Multilevel converter, control method and multilevel converter system

Abstract

The application provides a multi-level converter, a control method and a multi-level converter system. Each bridge arm of the multi-level converter comprises a plurality of sub-modules. Each sub-module comprises a resonance unit, an energy storage unit, a first switching unit, a second switching unit, a first capacitor unit and a second capacitor unit. The resonance unit comprises a coupling sub-unit. The first switching unit comprises a first sub-switching unit and a second sub-switching unit. The second switching unit comprises a third sub-switching unit and a fourth sub-switching unit. In each sub-module, the first end of the first sub-switching unit is electrically connected with the first end of the energy storage unit and the first end of the resonance unit respectively. The connection branch of the second end of the first sub-switching unit and the first end of the second sub-switching unit is electrically connected with the third end of the resonance unit. The first end of the third sub-switching unit is electrically connected with the fourth end of the resonance unit. The connection branch of the second end of the third sub-switching unit and the first end of the fourth sub-switching unit is electrically connected with the sixth end of the resonance unit.

Description

Technical Field

[0001] This application relates to the field of flexible DC transmission technology, and more specifically, to a multilevel converter, a control method, and a multilevel converter system. Background Technology

[0002] Flexible DC technology, based on modular multilevel converters, achieves precise voltage and power control through fully controllable devices, overcoming the inherent defects of traditional DC. However, the submodules of flexible DC technology suffer from high conduction losses, limiting their long-term economic viability. Furthermore, the turn-on speed of some submodules is uncontrollable, and significant internal commutation processes can damage the switching devices within these submodules. Currently, there is a need for a converter structure that can both avoid uncontrollable turn-on speeds and reduce switching losses in the submodules. Summary of the Invention

[0003] The main objective of this application is to provide a multilevel converter, a control method, and a multilevel converter system to at least solve the problem of high losses in sub-modules of multilevel converters during operation in the prior art.

[0004] To achieve the above objectives, according to one aspect of this application, a multilevel converter is provided, comprising: multiple bridge arms, each bridge arm including multiple sub-modules connected in series, each sub-module including a resonant unit, an energy storage unit, a first switching unit, a second switching unit, a first capacitor unit, and a second capacitor unit; the first switching unit including a first sub-switching unit and a second sub-switching unit connected in series, the second switching unit including a third sub-switching unit and a fourth sub-switching unit connected in series; the first capacitor unit and the third sub-switching unit connected in parallel, the second capacitor unit and the fourth sub-switching unit connected in parallel; the resonant unit including a first terminal, a second terminal, a third terminal, a fourth terminal, a fifth terminal, and a sixth terminal; the first terminal, the second terminal, and the third terminal are located on a first side of the resonant unit, and the fourth terminal, the fifth terminal, and the sixth terminal are located on a second side of the resonant unit opposite to the first side. The third end is located between the first end and the second end. The third end and the sixth end of the resonant unit are connected through the coupling sub-unit. In each sub-module, the first end of the first sub-switch unit is electrically connected to the first end of the energy storage unit and the first end of the resonant unit, respectively. The second end of the second sub-switch unit is electrically connected to the second end of the energy storage unit and the second end of the resonant unit, respectively. The connection branch between the second end of the first sub-switch unit and the first end of the second sub-switch unit is electrically connected to the third end of the resonant unit. The first end of the third sub-switch unit is electrically connected to the fourth end of the resonant unit. The second end of the fourth sub-switch unit is electrically connected to the fifth end of the resonant unit. The connection branch between the second end of the third sub-switch unit and the first end of the fourth sub-switch unit is electrically connected to the sixth end of the resonant unit.

[0005] Optionally, the coupling subunit is a first coupling subunit, and the resonant unit further includes: a first unidirectional conducting subunit and a second unidirectional conducting subunit. The first end of the first unidirectional conducting subunit is electrically connected to the first end of the first sub-switching unit and the first end of the third sub-switching unit, respectively. The second end of the second unidirectional conducting subunit is electrically connected to the second end of the second sub-switching unit and the second end of the fourth sub-switching unit, respectively. The second end of the first unidirectional conducting subunit is electrically connected to the second end of the first sub-switching unit, the first end of the second sub-switching unit, the first end of the second unidirectional conducting subunit, and the first coupling subunit, respectively.

[0006] Optionally, the first coupling subunit includes a first coupling inductor, the first end of the primary side of the first coupling inductor is electrically connected to the second end of the first sub-switching unit and the first end of the second sub-switching unit, the second end of the primary side is electrically connected to the second end of the third sub-switching unit and the first end of the fourth sub-switching unit, the first end of the secondary side of the first coupling inductor is electrically connected to the second end of the primary side, and the second end of the secondary side is electrically connected to the second end of the first unidirectional conducting subunit and the first end of the second unidirectional conducting subunit.

[0007] Optionally, the first unidirectional conducting subunit includes a first diode, the second unidirectional conducting subunit includes a second diode, the cathode of the first diode is electrically connected to the first terminal of the first sub-switching unit and the first terminal of the third sub-switching unit, the anode of the second diode is electrically connected to the second terminal of the second sub-switching unit and the second terminal of the fourth sub-switching unit, and the anode of the first diode is electrically connected to the second terminal of the first sub-switching unit, the first terminal of the second sub-switching unit, the cathode of the second diode, and the first coupling subunit.

[0008] Optionally, the coupling subunit is a second coupling subunit, and the resonant unit further includes a fifth switching subunit and a first resonant subunit connected in series with the second coupling subunit. One end of the second coupling subunit is electrically connected to the second end of the first sub-switching unit and the first end of the second sub-switching unit, respectively. One end of the second coupling subunit is electrically connected to the second end of the third sub-switching unit and the first end of the fourth sub-switching unit, respectively.

[0009] Optionally, the second coupling subunit includes a second coupling inductor, the first end of the primary side of the second coupling inductor is electrically connected to the second end of the first sub-switching unit and the first end of the second sub-switching unit, the second end of the primary side of the second coupling inductor is electrically connected to the second end of the third sub-switching unit and the first end of the fourth sub-switching unit, and the secondary side of the second coupling inductor is connected in series with the fifth switching subunit and the first resonant subunit.

[0010] Optionally, the fifth switching subunit includes a first switching device and a third diode connected in anti-parallel, the anode of the first switching device being electrically connected to the first resonant subunit, and the cathode of the first switching device being electrically connected to one end of the second coupling subunit.

[0011] Optionally, the first resonant subunit includes a resonant capacitor, the two ends of which are electrically connected to the second coupling subunit and the fifth switching subunit, respectively.

[0012] According to another aspect of this application, a control method for a multilevel converter is provided for controlling the multilevel converter. The control method includes: acquiring commutation information of multiple arms of the multilevel converter; when the commutation information indicates that a sub-module of the arm is engaged, sending a first signal to a first switching unit and a second switching unit, such that: when the current direction in the arm is positive, a first sub-switching unit of the first switching unit immediately turns on, a third sub-switching unit of the second switching unit turns on after a first preset time period, and after the third sub-switching unit turns on, the first sub-switching unit delays for a second preset time period. After a period of time, the third sub-switch unit is turned off; when the current direction in the bridge arm is reversed, the second sub-switch unit is turned on immediately, and the fourth sub-switch unit is turned on after a delay of the first preset time period. After the fourth sub-switch unit is turned on, the second sub-switch unit is turned off after a delay of the second preset time period. When the commutation information indicates that the sub-module of the bridge arm is cut off, a second signal is sent to the first and second switch units so that: when the current direction in the bridge arm is positive, the third sub-switch unit is turned off immediately; when the current direction in the bridge arm is reversed, the fourth sub-switch unit is turned off immediately.

