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WO2021017170A1 - 多端口直流潮流控制的模块化多电平变流器及控制方法 - Google Patents

多端口直流潮流控制的模块化多电平变流器及控制方法 Download PDF

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Publication number
WO2021017170A1
WO2021017170A1 PCT/CN2019/110679 CN2019110679W WO2021017170A1 WO 2021017170 A1 WO2021017170 A1 WO 2021017170A1 CN 2019110679 W CN2019110679 W CN 2019110679W WO 2021017170 A1 WO2021017170 A1 WO 2021017170A1
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WIPO (PCT)
Prior art keywords
power flow
flow controller
bridge
port
phase
Prior art date
Application number
PCT/CN2019/110679
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English (en)
French (fr)
Inventor
张建文
方玮昕
章一新
周剑桥
施刚
蔡小龙
王晗
蔡旭
汪家城
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上海交通大学
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Application filed by 上海交通大学 filed Critical 上海交通大学
Publication of WO2021017170A1 publication Critical patent/WO2021017170A1/zh
Priority to US17/197,071 priority Critical patent/US11133752B2/en

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    • 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
    • H02M1/0095—Hybrid converter topologies, e.g. NPC mixed with flying capacitor, thyristor converter mixed with MMC or charge pump mixed with buck
    • 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
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H02J1/10—Parallel operation of dc sources
    • H02J1/106—Parallel operation of dc sources for load balancing, symmetrisation, or sharing
    • 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
    • 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
    • H02M1/0003—Details of control, feedback or regulation circuits
    • H02M1/0006—Arrangements for supplying an adequate voltage to the control circuit of 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0003—Details of control, feedback or regulation circuits
    • H02M1/0012—Control circuits using digital or numerical techniques
    • 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
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
    • H02M3/158—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators including plural semiconductor devices as final control devices for a single load
    • 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
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33573—Full-bridge at primary side of an isolation transformer
    • 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
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33576—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
    • H02M3/33584—Bidirectional 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
    • H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
    • H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
    • H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/21—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/217—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M7/219—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only in a bridge configuration
    • 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
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
    • 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
    • H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
    • H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
    • H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • 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
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
    • 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
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/49—Combination of the output voltage waveforms of a plurality of converters
    • 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 invention relates to the fields of smart grid technology, power electronics technology and the like in power systems, and in particular to a modular multilevel converter with multi-port DC power flow control and a control method.
  • Multi-terminal DC and DC grid technology is one of the effective technical means to solve the problems of grid connection and consumption of new energy.
  • the ring and mesh structure have higher advantages than other topologies.
  • the multi-terminal DC transmission technology forms a gridded DC grid through the interconnection of the DC terminals of the converter. Compared with the traditional two-terminal DC transmission, it can realize multi-source power supply and multi-drop power reception, and the system reliability is enhanced. At the same time, the cost and operating expenses are lower and it is more economical and flexible.
  • the mesh structure of the multi-terminal DC distribution network has good flexibility and redundancy, which can realize flexible dispatch and management of distributed energy, realize a large number of acceptance, optimize configuration, and make full use of different types of distributed energy. In addition, once a local failure occurs, effective isolation and network reconstruction methods can be used to minimize the impact on the distribution network.
  • the ring and mesh structure play a role in improving the operational reliability of the DC system, realizing the interconnection and mutual benefit of electric energy, and improving the efficiency of resource utilization.
  • there may be multiple DC lines between controllable nodes making the number of lines greater than the number of controllable nodes-1, resulting in the line flow cannot rely solely on the voltage and current of each node Control to achieve effective adjustment.
  • the high voltage DC transmission system mostly uses voltage source converter (VSC) to form the converter station, and the medium voltage DC distribution network uses voltage source conversion.
  • VSC voltage source converter
  • a current transformer (VSC) or a solid-state transformer (SST) with a medium voltage DC port is the core device.
  • VSC voltage source converter
  • SST solid-state transformer
  • the traditional two-level or three-level structure is difficult to meet the needs of the system's high and medium voltage levels
  • the modular multi-level structure is modular in design, high output level, easy to expand in capacity and voltage level
  • Advanced advantages have been extensively researched and applied in the field of DC transmission and distribution, and become the realization method and topological basis of converter stations and solid-state transformers in DC systems.
  • the direct current system controls the direct current flow by adjusting the line resistance and direct current voltage.
  • a variable resistor can be connected in series to control the DC power flow. This method has large losses, high requirements for heat dissipation of the device, and can only adjust the flow in one direction.
  • one of the solutions is to connect a DC/DC converter with a transformation ratio of about 1 to the DC system of the same voltage level, and adjust the DC system power flow by fine-tuning the transformation ratio, but at this time all power is Need to pass through the DC/DC converter, the cost and loss are relatively large.
  • Another solution is to string an adjustable voltage source in the line to change the DC voltage, thereby controlling the DC power flow.
  • This circuit structure requires an additional power supply, a high-voltage isolation transformer for voltage isolation, and more switching devices.
  • a line-to-line DC power flow controller which does not require an external power source, and uses the power exchange between lines to be equivalent to connecting an adjustable voltage source in the line to achieve power flow control.
  • Balance control makes the structure and control of the device more complicated, and is limited to a certain extent by the transmission power of the line.
  • the purpose of the present invention is to propose a modular multi-level converter with multi-port DC power flow control and its modulation method to solve the power flow control problem in the DC system and ensure the stable operation of the DC system.
  • the present invention adopts the following technical solutions.
  • a modular multilevel converter for multi-port DC power flow control including: a modular multilevel converter topology and a DC power flow controller.
  • the converter topology is composed of multiple upper and lower bridge arms with the same structure, and each bridge arm is composed of multiple sub-modules cascaded, among which:
  • the input end of the DC power flow controller is connected in series with the uppermost sub-module of the upper bridge arm of the modular multilevel converter topology, and power is transferred to the AC power grid through the modular multilevel converter topology It interacts with energy, so that the converter station or solid-state transformer based on the modular multi-level converter topology has the DC power flow control capability.
  • the DC power flow controller includes an input terminal, a capacitor, and an output terminal, wherein the capacitor is connected in parallel between the input terminal and the output terminal, and the input terminal and the output terminal are realized by the capacitor. Energy transfer between terminals.
  • the input end of the DC power flow controller adopts a single-phase half-bridge or three-phase half-bridge structure.
  • the three-phase input terminal of the DC power flow controller is connected in series with the uppermost sub-module of the three-phase upper bridge arm in the modular multilevel converter topology.
  • the capacitor is connected in parallel with the single-phase half-bridge or three-phase half-bridge structure, and the single-phase half-bridge or three-phase half-bridge structure makes the capacitor exist by controlling the turn-on or turn-off of its upper and lower bridge arms.
  • the DC power flow controller has no other energy storage components except the capacitor, and by controlling the state of the single-phase half-bridge or three-phase half-bridge structure, the voltage of the capacitor is maintained Balancing is to realize the balance of input and output energy of the DC power flow controller.
  • the output terminal of the DC power flow controller adopts a DC/DC converter to adjust the output voltage, and a plurality of the DC/DC converters are connected in parallel to form a plurality of flexible and controllable DC ports, output voltage or output power
  • the size can be adjusted flexibly.
  • the output end of the DC power flow controller is provided with multiple DC ports, so that the converter station or solid-state transformer based on the modular multilevel converter topology provides multiple flexible and controllable DC ports, which can realize A mesh/ring multi-terminal flexible DC transmission system based on multiple converter stations or a mesh/ring multi-terminal flexible DC distribution system based on multiple solid-state transformers.
  • the method includes:
  • x 1 and y 1 are the voltage command DC component coefficients and fundamental frequency AC component coefficients of the single-phase half bridge of the DC power flow controller
  • x 2 , y 2 are the upper bridges of the modular multilevel converter topology.
  • the voltage command DC component coefficient and the fundamental frequency AC component coefficient of the arm u ap0 is the single-phase half-bridge AC side output voltage command
  • u ap , u an are the upper and lower arms of the modular multilevel converter topology
  • Output voltage command u aref is the AC side output voltage command of the modular multilevel converter topology
  • u aref U sin( ⁇ t)
  • U d is the DC bus voltage of the DC power flow controller;
  • U c0 is the DC component of the DC power flow controller capacitor voltage
  • U c1 is the DC component of the sub-module capacitor voltage of the modular multilevel converter topology
  • n is the DC component of the modular multilevel converter topology.
