Offshore wind power direct current transmission system

Abstract

The invention relates to the technical field of power electronics, in particular to an offshore wind power direct-current power transmission system, a direct-current submarine cable is provided with a submarine cable protection device, an offshore converter station is provided with an offshore protection device, and an onshore converter station is provided with an onshore protection device; the alternating current side of the offshore converter station is connected with the offshore wind power plant, and the direct current side of the offshore converter station is connected with the first end of the direct current submarine cable when the offshore wind power plant is in a starting stage; the direct current side of the land converter station is connected with the second end of the direct current submarine cable; wherein the submarine cable protection device is used for monitoring the operation state of the direct-current submarine cable and feeding back submarine cable abnormal information to the offshore protection device and the land protection device under the condition that the direct-current submarine cable operates abnormally; the offshore protection device is used for cutting off a connection part with the direct-current submarine cable according to the submarine cable abnormal information; and the onshore protection device is used for cooperating with the offshore protection device to analyze the submarine cable abnormal information to obtain a fault analysis result.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to an offshore wind power direct current transmission system. Background Art

[0002] As the pace of global decarbonization and emission reduction accelerates, vigorously promoting renewable and clean energy has become an important means to resolve the contradiction between energy supply and demand, and the development prospects of the offshore wind power industry are extremely broad.

[0003] At present, offshore wind power transmission technologies mainly include two types: high-voltage AC transmission and high-voltage DC transmission. Among them, high-voltage DC transmission technology is not affected by the charging current and charging power of the submarine cable, the transmission distance is not restricted, and the investment in submarine cables is relatively small. It is the preferred solution for large-scale long-distance offshore wind power transmission.

[0004] The existing offshore wind power transmission technology is greatly affected by the environment, so there is an urgent need to effectively protect the wind power transmission process. Summary of the invention

[0005] Based on this, it is necessary to provide an offshore wind power DC transmission system that can effectively protect the wind power transmission process in response to the above technical problems.

[0006] This embodiment provides an offshore wind power DC transmission system, which includes a DC submarine cable, an offshore converter station, and an onshore converter station; the DC submarine cable is equipped with a submarine cable protection device, the offshore converter station is equipped with an offshore protection device, and the onshore converter station is equipped with an onshore protection device;

[0007] The AC side of the offshore converter station is connected to the offshore wind farm, and the DC side of the offshore converter station is connected to the first end of the DC submarine cable when the offshore wind farm is in the startup phase; the DC side of the onshore converter station is connected to the second end of the DC submarine cable;

[0008] Among them, the submarine cable protection device is used to monitor the operating status of the DC submarine cable, and in the event of abnormal operation of the DC submarine cable, feedback the submarine cable abnormal information to the offshore protection device and the onshore protection device; the offshore protection device is used to cut off the connection with the DC submarine cable according to the submarine cable abnormal information; the onshore protection device is used to cooperate with the offshore protection device to analyze the submarine cable abnormal information and obtain the fault analysis result.

[0009] In one embodiment, a submarine cable protection device includes a current sensor, a voltage sensor, a temperature sensor, a signal processor, and a circuit breaker;

[0010] Among them, the current sensor and voltage sensor are used to monitor the current and voltage in the DC submarine cable; the temperature sensor is used to monitor the temperature of the DC submarine cable; the signal processor is used to receive signals from the current sensor, voltage sensor and current sensor, and process and analyze them to determine whether the DC submarine cable is in normal working condition; the circuit breaker is used to cut off the connection with the faulty DC submarine cable.

[0011] In one of the embodiments, the offshore protection device includes an offshore converter station monitoring circuit, a fault detection circuit, a protection control circuit, and a communication interface;

[0012] Among them, the converter station monitoring circuit is used to monitor the working status of the offshore converter station; the fault detection circuit is used to detect whether there is a fault in the converter station and the connection between it and the offshore wind farm; the protection control circuit is used to cut off the connection with the faulty part when a fault is detected; and the communication interface is used to communicate with the onshore protection device.

[0013] In one of the embodiments, the operating state of the offshore converter station is determined according to the voltage, current and power factor of the operating state of the offshore converter station.

[0014] In one of the embodiments, the onshore protection device includes an onshore converter station monitoring circuit, a grid protection circuit where the offshore wind power DC transmission system is located, a fault isolation circuit, and a control logic circuit;

[0015] Among them, the converter station monitoring circuit is used to monitor the working status of the onshore converter station; the grid protection circuit where the offshore wind power DC transmission system is located is used to monitor the voltage parameters and current parameters of the grid where the offshore wind power DC transmission system is located; the fault isolation circuit is used to cut off the connection with the faulty part; the control logic circuit is used to realize the automatic control function and logic judgment function of the onshore protection device;

[0016] Among them, the automatic control function of the onshore protection device is used to adjust the protection strategy and protection parameters of the protection device; the logical judgment function of the onshore protection device is used to determine the fault analysis results according to the voltage parameters and current parameters of the power grid where the offshore wind power DC transmission system is located.

[0017] In one of the embodiments, the logic judgment function of the onshore protection device is used to judge whether a fault occurs in the power grid based on the voltage parameters and current parameters of the power grid where the offshore wind power DC transmission system is located and the preset fault threshold or mode; after detecting the fault, the characteristics and information of the fault are analyzed to determine the type and nature of the fault and obtain the fault analysis result.