[0013] According to another aspect of this application, a multilevel converter system is provided, including a controller and the multilevel converter, wherein the controller is communicatively connected to the multilevel converter and is used to execute a control method for the multilevel converter.

[0014] The technical solution of this application can solve the technical problem of high losses in the operation of sub-modules in multilevel converters in related technologies. The multilevel converter arm of this application includes multiple sub-modules. Each sub-module includes a resonant unit, an energy storage unit, a first switching unit, a second switching unit, a first capacitor unit, and a second capacitor unit. When the multilevel converter arm is turned on, the first sub-switching unit turns on immediately. However, the presence of the coupling sub-module in the resonant unit prevents the current of the first sub-switching unit from rising immediately, keeping it in a zero-current soft-start state. As the current gradually increases, the resonant unit charges the second capacitor unit, causing its voltage to gradually rise. Simultaneously, the voltage of the first capacitor unit gradually decreases until it reaches zero, at which point the third sub-switching unit turns on, achieving zero-voltage soft-start. Because the coupling sub-module bears the voltage of the second capacitor unit, its current gradually decreases to zero. When the current of the coupling sub-module is zero, the first sub-switching unit soft-turns off under zero-current conditions. When the bridge arm of the multilevel converter is turned off, the first capacitor unit acts as a clamping capacitor to prevent the voltage of the third sub-switching unit from immediately rising, thus achieving soft turn-off at near-zero voltage. Through the synergistic effect of the aforementioned resonant unit, first switching unit, second switching unit, first capacitor unit, and second capacitor unit, the power submodule of the multilevel converter achieves a soft-switching state with zero voltage and zero current during commutation. Soft switching can reduce voltage and current overlap, thereby reducing or even eliminating switching losses, effectively reducing losses in the submodule during daily commutation. Attached Figure Description

[0015] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0016] Figure 1 A schematic diagram of a multilevel converter provided in an embodiment of this application is shown;

[0017] Figure 2 A schematic diagram of the structure of a submodule of a multilevel converter provided in an embodiment of this application is shown;

[0018] Figure 3 A schematic diagram of the structure of a first type of resonant unit provided according to an embodiment of this application is shown;

[0019] Figure 4 A schematic diagram of the structure of a second resonant unit provided according to an embodiment of this application is shown;

[0020] Figure 5 A schematic diagram of the structure of a third resonant unit provided according to an embodiment of this application is shown;

[0021] Figure 6 A schematic diagram of the structure of a fourth resonant unit provided according to an embodiment of this application is shown;

[0022] Figure 7 A flowchart illustrating a control method for a multilevel converter according to an embodiment of this application is shown.

[0023] The above figures include the following reference numerals:

[0024] 10. Submodule; 11. Resonant unit; 12. Energy storage unit; 13. First capacitor unit; 14. Second capacitor unit; 15. First sub-switch unit; 16. Second sub-switch unit; 17. Third sub-switch unit; 18. Fourth sub-switch unit; 110. Coupling sub-unit; 111. First coupling sub-unit; 112. First unidirectional conduction sub-unit; 113. Second unidirectional conduction sub-unit; 114. Second coupling sub-unit; 115. Fifth switch sub-unit; 116. First resonant sub-unit. Detailed Implementation

[0025] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0026] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0027] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of this application described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0028] As described in the background section, existing flexible DC technology is based on modular multilevel converters. It achieves precise voltage and power control through fully controllable devices, overcoming the inherent defects of traditional DC technology. However, the sub-modules of flexible DC technology suffer from high conduction losses, limiting the long-term economic viability of flexible DC. Furthermore, the turn-on speed of some sub-modules is uncontrollable, and large-scale internal commutation processes within sub-modules can damage internal switching devices. To address the issue of high losses in sub-modules during operation in existing multilevel converters, embodiments of this application provide a multilevel converter, a control method, and a multilevel converter system.

[0029] According to one embodiment of this application, a multilevel converter is provided, such as... Figure 1 As shown, it includes: multiple bridge arms, each bridge arm including multiple sub-modules 10 connected in series, each sub-module 10 including a resonant unit 11, an energy storage unit 12, a first switching unit, a second switching unit, a first capacitor unit 13, and a second capacitor unit 14. The first switching unit includes a first sub-switching unit 15 and a second sub-switching unit 16 connected in series, the second switching unit includes a third sub-switching unit 17 and a fourth sub-switching unit 18 connected in series, the first capacitor unit 13 and the third sub-switching unit 17 are connected in parallel, the second capacitor unit 14 and the fourth sub-switching unit 18 are connected in parallel, the resonant unit 11 has a first terminal, a second terminal, a third terminal, a fourth terminal, a fifth terminal, and a sixth terminal, the resonant unit 11 includes a coupling sub-unit 110, the first terminal of the resonant unit 11 is connected to the fourth terminal of the resonant unit 11, and the second terminal of the resonant unit 11 is connected to the fifth terminal of the resonant unit 11. Then, the third end and the sixth end of the resonant unit 11 are connected through the coupling sub-unit 110. In each sub-module 10, the first end of the first sub-switch unit 15 is electrically connected to the first end of the energy storage unit 12 and the first end of the resonant unit 11, respectively. The second end of the second sub-switch unit 16 is electrically connected to the second end of the energy storage unit 12 and the second end of the resonant unit 11, respectively. The connection branch between the second end of the first sub-switch unit 15 and the first end of the second sub-switch unit 16 is electrically connected to the third end of the resonant unit 11. The first end of the third sub-switch unit 17 is electrically connected to the fourth end of the resonant unit 11. The second end of the fourth sub-switch unit 18 is electrically connected to the fifth end of the resonant unit 11. The connection branch between the second end of the third sub-switch unit 17 and the first end of the fourth sub-switch unit 18 is electrically connected to the sixth end of the resonant unit 11.

[0030] By incorporating a resonant circuit comprising a resonant unit, an energy storage unit, a first switching unit, a second switching unit, a first capacitor unit, and a second capacitor unit in each submodule of a multilevel converter, the high losses during submodule operation in related technologies are solved. When the bridge arm of the multilevel converter is turned on, the first sub-switching unit turns on immediately, but the presence of the coupling submodule prevents the current of the first sub-switching unit from rising immediately, keeping it in a zero-current soft-start state. As the current gradually increases, the resonant unit charges the second capacitor unit, causing its voltage to gradually rise, while the voltage of the first capacitor unit gradually decreases until it reaches zero, at which point the third sub-switching unit turns on, achieving zero-voltage soft-start. Because the coupling submodule bears the voltage of the second capacitor unit, its current gradually decreases to zero. When the current of the coupling submodule is zero, the first sub-switching unit soft-turns off under zero-current conditions. When the bridge arm of the multilevel converter is turned off, the first capacitor unit acts as a clamping capacitor to prevent the voltage of the third sub-switching unit from immediately rising, thus achieving soft turn-off at near-zero voltage. Through the synergistic effect of the aforementioned resonant unit, first switching unit, second switching unit, first capacitor unit, and second capacitor unit, the power submodule of the multilevel converter achieves a soft-switching state with zero voltage and zero current during commutation. Soft switching can reduce voltage and current overlap, thereby reducing or even eliminating switching losses, effectively reducing losses in the submodule during daily commutation.

