Engineering Application of Iron Core in Oil-Immersed Power Transformers
As the core component of the magnetic circuit in oil-immersed power transformers, the iron core undertakes the key task of magnetic flux conduction and energy conversion. Oil-immersed power transformers are widely used in power transmission and distribution systems, industrial fields, and new energy power generation projects due to their excellent insulation performance, high heat dissipation efficiency, and strong adaptability to high-voltage and large-capacity scenarios. The design, material selection, processing, installation, and maintenance of the iron core directly determine the efficiency, stability, service life, and operating cost of the entire transformer. With the continuous development of the global power industry and the acceleration of the energy transition, the demand for high-efficiency, low-loss, and large-capacity oil-immersed power transformers is increasing, which puts forward higher requirements for the engineering application level of the iron core. This article comprehensively elaborates on the engineering application of the iron core in oil-immersed power transformers, covering its functional positioning, structural design, material selection, engineering design points, application scenarios, installation and maintenance, fault diagnosis, and development trends, providing a comprehensive and practical reference for engineering and technical personnel, with a total word count controlled at around 2000 words.
Functional Positioning and Core Role of the Iron Core in Oil-Immersed Power Transformers
The oil-immersed power transformer realizes the conversion of alternating current voltage and current through electromagnetic induction, and the iron core, as the core part of the magnetic circuit, plays an irreplaceable role in this process. Its main functions are reflected in three aspects: first, forming a closed magnetic circuit to guide the magnetic flux generated by the primary winding, minimizing magnetic leakage and improving the efficiency of electromagnetic energy conversion. Second, reducing energy loss: the iron core is made of high-permeability materials, which can reduce magnetic resistance, thereby reducing eddy current loss and hysteresis loss in the magnetic circuit, which are the main components of transformer no-load loss. Third, providing mechanical support for the winding: the iron core is used as the skeleton of the transformer, and the primary and secondary windings are wound on the iron core column, which ensures the relative position of the windings and improves the mechanical stability of the entire transformer. Without a reasonable iron core design, even high-quality windings and insulation oil cannot ensure the stable and efficient operation of the transformer.
In oil-immersed power transformers, the iron core is immersed in insulating oil together with the windings. The insulating oil not only plays an insulating role but also takes away the heat generated by the iron core during operation through natural convection or forced circulation, ensuring that the iron core works within the safe temperature range (generally not exceeding 105℃). This integration of the iron core and the oil-immersed system makes the transformer suitable for high-voltage, large-capacity power transmission and distribution scenarios, which is the key reason why oil-immersed transformers are widely used in the power system.
Structural Design of the Iron Core for Oil-Immersed Power Transformers
The structural design of the iron core for oil-immersed power transformers is closely related to the capacity, voltage level, and application scenario of the transformer. The core design principles are: minimizing magnetic leakage, reducing iron loss, ensuring mechanical strength, and facilitating processing and installation. The common structural types of iron cores in engineering applications are mainly core-type and shell-type, among which core-type iron cores are the most widely used in oil-immersed power transformers.
1. Core-Type Iron Core Structure
The core-type iron core is composed of iron core columns, yokes, and clamping parts. The iron core columns are the main parts for winding windings, and the yokes connect the upper and lower ends of the iron core columns to form a closed magnetic circuit. According to the number of iron core columns, it can be divided into single-phase two-column type, three-phase three-column type, and three-phase five-column type. Single-phase two-column iron cores are mainly used in single-phase oil-immersed transformers, which are widely used in residential distribution and small industrial power supply. Three-phase three-column iron cores are suitable for three-phase oil-immersed transformers with small and medium capacity (below 100MVA), and their structure is simple, economical, and easy to process. Three-phase five-column iron cores are mainly used in large-capacity, high-voltage oil-immersed transformers (above 100MVA, voltage level above 110kV). The two additional side columns can effectively reduce magnetic leakage, improve magnetic circuit symmetry, and reduce iron loss, which is especially suitable for ultra-high voltage and extra-high voltage power transmission scenarios.
