Direct-current transmission topological structure with parallel converters and control method of direct-current transmission topological structure

Abstract

The invention discloses a DC power transmission topological structure with parallel converters and a control method thereof. The DC power transmission topological structure comprises at least two converter stations. The converter station adopts a bipolar structure, and each pole comprises two groups of converters which are connected in parallel; each converter is formed by connecting a converter transformer and a converter valve group in series; the operation modes of the converter station comprise a single-pole single-valve group, a single-pole double-valve group, a double-pole single-valve group and a double-pole double-valve group, and the operation modes are switched by controlling on-off of different switches. The control method comprises the steps of determining an operation mode of a converter station according to a current load demand and current converter capacity information, so as to enable the converter station to be switched to a corresponding operation mode through switch control; and based on the determined operation mode, current distribution of each converter is optimized by taking load balancing as a target. According to the invention, the reliability and transmission capacity of the direct-current power transmission system can be improved, and flexible configuration of power resources among different power systems is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of direct current transmission, in particular to a direct current transmission topology with parallel-connected converters and a control method thereof. BACKGROUND

[0002] In recent years, with the rapid progress of power electronic technology and the decrease of direct current device cost, high-voltage direct current transmission technology has developed rapidly, and the transmission capacity has been continuously increased and the transmission distance has been continuously extended. The construction of small direct current transmission in regional power grids is also advancing, which provides more flexible and diverse ways for the optimal allocation of energy and resources, thereby putting forward higher requirements for the operation mode of direct current systems. Generally, the increase of transmission capacity will lead to the increase of voltage level, thereby increasing the system insulation requirement and the construction cost. In addition, the current direct current transmission mostly adopts the bipolar scheme or single-pole grounding / single-pole metal loop mode with series-connected converters, and such schemes have insufficient flexibility in operation mode, which is not conducive to the efficient allocation of resources. Therefore, there is an urgent need for a direct current transmission structure with high transmission capacity and flexible operation mode. SUMMARY

[0003] To solve the above technical problems, the present application provides a direct current transmission topology with parallel-connected converters and a control method thereof, which can improve the reliability and transmission capacity of the direct current transmission system and realize the flexible allocation of power resources between different power systems.

[0004] The present application provides a direct current transmission topology with parallel-connected converters, comprising:

[0005] At least two converter stations are connected through direct current lines between the converter stations;

[0006] The converter station adopts a bipolar structure, each pole comprising two groups of converters connected in parallel; each converter is composed of a series connection of a converter transformer and a converter valve group; in the same pole, the first ends of the two groups of converters of the converter station are respectively connected to an alternating current system, the second ends of the two groups of converters are respectively connected to the direct current lines of the corresponding poles through switches, the third ends of the two groups of converters are respectively connected to a first point through switches, the first point is connected to a ground wire through a switch, and the first point is also connected to the direct current line of another pole through a switch and a metal return line;

[0007] The operation modes of the converter station include single-pole single valve group, single-pole double valve group, bipolar single valve group and bipolar double valve group, and the operation modes are switched by controlling the on-off of different switches.

[0008] Further, in the bipolar double valve group operation mode:

[0009] Through switch control, the second ends of the two converters of the first pole of the converter station are connected with the first pole DC line respectively, and the second ends of the two converters of the second pole of the converter station are connected with the second pole DC line respectively, and the third end of each converter is connected with the ground wire.

[0010] Further, in the operation mode of the bipolar single-valve group:

[0011] Through switch control, in the first pole of the converter station, the second end of one converter is connected with the first pole DC line and the third end is connected with the ground wire, and the other converter is out of operation; in the second pole of the converter station, the second end of one converter is connected with the second pole DC line and the third end is connected with the ground wire, and the other converter is out of operation.

[0012] Further, in the operation mode of the bipolar single-valve group:

[0013] Through switch control, the second ends of the two converters of the first pole of the converter station are connected with the first pole DC line respectively, and the third end of each converter is connected with the ground wire and the DC line of the other pole through a metal return line; the converters of the other pole of the converter station are out of operation.