[0031] In the above embodiments, such as Figure 2As shown, the first sub-switching unit includes a second switching device T1 and a fourth diode D1 connected in anti-parallel; the second sub-switching unit includes a third switching device T2 and a fifth diode D2 connected in anti-parallel; the third sub-switching unit includes a fourth switching device T3 and a sixth diode D3 connected in anti-parallel; and the fourth sub-switching unit includes a fifth switching device T4 and a seventh diode D4 connected in anti-parallel. The aforementioned second switching device T1, third switching device T2, fourth switching device T3, and fifth switching device T4 can be integrated gate commutated thyristors (IGCTs) or insulated gate bipolar transistors (IGBTs). This combination of anti-parallel switching devices and diodes allows the sub-module to operate normally not only under forward current conditions (forward current flows from terminal A to terminal B) but also without damage under reverse current flow (reverse current flows from terminal B to terminal A). When the switching device is on, the diode is off; and when the switching device is off, the diode can provide a path for reverse current, enabling bidirectional flow of the bridge arm current in the multilevel converter. During the commutation process, the anti-parallel sixth diode D3 and seventh diode D4 ensure that the residual inductor energy can be naturally attenuated through the diodes when the fourth switch device T3 and the fifth switch device T4 are turned off, avoiding energy accumulation in the circuit that could lead to overvoltage or overcurrent problems. Simultaneously, the fourth diode D1 and the fifth diode D2 provide a freewheeling path when the second switch device T1 and the third switch device T2 are turned off, allowing energy to be smoothly transferred between the resonant unit and the energy storage unit, thus optimizing energy utilization efficiency.

[0032] In the above embodiments, the first capacitor unit and the second capacitor unit can be the first capacitor Cs1 and the second capacitor Cs2, respectively, for storing energy and stabilizing voltage, ensuring energy storage and voltage balance of the submodule under different switching states, and the dynamic adjustment of the submodule can be realized by controlling the switching devices of the submodule to turn on and off.

[0033] In the above embodiments, the presence of the coupling inductor can also limit the rate of current change of the switching devices of the submodule during the turn-on process, avoiding false turn-on caused by sudden current surges, and enabling better control of the turn-on and turn-off of the switching devices. When the switching devices of the submodule need to be turned on, the current on the primary and secondary sides of the coupling inductor is gradually changed by controlling other circuit elements, thereby achieving soft turn-on of the switching devices of the submodule.

[0034] The resonant unit in this application exists in two modes: separately excited resonant circuits (the energy of the resonant circuit is provided by an external power source or system, i.e., the initiation and maintenance of the resonance process are not spontaneously completed by the circuit itself) and self-excited resonant circuits (the resonant state is spontaneously initiated and maintained by the energy storage element inside the circuit, without the need for direct injection of external energy). Separately excited resonant circuits are as follows: In some optional embodiments, such as... Figure 3 As shown, the coupling subunit is the first coupling subunit 111. The resonant unit 11 also includes: a first unidirectional conducting subunit 112 and a second unidirectional conducting subunit 113. The first end of the first unidirectional conducting subunit 112 is electrically connected to the first end of the first sub-switching unit and the first end of the third sub-switching unit, respectively. The second end of the second unidirectional conducting subunit 113 is electrically connected to the second end of the second sub-switching unit and the second end of the fourth sub-switching unit, respectively. The second end of the first unidirectional conducting subunit 112 is electrically connected to the second end of the first sub-switching unit, the first end of the second sub-switching unit, the first end of the second unidirectional conducting subunit 113, and the first coupling subunit 111, respectively. Using the first coupling subunit 111, the first unidirectional conducting subunit 112, and the second unidirectional conducting subunit 113, a soft-switching path can be formed when the first sub-switching unit is turned on, limiting the current rise and achieving zero-current turn-on of the first sub-switching unit. Specifically, when the multilevel converter current is positive (from... Figure 2 As shown in the diagram (terminal A receives current, terminal B receives current), as the primary current of the first coupling subunit 111 increases, the induced current on the secondary side also continuously increases. When it reaches a certain level, the second unidirectional conducting subunit 113 turns on, and the current induced on the secondary side continues to charge the second capacitor unit through the second unidirectional conducting subunit 113. After the first sub-switching unit is turned on, the current of the fourth diode gradually decreases. When it drops to zero, the first and second capacitor units participate in resonance. The voltage of the second capacitor unit gradually increases, and the voltage of the first capacitor unit gradually decreases. When the voltage of the first capacitor unit is zero, the third sub-switching unit can be turned on under zero-voltage conditions. The above-mentioned zero-current, zero-voltage switching process significantly reduces the stress on the switching devices in the submodule and reduces switching losses.

[0035] In some alternative implementations, such as Figure 2 and Figure 4As shown, the first coupling sub-unit includes a first coupling inductor Ls1. The first end of the primary side of the first coupling inductor Ls1 is electrically connected to the second end of the first sub-switching unit and the first end of the second sub-switching unit, respectively. The second end of the primary side is electrically connected to the second end of the third sub-switching unit and the first end of the fourth sub-switching unit, respectively. The first end of the secondary side of the first coupling inductor Ls1 is electrically connected to the second end of the primary side, and the second end of the secondary side is electrically connected to the second end of the first unidirectional conducting sub-unit and the first end of the second unidirectional conducting sub-unit. The introduction of the first coupling inductor Ls1 ensures that when the first sub-switching unit T1 is turned on, the current does not suddenly increase, but rather transitions smoothly through the induced current on the secondary side of the first coupling inductor Ls1, creating conditions for the freewheeling current of the second unidirectional conducting sub-unit. When the voltage of the second capacitor Cs2 increases and the voltage of the first capacitor Cs1 decreases, reaching zero, the third sub-switching unit T3 turns on under zero-voltage conditions, achieving soft switching. Similarly, when the first sub-switching unit T1 is turned off, the natural zero-crossing of the secondary current of the first coupling inductor Ls1 provides a zero-current condition for the turn-off of the third sub-switching unit T3, thereby effectively reducing overvoltage and losses during the turn-off process. Introducing the first coupling inductor Ls1 as part of the resonant unit not only enables soft switching of the sub-module's switching devices but also allows the freewheeling current of the secondary inductor Ls1 to store energy for the second capacitor unit, achieving resonance where the voltage of the second capacitor unit increases and the voltage of the first capacitor unit decreases. This further optimizes the commutation process and reduces the overall operating losses of the sub-module.

