Power transformers are electrical equipment manufactured based on the principle of electromagnetic induction. Therefore, power transformers should at least have cores and windings that can efficiently utilize electromagnetic induction. The main parts of power transformers are cores, windings, insulation, tanks, and necessary components. Due to differences in capacity and voltage, the structural forms of the core, windings, insulation, tanks, and necessary components of power transformers can be different.
Core
Reduce eddy current loss: The silicon steel sheet itself is a conductor. Under the action of the alternating magnetic flux, an induced current will be generated in the iron core, which will cause eddy current loss. In severe cases, it will even cause the iron core to heat up and melt partially. By applying a layer of insulating paint on each silicon steel sheet, the iron core can be divided into many conductors with small cross-sections, thereby increasing resistance and significantly reducing eddy current loss
Prevent dielectric leakage: Ventilation slot silicon steel sheet surface insulation paint is widely used in transformers, motors, and power electronic components. Its main function is to form a uniform and dense insulation coating on the ventilation slots or other surfaces of the silicon steel sheet to prevent dielectric leakage, reduce insulation loss, and improve the insulation performance of the equipment.
Improve corrosion resistance: The insulation paint on the surface of the pole core is used to coat the silicon steel sheet to reduce the eddy current loss of the core and also improve the corrosion resistance. This coating has the characteristics of a thin coating, strong adhesion, high hardness, smoothness, uniform thickness, oil resistance, moisture resistance, and good electrical performance.
In summary, the insulation paint film on the silicon steel sheet not only helps to improve the efficiency and reliability of electrical equipment but also enhances its durability and safety.
Mechanical support： In addition, the iron core also provides essential mechanical support for the transformer. It serves as a rigid structure that maintains the precise positioning of the windings, helping to prevent displacement or deformation under mechanical stress, such as during transportation, installation, or short-circuit conditions. This structural stability is critical for ensuring reliable electrical performance and extending the operational life of the transformer.
Transformer Core Structures： Laminated vs. Wound Core
Laminated Core
The core is in the center of the winding, and the winding is wrapped around the core. This structure is usually used in high-voltage, high-power transformers
Wound Core
Silicon steel strip is wound to form the core, which is common in small and medium-sized transformers. Wound core can further reduce seams, improve magnetic circuit continuity and reduce losses
Windings
Winding Material: copper, aluminum, superconducting material
Copper winding： The vast majority of transformer windings use copper wire due to its high electrical conductivity and mechanical strength.
Aluminum winding： Although aluminum is not as conductive as copper, it is lighter and less expensive.
Winding structure
The structure of the winding is related to the capacity of the winding. Therefore, the current passing through the winding varies with the rated voltage, and the manufacturing convenience and possibility should be considered. The commonly used winding structures in power transformers are double-layer cylindrical, multi-layer cylindrical, segmented cylindrical, continuous, tangled, inserted capacitor shielding, spiral, foil, and staggered pancake (shell transformer). The winding adopts different structures to adapt to different winding voltages, currents and processing and manufacturing.
The circuit part of the transformer. Primary winding (primary winding): input electrical energy. Secondary winding (secondary winding): output electrical energy. The primary and secondary windings are mounted on the same core column. The primary and secondary windings have different numbers of turns. Through electromagnetic induction, the electrical energy of the primary winding can be transferred to the secondary winding, and the primary and secondary windings have different voltages and currents.
Insulation System
Insulating Varnish and Tapes: Materials such as polyester film tape and fiberglass tape are used for interlayer insulation and securing windings. They provide additional electrical insulation and enhance structural integrity.The performance of a transformer’s insulation system is directly linked to the safety and reliability of the equipment. High-quality insulation helps prevent electrical failures, protects components from overheating and mechanical stress, and ensures long-term operational stability. Over time, insulation materials can degrade due to aging, leading to reduced performance and an increased risk of failure
Common insulation materials
Tank
Transformer oil tanks can be classified by cooling method, such as flat-wall, corrugated, tubular (radiator), fin-type (radiator), and cooler-integrated tanks, and by shape, including single-phase cylindrical, barrel-type, and bell-type designs.
The oil tank is filled with transformer oil, and the entire transformer body is installed inside. It protects the core and winding from moisture, and also has the functions of insulation and heat dissipation. When the transformer is running, the heat generated by the transformer body is transferred to the tank wall and the heat dissipation pipe (sheet) outside the box by the transformer oil. The manufacturing process of the heat dissipation pipe is complicated, and the heat dissipation is poor. Now flat tubes, sheet radiators and corrugated oil tank structures are mostly used, especially for sealed transformers (without oil storage cabinets), which can cause a certain deformation with temperature changes, so that the transformer can “breathe”.
Tap changer
The tap changer is a mechanism used in certain transformers to adjust the transformer’s turns ratio. By altering the connection point on the winding, it enables precise voltage regulation, allowing the transformer to accommodate varying load conditions and maintain a stable output voltage. This feature is especially valuable in scenarios where the input voltage fluctuates or where different voltage levels are required for specific applications. Tap changers enhance transformer performance by providing greater adaptability and operational stability across diverse conditions.
On-Load Tap Changer (OLTC)
Definition: A tap changer that allows adjustment of the transformer turns ratio while the transformer is energized and under load.
Features: Enables real-time voltage regulation without interrupting power supply.
More complex in design and higher in cost, but essential for managing voltage fluctuations in power networks.
The adjustment range is large, such as ±8*1.25%, and it can be installed inside or outside the transformer tank. Widely used in applications that demand high voltage stability.
Typical Applications: Power transmission and distribution systems, large industrial installations, power plants, and substations.
No-Load Tap Changer (NLTC)
Definition: A type of tap changer that can only be operated when the transformer is de-energized or under no-load conditions.
