Hybrid DC power transmission system and power reverse transmission method thereof

Abstract

The invention discloses a hybrid DC power transmission system and a power reverse transmission method thereof. The system comprises a rectification converter station, a bipolar DC line and an inversion converter station. One side of the bipolar direct-current line is connected with the rectification converter station, and the other side of the bipolar direct-current line is connected with the inversion converter station; the rectification converter station comprises a first bipolar converter, each stage of converter comprises a first converter transformer and a first converter valve group, and the head end and the tail end of the first converter valve group are respectively provided with a switch; the switch comprises an isolating switch and a bypass switch; the inversion converter station comprises a second bipolar converter, each stage of converter comprises a second converter transformer and a second converter valve group, and the head end and the tail end of the second converter valve group are respectively provided with a switch; the first bipolar converter and the second bipolar converter are configured to realize different access modes, and the switch is configured to realize reverse power transmission during operation of the hybrid direct current power transmission system through opening and closing of the switch in different access modes, so that the flexibility and reliability of system operation are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power transmission systems, and in particular to a hybrid direct current power transmission system and a power reverse transmission method thereof. Background Art

[0002] In recent years, with the development of new energy in the Shagohuang area, the problem of new energy consumption has become increasingly prominent. For areas with weak power grids, flexible DC converter stations based on voltage source converters do not rely on the AC power grid, and are not prone to system overvoltage and other problems during operation, making them more suitable for the grid connection of new energy units. In addition, with the increasing number of DC projects, the phenomenon of multiple DC points in the same area has appeared in the eastern economically developed areas of the department. In order to avoid the fluctuation of multiple DC power caused by a unified fault, the use of flexible DC converter stations based on voltage source converters at the head end of the power grid can greatly improve the reliability and safety of multiple DC feed-in systems. As a result, a hybrid DC transmission system with a conventional DC (LCC) converter station at the sending end and a flexible DC (VSC) converter station at the receiving end or a flexible DC converter station at the sending end and a conventional DC converter station at the receiving end has emerged.

[0003] The structure of the traditional hybrid DC transmission system has the following problems: the LCC system uses thyristor commutator valves, in which the voltage can be reversed but the current cannot be reversed; the VSC system uses half-bridge and full-half-bridge hybrid IGBT devices, in which the current can be reversed but the voltage cannot be reversed, resulting in the entire hybrid DC system not having the power return function, reducing the flexibility and reliability of the system.

[0004] It can be seen that how to improve the structure of the hybrid DC transmission system to achieve power reversal has become a technical problem that needs to be urgently solved by those skilled in the art. Summary of the invention

[0005] The present invention provides a hybrid direct current power transmission system and a power reverse transmission method thereof, so as to solve the problem of how to improve the structure of the hybrid direct current power transmission system so as to enable the system to achieve power reversal.

[0006] In order to solve the above technical problems, an embodiment of the present invention provides a hybrid direct current transmission system, the system comprising:

[0007] A rectifier-converter station, a bipolar DC line and an inverter-converter station; one side of the bipolar DC line is connected to the rectifier-converter station, and the other side is connected to the inverter-converter station;

[0008] The rectifier converter station includes a first bipolar converter, each level of the converter includes a first converter transformer and a first converter valve group, and switches are respectively configured at the head and tail ends of the first converter valve group; the switches include an isolating switch and a bypass switch;

[0009] The inverse conversion station includes a second bipolar converter, each stage of the converter includes a second converter transformer and a second converter valve group, and the switches are respectively arranged at the head and tail ends of the second converter valve group;

[0010] The first bipolar converter and the second bipolar converter are configured to implement different access modes, and the switch is configured to implement power reverse transmission during operation of the hybrid direct current power transmission system by opening and closing the switch itself under different access modes.

[0011] Further, the first bipolar converter and the second bipolar converter are configured to implement different access modes, specifically:

[0012] Different access modes are achieved by controlling the first bipolar converter to be converted into a grid-commutated converter or a voltage source converter, and by controlling the second bipolar converter to be converted into a voltage source converter or a grid-commutated converter.

[0013] Furthermore, if the first bipolar converter and the second bipolar converter adopt the grid-commutated converter, the circuit characteristics are six-pulse type, twelve-pulse type or double twelve-pulse type.

