阅读说明：本技术 一种柔性直流背靠背系统及其运行方法 (Flexible direct current back-to-back system and operation method thereof ) 是由 赵彪 宋强 余占清 屈鲁 曾嵘 李海波 高浪 于 2020-04-24 设计创作，主要内容包括：本发明公开了一种柔性直流背靠背系统,包括第一MMC拓扑、DC-DC变换器模块及第二MMC拓扑,所述DC-DC变换器模块的两端分别与第一MMC拓扑、第二MMC拓扑连接,所述第一MMC拓扑和第二MMC拓扑均为具有故障自清除能力的高纹波MMC拓扑。本发明所述的柔性直流背靠背系统取消了联结变压器,采用直流变压器实现--电气隔离,可以大幅减小系统重量和体积。同时,本发明可以实现交直流故障在us级切断,和传统的柔性直流背靠背系统在ms级切断故障点相比,反应速度更快,系统可靠性更高。(The invention discloses a flexible direct current back-to-back system which comprises a first MMC topology, a DC-DC converter module and a second MMC topology, wherein two ends of the DC-DC converter module are respectively connected with the first MMC topology and the second MMC topology, and the first MMC topology and the second MMC topology are high-ripple MMC topologies with fault self-clearing capability. The flexible direct current back-to-back system provided by the invention cancels a coupling transformer, adopts a direct current transformer to realize-electrical isolation, and can greatly reduce the weight and the volume of the system. Meanwhile, the invention can realize the switching-off of the AC/DC fault at the us level, and compared with the switching-off fault point of the traditional flexible DC back-to-back system at the ms level, the invention has the advantages of higher response speed and higher system reliability.)

1. A flexible DC back-to-back system is characterized in that it comprises a first MMC topology, a DC-DC converter module and a second MMC topology,

two ends of the DC-DC converter module are respectively connected with the first MMC topology and the second MMC topology,

the first MMC topology and the second MMC topology are both high-ripple MMC topologies with fault self-clearing capability.

2. The flexible direct current back-to-back system of claim 1, wherein the DC-DC converter module comprises at least one dual active full bridge DC-DC converter.

3. The flexible direct current back-to-back system of claim 2, wherein the DC-DC converter module comprises a plurality of the dual active full-bridge DC-DC converters,

the input ends of the double-active full-bridge DC-DC converters are connected in series, and the output ends of the double-active full-bridge DC-DC converters are connected in series.

4. The flexible direct current back-to-back system of claim 1, wherein the DC-DC converter module is isolated at a medium frequency and has a switching frequency within 1 kHz.

5. The flexible direct current back-to-back system of claim 1, wherein the high ripple MMC topology comprises a plurality of MMC bridge arms,

each of the MMC bridge arms includes an upper bridge arm and a lower bridge arm,

the upper bridge arm and the lower bridge arm both comprise a plurality of MMC sub-modules which are sequentially connected in series.

6. The flexible direct current back-to-back system of claim 5, wherein the MMC sub-module employs a hybrid full-bridge and half-bridge topology, a clamped bi-sub-module topology, or a cross-clamped sub-module topology.

7. The flexible direct current back-to-back system of claim 1, wherein a capacitance ripple of the high ripple MMC topology is greater than 10%.

8. An operation method of the flexible direct current back-to-back system according to any one of claims 1 to 7, wherein the operation method comprises:

the DC-DC converter module controls power exchange between a transformer substation connected with the first MMC topology and a transformer substation connected with the second MMC topology;

the DC-DC converter module works in a direct-current voltage mode and controls direct-current voltage of a first MMC topology side, and the first MMC topology works in an alternating-current voltage control mode and supplies power to a load on an alternating-current bus feeder line connected with the first MMC topology;

and the first MMC topology and the second MMC topology independently work in a STATCOM operation mode.

9. The method of operating a flexible dc back-to-back system of claim 8, wherein the STATCOM mode further comprises:

the first MMC topology is locked, and the second MMC topology works in a STATCOM operation mode.

Technical Field

The invention belongs to the technical field of electric power, and particularly relates to a flexible direct current back-to-back system and an operation method thereof.

