Direct-current fault ride-through method and device for offshore wind power through heterogeneous direct-current sending-out system and storage medium

Abstract

The embodiment of the invention relates to the field of power systems, and provides a direct current fault ride-through method and device for offshore wind power through a heterogeneous direct current sending-out system and a storage medium, and the system comprises an offshore wind plant, an offshore alternating current side switch device, an offshore rectifier station, a land switch pooling station and a land inverter station. Wind turbine generators of an offshore wind plant are gathered to an alternating current bus of an offshore rectifier station through an alternating current collection submarine cable and offshore alternating current side switch equipment, the offshore rectifier station is connected to a conversion station through a high-voltage direct current submarine cable, and the conversion station is used for converting the direct current submarine cable into a direct current overhead line and is connected to an onshore switch gathering station through the direct current overhead line. The direct current fault follow current removing devices are arranged on the direct current overhead line in series in a series connection mode and arranged on the side close to the land switch pooling station, and the land switch pooling station is connected to the land inverter station through the direct current overhead line. The system can improve the operation stability of the power system.

Description

Technical Field

[0001] This application relates to the field of power system technology, and in particular to a method, apparatus and storage medium for DC fault ride-through of offshore wind power via a heterogeneous DC transmission system. Background Technology

[0002] In the field of power system technology, the flexible DC transmission scheme based on Modular Multilevel Converter (MMC) is currently the mainstream solution for offshore wind power transmission during the development of offshore wind power clusters.

[0003] However, as offshore wind power develops towards deep-sea and large-scale directions, using DRU as the main converter and supplementing it with small-capacity MMC to build a heterogeneous DC transmission system is a common way to achieve economical and efficient transmission of offshore wind power clusters. Based on this, some research has been conducted on heterogeneous DC transmission control and fault ride-through strategies. However, due to the scenario requirements of conventional offshore wind power flexible DC submarine cable combined with land cable transmission projects, DC fault ride-through research has not yet been carried out.

[0004] In summary, traditional offshore wind power transmission systems via heterogeneous DC transmission lack an effective and suitable fault ride-through scheme, which can easily lead to DC blocking due to transient energy surges during fault ride-through, thereby affecting the stability of the power system operation. Summary of the Invention

[0005] This application provides an offshore wind power transmission system via heterogeneous DC transmission, a DC fault ride-through method, an apparatus, and a computer storage medium, which enables DC blocking to occur during fault ride-through due to transient energy impacts, thereby improving the stability of power system operation.

[0006] In a first aspect, embodiments of this application provide a DC fault ride-through method, the DC fault ride-through method comprising:

[0007] In the event of a detected fault in the overhead DC line, the onshore inverter station controls the DC voltage to zero, activates the fault current limiting circuit of the offshore auxiliary converter, activates the DC side energy consumption circuit of the wind turbine, and controls the energy consumption resistor of the wind turbine to be connected; and disconnects the offshore AC side switching equipment.

[0008] When the DC fault follow-through clearing device detects a DC fault and the marine AC side switching equipment is disconnected, the fast power electronic switch is disconnected, the surge arrester is activated, and the transient current of the fault circuit is controlled to quickly cross zero within a preset time interval.

[0009] When the insulation strength of the DC line insulation medium recovers, the control DC voltage of the onshore inverter station is restored to its initial value;

[0010] Once the DC voltage returns to its initial value, the offshore AC-side switchgear is closed to disconnect the energy-consuming resistor of the wind turbine and restore DC transmission power.

[0011] Secondly, embodiments of this application provide an offshore wind power heterogeneous DC transmission system for implementing the DC fault ride-through method as described in the first aspect above. The offshore wind power heterogeneous DC transmission system includes:

[0012] Wind turbine units;

[0013] Offshore AC-side switchgear, which is connected to the wind turbine generator;

[0014] The offshore rectifier station, wherein the wind turbine generators are connected to the AC busbar of the offshore rectifier station via AC collector submarine cables and the offshore AC-side switchgear; the offshore rectifier station includes an auxiliary converter;

[0015] The onshore switchgear collection station is connected to the offshore rectifier station, which is used to convert the DC submarine cable into a DC overhead line and connect it to the onshore switchgear collection station through the DC overhead line; the DC fault follow-through clearing device is connected in series on the DC overhead line and arranged near the onshore switchgear collection station.

[0016] The onshore inverter station is connected to the onshore switchgear collection station. The onshore inverter station is used to convert DC power into AC power and transmit electrical energy to the onshore AC power grid.

[0017] In another aspect, this application provides a DC fault ride-through device, which includes at least one connected processor and a memory, wherein the memory is used to store program code, and the processor is used to call the program code in the memory to execute the methods described in the above aspects.

[0018] In another aspect, this application provides a computer storage medium including instructions that, when executed on a computer, cause the computer to perform the methods described in the above aspects.

[0019] In another aspect, this application provides a computer program product containing instructions that, when run on a computer, cause the computer to perform the methods described in the above aspects.

