A method for direct current fault ride through of a sea-land integrated multi-terminal flexible direct current transmission system and related device

Abstract

The application provides a kind of sea-land integrated multi-terminal flexible direct transmission system DC fault ride-through method and related device, belongs to DC fault ride-through technical field.The application when the DC fault current of land overhead line is completely removed, and the surplus power of sea-land integrated multi-terminal flexible direct transmission system reaches balance, sea-land integrated multi-terminal flexible direct transmission system enters deionization stage, after sea-land integrated multi-terminal flexible direct transmission system deionization ends, offshore converter station restores DC current, and sea-land integrated multi-terminal flexible direct transmission system restores stably, and sea-land integrated multi-terminal flexible direct transmission system restores to power before fault within a set time.The application solves the problem that after DC fault removal, there is lack of effective coordinated control means, and it is difficult to ensure the rapid recovery and stable operation of sea-land integrated multi-terminal flexible direct transmission system after DC fault removal.

Description

Technical Field

[0001] This invention belongs to the field of DC fault ride-through technology, specifically relating to a DC fault ride-through method and related devices for a multi-terminal flexible DC transmission system integrating land and sea. Background Technology

[0002] As my country's ultra-large-scale offshore wind power enters a stage of large-scale and clustered development, the ultra-large-scale offshore wind power integrated land-sea DC transmission scheme, based on "offshore wind power directly landing via multi-terminal flexible DC and submarine cables + onshore DC switching station aggregation + DC overhead line direct transmission to load centers," has become an important technological direction for ultra-large-scale offshore wind power transmission due to its significant economic advantages, flexible operation mode, and minimal land resource occupation. However, the main difficulty of this scheme lies in the fact that the integrated land-sea transmission mode, which combines multi-terminal hub topology with DC submarine cable-overhead line hybrid transmission, places higher demands on the rapid clearing, selective isolation, and system ride-through capabilities of DC faults.

[0003] Among existing DC fault clearing methods, the "half-bridge MMC (modular multilevel converter) + DCCB (direct current circuit breaker)" scheme utilizes the DC circuit breaker to clear DC faults, but it has high requirements for equipment size, weight, and platform occupancy, making it difficult to meet the development needs of lightweight and compact offshore platforms. The "hybrid MMC + HSS (high speed switch)" scheme utilizes the converter's own fault-blocking capability to achieve DC fault ride-through, possessing technical advantages such as reduced-voltage operation and multiple fault restarts.

[0004] However, none of the above solutions adequately consider the smooth recovery of the system after a DC fault is cleared. In a multi-terminal flexible DC system (hereinafter referred to as the system), the onshore converter station typically employs differentiated control strategies: one group uses Udc / Q control, and the other uses P / Q control. During the recovery process after a fault is cleared, the mismatch in the response characteristics of the two control groups leads to a large transient surge in the DC current at the receiving end (in severe cases, the peak value can reach more than 1.6 times the rated value), which can easily cause protection maloperation and system oscillation, prolonging the recovery time or even causing failure.

[0005] In summary, existing methods lack effective coordination and control measures after DC fault clearance, making it difficult to guarantee the rapid recovery and stable operation of the system after DC fault clearance.

[0006] Therefore, there is an urgent need to propose a DC fault ride-through method that takes into account fault clearing, surplus power absorption, and rapid and stable system recovery. This method should not only achieve rapid fault clearing but also effectively improve the recovery characteristics after fault clearing, suppress transient DC current surges, and ensure stable system recovery, thereby enhancing the system's power supply reliability and fault ride-through capability. Summary of the Invention

[0007] The purpose of this invention is to provide a DC fault ride-through method and related apparatus for an integrated land-sea multi-terminal flexible DC power transmission system, which solves the problem in the prior art that there is a lack of effective coordination and control means after the DC fault is cleared, making it difficult to ensure the rapid recovery and stable operation of the integrated land-sea multi-terminal flexible DC power transmission system after the DC fault is cleared.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a DC fault ride-through method for an integrated land-sea multi-terminal flexible DC transmission system, comprising the following steps: Real-time acquisition of DC-side voltage and current electrical quantity information of offshore converter stations and onshore converter stations, recorded as voltage and current electrical quantity information; Based on voltage and current electrical quantity information, fault identification and fault location are performed on the integrated land and sea multi-terminal flexible direct transmission system to obtain the fault type and fault location. Based on the fault type and location, when a DC fault occurs on the onshore overhead line, the full and half bridge hybrid MMC of the offshore converter station and the onshore converter station will be immediately switched to the fault clearing mode to clear the DC fault current of the onshore overhead line. During the process of clearing DC fault current of onshore overhead lines, power is continuously injected into the AC side of the MMC of the offshore converter station to absorb the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system. Once the DC fault current of the onshore overhead line is completely cleared and the surplus power of the integrated land-sea multi-terminal flexible DC transmission system reaches a balance, the integrated land-sea multi-terminal flexible DC transmission system enters the deionization stage. After the deionization of the integrated land-sea multi-terminal flexible DC transmission system is completed, the offshore converter station restores the DC current, and the integrated land-sea multi-terminal flexible DC transmission system smoothly recovers and returns to the power before the fault occurred within a set time.

