WO2016206547A1 - 一种混合直流输电*** - Google Patents

一种混合直流输电*** Download PDF

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Publication number
WO2016206547A1
WO2016206547A1 PCT/CN2016/085610 CN2016085610W WO2016206547A1 WO 2016206547 A1 WO2016206547 A1 WO 2016206547A1 CN 2016085610 W CN2016085610 W CN 2016085610W WO 2016206547 A1 WO2016206547 A1 WO 2016206547A1
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hybrid
transmission system
module
converter
mmc
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PCT/CN2016/085610
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English (en)
French (fr)
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刘欣和
吴金龙
王先为
张�浩
张军
行登江
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许继电气股份有限公司
西安许继电力电子技术有限公司
国家电网公司
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Publication of WO2016206547A1 publication Critical patent/WO2016206547A1/zh

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    • 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
    • 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 a high voltage direct current transmission technology, in particular to a hybrid direct current transmission system.
  • LCC-HVDC Line Commutated Converter based High Voltage Direct Current
  • the LCC-HVDC system has been widely used in submarine cable power transmission, large-capacity long-distance transmission, and asynchronous grid back-to-back interconnection.
  • the LCC-HVDC system has the failure of the inverter station to commutate, the power supply to the weak AC system, and the need to consume a large amount of reactive power during operation, which restricts its development to a certain extent.
  • VSC-HVDC Voltage Source Converter Based High Voltage Direct Current
  • MMC-HVDC Modular Multilevel Converter
  • the MMC-HVDC system also has the advantages of low switching frequency, low switching loss, no need for an AC filter bank and strong scalability, which makes it suitable for high DC voltage and high power transmission. occasion.
  • the MMC-HVDC system is expensive and cannot effectively deal with DC faults, but it restricts its application in long-distance high-power transmission.
  • Hybrid DC transmission technology combines the mature and low cost of traditional LCC-HVDC technology.
  • VSC-HVDC has no commutation failure, flexible control, and strong performance. It can effectively improve the commutation failure of the conventional DC transmission receiving end while satisfying the system transmission.
  • the flexible DC is not equivalent to the conventional DC.
  • the hybrid DC transmission technology is still in the initial stage of research in the world.
  • the research on hybrid DC transmission technology in China has also started late.
  • the embodiment of the present invention provides a hybrid direct current transmission system, which has the advantages of mature technology and low cost of LCC-HVDC, non-commutation failure of VSC-HVDC, flexible control, and strong performance. .
  • a hybrid DC power transmission system provided by an embodiment of the present invention has a bipolar structure, including a positive current converter transformer and a positive current converter, a negative current converter transformer, and a negative current converter.
  • a bipolar structure including a positive current converter transformer and a positive current converter, a negative current converter transformer, and a negative current converter.
  • the device is an MMC converter composed of MMC sub-modules, and the other is an LCC inverter system consisting of at least one LCC.
  • each bridge arm in the MMC is composed of a half bridge module and a full bridge submodule; or a cascade bridge module, a full bridge submodule, and a hybrid dual submodule.
  • the hybrid dual sub-module includes four power modules: T1, T2, T3, T4 and two capacitors: C1, C2; the anode of the T1 is connected to the anode of the T4, and the cathode of the T2 Connecting the cathode of T3, the cathode of T1 is connected to the anode of the T2, the cathode of the T4 is connected to the anode of the T3 through the capacitor C2, and the connection point of the T1 and T4 is connected to the T2 and T3
  • the capacitor C1 is connected between the points, and the connection point of the T1 and T2 is one port of the hybrid dual sub-module, and the connection point of the C2 and T4 is another port of the hybrid dual sub-module.
  • the power module is an insulated gate bipolar transistor (IGBT, An Insulated Gate Bipolar Transistor module
  • IGBT Insulated Gate Bipolar Transistor
  • the anode of the power module is a collector of the IGBT module
  • the cathode of the power module is an emitter of the IGBT module.
  • the hybrid DC power transmission system further includes a reactive power compensation device, and the reactive power compensation device is connected to the AC power grid.
  • the reactive power compensation device is one of the following: a parallel capacitor, a shunt reactor, a static reactive power compensation device, and a camera.
  • the hybrid DC power transmission system further includes an AC filter device, and the AC filter device is connected to the AC power grid.
  • the AC filtering device is composed of an AC filter bank.