[0018] In one embodiment, the logic judgment function of the onshore protection device is also used to determine the type of protection action according to the type of fault.

[0019] In one embodiment, the offshore converter station includes a first modular multilevel converter; the onshore converter station includes a second modular multilevel converter;

[0020] The rated voltage level of the second modular multilevel converter matches the rated voltage level of the offshore DC power transmission system, the DC side of the second modular multilevel converter is connected to the second end of the DC submarine cable, and when the offshore wind farm is in the startup stage, the voltage level of the second modular multilevel converter matches the rated voltage level of the first modular multilevel converter;

[0021] The second modular multilevel converter is a hybrid modular multilevel converter composed of half-bridge sub-modules and full-bridge sub-modules, and the number of full-bridge sub-modules of the second modular multilevel converter is determined by the rated voltage level of the first modular multilevel converter; the first modular multilevel converter is a modular multilevel converter composed of half-bridge sub-modules, and the ratio of the rated voltage level of the first modular multilevel converter to the rated voltage level of the offshore DC transmission system is 20% to 30%.

[0022] In one of the embodiments, the offshore wind power DC transmission system also includes a control module, which is used to adopt a current sharing voltage balancing control strategy, monitor the current of each half-bridge sub-module or full-bridge sub-module, and use equivalent capacitance and equivalent inductance methods to achieve voltage balancing for the half-bridge sub-modules and full-bridge sub-modules in the second modular multi-level converter, and to achieve voltage balancing for each half-bridge sub-module in the first modular multi-level converter.

[0023] In one of the embodiments, the offshore wind power DC transmission system also includes a monitoring module, which is used to monitor the operating status of the second modular multilevel converter and the operating status of the first modular multilevel converter using a Kalman filter algorithm to determine the faulty half-bridge submodule or full-bridge submodule in the second modular multilevel converter and the first modular multilevel converter.

[0024] The above-mentioned offshore wind power DC transmission system integrates submarine cable protection devices, offshore protection devices and onshore protection devices to form a complete protection system. This multi-level, distributed protection strategy can quickly respond to and handle various abnormal situations, effectively prevent the expansion of faults, and improve the reliability and stability of the entire wind power transmission system. The submarine cable protection device can monitor the operating status of the DC submarine cable in real time. Once an abnormality (such as overheating, short circuit, insulation damage, etc.) is found, the abnormal information will be immediately fed back to the offshore and onshore protection devices. Based on this information, the offshore protection device quickly cuts off the connection with the faulty submarine cable to prevent the fault from spreading to the entire system. At the same time, the onshore protection device collaboratively analyzes the fault information and quickly locates the fault point. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the conventional technology, the drawings required for use in the embodiments or the conventional technology descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0026] FIG1 is a schematic diagram of an offshore wind power direct current transmission system in one embodiment;

[0027] FIG2 is a schematic diagram of a submarine cable protection device in one embodiment;

[0028] FIG. 3 is a schematic diagram of an offshore protection device in one embodiment.

[0029] FIG4 is a schematic diagram of an onshore protection device in one embodiment;

[0030] FIG5 is a schematic diagram of an offshore wind power DC transmission system in another embodiment. DETAILED DESCRIPTION

[0031] In order to facilitate understanding of the present application, the present application will be described more fully below with reference to the relevant drawings. Embodiments of the present application are provided in the drawings. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0033] It can be understood that the terms "first", "second", etc. used in the present application can be used in this article to describe various elements, but these elements are not limited by these terms. These terms are only used to distinguish the first element from another element.

[0034] It should be noted that when an element is considered to be "connected" to another element, it can be directly connected to the other element, or connected to the other element through an intermediate element. In addition, the "connection" in the following embodiments should be understood as "electrical connection", "communication connection", etc. if there is transmission of electrical signals or data between the connected objects.

[0035] When used herein, the singular forms "a", "an" and " / the" may also include plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "include / comprise" or "have" etc. specify the presence of stated features, wholes, steps, operations, components, parts or combinations thereof, but do not exclude the possibility of the presence or addition of one or more other features, wholes, steps, operations, components, parts or combinations thereof.

[0036] At present, the mainstream topology of offshore converters in offshore wind power DC transmission systems is modular multilevel converters (MMC). Modular multilevel converters can supply power to passive systems, and at the same time, they have no characteristic harmonics and reactive power compensation problems, and can provide flexible reactive power support. They are very suitable for offshore wind power transmission scenarios in terms of technology. However, since the offshore modular multilevel converter is composed of a large number of half-bridge sub-modules connected in series, the volume and weight of the converter valve hall are very large, which directly affects the size and weight of the offshore platform, resulting in high construction costs and construction difficulties. In order to reduce the volume and weight of offshore converters, an offshore wind power transmission scheme based on diode rectifiers is proposed. This scheme can greatly reduce the cost of offshore converters. However, since the diode rectifier cannot reversely supply power during the start-up phase of the wind farm, the offshore wind farm needs to have the ability to self-start and establish AC voltage self-synchronous operation, and a large number of modifications to the current wind turbines are required, which is difficult. In order to solve the problems of wind farm black start and voltage synchronization, a hybrid DC transmission scheme using an auxiliary converter and a diode rectifier in parallel is proposed. In order to reduce the cost of auxiliary converter components, the auxiliary converter is composed of MMC, high-voltage sub-module string, and resonant branch. The topology structure and control strategy are relatively complex. At the same time, a large number of series-parallel capacitors and inductors need to be configured in the resonant circuit, which weakens the economic advantage of the diode rectifier to a certain extent.

