Wind storage channel flexible direct current power transmission system and inertia cooperative control method thereof

Abstract

The invention discloses a wind energy storage flexible direct current power transmission system and an inertia cooperative control method thereof, and relates to the field of new energy flexible direct current power transmission, and the system comprises a double-fed wind power plant, an energy storage device, a sending end MMC station, a receiving end MMC station, a direct current cable and a receiving end power grid. The energy storage device is arranged in the wind turbine generator set, so that the wind turbine generator set can simulate the inertia characteristic and the droop characteristic of a synchronous generator, and stored energy increases power in the rotating speed recovery process of a fan rotor to prevent frequency secondary drop; frequency change information is precisely restored through voltage-frequency restoration control and power loss compensation, so that the frequency information at the two ends of the direct-current power transmission system is transmitted in real time. Meanwhile, a capacitor in the MMC-HVDC system can release electrostatic energy due to the change of the direct-current voltage, the inertia supporting capacity of the system can be further improved through a corresponding control strategy, inertia cooperative supporting of source-network-storage heterogeneous resources is achieved, and the overall inertia level of the system is improved.

Description

Technical Field

[0001] This invention relates to the field of new energy transmission via flexible DC transmission, specifically to a wind-storage transmission system via flexible DC transmission and its inertia coordinated control method. Background Technology

[0002] As my country accelerates its energy transition and ensures energy supply security, renewable energy sources, represented by wind power, are developing rapidly, gradually replacing the traditional fossil fuel system. Simultaneously, the uneven distribution of energy resources and electricity load in my country has objectively promoted the rapid development of flexible direct current (DC) transmission. The proportion of power electronic equipment in new power systems is increasing, and the demand for power system inertial support capacity is also rising, yet research on inertial level enhancement control technologies that coordinate various heterogeneous inertial resources in new power systems is scarce.

[0003] The large-scale grid connection of renewable energy sources via DC transmission systems can adversely affect the frequency stability of AC systems. Firstly, the replacement of conventional synchronous generators by renewable energy power plants reduces the overall inertia level of the power system. Secondly, the electronic power equipment of renewable energy plants under traditional grid-connected control cannot actively provide inertia and primary frequency regulation support. Finally, the decoupling characteristics of DC transmission at both ends prevent the sending-end grid from responding to disturbances in the receiving-end grid to provide inertia support. Furthermore, previous frequency support control methods, including wind turbine rotor kinetic energy control and load shedding control, can lead to secondary frequency drops and wasted wind power resources. Summary of the Invention

[0004] The purpose of this invention is to solve the problems mentioned in the background above, and to propose a wind-storage flexible DC transmission system and its inertia coordinated control method.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] In a first aspect, the present invention provides a wind-storage flexible DC transmission system, comprising a doubly-fed wind farm, a rotor-side converter, a grid-side converter, an energy storage device, a sending-end MMC station, and a receiving-end MMC station. The doubly-fed wind farm includes several doubly-fed wind turbine generators. One end of each doubly-fed wind turbine generator is connected to one end of the rotor-side converter, and the other end of the rotor-side converter is connected to one end of the grid-side converter. The other end of the grid-side converter is connected in parallel with one end of the doubly-fed wind turbine generator and one end of the energy storage device, and then connected to a first transformer. The other end of the first transformer is connected to one end of the sending-end MMC station. The other end of the sending-end MMC station is connected to one end of the receiving-end MMC station via a DC cable. The other end of the receiving-end MMC station is connected to one end of a second transformer, and the other end of the second transformer is connected to the receiving-end power grid.

[0007] In a preferred embodiment of the present invention, there are 450 doubly-fed wind turbine generators, each with a rated capacity of 2.6MW; the rotor-side converter adopts a wind turbine inertia control strategy to provide inertia response when the frequency changes; the grid-side converter adopts DC voltage control to maintain the stability of the wind turbine DC voltage; the energy storage device is connected to the grid together with the wind turbine generators through the energy storage converter; the sending-end MMC station adopts constant frequency and constant AC voltage control to provide grid voltage for the wind farm; the receiving-end MMC station adopts constant DC voltage and constant reactive power control to maintain the voltage stability of the DC transmission system.

