Energy self-balancing flexible DC converter valve, control method and DC system

Abstract

This invention discloses an energy self-balancing flexible DC converter valve, a control method, and a DC system relating to the technical field of power transmission and distribution networks. The converter valve includes three phase units, each including an upper bridge arm and a lower bridge arm. Both the upper and lower bridge arms include several full-bridge energy self-balancing submodules and several half-bridge energy self-balancing submodules. Each full / half-bridge energy self-balancing submodule is an optimized MMC submodule that includes an energy self-balancing circuit. The energy self-balancing circuit consists of a power electronics switch and a bleeder resistor connected in series, which dissipates excess energy when an overvoltage warning occurs on the capacitor in the submodule. This invention does not affect the operating logic of traditional converter valves and solves the technical problem in new energy island ultra-long-distance DC output scenarios, where excess power cannot be immediately dissipated due to a failure in the DC transmission system, at a very low cost. Furthermore, it makes full use of the temporary energy storage of the capacitor in the optimized MMC submodule to reduce waste due to the dissipation of excess power in the form of heat.

Description

[Technical Field]

[0001] This application relates to the technical field of power transmission and distribution networks, and more particularly to an energy self-balancing flexible DC converter valve and a control method thereof.

[0002] This application claims priority to a Chinese patent application filed with the China National Intellectual Property Office on October 25, 2023, with application number 202311395835.7, titled "Energy Self-Balanced Flexible DC Converter Valve, Control Method, and DC System," and also claims priority to a Chinese patent application filed with the China National Intellectual Property Office on October 31, 2023, with application number 202311438766.3, titled "Energy Self-Balanced Flexible DC Converter Valve, Control Method, and DC System," the contents of which are incorporated into this application. [Background technology]

[0003] Against the backdrop of the global energy transformation, new energy sources are accessing the power network on a large scale. New energy bases are mostly built in remote locations with low load levels and weak grid structures, and there is a significant need for stable external power transmission to new energy islands. Flexible DC power transmission using Modular Multilevel Converters (MMCs) has become one of the important power supply methods for new energy external transmission due to its flexibility, controllability, and efficiency. When flexible DC power transmission is used to connect island power generation systems and receiving AC power networks, if the receiving AC power network fails, it will not be able to output electrical energy. If the supplying power generation system is not disconnected, a large amount of surplus power will appear in the DC system, leading to overvoltage in the DC system and threatening the safe operation of the system.

[0004] Conventional technologies primarily use two solutions to dissipate large amounts of surplus power. The first solution involves placing a DC energy consumption device on the DC side of the receiving-side converter station. In the event of a failure, this device consumes the excess power, enabling fault ride-through without disconnecting the supply-side island power generation system. However, the structure of the DC energy consumption device using this method is complex, involving a large number of controllable power devices, resulting in high costs and requiring additional land. The second solution involves installing an AC energy consumption device on the AC line of the supply-side converter station. This has a simpler topology and lower costs. However, because the AC energy consumption device is installed on the supply side, when the receiving side fails, the supply side must be notified by means of communication to activate the energy consumption device. For ultra-long-distance power transmission systems, this can lead to long communication delays and a continuous influx of large amounts of surplus power into the flexible DC converter valve during the failure period. If the supply side's energy consumption device is not activated in time, it can lead to an overvoltage lockout of the DC transmission system. [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] This application provides an energy self-balancing flexible DC converter valve and a control method thereof, thereby solving the technical problems of the prior art, which are complex in structure, expensive, or have excessively long energy consumption response times, making them prone to overvoltage and thus unable to economically and reliably address the application needs of new energy remote island ultra-long-distance DC output systems. [Means for solving the problem]

[0006] In view of this, a first aspect of the present application provides an energy self-balancing flexible DC converter valve comprising three phase units, the phase units comprising an upper bridge arm and a lower bridge arm, The upper bridge arm and the lower bridge arm each include several full-bridge energy self-balancing submodules, several half-bridge energy self-balancing submodules, and a bridge arm reactor. Both the full-bridge energy self-balancing submodule and the half-bridge energy self-balancing submodule are connected in series to the bridge arm reactor. Both the full-bridge energy self-balancing submodule and the half-bridge energy self-balancing submodule are optimized MMC submodules that include an energy self-balancing circuit. The aforementioned energy self-balancing circuit is configured by connecting a power electronics switch and a bleeder resistor in series, and dissipates excess energy when an overvoltage risk arises in the capacitor of the optimized MMC submodule due to a failure in the DC power transmission system.

