Understanding Transformer Core Structure

Introduction

As a core device for power transmission and voltage transformation in power systems, the performance of transformers directly affects the stable operation and energy transmission efficiency of power networks. The performance of a transformer is largely determined by the selection of its main materials and structural design.

From the magnetic energy conversion of the magnetic circuit to the power transmission of the circuit, from the safety guarantee of the insulation system to the stable support of structural components, the application of each material and the design of each structure assume specific functions. This article systematically sorts out the core materials and structural characteristics of transformer main bodies, helping to fully understand this key power equipment.

Magnetic Circuit Materials: Core Carriers for Magnetic Energy Conversion

The magnetic circuit is the key path for transformers to realize electromagnetic induction. Its core function is to guide and concentrate the magnetic field and reduce magnetic energy loss. Therefore, magnetic circuit materials need to have characteristics such as high magnetic permeability, low iron loss, and good magnetic stability. At present, the most widely used magnetic circuit materials in transformers are silicon steel sheets and amorphous alloy cores.

Silicon steel sheets, also known as electrical steel sheets, are the mainstream materials for transformer magnetic circuits. By adding silicon to pure iron, the resistivity of the material is effectively improved, eddy current loss is reduced, and the magnetic permeability is significantly enhanced, enabling more efficient transmission of magnetic field energy. According to the rolling process, silicon steel sheets can be divided into hot-rolled and cold-rolled types. Among them, cold-rolled silicon steel sheets have more excellent magnetic properties due to their more orderly grain orientation, and are more widely used in modern transformers. Silicon steel sheets are usually processed into thin sheets (with thicknesses of 0.35mm or 0.5mm) and coated with an insulating layer on the surface to further reduce interlaminar eddy current loss. When in use, multiple sheets are stacked to form a core, forming a closed magnetic circuit.

From the perspective of core characteristics, there are obvious differences between silicon steel sheets and amorphous alloy cores. The specific comparison is reflected in the following aspects:

Amorphous alloy cores are a new type of magnetic circuit material developed in recent years. They are prepared by rapid solidification technology, and their atomic arrangement presents a disordered amorphous structure. This characteristic is the core reason why their iron loss is much lower than that of silicon steel sheets. In addition to extremely low iron loss, amorphous alloy cores also have advantages such as high magnetic permeability and good corrosion resistance, making them suitable for power scenarios with extremely high energy efficiency requirements. However, amorphous alloy materials also have problems such as high brittleness, high processing difficulty, and relatively high cost, which limit their large-scale application in some fields. With the advancement of processing technology, the forming ability of amorphous alloy cores has gradually improved, and their application proportion in the field of distribution transformers is constantly expanding.

Circuit Materials: Efficient Conductors for Power Transmission

The circuit is the channel for transformers to realize power input, conversion and output. Its core requirement is low resistance and high conductivity to reduce power loss during transmission. The main conductor materials for transformer circuits are copper and aluminum. Each of the two materials has its own advantages and disadvantages, and a reasonable choice should be made according to factors such as the power level, application scenario, and cost budget of the transformer.

Copper conductors are widely used in transformer circuits due to their excellent electrical conductivity (second only to silver, with a conductivity of 58×10⁶ S/m at 20℃), good mechanical strength, and corrosion resistance. Copper has low resistivity. Under the same current load, copper conductors have smaller losses and less heat generation, which can effectively improve the operating efficiency and reliability of transformers. At the same time, copper has good ductility and is easy to process into wires and windings of various specifications, adapting to the design of transformers with different structures. In terms of characteristic comparison, copper is superior to aluminum in terms of conductivity efficiency, mechanical stability, and aging resistance, but the scarcity of copper resources leads to its high price, which will significantly increase the equipment manufacturing cost in large transformers or scenarios sensitive to cost.

Aluminum conductors have the core advantage of low cost and abundant resources. Their price is only about 1/3 of that of copper, which can significantly reduce the manufacturing cost of transformers. This is also an important reason for their application in the field of medium and low voltage transformers. Although the conductivity of aluminum is lower than that of copper (with a conductivity of about 37×10⁶ S/m at 20℃, only about 64% of that of copper), the deficiency in conductivity can be compensated by increasing the cross-sectional area of the conductor (usually 1.6 times that of copper conductors), meeting the circuit transmission requirements of transformers. However, aluminum conductors have obvious shortcomings: low mechanical strength, only half the tensile strength of copper; the surface is prone to oxidation to form a high-resistance oxide film, affecting conductive stability; and electrochemical corrosion is prone to occur when connected to other metals such as copper, leading to poor contact. Therefore, special processes must be adopted during processing and installation, such as using copper-aluminum transition joints and performing anti-corrosion treatment on the connection parts. In recent years, with the development of aluminum alloy material technology, high-strength aluminum alloy conductors made by adding magnesium, silicon and other elements to aluminum have greatly improved their mechanical strength and corrosion resistance, overcoming the defects of pure aluminum conductors to a certain extent, and their application range is gradually expanding.

