Hybrid direct current power transmission system black start method, power transmission system and storage medium

Abstract

The application provides a hybrid DC power transmission system black start method, a power transmission system and a storage medium. A receiving end converter is controlled to change the polarity of a receiving end DC voltage from positive to negative; in the case that the polarity of the receiving end DC voltage is negative, full-bridge sub-modules of a sending end converter are charged, and the full-bridge sub-modules are used to charge half-bridge sub-modules of the sending end converter, and the charging of the sending end converter is determined to be completed when the energy storage capacitor voltages of the full-bridge sub-modules and the half-bridge sub-modules reach a preset threshold; then, after the sending end converter generates a 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, a start signal is sent to a new energy field station at a zero-crossing point of the receiving end DC voltage, and in the case that the output power of the new energy field station reaches a rated value, the black start of the hybrid DC power transmission system is determined to be completed. The problem of high cost of the black start of the hybrid DC power transmission system is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of direct current transmission, in particular to a hybrid direct current transmission system black start method, a hybrid direct current transmission system and a computer readable storage medium. BACKGROUND

[0002] The multi-level converter (MMC) converter station under the 100% new energy sending scenario is passively started. In the actual operation process, the control and driving circuit of the IGBT is directly powered by the voltage division of the dispersed capacitors in each MMC sub-module. However, in the initial stage of starting, the initial voltage of the internal capacitor of the sub-module is zero. Therefore, the MMC lacks external energy and is in an uncontrolled blocking state. Before the MMC converter enters the steady state working mode, a suitable starting control strategy must be used to pre-charge the energy storage capacitors of these sub-modules to realize the starting of the MMC.

[0003] In view of the problem of passive starting of the sending end MMC of the hybrid direct current transmission, the existing MMC passive starting scheme needs to rely on additional devices, which increases the construction cost of the project. SUMMARY

[0004] The main purpose of the present application is to provide a hybrid direct current transmission system black start method, a hybrid direct current transmission system and a computer readable storage medium, so as to at least solve the problem of high cost of hybrid direct current transmission system black start in the prior art.

[0005] In order to achieve the above-mentioned purpose, according to one aspect of the present application, a hybrid direct current transmission system black start method is provided. Any bridge arm of the sending end converter of the hybrid direct current transmission system includes a plurality of full-bridge sub-modules and a plurality of half-bridge sub-modules connected in series. The method comprises: controlling the receiving end converter to change the polarity of the receiving end direct current voltage from positive to negative; in the case that the polarity of the receiving end direct current voltage is negative, charging the full-bridge sub-modules of the sending end converter, and charging the half-bridge sub-modules of the sending end converter by using the full-bridge sub-modules, and determining that the charging of the sending end converter is completed in the case that 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; after completing the charging of the sending end converter, controlling the sending end converter to generate a sending end alternating current bus voltage; after the sending end converter generates the sending end alternating current bus voltage, controlling the receiving end converter to change the polarity of the receiving end direct current voltage from negative to positive, sending a starting signal to the new energy field station at the zero crossing point of the receiving end direct current voltage, so that the output power of the new energy field station rises to a rated value at a predetermined rate, and determining that the black start of the hybrid direct current transmission system is completed in the case that the output power of the new energy field station reaches the rated value.

[0006] Optionally, in the case that the polarity of the receiving-end DC voltage is negative, the full-bridge sub-modules of the sending-end converter are charged, and the half-bridge sub-modules of the sending-end converter are charged by using the full-bridge sub-modules, comprising: in the case that the polarity of the receiving-end DC voltage is negative, the full-bridge sub-modules of the sending-end converter are uncontrolled charged, and the half-bridge sub-modules are uncontrolled charged by using the full-bridge sub-modules; after the uncontrolled charging of the full-bridge sub-modules and the half-bridge sub-modules, the full-bridge sub-modules and the half-bridge sub-modules are controlled charged.

[0007] Optionally, in the case that the polarity of the receiving-end DC voltage is negative, the full-bridge sub-modules of the sending-end converter are uncontrolled charged, and the half-bridge sub-modules are uncontrolled charged by using the full-bridge sub-modules, comprising: in the case that the polarity of the receiving-end DC voltage is negative, the full-bridge sub-modules are uncontrolled charged until the energy storage capacitor voltage of the full-bridge sub-modules 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 sub-modules, so that the full-bridge sub-modules uncontrolled charge the half-bridge sub-modules until the energy storage capacitor voltage of the half-bridge sub-modules 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 sub-modules, and the second preset voltage threshold is set based on the rated DC voltage of the hybrid DC power transmission system, the number of full-bridge sub-modules and the number of half-bridge sub-modules.

[0008] Optionally, after the uncontrolled charging of the full-bridge sub-modules and the half-bridge sub-modules, the full-bridge sub-modules and the half-bridge sub-modules are controlled charged, comprising: after the uncontrolled charging of the full-bridge sub-modules and the half-bridge sub-modules, the reference value of the circulating current component in the circulating current suppression control is adjusted to form a circulating current that meets the controllable charging requirement, and the full-bridge sub-modules and the half-bridge sub-modules are controlled charged by using the circulating current; in the case that the energy storage capacitor voltage of the full-bridge sub-modules and the energy storage capacitor voltage of the half-bridge sub-modules reach the preset threshold, it is determined that the charging of the sending-end converter is completed, and the preset threshold is set based on the rated DC voltage of the hybrid DC power transmission system, the number of full-bridge sub-modules and the number of half-bridge sub-modules.

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

[0010] Optionally, after the charging of the sending-end converter is completed, the sending-end converter is controlled to generate a sending-end AC bus voltage, including: in the case that the charging of the sending-end converter is completed, the sending-end converter is controlled to generate a target AC voltage signal, a voltage amplitude of the target AC voltage signal being a preset voltage amplitude, and a frequency of the target AC voltage signal being a preset frequency; and the target AC voltage signal is converted into an actual AC voltage, the actual AC voltage being the sending-end AC bus voltage.

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

[0012] Optionally, the control of the receiving-end converter to change the polarity of the receiving-end DC voltage from positive to negative includes: setting a DC voltage reference value of the receiving-end converter to a negative value; and 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 continues to decrease to the DC voltage reference value.

[0013] According to another aspect of the present application, there is provided a hybrid DC power transmission system, including a sending end, a receiving end, and a control unit. The sending end includes a sending-end converter, any bridge arm of the sending-end converter including a plurality of full-bridge sub-modules and a plurality of half-bridge sub-modules connected in series; the receiving end includes a receiving-end converter, a switching device of the receiving-end converter being a reverse blocking IGBT device or a reverse blocking IGCT device, for realizing change of polarity of a receiving-end DC voltage; and the control unit is in communication connection with the sending-end converter and the receiving-end converter, and is configured to execute any of the hybrid DC power transmission system black start methods.

[0014] According to still another aspect of the present application, there is provided a computer readable storage medium, including a stored program, wherein the program, when executed, controls a device in which the computer readable storage medium is located to execute any of the hybrid DC power transmission system black start methods.