[0007] Preferably, both ends of the energy self-balancing circuit are connected to the positive and negative terminals of the capacitor in the optimized MMC submodule, and the optimized MMC submodule is either the full-bridge energy self-balancing submodule or the half-bridge energy self-balancing submodule.

[0008] Preferably, one end of the upper bridge arm is connected to the positive terminal of the DC end of the converter valve, and the other end is connected to the AC end of the converter valve. One end of the lower bridge arm is connected to the negative terminal of the DC end of the converter valve, and the other end is connected to the AC end of the converter valve.

[0009] A second aspect of this application provides a control method for an energy self-balancing flexible DC converter valve, which is implemented by the flexible DC converter valve described in any one of the first aspects. The steps include: monitoring the capacitor voltage in the optimized MMC submodule in real time, When the capacitor voltage exceeds the ON threshold, the power electronics switch of the energy self-balancing circuit is triggered to turn ON, thereby dissipating excess energy. The steps include: if the capacitor voltage is lower than the off threshold, triggering the power electronics switch of the energy self-balancing circuit to turn off, thereby terminating the energy dissipation operation; When the total energy released from the bleeder resistor of the energy self-balancing circuit exceeds its bleeder threshold, the DC power transmission system supply-side AC energy consumption device is triggered to assist in energy consumption. The procedure includes the step of, if the total energy released from the bleeder resistor of the energy self-balancing circuit exceeds the maximum allowable energy of the bleeder resistor, triggering the power electronics switch of the energy self-balancing circuit to turn off, thereby ending the energy dissipation operation, and preventing the power electronics switch from turning on again until the bleeder resistor temperature and the ambient temperature reach equilibrium.

[0010] Preferably, the process for setting the on-threshold is as follows: U オン = (1-k) × U cut ; U オン is the aforementioned ON threshold, k is the first tolerance, the typical range is 10% to 20%, and U cut This is the lockout voltage for the submodule.

[0011] Preferably, the process for setting the off-threshold is as follows: U オフ = (1-m) × U オン ; U オフ is the off-threshold, m is the second tolerance, and the typical range is 5% to 10%.

[0012] Preferably, the total energy released from the single bleeder resistor should be below its maximum allowable energy, and the process of arranging the maximum allowable energy of the bleeder resistor is as follows:

Number

[0013] Preferably, the process of arranging the bleeder threshold of the bleeder resistor is as follows:

Number

[0014] The third aspect of this application provides a DC system, including a new energy station, a supply-side flexible DC converter station, a receiving-side flexible DC converter station, and an AC energy consumption device. The energy self-balancing flexible DC converter valve described in any one of the first aspects is arranged in both the supply-side flexible DC converter station and the receiving-side flexible DC converter station. The new energy station is connected to the supply-side flexible DC converter station by a three-phase AC bus. The supply-side flexible DC converter station and the receiving-side flexible DC converter station are connected by a DC line. The AC energy consumption device is connected between the new energy station and the supply-side flexible DC converter station, and assists in consuming surplus energy when the total energy released from the bleeder resistor in the energy self-balancing flexible DC converter valve exceeds its bleeder threshold.