[0014] Further, in the operation mode of the bipolar single-valve group:

[0015] Through switch control, in the first pole of the converter station, the second end of one converter is connected with the first pole DC line and the third end is connected with the DC line of the other pole through a metal return line, and the other converter is out of operation; the converters of the other pole of the converter station are out of operation.

[0016] Further, the converter station comprises a rectifier-side converter station and an inverter-side converter station; the rectifier-side converter station adopts a constant DC current control strategy, the constant DC current control strategy comprises controlling the DC current of the converter to reach a preset target value; in the inverter-side converter station, one of the two converters of each pole adopts the constant DC current control strategy, and the other converter adopts a constant DC voltage control strategy, the constant DC voltage control strategy comprises controlling the DC voltage of the converter to reach a preset target value.

[0017] Further, the wiring mode of the converter in the converter station comprises: complete bipolar, 1 / 2 bipolar, 3 / 4 bipolar, complete single-pole ground, incomplete single-pole ground, complete single-pole metal, and incomplete single-pole metal.

[0018] Further, the switch in the converter station adopts a disconnecting switch or a DC transfer switch.

[0019] The application also provides a control method of the DC power transmission topology structure with parallel converters, applied to the DC power transmission topology structure, and the control method comprises:

[0020] acquire current load demand and converter capacity information;

[0021] determine operation mode of the converter station according to the current load demand and the converter capacity information, so as to switch the converter station to corresponding operation mode through switch control;

[0022] optimize current distribution of each converter based on the determined operation mode, aiming at load balance.

[0023] Further, the operation mode includes single-pole single-valve group, single-pole double-valve group, double-pole single-valve group and double-pole double-valve group.

[0024] Compared with the prior art, the DC power transmission topology structure and the control method thereof provided by the application have the beneficial effects that: by adopting the parallel valve group mode, the transmission capacity is improved, and meanwhile the insulation level is not changed; by configuring corresponding switches at both ends of the converter valve group, the single-pole single-valve group, the single-pole double-valve group, the double-pole single-valve group and the double-pole double-valve group operation modes are realized, the reliability and availability of the DC power transmission system are greatly improved, and the flexible configuration of power resources between different power systems can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a schematic diagram of a DC power transmission topology structure of a parallel converter provided by an embodiment of the application;

[0026] Figure 2 is a schematic diagram of a DC power transmission topology structure of a bipolar operation mode provided by an embodiment of the application;

[0027] Figure 3 is a schematic diagram of another DC power transmission topology structure of a bipolar operation mode provided by an embodiment of the application;

[0028] Figure 4 is a schematic diagram of a DC power transmission topology structure of a single-pole operation mode provided by an embodiment of the application;

[0029] Figure 5 is a schematic diagram of another DC power transmission topology structure of a single-pole operation mode provided by an embodiment of the application. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the application.

[0031] The purpose of the embodiment of the present application is to provide a converter parallel DC power transmission topology, which enriches the operation mode of the DC power transmission system, improves the flexibility of the operation mode, and is beneficial to the demand of flexible resource allocation.

[0032] At least two converter stations are connected through DC lines;

[0033] The converter station adopts a bipolar structure, each pole includes two groups of converters connected in parallel; each converter is composed of a converter transformer and a converter valve group in series; in the same pole, the first ends of the two groups of converters of the converter station are respectively connected to the AC system, the second ends of the two groups of converters are respectively connected to the DC line of the corresponding pole through switches, the third ends of the two groups of converters are respectively connected to the first point through switches, the first point is connected to the ground wire through a switch, and the first point is also connected to the DC line of the other pole through a switch and a metal return line;

[0034] The operation mode of the converter station includes single-pole single-valve group, single-pole double-valve group, double-pole single-valve group and double-pole double-valve group, and the operation mode is switched by controlling the on-off of different switches.

