Hybrid power transmission system and operation method thereof

Abstract

The invention discloses a hybrid power transmission system, which comprises an offshore wind power transmission system and an energy storage power station power transmission system, and is characterized in that the offshore wind power transmission system comprises an offshore wind power plant, a power grid and a frequency conversion station; the frequency conversion station is connected with the low-frequency bus and the power-frequency bus. The energy storage power station power transmission system comprises an energy storage power station which is respectively connected with the low-frequency bus and the power frequency bus through a newly added circuit breaker.

Description

Technical Field

[0001] The present application relates to the fields of offshore wind power technology and energy storage technology, and is related to but not limited to a hybrid power transmission system and an operation method thereof. Background Art

[0002] The proportion of renewable energy power generation in my country's power system is increasing, and the inertia of the power grid is also slowly decreasing. Among them, the abundance of medium and long-distance offshore wind power resources has led to a rapid increase in the installed capacity of offshore wind power transmission systems in recent years. However, offshore wind power is intermittent and volatile, which makes the grid power unstable. When the grid-connected capacity is too large, it will inevitably have an impact on the power grid and even affect the stability of the grid voltage. Summary of the Invention

[0003] In view of this, embodiments of the present application at least provide a hybrid power transmission system and an operating method thereof.

[0004] The technical solution of the embodiment of the present application is implemented as follows:

[0005] In a first aspect, an embodiment of the present application provides a hybrid transmission system, comprising: an offshore wind power transmission system and an energy storage power station transmission system: wherein the offshore wind power transmission system includes an offshore wind farm, a power grid, and a frequency conversion station; the frequency conversion station is respectively connected to a low-frequency bus and an industrial frequency bus; the energy storage power station transmission system includes: an energy storage power station respectively connected to the low-frequency bus and the industrial frequency bus through newly added circuit breakers.

[0006] In an embodiment of the present application, an energy storage power station is added on the basis of the offshore wind power transmission system. The energy storage power station is connected to the low-frequency bus and the power frequency bus respectively through a newly added circuit breaker, thus forming a hybrid transmission system including the offshore wind power transmission system and the energy storage power station transmission system.

[0007] In a second aspect, an embodiment of the present application provides an operating method of a hybrid transmission system, wherein the hybrid transmission system includes an offshore wind power transmission system and an energy storage power station transmission system, wherein the offshore wind power transmission system includes an offshore wind farm, a power grid and a frequency conversion station, and the frequency conversion station is respectively connected to a low-frequency bus and an industrial frequency bus; the energy storage power station transmission system includes an energy storage power station connected to the low-frequency bus and the industrial frequency bus respectively through a newly added circuit breaker, and the method includes: determining a configuration point of the energy storage power station based on the working state of the newly added circuit breaker; the configuration point is used to characterize the configuration of the energy storage power station. is located at the low-frequency bus or at the power frequency bus; based on the configuration point of the energy storage power station, determining the object of the active power reference value, and determining the control strategy of the energy storage power station and the frequency conversion station in the offshore wind power transmission system; the object includes the energy storage power station or the frequency conversion station in the offshore wind power transmission system; based on the control strategy of the energy storage power station and the frequency conversion station, the operation process of the energy storage power station and the frequency conversion station is controlled respectively; during the access and operation of the offshore wind farm, the active power reference value of the object is adjusted based on the active power reference value of the offshore wind farm.

[0008] In the embodiment of the present application, the configuration point of the energy storage station represents whether the energy storage station is configured at the low-frequency bus or at the power frequency bus. Based on the configuration point of the energy storage station, it is determined whether the object of the active power reference value is the energy storage station or the frequency conversion station in the offshore wind power transmission system, and the control strategy of the energy storage station and the frequency conversion station in the offshore wind power transmission system is determined. Then, based on the control strategy of the energy storage station and the frequency conversion station, the operation process of the energy storage station and the frequency conversion station is controlled respectively. Finally, during the access and operation of the offshore wind farm, the active power reference value of the object is adjusted based on the active power reference value of the offshore wind farm. It can be seen from this that the operation mode of this hybrid transmission system can, in the case of fluctuations in the power input to the offshore wind power transmission system, prompt the newly added energy storage station system to supplement or absorb power based on the active power reference value of the object, thereby transmitting stable power to the power grid, effectively solving the problem of power fluctuations in the power input to the offshore wind power transmission system.

[0009] In some embodiments, determining the object of the active power reference value based on the configuration point of the energy storage power station includes: when the energy storage power station is configured at the low-frequency bus, determining the object of the active power reference value as the frequency conversion station; during the access and operation of the offshore wind farm, adjusting the active power reference value of the object based on the active power reference value of the offshore wind farm includes:

[0010] During the access and operation of the offshore wind farm, the active power reference value of the offshore wind farm is adjusted; the active power reference value of the frequency conversion station is adjusted to approach the active power reference value of the offshore wind farm through a virtual synchronous machine control mode started by the frequency conversion station on the power frequency side; or, the active power reference value of the frequency conversion station is adjusted to approach the active power reference value of the offshore wind farm through a virtual synchronous machine control mode started by the frequency conversion station on the low frequency side; when the active power reference value of the offshore wind farm approaches a preset power rated value, the adjustment of the active power reference value of the offshore wind farm and the active power reference value of the frequency conversion station is stopped.

[0011] In an embodiment of the present application, when the power transmitted by the offshore wind farm fluctuates, the active power reference value of the frequency conversion station is controlled to be close to the power rating. Based on the power difference between the transmitted power of the offshore wind farm and the active power reference value of the frequency conversion station, this power difference is automatically supplemented or absorbed by the energy storage station, so that stable power can be transmitted to the power grid.

[0012] In some embodiments, determining the object of the active power reference value based on the configuration point of the energy storage power station includes: when the energy storage power station is configured at the power frequency bus, determining the object of the active power reference value to be the energy storage power station; during the access and operation of the offshore wind farm, adjusting the active power reference value of the object based on the active power reference value of the offshore wind farm includes:

[0013] During the access and operation of the offshore wind farm, the active power reference value of the offshore wind farm is adjusted; when the active power reference value of the offshore wind farm approaches a preset power rated value, the adjustment of the active power reference value of the offshore wind farm is stopped; and based on the power rated value, the active power reference value of the energy storage power station is adjusted through the virtual synchronous machine control mode started by the energy storage power station.

[0014] In an embodiment of the present application, when the power delivered by the offshore wind farm fluctuates, the total output power of the offshore wind farm and the energy storage power station is controlled to approach the power rating. Therefore, when the offshore wind farm's delivered power is less than the power rating, the active power reference value of the energy storage power station is adjusted through virtual synchronous machine control to compensate for the power that does not reach the power rating. When the offshore wind farm's delivered power exceeds the power rating, the active power reference value of the energy storage power station is adjusted through virtual synchronous machine control to absorb the power that exceeds the power rating, thereby enabling stable power transmission to the power grid.

[0015] In some embodiments, the method further includes: when the active power reference value of the energy storage power station meets the power rating, stopping adjusting the active power reference value of the energy storage power station.

[0016] In an embodiment of the present application, when the active power reference value of the energy storage power station meets the power rating, the active power reference value of the energy storage power station is stopped from being adjusted. At this time, the active power is stably output by the energy storage power station, so that the total output power of the offshore wind farm and the energy storage power station is stabilized at a value close to the power rating, thereby solving the problem of fluctuations in the grid-connected power of the offshore wind power transmission system.

[0017] In some embodiments, based on the configuration point of the energy storage station, determining the control strategy of the energy storage station and the frequency conversion station in the offshore wind power transmission system includes: when the configuration point indicates that the energy storage station is configured at the low-frequency bus, determining that the control strategy of the energy storage station is a constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode, and the control strategy of the power frequency side of the frequency conversion station is a virtual synchronous machine control mode;

[0018] Alternatively, when the configuration point indicates that the energy storage power station is configured at the low-frequency bus, the control strategy of the energy storage power station is determined to be a constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is determined to be a virtual synchronous machine control mode, and the control strategy of the power frequency side of the frequency conversion station is determined to be a fixed bridge arm submodule average capacitor voltage control mode.

[0019] In an embodiment of the present application, when the energy storage power station is configured on a low-frequency bus, the control strategy of the energy storage power station is determined to be a constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode, and the control strategy of the power frequency side of the frequency conversion station is a virtual synchronous machine control mode; or, the control strategy of the energy storage power station is determined to be a constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is a virtual synchronous machine control mode, and the control strategy of the power frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode. In this way, when the hybrid transmission system is started, it can control the frequency conversion station and the energy storage power station to operate according to the control strategy according to the set control strategy.

[0020] In some embodiments, based on the configuration point of the energy storage power station, the control strategies of the energy storage power station and the frequency conversion station in the offshore wind power transmission system are determined, including: when the configuration point represents that the energy storage power station is configured at the power frequency bus, determining that the control strategy of the energy storage power station is a virtual synchronous machine control mode, the control strategy of the low-frequency side of the frequency conversion station is a constant voltage and constant frequency control mode, and the control strategy of the power frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode.