Background

A back-to-back dc transmission system is a dc transmission system with zero transmission line length. This type of dc transmission is mainly used for networking or power transmission between two ac power systems operating asynchronously (at different frequencies or at the same frequency but not synchronized), also called asynchronous tie stations. The rectifier station arrangement and the inverter station arrangement for back-to-back dc transmission are usually installed in one converter station, also called back-to-back converter station. In the back-to-back converter station, the direct current sides of the rectifier and the inverter are connected through a smoothing reactor to form a closed loop at the direct current side; the alternating current side of the power grid is connected with the connection points of the connected power grids respectively, so that asynchronous networking of two power systems is formed, and the size and the direction of the exchange power between the connected power grids are controlled by the control system quickly and conveniently.

Referring to fig. 1, at present, ac sides at two ends of a conventional flexible dc back-to-back system are connected to an ac/dc system through a power frequency isolated coupling transformer, and the flexible dc back-to-back system has the disadvantage of high cost due to excessive weight and volume of the connection transformer. Meanwhile, the traditional flexible back-to-back system also has the defects of low speed of cutting off fault points and poor reliability when faults occur.

Therefore, how to solve the problems of poor reliability, excessive weight and volume and high cost of the traditional flexible direct current back-to-back system is an urgent need in the field.

Disclosure of Invention

In view of the above problems, the present invention provides a flexible dc back-to-back system and a method for operating the same.

A flexible DC back-to-back system includes a first MMC topology, a DC-DC converter module and a second MMC topology,

two ends of the DC-DC converter module are respectively connected with the first MMC topology and the second MMC topology,

the first MMC topology and the second MMC topology are both high-ripple MMC topologies with fault self-clearing capability.

Preferably, the DC-DC converter module comprises at least one dual active full bridge DC-DC converter.

Preferably, the DC-DC converter module comprises a plurality of the dual-active full-bridge DC-DC converters,

and the input ends of the double-active full-bridge DC-DC converters are connected in series and the output ends of the double-active full-bridge DC-DC converters are connected in series.

Preferably, the DC-DC converter module adopts a medium frequency isolation mode, and the switching frequency is within 1 kHz.

Preferably, the high-ripple MMC topology with fault self-clearing capability comprises a plurality of MMC bridge arms,

each of the MMC bridge arms includes an upper bridge arm and a lower bridge arm,

the upper bridge arm and the lower bridge arm both comprise a plurality of MMC sub-modules which are sequentially connected in series.

Preferably, the MMC sub-module adopts a mixed topology of a full bridge and a half bridge, a clamped dual sub-module topology, or a cross-clamped sub-module topology.

Preferably, the high ripple MMC topology with fault self-clearing capability has a capacitance ripple greater than 10%.

An operation method based on the flexible direct current back-to-back system comprises the following steps:

the DC-DC converter module controls power exchange between a transformer substation connected with the first MMC topology and a transformer substation connected with the second MMC topology;

the DC-DC converter module works in a direct-current voltage mode to control direct-current voltage of the first MMC topology side, and the first MMC topology works in an alternating-current voltage control mode to supply power to a load on an alternating-current bus feeder line connected with the first MMC topology;

and the first MMC topology and the second MMC topology independently work in a STATCOM operation mode.

Preferably, the STATCOM mode further comprises:

the first MMC topology is locked, and the second MMC topology works in a STATCOM operation mode.

The flexible direct current back-to-back system provided by the invention cancels a coupling transformer, adopts a direct current transformer to realize-electrical isolation, and can greatly reduce the weight and the volume of the system. Meanwhile, the invention can realize the switching-off of the AC/DC fault at the us level, and compared with the switching-off fault point of the traditional flexible DC back-to-back system at the ms level, the invention has the advantages of higher response speed and higher system reliability.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and those skilled in the art can also obtain other drawings according to the drawings without creative efforts.

