Abstract:
At least one measurement value of a measurement variable characterizing the operating state of each of a plurality of system components that influence the operating conditions of an arc furnace is detected and compared to a respective currently permissible threshold value for the measurement variable. A maximum power that can be supplied to the arc furnace within a time window while satisfying all currently permissible threshold values is determined based on the result of the comparison.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. national stage of International Application No. PCT/EP2013/055951, filed Mar. 21, 2013 and claims the benefit thereof. The International Application claims the benefit of European Application No. 12163722 filed on Apr. 11, 2012, both applications are incorporated by reference herein in their entirety. 
     
    
     BACKGROUND 
       [0002]    Described below are a method for operating an electric arc furnace and a smelting plant with an electric arc furnace operated in accordance with this method. 
         [0003]    In an electric arc furnace, pieces of material to be smelted, in general steel scrap, if necessary together with further supplementary materials, are melted down by an electric arc which is struck between the material to be smelted and at least one electrode. For this purpose, electrical energy, which is converted into the low voltage range by a so-called furnace transformer, is fed to the electric arc furnace from a medium- or high-voltage supply. From this furnace transformer, the energy is fed to the electrode via a high current system. The scrap metal present in the furnace vessel is melted down by an electric arc which then burns at the tip of the electrode. 
         [0004]    The plant components which relate to the supply of electrical energy for the electric arc furnace include a so-called furnace substation, a furnace transformer and a high current system which, for example, incorporates current-conducting height-adjustable supporting arms, to which are attached the electrodes. The height setting of the electrodes, and thereby the energy input into the goods to be smelted, is regulated by an electrode regulation system. This produces a wide selection of working points, or operating conditions, which can differ substantially in terms of the performance demands on the plant components. 
         [0005]    The design and selection of the plant components is generally effected on the basis of values from experience for the electric arc furnace concerned. Thus, for example, the design of the furnace transformer is determined as a function of the size of the electric arc furnace, the nature of the steel scrap and the desired productivity, i.e. the smelting capacity (mass of the steel scrap to be melted down per unit time), which determines the required smelting time of the electric arc furnace. In doing this, apart from determining the complex power in the range up to about 300 MVA, the number of stages and the voltages of the stages in the furnace transformer, for example eighteen stages with voltages of up to about 1500 V, and the currents, which can be up to about 100 kA, are defined. 
         [0006]    However, in order to reliably avoid damage to those plant components which affect the operating conditions of the electric arc furnace, in particular the furnace transformer, the potential power which is actually available, at least for short periods, is not utilized to the full extent possible, and correspondingly allowance is made for a reduced productivity. 
       SUMMARY 
       [0007]    Described below are a method for the operation of an electric arc furnace with which it is possible to increase the productivity of an electric arc furnace and a smelting plant with an electric arc furnace operated in accordance with this method. 
         [0008]    For each of a plurality of the plant components which influence the operating conditions of the electric arc furnace, at least one measured value is sensed for a measurement variable which characterizes its operating status, and is compared with a currently permissible limiting value for the measurement variable concerned, and by reference to the result of this comparison a time window is determined, and a maximum power which may be fed to the electric arc furnace within this time window while adhering to all the currently permissible limiting values. 
         [0009]    These measures make it possible to operate an electric arc furnace so as to exploit its optimal productivity, because a loading range which is currently available can be utilized in order to have more power available for melting down, at least for a short period, i.e. within a time window which is shorter than the smelting time (time between the first charging of the electric arc furnace and the tapping off of the melt), or to compensate for unplanned production losses, without for example damaging the furnace transformer. 
         [0010]    The time window is determined automatically, making use of values from experience, where the length of the time window and the maximum power which can be fed in are generally dependent on each other, thus opening up for the operator the option of operating the electric arc furnace with a low maximum power and long time window or a high maximum power and a short time window. As an alternative to this, the duration of the time window can also be undefined, its end being determined by operating the electric arc furnace at the power which has been determined as the maximum when a currently permissible limiting value for a measurement variable at one of the plant components is reached. 
         [0011]    Because the plant components can be optimally exploited, in respect of their limiting values, for the purpose of achieving a prescribed productivity, the plant components in new plants can be designed so that they are optimized for the requirements, thus avoiding the cost of overdimensioning them. 
