Basic introduction of electric furnace transformer
Since the Second Industrial Revolution, electricity has become the energy of industry, and human beings have entered the "electric age". Power transformer is a important part of our heavy industry, power transformer will transfer the high voltage from the power transmission system, to low voltage for the electrical equipments in our industry.
Furnace transformer is one type of oil immersed power transformers, they are used for furances' power supply. So what is a furnace transformer? Compared with other transformers, is there any particularity in electric furnace transformers? Check out today’s article and find out the answers.
The electric furnace transformer is also called "smelting transformer", and its load is the electric furnace. Different from general IF equipment. This kind of transformer generally has a primary 10KV, and the secondary varies from 40 to 130 volts as needed. The current ranges from several thousand to tens of thousands of amperes, and the capacity is usually large. Both split and integrated. Use carbon rods as electrodes. Commonly used in electric furnace smelting of special steel, white corundum, electrolytic aluminum, etc.
(1) Primary rated voltage. This voltage is the voltage applied by the power supply network to the primary coil of the transformer, that is, the standard voltage of the power supply network, mainly including 6kV, 10kV, 35kV and 110kV.
(2) Secondary voltage. The secondary voltage is also known as the low-side voltage. Its size and its series mainly depend on the requirements of the steelmaking process, and its range is generally between 100 and 1200V.
(3) Rated current. No matter how the transformer is connected, the rated current in the low-voltage side coil remains unchanged; while the current in the high-voltage coil changes with the change of the secondary current.
(4) Rated capacity. The capacity at the highest secondary voltage is defined as the rated capacity of the electric arc furnace transformer.
(5) Power supply frequency. In China, the power frequency f=50Hz, but in other countries, the regulations of the power frequency are different based on local electrical system.
(6) The connection line and connection group of the coil. When the coils are connected in a Y-△ shape or a △-△ shape, the connection group label is Yd11 (11 means the primary and secondary phase angles differ by 11°) or Dd0 (0 means the primary and secondary phase angles differ by 0°).
(7) Efficiency. Generally, η = 95% to 98% for small and medium-sized transformers, and η = 99% for large transformers.
(1)Resistance Furnace Transformer
Mechanical parts can be heated in resistance furnaces and salt bath furnaces for heat treatment, powder metallurgy sintering, non-ferrous metal smelting, etc. To reduce and adjust the input voltage of the electric furnace, a resistance furnace transformer needs to be equipped between the furnace and the power grid because the resistance of the heating element is too small or changes too much during the heating process.
Low-voltage, small-capacity and salt-bath furnace transformers are usually dry-type transformers with box shells and natural cooling; resistance furnace transformers with medium capacity (hundreds to thousands of VA) are usually oil-immersed, self-cooling transformers; large-capacity transformers are forced oil circulation water-cooled.
(2)Electric arc furnace transformer
Special transformers used to power electric arc furnaces for steel smelting. These transformers are large, complex and have high technical requirements. The secondary side voltage is low, generally between tens and hundreds of volts, and it must be regulated within a wide range; the secondary side current typically reaches thousands to tens of thousands of amps. The melting period in iron and steel smelting also requires high power, and the transformer must have a 20% overload capacity within 2 hours. The primary side of an electric arc furnace transformer should be connected to a current limiting reactor or should have a large impedance to limit short circuit current in the process of steelmaking since the collapse of the charge is likely to cause electrode short circuit.
To operate the electric furnace, the power supply transformer must also be able to adjust the voltage. The voltage can be adjusted in two ways.
① Direct voltage regulation: the tap is connected to the tap changer on the primary winding of the transformer. A *-Y switchover can also be used between the three phases of the primary side to regulate the voltage on the secondary side. The *-Y switching technique is a very efficient way of regulating voltage, which can reduce the secondary voltage to 1/ of the original without changing the number of turns on the primary side. The 9 taps on each phase winding plus the *-Y switching give a total of 18 levels of voltage.
② Indirect voltage regulation method: indirect voltage regulation method with series transformer. This main transformer has three windings (see multi-winding transformer), winding 1 is a high-voltage winding connected to the grid, winding 2 is a winding with a tap connected to the primary winding 4 of the series transformer II through a tap changer, and winding 3 is connected to the secondary winding 5 of the series transformer II to provide power to the electric furnace load. Through the tap changer, the output voltage of windings 3 and 5 can be changed by changing the primary voltage of series transformer II. Usually, the 3 and 5 windings connected to the load are 1 to 2 turns and made from a figure-8 coil of copper or aluminum plate.
