1. Introduction Currently, the vast majority of distribution transformers used in China are traditional stacked core transformers, mostly of the S9 type, and even some S7 and SJ7 types. In recent years, the S11 type transformer has been widely used in the construction and renovation of urban and rural power grids, producing good energy-saving effects. However, due to the rapid increase in material prices, its economic advantage is not obvious. Therefore, providing more energy-saving, material-saving, and environmentally friendly products is the top priority for the transformer manufacturing industry. The three-dimensional triangular wound core transformer breaks through the traditional planar structure, overcomes structural defects, and brings a series of technical and economic advantages: more reasonable structure, superior performance, and significantly reduced manufacturing costs, meeting both the energy-saving requirements of the power industry and the cost and quality objectives of manufacturing enterprises. 2. Innovation in Core Structure In traditional stacked core transformers with air gaps, the coupling magnetic circuit between phases A and C is obviously 1/2 longer than that between phases A and B, and B and C, resulting in magnetic circuit imbalance and a larger magnetic reluctance in phases A and C (see Figure 1). When three-phase voltage is applied to a transformer, the core generates three-phase balanced magnetic fluxes φA, φB, and φC. When these balanced magnetic fluxes pass through an unbalanced magnetic circuit, the magnetic voltage drop in phases A and C is large, affecting the balance of the three-phase voltage. This imbalance in the magnetic circuit is an insurmountable structural defect for planar transformers. [ALIGN=CENTER] Figure 1 Planar Core Structure[/ALIGN] In principle, a completely symmetrical magnetic circuit structure is ideal for a three-phase transformer. This was recognized at the beginning of the transformer's invention, but industrial production was not possible due to technical reasons. With the continuous maturation of the core transformer technology, when we use three-dimensional thinking and consider the geometric characteristics of the three equal-length magnetic circuits, we can construct a symmetrical three-dimensional triangular core structure: using three identical single frames, each with a semi-circular cross-section, the diameter of the cross-section forming an exact 30° angle with the line connecting the center of the core column. Combining the three frames together, the cross-section of the core column forms a complete circle, while the cross-section of the yoke is semi-circular, and the three core columns are arranged in an equilateral three-dimensional triangle (see Figure 2). [ALIGN=CENTER] Figure 2 Three-dimensional triangular core arrangement and single frame[/ALIGN] 3. Comparison with planar stacked core technology Changes in core structure and manufacturing process have made the three-dimensional triangular core transformer technically similar to the traditional planar stacked core transformer. 1) The core frame is made of a few silicon steel strips continuously wound, without seams or air gaps; each frame is a closed magnetic circuit conductor, which can fully utilize the orientation properties of the cold-rolled silicon steel sheet, thus significantly improving the no-load performance. In particular, the active component in the no-load current is significantly reduced, which can improve the power factor and the power supply quality of the power grid. 2) The core is tightly wound, requiring no clamping, with a high lamination factor, resulting in significant noise reduction and meeting environmental protection requirements. Taking a 200 kVA transformer as an example, according to authoritative testing, the prototype of the three-dimensional triangular wound core transformer has a noise level of only 33.6 dB, while the prototype of the planar laminated core transformer has a noise level of 51 dB, while the national standard requires 54 dB. 3) The magnetic circuit between phases A and C is shortened, achieving the shortest arrangement with the same length as phases A and B, and phases B and C, realizing complete symmetry of the three-phase magnetic circuit and ensuring three-phase voltage balance. 4) The magnetic flux of the three frames forms its own loop and does not affect each other. The third harmonic magnetic flux can flow freely in its respective frame, and the quality of the combined magnetic flux in the core column is superior to that of the planar core. 5) The cross-section of the coiled iron core can take many forms, including stepped, circular, polygonal, or a "composite" shape combining a polygon and a section of arc (see Figure 3), while the cross-section of the stacked iron core can only be stepped. Therefore, the space filling coefficient within the circular section of the coiled iron core is 4%-6% higher than that of the stacked iron core. [ALIGN=CENTER] Figure 3 Core Cross-Section Forms[/ALIGN] 6) Waste is generated when the coiled iron core is cut into a "V" shape. The utilization rate of silicon steel sheets is up to 95%. When the cross-section of the coiled iron core is polygonal, the cutting utilization rate of silicon steel sheets is as high as 100%. 7) The production is highly mechanized, eliminating the need for manual operations such as cutting of stacked iron cores, manual stacking, and disassembling and inserting yokes, reducing 5-6 processes and making quality control easier. 4. Energy Saving Analysis (1) Energy Saving Analysis of No-Load Performance Using copper wire and silicon steel strip of the same material, with the same core diameter and similar cost, one 315 kVA three-dimensional triangular wound core transformer and one stacked core transformer were designed and manufactured, and their performance and cost were compared (see Table 1). It can be seen that the no-load loss of the triangular wound core transformer is reduced by about 30%. The main reasons are as follows. [ALIGN=CENTER]Table 1 Comparison of main materials and performance of 315 kVA planar stacked core and triangular wound core transformer[/ALIGN] 1) The core weight is reduced, and the no-load loss of the core will be reduced. This can be seen from the no-load loss calculation formula. In the formula, is the unit weight loss of the core, which is determined by the magnetic flux density; is the process coefficient; G is the total weight of the core. 2) Two factors reduce