[0037] Please refer to FIG1 , which shows an offshore wind power DC transmission system provided by the first embodiment of the present application. As shown in FIG1 , the offshore wind power DC transmission system may include a DC submarine cable 120, an offshore converter station 140, and an onshore converter station 160. The AC side of the offshore converter station 140 is connected to the offshore wind farm, and the DC side of the offshore converter station 140 is connected to the first end of the DC submarine cable 120 when the offshore wind farm is in the startup stage; the DC side of the onshore converter station 160 is connected to the second end of the DC submarine cable 120;

[0038] In this embodiment, the offshore wind power DC transmission system includes a DC submarine cable 120, an offshore converter station 140 and an onshore converter station 160; the DC submarine cable 120 is equipped with a submarine cable protection device, the offshore converter station 140 is equipped with an offshore protection device, and the onshore converter station 160 is equipped with an onshore protection device;

[0039] The submarine cable protection device is used to monitor the operating status of the DC submarine cable 120, and in the event of an operating abnormality of the DC submarine cable 120, feedback submarine cable abnormality information to the offshore protection device and the onshore protection device; the offshore protection device is used to cut off the connection with the DC submarine cable 120 based on the submarine cable abnormality information; the onshore protection device is used to analyze the submarine cable abnormality information in coordination with the offshore protection device to obtain a fault analysis result.

[0040] Optionally, when the offshore wind farm is ready to start, the AC side of the offshore converter station 140 is connected to the offshore wind farm, and the DC side is connected to the DC side of the onshore converter station 160 through the DC submarine cable 120. The submarine cable protection device starts to continuously monitor the operating status of the DC submarine cable 120, including but not limited to key parameters such as current, voltage, and temperature. If the submarine cable protection device detects that the DC submarine cable 120 has an operating abnormality (such as overcurrent, overvoltage, overtemperature, insulation failure, etc.), the submarine cable abnormality information is immediately sent to the offshore protection device and the onshore protection device.

[0041] The submarine cable abnormality information may include detailed information such as abnormality type, abnormality degree, abnormality location, etc., for subsequent analysis and processing. After receiving the submarine cable abnormality information, the offshore protection device quickly determines whether it is necessary to cut off the connection part with the DC submarine cable 120 according to the preset protection logic and threshold. If it is determined that it is necessary to cut off, the offshore protection device immediately performs the cutting operation to prevent the abnormality from expanding or affecting the overall operation of the offshore wind farm.

[0042] The onshore protection device simultaneously receives the abnormal information of the submarine cable and begins to analyze the abnormal information in coordination with the offshore protection device. The analysis process may include comparing historical data, analyzing abnormal trends, evaluating potential impacts, etc., to obtain fault analysis results. The fault analysis results may indicate the specific cause of the abnormality, possible fault points, recommended treatment measures, etc. Based on the fault analysis results, the offshore protection device and the onshore protection device may take further protection measures or recovery measures. For example, if it is determined to be a transient abnormality and has been eliminated, the DC submarine cable 120 may be reconnected; if it is determined to be a serious fault, it may be necessary to start the backup system or perform maintenance.

[0043] In an exemplary embodiment, as shown in FIG2 , the submarine cable protection device includes a current sensor, a voltage sensor, a temperature sensor, a signal processor and a circuit breaker; wherein the current sensor and the voltage sensor are used to monitor the current and voltage in the DC submarine cable 120; the temperature sensor is used to monitor the temperature of the DC submarine cable 120; the signal processor is used to receive signals from the current sensor, the voltage sensor and the current sensor, and process and analyze them to determine whether the DC submarine cable 120 is in a normal working state; the circuit breaker is used to cut off the connection with the faulty DC submarine cable 120.

[0044] Specifically, the current sensor continuously monitors the current passing through the submarine cable. The current sensor converts the current value into an electrical signal and sends it to the signal processor in real time. The voltage sensor is used to monitor the voltage at both ends of the submarine cable. The voltage sensor converts the voltage value into an electrical signal and sends it to the signal processor. The temperature sensor is arranged at key parts of the submarine cable, such as joints, areas prone to heat, etc., to monitor the temperature change of the DC submarine cable 120. The temperature sensor converts the temperature value into an electrical signal and sends it to the signal processor.

[0045] After receiving the signals from the current sensor, voltage sensor and temperature sensor, the signal processor performs preprocessing (such as filtering, amplification, etc.) to improve the accuracy and reliability of the signals. Then, the signal processor further analyzes and calculates the preprocessed signals, such as comparing the current value with the preset threshold, analyzing the waveform and phase relationship of the current and voltage, calculating the rate of change of the temperature, etc., to determine whether the DC submarine cable 120 is in a normal working state.

[0046] The working state of the offshore converter station 140 may also be determined based on the voltage, current and power factor of the working state of the offshore converter station 140 .