[0015] With the technical scheme, any bridge arm of the sending-end converter of the hybrid DC power transmission system comprises a plurality of full-bridge sub-modules and a plurality of half-bridge sub-modules connected in series, the receiving-end converter is controlled to change the polarity of the receiving-end DC voltage from positive to negative, the full-bridge sub-modules of the sending-end converter are charged in the case that the polarity of the receiving-end DC voltage is negative, the half-bridge sub-modules of the sending-end converter are charged by the full-bridge sub-modules, and the charging of the sending-end converter is determined to be completed in the case that 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, the sending-end converter generates the sending-end AC bus voltage after the charging of the sending-end converter is completed, the polarity of the receiving-end DC voltage is changed from negative to positive by the receiving-end converter after the sending-end converter generates the sending-end AC bus voltage, a start signal is sent to the new energy station at the zero-crossing point of the receiving-end DC voltage, so that the output power of the new energy station rises to the rated value at a predetermined rate, and the black start of the hybrid DC power transmission system is determined to be completed in the case that the output power of the new energy station reaches the rated value. In the scheme, the polarity of the receiving-end DC voltage is reversed to negative by controlling the receiving-end converter without additional equipment, and then the full-bridge and half-bridge sub-modules of the sending-end converter are effectively charged by the negative voltage, thereby overcoming the problem that the half-bridge sub-modules are difficult to be charged under negative voltage in the traditional method. After the charging is successful, the sending-end converter can generate stable AC voltage, and then the polarity of the receiving-end DC voltage is reversed to positive again, the new energy station is unlocked, the power of the new energy station is controlled to smoothly rise to the rated value, and finally the hybrid DC system is started smoothly, thereby solving the problem of high cost of black start of the hybrid DC power transmission system. BRIEF DESCRIPTION OF DRAWINGS

[0016] The drawings constituting a part of the specification of the present application are used to provide further understanding of the present application, the illustrative embodiments of the present application and the description thereof are used to explain the present application, and do not constitute improper limitation on the present application. In the drawings:

[0017] Figure 1 A hardware structure block diagram of a mobile terminal for executing a hybrid DC power transmission system black start method is shown according to an embodiment of the present application;

[0018] Figure 2 A flowchart of a hybrid DC power transmission system black start method is shown according to an embodiment of the present application;

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

[0020] Figure 4A MMC circulating current suppression controller block diagram of a hybrid DC power transmission system black start method provided by the embodiment of the application is shown;

[0021] Figure 5 A hybrid DC power transmission system topology of a hybrid DC power transmission system provided by the embodiment of the application is shown;

[0022] Figure 6 A receiving end CCSC converter topology structure diagram of a hybrid DC power transmission system provided by the embodiment of the application is shown;

[0023] Figure 7 An MMC uncontrolled charging principle diagram of a hybrid DC power transmission system provided by the embodiment of the application is shown;

[0024] Figure 8 A hybrid DC power transmission system start flow chart provided by the embodiment of the application is shown;

[0025] Figure 9 A structure block diagram of a hybrid DC power transmission system black start device provided by the embodiment of the application is shown.

[0026] Among them, the above-mentioned drawings include the following reference signs:

[0027] 102, processor; 104, memory; 106, transmission device; 108, input and output device. DETAILED DESCRIPTION

[0028] It should be noted that the embodiments and features in the embodiments of the present application can be combined with each other without conflict. The technical solutions in the embodiments of the present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0029] In order for those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should be 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 software programs of application software and modules, such as a computer program corresponding to the black start method of the HVDC system in the embodiments of the present application. The processor 102 can execute various functional applications and data processing, i.e., implement the method described above, by running the computer program stored in the memory 104. The memory 104 can include a high-speed random access memory, and can further include a non-volatile memory, such as one or more magnetic storage devices, flash memories, or other non-volatile solid-state memories. In some examples, the memory 104 can further include memories remotely arranged with respect to the processor 102, which can be connected to the mobile terminal through a network. Examples of the network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and a combination thereof. The transmission device 106 is used to receive or send data via a network. The specific examples of the network can include a wireless network provided by a communication provider of the mobile terminal. In one example, the transmission device 106 includes a network adapter (NIC), which can be connected to other network devices through a base station so as to communicate with the Internet. In one example, the transmission device 106 can be a radio frequency (RF) module, which is used to communicate with the Internet in a wireless manner.

[0035] In the embodiments, a black start method of a HVDC system running on a mobile terminal, a computer terminal or a similar computing device is provided. It should be noted that the steps shown in the flowchart of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and although a logical order is shown in the flowchart, in some cases, the steps shown or described herein can be executed in an order different from that shown.

[0036] Figure 2 is a flowchart of the black start method of the HVDC system according to the embodiments of the present application. As shown in Figure 2 , the method includes the following steps:

[0037] In step S201, the receiving end converter changes the polarity of the receiving end DC voltage from positive to negative.

[0038] Specifically, the receiving end converter in the embodiments is a fully controlled current source converter (CCSC), and the sending end converter is a multi-level 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 necessary charging energy for the sub-module capacitor of the sending end converter MMC, especially to realize the start of the sending end converter, i.e., the black start, without external auxiliary power supply.

[0039] Specifically, in the scenario of 100% new energy sending out, the sending end is a passive end, and the sending end MMC needs to be started up without the support of the traditional alternating current grid. Because the half-bridge sub-module of the MMC can only be charged under forward current, the sending end MMC cannot generate forward current under passive starting conditions. In this case, we use the unique characteristics of the receiving end CCSC to solve the charging problem. As a current source converter, the CCSC has the ability to independently control active and reactive power and maintain the stability of the DC current. In this embodiment, the CCSC outputs a negative voltage to enable the full-bridge sub-module to charge in the blocked state, because the structure of the full-bridge sub-module allows it to charge when the voltage is negative. After the full-bridge sub-module is fully charged and unlocked, active circulating current (containing forward current) is injected to provide charging conditions for the half-bridge sub-module.

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

[0041] In step S202, when the polarity of the DC voltage at the receiving end is negative, the full-bridge sub-module of the sending end converter is charged, and the half-bridge sub-module of the sending end converter is charged using the full-bridge sub-module. 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 predetermined threshold, it is determined that the charging of the sending end converter is complete.

[0042] Specifically, when the DC voltage polarity of the receiving end converter is set to a negative value, it provides conditions for charging the full-bridge sub-module capacitors of the sending end converter. After the full-bridge is charged, the half-bridge is charged using the circulating current injection method. In a hybrid DC transmission system, each bridge arm of the sending end MMC converter contains multiple full-bridge sub-modules and half-bridge sub-modules connected in series, which have energy storage capacitors inside to control the voltage level of the sub-modules and thus control the output voltage of the entire converter. When the energy storage capacitor voltages of the full-bridge sub-module and the half-bridge sub-module both reach a predetermined threshold, it is determined that the charging of the sending end MMC converter is complete, and the next stage of control is entered.

[0043] The full-bridge sub-module assists the half-bridge sub-module in charging, solving the problem of the half-bridge sub-module being unable to charge when the polarity of the DC voltage at the receiving end is negative. When the energy storage capacitor voltages of the full-bridge sub-module and the half-bridge sub-module both reach a predetermined threshold, it is determined that the charging process of the sending end converter is complete.

