Cascaded frequency converter, control method and computer readable storage medium

Abstract

The invention discloses a cascaded frequency converter, a control method and a computer readable storage medium. The cascaded frequency converter comprises a transformer which comprises a three-phase input end connected with a power grid and three groups of single-phase output ends connected with corresponding power unit groups. The input end of each power unit is connected with the corresponding single-phase output end; each power unit comprises a first branch and a second branch; the input sides of the first branch and the second branch are connected with corresponding single-phase output ends; the first chopping switch is connected in parallel with the output sides of the first branch and the second branch; or, a second chopping switch connected in parallel with the first diode or the second diode and a third chopping switch connected in parallel with the third diode or the fourth diode; the processing module is connected with the control ends of the first chopping switch or the second chopping switch and the third chopping switch and can determine the leakage inductance of the transformer; and the work of the first chopping switch or the second chopping switch and the third chopping switch is controlled based on the leakage inductance, so that the harmonic pollution to the power grid is reduced.

Description

Technical Field

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

[0002] A cascaded inverter (Variable-frequency Drive, VFD) is a power control device that controls an electric motor by changing the output voltage frequency and voltage amplitude of the motor. The cascaded inverter is widely used in scenarios such as fans, water pumps, belt conveyors, and experimental power supplies. The harmonic pollution of the cascaded inverter to the power grid has an adverse impact on the power supply quality of the power grid. How to reduce the harmonic pollution of the cascaded inverter to the power grid is a technical problem to be solved in this application.

[0003] The content of the background art section is only the technology known to the inventor and does not necessarily represent the prior art in this field. Summary of the Invention

[0004] In view of one or more of the problems existing in the prior art, the present invention provides a cascaded inverter, including: a transformer, including three-phase input terminals and three groups of single-phase output terminals, the three-phase input terminals being connectable to a power grid; and three power unit groups, connected to the three groups of single-phase output terminals in one-to-one correspondence, each power unit group including a plurality of cascaded power units, the input terminal of each power unit being connected to the corresponding single-phase output terminal; wherein each power unit includes: a rectifier bridge, including a first branch and a second branch, the first branch including a first diode and a second diode connected in series; the second branch including a third diode and a fourth diode connected in series; the input sides of the first branch and the second branch being connected to the corresponding single-phase output terminal; a first chopper switch, the first chopper switch being connected in parallel to the output sides of the first branch and the second branch; or, a second chopper switch and a third chopper switch, the second chopper switch being connected in parallel to the first diode or the second diode; the third chopper switch being connected in parallel to the third diode or the fourth diode; and a processing module, connected to the control terminal of the first chopper switch, or connected to the control terminals of the second chopper switch and the third chopper switch, and configured to determine the leakage inductance of the transformer; based on the leakage inductance, control the operation of the first chopper switch, or the second chopper switch and the third chopper switch, to reduce the 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 the corresponding single-phase output terminal; the power unit further 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, an anode of the fifth diode is connected to a cathode of the third diode, and a cathode of the fifth diode is connected to the capacitor.

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

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

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

[0010] Optionally, the on / off frequency is negatively correlated with the leakage inductance.

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

[0013] Optionally, the cascaded frequency converter further includes a main control module, the main control module is connected to a processing module of each power unit, and is configured to provide the synchronization signal; a processing module of each power unit is configured to adjust respective carrier phases based on the synchronization signal, so that 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 cascaded frequency converter is configured to provide the synchronization signal under preset conditions.

[0015] Optionally, the preset conditions include regularly and / or a deviation between a difference between carrier phases 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 the DC bus; the processing module is further configured to determine the input voltage, input current of the power unit, and the voltage across the capacitor; and based on the input voltage, input current, and the voltage across the capacitor, control the duty cycle of the control signal at the control end of the first chopper switch, or the second and third chopper switches.

[0017] Optionally, the cascaded frequency converter further includes a plurality of measuring windings disposed on the secondary side 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.

[0018] Optionally, the cascaded frequency converter further includes a main control module, the main control module is configured to collect the primary side phase voltage information of the transformer, and transmit the primary side phase voltage information to the processing module of the corresponding power unit, and the processing module determines the input voltage of the power unit based on the corresponding primary side 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 also provides a control method for the cascaded frequency converter as described above, including:

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

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

[0023] Optionally, step S330 includes: Based on the leakage inductance, control the on-off frequency of the first chopper switch, or the second and third chopper switches.

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

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

[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 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.

[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 frequency converter to provide the synchronization signal; 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.

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

[0030] Optionally, the preset conditions include periodically and / or the deviation between the difference in the carrier phases 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 of the power unit, and the voltage across the capacitor connected to the DC bus; based on the input voltage, input current, and the voltage across the capacitor, controlling the duty ratio of the control signal at the control end of the first chopper switch, or the second and third chopper switches.

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

[0034] The cascaded frequency converter or control method of the present invention makes clever use of 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, saving the volume, device cost and design complexity of the cascaded frequency converter. The power unit has single-phase input and single-phase output, greatly reducing the number of secondary coil windings of the transformer, the number of connecting cables between the transformer and the power unit, the wiring complexity and cost. The single-phase input side of the power unit uses 4 diodes to form a single-phase rectifier bridge, which reduces the number of diodes and saves costs compared with three-phase input.

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

[0036] The 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, but do not constitute a limitation to the present invention. In the drawings:

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

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

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

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

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

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

[0043] Figure 7 A schematic diagram of controlling the duty cycle of the chopper switch according to some embodiments of the present invention is shown.