[0035] Specifically, the DC power transmission topology includes rectifier converter stations, inverter converter stations, DC lines, conversion switches, etc. Among them, the rectifier converter station includes pole 1 and pole 2, each pole includes two groups of converters, and each group of converters includes a transformer and a converter valve group. The two converter valve groups in the same pole are connected in parallel, and switches are configured at the pole line end (i.e. the second end) and the neutral line end (i.e. the third end) of the two converter valve groups for switching the operation mode.

[0036] Among them, the converter station can be a land converter station or a sea converter station. The DC line can be an overhead line, a land cable or a submarine cable. The converter station includes a rectifier converter station and an inverter converter station, and the rectifier converter station and the inverter converter station can be converted to each other. The rectifier station can operate in inverter mode, and the inverter station can operate in rectifier mode.

[0037] Compared with the connection mode of valve group in series, the valve group in parallel mode of the embodiment of the present application can reduce the loss of the line without increasing the insulation level of the DC system. This mode not only maximizes the relative independence of the DC system and simplifies the design of the DC system control strategy, but also can realize the flexible distribution of power among the parallel converters. Specifically, the connection form of the converter in series needs to change the voltage level and operating current of the entire system when the system is expanded, which is complex and difficult to implement. Compared with the series connection mode, the connection mode of the parallel converter is more flexible and convenient when the system is expanded. Only the number of parallel branches needs to be increased, that is, a conventional double-end DC is first realized, and then the sending end and the receiving end are expanded to form a multi-converter parallel DC system.

[0038] In one specific example, refer to Figure 1 is a schematic diagram of a DC power transmission topology with parallel converters provided by an embodiment of the present application. The left part of the diagram is a rectifier station, and the right part is an inverter station. The DC line includes a first pole DC line (corresponding to the pole 1 line in the diagram) and a second pole DC line (corresponding to the pole 2 line in the diagram).

[0039] The rectifier station includes an AC system S11, a first pole converter P11, P12, and a second pole converter P21, P22. The first pole converter P11 includes a converter transformer T11 and a converter valve group V11. The upper part (i.e., the second end) of the converter valve group V11 is connected to the pole 1 line through a switch QS11. The lower part (i.e., the third end) of the converter valve group is connected to the pole 2 line through switches QF11, QS13, QS15, QF13, and QS26, and is connected to the ground through switches QF11, QS14, QS25, and QF23. The first pole converter P12 includes a converter transformer T12 and a converter valve group V12. The upper part (i.e., the second end) of the converter valve group V12 is connected to the pole 1 line through a switch QS12. The lower part (i.e., the third end) of the converter valve group is connected to the pole 2 line through switches QF12, QS13, QS15, QF13, and QS26, and is connected to the ground through switches QF12, QS14, QS25, and QF23. The second pole converter P21 includes a converter transformer T21 and a converter valve group V21. The lower part (i.e., the second end) of the converter valve group V21 is connected to the pole 2 line through a switch QS21. The upper part (i.e., the third end) of the converter valve group is connected to the pole 1 line through switches QF21, QS23, QS15, QF13, and QS16, and is connected to the ground through switches QF21, QS24, QS25, and QF23. The second pole converter P22 includes a converter transformer T22 and a converter valve group V22. The lower part (i.e., the second end) of the converter valve group V22 is connected to the pole 2 line through a switch QS22. The upper part (i.e., the third end) of the converter valve group is connected to the pole 1 line through switches QF22, QS23, QS15, QF13, and QS16, and is connected to the ground through switches QF22, QS24, QS25, and QF23. The first point in the first pole is located between QF11 and QS13, and the first point in the second pole is located between QF21 and QS23.