[0008] Secondly, the present invention provides a method for coordinated inertia control of wind power storage via a flexible DC transmission system, comprising:

[0009] S1: Construct a wind-storage-flexible DC transmission system;

[0010] S2: Through the wind-storage coordinated inertia control strategy, the wind farm is endowed with the inertia and droop characteristics of a synchronous machine, which effectively suppresses system frequency changes and improves steady-state frequency deviation; power compensation control is added to the energy storage coordinated control strategy.

[0011] S3: Through the inertia control strategy of the flexible DC transmission system, at the receiving-end MMC station, the frequency changes of the receiving-end grid are mapped to DC voltage changes, and the electrostatic energy stored in the submodule capacitors is used to provide inertia response, further enhancing the inertial support capability of the system; at the sending-end MMC station, through frequency restoration control, the DC voltage change information is restored to the frequency change information of the receiving-end grid, realizing the coupling of the sending / receiving grid frequencies; the sending-end wind farm senses the frequency changes of the receiving-end grid and coordinates frequency adjustment, realizing the overall inertia level improvement of the system with coordinated source-grid-storage heterogeneous resources.

[0012] In a preferred embodiment of the present invention, in step S2, under normal operating conditions, the fan operates in maximum power point tracking mode and the energy storage output power is zero; in the inertia response stage, the fan releases rotor kinetic energy to provide inertia response, and the energy storage generates power to provide droop response; in the rotor speed recovery stage, the fan absorbs power for rotor speed recovery, and the energy storage generates additional power to compensate.

[0013] In a preferred embodiment of the present invention, step S2 specifically includes:

[0014] S21: Under normal operating conditions, the wind turbine operates in maximum power point tracking mode and outputs power at steady-state operating point A. Switch S1 is in state one and switch S2 is in state two. At this time, the wind turbine maintains maximum active power output, the energy storage output power is zero, and it does not participate in system frequency regulation.

[0015] S22: When the system frequency change rate exceeds the threshold, the wind turbine enters the inertial response stage ABE. Switch S1 switches from state one to state two. The wind turbine rapidly releases rotor kinetic energy to participate in the inertial response. As the frequency change rate decreases, the output active power and rotor speed also change continuously. When the wind turbine reaches point C, the output active power decreases to the maximum power value P before the disturbance. A At this point, the wind turbine can no longer provide an effective power increment for system inertia support; therefore, the wind turbine rotor speed will decrease when it reaches ω. off The inertia response phase should be exited promptly, and switch S2 should switch from state two to state one to ensure maximum active power output and enter the rotor speed recovery phase; the wind turbine's active power output reference value P wind,ref for:

[0016]

[0017] In the formula, P MPPT This refers to the output power of the wind turbine operating in maximum power point tracking mode; K H This refers to the inertial control coefficient; The rate of change of frequency;

[0018] S23:ω off There are two cases regarding the timing: when it is the inertial response stage, the active power output of the wind turbine is equal to the active power output at the steady state, i.e., ω. C The point corresponds to the moment; when the fan is cut off to prevent instability due to excessively low speed or stall due to excessively high speed, the fan itself has a rotor speed limit [ω]. min ω max ], the actual ω off Take the larger of the two values, that is:

[0019] ω off =max(ω C ω min )

[0020] This determines the rotational speed at which the wind turbine exits the inertial response phase, which is also the corresponding time for energy storage to start up and compensate for power.

[0021] S24: When the system frequency deviation exceeds the threshold, the energy storage enters the droop response stage. Switch S3 is in state one, and the energy storage releases the stored energy, which, together with the rotor kinetic energy of the wind turbine, participates in the system frequency regulation, enabling the wind turbine to simultaneously possess inertia and droop characteristics. At this time, the reference value of the energy storage's output active power P... bess,ref for:

[0022] P bess,ref =-K D Δf

[0023] In the formula, K D Δf is the droop control coefficient; Δf is the frequency deviation.