[0047] Specifically, the signal processor determines whether there is an abnormality in the DC submarine cable 120, such as overcurrent, overvoltage, overtemperature, insulation failure, etc., based on the analysis and calculation results. If it is determined to be a fault, the protection mechanism is triggered immediately. After the protection mechanism is triggered, the signal processor sends a control signal to the circuit breaker, instructing it to cut off the connection with the faulty submarine cable. At the same time, the signal processor also sends submarine cable abnormality information to the offshore protection device and the onshore protection device for subsequent fault processing and analysis. After receiving the control signal from the signal processor, the circuit breaker responds quickly and cuts off the connection with the faulty submarine cable. This is usually achieved through internal mechanical or electronic mechanisms to ensure that the circuit can be quickly and reliably cut off in the event of a fault.

[0048] Furthermore, the offshore protection device may take further protection measures based on the received submarine cable abnormal information, such as shutting down related switches, starting backup systems, etc. At the same time, the onshore protection device also receives abnormal information and works with the offshore protection device to conduct detailed analysis and location of the fault for subsequent repair and recovery work. During the entire fault handling process, the signal processor can also record relevant fault data, such as the time of the fault, the type of fault, and the current, voltage, and temperature values ​​before and after the fault.

[0049] In an exemplary embodiment, as shown in FIG3 , the offshore protection device includes an offshore converter station 140 monitoring circuit, a fault detection circuit, a protection control circuit and a communication interface; wherein the converter station monitoring circuit is used to monitor the working status of the offshore converter station 140; the fault detection circuit is used to detect whether a fault occurs in the converter station and the connection part between the converter station and the offshore wind farm; the protection control circuit is used to cut off the connection with the faulty part when a fault is detected; and the communication interface is used to communicate with the onshore protection device.

[0050] The offshore wind power DC transmission system is started, the offshore converter station 140 starts working, and the offshore protection device enters the monitoring state. The working state of the offshore converter station 140 is continuously monitored, including but not limited to key parameters such as voltage, current, power, frequency, and the operating state of each component inside the converter station.

[0051] The fault detection circuit works in parallel with the converter station monitoring circuit to monitor in real time whether there are any abnormalities or faults in the converter station and its connection parts with the offshore wind farm (such as cables, connectors, transformers, etc.). The fault detection circuit analyzes the monitored data in real time based on the preset threshold and judgment logic to determine whether there is a fault or potential fault risk. Once the fault detection circuit determines that there is a fault, the protection control circuit is immediately triggered. The protection control circuit responds quickly and cuts off the connection with the faulty part to prevent the fault from expanding or affecting the stable operation of the entire system. This may include disconnecting specific switches, relays or circuit breakers.

[0052] At the same time or after the connection is cut off, the communication interface starts working and sends the fault information (including fault type, location, time, etc.) to the onshore protection device through the communication link. After receiving the fault information, the onshore protection device conducts further analysis and processing, and may work with the offshore protection device to determine the specific cause and solution of the fault. Based on the fault information, the offshore and onshore maintenance teams begin to locate the fault point and carry out necessary repair work. After the fault is repaired, the offshore protection device reconnects the disconnected part and restores the normal operation of the system after confirming that the system is safe.

[0053] In an exemplary embodiment, as shown in FIG4 , the onshore protection device includes an onshore converter station 160 monitoring circuit, a grid protection circuit where the offshore wind power DC transmission system is located, a fault isolation circuit and a control logic circuit; wherein the converter station monitoring circuit is used to monitor the working state of the onshore converter station 160; the grid protection circuit where the offshore wind power DC transmission system is located is used to monitor the voltage parameters and current parameters of the grid where the offshore wind power DC transmission system is located; the fault isolation circuit is used to cut off the connection with the faulty part; and the control logic circuit is used to realize the automatic control function and logic judgment function of the onshore protection device;

[0054] Among them, the automatic control function of the onshore protection device is used to adjust the protection strategy and protection parameters of the protection device; the logical judgment function of the onshore protection device is used to determine the fault analysis results according to the voltage parameters and current parameters of the power grid where the offshore wind power DC transmission system is located.

[0055] The specific onshore converter station 160 and onshore protection device are started and enter normal working state. The converter station monitoring circuit continuously monitors the working state of the onshore converter station 160, including key parameters such as voltage, current, power, temperature, and the operating state of the internal equipment of the converter station. The grid protection circuit where the offshore wind power DC transmission system is located simultaneously monitors the voltage parameters (such as voltage amplitude, phase, frequency, etc.) and current parameters (such as current amplitude, phase, harmonic content, etc.) of the grid where the offshore wind power DC transmission system is located to ensure the stable operation of the grid where the offshore wind power DC transmission system is located.

[0056] The monitoring circuit of the converter station and the grid protection circuit where the offshore wind power DC transmission system is located collect the monitored data in real time and send it to the control logic circuit. The control logic circuit preprocesses the received data, such as filtering, denoising, normalization, etc., to improve the accuracy and reliability of the data. The control logic circuit analyzes the processed data in real time according to the preset logical rules to determine whether the grid and converter station where the offshore wind power DC transmission system is located are in normal working condition. If an abnormality or fault is detected, the control logic circuit further analyzes the fault type, location, severity, etc., and generates a fault analysis result.

[0057] According to the fault analysis results and the current system status, the control logic circuit automatically adjusts the protection strategy and protection parameters of the protection device to adapt to different fault conditions and system requirements. The adjusted protection parameters are sent to the fault isolation circuit and other related circuits to ensure that the protection device can respond to the fault accurately and quickly.

