Flicker-free control of an arc furnace using a multilevel converter

Abstract

Electrodes (1) of a three-phase arc furnace (2) are supplied with electric energy from a supply network (5), comprising a plurality of input-side phases (6), by means of a multilevel converter (4) via a furnace transformer (3). A corresponding control device (15) receives measurement values for output-side phase currents (I1, I2, I3) flowing on the output side of the multilevel converter (4). Additionally, the present output-side target output (P*) for each current point in time is known to the control device (15). The control device (15) ascertains output-side target phase voltages (U1*, U2*, U3*) using the output-side phase currents (I1, I2, I3) and the output-side target output (P*) for the respective current point in time such that the sum of the products of the output-side target phase voltages (U1*, U2*, U3*) and output-side phase current (I1, I2, I3) target values (I1*, I2*, I3*), which are ascertained using the output-side phase currents (I1, I2, I3), equals the present output-side target output (P*). The control device (15) controls the multilevel converter (4) such that the multilevel converter (4) provides output-side phase voltages which correspond to the output-side target phase voltages (U1*, U2*, U3*). The control device repeatedly carries out the aforementioned steps with a cycle time (TZ).

Description

[0001] Description

[0002] Title of the invention

[0003] Flicker-free control of an arc furnace using a multilevel converter

[0004] field of technology

[0005] The present invention is based on a control method for a multilevel converter which supplies electrodes of a three-phase arc furnace with electrical energy via a furnace transformer from a supply network with several input-side phases, wherein a control device of the multilevel converter receives measured values ​​for output-side phase currents which flow on the output side of the multilevel converter.

[0006] The multilevel converter is therefore located between the supply network and the furnace transformer, and the furnace transformer is located between the multilevel converter and the electrodes. The electrical energy flows from the supply network to the multilevel converter, from there to the furnace transformer, and from there finally to the electrodes. This creates arcs between the electrodes and the scrap or molten metal, in which the electrical energy is converted into heat. The supply network typically carries a medium voltage of 11 kV, 30 kV, 33 kV, or 110 kV.

[0007] The output-side phase currents, the measured values ​​of which are received by the control device, can be the currents on the primary side or the secondary side of the furnace transformer, as required, with a primary-side measurement being preferred.

[0008] The present invention further relates to a control program for a control device for controlling a multilevel converter which supplies electrodes of a three-phase arc furnace with electrical energy via a furnace transformer from a supply network with a plurality of input-side phases, wherein the control program comprises machine code which can be directly processed by the control device, wherein the processing of the machine code by the control device causes the control device to execute such a control method.

[0009] The present invention further relates to a control device for controlling a multilevel converter that supplies the electrodes of a three-phase arc furnace with electrical energy via a furnace transformer from a supply network with multiple input-side phases, wherein the control device is programmed with such a control program so that the control device executes such a control method during operation. The present invention further relates to a multilevel converter that supplies the electrodes of a three-phase arc furnace with electrical energy via a furnace transformer from a supply network with multiple input-side phases, wherein the multilevel converter is controlled by such a control device.

[0010] State of the art

[0011] The above-mentioned objects are known, for example, from EP 2 329684 B1.

[0012] EP 3 124 903 A1 discloses a power supply device that supplies the electrodes of an arc furnace with electrical energy from a supply network with multiple input-side phases. The number of electrodes can be greater than 1. The electrodes form a three-phase system. Within the scope of the control method of EP 3 124 903 A1, a control device of the power supply device receives, among other things, measured values ​​for phase currents flowing on the output side of the power supply device. Furthermore, a target power can be known to the control device. The control device determines a target current value that ensures stable operation of the arc in the arc furnace. The control device can also determine a target voltage value.To determine the setpoints, the control device uses at least the current and voltage provided by the grid and the current and voltage flowing or present on the output side of the power supply. The control device determines the reference voltage as a function of the setpoint current, so that the power supplied to the electrodes follows the setpoint.

[0013] The paper "Negative Voltage Sequence Control for an Electric Arc Furnace Power Supply based on a Multilevel AC-AC Converter" by Andrea Volpini and Samuele Granata, IEEE Energy Conversion Congress and Exposition 2023, pages 2817 to 2824, describes a multilevel converter that supplies electrical energy to the electrodes of a three-phase arc furnace via a furnace transformer from a supply network with multiple input phases. The power supply to the electrodes of the arc furnace is provided via a furnace transformer. This paper presents a new approach for a control method.

[0014] Summary of the invention

[0015] In an electric steel mill, scrap is melted using an electric arc furnace. Several (usually three) electrodes are immersed in a container filled with scrap, and arcs are then generated between the scrap and the electrodes, heating and melting the scrap. The voltages applied to the electrodes typically range from several hundred volts to just over 1 kV, sometimes up to 2 kV. The power of an industrial electric arc furnace is often between 50 MW and 300 MW.