[0044] In summary, under the condition that the polarity of the receiving-end DC voltage is negative, the full-bridge sub-modules are first charged, and then the half-bridge sub-modules are charged by using the full-bridge sub-modules. Not only the charging of the full-bridge sub-modules is realized, but also the charging of the half-bridge sub-modules is solved, ensuring that the sending-end converter can complete the charging without external power support, which is helpful for realizing the efficient start and stable operation of the hybrid DC power transmission system. This charging method does not require additional charging equipment, reduces the system cost, and improves the reliability of new energy sending. It is especially suitable for the scene of 100% new energy sending, reduces the dependence on traditional starting power, reduces the system cost, and ensures the starting efficiency of the system and the stable sending of new energy.

[0045] Step S203, after completing the charging of the sending-end converter, the sending-end converter generates a sending-end AC bus voltage;

[0046] Specifically, after completing the charging of the sending-end converter, the sending-end converter generates its AC voltage output, which is crucial for the start and stable operation of the hybrid DC power transmission system. When the energy storage capacitors in the full-bridge sub-modules and the half-bridge sub-modules of the sending-end converter MMC complete charging and reach the preset threshold voltage, the sending-end converter has the necessary conditions for starting. At this time, the control strategy of the sending-end converter will be converted from the charging mode to the constant AC voltage and frequency control mode to establish a stable AC voltage output, i.e., the sending-end AC bus voltage, which will be used to support the voltage of the sending-end AC system and provide the necessary electrical conditions for the subsequent power transmission between the new energy station and the sending-end converter.

[0047] Generating a stable sending-end AC bus voltage is very important for the start and normal operation of the hybrid DC power transmission system. Not only does it ensure that the converter can work normally, but it also provides the necessary voltage support for the synchronous start of the receiving-end converter and the entire power transmission system, which is a key step to ensure the successful black start and subsequent stable operation of the hybrid DC power transmission system.

[0048] Step S204, after the sending-end converter generates the sending-end AC bus voltage, the polarity of the receiving-end DC voltage is changed from negative to positive, and a start signal is sent to the new energy station at the zero-crossing point of the receiving-end DC voltage, so that the output power of the new energy station rises to the rated value at a predetermined rate, and in the case that the output power of the new energy station reaches the rated value, it is determined that the black start of the hybrid DC power transmission system is completed.

[0049] Specifically, after the sending-end AC bus voltage is established by the sending-end converter, the receiving-end converter is then controlled to change the polarity of the receiving-end DC voltage from negative to positive. When the polarity of the receiving-end DC voltage is reversed and the output power of the new energy station steadily increases to its rated value, the black start process of the entire hybrid DC power transmission system is completed. Specifically, after the sending-end converter has been able to output stable AC voltage, it means that the new energy can be reliably connected to the grid, and the sending-end converter has the ability to receive current and transmit power, at which time the system has the condition to reverse the polarity of the receiving-end converter. The receiving-end converter adjusts the internal control strategy to change the polarity of its DC side voltage from negative to positive. The purpose of polarity reversal is to restore the normal power flow, i.e. from the sending-end converter to the receiving-end converter through the DC line, so that the system can operate normally.

[0050] During the polarity reversal process, especially when the receiving-end DC voltage approaches zero from a negative value and is ready to jump to a positive value, the new energy station (such as a wind farm or a photovoltaic power station) will be unlocked synchronously, starting the climbing process of its output power. It is ensured that at the moment when the polarity of the receiving-end DC voltage is just reversed and the system is about to re-enter the normal operating state, the new energy station can respond in time and start to smoothly increase its output power. With the gradual increase of the output power of the new energy station, its power output will be monitored. When the output power of the new energy station reaches the rated value, it indicates that the system has been able to bear the predetermined power load, and the power transmission between the sending-end and receiving-end converters has returned to the normal working state. At this time, the black start process of the entire hybrid DC power transmission system is completed.

[0051] When the sending-end converter establishes its AC voltage output, the polarity of the DC voltage of the receiving-end converter is reversed from negative to positive by control, and the new energy station gradually increases to its rated output power, realizing the completion of the black start of the system. This process can realize the self-starting and stable operation of the entire power transmission system without relying on the external AC grid, but only through the interaction between the converters in the system and the active response of the new energy station. The polarity reversal of the receiving-end converter provides a positive DC voltage for the sending-end converter, restores the normal power transmission direction of the system, and the gradual increase of the power of the new energy station to the rated value ensures that the system can bear the predetermined load, finally realizes the black start of the hybrid DC power transmission system, improves the flexibility and reliability of the new energy connected to the grid, and avoids the use of additional starting equipment, reduces the operation cost of the system.

[0052] By the embodiment, the control receiving-end converter realizes the polarity reversal of the receiving-end DC voltage to negative without additional equipment, and then effectively charges the full-bridge and half-bridge sub-modules of the sending-end converter with the negative voltage, thereby overcoming the problem that the half-bridge sub-modules are difficult to be charged under negative voltage in the traditional method. After successful charging, the sending-end converter can establish a stable AC voltage, and then reverses the polarity of the DC voltage of the receiving-end converter to positive again, controls the power of the new energy station to smoothly climb to the rated value, and finally realizes the smooth start of the hybrid DC system, thereby solving the problem of high cost of black start of the hybrid DC transmission system.

[0053] In the specific implementation process, under the condition that the polarity of the receiving-end DC voltage is negative, the full-bridge sub-modules of the sending-end converter are charged, and the half-bridge sub-modules of the sending-end converter are charged by using the full-bridge sub-modules, comprising: under the condition that the polarity of the receiving-end DC voltage is negative, the full-bridge sub-modules of the sending-end converter are uncontrolled charged, and the half-bridge sub-modules are uncontrolled charged by using the full-bridge sub-modules; after the uncontrolled charging of the full-bridge sub-modules and the half-bridge sub-modules, the full-bridge sub-modules and the half-bridge sub-modules are controlled charged.

[0054] The above describes the specific steps of charging the full-bridge sub-modules and the half-bridge sub-modules of the sending-end converter in the black start process of the hybrid DC transmission system, and the judgment condition of completing the charging. Specifically, the charging is divided into two stages of uncontrolled charging and controlled charging, which ensures that the energy storage capacitor voltage of the full-bridge sub-modules and the half-bridge sub-modules can reach the preset threshold value required for system start, thereby realizing the start preparation of the sending-end converter.

[0055] Further, under the condition that the polarity of the receiving-end DC voltage is negative, the full-bridge sub-modules of the sending-end converter are uncontrolled charged, and the half-bridge sub-modules are uncontrolled charged by using the full-bridge sub-modules, comprising: under the condition that the polarity of the receiving-end DC voltage is negative, the full-bridge sub-modules are uncontrolled charged until the energy storage capacitor voltage of the full-bridge sub-modules 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 circulating current in the full-bridge sub-modules, so that the full-bridge sub-modules uncontrolled charge the half-bridge sub-modules until the energy storage capacitor voltage of the half-bridge sub-modules 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 sub-modules, 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 sub-modules and the number of half-bridge sub-modules.

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

[0057] In the case where the polarity of the receiving end DC voltage is negative, the first step is to uncontrolled charge the full-bridge sub-modules. In this stage, the full-bridge sub-modules (consisting of four IGBTs and capable of bearing reverse voltage) will automatically charge until the energy storage capacitor voltage reaches a first preset voltage threshold. The first preset voltage threshold is set based on the polarity of the receiving end DC voltage and the number of full-bridge sub-modules, ensuring that the full-bridge sub-modules can store enough energy to support the subsequent steps.