         [0012]    Here, the currently permissible limiting value can be either a predefined fixed and unalterable value, for example the voltage endurance of an electrical component of the plant, or a value which varies with time, dependent on the current operating conditions of the electric arc furnace, as sensed by the measured values. An example of such a variable value could be the maximum power of the furnace transformer which, within a time window, can exceed a predefined basic value by a maximum possible predetermined value provided that another currently permissible limiting value, for example a temperature measured at the furnace transformer, is not exceeded. 
         [0013]    A particularly safe determination of the maximum available load range is achieved if the maximum power which can be fed in and/or the length of the time window is determined with the assistance of a prediction of the course over time of at least one of the measurement variables. This ensures that the plant component is not overloaded within the time window, so that a failure arising from an overload is avoided. 
         [0014]    The measurement variables which are sensed may be an electric current flowing through the plant component, an electrical voltage across the plant component and/or a temperature of the plant component. 
         [0015]    In a further embodiment, a piece of auxiliary equipment which indirectly influences the operating status of a plant component, such as in particular cooling equipment through which a cooling fluid flows, is controlled in that the input temperature and/or the throughput of the cooling fluid is regulated to maintain for the plant component a temperature, either currently defined or permanently prescribed as a limiting value. 
         [0016]    In a further embodiment of the method, the limiting value is a temperature of the plant component, wherein foaming slag is used as thermal screening to protect the plant components against the radiation from the electric arc. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The properties, characteristics and advantages described above, and the manner in which these are achieved, will become more clearly and more obviously comprehensible in conjunction with the following description of the exemplary embodiments, which are explained in more detail in conjunction with the accompanying drawings of which 
           [0018]      FIG. 1  is a schematic block diagram of a smelting plant with an electric arc furnace, 
           [0019]      FIG. 2  is a graph in which the power fed to the smelter is plotted against time. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0021]    As shown in  FIG. 1 , the electrical energy required for operation of the smelting plant is fed in from a cable or an overhead line  2  through a high voltage supply switch  4  to a high voltage busbar  6 . The high voltage across the high voltage busbar  6  is fed via a high voltage output switch  8  to a step-down transformer  10 , which transforms the high voltage to a medium voltage. The secondary side of the step-down transformer  10  is connected electrically via a medium voltage supply switch  12  to a medium voltage busbar  16 . The voltage across the medium voltage busbar  16  is fed via an output switch  18 , a furnace switch  20  and via a pre-furnace choke  22  to the primary side of a furnace transformer  24 , the secondary side of which is connected via a high current catenary  26  to an electrode  28  of an electric arc furnace  30 . The electrode  28  is arranged on a supporting column  32  so that its height can be adjusted, to enable the length of the arc which is burning between the electrode  28  and the goods to be smelted  34 , and correspondingly the energy input into the electric arc furnace  30 , to be adjusted. 
         [0022]    The plant components shown in  FIG. 1 , which are involved in the electrical supply to the electrode  28 , here represent only a selection by way of example, and do not include all the plant components actually present in practice. 
         [0023]    Apart from the electrical components of the plant, the smelting plant includes also non-electrical plant components, which are again illustrated in  FIG. 1  by way of example and not exhaustively, by cooling equipment  36 ,  38  and  40 , which effect cooling respectively of the step down transformer  10 , of the furnace transformer  24  or of wall panels of the electric arc furnace  30 . 
         [0024]      FIG. 1  also illustrates foaming slag  41 , with which the thermal radiation from the arc L can if necessary be screened off, in order to reduce the thermal load on the components located in the immediate vicinity of the arc L. 
         [0025]    Both on the primary side of the step down transformer  10  and also on the primary and secondary sides of the furnace transformer  24 , measurement equipment  42  and  44  is provided for the purpose of measuring the current or the voltage, as appropriate. 
         [0026]    Other measurement equipment which is illustrated is temperature measurement equipment  46 , with which the temperature at different thermally loaded places in the smelting plant can be measured directly as measurement variables, or can be determined indirectly on the basis of a thermal model. By way of example, but not an exhaustive list, such temperature measurement equipment  46  is shown on the step down transformer  10 , on the supply cabling system, on the primary and secondary sides of the furnace transformer  24 , and on the wall of the furnace vessel and on the electrode  28 . 
