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

Abstract

The present application discloses an energy self-balancing flexible DC converter valve, a control method, and a DC system. The converter valve includes three phase units, each of which includes an upper bridge arm and a lower bridge arm. The upper bridge arm and the lower bridge arm are both connected in series with a number of full-bridge submodules, a number of half-bridge submodules, and bridge arm reactors. The submodules of the upper bridge arm and the lower bridge arm are both optimized MMC submodules including an energy self-balancing circuit. The energy self-balancing circuit is composed of a power electronic switch and an energy dissipation resistor in series, and is used to dissipate surplus energy when the DC transmission system has a fault that causes an overvoltage risk in the capacitor of the optimized MMC submodule. The present application can solve the technical problem of the inability to dissipate surplus power in a DC transmission system due to a fault in a new energy island ultra-long-distance DC transmission scenario at a very low cost without affecting the operating logic of the traditional converter valve. It can make full use of the capacitor of the optimized MMC submodule to temporarily store energy and reduce the heat dissipation waste of surplus power.

Description

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on October 25, 2023, with application number 202311395835.7 and invention name “A Self-Balancing Energy Flexible DC Converter Valve, Control Method and DC System”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of power transmission and distribution networks, and in particular to an energy self-balancing flexible direct current (DC) converter valve, a control method, and a DC system. Background Art

[0003] Amidst global energy transformation, renewable energy is being connected to the power grid on a large scale. Most of these bases are located in remote areas with low load levels and weak grid structures. This creates a significant need for stable transmission of isolated renewable energy. Flexible DC transmission, based on modular multilevel converters (MMCs), has become a key means of transmitting renewable energy due to its flexibility, controllability, and efficiency. When flexible DC transmission is used to connect an isolated power generation system with a receiving AC grid, if a fault occurs in the receiving AC grid, the power cannot be transmitted. If the sending-end power generation system is not disconnected, a large amount of excess power will be generated in the DC system, causing severe overvoltages and endangering safe system operation.

[0004] Existing technologies mainly use two solutions to dissipate large amounts of surplus power. The first is to install a DC energy dissipation device on the DC side of the receiving-end converter station to consume excess power in the event of a fault, allowing the sending-end island power generation system to ride through the fault without disconnecting. However, the DC energy dissipation device in this method has a complex structure, involves a large number of controllable power devices, is expensive, and requires additional space. The second is to install an energy dissipation device in the AC line of the sending-end converter station. Its topology is simple and the cost is low. However, because the energy dissipation device is installed at the sending end, when a fault occurs at the receiving end, it is necessary to notify the sending end to activate the energy dissipation device through communication or other means. For ultra-long-distance power transmission systems, the communication delay is long, which may cause a large amount of surplus power to continue to flow into the flexible DC converter valve during the fault, causing the DC transmission system to overvoltage lock if the sending-end energy dissipation device is not activated in time. Summary of the Invention

[0005] The present application provides an energy self-balancing flexible DC converter valve, which is used to solve the technical problems that the existing technology is either complex in structure and high in cost, or has too long energy consumption response time and easily causes overvoltage, and cannot economically and reliably solve the current application needs of new energy island ultra-long-distance DC transmission systems.

[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, each phase unit comprising an upper bridge arm and a lower bridge arm;

[0007] The upper bridge arm and the lower bridge arm are both connected in series with a plurality of full-bridge sub-modules, a plurality of half-bridge sub-modules and bridge arm reactors;

[0008] The submodules of the upper bridge arm and the lower bridge arm are both optimized MMC submodules including energy self-balancing circuits;

[0009] The energy self-balancing circuit is composed of a power electronic switch and an energy dissipation resistor in series, and is used to dissipate surplus energy when a DC power transmission system failure causes an overvoltage risk in the optimized MMC submodule capacitor.

[0010] Preferably, the two ends of the energy self-balancing circuit are connected to the positive electrode and the negative electrode of the capacitor in the optimized MMC submodule, and the optimized MMC submodule is a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule.

[0011] Preferably, one end of the upper bridge arm is connected to the positive electrode of the DC terminal of the converter valve, and the other end is connected to the AC terminal of the converter valve;

[0012] One end of the lower bridge arm is connected to the negative electrode of the DC end of the converter valve, and the other end is connected to the AC end of the converter valve.

