A frequency control method and system for offshore wind power low-frequency power transmission

Abstract

The application provides a frequency control method and system for offshore wind power low-frequency power transmission, the method comprising: obtaining power data of an offshore wind farm; calculating three-phase output voltage reference values of a voltage regulating device according to the power data; inputting the three-phase output voltage reference values into the voltage regulating device, so that the voltage regulating device maintains voltage stability of the AC collection bus and the DC bus according to the three-phase output voltage reference values; transmitting voltage information of the DC bus to an onshore converter station, so that the onshore converter station generates a first output frequency according to the voltage information of the DC bus and onshore power data; and adjusting an actual output frequency of the onshore converter station according to the first output frequency, so as to control the offshore power grid system to always work at an optimal frequency value and improve the power transmission efficiency of offshore wind power.

Description

Technical Field

[0001] This invention relates to the field of large-scale new energy transmission technology, and in particular to a frequency control method and system for low-frequency power transmission from offshore wind power. Background Technology

[0002] Achieving large-scale, long-distance power transmission is a key challenge in current offshore wind power development. Due to the significant capacitive effect of submarine cables, conventional high-voltage AC transmission (HVAC) suffers from large capacitive reactive currents and a significant decrease in transmission capacity with increasing distance, making it unsuitable for offshore wind power transmission scenarios exceeding 70km. High-voltage DC transmission (HVDC) is theoretically unaffected by the capacitive effect of submarine cables, enabling efficient long-distance transmission of large-scale offshore wind power. However, it requires the construction of large and costly offshore converter stations, hindering grid parity for offshore wind power. Therefore, in offshore wind power transmission scenarios ranging from 70km to 300km, flexible low-frequency AC transmission technology has attracted widespread attention from academia and industry. In LFAC-based power systems, the 50Hz onshore grid is converted to low-frequency AC voltage at sea via an onshore converter station. Although LFAC systems have theoretical advantages in terms of economy, existing technologies still face the following challenges in practical applications:

[0003] 1) Due to the long transmission distance, the capacitive effect of submarine cables can generate overvoltages on the AC collection busbars of offshore wind farms. Currently, reactive power compensation is generally achieved through devices such as high-voltage reactance converters or static var compensators (STATCOMs). However, high-voltage reactance converters are inefficient and heavy. STATCOMs generally need to withstand high-voltage AC, resulting in lower cost and power density. This weakens the economic advantages of LFACs.

[0004] 2) The frequency of the offshore power grid system has a significant impact on system characteristics. Lower frequencies result in less capacitive effects from submarine cables, but also weaker transformer capacity. Meanwhile, onshore converter stations typically employ a modular multilevel structure, and their output frequency also significantly affects their transmission efficiency. Therefore, selecting the optimal offshore power grid frequency is a key issue that needs to be addressed in practical engineering. Current technologies generally pre-set the offshore wind power frequency based on system parameters; however, precise parameters for transformers, submarine cables, and onshore converter stations are difficult to obtain, making it difficult for actual offshore wind power frequencies to operate at their optimal values. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a frequency control method and system for low-frequency power transmission from offshore wind power, reducing the overvoltage risk of offshore wind farms, controlling the offshore power grid system to always operate at the optimal frequency value, and improving the power transmission efficiency of offshore wind power.

[0006] In a first aspect, the present invention provides a frequency control method for low-frequency power transmission from offshore wind power, comprising:

[0007] Acquire power data of an offshore wind farm, wherein the power data includes the three-phase grid voltage and three-phase grid current of the AC collection bus of the offshore wind farm and the voltage of the DC bus of the voltage regulation device;

[0008] The reference value of the three-phase output voltage of the voltage regulating device is calculated based on the power data.

[0009] The three-phase output voltage reference value is input to the voltage regulating device so that the voltage regulating device maintains the voltage stability of the AC bus and the DC bus according to the three-phase output voltage reference value.

[0010] The voltage information of the DC bus is transmitted to the onshore converter station, so that the onshore converter station generates a first output frequency based on the voltage information of the DC bus and the onshore power data;

[0011] The actual output frequency of the onshore converter station is adjusted according to the first output frequency.

[0012] This invention provides a frequency control method for low-frequency power transmission from offshore wind power. By acquiring power data from the offshore wind farm, a reference value for the three-phase output voltage of the corresponding voltage regulator is calculated. The voltage regulator then maintains the voltage stability of the AC collecting bus and the DC bus based on this reference value. Maintaining the stability of the AC collecting bus ensures that the voltage amplitude of the offshore wind farm is kept near a given value, thereby guaranteeing the safe and reliable power transmission from the offshore wind farm. Since the voltage regulator itself cannot generate or absorb active power for extended periods, maintaining the voltage stability of the DC bus is also necessary. This invention improves the stability of power transmission from offshore wind power by calculating the reference value for the three-phase output voltage and simultaneously using the voltage regulator to maintain the stability of the voltage amplitude of the offshore wind farm. Furthermore, the voltage information of the DC bus is transmitted to the onshore converter station, and a first output frequency is generated based on this information. This allows the onshore converter station to adjust its output frequency in real time according to the voltage information of the offshore wind farm, ensuring that the entire offshore power grid system always operates at the optimal frequency value, thus improving the power transmission efficiency of offshore wind power.

[0013] In one possible implementation, calculating the three-phase output voltage reference value of the voltage regulator based on the power data includes:

[0014] The instantaneous voltage amplitude of the AC busbar is calculated based on the three-phase grid voltage of the AC busbar.

[0015] Based on the instantaneous voltage amplitude, a reference value for the reactive component of the output voltage of the voltage regulating device is generated through closed-loop control;

[0016] Based on the voltage of the DC bus, a reference value for the active component of the output voltage of the voltage regulating device is generated through closed-loop control.

[0017] Based on the three-phase grid current, the corresponding current phase is calculated using a three-phase phase-locked loop algorithm;

[0018] Based on the reference values ​​of the reactive component and active component of the output voltage, and the current phase, the reference values ​​of the three-phase output voltage of the voltage regulating device are calculated using a coordinate transformation from a rotating coordinate system to a three-phase stationary coordinate system.