[0021] In an embodiment of the present application, when the energy storage power station is configured on the power frequency bus, the control strategy of the energy storage power station is determined to be a virtual synchronous machine control mode, the control strategy of the low-frequency side of the frequency conversion station is determined to be a constant voltage and constant frequency control mode, and the control strategy of the power frequency side of the frequency conversion station is determined to be a fixed bridge arm submodule average capacitor voltage control mode. In this way, when the hybrid transmission system is started, the frequency conversion station and the energy storage power station can be controlled according to the set control strategy to operate according to the control strategy.

[0022] In some embodiments, based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively, including: when the energy storage power station starts the constant voltage and constant frequency control mode, after the low-frequency side of the frequency conversion station is powered on, the low-frequency side of the frequency conversion station is controlled to start the fixed bridge arm sub-module average capacitor voltage control mode; through the fixed bridge arm sub-module average capacitor voltage control mode started on the low-frequency side of the frequency conversion station, the average capacitor voltage of all bridge arm sub-modules of the frequency conversion station is adjusted; when the average capacitor voltage approaches a preset reference voltage, after the industrial frequency side of the frequency conversion station is powered on, the industrial frequency side of the frequency conversion station is controlled to start the virtual synchronous machine control mode.

[0023] In an embodiment of the present application, when it is determined that the energy storage power station is controlled to start the constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is then started, and the average capacitor voltage of all bridge arm sub-modules of the frequency conversion station is adjusted. Finally, when the average capacitor voltage of all bridge arm sub-modules of the frequency conversion station approaches a preset reference voltage, the control strategy of the industrial frequency side of the frequency conversion station is started. This step-by-step method of starting each module can allow the hybrid transmission system to start smoothly and reduce the impact on the power grid.

[0024] In some embodiments, based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively, including: when the energy storage power station starts the constant voltage and constant frequency control mode, after the industrial frequency side of the frequency conversion station is powered on, the industrial frequency side of the frequency conversion station is controlled to start the average capacitor voltage control mode of the fixed bridge arm submodule; through the average capacitor voltage control mode of the fixed bridge arm submodule started on the industrial frequency side of the frequency conversion station, the average capacitor voltage of all bridge arm submodules of the frequency conversion station is adjusted; when the average capacitor voltage approaches a preset reference voltage, after the low frequency side of the frequency conversion station is powered on, the low frequency side of the frequency conversion station is controlled to start the virtual synchronous machine control mode.

[0025] In an embodiment of the present application, when it is determined that the energy storage power station is controlled to start the constant voltage and constant frequency control mode, the control strategy of the power frequency side of the frequency conversion station is then started, and the average capacitor voltage of all the bridge arm sub-modules of the frequency conversion station is adjusted. Finally, when the average capacitor voltage of all the bridge arm sub-modules of the frequency conversion station approaches the preset reference voltage, the control strategy of the low frequency side of the frequency conversion station is started. This step-by-step method of starting each module can allow the hybrid transmission system to start smoothly and reduce the impact on the power grid.

[0026] In some embodiments, based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively, including: when the virtual synchronous machine control mode is started in the energy storage power station, after the industrial frequency side of the frequency conversion station is powered on, the industrial frequency side of the frequency conversion station is controlled to start the average capacitor voltage control mode of the fixed bridge arm submodule; through the average capacitor voltage control mode of the fixed bridge arm submodule started on the industrial frequency side of the frequency conversion station, the average capacitor voltage of all bridge arm submodules of the frequency conversion station is adjusted; when the average capacitor voltage approaches a preset reference voltage, after the low frequency side of the frequency conversion station is powered on, the low frequency side of the frequency conversion station is controlled to start the constant voltage and constant frequency control mode.

[0027] In an embodiment of the present application, when it is determined that the energy storage power station is controlled to start the virtual synchronous machine control mode, the control strategy of the industrial frequency side of the frequency conversion station is then started, and the average capacitor voltage of all the bridge arm sub-modules of the frequency conversion station is adjusted. Finally, when the average capacitor voltage of all the bridge arm sub-modules of the frequency conversion station approaches a preset reference voltage, the control strategy of the low-frequency side of the frequency conversion station is started. This step-by-step method of starting each module can allow the hybrid transmission system to start smoothly and reduce the impact on the power grid.

[0028] In an embodiment of the present application, an energy storage power station is added on the basis of the offshore wind power transmission system. The energy storage power station is connected to the low-frequency bus and the power frequency bus respectively through a newly added circuit breaker, thus forming a hybrid transmission system including the offshore wind power transmission system and the energy storage power station transmission system.

[0029] The configuration point of the energy storage station in the embodiment of the present application represents whether the energy storage station is configured at the low-frequency bus or at the power frequency bus. Based on the configuration point of the energy storage station, it is determined whether the object of the active power reference value is the energy storage station or the frequency conversion station in the offshore wind power transmission system, and the control strategies of the energy storage station and the frequency conversion station in the offshore wind power transmission system are determined. Then, based on the control strategies of the energy storage station and the frequency conversion station, the operation processes of the energy storage station and the frequency conversion station are controlled respectively. Finally, during the access and operation of the offshore wind farm, the active power reference value of the object is adjusted based on the active power reference value of the offshore wind farm.

[0030] From this, it can be seen that the operation mode of this hybrid transmission system can, when there are fluctuations in the power entering the offshore wind power transmission system, prompt the newly added energy storage power station system to supplement or absorb power based on the active power reference value of the object, thereby transmitting stable power to the power grid, effectively solving the problem of fluctuations in the power entering the offshore wind power transmission system.

[0031] In addition, the newly added energy storage power station transmission system can smooth out the power fluctuations of offshore wind power entering the grid while providing more control target options for the frequency conversion station, thereby improving the stability of grid operation and the economic benefits of power transmission, while also providing a certain amount of inertia support for the grid.

[0032] It should be understood that the above general description and the following detailed description are merely exemplary and explanatory, and do not limit the technical solutions of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present application. Those skilled in the art can also derive other drawings based on these drawings without inventive work, among which:

[0034] Figure 1 A schematic diagram of an offshore wind power transmission system based on flexible low-frequency power transmission technology provided in an embodiment of the present application;

[0035] Figure 2 A schematic diagram of a hybrid power transmission system provided in an embodiment of the present application;

[0036] Figure 3 A schematic diagram of a circuit topology of an energy storage power station provided in an embodiment of the present application;

[0037] Figure 4 A schematic diagram of a circuit topology of a frequency conversion station provided in an embodiment of the present application;

[0038] Figure 5 A schematic diagram of a process flow for implementing an operating method of a hybrid power transmission system provided in an embodiment of the present application;

[0039] Figure 6A Schematic diagram of the implementation process of the operation method of a hybrid power transmission system in which an energy storage power station is configured at a low-frequency bus provided in an embodiment of the present application Figure 1 ;

[0040] Figure 6B Schematic diagram of the implementation process of the operation method of a hybrid power transmission system in which an energy storage power station is configured at a low-frequency bus provided in an embodiment of the present application Figure 2 ;

[0041] Figure 7 A schematic diagram of a flow chart of an implementation method of an operation method of a hybrid power transmission system in which an energy storage power station is configured at an industrial frequency bus provided in an embodiment of the present application. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solutions and advantages of this application clearer, the technical solutions of this application are further elaborated in detail below with reference to the accompanying drawings and embodiments. The described embodiments should not be regarded as limiting this application. All other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0043] In the following description, reference is made to “some embodiments”, which describes a subset of all possible embodiments, but it will be understood that “some embodiments” may be the same subset or different subsets of all possible embodiments and may be combined with each other without conflict.

[0044] It should be pointed out that the terms "first\second\third" involved in the embodiments of the present application are merely used to distinguish similar objects and do not represent a specific ordering of the objects. It can be understood that "first\second\third" can be interchanged with a specific order or sequence where permitted, so that the embodiments of the present application described here can be implemented in an order other than that illustrated or described here.

[0045] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art in the art to which the embodiments of the present application belong. It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art, and will not be interpreted in an idealized or overly formal sense unless specifically defined as herein.

[0046] Flexible Low-Frequency Transmission (LFAC) is a new AC transmission technology that uses power electronic converters to independently select low frequencies, such as 20Hz and 50 / 3Hz. Similar to flexible DC transmission, which converts industrial frequency AC power into DC, LFAC improves the system's transmission capacity and distance by reducing the reactance and capacitive charging current of transmission lines. LFAC utilizes AC transformers and circuit breakers. Compared to flexible DC transmission, LFAC eliminates the need for new electrical equipment development and facilitates voltage level changes, fault interruption, and networking.

[0047] Frequency converter: refers to a device that can effectively adjust frequency and voltage through the conversion and regulation of electric energy to adapt to changes in load and the needs of different operating conditions. Compared with the traditional fixed-frequency, fixed-voltage power supply method, the frequency converter can adjust the output current and voltage according to actual needs to achieve efficient energy utilization. This refined energy control method can significantly reduce energy consumption and improve the overall energy efficiency of the power system.

[0048] Grid-connected capacity: This refers to the maximum load a power system can withstand. It is related to the grid's operating status and transformer capacity, and is a key indicator of stable power system operation. It refers to the total amount of electrical energy the system can absorb, transport, and supply while operating safely, reliably, and stably.

[0049] Peak shaving and valley filling: A measure to adjust electricity load. Based on the electricity usage patterns of different users, the electricity usage schedules of different users are rationally and systematically arranged and organized. This reduces peak loads and fills valley loads. This reduces the difference between peak and valley loads on the power grid, thus balancing power generation and consumption.