  • m 1 , m 2 , and m are respectively the voltage modulation coefficients of the upper and lower arm sub-modules of the DC power flow controller single-phase half-bridge and the modular multilevel converter topology:
  • U is the amplitude of the AC side phase voltage
  • the sum of the current at the upper and lower input sides of the capacitor is the sum of the DC components in the three-phase bridge arm current of the modular multilevel converter topology.
  • the sum of the DC components in the three-phase bridge arm currents of the modular multilevel converter topology of the DC power flow controller is I d , then
  • I d i SMp0- +i SMp0+
  • i SMp0- is the current at the lower input side of the DC power flow controller capacitor
  • i SMp0+ is the current at the upper input side of the DC power flow controller capacitor
  • i a is the A-phase alternating current
  • Is the power factor angle
  • ⁇ is the AC voltage frequency
  • t is the time
  • I ap (t) and I an (t) are respectively the A-phase upper and lower arm currents of the modular multilevel converter topology
  • the capacitor input side current of the DC power flow controller is determined by the three-phase confluence after the multiplication of the bridge arm currents of each phase and the single-phase half-bridge switching function:
  • F kp0 (t) is the average switching function of the single-phase half-bridge of the DC power flow controller; I is the amplitude of the AC side phase current; U d is the DC bus voltage of the DC power flow controller; i kp (t ) Is the single-phase upper and lower arm currents of the modular multilevel converter topology; a, b, and c are three-phase;
  • the current on the capacitor input side contains a low-frequency AC component in addition to the DC component.
  • the modular multilevel converter topology connected to the DC power flow controller is set
  • the DC component of the single-phase bridge arm current is I d
  • the upper and lower bridge arm currents of the modular multilevel converter topology can be respectively set as:
  • the current on the capacitor input side of the DC power flow controller is determined by the multiplication of the current of the phase bridge arm and the single-phase half-bridge switching function:
  • the input and output power balance equation of the DC power flow controller is:
  • P PEC is the input active power of the DC power flow controller
  • P MVDC is the output active power of the medium voltage DC side
  • a, b, and c are three-phase
  • the appropriate voltage command DC component coefficient and fundamental frequency AC component coefficient are selected to enable the voltage of the upper and lower arms of the DC power flow controller single-phase half bridge and the modular multilevel converter topology
  • the modulation factor satisfies the energy balance of the DC power flow controller.
  • the output DC port of the DC power flow controller adopts an open-loop control strategy.
  • the output voltage command value Voi of each DC port is calculated, and then the output voltage command value Voi and DC/DC conversion are obtained from the specific topology of the output DC/DC converter the relationship between the duty ratio relationship of D i's, and thus obtain the output port of each difference value ⁇ D ij voltage difference ⁇ V ij of each duty cycle:
  • the duty cycle of a certain DC port is selected, and the duty cycle of all DC ports is determined, and then the carrier phase shift modulation is used to make each port output the transmission power command value.
  • a converter station which includes any of the above-mentioned modular multi-level converters for multi-port DC power flow control.
  • a solid-state transformer which includes any of the above-mentioned modular multi-level converters for multi-port DC power flow control.
  • a mesh/ring-shaped multi-terminal flexible DC transmission system based on multiple converter stations, including multiple converter stations, the converter stations including any of the above-mentioned multi-port DC Modular multilevel converter for power flow control.
  • a mesh/ring-shaped multi-terminal flexible DC power distribution system based on a plurality of solid-state transformers, including a plurality of solid-state transformers, the solid-state transformers including any of the above-mentioned multi-port DC power flow control
  • the modular multilevel converter based on a plurality of solid-state transformers, including a plurality of solid-state transformers, the solid-state transformers including any of the above-mentioned multi-port DC power flow control The modular multilevel converter.
  • the present invention has the following beneficial effects:
  • the existing power electronic converter or transformer topology based on the modular multilevel structure (MMC) does not have the DC power flow control function.
  • the present invention introduces a DC power flow controller,
  • the formation of multiple controllable DC ports increases the control freedom of DC power flow, achieves the effect of controlling the power flow of multiple lines, and can realize a networked/looped DC transmission and distribution system.
  • the effective control of the power flow of each line in the DC system is realized by increasing the degree of control freedom, and the stable operation of the DC system is guaranteed.
  • the modular multilevel converter with multi-port DC power flow control of the present invention has fewer switching devices and lower loss; no low-frequency isolation transformer and external power supply are required; power can flow in both directions, Wide adjustment range; easy and flexible to form a multi-port configuration.
  • the above-mentioned modular multi-level converter with multi-port DC power flow control of the present invention can be used in mesh/ring multi-terminal flexible DC transmission and distribution systems based on modular multi-level topology converters and solid-state
  • the transformer mesh/ring multi-terminal flexible DC power distribution system can achieve the effect of flexible control of the power flow of multiple DC lines through the multi-port control of the DC power flow, providing the equipment foundation for the networked DC system.
  • Figures 1a and 1b are schematic diagrams of a topology structure and a combination with a modular multilevel topology according to an embodiment of the present invention
  • FIG. 2 is a schematic diagram of modulation strategy parameters according to an embodiment of the present invention.
  • Fig. 3 is a block diagram of a control strategy according to an embodiment of the present invention.
  • Figure 4 An example of an existing ring network three-terminal HVDC power transmission system
  • Figure 5 is an example of a ring network three-terminal high-voltage direct current transmission system including the present invention
  • Figure 6 is a schematic diagram of the basic configuration of the second embodiment
  • Figure 7 is a block diagram of the control strategy of the DC power flow controller in the second embodiment
  • Figure 8 is the output power curve of the medium voltage DC side port 1 of the second embodiment
  • Fig. 9 is a DC voltage curve of medium voltage DC side port 1 in the second embodiment
  • Fig. 10 is a DC current curve of medium voltage DC side port 1 in the second embodiment
  • Figure 11 is the output power curve of the medium voltage DC side port 2 of the second embodiment
  • Fig. 12 is a DC voltage curve of medium voltage DC side port 2 of the second embodiment
  • Figure 13 is a DC current curve of medium voltage DC side port 2 of the second embodiment
  • Fig. 14 is the capacitor voltage curve of the MMC sub-module in the second embodiment
  • Figure 15 is the DC side capacitor voltage curve of the DC power flow controller in the second embodiment
  • Figure 16 is a curve of the average value of the low-voltage DC side voltage of the second embodiment
  • Figure 17 is a curve of the average value of the low-voltage DC side current of the second embodiment
  • FIG. 18 is a schematic diagram of the topology structure of an embodiment of the present invention and its application in a ring/mesh multi-terminal DC system.
  • FIG. 1a and 1b it is a topological diagram of a modular multilevel converter with multi-port DC power flow control in an embodiment of the present invention, including: a modular multilevel converter (MMC) topology and DC power flow controller, modular multilevel converter (MMC) topology is composed of multiple upper and lower bridge arms with the same structure, each bridge arm is composed of multiple sub-modules (SM) in series, among which: input of DC power flow controller The uppermost sub-module of the upper bridge arm of the modular multilevel converter (MMC) topology is connected in series, and the power transmission and energy interaction are carried out with the AC grid through the modular multilevel converter topology.
  • the converter station or solid-state transformer of the level converter topology has DC power flow control capability.
  • the modular multilevel converter (MMC) topology in the embodiment of the present invention may be any modular multilevel converter (MMC) in the prior art.
  • the DC power flow controller includes an input terminal, a capacitor, and an output terminal.
  • the capacitor is connected in parallel between the input terminal and the output terminal, and the energy transfer between the input terminal and the output terminal is realized through the capacitor.
  • the input terminal of the DC power flow controller can adopt a single-phase half bridge ( Figure 1b) or a three-phase bridge structure ( Figure 1a), and an additional capacitor is used to achieve energy transfer between the input and output.
  • the output end adopts a DC/DC converter, and multiple DC/DC converters are connected in parallel to realize multiple flexible and controllable DC ports, so that the output voltage or output power can be flexibly adjusted.
  • the DC power flow controller has multiple output ports (DC ports), so that the MMC converter station or MMC solid-state transformer device provides multiple flexible and controllable DC ports, which can realize a mesh based on multiple converter stations /Ring-shaped multi-terminal flexible DC transmission system or mesh/ring-shaped multi-terminal flexible DC power distribution system based on multiple solid-state transformers.