(3)Shell furnace transformer
① Iron core
The iron core of the shell-type electric furnace transformer is a frame structure with full miter joints. The core width is narrow, the heat dissipation condition is good, and the structure is simple.
The winding of the shell-type electric furnace transformer is a rectangle with the same cross-sectional shape as the stem. The low-voltage winding is made of a single piece of copper plate, with good heat dissipation conditions, and the head is a welded structure; the high-voltage winding is a pie structure. The winding arrangement shall be staggered. The high-voltage line segment and the low-voltage line segment in each group should have the same magnetic potential, and their radial dimensions should be basically the same. Theoretically, the low-voltage line segments are placed at both ends, because the insulation distance to the iron core is small. However, in order to make the short-circuit impedance of the transformer smaller, multiple magnetic flux leakage groups are required to meet the requirements, and placing the low-voltage line segments at both ends will limit the number of magnetic flux leakage groups, so sometimes the high-voltage line segments are placed at both ends. In the process of voltage regulation, in order to make the line segment configuration symmetrical and ensure the balance of the magnetic potential, the voltage regulation line segment usually adopts multiple parallel connections, so as to ensure that the impedance of each leakage flux group is equal, and the current of each low-voltage line segment is also equal.
③ Cooling method
Shell furnace transformers generally use forced oil-guided circulation, forced water cooling, forced oil-guided circulation, and forced air cooling. Since the shell-type transformer can conveniently set a partition between the oil tank and the body, the cooled transformer oil is forced to flow through the wire cake, the oil flow is uniform, the temperature difference of each part is small, the heat dissipation effect is good, and the temperature of the hottest spot can be reduced. Decrease it by about 5°C to increase the extra overload capacity of the transformer.
④ Fuel tank
Since the winding of the shell-type transformer is completely shielded by the iron core, it is less likely to be damaged by external force, so a suitable shape oil tank can be used according to the shape of the body, thereby greatly reducing the size and weight of the transformer.
① Small mechanical force and good strength
Theoretical calculations show that the radial electromagnetic force of the shell-type transformer is very small. Although the axial electromagnetic force is relatively large, it can also be significantly reduced when there are many leakage magnetic groups. The windings of the shell-type transformer are completely surrounded by insulating parts, and the iron core surrounds them. The short-circuit force is directly transmitted to the fuel tank through the insulating parts and the iron core, unlike the core-type structure with less winding support surface, so the shell-type transformer has high mechanical strength.
② Strong impact resistance of the winding
Because the shell-type transformer winding has few coils and large radial size, the capacitance between the coils is large, but the capacitance to ground is small, so when the impulse voltage acts on the shell-type transformer, the initial voltage is basically linear. distribution, the voltage gradient is greatly reduced. At the same time, due to the large inherent capacitance of the shell-type transformer, the oscillation time of the winding voltage is prolonged, and the transient voltage has been attenuated before the winding reaches the amplitude. Therefore, the shell-type transformer winding has a good performance of withstand overvoltage shock.
③ Low impedance
Each phase of the shell-type transformer can be divided into several leakage magnetic groups, and the radial size of the wire cake is large, the impedance can be designed to be 2% to 3%, and the mechanical force and load loss are also small. Since the reactive power of the transformer is greatly reduced, the power factor of the electric furnace naturally increases.
④ Split-phase voltage regulation has no effect on the magnetic circuit
Due to the asymmetry of the three-phase magnetic flux of the split-phase voltage regulation, the core-type transformer must use a five-column iron core. But in the shell-type transformer core, each phase already has an independent magnetic circuit, and the asymmetry of the magnetic circuit does not affect the design of the core.
⑤ Short leads and easy impedance balance
The wire end outlet and the branch wire are drawn as short as possible at the upper part of the winding, and the low voltage winding outlet can be of the same length, thereby eliminating the impedance imbalance of the low voltage lead and reducing the power transfer during the operation of the electric furnace.
⑥ Low loss
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