Converter, flexible dc power transmission system, method, medium and device

Abstract

The application provides a converter, a flexible direct-current power transmission system, a method, a medium and equipment. The converter comprises a plurality of sub-modules, a first switching device, a second switching device, a first diode, a second diode and an energy storage device, the first end of the first switching device is electrically connected with the first end of the second switching device, the first diode is connected in antiparallel with the first switching device, the second diode is connected in antiparallel with the second switching device, one end of the energy storage device is electrically connected with the second end of the first switching device, and the other end of the energy storage device is electrically connected with the second end of the first switching device; a plurality of energy consumption modules correspond to the plurality of sub-modules one by one, comprising a third switching device and an energy consumption device, the first end of the third switching device is electrically connected with one end of the energy consumption device, the second end of the third switching device is electrically connected with the first end of the second switching device, and the other end of the energy consumption device is electrically connected with the second end of the second switching device. The application solves the problem of excessively high DC bus voltage.

Description

Technical Field

[0001] This application relates to the field of power electronic conversion technology, and more specifically, to a modular multilevel converter, a flexible DC transmission system, its control method, a computer-readable storage medium, and an electronic device. Background Technology

[0002] In modern power systems, especially in offshore wind power flexible DC transmission systems, the Modular Multilevel Converter (MMC) is widely used as an advanced high-voltage direct current (HVDC) transmission technology due to its high efficiency, low harmonics, and ease of expansion. However, when the onshore AC system encounters severe faults, such as short circuits or voltage drops, the receiving end of the MMC may face extreme arm current surges, leading to the challenge of excessively high DC bus voltage. Under such conditions, traditional control strategies, i.e., blocking strategies, while preventing further current increases, cannot solve the problem of excessively high DC bus voltage. Summary of the Invention

[0003] The main objective of this application is to provide a modular multilevel converter, a flexible DC transmission system, its control method, a computer-readable storage medium, and electronic equipment, so as to at least solve the problem that the existing blocking strategies cannot solve the problem of excessive DC bus voltage.

[0004] To achieve the above objectives, according to one aspect of this application, a modular multilevel converter is provided, comprising: a plurality of sub-modules, each sub-module including a first switching device, a second switching device, a first diode, a second diode, and an energy storage device; a first terminal of the first switching device being electrically connected to a first terminal of the second switching device; the first diode being connected in anti-parallel to the first switching device; the second diode being connected in anti-parallel to the second switching device; one terminal of the energy storage device being electrically connected to a second terminal of the first switching device; and another terminal of the energy storage device being electrically connected to a second terminal of the first switching device; and a plurality of energy-consuming modules corresponding one-to-one with the plurality of sub-modules, each energy-consuming module including a third switching device and an energy-consuming device; a first terminal of the third switching device being electrically connected to a first terminal of the energy-consuming device; a second terminal of the third switching device being electrically connected to a first terminal of the second switching device; and another terminal of the energy-consuming device being electrically connected to a second terminal of the second switching device.

[0005] Optionally, the modular multilevel converter has a three-phase six-arm topology, with each phase including two arms, one upper and one lower, and each arm including multiple sub-modules, which are connected in series in each arm.

[0006] Optionally, each of the bridge arms further includes: a bridge arm inductor, one end of which is electrically connected to one end of a series branch of the plurality of sub-modules; and a bridge arm resistor, one end of which is electrically connected to the other end of the bridge arm inductor, and the other end of which is electrically connected to the midpoint of the upper and lower bridge arms.

[0007] Optionally, the first switching device, the second switching device, and the third switching device are independently selected from one of IGBT, IGCT, thyristor, and MOSFET.

[0008] Optionally, each of the submodules is independently selected from one of the full-bridge submodule, half-bridge submodule, and clamped dual-type submodule.

[0009] Optionally, the energy-consuming device includes a resistor, and the energy-storing device includes a capacitor.

[0010] According to another aspect of this application, a control method for a receiving-end converter is provided, wherein the receiving-end converter is any of the modular multilevel converters described above. The control method for the receiving-end converter includes: when the arm current of the receiving-end converter is greater than a device protection threshold, controlling all sub-modules in the receiving-end converter to lock out; when all sub-modules are locked out and the DC bus voltage of the receiving-end converter is greater than a first threshold, controlling the operating state of a third switching device in each energy-consuming module according to the DC side input power and AC side output power of the receiving-end converter, so that at least some of the energy-consuming devices in the energy-consuming modules are connected to the corresponding sub-modules; and when all sub-modules are locked out and the DC bus voltage is greater than a second threshold and less than or equal to the first threshold, controlling the operating state of the third switching device in each energy-consuming module to remain unchanged.

[0011] Optionally, the receiving-end converter has a three-phase six-arm topology, with each phase including two arms, each arm including multiple sub-modules, and the multiple sub-modules in each arm connected in series. The three phases are the first phase, the second phase, and the third phase. Based on the DC-side input power and the AC-side output power of the receiving-end converter, the operating state of the third switching device in each energy-consuming module is controlled, including: determining the difference between the DC-side input power and the AC-side output power as surplus energy; when the surplus energy is less than or equal to a third threshold, controlling all the third switching devices corresponding to the first phase to remain on for a first time, all the third switching devices corresponding to the second phase to remain on for a second time, and all the third switching devices corresponding to the third phase to remain on for a second time. The third switching device remains on during a third time period, and the first time period, the second time period, and the third time period do not overlap; when the surplus energy is greater than the third threshold and less than or equal to the fourth threshold, all the third switching devices corresponding to the first phase and the second phase are controlled to remain on during a fourth time period, all the third switching devices corresponding to the second phase and the third phase are controlled to remain on during a fifth time period, and the third switching devices corresponding to the third phase and the first phase are controlled to remain on during a sixth time period, and the fourth time period, the fifth time period, and the sixth time period do not overlap; when the surplus energy is greater than the fourth threshold, all the third switching devices corresponding to the first phase, the second phase, and the third phase are controlled to remain on.

[0012] Optionally, the method further includes: while controlling the latching of all submodules in the receiving-end converter, latching the modulation wave parameters of the submodules; when controlling the restart of all submodules in the receiving-end converter, processing the latched modulation wave parameters using a ramp function to obtain new modulation wave parameters, wherein the new modulation wave parameters gradually increase from a predetermined value to the original modulation wave parameters; and sending the new modulation wave parameters to the submodules.

[0013] Optionally, the third threshold is greater than or equal to 1 / 3 of the total energy consumption that the energy-consuming device can withstand and less than 2 / 3 of the total energy consumption, and the fourth threshold is greater than or equal to 2 / 3 of the total energy consumption and less than the total energy consumption.

