Fault ride-through method, equipment and medium

Abstract

The invention belongs to the technical field of offshore wind power direct current transmission, and discloses a fault ride-through method, equipment and a medium, and the method is applied to a hybrid offshore wind power direct current delivery system composed of an offshore hybrid commutation platform, a seabed direct current cable and an onshore inverter station. The land inverter station is an MMC converter station; the offshore hybrid commutation platform is formed by connecting a diode rectification unit and an MMC unit in parallel, the rated capacity of the diode rectification unit is 85%-95% of the rated capacity of the system, and the rated capacity of the MMC unit is 5%-15% of the rated capacity of the system; the method comprises the following steps: monitoring DC and AC bus voltages in real time to judge a system fault state; triggering a corresponding fault ride-through mode based on the system fault state; the fault ride-through mode comprises the step of changing the operation mode of the MMC unit of the offshore hybrid commutation platform so as to realize fault ride-through. The MMC unit is utilized to make up for the defect that a diode rectification unit lacks controllability, and optimal balance of economical efficiency and fault ride-through performance is achieved.

Description

A fault ride-through method, device and medium Technical Field

[0001] This invention belongs to the field of offshore wind power DC transmission technology, specifically relating to a fault ride-through method, equipment, and medium. Background Technology

[0002] Diode rectifier units have shown great application potential in offshore wind power transmission due to their significant advantages such as simple structure, low cost, and high operating efficiency. However, diode rectifier systems have inherent technical defects, namely a lack of controllability and fault ride-through capability. Specifically, when a fault occurs in the receiving-end grid causing a rise in DC voltage, the diode rectifier unit cannot actively adjust the power delivery. The accumulation of excess power will cause the DC voltage to continue to rise, eventually triggering the wind turbine protection to disconnect from the grid, seriously affecting the safe and stable operation of the system.

[0003] In existing technologies, the following technical solutions mainly address the fault ride-through problem of offshore wind power DC transmission systems: 1. Patent CN114696354B proposes a fault ride-through method for back-to-back flexible DC systems. This method provides reactive power support and improves system recovery characteristics by setting an inner loop current limit and restoring the system at a preset rate during the fault recovery phase. While this technical solution can achieve fault ride-through, it is only applicable to purely flexible DC systems and requires a full-capacity converter, resulting in high investment costs.

[0004] 2. A joint fault ride-through control strategy for receiving-end AC faults in offshore wind power flexible DC transmission systems improves fault ride-through capability and reduces the capacity of DC energy-consuming devices through rapid voltage reduction and load shedding at wind farms and coordinated operation of DC energy-consuming devices. However, this technical solution still relies on a full-capacity flexible DC system, and the DC energy-consuming devices suffer from energy waste.

[0005] 3. Fault ride-through control strategy combining DC energy-consuming devices with wind farm-side converter voltage reduction and active load shedding in wind farms, as well as specific ride-through methods for onshore AC single-phase grounding faults.

[0006] While these technical solutions have improved the fault ride-through performance of the system to some extent, they still have the following limitations: 1. Limitations in technical applicability: Existing solutions mainly focus on the optimization of pure flexible DC systems or single devices (such as DC power consumption devices), and lack effective fault ride-through solutions for hybrid systems composed of diode rectifier units and MMC.

[0007] 2. Low energy utilization efficiency: Existing technologies mostly use DC energy-consuming devices to convert surplus energy into heat energy consumption, resulting in significant energy waste.

[0008] 3. High system cost: Pure flexible DC systems require full-capacity converters, and the investment cost is significantly higher than that of diode rectification solutions.

[0009] 4. Lack of collaborative control: For collaborative control of fault processes in hybrid systems, especially how to use small-capacity MMC to achieve proactive management of fault energy and system stability, existing technologies have not provided effective solutions.

[0010] In summary, existing technologies have significant gaps in fault ride-through capabilities for hybrid offshore wind power DC transmission systems, and there is an urgent need to develop a fault ride-through strategy that can effectively improve the system's fault ride-through capability and stability while taking into account economic efficiency. Summary of the Invention

[0011] To address the shortcomings of existing technologies, this invention provides a fault ride-through method, device, and medium. It rapidly identifies fault states based on real-time monitoring of DC and AC voltages, and changes the operating mode of small-capacity marine MMC units based on the fault states to achieve fault ride-through, thereby ensuring the safe and stable operation of the system.