Advantages of the Invention

[0015] As can be seen from the above technical solutions, the embodiments of this application have the following advantages: In contrast to the two conventional methods for balancing surplus energy used in DC and AC energy consumption devices, this invention eliminates the need for DC energy consumption devices widely used in current construction projects. While achieving the same fault ride-through functionality, it significantly reduces construction costs, saves conversion station area, improves economic efficiency, and offers a greater cost advantage. Furthermore, in large-scale new energy island ultra-long-distance DC output scenarios, if an AC fault occurs on the receiving side, relying solely on the supply-side AC energy consumption device would result in insufficient dissipation of surplus energy, leading to system overvoltage. This problem is solved by actively controlling the energy self-balancing circuit to immediately release surplus power within the module, thus avoiding overvoltage in sub-module capacitors due to power surplus, which would threaten the safety and reliability of the system.

[0016] According to the design of the energy self-balancing flexible DC converter valve of the present invention, each optimized MMC submodule is provided with an energy self-balancing circuit, thereby ensuring that the charging and discharging frequencies of the capacitors in each submodule are similar during the voltage equalization process. Since the excess energy of the system is jointly shared by the bleeder resistors in all energy self-balancing modules, even bleeder resistors with small volume can meet the bleeder needs, and the impact on the original volume and arrangement of the submodules is small.

[0017] According to the energy self-balancing control method proposed by the present invention, when surplus power appears in a DC system, the surplus power is first stored in the capacitor of the optimized MMC submodule of the flexible DC converter. When the capacitor voltage of the submodule rises to the ON threshold, the surplus power is released through the bleeder resistor in the energy balance circuit. The surplus power of the system is recovered by making full use of the energy margin of the capacitor of the converter-optimized MMC submodule, and the surplus power is dissipated in the form of heat by directly using the bleeder resistor, thereby reducing the resulting waste.

[0018] According to the fault ride-through method for a novel new energy island ultra-long distance DC output system proposed by the present invention, the energy self-balancing flexible DC converter valve discharges only the excess power when the module's capacitor voltage reaches the ON threshold. Simultaneously, in scenarios with long failure durations, when the total energy discharged from a single bleeder resistor reaches the maximum allowable energy of the equipment, an AC energy consumption device is introduced on the supply side to cooperate. This further reduces the bleeder resistance value in the energy self-balancing passage, decreasing the volume of the resistor and reducing its impact on the original placement and water-cooling design of the flexible DC converter valve.

[0019] The novel new energy island ultra-long distance DC output system and fault ride-through method proposed by the present invention solves the problem of overvoltage of the capacitor in the full-bridge module during the DC fault period of the system, reduces the ratio of the full-bridge module in the full-half-bridge hybrid flexible DC converter valve, and further reduces equipment costs. [Brief explanation of the drawing]

[0020] [Figure 1] This is a schematic diagram of the structure of an energy self-balancing flexible DC converter valve provided by an embodiment of this application. [Figure 2] This is a schematic flow diagram of a control method for an energy self-balancing flexible DC converter valve provided in an embodiment of this application. [Figure 3] This is a schematic diagram of the structure of a DC system in which an AC energy consumption device is connected to the supply side, as provided in the embodiment of this application. [Modes for carrying out the invention]

[0021] To enable those skilled in the art to better understand the solutions of this application, the drawings of the embodiments of this application are combined below to clearly and completely describe the technical solutions of the embodiments of this application, and the embodiments described are not all but some of the embodiments of this application. Based on the embodiments of this application, all other embodiments obtained, on the premise that those skilled in the art do not perform work worthy of inventive step, are all within the scope of protection of this application.

[0022] To facilitate understanding, refer to Figure 1, and the embodiment of the energy self-balancing flexible DC converter valve provided in this application includes three phase units, each phase unit including an upper bridge arm and a lower bridge arm. Both the upper and lower bridge arms include several full-bridge energy self-balancing submodules, several half-bridge energy self-balancing submodules, and a bridge arm reactor. Both the full-bridge energy self-balancing submodule and the half-bridge energy self-balancing submodule are connected in series to the bridge arm reactor. Both the full-bridge energy self-balancing submodule and the half-bridge energy self-balancing submodule are optimized MMC submodules that include an energy self-balancing circuit. The energy self-balancing circuit consists of a power electronics switch and a bleeder resistor connected in series. When the DC system fails and an overvoltage risk arises in the capacitors of the optimized MMC submodule, it dissipates excess energy.

