A flexible DC back-to-back system and operation method thereof

Abstract

The present invention discloses a flexible DC back-to-back system, including a first MMC topology, a DC-DC converter module and a second MMC topology, wherein the two ends of the DC-DC converter module are respectively connected to the first MMC topology and the second MMC topology, and both the first MMC topology and the second MMC topology are high-ripple MMC topologies with fault self-clearing capabilities. The flexible DC back-to-back system of the present invention eliminates the coupling transformer and adopts a DC transformer to achieve electrical isolation, which can greatly reduce the weight and volume of the system. At the same time, the present invention can realize the cutting off of AC and DC faults at the us level, which has a faster response speed and higher system reliability than the traditional flexible DC back-to-back system that cuts off the fault point at the ms level.

Description

Technical Field

[0001] The invention belongs to the field of electric power technology, and in particular relates to a flexible direct current back-to-back system and an operation method thereof. Background Art

[0002] 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 that operate asynchronously (different frequencies or the same frequency but asynchronous), also known as an asynchronous interconnection station. The rectifier station equipment and inverter station equipment of back-to-back DC transmission are usually installed in a converter station, also known as a back-to-back converter station. In the back-to-back converter station, the DC sides of the rectifier and inverter are connected through smoothing reactors to form a closed loop on the DC side; while their AC sides are respectively connected to the connection points of the connected power grid, thereby forming an asynchronous networking of the two power systems. The size and direction of the power exchange between the connected power grids are quickly and conveniently controlled by the control system.

[0003] Please refer to Figure 1 At present, the AC sides of the traditional flexible DC back-to-back system are connected to the AC / DC system through the power frequency isolation connection transformer. Due to the connection of the connection transformer, the flexible DC back-to-back system has the disadvantages of being too heavy and large, resulting in high costs. At the same time, the traditional flexible back-to-back system also has the disadvantages of slow disconnection of the fault point and poor reliability when a fault occurs.

[0004] Therefore, how to solve the problems of poor reliability, excessive weight and volume, and high cost of traditional flexible DC back-to-back systems is an urgent problem to be solved in this field. Summary of the invention

[0005] In view of the above problems, the present invention provides a flexible DC back-to-back system and an operation method thereof.

[0006] A flexible direct current back-to-back system comprises a first MMC topology, a DC-DC converter module and a second MMC topology.

[0007] The two ends of the DC-DC converter module are respectively connected to the first MMC topology and the second MMC topology.

[0008] The first MMC topology and the second MMC topology are both high-ripple MMC topologies with fault self-clearing capabilities.

[0009] Preferably, the DC-DC converter module includes at least one dual active full-bridge DC-DC converter.

[0010] Preferably, the DC-DC converter module comprises a plurality of the dual active full-bridge DC-DC converters.

[0011] The multiple dual active full-bridge DC-DC converters are connected in series in input and in series in output.

[0012] Preferably, the DC-DC converter module adopts medium frequency isolation, and the switching frequency is within 1 kHz.

[0013] Preferably, the high ripple MMC topology with fault self-clearing capability includes a plurality of MMC bridge arms,

[0014] Each of the MMC bridge arms comprises an upper bridge arm and a lower bridge arm,

[0015] The upper bridge arm and the lower bridge arm each include a plurality of MMC submodules connected in series.

[0016] Preferably, the MMC submodule adopts a hybrid topology of a full-bridge and a half-bridge, a clamped dual submodule topology or a cross-clamped submodule topology.

[0017] Preferably, the capacitor ripple of the high ripple MMC topology with fault self-clearing capability is greater than 10%.

[0018] An operation method based on the flexible DC back-to-back system comprises:

[0019] Back-to-back mode, the first MMC topology and the second MMC topology both operate in a DC voltage control mode, and the DC-DC converter module controls power exchange between the substation connected to the first MMC topology and the substation connected to the second MMC topology;

[0020] In the uninterruptible power supply mode, the second MMC topology operates in the DC voltage control mode, the DC-DC converter module operates in the DC voltage mode to control the DC voltage on the first MMC topology side, and the first MMC topology operates in the AC voltage control mode to supply power to the load on the AC bus feeder connected to the first MMC topology;

[0021] STATCOM mode, the first MMC topology and the second MMC topology both independently operate in the STATCOM operation mode.