[0016] The constantly flickering arc causes voltage fluctuations and—if no countermeasures are taken—significant grid disturbances. Historically, it has been common practice to connect compensators in parallel with the arc furnace, such as a so-called SVC (static VAR compensator) or a so-called STATCOM (static compensator). Such compensators are large, heavy, and expensive. Furthermore, they generate power losses and also affect other units connected to the supply grid.

[0017] EP 2 329684 B1 discloses feeding the arc furnace via a multilevel converter located on the input side of the furnace transformer. Such a converter makes it possible to regulate the active power consumed by the electric arc furnace in addition to precisely adjusting the reactive power on the input side. The cited EP document states that, beyond the symmetrical loading of the phases of the supply grid with active power, the effects of the arc furnace phases on the supply grid are minimized. The exact method by which this is achieved is not disclosed in the cited EP document.

[0018] The object of the present invention is to provide precisely such possibilities.

[0019] The object is achieved by a control method having the features of claim 1. Advantageous embodiments of the control method are the subject of dependent claims 2 to 7.

[0020] According to the invention, a control method of the type mentioned at the outset is created in which the control device of the multilevel converter - in addition to the already mentioned receipt of the measured values ​​for the output-side phase currents - repeatedly carries out the following steps with the cycle time:

[0021] - the control device is informed of a current output target power for a given time,

[0022] - the control device determines output-side target phase voltages on the basis of the output-side phase currents and the output-side target power for the respective current point in time, so that the sum of the products of the output-side target phase voltages and the target values ​​for the output-side phase currents determined on the basis of the output-side phase currents is equal to the current output-side target power, the control device controls the multilevel converter in such a way that the multilevel converter provides output-side phase voltages corresponding to the output-side target phase voltages.

[0023] Within the scope of the present invention, the output-side phase voltages are thus provided in such a way that the instantaneous output-side power is equal to the desired instantaneous output-side target power. The instantaneous output-side power is determined by the sum of the products of the output-side phase voltages and the output-side phase currents. The respective sign of the respective phase voltage and phase current is also taken into account.

[0024] Within the scope of the procedure according to the invention, the instantaneous output-side power is regulated as it results for the respective time from the sum of the products of the output-side phase voltages and the output-side phase currents.

[0025] This approach contrasts with prior art approaches, where control appears to occur over several periods of the grid frequency. This is particularly indicated by the terms "active power" and "reactive power," which are repeatedly used in the prior art—including in EP 2 329 684 B1. For this reason, reactive power compensators continue to be present in the prior art. Within the scope of the present invention, however, such compensators can be completely omitted. Furthermore, a control system is possible that can compensate for disturbances very quickly—within 1 ms or less.

[0026] The current target power can be known to the control system as a corresponding power curve. The power curve can be constant, but can also vary. If it does vary, the target power should preferably vary only slowly.

[0027] Over a full period of the grid frequency, the shares will generally be equal on average. However, a different distribution is also conceivable.

[0028] The control of the multilevel converter is therefore advantageously carried out in such a way that the required input voltages for the furnace transformer are obtained and the multilevel converter draws the respective portions of the active power (and usually only the active power) from the supply network from the input-side phases.

[0029] Controlling the multilevel converter so that it provides predefined phase voltages on the output side is easily possible. The options for such control are known to those skilled in the art. The decisive factor here is determining the phase voltages on the output side.

[0030] Preferably, the multilevel converter is designed as a DC link converter with an input-side rectifier and an output-side inverter. In this case, the control device controls the inverter in such a way that the inverter provides the output-side phase voltages. Thus, the inverter control can be determined virtually independently of the rectifier control.

[0031] Preferably, the control device also knows the proportions of the current target power for the input-side phases at the current time. In this case, the control device can control the multilevel converter in such a way that the multilevel converter draws input-side power from the input-side phases according to the proportions known to it. If the multilevel converter is divided into an input-side rectifier and an output-side inverter, this specifically concerns the corresponding control of the rectifier.

[0032] Thus, in addition to regulating the power consumption of the furnace transformer, the distribution of the total power currently drawn from the supply network to the input-side phases is regulated on the input side, whereby the distribution is determined by the known proportions.

[0033] The phase-related powers on the input side are calculated from the product of the respective input phase voltage and the respective input phase current. The respective sign of the respective phase voltage and phase current is taken into account. The power drawn from the supply grid on the input side will naturally be somewhat greater than the target power due to losses in the multilevel converter.

[0034] The shares of the input-side phases in the target power preferably vary without phase offset with the input-side voltages. This ensures that the multilevel converter - over a longer period of time - draws only active power from the supply grid. The multilevel converter therefore preferably does not draw any reactive power from the supply grid. Alternatively, it is possible to specifically set a phase offset - which may vary over time - and thus draw reactive power from the supply grid in a defined manner. Drawing a defined amount of reactive power can be useful, for example, to compensate for the reactive power of other consumers connected to the supply grid. The supply grid is operated at a grid frequency. The grid frequency is usually 50 Hz or 60 Hz. Other grid frequencies are of course also possible.Preferably, the inverse of the cycle time is at least twenty times the grid frequency, in particular at least fifty times. For example, if the grid frequency is 50 Hz, the control device updates the control of the multilevel converter preferably at least every ms, and particularly preferably at least every 0.4 ms. Of course, other cycle times are also possible. Furthermore, the inverse of the cycle time can also be greater than fifty times the grid frequency.

