Precise power matching fault ride-through control method based on submodule energy control

Abstract

This invention provides a precise power matching fault ride-through control method based on submodule energy control, belonging to the field of wind power grid connection technology. The technical solution includes the following steps: S1: Onshore converters first enter a passive energy absorption stage; S2: The offshore converter station enters an active energy absorption state by transmitting a signal via DC voltage, and the power absorption command value is given according to the surplus power information transmitted by the DC voltage; S3: When the energy of the submodules of the offshore converter station reaches its maximum value, the flexible DC system enters precise power matching control, extending the non-disconnection time of the wind turbine after a fault. The beneficial effects of this invention are: By fully exploring the potential for functional energy recovery of the internal energy storage devices of the MMC, this invention provides a reliable solution for buying time for energy-consuming equipment to be put into operation or for wind farm load reduction, and for resolving the surplus power of the MMC-HVDC.

Description

Technical Field

[0001] This invention relates to the field of wind power grid connection technology, and in particular to a precise power matching fault ride-through control method based on submodule energy control. Background Technology

[0002] Wind energy is currently the renewable energy source with the broadest application potential and the most mature application technology, addressing issues such as fossil fuel shortages, optimizing energy structure, and reducing environmental pollution. High-voltage direct current (HVDC) transmission technology based on voltage source converters (VSCs), especially modular multilevel converters (MMCs), is recognized as a suitable technology for deep-sea offshore wind power projects. In traditional fault ride-through methods, the cascaded wind power and flexible DC converter equipment rely on the judgment threshold and response time of the previous stage for operation. This results in a long and difficult-to-control time required for power matching on both sides of the flexible DC system, causing the flexible DC system to operate at high voltage during faults, potentially leading to system shutdown. Furthermore, the installation of energy-consuming devices typically requires designing for full power, resulting in significant investment. Therefore, researching more reliable and economical grid-side fault ride-through methods for offshore wind power flexible DC grid-connected systems is of great importance.

[0003] Currently, methods for grid-side fault ride-through can be categorized into three types based on different control strategies: deploying energy-consuming devices, reducing wind farm output control, and a combination of the above two methods. Energy-consuming devices offer advantages such as rapid dissipation of unbalanced power and simple control strategies, but their economic efficiency is poor due to drawbacks such as large resistance values ​​and installation capacity. Reducing wind farm output control can be divided into two methods: communication-based and offshore converter station-based. Communication suffers from latency and potential failures, resulting in poor reliability. Offshore converter station-based control strategies can be divided into voltage reduction and frequency increase methods. Voltage reduction offers fast response, but excessively low voltage reduction can lead to significant electrical and mechanical stress on the wind turbine, affecting its safe and reliable operation. Frequency increase methods have a slower response and a linear constant droop coefficient, making rapid power matching impossible.

[0004] How to solve the above-mentioned technical problems is the challenge facing this invention. Summary of the Invention

[0005] The purpose of this invention is to provide a precise power matching fault ride-through control method based on submodule energy control. This method is based on the three-degree-of-freedom control of offshore wind power flexible DC grid connection. By fully exploring the potential of the active energy recovery of the internal energy storage devices (such as submodule capacitors) of the modular multilevel converter (MMC), it provides a reliable solution for buying time for the commissioning of energy-consuming equipment or the reduction of wind farm load and solving the surplus power of MMC-HVDC.

[0006] To achieve the above-mentioned objectives, the present invention employs the following technical solution: a precise power matching fault ride-through control method based on submodule energy control, comprising the following steps:

[0007] S1: When the onshore AC grid of the offshore wind power flexible DC system fails, the output power of the offshore wind power flexible DC system decreases while the input power remains unchanged, resulting in a power imbalance and surplus power. The onshore converter station (grid side MMC, GSMMC) first enters the passive energy absorption stage, and the DC voltage rises according to the energy of the onshore converter station sub-module.

[0008] S2: When the energy of the onshore converter station submodule reaches the warning value, the offshore converter station (wind farm MMC, WFMMC) enters the active energy absorption state through the DC voltage transmission signal, and the absorption power command value is given according to the surplus power information transmitted by the DC voltage.

[0009] S3: When the energy of the submodule of the offshore converter station reaches its maximum value, the offshore wind power flexible DC system enters precise power matching control, extending the time that the wind turbine can operate without disconnecting from the grid after a fault.