[0014] Furthermore, the rectifier converter station and the inverter converter station both include an onshore converter station or an offshore converter station.

[0015] Furthermore, both the first bipolar converter and the second bipolar converter may be integrated gate commutated thyristor based converters, insulated gate bipolar transistor based converters, multi-source commutated converters or controllable commutated converters.

[0016] Furthermore, the bipolar DC line adopts an overhead line, a land cable line or a submarine cable line.

[0017] Furthermore, if the first bipolar converter and the second bipolar converter adopt the grid-commutated converter, they also include correspondingly connected DC filters, AC filters and reactive power compensation devices.

[0018] Furthermore, the switch is also configured to realize back-to-back operation between the high-end and low-end converter valve groups of the same pole when the hybrid DC transmission system is in operation by opening and closing the switch itself under different access modes.

[0019] Furthermore, both ends of the first converter valve group and the second converter valve group may be configured as DC conversion switches.

[0020] Another embodiment of the present invention further provides a power reverse transmission method of a hybrid DC power transmission system, which is applied to the above-mentioned hybrid DC power transmission system, comprising:

[0021] Performing on / off control on a plurality of switches at the positive pole of the bipolar converter valve group of the hybrid DC power transmission system, and setting the voltage at the upper and lower bridge arms of the high converter valve group at the positive pole to the first target voltage, and the voltage at the upper and lower bridge arms of the low converter valve group at the positive pole to the second target voltage;

[0022] A plurality of switches at the negative pole of the bipolar converter valve group of the hybrid DC power transmission system are controlled to be turned on / off, and the voltage at the upper and lower bridge arms of the high converter valve group at the negative pole is set to a third target voltage, and the voltage at the upper and lower bridge arms of the low converter valve group at the negative pole is set to a fourth target voltage.

[0023] Compared with the prior art, the embodiments of the present invention have the following advantages:

[0024] (1) By configuring an additional isolating switch near the bypass switch of the hybrid DC transmission system converter, the voltage polarity of the converter can be reversed, so that the entire hybrid DC transmission system has the function of power reverse transmission, thereby improving the flexibility of system operation.

[0025] (2) By configuring an isolating switch and a bypass switch at the head and tail ends of the converter valve group, the high-end and low-end valve groups of the single pole can be operated back-to-back, so that during the system debugging, the high-end and low-end valve groups of the single pole can be debugged, and the power mutual assistance between the AC systems connected to the high-end and low-end valve groups at the receiving end can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a schematic diagram of the topological structure of a hybrid DC system in one embodiment of the present invention;

[0027] Figure 2 It is a schematic diagram of a topological structure of a hybrid DC system in one embodiment of the present invention to realize bipolar operation with positive power transmission;

[0028] Figure 3 It is a schematic diagram of a topological structure of a hybrid DC system in one embodiment of the present invention that realizes bipolar operation of power reverse transmission;

[0029] Figure 4 It is a schematic diagram of the topological structure of a hybrid DC system in one embodiment of the present invention to realize unipolar full-voltage power reverse transmission operation;

[0030] Figure 5 It is a schematic diagram of the topological structure of a hybrid DC system in one embodiment of the present invention to realize the power reverse operation of a single-pole high-end valve group;

[0031] Figure 6 It is a schematic diagram of a topological structure of a hybrid DC system in one embodiment of the present invention to realize power reverse operation of a single-pole low-end converter valve group;

[0032] Figure 7 It is a schematic diagram of the topological structure of a hybrid DC system in one embodiment of the present invention to achieve back-to-back operation of single-pole high-end and low-end converter valve groups. DETAILED DESCRIPTION

[0033] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. 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.

[0034] In the description of this application, the terms "first", "second", "third", etc. are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first", "second", "third", etc. may explicitly or implicitly include one or more of the feature. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0035] In the description of the present application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be a connection between the two elements. The terms "vertical", "horizontal", "left", "right", "upper", "lower" and similar expressions used herein are only for illustrative purposes, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0036] In the description of this application, it should be noted that, unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as those commonly understood by those skilled in the art. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood by specific circumstances.

