Electric heating – Heating devices – With power supply and voltage or current regulation or...
Reexamination Certificate
1999-08-31
2001-08-14
Paschall, Mark (Department: 3742)
Electric heating
Heating devices
With power supply and voltage or current regulation or...
C219S497000, C219S506000, C373S018000, C373S135000, C323S205000, C323S235000
Reexamination Certificate
active
06274851
ABSTRACT:
FIELD OF THE INVENTION
This present invention relates in general to electric arc furnaces. In particular, the present invention relates to a method and apparatus for reducing flicker in power lines when supplying an electric arc furnace.
BACKGROUND OF THE INVENTION
Electric arc furnaces are well known for melting scrap metal for recycling purposes. An arc furnace typically comprises a container for receiving a scrap metal charge, electrodes spaced a distance from the container, and an electrical power source coupled to the electrodes. The power source induces an electrical discharge between the electrode and the metal charge which produces sufficient heat energy to melt the metal charge.
One common problem associated with electric arc furnaces is that the furnace can produce voltage and current disturbances in the power supply network which supplies the arc furnace. This phenomenon, often called “flicker”, arises from large erratic fluctuations in reactive load current through the arc furnace at frequencies up to 25 Hz. When flicker is severe, it can impact on the proper operation of sensitive process loads having a point of common coupling with the arc furnace. Further, due to the sensitivity of the human eye, flicker levels which may not have an impact upon process loads may produce annoying fluctuations in incandescent and fluorescent lamp luminescent levels. Accordingly, North American IEEE and international IEC standards of power quality have been established for flicker and harmonic emission levels acceptable for process loads and incandescent and fluorescent lamps.
Systems have been proposed to compensate for the erratic reactive current swings of the typical arc furnace. In one such system, proposed by L. Gyugyi and R. H. Otto in “Principles and Applications of Static, Thyristor-Controlled Shunt Compensators”, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 5, September/October 1978, a shunt-type VAR generator is used to reduce flicker by introducing reactive currents into the load current to cancel the reactive components of the load current. Each leg of the VAR generator comprises a fixed capacitor in parallel with a thyristor-controlled fixed inductor, and the conduction angle of the thyristor is varied in response to the magnitude of measured reactive load current. However, as the thyristors are positioned in parallel with the power source and the load, it is difficult to control the magnitude of the load current. Therefore, there remains a need for an electric arc furnace which allows the user to control the operating temperature of the furnace without having a deleterious impact on power quality.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an arc furnace flicker controller which addresses the problems associated with the conventional arc furnace flicker controllers.
The flicker controller, in accordance with the present invention, reduces flicker induced by an electric arc furnace. The arc furnace includes a container for receiving a metal charge, an electrode spaced apart from the container, and an electrical power source coupled to the electrode and the container for generating an electrical discharge between the electrode and the metal charge. The flicker controller comprises a switch electrically connected between the power source and the electrode, and a control system coupled to the switch. The switch includes a gating input for controlling a conduction interval of the switch, and the control system applies gating signals to the gating input for maintaining a magnitude of reactive current flow through the arc furnace substantially constant.
In a preferred embodiment of the invention, the arc furnace comprises a three-phase arc furnace having three arc furnace electrodes, a three-phase power source, and three thyristor sets each connected between a respective one of the power source phases and a respective one of the arc furnace electrodes. The flicker controller includes three reactors, with each reactor being coupled between a phase of the power source and an electrode for providing a reactive current path between the power source and the electrode independent of the conductive state of the thyristors. The reactance of each reactor is selected such that the reactive current drawn by the reactor while the associated electrode is short circuited to ground and the thyristor is off has a maximum value equal to the reactive current drawn by the associated power source phase at full load when the discharge is established and the thyristor is fully conducting. As a result, reactive current drawn by the arc furnace remains substantially constant between short circuit and full load conditions.
Preferably, the arc furnace includes a three-phase step-down transformer having a primary winding electrically connected to the thyristors and the reactors, and a secondary winding connected to the electrodes. The control system comprises voltage sensors for sensing the voltages at the secondary winding, an analog-to-digital converter coupled to the sensors for digitizing the sensed voltages, a signal processor for determining the average voltage at the secondary windings from the sensed voltages, and a microcontroller for generating the appropriate gating signals for the thyristors from the determined average secondary voltages. More specifically, the signal processor removes any DC offset and noise from the digitized samples, calculates the average secondary voltages from the digitized samples each half cycle, and then outputs the calculated secondary voltages to the microcontroller. The microcontroller includes a look-up table which stores conduction angle values for delaying gating signals to the thyristors in accordance with the conduction angle values. The microcontroller receives the calculated secondary voltages from the signal processor, calculates index values from the calculated secondary voltages, and then uses the index values as address inputs to the look-up table to determine the appropriate conduction angle values for the thyristors.
In one variation, the flicker controller also includes a resistive element coupled to each power source phase and the associated electrode for retarding the flow of real current through the arc furnace when the switch is in an non-conductive state. Preferably, the resistive element comprises a resistor coupled across each thyristor through a transformer. As a result, when the thyristors are off and an electrical discharge is initially formed between the electrode and the container, current flows through the transformers, causing each resistor to appear in series with the respective power source phase and the respective electrode and a real power demand to be imposed on the power source. However, when the thyristors are on, essentially no current flows through the transformers, thereby effectively eliminating the effect of the resistors. Preferably, the magnitude of the resistors is selected so that the real power load demand imposed on the power source by the presence of the resistors when the thyristors are open is half the real power demand imposed at full load when the thyristors are closed. As a result, the power source is exposed to a stepped increase in real power demand at startup, and a stepped decrease in real power demand at shutdown.
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W.E. Staib et al., “Neural Network Conversion of the Electric Arc Furnace into the Intelligent Arc Furnce”, 1991, pp. 749-756.
Laszlo Gyugyi, et al., American Power Confer
Kojori Hassan Ali
Mulcahy Joseph A.
Rajda Janos
Tadesse Dawit
Gowling Lafleur Henderson LLP
Inverpower Controls Ltd.
Paschall Mark
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