Electric arc furnaces (EAFs) employed in smelting operations typically undergo three distinct characteristic working stages.
At the outset of the melting phase, solid charge materials are transformed into a molten state, necessitating the highest energy input among all stages.
Subsequently, during the initial refining and heating stage, the energy supplied is primarily utilized to compensate for heat losses, maintaining the desired temperature for the refining process.
The smelting cycle of a conventional alternating current (AC) electric arc furnace spans approximately 3 to 8 hours, contingent upon factors such as power supply circuit parameters, furnace capacity, and the specific smelting process employed. Notably, the melting period, lasting roughly 0.5 to 2 hours, represents a three-phase asymmetrical impact load characterized by highly unstable currents and substantial power consumption, accounting for approximately 60% to 70% of the total energy expended throughout the smelting cycle. In contrast, during the oxidation and reduction refining periods, voltage fluctuations and power consumption are markedly reduced.
The operational characteristics of electric arc furnaces during scrap smelting are as follows:
Initially, as melting commences, the arc frequently extinguishes and re-ignites, leading to erratic current patterns. Throughout the full melting period, arc fluctuations persist, resulting in rapid current changes, arc collapses, and short circuits.
The power factor range of the operating point for a standard electric arc furnace circuit typically falls between 0.8 and 0.85, whereas for high-power electric arc furnaces, it ranges from 0.7 to 0.8. A lower power factor inevitably translates to reduced power efficiency.
The wastage of electrical energy in electric arc furnaces primarily manifests in two key areas: a low power factor and the generation of substantial flicker and harmonics during the melting process.
Flicker serves as the primary catalyst for a host of adverse effects, including harmonic distortion, voltage and current phase loss, among others. "Flicker" (transients) refers to transient distortions in current and voltage within AC sine wave circuits, characterized by surges, spikes, harmonics, and other phenomena. According to Dr. Hesfield, a renowned American energy theorist, the defining features of this distortion are ultra-high pressure, ultra-high speed, and ultra-high frequency.
Despite the well-documented fact that these high-voltage and high-frequency flickers are a contributing factor to the destruction of sensitive electrical equipment, this issue has often been overlooked. Moreover, electrical work is defined as the product of current and voltage, and any instantaneous increases in voltage or current result in greater instantaneous power consumption.
Given that the heating end of the electric arc furnace constitutes a resistive load, these instantaneous voltages or currents are incapable of participating in arc initiation and heating within the furnace. Instead, they are distributed as iron loss and line loss in the form of reactive power, feeding back into the inductive load. In the furnace system, the inductive load is primarily the transformer, and the instantaneous reactive power consumption within the transformer does not contribute to the smelting process—a factor that has long been disregarded during EAF operations.
Even without considering the electrical energy waste caused by the low power factor of electric arc furnaces and focusing solely on the substantial flicker generated during the smelting and melting periods, it becomes evident that the electrical energy efficiency of electric arc furnaces is relatively lower compared to devices with the same rated power operating smoothly (with fewer flickers generated).
By suppressing or reducing the number of flickers and flicker peaks generated by electric arc furnaces during the smelting and melting periods and converting this portion of ineffective power into effective power, not only can the electrical energy efficiency of electric arc furnaces be enhanced, leading to energy savings, but also their impact on and pollution of the power grid can be eliminated, thereby safeguarding sensitive power equipment.
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