Black-start method of hybrid direct-current power transmission system, power transmission system and storage medium

Abstract

The invention provides a black-start method of a hybrid direct-current power transmission system, the power transmission system and a storage medium. Controlling a receiving-end converter to change the polarity of the receiving-end direct-current voltage from positive to negative; under the condition that the polarity of the receiving-end direct-current voltage is negative, a full-bridge sub-module of the transmitting-end converter is charged, a half-bridge sub-module of the transmitting-end converter is charged through the full-bridge sub-module, and when the energy-storage capacitor voltage of the full-bridge sub-module and the energy-storage capacitor voltage of the half-bridge sub-module reach a preset threshold value, it is determined that charging of the transmitting-end converter is completed; and then controlling a sending end converter to generate a sending end alternating current bus voltage, then controlling a receiving end converter to change the polarity of a receiving end direct current voltage from negative to positive, sending a starting signal to the new energy station when the receiving end direct current voltage crosses a zero point, and starting the new energy station when the output power of the new energy station reaches a rated value. And black start completion of the hybrid DC power transmission system is determined. The problem that the black start cost of the hybrid DC power transmission system is high is solved.

Description

Technical Field

[0001] This application relates to the field of DC power transmission technology, and more specifically, to a black start method for a hybrid DC power transmission system, a hybrid DC power transmission system, and a computer-readable storage medium. Background Technology

[0002] In a 100% renewable energy transmission scenario, the multilevel converter (MMC) station operates with passive startup. During actual operation, the IGBT control and drive circuits are directly powered by voltage division from the distributed capacitors within each MMC submodule. However, in the initial startup phase, the initial voltage of the capacitors within the submodules is zero. Therefore, the MMC lacks external energy and is in an uncontrolled, locked-out state. Before entering steady-state operation, the MMC converter must employ a suitable startup control strategy to pre-charge the energy storage capacitors of these submodules to achieve MMC startup.

[0003] To address the issue of passive starting of MMC at the sending end of hybrid DC transmission, existing passive starting solutions all require additional equipment, increasing the construction cost of the project. Summary of the Invention

[0004] The main objective of this application is to provide a black start method for a hybrid DC transmission system, a hybrid DC transmission system, and a computer-readable storage medium, so as to at least solve the problem of high cost of black start in the prior art for hybrid DC transmission systems.

[0005] To achieve the above objectives, according to one aspect of this application, a black-start method for a hybrid DC transmission system is provided. Each arm of the sending-end converter in the hybrid DC transmission system includes multiple full-bridge submodules and multiple half-bridge submodules connected in series. The method includes: controlling the receiving-end converter to change the polarity of the receiving-end DC voltage from positive to negative; charging the full-bridge submodules of the sending-end converter when the receiving-end DC voltage is negative, and using the full-bridge submodules to charge the half-bridge submodules of the sending-end converter, and charging the half-bridge submodules of the sending-end converter when the energy storage capacitor voltages of the full-bridge submodules and the half-bridge submodules reach a predetermined value. Under a given threshold, the charging of the sending-end converter is determined to be complete. After the charging of the sending-end converter is completed, the sending-end converter is controlled to generate the sending-end AC bus voltage. After the sending-end converter generates the sending-end AC bus voltage, the receiving-end converter is controlled to change the polarity of the receiving-end DC voltage from negative to positive. When the receiving-end DC voltage crosses zero, a start-up signal is sent to the new energy power station so that the output power of the new energy power station rises to the rated value at a predetermined rate. When the output power of the new energy power station reaches the rated value, the black start of the hybrid DC transmission system is determined to be complete.

[0006] Optionally, when the polarity of the receiving-end DC voltage is negative, charging the full-bridge submodule of the sending-end converter and using the full-bridge submodule to charge the half-bridge submodule of the sending-end converter includes: performing uncontrolled charging on the full-bridge submodule of the sending-end converter when the polarity of the receiving-end DC voltage is negative, and using the full-bridge submodule to perform uncontrolled charging on the half-bridge submodule; after performing the uncontrolled charging on the full-bridge submodule and the half-bridge submodule, performing controlled charging on the full-bridge submodule and the half-bridge submodule.

[0007] Optionally, when the polarity of the receiving-end DC voltage is negative, uncontrolled charging is performed on the full-bridge submodule of the sending-end converter, and the full-bridge submodule is used to uncontrolled charge the half-bridge submodule, including: when the polarity of the receiving-end DC voltage is negative, uncontrolled charging is performed on the full-bridge submodule until the energy storage capacitor voltage of the full-bridge submodule reaches a first preset voltage threshold; the reference value of the circulating current component in the circulating current suppression control is set to a non-zero value to form a circulating current in the full-bridge submodule, so that the full-bridge submodule performs uncontrolled charging on the half-bridge submodule until the energy storage capacitor voltage of the half-bridge submodule reaches a second preset voltage threshold; wherein, the first preset voltage threshold is set based on the receiving-end DC voltage and the number of full-bridge submodules, and the second preset voltage threshold is set based on the rated DC voltage of the hybrid DC transmission system, the number of full-bridge submodules, and the number of half-bridge submodules.

[0008] Optionally, after performing uncontrolled charging on the full-bridge submodule and the half-bridge submodule, performing controlled charging on the full-bridge submodule and the half-bridge submodule includes: after performing uncontrolled charging on the full-bridge submodule and the half-bridge submodule, adjusting the reference value of the circulating current component in the circulating current suppression control to form a circulating current that meets the requirements of controlled charging, and using the circulating current to perform controlled charging on the full-bridge submodule and the half-bridge submodule; when the energy storage capacitor voltage of the full-bridge submodule and the energy storage capacitor voltage of the half-bridge submodule reach the preset threshold, determining that the charging of the sending-end converter is complete, wherein the preset threshold is set based on the rated DC voltage of the hybrid DC transmission system, the number of full-bridge submodules, and the number of half-bridge submodules.

[0009] Optionally, the first preset voltage threshold is a first proportional coefficient multiplied by the ratio of the receiving-end DC voltage to the number of full-bridge submodules, and the second preset voltage threshold is the ratio of the rated DC voltage of the hybrid DC transmission system to the total number of full-bridge submodules and half-bridge submodules, multiplied by a second proportional coefficient.

[0010] Optionally, after charging the sending-end converter is completed, controlling the sending-end converter to generate a sending-end AC bus voltage includes: after charging the sending-end converter is completed, controlling the sending-end converter to generate a target AC voltage signal, wherein the voltage amplitude of the target AC voltage signal is a preset voltage amplitude and the frequency of the target AC voltage signal is a preset frequency; and converting the target AC voltage signal into an actual AC voltage, wherein the actual AC voltage is the sending-end AC bus voltage.

[0011] Optionally, after charging the sending-end converter is completed and before controlling the sending-end converter to generate the sending-end AC bus voltage, the method further includes: setting the reference value of the circulating current component in the circulating current suppression control to zero to stop the formation of circulating current in the full-bridge submodule and the half-bridge submodule.

[0012] Optionally, controlling the receiving-end converter to change the polarity of the receiving-end DC voltage from positive to negative includes: setting the DC voltage reference value of the receiving-end converter to a negative value; controlling the polarity of the receiving-end DC voltage of the receiving-end converter to change from positive to negative until the actual measured value of the receiving-end DC voltage gradually decreases to a negative value, and then continuing to decrease it to the DC voltage reference value.

[0013] According to another aspect of this application, a hybrid DC transmission system is provided, including a sending end, a receiving end, and a control unit. The sending end includes a sending-end converter, each arm of which includes multiple full-bridge submodules and multiple half-bridge submodules connected in series. The receiving end includes a receiving-end converter, the switching devices of which are reverse-resistance IGBT devices or reverse-resistance IGCT devices, used to change the polarity of the receiving-end DC voltage. The control unit is communicatively connected to the sending-end converter and the receiving-end converter, and is used to execute any of the described hybrid DC transmission system black-start methods.

[0014] 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 described hybrid DC transmission system black-start methods.

