Cascade inverter, control method and computer readable storage medium

Abstract

A cascade frequency converter, control method, and computer-readable storage medium. The cascade frequency converter includes a transformer, including a three-phase input terminal connected to a power grid and three groups of single-phase output terminals connected to corresponding power unit groups. Each power unit input terminal is connected to a corresponding single-phase output terminal; each power unit includes a first branch and a second branch; the input sides of the first branch and the second branch are connected to the corresponding single-phase output terminals; a first chopper switch is connected in parallel to the output sides of the first branch and the second branch; or, a second chopper switch is connected in parallel to the first diode or the second diode and a third chopper switch is connected in parallel to the third diode or the fourth diode; a processing module is connected to the control terminals of the first chopper switch, the second chopper switch, and the third chopper switch, and can determine the leakage inductance of the transformer; based on the leakage inductance, the operation of the first chopper switch, the second chopper switch, and the third chopper switch is controlled to reduce harmonic pollution to the power grid.

Description

Technical Field

[0001] The present invention generally relates to the technical field of cascade frequency converters, and in particular to a cascade frequency converter, a control method, and a computer-readable storage medium. Background Art

[0002] A cascade variable-frequency drive (VFD) is a power control device that controls a motor by varying the frequency and amplitude of its output voltage. Cascade VFDs are widely used in applications such as fans, water pumps, belt conveyors, and experimental power supplies. The harmonic pollution caused by cascade VFDs in the power grid adversely affects the quality of power supply. This application addresses the technical problem of reducing harmonic pollution caused by cascade VFDs.

[0003] The contents of the background technology section are merely the technologies known to the inventors and do not necessarily represent the existing technologies in this field. Summary of the Invention

[0004] In response to one or more problems in the prior art, the present invention provides a cascaded frequency converter, comprising: a transformer, comprising a three-phase input terminal and three groups of single-phase output terminals, wherein the three-phase input terminal can be connected to a power grid; and three power unit groups, connected one-to-one with the three groups of single-phase output terminals, each power unit group comprising a plurality of cascaded power units, and the input terminal of each power unit being connected to the corresponding single-phase output terminal; wherein each power unit comprises: a rectifier bridge, comprising a first branch and a second branch, wherein the first branch comprises a first diode and a second diode connected in series; the second branch comprises a third diode and a fourth diode connected in series; the input sides of the first branch and the second branch are connected to the corresponding single-phase output terminals. a first chopping switch connected in parallel to the output sides of the first branch and the second branch; or, a second chopping switch and a third chopping switch, wherein the second chopping switch is connected in parallel to the first diode or the second diode; and the third chopping switch is connected in parallel to the third diode or the fourth diode; and a processing module connected to the control end of the first chopping switch, or to the control ends of the second chopping switch and the third chopping switch, and configured to determine the leakage inductance of the transformer; and control the operation of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance, so as to reduce harmonic pollution of the cascaded inverter to the power grid.

[0005] Optionally, the cathode of the first diode is connected to the cathode of the third diode; the anode of the second diode is connected to the anode of the fourth diode; the cathode of the second diode and the anode of the third diode are connected to corresponding single-phase output terminals; the power unit also includes a capacitor connected to the DC bus and an inverter bridge connected in parallel to the capacitor.

[0006] Optionally, when the power unit includes the first chopper switch, the power unit further includes a fifth diode, the anode of the fifth diode is connected to the cathode of the third diode, and the cathode of the fifth diode is connected to the capacitor.

[0007] Optionally, the processing module is configured to control the on-off frequency of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance.

[0008] Optionally, the processing module is configured to control a period of a control signal at a control end of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance, so as to control the on-off frequency.

[0009] Optionally, the control signal includes a PWM wave signal; and the processing module is configured to control a carrier period of the PWM wave signal based on leakage inductance to control a period of the PWM wave signal.

[0010] Optionally, the switching frequency is negatively correlated with the leakage inductance.

[0011] Optionally, the processing module of each power unit is configured to control the respective carrier phases based on the synchronization signal, so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0012] Optionally, in each power unit group, the processing module of one power unit is configured to provide the synchronization signal, and the processing modules of other power units in the same group are configured to adjust their respective carrier phases based on the synchronization signal so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0013] Optionally, the cascade inverter also includes a main control module, which is connected to the processing module of each power unit and configured to provide the synchronization signal; the processing module of each power unit is configured to adjust its respective carrier phase based on the synchronization signal so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0014] Optionally, the processing module and / or the main control module of the cascade frequency converter is configured to provide the synchronization signal under preset conditions.

[0015] Optionally, the preset condition includes periodically and / or the deviation between the difference in carrier phase of any two adjacent power units in the power unit group and the preset angle is greater than or equal to an angle threshold.

[0016] Optionally, the power unit further includes a capacitor connected to a DC bus; the processing module is further configured to determine an input voltage, an input current, and a voltage across the capacitor of the power unit; and control a duty cycle of a control signal at a control end of the first chopper switch, or the second chopper switch and the third chopper switch based on the input voltage, the input current, and the voltage across the capacitor.

[0017] Optionally, the cascade inverter further includes a plurality of measuring windings arranged on the secondary coil of the transformer, each measuring winding is connected to a processing module of a corresponding power unit, and the processing module of each power unit determines the input voltage based on the corresponding measuring winding.

[0018] Optionally, the cascade inverter also includes a main control module, which is configured to collect primary phase voltage information of the transformer and transmit the primary phase voltage information to a processing module of the corresponding power unit, and the processing module determines the input voltage of the power unit based on the corresponding primary voltage information.

[0019] Optionally, the chopper switch includes at least one of a metal oxide semiconductor field effect transistor, an insulated gate bipolar transistor, or an electron injection enhanced gate transistor.

[0020] The present invention further provides a control method for the cascade frequency converter as described above, comprising:

[0021] S310: Determine the leakage inductance of the transformer; and

[0022] S330: Based on the leakage inductance, control the operation of the first chopping switch, or the second chopping switch and the third chopping switch to reduce harmonic pollution of the cascaded frequency converter to the power grid.

[0023] Optionally, step S330 includes: controlling the on-off frequency of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance.

[0024] Optionally, controlling the on-off frequency includes: controlling a period of a control signal at a control end of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance.

[0025] Optionally, the control signal includes a PWM wave signal; and controlling the period of the PWM wave signal includes: controlling a carrier period of the PWM wave signal based on leakage inductance.

[0026] Optionally, step S330 further includes: controlling the processing module of each power unit to control the respective carrier phases based on the synchronization signal, so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0027] Optionally, the processing module that controls each power unit controls their respective carrier phases based on the synchronization signal, including: controlling the processing module of one of the power units in each power unit group to provide the synchronization signal, and controlling the processing modules of other power units in the same group to adjust their respective carrier phases based on the synchronization signal, so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0028] Optionally, the processing module that controls each power unit controls the respective carrier phases based on the synchronization signal, including: controlling the main control module of the cascaded inverter to provide the synchronization signal; and controlling the processing module of each power unit to adjust the respective carrier phases based on the synchronization signal so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0029] Optionally, controlling the processing module of each power unit to control the respective carrier phases based on the synchronization signal includes: controlling the processing module and / or the main control module of the cascade converter to provide the synchronization signal under preset conditions.

[0030] Optionally, the preset condition includes periodically and / or the deviation between the difference in carrier phase of any two adjacent power units in the power unit group and the preset angle is greater than or equal to an angle threshold.

[0031] Optionally, the control method further includes: determining the input voltage, input current and voltage across the capacitor connected to the DC bus of the power unit; and controlling the duty cycle of the control signal of the control end of the first chopper switch, or the second chopper switch and the third chopper switch based on the input voltage, input current and voltage across the capacitor.

[0032] Optionally, the control method further includes: controlling the processing module of each power unit to determine the input voltage based on the corresponding measurement winding.

[0033] Optionally, the control method further includes: collecting transformer primary phase voltage information via a main control module, transmitting the primary phase voltage information to a processing module of a corresponding power unit, and controlling the processing module to determine the input voltage of the power unit based on the corresponding primary voltage information. The present invention also provides a computer-readable storage medium including computer-executable instructions stored thereon, which, when executed by a processor, implement the control method described above.

