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Furnace Transformers
Introduction
Electric Arc Furnace (EAF) transformers supply the power needed for steel production and special ferroalloys. Most such transformers are custom built to fit both mechanically and electrically In practice, these applications demand very high power levels and operate under severe conditions. As a result, short circuits in the furnace and switching operations in HV circuit breakers frequently cause overcurrents and overvoltages.
To address these demands, many installations use three single-phase furnace transformers instead of one three-phase unit. Consequently, each transformer must withstand extreme thermal, mechanical, and electrical stress. Over time, decades of operation have proven the shell-type EAF transformer as a strong and reliable technology.
In addition, an EAF transformer delivers exceptionally high secondary current—often 10 kA or more—and provides adjustable current and voltage. Therefore, operators optimize furnace performance by using a shell-type or core-type transformer with an on-load or off-load tap changer on the primary winding.
Finally, when installations require compact and cost-efficient solutions, engineers can integrate a series reactor into the same tank as the EAF transformer. Furthermore, operators can apply a tap changer on the reactor to fine-tune operating values and improve process control. Get in touch to learn more.
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Construction Principles, Comparison, and Selection
There are two fundamental construction principles for furnace transformers, based on how engineers configure the magnetic core and windings. In general, these principles define both the physical layout and the electrical behavior of the unit.
Shell-type (Fig. 2a)
First, this design uses a rectangular core cross-section and windings with a matching shape. Next, engineers interleave high-voltage (HV) and low-voltage (LV) discs and separate them with insulating shields. At the same time, they tap selected HV discs to regulate voltage. Moreover, they build the LV coils from solid copper plates and divide them into parallel groups so they match the bus-bar system feeding the furnace.
Core-type (Fig. 2b)
Alternatively, this design uses two core legs with a roughly circular cross-section. Then, engineers mount the HV and LV windings concentrically on the core legs and typically place the LV winding outside the HV winding. In addition, the LV windings use multiple disc coils, and engineers connect them in parallel to bars that lead to the LV bushings.
Advantages and Disadvantages
Engineers can adapt both concepts to furnace applications. However, the two designs differ in their electrical characteristics. For that reason, selection often depends on the operating philosophy and process demands.
Shell-type advantages
Primarily, the transformer delivers low reactance (1,5–3%), which is typically two to three times lower than a comparable core-type transformer.
Furthermore, the design offers greater flexibility in voltage steps and distributes voltage more uniformly.
Equally important, the active part provides high mechanical strength, which suits heavy-duty furnace service.
Core-type characteristics
By contrast, the transformer operates with higher reactance (commonly 4–12%).
Nevertheless, the concentric winding layout simplifies the design and can fit certain installation constraints.
Overall, because furnace operations generally require low reactance, users usually choose shell-type transformers. In other words, the shell-type concept often delivers the electrical behavior the process needs, while still providing robust mechanical performance.
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