[0020] Compared to traditional DC fault ride-through methods in conventional technologies, this application embodiment constructs a heterogeneous DC transmission system for offshore wind power, in which diode rectifier units and auxiliary converters are connected in parallel on the AC side and operate independently on the DC side. Through the coordinated operation of onshore inverter stations, auxiliary converters, wind turbines of offshore wind farms, offshore AC-side switchgear, and DC fault follow-through devices, in the event of a short-circuit fault in the DC overhead line of the offshore wind power transmission system via the heterogeneous DC transmission system, the DC fault current rapidly crosses zero, preventing DC blocking due to transient energy impacts during fault ride-through, thereby improving the stability of power system operation. Attached Figure Description

[0021] Figure 1 A topology diagram of an offshore wind power transmission system via heterogeneous DC transmission;

[0022] Figure 2 This is a schematic diagram of the control logic of an offshore wind turbine in one embodiment;

[0023] Figure 3 This is a schematic diagram of the control logic for the DC-side energy consumption of a wind turbine in one embodiment.

[0024] Figure 4 This is a schematic diagram of the auxiliary MMC control strategy for a marine rectifier station in one embodiment;

[0025] Figure 5 This is a schematic diagram of the topology and control logic of a DC fault freewheeling clearing device in one embodiment;

[0026] Figure 6 This is a flowchart illustrating a DC fault ride-through method in one embodiment;

[0027] Figure 7 A schematic diagram of the steady-state operating waveform of offshore wind power transmitted via a heterogeneous DC transmission system on the offshore side.

[0028] Figure 8 A schematic diagram of the steady-state operating waveform of the onshore side of the offshore wind power transmission system via a heterogeneous DC transmission system.

[0029] Figure 9 A schematic diagram of DC fault occurrence in an offshore wind power transmission system via a heterogeneous DC transmission system.

[0030] Figure 10 A schematic diagram of DC fault occurrence in an offshore wind power transmission system via a heterogeneous DC transmission system.

[0031] Figure 11 A schematic diagram showing the changes in DC fault current of offshore wind power transmitted through a heterogeneous DC transmission system.

[0032] Figure 12 This is an internal structural diagram of a DC fault ride-through device in one embodiment;

[0033] Figure 13 This is an internal structural diagram of the DC fault ride-through device in another embodiment. Detailed Implementation

[0034] The terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments described herein can be implemented in a sequence other than that illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or modules is not necessarily limited to those explicitly listed, but may include other steps or modules not explicitly listed or inherent to these processes, methods, products, or devices. The division of modules in the embodiments of this application is merely a logical division; in actual applications, there may be other division methods. For example, multiple modules may be combined into or integrated into another system, or some features may be ignored or not performed. Additionally, the shown or discussed mutual coupling or direct coupling or communication connection may be through some interface, and the indirect coupling or communication connection between modules may be electrical or other similar forms, none of which are limited in the embodiments of this application. Furthermore, the modules or sub-modules described as separate components may or may not be physically separated, may or may not be physical modules, or may be distributed among multiple circuit modules. Some or all of the modules may be selected according to actual needs to achieve the purpose of the embodiments of this application.

[0035] Figure 1 This is a topology diagram of an offshore wind power transmission system via heterogeneous DC transmission, also known as an offshore wind power transmission system via heterogeneous DC transmission based on weak AC interconnection. The following is a reference to... Figure 1 This application provides a topology diagram of a heterogeneous DC transmission system for offshore wind power, including: an offshore wind farm, offshore AC-side switchgear, an offshore rectifier station, an onshore switchgear collection station, and an onshore inverter station.

[0036] Offshore wind farms include at least one wind turbine unit. Specifically, offshore wind farms consist of grid-connected or grid-connected wind turbine units, which can be adapted to different turbine models.

[0037] Specifically, the offshore AC-side switchgear can be a 66kV gas-insulated switchgear (GIS) offshore, which is connected to the wind turbine.

[0038] In the offshore rectifier station, the wind turbine units are connected to the AC bus of the offshore rectifier station via AC collector submarine cables and offshore AC side switchgear. The offshore rectifier station can be an offshore DRU-MMC rectifier station, and the AC bus of the offshore rectifier station is denoted as the point of common coupling (PCC).

[0039] The offshore rectifier station includes a diode rectifier unit and an auxiliary converter. The diode rectifier unit (DRU) and the auxiliary converter (MMC) are connected in parallel on the AC side and operate independently on the DC side. The auxiliary converter is also known as the auxiliary MMC.

[0040] The offshore rectifier station is connected to a conversion station via a high-voltage direct current (HVDC) submarine cable. The conversion station converts the HVDC submarine cable into an overhead HVDC line, which is then connected to the onshore switchgear station. HVDC fault follow current clearing devices are connected in series on the HVDC submarine cable and positioned near the onshore switchgear station. The HVDC submarine cable is also known as a long-distance HVDC submarine cable, and the HVDC fault follow current clearing device is also called a DRU (Dual-Relay Run-Up) device. The conversion station's main function is to convert the HVDC submarine cable into an overhead HVDC line, while the DRU follow current clearing device is connected to the overhead HVDC line.

[0041] Among them, the onshore switchgear collection station is connected to the onshore inverter station via DC overhead line. The onshore inverter station is also known as the onshore MMC inverter station. The onshore inverter station is used to convert DC power into AC power and transmit electrical energy to the onshore AC power grid.

[0042] For example, the electrical energy output by the wind turbine is collected by a 66kV gas-insulated switchgear (GIS) and an AC collector cable at sea to the AC bus of the offshore DRU-MMC rectifier station, and then transmitted to the onshore AC power grid via the offshore DRU-MMC rectifier station, a long-distance high-voltage DC submarine cable, an onshore switchgear collection station, an overhead DC line, and an onshore MMC inverter station.