[0036] In some alternative implementations, such as Figure 4 As shown, the first unidirectional conducting subunit includes a first diode D5, and the second unidirectional conducting subunit includes a second diode D6. The cathode of the first diode D5 is electrically connected to the first terminal of the first sub-switching unit and the first terminal of the third sub-switching unit, respectively. The anode of the second diode D6 is electrically connected to the second terminal of the second sub-switching unit and the second terminal of the fourth sub-switching unit, respectively. The anode of the first diode D5 is electrically connected to the second terminal of the first sub-switching unit, the first terminal of the second sub-switching unit, the cathode of the second diode D6, and the first coupling subunit. The first coupling subunit includes a first coupling inductor Ls1. The anode of the first diode D5 is first electrically connected to one end of the secondary side and one end of the primary side of the first coupling inductor Ls1, and then electrically connected to the cathode of the second diode D6. The other end of the secondary side of the first coupling inductor Ls1 is electrically connected to the other end of the primary side. The first diode D5 is used when the current in the multilevel converter is the reverse current (from...). Figure 2 When the current flows into terminal B and out terminal A (as shown in the diagram), the first coupling inductor Ls1 provides freewheeling current to charge the first capacitor unit. The second diode D6 is used when the current in the multilevel converter is forward current (from...). Figure 2When current flows into terminal A and out terminal B (as shown in the diagram), it freewheels through the first coupling inductor Ls1 to charge the second capacitor unit. The first diode D5 and the second diode D6 form a unidirectional conduction path in the resonant unit, which allows the current to flow along a preset path during soft switching without causing overvoltage or overcurrent damage to the device.

[0037] In a multilevel converter with the aforementioned separately excited resonant circuit, when the bridge arm is engaged, the third sub-switch unit does not turn on immediately due to a rise delay, while the first sub-switch unit turns on immediately. The load current flows through the seventh diode and the first sub-switch unit. Due to the presence of the coupling inductor, the current does not rise immediately, and the first sub-switch unit is in a soft-switching state with zero current turn-on. As the primary current of the first coupling inductor rises, the current induced on the secondary side freewheels through the second diode, at which point the current in the second diode begins to decay. When the current in the second diode decays to zero, the first and second capacitors begin to resonate. The voltage of the second capacitor gradually increases, while the voltage of the first capacitor gradually decreases. When the voltage of the first capacitor drops to zero, the turn-on signal, delayed by τ1, is transmitted to the third sub-switch unit, allowing the third sub-switch unit to turn on under zero-voltage conditions. Because the secondary side of the first coupled inductor is subjected to the voltage of the second capacitor, this voltage forces the secondary current of the first coupled inductor to gradually cross zero. Similarly, the primary current of the first coupled inductor also crosses zero. The first sub-switching unit naturally turns off when the resonant current crosses zero. Its turn-off is delayed by τ2 after the third sub-switching unit turns on. When the bridge arm is cut off, the first sub-switching unit is already in the off state, and the third sub-switching unit immediately turns off. The current here is the bridge arm current. When the third sub-switching unit turns off, due to the clamping effect of the first capacitor, the voltage of the third sub-switching unit cannot change abruptly, but rises slowly. This makes the third sub-switching unit have an effect of near-zero voltage turn-off. At the same time, the bridge arm current causes the first and second capacitors to complete voltage reset, and the seventh diode can be turned on smoothly. The above-mentioned delay times τ1 and τ2 can be calculated through resonance.

[0038] A self-excited resonant circuit is as follows: In some optional implementations, such as Figure 5As shown, the coupling subunit is the second coupling subunit 114. The resonant unit 11 also includes a fifth switching subunit 115 and a first resonant subunit 116 connected in series with the second coupling subunit 114. One end of the second coupling subunit 114 is electrically connected to the second end of the first sub-switching unit and the first end of the second sub-switching unit, respectively. Another end of the second coupling subunit 114 is electrically connected to the second end of the third sub-switching unit and the first end of the fourth sub-switching unit, respectively. During the turn-on and turn-off process of the submodule, the second coupling subunit 114 utilizes its inductive characteristics to limit the rate of change of current, thereby avoiding malfunctions and current surges in the switching units within the submodule and ensuring soft turn-on and soft turn-off of the device. Simultaneously, by connecting the second coupling subunit 114 in series with the first resonant subunit 116 and the fifth switching subunit 115, the energy distribution of the resonance process can be effectively controlled, achieving smooth switching of the switching units within the submodule under low stress conditions. When the switching unit in the submodule needs to be turned off, the fifth switching subunit 115 can control the local current flow of the second coupling subunit 114 to avoid excessively high turn-off voltage and achieve zero-current turn-off. The first resonant subunit 116 can resonate with the second coupling subunit 114, making the turn-on process of the switching unit in the submodule smoother.

[0039] In some alternative implementations, such as Figure 6 As shown, the second coupling subunit includes a second coupling inductor Ls2. The first end of the primary side of the second coupling inductor Ls2 is electrically connected to the second end of the first sub-switching unit and the first end of the second sub-switching unit, respectively. The second end of the primary side of the second coupling inductor Ls2 is electrically connected to the second end of the third sub-switching unit and the first end of the fourth sub-switching unit, respectively. The secondary side of the second coupling inductor Ls2 is connected in series with the fifth switching subunit and the first resonant subunit. The second coupling inductor Ls2 can enhance the control capability of the resonant circuit. Through its inductance characteristics, it effectively limits the rate of current rise of the switching devices of the submodule during the turn-on process, avoiding uncontrollable turn-on of the switching devices of the submodule due to sudden current changes. During the turn-off process, the induced current on the secondary side of the second coupling inductor Ls2 can form a freewheeling path through the fifth switching subunit and the first resonant subunit, smoothing the turn-off process of the switching devices of the submodule, reducing the voltage peak during turn-off, and thus reducing turn-off losses.

[0040] In some alternative implementations, such as Figure 6As shown, the fifth switching subunit includes a first switching device T5 and a third diode D7 connected in anti-parallel. The anode of the first switching device T5 is electrically connected to the first resonant subunit, and the cathode of the first switching device T5 is electrically connected to one end of the second coupling subunit. The anti-parallel connection of the first switching device T5 and the third diode D7 enables bidirectional energy flow control in the circuit, providing an effective way to recover energy and prevent reverse voltage. When the switching devices of the submodule are turned off, the first switching device T5 can control the direction of the secondary current of the second coupling inductor Ls2, avoiding overvoltage caused by direct current disconnection. Before the switching devices of the submodule are turned on, the third diode D7 ensures proper current guidance, allowing the switching devices of the submodule to turn on under zero-voltage conditions, reducing losses.

[0041] In some alternative implementations, such as Figure 6 As shown, the first resonant subunit includes a resonant capacitor Cs3. The two ends of the resonant capacitor Cs3 are electrically connected to the secondary side of the second coupling subunit and the anode of the switching device in the fifth switching subunit, respectively. As the core of the first resonant subunit, the resonant capacitor Cs3 forms a circuit with the secondary side of the second coupling subunit and the input terminal of the first switching device T5 in the fifth switching subunit, participating in the resonance process and effectively regulating the voltage stress of the switching device in the submodule during turn-on and turn-off.