[0058] Once a fault is detected, the control logic circuit immediately triggers the fault isolation circuit to cut off the connection with the faulty part to prevent the fault from expanding or affecting the operation of other normal parts. At the same time, the control logic circuit may also need to trigger other protection actions, such as sending an alarm signal, starting the backup power supply, adjusting the system operation mode, etc.

[0059] During the fault handling process, the onshore protection device may need to communicate with the offshore protection device, other onshore protection devices or the central control system to share fault information, protection strategies, etc. Through the communication interface, the onshore protection device can work with other equipment to jointly deal with fault conditions and improve the overall reliability and stability of the system. After the fault is isolated and handled, the control logic circuit will automatically adjust the system operation mode according to the system status and gradually restore the normal operation of the system.

[0060] Specifically, the logic judgment function of the onshore protection device is used to determine whether a fault has occurred in the power grid based on the voltage parameters and current parameters of the power grid where the offshore wind power DC transmission system is located and the preset fault threshold or mode; after detecting a fault, the fault characteristics and information are analyzed to determine the type and nature of the fault and obtain the fault analysis results.

[0061] Furthermore, the logic judgment function of the onshore protection device is also used to determine the type of protection action according to the type of fault.

[0062] Optionally, the onshore protection device is started, and the preset protection strategy, fault threshold and fault mode are loaded. According to the characteristics and requirements of the power system, the monitoring thresholds of parameters such as voltage and current, as well as different types of fault modes and corresponding protection actions are set.

[0063] The grid protection circuit of the offshore wind power DC transmission system is used to monitor the voltage parameters (such as voltage amplitude, phase difference, frequency deviation, etc.) and current parameters (such as current amplitude, phase difference, harmonic content, etc.) of the grid where the offshore wind power DC transmission system is located in real time. The monitored voltage and current data are collected in real time and sent to the control logic circuit for processing.

[0064] The control logic circuit preprocesses the received voltage and current data, such as filtering, denoising, and normalization, to improve the accuracy and reliability of the data. The preprocessed data is compared with the preset fault threshold or fault mode to determine whether the power grid has a fault. If the data exceeds the threshold or meets the fault mode, the fault detection process is triggered. The fault data is further analyzed and processed to extract the characteristic information of the fault, such as the time, location, duration, and voltage and current change trends of the fault. According to the extracted fault characteristic information, it is matched with the preset fault type library to determine the type and nature of the fault (such as short circuit fault, overload fault, ground fault, etc.).

[0065] According to the type of fault and the current state of the power system, determine the appropriate protection action type and protection mode (such as tripping, isolation, alarm, etc.). If necessary, automatically adjust the protection strategy and protection parameters according to the fault situation to adapt to different fault situations and system requirements.

[0066] Through the fault isolation circuit or other actuators, the determined protection action is executed, such as cutting off the connection of the faulty part, starting the backup power supply, sending an alarm signal, etc. The execution of the protection action is monitored, and the feedback signal is sent to the control logic circuit for subsequent processing and decision-making.

[0067] Send fault information, protection action information, etc. to other related equipment or systems (such as offshore protection devices, central control systems, etc.) through the communication interface for collaborative work and information sharing. According to needs, coordinate with other equipment or systems to handle fault conditions and improve the overall reliability and stability of the system.

[0068] In an exemplary embodiment, the offshore converter station 140 includes a first modular multilevel converter; the onshore converter station 160 includes a second modular multilevel converter;

[0069] The rated voltage level of the second modular multilevel converter matches the rated voltage level of the offshore DC power transmission system, the DC side of the second modular multilevel converter is connected to the second end of the DC submarine cable 120, and when the offshore wind farm is in the startup stage, the voltage level of the second modular multilevel converter matches the rated voltage level of the first modular multilevel converter;

[0070] The second modular multilevel converter is a hybrid modular multilevel converter composed of half-bridge sub-modules and full-bridge sub-modules, and the number of full-bridge sub-modules of the second modular multilevel converter is determined by the rated voltage level of the first modular multilevel converter; the first modular multilevel converter is a modular multilevel converter composed of half-bridge sub-modules, and the ratio of the rated voltage level of the first modular multilevel converter to the rated voltage level of the offshore DC transmission system is 20% to 30%.

[0071] It is understandable that the second modular multilevel converter (MMC2) in the onshore converter station 160 is designed to ensure that its rated voltage level matches the rated voltage level of the offshore DC transmission system. At the same time, the first modular multilevel converter (MMC1) in the offshore converter station 140 is designed to have a rated voltage level of 20% to 30% of the rated voltage level of the offshore DC transmission system. MMC2 adopts a hybrid design, including a half-bridge submodule and a full-bridge submodule. According to the rated voltage level of MMC1, the number of full-bridge submodules in MMC2 is determined to ensure that the two can match and work during the startup phase. The control module is designed to adopt a current sharing voltage balancing control strategy to achieve voltage balancing of the half-bridge submodule and the full-bridge submodule inside MMC2, as well as voltage balancing of each half-bridge submodule inside MMC1.

[0072] Furthermore, the offshore wind power DC transmission system also includes a control module, which is used to adopt a current sharing voltage balancing control strategy, monitor the current of each half-bridge sub-module or full-bridge sub-module, and use equivalent capacitance and equivalent inductance methods to achieve voltage balancing for the half-bridge sub-modules and full-bridge sub-modules in the second modular multi-level converter, and to achieve voltage balancing for each half-bridge sub-module in the first modular multi-level converter.