  • the capacitor is connected in parallel with the single-phase half-bridge or three-phase half-bridge structure, and the DC power flow controller input terminal controls the conduction and shutdown of the upper and lower arms of the single-phase half-bridge or three-phase bridge structure to make the capacitor exist In charging, discharging or bypass state, since there are no other energy storage components in the DC power flow controller except for the intermediate capacitor, the DC power flow controller is realized by controlling the switch of the single-phase half-bridge or three-phase bridge structure to keep the capacitor voltage in balance. The input and output energy balance.
  • the following describes the modulation method of the modular multi-level converter with multi-port DC power flow control.
  • the DC power flow controller and the modular multilevel converter topology in the modular multilevel converter with multi-port DC power flow control adopt the carrier phase shift modulation strategy.
  • the input single-phase half-bridge of the DC power flow controller and the modular multilevel topology (MMC) sub-module respectively adopt separate modulation strategies.
  • the modulation strategy parameter diagram is shown in Figure 2, ignoring the correction amount of the bridge arm voltage command by the bridge arm circulation control and other balance control, there is
  • x 1 and y 1 are the voltage command DC component coefficients and fundamental frequency AC component coefficients of the single-phase half-bridge of the DC power flow controller
  • x 2 , y 2 are the voltage command DC component coefficients and fundamental frequency AC component coefficients of the MMC upper bridge arm Component coefficient
  • u ap0 is a single-phase half-bridge AC side output voltage command
  • u ap and u an are MMC upper and lower arm output voltage commands
  • u aref MMC AC side output voltage command
  • u aref U sin( ⁇ t)
  • U c0 is the DC component of the capacitor voltage of the DC power flow controller
  • U c1 is the DC component of the capacitor voltage of the MMC sub-module.
  • m 1 , m 2 , m are the voltage modulation coefficients of the DC power flow controller single-phase half-bridge and the upper and lower arm sub-modules of the MMC:
  • the sum of the current at the upper and lower input sides of the capacitor is the sum of the DC components in the current of the MMC three-phase bridge arm. Therefore, it can be set
  • I d i SMp0- +i SMp0+
  • the capacitor input side current of the DC power flow controller is determined by the three-phase confluence after the multiplication of the bridge arm currents of each phase and the single-phase half-bridge switching function:
  • the current on the input side of the capacitor contains a low-frequency AC component in addition to the DC component.
  • the DC component of the MMC single-phase bridge arm current connected to the DC power flow controller is I d
  • the upper and lower bridge arm currents of the MMC can be respectively set as:
  • the current on the capacitor input side of the DC power flow controller is determined by the multiplication of the current of the phase bridge arm and the single-phase half-bridge switching function:
  • the input and output power balance equation of the DC power flow controller is:
  • selecting appropriate voltage command DC component coefficients and fundamental frequency AC component coefficients can make the DC power flow controller single-phase half-bridge and the voltage modulation coefficients of the upper and lower arms of the MMC meet the energy balance of the DC power flow controller.
  • the DC power flow controller and the modular multilevel topology can solve the problem of introducing the DC power flow controller into the converter station or solid-state transformer based on the modular multilevel topology through the carrier phase shift modulation strategy described above. After the power balance problem, the steady-state operation of the device is realized.
  • the duty cycle of a certain DC port is selected, and the duty cycle of all DC ports is determined, and then the carrier phase shift modulation is used to make each port output the transmission power command value.
  • the multi-port DC power flow controller is connected to the MMC topology through the input terminal and the uppermost submodule of the MMC upper bridge arm or the lowermost submodule of the lower bridge arm in series connection, and the input end can be a single-phase half bridge or
  • the three-phase bridge structure buffers the energy transfer between the input terminal and the output terminal through a capacitor; the output terminal adopts a non-isolated DC/DC converter to adjust the output voltage, and multiple DC/DC converters share a DC bus and connect different ports DC outlets to achieve multiple flexible and controllable DC ports.
  • the topology of the above-mentioned embodiment of the present invention can be used in a mesh or ring multi-terminal flexible DC transmission and distribution system based on a modular multi-level topology converter and a mesh or ring multi-terminal flexible system based on a solid-state transformer
  • the DC power distribution system achieves the effect of flexible control of the power flow of multiple DC lines through the multi-port control of the DC power flow, providing the equipment foundation for the networked DC system.
  • This application scheme is based on the above-mentioned ring mesh (ring) three-terminal HVDC transmission system with a modular multilevel converter with multi-port DC power flow control.
  • the specific implementation is as follows.
  • FIG. 4 it is an example of an existing ring network three-terminal HVDC transmission system.
  • the MMC converter station 3 operates in a constant voltage mode, the control voltage V 3 is 150kV, and the power is transmitted from MMC 1 and MMC 2 to MMC 3.
  • FIG. 5 it is an example of a ring-netted three-terminal HVDC power transmission system including the above-mentioned modular multi-level converter with multi-port DC power flow control of the present invention.
  • the MMC converter station 1 can independently control the transmission power P 12 and P 13 on the lines L 12 and L 13 by introducing a DC power flow controller, and the remaining operating conditions remain unchanged.
  • the line parameters are shown in the table below.
  • V o1 and V o2 are the two medium voltage DC port voltages of MMC converter station 1
  • R 12 , R 13 , and R 23 are MMC converter station 1 and MMC converter station 2, MMC converter station 1 and MMC The line resistance between converter station 3, MMC converter station 2 and MMC converter station 3
  • I 12 , I 13 , I 13 are MMC converter station 1 and MMC converter station 2, MMC converter station 1 and MMC Line current between converter station 3, MMC converter station 2 and MMC converter station 3
  • P 12 , P 13 and P 23 are MMC converter station 1 and MMC converter station 2, MMC converter station 1 and Transmission power between MMC converter station 3, MMC converter station 2 and MMC converter station 3.
  • the MMC converter station 1 adopting the modular multilevel converter with multi-port DC power flow control of the present invention has two high-voltage DC ports, and the power flow optimization with minimum line loss can be realized by adjusting the power of the two ports.
  • the results of power flow optimization are shown in the table below.
  • the application scheme is based on the above-mentioned solid-state transformer of the modular multi-level converter with multi-port DC power flow control.
  • the specific implementation is as follows.
  • FIG. 6 a schematic diagram of the basic configuration of an embodiment of a solid-state transformer including a modular multi-level converter with multi-port DC power flow control of the present invention, in which: a DC power flow controller and a modular multi-level topology-based The solid-state transformer is connected, and the three-phase input of the DC power flow controller is connected in series with the uppermost sub-module of the three-phase upper bridge arm of the modular multilevel converter (MMC) topology.
  • MMC modular multilevel converter
  • the energy transfer between the input end and the output end, the output end adopts a half-bridge (two-quadrant) DC/DC converter structure, and two half-bridge (two-quadrant) DC/DC converters are connected in parallel to realize two flexible Controlled medium voltage DC port.
  • the sub-modules of the modular multilevel converter (MMC) topology are interconnected with the isolated dual active bridge converter (DAB), and the sub-modules of the modular multilevel converter (MMC) topology use two Level half-bridge structure, isolated dual active bridge converter (DAB) input end and output end converter adopt two-level full bridge structure, through multiple isolated dual active bridge converter (DAB) output ends in parallel Form a low-voltage DC bus.
  • the topology of the DC power flow controller is shown in the detailed diagram in Figure 6.
  • the three-phase half bridge is composed of three-phase half-bridge sub-modules in parallel. Its AC port can output two-level three-phase AC voltage, while the DC port outputs Capacitor voltage:
  • the half-bridge (two-quadrant) DC/DC converter regulates the output DC voltage by controlling the upper and lower arm switches to conduct.
  • a three-phase half bridge is used at the input end of the DC power flow controller, and a half-bridge (two-quadrant) DC/DC converter is used at the output end of the DC power flow controller.
  • the AC port of the solid-state transformer based on the modular multilevel converter with multi-port DC power flow control is connected to the medium voltage AC distribution network, the low voltage DC port is connected to the low voltage DC distribution network, and the DC power flow
  • the controller enables the solid-state transformer device to provide two medium-voltage DC ports, which can be connected to the medium-voltage DC distribution network.
  • the control strategy block diagram of the DC power flow controller is shown in Figure 7. According to the rated output power of the two medium voltage DC ports, the output DC voltage difference of the two ports is calculated, and then the output terminal two half-bridge type (two quadrants) are obtained from the difference. The duty cycle of the DC/DC converter and the carrier phase shift modulation.