[0019] This invention provides a method for calculating a three-phase output voltage reference value. First, the instantaneous voltage amplitude of the AC collecting bus is calculated using the three-phase grid voltage. The voltage regulation device needs to compensate the AC collecting bus voltage according to the magnitude of this instantaneous voltage amplitude; therefore, the calculated instantaneous voltage amplitude provides the necessary data support for the subsequent voltage compensation by the voltage regulation device. Then, the active component reference value of the output voltage of the voltage regulation device is calculated based on the instantaneous voltage amplitude, used to compensate the AC collecting bus voltage and maintain the stability of the amplitude of the offshore wind farm. The active component reference value of the output voltage of the voltage regulation device is also calculated based on the DC bus voltage, used to compensate the voltage of its own DC bus, ensuring that the voltage regulation device always operates at the optimal modulation ratio and reducing its own losses. Finally, the comprehensive three-phase output voltage reference value is calculated by combining the reactive component reference value, the active component reference value, and the current phase. The voltage regulation device can then output voltage based on this three-phase output voltage reference value, simultaneously maintaining the voltage stability of the offshore wind farm and itself, thereby improving the stability of power transmission from offshore wind power.

[0020] Furthermore, the instantaneous voltage amplitude of the AC collecting bus is calculated based on the three-phase grid voltage and three-phase grid current of the AC collecting bus, using the following formula:

[0021]

[0022] Among them, u wA u wB u wC Let u be the voltage of the three-phase power grid. wm The instantaneous amplitude of the voltage at the AC busbar;

[0023] The reference value for the reactive component of the output voltage of the voltage regulation device is generated through closed-loop control based on the instantaneous voltage amplitude. The specific formula is as follows:

[0024]

[0025] Among them, v wq * Let s be the reference value for the reactive component of the output voltage of the voltage regulating device, s be the Laplace operator, and k be the reference value. p1 and k i1 These are the proportional and integral control coefficients of the voltage controller for the AC bus, respectively. wm * This is a reference value for the voltage amplitude of the AC busbar;

[0026] The reference value for the active component of the output voltage of the voltage regulation device is generated through closed-loop control based on the voltage of the DC bus, and the specific formula is as follows:

[0027]

[0028] Among them, v wd * k is the reference value for the active component of the output voltage of the voltage regulating device. p2 and k i2 These are the proportional and integral control coefficients of the voltage controller for the DC bus, respectively; U dcw * u is the reference value for the voltage of the DC bus. dcw The voltage of the DC bus;

[0029] Based on the reference values ​​of the reactive component and active component of the output voltage, and the current phase, the three-phase output voltage reference value of the voltage regulating device is calculated using a coordinate transformation from a rotating coordinate system to a three-phase stationary coordinate system. The specific formula is as follows:

[0030]

[0031] Among them, v wA * v wA * v wA * θ is the reference value for the three-phase output voltage of the voltage regulating device. wI The current phase corresponding to the three-phase power grid current.

[0032] In one possible implementation, the onshore converter station generates a first output frequency based on the voltage information of the DC bus and onshore power data, including:

[0033] Acquire onshore power data, which includes three-phase onshore grid voltage and three-phase onshore grid current;

[0034] The active power flowing into the onshore power grid is calculated based on the onshore power data.

[0035] Based on the active power flowing into the onshore power grid and the voltage information of the DC bus, a first output frequency is generated using a preset optimized target transfer function and frequency optimization adjustment algorithm.

[0036] This invention provides a method for generating a first output frequency based on voltage information. First, the active power flowing into the onshore power grid is calculated based on onshore power data. Then, the optimal output frequency is calculated based on the active power and the DC bus voltage information. This invention achieves joint optimization of voltage and frequency. While optimizing the voltage of the offshore wind farm using a voltage regulation device, it also transmits the voltage information to the onshore converter station, enabling the onshore converter station to perform real-time frequency adjustment. This determines the output frequency most suitable for the offshore wind farm voltage while meeting the active power requirements, ensuring the entire offshore power grid system always operates at the optimal frequency value and improving the power transmission efficiency of offshore wind power.

[0037] Furthermore, the active power flowing into the onshore power grid is calculated based on the onshore power data using the following formula:

[0038] p g =u gA i gA +u gB i gB +u gC i gC

[0039] Among them, u gA u gB u gC Let i be the voltage of the three-phase onshore power grid. gA i gB i gC The three-phase onshore power grid current;

[0040] The specific formula for the optimized objective transfer function is as follows:

[0041] h f =k pg p g +k dc (u dcw -U dcw,normal ) 2

[0042] Where, k pg and k dc These are the weighting factors for active power and DC bus voltage, respectively, U dcw,normal This refers to the rated DC bus voltage of the voltage regulating device.

[0043] In one possible implementation, the frequency optimization adjustment algorithm includes:

[0044] Initialize the output frequency, basic step size, step size factor, correction direction, and exit threshold;

[0045] The output frequency is corrected based on the basic step size, step size factor, and correction direction, and the difference and absolute value of the optimized objective transfer function before and after the output frequency correction are calculated.

[0046] If the absolute value is less than the exit threshold, the output frequency is determined as the average of the output frequencies before and after correction, and the algorithm exits.

[0047] If the absolute value is greater than the exit threshold, the correction direction is updated according to the sign of the difference. Specifically, if the sign of the difference is negative, the correction direction remains unchanged; if the sign of the difference is positive, the correction direction is reversed.

[0048] The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and the preset step size factor update formula, and the output frequency is further corrected until the algorithm exits.

[0049] This invention provides a frequency optimization and regulation algorithm. Based on the active power flowing into the onshore power grid, the initial output frequency is continuously corrected through a target transfer function until the output frequency meets the conditions for exiting the algorithm. This invention achieves automatic adjustment through multiple corrections of the initial output frequency, obtaining the optimal output frequency that matches the voltage and active power of the offshore wind farm. It eliminates the need for pre-setting the offshore wind farm frequency, thus improving the flexibility and efficiency of power transmission from offshore wind power.

[0050] Furthermore, the output frequency is corrected based on the basic step size, step size factor, and correction direction, using the following formula:

[0051] f ok+1 =f ok +k·d·Δf0

[0052] Among them, f ko+1 with f ok These are the output frequencies before and after correction, respectively; k is the step size factor; d is the correction direction; and Δf0 is the basic step size.

[0053] The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and a preset step size factor update formula. The specific formula is as follows:

[0054]

[0055] Among them, T ch is the calculation cycle for the frequency optimization adjustment algorithm. fk+1 with h fk These are the optimized target transfer functions before and after output frequency correction, respectively.