[0043] The implementation of the offshore wind power transmission system via heterogeneous DC in this embodiment relies on a DC fault ride-through method. When a fault is detected in the overhead DC line, the onshore inverter station controls the DC voltage to zero, activates the fault current limiting circuit of the offshore auxiliary converter, activates the DC-side energy consumption circuit of the wind turbine, controls the energy consumption resistor of the wind turbine to be connected, and then disconnects the offshore AC-side switching equipment. Then, when the DC fault follow-through clearing device detects the occurrence of a DC fault, it disconnects the fast power electronic switch, connects the surge arrester, and controls the transient current of the fault circuit to quickly cross zero within a preset time interval. Then, the onshore inverter station controls the DC voltage to return to its initial value, and finally closes the offshore AC-side switching equipment, disconnects the energy consumption resistor of the wind turbine, and restores the DC transmission power.

[0044] The DC fault ride-through method in this embodiment will be described in detail in subsequent embodiments, so it will not be repeated here.

[0045] Compared to traditional DC fault ride-through methods in conventional technologies, this application embodiment constructs a heterogeneous DC transmission system for offshore wind power, in which diode rectifier units and auxiliary converters are connected in parallel on the AC side and operate independently on the DC side. Through the coordinated operation of onshore inverter stations, auxiliary converters, wind turbines of offshore wind farms, offshore AC-side switchgear, and DC fault follow-through devices, in the event of a short-circuit fault in the DC overhead line of the offshore wind power transmission system via the heterogeneous DC transmission system, the DC fault current rapidly crosses zero, preventing DC blocking due to transient energy impacts during fault ride-through, thereby improving the stability of power system operation.

[0046] Optionally, in some embodiments of this application, the DC side of the auxiliary converter is connected to the target wind farm via a small uncontrolled rectifier module, a transformer, and an AC weak tie line. The target wind farm is a wind farm other than an offshore wind farm connected to the auxiliary converter. The tie line has a smaller connection capacity than the transmission capacity of the offshore wind farm.

[0047] Among them, the uncontrolled rectifier module can be a small uncontrolled rectifier module. Generally, the capacity of a small uncontrolled rectifier module is usually only in the tens of megawatts range. Its cabinet size can usually be directly embedded in the matching control cabinet of the auxiliary MMC, without the need to reserve a large area of ​​installation space separately.

[0048] In this embodiment, "weak interconnection" refers to a relatively small interconnection capacity between the auxiliary MMC and other wind farms relative to the total transmission capacity of the DRU. Furthermore, the AC weak interconnection line can provide an active power source for the DRU and offshore wind turbines, thereby addressing the startup problem of offshore wind farms.

[0049] The operating logic of the auxiliary converter is as follows: other offshore wind farms (AC) are connected to the transformer via AC weak tie lines, then connected to the auxiliary converter (AC) via small uncontrolled rectifiers (DC), and finally connected to the AC bus to provide voltage and frequency support for the AC bus. Because no power exchange is involved, only a very small tie capacity is required.

[0050] Optionally, in some embodiments of this application, the wind turbines of the offshore wind farm operate in a grid-connected control mode, and the implementation of the control logic of the wind turbines includes: a turbine-side converter control link, a grid-side converter control link, a fault current limiting link, and a wind turbine DC-side energy consumption link.

[0051] Among them, the turbine-side converter control circuit is used to achieve maximum power tracking control of the wind turbine; the grid-side converter control circuit is used to maintain DC voltage and reactive power control; the fault current limiting circuit is used to limit the current amplitude under fault conditions; and the wind turbine DC-side energy dissipation circuit is used to dissipate the transient excess energy generated by the fault.

[0052] Figure 2 This is a schematic diagram of the control logic of an offshore wind turbine in one embodiment, such as... Figure 2 As shown, ω m θ is the generator rotor speed. s For the phase of the converter voltage on the machine side, u s and i s These represent the AC side voltage and current of the machine-side converter, u. v and i v These represent the AC side voltage and current of the machine-side converter, respectively. g and Q g This refers to the active and reactive power output of the wind turbine. θ g This represents the voltage phase of the grid-side converter. The superscript * indicates the command value, and the subscripts d and q represent the d-axis and q-axis components, respectively.

[0053] The implementation equipment module for the generator-side converter includes Maximum Power Point Tracking (MPPT), a current controller, coordinate transformation, and PWM modulation modules. It employs a zero d-axis current control strategy to control the d-axis current i of the permanent magnet synchronous generator stator winding. sd The value is 0. The q-axis current reference value i is... sq * Generated by the MPPT module to maximize wind energy capture. The current controller module uses proportional-integral control to generate a three-phase voltage reference value u. s The data is input to the PWM modulation stage. The PWM modulation stage receives the three-phase voltage reference values ​​and generates the drive pulse signals for the converter. The coordinate transformation module is used to convert between the variables on the abc three-phase rotating coordinate axis and the variables on the dq stationary coordinate axis.

[0054] The grid-side converter stage includes a phase-locked loop (PLL), a DC voltage controller, a reactive power controller, a current controller, and a PWM modulation module.

[0055] In the grid-side converter stage, the PLL module is used to track the AC port voltage vector u of the wind turbine. g Generate phase angle θ g Maintain converter synchronous operation. The DC voltage controller module is used to control the DC bus voltage U. dcwt The d-axis current reference value i required by the current controller module is generated. vd* The reactive power controller module is used to control the output reactive power Q. g The q-axis current reference value i required by the current controller module is generated. vq * The current controller module is used to generate the three-phase voltage reference value u. v * and input to the PWM modulation stage. The functions of the PWM modulation module and the coordinate transformation module are the same as those of the machine-side converter stage.

