Variable parameter control method and device for flexible DC transmission systems in wind farms

Abstract

This invention provides a variable parameter control method and apparatus for a flexible DC transmission system in a wind farm. The flexible DC transmission system includes a sending-end grid, a receiving-end grid, a flexible DC transmission network, and a high-voltage AC transmission network. The sending-end grid is connected to different receiving-end grids through the flexible DC transmission network and the high-voltage AC transmission network. The sending-end grid consists of a doubly-fed asynchronous wind turbine and a battery energy storage system controlled by a voltage source converter, connected in sequence. The flexible DC transmission system also includes a virtual synchronizing machine connected to the sending-end grid. The virtual synchronizing machine includes: monitoring the system bus voltage of the sending-end grid; if the system bus voltage exceeds a voltage threshold, sending a first control signal to the virtual synchronizing machine to adjust the virtual inertia and virtual damping to a first standard value; if the system bus voltage is lower than the voltage threshold, sending a second control signal to the virtual synchronizing machine to adjust the virtual inertia and virtual damping to a second standard value.

Description

Technical Field

[0001] This invention relates to the field of energy storage systems, and more specifically to a variable parameter control method and apparatus for flexible DC transmission systems in wind farms. Background Technology

[0002] With increasing societal electricity demand, traditional AC transmission methods often suffer from long-distance power loss and declining power quality, making it increasingly difficult to meet users' high power quality requirements. DC transmission, with its characteristics of effectively addressing harmonic interference and three-phase imbalance and instability—problems closely related to power quality—has received widespread attention in recent years. Furthermore, energy and load exhibit an inverse distribution. With the widespread adoption of new energy power equipment such as wind and solar power generation and electric vehicles, long-distance, high-capacity ultra-high-voltage (UHV) transmission technology has been vigorously developed to achieve rational resource allocation, improve the utilization rate of new energy sources, and meet the urgent needs of energy conservation and emission reduction. Constructing AC / DC hybrid systems based on traditional AC power systems will be a future trend in power system development.

[0003] With the vigorous development of wind power, many regions rich in wind resources typically employ flexible direct current (HVDC) transmission to transfer electricity generated by wind farms into the power grid. Voltage source converters (VSCs) play a crucial role in HVDC transmission. In HVDC (Voltage Source Converter based High Voltage Direct Current Transmission, VSC-HVDC) structures, VSCs are simpler to install and less expensive. They utilize insulated-gate bipolar transistor (IGBT) valves, enabling bidirectional current flow and facilitating the reversal of power flow direction. They can control both active and reactive power on the AC side, thus allowing power to be supplied to passive loads and providing black-start capability.

[0004] However, DC transmission involves long distances, difficult maintenance, and complex transmission line conditions, often facing DC blocking events caused by line faults or commutation failures. Furthermore, for sending-end power grids that primarily rely on renewable energy generation, the system inertia is low, making them more susceptible to instability when subjected to DC blocking faults. Summary of the Invention

[0005] Based on this, the present invention provides a variable parameter control method and apparatus for a flexible DC transmission system for wind farms. By constructing a virtual synchronous machine in the grid at the wind power sending end to provide virtual inertia and virtual damping, the method actively supports the stability of the frequency and voltage of wind power connection to the grid, and reduces grid instability caused by DC blocking faults.

[0006] In a first aspect, the present invention provides a variable parameter control method for a flexible DC transmission system in a wind farm. The flexible DC transmission system includes a sending-end grid, a receiving-end grid, a flexible DC transmission network, and a high-voltage AC transmission network. The sending-end grid is connected to different receiving-end grids via the flexible DC transmission network and the high-voltage AC transmission network. The sending-end grid consists of a doubly-fed asynchronous wind turbine and a battery energy storage system controlled by a voltage source converter, connected in sequence. The flexible DC transmission system also includes a virtual synchronous machine connected to the sending-end grid. The variable parameter control method includes the following steps:

[0007] Monitor the system bus voltage of the sending-end power grid;

[0008] If the system bus voltage exceeds the voltage threshold, a first control signal is sent to the virtual synchronizing machine to adjust the virtual inertia and virtual damping to the first standard value.

[0009] If the system bus voltage is lower than the voltage threshold, a second control signal is sent to the virtual synchronizing machine to adjust the virtual inertia and virtual damping to the second standard value.

[0010] Furthermore, if the system bus voltage exceeds the voltage threshold, a first control signal is sent to the virtual synchronizer to adjust the virtual inertia and virtual damping to the first standard value, specifically:

[0011] If the system bus voltage exceeds the voltage threshold, a first control signal is sent to the virtual machine synchronizer.

[0012] The virtual synchronizer increases the virtual inertia to the maximum value according to the first control signal, and decreases the virtual damping to the minimum value according to the first control signal.

[0013] Furthermore, if the system bus voltage is lower than the voltage threshold, a second control signal is sent to the virtual synchronizer to adjust the virtual inertia and virtual damping to the second standard value, specifically:

[0014] If the system bus voltage is lower than the voltage threshold, a second control signal is sent to the virtual synchronizer;

[0015] The virtual synchronizer reduces the virtual inertia to the minimum value according to the second control signal, and increases the virtual damping to the maximum value according to the second control signal.

[0016] Furthermore, the specific expression for the control strategy of the virtual synchronizer is as follows:

[0017] ,

[0018] ,

[0019] in, For the virtual inertia of the virtual synchronizer, This represents the maximum virtual inertia. This represents the minimum value of the virtual inertia. The system bus voltage of the sending-end power grid. This is the voltage critical value. For virtual damping of the virtual synchronizer, This represents the virtual damping minimum. This represents the maximum virtual damping value.

[0020] Furthermore, the specific expressions for adjusting the virtual inertia and virtual damping of the virtual synchronizer are as follows:

[0021] ,

[0022] in, This refers to the mechanical power of the voltage source converter. For the electromagnetic power of the voltage source converter, For virtual damping of the virtual synchronizer, For the virtual inertia of the virtual synchronizer, The synchronization frequency of the virtual synchronizer. The synchronization frequency rating of the virtual synchronizer. This refers to the power angle during actual operation of the voltage source converter.

[0023] Furthermore, the specific expression for the output power of the doubly-fed asynchronous wind turbine is as follows:

[0024] ,

[0025] in, This refers to the output power of a doubly-fed asynchronous wind turbine. The air density of the environment in which the doubly-fed asynchronous wind turbine is located. The wind energy capture factor, This refers to the blade tip velocity of a doubly-fed asynchronous wind turbine. blade tip velocity and pitch angle Intermediate variables formed together This refers to the pitch angle of a doubly-fed asynchronous wind turbine. Let be the radius of the wind turbine rotor of the doubly-fed asynchronous wind turbine. The wind speed in the environment where the doubly-fed asynchronous wind turbine is located. This refers to the turbine speed of the doubly-fed asynchronous wind turbine.