[0035] A completely analogous approach can be taken with the furnace transformer. In particular, the furnace transformer is operated at a furnace frequency, and the inverse of the cycle time is preferably at least twenty times the furnace frequency, in particular at least fifty times.

[0036] This definition of the cycle time takes into account the fact that for a robust and smooth control, several samples are required during which the output phase currents do not change significantly.

[0037] The determination of the output-side phase voltages is preferably carried out by the control device

[0038] - a complex target current vector is determined based on the measured values ​​for the output phase currents,

[0039] - by dividing the instantaneous target power by the real part of the product of the conjugate complex of the complex target current vector and a complex space vector, a real output-side target voltage value is determined, wherein the complex space vector has a unit length in the complex plane and changes its value according to a predetermined determination rule, in particular periodically, and

[0040] - a complex output-side target voltage is determined based on the determined real output-side target voltage value and the complex space vector, and the output-side target phase voltages are determined based on the complex output-side target voltage.

[0041] This method of determining the output phase voltages is simple, fast and can be implemented with little computational effort.

[0042] There are different ways of determining the complex space vector.

[0043] For example, the control device can determine the complex space vector such that the complex space vector continuously rotates in the complex plane at a constant frequency. This approach would correspond to sinusoidal waveforms of the output-side target phase voltages if the real output-side target voltage value were constant.

[0044] It is also possible for the control device to determine the complex space vector in such a way that the complex space vector has a constant value in the complex plane during a respective holding time of T / n and, after the respective holding time has elapsed, changes its value abruptly by an angle of 2n7n, where n is the number of output-side phase currents and T is a period with which the complex space vector changes its value. If the real output-side target voltage value were constant, this procedure would correspond to rectangular waveforms of the output-side target phase voltages.

[0045] It is possible that in the last-mentioned case, the voltage slopes turn out to be too steep. In this case, the control device can adopt a slightly modified approach and determine the space vector in such a way that the output-side target phase voltages exhibit a trapezoidal curve in the case of a constant real output-side target voltage value.

[0046] The above examples are not the only possible ones. It is equally possible to assume any other temporal progression of the angle of the complex space vector (which, however, must always have a length of 1, i.e., move on the unit circle in the complex plane) and determine the corresponding progression of the output-side target phase voltages for a constant, real output-side target voltage value. The corresponding progressions for the angle of the space vector can be determined, for example, by simple trial and error until the shape of the output-side target phase voltages for a constant, real output-side target voltage value meets specified requirements.Such requirements may, for example, be (to reduce the risk of arcing) a fast but not too steep zero crossing and / or otherwise (to maximize the power that can be transmitted by the converter) a range that is as flat as possible, similar to a trapezoidal curve.

[0047] Furthermore, it is possible to vary the period with which the complex space vector changes its value from time to time. For example, in an early melting phase, when there is still a risk of arcs breaking off, a shorter period can be set, and the period can be increased continuously or in stages as the melting process progresses.

[0048] In the simplest case, the control device adopts the output phase currents as setpoints for the output phase currents. Alternatively, the control device can further develop the setpoints for the output phase currents by updating the output phase currents according to a desired sinusoidal curve with constant amplitudes and intervene to correct any deviations.

[0049] The object is further achieved by a control program having the features of claim 12. According to the invention, the processing of the control program by the control device causes the control device to execute a control method according to the invention.

[0050] The object is further achieved by a control device having the features of claim 13. According to the invention, the control device is programmed with a control program according to the invention, so that the control device executes a control method according to the invention during operation.

[0051] The object is further achieved by a multilevel converter having the features of claim 14. According to the invention, the multilevel converter is controlled by a control device according to the invention.

[0052] Short description of the drawings

[0053] The above-described properties, features, and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood in connection with the following description of an embodiment, which is explained in more detail in conjunction with the drawings.

[0054] FIG 1 an arc furnace and its power supply,

[0055] FIG 2 an arm of a multilevel converter,

[0056] FIG 3 a submodule,

[0057] FIG 4 another submodule,

[0058] FIG 5 a flow chart,

[0059] FIG 6 a timing diagram,

[0060] FIG 7 a flow chart,

[0061] FIG 8 a complex plane,

[0062] FIG 9 shows a step of a flow chart,

[0063] FIG 10 a timing diagram and

[0064] FIG 11 shows a step of a flow chart.