[0044] Figure 8 A schematic diagram of controlling the duty cycle of the chopper switch according to some embodiments of the present invention is shown.

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

[0046] Figure 10Shows a partial schematic diagram of a cascaded frequency converter according to some embodiments of the present invention.

[0047] Figure 11 Shows a schematic flow diagram of a control method according to some embodiments of the present invention.

[0048] Figure 12 Shows a waveform schematic diagram of the input voltage and input current of a single power unit according to some embodiments of the present invention.

[0049] Figure 13 Shows Figure 12 A spectrum schematic diagram of the input current harmonics of a single power unit.

[0050] Figure 14 Shows a waveform schematic diagram of the input current of multiple single-phase power units and the input current of a cascaded frequency converter according to some embodiments of the present invention.

[0051] Figure 15 Shows Figure 14 A partial enlarged view.

[0052] Figure 16 Shows Figure 14 A spectrum schematic diagram of the input current harmonics of a single power unit and a cascaded frequency converter. Detailed implementation manners

[0053] In the following, only some exemplary embodiments are simply described. As those skilled in the art can recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the present invention. Therefore, the drawings and the description are considered to be exemplary in nature and not restrictive.

[0054] In the description of the present invention, it should be understood that the orientation or positional relationships indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc. are based on the orientation or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as a limitation to the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of the described features. In the description of the present invention, "a plurality" means two or more unless otherwise specifically defined.

[0055] In the description of the present invention, it should be noted that, unless otherwise clearly defined and limited, the terms "installed", "connected", "coupled" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or a connection that allows mutual communication; it can be directly connected, or indirectly connected through an intermediate medium, and can be the internal communication of two components or the interaction relationship between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0056] In the present invention, unless otherwise clearly defined and limited, the first feature being "above" or "below" the second feature may include the direct contact between the first and second features, or may include the situation where the first and second features are not in direct contact but in contact through other features therebetween. Moreover, the first feature being "above", "over" and "on" the second feature includes that the first feature is directly above and obliquely above the second feature, or merely indicates that the horizontal height of the first feature is higher than that of the second feature. The first feature being "below", "under" and "beneath" the second feature includes that the first feature is directly below and obliquely below the second feature, or merely indicates that the horizontal height of the first feature is lower than that of the second feature.

[0057] Many different embodiments or examples are provided below to implement different structures of the present invention. 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 numerals and / or reference letters in different examples. This repetition is for the purpose of simplification and clarity, and does not itself indicate the relationship between the various embodiments and / or settings discussed. In addition, various specific examples of processes and materials are provided in the present invention, but those of ordinary skill in the art can be aware of 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 cascaded frequency converter. The cascaded frequency converter includes a transformer and three power unit groups. The transformer includes three-phase input terminals and three groups of single-phase output terminals. The three-phase input terminals can be connected to the power grid. The three groups of single-phase output terminals are connected to the three power unit groups in 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. 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 terminal. 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 the control terminals of the second chopper switch and the third chopper switch. The processing module can determine the leakage inductance of the transformer. Based on the leakage inductance of the transformer, the processing module can control the operation of the first chopper switch. Alternatively, the processing module can control the operation of the second chopper switch and the third chopper switch to reduce the harmonic pollution of the cascaded frequency converter to the power grid.

[0060] Figure 1 FIG. shows a schematic diagram of a cascaded frequency converter 100 according to some embodiments of the present invention. As Figure 1 shown, the cascaded frequency converter 100 includes a transformer 10 and three power unit groups G1, G2, and G3. The transformer 10 includes three-phase input terminals A, B, and C and three groups of single-phase output terminals. The three-phase input terminals of the transformer 10 are provided on the three-phase primary coils 101A, 101B, and 101C of the transformer 10 and can be connected to the power grid E (for example, 10 kV, etc.). The three groups of single-phase output terminals are provided on the three-phase secondary coils 103A, 103B, and 103C of the transformer 10. The transformer 10 further includes iron cores 102A, 102B, and 102C connecting the three-phase primary coils and the three-phase secondary coils. The three groups of single-phase output terminals of the transformer 10 are connected to the three power unit groups G1, G2, and G3 in one-to-one correspondence. Exemplarily, the A-phase output terminal of the transformer 10 is connected to the power unit group G1. The B-phase output terminal of the transformer 10 is connected to the power unit 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. A column of cascaded power units constitutes a power unit group, and the input terminal of each power unit is connected to the corresponding single-phase output terminal of the transformer. Exemplarily, as Figure 1As shown in the figure, the power unit group G1 includes a plurality of cascaded power units 21, and the input end of each power unit 21 is connected to the A-phase output end of the transformer 10. The power unit group G2 includes a plurality of cascaded power units 22, and the input end of each power unit 22 is connected to the B-phase output end of the transformer 10. The power unit group G3 includes a plurality of cascaded power units 23, and the input end of each power unit 23 is connected to the C-phase output end of the transformer 10. In each power unit group, one of the output ends (such as the V output end) of the power unit at the head end (e.g., close to the transformer 10) is short-circuited to form the neutral point O, and one of the output ends (such as the U output end) of the power unit at the tail end (close to the motor M) is connected to the motor M. The adjacent power units are connected through the U and V output ends. Among the three power unit groups G1, G2, and G3, each power unit has single-phase input and single-phase output. It should be noted that the present invention does not limit the number of power units in the power unit group, nor the number of primary and secondary coils and windings of the transformer. In practical applications, it can be set according to requirements.