  • the reactive power compensation device and the AC filter bank and the MMC provide reactive power compensation for the LCC.
  • the technical solution of the embodiment of the invention provides a novel hybrid DC transmission system, which not only has a simple and reliable structure, but also combines the respective advantages of the LCC and the MMC and overcomes their respective shortcomings: using the active and reactive independent adjustment capability of the MMC to Adjust the AC voltage, thereby increasing the maximum transmission active power capability of the LCC and reducing the possibility of its commutation failure; and the MMC can access various types of sub-modules according to actual needs, the control is more flexible and variable, and the DC side fails.
  • the DC output of the DC terminal can be controlled, the shortcoming of the normal MMC can not effectively deal with the DC fault.
  • FIG. 1 is a schematic structural view of a hybrid DC power transmission system according to an embodiment of the present invention.
  • LCC power grid commutator
  • FIG. 3 is a topological structural diagram of a module hybrid modular multilevel converter MMC according to an embodiment of the present invention.
  • FIG. 4 is a schematic diagram of a topology structure of a half bridge submodule according to an embodiment of the present invention.
  • FIG. 5 is a schematic diagram of a topology structure of a full bridge submodule according to an embodiment of the present invention.
  • FIG. 6 is a schematic diagram of a topology structure of a hybrid dual sub-module according to an embodiment of the present invention.
  • 7-1 is a schematic diagram of a first working state in a normal working mode of a hybrid dual sub-module according to an embodiment of the present invention
  • FIG. 7-2 is a schematic diagram of a second working state in a normal working mode of a hybrid dual sub-module according to an embodiment of the present invention.
  • FIG. 7-3 is a schematic diagram of a third working state in a normal working mode of a hybrid dual sub-module according to an embodiment of the present invention.
  • 7-4 is a fourth working state diagram of a hybrid dual sub-module in a normal working mode according to an embodiment of the present invention.
  • 8-1 is a schematic diagram of one of the working states of the hybrid dual sub-module in the latching mode according to the embodiment of the present invention.
  • FIG. 8-2 is a schematic diagram of another working state in the interlocking mode of the hybrid dual sub-module according to the embodiment of the present invention.
  • the hybrid DC transmission system includes a power grid commutating converter (LCC), a modular multilevel converter (MMC), a converter transformer, an alternating current filter bank (ACF), and an alternating current reactive power compensation system.
  • LCC power grid commutating converter
  • MMC modular multilevel converter
  • ACF alternating current filter bank
  • DCF DC filter
  • the LCC and MMC form a bipolar structure, and an AC filter bank (ACF) and an AC reactive power compensation device are also connected to the AC grid.
  • the AC reactive power compensation device can be a parallel capacitor, or one of a shunt reactor, a static reactive power compensation device, and a camera. It cooperates with an AC filter bank (ACF) and an MMC to provide reactive power compensation for the LCC. .
  • ACF AC filter bank
  • a DC filter is connected between the two DC terminals of the grid commutating converter, and the DC negative terminal of the grid commutating converter is connected to the DC positive terminal of the modular multilevel converter, and the connection point is
  • a switch is connected in series between the DC negative terminal of the grid commutated converter and the DC positive terminal of the modular multilevel converter, and the contact is grounded through a switch (MRTB).
  • the DC positive terminal of the grid commutating converter is the DC positive terminal of the hybrid DC transmission system
  • the DC cathode terminal of the modular multilevel converter is the DC cathode terminal of the hybrid DC transmission system
  • the hybrid DC transmission system A DC smoothing reactor is connected in series on both DC positive and negative lines.
  • the most basic LCC is a three-phase bridge circuit based on a semi-controlled thyristor, but the LCC can be a 6-pulse converter composed of 6 bridge arms, or it can be a 6-pulse converter.
  • the 12-pulse wave converter can also be a multi-pulse converter composed of a plurality of 6-pulse converters.
  • an embodiment of LCC is given. That is to say, the LCC can be a basic three-phase bridge circuit or a combination of a plurality of three-phase bridge circuits.
  • the topology of the MMC is shown in Figure 2.
  • the MMC consists of three-phase six-bridge arms. Each bridge arm is composed of a half-bridge topology sub-module and a full-bridge topology sub-module in a one-to-one ratio.
  • the bridge arm is also connected in series with the bridge arm reactor L.