[0013] A second aspect of the present application provides a control method for an energy self-balancing flexible DC converter valve, which is implemented by any one of the energy self-balancing flexible DC converter valves described in the first aspect, comprising:

[0014] Real-time monitoring and optimization of capacitor voltage in MMC submodule;

[0015] If the capacitor voltage exceeds the conduction threshold, the power electronic switch of the energy self-balancing circuit is triggered to turn on, thereby dissipating the surplus energy;

[0016] If the capacitor voltage is lower than the shutdown threshold, the power electronic switch of the energy self-balancing circuit is triggered to turn off, thereby terminating the energy dissipation operation;

[0017] If the total energy discharged by the energy dissipation resistor of the energy self-balancing circuit exceeds its energy dissipation threshold, the energy consumption device on the AC side of the sending end of the DC transmission system is triggered to assist in energy consumption;

[0018] If the total energy discharged by the energy dissipation resistor of the energy self-balancing circuit exceeds the maximum tolerable energy of the energy dissipation resistor, the power electronic switch of the energy self-balancing circuit is triggered to turn off, the energy dissipation operation is terminated, and the power electronic switch is prohibited from turning on again.

[0019] Preferably, the configuration process of the conduction threshold is:

[0020]

[0021] in, is the conduction threshold, is the first margin, and the typical value range is 10%-20%. The blocking voltage of the MMC submodule is optimized.

[0022] Preferably, the configuration process of the shutdown threshold is:

[0023]

[0024] in, is the shut-off threshold, This is the second margin, and the typical value range is 5%-10%.

[0025] Preferably, the total energy discharged by the single energy dissipation resistor shall not exceed its maximum tolerance energy. The maximum tolerance energy configuration process of the energy dissipation resistor is as follows:

[0026] [( U conduction + U Shutdown ) / 2 ] 2 R ⋅ n ⋅ Δ T ≤ E R

[0027] Wherein, R is the resistance value of the energy dissipation resistor; is the maximum tolerable energy of the energy dissipation resistor; is the duration of the AC fault under consideration; and n is the on-duty cycle of the energy dissipation resistor.

[0028] Preferably, the configuration process of the energy dissipation threshold of the energy dissipation resistor is:

[0029] E Energy Dissipation Threshold ≤ E R − [( U conduction + U Shutdown ) / 2 ] 2 R ⋅ n ⋅ （ t 1 + t 2 )

[0030] in, is the energy dissipation threshold of the energy dissipation resistor, t1 is the communication time from the sending end to the receiving end of the DC transmission system, and t2 is the enabling delay of the AC side energy consumption device at the sending end.

[0031] A third aspect of the present application provides a DC system, comprising: a new energy station, a sending-end flexible DC converter station, a receiving-end flexible DC converter station, and an AC-side energy consumption device;

[0032] The sending-end flexible DC converter station and the receiving-end flexible DC converter station are both equipped with any one of the energy self-balancing flexible DC converter valves described in the first aspect;

[0033] The new energy station is connected to the sending-end flexible DC converter station via a three-phase AC bus;

[0034] The sending-end flexible DC converter station and the receiving-end flexible DC converter station are connected via a DC line;

[0035] The AC side energy consumption device is used to assist in consuming surplus energy when the total energy discharged by the energy discharge resistor in the energy self-balancing flexible DC converter valve exceeds its energy discharge threshold.

[0036] It can be seen from the above technical solutions that the embodiments of the present application have the following advantages:

[0037] Compared with the two existing technical routes of balancing surplus energy by using DC energy dissipation devices and AC side energy dissipation devices, the present invention does not require the DC energy dissipation devices widely used in current projects. On the basis of achieving the same fault ride-through performance, it can greatly reduce the project cost, save the area of ​​the converter station, improve the economy, and have more cost advantages. At the same time, in the scenario of large-scale new energy island ultra-long-distance DC transmission, it solves the problem of system overvoltage caused by relying solely on the AC side energy dissipation device at the sending end when an AC fault occurs at the receiving end. The energy self-balancing circuit actively controls and timely discharges the surplus power in the module, avoiding the threat of submodule capacitor overvoltage caused by power surplus to the safety and reliability of the system. In addition, in the present invention, all submodules of the bridge arm include an energy self-balancing circuit, that is, each submodule can dissipate the surplus energy, and each submodule can share the energy dissipation pressure. The pressure borne by each energy dissipation resistor is smaller, so the required resistor volume is smaller, the floor space is also smaller, and the overall value is more economical. While existing technologies involve using energy-dissipating resistors to dissipate excess energy, these only employ a small number of these resistors within the converter valves, rather than deploying energy-dissipating modules throughout the entire DC system. This approach aims to limit the number of devices involved and reduce costs. Practical engineering applications of this invention demonstrate that the value generated by the full energy-dissipating submodule far exceeds the cost of deploying the energy-dissipating circuits, demonstrating significant advantages over existing technologies.

[0038] In the energy self-balancing flexible DC converter valve of the present invention, each optimized MMC submodule is equipped with an energy self-balancing circuit, which ensures that the charging and discharging frequencies of the capacitors of each submodule are similar during the voltage balancing sorting process. The surplus energy of the system can be shared by the energy dissipation resistors in all the energy self-balancing submodules. Therefore, the energy dissipation requirements can be met by the energy dissipation resistors with a smaller volume, which has little impact on the original volume and layout of the submodules.

[0039] The control method of the energy self-balancing flexible DC converter valve proposed in the present invention first stores the surplus power in the optimized MMC sub-module capacitor of the flexible DC converter station when surplus power appears in the DC system, until the voltage of the sub-module capacitor rises to the conduction threshold, and then discharges the surplus power through the energy dissipation resistor in the energy balancing circuit. This can fully utilize the energy margin of the optimized MMC sub-module capacitor of the converter station to recover the system surplus power, reducing the waste caused by directly dissipating the surplus power in the form of heat through the energy dissipation resistor.

[0040] Based on the fault ride-through method for the novel new energy isolated island ultra-long-distance DC transmission system proposed in the present invention, the energy self-balancing flexible DC converter valve only discharges the surplus power after the module capacitor voltage reaches the conduction threshold. At the same time, in scenarios where the fault duration is long, when the total energy discharged by a single energy discharge resistor reaches the maximum tolerance energy of the equipment, it will cooperate with the AC-side energy consumption device at the sending end. Therefore, the energy discharge resistor value in the energy self-balancing circuit can be further reduced, thereby reducing the resistor volume and minimizing the impact on the original layout and water cooling design of the flexible DC converter valve.

[0041] The new type of new energy isolated island ultra-long-distance DC transmission system and fault ride-through method proposed in the present invention can also solve the problem of full-bridge module capacitor overvoltage during system DC faults, reduce the proportion of full-bridge modules in the full-half-bridge hybrid flexible DC converter valve, and further reduce equipment costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 A schematic structural diagram of an energy self-balancing flexible DC converter valve provided in an embodiment of the present application;

[0043] Figure 2 A flow chart of a control method for an energy self-balancing flexible DC converter valve provided in an embodiment of the present application;

[0044] Figure 3 Schematic diagram of the DC system structure with AC-side energy consumption devices input at the sending end provided in an embodiment of the present application. DETAILED DESCRIPTION

[0045] In order to help those skilled in the art better understand the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of this application.

[0046] For easier understanding, see Figure 1, an embodiment of an energy self-balancing flexible DC converter valve provided in the present application includes: three phase units, each phase unit including an upper bridge arm and a lower bridge arm;

[0047] Both the upper bridge arm and the lower bridge arm are connected in series with several full-bridge sub-modules, several half-bridge sub-modules and bridge arm reactors;

[0048] The submodules of the upper bridge arm and the lower bridge arm are both optimized MMC submodules including energy self-balancing circuits;

[0049] The energy self-balancing circuit is composed of a power electronic switch and an energy dissipation resistor in series, and is used to dissipate surplus energy when the DC transmission system fails and the optimized MMC submodule capacitor is at risk of overvoltage.