[0062] In some traditional cascaded high-voltage inverters, the secondary side of the transformer uses the extended delta connection method. The three-phase output terminals of the low-voltage side of the transformer are connected to the three-phase input ends of each power unit to achieve multi-pulse rectification, reduce the harmonic current of the primary side of the transformer, and reduce the harmonic pollution of the cascaded inverter to the power grid. This extended delta connection method requires the three-phase output of the transformer and the three-phase input of the power unit. The three-phase output terminals of the transformer need to be connected to the three-phase input ends of each power unit. Each power unit needs to be connected to 3 secondary windings of the three-phase of the transformer, and 3 cables are also required to connect between the transformer and each power unit. As a result, the number of secondary coil windings of the transformer and the number of connection cables between the transformer and the power unit are very large, the wiring is complex, and the cost is high. At the same time, when the transformer uses the extended delta connection method, the secondary side of the transformer needs to be connected in a proportional combination of delta connection and star connection. The connection process requires welding technology and the technology of insulating damage and restoration, which greatly improves the process complexity and labor input, is not conducive to automated production, increases the comprehensive cost of the transformer, and further increases the comprehensive cost of the cascaded inverter.

[0063] Different from the traditional scheme, in the cascaded inverter of the present invention, the transformer has three groups of single-phase outputs, and each power unit has single-phase input. The three groups of single-phase output terminals of the transformer are connected to the single-phase input ends of the power units of the three power unit groups one by one. That is to say, the single-phase output terminal of the transformer is connected to the single-phase input end of each power unit of the corresponding power unit group. In this way, each power unit can be connected to 1 secondary winding of the single-phase of the transformer, and 2 cables can be used to connect between the transformer and each power unit, greatly reducing the number of secondary coil windings of the transformer, the number of connection cables between the transformer and the power unit, the wiring complexity and the cost.

[0064] Exemplarily, taking the 10kV cascaded frequency converter as an example, each power unit group includes 8 levels of 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, 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 3 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 3 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, 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 3 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 3 power unit groups to the transformer is 16 * 3 = 48. It can be seen that in the technical solution of the present invention, compared with the traditional technical solution, both the number of secondary windings of the transformer and the number of cables are significantly reduced, and the wiring complexity and cost are also reduced accordingly. It should be understood that only the 10kV cascaded frequency converter is taken as an example for exemplary introduction here, and the present invention is not limited thereto. For other high-voltage cascaded frequency converters above kV, adopting the technical solution of the present invention can also achieve similar technical effects such as reducing the number of secondary windings of the transformer, the number of cables, the wiring complexity, and the cost.

[0065] Figure 2 Schematic diagrams showing power units 21 / 22 / 23 according to some embodiments of the present invention. As Figure 2 shown, the power units 21 / 22 / 23 include 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 series-connected first diode D1 and second diode D2. The second branch includes a series-connected third diode D3 and fourth diode D4. 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 terminal S where the first diode D1 and the second diode D2 are connected constitutes the input side of the first branch. The common terminal R where the third diode D3 and the fourth diode D4 are connected 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 terminal of the power unit 21 / 22 / 23. The single-phase input terminal of the power unit is connected to the corresponding single-phase output terminal of the transformer. As Figure 1 and Figure 2As shown, the input sides of the first and second branches of the power cell 21 of the power cell group G1 are connected to the A-phase output terminal. The input sides of the first and second branches of the power cell 22 of the power cell group G2 are connected to the B-phase output terminal. The input sides of the first and second branches of the power cell 23 of the power cell group G3 are connected to the C-phase output terminal. That is, the cathode of the second diode D2 and the anode of the third diode D3 are connected to the corresponding single-phase output terminal of the transformer. That is, the anode of the first diode D1 and the cathode of the fourth diode D4 are connected to the corresponding single-phase output terminal of the transformer.

[0066] As Figure 2 shown, the first chopper switch S1 is connected in parallel to the output sides of the first and second branches. The cathode of the third diode D3 and the anode of the fourth diode D4 form the output sides of the first and second branches. 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, based on the leakage inductance L, control the operation of the first chopper switch S1 to reduce the harmonic pollution of the cascaded frequency converter 100 to the power grid E. The present invention makes good use of the leakage inductance of the transformer as an inductor and controls the operation of the chopper switch based on the leakage inductance, saving the volume, device cost, and design complexity of the cascaded frequency converter while reducing the harmonic pollution of the cascaded frequency converter to the power grid.

[0067] As Figure 2 shown, the power cells 21 / 22 / 23 further include a capacitor C connected to the DC bus and an inverter bridge connected in parallel to the capacitor C. The inverter bridge includes a first switching tube K1 and a second switching tube K2 connected in series, and a third switching tube K3 and a fourth switching tube K4 connected in series. One ends of the first switching tube K1 and the third switching tube K3 are connected to the positive DC bus DC+. One ends of the second switching tube K2 and the fourth switching tube K4 are connected to the negative DC bus DC-. The common terminal V connected by the first switching tube K1 and the second switching tube K2, and the common terminal U connected by the third switching tube K3 and the fourth switching tube K4 form the single-phase output terminals of the power cells 21 / 22 / 23. It should be noted that although not shown in the figure, the first / second / third / fourth switching tubes K1 / K2 / K3 / K4 can be connected to the processing module 24 and respond to the commands of the main control module of the cascaded frequency converter, and finally a sinusoidal pulse width modulation wave (Sine pulse width modulation, SPWM) with adjustable frequency and / or voltage can be output at UV.