  • the same AC grid system as the LCC is connected to the AC side of the three-phase bridge circuit of the MMC through a converter transformer and a soft-start resistor.
  • the half bridge submodule and the full bridge submodule in the MMC are based on a fully controlled power electronic device such as an IGBT, and the topology thereof is shown in FIGS. 3 and 4.
  • the LCC and the MMC are simultaneously connected to an AC system to form a hybrid multi-infeed DC transmission system.
  • the system can fully utilize the active and reactive power independent adjustment capability of the MMC system, effectively adjust the AC bus voltage, increase the maximum transmission active power capability of the LCC system, reduce the transient overvoltage of the LCC system, and also reduce the LCC. Inverter commutation failure may be possible.
  • the LCC system can overcome the DC fault by adjusting the trigger phase angle.
  • the MMC system can realize the DC voltage control on the DC side by controlling the full bridge submodule, thereby effectively overcoming the DC fault.
  • the topology of the hybrid DC power transmission system of the present invention is not limited to the above embodiment.
  • the reactive power compensation device is cancelled.
  • the DC negative terminal of the grid commutated inverter is connected to the DC positive terminal of the modular multilevel converter and grounded, and the DC positive terminal of the grid commutated converter is the DC of the hybrid DC transmission system.
  • the DC negative terminal of the modular multilevel converter is the DC negative terminal of the hybrid DC transmission system; as another embodiment, the positions of the LCC and the MMC can be exchanged, and the DC positive terminal of the grid commutation converter Connected to the DC negative terminal of the modular multilevel converter and grounded, the DC positive terminal of the modular multilevel converter is the DC positive terminal of the hybrid DC transmission system, and the DC negative terminal of the grid commutated converter It is the DC negative terminal of the hybrid DC transmission system.
  • the MMC includes a half bridge submodule and a full bridge submodule, and the ratio of the two in each bridge arm is 1:1.
  • the configuration ratio of the half bridge submodule and the full bridge submodule It can be set according to the actual situation. It is also possible to change the full bridge submodule to a clamped dual submodule depending on the situation.
  • the hybrid dual sub-module includes four IGBT modules: T1, T2, T3, T4 and two capacitors: C1, C2, the collector of T1 is connected to the collector of T4, and the emitter of T2 is connected to the emission of T3.
  • the emitter of T1 is connected to the collector of T2, the emitter of T4 is connected to the collector of T3 through capacitor C2, the connection point of T1 and T4 is connected with the connection point of T2 and T3, and the connection point of T1 and T2 is connected.
  • connection point of C2 and T4 is the other port of the hybrid dual submodule.
  • All the IGBTs (T1, T2, T3, T4) in the hybrid dual sub-module are connected in parallel with the freewheeling diode.
  • the bases of T1, T2, T3 and T4 respectively receive the control signals provided by the external device.
  • the hybrid dual submodule has two operating modes, normal operating mode and blocking mode.
  • normal operating mode In row mode, only one IGBT can be turned on between T1 and T2. In order to prevent capacitor C1 from short-circuiting, T1 and T2 cannot be turned on at the same time; at most one IGBT can be turned on between T3 and T4.
  • the hybrid dual submodule has four operating states in the normal operating mode, and the four operating states in the normal operating mode are shown in Figures 7-1 to 7-4.
  • the port output voltage is two capacitor voltages; when T1 and T4 are turned on, the port output voltage is zero; when T2 and T3 are turned on, the port output voltage is Capacitor C2 voltage; when T2 and T4 are turned on, the port output voltage is the reverse voltage of the capacitor C1, that is, the output negative voltage.
  • the current direction does not affect the port output voltage.
  • Usm represents the submodule port output voltage.
  • the sub-module can output four kinds of voltages, which are twice the capacitance voltage, the capacitor voltage, the zero voltage and the negative capacitance voltage.
  • This sub-module can replace the two half-bridge modules to output 2 times the capacitor voltage, and has the negative voltage characteristic of the full bridge sub-module, which can improve the DC voltage utilization and increase the system capacity.
  • the hybrid dual submodule has two operating states in the blocking mode, and the two operating states in the blocking mode are shown in Figures 8-1 and 8-2.
  • the latched state all IGBTs are in the off state.
  • the port output voltage is two capacitor voltages; when the negative current flows, the port output voltage is the negative voltage of the capacitor C1, that is, opposite to the current The voltage to the direction.
