WO2025218283A1 - 一种具有卸荷功能的柔直换流器及控制方法 - Google Patents

一种具有卸荷功能的柔直换流器及控制方法

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Publication number
WO2025218283A1
WO2025218283A1 PCT/CN2025/070916 CN2025070916W WO2025218283A1 WO 2025218283 A1 WO2025218283 A1 WO 2025218283A1 CN 2025070916 W CN2025070916 W CN 2025070916W WO 2025218283 A1 WO2025218283 A1 WO 2025218283A1
Authority
WO
WIPO (PCT)
Prior art keywords
bridge arm
unloading
converter station
converter
phase
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/CN2025/070916
Other languages
English (en)
French (fr)
Inventor
张建文
施刚
杨仁炘
周剑桥
王晗
蔡旭
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.)
Shanghai Jiao Tong University
Original Assignee
Shanghai Jiao Tong University
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Shanghai Jiao Tong University filed Critical Shanghai Jiao Tong University
Publication of WO2025218283A1 publication Critical patent/WO2025218283A1/zh
Pending legal-status Critical Current
Anticipated expiration legal-status Critical

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Classifications

    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K7/00—Constructional details common to different types of electric apparatus
    • H05K7/20—Modifications to facilitate cooling, ventilating, or heating
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—ELECTRIC POWER NETWORKS; 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
    • H02J3/001—Arrangements for handling faults or abnormalities, e.g. emergencies or contingencies
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—ELECTRIC POWER NETWORKS; 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
    • H02J3/001—Arrangements for handling faults or abnormalities, e.g. emergencies or contingencies
    • H02J3/0014—Arrangements for handling faults or abnormalities, e.g. emergencies or contingencies for preventing or reducing power oscillations in networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—ELECTRIC POWER NETWORKS; 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 high-voltage DC [HVDC] links; Arrangements for transfer of electric power between generators and networks via HVDC links
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—ELECTRIC POWER NETWORKS; 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 feeding a single network from two or more generators or sources in parallel; Arrangements for feeding already energised networks from additional generators or sources in parallel
    • H02J3/46—Controlling the sharing of generated power between the generators, sources or networks
    • H02J3/48—Controlling the sharing of active power
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—ELECTRIC POWER NETWORKS; 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 feeding a single network from two or more generators or sources in parallel; Arrangements for feeding already energised networks from additional generators or sources in parallel
    • H02J3/46—Controlling the sharing of generated power between the generators, sources or networks
    • H02J3/50—Controlling the sharing of reactive power
    • 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
    • H02M1/08—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static 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
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • 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
    • 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 invention relates to the field of flexible direct current (DC) power transmission and distribution and power electronics technology, and in particular to a flexible DC converter with a load shedding function and a control method thereof.
  • DC direct current
  • Flexible direct current transmission technology has the characteristics of independent control of active and reactive power, no need for reactive power compensation, ability to supply power to passive networks, and unchanged voltage polarity when the power flow reverses. It is very suitable for long-distance and large-scale renewable energy grid connection.
  • Existing DC unloading devices can be divided into three types: centralized, distributed, and hybrid.
  • Centralized unloading devices have difficulty equalizing the voltage of series-connected switching devices, and they experience large voltage fluctuations during operation.
  • Distributed unloading devices offer smaller voltage fluctuations, but the large number of submodules requires more fully controlled switching devices, such as thyristors, and additional water cooling, significantly increasing costs.
  • Hybrid unloading devices combine centralized and distributed unloading devices, reducing device costs to a certain extent compared to distributed unloading devices, but still require a large number of switching devices and water cooling.
  • the object of the present invention is to provide a flexible DC converter with unloading function and a control method.
  • a flexible DC converter with a load shedding function comprising: a modular multi-level flexible DC converter, and a DC load shedding device installed in the modular multi-level flexible DC converter;
  • the modular multi-level flexible DC converter includes an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor and a lower bridge arm reactor;
  • the DC unloading device includes an unloading bridge arm and an auxiliary shutoff bridge arm, wherein the unloading bridge arm can consume surplus power in the system, and the auxiliary shutoff bridge arm can assist in shutting off the switch device in the unloading bridge arm;
  • the flexible DC converter topology with the load shedding function includes one or more of the following:
  • the DC unloading device is single-phase and comprises two identical upper and lower parts, which are respectively installed on the upper converter station bridge arm and the lower converter station bridge arm of any phase of the flexible DC converter; each part comprises: an unloading bridge arm and an auxiliary shutdown bridge arm; the auxiliary shutdown bridge arm is connected in series with the upper converter station bridge arm and the lower converter station bridge arm, and then connected in parallel with the unloading bridge arm;
  • the DC unloading device is three-phase, and each phase is the same as the DC unloading device in the first topology;
  • the DC unloading device is single-phase, and each phase includes two unloading bridge arms and one auxiliary shutdown bridge arm.
  • the two unloading bridge arms are connected in series and then connected in parallel with the upper converter station bridge arm and the lower converter station bridge arm of any phase of the flexible DC converter.
  • the auxiliary shutdown bridge arm is connected between the upper and lower unloading bridge arms and between the upper converter station bridge arm and the lower converter station bridge arm.
  • the DC unloading device is three-phase, and each phase is the same as the DC unloading device in the third topology;
  • the DC unloading device is three-phase; each phase includes a unloading bridge arm and two auxiliary shutdown bridge arms; the two auxiliary shutdown bridge arms are respectively connected between the AC port and the upper converter station bridge arm and between the AC port and the lower converter station bridge arm; the unloading bridge arm is connected between the upper converter station bridge arm and the lower converter station bridge arm;
  • the DC unloading device is three-phase; each phase includes an unloading bridge arm and an auxiliary shutdown bridge arm; the unloading bridge arm and the auxiliary shutdown bridge arm are connected in series and connected in the middle of the upper converter station bridge arm and the middle of the lower converter station bridge arm.
  • each phase of the modular multi-level flexible DC converter includes: an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor;
  • One end of the upper bridge arm reactor adopts a star connection, wherein the neutral point is connected to the positive pole of the DC port, and the other end of the upper bridge arm reactor of each phase is connected to the upper end of the upper converter station bridge arm of the corresponding phase; for each phase converter station bridge arm that is not equipped with a DC unloading device, the lower end of the upper converter station bridge arm is connected to the upper end of the lower converter station bridge arm; one end of the lower bridge arm reactor adopts a star connection, wherein the neutral point is connected to the negative pole of the DC port, and the other end of the lower bridge arm reactor of each phase is connected to the lower end of the lower converter station bridge arm of the corresponding phase.
  • the flow unloading device is installed only on one-phase bridge arm of the flexible DC converter
  • the DC unloading device includes an upper unloading bridge arm, an upper auxiliary shutoff bridge arm, a lower unloading bridge arm, and a lower auxiliary shutoff bridge arm;
  • One end of the upper auxiliary shutdown bridge arm is connected to the lower end of the upper converter station bridge arm, and the other end is connected to the AC port; one end of the lower auxiliary shutdown bridge arm is connected to the upper end of the lower converter station bridge arm, and the other end is connected to the AC port; one end of the upper unloading bridge arm is connected to the upper end of the upper converter station bridge arm, and the other end is connected to the AC port; one end of the lower unloading bridge arm is connected to the lower end of the lower converter station bridge arm, and the other end is connected to the AC port.
  • one end of the upper converter station bridge arm is connected to the lower end of the upper auxiliary shutdown bridge arm, and the other end is connected to the AC port;
  • One end of the lower converter station bridge arm is connected to the upper end of the lower auxiliary shutdown bridge arm, and the other end is connected to the AC port;
  • One end of the upper unloading bridge arm is connected to the upper end of the upper auxiliary shut-off bridge arm, and the other end is connected to the AC port;
  • one end of the lower unloading bridge arm is connected to the lower end of the lower auxiliary shut-off bridge arm, and the other end is connected to the AC port;
  • the flow unloading device is installed on the three-phase bridge arm of the flexible DC converter; each phase is the same as the first topology.
  • the DC unloading device is installed only on one-phase bridge arm of the flexible DC converter
  • the DC unloading device includes: an upper unloading bridge arm, a lower unloading bridge arm, and an auxiliary shutoff bridge arm;
  • the upper end of the upper unloading bridge arm is connected to the upper end of the upper converter station bridge arm, the lower end of the upper unloading bridge arm is connected to the upper end of the lower unloading bridge arm, and the lower end of the lower unloading bridge arm is connected to the lower end of the lower converter station bridge arm; one end of the auxiliary shutdown bridge arm is connected to the lower end of the upper unloading bridge arm, and the other end is connected to the lower end of the upper converter station bridge arm.
  • the flow unloading device is installed on the three-phase bridge arm of the flexible DC converter; each phase is the same as the third topology.
  • each phase of the modular multi-level flexible DC converter includes: a first upper converter station bridge arm, a second upper converter station bridge arm, a first lower converter station bridge arm, a second lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor;
  • One end of the upper bridge arm reactor is connected in a star configuration, with the neutral point connected to the positive pole of the DC port.
  • the other end of the upper bridge arm reactor of each phase is connected to the upper end of the first upper converter station bridge arm of the corresponding phase; the upper end of the second upper converter station bridge arm is connected to the lower end of the first upper converter station bridge arm.
  • One end of the lower bridge arm reactor is star-connected, with the neutral point connected to the negative pole of the DC port, and the other end of the lower bridge arm reactor of each phase is connected to the lower end of the second lower converter station bridge arm of the corresponding phase; the lower end of the first lower converter station bridge arm is connected to the upper end of the second lower converter station bridge arm.
  • the DC unloading device is installed on the three-phase bridge arm of the flexible DC converter
  • Each phase of the DC unloading device includes: an unloading bridge arm, an upper auxiliary shutoff bridge arm, and a lower auxiliary shutoff bridge arm;
  • the upper end of the unloading bridge arm is connected to the lower end of the first upper converter station bridge arm, and the lower end of the unloading bridge arm is connected to the upper end of the second lower converter station bridge arm; one end of the upper auxiliary shutdown bridge arm is connected to the lower end of the second upper converter station bridge arm, and the other end is connected to the AC port; one end of the lower auxiliary shutdown bridge arm is connected to the upper end of the first lower converter station bridge arm, and the other end is connected to the AC port.
  • the DC unloading device is installed on the three-phase bridge arm of the flexible DC converter
  • Each phase of the DC unloading device includes: an unloading bridge arm and an auxiliary shutdown bridge arm; the upper end of the unloading bridge arm is connected to the lower end of the first upper converter station bridge arm, the lower end of the unloading bridge arm is connected to the upper end of the auxiliary shutdown bridge arm, and the lower end of the auxiliary shutdown bridge arm is connected to the upper end of the second lower converter station bridge arm.