[0015] Applying the technical solution of this application, each arm of the sending-end converter of the hybrid DC transmission system includes multiple full-bridge submodules and multiple half-bridge submodules connected in series. The receiving-end converter is controlled to change the polarity of the receiving-end DC voltage from positive to negative. When the polarity of the receiving-end DC voltage is negative, the full-bridge submodule of the sending-end converter is charged, and the half-bridge submodule of the sending-end converter is charged using the full-bridge submodule. When the energy storage capacitor voltages of the full-bridge submodule and the half-bridge submodule reach preset thresholds, the charging of the sending-end converter is determined to be complete. After the charging of the sending-end converter is completed, the sending-end converter is controlled to generate the sending-end AC bus voltage. After the sending-end converter generates the sending-end AC bus voltage, the receiving-end converter is controlled to change the polarity of the receiving-end DC voltage from negative to positive. When the receiving-end DC voltage crosses zero, a start-up signal is sent to the renewable energy power station so that the output power of the renewable energy power station rises to the rated value at a predetermined rate. When the output power of the renewable energy power station reaches the rated value, the black start of the hybrid DC transmission system is determined to be complete. This scheme controls the receiving-end converter to reverse the polarity of the receiving-end DC voltage to negative without additional equipment. This negative voltage is then used to effectively charge the full-bridge and half-bridge sub-modules of the sending-end converter, overcoming the problem of charging the half-bridge sub-modules under negative voltage in traditional methods. After successful charging, the sending-end converter generates a stable AC voltage. The polarity of the receiving-end converter's DC voltage is then reversed back to positive, unlocking the renewable energy power station and controlling its power to steadily climb to its rated value. This ultimately achieves a smooth start-up of the hybrid DC system, solving the problem of high black-start costs in hybrid DC transmission systems. Attached Figure Description

[0016] 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:

[0017] Figure 1 A hardware structure block diagram of a mobile terminal for performing a black start method for a hybrid DC transmission system according to an embodiment of this application is shown.

[0018] Figure 2 A schematic flowchart of a black start method for a hybrid DC transmission system according to an embodiment of this application is shown.

[0019] Figure 3 A single-phase circuit equivalent diagram of the sending-end converter MMC of a black-start method for a hybrid DC transmission system according to an embodiment of this application is shown.

[0020] Figure 4A block diagram of an MMC circulating current suppression controller for a black start method of a hybrid DC transmission system according to an embodiment of this application is shown.

[0021] Figure 5 A hybrid DC transmission system topology according to an embodiment of this application is illustrated;

[0022] Figure 6 A receiving-end CCSC converter topology diagram of a hybrid DC transmission system according to an embodiment of this application is shown.

[0023] Figure 7 A schematic diagram of the MMC uncontrolled charging principle of a hybrid DC transmission system according to an embodiment of this application is shown.

[0024] Figure 8 A flowchart illustrating the startup process of a hybrid DC transmission system according to an embodiment of this application is shown.

[0025] Figure 9 A structural block diagram of a black start device for a hybrid DC transmission system provided according to an embodiment of this application is shown.

[0026] The above figures include the following reference numerals:

[0027] 102. Processor; 104. Memory; 106. Transmission device; 108. Input / output device. Detailed Implementation

[0028] 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.

[0029] 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.

[0030] 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.

[0031] As described in the background section, existing MMC passive start-up solutions all require additional devices, increasing the construction cost of the project. To address the high cost of black start-up in hybrid DC transmission systems, embodiments of this application provide a black start-up method for hybrid DC transmission systems, a hybrid DC transmission system, and a computer-readable storage medium.

[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] The methods and 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 1 This is a hardware structure block diagram of a mobile terminal for a black start method of a hybrid DC transmission system according to an embodiment of the present invention. Figure 1 As shown, a mobile terminal may include one or more ( Figure 1 Only one is shown in the diagram. A processor 102 (which may include, but is not limited to, a microprocessor MCU or a programmable logic device FPGA, etc.) and a memory 104 for storing data are also shown. The mobile terminal may further include a transmission device 106 for communication functions and an input / output device 108. Those skilled in the art will understand that... Figure 1 The structure shown is for illustrative purposes only and does not limit the structure of the mobile terminal described above. For example, the mobile terminal may also include components that are more... Figure 1 The more or fewer components shown, or having the same Figure 1 The different configurations shown.

[0034] The memory 104 can be used to store computer programs, such as application software programs and modules, like the computer program corresponding to the black start method of the hybrid DC transmission system in this embodiment of the invention. The processor 102 executes various functional applications and data processing by running the computer program stored in the memory 104, thereby implementing the above-described method. The memory 104 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 104 may further include memory remotely located relative to the processor 102, and these remote memories can be connected to the mobile terminal via a network. Examples of the aforementioned networks include, but are not limited to, the Internet, corporate intranets, local area networks, mobile communication networks, and combinations thereof. The transmission device 106 is used to receive or send data via a network. Specific examples of the aforementioned networks may include wireless networks provided by the mobile terminal's communication provider. In one example, the transmission device 106 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 106 may be a radio frequency (RF) module, which is used to communicate with the Internet wirelessly.

[0035] This embodiment provides a black start method for a hybrid DC transmission system that runs on a mobile terminal, computer terminal, or similar computing device. 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. Also, although the logical order is shown in the flowchart, in some cases, the steps shown or described can be executed in a different order than that shown here.

[0036] Figure 2 This is a schematic flowchart of a black-start method for a hybrid DC transmission system according to an embodiment of this application. Figure 2 As shown, the method includes the following steps:

[0037] Step S201: Control the receiving-end converter to change the polarity of the receiving-end DC voltage from positive to negative;

[0038] Specifically, in this embodiment, the receiving-end converter is a fully controlled current source converter (CCSC), and the sending-end converter is a multilevel converter (MMC). The purpose of changing the polarity of the receiving-end DC voltage from positive to negative is to utilize the polarity reversal capability of the receiving-end converter CCSC to provide the necessary charging energy for the submodule capacitors of the sending-end converter MMC, especially to achieve the startup of the sending-end converter without external auxiliary power supply, that is, black start.

[0039] Specifically, in a scenario where 100% renewable energy is transmitted, the sending end is passive, and the sending-end MMC needs to self-start without the support of a traditional AC power grid. This is because the half-bridge submodules of the MMC can only charge under positive current, and the sending-end MMC cannot generate positive current independently under passive startup conditions. In this case, we utilize the unique characteristics of the receiving-end CCSC to solve the charging problem. The receiving-end converter (CCSC), as a current-source converter, has the ability to independently control active and reactive power and maintain stable DC current. In this embodiment, the CCSC outputs a negative voltage, enabling the full-bridge submodules to charge in a locked state, as the structure of the full-bridge submodules allows them to charge even with a negative voltage. After the full-bridge submodules are fully charged and unlocked, a circulating current (containing positive current) is actively injected, providing the conditions for charging the half-bridge submodules.

[0040] This initial polarity reversal provides a charging opportunity for the capacitors of the full-bridge and half-bridge submodules of the sending-end MMC. This process not only avoids dependence on additional energy storage devices or starting power supplies, reducing system complexity and cost, but also fully utilizes the current source characteristics of the CCSC and the hybrid topology advantages of the MMC, facilitating the self-starting of the hybrid DC transmission system under extreme conditions. This polarity reversal control strategy is the foundation for achieving black start of the sending-end MMC in a 100% renewable energy transmission scenario in this embodiment.

[0041] Step S202: When the polarity of the receiving-end DC voltage is negative, the full-bridge submodule of the sending-end converter is charged, and the half-bridge submodule of the sending-end converter is charged using the full-bridge submodule. When the energy storage capacitor voltage of the full-bridge submodule and the energy storage capacitor voltage of the half-bridge submodule reach a preset threshold, the charging of the sending-end converter is determined to be complete.

[0042] Specifically, when the DC voltage polarity of the receiving-end converter is set to negative, this provides the conditions for charging the full-bridge submodule capacitors of the sending-end converter. After full-bridge charging, a circulating current injection method is used to charge the half-bridge. In a hybrid DC transmission system, each arm of the sending-end MMC converter contains multiple series-connected full-bridge and half-bridge submodules. These submodules contain energy storage capacitors used to control the voltage level of the submodules, thereby controlling the output voltage of the entire converter. When the energy storage capacitor voltages of both the full-bridge and half-bridge submodules reach a preset threshold, it is determined that the charging of the sending-end MMC converter is complete, and the next stage of control begins.

[0043] The full-bridge submodule assists in charging the half-bridge submodule, solving the problem that the half-bridge submodule cannot be charged when the polarity of the DC voltage at the receiving end is negative. When the energy storage capacitor voltages of both the full-bridge submodule and the half-bridge submodule reach the preset threshold, the charging process of the sending-end converter is determined to be complete.

[0044] In summary, under the condition that the receiving-end DC voltage polarity is negative, the full-bridge submodule is charged first, and then the half-bridge submodule is charged using the full-bridge submodule. This not only achieves the charging of the full-bridge submodule but also solves the problem of charging the half-bridge submodule, ensuring that the sending-end converter can be charged without external power support. This contributes to the efficient startup and stable operation of the hybrid DC transmission system. This charging method eliminates the need for additional charging equipment, reducing system costs while improving the reliability of renewable energy transmission. It is particularly suitable for scenarios involving 100% renewable energy transmission, reducing reliance on traditional starting power supplies, lowering system costs, and ensuring system startup efficiency and stable renewable energy transmission.

[0045] Step S203: After charging the aforementioned sending-end converter is completed, control the aforementioned sending-end converter to generate the sending-end AC bus voltage.