[0023] Furthermore, the full-bridge energy self-balancing submodule, half-bridge energy self-balancing submodule, and bridge arm reactor on the bridge arm are connected in series. The energy self-balancing circuit either connects or disconnects a bleeder resistor from the system to switch the circuit on / off. When the bleeder resistor is connected to the system, it dissipates excess energy, addressing excess power faults in the system. In addition, both the full-bridge energy self-balancing submodule and the half-bridge energy self-balancing submodule in this embodiment are optimized MMC submodules obtained by adding an energy self-balancing circuit to the traditional MMC submodule structure. Since each improved optimized MMC submodule includes an energy self-balancing circuit, there is sufficient bleeder resistor to jointly share the energy dissipation, thus meeting the system's needs.

[0024] Furthermore, both ends of the energy self-balancing circuit are connected to the positive and negative terminals of the capacitor in the optimized MMC submodule, and the optimized MMC submodule is either a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule.

[0025] Furthermore, one end of the upper bridge arm is connected to the positive terminal of the DC end of the converter valve, and the other end is connected to the AC end of the converter valve. One end of the lower bridge arm is connected to the negative terminal of the DC end of the converter valve, and the other end is connected to the AC end of the converter valve.

[0026] Here, the capacitor voltage value in the optimized MMC submodule can reflect the operating state of the system, that is, it analyzes whether the system has failed and reacts based on the analyzed failure. The energy self-balancing circuit is connected to both ends of the capacitor in the optimized MMC submodule, so that when the capacitor voltage exceeds the on threshold, it triggers the power electronics switch in the energy self-balancing circuit, connecting the bleeder resistor to the system and dissipating excess power. Here, one end of the upper bridge arm of each of the three phase units must be connected to the positive terminal of the DC end of the converter valve, and one end of each lower bridge arm must be connected to the negative terminal of the DC end of the converter valve. In addition, the optimized MMC submodule in this embodiment is either a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule, and each submodule acquires its respective capacitor voltage, performs voltage-based failure analysis, and then performs the corresponding trigger operation.

[0027] According to the energy self-balancing flexible DC converter valve provided in the embodiment of this application, each phase unit in its structure includes two bridge arms, each bridge arm includes several full-bridge energy self-balancing submodules and half-bridge energy self-balancing submodules, each submodule equipped with an energy self-balancing circuit, and in the event of a power transmission system failure, the power electronics switches and bleeder resistors in the circuit can dissipate excess energy, and the bleeder resistors in all submodules can be shared jointly, meeting the actual need for dissipating excess power, and the structure of the devices involved is simple and regular, easy to arrange and implement, and has practical applicability significance. Therefore, the embodiment of this application solves the technical problem in the prior art where the structure is complex and expensive, or the response time for energy consumption is too long, making it prone to overvoltage, and currently the application needs for new energy island ultra-long distance DC output systems cannot be economically and reliably solved.

[0028] To facilitate understanding, refer to Figure 2. This application provides an embodiment of a control method for an energy self-balancing flexible DC converter valve, which includes the following steps: Step 201: Monitor the capacitor voltage in the optimized MMC submodule in real time; Step 202: When the capacitor voltage exceeds the ON threshold, trigger the power electronics switch of the energy self-balancing circuit to turn ON to dissipate excess energy; Step 203: When the capacitor voltage is below the off threshold, trigger the power electronics switch of the energy self-balancing circuit to turn off, thereby terminating the energy dissipation operation; Step 204: When the total energy released from the bleeder resistor of the energy self-balancing circuit exceeds its bleeder threshold, the DC power transmission system supply-side AC energy consumption device is triggered to assist in energy consumption; Step 205: When the total energy released from the bleeder resistor of the energy self-balancing circuit exceeds the maximum allowable energy of the bleeder resistor, the power electronics switch of the energy self-balancing circuit is triggered to turn off, ending the energy dissipation operation and preventing the power electronics switch from turning on again until the bleeder resistor temperature and the ambient temperature reach equilibrium.