[0050] Inertia support refers to the ability of a power system's internal structure to delay its dynamic response and maintain stability in the face of emergencies such as sudden loads or power fluctuations. Power system inertia support is a crucial element in ensuring system stability. It ensures that when one or more faults occur during system operation, the system remains within a certain range and prevents grid collapse.

[0051] Modular Multilevel Matrix Converters (M3C) are a new type of direct AC-AC converter topology based on cascaded H-bridges. They can be used in medium / high voltage power frequency conversion applications, but their application range is limited by the ripple voltage of the DC-side capacitors of the H-bridge submodules.

[0052] my country boasts abundant offshore wind power resources, particularly in mid- and long-distance waters. This has led to rapid growth in the installed capacity of offshore wind power transmission systems in recent years. However, due to the high ground capacitance of submarine cables in offshore wind power transmission systems, 50Hz AC transmission technology is unable to support submarine cable transmission over distances exceeding 70 kilometers. The use of flexible DC transmission inevitably requires the construction of offshore converter stations, which is costly. However, applying flexible low-frequency transmission technology to the long-distance aggregation and transmission of offshore wind farms could enable large-scale offshore wind power transmission without the need for the construction and maintenance of large-scale offshore converter platforms. This reduces overall construction time, reduces costs, and improves efficiency.

[0053] As renewable energy generation and power electronics devices account for an increasing proportion of the power system, my country's power system is experiencing a "double high" situation, with both system inertia and stability gradually declining. Due to the intermittent and unstable nature of offshore wind power, it is unable to deliver high-quality electricity to the grid. Even excessive grid-connected capacity can further threaten the stability of the power system. Therefore, energy storage devices are needed to smooth offshore wind power output while providing a certain amount of inertia support for the grid, ensuring safe and stable operation of the power system.

[0054] The following is an introduction to the offshore wind power transmission system. Figure 1 As shown, an offshore wind power transmission system based on flexible low-frequency transmission technology is demonstrated. The offshore wind power transmission system mainly includes an offshore wind farm 11, a wind turbine end booster 12, an offshore wind farm busbar 121, an offshore boost transformer 13, a submarine cable 14, a frequency conversion station 17 and a power grid 21. Among them, the electric energy generated by the offshore wind turbine 111 in the offshore wind farm 11 is boosted by the wind turbine end booster 12 and then merged into the offshore wind farm busbar 121. Then, it is boosted again by the offshore boost transformer 13 and enters the submarine cable 14. Finally, it is converted by the frequency conversion station 17 and then merged into the power grid 21. In this way, not only can large-capacity offshore wind power be transmitted, but the entire system can also operate at a unified frequency. There is no need to build and operate a large-scale offshore converter platform, and the overall construction time is shorter, the cost is lower, and the efficiency is higher.

[0055] The embodiment of the present application provides a hybrid power transmission system, such as Figure 2 As shown, it includes: an offshore wind power transmission system and an energy storage power station transmission system; wherein, the offshore wind power transmission system is implemented based on flexible low-frequency transmission technology; the offshore wind power transmission system includes an offshore wind farm 11, a power grid 21 and a frequency conversion station 17; the frequency conversion station is respectively connected to the low-frequency bus 16 and the power frequency bus 19; the energy storage power station transmission system includes an energy storage power station 18 connected to the low-frequency bus 16 and the power frequency bus 19 respectively through newly added circuit breakers (QS1 and QS2).

[0056] Specifically, the hybrid power transmission system includes an offshore part 1 and an onshore part 2 .

[0057] In the offshore part 1, the offshore wind farm 11, the wind turbine generator-side booster 12, the offshore wind farm busbar 121, the circuit breaker QS7, the offshore boost transformer 13, the submarine cable busbar M, the circuit breaker QS5 and the submarine cable 14 are connected in series in sequence. Among them, the offshore wind farm 11 includes a wind turbine 111, a wind turbine-side converter 112, a wind turbine converter DC-side capacitor 113, and a wind turbine-side converter 114.

[0058] In the onshore portion 2, the circuit breaker QS6, the submarine cable busbar N, the low-frequency side transformer 15 of the frequency conversion station, the low-frequency busbar 16, the power frequency busbar 19, the power frequency side transformer 20 of the frequency conversion station, and the power grid 21 are connected in series in sequence. The frequency conversion station 17 is connected to the low-frequency busbar 16 via the circuit breaker QS3 and to the power frequency busbar 19 via the circuit breaker QS4. The energy storage station 18 is connected to the low-frequency busbar 16 via the circuit breaker QS1 and to the power frequency busbar 19 via the circuit breaker QS2. The energy storage station 18 includes an energy storage converter 181 and an energy storage battery module 182.

[0059] In an embodiment of the present application, an energy storage power station is added on the basis of the offshore wind power transmission system. The energy storage power station is connected to the low-frequency bus and the power frequency bus respectively through a newly added circuit breaker, thus forming a hybrid transmission system including the offshore wind power transmission system and the energy storage power station transmission system.

[0060] In some embodiments, the energy storage station 18 may adopt a chain H-bridge topology, such as Figure 3 As shown, U a 、U b and U c They represent the A phase system voltage, B phase system voltage and C phase system voltage respectively; L a 、L b and L c They respectively represent the system inductance of phase A, the system inductance of phase B and the system inductance of phase C; SMA1, SMA2, SMA3, ..., SMAn represent the n H-bridge sub-modules of phase A; SMB1, SMB2, SMB3, ..., SMBn represent the n H-bridge sub-modules of phase B; SMC1, SMC2, SMC3, ..., SMCn represent the n H-bridge sub-modules of phase C.

[0061] Each phase of this circuit topology is composed of n identical H-bridge submodules connected in cascade. Each submodule includes a storage battery 1811, an absorption capacitor 1812, and an H-bridge converter in parallel. One of the H-bridge converters includes four identical transistors 1813. In some embodiments, the transistors can be insulated-gate bipolar transistors (IGBTs).

[0062] Frequency conversion stations using large-capacity, fully controlled components are the core device for implementing flexible low-frequency power transmission technology. Initially, low-frequency power transmission was achieved using frequency-doubling transformers. However, these transformers suffer from high operating losses and poor controllability, making them difficult to implement in offshore wind power grid-connected systems. With the advancement of power electronics technology, phase-controlled cycloconverters using thyristors have become the primary choice for frequency conversion stations. However, these phase-controlled AC-AC converters generate a large number of difficult-to-filter low-order harmonics and intermittent waves, leading to high filtering costs and poor system stability.

[0063] Back-to-back modular multilevel converters (MMCs), modular multilevel matrix converters, and hexagonal modular multilevel AC-AC converters (Hexverters) offer advantages such as good output harmonic characteristics, low equivalent switching frequency, and a high degree of modularity. These are important development directions for AC-AC frequency conversion stations. M3Cs have been a research hotspot in recent years. Compared to MMCs, M3Cs eliminate the intermediate DC conversion stage and directly implement AC-AC conversion. They offer more flexible control, enabling four-quadrant operation of both input and output power, and are simpler to control than Hexverters. Therefore, M3Cs are often chosen as the frequency converters for hybrid transmission systems.

[0064] In some embodiments, the frequency conversion station 17 may adopt an M3C topology, such as Figure 4 As shown, M3C includes 9 bridge arms, each of which is composed of N H-bridge submodules and a bridge arm inductor L in series. An H-bridge submodule contains four transistors (Q1, Q2, Q3, Q4) and a storage capacitor. In some embodiments, the transistor can be an IGBT type. The power frequency side phase voltage and power frequency side phase current of the M3C frequency conversion station are respectively denoted as v gx and i sx (x=u,v,w), the low-frequency side phase voltage and low-frequency side phase current of M3C frequency conversion station are recorded as v my and i my (y=a,b,c), the current flowing through the bridge arm is recorded as (x=u,v,w;y=a,b,c), the voltage flowing through the bridge arm is recorded as (x=u, v, w; y=a, b, c).

[0065] M3C frequency conversion station can interconnect two AC systems with different frequencies, where the AC system refers to Figure 2In the offshore portion 1 and grid 21 of the M3C inverter, flexible low-frequency transmission technology is implemented using a two-sided control system: the low-frequency side and the power frequency side. Active and reactive power control on both sides can be independent and independent of each other. Because the submodule capacitor voltage of the M3C inverter must be controlled to a given value, active power control on either the low-frequency side or the power frequency side must operate in the fixed-arm submodule average capacitor voltage mode.

[0066] The embodiment of the present application provides an operating method of a hybrid power transmission system, wherein the method is applied to a hybrid power transmission system, such as Figure 5 As shown, the method may include steps S501 to S504:

[0067] Step S501: determining a configuration point of the energy storage power station based on the working state of the newly added circuit breaker; the configuration point is used to indicate whether the energy storage power station is configured at the low-frequency bus or at the power-frequency bus;

[0068] Based on their topology and grid connection methods, energy storage stations can be categorized as AC step-down parallel-connected, AC direct-connected, and DC direct-connected. In some embodiments, the AC direct-connected type can be selected. This type of station not only reduces power loss but also enables rapid storage and release of energy.

[0069] A circuit breaker is a switching device capable of closing, carrying, and interrupting current under normal circuit conditions, and opening, carrying, and interrupting current under abnormal circuit conditions within a specified timeframe. A busbar is a conductor in a power system that transmits electrical energy to various electrical devices (such as transformers and generators), typically made of copper and aluminum.

