A unidirectional power flow rectifier bridge and a method of controlling the same

Abstract

The embodiment of the present disclosure provides a unidirectional power flow rectifier bridge and a control method thereof, and is applied to the technical field of power supply. The rectifier bridge comprises: three-phase rectifier bridge arms, each phase rectifier bridge arm being connected in parallel between positive and negative DC buses; each phase rectifier bridge arm is of the same structure and comprises a diode bridge arm and a modular multilevel bridge arm, the diode bridge arm and the modular multilevel bridge arm being connected in series with each other and connected between the positive and negative DC buses, and the connection points of the diode bridge arm and the modular multilevel bridge arm being connected with corresponding phases of an AC power grid. On this basis, the rectifier bridge is controlled to stabilize the DC voltage and reduce AC power grid flicker and harmonic interference, so that energy flows from the AC power grid to the DC bus through the rectifier bridge.

Description

Technical Field

[0001] This disclosure relates to the field of power supply technology, and in particular to a rectifier bridge with unidirectional power flow and its control method. Background Technology

[0002] An AC electric arc furnace is a steel smelting equipment that uses scrap steel as the main raw material. It uses an AC electric arc generated between the electrodes and the furnace charge as a heat source. The high temperature of the arc rapidly melts the scrap steel and completes the oxidation-reduction reaction, thereby realizing the recycling of scrap steel.

[0003] In the operation of an AC electric arc furnace, the power supply system is the core device for realizing the conversion and transmission of electrical energy. A typical AC electric arc furnace power supply system includes an arc furnace transformer, reactor, high-voltage switchgear, reactive power compensation device, and corresponding control and protection system. However, the arc load exhibits severe nonlinearity, time-varying characteristics, and asymmetry. This complex load characteristic brings a series of technical challenges to the AC electric arc furnace power supply equipment: First, the arc current fluctuates frequently and significantly, generating significant voltage flicker and harmonic interference, affecting the power quality of the power grid and the safe operation of the power supply equipment itself; second, it is difficult to maintain a balance of three-phase arc power, which can easily cause long-term overload of one phase winding of the transformer, shortening the equipment life; third, the power factor of the power supply system is low, requiring the configuration of a large-capacity reactive power compensation device.

[0004] To address the aforementioned issues, researchers have conducted extensive work on aspects such as the topology of power supply equipment, control strategies, and the design of key components. In existing technologies, AC electric arc furnace power supply systems have gradually adopted Static Var Compensators (SVCs) or Static Synchronous Compensators (STATCOMs) for dynamic reactive power compensation and flicker suppression, as illustrated in Static Synchronous Compensator STATCOM for Arc Furnace and Flicker Compensation, CIGRE Working Group B4.9, 2003.12. While these methods can correct power factors and reduce harmonics, flicker, and imbalances, achieving the highest power quality performance and furnace productivity requires large-capacity STATCOM equipment, and the flicker suppression effect is limited. The paper "Grid Friendly Supply of Nonlinear Dynamic Loads by a Scalable Hybrid Multilevel Converter Renewable Energy and Energy Management," published in Nürnberg, Germany, 2025, proposes a hybrid topology for the rectifier side. The AC side of the diode rectifier bridge is connected to the AC grid via a half-bridge modular multilevel converter (MMC) arm, improving the diode rectification characteristics. However, for this AC-AC converter composed of a rectifier on the rectifier side and an inverter on the inverter side, there is still room for further improvement in terms of size and cost. Summary of the Invention

[0005] In a first aspect, embodiments of this disclosure provide a rectifier bridge with unidirectional power flow, the rectifier bridge comprising: Three-phase rectifier bridge arms, with each phase rectifier bridge arm connected in parallel between the positive and negative DC buses; Each phase of the rectifier bridge arm has the same structure, including a diode bridge arm and a modular multilevel bridge arm. The diode bridge arm and the modular multilevel bridge arm are connected in series between the positive and negative DC buses. The connection point of the diode bridge arm and the modular multilevel bridge arm is connected to the corresponding phase of the AC power grid.

[0006] In some possible implementations of the first aspect, the modular multilevel bridge arm is composed of a reactor and at least two power modules connected in series; or, the modular multilevel bridge arm is composed of at least two power modules connected in series.

[0007] In some possible implementations of the first aspect, the power module is a half-bridge module, including: The first IGBT to the second IGBT and the first capacitor are connected together. The emitter of the first IGBT is connected to the collector of the second IGBT, and the connection point is used as the positive terminal of the half-bridge module. The collector of the first IGBT is connected to the positive terminal of the first capacitor, and the negative terminal of the first capacitor is connected to the emitter of the second IGBT, and the connection point is used as the negative terminal of the half-bridge module. Each of the first IGBTs to the second IGBT is connected to a reverse-connected diode.

[0008] In some possible implementations of the first aspect, the power module is a full-bridge module, including: The first to fourth IGBTs and the first capacitor are connected together. The emitter of the first IGBT is connected to the collector of the second IGBT, and the connection point is used as the positive terminal of the full-bridge module. The collector of the first IGBT is connected to the collector of the third IGBT and the positive terminal of the first capacitor. The emitter of the third IGBT is connected to the collector of the fourth IGBT, and the connection point is used as the negative terminal of the full-bridge module. The emitter of the fourth IGBT, the negative terminal of the first capacitor, and the emitter of the second IGBT are connected together. Each of the first to fourth IGBTs is connected to a reverse-connection diode.

[0009] In some possible implementations of the first aspect, a reactor is connected in series in the diode bridge arm near the connection point, and a reactor is connected in series between the AC power grid and the rectifier bridge.