[0068] As Figure 2As shown, the power units 21 / 22 / 23 include a first chopper switch S1. The power units 21 / 22 / 23 also include 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 can be a fast recovery diode (FRD) to prevent the energy of the capacitor C from flowing backward (flowing from right to left in the figure) and causing a short circuit of the first chopper switch S1.

[0069] Figure 2 The embodiments of [description] introduce an example where the power unit includes a first chopper switch S1. In some embodiments, the power unit may include a second chopper switch and a third chopper switch. The second chopper switch can be connected in parallel with the first diode or the second diode. The third chopper switch can be connected in parallel with the third diode or the fourth diode.

[0070] Figure 3 The schematic diagram of the power units 21 / 22 / 23 according to some embodiments of the present invention is shown. As Figure 3 shown, the power units 21 / 22 / 23 include a second chopper switch S2 and a third chopper switch S3. The second chopper switch S2 is connected in parallel with the first diode D1. The third chopper switch S3 is connected in parallel with the third diode D3. Figure 4 The schematic diagram of the power units 21 / 22 / 23 according to some embodiments of the present invention is shown. As Figure 4 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; 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; the third chopper switch S3 can be connected in parallel with the third diode D3. These are all within the protection scope of the present invention. In practical applications, the connection manner between the chopper switch and the diode can be set according to requirements.

[0071] As Figure 3 and Figure 4 shown, the processing module 24 is connected to the control terminals g2, g3 of the second chopper switch S2 and the third chopper 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 chopper switch S2 and the third chopper switch S3 to reduce the harmonic pollution of the cascaded frequency converter 100 to the power grid E. It should be noted that although not shown in the figure, it should be understood that the second chopper switch S2 can be connected in parallel with the third diode D3 or the fourth diode D4. The third chopper switch S3 can be connected in parallel with the first diode D1 or the second diode D2. These are all within the protection scope of the present invention. It should be noted that Figure 3 and Figure 4The power unit introduced in the embodiment may not include Figure 2 the fifth diode D5 of the embodiment, and the others are Figure 2 substantially the same as the power unit introduced in the embodiment.

[0072] It should be noted that the specifications of each chopper switch (S1~S3), each diode (D1~D5), and each switching transistor (K1~K4) may be the same or different, and can be set according to requirements in actual applications.

[0073] In some embodiments, the chopper switch (S1~S3) / switching transistor (K1~K4) may include at least one of a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), an Insulate-Gate Bipolar Transistor (IGBT), or an 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). Optionally, the MOSFET may employ power devices such as silicon carbide (SIC) MOSFET and gallium nitride (GaN) MOSFET, which have lower losses, higher efficiency, and better performance. In actual applications, it can be set according to requirements.

[0074] In some embodiments, the processing module 24 may include components or circuits such as processing circuitry, 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 Micro Control 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, etc.

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

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

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

[0078] In some embodiments, the processing module may control the period of the control signal at the control terminal of the first chopper switch, or the second and third chopper switches, based on the leakage inductance of the transformer to control its on-off frequency. As Figure 1 and Figure 2 shown, the processing module 24 may control the period of the control signal at the control terminal g1 of the first chopper switch S1 based on the leakage inductance L of the transformer 10 to control the on-off frequency of the first chopper switch S1. Another example isFigure 1 , Figure 3 or Figure 4 As shown, the processing module 24 can control the period of the control signals of the control terminal g2 of the second chopper switch S2 and the control terminal g3 of the third chopper switch S3 based on the leakage inductance L of the transformer 10, so as to control the on-off frequency of the second chopper switch S2 and the third chopper switch S3. It should be understood that the on-off frequency is the reciprocal 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 triangular wave or a sawtooth wave, etc.) with a modulated wave signal (e.g., a sine wave). The carrier signal and the PWM wave signal can be generated by the PWM wave signal generator of the processing module 24. The processing module 24 can control the carrier period for generating the PWM wave signal based on the leakage inductance L of the transformer 10, so as 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 frequency of the first / second / third chopper switch is negatively correlated with the leakage inductance L of the transformer 10. The processing module 24 can control the on-off frequency of the first / second / third chopper switch based on such a correlation.

[0081] In some embodiments, the processing module 24 can control the minimum on-off frequency fmin of the first / second / third chopper switch 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 current allowed in the coil, the rectifier bridge, other components in the IGBT circuit, etc. 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] That is: ,

[0084] Then: .

[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 switches of each power unit can be controlled in a phase-shifting manner, such that the carrier phases of any two adjacent power units in the same power unit group differ by a preset angle. Eventually, the single-phase input current of the cascaded frequency converter 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 more beneficial grid-side current harmonic indexes. Through the phase-shifted superposition technology, the on-off frequency and current stress of the chopper switches can be further reduced. It should be noted that the "adjacent" here should be understood in a broad sense, which can be physically adjacent or logically adjacent.

[0086] The chopper switch of the power unit is controlled by the processing module inside the power unit. The carrier wave for controlling the on-off of the chopper switch is generated by the PWM wave timer in the processing module. Therefore, the carrier phase can be controlled by the processing module to make 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 10 kV cascaded high-voltage frequency converter, each power unit group includes 8 levels of power units. The preset angle by which the carrier phases of any two adjacent power units in each power unit group differ is 360 / 8 = 45°. If the initial angle of the carrier phase of the first power unit (the head-end power unit) in each power unit group is 0°, then the carrier phase angles of the other 7 power units are 45°, 90°, 135°, 180°, 225°, 270°, and 315° respectively. It should be noted that the carrier phase-shifting angles of the 8 power units may not be arranged in adjacent order, as long as 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315° are covered. Here, only the 10 kV cascaded frequency converter is used as an example for illustrative introduction. The present invention is not limited thereto. For other high-voltage cascaded frequency converters above a certain kV, the preset angle by which the carrier phases of any two adjacent power units in each power unit group differ can be appropriately adjusted according to the actual situation, and these are all within the protection scope of the present invention.