[0056] Secondly, correspondingly, the present invention provides a frequency control system for low-frequency power transmission of offshore wind power, including an acquisition module, a calculation module, a voltage regulation module, a frequency generation module and a control module;

[0057] The acquisition module is used to acquire power data of the offshore wind farm, wherein the power data includes the three-phase grid voltage, three-phase grid current and DC bus voltage of the AC collection bus of the offshore wind farm.

[0058] The calculation module is used to calculate the reference value of the three-phase output voltage of the voltage regulating device based on the power data;

[0059] The voltage regulation module is used to input the three-phase output voltage reference value to the voltage regulation device, so that the voltage regulation device maintains the voltage stability of the AC bus and the DC bus according to the three-phase output voltage reference value.

[0060] The frequency generation module is used to transmit the voltage information of the DC bus to the onshore converter station, so that the onshore converter station generates a first output frequency based on the voltage information of the DC bus and the onshore power data;

[0061] The control module is used to adjust the actual output frequency of the onshore converter station according to the first output frequency.

[0062] In one possible implementation, the calculation module includes a voltage instantaneous amplitude calculation unit, a reactive component reference value calculation unit, an active component reference value calculation unit, a current phase calculation unit, and an output voltage reference value calculation unit.

[0063] The instantaneous voltage amplitude calculation unit is used to calculate the instantaneous voltage amplitude of the AC busbar based on the three-phase grid voltage of the AC busbar.

[0064] The reactive component reference value calculation unit is used to generate the output voltage reactive component reference value of the voltage regulation device through closed-loop control based on the instantaneous voltage amplitude.

[0065] The active component reference value calculation unit is used to generate the active component reference value of the output voltage of the voltage regulation device through closed-loop control based on the voltage of the DC bus.

[0066] The current phase calculation unit is used to calculate the corresponding current phase based on the three-phase power grid current using a three-phase phase-locked loop algorithm.

[0067] The output voltage reference value calculation unit is used to calculate the three-phase output voltage reference value of the voltage regulating device based on the output voltage reactive component reference value, the output voltage active component reference value, and the current phase, and based on the coordinate transformation from the rotating coordinate system to the three-phase stationary coordinate system.

[0068] Furthermore, the instantaneous voltage amplitude calculation unit calculates the instantaneous voltage amplitude of the AC busbar based on the three-phase grid voltage of the AC busbar, using the following formula:

[0069]

[0070] Among them, u wA u wB u wC Let u be the voltage of the three-phase power grid. wm The instantaneous amplitude of the voltage at the AC busbar;

[0071] The reactive power component reference value calculation unit generates the output voltage reactive power component reference value of the voltage regulation device through closed-loop control based on the instantaneous voltage amplitude. The specific formula is as follows:

[0072]

[0073] Among them, v wq * Let s be the reference value for the reactive component of the output voltage of the voltage regulating device, s be the Laplace operator, and k be the reference value. p1 and k i1 These are the proportional and integral control coefficients of the voltage controller for the AC bus, respectively. wm * This is a reference value for the voltage amplitude of the AC busbar;

[0074] The active component reference value calculation unit generates the active component reference value of the output voltage of the voltage regulation device through closed-loop control based on the voltage of the DC bus. The specific formula is as follows:

[0075]

[0076] Among them, v wd * k is the reference value for the active component of the output voltage of the voltage regulating device. p2 and k i2 These are the proportional and integral control coefficients of the voltage controller for the DC bus, respectively; U dcw *u is the reference value for the voltage of the DC bus. dcw The voltage of the DC bus;

[0077] The output voltage reference value calculation unit calculates the three-phase output voltage reference value of the voltage regulating device based on the output voltage reactive component reference value, the output voltage active component reference value, and the current phase, using a coordinate transformation from a rotating coordinate system to a three-phase stationary coordinate system. The specific formula is as follows:

[0078]

[0079] Among them, v wA * v wA * v wA * θ is the reference value for the three-phase output voltage of the voltage regulating device. wI The current phase corresponding to the three-phase power grid current.

[0080] In one possible implementation, the onshore converter station generates a first output frequency based on the voltage information of the DC bus and onshore power data, including:

[0081] Acquire onshore power data, which includes three-phase onshore grid voltage and three-phase onshore grid current;

[0082] The active power flowing into the onshore power grid is calculated based on the onshore power data.

[0083] Based on the active power flowing into the onshore power grid and the voltage information of the DC bus, a first output frequency is generated using a preset optimized target transfer function and frequency optimization adjustment algorithm.

[0084] Furthermore, the active power flowing into the onshore power grid is calculated based on the onshore power data using the following formula:

[0085] p g =u gA i gA +u gB i gB +u gC i gC

[0086] Among them, u gA u gB u gC Let i be the voltage of the three-phase onshore power grid. gA i gB i gC The three-phase onshore power grid current;

[0087] The specific formula for the optimized objective transfer function is as follows:

[0088] h f =k pg p g +k dc (u dcw -U dcw,normal ) 2

[0089] Where, k pg and k dc These are the weighting factors for active power and DC bus voltage, respectively, U dcw,normal This refers to the rated DC bus voltage of the voltage regulating device.

[0090] In one possible implementation, the frequency optimization adjustment algorithm includes:

[0091] Initialize the output frequency, basic step size, step size factor, correction direction, and exit threshold;

[0092] The output frequency is corrected based on the basic step size, step size factor, and correction direction, and the difference and absolute value of the optimized objective transfer function before and after the output frequency correction are calculated.

[0093] If the absolute value is less than the exit threshold, the output frequency is determined as the average of the output frequencies before and after correction, and the algorithm exits.

[0094] If the absolute value is greater than the exit threshold, the correction direction is updated according to the sign of the difference. Specifically, if the sign of the difference is negative, the correction direction remains unchanged; if the sign of the difference is positive, the correction direction is reversed.

[0095] The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and the preset step size factor update formula, and the output frequency is further corrected until the algorithm exits.

[0096] Furthermore, the output frequency is corrected based on the basic step size, step size factor, and correction direction, using the following formula:

[0097] f ok+1 =f ok +k·d·Δf0

[0098] Among them, f ko+1 with f ok These are the output frequencies before and after correction, respectively; k is the step size factor; d is the correction direction; and Δf0 is the basic step size.