[0026] Furthermore, the variable parameter control method for flexible DC transmission systems in wind farms also includes:

[0027] When the wind energy capture coefficient is at its maximum value, the doubly fed asynchronous wind turbine adopts the maximum power point tracking control mode.

[0028] When the wind capture coefficient is less than the maximum value, the control mode of the doubly fed asynchronous wind turbine is switched to the power load reduction control mode.

[0029] Secondly, the present invention also provides a variable parameter control device for a flexible DC transmission system in a wind farm. The flexible DC transmission system includes a sending-end grid, a receiving-end grid, a flexible DC transmission network, and a high-voltage AC transmission network. The sending-end grid is connected to different receiving-end grids via the flexible DC transmission network and the high-voltage AC transmission network. The sending-end grid consists of a battery energy storage system controlled by a doubly-fed asynchronous wind turbine and a voltage source converter, connected in sequence. The flexible DC transmission system also includes a virtual synchronous machine connected to the sending-end grid. The variable parameter control device includes the following modules:

[0030] The voltage monitoring module is used to monitor the system bus voltage of the sending-end power grid;

[0031] The first parameter adjustment module is used to send a first control signal to the virtual synchronizing machine if the system bus voltage exceeds the voltage threshold value, so as to adjust the virtual inertia and virtual damping to the first standard value.

[0032] The second parameter adjustment module is used to send a second control signal to the virtual synchronizing machine to adjust the virtual inertia and virtual damping to a second standard value if the system bus voltage is lower than the voltage threshold.

[0033] Thirdly, the present invention also provides a computer-readable storage medium having a computer program stored thereon, wherein the computer program, when executed by a processor, implements the steps of any of the variable parameter control methods for a flexible DC transmission system for a wind farm in the first aspect.

[0034] Fourthly, the present invention also provides a computer device, including a memory and a processor, wherein the memory stores a computer program, and when the processor executes the computer program, it executes any one of the variable parameter control methods for a flexible DC transmission system for a wind farm in the first aspect.

[0035] The beneficial effects of adopting the above technical solution are as follows: This embodiment constructs a virtual synchronous machine in the wind power generation grid at the sending end to provide virtual inertia and virtual damping, actively supporting the stability of the frequency and voltage of wind power generation connected to the grid, and reducing grid instability caused by DC blocking faults. As the virtual inertia of the grid-connected energy storage at the sending end increases, after DC bipolar blocking occurs in the AC / DC hybrid system, the rate of frequency rise in the sending end system decreases significantly, and the peak value of the final frequency rise is suppressed, but the system recovery time increases continuously. The addition of virtual damping can significantly improve the recovery time of the sending end system, and the increase in virtual damping can also slightly reduce the peak value of the sudden frequency rise. When the virtual inertia of the grid-connected energy storage increases, the peak value of transient overvoltage at the sending end bus after DC blocking decreases significantly, but the system recovery time also increases simultaneously. The addition of virtual damping significantly enhances the system's ability to absorb oscillations, thus the recovery rate is also faster. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0037] Figure 1 This is a schematic diagram of a variable parameter control method for a flexible DC transmission system in a wind farm, as shown in one embodiment of this application.

[0038] Figure 2 This is a schematic diagram of the topology of a doubly-fed asynchronous wind turbine in one embodiment of this application;

[0039] Figure 3 This is a schematic diagram of the control principle of the rotor-side converter of a doubly-fed asynchronous wind turbine in one embodiment of this application;

[0040] Figure 4 This is a schematic diagram of the grid-side converter control principle of a doubly-fed asynchronous wind turbine in one embodiment of this application;

[0041] Figure 5 This is a topology diagram of a virtual synchronizer in one embodiment of this application;

[0042] Figure 6 This is a schematic diagram of the mathematical model of a virtual synchronizer in one embodiment of this application;

[0043] Figure 7 This is a schematic diagram of the virtual synchronous machine inner loop current controller in one embodiment of this application;

[0044] Figure 8 This is a schematic diagram of the control structure of the wind farm-side voltage source converter in one embodiment of this application;

[0045] Figure 9This is a schematic diagram of the grid-side voltage source converter control structure in one embodiment of this application;

[0046] Figure 10 This is a schematic diagram of the topology of a flexible DC transmission system for a wind farm in one embodiment of this application;

[0047] Figure 11 This is a schematic diagram illustrating the impact of different virtual inertia on the frequency stability of the sending system in one embodiment of this application;

[0048] Figure 12 This is a schematic diagram illustrating the impact of different virtual damping on the frequency stability of the sending-end system in one embodiment of this application;

[0049] Figure 13 A schematic diagram showing the voltage variation of the sending-end bus when different virtual inertia values ​​are set for the grid-connected energy storage installed at the sending-end power grid in one embodiment of this application.

[0050] Figure 14 A schematic diagram showing the voltage variation of the sending-end bus when different virtual damping settings are applied to the grid-connected energy storage installed at the sending-end power grid in one embodiment of this application.

[0051] Figure 15 This is a schematic diagram of the frequency stability of the sending-end system in one embodiment of the present application, incorporating a variable parameter control method.

[0052] Figure 16 This is a schematic diagram of the voltage stability of the sending-end system in one embodiment of the present application, incorporating a variable parameter control method;

[0053] in, Figure 2 In the diagram, 1 represents a wind turbine, 2 represents an asynchronous generator, 3 represents a step-up transformer, 4 represents a rotor-side converter, and 5 represents a grid-side converter. Figure 10 In the diagram, 11 represents a doubly-fed asynchronous wind turbine, 12 represents a grid-connected converter, 13 represents a flexible DC transmission system, 14 represents a high-voltage AC transmission system, 15 represents receiving-end grid one, 16 represents receiving-end grid two, 131 represents wind turbine-side converter VSC one, and 132 represents grid-side converter VSC two. Detailed Implementation

[0054] 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, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention. To describe the present invention in more detail, the variable parameter control method and apparatus for flexible DC transmission systems in wind farms provided by the present invention will be specifically described below with reference to the accompanying drawings.