[0010] Furthermore, in step S1, when a fault occurs in the AC power grid, the onshore converter station is the first to activate passive energy absorption. The rated capacitor energy of the capacitor energy storage element in the offshore converter station or the onshore converter station module is expressed as:

[0011] (1)

[0012] In the formula Rated capacitor energy for offshore or onshore converter stations; This refers to the capacitance value of the submodule. This refers to the rated operating voltage of the submodule capacitor. The number of individual bridge arm sub-modules for offshore or onshore converter stations;

[0013] The energy margin of an energy storage element is defined as the difference between the maximum energy storage capacity and the rated energy storage capacity. Therefore, the energy margin of the energy storage element in a submodule of an offshore or onshore converter station is expressed as follows:

[0014] (2)

[0015] In the formula Set the maximum value of the capacitor voltage of the submodule at the offshore or onshore converter station. Maximum capacitor energy storage =2.25 pu;

[0016] The expression for the DC voltage reference value during this period is:

[0017] (3)

[0018] In the formula This is the per-unit value of the DC voltage reference value; Set the maximum operating voltage of the DC system. ; This refers to the capacitor energy of the onshore converter station submodule.

[0019] Furthermore, in step S2, when the energy of the onshore converter station submodule reaches the warning value, the offshore converter station enters an active energy absorption state. For the energy early warning value of the onshore converter station, set the current ,Right now DC voltage reaches warning value The power absorption command value is given based on the surplus power information transmitted by the DC voltage:

[0020] (4)

[0021] In the formula and The two preset thresholds are 1.08 pu and 1.02 pu; and For DC voltage to reach and At that moment.

[0022] Furthermore, in step S3, when the energy of the offshore converter station submodule reaches its maximum value of 2.25 pu, the flexible DC system enters the precise power matching control stage. At this time, a small portion of the surplus energy is continued to be absorbed by the onshore converter station, leading to... ,Will As a control switching action signal, it disconnects the voltage outer loop between the grid-side converter station (grid-side VSC, GSVSC) and the onshore converter station (grid-side MMC, GSMMC). d , q The inner loop reference value of the shaft current is given based on the voltage drop at the grid connection point and takes into account limiting, and should meet the following requirements:

[0023] (5)

[0024] In the formula: , They are respectively d , q Inner loop reference value for shaft current; Voltage at grid connection point d Axial components; The rated current of the converter equipment; The maximum allowable current for the converter equipment;

[0025] Based on instantaneous power theory and combined with the formula, the analytical expressions for the output power of the onshore converter station and the point of common coupling (PCC) fault voltage during this control strategy are as follows:

[0026] (6)

[0027] In the formula: and PCC points d, q Shaft voltage; L It is the sum of the bridge arm equivalent inductance and the AC equivalent inductance. R It is the sum of the equivalent resistance of the bridge arm and the equivalent resistance of the AC system;

[0028] Equation (6) shows that when the voltage is between 0.2 pu and 0.8 pu, the active power output of the onshore converter station is directly proportional to the voltage. Since grid-side converter stations (GSVSCs) have similar output characteristics, for ease of subsequent derivation, the active power outputs of the two converter devices are linearly fitted as follows:

[0029] (7)

[0030] In the formula: This refers to the fault voltage at the onshore converter station. This refers to the fault voltage at the grid-side converter station. The slope in the fitting equation for the onshore converter station; The slope in the fitting equation for the grid-side converter station; The intercept in the fitting equation for the onshore converter station is denoted as . The intercept in the fitting equation for the grid-side converter station;

[0031] The expression for the DC voltage reference value during this period is as follows:

[0032] (8)

[0033] In the formula: This is the voltage value at which the system enters precise power matching control, i.e. ,set up ;

[0034] Substituting equation (8) into equation (7) yields

[0035] (9)

[0036] In the formula This is the sum of the active power losses of the DC system and the AC system; Let be the system DC voltage; , ;

[0037] When the voltage at the wind turbine grid connection point drops, the grid-side converter station (GSVSC) generates reactive power to support the voltage. According to equation (6), the reactive power of the grid-side converter station is:

[0038] (10)

[0039] In the formula These are the rated current and maximum allowable output current of the wind turbine when connected to the grid, respectively. For wind farm line reactance; The equivalent inductance on the AC side of the grid-side converter station;

[0040] Based on equations (9) and (10), the expression for the precise power matching control command value of the offshore converter station (wind farm MMC, WFMMC) is as follows:

[0041] (11).