[0040] The inverting flow station comprises an alternating current system S21, a first-pole converter P31, P32, and a second-pole converter P41, P42. The first-pole converter P31 comprises a transformer T31 and a converter valve group V31, the upper part (i.e. the second end) of the converter valve group V31 is connected to the pole 1 line through a switch QS31; the lower part (i.e. the third end) of the converter valve group is connected to the pole 2 line through switches QF31, QS33, QS35, QS46 on one hand, and connected to the ground line through switches QF31, QS34, QS45 on the other hand. The first-pole converter P32 comprises a transformer T32 and a converter valve group V32, the upper part (i.e. the second end) of the converter valve group V32 is connected to the pole 1 line through a switch QS32; the lower part (i.e. the third end) of the converter valve group is connected to the pole 2 line through switches QF32, QS33, QS35, QS46 on one hand, and connected to the ground line through switches QF32, QS34, QS45 on the other hand. The second-pole converter P41 comprises a transformer T41 and a converter valve group V41, the lower part (i.e. the second end) of the converter valve group V41 is connected to the pole 2 line through a switch QS41; the upper part (i.e. the third end) of the converter valve group is connected to the pole 1 line through switches QF41, QS43, QS35, QS36 on one hand, and connected to the ground line through switches QF41, QS44, QS45 on the other hand. The second-pole converter P42 comprises a transformer T42 and a converter valve group V42, the lower part (i.e. the second end) of the converter valve group V42 is connected to the pole 2 line through a switch QS42; the upper part (i.e. the third end) of the converter valve group is connected to the pole 1 line through switches QF42, QS43, QS35, QS36 on one hand, and connected to the ground line through switches QF42, QS44, QS45 on the other hand. The first point in the first pole is located between QF31 and QS33, and the first point in the second pole is located between QF41 and QS43.

[0041] Further, the inverting flow station can realize parallel connection of the pole converters or single converter operation through control of the switches, and the DC power transmission topology of the embodiment of the present application can realize asymmetric operation of the pole 1 and pole 2 converters. Through control of the switches, the inverting flow station can flexibly switch between multiple operation modes, including but not limited to single-pole single valve group, single-pole double valve group, double-pole single valve group, and double-pole double valve group.

[0042] As one of the optional embodiments, in the double-pole double valve group operation mode:

[0043] Through control of the switches, the second ends of the two converters of the first pole of the inverting flow station are connected to the DC line of the first pole respectively, the second ends of the two converters of the second pole of the inverting flow station are connected to the DC line of the second pole respectively, and the third end of each converter is connected to the ground line.

[0044] Specifically, please refer to Figure 2 , Figure 2is a schematic diagram of a DC power transmission topology in a bipolar operation mode provided by an embodiment of the present application. In which, the left rectifier converter station and the right inverter converter station are both in a bipolar double valve group operation mode, and the switch input conditions are as follows:

[0045] In the rectifier converter station, switches QS11, QS12, QF11, QF12, QS14, QS24, QS21, QS22, QF21, QF22, QS25, and QF23 are closed, switches QS15, QF13, QS16, and QS26 are open, and the converter P11 and P12 of the first pole and the converter P21 and P22 of the second pole are all put into operation.

[0046] In the inverter converter station, switches QS31, QS32, QF31, QF32, QS34, QS44, QS41, QS42, QF41, QF42, and QS45 are closed, switches QS35, QS36, and QS46 are open, and the converter P31 and P32 of the first pole and the converter P41 and P42 of the second pole are all put into operation.

[0047] In which, the DC power of the AC system S11 is transmitted to the AC system S21 through the pole 1 line and the ground loop on one hand, and is transmitted to the AC system S21 through the pole 2 line and the ground loop on the other hand.

[0048] As one of the optional embodiments, in the bipolar single valve group operation mode:

[0049] Through switch control, in the first pole of the converter station, the second end of one converter is connected to the first pole DC line and the third end is connected to the ground wire, and the other converter is out of operation; in the second pole of the converter station, the second end of one converter is connected to the second pole DC line and the third end is connected to the ground wire, and the other converter is out of operation.