  • the modular multi-level flexible DC converter is a medium voltage or high voltage level, three-phase, voltage source converter with a modular multi-level structure, which has the function of AC/DC power conversion and can realize the connection between medium and high voltage AC power grids and DC systems.
  • the upper converter station bridge arm and the lower converter station bridge arm include: a plurality of converter station submodules connected in series;
  • the converter station bridge arm includes a bipolar submodule, a monopolar submodule, or a mixture of bipolar submodule and monopolar submodule;
  • the unloading bridge arm includes an unloading resistor R and a half-controlled switch device string connected in series, or an unloading resistor R and a capacitor string C connected in series;
  • the direction of the half-controlled switch device string is consistent with the voltage polarity direction of the converter station bridge arm;
  • the resistor is a power resistor for dissipating surplus power;
  • the auxiliary shutdown bridge arm includes a bipolar submodule or a plurality of bipolar submodules connected in series.
  • a control method for a flexible DC converter with unloading capability is provided.
  • the flexible DC converter with unloading capability has two operating modes, including:
  • Mode 1 Normal operation mode. In this mode, the AC system operates normally, the converter is controlled to perform reactive power control, and the unloading bridge arm does not operate; all the unloading bridge arms are in the off state, and the current flows only through the bridge arm reactor and the converter station bridge arm;
  • Mode 2 Fault unloading mode. In this mode, if a short circuit or ground fault occurs in the AC system and the DC grid has surplus power, the converter station is controlled to perform active power control and unloading power control, including: controlling the AC current in the converter station and controlling the opening and closing of the thyristors in the unloading bridge arm;
  • the output voltages of the auxiliary shutoff bridge arm and the converter station bridge arm are controlled to realize the shutoff of the thyristor in the unloading bridge arm.
  • the output voltages of the auxiliary turn-off bridge arm and the converter station bridge arm are controlled to change simultaneously in each power frequency cycle, so as to realize the turn-off of the thyristor in the unloading bridge arm in each power frequency cycle.
  • the voltage of the upper converter station bridge arm of the corresponding phase is controlled to be zero
  • the voltage of the lower converter station bridge arm of the corresponding phase is controlled to be the DC grid voltage
  • the voltage of the auxiliary shutoff bridge arm is controlled to be a positive polarity voltage not greater than 0.05 pu, so that the thyristor in the upper unloading bridge arm is subjected to reverse voltage and shuts down;
  • the voltage of the lower converter station bridge arm of the corresponding phase is controlled to be zero
  • the voltage of the upper converter station bridge arm of the corresponding phase is controlled to be the DC grid voltage
  • the voltage of the auxiliary shut-off bridge arm is controlled to be a negative polarity voltage not greater than 0.05pu, so that the thyristor in the lower unloading bridge arm is subjected to reverse voltage and shuts down.
  • the voltages of the bridge arms of the converter stations of other phases are controlled to follow the changes to compensate for the changes in the AC line voltage of the converter station. Assuming that the time of turning off the unloading bridge arm of phase A is t 0 , the voltages of the bridge arms of each converter station are controlled to be:
  • u'ap , u'an , u'bp , u'bn , u'cp , and u'cn are the modulation voltages of the upper and lower converter station arms of phase A, the upper and lower converter station arms of phase B, and the upper and lower converter station arms of phase C, respectively, under the fault unloading condition;
  • uap, uan, ubp, ubn, ucp, and ucn are the modulation voltages of the upper and lower converter station arms of phase A, the upper and lower converter station arms of phase B, and the upper and lower converter station arms of phase C, respectively, under normal operation;
  • UDC is the DC grid voltage;
  • the DC side output voltage and the AC side output voltage of the converter station are respectively:
  • U DC and U' DC are the DC grid voltages before and after the fault respectively;
  • U' AB , U' BC , U' CA and U AB , U BC , U CA are the AC grid line voltages before and after the fault respectively; changing the converter station bridge arm voltage does not change the DC voltage and AC line voltage output by the converter station.
  • the embodiments of the present invention have at least one of the following beneficial effects:
  • the flexible DC converter with unloading capability in the embodiment of the present invention has an unloading function itself, which satisfies the fault ride-through process of the flexible DC transmission system and ensures safe and stable operation of the system.
  • the flexible DC converter with unloading capability in the embodiment of the present invention significantly reduces the number of switching devices and uses half-controlled thyristor switching devices instead of fully controlled switching devices, thus having a significant economic advantage of low construction cost.
  • the flexible DC converter with unloading capability in the embodiments of the present invention uses series thyristors to control the unloading bridge arm, making it easier to achieve device voltage balancing than traditional solutions using series fully controlled switching devices. Furthermore, the unloading power can be precisely and continuously controlled by controlling the conduction time of the thyristors.
  • the flexible DC converter with unloading capability of the embodiment of the present invention has fewer switching devices and uses centralized resistors, which facilitates heat dissipation and reduces the heat dissipation pressure of the converter station, thereby significantly reducing the cost of the device.
  • the control method of the flexible DC converter with unloading capability controls the coordination of the active power output of the converter station and the unloading power of the unloading device, thereby achieving the stability of the DC grid voltage when a fault occurs on the AC side of the receiving converter station.
  • the control method of the flexible DC converter with unloading capability controls the output voltage of the converter station bridge arm and the auxiliary shutdown bridge arm, realizes the controllable shutdown of the unloading bridge arm, and does not affect the DC output voltage and AC output line current of the converter station during the control process.
  • the DC voltage fluctuation is smaller than that of the traditional DC unloading device.
  • FIG1 is a schematic diagram of an application scenario of a flexible DC converter with load shedding capability according to an embodiment of the present invention
  • FIG2 is a schematic diagram of a topological structure of a first flexible DC converter with load shedding capability in a preferred embodiment of the present invention
  • FIG3 is a schematic diagram of a second topological structure of a flexible DC converter with load shedding capability in a preferred embodiment of the present invention
  • FIG4 is a schematic diagram of a third topological structure of a flexible DC converter with load shedding capability in a preferred embodiment of the present invention.
  • FIG5 is a schematic diagram of a fourth topological structure of a flexible DC converter with load shedding capability in a preferred embodiment of the present invention.
  • FIG6 is a schematic diagram of a fifth topological structure of a flexible DC converter with load shedding capability in a preferred embodiment of the present invention.
  • FIG7 is a schematic diagram of a sixth topological structure of a flexible DC converter with load shedding capability in a preferred embodiment of the present invention.
  • FIG8 is a schematic diagram of a typical topological structure of an unloading bridge arm, an auxiliary shutdown bridge arm, a converter station bridge arm, and a submodule in a preferred embodiment of the present invention
  • FIG9 is a diagram showing simulated waveforms of AC-side grid voltage and current of a flexible DC converter with load shedding capability according to a specific embodiment of the present invention.
  • FIG10 is a diagram showing simulated waveforms of voltage and current on the DC side of a flexible DC converter with unloading capability according to a specific embodiment of the present invention
  • FIG11 is a simulation waveform diagram of the modulation voltage of the converter station bridge arm of a flexible DC converter with unloading capability in a specific embodiment of the present invention.
  • FIG. 12 is a simulation waveform diagram of the unloading bridge arm switch signal, the A-phase auxiliary shutdown bridge arm voltage simulation waveform, and the unloading bridge arm current simulation waveform of a flexible DC converter with unloading capability in a specific embodiment of the present invention.
  • a flexible DC converter with a load shedding function comprising: a modular multi-level flexible DC converter, and a DC load shedding device installed in the modular multi-level flexible DC converter;
  • the modular multi-level flexible DC converter includes an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor;
  • the DC unloading device includes an unloading bridge arm and an auxiliary shutoff bridge arm.
  • the unloading bridge arm can consume the surplus power in the system, and the auxiliary shutoff bridge arm can assist in shutting down the switching devices in the unloading bridge arm.
  • the flexible DC converter topologies with load shedding function include one or more of the following:
  • the DC unloading device is single-phase and consists of two identical upper and lower parts, installed on the upper and lower converter station bridge arms respectively. Each part includes an unloading bridge arm and an auxiliary shutdown bridge arm.
  • the auxiliary shutdown bridge arm is connected in series with the upper and lower converter station bridge arms and then in parallel with the unloading bridge arm.
  • the DC unloading device is three-phase, and each phase is the same as the DC unloading device in the first topology;
  • the DC unloading device is single-phase, with each phase comprising two unloading bridge arms and one auxiliary shutdown bridge arm.
  • the two unloading bridge arms are connected in series and then in parallel with the upper and lower converter station bridge arms.
  • the auxiliary shutdown bridge arm is connected between the upper and lower unloading bridge arms and between the upper and lower converter station bridge arms.
  • the DC unloading device is three-phase, and each phase has the same DC unloading device as the third topology;
  • the DC unloading device is three-phase; each phase includes a unloading bridge arm and two auxiliary shutdown bridge arms; the two auxiliary shutdown bridge arms are connected between the AC port and the upper converter station bridge arm and between the AC port and the lower converter station bridge arm respectively; the unloading bridge arm is connected between the upper converter station bridge arm and the lower converter station bridge arm;
  • the DC unloading device is three-phase; each phase includes a unloading bridge arm and an auxiliary shutdown bridge arm; the unloading bridge arm and the auxiliary shutdown bridge arm are connected in series and connected in the middle of the upper converter station bridge arm and the middle of the lower converter station bridge arm.
  • the flexible DC converter in the above embodiment itself has a load unloading function, which satisfies the fault ride-through process of the flexible DC transmission system and ensures safe and stable operation of the system.
  • each phase of the modular multi-level flexible DC converter includes: an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor.
  • One end of the upper bridge arm reactor is connected in a star configuration, with its neutral point connected to the positive pole of the DC port, and the other end of the upper bridge arm reactor of each phase is connected to the upper end of the upper converter station bridge arm of the corresponding phase.
  • the lower end of the upper converter station bridge arm is connected to the upper end of the lower converter station bridge arm.
  • One end of the lower bridge arm reactor is connected in a star configuration, with its neutral point connected to the negative pole of the DC port, and the other end of the lower bridge arm reactor of each phase is connected to the lower end of the lower converter station bridge arm of the corresponding phase.
  • the DC unloading device is installed only on one phase arm of the flexible DC converter. It consists of an upper unloading arm, an upper auxiliary disconnect arm, a lower unloading arm, and a lower auxiliary disconnect arm.
  • the upper auxiliary disconnect arm is connected to the lower end of the upper converter station arm at one end and to the AC port at the other end.
  • the lower auxiliary disconnect arm is connected to the upper end of the lower converter station arm at one end and to the AC port at the other end.
  • the upper unloading arm is connected to the upper end of the upper converter station arm at one end and to the AC port at the other end.