[0099] The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and a preset step size factor update formula. The specific formula is as follows:

[0100]

[0101] Among them, T c h is the calculation cycle for the frequency optimization adjustment algorithm. fk+1 with h fk These are the optimized target transfer functions before and after output frequency correction, respectively. Attached Figure Description

[0102] Figure 1 This is a schematic diagram of the power system structure of an offshore wind farm and an onshore converter station in this invention.

[0103] Figure 2 This is a schematic diagram of the circuit structure of an onshore converter station according to the present invention.

[0104] Figure 3 This is a schematic diagram of the circuit structure of a transformer low-voltage side and a three-phase voltage regulation device in this invention.

[0105] Figure 4 This is a circuit topology diagram of a voltage regulation device for a 2-level converter in this invention.

[0106] Figure 5 This is a circuit topology diagram of a voltage regulation device for a 3-level converter in this invention.

[0107] Figure 6 This is a circuit topology diagram of a voltage regulation device for a modular multilevel converter in this invention.

[0108] Figure 7 This is a flowchart illustrating an embodiment of a frequency control method for low-frequency power transmission from offshore wind power provided by the present invention.

[0109] Figure 8 : A closed-loop control block diagram of the voltage regulation device in a frequency control method for low-frequency power transmission of offshore wind power provided by the present invention.

[0110] Figure 9 : Phasor diagram of the voltage regulation principle of the voltage regulation device in the frequency control method for low-frequency power transmission of offshore wind power provided by the present invention.

[0111] Figure 10 This is a schematic diagram of the process for generating the first output frequency in a frequency control method for low-frequency power transmission of offshore wind power provided by the present invention.

[0112] Figure 11This is a flowchart illustrating the frequency optimization adjustment algorithm in a frequency control method for low-frequency power transmission from offshore wind power provided by the present invention.

[0113] Figure 12 This is a schematic diagram of an embodiment of a frequency control system for low-frequency power transmission in offshore wind power provided by the present invention.

[0114] Figure 13 This is a schematic diagram of the calculation module in a frequency control system for low-frequency power transmission of offshore wind power provided by the present invention. Detailed Implementation

[0115] 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0116] It should be noted that the step numbers in this document are only for the convenience of explaining the specific embodiments and are not intended to limit the order in which the steps are performed.

[0117] To further illustrate the power transmission process between offshore wind farms and onshore converter stations described in this invention, Figure 1 A schematic diagram of the power system structure of an offshore wind farm and an onshore converter station is presented. The offshore wind farm is connected to the low-voltage side of an offshore step-up transformer via an AC collecting bus. The high-voltage side of the transformer is connected to the onshore converter station via a submarine power cable. A voltage regulator is connected in series with the low-voltage side of the offshore step-up transformer via a DC bus and maintains the voltage of the offshore wind farm at a constant value through closed-loop control. The voltage regulator outputs a compensation voltage through the DC bus, thereby maintaining the voltage amplitude of the AC collecting bus at a constant value. Simultaneously, the DC bus voltage information is transmitted to the real-time frequency regulation module via a communication submarine cable. The real-time frequency regulation module calculates the active power flowing into the onshore power grid and, combined with the DC bus voltage of the voltage regulator, generates the output frequency signal of the onshore converter station in real time, realizing frequency optimization control of the onshore converter station. Furthermore, the onshore converter station uses a modular multilevel matrix converter (M3C), such as... Figure 2 As shown. The M3C consists of 9 bridge arms, each composed of multiple sub-modules and one bridge arm reactor connected in series. The M3C is used to convert onshore power frequency AC voltage to offshore low-frequency AC voltage. The low-voltage side of the offshore step-up transformer has an open winding structure, while the high-voltage side has a neutral-grounded structure. The three-phase windings on the low-voltage side of the transformer are connected to a three-phase voltage regulator, and the circuit structure is as follows. Figure 3As shown in the attached diagram. The circuit structure of the voltage regulation device includes a 2-level converter, a 3-level converter, or a modular multilevel static var compensator (MMC-STATCOM), etc., as shown in the attached diagram. Figures 4-6 As shown. Among them, the 2-level converter or 3-level converter has a common DC bus, while the MMC-STATCOM does not. Similar to the M3C, the MMC-STATCOM includes three bridge arms, each consisting of multiple full-bridge submodules and one filter inductor connected in series. It should be noted that the above description is for illustrative purposes only and does not constitute a limitation on the application scenario or structure of this invention.

[0118] Example 1:

[0119] like Figure 7 As shown, Embodiment 1 provides a frequency control method for low-frequency power transmission from offshore wind power, including steps S1-S5:

[0120] Step S1: Obtain power data of the offshore wind farm, wherein the power data includes the three-phase grid voltage, three-phase grid current and DC bus voltage of the AC collection bus of the offshore wind farm;

[0121] Step S2: Calculate the reference value of the three-phase output voltage of the voltage regulating device based on the power data;

[0122] Step S3: Input the three-phase output voltage reference value to the voltage regulating device so that the voltage regulating device maintains the voltage stability of the AC bus and the DC bus according to the three-phase output voltage reference value;

[0123] Step S4: Transmit the voltage information of the DC bus to the onshore converter station, so that the onshore converter station generates a first output frequency based on the voltage information of the DC bus and the onshore power data;

[0124] Step S5: Adjust the actual output frequency of the onshore converter station according to the first output frequency.

[0125] In step S1, the voltage detection method of the DC bus of the voltage regulating device is determined by the circuit topology of the voltage regulating device. If the power circuit topology of the voltage regulating device is a two-level or multi-level circuit with a three-phase common DC bus (e.g., ...), ... Figure 4 and Figure 5 (as shown), then the voltage of the DC bus of the voltage regulation device is the common DC bus voltage; if the power circuit topology is a modular multilevel converter topology (such as...) Figure 6 As shown in the figure, the voltage of the DC bus of the voltage regulating device is the average value of the DC bus voltages of all sub-modules.