[0056] In addition, in the grid-side converter stage, the DC voltage controller, reactive power controller, and current controller modules of the grid-side converter all adopt proportional-integral control.

[0057] The fault current limiting circuit is mainly used to control the short-circuit fault current to protect the switching devices of the grid-side converter when a DC fault occurs in the system. Upon receiving a fault signal, the fault current limiting circuit provides a current limiting value to the current controller according to the principle of reactive power priority output, thereby limiting the grid-side output current i. v Limited to the set current limit value.

[0058] The DC-side energy dissipation component of the wind turbine consists of a switch and a power dissipation resistor, which provides a transient energy dissipation path for offshore wind power to pass through the heterogeneous DC transmission system during DC faults. When the wind turbine is running normally, the switch is open; when the DC bus voltage is overvoltage, the switch is closed, the power dissipation resistor is engaged, and the transient energy is dissipated.

[0059] Figure 3 This is a schematic diagram of the control logic for the DC-side energy consumption of a wind turbine in one embodiment. The control logic for the DC-side energy consumption of the wind turbine mainly includes a DC bus voltage detection stage, a hysteresis comparison stage, an energy-consuming resistor activation control stage, and an energy-consuming resistor deactivation control stage, such as... Figure 3 As shown. The DC bus voltage detection unit is used to detect the DC bus voltage U in real time. dcwt The voltage signal is then output to the hysteresis comparison circuit. The hysteresis comparison circuit compares the DC bus voltage with the hysteresis range and outputs the signal to the energy-consuming resistor activation and deactivation control circuit. If the voltage is lower than the hysteresis range, the energy-consuming resistor is deactivated; if the voltage is higher than the hysteresis range, the energy-consuming resistor is activated.

[0060] Optionally, in some embodiments of this application, the auxiliary converter of the offshore rectifier station adopts a half-bridge topology. The auxiliary converter operates in voltage / frequency control mode and is used to: construct the amplitude and frequency of the offshore AC voltage, dynamically compensate the reactive power required by the diode rectifier unit and the offshore wind farm, and control the active power of the offshore wind power to be transmitted by the diode rectifier unit.

[0061] The implementation of the control logic for the auxiliary converter includes the active power controller, voltage controller, current controller, modulation, and fault current limiting.

[0062] Among them, the auxiliary converter is also known as the auxiliary MMC, and the voltage / frequency control mode refers to the voltage / frequency (V / f) control mode.

[0063] The active power controller is used to control the output active power; the voltage controller is used to control the output voltage; the current controller is used to generate three-phase voltage reference values; the modulation stage is used to receive differential voltage and generate drive pulses based on the nearest level approximation method; and the fault current limiting stage is used to control the short-circuit fault current to protect the switching devices of the auxiliary converter when a DC fault occurs in the system.

[0064] Figure 4 This is a schematic diagram of the auxiliary MMC control strategy for an offshore rectifier station in one embodiment, as shown below. Figure 4 As shown, i mv To assist the MMC in outputting AC current, U r and u r P represents the effective and instantaneous values ​​of the AC voltage at point PCC. m and Q m To assist the MMC in outputting active and reactive power. N and θ N These are the rated fundamental frequency and fundamental phase of the marine AC system, u diff This is the differential-mode voltage used to assist the MMC. The superscript * indicates a reference value, and the subscripts d and q indicate the d-axis and q-axis components, respectively.

[0065] For example, such as Figure 4 The auxiliary MMC of the offshore rectifier station adopts a half-bridge topology and operates in voltage / frequency (V / f) control mode. It is used to support the amplitude and frequency of the offshore AC voltage PCC, while dynamically compensating for the reactive power required by the DRU and the offshore wind farm, and controlling the active power of the offshore wind power to be completely transmitted by the DRU.

[0066] The implementation of the control logic of the auxiliary converter mainly includes the active power controller, voltage controller, current controller, modulation, and fault current limiting.

[0067] The principles and functions of the active power controller, voltage controller, and current controller are basically the same as those of the grid-side converter control process of wind turbine generators, and will not be elaborated here.

[0068] The modulation stage is used to receive the differential mode voltage of the MMC and generate drive pulses for the auxiliary MMC half-bridge sub-module based on the nearest level approximation method.

[0069] The fault current limiting circuit is mainly used to control the short-circuit fault current to protect the switching devices of the auxiliary MMC when a DC fault occurs in the system. Upon receiving a fault signal, the fault current limiting circuit provides a current limiting value to the current controller according to the principle of reactive power priority output, thereby limiting the grid-side output current i. mv Limited to the set current limit value.

[0070] Optionally, in some embodiments of this application, the onshore inverter station adopts a hybrid topology combining full-bridge and half-bridge. The onshore inverter station operates in a constant DC voltage and constant reactive power control mode. The onshore inverter station is used to: control the DC voltage of the high-voltage DC transmission system to maintain a constant value, and provide reactive power or AC voltage support to the onshore power grid.

[0071] Among them, the onshore inverter station is also known as the onshore MMC inverter station. The onshore MMC inverter station adopts a full-bridge-half-bridge hybrid topology and operates in constant DC voltage and constant reactive power control mode. It is used to control the DC voltage of the high voltage DC transmission system to maintain a constant value, while providing reactive power or AC voltage support to the onshore power grid.