[0034] The cascaded frequency converter or control method of the present invention cleverly utilizes the leakage inductance of the transformer as an inductor and controls the operation of the chopper switch based on the leakage inductance. This reduces the harmonic pollution of the cascaded frequency converter to the power grid while saving the volume, device cost, and design complexity of the cascaded frequency converter. The power unit has a single-phase input and single-phase output, which greatly reduces the number of transformer secondary coil windings, the number of cables connecting the transformer and the power unit, and the wiring complexity and cost. The single-phase input side of the power unit uses four diodes to form a single-phase rectifier bridge. Compared with three-phase input, this reduces the number of diodes and saves costs.

[0035] The cascade frequency converter or control method of the present invention helps to improve the stability, reliability, efficiency and safety of power supply of the power grid by reducing the harmonic pollution of the cascade frequency converter to the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0037] Figure 1 A schematic diagram of a cascade frequency converter according to some embodiments of the present invention is shown.

[0038] Figure 2 A schematic diagram illustrating a power unit according to some embodiments of the present invention is shown.

[0039] Figure 3 A schematic diagram illustrating a power unit according to some embodiments of the present invention is shown.

[0040] Figure 4 A schematic diagram illustrating a power unit according to some embodiments of the present invention is shown.

[0041] Figure 5 A schematic diagram illustrating carrier synchronization control according to some embodiments of the present invention is shown.

[0042] Figure 6 A schematic diagram illustrating carrier synchronization control according to some embodiments of the present invention is shown.

[0043] Figure 7 A schematic diagram illustrating controlling the duty cycle of a chopping switch according to some embodiments of the present invention.

[0044] Figure 8 A schematic diagram illustrating controlling the duty cycle of a chopping switch according to some embodiments of the present invention.

[0045] Figure 9 A schematic diagram illustrating the working principle of controlling the duty cycle of a chopper switch according to some embodiments of the present invention is shown.

[0046] Figure 10A partial schematic diagram of a cascade frequency converter according to some embodiments of the present invention is shown.

[0047] Figure 11 A flow chart of a control method according to some embodiments of the present invention is shown.

[0048] Figure 12 Schematic diagram showing waveforms of input voltage and input current of a single power unit according to some embodiments of the present invention.

[0049] Figure 13 Show Figure 12 Schematic diagram of the spectrum of the input current harmonics of a single power unit.

[0050] Figure 14 Schematic diagram showing waveforms of input currents of single-phase multiple power units and input currents of a cascaded inverter according to some embodiments of the present invention.

[0051] Figure 15 Show Figure 14 A partial enlarged view of .

[0052] Figure 16 Show Figure 14 Schematic diagram of the spectrum of the input current harmonics of a single power unit and a cascaded inverter. DETAILED DESCRIPTION

[0053] Hereinafter, only certain exemplary embodiments are briefly described. As will be appreciated by those skilled in the art, the described embodiments may be modified in various ways without departing from the spirit or scope of the present invention. Therefore, the drawings and description are to be considered as illustrative in nature and not restrictive.

[0054] In the description of the present invention, it should be understood that the terms "center," "longitudinal," "transverse," "length," "width," "thickness," "up," "down," "front," "back," "left," "right," "vertical," "horizontal," "top," "bottom," "inside," "outside," "clockwise," "counterclockwise," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely for the purpose of facilitating the description of the present invention and simplifying the description. They do not indicate or imply that the devices or components referred to must have a specific orientation, be constructed, or operate in a specific orientation, and therefore should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features indicated. Thus, features defined as "first" or "second" may explicitly or implicitly include one or more of the aforementioned features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0055] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," "connected," and "coupled" should be understood broadly. For example, they may refer to fixed, removable, or integral connections; mechanical, electrical, or intercommunication connections; direct or indirect connections through an intermediary; and internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0056] In the present invention, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may also include the first and second features not being in direct contact but being in contact via another feature between them. Furthermore, a first feature being "above," "above," and "above" a second feature may include the first feature being directly above or diagonally above the second feature, or may simply mean that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may include the first feature being directly below or diagonally below the second feature, or may simply mean that the first feature is lower in level than the second feature.

[0057] Many different embodiments or examples are provided below to implement different structures of the present invention. In order to simplify the present invention, the components and settings of specific examples are described below. Of course, they are only examples and are not intended to limit the present invention. In addition, the present invention may repeat reference numbers and / or reference letters in different examples. Such repetition is for the purpose of simplicity and clarity and does not in itself indicate the relationship between the various embodiments and / or settings discussed. In addition, the present invention provides examples of various specific processes and materials, but those of ordinary skill in the art will recognize the application of other processes and / or the use of other materials.

[0058] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0059] The present invention provides a cascade frequency converter. The cascade frequency converter includes a transformer and three power unit groups. The transformer includes a three-phase input terminal and three sets of single-phase output terminals. The three-phase input terminal can be connected to a power grid. The three sets of single-phase output terminals are connected one-to-one with the three power unit groups. Each power unit group includes multiple cascaded power units, with the input terminal of each power unit connected to a corresponding single-phase output terminal. Each power unit includes a rectifier bridge. The rectifier bridge includes a first branch and a second branch. The first branch includes a first diode and a second diode connected in series. The second branch includes a third diode and a fourth diode connected in series. The input sides of the first branch and the second branch are connected to the corresponding single-phase output terminals. Each power unit includes a first chopper switch. The first chopper switch is connected in parallel to the output sides of the first branch and the second branch. Alternatively, each power unit includes a second chopper switch and a third chopper switch. The second chopper switch is connected in parallel to the first diode or the second diode. The third chopper switch is connected in parallel to the third diode or the fourth diode. Each power unit includes a processing module. The processing module is connected to the control terminal of the first chopper switch. Alternatively, the processing module is connected to control terminals of the second chopper switch and the third chopper switch. The processing module may determine the leakage inductance of the transformer. Based on the leakage inductance of the transformer, the processing module may control the operation of the first chopper switch. Alternatively, the processing module may control the operation of the second chopper switch and the third chopper switch to reduce harmonic pollution of the cascaded frequency converter on the power grid.

[0060] Figure 1 FIG. 1 shows a schematic diagram of a cascade frequency converter 100 according to some embodiments of the present invention. Figure 1 As shown, the cascade inverter 100 includes a transformer 10 and three power cell groups G1, G2, and G3. Transformer 10 includes three-phase input terminals (A, B, and C) and three sets of single-phase output terminals. The three-phase input terminals of transformer 10 are connected to the transformer's three-phase primary coils 101A, 101B, and 101C, which can be connected to the power grid E (e.g., 10 kV). The three sets of single-phase output terminals are connected to the transformer's three-phase secondary coils 103A, 103B, and 103C. Transformer 10 also includes iron cores 102A, 102B, and 102C that connect the three-phase primary coils to the three-phase secondary coils. The three sets of single-phase output terminals of transformer 10 are connected to the three power cell groups G1, G2, and G3 in a one-to-one correspondence. For example, the A-phase output terminal of transformer 10 is connected to power cell group G1. The B-phase output terminal of transformer 10 is connected to power cell group G2. The C-phase output terminal of the transformer 10 is connected to the power unit group G3 .

[0061] Each power unit group includes a plurality of cascaded power units, and a column of cascaded power units constitutes a power unit group, and the input end of each power unit is connected to the single-phase output end of the corresponding transformer. Figure 1As shown, power cell group G1 includes multiple cascaded power cells 21, with the input of each power cell 21 connected to the A-phase output of transformer 10. Power cell group G2 includes multiple cascaded power cells 22, with the input of each power cell 22 connected to the B-phase output of transformer 10. Power cell group G3 includes multiple cascaded power cells 23, with the input of each power cell 23 connected to the C-phase output of transformer 10. In each power cell group, one of the output terminals (e.g., the V output terminal) of the power cells at the head end (e.g., near transformer 10) is short-circuited to form a neutral point O, and one of the output terminals (e.g., the U output terminal) of the power cells at the tail end (e.g., near motor M) is connected to motor M. Adjacent power cells are connected via the U and V output terminals. Each power cell in the three power cell groups G1, G2, and G3 has a single-phase input and a single-phase output. It should be noted that the present invention does not limit the number of power cells in a power cell group, nor does it limit the number of primary and secondary coils or windings of the transformer. In practical applications, these can be set according to requirements.