[0126] This invention provides a frequency control method for low-frequency power transmission from offshore wind power. By acquiring power data from the offshore wind farm, a reference value for the three-phase output voltage of the corresponding voltage regulator is calculated. The voltage regulator then maintains the voltage stability of the AC collecting bus and the DC bus based on this reference value. Maintaining the stability of the AC collecting bus ensures that the voltage amplitude of the offshore wind farm is kept near a given value, thereby guaranteeing the safe and reliable power transmission from the offshore wind farm. Since the voltage regulator itself cannot generate or absorb active power for extended periods, maintaining the voltage stability of the DC bus is also necessary. This invention improves the stability of power transmission from offshore wind power by calculating the reference value for the three-phase output voltage and simultaneously using the voltage regulator to maintain the stability of the voltage amplitude of the offshore wind farm. Furthermore, the voltage information of the DC bus is transmitted to the onshore converter station, and a first output frequency is generated based on this information. This allows the onshore converter station to adjust its output frequency in real time according to the voltage information of the offshore wind farm, ensuring that the entire offshore power grid system always operates at the optimal frequency value, thus improving the power transmission efficiency of offshore wind power.

[0127] In step S2, the closed-loop control block diagram of the voltage regulation device is as follows: Figure 8 As shown, calculating the three-phase output voltage reference value of the voltage regulating device based on the power data includes:

[0128] The instantaneous voltage amplitude u of the AC busbar is calculated based on the three-phase grid voltage of the AC busbar. wm ;

[0129] Based on the instantaneous voltage amplitude, a reference value v for the reactive component of the output voltage of the voltage regulating device is generated through closed-loop control. wq * ;

[0130] Based on the voltage of the DC bus, a reference value v for the active component of the output voltage of the voltage regulating device is generated through closed-loop control. wd * ;

[0131] Based on the three-phase grid current, the corresponding current phase θ is calculated using a three-phase phase-locked loop algorithm. wI ;

[0132] Based on the reference values ​​of the reactive component and active component of the output voltage, and the current phase, the three-phase output voltage reference value v of the voltage regulating device is calculated using a coordinate transformation from a rotating coordinate system to a three-phase stationary coordinate system. wA * v wA * v wA * .

[0133] The three-phase phase-locked loop algorithm can employ a common phase-locked loop in a synchronous rotating coordinate system, or it can incorporate a pre-filter or an in-loop filter to remove harmonics and unbalanced components. Phase θ wI This represents the phase of the fundamental component.

[0134] This invention provides a method for calculating a three-phase output voltage reference value. First, the instantaneous voltage amplitude of the AC collecting bus is calculated using the three-phase grid voltage. The voltage regulation device needs to compensate the AC collecting bus voltage according to the magnitude of this instantaneous voltage amplitude; therefore, the calculated instantaneous voltage amplitude provides the necessary data support for the subsequent voltage compensation by the voltage regulation device. Then, the active component reference value of the output voltage of the voltage regulation device is calculated based on the instantaneous voltage amplitude, used to compensate the AC collecting bus voltage and maintain the stability of the amplitude of the offshore wind farm. The active component reference value of the output voltage of the voltage regulation device is also calculated based on the DC bus voltage, used to compensate the voltage of its own DC bus, ensuring that the voltage regulation device always operates at the optimal modulation ratio and reducing its own losses. Finally, the comprehensive three-phase output voltage reference value is calculated by combining the reactive component reference value, the active component reference value, and the current phase. The voltage regulation device can then output voltage based on this three-phase output voltage reference value, simultaneously maintaining the voltage stability of the offshore wind farm and itself, thereby improving the stability of power transmission from offshore wind power.

[0135] Furthermore, the instantaneous voltage amplitude of the AC collecting bus is calculated based on the three-phase grid voltage and three-phase grid current of the AC collecting bus, using the following formula:

[0136]

[0137] Among them, u wA u wB u wC Let u be the voltage of the three-phase power grid. wm The instantaneous amplitude of the voltage at the AC busbar;

[0138] The reference value for the reactive component of the output voltage of the voltage regulation device is generated through closed-loop control based on the instantaneous voltage amplitude. The specific formula is as follows:

[0139]

[0140] Among them, v wq * The reactive component of the output voltage of the voltage regulating device is the reference value used to maintain the stability of the AC bus voltage. s is the Laplace operator, and k... p1 and k i1These are the proportional and integral control coefficients of the voltage controller for the AC bus, respectively. wm * This is a reference value for the voltage amplitude of the AC busbar;

[0141] The reference value for the active component of the output voltage of the voltage regulation device is generated through closed-loop control based on the voltage of the DC bus, and the specific formula is as follows:

[0142]

[0143] Among them, v wd * This is the reference value for the active component of the output voltage of the voltage regulation device, generally used to balance the losses generated by the device. Its amplitude is very small under steady-state conditions. p2 and k i2 These are the proportional and integral control coefficients of the voltage controller for the DC bus, respectively; U dcw * u is the reference value for the voltage of the DC bus. dcw The voltage of the DC bus;

[0144] Furthermore, the DC bus voltage u of the voltage regulation device dcw It is necessary to ensure that the circuit operates in the linear modulation region (otherwise, significant harmonic content will be generated due to nonlinear modulation), but it should not be too large, resulting in low device utilization. Therefore, this invention employs an optimization algorithm to design U dcw * ,Right now:

[0145]

[0146] Where, k wm This is a pre-set voltage proportionality coefficient. The meaning of this formula is: the DC bus voltage amplitude of the voltage regulating device should be a fixed multiple of the AC side voltage amplitude. For example, for a 2-level or 3-level converter, the DC bus voltage should theoretically not be lower than the AC side line voltage amplitude. Dead-zone commutation and dynamic regulation require an additional margin of at least 10%, therefore k... wm Possible options:

[0147]

[0148] Based on the reference values ​​of the reactive component and active component of the output voltage, and the current phase, the three-phase output voltage reference value of the voltage regulating device is calculated using a coordinate transformation from a rotating coordinate system to a three-phase stationary coordinate system. The specific formula is as follows:

[0149]

[0150] Among them, v wA * v wA * v wA * θ is the reference value for the three-phase output voltage of the voltage regulating device. wI The current phase corresponding to the three-phase power grid current.