[0072] Optionally, in some embodiments of this application, the DC fault follow current clearing device includes a fast power electronic switch and a surge arrester; wherein, the implementation steps of the control logic of the DC fault follow current clearing device include a DC fault signal receiving step, a DC fault occurrence judgment step, a fast power electronic switch activation control step, and a fast power electronic switch deactivation control step.

[0073] Figure 5 This is a schematic diagram of the topology and control logic of a DC fault freewheeling clearing device in one embodiment, as shown below. Figure 5 As shown, this application proposes a topology scheme for a DC fault follow current clearing device. The DC fault follow current clearing device consists of a fast power electronic switch and a surge arrester, which are connected in series to a high-voltage DC submarine cable. When the offshore wind power is operating normally through the heterogeneous DC transmission system, the power electronic switch is in the ON / OFF state, and the surge arrester is bypassed, with no current path.

[0074] When a DC fault occurs, the receiving-end MMC converter station detects the DC fault and sends a DC fault signal and action command to the DRU DC freewheeling clearing device. At this time, the fast power electronic switch will be disconnected within 5ms, and the transient current on the DC side will generate an overvoltage on the surge arrester. The surge arrester exhibits a low resistance value under transient overvoltage conditions, so the freewheeling time constant of the DC fault circuit can be significantly reduced. If the 66kV AC side switch at sea has already been disconnected at this time, and there is no energy feed path to the short circuit point, the fault transient current can quickly cross zero within 10ms.

[0075] like Figure 5The control logic of the DRU DC freewheeling clearing device mainly includes a DC fault signal receiving stage, a DC fault occurrence judgment stage, a fast power electronic switch on / off control stage, and a fast power electronic switch off / off control stage.

[0076] The DC fault signal receiving stage is used to receive DC fault signals; the DC fault occurrence judgment stage is used to make judgments based on the DC fault signals; the fast power electronic switch activation control stage is used to control the activation of the power electronic switch; and the fast power electronic switch deactivation control stage is used to control the deactivation of the fast power electronic switch.

[0077] Figure 6 This is a flowchart illustrating a DC fault ride-through method in one embodiment, as shown below. Figure 6 As shown, the DC fault ride-through method includes:

[0078] S601, when a fault is detected in the DC overhead line, the onshore inverter station controls the DC voltage to zero, activates the fault current limiting circuit of the offshore auxiliary converter, activates the DC side energy consumption circuit of the wind turbine, and controls the energy consumption resistor of the wind turbine to be connected.

[0079] Among them, the onshore inverter station is the onshore MMC inverter station. The offshore auxiliary converter is the auxiliary converter, also known as auxiliary MMC or offshore auxiliary MMC.

[0080] S602, Disconnect the AC side switchgear at sea.

[0081] Among them, offshore AC side switchgear refers to offshore 66kV AC side switchgear.

[0082] S603: When the DC fault follow-through clearing device detects a DC fault and the offshore AC side switchgear is disconnected, the fast power electronic switch is disconnected, the surge arrester is engaged, and the transient current of the fault circuit is controlled to quickly cross zero within a preset time interval.

[0083] The preset time interval can be 10ms.

[0084] S604, when the insulation strength of the DC line insulation medium is restored, the onshore inverter station controls the DC voltage to return to its initial value.

[0085] The initial value refers to the voltage value at which the DC voltage ramps up and recovers to its initial state.

[0086] S605, when the DC voltage returns to its initial value, closes the AC side switchgear on the sea surface, disconnects the energy-consuming resistor of the wind turbine, and restores the DC transmission power.

[0087] Compared to traditional DC fault ride-through technology, in this embodiment, when a fault is detected in the DC overhead line, the onshore inverter station controls the DC voltage to zero, activates the fault current limiting circuit of the offshore auxiliary converter, activates the DC-side energy consumption circuit of the wind turbine, controls the energy consumption resistor of the wind turbine to be connected, and then disconnects the offshore AC-side switching equipment. Then, when the DC fault follow-through clearing device detects a DC fault, it disconnects the fast power electronic switch, connects the surge arrester, and controls the transient current of the fault circuit to quickly cross zero within a preset time interval. The onshore inverter station then controls the DC voltage to return to its initial value, and finally closes the offshore AC-side switching equipment, disconnects the energy consumption resistor of the wind turbine, and restores the DC transmission power. The technical solution of this embodiment, through the coordinated operation of various parts of the system, enables the DC fault current to quickly cross zero when a short-circuit fault occurs in the DC overhead line of the offshore wind power transmission system via the heterogeneous DC transmission system. This prevents DC blocking due to transient energy impacts during fault ride-through, thereby improving the stability of the power system operation.

[0088] For example, this application proposes a DC fault ride-through method for offshore wind power transmitted through a heterogeneous DC transmission system. Through the coordinated operation of the onshore receiving-end MMC inverter station, the offshore sending-end auxiliary MMC converter, the offshore wind turbine, the offshore 66kV AC side switch, and the DRU DC fault follow-through clearing device, in the event of a short-circuit fault on the DC overhead line of the offshore wind power transmitted through the heterogeneous DC transmission system, the DC fault current rapidly crosses to zero, preventing DC blocking due to transient energy surges during fault ride-through. The specific details are as follows.

[0089] In step S601, after a fault is detected in the DC overhead line (2.5ms), the onshore MMC inverter station controls the DC voltage to zero (or outputs negative voltage). At the same time, the fault current limiting circuit of the offshore auxiliary MMC is activated. Simultaneously, the wind turbine is used to consume energy through rapid communication between the DC and the wind turbine to achieve transient energy balance (approximately 5ms). Specifically, the DC side energy consumption circuit of the wind turbine is activated, and the energy consumption resistor of the wind turbine is controlled to be connected to achieve transient energy balance on the wind turbine side.