  • the following uses MATLAB/Simulink software to conduct open-loop simulation verification for this topology. Connect the medium voltage DC power supply and load at the medium voltage DC port, and connect the power supply and the load in series. In the medium voltage AC port access Voltage AC power supply, low-voltage DC port connected to the load, the simulation parameters are shown in the table below.
  • the solid-state transformer based on the modular multi-level converter with multi-port DC power flow control is composed of a medium-voltage side MMC and multiple DABs in the intermediate stage.
  • the non-isolated multi-port DC power flow controller with flat topology is composed of three-phase half-bridge and half-bridge (two-quadrant) DC/DC converters. Different converters need to adopt different modulation methods to achieve steady-state operation.
  • the carrier phase shift modulation method is adopted; for the intermediate stage DAB, the square wave phase shift modulation method is adopted.
  • Average value of DC voltage at port 1 of the medium voltage DC side 20kV DC voltage
  • Average value of DC current at port 1 of the medium voltage DC side 15A DC current
  • Average value of DC voltage at port 2 on the medium voltage DC side 20kV DC voltage
  • Average value of DC current at port 2 on the medium voltage DC side 35A DC current
  • Average capacitor voltage of MMC sub-module 1.67kV average capacitor voltage
  • DC power flow controller DC side capacitor voltage average value 400V average capacitor voltage
  • Low-voltage DC side output power 1MW active power output
  • Fig. 9 is the DC voltage curve of the medium voltage DC port 1 in the second embodiment of the present invention, in which: in the steady state, the DC voltage of the port is stabilized at 20kV.
  • Fig. 10 is the DC current curve of the medium voltage DC port 1 in the second embodiment of the present invention, in which: in the steady state, the DC current of the port is about 15A.
  • Figure 11 shows the active power output curve of the medium voltage DC port 2 of the second embodiment, where the active power curve of the port is stable at about 0.7MW in the steady state.
  • Fig. 12 is the DC voltage curve of the medium voltage DC port 2 in the second embodiment of the present invention, in which: in the steady state, the DC voltage of the port is stabilized at 20kV.
  • Fig. 13 is the DC current curve of the medium voltage DC port 2 of the second embodiment of the present invention, where: in the steady state, the DC current of the port is about 35A.
  • Figure 14 shows the capacitance voltage curve of the MMC sub-module in the second embodiment, where the capacitance voltage of the MMC sub-module fluctuates at an average value of 1.67 kV in the steady state.
  • Figure 15 shows the DC side capacitor voltage curve of the DC power flow controller in the second embodiment, where the DC side capacitor voltage of the power flow control module fluctuates at an average value of 400V in the steady state.
  • FIG. 16 is a curve of the average value of the low-voltage side DC bus voltage in the second embodiment of the present invention, in which: in a steady state, the low-voltage DC voltage is stable at 750V.
  • Figure 17 is a curve of the average value of the DC bus current on the medium and low voltage side of the second embodiment of the present invention, in which: in the steady state, the low voltage DC bus current is about 1331A, that is, the active power transmitted from the medium voltage AC side to the low voltage DC side in this embodiment The output is about 1MW.
  • the present invention by introducing a DC power flow controller, makes the solid-state transformer based on the modular multi-level topology form multiple medium voltage DC ports, which increases the degree of freedom of DC power flow control and achieves the control of multiple lines
  • the effect of the power flow; the DC power flow controller of the present invention has fewer switching devices and lower loss; no low-frequency isolation transformer and external power supply are required; power can flow in both directions, and the power flow adjustment range is wide.