  • connection topology can also be as follows: one end of the upper converter station bridge arm is connected to the lower end of the upper auxiliary shutdown bridge arm, and the other end is connected to the AC port; one end of the lower converter station bridge arm is connected to the upper end of the lower auxiliary shutdown bridge arm, and the other end is connected to the AC port; one end of the upper unloading bridge arm is connected to the upper end of the upper auxiliary shutdown bridge arm, and the other end is connected to the AC port; one end of the lower unloading bridge arm is connected to the lower end of the lower auxiliary shutdown bridge arm, and the other end is connected to the AC port.
  • each phase of the modular multilevel flexible DC converter includes an upper converter station arm, a lower converter station arm, an upper arm reactor, and a lower arm reactor.
  • One end of the upper arm reactor is connected in a star configuration, with its neutral point connected to the positive pole of the DC port.
  • the other end of the upper arm reactor of each phase is connected to the upper end of the upper converter station arm of the corresponding phase.
  • One end of the lower arm reactor is connected in a star configuration, with its neutral point connected to the negative pole of the DC port.
  • the other end of the lower arm reactor of each phase is connected to the lower end of the lower converter station arm of the corresponding phase.
  • the DC unloading device is installed on the three-phase bridge arms of the flexible DC converter.
  • Each phase of the DC unloading device includes an upper unloading bridge arm, an upper auxiliary shutdown bridge arm, a lower unloading bridge arm, and a lower auxiliary shutdown bridge arm.
  • the upper auxiliary shutdown bridge arm has one end connected to the lower end of the upper converter station bridge arm and the other end connected to the AC port.
  • the lower auxiliary shutdown bridge arm has one end connected to the upper end of the lower converter station bridge arm and the other end connected to the AC port.
  • the upper unloading bridge arm has one end connected to the upper end of the upper converter station bridge arm and the other end connected to the AC port.
  • the lower unloading bridge arm has one end connected to the lower end of the lower converter station bridge arm and the other end connected to the AC port.
  • the positions of the converter station bridge arm and the auxiliary shutdown bridge arm can also be swapped.
  • One end of the upper converter station bridge arm is connected to the lower end of the upper auxiliary shutdown bridge arm, and the other end is connected to the AC port;
  • one end of the lower converter station bridge arm is connected to the upper end of the lower auxiliary shutdown bridge arm, and the other end is connected to the AC port;
  • one end of the upper unloading bridge arm is connected to the upper end of the upper auxiliary shutdown bridge arm, and the other end is connected to the AC port;
  • one end of the lower unloading bridge arm is connected to the lower end of the lower auxiliary shutdown bridge arm, and the other end is connected to the AC port.
  • each phase of the modular multilevel flexible DC converter includes: an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor.
  • One end of the upper bridge arm reactor is connected in a star configuration, with its neutral point connected to the positive pole of the DC port.
  • the other end of the upper bridge arm reactor of each phase is connected to the upper end of the upper converter station bridge arm of the corresponding phase.
  • the lower end of the upper converter station bridge arm is connected to the upper end of the lower converter station bridge arm.
  • One end of the lower bridge arm reactor is connected in a star configuration, with its neutral point connected to the negative pole of the DC port.
  • the other end of the lower bridge arm reactor of each phase is connected to the lower end of the lower converter station bridge arm of the corresponding phase.
  • the DC unloading device is installed only on one phase arm of the flexible DC converter. It consists of an upper unloading arm, a lower unloading arm, and an auxiliary shutdown arm.
  • the upper end of the upper unloading arm is connected to the upper end of the upper converter station arm
  • the lower end of the upper unloading arm is connected to the upper end of the lower unloading arm
  • the lower end of the lower unloading arm is connected to the lower end of the lower converter station arm.
  • One end of the auxiliary shutdown arm is connected to the lower end of the upper unloading arm, and the other end is connected to the lower end of the upper converter station arm.
  • each phase of the modular multilevel flexible DC converter includes: an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor.
  • One end of the upper bridge arm reactor is connected in a star configuration, with its neutral point connected to the positive pole of the DC port.
  • the other end of the upper bridge arm reactor of each phase is connected to the upper end of the upper converter station bridge arm of the corresponding phase.
  • the lower end of the upper converter station bridge arm is connected to the upper end of the lower converter station bridge arm.
  • One end of the lower bridge arm reactor is connected in a star configuration, with its neutral point connected to the negative pole of the DC port.
  • the other end of the lower bridge arm reactor of each phase is connected to the lower end of the lower converter station bridge arm of the corresponding phase.
  • the DC unloading device is installed on the three-phase bridge arms of the flexible DC converter.
  • Each phase of the DC unloading device consists of an upper unloading arm, a lower unloading arm, and an auxiliary shutdown arm.
  • the upper end of the upper unloading arm is connected to the upper end of the upper converter station arm
  • the lower end of the upper unloading arm is connected to the upper end of the lower unloading arm
  • the lower end of the lower unloading arm is connected to the lower end of the lower converter station arm.
  • One end of the auxiliary shutdown arm is connected to the lower end of the upper unloading arm, and the other end is connected to the lower end of the upper converter station arm.
  • each phase of the modular multilevel flexible DC converter includes: a first upper converter station arm, a second upper converter station arm, a first lower converter station arm, a second lower converter station arm, an upper arm reactor, and a lower arm reactor.
  • One end of the upper arm reactor is connected in a star configuration, with its neutral point connected to the positive terminal of the DC port.
  • the other end of each phase's upper arm reactor is connected to the upper end of the first upper converter station arm of the corresponding phase.
  • the upper end of the second upper converter station arm is connected to the lower end of the first upper converter station arm.
  • One end of the lower arm reactor is connected in a star configuration, with its neutral point connected to the negative terminal of the DC port.
  • the other end of each phase's lower arm reactor is connected to the lower end of the second lower converter station arm of the corresponding phase.
  • the lower end of the first lower converter station arm is connected to the upper end of the second lower converter station arm.
  • the DC unloading device is installed on the three-phase bridge arms of the flexible DC converter.
  • Each phase of the DC unloading device includes a unloading bridge arm, an upper auxiliary shutdown bridge arm, and a lower auxiliary shutdown bridge arm.
  • the upper end of the unloading bridge arm is connected to the lower end of the first upper converter station bridge arm, and the lower end of the unloading bridge arm is connected to the upper end of the second lower converter station bridge arm.
  • One end of the upper auxiliary shutdown bridge arm is connected to the lower end of the second upper converter station bridge arm, and the other end is connected to the AC port.
  • One end of the lower auxiliary shutdown bridge arm is connected to the upper end of the first lower converter station bridge arm, and the other end is connected to the AC port.
  • each phase of the modular multilevel flexible DC converter includes: a first upper converter station arm, a second upper converter station arm, a first lower converter station arm, a second lower converter station arm, an upper arm reactor, and a lower arm reactor.
  • One end of the upper arm reactor is connected in a star configuration, with its neutral point connected to the positive terminal of the DC port.
  • the other end of each phase's upper arm reactor is connected to the upper end of the first upper converter station arm of the corresponding phase.
  • the upper end of the second upper converter station arm is connected to the lower end of the first upper converter station arm, and the lower end of the second upper converter station arm is connected to the AC port.
  • One end of the lower arm reactor is connected in a star configuration, with its neutral point connected to the negative terminal of the DC port.
  • the other end of each phase's lower arm reactor is connected to the lower end of the second lower converter station arm of the corresponding phase.
  • the lower end of the first lower converter station bridge arm is connected to the upper end of the second lower converter station bridge arm, and the upper end of the first lower converter station bridge arm is connected to the AC port.
  • the DC unloading device is installed on the three-phase bridge arms of the flexible DC converter.
  • Each phase of the DC unloading device includes a unloading bridge arm and an auxiliary shutdown bridge arm.
  • the upper end of the unloading bridge arm is connected to the lower end of the bridge arm of the first upper converter station, the lower end of the unloading bridge arm is connected to the upper end of the auxiliary shutdown bridge arm, and the lower end of the auxiliary shutdown bridge arm is connected to the upper end of the bridge arm of the second lower converter station.
  • the converter station bridge arm includes: a plurality of converter station submodules connected in series.
  • the converter station bridge arm can be composed of bipolar submodules, monopolar submodules, or a mixture of bipolar and monopolar submodules.
  • the unloading arm is used to dissipate excess power in the DC system in the event of an AC system fault. It can consist of a series connection of a unloading resistor R and a string of half-controlled switching devices T, or a series connection of a unloading resistor R and a capacitor string C. The orientation of the thyristor string aligns with the voltage polarity of the converter station arm.
  • the resistor is a power resistor that dissipates excess power.
  • the auxiliary shutdown bridge arm is composed of one or more bipolar sub-modules connected in series.
  • the flexible DC converter with unloading capability in the embodiment of the present invention significantly reduces the number of switching devices and uses half-controlled thyristor switching devices instead of fully controlled switching devices, thus having a significant economic advantage of low construction cost.
  • the flexible DC converter with unloading capability of the present invention uses series thyristors to control the unloading bridge arm. This makes it easier to achieve device voltage balancing than the traditional solution of series fully controlled switching devices. It can also accurately and continuously control the unloading power by controlling the conduction time of the thyristors.
  • the flexible DC converter with unloading capability of the present invention has fewer switching devices and uses centralized resistors, which facilitates heat dissipation and reduces the heat dissipation pressure of the converter station, thereby significantly reducing the cost of the device.
  • Another embodiment of the present invention provides a control method for a flexible DC converter with unloading capability.
  • the flexible DC converter with unloading capability has two operating modes, including:
  • Mode 1 Normal operation mode. In this mode, the AC system operates normally, the converter controls reactive power, and the unloading bridge arm does not operate.
  • Reactive power control is an interactive reactive power control between the converter station and the AC system. All unloading bridge arms are shut down, and current flows only through the bridge arm reactors and the converter station bridge arm.
  • Mode 2 Fault shedding mode.
  • the converter station In this mode, if a short circuit or ground fault occurs in the AC system, resulting in surplus power in the DC grid, the converter station is controlled to implement active power control and shedding power control. This includes controlling the AC current in the converter station and switching on and off the thyristors in the shedding arm. Current flows through the converter station arm, the shedding arm, and the auxiliary shutdown arm, dissipating the surplus power.
  • the active power output of the converter station and the unloading power of the unloading device are controlled to coordinate with each other to achieve the stability of the DC grid voltage when a fault occurs on the AC side of the receiving converter station.
  • the output voltage of the auxiliary shutdown bridge arm and the converter station bridge arm is controlled to realize the shutdown of the thyristor in the unloading bridge arm.
  • the output voltages of the auxiliary shutdown bridge arm and the converter station bridge arm are controlled to change simultaneously in each power frequency cycle, thereby realizing the shutdown of the thyristor in the unloading bridge arm in each power frequency cycle.