[0046] Specifically, after charging the sending-end converter is complete, the next step is to control the sending-end converter to generate its AC voltage output. This step is crucial for the startup and stable operation of the hybrid DC transmission system. Once the energy storage capacitors in the full-bridge and half-bridge submodules of the sending-end converter's MMC have completed charging and reached the preset threshold voltage, the sending-end converter meets the necessary conditions for startup. At this point, the control strategy of the sending-end converter will switch from charging mode to constant AC voltage and frequency control mode to establish a stable AC voltage output, namely the aforementioned sending-end AC bus voltage. This sending-end AC bus voltage will be used to support the voltage of the sending-end AC system, providing the necessary electrical conditions for power transmission between the subsequent renewable energy power plants and the sending-end converter.

[0047] Generating a stable AC bus voltage at the sending end is crucial for the startup and normal operation of a hybrid DC transmission system. It not only ensures the proper functioning of the converter but also provides the necessary voltage support for the synchronous startup of the receiving-end converter and the entire transmission system. This is a key step in ensuring a successful black start and subsequent stable operation of the hybrid DC transmission system.

[0048] Step S204: After the sending-end converter generates the sending-end AC bus voltage, the receiving-end converter is controlled to change the polarity of the receiving-end DC voltage from negative to positive. When the receiving-end DC voltage crosses zero, a start signal is sent to the new energy power station so that the output power of the new energy power station rises to the rated value at a predetermined rate. When the output power of the new energy power station reaches the rated value, the black start of the hybrid DC transmission system is determined to be complete.

[0049] Specifically, after the sending-end converter establishes the sending-end AC bus voltage, the next step is to control the receiving-end converter to change the polarity of the receiving-end DC voltage from negative to positive. When the polarity reversal of the receiving-end DC voltage is complete, and the output power of the renewable energy plant steadily increases to its rated value, the black start process of the entire hybrid DC transmission system is completed. In other words, once the sending-end converter can output a stable AC voltage, it means that the renewable energy can be reliably connected to the grid, and the sending-end converter has the capability to receive current and transmit power. At this point, the system meets the conditions for polarity reversal of the receiving-end converter. The receiving-end converter adjusts its internal control strategy, changing the polarity of its DC-side voltage from negative to positive. The purpose of polarity reversal is to restore the normal power flow direction, that is, from the sending-end converter through the DC line to the receiving-end converter, so that the system can operate normally.

[0050] During the polarity reversal process, especially at the moment when the receiving-end DC voltage approaches zero from a negative value and is about to jump to a positive value, the renewable energy power station (such as a wind farm or photovoltaic power station) will simultaneously unlock and initiate its output power ramp-up process. This ensures that the renewable energy power station can respond promptly and begin to steadily increase its output power just as the receiving-end DC voltage polarity reversal is completed and the system is about to re-enter normal operation. As the output power of the renewable energy power station gradually increases, its power output will be monitored. When the output power of the renewable energy power station reaches its rated value, it indicates that the system is capable of handling the predetermined power load, and the power transmission between the converters at both ends has returned to normal operation. At this point, the black start process of the entire hybrid DC transmission system is complete.

[0051] Once the sending-end converter establishes its AC voltage output, the DC voltage polarity of the receiving-end converter is reversed from negative to positive. This, combined with the gradual ramp-up of the renewable energy power plants to their rated output power, completes the system's black start. This process enables the entire transmission system to achieve self-start and stable operation without relying on an external AC grid, solely through the interaction between converters within the system and the active response of the renewable energy power plants. The polarity reversal of the receiving-end converter provides a positive DC voltage to the sending-end converter, restoring the system's normal power transmission direction. The gradual ramp-up of the renewable energy power plants to their rated power ensures the system can carry the predetermined load, ultimately achieving a black start for the hybrid DC transmission system. This improves the flexibility and reliability of renewable energy grid connection, while avoiding the use of additional starting equipment and reducing system operating costs.

[0052] This embodiment controls the receiving-end converter to reverse the polarity of the receiving-end DC voltage to negative without additional equipment. This negative voltage is then used to effectively charge the full-bridge and half-bridge sub-modules of the sending-end converter, overcoming the problem of charging the half-bridge sub-modules under negative voltage in traditional methods. After successful charging, the sending-end converter can establish a stable AC voltage. Subsequently, the polarity of the receiving-end converter's DC voltage is reversed back to positive, controlling the power of the renewable energy power station to smoothly climb to its rated value, ultimately achieving a smooth start-up of the hybrid DC system. This solves the problem of high black-start costs in hybrid DC transmission systems.

[0053] In the specific implementation process, when the polarity of the receiving-end DC voltage is negative, the full-bridge submodule of the sending-end converter is charged, and the half-bridge submodule of the sending-end converter is charged using the full-bridge submodule. This includes: when the polarity of the receiving-end DC voltage is negative, performing uncontrolled charging on the full-bridge submodule of the sending-end converter, and performing uncontrolled charging on the half-bridge submodule using the full-bridge submodule; after performing the uncontrolled charging on the full-bridge submodule and the half-bridge submodule, performing controlled charging on the full-bridge submodule and the half-bridge submodule.

[0054] The above describes the specific steps for charging the full-bridge and half-bridge submodules of the sending-end converter during the black start process of a hybrid DC transmission system, as well as the conditions for determining the completion of charging. Specifically, charging is divided into two stages: uncontrolled charging and controlled charging, ensuring that the energy storage capacitor voltages of the full-bridge and half-bridge submodules can reach the preset threshold required for system startup, thereby preparing the sending-end converter for startup.

[0055] Furthermore, when the polarity of the receiving-end DC voltage is negative, uncontrolled charging is performed on the full-bridge submodule of the sending-end converter, and the full-bridge submodule is used to uncontrolled charge the half-bridge submodule, including: when the polarity of the receiving-end DC voltage is negative, uncontrolled charging is performed on the full-bridge submodule until the energy storage capacitor voltage of the full-bridge submodule reaches a first preset voltage threshold; the reference value of the circulating current component in the circulating current suppression control is set to a non-zero value to form a circulating current in the full-bridge submodule, so that the full-bridge submodule uncontrolled charging the half-bridge submodule is performed until the energy storage capacitor voltage of the half-bridge submodule reaches a second preset voltage threshold; wherein, the first preset voltage threshold is set based on the receiving-end DC voltage and the number of the full-bridge submodules, and the second preset voltage threshold is set based on the rated DC voltage of the hybrid DC transmission system, the number of the full-bridge submodules, and the number of the half-bridge submodules.

[0056] Specifically, when the polarity of the DC voltage at the receiving end is negative, uncontrolled charging is performed on the full-bridge sub-module and half-bridge sub-module in the sending-end converter. This charging process is divided into two main stages, targeting the full-bridge sub-module and the half-bridge sub-module respectively, to ensure that the energy storage capacitor voltage of the full-bridge sub-module and the half-bridge sub-module can reach specific preset voltage thresholds respectively.

[0057] When the polarity of the receiving-end DC voltage is negative, the first step is uncontrolled charging of the full-bridge submodule. During this stage, the full-bridge submodule (composed of four IGBTs, capable of withstanding reverse voltage) automatically charges until its energy storage capacitor voltage reaches a first preset voltage threshold. This first preset voltage threshold is set based on the polarity of the receiving-end DC voltage and the number of full-bridge submodules, ensuring that the full-bridge submodule can store sufficient energy to support subsequent steps.

[0058] After the energy storage capacitor voltage of the full-bridge submodule reaches the first preset voltage threshold, the reference value of the circulating current component in the circulating current suppression control needs to be set to a non-zero value to form a circulating current in the full-bridge submodule, allowing the full-bridge submodule to uncontrolled charge the half-bridge submodule. This circulating current includes both forward and reverse AC components. When the current direction is positive, the half-bridge submodule (consisting of two IGBTs and one capacitor, which can only be charged under forward voltage) can begin charging. Through this process, the energy storage capacitor voltage of the half-bridge submodule will gradually rise until it reaches the second preset voltage threshold. The second preset voltage threshold is set based on the rated DC voltage of the hybrid DC transmission system, the number of full-bridge submodules, and the number of half-bridge submodules, to ensure that the half-bridge submodules can also store sufficient energy to prepare for system startup.

[0059] The principle of circulating current generation in the MMC of the sending-end converter is explained below. The equivalent circuit diagram of the single-phase MMC of the sending-end converter is shown below. Figure 3 As shown, i a and u a These represent the AC current and voltage of phase a, respectively. Points O' and O represent the neutral points on both sides, respectively. arm and R arm These are the bridge arm inductance and resistance, u pa Let u be the voltage of the upper bridge arm of phase a. na U is the lower bridge arm voltage of phase a. dc This is the DC line voltage.

[0060] The relationship between phase a output current and bridge arm current is: , where i pa Let i be the upper arm current of phase a. na Let be the lower arm current of phase a. Consider the circulating current caused by phase-to-phase voltage imbalance, defined as... .