[0009] A further improvement of this invention lies in that, based on voltage and current electrical quantity information, fault identification and fault location are performed on the integrated land-sea multi-terminal flexible direct transmission system to obtain the fault type and fault location, specifically as follows: Based on voltage and current electrical quantity information, the fault identification and fault location of the integrated land and sea multi-terminal flexible direct transmission system are performed using protection criteria to obtain the fault type and fault location.

[0010] A further improvement of the present invention is that the clearing of DC fault current in overhead power lines specifically involves: When a DC fault occurs in an overhead line, the negative voltage output capability of the full-bridge submodule in the full-half-bridge hybrid MMC is utilized to first reduce the DC voltage to zero or even a negative value, and then a high-speed switch is used to trip the overhead line, thereby clearing the DC fault current of the overhead line.

[0011] A further improvement of the present invention is that the proportion of the full-bridge submodule in the full-half-bridge hybrid MMC is not less than a set threshold.

[0012] A further improvement of this invention is that the surplus power of the integrated land-sea multi-terminal flexible direct transmission system is specifically as follows: The surplus power of the integrated land-sea multi-terminal flexible direct transmission system is absorbed through the wind turbine energy dissipation device.

[0013] A further improvement of the present invention is that the offshore converter station restores DC current, specifically as follows: The offshore converter station restores DC current at a set rate ramp.

[0014] A further improvement of this invention lies in the smooth recovery of the integrated land-sea multi-terminal flexible direct transmission system, specifically as follows: Obtain the real-time output voltage of the DC controller; Based on the real-time output voltage of the DC controller, the AC quantity control command value of the onshore converter station is dynamically adjusted. By dynamically adjusting the AC output control command value of the onshore converter station, the integrated land-sea multi-terminal flexible direct transmission system can be smoothly restored.

[0015] Secondly, the present invention provides a DC fault ride-through system for a multi-terminal flexible DC power transmission system integrating land and sea, comprising: The data acquisition module is used to collect real-time voltage and current electrical quantity information of the DC side of the offshore converter station and the onshore converter station, and record it as voltage and current electrical quantity information; The fault diagnosis module is used to identify and locate faults in the integrated land and sea multi-terminal flexible direct transmission system based on voltage and current electrical quantity information, and to obtain the fault type and fault location. The fault clearing module is used to immediately switch the full-bridge hybrid MMC of the offshore converter station and the onshore converter station to the fault clearing mode when a DC fault occurs on the onshore overhead line based on the fault type and fault location, thereby clearing the DC fault current of the onshore overhead line. The surplus power absorption module is used to continuously inject power into the AC side of the MMC of the offshore converter station during the process of clearing DC fault current of onshore overhead lines, and to absorb the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system. The fault recovery module is used to ensure that when the DC fault current of the onshore overhead line is completely cleared and the surplus power of the integrated onshore and offshore flexible DC transmission system reaches a balance, the integrated onshore and offshore flexible DC transmission system enters the deionization stage. After the deionization of the integrated onshore and offshore flexible DC transmission system is completed, the offshore converter station restores the DC current, and the integrated onshore and offshore flexible DC transmission system recovers smoothly and restores to the power before the fault occurred within a set time.

[0016] Thirdly, the present invention provides an electronic device, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps of the DC fault ride-through method for the integrated land and sea multi-terminal flexible DC transmission system described above.

[0017] Fourthly, the present invention provides a storage medium storing a computer program thereon, wherein the computer program, when executed by a processor, implements the steps of the DC fault ride-through method for the integrated land-sea multi-terminal flexible DC transmission system described above.