[0037] In large-scale grid interconnection, hybrid DC transmission systems can realize power exchange between grids in different regions. Through power reversal, the direction and size of power transmission between different regions can be easily adjusted to achieve optimized dispatching and stable operation of the grid. And with the large-scale development and application of renewable energy, hybrid DC transmission systems play an important role in renewable energy grid connection. The power reversal function can adapt to the volatility and uncertainty of renewable energy generation, ensuring the stable operation of the grid and the reliability of power supply. It can be understood that the hybrid DC transmission system can perform power reversal, which can not only improve the flexibility and adaptability of the system, but also meet the needs of application scenarios such as grid interconnection and dispatching, renewable energy access, and load balancing and regulation. At the same time, it can also improve energy utilization efficiency, promote economic development and enhance grid security. Therefore, improving the traditional transmission system to achieve system power reversal is of great significance to maintaining the security of the grid and improving the utilization of resources.

[0038] An embodiment of the present invention provides a hybrid direct current transmission system. For details, see Figure 1 , Figure 1 The figure shows a structural topology diagram of a hybrid DC power transmission system in one embodiment of the present invention. It can be seen that the system includes: a rectifier converter station, a bipolar DC line and an inverter station.

[0039] Among them, one side of the bipolar DC line is connected to the rectifier converter station, and the other side is connected to the inverter station. The rectifier converter station, the bipolar DC line and the inverter station work together to achieve efficient operation of the hybrid DC transmission system. The rectifier converter station converts AC power into DC power and transmits it to the inverter station through the bipolar DC line. The inverter station then converts DC power back into AC power and supplies it to end users or the power grid.

[0040] As an example, in the embodiment of the present invention, the bipolar DC line can adopt an overhead line, a land cable line or a submarine cable line to meet the transmission performance of the power transmission system in different scenarios. The rectifier and inverter stations can be conventional DC converter stations or flexible DC converter stations.

[0041] It can be understood that conventional DC converter stations refer to converter stations that use conventional DC transmission technology (Line Commutated Converter-based High Voltage Direct Current, LCC-HVDC). This type of converter station mainly uses semi-controlled power electronic devices such as thyristors to achieve conversion between AC and DC through converter valves. Corresponding to conventional DC converter stations is the flexible DC converter station (Voltage Source Converter-based High Voltage Direct Current, VSC-HVDC), which uses fully controlled power electronic devices (such as IGBT) and modular multilevel converter (MMC) technology, and has higher flexibility and controllability.

[0042] In some embodiments of the present invention, the rectifier converter station and the inverter converter station are also configured as converter stations including onshore converter stations or offshore converter stations to adapt to the needs of different scenarios such as offshore wind farms or other offshore energy projects of different sizes and types according to different geographical locations.

[0043] The rectifier converter station includes a first bipolar converter, each stage of the converter includes a first converter transformer and a first converter valve group, and switches are respectively configured at the first and last ends of the first converter valve group;

[0044] The inverter station includes a second bipolar converter, each stage of the converter includes a second converter transformer and a second converter valve group, and the switches are respectively arranged at the head and tail ends of the second converter valve group.

[0045] The first bipolar converter and the second bipolar converter are configured to implement different access modes, and the switch is configured to implement power reverse transmission during operation of the hybrid direct current power transmission system by opening and closing the switch itself under different access modes.

[0046] A bipolar converter is usually formed by two unipolar converters connected in parallel, and each unipolar converter can independently perform AC-DC conversion. In this embodiment, the first bipolar converter and the second bipolar converter can both be grid-commutated converters (LCC) or voltage source converters (VSC). Different access methods are implemented according to the types of converters configured for the rectifier and inverter-side bipolar converters. That is, the bipolar converters are connected to the same AC system or different AC systems. Exemplarily, when the rectifier-side converter station is a conventional DC converter station, the corresponding first bipolar converter can adopt a grid-commutated converter and connect it to the same AC system; and when the inverter side is a flexible DC converter station, the corresponding second bipolar converter can adopt a voltage source converter and connect it to different AC systems.

[0047] If the bipolar converter uses a grid-commutated converter, it is necessary to configure corresponding DC filters, AC filters and reactive power compensation equipment. The specific configuration method is determined according to the actual project.