[0090] In step S602, the 66kV AC side switch at sea (also known as the 66kV AC side grid connection point switch at sea) is disconnected (it takes about 60~80ms to completely disconnect).

[0091] In step S603, after the 66kV AC switch is opened, the DRU DC fault follow current clearing device is activated, and the transient current of the DRU fault circuit is rapidly zeroed within 10ms. Specifically, when the DRU DC fault follow current clearing device determines that a DC fault has occurred, the fast power electronic switch will be disconnected, the surge arrester will be activated, and the transient current of the DRU fault circuit will rapidly cross zero and the DC overhead line will be extinguished within 10ms.

[0092] In step S604, after the DC overhead line undergoes arc extinction and deionization, the insulation strength of the DC line insulation medium recovers, and the onshore MMC inverter station's control DC voltage ramp-up returns to its initial state. The deionization waiting time is approximately 200-300 ms.

[0093] In step S605, the DC voltage ramp-up recovery controlled by the downstream flexible DC converter takes approximately 100ms. After the DC voltage is restored, the 66kV switch must be reclosed. Specifically, closing the offshore AC switch disconnects the energy-consuming resistor of the wind turbine, and the DC transmission power of the offshore wind power through the heterogeneous DC transmission system is restored, ensuring that DC blocking does not occur due to transient energy impacts during fault ride-through.

[0094] The research process and other technical details of the DC fault ride-through method provided in this application are described below with a specific embodiment.

[0095] In the development of offshore wind power clusters, the flexible DC transmission scheme based on Modular Multilevel Converter (MMC) is currently the mainstream scheme for offshore wind power transmission. The transmission system mainly consists of an offshore MMC rectifier station, a high-voltage DC submarine cable, and an onshore MMC inverter station. This scheme has been adopted in some offshore wind power projects.

[0096] However, as offshore wind power develops towards deeper waters and larger scales, MMC-based offshore converter equipment faces challenges such as high cost, heavy weight, and difficult construction. Diode rectifier units (DRUs) offer advantages like simple structure, low cost, and light weight; however, DRUs lack AC voltage generation capabilities and have high harmonic and reactive power requirements. The solution of using DRUs as the main converter, supplemented by small-capacity MMCs to construct a heterogeneous DC transmission system, is the preferred approach for achieving economical and efficient transmission from offshore wind power clusters.

[0097] The main implementation schemes include: 1) a hybrid topology in which DRU and MMC are connected in parallel on the AC side and operate independently on the DC side; 2) DRU and MMC are connected in parallel on the DC side and operate independently on the AC side; 3) DRU and MMC are connected in parallel on both the AC and DC sides.

[0098] Based on this, some research has been conducted on heterogeneous DC transmission control and fault ride-through strategies. However, due to the scenario requirements of conventional offshore wind power flexible DC submarine cable + land cable transmission projects, research on DC fault ride-through has not yet been carried out.

[0099] In some traditional technologies, only AC fault ride-through scenarios at the sending end are studied, without providing fault ride-through strategies for hybrid DC transmission systems when DC overhead lines fail.

[0100] Furthermore, with the large-scale grid connection of offshore wind power, the traditional wind power transmission method relying on "landing point converter station + AC overhead line" faces the problem of transmission corridor strain. Adopting "power aggregation + DC overhead line direct transmission to load center" can effectively alleviate the pressure on onshore transmission corridors. However, the working environment of overhead lines is harsh, and the probability of failure is high. Reliable protection technology is needed to isolate or clear faults, so that DC blocking and wind power interruption are not caused by transient energy impacts during system fault crossing.

[0101] In summary, existing offshore wind power heterogeneous DC transmission systems are characterized by the presence of unidirectional DRUs and a hybrid transmission of DC submarine cables and overhead lines. Their transient characteristics are complex, and the experience gained from conventional offshore wind power flexible DC submarine cable + land cable transmission projects is difficult to draw upon. There is currently no DC fault ride-through solution for offshore wind power heterogeneous DC transmission systems.

[0102] Based on this, this application provides a DC fault ride-through method, namely a DC fault ride-through method for offshore wind power via a heterogeneous DC transmission system. It constructs a topology for offshore wind power via heterogeneous DC transmission with DRU and MMC connected in parallel on the AC side and operating independently on the DC side. Through the coordinated cooperation of the onshore receiving-end MMC inverter station, the offshore sending-end auxiliary MMC converter, the offshore wind turbine, the offshore 66kV AC side switch and the DRU DC fault follow current clearing device, in the event of a short-circuit fault in the DC overhead line of the offshore wind power via heterogeneous DC transmission system, the DC fault current is made to quickly cross zero, and DC blocking is not caused by transient energy impact during fault ride-through.

[0103] In terms of numerical verification, this application utilizes the PSCAD / EMTDC simulation platform to build a ±500kV / 2000MW offshore wind power transmission system via heterogeneous DC transmission. The system topology is as follows: Figure 1 As shown. At 5 seconds, a short-circuit fault between the positive and negative poles was introduced into the DC overhead line of the system, with a fault duration of 200 milliseconds. Its steady-state operation and DC fault ride-through waveforms are shown below. Figures 8 to 12 It can be seen that after the 66kV AC switch is disconnected, the DC fault current drops to about 2kA. The DRU DC fault freewheeling current clearing device is quickly activated, and the transient current of the DRU fault circuit reaches zero rapidly within 10ms.