  • the submodules in the MMC may all be the hybrid dual submodules, or the following cases: the bridge arm in the MMC is composed of a hybrid submodule and one or more existing submodules (half bridge submodules, all The bridge module and the clamped double sub-module are cascaded. Then, the MMC is a hybrid MMC inverter.
  • the hybrid MMC converter has a wide range of applications when it is expanded according to actual conditions, such as improving the modulation degree and having the STATCOM operation fault traversing capability and saving system hardware cost.
  • the power module is an IGBT module.
  • the power module may be other fully controlled devices.
  • the technical solution of the embodiment of the invention not only has a simple and reliable structure, but also combines the respective advantages of the LCC and the MMC and overcomes the respective disadvantages: the active voltage and reactive independent adjustment capability of the MMC is used to adjust the AC voltage, thereby increasing the maximum transmission active power of the LCC. Power capability and reduce the possibility of its commutation failure; and MMC can access various types of sub-modules according to actual needs, the control is more flexible and variable, and the DC-side output zero voltage can be controlled when the DC side fails.
  • Overcoming the shortcomings of ordinary MMC can not effectively deal with DC faults.

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Abstract

一种混合直流输电***，该混合直流输电***为双极结构，包括正极换流变压器（T1）和正极换流器（LCC）、负极换流变压器（T2）和负极换流器（MMC），双极结构中，其中一极换流器为由MMC子模块构成的MMC换流器，另一极为由至少一个LCC构成的LCC换流器***。混合直流输电***同时具有LCC-HVDC的技术成熟、成本低的特点以及VSC-HVDC的无换相失败，控制灵活，拓展性能强的优点。

Description

一种混合直流输电*** 技术领域
本发明涉及高压直流输电技术，尤其涉及一种混合直流输电***。
背景技术
随着电力科学技术的发展，传统直流输电***(又称电网换相换流器高压直流输电***(LCC-HVDC，Line Commutated Converter based High Voltage Direct Current))的技术已经非常成熟。LCC-HVDC***目前已经被广泛地应用于海底电缆送电、大容量远距离输电以及异步电网背靠背互联等场合。但是，LCC-HVDC***存在着的逆变站换相失败、无法对弱交流***供电、运行过程中需要消耗大量无功功率等缺陷，在一定程度上制约它的发展。