[0018] Compared with the prior art, the present invention has the following beneficial effects: The proposed DC fault ride-through method for an integrated land-sea multi-terminal flexible DC power transmission system addresses two key issues. First, during the clearing of DC fault current from the onshore overhead line, continuous power injection is performed on the AC side of the MMC (Multi-Channel Controller) of the offshore converter station to absorb the surplus power of the integrated land-sea multi-terminal flexible DC power transmission system. This operation not only avoids overvoltage lockout of the sending-end converter valve but also ensures the transient stability of the integrated land-sea multi-terminal flexible DC power transmission system during fault clearing. Second, after the DC fault current from the onshore overhead line is cleared, a synchronous recovery strategy is designed (the offshore converter station restores DC current while the integrated land-sea multi-terminal flexible DC power transmission system smoothly recovers). This design effectively suppresses the transient peak value of the receiving-end DC current, avoids the risk of protection maloperation caused by current surges, and improves the recovery characteristics and operational stability of the integrated land-sea multi-terminal flexible DC power transmission system after fault clearing. These technical measures effectively solve the problem of lacking effective coordination and control methods after DC fault clearing, making it difficult to guarantee the rapid recovery and stable operation of the integrated land-sea multi-terminal flexible DC power transmission system after fault clearing. Attached Figure Description

[0019] Figure 1 This is a flowchart of the DC fault ride-through method for the integrated land-sea multi-terminal flexible DC transmission system of the present invention; Figure 2 This is a schematic diagram of the DC fault ride-through system of the integrated land and sea multi-terminal flexible DC transmission system of the present invention; Figure 3 This is a single-line diagram of the ±500kV / 6GW offshore wind power DC transmission system in Embodiment 4 of the present invention; Figure 4This is a flowchart of the DC fault ride-through method for the integrated land-sea multi-terminal flexible DC transmission system in Embodiment 4 of the present invention; Figure 5 This is a diagram of the energy-consuming device for offshore wind turbines in Embodiment 4 of the present invention; Figure 6 This is a block diagram of the onshore converter station fault recovery synchronization control strategy in Embodiment 4 of the present invention; Figure 7 The simulation results of DC fault ride-through for offshore and onshore converter stations in Embodiment 4 of the present invention are shown. Figure 8 This is a comparison of simulation results before and after adopting the fault recovery synchronization control strategy for the DC current of the onshore converter station in Embodiment 4 of the present invention; Figure 9 This is a schematic diagram of the structure of the electronic device of the present invention. Detailed Implementation

[0020] To further understand the content of this invention, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments are merely illustrative and not limiting of the invention.

[0021] Example 1: The flowchart of the DC fault ride-through method for the integrated land-sea multi-terminal flexible DC transmission system of the present invention is as follows: Figure 1 As shown, the DC fault ride-through method for the integrated land-sea multi-terminal flexible DC transmission system of the present invention includes the following steps: S1. Real-time acquisition of DC side voltage and current electrical quantity information of offshore converter station and onshore converter station, recorded as voltage and current electrical quantity information; S2. Based on voltage and current electrical quantity information, perform fault identification and fault location on the integrated land-sea multi-terminal flexible direct transmission system to obtain the fault type and fault location; S3. Based on the fault type and fault location, when a DC fault occurs on the onshore overhead line, the full and half bridge hybrid MMC of the offshore converter station and the onshore converter station will be immediately switched to the fault clearing mode to clear the DC fault current of the onshore overhead line. S4. During the process of clearing DC fault current of onshore overhead lines, power is continuously injected into the AC side of MMC of offshore converter station to absorb the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system. S5. When the DC fault current of the onshore overhead line is completely cleared and the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system reaches balance, the integrated onshore and offshore multi-terminal flexible DC transmission system enters the deionization stage. After the deionization of the integrated onshore and offshore multi-terminal flexible DC transmission system is completed, the offshore converter station restores DC current, and the integrated onshore and offshore multi-terminal flexible DC transmission system recovers smoothly and restores to the power before the fault within a set time.

[0022] Example 2: A schematic diagram of the DC fault ride-through system of the integrated land-sea multi-terminal flexible DC transmission system of this invention is shown below. Figure 2 As shown, the DC fault ride-through system of the integrated land-sea multi-terminal flexible DC transmission system of the present invention includes: The data acquisition module is used to collect real-time voltage and current electrical quantity information of the DC side of the offshore converter station and the onshore converter station, and record it as voltage and current electrical quantity information; The fault diagnosis module is used to identify and locate faults in the integrated land and sea multi-terminal flexible direct transmission system based on voltage and current electrical quantity information, and to obtain the fault type and fault location. The fault clearing module is used to immediately switch the full-bridge hybrid MMC of the offshore converter station and the onshore converter station to the fault clearing mode when a DC fault occurs on the onshore overhead line based on the fault type and fault location, thereby clearing the DC fault current of the onshore overhead line. The surplus power absorption module is used to continuously inject power into the AC side of the MMC of the offshore converter station during the process of clearing DC fault current of onshore overhead lines, and to absorb the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system. The fault recovery module is used to ensure that when the DC fault current of the onshore overhead line is completely cleared and the surplus power of the integrated onshore and offshore flexible DC transmission system reaches a balance, the integrated onshore and offshore flexible DC transmission system enters the deionization stage. After the deionization of the integrated onshore and offshore flexible DC transmission system is completed, the offshore converter station restores the DC current, and the integrated onshore and offshore flexible DC transmission system recovers smoothly and restores to the power before the fault occurred within a set time.