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Abstract

一种多端口直流潮流控制的模块化多电平变流器及控制方法，包括：模块化多电平换流器拓扑、直流潮流控制器，其中：所述直流潮流控制器的输入端与所述模块化多电平换流器拓扑的上桥臂最上层子模块串联连接，通过所述模块化多电平换流器拓扑与交流电网进行功率传递和能量交互，使基于所述模块化多电平换流器拓扑的换流站或固态变压器具备直流潮流控制能力。输出端采用直流/直流变换器以调节输出电压，多个直流/直流变换器共直流母线且端口连接不同的直流出线，以实现多个灵活可控的直流端口。通过直流潮流的多端口控制使其达到多条直流线路潮流灵活调控的效果，为网络化直流系统提供装备基础。

Description

多端口直流潮流控制的模块化多电平变流器及控制方法 技术领域
本发明涉及电力系统中智能电网技术、电力电子技术等领域，具体地，涉及一种具有多端口直流潮流控制的模块化多电平变流器及控制方法。
背景技术
近年来随着可再生能源技术和产业的快速发展，新能源发电的间歇性给电力系统带来了许多挑战。在大规模新能源并网方面，交流电网不利于高比例接入新能源的接纳，需要新的技术、装备和电网结构来提高对可再生能源的利用率。多端直流和直流电网技术是解决新能源并网和消纳问题的有效技术手段之一。
在多端直流系统中，环、网状结构相比其他拓扑具有更高的优越性。多端直流输电技术通过换流器直流端互联构成网格化的直流电网，相对传统两端直流输电，可实现多电源供电和多落点受电，系统可靠性增强，在实现多个电网互联的同时成本与运行费用较低，更为经济灵活。多端直流配电网中网状结构具有较好的灵活性和冗余度，可实现对分布式能源的灵活调度与管理，实现大量接纳、优化配置、充分利用不同类型的分布式能源。此外，一旦局部发生故障，可通过有效的隔离手段和网络重构手段，使配电网受到最小的影响。因此环、网状结构对于提高直流系统运行可靠性、实现电能的互联互济、提高资源利用效率均具有促进作用。但在含有环、网状结构的直流系统中，可控节点之间可能存在多条直流线路，使得线路的数目大于可控节点个数-1，导致线路潮流无法仅靠各节点的电压、电流控制实现有效调节。
随着电力电子器件与控制技术的发展，在目前的多端柔直系统中，高压直流输电系统多采用电压源换流器(VSC)构成换流站，中压直流配电网则采用电压源换流器(VSC)或者具有中压直流端口的固态变压器(SST)作为核心装置。受功率器件和串联技术限制，传统两电平或三电平结构难以满足系统高中压等级的需求，而模块化多电平结构以其设计模块化、输出电平高、容量易扩展和电压等级高等优势在直流输配电领域得到了广泛的研究与应用，成为直流系统中换流站与固态变压器的实现方式及拓扑基础。
发明内容
基于现有多端直流系统中换流站或固态变压器的拓扑基础和直流潮流控制的技术需求，为解决直流系统中的潮流控制问题，需要增加直流潮流控制功能，通过增加控制自由度来实现对直流系统内每条线路潮流的有效控制，保障直流系统的稳定运行。
由于不存在交流电的无功功率、电抗和相角，直流系统通过调节线路电阻和直流电压来控制直流潮流。在控制线路电阻方面，可采用在线路中串入可变电阻的方式来控制直流潮流。这种方式损耗大，对装置散热要求高，且只能单向调节潮流。在控制直流电压方面，其中一种方案是将变比为1左右的DC/DC变换器接入同一电压等级的直流系统中，通过微调变比来调节直流系统潮流，但此时所有的功率都需通过DC/DC变换器，成本、损耗较大。另一种方案是在线路中串入可调电压源来改变直流电压，进而控制直流潮流。这种电路结构需要额外电源，还需要高压隔离变压器进行电压隔离，且所需开关器件较多。还有一种方案称为线间直流潮流控制器，它无需外部电源，利用线路之间的功率交换即可等效为在线路中串入可调电压源，从而实现潮流控制，为了实现装置内部能量平衡控制，造成装置的结构与控制较为复杂，而且在一定程度上受到线路传输功率的限制。
针对上述存在的问题，本发明的目的是提出一种具有多端口直流潮流控制的模块化多电平变流器及其调制方法，解决直流系统中的潮流控制问题，保障直流系统的稳定运行。
为实现上述目的，本发明采用以下技术方案。
根据本发明的第一方面，提供一种多端口直流潮流控制的模块化多电平变流器，包括：模块化多电平换流器拓扑和直流潮流控制器，所述模块化多电平换流器拓扑由多个结构相同的上下桥臂组成，每个桥臂由多个子模块级联构成，其中：
所述直流潮流控制器的输入端与所述模块化多电平换流器拓扑的上桥臂最上层子模块串联连接，通过所述模块化多电平换流器拓扑与交流电网进行功率传递和能量交互，使基于所述模块化多电平换流器拓扑的换流站或固态变压器具备直流潮流控制能力。
优选地，所述直流潮流控制器包括输入端、电容以及输出端，其中：所述电容并联于所述输入端与所述输出端之间，通过所述电容实现所述输入端与所述输 出端之间的能量传递。
优选地，所述直流潮流控制器的输入端采用单相半桥或三相半桥结构。
优选地，所述直流潮流控制器的三相输入端与所述模块化多电平换流器拓扑中的三相上桥臂最上层子模块串联。
优选地，所述电容与所述单相半桥或三相半桥结构并联，所述单相半桥或三相半桥结构通过控制其上下桥臂的导通或关断使所述电容存在充电、放电或旁路状态，所述直流潮流控制器中除所述电容外无其他储能元件，通过控制所述单相半桥或三相半桥结构的状态，使得所述电容的电压保持平衡即实现所述直流潮流控制器的输入输出能量平衡。
优选地，所述直流潮流控制器的输出端采用直流/直流变换器，以调节输出电压，多个所述直流/直流变换器并联，形成多个灵活可控的直流端口，输出电压或输出功率大小可灵活调节。进一步的，所述直流潮流控制器的输出端具备多个直流端口，使基于所述模块化多电平换流器拓扑的换流站或固态变压器提供多个灵活可控的直流端口，能实现基于多个换流站的网状/环状多端柔性直流输电系统或基于多个固态变压器的网状/环状多端柔性直流配电系统。
根据本发明的第二方面，提供一种上述多端口直流潮流控制的模块化多电平变流器的调制方法，所述直流潮流控制器和所述模块化多电平变流器拓扑采用载波移相调制策略，其中：所述直流潮流控制器的输入端与所述模块化多电平变流器拓扑的子模块分别采用单独的调制策略。
具体的，所述方法包括：
忽略所述直流潮流控制器与所述模块化多电平变流器拓扑的桥臂环流控制及其他平衡控制对桥臂电压指令修正量，有
Figure PCTCN2019110679-appb-000001
式中，x 1、y 1为直流潮流控制器单相半桥的电压指令直流分量系数和基频交流分量系数，x 2、y 2为所述模块化多电平变流器拓扑的上桥臂的电压指令直流分量系数和基频交流分量系数，u ap0为单相半桥交流侧输出电压指令，u ap、u an为所述模块化多电平变流器拓扑的上、下桥臂输出电压指令，u aref为所述模块 化多电平变流器拓扑的交流侧输出电压指令，u aref＝U sin(ωt)，U d为直流潮流控制器的直流母线电压；
显然，指令系数满足：
Figure PCTCN2019110679-appb-000002
其中：U c0为直流潮流控制器电容电压直流分量，U c1为所述模块化多电平变流器拓扑的子模块电容电压直流分量，n为所述模块化多电平变流器拓扑的桥臂子模块数，从而得到直流潮流控制器单相半桥和所述模块化多电平变流器拓扑的上下桥臂子模块平均开关函数为：
Figure PCTCN2019110679-appb-000003
m 1、m 2、m分别为直流潮流控制器单相半桥和所述模块化多电平变流器拓扑的上、下桥臂子模块的电压调制系数：
Figure PCTCN2019110679-appb-000004
Figure PCTCN2019110679-appb-000005
Figure PCTCN2019110679-appb-000006
其中：U为交流侧相电压幅值；
对于输入端采用三相半桥结构的直流潮流控制器，电容上下端输入侧电流之和为所述模块化多电平变流器拓扑的三相桥臂电流中直流分量之和，设接入直流潮流控制器的所述模块化多电平变流器拓扑的三相桥臂电流中直流分量之和为I d，则
I d＝i SMp0-+i SMp0+
其中：i SMp0-为直流潮流控制器电容下端输入侧电流；i SMp0+为直流潮流控制器电容上端输入侧电流；
则所述模块化多电平变流器拓扑的上下桥臂电流分别为：
Figure PCTCN2019110679-appb-000007
Figure PCTCN2019110679-appb-000008
其中：i a为A相交流电流；
Figure PCTCN2019110679-appb-000009
为功率因数角；ω为交流电压频率；t为时间；I ap(t)、I an(t)分别为所述模块化多电平变流器拓扑的A相上、下桥臂电流；
直流潮流控制器的电容输入侧电流由各相桥臂电流和单相半桥开关函数相乘后三相汇合而决定：
Figure PCTCN2019110679-appb-000010
Figure PCTCN2019110679-appb-000011
其中：F kp0(t)为所述直流潮流控制器的单相半桥平均开关函数；I为交流侧相电流幅值；U d为所述直流潮流控制器的直流母线电压；i kp(t)为所述模块化多电平变流器拓扑的单相上、下桥臂电流；a,b,c为三相；