近年来以全控型电力电子器件为基础的电压源型换流器高压直流输电(VSC-HVDC，Voltage Source Converter Based High Voltage Direct Current)因其可独立控制有功无功功率、不存在换相失败、可为无源孤岛供电等诸多优点得到学术界与工业界的青睐。作为VSC-HVDC拓扑的一种，模块化多电平换流器高压直流输电(MMC-HVDC，Modular Multilevel Converter)具备VSC-HVDC所有的优点。同时，因MMC拓扑结构的特点，MMC-HVDC***还具备开关频率较低、开关损耗小、无需交流滤波器组和扩展性强等优点，这使得其可以运用于高直流电压、大功率输电的场合。但是，MMC-HVDC***造价昂贵、无法有效地处理直流故障等缺点却制约其在长距离大功率输电场合的运用。
针对上述现有技术的不足，混合直流输电技术成为新的研究热点。混合直流输电技术结合了传统LCC-HVDC技术成熟、成本低的特点以及 VSC-HVDC无换相失败，控制灵活，拓展性能强的优点，在满足***输电的同时，能够有效改善目前常规直流输电受端的换相失败等问题，在柔性直流尚不具备与常规直流相当的输电容量的现状下，是一种具备较高技术经济性的优化配置方案。但是该混合直流输电技术作为一项全新的技术，目前世界范围内仍处于研究起步阶段，国内关于混合直流输电技术的研究也起步较晚，目前现有技术中并没有一种有效地混合直流输电***以实现上述优点。
发明内容
为解决上述技术问题，本发明实施例提供了一种混合直流输电***，同时具有LCC-HVDC的技术成熟、成本低的特点以及VSC-HVDC的无换相失败，控制灵活，拓展性能强的优点。
本发明实施例提供的一种混合直流输电***为双极结构，包括正极换流变压器和正极换流器、负极换流变压器和负极换流器，所述双极结构中，其中一极换流器为由MMC子模块构成的MMC换流器，另一极为由至少一个LCC构成的LCC换流器***。
本发明实施例中，所述MMC中的每个桥臂由半桥子模块和全桥子模块级联构成；或者，由半桥子模块、全桥子模和混合双子模块级联构成。
本发明实施例中，所述混合双子模块包括4个功率模块：T1、T2、T3、T4和2个电容：C1、C2；所述T1的阳极连接所述T4的阳极，所述T2的阴极连接T3的阴极，所述T1的阴极连接所述T2的阳极，所述T4的阴极通过所述电容C2连接所述T3的阳极，所述T1和T4的连接点与所述T2和T3的连接点之间连接所述电容C1，所述T1和T2的连接点为所述混合双子模块的一个端口，所述C2和T4的连接点为所述混合双子模块的另一个端口。
本发明实施例中，所述功率模块为绝缘栅双极型晶体管(IGBT， Insulated Gate Bipolar Transistor)模块，所述功率模块的阳极为IGBT模块的集电极，所述功率模块的阴极为IGBT模块的发射极。
本发明实施例中，所述混合直流输电***还包括无功补偿装置，所述无功补偿装置连接交流电网。
本发明实施例中，所述无功补偿装置为以下的一种：并联电容器、并联电抗器、静止无功补偿装置和调相机。
本发明实施例中，所述混合直流输电***还包括交流滤波装置，所述交流滤波装置连接交流电网。
本发明实施例中，所述交流滤波装置由交流滤波器组构成。
本发明实施例中，所述无功补偿装置与交流滤波器组以及MMC，为所述LCC提供无功补偿。
本发明实施例的技术方案提供了一种较为新颖的混合直流输电***，其不仅结构简单可靠，而且还能结合LCC和MMC的各自优点并克服各自缺点：利用MMC的有功无功独立调节能力来调节交流电压，从而增加LCC的最大传输有功功率能力并减小其换相失败的可能；而且MMC中可以根据实际需要接入各种类型的子模块，控制更加灵活多变，在直流侧发生故障时可以控制其直流端输出零电压，从而克服普通MMC无法有效处理直流故障的缺点。
附图说明
图1是本发明实施例的混合直流输电***的结构示意图；
图2是本发明实施例的电网换相换流器LCC的一种拓扑结构示意图；
图3是本发明实施例的模块混合型模块化多电平换流器MMC的拓扑结构图；
图4是本发明实施例的半桥子模块的拓扑结构示意图；
图5是本发明实施例的全桥子模块的拓扑结构示意图；
图6是本发明实施例的混合双子模块的拓扑结构示意图；
图7-1是本发明实施例的混合双子模块正常工作模式下的第一种工作状态示意图；
图7-2是本发明实施例的混合双子模块正常工作模式下的第二种工作状态示意图；
图7-3是本发明实施例的混合双子模块正常工作模式下的第三种工作状态示意图；
图7-4是本发明实施例的混合双子模块正常工作模式下的第四种工作状态示意图；
图8-1是本发明实施例的混合双子模块闭锁模式下的其中一种工作状态示意图；
图8-2是本发明实施例的混合双子模块闭锁模式下的另一种工作状态示意图。
