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WO2025086441A1 - 一种能量自平衡柔性直流换流阀、控制方法及直流系统 - Google Patents

一种能量自平衡柔性直流换流阀、控制方法及直流系统 Download PDF

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Publication number
WO2025086441A1
WO2025086441A1 PCT/CN2023/140072 CN2023140072W WO2025086441A1 WO 2025086441 A1 WO2025086441 A1 WO 2025086441A1 CN 2023140072 W CN2023140072 W CN 2023140072W WO 2025086441 A1 WO2025086441 A1 WO 2025086441A1
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WO
WIPO (PCT)
Prior art keywords
energy
balancing
self
flexible
converter valve
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
PCT/CN2023/140072
Other languages
English (en)
French (fr)
Inventor
周月宾
蔡希鹏
袁智勇
曹琬钰
饶宏
李岩
许树楷
杨柳
徐义良
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
CSG Electric Power Research Institute
Original Assignee
CSG Electric Power Research Institute
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Filing date
Publication date
Application filed by CSG Electric Power Research Institute filed Critical CSG Electric Power Research Institute
Publication of WO2025086441A1 publication Critical patent/WO2025086441A1/zh
Pending legal-status Critical Current
Anticipated expiration legal-status Critical

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for AC mains or AC distribution networks
    • H02J3/36—Arrangements for transfer of electric power between AC networks via a high-tension DC link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for AC mains or AC distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for AC mains or AC distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/388—Islanding, i.e. disconnection of local power supply from the network
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • H02M1/325—Means for protecting converters other than automatic disconnection with means for allowing continuous operation despite a fault, i.e. fault tolerant converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
    • H02M7/003—Constructional details, e.g. physical layout, assembly, wiring or busbar connections
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Definitions

  • 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 converter valve and a control method thereof.
  • the existing technology mainly uses two solutions to dissipate a large amount of surplus power.
  • the first is to deploy a DC energy-consuming device on the DC side of the receiving-end converter station to consume excess power in the event of a fault, so as to achieve fault riding without cutting off the isolated island power generation system at the sending-end.
  • the DC energy-consuming device in this method has a complex structure and involves a large number of controllable power devices. It is expensive and requires additional space.
  • the second is to install an AC energy-consuming device in the AC line of the sending-end converter station, which has a simple topology and low cost.
  • the AC energy-consuming device since the AC energy-consuming device is installed at the sending end, when a fault occurs at the receiving end, it is necessary to notify the sending end to put the energy-consuming device into operation based on communication and other means.
  • the communication delay is relatively long, which may cause a large amount of surplus power to continue to flow into the flexible DC converter valve during the fault period.
  • the energy-consuming device at the sending end is not put into operation in time, the DC transmission system will be locked due to overvoltage.
  • the present application provides an energy self-balancing flexible DC converter valve and a control method thereof, which are used to solve the technical problems that the prior art is either complex in structure and high in cost, or has too long an energy consumption response time and easily causes overvoltage, and cannot economically and reliably solve the current application requirements of new energy isolated island ultra-long-distance DC transmission systems.
  • the 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;
  • the upper bridge arm and the lower bridge arm each include a plurality of full-bridge energy self-balancing submodules, a plurality of half-bridge energy self-balancing submodules and a bridge arm reactor;
  • the full-bridge energy self-balancing submodule and the half-bridge energy self-balancing submodule are both connected in series with the bridge arm reactor;
  • the full-bridge energy self-balancing submodule and the half-bridge energy self-balancing submodule 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 power transmission system fails and causes an overvoltage risk to the optimized MMC submodule capacitor.
  • 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 the full-bridge energy self-balancing submodule or the half-bridge energy self-balancing submodule.
  • one end of the upper bridge arm is connected to the positive electrode 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 pole of the DC end of the converter valve, and the other end is connected to the AC end of the converter valve.
  • 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 flexible DC converter valves described in the first aspect, and includes:
  • the power electronic switch of the energy self-balancing circuit is triggered.
  • the switch is turned on to dissipate the surplus energy
  • the power electronic switch of the energy self-balancing circuit is triggered to shut down, thereby terminating the energy dissipation operation;
  • the AC energy consumption device at the sending end of the DC power transmission system is triggered to assist in energy consumption;
  • 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 until the temperature of the energy dissipation resistor is balanced with the ambient temperature.
  • Uconduction is the conduction threshold
  • k is the first margin
  • a typical value range is 10%-20%
  • Ucut is the submodule locking voltage
  • Uoff is the off threshold
  • m is the second margin
  • a typical value range is 5%-10%.
  • the total energy discharged by a single energy-discharging resistor shall not exceed its maximum tolerable energy, and the maximum tolerable energy configuration process of the energy-discharging resistor is:
  • R is the resistance value of the energy-discharging resistor
  • ER is the maximum tolerable energy of the energy-discharging resistor
  • ⁇ T is the duration of a single AC fault under consideration
  • n is the on-duty cycle of the energy-discharging resistor.
  • the configuration process of the energy dissipation threshold of the energy dissipation resistor is:
  • E energy discharge threshold is the energy discharge threshold of the energy discharge resistor
  • t1 is the communication time from the sending end to the receiving end of the DC power transmission system
  • t2 is the enabling delay of the AC energy consumption device at the sending end.
