Multi-energy true bipolar direct current collection system and direct current transmission system

Abstract

This invention discloses a multi-energy true bipolar DC collection system and a DC transmission system, relating to the field of high-voltage DC transmission. In this collection system, the DRU module is used to realize the main rectified output of active power from offshore wind power; the first MMC module is used to realize the secondary rectified output of active power from offshore wind power, and the first MMC module adopts grid-type control to provide AC voltage and frequency support at sea; the receiving-end converter module absorbs the power collected by the true bipolar DC bus and outputs power to different electrical loads when the multi-energy true bipolar DC collection system is operating stably. In this scheme, the DRU module undertakes the main rectified output of active power, while the first MMC module, as an auxiliary rectified output, only bears a portion of the DC voltage. Furthermore, the first MMC module can also provide AC voltage and frequency support at sea, enabling this application to maintain the controllability of offshore wind power DC transmission while reducing the size, weight, and cost of the offshore platform.

Description

Technical Field

[0001] This invention relates to the field of high voltage direct current transmission technology, and in particular to a multi-energy true bipolar DC collection system and a DC transmission system. Background Technology

[0002] As offshore wind power develops towards deeper waters, larger capacity, and longer transmission distances, the transmission distance between offshore wind farms and onshore load centers is constantly increasing. Although traditional power frequency AC transmission methods are mature in structure and relatively simple in control, in long-distance submarine cable transmission scenarios, the reactive power of submarine cable charging increases significantly, occupying transmission capacity and providing reactive power compensation requirements. When the capacity of offshore wind farms further increases, the transmission distance is further extended, or offshore wind power needs to be transmitted together with other clean energy sources, high voltage direct current transmission systems have greater advantages in terms of long-distance transmission capacity, transmission corridor utilization, and multi-terminal interconnection capabilities.

[0003] Under offshore wind power DC transmission technology, flexible DC transmission systems based on MMC (Modular Multilevel Converter) can independently control active and reactive power, and are capable of supporting weak AC systems and passive AC systems, making them suitable for offshore wind power grid connection. However, for large-capacity offshore wind farms in deep waters, if a full-capacity offshore MMC converter station is used, a large number of sub-modules, capacitors, bridge arm reactors, valve control systems, cooling systems, and offshore platform auxiliary equipment are required, resulting in large size and weight of the offshore converter platform, high cost, and high operation and maintenance difficulty.

[0004] Therefore, how to provide a more effective DC transmission solution for offshore wind power is an urgent problem to be solved. Summary of the Invention

[0005] In view of this, the present invention provides a multi-energy true bipolar DC collection system and DC transmission system, which can reduce the size, weight and cost of offshore platforms while maintaining the controllability of offshore wind power DC transmission.

[0006] To address the aforementioned technical problems, this application provides a multi-energy true bipolar DC collection system, including an offshore wind power sending-end converter module, a true bipolar DC bus, and a receiving-end converter module; the offshore wind power sending-end converter module includes a DRU module, a first MMC module, and a polarity interface conversion device; The AC input terminal of the DRU module is connected to the AC output terminal of the offshore wind farm. The positive DC output terminal is connected to the positive bus of the true bipolar DC bus through the polarity interface conversion device. The negative DC output terminal is connected to the positive DC output terminal of the first MMC module. It is used to adjust the output according to the preset DC voltage distribution relationship to realize the main rectified output of the active power of the offshore wind power. The AC input terminal of the first MMC module is connected to the AC output terminal of the offshore wind farm, and the DC output negative terminal is connected to the negative bus of the true bipolar DC bus through the polarity interface conversion device. This is used to adjust the output according to the preset DC voltage distribution relationship, so as to realize the rectified output of the active power of the offshore wind power. The first MMC module adopts grid-type control to provide offshore AC voltage and frequency support. The polarity interface conversion device is used for polarity conversion to obtain a true bipolar DC topology structure composed of the DRU module, the first MMC module and the polarity interface conversion device connected to the true bipolar DC bus. The input terminal of the receiving-end converter module is connected to the true bipolar DC bus, and the output terminal is connected to the electrical load. It is used to absorb the power collected by the true bipolar DC bus and output power to different electrical loads when the multi-energy true bipolar DC collection system is running stably.

[0007] Furthermore, the multi-energy true bipolar DC collection system also includes a coastal energy converter module; the coastal energy converter module includes a nuclear power sending-end converter module, a photovoltaic sending-end converter module, and a pumped storage sending-end converter module; The nuclear power unit is connected to the true bipolar DC bus through the nuclear power transmission end converter module. The nuclear power transmission end converter module is used to send the output power of the nuclear power unit into the true bipolar DC bus in a stable power transmission mode. The photovoltaic power generation unit is connected to the true bipolar DC bus through the photovoltaic sending-end converter module. The photovoltaic sending-end converter module is used to send the output power of the photovoltaic power generation unit into the true bipolar DC bus. The pumped storage module is connected to the true bipolar DC bus through the pumped storage sending-end converter module, and the pumped storage sending-end converter module is used to send the output power of the pumped storage module into the true bipolar DC bus.

[0008] Furthermore, the nuclear power transmission-end converter module includes a first LCC module and a second LCC module; The AC input terminal of the first LCC module is connected to the AC output terminal of the nuclear power unit, the DC output positive terminal is connected to the positive bus, and the DC output negative terminal is connected to the DC output positive terminal of the second LCC module, with the common terminal of the connection grounded. The AC input terminal of the second LCC module is connected to the AC output terminal of the nuclear power unit, and the negative DC output terminal is connected to the negative bus.

[0009] Furthermore, the photovoltaic sending-end converter module includes a second MMC module and a third MMC module; The AC input terminal of the second MMC module is connected to the AC output terminal of the photovoltaic power generation unit, the DC output positive terminal is connected to the positive bus, and the DC output negative terminal is connected to the DC output positive terminal of the third MMC module and the common terminal of the connection is grounded. The AC input terminal of the third MMC module is connected to the AC output terminal of the photovoltaic power generation unit, and the DC output negative terminal is connected to the negative bus.