具体实施方式
下面结合附图对本发明做进一步详细的说明。
实施例1
如图1所示，该混合直流输电***包括电网换相换流器(LCC)、模块化多电平换流器(MMC)、换流变压器、交流滤波器组(ACF)、交流无功补偿装置组、直流平波电抗器、直流滤波器(DCF)。LCC和MMC构成双极结构，交流电网上还连接交流滤波器组(ACF)和交流无功补偿装置。
交流无功补偿装置可以为并联电容器，也可以为并联电抗器、静止无功补偿装置和调相机中的一种，它与交流滤波器组(ACF)、MMC配合，共同为LCC提供无功补偿。
电网换相换流器的两个直流端之间连接直流滤波器(DCF)，电网换相换流器的直流负极端与模块化多电平换流器的直流正极端连接，连接点与 电网换相换流器的直流负极端之间和连接点与模块化多电平换流器的直流正极端之间均串接有开关，并且该接点通过开关(MRTB)接地。电网换相换流器的直流正极端为该混合直流输电***的直流正极端，模块化多电平换流器的直流负极端为该混合直流输电***的直流负极端，该混合直流输电***的两个直流正负极线路上均串接有直流平波电抗器。
最基本的LCC是由半控型的晶闸管为基础构成的三相桥式电路，但是LCC可以是6个桥臂构成的6脉波换流器，也可以是由两个6脉波换流器组成的12脉波换流器，还可以是由多个6脉波换流器组成的多脉波换流器，如图2所示，给出了一种LCC的实施方式。也就是说，该LCC可以是一个最基本的三相桥式电路，也可以由多个三相桥式电路组合构成。
MMC的拓扑结构如图2所示，该MMC由三相六桥臂构成，每个桥臂都由半桥拓扑子模块和全桥拓扑子模块以一比一的比例级联组成，同时每个桥臂均还串联有桥臂电抗器L。在该MMC的交流侧，与LCC相同的交流电网***依次通过换流变压器和软启电阻装置连接该MMC的三相桥式电路的交流侧。
该MMC中的半桥子模块和全桥子模块以全控型电力电子器件如IGBT为基础构成，其拓扑结构如图3、图4所示。
该混合直流输电***工作时，LCC和MMC同时接入一个交流***，形成了混合多馈入直流输电***。***运行时，可以充分利用MMC***的有功无功功率独立调节能力，有效地调节交流母线电压，增加LCC***的最大传输有功功率能力，降低LCC***的暂态过电压，同时也可以减小LCC换流器换相失败可能。当直流侧出现故障时，LCC***可以通过调整触发相角来克服直流故障，MMC***可以通过对全桥子模块的控制，实现直流侧零电压控制，从而有效的克服直流故障。
当然，本发明的混合直流输电***的拓扑结构并不局限于上述实施例， 例如通过合理的搭配交流滤波器组(ACF)，同时配合MMC控制交流***的无功功率，从而取消无功功率补偿装置。
上述实施例中，电网换相换流器的直流负极端与模块化多电平换流器的直流正极端连接并且接地，电网换相换流器的直流正极端为该混合直流输电***的直流正极端，模块化多电平换流器的直流负极端为该混合直流输电***的直流负极端；作为其他的实施例，可以交换LCC和MMC的位置，电网换相换流器的直流正极端与模块化多电平换流器的直流负极端连接并且接地，模块化多电平换流器的直流正极端为该混合直流输电***的直流正极端，电网换相换流器的直流负极端为该混合直流输电***的直流负极端。
上述实施例中，MMC包括半桥子模块和全桥子模块，两者在每个桥臂中的比例为1:1，作为其他的实施例，半桥子模块和全桥子模块的配置比例可以按照实际情况进行设置。而且还可以根据具体情况将全桥子模块更改为箝位双子模块。
实施例2
本实施例与实施例1的区别在于：本实施例中的MMC中，每个桥臂至少有一个子模块为混合双子模块。如图6所示，该混合双子模块包括4个IGBT模块：T1、T2、T3、T4和2个电容：C1、C2，T1的集电极连接T4的集电极，T2的发射极连接T3的发射极，T1的发射极连接T2的集电极，T4的发射极通过电容C2连接T3的集电极，T1和T4的连接点与T2和T3的连接点之间连接电容C1，T1和T2的连接点为该混合双子模块的一个端口，C2和T4的连接点为该混合双子模块的另一个端口。混合双子模块内所有的IGBT(T1、T2、T3、T4)均反并联了续流二极管，T1、T2、T3、T4的基极分别接受外部设备提供的控制信号。
混合双子模块有两种工作模式，正常运行模式和闭锁模式。在正常运 行模式下，T1和T2之间最多只能有一个IGBT导通，为了防止电容C1短路，T1和T2均不能同时导通；T3和T4之间最多只能有一个IGBT导通。