[0087] Since the processing modules of each power unit operate independently and use independent clocks, there may be differences in clock errors. As time goes by, the carrier phases of each power unit may change, causing the difference between the carrier phases of two adjacent power units in the same power unit group to deviate from the preset angle, which will also cause harmonic pollution of the cascaded frequency converter 100 to the power grid E.

[0088] To solve this problem, the inventors of the present application ingeniously conceived that it can be solved through carrier synchronization control. The processing module of each power unit can control 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.

[0089] In some embodiments, the synchronization signal may be provided by the processing module of the power unit. Specifically, in each power unit group, the processing module of one of the power units 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 may be provided by the processing module of the first power unit in each power unit group and sequentially transmitted by the processing modules of the upstream power units to the processing modules of the downstream power units. The processing modules of each power unit in the same group may 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 A schematic diagram showing carrier synchronization control according to some embodiments of the present invention is shown. As Figure 5 shown, the power unit group G1 includes a plurality of cascaded power units 21-1, 21-2... 21-N. The power units 21-1, 21-2... 21-N respectively include processing modules 24-11, 24-12... 24-1N, where N is a positive integer. The processing module 24-11 provides the synchronization signal CLK. The processing module 24-11 transmits the step signal CLK to the processing module 24-12, and the processing module 24-12 transmits the synchronization signal CLK to the processing module 24-13,..., until the synchronization signal CLK is transmitted to the processing module 24-1N. The processing modules 24-11 to 24-1N may adjust their respective carrier phases based on the synchronization signal CLK so that the carrier phases of any two adjacent power units in the power unit group G1 differ by a preset angle.

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

[0092] Similarly, the power unit group G3 includes a plurality of cascaded power units 23-1, 23-2... 23-N, and the power units 23-1, 23-2... 23-N respectively include processing modules 24-31, 24-32... 24-3N, where N is a positive integer. The processing module 24-31 provides a synchronization signal CLK. The processing module 24-31 transmits the step signal CLK to the processing module 24-32, and the processing module 24-32 transmits the synchronization signal CLK to the processing module 24-33,..., until the synchronization signal CLK is transmitted to the processing module 24-3N. The 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 the 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 sequentially transmitted by the processing modules of the downstream power units to the processing modules of the upstream power units. 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 yet another example, the synchronization signal can be provided by the processing module of any other power unit except the first and terminal power units of each power unit group, and sequentially transmitted by this processing module to the processing modules of the upstream power units and the processing modules of the downstream power units. 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 the main control module of the cascaded frequency converter. The cascaded frequency converter further 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 A schematic diagram showing carrier synchronization control according to some embodiments of the present invention. As Figure 6As shown, the cascaded frequency converter 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, and 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 the 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 the 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 the 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. Exemplarily, the above synchronization signal CLK can be a level signal or a communication command word signal, such as: 0x88, etc.

[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, ASICs, FPGAs, CPLDs, 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 cascaded frequency converter may provide a synchronization signal under preset conditions. That is to say, when the preset conditions are met, the synchronization signal for carrier phase adjustment is provided, which can reduce the system complexity and communication rate requirements.

[0098] In some embodiments, the preset conditions include that the deviation between the difference in carrier phases of any two adjacent power units in the power unit group at regular intervals and / or with respect to a preset angle is greater than or equal to an angle threshold. For example, the processing module of the power unit and / or the main control module of the cascaded frequency converter can provide a synchronization signal at regular intervals, so that the processing modules of the respective power units 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 cascaded frequency converter can provide a synchronization signal when the deviation between the difference in carrier phases of any two adjacent power units in the power unit group and the preset angle is greater than or equal to the angle threshold, so that the processing modules of the respective power units can adjust their respective carrier phases based on the synchronization signal. Or, the processing module of the power unit and / or the main control module of the cascaded frequency converter can provide a synchronization signal at regular intervals and when the deviation between the difference in carrier phases of any two adjacent power units in the power unit group and the preset angle is greater than or equal to the angle threshold, so that the processing modules of the respective power units 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 can be set according to requirements in practical applications.

[0100] In some embodiments, the processing module is further configured to determine the input voltage, input current, or the 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 at the control end of the first chopper switch, or the second and third chopper switches. 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, which is the equivalent 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 schematically), the input current (such as Figure 7 or Figure 8 the input current Iin shown schematically), optimize the input harmonic index of the power unit and even the frequency converter, and optimize the performance of the cascaded high-voltage frequency converter.

[0101] Figure 7 A schematic diagram showing the control of the duty cycle of the chopper switch according to some embodiments of the present invention. As Figure 7As shown, the processing module 24 can determine the input voltage Vin of the power units 21 / 22 / 23, the input current Iin (or the absolute value of the input current Iin), 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 of the control terminal g1 of the first chopper switch S1.

[0102] Figure 8 A schematic diagram showing the control of the duty cycle of a chopper switch according to some embodiments of the present invention is shown. As Figure 8 shown, the processing module 24 can determine the input voltage Vin of the power units 21 / 22 / 23, the input current Iin (or the absolute value of the input current Iin), 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 cycles of the control signals of the control terminals g2 and g3 of the second chopper switch S2 and the third chopper switch S3. The duty cycles of the control signals of the control terminals g2 and g3 are the same.