[0061] Regarding the circulating current suppression control of the sending-end converter MMC, since the capacitor voltages of different submodules are not the same at the same time, unbalanced currents will be generated between the bridge arms. The unbalanced current includes both DC and AC components, and the DC component will distort the current flowing between the bridge arms, affecting the life of the switching transistors. Therefore, it is necessary to suppress the circulating current between the bridge arms.

[0062] Since the PI controller can achieve zero steady-state error control of the current component, a block diagram of an MMC circulating current suppression controller based on PI control can be obtained, such as... Figure 4 As shown. i cird i cirq Let i represent the d-axis and q-axis components of the three-phase circulating current, respectively. cird_ref i cirq_ref These are the reference values ​​for the d-axis and q-axis components of the circulation, u. cird The modulated signal component output after controlling the d-axis component of the circulating current, u cirq This is the modulated signal component output after controlling the q-axis component of the circulating current, where ω is the angular frequency of the power grid and L is the bridge arm inductance. During steady-state operation, it is necessary to suppress the circulating current; in this case, i... cird_ref =0, i cirq_ref =0, suppressing circulating current to 0, thus achieving the effect of circulating current suppression. However, when circulating current is needed, i needs to be set to 0. cird_ref and i cirq_ref If we set it to a non-zero value, such as 0.1, then there will be circulating current flowing in the three bridge arms, which is also the way to achieve charging later.

[0063] The key to the entire uncontrolled charging process lies in utilizing the polarity reversal of the DC voltage at the receiving end and the characteristics of the full-bridge submodule to charge the sending-end converter submodule. No additional starting power supply or equipment is required; relying solely on internal system coordination control—specifically, the negative DC voltage established by the receiving-end converter and the circulating current formed in the full-bridge submodule of the sending-end converter—effectively charges the energy storage capacitors of both the full-bridge and half-bridge submodules, thus providing the necessary power support for the system's black start. When the energy storage capacitor voltage of the full-bridge submodule reaches the first preset voltage threshold, the uncontrolled charging of the full-bridge submodule is complete. Similarly, when the energy storage capacitor voltage of the half-bridge submodule reaches the second preset voltage threshold, the uncontrolled charging of the half-bridge submodule is complete, and the system enters the next stage of start-up control.

[0064] Furthermore, after performing uncontrolled charging on the full-bridge submodule and the half-bridge submodule, performing controlled charging on the full-bridge submodule and the half-bridge submodule includes: after performing uncontrolled charging on the full-bridge submodule and the half-bridge submodule, adjusting the reference value of the circulating current component in the circulating current suppression control to form a circulating current that meets the requirements of controlled charging, and using the circulating current to perform the controlled charging on the full-bridge submodule and the half-bridge submodule; when the energy storage capacitor voltage of the full-bridge submodule and the energy storage capacitor voltage of the half-bridge submodule reach the preset threshold, determining that the charging of the sending-end converter is complete, wherein the preset threshold is set based on the rated DC voltage of the hybrid DC transmission system, the number of the full-bridge submodules, and the number of the half-bridge submodules.

[0065] After uncontrolled charging of the full-bridge and half-bridge submodules, controlled charging is performed to ensure that the energy storage capacitor voltages of the full-bridge and half-bridge submodules reach the rated values ​​required for stable system operation. Specifically, after the uncontrolled charging phase, i.e., when the energy storage capacitor voltages of the full-bridge and half-bridge submodules reach the first and second preset voltage thresholds respectively, but have not yet reached the capacitor voltage values ​​required for rated operation, the controlled charging phase begins. The goal of this phase is to further increase the energy storage voltages of the full-bridge and half-bridge submodule capacitors to the preset thresholds through control strategies, ensuring that the sending-end converter can reliably enter steady-state operation mode.

[0066] During the controlled charging phase, the reference value of the circulating current component in the circulating current suppression control is reset. Unlike the non-zero value during the uncontrolled charging phase, the reference value of the circulating current component here is adjusted to a specific value that meets the controlled charging requirements, forming an appropriate AC circulating current. The generation of this circulating current is controllable, and its waveform and intensity are designed to meet the requirements for controlled charging of the full-bridge and half-bridge submodules. During the startup phase of the hybrid DC transmission system, a current that can effectively flow between the full-bridge and half-bridge submodules is formed through precise control of the circulating current component, i.e., a circulating current that meets the controlled charging requirements. The characteristics of this circulating current are optimized to ensure that the submodule capacitors are uniformly charged to a preset threshold without being affected by external energy sources, thereby achieving a smooth startup of the sending-end MMC converter. By dynamically adjusting the intensity and direction of the circulating current, the charging requirements can be precisely matched while maintaining operational stability and avoiding unnecessary energy loss. Specifically, the circulating current that meets the controlled charging requirements refers to the specific ring current formed inside the bridge arm during the charging process of the MMC through precise control. The magnitude, direction, and frequency of the circulating current can be dynamically adjusted, changing in real time according to the charging progress and the state of the submodules to ensure that the capacitors of each submodule are charged smoothly within a preset voltage threshold range. The AC component of the circulating current is optimized to maximize the balanced rise of the capacitor voltages of both full-bridge and half-bridge submodules. This means that during charging, regardless of whether the submodule is a full-bridge or half-bridge, it can obtain an appropriate and balanced charging current, avoiding capacitor voltage instability caused by uneven charging. Considering the differences in the charging mechanisms of full-bridge and half-bridge submodules, the circulating current is designed to effectively charge the full-bridge submodules and, through changes in its AC component, achieve controllable charging of the half-bridge submodules, overcoming the charging difficulties of half-bridge submodules in traditional charging methods. Throughout the charging process, the characteristics of the circulating current are continuously monitored. Once the energy storage capacitor voltages of the full-bridge and half-bridge submodules are detected to be close to the preset threshold, the characteristics of the circulating current will be adjusted in a timely manner until the energy storage capacitor voltages of all full-bridge and half-bridge submodules reach the preset threshold, ensuring the safety of the charging process and the normal startup of the submodules. Through the above adjustments, the positive current component in the circulating current will form a controllable charge in the full-bridge submodule, while the negative current component will form a controllable charge in the half-bridge submodule. This process is precisely controlled by the converter's control logic, ensuring that the capacitor voltage of each submodule can be stably increased under controllable conditions. During the controllable charging phase, the full-bridge and half-bridge submodules work together, charging through the formed circulating current. This collaborative charging mechanism ensures that all full-bridge and half-bridge submodules can reach the same capacitor voltage level, which is a necessary condition for the startup and normal operation of the hybrid DC transmission system.

[0067] Through the above-mentioned controllable charging, not only can the voltage of the energy storage capacitors in the full-bridge and half-bridge submodules of the sending-end converter reach the preset threshold, but the charging process can also be optimized to avoid unnecessary energy loss or unstable states during charging. The implementation of controllable charging not only improves the energy conversion efficiency and stability during system startup, but also further enhances the energy management capability of the sending-end converter under passive startup conditions, which is conducive to the efficient and reliable access of new energy sources.

[0068] During the controlled charging phase, the energy storage capacitor voltages of both the full-bridge and half-bridge submodules are continuously monitored. When both the full-bridge and half-bridge submodule energy storage capacitor voltages reach preset thresholds, the charging of the sending-end converter is confirmed to be complete. The preset thresholds are set considering the rated DC voltage of the hybrid DC transmission system, the number of full-bridge and half-bridge submodules, to ensure that the submodule capacitor voltages meet the conditions for startup and normal operation.

[0069] A two-stage charging strategy—first uncontrolled charging of the full-bridge and half-bridge submodules, then transitioning to controlled charging—ensures that the energy storage capacitors of both submodules reach preset thresholds without requiring additional start-up power, thus enabling reliable startup of the sending-end converter. The preset thresholds fully consider the impact of the rated DC voltage of the hybrid DC transmission system, the number of full-bridge and half-bridge submodules, making the charging process more precise and efficient. This charging mechanism, based on its own system resources, not only improves the startup flexibility and stability of the renewable energy access system but also significantly reduces reliance on auxiliary startup equipment, lowering system construction and operation costs, and providing technical support for hybrid DC transmission systems in 100% renewable energy transmission scenarios.

[0070] In some embodiments of this application, the first preset voltage threshold is a first proportional coefficient multiplied by the ratio of the receiving-end DC voltage to the number of full-bridge submodules, and the second preset voltage threshold is the ratio of the rated DC voltage of the hybrid DC transmission system to the total number of full-bridge submodules and half-bridge submodules, multiplied by a second proportional coefficient.

[0071] Specifically, formula U can be used. dc1 / a1×N F Calculate the first preset voltage threshold, where U dc1 N is the DC voltage at the receiving end. F Where is the number of full-bridge submodules, and a1 is the first scaling factor, which is set to 2 in this embodiment. The formula [U] can be used. dcN / (N F +N H The second preset voltage threshold is calculated by multiplying U by a2, where U dcNN is the rated DC voltage of the hybrid DC transmission system. F N represents the number of full-bridge submodules. H a1 represents the number of half-bridge sub-modules, and a2 represents the second scaling factor. In this embodiment, the second scaling factor can be set to ×25%.