[0058] After the energy storage capacitor voltage of the full-bridge sub-modules 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 circulating current in the full-bridge sub-modules, so that the full-bridge sub-modules uncontrolled charge the half-bridge sub-modules. This circulating current contains forward and reverse alternating components, when the current direction is positive, the half-bridge sub-modules (consisting of two IGBTs and one capacitor, and only capable of charging under forward voltage) can start charging. Through this process, the energy storage capacitor voltage of the half-bridge sub-modules will gradually rise until it reaches a second preset voltage threshold. The second preset voltage threshold is set according to the rated DC voltage of the hybrid DC power transmission system, the number of full-bridge sub-modules and the number of half-bridge sub-modules, and is used to ensure that the half-bridge sub-modules can also store enough energy to prepare for system startup.

[0059] Next, the principle of generating circulating current in the sending end converter MMC is described. The single-phase circuit equivalent diagram of the sending end converter MMC is shown in Figure 3 . a and u a represent the a-phase AC current and voltage, respectively, O' and O points represent the neutral points on both sides, L arm and R arm are the bridge arm inductance and resistance, respectively, u pa is the upper bridge arm voltage of the a-phase, u na is the lower bridge arm voltage of the a-phase, and U dc is the DC line voltage.

[0060] The relationship between the a-phase output current and the bridge arm current is , wherein i pa is the upper bridge arm current of the a-phase, and i na is the lower bridge arm current of the a-phase. Considering the circulating current caused by inter-phase voltage imbalance, the circulating current is defined as .

[0061] Regarding the MMC circulating current suppression control of the sending converter, the different sub-module capacitor voltages at the same time will cause unbalanced current between the bridge arms. The unbalanced current includes a DC component and an AC component, and the DC component will distort the current flowing between the bridge arms, affecting the service life of the switch tube, so the circulating current between the bridge arms needs to be suppressed.

[0062] Since the PI controller can achieve zero static error control of the current component, the MMC circulating current suppression controller block diagram based on PI control can be obtained, as shown in Figure 4 i cird i cirq , i cird_ref , i cirq_ref respectively represent the d-axis and q-axis components of the three-phase circulating current, i cird , i cirq are the modulation signal components output after the d-axis and q-axis component control of the circulating current, ω is the angular frequency of the power grid, and L is the bridge arm inductance. When the steady-state operation is required, the circulating current needs to be suppressed, at which time i cird_ref = 0, i cirq_ref = 0, and the circulating current is 0, i.e. the circulating current suppression effect is achieved. When the circulating current is needed, i cird_ref and i cirq_ref are set to non-zero values, such as 0.1, at which time the circulating current will flow in the three bridge arms, which is also the way to realize the subsequent charging.

[0063] The key to the entire uncontrolled charging process is to use the polarity reversal of the receiving end DC voltage and the characteristics of the full-bridge sub-module to charge the sending converter sub-module. Without additional starting power or equipment, only relying on the internal coordination control of the system, through the negative DC voltage established by the receiving converter and the circulating current formed in the full-bridge sub-module of the sending converter, the charging of the energy storage capacitor of the full-bridge sub-module and the half-bridge sub-module can be effectively completed, thereby providing necessary power support for the black start of the system. When the energy storage capacitor voltage of the full-bridge sub-module reaches the first preset voltage threshold, the uncontrolled charging of the full-bridge sub-module is completed, and when the energy storage capacitor voltage of the half-bridge sub-module reaches the second preset voltage threshold, the uncontrolled charging of the half-bridge sub-module is completed, and the next stage of starting control is entered.

[0064] Further, after the uncontrolled charging of the full bridge sub-modules and the half bridge sub-modules, controlled charging is performed on the full bridge sub-modules and the half bridge sub-modules, including: after the uncontrolled charging of the full bridge sub-modules and the half bridge sub-modules, adjusting the reference value of the circulating current component in circulating current suppression control to form a circulating current that meets the demand of controlled charging, and using the circulating current to perform the controlled charging on the full bridge sub-modules and the half bridge sub-modules; in the case that the energy storage capacitor voltage of the full bridge sub-modules and the energy storage capacitor voltage of the half bridge sub-modules reach the preset threshold value, determining that the charging of the sending end converter is completed, and the preset threshold value is set based on the rated DC voltage of the hybrid DC power transmission system, the number of the full bridge sub-modules and the number of the half bridge sub-modules.

[0065] After the uncontrolled charging of the full bridge sub-modules and the half bridge sub-modules is completed, controlled charging is performed to ensure that the energy storage capacitor voltage of the full bridge sub-modules and the half bridge sub-modules reaches the rated value required for stable operation of the system. Specifically, after the uncontrolled charging phase, that is, when the energy storage capacitor voltage of the full bridge sub-modules and the half bridge sub-modules reaches the first preset voltage threshold and the second preset voltage threshold respectively, but has not yet reached the capacitor voltage value required for rated operation, the controlled charging phase is entered. The goal of this phase is to further improve the energy storage voltage of the full bridge sub-modules and the half bridge sub-module capacitors to the preset threshold value through a control strategy, to ensure that the sending end converter can reliably enter the steady state operation mode.

[0066] In the controllable charging phase, the reference value of the circulating current component in the circulating current suppression control will be reset. Unlike the non-zero value in the uncontrolled charging phase, the circulating current component reference value here will be adjusted to a specific value that meets the controllable charging requirements to form an appropriate alternating circulating current. The generation of this circulating current is controllable, and its waveform and strength are designed to meet the controllable charging requirements of the full-bridge submodules and half-bridge submodules. During the startup phase of the hybrid DC transmission system, through precise control of the circulating current component, a current is formed that can effectively circulate between the full-bridge submodules and half-bridge submodules, i.e., a circulating current that meets the controllable charging requirements. The characteristics of this circulating current are optimized to ensure that the submodule capacitors are uniformly charged to the preset threshold without being affected by external energy sources, thereby achieving a smooth startup of the sending-end MMC converter. By dynamically adjusting the strength and direction of the circulating current, the charging requirements can be accurately matched while maintaining operational stability and avoiding unnecessary energy loss. Specifically, the circulating current that meets the controllable charging requirements refers to a specific circulating current formed within the bridge arm during the charging process of the MMC, which is precisely controlled. The size, direction, and frequency of this circulating current can be dynamically adjusted to change in real time according to the charging progress and the state of the submodules, ensuring that the capacitors of each submodule are smoothly charged within the preset voltage threshold range. The alternating component of the circulating current is optimized to maximize the balanced rise of the full-bridge and half-bridge submodule capacitor voltages. This means that during the charging process, both full-bridge and half-bridge submodules can obtain appropriate and balanced charging current, avoiding unstable capacitor voltages caused by uneven charging. Considering the differences in charging mechanisms between full-bridge submodules and half-bridge submodules, the circulating current is designed to effectively charge both full-bridge submodules and, through changes in its alternating component, to achieve controllable charging of 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, and once it is found that the energy storage capacitor voltages of the full-bridge submodules and half-bridge submodules approach the preset threshold, the circulating current characteristics will be adjusted in a timely manner until the energy storage capacitor voltages of all full-bridge submodules 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 controllable charging in the full-bridge submodules, while the negative current component will form controllable charging in the half-bridge submodules. This process is precisely controlled by the control logic of the converter to ensure that the capacitor voltage of each submodule can be stably increased in a controllable state. In the controllable charging phase, the full-bridge submodules and half-bridge submodules work together to charge through the formed circulating current. This cooperative charging mechanism ensures that all full-bridge submodules 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] By the above controllable charging, not only can the full-bridge sub-modules and the half-bridge sub-modules of the sending-end converter ensure that the energy storage capacitor voltage reaches the preset threshold, but also can optimize the charging process and avoid unnecessary energy loss or unstable state in the charging process. The implementation of controllable charging not only improves the energy conversion efficiency and stability in the system starting process, but also further enhances the energy management capability of the sending-end converter under the passive starting condition, which is helpful for efficient and reliable access of new energy.