[0024] S25: When the wind turbine exits the inertia response stage, its operating state drops from point C to point D, entering the rotor speed recovery stage. At this time, the active power output of the wind turbine unit decreases sharply, resulting in a power deficit. Switch S3 switches from state one to state two, initiating energy storage power compensation control to compensate for this power deficit. This power deficit is the energy storage compensation power ΔP. cp , represented as:

[0025] ΔP cp =P A -P MPPT

[0026] That is, the reference value for the output active power of the energy storage at this time is:

[0027] P bess,ref =-K D Δf+ΔP cp

[0028] Among them, the energy storage compensation power ΔP cp It will adaptively decrease as the output active power of the wind turbine gradually recovers.

[0029] In a preferred embodiment of the present invention, step S3 specifically includes:

[0030] S31: When the receiving-end system is subjected to an active power disturbance, the flexible DC system absorbs or releases energy through energy storage elements to provide inertial support for the receiving-end system. When the sending-end MMC station and the receiving-end MMC station have the same configuration, the capacitance of all submodules in the MMC-HVDC system is equivalent to the equivalent capacitance C. eq Since both store the same amount of energy, the following equation applies:

[0031]

[0032] V dc =nV sm

[0033] In the formula, C sm V represents the capacitance value of the submodule. sm V is the submodule capacitor voltage; N is the number of MMC stations; n is the number of submodules in a single bridge arm; dc It is a DC voltage;

[0034] Therefore, the equivalent capacitance C of MMC-HVDC eq Represented as:

[0035]

[0036] S22: Analogous to the inertial response of a synchronous generator, a coupling relationship between DC voltage and AC frequency is established, allowing the electrostatic power of the capacitor to simulate the virtual inertial characteristics similar to a synchronous machine, thus obtaining the DC voltage reference value V of the MMC-HVDC. dc,ref As shown in the following formula:

[0037]

[0038] Where H mmc It is the virtual equivalent inertial time constant of MMC; S mmc The rated power of the MMC station; V dc0 f0 represents the rated DC voltage amplitude of the MMC-HVDC system; f0 represents the rated frequency of the AC system.

[0039] DC voltage V dc,ref It is coupled with AC frequency f and is positively correlated. By changing DC voltage, the MMC submodule capacitor releases energy to achieve a certain inertia support. At the same time, by mapping the frequency change of the receiving end grid to the DC voltage change, this frequency change information is restored at the sending end MMC station, thereby realizing the cross-regional inertia support of the sending end wind farm to the receiving end grid.

[0040] S23: For power loss compensation in DC cable lines under long-distance power transmission, the DC voltage relationship between the sending / receiving MMC stations considering the equivalent impedance of the MMC-HVDC DC line is as follows:

[0041]

[0042] In the formula, ΔV dc It is the DC line voltage drop of the MMC-HVDC; V dc1 and V dc2 These are the DC voltages at the ports of the sending-end MMC station and the receiving-end MMC station, respectively; R dc and L dc These are the equivalent resistance and equivalent reactance of the MMC-HVDC DC line, respectively; I dc The DC current of MMC-HVDC;

[0043] S24: Through frequency restoration control and power loss compensation, the receiving-end MMC station accurately restores the frequency information of the sending-end power grid without communication. At this time, the frequency reference value f of the receiving-end power grid is... ref for:

[0044]

[0045] Based on the reference frequency information restored by the sending-end MMC station, the wind farm senses the frequency change of the receiving-end grid and initiates a wind-storage coordinated inertia control strategy to provide inertial support, thereby achieving a system inertia level improvement for coordinated cross-regional source-grid-storage heterogeneous resources.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] 1. This invention utilizes the substantial rotor kinetic energy inherent in the wind turbine itself to support the system's inertia, simulating the inertia response characteristics of a synchronous generator set through an inertia control strategy. Energy storage offers advantages in speed and flexibility, simulating the droop frequency regulation characteristics of a synchronous generator set through droop control. The coordination of these two elements enables the wind farm to suppress the system's rate of frequency change and improve the system's steady-state frequency deviation.

