Optimized control of an electric arc furnace using a multilevel converter

Abstract

The electrodes (1) of the three-phase electric arc furnace (2) are powered by a multilevel converter (4) via a furnace transformer (3). The control device (15) of the multilevel converter (4) is aware of the output-side phase currents (I1, I2, I3) flowing on the output side of the multilevel converter (4). The control device (15) determines the first output-side phase voltage components (U11, U12, U13) for each current time point, such that the complex first output-side voltage (U1) determined by the first output-side phase voltage components (U11, U12, U13) is oriented in the complex plane parallel to the complex output-side current (I) determined by the output-side phase currents (I1, I2, I3). The control device (15) determines the second output-side phase voltage components (U21, U22, U23) such that the complex second output-side voltage (U2) determined by the second output-side phase voltage components (U21, U22, U23) is orthogonal to the orientation of the complex first output-side voltage (U1) in the complex plane. The control device (15) drives the multilevel converter (4) such that the multilevel converter (4) provides output-side phase voltages corresponding to the sums of the first and second output-side phase voltage components (U11, U12, U13, U21, U22, U23). The control device (15) repeats the aforementioned steps for a periodic time (TZ).

Description

Technical Field

[0001] The present invention is based on a control method for a multilevel converter, wherein the multilevel converter supplies electrical energy to the electrodes of a three-phase electric arc furnace via a furnace transformer, wherein the output phase current flowing on the output side of the multilevel converter is repeatedly detected by the control device of the multilevel converter at periodic intervals.

[0002] The present invention is also based on a control program for a control device for controlling a multilevel converter that supplies electrical energy to the electrodes of a three-phase electric arc furnace via a furnace transformer. The control program includes machine code that can be directly processed by the control device, wherein processing the machine code by the control device causes the control device to execute such a control method.

[0003] The present invention is also based on a control device for controlling a multilevel converter that supplies electrical energy to the electrodes of a three-phase electric arc furnace via a furnace transformer, wherein the control device is programmed with such a control program that the control device executes such a control method during operation.

[0004] The present invention is also based on a multilevel converter that supplies electrical energy to the electrodes of a three-phase electric arc furnace via a furnace transformer, wherein the multilevel converter is controlled by such a control device. Background Technology

[0005] The subject mentioned at the beginning is known, for example, from EP 2 329 684 B1.

[0006] A power supply device for a nonlinear load is known from EP 2 947 766 A1. The load can be an electric arc furnace. The power supply device is configured as a multi-stage matrix converter. It can detect the voltage and current present on the secondary side and take these into account when determining the target current and / or voltage values ​​for the converter units used in the power supply device.

[0007] Similar, but not equally extensive, disclosures can be found in EP 3 124 903 A1, FR 2 926 182 A1 and CN 110 957 903 A.

[0008] An apparatus is known from US 2022 / 0115 888 A1, which has an energy storage battery on one side and a three-phase power grid on the other side, the energy storage battery and the three-phase power grid being interconnected via a converter. This converter can be a multi-level converter. The battery can be alternately charged or discharged. Voltage and current present on the output side of the converter towards the three-phase power grid are supplied to a control device for the converter. Active power and reactive power are determined based on the voltage and current. A control signal for the converter is determined based on the difference from a corresponding target value, and the converter is driven accordingly. Specifically, in order to discharge the battery, a target voltage can be determined from the voltage starting on the grid side, the target voltage being provided by the converter towards the three-phase power grid. The difference between the target voltage and the grid voltage is orthogonal to the grid voltage orientation. A current parallel to the grid voltage orientation is produced by providing the target voltage.

[0009] In electric arc furnace steelmaking plants, scrap steel is melted using an electric arc furnace. To do this, three electrodes are typically immersed in a container filled with scrap steel, and then an electric arc is generated between the scrap steel and the electrodes, which heats and melts the scrap steel.

[0010] The voltage applied to the electrodes is typically in the range of several hundred volts to slightly above 1 kV, sometimes reaching 2 kV. The power of industrial electric arc furnaces often ranges from 50 MW to 300 MW.

[0011] The continuous flickering of the electric arc causes voltage fluctuations and, if left unchecked, significant grid backlash. Historically, it has been known to connect compensators in parallel with the electric arc furnace, such as so-called SVCs (Static Var Compensators) or STATCOMs (Static Synchronous Compensators) or passive filters. These compensators are large, bulky, and expensive. Furthermore, these compensators generate power losses and also affect other units connected to the supply network.

[0012] As known from EP 2 329 684 B1, power is supplied to an electric arc furnace via a multilevel converter located on the input side of the furnace transformer. With this converter, it is possible to regulate not only the reactive power on the input side but also the active power absorbed by the electric arc furnace. The aforementioned EP document states that the reaction force of the electric arc furnace relative to the supply network, exceeding the symmetrical loading range of the active power on the supply network phases, is minimized. The exact manner and method of driving and controlling the multilevel converter cannot be derived from this EP document. Summary of the Invention

[0013] The purpose of this invention is to demonstrate this feasibility.

[0014] This objective is achieved by a control method having the features of claim 1. An advantageous design of this control method is the subject of dependent claims 2 to 11.

[0015] According to the present invention, a control method of the type mentioned at the beginning is provided, wherein, in addition to receiving the measured value as already mentioned, the control device of the multilevel converter repeatedly performs the following steps at the specified cycle time: - The control device determines the first output-side phase voltage component for the corresponding current time point, such that the complex first output-side voltage determined by the first output-side phase voltage component is oriented parallel to the complex output-side current determined by the output-side phase current in the complex plane. The control device determines the second output-side phase voltage component such that the complex second output-side voltage determined by the second output-side phase voltage component is orthogonal to the orientation of the complex first output-side voltage in the complex plane. - The control device drives the multilevel converter so that the multilevel converter provides an output phase voltage corresponding to the sum of the first and second output phase voltage components.