[0103] Figure 9 A schematic diagram showing the working principle of controlling the duty cycle of a chopper switch according to some embodiments of the present invention is shown. As Figure 9 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 voltage signal Vc across the capacitor after digital sampling 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 by the multiplier 242, a reference current signal Iref is output to the second error amplifier U2. The input current Iin after digital sampling 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 after adjustment by the current inner loop PI regulator 243 is fed into the PWM wave signal generator 244. The PWM wave signal generator 244 compares it with a carrier signal to generate PWM wave control signals Sig1 or Sig2 and Sig3, and then generates drive signals Sig1' or Sig2' and Sig3' through the driver 245 to the control terminals g1 or g2 and g3 to control the duty cycles of the first chopper switch S1 or the second chopper switch S2 and the third chopper switch S3.

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

[0105] The sampling of the input voltage Vin is to a certain extent affected by the operation of the first chopper switch S1 or the second chopper switch S2 and the third chopper switch S3, resulting in a certain sampling error. In order to more accurately obtain the input voltage Vin of the power unit (such as voltage phase information, voltage amplitude information, etc.), in some embodiments, a measuring winding for measuring the input voltage Vin of the power unit (such as voltage phase information, voltage amplitude information, etc.) may be provided on the secondary side of the transformer. Almost no current flows through the measuring winding, so a very thin wire can be selected, and the measuring winding only needs one turn. Therefore, the introduction of the measuring winding has a negligible impact on the overall cost. Without substantially increasing the cost, the phase and / or amplitude measurement errors caused by the operation of the first chopper switch S1 or the second chopper S2 and the third chopper switch S3 can be reduced, the sampling accuracy of the input voltage Vin is improved, and it helps to more reliably and effectively reduce the harmonics on the input side of the transformer.

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

[0107] In some embodiments, the input voltage Vin of the power unit can also be obtained by measuring the input phase voltage of the transformer. Since the transformer has no phase shift, the phase of the input voltage is the same as that of the output voltage. Exemplarily, the main control module 30 can measure and communicate the input phase voltage of the transformer to the power unit, and the power unit can use this voltage as input information and convert it into the input voltage Vin. Generally, the main control module can measure the input voltage and current of the cascaded frequency converter for control and protection, and use the input voltage as the input of Vin without adding any hardware cost.

[0108] In some embodiments, the cascaded frequency converter further includes a main control module. The main control module is configured to collect the primary side phase voltage information of the transformer and transmit the primary side phase voltage 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 side voltage information. For example, the main control module 30 can collect the primary side A-phase voltage information of the transformer and transmit the primary side A-phase voltage information to the processing module of the power unit in the power unit group G1. The processing module of the power unit in the power unit group G1 can determine its input voltage Vin based on the primary side A-phase voltage information. Again, for example, the main control module 30 can collect the primary side B-phase voltage information of the transformer and transmit the primary side B-phase voltage information to the processing module of the power unit in the power unit group G2. The processing module of the power unit in the power unit group G2 can determine its input voltage Vin based on the primary side B-phase voltage information. Still again, for example, the main control module 30 can collect the primary side C-phase voltage information of the transformer and transmit the primary side C-phase voltage information to the processing module of the power unit in the power unit group G3. The processing module of the power unit in the power unit group G3 can determine its input voltage Vin based on the primary side C-phase voltage information.

[0109] In some embodiments, although not shown in the figures, the cascaded frequency converter 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 information.

[0110] In some embodiments, the cascaded frequency converter 100 may include a storage module (not shown in the figures). The storage module is used to store the operation data of the cascaded frequency converter 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), the bus capacitor voltage, and other information. The storage module may include a memory. The memory may include a Random Access Memory (RAM), or may also include a Non-Volatile Memory (NVM). Further, the memory may include at least one of a Phase-Change Random Access Memory (PRAM), a Static Random Access Memory (SRAM), a Dynamic Random Access Memory (DRAM), a Read-Only Memory (ROM), and an Electrically Erasable Programmable Read-Only Memory (EEPROM). Alternatively, the storage module may include a cloud memory. The storage module may be implemented by software and / or hardware means.

[0111] In some embodiments, the cascaded frequency converter 100 may include a display module (not shown in the figures). The display module may be connected to at least one of the main control module 30 and the processing module 24 and can be used to visually output the operation data of the cascaded frequency converter 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), the bus capacitor voltage, and other information. The display module may include a display screen. The display screen may include at least one of a liquid-crystal display (LCD), a light emitting diode (LED) display, and an organic light emitting diode (OLED) display.

[0112] The present invention also provides a control method for the cascaded frequency converter 100 as described above. Figure 11 The flowchart of the control method 300 according to some embodiments of the present invention is shown. As Figure 11 shown, the control method 300 includes steps S310 to S330. Step S310, determining the leakage inductance L of the transformer 10. Step S330, based on the leakage inductance L, controlling the operation of the first chopper switch S1, or the second chopper switch S2 and the third chopper switch S3, to reduce the harmonic pollution of the cascaded frequency converter 100 to the power grid E.

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

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

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

[0116] In some embodiments, step S330 further includes: controlling the processing module of each power unit to control 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.

[0117] In some embodiments, controlling the processing module of each power unit to control its respective carrier phase based on the synchronization signal includes: 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.