[0010] Secondly, embodiments of this disclosure provide a control method for a rectifier bridge as described above, the method being executed by a rectifier bridge controller, including: The voltage on the grid side of the rectifier bridge is detected, and the phase of the AC grid is locked through a phase-locked loop. Based on the capacitor voltage of the rectifier bridge module With rectifier bridge module capacitor voltage command The relationship is such that the active current command of the rectifier bridge is generated through the module capacitor voltage controller. ; Based on rectifier bridge active current command Calculate the reactive power consumed on the grid-side impedance and generate the rectifier bridge reactive current command. To compensate for the reactive power consumed by the grid-side impedance; The rectifier bridge active current command Reactive current command of rectifier bridge The current regulator generates the rectifier bridge dq voltage command. , Based on the AC grid phase, the AC modulation command of the rectifier bridge is generated after a 2 / 3 transformation. , , ; Based on the DC bus voltage of the rectifier bridge With rectifier bridge DC bus voltage command The relationship is such that the DC bus voltage controller generates DC modulation commands for the rectifier bridge. ; AC modulation command for rectifier bridge , , and rectifier bridge DC modulation command The pulses are summed, and the trigger pulses for each phase rectifier bridge arm are obtained by using the nearest level approximation method based on the direction of the alternating current.

[0011] Thirdly, embodiments of this disclosure provide an AC-AC converter, which includes: The rectifier bridge and inverter bridge are connected in parallel on the DC side between the positive and negative DC buses. The rectifier bridge is as described above.

[0012] Among the possible implementations of the third aspect, the inverter bridge includes: At least one phase inverter arm, and each phase inverter arm is connected in parallel between the positive and negative DC buses; Each phase inverter arm has the same structure, including an upper arm and a lower arm. The upper arm and the lower arm are connected in series between the positive and negative DC buses. The connection point of the upper arm and the lower arm is connected to the corresponding phase of the electric arc furnace transformer.

[0013] Among the possible implementations of the third aspect, the inverter bridge includes: At least one phase inverter arm, and each phase inverter arm is connected in parallel between the positive and negative DC buses; Each phase inverter arm has the same structure, including a thyristor arm and a modular multilevel arm. The thyristor arm and the modular multilevel arm are connected in series between the positive and negative DC buses. The connection point of the thyristor arm and the modular multilevel arm is connected to the corresponding phase of the electric arc furnace transformer.

[0014] Fourthly, embodiments of this disclosure provide a flexible power supply system for an electric arc furnace, the flexible power supply system for the electric arc furnace comprising: AC-AC converter, electric arc furnace transformer, rectifier bridge controller, inverter bridge controller, electric arc furnace electrode position controller, the AC-AC converter is the AC-AC converter as described above.

[0015] In summary, the embodiments disclosed herein achieve at least the following technical effects compared to the prior art: In existing technologies, phase-shifting transformers are typically used to cascade multi-stage rectifier modules in medium- and high-voltage rectifier equipment. However, the rectifier bridge in this embodiment does not require the addition of multi-winding transformers or phase-shifting transformers. Furthermore, due to the effect of the cascaded power modules in the modular multilevel bridge arms, the harmonics flowing into the power grid are very small, eliminating the need for any additional AC filtering equipment. Simultaneously, in this embodiment, the maximum voltage borne by the modular multilevel bridge arms of the rectifier bridge is only equivalent to the DC voltage of the rectifier bridge. By introducing diodes, the number of power modules in the rectifier bridge can be significantly reduced, lowering its size and cost.

[0016] It should be understood that the description in the Summary of the Invention is not intended to limit the key or essential features of the embodiments of this disclosure, nor is it intended to restrict the scope of this disclosure. Other features of this disclosure will become readily apparent from the following description. Attached Figure Description

[0017] The above and other features, advantages, and aspects of the embodiments of this disclosure will become more apparent from the accompanying drawings and the following detailed description. The drawings are provided for a better understanding of the invention and are not intended to limit the scope of this disclosure. In the drawings, the same or similar reference numerals denote the same or similar elements, wherein: Figure 1 A structural diagram of a rectifier bridge providing unidirectional power flow according to an embodiment of this disclosure; Figure 2 A structural diagram of a modular multilevel bridge arm provided in an embodiment of this disclosure; Figure 3 A structural diagram of another modular multilevel bridge arm provided in this embodiment of the disclosure; Figure 4 A structural diagram of a power module provided in an embodiment of this disclosure; Figure 5 A structural diagram of another power module provided in an embodiment of this disclosure; Figure 6 A schematic diagram showing another arrangement of the diode bridge arm and the modular multilevel bridge arm in the rectifier bridge provided in the embodiments of this disclosure, as well as a reactor arrangement. Figure 7 A schematic diagram of a control method for a rectifier bridge with unidirectional power flow provided in an embodiment of this disclosure; Figure 8 A schematic diagram of the three-phase voltage waveform at the AC port of a rectifier bridge provided in this embodiment of the present disclosure; Figure 9 A schematic diagram of a current path of a three-phase rectifier bridge arm when the voltage of phase A on the AC side of the rectifier bridge is the highest and the voltage of phase C is the lowest, provided in an embodiment of this disclosure. Figure 10This is a schematic diagram of another current path for the three-phase rectifier bridge arm when the voltage of phase A on the AC side of the rectifier bridge is the highest and the voltage of phase C is the lowest, as provided in an embodiment of this disclosure. Figure 11 A structural diagram of an AC-AC converter consisting of a rectifier bridge and an MMC inverter bridge is provided for an embodiment of this disclosure; Figure 12 A schematic diagram of a control method for an MMC inverter bridge provided in an embodiment of this disclosure; Figure 13 A schematic diagram of an AC-AC converter consisting of a rectifier bridge and a low-cost inverter bridge, provided as an embodiment of this disclosure; Figure 14 This disclosure provides an embodiment of a current path for a three-phase inverter bridge arm when the voltage of phase A on the AC side of the inverter bridge is at its highest. Figure 15 A schematic diagram illustrating a control method for a modular multilevel bridge arm in a low-cost inverter bridge provided in this embodiment of the present disclosure; Figure 16 A schematic diagram illustrating a control method for a thyristor bridge arm in a low-cost inverter bridge provided in this embodiment of the disclosure; Figure 17 A schematic diagram of the three-phase voltage, thyristor bridge arm trigger pulse, and AC current waveforms at the AC port of an inverter bridge provided in an embodiment of this disclosure; Figure 18 This is a schematic diagram of another arrangement of the diode bridge arm, thyristor bridge arm, and modular multilevel bridge arm in an AC-AC converter composed of a rectifier bridge and a low-cost inverter bridge, provided in an embodiment of this disclosure. Figure 19 A schematic diagram illustrating the configuration of diode bridge arm reactance, thyristor bridge arm reactance, AC grid-side reactance, DC smoothing reactance, and DC capacitor in an AC-AC converter composed of a rectifier bridge and a low-cost inverter bridge, provided in an embodiment of this disclosure. Figure 20 A structural diagram of a flexible power supply system for an electric arc furnace, comprising an AC-AC converter consisting of a rectifier bridge and a low-cost inverter bridge, is provided for embodiments of this disclosure. Figure 21 This is a schematic diagram of a control method for an electric arc furnace electrode manipulation mechanism provided in an embodiment of this disclosure.