[0050] Specifically, please refer to Figure 3 , Figure 3 is another schematic diagram of a DC power transmission topology in a bipolar operation mode provided by an embodiment of the present application. In which, the left rectifier converter station and the right inverter converter station are both in a bipolar single valve group operation mode, and the switch input conditions are as follows:

[0051] In the rectifier converter station, switches QS11, QF11, QS14, QS24, QS21, QF21, QS25, and QF23 are closed, switches QS15, QS12, QF12, QF13, QS16, QS26, QS22, QF22, QS13, and QS23 are open, the converter P11 of the first pole and the converter P21 of the second pole are put into operation, and the converter P12 of the first pole and the converter P22 of the second pole are out of operation.

[0052] In the rectifier-inverter station, switches QS31, QF31, QS34, QS44, QS41, QF41, QS45 are closed, switches QS32, QF32, QS42, QF42, QS35, QS36, QS46, QS33, QS43 are disconnected, the converter P31 of the first pole and the converter P41 of the second pole are put into operation, and the converter P32 of the first pole and the converter P42 of the second pole are taken out of operation.

[0053] In the rectifier-inverter station, switches QS31, QF31, QS34, QS44, QS41, QF41, QS45 are closed, switches QS32, QF32, QS42, QF42, QS35, QS36, QS46, QS33, QS43 are disconnected, the converter P31 of the first pole and the converter P41 of the second pole are put into operation, and the converter P32 of the first pole and the converter P42 of the second pole are taken out of operation.

[0054] As one of the optional embodiments, in the single-pole double-valve group operation mode:

[0055] Through switch control, the second end of the two converters of the first pole of the converter station is connected with the DC line of the corresponding pole, and the third end is connected with the DC line of the other pole through the metal return line; the converters of the other pole of the converter station are all taken out of operation.

[0056] Specifically, please refer to Figure 4 , Figure 4 is a schematic diagram of a single-pole operation mode of a DC power transmission topology provided by the embodiment of the application. The left rectifier-converter station and the right inverter-converter station are both in a single-pole double-valve group operation mode, and the switch input conditions are as follows:

[0057] In the rectifier-converter station, switches QS11, QS12, QF11, QF12, QS13, QS15, QF13, QS26 are closed, switches QS21, QS22, QF21, QF22, QS23, QS24, QS25, QF23, QS16 are disconnected, the converters P11, P12 of the first pole are put into operation, and the converters P21, P22 of the second pole are not operated.

[0058] In the rectifier-inverter station, switches QS31, QS32, QF31, QF32, QS33, QS35, QS46 are closed, switches QS41, QS42, QF41, QF42, QS43, QS44, QS45, QS36 are disconnected, the converters P31, P32 of the first pole are put into operation, and the converters P41, P42 of the second pole are taken out of operation.

[0059] In the rectifier-inverter station, switches QS31, QS32, QF31, QF32, QS33, QS35, QS46 are closed, switches QS41, QS42, QF41, QF42, QS43, QS44, QS45, QS36 are disconnected, the converters P31, P32 of the first pole are put into operation, and the converters P41, P42 of the second pole are taken out of operation.

[0060] As one of the optional embodiments, in the single-pole double-valve group operation mode:

[0061] By switch control, in the first pole of the converter station, the second end of one converter is connected with the first pole DC line, the third end is connected with the DC line of another pole through the metal return line, and the other converter is out of operation; the converters in the other pole of the converter station are all out of operation.

[0062] Specifically, refer to Figure 5 , Figure 5 is another DC power transmission topology structure diagram of the single-pole operation mode provided by the embodiment of the application. The left rectifier converter station is in the single-pole double-valve group operation mode, and the right inverter converter station is in the single-pole single-valve group operation mode, and the switch input conditions are as follows:

[0063] In the rectifier converter station, switches QS11, QS12, QF11, QF12, QS13, QS15, QF13, QS26 are closed, QS21, QS22, QF21, QF22, QS23, QS24, QS25, QF23, QS16 are disconnected, the converters P11, P12 of the first pole are put into operation, and the converters P21, P22 of the second pole are out of operation.