[0118] In some embodiments, controlling the processing module of each power unit to control its respective carrier phase based on the synchronization signal includes: controlling the main control module of the cascaded frequency converter to provide the synchronization signal; 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 its respective carrier phase based on the synchronization signal includes: controlling the processing module and / or the main control module of the cascaded frequency converter to provide the synchronization signal under a preset condition.

[0120] In some embodiments, the preset conditions include that the deviation between the difference in the carrier phases of any two adjacent power units in the regular and / or power unit group and the preset angle is greater than or equal to the angle threshold.

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

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

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

[0124] In some embodiments, the control method 300 and its respective steps can be executed by one or more of the processing module of the power unit, the main control module of the cascaded frequency converter, the HMI, the DA, the PLC, etc., or other remote controllers.

[0125] Figure 12 The waveform schematic diagram of the input voltage Vin and input current Iin of a single power unit according to some embodiments of the present invention is shown. As Figure 12 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 sine wave. The input current Iin is approximately a sine wave. It can be seen that by adopting the technical solution of the present invention, the input current harmonics of a single power unit can be suppressed.

[0126] Figure 13 Shown Figure 12 is the spectrum schematic diagram of the input current harmonics of a single power unit. As Figure 12 shown, through the Fourier analysis of the input current harmonics of a single power unit, it can be known that there are high-frequency components in the input current harmonics (as Figure 13 shown, around 2000 Hz and around 4000 Hz). The reasons for the existence of these high-frequency components are caused by, for example, the on-off instant of chopper switches, etc., but are not limited thereto.

[0127] Figure 14 The waveform schematic diagram of the input current of multiple single-phase power units and the input current of the cascaded frequency converter according to some embodiments of the present invention is shown, Figure 15 ShownFigure 14 Partial enlarged view. Exemplarily, taking 4 cascaded power units 21-1 to 21-4 of phase A as an example, as Figure 14 and Figure 15 shown, (a) represents the input current waveform of power unit 21-1, (b) represents the input current waveform of power unit 21-2, (c) represents the input current waveform of power unit 21-3, (d) represents the input current waveform of power unit 21-4, and (e) represents the input current waveform of cascaded frequency converter 100. It can be seen from the figure that the input current waveforms of multiple single-phase power units are basically sinusoidal waves, and after superposition, the input current waveform of the cascaded frequency converter becomes less harmonic and closer to a sinusoidal wave.

[0128] Figure 16 Shows Figure 14 The spectrum schematic diagram of the input current harmonics of a single power unit and a cascaded frequency converter. As Figure 16 shown, through Fourier analysis of the input current harmonics of multiple single-phase power units and the input current harmonics of the cascaded frequency converter, it can be seen that although there are some high-frequency harmonic components in the input current of a single power unit, they can cancel each other out after superposition, and finally there are almost no high-frequency harmonic components in the input current of the cascaded frequency converter. In particular, the high-frequency harmonic components near 2000 Hz and 4000 Hz are effectively suppressed. It can be seen that by adopting the technical solution of the present invention, the technical effect of suppressing input current harmonics is ideally presented.

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

[0130] Table 1

[0131] Input current of power unit 21-1 Input current of power unit 21-1 Input current of power unit 21-1 Input current of power unit 21-1 Input current of cascaded frequency converter Total harmonic distortion THD 0.28147 0.28149 0.28146 0.28144 0.00525

[0132] It can be seen that by adopting the technical solution of the present invention, the input current harmonics of the cascaded frequency converter are well suppressed. The total harmonic distortion (Total Harmonic Distortion, THD) of a single power unit is about 28%, and the total harmonic distortion of the cascaded frequency converter is about 0.525%, which can well meet the national standard requirement within 5%.

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

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

[0135] The traditional cascaded frequency converter uses a phase-shifting transformer to achieve multi-pulse rectification and reduce 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 terminals of the transformer can be connected to the power grid, the three groups of single-phase output terminals of the transformer are connected to three power unit groups in one-to-one correspondence, the input terminal of each power unit is connected to the corresponding single-phase output terminal, and based on the leakage inductance of the transformer, the operation of the first chopper switch, or the second and third chopper switches of the power unit is controlled to reduce the harmonic pollution of the cascaded frequency converter to the power grid. In other words, the present invention uses single-phase active power factor correction (APFC) technology to reduce the harmonic current on the grid side, and can achieve the same or even better technical effects without performing multi-pulse rectification through a phase-shifting transformer as in the traditional technical solution.

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

[0137] The present invention may take the form of a computer program product implemented on one or more storage media that contain program code. Computer-usable storage media include both permanent and non-permanent, removable and non-removable media, and may implement the storage of information by any method or technology. 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 technologies, compact disc read-only memory (CD-ROM), digital video disc (DVD) or other optical storage, magnetic cassette tapes, magnetic disk storage or other magnetic storage devices, or any other non-transmission media that can be used to store information accessible by a computing device.

[0138] It should be noted that this specification provides method operation steps such as in the embodiments or schematic diagrams, but based on routine or non-creative labor, there may be more or fewer operation steps. The step sequences listed in the embodiments are only one of the many ways of the execution sequences of the steps, and do not represent the only execution sequence. When the actual system or device product executes, it may execute in the method sequence shown in the embodiments or flowcharts or execute in parallel.

[0139] It should be noted that although several modules of the cascaded frequency converter are mentioned in the above detailed description, this division is not mandatory. In fact, according to the embodiments of the present invention, the features and functions of the two or more modules described above can be implemented in one module. Conversely, the features and functions of one module described above can be further divided and embodied by multiple modules.