[0151] In step S3, after the three-phase output voltage reference value is input to the voltage regulator, the AC side circuit closed-loop control of the voltage regulator generates the modulation signal for the power device. Closed-loop control methods include common voltage-current dual closed-loop control. For topologies such as MMC-STATCOM, voltage balance control algorithms between and within bridge arms are also required. The voltage regulator ultimately generates a signal such as... Figure 8 The Δu shown w This allows for the simultaneous maintenance of stable AC bus voltage and stable DC bus voltage at the voltage regulator. The principle by which the voltage regulator maintains stable AC bus voltage is explained in [reference needed]. Figure 9 The phasor equilibrium relationship shown in the figure, u T This refers to the low-voltage open-winding voltage phasor of a step-up transformer. Offshore wind farms typically operate at unity power factor, i.e., ii w and u w The phases are the same, and the output voltage Δu of the voltage regulator is the same. w Always with i w Vertical, u w and Δu w phasors and composition u T ,Depend on Figure 9 It can be seen that u w The magnitude (phasor length) is less than u T The voltage regulation device can compensate for overvoltages caused by the capacitive effect of the submarine cable by adjusting the amplitude of the voltage. After the voltage regulation device achieves optimal regulation of the voltage of the offshore AC bus, the voltage information can be transmitted to the onshore converter station via the submarine communication cable to achieve optimal frequency control. In actual large-scale offshore wind power transmission systems, communication cables and power cables are generally installed coaxially, so this invention does not require the addition of an extra communication cable, and the hardware cost is not increased.

[0152] After receiving the voltage signal from the offshore voltage regulator, the onshore converter station can perform frequency optimization control. The basic principle of the frequency optimization method is as follows: the lower the frequency of the offshore AC grid, the smaller the capacitive reactive current generated by the submarine cable, the smaller the cable loss, and the more active power received by the onshore grid (i.e., the higher the power generation efficiency of offshore wind power). However, the transformation capacity of the offshore step-up transformer is weaker, meaning the voltage regulator needs to compensate for more voltage. Therefore, it is necessary to find an optimal frequency value to achieve optimized system operation, i.e., steps S4 and S5 in Example 1.

[0153] In one possible implementation, in step S4, the onshore converter station generates a first output frequency based on the voltage information of the DC bus and the onshore power data, such as... Figure 10 As shown, steps S401-S403 are included:

[0154] Step S401: Obtain onshore power data, which includes three-phase onshore grid voltage and three-phase onshore grid current;

[0155] Step S402: Calculate the active power flowing into the onshore power grid based on the onshore power data;

[0156] Step S403: Based on the active power flowing into the onshore power grid and the voltage information of the DC bus, generate the first output frequency using a preset optimized target transfer function and frequency optimization adjustment algorithm.

[0157] This invention provides a method for generating a first output frequency based on voltage information. First, the active power flowing into the onshore power grid is calculated based on onshore power data. Then, the optimal output frequency is calculated based on the active power and the DC bus voltage information. This invention achieves joint optimization of voltage and frequency. While optimizing the voltage of the offshore wind farm using a voltage regulation device, it also transmits the voltage information to the onshore converter station, enabling the onshore converter station to perform real-time frequency adjustment. This determines the output frequency most suitable for the offshore wind farm voltage while meeting the active power requirements, ensuring the entire offshore power grid system always operates at the optimal frequency value and improving the power transmission efficiency of offshore wind power.

[0158] Furthermore, in step S402, the active power flowing into the onshore power grid is calculated based on the onshore power data, using the following formula:

[0159] p g =u gA i gA +u gB i gB +u gC i gC

[0160] Among them, ugA u gB u gC Let i be the voltage of the three-phase onshore power grid. gA i gB i gC The three-phase onshore power grid current;

[0161] In step S403, the specific formula for optimizing the objective transfer function is as follows:

[0162] h f =k pg p g +k dc (u dcw -U dcw,normal ) 2

[0163] Where, k pg and k dc These are the weighting factors for active power and DC bus voltage, respectively, U dcw,normal This refers to the rated DC bus voltage of the voltage regulating device.

[0164] Furthermore, due to p g Taking the inflow into the onshore power grid as positive, therefore k pg k is a value less than 0. dc Then it is a value greater than 0. This can be determined by k. pg and k dc Different optimization goals can be achieved by setting k. pg =-1, k dc =0 indicates that frequency optimization aims to maximize output power; k pg =0, k dc =1 indicates that frequency optimization aims to optimize the operating performance of the voltage regulator; k pg =-1, k dc =1 or other non-zero values ​​indicate that frequency optimization simultaneously maximizes output power and optimizes the operation of the voltage regulation device.

[0165] In one possible implementation, such as Figure 11 As shown, the frequency optimization adjustment algorithm includes:

[0166] Initialize the output frequency, basic step size, step size factor, correction direction, and exit threshold;

[0167] The output frequency is corrected based on the basic step size, step size factor, and correction direction, and the difference and absolute value of the optimized objective transfer function before and after the output frequency correction are calculated.

[0168] If the absolute value is less than the exit threshold, the output frequency is determined as the average of the output frequencies before and after correction, and the algorithm exits.

[0169] If the absolute value is greater than the exit threshold, the correction direction is updated according to the sign of the difference. Specifically, if the sign of the difference is negative, the correction direction remains unchanged; if the sign of the difference is positive, the correction direction is reversed.

[0170] The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and the preset step size factor update formula, and the output frequency is further corrected until the algorithm exits.

[0171] This invention provides a frequency optimization and regulation algorithm. Based on the active power flowing into the onshore power grid, the initial output frequency is continuously corrected through a target transfer function until the output frequency meets the conditions for exiting the algorithm. This invention achieves automatic adjustment through multiple corrections of the initial output frequency, obtaining the optimal output frequency that matches the voltage and active power of the offshore wind farm. It eliminates the need for pre-setting the offshore wind farm frequency, thus improving the flexibility and efficiency of power transmission from offshore wind power.

[0172] Furthermore, the output frequency is corrected based on the basic step size, step size factor, and correction direction, using the following formula:

[0173] f ok+1 =f ok +k·d·Δf0

[0174] Among them, f ko+1 with f ok These are the output frequencies before and after correction, respectively; k is the step size factor; d is the correction direction; and Δf0 is the basic step size. The correction direction d has two states: 1 and -1, where 1 represents the positive direction and -1 represents the negative direction, and its initial value is 1.

[0175] The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and a preset step size factor update formula. The specific formula is as follows:

[0176]

[0177] Among them, T c h is the calculation cycle for the frequency optimization adjustment algorithm. fk+1 with h fk These are the optimized target transfer functions before and after output frequency correction, respectively.