2. Shell-Type Iron Core Structure
The shell-type iron core is characterized by the iron core wrapping the winding, forming a "shell" shape. Its magnetic circuit is relatively short, the magnetic leakage is small, and the mechanical strength is high, but the structure is complex, the processing difficulty is large, and the material consumption is more. In oil-immersed power transformers, shell-type iron cores are mainly used in special scenarios such as small-capacity precision transformers and test transformers, and their application scope is narrower than that of core-type iron cores.
3. Key Structural Components
In addition to the iron core columns and yokes, the iron core structure also includes clamping parts, insulation parts, and grounding parts. The clamping parts (upper and lower clamping plates, pull screws) are used to fix the iron core lamination, ensuring the mechanical stability of the iron core and preventing vibration and noise during operation. The insulation parts are arranged between the iron core lamination and the clamping parts, between the iron core and the winding, to prevent short circuits between the iron core and the clamping parts or the winding, ensuring the insulation performance of the magnetic circuit. The iron core must be reliably grounded (usually single-point grounding) to prevent the generation of induced voltage on the iron core surface, which may lead to insulation breakdown and iron core burning.
Material Selection of the Iron Core for Oil-Immersed Power Transformers
The material of the iron core is the key factor affecting the iron loss and magnetic permeability of the transformer. The core material must have high magnetic permeability, low coercivity, low iron loss, and good mechanical properties to meet the requirements of efficient and stable operation of oil-immersed power transformers. At present, the main materials used in the engineering application of oil-immersed power transformer iron cores are cold-rolled silicon steel sheets, which can be divided into oriented silicon steel sheets and non-oriented silicon steel sheets according to the grain orientation.
1. Oriented Silicon Steel Sheets
Oriented silicon steel sheets (also known as grain-oriented silicon steel sheets) have excellent magnetic properties in the rolling direction, with high magnetic permeability and low iron loss, which is the preferred material for large-capacity, high-voltage oil-immersed power transformer iron cores. The iron loss of oriented silicon steel sheets is 30%-50% lower than that of non-oriented silicon steel sheets, which can significantly reduce the no-load loss of the transformer and improve the operating efficiency. According to the iron loss level, it can be divided into ordinary oriented silicon steel sheets and high-grade oriented silicon steel sheets (HiB silicon steel sheets). HiB silicon steel sheets have lower iron loss and higher magnetic permeability, which are widely used in ultra-high voltage (UHV) oil-immersed transformers (such as 500kV, 750kV, 1000kV) and large-capacity energy-saving transformers. The thickness of oriented silicon steel sheets used in engineering is usually 0.23mm, 0.27mm, or 0.30mm, and the thinner the thickness, the lower the eddy current loss, but the higher the processing cost.
2. Non-Oriented Silicon Steel Sheets
Non-oriented silicon steel sheets have uniform magnetic properties in all directions, simple processing technology, and low cost, which are mainly used in small and medium-capacity oil-immersed power transformers (below 35kV, below 50MVA), such as distribution transformers in residential areas and small industrial plants. The thickness of non-oriented silicon steel sheets is usually 0.35mm or 0.50mm, and their iron loss is higher than that of oriented silicon steel sheets, but they can meet the performance requirements of small and medium-capacity transformers. In addition, non-oriented silicon steel sheets are also used in the yokes of some large-capacity transformers to reduce costs on the premise of ensuring performance.
3. Material Processing Requirements
The processing quality of silicon steel sheets directly affects the performance of the iron core. In engineering applications, the silicon steel sheets need to be cut, stacked, and insulated. The cutting must be accurate to ensure the size and shape of the lamination, avoiding gaps between laminations which will increase magnetic resistance. The surface of the silicon steel sheet is coated with an insulating film (usually phosphate film or oxide film) to prevent eddy current between laminations. When stacking, the laminations are usually stacked in a staggered manner (lap joint or butt joint) to reduce magnetic leakage. The stacking coefficient (the ratio of the actual cross-sectional area of the iron core to the theoretical cross-sectional area) is generally 0.93-0.97, which is an important indicator to measure the stacking quality of the iron core.