[0014] According to another aspect of this application, a DC transmission system is provided, comprising: a sending-end converter; a DC line, one end of which is electrically connected to the sending-end converter; a receiving-end converter, the other end of which is electrically connected to the receiving-end converter, wherein the receiving-end converter is any of the modular multilevel converters described above; and a controller for the receiving-end converter, configured to execute any of the control methods for the receiving-end converter described above.

[0015] According to another aspect of this application, a computer-readable storage medium is provided, the computer-readable storage medium including a stored program, wherein, when the program is executed, it controls the device where the computer-readable storage medium is located to perform any of the control methods of the receiving-end converter described above.

[0016] According to another aspect of this application, an electronic device is also provided, comprising: one or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, the one or more programs including a control method for performing any of the described receiving-end converters.

[0017] By applying the technical solution of this application, a power dissipation module corresponding to each submodule is set in the modular multilevel converter. The power dissipation module includes a third switching device and a power dissipation device connected in series. These are electrically connected to the AC side of the submodule. When a fault causes a sudden increase in the arm current of the modular multilevel converter, the switching state of the third switching device in the power dissipation module is changed, connecting the power dissipation device to the submodule. This changes the current flow from charging the energy storage device to discharging through the power dissipation device, not only cutting off the charging path of the energy storage device but also directly consuming AC energy. In other words, this application directly suppresses or even eliminates the influence of AC voltage components on the DC bus voltage through the power dissipation module, achieving the effect of reducing the DC bus voltage. This solves the problem of excessively high DC bus voltage under converter lockout conditions, significantly improving the stability of the converter during fault ride-through and thus protecting the safe operation of the entire system. Furthermore, the power dissipation module does not consume energy from the energy storage devices in the submodule, avoiding energy consumption on the DC side of the submodule during lockout. Attached Figure Description

[0018] 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:

[0019] Figure 1 A schematic diagram of the circuit structure of a modular multilevel converter provided in an embodiment of this application is shown;

[0020] Figure 2 An equivalent circuit diagram of a modular multilevel converter provided in an embodiment of this application is shown;

[0021] Figure 3 A hardware structure block diagram of a mobile terminal for performing a control method for a receiving-end converter according to an embodiment of this application is shown.

[0022] Figure 4 A schematic flowchart of a control method for a receiving-end converter according to an embodiment of this application is shown;

[0023] Figure 5 A control flowchart of a flexible DC transmission system provided according to an embodiment of this application is shown.

[0024] The above figures include the following reference numerals:

[0025] 10. Submodule; 101. First switching device; 102. Second switching device; 103. First diode; 104. Second diode; 105. Energy storage device; 106. Equivalent switching transistor; 20. Energy-consuming module; 201. Third switching device; 202. Energy-consuming device; 203. Third diode; 30. Bridge arm inductor; 40. Bridge arm resistor; 50. Module; 602. Processor; 604. Memory; 606. Transmission device; 608. Input / output device. Detailed Implementation

[0026] The inventors discovered that current blocking strategies typically use parallel energy-dissipating branches on the DC side of submodules to limit DC bus voltage. While this method can release the voltage across the submodule capacitors to some extent and achieve local control, it does not solve the fundamental problem from the perspective of the entire converter. This is because when a converter arm is blocked, the DC bus voltage equals Σ submodule capacitor voltage plus the peak value of the AC side line voltage. This means that even after the energy-dissipating branches release the capacitor voltage, the DC bus voltage still equals the sum of the remaining capacitor voltages of all submodules and the uncontrolled AC side voltage. In this case, the DC bus is still overvoltage.

[0027] Based on the above problems, this application provides a technical solution to address the issue that the existing blocking strategies cannot fundamentally solve the problem of excessively high DC bus voltage.

[0028] 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.

[0029] 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.

[0030] It should be noted that the terms "first," "second," etc., used 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 used interchangeably 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.

[0031] To address the technical problems described in the background section, embodiments of this application provide a modular multilevel converter, a flexible DC transmission system, a control method thereof, a computer-readable storage medium, and an electronic device.

[0032] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0033] Embodiments of this application provide a modular multilevel converter. Figure 1 A schematic diagram of the circuit structure of a modular multilevel converter according to an embodiment of this application is shown as an example. Figure 1 As shown, the modular multilevel converter includes:

[0034] Multiple sub-modules 10, each sub-module 10 including a first switching device 101, a second switching device 102, a first diode 103, a second diode 104, and an energy storage device 105. The first terminal of the first switching device 101 is electrically connected to the first terminal of the second switching device 102. The first diode 103 is connected in anti-parallel to the first switching device 101. The second diode 104 is connected in anti-parallel to the second switching device 102. One terminal of the energy storage device 105 is electrically connected to the second terminal of the first switching device 101, and the other terminal of the energy storage device 105 is electrically connected to the second terminal of the first switching device 101.

[0035] Specifically, when the first switching device 101 (or the second switching device 102) is turned on, the current can flow along the path of the first switching device 101 (or the second switching device 102). When the first switching device 101 (or the second switching device 102) is turned off, since there is an energy storage device 105 in the circuit, the current cannot stop immediately and a freewheeling path is needed. At this time, the first diode 103 (or the second diode 104) connected in reverse parallel with the first switching device 101 (or the second switching device 102) provides such a path, allowing the current in the energy storage device 105 to freewheel or recover energy through the first diode 103 (or the second diode 104), avoiding the impact of sudden current changes on the circuit. By controlling the switching state of the first switching device 101 (or the second switching device 102) and combining the natural conduction characteristics of the first diode 103 (or the second diode 104), it is possible to change the direction of the current while keeping the DC voltage polarity unchanged, thereby changing the direction of power flow and realizing bidirectional energy flow. In the submodule 10, the two ends of the energy storage device 105 are the DC side of the submodule 10, and the two ends of the first switching device 101 or the second switching device 102 are the AC side of the submodule 10. The anode of the first diode 103 is electrically connected to the cathode of the second diode 104, the cathode of the first diode 103 is electrically connected to one end of the energy storage device 105, and the anode of the second diode 104 is electrically connected to the other end of the energy storage device 105.

[0036] Multiple energy-consuming modules 20 correspond one-to-one with multiple sub-modules 10. Each energy-consuming module 20 includes a third switching device 201 and an energy-consuming device 202. The first end of the third switching device 201 is electrically connected to one end of the energy-consuming device 202, the second end of the third switching device 201 is electrically connected to the first end of the second switching device 102, and the other end of the energy-consuming device 202 is electrically connected to the second end of the second switching device 102.

