[align=left]As low loss is one of the important performance indicators of power transmission equipment, amorphous alloy core transformers have attracted much attention from manufacturers and power users due to their significant low iron loss (no-load loss) performance. In recent years, through continuous improvement of the quality and performance of amorphous alloys and the increase in the production scale of amorphous alloy cores, the price of amorphous alloy cores has been greatly reduced. Moreover, suppliers can provide wound cores of different specifications according to requirements, which has greatly promoted the manufacturing and use of amorphous alloy cores in distribution transformers. So what is an amorphous alloy core? What are its physical properties? How well does it perform in terms of low loss? These are all questions that deserve the attention of our power equipment operation management departments and testing technicians. [b]1 What is "Amorphous Alloy"[/b] "Amorphous alloy" was first discovered by P. Duwez et al. of the California Institute of Technology in 1960. At that time, it was used for magnetic sensors, such as in supermarkets and libraries. Subsequently, Luborsky of GE (General Electric Company) further discovered its low iron loss performance (0.44 W/kg), which attracted attention and led to research on its application in transformer core manufacturing. The "amorphous alloy" used in transformer core manufacturing is mainly synthesized from elements such as iron (Fe), cobalt (Co), silicon (Si), boron (B), and carbon (C) in a specific ratio. Under high-temperature melting conditions, it is spun into alloy foil through a high-speed rotating wheel. Due to the action of the high-speed rotating wheel and the rapid temperature drop during cooling, the atomic structure of the alloy foil produced by this process is similar to the random arrangement of glass, rather than the crystalline structure characterized by typical metallic alloys. Therefore, it is called "amorphous alloy," and some also refer to it as "metallized glass" based on its atomic structure characteristics. Because "amorphous alloy" lacks obvious lattice interfaces, the random amorphous structure has very low coercivity under the influence of a magnetic field, thus significantly reducing "hysteresis loss." In addition, the "amorphous alloy" material itself is a foil strip, typically 0.003 mm thick, only one-tenth the thickness of ordinary silicon steel sheets, and has a high resistivity, making it a low-loss soft magnetic material. Research on "amorphous alloys" in my country began around 1977. Through nearly 20 years of effort, the Beijing Iron and Steel Research Institute and the Shanghai Iron and Steel Research Institute under the Ministry of Metallurgical Industry now have a certain production capacity for "amorphous alloy" foil strips, with a maximum width of approximately 150-200 mm. Foreign manufacturers (alloys) have also invested in production and processing bases in China and can provide wound open or closed cores according to design requirements. In the past decade, many domestic manufacturers have used "amorphous alloy" cores to develop 10 kV 30-250 kVA oil-immersed or epoxy resin-insulated distribution transformers and 1600 kVA medium-frequency transformers. Most of these products are currently in operation. Furthermore, there are indications that some high-voltage transformers using "amorphous alloy" cores are under development or production. Currently, most power equipment using "amorphous alloy" cores lacks complete technical standards, and its main impression is "low energy consumption." However, what are its overall technical performance indicators in actual use? As on-site engineering technicians, we should have a grasp of the differences in technical performance between power equipment using "amorphous alloy" cores and those using ordinary silicon steel cores. [b]2 Electromagnetic and Physical Properties of "Amorphous Alloy" Materials[/b] To fully understand the differences in the electromagnetic and physical properties of "amorphous alloy" materials compared to silicon steel cores, Table 1 lists a performance comparison of the two types of materials. Under the same magnetic flux density, the loss of an amorphous alloy core is only one-quarter that of a silicon steel core, and its excitation power is about half that of a silicon steel core; its resistivity is three times that of a silicon steel core. Furthermore, the production process of amorphous alloy foil strips simplifies the manufacturing process compared to silicon steel sheets by eliminating ingot casting, initial rolling (brushing), hot rolling, cold rolling, and intermediate steps. Calculations show that the energy consumption of the entire process is only 20%–25% of that of silicon steel core production. The resulting economic and social benefits are considerable. However, amorphous alloy materials also have several drawbacks. For example, as a core material, its working magnetic flux density (1.4 T) is lower than that of silicon steel sheets (1.76 T); its lamination fill factor is relatively small, typically 0.75–0.80, about 84% of the lamination fill factor of silicon steel sheets (typically 0.95–0.97); the material is very thin, has high hardness, and is brittle, making it difficult to process; its thermal stability is poor, and if local overheating exceeds a certain range, the material's magnetic permeability will severely deteriorate; in addition, amorphous alloy cores must be annealed under inert gas protection in a magnetic field. [b]3 Process Factors Affecting the Low-Loss Performance of Amorphous Alloy Cores[/b] The low-loss performance of amorphous alloy cores depends on the entire production process, primarily the complete elimination of internal stress. So how does internal stress affect the low-loss performance of amorphous alloy cores? [b]3.1 Influence of the Bending Radius of the Winded Core[/b] Because the thickness of amorphous alloy foil is very thin, about one-tenth that of silicon steel