[0068] During the controllable charging phase, the energy storage capacitor voltages of the full-bridge sub-modules and the half-bridge sub-modules are continuously monitored. When it is detected that the energy storage capacitor voltages of the full-bridge sub-modules and the half-bridge sub-modules both reach the preset threshold, it is confirmed that the charging of the sending-end converter is completed. The preset threshold is set considering the rated DC voltage of the hybrid DC power transmission system, the number of full-bridge sub-modules and the number of half-bridge sub-modules, so as to ensure that the sub-module capacitor voltage can meet the conditions of starting and normal operation.

[0069] By the two-stage charging strategy, i.e. first uncontrolled charging of the full-bridge sub-modules and the half-bridge sub-modules, and then entering the controllable charging phase, it is ensured that the energy storage capacitors of the full-bridge sub-modules and the half-bridge sub-modules can reach the preset threshold without the need for additional starting power, thereby realizing reliable starting of the sending-end converter. The setting of the preset threshold fully considers the influence of the rated DC voltage of the hybrid DC power transmission system, the number of full-bridge sub-modules and the number of half-bridge sub-modules, so that the charging process is more accurate and efficient. This charging mechanism based on the system resources itself not only improves the starting flexibility and stability of the new energy access system, but also significantly reduces the dependence on auxiliary starting equipment, reduces the system construction and operation cost, and provides technical support for the hybrid DC power transmission system under the 100% new energy sending scenario.

[0070] In some embodiments of the present application, the first preset voltage threshold is a ratio of the above receiving-end DC voltage and the number of the full-bridge sub-modules multiplied by a first proportionality coefficient, and the second preset voltage threshold is a ratio of the rated DC voltage of the hybrid DC power transmission system and the total number of the full-bridge sub-modules and the half-bridge sub-modules multiplied by a second proportionality coefficient.

[0071] Specifically, the first preset voltage threshold can be calculated by the formula U dc1 / a1×N F , wherein U dc1 is the receiving-end DC voltage, N F is the number of full-bridge sub-modules, and a1 is the first proportionality coefficient, which is set to 2 in the present embodiment. The second preset voltage threshold can be calculated by the formula [U dcN / (N F +N H )]×a2, wherein U dcNN is the number of full-bridge submodules F N is the number of full-bridge submodules H N is the number of half-bridge submodules, a2 is a second proportional coefficient, and in the embodiment, the second proportional coefficient can be set to × 25%.

[0072] The setting of the first preset voltage threshold and the second preset voltage threshold is the key to ensuring that the full-bridge submodules and the half-bridge submodules of the sending converter can be effectively charged. The calculation of these thresholds fully considers the influence of the receiving end DC voltage, the system rated DC voltage, and the number of submodules, and simultaneously fine-tunes through a proportional coefficient to adapt to the design requirements of a specific system. The process of realizing the two thresholds first performs uncontrolled charging for the full-bridge submodules through the negative polarity of the receiving end DC voltage, and then performs controllable charging for the half-bridge submodules through controllable circulating current injection until the respective preset thresholds are met, which provides a reliable and efficient starting strategy for the hybrid DC power transmission system and guarantees the stable operation of the system.

[0073] In some embodiments of the present application, after the charging of the sending converter is completed, the sending converter generates a sending AC bus voltage, including: in the case where the charging of the sending converter is completed, the sending converter generates 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; and the target AC voltage signal is converted into an actual AC voltage, and the actual AC voltage is the sending AC bus voltage.

[0074] The above illustrates how to generate the sending end AC bus voltage after completing the charging of the sending end converter, thereby further promoting the black start process of the entire hybrid DC power transmission system. Specifically, after the full-bridge sub-modules and half-bridge sub-modules of the sending end converter are charged and the voltages of the respective energy storage capacitors reach the preset threshold, the stage of controlling the sending end converter to establish an AC voltage is entered. In this stage, a preset AC voltage and frequency control strategy is used to generate a target AC voltage signal. That is, the sending end converter will generate a stable AC voltage output signal according to the preset voltage amplitude and preset frequency. The setting of the preset voltage amplitude and preset frequency takes into account the system design requirements, grid standards and output characteristics of the new energy station, ensuring that the generated AC voltage signal can meet the conditions for subsequent stable operation of the system. The target AC voltage signal generated by the above control strategy will guide the switching action of each sub-module inside the sending end converter to produce the desired AC. This process involves converting DC power into AC power through inversion, and the target AC voltage signal provides an accurate reference for the inversion process, ensuring that the output AC voltage has a stable and required amplitude and frequency. Finally, the sending end converter converts the target AC voltage signal into an actual AC voltage, thereby generating a sending end AC bus voltage at the sending end. The generation of this sending end AC bus voltage not only provides the grid reference required for grid connection of the new energy station, but also further promotes the start-up process of the entire hybrid DC power transmission system, providing conditions for subsequent steps such as polarity reversal of the receiving end converter and power ramp-up of the new energy station.

[0075] After the sending end converter is charged, the target AC voltage signal is generated by using the fixed AC voltage and frequency control strategy, and is converted into an actual AC voltage, thereby generating a sending end AC bus voltage at the sending end. The key to this process is that the sending end converter stabilizes the required AC voltage output through internal precise control according to the preset fixed amplitude and frequency parameters. This improves the self-starting capability of the system without external power support, ensures the smooth progress of the grid connection process of the new energy station, reduces the dependence on traditional starting power, reduces the starting cost, and improves the flexibility and reliability of new energy power transmission. By generating the AC voltage autonomously through the sending end converter, the entire hybrid DC power transmission system can transition to normal operation more orderly and efficiently.

[0076] Further, after completing the charging of the above sending end converter and before controlling the above sending end converter to generate a 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 circulating current from being formed in the full-bridge sub-modules and the half-bridge sub-modules.

[0077] Specifically, after the completion of the charging of the sending converter, and before the sending converter is controlled to generate the sending AC bus voltage, the reference value of the circulating current component in the circulating current suppression control is set to zero to stop the circulating current from being formed in the full-bridge sub-modules and the half-bridge sub-modules. This step is intended to ensure that the operating state inside the sending converter has reached stability when entering the next stage, i.e., the stage of generating the sending AC bus voltage, so as to avoid the interference of the circulating current on the process of generating the AC voltage. During the controllable charging stage, the full-bridge sub-modules and the half-bridge sub-modules are charged by forming the circulating current, so as to reach the required energy storage capacitor voltage. When the charging is completed, and before the sending converter is prepared to generate the sending AC bus voltage by using the fixed AC voltage and frequency control strategy, the full-bridge sub-modules and the half-bridge sub-modules are controlled to stop forming the circulating current, which means that the circulating current control inside the sending converter will be turned off, and the full-bridge sub-modules and the half-bridge sub-modules enter the preparation state, in preparation for the subsequent generation of the AC voltage.

