Industrial electric heating furnaces – Arc furnace device – Power supply system
Reexamination Certificate
2000-09-26
2002-06-25
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Arc furnace device
Power supply system
C373S105000
Reexamination Certificate
active
06411643
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electric arc furnaces and, more particularly, to a control system and method for controlling electrodes in electric arc furnaces based on direct power factor regulation.
2. Description of the Prior Art
Typically, electric arc and, therefore, power input to an electric arc furnace is regulated by controlling the positioning of the electrodes in the electric arc furnace either manually or automatically. However, to provide consistent operation of the electric arc furnace, automatic regulation of the electrodes is preferred. In particular, the electric arc, or arc length, of the electrode is controlled by controlling the position of the electrode with respect to the level of molten metal contained in the electric arc furnace. Various types of electrode regulating systems are known in the art such as current regulating systems, impedance regulating systems, resistance regulating systems and power regulating systems. Impedance regulating systems are the most widespread and well-established in the industry. In impedance regulating systems, it is common practice to measure the current and voltage of the electrodes at the tertiary or secondary side of the furnace transformer to determine electrode impedance. The electrode impedance as a process input is then compared with a theoretical impedance value to maintain a constant electric arc, or arc length.
A principal disadvantage with impedance regulating systems is that an “optimum” power factor and, thus, an optimum working/operating condition of the electric arc furnace is difficult to achieve and maintain in practice. This is primarily due to the inherent complexity and time consuming efforts required for adjusting the theoretical impedance value to the required or “optimum” power factor of the electrode. In addition, the theoretical impedance is a constant and does not consider the frequently changing conditions in the electric arc furnace which necessitates continuous adjustments of the theoretical impedance. Optimum working/operating conditions are typically never met in electric arc furnaces that utilize impedance regulating systems. A typical impedance regulating system for electrodes in an electric furnace is disclosed by U.S. Pat. No. 5,255,285 to Aberl et al. (hereinafter “the Aberl patent”).
The Aberl patent generally discloses an impedance based control system for an electric arc furnace that includes an electrode actuator, a controller for controlling the electrode actuator based on electrode impedance, and an impedance signal generator in which the electrode impedance is calculated. The controller is utilized to compare the electrode impedance with a desired impedance value. The impedance regulating system disclosed by the Aberl patent attempts to overcome the inherent deficiencies with impedance regulating systems by providing a feed back control loop or “correcting” signal to the controller. However, the “correcting” signal is nothing more than an indirect measure of the electrode impedance with an allowance for the resistance and inductive reactance of an electric lead connected to the furnace transformer of the electric arc furnace, and thus does little to improve the performance of the overall control system. Impedance type electrode regulating systems are generally time consuming and inefficient methods of control for electrodes in electric arc furnaces.
As discussed previously, electrode regulating systems may be based on such criteria as current, impedance and resistance. However, these values arc only indirect measurements providing indirect information on the power input to the individual electrodes. The most important value, or control criteria, requiring scrutiny and generally ignored in the prior art is electrode power factor.
FIG. 1
illustrates how operating or “secondary” current, operating or “secondary” voltage, impedance and power input are related in electric arc furnaces.
FIG. 1
is a circle diagram of operating voltage versus operating current and shows power factor values for five power factor set points. During normal operating conditions, the power input should be maximized and the electrode consumption minimized. The arc length should be stable and over-currents should be avoided. The circle diagram of
FIG. 1
shows that maximum available power occurs at a power factor of cos ∝=0.707. However, maximum power input does not necessarily result in a maximum rate of heating or optimal operation of the furnace when other factors such as electrode consumption or carbon pickup are considered. For this reason, most electric arc furnaces operate at slightly higher power factors ranging between cos ∝=0.72 and 0.78.
Attempts have been made in the prior art to incorporate power factor into a control system for controlling electrodes in electric arc furnaces. However, these attempts have centered on utilizing power factor to control the power source for the electric arc furnace or as a secondary or “correcting” signal in what otherwise are well-known current or impedance based regulating systems. At best, these attempts have only succeeded in utilizing power factor as an indirect or secondary process input and, hence, are not true power factor based regulating systems.
For example, U.S. Pat. No. 3,435,121 to Jackson discloses an arc power responsive control system for consumable electrode furnaces that utilizes power factor as a control criteria or value for controlling a transformer power source of the furnace, rather than as a control criteria for controlling the positioning of the furnace electrodes. In particular, the transformer power source control circuit disclosed by the Jackson patent includes a power factor transducer which receives an arc current signal and an arc voltage signal from the transformer power source. From the power factor transducer, a signal indicative of the phase angle between the arc current signal and arc voltage signal is fed to a power factor level detector that compares the actual power factor with a desired power factor range. If actual power factor falls outside of the desired power factor range, the power factor level detector provides an output signal proportional to the difference directly to the transformer power source. The output signal provided to the transformer power source identifies which transformer tap should be used on the transformer power source. The output signal is not used as a process input to the electrode control circuit disclosed by the Jackson patent that actually controls the positioning of the electrodes in the furnace. The electrode control circuit disclosed by the Jackson patent is defined by a power transducer which receives the arc current and arc voltage signals from the transformer power source, a power averaging circuit, a power comparison circuit, a power reference source, an amplidyne, and an electrode drive motor. The output signal generated by the power factor level detector is not provided as a process input to this control circuit. Hence, the control system disclosed by the Jackson patent is not configured to change the positioning of the electrodes in the furnace based directly on power factor as a process input to the electrode control circuit.
Another prior art system which attempts to incorporate power factor in a control system for controlling electrodes in electric arc furnaces is disclosed by U.S. Pat. No. 3,662,075 to Sakai et al. (hereinafter “the Sakai patent”). The electrode control system disclosed by the Sakai patent includes an electrode driving mechanism, an automatic current regulator responsive to the current flowing through the electrode, and a program control unit connected between the automatic current regulator and the electrode driving mechanism for transmitting electrode control signals to the electrode driving mechanism. A power factor regulator responsive to the actual power factor of the furnace is connected to the program control unit to provide an error-correcting signal to the program control unit.
Eiermann Jörg
Schreiter Till A.
Hoang Tu Ba
Poff Clifford A.
SMS Demag, Inc
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