[0070] In some embodiments, when the operating state of the circuit breaker connected to the low-frequency bus of the energy storage station is closed, and the operating state of the circuit breaker connected to the power frequency bus is open, the energy storage station is determined to be configured at the low-frequency bus; when the operating state of the circuit breaker connected to the power frequency bus of the energy storage station is closed, and the operating state of the circuit breaker connected to the low-frequency bus of the energy storage station is open, the energy storage station is determined to be configured at the power frequency bus; when the operating states of the circuit breaker connected to the low-frequency bus of the energy storage station and the circuit breaker connected to the power frequency bus of the energy storage station are both open, the energy storage station is determined to be disconnected from the hybrid transmission system. Generally, when a fault occurs on the power frequency bus and the low-frequency bus, or when a fault occurs within the energy storage station, a simultaneous disconnection situation occurs. It should be noted that the operating states of the circuit breaker connected between the low-frequency bus and the circuit breaker connected between the power frequency bus of the energy storage station will not be closed at the same time.

[0071] Step S502: Based on the configuration point of the energy storage power station, determine the object of the active power reference value, and determine the control strategy of the energy storage power station and the frequency conversion station in the offshore wind power transmission system; the object includes the energy storage power station or the frequency conversion station in the offshore wind power transmission system;

[0072] Here, active power refers to the electrical power required to maintain the normal operation of electrical equipment. This refers to the electrical power required to convert electrical energy into other forms of energy (mechanical energy, solar energy, thermal energy). A frequency conversion station is a substation that regulates the frequency of electricity. Its primary purpose is to meet the power quality requirements of industrial and commercial users. Its main components include transformers, inverters, and filters.

[0073] In some embodiments, the offshore wind power transmission system may be implemented based on flexible low-frequency transmission technology. When the energy storage station is located at the low-frequency bus, the frequency conversion station is used to determine the active power reference value; when the energy storage station is located at the power frequency bus, the energy storage station is used to determine the active power reference value.

[0074] Step S503: Based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively;

[0075] Step S504: During the access and operation of the offshore wind farm, the active power reference value of the object is adjusted based on the active power reference value of the offshore wind farm.

[0076] Offshore wind farms, as used here, refer to offshore wind farms located in water depths of approximately 10 meters. Compared to onshore wind farms, offshore wind farms offer the following advantages: they occupy no land resources, are largely unaffected by topography, offer higher wind speeds, offer more abundant wind energy resources, have larger turbine capacities (3-5 MW), and offer more annual utilization hours. However, offshore wind farm construction is also technically challenging, with construction costs generally being two to three times higher than onshore wind farms.

[0077] In an embodiment of the present application, an energy storage power station is added on the basis of the offshore wind power transmission system. The energy storage power station is connected to the low-frequency bus and the power frequency bus respectively through a newly added circuit breaker, thus forming a hybrid transmission system including the offshore wind power transmission system and the energy storage power station transmission system.

[0078] The configuration point of the energy storage station in the embodiment of the present application represents whether the energy storage station is configured at the low-frequency bus or the power frequency bus. Based on the configuration point of the energy storage station, the object of the active power reference value is determined, as well as the control strategy of the energy storage station and the frequency conversion station in the offshore wind power transmission system. Then, based on the control strategy of the energy storage station and the frequency conversion station, the operation process of the energy storage station and the frequency conversion station is controlled respectively. Finally, during the access and operation of the offshore wind farm, the active power reference value of the object is adjusted based on the active power reference value of the offshore wind farm.

[0079] From this, it can be seen that the operation mode of this hybrid transmission system, when there are fluctuations in the power entering the offshore wind power transmission system, prompts the newly added energy storage power station system to supplement or absorb power based on the active power reference value of the object, thereby transmitting stable power to the power grid, effectively solving the problem of fluctuations in the power entering the offshore wind power transmission system.

[0080] In addition, the newly added energy storage power station transmission system can smooth out the power fluctuations of offshore wind power entering the grid while providing more control target options for the frequency conversion station, thereby improving the stability of grid operation and the economic benefits of power transmission, while also providing a certain amount of inertia support for the grid.

[0081] In the embodiment of the present application, the configuration point of the energy storage power station represents that the energy storage power station is configured at the low-frequency bus, such as Figure 6A and 6B As shown, two different control strategies are provided, each of which is specific to the control mode of the energy storage power station and the frequency conversion station. These two strategies correspond to two different operating methods, which are described below.

[0082] The embodiment of the present application provides an operation method of a hybrid power transmission system in which an energy storage power station is configured at a low-frequency bus. Figure 6A As shown, the method may include steps S601 to S606:

[0083] Step S601: Determine the configuration point of the energy storage power station based on the working state of the newly added circuit breaker; the configuration point is used to indicate whether the energy storage power station is configured at the low-frequency bus or the power-frequency bus;

[0084] Step S602: When the configuration point indicates that the energy storage station is configured at the low-frequency bus, determining that the object of the active power reference value is the frequency conversion station, determining that the control strategy of the energy storage station is a constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode, and the control strategy of the power frequency side of the frequency conversion station is a virtual synchronous machine control mode;

[0085] Here, constant voltage / frequency (V / F) control is a power control technology that can maintain the voltage and frequency of alternating current (AC) stable. The basic principle of constant voltage and frequency control is to effectively control electrical equipment by adjusting the voltage and frequency in the power system. Low frequency generally refers to a signal or current with a narrow frequency range, characterized by a frequency lower than medium or high frequency. Specifically, low frequency can be defined as radio waves from approximately 30 kHz to 300 kHz. Signals or currents within this range are mainly used in power transmission, audio signals, and low-speed data transmission. In the electronics field, low-frequency circuits are mainly used in audio amplifiers, power supply circuits, logic circuits, etc.; in the communications field, low-frequency communication is mainly used in shortwave communication, submarine communication, and voice signal transmission.

[0086] Virtual Synchronous Generator (VSG) control involves controlling the inverter based on the electrical and mechanical characteristics of a synchronous generator, giving it the same operating characteristics. Based on the principles of the synchronous generator's speed regulator and excitation regulator, a frequency controller and voltage controller are designed to control the inverter's output frequency and voltage, respectively. Industrial frequency refers to the standard AC power frequency used in power systems. It determines the rated operating frequency of power generation, transmission, transformation, and distribution equipment, as well as industrial and civilian electrical equipment within the power system. In China, the industrial frequency is typically 50 Hz, while in other countries, such as the United States, it is 60 Hz. Average capacitor voltage control in the fixed bridge arm submodule involves adjusting the system's active current to maintain the system's average bridge arm capacitor voltage at the desired level.

[0087] Step S603: Based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively;

[0088] Here, the energy storage power station operates according to the constant voltage and constant frequency control mode determined in the above steps, the low frequency side of the frequency conversion station operates according to the fixed bridge arm submodule average capacitor voltage control mode determined in the above steps, and the industrial frequency side of the frequency conversion station operates according to the virtual synchronous machine control mode determined in the above steps.

[0089] Step S604: During the access and operation of the offshore wind farm, adjusting the active power reference value of the offshore wind farm;

[0090] Here, the active power reference value of the offshore wind farm can increase in a ramp-like manner. Such a change trend can avoid system instability caused by too rapid changes in the active power reference value of the offshore wind farm. In some embodiments, the adjustment rate of the active power reference value of the offshore wind farm can be selected to change by 1 megawatt per second.

[0091] Step S605: adjusting the active power reference value of the frequency conversion station to approach the active power reference value of the offshore wind farm through the virtual synchronous machine control mode started by the frequency conversion station on the power frequency side;

[0092] Here, it is necessary to establish communication between the offshore wind farm and the frequency conversion station so that the active power reference value of the frequency conversion station can change synchronously based on the change of the active power reference value of the offshore wind farm.

[0093] Step S606: when the active power reference value of the offshore wind farm approaches a preset power rating, stop adjusting the active power reference value of the offshore wind farm and the active power reference value of the frequency conversion station.

[0094] An embodiment of the present application provides an operating method for a hybrid transmission system. When an energy storage station is determined to be configured on a low-frequency bus, the active power reference value is determined to be the frequency conversion station. Furthermore, the control strategy of the energy storage station in the hybrid transmission system is determined to be a constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is determined to be a fixed-arm submodule average capacitor voltage control mode, and the control strategy of the power frequency side of the frequency conversion station is determined to be a virtual synchronous machine control mode. The energy storage station is then controlled to operate in the constant voltage and constant frequency control mode, the low-frequency side of the frequency conversion station is controlled to operate in the fixed-arm submodule average capacitor voltage control mode, and the power frequency side of the frequency conversion station is controlled to operate in the virtual synchronous machine control mode. During access and operation of an offshore wind farm, the active power reference value of the frequency conversion station is adjusted based on the active power reference value of the offshore wind farm. Finally, when the active power reference value of the offshore wind farm approaches a preset power rating, adjustment of the active power reference values ​​of the offshore wind farm and the frequency conversion station is stopped.

[0095] This hybrid transmission system operates by controlling the active power reference of the frequency converter station to approach the rated power value when the power delivered by the offshore wind farm fluctuates. Based on the power difference between the offshore wind farm's delivered power and the frequency converter station's active power reference value, the energy storage station automatically compensates or absorbs this power difference, thereby ensuring stable power transmission to the grid. In the event of a decrease in grid frequency, the energy storage station automatically increases a certain amount of active power to the grid based on the degree of grid frequency decrease, using the active power reference value controlled by the virtual synchronous machine and the offshore wind farm's delivered power, providing active inertia support for the grid.