[0023] Example 3: The DC fault ride-through method for the integrated land-sea multi-terminal flexible DC transmission system of the present invention includes the following steps: S1. Real-time acquisition of DC side voltage and current electrical quantity information of offshore converter stations and onshore converter stations, recorded as voltage and current electrical quantity information.

[0024] S2. Based on voltage and current electrical quantity information, perform fault identification and fault location on the integrated land and sea multi-terminal flexible direct transmission system to obtain the fault type and fault location.

[0025] This step involves fault identification and location of the integrated land-sea multi-terminal flexible direct transmission system based on voltage and current electrical quantity information, obtaining the fault type and location, specifically: Based on voltage and current electrical quantity information, the fault identification and fault location of the integrated land and sea multi-terminal flexible direct transmission system are performed using protection criteria to obtain the fault type and fault location.

[0026] S3. Based on the fault type and fault location, when a DC fault occurs on the onshore overhead line, the full and half bridge hybrid MMC of the offshore converter station and the onshore converter station is immediately switched to the fault clearing mode to clear the DC fault current of the onshore overhead line.

[0027] This step involves clearing the DC fault current from the overhead power line, specifically: When a DC fault occurs in an overhead line, the negative voltage output capability of the full-bridge submodule in the full-half-bridge hybrid MMC is utilized to first reduce the DC voltage to zero or even a negative value, and then a high-speed switch is used to trip the overhead line, thereby clearing the DC fault current of the overhead line.

[0028] In this step, the proportion of the full-bridge submodule in the full-half-bridge hybrid MMC is not less than a set threshold (the set threshold in this embodiment is 60%), so as to ensure that the output capability of the negative voltage can meet the fault current clearing requirements.

[0029] S4. During the process of clearing DC fault current of onshore overhead lines, power is continuously injected into the AC side of the MMC of the offshore converter station to absorb the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system.

[0030] Specifically, during the process of clearing DC fault current in onshore overhead lines, offshore wind farms continuously inject power into the AC side of the offshore converter station's MMC, absorbing surplus power from the integrated onshore and offshore multi-terminal flexible DC transmission system through the wind turbine's energy dissipation devices. S5. When the DC fault current of the onshore overhead line is completely cleared and the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system reaches a balance, the integrated onshore and offshore multi-terminal flexible DC transmission system enters the deionization stage. After the deionization of the integrated onshore and offshore multi-terminal flexible DC transmission system is completed, the offshore converter station restores the DC current, and the integrated onshore and offshore multi-terminal flexible DC transmission system recovers smoothly and within a set time (200ms in this embodiment), the integrated onshore and offshore multi-terminal flexible DC transmission system recovers to the power before the fault (specifically, the rated operating power before the fault).

[0031] In this step, the offshore converter station restores DC current, specifically as follows: The offshore converter station restores DC current at a set rate ramp (the set rate ramp in this embodiment is 0.01 pu / ms).

[0032] In this step, the integrated land-sea multi-terminal flexible direct transmission system smoothly recovers, specifically as follows: Obtain the real-time output voltage of the DC controller; Based on the real-time output voltage of the DC controller, the AC quantity control command value of the onshore converter station is dynamically adjusted. By dynamically adjusting the AC output control command value of the onshore converter station, the integrated land-sea multi-terminal flexible direct transmission system can be smoothly restored.

[0033] This embodiment of the integrated land-sea multi-terminal flexible DC transmission system includes an offshore converter station, an onshore converter station, a high-speed DC switch (HSS), and an overhead DC line.

[0034] Example 4: This embodiment uses a ±500kV / 6GW symmetrical monopolar offshore wind power DC transmission system as an example to illustrate the method of the present invention in detail.

[0035] Figure 3 This is a single-line diagram of the ±500kV / 6GW symmetrical monopolar offshore wind power DC transmission system (also known as an integrated onshore and offshore multi-terminal flexible DC transmission system) involved in this invention. The system adopts a symmetrical monopolar topology, with a rated DC line voltage of ±500kV and a total transmission capacity of 6GW. The offshore converter area consists of three sets of converter stations connected in parallel, each with a capacity of 2GW. Each converter station adopts a symmetrical monopolar structure, and each station is connected to the 66kV AC bus via three three-phase, three-winding transformers. The offshore converter stations are connected to the onshore converter stations via DC submarine cables, DC high-speed switches, and DC overhead lines. The onshore converter area consists of two sets of converter stations connected in parallel, each with a capacity of 3GW. Each converter station also adopts a symmetrical monopolar structure, and each station is connected to the AC bus via two sets of three single-phase, two-winding transformers. Both the offshore and onshore sides adopt a full-half-bridge hybrid MMC topology and are equipped with high-speed switches for fault isolation.