[0016] Driving a multilevel converter to provide a preset phase voltage on the output side is inherently feasible. The feasibility of such driving is known to those skilled in the art. The key point here is to determine the output phase voltages such that the complex first and complex second output phase voltages are parallel to or orthogonal to the complex output current orientation, as defined by the output phase currents known from the control device.

[0017] The instantaneous active power supplied to the furnace transformer by the multilevel converter is regulated by the first output-side phase voltage component. The rate of change of the instantaneous operating point of the electric arc furnace is regulated by the second output-side phase voltage component, which is determined by the output-side phase current known from the control device and the first output-side phase voltage component determined by the control device. Because these two phase voltage components for each phase are determined independently, completely flexible control of the multilevel converter is achieved, which was previously impossible.

[0018] The first and second output phase voltage components are redefined in each cycle. Therefore, it is feasible to regulate the electric arc furnace, which can eliminate interference very quickly (within 1 ms or less).

[0019] The sum of the products of the second output-side phase voltage component and the output-side phase current can be termed instantaneous reactive power. However, this terminology is inaccurate and even misleading because reactive power, like active power, only yields meaningful values ​​for operation at a fixed frequency and only for integer multiples of half-cycles (cycle = reciprocal of frequency). In this invention, like instantaneous active power, the sum of the products of the second output-side phase voltage component and the output-side phase current is an instantaneous value, calculated only based on the instantaneous values ​​of the corresponding cycle and valid only for the corresponding cycle. The determination of the first and second output-side phase voltage components does not necessarily, methodologically, result in periodic drive control of the multilevel converter. While achieving this result is feasible, it is not mandatory.

[0020] The two criteria used to determine the first and second output phase voltage components can be correlated with each other or independent of each other as needed.

[0021] Therefore, it is feasible for the control device to determine the second output-side phase voltage component while considering the output-side phase current but not the first output-side phase voltage component. For example, the control device can know the maximum value of the output-side phase current. In this case, the control device can determine the second output-side phase voltage component by considering the distance between the output-side phase current and its maximum value. More preferably, the control device knows the current region in the complex plane, within which the complex output-side currents should lie, and the control device determines the second output-side phase voltage component by considering the distance between the complex output-side currents and the boundary of the current region. By defining the current region, it is possible to take into account that the complex output-side currents must also have a minimum length so that the complex voltages of the output sides, which are meaningfully divided into a first parallel and a second perpendicular to it, are fundamentally stable. Furthermore, the constraints are often not constant in time, but are very specifically based on the precise design and instantaneous operating state of the multilevel converter. Therefore, the definition of the current region can also vary in time.

[0022] Alternatively, the control device can determine the second output-side phase voltage component while considering the first output-side phase voltage component but not the output-side phase current. For example, the control device can determine the maximum value of the output-side phase voltage. In this case, the control device can first determine the first output-side phase voltage component, and then determine the second output-side phase voltage component considering the distance between the first output-side phase voltage component and the maximum value of the output-side phase voltage. More preferably, the control device knows the voltage region in the complex plane, within which the complex total output-side voltage determined by the output-side phase voltage should lie, and the control device determines the second output-side phase voltage component considering the distance between the complex first output-side voltage and the boundary of the voltage region. Furthermore, the constraints are often not constant in time, but are very specific to the instantaneous operating state of the multilevel converter. By defining a voltage region that depends on the corresponding operating state, it is also possible to consider that the constraints are very specific to the precise design of the multilevel converter.

[0023] Finally, it is also feasible for the control device to determine the second output-side phase voltage component while considering both the output-side phase current and the first output-side phase voltage component. Specifically, the feasibility of considering only the output-side phase current and only the first output-side phase voltage component can be combined with each other, and they can also be interdependent.

[0024] Preferably, the control device knows the maximum value of the output-side phase voltage. In this case, the control device can determine the first output-side phase voltage component while taking the maximum value into account.

[0025] Even better, the control device learns that the total complex output voltage, determined by the output phase voltage, should lie within a voltage region in the complex plane. In this case, the control device can determine the first output phase voltage component while taking into account the boundaries of the voltage region. Furthermore, the constraints are often not constant in time, but rather very specifically based on the instantaneous operating state of the multilevel converter. By defining a voltage region that depends on the corresponding operating state, it is also possible to take into account the fact that the constraints are very specifically based on the precise design of the multilevel converter.

[0026] Alternatively, the control device can determine the flux region in the complex plane, within which the complex flux vector of the furnace transformer should lie. In this case, the control device can determine the first and / or second output-side phase voltage components, taking into account the distance between the complex flux vector and the boundary of the flux region. The flux vector can be determined by the control device by integrating the space vector (weighted by the corresponding number of turns) of the primary or secondary voltage after deducting ohmic voltage loss and, if necessary, inductor voltage loss. The space vector must adhere to specific values, otherwise core saturation will occur. In particular, the absolute value of the flux vector must not become too large, otherwise core saturation will occur in the furnace transformer. Conversely, the absolute value of the flux vector must not become too small, as this would result in excessively high frequencies of the output-side phase voltages or output-side phase currents.

[0027] The regions mentioned for the complex space vectors of the output-side voltage, output-side current, and magnetic flux are not necessarily simply continuous. More precisely, it is possible for them to have "voids," i.e., forbidden regions completely surrounded by allowed regions in the complex plane. It is also possible for the allowed region of one of the complex space vectors to vanish as a surface, i.e., degenerate into a closed line, for example. This is particularly possible in the case of the magnetic flux vector. Furthermore, it is possible for the mentioned regions to vary over time. This can occur, for example, when the boundary conditions change or new boundary conditions are introduced.

[0028] Alternatively, the control device may receive the output-side phase current (i.e., the output-side phase current used within the scope of the control method according to the invention) as a measured value. Alternatively, the control device may determine the output-side phase current using a model of the electric arc furnace. Such models are known to those skilled in the art.

[0029] Even when the output phase current is determined in a model-supported manner, it is feasible for the control device to (additionally) receive a measured value of the output phase current. In the case of determining the first and, if necessary, a second output phase voltage component, the model-supported determined output phase current is also used. However, it is feasible for the control device to adapt the electric arc furnace model based on the deviation between the output phase current determined using the electric arc furnace model and the measured value.