  • the bridge arm voltage of the upper converter station of the corresponding phase is controlled to be zero
  • the bridge arm voltage of the lower converter station of the corresponding phase is controlled to be the DC grid voltage
  • the voltage of the auxiliary shut-off bridge arm is controlled to be a positive polarity voltage not greater than 0.05pu, so that the thyristor in the upper unloading bridge arm is subjected to reverse voltage and turned off
  • the bridge arm voltage of the lower converter station of the corresponding phase is controlled to be zero
  • the bridge arm voltage of the upper converter station of the corresponding phase is controlled to be the DC grid voltage
  • the voltage of the auxiliary shut-off bridge arm is controlled to be a negative polarity voltage not greater than 0.05pu, so that the thyristor in the lower unloading bridge arm is subjected to reverse voltage and turned off.
  • the voltage of the bridge arm of the converter station of other phases is controlled to follow the change to compensate for the change of the AC line voltage of the converter station. Assuming that the time of turning off the unloading bridge arm of phase A is t 0 , the voltage of the bridge arm of each converter station is controlled to be:
  • u'ap , u'an , u'bp , u'bn , u'cp , and u'cn are the modulation voltages of the upper and lower converter station arms of phase A, the upper and lower converter station arms of phase B, and the upper and lower converter station arms of phase C, respectively, under the fault unloading condition;
  • uap , uan , ubp, ubn , ucp , and ucn are the modulation voltages of the upper and lower converter station arms of phase A, the upper and lower converter station arms of phase B, and the upper and lower converter station arms of phase C, respectively, under normal operation;
  • UDC is the DC grid voltage.
  • the DC side output voltage and AC side output voltage of the converter station are:
  • U DC and U' DC are the DC grid voltages before and after the fault, respectively ;
  • U' AB , U' BC , and U' CA are the AC grid line voltages before and after the fault, respectively. Therefore, changing the converter station arm voltage does not change the DC voltage and AC line voltage at the converter station output.
  • the turn-on time of the thyristor in the unloading bridge arm in each power frequency cycle is controlled to control the power consumed by the unloading bridge arm.
  • the system is simulated and verified using MATLAB/Simulink software.
  • the simulation parameters are shown in Table 1.
  • FIG. 1 A typical application scenario of a flexible DC converter with unloading function in a DC system is shown in Figure 1 of the specification.
  • the sending-end converter station collects power and sends it to the DC grid, and the receiving-end converter station then converts the DC power into AC power.
  • the flexible DC system In phase 1, the flexible DC system is in normal operation, the power of the sending-end converter station and the receiving-end converter station are equal, and the voltage and current of the DC grid are both rated values.
  • the flexible DC converter with unloading function operates in mode 1 and does not unload.
  • the simulation assumes a fault on the AC side of the receiving converter station.
  • the AC voltage drops to 0.2 pu at the 1.5th second of the simulation, reducing the power at the receiving converter station to 0.2 times its rated value.
  • the output power at the sending converter station remains unchanged, resulting in surplus power in the DC system.
  • the DC unloading device transitions to Mode 2 and begins consuming surplus power.
  • the total power consumed by the unloading bridge arm and the AC active power output of the converter station are controlled to equal the DC grid power, preventing overvoltage events.
  • Figure 9 is a simulation waveform diagram of the AC side grid voltage and current of the flexible DC converter with unloading capability during the implementation process, which contains a total of 6 waveform diagrams, from top to bottom: the AC grid voltage waveform and AC grid current waveform during the entire simulation process; the AC grid voltage waveform and AC grid current waveform in normal operating mode; the AC grid voltage waveform and AC grid current waveform in fault unloading mode.
  • normal operating mode the AC grid voltage and current are both rated values.
  • fault unloading mode the AC grid voltage is reduced to 0.2pu, and the AC grid current amplitude remains unchanged.
  • the control method for the flexible DC converter with unloading function provided by the embodiment of the present invention can change the bridge arm voltage of the converter station to unload while ensuring that the line voltage and phase current waveforms output on the AC side are not significantly distorted.
  • Figure 10 shows simulated waveforms of the DC grid voltage and current during the implementation of a flexible DC converter with load shedding capability.
  • the waveforms are divided into two sections: the DC grid voltage waveform and the DC grid current waveform during the entire simulation.