[0037] Specifically, the energy-consuming module 20 is connected in parallel across the two ends of the second switching device 102, which is equivalent to connecting the energy-consuming module 20 to the AC side of the submodule 10.

[0038] In the above embodiment, an energy-consuming module 20 is set up to correspond one-to-one with the sub-module 10. The energy-consuming module 20 includes a third switching device 201 and an energy-consuming device 202 connected in series. The third switching device 201 and the energy-consuming device 202 are electrically connected to the AC side of the sub-module 10. When the arm current of the modular multilevel converter suddenly increases due to a fault, the switching state of the third switching device 201 in the energy-consuming module 20 is changed to connect the energy-consuming device 202 to the sub-module 10. This changes the current from charging the energy storage device 105 to discharging through the energy-consuming device 202. This not only cuts off the charging path of the energy storage device 105, but also realizes the direct consumption of AC energy. In other words, this application directly suppresses or even eliminates the influence of AC voltage components on DC bus voltage through the energy-consuming module 20, thereby reducing the DC bus voltage and solving the problem of excessively high DC bus voltage under the converter lockout condition. This significantly improves the stability of the converter during fault ride-through, thus protecting the safe operation of the entire system. In addition, the energy-consuming module 20 will not consume the energy of the energy storage device 105 in the sub-module 10, thus avoiding the consumption of DC side energy of the sub-module 10 during the lockout process.

[0039] According to some optional embodiments of this application, the modular multilevel converter has a three-phase six-arm topology, with each phase including two arms (upper and lower), and each arm including multiple sub-modules 10, which are connected in series. This embodiment adopts a three-phase six-arm MMC topology, and through the switching control of each arm sub-module 10, effective rectification and inversion of AC power are achieved.

[0040] In addition, such as Figure 1 As shown, the energy-consuming module further includes a third diode 203, which is connected in reverse parallel with the third switching device 201, and the cathode of the third diode 203 is electrically connected to the anode of the first diode 103 and the cathode of the second diode 104, respectively.

[0041] Optionally, such as Figure 1As shown, each bridge arm further includes: a bridge arm inductor 30, one end of which is electrically connected to one end of a series branch of the multiple sub-modules 10; and a bridge arm resistor 40, one end of which is electrically connected to the other end of the bridge arm inductor 30, and the other end of which is electrically connected to the midpoint between the upper and lower bridge arms. The introduction of the bridge arm inductor 30 and the bridge arm resistor 40 provides the necessary filtering and damping functions for the converter, ensuring a smooth transition of current and voltage. Specifically, the bridge arm inductor 30 smooths the current waveform and reduces current fluctuations, while the bridge arm resistor 40 provides additional damping to prevent sudden current changes. This effectively reduces the current surge during fault ride-through, protects the sub-modules 10 and energy-consuming modules 20 from overcurrent damage, and improves the overall stability of the system.

[0042] Specifically, such as Figure 1 As shown, in the modular multilevel converter, one submodule 10 and its corresponding energy-consuming module 20 constitute one module 50 of the modular multilevel converter. In one bridge arm, multiple submodules 10 are connected in series and then electrically connected to the bridge arm resistor 40 and the bridge arm inductor 30. In the six bridge arms, the submodules 10 at the beginning of the series connection of the three upper bridge arms are electrically connected and then electrically connected to one end of the DC line; the submodules 10 at the beginning of the series connection of the three lower bridge arms are electrically connected and then electrically connected to the other end of the DC line. The midpoint of the three bridge arms is connected to the AC source (u). a u b u c Electrical connection.

[0043] Those skilled in the art can choose any suitable switching device or combination of switching devices as the first switching device 101, the second switching device 102, and the third switching device 201. The switching types of the first switching device 101, the second switching device 102, and the third switching device 201 can be the same or different. In some embodiments of this application, the first switching device 101, the second switching device 102, and the third switching device 201 are independently selected from one of IGBT, IGCT, thyristor, and MOSFET. By controlling the on and off states of these switching devices, the current path is switched, thereby controlling the flow of energy.

[0044] In one specific embodiment, the first switching device 101, the second switching device 102, and the third switching device 201 are all IGCTs.

[0045] Optionally, each submodule 10 is independently selected from a full-bridge submodule, a half-bridge submodule, a clamped dual-type submodule, and a hybrid submodule. In this embodiment, the full-bridge, half-bridge, clamped dual-type, and hybrid submodules 10 have different voltage control capabilities and energy dissipation characteristics, and can be selected according to the specific requirements of the converter and the fault ride-through strategy. For example, the full-bridge submodule can provide bidirectional energy flow, while the half-bridge and clamped dual-type submodules can provide more efficient energy management under specific conditions.

[0046] In one specific embodiment, each of the submodules 10 is of the same type; they can all be full-bridge submodules, half-bridge submodules, or clamped dual-type submodules. Figure 1 An exemplary circuit structure diagram is shown for each of the submodules 10 being a half-bridge type submodule.

[0047] In other embodiments, the energy-consuming device 202 includes a resistor, and the energy storage device 105 includes a capacitor. The resistor can convert surplus energy into heat energy for direct discharge, while the capacitor can store energy to provide voltage support for the converter. The technical solution in this embodiment can effectively dissipate surplus energy during faults, and at the same time, maintain the voltage stability of the converter during fault ride-through through the energy storage function of the capacitor.

[0048] Furthermore, the energy-consuming device 202 can be a resistor. The energy storage device 105 can be a capacitor.

[0049] In other embodiments, the efficiency of energy dissipation and storage can be optimized by using other types of energy-consuming devices 202 (such as inductors) and energy storage devices 105 (such as supercapacitors) to solve energy management problems under different fault scenarios.

[0050] In practical applications, severe faults (such as short circuits or voltage drops) in the onshore AC system can cause a sudden increase in bridge arm current, necessitating the implementation of a blocking strategy by the receiving-end converter in extreme conditions. Therefore, the solution described in this application can be applied to fault ride-through control scenarios of the receiving-end converter in offshore wind power flexible DC transmission systems.

[0051] Figure 2 An equivalent circuit diagram of a modular multilevel converter with a three-phase six-arm topology as exemplarily shown in this application is illustrated, such as... Figure 2As shown, each bridge arm includes an equivalent switching transistor 106, an energy storage device 105, a bridge arm resistor 40, and a bridge arm inductor 30 connected in series. Each also includes an energy dissipation module 20 connected in parallel across the energy storage device 105. The energy dissipation module 20 includes a third switching device 201, an energy dissipation device 202, and a third diode 203 connected in series. The third diode 203 is connected in reverse parallel across the third switching device 201. When the submodule 10 is in the locked-out state, and the energy dissipation module 20 is not activated, the current i... d The path is: AC source (u a u b u c → The equivalent switching transistor 106 of submodule 10 → charges the energy storage device 105; while when the energy-consuming module 20 is turned on, the current i d The path is: AC source (u a u b u c → Power consumption module 20 → Direct discharge, cutting off the capacitor charging path.