[0036] Figure 4 This is a flowchart of the DC fault ride-through method for an integrated land-sea multi-terminal flexible DC power transmission system. The DC fault ride-through method for such systems is applied to applications such as... Figure 3 The offshore wind power integrated onshore-sea DC system shown is also called an integrated onshore-sea multi-terminal flexible DC transmission system. The invention will now be described in further detail: Under steady-state operation of the integrated land-sea multi-terminal flexible direct power transmission system, all three offshore converter stations adopt a V / F control strategy, while one onshore converter station adopts... U dc One group uses P / Q control, and the other group uses P / Q control. When a transient ground fault occurs on the DC overhead line, in order to clear the DC fault, both offshore and onshore converter stations are controlled to operate at zero voltage after the fault is detected. The specific control strategy is as follows: The three offshore converter stations switched from "AC quantity constant V / F + DC quantity open loop" before the fault to "AC quantity constant V / F + DC quantity constant DC current" during the fault, with the DC current command being zero.

[0037] One onshore converter station switched from "AC stator module voltage + DC current open loop" to "AC stator module voltage + DC current", with the DC current command set to zero. The other onshore converter station switched from "AC stator voltage + DC current open loop" to "AC stator module voltage + DC current open loop".U dc The command " / Q+DC open loop" switches to "AC stator module voltage + DC stator DC voltage", with the DC voltage command set to zero. Since the offshore wind farm continues to inject power into the AC side of the offshore converter station, the surplus power balancing issue must also be considered. 10ms after a fault occurs, the offshore wind farm's turbine energy dissipation device is activated to absorb the surplus power. The main surplus power is absorbed by the offshore wind farm's energy dissipation branch. The offshore wind farm's turbine energy dissipation device... Figure 5 As shown.

[0038] Figure 6 This is a block diagram of the AC / DC flow fault recovery synchronization control strategy for onshore P / Q converter stations and onshore Udc / Q converter stations in an embodiment of the present invention. Figure 6 The upper part is the control architecture of the onshore P / Q converter station. Figure 6 The lower half is the control architecture of the onshore Udc / Q converter station. Both control architectures are connected to the AC power grid and generate drive signals through NLM (Nearest Level Modulation). di / dt The differential limiting stage limits the rate of change of DC current by acquiring it. Min is the smallest unit, and NLM is the nearest-level approximation modulation module. The tracking synchronization strategy dynamically corrects the calculation formula and adjusts the control command based on the real-time DC voltage. Voltage and current dual closed-loop control generates the AC modulation voltage. V abc The DC voltage control loop and DC current control loop generate a DC bias voltage by acquiring DC voltage and DC current. u dc After being divided by 1 / 2, it is mixed with AC modulation voltage. V abc The summation generates the final modulated wave.

[0039] After the fault is cleared and the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system is balanced, the DC deionization phase begins. Once the deionization process is complete, DC current ramp recovery control is executed. Both the offshore converter station and the P / Q control converter station ramp recovery of DC current at a rate of 0.01 pu / ms. The DC current sample is then sent to... di / dt In the differential limiting stage, the DC current rise rate is synchronously constrained from the land side inner loop through the DC current control loop. The land and sea sides work together to limit the rate of change of DC current, so as to avoid the DC current recovery speed being too fast and causing a violent transient impact.

[0040] After the ionization process ends, the onshore converter station implements a fault recovery synchronization control strategy, and the AC side control commands are dynamically and adaptively adjusted according to the real-time output voltage of the DC controller.

[0041] For onshore P / Q converter stations, DC current sampling is fed into the DC current control loop to output DC bias voltage. u dc Relying on DC real-time voltage u dc Dynamic scaling of active power commands, rated active power command value P ref0 Real-time active power command values ​​are generated through a dynamically corrected formula. P ref : , P ref0 With dynamic correction P ref The minimum value is taken by the Min unit as the input command value for the voltage and current dual closed-loop controller, generating an AC modulated voltage. V abc Add 1 / 2 u dc The NLM modulation module then generates the drive signal.