[0078] The operation of stopping the circulating current ensures that the sending converter can convert the direct current into alternating current according to the target AC voltage signal by the inversion process in an environment free of the interference of the circulating current, so as to generate a stable AC voltage at the sending end. This step improves the accuracy and stability of the establishment of the AC voltage, avoids the voltage fluctuation caused by the circulating current, and is very important for the smooth performance of the entire hybrid DC power transmission system black start process, and ensures the energy management and control accuracy during the transition from the start to the stable operation state of the system.

[0079] In still another embodiment of the present application, the control of the polarity of the receiving DC voltage of the receiving converter from positive to negative comprises: setting the DC voltage reference value of the receiving converter to a negative value; and controlling the polarity of the receiving DC voltage of the receiving converter from positive to negative, until the actual measured value of the receiving DC voltage gradually decreases to a negative value, and then continues to decrease to the DC voltage reference value.

[0080] Specifically, the DC voltage reference value of the receiving converter is set to a negative value. This setting indicates that the control mechanism inside the receiving converter should adjust the direction of the output voltage from positive to negative to meet the specific operating requirements of the system. Setting the DC voltage reference value to a negative value is the primary condition for realizing the polarity reversal of the receiving DC voltage. The polarity reversal is realized by setting the DC voltage reference value to a negative value, controlling the polarity of the receiving DC voltage of the receiving converter from positive to negative, and if the receiving DC voltage becomes negative (not necessarily the reference value), then the polarity has changed from positive to negative at this time, the polarity reversal is realized, and the actual measured value of the receiving DC voltage changes further until the DC voltage reference value is reached, at which time the polarity reversal is completed. The polarity reversal is a gradual process, and the voltage decreases from positive to pass through 0, and then becomes negative, and is finally stabilized 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 the present application, after sending the start signal to the new energy station, the above method further comprises: monitoring the power transmission state of the receiving end DC line and the power change of the new energy station; in the case where the change range of the power of the new energy station is detected to exceed the preset change range, calculating the voltage adjustment amount required for compensation; and adjusting the AC voltage instruction value of the sending end converter based on the voltage adjustment amount.

[0089] Specifically, the power transmission state of the receiving end DC line and the power output change of the new energy station are monitored. This monitoring can be achieved by various sensors and data acquisition devices installed in the system, which can collect power transmission data and power output information of the new energy station in real time. A preset change range is set, and the system needs to respond when the power change of the new energy station exceeds this preset range. The setting of the preset change range is based on the comprehensive consideration of system stability and energy transmission efficiency, and is usually a safety threshold. Exceeding this threshold may cause disturbance to the system. Once the change amplitude of the new energy power exceeds the preset change range, the required voltage adjustment amount is calculated and determined. This adjustment amount is calculated based on the current power change, and the goal is to offset the impact of power change by adjusting the voltage to maintain system stability. Based on the calculated voltage adjustment amount, the AC voltage instruction value of the sending end converter is adjusted. By changing the AC voltage instruction, the sending end converter can adjust its output voltage to compensate for the impact of new energy power change on system voltage stability.

[0090] The core of the present embodiment lies in its continuous monitoring of system state and rapid response to power change. When the new energy power fluctuates greatly, the voltage output of the sending end converter can be dynamically adjusted to ensure the stable operation of the hybrid DC power transmission system and avoid system disturbance caused by sudden power change. In addition, through accurate calculation and timely adjustment, this function also helps to improve energy transmission efficiency and reduce energy loss caused by power fluctuation.

[0091] In some other embodiments of the present application, the above method further comprises: 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 instruction 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 converter and the receiving converter is achieved. This platform allows the two converters to timely transmit key system state data, including but not limited to power transmission conditions, voltage levels, submodule states, etc. The sending converter and the receiving converter continuously monitor their own and the other's operating state and transmit these information to the other in real time. For example, when the sending converter detects new energy power fluctuations, it will quickly send this change information to the receiving converter so that the receiving converter can respond in time. Based on the real-time data received from the sending converter, the receiving converter can dynamically adjust its DC voltage instruction value. This adjustment is in response to the AC voltage changes and new energy power fluctuations of the sending converter, aiming to maintain power balance between the sending and receiving ends and prevent power surges from negatively affecting system stability.

[0093] The real-time bidirectional information exchange mechanism greatly improves the response speed of the hybrid DC power transmission system to new energy power fluctuations, enabling the receiving converter to quickly adjust the control strategy and maintain stable system operation. Through real-time data transmission through the communication network, the coordination control capability between the sending converter and the receiving converter is enhanced, ensuring power balance and operational stability of the entire system when facing complex working conditions. Based on real-time data-based control instruction adjustment, the operating parameters of the receiving converter can be optimized, energy loss caused by power fluctuations can be reduced, and energy transmission efficiency can be improved.

[0094] In order to enable those skilled in the art to more clearly understand the technical solutions of the present application, the implementation process of the hybrid DC power transmission system black start method of the present application will be described in detail below in conjunction with specific embodiments.

[0095] The present embodiment relates to a specific hybrid DC power transmission system black start method, which is based on the above-mentioned hybrid DC power transmission system implementation, as shown in Figure 8 , including the following several stages:

[0096] Stage 1: At this time, both full-bridge submodules 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 full-bridge submodule is not controlled to charge, the full-bridge submodule meets the energy requirement and can be unlocked for control. In stage 1, the half-bridge submodule capacitor voltage is always 0. The full-bridge submodule charging voltage is U dc1 / 2N F (i.e. the first preset voltage threshold), wherein U dc1 is the receiving end DC voltage, and N F is the number of full-bridge submodules.

[0097] Phase 2: uncontrolled charging is performed on the half-bridge sub-modules, and the specific method principle is as follows: the full-bridge sub-modules are unlocked, the active circulating current injection is enabled (the circulating current suppression control is disabled), the circulating current contains an alternating component (the current has positive and negative), and the uncontrolled charging can be performed on the half-bridge sub-modules (the current is positive, and the half-bridge is charged), and when the uncontrolled charging is completed, the half-bridge sub-modules reach the energy requirement, that is, the energy storage capacitor voltage of the half-bridge sub-modules reaches the second preset voltage threshold, and the half-bridge can be unlocked. In the embodiment, the second preset voltage threshold is set to [U dcN / (N F +N H )]×25%, wherein U dcN is the rated DC voltage of the hybrid DC power transmission system, N F is the number of full-bridge sub-modules, and N H is the number of half-bridge sub-modules.

[0098] Phase 3: after the full-bridge sub-modules and the half-bridge sub-modules are unlocked, controllable charging is performed, and the specific method is as follows: in this phase, the full-bridge sub-modules and the half-bridge sub-modules are unlocked, and the active circulating current injection is used to charge the capacitor voltage of the full-bridge sub-modules and the half-bridge sub-modules to the rated voltage value (the preset threshold); the preset threshold is U dcN / (N F +N H ), wherein U dcN is the rated DC voltage of the hybrid DC power transmission system, N F is the number of full-bridge sub-modules, and N H is the number of half-bridge sub-modules.

