The core materials of transformers directly determine their energy efficiency, stability and service life. Among the main materials of transformer main bodies (magnetic circuit materials, circuit materials, insulating materials, structural materials, etc.), silicon steel sheets and amorphous alloy cores are the core of magnetic circuit materials, which are crucial to the overall performance of transformers. Whether it is the 150 kva isolation transformer widely used in industrial production or the step up isolation transformer for voltage regulation and transmission, the selection of high-quality core materials is the basis for ensuring stable operation. Below, JINSHANMEN TECHNOLOGY CO., LTD will take you to in-depth understanding of these two core materials.
1. Silicon Steel Sheet (Silicon Steel Laminations)
Silicon steel sheet is a kind of ferrosilicon alloy with specific magnetic properties, which is the most commonly used core material in traditional transformers. For transformers of different types and specifications, such as the 150 kva isolation transformer and step up isolation transformer, the performance requirements of silicon steel sheets are basically consistent, mainly including the following aspects:
a) Low iron loss: This is the most important indicator of silicon steel sheet quality. All countries classify grades according to iron loss value; the lower the iron loss, the higher the grade. b) High magnetic induction intensity (magnetic induction) under strong magnetic field: This reduces the volume and weight of the iron core of motors and transformers, saving silicon steel sheets, copper wires, insulating materials, etc. c) Smooth, flat surface and uniform thickness: It can improve the filling factor of the iron core. d) Good punchability: Easy to process and form. e) Good adhesion and weldability of surface insulation film: It can prevent corrosion and improve punchability. f) Basically no magnetic aging: Ensure stable magnetic properties during long-term use.
1.1 Classification and Grade Definition of Silicon Steel Sheets
Transformers usually adopt cold-rolled grain-oriented silicon steel sheets to ensure their no-load energy efficiency. According to performance and processing methods, cold-rolled grain-oriented silicon steel sheets can be divided into ordinary cold-rolled grain-oriented silicon steel sheets, high magnetic permeability silicon steel sheets (or high magnetic induction silicon steel sheets), and laser-scribed silicon steel sheets. Generally, silicon steel sheets with a minimum magnetic polarization intensity B800A = 1.78T ~ 1.85T under a 50Hz, 800A alternating magnetic field (peak value) are called ordinary silicon steel sheets, denoted as "CGO", while those with B800A = 1.85T or more are called high magnetic permeability silicon steel sheets (high magnetic induction silicon steel sheets), denoted as "Hi-B steel". The main difference between Hi-B steel and conventional silicon steel sheets is that Hi-B steel has a very high Goss texture degree, that is, the alignment of silicon steel grains in the easy magnetization direction is very high. Industrially, a secondary recrystallization process is used to manufacture silicon steel sheets with a silicon content of 3%. The average deviation of the grain orientation of Hi-B steel from the rolling direction is 3°, while that of ordinary silicon steel sheets is 7°, which makes Hi-B steel have higher magnetic permeability. Usually, its B800A can reach more than 1.88T. Improving the Goss texture degree and magnetic permeability can reduce iron loss.
Another feature of Hi-B steel is that the elastic tension of the glass film and insulating coating attached to the surface of the steel sheet is 3~5N/mm2, which is better than 1~2 N/mm2 of ordinary oriented silicon steel sheets. The high-tension layer on the surface of the steel strip can reduce the magnetic domain width and abnormal eddy current loss. Therefore, Hi-B steel has a lower iron loss value than conventional oriented silicon steel sheets.
Laser-scribed silicon steel sheets are based on Hi-B steel. Through laser beam irradiation technology, tiny strains are generated on the surface, further refining the magnetic axis and achieving lower iron loss. Laser-scribed silicon steel sheets cannot be annealed, because increasing the temperature will make the laser treatment effect disappear.