  • the third aspect of the present application provides a DC system, including: a new energy station, a sending-end flexible DC converter station, a receiving-end flexible DC converter station, and an AC energy consumption device;
  • 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;
  • 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 energy consumption device is connected between the new energy station and the sending-end flexible DC converter station, and 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.
  • the present invention does not require a DC energy consuming device 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 economy, and have more cost advantages.
  • it solves the problem of system overvoltage caused by relying solely on the AC energy consuming device at the sending end to dissipate surplus energy in time when an AC fault occurs at the receiving end.
  • the surplus power in the module is discharged in time through active control of the energy self-balancing circuit, thereby avoiding the threat of overvoltage of the sub-module capacitor caused by power surplus to the safety and reliability of the system.
  • the energy self-balancing flexible DC converter valve design of the present invention is provided with an energy self-balancing circuit in each optimized MMC submodule, which ensures that the charging and discharging frequencies of the capacitors of each submodule are similar during the voltage balancing sorting process, and the surplus energy of the system can be shared by the energy dissipation resistors in all the energy self-balancing modules. Therefore, smaller energy dissipation resistors can meet the energy dissipation requirements, with little impact on the original volume and layout of the submodules.
  • the energy self-balancing control method proposed in the present invention first stores the surplus power in the optimized MMC submodule capacitor of the flexible DC converter station when surplus power appears in the DC system, until the voltage of the submodule capacitor rises to the conduction threshold, and then discharges the surplus power through the energy discharge resistor in the energy balance circuit.
  • the energy margin of the optimized MMC submodule capacitor of the converter station can be fully utilized to recover the system surplus power, reducing the waste caused by directly dissipating the surplus power in the form of heat through the energy discharge resistor.
  • the energy self-balancing flexible DC converter valve only discharges the surplus power after the module capacitor voltage reaches the conduction threshold.
  • the sending-end AC energy consumption device will be put into use. Therefore, the energy discharge resistor value in the energy self-balancing path can be further reduced, thereby reducing the resistor volume and reducing the impact on the original layout and water cooling design of the flexible DC converter valve.
  • 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 overvoltage of full-bridge module capacitors during system DC faults, reduce the proportion of full-bridge modules in full-half-bridge hybrid flexible DC converter valves, and further reduce equipment costs.
  • FIG1 is a schematic structural diagram of an energy self-balancing flexible DC converter valve provided in an embodiment of the present application.
  • FIG2 is a schematic flow chart of a control method for an energy self-balancing flexible DC converter valve provided in an embodiment of the present application
  • FIG3 is a schematic diagram of the structure of a DC system with an AC energy consuming device at the sending end provided in an embodiment of the present application.
  • 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;
  • the full-bridge energy self-balancing submodule and the half-bridge energy self-balancing submodule are both connected in series with the bridge arm reactor;
  • 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 system fault occurs, resulting in an overvoltage risk in the optimized MMC submodule capacitor.
  • the full-bridge energy self-balancing submodule, the half-bridge energy self-balancing submodule and the bridge arm reactor on the bridge arm are connected in series.
  • the energy self-balancing circuit 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, the surplus energy can be dissipated, thereby coping with the surplus power failure of the system.
  • 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 jointly bear the energy dissipation, which can meet the needs of the system.
  • the optimized MMC submodule is a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule.
  • one end of the upper bridge arm is connected to the positive electrode 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 pole of the DC end of the converter valve, and the other end is connected to the AC end of the converter valve.
  • the optimized MMC submodule in this embodiment is a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule, and each submodule can obtain its own capacitor voltage, perform voltage-based fault analysis, and then make corresponding triggers. operate.
  • 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 submodules and half-bridge energy self-balancing submodules, each submodule has an energy self-balancing circuit, and the power electronic switches and energy dissipation resistors in the circuit can dissipate surplus energy when the transmission system fails; the energy dissipation resistors in all submodules can be shared, which can meet the actual problem of surplus power dissipation; and the device structure involved is simple and regular, easy to configure and execute, and has practical application significance.
  • the embodiment of the present application can solve the technical problems that the prior art is either complex in structure and high in cost, or the energy consumption response time is too long to easily cause overvoltage, and cannot economically and reliably solve the current new energy island ultra-long-distance DC transmission system application needs.
  • Step 201 real-time monitoring and optimization of capacitor voltage in the MMC submodule
  • Step 202 If the capacitor voltage exceeds the conduction threshold, the power electronic switch of the energy self-balancing circuit is triggered to conduct, thereby dissipating the surplus energy;
  • 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 shut down, thereby terminating the energy dissipation operation;
  • Step 204 If the total energy discharged by the energy dissipation resistor of the energy self-balancing circuit exceeds its energy dissipation threshold, the AC energy consumption device at the sending end of the DC power transmission system is triggered to assist in energy consumption;
  • Step 205 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 until the temperature of the energy dissipation resistor is balanced with the ambient temperature.
  • this process is a control method for the above-mentioned energy self-balancing flexible DC converter valve embodiment, and the execution subject is not limited, as long as the energy self-balancing flexible DC converter valve can be controlled based on this method, such as a computer or other device.
  • the optimized MMC submodule in this embodiment is a full-bridge energy self-balancing submodule or a half-bridge energy self-balancing submodule, and each submodule can obtain its own module capacitor voltage, and perform threshold judgment respectively to make corresponding trigger operations.