[0012] This invention provides the following technical solution: The first objective of this invention is to provide a fault ride-through method, which is applied to a hybrid offshore wind power DC transmission system composed of an offshore hybrid converter platform, a submarine DC cable, and an onshore inverter station; the onshore inverter station is an MMC converter station; the offshore hybrid converter platform is composed of a diode rectifier unit and an MMC unit connected in parallel; the rated capacity of the diode rectifier unit is 85%~95% of the rated capacity of the system, and the rated capacity of the MMC unit is 5%~15% of the rated capacity of the system; the method includes: real-time monitoring of DC bus voltage and AC bus voltage, and judging the system fault state based on the real-time voltage; based on the system fault state, triggering a corresponding fault ride-through mode; the fault ride-through mode includes changing the operating mode of the MMC unit of the offshore hybrid converter platform to achieve fault ride-through.

[0013] A diode rectifier unit serves as the main power path, while a small-capacity MMC unit (5%~15%) serves as an auxiliary control path. This compensates for the lack of controllability in the diode rectifier unit, achieving an optimized balance between economy and fault ride-through performance. The method relies on real-time monitoring of DC and AC voltages to ensure rapid fault identification. Based on the fault condition, the operating mode of the MMC unit is changed to achieve fault ride-through, thereby ensuring the safe and stable operation of the system.

[0014] As a further improvement of the present invention, the system fault state determination based on real-time voltage includes: when the DC bus voltage is greater than a first set threshold and the duration is greater than a first delay, the system fault state is a DC side overvoltage fault; when the AC bus voltage is less than a second set threshold and the duration is greater than a second delay, the system fault state is an AC side low voltage fault.

[0015] By setting voltage thresholds and delays, transient disturbances are distinguished from actual faults, reducing false alarms and improving the accuracy and reliability of fault detection. It also covers DC-side overvoltage (typically caused by onshore power grid faults) and AC-side undervoltage (caused by internal wind farm faults), ensuring comprehensive system protection against typical faults and enhancing the method's adaptability.

[0016] As a further improvement of the present invention, the triggering of the corresponding fault ride-through mode includes: when the system fault state is a DC-side overvoltage fault, triggering a first fault ride-through mode to control the MMC unit of the offshore hybrid converter platform to switch from rectification mode to inverter mode; when the system fault state is an AC-side low voltage fault, triggering a second fault ride-through mode to control the MMC unit of the offshore hybrid converter platform to switch from rectification mode to reactive power support mode.

[0017] When a fault occurs, the diode rectifier unit continues to transmit active power. At this time, the MMC unit changes its operating mode according to the fault type. The MMC unit actively manages the power difference in the system during the fault by providing inverter or reactive power support, so as to quickly respond to the fault and achieve fault ride-through.

[0018] As a further improvement of the present invention, the inverter mode includes: the MMC unit of the offshore hybrid converter platform absorbs active power from the DC bus and inverts the absorbed active power into AC power to feed back to the offshore AC power grid.

[0019] Compared to traditional DC power-consuming devices that convert surplus power into heat consumption, this invention uses an MMC unit to invert and feed surplus power back to the offshore AC power grid, forming a local power discharge channel. This effectively suppresses the rise of DC bus voltage, reduces energy waste, and lowers system costs.

[0020] As a further improvement of this invention, the active power value is dynamically calculated by the proportional-integral controller based on the deviation of the DC bus voltage. This ensures precise matching between the absorbed power and the remaining power of the wind farm, avoiding over-adjustment or under-adjustment and improving control accuracy.

[0021] As a further improvement of the present invention, the reactive power support mode includes: the MMC unit of the offshore hybrid converter platform preferentially outputs reactive current to support and boost the AC bus voltage.

[0022] As a further improvement of the present invention, the reactive current value is calculated based on the voltage drop depth of the AC bus.

[0023] As a further improvement of this invention, once the DC bus voltage / AC bus voltage recovers to the normal range and remains stable, the MMC unit transitions from inverter mode / reactive power support mode to rectification mode at a preset rate. This gradual recovery strategy effectively prevents voltage or current oscillations caused by sudden power changes, ensuring the stability of the system recovery process.