  • the DC grid voltage and current were both rated at 500 kV and 3000 A, respectively.
  • the control method for a flexible DC converter with load shedding capability provided by the present invention maintained the DC grid voltage at a stable 500 kV. Therefore, after the fault, the DC grid current remained at the rated value of 3000 A.
  • Figure 11 is a simulation waveform of the modulation voltage of the converter station bridge arm and the current of the unloading bridge arm of the flexible direct current converter with unloading capability during the implementation process. It contains a total of 6 waveforms, which are as follows from top to bottom: the modulation voltage per unit waveform of the upper and lower converter station bridge arms of phases A, B, and C of the converter station before the fault occurs; the modulation voltage per unit waveform of the upper and lower converter station bridge arms of phases A, B, and C of the converter station after the fault occurs. Before the fault occurs, the voltage of the converter station bridge arm is a three-phase sine wave.
  • the control method of the flexible direct current converter with unloading function provided by the embodiment of the present invention superimposes a three-phase symmetrical pulse on the converter station bridge arm voltage to shut down each unloading bridge arm.
  • the unloading arm of phase A is shut down, the voltage of the upper converter arm of phase A is zero, the voltage of the lower converter arm of phase A is equal to the DC grid voltage, and the modulated voltage of the upper converter arm of phase A before the fault is subtracted from the upper converter arm of phase B and C.
  • the modulated voltage of the upper converter arm of phase A before the fault is added to the lower converter arms of phases B and C.
  • the same amount of change is symmetrically increased or decreased in the three converter arms of phases A, B, and C.
  • Figure 12 contains three waveforms: a waveform showing the drive signal waveform for the Phase A unloading arm of a flexible DC converter with unloading capability, a waveform showing the voltage on the Phase A auxiliary shutdown arm, and a waveform showing the current simulation of the unloading arm on Phase A during the implementation of the control method of the above embodiment after a fault occurs.
  • the discontinuous current in the unloading arm demonstrates that the control method provided by the present invention effectively shuts down the unloading arm and achieves accurate control of the unloading power.
  • Simulation waveform results show that the flexible DC converter with unloading function provided by the embodiment of the present invention can effectively consume surplus power and maintain DC grid voltage stability when a fault occurs on the AC side of the receiving converter station.
  • the control method of the flexible DC converter with unloading function provided by the embodiment of the present invention can coordinate the control of the active power of the converter station and the dissipated power of the unloading bridge arm when a fault occurs, and can accurately and smoothly control the unloading power.
  • the controller of the flexible DC converter with unloading function provided by the embodiment of the present invention controls the output voltage of the converter station bridge arm and the auxiliary shutdown bridge arm to achieve controlled shutdown of the unloading bridge arm, and the control process does not affect the quality of the converter station DC output voltage and AC output line current waveform.
  • the flexible DC converter with unloading function provided by the embodiment of the present invention produces smaller DC voltage ripple during unloading.
  • the unloading device provided by the embodiment of the present invention reduces the number of switching devices, uses thyristors instead of fully controlled switching devices, and uses centralized unloading resistors to reduce the requirements for heat dissipation devices, resulting in lower cost.

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Abstract

提供一种具有卸荷功能的柔直换流器及其控制方法，包括模块化多电平柔直换流器，以及安装于模块化多电平柔直换流器中的直流卸荷装置；模块化多电平柔直换流器包括上换流站桥臂、下换流站桥臂、上桥臂电抗器和下桥臂电抗器；直流卸荷装置包括卸荷桥臂和辅助关断桥臂，卸荷桥臂能够消耗系统中的盈余功率，辅助关断桥臂能够协助关断卸荷桥臂中的开关器件；卸荷功能的柔直换流器具有六种拓扑。柔直换流器本身具有卸荷功能，满足柔直送出系统故障穿越过程，保证系统安全稳定运行。

Description

一种具有卸荷功能的柔直换流器及控制方法 技术领域
本发明涉及柔性直流输配电、电力电子技术领域，具体地，涉及一种具有卸荷功能的柔直换流器及控制方法。
背景技术
柔性直流输电技术，具有有功无功独立可控、无需无功补偿、可向无源网络供电、潮流反转时电压极性不变等特点，非常适用于远距离大规模新能源并网。
以大规模海上风电并网系统为例，由于风电机组惯性大，海上换流站(送端)的送出功率不能突变。当岸上换流站(受端)交流并网侧发生故障时，岸上换流站(受端)并网功率受到限制，无法将海上换流站(送端)的送出功率完全消纳。盈余功率会对换流器中的电容器以及直流海缆寄生电容进行充电，导致过电压。直流卸荷装置能够消耗直流电网中的盈余功率，避免发生过电压或风机大规模脱网。
现有的直流卸荷装置，可以分为集中式卸荷装置、分布式卸荷装置和混合式卸荷装置三种。其中，集中式卸荷装置存在串联开关器件均压困难问题，且工作时电压波动大；分布式卸荷装置的电压波动小，但是大量子模块需要更多的全控型开关器件、晶闸管等开关器件，同时需要额外的水冷装置，显著增加了成本；混合式卸荷装置是集中式和分布式卸荷装置的结合，与分布式卸荷装置相比，一定程度上减少了装置成本，但仍需要大量开关器件和水冷装置。
发明内容
针对现有技术中的缺陷，本发明的目的是提供一种具有卸荷功能的柔直换流器及控制方法。
根据本发明的一个方面，提供一种具有卸荷功能的柔直换流器，包括：模块化多电平柔直换流器，以及安装于所述模块化多电平柔直换流器中的直流卸荷装置；
所述模块化多电平柔直换流器包括上换流站桥臂、下换流站桥臂、上桥臂电抗器和下桥臂电抗器；
所述直流卸荷装置包括卸荷桥臂和辅助关断桥臂，卸荷桥臂能够消耗系统中的盈余功率，辅助关断桥臂能够协助关断卸荷桥臂中的开关器件；
所述卸荷功能的柔直换流器拓扑包括如下的一种或者多种：