[0030] Preferably, the control device determines the first output-side phase voltage component such that the sum of the products of the first output-side phase voltage component and the output-side phase current equals the instantaneous target output power known to the control device and valid only for the corresponding period. This allows for targeted power adjustment of the electric arc furnace.

[0031] This approach contrasts with existing technologies where regulation appears to occur over multiple cycles of the grid frequency. This is particularly evident in the repeated use of the terms "active power" and "reactive power" in existing technologies (also in EP 2 329 684 B1). The instantaneous target power on the output side can be known by the control device as a corresponding power trend. The power curve can be constant, but it can also vary. In the case of variation, the instantaneous target power on the output side should preferably change only slowly.

[0032] Typically, the multilevel converter obtains the input phase current from the input phase of the supply network through corresponding drive control of the control device. The supply network operates at the grid frequency, which is typically 50 Hz or 60 Hz. Of course, other grid frequencies are also feasible. This supply network typically carries medium voltage of 11 kV, 30 kV, 33 kV, or 110 kV.

[0033] Preferably, the control device repeatedly determines the drive of the multilevel converter at periodic intervals, such that the complex input-side current, determined by the input-side phase current, is oriented in the complex plane at a predetermined phase angle relative to the complex input-side voltage, determined by the input-side phase voltage of the input-side phase of the supply network, particularly parallel to the complex input-side voltage. Thus, observed over the entire cycle or at least half a cycle, a defined amount of reactive power is obtained from the supply network in addition to active power, or (in the case of parallel orientation) no reactive power is obtained. It is feasible to observe from the supply network, respectively, over the entire cycle or at least half a cycle, only active power is obtained, or reactive power is selectively set to be obtained from the supply network for the specified time period. Obtaining the defined reactive power can be meaningful, for example, to compensate for the reactive power of other appliances connected to the supply network. However, observed over multiple half cycles, the acquisition of reactive power can also vary over time, preferably slowly.

[0034] Driving a multilevel converter to obtain a preset current from the supply network at the input side (and, in combination with the input phase voltage, also obtain instantaneous power) is itself a straightforward feasibility. The feasibility of such driving is known to those skilled in the art. The crucial point here is determining the input phase current.

[0035] Preferably, the control device repeatedly determines the drive of the multilevel converter at periodic intervals, such that the instantaneous power obtained by the multilevel converter from the input phase matches the component of the input phase valid for the corresponding period. A completely symmetrical loading of the supply network is also performed as the components of the input phase are preset accordingly over multiple cycles. In this case, the components of the input phase vary with the input phase voltage with a constant phase shift, particularly in the absence of a phase shift.

[0036] By pre-setting the instantaneous active power on the output side (and, if necessary, the reactive power on the input side) to change only slowly over time, near-flicker-free operation of the electric arc furnace is possible from the perspective of the supply network. Consequently, the electric arc furnace becomes an easily operable (one might say "tamed") load. This also applies to situations where no active or passive compensators are arranged on either the input or output side of the multilevel converter. Furthermore, it also applies to extreme operating conditions of the electric arc furnace, such as a brief interruption of one of the arcs.

[0037] Preferably, the multilevel converter is configured as a DC-link converter, which has an input side to the supply network and an inverter with an output side to the furnace transformer, and they are interconnected via a DC voltage circuit. In this case, a near-decoupled determination of the drive control of the inverter and the rectifier can be achieved.

[0038] This objective is also 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 the control method according to the invention.

[0039] This objective is also 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, such that the control device executes the control method according to the invention during operation.

[0040] This objective is also 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. Attached Figure Description

[0041] The above-described features, characteristics, and advantages of the present invention, as well as the ways and means of achieving them, become clearer and more readily understood in conjunction with the following description of embodiments, which are illustrated in detail with reference to the accompanying drawings. Hereinafter: Figure 1 The electric arc furnace and its energy supply are shown. Figure 2 The bridge arm of the multilevel converter is shown. Figure 3 Showing a submodule, Figure 4 Another submodule is shown. Figure 5 The flowchart is shown. Figure 6 A timeline is shown. Figure 7 The diagram shows a vector graph of the electrical quantities on the complex output side. Figure 8The flowchart is shown. Figure 9 The complex plane of the complex output-side current is shown. Figure 10 The complex plane of the complex first output side voltage is shown. Figure 11 The flowchart is shown. Figure 12 The flowchart is shown. Figure 13 The flowchart is shown. Figure 14 A vector diagram showing the electrical quantities on the complex output side, and Figure 15 The electric arc furnace and its energy supply are shown. Detailed Implementation

[0042] according to Figure 1 The electrodes 1 of the three-phase electric arc furnace 2 are powered by a multi-level converter 4 via a furnace transformer 3. The electric arc furnace 2 can be of any type. It can be an electric arc furnace (EAF) in the narrow sense, but it can also be a submerged arc furnace (SAF) or a SMELTER. Iron, steel, aluminum, or other metals can be melted in the electric arc furnace 2 as needed.

[0043] The multilevel converter 4 is typically powered by a supply network 5 having multiple input-side phases 6 (usually three phases 6). The supply network 5 can be, for example, a medium-voltage grid with a target voltage of 11 kV, 30 kV, 33 kV, or (in some cases) 110 kV. The supply network 5 operates at the grid frequency fN. The grid frequency fN is typically 50 Hz or 60 Hz.

[0044] A multilevel converter 4 is arranged between the supply network 5 and the furnace transformer 3, and the furnace transformer 3 is arranged between the multilevel converter 4 and the electrode 1. Therefore, electrical energy flows from the supply network 5 to the multilevel converter 4, from there to the furnace transformer 3, and from there finally to the electrode 1.

[0045] Specifically Figure 1 In the design, the multilevel converter 4 is configured as a DC-DC converter, which has a rectifier 7 on the input side facing the supply network 5 and an inverter 8 on the output side facing the furnace transformer 3. The rectifier 7 and the inverter 8 are interconnected via a DC voltage circuit.