[0029] Furthermore, this process applies to the control method of the above-described embodiment of the energy self-balancing flexible DC converter valve, and is not limited to the main unit. Any method that enables control of the energy self-balancing flexible DC converter valve based on this method may be, for example, a computer or other device. In addition, the optimized MMC submodule in this embodiment is either a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule. Each submodule acquires the capacitor voltage of its respective module, determines the threshold value, and performs the corresponding trigger operation. The bleeder threshold may be set based on the actual situation, and is not limited thereto.

[0030] Furthermore, the on-threshold placement process is as follows: U オン = (1-k) × U cut ; U オン is the ON threshold, k is the first margin, the typical range is 10% to 20%, and U cut This is the lockout voltage for the submodule.

[0031] Furthermore, the off-threshold placement process is as follows: U オフ = (1-m) × U オン ; U オフ is the off-threshold, m is the second margin, and the typical range is 5% to 10%.

[0032] Furthermore, the total energy emitted from a single bleeder resistor must be less than or equal to its maximum allowable energy, and the arrangement process for the maximum allowable energy of the bleeder resistor is as follows:

number

[0033] In the selection process for bleeder resistors, they should not be selected and placed arbitrarily, but rather the above resistor selection constraints must be met.

[0034] Furthermore, the process for arranging the bleeder threshold of the bleeder resistor is as follows:

number

[0035] If the above control method is adopted, when the DC system fails and the transmitting and receiving energy becomes unbalanced, resulting in surplus power in the system, the surplus power is first stored in the capacitor of the optimized MMC submodule of the energy self-balancing converter valve. At this time, the capacitor voltage of the submodule is set to the upper limit of the capacitor voltage, U オン The voltage rises steadily until it reaches a certain level, and then the excess power is released by the bleeder resistor in the energy balance circuit. This method can recover excess power from the system by making full use of the energy margin of the capacitors in the optimized MMC submodule, and reduce the resulting waste by directly using the bleeder resistor to dissipate the excess power in the form of heat.

[0036] When the voltage is monitored to exceed the ON threshold, the power electronics switch is triggered to turn on, allowing the bleeder resistor to dissipate excess energy. When the voltage is monitored to fall to the OFF threshold, the power electronics switch is triggered to turn off, ending the excess energy dissipation operation.

[0037] However, if the total energy released from the bleeder resistor during a failure period reaches its own bleeder threshold, the receiving side must indicate that it cannot dissipate the excess energy with its own equipment and notify the supply side to activate a specific energy-consuming device to support energy consumption. Here, the bleeder threshold is determined based on the total energy released from the bleeder resistor, taking into account the communication period from the DC power transmission system supply side to the receiving side, and the enable delay of the AC energy-consuming device on the supply side, for example, if an allowable energy is set based on the equipment's maximum allowable energy.

[0038] If the total energy released from the bleeder resistor exceeds the maximum allowable energy of the bleeder resistor, the power electronics switch of the energy self-balancing circuit is triggered to turn off, ending the energy dissipation operation and preventing the power electronics switch from turning on again until the bleeder resistor temperature and the ambient temperature reach equilibrium.

[0039] As can be seen from the above, in the monitoring and surplus energy dissipation process, the on threshold, off threshold, and bleeder threshold are involved, and these thresholds must be adaptively positioned to satisfy the conditions of predetermined values.