[0065] Description of the embodiments

[0066] According to FIG. 1, electrodes 1 of a three-phase arc furnace 2 are supplied with electrical energy via a furnace transformer 3 and a multilevel converter 4 from a supply network 5 with several input phases 6 (usually three phases 6). The supply network 5 can, for example, be a medium-voltage network with a nominal voltage of 11 kV, 30 kV, 33 kV, or—in individual cases—110 kV. The supply network 5 is operated at a network frequency fN. The network frequency fN is typically 50 Hz or 60 Hz.

[0067] Specifically, in the embodiment of FIG 1, the multilevel converter 4 is designed as an intermediate circuit converter with an input-side rectifier 7 and an output-side inverter 8.

[0068] Multilevel converters are generally known to those skilled in the art. Multilevel converters have arms 9 (see also FIG. 2). Only a few of the arms 9 are provided with their reference symbols in FIG. 1, as examples.

[0069] The arms 9, as shown in FIG. 2, consist of a multi-stage series connection of submodules 10. Typically, there are eight or more such submodules 10 per arm 9. The submodules 10 are constructed identically to one another. Typically, a choke 11 is located at the end of the series connection. In the case of an arm 9 of the rectifier 7, the choke 11 is usually located at the end of the arm 9 facing a phase 6. Similarly, in the case of an arm 9 of the inverter 8, the choke 11 is located at the end of the respective arm 9 facing an output-side phase 12 (see FIG. 1).

[0070] According to FIG. 3, each submodule 10 comprises a storage capacitor 13 and self-commutated semiconductor switches 14. The term "self-commutated" means that the semiconductor switches 14 can be both switched on and off by control signals supplied to the semiconductor switches 14 from outside. For example, the self-commutated semiconductor switches 14 can be designed as IGBTs. The term "self-commutated" contrasts with the term "line-commutated." This term means that the respective semiconductor switch 14 can be specifically switched on, but cannot be switched off by an external control signal. An example of a line-commutated semiconductor switch is a "normal" thyristor.

[0071] According to FIG. 3, the submodules 10 each have a single storage capacitor 13 and exactly two semiconductor switches 14. This configuration is the minimum configuration of the submodules 10. Alternatively, the submodules 10 could, for example, have one storage capacitor 13 and four semiconductor switches 14 in a bridge circuit, as shown in FIG. 4. The submodules 10 could also have several individually switchable storage capacitors 13. In this case, at least two semiconductor switches 14 must be present for each independently switchable storage capacitor 13.

[0072] The semiconductor switches 14 of each submodule 10 can be switched independently of the semiconductor switches 14 of the other submodules 10. This applies regardless of whether the other submodules 10 are arranged in the same or in a different arm 9 of the converter 6 as the relevant submodule 10. Depending on the switching state of the semiconductor switches 14 of the respective submodule 10, the storage capacitor 13 of the respective submodule 10 is alternatively bridged or active. If the upper semiconductor switch 14 of a submodule 10 in FIG. 3 is closed and the other semiconductor switch 14 is open, the storage capacitor 13 of the respective submodule 10 is active. Conversely, if the upper semiconductor switch 14 in FIG. 3 is open and the lower semiconductor switch 14 is closed, the storage capacitor 13 of the respective submodule 10 is bridged.When the storage capacitor 13 is active, the voltage drop across the storage capacitor 13 contributes to the resulting voltage of the respective arm 9. A current flowing in the respective arm 9 causes the storage capacitor 13 to be charged or discharged, depending on the direction of the current flowing in the arm 9 and the charge state of the storage capacitor 13. If, however, the storage capacitor 13 is bridged, the voltage drop across the storage capacitor 13 does not contribute to the resulting voltage of the respective arm 9. A current flowing in the respective arm 9 causes neither charging nor discharging of the storage capacitor 13.

[0073] In an analogous manner, the storage capacitor 13 of the submodule 10 of FIG. 4 is also active or bridged depending on the control state of the semiconductor switches 14. However, in the case of the embodiment of FIG. 4, there is a further switching state in which the voltage drop across the storage capacitor 13 contributes with inverse polarity to the resulting voltage of the respective arm 9.

[0074] The multilevel converter 4 is controlled by a control device 15 according to FIG. 1. The control device 15 is used, in particular, to control the semiconductor switches 14 of the submodules 10. The manner in which the semiconductor switches 14 of the submodules 10 are controlled is known to those skilled in the art.

[0075] The control device 15 is programmed with a control program 16. The control program 16 includes machine code 17, which can be directly executed by the control device 15. The programming of the control device 15 with the control program 16 or—equally effective—the execution of the machine code 17 by the control device 15 causes the control device 15 to execute a control method, which is explained in more detail below in conjunction with FIG. 5.

