Controlled current feed device for electric arc furnace

Abstract: Controlled current feed device for electric arc furnace employed to melt metals, wherein the feed line comprises at least a medium tension bar (13), a feed control device and a transformer (17) serving the furnace, the feed control device (16) being arranged between the medium tension bar (13) and the transformer of the furnace (17) and comprising a mutual inductor (19) consisting of a primary coil (19a) and a secondary (19b) coil, the primary coil (19a) being arranged in serial connection on the feed line (10) and the secondary coil (19b) being connected to at least a re-phasing filter (20) comprising at least a condenser (22).

Description: FIELD OF THE INVENTION This invention concerns a controlled current feed device for electric arc furnaces as set forth in the main claim. The invention is applied in the field of electric melting furnaces to produce steel or alloys thereof, in order to guarantee minimum disturbances on the mains supply (flicker) caused by peaks of load absorption and phase displacements on the line due to the strongly inductive characteristics of the loads to be supplied. BACKGROUND OF THE INVENTION In recent years there has been a considerable increase in the power supplied to electric arc furnaces employed for melting metals, passing from unitary powers of 16 MW and 20 MVA to powers of more than 85 MW and over 120 MVA. These high powers create serious problems of disturbance in the tension (flicker) for the mains supply, and also considerable phase displacements due to the inductive characteristics of the loads. There are various techniques known to the state of the art which are used to reduce such tension fluctuations and/or to re-phase the inductive loads. Some examples of these techniques are shown in the article by L.C. Elliott "Electric Power Systems for large Arc Furnaces", WESTINGHOUSE ENGINEER, vol. 29, December 1969, pages 143-149, in the article by Yngve Sundberg "The arc furnace as a load in the network", ASEA journal, vol. 49, 1976, pages 75-87, in the article by Richard F. Dudley "Special Design Considerations ...", IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, vol. 33, n°. 1, January 1997 - February 1997 and in the patent US-A-3567837. A first technique is to place a stationary inductor on each branch of the feed line of the furnace, with the function of limiting the maximum current absorbed by the furnace. This solution has the advantage of being simple and economical, but has great limits because it cannot be regulated and cannot therefore be adapted to the variable conditions which occur during the melting cycle. In co-operation with these stationary inductors, there are re-phasing filters on the medium tension bar which generally consist of a condenser and an inductor in serial connection; these have the function of re-phasing and filtering the harmonics introduced onto the line by the furnace and by the feed control system. Another solution provides to arrange on the feed line, parallel to the re-phasing filters, a system of inductors which can be inserted by means of thyristors. When the inductive-type power required by the furnace varies, the thyristors insert the system of inductors into, or remove said system from, the line, so as to compensate this variation. This adjustment system balances the inductive component, and maintains at a low value the total reactive power employed by the furnace, the reactors and the series of re-phasing filters. This solution has a certain flexibility and versatility, but it also has the disadvantage that it is very expensive and complex to manage, and that it introduces harmonics into the line. Moreover, the active power of the arcs of the furnace is adjusted only by varying the height of the electrodes by means of appropriate hydraulic units, trying to maintain the resistances of the arcs themselves constant. This entails long response times and limited accuracy due to the mechanical sizes being acted on. Another technique is to act directly on the current of the arc of the furnace so as to determine the working point and reduce the disturbances. With this technique, a saturable reactor is arranged in serial connection on the feed line of the furnace which, when excited by an appropriate continuous current, has the characteristic that it has a low reactance value for small values of current required by the furnace and a high value of reactance for high values of current. Therefore, once the working point has been set by choosing the polarisation current, the saturable reactor automatically limits the overcurrents, with a consequent reduction of flicker on the mains supply. This solution has the advantage that it does not need a complicated control system and is potentially valid in that it acts directly on the electric sizes feeding the furnace, limiting drops in tension caused by the unstable functioning of the arcs of the furnace. However, the inclusion of saturable or dividable inductors on the feed line introduces a further lack of linearity into the system in addition to that caused by the furnace itself, and therefore causes a high number of harmonics to be generated and introduced into the line, with the consequent negative effects. Moreover, these systems are expensive and burdensome to install and manage. The present applicant has devised, tested and embodied this invention to overcome all these shortcomings with a simple but at the same time very efficient solution. SUMMARY OF THE INVENTION The invention is set forth and characterised in the main claim, while the dependent claims describe other characteristics of the main embodiment. The purpose of the invention is to provide a controlled current feed device for an electric arc furnace with the following characteristics: maximum efficiency in reducing flickering on the mains supply due to instantaneous drops in tension caused by the absorption peaks of the furnace; substantially zero response times; lower costs than the solutions described above (controlled inductors with thyristors and saturable reactor) and not much more than the solution with a single stationary reactor located in serial connection on the feed line; it exerts an automatic re-phasing action of the load with respect to the feed line; it does not generate its own harmonics in the system since there are no non-linear components; therefore it is not necessary to provide additional filters besides those required by the furnace; it can be installed almost universally on any pre-existing plant; reduced size with the same effect and ease of installation and management as solutions with, for example, a saturable reactor; requires fewer manoeuvres of the tap-changer compared with solutions with stationary or divided reactor. The invention provides to locate on the feed line of the furnace a mutual inductor, whose secondary coil is connected with at least a re-phasing filter comprising at least a condenser. In the preferential embodiment of the invention there are two or three re-phasing filters arranged in parallel. In a preferential but non-restrictive embodiment, each re-phasing filter comprises a condenser and an inductor in serial connection. According to a variant, the mutual inductor is of the type with an air gap. The mutual inductor has two functions. A first function is obtained due to the fact that the primary coil, due to the effect of the air gap and the consequent dispersion inductance, performs the known function of fixed reactance in serial connection with the furnace. A second function is obtained due to the fact that, as the current required by the furnace varies, the mutual inductor acts on the re-phasing filters and gives rise to a variation of equal sign of the reactive capacitive power. In other words, if the current required by the furnace increases, due to the effect of the mutual inductor the tension on the condensers also increases in a correlated manner, and consequently the capacitive component of the reactive power transmitted on the line also increases. In this way there is an automatic compensation, caused by the capacitive component, of the induction-type reactive power required by the furnace. The percentage of compensation can be adjusted by acting on the size of the mutual inductor and/or the re-phasing filters. In this way the invention ensures an automatic and immediate adjustment, with a circuit which is linear and which therefore does not introduce its own harmonics, and which furthermore exerts an automatic re-phasing function on the reactive power, compensating the variations in the inductive component in the power with correlated variations in the capacitive component. BRIEF DESCRIPTION OF THE DRAWING The attached Figure is given as a non-restrictive example and shows a preferential embodiment of the controlled current feed device for electric arc furnace according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows, in a simplified form, a diagram of the feed line, indicated generally by the reference number 10, of a three-phase electric furnace 11. The Figure represents one of the branches of the three-phase furnace 11, but it is understood that this representation is valid for each of the three branches of the furnace 11. The reference number 12 indicates the high tension bar which is connected to the medium tension bar 13 through a mains transformer 14. Reference number 15 denotes the equivalent network impedance. The medium tension bar 13 is connected to the furnace 11 through the device 16 according to the invention, which is located upstream of the furnace transformer 17. Number 18 denotes the equivalent inductive-type impedance of the furnace 11. The device 16 consists of a mutual inductor 19 whose primary coil 19a is located directly in serial connection on the feed line of the furnace 11 and whose secondary coil 19b is connected to a series of re-phasing filters 20, respectively 20a, 20b and 20c, which are arranged parallel and each of which comprises at least a condenser 22. In the embodiment shown here, there are three re-phasing filters 20, but it remains within the scope of the invention if one, two, four or more re-phasing filters 20 are provided. Moreover, in the embodiment shown here, the re-phasing filters 20 consist of the condenser 22 and a relative inductor in serial connection 23, but it remains within the scope of the invention if other components associated with the condenser 22 are provided. In the preferential embodiment of the invention, the core 21 of the mutual inductor 19 is of the type with an air gap. With the device 16 according to the invention, every fluctuation in the current on the feed line of the furnace 11 produces a concomitant variation of the same sign as the capacitive component of the reactive power, due to the automatic variation in the tension applied to the re-phasing filters 20a, 20b and 20c, and in particular to the respective condensers 22. This derives from the fact that the current flowing in the secondary 19b and the current flowing in the primary 19a are of opposite signs, as indicated by the symbols in the Figure. Therefore, there is an automatic increase in the re-phasing of the load without any harmonics being introduced, wherein every increase in the inductive component of the reactive power required by the furnace 11 is automatically compensated by a correlated increase in the capacitive component due to the re-phasing filters 20a, 20b and 20c. Due to the air gap in the core 21, the primary coil 19a of the mutual inductor 19 also performs the function of a serial reactor located on the feed line 10 of the furnace 11 because of the dispersion inductance. In a variant which is not shown here, the mutual inductor 19 is of the type with connectors applied to the primary coil 19a and/or the secondary coil 19b, which can be selected as an alternative. In this way, by adjusting the working parameters it is possible to make the device 16 more flexible and better able to adapt to those anomalous situations which occur in electric furnaces in a melting cycle. It should also be noted that the action of the mutual inductor 19 is specific and autonomous for each of the three arcs, which guarantees a balance in the feed to the three branches of the three-phase circuit even in the event of an asymmetrical behaviour of the furnace; it therefore also ensures regular and uniform functioning with respect to the feed line.