[0010] Furthermore, the pumped storage power transmission converter module includes a third LCC module, a fourth LCC module, a fourth MMC module, and a fifth MMC module; The AC input terminal of the third LCC module is connected to the AC output terminal of the pumped storage module, the DC output positive terminal is connected to the positive bus, and the DC output negative terminal is connected to the DC output positive terminal of the fourth MMC module. The AC input terminal of the fourth MMC module is connected to the AC output terminal of the pumped storage module, and the DC output negative terminal is connected to the DC output positive terminal of the fifth MMC module, with the common terminal of the connection grounded. The AC input terminal of the fifth MMC module is connected to the AC output terminal of the pumped storage module, and the negative DC output terminal is connected to the positive DC output terminal of the fourth LCC module. The AC input terminal of the fourth LCC module is connected to the AC output terminal of the pumped storage module, and the negative DC output terminal is connected to the negative bus.

[0011] Furthermore, the receiving-end converter module is also used to adjust its absorbed power according to the system power balance relationship, the target control voltage of the true bipolar DC bus, and the power fluctuation of the disturbed branch when the multi-energy true bipolar DC collection system is under disturbance conditions, so as to maintain the coordination of the positive and negative voltages of the true bipolar DC bus.

[0012] Furthermore, the receiving-end converter module includes at least two receiving-end MMC converter modules; The receiving-end MMC converter module is connected in parallel to the true bipolar DC bus, and the output terminal of each receiving-end MMC converter module is connected to the corresponding electrical load.

[0013] Furthermore, the receiving-end MMC converter module includes a sixth MMC module and a seventh MMC module; The positive DC input terminal of the sixth MMC module is connected to the positive bus, the negative DC input terminal is connected to the positive DC input terminal of the seventh MMC module and the common terminal of the connection is grounded, and the AC output terminal is connected to the corresponding electrical load. The negative DC input terminal of the seventh MMC module is connected to the negative bus, and the AC output terminal is connected to the electrical load.

[0014] Furthermore, the multi-energy true bipolar DC collection system also includes a DC circuit breaker; The true bipolar DC bus is connected to the receiving-end converter module through the DC circuit breaker.

[0015] To address the aforementioned technical problems, the present invention also provides a DC power transmission system, including the multi-energy true bipolar DC collection system as described above.

[0016] The beneficial effects of this invention are as follows: This application provides a multi-energy true bipolar DC collection system and a DC transmission system. The collection system includes an offshore wind power sending-end converter module, a true bipolar DC bus, and a receiving-end converter module. The offshore wind power sending-end converter module includes a DRU module, a first MMC module, and a polarity interface conversion device. The AC input terminal of the DRU module is connected to the AC output terminal of the offshore wind farm, and the DC output positive terminal is connected to the positive bus of the true bipolar DC bus through the polarity interface conversion device. The DC output negative terminal is connected to the DC output positive terminal of the first MMC module, which is used to adjust the output according to a preset DC voltage distribution relationship to realize the main rectified output of the active power of the offshore wind power. The AC input terminal of the first MMC module is connected to the AC output terminal of the offshore wind farm, and the DC output positive terminal is connected to the positive bus of the true bipolar DC bus. The negative terminal of the current output is connected to the negative bus of the true bipolar DC bus through a polarity interface conversion device. This device is used to regulate the output according to the preset DC voltage distribution relationship, so as to realize the active power output of offshore wind power from the rectifier. The first MMC module adopts a grid-type control to provide AC voltage and frequency support for the offshore area. The polarity interface conversion device is used for polarity conversion to obtain a true bipolar DC topology structure composed of the DRU module, the first MMC module and the polarity interface conversion device connected to the true bipolar DC bus. The input terminal of the receiving end converter module is connected to the true bipolar DC bus and the output terminal is connected to the electrical load. This module is used to absorb the power collected by the true bipolar DC bus and output power to different electrical loads when the multi-energy true bipolar DC collection system is running stably. As can be seen, this scheme adopts a DRU-MMC series hybrid topology for the power output of offshore wind farms. The DRU module is responsible for the main rectified output of active power, while the first MMC module serves as the auxiliary rectified output, bearing only a portion of the DC voltage. Furthermore, the first MMC module can also provide offshore AC voltage and frequency support, enabling this application to maintain the controllability of offshore wind power DC transmission while reducing the size, weight, and cost of the offshore platform, which is beneficial for practical applications.

[0017] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application, it can be implemented according to the contents of the specification. In order to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0018] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 A schematic diagram of the structure of a multi-energy true bipolar DC collection system provided by the present invention; Figure 2 A schematic diagram of another multi-energy true bipolar DC collection system provided by the present invention; Figure 3 A schematic diagram showing the changes in DC voltage of the DRU module and the first MMC module under the condition of a step change in wind speed in offshore wind power provided by the present invention. Figure 4 A schematic diagram of the power change of the DC transmission branch of offshore wind power under the condition of step change in offshore wind speed, provided by the present invention. Figure 5 This invention provides a schematic diagram of voltage variation of a true bipolar DC bus under the condition of step change in wind speed in offshore wind power. Figure 6 This invention provides a schematic diagram of power changes in a nuclear power DC access branch, a photovoltaic DC access branch, and a pumped storage DC access branch under the condition of a step change in wind speed in offshore wind power. Figure 7 This invention provides a schematic diagram of power sharing between two receiving-end MMC converter modules in a receiving-end converter module under the condition of step change in wind speed in offshore wind power. Detailed Implementation

[0019] The core of this invention is to provide a multi-energy true bipolar DC collection system and DC transmission system, which can reduce the size, weight and cost of offshore platforms while maintaining the controllability of offshore wind power DC transmission.