[0073] Optionally, the offshore wind power DC transmission system starts to start, and MMC1 and MMC2 enter the initialization state. According to the system design requirements, set the working parameters of MMC1 and MMC2, including rated voltage, rated current, control strategy parameters, etc. Check the connection between MMC1 and MMC2 through the DC submarine cable 120 to ensure that the connection is normal and correct.

[0074] When the offshore wind farm is in the startup phase, the voltage level of MMC2 is adjusted to match the rated voltage level of MMC1 to ensure that the two can be connected and work smoothly. The control module starts working, adopting the current sharing voltage balancing control strategy to monitor the current of each module in MMC2 and MMC1.

[0075] According to the monitored current data, the control quantity required for voltage balancing of the half-bridge submodule and the full-bridge submodule in MMC2 is calculated using the method of equivalent capacitance and equivalent inductance. According to the calculated control quantity, the voltage of each submodule in MMC2 is adjusted to achieve voltage balancing. At the same time, the voltage of each half-bridge submodule in MMC1 is also adjusted to ensure its voltage balancing. On the basis of voltage balancing, MMC1 transmits power to MMC2 through the DC submarine cable 120, and then transmits the electricity of the offshore wind farm to the power grid where the offshore wind power DC transmission system is located on land. The control module continuously monitors the working status of MMC1 and MMC2, including parameters such as voltage, current, and power, and makes adjustments as needed to ensure the stable operation of the system.

[0076] If a system fault occurs, such as overvoltage, overcurrent, short circuit, etc., the control module will immediately detect it and trigger the protection mechanism. According to the type and severity of the fault, the control module will instruct MMC1 and MMC2 to take corresponding protection actions, such as cutting off the fault circuit, starting the backup equipment, etc., to ensure the safe and reliable operation of the system.

[0077] Furthermore, based on the above-mentioned control module, when the offshore wind farm is in the startup stage, the control module generates a control instruction according to the voltage difference between the current output voltage of the onshore converter station and the rated voltage of the offshore converter station, and adjusts the switching state of the power electronic converter of the onshore converter station according to the control instruction; wherein the control instruction includes the target voltage adjustment amount and the switching frequency of the power electronic converter.

[0078] Exemplarily, the control module includes: a data acquisition unit: responsible for real-time acquisition of current and voltage data of onshore converter stations, offshore converter stations and DC cables. An initial voltage adjustment calculation unit: calculates the initial voltage adjustment according to the difference between the current output voltage of the onshore converter station and the rated voltage of the offshore converter station. A correction adjustment and current upper limit threshold calculation unit: uses real-time current and voltage data to calculate the correction adjustment, current upper limit threshold and switching frequency. A closed-loop control algorithm unit: uses a closed-loop control algorithm including virtual impedance to generate control instructions based on input data. A feature extraction and state evaluation unit: extracts key features and evaluates the operating status of the system. An environmental factor consideration unit: adjusts the control strategy according to environmental factors and generates final control instructions.

[0079] Further, the control module is specifically used for: the control module is used to generate an initial voltage adjustment amount according to the voltage difference between the current output voltage of the onshore converter station and the rated voltage of the offshore converter station, and generate a correction adjustment amount, a current upper limit threshold and a switching frequency according to the first real-time current and the first real-time voltage of the DC cable, the second real-time current and the second real-time voltage of the offshore converter station, and the third real-time current and the third real-time voltage of the onshore converter station; the current upper limit threshold is the current upper limit value between the DC submarine cable, the offshore converter station and the onshore converter station. The control module is also used to obtain a target voltage adjustment amount based on the correction adjustment amount for the initial voltage adjustment amount.

[0080] Furthermore, the control module is specifically used to: use the voltage difference between the current output voltage of the onshore converter station and the rated voltage of the offshore converter station, as well as the first real-time current and the first real-time voltage of the DC cable, the second real-time current and the second real-time voltage of the offshore converter station, and the third real-time current and the third real-time voltage of the onshore converter station as inputs to the closed-loop control algorithm, and generate control instructions according to the output of the closed-loop control algorithm; wherein the closed-loop control algorithm includes virtual impedance.

[0081] Furthermore, the control module is specifically used to: extract key features from a first real-time current and a first real-time voltage of the DC cable, a second real-time current and a second real-time voltage of the offshore converter station, and a third real-time current and a third real-time voltage of the onshore converter station; the key features include the mean, volatility and phase difference of the current and voltage; based on the key features, evaluate the operating status of the offshore wind power DC transmission system; wherein the operating status includes power transmission efficiency, stability, and load conditions.

[0082] Further, the control module is specifically used to: generate a control instruction according to environmental factors and the voltage difference between the current output voltage of the onshore converter station and the rated voltage of the offshore converter station; wherein the environmental factors include at least wind speed, wind direction and sea conditions. According to the environmental factors, determine the onshore operation strategy of the onshore converter station and the offshore operation strategy of the offshore converter station, and generate a control instruction based on the onshore operation strategy, the offshore operation strategy, and the voltage difference between the current output voltage of the onshore converter station and the rated voltage of the offshore converter station;

[0083] Among them, the onshore operation strategy is used for the output voltage and current values ​​of the converter in the onshore converter station, and the onshore operation strategy includes pulse width modulation parameters, reactive power compensation amount and trigger angle; the offshore operation strategy is used for the output voltage and current values ​​of the converter in the offshore converter station, and the offshore operation strategy includes pulse width modulation parameters, reactive power compensation amount and trigger angle.