[0076] According to FIG 5, the control device 15 repeatedly executes steps S1 to S6 in a cyclic manner. The execution takes place with a cycle time TZ. The cycle time TZ is very short, as shown in FIG 6. As a rule, the cycle time TZ is less than 1 ms. In particular, the cycle time TZ is considerably shorter than a period time TN = 1 / fN of the supply network 5. Specifically, FIG 6 shows an individual period of one of the voltages present at the input-side phases 6, the associated period time TN and the cycle time TZ. The inverse of the cycle time TZ is preferably at least twenty times the network frequency fN, preferably at least fifty times. For a network frequency fN of 50 Hz, the cycle time TZ is therefore preferably 1.0 ms or less, for example 0.4 ms, 0.2 ms or 0.1 ms. Specifically, for example, a control clock of 8 kHz or 16 kHz can be realized, which corresponds to a cycle time TZ of 125 ps or 160 ps.62.5 hp corresponds.

[0077] Analogous statements apply to the ratio of the cycle time TZ to a period time TO of the furnace transformer 3. The period time TO is the inverse of the frequency fO at which the furnace transformer 3 operates. The furnace frequency fO can – purely by chance – have the same value as the grid frequency fN. However, the two frequencies fO and fN are usually different from each other.

[0078] According to FIG. 5, in step S1, the control device 15 receives output-side phase currents I1 to I3 (more precisely: the corresponding measured values) from a measuring device 18 (see FIG. 1). The output-side phase currents I1 to I3 are the currents flowing on the output side of the multilevel converter 4. These are generally the currents on the input side of the furnace transformer 3. Conversely, if these are the currents on the output side of the furnace transformer 3, the transformation ratio of the furnace transformer 3 can be used to easily convert them to the currents on the input side of the furnace transformer 3. The measured values ​​I1 to I3 are only valid for the respective cycle.

[0079] Strictly speaking, it is sufficient if the control device 15 receives the measured values ​​for two of the output-side phase currents I1 to I3, since the relationship

[0080] 11 + 12 + 13 = 0 (1) must apply. Therefore, the remaining output-side phase current 11 to I3 can always be determined using two of the measured values ​​11 to I3.

[0081] In step S2, at least one current output-side target power P* is known to the control device 15. Preferably, the control device 15 also simultaneously knows the components a1, a2, and a3 of the current target power P* for the input-side phases 6. Both the current output-side target power P* and the components a1, a2, and a3 are only valid for the current cycle. The relationship a1 + a2 + a3 - 1 always applies to the components a1, a2, and a3. (2)

[0082] Furthermore, the components a1, a2, a3 are generally non-negative, i.e., greater than 0 or equal to 0. It is possible that the control device 15 knows in advance the temporal profiles for the instantaneous output-side target power P* and / or the components a1, a2, a3 for a plurality of cycles. However, in this case, too, the control device 15 determines the current values ​​P*, a1, a2, a3 for the respective cycle based on the temporal profiles known to it in step S2.

[0083] The components a1, a2 and a3 can in particular have curves that each correspond to the square of a sinusoidal curve, wherein the respective temporal curve is in phase with the corresponding voltage curve of the respective input-side phase 6. Under the assumption that the output-side target power P* does not change or changes only slowly over time, this results in a completely flicker-free load of the supply network 5 exclusively with active power.

[0084] In step S3, the control device 15 determines the output-side target phase voltages U1*, U2*, U3* for the output-side phases 12. The determination is made using target values ​​I1*, I2*, I3* for the output-side phase currents I1, I2, I3 and the output-side target power P*. The control device 15 previously determines the target values ​​I1*, I2* based on the output-side phase currents I1, I2, I3. Due to the fact that both the measured values ​​I1 to I3 and the instantaneous output-side target power P* are only valid for the respective cycle, the output-side target phase voltages U1*, U2*, U3* are also only determined for the respective cycle and thus only for the current point in time. The determination of step S3 is carried out in such a way that the sum of the products of the output-side target phase voltages U1*, U2*, U3* and the output-side phase currents I1, I2, I3 is equal to the instantaneous output-side target power P*.The determination of step S3 is therefore carried out in such a way that the relationship.

[0085] I1* U1* +I2* U2*+I3 * U3 * = P * (3) applies. Step S3 will be explained in more detail later.

[0086] Step S4 is only present if, in step S2, the control device 15 knows not only the instantaneous output-side target power P*, but also the components a1, a2, a3. In step S4, the control device 15 determines the input-side target phase currents i1*, i2*, i3*. The determination is performed in such a way that the relationships il*-ul = al-P*, (4)

[0087] 12 * -u2 = a2 • P * and (5)

[0088] 13 * -u3 = a3 • P * . (6) apply. u1, u2, u3 are the input-side phase voltages. The input-side phase voltages u1, u2, u3 can be measured, for example, by means of a measuring device 19 (see FIG. 1) and transmitted to the control device 15.

[0089] Strictly speaking, the losses occurring in the multilevel converter 4 would also have to be taken into account when determining step S4. However, this is readily known and familiar to experts. The value used for the target power P* in equations (4) to (6) would therefore have to be slightly larger than the current output-side target power P*.