[0020] The technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application are within the scope of protection of this application.

[0021] The terms "first," "second," etc., used in this application are used to distinguish similar objects and not to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of this application can be implemented in orders other than those illustrated or described herein, and the objects distinguished by "first," "second," etc., are generally of the same class, without limiting the number of objects; for example, a first object can be one or more. Furthermore, in this application, "and / or" indicates at least one of the connected objects, and the character " / " generally indicates that the preceding and following objects have an "or" relationship.

[0022] Please refer to Figure 1 , Figure 1 This is a schematic diagram of a multi-energy true bipolar DC collection system provided by the present invention.

[0023] The multi-energy true bipolar DC collection system includes an offshore wind power sending-end converter module 1, a true bipolar DC bus 2, and a receiving-end converter module 3; the offshore wind power sending-end converter module 1 includes a DRU module 11, a first MMC module 12, and a polarity interface conversion device 13. The AC input terminal of the DRU module 11 is connected to the AC output terminal of the offshore wind farm. The positive DC output terminal is connected to the positive bus of the true bipolar DC bus 2 through the polarity interface conversion device 13. The negative DC output terminal is connected to the positive DC output terminal of the first MMC module 12. It is used to adjust the output according to the preset DC voltage distribution relationship to realize the main rectified output of the active power of the offshore wind power. The AC input terminal of the first MMC module 12 is connected to the AC output terminal of the offshore wind farm, and the DC output negative terminal is connected to the negative bus of the true bipolar DC bus 2 through the polarity interface conversion device 13. This is used to adjust the output according to the preset DC voltage distribution relationship, so as to realize the active power output of offshore wind power from the rectifier. The first MMC module 12 adopts grid-type control to provide offshore AC voltage and frequency support. The polarity interface conversion device 13 is used for polarity conversion to obtain a true bipolar DC topology structure composed of DRU module 11, first MMC module 12 and polarity interface conversion device 13 connected to true bipolar DC bus 2. The input terminal of the receiving-end converter module 3 is connected to the true bipolar DC bus 2, and the output terminal is connected to the electrical load. It is used to absorb the power collected by the true bipolar DC bus 2 and output power to different electrical loads when the multi-energy true bipolar DC collection system is running stably.

[0024] In this embodiment, to address the issue of large-scale transmission platforms for deep-sea offshore wind farms in existing technologies, and considering the advantages of the DRU module 11 (Diode Rectifier Unit) such as simple structure, few components, low loss, and high reliability, it is suitable for lightweight DC transmission of offshore wind power. However, the DRU module 11 is an uncontrollable rectifier device that cannot independently establish offshore AC voltage and frequency, nor can it provide black start and dynamic support capabilities. Therefore, if the DRU module 11 is used alone as the DC transmission interface for offshore wind power, the voltage and frequency support capabilities and dynamic adaptability of the collection system are insufficient when offshore wind power output fluctuates, AC voltage disturbances occur, or DC-side power changes. To address this, a DRU-MMC hybrid converter topology is adopted, which can retain the lightweight advantages of the DRU module 11 while introducing the active control capabilities of the first MMC module 12.

[0025] Specifically, the offshore wind power sending-end converter module 1 includes a DRU module 11, a first MMC module 12, and a polarity interface conversion device 13. The AC sides of the DRU module 11 and the first MMC module 12 are used to connect to the AC output terminal of the offshore wind farm. The DC sides of the DRU module 11 and the first MMC module 12 are connected in series and then connected to the true bipolar DC bus 2 after passing through the polarity interface conversion device 13. The positive input terminal of the receiving-end converter module 3 is connected to the positive bus of the true bipolar DC bus 2, and the negative input terminal is connected to the negative bus. The three-phase AC output terminal is used to match and connect with the three-phase power input terminal of the electrical load. The electrical load can be various receiving-end AC systems or load centers, and the number of electrical loads can be one or more, without any particular limitation.

[0026] The offshore wind farm here may include multiple wind turbine generators, each of which is connected to the offshore AC collection bus via an offshore AC collection line; the AC input terminals of DRU module 11 and the first MMC module 12 are both connected to the offshore AC collection bus; it can be understood that the AC input terminals of DRU module 11 and the first MMC module 12 are essentially three-phase AC input terminals, and the two are respectively connected to two mutually isolated three-phase branches through independent Y / Δ isolation transformers, and can be connected to the three-phase lines of the offshore wind power AC collection bus in a one-to-one correspondence according to the A / B / C three-phase sequence.

[0027] Please refer to Figure 2 , Figure 2This is a schematic diagram of another multi-energy true bipolar DC collection system provided by the present invention; the positive DC output terminal of the DRU module 11 is connected to the positive input terminal of the polarity interface conversion device 13 via a submarine cable, and the positive output terminal of the polarity interface conversion device 13 is connected to the positive bus; the negative DC output terminal of the first MMC module 12 is connected to the negative input terminal of the polarity interface conversion device 13 via a submarine cable, and the negative output terminal of the polarity interface conversion device 13 is connected to the negative bus. It should be noted that... Figure 2 Due to limitations in the focus of the images and to avoid confusing the wiring, the specific structure of the true bipolar DC bus will not be elaborated here.

[0028] The reason for setting up the polarity interface conversion device 13 is that the DRU module 11 and the first MMC module 12 essentially constitute a local pseudo-bipolar DC topology. This pseudo-bipolar DC topology is not compatible with the back-end true bipolar DC bus 2 in terms of polarity, voltage level, and access boundary. Therefore, the polarity interface conversion device 13 is set up to perform polarity conversion, so that the DRU module 11, the first MMC module 12, and the polarity interface conversion device 13 constitute a true bipolar DC topology. This true bipolar DC topology can be matched with the true bipolar DC bus 2 in terms of polarity, voltage level, and access boundary. Then, relying on this topology, the power of the offshore wind farm can be reliably sent to the true bipolar DC bus 2.