[0062] Some traditional cascaded high-voltage inverters use a delta-shaped transformer secondary connection. The three-phase output of the transformer's low-voltage side is connected to the three-phase input of each power unit, achieving multi-pulse rectification, reducing the harmonics of the transformer's primary current, and reducing the harmonic pollution of the cascaded inverter to the power grid. This delta-shaped connection requires a three-phase transformer output and a three-phase power unit input. The three-phase transformer output must be connected to the three-phase input of each power unit. Each power unit must be connected to the transformer's three secondary windings, and three cables are required between the transformer and each power unit. This results in a large number of transformer secondary windings and connecting cables between the transformer and power units, making wiring complex and costly. Furthermore, the delta-shaped transformer connection requires a proportional combination of delta and star connections on the transformer's secondary side. The connection process requires welding and insulation damage recovery techniques, which greatly increases process complexity and labor input, hindering automated production and increasing the overall cost of the transformer, and thus the overall cost of the cascaded inverter.

[0063] Unlike traditional solutions, the cascaded inverter of the present invention features three sets of single-phase transformer outputs and a single-phase input for each power unit. The three sets of single-phase transformer outputs are connected one-to-one with the single-phase inputs of the power units in the three power unit groups. In other words, the single-phase transformer output is connected to the single-phase input of each power unit in the corresponding power unit group. This allows each power unit to be connected to a single-phase secondary winding of the transformer, and the transformer and each power unit can be connected via two cables, significantly reducing the number of transformer secondary windings, the number of cables connecting the transformer and power units, and the complexity and cost of wiring.

[0064] For example, taking a 10kV cascade frequency converter as an example, each power unit group includes 8 power units, and 3 power unit groups include a total of 24 power units. If the traditional technical solution is adopted, each power unit has three-phase input, and the number of secondary windings of the transformer corresponding to each power unit group is 8*3=24, and the number of secondary windings corresponding to the three power unit groups is 24*3=72; the number of cables connecting each power unit group to the transformer is 8*3=24, and the number of cables connecting the three power unit groups to the transformer is 24*3=72. In the technical solution of the present invention, each power unit has single-phase input, and the number of secondary windings of the transformer corresponding to each power unit group is 8*1, and the number of secondary windings corresponding to the three power unit groups is 8*3; the number of cables connecting each power unit group to the transformer is 8*2=16, and the number of cables connecting the three power unit groups to the transformer is 16*3=48. As can be seen, the technical solution of the present invention significantly reduces the number of transformer secondary windings and cables compared to traditional technical solutions, and also reduces wiring complexity and costs. It should be understood that this is only an example of a 10kV cascade inverter, and the present invention is not limited to this. For high-voltage cascade inverters of other kV and above, the technical solution of the present invention can also achieve similar technical effects such as reducing the number of transformer secondary windings, the number of cables, wiring complexity, and costs.

[0065] Figure 2 Schematic diagram of power units 21 / 22 / 23 according to some embodiments of the present invention is shown. Figure 2 As shown, the power unit 21 / 22 / 23 includes a rectifier bridge, a first chopper switch S1 and a processing module 24. The rectifier bridge is used to convert alternating current into direct current. The rectifier bridge includes a first branch and a second branch. The first branch includes a first diode D1 and a second diode D2 connected in series. The second branch includes a third diode D3 and a fourth diode D4 connected in series. The cathode of the first diode D1 is connected to the cathode of the third diode D3. The anode of the second diode D2 is connected to the anode of the fourth diode D4. The common end S connected to the first diode D1 and the second diode D2 constitutes the input side of the first branch. The common end R connected to the third diode D3 and the fourth diode D4 constitutes the input side of the second branch. The input side of the first branch and the input side of the second branch constitute the single-phase input end of the power unit 21 / 22 / 23. The single-phase input end of the power unit is connected to the corresponding single-phase output end of the transformer. As shown Figure 1 and Figure 2As shown, the input sides of the first and second branches of power cell 21 in power cell group G1 are connected to the A-phase output terminal. The input sides of the first and second branches of power cell 22 in power cell group G2 are connected to the B-phase output terminal. The input sides of the first and second branches of power cell 23 in power cell group G3 are connected to the C-phase output terminal. In other words, the cathode of second diode D2 and the anode of third diode D3 are connected to the corresponding single-phase output terminals of the transformer. In other words, the anode of first diode D1 and the cathode of fourth diode D4 are connected to the corresponding single-phase output terminals of the transformer.

[0066] like Figure 2 As shown, the first chopper switch S1 is connected in parallel to the output side of the first branch and the second branch. The cathode of the third diode D3 and the anode of the fourth diode D4 constitute the output side of the first branch and the second branch. In other words, the first chopper switch S1 is connected in parallel to the output side of the rectifier bridge. The processing module 24 is connected to the control terminal g1 of the first chopper switch S1. The processing module 24 can determine the leakage inductance L of the transformer 10, and control the operation of the first chopper switch S1 based on the leakage inductance L to reduce the harmonic pollution of the cascaded frequency converter 100 to the power grid E. The present invention cleverly uses the leakage inductance of the transformer as an inductor and controls the operation of the chopper switch based on the leakage inductance. While reducing the harmonic pollution of the cascaded frequency converter to the power grid, it saves the volume, device cost and design complexity of the cascaded frequency converter.

[0067] like Figure 2 As shown, power units 21 / 22 / 23 also include a capacitor C connected to the DC bus and an inverter bridge connected in parallel with capacitor C. The inverter bridge includes a first switching transistor K1, a second switching transistor K2, and a third switching transistor K3 and a fourth switching transistor K4 connected in series. One end of the first switching transistor K1 and the third switching transistor K3 is connected to the positive DC bus DC+. One end of the second switching transistor K2 and the fourth switching transistor K4 is connected to the negative DC bus DC-. The common terminal V connecting the first and second switching transistors K1 and K2, and the common terminal U connecting the third and fourth switching transistors K3 and K4, constitute the single-phase output terminals of power units 21 / 22 / 23. It should be noted that, although not shown in the figure, the first / second / third / fourth switching transistors K1 / K2 / K3 / K4 can be connected to the processing module 24 and respond to commands from the main control module of the cascaded inverter, ultimately outputting a sinusoidal pulse width modulation (SPWM) wave with adjustable frequency and / or voltage at UV.

[0068] like Figure 2As shown, the power unit 21 / 22 / 23 includes a first chopper switch S1 and a fifth diode D5. The anode of the fifth diode D5 is connected to the cathode of the third diode D3, and the cathode of the fifth diode D5 is connected to the capacitor C. The fifth diode D5 may be a fast recovery diode (FRD) to prevent energy from flowing back (from right to left in the figure) from the capacitor C and causing a short circuit in the first chopper switch S1.

[0069] Figure 2 The embodiment described above describes an example in which the power unit includes a first chopping switch S1. In some embodiments, the power unit may include a second chopping switch and a third chopping switch. The second chopping switch may be connected in parallel with the first diode or the second diode. The third chopping switch may be connected in parallel with the third diode or the fourth diode.

[0070] Figure 3 Schematic diagram of power units 21 / 22 / 23 according to some embodiments of the present invention is shown. Figure 3 As shown, the power unit 21 / 22 / 23 includes a second chopping switch S2 and a third chopping switch S3. The second chopping switch S2 is connected in parallel to the first diode D1. The third chopping switch S3 is connected in parallel to the third diode D3. Figure 4 Schematic diagram of power units 21 / 22 / 23 according to some embodiments of the present invention is shown. Figure 4 As shown, the second chopper switch S2 is connected in parallel with the second diode D2. The third chopper switch S3 is connected in parallel with the fourth diode D4. Although not shown in the figure, in some embodiments, the second chopper switch S2 can be connected in parallel with the first diode D1, and the third chopper switch S3 can be connected in parallel with the fourth diode D4. In some embodiments, the second chopper switch S2 can be connected in parallel with the second diode D2, and the third chopper switch S3 can be connected in parallel with the third diode D3. These are all within the scope of protection of the present invention. In actual applications, the connection method of the chopper switches and diodes can be customized as needed.