[0099] The above completes the charging of the MMC, and the starting condition is met. Further, the starting of the MMC-CCSC system is completed through coordination, including the following phases:

[0100] Phase 4: the sending end MMC adopts the constant AC voltage and frequency control, the circulating current suppression control is enabled (the active circulating current injection is no longer put into operation), and the sending end AC bus voltage is established;

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

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

[0103] Through the above phases, the starting of the MCC-CCSC system is realized.

[0104] In this embodiment, for the problem that the half-bridge sub-modules in the MMC cannot be charged under negative pressure, the charging and energy extraction of the half-bridge sub-modules in the MMC are realized through the full-bridge active circulating current injection method; without relying on additional equipment, the sub-module capacitor pre-charging problem of the sending-end MMC converter station of the hybrid DC power transmission system under 100% new energy sending is solved, and the reliable start of the MMC is ensured; through the coordinated control of the new energy field station, the sending-end and receiving-end converter stations, the system is quickly and orderly started to the rated operating condition, which is conducive to the stable sending of new energy.

[0105] The embodiment of the present application also provides a black start device of a hybrid DC power transmission system. It should be noted that the black start device of the hybrid DC power transmission system of the embodiment of the present application can be used to execute the method for black start of the hybrid DC power transmission system provided by the embodiment of the present application. The device is used to realize the above-mentioned embodiments and preferred embodiments, and will not be described here. As used below, the term "module" can be a combination of software and / or hardware that realizes a predetermined function. Although the device described in the following embodiments is preferably realized in software, hardware or a combination of software and hardware is also possible and is contemplated.

[0106] The black start device of the hybrid DC power transmission system provided by the embodiment of the present application is introduced below.

[0107] Figure 9 is a structural block diagram of the black start device of the hybrid DC power transmission system according to the embodiment of the present application. As Figure 9 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 determination unit is used to charge the full-bridge sub-modules of the sending-end converter when the polarity of the receiving-end DC voltage is negative, and use the full-bridge sub-modules to charge the half-bridge sub-modules of the sending-end converter, and determine that the charging of the sending-end converter is completed when the energy storage capacitor voltage of the full-bridge sub-modules and the energy storage capacitor voltage of the half-bridge sub-modules reach a preset threshold; the second control unit is used to generate a sending-end AC bus voltage after completing the charging of the sending-end converter; the second determination 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, send a start signal to the new energy field station at the zero-crossing point of the receiving-end DC voltage, so that the output power of the new energy field station rises to a rated value at a predetermined rate, and determine that the black start of the hybrid DC power transmission system is completed when the output power of the new energy field station reaches the rated value.

[0108] By the embodiment, the control receiving-end converter realizes polarity reversal of the receiving-end DC voltage to negative without additional equipment, and then effectively charges the full-bridge and half-bridge sub-modules of the sending-end converter with the negative voltage, thereby overcoming the problem that the half-bridge sub-modules are difficult to be charged under negative voltage in the traditional method. After successful charging, the sending-end converter can establish a stable AC voltage, and then reverses the polarity of the DC voltage of the receiving-end converter to positive again, controls the power of the new energy station to smoothly rise to the rated value, and finally realizes the smooth start of the hybrid DC system, thereby solving the problem of high cost of black start of the hybrid DC power transmission system.

[0109] In the implementation process, the first determination unit includes a first charging module and a second charging module. The first charging module is configured to perform uncontrolled charging on the full-bridge sub-modules of the sending-end converter when the polarity of the receiving-end DC voltage is negative, and perform uncontrolled charging on the half-bridge sub-modules by using the full-bridge sub-modules. The second charging module is configured to perform controllable charging on the full-bridge sub-modules and the half-bridge sub-modules after the uncontrolled charging.

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

[0111] The key of the whole uncontrolled charging process is to use the polarity reversal of the receiving-end DC voltage and the characteristics of the full-bridge sub-modules to realize the charging of the sub-modules of the sending-end converter. Without additional start-up power or equipment, only relying on the internal coordinated control of the system, the negative DC voltage established by the receiving-end converter and the circulating current formed in the full-bridge sub-modules of the sending-end converter can effectively complete the charging of the energy storage capacitors of the full-bridge sub-modules and the half-bridge sub-modules, thereby providing necessary power support for the black start of the system. When the energy storage capacitor voltage of the full-bridge sub-modules reaches the first preset voltage threshold, the uncontrolled charging of the full-bridge sub-modules is completed, and when the energy storage capacitor voltage of the half-bridge sub-modules reaches the second preset voltage threshold, the uncontrolled charging of the half-bridge sub-modules is completed, and the next stage of start-up control is entered.

[0112] Further, the second charging module comprises a third charging submodule and a determination submodule. The third charging submodule is configured to adjust the reference value of the circulating current component in the circulating current suppression control to form a circulating current meeting the controllable charging requirement after the uncontrolled charging of the full-bridge submodule and the half-bridge submodule, and perform the controllable charging on the full-bridge submodule and the half-bridge submodule by using the circulating current. The determination submodule is configured to determine that the charging of the sending-end converter is completed 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 value, which is set based on the rated DC voltage of the hybrid DC power transmission system, the number of the full-bridge submodules and the number of the half-bridge submodules.

[0113] Through the controllable charging, the energy storage capacitor voltage of the full-bridge submodule and the half-bridge submodule of the sending-end converter can reach the preset threshold value, and the charging process can be optimized to avoid unnecessary energy loss or unstable state in the charging process. The implementation of the controllable charging not only improves the energy conversion efficiency and stability in the system starting process, but also further enhances the energy management capability of the sending-end converter under the passive starting condition, which is helpful for efficient and reliable access of new energy.

[0114] Through the two-stage charging strategy, i.e., first performing the uncontrolled charging on the full-bridge submodule and the half-bridge submodule, and then entering the controllable charging stage, it is ensured that the energy storage capacitor of the full-bridge submodule and the half-bridge submodule can reach the preset threshold value without the need of additional starting power supply, thereby realizing the reliable starting of the sending-end converter. The setting of the preset threshold value fully considers the influence of the rated DC voltage of the hybrid DC power transmission system, the number of the full-bridge submodules and the number of the half-bridge submodules, so that the charging process is more accurate and efficient. This charging mechanism based on the system resources improves the starting flexibility and stability of the new energy access system, significantly reduces the dependence on auxiliary starting equipment, reduces the system construction and operation cost, and provides technical support for the hybrid DC power transmission system under the 100% new energy sending scenario.

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

[0116] The setting of the first preset voltage threshold and the second preset voltage threshold is the key to ensure that the full-bridge sub-modules and the half-bridge sub-modules of the sending converter can be effectively charged. The calculation of these thresholds fully considers the influence of the receiving end DC voltage, the system rated DC voltage and the number of sub-modules, and simultaneously fine-tunes through a proportional coefficient to adapt to the design requirements of a specific system. The process of realizing the two thresholds first performs uncontrolled charging for the full-bridge sub-modules through the negative polarity of the receiving end DC voltage, and then performs controllable charging for the half-bridge sub-modules through controllable circulating current injection until the respective preset thresholds are met, which provides a reliable and efficient starting strategy for the hybrid DC power transmission system and guarantees the stable operation of the system.

[0117] In some embodiments of the present application, the second control unit includes a generation module and a conversion module. The generation module is configured to control the sending converter to generate a target AC voltage signal when the charging of the sending converter is completed, the voltage amplitude of the target AC voltage signal being a preset voltage amplitude, and the frequency of the target AC voltage signal being a preset frequency; and the conversion module is configured to convert the target AC voltage signal into an actual AC voltage, the actual AC voltage being the sending AC bus voltage.