[0046] Multilevel converters are well known to those skilled in the art. (See also: Multilevel converters) Figure 2 It has 9 bridge arms. Figure 1In the figure, only a few of the bridge arms 9 are provided with reference numerals as an example.

[0047] according to Figure 2 Each bridge arm 9 consists of a multi-stage series circuit of sub-modules 10. Typically, each bridge arm 9 has eight or more such sub-modules 10. The sub-modules 10 are constructed identically to each other. Typically, chokes 11 are also present at the ends of the series circuits. In the bridge arms 9 of the rectifier 7, the chokes 11 are mostly located at the end of the corresponding bridge arm 9 facing the input side phase 6. Similarly, in the bridge arms 9 of the inverter 8, the chokes 11 are located at the end of the corresponding bridge arm 9 facing the output side phase 12. (See figure) Figure 1 At the end.

[0048] according to Figure 3 Each submodule 10 includes an energy storage capacitor 13 and a self-booting semiconductor switch 14. The term "self-booting" means that the semiconductor switch 14 can be both turned on and off by a control signal supplied to it from an external source. For example, the self-booting semiconductor switch 14 can be configured as an IGBT. The term "self-booting" is contrasted with the term "grid-booting." This term means that the corresponding semiconductor switch can be selectively turned on but cannot be turned off by an external control signal. An example of a grid-booted semiconductor switch is a "common" thyristor.

[0049] according to Figure 3 Submodule 10 has a single energy storage capacitor 13 and exactly two semiconductor switches 14. This configuration is the minimum configuration of submodule 10. Alternatively, according to... Figure 4 As illustrated, submodule 10 can, for example, have a storage capacitor 13 and four semiconductor switches 14 in a bridge circuit. Submodule 10 can also have multiple storage capacitors 13 that can be switched individually. In this case, for each storage capacitor 13 that can be switched independently, there must be at least two semiconductor switches 14.

[0050] Each submodule 10's semiconductor switch 14 can be switched independently of the semiconductor switches 14 of other submodules 10. This applies regardless of whether other submodules 10 are arranged in the same or different bridge arms of the converter 6 as the relevant submodule 10. Depending on the switching state of the semiconductor switch 14 of the corresponding submodule 10, the energy storage capacitor 13 of the corresponding submodule 10 is alternatively bridged or activated. When the submodule 10 is in Figure 3 When the upper semiconductor switch 14 is closed and the other semiconductor switch 14 is open, the energy storage capacitor 13 of the corresponding submodule 10 is activated. Conversely, when... Figure 3When the upper semiconductor switch 14 is open and the lower semiconductor switch 14 is closed, the energy storage capacitor 13 of the corresponding submodule 10 is bridged. When the energy storage capacitor 13 is active, the voltage drop across it contributes to the composite voltage of the corresponding bridge arm 9. The current flowing in the corresponding bridge arm 9 (depending on the direction of the current flowing in the bridge arm 9 and the charging state of the energy storage capacitor 13) causes the energy storage capacitor 13 to charge or discharge. Conversely, when the energy storage capacitor 13 is bridged, the voltage drop across it does not contribute to the composite voltage of the corresponding bridge arm 9. The current flowing in the corresponding bridge arm 9 neither causes the energy storage capacitor 13 to charge nor discharge.

[0051] In a similar manner, based on the driving state of semiconductor switch 14, Figure 4 The energy storage capacitor 13 of submodule 10 is also activated or bridged. However, in Figure 4 In the case of the design scheme, there is another switching state in which the voltage dropped across the energy storage capacitor 13 contributes to the composite voltage of the corresponding bridge arm 9 with the opposite polarity.

[0052] according to Figure 1 The multilevel converter 4 is controlled by the control device 15. The control device 15 specifically drives the semiconductor switch 14 of the submodule 10. The methods and approaches for driving the semiconductor switch 14 of the submodule 10 are known to those skilled in the art.

[0053] Control device 15 is programmed with control program 16. Control program 16 includes machine code 17, which can be directly processed by control device 15. Programming control device 15 with control program 16, or (equivalently) processing machine code 17 by control device 15, causes control device 15 to execute control method, which is described below in conjunction with... Figure 5 To elaborate in detail.

[0054] according to Figure 5 The control device 15 cyclically repeats steps S1 to S6. This execution is performed over a period of time TZ. According to... Figure 6 The diagram shows that the cycle time TZ is very small. Typically, the cycle time TZ is less than 1 ms. Specifically, the cycle time TZ is significantly smaller than the cycle time TN = 1 / fN of supply network 5. Figure 6 The diagram shows a single cycle of one of the voltages present at phase 6 on the input side, along with the corresponding cycle time TN and cycle time TZ.

[0055] Preferably, the reciprocal of the cycle time TZ is at least twenty times, and preferably at least fifty times, the grid frequency fN. Therefore, when the grid frequency fN is 50 Hz, the cycle time TZ is preferably 1.0 ms or less, for example, 0.4 ms, 0.2 ms, or 0.1 ms. Specifically, for example, an adjustment cycle of 8 kHz or 16 kHz can be achieved, corresponding to a cycle time TZ of 125 μs or 62.5 μs. This confirmation of the cycle time TZ takes into account the fact that for robust and smooth adjustment, multiple samplings are required, during which the output phase currents I1, I2, and I3 supplied by the multilevel converter 4 to the furnace transformer 3 do not change significantly.

[0056] Similarly, the ratio of cycle time TZ to the cycle time TO of furnace transformer 3 applies. Cycle time TO is the reciprocal of the operating frequency fO of furnace transformer 3. The furnace frequency fO can (purely randomly) have the same value as the grid frequency fN. However, these two frequencies, fO and fN, are usually different from each other.