[0050] It should be noted that the full-bridge submodule, half-bridge submodule and bridge arm reactor on the bridge arm are connected in series; the full-bridge submodule that deploys the energy self-balancing circuit can be called a full-bridge energy self-balancing submodule, and similarly, the half-bridge submodule that deploys the energy self-balancing circuit can be called a half-bridge energy self-balancing submodule. Therefore, the optimized MMC submodule of this embodiment refers to a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule that can achieve surplus energy consumption; or it can be understood that the full-bridge energy self-balancing submodule and the half-bridge energy self-balancing submodule in this embodiment are both optimized MMC submodules obtained by adding an energy self-balancing circuit to the traditional MMC submodule structure. Since each of the improved optimized MMC submodules includes an energy self-balancing circuit, there are enough energy dissipation resistors to share the energy dissipation, which can meet the system's surplus energy dissipation requirements. The energy self-balancing circuit in the optimized MMC submodule connects the energy dissipation resistor to the system or cuts it out of the system by turning the circuit on and off; when the energy dissipation resistor is connected to the system, surplus energy can be dissipated, thereby responding to the system's surplus power failure.

[0051] Furthermore, the two ends of the energy self-balancing circuit are connected to the positive and negative electrodes of the capacitor in the optimized MMC submodule, and the optimized MMC submodule is a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule.

[0052] Furthermore, one end of the upper bridge arm is connected to the positive electrode of the DC terminal of the converter valve, and the other end is connected to the AC terminal of the converter valve;

[0053] One end of the lower bridge arm is connected to the negative electrode of the DC terminal of the converter valve, and the other end is connected to the AC terminal of the converter valve.

[0054] It's important to note that the voltage of the capacitor in the optimized MMC submodule reflects the system's operating status, allowing it to analyze whether the system has experienced a fault and respond accordingly. The energy self-balancing circuit is connected to both ends of the capacitor in the optimized MMC submodule. When the capacitor voltage exceeds the conduction threshold, the power electronic switch in the energy self-balancing circuit is triggered, thereby connecting the energy dissipation resistor to the system and dissipating excess power.

[0055] It is understood that the upper bridge arm of each of the three phase units must be connected to the positive terminal of the converter valve DC terminal, while the lower bridge arm must be connected to the negative terminal of the converter valve DC terminal. Furthermore, the optimized MMC submodule in this embodiment is a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule. Each submodule can independently obtain its own capacitor voltage, perform voltage-based fault analysis, and then perform corresponding triggering operations.

[0056] The energy self-balancing flexible DC converter valve provided in the embodiment of the present application has a structure in which each phase unit includes two bridge arms, and each bridge arm includes a number of full-bridge energy self-balancing sub-modules and half-bridge energy self-balancing sub-modules. Each sub-module has an energy self-balancing circuit, and the power electronic switches and energy dissipation resistors in the circuit can dissipate surplus energy when a transmission system failure occurs; the energy dissipation resistors in all sub-modules can be shared, which can meet the actual problem of surplus power dissipation; and the device structure involved is simple and regular, with strong modularity, easy to configure and execute, and has practical application significance. This means that each module can be mass-customized with the same standard, which is more practical in engineering applications. Therefore, the embodiment of the present application can solve the technical problems of the existing technology, which is either complex in structure and high in cost, or too long in energy consumption response time, which easily causes overvoltage, and cannot economically and reliably solve the current new energy island ultra-long-distance DC transmission system application needs.

[0057] For easier understanding, see Figure 2 The present application provides an embodiment of a control method for an energy self-balancing flexible DC converter valve, comprising:

[0058] Step 201: monitor and optimize the capacitor voltage in the MMC submodule in real time;

[0059] Step 202: If the capacitor voltage exceeds the conduction threshold, the power electronic switch of the energy self-balancing circuit is triggered to turn on, thereby dissipating the surplus energy;

[0060] Step 203: If the capacitor voltage is lower than the shutdown threshold, the power electronic switch of the energy self-balancing circuit is triggered to turn off, terminating the energy dissipation operation;

[0061] Step 204: If the total energy discharged by the energy dissipation resistor of the energy self-balancing circuit exceeds its energy dissipation threshold, the energy dissipation device on the AC side of the sending end of the DC transmission system is triggered to assist in energy dissipation;

[0062] Step 205: If the total energy discharged by the energy dissipation resistor of the energy self-balancing circuit exceeds the maximum tolerance energy of the energy dissipation resistor, the power electronic switch of the energy self-balancing circuit is triggered to turn off, terminate the energy dissipation operation, and prohibit the power electronic switch from turning on again.