[0048] 2. The energy storage coordination control strategy of this invention includes additional power compensation control, which changes the active power output mode of the energy storage during the process of participating in system frequency regulation, to make up for the power deficit caused by the wind turbine exiting the inertial response, and effectively avoids the secondary drop in system frequency.

[0049] 3. The inertia control strategy of the MMC-HVDC of the present invention maps the frequency change of the receiving end grid to the DC voltage change through frequency-voltage mapping control at the receiving end MMC station, thereby using the energy of the sub-module capacitor to provide a certain inertia support; at the sending end MMC station, the voltage change is accurately restored to the frequency change through voltage-frequency restoration control and power loss compensation, so that the wind farm can obtain frequency reference information and provide inertia support for the system. Attached Figure Description

[0050] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0051] Figure 1 This is a schematic diagram of the topology of a wind-storage-flexible DC transmission system;

[0052] Figure 2 It is a graph showing the power-speed variation of a wind turbine.

[0053] Figure 3 This is a block diagram of the inertia control strategy for wind turbine units;

[0054] Figure 4 This is a block diagram of the coordinated control strategy for energy storage;

[0055] Figure 5 This is a flowchart of the inertia coordination control of the wind and energy storage combined system;

[0056] Figure 6 These are dynamic response diagrams of the wind-storage integrated system under different control strategies;

[0057] Figure 7 This is a diagram of the inertia support control strategy for MMC-HVDC.

[0058] Figure 8 It is a frequency restoration comparison chart;

[0059] Figure 9 This is a schematic diagram of the simulation testing system;

[0060] Figure 10 This is a simulation result of the system response after a sudden increase in load. Detailed Implementation

[0061] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0062] Example 1

[0063] Please see Figure 1 As shown, a wind-storage-flexible DC transmission system has the following topology: Figure 1 As shown, the system mainly includes a doubly-fed induction generator (DFIG) wind farm comprising 450 DFIG wind turbines with a rated capacity of 2.6MW. The rotor-side converters employ a turbine inertia control strategy to provide inertia response during frequency changes, while the grid-side converters use DC voltage control to maintain stable DC voltage for the turbines. Each wind turbine is equipped with a 0.26MW energy storage device, which is connected to the grid via an energy storage converter. A 1000MW flexible DC transmission system is implemented using modular multilevel converters (MMCs) to achieve high-voltage DC transmission. The sending-end MMC station uses constant frequency and constant AC voltage control to provide grid connection voltage for the DFIG wind farm. The receiving-end MMC station uses constant DC voltage and constant reactive power control to maintain voltage stability and power output of the DC transmission system. In traditional control modes, neither the wind farm nor the flexible DC transmission system participates in the frequency regulation process of the receiving-end grid, and the decoupling characteristics of the flexible DC transmission system prevent the wind farm from providing an inertia response to the receiving-end grid.

[0064] Example 2

[0065] Please see Figure 1-10 As shown, a method for coordinated inertia control of wind power storage via a flexible DC transmission system includes:

[0066] The wind-storage flexible DC transmission system consists of a doubly-fed wind farm power grid, an energy storage device, a flexible DC transmission system based on a modular multilevel converter, and a receiving-end power grid connected in sequence.

[0067] By employing a wind-storage coordinated inertia control strategy, the wind farm acquires the inertia and droop characteristics of a synchronous machine, effectively suppressing system frequency variations and improving steady-state frequency deviation. The additional power compensation control within the energy storage coordinated control strategy addresses the secondary frequency drop issue caused by the wind turbine exiting the inertia response phase.

[0068] By employing an inertia control strategy for the flexible DC transmission system, frequency changes in the receiving-end grid are mapped to DC voltage changes at the receiving-end MMC station. The electrostatic energy stored in the submodule capacitors provides the inertia response, further enhancing the system's inertial support capability. At the sending-end MMC station, frequency restoration control restores DC voltage changes to receiving-end grid frequency changes, achieving frequency coupling between the sending and receiving grids. Furthermore, line power losses in the MMC-HVDC system are considered, reducing errors in frequency restoration. The sending-end wind farm senses frequency changes in the receiving-end grid and coordinates frequency regulation, achieving an overall improvement in the system's inertia level through coordinated source-grid-storage heterogeneous resources.