[0064] In the rectifier inverter station, switches QS31, QF31, QS33, QS35, QS46 are closed, QS32, QF32, QS41, QS42, QF41, QF42, QS43, QS44, QS45, QS36 are disconnected, the converter P31 of the first pole is put into operation, the converter P32 of the first pole is out of operation, and the converters P41, P42 of the second pole are out of operation.

[0065] The DC power of the AC system S11 is transmitted to the AC system S21 through the first pole line and the metal return line circuit.

[0066] As one of the optional embodiments, the wiring mode of the converter in the converter station includes: complete double-pole, 1 / 2 double-pole, 3 / 4 double-pole, complete single-pole ground, incomplete single-pole ground, complete single-pole metal, and incomplete single-pole metal.

[0067] Specifically, as shown in Figure 1The operation mode category of each converter station in the shown DC power transmission topology includes bipolar, monopole ground and monopole metal. The bipolar corresponds to the connection mode including complete bipolar, 1 / 2 bipolar and 3 / 4 bipolar. The number of connection modes of complete bipolar is 1, the number of connection modes of 1 / 2 bipolar is 16, and the number of connection modes of 3 / 4 bipolar is 8. The monopole ground corresponds to the connection mode including complete monopole ground and incomplete monopole ground. The number of connection modes of complete monopole ground is 2, and the number of connection modes of incomplete monopole ground is 16. The monopole metal corresponds to the connection mode including complete monopole metal and incomplete monopole metal. The number of connection modes of complete monopole metal is 2, and the number of connection modes of incomplete monopole metal is 16. The total number of connection modes is 61, and different connection modes can be switched flexibly according to actual needs in actual application.

[0068] As one of the optional embodiments, the switch in the converter station adopts an isolating switch or a DC transfer switch.

[0069] Specifically, the isolating switch in the application can also be a DC transfer switch, which has a faster response speed and can realize online conversion of the operation mode.

[0070] Further, the converter can be a voltage source converter, i.e., an IGBT (insulated gate bipolar transistor) based converter IGBT-MMC or an IGCT (integrated gate-commutated thyristor) based converter IGCT-MMC; or a line commutated converter, i.e., a thyristor based converter LCC, an IGCT based converter IGCT-LCC, a multi-source line commutated converter SLCC or a controllable line commutated converter CLCC. The line commutated converter needs to be configured with a DC filter, an AC filter and a reactive power compensation device, and the specific scheme is determined according to the actual engineering. If the converter is designed based on the voltage source converter, the insulated gate bipolar transistor is used to form a modular multilevel structure, the voltage level is matched through transformer voltage regulation, and the real-time switching is satisfied by combining the dynamic response speed to determine the converter adaptation structure. If the converter is designed based on the line commutated converter, the DC filter is configured to smooth fluctuations, the reactive power compensation is configured to stabilize the AC side voltage, and the switching combination control is used to meet the load balancing control demand.

[0071] As one of the optional embodiments, the converter station includes a rectifier side converter station and an inverter side converter station. The rectifier side converter station adopts a fixed DC current control strategy, and the fixed DC current control strategy includes controlling the DC current of the converter to reach a preset target value. In the inverter side converter station, one of the two converters in each pole adopts a fixed DC current control strategy, and the other converter adopts a fixed DC voltage control strategy, and the fixed DC voltage control strategy includes controlling the DC voltage of the converter to reach a preset target value.