[0024] A second objective of this invention is to provide a computer device comprising at least one processing unit and at least one storage unit, wherein the storage unit stores a computer program, and when the program is executed by the processing unit, the processing unit performs the aforementioned method.

[0025] A third objective of this invention is to provide a computer-readable storage medium storing a computer program executable by an electronic device, which, when run on the electronic device, causes the electronic device to perform the above-described method.

[0026] Compared with existing technologies, the beneficial effects of this invention are as follows: 1. Balance between economy and high performance: The diode rectifier unit serves as the main power path, while the small-capacity MMC unit serves as the auxiliary control path, compensating for the lack of controllability of the diode rectifier unit and achieving an optimized balance between economy and fault ride-through performance. The method is based on real-time monitoring of DC and AC bus voltages to ensure rapid identification of fault conditions, and changes the operating mode of the MMC unit based on the fault condition to achieve fault ride-through, thereby ensuring the safe and stable operation of the system.

[0027] 2. Adaptability to multiple fault scenarios: It can simultaneously cope with two typical fault scenarios, DC side overvoltage and AC side undervoltage, and protect the system safety under different disturbances.

[0028] 3. Active fault energy management: When a fault occurs on the DC side, compared with the passive energy consumption scheme of simply putting on DC energy consumption devices, this invention feeds the surplus power of the fault back to the local power grid through the active inverter conversion of the small-capacity MMC unit at sea, which effectively suppresses the rise of DC voltage and improves energy utilization efficiency.

[0029] 4. Smoother recovery process: A rate-controlled recovery logic is designed to avoid system oscillations caused by sudden changes in control commands after fault clearing, ensuring the stability of the system recovery process. Attached Figure Description

[0030] Figure 1 is a schematic diagram of the structure of the hybrid offshore wind power DC transmission system; Figure 2 is an overall flowchart of the fault ride-through method; Figure 3 is a control logic diagram of the fault ride-through method. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] The present invention will be further described in detail below with reference to the accompanying drawings: This embodiment provides a fault ride-through method, which is applied to a hybrid offshore wind power DC transmission system consisting of an offshore hybrid converter platform, a submarine DC cable, and an onshore inverter station. The system structure is shown in Figure 1. Specifically, the onshore inverter station is an MMC converter station; the offshore hybrid converter platform consists of a diode rectifier unit and an MMC unit connected in parallel; the rated capacity of the diode rectifier unit is 85% to 95% of the rated capacity of the system, and the rated capacity of the MMC unit is 5% to 15% of the rated capacity of the system.

[0033] As shown in Figures 2 and 3, the fault ride-through method includes: real-time monitoring of DC bus voltage and AC bus voltage, and determining the system fault state based on the real-time voltage; determining the system fault state based on the real-time voltage includes: when the DC bus voltage is greater than a first set threshold (e.g., 1.1 times the rated voltage) and the duration is greater than a first delay (e.g., 10ms), the system fault state is a DC side overvoltage fault; when the AC bus voltage is less than a second set threshold (e.g., 0.8 times the rated voltage) and the duration is greater than a second delay (e.g., 10ms), the system fault state is an AC side low voltage fault.

[0034] Based on the system fault state, a corresponding fault ride-through mode is triggered. In the fault ride-through mode, the operating mode of the MMC unit of the offshore hybrid converter platform is changed to achieve fault ride-through in coordination with the diode rectifier unit. Specifically, when the system fault state is a DC-side overvoltage fault, the first fault ride-through mode is triggered, controlling the MMC unit of the offshore hybrid converter platform to switch from rectification mode to inverter mode. In inverter mode, the MMC unit of the offshore hybrid converter platform absorbs active power from the DC bus and inverts the absorbed active power into AC power to feed back to the offshore AC grid. The absorbed active power value is dynamically calculated by the proportional-integral controller based on the deviation of the DC bus voltage. When the system fault state is an AC-side undervoltage fault, the second fault ride-through mode is triggered, controlling the MMC unit of the offshore hybrid converter platform to switch from rectification mode to reactive power support mode. In reactive power support mode, the MMC unit of the offshore hybrid converter platform prioritizes outputting reactive current to support and raise the AC bus voltage. The reactive current value is calculated based on the drop depth of the AC bus voltage.