[0022] Preferably, the STATCOM mode further includes:

[0023] The first MMC topology is locked, and the second MMC topology operates in a STATCOM operation mode.

[0024] The flexible DC back-to-back system of the present invention eliminates the connection transformer and uses a DC transformer to achieve electrical isolation, which can greatly reduce the weight and volume of the system. At the same time, the present invention can achieve AC and DC fault cut-off at the us level, which is faster than the traditional flexible DC back-to-back system that cuts off the fault point at the ms level, and has higher system reliability.

[0025] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0027] Figure 1 The structure of a flexible DC back-to-back system according to the prior art is shown;

[0028] Figure 2 The structure of a flexible DC back-to-back system according to an embodiment of the present invention is shown;

[0029] Figure 3 The topological structure of the flexible DC back-to-back system according to an embodiment of the present invention is shown;

[0030] Figure 4 A first topological structure of the MMC submodule is shown;

[0031] Figure 5 A second topology of the MMC submodule is shown;

[0032] Figure 6 A third topological structure of the MMC submodule is shown;

[0033] Figure 7 The topology of the dual active full-bridge DC-DC converter is shown. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] Please refer to Figure 2The present invention discloses a flexible DC back-to-back system, which includes a first MMC topology, a DC-DC converter module and a second MMC topology. The two ends of the DC-DC converter module are respectively connected to 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 capabilities. The MMC refers to a modular multilevel converter. The flexible DC back-to-back system described in this embodiment has a symmetrical circuit structure, can realize bidirectional power transmission, and can lead out a DC port, which is suitable for high-voltage and large-capacity occasions.

[0036] The first MMC topology and the second MMC topology described in this embodiment are also connected to the busbar through line 1 and line 2 respectively. Among them, the busbar connected to the first MMC topology is named as the first busbar, i.e., busbar 1, and the busbar connected to the second MMC topology is named as the second busbar, i.e., busbar 2. Among them, the first MMC topology is used to realize the mutual conversion between the alternating current of the first busbar and the direct current of the DC-DC converter module, and the second MMC topology is used to realize the mutual conversion between the alternating current of the second busbar and the direct current of the DC-DC converter module. The DC-DC converter module is used to realize the voltage conversion between the first MMC topology and the second MMC topology, control the power exchange of the AC substations at both ends, ensure load balancing, and improve power supply efficiency and equipment utilization.

[0037] Compared with the traditional flexible DC back-to-back system, the flexible DC back-to-back system described in this embodiment eliminates the connection transformer and uses a DC transformer to achieve electrical isolation, which can greatly reduce the weight of the system and reduce costs. At the same time, the first MMC topology and the second MMC topology in the flexible DC back-to-back system described in this embodiment are both high-ripple MMC topologies with fault self-clearing capabilities. After a fault occurs on the DC side or the AC side, the switch tube can be quickly locked to quickly cut off the fault point. The flexible DC back-to-back system described in this embodiment can achieve AC and DC fault cutting at the us level. Compared with the traditional flexible DC back-to-back system that cuts off the fault point at the ms level, it has a faster response speed and higher system reliability.

[0038] Specifically, the DC-DC converter module adopts medium frequency isolation, and the switching frequency is within 1kHz. The DC-DC converter module based on medium frequency isolation is adopted to greatly reduce the number of DC transformer modules, and reduce the system weight, volume and cost while ensuring high-voltage and large-capacity applications, and reduce the volume and weight of power electronic transformers.

[0039] Please refer to Figure 3The DC-DC converter module described in this embodiment includes at least one dual-active full-bridge DC-DC converter. The DC-DC converter module may also include a plurality of the dual-active full-bridge DC-DC converters, wherein the input and output of the plurality of dual-active full-bridge DC-DC converters are connected in series.

[0040] The high-ripple MMC topology with fault self-clearing capability described in this embodiment includes multiple MMC bridge arms. This embodiment exemplarily shows that the first MMC topology and the second MMC topology are both composed of three MMC bridge arms. Each of the MMC bridge arms includes an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm each include multiple MMC sub-modules SM connected in series in sequence. 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. Among them, the capacitor ripple of the high-ripple MMC topology with fault self-clearing capability is greater than 10%, which can reduce the capacitor usage of the MMC sub-module SM.