The physical properties of different grades of silicon steel sheets are basically equivalent, and the density is basically 7.65g/cm3. For the same type of silicon steel sheets, the main differences in performance and quality lie in the silicon content and the influence of the production process. The 150 kva isolation transformer and step up isolation transformer produced by JINSHANMEN TECHNOLOGY CO., LTD select high-grade cold-rolled grain-oriented silicon steel sheets according to actual working conditions, ensuring excellent energy efficiency and stable operation performance.
2. Amorphous Alloy Core
Amorphous alloy material is a new type of alloy material developed in the 1970s. It uses internationally advanced ultra-rapid cooling technology to directly cool liquid metal into solid thin strips with a thickness of 0.02～0.03mm at a cooling rate of 106℃/S, which solidifies before it has time to crystallize. Similar to glass, this alloy material has an irregular atomic arrangement and no crystal structure characterized by metals. Its basic elements include iron (Fe), nickel (Ni), cobalt (Co), silicon (Si), boron (B), carbon (C), etc. Due to its excellent magnetic properties and energy-saving effects, it has been widely used in the core manufacturing of transformers, especially in amorphous alloy power transformers. Compared with traditional silicon steel sheet cores, amorphous alloy cores have obvious advantages, which also make the 150 kva isolation transformer and step up isolation transformer equipped with them have better energy-saving performance.
2.1 Advantages of Amorphous Alloy Materials
a) Isotropic soft magnetic material: Amorphous alloy material has no crystal structure and is an isotropic soft magnetic material; it has small magnetization power and good temperature stability. Since amorphous alloy is a non-oriented material, direct seams can be used, making the process of manufacturing the iron core relatively simple. b) Low hysteresis loss: There are no structural defects that hinder the movement of magnetic domains, so the hysteresis loss is smaller than that of silicon steel sheets. c) Ultra-thin strip: The thickness of the strip is only 0.02～0.03mm, which is about 1/10 of that of silicon steel sheets. d) High resistivity and low eddy current loss: The resistivity is about 3 times that of oriented silicon steel sheets; the eddy current loss of amorphous alloy materials is greatly reduced, so the unit loss is about 20%～30% of that of oriented silicon steel sheets. e) Excellent no-load performance: The annealing temperature is low, about 1/2 of that of oriented silicon steel sheets; the no-load performance of amorphous alloy cores is superior. Transformers manufactured with amorphous alloy cores have a no-load loss reduction of 70～80% and a no-load current reduction of more than 50% compared with conventional transformers, with outstanding energy-saving effects. For the purpose of energy conservation, emission reduction and reducing network line losses, State Grid and China Southern Power Grid have greatly increased the procurement ratio of amorphous alloy transformers since 2012, and currently the procurement ratio of amorphous alloy distribution transformers has basically reached more than 50%.
2.2 Disadvantages and Countermeasures of Amorphous Alloy Transformers
While having outstanding energy-saving advantages, amorphous alloy transformers also have some shortcomings, which need to be optimized in design and manufacturing:
1) Low saturation magnetic density: The saturation magnetic density of amorphous alloy cores is usually around 1.56T, which is about 20% lower than the 1.9T of conventional silicon steel sheets. Therefore, the design magnetic density of the transformer also needs to be reduced by 20%. The design magnetic density of amorphous alloy oil-immersed transformers is usually below 1.35T, and that of amorphous alloy dry-type transformers is usually below 1.2T.
2) Sensitive to stress: The strip of the amorphous alloy core is sensitive to stress. After being stressed, the no-load performance is easy to deteriorate. Therefore, special attention should be paid to the structure: the iron core should be hung on the support frame and the coil, only bearing its own weight. At the same time, special attention should be paid during the assembly process: the iron core should not be stressed, and the way of knocking should be reduced.
3) Large magnetostriction: The magnetostriction is about 10% larger than that of conventional silicon steel sheets, so its noise is difficult to control, which is one of the main reasons limiting the wide promotion of amorphous alloy transformers. At present, both China Southern Power Grid and State Grid have put forward higher requirements for the noise of amorphous alloy transformers in tenders, dividing them into sensitive areas and non-sensitive areas, and putting forward targeted sound level requirements, which requires further reducing the design magnetic density of the iron core.