[0057] according to Figure 5 In step S1, the control device 15 obtains the output-side phase currents I1 to I3. For example, the control device 15 can obtain the information from the measuring device 18 (see...) in step S1. Figure 1 The multilevel converter 4 receives the output phase currents I1 to I3 (more precisely, the corresponding measured values). The output phase currents I1 to I3 are the instantaneous currents flowing on the output side of the multilevel converter 4. Typically, they are the input currents of the furnace transformer 3. Conversely, if they are the output currents of the furnace transformer 3, they can be easily converted to the input currents of the furnace transformer 3 based on its turns ratio. The measured values ​​I1 to I3 are only valid for the corresponding period.

[0058] To be precise, it is sufficient that the control device 15 receives two of the three output-side phase currents I1 to I3, because the relationship must always be satisfied at every point in time.

[0059] I1 + I2 + I3 = 0 (1).

[0060] Therefore, based on two of the output-side phase currents I1 to I3, the remaining output-side phase currents I1 to I3 can always be determined.

[0061] In step S2, the control device 15 determines the relationship according to the formula.

[0062] Determine the complex output side current I . Figure 7 The complex plane and the complex output-side current in the complex plane are shown. I .

[0063] In step S3, the control device 15 determines the first output side phase voltage component U11. U12 U13 This determination is performed such that the complex first output side voltage... U1 The current I is oriented parallel to the complex output side in the complex plane, wherein the voltage is determined according to the relation...

[0064] Through the first output side phase voltage component U11 U12 U13 It is determined that the current has either been explicitly communicated to the control device 15, or at least has been explicitly determined by informing the control device 15 of the output phase currents I1 to I3. Figure 7 The complex first output side voltage is also shown. U1 Complex first output side voltage U1 exist Figure 7 The middle is described as being related to the complex output side current. I It forms a very small angle. However, this angle is only used when it is possible to... Figure 7 In order to better distinguish complex output side current I and complex first output side voltage U1 Therefore, this angle is only used for better illustration. In reality, the angle is 0°.

[0065] It is feasible for the control device 15 to first determine the first output side phase voltage component U11 U12 U13 And then determine the complex first output side voltage. U1 However, it is usually simpler for the control device 15 to first determine the complex first output side voltage. U1 And then according to the relation (4) , and (5) (6) Determine the first output phase voltage component U11 U12 U13 .

[0066] Furthermore, in step S4, the control device 15 determines the second output-side phase voltage component U21. U22 U23 This determination is performed such that the complex second output side voltage... U2 In the complex plane, it is orthogonal to the complex output-side current I (and consequently also orthogonal to the complex first output-side voltage). U1 (directional), wherein the voltage is transmitted through the second output side phase voltage component U21 U22 U23 Sure. Figure 7 The complex second output side voltage is also shown. U2 .

[0067] Similarly, as in step S3, it is feasible for the control device 15 to first determine the second output side phase voltage component U21. U22 U23 And then determine the complex second output side voltage. U2 However, it is usually simpler for the control device 15 to first determine the complex second output side voltage. U2 And then determine the second output phase voltage component U21. U22 U23 The conversion is related to the phase voltage component U11 on the first output side. U12 U13 and complex first output side voltage U1 The conversions between them are exactly the same.

[0068] Alternatively, the control device 15 can first add the two complex output voltages in the complex plane. U1 , U2 And then determine the output phase voltage based on that.

[0069] In step S5, the control device 15 determines the drive C for the multilevel converter 4. The determination in step S5 is based on the corresponding first and second output-side phase voltage components U11. and U21 or U12 and U22 Or U13 and U23 The process is carried out by the sum of the values. In step S6, the control device 15 drives the multilevel converter 4 according to the drive control C determined in step S5. As a result, the multilevel converter 4 provides the corresponding output phase voltage.

[0070] It is feasible for the control devices 15 to independently determine the first output-side phase voltage component U11. U12 U13 Second output side phase voltage component U21 U22 U23 However, dependencies can also exist.

[0071] Therefore, according to Figure 8 As illustrated in the diagram, for example, it is feasible for the control device 15 to determine the second output-side phase voltage component U21 by taking into account the output-side phase currents I1, I2, and I3. U22 U23 This is in Figure 8 This is illustrated in step S11. Alternatively, it is possible for the control device 15 to take into account the first output-side phase voltage component U11. U12 U13 Determine the second output side phase voltage component U21 under the following conditions U22 U23 This is in Figure 8 This is illustrated in step S12. Furthermore, it is feasible for the control device 15 to consider both the output-side phase currents I1, I2, and I3 and the first output-side phase voltage component U11. U12 U13 Determine the second output side phase voltage component U21 under the following conditions U22 U23 This is in Figure 8 This is shown in step S13.

[0072] Steps S11 to S13 are Figure 5 The feasible design scheme for step S4. Only one of steps S11 to S13 always exists. Therefore, steps S11 to S13 are... Figure 8 Only dashed lines are shown. Furthermore, the measures taken in steps S11 to S13 relate to the complex output side current in steps S11 to S13. I and voltage U1 ,U2 This is illustrated. This processing method is equivalent to the phase-based processing method. Furthermore, this determination only involves the complex second output-side voltage U2. The (signed) length. Because its orientation is pre-defined by the following condition: that is, the orientation must be orthogonal to the complex first output voltage U1. Or orthogonal to the complex output current I Orientation.

[0073] To account for the output-side phase currents I1, I2, and I3, the control device 15 can, for example, determine the maximum values ​​of the output-side phase currents I1, I2, and I3. If simple, independent maximum values ​​are preset, then according to... Figure 9 The diagram shows that the maximum value corresponds to a hexagon. In this case, the control device 15 can, for example, determine the distance between the output-side phase currents I1, I2, I3 and their maximum values ​​in step S11 or S13. Figure 9 The spacing δI is simply exemplarily labeled for the output-side phase current I1. Then, the control device 15 is able to determine the second output-side phase voltage component U21. U22 U23 Spacing should be considered.