[0035] In fault ride-through mode, the diode rectifier unit continues to operate uncontrolled, while the MMC unit provides critical inertial damping and voltage support to the system through rapid response.

[0036] Once the fault is detected as cleared (DC or AC bus voltage returns to normal and remains stable), the MMC unit of the offshore hybrid converter platform does not immediately shut down. Instead, it gradually reduces the absorbed active power or output reactive current according to a preset smooth transition rate, gradually transitioning from inverter / reactive power support mode to rectification mode, until the system finally returns to normal operation. Compared to instantaneous recovery, this gradual recovery strategy avoids secondary power and voltage surges, ensuring the stability of system recovery.

[0037] The following specific examples illustrate the detailed implementation of the fault ride-through method provided in this embodiment; wherein, the rated capacity of the system is 1000MW, and the rated capacity of the MMC unit of the offshore hybrid converter platform is 50MW.

[0038] 1. Parameter initialization: Set the DC overvoltage fault trigger threshold. Set the AC low voltage fault trigger threshold to 1.15 (pu). Set the fault detection delay to 0.8 (pu). It takes 10ms.

[0039] 2. Fault Ride-through Process: Scenario 1: A short circuit occurs in the onshore power grid, and the DC bus voltage rises to 1.18 (pu) and lasts for more than 10ms. At this time, the first fault ride-through (FRT) mode is immediately triggered. The MMC unit of the offshore hybrid converter platform switches from rectification mode to inverter mode within 2ms. The MMC unit of the offshore hybrid converter platform absorbs active power from the DC bus and inverts the absorbed active power into AC power to feed back to the offshore AC power grid.

[0040] The absorbed active power value is dynamically calculated by the proportional-integral (PI) controller based on the deviation of the DC bus voltage. The calculation formula is as follows:

[0041] in, This is the active power value, representing the target active power that the MMC unit needs to absorb; This is the proportional gain coefficient. This is the integral gain coefficient. Measure the voltage of the DC bus. The DC bus reference voltage is used as the reference voltage. The calculated active power value is used as the reference for the active current inner loop after being passed through the limiter. This drives the MMC unit to absorb active power from the DC bus and invert the absorbed active power into AC power to feed back to the offshore AC power grid, effectively curbing the further rise of DC voltage. The limiter is used to limit the upper limit of the absorbed active power value, which is the rated capacity of the MMC unit.

[0042] Scenario 2: A momentary voltage drop occurs inside the offshore wind farm, with the AC bus voltage dropping to 0.8 (pu) and lasting for more than 10ms. At this time, the second fault ride-through (FRT) mode is immediately triggered. The MMC unit of the offshore hybrid converter platform switches from rectification mode to reactive power support mode within 2ms. The MMC unit of the offshore hybrid converter platform prioritizes the output of reactive current to support and raise the AC bus voltage.

[0043] The reactive current value is calculated based on the voltage sag depth of the AC bus, and the calculation formula is as follows:

[0044] in, This is the reactive current value; The reactive power support factor ensures that the MMC unit can respond quickly to voltage changes; This is the AC bus reference voltage. Measure the voltage of the AC bus.

[0045] The MMC unit prioritizes the output of reactive current to ensure rapid injection of reactive power, thereby quickly supporting the AC bus voltage and helping the AC bus voltage to quickly return to the normal range, thus ensuring the normal operation of the wind turbine.

[0046] 3. Fault recovery: After the system detects the fault clearing signal (DC bus voltage stabilizes below 1.05 pu and remains stable for 100 ms, or AC bus voltage stabilizes above 0.95 pu and remains stable for 100 ms), it starts the recovery sequence; the MMC unit of the offshore hybrid converter platform does not immediately exit, but smoothly transitions from inverter mode or reactive power support mode to rectifier mode at a preset rate.

[0047] Specifically, the active power absorbed or the reactive current output by the MMC unit decreases linearly at a rate of 20% of its rated value per second, smoothly transitioning to zero within 5 seconds. Once the absorbed active power or output reactive current drops to zero, the MMC unit automatically switches back to normal rectification and voltage regulation mode, and the system resumes normal operation. This gradual recovery strategy avoids secondary power and voltage surges, ensuring the stability of the system recovery process.