[0018] Figure label: 1: Rectifier bridge; 2: Inverter bridge; 3: Rectifier bridge arm; 4: Inverter bridge arm; 5: Diode bridge arm; 6: Modular multilevel bridge arm; 7: Upper bridge arm; 8: Lower bridge arm; 9: Power module; 10: Thyristor bridge arm; 11: Modular multilevel bridge arm; 5-A: Rectifier bridge A-phase diode bridge arm; 5-B: Rectifier bridge B-phase diode bridge arm; 5-C: Rectifier bridge C-phase diode bridge arm; 6-A: Rectifier bridge A-phase module Modular multilevel bridge arm; 6-B: Rectifier bridge B-phase modular multilevel bridge arm; 6-C: Rectifier bridge C-phase modular multilevel bridge arm; 10A: Inverter bridge A-phase thyristor bridge arm; 10B: Inverter bridge B-phase thyristor bridge arm; 10C: Inverter bridge C-phase thyristor bridge arm; 11A: Inverter bridge A-phase modular multilevel bridge arm; 11B: Inverter bridge B-phase modular multilevel bridge arm; 11C: Inverter bridge C-phase modular multilevel bridge arm. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. Based on the embodiments of this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure.

[0020] Furthermore, the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0021] Figure 1 A structural diagram of a rectifier bridge providing unidirectional power flow is provided in an embodiment of this disclosure, as shown below. Figure 1 As shown, rectifier bridge 1 includes: Three-phase rectifier bridge arm 3, each phase rectifier bridge arm 3 is connected in parallel between the positive and negative DC bus.

[0022] Each phase of the rectifier bridge arm 3 has the same structure, including a diode bridge arm 5 and a modular multilevel bridge arm 6. The diode bridge arm 5 and the modular multilevel bridge arm 6 are connected in series between the positive and negative DC buses. The connection point of the diode bridge arm 5 and the modular multilevel bridge arm 6 is connected to the corresponding phase of the AC power grid.

[0023] In some embodiments, the modular multilevel bridge arm 6 can be as follows: Figure 2 As shown, it consists of a reactor and at least two power modules 9 connected in series.

[0024] In other embodiments, the modular multilevel bridge arm 6 can be as follows: Figure 3 As shown, it consists of at least two power modules 9 connected in series.

[0025] Furthermore, the power module 9 can be a half-bridge module or a full-bridge module.

[0026] Among them, the half-bridge module can be as follows: Figure 4 As shown, it includes: a first IGBT T1 to a second IGBT T2 and a first capacitor C1. The emitter of the first IGBT T1 is connected to the collector of the second IGBT T2, and the connection point is used as the positive terminal of the half-bridge module. The collector of the first IGBT T1 is connected to the positive terminal of the first capacitor C1, and the negative terminal of the first capacitor C1 is connected to the emitter of the second IGBT T2, and the connection point is used as the negative terminal of the half-bridge module. Each of the first IGBT T1 to the second IGBT T2 is connected to a reverse-connected diode, namely diode D1 to diode D2.

[0027] The full-bridge module can be like Figure 5 As shown, the system includes: a first IGBT T1 to a fourth IGBT T4 and a first capacitor C1. The emitter of the first IGBT T1 is connected to the collector of the second IGBT T2, and the connection point serves as the positive terminal of the full-bridge module. The collector of the first IGBT T1 is connected to the collector of the third IGBT T3 and the positive terminal of the first capacitor C1, respectively. The emitter of the third IGBT T3 is connected to the collector of the fourth IGBT T4, and the connection point serves as the negative terminal of the full-bridge module. The emitter of the fourth IGBT T4, the negative terminal of the first capacitor C1, and the emitter of the second IGBT T2 are connected together. Each of the first IGBT T1 to the fourth IGBT T4 is connected to a reverse-biased diode, namely diode D1 to diode D4.