[0052] This application, by setting an energy-consuming module 20 on the AC side of submodule 10, can directly eliminate the influence of AC voltage components on the DC bus, and for the first time achieve complete suppression of DC bus overvoltage under the blocking condition. It solves the decoupling problem of converter system-level and local-level voltage control, and simultaneously solves the dual problems of capacitor overvoltage and DC bus overvoltage in submodule 10.

[0053] This application also provides a control method for the receiving-end converter described above. The method embodiments provided in this application can be executed on a mobile terminal, computer terminal, or similar computing device. Taking running on a mobile terminal as an example... Figure 3 This is a hardware structure block diagram of a mobile terminal for a control method of a receiving-end converter according to an embodiment of the present invention. Figure 3 As shown, a mobile terminal may include one or more ( Figure 3 Only one is shown in the diagram. A processor 602 (which may include, but is not limited to, a microprocessor MCU or a programmable logic device FPGA, etc.) and a memory 604 for storing data are also shown. The mobile terminal may further include a transmission device 606 for communication functions and an input / output device 608. Those skilled in the art will understand that... Figure 3 The structure shown is for illustrative purposes only and does not limit the structure of the mobile terminal. For example, the mobile terminal may also include components that are larger than... Figure 3 The more or fewer components shown, or having the same Figure 3 The different configurations shown.

[0054] The memory 604 can be used to store computer programs, such as application software programs and modules, like the computer program corresponding to the control method of the receiving-end converter in this embodiment of the invention. The processor 602 executes various functional applications and data processing by running the computer program stored in the memory 604, thereby implementing the method described. The memory 604 may include high-speed random access memory and may also include non-volatile memory, such as one or more magnetic storage devices, flash memory, or other non-volatile solid-state memory. In some instances, the memory 604 may further include memory remotely located relative to the processor 602, and these remote memories can be connected to the mobile terminal via a network. Examples of such networks include, but are not limited to, the Internet, corporate intranets, local area networks, mobile communication networks, and combinations thereof. The transmission device 606 is used to receive or send data via a network. Specific examples of such networks may include wireless networks provided by the mobile terminal's communication provider. In one example, the transmission device 606 includes a network interface controller (NIC), which can be connected to other network devices via a base station to communicate with the Internet. In one example, the transmission device 606 may be a radio frequency (RF) module, which is used to communicate with the Internet wirelessly.

[0055] This embodiment provides a control method for a receiving-end converter operating on a mobile terminal, computer terminal, or similar computing device. The receiving-end converter is any of the modular multilevel converters described herein. It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions. Furthermore, although a logical order is shown in the flowchart, in some cases, the steps shown or described may be executed in a different order than that shown here.

[0056] Figure 4 This is a flowchart of a control method for a receiving-end converter according to an embodiment of this application. For example... Figure 4 As shown, the method includes the following steps:

[0057] Step S701: When the arm current of the receiving-end converter is greater than the device protection threshold, control all sub-modules in the receiving-end converter to be locked out.

[0058] Specifically, when the arm current of the receiving-end converter exceeds a preset device protection threshold, the normal energy transmission path is cut off through the blocking submodule. This device protection threshold can be determined through empirical accumulation or through a limited number of experiments.

[0059] Step S702: When all the sub-modules are locked and the DC bus voltage of the receiving-end converter is greater than the first threshold, the operating state of the third switching device in each energy-consuming module is controlled according to the DC side input power and AC side output power of the receiving-end converter, so that at least some of the energy-consuming devices in the energy-consuming modules are connected to the corresponding sub-modules.

[0060] Specifically, the first threshold is a preset maximum value of the DC bus voltage, which can be determined through empirical accumulation or through a limited number of experiments. In step S702, the operating state of the third switching device in each energy-consuming module is controlled so that at least some of the third switching devices have different operating states.

[0061] Step S703: When all the sub-modules are locked and the DC bus voltage is greater than the second threshold and less than or equal to the first threshold, the operating state of the third switching device in each of the energy-consuming modules is kept unchanged.

[0062] Specifically, the second threshold is a preset minimum value of the DC bus voltage, which can be determined through empirical accumulation or through a limited number of experiments.

[0063] In this embodiment, when the arm current of the receiving-end converter exceeds the device protection threshold, all sub-modules are first locked out. Then, based on the magnitude of the DC bus voltage, if it exceeds the first threshold, the switching of the third switching device in each energy-consuming module is controlled according to the DC side input power and AC side output power of the receiving-end converter. This controls the number of energy-consuming devices connected to the sub-modules, thereby suppressing or even eliminating the influence of AC voltage components on the DC bus voltage, achieving the effect of reducing the DC bus voltage. When the DC bus voltage is less than or equal to the first threshold and greater than the second threshold, the switching state of the energy-consuming devices in all the energy-consuming modules is kept consistent with the previous state. This ensures that, in the locked-out state, the capacitor voltage and DC bus voltage of all sub-modules are maintained near their rated values, while also fully absorbing all the surplus power from the sending end.

[0064] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0065] For example, the method further includes: when the DC bus voltage is less than or equal to the second threshold, controlling the energy-consuming device in each of the energy-consuming modules not to be connected to the corresponding sub-module.

[0066] According to some exemplary embodiments of this application, the receiving-end converter has a three-phase six-arm topology, each phase includes two arms (upper and lower), each arm includes multiple sub-modules, and the multiple sub-modules in each arm are connected in series. The three phases are the first phase, the second phase, and the third phase. Step S702: Based on the DC-side input power and AC-side output power of the receiving-end converter, control the operating state of the third switching device in each energy-consuming module, including:

[0067] Step S7021: Determine the difference between the DC side input power and the AC side output power as surplus energy;

[0068] Step S7022: When the surplus energy is less than or equal to the third threshold, control all the third switching devices corresponding to the first to remain on for a first time, all the third switching devices corresponding to the second to remain on for a second time, and all the third switching devices corresponding to the third to remain on for a third time, with no overlap between the first time, the second time, and the third time;

[0069] Specifically, the first time, the second time, and the third time are all specific time periods. The first time, the second time, and the third time are different, so that when the surplus energy is less than or equal to the third threshold, the three-phase energy-consuming modules are alternately turned on. For example, the first time can be 8:00~8:15, the second time can be 8:20~8:35, and the third time can be 8:40~8:55. Optionally, the durations of the first time, the second time, and the third time can be the same or different. Optionally, any two of the first time, the second time, and the third time can be non-overlapping; optionally, at least two of the first time, the second time, and the third time partially overlap.