[0042] In a multilevel converter with the aforementioned self-excited resonant circuit, when the bridge arm is engaged, the third sub-switch unit will not turn on immediately due to the rise delay, while the first sub-switch unit will turn on immediately. The load current flows through the seventh diode and the third sub-switch unit. Due to the presence of the coupling inductor, the current will not rise immediately, and the third sub-switch unit is in a soft-switching state with zero current turn-on. As the primary current of the second coupling inductor rises, the current induced on the secondary side freewheels through the third diode to charge the resonant capacitor. At this time, the current of the seventh diode begins to decay. When the current of the seventh diode decays to zero, the first and second capacitors begin to resonate. The voltage of the second capacitor gradually increases, while the voltage of the first capacitor gradually decreases. When the voltage of the first capacitor decreases to zero, the turn-on signal, delayed by τ1, is transmitted to the third sub-switch unit. The third sub-switch unit can turn on under zero-voltage conditions. Since the secondary side of the second coupled inductor bears the voltage of the resonant capacitor, this voltage forces the secondary current of the second coupled inductor to gradually cross zero. Similarly, the primary current of the second coupled inductor also crosses zero. The first sub-switch unit naturally turns off when the resonant current crosses zero. Its turn-off is executed after a delay of τ2 from the turn-on of the first sub-switch unit. When the bridge arm is disconnected, the first sub-switch unit is already in the off state, and the third sub-switch unit immediately turns off. When the third sub-switch unit turns off, it is clamped by the first capacitor, so the voltage of the third sub-switch unit cannot change abruptly but rises slowly. This makes the third sub-switch unit have an effect of near-zero-voltage turn-off. The aforementioned delay times τ1 and τ2 can be calculated through resonance.

[0043] The delay time is calculated using the following formula, and this calculated value determines the rise time of the pulse width:

[0044] ,

[0045] ,

[0046] ,

[0047] ,

[0048] Among them, I a I is the bridge arm current before commutation. rrm For diodes ( Figure 2 The reverse recovery current of the diodes is the same (any diode can be selected). V dc Ls is the voltage of the energy storage capacitor of the submodule, Ls is the inductance value of the coupling inductor (the inductance value of the first coupling inductor in the externally excited resonant circuit, and the inductance value of the second coupling inductor in the self-excited resonant circuit), and Cs is the capacitance value of the first capacitor unit or the second capacitor unit (the capacitance values ​​of the two are the same).

[0049] In some optional implementations, a magnetic core material with adjustable permeability is selected as the material for the coupling inductor. This core material can be a magnetostrictive material or a composite material containing magnetizable particles. The permeability of these materials can be controlled by the magnitude of an external magnetic field or current, thereby achieving dynamic adjustment of the coupling coefficient. An electromagnetic adjustment device, such as a coil or yoke, is integrated into the coupling inductor, which can generate a variable magnetic field by changing the current intensity in the device, thereby controlling the permeability of the core material. A flux control module is added to the control system of the multilevel converter. This module is used to adjust the coupling coefficient of the coupling inductor by calculating the current required by the electromagnetic adjustment device. The control system controls sensors to monitor the flux changes and coupling coefficient of the coupling inductor in real time, feeding this data back to the flux control module to ensure that the control system can adjust the current of the electromagnetic adjustment device in real time, maintaining precise control of the coupling coefficient. Through the above-mentioned dynamic adjustment of the coupling coefficient of the coupling inductor, adaptive control of the resonance process is achieved. This allows for intelligent adjustment of the resonant frequency and period based on real-time system conditions, such as current magnitude, voltage level, and load characteristics, thereby optimizing the energy conversion efficiency during soft switching.

[0050] For example, when the system detects an increase in load current or anticipates a peak current demand, the flux control module increases the permeability of the coupling inductor, effectively reducing the current rise rate and thus decreasing the turn-on losses and thermal stress of the submodule's switching devices. By increasing the coupling coefficient, energy exchange during resonance becomes smoother, contributing to better soft-switching performance. When a decrease in load current or a light load condition is detected, the flux control module decreases the permeability of the coupling inductor, accelerating the circuit's response speed and reducing energy retention during turn-off. This facilitates rapid turn-off of the submodule's switching devices, reduces turn-off losses, and improves system efficiency under low load conditions.

[0051] In some embodiments, the multilevel converter may further include an energy recovery device comprising an electrically connected energy storage element and a conversion control module. The energy recovery device has multiple input and output ports, including: an input port for connecting to the main circuit of a switching device to capture recoil energy during the switching process; and an output port connected to a first capacitor unit and / or a second capacitor unit for releasing the stored energy. The conversion control module can monitor the switching state of the switching device and the system energy demand in real time, automatically determining when to perform energy recovery and release. The conversion control module includes an electrically connected inverter circuit and a fast detection unit. The fast detection unit can detect the on / off state of the switching device. When the switching device is on, the inverter circuit converts the recovered energy from DC to AC for storage in the energy storage element. When the switching device is on: utilizing the self-inductance of the coupled inductor, when the switching device transitions from the off state to the on state, the inductor generates a back electromotive force (EMF), which can be captured by the energy recovery device. The captured energy is converted into a suitable form by the inverter circuit and then stored in the energy storage element. When the switching device is turned off: a freewheeling diode (either the first or second diode in this application) is used to capture the recoil energy during the switching device's turn-off. The energy is converted and stored in an energy storage element by a conversion control module.

[0052] The above description describes the multilevel converter in the case of forward current. In the case of reverse current, the second and fourth sub-switching units achieve soft switching. The initial current flows through terminal B, the sixth diode D3, and the energy storage unit C1 to terminal A. When the sub-module is engaged, the second sub-switching unit T2 immediately turns on. Due to the presence of the coupling inductance, the current does not rise immediately, and the second sub-switching unit T2 is in a soft-switching state with zero current turn-on. The current flows through the sixth diode D3 and the second sub-switching unit T2. The current in the sixth diode D3 gradually decays to zero, and the first and second capacitor units begin to resonate. The resonant unit charges the first capacitor unit, and the voltage of the first capacitor unit gradually increases while the voltage of the second capacitor unit gradually decreases. When the voltage of the second capacitor unit decreases to zero, the fourth sub-switching device T4 turns on with a delay, achieving zero-voltage turn-on. Since the resonant unit is subjected to the voltage of the first capacitor unit, this voltage forces the current of the resonant unit to gradually cross zero. The second sub-switching unit T2 naturally turns off when the resonant current crosses zero. When the bridge arm is disconnected, the second sub-switch unit T2 is already in the off state. The fourth sub-switch unit T4 immediately turns off. When the fourth sub-switch unit T4 turns off, it is clamped by the second capacitor unit, so the voltage of the fourth sub-switch unit T4 cannot change abruptly, but rises slowly. This makes the fourth sub-switch unit T4 have an effect of near-zero voltage turn-off. The same resonant unit can also be a separately excited resonant circuit or a self-excited resonant circuit, which will not be described again.