[0028] The following uses AC mains voltage (Line-to-line voltage RMS value), Voltage between positive and negative DC buses (DC bus voltage) Taking the above rectifier bridge as an example, let me introduce its working principle: During normal operation, Figure 4 The half-bridge module shown also has a stable DC voltage across its capacitors. For example, when using a 4500V IGBT, the capacitor voltage is... Left and right, through the turn-on and turn-off actions of the first IGBT T1 and the second IGBT T2, Figure 4 The half-bridge module shown can output 0 voltage or capacitor voltage. Figure 5 The full-bridge module shown also has a stable DC voltage across its capacitors. For example, when using 4500V IGBTs, the capacitor voltage is... Left and right, through the turn-on and turn-off actions of the first IGBT T1 to the fourth IGBT T4, Figure 5The full-bridge module shown can output 0 voltage, capacitor voltage, or negative capacitor voltage. The modular multilevel bridge arm 6, composed of half-bridge and / or full-bridge modules connected in series, requires a number of modules connected in series, depending on the maximum voltage across the bridge arm and the capacitor voltage of the modules. Considering the case of each phase rectifier bridge arm 3, since it is in rectification mode, when the voltage of phase A on the AC side of the rectifier bridge is highest, the diode bridge arm 5 of phase A of the rectifier bridge is turned on, and the modular multilevel bridge arm 6 of phase A of the rectifier bridge bears the entire DC bus voltage. This is the highest voltage that modular multilevel bridge arm 6 can withstand. Therefore, modular multilevel bridge arm 6 of the rectifier bridge can be designed in series: Therefore, the total number of power modules required for the three phases of rectifier bridge 1 is 27. 3 = 81. For existing standard MMC topologies used as rectifier bridges, the maximum voltage a single rectifier bridge arm can withstand is also DC voltage. For power modules with the same capacitor voltage, the number of power modules in a single rectifier bridge arm is 27, while a rectifier bridge with a typical MMC topology has 6 rectifier bridge arms, thus requiring a total of 162 power modules for three phases. In this embodiment, the number of power modules in the rectifier bridge is only half that of a typical MMC topology rectifier bridge, and the addition of diode bridge arms can significantly reduce the cost and size of the rectifier bridge.

[0029] It is worth noting that in the rectifier bridge arm 3 of this embodiment, the positions of the diode bridge arm 5 and the modular multilevel bridge arm 6 can be freely arranged, such as... Figure 1 As shown, the upper bridge arm is diode bridge arm 5, and the lower bridge arm is modular multilevel bridge arm 6; it can also be as follows: Figure 6 As shown, the two are reversed, but this is not a restriction.

[0030] Furthermore, in the rectifier bridge of this embodiment, necessary reactors can be added according to the requirements of fault handling and protection. For example... Figure 6 As shown, a recommended reactor arrangement is illustrated: a reactor is connected in series in the diode arm near the connection point, and another reactor is connected in series between the AC grid and the rectifier bridge. This allows the voltage of the modular multilevel bridge arm to directly determine the current flowing into the AC grid, reducing the impact of sudden load changes on the AC grid.

[0031] In summary, the embodiments disclosed herein achieve at least the following technical effects compared to the prior art: In existing technologies, phase-shifting transformers are typically used to cascade multi-stage rectifier modules in medium- and high-voltage rectifier equipment. However, the rectifier bridge in this embodiment does not require the addition of multi-winding transformers or phase-shifting transformers. Furthermore, due to the effect of the cascaded power modules in the modular multilevel bridge arms, the harmonics flowing into the power grid are very small, eliminating the need for any additional AC filtering equipment. Simultaneously, in this embodiment, the maximum voltage borne by the modular multilevel bridge arms of the rectifier bridge is only equivalent to the DC voltage of the rectifier bridge. By introducing diodes, the number of power modules in the rectifier bridge can be significantly reduced, lowering its size and cost.

[0032] The above is an introduction to the rectifier bridge structure. Based on this, the present disclosure also proposes a control method for the aforementioned rectifier bridge. This rectifier bridge control method is similar to that of a traditional MMC rectifier bridge, employing a nearest-level modulation method. The difference lies in that the AC modulation command generated by the current regulator needs to be superimposed with a changing DC modulation command to generate trigger pulses for each phase rectifier bridge arm. This changing DC modulation command originates from the DC bus voltage control loop. In other words, the rectifier bridge in this disclosure requires simultaneous control of both the DC bus voltage and the average capacitor voltage of the rectifier bridge module. Furthermore, the DC modulation command of the rectifier bridge in this disclosure is a time-varying quantity. The following detailed method embodiments illustrate the rectifier bridge control method. Figure 1 The rectifier bridge controller in the middle performs the function of stabilizing the DC voltage, specifically as follows: Figure 7 As shown, it includes: Detect the voltage on the grid side of the rectifier bridge and lock the AC grid phase through a phase-locked loop (PLL). .

[0033] Based on the capacitor voltage of the rectifier bridge module With rectifier bridge module capacitor voltage command The relationship is such that the active current command of the rectifier bridge is generated through the module capacitor voltage controller. Among them, if the rectifier bridge module capacitor voltage If the current is too high, the active current command of the rectifier bridge will be reduced. If the rectifier bridge module capacitor voltage If the current is too low, increase the active current command of the rectifier bridge. For example, the module capacitor voltage controller employs a typical PI regulator, whose transfer function is shown below: in, This is the proportional gain coefficient. This is the integral gain coefficient. is the frequency variable in the complex function.

[0034] Based on rectifier bridge active current command Calculate the reactive power consumed on the grid-side impedance and generate the rectifier bridge reactive current command. To compensate for the reactive power consumed on the grid-side impedance. For example, the rectifier bridge reactive current command... The generation of can be represented as: in, Indicates the grid-side equivalent reactance. This indicates the grid-side voltage.