[0074] In a similar manner, in order to take into account the first output-side phase voltage component U11 U12 U13 For example, control device 15 can know the maximum value of the output phase voltage. If simple, independent maximum values ​​are preset, then according to... Figure 10 The diagram shows that the maximum value corresponds to a hexagon. In this case, the control device 15 can, for example, determine the first output-side phase voltage component U11 in step S12 or S13. U12 U13 The distance from the maximum value of the output phase voltage. Figure 10 U11 is the first output-side phase voltage component, which is purely exemplary. Annotation spacing δU Then, the control device 15 is able to determine the second output side phase voltage component U21. U22 U23 The spacing is taken into account. Specifically, when the spacing is below a threshold, the control device 15 can determine a larger corresponding second output-side phase voltage component U21 than when the spacing exceeds the threshold. U22 U23 .

[0075] Currently, based on the complex output side current in the corresponding complex plane... I Or complex first output side voltage U1 The diagram illustrates how to consider the maximum values ​​of the output-side phase currents I1, I2, I3, or the output-side phase voltages. Specifically... Figure 9 The diagram shows the current regions corresponding to the maximum values ​​of the output-side phase currents I1, I2, and I3, and the complex output-side currents. I It should be located within the aforementioned current region. Similarly, in... Figure 10 The diagram shows the voltage region corresponding to the maximum value of the output-side phase voltage, and the complex total output-side voltage determined by the output-side phase voltage should lie within said voltage region. Alternatively, the control device 15 may be directly informed of the current region and / or voltage region in the corresponding complex plane. In this case, the same processing method can be performed in principle; however, the output-side phase currents I1, I2, I3 or the first output-side phase voltage component U11 are not considered. U12 U13 Instead of considering the distance from its maximum value, consider the complex output-side current. I The distance from the boundary of the current region, or the complex first output side voltage. U1 The distance from the boundary of the voltage region. This processing method is therefore particularly advantageous because it also allows for the pre-setting of permissible current or voltage regions for the control device 15, which are not simply defined by hexagons in the complex plane. Thus, for example, the interdependence between phases 12 on the output side can be taken into account.

[0076] Furthermore, in determining the first output-side phase voltage component U11 U12 U13 At the same time, it is also possible to consider the maximum value of the output-side phase voltage. Similarly, when determining this complex first output-side voltage... U1 At the same time, it is also possible to consider the voltage region in the complex plane. For example, according to Figure 11 As shown in the diagram, the control device 15 can first determine the complex first output side voltage according to a preset criterion in step S21. U1 The determination in step S21 is performed without considering the voltage region and is therefore only provisional. In this case, the control device 15 can check in step S22 the complex first output side voltage determined in step S21. U1 Is it within the voltage region of the total voltage of the complex output side? If not, the control device 15 corrects the complex first output side voltage determined in step S21 in step S23. U1 For example, scaling can be achieved using a (real) factor α, which is between 0 and 1. In this case, the factor α is determined as follows: the complex first output-side voltage determined in step S23. U1 Located within the voltage region of the total voltage on the complex output side. If the determination is performed by phase instead of in the complex plane, an equivalent result is obtained. Therefore, steps S21 to S23 correspond to... Figure 5 The design scheme for step S3.

[0077] For in Figure 5 In step S3 or Figure 11 In step S21, the complex first output side voltage is determined. U1 Different processing methods are feasible. Currently, the preferred method is for the control device 15 to determine the first output side phase voltage component U11. U12 U13 : Make the first output side phase voltage component U11 U12 U13 The sum of the products of the output-side phase currents I1, I2, and I3 equals the instantaneous target power P on the output side obtained by the control device 15. This is combined below. Figure 12 A detailed explanation is provided.

[0078] Similarly, control device 15 can determine the complex flux region in the complex plane, within which the complex flux vector of furnace transformer 3 should lie. In this case, control device 15 can determine the first output-side phase voltage component U11, taking into account the distance between the complex flux vector and the boundary of the flux region. U12 U13 and / or the second output phase voltage component U21 U22 U23 This is not shown separately in the accompanying drawings.

[0079] according to Figure 12 In step S31, the control device 15 obtains the instantaneous target power P on the output side. Output-side instantaneous target power P Is only valid for the current cycle. It is feasible to inform the control device 15 in advance for a large number of cycles about the time profile of the output-side instantaneous target power P . However, in this case, the control device 15 also determines the current value P for the respective period in step S31 according to the informed time profile . Whether or not the output-side instantaneous target power P is explicitly preset for the respective period, or the corresponding time profile is preset, however, the output-side instantaneous target power P should not change or should only change slowly over time.

[0080] In step S32, the control device 15 determines the first output-side phase voltage components U11 , U12 , U13 for the phase 12 on the output side. This determination is only made for the respective period and thus only for each current point in time. The determination in step S32 is made such that the sum of the products of the first output-side phase voltage components U11 , U12 , U13 and the output-side phase currents I1, I2, I3 is equal to the output-side instantaneous target power P . Therefore, the determination in step S3 is made such that the relationship

[0081] I1·U11 + I2·U12 + I3·U13 = P (7)

[0082] holds. As also Figure 12 shown, equivalent to the determination according to equation (7) is the direct determination according to the associated complex values I , U1 . By means of I’ in Figure 12 the conjugate complex value of the complex output-side current I is represented. Since the real part is no longer explicitly formed according to step S32, the imaginary part is required to have the value 0. Thereby, it is ensured at the same time that the first complex output-side voltage U1 is oriented parallel to this complex output-side current I . If in addition to the first complex output-side voltage U1 there is also a second complex output-side voltage U2 , the determined power does not change either. Because, due to the second complex output-side voltage U2 Orthogonal to the complex output side current I In this case, the voltage does not contribute to the power.

[0083] By using the drive C of the multilevel converter 4, the input-side phase currents i1, i2, and i3 are typically also confirmed. Conversely, due to the operation of the supply network 5, the input-side phase voltages u1, u2, and u3 are preset. These phase voltages can be determined, for example, by using a corresponding measuring device 19 (see...). Figure 1 ) detection notification control device 15.