  • the energy discharge threshold can be set according to the actual situation. No limitation.
  • Uconduction is the conduction threshold
  • k is the first margin
  • the typical value range is 10%-20%
  • Ucut is the submodule locking voltage.
  • Uoff is the off threshold
  • m is the second margin
  • a typical value range is 5%-10%.
  • the total energy discharged by a 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:
  • R is the resistance value of the energy-discharging resistor
  • ER is the maximum withstand energy of the energy-discharging resistor
  • ⁇ T is the duration of the single AC fault under consideration
  • n is the on-duty cycle of the energy-discharging resistor.
  • the configuration process of the energy dissipation threshold of the energy dissipation resistor is as follows:
  • E energy discharge threshold is the energy discharge threshold of the energy discharge resistor
  • t1 is the communication time from the sending end to the receiving end of the DC transmission system
  • t2 is the enabling delay of the AC energy consumption device at the sending end.
  • the surplus power when a fault occurs in the DC system, resulting in energy imbalance at 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, and at this time, the submodule capacitor voltage will continue to increase until it rises to the capacitor voltage upper limit U and is turned on , and then the surplus power is discharged through the energy balance circuit through the energy discharge resistor.
  • This method makes full use of the energy margin of the optimized MMC submodule capacitor to recover the system surplus power, which can reduce the waste caused by directly dissipating the surplus power in the form of heat through the energy discharge resistor.
  • the power electronic switch 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.
  • the energy dissipation threshold is a value set according to the maximum tolerance energy of the equipment, such as the tolerance energy. Then the energy dissipation threshold can be selected based on the total energy discharged by the energy dissipation resistor under the communication time from the sending end to the receiving end of the DC transmission system and the enabling delay of the AC energy consumption device at the sending end.
  • 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 until the temperature of the energy dissipation resistor is balanced with the ambient temperature.
  • the on-threshold, off-threshold and energy dissipation threshold are involved in the monitoring and surplus energy dissipation process, and these thresholds are adaptively configured and meet certain value conditions.
  • the detailed energy self-balancing control scheme is described as follows: real-time monitoring of the capacitor voltage Uc of each submodule of the converter valve.
  • 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.
  • the power of the new energy generation fails to change in time, resulting in an unbalanced power transmission between the sending end and the receiving end.
  • the full/half-bridge energy self-balancing submodule capacitor of the converter valve preferentially recovers the surplus power of the DC system, and the capacitor voltage continues to rise.
  • the power electronic switch in the energy self-balancing circuit When it is monitored that the submodule capacitor voltage rises above 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 submodule capacitor voltage gradually decreases.
  • the switch in the energy self-balancing circuit is turned off.
  • the sending-end AC energy consumption device is put into operation, please refer to Figure 3, until the fault is cleared, and the AC energy consumption device is gradually exited.
  • the naming of the sub-modules in Figure 3 is the same as explained above, and corresponds one-to-one with the specified names in this embodiment, and the details will not be repeated.
  • the present application also provides an embodiment of a DC system, including: a new energy station, a sending-end flexible DC converter station, a receiving-end flexible DC converter station, and an AC energy consumption device;
  • 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;
  • the new energy station is connected to the sending-end flexible DC converter station through 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 energy consumption device is connected between the new energy station and the sending-end flexible DC converter station, and 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.
  • the surplus power can be dissipated first through the energy self-balancing flexible DC converter valve configured in the flexible DC converter station.
  • the total energy discharged by the energy dissipation resistor in the flexible DC converter valve during the fault period reaches its own energy dissipation threshold, it is necessary to notify the sending end of the DC system to put its AC energy consumption device into the system until the fault is cleared and gradually withdraw from AC energy consumption.
  • the system provided in this embodiment solves the problem of system overvoltage caused by the AC energy-consuming device at the sending end not being able to dissipate the surplus energy in time when an AC fault occurs at the receiving end.
  • the surplus power in the module is released in time through active control of the energy self-balancing circuit, so as to avoid the overvoltage of the submodule capacitor voltage caused by the serious power surplus, which threatens the safety and reliability of the system.
  • the flexible DC converter valve only releases the surplus power after the module capacitor voltage reaches the warning value, that is, exceeds the conduction threshold.
  • the energy dissipation resistor value in the flexible DC converter valve can be further reduced, thereby reducing the resistor volume and reducing the impact on the original layout and water cooling design of the flexible DC converter valve.
  • the disclosed devices and methods can be implemented in other ways.
  • the device embodiments described above are only schematic.
  • the division of the units is only a logical function division. There may be other division methods in actual implementation, 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.
  • the units described as separate components may or may not be physically separate.
  • the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.
  • each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit.
  • the above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.
  • 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.
  • the technical solution of the present application is essentially 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, including a number of 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 in English: Read-Only Memory, English abbreviation: ROM), random access memory (full name in English: Random Access Memory, English abbreviation: RAM), disk or optical disk and other media that can store program codes.