[0071] like Figure 3 and Figure 4 As shown, the processing module 24 is connected to the control terminals g2 and g3 of the second chopping switch S2 and the third chopping switch S3. The processing module 24 can determine the leakage inductance L of the transformer 10, and based on the leakage inductance L, control the operation of the second chopping switch S2 and the third chopping switch S3 to reduce the harmonic pollution of the cascaded inverter 100 to the power grid E. It should be noted that, although not shown in the figure, it should be understood that the second chopping switch S2 can be connected in parallel to the third diode D3 or the fourth diode D4. The third chopping switch S3 can be connected in parallel to the first diode D1 or the second diode D2. These are all within the scope of protection of the present invention. It should be noted that, Figure 3 and Figure 4The power unit of the embodiment described may not include Figure 2 The fifth diode D5 of the embodiment, the other Figure 2 The power units described in the embodiments are basically the same.

[0072] It should be noted that the specifications of the chopper switches (S1-S3), the diodes (D1-D5), and the switching tubes (K1-K4) may be the same or different, and may be set according to requirements in actual applications.

[0073] In some embodiments, the chopper switches (S1-S3) / switching transistors (K1-K4) may include at least one of a metal-oxide-semiconductor field-effect transistor (MOSFET), an insulated-gate bipolar transistor (IGBT), an electron injection-enhanced gate transistor (IEGT), or other similar devices. The MOSFET may be a P-channel metal-oxide-semiconductor field-effect transistor (PMOS). Alternatively, the MOSFET may be an N-channel metal-oxide-semiconductor field-effect transistor (NMOS). Alternatively, the MOSFET may be a power device such as a silicon carbide (SiC) MOSFET or a gallium nitride (GaN) MOSFET, which offers lower losses, higher efficiency, and better performance. In practical applications, this configuration can be tailored to specific needs.

[0074] In some embodiments, the processing module 24 may include a processing circuit, a processor, a PWM wave signal generator, a PWM wave timer, a driver, an error amplifier, other general amplifiers, a PI regulator, a multiplier, a sampler, a central processing unit (CPU), a microprocessor unit (MCU), a digital signal processor (DSP), other general processors, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a complex programmable logic device (CPLD) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components and other components or circuits.

[0075] In some embodiments, the processing module 24 can collect information such as the temperature, input voltage, current, bus voltage, etc. of the power unit. The processing module can communicate with the main control module and control the switches in the power unit to perform corresponding actions in response to the commands of the main control module.

[0076] The following describes the detailed process of the processing module controlling the chopper switch of the power unit.

[0077] In some embodiments, the processing module can control the on-off frequency of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance of the transformer. Figure 1 and Figure 2 As shown, the processing module 24 can control the on-off frequency of the first chopping switch S1 based on the leakage inductance L of the transformer 10. Figure 1 、 Figure 3 or Figure 4 As shown, the processing module 24 can control the on-off frequencies of the second chopping switch S2 and the third chopping switch S3 based on the leakage inductance L of the transformer 10. The on-off frequencies of the second chopping switch S2 and the third chopping switch S3 can be controlled synchronously.

[0078] In some embodiments, the processing module may control the period of the control signal of the control terminal of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance of the transformer to control the on-off frequency thereof. Figure 1 and Figure 2 As shown, the processing module 24 can control the period of the control signal of the control terminal g1 of the first chopping switch S1 based on the leakage inductance L of the transformer 10 to control the on-off frequency of the first chopping switch S1. Figure 1 、 Figure 3 or Figure 4 As shown, the processing module 24 can control the period of the control signal at the control terminal g2 of the second chopping switch S2 and the control terminal g3 of the third chopping switch S3 based on the leakage inductance L of the transformer 10 to control the on-off frequency of the second chopping switch S2 and the third chopping switch S3. It should be understood that the on-off frequency is the inverse of the period.

[0079] In some embodiments, the control signal may include a PWM wave signal. The PWM wave signal is generated by comparing a carrier signal (e.g., a triangle wave or sawtooth wave) with a modulated wave signal (e.g., a sine wave). The carrier signal and the PWM wave signal may be generated by a PWM wave signal generator in processing module 24. Processing module 24 may control the carrier period of the generated PWM wave signal based on the leakage inductance L of transformer 10 to control the period of the PWM wave signal. The period of the PWM wave signal is the same as the carrier period.

[0080] In some embodiments, the on-off frequencies of the first / second / third chopping switches are negatively correlated with the leakage inductance L of the transformer 10. The processing module 24 may control the on-off frequencies of the first / second / third chopping switches based on this correlation.

[0081] In some embodiments, the processing module 24 may control the minimum on-off frequency fmin of the first / second / third chopper switches based on the minimum leakage inductance Lmin of the transformer 10. Assuming that the minimum leakage inductance of each secondary winding of the transformer 10 is Lmin, the maximum allowable current of the coil and other components in the circuit, such as the rectifier bridge and IGBT, is Ipeak, the input peak voltage of the power unit is Vinpeak, and the maximum duty cycle of the control signal of the first / second / third chopper switch is Dmax, then the minimum on-off frequency fmin of the first / second / third chopper switch satisfies the following relationship:

[0082] ,

[0083] Right now: ,

[0084] but: .

[0085] In some embodiments, the carrier phases of any two adjacent power units in each power unit group differ by a preset angle. In each power unit group, the chopper switch of each power unit can be controlled in a phase-shifted manner, so that the carrier phases of any two adjacent power units in the same power unit group differ by a preset angle, and ultimately the single-phase input current of the cascaded inverter is the result of the phase-shifted superposition of the input currents of the respective power units in the corresponding power unit group, so as to obtain a more beneficial grid-side current harmonic index. The on-off frequency and current stress of the chopper switch can be further reduced by the phase-shifted superposition technology. It should be noted that the "adjacent" here should be understood in a broad sense, and can be physically adjacent or logically adjacent.

[0086] The chopper switching of the power unit is controlled by the processing module within the power unit. The carrier wave that controls the on-off switching is generated by the PWM wave timer in the processing module. Therefore, the carrier phase can be controlled by the processing module so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle. Continuing with the example of a 10kV cascaded high-voltage inverter, each power unit group includes eight power units. The carrier phase difference between any two adjacent power units in each power unit group is a preset angle of 360 / 8 = 45°. If the initial carrier phase angle of the first power unit (the head-end power unit) in each power unit group is 0°, the carrier phase angles of the other seven power units are 45°, 90°, 135°, 180°, 225°, 270°, and 315°, respectively. It should be noted that the carrier phase shift angles of the eight power units do not need to be arranged in an adjacent order, as long as they cover 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°. Here, only a 10kV cascade inverter is used as an example for illustration, and the present invention is not limited thereto. For high-voltage cascade inverters of other kV or above, the preset angle of the carrier phase difference between any two adjacent power units in each power unit group can be appropriately adjusted according to actual conditions, and all of these are within the scope of protection of the present invention.

[0087] Since the processing module of each power unit operates independently and uses an independent clock, there may be differences in clock errors. Over time, the carrier phase of each power unit may change, causing the difference in the carrier phase between two adjacent power units in the same power unit group to deviate from the preset angle, which may also cause the cascaded inverter 100 to cause harmonic pollution to the power grid E.