[0070] Step S7023: When the surplus energy is greater than the third threshold and less than or equal to the fourth threshold, control all the third switching devices corresponding to the first phase and the second phase to remain on for a fourth time period, control all the third switching devices corresponding to the second phase and the third phase to remain on for a fifth time period, and control the third switching devices corresponding to the third phase and the first phase to remain on for a sixth time period, wherein the fourth time period, the fifth time period, and the sixth time period do not overlap;

[0071] Specifically, the fourth, fifth, and sixth times are different, such that when the surplus energy is greater than the third threshold and less than or equal to the fourth threshold, the energy-consuming modules of the first and second phases, the second and third phases, and the third and first phases are alternately switched on. For example, the fourth time can be 8:00-8:15, the fifth time can be 8:20-8:35, and the sixth time can be 8:40-8:55. Optionally, the fourth, fifth, and sixth times can be the same or different. Optionally, any two of the fourth, fifth, and sixth times can be non-overlapping; optionally, at least two of the fourth, fifth, and sixth times partially overlap.

[0072] Step S7024: When the surplus energy is greater than the fourth threshold, control all the third switching devices corresponding to the first phase, the second phase, and the third phase to be turned on.

[0073] Specifically, when the surplus energy is greater than the fourth threshold, all the third switching devices are controlled to operate simultaneously and be fully turned on.

[0074] The embodiment described provides an energy dissipation module control strategy based on surplus energy and a three-phase bridge arm topology. By calculating the surplus energy and dynamically adjusting the conduction time of the third switching device in each phase according to its magnitude, precise control of energy dissipation is achieved. By correlating the surplus energy with the conduction time of the energy dissipation module, reasonable energy dissipation is ensured. Furthermore, the staggered control of the three-phase bridge arms further improves energy dissipation efficiency and system stability. The technical solution in this embodiment can further effectively suppress DC bus voltage overvoltage, protecting the system from overvoltage damage. Simultaneously, through the control of the energy dissipation module, reasonable energy management is achieved, improving system stability. In addition, this embodiment, through time-non-overlapping control, ensures that the energy dissipation module dissipates energy sequentially in the bridge arms of different phases according to a predetermined order. By controlling the conduction state of the third switching device at different times, precise control of energy dissipation is further achieved, ensuring energy dissipation efficiency and system stability. This improves energy dissipation efficiency, reduces voltage fluctuations during fault ride-through, and enhances overall system stability through time-non-overlapping control.

[0075] In some alternative embodiments, the method further includes: latching the modulation wave parameters of the sub-modules while controlling the latching of all sub-modules in the receiving-end converter; when controlling the restart of all sub-modules in the receiving-end converter, processing the latched modulation wave parameters using a ramp function according to the latched modulation wave parameters to obtain new modulation wave parameters, the new modulation wave parameters gradually rising from a predetermined value to the original modulation wave parameters; and sending the new modulation wave parameters to the sub-module. This embodiment provides a control method for the converter restart process, achieving a smooth restart of the converter by combining latched modulation wave parameters and a ramp function. Latching the modulation wave parameters ensures that the control parameters at restart are consistent with those before the fault, while the ramp function enables the gradual recovery of the modulation wave parameters, avoiding current surges during the restart process. The technical solution in this embodiment can significantly reduce current surges during the converter restart process, protecting the system from damage caused by restart surges, and improving system stability through the smooth recovery of modulation wave parameters.

[0076] In other embodiments, energy management during the restart process can be further optimized by introducing other types of restart control strategies, such as current prediction-based control strategies, to solve the system restart problem under different fault scenarios.

[0077] In practical applications, the specific values ​​of the first time, the second time, the third time, the fourth time, the fifth time, the sixth time, the first threshold, the second threshold, the third threshold, and the fourth threshold can be set based on experience or obtained through a limited number of experiments. Those skilled in the art can flexibly choose the method of determining the above values ​​according to the actual situation, and this application does not impose specific restrictions on this.

[0078] According to some alternative embodiments of this application, the third threshold is greater than or equal to 1 / 3 of the total energy consumption that the energy-consuming device can withstand and less than 2 / 3 of the total energy consumption, and the fourth threshold is greater than or equal to 2 / 3 of the total energy consumption and less than the total energy consumption. This embodiment achieves fine-grained control of the energy dissipation process by setting the third and fourth thresholds. By correlating surplus energy with the conduction time of the energy-consuming module, reasonable energy dissipation is ensured. Simultaneously, by setting the thresholds, overuse of the energy-consuming module is avoided, protecting the energy-consuming device from damage. This not only improves the efficiency of energy dissipation and reduces voltage fluctuations during fault ride-through, but also enhances the overall stability of the system and the lifespan of the energy-consuming module through the threshold settings.

[0079] This application achieves an optimal balance between switching losses and control accuracy through a multi-phase dynamic scheduling mechanism based on energy demand. The multi-phase staggered scheduling strategy reduces the fluctuation rate of DC bus and capacitor voltage. By combining modulation wave latching with ramp feedforward, the restart impact is eliminated and recovery is accelerated. The sub-module capacitor voltage is stabilized. The superimposed combination of modulation wave latching and ramp feedforward significantly reduces the system restart time.

[0080] In some other embodiments of this application, the energy-consuming device may employ an adjustable-resistance electronic component (such as an adjustable-resistance resistor network, a magnetron resistor, or a nonlinear resistor) to control the resistance value, thereby controlling the rate and total amount of energy dissipation. When surplus energy is detected to be greater than a fifth threshold, the resistance value of the energy-consuming module is automatically reduced to enhance energy dissipation capability; conversely, when surplus energy is detected to be less than or equal to a sixth threshold, the resistance value is increased to reduce energy loss. The fifth threshold is greater than the sixth threshold.

[0081] In some exemplary embodiments, a machine learning model is used to predict the DC bus voltage and capacitance value. When the DC bus voltage is predicted to be greater than the rated bus voltage, and the difference between the time when the DC bus voltage is predicted to be greater than the rated bus voltage and the current time is less than a predetermined time threshold, the energy consumption module is turned on in advance, thereby realizing the early activation of the energy consumption module.

[0082] Optionally, the machine learning model is trained using multiple sets of time series data. Each set of time series data includes: historical AC voltage, historical current, AC frequency, switching status of each submodule, historical DC bus voltage, and historical capacitance value of the energy storage device at each time point.