[0053] According to another embodiment of this application, a control method for a multilevel converter is provided for controlling the multilevel converter. Figure 7 This is a flowchart of a control method for a multilevel converter according to an embodiment of this application. Figure 7 As shown, the method includes the following steps:

[0054] Step S1: Obtain commutation information of multiple arms of the multilevel converter;

[0055] Specifically, commutation information is a key indicator of the operating status of a Modular Multilevel Converter (MMC), indicating the engagement and disengagement status of each bridge arm submodule. In practice, commutation information can be obtained through bridge arm current monitoring, system operating mode determination, or host computer commands, providing a basis for soft-switching control of submodules.

[0056] Step S2: When the commutation information indicates that a sub-module of the bridge arm is engaged, a first signal is sent to the first switching unit and the second switching unit so that: when the current direction in the bridge arm is positive, the first sub-switching unit of the first switching unit is immediately turned on, the third sub-switching unit of the second switching unit is turned on after a first preset time period, and after the third sub-switching unit is turned on, the first sub-switching unit is turned off after a second preset time period; when the current direction in the bridge arm is reverse, the second sub-switching unit is immediately turned on, the fourth sub-switching unit is turned on after a first preset time period, and after the fourth sub-switching unit is turned on, the second sub-switching unit is turned off after a second preset time period.

[0057] Specifically, firstly, a first signal is sent to the first switching unit (including the first and second sub-switching units). If the current direction in the bridge arm is positive, the first sub-switching unit is turned on promptly. Then, after a first preset time (τ1 mentioned above), the signal is transmitted to the third sub-switching unit, causing it to turn on with a delay. If the current direction in the bridge arm is reversed, the second sub-switching unit immediately turns on. After a second preset time (τ1 mentioned above), the signal is transmitted to the fourth sub-switching unit, causing it to turn on with a delay. The positive current direction refers to the current flowing from the power supply of the multilevel converter to the bus, and the reverse direction is the opposite. This delay time is calculated based on the parameters of the coupling inductor and resonant capacitor, ensuring that the first or second sub-switching unit is turned on in a zero-current state, and the third or fourth sub-switching unit is turned on in a zero-voltage state. This soft-turn-on state reduces the switching losses of the devices.

[0058] Specifically, when the third or fourth sub-switching unit is turned on under zero-voltage conditions, the voltage of the resonant capacitor on the secondary side of the second coupled inductor forces the secondary current of the second coupled inductor to gradually cross zero. Similarly, the primary current of the second coupled inductor also crosses zero. The first or second sub-switching unit naturally turns off when the resonant current crosses zero. Its turn-off is performed after a delay of τ2 from the start of the first or second sub-switching unit being turned on.

[0059] Step S3: When the commutation information indicates that a sub-module of the bridge arm has been cut off, a second signal is sent to the first and second switching units so that: when the current direction in the bridge arm is positive, the third sub-switching unit is immediately turned off; when the current direction in the bridge arm is negative, the fourth sub-switching unit is immediately turned off.

[0060] Specifically, a second signal is sent to the third sub-switch unit. If the current direction in the bridge arm is positive, and the first sub-switch unit is already off, the third sub-switch unit is immediately turned off, stopping the current flow between the submodule and the bridge arm. If the current direction in the bridge arm is negative, and the second sub-switch unit is already off, the fourth sub-switch unit is immediately turned off, stopping the current flow between the submodule and the bridge arm. The current here is the bridge arm current. When the third (or fourth) sub-switch unit is turned off, it is clamped by the first (or second) capacitor unit, preventing a sudden voltage change. Instead, the voltage rises slowly, resulting in a near-zero voltage turn-off effect. Simultaneously, the bridge arm current resets the voltage of the first and second capacitor units, allowing operation to resume the next time the bridge arm is engaged.

[0061] In this embodiment, the multilevel converter can be controlled using the aforementioned control method. When the bridge arm is turned on, a first signal is sent to the first switching unit. Due to the presence of the coupling sub-unit, the current of the first sub-switching unit does not rise immediately, and the first sub-switching unit is in a zero-current soft-turn-on state. As the current gradually increases, the resonant unit charges the second capacitor unit, causing the voltage of the second capacitor unit to gradually increase. Simultaneously, the voltage of the first capacitor unit gradually decreases until it reaches zero, at which point the third sub-switching unit achieves zero-voltage soft-turn-on. Because the coupling sub-unit bears the voltage of the second capacitor unit, the current of the coupling sub-unit gradually decreases to zero. When the current of the coupling sub-unit is zero, the first sub-switching unit soft-turns off under zero-current conditions. When the bridge arm of the multilevel converter is turned off, a second signal is sent to the second switching unit, and the third sub-switching unit immediately turns off. Because the first capacitor unit acts as a clamping capacitor, the voltage of the third sub-switching unit does not immediately rebound, and the third sub-switching unit achieves soft turn-off at approximately zero voltage. By employing the aforementioned control method to control the synergistic effect of the resonant unit, the first switching unit, the second switching unit, the first capacitor unit, and the second capacitor unit, the soft-switching state of the power submodule with zero voltage and zero current during the commutation process of the multilevel converter arm is achieved. Soft switching can reduce the overlap of voltage and current, thereby reducing or even eliminating switching losses, and can effectively reduce the losses of the submodule during daily commutation.

[0062] This application also provides a multilevel converter system, including a controller and a multilevel converter. The controller is communicatively connected to the multilevel converter and is used to execute a control method for the multilevel converter.

[0063] This application also provides a control device for a multilevel converter. It should be noted that the control device for the multilevel converter in this application can be used to execute the control method for a multilevel converter provided in this application. This device is used to implement the above embodiments and preferred embodiments; details already described will not be repeated. As used below, the term "module" can refer to a combination of software and / or hardware that implements a predetermined function. Although the device described in the following embodiments is preferably implemented in software, hardware implementation, or a combination of software and hardware, is also possible and contemplated.

[0064] The control device for the multilevel converter provided in the embodiments of this application is described below.

[0065] The control device for a multilevel converter according to an embodiment of this application includes: an acquisition module for acquiring commutation information of multiple arms of the multilevel converter; a first control module for sending a first signal to a first switching unit and a second switching unit when the commutation information indicates that a sub-module of an arm is engaged, such that when the current direction in the arm is positive, a first sub-switching unit of the first switching unit is immediately turned on, a third sub-switching unit of the second switching unit is turned on after a first preset time period, and after the third sub-switching unit is turned on, the first sub-switching unit is turned off after a second preset time period; when the current direction in the arm is reversed, a second sub-switching unit is immediately turned on, a fourth sub-switching unit is turned on after a first preset time period, and after the fourth sub-switching unit is turned on, the second sub-switching unit is turned off after a second preset time period; and a second control module for sending a second signal to the first switching unit and the second switching unit when the commutation information indicates that a sub-module of an arm is disconnected, such that when the current direction in the arm is positive, the third sub-switching unit is immediately turned off, and when the current direction in the arm is reversed, the fourth sub-switching unit is immediately turned off.

[0066] The control unit of the multilevel converter includes a processor and a memory. The aforementioned acquisition modules are all stored as program units in the memory, and the processor executes these program units to achieve the corresponding functions. All of the aforementioned modules reside in the same processor; alternatively, the modules may be located in different processors in any combination.

[0067] The processor contains a kernel, which retrieves the corresponding program units from memory. One or more kernels can be configured, and adjusting kernel parameters can address the issue of high losses in submodules of existing multilevel converters during operation.