Features: Simple in structure and relatively low in cost.
The voltage regulation range is usually ±2*2.5%. Suitable for applications where voltage changes are infrequent and voltage stability is not critical.
Adjustment requires taking the transformer offline, making it ideal for systems with stable or predictable loads.
Typical Applications: Distribution transformers, small industrial equipment, or systems with fixed voltage requirements.
oil conservator
The oil conservator is connected to the oil tank. When the oil expands and contracts and causes the oil level to change up and down, the oil level in the oil conservator will rise and fall accordingly, so that the oil tank will not be squeezed or the oil level will drop to allow air to enter the oil tank. In order to keep the air in the oil conservator dry, a respirator (dehumidifier) is installed at the end of the air inlet pipe of the oil conservator. A glass oil gauge is installed on the side of the oil conservator to observe the height of the oil level. The oil level should be half the height. If a fully sealed transformer is used, the oil conservator can be omitted. Generally, it can be maintained for 15 years. The volume is also small, which is very suitable for urban power supply.
Function: The function of the oil conservator is to adjust the thermal expansion and contraction of the transformer oil in the transformer, and at the same time, it can reduce and prevent the transformer oil from being oxidized and dampened. There is a precipitator at the bottom of the oil conservator to precipitate the water and dirt that invade the oil conservator.
Bushing
The insulating bushing passes through the oil tank cover and leads the input and output lines of the transformer winding in the oil tank from the inside of the box to the outside of the box to connect to the power grid. The insulating bushing consists of an outer porcelain sleeve and a conductive rod in the middle. The main requirements for it are good insulation and sealing properties. According to the different operating voltages, it is divided into gas-filled and oil-filled types. The latter is for high voltage (60kV uses oil-filled). When used for higher voltages (above 110 kV), multiple layers of insulation and aluminum foil are also wrapped in the oil-filled insulating bushing to evenly distribute the electric field and enhance insulation performance. According to different operating environments, it can be divided into indoor and outdoor types. The purpose of gas filling and oil filling is to reduce the temperature.
Buchholz relay
The buchholz relay is installed in the pipeline between the oil tank and the oil storage cabinet. When the transformer fails, the body will overheat and the oil will decompose to produce gas. The gas enters the relay, causing one of the mercury switches to turn on (upper float action), and an alarm signal is issued (light gas protection). When the accident is serious, the transformer oil expands and impacts the baffle in the relay, causing another mercury switch to turn on the trip circuit (lower float action), cutting off the power supply to prevent the fault from expanding (heavy gas protection). This is also the working principle of the float gas relay.
The gas protection of the transformer is divided into light gas protection and heavy gas protection.
Transformer overload and temperature protection generally act on tripping.
Pressure relief valve
The pressure relief valve is a pressure protection device for transformers. When there is a serious fault inside the transformer, the oil decomposes to produce a large amount of gas. Since the transformer is a closed object, the diameter of the connecting pipe connecting the oil conservator is relatively small. The connecting pipe connecting the oil conservator alone cannot effectively and quickly reduce the pressure of the pressure relief valve, causing the pressure in the oil tank to rise sharply, which will cause the transformer oil tank to rupture. The pressure relief valve will open in time to discharge part of the transformer oil and reduce the pressure in the oil tank. After the pressure in the oil tank is reduced, the pressure relief valve will automatically close to keep the oil tank sealed.
Radiator
The radiator is a vital component of an oil-immersed transformer, designed to dissipate heat generated during operation. As the transformer load increases, the temperature of the insulating oil rises. The hot oil circulates through the radiator, where it releases heat to the surrounding air, helping maintain a stable operating temperature.
Radiators are typically made of steel and consist of a series of fins or tubes to maximize surface area for effective heat dissipation. Some transformers use natural air cooling (ONAN), while others may include forced air or oil circulation systems for enhanced cooling efficiency.
Efficient radiator performance is essential to prevent overheating, ensure long-term reliability, and extend the service life of the transformer.
Breather/Moisture absorber
The breather is installed on the transformer’s oil conservator to prevent moisture from entering during air exchange caused by oil volume changes. It contains silica gel, which absorbs moisture from incoming air. The gel turns from blue to pink as it becomes saturated and can be reused by heating to 120°C.
By keeping the air dry, the breather protects the insulating oil, ensuring reliable transformer operation and extending its service life.
Transformer Terminals
Transformer terminals are the external connection points that link the transformer windings-both primary and secondary-to the external power system, such as a grid station or local electrical installation. One end of the terminal is internally connected to the transformer winding, while the other end is exposed on the top or side of the transformer for external access.
These terminals are housed within a terminal box, which is specifically designed to provide mechanical protection, electrical insulation, and ease of access during installation or maintenance. The terminal box ensures that connections to the power lines are made safely and securely, reducing the risk of environmental damage, short circuits, or accidental contact.
By enabling a safe and efficient interface between the transformer and the external electrical network, the terminal and its enclosure play a critical role in the transformer’s overall reliability and safety.
In addition to the main structure and accessories mentioned above, the transformer has additional accessories to ensure its safe and stable operation. These accessories have various functions including cooling, protection, monitoring and control etc. By properly configuring and maintaining these accessories, the transformer can operate more efficiently, safely and stably, ensuring the reliability and continuity of the power system.
Power transformers are essential components of high-voltage power grids, playing a critical role in minimizing energy losses during long-distance electrical transmission. Power transformers include several key components such as the core, windings, insulation, tap changers, conservator tanks, buchholz relays, pressure relief valve, radiator, moisture absorber, transformer Terminals… Each of these components works in coordination to maintain the transformer’s performance, reliability, and safety under various load and environmental conditions.