[0048] It is worth noting that when the bipolar converter uses a grid-commutated converter, its circuit characteristics can be selected as six-pulse, twelve-pulse or double twelve-pulse. Six-pulse, twelve-pulse and double twelve-pulse refer to the pulsating waveforms and their numbers generated by the converter valve group when it is working. These pulsating waveforms are closely related to the configuration and control method of the thyristors in the converter valve group. Different pulsating waveforms correspond to different circuit structures and performance characteristics.

[0049] It can be understood that the first bipolar converter and the second bipolar converter can be grid-commutated converters or voltage source converters, and correspondingly, as an example, the voltage source converter can be a converter IGBT-MMC based on an insulated gate bipolar transistor (IGBT) or a converter IGCT-MMC based on an integrated gate-commutated thyristor (IGCT). The grid-commutated converter can be a converter LCC based on a thyristor, a converter based on an integrated gate-commutated thyristor (IGCT-LCC) or a multi-source commutated converter (SLCC), or a controllable commutated converter (CLCC).

[0050] In the embodiment of the present invention, it can be understood that the switch includes an isolating switch and a bypass switch. Figure 1 As shown, an isolating switch is provided near the bypass switch. By controlling the closing and opening of these switches, the voltage polarity of each valve group in the system can be reversed, thereby having power reverse transmission in various operating modes such as single-pole, double-pole, single-stage single-valve group, etc., and when the high and low pressure valve groups are connected to two different AC systems, power mutual assistance between the two AC systems can be achieved.

[0051] In some implementations of the embodiments of the present invention, both ends of the converter valve group may also be configured as DC conversion switches to achieve online conversion of the system operation mode.

[0052] It is worth noting that the switch is also configured to realize the back-to-back operation between the high-end and low-end converter valve groups of the same polarity when the hybrid DC transmission system is in operation by opening and closing itself under different access modes. In the hybrid DC transmission system, by controlling the opening and closing of the corresponding isolation switches configured at both ends of the converter valve group, the back-to-back operation of the high-end and low-end valve groups of the same valve group polarity is realized, and the live operation debugging of the unipolar valve group is further realized, which can improve the system flexibility, enhance the system reliability, reduce harmonic pollution and realize power reverse transmission.

[0053] Example 1

[0054] As an example, please refer to Figure 1 As shown, a topological structure of a hybrid DC system is provided in an embodiment of the present invention, wherein:

[0055] The rectifier and converter station adopts true bipolar wiring. The true bipolar topology has grounding electrodes set on the rectifier and inverter sides, which can realize unipolar operation. The bipolar converter on the rectifier side adopts the thyristor-based converter LCC, which is connected to the same AC system S11. Each pole adopts double twelve-pulse wiring, which is divided into high and low end valve groups, and each end valve group is twelve-pulse wiring. The DC voltage is set to ±800kV. The positive and negative valve group lines on the rectifier side share a grounding electrode, and a metal return line is set, so that the system can realize unipolar metal return line operation.

[0056] like Figure 1 The topological structure of the inverter side is shown. It can be seen that similarly, the inverter side includes a bipolar converter and several groups of converter transformers. The converter transformer refers to the topology shown in C. The bipolar converter on the inverter side adopts a voltage source converter-MCC. Among them, it can be understood that the bipolar is divided into positive and negative poles. According to the different polarities, the bipolar converters are respectively recorded as converter A (positive pole) and converter B (negative pole). Converters A and B are respectively provided with high and low end converter valve groups, and each end converter valve group is provided with upper and lower bridge arms. The high-end valve groups of the two levels are connected to the AC system S21, and the low-end valve groups are connected to the AC system S22.

[0057] Furthermore, four additional isolation switches are configured near the bypass switches of each end valve group of converter A and converter B. As an example, it can be seen from the structure of converter A that the isolation switches of its high-end valve group are configured with switches P11, P12, P13 and P14, while the isolation switches of the low-end valve group are configured with switches P21, P22, P23 and P24. Correspondingly, the isolation switches of the high-end valve group of converter B are configured with switches P31, P32, P33 and P34, while the isolation switches of the low-end valve group are configured with switches P41, P42, P43 and P44.

[0058] As an example, the connection relationship of the isolating switch at the high-end valve group of converter A is as follows: one end of P11 is connected to one end of P12 through switch P15, the other end of P11 is connected to one end of P14, the other end of P12 is connected to one end of P13, and the other end of P13 is connected to the other end of P14 through switch P16. The connection relationship of the isolating switch at the low-end valve group is similar to that at the high-end valve group, and will not be repeated here.