Key Engineering Design Points of the Iron Core
The engineering design of the iron core for oil-immersed power transformers needs to comprehensively consider factors such as magnetic flux density, iron loss, mechanical strength, insulation performance, and heat dissipation, to achieve the balance between performance, cost, and reliability. The key design points are as follows:
1. Magnetic Flux Density Design
Magnetic flux density is the core design parameter of the iron core, which directly affects the iron loss and the volume of the iron core. The selection of magnetic flux density should be based on the material of the silicon steel sheet, the capacity of the transformer, and the operating conditions. For oriented silicon steel sheets, the designed magnetic flux density is usually 1.5-1.7T, which can give full play to the magnetic properties of the material and reduce the volume of the iron core. For non-oriented silicon steel sheets, the magnetic flux density is usually 1.3-1.5T, to avoid excessive iron loss caused by high magnetic flux density. If the magnetic flux density is too high, the iron loss will increase sharply, leading to overheating of the iron core; if it is too low, the volume of the iron core will increase, increasing the material cost and the overall volume of the transformer.
2. Iron Loss Control
Iron loss is the main component of the no-load loss of the transformer, accounting for 70%-80% of the no-load loss. Reducing iron loss is the key to improving the efficiency of the transformer. In engineering design, iron loss control can be achieved through three aspects: first, selecting high-quality silicon steel sheets with low iron loss; second, optimizing the magnetic flux density design to avoid excessive magnetic flux density; third, improving the processing and stacking quality of the iron core, reducing the gap between laminations, and ensuring the insulation performance of the lamination surface. In addition, the use of stepped joints between the iron core columns and yokes can also reduce magnetic leakage loss, further optimizing the iron loss level.
3. Mechanical Strength Design
The iron core of oil-immersed power transformers needs to bear the mechanical stress caused by the electromagnetic force during operation, as well as the own weight of the iron core and windings. Therefore, the mechanical strength design of the iron core is crucial to ensure the stable operation of the transformer. The clamping parts of the iron core should be made of high-strength steel plates, and the pull screws should be selected according to the load-bearing requirements to ensure that the iron core does not deform or loosen during operation. For large-capacity transformers, the iron core yoke can be reinforced with reinforcing ribs to improve the overall mechanical strength. In addition, the vibration and noise of the iron core should be considered in the design. The vibration of the iron core is caused by the magnetostriction effect of the silicon steel sheet, which can be reduced by optimizing the stacking method and increasing the clamping force.
4. Insulation and Heat Dissipation Design
The insulation design of the iron core mainly includes the insulation between the iron core lamination and the clamping parts, between the iron core and the winding, and the insulation of the iron core grounding point. The insulation material should have good insulation performance and heat resistance, adapting to the operating temperature of the oil-immersed transformer. Common insulation materials include insulation paper, insulating cardboard, and epoxy resin. The heat dissipation design of the iron core is closely combined with the oil-immersed system. The iron core is immersed in insulating oil, and the heat generated by the iron core is transferred to the insulating oil, which is then dissipated through the radiator or cooling pipe. In the design, the surface area of the iron core should be reasonably designed to ensure that the heat can be dissipated in time, and the temperature rise of the iron core does not exceed the standard limit.
Engineering Application Scenarios of Iron Core in Oil-Immersed Power Transformers
Oil-immersed power transformers with different iron core designs are widely used in various fields of the power industry, industrial production, and new energy power generation. The application scenarios of the iron core are closely related to the capacity, voltage level, and performance requirements of the transformer, mainly including the following aspects:
1. Power Transmission and Distribution System
This is the most important application scenario of oil-immersed power transformers and their iron cores. In the power transmission link, ultra-high voltage (UHV) and extra-high voltage oil-immersed transformers (500kV, 750kV, 1000kV) adopt three-phase five-column core-type iron cores made of high-grade oriented silicon steel sheets, which can reduce iron loss and magnetic leakage, ensuring efficient and stable power transmission over long distances. For example, in the "West-East Power Transmission" project, a large number of UHV oil-immersed transformers are used, and their iron cores are designed with low loss and high magnetic permeability to adapt to the high-voltage, large-capacity power transmission requirements. In the power distribution link, medium and low voltage oil-immersed transformers (10kV, 35kV) adopt three-phase three-column or single-phase two-column iron cores, which are cost-effective and meet the power supply needs of residential areas, commercial districts, and industrial parks.