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Abstract

一种能量自平衡柔性直流换流阀、控制方法及直流系统，涉及输配电网技术领域，换流阀包括三个相单元，相单元包括上桥臂和下桥臂；上桥臂和下桥臂均包含若干全桥能量自平衡子模块和若干半桥能量自平衡子模块；全/半桥能量自平衡子模块均为包括能量自平衡电路的优化MMC子模块；能量自平衡电路由电力电子开关和泄能电阻串联构成，用于在子模块中的电容电压发生过压预警时进行盈余能量消散。本申请能在不影响传统换流阀运行逻辑的基础上，以极小的成本代价解决新能源孤岛超远距离直流送出场景中，直流输电系统因发生故障导致盈余功率无法及时消散的技术难题；同时能充分利用优化MMC子模块中的电容暂存能量，减少盈余功率以热量耗散产生的浪费。

Description

一种能量自平衡柔性直流换流阀、控制方法及直流系统
本申请要求于2023年10月25日提交中国专利局、申请号为202311395835.7、发明名称为“一种能量自平衡柔性直流换流阀、控制方法及直流系统”的中国专利申请的优先权，以及要求于2023年10月31日提交中国专利局、申请号为202311438766.3、发明名称为“一种能量自平衡柔性直流换流阀、控制方法及直流系统”的中国专利申请的优先权，其内容通过引用结合在本申请中。
技术领域
本申请涉及输配电网技术领域，尤其涉及一种能量自平衡柔性直流换流阀及其控制方法。
背景技术
在全球能源变革背景下，新能源大规模接入电网，而且新能源基地大多建设在偏远地区，负荷水平低、网架结构薄弱，新能源孤岛稳定外送需求明显。基于模块化多电平换流器(Modular Multilevel Converter，MMC)的柔性直流输电以其灵活、可控、高效等特点，成为新能源外送的重要电力输送手段之一。当柔性直流输电应用于连接孤岛发电系统和受端交流电网时，在受端交流电网发生故障后，由于电能无法送出，若不切除送端发电系统，会引起直流系统出现大量盈余功率，造成直流系统出现严重过电压，危及系统安全运行。
现有技术主要通过两种方案消散大量的盈余功率，第一种是在受端换流站的直流侧布设直流耗能装置，在故障时消耗多余功率，实现送端孤岛发电系统不切除即可实现故障穿越；但是这种方法中的直流耗能装置结构复杂，涉及大量的可控型功率器件，造价昂贵，且需要额外的占地。第二种是在送端换流站的交流线路中安装交流耗能装置，其拓扑结构简单，成本较低。但是由于交流耗能装置安装在送端，当受端发生故障时，需要基于通讯等手段通知送端投入耗能装置，对于超远距离输电系统而言，通讯延时较长，在故障期间可能导致大量盈余功率持续涌入柔性直流换流阀， 在送端耗能装置来不及投入的情况下引起直流输电系统过压闭锁。
发明内容
本申请提供了一种能量自平衡柔性直流换流阀及其控制方法，用于解决现有技术要么结构复杂且成本高，要么耗能响应时间太长容易引发过电压，无法经济可靠解决当前新能源孤岛超远距离直流送出系统应用需求的技术问题。
有鉴于此，本申请第一方面提供了一种能量自平衡柔性直流换流阀，包括：三个相单元，所述相单元包括上桥臂和下桥臂；
所述上桥臂和所述下桥臂均包括若干全桥能量自平衡子模块、若干半桥能量自平衡子模块和桥臂电抗器；
所述全桥能量自平衡子模块和所述半桥能量自平衡子模块均与所述桥臂电抗器串联；
所述全桥能量自平衡子模块和所述半桥能量自平衡子模块均为包括能量自平衡电路的优化MMC子模块；
所述能量自平衡电路由电力电子开关和泄能电阻串联构成，用于在直流输电系统因发生故障导致所述优化MMC子模块电容出现过压风险时进行盈余能量消散。
优选地，所述能量自平衡电路的两端与所述优化MMC子模块中电容的正极和负极连接，所述优化MMC子模块为所述全桥能量自平衡子模块或所述半桥能量自平衡子模块。
优选地，所述上桥臂的一端与换流阀直流端的正极连接，另一端与所述换流阀的交流端连接；
所述下桥臂的一端与所述换流阀直流端的负极连接，另一端与所述换流阀的交流端连接。
本申请第二方面提供了一种能量自平衡柔性直流换流阀的控制方法，通过第一方面中任意一种所述的柔性直流换流阀实现，包括：
实时监测优化MMC子模块中的电容电压；
若所述电容电压超过导通阈值，则触发能量自平衡电路的电力电子开 关导通，实现盈余能量消散；
若所述电容电压低于关断阈值，则触发所述能量自平衡电路的所述电力电子开关关断，终止能量消散操作；
若所述能量自平衡电路的所述泄能电阻泄放的总能量超过其泄能阈值，则触发直流输电系统送端交流耗能装置辅助耗能；
若所述能量自平衡电路的泄能电阻泄放的总能量超过所述泄能电阻的最大耐受能量，则触发所述能量自平衡电路的所述电力电子开关关断，终止能量消散操作，并禁止所述电力电子开关再次导通，直至泄能电阻温度与环境温度平衡。