[0059] In the present embodiment 1, the voltage polarity reversal of the bipolar converter is achieved by controlling the opening and closing of the isolating switch configured on the inverter side, so that the entire hybrid DC power transmission system has the function of power reverse transmission.

[0060] The following provides some operation modes of various hybrid DC systems specifically based on the topological structure shown in Example 1:

[0061] Example 2

[0062] Based on the topology structure shown in Example 1, a bipolar operation mode of system power forward transmission is provided. For details, please refer to Figure 2 , Figure 2 The topological structure diagram of the hybrid DC system in one embodiment of the present invention to achieve bipolar operation with positive power transmission is shown. As an example, the inverter side switch input conditions are as follows:

[0063] At the positive converter: switches P15, P16, P25, and P26 are closed, P11, P12, P13, P14, P21, P22, P23, P24, Q1, Q11, Q2, and Q21 are opened, the upper arm of the high-end converter valve group is subjected to a voltage of +800 kV, and the lower arm is subjected to a voltage of +400 kV; the upper arm of the low-end converter valve group is subjected to a voltage of +400 kV, and the lower arm is subjected to the neutral line voltage.

[0064] At the negative converter: switches P35, P36, P35, and P36 are closed, P31, P32, P33, P34, P41, P42, P43, P44, Q3, Q31, Q4, and Q41 are opened, the upper arm of the high-end converter valve group is subjected to a voltage of -800 kV, and the lower arm is subjected to a voltage of -400 kV; the upper arm of the negative low-end converter valve group is subjected to a voltage of -400 kV, and the lower arm is subjected to the neutral line voltage.

[0065] The DC power flows from the rectifier side system S11 to the inverter side systems S21 and S22 respectively.

[0066] It can be understood that the above-mentioned input mode can realize the maximum power operation mode of the DC system. This operation mode can be adopted when both the rectifier and inverter side converters of the converter station are available.

[0067] Example 3

[0068] Based on the topology shown in Example 1, a bipolar operation mode of system power reverse transmission is provided. For details, please refer to Figure 3 , Figure 3 The topological structure diagram of a hybrid DC system in one embodiment of the present invention for realizing bipolar operation of power reverse transmission is shown. As an example, the inverter side switch input conditions are as follows:

[0069] At the positive converter: switches P15, P16, P25, P26, Q1, Q11, Q2, and Q21 are opened, and P11, P12, P13, P14, P21, P22, P23, and P24 are closed. The upper arm of the high-end converter valve group is subjected to a voltage of +400 kV, and the lower arm is subjected to a voltage of +800 kV; the lower arm of the low-end converter valve group is subjected to a voltage of +400 kV, and the upper arm is subjected to the neutral line voltage.

[0070] At the negative converter: switches P35, P36, P35, P36, Q3, Q31, Q4, and Q41 are open, and P31, P32, P33, P34, P41, P42, P43, and P44 are closed. The upper arm of the high-end converter valve group is subjected to a voltage of -400kV, and the lower arm is subjected to a voltage of -800kV; the lower arm of the negative low-end converter valve group is subjected to a voltage of -400kV, and the upper arm is subjected to the neutral line voltage.

[0071] And direct current power flows from the inverter side systems S21 and S22 into the rectifier side system S11.

[0072] It can be understood that the above-mentioned input mode can realize the maximum power reverse operation mode of the DC system. This operation mode can be adopted when both the rectifier and inverter side converters of the converter station are available.

[0073] Example 4

[0074] Based on the topology shown in Example 1, a system unipolar full-voltage power reverse transmission operation mode is provided. For details, please refer to Figure 4 , Figure 4 The topological structure diagram of a hybrid DC system in one embodiment of the present invention realizing unipolar full-voltage power reverse transmission operation is shown. As an example, the inverter side switch input conditions are as follows:

[0075] Exemplarily, the switches at the positive converter are controlled: switches P15, P16, Q1, and Q11 are opened, and P11, P12, P13, and P14 are closed. The upper bridge arm of the high-end converter valve group is subjected to a voltage of +400 kV, and the lower bridge arm is subjected to a voltage of +800 kV.