The detailed performance comparison between copper and aluminum conductors is shown in the following table:

Insulating Materials: Key Barriers for Safe Operation

During the operation of transformers, there is a high potential difference between the circuit and the magnetic circuit, and between the circuit and structural components. The function of insulating materials is to isolate these potential differences, prevent faults such as leakage and short circuits, and ensure the safe and stable operation of the equipment. Insulating materials need to have excellent insulation performance, high temperature resistance, aging resistance, and mechanical strength. According to the application parts and functions, they can be divided into three categories: solid insulating materials, liquid insulating materials, and gas insulating materials.

Solid insulating materials are the core of the transformer insulation system, characterized by stable shape and long-lasting insulation performance. They mainly include insulating paper, insulating cardboard, insulating varnish, epoxy glass cloth board, spacers, angle rings, etc. Insulating paper and insulating cardboard are the most basic solid insulating materials, which can be divided into plant fiber (such as wood pulp) and synthetic fiber (such as aramid fiber) according to raw materials. Plant fiber insulating paper has low cost and good oil absorption performance, and can form a synergistic insulation system with insulating oil. It is often used for inter-turn and inter-layer insulation of windings and isolation between cores and windings; synthetic fiber insulating paper (such as Nomex paper) has the advantages of high temperature resistance (long-term operating temperature up to 180℃), aging resistance, and high mechanical strength, and is suitable for key insulation parts of dry-type transformers or oil-immersed transformers in high-temperature environments. Insulating varnish is divided into oil-based insulating varnish and resin insulating varnish. Oil-based varnish has low cost but poor temperature resistance, and is mostly used for small and low-voltage transformers; resin varnish (such as epoxy resin varnish, polyurethane varnish) has higher temperature resistance and insulation strength. By impregnating the windings, it can fill the gaps of the windings and wrap the wires to form a continuous insulation layer, which not only improves the insulation performance but also enhances the integrity of the windings and prevents vibration and wear. Epoxy glass cloth board is made of glass cloth impregnated with epoxy resin and hot-pressed, which has high strength, high temperature resistance, and excellent insulation properties. It is often used to make structural insulation components such as supports, partitions, and terminal blocks of transformers; spacers and angle rings are mostly made of cardboard or epoxy materials, which are used for internal support of windings and insulation of winding ends respectively, ensuring the stable structure of windings and sufficient insulation distance.

Liquid insulating materials, also known as insulating oil, are mainly used in oil-immersed transformers and have three core functions: insulation, cooling, and arc extinction. They can effectively improve the capacity and service life of transformers. Commonly used insulating oils are mainly divided into three categories: mineral insulating oil, synthetic insulating oil, and vegetable insulating oil. Mineral insulating oil is refined from petroleum, with advantages such as excellent insulation performance (breakdown voltage up to 40kV or more), high heat dissipation efficiency, good fluidity, and low cost. It is compatible with solid insulating materials and can fully impregnate insulating paper to form a composite insulation system. It is currently the most widely used liquid insulating material in oil-immersed transformers worldwide. Synthetic insulating oil is an insulating oil prepared by chemical synthesis methods, such as polyalphaolefin and silicone oil. Its biggest feature is high flash point (usually above 300℃), strong aging resistance, and good low-temperature fluidity. It is widely used in scenarios with high fire protection requirements (such as high-rise buildings, underground substations), but its high cost limits its large-scale popularization. Vegetable insulating oil is an environmentally friendly insulating oil refined from vegetable oils such as soybean oil and rapeseed oil. It has the advantages of good biodegradability, high flash point, and renewable resources, which is in line with the development trend of green environmental protection. However, its aging resistance and low-temperature fluidity still need to be improved, and it is currently mainly pilot-applied in small equipment such as distribution transformers.