[0035] The rectifier bridge active current command Reactive current command of rectifier bridge The current regulator generates the rectifier bridge dq voltage command. , Based on AC power grid phase After a 2 / 3 transformation, the AC modulation command for the rectifier bridge is generated. , , .

[0036] Based on the DC bus voltage of the rectifier bridge With rectifier bridge DC bus voltage command The relationship is such that the DC bus voltage controller generates DC modulation commands for the rectifier bridge. Among them, if the DC bus voltage of the rectifier bridge If the value is too high, reduce the DC modulation command of the rectifier bridge. If the DC bus voltage of the rectifier bridge If the value is too low, increase the DC modulation command of the rectifier bridge. For example, the DC bus voltage controller employs a typical PI regulator, whose transfer function is shown below: in, This is the proportional gain coefficient. This is the integral gain coefficient. is the frequency variable in the complex function.

[0037] AC modulation command for rectifier bridge , , and rectifier bridge DC modulation command The pulses are summed, and the trigger pulses for each phase rectifier bridge arm are obtained by using the nearest level approximation method based on the direction of the alternating current.

[0038] It should be noted that the above method can further consider the DC side current of the rectifier bridge and then add an inner loop control for the DC current.

[0039] In some embodiments of this disclosure, the rectifier bridge power factor can be actively adjusted over a wide range so that the three-phase voltage at the AC port of the rectifier bridge is within a certain range. Figure 8 Taking the interval 12 as an example, where the AC phase A voltage is the highest and the phase C voltage is the lowest, the direction of the AC current at this time can be controlled as follows: Figure 9 As shown, the B-phase current flows from the rectifier bridge to the AC power grid, and can also be controlled as follows: Figure 10 The B-phase current shown flows from the AC grid to the rectifier bridge. This is because the current direction of the modular multilevel arm of the rectifier bridge can be bidirectionally controllable. Similarly, in... Figure 8 In interval 13 shown, although the current direction of the diode arm 5-A of the rectifier bridge phase A can only flow from the AC grid to the DC bus, the current direction of the modular multilevel arm 6-A of the rectifier bridge phase A can flow bidirectionally, such as... Figure 9 and Figure 10 As shown. For simplicity, the current directions of phase B diode bridge arm 5-B, phase B modular multilevel bridge arm 6-B, phase C diode bridge arm 5-C, and phase C modular multilevel bridge arm 6-C are not described in detail here. Therefore, the degree of freedom in current flow direction ensures that the rectifier bridge of this embodiment can operate normally over a wide power factor range.

[0040] The above is an introduction to the rectifier bridge structure and its control method. Based on this, the present disclosure also proposes an AC-AC converter based on the above-mentioned rectifier bridge, including: The rectifier bridge and inverter bridge are connected in parallel on the DC side between the positive and negative DC buses.

[0041] In some embodiments, the AC-AC converter can be composed of a rectifier bridge and an MMC inverter bridge connected in parallel on the DC side, specifically as follows: Figure 11 As shown, rectifier bridge 1 is the aforementioned rectifier bridge, and inverter bridge 2 is the MMC inverter bridge, which includes: At least one phase inverter arm 4, and each phase inverter arm 4 is connected in parallel between the positive and negative DC buses.

[0042] Each phase inverter arm 4 has the same structure, including upper arm 7 and lower arm 8. Upper arm 7 and lower arm 8 are connected in series between the positive and negative DC buses. The connection point of upper arm 7 and lower arm 8 is connected to the corresponding phase of the AC load (electric arc furnace transformer).

[0043] It should be noted that the upper bridge arm 7 and the lower bridge arm 8 can adopt the same structure as the modular multilevel bridge arm 6 in the rectifier bridge 1, consisting of at least two power modules 9 connected in series, which are the aforementioned power modules.

[0044] The following uses the inverter-side AC output voltage (Line-to-line voltage RMS value), Voltage between positive and negative DC buses (DC bus voltage) Taking the above inverter bridge as an example, the working principle is as follows: For inverter bridge 2, the maximum inter-terminal voltage of each upper arm 7 and lower arm 8 is equivalent to 50% of the DC bus voltage plus the peak value of the inverter-side AC output voltage, that is: The required number of series modules in the upper arm 7 and lower arm 8 of inverter bridge 2 is: The inverter bridge requires a total of 25 power modules for its three phases. 6 = 150.

[0045] The above is an introduction to the MMC inverter bridge structure. Based on this, the present disclosure also proposes a control method for the aforementioned MMC inverter bridge. This control method can be a nearest-level approximation modulation method. The core of this method is: after the DC modulation command and AC modulation command are subjected to nearest-level approximation modulation, the trigger pulses of each power module in the upper bridge arm 7 are obtained; after the DC modulation command and AC modulation command are subjected to nearest-level approximation modulation, the trigger pulses of each power module in the lower bridge arm 8 are obtained. The following detailed description of the inverter bridge control method is provided through specific method embodiments. This method is... Figure 11 The inverter bridge controller in the middle performs this function to stabilize the AC output current, specifically as follows: Figure 12 As shown, it includes: According to the active current command of the inverter bridge Inverter bridge reactive current command Achieved via current regulator Current, Current control and generation of inverter bridge dq-axis voltage commands. , After a 2 / 3 conversion, an inverter bridge AC modulation command is generated. , , Among them, the inverter bridge active current command The inverter bridge reactive current command is the preset value. The default value is 0.

[0046] Calculate the DC modulation command of the inverter bridge The default value is 0.5pu (pu is a per-unit value, and the reference value is the rated DC voltage).