[0040] The energy self-balancing control solution is explained in detail below, namely, the capacitor voltage U of each submodule of the converter valve. cThe converter valve is monitored in real time, and when it is operating normally, the power electronics switch in the energy self-balancing circuit is in the off state, and the converter valve only has an energy exchange function. After the system fails, the new energy generation power cannot be changed immediately, so the power transmission between the supply side and the receiving side is not balanced. To reduce the waste due to the dissipation of excess power in the form of heat, the capacitor of the converter valve's full / half-bridge energy self-balancing submodule preferentially recovers the excess power of the DC system, and in this case the capacitor voltage rises continuously. After monitoring that the capacitor voltage of the submodule has exceeded the on threshold, the power electronics switch in the energy self-balancing circuit is turned on, and the excess power is dissipated through the bleeder resistor in the energy self-balancing circuit, and the capacitor voltage of the submodule gradually decreases. After detecting that the capacitor voltage has fallen to the off threshold, the switch in the energy self-balancing circuit is turned off. During the ON period of the bleeder circuit, when the bleeder resistance reaches the bleeder threshold of the equipment's allowable energy, an AC energy consumption device is connected to the supply side. Referring to Figure 3, the AC energy consumption device is gradually removed until the fault is eliminated. Regarding the naming of the submodules in Figure 3, refer to the above interpretation, and since each name is specific to this embodiment, no further explanation is given here.

[0041] To facilitate understanding, this application further provides embodiments of a DC system, including a new energy station, a supply-side flexible DC converter, a receiving-side flexible DC converter, and an AC energy consumption device. Both the supply-side flexible DC converter station and the receiving-side flexible DC converter station are equipped with one of the energy self-balancing flexible DC converter valves described in the above embodiment. The new energy station is connected to the supply-side flexible DC converter station by a three-phase AC busbar. The supply-side flexible DC converter station and the receiving-side flexible DC converter station are connected by a DC line. The AC energy consumption device is connected between the new energy station and the supply-side flexible DC converter station, and assists in the consumption of excess energy when the total energy released from the bleeder resistor in the energy self-balancing flexible DC converter valve exceeds its bleeder threshold.

[0042] In this scenario, when an AC or DC fault occurs on the receiving side of the system, the excess power is first dissipated by an energy self-balancing flexible DC converter valve located in the flexible DC converter station. However, if the total energy released from the bleeder resistor in the flexible DC converter valve during the fault period reaches its own bleeder threshold, the system must be notified to activate the AC energy consumption device and then gradually remove the AC energy consumption device until the fault is resolved.

[0043] The system provided in this embodiment solves the problem of system overvoltage that occurs when an AC fault occurs on the receiving side, because relying solely on the supply-side AC energy consumption device would not be able to dissipate excess energy in time. By using an energy self-balancing circuit, the system actively controls to immediately release excess power within the module, avoiding overvoltage of submodule capacitors due to excessive power surplus, which would threaten the safety and reliability of the system. Furthermore, the flexible DC converter valve releases only excess power that has exceeded the on threshold when the module's capacitor voltage reaches a warning value. Simultaneously, in scenarios with a long fault duration, it cooperates to activate the supply-side AC energy consumption device when the bleeder resistor reaches the equipment's allowable energy bleeder threshold. This further reduces the bleeder resistance value in the flexible DC converter valve, decreasing the volume of the resistor and reducing its impact on the original placement and water cooling design of the flexible DC converter valve.

[0044] In some embodiments provided in this application, the disclosed apparatus and methods may be implemented in other forms. For example, the apparatus embodiments described above are merely illustrative, and for example, the division of the units is merely a logical functional division, and in actual implementation, other division forms may exist, for example, multiple units or components may be combined or integrated with other systems, or certain features may be ignored or not performed. Also, the connections between them, whether direct or communicative, as indicated or considered may be indirect or communicative connections by several interfaces, devices or units, and may be in electrical, mechanical or other forms.

[0045] Units described as individual components may or may not be physically separated, and components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Based on actual needs, some or all of these units can be selected to achieve the objectives of the technical proposal of this embodiment.

[0046] Furthermore, each functional unit in each embodiment of this application may be integrated into a single processing unit, each unit may exist individually and physically, or two or more units may be integrated into a single unit. The above integrated unit may be implemented in hardware form or in the form of a software functional unit.