Gas insulating materials have the characteristics of little influence by the environment, uniform heat dissipation, and no leakage risk. They are mainly used for auxiliary insulation of gas-insulated transformers (GIT) and dry-type transformers. Commonly used gases include sulfur hexafluoride (SF₆), nitrogen (N₂), and dry air. Sulfur hexafluoride is one of the gas insulating materials with the best insulation performance currently. Its breakdown field strength is more than three times that of air, and it has excellent arc-extinguishing performance, stable chemical properties, and is not easy to age. Therefore, it is widely used in gas-insulated transformers and gas-insulated switchgear. However, sulfur hexafluoride is a strong greenhouse gas with an extremely high global warming potential (GWP) and a long retention time in the atmosphere. With the increasingly strict environmental protection regulations, its application is subject to more and more restrictions. At present, recycling technology is mostly used to reduce emissions. Nitrogen, as an inert gas, has the advantages of wide source, low cost, environmental protection, and no pollution. Although its insulation performance is lower than that of sulfur hexafluoride, it can meet the insulation requirements of transformers by increasing the gas pressure (usually 0.3-0.5MPa). It is often used for nitrogen-filled protection of dry-type transformers and alternative gas for gas-insulated transformers. Dry air is dehumidified air, with insulation performance similar to nitrogen and lower cost. It is mainly used for insulation and cooling of small dry-type transformers, but its insulation performance is greatly affected by humidity, so the water content must be strictly controlled.

The comparison of key characteristics of various insulating materials is shown in the following table:

Structural Materials and Accessories: Important Guarantees for Support and Protection

Structural materials and accessories are important components of transformers that realize mechanical support, structural reinforcement, performance monitoring, and safety protection. Their design and material selection directly affect the mechanical stability, operational reliability, and service life of transformers.

Structural materials mainly undertake the functions of transformer support, magnetic circuit and circuit reinforcement, and insulating fluid encapsulation. Core components include clamps, oil tanks, radiators, oil conservators, etc. Clamps are usually made of steel and used to fix the core and windings, ensuring the stability of the magnetic circuit and circuit structure and preventing vibration and displacement of the transformer due to electromagnetic force during operation; the oil tank is the core encapsulation component of oil-immersed transformers, welded with steel plates, used to accommodate insulating oil and the main structure of the transformer, and at the same time play the roles of sealing, corrosion prevention, and mechanical protection; radiators are divided into various types such as finned and tubular types, which transfer the heat generated by the transformer during operation to the air by increasing the heat dissipation area to achieve equipment cooling; the oil conservator is connected to the top of the oil tank, used to compensate for the volume expansion and contraction of insulating oil due to temperature changes, and at the same time reduce the contact area between insulating oil and air to delay oil aging.

Transformer accessories vary according to the type of transformer (dry-type or oil-immersed) and mainly undertake performance monitoring and protection functions. Core accessories of dry-type transformers include temperature controllers, fans, instrument transformers, etc.: the temperature controller is used to monitor the temperature of windings and cores in real time and issue an alarm signal when the temperature exceeds the threshold; the fan is linked with the temperature controller and automatically starts when the temperature rises to enhance the heat dissipation effect; the instrument transformer is used to measure the voltage and current of the transformer and provide data support for the measurement and protection of the power system. In addition to the temperature controller, accessories of oil-immersed transformers also include gas relays, pressure relief valves, tap changers, etc.: the gas relay is the core protection component of oil-immersed transformers. When a fault occurs inside the transformer to generate gas or the flow rate of insulating oil is abnormal, it timely issues an alarm signal or cuts off the power supply; the pressure relief valve is used to automatically release pressure when the pressure inside the oil tank rises to a certain value due to a fault to prevent the oil tank from bursting; the tap changer is used to adjust the number of winding turns of the transformer to realize the adjustment of the output voltage to adapt to the fluctuation of the power grid voltage.

The main structure of a transformer is the organic combination of various materials, forming a synergistic system integrating “magnetic circuit – circuit – insulation – structure”. The core, as the core of the magnetic circuit, is fixed on the oil tank (oil-immersed transformer) or bracket (dry-type transformer) through clamps. The windings are wound on the core columns, forming the core unit of electromagnetic induction; solid insulating materials are used to isolate between the windings and the core, and between the windings and the windings. In oil-immersed transformers, insulating oil fills the gaps between various components to achieve insulation and cooling at the same time; structural components such as oil tanks and clamps provide mechanical support for the core components, and accessories real-time monitor the operation status of the equipment and start the protection mechanism in case of faults.

This structural design not only ensures the efficient realization of electromagnetic induction but also ensures operational safety through the insulation system and protective accessories. At the same time, with the support of structural materials and the role of heat dissipation components, the service life of the equipment is extended. In different application scenarios, the structure of the transformer will be adjusted targetedly. For example, dry-type transformers eliminate the oil tank and insulating oil, adopt air cooling and solid insulation, and are suitable for the interior of buildings with high fire protection requirements; oil-immersed transformers, with their excellent heat dissipation performance, are suitable for outdoor large-scale power transmission scenarios.

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