[0069] like Figure 2 , Figure 3 As shown, under normal operating conditions, the wind turbine operates in maximum power point tracking mode and outputs power at steady-state operating point A. Switch S1 is in state 1 and switch S2 is in state 2. At this time, the wind turbine maintains maximum active power output, and the energy storage output power is 0, and it does not participate in system frequency regulation.

[0070] When the system frequency change rate exceeds the threshold, the wind turbine enters the inertial response stage ABE. The state of switch S1 changes from 1 to 2, and the wind turbine rapidly releases rotor kinetic energy to participate in the inertial response. As the frequency change rate decreases, the output active power and rotor speed also change continuously. When the wind turbine reaches point C, the output active power decreases to the maximum power value P before the disturbance. A At this point, the wind turbine can no longer provide an effective power increment for system inertia support, therefore the wind turbine rotor speed reaches ω. off The inertia response phase should be exited in a timely manner, and the S2 state should be switched from 2 to 1 to ensure the maximum output power and enter the rotor speed recovery phase.

[0071] The active power reference value P of the wind turbine generator set wind,ref for:

[0072]

[0073] In the formula, P MPPT This refers to the output power of the wind turbine operating in maximum power point tracking mode; K H This refers to the inertial control coefficient; This represents the rate of change of frequency.

[0074] The ω offThe timing is discussed in two cases: 1) Inertial response stage, the active power output of the wind turbine is equal to the active power output at steady state, i.e., ω C Point corresponding to the time; 2) In order to prevent the fan from becoming unstable due to excessively low speed or stalling due to excessively high speed, the fan itself has a rotor speed limit [ω]. min ω max Therefore, the actual ω off The larger of the two values ​​should be taken, that is:

[0075] ω off =max(ω C ω min )

[0076] This allows us to determine the rotational speed at which the wind turbine exits the inertial response phase, which is also the corresponding time for energy storage to start up and compensate for power.

[0077] like Figure 4 As shown, during normal steady-state operation, the energy storage outputs zero active power and does not participate in system frequency regulation.

[0078] When the system frequency deviation exceeds the threshold, the energy storage enters the droop response stage, the switch S3 is in state 1, the energy storage releases the stored energy, and works with the wind turbine rotor kinetic energy to participate in the system frequency regulation, so that the wind turbine has both inertia characteristics and droop characteristics.

[0079] At this time, the reference value of the active power output of the energy storage is P. bess,ref for:

[0080] P bess,ref =-K D Δf

[0081] In the formula, K D Δf is the droop control coefficient; Δf is the frequency deviation.

[0082] When the wind turbine exits the inertia response phase, its operating state drops from point C to point D, entering the rotor speed recovery phase. At this time, the active power output of the wind turbine unit decreases sharply, resulting in a power deficit. Switch S3 switches from state 1 to 2, initiating energy storage power compensation control to compensate for this power deficit and prevent the occurrence of a secondary frequency drop. This power deficit is the energy storage compensation power ΔP. cp It can be represented as:

[0083] ΔP cp =P A -P MPPT

[0084] That is, the reference value for the output active power of the energy storage at this time is:

[0085] P bess,ref =-K D Δf+ΔPcp

[0086] Among them, the compensation power ΔP cp It will adaptively decrease as the output active power of the wind turbine gradually recovers, effectively mitigating the frequency overshoot caused by the fixed compensation power output of the energy storage.

[0087] By incorporating energy storage into wind turbine units, the main solution addresses the power drop during the rotor speed recovery phase, preventing secondary frequency drops and reducing wind resource waste. The control process for coordinating the wind-storage integrated system for inertia support is as follows: Figure 5 As shown:

[0088] 1) When the system frequency changes, if the rotor speed of the wind turbine is within the range of [0.7pu, 1.2pu], the wind turbine releases or absorbs rotor kinetic energy to provide inertial support and suppress the rate of frequency change; if the system frequency deviation is greater than 0.033Hz and the state of charge (SOC) of the energy storage is within the limit range, the energy storage releases energy to provide droop support and reduce the frequency deviation.