[0047] Subtracting the inverter bridge's DC modulation command from its AC modulation command yields the upper arm modulation command. Adding the DC modulation command to the AC modulation command yields the lower arm modulation command. Finally, the trigger pulses for each phase of the inverter bridge arm are obtained using the nearest-level approximation method. The phase θ of the 2 / 3 transformation varies depending on the application and can be calculated based on the rated frequency of the AC load. For example, in a typical application supplying power to an electric arc furnace load, it is calculated using the furnace frequency constant of 50Hz. For connections to the AC grid, it can also be derived from a phase-locked loop (PLL).

[0048] In other embodiments, the AC-AC converter can be composed of a rectifier bridge and a low-cost inverter bridge connected in parallel on the DC side, specifically as follows: Figure 13 As shown, rectifier bridge 1 is the aforementioned rectifier bridge, and inverter bridge 2 is a low-cost inverter bridge, including: At least one phase inverter arm 4, and each phase inverter arm 4 is connected in parallel between the positive and negative DC buses; Each phase inverter bridge arm 4 has the same structure, including thyristor bridge arm 10 and modular multilevel bridge arm 11. Thyristor bridge arm 10 and modular multilevel bridge arm 11 are connected in series between the positive and negative DC buses. The connection point of thyristor bridge arm 10 and modular multilevel bridge arm 11 is connected to the corresponding phase of the AC load (electric arc furnace transformer).

[0049] It should be noted that the modular multilevel bridge arm 11 can adopt the same structure as the modular multilevel bridge arm 6 in the rectifier bridge 1, which will not be elaborated here.

[0050] The following uses the inverter-side AC output voltage (Line-to-line voltage RMS value), Voltage between positive and negative DC buses (DC bus voltage) Taking the above inverter bridge as an example, the working principle is as follows: For the thyristor bridge arm 10 and the modular multilevel bridge arm 11 in inverter bridge 2, the maximum inter-terminal voltage of each thyristor bridge arm 10 and modular multilevel bridge arm 11 is the DC bus voltage of 66kV.

[0051] For the thyristor bridge arm in inverter bridge 2, considering that when the voltage of phase A on the AC side of the inverter bridge is at its highest, it triggers the 10A thyristor bridge arm of phase A to conduct, such as... Figure 14 As shown in the current direction of phase A, the 11A modular multilevel bridge arm of phase A of the inverter bridge bears the entire DC bus voltage. (This is the highest voltage that the modular multilevel bridge arm of the inverter bridge can withstand.) The situations of the other phase B thyristor bridge arms 10B, 11B, 10C, and 11C are not detailed here. At this point, using a single thyristor with a rated voltage of 6kV, it withstands 3kV during normal operation. The number of thyristor series stages in inverter bridge 2 is 66kV / 3kV = 22, totaling 66 thyristors across the three inverter bridge arms. The number of series modules in the modular multilevel bridge arm of inverter bridge 2 is: The inverter bridge requires a total of 27 power modules across its three modular multilevel bridge arms. 3 = 81.

[0052] The above is an introduction to the low-cost inverter bridge structure. Based on this, the embodiments of this disclosure also propose a control method for the above-mentioned low-cost inverter bridge. This method consists of... Figure 13 The inverter bridge controller in the process is executed in two parts, as follows: Figure 15 , Figure 16 As shown.

[0053] exist Figure 15 In the modular multilevel bridge arm control method shown: According to the active current command of the inverter bridge Inverter bridge reactive current command Achieved via current regulator Current, Current control and generation of inverter bridge dq-axis voltage commands. , After a 2 / 3 conversion, an inverter bridge AC modulation command is generated. , , Among them, the inverter bridge active current command The inverter bridge reactive current command is the preset value. The default value is 0. The phase θ of the 3 / 2 and 2 / 3 transformations can be calculated based on the rated frequency of the AC load, depending on the application. For example, in a typical application supplying power to an electric arc furnace load, it is calculated using the electric arc furnace frequency constant of 50Hz. For connections to the AC power grid, it can also be derived from a phase-locked loop (PLL).

[0054] Considering voltage balance control among the modular multilevel bridge arms, a zero-sequence voltage injection method is used to achieve voltage balance among the three-phase modular multilevel bridge arms. Specifically, the average capacitor voltage of the three-phase modular multilevel bridge arms is calculated. , , And respectively compared with the total average value of the modular multilevel bridge arm capacitor voltage. The difference is calculated, and after passing through a PI regulator, the d-axis component is extracted using a 3 / 2 transformation as the zero-sequence voltage amplitude. Set the zero-sequence q-axis component to 0, then... Connecting 0 to the coordinate inverse transformation module for a 2 / 3 transformation, and then inversely transforming back to the three-phase stationary coordinate system, yields the zero-sequence voltage of phase A. .

[0055] Fixed inverter bridge DC modulation command =0.5pu (pu is a per-unit value, the base value is the rated DC voltage) and the zero-sequence voltage of phase A. Together they are superimposed on the three-phase AC modulation signal of the inverter bridge. , , The modulation commands for each phase of the inverter bridge modular multilevel bridge arm are obtained, and the trigger pulses for each phase of the inverter bridge modular multilevel bridge arm are obtained by the nearest level approximation method.

[0056] exist Figure 16 In the thyristor bridge arm control method shown, the DC input current of the inverter bridge is controlled by adjusting the thyristor trigger phase, so that the input power and the output power of the inverter bridge are kept in balance, and the voltage of the modular multilevel bridge arm capacitors in the inverter bridge is kept constant. Specifically: Modular multilevel bridge arm capacitor voltage setpoint With modular multilevel bridge arm capacitor voltage feedback value The difference is calculated, and the error is used to obtain the inverter bridge DC current command via the module capacitor voltage controller. Then, the inverter bridge DC current command DC current feedback value of inverter bridge The difference is calculated, and the error is used to trigger the phase adjustment amount via the DC current controller. .