[0088] To address this problem, the inventors of this application have cleverly devised a solution through carrier synchronization control. The processing module of each power unit can control its own carrier phase based on a synchronization signal, so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0089] In some embodiments, the synchronization signal may be provided by a processing module of the power unit. Specifically, in each power unit group, the processing module of one power unit is configured to provide the synchronization signal, and the processing modules of the other power units in the same group are configured to adjust their respective carrier phases based on the synchronization signal, so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0090] For example, the synchronization signal can be provided by the processing module of the head-end power unit of each power unit group, and transmitted in turn by the processing module of the upstream power unit to the processing module of the downstream power unit. The processing modules of each power unit in the same group can adjust their respective carrier phases based on the synchronization signal so that the carrier phases of any two adjacent power units in the same power unit group differ by a preset angle. Figure 5 Schematic diagram of carrier synchronization control according to some embodiments of the present invention is shown. Figure 5 As shown, power unit group G1 includes multiple cascaded power units 21-1, 21-2, ..., 21-N. Power units 21-1, 21-2, ..., 21-N each include processing modules 24-11, 24-12, ..., 24-1N, where N is a positive integer. Processing module 24-11 provides a synchronization signal CLK, which is then passed to processing module 24-12. Processing module 24-12 then passes the synchronization signal CLK to processing module 24-13, ..., until the synchronization signal CLK is passed to processing module 24-1N. Processing modules 24-11 to 24-1N can adjust their respective carrier phases based on the synchronization signal CLK, so that the carrier phases of any two adjacent power units in power unit group G1 differ by a preset angle.

[0091] Similarly, power unit group G2 includes multiple cascaded power units 22-1, 22-2, ..., 22-N, each of which includes processing modules 24-21, 24-22, ..., 24-2N, where N is a positive integer. Processing module 24-21 provides a synchronization signal CLK, which is then transmitted to processing module 24-22. Processing module 24-22 transmits the synchronization signal CLK to processing module 24-23, ..., until the synchronization signal CLK is transmitted to processing module 24-2N. Processing modules 24-21 to 24-2N can adjust their respective carrier phases based on the synchronization signal CLK so that the carrier phases of any two adjacent power units in power unit group G2 differ by a preset angle.

[0092] Similarly, power unit group G3 includes multiple cascaded power units 23-1, 23-2, ..., 23-N, each of which includes processing modules 24-31, 24-32, ..., 24-3N, where N is a positive integer. Processing module 24-31 provides a synchronization signal CLK, which is then passed to processing module 24-32. Processing module 24-32 then passes the synchronization signal CLK to processing module 24-33, ..., until the synchronization signal CLK is passed to processing module 24-3N. Processing modules 24-31 to 24-3N can adjust their respective carrier phases based on the synchronization signal CLK so that the carrier phases of any two adjacent power units in power unit group G3 differ by a preset angle.

[0093] For another example, the synchronization signal can be provided by the processing module of the terminal power unit of each power unit group, and is sequentially transmitted by the processing module of the downstream power unit to the processing module of the upstream power unit. The processing modules of each power unit in the same group can adjust their respective carrier phases based on the synchronization signal, so that the carrier phases of any two adjacent power units in the same power unit group differ by a preset angle. For another example, the synchronization signal can be provided by the processing module of any power unit other than the head end and the end end of each power unit group, and is sequentially transmitted by the processing module to the processing module of the upstream power unit and the processing module of the downstream power unit. The processing modules of each power unit in the same group can adjust their respective carrier phases based on the synchronization signal, so that the carrier phases of any two adjacent power units in the same power unit group differ by a preset angle.

[0094] In some embodiments, the synchronization signal can be provided by a main control module of the cascade frequency converter. The cascade frequency converter also includes a main control module. The main control module is connected to the processing module of each power unit and is configured to provide a synchronization signal. The processing module of each power unit is configured to adjust its respective carrier phase based on the synchronization signal so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle. Figure 6 Schematic diagram of carrier synchronization control according to some embodiments of the present invention is shown. Figure 6As shown, the cascade inverter includes a main control module 30. The main control module 30 is connected to processing modules 24-11 to 24-1N, 24-21 to 24-2N, and 24-31 to 24-3N. The main control module 30 provides a synchronization signal CLK. Based on the synchronization signal CLK, the processing modules 24-11 to 24-1N adjust their respective carrier phases so that the carrier phases of any two adjacent power units in power unit group G1 differ by a preset angle. Based on the synchronization signal CLK, the processing modules 24-21 to 24-2N adjust their respective carrier phases so that the carrier phases of any two adjacent power units in power unit group G2 differ by a preset angle. Based on the synchronization signal CLK, the processing modules 24-31 to 24-3N adjust their respective carrier phases so that the carrier phases of any two adjacent power units in power unit group G3 differ by a preset angle.

[0095] It should be noted that the present invention does not limit the specific form of the synchronization signal CLK. For example, the synchronization signal CLK can be a level signal or a communication command word signal, such as 0x88.

[0096] In some embodiments, the main control module 30 may include components or circuits such as a control circuit, a controller, a PWM wave signal generator, a driver, an error amplifier, a PI regulator, a multiplier, a CPU, an MCU, a DSP, other general-purpose processors, an ASIC, an FPGA, a CPLD or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc.

[0097] In some embodiments, the processing module of the power unit and / or the main control module of the cascade inverter can provide a synchronization signal under preset conditions. In other words, when the preset conditions are met, a synchronization signal with carrier phase adjustment is provided, which can reduce system complexity and communication rate requirements.

[0098] In some embodiments, the preset conditions include periodicity and / or when the difference between the carrier phases of any two adjacent power units in the power unit group deviates from a preset angle by more than or equal to an angle threshold. For example, the processing module of the power unit and / or the main control module of the cascade frequency converter can periodically provide a synchronization signal so that the processing modules of each power unit can adjust their respective carrier phases based on the synchronization signal. Alternatively, the processing module of the power unit and / or the main control module of the cascade frequency converter can provide a synchronization signal when the difference between the carrier phases of any two adjacent power units in the power unit group deviates from a preset angle by more than or equal to an angle threshold, so that the processing modules of each power unit can adjust their respective carrier phases based on the synchronization signal. Alternatively, the processing module of the power unit and / or the main control module of the cascade frequency converter can periodically and when the difference between the carrier phases of any two adjacent power units in the power unit group deviates from a preset angle by more than or equal to an angle threshold, so that the processing modules of each power unit can adjust their respective carrier phases based on the synchronization signal.

[0099] It should be noted that the present invention does not limit the provider, transmission method and triggering method (such as preset conditions) of the synchronization signal, and in actual application, they can be set according to needs.

[0100] In some embodiments, the processing module is further configured to determine the input voltage, input current or output current of the rectifier bridge and the voltage across the capacitor of the power unit; based on the input voltage, input current and the voltage across the capacitor, control the duty cycle of the control signal of the control end of the first chopper switch, or the second chopper switch and the third chopper switch. It should be noted that the input voltage of the power unit is the output voltage of the transformer. The input current of the power unit is the output current of the transformer, equivalent to the leakage inductance current. It can be understood that the purpose of controlling the duty cycle of the control signal is to control the bus voltage of the power unit (such as Figure 7 or Figure 8 The voltage Vc across the capacitor C shown), the input current (such as Figure 7 or Figure 8 Schematic representation of the input current Iin), optimize the power unit and even the inverter input harmonic indicators, optimize the performance of the cascaded high-voltage inverter.

[0101] Figure 7 Schematic diagram showing the duty cycle of the control chopper switch according to some embodiments of the present invention. Figure 7As shown, the processing module 24 can determine the input voltage Vin, the input current Iin (or the absolute value of the input current Iin) of the power units 21 / 22 / 23, and the voltage Vc across the capacitor C. Based on the input voltage Vin, the input current Iin, and the voltage Vc across the capacitor C, the processing module 24 can control the duty cycle of the control signal at the control terminal g1 of the first chopping switch S1.

[0102] Figure 8 Schematic diagram showing the duty cycle of the control chopper switch according to some embodiments of the present invention. Figure 8 As shown, the processing module 24 can determine the input voltage Vin, the input current Iin (or the absolute value of the input current Iin) of the power units 21 / 22 / 23, and the voltage Vc across the capacitor C. Based on the input voltage Vin, the input current Iin, and the voltage Vc across the capacitor C, the processing module 24 can control the duty ratios of the control signals at the control terminals g2 and g3 of the second chopping switch S2 and the third chopping switch S3. The duty ratios of the control signals at the control terminals g2 and g3 are the same.

