air cooling to maintain safe operating temperatures. Ventilation ducts and cooling fins are intricately designed to allow air to circulate around the windings and core, aiding in heat dissipation through natural convection currents. Some dry-type transformers incorporate forced-air cooling with fans or blowers, activated by temperature or load sensors when needed. These cooling systems maintain a low environmental impact thanks to the absence of insulating oil.

Transformer parts exclusive to liquid-filled transformers 

Thanks to their specific characteristics, liquid-filled transformers possess a range of vital components, such as the tank and breather, which are unique to this type of transformer. In this section, we will explore these components and elaborate on their significance.


The primary function of the main tank in a liquid-filled transformer serves two crucial roles: safeguarding the core and windings against external environmental factors and functioning as the reservoir for insulating liquid while providing structural support for various transformer components. The tank’s construction involves the fabrication of rolled steel plates into robust enclosures, complete with lifting hooks and cooling tubes. To enhance efficiency by minimizing weight and reducing stray losses, aluminum sheets are sometimes employed as an alternative to steel plates, albeit at a higher cost than their steel counterparts.


A conservator in a liquid-filled transformer is designed to maintain the optimal level of insulating oil within the transformer’s tank. Typically, the conservator is a cylindrical or rectangular expansion chamber mounted above the main tank and connected to it via a pipe. Its primary function is to accommodate variations in the volume of insulating oil caused by temperature fluctuations. As the transformer operates and the oil temperature rises, causing expansion, excess oil flows into the conservator, preventing overpressurization of the main tank. Conversely, during cooldown periods, when the oil contracts, the conservator returns the oil to the main tank, ensuring a consistent and safe oil level. This dynamic process not only safeguards the transformer against potential damage due to oil expansion but also aids in maintaining the insulating properties and temperature stability of the transformer.


A breather is added to the transformer’s oil conservator to handle oil volume fluctuations resulting from temperature shifts during regular operation. Inside the breather is silica gel, initially blue, which absorbs moisture from the surroundings. As it absorbs moisture, the silica gel changes color to pink. It’s possible to recycle the silica gel by heating it to 120 degrees Celsius until it reverts to its original blue hue.

Buchholz Relay

The Buchholz relay holds significant importance in oil-immersed transformers with a rating exceeding 500kVA. It operates as an oil and gas-activated device designed to detect faults occurring within the components immersed in the transformer’s insulating oil.

Short circuits happening beneath the transformer oil generate heat, causing the oil to break down into gases like hydrogen, carbon monoxide, and methane. These gases gradually travel through the connecting pipe toward the conservator tank. Positioned on the pipe linking the conservator tank and the main tank, the Buchholz relay senses these gases and triggers the trip and alarm systems. The trip system functions by opening the circuit breaker that supplies current to the primary winding, effectively cutting off the current flow.

Explosion vent

An explosion vent, found exclusively in high-rated liquid-filled power transformers, consists of a metallic pipe with a diaphragm at one end. Positioned slightly above the conservator tank on the main tank, its primary purpose is to safeguard the power transformer from explosions resulting from excessive pressure build-up within the main tank due to severe internal faults. Essentially, it serves as an emergency outlet for releasing oil and hot air gases from the transformer’s main tank. 


Transformers are intricate electrical devices with a myriad of components, each playing a critical role in ensuring efficient energy transmission and electrical safety. As technology advances and environmental concerns grow, transformer designs continue to evolve, striving for higher efficiency, safety, and sustainability.

At TTES, we have over 100 combined years of experience manufacturing and repairing transformers. If you’re looking for industry-leading lead times of just 20 weeks on average, don’t hesitate and reach out to us for a free quote!

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