[0076] Switches P25, P26, Q2, and Q21 are opened, and P21, P22, P23, and P24 are closed. The upper bridge arm of the low-end converter valve group bears the neutral line voltage, and the lower bridge arm bears the +400kV voltage.

[0077] And direct current power flows from the inverter side systems S21 and S22 into the rectifier side system S11.

[0078] It can be understood that this operation mode can be adopted when the negative pole equipment of the converter station fails or is under maintenance.

[0079] Example 5

[0080] Based on the topology structure shown in Example 1, a system single-pole high-end converter power reverse transmission operation mode is provided. For details, please refer to Figure 5 , Figure 5 The topological structure diagram of a hybrid DC system in one embodiment of the present invention to realize the power reverse operation of a single-pole high-end valve group is shown. As an example, the inverter side switch input conditions are as follows:

[0081] Exemplarily, the switches at the positive converter are controlled: switches P15, P16, Q1, and Q11 are opened, and P11, P12, P13, and P14 are closed. The upper bridge arm of the high-end converter valve group is subjected to the neutral point voltage, and the lower bridge arm is subjected to the +400 kV voltage.

[0082] Switches P25, P26, Q2, P21, P22, P23, and P24 are opened, Q21 is closed, and the low-end converter valve group is bypassed.

[0083] And direct current power flows from the inverter side system S21 into the rectifier side system S11.

[0084] It can be understood that this operation mode can be adopted when the negative pole equipment and the positive pole low-end converter of the converter station fail or are under maintenance.

[0085] Example 6

[0086] Based on the topology structure shown in Example 1, a system single-pole low-end converter power reverse transmission operation mode is provided. For details, please refer to Figure 6 , Figure 6 The topological structure diagram of a hybrid DC system in one embodiment of the present invention to realize the power reverse operation of a single-pole low-end converter valve group is shown. As an example, the inverter side switch input conditions are as follows:

[0087] Exemplarily, the switches at the positive converter are controlled: switches P15, P16, Q1, P11, P12, P13, and P14 are opened, Q11 is closed, and the high-end converter valve group is bypassed.

[0088] Switches P25, P26, Q2, and Q21 are opened, and P21, P22, P23, and P24 are closed. The lower bridge arm of the low-end converter valve group is subjected to a +400kV voltage, and the upper bridge arm is subjected to a neutral line voltage.

[0089] And direct current power flows from the inverter side system S22 into the rectifier side system S11.

[0090] It can be understood that this operation mode can be adopted when the negative equipment and the positive high-end converter of the converter station fail or are under maintenance.

[0091] Example 7

[0092] Based on the topology structure shown in Example 1, a system flexible converter station single-pole high-end and low-end converter back-to-back operation mode is provided. For details, please refer to Figure 7 , Figure 7 The topological structure diagram of the hybrid DC system in one embodiment of the present invention realizing the back-to-back operation of the unipolar high-end and low-end converter valve groups is shown. As an example, the inverter side switch input conditions are as follows:

[0093] Exemplarily, the switches at the positive converter are controlled: switches P15, P16, Q1, and Q11 are opened, and P11, P12, P13, and P14 are closed. The upper bridge arm of the high-end converter valve group is subjected to +400 kV, and the lower bridge arm is subjected to the neutral line voltage.

[0094] Switches P25, P26, Q2, and Q21 are opened, and P21, P22, P23, and P24 are closed. The lower bridge arm of the low-end converter valve group bears -400kV and the upper bridge arm bears the neutral line voltage.

[0095] And direct current power flows from the inverter side system S22 to the rectifier side system S21.

[0096] It can be understood that this operation mode can be adopted when the DC system is out of service and the inverter-side AC system S21 urgently needs power support.

[0097] In summary, the embodiment of the present invention improves the structure of the hybrid DC system, controls the isolating switch configured for the bipolar converter of the converter station to open or close, and realizes the reversal of the voltage polarity of the valve group, so that the hybrid DC power transmission system can realize power reverse transmission in various operating modes such as single, bipolar, single-stage single valve group, and when the high-end and low-end converter valve groups are connected to two different AC systems, power mutual assistance between the two AC systems is realized. The reliability, safety and resource utilization of the DC power transmission system are improved.