[0042] Furthermore, in S3, after the fault ends, the onshore converter station first releases the stored energy, and the DC voltage command value is given according to equation (3). When the offshore converter station detects that the DC voltage has dropped to the rated value, it releases the stored energy. The energy release power of the two is:

[0043] (12)

[0044] In the formula The rated power output of the wind farm; It provides the maximum AC output power for the onshore converter station.

[0045] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0046] 1. This invention addresses the technical problem of power mismatch at both ends of the converter station after a fault by controlling the sub-modules of the offshore converter station and the onshore converter station to actively absorb energy, thereby absorbing surplus power to ensure stable system operation.

[0047] 2. This invention transmits fault information via DC voltage and performs precise power matching of the wind turbine based on surplus power to reduce surplus power and alleviate system burden, thus having high feasibility and application value.

[0048] 3. By using a three-degree-of-freedom control strategy, this invention can actively absorb the surplus power caused by faults using the submodule capacitors, thereby stabilizing the DC voltage of the system and ensuring that the wind turbines do not disconnect from the grid for a period of time. At the same time, by using flexible DC voltage control, surplus power information is transmitted to the offshore converter station through DC voltage. When the energy of the submodules at the offshore and onshore converter stations reaches the limit value, precise power matching control is initiated to improve the problem of power imbalance at both ends.

[0049] 4. Based on the three-degree-of-freedom control of the offshore wind power flexible DC system, this invention fully explores the potential for energy recovery of internal energy storage devices (such as submodule capacitors) in the MMC, providing a reliable solution for buying time for energy-consuming equipment to be put into operation or for wind farm load reduction and solving the surplus power of MMC-HVDC. Attached Figure Description

[0050] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0051] Figure 1 This is an overall flowchart of the precise power matching fault ride-through control method based on submodule active energy control of the present invention.

[0052] Figure 2 This is a block diagram of the MMC AC / DC current dual control in this invention.

[0053] Figure 3 This is a control block diagram of the MMC type offshore and onshore converter station in this invention.

[0054] Figure 4 This is a waveform diagram of the power output of the wind turbine after load reduction following a fault, as shown in this invention.

[0055] Figure 5 This is a waveform diagram of the d-axis voltage at the grid connection point in this invention. Detailed Implementation

[0056] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. Of course, the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0057] Example 1:

[0058] See Figure 1-3 , Figure 1 middle This refers to the energy command value for the GSMMC capacitor at the onshore converter station. This refers to the WFMMC capacitor energy command value for the offshore converter station. Energy for WFMMC capacitors at offshore converter stations; Figure 2 middle , They are respectively d , q shaft current, The equivalent inductance on the AC side of the GSMMC for onshore converter stations or the WFMMC for offshore converter stations. For the bridge arm inductors of GSMMC for onshore converter stations or WFMMC for offshore converter stations, This is the DC current command value. It is direct current. This is the output modulation ratio of the circulating current suppressor; Figure 3 middle , These are the WFMMC offshore converter stations. d , q Shaft voltage command value, , WFMMC, a marine converter station d , q shaft voltage, This refers to the reactive power command value of the GSMMC at the onshore converter station. This embodiment discloses a precise power matching fault ride-through control method based on submodule energy control, including the following steps:

[0059] S1: When the onshore AC grid of the offshore wind power flexible DC system fails, the output power of the offshore wind power flexible DC system decreases while the input power remains unchanged, resulting in a power imbalance and surplus power. The onshore converter station first enters the passive energy absorption stage, and the DC voltage rises according to the energy of the onshore converter station sub-module.

[0060] S2: When the energy of the onshore converter station submodule reaches the warning value, the offshore converter station enters the active energy absorption state by transmitting a signal through DC voltage, and the absorption power command value is given according to the surplus power information transmitted by DC voltage.

[0061] S3: When the energy of the submodule of the offshore converter station reaches its maximum value, the flexible DC system enters precise power matching control to extend the time that the wind turbine can operate without disconnecting from the grid after a fault.