优选地，所述导通阈值的配置过程为：
U导通＝(1-k)·Ucut；
其中，U导通为所述导通阈值，k为第一裕度，典型值取值范围是10％-20％，Ucut为子模块闭锁电压。
优选地，所述关断阈值的配置过程为：
U关断＝(1-m)·U导通；
其中，U关断为所述关断阈值，m为第二裕度，典型值取值范围是5％-10％。
优选地，单个所述泄能电阻泄放的总能量不得超过其最大耐受能量，所述泄能电阻的最大耐受能量配置过程为：
其中，R为所述泄能电阻的电阻值；ER为所述泄能电阻的最大耐受能量；ΔT为所考虑的单次交流故障的持续时间；n为泄能电阻的导通占空比。
优选地，所述泄能电阻的所述泄能阈值的配置过程为：
其中，E泄能阈值为所述泄能电阻的所述泄能阈值，t1为直流输电系统送端至受端的通讯时长，t2为送端交流耗能装置使能延时。
本申请第三方面提供了一种直流系统，包括：新能源场站、送端柔性直流换流站、受端柔性直流换流站和交流耗能装置；
所述送端柔性直流换流站和所述受端柔性直流换流站均配置第一方面中任意一种所述能量自平衡柔性直流换流阀；
所述新能源场站通过三相交流母线与所述送端柔性直流换流站连接；
所述送端柔性直流换流站与所述受端柔性直流换流站之间通过直流线路连接；
所述交流耗能装置连接在所述新能源场站和所述送端柔性直流换流站之间，在所述能量自平衡柔性直流换流阀中的所述泄能电阻泄放的总能量超过其泄能阈值时，用于辅助盈余能量消耗。
从以上技术方案可以看出，本申请实施例具有以下优点：
本发明相对于现有的直流耗能装置和交流耗能装置平衡盈余能量两种技术路线，可无需目前工程中广泛采用的直流耗能装置，在实现相同故障穿越效能的基础上，可大大降低工程造价，节省换流站面积，提高经济性，更具成本优势；同时，在大规模新能源孤岛超远距离直流送出场景中，解决了发生受端交流故障时，仅依靠送端交流耗能装置来不及耗散盈余能量引起系统过压的问题，通过能量自平衡电路主动控制及时泄放模块内盈余功率，避免由于功率盈余造成子模块电容过电压威胁系统的安全与可靠性。
本发明的能量自平衡柔性直流换流阀设计，每个优化MMC子模块中均带有能量自平衡电路，保证了每个子模块电容在均压排序过程中的充放电频率相近，系统的盈余能量可由所有能量自平衡模块中的泄能电阻共同分担，因此较小体积的泄能电阻可满足泄能需求，对子模块的原有体积及布置影响小。
本发明提出的能量自平衡控制方法，当直流系统出现盈余功率时，先将盈余功率存储在柔性直流换流站的优化MMC子模块电容内，直到子模块电容电压上升至导通阈值时，再通过能量平衡电路中的泄能电阻泄放盈余功率，可以充分利用换流站优化MMC子模块电容的能量裕度对系统盈余功率进行回收，减少直接通过泄能电阻将盈余功率以热量形式耗散产生的浪费。
基于本发明提出的新型新能源孤岛超远距离直流送出系统的故障穿越方法，能量自平衡柔性直流换流阀只泄放模块电容电压达到导通阈值后的盈余功率，同时在故障持续时间较长的场景中，当单个泄能电阻泄放的总能量达到设备最大耐受能量时，将配合投入送端交流耗能装置，因此，可进一步减小能量自平衡通路中的泄能电阻值，从而减小电阻体积，减小对柔性直流换流阀的原有布置及水冷设计影响。
基于本发明提出的新型新能源孤岛超远距离直流送出系统及故障穿越方法，还可以解决系统直流故障期间全桥模块电容过压问题，降低全半桥混合型柔性直流换流阀中的全桥模块比例，进一步降低设备成本。
附图说明
图1为本申请实施例提供的一种能量自平衡柔性直流换流阀的结构示意图；
图2为本申请实施例提供的一种能量自平衡柔性直流换流阀的控制方法的流程示意图；
图3为本申请实施例提供的送端投入交流耗能装置的直流系统结构示意图。
具体实施方式
为了使本技术领域的人员更好地理解本申请方案，下面将结合本申请实施例中的附图，对本申请实施例中的技术方案进行清楚、完整地描述，显然，所描述的实施例仅是本申请一部分实施例，而不是全部的实施例。基于本申请中的实施例，本领域普通技术人员在没有做出创造性劳动前提下所获得的所有其他实施例，都属于本申请保护的范围。
为了便于理解，请参阅图1，本申请提供的一种能量自平衡柔性直流换流阀的实施例，包括：三个相单元，相单元包括上桥臂和下桥臂；
上桥臂和下桥臂均包括若干全桥能量自平衡子模块、若干半桥能量自平衡子模块和桥臂电抗器；
全桥能量自平衡子模块和半桥能量自平衡子模块均与桥臂电抗器串联；
全桥能量自平衡子模块和半桥能量自平衡子模块均为包括能量自平衡电路的优化MMC子模块；
能量自平衡电路由电力电子开关和泄能电阻串联构成，用于在发生直流系统故障导致优化MMC子模块电容出现过压风险时进行盈余能量消散。