         [0027]    Over and above this, the input and exit temperatures of the coolant flowing through the cooling equipment,  36 ,  38  and  40 , are also sensed, together with its throughput. 
         [0028]    In principle, measurement equipment  48  can also be provided with which mechanical measurement variables, for example vibrations of the supporting arm on the supporting column  32 , are sensed. 
         [0029]    The measured values, M i , i=1 to n, which are sensed by the measurement equipment  42 ,  44 ,  46 ,  48  for the relevant electrical, thermal or mechanical measurement variables, as applicable, are fed—as shown symbolically by the dashed arrows which have been drawn in—to a control and analysis facility  50 . In the control and analysis facility  50 , the relevant permissible limiting values for the measurement variables in the plant components which are being monitored are held in the form of a look-up table or a dynamic model. These permissible limiting values can be permanently predefined for certain plant components, but can in addition also be dependent on the measured values of other measurement variables, in particular on the same plant component. Thus, for example, the permissible limiting value for the power transmitted by the furnace transformer  24  can be a function of its temperature, and can reduce with increasing temperature. The temperature of the furnace transformer  24  can in turn be influenced by the input temperature and the throughput of the coolant through the cooling equipment  38 . In addition, this permissible limiting value can also be further dependent on the length of the time window within which the working point is to be adjusted to this limiting value. Thus, for example, the shorter is the time window, the higher is the permissible limiting value for a measured temperature. 
         [0030]    By comparing the measured values which have been measured against the limiting values which have been determined as currently permissible, if necessary taking into consideration the measured values themselves, a time window and the maximum power which can be fed to the electric arc furnace  30  within this time window are now determined. These are, for example, displayed in the control center to a user, who is thus in a position to increase the productivity, if necessary, while the analysis and control facility  50  will in addition block any breach of this maximum power even if manual control is being exercised. As an alternative to this, an automated way of running can be provided by which, within the time window which has been determined, the electric arc furnace  30  is automatically operated with the maximum power which can be fed in. 
         [0031]    Here, it is important for operational safety that the currently permissible limiting values are not exceeded for any of the plant components. If, for example, the operation of the smelting plant at the maximum power which can be fed to it leads to the currently permissible limiting value being reached for one plant component, the control facility  50  automatically generates for the smelting plant control signals S k , k=1 to m which have the result that this limiting value is not exceeded. For example, if the thermal load on the electrode  28  reaches a currently applicable limiting value even though the furnace transformer is being operated with a power which does not exceed the permissible limiting value which applies for the measured temperature and the time window which is set, then either this power is reduced in order to prevent the currently permissible limiting value for the temperature of the electrode  28  being exceeded or, for example, measures are initiated which effect a more efficient cooling of the electrode  28 . 
         [0032]      FIG. 2  shows, in a simplified diagram, one possible way of running when the electric arc furnace is in operation. In this diagram, the power P which is fed in is plotted against the time t. From the diagram it will be seen that, within several short time windows Δt 1 , Δt 2 , Δt 3 , Δt 4 , the durations of which are shorter than the smelting time, the power P fed to the electric arc furnace is significantly above a basic power P 0  which would be possible for continuous operation. In these time windows Δt i , the currently available limiting values are determined by reference to the current operating conditions, i.e. the measured values currently sensed by the measurement equipment, from which are deduced the implied maximum possible powers within these time windows Δt i . Here, these time windows Δt i  are either determined and appropriately prescribed by the analysis and control facility, on the basis of the current operating state and/or an expected course of the changes in this operating state, or are defined by an abort criterion, for example the reaching of a currently permissible limiting value. 
         [0033]    Although the details of the invention have been illustrated and described in more detail by the exemplary embodiments, the examples disclosed do not thereby restrict the invention, and other variations can be deduced from it by a person skilled in the art without going outside the scope of protection of the spirit and scope of the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 358 F3d 870, 69 USPQ2d 1865 (Fed. Cir. 2004).