[0178] Furthermore, in step S5, after obtaining the first output frequency f0, f0 is integrated to generate the output phase angle θ. uo ,Right now

[0179] θ uo =2π∫f o dt

[0180] Combined with the set reference amplitude U of the offshore AC power grid voltage om * Control of the onshore converter station. Among them, U... om * Determined by the rated voltage of the transmission system, for a 220kV (line voltage RMS) transmission system, U om * The value is 180kV. Then, according to U... om * and θ uo Closed-loop control is implemented on the output side circuit of the onshore converter station (such as M3C). This involves voltage-current dual closed-loop control to ensure... Figure 2 u in U u V u W The amplitude and frequency reach the set U om * And f0. The control of onshore converter stations also includes power frequency grid current control, bridge arm current decoupling control, capacitor voltage balance control, etc.

[0181] Example 2:

[0182] like Figure 12 As shown, correspondingly, Embodiment 2 provides a frequency control system for low-frequency power transmission of offshore wind power, including an acquisition module 10, a calculation module 20, a voltage regulation module 30, a frequency generation module 40, and a control module 50.

[0183] The acquisition module 10 is used to acquire power data of the offshore wind farm, wherein the power data includes the three-phase grid voltage, three-phase grid current and DC bus voltage of the AC collection bus of the offshore wind farm.

[0184] The calculation module 20 is used to calculate the reference value of the three-phase output voltage of the voltage regulating device based on the power data;

[0185] The voltage regulation module 30 is used to input the three-phase output voltage reference value to the voltage regulation device, so that the voltage regulation device maintains the voltage stability of the AC bus and the DC bus according to the three-phase output voltage reference value.

[0186] The frequency generation module 40 is used to transmit the voltage information of the DC bus to the onshore converter station, so that the onshore converter station generates a first output frequency based on the voltage information of the DC bus and the onshore power data;

[0187] The control module 50 is used to adjust the actual output frequency of the onshore converter station according to the first output frequency.

[0188] In one possible implementation, such as Figure 13 As shown, the calculation module 20 includes a voltage instantaneous amplitude calculation unit 201, a reactive component reference value calculation unit 202, an active component reference value calculation unit 203, a current phase calculation unit 204, and an output voltage reference value calculation unit 205.

[0189] The instantaneous voltage amplitude calculation unit 201 is used to calculate the instantaneous voltage amplitude of the AC busbar based on the three-phase grid voltage of the AC busbar.

[0190] The reactive component reference value calculation unit 202 is used to generate the output voltage reactive component reference value of the voltage regulating device through closed-loop control based on the instantaneous voltage amplitude.

[0191] The active component reference value calculation unit 203 is used to generate the active component reference value of the output voltage of the voltage regulating device through closed-loop control based on the voltage of the DC bus.

[0192] The current phase calculation unit 204 is used to calculate the corresponding current phase based on the three-phase grid current using a three-phase phase-locked loop algorithm.

[0193] The output voltage reference value calculation unit 205 is used to calculate the three-phase output voltage reference value of the voltage regulating device based on the output voltage reactive component reference value, the output voltage active component reference value, and the current phase, and based on the coordinate transformation from the rotating coordinate system to the three-phase stationary coordinate system.

[0194] Furthermore, the instantaneous voltage amplitude calculation unit 201 calculates the instantaneous voltage amplitude of the AC busbar based on the three-phase grid voltage of the AC busbar, using the following formula:

[0195]

[0196] Among them, u wA u wB u wC Let u be the voltage of the three-phase power grid. wm The instantaneous amplitude of the voltage at the AC busbar;

[0197] The reactive power component reference value calculation unit 202 generates the reactive power component reference value of the output voltage of the voltage regulation device through closed-loop control based on the instantaneous voltage amplitude. The specific formula is as follows:

[0198]

[0199] Among them, v wq * Let s be the reference value for the reactive component of the output voltage of the voltage regulating device, s be the Laplace operator, and k be the reference value. p1 and k i1 These are the proportional and integral control coefficients of the voltage controller for the AC bus, respectively. wm * This is a reference value for the voltage amplitude of the AC busbar;

[0200] The active component reference value calculation unit 203 generates the active component reference value of the output voltage of the voltage regulation device through closed-loop control based on the voltage of the DC bus. The specific formula is as follows:

[0201]

[0202] Among them, v wd * k is the reference value for the active component of the output voltage of the voltage regulating device. p2 and k i2 These are the proportional and integral control coefficients of the voltage controller for the DC bus, respectively; U dcw * u is the reference value for the voltage of the DC bus. dcw The voltage of the DC bus;

[0203] The output voltage reference value calculation unit 205 calculates the three-phase output voltage reference value of the voltage regulating device based on the output voltage reactive component reference value, the output voltage active component reference value, and the current phase, using a coordinate transformation from a rotating coordinate system to a three-phase stationary coordinate system. The specific formula is as follows:

[0204]

[0205] Among them, v wA * v wA * v wA * θ is the reference value for the three-phase output voltage of the voltage regulating device. wI The current phase corresponding to the three-phase power grid current.

[0206] In one possible implementation, the onshore converter station generates a first output frequency based on the voltage information of the DC bus and onshore power data, including:

[0207] Acquire onshore power data, which includes three-phase onshore grid voltage and three-phase onshore grid current;

[0208] The active power flowing into the onshore power grid is calculated based on the onshore power data.

[0209] Based on the active power flowing into the onshore power grid and the voltage information of the DC bus, a first output frequency is generated using a preset optimized target transfer function and frequency optimization adjustment algorithm.

[0210] Furthermore, the active power flowing into the onshore power grid is calculated based on the onshore power data using the following formula:

[0211] p g =u gA i gA +u gB i gB +u gC i gC

[0212] Among them, u gA u gB u gC Let i be the voltage of the three-phase onshore power grid. gA i gB i gC The three-phase onshore power grid current;

[0213] The specific formula for the optimized objective transfer function is as follows:

[0214] h f =k pg p g +k dc (u dcw -U dcw,normal ) 2

[0215] Where, k pg and k dc These are the weighting factors for active power and DC bus voltage, respectively, U dcw,normal This refers to the rated DC bus voltage of the voltage regulating device.

[0216] In one possible implementation, the frequency optimization adjustment algorithm includes:

[0217] Initialize the output frequency, basic step size, step size factor, correction direction, and exit threshold;

[0218] The output frequency is corrected based on the basic step size, step size factor, and correction direction, and the difference and absolute value of the optimized objective transfer function before and after the output frequency correction are calculated.