[0029] Furthermore, in some embodiments, the multi-energy true bipolar DC collection system also includes a coastal energy converter module; the coastal energy converter module includes a nuclear power sending-end converter module 4, a photovoltaic sending-end converter module 5, and a pumped storage sending-end converter module 6; The nuclear power unit is connected to the true bipolar DC bus 2 through the nuclear power sending end converter module 4. The nuclear power sending end converter module 4 is used to send the output power of the nuclear power unit into the true bipolar DC bus 2 in a stable power output manner. The photovoltaic power generation unit is connected to the true bipolar DC bus 2 through the photovoltaic sending-end converter module 5. The photovoltaic sending-end converter module 5 is used to send the output power of the photovoltaic power generation unit into the true bipolar DC bus 2. The pumped storage module is connected to the true bipolar DC bus 2 through the pumped storage sending-end converter module 6. The pumped storage sending-end converter module 6 is used to send the output power of the pumped storage module into the true bipolar DC bus 2.

[0030] Specifically, considering that the transmission demand in practical applications is not limited to offshore wind farms, nuclear power, onshore photovoltaics, and pumped storage power sources may also be jointly developed and centrally transmitted with offshore wind farms, existing solutions usually build complete and independent DC transmission channels for each of the above-mentioned power sources. That is, offshore wind power, nuclear power, onshore photovoltaics, and pumped storage will each be equipped with independent sending-end converter units, independent DC transmission lines, and independent receiving-end converter facilities. This can easily lead to the duplication of resources in transmission corridors, landing points, and receiving-end converter stations, which is not conducive to the intensive development of clean energy bases and grid absorption. In this application, the above-mentioned configuration enables the multi-energy true bipolar DC collection system to be used in scenarios involving the coordinated transmission of offshore wind power and coastal clean energy. Offshore wind power, nuclear power, onshore photovoltaic power, and pumped storage are connected to the same true bipolar DC bus 2 through differentiated sending-end converter modules. In other words, only the sending-end access branch of the bus needs to be configured for each type of power source. Power collection is achieved by relying on the common true bipolar DC bus 2, and DC power absorption and distribution are carried out through the shared receiving-end converter module 3. This reduces the duplication of independent transmission channels and improves the utilization rate of the transmission corridor.

[0031] Understandably, nuclear power, with its large capacity, stable output, and high utilization hours, is connected to the true bipolar DC bus 2 as a stable power transmission resource. Onshore photovoltaic (i.e., photovoltaic power generation units) is characterized by its output being affected by irradiance and weather conditions, rapid power fluctuations, and fast power electronic interface response speed; it is connected to the true bipolar DC bus 2 as a fast and controllable new energy source. Pumped storage, with its power generation, pumping, and phase regulation operation modes, is connected to the true bipolar DC bus 2 as a system support and dynamic adjustment resource. The topology of each module under the coastal energy converter module is set based on the operating characteristics of the corresponding coastal clean energy. Here, the nuclear power units and nuclear power transmission-end converters... Module 4 constitutes the nuclear power DC access branch; the photovoltaic power generation unit (i.e., the onshore photovoltaic power generation unit) and photovoltaic sending-end converter module 5 constitute the photovoltaic DC access branch; the pumped storage module and pumped storage sending-end converter module 6 constitute the pumped storage DC access branch; and the offshore wind farm and offshore wind power sending-end converter module 1 constitute the offshore wind power DC transmission branch. The true bipolar DC bus 2 can collect the DC power from the above four branches at the same DC voltage level. Therefore, the true bipolar DC bus 2 provides a unified DC access platform for multiple types of power sources, which is beneficial for the centralized collection of multiple energy sources, the intensive utilization of transmission corridors, and the unified distribution of receiving-end power. For example, the inter-pole voltage of the true bipolar DC bus 2 can be 1600 kV, where the voltage of the positive bus is +800 kV and the voltage of the negative bus is -800 kV.

[0032] From the perspective of implementation principle, assuming that the DC voltage of the unit composed of DRU module 11 and first MMC module 12 is U dcrThe DC voltage of DRU module 11 is U DRU The DC voltage of the first MMC module 12 is U MMC The DC current of the offshore wind power DC transmission branch is I. dc P DRU The active power transmitted by DRU module 11, P MMC For the active power transmitted by the first MMC module 12, then we have , , Since the DRU module 11 and the first MMC module 12 are connected in series on the DC side, they both carry the same DC current, which is I. dc The total power of the unit composed of DRU module 11 and the first MMC module 12 is the sum of the power undertaken by DRU module 11 and the power undertaken by the first MMC module 12. The power sharing between DRU module 11 and the first MMC module 12 is mainly determined by their DC voltage division relationship. Therefore, the preset DC voltage distribution relationship mentioned here pre-sets that the DC output voltage ratio of DRU module 11 is higher than that of the first MMC module 12, so that DRU module 11 undertakes the main rectification and active power transmission tasks of offshore wind power, and the first MMC module 12 undertakes the secondary rectification and active power transmission tasks of offshore wind power. That is, the DC voltage U undertaken by the first MMC module 12 is... MMC Below U dcr Therefore, the number of sub-modules required for the first MMC module 12 can be reduced, thus decreasing the size of the offshore platform. In one specific embodiment, the rated power of the offshore wind power DC transmission branch is 2000MW, the rated power of the DRU module 11 is 1500MW, the rated DC voltage of the DRU module 11 is 750 kV, the rated power of the first MMC module 12 is 500MW, and the rated DC voltage of the first MMC module 12 is 250 kV. In this configuration, the DRU module 11 undertakes the main rectification voltage and main active power transmission tasks, while the first MMC module 12 undertakes the tasks of lower DC voltage and dynamic support. This configuration can provide the necessary offshore AC system support for the rectification operation of the DRU module 11 while reducing the size of the offshore platform equipment.