[0062] Specifically, in step S1, when a fault occurs in the AC power grid, the onshore converter station first activates passive energy absorption, and the rated capacitor energy of the capacitor energy storage element in the offshore converter station or onshore converter station module is expressed as:

[0063] (1)

[0064] In the formula Rated capacitor energy for offshore or onshore converter stations; This refers to the capacitance value of the submodule. This refers to the rated operating voltage of the submodule capacitor. The number of individual bridge arm sub-modules for offshore or onshore converter stations;

[0065] The energy margin of an energy storage element is defined as the difference between the maximum energy storage capacity and the rated energy storage capacity. Therefore, the energy margin of the energy storage element in a submodule of an offshore or onshore converter station is expressed as follows:

[0066] (2)

[0067] In the formula Set the maximum value of the capacitor voltage of the submodule at the offshore or onshore converter station. Maximum capacitor energy storage =2.25 pu;

[0068] The expression for the DC voltage reference value during this period is:

[0069] (3)

[0070] In the formula This is the per-unit value of the DC voltage reference value; Set the maximum operating voltage of the DC system. ; This refers to the capacitor energy of the onshore converter station submodule.

[0071] Specifically, in step S2, when the energy of the onshore converter station submodule reaches the warning value, the offshore converter station enters an active energy absorption state. For the energy early warning value of the onshore converter station, set the current ,Right now DC voltage reaches warning value The power absorption command value is given based on the surplus power information transmitted by the DC voltage:

[0072] (4)

[0073] In the formula and The two preset thresholds are 1.08 pu and 1.02 pu; and For DC voltage to reach and At that moment.

[0074] Specifically, in step S3, when the energy of the offshore converter station submodule reaches its maximum value of 2.25 pu, the flexible DC system enters the precise power matching control stage. At this time, a small portion of surplus energy is still absorbed by the onshore converter station, leading to... ,Will As a control switching action signal, it disconnects the voltage outer loop between the grid-side converter station (grid-side VSC, GSVSC) and the onshore converter station (grid-side MMC, GSMMC). d , q The inner loop reference value of the shaft current is given based on the voltage drop at the grid connection point and takes into account limiting, and should meet the following requirements:

[0075] (5)

[0076] In the formula: , They are respectively d , q Inner loop reference value for shaft current; Voltage at grid connection point d Axial components; The rated current of the converter equipment; The maximum allowable current for the converter equipment;

[0077] Based on instantaneous power theory and combined with the formula, the analytical expression for the output power and point of common coupling (PCC) fault voltage of the grid side converter station (GSMMC) during this control strategy is as follows: (6)

[0078] In the formula: and PCC points d, q Shaft voltage; L It is the sum of the bridge arm equivalent inductance and the AC equivalent inductance. R It is the sum of the equivalent resistance of the bridge arm and the equivalent resistance of the AC system;

[0079] Equation (6) shows that when the voltage is between 0.2 pu and 0.8 pu, the active power output of the onshore converter station (grid-side MMC, GSMMC) is directly proportional to the voltage. Since the grid-side converter station (grid-side VSC, GSVSC) has similar output characteristics, for ease of subsequent derivation, the active power output of the two converter devices is linearly fitted as follows:

[0080] (7)

[0081] In the formula: This refers to the fault voltage of the GSMMC at the onshore converter station. This refers to the fault voltage of the grid-side converter station GSVSC. The slope in the GSMMC fitting equation for the onshore converter station; The slope in the fitting equation for the grid-side converter station GSVSC is denoted as ; The intercept in the GSMMC fitting equation for the onshore converter station is... The intercept in the GSVSC fitting formula for the grid-side converter station is denoted as ;

[0082] The expression for the DC voltage reference value during this period is as follows:

[0083] (8)

[0084] In the formula: This is the voltage value at which the system enters precise power matching control, i.e. ,set up ;

[0085] Substituting equation (8) into equation (7) yields

[0086] (9)

[0087] In the formula This is the sum of the active power losses of the DC system and the AC system; Let be the system DC voltage; , ;

[0088] When the voltage at the wind turbine grid connection point drops, the grid-side converter station (grid-side VSC, GSVSC) generates reactive power to support the voltage. According to equation (6), the reactive power of the grid-side converter station (grid-side VSC, GSVSC) is:

[0089] (10)

[0090] In the formula These are the rated current and maximum allowable output current of the wind turbine when connected to the grid, respectively. For wind farm line reactance; The equivalent inductance on the AC side of the grid-side converter station (grid-side VSC, GSVSC);

[0091] Based on equations (9) and (10), the expression for the precise power matching control command value of the offshore converter station (wind farm MMC, WFMMC) is as follows:

[0092] (11).

[0093] Specifically, in S3, after the fault ends, the onshore converter station first releases the stored energy, and the DC voltage command value is given according to equation (3). When the offshore converter station detects that the DC voltage has dropped to the rated value, it releases the stored energy. The energy release power of the two is:

[0094] (12)

[0095] In the formula The rated power output of the wind farm; It provides the maximum AC output power for the onshore converter station.