[0063] It should be noted that this process is a control method for the above-mentioned energy self-balancing flexible DC converter valve embodiment. The execution subject is not limited, as long as it can achieve control of the energy self-balancing flexible DC converter valve based on this method, such as a computer or other device. Moreover, the optimized MMC submodule in this embodiment is a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule. Each submodule can obtain its own module capacitor voltage, perform threshold judgment, and perform corresponding triggering operations. In addition, the energy discharge threshold can be set according to actual conditions and is not limited here.

[0064] Furthermore, the configuration process of the conduction threshold is:

[0065]

[0066] in, is the conduction threshold, is the first margin, and the typical value range is 10%-20%. To optimize the blocking voltage of the MMC submodule.

[0067] Furthermore, the configuration process of the shutdown threshold is:

[0068]

[0069] in, is the shutdown threshold, This is the second margin, and the typical value range is 5%-10%.

[0070] Furthermore, the total energy discharged by a single energy-discharging resistor must not exceed its maximum tolerance energy. The maximum tolerance energy configuration process of the energy-discharging resistor is as follows:

[0071] [( U conduction + U Shutdown ) / 2 ] 2 R ⋅ n ⋅ Δ T ≤ E R

[0072] Wherein, R is the resistance value of the energy dissipation resistor; is the maximum tolerable energy of the energy dissipation resistor; is the duration of the AC fault under consideration; n is the on-duty cycle of the energy dissipation resistor.

[0073] The above resistor selection constraints must be met during the energy dissipation resistor selection process, rather than randomly selecting the configuration.

[0074] Furthermore, the configuration process of the energy dissipation threshold of the energy dissipation resistor is as follows:

[0075] E Energy Dissipation Threshold ≤ E R − [( U conduction + U Shutdown ) / 2 ] 2 R ⋅ n ⋅ （ t 1 + t 2 )

[0076] in, is the energy dissipation threshold of the energy dissipation resistor, t1 is the communication time from the sending end to the receiving end of the DC transmission system, and t2 is the enabling delay of the energy consumption device on the AC side of the sending end.

[0077] It should be noted that, using the above control method, when a fault occurs in the DC system, resulting in energy imbalance between the sending and receiving ends and thus surplus power in the system, the surplus power will first be stored in the optimized MMC submodule capacitor of the energy self-balancing converter valve. At this time, the submodule capacitor voltage will continue to increase until it reaches the capacitor voltage upper limit. When the power is fully utilized, the excess power is discharged through the energy balance circuit through the energy dissipation resistor. This method fully utilizes the energy margin of the optimized MMC submodule capacitor to recover the system's excess power, which can reduce the waste caused by directly dissipating the excess power as heat through the energy dissipation resistor.

[0078] When the monitoring voltage exceeds the turn-on threshold, the power electronic switch can be triggered to turn on, thereby dissipating the surplus energy through the energy dissipation resistor. Until the monitoring voltage drops to the turn-off threshold, the power electronic switch can be triggered to turn off, terminating the surplus energy dissipation operation.

[0079] However, if the total energy discharged by the energy-discharging resistor during a fault reaches its own energy-discharging threshold, the receiving end cannot dissipate the surplus energy through its own devices and must notify the sending end to activate specific energy-dissipating devices to assist in energy consumption. The energy-discharging threshold is set based on the maximum energy tolerance of the device, such as the tolerance energy. The energy-discharging threshold can be selected based on the total energy discharged by the energy-discharging resistor, taking into account the communication time from the sending end to the receiving end of the DC transmission system and the delay in enabling the energy-dissipating devices on the sending end's AC side.

[0080] When the total energy discharged by the energy dissipation resistor exceeds the maximum tolerable energy of the energy dissipation resistor, the power electronic switch of the energy self-balancing circuit is triggered to turn off, terminating the energy dissipation operation and prohibiting the power electronic switch from turning on again.