FIG. 1 shows the structure of a flexible DC back-to-back system according to the prior art;

FIG. 2 illustrates the structure of a flexible DC back-to-back system according to an embodiment of the present invention;

FIG. 3 illustrates a topology of a flexible DC back-to-back system according to an embodiment of the present invention;

FIG. 4 shows a first topology of the MMC sub-module;

FIG. 5 shows a second topology of the MMC sub-module;

FIG. 6 shows a third topology of the MMC sub-module;

fig. 7 shows the topology of the dual active full bridge DC-DC converter.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Referring to fig. 2, the invention discloses a flexible direct current back-to-back system, which includes a first MMC topology, a DC-DC converter module and a second MMC topology, wherein two ends of the DC-DC converter module are respectively connected with the first MMC topology and the second MMC topology, and the first MMC topology and the second MMC topology are both high ripple MMC topologies with fault self-clearing capability. The MMC refers to a modular multilevel converter. The flexible direct current back-to-back system has a symmetrical circuit structure, can realize bidirectional power transmission, can lead out a direct current port, and is suitable for occasions with high voltage and large capacity.

The first MMC topology and the second MMC topology described in this embodiment are also connected to the bus bar through a line 1 and a line 2, respectively. Wherein the bus bar connected to the first MMC topology is named first bus bar 1 and the bus bar connected to said second MMC topology is named second bus bar 2. The first MMC topology is used for realizing mutual conversion of alternating current of a first bus and direct current of the DC-DC converter module, and the second MMC topology is used for realizing mutual conversion of alternating current of a second bus and direct current of the DC-DC converter module. The DC-DC converter module is used for realizing voltage conversion between the first MMC topology and the second MMC topology, controlling power exchange of alternating-current transformer substations at two ends, ensuring load sharing and improving power supply efficiency and equipment utilization rate.

Compared with the traditional flexible direct current back-to-back system, the flexible direct current back-to-back system provided by the embodiment cancels the coupling transformer, adopts the direct current transformer to realize electrical isolation, and can greatly reduce the weight of the system and reduce the cost. Simultaneously, this embodiment first MMC topology and second MMC topology in the flexible direct current back-to-back system are the high ripple MMC topology that has the trouble from clearing away the ability, and after direct current side or interchange side broke down, the switch tube can block rapidly to the quick fault point of amputating. The flexible direct current back-to-back system described in this embodiment can realize that the alternating current-direct current trouble cuts off at us level, compares with traditional flexible direct current back-to-back system at ms level cut off fault point, and the reaction rate is faster, and the system reliability is higher.

Specifically, the DC-DC converter module adopts a medium-frequency isolation mode, and the switching frequency is within 1 kHz. The DC-DC converter module based on intermediate frequency isolation is adopted, the module number of the direct current transformer is greatly reduced, the weight, the volume and the cost of a system are reduced while high-voltage and high-capacity application is ensured, and the volume and the weight of the power electronic transformer are reduced.

Referring to fig. 3, the DC-DC converter module of the present embodiment includes at least one dual-active full-bridge DC-DC converter. The DC-DC converter module may further include a plurality of the dual-active full-bridge DC-DC converters, and the dual-active full-bridge DC-DC converters are connected in series with each other in input and output.

The high-ripple MMC topology with fault self-clearing capability described in this embodiment includes a plurality of MMC bridge arms. The embodiment exemplarily shows that the first MMC topology and the second MMC topology are both composed of 3 MMC bridge arms. Each MMC bridge arm comprises an upper bridge arm and a lower bridge arm, wherein the upper bridge arm and the lower bridge arm respectively comprise a plurality of MMC sub-modules SM which are sequentially connected in series. The MMC topology described in this embodiment can operate in a dc voltage control mode, an ac voltage control mode, and a power control mode. Wherein, the electric capacity ripple of the high ripple MMC topology that has trouble self-cleaning ability is greater than 10%, can reduce MMC submodule piece SM's electric capacity quantity.

Referring to fig. 4, fig. 4 shows an exemplary first topology of the MMC sub-module SM, which adopts a mixed topology of a full bridge and a half bridge.

