Measurement of electrical variables on a DC furnace

Abstract

A method of detecting an open arc in a DC plasma arc furnace which is based on detecting a linear decrease in the log of the magnitude of the frequency spectrum of the voltage between the anode and cathode.

Claims

1. A method of detecting an open arc in a DC plasma arc furnace which includes a power supply, and an anode and a cathode connected to the power supply, the method including the steps of: a) measuring the magnitude of a voltage between the anode and the cathode over a time interval wherein the magnitude of the voltage is sampled at a sampling rate of not less than 15 kilo-samples per second; b) expressing the voltage magnitude as a function of time, over the time interval; c) deriving from the function of time a frequency spectrum of the voltage for the time interval; and d) examining the frequency spectrum of the voltage to determine a characteristic in the frequency spectrum which is indicative of an open arc in the furnace, wherein the characteristic is a substantially linear decrease in the log of the magnitude of the frequency spectrum as a function of the log of the frequency.

2. The method according to claim 1, wherein the slope of the linear decrease is of the order of −2.

3. The method according to claim 1, wherein steps (a), (b), (c) and (d) are repeated in each of a plurality of successive time intervals.

4. The method according to claim 3, wherein in each time interval the magnitude of the voltage is sampled to detect fluctuations in the magnitude of the voltage which are due, at least, to the establishment of an open arc in the furnace.

5. An apparatus for detecting an open arc produced by an electrode in a DC plasma arc furnace which includes a power source and an anode and a cathode which are connected to the power source, the apparatus including a circuit for measuring the magnitude of a voltage between the anode and the cathode over a time interval wherein the circuit is adapted to sample the magnitude of the voltage at a sampling rate of not less than 15 kilo-samples per second, a processor which produces a frequency spectrum of the voltage over the time interval and a discriminator which examines the frequency spectrum of the voltage to determine a characteristic in the frequency spectrum which is indicative of an open arc in the furnace, and wherein the characteristic is a substantially linear decrease in the log of the magnitude of the frequency spectrum as a function of the log of the frequency.

6. The apparatus according to claim 5 in combination with a hoist which is operable in response to an output signal of the discriminator to adjust the position of an electrode, which is connected to the cathode, in the furnace.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is further described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1 illustrates a DC plasma arc furnace with apparatus for carrying out the method of the invention;

(3) FIG. 2 illustrates voltage magnitude samples plotted as a function of time; and

(4) FIG. 3 is a Fourier spectrum on a log basis of the voltage values shown in FIG. 2 versus frequency, with a gradient fit.

DESCRIPTION OF PREFERRED EMBODIMENT

(5) FIG. 1 of the accompanying drawings illustrates a DC plasma arc furnace 10, and components which are used to implement the method of the invention.

(6) The DC arc furnace, which is of conventional construction, includes a vessel 12 with an internal operating volume 14. The vessel has a refractory lining 16. An anode 18 is provided in a base of the vessel. The anode is connected to a DC power supply 20 which is controlled, using various techniques, by a control unit 22.

(7) A negative terminal of the power supply is connected to clamp 26 which is coupled to an elongate graphite electrode 30 which extends downwardly, through an opening 34 in roof 36 of the vessel, into the operating volume 14.

(8) The invention is described herein with reference to a furnace which makes use of a single electrode. This however is by way of example only. The principles of the invention are fully applicable to a furnace which includes two or more electrodes.

(9) At least one feed port 38 is formed the roof. Material to be smelted is introduced into the volume 14 through this port.

(10) In operation of the smelter an arc 40, produced in the operating volume between the feed material and tip 30A of the electrode, is used to smelt the material. Typically this causes the feed material to form a metal bath 42 at a base of the vessel and an overlying slag bath 44.

(11) As the electrode is gradually consumed during operation it is moved by means of an electrode hoist 48 to effect a change in the position of the electrode tip inside the operating volume 14, with the intention of keeping the arc 40 in an optimum state.