2. Industrial Field
In large industrial enterprises such as steel, chemical, metallurgy, and mining, oil-immersed power transformers are widely used to provide stable power for production equipment. These transformers usually have large capacity and harsh operating environments (high temperature, high humidity, dust), so the iron core is designed with high mechanical strength and strong environmental adaptability. For example, in steel plants, the iron core of the oil-immersed transformer is made of high-grade oriented silicon steel sheets, with a reasonable magnetic flux density design, which can withstand the impact of load fluctuations and ensure the stable operation of the production line. In mining areas, the iron core is treated with anti-corrosion and dust-proof measures to adapt to the harsh underground environment.
3. New Energy Power Generation Projects
With the rapid development of new energy power generation (photovoltaic, wind power, hydropower), oil-immersed power transformers are widely used in new energy power stations as booster transformers. The iron core of these transformers needs to adapt to the characteristics of unstable new energy power generation and frequent load fluctuations. For example, in large-scale photovoltaic power stations and wind farms, the booster transformers adopt core-type iron cores with low loss and high reliability, which can reduce the no-load loss during low-power operation and improve the overall efficiency of the power station. The iron core is also designed with strong anti-interference performance to adapt to the electromagnetic environment of the new energy power station.
4. Special Environmental Applications
In special environments such as high altitude, cold areas, and coastal areas, the iron core of the oil-immersed transformer needs to be specially designed. For high-altitude areas (above 2000m), the air pressure is low, and the insulation performance of the insulating oil decreases. The iron core is designed with a larger insulation distance and higher insulation level to ensure safe operation. For cold areas, the iron core is made of silicon steel sheets with good low-temperature performance, and the clamping parts are treated with anti-freezing measures to prevent the iron core from loosening due to low temperature. For coastal areas, the iron core and clamping parts are treated with anti-corrosion and anti-salt spray measures to avoid corrosion caused by salt spray and moisture, extending the service life of the iron core.
Installation, Maintenance and Fault Diagnosis of the Iron Core
The installation and maintenance quality of the iron core directly affects the service life and operating stability of the oil-immersed power transformer. In engineering applications, strict operating specifications must be followed to ensure the safe and reliable operation of the iron core.
1. Installation Requirements
Before installing the iron core, the installation site should be cleaned to avoid dust, debris, and moisture entering the iron core. The iron core lamination should be inspected for damage, deformation, and insulation film damage, and unqualified laminations should be replaced. During installation, the stacking of the iron core should be accurate, the gaps between laminations should be uniform, and the clamping force should be appropriate to avoid deformation of the iron core. The iron core grounding should be reliable, and the grounding point should be checked to ensure that there is no poor contact. After installation, the iron core should be tested for insulation resistance to ensure that the insulation performance meets the standard. In addition, the lifting of the iron core must be carried out by professional technicians, using lifting equipment that can bear the weight of the iron core, and the lifting angle should not exceed 60 degrees to avoid damage to the iron core.
2. Daily Maintenance
The daily maintenance of the iron core mainly includes regular inspection and cleaning. Regularly check the operating temperature of the iron core (through the temperature sensor or oil temperature detection) to ensure that it is within the safe range. Check the clamping parts of the iron core for looseness, and tighten the pull screws in time if there is looseness. Clean the surface of the iron core and the insulation parts regularly to avoid dust accumulation, which may affect heat dissipation and insulation performance. For transformers in harsh environments, the iron core should be inspected more frequently, and anti-corrosion and dust-proof measures should be strengthened. In addition, the iron core grounding point should be inspected regularly to ensure reliable grounding, and the insulation resistance of the iron core should be tested annually to detect potential insulation faults in time.