[0047] The integrated unit may be implemented in the form of a software function unit and, when sold or used as an independent product, may be stored on a computer-readable storage medium. Based on this understanding, the essence of the proposed technology of this application, or a portion that contributes to the prior art, or all or part of the proposed technology, may be embodied in the form of a software product, which is stored on a storage medium and includes several instructions for performing all or part of the steps of the methods described in each embodiment of this application on a single computer device (which may be a personal computer, server, or network device, etc.). The storage medium may include various media capable of storing program code, such as U disks, portable hard disks, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0048] The above embodiments are for illustrative purposes only and do not limit the technical solutions of this application. Although this application has been described in detail with reference to the above embodiments, it is still possible to amend the technical solutions described in each of the above embodiments or to replace some of their technical features with equivalent substitutions, as will be understood by those skilled in the art. The essence of the corresponding technical solutions will not deviate from the spirit and scope of the technical solutions of each embodiment of this application due to such amendments or substitutions.

Claims

1. An energy self-balancing flexible DC converter valve comprising three phase units, each phase unit comprising an upper bridge arm and a lower bridge arm, Both the upper and lower bridge arms are connected in series with several full-bridge energy self-balancing submodules, several half-bridge energy self-balancing submodules, and a bridge arm reactor, and all of the submodules of the upper and lower bridge arms are capable of achieving energy self-balancing. Both the full-bridge energy self-balancing submodule and the half-bridge energy self-balancing submodule are optimized MMC submodules that include an energy self-balancing circuit. The aforementioned energy self-balancing circuit is configured by connecting a power electronics switch and a bleeder resistor in series, and when an overvoltage risk occurs in the capacitor of the optimized MMC submodule due to a failure in the DC power transmission system, it dissipates the excess energy. The ends of the energy self-balancing circuit are connected to the positive and negative terminals of the capacitor in the optimized MMC submodule. One end of the upper bridge arm is connected to the positive terminal of the DC end of the converter valve, and the other end is connected to the AC end of the converter valve. One end of the lower bridge arm is connected to the negative terminal of the DC end of the converter valve, and the other end is connected to the AC end of the converter valve. The control method for the energy self-balancing flexible DC converter valve is as follows: The steps include: monitoring the capacitor voltage in the optimized MMC submodule in real time; When the capacitor voltage exceeds the ON threshold, the power electronics switch of the energy self-balancing circuit is triggered to turn ON, thereby dissipating excess energy. The steps include: if the capacitor voltage is lower than the off threshold, triggering the power electronics switch of the energy self-balancing circuit to turn off, thereby terminating the energy dissipation operation; The step of triggering a DC power transmission system supply-side AC energy consumption device to assist energy consumption when the total energy released from the bleeder resistor of the energy self-balancing circuit exceeds its bleeder threshold. An energy-self-balancing flexible DC converter valve characterized by the following features.

2. The step of triggering a DC power transmission system supply-side AC energy consumption device to assist energy consumption when the total energy released from the bleeder resistor of the energy self-balancing circuit exceeds its bleeder threshold, The energy self-balancing circuit further includes the step of triggering the power electronics switch of the energy self-balancing circuit to turn off when the total energy released from the bleeder resistor exceeds the maximum allowable energy of the bleeder resistor, thereby ending the energy dissipation operation and preventing the power electronics switch from turning on again until the bleeder resistor temperature and the ambient temperature reach equilibrium. The energy self-balancing flexible DC converter valve according to feature 1.

3. The process for setting the ON threshold is as follows: U on = (1 - k) × U cut; The energy self-balancing flexible DC converter valve according to claim 2, characterized in that U ON is the ON threshold, k is a first tolerance with a range of values ​​from 10% to 20%, and U cut is the lockout voltage of the submodule.

4. The process for setting the off-threshold is as follows: U Off = (1 - m) × U On; The energy self-balancing flexible DC converter valve according to claim 3, characterized in that U Off is the off threshold, m is the second margin, and the range of values ​​is 5% to 10%.