[0104] Figure 7 This is a schematic diagram of the steady-state operating waveforms of offshore wind power transmitted via a heterogeneous DC transmission system on the offshore side. Figure 7 The image shows, from top to bottom, the effective value of the line voltage of the PCC point, the active power output of the auxiliary MMC, the reactive power output of the auxiliary MMC, and the AC current output of the offshore wind farm.

[0105] Figure 8 This is a schematic diagram of the steady-state operating waveform of the onshore side of the heterogeneous DC transmission system for offshore wind power. Figure 8 In the middle, from top to bottom, are images of DC voltage, active power received by the onshore MMC inverter station, reactive power received by the onshore MMC inverter station, and AC current on the onshore grid side.

[0106] Figure 9 This is a schematic diagram illustrating the occurrence of DC faults in an offshore wind power transmission system via a heterogeneous DC transmission network. Figure 9 The image shows, from top to bottom, the effective value of the line voltage of the PCC point, the active power output of the auxiliary MMC, the reactive power output of the auxiliary MMC, and the AC current output of the offshore wind farm.

[0107] Figure 10 This is a schematic diagram illustrating the occurrence of DC faults in an offshore wind power transmission system via a heterogeneous DC transmission network. Figure 10 In the middle, from top to bottom, are images of DC voltage, active power received by the onshore MMC inverter station, reactive power received by the onshore MMC inverter station, and AC current on the onshore grid side.

[0108] Figure 11 This is a schematic diagram showing the changes in DC fault current of offshore wind power transmitted through a heterogeneous DC transmission system.

[0109] In another embodiment, a DC fault ride-through device is provided. This device can be a computer device, such as a server, and its internal structure diagram can be as follows: Figure 12 As shown, the device includes a processor, memory, input / output (I / O) interfaces, and a communication interface. The processor, memory, and I / O interfaces are connected via a system bus, and the communication interface is also connected to the system bus via the I / O interfaces. The processor provides computing and control capabilities. The memory includes non-volatile storage media and internal memory. The non-volatile storage media stores the operating system, computer programs, and a database. The internal memory provides an environment for the operation of the operating system and computer programs stored in the non-volatile storage media. The device's database stores relevant data. The I / O interfaces are used for exchanging information between the processor and external devices. The device's communication interface is used for communicating with external terminals via a network connection. When the computer program is executed by the processor, it implements the various methods described in the above embodiments.

[0110] In yet another embodiment, a DC fault ride-through device is provided. This device can be a computer device, such as a terminal, and its internal structure diagram can be as follows: Figure 13As shown, the device includes a processor, memory, input / output interface, communication interface, display unit, and input device. The processor, memory, and input / output interface are connected via a system bus, and the communication interface, display unit, and input device are also connected to the system bus via the input / output interface. The processor provides computing and control capabilities. The memory includes a non-volatile storage medium and internal memory. The non-volatile storage medium stores the operating system and computer programs. The internal memory provides an environment for the operation of the operating system and computer programs in the non-volatile storage medium. The input / output interface is used for exchanging information between the processor and external devices. The communication interface is used for wired or wireless communication with external terminals; wireless communication can be achieved through Wi-Fi, mobile cellular networks, NFC (Near Field Communication), or other technologies. When the computer program is executed by the processor, it implements the various methods described in the above embodiments.

[0111] Those skilled in the art will understand that Figure 12 and Figure 13 The structure shown is merely a block diagram of a portion of the structure related to the present application and does not constitute a limitation on the DC fault ride-through device to which the present application is applied. Specifically, the device may include more or fewer components than shown in the figure, or combine certain components, or have different component arrangements, in order to realize the functions of computer equipment such as terminals or servers.

[0112] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0113] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and modules described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0114] In the embodiments provided in this application, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of modules is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple modules or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be indirect coupling or communication connection through some interfaces, apparatuses, or modules, and may be electrical, mechanical, or other forms.

[0115] The modules described as separate components may or may not be physically separate. The components shown as modules may or may not be physical modules; that is, they may be located in one place or distributed across multiple network modules. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs.

[0116] Furthermore, the functional modules in the various embodiments of this application can be integrated into one processing module, or each module can exist physically separately, or two or more modules can be integrated into one module. The integrated module can be implemented in hardware or as a software functional module. If the integrated module is implemented as a software functional module and sold or used as an independent product, it can be stored in a computer-readable storage medium.

[0117] In the above embodiments, implementation can be achieved, in whole or in part, through software, hardware, firmware, or any combination thereof. When implemented in software, it can be implemented, in whole or in part, as a computer program product.

[0118] The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, all or part of the processes or functions described in the embodiments of this application are generated. The computer may be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions may be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another. For example, the computer instructions may be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wired (e.g., coaxial cable, fiber optic, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) means. The computer-readable storage medium may be any available medium that a computer can store or a data storage device such as a server or data center that integrates one or more available media. The available medium may be a magnetic medium (e.g., floppy disk, hard disk, magnetic tape), an optical medium (e.g., DVD), or a semiconductor medium (e.g., a solid-state disk (SSD)).

[0119] The technical solutions provided in the embodiments of this application have been described in detail above. Specific examples have been used in the embodiments of this application to illustrate the principles and implementation methods of the embodiments of this application. The description of the above embodiments is only for the purpose of helping to understand the methods and core ideas of the embodiments of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the embodiments of this application. Therefore, the content of this specification should not be construed as a limitation on the embodiments of this application.