[0055] Unless otherwise defined, the technical or scientific terms used in this application shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, the terms "an," "a," or "the" do not indicate a quantity limitation, but rather indicate the presence of at least one. The terms "comprising" or "including" mean that the preceding element or object encompasses the elements or objects listed after the word and their equivalents, without excluding other elements or objects. The terms "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. "Up," "down," "left," "right," etc., are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0056] This invention provides an application scenario for a variable parameter control method for a flexible DC transmission system in a wind farm. This application scenario includes the terminal equipment provided in the embodiments, which includes, but is not limited to, smartphones and computer devices. The computer device can be at least one of a desktop computer, portable computer, laptop computer, mainframe computer, tablet computer, etc. By monitoring the system bus voltage of the sending-end power grid, the virtual inertia and virtual damping of the virtual synchronous machine control are adjusted to ensure the DC transmission system is in a stable state. (See attached...) Figure 1 The diagram shows a variable parameter control method for a flexible DC transmission system in a wind farm. For details, please refer to the embodiment of the variable parameter control method for a flexible DC transmission system in a wind farm.

[0057] The flexible DC transmission system for wind farms in this embodiment includes a sending-end grid, a receiving-end grid, a flexible DC transmission network, and a high-voltage AC transmission network. The sending-end grid is connected to different receiving-end grids through the flexible DC transmission network and the high-voltage AC transmission network, respectively. The sending-end grid consists of a doubly-fed asynchronous wind turbine and a battery energy storage system controlled by a voltage source converter, connected in sequence. Additionally, the flexible DC transmission system for wind farms also includes a virtual synchronous machine, which is connected to the sending-end grid.

[0058] Among them, combined with the appendix Figure 2 The diagram shows the topology of a doubly-fed asynchronous wind turbine. The turbine 1 converts external wind energy into mechanical energy by rotating its blades, which is then converted into power output by the turbine. The power output is related to factors such as wind speed and blade pitch angle. The specific expression for the output power of the doubly-fed asynchronous wind turbine is as follows:

[0059] ,

[0060] in, This refers to the output power of a doubly-fed asynchronous wind turbine. The air density of the environment in which the doubly-fed asynchronous wind turbine is located. The wind energy capture factor, This refers to the blade tip velocity of a doubly-fed asynchronous wind turbine. blade tip velocity and pitch angle Intermediate variables formed together This refers to the pitch angle of a doubly-fed asynchronous wind turbine. Let be the radius of the wind turbine rotor of the doubly-fed asynchronous wind turbine. The wind speed in the environment where the doubly-fed asynchronous wind turbine is located. This refers to the turbine speed of the doubly-fed asynchronous wind turbine.

[0061] Based on the above expression, it can be concluded that for a given wind farm model, the radius of its wind turbine rotor and the air density of its region are uniquely determined. When the wind speed is constant, the mechanical power that the wind turbine can convert is only related to the wind energy capture coefficient. The correlation is with the pitch angle and the tip speed ratio. Adjusting the pitch angle and tip speed ratio adjusts the wind energy capture coefficient. When the wind energy capture coefficient At maximum, the wind turbine adopts maximum power point tracking (MPPT) control mode; when the wind energy capture coefficient... When the value is less than the maximum value, the wind turbine control mode can be switched to power load reduction control.

[0062] When the wind turbine adopts maximum power point tracking (MPPT) control mode, the pitch angle and tip speed ratio are adjusted based on the wind speed of the doubly-fed asynchronous wind turbine's environment to keep the turbine at its maximum power point. When the wind turbine's control mode is switched to power load shedding control, the pitch angle and tip speed ratio are adjusted based on the wind speed of the doubly-fed asynchronous wind turbine's environment to reduce the active power output of the wind turbine, thereby preventing overload operation and protecting the wind turbine equipment from damage.

[0063] The asynchronous generator 2 inside the doubly fed induction generator (DFIG) can convert the mechanical energy generated by the wind turbine 1 back into electrical energy. Its structure is basically the same as that of a conventional wound-rotor induction motor. The voltage equation and flux linkage equation of the asynchronous generator inside the DFIG are expressed as follows:

[0064] ,

[0065] ,

[0066] in, For the stator side of the asynchronous generator Voltage components on the axis, For the stator side of the asynchronous generator Magnetic flux component on the axis, The synchronous angular velocity of the power system. For the stator side of the asynchronous generator Magnetic flux component on the axis, The resistance on the stator side of the asynchronous generator. For the stator side of the asynchronous generator Current components on the shaft, For the stator side of the asynchronous generator Voltage components on the axis, For the stator side of the asynchronous generator Current components on the shaft, For the rotor side of the asynchronous generator Voltage components on the axis, For the rotor side of the asynchronous generator Magnetic flux component on the axis, The slip of the asynchronous generator. For the rotor side of the asynchronous generator Magnetic flux component on the axis, The resistance on the rotor side of the asynchronous generator. For the rotor side of the asynchronous generator Current components on the shaft, For the rotor side of the asynchronous generator Voltage components on the axis, For the rotor side of the asynchronous generator Current components on the shaft, For the self-inductance on the stator side of the asynchronous generator, For the mutual inductance between the stator and rotor sides of the asynchronous generator, This refers to the self-inductance on the rotor side of the asynchronous generator.

[0067] Based on the voltage equation and flux linkage equation of the asynchronous generator mentioned above, the voltage drop of the stator winding is often ignored in actual calculations. Therefore, the active power of the asynchronous generator on the stator side in a doubly-fed asynchronous wind turbine is... The specific expression is:

[0068] ;

[0069] Active power of asynchronous generator on rotor side The specific expression is:

[0070] ;

[0071] Combining the active power expressions for the stator and rotor sides of a doubly-fed asynchronous wind turbine, the total active power generated by the doubly-fed asynchronous wind turbine is... The specific expression is:

[0072] .

[0073] Combined with appendix Figure 3 The diagram shown illustrates the control principle of the rotor-side converter of a doubly-fed asynchronous wind turbine. During actual operation, the magnetic flux linkage and voltage direction on the stator side of the doubly-fed asynchronous wind turbine are exactly orthogonal. In practical calculations, either the magnetic flux linkage or the voltage direction can be selected. With the axis as the positive direction, when the flux linkage direction is chosen as positive, the stator flux linkage... Axial direction and voltage The values ​​in the axial direction are converted to constants, while the magnetic flux linkage... Shaft and voltage The component values ​​of the shaft can be converted to 0, greatly reducing the difficulty of calculation. Therefore, the stator-side voltage flux linkage equation of the doubly-fed asynchronous wind turbine can be obtained as follows:

[0074] ,

[0075] By combining the voltage flux linkage equation on the stator side of a doubly-fed asynchronous wind turbine, the relationship between the stator-side current and the rotor-side current can be obtained as follows:

[0076] ,

[0077] After Park's transformation, the stator-side power equation in the dq coordinate system is obtained as follows:

[0078] ,

[0079] In the above formula, and Let be the instantaneous values ​​of the d-axis and q-axis components of the stator voltage. Substituting the stator voltage flux linkage equation and the relationship between the stator current and rotor current of the doubly-fed asynchronous wind turbine into the stator power equation in the dq coordinate system, we obtain the stator power equation as follows:

[0080] ,

[0081] in, This refers to the reactive power on the stator side. This refers to the voltage on the stator side of the asynchronous generator. This refers to the magnetic flux linkage on the stator side of an asynchronous generator.