[0081] As can be seen above, the monitoring and excess energy dissipation process involves turn-on thresholds, turn-off thresholds, and energy dissipation thresholds. These thresholds are adaptively configured and meet certain conditions. Furthermore, specific recovery conditions can be set after the power electronic switch is prohibited from turning on again, such as waiting until the temperature of the energy dissipation resistor reaches equilibrium with the ambient temperature. Other conditions can also be set based on actual conditions, and the specifics are not limited here.

[0082] The detailed energy self-balancing control scheme is described as follows: real-time monitoring of the capacitor voltage U of each sub-module of the converter valve c When the converter valve operates normally, the power electronic switch in the energy self-balancing circuit is in the off state, and the converter valve only has the energy exchange function. When a system fault occurs, the power generated by renewable energy fails to change in time, resulting in an unbalanced power transmission between the sending and receiving ends. In order to reduce the waste of surplus power dissipated in the form of heat, the full / half-bridge energy self-balancing sub-module capacitor of the converter valve prioritizes recovering the surplus power of the DC system. At this time, the capacitor voltage continues to rise. When it is monitored that the sub-module capacitor voltage rises and exceeds the conduction threshold, the power electronic switch in the energy self-balancing circuit is turned on, and the surplus power is dissipated by the energy dissipation resistor in the energy self-balancing circuit. The sub-module capacitor voltage gradually decreases. When it is monitored that the capacitor voltage drops to the disconnection threshold, the switch in the energy self-balancing circuit is turned off. During the conduction period of the energy dissipation circuit, if the energy dissipation resistor reaches the energy dissipation threshold of the equipment's tolerance energy, the energy consumption device on the sending-end AC side is put into operation. Please refer to Figure 3 , until the fault is cleared, gradually exit the AC side energy consumption device, Figure 3 The submodule naming in is the same as explained above, and corresponds one-to-one with the limited names in this embodiment, and the specific details are not repeated here.

[0083] For ease of understanding, the present application also provides an embodiment of a DC system, comprising: a new energy station, a sending-end flexible DC converter station, a receiving-end flexible DC converter station, and an AC-side energy consumption device;

[0084] The sending-end flexible DC converter station and the receiving-end flexible DC converter station are both equipped with any one of the energy self-balancing flexible DC converter valves in the above embodiments;

[0085] The new energy station is connected to the sending-end flexible DC converter station through a three-phase AC bus;

[0086] The sending-end flexible DC converter station and the receiving-end flexible DC converter station are connected via a DC line;

[0087] The AC side energy consumption device is used to assist in consuming surplus energy when the total energy discharged by the energy discharge resistor in the energy self-balancing flexible DC converter valve exceeds its energy discharge threshold.

[0088] It should be noted that the AC-side energy-consuming device can be an independent device connected between the new energy station and the sending-end flexible DC converter station, or it can be a device distributed in the new energy power generation unit. When an AC fault or DC fault occurs at the receiving end of the system, the surplus power can be dissipated first through the energy self-balancing flexible DC converter valve configured in each flexible DC converter station. However, if the total energy discharged by the energy-discharging resistor in the flexible DC converter valve during the fault reaches its own energy-discharging threshold, it is necessary to notify the sending end of the DC system to put its AC-side energy-consuming device into the system until the fault is cleared and gradually withdraw from AC-side energy consumption.

[0089] Furthermore, the system of this embodiment, combined with the sending-end AC-side energy dissipation device and the DC converter valves at both ends, not only solves DC fault ride-through issues, but also addresses sending-end AC faults and can assist in resolving receiving-end AC faults. In this process, the energy dissipation resistors distribute the pressure, reducing the amount of energy they must dissipate and further reducing the resistor size. This reduction in resistor size can also reduce the size of the converter valves, further reducing the overall footprint of the valve tower and valve hall, thus offering significant engineering applications.