[0083] Optionally, the machine learning model includes, but is not limited to, recurrent neural network models and long short-term memory network models. The machine learning module may include a sequentially connected feature extraction layer, a recurrent hidden layer, and a prediction layer. The feature extraction layer is used to preprocess the input time-series data, including standardization, denoising, and windowing, to extract useful features. The recurrent hidden layer is used to ignore irrelevant information while remembering and outputting long-term information, thereby processing time-dependent data. The prediction layer is used to predict future DC bus voltage and capacitance values ​​based on the output of the recurrent hidden layer.

[0084] To enable those skilled in the art to better understand the technical solution of this application, the implementation process of the control method of the receiving-end converter of this application will be described in detail below with reference to specific embodiments.

[0085] This embodiment relates to a specific control method for a receiving-end converter, such as... Figure 5 As shown, it includes the following steps:

[0086] Step S1: Monitor the arm current of the receiving-end converter. If it exceeds the device protection threshold, control the receiving-end converter to lock out; otherwise, end the lockout.

[0087] Step S2: Determine whether the interlocking condition has ended. If the interlocking condition has ended, disable all power-consuming modules.

[0088] Step S3: If the interlocking condition has not ended, the energy consumption module adopts dual-threshold Bang-Bang control. That is, when the relationship between the DC bus voltage ud and the first threshold udmax satisfies ud>udmax, the energy consumption module is enabled and energy consumption is turned on (i.e., at least some of the third switching devices are turned on, so that the energy consumption module is connected to the sub-module to work); when the DC bus voltage ud satisfies udmin<ud≤udmax, the switching state of the third switching devices in the energy consumption module is not changed, that is, each third switching device is controlled to maintain the switching state of the previous moment; when the relationship between the DC bus voltage ud and the second threshold udmin satisfies ud≤udmin, the energy consumption devices in each energy consumption module are controlled not to be connected to the corresponding sub-module, that is, all third switching devices are controlled to be turned off.

[0089] To improve control accuracy and reduce switching frequency under interlocking conditions, a multi-phase staggered energy consumption control strategy can be implemented based on the energy consumption after interlocking. For example, if the energy required after interlocking the entire machine is less than one-third of the total capacity, the energy consumption modules for phases A, B, and C are activated separately, with each phase branch simultaneously turned on and off. Specifically, the energy consumption control strategy is as follows:

[0090] If Ediss > 2 / 3Etotal, then the energy-consuming modules corresponding to the three-phase sub-modules will operate simultaneously.

[0091] If 2 / 3Etotal≥Ediss≥1 / 3Etotal, then the energy-consuming modules corresponding to the two-phase sub-modules A (first phase)B (second phase) / B (second phase)C (third phase) / C (third phase)A (first phase) will operate alternately.

[0092] If 1 / 3Etotal≥Ediss≥0, then the energy-consuming modules corresponding to the three-phase sub-modules A / B / C will operate in a time-sharing and interleaved manner.

[0093] Wherein, Ediss is the surplus energy, which is the difference between the DC side input power and the AC side output power of the receiving-end converter. The DC side input power is the product of the DC side current and the DC side voltage, and the AC side output power is the product of the AC side current and the AC side voltage; Etotal is the total energy that the energy-consuming device can withstand, that is, the equivalent cumulative energy value after the resistors are cascaded in the energy-consuming module.

[0094] Furthermore, during the lockout execution, the modulated wave output to the switching devices in the submodule and the output of the PI controller are latched. The modulated wave is the control signal output by the polarity controller, and the latched values ​​include the values ​​of various voltage and current control components within the MMC polarity controller. During the lockout strategy execution, the control is stopped, and only the fault ride-through strategy and phase-locked loop are executed. After the lockout ends, the modulated wave is used as a feedforward component and multiplied by a descending ramp coefficient to achieve rapid recovery after lockout. For example, when exiting the lockout condition and restarting the submodule, the modulated wave multiplied by the ramp function is sent as a feedforward quantity to the submodule.

[0095] This application also provides a control device for a flexible DC transmission system. It should be noted that the control device for the flexible DC transmission system in this application can be used to execute the control method for the receiving-end converter provided in this application. This device is used to implement the embodiments and preferred embodiments, and 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.

[0096] The control device of the flexible DC transmission system includes a processor and a memory. Program units for executing the method are stored in the memory, and the processor executes these program units to achieve the corresponding functions. All modules reside in the same processor; alternatively, the modules may be located in different processors in any combination.

[0097] 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 at least address the problem of excessive DC bus voltage that current latch-up strategies cannot solve.

[0098] 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.

[0099] This application also provides a flexible DC transmission system, including:

[0100] Sending-end converter;

[0101] A DC line, one end of which is electrically connected to the sending-end converter;

[0102] The receiving-end converter is electrically connected to the other end of the DC line, and the receiving-end converter is any of the modular multilevel converters described above.

[0103] Specifically, the sending-end converter converts AC power into DC power, which is then transmitted through DC lines. The receiving-end converter then converts the DC power back into AC power for use by the onshore power grid.

[0104] The controller of the receiving-end converter includes one or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, and the one or more programs include a control method for performing any of the receiving-end converters described above.

[0105] In the aforementioned embodiment, in the flexible DC transmission system, the sending-end converter, DC line, and receiving-end converter are connected sequentially. The receiving-end converter is any of the modular multilevel converters described above. Within this receiving-end converter, energy-consuming modules are configured, each corresponding to a sub-module. Each energy-consuming module includes a third switching device and an energy-consuming device connected in series. These devices are electrically connected to the AC side of the sub-module. When a fault causes a sudden increase in the arm current of the modular multilevel converter, the switching state of the third switching device in the energy-consuming module is changed, connecting the energy-consuming device to the sub-module, thus redirecting the current from its original direction to the energy storage. The charging of energy storage devices is changed to discharging through energy-consuming devices, which not only cuts off the charging path of energy storage devices, but also realizes the direct consumption of AC energy. Therefore, the receiving-end converter of this application directly suppresses or even eliminates the influence of AC voltage components on DC bus voltage through the energy-consuming module, thereby achieving the effect of reducing DC bus voltage. This solves the problem of excessively high DC bus voltage under converter lockout conditions, significantly improves the stability of the receiving-end converter during fault ride-through, and thus protects the safe operation of the entire flexible DC transmission system, improves the reliability of the flexible DC transmission system, and reduces the impact of faults on the flexible DC transmission system.

[0106] In practical applications, the sending-end converter can be any suitable type of converter.