[0082] Based on this, when Mutual inductance between stator and rotor sides Self-inductance on the stator side When both are constant values, the active power on the stator side is only related to the rotor. The axis current component is uniquely correlated, while when Mutual inductance between stator and rotor sides Self-inductance on the stator side and magnetic flux on the stator side of the asynchronous generator When both are constant values, the reactive power on the stator side is only related to that on the rotor. The shaft current component is uniquely correlated. Therefore, through the derivation of the above formula, the stator-side reactive power and active power control of the doubly-fed asynchronous wind turbine generator are successfully decoupled.

[0083] Based on the above theory, a rotor-side converter (RSC) control system for a doubly-fed asynchronous wind turbine can be constructed (such as...). Figure 3 (As shown). In the rotor-side converter control system, Maximum Power Point Tracking (MPPT) control and control based on reactive voltage droop characteristics convert the active power reference value and stator voltage output by the wind turbine into the rotor's angular frequency reference value and reactive power reference value, respectively. These values ​​are then subtracted from the actual angular frequency and actual reactive power. This difference is used by proportional-integral control to obtain the rotor... shaft and The reference value of the shaft component current is then compared with the actual value and input into proportional-integral control to apply a control effect to the converter on the rotor side.

[0084] Furthermore, in conjunction with the appendix Figure 4 The diagram shown illustrates the control principle of the grid-side converter (GSC) for a doubly-fed asynchronous wind turbine. The core purpose of GSC control is to ensure the stability of the DC voltage between the back-to-back converters while simultaneously setting the reactive power exchanged between the GSC and the grid. Compared to the control system of the rotor-side converter, the main difference lies in that the GSC sets the voltage direction on the grid side to be... If the axis is in the positive direction, then the following expression can be obtained:

[0085] ,

[0086] ,

[0087] in, For grid-side active power, This refers to the grid voltage on the grid side. For grid-side current Axial components, For grid-side current Axial components, For grid-side reactive power, The simplified equivalent DC capacitor for a back-to-back converter. This is the DC voltage value. This represents the DC component of the current output from the grid side. This represents the DC component of the current output from the rotor side. This refers to the active power on the rotor side. Therefore, it can be interpreted as the active power on the grid side voltage. At a given time, the active power on the grid side Only with grid-side current Axial components Related, grid-side reactive power Only with grid-side current Axial components Related. Therefore, it can be seen that the grid-side current in the grid-side converter... shaft and The axial components no longer influence each other, meaning the coupling is broken, allowing for separate and effective control of active and reactive power on the grid side. Similarly, the above equation shows that if the DC voltage can reach a stable state, effective control of active power on the grid side can be achieved.

[0088] In the model built in this embodiment, such as Figure 4 The grid-side converter control structure of the doubly-fed asynchronous wind turbine is shown. Reference value for DC voltage. The difference between the actual DC voltage and the input voltage is calculated and then input into the proportional-integral control loop for output. Shaft current reference value and with the grid-side current The difference between the shaft components is then input into the proportional-integral module and output to the grid-side converter. The reactive power control method is the same as the above process, so that the DC voltage and reactive power of the grid-side converter can be stabilized after control.

[0089] Combined with appendix Figure 5 The diagram shows the topology of the Virtual Synchronous Generator (VSG). The equivalent resistance of the AC side of the converter, Reactance measured for AC converter For the phase voltage of the AC side grid of the converter, For measuring the phase current of the power grid in AC mode for the converter, For the active power input to the AC side converter, For the reactive power input to the AC side converter, For the DC-side line-to-line capacitance of the converter, For the DC side outflow current of the converter, This refers to the voltage between DC-side lines.

[0090] Combined with appendix Figure 6 The schematic diagram of the mathematical model of the virtual synchronizer shown, combined with the topology of the virtual synchronizer and Kirchhoff's Voltage Laws (KVL), yields the following expression for the time-domain mathematical model of the virtual synchronizer:

[0091] ,

[0092] in, For the AC side voltage source of the converter Phase voltage components, For AC side converter Phase port voltage components, For the inflow of the converter Phase current components, For the AC side voltage source of the converter Phase voltage components, For AC side converter Phase port voltage components, For the flow into the converter Phase current components, For the AC side voltage source of the converter Phase voltage components, For AC side converter Phase port voltage components, For the inflow of the converter Phase current components, This is the equivalent resistance on the AC side of the converter. The reactance measured for AC converter.

[0093] Performing a Parker transformation on the above expression makes the original three-phase... Variables in the coordinate system are transformed to those rotating at synchronous speed. With the coordinate system as the reference axis, the following expression is obtained:

[0094] ,

[0095] In the above formula, and respectively The time-domain mathematical model of the converter in the virtual synchronous machine is shown in Functional expression of coordinate system, The time-domain mathematical model of the converter in the virtual synchronous machine is shown in Functional expression in coordinate system It is the Park transformation matrix. It is in the line Phase angle of phase voltage, This can be represented as:

[0096] ,

[0097] After the Parker transformation, rotating at synchronous speed The time-domain mathematical model of a virtual synchronizer in the coordinate system can be written as:

[0098] ,

[0099] in, The d-axis current component flowing into the converter from the AC side in the time domain. The q-axis current component flowing into the converter from the AC side in the time domain. The d-axis voltage component in the time domain of the AC side voltage source of the converter. The q-axis voltage component in the time domain of the AC side voltage source of the converter. This represents the d-axis component of the port voltage in the time domain of the AC-side converter. This represents the q-axis component of the port voltage in the time domain of the AC-side converter. This is the synchronization frequency of the virtual synchronizer.

[0100] right After performing a Laplace transform on the time-domain mathematical model of the virtual synchronizer in the coordinate system, the mathematical model expression of the virtual synchronizer in the frequency domain is obtained as follows:

[0101] ,

[0102] in, The frequency domain d-axis current component flowing into the converter from the AC side. The frequency domain q-axis current component flowing into the converter from the AC side. The d-axis voltage component in the frequency domain of the AC side voltage source of the converter. The q-axis voltage component in the frequency domain of the AC side voltage source of the converter. This represents the d-axis component of the port voltage in the frequency domain of the AC-side converter. This represents the q-axis component of the port voltage in the frequency domain of the AC-side converter.