[0084] Exemplarily, the current and voltage data of the onshore converter station, the offshore converter station and the DC cable are collected in real time through sensors and communication equipment. The collected data is transmitted to the data acquisition unit of the control module. The data acquisition unit transmits the difference between the current output voltage of the onshore converter station and the rated voltage of the offshore converter station to the initial voltage adjustment amount calculation unit. The initial voltage adjustment amount calculation unit calculates the initial voltage adjustment amount according to the difference. The data acquisition unit transmits the real-time current and voltage data of the DC cable, the offshore converter station and the onshore converter station to the correction adjustment amount and current upper limit threshold value calculation unit. The unit uses these data to calculate the correction adjustment amount, the current upper limit threshold value and the switching frequency. The correction adjustment amount is used to adjust the initial voltage adjustment amount to obtain the target voltage adjustment amount.

[0085] The current output voltage of the onshore converter station, the rated voltage of the offshore converter station, the real-time current and voltage of the DC cable, and the real-time current and voltage of the offshore and onshore converter stations are used as inputs to the closed-loop control algorithm. The closed-loop control algorithm unit contains virtual impedance to simulate the electrical characteristics of the actual system. The algorithm outputs control instructions, including the target voltage adjustment amount and the switching frequency of the power electronic converter.

[0086] The data acquisition unit transmits real-time current and voltage data to the feature extraction and state evaluation unit. This unit extracts key features such as the mean, fluctuation rate, and phase difference of current and voltage. Based on these features, the operating status of the system is evaluated, including power transmission efficiency, stability, and load conditions.

[0087] The environmental factor consideration unit receives environmental factor data such as wind speed, wind direction and sea conditions. Based on these factors, the onshore operation strategy of the onshore converter station and the offshore operation strategy of the offshore converter station are determined. The onshore operation strategy includes pulse width modulation parameters, reactive power compensation amount and trigger angle, etc., which are used to adjust the output voltage and current values ​​of the converter in the onshore converter station. The offshore operation strategy also includes these parameters, which are used to adjust the output voltage and current values ​​of the converter in the offshore converter station. The final control instruction is obtained by combining the onshore operation strategy, the offshore operation strategy and the control instruction generated by the voltage difference.

[0088] The control module transmits the final control command to the power electronic converter of the onshore converter station. The power electronic converter adjusts its switching state according to the control command, including the switching frequency and the target voltage adjustment. Through continuous adjustment, the efficient and stable operation of the offshore wind power DC transmission system is ensured. The control module should have a monitoring function that can display the operating status of the system and the execution of the control command in real time. In the event of an abnormality or failure, the control module should be able to respond quickly and take corresponding protection measures.

[0089] In an exemplary embodiment, the offshore wind power DC transmission system also includes a monitoring module for monitoring the operating status of the second modular multilevel converter and the operating status of the first modular multilevel converter using a Kalman filter algorithm to determine a faulty half-bridge submodule or a full-bridge submodule in the second modular multilevel converter and the first modular multilevel converter.

[0090] Specifically, the offshore wind power DC transmission system is started, and MMC1 and MMC2 enter the initialization state. The operating parameters of MMC1 and MMC2 are set, including the rated voltage, rated current, control strategy parameters, etc. At the same time, the parameters of the monitoring module are initialized, such as the initial state estimation and covariance matrix of the Kalman filter. Check the connection between MMC1 and MMC2 through the DC submarine cable 120 to ensure that the connection is normal. Adjust the voltage level of MMC2 to match MMC1, and the control module begins to adopt the voltage balancing control strategy of current sharing. The monitoring module is started, and the Kalman filter algorithm is used to monitor the operating status of MMC1 and MMC2 in real time, including voltage, current and sub-module status.

[0091] Collect real-time operating data of MMC1 and MMC2, such as voltage, current, and submodule status. Use the Kalman filter algorithm to process the collected data and estimate the current status of MMC1 and MMC2, including the voltage and current of each submodule. According to the state estimation results, detect whether there are abnormal or faulty submodules in MMC1 and MMC2. Determine whether a fault has occurred by comparing the estimated value with the preset threshold or mode. If a fault is detected, the monitoring module will further locate the faulty half-bridge submodule or full-bridge submodule.

[0092] Those of ordinary skill in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program, and the computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to memory, storage, database or other media used in the embodiments provided in this application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), tape, floppy disk, flash memory or optical memory, etc. Volatile memory can include random access memory (RAM) or external cache memory. As an illustration and not limitation, RAM can be in various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM).

[0093] In the description of this specification, the description with reference to the terms "some embodiments", "other embodiments", "ideal embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic descriptions of the above terms do not necessarily refer to the same embodiment or example.