[0072] Specifically, when the DC power transmission system is in normal operation, the two rectifier converters adopt the same constant DC control strategy, and the DC current of each converter is controlled to reach the target value through the current regulator. The control strategy adopted by the two inverters is different, one inverter adopts constant DC voltage control strategy, and the other inverter adopts constant DC current control strategy. The constant DC voltage control strategy is similar to the control strategy of the inverter side of the conventional DC project, and the voltage regulator in normal operation plays a role in stabilizing the size of the DC voltage. In a specific example, the constant DC current control strategy controls the DC current of the converter to reach the target value, which is generally half of the pole current command, to achieve the average distribution of the DC current between the two inverters on the inverter side. Through the coordinated control of the regulators of the two converters, the target DC voltage can be achieved, and the long-term safe and reliable operation of the two converters can be ensured through the average distribution of the current.

[0073] Further, in the multi-parallel converter UHV DC power transmission project, each pole is equipped with double 12-pulse converters. In actual operation, double converters can be operated, and single converter can also be operated, and the online conversion of the operation mode can be realized through the online input and output sequence of the converter. The input and output of one converter do not interrupt the normal operation of the other converter, and at the same time, the disturbance caused by the DC power transmission should be as small as possible to avoid excessive impact on the entire power grid.

[0074] Further, in the bipolar power distribution, the two pole DC voltages are distributed to minimize the current of the grounding connection under the condition of meeting the power. In the pole power distribution control, when the parameters of the two converters are the same, the constant current control target of each converter is half of the pole current command; when the parameters of the two converters are different, the proportion can be distributed according to the preset proportion.

[0075] The embodiment of the application improves the transmission capacity by adopting the parallel valve group mode, and at the same time does not change the insulation level; by configuring corresponding switches at both ends of the converter valve group to realize the operation modes of single-pole single-valve group, single-pole double-valve group, double-pole single-valve group and double-pole double-valve group, the reliability and availability of the DC power transmission system are greatly improved, and the flexible configuration of power resources between different power systems can be realized.

[0076] Correspondingly, the application also provides a control method of a DC power transmission topology structure with parallel converters, which is applied to the DC power transmission topology structure with parallel converters, and the control method comprises the following steps:

[0077] Obtaining current load demand and converter capacity information;

[0078] According to the current load demand and the converter capacity information, the operation mode of the converter station is determined, so that the converter station is switched to the corresponding operation mode through switch control;

[0079] Based on the determined operation mode, the current distribution of each converter is optimized for load balancing.

[0080] Further, the operation mode includes a single-pole single-valve group, a single-pole double-valve group, a double-pole single-valve group, and a double-pole double-valve group.

[0081] Specifically, by obtaining the load demand of the rectifier station and the inverter station and the capacity information of each converter, and then comparing the load demand and the corresponding threshold range of the converter capacity, the number of required converter valve groups is determined and mapped to four operation modes, the operation mode of the rectifier station that needs to be switched is determined, based on the determined operation mode, the current distribution of each converter is optimized for load balancing or for minimizing power loss, to obtain the current reference value of each valve group, using a preset algorithm.

[0082] Further, when the load demand exceeds the single-converter capacity, the double-pole operation mode is switched by the transfer switch, and the running state is adjusted in combination with resource allocation optimization; when the load demand decreases to the single-pole operation threshold, part of the converter valve groups can be disconnected by the neutral line isolation, the single-converter operation is switched by the switch control, and the power transmission efficiency is maintained in a preset range to obtain a single-pole stable output.

[0083] Further, when the load fluctuates, the current and voltage data on the DC side are obtained, the support vector machine is used to generate a predicted load fluctuation trend, and the rectifier station operation mode and current distribution result are determined based on the predicted load fluctuation trend, realizing dynamic response and ensuring system stability and improving transmission efficiency.

[0084] The embodiments of the present application dynamically adjust the operation mode according to the load demand, smoothly transition between single-converter and double-pole operation, effectively expand the transmission capacity, and ensure system stability through operation mode switching and current distribution optimization, realizing flexible regulation and optimal operation of the DC power transmission system.

[0085] The above describes the preferred embodiments of the present application, and it should be noted that for those skilled in the art, without departing from the principles of the present application, several improvements and refinements can be made, which are also considered within the protection scope of the present application.