[0103] Combination Figure 7The diagram shows the inner-loop current controller of a virtual synchronous machine. To achieve rapid current response, direct current control strategy based on vector control technology is widely used in virtual synchronous machine control systems. This control strategy consists of two parts: an inner-loop current controller and an outer-loop controller. The inner-loop current controller can quickly track the current reference value generated by the outer-loop controller and generate... The reference value for the shaft voltage is ultimately input into the mathematical model of the virtual synchronizer. Figure 6 It can be seen that, in coordinate system shaft current and The shaft currents are coupled. To decouple them, a coupling compensation term can be added to the inner current loop controller. , and the AC side voltage of the converter , And by employing a proportional-integral control module, the inner loop current controller is obtained. , The representation is as follows:

[0104] ,

[0105] in, The d-axis component of the voltage reference value generated by the control section of the converter inner loop current controller. The q-axis component of the voltage reference value generated by the control mid-section of the converter inner loop current controller. The first step for the inner loop current controller to control the d-axis current. parameter, The first step for the inner loop current controller to control the q-axis current. parameter, The d-axis component of the current reference value input to the outer loop controller. The q-axis component of the current reference value input to the outer loop controller. The second control of the d-axis current by the inner loop current controller parameter, The second control of the q-axis current by the inner loop current controller parameter.

[0106] After the above steps, the final control principle of the inner loop current controller is as follows: Figure 7 As shown, this achieves the effect of current decoupling control.

[0107] Based on the working principles of each component in the aforementioned flexible DC transmission system for wind farms, combined with Figure 8The control structure of the wind farm-side voltage source converter shown is crucial in flexible DC transmission systems. It plays a key role in regulating voltage, controlling current, providing reactive power support, suppressing harmonics, and responding to grid disturbances to maintain system stability and reliability. By adjusting the output voltage and current, the wind farm-side voltage source converter ensures grid matching and flexibly responds to the system's active and reactive power demands. Simultaneously, it suppresses harmonic components in the system through a harmonic suppression controller, improving power quality. The converter can quickly respond to voltage and frequency disturbances in the grid, ensuring stable system operation. In wind farms where only renewable energy generators serve as power sources, their grid connection capability is weak, resembling an isolated passive system. The wind farm-side voltage source converter can use constant frequency and constant AC voltage control, also known as constant power control. The controller is shown in the attached diagram. Figure 8 The diagram shows a two-level structure with an inner ring and an outer ring. Among them, and The three-phase voltage of the line is obtained after Parker transformation. Reference values ​​in a coordinate system; The reference value for the frequency on the wind farm side of the system is used. By integrating the frequency and then performing a remainder operation, the phase angle controlled by the voltage source converter can be calculated.

[0108] As attached Figure 9 As shown in the schematic diagram of the grid-side voltage source converter control structure, the grid-side voltage source converter (GVSC) in a flexible DC transmission system is one of the key components of the system. Its main functions include controlling and regulating the power flow on the grid side, maintaining the stability of the grid-side voltage, and responding to grid disturbances. The GVSC controls the active and reactive power on the grid side by adjusting its output voltage and current, and ensures power flow matching with the grid through a current controller. Furthermore, the GVSC maintains grid-side voltage stability through a voltage controller and quickly adjusts its output to ensure stable system operation when grid disturbances occur. In summary, the GVSC plays a crucial role in ensuring stable interconnection between the grid and the system in a flexible DC transmission system. Generally, the GVSC adopts a constant DC voltage control method, that is, a grid voltage-oriented vector control method.

[0109] like Figure 10The diagram shows the topology of a flexible DC transmission system for a wind farm. The sending-end grid on the left consists of a doubly-fed asynchronous wind turbine generator 11 and a battery energy storage system controlled by a grid converter 12. The power is transmitted to receiving-end grid 15 and receiving-end grid 2 16 via a flexible DC transmission system 13 and a high-voltage AC transmission system 14, respectively. The wind turbine-side converter VSC-131 uses constant power control, and the grid-side converter VSC-132 uses constant DC voltage control.

[0110] In AC / DC power transmission systems containing only renewable energy generation, the lack of a traditional synchronous generator weakens system strength and reduces inertia. This embodiment addresses this issue by adding a virtual synchronous generator-controlled energy storage power station to the transmission system, effectively resolving the insufficient system inertia. By simulating the mechanical characteristics of a traditional synchronous generator, the control equations for adjusting virtual inertia and virtual damping using the virtual synchronous generator can be derived as follows:

[0111] ,

[0112] in, This refers to the mechanical power of the voltage source converter. For the electromagnetic power of the voltage source converter, For virtual damping of the virtual synchronizer, For the virtual inertia of the virtual synchronizer, The synchronization frequency of the virtual synchronizer. The synchronization frequency rating of the virtual synchronizer. as well as The difference can reflect the change in system frequency. This represents the power angle during actual operation of the voltage source converter. By simplifying the sending-end model into a combined module of a wind turbine and a virtual synchronous machine, it is possible to study the influence mechanism of the virtual inertia and damping of the virtual synchronous machine on the sending-end system.

[0113] The electromagnetic power of a voltage source converter can be expressed as:

[0114] ,

[0115] in, This represents the electromotive force amplitude at the connection point between the wind turbine and the power grid. The amplitude of the voltage in the remote large power grid system. It is the equivalent reactance of the transmission line between the sending system and the remote system.

[0116] To simplify calculations, you can set... The above equation can be derived as follows:

[0117] ,

[0118] Further results were obtained:

[0119] ,

[0120] After performing a Laplace transform on the above equation, the expressions for the natural oscillation angular frequency and the system damping ratio of the second-order system are obtained as follows:

[0121]

[0122] The natural oscillation frequency of the second-order system under small oscillation disturbances in the undamped system. Let be the damping ratio of this second-order system. Analyzing the above expression, the virtual inertia... Appears in natural oscillation angular frequency and damping ratio The denominator of the expression indicates that the larger the inertia, the smaller the undamped angular frequency of the system during oscillation, and the smaller the system's damping ratio, meaning it takes longer for the system to recover to stability. Damping The characteristic roots of the system are positively correlated with the system's damping ratio; the larger the characteristic root, the greater the damping ratio, and the faster the system can reach a steady state. Furthermore, the eigenvalues ​​of the system can be derived from the above expression. for:

[0123] .