[0084] In order to (completely) determine the drive control C for the multilevel converter 4, according to Figure 13 , Figure 5 The processing is preferably supplemented by additional steps S41 to S43. Steps S41 to S43 are also re-executed in each loop, and in any case, they are executed before step S5. Typically, they are executed after steps S1 to S4.

[0085] In step S41, the control device 15 learns the components a1, a2, and a3 for the input phase 6, and the power p instantaneously obtained from the supply network 5 (hereinafter referred to as the input instantaneous power p) should be distributed to the input phase 6 according to the components. Components a1, a2, and a3 are also only valid for the current cycle.

[0086] Similar to the instantaneous target power P on the output side It is feasible to inform the control device 15 in advance of the time direction for components a1, a2, and a3 for a large number of cycles. However, in this case, the control device 15 also determines the current values ​​a1, a2, and a3 for the corresponding cycle in step S41 based on the time direction informed to it.

[0087] For the instantaneous input power p, it is applicable that it is structurally equal to the power instantaneously delivered to the furnace transformer 3 by the multilevel converter 4. Only the losses occurring within the multilevel converter 4 need to be considered. Therefore, when the target instantaneous output power P is preset... At that time, the control device 15 can determine the instantaneous target power P on the output side. In step S42, the instantaneous power p on the input side is determined. When the instantaneous target power p on the output side is not preset... At that time, the control device 15 can determine the complex first output side voltage. U1 and complex output side current I In step S42, the instantaneous power p on the input side is determined. The latter represents the general case, and is therefore described using a complex value in step S42. In both cases, it is perfectly feasible to determine the instantaneous power p on the input side in step S42.

[0088] In step S43, the control device 15 determines the input-side phase currents i1, i2, and i3. This determination is performed such that the relationship is as follows: (8) , and (9) (10) Established.

[0089] Similar to the processing method on the output side of a multilevel converter, the input phase currents i1, i2, i3 and the input phase voltages u1, u2, u3 can also be expressed using the following formulas.

[0090] and

[0091] Convert to the corresponding complex value i , u Preferably, components a1, a2, and a3 are determined such that the complex input-side current... i according to Figure 14 The diagram shows the voltage parallel to the complex input side in the complex plane. u Orientation. Figure 14 The current on the complex input side is given in the figure. i and complex input side voltage u A very small angle between them (similar to) Figure 7 (Only for use in situations where) Figure 14 In order to better distinguish complex input side currents i and complex input side voltage u Therefore, the angle is only used for better illustration. In reality, the angle is 0°.

[0092] Complex input side current i and complex input side voltage u The parallel orientation is derived, particularly when components a1, a2, and a3 have orientations corresponding to the square of a sinusoidal orientation, where the corresponding time orientation is in phase with the corresponding voltage orientation of the input phase 6. Thus, observing over the entire or half-cycle, it is concluded that the supply network 5 is only loaded with active power. Under the additional condition that the instantaneous target power P on the output side... Furthermore, the supply network 5 experiences completely flicker-free loading, with no or only slow changes in time. If necessary, a constant or time-varying current on the complex input side can also be set according to predetermined criteria. i and complex input side voltage uThe phase angle between them. Thus, by observing again over the entire or half cycle, it is possible to specifically set the reactive power to be obtained from the supply network 5 in a limited manner.

[0093] The above text combined Figure 1 and 5 The following processing method is described, in which the control device 15 receives the output-side phase currents I1, I2, and I3 as measured values. Alternatively, according to Figure 15 As illustrated in the diagram, control device 15 can determine the output-side phase currents I1, I2, and I3 using model 20 of the electric arc furnace 2. The corresponding model is known to those skilled in the art. Furthermore, Figure 1 The design remains unchanged, and Figure 5 The processing method can also be preserved unchanged.

[0094] This invention has many advantages. In particular, it can almost completely avoid flickering, even when one of the arcs is briefly interrupted. Furthermore, the energy supply of the electric arc furnace 2 can be set very flexibly. In particular, the power limits of the furnace transformer 3 and the multi-stage converter 4 can be fully utilized.

[0095] Although the invention has been described and illustrated in more detail through preferred embodiments, the invention is not limited to the disclosed examples, and those skilled in the art can derive other variations therefrom without departing from the scope of protection of the invention.

[0096] List of reference numerals

[0097] 1 Electrode

[0098] 2. Electric Arc Furnace

[0099] 3 transformers

[0100] 4 Multilevel converter

[0101] 5. Supply Network

[0102] 6 Input phase

[0103] 7 Rectifier

[0104] 8 Inverters

[0105] 9 bridge arms

[0106] 10 submodules

[0107] 11. Choke

[0108] 12 Output side phase

[0109] 13 Energy storage capacitors

[0110] 14 Semiconductor Switches

[0111] 15. Control device

[0112] 16 Control Procedures

[0113] 17 Machine Code

[0114] 18, 19 Measuring devices

[0115] Model of 20 electric arc furnace

[0116] Components a1, a2, a3

[0117] C drive control

[0118] fN Grid frequency

[0119] I , i Complex current

[0120] I’ Conjugate complex value

[0121] I1, I2, I3 Output phase currents

[0122] i1, i2, i3 Input phase currents

[0123] P Output side instantaneous target power

[0124] p Input-side instantaneous power

[0125] Steps S1 to S43

[0126] TN cycle time

[0127] TZ cycle time

[0128] U1 , U2 , u Complex voltage

[0129] U11 U12 U13 First output side phase voltage component

[0130] U21 U22 U23 Second output side phase voltage component

[0131] Input phase voltages u1, u2, u3

[0132] alpha factor

[0133] δI, δU1 spacing.