第一种拓扑，所述直流卸荷装置为单相，包括上下两个相同的部分，分别安装于柔直换流器任意一相的上换流站桥臂、下换流站桥臂上；每部分包括：卸荷桥臂、辅助关断桥臂；所述辅助关断桥臂与上换流站桥臂、下换流站桥臂串联后再与所述卸荷桥臂并联；
第二种拓扑，所述直流卸荷装置为三相，每相与第一种拓扑中的直流卸荷装置相同；
第三种拓扑，所述直流卸荷装置为单相，每相包括两个卸荷桥臂、一个辅助关断桥臂；两个卸荷桥臂串联后，与柔直换流器任意一相的上换流站桥臂、下换流站桥臂并联，所述辅助关断桥臂连接在上下的卸荷桥臂中间和上换流站桥臂、下换流站桥臂中间；
第四种拓扑，所述直流卸荷装置为三相，每相与第三种拓扑中的直流卸荷装置相同；
第五种拓扑，所述直流卸荷装置为三相；每相包括一个卸荷桥臂、两个辅助关断桥臂；两个所述辅助关断桥臂分别连接在交流端口和上换流站桥臂之间、交流端口和下换流站桥臂之间；所述卸荷桥臂连接在上换流站桥臂中间和下换流站桥臂中间；
第六种拓扑，所述直流卸荷装置为三相；每相包括一个卸荷桥臂、一个辅助关断桥臂；所述卸荷桥臂和所述辅助关断桥臂串联后，连接在上换流站桥臂中间和下换流站桥臂中间。
优选地，所述模块化多电平柔直换流器的每相包括：上换流站桥臂、下换流站桥臂、上桥臂电抗器、下桥臂电抗器；
所述上桥臂电抗器一端采用星型连接，其中性点连接直流端口的正极，各相所述上桥臂电抗器的另一端连接对应相的所述上换流站桥臂上端；对于未安装直流卸荷装置的每相换流站桥臂，所述上换流站桥臂下端连接所述下换流站桥臂上端；所述下桥臂电抗器一端采用星型连接，其中性点连接直流端口的负极，各相所述下桥臂电抗器的另一端连接对应相的所述下换流站桥臂下端。
优选地，第一种拓扑，所述流卸荷装置仅安装在柔直换流器的一相桥臂上；
所述直流卸荷装置包括上卸荷桥臂、上辅助关断桥臂、下卸荷桥臂、下辅助关断桥臂；
所述上辅助关断桥臂一端连接上换流站桥臂的下端，另一端连接交流端口；所述下辅助关断桥臂一端连接下换流站桥臂的上端，另一端连接交流端口；所述上卸荷桥臂的一端连接上换流站桥臂的上端，另一端连接交流端口；所述下卸荷桥臂的一端连接下换流站桥臂的下端，另一端连接交流端口。
或者，所述上换流站桥臂一端连接上辅助关断桥臂的下端，另一端连接交流端口；
所述下换流站桥臂一端连接下辅助关断桥臂的上端，另一端连接交流端口；
所述上卸荷桥臂的一端连接上辅助关断桥臂的上端，另一端连接交流端口；所述下卸荷桥臂的一端连接下辅助关断桥臂的下端，另一端连接交流端口；
优选地，第二种拓扑，所述流卸荷装置安装在柔直换流器的三相桥臂上；每相与第一种拓扑相同。
优选地，第三种拓扑，所述直流卸荷装置仅安装在柔直换流器的一相桥臂上；
所述直流卸荷装置包括：上卸荷桥臂、下卸荷桥臂、辅助关断桥臂；
所述上卸荷桥臂上端和上换流站桥臂上端连接，所述上卸荷桥臂下端和所述下卸荷桥臂上端连接，所述下卸荷桥臂下端和所述下换流站桥臂下端连接；所述辅助关断桥臂的一端连接所述上卸荷桥臂下端，另一端连接所述上换流站桥臂下端。
优选地，第四种拓扑，所述流卸荷装置安装在柔直换流器的三相桥臂上；每相与第三种拓扑相同。
优选地，所述具有模块化多电平柔直换流器的每相包括：第一上换流站桥臂、第二上换流站桥臂、第一下换流站桥臂、第二下换流站桥臂、上桥臂电抗器、下桥臂电抗器；
所述上桥臂电抗器一端采用星型连接，其中性点连接直流端口的正极，各相所述上桥臂电抗器的另一端连接对应相的所述第一上换流站桥臂上端；所述第二上换流站桥臂的上端连接所述第一上换流站桥臂的下端；
所述下桥臂电抗器一端采用星型连接，其中性点连接直流端口的负极，各相所述下桥臂电抗器的另一端连接对应相的所述第二下换流站桥臂下端；所述第一下换流站桥臂的下端连接所述第二下换流站桥臂的上端。
优选地，第五种拓扑，所述直流卸荷装置安装在柔直换流器的三相桥臂上；
所述直流卸荷装置中每一相包括：卸荷桥臂、上辅助关断桥臂、下辅助关断桥臂；
所述卸荷桥臂上端和第一上换流站桥臂的下端连接，所述卸荷桥臂下端和所述第二下换流站桥臂的上端连接；所述上辅助关断桥臂的一端连接所述第二上换流站桥臂的下端，另一端连接交流端口；所述下辅助关断桥臂的一端连接所述第一下换流站桥臂的上端，另一端连接交流端口。
优选地，第六种拓扑，所述直流卸荷装置安装在柔直换流器的三相桥臂上；
所述直流卸荷装置中每一相包括：卸荷桥臂、辅助关断桥臂；所述卸荷桥臂上端和第一上换流站桥臂的下端连接，所述卸荷桥臂下端和所述辅助关断桥臂的上端连接，所述辅助关断桥臂的下端和所述第二下换流站桥臂的上端连接。
优选地，所述模块化多电平柔直换流器为中压级或高压级、具有模块化多电平结构的、三相的、电压源型变流器，具备交直流电能变换的功能，能够实现中高压交流电网和直流系统的联接。
优选地，所述上换流站桥臂、下换流站桥臂包括：多个串联连接的换流站子模块；
所述换流站桥臂包括双极型子模块、单极型子模块或混合的双极型子模块与单极型子模块；
所述卸荷桥臂包括串联的卸荷电阻R和半控型开关器件串，或是串联的卸荷电阻R和电容串C；
所述半控型开关器件串的方向与所述换流站桥臂的电压极性方向一致；所述电阻为耗散盈余功率的功率电阻；
所述辅助关断桥臂包括一个双极性子模块或多个串联的双极性子模块。
根据本发明的第二个方面，提供一种具有卸荷能力的柔直换流器的控制方法，具有卸荷能力的柔直换流器具有两种运行模式，包括：
模式1：正常运行模式，在此模式下，交流系统正常运行，控制所述换流器进行无功功率控制，卸荷桥臂不工作；所有所述卸荷桥臂处于关断状态，电流只从所述桥臂电抗器和所述换流站桥臂流过；
模式2：故障卸荷模式，在此模式下，交流系统发生短路或接地故障，直流电网出现盈余功率，控制所述换流站进行有功功率控制和卸荷功率控制，包括：控制换流站交流电流，控制卸荷桥臂中晶闸管的开通和关断；
电流既从所述换流站桥臂流过，也从所述卸荷桥臂和辅助关断桥臂流过，消耗盈余功率。
优选地，针对第四种具有卸荷功能的柔直换流器拓扑，控制所述辅助关断桥臂和所述换流站桥臂的输出电压，实现所述卸荷桥臂中晶闸管的关断。
优选地，控制所述辅助关断桥臂和所述换流站桥臂的输出电压在每个工频周期内同时改变，实现所述卸荷桥臂中晶闸管在每个工频周期内的关断。
优选地，在关断某一相所述上卸荷桥臂时，控制对应相所述上换流站桥臂电压为零，控制对应相所述下换流站桥臂电压为直流电网电压，控制辅助关断桥臂电压为不大于0.05pu的正极性电压，让所述上卸荷桥臂中的晶闸管承受反向电压而关断；
在关断某一相所述下卸荷桥臂时，控制对应相所述下换流站桥臂电压为零，控制对应相所述上换流站桥臂电压为直流电网电压，控制辅助关断桥臂电压为不大于0.05pu的负极性电压，让所述下卸荷桥臂中的晶闸管承受反向电压而关断。
优选地，在关断某一相所述卸荷桥臂时，控制其他相所述换流站桥臂电压跟随变化，补偿换流站交流测线电压的变化；假设关断A相上卸荷桥臂的时刻为t0，此时控制各换流站桥臂电压为：
其中，u’ap、u’an、u’bp、u’bn、u’cp、u’cn分别是故障卸荷情况下A相上下换流站桥臂、B相上下换流站桥臂、C相上下换流站桥臂的调制电压；uap、uan、ubp、ubn、ucp、ucn分别是正常工作情况下A相上下换流站桥臂、B相上下换流站桥臂、C相上下换流站桥臂的调制电压，UDC是直流电网电压；
优选地，根据方程式(1)计算出的调制电压，换流站直流侧输出电压和交流侧输出电压分别为：
其中，UDC和U’DC分别是故障发生前、故障发生后的直流电网电压；U’AB、U’BC、U’CA和UAB、UBC、UCA分别是故障发生前、故障发生后的交流电网线电压；改变换流站桥臂电压没有改变换流站输出的直流电压和交流线电压。
与现有技术相比，本发明实施例至少具有如下的一项有益效果：
本发明实施例的具有卸荷能力的柔直换流器，柔直换流器本身具有卸荷功能，满足柔直送出系统故障穿越过程，保证系统安全稳定运行。
本发明实施例的具有卸荷能力的柔直换流器，与现有直流卸荷装置相比，大幅减少了开关器件数量，使用半控型晶闸管开关器件代替全控型开关器件，具有建造成本低的显著经济性优势；
本发明实施例的具有卸荷能力的柔直换流器，与现有并联型集中式直流卸荷装置相比，本发明的具有卸荷能力的柔直换流器，采用串联晶闸管控制卸荷桥臂，比传统串联全控型开关器件的方案更容易实现器件均压，还可以通过控制晶闸管的导通时间，精准、连续的控制卸荷功率；
本发明实施例的具有卸荷能力的柔直换流器，与现有并联型分布式或混合式直流卸荷装置相比，本发明的具有卸荷能力的柔直换流器，开关器件更少，使用集中式电阻，便于散热，降低了换流站的散热压力，因此显著降低了装置的成本；
本发明实施例提供的具有卸荷能力的柔直换流器的控制方法，控制换流站的有功功率输出和卸荷装置的卸荷功率配合，实现受端换流站交流侧故障时，直流电网电压的稳定。
本发明实施例提供的具有卸荷能力的柔直换流器的控制方法，控制换流站桥臂和辅助关断桥臂的输出电压，实现卸荷桥臂的可控关断，且控制过程中不影响换流站直流输出电压和交流输出线电流，直流电压波动比传统直流卸荷装置小。
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1为本发明一实施例的具有卸荷能力的柔直换流器应用场景示意图；
图2为本发明一优选实施例中的第一种具有卸荷能力的柔直换流器拓扑结构示意图；
图3为本发明一优选实施例中的第二种具有卸荷能力的柔直换流器拓扑结构示意图；
图4为本发明一优选实施例中的第三种具有卸荷能力的柔直换流器拓扑结构示意图；
图5为本发明一优选实施例中的第四种具有卸荷能力的柔直换流器拓扑结构示意图；
图6为本发明一优选实施例中的第五种具有卸荷能力的柔直换流器拓扑结构示意图；
图7为本发明一优选实施例中的第六种具有卸荷能力的柔直换流器拓扑结构示意图；
图8为本发明一优选实施例中的卸荷桥臂、辅助关断桥臂、换流站桥臂和子模块的典型拓扑结构示意图；
图9为本发明一具体实施例中具有卸荷能力的柔直换流器交流侧电网电压和电流仿真波形图；
图10为本发明一具体实施例中具有卸荷能力的柔直换流器直流侧电网电压和电流仿真波形图；
图11为本发明一具体实施例中具有卸荷能力的柔直换流器的换流站桥臂调制电压仿真波形图。
图12为本发明一具体实施例中具有卸荷能力的柔直换流器的卸荷桥臂开关信号仿真波形图、A相辅助关断桥臂电压仿真波形图、卸荷桥臂电流仿真波形图。
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。
本发明一实施例中，提供一种具有卸荷功能的柔直换流器，如图1所示，包括：模块化多电平柔直换流器，以及安装于模块化多电平柔直换流器中的直流卸荷装置；
模块化多电平柔直换流器包括上换流站桥臂、下换流站桥臂、上桥臂电抗器和下桥臂电抗器；
直流卸荷装置包括卸荷桥臂和辅助关断桥臂，卸荷桥臂能够消耗系统中的盈余功率，辅助关断桥臂能够协助关断卸荷桥臂中的开关器件；
卸荷功能的柔直换流器拓扑包括如下的一种或者多种：
第一种拓扑，直流卸荷装置为单相，包括上下两个相同的部分，分别安装于上换流站桥臂、下换流站桥臂上；每部分包括：卸荷桥臂、辅助关断桥臂；辅助关断桥臂与上换流站桥臂、下换流站桥臂串联后再与卸荷桥臂并联；
第二种拓扑，直流卸荷装置为三相，每相与第一种拓扑中的直流卸荷装置相同；
第三种拓扑，直流卸荷装置为单相，每相包括两个卸荷桥臂、一个辅助关断桥臂；两个卸荷桥臂串联后，与上换流站桥臂、下换流站桥臂并联，辅助关断桥臂连接在上下的卸荷桥臂中间和上换流站桥臂、下换流站桥臂中间；
第四种拓扑，直流卸荷装置为三相，每相与第三种拓扑中的直流卸荷装置相同；
第五种拓扑，直流卸荷装置为三相；每相包括一个卸荷桥臂、两个辅助关断桥臂；两个辅助关断桥臂分别连接在交流端口和上换流站桥臂之间、交流端口和下换流站桥臂之间；卸荷桥臂连接在上换流站桥臂中间和下换流站桥臂中间；
第六种拓扑，直流卸荷装置为三相；每相包括一个卸荷桥臂、一个辅助关断桥臂；卸荷桥臂和辅助关断桥臂串联后，连接在上换流站桥臂中间和下换流站桥臂中间。
上述实施例中的柔直换流器本身具有卸荷功能，满足柔直送出系统故障穿越过程，保证系统安全稳定运行。
一优选实施例中，进一步提供了具有卸荷功能的柔直换流器的第一种拓扑，如图2所示，具有模块化多电平柔直换流器的每相包括：上换流站桥臂、下换流站桥臂、上桥臂电抗器、下桥臂电抗器。上桥臂电抗器一端采用星型连接，其中性点连接直流端口的正极，各相上桥臂电抗器的另一端连接对应相的上换流站桥臂上端。对于未安装直流卸荷装置的每相换流站桥臂，上换流站桥臂下端连接下换流站桥臂上端。下桥臂电抗器一端采用星型连接，其中性点连接直流端口的负极，各相下桥臂电抗器的另一端连接对应相的下换流站桥臂下端。