需要说明的是，桥臂上的全桥能量自平衡子模块、半桥能量自平衡子模块和桥臂电抗器之间通过串联的方式连接。而能量自平衡电路通过电路的导通和断开的方式将泄能电阻接入系统或者切除出系统；在泄能电阻接入系统时就可以进行盈余能量消散，从而应对系统的盈余功率故障。此外，本实施例中的全桥能量自平衡子模块和半桥能量自平衡子模块都是在传统MMC子模块结构上增加了能量自平衡电路之后得到的优化MMC子模块，由于改进后的每个优化MMC子模块中均包括能量自平衡电路，所以存在足够的泄能电阻共同承担能量消散，能够满足系统的需求。
进一步地，能量自平衡电路的两端与优化MMC子模块中电容的正极和负极连接，优化MMC子模块为全桥能量自平衡子模块或半桥能量自平衡子模块。
进一步地，上桥臂的一端与换流阀直流端的正极连接，另一端与换流阀的交流端连接；
下桥臂的一端与换流阀直流端的负极连接，另一端与换流阀的交流端连接。
需要说明的是，优化MMC子模块中电容的电压值可以反映系统的运行状态，即可以用于分析系统是否出现故障，并根据分析的故障作出反应。能量自平衡电路分别接在优化MMC子模块中电容的两端，便于在电容的电压超过导通阈值时，触发能量自平衡电路中的电力电子开关，从而将泄能电阻接入系统，实现盈余功率的消散。可以理解的是，三个相单元的上桥臂一端均需要与换流阀直流端的正极相连，而下桥臂的一端均需要与换流阀直流端的负极相连。而且，本实施例中的优化MMC子模块为全桥能量自平衡子模块或者半桥能量自平衡子模块，各个子模块均可以分别获取各自的电容电压，并分别进行基于电压的故障分析，然后作出对应的触发 操作。
本申请实施例提供的能量自平衡柔性直流换流阀，其结构中的每个相单元均包含两个桥臂，而每个桥臂上包含若干全桥能量自平衡子模块和半桥能量自平衡子模块，每个子模块都有能量自平衡电路，该电路中的电力电子开关和泄能电阻能够在输电系统故障时进行盈余能量消散；所有子模块中的泄能电阻都可以共同分担，可以满足实际的盈余功率消散问题；而且涉及的器件结构简单有规律，容易配置执行，有实际应用意义。因此，本申请实施例能够解决现有技术要么结构复杂且成本高，要么耗能响应时间太长容易引发过电压，无法经济可靠解决当前新能源孤岛超远距离直流送出系统应用需求的技术问题。
为了便于理解，请参阅图2，本申请提供了一种能量自平衡柔性直流换流阀的控制方法的实施例，包括：
步骤201、实时监测优化MMC子模块中的电容电压；
步骤202、若电容电压超过导通阈值，则触发能量自平衡电路的电力电子开关导通，实现盈余能量消散；
步骤203、若电容电压低于关断阈值，则触发能量自平衡电路的电力电子开关关断，终止能量消散操作；
步骤204、若能量自平衡电路的泄能电阻泄放的总能量超过其泄能阈值，则触发直流输电系统送端交流耗能装置辅助耗能；
步骤205、若能量自平衡电路的泄能电阻泄放的总能量超过泄能电阻的最大耐受能量，则触发能量自平衡电路的电力电子开关关断，终止能量消散操作，并禁止电力电子开关再次导通，直至泄能电阻温度与环境温度平衡。
需要说明的是，该过程是用于上述能量自平衡柔性直流换流阀实施例的控制方法，执行主体不作限定，只要能够基于该方法实现对能量自平衡柔性直流换流阀的控制即可，例如计算机或者其他设备。而且，本实施例中的优化MMC子模块为全桥能量自平衡子模块或者半桥能量自平衡子模块，各个子模块均可以分别获取各自的模块电容电压，并分别进行阈值判断，作出对应的触发操作。此外，泄能阈值可以根据实际情况设置，在此 不作限定。
进一步地，导通阈值的配置过程为：
U导通＝(1-k)·Ucut；
其中，U导通为导通阈值，k为第一裕度，典型值取值范围是10％-20％，Ucut为子模块闭锁电压。
进一步地，关断阈值的配置过程为：
U关断＝(1-m)·U导通；
其中，U关断为关断阈值，m为第二裕度，典型值取值范围是5％-10％。
进一步地，单个泄能电阻泄放的总能量不得超过其最大耐受能量，泄能电阻的最大耐受能量配置过程为：
其中，R为泄能电阻的电阻值；ER为泄能电阻的最大耐受能量；ΔT为所考虑的单次交流故障的持续时间；n为泄能电阻的导通占空比。
泄能电阻选取过程中需要满足以上电阻选取约束条件，而不是随意选取配置。
进一步地，泄能电阻的泄能阈值的配置过程为：
其中，E泄能阈值为泄能电阻的泄能阈值，t1为直流输电系统送端至受端的通讯时长，t2为送端交流耗能装置使能延时。
需要说明的是，采用上述控制方法，当直流系统发生故障导致送受端能量不平衡从而出现系统盈余功率时，盈余功率先会存储在能量自平衡换流阀的优化MMC子模块电容内，此时子模块电容电压将不断升高，直到上升至电容电压上限U导通时，再通过能量平衡电路中的泄能电阻泄放盈余功率。此方法充分利用优化MMC子模块电容的能量裕度对系统盈余功率进行回收，可以减少直接通过泄能电阻将盈余功率以热量形式耗散产生的浪费。