[0033] Furthermore, the first MMC module 12 adopts a grid-type control method, thus providing voltage and frequency support for the offshore AC bus, solving the problem that the DRU module 11 alone cannot actively establish AC voltage and frequency. Moreover, based on the inherent functions of the MMC module, the first MMC module 12 can also provide black start, dynamic reactive power support, and power fluctuation adaptability. Specifically, black start refers to the unidirectional conductivity of the DRU module 11; the establishment of the offshore AC voltage requires the MMC module in the onshore receiving-end converter module 3 to send power back to the first MMC module 12 at sea for pre-charging, and then the first MMC module 12 establishes the offshore AC voltage to achieve black start. Dynamic reactive power support refers to the first MMC module 12 injecting or absorbing reactive power by adjusting the reactive component of its own AC side current. Power fluctuation adaptability refers to the ability of the first MMC module 12 to temporarily absorb or release power using the energy of the capacitors in its sub-modules under disturbance conditions such as wind speed jumps, buffering sudden changes in wind turbine power and helping to maintain the amplitude and frequency of the offshore bus voltage.

[0034] When the collection system is running stably, each sending branch, namely the offshore wind power DC transmission branch, the nuclear power DC access branch, the onshore photovoltaic DC access branch, and the pumped storage DC access branch, injects power into the true bipolar DC bus 2 according to its own power command. The receiving-end converter module 3 is used to absorb the power collected by the true bipolar DC bus 2 and output power to different electrical loads. Specifically, the receiving-end converter module 3 essentially uses the system power balance relationship as a control constraint to dynamically adjust its absorbed power in order to maintain the coordination of the positive and negative voltages of the true bipolar DC bus 2, so that the positive and negative buses are kept near the corresponding target control voltages, and then output power to different electrical loads.

[0035] In some embodiments, the receiving-end converter module 3 is further configured to adjust its absorbed power under disturbance conditions of the multi-energy true bipolar DC-DC converter system, based on the system power balance relationship, the target control voltage of the true bipolar DC bus 2, and the power fluctuation of the disturbed branch, in order to maintain the coordination of the positive and negative voltages of the true bipolar DC bus 2. The disturbed branch refers to any one or more branches among the aforementioned sending-end branches that experience power disturbances. Corresponding to the power fluctuation, the sending-end branches that are not disturbed maintain continuous operation, and the MMC modules on the disturbed sending-end branches can perform adaptive power fluctuation adjustment. Therefore, by adjusting the absorbed power of the receiving-end converter module 3, in conjunction with the coordinated operation of each sending-end branch, it is beneficial to achieve stable operation of the multi-energy true bipolar DC-DC converter system under disturbance conditions, especially under typical disturbance conditions.

[0036] In detail, the total injected power at the sending end, the total absorbed power at the receiving end, the system losses, and the equivalent electromagnetic energy storage of the DC system (formed by the true bipolar DC bus 2, the capacitance of each sending-end converter module, and the distributed capacitance of the line, rather than an additional energy storage device) satisfy the following power balance relationship:

[0037] Among them, C eq U is the equivalent capacitance of the DC system. dc The voltage at true bipolar DC bus 2; P in Total injected power at the sending end, and P in It consists of the injected power from the offshore wind power DC transmission branch, the nuclear power DC access branch, the onshore photovoltaic DC access branch, and the pumped storage DC access branch; P out P represents the total absorbed power of the receiving end converter module 3 (i.e., composed of the absorbed power of each receiving end MMC converter module under the receiving end converter module 3). loss Because of system losses, the system power balance relationship here refers to the sum of the total injected power at the sending end, the total absorbed power at the receiving end, system losses, and the rate of change of the equivalent electromagnetic energy storage of the DC system. When the total injected power at the sending end is greater than the sum of the total absorbed power at the receiving end and system losses, the equivalent electromagnetic energy storage of the DC system increases, and the voltage of the true bipolar DC bus 2 shows an upward trend; when the total injected power at the sending end is less than the sum of the total absorbed power at the receiving end and system losses, the equivalent electromagnetic energy storage of the DC system decreases, and the voltage of the true bipolar DC bus 2 shows a downward trend. Thus, the multi-energy true bipolar DC collection system achieves positive and negative voltage coordination and stable operation under multi-energy DC collection conditions through the power absorption of the receiving end converter module 3 and the continuous operation of each sending end branch.

[0038] In summary, this application provides a multi-energy true bipolar DC collection system. For the power output of offshore wind farms, a DRU-MMC series hybrid topology is adopted. The DRU module 11 is responsible for the main rectification output of active power, while the first MMC module 12 serves as the auxiliary rectification output, bearing only a portion of the DC voltage. Furthermore, the first MMC module 12 can also provide offshore AC voltage and frequency support, enabling this application to maintain the controllability of offshore wind power DC transmission while reducing the size, weight, and cost of the offshore platform. It also configures differentiated sending-end converter modules according to the different power characteristics of nuclear power, onshore photovoltaics, and pumped storage, realizing the functional division and centralized transmission of multiple types of clean energy on the same true bipolar DC bus 2.

[0039] Based on the above embodiments: In some embodiments, the nuclear power transmission end converter module 4 includes a first LCC module 41 and a second LCC module 42; The AC input terminal of the first LCC module 41 is connected to the AC output terminal of the nuclear power unit, the positive DC output terminal is connected to the positive bus, and the negative DC output terminal is connected to the positive DC output terminal of the second LCC module 42, with the common terminal of the connection grounded. The AC input terminal of the second LCC module 42 is connected to the AC output terminal of the nuclear power unit, and the negative DC output terminal is connected to the negative bus.