[0107] To further enhance the performance of flexible DC transmission systems, in some embodiments, the sending-end converter is any of the modular multilevel converters described above. In this embodiment, the sending-end converter also adopts the modular multilevel converter design, forming a symmetrical structure with the receiving-end converter, thus improving the overall system performance and reliability. The sending-end converter achieves rectification of the AC side current through the switching control of sub-modules, and simultaneously ensures energy management during fault ride-through through the control of energy consumption modules. The technical solution in this embodiment can improve the power conversion efficiency of the sending-end converter, reduce the impact of faults on the system, and improve the system's flexibility and maintainability through modular design.

[0108] This invention provides a computer-readable storage medium including a stored program, wherein the program, when running, controls the device containing the computer-readable storage medium to execute a control method for the receiving-end converter.

[0109] This invention provides a processor for running a program, wherein the program executes a control method for the receiving-end converter during runtime.

[0110] This invention 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:

[0111] Step S701: When the arm current of the receiving-end converter is greater than the device protection threshold, control all sub-modules in the receiving-end converter to be locked out.

[0112] Step S702: When all the sub-modules are locked and the DC bus voltage of the receiving-end converter is greater than the first threshold, the operating state of the third switching device in each energy-consuming module is controlled according to the DC side input power and AC side output power of the receiving-end converter, so that at least some of the energy-consuming devices in the energy-consuming modules are connected to the corresponding sub-modules.

[0113] Step S703: When all the sub-modules are locked and the DC bus voltage is greater than the second threshold and less than or equal to the first threshold, the operating state of the third switching device in each of the energy-consuming modules is kept unchanged.

[0114] The devices mentioned in this article can be servers, PCs, tablets, mobile phones, etc.

[0115] Optionally, the receiving-end converter has a three-phase six-arm topology, with each phase including two arms (upper and lower), and each arm including multiple sub-modules. These sub-modules are connected in series, and the three phases are designated as the first, second, and third phases. Based on the DC-side input power and AC-side output power of the receiving-end converter, the operating state of the third switching device in each energy-consuming module is controlled, including: determining the difference between the DC-side input power and the AC-side output power as surplus energy; and when the surplus energy is less than or equal to a third threshold, controlling all third switching devices corresponding to the first phase to remain on for a first time, and controlling all third switching devices corresponding to the second phase to remain on for a second time. The device remains on for a second time period, and all the third corresponding devices remain on for a third time period; if the surplus energy is greater than the third threshold and less than or equal to the fourth threshold, the first phase and all the third corresponding devices are controlled to remain on for a fourth time period, the second phase and all the third corresponding devices are controlled to remain on for a fifth time period, and the third phase and the first corresponding device are controlled to remain on for a sixth time period; if the surplus energy is greater than the fourth threshold, the first phase, the second phase, and all the third corresponding devices are controlled to remain on.

[0116] Optionally, the method further includes: while controlling the latching of all submodules in the receiving-end converter, latching the modulation wave parameters of the submodules; and when controlling the restart of all submodules in the receiving-end converter, generating new modulation wave parameters through a ramp function based on the latched modulation wave parameters and sending them to the submodules, so that the transmitted new modulation wave parameters gradually rise from a predetermined value to the modulation wave parameters.

[0117] Optionally, the first time, the second time, and the third time do not overlap, and the fourth time, the fifth time, and the sixth time do not overlap.

[0118] Optionally, the third threshold is greater than or equal to 1 / 3 of the total energy consumption that the energy-consuming device can withstand and less than 2 / 3 of the total energy consumption, and the fourth threshold is greater than or equal to 2 / 3 of the total energy consumption and less than the total energy consumption.

[0119] 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:

[0120] Step S701: When the arm current of the receiving-end converter is greater than the device protection threshold, control all sub-modules in the receiving-end converter to be locked out.

[0121] Step S702: When all the sub-modules are locked and the DC bus voltage of the receiving-end converter is greater than the first threshold, the operating state of the third switching device in each energy-consuming module is controlled according to the DC side input power and AC side output power of the receiving-end converter, so that at least some of the energy-consuming devices in the energy-consuming modules are connected to the corresponding sub-modules.

[0122] Step S703: When all the sub-modules are locked and the DC bus voltage is greater than the second threshold and less than or equal to the first threshold, the operating state of the third switching device in each of the energy-consuming modules is kept unchanged.

[0123] Optionally, the receiving-end converter has a three-phase six-arm topology, with each phase including two arms (upper and lower), and each arm including multiple sub-modules. These sub-modules are connected in series, and the three phases are designated as the first, second, and third phases. Based on the DC-side input power and AC-side output power of the receiving-end converter, the operating state of the third switching device in each energy-consuming module is controlled, including: determining the difference between the DC-side input power and the AC-side output power as surplus energy; and when the surplus energy is less than or equal to a third threshold, controlling all third switching devices corresponding to the first phase to remain on for a first time, and controlling all third switching devices corresponding to the second phase to remain on for a second time. The device remains on for a second time period, and all the third corresponding devices remain on for a third time period; if the surplus energy is greater than the third threshold and less than or equal to the fourth threshold, the first phase and all the third corresponding devices are controlled to remain on for a fourth time period, the second phase and all the third corresponding devices are controlled to remain on for a fifth time period, and the third phase and the first corresponding device are controlled to remain on for a sixth time period; if the surplus energy is greater than the fourth threshold, the first phase, the second phase, and all the third corresponding devices are controlled to remain on.

[0124] Optionally, the method further includes: while controlling the latching of all submodules in the receiving-end converter, latching the modulation wave parameters of the submodules; and when controlling the restart of all submodules in the receiving-end converter, generating new modulation wave parameters through a ramp function based on the latched modulation wave parameters and sending them to the submodules, so that the transmitted new modulation wave parameters gradually rise from a predetermined value to the modulation wave parameters.

[0125] Optionally, the first time, the second time, and the third time do not overlap, and the fourth time, the fifth time, and the sixth time do not overlap.

[0126] Optionally, the third threshold is greater than or equal to 1 / 3 of the total energy consumption that the energy-consuming device can withstand and less than 2 / 3 of the total energy consumption, and the fourth threshold is greater than or equal to 2 / 3 of the total energy consumption and less than the total energy consumption.

[0127] It will be apparent to those skilled in the art that the modules or steps of the present invention 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 device-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 herein, 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, the present invention is not limited to any particular hardware and software combination.

[0128] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0129] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0130] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0131] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0132] In a typical configuration, a computing device includes one or more processors (CPU), input / output interfaces, network interfaces, and memory.

[0133] Memory may include non-persistent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, like read-only memory (ROM) or flash RAM. Memory is an example of computer-readable media.