[0124] Based on the above derivation, the following relationship exists between virtual inertia and virtual damping: The existence of virtual inertia significantly reduces the rate of change of frequency (RoCoF) when the system faces frequency fluctuations, and significantly reduces the peak overvoltage caused by DC blocking. However, excessive virtual inertia prolongs the time for the system to reach steady state and can cause oscillations. Virtual damping can work in conjunction with virtual inertia for control. When virtual damping increases, the system damping ratio increases, the system's ability to absorb oscillations is stronger, and it means the system can reach steady state more quickly. Therefore, when setting virtual inertia and virtual damping parameters, factors such as the rate of change of frequency, peak overvoltage, and recovery time to steady state should be considered to design a control strategy that takes into account multiple indicators as much as possible.

[0125] When a severe fault in a DC line leads to DC bipolar blocking, the reactive power consumed by the DC line decreases sharply. Because filters and capacitors on the rectifier bus cannot be switched on and off in time, a large amount of reactive power becomes surplus and is fed back to the sending-end grid, causing the sending-end bus voltage to rise. Simultaneously, the DC blocking operation causes the current flowing into the rectifier station to drop rapidly to zero, similar to a sudden load reduction in the receiving-end grid, i.e., a "load shedding" phenomenon, causing the bus voltage to rise rapidly to the level of the wind turbine terminal voltage. The combination of these two situations results in a transient overvoltage on the sending-end bus, and the surplus active power causes the sending-end grid frequency to rise. Due to the blocking of the DC transmission line, a large amount of power flows to another AC line. The surplus reactive and active power is absorbed and transferred by energy storage devices and higher-capacity high-voltage AC transmission lines. At this point, a new steady state can be reached without the need for a generator switching system.

[0126] Since the sending-end system consists entirely of wind turbines, its frequency regulation capability is poor and its system inertia is insufficient. When faced with frequency increases and transient overvoltages caused by DC blocking, the system is highly susceptible to instability, leading to turbine shutdown and severely impacting the power transmission quality of the wind farm. Grid-connected energy storage at the sending end provides virtual inertia and virtual damping energy to offer damping and inertial support to the system, improving the transient stability of the AC / DC transmission system under DC blocking conditions.

[0127] Based on this, the variable parameter control method in this application includes the following steps:

[0128] Step S100: Monitor the system bus voltage of the sending-end power grid.

[0129] In step S200, if the system bus voltage exceeds the voltage threshold, a first control signal is sent to the virtual synchronizing machine to adjust the virtual inertia and virtual damping to the first standard value.

[0130] In step S200, if the system bus voltage exceeds the voltage threshold, a first control signal is sent to the virtual synchronizer to adjust the virtual inertia and virtual damping to the first standard value. Specifically:

[0131] Step S201: If the system bus voltage exceeds the voltage threshold, a first control signal is sent to the virtual machine synchronizer.

[0132] In step S202, the virtual synchronizer increases the virtual inertia to the maximum value of virtual inertia according to the first control signal, and decreases the virtual damping to the minimum value of virtual damping according to the first control signal.

[0133] In step S300, if the system bus voltage is lower than the voltage threshold, a second control signal is sent to the virtual synchronizing machine to adjust the virtual inertia and virtual damping to the second standard value.

[0134] In step S300, if the system bus voltage is lower than the voltage threshold, a second control signal is sent to the virtual synchronizer to adjust the virtual inertia and virtual damping to the second standard value, specifically:

[0135] Step S301: If the system bus voltage is lower than the voltage threshold, a second control signal is sent to the virtual synchronizer;

[0136] In step S302, the virtual synchronizer reduces the virtual inertia to the minimum value of virtual inertia according to the second control signal, and increases the virtual damping to the maximum value of virtual damping according to the second control signal.

[0137] The specific expression for the control strategy of the virtual synchronizer is as follows:

[0138] ,

[0139] ,

[0140] in, For the virtual inertia of the virtual synchronizer, This represents the maximum virtual inertia. This represents the minimum value of the virtual inertia. The system bus voltage of the sending-end power grid. This is the voltage critical value. For virtual damping of the virtual synchronizer, This represents the virtual damping minimum. This represents the maximum virtual damping value.

[0141] To better demonstrate the effectiveness of the aforementioned variable parameter control method in stabilizing the voltage and frequency of the sending-end grid by adjusting virtual inertia and virtual damping, and to reduce the occurrence of DC blocking, this embodiment uses the aforementioned flexible DC transmission system for the wind farm built in MATLAB / Simulink for DC blocking simulation analysis. (See attached...) Figure 11 The diagram showing the impact of different virtual inertia on the frequency stability of the sending system, and the attached diagram. Figure 12 The diagram illustrates the impact of different virtual damping parameters on the frequency stability of the sending-end system. The system operates in steady state at time t = 1.5 s. At time t = 3 s, DC bipolar blocking causes a sudden increase in both frequency and voltage in the sending-end system. The mechanism of these parameters' effects on the frequency and voltage stability of the sending-end system is compared by setting different virtual inertia and virtual damping parameters for grid-connected energy storage. Analysis of the simulation curves shows that as the virtual inertia of the grid-connected energy storage at the sending end increases... With the continuous increase of the DC bipolar blocking in the AC / DC hybrid system, the rate of frequency rise in the sending-end system significantly decreases, and the peak value of the final frequency rise is suppressed. However, the system recovery time continuously increases. The addition of virtual damping can significantly improve the recovery time of the sending-end system, and the increase of virtual damping can also slightly reduce the peak value of the frequency surge. Therefore, in order to improve frequency stability after DC bipolar blocking, it is necessary to comprehensively consider the factors of frequency change rate, frequency rise peak, and frequency recovery time, and set reasonable virtual inertia and virtual damping parameters to improve frequency stability. Furthermore, when DC blocking occurs, the sending-end bus voltage will experience transient overvoltage due to the inability of the reactive power compensation equipment at the converter station to respond quickly enough to cut off the reactive power, resulting in a large reactive power surplus at the sending end. Generally, the reactive power compensation equipment is cut off after a delay of 0.2 seconds, and the energy storage on the sending-end grid side can also help absorb some reactive power. During the transient voltage recovery process, the virtual inertia and virtual damping of the grid energy storage also play a supporting role.

[0142] Furthermore, in conjunction with the appendix Figure 13 The diagram shows the voltage variation of the sending-end bus when the grid-connected energy storage is configured with different virtual inertia values. (See attached diagram.) Figure 14 The diagram shows the voltage changes of the sending-end bus after the system experiences DC bipolar blocking at t=3s, with different virtual damping settings for the grid-connected energy storage. When the virtual inertia of the grid-connected energy storage increases, the peak value of the transient overvoltage at the sending-end bus after DC blocking decreases significantly, but the system recovery time also increases. The addition of virtual damping significantly enhances the system's ability to absorb oscillations, thus resulting in a faster recovery rate, while its effect on reducing the peak value of transient overvoltage is not significant.