[0040] Specifically, such as Figure 2 As shown, to adapt to the operating characteristics of nuclear power units, which have large capacity, stable output, and are not suitable for frequent and rapid deep adjustments, the nuclear power sending-end converter module 4 includes two LCC modules (Line Commutated Converters). This allows the output power of the nuclear power unit to be sent to the true bipolar DC bus 2 in a stable power delivery mode. Here, stable power delivery mode refers to continuously sending the constant base load active power of the nuclear power unit to the true bipolar DC bus 2 with minimal fluctuations, so as to maintain the total power sent to the DC bus stable under various operating conditions and not trigger large and rapid power changes of the nuclear power unit, thus adapting to the operating characteristics of nuclear power that cannot be deeply peak-shaving.

[0041] It should also be noted that the AC input terminals of the first LCC module 41 and the second LCC module 42 are essentially three-phase AC input terminals. They are formed into two mutually isolated three-phase branches through independent Y / Δ isolation transformers, and can be connected to the three-phase AC output line of the nuclear power unit according to the A / B / C three-phase sequence.

[0042] In some embodiments, the photovoltaic sending-end converter module 5 includes a second MMC module 51 and a third MMC module 52; The AC input terminal of the second MMC module 51 is connected to the AC output terminal of the photovoltaic power generation unit, the DC output positive terminal is connected to the positive bus, and the DC output negative terminal is connected to the DC output positive terminal of the third MMC module 52, and the common terminal of the connection is grounded. The AC input terminal of the third MMC module 52 is connected to the AC output terminal of the photovoltaic power generation unit, and the negative DC output terminal is connected to the negative bus.

[0043] Specifically, such as Figure 2 As shown, the photovoltaic power generation unit injects photovoltaic power into the true bipolar DC bus 2 through the second MMC module 51 and the third MMC module 52; in addition, the MMC module in the photovoltaic sending-end converter module 5 can also realize active power and reactive power regulation, so that the branch can adapt to the characteristics of the output power of the onshore photovoltaic power generation unit changing due to the influence of irradiance and meteorological conditions.

[0044] It should also be noted that the AC input terminals of the second MMC module 51 and the third MMC module 52 are essentially three-phase AC input terminals. They are respectively connected to the three-phase AC output lines of the photovoltaic power generation unit through independent Y / Δ isolation transformers.

[0045] In some embodiments, the pumped storage power transmission converter module 6 includes a third LCC module 61, a fourth LCC module 62, a fourth MMC module 63, and a fifth MMC module 64; The AC input terminal of the third LCC module 61 is connected to the AC output terminal of the pumped storage module, the positive DC output terminal is connected to the positive bus, and the negative DC output terminal is connected to the positive DC output terminal of the fourth MMC module 63. The AC input terminal of the fourth MMC module 63 is connected to the AC output terminal of the pumped storage module, and the negative DC output terminal is connected to the positive DC output terminal of the fifth MMC module 64, with the common terminal of the connection grounded. The AC input terminal of the fifth MMC module 64 is connected to the AC output terminal of the pumped storage module, and the negative DC output terminal is connected to the positive DC output terminal of the fourth LCC module 62. The AC input terminal of the fourth LCC module 62 is connected to the AC output terminal of the pumped storage module, and the negative DC output terminal is connected to the negative bus.

[0046] Specifically, such as Figure 2 As shown, the LCC module in the pumped storage power transmission converter module 6 is used for high-capacity power transmission, and the MMC module is used for dynamic adjustment and reactive power support. This configuration is conducive to balancing the high-capacity, low-loss power transmission capacity of the LCC module with the flexible adjustment capability of the MMC module.

[0047] In some embodiments, the receiving-end converter module 3 includes at least two receiving-end MMC converter modules; The receiving-end MMC converter modules are connected in parallel to the true bipolar DC bus 2, and the output terminal of each receiving-end MMC converter module is connected to the corresponding electrical load.

[0048] Specifically, different receiving-end MMC converter modules are connected to different electrical loads; such as Figure 2 As shown, the circuit connection is illustrated using two receiving-end MMC converter modules (i.e., the first receiving-end MMC converter module 31 and the second receiving-end MMC converter module 32) as an example.

[0049] More specifically, in some embodiments, the receiving-end MMC converter module includes a sixth MMC module 71 and a seventh MMC module 72; The positive DC input terminal of the sixth MMC module 71 is connected to the positive bus, the negative DC input terminal is connected to the positive DC input terminal of the seventh MMC module 72 and the common terminal of the connection is grounded, and the AC output terminal is connected to the corresponding electrical load. The negative DC input terminal of the seventh MMC module 72 is connected to the negative bus, and the AC output terminal is connected to the electrical load.

[0050] As can be seen, the above settings facilitate the reliable absorption and distribution of the collected power of the true bipolar DC bus 2, improve the flexibility of power absorption at the receiving end and the operational redundancy, and enable different electrical loads to share the collected power.

[0051] In some embodiments, the multi-energy true bipolar DC collection system further includes a DC circuit breaker 8; The true bipolar DC bus 2 is connected to the receiving-end converter module 3 through the DC circuit breaker 8.

[0052] Specifically, the DC circuit breaker 8 can be installed at a key aggregation node between the true bipolar DC bus 2 and the receiving-end converter module 3, for example, such as... Figure 2 As shown, the true bipolar DC bus 2 is connected to the receiving-end converter module 3 via a DC circuit breaker 8 and an overhead line; alternatively, DC circuit breakers can be installed at the connection points of each DC access branch (i.e., nuclear power DC access branch, photovoltaic DC access branch, pumped storage DC access branch) to the true bipolar DC bus 2, without any particular limitation here; the DC circuit breaker in this application is used to limit current, interrupt and selectively isolate the faulty branch when a fault occurs in the DC line or DC access branch, so as to maintain the power transmission capacity of the non-faulty branch, reduce the impact of local DC faults on the non-faulty DC access branch, and improve the continuous operation capability of the multi-energy DC collection system.