The half-bridge submodule is composed of switching tubes S9 and S10, diodes D9 and D10 and a capacitor Cd. The switching tube S9 is connected in parallel with the diode D9 in an opposite direction, the switching tube S10 is connected in parallel with the diode D10 in an opposite direction, and the switching tube S9 is connected in series with the switching tube S10 and then connected in parallel with the capacitor Cd to form the half-bridge submodule. The connecting point of the series connection of the switching tubes S9 and S10 and one end point of the capacitor Cd serve as the input and output ends of the half-bridge submodule.

The full-bridge submodule is composed of switching tubes S11, S12, S13 and S14, diodes D11, D12, D13 and D14 and a capacitor Cd. The switch tube S11 is connected in inverse parallel with the diode D11, the switch tube S12 is connected in inverse parallel with the diode D12, the switch tube S13 is connected in inverse parallel with the diode D13, and the switch tube S14 is connected in inverse parallel with the diode D14. The switching tubes S11 and S12 are connected in series, the switching tubes S13 and S14 are connected in series to form two half bridges, and the two half bridges are connected with the capacitor Cd in parallel to form the full bridge submodule. The connection point of the series connection of the switching tubes S11 and S12 and the connection point of the series connection of the switching tubes S13 and S14 serve as the input and output terminals of the full-bridge submodule.

Referring to fig. 5, fig. 5 shows an exemplary second topology of the MMC sub-module SM, where the MMC sub-module SM employs a clamped dual sub-module topology.

The clamping dual-sub-module topology comprises switching tubes S15, S16, S17, S18 and S19, capacitors Cd1 and Cd2 and additional diodes D1 and D2, wherein the switching tubes S15, S16, S17, S18 and S19 are respectively connected with a diode in series in an inverted mode.

The switching tube S15 is connected in series with S16 and then in parallel with the capacitor Cd 1. The switching tubes S15, S16 and the capacitor Cd1 are connected in series with the diode D1 as a whole to form a first branch. Specifically, the second terminal of the capacitor Cd1 is connected to the anode of the diode D1.

The switching tube S17 is connected in series with S18 and then in parallel with the capacitor Cd 2. The switching tubes S17 and S18 and the capacitor Cd2 are connected in series with the diode D2 as a whole to form a second branch. Specifically, a first terminal of the capacitor Cd2 is connected to a cathode of the diode D2.

The first branch, the second branch and the switch tube S19 are connected in parallel. Specifically, a first end of the capacitor Cd1 is connected to a first pole of the switch transistor S19 and an anode of the diode D2, and a cathode of the diode D1 is connected to a second pole of the switch transistor S19 and a second end of the capacitor Cd 2.

The serial connection point of the switch tubes S15 and S16 and the serial connection point of the switch tubes S17 and S18 are used as the input and output ends of the clamping double-submodule topology.

Referring to fig. 6, fig. 6 shows an exemplary third topology of the MMC sub-module SM, where the MMC sub-module SM employs a cross-clamping sub-module topology.

The cross-clamping submodule topology comprises switching tubes S20, S21, S22 and S23, diodes D15 and D16, an Integrated Gate Commutated Thyristor (IGCT) S24 and capacitors Cd3 and Cd4, wherein the switching tubes S20, S21, S22 and S23 are respectively connected with a diode in series in a reverse direction.

Wherein, the switch tubes S20 and S21 are connected in series and then connected in parallel with the capacitor Cd3, and the switch tube S22 is connected in series with the switch tube S23 and then connected in parallel with the capacitor Cd 4.

A first pole of the diode D15 is connected to the first terminal of the capacitor Cd3 and the first pole of the diode D16, and a second pole of the diode D15 is connected to the second terminal of the capacitor Cd 3.

The first pole of the diode D16 is further connected to the second pole of the IGCT S24 and the first end of the capacitor Cd4, and the second pole of the diode D16 is connected to the first pole of the IGCT S24 and the second end of the capacitor Cd 4.

The serial connection point of the switch tubes S20 and S21 and the serial connection point of the switch tubes S22 and S23 are used as the input and output ends of the cross clamping submodule topology.