[0134] Computer-readable media includes both permanent and non-permanent, removable and non-removable media that can store information using any method or technology. Information can be computer-readable instructions, data structures, modules of programs, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, disk storage or other magnetic storage devices, or any other non-transferable medium that can be used to store information accessible by a computing device. As defined herein, computer-readable media does not include transient computer-readable media, such as modulated data signals and carrier waves.

[0135] 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 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.

[0136] 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.

[0137] As can be seen from the above description, the embodiments described in this application achieve the following technical effects:

[0138] In the modular multilevel converter of this application, an energy dissipation module is provided, corresponding one-to-one with each submodule. The energy dissipation module includes a third switching device and an energy dissipation device connected in series. These are electrically connected to the AC side of the submodule. When a fault causes a sudden increase in the arm current of the modular multilevel converter, the switching state of the third switching device in the energy dissipation module is changed, connecting the energy dissipation device to the submodule. This changes the current flow from charging the energy storage device to discharging through the energy dissipation device, not only cutting off the charging path of the energy storage device but also directly consuming AC energy. In other words, this application directly suppresses or even eliminates the influence of AC voltage components on the DC bus voltage through the energy dissipation module, achieving a reduction in DC bus voltage. This solves the problem of excessively high DC bus voltage under converter lockout conditions, significantly improving the stability of the converter during fault ride-through and thus protecting the safe operation of the entire system. Furthermore, the energy dissipation module does not consume energy from the energy storage devices in the submodule, avoiding energy consumption on the DC side of the submodule during lockout.

[0139] 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 flexible DC transmission system, characterized in that, include: Sending-end converter; A DC line, one end of which is electrically connected to the sending-end converter; A receiving-end converter, the other end of the DC line being electrically connected to the receiving-end converter, the receiving-end converter being a modular multilevel converter, the modular multilevel converter comprising: multiple sub-modules, each sub-module comprising a first switching device, a second switching device, a first diode, a second diode, and an energy storage device, wherein a first terminal of the first switching device is electrically connected to a first terminal of the second switching device, the first diode is connected in anti-parallel to the first switching device, the second diode is connected in anti-parallel to the second switching device, one terminal of the energy storage device is electrically connected to a second terminal of the first switching device, and the other terminal of the energy storage device is electrically connected to a second terminal of the second switching device; and multiple energy-consuming modules, corresponding one-to-one with the multiple sub-modules, each energy-consuming module comprising a third switching device and an energy-consuming device, wherein a first terminal of the third switching device is electrically connected to a first terminal of the energy-consuming device, the second terminal of the third switching device is electrically connected to a first terminal of the second switching device, and the other terminal of the energy-consuming device is electrically connected to a second terminal of the second switching device; The controller of the receiving-end converter is used to execute a control method for the receiving-end converter, the method comprising: controlling all sub-modules in the receiving-end converter to lock out when the arm current of the receiving-end converter is greater than a device protection threshold; controlling the operating state of a third switching device in each energy-consuming module according to the DC side input power and AC side output power of the receiving-end converter when all sub-modules are locked out and the DC bus voltage of the receiving-end converter is greater than a first threshold, so that at least some of the energy-consuming devices in the energy-consuming modules are connected to the corresponding sub-modules; and controlling the operating state of the third switching device in each energy-consuming module to remain unchanged when all sub-modules are locked out and the DC bus voltage is greater than a second threshold and less than or equal to the first threshold.

2. The flexible DC transmission system according to claim 1, characterized in that, The modular multilevel converter has a three-phase six-arm topology, with each phase including two arms, one upper and one lower. Each arm includes multiple sub-modules, and the multiple sub-modules in each arm are connected in series.

3. The flexible DC transmission system according to claim 2, characterized in that, Each of the bridge arms also includes: A bridge arm inductor, one end of which is electrically connected to one end of a series branch of the plurality of sub-modules; The bridge arm resistor has one end electrically connected to the other end of the bridge arm inductor, and the other end electrically connected to the midpoint of the upper and lower bridge arms.

4. The flexible DC transmission system according to claim 1, characterized in that, The first switching device, the second switching device, and the third switching device are independently selected from one of IGBT, IGCT, thyristor, and MOSFET.

5. The flexible DC transmission system according to claim 1, characterized in that, Each of the submodules is independently selected from one of the full-bridge submodule, half-bridge submodule, and clamped dual-type submodule.

6. The flexible DC transmission system according to claim 1, characterized in that, The energy-consuming device includes a resistor, and the energy-storing device includes a capacitor.

7. The flexible DC transmission system according to claim 1, characterized in that, The receiving-end converter has a three-phase six-arm topology, with each phase including two arms (upper and lower). Each arm includes multiple sub-modules, which are connected in series. The three phases are designated as the first, second, and third phases. Based on the DC-side input power and AC-side output power of the receiving-end converter, the operating state of the third switching device in each energy-consuming module is controlled, including: The difference between the DC-side input power and the AC-side output power is defined as surplus energy. When the surplus energy is less than or equal to the third threshold, all the third switching devices corresponding to the first threshold are controlled to remain on for a first time, all the third switching devices corresponding to the second threshold are controlled to remain on for a second time, and all the third switching devices corresponding to the third threshold are controlled to remain on for a third time, with no overlap between the first time, the second time, and the third time. When the surplus energy is greater than the third threshold and less than or equal to the fourth threshold, all the third switching devices corresponding to the first phase and the second phase are controlled to remain on for a fourth time period, all the third switching devices corresponding to the second phase and the third phase are controlled to remain on for a fifth time period, and the third switching devices corresponding to the third phase and the first phase are controlled to remain on for a sixth time period, wherein the fourth time period, the fifth time period, and the sixth time period do not overlap; When the surplus energy is greater than the fourth threshold, control all the third switching devices corresponding to the first phase, the second phase, and the third phase to be turned on.

8. The flexible DC transmission system according to claim 1, characterized in that, The method further includes: While controlling the latching of all submodules in the receiving-end converter, the modulation wave parameters of the submodules are latched. When all submodules in the receiving-end converter are restarted, the latched modulation wave parameters are processed using a ramp function to obtain new modulation wave parameters, which gradually increase from a predetermined value to the original modulation wave parameters. The new modulation wave parameters are sent to the submodule.

9. The flexible DC transmission system according to claim 7, characterized in that, The third threshold is greater than or equal to 1 / 3 of the total energy consumption that the energy-consuming device can withstand and less than 2 / 3 of the total energy consumption, and the fourth threshold is greater than or equal to 2 / 3 of the total energy consumption and less than the total energy consumption.

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