[0053] To verify the operational adaptability of the multi-energy true bipolar DC collection system described in this invention under new energy power fluctuation conditions, corresponding to Figure 2 The system structure shown here is illustrated using the step change of wind speed in offshore wind power as a representative simulation condition.

[0054] In the wind speed step simulation, the system first enters steady-state operation. The offshore wind power DC transmission branch, the nuclear power DC access branch, the onshore photovoltaic DC access branch, and the pumped storage DC access branch inject power into the true bipolar DC bus 2, and the receiving-end converter module 3 absorbs the collected power of the DC bus. During steady-state operation, the inter-pole voltage of the true bipolar DC bus 2 stabilizes near the rated value, and the positive and negative buses stabilize near their respective rated voltages. The offshore wind power DC transmission branch, the nuclear power DC access branch, the onshore photovoltaic DC access branch, and the pumped storage DC access branch stabilize at their respective power operating points. The two receiving-end MMC converter modules under the receiving-end converter module 3 jointly absorb the collected power of the true bipolar DC bus 2 and perform power distribution.

[0055] Subsequently, the wind speed of the offshore wind farm was set to undergo a step change. Under rated operating conditions, the wind speed was 12 m / s, at which point the offshore wind farm output 2000 MW of active power. Assuming that the system had reached steady state under rated operating conditions at 2 seconds, the wind speed was set to change from 12 m / s to 10 m / s at 4 seconds, from 10 m / s to 8 m / s at 6 seconds, from 8 m / s to 6 m / s at 8 seconds, from 6 m / s to 4 m / s at 10 seconds, and from 4 m / s to 12 m / s at 12 seconds. As the wind speed decreased, the active power of the offshore wind power DC transmission branch decreased accordingly and entered the corresponding power operating point. When the wind speed returned to the rated wind speed, the active power of the offshore wind power DC transmission branch returned to the rated operating level.

[0056] Please refer to Figure 3 and Figure 4 , Figure 3 This is a schematic diagram illustrating the changes in the DC voltage of the DRU module and the DC voltage of the first MMC module under the condition of a step change in wind speed in offshore wind power, as provided by the present invention. Figure 4 This is a schematic diagram illustrating the power variation of the offshore wind power DC transmission branch under the condition of a step change in offshore wind speed, as provided by the present invention. It can be seen that during the fluctuation of offshore wind power with wind speed, the DRU-MMC series topology maintains its basic voltage sharing relationship. The DRU module 11 continues to undertake the main DC voltage and main power rectification and transmission tasks, while the first MMC module 12 maintains a lower DC voltage level and undertakes the functions of offshore AC voltage frequency support and dynamic adjustment. It should be noted that... Figure 3 The blue line in the diagram corresponds to the DC voltage U of DRU module 11. DRU The orange line corresponds to the DC voltage U of the first MMC module 12. MMC .

[0057] Please refer to Figure 5 , Figure 5This invention provides a schematic diagram of the voltage change of a true bipolar DC bus under the condition of a step change in wind speed in offshore wind power. In the diagram, the blue line represents the inter-pole voltage Udc of the true bipolar DC bus, the orange line represents the voltage Up of the positive bus to ground, and the green line represents the voltage Un of the negative bus to ground. It can be seen that the positive and negative voltages of the true bipolar DC bus 2 can remain coordinated, with only short-term transient fluctuations at the moment of wind speed step change, and no continuous voltage shift or oscillation instability.

[0058] Please refer to Figure 6 , Figure 6 This invention provides a schematic diagram illustrating the power changes of a nuclear power DC access branch, a photovoltaic DC access branch, and a pumped storage DC access branch under a step change in offshore wind speed. The orange line represents the power P injected from the nuclear power DC access branch into the true bipolar DC bus 2. N The yellow line represents the power P injected into the true bipolar DC bus 2 by the photovoltaic DC access branch. PV The purple line represents the power P injected into the true bipolar DC bus 2 by the pumped storage DC access branch. PS It can be seen that the nuclear power DC access branch, photovoltaic DC access branch and pumped storage DC access branch maintained continuous operation during the change of offshore wind speed, and no continuous power interruption occurred.

[0059] Please refer to Figure 7 , Figure 7 This invention provides a schematic diagram of power sharing between two receiving-end MMC converter modules in an offshore wind power receiving-end converter module under conditions of abrupt wind speed changes. The blue line represents the change in absorbed power PA of the first receiving-end MMC converter module 31, and the orange line represents the change in absorbed power PB of the second receiving-end MMC converter module 32. It should be noted that since the two receiving-end MMC converter modules are connected in parallel, and this application adjusts the absorbed power under fluctuating conditions, the two receiving-end MMC converter modules can still maintain their power sharing relationship. Figure 7 In the middle, the blue line and the orange line overlap; in addition, Figure 7 The green line in the diagram represents the change in the total absorbed power PA+PB from the true bipolar DC bus 2.

[0060] In summary, the multi-energy true bipolar DC aggregation system provided in this application can achieve multi-energy DC aggregation, true bipolar DC bus voltage coordination, and continuous operation of non-disturbance DC access branches under offshore wind power fluctuation conditions.

[0061] The present invention also provides a DC power transmission system, including the multi-energy true bipolar DC collection system as described above.

[0062] For a description of the DC transmission system provided in this application, please refer to the above-described embodiment of the multi-energy true bipolar DC collection system; further details will not be repeated here.