[0089] 2) When the output power of the wind turbine is equal to the steady-state output power before the fault, the wind turbine exits the inertia response stage and enters the rotor speed recovery stage. The energy storage generates active power to help the rotor recover its speed and maintain the system power balance.

[0090] like Figure 7 As shown, when the receiving-end system is subjected to an active power disturbance, the flexible DC system can absorb or release energy through energy storage elements to support the inertia of the receiving-end system. When the sending-end MMC station and the receiving-end MMC station have the same configuration, the capacitance of all submodules in the MMC-HVDC system can be equivalent to the equivalent capacitance C. eq Since both store the same amount of energy, the following equation applies:

[0091]

[0092] V dc =nV sm

[0093] In the formula, C sm V represents the capacitance value of the submodule. sm V is the submodule capacitor voltage; N is the number of MMC stations; n is the number of submodules in a single bridge arm; dc It is a DC voltage.

[0094] Therefore, the equivalent capacitance C of MMC-HVDC eq It can be represented as:

[0095]

[0096] By analogy with the inertial response of a synchronous generator, a coupling relationship between DC voltage and AC frequency is established, allowing the electrostatic power of the capacitor to simulate the virtual inertial characteristics similar to a synchronous machine. This yields the DC voltage reference value V for the MMC-HVDC. dc,ref As shown in the following formula:

[0097]

[0098] DC voltage V dc,ref Coupled with and positively correlated with the AC frequency f, it can not only release energy from the capacitor of the MMC submodule by changing the DC voltage to achieve a certain inertia support, but also restore this frequency change information at the sending-end MMC station by mapping the frequency change of the receiving-end grid to the DC voltage change, thereby realizing the cross-regional inertia support of the sending-end wind farm to the receiving-end grid.

[0099] like Figure 7 As shown, to compensate for the power loss of DC cable lines under long-distance power transmission, the DC voltage relationship between the sending-end MMC station and the receiving-end MMC station, considering the equivalent impedance of the MMC-HVDC DC line, is as follows:

[0100]

[0101] In the formula, ΔV dc It is the DC line voltage drop of the MMC-HVDC; V dc1 and V dc2 These are the DC voltages at the ports of the sending-end MMC station and the receiving-end MMC station, respectively; R dc and L dc These are the equivalent resistance and equivalent reactance of the MMC-HVDC DC line, respectively; I dc This refers to the DC current of the MMC-HVDC.

[0102] Through frequency restoration control and power loss compensation, the receiving-end MMC station can accurately restore the frequency information of the sending-end power grid without communication. At this time, the frequency reference value f of the receiving-end power grid is... ref for:

[0103]

[0104] Based on the reference frequency information restored by the sending-end MMC station, the wind farm can sense the frequency change of the receiving-end grid and thus initiate the wind-storage joint system inertia coordination control strategy to provide inertial support. This enables the coordinated improvement of the system inertia level of heterogeneous resources across regions (source, grid, and storage) and ensures the frequency stability of the system.

[0105] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to any specific implementation. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A wind-storage flexible DC transmission system, comprising a doubly-fed wind farm, a rotor-side converter, a grid-side converter, an energy storage device, a sending-end MMC station, and a receiving-end MMC station, characterized in that, The doubly-fed wind farm includes several doubly-fed wind turbine generators. One end of each doubly-fed wind turbine generator is connected to one end of a rotor-side converter. The other end of the rotor-side converter is connected to one end of a grid-side converter. The other end of the grid-side converter is connected in parallel with one end of the doubly-fed wind turbine generator and one end of the energy storage device, and then connected to a first transformer. The other end of the first transformer is connected to one end of a sending-end MMC station. The other end of the sending-end MMC station is connected to one end of a receiving-end MMC station via a DC cable. The other end of the receiving-end MMC station is connected to one end of a second transformer. The other end of the second transformer is connected to the receiving-end grid.