当监测电压超过导通阈值可以触发电力电子开关导通，从而通过泄能电阻实现盈余能量消散，直至监测电压下降到关断阈值，即可触发电力电子开关断开，终止盈余能量消散操作。
但是，若是在故障期间，泄能电阻泄放的总能量达到了其自身的泄能阈值，则说明受端无法通过自身装置满足盈余能量消散，需要通知送端投入具体的耗能装置辅助耗能。此处的泄能阈值根据设备最大耐受能量设置的一个值，例如耐受能量，那么泄能阈值可以基于考虑直流输电系统送端至受端的通讯时长和送端交流耗能装置使能延时下泄能电阻泄放的总能量选取。
当泄能电阻泄放的总能量超过泄能电阻的最大耐受能量，则触发能量自平衡电路的电力电子开关关断，终止能量消散操作，并禁止电力电子开关再次导通，直至泄能电阻温度与环境温度平衡。
基于以上可知，在监测和盈余能量消散过程中涉及到导通阈值、关断阈值和泄能阈值，这些阈值都各自进行自适应配置并且满足一定的取值条件。
详细的能量自平衡控制方案描述为：实时监测换流阀各子模块的电容电压Uc，当换流阀正常运行时，能量自平衡电路中的电力电子开关为关断状态，换流阀只具备能量交换功能。当系统发生故障后，因新能源发电功率未能及时改变，导致送端受端功率传输不平衡，为减小盈余功率以热量形式耗散产生浪费，由换流阀的全/半桥能量自平衡子模块电容优先回收直流系统的盈余功率，此时电容电压持续上升。当监测到子模块电容电压升超过导通阈值后，导通能量自平衡电路中的电力电子开关，盈余功率由能量自平衡电路中的泄能电阻耗散，子模块电容电压逐渐下降，当检测到电容电压下降至断开阈值后，关断能量自平衡电路中的开关。在泄能电路导通期间，若泄能电阻达到设备耐受能量的泄能阈值时，送端交流耗能装置投入，请参阅图3，直到故障清除，逐步退出交流耗能装置，图3中的子模块命名同上文解释，与本实施例中限定名称一一对应，具体的不再赘述。
为了便于理解，本申请还提供了一种直流系统的实施例，包括：新能源场站、送端柔性直流换流站、受端柔性直流换流站和交流耗能装置；
送端柔性直流换流站和受端柔性直流换流站均配置上述实施例中任意一种能量自平衡柔性直流换流阀；
新能源场站通过三相交流母线与送端柔性直流换流站连接；
送端柔性直流换流站与受端柔性直流换流站之间通过直流线路连接；
交流耗能装置连接在新能源场站和送端柔性直流换流站之间，在能量自平衡柔性直流换流阀中的泄能电阻泄放的总能量超过其泄能阈值时，用于辅助盈余能量消耗。
需要说明的是，在系统的受端发生交流故障或者直流故障时，可以先通过柔性直流换流站均配置的能量自平衡柔性直流换流阀进行盈余功率消散。但是，若柔性直流换流阀中的泄能电阻在故障期间泄放的总能量达到了其自身的泄能阈值，则需要通知直流系统送端将其交流耗能装置投入到系统中，直至故障清除，逐步退出交流耗能。
本实施例提供的系统解决了发生受端交流故障时，仅依靠送端交流耗能装置来不及耗散盈余能量引起系统过压的问题，通过能量自平衡电路主动控制及时泄放模块内盈余功率，避免由于功率盈余严重造成子模块电容电压过电压威胁系统的安全与可靠性。而且，柔性直流换流阀只泄放模块电容电压达到预警值，即超过导通阈值后的盈余功率，同时在故障持续时间较长的场景中，当泄能电阻达到设备耐受能量的泄能阈值时，将配合投入送端交流耗能装置，因此，可进一步减小柔性直流换流阀中的泄能电阻值，从而减小电阻体积，减小对柔性直流换流阀的原有布置及水冷设计影响。
在本申请所提供的几个实施例中，应该理解到，所揭露的装置和方法，可以通过其它的方式实现。例如，以上所描述的装置实施例仅仅是示意性的，例如，所述单元的划分，仅仅为一种逻辑功能划分，实际实现时可以有另外的划分方式，例如多个单元或组件可以结合或者可以集成到另一个系统，或一些特征可以忽略，或不执行。另一点，所显示或讨论的相互之间的耦合或直接耦合或通信连接可以是通过一些接口，装置或单元的间接耦合或通信连接，可以是电性，机械或其它的形式。
所述作为分离部件说明的单元可以是或者也可以不是物理上分开的， 作为单元显示的部件可以是或者也可以不是物理单元，即可以位于一个地方，或者也可以分布到多个网络单元上。可以根据实际的需要选择其中的部分或者全部单元来实现本实施例方案的目的。
另外，在本申请各个实施例中的各功能单元可以集成在一个处理单元中，也可以是各个单元单独物理存在，也可以两个或两个以上单元集成在一个单元中。上述集成的单元既可以采用硬件的形式实现，也可以采用软件功能单元的形式实现。
