1. Overview As is well known, electricity is a clean and high-quality secondary energy source. With the development of China's socialist modernization, the proportion of electricity in final energy consumption will continue to increase. In terms of energy conversion efficiency, China's current power generation efficiency is only 33%. Therefore, saving 1 kWh of electricity is equivalent to saving approximately three times the amount of primary energy. The "Eleventh Five-Year Plan for National Economic and Social Development," adopted at the Fourth Session of the Tenth National People's Congress, clearly stated the goal of reducing energy consumption per unit of GDP by about 20% by 2010 compared to 2005. Calculated at comparable prices in 2000, from 1995 to 1999, China's power consumption per 10,000 yuan of GDP decreased annually from 1644 kWh/10,000 yuan to 1460 kWh/10,000 yuan, a decrease of 12.6%. However, during the "Tenth Five-Year Plan" period, it increased instead of decreasing, reaching 1746 kWh/10,000 yuan in 2004, 16% higher than in 1999 and exceeding the level of 1995. In 2005, my country's total electricity consumption was 2,176.1 billion kWh, of which the steel, non-ferrous metals, chemical, building materials, power, commercial, and residential sectors accounted for 58.3% of the total, making them key areas for energy conservation. On January 9, 2006, the state issued and officially implemented the GB20052-2006 standard, "Minimum Allowable Values ​​of Energy Efficiency and Energy Saving Evaluation Values ​​for Three-Phase Distribution Transformers," on July 1, 2006. This standard specifies the minimum allowable values ​​of energy efficiency and target energy efficiency values ​​for distribution transformers, stipulating that the target energy efficiency values ​​would be implemented four years after the standard's implementation date. On April 24, 2008, the state issued and implemented the GB/T10228-2008 standard, "Technical Parameters and Requirements for Dry-Type Power Transformers," on December 1, 2008. This standard reduced the no-load and load losses of dry-type transformers and merged the technical parameters of enclosed and unenclosed dry-type transformers into unified technical parameters. Jiangxi Dazhu Power Technology Co., Ltd. is a high-tech enterprise specializing in the production of distribution transformers. Since 1999, the company has successively designed and developed the S11-M.RL series of high-efficiency and energy-saving three-dimensional wound core oil-immersed transformers and the SGB11-RL series of three-dimensional wound core three-dimensional non-encapsulated dry-type transformers with independent intellectual property rights. Compared with similar products, the no-load loss is reduced by 30% and the load loss is reduced by 20%. The company has completely independent intellectual property rights and owns nine national patents related to various three-dimensional wound core transformers (ZL99236753.0, ZL99240515.7, ZL99240750.8, ZL99240653.6, ZL01215038.X, ZL01315223.8, ZL200620163597.2, ZL200620163598.7, ZL200820137669.5). The no-load and load losses of the three-dimensional wound core transformer are superior to the energy-saving evaluation values ​​in the GB2005-2006 standard "Energy Efficiency Limits and Energy Saving Evaluation Values ​​for Three-Phase Distribution Transformers," and it has passed the energy-saving certification of the National CQC Certification Center. 2. Technological Developments of Similar Products at Home and Abroad The reduction of transformer losses at home and abroad is mainly achieved through the development of magnetic materials (silicon steel sheets) and conductive materials (oxygen-free copper rods or copper foil). In recent years, in addition to breakthroughs in transformer capacity structure and manufacturing processes, the main improvement and development has been in silicon steel sheets. Currently, the thickness of silicon steel sheets used for core magnetic materials is generally 0.23-0.30 mm; the future trend is to use thinner silicon steel sheets, with 0.05-0.18 mm thick silicon steel sheets already being used. Furthermore, the development of amorphous alloy materials has also promoted the development of transformers. Amorphous alloy cores can reduce no-load losses in distribution transformers by 70% compared to silicon steel cores. However, amorphous alloy cores have fewer laminations and a rectangular cross-section, requiring the coils to be wound into rectangular coils. Rectangular coil winding is complex, has poor mechanical strength, and results in high noise levels. The cores typically use a three-phase four-frame structure, leading to high material consumption and cost, and are currently mostly used in oil-immersed transformers. A few domestic transformer manufacturers have developed amorphous alloy dry-type transformers, but these products are noisy, and noise and no-load losses increase significantly after stress or long-distance transportation. Statistics from the National Transformer Quality Inspection Center and the Jiangsu Provincial Power Bureau fully illustrate this point. Currently, domestic and international dry-type transformers still primarily use traditional laminated cores to produce non-encapsulated or resin-cast dry-type transformers. To produce lower-loss, lower-noise dry-type transformers, it is necessary to use thinner and better silicon steel sheets or increase material usage. 3. Technical and Economic Analysis of Various Transformer Core Structures The traditional core (planar laminated) structure and magnetic circuit are shown in Figure 1, and the magnetic flux vector diagram is shown in Figure 2. [align=center] Figure 1 Traditional iron core (planar laminated type) structure and magnetic circuit diagram Figure 2 Planar laminated type iron core magnetic flux vector diagram[/align] The structure and magnetic flux distribution of the three-phase three-frame wound iron core are shown in Figure 3, and the magnetic flux vector diagram is shown in Figure 4. [align=center] Figure 3 Three-phase three-frame wound iron core structure and magnetic flux distribution diagram Figure 4 Three-phase three-frame wound iron core magnetic flux vector diagram[/align] The structure and magnetic flux distribution of the three-phase four-frame five-column wound iron core are shown in Figure 5, and the magnetic flux vector diagram is shown in Figure 6. It is very similar to the three-dimensional D-shaped wound iron core. In terms of magnetic circuit structure, the former has one more single-frame iron core than the latter, which is less economical. [align=center] Figure 5 Three-phase four-frame wound iron core structure