[0143] Regarding the frequency stability of the flexible DC transmission system for wind farms constructed in this embodiment under DC bipolar blocking conditions, it is desirable to have the lowest possible rate of frequency change and the lowest possible peak frequency at high frequencies, while also achieving the fastest possible rate of recovery to stability. Similarly, regarding the voltage stability of the system under the same transient conditions, it is desirable to have the lowest possible peak transient overvoltage and the fastest possible rate of voltage recovery to stability. Based on the mechanism of virtual inertia's influence on system stability described above, when the system bus voltage exceeds the critical value, it is determined that a DC blocking fault has occurred, and the virtual inertia setpoint is increased to the maximum virtual inertia value. The virtual damping is adjusted to the minimum virtual damping value. A higher virtual inertia can significantly reduce the rate of frequency change and also significantly reduce the peak value of transient overvoltages. When the system bus voltage is detected to drop below the critical value, it is determined that the system is gradually recovering from a high-frequency and high-voltage state to a steady state. At this time, the virtual inertia setpoint is reduced to the minimum virtual inertia value. The virtual damping is adjusted to the maximum virtual damping value. This makes the system recover to a steady state faster.

[0144] Through multiple tests and adjustments of the above-described variable parameter control method, the optimal parameter values ​​in the variable parameter control method of this embodiment are: the maximum value of virtual inertia. Set to 80 p.u., minimum virtual inertia. Set to 20 p.u., minimum virtual damping. The virtual damping maximum value is set to 1500 p.u. Set to 2500 p.u., voltage threshold Set to 1.2 pu.

[0145] Based on the above parameter settings, virtual inertia For 20 p.u., virtual damping A conventional control strategy for 1500 p.u. was simulated in the constructed model. The frequency stability and voltage stability of the sending-end system after DC blocking were compared. The results are as follows: Figure 15 and Figure 16 As shown in the figure. Analysis reveals that the control strategy proposed in this embodiment, compared to conventional control strategies, not only reduces the frequency change rate (lowering the peak frequency rise from 51.02Hz to 50.79Hz) in terms of frequency stability, but also shortens the system's stabilization time from 7.92s to 6.74s, significantly accelerating the system's recovery speed. Simultaneously, in terms of voltage stability, it reduces the transient overvoltage peak value (from 1.33pu to 1.27pu) and shortens the system's stabilization time from 9.74s to 7.47s, again reducing the overall system stabilization time.

[0146] It should be understood that, although attached Figure 1 The steps in the flowchart are shown sequentially according to the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified in this document, there is no strict order requirement for the execution of these steps, and they can be executed in other orders. Furthermore, [the following is a list of steps]. Figure 1 At least some of the steps in the process may include multiple sub-steps or sub-stages. These sub-steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these sub-steps or stages is not necessarily sequential, but can be executed in turn or alternately with other steps or at least some of the sub-steps or stages of other steps.

[0147] The above-described embodiments of the present invention describe in detail a variable parameter control method for a flexible DC transmission system in a wind farm. The above-described method can be implemented using various types of equipment. Therefore, the present invention also discloses a variable parameter control device for a flexible DC transmission system in a wind farm. Specific embodiments are given below for detailed description.

[0148] The voltage monitoring module is used to monitor the system bus voltage of the sending-end power grid;

[0149] The first parameter adjustment module is used to send a first control signal to the virtual synchronizing machine if the system bus voltage exceeds the voltage threshold value, so as to adjust the virtual inertia and virtual damping to the first standard value.

[0150] The second parameter adjustment module is used to send a second control signal to the virtual synchronizing machine to adjust the virtual inertia and virtual damping to a second standard value if the system bus voltage is lower than the voltage threshold.

[0151] For the variable parameter control device used in the flexible DC transmission system of wind farms, please refer to the above description of the method limitations, which will not be repeated here. Each module in the above device can be implemented entirely or partially through software, hardware, or a combination thereof. These modules can be embedded in the processor of the terminal device in hardware form or independent of it, or stored in the memory of the terminal device in software form, so that the processor can call and execute the corresponding operations of each module.

[0152] In one embodiment, the present invention also provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the above-described variable parameter control method for a flexible DC transmission system for wind farms.

[0153] The computer-readable storage medium may be an electronic storage device such as flash memory, EEPROM (Electrically Erasable Programmable Read-Only Memory), EPROM (Erasable Programmable Read-Only Memory), hard disk, or ROM. Optionally, the computer-readable storage medium includes a non-transitory computer-readable storage medium. The computer-readable storage medium has storage space for program code that performs any of the method steps described above. This program code can be read from or written to one or more computer program products, and the program code may be compressed in an appropriate form.

[0154] In one embodiment, the present invention provides a computer device including a memory and a processor, the memory storing a computer program, and the processor executing the computer program to perform the above-described variable parameter control method for a flexible DC transmission system for wind farms.

[0155] The computer device includes a memory, a processor, and one or more computer programs, wherein the one or more computer programs can be stored in the memory and configured to be executed by one or more processors, and the one or more application programs are configured to execute the above-described variable parameter control method for flexible DC transmission systems in wind farms.

[0156] A processor may include one or more processing cores. The processor connects to various parts of the computer device using various interfaces and lines, and performs various functions and processes data by running or executing instructions, programs, code sets, or instruction sets stored in memory, and by calling data stored in memory. Optionally, the processor may be implemented using at least one hardware form of Digital Signal Processing (DSP), Field-Programmable Gate Array (FPGA), or Programmable Logic Array (PLA). The processor may integrate one or a combination of several of the following: Central Processing Unit (CPU), Graphics Processing Unit (GPU), and modem. The CPU primarily handles the operating system, user interface, and applications; the GPU is responsible for rendering and drawing the displayed content; and the modem handles wireless communication. It is understood that the modem may also be implemented separately as a communication chip, without being integrated into the processor.

[0157] The memory may include random access memory (RAM) or read-only memory (ROM). The memory can be used to store instructions, programs, code, code sets, or instruction sets. The memory may include a program storage area and a data storage area. The program storage area may store instructions for implementing an operating system, instructions for implementing at least one function (such as touch functionality, sound playback functionality, image playback functionality, etc.), and instructions for implementing the various method embodiments described above. The data storage area may also store data created by the terminal device during use.