[0057] To maintain synchronization between the thyristor bridge arm triggering and the modular multilevel bridge arm modulation signal, the phase of the modular multilevel bridge arm voltage vector is adjusted. With trigger phase adjustment amount The difference is calculated, and then the phase offset π / 6 is added to obtain the trigger phase of the A-phase thyristor bridge arm. ; Trigger phase of phase A thyristor bridge arm The trigger phases of the B-phase and C-phase thyristor bridge arms are obtained by successively delaying by 2π / 3 and 4π / 3. and .

[0058] It is worth noting that the modular multilevel bridge arm voltage vector phase Depend on Figure 15 The phase angle calculation stage is generated. The module capacitor voltage controller and DC current controller use conventional PI regulators, which will not be described in detail here.

[0059] Based on the above control method, the embodiments of this disclosure can realize flexible power supply for AC loads, and the inverter side is usually set with a q-axis setpoint of 0. Figure 17 The waveforms of the three-phase voltage, thyristor bridge arm trigger pulses, and AC current at the AC port of the inverter bridge are shown. The three-phase voltage is at... Figure 17 Taking the interval 14 as an example, the AC phase A voltage is the highest, triggering the phase A thyristor bridge arm 10A. Figure 14 The DC current flows through the A-phase thyristor bridge arm 10A and the A-phase modular multilevel bridge arm 11A back to the negative DC bus, replenishing the energy of the modular multilevel bridge arms and maintaining a constant module capacitor voltage. Simultaneously, the A-phase modular multilevel bridge arm 11A outputs current to the AC load, which then flows through the B-phase modular multilevel bridge arm 11B and the C-phase modular multilevel bridge arm 11C back to the A-phase modular multilevel bridge arm 11A. The modular multilevel bridge arms buffer the DC input power and supply power to the AC load. The modular multilevel bridge arms can control the three-phase load current to be approximately sinusoidal.

[0060] It should be noted that in inverter bridge arm 4, the positions of thyristor bridge arm 10 and modular multilevel bridge arm 11 can be freely arranged, and the inverter bridge can be configured as follows: Figure 13 As shown, the upper bridge arm is thyristor bridge arm 10, and the lower bridge arm is modular multilevel bridge arm 11; it can also be as follows: Figure 18 As shown, the lower bridge arm is the thyristor bridge arm 10, and the upper bridge arm is the modular multilevel bridge arm 11.

[0061] Furthermore, in the inverter bridge of this embodiment, necessary reactors can be added according to the requirements of fault handling and protection. For example... Figure 19 As shown, a recommended reactor arrangement is illustrated: reactors are connected in series in the thyristor arms of the inverter bridge, and another reactor is connected in series between the AC grid and the rectifier bridge. This allows the voltage of the modular multilevel arms of the rectifier bridge to directly determine the current flowing into the AC grid, reducing potential flicker. A smoothing reactor can be connected in series on the DC side between the rectifier bridge and the inverter bridge, or a DC capacitor can be connected in parallel to further suppress the impact of sudden changes in AC load on the rectifier bridge.

[0062] The above is an introduction to AC-AC converters. Based on this, the embodiments of this disclosure also propose a flexible power supply system for electric arc furnaces based on the above-mentioned AC-AC converters, including: AC-AC converter, electric arc furnace transformer, rectifier bridge controller, inverter bridge controller, electric arc furnace electrode position controller.

[0063] Taking an AC-AC converter composed of a rectifier bridge and a low-cost inverter bridge as an example, the flexible power supply system for an electric arc furnace can be as follows: Figure 20 As shown, its control methods include: The rectifier bridge controller stabilizes the DC voltage and reduces AC grid flicker, allowing energy to flow from the AC grid to the DC bus through the rectifier bridge.

[0064] The inverter bridge controller stabilizes the primary current of the electric arc furnace transformer, allowing energy to flow from the DC bus to the electric arc furnace through the inverter bridge.

[0065] The electrode position controller controls the electrode manipulation mechanism of the electric arc furnace and stabilizes the three-phase arc impedance. Specifically, it can be implemented as follows: Figure 21 As shown, the process includes: detecting the secondary voltage and current of the electric arc furnace transformer, then calculating the current three-phase arc impedance of the electric arc furnace, and based on this, generating and sending control commands to manipulate the electric arc furnace electrode manipulation mechanism according to the relationship between the current three-phase arc impedance and the arc impedance command: wherein, if the arc impedance is lower than the arc impedance command, the electrode position is raised to increase the arc impedance; if the arc impedance is higher than the arc impedance command, the electrode position is lowered to decrease the arc impedance; if a current interruption occurs, the electrode position is lowered to trigger arc reignition.

[0066] As can be seen from the above, when the rectifier bridge and inverter bridge of this disclosure are used in the flexible power supply system of an electric arc furnace, they achieve two-stage isolation between the electric arc furnace fluctuations and the power supply system, significantly reducing the impact of the electric arc furnace smelting process on the AC power grid and significantly reducing the resulting flicker. Meanwhile, this disclosure embodiment does not require the addition of a multi-winding transformer or a phase-shifting transformer, and due to the effect of the cascaded power modules, the harmonics flowing into the power grid are very small, eliminating the need for any additional AC filtering equipment. Furthermore, in this disclosure embodiment, the maximum voltage borne by the modular multilevel bridge arms of the rectifier bridge and the modular multilevel bridge arms of the inverter bridge is only equivalent to the DC bus voltage, which can significantly reduce costs. Additionally, the flexible power supply system for the electric arc furnace of this disclosure embodiment can directly output medium voltage, making it suitable for the retrofitting of existing AC electric arc furnaces.