[0090] In step S5, the control device 15 determines a control C for the multilevel converter 4. The determination in step S5 is based on the output-side target phase voltages U1*, U2*, U3* and, if applicable, also the input-side target phase currents i1*, i2*, i3*. In step S6, the control device 15 controls the multilevel converter 4 according to the control C determined in step S5.

[0091] The result is that the multilevel converter provides 4 output-side phase voltages that correspond to the output-side target phase voltages U1*, U2*, U3*, and, if necessary, also draws 6 input-side powers from the input-side phases corresponding to the proportions a1, a2, a3.

[0092] As already mentioned, the multilevel converter 4 is divided into a rectifier 7 and an inverter 8 in the embodiment of FIG 1. In this case, a separate determination is made in step S5 for the control of the rectifier 7 and the control of the inverter 8. Specifically, the determination is made by the control device 15 in step S5 such that the inverter 8 provides the output-side phase voltages. This is made possible by the decoupling of the individual input-side phases 6 and the output-side phases 12 by the DC voltage intermediate circuit between the rectifier 7 and the inverter 8. If the components a1, a2, a3 are specified, the determination of the control C is also made at the same time such that the rectifier 7 draws the input-side power from the input-side phases 6 according to the components a1, a2, a3.

[0093] A possible and currently preferred implementation of step S3 of FIG. 5 is explained below in conjunction with FIG. 7.

[0094] According to FIG 7, the control device 15 first determines setpoints 11*, I2*, I3* for the output-side phase currents 11, I2, I3 in a step S11 based on the measured values ​​for the output-side phase currents 11, I2, I3. In the simplest case, the control device 15 directly adopts the output-side phase currents 11, I2, I3 as setpoints 11*, I2*, I3*. Alternatively, it is possible for the control device 15 to take into account that the phase currents 11, I2, I3 are ideally sinusoidal and have equal amplitudes. In this case, the control device 15 can, on the one hand, in principle further develop the setpoints 11*, I2*, I3* by updating the output-side phase currents 11, I2, I3 in accordance with the desired sinusoidal curve and can also intervene to correct deviations.In any case, however, the control device 15 takes into account that due to the output-side inductances (in particular of the furnace transformer 3), the output-side phase currents I1, I2, I3 cannot change their values ​​abruptly from cycle to cycle.

[0095] In a step S12, the control device 15 determines a complex target current vector R based on the target values ​​I1*, I2*, I3*. The determination is carried out according to the relationship

[0096] In a step S13, the control device 15 determines a complex space vector RZ. The complex space vector RZ has the unit length in the complex plane, i.e., the length 1. Furthermore, the complex space vector RZ changes its value periodically. The complex space vector RZ therefore runs around the origin on the unit circle 20 in the complex plane as shown in FIG. 8. In the embodiment according to FIG. 7, the complex space vector RZ rotates continuously in the complex plane at a constant frequency, where the frequency is determined by W / 2TT and w is the angular frequency. The complex space vector RZ thus repeatedly moves along the unit circle 20, so to speak. The frequency at which the complex space vector RZ rotates can, in principle, be freely selected. However, the frequency will generally be different from the mains frequency fN.

[0097] In a step S14, the control device 15 determines a real output-side target voltage value U0*. The determination is made by dividing the instantaneous target power P* by the real part of the product of the complex conjugate Icc* of the complex target current vector R and the complex space vector RZ:

[0098] Based on this, the control device 15 determines a complex output-side target voltage U* in a step S15. To do this, it multiplies the real output-side target voltage value U0* by the complex space vector RZ:

[0099] U * = U0* -7?Z . (9) Finally, in a step S16, the control device 15 determines the output-side target phase voltages U1*, U2*, U3* based on the complex output-side target voltage U*. This is done using the relationships

[0100] (10) and (11) (12)

[0101] Above, a procedure was explained in which the complex space vector RZ rotates continuously in the complex plane at the constant frequency W / 2TT. If the real output-side target voltage value UO* were not to change over time, this would correspond to a sinusoidal curve of the output-side target phase voltages U1*, U2*, U3* (a sinusoidal curve is shown in FIG. 6 for one of the input-side phase voltages).

[0102] However, other procedures for the functional course of the complex space vector RZ as a function of time are also possible. For example, it is possible to retain the approach of FIG. 7, but replace step S13 with a step S21 (see FIG. 9). The other steps of FIG. 7, i.e., steps S11, S12, and S14 to S16, can be retained unchanged. In step S21, the control device 15 determines the complex space vector RZ according to the relationship

[0103] RZ = exp[(2i^ / 3) • INT(t / T)]. (13)

[0104] The INT operator represents the usual INTEGER operation, i.e. rounding down to the nearest integer value.