[0072] Setting the first and second preset voltage thresholds is crucial to ensuring the effective charging of the full-bridge and half-bridge submodules of the sending-end converter. These thresholds are calculated by fully considering the influence of the receiving-end DC voltage, the system's rated DC voltage, and the number of submodules, and are fine-tuned using proportional coefficients to adapt to the specific system design requirements. The process of achieving these two thresholds involves first uncontrolled charging of the full-bridge submodules using the negative polarity of the receiving-end DC voltage, followed by controlled charging of the half-bridge submodules using controlled circulating current injection, until their respective preset thresholds are met. This provides a reliable and efficient startup strategy for hybrid DC transmission systems, ensuring stable system operation.

[0073] In some embodiments of this application, after charging the aforementioned sending-end converter is completed, controlling the sending-end converter to generate a sending-end AC bus voltage includes: after charging the aforementioned sending-end converter is completed, controlling the sending-end converter to generate a target AC voltage signal, wherein the voltage amplitude of the target AC voltage signal is a preset voltage amplitude and the frequency of the target AC voltage signal is a preset frequency; and converting the target AC voltage signal into an actual AC voltage, wherein the actual AC voltage is the aforementioned sending-end AC bus voltage.

[0074] The above describes how, after charging the sending-end converter, the AC bus voltage is generated to further drive the black start process of the entire hybrid DC transmission system. Specifically, after both the full-bridge and half-bridge submodules of the sending-end converter have completed charging and the voltage of their respective energy storage capacitors has reached a preset threshold, the stage of controlling the sending-end converter to establish AC voltage begins. In this stage, a preset constant AC voltage and frequency control strategy is used to generate the target AC voltage signal. That is, the sending-end converter will generate a stable AC voltage output signal based on the preset voltage amplitude and preset frequency. The preset voltage amplitude and preset frequency settings take into account system design requirements, grid standards, and the output characteristics of new energy power plants, ensuring that the generated AC voltage signal meets the conditions for stable subsequent system operation. The target AC voltage signal generated by the above control strategy will guide the switching actions of each submodule within the sending-end converter to generate the desired AC power. This process involves converting DC power to AC power through an inverter, and the target AC voltage signal provides a precise reference for the inverter process, ensuring that the output AC voltage has a stable amplitude and frequency that meets the requirements. Finally, the sending-end converter converts the target AC voltage signal into the actual AC voltage, thereby generating the sending-end AC bus voltage. This generation of the sending-end AC bus voltage not only provides the grid reference required for grid connection of the renewable energy power plants, but also further promotes the startup process of the entire hybrid DC transmission system, providing conditions for subsequent stages such as polarity reversal of the receiving-end converter and the ramp-up of the renewable energy power plant's output power.

[0075] After charging, the sending-end converter generates a target AC voltage signal using a constant AC voltage and frequency control strategy, converts it into an actual AC voltage, and thus generates the sending-end AC bus voltage. The key to this process is that the sending-end converter, based on preset fixed amplitude and frequency parameters, uses precise internal control to stably output the required AC voltage. This enhances the system's self-starting capability without external power support, ensuring the smooth grid connection of renewable energy power plants, while reducing reliance on traditional starting power sources, lowering start-up costs, and improving the flexibility and reliability of renewable energy power transmission. By autonomously generating AC voltage through the sending-end converter, the entire hybrid DC transmission system can transition to normal operation more orderly and efficiently.

[0076] Furthermore, after charging the aforementioned sending-end converter is completed, and before controlling the aforementioned sending-end converter to generate the sending-end AC bus voltage, the method further includes: setting the reference value of the circulating current component in the circulating current suppression control to zero to stop the formation of circulating current in the aforementioned full-bridge submodule and the aforementioned half-bridge submodule.

[0077] Specifically, after charging the sending-end converter is complete, and before controlling the sending-end converter to generate the sending-end AC bus voltage, the reference value of the circulating current component in the circulating current suppression control is set to zero to stop the formation of circulating current in the full-bridge and half-bridge submodules. This step aims to ensure that the internal operating state of the sending-end converter has reached stability before entering the next stage, namely the generation of the sending-end AC bus voltage, thus avoiding interference from circulating current in the AC voltage generation process. During the controllable charging stage, the full-bridge and half-bridge submodules charge by forming circulating current to achieve the required energy storage capacitor voltage. When charging is complete, and a constant AC voltage and frequency control strategy is adopted, before the sending-end converter is ready to generate the sending-end AC bus voltage, the full-bridge and half-bridge submodules are controlled to stop forming circulating current. This means that the circulating current control inside the sending-end converter will be turned off, and the full-bridge and half-bridge submodules enter a ready state to prepare for the subsequent AC voltage generation.

[0078] The cessation of circulating current formation ensures that the sending-end converter can convert DC to AC through an inversion process based on the target AC voltage signal in an environment free from circulating current interference, thereby generating a stable AC voltage at the sending end. This step improves the accuracy and stability of AC voltage establishment, avoids voltage fluctuations that may be caused by circulating current, and is crucial for the smooth black start process of the entire hybrid DC transmission system, ensuring energy management and control precision during the transition from startup to stable operation.

[0079] In some embodiments of this application, controlling the receiving-end converter to change the polarity of the receiving-end DC voltage from positive to negative includes: setting the DC voltage reference value of the receiving-end converter to a negative value; controlling the polarity of the receiving-end DC voltage of the receiving-end converter to change from positive to negative until the actual measured value of the receiving-end DC voltage gradually decreases to a negative value, and then continuing to decrease it to the DC voltage reference value.

[0080] Specifically, the DC voltage reference value of the receiving-end converter is set to a negative value. This setting instructs the internal control mechanism of the receiving-end converter to adjust the direction of the output voltage from positive to negative to meet the specific operational requirements of the system. Setting the DC voltage reference value to negative is the primary condition for achieving polarity reversal of the receiving-end DC voltage. Polarity reversal is achieved by setting the DC voltage reference value to negative, controlling the polarity of the receiving-end DC voltage of the receiving-end converter from positive to negative. If the receiving-end DC voltage becomes negative (not necessarily the reference value), then the polarity has already changed from positive to negative, achieving polarity reversal. The actual measured value of the receiving-end DC voltage continues to change until it reaches the DC voltage reference value, at which point the polarity reversal is complete. Polarity reversal is a gradual process; the voltage decreases from positive to below zero, until it becomes negative, and then stabilizes at the set negative DC voltage reference value.

[0081] In the above process, on the one hand, by setting the DC voltage reference value of the receiving-end converter to a negative value, the target direction of voltage polarity reversal is clearly given; on the other hand, this involves not only setting the DC voltage reference value, but also continuously adjusting the actual value of the DC voltage of the receiving-end converter until it stabilizes at the negative DC voltage reference value, thereby achieving the polarity reversal from positive to negative. This effectively improves the flexibility and accuracy of receiving-end DC voltage control during the black start process of the hybrid DC transmission system, laying a solid foundation for the stable operation of the system and the reliable access of new energy power, ensuring the controllable reversal of the receiving-end DC voltage polarity, and enhancing the operational stability and efficiency optimization capabilities of the hybrid DC transmission system.

[0082] This application also provides a hybrid DC transmission system, including a sending end, a receiving end, and a control unit. The sending end includes a sending-end converter, each arm of which includes multiple full-bridge submodules and multiple half-bridge submodules connected in series; the receiving end includes a receiving-end converter, whose switching devices are fully controlled devices used to change the polarity of the receiving-end DC voltage; the control unit is communicatively connected to the sending-end converter and the receiving-end converter, and is used to execute any of the above-described hybrid DC transmission system black-start methods.

[0083] Specifically, under 100% renewable energy transmission, renewable energy power plants cannot provide a stable AC power supply. To meet the demand for renewable energy transmission, the sending-end converter station (MMC converter station) uses MMC converters for flexible DC transmission to support the sending-end AC bus voltage. Meanwhile, the receiving-end converter station (CCSC converter) uses CCSC converters based on reverse-resistance IGBTs or reverse-resistance IGCT devices to solve the commutation failure problem, and can independently control active and reactive power. Figure 5 The topology of the hybrid DC transmission system is as follows: the sending end MMC adopts a unit cascade method to form a three-phase six-arm bridge, and each bridge arm is composed of several full-bridge and half-bridge sub-modules connected in series.

[0084] The receiving-end converter CCSC adopts a six-pulse converter bridge structure, such as Figure 6 As shown, the commutation devices are reverse-blocking integrated gate-commutated thyristors (RB-IGCT) or reverse-blocking insulated-gate bipolar transistors (RB-IGBT). S1 to S6 represent the commutation devices in the commutation bridge, respectively. An LC filter circuit (inductor L, capacitor C) is configured on the AC side to eliminate harmonics, and a smoothing reactor L... dc Used to stabilize DC current, employing constant DC voltage and constant reactive power control. L is the filter inductor; C is the filter capacitor; udc U is the DC output voltage of the CCSC converter. dc For DC line voltage; i dc For DC line current; i a i b i c This represents the three-phase current at the outlet of the converter valve; u a u b u c Indicates the three-phase voltage of the filter capacitor; i ga i gb i gc This represents the three-phase current fed into the AC system by the CCSC; u ga u gb u gc This indicates the three-phase voltage of an AC power grid.