[0067] Furthermore, the control method of the electric arc furnace flexible power supply system in this embodiment realizes independent decoupling control of the rectifier bridge, inverter bridge and electric arc furnace electrode manipulation mechanism, with minimal mutual influence and simple and reliable system control.

[0068] It should be understood that the specific embodiments described above do not constitute a limitation on the scope of protection of this disclosure. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this disclosure should be included within the scope of protection of this disclosure.

Claims

1. A rectifier bridge with unidirectional power flow, characterized in that, The rectifier bridge includes: Three-phase rectifier bridge arms, with each phase rectifier bridge arm connected in parallel between the positive and negative DC buses; Each phase of the rectifier bridge arm has the same structure, including a diode bridge arm and a modular multilevel bridge arm. The diode bridge arm and the modular multilevel bridge arm are connected in series between the positive and negative DC buses. The connection point of the diode bridge arm and the modular multilevel bridge arm is connected to the corresponding phase of the AC power grid.

2. The rectifier bridge according to claim 1, characterized in that, The modular multilevel bridge arm is composed of a reactor and at least two power modules connected in series; or, the modular multilevel bridge arm is composed of at least two power modules connected in series.

3. The rectifier bridge according to claim 2, characterized in that, The power module is a half-bridge module, comprising: The first IGBT to the second IGBT and the first capacitor are connected together. The emitter of the first IGBT is connected to the collector of the second IGBT, and the connection point is used as the positive terminal of the half-bridge module. The collector of the first IGBT is connected to the positive terminal of the first capacitor, and the negative terminal of the first capacitor is connected to the emitter of the second IGBT, and the connection point is used as the negative terminal of the half-bridge module. Each of the first IGBT to the second IGBT is connected to a reverse-connected diode.

4. The rectifier bridge according to claim 2, characterized in that, The power module is a full-bridge module, comprising: The first to fourth IGBTs and the first capacitor are connected together. The emitter of the first IGBT is connected to the collector of the second IGBT, and the connection point is used as the positive terminal of the full-bridge module. The collector of the first IGBT is connected to the collector of the third IGBT and the positive terminal of the first capacitor. The emitter of the third IGBT is connected to the collector of the fourth IGBT, and the connection point is used as the negative terminal of the full-bridge module. The emitter of the fourth IGBT, the negative terminal of the first capacitor, and the emitter of the second IGBT are connected together. Each of the first to fourth IGBTs is connected to a reverse-connected diode.

5. The rectifier bridge according to claim 2, characterized in that, A reactor is connected in series in the diode bridge arm near the connection point, and a reactor is connected in series between the AC power grid and the rectifier bridge.

6. A control method for a rectifier bridge according to any one of claims 1-5, characterized in that, The method is executed by the rectifier bridge controller and includes: The voltage on the grid side of the rectifier bridge is detected, and the phase of the AC grid is locked through a phase-locked loop. Based on the capacitor voltage of the rectifier bridge module With rectifier bridge module capacitor voltage command The relationship is such that the active current command of the rectifier bridge is generated through the module capacitor voltage controller. ; Based on rectifier bridge active current command Calculate the reactive power consumed on the grid-side impedance and generate the rectifier bridge reactive current command. To compensate for the reactive power consumed by the grid-side impedance; The rectifier bridge active current command Reactive current command of rectifier bridge The current regulator generates the rectifier bridge dq voltage command. , Based on the AC grid phase, the AC modulation command of the rectifier bridge is generated after a 2 / 3 transformation. , , ; Based on the DC bus voltage of the rectifier bridge With rectifier bridge DC bus voltage command The relationship is such that the DC bus voltage controller generates DC modulation commands for the rectifier bridge. ; AC modulation command for rectifier bridge , , and rectifier bridge DC modulation command The pulses are summed, and the trigger pulses for each phase rectifier bridge arm are obtained by using the nearest level approximation method based on the direction of the alternating current.

7. An AC-AC converter, characterized in that, The AC-AC converter includes: A rectifier bridge and an inverter bridge are provided, wherein the DC side of the rectifier bridge and the inverter bridge are connected in parallel between the positive and negative DC buses, and the rectifier bridge is the rectifier bridge according to any one of claims 1-5.

8. The AC-AC converter according to claim 7, characterized in that, The inverter bridge includes: At least one phase inverter arm, and each phase inverter arm is connected in parallel between the positive and negative DC buses; Each phase inverter arm has the same structure, including an upper arm and a lower arm. The upper arm and the lower arm are connected in series between the positive and negative DC buses. The connection point of the upper arm and the lower arm is connected to the corresponding point of the electric arc furnace transformer.

9. The AC-AC converter according to claim 7, characterized in that, The inverter bridge includes: At least one phase inverter arm, and each phase inverter arm is connected in parallel between the positive and negative DC buses; Each phase inverter arm has the same structure, including a thyristor arm and a modular multilevel arm. The thyristor arm and the modular multilevel arm are connected in series between the positive and negative DC buses. The connection point of the thyristor arm and the modular multilevel arm is connected to the corresponding phase of the electric arc furnace transformer.

10. A flexible power supply system for an electric arc furnace, characterized in that, The flexible power supply system for the electric arc furnace includes: AC-AC converter, electric arc furnace transformer, rectifier bridge controller, inverter bridge controller, electric arc furnace electrode position controller, wherein the AC-AC converter is the AC-AC converter according to any one of claims 7-9.

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