[0105] The procedure of step S21 results in the complex plane in the complex space vector RZ having a constant value during a respective holding time and, after the respective holding time has elapsed, changing its value abruptly by an angle of 36073 = 120° (or 2TT / 3). The three values ​​lie on the unit circle 20. They are marked in FIG 8 by small crosses 21 on the unit circle 20. The representation corresponds to the usual procedure in which three output-side phases 12 are present. In general, the holding time has the value T / n, where n is the number of output-side phase currents I1, I2, I3 and T is the period with which the complex space vector RZ changes its value.With respect to the output-side target phase voltages U1*, U2*, U3*, the procedure with step S21 corresponds to the situation where the output-side target phase voltages U1*, U2*, U3* would exhibit a rectangular waveform if the real target voltage value U0* were kept constant. The corresponding waveform is shown in solid lines in FIG. 10 as an example for the target phase voltage U1*. The procedure of FIG. 9 has the particular advantage that the multilevel converter 4 can be utilized to the greatest possible extent.

[0106] Alternatively, it is possible to retain the approach of FIG. 7 in principle, but replace step S13 with step S22 (see FIG. 11). The other steps of FIG. 7, i.e., steps S11, S12, and S14 to S16, can again be retained unchanged. In step S22, the control device 15 determines the complex space vector RZ such that the output-side target phase voltages U1*, U2*, U3* have a trapezoidal curve in the case of a constant real output-side target voltage value UO*. The corresponding curve is shown in FIG. 10 as an example for the target phase voltage U1* in dashed lines.

[0107] In this case, the time course of the complex space vector RZ can be calculated in advance and "thinking backwards" within the framework of this calculation. In this case, the (theoretically) desired time course of the output-side target phase voltages U1*, U2*, U3* is first set for constant and equal amplitudes of the output-side target phase voltages U1*, U2*, U3*. Then, according to the relationship the resulting curve of the associated complex output-side setpoint voltage U* is determined. Finally, the complex output-side setpoint voltage U* is divided into its real setpoint voltage value UO* and the associated complex space vector RZ. This determines the desired time curve of the complex space vector RZ. The time curve of the complex space vector RZ can thus be used within the framework of the procedure of FIG. 7, modified according to FIG. 11. The procedure of FIG. 9 is particularly advantageous if the multilevel converter 4 is to be utilized as fully as possible, but an abrupt switching of the output-side phase voltages should prove problematic.

[0108] Other determination rules for the determination of the complex space vector RZ are also conceivable.

[0109] The present invention has many advantages. In particular, flicker can be almost completely avoided. The inventive approach turns the arc furnace 2 into an easily manageable—so to speak, "tamed"—load. This applies even if no active or passive compensators are arranged on either the input or output side of the multilevel converter 4. This also applies if extreme operating conditions occur in the arc furnace 2, for example, if one of the arcs breaks off briefly. In this case, the power levels for the two remaining arcs can be adjusted by appropriately adjusting the output-side phase voltages and / or the output-side phase currents I1, I2, I3.

[0110] Although the invention has been illustrated and described in detail by the preferred embodiments, the invention is not limited by the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

[0111] List of reference symbols

[0112] 1 electrodes

[0113] 2 arc furnaces

[0114] 3 Furnace transformer

[0115] 4 multilevel converters

[0116] 5 Supply network

[0117] 6 input-side phases

[0118] 7 rectifiers

[0119] 8 inverters

[0120] 9 arms

[0121] 10 submodules

[0122] 11 throttles

[0123] 12 output phases

[0124] 13 storage capacitors

[0125] 14 semiconductor switches

[0126] 15 Control device

[0127] 16 Control program

[0128] 17 Machine code

[0129] 18, 19 Measuring devices

[0130] 20 unit circle

[0131] 21 crosses a1, a2, a3 shares of the target performance

[0132] C Control fN, fO Frequencies complex target current vector

[0133] Icc* conjugate complex of the complex desired current vector

[0134] 11 , I2, I3 Measured values ​​for output phase currents i1*, i2', i3* Input-side target phase currents

[0135] 11*, I2*, I3* output-side target phase currents

[0136] P* Target power

[0137] RZ space vector

[0138] S1 to S22 steps

[0139] T period

[0140] TN, TO period times

[0141] TZ cycle time

[0142] U1*, U2*, U3* output-side target phase voltages

[0143] U0* real target voltage value

[0144] LT complex output voltage

[0145] Cü angular frequency

Claims

Claims 1. Control method for a multilevel converter (4) which supplies electrodes (1) of a three-phase arc furnace (2) with electrical energy via a furnace transformer (3) from a supply network (5) having a plurality of input-side phases (6), comprising the following steps which are repeatedly executed with a cycle time (TZ): - a control device (15) of the multilevel converter (4) receives measured values ​​for output-side phase currents (11, I2, I3) flowing on the output side of the multilevel converter (4), - the control device (15) is informed of a current output-side target power (P*) for a current point in time, - the control device (15) determines output-side target phase voltages (U1*, U2*, U3*) for the respective current time based on the output-side phase currents (I1, I2, I3) and the output-side target power (P*), so that the sum of the products of the output-side target phase voltages (U1*, U2*, U3*) and the target values ​​(I1*, I2*, I3*) for the output-side phase currents (I1, I2, I3) determined based on the output-side phase currents (I1, I2, I3) is equal to the current output-side target power (P*), - the control device (15) controls the multilevel converter (4) in such a way that the multilevel converter (4) provides output-side phase voltages corresponding to the output-side target phase voltages (U1*, U2*, U3*).