[0096] When the power grid needs to reduce peak loads and fill valley loads, the active power reference value of the frequency conversion station controlled by the virtual synchronous machine is adjusted based on the actual needs of the power grid. Then, the energy storage power station automatically outputs or absorbs the corresponding power through the power difference between the transmission power of the offshore wind farm and the active power reference value of the frequency conversion station.

[0097] In some embodiments, the implementation of step S603 may include steps S6031 to S6033:

[0098] Step S6031: When the energy storage station starts the constant voltage and constant frequency control mode, after the low frequency side of the frequency conversion station is powered on, the low frequency side of the frequency conversion station is controlled to start the fixed bridge arm submodule average capacitor voltage control mode;

[0099] In some embodiments, after the low-frequency side of the frequency conversion station is powered on, the capacitor voltages of all bridge arms of the frequency conversion station enter a non-rechargeable stage, wherein the non-rechargeable stage means that when the frequency conversion station is not unlocked, that is, the transistors in the circuit of the frequency conversion station are in the off state, the capacitor voltages of all bridge arms of the frequency conversion station are charged through diodes.

[0100] Step S6032: adjusting the average capacitor voltage of all bridge arm submodules of the frequency conversion station by using the fixed bridge arm submodule average capacitor voltage control mode started on the low-frequency side of the frequency conversion station;

[0101] Here, a bridge arm refers to a portion of a commutation circuit that is connected between AC and DC terminals and has unidirectional or bidirectional conductivity. In some embodiments, the average capacitor voltage of the bridge arm submodule can be obtained by first determining the number of bridge arms in the frequency conversion station and the number of submodules comprising each bridge arm based on the circuit topology of the frequency conversion station; then, based on the actual capacitor voltage value of each submodule in each bridge arm of the frequency conversion station, determining the total capacitor voltage value of all submodules in all bridge arms of the frequency conversion station; and finally, dividing the total capacitor voltage value by the number of all submodules in the frequency conversion station to obtain the average capacitor voltage of all bridge arm submodules.

[0102] In some embodiments, when the average capacitor voltage is less than a preset reference voltage, the active current output from the AC side to the frequency conversion station can be increased through the average capacitor voltage control mode of the fixed bridge arm submodule, thereby increasing the average capacitor voltage; when the average capacitor voltage is greater than the preset reference voltage, the active current output from the AC side to the frequency conversion station can be reduced through the average capacitor voltage control mode of the fixed bridge arm submodule, thereby reducing the average capacitor voltage. The AC side here refers to the AC system connected to both sides of the frequency conversion station. In the case of a hybrid transmission system, the AC system refers to the power grid and the offshore module in the hybrid transmission system.

[0103] Step S6033: When the average capacitor voltage approaches a preset reference voltage, after powering on the power frequency side of the frequency conversion station, the power frequency side of the frequency conversion station is controlled to start a virtual synchronous machine control mode.

[0104] In some embodiments, the reference voltage is determined based on the number of AC systems and bridge arm submodules connected to the frequency conversion station. When the virtual synchronous machine control mode is enabled on the power frequency side of the frequency conversion station, the initial value of the active power reference value of the frequency conversion station is set to 0.

[0105] In an embodiment of the present application, when it is determined that the energy storage power station is controlled to start the constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is then started, and the average capacitor voltage of all bridge arm sub-modules of the frequency conversion station is adjusted. Finally, when the average capacitor voltage of all bridge arm sub-modules of the frequency conversion station approaches a preset reference voltage, the control strategy of the industrial frequency side of the frequency conversion station is started. This step-by-step method of starting each module can allow the hybrid transmission system to start smoothly and reduce the impact on the power grid.

[0106] Based on the above embodiments, Figure 6B As shown, the embodiment of the present application further provides an operation method of a hybrid power transmission system in which an energy storage power station is configured at a low-frequency bus, which may include steps S611 to S616:

[0107] Step S611: Determine the configuration point of the energy storage power station based on the working status of the newly added circuit breaker; the configuration point is used to indicate whether the energy storage power station is configured at the low-frequency bus or the power-frequency bus;

[0108] Step S612: When the configuration point indicates that the energy storage station is configured at the low-frequency bus, determining that the object of the active power reference value is the frequency conversion station, and determining that the control strategy of the energy storage station is a constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is a virtual synchronous machine control mode, and the control strategy of the power frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode;

[0109] Step S613: Based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively;

[0110] Here, the energy storage power station operates according to the constant voltage and constant frequency control mode determined in the above steps, the low frequency side of the frequency conversion station operates according to the virtual synchronous machine control mode determined in the above steps, and the industrial frequency side of the frequency conversion station operates according to the fixed bridge arm submodule average capacitor voltage control mode determined in the above steps.

[0111] Step S614: During the access and operation of the offshore wind farm, adjusting the active power reference value of the offshore wind farm;

[0112] Here, the active power reference value of the offshore wind farm may increase in a ramp-like manner. Such a change trend may avoid system instability caused by too rapid changes in the active power reference value of the offshore wind farm.

[0113] Step S615: adjusting the active power reference value of the frequency conversion station to approach the active power reference value of the offshore wind farm through the virtual synchronous machine control mode started by the frequency conversion station on the low-frequency side;

[0114] Here, it is necessary to establish communication between the offshore wind farm and the frequency conversion station so that the active power reference value of the frequency conversion station can change synchronously based on the change of the active power reference value of the offshore wind farm.

[0115] Step S616: when the active power reference value of the offshore wind farm approaches a preset power rating, stop adjusting the active power reference value of the offshore wind farm and the active power reference value of the frequency conversion station.

[0116] An embodiment of the present application provides an operating method for a hybrid transmission system. When an energy storage station is determined to be configured on a low-frequency bus, the active power reference value is determined to be the frequency conversion station. Furthermore, the control strategy of the energy storage station in the hybrid transmission system is determined to be a constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is determined to be a virtual synchronous machine control mode, and the control strategy of the power frequency side of the frequency conversion station is determined to be a fixed-bridge arm submodule average capacitor voltage control mode. The energy storage station is then controlled to operate in the constant voltage and constant frequency control mode, the low-frequency side of the frequency conversion station is controlled to operate in the virtual synchronous machine control mode, and the power frequency side of the frequency conversion station is controlled to operate in the fixed-bridge arm submodule average capacitor voltage control mode. During the connection and operation of an offshore wind farm, the active power reference value of the frequency conversion station is adjusted based on the active power reference value of the offshore wind farm. Finally, when the active power reference value of the offshore wind farm approaches a preset power rating, the adjustment of the active power reference values ​​of the offshore wind farm and the frequency conversion station is stopped.

[0117] This hybrid transmission system operates by controlling the active power reference of the frequency converter station at its rated power when the power delivered by the offshore wind farm fluctuates. The energy storage station automatically compensates or absorbs the power difference between the offshore wind farm's delivered power and the frequency converter station's active power reference, thereby ensuring stable power transmission to the grid. In the event of a decrease in grid frequency, the energy storage station automatically increases active power to the grid based on the degree of frequency decrease, combining the active power reference controlled by the virtual synchronous machine with the offshore wind farm's delivered power, providing active inertia support.

[0118] When the power grid needs to reduce peak loads and fill valley loads, the active power reference value of the frequency conversion station controlled by the virtual synchronous machine is adjusted based on the actual needs of the power grid. Then, through the power difference between the transmission power of the offshore wind farm and the active power reference value of the frequency conversion station, the energy storage power station automatically outputs or absorbs the corresponding power to the power grid.

[0119] In some embodiments, the implementation of step S613 may include steps S6131 to S6133:

[0120] Step S6131: When the energy storage station starts the constant voltage and constant frequency control mode, after the power frequency side of the frequency conversion station is powered on, the power frequency side of the frequency conversion station is controlled to start the fixed bridge arm submodule average capacitor voltage control mode;

[0121] In some embodiments, after the power frequency side of the frequency conversion station is powered on, the capacitor voltages of all bridge arms of the frequency conversion station enter a non-chargeable stage.

[0122] Step S6132: adjusting the average capacitor voltage of all bridge arm submodules of the frequency conversion station by using the fixed bridge arm submodule average capacitor voltage control mode started on the power frequency side of the frequency conversion station;

[0123] Step S6133: When the average capacitor voltage approaches a preset reference voltage, after powering on the low-frequency side of the frequency conversion station, controlling the low-frequency side of the frequency conversion station to start a virtual synchronous machine control mode.

[0124] Here, the reference voltage is determined based on the AC system and the number of bridge arm submodules connected on both sides of the frequency conversion station. When the virtual synchronous machine control mode is enabled on the low-frequency side of the frequency conversion station, the initial value of the active power reference value of the frequency conversion station is set to 0.

[0125] In an embodiment of the present application, when it is determined that the energy storage power station is controlled to start the constant voltage and constant frequency control mode, the control strategy of the power frequency side of the frequency conversion station is then started, and the average capacitor voltage of all the bridge arm sub-modules of the frequency conversion station is adjusted. Finally, when the average capacitor voltage of all the bridge arm sub-modules of the frequency conversion station approaches the preset reference voltage, the control strategy of the low frequency side of the frequency conversion station is started. This step-by-step method of starting each module can allow the hybrid transmission system to start smoothly and reduce the impact on the power grid.