[0048] The fault ride-through method provided in this embodiment is for a hybrid offshore wind power DC transmission system. The offshore hybrid circulating platform uses a diode rectifier unit as the main power channel and utilizes the characteristics of a small-capacity MMC unit as an auxiliary control path to compensate for the lack of controllability of the diode rectifier unit and achieve an optimized balance between economy and fault ride-through performance.

[0049] The fault ride-through method provided in this embodiment quickly identifies fault states based on real-time monitoring of DC and AC bus voltages, and changes the operating mode of the MMC unit based on the fault state to achieve fault ride-through. It can simultaneously cope with two typical fault scenarios: DC side overvoltage and AC side undervoltage, protecting the system's safety under different disturbances, thereby ensuring the safe and stable operation of the system.

[0050] This embodiment provides a computer device, including at least one processing unit and at least one storage unit, wherein the storage unit stores a computer program, and when the program is executed by the processing unit, the processing unit performs the above-described method.

[0051] This embodiment provides a computer-readable storage medium storing a computer program executable by an electronic device, which, when run on the electronic device, causes the electronic device to perform the above-described method.

[0052] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A fault ride-through method, the method being applied to a hybrid offshore wind power DC transmission system consisting of an offshore hybrid converter platform, a submarine DC cable, and an onshore inverter station; wherein the onshore inverter station is an MMC converter station; and the offshore hybrid converter platform is composed of a diode rectifier unit and an MMC unit connected in parallel; characterized in that, The rated capacity of the diode rectifier unit is 85% to 95% of the rated capacity of the system, and the rated capacity of the MMC unit is 5% to 15% of the rated capacity of the system. The method includes: real-time monitoring of DC bus voltage and AC bus voltage, and judging the system fault status based on the real-time voltage; triggering the corresponding fault ride-through mode based on the system fault status; the fault ride-through mode includes changing the operating mode of the MMC unit of the offshore hybrid converter platform to achieve fault ride-through.

2. The method according to claim 1, characterized in that, The system fault state determination based on real-time voltage includes: when the DC bus voltage is greater than a first set threshold and the duration is greater than a first delay, the system fault state is a DC side overvoltage fault; when the AC bus voltage is less than a second set threshold and the duration is greater than a second delay, the system fault state is an AC side low voltage fault.

3. The method according to claim 2, characterized in that, The triggering of the corresponding fault ride-through modes includes: when the system fault state is a DC-side overvoltage fault, triggering the first fault ride-through mode and controlling the MMC unit of the offshore hybrid converter platform to switch from rectification mode to inverter mode; when the system fault state is an AC-side low voltage fault, triggering the second fault ride-through mode and controlling the MMC unit of the offshore hybrid converter platform to switch from rectification mode to reactive power support mode.

4. The method according to claim 3, characterized in that, The inverter mode includes: the MMC unit of the offshore hybrid converter platform absorbs active power from the DC bus and inverts the absorbed active power into AC power to feed back to the offshore AC power grid.

5. The method according to claim 4, characterized in that, The active power value is dynamically calculated by the proportional-integral controller based on the deviation of the DC bus voltage.

6. The method according to claim 3, characterized in that, The reactive power support mode includes: the MMC unit of the offshore hybrid converter platform prioritizes the output of reactive current to support and boost the AC bus voltage.

7. The method according to claim 6, characterized in that, The reactive current value is calculated based on the voltage drop depth of the AC bus.

8. The method according to claim 3, characterized in that, Once the DC bus voltage / AC bus voltage returns to the normal range and remains stable, the MMC unit transitions from inverter mode / reactive power support mode to rectifier mode at a preset rate.

9. A computer device, characterized in that, It includes at least one processing unit and at least one storage unit, wherein the storage unit stores a computer program that, when executed by the processing unit, causes the processing unit to perform the method as described in any one of claims 1 to 8.

10. A computer-readable storage medium, characterized in that, It stores a computer program executable by an electronic device, which, when run on the electronic device, causes the electronic device to perform the method as described in any one of claims 1 to 8.

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