[0094] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0095] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. An offshore wind power direct current transmission system, characterized in that: The offshore wind power DC transmission system comprises a DC submarine cable, an offshore converter station and an onshore converter station; the DC submarine cable is equipped with a submarine cable protection device, the offshore converter station is equipped with an offshore protection device, and the onshore converter station is equipped with an onshore protection device; The AC side of the offshore converter station is connected to the offshore wind farm, and the DC side of the offshore converter station is connected to the first end of the DC submarine cable when the offshore wind farm is in the startup phase; the DC side of the onshore converter station is connected to the second end of the DC submarine cable; Among them, the submarine cable protection device is used to monitor the operating status of the DC submarine cable, and when the DC submarine cable has an operating abnormality, it feeds back the submarine cable abnormality information to the offshore protection device and the onshore protection device; the offshore protection device is used to cut off the connection with the DC submarine cable according to the submarine cable abnormality information; the onshore protection device is used to cooperate with the offshore protection device to analyze the submarine cable abnormality information to obtain a fault analysis result.

2. The offshore wind power direct current transmission system according to claim 1, characterized in that: The submarine cable protection device includes a current sensor, a voltage sensor, a temperature sensor, a signal processor and a circuit breaker; Among them, the current sensor and the voltage sensor are used to monitor the current and voltage in the DC submarine cable; the temperature sensor is used to monitor the temperature of the DC submarine cable; the signal processor is used to receive signals from the current sensor, the voltage sensor and the temperature sensor, and process and analyze them to determine whether the DC submarine cable is in a normal working state; the circuit breaker is used to cut off the connection with the faulty DC submarine cable.

3. The offshore wind power direct current transmission system according to claim 1, characterized in that: The offshore protection device comprises an offshore converter station monitoring circuit, a fault detection circuit, a protection control circuit and a communication interface; Among them, the converter station monitoring circuit is used to monitor the working status of the offshore converter station; the fault detection circuit is used to detect whether there is a fault in the offshore converter station and the connection part between it and the offshore wind farm; the protection control circuit is used to cut off the connection with the faulty part when a fault is detected; and the communication interface is used to communicate with the onshore protection device.

4. The offshore wind power direct current transmission system according to claim 3, characterized in that: The operating state of the offshore converter station is determined according to the voltage, current and power factor of the operating state of the offshore converter station.

5. The offshore wind power direct current transmission system according to claim 1, characterized in that: The onshore protection device includes an onshore converter station monitoring circuit, a grid protection circuit, a fault isolation circuit and a control logic circuit where the offshore wind power DC transmission system is located; Among them, the converter station monitoring circuit is used to monitor the working status of the onshore converter station; the grid protection circuit where the offshore wind power DC transmission system is located is used to monitor the voltage parameters and current parameters of the grid where the offshore wind power DC transmission system is located; the fault isolation circuit is used to cut off the connection with the faulty part; the control logic circuit is used to realize the automatic control function and logic judgment function of the onshore protection device; Among them, the automatic control function of the onshore protection device is used to adjust the protection strategy and protection parameters of the protection device; the logical judgment function of the onshore protection device is used to determine the fault analysis result according to the voltage parameters and current parameters of the power grid where the offshore wind power DC transmission system is located.

6. The offshore wind power direct current transmission system according to claim 4, characterized in that: The logic judgment function of the onshore protection device is used to judge whether a fault occurs in the power grid based on the voltage parameters and current parameters of the power grid where the offshore wind power DC transmission system is located and the preset fault threshold or mode, and after detecting the fault, analyze the characteristics and information of the fault to determine the type and nature of the fault and obtain the fault analysis result.

7. The offshore wind power direct current transmission system according to claim 5, characterized in that: The logic judgment function of the onshore protection device is also used to determine the type of protection action according to the type of fault.

8. The offshore wind power direct current transmission system according to claim 7, characterized in that: The offshore converter station includes a first modular multilevel converter; the onshore converter station includes a second modular multilevel converter; The rated voltage level of the second modular multilevel converter matches the rated voltage level of the offshore DC power transmission system, the DC side of the second modular multilevel converter is connected to the second end of the DC submarine cable, and when the offshore wind farm is in the startup stage, the voltage level of the second modular multilevel converter matches the rated voltage level of the first modular multilevel converter; The second modular multilevel converter is a hybrid modular multilevel converter composed of half-bridge sub-modules and full-bridge sub-modules, and the number of full-bridge sub-modules of the second modular multilevel converter is determined by the rated voltage level of the first modular multilevel converter; the first modular multilevel converter is a modular multilevel converter composed of half-bridge sub-modules, and the ratio of the rated voltage level of the first modular multilevel converter to the rated voltage level of the offshore direct current transmission system is 20% to 30%.

9. The offshore wind power direct current transmission system according to claim 8, characterized in that: The offshore wind power direct current transmission system also includes a control module, which is used to adopt a current sharing voltage balancing control strategy, monitor the current of each half-bridge sub-module or full-bridge sub-module, and use the method of equivalent capacitance and equivalent inductance to achieve voltage balancing of the half-bridge sub-modules and the full-bridge sub-modules in the second modular multi-level converter, and to achieve voltage balancing of each half-bridge sub-module in the first modular multi-level converter.

10. The offshore wind power direct current transmission system according to claim 8, characterized in that: The offshore wind power DC transmission system also includes a monitoring module, which is used to monitor the operating status of the second modular multilevel converter and the operating status of the first modular multilevel converter using a Kalman filter algorithm to determine a faulty half-bridge submodule or a full-bridge submodule in the second modular multilevel converter and the first modular multilevel converter.

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