[0041] Please refer to Figure 4 , Figure 4 A first topology structure of the MMC submodule SM is given as an example. The MMC submodule SM adopts a mixed topology of a full-bridge and a half-bridge.

[0042] The half-bridge submodule is composed of switch tubes S9 and S10, diodes D9 and D10, and capacitor Cd. The switch tube S9 is connected in reverse parallel with the diode D9, the switch tube S10 is connected in reverse parallel with the diode D10, the switch tubes S9 and S10 are connected in series, and then connected in parallel with the capacitor Cd to form the half-bridge submodule. The connection point of the switch tubes S9 and S10 in series and one end point of the capacitor Cd serve as the input and output ends of the half-bridge submodule.

[0043] The full-bridge submodule is composed of switch tubes S11, S12, S13, S14, diodes D11, D12, D13, D14 and capacitor Cd. The switch tube S11 is connected in reverse parallel with the diode D11, the switch tube S12 is connected in reverse parallel with the diode D12, the switch tube S13 is connected in reverse parallel with the diode D13, and the switch tube S14 is connected in reverse parallel with the diode D14. The switch tubes S11 and S12 are connected in series, and the switch tubes S13 and S14 are connected in series to form two half bridges, and the two half bridges are connected in parallel with the capacitor Cd to form the full-bridge submodule. The connection point of the switch tubes S11 and S12 in series and the connection point of the switch tubes S13 and S14 in series serve as the input and output ends of the full-bridge submodule.

[0044] Please refer to Figure 5 , Figure 5The second topology structure of the MMC submodule SM is given as an example. The MMC submodule SM adopts a clamped dual submodule topology.

[0045] The clamping dual module topology includes switch tubes S15, S16, S17, S18, S19, capacitors Cd1, Cd2 and additional diodes D1 and D2, and the switch tubes S15, S16, S17, S18, S19 are respectively connected in reverse series with a diode.

[0046] The switch tubes S15 and S16 are connected in series and then connected in parallel with the capacitor Cd1. The switch 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 end of the capacitor Cd1 is connected to the anode of the diode D1.

[0047] The switch tubes S17 and S18 are connected in series and then connected in parallel with the capacitor Cd2. The switch tubes S17, S18 and the capacitor Cd2 are connected in series with the diode D2 as a whole to form a second branch. Specifically, the first end of the capacitor Cd2 is connected to the cathode of the diode D2.

[0048] The first branch, the second branch and the switch tube S19 are connected in parallel. Specifically, the first end of the capacitor Cd1 is connected to the first electrode of the switch tube S19 and the anode of the diode D2, and the cathode of the diode D1 is connected to the second electrode of the switch tube S19 and the second end of the capacitor Cd2.

[0049] The connection point of the switch tubes S15 and S16 connected in series and the connection point of the switch tubes S17 and S18 connected in series serve as the input and output ends of the clamping twin module topology.

[0050] Please refer to Figure 6 , Figure 6 A third topology structure of the MMC submodule SM is given as an example. The MMC submodule SM adopts a cross-clamping submodule topology.

[0051] The cross-clamping submodule topology includes switch tubes S20, S21, S22 and S23, diodes D15, D16, integrated gate-commutated thyristor (IGCT) S24 and capacitors Cd3, Cd4, and the switch tubes S20, S21, S22 and S23 are respectively connected in reverse series with a diode.

[0052] The switch tubes S20 and S21 are connected in series and then connected in parallel with the capacitor Cd3 , and the switch tubes S22 and S23 are connected in series and then connected in parallel with the capacitor Cd4 .

[0053] A first electrode of the diode D15 is connected to a first end of the capacitor Cd3 and a first electrode of the diode D16 , and a second electrode of the diode D15 is connected to a second end of the capacitor Cd3 .

[0054] The first electrode of the diode D16 is also connected to the second electrode of the IGCT S24 and the first end of the capacitor Cd4 , and the second electrode of the diode D16 is connected to the first electrode of the IGCT S24 and the second end of the capacitor Cd4 .

[0055] The connection point of the switch tubes S20 and S21 connected in series and the connection point of the switch tubes S22 and S23 connected in series serve as input and output ends of the cross-clamping submodule topology.