[0126] In the embodiment of the present application, the configuration point of the energy storage power station represents that the energy storage power station is configured at the power frequency bus, such as Figure 7 As shown, a control strategy for an energy storage power station and a frequency conversion station is provided. An operation method based on this control strategy may include steps S701 to S706:

[0127] Step S701: Determine the configuration point of the energy storage power station based on the working state of the newly added circuit breaker; the configuration point is used to indicate whether the energy storage power station is configured at the low-frequency bus or the power-frequency bus;

[0128] Step S702: When the configuration point indicates that the energy storage station is configured at the power frequency bus, determining that the object of the active power reference value is the energy storage station, and determining that the control strategy of the energy storage station is a virtual synchronous machine control mode, the control strategy of the low-frequency side of the frequency conversion station is a constant voltage and constant frequency control mode, and the control strategy of the power frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode;

[0129] Step S703: Based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively;

[0130] Here, the energy storage power station operates according to the virtual synchronous machine control mode determined in the above steps, the low-frequency side of the frequency conversion station operates according to the constant voltage and constant frequency control mode determined in the above steps, and the industrial frequency side of the frequency conversion station operates according to the fixed bridge arm submodule average capacitor voltage control mode determined in the above steps.

[0131] Step S704: During the access and operation of the offshore wind farm, adjusting the active power reference value of the offshore wind farm;

[0132] Here, the active power reference value of the offshore wind farm may increase in a ramp-like manner. Such a change trend may avoid system instability caused by too rapid changes in the active power reference value of the offshore wind farm.

[0133] Step S705: When the active power reference value of the offshore wind farm approaches a preset power rating, stop adjusting the active power reference value of the offshore wind farm; and adjust the active power reference value of the energy storage power station based on the power rating by using the virtual synchronous machine control mode started by the energy storage power station;

[0134] Step S706: When the active power reference value of the energy storage power station meets the current demand of the power grid, stop adjusting the active power reference value of the energy storage power station.

[0135] An embodiment of the present application provides an operating method for a hybrid transmission system. When it is determined that an energy storage power station is configured on a power frequency bus, the object of the active power reference value is determined to be the energy storage power station, and the control strategy of the energy storage power station in the hybrid transmission system is determined to be a virtual synchronous machine control mode, the control strategy of the low-frequency side of the frequency conversion station is a constant voltage and constant frequency control mode, and the control strategy of the power frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode. Then, the energy storage power station is controlled to operate in the virtual synchronous machine control mode, the low-frequency side of the frequency conversion station is controlled to operate in the constant voltage and constant frequency control mode, and the power frequency side of the frequency conversion station is controlled to operate in the fixed bridge arm submodule average capacitor voltage mode. During the access and operation of the offshore wind farm, the active power reference value of the offshore wind farm is adjusted. Finally, when the active power reference value of the offshore wind farm approaches a preset power rated value, the active power reference value of the energy storage power station is adjusted.

[0136] This hybrid transmission system operates by controlling the combined output power of the offshore wind farm and energy storage power station to approach the rated power value when the power transmitted by the offshore wind farm fluctuates. Therefore, when the offshore wind farm's transmitted power is less than the rated power value, the virtual synchronous machine controls the active power reference value of the energy storage power station to compensate for the power that does not reach the rated power value. When the offshore wind farm's transmitted power exceeds the rated power value, the virtual synchronous machine controls the active power reference value of the energy storage power station to absorb the power that exceeds the rated power value, thereby transmitting stable power to the grid. When the grid frequency decreases, the active power reference value of the energy storage power station controlled by the virtual synchronous machine is adjusted based on the degree of grid frequency reduction, allowing the energy storage power station to automatically increase a certain amount of active power to the grid, providing active inertia support for the grid.

[0137] When the power grid needs to reduce peak loads and fill valley loads, based on the actual needs of the power grid, the active power reference value of the energy storage power station controlled by the virtual synchronous machine is adjusted, and then the transmission power of the offshore wind farm and the active power reference value of the energy storage power station are combined to allow the energy storage power station to output or absorb the corresponding power.

[0138] In addition, the operation mode of the hybrid power transmission system provided in this application can also supply power to local loads.

[0139] In some embodiments, the implementation of step S703 may include steps S7031 to S7033:

[0140] Step S7031: When the energy storage power station starts the virtual synchronous machine control mode, after the power frequency side of the frequency conversion station is powered on, the power frequency side of the frequency conversion station is controlled to start the fixed bridge arm submodule average capacitor voltage control mode;

[0141] Here, when the energy storage station starts the virtual synchronous machine control mode, the initial value of the active power reference value of the energy storage station is set to 0. After the power frequency side of the frequency conversion station is powered on, the capacitor voltage of all the bridge arms of the frequency conversion station enters the non-rechargeable stage.

[0142] Step S7032: adjusting the average capacitor voltage of all bridge arm submodules of the frequency conversion station by using the fixed bridge arm submodule average capacitor voltage control mode started on the power frequency side of the frequency conversion station;

[0143] Step S7033: When the average capacitor voltage approaches a preset reference voltage, after powering on the low-frequency side of the frequency conversion station, the low-frequency side of the frequency conversion station is controlled to start a constant voltage and constant frequency control mode.

[0144] Here, the reference voltage is determined according to the number of AC systems and bridge arm submodules connected on both sides of the frequency conversion station.

[0145] In an embodiment of the present application, when it is determined that the energy storage power station is controlled to start the virtual synchronous machine control mode, the control strategy of the industrial frequency side of the frequency conversion station is then started, and the average capacitor voltage of all the bridge arm sub-modules of the frequency conversion station is adjusted. Finally, when the average capacitor voltage of all the bridge arm sub-modules of the frequency conversion station approaches a preset reference voltage, the control strategy of the low-frequency side of the frequency conversion station is started. This step-by-step method of starting each module can allow the hybrid transmission system to start smoothly and reduce the impact on the power grid.

[0146] The operation method of the hybrid power transmission system is described in detail below with reference to a specific embodiment. However, it should be noted that this specific embodiment is only for better illustrating the present application and does not constitute an improper limitation to the present application.

[0147] The embodiment of the present application proposes a hybrid transmission system of an offshore wind power transmission system and an energy storage power station transmission system, wherein the offshore wind power transmission system is implemented based on flexible low-frequency transmission technology, and the energy storage power station is an AC direct-mounted type. Figure 2 As shown, energy storage station 18 is deployed between low-frequency bus 16 and power-frequency bus 19 of onshore portion 2, connected via circuit breakers QS1 and QS2, respectively. The deployment point of energy storage station 18 is changed by controlling the closing and opening of circuit breakers QS1 and QS2. For example, when QS1 is closed and QS2 is open, energy storage station 18 is deployed at low-frequency bus 16; when QS2 is closed and QS1 is open, energy storage station 18 is deployed at power-frequency bus 19. Depending on the deployment point, the operating frequency and control strategy of energy storage station 18 also vary.

[0148] Then, based on typical converter grid-type and grid-following control strategies, including: VSG control, V / F control, PQ control, constant capacitance voltage control, constant reactive power and constant AC voltage control, the control strategies of the energy storage power station 18 and the control strategies of the inverter 17 on the low-frequency side and the industrial frequency side were designed respectively.

[0149] Based on this, an operation method of the overall system in which the energy storage power station is configured at the low-frequency bus and the industrial frequency bus is further designed. This hybrid system and operation method can absorb offshore wind power and provide frequency and peak regulation services for the power grid.

[0150] 1. The energy storage power station is configured at low frequency;

[0151] In this embodiment of the present application, when the energy storage station is configured at the low-frequency bus, the operating frequency of the energy storage station is 20Hz. In this hybrid system, the energy storage battery acts as a stable DC voltage source, providing a stable low-frequency AC voltage for the offshore wind farm, and thus can operate in V / F control mode.

[0152] The low-frequency side active power (P) control of the M3C frequency conversion station can operate in the fixed bridge arm submodule average capacitor voltage control mode or VSG control mode, and the reactive power (Q) control can operate in the constant reactive power control mode or constant AC voltage control mode. Furthermore, when the low-frequency side active power (P) control of the M3C frequency conversion station operates in the fixed bridge arm submodule average capacitor voltage control mode, the power frequency side active power (P) control of the M3C frequency conversion station operates in the VSG control mode; when the low-frequency side active power (P) control of the M3C frequency conversion station operates in the VSG control mode, the power frequency side active power (P) control of the M3C frequency conversion station operates in the fixed bridge arm submodule average capacitor voltage control mode. As shown in Table 1 below, it shows the control strategy when the energy storage station is configured on the low-frequency bus:

[0153] Table 1: Control strategy for energy storage power stations configured on low-frequency buses

[0154]

[0155] In some embodiments, when the configuration point indicates that the energy storage power station is configured at the low-frequency bus, determining the control strategy of the energy storage power station and the frequency conversion station includes:

[0156] Determining that the control strategy of the energy storage power station is a constant voltage and constant frequency control mode, the control strategy of the low frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode, and the control strategy of the power frequency side of the frequency conversion station is a virtual synchronous machine control mode;

[0157] Alternatively, the control strategy of the energy storage power station is determined to be a constant voltage and constant frequency control mode, the control strategy of the low frequency side of the frequency conversion station is a virtual synchronous machine control mode, and the control strategy of the industrial frequency side of the frequency conversion station is determined to be a fixed bridge arm submodule average capacitor voltage control mode.