[0085] The characteristics of the CCSC dictate that the current direction in a hybrid DC system cannot be changed. To charge the sending-end MMC, the polarity of the CCSC is reversed. The CCSC employs constant DC voltage and constant reactive power control, with an initial command value (receiving-end DC voltage) U. dc1 If the value is negative, under constant DC voltage control, a negative DC voltage is established at the receiving end, which charges the sending end MMC through the DC line, such as... Figure 7 As shown, N H and N F These represent the number of sub-modules for a single bridge arm half-bridge and a full-bridge, respectively.

[0086] The control unit is connected to the sending-end converter and the receiving-end converter. It can send corresponding control commands to the sending-end converter and the receiving-end converter according to the current status and objectives of the system, and guide the sending-end converter and the receiving-end converter to perform the correct power conversion operation, thereby ensuring that the entire hybrid DC transmission system can smoothly and orderly start up from the passive state to the normal operation state.

[0087] The hybrid DC transmission system in this embodiment includes a sending end, a receiving end, and a control unit responsible for coordination and control. The sending-end converter is designed with a series combination of full-bridge and half-bridge submodules, improving adaptability and handling capabilities under different power conditions. The receiving-end converter utilizes the unique properties of reverse-resistance IGBTs or IGCT devices to change the polarity of the receiving-end DC voltage, providing a flexible control method for the system. The control unit communicates with both the sending-end and receiving-end converters, enabling the aforementioned black-start method for the hybrid DC transmission system. This ensures that the system can start autonomously and smoothly transition to a stable operating state without external power support, improving the efficiency and stability of new energy power integration. In summary, the hybrid DC transmission system architecture based on this embodiment reduces the cost of black-starting the hybrid DC transmission system.

[0088] In some embodiments of this application, after sending a start-up signal to the new energy power station, the method further includes: monitoring the power transmission status of the receiving-end DC line and the power change of the new energy power station; if the power change range of the new energy power station is detected to exceed a preset change range, calculating the voltage adjustment amount required for compensation; and adjusting the AC voltage command value of the sending-end converter based on the voltage adjustment amount.

[0089] Specifically, the system monitors the power transmission status of the receiving-end DC line and changes in the power output of renewable energy plants. This monitoring utilizes various sensors and data acquisition devices installed within the system to collect power transmission data and renewable energy plant power output information in real time. A preset range of variation is set; when a power change at a renewable energy plant is detected to exceed this preset range, the system needs to respond. The preset range is set based on a comprehensive consideration of system stability and energy transmission efficiency, typically a safety threshold exceeding which may cause disturbances to the system. Once a power change in renewable energy is detected to exceed the preset range, the required voltage adjustment is calculated. This adjustment is calculated based on the current power change, aiming to offset the impact of the power change and maintain system stability by adjusting the voltage. Based on the calculated voltage adjustment, the AC voltage command value of the sending-end converter is adjusted. By changing the AC voltage command, the sending-end converter can adjust its output voltage to compensate for the impact of renewable energy power changes on system voltage stability.

[0090] The core of this embodiment lies in its continuous monitoring of system status and rapid response to power changes. When the power of new energy sources fluctuates significantly, it can dynamically adjust the voltage output of the sending-end converter to ensure the stable operation of the hybrid DC transmission system and avoid system disturbances caused by sudden power changes. In addition, through precise calculation and timely adjustment, this function also helps to improve energy transmission efficiency and reduce energy losses caused by power fluctuations.

[0091] In some other embodiments of this application, the method further includes: using a communication network to perform real-time bidirectional information exchange between the sending-end converter and the receiving-end converter; and adjusting the DC voltage command value of the receiving-end converter based on the real-time bidirectional information exchange.

[0092] Specifically, by constructing an efficient communication network, real-time bidirectional information sharing between the sending-end and receiving-end converters is achieved. This platform allows both converters to promptly transmit critical system status data, including but not limited to power transmission status, voltage levels, and submodule status. Both the sending-end and receiving-end converters continuously monitor their own and each other's operating status and transmit this information to the other in real time. For example, when the sending-end converter detects fluctuations in renewable energy power, it quickly sends this change information to the receiving-end converter so that the receiving-end converter can respond promptly. Based on the real-time data received from the sending-end converter, the receiving-end converter can dynamically adjust its DC voltage command value. This adjustment responds to changes in the sending-end converter's AC voltage and fluctuations in renewable energy power, aiming to maintain power balance between the sending and receiving ends and prevent the negative impact of sudden power changes on system stability.

[0093] The real-time two-way information exchange mechanism greatly improves the response speed of the hybrid DC transmission system to power fluctuations from new energy sources, enabling the receiving-end converter to quickly adjust its control strategy and maintain stable system operation. Through real-time data transmission over the communication network, the coordinated control capability between the sending-end and receiving-end converters is enhanced, ensuring power balance and operational stability of the entire system under complex operating conditions. Control command adjustments based on real-time data help optimize the operating parameters of the receiving-end converter, reduce energy losses caused by power fluctuations, and improve energy transmission efficiency.

[0094] To enable those skilled in the art to better understand the technical solution of this application, the implementation process of the black start method for hybrid DC transmission systems of this application will be described in detail below with reference to specific embodiments.

[0095] This embodiment relates to a specific black-start method for a hybrid DC transmission system. This method is implemented based on the aforementioned hybrid DC transmission system, such as... Figure 8 As shown, it includes the following segments:

[0096] Phase 1: At this stage, both the full-bridge and half-bridge submodules are locked, and the submodules are uncontrollable. Since the current flowing through the half-bridge submodule is negative, it cannot be charged. Only the full-bridge submodule can be charged. After the uncontrolled charging of the full-bridge submodule is completed, the full-bridge submodule reaches the energy extraction requirement and can be unlocked. In Phase 1, the capacitor voltage of the half-bridge submodule remains 0. The charging voltage of the full-bridge submodule is U. dc1 / 2N F (i.e., the first preset voltage threshold), where U dc1 N is the DC voltage at the receiving end. F This represents the number of full-bridge submodules.

[0097] Phase 2: Uncontrolled charging of the half-bridge submodule. The specific principle is as follows: Unlock the MMC full-bridge submodule, enable active circulating current injection (disable circulating current suppression control), the circulating current contains an AC component (current can be positive or negative), which can perform uncontrolled charging of the half-bridge submodule (half-bridge charging when current is positive). After uncontrolled charging ends, the half-bridge submodule reaches the energy harvesting requirement, that is, the energy storage capacitor voltage of the half-bridge submodule reaches the second preset voltage threshold, and the half-bridge can be unlocked. In this embodiment, the second preset voltage threshold is set to [U dcN / (N F +N H )]×25%, of which, U dcN N is the rated DC voltage of the hybrid DC transmission system. F N represents the number of full-bridge submodules. H This represents the number of half-bridge sub-modules.

[0098] Phase 3: After unlocking the full-bridge and half-bridge submodules, controllable charging is performed, as follows: In this phase, both the full-bridge and half-bridge submodules are unlocked, and active circulating current injection is performed to charge the capacitor voltages of the full-bridge and half-bridge submodules to their rated voltage values ​​(preset thresholds); the preset threshold is U. dcN / (N F +N H ), where U dcN N is the rated DC voltage of the hybrid DC transmission system. F N represents the number of full-bridge submodules. H This represents the number of half-bridge sub-modules.

[0099] The above steps complete the charging of the MMC, making it ready for startup. Further coordination is needed to start the MMC-CCSC system, which includes the following stages:

[0100] Phase 4: The sending-end MMC adopts constant AC voltage and frequency control, and the circulating current suppression control is enabled (active circulating current injection is no longer activated) to establish the sending-end AC bus voltage;

[0101] Phase 5: The CCSC polarity begins to reverse, and the DC voltage command value changes from negative U at a certain rate. dc1 To the positive rated DC voltage command value U dcN (Rated DC voltage of the hybrid DC transmission system);

[0102] Phase 6: During the polarity reversal process, at the moment when the voltage changes from negative to positive (when the voltage is 0), the new energy power station is simultaneously unlocked and its power steadily increases.

[0103] The above steps enabled the startup of the MCC-CCSC system.

[0104] In this embodiment, to address the issue of the inability of the half-bridge submodule in the MMC to charge under negative pressure, a full-bridge active circulating current injection method was used to enable the charging and energy harvesting of the half-bridge submodule in the MMC. Without relying on additional equipment, the problem of pre-charging the submodule capacitor in the sending-end MMC converter station of the hybrid DC transmission system under 100% renewable energy transmission was solved, ensuring the reliable start-up of the MMC. By coordinating the control of the renewable energy power plant and the sending-receiving converter station, the system was able to start up quickly and orderly to the rated operating conditions, which is conducive to the stable transmission of renewable energy.