[0096] Example 2:

[0097] In the same wind farm grid-connected model, the fault ride-through performance of the proposed precise power matching fault ride-through control method based on submodule energy control and the traditional method were compared. The test conditions included voltage drops to 0.2 pu, 0.5 pu, and 0.8 pu during faults.

[0098] Table 1 compares traditional fault ride-through control methods.

[0099]

[0100] The experimental results are compared in Table 1, and the following conclusions can be drawn:

[0101] 1. Fault information transmission: In traditional methods, the DC voltage passively rises during a fault due to surplus power, but this cannot accurately transmit fault information. However, the method of this invention can transmit fault information by controlling the DC voltage through a DC voltage loop.

[0102] 2. Impact of the fault: Due to the fault, the voltage will overshoot before the system stabilizes. The overshoot of traditional control methods is greater than that of the method of this invention, and the difference between the two can be as high as 3%.

[0103] Therefore, this embodiment verifies that the method of the present invention is superior to the traditional fault-crossing method in terms of fault information transmission and reducing the impact of faults, and the advantages are more obvious as the fault becomes more severe.

[0104] Example 3:

[0105] In the same wind farm grid-connected model, the surplus power solutions proposed in this invention are compared with those of traditional methods. Test conditions include voltage drops to 0.2 pu, 0.5 pu, and 0.8 pu during faults.

[0106] Table 2 compares the surplus power solutions of traditional methods.

[0107]

[0108] The experimental results are shown in Table 2, and the following experimental conclusions can be drawn:

[0109] 1. Surplus Power Solution: Traditional methods require the installation of energy-consuming devices before the wind turbine operates to ensure system stability. The method of this invention utilizes the energy margin of the energy storage device in the sub-module to absorb surplus power and provide operating time for wind turbine load reduction. It can be seen that the method of this invention not only saves the investment cost of energy-consuming devices, but also makes full use of surplus power, and its economic efficiency is better than that of traditional methods.

[0110] 2. Fan start time: In the traditional method, regardless of the severity of the fault, the fan will start 10ms after the energy-consuming device is put into operation to stabilize the voltage. The method of this invention uses sub-modules to absorb surplus power. The power level varies depending on the severity of the fault. The maximum power can make the fan run normally for 240ms. If the fault is cleared during this period, the fan does not need to be unloaded. It can be seen that the system stability is significantly better than the traditional method.

[0111] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A precise power matching fault ride-through control method based on submodule energy control, characterized in that, Includes the following steps: S1: When the onshore AC grid of the offshore wind power flexible DC system fails, the output power of the offshore wind power flexible DC system decreases while the input power remains unchanged, resulting in power imbalance and surplus power. The onshore converter station GSMMC first enters the passive energy absorption stage, and the DC voltage increases with the increase of energy storage in the sub-modules of the onshore converter station GSMMC. In step S1, when a fault occurs in the AC power grid, the onshore converter station GSMMC is the first to activate passive energy absorption. The rated capacitor energy of the capacitor energy storage element in the offshore converter station WFMMC or the onshore converter station GSMMC module is expressed as: （1）； In the formula The rated capacitor energy for offshore converter station WFMMC or onshore converter station GSMMC; This refers to the capacitance value of the submodule. This refers to the rated operating voltage of the submodule capacitor. The number of individual bridge arm submodules for offshore converter station WFMMC or onshore converter station GSMMC; Define the energy margin of an energy storage element as the difference between the maximum capacitor energy storage and the rated capacitor energy storage. Then, the energy margin of the energy storage element in the submodule of the WFMMC in an offshore converter station or the GSMMC in an onshore converter station is expressed as: （2）； In the formula This refers to the energy margin of the WFMMC capacitor in offshore converter stations or the GSMMC capacitor in onshore converter stations. Set the maximum capacitor voltage of the WFMMC submodule in the offshore converter station or the GSMMC submodule in the onshore converter station. Maximum capacitor energy storage =2.25 pu; The expression for the DC voltage reference value during this period is: （3）； In the formula This is the per-unit value of the DC voltage reference value; Set the maximum operating voltage of the DC system. ; Energy for the GSMMC submodule capacitors of the onshore converter station; S2: When the energy of the GSMMC submodule of the onshore converter station reaches the warning value, the WFMMC of the offshore converter station enters the active energy absorption state through the DC voltage transmission signal, and the absorption power command value is given according to the surplus power information transmitted by the DC voltage. S3: When the energy of the WFMMC submodule of the offshore converter station reaches its maximum value, the offshore wind power flexible DC system enters precise power matching control, extending the time that the wind turbine can operate without disconnecting from the grid after a fault.