[0042] For onshore UDC / Q converter stations, DC voltage sampling is fed into the DC voltage control loop to output DC bias voltage. u dc Rated DC voltage command value U dcref0 Real-time DC voltage command values ​​are generated using a dynamic correction formula. U dcref : , U dcref0 With dynamic correction U dcref The input voltage and current are taken by the Min unit and generated by the dual closed-loop AC modulation voltage. V abc After 1 / 2 u dc The superimposed signal is then input into the NLM modulation module to generate the driving signal.

[0043] The two converter stations have achieved real-time linkage adjustment of AC control commands with DC bus voltage, solving the defects of AC / DC power mismatch and DC current oscillation in the traditional fixed command recovery strategy.

[0044] Relying on Figure 6 The mid-loop control architecture, a full-bridge hybrid MMC, is based on dynamic correction. P ref , U dcref Gradually increase active power and DC voltage, following the DC bus voltage. u dc Continued recovery, dynamic adjustment coefficient u dc / V dcN Increase synchronously. P ref , U dcref As the wind power output approaches its rated value, the output power of offshore wind power is also recovering.

[0045] The synchronous control strategy of this invention ensures that the integrated onshore and offshore multi-terminal flexible DC power transmission system recovers to its pre-fault rated operating power within 200ms after the deionization phase ends, and the system recovers smoothly and quickly after the fault. Ultimately, the three offshore converter stations switch back to "AC constant V / F + DC open loop". The onshore converter station switches back to "AC constant P / Q + DC open loop". The other onshore converter station switches to "AC constant P / Q + DC open loop". U dc / Q+ direct flow open loop.

[0046] Figure 7 'a' represents the simulation results of fault ride-through at the offshore converter station. Figure 7 b and Figure 7 c represents the simulation results of fault ride-through at the onshore converter station, from... Figure 7 a, Figure 7 b and Figure 7 c shows that after a ground fault occurs in the DC line, both the offshore and onshore converter stations can reliably ride through the fault, enabling the integrated offshore and onshore multi-terminal flexible DC transmission system to have a zero-voltage fault ride-through capability of no less than 650ms under DC fault conditions.

[0047] Figure 8 'a' represents the simulation results of the DC current of the onshore converter station before adopting the fault recovery synchronization control strategy. Figure 8 As can be seen, there are problems such as DC current oscillation, large transient current impact, and poor recovery characteristics during the recovery process.

[0048] Figure 8 b represents the simulation results of the DC current of the onshore converter station after adopting a fault recovery synchronization control strategy. Figure 8 As can be seen from b, the method of the present invention effectively suppresses the transient peak value of the DC current at the receiving end, enables the two sets of converter stations to maintain power coordination during the recovery process, and improves the recovery characteristics and operational stability of the integrated land-sea multi-terminal flexible DC transmission system after fault clearance.

[0049] Compared with the prior art, the present invention has the following advantages: 1. Addressing the surplus power balance issue during DC faults in offshore wind power integrated onshore-offshore systems (also known as onshore-offshore multi-terminal flexible DC transmission systems, hereinafter referred to as the system), this invention utilizes energy dissipation devices on offshore wind turbines to absorb the surplus power continuously injected from the offshore side during DC faults. These energy dissipation devices are distributed at the outlet ends of each wind turbine, achieving reliable surplus power dissipation without relying on large-capacity centralized energy dissipation devices, thus avoiding overvoltage lockout of the sending-end converter valve. This invention enables the system to possess a zero-voltage fault ride-through capability of no less than 650ms under DC fault conditions, significantly improving the fault ride-through capability and power supply reliability of the onshore-offshore multi-terminal flexible DC transmission system.

[0050] 2. This invention addresses the problem of large DC current surges and poor recovery characteristics during the recovery period caused by the mismatch between the dynamic characteristics of Udc / Q control and P / Q control in onshore converter stations. It proposes a synchronous control strategy for ramp recovery and fault recovery. The offshore converter station ramps the DC current at a rate of 0.01 pu / ms. Simultaneously, the AC control command value of the onshore converter station is dynamically adjusted based on the real-time output voltage of the DC controller. That is, the onshore Udc / Q control converter station... U dcref The command value is dynamically tracked and adjusted to gradually restore the DC voltage; the P / Q control converter station uses... P dcref The command value is dynamically tracked and adjusted to gradually restore active power at a rate of 0.01 pu / ms, so that the two converter stations maintain power coordination during the recovery process. This effectively suppresses the transient peak value of the DC current at the receiving end, avoids the risk of protection maloperation caused by current surge, and improves the recovery characteristics and operational stability of the system after fault clearance.

[0051] 3. In existing technologies, when restoring power from a fault, onshore converter stations directly issue rated active power commands, causing a sudden surge in DC current and resulting in a severe transient impact. This invention addresses this by limiting the DC current recovery rate of offshore converter stations to a 0.01 pu / ms ramp, and by adding a feature to onshore P / Q converter stations... di / dt The differential limiting inner loop, with coordinated constraint on both land and sea sides, limits the rate of rise of DC current, significantly reducing transient overvoltage and overcurrent during fault recovery.