Claims

1. A DC transmission topology with parallel converters, characterized in that, include: At least two converter stations are connected by DC lines; The converter station adopts a bipolar structure, with each pole including two sets of converters connected in parallel. Each converter is composed of a converter transformer and a converter valve group connected in series. In the same pole, the first ends of the two sets of converters are respectively connected to the AC system, the second ends of the two sets of converters are respectively connected to the DC line of the corresponding pole through switches, and the third ends of the two sets of converters are respectively connected to a first point through switches. The first point is connected to the ground wire through a switch, and the first point is also connected to the DC line of the other pole through a switch and a metal return wire. The converter station operates in several modes, including single-pole single-valve group, single-pole double-valve group, double-pole single-valve group, and double-pole double-valve group. These modes are switched by controlling the on / off state of different switches.

2. The DC transmission topology with parallel converters as described in claim 1, characterized in that, The operating mode of the bipolar dual-valve assembly: Through switch control, the second ends of the two converters of the first pole of the converter station are connected to the first pole DC line respectively, the second ends of the two converters of the second pole of the converter station are connected to the second pole DC line respectively, and the third end of each converter is connected to the ground wire.

3. The DC transmission topology with parallel converters as described in claim 1, characterized in that, The operating mode of the bipolar single valve assembly: Through switch control, in the first pole of the converter station, the second terminal of one converter is connected to the first pole DC line and the third terminal is connected to the ground wire, while the other converter is taken out of operation; in the second pole of the converter station, the second terminal of one converter is connected to the second pole DC line and the third terminal is connected to the ground wire, while the other converter is taken out of operation.

4. The DC transmission topology with parallel converters as described in claim 1, characterized in that, The single-stage dual-valve assembly operates as follows: Through switch control, the second ends of the two converters on one pole of the converter station are connected to the DC line of the corresponding pole, and the third ends are connected to the DC line of the other pole through the metal return line; the converters on the other pole of the converter station are all out of operation.

5. The DC transmission topology with parallel converters as described in claim 1, characterized in that, The single-stage single-valve assembly operates in the following mode: Through switch control, in the first pole of the converter station, the second end of one converter is connected to the DC line of the first pole, and the third end is connected to the DC line of the other pole through a metal return line, while the other converter is taken out of operation; all converters in the other pole of the converter station are taken out of operation.

6. The DC transmission topology with parallel converters as described in claim 1, characterized in that, The converter station includes a rectifier-side converter station and an inverter-side converter station; the rectifier-side converter station adopts a constant DC current control strategy, which includes controlling the DC current of the converter to reach a preset target value; in the inverter-side converter station, one of the two converters in each pole adopts a constant DC current control strategy, and the other converter adopts a constant DC voltage control strategy, which includes controlling the DC voltage of the converter to reach a preset target value.

7. The DC transmission topology with parallel converters as described in claim 1, characterized in that, The wiring methods of the converters in the converter station include: complete bipolar, 1 / 2 bipolar, 3 / 4 bipolar, complete monopolar, incomplete monopolar, complete monopolar metal, and incomplete monopolar metal.

8. The DC transmission topology with parallel converters as described in claim 1, characterized in that, The switches in the converter station are either disconnect switches or DC-DC conversion switches.

9. A control method for a DC transmission topology with parallel converters, characterized in that, The control method, applied to the DC transmission topology with parallel converters as described in any one of claims 1 to 8, comprises: Obtain current load demand and converter capacity information; Based on the current load demand and the converter capacity information, the operating mode of the converter station is determined, and the converter station is switched to the corresponding operating mode through switch control. Based on the determined operating mode, the current distribution of each converter is optimized with load balancing as the goal.

10. The control method for a DC transmission topology with parallel converters as described in claim 9, characterized in that, The operating modes include single-stage single-valve group, single-stage double-valve group, double-stage single-valve group and double-stage double-valve group.

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