[0105] This application also provides a black-start device for a hybrid DC transmission system. It should be noted that the black-start device for a hybrid DC transmission system in this application can be used to execute the black-start method for a hybrid DC transmission system 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 performs 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.

[0106] The following describes the black start device for the hybrid DC transmission system provided in the embodiments of this application.

[0107] Figure 9 This is a structural block diagram of the black-start device of a hybrid DC transmission system according to an embodiment of this application. Figure 9 As shown, the device includes a first control unit 10, a first determination unit 20, a second control unit 30, and a second determination unit 40. The first control unit is used to control the receiving-end converter to change the polarity of the receiving-end DC voltage from positive to negative; the first determining unit is used to charge the full-bridge submodule of the sending-end converter when the polarity of the receiving-end DC voltage is negative, and to charge the half-bridge submodule of the sending-end converter using the full-bridge submodule, and to determine that the charging of the sending-end converter is complete when the energy storage capacitor voltage of the full-bridge submodule and the energy storage capacitor voltage of the half-bridge submodule reach a preset threshold; the second control unit is used to control the sending-end converter to generate the sending-end AC bus voltage after the charging of the sending-end converter is completed; the second determining unit is used to control the receiving-end converter to change the polarity of the receiving-end DC voltage from negative to positive after the sending-end converter generates the sending-end AC bus voltage, and to send a start signal to the new energy power station when the receiving-end DC voltage crosses zero, so that the output power of the new energy power station rises to the rated value at a predetermined rate, and to determine that the black start of the hybrid DC transmission system is complete when the output power of the new energy power station reaches the rated value.

[0108] This embodiment controls the receiving-end converter to reverse the polarity of the receiving-end DC voltage to negative without additional equipment. This negative voltage is then used to effectively charge the full-bridge and half-bridge sub-modules of the sending-end converter, overcoming the problem of charging the half-bridge sub-modules under negative voltage in traditional methods. After successful charging, the sending-end converter can establish a stable AC voltage. Subsequently, the polarity of the receiving-end converter's DC voltage is reversed back to positive, controlling the power of the renewable energy power station to smoothly climb to its rated value, ultimately achieving a smooth start-up of the hybrid DC system. This solves the problem of high black-start costs in hybrid DC transmission systems.

[0109] In specific implementation, the first determining unit includes a first charging module and a second charging module. The first charging module is used to perform uncontrolled charging on the full-bridge submodule of the sending-end converter when the polarity of the receiving-end DC voltage is negative, and to perform uncontrolled charging on the half-bridge submodule using the full-bridge submodule; the second charging module is used to perform controlled charging on the full-bridge submodule and the half-bridge submodule after performing the uncontrolled charging on the full-bridge submodule and the half-bridge submodule.

[0110] Further, the first charging module includes a first charging submodule and a second charging submodule. The first charging submodule is used to perform uncontrolled charging on the full-bridge submodule when the polarity of the receiving-end DC voltage is negative until the energy storage capacitor voltage of the full-bridge submodule reaches a first preset voltage threshold. The second charging submodule is used to set the reference value of the circulating current component in the circulating current suppression control to a non-zero value to form a circulating current in the full-bridge submodule, so that the full-bridge submodule performs uncontrolled charging on the half-bridge submodule until the energy storage capacitor voltage of the half-bridge submodule reaches a second preset voltage threshold. The first preset voltage threshold is set based on the receiving-end DC voltage and the number of full-bridge submodules, and the second preset voltage threshold is set based on the rated DC voltage of the hybrid DC transmission system, the number of full-bridge submodules, and the number of half-bridge submodules.

[0111] The key to the entire uncontrolled charging process lies in utilizing the polarity reversal of the DC voltage at the receiving end and the characteristics of the full-bridge submodule to charge the sending-end converter submodule. No additional starting power supply or equipment is required; relying solely on internal system coordination control—specifically, the negative DC voltage established by the receiving-end converter and the circulating current formed in the full-bridge submodule of the sending-end converter—effectively charges the energy storage capacitors of both the full-bridge and half-bridge submodules, thus providing the necessary power support for the system's black start. When the energy storage capacitor voltage of the full-bridge submodule reaches the first preset voltage threshold, the uncontrolled charging of the full-bridge submodule is complete. Similarly, when the energy storage capacitor voltage of the half-bridge submodule reaches the second preset voltage threshold, the uncontrolled charging of the half-bridge submodule is complete, and the system enters the next stage of start-up control.

[0112] Furthermore, the second charging module includes a third charging submodule and a determining submodule. The third charging submodule is used to adjust the reference value of the circulating current component in the circulating current suppression control to form a circulating current that meets the requirements for controlled charging after performing uncontrolled charging on the full-bridge submodule and the half-bridge submodule, and then use the circulating current to perform controlled charging on the full-bridge submodule and the half-bridge submodule. The determining submodule is used to determine that the charging of the sending-end converter is complete when the energy storage capacitor voltage of the full-bridge submodule and the energy storage capacitor voltage of the half-bridge submodule reach the preset threshold. The preset threshold is set based on the rated DC voltage of the hybrid DC transmission system, the number of full-bridge submodules, and the number of half-bridge submodules.

[0113] Through the above-mentioned controllable charging, not only can the voltage of the energy storage capacitors in the full-bridge and half-bridge submodules of the sending-end converter reach the preset threshold, but the charging process can also be optimized to avoid unnecessary energy loss or unstable states during charging. The implementation of controllable charging not only improves the energy conversion efficiency and stability during system startup, but also further enhances the energy management capability of the sending-end converter under passive startup conditions, which is conducive to the efficient and reliable access of new energy sources.

[0114] A two-stage charging strategy—first uncontrolled charging of the full-bridge and half-bridge submodules, then transitioning to controlled charging—ensures that the energy storage capacitors of both submodules reach preset thresholds without requiring additional start-up power, thus enabling reliable startup of the sending-end converter. The preset thresholds fully consider the impact of the rated DC voltage of the hybrid DC transmission system, the number of full-bridge and half-bridge submodules, making the charging process more precise and efficient. This charging mechanism, based on its own system resources, not only improves the startup flexibility and stability of the renewable energy access system but also significantly reduces reliance on auxiliary startup equipment, lowering system construction and operation costs, and providing technical support for hybrid DC transmission systems in 100% renewable energy transmission scenarios.

[0115] In some embodiments of this application, the first preset voltage threshold is a first proportional coefficient multiplied by the ratio of the receiving-end DC voltage to the number of full-bridge submodules, and the second preset voltage threshold is the ratio of the rated DC voltage of the hybrid DC transmission system to the total number of full-bridge submodules and half-bridge submodules, multiplied by a second proportional coefficient.

[0116] Setting the first and second preset voltage thresholds is crucial to ensuring the effective charging of the full-bridge and half-bridge submodules of the sending-end converter. These thresholds are calculated by fully considering the influence of the receiving-end DC voltage, the system's rated DC voltage, and the number of submodules, and are fine-tuned using proportional coefficients to adapt to the specific system design requirements. The process of achieving these two thresholds involves first uncontrolled charging of the full-bridge submodules using the negative polarity of the receiving-end DC voltage, followed by controlled charging of the half-bridge submodules using controlled circulating current injection, until their respective preset thresholds are met. This provides a reliable and efficient startup strategy for hybrid DC transmission systems, ensuring stable system operation.

[0117] In some embodiments of this application, the second control unit includes a generation module and a conversion module. The generation module is used to control the sending-end converter to generate a target AC voltage signal after charging of the sending-end converter is completed. The voltage amplitude of the target AC voltage signal is a preset voltage amplitude, and the frequency of the target AC voltage signal is a preset frequency. The conversion module is used to convert the target AC voltage signal into an actual AC voltage, which is the sending-end AC bus voltage.

[0118] After charging, the sending-end converter generates a target AC voltage signal using a constant AC voltage and frequency control strategy, and converts it into an actual AC voltage output, thus generating the sending-end AC bus voltage. The key to this process is that the sending-end converter, based on preset fixed amplitude and frequency parameters, uses precise internal control to stably output the required AC voltage. This enhances the system's self-starting capability without external power support, ensuring the smooth grid connection of renewable energy power plants, while reducing reliance on traditional starting power sources, lowering start-up costs, and improving the flexibility and reliability of renewable energy power transmission. By autonomously generating AC voltage through the sending-end converter, the entire hybrid DC transmission system can transition to normal operation more orderly and efficiently.

[0119] Furthermore, the above-mentioned device also includes a third control unit, which, after completing the charging of the sending-end converter and before controlling the sending-end converter to generate the sending-end AC bus voltage, sets the reference value of the circulating current component in the circulating current suppression control to zero so as to stop the formation of circulating current in the full-bridge submodule and the half-bridge submodule.