[0158] Here, it should be noted that the above two strategies are applicable to the same scenario. The operation modes of the two strategies in the above embodiment are specifically described below:

[0159] 1. Operation mode 1

[0160] Here, operation mode 1 is combined with Figure 2 To illustrate the operation process of the control strategy in the first row of Table 1.

[0161] Step 11: Close QS1 to connect the energy storage station 18 to the low-frequency bus 16;

[0162] Step 12: The energy storage power station 18 starts the V / F control mode to establish the low-frequency bus voltage;

[0163] Step 13: Close QS3 to power on the low-frequency side of the M3C frequency conversion station 17. At this time, the bridge arm capacitor voltage of the frequency conversion station 17 enters an uncontrollable charging stage;

[0164] Step 14: The low-frequency side of the frequency conversion station 17 starts the fixed bridge arm submodule average capacitor voltage control mode to adjust the average capacitor voltage of all bridge arm submodules of the frequency conversion station until the average capacitor voltage approaches the preset reference voltage;

[0165] Step 15: After closing QS4 to power on the power frequency side of the frequency conversion station 17, the power frequency side of the frequency conversion station 17 starts the VSG control mode, wherein the initial value of the active power reference value of the frequency conversion station 17 is set to 0;

[0166] Step 16: Close QS5 and QS6, and connect the submarine cable to 14;

[0167] Step 17: Close QS7, connect the offshore wind farm 11, and adjust the active power reference value of the offshore wind farm 11 to increase it in a ramp-like manner until the active power reference value of the offshore wind farm 11 approaches the preset power rating, and then stop adjusting;

[0168] Step 18: The VSG control mode started by the frequency conversion station 17 on the power frequency side is used to adjust the active power reference value of the frequency conversion station so that it is always consistent with the active power reference value of the offshore wind farm 11 until the rated power value is reached.

[0169] 2. Operation mode 2

[0170] Here, operation mode 1 is combined with Figure 2To illustrate the operation process of the control strategy in the second row of Table 1.

[0171] Step 21: Close QS1, and the energy storage station 18 is connected to the low-frequency bus 16;

[0172] Step 22: The energy storage power station 18 starts the V / F control mode to establish the low-frequency bus voltage;

[0173] Step 23: Close QS4 to power on the power frequency side of the frequency conversion station 17. At this time, the bridge arm capacitor voltage of the frequency conversion station enters the uncontrollable charging stage;

[0174] Step 24: The power frequency side of the frequency conversion station 17 starts the fixed bridge arm submodule average capacitor voltage control mode to adjust the average capacitor voltage of all bridge arm submodules of the frequency conversion station 17 until the average capacitor voltage approaches the preset reference voltage;

[0175] Step 25: After closing QS3 to power on the low-frequency side of the frequency conversion station 17, the low-frequency side of the frequency conversion station 17 starts the VSG control mode, wherein the initial value of the active power reference value of the frequency conversion station 17 is set to 0;

[0176] Step 26: Close QS5 and QS6, and connect the submarine cable 14;

[0177] Step 27: Close QS7, connect the offshore wind farm 11, and adjust the active power reference value of the offshore wind farm 11 to increase it in a ramp-like manner until the active power reference value of the offshore wind farm 11 approaches the preset rated value, and then stop adjusting;

[0178] Step 28: The VSG control mode started by the frequency conversion station 17 on the low frequency side is used to adjust the active power reference value of the frequency conversion station 17 so that it is always consistent with the active power reference value of the offshore wind farm 11 until the rated power value is reached.

[0179] It is worth noting that, between steps 17 and 18 in the above-mentioned operation mode 1, and between steps 27 and 28 in the above-mentioned operation mode 2, communication needs to be established between the offshore wind farm 11 and the frequency conversion station 17, so that the active power reference value of the frequency conversion station 17 can always be kept consistent with the active power reference value of the offshore wind farm 11.

[0180] Through either of the two aforementioned operating modes, if the power delivered by the offshore wind farm fluctuates, the active power reference value of the frequency converter station is controlled at the rated power value. Based on the power difference between the offshore wind farm's delivered power and the frequency converter station's active power reference value, the energy storage station automatically compensates for or absorbs this power difference, thereby enabling stable power transmission to the grid. In the event of a decrease in grid frequency, based on the degree of grid frequency decrease, the energy storage station automatically outputs a certain amount of active power to the grid, providing active inertia support, by adjusting the active power reference value controlled by the virtual synchronous machine and the offshore wind farm's delivered power. In the event of peak load shaving and valley filling, the active power reference value of the frequency converter station controlled by the virtual synchronous machine is adjusted based on the actual grid demand. Based on the power difference between the offshore wind farm's delivered power and the frequency converter station's active power reference value, the energy storage station automatically outputs or absorbs the corresponding power to the grid.

[0181] 2. The energy storage power station is configured at the industrial frequency.

[0182] In the embodiment of the present application, when the AC direct-hung energy storage power station is configured at the power frequency bus, the operating frequency of the energy storage power station needs to be 50Hz. In this hybrid system, the energy storage battery acts as a stable DC voltage source and can operate in VSG control mode; the low-frequency side of the M3C frequency conversion station needs to provide a stable low-frequency AC voltage for the offshore wind farm, so it needs to operate in V / F control mode; the active power (P) control of the power frequency side of the M3C frequency conversion station needs to operate in the fixed bridge arm submodule average capacitor voltage control mode, and the reactive power (Q) control of the power frequency side of the M3C frequency conversion station can operate in a fixed AC voltage or fixed reactive power control mode, as shown in Table 2 below, which shows the control strategy when the energy storage power station is configured at the power frequency bus:

[0183] Table 2: Control strategy for energy storage power stations configured on the power frequency bus

[0184]

[0185] In some embodiments, when the configuration point indicates that the energy storage station is configured at the power frequency bus, determining the control strategy of the energy storage station and the frequency conversion station includes: determining that the control strategy of the energy storage station is a virtual synchronous machine control mode, the control strategy of the low-frequency side of the frequency conversion station is a constant voltage and constant frequency control mode, and the control strategy of the power frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode. The specific operation of this embodiment is described below:

[0186] 1. Operation mode

[0187] Here, the operation mode is combined with Figure 2 To illustrate the operation process of the control strategy in Table 2.

[0188] Step 31: Close QS2 to connect the energy storage station 18 to the power frequency bus 19;

[0189] Step 32: The energy storage station 18 starts the VSG control mode, wherein the initial value of the active power reference value of the energy storage station 18 is set to 0;

[0190] Step 33: Close QS4 to power on the power frequency side of the frequency conversion station 17. At this time, the bridge arm capacitor voltage of the frequency conversion station 17 enters an uncontrollable charging stage;

[0191] Step 34: The power frequency side of the frequency conversion station 17 starts the fixed bridge arm submodule average capacitor voltage control mode to adjust the average capacitor voltage of all bridge arm submodules of the frequency conversion station 17 until the average capacitor voltage approaches the preset reference voltage;

[0192] Step 35: After closing QS3 to power on the low-frequency side of the frequency conversion station 17, the low-frequency side of the frequency conversion station 17 starts the V / F control mode and gradually builds up the low-frequency AC voltage;

[0193] Step 36: Close QS5 and QS6, and connect the submarine cable 14;

[0194] Step 37: Close QS7, connect the offshore wind farm 11, and adjust the active power reference value of the offshore wind farm 11 to increase it in a ramp-like manner until the active power reference value of the offshore wind farm 11 reaches the rated power value, and then stop adjusting;

[0195] Step 38 : According to the power rating or the current demand of the grid 21 , the active power reference value of the energy storage station 18 is adjusted by the VSG control mode started by the energy storage station 18 .

[0196] Through the above-mentioned operating mode, when the power transmitted by the offshore wind farm fluctuates, the total output power of the offshore wind farm and the energy storage power station is controlled to be close to the power rating. Therefore, when the transmission power of the offshore wind farm is less than the power rating, the active power reference value of the energy storage power station is adjusted by the virtual synchronous machine control to make up for the power that does not reach the power rating. When the transmission power of the offshore wind farm is greater than the power rating, the active power reference value of the energy storage power station is adjusted by the virtual synchronous machine control to absorb the power that exceeds the power rating, thereby transmitting stable power to the power grid. The operating mode of the hybrid transmission system provided in this application can also provide inertia support and peak-shaving and valley-filling services for the power grid, and can also supply power to local loads.

[0197] Based on the above implementation process, the embodiments of the present application propose a hybrid transmission system and its operation method. By configuring an energy storage power station at a low-frequency bus or a power-frequency bus, it can not only smooth the output of offshore wind power but also provide peak shaving and valley filling and inertia support functions for the power grid. The joint collaboration of offshore wind farms and energy storage power stations can achieve stable delivery of clean energy. Based on the embodiments of the present application, the following beneficial effects can be achieved:

[0198] 1. A hybrid transmission system is provided, comprising an offshore wind power transmission system and an energy storage power station transmission system. Specifically, the hybrid transmission system is based on an offshore wind power transmission system implemented using flexible low-frequency transmission technology, with the addition of an AC direct-mounted energy storage power station and two circuit breakers. The energy storage power station configuration point can be flexibly switched between the power frequency bus and the low-frequency bus via the circuit breaker. Compared to traditional offshore wind flexible low-frequency transmission solutions, the addition of an onshore AC direct-mounted energy storage power station can smooth offshore wind output fluctuations while providing more control target options for the frequency conversion station.