Claims

1. A DC fault ride-through method, characterized in that, The DC fault ride-through method includes: In the event of a detected fault in the overhead DC line, the onshore inverter station controls the DC voltage to zero, activates the fault current limiting circuit of the offshore auxiliary converter, activates the DC side energy consumption circuit of the wind turbine, and controls the energy consumption resistor of the wind turbine to be connected; and disconnects the offshore AC side switching equipment. When the DC fault follow-through clearing device detects a DC fault and the marine AC side switching equipment is disconnected, the fast power electronic switch is disconnected, the surge arrester is activated, and the transient current of the fault circuit is controlled to quickly cross zero within a preset time interval. When the insulation strength of the DC line insulation medium recovers, the control DC voltage of the onshore inverter station is restored to its initial value; Once the DC voltage returns to its initial value, the offshore AC-side switchgear is closed to disconnect the energy-consuming resistor of the wind turbine and restore DC transmission power.

2. A system for transmitting offshore wind power via heterogeneous DC transmission, characterized in that, For implementing the DC fault ride-through method as described in claim 1, the offshore wind power via a heterogeneous DC transmission system comprises: Wind turbine units; Offshore AC-side switchgear, which is connected to the wind turbine generator; The offshore rectifier station, wherein the wind turbine generators are connected to the AC busbar of the offshore rectifier station via AC collector submarine cables and the offshore AC-side switchgear; the offshore rectifier station includes an auxiliary converter; The onshore switchgear collection station is connected to the offshore rectifier station, which is used to convert the DC submarine cable into a DC overhead line and connect it to the onshore switchgear collection station through the DC overhead line; the DC fault follow-through clearing device is connected in series on the DC overhead line and arranged near the onshore switchgear collection station. The onshore inverter station is connected to the onshore switchgear collection station. The onshore inverter station is used to convert DC power into AC power and transmit electrical energy to the onshore AC power grid.

3. The offshore wind power heterogeneous DC transmission system according to claim 2, characterized in that, The DC side of the auxiliary converter is connected to the target wind farm via an uncontrolled rectifier module, a transformer, and an AC weak tie line. The target wind farm is any wind farm other than an offshore wind farm that is connected to the auxiliary converter. The connection capacity of the AC weak connection line is less than the transmission capacity of the offshore wind farm.

4. The offshore wind power heterogeneous DC transmission system according to claim 2, characterized in that, The wind turbine operates in grid-connected control mode. The implementation of the control logic of the wind turbine includes: turbine-side converter control, grid-side converter control, fault current limiting, and wind turbine DC-side energy consumption. The turbine-side converter control circuit is used to achieve maximum power point tracking control of the wind turbine; the grid-side converter control circuit is used to maintain DC voltage and reactive power control; the fault current limiting circuit is used to limit the current amplitude under fault conditions; and the wind turbine DC-side energy dissipation circuit is used to dissipate the transient excess energy generated by the fault.

5. The offshore wind power heterogeneous DC transmission system according to claim 2, characterized in that, The auxiliary converter of the offshore rectifier station adopts a half-bridge topology. The auxiliary converter operates in voltage / frequency control mode and is used for: The amplitude and frequency of the offshore AC voltage are constructed to dynamically compensate the reactive power required by the diode rectifier unit of the offshore rectifier station and the offshore wind farm, and to control the active power of the offshore wind power to be transmitted by the diode rectifier unit. The implementation of the control logic of the auxiliary converter includes an active power controller, a voltage controller, a current controller, a modulation stage, and a fault current limiting stage. The active power controller is used to control the output active power; the voltage controller is used to control the output voltage; the current controller is used to generate a three-phase voltage reference value; the modulation stage is used to receive the differential voltage and generate a drive pulse based on the nearest level approximation method; and the fault current limiting stage is used to control the short-circuit fault current to protect the switching devices of the auxiliary converter when a DC fault occurs in the system.

6. The offshore wind power heterogeneous DC transmission system according to claim 2, characterized in that, The onshore inverter station adopts a hybrid topology combining full-bridge and half-bridge converters. The onshore inverter station operates in a constant DC voltage and constant reactive power control mode. The onshore inverter station is used for: The DC voltage of the high-voltage direct current transmission system where the onshore AC power grid is located is kept constant, and reactive power or AC voltage support is provided to the onshore power grid.

7. The offshore wind power heterogeneous DC transmission system according to claim 2, characterized in that, The DC fault follow current clearing device includes a fast power electronic switch and a surge arrester; The implementation steps of the control logic of the DC fault freewheeling clearing device include a DC fault signal receiving step, a DC fault occurrence judgment step, a fast power electronic switch activation control step, and a fast power electronic switch deactivation control step. The DC fault signal receiving stage is used to receive DC fault signals; the DC fault occurrence judgment stage is used to make judgments based on the DC fault signals; the fast power electronic switch activation control stage is used to control the activation of the power electronic switch; and the fast power electronic switch deactivation control stage is used to control the deactivation of the fast power electronic switch.

8. A DC fault ride-through device, characterized in that, The device includes: At least one processor and memory; The memory is used to store program code, and the processor is used to call the program code stored in the memory to execute the method as described in claim 1.

9. A computer storage medium, characterized in that, It includes instructions that, when run on a computer, cause the computer to perform the method as described in claim 1.

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