(12) The various components which have been referred to and described are conventional and are given by way of background only.

(13) The position of the tip 30A of the electrode and conditions inside the operating volume 14, are not easily visually discernible to an operator. What can occur, under these conditions, is that the process is operated with at least the electrode tip 30A in direct contact with the molten bath 44. In this instance the arc is extinguished and power is supplied to the furnace by resistance heating of the molten bath. This may be an undesirable situation.

(14) If the interior of the operating volume 14 were visible externally of the vessel then it would be possible, visually to ascertain the existence of the arc 40. However, the temperature in the operating volume reaches a high value, in excess of 1500° C. during operation of the furnace. Large quantities of dust and fumes are present and obscure vision. Also, high levers of ultraviolet radiation exist in the volume. Visual determination of the existence of the arc 40 is thus not reliably possible. Additionally, other techniques which might possibly be considered and which are based, for example, on the use of vibration transducers, microphones etc., cannot reliably and safely be resorted to. Also if these approaches were technically feasible the likelihood is that they would be complex and expensive.

(15) To address the aforementioned situation and to enable the existence of an open arc in the operating volume to be detected the invention provides the various additional components shown in FIG. 1.

(16) A voltage measurement module 50 is connected between the cathode and the anode. The module is constructed using high voltage insulated cables and strategically placed surge arresters to provide protection to sensitive elements during the measurement process.

(17) It is possible to monitor the voltage across the furnace continuously. However, satisfactory data can be obtained by sampling the magnitude of the voltage, but at a rate which is high enough to be able to measure fluctuations which are attributable to the existence of an open arc 40 in the electric circuit. Typically a sampling rate of at least 15 kilo-samples per second is used.

(18) The magnitude of the furnace voltage is sampled at the chosen rate over a time interval of a defined duration. The samples of the magnitude of the voltage are fed to a recorder and processor 52 which has an output connected to a further processor 54 for carrying out a Fast Fourier Transform on a basis which is described hereinafter. A discriminator 56 is used to examine the output of the processor 54.

(19) FIG. 2 is curve, derived from experimental processes, to represent the magnitude of the furnace voltage as a function of time. Under laboratory conditions a test was carried out using an arc, established in open air, between a graphite electrode and an anode surface. The voltage was sampled at a rate of 15 kilo-samples per second over a time interval of defined duration T for a situation in which an arc is struck at the beginning of the interval T. The arc was thereafter maintained between the tip of the graphite electrode and the anode surface. The existence of the arc was confirmed visually using a high speed camera, positioned at a safe distance of about 10 meters from the experimental area. The duration of the time interval T is typically of the order of 10 milliseconds.

(20) The data produced by the sampling process is, as noted, recorder in the component 52.

(21) The processor 54 is used to implement a Fast Fourier Transform function, thereby to produce the frequency spectrum the furnace voltage.

(22) FIG. 3 illustrates a log value of the furnace voltage (on a vertical axis) as a function log value of the frequency spectrum (on a horizontal axis). Over a frequency range of from about 500 Hz to 5 kHz the magnitude of the frequency spectrum drops off at rate of about 2 decades of magnitude for each decade of frequency. It is possible that the frequency spectrum may continue to drop off beyond about 5 kHz but this has not yet been experimentally determined.

(23) A fitted gradient line 60 in FIG. 3 has a slope of about −2 and represents a falloff in the magnitude of the furnace voltage of about 2 decades per decade.

(24) The discriminator 56 is capable of recognizing the negative slope referred to. By the use of appropriate control techniques, and in response to an output from the discriminator 56, the control device 48 is capable of automatically manipulating the position of the electrode 30 to maintain the open arc 40 inside the operating volume 14. It is also possible to make use of the information produced by the apparatus to provide one or more guidance parameters which are usable by an operator in manual or semi-automatic control of operating aspects of the furnace.