[0120] The cessation of circulating current formation ensures that the sending-end converter can convert DC to AC through an inversion process based on the target AC voltage signal in an environment free from circulating current interference, thereby generating a stable AC voltage at the sending end. This step improves the accuracy and stability of AC voltage establishment, avoids voltage fluctuations that may be caused by circulating current, and is crucial for the smooth black start process of the entire hybrid DC transmission system, ensuring energy management and control precision during the transition from startup to stable operation.

[0121] In some embodiments of this application, the first control unit includes a setting module and a control module. The setting module is used to set the DC voltage reference value of the receiving-end converter to a negative value; the control module is used to control the polarity of the receiving-end DC voltage of the receiving-end converter to change from positive to negative, until the actual measured value of the receiving-end DC voltage gradually decreases to a negative value, and then continues to decrease to the DC voltage reference value.

[0122] On the one hand, by setting the DC voltage reference value of the receiving-end converter to a negative value, the target direction of voltage polarity reversal is clearly given. On the other hand, this involves not only setting the DC voltage reference value but also continuously adjusting the actual value of the DC voltage of the receiving-end converter until it stabilizes at the negative DC voltage reference value, thereby achieving a polarity reversal from positive to negative. This effectively improves the flexibility and accuracy of receiving-end DC voltage control during the black start process of the hybrid DC transmission system, laying a solid foundation for the stable operation of the system and the reliable access of new energy power, ensuring the controllable reversal of the receiving-end DC voltage polarity, and enhancing the operational stability and efficiency optimization capabilities of the hybrid DC transmission system.

[0123] The black-start device of the aforementioned hybrid DC transmission system includes a processor and a memory. The first control unit, the first determining unit, the second control unit, and the second determining unit are all stored as program units in the memory. The processor executes the program units stored in the memory to achieve the corresponding functions. All of the above modules are located in the same processor; or, the above modules are located in different processors in any combination.

[0124] 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.

[0125] This invention provides a computer-readable storage medium including a stored program, wherein, when the program is executed, it controls the device containing the computer-readable storage medium to perform the hybrid DC transmission system black start method.

[0126] This invention provides a processor for running a program, wherein the program executes the aforementioned black-start method for a hybrid DC transmission system.

[0127] This invention provides an electronic 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 implements the steps of the aforementioned black-start method for a hybrid DC transmission system. The device described herein can be a server, PC, PAD, mobile phone, etc.

[0128] This application also provides a computer program product that, when executed on a data processing device, is adapted to perform the steps of initializing the above-described black-start method for a hybrid DC transmission system.

[0129] It is obvious to those skilled in the art that the modules or steps of the present invention 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 described 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 combination of hardware and software.

[0130] 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.

[0131] 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 1A device that provides the functions specified in one or more boxes.

[0132] 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.

[0133] 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.

[0134] In a typical configuration, a computing device includes one or more processors (CPU), input / output interfaces, network interfaces, and memory.

[0135] Memory may include non-persistent 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. Memory is an example of computer-readable media.

[0136] 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.

[0137] 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.

[0138] 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.

[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 black-start method for a hybrid DC transmission system, characterized in that, Each arm of the sending-end converter of the hybrid DC transmission system includes multiple full-bridge submodules and multiple half-bridge modules connected in series, including: The receiving-end converter is controlled to change the polarity of the receiving-end DC voltage from positive to negative; When the polarity of the DC voltage at the receiving end is negative, the full-bridge submodule of the sending-end converter is charged, and the half-bridge submodule of the sending-end converter is charged using the full-bridge submodule. When the energy storage capacitor voltage of the full-bridge submodule and the energy storage capacitor voltage of the half-bridge submodule reach a preset threshold, the charging of the sending-end converter is determined to be complete. After charging the sending-end converter is completed, the sending-end converter is controlled to generate the sending-end AC bus voltage. After the sending-end converter generates the sending-end AC bus voltage, the receiving-end converter is controlled to change the polarity of the receiving-end DC voltage from negative to positive. When the receiving-end DC voltage crosses zero, a start signal is sent to the new energy power station so that the output power of the new energy power station rises to the rated value at a predetermined rate. When the output power of the new energy power station reaches the rated value, the black start of the hybrid DC transmission system is determined to be complete.

2. The method according to claim 1, characterized in that, When the polarity of the receiving-end DC voltage is negative, the full-bridge submodule of the sending-end converter is charged, and the half-bridge submodule of the sending-end converter is charged using the full-bridge submodule, including: When the polarity of the DC voltage at the receiving end is negative, the full-bridge submodule of the sending-end converter is uncontrolled charged, and the half-bridge submodule is uncontrolled charged using the full-bridge submodule. After uncontrolled charging of the full-bridge submodule and the half-bridge submodule, controlled charging is performed on the full-bridge submodule and the half-bridge submodule.

3. The method according to claim 2, characterized in that, When the polarity of the receiving-end DC voltage is negative, uncontrolled charging is performed on the full-bridge submodule of the sending-end converter, and the half-bridge submodule is uncontrolled charged using the full-bridge submodule, including: When the polarity of the DC voltage at the receiving end is negative, the uncontrolled charging of the full-bridge submodule is performed until the energy storage capacitor voltage of the full-bridge submodule reaches the first preset voltage threshold. The reference value of the circulating current component in the circulating current suppression control is set to a non-zero value to form a circulating current in the full-bridge submodule, so that the full-bridge submodule performs the uncontrolled charging of the half-bridge submodule until the energy storage capacitor voltage of the half-bridge module reaches the second preset voltage threshold. The first preset voltage threshold is set based on the receiving-end DC voltage and the number of full-bridge submodules, while the second preset voltage threshold is set based on the rated DC voltage of the hybrid DC transmission system, the number of full-bridge submodules, and the number of half-bridge submodules.

4. The method according to claim 2, characterized in that, After performing uncontrolled charging on the full-bridge submodule and the half-bridge submodule, performing controlled charging on the full-bridge submodule and the half-bridge submodule includes: After performing uncontrolled charging on the full-bridge submodule and the half-bridge submodule, the reference value of the circulating current component in the circulating current suppression control is adjusted to form a circulating current that meets the requirements of controlled charging, and the circulating current is used to perform controlled charging on the full-bridge submodule and the half-bridge submodule. When the energy storage capacitor voltage of the full-bridge submodule and the energy storage capacitor voltage of the half-bridge submodule reach the preset threshold, it is determined that the charging of the sending-end converter is complete. The preset threshold is set based on the rated DC voltage of the hybrid DC transmission system, the number of full-bridge submodules, and the number of half-bridge submodules.

5. The method according to claim 3, characterized in that, The first preset voltage threshold is the ratio of the first proportional coefficient to the DC voltage at the receiving end and the number of the full-bridge submodules. The second preset voltage threshold is the ratio of the rated DC voltage of the hybrid DC transmission system to the total number of the full-bridge submodules and the half-bridge submodules, multiplied by the second proportional coefficient.

6. The method according to claim 1, characterized in that, After charging the sending-end converter is completed, controlling the sending-end converter to generate the sending-end AC bus voltage includes: When the charging of the sending-end converter is completed, the sending-end converter is controlled to generate a target AC voltage signal. The voltage amplitude of the target AC voltage signal is a preset voltage amplitude, and the frequency of the target AC voltage signal is a preset frequency. The target AC voltage signal is converted into an actual AC voltage, which is the AC bus voltage at the sending end.

7. The method according to claim 6, characterized in that, After charging the sending-end converter is completed, and before controlling the sending-end converter to generate the sending-end AC bus voltage, the method further includes: The reference value of the circulating component in the circulating suppression control is set to zero to stop the formation of circulating current in the full-bridge submodule and the half-bridge submodule.

8. The method according to claim 1, characterized in that, Controlling the receiving-end converter to change the polarity of the receiving-end DC voltage from positive to negative includes: Set the DC voltage reference value of the receiving-end converter to a negative value; The polarity of the DC voltage at the receiving end of the converter is controlled to change from positive to negative until the actual measured value of the DC voltage at the receiving end gradually decreases to a negative value, and then continues to decrease to the reference value of the DC voltage.

9. A hybrid DC transmission system, characterized in that, include: The sending end includes a sending end converter, wherein any arm of the sending end converter includes multiple full-bridge sub-modules and multiple half-bridge modules connected in series; The receiving end includes a receiving-end converter, wherein the switching device of the receiving-end converter is a fully controllable device, used to realize the change of the polarity of the receiving-end DC voltage; The control unit, which is communicatively connected to the sending-end converter and the receiving-end converter, is used to execute the black-start method of the hybrid DC transmission system according to any one of claims 1 to 8.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium includes a stored program, wherein, when the program is executed, it controls the device containing the computer-readable storage medium to perform the black-start method for a hybrid DC transmission system as described in any one of claims 1 to 8.

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