[0098] An embodiment of the present invention provides a power reverse transmission method for a hybrid direct current power transmission system, which is applied to the above hybrid direct current power transmission system and specifically includes the following steps:

[0099] S1, performing on / off control on a plurality of switches at the positive pole of the bipolar converter valve group of the hybrid DC power transmission system, and setting the voltage at the upper and lower bridge arms of the high converter valve group at the positive pole to a first target voltage, and the voltage at the upper and lower bridge arms of the low converter valve group at the positive pole to a second target voltage;

[0100] S2. Perform on / off control on a plurality of switches at the negative pole of the bipolar converter valve group of the hybrid DC power transmission system, and set the voltage at the upper and lower bridge arms of the high converter valve group at the negative pole to a third target voltage, and the voltage at the upper and lower bridge arms of the low converter valve group at the negative pole to a fourth target voltage.

[0101] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A hybrid direct current transmission system, characterized in that: The system comprises: a rectifier converter station, a bipolar DC line and an inverter station; one side of the bipolar DC line is connected to the rectifier converter station, and the other side is connected to the inverter station; The rectifier converter station includes a first bipolar converter, each level of the converter includes a first converter transformer and a first converter valve group, and switches are respectively configured at the head and tail ends of the first converter valve group; the switches include an isolating switch and a bypass switch; The inverse conversion station includes a second bipolar converter, each stage of the converter includes a second converter transformer and a second converter valve group, and the switches are respectively arranged at the head and tail ends of the second converter valve group; The first bipolar converter and the second bipolar converter are configured to implement different access modes, and the switch is configured to implement power reverse transmission during operation of the hybrid direct current power transmission system by opening and closing the switch itself under different access modes.

2. The hybrid direct current transmission system according to claim 1, characterized in that: The first bipolar converter and the second bipolar converter are configured to implement different access modes, specifically: Different access modes are achieved by controlling the first bipolar converter to be converted into a grid-commutated converter or a voltage source converter, and by controlling the second bipolar converter to be converted into a voltage source converter or a grid-commutated converter.

3. The hybrid direct current transmission system according to claim 2, characterized in that: If the first bipolar converter and the second bipolar converter adopt the grid-commutated converter, the circuit characteristics are six-pulse type, twelve-pulse type or double twelve-pulse type.

4. The hybrid direct current transmission system according to claim 1, characterized in that: The rectifier and converter stations and the inverter and converter stations also include onshore converter stations or offshore converter stations.

5. The hybrid direct current transmission system according to claim 1, characterized in that: The first bipolar converter and the second bipolar converter may both be integrated gate commutated thyristor based converters, insulated gate bipolar transistor based converters, multi-source commutated converters or controllable commutated converters.

6. The hybrid direct current transmission system according to claim 1, characterized in that: The bipolar DC line adopts an overhead line, a land cable line or a submarine cable line.

7. The hybrid direct current transmission system according to claim 1, characterized in that: If the first bipolar converter and the second bipolar converter adopt the grid phase-commutation converter, they also include a DC filter, an AC filter and a reactive power compensation device connected accordingly.

8. The hybrid direct current transmission system according to claim 1, characterized in that: The switch is also configured to realize back-to-back operation between the high-end and low-end converter valve groups of the same pole when the hybrid DC power transmission system is in operation by opening and closing the switch itself under different access modes.

9. The hybrid direct current transmission system according to claim 1, characterized in that: Both ends of the first converter valve group and the second converter valve group may also be configured as DC conversion switches.

10. A power reverse transmission method for a hybrid DC power transmission system, applied to the hybrid DC power transmission system according to claims 1 to 9, characterized in that: include: Performing on / off control on a plurality of switches at the positive pole of the bipolar converter valve group of the hybrid DC power transmission system, and setting the voltage at the upper and lower bridge arms of the high converter valve group at the positive pole to the first target voltage, and the voltage at the upper and lower bridge arms of the low converter valve group at the positive pole to the second target voltage; A plurality of switches at the negative pole of the bipolar converter valve group of the hybrid DC power transmission system are controlled to be turned on / off, and the voltage at the upper and lower bridge arms of the high converter valve group at the negative pole is set to a third target voltage, and the voltage at the upper and lower bridge arms of the low converter valve group at the negative pole is set to a fourth target voltage.

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