[0090] Specifically, the system provided by this embodiment solves the problem of system overvoltage caused by the inability of the sending-end AC energy dissipation device to dissipate excess energy in a timely manner when a receiving-end AC fault occurs. By actively controlling the timely discharge of excess power within the module through the energy self-balancing circuit, the system avoids the threat of overvoltage in the submodule capacitors caused by severe power surplus, which threatens the safety and reliability of the system. Furthermore, the flexible DC converter valve only discharges excess power after the module capacitor voltage reaches the warning value, that is, exceeds the conduction threshold. Furthermore, in scenarios where the fault lasts for a long time, when the energy dissipation resistor reaches the energy dissipation threshold of the device's tolerance, the sending-end AC energy dissipation device is activated. This further reduces the energy dissipation resistor value in the flexible DC converter valve, thereby reducing the resistor size and the impact on the original layout and water cooling design of the flexible DC converter valve. In actual projects, it has even been found that the energy dissipation method provided by this embodiment requires a smaller resistor size, essentially achieving natural cooling and eliminating the need for water cooling, which, to a certain extent, saves water cooling design costs and reduces engineering design complexity.

[0091] In the several embodiments provided in this application, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are merely schematic. For example, the division of the units is merely a logical function division. In actual implementation, there may be other division methods, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0092] The units described as separate components may or may not be physically separate, and the 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. Some or all of these units may be selected to achieve the purpose of this embodiment according to actual needs.

[0093] In addition, the functional units in the various embodiments of the present application may be integrated into a single processing unit, or each unit may exist physically separately, or two or more units may be integrated into a single unit. The aforementioned integrated units may be implemented in the form of hardware or software functional units.

[0094] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application, or the part that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for executing all or part of the steps of the method described in each embodiment of the present application through a computer device (which can be a personal computer, server, or network device, etc.). The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (full name: Read-Only Memory, English abbreviation: ROM), random access memory (full name: Random Access Memory, English abbreviation: RAM), disk or optical disk, and other media that can store program code.

[0095] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. An energy self-balancing flexible DC converter valve, characterized in that: include: Three phase units, each phase unit comprising an upper bridge arm and a lower bridge arm; The upper bridge arm and the lower bridge arm are both connected in series with a plurality of full-bridge sub-modules, a plurality of half-bridge sub-modules and bridge arm reactors; The submodules of the upper bridge arm and the lower bridge arm are both optimized MMC submodules including energy self-balancing circuits; The energy self-balancing circuit is composed of a power electronic switch and an energy dissipation resistor connected in series, and is used to dissipate surplus energy when the DC transmission system fails and the optimized MMC submodule capacitor has an overvoltage risk; The control method of the energy self-balancing flexible DC converter valve is: Real-time monitoring of the capacitor voltage in the optimized MMC submodule; If the capacitor voltage exceeds the conduction threshold, the power electronic switch of the energy self-balancing circuit is triggered to turn on, thereby dissipating the surplus energy; If the capacitor voltage is lower than the shutdown threshold, the power electronic switch of the energy self-balancing circuit is triggered to turn off, thereby terminating the energy dissipation operation; If the total energy discharged by the energy discharge resistor of the energy self-balancing circuit exceeds its energy discharge threshold, the energy consumption device at the AC side of the sending end of the direct current transmission system is triggered to assist in energy consumption.

2. The energy self-balancing flexible DC converter valve according to claim 1, characterized in that: The two ends of the energy self-balancing circuit are connected to the positive electrode and the negative electrode of the capacitor in the optimized MMC submodule, and the optimized MMC submodule is a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule.

3. The energy self-balancing flexible DC converter valve according to claim 1, characterized in that: One end of the upper bridge arm is connected to the positive electrode of the DC terminal of the converter valve, and the other end is connected to the AC terminal of the converter valve; One end of the lower bridge arm is connected to the negative electrode of the DC end of the converter valve, and the other end is connected to the AC end of the converter valve.

4. The energy self-balancing flexible DC converter valve according to claim 1, characterized in that: Also includes: If the total energy discharged by the energy dissipation resistor of the energy self-balancing circuit exceeds the maximum tolerable energy of the energy dissipation resistor, the power electronic switch of the energy self-balancing circuit is triggered to turn off, the energy dissipation operation is terminated, and the power electronic switch is prohibited from turning on again.

5. The energy self-balancing flexible DC converter valve according to claim 4, characterized in that: The configuration process of the conduction threshold is: ； in, is the conduction threshold, is the first margin, and its value range is 10%-20%. The blocking voltage of the MMC submodule is optimized.