[0103] Figure 9 FIG. 1 is a schematic diagram showing the working principle of controlling the duty cycle of a chopper switch according to some embodiments of the present invention. Figure 9 As shown, the processing module 24 includes a first error amplifier U1, a voltage outer loop PI regulator 241, a multiplier 242, a second error amplifier U2, a current inner loop PI regulator 243, a PWM wave signal generator 244, and a driver 245. The digitally sampled voltage signal Vc across the capacitor and a preset reference voltage signal Vref are input to the first error amplifier U1. The first error amplifier U1 calculates the signal difference Verr between the voltage signal Vc and the reference voltage signal Vref and outputs it to the voltage outer loop PI regulator 241. The output signal of the voltage outer loop PI regulator 241 and the sampled value of the input voltage signal Vin are input to the multiplier 242. After multiplication, the multiplier 242 outputs a reference current signal Iref to the second error amplifier U2. The digitally sampled input current Iin is also input to the second error amplifier U2. The second error amplifier U2 calculates the current signal difference Ierr between the input current Iin and the reference current signal Iref and outputs it to the current inner loop PI regulator 243. The output signal regulated by the current inner loop PI regulator 243 is sent to the PWM wave signal generator 244. The PWM wave signal generator 244 compares the output signal with the carrier signal and generates a PWM wave control signal Sig1 or Sig2 and Sig3. Then, the driver 245 generates a drive signal Sig1' or Sig2' and Sig3' to the control terminal g1 or g2 and g3 to control the duty cycle of the first chopping switch S1 or the second chopping switch S2 and the third chopping switch S3.

[0104] In some embodiments, the processing module and / or the main control module may control only the on-off frequency of the first chopper switch S1 or the second chopper switch S2 and the third chopper switch S3, or may control only the duty cycle, or may control both the on-off frequency and the duty cycle. The on-off frequency of the second chopper switch and the third chopper switch may be controlled synchronously. The duty cycle of the second chopper switch and the third chopper switch may be controlled synchronously. It should be noted that, in the present invention, controlling the on-off frequency and / or the duty cycle should be understood in a broad sense, and may be maintained unchanged or adjusted, depending on the actual situation.

[0105] The sampling of the input voltage Vin is affected to some extent by the operation of the first chopper switch S1 or the second chopper switch S2 and the third chopper switch S3, resulting in certain sampling errors. To more accurately obtain the input voltage Vin of the power unit (e.g., voltage phase information, voltage amplitude information, etc.), in some embodiments, a measurement winding for measuring the power unit input voltage Vin (e.g., voltage phase information, voltage amplitude information, etc.) can be provided on the secondary side of the transformer. Since almost no current flows through the measurement winding, a very thin wire can be selected, and the measurement winding may only require a single turn. Therefore, the introduction of the measurement winding has a negligible impact on the overall cost. With virtually no increase in cost, measurement errors such as phase and / or amplitude caused by the operation of the first chopper switch S1 or the second chopper switch S2 and the third chopper switch S3 can be reduced, thereby improving the sampling accuracy of the input voltage Vin and facilitating more reliable and effective harmonic reduction on the transformer input side.

[0106] In some embodiments, the cascade converter may include multiple measurement windings arranged on the secondary coil of the transformer, each measurement winding is connected to a processing module of a corresponding power unit, and the processing module of each power unit determines the input voltage based on the corresponding measurement winding. Figure 10 FIG. 1 shows a partial schematic diagram of a cascade frequency converter 100 according to some embodiments of the present invention. Figure 1 、 Figure 6 and Figure 10As shown, the cascade inverter 100 includes multiple measurement windings tA1 to tCN, which are provided on the secondary coils 103A to 103C of the transformer 10. Measurement windings tA1 to tAN are provided on the secondary coils 103A1 to 103AN, respectively. Measurement windings tB1 to tBN are provided on the secondary coils 103B1 to 103BN, respectively. Measurement windings tC1 to tCN are provided on the secondary coils 103C1 to 103CN, respectively. Each measurement winding is connected to the processing module of the corresponding power unit. For example, measurement windings tA1 to tAN are connected to processing modules 24-11 to 24-1N, respectively; measurement windings tB1 to tBN are connected to processing modules 24-21 to 24-2N, respectively; and measurement windings tC1 to tCN are connected to processing modules 24-31 to 24-3N, respectively. The processing module of each power unit determines the input voltage based on the corresponding measurement winding.

[0107] In some embodiments, the power unit's input voltage Vin can also be obtained by measuring the transformer's input phase voltage. Because the transformer does not have phase shifting, the input voltage phase and the output voltage phase are consistent. For example, the main control module 30 can measure and communicate the transformer's input phase voltage to the power unit, which can use this voltage as input information and convert it into the input voltage Vin. Typically, the main control module can measure the input voltage and current of the cascade inverter for control and protection purposes, using the input voltage as the input for Vin without increasing any hardware costs.

[0108] In some embodiments, the cascade inverter further includes a main control module. The main control module is configured to collect transformer primary phase voltage information and transmit this information to the processing module of the corresponding power unit. The processing module determines the input voltage of the power unit based on the corresponding primary voltage information. For example, the main control module 30 may collect transformer primary phase A voltage information and transmit this information to the processing module of the power unit in power unit group G1. The processing module of the power unit in power unit group G1 may determine its input voltage Vin based on the primary phase A voltage information. For another example, the main control module 30 may collect transformer primary phase B voltage information and transmit this information to the processing module of the power unit in power unit group G2. The processing module of the power unit in power unit group G2 may determine its input voltage Vin based on the primary phase B voltage information. For another example, the main control module 30 may collect transformer primary phase C voltage information and transmit this information to the processing module of the power unit in power unit group G3. The processing module of the power unit of the power unit group G3 can determine its input voltage Vin based on the primary C-phase voltage information.

[0109] In some embodiments, although not shown, the cascade inverter 100 may further include one or more of a human-machine interface (HMI), a drive advisor (DA), and a programmable logic controller (PLC). The HMI, DA, and PLC are connected to the main control module 30 and can communicate with the main control module 30 and the processing module 24 of the power unit to share data.

[0110] In some embodiments, the cascade inverter 100 may include a storage module (not shown). The storage module is used to store information such as the operating data of the cascade inverter 100, the input voltage of the power unit (i.e., the output voltage of the transformer), the input current (i.e., the output current of the transformer), and the bus capacitor voltage. The storage module may include a memory. The memory may include random access memory (RAM) or non-volatile memory (NVM). Furthermore, the memory may include at least one of phase-change random access memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), read-only memory (ROM), and electrically erasable programmable read-only memory (EEPROM). Alternatively, the storage module may include cloud storage. The storage module may be implemented using software and / or hardware.

[0111] In some embodiments, the cascade inverter 100 may include a display module (not shown). The display module can be connected to at least one of the main control module 30 and the processing module 24 and can be used to visually output information such as the operating data of the cascade inverter 100, the input voltage of the power unit (i.e., the output voltage of the transformer), the input current (i.e., the output current of the transformer), and the bus capacitor voltage. The display module may include a display screen. The display screen may include at least one of an LCD (liquid-crystal display), an LED (light-emitting diode), and an OLED (organic light-emitting diode).

[0112] The present invention also provides a control method for the cascade inverter 100 as described above. Figure 11 FIG. 3 is a flow chart showing a control method 300 according to some embodiments of the present invention. Figure 11 As shown, the control method 300 includes steps S310 to S330. In step S310, the leakage inductance L of the transformer 10 is determined. In step S330, based on the leakage inductance L, the operation of the first chopping switch S1, or the second chopping switch S2 and the third chopping switch S3 is controlled to reduce harmonic pollution of the cascade inverter 100 to the power grid E.

[0113] In some embodiments, step S330 includes: controlling the on-off frequency of the first chopping switch S1 , or the second chopping switch S2 and the third chopping switch S3 based on the leakage inductance L.

[0114] In some embodiments, controlling the on-off frequency includes: controlling the period of the control signal at the control terminals g1 , or g2 and g3 of the first chopping switch S1 , or the second chopping switch S2 and the third chopping switch S3 , based on the leakage inductance L.