sheets, amorphous alloy materials are very suitable for manufacturing wound core transformers. Figure 1 shows the results of the trial production. It can be seen that the smaller the diameter of the wound core, the greater the core loss. According to reports, annealing amorphous alloy at 365 ℃ under certain magnetic field conditions can restore the loss performance of amorphous alloy cores to their original level. [img=192,189]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gddl/gddl99/gddl9906/image06/1301.gif[/img] Figure 1. Core loss under different winding bending radii [b]3.2 Influence of joint method on core loss[/b] When manufacturing wound core transformers, coils can be wound directly on the coil mold on the core; alternatively, an "open-wound core" can be used, followed by inserting the formed coil. Since annealing cannot be applied after the core is fitted with the coil, the "open-wound core" is a more practical method for manufacturing larger capacity transformers. Figure 2 shows the degree of influence of different core overlapping methods on core loss. According to relevant foreign reports, when the open-wound core has undergone annealing, or when the core size is large, the impact of the coil assembly operation on the loss of the amorphous alloy core can be ignored. However, based on the current domestic production processes and management practices, no reports have been found regarding changes in the loss level after the core is opened. [img=232,181]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gddl/gddl99/gddl9906/image06/1302.gif[/img] Figure 2. Influence of different joint methods on the loss of the open-wound core . 3.3 Annealing Process Since the material used to make the amorphous alloy core is formed into foil strips under a state of rapid cooling, there are still slight residual stresses in the material. Annealing can eliminate these residual stresses. However, the annealing process requires a nitrogen protective atmosphere and appropriate temperature and sufficient magnetic field strength along the length of the foil strip. The recommended annealing temperature for 2605Sc is 345 ℃; comparative experiments show that when the annealing temperature is below 345 ℃, the iron loss will increase by 25%. The recommended magnetic field strength during annealing is 2–8 A/cm. When the magnetic field strength is below 0.5 A/cm, the effect on reducing iron loss is not significant. 3.4 Influence of External Stress Based on the above-mentioned influence on iron loss, even with various stress-relief measures, the stress caused by the overall assembly of the coiled amorphous alloy core will inevitably increase core loss. Binding and support during production assembly, vibration during transport, and stress during accidents will all cause changes in core loss. The magnitude of core loss is a technical indicator for judging the operating status of the transformer. Due to the significant randomness of loss changes, judging the equipment status is somewhat difficult. 4 Performance Comparison of Amorphous Alloy Core Transformers and Silicon Steel Sheet Core Transformers According to the existing processes in China, the performance ratio of amorphous alloy core transformers varies with the transformer capacity. Table 2 compares the performance of amorphous alloy core transformers and silicon steel sheet core transformers. As shown in Table 2, amorphous alloy core transformers have superior performance for small-capacity transformers. However, as the capacity increases, the performance of amorphous alloy core transformers becomes comparable to that of silicon steel core transformers. The savings in iron loss from amorphous alloy core transformers offset the excess copper loss compared to silicon steel core transformers. Although relevant quality data is unavailable, it is certain that large-capacity amorphous alloy core transformers will have a larger mass and volume than their silicon steel core counterparts due to the lower operating magnetic flux density of amorphous alloy core transformers compared to silicon steel core transformers of the same capacity. Therefore, a comprehensive economic efficiency ratio should be calculated when evaluating amorphous alloy core transformers. Furthermore, the safety level of equipment operation should be given greater consideration in their use, and the overall safety status of the equipment should be fully assessed. [b]5 Recommendations[/b] Amorphous alloy core transformers have significantly lower core losses than those of oriented cold-rolled silicon steel sheets, resulting in considerable economic benefits in energy saving and consumption reduction. However, the low-loss performance of amorphous alloy cores is affected by various factors. Based on existing reports, a comprehensive economic performance evaluation under equivalent technological levels has not been conducted. Some products undergoing trial operation showed a 25% change in no-load loss after one year of operation. Whether this is a general trend requires further statistical analysis and observation. According to the design principles of some domestic manufacturers, the maximum magnetic flux density is selected based on 80%–85% of the equipment's nominal capacity. Therefore, the equipment operates at full load in the non-linear segment of the magnetization curve. Since the magnetic permeability of amorphous alloy cores can suffer irreversible damage under high temperature and overmagnetic conditions, the possibility of core overmagnetization should be rigorously evaluated for electrical equipment sensitive to magnetic permeability. Currently, the price of amorphous alloy cores is relatively high (40 yuan/kg), four times that of grain-oriented silicon steel sheets. A comprehensive evaluation of the economic performance and safety performance of amorphous alloy core transformers (current transformers) is essential. [b]References[/b] [1] IEEE CH2133-7/85/0000-0205, Evaluation of Electrical Insulation Exposed to Amorphous Metal Transformer Core Annealing Temperature［S］