2. The wind-storage flexible DC transmission system according to claim 1, characterized in that, The system comprises 450 doubly-fed wind turbines, each with a rated capacity of 2.6MW. The rotor-side converter employs a wind turbine inertia control strategy to provide inertia response when the frequency changes. The grid-side converter uses DC voltage control to maintain stable DC voltage for the wind turbines. The energy storage device is connected to the grid with the wind turbines through an energy storage converter. The sending-end MMC station uses constant frequency and constant AC voltage control to provide grid connection voltage for the wind farm. The receiving-end MMC station uses constant DC voltage and constant reactive power control to maintain stable voltage in the DC transmission system.

3. A method for coordinated inertia control of wind power storage via a flexible DC transmission system, characterized in that... Implementing a wind-storage-flexible DC transmission system according to any one of claims 1-2, comprising: S1: Construct a wind-storage-flexible DC transmission system; S2: Through the wind-storage coordinated inertia control strategy, the wind farm is endowed with the inertia and droop characteristics of a synchronous machine, which effectively suppresses system frequency changes and improves steady-state frequency deviation; power compensation control is added to the energy storage coordinated control strategy. S3: Through the inertia control strategy of the flexible DC transmission system, at the receiving-end MMC station, the frequency changes of the receiving-end grid are mapped to DC voltage changes, and the electrostatic energy stored in the submodule capacitors is used to provide inertia response, further enhancing the inertial support capability of the system; at the sending-end MMC station, through frequency restoration control, the DC voltage change information is restored to the frequency change information of the receiving-end grid, realizing the coupling of the sending / receiving grid frequencies; the sending-end wind farm senses the frequency changes of the receiving-end grid and coordinates frequency adjustment, realizing the overall inertia level improvement of the system with coordinated source-grid-storage heterogeneous resources.

4. The inertia coordinated control method of wind power storage through a flexible DC transmission system according to claim 3, characterized in that, In step S2, under normal operating conditions, the wind turbine operates in maximum power point tracking mode, and the energy storage output power is zero; in the inertia response stage, the wind turbine releases rotor kinetic energy to provide inertia response, and the energy storage generates power to provide droop response. During the rotor speed recovery phase, the wind turbine absorbs power to restore the rotor speed, and the energy storage generates additional power for compensation.

5. The inertia coordinated control method of wind-storage-flexible DC transmission system according to claim 4, characterized in that, Step S2 specifically includes: S21: Under normal operating conditions, the wind turbine operates in maximum power point tracking mode and outputs power at steady-state operating point A. Switch S1 is in state one and switch S2 is in state two. At this time, the wind turbine maintains maximum active power output, the energy storage output power is zero, and it does not participate in system frequency regulation. S22: When the system frequency change rate exceeds the threshold, the wind turbine enters the inertial response stage ABE. Switch S1 switches from state one to state two. The wind turbine rapidly releases rotor kinetic energy to participate in the inertial response. As the frequency change rate decreases, the output active power and rotor speed also change continuously. When the wind turbine reaches point C, the output active power decreases to the maximum power value P before the disturbance. A At this point, the wind turbine can no longer provide an effective power increment for system inertia support; therefore, the wind turbine rotor speed will decrease when it reaches ω. off The inertia response phase should be exited promptly, and switch S2 should switch from state two to state one to ensure maximum active power output and enter the rotor speed recovery phase; the wind turbine's active power output reference value P wind,ref for: In the formula, P MPPT This refers to the output power of the wind turbine operating in maximum power point tracking mode; K H This refers to the inertial control coefficient; The rate of change of frequency; S23:ω off There are two cases regarding the timing: when it is the inertial response stage, the active power output of the wind turbine is equal to the active power output at the steady state, i.e., ω. C The point corresponds to the moment; when the fan is cut off to prevent instability due to excessively low speed or stall due to excessively high speed, the fan itself has a rotor speed limit [ω]. min ω max ], the actual ω off Take the larger of the two values, that is: oh off =max(ω C Oh, oh min ) Therefore, the rotational speed at which the wind turbine exits the inertial response phase is determined is also the corresponding time for energy storage to start up and compensate for power. S24: When the system frequency deviation exceeds the threshold, the energy storage enters the droop response stage. Switch S3 is in state one, and the energy storage releases the stored energy, which, together with the rotor kinetic energy of the wind turbine, participates in the system frequency regulation, enabling the wind turbine to simultaneously possess inertia and droop characteristics. At this time, the reference value of the energy storage's output active power P... bess,ref for: P bess,ref =-K D Δf In the formula, K D Δf is the droop control coefficient; Δf is the frequency deviation. S25: When the wind turbine exits the inertia response stage, its operating state drops from point C to point D, entering the rotor speed recovery stage. At this time, the active power output of the wind turbine unit decreases sharply, resulting in a power deficit. Switch S3 switches from state one to state two, initiating energy storage power compensation control to compensate for this power deficit. This power deficit is the energy storage compensation power ΔP. cp , represented as: ΔP cp =P A -P MPPT That is, the reference value for the output active power of the energy storage at this time is: P bess,ref =-K D Δf+ΔP cp Among them, the energy storage compensation power ΔP cp It will adaptively decrease as the output active power of the wind turbine gradually recovers.