[0115] In some embodiments, the control signal includes a PWM wave signal. Controlling the period of the PWM wave signal includes: controlling the carrier period of the PWM wave signal based on the leakage inductance L.

[0116] In some embodiments, step S330 further includes: controlling the processing module of each power unit to control the respective carrier phases based on the synchronization signal, so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0117] In some embodiments, the processing module that controls each power unit controls its respective carrier phase based on a synchronization signal, including: controlling the processing module of one of the power units in each power unit group to provide a synchronization signal, and controlling the processing modules of other power units in the same group to adjust their respective carrier phases based on the synchronization signal, so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0118] In some embodiments, the processing module that controls each power unit controls its respective carrier phase based on a synchronization signal, including: controlling the main control module of the cascaded inverter to provide a synchronization signal; and controlling the processing module of each power unit to adjust its respective carrier phase based on the synchronization signal, so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

[0119] In some embodiments, controlling the processing module of each power unit to control the respective carrier phase based on the synchronization signal includes: controlling the processing module and / or the main control module of the cascade inverter to provide the synchronization signal under preset conditions.

[0120] In some embodiments, the preset condition includes periodically and / or the deviation between the difference in carrier phase between any two adjacent power units in the power unit group and a preset angle is greater than or equal to an angle threshold.

[0121] In some embodiments, the control method 300 also includes: determining the input voltage, input current and voltage across the capacitor connected to the DC bus of the power unit; and controlling the duty cycle of the control signal of the control end of the first chopping switch, or the second chopping switch and the third chopping switch based on the input voltage, input current and voltage across the capacitor.

[0122] In some embodiments, the control method 300 further includes controlling the processing module of each power unit to determine an input voltage based on a corresponding measurement winding.

[0123] In some embodiments, the control method 300 further includes: collecting transformer primary phase voltage information through a main control module, and transmitting the primary phase voltage information to a processing module of a corresponding power unit, and controlling the processing module to determine the input voltage of the power unit based on the corresponding primary voltage information.

[0124] In some embodiments, the control method 300 and its various steps may be executed by one or more of a processing module of a power unit, a main control module of a cascade inverter, an HMI, a DA, a PLC, or other remote controllers.

[0125] Figure 12 Schematic diagram showing waveforms of input voltage Vin and input current Iin of a single power unit according to some embodiments of the present invention. Figure 12 As shown, the red waveform represents the input voltage Vin of a single power unit. The green waveform represents the input current Iin of a single power unit. The input voltage Vin is a sinusoidal wave. The input current Iin is also roughly sinusoidal. As can be seen, the technical solution of the present invention can suppress the input current harmonics of a single power unit.

[0126] Figure 13 Show Figure 12 Schematic diagram of the spectrum of the input current harmonics of a single power unit. Figure 12 As shown in the figure, the Fourier analysis of the input current harmonics of a single power unit shows that the input current harmonics have high-frequency components (such as Figure 13 As shown, around 2000 Hz and around 4000 Hz). The presence of these high-frequency components may be caused by, but is not limited to, the on-off transients of a switch such as a chopper switch.

[0127] Figure 14 Schematic diagram showing waveforms of input currents of single-phase multiple power units and input currents of cascaded frequency converters according to some embodiments of the present invention. Figure 15 Show Figure 14 For example, take the four cascaded power units 21-1 to 21-4 of phase A as an example. Figure 14 and Figure 15 As shown in the figure, (a) shows the input current waveform of power unit 21-1, (b) shows the input current waveform of power unit 21-2, (c) shows the input current waveform of power unit 21-3, (d) shows the input current waveform of power unit 21-4, and (e) shows the input current waveform of cascade inverter 100. As can be seen from the figure, the input current waveforms of multiple single-phase power units are essentially sinusoidal. After superposition, the input current waveform of the cascade inverter becomes less harmonic and closer to a sine wave.

[0128] Figure 16 Show Figure 14 Schematic diagram of the spectrum of the input current harmonics of a single power unit and a cascaded inverter. Figure 16 As shown, Fourier analysis of the input current harmonics of multiple single-phase power units and the cascaded inverter reveals that while the input current of a single power unit contains some high-frequency harmonic components, these components cancel each other out after superposition, ultimately resulting in virtually no high-frequency harmonic components in the input current of the cascaded inverter. In particular, high-frequency harmonic components around 2000Hz and 4000Hz are effectively suppressed. This demonstrates that the technical solution of the present invention achieves an ideal result in suppressing input current harmonics.

[0129] Table 1 exemplarily shows data for suppressing input current harmonics according to some embodiments of the present invention.

[0130] Table 1

[0131] Power unit 21-1 input current Power unit 21-1 input current Power unit 21-1 input current Power unit 21-1 input current Cascade inverter input current Total Harmonic Distortion THD 0.28147 0.28149 0.28146 0.28144 0.00525

[0132] It can be seen that the technical solution of the present invention effectively suppresses the input current harmonics of the cascaded inverter. The total harmonic distortion (THD) of a single power unit is approximately 28%, and the total harmonic distortion of the cascaded inverter is approximately 0.525%, which can well meet the national standard requirement of less than 5%.

[0133] It should be noted that Figures 14 to 16 Table 1 uses four cascaded power units 21 - 1 to 21 - 4 of phase A as an example, but the present invention is not limited thereto, and the same or similar technical effects can be achieved for phase B, phase C, and other numbers of power units.

[0134] The cascade frequency converter or control method of the present invention cleverly uses the leakage inductance of the transformer as inductance, and controls the operation of the chopper switch based on the leakage inductance, thereby reducing the harmonic pollution of the cascade frequency converter to the power grid, and saving the volume, device cost and design complexity of the cascade frequency converter. The single-phase input and single-phase output of the power unit greatly reduces the number of secondary coil windings of the transformer, the number of cables connecting the transformer and the power unit, the wiring complexity and cost. The single-phase input side of the power unit adopts 4 diodes to form a single-phase rectifier bridge, which reduces the number of diodes and saves costs compared to the three-phase input. The cascade frequency converter or control method of the present invention helps to improve the stability, reliability, efficiency and safety of the power supply of the power grid by reducing the harmonic pollution of the cascade frequency converter to the power grid.

[0135] Traditional cascade frequency converters use phase-shifting transformers to achieve multi-pulse rectification, which reduces the grid-side current harmonics at one time. Different from the traditional technical solution, in the technical solution of the present invention, the three-phase input end of the transformer can be connected to the grid, and the three groups of single-phase output ends of the transformer are connected one-to-one with the three power unit groups. The input end of each power unit is connected to the corresponding single-phase output end, and based on the leakage inductance of the transformer, the operation of the first chopper switch, or the second chopper switch and the third chopper switch of the power unit is controlled to reduce the harmonic pollution of the cascade frequency converter to the grid. In other words, the present invention uses single-phase active power factor correction (APFC) technology to reduce the grid-side harmonic current without the need for multi-pulse rectification through a phase-shifting transformer as in the traditional technical solution, and can also achieve the same or even better technical effects.

[0136] The present invention further provides a computer-readable storage medium, which includes computer-executable instructions stored thereon, and the computer-executable instructions implement the control method 300 described above when executed by a processor.

[0137] The present invention may take the form of a computer program product embodied in one or more storage media containing program code. Computer-usable storage media include permanent and non-permanent, removable and non-removable media, and may be implemented by any method or technology for storing information. The information may be computer-readable instructions, data structures, program modules, or other data. Examples of computer storage media include, but are not limited to, PRAM, SRAM, DRAM, other types of RAM, ROM, EEPROM, flash memory or other memory technology, compact disc read-only memory (CD-ROM), digital versatile disc (DVD) or other optical storage, magnetic cassettes, magnetic tape, disk storage or other magnetic storage devices, or any other non-transmission medium that can be used to store information that can be accessed by a computing device.

[0138] It should be noted that this specification provides method operation steps such as embodiments or schematic diagrams, but based on routine or non-creative work, more or fewer operation steps may be included. The order of steps listed in the embodiments is only one way of executing the steps among many, and does not represent the only execution order. When implemented in actual systems or device products, the methods shown in the embodiments or flowcharts can be executed sequentially or in parallel.