Referring to fig. 7, fig. 7 exemplarily shows a topology of the dual-active full-bridge DC-DC converter, where the dual-active full-bridge DC-DC converter includes an input-side full-bridge circuit, a transformer, and an output-side full-bridge circuit, the transformer is connected to the input-side full-bridge circuit and the output-side full-bridge circuit, and an inductor L is connected between the input-side full-bridge circuit and the transformer in series. The dual-active full-bridge DC-DC converter described in this embodiment can operate in a DC voltage control mode and a power control mode.

The input side full-bridge circuit comprises four switching tubes S1, S2, S3 and S4, wherein the switching tubes S1, S2, S3 and S4 are respectively connected with diodes in an inverse parallel mode; the first switch tube S1 is connected in series with the second switch tube S2 to form a first half-bridge circuit; the fourth switching tube S4 and the third switching tube S3 are connected in series to form a second half-bridge circuit; the first half-bridge circuit and the second half-bridge circuit are connected in parallel to form an input side full-bridge circuit. Specifically, the input side full bridge circuit further comprises a first capacitor C1, and the first capacitor C1 is connected in parallel with the first half bridge circuit and the second half bridge circuit.

The input end of the input side full-bridge circuit is led out from two connection points of the first half-bridge circuit and the second half-bridge circuit, and the input end of the input side full-bridge circuit is connected with the first MMC topology. The output end of the input side full bridge circuit is led out from the connection point of the first switch tube S1 and the second switch tube S2 and the connection point of the fourth switch tube S4 and the third switch tube S3. And the output end of the input side full bridge circuit is connected with the primary side of the transformer.

The output side full bridge circuit comprises four switching tubes S5, S6, S7 and S8, wherein the four switching tubes S5, S6, S7 and S8 are respectively connected with diodes in an anti-parallel mode. The fifth switching tube S5 is connected in series with the sixth switching tube S6 to form a third half-bridge circuit; the eighth switching tube S8 is connected in series with the seventh switching tube S7 to form a fourth half-bridge circuit; and the third half-bridge circuit and the fourth half-bridge circuit are connected in parallel to form an output side full-bridge circuit. Specifically, the output-side full-bridge circuit further includes a second capacitor C2, and the second capacitor C2 is connected in parallel with the third half-bridge circuit and the fourth half-bridge circuit.

The output end of the output side full-bridge circuit is led out from two connection points of the third half-bridge circuit and the fourth half-bridge circuit, and the output end of the output side full-bridge circuit is connected with the second MMC topology.

The input end of the output side full-bridge circuit is led out from the connection point of the fifth switch tube S5 and the sixth switch tube S6 and the connection point of the eighth switch tube S8 and the seventh switch tube S7. And the input end of the output side full bridge circuit is connected with the secondary side of the transformer.

The present embodiment further provides an operation method of the flexible dc back-to-back system, where the MMC topology described in the present embodiment may operate in a dc voltage control mode, an ac voltage control mode, and a power control mode; the DC-DC converter module can work in a direct current voltage control mode and a power control mode, and the operation method comprises the following steps:

the DC-DC converter module controls power exchange between a transformer substation connected with the first MMC topology and a transformer substation connected with the second MMC topology;

the DC-DC converter module works in a direct-current voltage mode to control direct-current voltage of the first MMC topology side, and the first MMC topology works in an alternating-current voltage control mode to supply power to a load on an alternating-current bus feeder line connected with the first MMC topology;

and the first MMC topology and the second MMC topology independently work in a STATCOM operation mode.

It should be noted that, the first MMC topology and the second MMC topology may be replaced with each other, and therefore, the uninterruptible power supply mode may also be such that the first MMC topology operates in a direct-current voltage control mode, the DC-DC converter module operates in a direct-current voltage control mode to control direct-current voltage at the second MMC topology side, and the second MMC topology operates in an alternating-current voltage control mode to supply power to a load on an alternating-current bus feeder line connected to the second MMC topology.

Wherein the STATCOM mode further comprises:

the first MMC topology is locked, and the second MMC topology works in a STATCOM operation mode.

The operation method of the flexible direct current back-to-back system has multiple operation modes including a back-to-back mode, an uninterruptible power supply mode, namely a UPS power supply mode and a STATCOM mode, has good redundancy performance for an alternating current system, and can effectively improve the reliability of the alternating current system.

Although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.