对于输入端采用单相半桥结构的直流潮流控制器，电容输入侧电流除了包含直流分量外还含有低频交流分量，设接入直流潮流控制器的所述模块化多电平变流器拓扑的单相桥臂电流中直流分量为I d，则所述模块化多电平变流器拓扑的上下桥臂电流可分别设为：
Figure PCTCN2019110679-appb-000012
Figure PCTCN2019110679-appb-000013
直流潮流控制器的电容输入侧电流由该相桥臂电流和单相半桥开关函数相乘决定：
Figure PCTCN2019110679-appb-000014
Figure PCTCN2019110679-appb-000015
直流潮流控制器的输入输出功率平衡方程为：
P PFC＝I di SMp0++(U d-U c0)i SMp0-＝P MVDC
其中：P PEC为直流潮流控制器的输入有功功率；P MVDC为中压直流侧输出有功功率；a,b,c为三相；
通过计算分析，选取合适的电压指令直流分量系数和基频交流分量系数，即能使直流潮流控制器单相半桥和所述模块化多电平变流器拓扑的上、下桥臂的电压调制系数满足直流潮流控制器的能量平衡。
优选地，所述直流潮流控制器输出直流端口采用开环控制策略，由
Figure PCTCN2019110679-appb-000016
根据各直流端口的输出功率指令值P i，计算得到各直流端口的输出电压指令值V oi，再由输出端直流/直流变换器的具体拓扑，得到输出电压指令值V oi与直流/直流变换器的占空比D i的关系，进而得到各端口输出电压的差值ΔV ij与各占空比的差值ΔD ij之间的关系：
V oi＝f(U d,U c0,D i)  i＝1,2,…n
ΔV ij＝f(U c0,ΔD ij)  i,j＝1,2,…n,i≠j
根据直流/直流变换器的具体拓扑的特性，选取某一直流端口的变换器占空比，则所有直流端口的占空比得以确定，再通过载波移相调制使各端口输出传输功率指令值。
根据本发明的第三方面，提供一种换流站，包括上述的任一种多端口直流潮流控制的模块化多电平变流器。
根据本发明的第四方面，提供一种固态变压器，包括上述的任一项多端口直流潮流控制的模块化多电平变流器。
根据本发明的第五方面，提供一种基于多个换流站的网状/环状多端柔性直流输电系统，包括多个换流站，所述换流站包括上述的任一项多端口直流潮流控制的模块化多电平变流器。
根据本发明的第六方面，提供一种基于多个固态变压器的网状/环状多端柔性直流配电系统，包括多个固态变压器，所述固态变压器包括上述的任一项多端口直流潮流控制的模块化多电平变流器。
与现有拓扑相比，本发明具有以下有益效果：
1、现有的基于模块化多电平结构(MMC)的电力电子换流器或变压器拓扑不具备直流潮流控制功能，为解决直流系统中的潮流控制问题，本发明通过引入直流潮流控制器，形成多个可控直流端口，增加了直流潮流的控制自由度，达到了控制多条线路潮流的效果，可实现网络化/环状的直流输配电系统。进一步的，在增加直流潮流控制功能的同时，通过增加控制自由度来实现对直流系统内每条线路潮流的有效控制，保障直流系统的稳定运行。
2、与现有技术相比，本发明具有多端口直流潮流控制的模块化多电平变流器的开关器件较少，损耗较低；无需低频隔离变压器以及外部电源；功率可双向流动，潮流调节范围广；易于且灵活形成多端口的配置。
3、本发明上述的具有多端口直流潮流控制的模块化多电平变流器，可用于基于模块化多电平拓扑变流器的网状/环状多端柔性直流输配电系统与基于固态变压器的网状/环状多端柔性直流配电系统，通过直流潮流的多端口控制使其达到多条直流线路潮流灵活调控的效果，为网络化直流系统提供装备基础。
附图说明
通过阅读参照以下附图对非限制性实施例所作的详细描述，本发明的其它特征、目的和优点将会变得更明显：
图1a、1b为本发明实施例的拓扑结构以及与模块化多电平拓扑组合方式示意图；
图2为本发明一实施例的调制策略参数示意图；
图3为本发明一实施例的控制策略框图；
图4现有的环网状三端高压直流输电系统实例；
图5为包含本发明的环网状三端高压直流输电系统实例；
图6为实施方案二的基本配置示意图；
图7为实施方案二中直流潮流控制器的控制策略框图；
图8为实施方案二中压直流侧端口1输出功率曲线；
图9为实施方案二中压直流侧端口1直流电压曲线；
图10为实施方案二中压直流侧端口1直流电流曲线；
图11为实施方案二中压直流侧端口2输出功率曲线；
图12为实施方案二中压直流侧端口2直流电压曲线；
图13为实施方案二中压直流侧端口2直流电流曲线；
图14为实施方案二MMC子模块电容电压曲线；
图15为实施方案二直流潮流控制器直流侧电容电压曲线；
图16为实施方案二低压直流侧电压平均值曲线；
图17为实施方案二低压直流侧电流平均值曲线；
图18为本发明实施例的拓扑结构及其在环状/网状多端直流系统中的应用示意图。
具体实施方式
下面结合具体实施例对本发明进行详细说明。以下实施例将有助于本领域的技术人员进一步理解本发明，但不以任何形式限制本发明。应当指出的是，对本领域的普通技术人员来说，在不脱离本发明构思的前提下，还可以做出若干变形和改进。这些都属于本发明的保护范围。
参照图1a、图1b所示，为本发明一实施例中具有多端口直流潮流控制的模块化多电平变流器拓扑图，其中包括：模块化多电平换流器(MMC)拓扑和直流潮流控制器，模块化多电平换流器(MMC)拓扑由多个结构相同的上下桥臂组成，每个桥臂由多个子模块(SM)串联构成，其中：直流潮流控制器的输入端与模块化多电平换流器(MMC)拓扑的上桥臂最上层子模块串联连接，通过模块化多电平换流器拓扑与交流电网进行功率传递和能量交互，使基于模块化多电平换流器拓扑的换流站或固态变压器具备直流潮流控制能力。本发明实施例中模块化多电平换流器(MMC)拓扑可以是现有技术任一模块化多电平换流站(MMC)。
如图1a、图1b所示，直流潮流控制器包括输入端、电容以及输出端，其中：电容并联于输入端与所述输出端之间，通过电容实现输入端与输出端之间的能量传递。在一具体实施例中，直流潮流控制器的输入端可采用单相半桥(图1b)或三相桥结构(图1a)，通过一个附加电容实现输入端与输出端之间的能量传递， 输出端采用直流/直流变换器，多个直流/直流变换器并联，实现多个灵活可控的直流端口，使得输出电压或输出功率大小可灵活调节。
上述实施例中，直流潮流控制器具备多个输出端口(直流端口)，使MMC换流站或MMC固态变压器装置提供多个灵活可控的直流端口，可实现基于多个换流站的网状/环状多端柔性直流输电系统或基于多个固态变压器的网状/环状多端柔性直流配电系统。
上述实施例中，电容与单相半桥或三相半桥结构并联，直流潮流控制器输入端，通过控制单相半桥或三相桥结构的上下桥臂的导通、关断使电容存在充电、放电或旁路状态，由于直流潮流控制器中除中间级电容外无其他储能元件，通过控制单相半桥或三相桥结构的开关，使得电容电压保持平衡即实现直流潮流控制器的输入输出能量平衡。
针对上述的具有多端口直流潮流控制的模块化多电平变流器，以下说明该具有多端口直流潮流控制的模块化多电平变流器的调制方法。总体来说，具有多端口直流潮流控制的模块化多电平变流器中的直流潮流控制器和模块化多电平变流器拓扑采用载波移相调制策略。
具体的，直流潮流控制器的输入端单相半桥与所述模块化多电平拓扑(MMC)子模块分别采用单独的调制策略。以a相为例(其他两相也同样适用)，调制策略参数示意图如图2所示，忽略桥臂环流控制及其他平衡控制对桥臂电压指令修正量，有
Figure PCTCN2019110679-appb-000017
式中，x 1、y 1为直流潮流控制器单相半桥的电压指令直流分量系数和基频交流分量系数，x 2、y 2为MMC上桥臂的电压指令直流分量系数和基频交流分量系数，u ap0为单相半桥交流侧输出电压指令，u ap、u an为MMC上、下桥臂输出电压指令，u aref为MMC交流侧输出电压指令，u aref＝U sin(ωt),显然，指令系数满足：
Figure PCTCN2019110679-appb-000018
U c0为直流潮流控制器电容电压直流分量，U c1为MMC子模块电容电压直流分量，从而得到直流潮流控制器单相半桥和MMC上下桥臂子模块平均开关函数为：
Figure PCTCN2019110679-appb-000019
m 1、m 2、m分别为直流潮流控制器单相半桥和MMC上、下桥臂子模块的电压调制系数：
Figure PCTCN2019110679-appb-000020
Figure PCTCN2019110679-appb-000021
Figure PCTCN2019110679-appb-000022
对于输入端采用三相半桥结构的直流潮流控制器，电容上下端输入侧电流之和为MMC三相桥臂电流中直流分量之和，因此，可设
I d＝i SMp0-+i SMp0+
则MMC上下桥臂电流分别为：
Figure PCTCN2019110679-appb-000023
Figure PCTCN2019110679-appb-000024
直流潮流控制器的电容输入侧电流由各相桥臂电流和单相半桥开关函数相乘后三相汇合而决定：
Figure PCTCN2019110679-appb-000025
Figure PCTCN2019110679-appb-000026
对于输入端采用单相半桥结构的直流潮流控制器，电容输入侧电流除了包含 直流分量外还含有低频交流分量。设接入直流潮流控制器的MMC单相桥臂电流中直流分量为I d，则MMC上下桥臂电流可分别设为：