混合双子模块在正常工作模式下，有4种工作状态，正常运行模式下的4种运行状态如图7-1至7-4所示，(1)为当T1、T3导通时的电流流向，(2)为当T1、T4导通时的电流流向，(3)为当T2、T3导通时的电流流向，(4)为当T2、T4导通时的电流流向。如表1所示，当T1、T3导通时，端口输出电压为两个电容电压和；当T1、T4导通时，端口输出电压为零；当T2、T3导通时，端口输出电压为电容C2电压；当T2、T4导通时，端口输出电压为电容C1的反向电压，即为输出负电压。电流方向不影响端口输出电压。表1中，Usm表示子模块端口输出电压。
Figure PCTCN2016085610-appb-000001
表1
由混合双子模块的正常工作模式可知，该子模块能够输出4种电压，分别为两倍电容电压、电容电压、零电压和负向电容电压。说明该子模块能够代替2个半桥子模块输出2倍电容电压的同时，具备全桥子模块的负电压特性，能提高直流电压利用率，提升***容量。
混合双子模块在闭锁模式下，有2种工作状态，闭锁模式下的2种运行状态如图8-1和8-2所示。在闭锁状态下，所有IGBT均处于关断状态。当流过正向电流(电流方向由A至B)时，端口输出电压为两个电容电压和；当流过负向电流时，端口输出电压为电容C1的负向电压，即与电流反 向的电压。
混合双子模块内部的两个电容器，可以根据需要合理配置这两个电容电压为不同值。该方式下，能够有效扩展所属MMC的应用范围，例如合理配置子模块中的两个电容电压以实现该MMC提高调制度的同时具备STATCOM运行故障穿越能力。
还有就是，MMC中的子模块可以全部是该混合双子模块，还可以是以下情况：MMC中的桥臂由混合子模块与其他一种或者几种现有子模块(半桥子模块、全桥子模块、钳位双子模块)级联构成，那么，该MMC为混合式MMC换流器。该混合式MMC换流器按照实际情况进行拓展时具有广泛的应用，如提高调制度的同时具备STATCOM运行故障穿越能力、节约***硬件成本等。
上述实施例中，功率模块为IGBT模块，作为其他的实施例，功率模块该可以是其他的全控型器件。
以上给出了具体的实施方式，但本发明不局限于所描述的实施方式。本发明的基本思路在于上述基本方案，对本领域普通技术人员而言，根据本发明的教导，设计出各种变形的模型、公式、参数并不需要花费创造性劳动。在不脱离本发明的原理和精神的情况下对实施方式进行的变化、修改、替换和变型仍落入本发明的保护范围内。
工业实用性
本发明实施例的技术方案，其不仅结构简单可靠，而且还能结合LCC和MMC的各自优点并克服各自缺点：利用MMC的有功无功独立调节能力来调节交流电压，从而增加LCC的最大传输有功功率能力并减小其换相失败的可能；而且MMC中可以根据实际需要接入各种类型的子模块，控制更加灵活多变，在直流侧发生故障时可以控制其直流端输出零电压，从而克服普通MMC无法有效处理直流故障的缺点。

Claims (9)

  1. 一种混合直流输电***，该混合直流输电***为双极结构，包括正极换流变压器和正极换流器、负极换流变压器和负极换流器，所述双极结构中，其中一极换流器为由模块化多电平换流器MMC子模块构成的MMC换流器，另一极为由至少一个电网换相换流器LCC构成的LCC换流器***。
  2. 根据权利要求1所述的混合直流输电***，其中，所述MMC中的每个桥臂由半桥子模块和全桥子模块级联构成；或者，由半桥子模块、全桥子模和混合双子模块级联构成。
  3. 根据权利要求2所述的混合直流输电***，其中，所述混合双子模块包括4个功率模块：T1、T2、T3、T4和2个电容：C1、C2；所述T1的阳极连接所述T4的阳极，所述T2的阴极连接T3的阴极，所述T1的阴极连接所述T2的阳极，所述T4的阴极通过所述电容C2连接所述T3的阳极，所述T1和T4的连接点与所述T2和T3的连接点之间连接所述电容C1，所述T1和T2的连接点为所述混合双子模块的一个端口，所述C2和T4的连接点为所述混合双子模块的另一个端口。
  4. 根据权利要求3所述的混合直流输电***，其中，所述功率模块为绝缘栅双极型晶体管IGBT模块，所述功率模块的阳极为IGBT模块的集电极，所述功率模块的阴极为IGBT模块的发射极。
  5. 根据权利要求1所述的混合直流输电***，其中，所述混合直流输电***还包括无功补偿装置，所述无功补偿装置连接交流电网。
  6. 根据权利要求5所述的混合直流输电***，其中，所述无功补偿装置为以下的一种：并联电容器、并联电抗器、静止无功补偿装置和调相机。
  7. 根据权利要求5所述的混合直流输电***，其中，所述混合直流输电***还包括交流滤波装置，所述交流滤波装置连接交流电网。
  8. 根据权利要求7所述的混合直流输电***，其中，所述交流滤波装 置由交流滤波器组构成。
  9. 根据权利要求8所述的混合直流输电***，其中，所述无功补偿装置与交流滤波器组以及MMC，为所述LCC提供无功补偿。
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