直流卸荷装置仅安装在柔直换流器的一相桥臂上。直流卸荷装置包括：上卸荷桥臂、上辅助关断桥臂、下卸荷桥臂、下辅助关断桥臂。上辅助关断桥臂一端连接上换流站桥臂的下端，另一端连接交流端口。下辅助关断桥臂一端连接下换流站桥臂的上端，另一端连接交流端口。上卸荷桥臂的一端连接上换流站桥臂的上端，另一端连接交流端口。
需要说明的是，换流站桥臂和辅助关断桥臂位置对调也能实现同样功能。即它们的连接拓扑也可以如下：上换流站桥臂一端连接上辅助关断桥臂的下端，另一端连接交流端口；下换流站桥臂一端连接下辅助关断桥臂的上端，另一端连接交流端口；上卸荷桥臂的一端连接上辅助关断桥臂的上端，另一端连接交流端口；所述下卸荷桥臂的一端连接下辅助关断桥臂的下端，另一端连接交流端口。
一优选实施例中，进一步提供了具有卸荷功能的柔直换流器的第二种拓扑，如图3所示，具有模块化多电平柔直换流器的每相包括：上换流站桥臂、下换流站桥臂、上桥臂电抗器、下桥臂电抗器。上桥臂电抗器一端采用星型连接，其中性点连接直流端口的正极，各相上桥臂电抗器的另一端连接对应相的上换流站桥臂上端。下桥臂电抗器一端采用星型连接，其中性点连接直流端口的负极，各相下桥臂电抗器的另一端连接对应相的下换流站桥臂下端。
直流卸荷装置安装在柔直换流器的三相桥臂上。直流卸荷装置的每相包括：上卸荷桥臂、上辅助关断桥臂、下卸荷桥臂、下辅助关断桥臂。上辅助关断桥臂一端连接上换流站桥臂的下端，另一端连接交流端口。下辅助关断桥臂一端连接下换流站桥臂的上端，另一端连接交流端口。上卸荷桥臂的一端连接上换流站桥臂的上端，另一端连接交流端口。下卸荷桥臂的一端连接下换流站桥臂的下端，另一端连接交流端口。
同样的，也可以将换流站桥臂和辅助关断桥臂位置对调。上换流站桥臂一端连接上辅助关断桥臂的下端，另一端连接交流端口；下换流站桥臂一端连接下辅助关断桥臂的上端，另一端连接交流端口；上卸荷桥臂的一端连接上辅助关断桥臂的上端，另一端连接交流端口；所述下卸荷桥臂的一端连接下辅助关断桥臂的下端，另一端连接交流端口。
一优选实施例中，进一步提供了具有卸荷功能的柔直换流器的第三种拓扑，如图4所示，模块化多电平柔直换流器的每一相包括：上换流站桥臂、下换流站桥臂、上桥臂电抗器、下桥臂电抗器。上桥臂电抗器一端采用星型连接，其中性点连接直流端口的正极，各相上桥臂电抗器的另一端连接对应相的上换流站桥臂上端。上换流站桥臂下端连接下换流站桥臂上端。下桥臂电抗器一端采用星型连接，其中性点连接直流端口的负极，各相下桥臂电抗器的另一端连接对应相的下换流站桥臂下端。
直流卸荷装置仅安装在柔直换流器的一相桥臂上。直流卸荷装置包括：上卸荷桥臂、下卸荷桥臂、辅助关断桥臂。上卸荷桥臂上端和上换流站桥臂上端连接，上卸荷桥臂下端和下卸荷桥臂上端连接，下卸荷桥臂下端和下换流站桥臂下端连接。辅助关断桥臂的一端连接上卸荷桥臂下端，另一端连接上换流站桥臂下端。
一优选实施例中，进一步提供了具有卸荷功能的柔直换流器的第四种拓扑，如图5所示，模块化多电平柔直换流器的每一相包括：上换流站桥臂、下换流站桥臂、上桥臂电抗器、下桥臂电抗器。上桥臂电抗器一端采用星型连接，其中性点连接直流端口的正极，各相上桥臂电抗器的另一端连接对应相的上换流站桥臂上端。上换流站桥臂下端连接下换流站桥臂上端。下桥臂电抗器一端采用星型连接，其中性点连接直流端口的负极，各相下桥臂电抗器的另一端连接对应相的下换流站桥臂下端。
直流卸荷装置安装在柔直换流器的三相桥臂上。直流卸荷装置中每一相包括：上卸荷桥臂、下卸荷桥臂、辅助关断桥臂。上卸荷桥臂上端和上换流站桥臂上端连接，上卸荷桥臂下端和下卸荷桥臂上端连接，下卸荷桥臂下端和下换流站桥臂下端连接。辅助关断桥臂的一端连接上卸荷桥臂下端，另一端连接上换流站桥臂下端。
一优选实施例中，进一步提供具有卸荷功能的柔直换流器的第五种拓扑，如图6所示，具有模块化多电平柔直换流器的每相包括：第一上换流站桥臂、第二上换流站桥臂、第一下换流站桥臂、第二下换流站桥臂、上桥臂电抗器、下桥臂电抗器。上桥臂电抗器一端采用星型连接，其中性点连接直流端口的正极，各相上桥臂电抗器的另一端连接对应相的第一上换流站桥臂上端。第二上换流站桥臂的上端连接第一上换流站桥臂的下端。下桥臂电抗器一端采用星型连接，其中性点连接直流端口的负极，各相下桥臂电抗器的另一端连接对应相的第二下换流站桥臂下端。第一下换流站桥臂的下端连接第二下换流站桥臂的上端。
直流卸荷装置安装在柔直换流器的三相桥臂上。直流卸荷装置中每一相包括：卸荷桥臂、上辅助关断桥臂、下辅助关断桥臂。卸荷桥臂上端和第一上换流站桥臂的下端连接，卸荷桥臂下端和第二下换流站桥臂的上端连接。上辅助关断桥臂的一端连接第二上换流站桥臂的下端，另一端连接交流端口。下辅助关断桥臂的一端连接第一下换流站桥臂的上端，另一端连接交流端口。
一优选实施例中，进一步提供具有卸荷功能的柔直换流器的第六种拓扑，如图7所示，具有模块化多电平柔直换流器的每相包括：第一上换流站桥臂、第二上换流站桥臂、第一下换流站桥臂、第二下换流站桥臂、上桥臂电抗器、下桥臂电抗器。上桥臂电抗器一端采用星型连接，其中性点连接直流端口的正极，各相上桥臂电抗器的另一端连接对应相的第一上换流站桥臂上端。第二上换流站桥臂的上端连接第一上换流站桥臂的下端，第二上换流站桥臂的下端连接交流端口。下桥臂电抗器一端采用星型连接，其中性点连接直流端口的负极，各相下桥臂电抗器的另一端连接对应相的第二下换流站桥臂下端。第一下换流站桥臂的下端连接第二下换流站桥臂的上端，第一下换流站桥臂的上端连接交流端口。
直流卸荷装置安装在柔直换流器的三相桥臂上。直流卸荷装置中每一相包括：卸荷桥臂、辅助关断桥臂。卸荷桥臂上端和第一上换流站桥臂的下端连接，卸荷桥臂下端和辅助关断桥臂的上端连接，辅助关断桥臂的下端和第二下换流站桥臂的上端连接。
一些优选实施例中，如图8所示，换流站桥臂包括：多个串联连接的换流站子模块。换流站桥臂可以由双极型子模块构成、可以由单极型子模块构成，也可以由双极型子模块与单极型子模块混合构成。
卸荷桥臂用于在交流系统发生故障的情况下，消耗直流系统中的盈余功率。卸荷桥臂可以是卸荷电阻R和半控型开关器件串T串联组成，也可以是卸荷电阻R和电容串C串联组成。晶闸管串的方向与换流站桥臂的电压极性方向一致。电阻为耗散盈余功率的功率电阻。
辅助关断桥臂是由一个或多个双极性子模块串联组成。
本发明实施例的具有卸荷能力的柔直换流器，与现有直流卸荷装置相比，大幅减少了开关器件数量，使用半控型晶闸管开关器件代替全控型开关器件，具有建造成本低的显著经济性优势；
与现有并联型集中式直流卸荷装置相比，本发明的具有卸荷能力的柔直换流器，采用串联晶闸管控制卸荷桥臂，比传统串联全控型开关器件的方案更容易实现器件均压，还可以通过控制晶闸管的导通时间，精准、连续的控制卸荷功率；
与现有并联型分布式或混合式直流卸荷装置相比，本发明的具有卸荷能力的柔直换流器，开关器件更少，使用集中式电阻，便于散热，降低了换流站的散热压力，因此显著降低了装置的成本。
基于相同的发明构思，本发明其他实施例中，提供一种具有卸荷能力的柔直换流器的控制方法，具有卸荷能力的柔直换流器具有两种运行模式，包括：
模式1：正常运行模式，在此模式下，交流系统正常运行，控制换流器进行无功功率控制，卸荷桥臂不工作。无功功率控制为换流站和交流系统之间交互的无功功率控制。所有卸荷桥臂处于关断状态，电流只从桥臂电抗器和换流站桥臂流过；
模式2：故障卸荷模式，在此模式下，交流系统发生短路或接地故障，直流电网出现盈余功率，控制换流站进行有功功率控制和卸荷功率控制，包括：控制换流站交流电流，控制卸荷桥臂中晶闸管的开通和关断。电流既从换流站桥臂流过，也从卸荷桥臂和辅助关断桥臂流过，消耗盈余功率。
上述实施例中，控制换流站的有功功率输出和卸荷装置的卸荷功率配合，实现受端换流站交流侧故障时，直流电网电压的稳定。
进一步的，一较佳实施例中，针对第四种具有卸荷功能的柔直换流器拓扑，控制辅助关断桥臂和换流站桥臂的输出电压，实现卸荷桥臂中晶闸管的关断。
更进一步的，一较佳实施例中控制辅助关断桥臂和换流站桥臂的输出电压在每个工频周期内同时改变，实现卸荷桥臂中晶闸管在每个工频周期内的关断。
进一步的，一较佳实施例中在关断某一相上卸荷桥臂时，控制对应相上换流站桥臂电压为零，控制对应相下换流站桥臂电压为直流电网电压，控制辅助关断桥臂电压为不大于0.05pu的正极性电压，让上卸荷桥臂中的晶闸管承受反向电压而关断；在关断某一相下卸荷桥臂时，控制对应相下换流站桥臂电压为零，控制对应相上换流站桥臂电压为直流电网电压，控制辅助关断桥臂电压为不大于0.05pu的负极性电压，让下卸荷桥臂中的晶闸管承受反向电压而关断。
进一步的，一较佳实施例中，为了避免影响换流站交流侧线电流，在关断某一相卸荷桥臂时，控制其他相换流站桥臂电压跟随变化，补偿换流站交流测线电压的变化；假设关断A相上卸荷桥臂的时刻为t0，此时控制各换流站桥臂电压为：
其中，u’ap、u’an、u’bp、u’bn、u’cp、u’cn分别是故障卸荷情况下A相上下换流站桥臂、B相上下换流站桥臂、C相上下换流站桥臂的调制电压；uap、uan、ubp、ubn、ucp、ucn分别是正常工作情况下A相上下换流站桥臂、B相上下换流站桥臂、C相上下换流站桥臂的调制电压，UDC是直流电网电压。
根据方程式(1)计算出的调制电压，换流站直流侧输出电压和交流侧输出电压分别为：
其中，UDC和U’DC分别是故障发生前、故障发生后的直流电网电压；U’AB、U’BC、U’CA和UAB、UBC、UCA分别是故障发生前、故障发生后的交流电网线电压。因此，改变换流站桥臂电压没有改变换流站输出的直流电压和交流线电压。
进一步的，一较佳实施例中，控制卸荷桥臂中晶闸管在每个工频周期的开通时间，控制卸荷桥臂消耗的功率。
以下结合具体的仿真实例来对上述实施例中的具有卸荷功能的柔直换流器及控制方法的应用分别进行进一步说明。
结合上述实施例，以下采用MATLAB/Simulink软件针对系统进行仿真验证，仿真参数如表1所示。
表1仿真验证参数
仿真实例一：
具有卸荷功能的柔直换流器在直流系统中的典型应用场景如说明书附图1，送端换流站将功率汇集，发送到直流电网中，受端换流站再将直流功率转换为交流功率。
为验证具有卸荷功能的柔直换流器及其控制方法的可行性与合理性，仿真设定两个阶段，具体工况设计如下：
阶段一(0-1.5s，正常运行模式)：
在阶段一，柔直系统处于正常运行状态，送端换流站和受端换流站的功率相等，且直流电网的电压和电流均为额定值，具有卸荷功能的柔直换流器运行于模式1，不进行卸荷。
阶段二(1.5s-4s故障卸荷模式)：
仿真假设受端换流站交流侧发生故障，交流电压在仿真第1.5s跌落至0.2pu，受端换流站的功率因此降为额定值0.2倍，而送端换流站的输出功率不变。直流系统中出现盈余功率。
随后采用本发明提供的具有卸荷功能的柔直换流器的控制方法，直流卸荷装置过渡到模式2，开始消耗盈余功率。控制卸荷桥臂消耗的总功率和换流站的输出的交流有功功率之和为直流电网功率，避免出现过电压事故。
图9为实施过程中具有卸荷能力的柔直换流器交流侧电网电压和电流仿真波形图，总共包含6幅波形图，从上到下依次为：整个仿真过程中，交流电网电压波形、交流电网电流波形；正常运行模式下，交流电网电压波形、交流电网电流波形；故障卸荷模式下，交流电网电压波形、交流电网电流波形。在正常运行模式下，交流电网电压和电流均为额定值，在故障卸荷模式下，交流电网电压降低为0.2pu，交流电网电流幅值保持不变。在故障卸荷模式下，交流电网的线电压和相电流仍保持为正弦波，证明本发明实施例提供的具有卸荷功能的柔直换流器控制方法能够在改变换流站桥臂电压进行卸荷的同时，保证交流侧输出的线电压和相电流波形不发生明显畸变。
图10为实施过程中具有卸荷能力的柔直换流器直流侧电网电压和电流仿真波形图，总共包含2幅波形图，从上到下依次为：整个仿真过程中，直流电网电压波形、直流电网电流波形。在故障发生前，直流电网电压和直流电网电流均为额定值，分别为500kV和3000A，在故障发生后，通过本发明提供的具有卸荷能力的柔直换流器的控制方法，保持直流电网电压稳定在500kV，因此在故障后，直流电网的电流始终保持为额定值3000A。