[0052] Existing onshore converter stations use fixed rated values ​​for active power and DC voltage control commands, and the AC / DC control loops are completely decoupled. During DC bus reconstruction, AC / DC power mismatch easily induces continuous DC current oscillations. This invention innovatively introduces a dynamic DC voltage correction coefficient. u dc / V dcN This enables the active power and DC voltage control commands to be linearly and adaptively adjusted according to the real-time DC voltage, achieving strong coupling and tracking synchronization between AC control commands and DC bus voltage status, thus eliminating the AC / DC power imbalance problem from the root of the control.

[0053] Example 5: Please see Figure 9 As shown, the present invention also provides an electronic device 100 for a DC fault ride-through method of a multi-terminal flexible DC transmission system integrating land and sea; the electronic device 100 includes a memory 101, at least one processor 102, a computer program 103 stored in the memory 101 and executable on the at least one processor 102, and at least one communication bus 104.

[0054] The memory 101 can be used to store the computer program 103. The processor 102 implements the steps of the DC fault ride-through method for the integrated land and sea multi-terminal flexible DC transmission system described in Embodiment 1 by running or executing the computer program stored in the memory 101 and calling the data stored in the memory 101. The memory 101 may mainly include a program storage area and a data storage area. The program storage area may store the operating system, at least one application program required for a function (such as sound playback function, image playback function, etc.), etc.; the data storage area may store data created according to the use of the electronic device 100 (such as audio data), etc. In addition, the memory 101 may include non-volatile memory, such as hard disk, memory, plug-in hard disk, smart media card (SMC), secure digital (SD) card, flash card, at least one disk storage device, flash memory device, or other non-volatile solid-state storage device.

[0055] The at least one processor 102 may be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The processor 102 may be a microprocessor or any conventional processor. The processor 102 is the control center of the electronic device 100, connecting various parts of the electronic device 100 via various interfaces and lines.

[0056] The memory 101 in the electronic device 100 stores multiple instructions to implement a DC fault ride-through method for a multi-terminal flexible DC transmission system integrating land and sea, and the processor 102 can execute the multiple instructions to achieve the following: Real-time acquisition of DC-side voltage and current electrical quantity information of offshore converter stations and onshore converter stations, recorded as voltage and current electrical quantity information; Based on voltage and current electrical quantity information, fault identification and fault location are performed on the integrated land and sea multi-terminal flexible direct transmission system to obtain the fault type and fault location. Based on the fault type and location, when a DC fault occurs on the onshore overhead line, the full and half bridge hybrid MMC of the offshore converter station and the onshore converter station will be immediately switched to the fault clearing mode to clear the DC fault current of the onshore overhead line. During the process of clearing DC fault current of onshore overhead lines, power is continuously injected into the AC side of the MMC of the offshore converter station to absorb the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system. Once the DC fault current of the onshore overhead line is completely cleared and the surplus power of the integrated land-sea multi-terminal flexible DC transmission system reaches a balance, the integrated land-sea multi-terminal flexible DC transmission system enters the deionization stage. After the deionization of the integrated land-sea multi-terminal flexible DC transmission system is completed, the offshore converter station restores the DC current, and the integrated land-sea multi-terminal flexible DC transmission system smoothly recovers and returns to the power before the fault occurred within a set time.

[0057] Example 6: If the modules / units integrated in the electronic device 100 are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, all or part of the processes in the methods of the above embodiments can also be implemented by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium, and when executed by a processor, it can implement the steps of the various method embodiments described above. The computer program includes computer program code, which can be in the form of source code, object code, executable files, or certain intermediate forms. The computer-readable medium can include: any entity or device capable of carrying the computer program code, a recording medium, a USB flash drive, a portable hard drive, a magnetic disk, an optical disk, a computer memory, and a read-only memory (ROM).