2. The precise power matching fault ride-through control method based on submodule energy control according to claim 1, characterized in that, In step S2, when the energy of the GSMMC submodule of the onshore converter station reaches the warning value, the WFMMC of the offshore converter station enters the active energy absorption state. Set the GSMMC energy early warning value for the onshore converter station. ,Right now DC voltage reaches warning value The power absorption command value is given based on the surplus power information transmitted by the DC voltage: （4）； In the formula The power absorbed by the WFMMC offshore converter station. and The two preset thresholds are 1.08 pu and 1.02 pu; and For DC voltages, respectively, when they reach and At that moment.

3. The precise power matching fault ride-through control method based on submodule energy control according to claim 2, characterized in that, In step S3, when the energy of the WFMMC submodule at the offshore converter station reaches its maximum value of 2.25 pu, the offshore wind power flexible DC system enters the precise power matching control stage. At this time, surplus energy is continued to be absorbed by the GSMMC at the onshore converter station, leading to... ,Will As a control switching action signal, it disconnects the voltage outer loop between the grid-side converter station GSVSC and the onshore converter station GSMMC. d , q The inner loop reference value of the shaft current is given based on the voltage drop at the grid connection point and takes into account limiting, and should meet the following requirements: （5）； In the formula: , They are respectively d , q Inner loop reference value for shaft current; Voltage at grid connection point d Axial components; The rated current of the converter equipment; The maximum allowable current for the converter equipment; Based on instantaneous power theory and combined with equation (5), the analytical expressions for the output power of the onshore converter station GSMMC and the fault voltage of the point of common coupling PCC during this control strategy are as follows: （6）； In the formula: and These represent the active and reactive power outputs of the GSMMC at the onshore converter station, respectively. and These are the common connection points PCC. d, q Shaft voltage; L It is the sum of the bridge arm equivalent inductance and the AC equivalent inductance. R It is the sum of the equivalent resistance of the bridge arm and the equivalent resistance of the AC system. The angular frequency of the AC system; The output active power of the two converter devices is linearly fitted as follows: （7）； In the formula: This refers to the active power output of the grid-side converter station GSVSC. This refers to the fault voltage of the GSMMC at the onshore converter station. This refers to the fault voltage of the grid-side converter station GSVSC. The slope in the GSMMC fitting equation for the onshore converter station; The slope in the fitting equation for the grid-side converter station GSVSC is denoted as ; The intercept in the GSMMC fitting equation for the onshore converter station is... The intercept in the GSVSC fitting formula for the grid-side converter station is denoted as ; The expression for the DC voltage reference value during this period is as follows: （8）； In the formula: This is the voltage value at which the system enters precise power matching control, i.e. ,set up ; Substituting equation (8) into equation (7) yields （9）； In the formula This is the sum of the active power losses of the DC system and the AC system; Let be the system DC voltage; , ; When the voltage at the wind turbine grid connection point drops, the grid-side converter station GSVSC generates reactive power to support the voltage. According to equation (6), the reactive power of the grid-side converter station GSVSC is: （10）； In the formula This refers to the reactive power output of the grid-side converter station GSVSC. These are the rated current measured when the wind turbine is connected to the grid and the maximum allowable output current measured when the wind turbine is connected to the grid, respectively. The equivalent inductance of the AC side of the grid-side converter station GSVSC; Based on equations (9) and (10), the expression for the WFMMC precision power matching control command value of the offshore converter station is: （11）； In the formula To ensure precise power matching control command values ​​for the WFMMC at the offshore converter station. For wind farm line reactance.

4. The precise power matching fault ride-through control method based on submodule energy control according to claim 3, characterized in that, In S3, after the fault ends, the onshore converter station GSMMC first releases the stored energy, and the DC voltage command value is given according to equation (3). When the offshore converter station WFMMC detects that the DC voltage has dropped to the rated value, it releases the stored energy. The energy release power of the two is: （12）； In the formula The power output of the onshore converter station GSMMC and the offshore converter station WFMMC. The rated power output of the wind farm; This provides the maximum AC output power for the GSMMC converter station on shore.

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