图11为实施过程中具有卸荷能力的柔直换流器内部换流站桥臂调制电压仿真波形图、卸荷桥臂电流仿真波形图，总共包含6幅波形图，从上到下依次为：故障发生前，换流站A相、B相、C相上下换流站桥臂的调制电压标幺值波形图；故障发生后，换流站A相、B相、C相上下换流站桥臂的调制电压标幺值波形图。在故障发生前，换流站桥臂的电压为三相正弦波。在故障发生后，本发明实施例提供的具有卸荷功能的柔直换流器控制方法在换流站桥臂电压中叠加三相对称的脉冲，以关断各个卸荷桥臂。当关断A相上卸荷桥臂时，A相上换流站桥臂电压为零，A相下换流站桥臂电压为直流电网电压，B、C相上换流站桥臂同时减去A相上换流站桥臂故障前的调制电压，B、C相下换流站桥臂同时加上A相上换流站桥臂故障前的调制电压。类似的，关断其他卸荷桥臂时也在A、B、C三相换流站桥臂中对称增减相同的变化量。
图12包含3幅波形图，分别为故障发生后，实施上述实施例的控制方法过程中，具有卸荷能力的柔直换流器的A相卸荷桥臂驱动信号波形图、A相辅助关断桥臂电压、A相上卸荷桥臂的电流仿真波形图。卸荷桥臂中的电流断续，证明本发明所提供的控制方法有效关断卸荷桥臂、实现卸荷功率的准确控制。
仿真波形结果表明，本发明实施例所提供的具有卸荷功能的柔直换流器能够有效在受端换流站交流侧故障时，消耗盈余功率，维持直流电网电压稳定。本发明实施例所提供的具有卸荷功能的柔直换流器控制方法，能够在故障发生时协调控制换流站的有功功率与卸荷桥臂的耗散功率，能够精确平滑控制卸荷功率。本发明实施例提供的具有卸荷功能的柔直换流器控制方，控制换流站桥臂和辅助关断桥臂的输出电压，实现卸荷桥臂的可控关断，且控制过程中不影响换流站直流输出电压和交流输出线电流波形质量。相比现有的并联型集中式直流卸荷装置，本发明实施例所提供的具有卸荷功能的柔直换流器在卸荷时产生的直流电压纹波更小；相比现有的并联型分布式直流卸荷装置，本发明实施例所提供的卸荷装置减小开关器件的数量、使用晶闸管而不是全控型开关器件、集中的卸荷电阻降低散热装置要求，成本更低。
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。上述各优选特征在互不冲突的情况下，可以任意组合使用。

Claims (17)

  1. 一种具有卸荷功能的柔直换流器，其特征在于，包括：模块化多电平柔直换流器，以及安装于所述模块化多电平柔直换流器中的直流卸荷装置；
    所述模块化多电平柔直换流器包括上换流站桥臂、下换流站桥臂、上桥臂电抗器和下桥臂电抗器；
    所述直流卸荷装置包括卸荷桥臂和辅助关断桥臂，卸荷桥臂能够消耗系统中的盈余功率，辅助关断桥臂能够协助关断卸荷桥臂中的开关器件；
    所述卸荷功能的柔直换流器拓扑包括如下的一种或者多种：
    第一种拓扑，所述直流卸荷装置为单相，包括上下两个相同的部分，分别安装于柔直换流器任意一相的上换流站桥臂、下换流站桥臂上；每部分包括：卸荷桥臂、辅助关断桥臂；所述辅助关断桥臂与上换流站桥臂、下换流站桥臂串联后再与所述卸荷桥臂并联；
    第二种拓扑，所述直流卸荷装置为三相，每相与第一种拓扑中的直流卸荷装置相同；
    第三种拓扑，所述直流卸荷装置为单相，每相包括两个卸荷桥臂、一个辅助关断桥臂；两个卸荷桥臂串联后，与柔直换流器任意一相的上换流站桥臂、下换流站桥臂并联，所述辅助关断桥臂连接在上下的卸荷桥臂中间和上换流站桥臂、下换流站桥臂中间；
    第四种拓扑，所述直流卸荷装置为三相，每相与第三种拓扑中的直流卸荷装置相同；
    第五种拓扑，所述直流卸荷装置为三相；每相包括一个卸荷桥臂、两个辅助关断桥臂；两个所述辅助关断桥臂分别连接在交流端口和上换流站桥臂之间、交流端口和下换流站桥臂之间；所述卸荷桥臂连接在上换流站桥臂中间和下换流站桥臂中间；
    第六种拓扑，所述直流卸荷装置为三相；每相包括一个卸荷桥臂、一个辅助关断桥臂；所述卸荷桥臂和所述辅助关断桥臂串联后，连接在上换流站桥臂中间和下换流站桥臂中间。
  2. 根据权利要求1所述的一种具有卸荷功能的柔直换流器，其特征在于，所述模块化多电平柔直换流器的每相包括：上换流站桥臂、下换流站桥臂、上桥臂电抗器、下桥臂电抗器；
    所述上桥臂电抗器一端采用星型连接，其中性点连接直流端口的正极，各相所述上桥臂电抗器的另一端连接对应相的所述上换流站桥臂上端；对于未安装直流卸荷装置的每相换流站桥臂，所述上换流站桥臂下端连接所述下换流站桥臂上端；所述下桥臂电抗器一端采用星型连接，其中性点连接直流端口的负极，各相所述下桥臂电抗器的另一端连接对应相的所述下换流站桥臂下端。
  3. [根据细则26改正 12.02.2025]
    根据权利要求2所述的一种具有卸荷功能的柔直换流器，其特征在于，第一种拓扑，
    所述流卸荷装置仅安装在柔直换流器的一相桥臂上；
    所述直流卸荷装置包括上卸荷桥臂、上辅助关断桥臂、下卸荷桥臂、下辅助关断桥臂；
    所述上辅助关断桥臂一端连接上换流站桥臂的下端，另一端连接交流端口；所述下辅助关断桥臂一端连接下换流站桥臂的上端，另一端连接交流端口；所述上卸荷桥臂的一端连接上换流站桥臂的上端，另一端连接交流端口；所述下卸荷桥臂的一端连接下换流站桥臂的下端，另一端连接交流端口；
    或者，所述上换流站桥臂一端连接上辅助关断桥臂的下端，另一端连接交流端口；所述下换流站桥臂一端连接下辅助关断桥臂的上端，另一端连接交流端口；所述上卸荷桥臂的一端连接上辅助关断桥臂的上端，另一端连接交流端口；所述下卸荷桥臂的一端连接下辅助关断桥臂的下端，另一端连接交流端口。
  4. 根据权利要求3所述的一种具有卸荷功能的柔直换流器，其特征在于，第二种拓扑，所述流卸荷装置安装在柔直换流器的三相桥臂上；每相与第一种拓扑相同。
  5. 根据权利要求2所述的一种具有卸荷功能的柔直换流器，其特征在于，第三种拓扑，
    所述直流卸荷装置仅安装在柔直换流器的一相桥臂上；
    所述直流卸荷装置包括：上卸荷桥臂、下卸荷桥臂、辅助关断桥臂；
    所述上卸荷桥臂上端和上换流站桥臂上端连接，所述上卸荷桥臂下端和所述下卸荷桥臂上端连接，所述下卸荷桥臂下端和所述下换流站桥臂下端连接；所述辅助关断桥臂的一端连接所述上卸荷桥臂下端，另一端连接所述上换流站桥臂下端。
  6. 根据权利要求5所述的一种具有卸荷功能的柔直换流器，其特征在于，第四种拓扑，所述流卸荷装置安装在柔直换流器的三相桥臂上；每相与第三种拓扑相同。
  7. 根据权利要求1所述的一种具有卸荷功能的柔直换流器，其特征在于，所述具有模块化多电平柔直换流器的每相包括：第一上换流站桥臂、第二上换流站桥臂、第一下换流站桥臂、第二下换流站桥臂、上桥臂电抗器、下桥臂电抗器；
    所述上桥臂电抗器一端采用星型连接，其中性点连接直流端口的正极，各相所述上桥臂电抗器的另一端连接对应相的所述第一上换流站桥臂上端；所述第二上换流站桥臂的上端连接所述第一上换流站桥臂的下端；
    所述下桥臂电抗器一端采用星型连接，其中性点连接直流端口的负极，各相所述下桥臂电抗器的另一端连接对应相的所述第二下换流站桥臂下端；所述第一下换流站桥臂的下端连接所述第二下换流站桥臂的上端。
  8. 根据权利要求7所述的一种具有卸荷功能的柔直换流器，其特征在于，第五种拓扑，所述直流卸荷装置安装在柔直换流器的三相桥臂上；
    所述直流卸荷装置中每一相包括：卸荷桥臂、上辅助关断桥臂、下辅助关断桥臂；
    所述卸荷桥臂上端和第一上换流站桥臂的下端连接，所述卸荷桥臂下端和所述第二下换流站桥臂的上端连接；所述上辅助关断桥臂的一端连接所述第二上换流站桥臂的下端，另一端连接交流端口；所述下辅助关断桥臂的一端连接所述第一下换流站桥臂的上端，另一端连接交流端口。
  9. 根据权利要求1所述的一种具有卸荷功能的柔直换流器，其特征在于，第六种拓扑，所述直流卸荷装置安装在柔直换流器的三相桥臂上；
    所述直流卸荷装置中每一相包括：卸荷桥臂、辅助关断桥臂；所述卸荷桥臂上端和第一上换流站桥臂的下端连接，所述卸荷桥臂下端和所述辅助关断桥臂的上端连接，所述辅助关断桥臂的下端和所述第二下换流站桥臂的上端连接。
  10. 根据权利要求1-9任一项所述的一种具有卸荷功能的柔直换流器，其特征在于，所述模块化多电平柔直换流器为中压级或高压级、具有模块化多电平结构的、三相的、电压源型变流器，具备交直流电能变换的功能，能够实现中高压交流电网和直流系统的联接。
  11. 根据权利要求1-9任一项所述的一种具有卸荷功能的柔直换流器，其特征在于，所述上换流站桥臂、下换流站桥臂包括：多个串联连接的换流站子模块；
    所述换流站桥臂包括双极型子模块、单极型子模块或混合的双极型子模块与单极型子模块；
    所述卸荷桥臂包括串联的卸荷电阻R和半控型开关器件串，或是串联的卸荷电阻R和电容串C；
    所述半控型开关器件串的方向与所述换流站桥臂的电压极性方向一致；所述电阻为耗散盈余功率的功率电阻；
    所述辅助关断桥臂包括一个双极性子模块或多个串联的双极性子模块。
  12. 一种具有卸荷能力的柔直换流器的控制方法，其特征在于，具有卸荷能力的柔直换流器具有两种运行模式，包括：
    模式1：正常运行模式，在此模式下，交流系统正常运行，控制所述换流器进行无功功率控制，卸荷桥臂不工作；所有所述卸荷桥臂处于关断状态，电流只从所述桥臂电抗器和所述换流站桥臂流过；
    模式2：故障卸荷模式，在此模式下，交流系统发生短路或接地故障，直流电网出现盈余功率，控制所述换流站进行有功功率控制和卸荷功率控制，包括：控制换流站交流电流，控制卸荷桥臂中晶闸管的开通和关断；
    电流既从所述换流站桥臂流过，也从所述卸荷桥臂和辅助关断桥臂流过，消耗盈余功率。
  13. 根据权利要求12所述的一种具有卸荷能力的柔直换流器的控制方法，其特征在于，针对第四种具有卸荷功能的柔直换流器拓扑，控制所述辅助关断桥臂和所述换流站桥臂的输出电压，实现所述卸荷桥臂中晶闸管的关断。
  14. 根据权利要求13所述的一种具有卸荷能力的柔直换流器的控制方法，其特征在于，控制所述辅助关断桥臂和所述换流站桥臂的输出电压在每个工频周期内同时改变，实现所述卸荷桥臂中晶闸管在每个工频周期内的关断。
  15. 根据权利要求14所述的一种具有卸荷能力的柔直换流器的控制方法，其特征在于，在关断某一相上卸荷桥臂时，控制对应相上换流站桥臂电压为零，控制对应相下换流站桥臂电压为直流电网电压，控制辅助关断桥臂电压为不大于0.05pu的正极性电压，让上卸荷桥臂中的晶闸管承受反向电压而关断；
    在关断某一相下卸荷桥臂时，控制对应相下换流站桥臂电压为零，控制对应相上换流站桥臂电压为直流电网电压，控制辅助关断桥臂电压为不大于0.05pu的负极性电压，让下卸荷桥臂中的晶闸管承受反向电压而关断。
  16. 根据权利要求15所述的一种具有卸荷能力的柔直换流器的控制方法，其特征在于，在关断某一相卸荷桥臂时，控制其他相所述换流站桥臂电压跟随变化，补偿换流站交流测线电压的变化；假设关断A相上卸荷桥臂的时刻为t0，此时控制各换流站桥臂电压为：
    其中，u’ap、u’an、u’bp、u’bn、u’cp、u’cn分别是故障卸荷情况下A相上下换流站桥臂、B相上下换流站桥臂、C相上下换流站桥臂的调制电压；uap、uan、ubp、ubn、ucp、ucn分别是正常工作情况下A相上下换流站桥臂、B相上下换流站桥臂、C相上下换流站桥臂的调制电压，UDC是直流电网电压。
  17. 根据权利要求16所述的一种具有卸荷能力的柔直换流器的控制方法，其特征在于，改变换流站桥臂电压没有改变换流站输出的直流电压和交流线电压，具体的，
    根据方程式(1)计算出的调制电压，换流站直流侧输出电压和交流侧输出电压分别为：
    其中，UDC和U’DC分别是故障发生前、故障发生后的直流电网电压；U’AB、U’BC、U’CA和UAB、UBC、UCA分别是故障发生前、故障发生后的交流电网线电压。
PCT/CN2025/070916 2024-04-15 2025-01-07 一种具有卸荷功能的柔直换流器及控制方法 Pending WO2025218283A1 (zh)

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