Industrial electric heating furnaces – Arc furnace device – Power supply system
Reexamination Certificate
1999-10-28
2001-02-20
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Arc furnace device
Power supply system
C373S104000
Reexamination Certificate
active
06192065
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention is concerned generally with controlling the operation of a DC arc furnace and is more particularly concerned with controlling and limiting the incidence of arcing between furnace components.
The processes which give rise to unwanted arcing between furnace components in a DC arc furnace are not readily understood nor predictable. A voltage gradient should however be established between various components of the furnace in such a way that the voltage difference between any two components is below a threshold voltage at which arcing could occur.
The threshold voltage is dependent on a number of variable parameters. Also the voltage between any two components may also be affected by operating conditions. Consequently although a particular voltage difference between two components of a furnace may not, initially, be in excess of the relevant threshold voltage the operating conditions may change in such a way that the threshold voltage is in fact exceeded and arcing occurs.
The effect of unwanted arcing can be catastrophic. It is essential therefore to detect an arc as soon as possible after it occurs and to take action to extinguish the arc.
Although the incidence of arcing can be minimised by operating a furnace at rated power with a low voltage and high current it is difficult to justify this type of approach on economic grounds. On the contrary it is desirable to operate a furnace at its rated power output with a high voltage but under operating conditions such that arcing is unlikely to occur. If however unwanted arcing does take place action must be initiated as soon as possible to limit damage and thereafter to restore the furnace to an effective operating situation so that down-time is reduced.
SUMMARY OF THE INVENTION
The invention provides a method of controlling and limiting the incidence of arcing in a DC arc furnace which includes the steps of establishing a first path with a low impedance between at least two components of the furnace, monitoring the components to detect an unwanted arc between the components and, when an arc is detected, of establishing a second path with a predetermined impedance between the components, in parallel to the arc, to extinguish the arc.
Thus the arc is extinguished by reducing the voltage across the arc.
The method may include the step of reducing the impedance of the first path from a first impedance value to a second impedance value. The second impedance value is preferably approximately equal to the said predetermined impedance.
The low impedance path may be a low resistance path.
The components may be monitored by monitoring at least one of the following: the voltage between the components; the current between the components, and any combination thereof.
The existence of an unwanted arc between the two components may be indicated by a decrease, of a predetermined magnitude, of the voltage between the components, or an increase, of a predetermined magnitude, of the current between the components, or a change in any other electrical property.
The rate of change of the voltage, or the current, or other electrical property between the two components may also be used to indicate the existence of an unwanted arc.
After the arc has been extinguished the method may include the steps of removing the second path and then increasing the impedance of the first path from the second impedance value to the first impedance value, while maintaining the voltage across the components below a threshold value at which arcing occurs.
The impedance of the first path may be increased in a gradual manner which is consistent with the establishment of a high operating power level of the furnace whilst maintaining a reduced likelihood of unwanted arcing taking place.
The second path may be open circuited when the impedance of the first path is reduced to the second impedance value. The impedance of the first path may be varied if unwanted arcing continues to occur.
The aforesaid method may be used, substantially independently, with different sets of components of the furnace. Thus the method may be used for controlling the occurrence of unwanted arcs between the electrode of the furnace and a furnace seal; between the electrode and a centre ring of the furnace; and between the electrode and one or more conductive pins which are embedded in the roof of the furnace.
Arcing may also occur between the furnace seal and the centre ring. Thus, more generally, the method may be used to provide protection against arcing between any two components even if the voltage between the components is not measured directly but is calculated in some other way.
The use of conductive pins, in the furnace roof, is described for example in the specification of South African patent application No. 97/11080 and for this reason this aspect is not further elaborated on herein.
The invention also extends to apparatus for controlling and limiting the incidence of arcing in a DC arc furnace which includes means for establishing a first path with a low impedance between at least two components of the furnace, means to detect the existence of an unwanted arc between the components, and means responsive to the detecting means for establishing a second path with a predetermined impedance between the components, when an arc is detected, to extinguish the arc.
The detecting means may be responsive to at least one of the following: the voltage between the components; the current between the components.
The detecting means may be responsive to a change, of a predetermined magnitude, in the voltage or the current between the components, as the case may be, or in both parameters. Such change is indicative of the occurrence of an unwanted arc.
The detecting means may be responsive to the rate of change of the voltage or the current or, again, of both these parameters.
The means for establishing the said first path may include switch means and an impedance of low value and high power handling capability.
The switch means may operate at a high frequency. The said low value impedance may be a variable and controllable impedance.
The apparatus may include means for increasing the impedance of the said low value impedance, in a controlled manner, so that once the arc between the components has been extinguished the voltage across the components may be increased to a value which is consistent with a high power operating level of the furnace under operating conditions which do not readily give rise to the generation of unwanted arcing.
The apparatus may be employed for controlling the occurrence of unwanted arcing between the electrode of the furnace and a seal of the furnace; between the electrode and a centre ring of the furnace; between the electrode and one or more conductive pins which are embedded in the roof of the furnace and to which reference has been made hereinbefore, and between any two other components of the furnace.
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Ferreira Jan Abraham
Wewege Anthony Wayne
Hoang Tu Ba
Jones Tullar & Cooper PC
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