and magnetic flux distribution diagram Figure 6 Three-phase four-frame wound iron core magnetic flux vector diagram[/align] The three-dimensional wound iron core structure of the three-dimensional wound iron core dry-type transformer consists of three single-frame semi-circular cross-section wound iron cores with identical geometric dimensions, which are combined at 60° to each other. The core column cross-section consists of two semicircles forming a complete circle, and the yoke cross-section is semicircular, half the size of the core column. The three-dimensional wound core structure and magnetic flux distribution are shown in Figure 7, and the magnetic flux phasor diagram is shown in Figure 8. [align=center] Figure 7 Three-dimensional wound core structure and magnetic circuit diagram Figure 8 Three-dimensional wound core magnetic flux vector diagram[/align] From the above magnetic circuit structures and magnetic flux distribution diagrams of various cores, it can be seen that, except for the three-dimensional wound core, the other three planar core arrangements all exhibit three-phase magnetic circuit asymmetry and unequal magnetic reluctance, resulting in an unbalanced three-phase no-load current. Compared with planar cores, the three-dimensional wound core can reduce the yoke by 25%, and compared with planar wound cores, it is more economical and reasonable in terms of magnetic circuit symmetry, no-load loss, and core usage. Compared with laminated cores, in addition to the above advantages, it can also reduce the core column diameter and coil size, thus achieving better technical and economic efficiency. From Figures 1-8, we can see that: (1) Traditional core (planar laminated) structures and magnetic circuits suffer from three-phase magnetic circuit asymmetry and unequal magnetic reluctance, leading to an imbalance in the three-phase no-load current. In contrast, the three-dimensional wound core structure exhibits completely symmetrical three-phase magnetic circuits and equal magnetic reluctance, resulting in a perfectly balanced three-phase no-load current. (2) Traditional core structures have six seams per layer, totaling tens of millions of seams, increasing magnetic reluctance. In the three-dimensional wound core structure, each frame is continuously wound from a single silicon steel sheet, eliminating seams in the magnetic circuit. Therefore, compared to traditional cores, the three-dimensional wound core significantly reduces no-load loss, no-load current, and core noise. (3) Under the same core cross-section, window height, and core column spacing, the traditional core structure requires 25% more yoke material than the three-dimensional wound core structure (as shown in Figure 1). The fill factor of the three-dimensional wound core is 0.96–0.98, while that of the traditional core is only 0.87–0.90. This allows for a reduction in core diameter while maintaining the same net cross-sectional area. Considering the above factors and the angular weight of the traditional core, the weight of the three-dimensional wound core can be reduced by 15–20%. As is well known, the no-load loss of the core is directly proportional to its weight; that is, reducing the core weight alone can reduce the no-load loss by 15–20%. (4) Vacuum annealing of the core not only eliminates mechanical stress and reduces no-load loss and noise, but a suitable vacuum annealing process can further refine the magnetic domains of the silicon steel sheets, improve the secondary recrystallization ability of the silicon steel sheets, further reduce silicon steel sheet loss, and of course, also reduce the no-load loss of the core. Table 1. Comparison of Performance Data of Silicon Steel Sheets at the Factory and After Annealing. The data shows that, through appropriate core vacuum annealing, silicon steel sheets can not only restore their original performance data but also improve their secondary recrystallization ability by refining the magnetic domains, resulting in significantly superior performance compared to their factory state. Due to the seamless winding and three-dimensional structure of the core, coupled with secondary annealing, the no-load loss can be reduced by 20-30% compared to traditional products. Because the core cross-section is a perfect circle, the core diameter and coil diameter can be reduced by 5-10% while maintaining the same net cross-sectional area, resulting in a corresponding reduction of 5-10% in load loss. Therefore, in addition to the excellent no-load performance mentioned above, the three-dimensional wound core transformer also has the following outstanding advantages: (1) The three-dimensional wound core structure of the transformer results in completely symmetrical three-phase magnetic circuits and completely equal magnetic reluctances in the three-phase magnetic circuits, thus achieving complete balance of the three-phase no-load current, which is unattainable by any other core structure transformer. (2) Product noise is reduced by 3-5 dB compared to similar products. (3) Small size, small footprint, saving installation space. (4) Low spatial leakage flux and no electromagnetic radiation. (5) Good voltage waveform and no pollution to the power grid. The table below shows the statistical results of voltage harmonics in the SGB10-RL-2000/10 three-dimensional wound-core dry-type transformer, including the phase B and phase C voltage harmonic content rates. From these results, it can be seen that the maximum values ​​of each harmonic are far below the limits, and the total voltage distortion rate is 2.97%, a significant reduction compared to the previous rate of 6.77% (using SCB9 products). This indicates that high-order harmonics are well controlled, and the power supply quality is greatly improved. Taking SGB11-RL-2000/10 and SB13-M.RL-630/10 as examples, the socio-economic benefits of using three-dimensional wound-core transformers are calculated. The table shows that three-dimensional wound-core transformers have significant social benefits. 5. Product Operation Jiangxi Dazhu Power Technology Co., Ltd.'s three-dimensional wound core transformers have achieved outstanding results and received widespread praise from key enterprises and projects, including the five major power generation groups in China, the State Grid, key national projects, government projects, aerospace groups, and the Second Artillery Corps, due to their high efficiency, energy saving, and environmental protection. Nearly 4,000 units of the product are currently in operation. The company's sales network covers more than 30 provinces, municipalities, and autonomous regions across China and is exported to Southeast Asia.