Figure PCTCN2019110679-appb-000027
Figure PCTCN2019110679-appb-000028
直流潮流控制器的电容输入侧电流由该相桥臂电流和单相半桥开关函数相乘决定：
Figure PCTCN2019110679-appb-000029
Figure PCTCN2019110679-appb-000030
直流潮流控制器的输入输出功率平衡方程为：
P I＝U dI SMp0++(U d-U c0)I SMp0-＝P O
通过计算分析，选取合适的电压指令直流分量系数和基频交流分量系数，即可使直流潮流控制器单相半桥和MMC上、下桥臂的电压调制系数满足直流潮流控制器的能量平衡。
在具体实施例中，直流潮流控制器和模块化多电平拓扑(MMC)通过上述的载波移相调制策略，可解决基于模块化多电平拓扑的换流站或固态变压器引入直流潮流控制器后的功率平衡问题，实现装置的稳态运行。
如图3所示，直流潮流控制器输出直流端口的开环控制策略框图，由
Figure PCTCN2019110679-appb-000031
可根据各直流端口的输出功率指令值P i，计算得到各直流端口的输出电压指令值V oi。再由输出端直流/直流变换器的具体拓扑选择，得到输出电压指令值V oi与直流/直流变换器的占空比D i的关系，进而得到各端口输出电压的差值ΔV ij与各 占空比的差值ΔD ij之间的关系：
V oi＝f(U d,U c0,D i)i＝1,2,…n
ΔV ij＝f(U c0,ΔD ij)i,j＝1,2,…n,i≠j
根据直流/直流变换器的具体拓扑的特性，选取某一直流端口的变换器占空比，则所有直流端口的占空比得以确定，再通过载波移相调制使各端口输出传输功率指令值。
本发明上述实施例中，多端口直流潮流控制器通过输入端与MMC上桥臂最上层子模块或者下桥臂最下层子模块串联的方式接入MMC拓扑，输入端可采用单相半桥或三相桥结构，通过电容缓冲输入端与输出端之间的能量传递；输出端采用非隔离型直流/直流变换器以调节输出电压，多个直流/直流变换器共直流母线且端口连接不同的直流出线，以实现多个灵活可控的直流端口。
参照图18所示，本发明上述实施例拓扑，可用于基于模块化多电平拓扑变流器的网状或环状多端柔性直流输配电系统与基于固态变压器的网状或环状多端柔性直流配电系统，通过直流潮流的多端口控制使其达到多条直流线路潮流灵活调控的效果，为网络化直流系统提供装备基础。为了更好理解本发明上述实施例，以下对具体应用案例进行说明。
应用方案一
本应用方案是基于上述具有多端口直流潮流控制的模块化多电平变流器的环网状(环状)三端高压直流输电系统，具体实施如下所述。
如图4所示，为现有的环网状三端高压直流输电系统实例。假设采用主从控制模式，MMC换流站1和MMC换流站2为定功率模式运行，向系统注入功率P 1＝200MW，P 2＝80MW。MMC换流站3为定电圧模式运行，控制电压V 3为150kV，功率从MMC 1和MMC 2向MMC 3传输。
如图5所示，为包含本发明上述具有多端口直流潮流控制的模块化多电平变流器的环网状三端高压直流输电系统实例。MMC换流站1通过引入直流潮流控制器可独立控制线路L 12和L 13上的传输功率P 12和P 13，其余运行条件不变。线路参数如下表所示。
直流线路 距离/km 电阻/Ω
L 12 400 4
L 13 500 5
L 23 200 2
由图5，可得
Figure PCTCN2019110679-appb-000032
Figure PCTCN2019110679-appb-000033
Figure PCTCN2019110679-appb-000034
其中：V o1和V o2是MMC换流站1的两个中压直流端口电压，R 12、R 13、R 23是MMC换流站1与MMC换流站2、MMC换流站1与MMC换流站3、MMC换流站2与MMC换流站3之间的线路电阻；I 12、I 13、I 13是MMC换流站1与MMC换流站2、MMC换流站1与MMC换流站3、MMC换流站2与MMC换流站3之间的线路电流；P 12、P 13、P 23分别为MMC换流站1与MMC换流站2、MMC换流站1与MMC换流站3、MMC换流站2与MMC换流站3之间的传输功率。
采用本发明具有多端口直流潮流控制的模块化多电平变流器的MMC换流站1具备两个高压直流端口，通过调节两个端口的出线功率，可实现最小线路损耗的潮流优化。潮流优化结果如下表所示。通过引入潮流控制器，可对潮流进行主动控制，实现潮流优化。
Figure PCTCN2019110679-appb-000035
实施方案二
本应用方案是基于上述具有多端口直流潮流控制的模块化多电平变流器的固态变压器，具体实施如下所述。
如图6所示，包含本发明具有多端口直流潮流控制的模块化多电平变流器的固态变压器实施例的基本配置示意图，其中：直流潮流控制器与基于模块化多电平拓扑结构的固态变压器相连接，直流潮流控制器的三相输入端与模块化多电平变流器(MMC)拓扑的三相上桥臂最上层子模块串联，采用三相桥结构，通过一个附加电容实现输入端与输出端之间的能量传递，输出端采用半桥型(两象限)直流/直流变换器结构，两个半桥型(两象限)直流/直流变换器并联，以实现两个灵活可控的中压直流端口。固态变压器中，模块化多电平变流器(MMC)拓扑的子模块和隔离型双有源桥变换器(DAB)互联，模块化多电平变流器(MMC)拓扑的子模块采用两电平半桥结构，隔离型双有源桥变换器(DAB)输入端和输出端变换器均采用两电平全桥结构，通过多个隔离型双有源桥变换器(DAB)输出端并联形成低压直流母线。
直流潮流控制器拓扑如图6中的细节图所示，三相半桥通过三相的半桥子模块并联组合而成，其交流端口可输出两电平三相交流电压，而直流端口则输出电容电压；半桥型(两象限)直流/直流变换器通过控制上下桥臂开关管导通来调节输出直流电压大小。在本发明实施例中，三相半桥用于直流潮流控制器输入端，半桥型(两象限)直流/直流变换器用于直流潮流控制器输出端。
在上述实施方案二中，基于具有多端口直流潮流控制的模块化多电平变流器的固态变压器交流端口与中压交流配电网相连，低压直流端口与低压直流配电网相连，直流潮流控制器使固态变压器装置提供两个中压直流端口，可与中压直流配电网相连。直流潮流控制器控制策略框图如图7所示，根据两个中压直流端口额定输出功率计算出两端口输出直流电压差值，再由该差值得到输出端两个半桥型(两象限)直流/直流变换器的占空比，并进行载波移相调制。
结合上述实施方案二，以下采用MATLAB/Simulink软件针对该拓扑进行开环仿真验证，在中压直流端口接入中压直流电源与负载，且电源与负载串联连接，在中压交流端口接入中压交流电源，低压直流端口接负载，仿真参数如下表所示。
Figure PCTCN2019110679-appb-000036
Figure PCTCN2019110679-appb-000037
在仿真实例中，基于具有多端口直流潮流控制的模块化多电平变流器的固态变压器由中压侧MMC，以及中间级多个DAB等多个变换器构成，所述基于模块化多电平拓扑结构的非隔离性多端口直流潮流控制器由三相半桥和半桥型(两象限)直流/直流变换器构成。不同变换器需采用不同的调制方式实现其稳态运行。对于中压侧MMC、三相半桥和半桥型(两象限)直流/直流变换器，采用的是载波移相调制方式；对于中间级DAB，采用的是方波移相调制方式。
在该仿真参数下，理论结果为：
中压直流侧端口1输出功率：0.3MW有功功率输出；
中压直流侧端口1直流电压平均值：20kV直流电压；
中压直流侧端口1直流电流平均值：15A直流电流；
中压直流侧端口2输出功率：0.7MW有功功率输出；
中压直流侧端口2直流电压平均值：20kV直流电压；
中压直流侧端口2直流电流平均值：35A直流电流；
MMC子模块电容电压平均值：1.67kV平均电容电压；
直流潮流控制器直流侧电容电压平均值：400V平均电容电压；
低压直流侧输出功率：1MW有功功率输出；
低压直流侧直流电压平均值：750V直流电压；
低压直流侧直流电流平均值：1331A直流电流；
附图的仿真结果与理论计算结果一致，因此通过以上调制方式，可实现多个交直流配电端口间的稳态功率传输与变压功能，可实现各中压直流端口出线功率灵活可控。
如图8所示，为实施方案二中中压直流端口1有功功率输出曲线，其中：稳态时该端口输出有功功率曲线稳定在0.3MW左右。
图9为本发明实施方案二中中压直流端口1直流电压曲线，其中：稳态时，该端口直流电压稳定在20kV。
图10为本发明实施方案二中中压直流端口1直流电流曲线，其中：稳态时，该端口直流电流在15A左右。
图11所示，为实施方案二中中压直流端口2有功功率输出曲线，其中：稳态时该端口输出有功功率曲线稳定在0.7MW左右。
图12为本发明实施方案二中中压直流端口2直流电压曲线，其中：稳态时，该端口直流电压稳定在20kV。
图13为本发明实施方案二中中压直流端口2直流电流曲线，其中：稳态时，该端口直流电流在35A左右。
图14所示，为实施方案二中MMC子模块电容电压曲线，其中：稳态时MMC子模块电容电压在平均值1.67kV处波动。
图15所示，为实施方案二中直流潮流控制器直流侧电容电压曲线，其中：稳态时潮流控制模块直流侧电容电压在平均值400V处波动。
图16为本发明实施方案二中低压侧直流母线电压平均值曲线，其中：稳态时，该低压直流电压稳定在750V。
图17为本发明实施方案二中低压侧直流母线电流平均值曲线，其中：稳态时，低压直流母线电流在1331A左右，即该实施例中，中压交流侧传递给低压直流侧的有功功率输出约为1MW。