[0056] Please refer to Figure 7 , Figure 7 An exemplary topology of the dual-active full-bridge DC-DC converter is given, wherein the dual-active full-bridge DC-DC converter includes an input-side full-bridge circuit, a transformer, and an output-side full-bridge circuit, wherein the transformer connects the input-side full-bridge circuit and the output-side full-bridge circuit, and an inductor L is connected in series between the input-side full-bridge circuit and the transformer. 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.

[0057] The input side full-bridge circuit includes four switch tubes S1, S2, S3, and S4, and the switch tubes S1, S2, S3, and S4 are respectively connected in reverse parallel with diodes; the first switch tube S1 and the second switch tube S2 are connected in series to form a first half-bridge circuit; the fourth switch tube S4 and the third switch 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 also includes 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.

[0058] The input end of the input side full bridge circuit is led out from the 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 to the first MMC topology. The output end of the input side full bridge circuit is led out from the connection point between the first switch tube S1 and the second switch tube S2 and the connection point between the fourth switch tube S4 and the third switch tube S3. The output end of the input side full bridge circuit is connected to the primary side of the transformer.

[0059] The output side full bridge circuit includes four switch tubes S5, S6, S7 and S8, and the four switch tubes S5, S6, S7 and S8 are respectively connected in reverse parallel with diodes. The fifth switch tube S5 is connected in series with the sixth switch tube S6 to form a third half-bridge circuit; the eighth switch tube S8 is connected in series with the seventh switch tube S7 to form a fourth half-bridge circuit; the third half-bridge circuit is connected in parallel with the fourth half-bridge circuit to form an output side full bridge circuit. Specifically, the output side full bridge circuit also 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.

[0060] 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 to the second MMC topology.

[0061] The input end of the output side full bridge circuit is led out from the connection point between the fifth switch tube S5 and the sixth switch tube S6 and the connection point between the eighth switch tube S8 and the seventh switch tube S7. The input end of the output side full bridge circuit is connected to the secondary side of the transformer.

[0062] This embodiment also provides an operation method of the flexible DC back-to-back system. 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; the DC-DC converter module can operate in a DC voltage control mode and a power control mode. The operation method includes:

[0063] Back-to-back mode, the first MMC topology and the second MMC topology both operate in a DC voltage control mode, and the DC-DC converter module controls power exchange between the substation connected to the first MMC topology and the substation connected to the second MMC topology;

[0064] In the uninterruptible power supply mode, the second MMC topology operates in the DC voltage control mode, the DC-DC converter module operates in the DC voltage mode to control the DC voltage on the first MMC topology side, and the first MMC topology operates in the AC voltage control mode to supply power to the load on the AC bus feeder connected to the first MMC topology;

[0065] STATCOM mode, the first MMC topology and the second MMC topology both independently operate in the STATCOM operation mode.

[0066] It should be noted that the first MMC topology and the second MMC topology can be replaced with each other. Therefore, the uninterruptible power supply mode can also be that the first MMC topology works in the DC voltage control mode, the DC-DC converter module works in the DC voltage mode to control the DC voltage on the second MMC topology side, and the second MMC topology works in the AC voltage control mode to supply power to the load on the AC bus feeder connected to the second MMC topology.

[0067] Wherein, the STATCOM mode also includes:

[0068] The first MMC topology is locked, and the second MMC topology operates in a STATCOM operation mode.

[0069] The operation method of the flexible DC back-to-back system described in this embodiment has multiple operation modes, including back-to-back mode, uninterruptible power supply mode, i.e. UPS power supply mode and STATCOM mode. It has good redundancy performance for the AC system and can effectively improve the reliability of the AC system.