2. Control method according to claim 1, characterized in that - that the multilevel converter (4) is designed as an intermediate circuit converter with an input-side rectifier (7) and an output-side inverter (8) and - that the control device (15) controls the inverter (8) in such a way that the inverter (8) provides the output-side phase voltages.

3. Control method according to claim 2, characterized in that - that the control device (15) also knows the proportions (a1, a2, a3) of the instantaneous nominal power (P*) for the respective current time for the input-side phases (6) and - that the control device (15) controls the rectifier (7) in such a way that the rectifier (7) draws input-side power from the input-side phases (6) in accordance with the components (a1, a2, a3) known to it.

4. Control method according to claim 1, characterized in that - that the control device (15) also knows the proportions (a1, a2, a3) of the instantaneous nominal power (P*) for the respective current time for the input-side phases (6) and - that the control device (15) controls the multilevel converter (4) in such a way that the multilevel converter (4) draws input-side power from the input-side phases (6) in accordance with the components (a1, a2, a3) known to it.

5. Control method according to one of the above claims, characterized in that the supply network (5) is operated at a network frequency (fN) and that the reciprocal of the cycle time (TZ) is at least twenty times as large as the network frequency (fN), preferably at least fifty times as large.

6. Control method according to one of the above claims, characterized in that the furnace transformer (3) is operated at a furnace frequency (fO) and that the reciprocal of the cycle time (TZ) is at least twenty times as large as the furnace frequency (fO), preferably at least fifty times as large.

7. Control method according to one of the above claims, characterized in that the control device (15) - a complex target current vector (l^) is determined based on the measured values ​​for the output phase currents (11, I2, I3), - by dividing the instantaneous nominal power (P*) by the real part of the product of the conjugate complex (Icc*) of the complex nominal current vector (T) and a complex space vector (RZ), a real output-side nominal voltage value (UO*) is determined, wherein the complex space vector (RZ) has the unit length in the imaginary plane and changes its value according to a predetermined determination rule, in particular periodically, and - a complex output-side target voltage (U*) is determined based on the determined real output-side target voltage value (UO*) and the complex space vector (RZ), and the output-side target phase voltages (U1*, U2*, U3*) are determined based on the complex output-side target voltage (U*).

8. Control method according to claim 7, characterized in that the control device (15) determines the complex space vector (RZ) in such a way that the complex space vector (RZ) rotates continuously in the imaginary plane at a constant frequency.

9. Control method according to claim 7, characterized in that the control device (15) determines the complex space vector (RZ) in such a way that the complex space vector (RZ) has a constant value in the imaginary plane during a respective holding time of T / n and, after the respective holding time has elapsed, changes its value abruptly by an angle of 2n7n, where n is the number of output-side phase currents (11, I2, I3) and T is a period with which the complex space vector (RZ) changes its value.

10. Control method according to claim 7, characterized in that the control device (15) determines the space vector (RZ) in such a way that the output-side target phase voltages (U1*, U2*, U3*) have a trapezoidal profile in the case of a constant real output-side target voltage value (UO*).

11. Control method according to one of the above claims, characterized in that the control device (15) adopts the output-side phase currents (11, I2, I3) as setpoint values ​​(11*, I2*, I3*) for the output-side phase currents (11, I2, I3) or further develops the setpoint values ​​(11*, I2*, I3*) for the output-side phase currents (11, I2, I3) by updating the output-side phase currents (11, I2, I3) in accordance with a desired sinusoidal curve with equal amplitudes and intervenes to correct deviations.

12. Control program for a control device (15) for controlling a multilevel converter (4) which supplies electrodes (1) of a three-phase arc furnace (2) with electrical energy via a furnace transformer (3) from a supply network (5) having a plurality of input-side phases (6), wherein the control program comprises machine code (17) which can be processed directly by the control device (15), wherein the processing of the machine code (17) by the control device (15) causes the control device (15) to carry out a control method according to one of the above claims.

13. Control device for controlling a multilevel converter (4) which supplies electrodes (1) of a three-phase arc furnace (2) with electrical energy via a furnace transformer (3) from a supply network (5) having a plurality of input-side phases (6), wherein the control device is programmed with a control program (16) according to claim 11, so that the control device carries out a control method according to one of claims 1 to 11 during operation.

14. Multilevel converter, which connects the electrodes (1) of a three-phase arc furnace (2) to a Furnace transformer (3) from a supply network (5) with several input phases (6) is supplied with electrical energy, wherein the multilevel converter is controlled by a control device (15) is controlled according to claim 13.

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