4) Special core structure: Amorphous alloy strips are thin, only 0.03mm thick, so they cannot be made into lamination form like conventional silicon steel sheets, but only into wound core form. Therefore, the iron core structure cannot be processed by conventional transformer manufacturers and usually needs to be purchased as a whole. Corresponding to the rectangular cross-section of the wound core strip, the coil of the amorphous alloy transformer is usually made into a rectangular structure.
5) Insufficient localization: At present, it is mainly imported amorphous alloy strips from Hitachi Metals, and localization is being gradually realized. Domestic companies such as Advanced Technology & Materials Co., Ltd. and Qingdao Yunlu have amorphous alloy wide strips (213mm, 170mm and 142mm), but their performance still has a certain gap in stability compared with imported strips.
6) Limit on maximum strip length: Previously, the maximum outer circumference strip length of amorphous alloy strips was greatly limited due to the size of the annealing furnace. However, this problem has been basically solved at present, and amorphous alloy core frames with a maximum outer circumference strip length of 10m can be made, which can be used to manufacture amorphous alloy dry-type transformers below 3150kVA and amorphous alloy oil-immersed transformers below 10000kVA.
2.3 Cost Comparison of Amorphous Alloy Transformers
Due to the excellent energy-saving effect of amorphous alloy transformers, coupled with the promotion of national energy conservation and emission reduction policies, the market share of amorphous alloy transformers is increasing. Considering that the price of amorphous alloy strips (currently 26.5 yuan/kg) is about twice that of conventional silicon steel sheets (30Q120 or 30Q130), and the gap with copper is relatively small, and considering the quality and tender requirements of power grid products, amorphous alloy transformers usually use copper conductors. Compared with conventional silicon steel sheets, the main cost differences of amorphous alloy transformers are as follows:
1) Due to the adoption of wound core structure, the transformer core type should adopt three-phase five-limb structure, which can reduce the weight of a single frame core and reduce assembly difficulty. The three-phase five-limb structure and three-phase three-limb structure have their own advantages and disadvantages in cost. At present, most manufacturers adopt three-phase five-limb structure.
2) Since the cross-section of the core column is rectangular, in order to maintain consistent insulation distance, the high and low voltage coils are correspondingly made into rectangular structures.
3) Since the design magnetic density of the iron core is about 25% lower than that of conventional silicon steel sheet transformers, and its iron core lamination factor is about 0.87, which is much lower than 0.97 of conventional silicon steel sheets, its design cross-sectional area needs to be more than 25% larger than that of conventional silicon steel sheet transformers. Correspondingly, the perimeter of the high and low voltage coils will also increase. At the same time, it is necessary to consider the increase in the length of the wire turns of the high and low voltage coils. To ensure that the load loss of the coil does not change, the cross-sectional area of the wire needs to be correspondingly increased. Therefore, the copper consumption of amorphous alloy transformers is about 20% more than that of conventional transformers.
About JINSHANMEN TECHNOLOGY CO., LTD
JINSHANMEN TECHNOLOGY CO., LTD is a professional manufacturer of power transmission and distribution equipment. The company mainly produces oil immersed power transformers, dry-type power transformers, oil immersed three-dimensional coiled power transformers, dry-type three-dimensional coiled power transformers, mining explosion-proof dry-type transformers, mining explosion-proof mobile substations, amorphous alloy power transformers, on load capacity regulating power transformers, locomotive dry-type transformers, as well as prefabricated substations, modular substations, wind energy box type substations, high and low voltage switchgear and other transmission and distribution equipment. Whether it is transformers using silicon steel sheet cores or amorphous alloy cores, such as 150 kva isolation transformer and step up isolation transformer, we strictly control the selection of core materials and the manufacturing process to ensure that the products have excellent performance, stable operation and energy saving and environmental protection. We are committed to providing high-quality transmission and distribution equipment solutions for global customers.