[0068] The memory may include non-permanent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM, and the memory includes at least one memory chip.

[0069] This application provides a computer-readable storage medium including a stored program, wherein, when the program is running, it controls the device where the computer-readable storage medium is located to perform a control method for a multilevel converter.

[0070] Specifically, the control methods for multilevel converters include:

[0071] Step S1: Obtain commutation information of multiple arms of the multilevel converter;

[0072] Specifically, commutation information is a key indicator of the operating status of a Modular Multilevel Converter (MMC), indicating the engagement and disengagement status of each bridge arm submodule. In practice, commutation information can be obtained through bridge arm current monitoring, system operating mode determination, or host computer commands, providing a basis for soft-switching control of submodules.

[0073] Step S2: When the commutation information indicates that a sub-module of the bridge arm is engaged, a first signal is sent to the first switching unit and the second switching unit. When the current direction in the bridge arm is positive, the first sub-switching unit of the first switching unit is immediately turned on, and the third sub-switching unit of the second switching unit is turned on after a first preset time period. After the third sub-switching unit is turned on, the first sub-switching unit is turned off after a second preset time period. When the current direction in the bridge arm is reversed, the second sub-switching unit is immediately turned on, and the fourth sub-switching unit is turned on after a first preset time period. After the fourth sub-switching unit is turned on, the second sub-switching unit is turned off after a second preset time period.

[0074] Specifically, firstly, a first signal is sent to the first switching unit (including the first and second sub-switching units). If the current direction in the bridge arm is positive, the first sub-switching unit is turned on promptly. Then, after a first preset time (τ1 mentioned above), the signal is transmitted to the third sub-switching unit, causing it to turn on with a delay. If the current direction in the bridge arm is reversed, the second sub-switching unit immediately turns on. After a second preset time (τ1 mentioned above), the signal is transmitted to the fourth sub-switching unit, causing it to turn on with a delay. The positive current direction refers to the current flowing from the power supply of the multilevel converter to the bus, and the reverse direction is the opposite. This delay time is calculated based on the parameters of the coupling inductor and resonant capacitor, ensuring that the first or second sub-switching unit is turned on in a zero-current state, and the third or fourth sub-switching unit is turned on in a zero-voltage state. This soft-turn-on state reduces the switching losses of the devices.

[0075] Specifically, when the third or fourth sub-switching unit is turned on under zero-voltage conditions, the voltage of the resonant capacitor on the secondary side of the second coupled inductor forces the secondary current of the second coupled inductor to gradually cross zero. Similarly, the primary current of the second coupled inductor also crosses zero. The first or second sub-switching unit naturally turns off when the resonant current crosses zero. Its turn-off is performed after a delay of τ2 from the start of the first or second sub-switching unit being turned on.

[0076] Step S3: When the commutation information indicates that the sub-module of the bridge arm is cut off, a second signal is sent to the first and second switching units so that when the current direction in the bridge arm is positive, the third sub-switching unit is immediately turned off, and when the current direction in the bridge arm is negative, the fourth sub-switching unit is immediately turned off.

[0077] Specifically, a second signal is sent to the third sub-switch unit. If the current direction in the bridge arm is positive, and the first sub-switch unit is already off, the third sub-switch unit is immediately turned off, stopping the current flow between the submodule and the bridge arm. If the current direction in the bridge arm is negative, and the second sub-switch unit is already off, the fourth sub-switch unit is immediately turned off, stopping the current flow between the submodule and the bridge arm. The current here is the bridge arm current. When the third (or fourth) sub-switch unit is turned off, it is clamped by the first (or second) capacitor unit, preventing a sudden voltage change. Instead, the voltage rises slowly, resulting in a near-zero voltage turn-off effect. Simultaneously, the bridge arm current resets the voltage of the first and second capacitor units, allowing operation to resume the next time the bridge arm is engaged.

[0078] This application provides a processor for running a program, wherein the program executes a control method for a multilevel converter during runtime.

[0079] Specifically, the control methods for multilevel converters include:

[0080] Step S1: Obtain commutation information of multiple arms of the multilevel converter;

[0081] Specifically, commutation information is a key indicator of the operating status of a Modular Multilevel Converter (MMC), indicating the engagement and disengagement status of each bridge arm submodule. In practice, commutation information can be obtained through bridge arm current monitoring, system operating mode determination, or host computer commands, providing a basis for soft-switching control of submodules.

[0082] Step S2: When the commutation information indicates that a sub-module of the bridge arm is engaged, a first signal is sent to the first switching unit and the second switching unit. When the current direction in the bridge arm is positive, the first sub-switching unit of the first switching unit is immediately turned on, and the third sub-switching unit of the second switching unit is turned on after a first preset time period. After the third sub-switching unit is turned on, the first sub-switching unit is turned off after a second preset time period. When the current direction in the bridge arm is reversed, the second sub-switching unit is immediately turned on, and the fourth sub-switching unit is turned on after a first preset time period. After the fourth sub-switching unit is turned on, the second sub-switching unit is turned off after a second preset time period.

[0083] Specifically, firstly, a first signal is sent to the first switching unit (including the first and second sub-switching units). If the current direction in the bridge arm is positive, the first sub-switching unit is turned on promptly. Then, after a first preset time (τ1 mentioned above), the signal is transmitted to the third sub-switching unit, causing it to turn on with a delay. If the current direction in the bridge arm is reversed, the second sub-switching unit immediately turns on. After a second preset time (τ1 mentioned above), the signal is transmitted to the fourth sub-switching unit, causing it to turn on with a delay. The positive current direction refers to the current flowing from the power supply of the multilevel converter to the bus, and the reverse direction is the opposite. This delay time is calculated based on the parameters of the coupling inductor and resonant capacitor, ensuring that the first or second sub-switching unit is turned on in a zero-current state, and the third or fourth sub-switching unit is turned on in a zero-voltage state. This soft-turn-on state reduces the switching losses of the devices.

[0084] Specifically, when the third or fourth sub-switching unit is turned on under zero-voltage conditions, the voltage of the resonant capacitor on the secondary side of the second coupled inductor forces the secondary current of the second coupled inductor to gradually cross zero. Similarly, the primary current of the second coupled inductor also crosses zero. The first or second sub-switching unit naturally turns off when the resonant current crosses zero. Its turn-off is performed after a delay of τ2 from the start of the first or second sub-switching unit being turned on.

[0085] Step S3: When the commutation information indicates that the sub-module of the bridge arm is cut off, a second signal is sent to the first and second switching units so that when the current direction in the bridge arm is positive, the third sub-switching unit is immediately turned off, and when the current direction in the bridge arm is negative, the fourth sub-switching unit is immediately turned off.

[0086] Specifically, a second signal is sent to the third sub-switch unit. If the current direction in the bridge arm is positive, and the first sub-switch unit is already off, the third sub-switch unit is immediately turned off, stopping the current flow between the submodule and the bridge arm. If the current direction in the bridge arm is negative, and the second sub-switch unit is already off, the fourth sub-switch unit is immediately turned off, stopping the current flow between the submodule and the bridge arm. The current here is the bridge arm current. When the third (or fourth) sub-switch unit is turned off, it is clamped by the first (or second) capacitor unit, preventing a sudden voltage change. Instead, the voltage rises slowly, resulting in a near-zero voltage turn-off effect. Simultaneously, the bridge arm current resets the voltage of the first and second capacitor units, allowing operation to resume the next time the bridge arm is engaged.