3. Common Faults and Diagnosis Methods
The common faults of the iron core in engineering applications mainly include iron core overheating, iron core insulation damage, iron core grounding fault, and iron core vibration and noise. The diagnosis methods are as follows:
Iron core overheating: The main manifestations are the increase of transformer oil temperature, the abnormal rise of no-load loss, and the discoloration of the iron core surface. The cause may be excessive magnetic flux density, poor stacking quality of the iron core, damage to the insulation film of the silicon steel sheet, or poor heat dissipation. The diagnosis can be carried out through oil temperature detection, no-load loss test, and infrared thermal imaging, and the fault can be eliminated by adjusting the magnetic flux density, re-stacking the iron core, or replacing the silicon steel sheet.
Iron core insulation damage: The main manifestations are the increase of leakage current, the decrease of insulation resistance, and even short circuit between the iron core and the winding. The cause may be damage to the insulation film of the silicon steel sheet, mixing of conductive debris between laminations, or poor insulation between the iron core and the clamping parts. The diagnosis can be carried out through insulation resistance test and leakage current test, and the fault can be eliminated by cleaning the iron core, replacing the insulation parts, or re-coating the insulation film.
Iron core grounding fault: The main manifestations are the generation of induced current in the iron core, overheating of the grounding point, and even burning of the iron core. The cause may be multiple grounding points of the iron core, poor contact of the grounding point, or damage to the grounding wire. The diagnosis can be carried out through grounding current detection and insulation resistance test, and the fault can be eliminated by checking the grounding point, ensuring single-point grounding, and replacing the grounding wire.
Iron core vibration and noise: The main manifestations are excessive vibration of the transformer and abnormal noise during operation. The cause may be insufficient clamping force of the iron core, uneven stacking of laminations, or magnetostriction of the silicon steel sheet. The diagnosis can be carried out through vibration measurement and noise detection, and the fault can be eliminated by increasing the clamping force, re-stacking the iron core, or adding vibration-damping pads.
Development Trend of Iron Core in Oil-Immersed Power Transformers
With the continuous development of power technology and the increasing demand for energy conservation and emission reduction, the engineering application of the iron core in oil-immersed power transformers is developing towards high efficiency, low loss, intelligence, and large capacity. The main development trends are as follows: first, the wide application of high-grade oriented silicon steel sheets (HiB silicon steel sheets) and new magnetic materials (such as amorphous alloy), which can further reduce iron loss and improve the efficiency of the transformer. Amorphous alloy iron cores have lower iron loss than oriented silicon steel sheets, which are widely used in energy-saving distribution transformers. Second, the optimization of the iron core structure, such as the adoption of seamless iron cores and segmented iron cores, which can reduce magnetic leakage and iron loss, and improve the mechanical strength of the iron core. Third, the intelligent monitoring of the iron core, through the installation of temperature sensors, vibration sensors, and current sensors, the real-time monitoring of the operating status of the iron core is realized, and the fault early warning and diagnosis are carried out, improving the reliability of the transformer. Fourth, the integration of the iron core and the energy storage system, in new energy power stations, the iron core of the oil-immersed transformer is designed with energy storage functions, which can realize peak shaving and valley filling, improving the stability of the power grid.
Conclusion
The iron core is the core component of the oil-immersed power transformer, and its engineering application level directly determines the performance, reliability, and economic benefits of the transformer. From material selection, structural design, and engineering design to installation, maintenance, and fault diagnosis, every link is crucial to ensuring the safe and efficient operation of the iron core. In engineering practice, it is necessary to select appropriate iron core materials and structural types according to the application scenario, optimize the design parameters, strictly follow the installation and maintenance specifications, and timely diagnose and eliminate faults, to give full play to the role of the iron core in the transformer. With the continuous advancement of technology, the iron core of oil-immersed power transformers will develop towards higher efficiency, lower loss, and smarter, providing strong support for the high-quality development of the power industry and the energy transition. This article comprehensively summarizes the engineering application of the iron core in oil-immersed power transformers, which can provide practical reference for engineering and technical personnel, helping to improve the design, installation, and maintenance level of the iron core and promote the healthy development of the oil-immersed power transformer industry.