[0118] After the sending converter is charged, a target AC voltage signal is generated by adopting a fixed AC voltage and frequency control strategy, and the target AC voltage signal is converted into an actual AC voltage for output, so as to generate a sending AC bus voltage at the sending end. The key of this process is that the sending converter stabilizes the output of the required AC voltage through internal accurate control according to the preset fixed amplitude and frequency parameters. The self-starting capability of the system without external power support is improved, the grid connection process of the new energy station is ensured to proceed smoothly, the dependence on the traditional starting power is reduced, the starting cost is reduced, and the flexibility and reliability of the new energy power transmission are improved. By generating the AC voltage by the sending converter, the entire hybrid DC power transmission system can more orderly and efficiently transit to a normal operating state.

[0119] Further, the device further includes a third control unit configured to set 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 sub-modules and the half-bridge sub-modules after the charging of the sending converter is completed and before the sending converter is controlled to generate the sending AC bus voltage.

[0120] The operation of forming the circulating current is stopped, and the sending end converter can convert the direct current into alternating current through the inversion process according to the target alternating current voltage signal in a circulating current interference-free environment, so as to generate a stable alternating current voltage at the sending end. This step improves the accuracy and stability of the alternating current voltage establishment, avoids the voltage fluctuation caused by the circulating current, is very important for the smooth progress of the black start process of the whole hybrid DC power transmission system, and ensures the energy management and control accuracy during the transition from the start to the stable operation state.

[0121] In some embodiments of the application, the first control unit includes a setting module and a control module. The setting module is configured to set the direct current voltage reference value of the receiving end converter to a negative value; and the control module is configured to control the polarity of the receiving end direct current voltage of the receiving end converter to change from positive to negative, and continue to decrease to the direct current voltage reference value after the actual measured value of the receiving end direct current voltage gradually decreases to a negative value.

[0122] On the one hand, by setting the direct current voltage reference value of the receiving end converter to a negative value, the target direction of the voltage polarity inversion is clearly given; on the other hand, this not only involves the setting of the direct current voltage reference value, but also includes the continuous adjustment of the actual value of the receiving end converter direct current voltage until it stabilizes at the negative direct current voltage reference value, thereby realizing the polarity conversion from positive to negative. The flexibility and accuracy of the control of the receiving end direct current voltage during the black start process of the hybrid DC power transmission system are effectively improved, which lays a solid foundation for the stable operation of the system and the reliable access of new energy power, ensures the controllable inversion of the polarity of the receiving end direct current voltage, and enhances the operation stability and efficiency optimization capability of the hybrid DC power transmission system.

[0123] The black start device of the hybrid DC power transmission system includes a processor and a memory, and the first control unit, the first determination unit, the second control unit, the second determination unit and the like are stored in the memory as program units. The processor executes the program units stored in the memory to realize the corresponding functions. The modules are located in the same processor; or the modules are located in different processors in any combination.

[0124] The memory can include a non-persistent memory in a computer readable medium, a random access memory (RAM) and / or a non-volatile memory such as a read-only memory (ROM) or a flash memory (flash RAM), and the memory includes at least one memory chip.

[0125] The embodiment of the application provides a computer readable storage medium, and the computer readable storage medium includes a stored program. When the program runs, the device where the computer readable storage medium is located executes the black start method of the hybrid DC power transmission system.

[0126] The embodiment of the present application provides a processor used for running a program, wherein the processor is used for executing the black start method of the hybrid DC power transmission system when the program is running.

[0127] The embodiment of the present application provides an electronic device, including a processor, a memory, and a program stored in the memory and capable of running on the processor, and the processor implements the steps of the black start method of the hybrid DC power transmission system when the program is running. The device herein can be a server, a PC, a PAD, a mobile phone, and the like.

[0128] The present application also provides a computer program product, which is suitable for executing the program of the steps of the black start method of the hybrid DC power transmission system when executed on a data processing device.

[0129] Obviously, those skilled in the art should understand that each module or each step of the present application described above can be realized by a general computing device, which can be concentrated on a single computing device, or distributed on a network composed of multiple computing devices, which can be realized by program codes executable by the computing device, so that they can be stored in a storage device and executed by the computing device, and in some cases, the steps shown or described can be executed in different sequences, or they can be manufactured into each integrated circuit module respectively, or multiple modules or steps among them can be manufactured into a single integrated circuit module to realize. Thus, the present application is not limited to any specific combination of hardware and software.

[0130] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can adopt a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can adopt a computer program product implemented on one or more computer usable storage media containing computer usable program codes (including but not limited to disk storage, CD-ROM, optical storage, etc.).

[0131] The present application is described with reference to the flowcharts and / or block diagrams according to the method, device (system), and computer program product of the embodiments of the present application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of the flows and / or blocks in the flowcharts and / or block diagrams can be realized by computer program instructions. These computer program instructions can be provided to a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to produce a machine, so that the instructions executed by the computer or other programmable data processing devices produce a device for implementing the functions described in the flowcharts and / or block diagrams. Figure 1 Each flow or multiple flows and / or blocks Figure 1an apparatus to perform each function recited in a block or blocks.

[0132] These computer program instructions can also be stored in a computer- readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the flow Figure 1 one or more flows and / or blocks Figure 1 an apparatus to perform each function recited in a block or blocks.

[0133] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the flow Figure 1 one or more flows and / or blocks Figure 1 an apparatus to perform each function recited in a block or blocks.

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

[0135] The memory can include non-persistent memory and / or volatile memory, such as random access memory (RAM) about which the processor can execute instructions. The memory can also include non-volatile memory, such as read only memory (ROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), flash memory, or other memory technologies, about which the processor can execute instructions. The memory is an example of computer readable media.

[0136] Computer readable media includes permanent and non-permanent, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules 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 programmable read only memory (EEPROM), flash memory or other memory technologies, compact disc read only memory (CD-ROM), digital versatile discs (DVDs) or other optical storage, magnetic cassettes, magnetic disks storage or other magnetic storage devices, or any other non-transmission medium that can be used to store information accessible to computing devices. According to the definition herein, computer readable media does not include transitory media, such as modulated data signals and carrier waves.

[0137] Any technical features in the above-described embodiments can be combined in any manner, and for the sake of brevity, not all possible combinations are described, however, it is to be understood that the application encompasses all possible combinations.

[0138] It should also be noted that the terms "comprising", "comprises", "including", "includes" or any other variation thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can also include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "comprises a... " does not, without more constraints, exclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

[0139] The preferred embodiments of the application are described above in detail. The application is not limited to the embodiments described above, but can be modified and changed by those skilled in the art without departing from the scope of the application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the application shall be included in the scope of the 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. The receiving-end converter is a fully controlled current source converter. When the polarity of the DC voltage at the receiving end is negative, the full-bridge submodule is charged uncontrolled 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. 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; After performing uncontrolled charging on the full-bridge submodule and the half-bridge submodule, the full-bridge submodule and the half-bridge submodule are then subjected to controlled charging. 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-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.

2. The method according to claim 1, 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.

3. The method according to claim 1, 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.

4. 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.

5. The method according to claim 4, 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.

6. 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.

7. 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 6.

8. 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 6.

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