由上述实施例可见，本发明通过引入直流潮流控制器，使基于模块化多电平拓扑结构的固态变压器形成多个中压直流端口，增加了直流潮流的控制自由度，达到了控制多条线路潮流的效果；本发明直流潮流控制器开关器件较少，损耗较低；无需低频隔离变压器以及外部电源；功率可双向流动，潮流调节范围广。
以上对本发明的具体实施例进行了描述。需要理解的是，本发明并不局限于上述特定实施方式，本领域技术人员可以在权利要求的范围内做出各种变形或修改，这并不影响本发明的实质内容。

Claims (14)

  1. 一种多端口直流潮流控制的模块化多电平变流器，包括：模块化多电平换流器拓扑，所述模块化多电平换流器拓扑由多个结构相同的上下桥臂组成，每个桥臂由多个子模块级联构成，其特征在于：还包括：直流潮流控制器，其中：
    所述直流潮流控制器的输入端与所述模块化多电平换流器拓扑的上桥臂最上层子模块串联连接，通过所述模块化多电平换流器拓扑与交流电网进行功率传递和能量交互，使基于所述模块化多电平换流器拓扑的换流站或固态变压器具备直流潮流控制能力。
  2. 根据权利要求1所述的多端口直流潮流控制的模块化多电平变流器，其特征在于：所述直流潮流控制器包括输入端、电容以及输出端，其中：所述电容并联于所述输入端与所述输出端之间，通过所述电容实现所述输入端与所述输出端之间的能量传递。
  3. 根据权利要求2所述的多端口直流潮流控制的模块化多电平变流器，其特征在于：所述直流潮流控制器的输入端采用单相半桥或三相半桥结构。
  4. 根据权利要求3所述的多端口直流潮流控制的模块化多电平变流器，其特征在于：所述直流潮流控制器的三相输入端与所述模块化多电平换流器拓扑中的三相上桥臂最上层子模块串联。
  5. 根据权利要求4所述的多端口直流潮流控制的模块化多电平变流器，其特征在于：所述电容与所述单相半桥或三相半桥结构并联，所述单相半桥或三相半桥结构通过控制其上下桥臂的导通或关断使所述电容存在充电、放电或旁路状态，所述直流潮流控制器中除所述电容外无其他储能元件，通过控制所述单相半桥或三相半桥结构的状态，使得所述电容的电压保持平衡即实现所述直流潮流控制器的输入输出能量平衡。
  6. 根据权利要求1-5任一项所述的多端口直流潮流控制的模块化多电平变流器，其特征在于：所述直流潮流控制器的输出端采用直流/直流变换器，以调节输出电压，多个所述直流/直流变换器并联，形成多个灵活可控的直流端口。
  7. 根据权利要求6所述的多端口直流潮流控制的模块化多电平变流器，其特征在于：所述直流潮流控制器的输出端具备多个直流端口，使基于所述模 块化多电平换流器拓扑的换流站或固态变压器提供多个灵活可控的直流端口，能实现基于多个换流站的网状/环状多端柔性直流输电系统或基于多个固态变压器的网状/环状多端柔性直流配电系统。
  8. 一种权利要求1-7任一项所述多端口直流潮流控制的模块化多电平变流器的调制方法，其特征在于：所述直流潮流控制器和所述模块化多电平变流器拓扑采用载波移相调制策略，其中：所述直流潮流控制器的输入端与所述模块化多电平变流器拓扑的子模块分别采用单独的调制策略。
  9. 根据权利要求1所述的具有多端口直流潮流控制的模块化多电平变流器的调制方法，其特征在于：
    忽略所述直流潮流控制器与所述模块化多电平变流器拓扑的桥臂环流控制及其他平衡控制对桥臂电压指令修正量，有
    Figure PCTCN2019110679-appb-100001
    式中，x 1、y 1为直流潮流控制器单相半桥的电压指令直流分量系数和基频交流分量系数，x 2、y 2为所述模块化多电平变流器拓扑的上桥臂的电压指令直流分量系数和基频交流分量系数，u ap0为单相半桥交流侧输出电压指令，u ap、u an为所述模块化多电平变流器拓扑的上、下桥臂输出电压指令，u aref为所述模块化多电平变流器拓扑的交流侧输出电压指令，u aref＝U sin(ωt)，U d为直流潮流控制器的直流母线电压；
    显然，指令系数满足：
    Figure PCTCN2019110679-appb-100002
    U c0为直流潮流控制器电容电压直流分量，U c1为所述模块化多电平变流器拓扑的子模块电容电压直流分量，n为所述模块化多电平变流器拓扑的桥臂子模块数，从而得到直流潮流控制器单相半桥和所述模块化多电平变流器拓扑的上下桥臂子模块平均开关函数为：
    Figure PCTCN2019110679-appb-100003
    m 1、m 2、m分别为直流潮流控制器单相半桥和所述模块化多电平变流器拓扑的上、下桥臂子模块的电压调制系数：
    Figure PCTCN2019110679-appb-100004
    Figure PCTCN2019110679-appb-100005
    Figure PCTCN2019110679-appb-100006
    其中：U为交流侧相电压幅值；
    对于输入端采用三相半桥结构的直流潮流控制器，电容上下端输入侧电流之和为所述模块化多电平变流器拓扑的三相桥臂电流中直流分量之和，设接入直流潮流控制器的所述模块化多电平变流器拓扑的三相桥臂电流中直流分量之和为I d，则
    I d＝i SMp0-+i SMp0+
    其中：i SMp0-为直流潮流控制器电容下端输入侧电流；i SMp0+为直流潮流控制器电容上端输入侧电流；
    则所述模块化多电平变流器拓扑的上下桥臂电流分别为：
    Figure PCTCN2019110679-appb-100007
    Figure PCTCN2019110679-appb-100008
    其中：i a为A相交流电流；
    Figure PCTCN2019110679-appb-100009
    为功率因数角；ω为交流电压频率；t为时间；I ap(t)、I an(t)分别为所述模块化多电平变流器拓扑的A相上、下桥臂电流；
    直流潮流控制器的电容输入侧电流由各相桥臂电流和单相半桥开关函数相乘后三相汇合而决定：
    Figure PCTCN2019110679-appb-100010
    Figure PCTCN2019110679-appb-100011
    其中：F kp0(t)为所述直流潮流控制器的单相半桥平均开关函数；I为交流侧相电流幅值；U d为所述直流潮流控制器的直流母线电压；i kp(t)为所述模块化多电平变流器拓扑的单相上、下桥臂电流；a,b,c为三相；
    对于输入端采用单相半桥结构的直流潮流控制器，电容输入侧电流除了包含直流分量外还含有低频交流分量，设接入直流潮流控制器的所述模块化多电平变流器拓扑的单相桥臂电流中直流分量为I d，则所述模块化多电平变流器拓扑的上下桥臂电流可分别设为：
    Figure PCTCN2019110679-appb-100012
    Figure PCTCN2019110679-appb-100013
    直流潮流控制器的电容输入侧电流由该相桥臂电流和单相半桥开关函数相乘决定：
    Figure PCTCN2019110679-appb-100014
    Figure PCTCN2019110679-appb-100015
    直流潮流控制器的输入输出功率平衡方程为：
    P PFC＝U di SMp0++(U d-U c0)i SMp0-＝P MVDC
    其中：P PEC为直流潮流控制器的输入有功功率；P MVDC为中压直流侧输出有功功率；a,b,c为三相；
    通过计算分析，选取合适的电压指令直流分量系数和基频交流分量系数，即能使直流潮流控制器单相半桥和所述模块化多电平变流器拓扑的上、下桥臂的电压调制系数满足直流潮流控制器的能量平衡。
  10. 根据权利要求9所述的多端口直流潮流控制的模块化多电平变流器的调制方法，其特征在于：所述直流潮流控制器输出直流端口采用开环控制策 略，由
    Figure PCTCN2019110679-appb-100016
    根据各直流端口的输出功率指令值P i，计算得到各直流端口的输出电压指令值V oi，再由输出端直流/直流变换器的具体拓扑，得到输出电压指令值V oi与直流/直流变换器的占空比D i的关系，进而得到各端口输出电压的差值ΔV ij与各占空比的差值ΔD ij之间的关系：
    V oi＝f(U d,U c0,D i)i＝1,2,…n
    ΔV ij＝f(U c0,ΔD ij)i,j＝1,2,…n,i≠j
    根据直流/直流变换器的具体拓扑的特性，选取某一直流端口的变换器占空比，则所有直流端口的占空比得以确定，再通过载波移相调制使各端口输出传输功率指令值。
  11. 一种换流站，其特征在于，包括权利要求1-7任一项所述的多端口直流潮流控制的模块化多电平变流器。
  12. 一种固态变压器，其特征在于，包括权利要求1-7任一项所述的多端口直流潮流控制的模块化多电平变流器。
  13. 一种基于多个换流站的网状/环状多端柔性直流输电系统，其特征在于，包括多个换流站，所述换流站包括权利要求1-7任一项所述的多端口直流潮流控制的模块化多电平变流器。
  14. 一种基于多个固态变压器的网状/环状多端柔性直流配电系统，其特征在于，包括多个固态变压器，所述固态变压器包括权利要求1-7任一项所述的多端口直流潮流控制的模块化多电平变流器。
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