6. The inertia coordinated control method of wind power storage through a flexible DC transmission system according to claim 3, characterized in that, Step S3 specifically includes: S31: When the receiving-end system is subjected to an active power disturbance, the flexible DC system absorbs or releases energy through energy storage elements to provide inertial support for the receiving-end system. When the sending-end MMC station and the receiving-end MMC station have the same configuration, the capacitance of all submodules in the MMC-HVDC system is equivalent to the equivalent capacitance C. eq Since both store the same amount of energy, the following equation applies: In dc =nV sm In the formula, C sm V represents the capacitance value of the submodule. sm V is the submodule capacitor voltage; N is the number of MMC stations; n is the number of submodules in a single bridge arm; dc It is a DC voltage; Therefore, the equivalent capacitance C of MMC-HVDC eq Expressed as: S22: Analogous to the inertial response of a synchronous generator, a coupling relationship between DC voltage and AC frequency is established, allowing the electrostatic power of the capacitor to simulate the virtual inertial characteristics similar to a synchronous machine, thus obtaining the DC voltage reference value V of the MMC-HVDC. dc,ref As shown in the following formula: In the formula, H mmc It is the virtual equivalent inertial time constant of MMC; S mmc The rated power of the MMC station; V dc0 f0 represents the rated DC voltage amplitude of the MMC-HVDC system; f0 represents the rated frequency of the AC system. DC voltage V dc,ref It is coupled with AC frequency f and is positively correlated. By changing DC voltage, the MMC submodule capacitor releases energy to achieve a certain inertia support. At the same time, by mapping the frequency change of the receiving end grid to the DC voltage change, this frequency change information is restored at the sending end MMC station, thereby realizing the cross-regional inertia support of the sending end wind farm to the receiving end grid. S23: For power loss compensation in DC cable lines under long-distance power transmission, the DC voltage relationship between the sending / receiving MMC stations considering the equivalent impedance of the MMC-HVDC DC line is as follows: In the formula, ΔV dc It is the DC line voltage drop of the MMC-HVDC; V dc1 and V dc2 These are the DC voltages at the ports of the sending-end MMC station and the receiving-end MMC station, respectively; R dc and L dc These are the equivalent resistance and equivalent reactance of the MMC-HVDC DC line, respectively; I dc The DC current of MMC-HVDC; S24: Through frequency restoration control and power loss compensation, the receiving-end MMC station accurately restores the frequency information of the sending-end power grid without communication. At this time, the frequency reference value f of the receiving-end power grid is... ref for: Based on the reference frequency information restored by the sending-end MMC station, the wind farm senses the frequency change of the receiving-end grid and initiates a wind-storage coordinated inertia control strategy to provide inertial support, thereby achieving a system inertia level improvement for coordinated cross-regional source-grid-storage heterogeneous resources.

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