[0087] This application provides a device including a processor, a memory, and a program stored in the memory and executable on the processor. When the processor executes the program, it performs at least the following steps: acquiring commutation information of multiple arms of a multilevel converter; when the commutation information indicates that a sub-module of an arm is engaged, sending a first signal to a first switching unit and a second switching unit to immediately turn on the first sub-switching unit, delaying the turn-on of a third sub-switching unit, and delaying the turn-off of the first sub-switching unit after the third sub-switching unit is engaged; when the commutation information indicates that a sub-module of an arm is disengaged, sending a second signal to the first and second switching units to immediately turn off the third sub-switching unit. The device described herein can be a server, PC, PAD, mobile phone, etc.

[0088] This application also provides a computer program product, which, when executed on a data processing device, is suitable for executing an initialization program having at least the following method steps: acquiring commutation information of multiple arms of a multilevel converter; when the commutation information indicates that a sub-module of an arm is engaged, sending a first signal to a first switching unit and a second switching unit to immediately turn on the first sub-switching unit, delaying the turn-on of a third sub-switching unit, and delaying the turn-off of the first sub-switching unit after the third sub-switching unit is turned on; when the commutation information indicates that a sub-module of an arm is disengaged, sending a second signal to the first switching unit and the second switching unit to immediately turn off the third sub-switching unit.

[0089] Obviously, those skilled in the art should understand that the modules or steps of this application described above can be implemented using general-purpose computing devices. They can be centralized on a single computing device or distributed across a network of multiple computing devices. They can be implemented using computer-executable program code, and thus can be stored in a storage device for execution by a computing device. In some cases, the steps shown or described can be performed in a different order than those presented here, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. Thus, this application is not limited to any particular combination of hardware and software.

[0090] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0091] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0092] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A multilevel converter, characterized in that, include: The system comprises multiple bridge arms, each including multiple sub-modules connected in series. Each sub-module includes a resonant unit, an energy storage unit, a first switching unit, a second switching unit, a first capacitor unit, and a second capacitor unit. The first switching unit includes a first sub-switching unit and a second sub-switching unit connected in series. The second switching unit includes a third sub-switching unit and a fourth sub-switching unit connected in series. The first capacitor unit and the third sub-switching unit are connected in parallel, and the second capacitor unit and the fourth sub-switching unit are connected in parallel. The resonant unit has a first terminal, a second terminal, a third terminal, a fourth terminal, a fifth terminal, and a sixth terminal. The resonant unit includes a coupling sub-unit. The first terminal of the resonant unit is connected to the fourth terminal of the resonant unit, the second terminal of the resonant unit is connected to the fifth terminal of the resonant unit, and the third terminal and the sixth terminal of the resonant unit are connected through the coupling sub-unit. The coupling subunit includes a second coupling subunit, and the resonant unit further includes a fifth switching subunit and a first resonant subunit connected in series with the second coupling subunit. One end of the second coupling subunit is electrically connected to the second end of the first sub-switching unit and the first end of the second sub-switching unit, respectively. One end of the second coupling subunit is electrically connected to the second end of the third sub-switching unit and the first end of the fourth sub-switching unit, respectively. In each of the sub-modules, the first terminal of the first sub-switch unit is electrically connected to the first terminal of the energy storage unit and the first terminal of the resonant unit, respectively; the second terminal of the second sub-switch unit is electrically connected to the second terminal of the energy storage unit and the second terminal of the resonant unit, respectively; the connection branch between the second terminal of the first sub-switch unit and the first terminal of the second sub-switch unit is electrically connected to the third terminal of the resonant unit; the first terminal of the third sub-switch unit is electrically connected to the fourth terminal of the resonant unit; the second terminal of the fourth sub-switch unit is electrically connected to the fifth terminal of the resonant unit; and the connection branch between the second terminal of the third sub-switch unit and the first terminal of the fourth sub-switch unit is electrically connected to the sixth terminal of the resonant unit. When the sub-module is engaged and the current direction in the bridge arm is positive, the first sub-switch unit of the first switch unit immediately conducts; the third sub-switch unit of the second switch unit conducts after a first preset time period; after the third sub-switch unit conducts, the first sub-switch unit turns off after a second preset time period. The first preset time τ1 and the second preset time τ2 are calculated as follows: , ,in, ， ， in, I a The current of the bridge arm before commutation, I rrm This refers to the reverse recovery current of any diode in either the first or the second switching unit. V dc Ls is the voltage of the energy storage unit, Ls is the inductance of the second coupling subunit, and Cs is the capacitance of the first capacitor unit or the second capacitor unit.

2. The multilevel converter according to claim 1, characterized in that, The second coupling subunit includes a second coupling inductor. The first end of the primary side of the second coupling inductor is electrically connected to the second end of the first sub-switching unit and the first end of the second sub-switching unit, respectively. The second end of the primary side of the second coupling inductor is electrically connected to the second end of the third sub-switching unit and the first end of the fourth sub-switching unit, respectively. The secondary side of the second coupling inductor is connected in series with the fifth switching subunit and the first resonant subunit.

3. The multilevel converter according to claim 1, characterized in that, The fifth switching subunit includes a first switching device and a third diode connected in anti-parallel. The anode of the first switching device is electrically connected to the first resonant subunit, and the cathode of the first switching device is electrically connected to one end of the second coupling subunit.

4. The multilevel converter according to claim 1, characterized in that, The first resonant subunit includes a resonant capacitor, the two ends of which are electrically connected to the second coupling subunit and the fifth switching subunit, respectively.

5. A control method for a multilevel converter, characterized in that, The control method for controlling the multilevel converter according to any one of claims 1 to 4 includes: Obtain commutation information of multiple arms of the multilevel converter; When the commutation information indicates that a sub-module of the bridge arm is engaged, a first signal is sent to the first switching unit and the second switching unit so that: when the current direction in the bridge arm is positive, the first sub-switching unit of the first switching unit is immediately turned on, the third sub-switching unit of the second switching unit is turned on after a first preset time period, and after the third sub-switching unit is turned on, the first sub-switching unit is turned off after a second preset time period; when the current direction in the bridge arm is reversed, the second sub-switching unit is immediately turned on, the fourth sub-switching unit is turned on after a first preset time period, and after the fourth sub-switching unit is turned on, the second sub-switching unit is turned off after a second preset time period. When the commutation information indicates that a sub-module of the bridge arm is cut off, a second signal is sent to the first switching unit and the second switching unit so that: when the current direction in the bridge arm is positive, the third sub-switching unit is immediately turned off; when the current direction in the bridge arm is negative, the fourth sub-switching unit is immediately turned off.

6. A multilevel converter system, characterized in that, The device includes a controller and a multilevel converter as described in any one of claims 1 to 4, wherein the controller is communicatively connected to the multilevel converter and is used to execute the control method of the multilevel converter as described in claim 5.

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