the no-load loss process coefficient: The core frame is made of continuously wound silicon steel strips, without seams or air gaps, which better utilizes the orientation of cold-rolled silicon steel sheets compared to stacked cores; the core uses a full annealing process, which completely eliminates internal stress generated during machining, restoring the original magnetic conductivity of the silicon steel sheets and eliminating high magnetic resistance regions. Statistical calculations show that the process coefficient for a three-dimensional triangular wound core can reach 1.05–1.1, while the process coefficient for a stacked core is 1.15–1.3. The no-load loss of a transformer is a fixed value independent of the load size; it occurs whenever electricity is applied and accumulates over many years. A reduction of approximately 30% in no-load loss translates to cost reduction for power supply departments and energy savings for the country. For example, based on China's annual production of distribution transformers of approximately 240 million kVA, and assuming each transformer is 200 kVA, the annual production is 1.2 million units. All transformers using the SJ1 type delta-wound core transformer reduce no-load losses by 180,000 kW compared to the S9 planar stacked core transformer, saving 1.548 billion kWh of electricity per year. Correspondingly, this can save on line losses and investment in power generation equipment. At the same time, the no-load current is reduced by 75%, which can save on investment in reactive power compensation equipment. (2) Analysis of material savings in transformers Using the same material, and with the same effective core area, window, and other parameters, a comparison is made between a three-dimensional delta-wound core transformer and a planar stacked core transformer with the same performance level. 1) Savings in core material The magnetic circuit between AC in the delta-wound core is shortened by 1/2 in the yoke section compared to the planar core, and the yoke area is 1/2 of the cross-section of each phase column, so the weight of the yoke section is reduced by 1/4. The weight ratio of the yoke to the core column is generally 2:3, so the total weight of the core should theoretically be reduced by about 10%; the delta-wound core has no extra corner weight and is about 5% lighter than the planar stacked core. In addition, the utilization rate of silicon steel strip cutting is 5% higher than that of the stacked core. Therefore, under the same parameters, the three-dimensional triangular wound core saves about 20% of silicon steel strip compared to the stacked core. 2) Reduction of overall cost The transformer performance level is the same, and the standard value of no-load loss is the same. The weight G and process coefficient of the triangular wound core transformer are reduced. According to formula (1), the magnetic flux density of the core can be appropriately increased, so that the unit weight loss of the core increases while keeping the no-load loss unchanged. In the electromagnetic calculation formula, the phase voltage u and frequency f are constant, w is the number of coil turns, is the magnetic flux density of the core, and s is the effective area of ​​the magnetic flux core column. It can be seen that the number of coil turns w is inversely proportional to the magnetic flux density. Therefore, increasing the magnetic flux density can reduce the number of wire turns w and save copper wire usage. In addition, the space filling coefficient inside the cross section of the wound core column is 4% to 6% higher than that of the stacked core. When the effective area S is the same, the diameter of the wound core will be 2% to 3% smaller than that of the stacked core. This can shorten the copper circumference, reduce the usage, and further reduce the cost of the entire transformer. Therefore, when designing transformers, different cross-sectional forms are selected to minimize the overall cost of the transformer, taking into account factors such as the fill factor within the circumscribed circle, the material utilization rate of the cut material, and the unit price ratio of copper and silicon steel strip, depending on the product capacity. Table 2 compares the materials of S11 type planar laminated core transformers and triangular wound core transformers. [ALIGN=CENTER]Table 2 Comparison of Main Materials of S11 Type Planar Laminated Core Transformers and Triangular Wound Core Transformers[/ALIGN] It can be seen that the cost of the S11 type triangular wound core transformer is similar to that of the S9 type planar laminated core transformer, while the core weight is reduced by an average of 14.13% and the copper material by an average of 14.49% compared to the S11 type planar laminated core transformer. This demonstrates significant material savings, which can conserve substantial national resources and energy. Given the current high prices of raw materials, its advantages should be given greater social attention. (3) Theft Prevention of Winded Core Transformers Preventing transformer theft is saving social resources. Due to technical reasons, laminated core transformers are easily stolen. Theft methods involve separating the transformer tank from the transformer body, disassembling the fasteners, and knocking off the silicon steel sheets. At this point, the three windings of the transformer immediately separate from the silicon steel sheets, allowing valuable materials such as copper wire to be stolen and transported away. Since the core of a wound core transformer is a single unit, thieves cannot easily break it apart, and the coils are not easily removed. Due to its weight, it is quite difficult for thieves to move the entire core and coils. Even if the entire unit is stolen, it is not easy to separate the silicon steel sheets and copper wires. Therefore, wound core transformers have better theft prevention. 5. Conclusion Through nearly two years of grid operation, the delta-wound core transformer has performed well, fully meeting design requirements. It has advantages such as energy saving, material saving, low noise, and environmental friendliness, meeting the requirements of technological progress and innovation in China's power industry, aligning with the technical direction of urban and rural power distribution development, and has broad application prospects. This article is from "Energy Saving Innovation 2006 - Proceedings of the First National Electrical Energy Saving Competition".