6. The energy self-balancing flexible DC converter valve according to claim 5, characterized in that: The configuration process of the shutdown threshold is as follows: ； in, is the shut-off threshold, is the second margin, and its value range is 5%-10%.

7. The energy self-balancing flexible DC converter valve according to claim 6, characterized in that: The total energy discharged by the single energy dissipation resistor shall not exceed its maximum tolerance energy. The maximum tolerance energy configuration process of the energy dissipation resistor is as follows: ； Wherein, R is the resistance value of the energy dissipation resistor; is the maximum tolerable energy of the energy dissipation resistor; is the duration of the AC fault under consideration; and n is the on-duty cycle of the energy dissipation resistor.

8. The energy self-balancing flexible DC converter valve according to claim 7, characterized in that: The configuration process of the energy dissipation threshold of the energy dissipation resistor is as follows: ； in, is the energy dissipation threshold of the energy dissipation resistor, t1 is the communication time from the sending end to the receiving end of the DC transmission system, and t2 is the enabling delay of the AC side energy consumption device at the sending end.

9. A method for controlling an energy self-balancing flexible DC converter valve, implemented by the energy self-balancing flexible DC converter valve according to any one of claims 1 to 8, characterized in that: include: Real-time monitoring of the capacitor voltage in the optimized MMC submodule; If the capacitor voltage exceeds the conduction threshold, the power electronic switch of the energy self-balancing circuit is triggered to turn on, thereby dissipating the surplus energy; If the capacitor voltage is lower than the shutdown threshold, the power electronic switch of the energy self-balancing circuit is triggered to turn off, thereby terminating the energy dissipation operation; If the total energy discharged by the energy dissipation resistor of the energy self-balancing circuit exceeds its energy dissipation threshold, the energy consumption device on the AC side of the sending end of the DC transmission system is triggered to assist in energy consumption; If the total energy discharged by the energy dissipation resistor of the energy self-balancing circuit exceeds the maximum tolerable energy of the energy dissipation resistor, the power electronic switch of the energy self-balancing circuit is triggered to turn off, the energy dissipation operation is terminated, and the power electronic switch is prohibited from turning on again.

10. The control method of the energy self-balancing flexible DC converter valve according to claim 9, characterized in that: The configuration process of the conduction threshold is: ； in, is the conduction threshold, is the first margin, and its value range is 10%-20%. The blocking voltage of the MMC submodule is optimized.

11. The control method of the energy self-balancing flexible DC converter valve according to claim 10, characterized in that: The configuration process of the shutdown threshold is as follows: ； in, is the shut-off threshold, is the second margin, and its value range is 5%-10%.

12. The control method of the energy self-balancing flexible DC converter valve according to claim 11, characterized in that: The total energy discharged by the single energy dissipation resistor shall not exceed its maximum tolerance energy. The maximum tolerance energy configuration process of the energy dissipation resistor is as follows: ； Wherein, R is the resistance value of the energy dissipation resistor; is the maximum tolerable energy of the energy dissipation resistor; is the duration of the AC fault under consideration; and n is the on-duty cycle of the energy dissipation resistor.

13. The control method of the energy self-balancing flexible DC converter valve according to claim 12, characterized in that: The configuration process of the energy dissipation threshold of the energy dissipation resistor is as follows: ； in, is the energy dissipation threshold of the energy dissipation resistor, t1 is the communication time from the sending end to the receiving end of the DC transmission system, and t2 is the enabling delay of the AC side energy consumption device at the sending end.

14. A DC system, characterized in that: include: New energy stations, sending-end flexible DC converter stations, receiving-end flexible DC converter stations and AC-side energy consumption devices; The sending-end flexible DC converter station and the receiving-end flexible DC converter station are both equipped with the energy self-balancing flexible DC converter valve according to any one of claims 1 to 8; The new energy station is connected to the sending-end flexible DC converter station via a three-phase AC bus; The sending-end flexible DC converter station and the receiving-end flexible DC converter station are connected via a DC line; The AC side energy consumption device is used to assist in consuming surplus energy when the total energy discharged by the energy discharge resistor in the energy self-balancing flexible DC converter valve exceeds its energy discharge threshold.

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