[0140] It should be noted that the present invention may only include Figures 1-16 any one or more features of any one or more of the embodiments. In other words, not all of the shown features need to be implemented simultaneously in the cascaded frequency converter / control method of the present invention.

[0141] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A cascaded frequency converter, characterized in that, Comprising: A transformer, including a three-phase input end and three groups of single-phase output ends, and the three-phase input end can be connected to the power grid; And Three power unit groups, which are respectively and correspondingly connected to the three groups of single-phase output ends. Each power unit group includes a plurality of cascaded power units, and the input end of each power unit is connected to the corresponding single-phase output end; Wherein, each power unit includes: A rectifier bridge, including 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 end; A first chopper switch, which is connected in parallel to the output sides of the first branch and the second branch; or, 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; and A processing module, connected to the control end of the first chopper switch, or connected to the control ends of the second chopper switch and the third chopper switch, and configured to determine the leakage inductance of the transformer; based on the leakage inductance, control the operation of the first chopper switch, or the second chopper switch and the third chopper switch, so as to reduce the harmonic pollution of the cascaded frequency converter to the power grid.

2. The cascaded 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 further includes a capacitor connected to the DC bus and an inverter bridge connected in parallel to the capacitor.

3. The cascaded frequency converter according to claim 2, wherein 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.

4. The cascaded frequency converter according to claim 1, wherein The processing module is configured to control the on-off frequency of the first chopper switch, or the second chopper switch and the third chopper switch based on the leakage inductance.

5. The cascaded frequency converter according to claim 4, wherein The processing module is configured to control the period 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 leakage inductance, so as to control the on-off frequency.

6. The cascaded frequency converter according to claim 5, wherein The control signal includes a PWM wave signal; the processing module is configured to control the carrier period for generating the PWM wave signal based on the leakage inductance, so as to control the period of the PWM wave signal.

7. The cascaded frequency converter according to claim 4, wherein The on-off frequency is negatively correlated with the leakage inductance.

8. The cascaded frequency converter according to claim 6, 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.

9. The cascaded frequency converter according to claim 8, wherein, In each power unit group, the processing module of one of the power units 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.

10. The cascaded frequency converter according to claim 8, characterized in that, The cascaded frequency converter further 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 the carrier phase of each 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.

11. The cascaded frequency converter according to claim 8, wherein, The processing module and / or the main control module of the cascaded frequency converter is configured to provide the synchronization signal under preset conditions.

12. The cascaded frequency converter according to claim 11, characterized in that, The preset conditions include regularly and / or the deviation between the difference in the carrier phases of any two adjacent power units in the power unit group and the preset angle is greater than or equal to an angle threshold.

13. The cascaded frequency converter according to any one of claims 1-12, characterized in that, The power unit further includes a capacitor connected to the DC bus; the processing module is further configured to determine the input voltage, input current of the power unit, and the voltage across the capacitor; based on the input voltage, input current, and the voltage across the capacitor, control the duty cycle of the control signal at the control end of the first chopper switch, or the second and third chopper switches.

14. The cascaded frequency converter according to claim 13, characterized in that, It further includes a plurality of measurement windings provided on the secondary side coil of the transformer, each measurement 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 measurement winding.

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

16. The cascaded frequency converter according to any one of claims 1-12, 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.

17. A control method for a cascaded frequency converter as described in any one of claims 1-16, characterized in that, Includes: 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 and third chopper switches, so as to reduce the harmonic pollution of the cascaded frequency converter to the power grid.

18. The control method according to claim 17, wherein Step S330 includes: Based on the leakage inductance, control the on-off frequency of the first chopper switch, or the second and third chopper switches.

19. The control method according to claim 18, wherein Controlling the on-off frequency includes: Based on the leakage inductance, control the period of the control signal at the control end of the first chopper switch, or the second and third chopper switches.

20. The control method according to claim 19, wherein The control signal includes a PWM wave signal; controlling the period of the PWM wave signal includes: Based on the leakage inductance, control the carrier period for generating the PWM wave signal.

21. The control method according to claim 20, characterized in that, Step S330 further includes: controlling the processing module of each power unit to control 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.

22. The control method according to claim 21, wherein 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.

23. The control method according to claim 21, 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 frequency converter to provide the synchronization signal; 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.

24. The control method according to claim 21, wherein The processing module that controls each power unit controls the respective carrier phases based on the synchronization signal, including: controlling the processing module and / or the main control module of the cascaded frequency converter to provide the synchronization signal under preset conditions.

25. The control method according to claim 24, wherein The preset conditions include periodically and / or the deviation between the difference in the carrier phases of any two adjacent power units in the power unit group and the preset angle being greater than or equal to an angle threshold.

26. The control method according to any one of claims 17-25, characterized in that, Further comprising: Determining the input voltage, input current of the power unit, and the voltage across the capacitor connected to the DC bus; based on the input voltage, input current, and the voltage across the capacitor, controlling the duty ratio of the control signal at the control end of the first chopper switch, or the second and third chopper switches.

27. The control method according to claim 26, wherein Further comprising: Controlling the processing module of each power unit to determine the input voltage based on the corresponding measuring winding.

28. The control method according to claim 26, wherein Further comprising: Collecting the primary side phase voltage information of the transformer through the main control module, and transmitting the primary side phase voltage information to the processing module of the corresponding power unit, controlling the processing module to determine the input voltage of the power unit based on the corresponding primary side voltage information.

29. A computer-readable storage medium, characterized in that, Comprising computer-executable instructions stored thereon, the executable instructions, when executed by a processor, implement the control method according to any one of claims 17-28.

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