Claims

1. A control method for a multilevel converter (4) that supplies electrical energy to the electrodes (1) of a three-phase electric arc furnace (2) via a furnace transformer (3), the control method comprising the following steps repeated over a periodic time (TZ): The control device (15) of the multilevel converter (4) learns the output phase currents (I1, I2, I3) flowing on the output side of the multilevel converter (4). -The control device (15) determines the first output phase voltage component (U11) for the corresponding current time point. U12 U13 ), so that the first output side phase voltage component (U11) U12 U13 The complex first output side voltage () is defined U1 ) in the complex plane parallel to the complex output-side currents defined by the output-side phase currents (I1, I2, I3) I ) orientation, -The control device (15) determines the second output phase voltage component (U21) U22 U23 ), so that the second output phase voltage component (U21) U22 U23 The complex second output side voltage () is defined U2 ) orthogonal to the complex first output side voltage in the complex plane ( U1 ) orientation, The control device (15) drives the multilevel converter (4) so ​​that the multilevel converter (4) provides the phase voltage component of the first output side and the phase voltage component of the second output side (U11). U12 U13 U21 U22 U23 The output phase voltage is consistent with the phase voltage of the phase.

2. The control method according to claim 1, characterized in that, The control device (15) takes into account the output side phase currents (I1, I2, I3) and / or the first output side phase voltage component (U11). U12 U13 In the case of determining the second output side phase voltage component (U21) U22 U23 ).

3. The control method according to claim 2, characterized in that, The control device (15) obtains the maximum value of the output-side phase currents (I1, I2, I3), and the control device (15) determines the second output-side phase voltage component (U21) after considering the distance between the output-side phase currents (I1, I2, I3) and the maximum value of the output-side phase currents. U22 U23 ), and / or The control device (15) obtains the maximum value of the output phase voltage, and the control device (15) takes into account the determined first output phase voltage component (U11). U12 U13 The second output side phase voltage component (U21) is determined based on the distance between the output side phase voltage and the maximum value of the output side phase voltage. U22 U23 ).

4. The control method according to claim 2, characterized in that, The control device (15) learns of a current region in the complex plane that can include the complex output-side current (I), and the control device (15) determines the second output-side phase voltage component (U21) taking into account the distance between the complex output-side current (I) and the boundary of the current region. U22 U23 ), and / or The control device (15) learns that there is a voltage region in the complex plane that can include the complex total output voltage defined by the output phase voltage, and the control device (15) takes into account the complex first output voltage (U1). The second output-side phase voltage component (U21) is determined based on the distance between the voltage region and the boundary of the voltage region. U22 U23 ).

5. The control method according to any one of the preceding claims, characterized in that, The control device (15) obtains the maximum value of the output phase voltage, and the control device (15) determines the first output phase voltage component (U11) taking into account the maximum value. U12 U13 ), and / or The control device (15) learns of a voltage region in the complex plane that includes a complex total output voltage defined by the output phase voltage, and the control device (15) determines the first output phase voltage component (U11) taking into account the boundary of the voltage region. U12 U13 ).

6. The control method according to any one of the preceding claims, characterized in that, The control device (15) identifies a flux region in the complex plane that includes the complex flux vector of the furnace transformer (3), and the control device (15) determines the first output-side phase voltage component and / or the second output-side phase voltage component (U11) taking into account the distance between the complex flux vector and the boundary of the flux region. U12 U13 U21 U22 U23 ).

7. The control method according to any one of the preceding claims, characterized in that, The control device (15) receives the output side phase current (I1, I2, I3) as a measurement value, or the control device (15) determines the output side phase current (I1, I2, I3) by means of the model (20) of the electric arc furnace (2).

8. The control method according to any one of the preceding claims, characterized in that, The control device (15) obtains the first output side phase voltage component (U11). U12 U13 ), causing the first output-side phase voltage component (U11) U12 U13 The sum of the products of the phase currents (I1, I2, I3) on the output side and the phase currents (I3, I1, I2, I3) on the output side is equal to the instantaneous target power (P) on the output side obtained by the control device (15). ).

9. The control method according to any one of the preceding claims, characterized in that, Due to the corresponding drive control by the control device (15), the multilevel converter (4) obtains input-side phase currents (i1, i2, i3) from the input-side phase (6) of the supply network (5), and the control device (15) repeatedly determines the drive control of the multilevel converter (4) with the periodic time (TZ), such that the complex input-side currents (i1, i2, i3) defined by the input-side phase currents (i1, i2, i3) i In the complex plane, the complex input-side voltage (u1, u2, u3) is defined at a predetermined phase angle relative to the input-side phase voltage (u1, u2, u3) of the input-side phase (6) of the supply network (5). u Orientation, particularly parallel to the complex input side voltage ( u ) orientation.

10. The control method according to any one of the preceding claims, characterized in that, Due to the corresponding drive control by the control device (15), the multilevel converter (4) obtains the input phase current (i1, i2, i3) from the input phase (6) of the supply network (5), and the control device (15) repeatedly determines the drive control of the multilevel converter (4) with the period time (TZ), such that the multilevel converter (4) obtains the input instantaneous power from the input phase (6) according to the periodic effective components (a1, a2, a3) of the input phase (6).

11. The control method according to any one of the preceding claims, characterized in that, The multilevel converter (4) is designed as a DC-link converter, which has an input-side rectifier (7) in the direction toward the supply network (5) and an output-side inverter (8) in the direction toward the furnace transformer (3), the rectifier and the inverter being interconnected via a DC voltage circuit.

12. A control program for a control device (15) for controlling a multilevel converter (4) that supplies electrical energy via a furnace transformer (3) to the electrodes (1) of a three-phase electric arc furnace (2), wherein, The control program has machine code (17) that can be directly processed by the control device (15), wherein the processing of the machine code (17) by the control device (15) causes the control device (15) to execute the control method according to any one of the preceding claims.

13. A control device for controlling a multilevel converter (4) that supplies electrical energy to the electrodes (1) of a three-phase electric arc furnace (2) via a furnace transformer (3), wherein, The control device is programmed using the control program (16) according to claim 12, such that the control device executes the control method according to any one of claims 1 to 11 during operation.

14. A multilevel converter that supplies electrical energy to the electrodes (1) of a three-phase electric arc furnace (2) via a furnace transformer (3), wherein, The multilevel converter is controlled by the control device (15) according to claim 13.

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