所述集成的单元如果以软件功能单元的形式实现并作为独立的产品销售或使用时，可以存储在一个计算机可读取存储介质中。基于这样的理解，本申请的技术方案本质上或者说对现有技术做出贡献的部分或者该技术方案的全部或部分可以以软件产品的形式体现出来，该计算机软件产品存储在一个存储介质中，包括若干指令用以通过一台计算机设备(可以是个人计算机，服务器，或者网络设备等)执行本申请各个实施例所述方法的全部或部分步骤。而前述的存储介质包括：U盘、移动硬盘、只读存储器(英文全称：Read-Only Memory，英文缩写：ROM)、随机存取存储器(英文全称：Random Access Memory，英文缩写：RAM)、磁碟或者光盘等各种可以存储程序代码的介质。
以上所述，以上实施例仅用以说明本申请的技术方案，而非对其限制；尽管参照前述实施例对本申请进行了详细的说明，本领域的普通技术人员应当理解：其依然可以对前述各实施例所记载的技术方案进行修改，或者对其中部分技术特征进行等同替换；而这些修改或者替换，并不使相应技术方案的本质脱离本申请各实施例技术方案的精神和范围。

Claims (9)

  1. 一种能量自平衡柔性直流换流阀，其特征在于，包括：三个相单元，所述相单元包括上桥臂和下桥臂；
    所述上桥臂和所述下桥臂均串联有若干全桥能量自平衡子模块、若干半桥能量自平衡子模块和桥臂电抗器；
    所述全桥能量自平衡子模块和所述半桥能量自平衡子模块均为包括能量自平衡电路的优化MMC子模块；
    所述能量自平衡电路由电力电子开关和泄能电阻串联构成，用于在直流输电系统因发生故障导致所述优化MMC子模块电容出现过压风险时进行盈余能量消散。
  2. 根据权利要求1所述的能量自平衡柔性直流换流阀，其特征在于，所述能量自平衡电路的两端与所述优化MMC子模块中电容的正极和负极连接，所述优化MMC子模块为所述全桥能量自平衡子模块或所述半桥能量自平衡子模块。
  3. 根据权利要求1所述的能量自平衡柔性直流换流阀，其特征在于，所述上桥臂的一端与换流阀直流端的正极连接，另一端与所述换流阀的交流端连接；
    所述下桥臂的一端与所述换流阀直流端的负极连接，另一端与所述换流阀的交流端连接。
  4. 一种能量自平衡柔性直流换流阀的控制方法，通过权利要求1-3中任意一种所述的柔性直流换流阀实现，其特征在于，包括：
    实时监测优化MMC子模块中的电容电压；
    若所述电容电压超过导通阈值，则触发能量自平衡电路的电力电子开关导通，实现盈余能量消散；
    若所述电容电压低于关断阈值，则触发所述能量自平衡电路的所述电力电子开关关断，终止能量消散操作；
    若所述能量自平衡电路的所述泄能电阻泄放的总能量超过其泄能阈值，则触发直流输电系统送端交流耗能装置辅助耗能；
    若所述能量自平衡电路的泄能电阻泄放的总能量超过所述泄能电阻的 最大耐受能量，则触发所述能量自平衡电路的所述电力电子开关关断，终止能量消散操作，并禁止所述电力电子开关再次导通，直至泄能电阻温度与环境温度平衡。
  5. 根据权利要求4所述的能量自平衡柔性直流换流阀的控制方法，其特征在于，所述导通阈值的配置过程为：
    U导通＝(1-k)·Ucut；
    其中，U导通为所述导通阈值，k为第一裕度，典型值取值范围是10％-20％，Ucut为子模块闭锁电压。
  6. 根据权利要求5所述的能量自平衡柔性直流换流阀的控制方法，其特征在于，所述关断阈值的配置过程为：
    U关断＝(1-m)·U导通；
    其中，U关断为所述关断阈值，m为第二裕度，典型值取值范围是5％-10％。
  7. 根据权利要求6所述的能量自平衡柔性直流换流阀的控制方法，其特征在于，单个所述泄能电阻泄放的总能量不得超过其最大耐受能量，所述泄能电阻的最大耐受能量配置过程为：
    其中，R为所述泄能电阻的电阻值；ER为所述泄能电阻的最大耐受能量；ΔT为所考虑的单次交流故障的持续时间；n为泄能电阻的导通占空比。
  8. 根据权利要求7所述的能量自平衡柔性直流换流阀的控制方法，其特征在于，所述泄能电阻的所述泄能阈值的配置过程为：
    其中，E泄能阈值为所述泄能电阻的所述泄能阈值，t1为直流输电系统送端至受端的通讯时长，t2为送端交流耗能装置使能延时。
  9. 一种直流系统，其特征在于，包括：新能源场站、送端柔性直流换流站、受端柔性直流换流站和交流耗能装置；
    所述送端柔性直流换流站和所述受端柔性直流换流站均配置权利要求1-3中任意一种所述能量自平衡柔性直流换流阀；
    所述新能源场站通过三相交流母线与所述送端柔性直流换流站连接；
    所述送端柔性直流换流站与所述受端柔性直流换流站之间通过直流线路连接；
    所述交流耗能装置连接在所述新能源场站和所述送端柔性直流换流站之间，在所述能量自平衡柔性直流换流阀中的所述泄能电阻泄放的总能量超过其泄能阈值时，用于辅助盈余能量消耗。
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