[0139] It should be noted that while the detailed description above mentions several modules of the cascaded inverter, this division is not mandatory. In fact, according to embodiments of the present invention, the features and functions of two or more modules described above can be implemented in a single module. Conversely, the features and functions of a single module described above can be further divided and implemented by multiple modules.

[0140] It should be noted that the present invention may only include Figure 1-16 In other words, not all of the features shown need to be implemented simultaneously in the cascade inverter / control method of the present invention.

[0141] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A cascade frequency converter, characterized in that: include: A transformer comprising a three-phase input terminal and three sets of single-phase output terminals, wherein the three-phase input terminal can be connected to a power grid; and Three power unit groups are connected to the three groups of single-phase output terminals in a one-to-one correspondence, each power unit group includes a plurality of cascaded power units, and the input terminal of each power unit is connected to the corresponding single-phase output terminal; Each power unit includes: A rectifier bridge includes a first branch and a second branch, wherein the first branch includes a first diode and a second diode connected in series; the second branch includes a third diode and a fourth diode connected in series; the input sides of the first branch and the second branch are connected to corresponding single-phase output terminals; a first chopping switch, the first chopping switch being connected in parallel to the output sides of the first branch and the second branch; or a second chopping switch and a third chopping switch, the second chopping switch being connected in parallel to the first diode or the second diode; and the third chopping switch being connected in parallel to the third diode or the fourth diode; and a processing module connected to a control terminal of the first chopper switch, or connected to control terminals of the second chopper switch and the third chopper switch, and configured to determine a leakage inductance of the transformer; and control operations of the first chopper switch, or the second chopper switch and the third chopper switch based on the leakage inductance to reduce harmonic pollution of the cascaded frequency converter on the power grid; The processing module is configured to control the on-off frequency of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance; the on-off frequency is negatively correlated with the leakage inductance.

2. The cascade frequency converter according to claim 1, characterized in that: The cathode of the first diode is connected to the cathode of the third diode; the anode of the second diode is connected to the anode of the fourth diode; the cathode of the second diode and the anode of the third diode are connected to the corresponding single-phase output end of the transformer; the power unit also includes a capacitor connected to the DC bus and an inverter bridge connected in parallel with the capacitor.

3. The cascade frequency converter according to claim 2, characterized in that: When the power unit includes the first chopper switch, the power unit further includes a fifth diode, an anode of the fifth diode is connected to the cathode of the third diode, and a cathode of the fifth diode is connected to the capacitor.

4. The cascade frequency converter according to claim 1, characterized in that: The processing module is configured to control a period of a control signal at a control terminal of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance, so as to control the on-off frequency.

5. The cascade frequency converter according to claim 4, characterized in that: The control signal includes a PWM wave signal; the processing module is configured to control a carrier cycle of the PWM wave signal based on leakage inductance to control a cycle of the PWM wave signal.

6. The cascade frequency converter according to claim 5, characterized in that: The processing module of each power unit is configured to control the respective carrier phases based on the synchronization signal so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

7. The cascade frequency converter according to claim 6, characterized in that: In each power unit group, the processing module of one power unit is configured to provide the synchronization signal, and the processing modules of other power units in the same group are configured to adjust their respective carrier phases based on the synchronization signal so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

8. The cascade frequency converter according to claim 6, characterized in that: The cascade frequency converter also includes a main control module, which is connected to the processing module of each power unit and configured to provide the synchronization signal; the processing module of each power unit is configured to adjust its respective carrier phase based on the synchronization signal so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

9. The cascade frequency converter according to claim 6, characterized in that: The processing module and / or the main control module of the cascade frequency converter is configured to provide the synchronization signal under preset conditions.

10. The cascade frequency converter according to claim 9, characterized in that: The preset conditions include periodicity and / or a deviation between a difference in carrier phase between any two adjacent power units in the power unit group and the preset angle being greater than or equal to an angle threshold.

11. The cascade frequency converter according to any one of claims 1 to 10, characterized in that: The power unit also includes a capacitor connected to the DC bus; the processing module is further configured to determine the input voltage, input current and voltage across the capacitor of the power unit; and control the duty cycle of the control signal at the control end of the first chopper switch, or the second chopper switch and the third chopper switch based on the input voltage, input current and voltage across the capacitor.

12. The cascade frequency converter according to claim 11, characterized in that: It also includes a plurality of measuring windings arranged on the secondary coil of the transformer, each measuring winding is connected to the processing module of the corresponding power unit, and the processing module of each power unit determines the input voltage based on the corresponding measuring winding.

13. The cascade frequency converter according to claim 11, characterized in that: The cascade inverter also includes a main control module, which is configured to collect primary phase voltage information of the transformer and transmit the primary phase voltage information to a processing module of the corresponding power unit. The processing module determines the input voltage of the power unit based on the corresponding primary voltage information.

14. The cascade frequency converter according to any one of claims 1 to 10, characterized in that: The chopper switch includes at least one of a metal oxide semiconductor field effect transistor, an insulated gate bipolar transistor, or an electron injection enhanced gate transistor.

15. A control method for a cascade frequency converter according to any one of claims 1 to 14, characterized in that: include: S310: Determine the leakage inductance of the transformer; and S330: Based on the leakage inductance, determine the operating frequency of the first chopper switch, or the second chopper switch and the third chopper switch to reduce harmonic pollution of the cascaded frequency converter to the power grid. Step S330 includes: controlling the on-off frequency of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance; The switching frequency is negatively correlated with the leakage inductance.

16. The control method according to claim 15, characterized in that: Controlling the on-off frequency includes controlling a period of a control signal at a control end of the first chopping switch, or the second chopping switch and the third chopping switch based on the leakage inductance.

17. The control method according to claim 16, characterized in that: The control signal includes a PWM wave signal; controlling the period of the PWM wave signal includes: controlling the carrier period of the PWM wave signal based on leakage inductance.

18. The control method according to claim 17, characterized in that: Step S330 further includes: controlling the processing module of each power unit to control the respective carrier phases based on the synchronization signal, so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

19. The control method according to claim 18, characterized in that: The processing module that controls each power unit controls the respective carrier phases based on the synchronization signal, including: controlling the processing module of one power unit in each power unit group to provide the synchronization signal, and controlling the processing modules of other power units in the same group to adjust their respective carrier phases based on the synchronization signal, so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

20. The control method according to claim 18, characterized in that: The processing module that controls each power unit controls the respective carrier phases based on the synchronization signal, including: controlling the main control module of the cascaded inverter to provide the synchronization signal; and controlling the processing module of each power unit to adjust the respective carrier phases based on the synchronization signal so that the carrier phases of any two adjacent power units in each power unit group differ by a preset angle.

21. The control method according to claim 18, characterized in that: The controlling the processing module of each power unit to control the respective carrier phase based on the synchronization signal includes: controlling the processing module and / or the main control module of the cascade converter to provide the synchronization signal under preset conditions.

22. The control method according to claim 21, characterized in that: The preset conditions include periodicity and / or a deviation between a difference in carrier phase between any two adjacent power units in the power unit group and the preset angle being greater than or equal to an angle threshold.

23. The control method according to any one of claims 15 to 22, characterized in that: Also includes: Determine an input voltage, an input current, and a voltage across a capacitor connected to a DC bus of the power unit; and control a duty cycle of a control signal at a control end of the first chopper switch, or the second chopper switch and the third chopper switch based on the input voltage, the input current, and the voltage across the capacitor.

24. The control method according to claim 23, characterized in that: Also includes: A processing module controlling each power unit determines the input voltage based on the corresponding measurement winding.

25. The control method according to claim 23, characterized in that: Also includes: The main control module collects the primary phase voltage information of the transformer and transmits the primary phase voltage information to the processing module of the corresponding power unit, and controls the processing module to determine the input voltage of the power unit based on the corresponding primary phase voltage information.

26. A computer-readable storage medium, characterized in that The device comprises computer executable instructions stored thereon, which implement the control method according to any one of claims 15 to 25 when executed by a processor.

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