Claims: Controlled current feed device for electric arc furnace employed to melt metals, wherein the feed line comprises at least a medium tension bar (13), a feed control device and a transformer (17) serving the furnace, the device being characterised in that the feed control device (16) is arranged between the medium tension bar (13) and the transformer of the furnace (17) and comprises a mutual inductor (19) consisting of a primary coil (19a) and a secondary (19b) coil, the primary coil (19a) being arranged in serial connection on the feed line (10) and the secondary coil (19b) being connected to at least a re-phasing filter (20) comprising at least a condenser (22). Device as in Claim 1, characterised in that the secondary coil (19b) of the mutual inductor (19) is connected to two or three re-phasing filters (20a, 20b, 20c) arranged parallel to each other. Device as in Claim 1 or 2, characterised in that each re-phasing filter (20a, 20b, 20c) comprises an inductor (23) arranged in serial connection with the relative condenser (22). Device as in Claim 1 or 2, characterised in that the core (21) of the mutual inductor (19) is of the type with an air gap and the primary coil (19a) performs the function of a stationary reactor in serial connection on the feed line (10). Device as in any claim hereinbefore, characterised in that the mutual inductor (19) is associated with a plurality of sockets which can be selected as alternatives and have the function of adjusting the working parameters of the control device (16).

Inventors: DELLA VEDOVA FERRUCCIO, FRAGIACOMO RICCARDO, GENSINI GIANNI, DELLA VEDOVA, FERRUCCIO, FRAGIACOMO, RICCARDO, GENSINI, GIANNI
Applicants: DANIELI AUTOMATION SPA
Application year: 1999
Main classification: H05B7
IPC codes: H05B7