[0199] 2. A control strategy for a hybrid transmission system is provided. Based on typical converter grid-type and grid-following control strategies, detailed control strategies for frequency conversion stations configured on the power frequency side and low-frequency side, as well as control strategies for energy storage power stations, are designed.

[0200] 3. It provides an operating mode for a hybrid power transmission system, which enables the system to start smoothly by starting each module in steps, thus reducing the impact on the power grid.

[0201] It should be noted that the above descriptions of the various embodiments tend to emphasize the differences between the embodiments, and reference can be made to the similarities or similarities between them. The description of the above system embodiment is similar to the description of the above method embodiment and has similar beneficial effects as the method embodiment. For technical details not disclosed in the system embodiment of this application, please refer to the description of the method embodiment of this application for understanding.

[0202] It should be understood that "one embodiment" or "an embodiment" mentioned throughout the specification means that the specific features, structures or characteristics related to the embodiment are included in at least one embodiment of the present application. Therefore, "in one embodiment" or "in an embodiment" appearing throughout the specification does not necessarily refer to the same embodiment. In addition, these specific features, structures or characteristics can be combined in one or more embodiments in any suitable manner. It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned steps / processes does not mean the order of execution, and the execution order of each step / process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application. The above-mentioned serial numbers of the embodiments of the present application are for description only and do not represent the advantages and disadvantages of the embodiments.

[0203] It should be noted that, in this document, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or apparatus comprising the element.

[0204] In the several embodiments provided in this application, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are merely schematic. For example, the division of the units is merely a logical function division. In actual implementation, there may be other division methods, such as: multiple units or components can be combined, or can be integrated into another system, or some features can be ignored or not executed. In addition, the coupling, direct coupling, or communication connection between the components shown or discussed can be through some interfaces, and the indirect coupling or communication connection of the devices or units can be electrical, mechanical or other forms.

[0205] The units described above as separate components may or may not be physically separate, and the components displayed as units may or may not be physical units; they may be located in one place or distributed across multiple network units; some or all of the units may be selected according to actual needs to achieve the purpose of the scheme of this embodiment. In addition, the functional units in the various embodiments of the present application may all be integrated into one processing unit, or each unit may be separately used as a unit, or two or more units may be integrated into one unit; the above-mentioned integrated units may be implemented in the form of hardware or in the form of hardware plus software functional units.

[0206] The above is only an implementation method of the present application, but the scope of protection of the present application is not limited thereto. Any technician familiar with this technical field can easily think of changes or replacements within the technical scope disclosed in this application, which should be covered by the scope of protection of the present application.

Claims

1. A hybrid power transmission system, characterized in that: The system includes: an offshore wind power transmission system and an energy storage power station transmission system, wherein: The offshore wind power transmission system includes an offshore wind farm, a power grid and a frequency conversion station; the frequency conversion station is connected to the low-frequency bus and the power frequency bus respectively; The energy storage power station transmission system includes: an energy storage power station connected to the low-frequency bus and the power frequency bus respectively through newly added circuit breakers.

2. A method for operating a hybrid power transmission system according to claim 1, characterized in that: The method comprises: Determining a configuration point of the energy storage power station based on the working state of the newly added circuit breaker; the configuration point is used to indicate whether the energy storage power station is configured at the low-frequency bus or at the power-frequency bus; Based on the configuration point of the energy storage power station, determining an object of an active power reference value, and determining a control strategy for the energy storage power station and a frequency conversion station in the offshore wind power transmission system; the object includes the energy storage power station or the frequency conversion station in the offshore wind power transmission system; Based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively; During access and operation of the offshore wind farm, the active power reference value of the object is adjusted based on the active power reference value of the offshore wind farm.

3. The method according to claim 2, characterized in that Determining an object of an active power reference value based on a configuration point of the energy storage power station includes: when the energy storage power station is configured at the low-frequency bus, determining the object of the active power reference value as the frequency conversion station; During access and operation of the offshore wind farm, adjusting the active power reference value of the object based on the active power reference value of the offshore wind farm includes: During the access and operation of the offshore wind farm, adjusting the active power reference value of the offshore wind farm; By means of a virtual synchronous machine control mode started by the frequency conversion station on the power frequency side, the active power reference value of the frequency conversion station is adjusted to be close to the active power reference value of the offshore wind farm; or by means of a virtual synchronous machine control mode started by the frequency conversion station on the low frequency side, the active power reference value of the frequency conversion station is adjusted to be close to the active power reference value of the offshore wind farm; When the active power reference value of the offshore wind farm approaches a preset power rating, adjusting the active power reference value of the offshore wind farm and the active power reference value of the frequency conversion station is stopped.

4. The method according to claim 2, characterized in that Determining an object of the active power reference value based on a configuration point of the energy storage power station includes: when the energy storage power station is configured at the power frequency bus, determining the object of the active power reference value as the energy storage power station; During access and operation of the offshore wind farm, adjusting the active power reference value of the object based on the active power reference value of the offshore wind farm includes: During the access and operation of the offshore wind farm, adjusting the active power reference value of the offshore wind farm; When the active power reference value of the offshore wind farm approaches a preset power rated value, stop adjusting the active power reference value of the offshore wind farm; and adjust the active power reference value of the energy storage power station based on the power rated value through the virtual synchronous machine control mode started by the energy storage power station.

5. The method according to claim 4, characterized in that The method further comprises: When the active power reference value of the energy storage power station meets the power rated value, the adjustment of the active power reference value of the energy storage power station is stopped.

6. The method according to any one of claims 2 to 5, characterized in that Determining control strategies for the energy storage station and the frequency conversion station in the offshore wind power transmission system based on the configuration point of the energy storage station includes: When the configuration point indicates that the energy storage station is configured at the low-frequency bus, determining that the control strategy of the energy storage station is a constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode, and the control strategy of the power frequency side of the frequency conversion station is a virtual synchronous machine control mode; or, When the configuration point indicates that the energy storage power station is configured at the low-frequency bus, it is determined that the control strategy of the energy storage power station is a constant voltage and constant frequency control mode, the control strategy of the low-frequency side of the frequency conversion station is a virtual synchronous machine control mode, and the control strategy of the power frequency side of the frequency conversion station is a fixed bridge arm submodule average capacitor voltage control mode.

7. The method according to any one of claims 2 to 5, characterized in that Determining control strategies for the energy storage station and the frequency conversion station in the offshore wind power transmission system based on the configuration point of the energy storage station includes: When the configuration point indicates that the energy storage power station is configured at the power frequency bus, it is determined that the control strategy of the energy storage power station is a virtual synchronous machine control mode, the control strategy of the low-frequency side of the frequency conversion station is a constant voltage and constant frequency control mode, and the control strategy of the power frequency side of the frequency conversion station is an average capacitor voltage control mode of the fixed bridge arm submodule.

8. The method according to any one of claims 2 to 5, characterized in that Based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively, including: When the energy storage power station starts the constant voltage and constant frequency control mode, after the low frequency side of the frequency conversion station is powered on, the low frequency side of the frequency conversion station is controlled to start the fixed bridge arm submodule average capacitor voltage control mode; Adjusting the average capacitor voltage of all bridge arm submodules of the frequency conversion station by using the average capacitor voltage control mode of the fixed bridge arm submodule started on the low-frequency side of the frequency conversion station; When the average capacitor voltage approaches a preset reference voltage, after the power frequency side of the frequency conversion station is powered on, the power frequency side of the frequency conversion station is controlled to start a virtual synchronous machine control mode.

9. The method according to any one of claims 2 to 5, characterized in that Based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively, including: When the energy storage power station starts the constant voltage and constant frequency control mode, after the power frequency side of the frequency conversion station is powered on, the power frequency side of the frequency conversion station is controlled to start the fixed bridge arm submodule average capacitor voltage control mode; Adjusting the average capacitor voltage of all bridge arm submodules of the frequency conversion station by using the fixed bridge arm submodule average capacitor voltage control mode started on the power frequency side of the frequency conversion station; When the average capacitor voltage approaches a preset reference voltage, after the low-frequency side of the frequency conversion station is powered on, the low-frequency side of the frequency conversion station is controlled to start a virtual synchronous machine control mode.

10. The method according to any one of claims 2 to 5, characterized in that Based on the control strategies of the energy storage power station and the frequency conversion station, the operation processes of the energy storage power station and the frequency conversion station are controlled respectively, including: When the energy storage power station starts the virtual synchronous machine control mode, after the power frequency side of the frequency conversion station is powered on, the power frequency side of the frequency conversion station is controlled to start the fixed bridge arm submodule average capacitor voltage control mode; Adjusting the average capacitor voltage of all bridge arm submodules of the frequency conversion station by using the fixed bridge arm submodule average capacitor voltage control mode started on the power frequency side of the frequency conversion station; When the average capacitor voltage approaches a preset reference voltage, after the low-frequency side of the frequency conversion station is powered on, the low-frequency side of the frequency conversion station is controlled to start a constant voltage and constant frequency control mode.

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