[0070] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A flexible DC back-to-back system, It is characterized in that It includes a first MMC topology, a DC-DC converter module and a second MMC topology, The two ends of the DC-DC converter module are respectively connected to 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; The high ripple MMC topology includes a plurality of MMC bridge arms, each of the MMC bridge arms includes an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm each include a plurality of MMC sub-modules connected in series in sequence; The MMC submodule adopts a clamped dual module topology, including: switch tube S15, switch tube S16, switch tube S17, switch tube S18, switch tube S19, capacitor Cd1, capacitor Cd2, diode D1 and diode D2, the switch tube S15, switch tube S16, switch tube S17, switch tube S18, switch tube S19 are respectively connected in reverse series with a diode; the switch tube S15 is connected in series with the switch tube S16 and connected in parallel with the capacitor Cd1; the switch tube S15, the switch tube S16 and the capacitor Cd1 are connected in series with the diode D1 as a whole to form a first branch, and the second end of the capacitor Cd1 is connected to the anode of the diode D1; the switch tube S17 and the switch tube S18 is connected in series and in parallel with the capacitor Cd2; the switch tube S17, the switch tube S18 and the capacitor Cd2 are connected in series with the diode D2 as a whole to form a second branch; the first end of the capacitor Cd2 is connected to the cathode of the diode D2; the first branch, the second branch and the switch tube S19 are connected in parallel; the first end of the capacitor Cd1 is connected to the first pole of the switch tube S19 and the anode of the diode D2, and the cathode of the diode D1 is connected to the second pole of the switch tube S19 and the second end of the capacitor Cd2; the connection point of the switch tube S15 and the switch tube S16 in series and the connection point of the switch tube S17 and the switch tube S18 in series serve as the input and output ends of the clamping twin module topology.

2. A flexible DC back-to-back system, It is characterized in that It includes a first MMC topology, a DC-DC converter module and a second MMC topology, The two ends of the DC-DC converter module are respectively connected to 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; The high ripple MMC topology includes a plurality of MMC bridge arms, each of the MMC bridge arms includes an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm each include a plurality of MMC sub-modules connected in series in sequence; The MMC submodule adopts a cross-clamp submodule topology, including: switch tube S20, switch tube S21, switch tube S22, switch tube S23, diode D15, diode D16, integrated gate commutation thyristor S24, capacitor Cd3 and capacitor Cd4, the switch tube S20, switch tube S21, switch tube S22 and switch tube S23 are respectively connected in reverse series with a diode; wherein the switch tube S20 is connected in series with the switch tube S21 and in parallel with the capacitor Cd3, the switch tube S22 is connected in series with the switch tube S23 and in parallel with the capacitor Cd4; the diode D15 is connected in parallel with the capacitor Cd4; the diode D16 ... 5 is connected to the first end of the capacitor Cd3 and the first end of the diode D16, and the second end of the diode D15 is connected to the second end of the capacitor Cd3; the first end of the diode D16 is also connected to the second end of the integrated gate-commutated thyristor S24 and the first end of the capacitor Cd4, and the second end of the diode D16 is connected to the first end of the integrated gate-commutated thyristor S24 and the second end of the capacitor Cd4; the connection point of the switch tube S20 and the switch tube S21 in series and the connection point of the switch tube S22 and the switch tube S23 in series serve as the input and output ends of the cross-clamping sub-module topology.

3. The flexible DC back-to-back system according to claim 1 or 2, It is characterized in that The DC-DC converter module includes at least one dual active full-bridge DC-DC converter.

4. The flexible DC back-to-back system according to claim 3, It is characterized in that The DC-DC converter module includes a plurality of dual active full-bridge DC-DC converters. The inputs of the plurality of dual active full-bridge DC-DC converters are connected in series and the outputs are connected in series.

5. The flexible DC back-to-back system according to claim 1 or 2, It is characterized in that The DC-DC converter module adopts a medium frequency isolation method, and the switching frequency is within 1kHz.

6. The flexible DC back-to-back system according to claim 1 or 2, It is characterized in that The capacitor ripple of the high ripple MMC topology is greater than 10%.

7. An operating method of a flexible DC back-to-back system according to any one of claims 1 to 6, It is characterized in that The operation method comprises: Back-to-back mode, the first MMC topology and the second MMC topology both operate in a DC voltage control mode, and the DC-DC converter module controls power exchange between the substation connected to the first MMC topology and the substation connected to the second MMC topology; Uninterruptible power supply mode, the second MMC topology operates in a DC voltage control mode, the DC-DC converter module operates in a DC voltage mode and controls the DC voltage on the first MMC topology side, the first MMC topology operates in an AC voltage control mode, and supplies power to the load on the AC bus feeder connected to the first MMC topology; STATCOM mode, the first MMC topology and the second MMC topology both independently operate in the STATCOM operation mode.

8. The method for operating the flexible DC back-to-back system according to claim 7, It is characterized in that The STATCOM mode also includes: The first MMC topology is locked, and the second MMC topology operates in a STATCOM operation mode.

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