The feasibility study for the choice of steel grade and annealing applies to power transformers in accordance with the requirements
standards for specific types and groups of transformers of all voltage classes with a capacity from 25 to 1 250 000 kV • A with magnetic cores made of 0.27 thick electrical steel; 0.30 and 0.35 mm grades 3404, 3405 and 3406 and having a flat laminated hairpin-free design. Cold-rolled steel is much more sensitive to mechanical stress than hot-rolled steel. As a result of machining during the blanking of magnetic system plates - longitudinal and transverse cutting, rolling or cutting of burrs, punching holes (in reactor structures), specific losses and specific magnetizing power of steel increase. This deterioration in the magnetic properties of steel can be fully or largely compensated by reducing annealing of the prepared plates at 800-820 ° C. In modern plants, such annealing is necessarily included in the technological process of making plates after their mechanical treatment. In the absence of annealing, a possible increase in no-load losses by 8-10% and no-load current by 25 30% should be considered.
Especially strongly the magnetic properties of steel deteriorate in the manufacture of parts of the magnetic system by winding from a cold-rolled strip. Such parts should be annealed after winding. During further transportation after annealing to the assembly, as well as in the process of assembling the core and tie rods and yokes, the plates can be subjected to various mechanical influences. In this case, a deterioration in the magnetic properties of steel occurs, which cannot be removed by annealing in the finished transformer core. To avoid deterioration of the magnetic properties of steel and idling parameters of the transformer when performing these operations, the plates should not be subjected to shocks, bends, impacts and pressures. Plates of electrical steel, prepared for the assembly of the magnetic system, in order to avoid the occurrence of eddy currents between them, must be reliably isolated from one another. The modern heat-resistant electrical insulating coating provides sufficiently strong and reliable insulation of the plates with a high filling factor of the section of the package of plates with the section of pure steel. With the power of transformers. exceeding 100 MVA sometimes reinforce the insulation of the plates by applying one layer of varnish film over the heat-resistant coating.
Filling factor of the cross-section of the bar (yoke) with steel KZ equal to the ratio of the active section of the bar or yoke Ps to the area of its actual cross-section PFS, i.e. KZ = Ps / PFS , it is desirable to have the highest, because a decrease in this coefficient leads to an increase in the cross section of the magnetic system and the mass of the metal of the windings. Duty factor KZ , depends on the thickness of the steel plates (0.35; 0.30 or 0.27 mm), the type of plate insulation, the compression force of the plates and the presence of such a defect as non-flatness, i.e. deviation from flatness. Duty factor KZ for steel with modern core assembly technology, is given in table 1
When choosing the grade and thickness of steel for magnetic system of power transformer it should be borne in mind that steel with higher magnetic properties has a significantly higher price, and steel of thinner thickness with higher magnetic properties has a lower filling factor.
To obtain a package of specified dimensions, this steel requires manufacturing, annealing and laying during the assembly of the magnetic system, a larger number of plates compared to steel of greater thickness. Table 2 shows comparative indicators for steel with a thickness of 0.35; 0.3 and 0.27 mm In the bulk of power transformers, taking into account the complexity of individual technological operations, magnetic properties and steel prices, steel grades 3404 and 3405 with a thickness of 0.35 and 0.30 mm are used. Where low loss is critical, 0.27 mm steel can be used. To select a steel grade and, which is important in educational design, to determine the economic efficiency of using new steel grades or amorphous alloys, a simplified calculation should be used according to the reduced annual costs, replacing one steel grade with another. Replacing steel with a higher quality one leads to higher prices transformer cost, since the price of such steel is higher, but at the same time, no-load losses are reduced, which pay off during the entire life of the transformer.