5. The total energy emitted from a single bleeder resistor must be less than or equal to its maximum allowable energy, and the arrangement process for the maximum allowable energy of the bleeder resistor is as follows: [Math 1] The energy self-balancing flexible DC converter valve according to claim 4, characterized in that R is the resistance value of the bleeder resistor, E R is the maximum allowable energy of the bleeder resistor, ΔT is the duration of a single AC-side fault, and n is the on-duty cycle of the bleeder resistor.

6. The process for arranging the bleeder threshold of the bleeder resistor is as follows: [Math 2] The energy self-balancing flexible DC converter valve according to claim 5, characterized in that E is the bleeder threshold of the bleeder resistance, t1 is the communication period from the DC power transmission system supply side to the power receiving side, and t2 is the enable delay of the AC energy consumption device on the supply side.

7. A method for controlling an energy self-balancing flexible DC converter valve realized by the flexible DC converter valve described in any one of Claims 1 to 6, The steps include: monitoring the capacitor voltage in the optimized MMC submodule in real time, When the capacitor voltage exceeds the ON threshold, the power electronics switch of the energy self-balancing circuit is triggered to turn ON, thereby dissipating excess energy. The steps include: if the capacitor voltage is lower than the off threshold, triggering the power electronics switch of the energy self-balancing circuit to turn off, thereby terminating the energy dissipation operation; When the total energy released from the bleeder resistor of the energy self-balancing circuit exceeds its bleeder threshold, the DC power transmission system supply-side AC energy consumption device is triggered to assist in energy consumption. A method for controlling an energy self-balancing flexible DC converter valve, comprising the steps of: triggering the power electronics switch of the energy self-balancing circuit to turn off when the total energy released from the bleeder resistor of the energy self-balancing circuit exceeds the maximum allowable energy of the bleeder resistor, thereby ending the energy dissipation operation and preventing the power electronics switch from turning on again until the bleeder resistor temperature and the ambient temperature reach equilibrium.

8. The process for setting the ON threshold is as follows: U on = (1 - k) × U cut; The control method for an energy self-balancing flexible DC converter valve according to claim 7, characterized in that U ON is the ON threshold, k is a first tolerance with a range of values ​​from 10% to 20%, and U cut is the lockout voltage of the submodule.

9. The process for setting the off-threshold is as follows: U Off = (1 - m) × U On; The control method for an energy self-balancing flexible DC converter valve according to claim 8, characterized in that U Off is the off threshold, m is the second margin, and the range of values ​​is 5% to 10%.

10. The total energy emitted from a single bleeder resistor must be less than or equal to its maximum allowable energy, and the arrangement process for the maximum allowable energy of the bleeder resistor is as follows: [Math 3] The control method for an energy self-balancing flexible DC converter valve according to claim 9, characterized in that R is the resistance value of the bleeder resistor, E R is the maximum allowable energy of the bleeder resistor, ΔT is the duration of a single AC-side fault, and n is the on-duty cycle of the bleeder resistor.

11. The process for arranging the bleeder threshold of the bleeder resistor is as follows: [Math 4] The control method for an energy self-balancing flexible DC converter valve according to claim 10, characterized in that E is the bleeder threshold of the bleeder resistor, t1 is the communication period from the DC power transmission system supply side to the power receiving side, and t2 is the enable delay of the supply side AC energy consumption device.

12. A DC system comprising a new energy station, a supply-side flexible DC converter, a receiving-side flexible DC converter, and an AC energy consumption device, Both the supply-side flexible DC converter station and the receiving-side flexible DC converter station are equipped with an energy self-balancing flexible DC converter valve as described in any one of claims 1 to 6. The aforementioned new energy station is connected to the supply-side flexible DC converter by a three-phase AC busbar. The supply-side flexible DC converter and the receiving-side flexible DC converter are connected by a DC line. The aforementioned AC energy consumption device is connected between the new energy station and the supply-side flexible DC converter station, and is characterized in that it assists in the consumption of excess energy when the total energy released from the bleeder resistor in the energy self-balancing flexible DC converter valve exceeds its bleeder threshold.

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