[0158] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A variable parameter control method for a wind farm flexible HVDC power transmission system, the wind farm flexible HVDC power transmission system comprising a sending end power grid, a receiving end power grid, a flexible HVDC power transmission network and a high voltage AC power transmission network, the sending end power grid being connected to different receiving end power grids through the flexible HVDC power transmission network and the high voltage AC power transmission network respectively, wherein the sending end power grid is composed of a doubly-fed asynchronous wind generator and a voltage source converter controlled battery energy storage system connected in sequence; characterized in that, The wind farm flexible HVDC system further comprises a virtual synchronous machine connected with the sending-end power grid, and the variable parameter control method comprises the following steps: monitoring the system bus voltage of the sending-end power grid; if the system bus voltage exceeds a voltage threshold, sending a first control signal to the virtual synchronous machine to adjust the virtual inertia and the virtual damping to first standard values; if the system bus voltage is lower than the voltage threshold, sending a second control signal to the virtual synchronous machine to adjust the virtual inertia and the virtual damping to second standard values; if the system bus voltage exceeds the voltage threshold, sending a first control signal to the virtual machine synchronous machine; the virtual synchronous machine increases the virtual inertia to a virtual inertia maximum value according to the first control signal and decreases the virtual damping to a virtual damping minimum value according to the first control signal; if the system bus voltage is lower than the voltage threshold, sending a second control signal to the virtual synchronous machine; the virtual synchronous machine decreases the virtual inertia to a virtual inertia minimum value according to the second control signal and increases the virtual damping to a virtual damping maximum value according to the second control signal; a specific expression of the virtual synchronous machine adjusting the virtual inertia and the virtual damping is: a specific expression of the control strategy of the virtual synchronous machine is: a specific expression of the output power of the doubly-fed asynchronous wind generator is: ， wherein, is the mechanical power of the voltage source converter, is the electromagnetic power of the voltage source converter, is the virtual damping of the virtual synchronous machine, is the virtual inertia of the virtual synchronous machine, is the synchronous frequency of the virtual synchronous machine, is the synchronous frequency rating of the virtual synchronous machine, is the power angle of the voltage source converter in actual operation.

2. The variable parameter control method for a flexible HVDC power transmission system of a wind farm according to claim 1, wherein, further comprising: ， ， wherein, is a virtual inertia of the virtual synchronous machine, is a virtual inertia maximum value, is a virtual inertia minimum value, is a system bus voltage of the sending end power grid, is a voltage threshold value, is a virtual damping of the virtual synchronous machine, is a virtual damping minimum value, is a virtual damping maximum value.

3. The variable parameter control method for a flexible HVDC power transmission system of a wind farm according to claim 1, wherein, when the wind energy capture coefficient is a maximum value, the wind turbine of the doubly-fed asynchronous wind generator adopts a maximum power point tracking control mode; ， wherein is the output power of the doubly-fed asynchronous wind generator, is the air density of the environment in which the doubly-fed asynchronous wind generator is located, is the wind energy capture coefficient, is the tip speed of the blades of the doubly-fed asynchronous wind generator, is the tip speed of the blades and the pitch angle together constitute an intermediate variable, is the pitch angle of the doubly-fed asynchronous wind generator, is the radius of the fan rotor of the doubly-fed asynchronous wind generator, is the wind speed of the environment in which the doubly-fed asynchronous wind generator is located, is the fan rotational speed of the doubly-fed asynchronous wind generator.

4. The variable parameter control method for a flexible HVDC power transmission system of a wind farm according to claim 3, wherein, when the wind energy capture coefficient is less than the maximum value, the control mode of the wind turbine of the doubly-fed asynchronous wind generator is switched to a power reduction control mode. The wind farm flexible HVDC system further comprises a virtual synchronous machine connected with the sending-end power grid, and the variable parameter control device comprises the following modules: a voltage monitoring module for monitoring the system bus voltage of the sending-end power grid; 5. A variable parameter control device for a wind farm flexible HVDC power transmission system, the wind farm flexible HVDC power transmission system comprising a sending end power grid, a receiving end power grid, a flexible HVDC power transmission network and a high voltage AC power transmission network, the sending end power grid being connected to different receiving end power grids through the flexible HVDC power transmission network and the high voltage AC power transmission network respectively, wherein the sending end power grid is composed of a doubly-fed asynchronous wind generator and a voltage source converter controlled battery energy storage system connected in sequence; characterized in that, a first parameter adjusting module for sending a first control signal to the virtual synchronous machine to adjust the virtual inertia and the virtual damping to first standard values if the system bus voltage exceeds a voltage threshold; a second parameter adjusting module for sending a second control signal to the virtual synchronous machine to adjust the virtual inertia and the virtual damping to second standard values if the system bus voltage is lower than the voltage threshold; if the system bus voltage exceeds the voltage threshold, sending a first control signal to the virtual machine synchronous machine; the virtual synchronous machine increases the virtual inertia to a virtual inertia maximum value according to the first control signal and decreases the virtual damping to a virtual damping minimum value according to the first control signal; if the system bus voltage is lower than the voltage threshold, sending a second control signal to the virtual synchronous machine; the virtual synchronous machine decreases the virtual inertia to a virtual inertia minimum value according to the second control signal and increases the virtual damping to a virtual damping maximum value according to the second control signal; ​ If the system bus voltage is lower than the voltage threshold value, a second control signal is sent to the virtual synchronous machine to adjust the virtual inertia and the virtual damping to second standard values, specifically: If the system bus voltage is lower than the voltage threshold value, a second control signal is sent to the virtual synchronous machine; The virtual synchronous machine reduces the virtual inertia to a virtual inertia minimum value according to the second control signal, and increases the virtual damping to a virtual damping maximum value according to the second control signal; The specific expression of the virtual synchronous machine adjusting the virtual inertia and the virtual damping is: ， wherein, is the mechanical power of the voltage source converter, is the electromagnetic power of the voltage source converter, is the virtual damping of the virtual synchronous machine, is the virtual inertia of the virtual synchronous machine, is the synchronous frequency of the virtual synchronous machine, is the synchronous frequency rating of the virtual synchronous machine, is the power angle of the voltage source converter in actual operation.

6. A computer-readable storage medium having stored thereon a computer program, characterized in that, The computer program is executed by the processor to realize the steps of the variable parameter control method for the wind farm flexible HVDC power transmission system in any one of claims 1-4. 7.A computer device, comprising a memory and a processor, wherein the memory stores a computer program, and the computer device is configured to perform the method according to any one of claims 1-6 when the computer program is executed by the processor. The processor executes the computer program to execute the variable parameter control method for the wind farm flexible HVDC power transmission system in any one of claims 1-4.

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