[0063] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since it corresponds to the method disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to in the method section. Relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0064] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A multi-energy true bipolar DC collection system, characterized in that, It includes an offshore wind power sending-end converter module, a true bipolar DC bus, and a receiving-end converter module; the offshore wind power sending-end converter module includes a DRU module, a first MMC module, and a polarity interface conversion device; The AC input terminal of the DRU module is connected to the AC output terminal of the offshore wind farm. The positive DC output terminal is connected to the positive bus of the true bipolar DC bus through the polarity interface conversion device. The negative DC output terminal is connected to the positive DC output terminal of the first MMC module. It is used to adjust the output according to the preset DC voltage distribution relationship to realize the main rectified output of the active power of the offshore wind power. The AC input terminal of the first MMC module is connected to the AC output terminal of the offshore wind farm, and the DC output negative terminal is connected to the negative bus of the true bipolar DC bus through the polarity interface conversion device. This is used to adjust the output according to the preset DC voltage distribution relationship, so as to realize the rectified output of the active power of the offshore wind power. The first MMC module adopts grid-type control to provide offshore AC voltage and frequency support. The polarity interface conversion device is used for polarity conversion to obtain a true bipolar DC topology structure composed of the DRU module, the first MMC module and the polarity interface conversion device connected to the true bipolar DC bus. The input terminal of the receiving-end converter module is connected to the true bipolar DC bus, and the output terminal is connected to the electrical load. It is used to absorb the power collected by the true bipolar DC bus and output power to different electrical loads when the multi-energy true bipolar DC collection system is running stably.

2. The multi-energy true bipolar DC collection system as described in claim 1, characterized in that, The multi-energy true bipolar DC collection system also includes a coastal energy converter module; the coastal energy converter module includes a nuclear power sending-end converter module, a photovoltaic sending-end converter module, and a pumped storage sending-end converter module. The nuclear power unit is connected to the true bipolar DC bus through the nuclear power transmission end converter module. The nuclear power transmission end converter module is used to send the output power of the nuclear power unit into the true bipolar DC bus in a stable power transmission mode. The photovoltaic power generation unit is connected to the true bipolar DC bus through the photovoltaic sending-end converter module. The photovoltaic sending-end converter module is used to send the output power of the photovoltaic power generation unit into the true bipolar DC bus. The pumped storage module is connected to the true bipolar DC bus through the pumped storage sending-end converter module, and the pumped storage sending-end converter module is used to send the output power of the pumped storage module into the true bipolar DC bus.

3. The multi-energy true bipolar DC collection system as described in claim 2, characterized in that, The nuclear power transmission-end converter module includes a first LCC module and a second LCC module; The AC input terminal of the first LCC module is connected to the AC output terminal of the nuclear power unit, the DC output positive terminal is connected to the positive bus, and the DC output negative terminal is connected to the DC output positive terminal of the second LCC module, with the common terminal of the connection grounded. The AC input terminal of the second LCC module is connected to the AC output terminal of the nuclear power unit, and the negative DC output terminal is connected to the negative bus.

4. The multi-energy true bipolar DC collection system as described in claim 2, characterized in that, The photovoltaic sending-end converter module includes a second MMC module and a third MMC module; The AC input terminal of the second MMC module is connected to the AC output terminal of the photovoltaic power generation unit, the DC output positive terminal is connected to the positive bus, and the DC output negative terminal is connected to the DC output positive terminal of the third MMC module and the common terminal of the connection is grounded. The AC input terminal of the third MMC module is connected to the AC output terminal of the photovoltaic power generation unit, and the DC output negative terminal is connected to the negative bus.

5. The multi-energy true bipolar DC collection system as described in claim 2, characterized in that, The pumped storage power transmission converter module includes a third LCC module, a fourth LCC module, a fourth MMC module, and a fifth MMC module; The AC input terminal of the third LCC module is connected to the AC output terminal of the pumped storage module, the DC output positive terminal is connected to the positive bus, and the DC output negative terminal is connected to the DC output positive terminal of the fourth MMC module. The AC input terminal of the fourth MMC module is connected to the AC output terminal of the pumped storage module, and the DC output negative terminal is connected to the DC output positive terminal of the fifth MMC module, with the common terminal of the connection grounded. The AC input terminal of the fifth MMC module is connected to the AC output terminal of the pumped storage module, and the negative DC output terminal is connected to the positive DC output terminal of the fourth LCC module. The AC input terminal of the fourth LCC module is connected to the AC output terminal of the pumped storage module, and the negative DC output terminal is connected to the negative bus.

6. The multi-energy true bipolar DC collection system as described in claim 2, characterized in that, The receiving-end converter module is also used to adjust its absorbed power according to the system power balance relationship, the target control voltage of the true bipolar DC bus, and the power fluctuation of the disturbed branch when the multi-energy true bipolar DC collection system is under disturbance conditions, so as to maintain the coordination of the positive and negative voltages of the true bipolar DC bus.

7. The multi-energy true bipolar DC collection system as described in claim 6, characterized in that, The receiving-end converter module includes at least two receiving-end MMC converter modules; The receiving-end MMC converter module is connected in parallel to the true bipolar DC bus, and the output terminal of each receiving-end MMC converter module is connected to the corresponding electrical load.

8. The multi-energy true bipolar DC collection system as described in claim 7, characterized in that, The receiving-end MMC converter module includes a sixth MMC module and a seventh MMC module; The positive DC input terminal of the sixth MMC module is connected to the positive bus, the negative DC input terminal is connected to the positive DC input terminal of the seventh MMC module and the common terminal of the connection is grounded, and the AC output terminal is connected to the corresponding electrical load. The negative DC input terminal of the seventh MMC module is connected to the negative bus, and the AC output terminal is connected to the electrical load.

9. The multi-energy true bipolar DC collection system as described in any one of claims 1 to 8, characterized in that, The multi-energy true bipolar DC collection system also includes a DC circuit breaker. The true bipolar DC bus is connected to the receiving-end converter module through the DC circuit breaker.

10. A direct current transmission system, characterized in that, Including the multi-energy true bipolar DC collection system as described in any one of claims 1 to 9.

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