[0058] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0059] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0060] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0061] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A DC fault ride-through method for a multi-terminal flexible DC power transmission system integrating land and sea, characterized in that, Includes the following steps: Real-time acquisition of DC-side voltage and current electrical quantity information of offshore converter stations and onshore converter stations, recorded as voltage and current electrical quantity information; Based on voltage and current electrical quantity information, fault identification and fault location are performed on the integrated land and sea multi-terminal flexible direct transmission system to obtain the fault type and fault location. Based on the fault type and location, when a DC fault occurs on the onshore overhead line, the full and half bridge hybrid MMC of the offshore converter station and the onshore converter station will be immediately switched to the fault clearing mode to clear the DC fault current of the onshore overhead line. During the process of clearing DC fault current of onshore overhead lines, power is continuously injected into the AC side of the MMC of the offshore converter station to absorb the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system. Once the DC fault current of the onshore overhead line is completely cleared and the surplus power of the integrated land-sea multi-terminal flexible DC transmission system reaches a balance, the integrated land-sea multi-terminal flexible DC transmission system enters the deionization stage. After the deionization of the integrated land-sea multi-terminal flexible DC transmission system is completed, the offshore converter station restores the DC current, and the integrated land-sea multi-terminal flexible DC transmission system smoothly recovers and returns to the power before the fault occurred within a set time.

2. The DC fault ride-through method for a multi-terminal flexible DC transmission system integrating land and sea as described in claim 1, characterized in that, The method for fault identification and location of the integrated land-sea multi-terminal flexible direct transmission system based on voltage and current electrical quantity information is as follows: Based on voltage and current electrical quantity information, the fault identification and fault location of the integrated land and sea multi-terminal flexible direct transmission system are performed using protection criteria to obtain the fault type and fault location.

3. The DC fault ride-through method for an integrated land-sea multi-terminal flexible DC transmission system according to claim 1, characterized in that, The process of clearing DC fault current from overhead power lines specifically involves: When a DC fault occurs in an overhead line, the negative voltage output capability of the full-bridge submodule in the full-half-bridge hybrid MMC is utilized to first reduce the DC voltage to zero or even a negative value, and then a high-speed switch is used to trip the overhead line, thereby clearing the DC fault current of the overhead line.

4. The DC fault ride-through method for an integrated land-sea multi-terminal flexible DC transmission system according to claim 3, characterized in that, The proportion of full-bridge submodules in the full-half-bridge hybrid MMC is not less than a set threshold.

5. The DC fault ride-through method for an integrated land-sea multi-terminal flexible DC transmission system according to claim 1, characterized in that, The surplus power of the integrated land-sea multi-terminal flexible direct transmission system is specifically described as follows: The surplus power of the integrated land-sea multi-terminal flexible direct transmission system is absorbed through the wind turbine energy dissipation device.

6. The DC fault ride-through method for an integrated land-sea multi-terminal flexible DC transmission system according to claim 1, characterized in that, The restoration of DC current at the offshore converter station is specifically as follows: The offshore converter station restores DC current at a set rate ramp.

7. The DC fault ride-through method for an integrated land-sea multi-terminal flexible DC transmission system according to claim 1, characterized in that, The integrated land-sea multi-terminal flexible direct transmission system has been stably restored, specifically as follows: Obtain the real-time output voltage of the DC controller; Based on the real-time output voltage of the DC controller, the AC quantity control command value of the onshore converter station is dynamically adjusted. By dynamically adjusting the AC output control command value of the onshore converter station, the integrated land-sea multi-terminal flexible direct transmission system can be smoothly restored.

8. A DC fault ride-through system for a multi-terminal flexible DC power transmission system integrating land and sea, characterized in that, include: The data acquisition module is used to collect real-time voltage and current electrical quantity information of the DC side of the offshore converter station and the onshore converter station, and record it as voltage and current electrical quantity information; The fault diagnosis module is used to identify and locate faults in the integrated land and sea multi-terminal flexible direct transmission system based on voltage and current electrical quantity information, and to obtain the fault type and fault location. The fault clearing module is used to immediately switch the full-bridge hybrid MMC of the offshore converter station and the onshore converter station to the fault clearing mode when a DC fault occurs on the onshore overhead line based on the fault type and fault location, thereby clearing the DC fault current of the onshore overhead line. The surplus power absorption module is used to continuously inject power into the AC side of the MMC of the offshore converter station during the process of clearing DC fault current of onshore overhead lines, and to absorb the surplus power of the integrated onshore and offshore multi-terminal flexible DC transmission system. The fault recovery module is used to ensure that when the DC fault current of the onshore overhead line is completely cleared and the surplus power of the integrated onshore and offshore flexible DC transmission system reaches a balance, the integrated onshore and offshore flexible DC transmission system enters the deionization stage. After the deionization of the integrated onshore and offshore flexible DC transmission system is completed, the offshore converter station restores the DC current, and the integrated onshore and offshore flexible DC transmission system recovers smoothly and restores to the power before the fault occurred within a set time.

9. An electronic device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the DC fault ride-through method for the integrated land and sea multi-terminal flexible DC transmission system as described in any one of claims 1 to 7.

10. A storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the DC fault ride-through method for the integrated land and sea multi-terminal flexible DC transmission system as described in any one of claims 1 to 7.

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