Industrial electric heating furnaces – Arc furnace device – Power supply system
Reexamination Certificate
2002-09-09
2004-02-03
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Arc furnace device
Power supply system
C373S104000, C219S121110
Reexamination Certificate
active
06687284
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to arc-furnace equipment and more specifically to means and methods to help an electrical arc to strike thus improving productivity, reducing operating cost and reducing flicker.
BACKGROUND OF THE INVENTION
Industrial arc-furnaces are huge furnaces which are typically used to melt different metallurgical elements such as bulk iron coming from scrap. The bulk metal is melted by the intense heat radiating from a hot gas column produced between an electrode and the scrap by an electric arc. The arc-furnace is basically composed of a vat to retain the scrap and the melted metal; a set of electrodes to spark the arcs; a set of actuators to control the electrodes distance from the scrap; and a large current power supply (including a transformer equipped with a tap changer to select a voltage level) to supply the arc currents. When the melting is completed, impurities floating on the surface are skimmed or scraped from the surface and then, the liquid metal is retrieved from the vat for further processing.
Creation of an electric arc requires an ignition normally performed by making a contact between two electrodes: a cathode and an anode. The cathode then emits electrons that are accelerated towards the anode by an electric field applied between the electrodes. These electrons collide with the gas molecules within the gap to generate positively charged ions and negatively charged free electrons to form a conductive gas column between the electrodes allowing the current to flow. A gas conductive enough to allow a current to flow will be referred in this document as a plasma. As the current increases, more collisions are made and more ions and electrons are freed, thus increasing the conductivity and temperature of the plasma column. At the same time, the cathode is bombarded with more ions and heats up thus maintaining electron emission. The anode also heats up due to the impact of the incoming electrons. The emission, the bombardments and the series of collisions generate a voltage-drop that can be divided in three zones: the cathode voltage-drop; the anode voltage-drop; and the plasma column voltage-drop. An arc-furnace arc has a voltage-drop distributed, for the most part, along the plasma column. Therefore, the arc voltage-drop will mainly increase with the arc length, will diminish inversely to the plasma temperature, and will depend on the plasma gas composition.
When the furnace electric arc is interrupted, it leaves the plasma column in an initial ionized state whose lifetime is influenced by the rate of plasma temperature drop and composition. The ignition-voltage required to re-strike the electrical arc will increase with the degradation of the plasma state. If the plasma is lost, a dielectric breakdown or a temporary electrical contact will be required to recreate the plasma and restrike the arc.
The most commonly used arc-furnace is the three-phase AC current type. The furnace comprises an electrode for each phase, all disposed according to a triangular pattern in the vat. During operation, each electrode produces an arc having its other end in contact with the load of metal. All the electrodes of the AC arc-furnace are alternately anode and cathode. At each half cycle, the arc current must pass through a zero point in order to reverse. The intense heat radiating from each plasma column is proportional to the arc current and therefore will fluctuate in a synchronous manner. At the line frequency of 50 or 60 Hz and in a cold environment, there is not enough heat inertia to maintain the plasma temperature to preserve the ionized state. In this case, the plasma temperature will fluctuate according to the current flow and will affect its conductivity. This change in conductivity will then affect the voltage-drop as the current fluctuates. If we consider the state following a current peak while the arc burns in a cold environment, there will be a progressive increase of the voltage-drop at the electrodes end. This voltage-drop will rise up to the extinction-voltage value where the current reaches zero and the arc extinguishes. For the reverse arc current to ignite, the alternating voltage supply must then, in the reverse polarity, exceed the ignition-voltage, which is dependent on the plasma column ionized state (temperature) and on the anode and cathode condition. After re-ignition, as the arc current increases back, the gas column warms up again, and the voltage-drop progressively regains, in reverse polarity, a lower value equivalent to the voltage-drop of the precedent current peak. If we draw the evolution of the arc voltage, the ignition-voltage will be higher than the extinction-voltage because in between events, the plasma column has continued to cool down.
An AC arc at a frequency of 50 or 60 Hz and burning in a hot environment behaves differently. The plasma column remains hot therefore sufficiently ionized when the arc current reaches zero and extinguishes. The extinction/ignition-voltage level will be weakly affected and the evolution of the voltage-drop will show a shape between a sinusoidal and a square wave.
An AC arc-furnace works with a sinusoidal voltage power supply. In order to ignite the arc shortly after its extinction, the arc-furnace operates at a lower power factor making the voltage leading the current due to the leakage inductance in the supply path of the furnace. In many cases, a series inductance is even inserted on the primary side of the furnace transformer. Then, when the current reaches zero and the extinction occurs, there is an immediate application of the reverse polarity voltage from the supply source with the vanishing of the back emf in the inductance. If the supply voltage is higher than the ignition-voltage at this instant, the arc will strike immediately. If it is not the case, a delay will be introduced until the voltage supply catches up the ignition-voltage level. This delay introduces dead time periods in the arc current, which creates current-less time intervals. Even the amplitude of the current, as well as its RMS value is reduced in a way similar to a phase controlled dimmer. The impact on the power input of the furnace is impressive.
The behavior of the arc-furnace depends strongly on the environment in which the arc is burning. Normally, a melting process involves two phases. In the first phase, subsequent loads of scrap are poured in the vat for melting down. During that phase, the furnace operates mainly in a cold environment. The arcs are not stable as they move erratically and jump from one piece of scrap to another. Also, the continuous slipping and melting of the scrap affects the arc length and generates frequent short-circuits of the electrodes. The arc behavior continuously changes the plasma column length, which also introduces a continuous variation in the dead time period and the short circuits creates inrush currents in the furnace high current power supply. If the dead time period is prolonged, the ignition-voltage will eventually become too high for the furnace supply voltage to strike an arc and the plasma will be lost. When a complete extinction of an arc occurs, the electrode must then be moved towards the scrap to make a contact and reinitiate the arc. The touch of the contact generates a high inrush current until the electrode is moved away to have enough plasma length for the current to reduce. For the second phase of the process the arcs behave differently. The scrap is completely melted in a hot liquid bath and the arcs are burning in a hotter and more stable environment. Moreover, a foamy slag is used to improve arc stability. Contrary to the first phase, the arc length is more stable and easier to control even if the arc contains current-less time intervals.
In a DC arc-furnace, there is no change in the direction of the arc current so that only the dead time periods described above do not exist. However, in a similar way to the AC arc-furnace, the erratic movement of the arc in the first phase may stretch the arc length to a limit where the furna
Beauregard François
Francoeur Bruno
(Ogilvy Renault)
Benoît C. Marc
Centre d'Innovation sur le Transport d'Energie du Québ
Hoang Tu Ba
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