[0219] If the absolute value is less than the exit threshold, the output frequency is determined as the average of the output frequencies before and after correction, and the algorithm exits.

[0220] If the absolute value is greater than the exit threshold, the correction direction is updated according to the sign of the difference. Specifically, if the sign of the difference is negative, the correction direction remains unchanged; if the sign of the difference is positive, the correction direction is reversed.

[0221] The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and the preset step size factor update formula, and the output frequency is further corrected until the algorithm exits.

[0222] Furthermore, the output frequency is corrected based on the basic step size, step size factor, and correction direction, using the following formula:

[0223] f ok+1 =f ok +k·d·Δf0

[0224] Among them, f ko+1 with f ok These are the output frequencies before and after correction, respectively; k is the step size factor; d is the correction direction; and Δf0 is the basic step size.

[0225] The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and a preset step size factor update formula. The specific formula is as follows:

[0226]

[0227] Among them, T c h is the calculation cycle for the frequency optimization adjustment algorithm. fk+1 with h fk These are the optimized target transfer functions before and after output frequency correction, respectively.

[0228] For a more detailed explanation of the working principle and procedures of this embodiment, please refer to the relevant description in Embodiment 1.

[0229] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention for those skilled in the art.

Claims

1. A frequency control method for low-frequency power transmission from offshore wind power, characterized in that, include: Acquire power data of an offshore wind farm, wherein the power data includes the three-phase grid voltage and three-phase grid current of the AC collection bus of the offshore wind farm and the voltage of the DC bus of the voltage regulation device; The reference value of the three-phase output voltage of the voltage regulating device is calculated based on the power data, including: calculating the instantaneous voltage amplitude of the AC collecting bus based on the three-phase grid voltage of the AC collecting bus, using the following formula: Among them, u wA u wB u wC Let u be the voltage of the three-phase power grid. wm The instantaneous amplitude of the voltage at the AC busbar; Based on the instantaneous voltage amplitude, a reference value for the reactive component of the output voltage of the voltage regulating device is generated through closed-loop control, using the following formula: Among them, v wq * Let s be the reference value for the reactive component of the output voltage of the voltage regulating device, s be the Laplace operator, and k be the reference value. p1 and k i1 These are the proportional and integral control coefficients of the voltage controller for the AC bus, respectively. wm * This is a reference value for the voltage amplitude of the AC busbar; Based on the voltage of the DC bus, a reference value for the active component of the output voltage of the voltage regulation device is generated through closed-loop control, using the following formula: Among them, v wd * k is the reference value for the active component of the output voltage of the voltage regulating device. p2 and k i2 These are the proportional and integral control coefficients of the voltage controller for the DC bus, respectively; U dcw * u is the reference value for the voltage of the DC bus. dcw The voltage of the DC bus; Based on the three-phase grid current, the corresponding current phase is calculated using a three-phase phase-locked loop algorithm; Based on the reference values ​​of the reactive component and active component of the output voltage, and the current phase, and using a coordinate transformation from a rotating coordinate system to a three-phase stationary coordinate system, the reference values ​​of the three-phase output voltage of the voltage regulating device are calculated. The specific formula is as follows: Among them, v wA * v wA * v wA * This is the reference value for the three-phase output voltage of the voltage regulating device. The current phase corresponding to the three-phase power grid current; The three-phase output voltage reference value is input to the voltage regulating device so that the voltage regulating device maintains the voltage stability of the AC bus and the DC bus according to the three-phase output voltage reference value. The voltage information of the DC bus is transmitted to the onshore converter station, so that the onshore converter station generates a first output frequency based on the voltage information of the DC bus and the onshore power data; The actual output frequency of the onshore converter station is adjusted according to the first output frequency.

2. The frequency control method for low-frequency power transmission from offshore wind power as described in claim 1, characterized in that, The onshore converter station generates a first output frequency based on the voltage information of the DC bus and the onshore power data, including: Acquire onshore power data, which includes three-phase onshore grid voltage and three-phase onshore grid current; The active power flowing into the onshore power grid is calculated based on the onshore power data. Based on the active power flowing into the onshore power grid and the voltage information of the DC bus, a first output frequency is generated using a preset optimized target transfer function and frequency optimization adjustment algorithm.

3. The frequency control method for low-frequency power transmission from offshore wind power as described in claim 2, characterized in that, The active power flowing into the onshore power grid is calculated based on the onshore power data using the following formula: Among them, u gA u gB u gC Let i be the voltage of the three-phase onshore power grid. gA i gB i gC The three-phase onshore power grid current; The specific formula for the optimized objective transfer function is as follows: Where, k pg and k dc These are the weighting factors for active power and DC bus voltage, respectively, U dcw,normal This refers to the rated DC bus voltage of the voltage regulating device.

4. The frequency control method for low-frequency power transmission from offshore wind power as described in claim 2, characterized in that, The frequency optimization adjustment algorithm includes: Initialize the output frequency, basic step size, step size factor, correction direction, and exit threshold; The output frequency is corrected based on the basic step size, step size factor, and correction direction, and the difference and absolute value of the optimized objective transfer function before and after the output frequency correction are calculated. If the absolute value is less than the exit threshold, the output frequency is determined as the average of the output frequencies before and after correction, and the algorithm exits. If the absolute value is greater than the exit threshold, the correction direction is updated according to the sign of the difference. Specifically, if the sign of the difference is negative, the correction direction remains unchanged; if the sign of the difference is positive, the correction direction is reversed. The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and the preset step size factor update formula, and the output frequency is further corrected until the algorithm exits.

5. The frequency control method for low-frequency power transmission from offshore wind power as described in claim 4, characterized in that, The specific formula for correcting the output frequency based on the basic step size, step size factor, and correction direction is as follows: Among them, f ko+1 with f ok These are the output frequencies before and after correction, respectively; k is the step size factor; d is the correction direction; and Δf0 is the basic step size. The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and a preset step size factor update formula. The specific formula is as follows: Among them, T c h is the calculation cycle for the frequency optimization adjustment algorithm. fk+1 with h fk These are the optimized target transfer functions before and after output frequency correction, respectively.

6. A frequency control system for low-frequency power transmission from offshore wind power, characterized in that, It includes an acquisition module, a calculation module, a voltage regulation module, a frequency generation module, and a control module; The acquisition module is used to acquire power data of the offshore wind farm, wherein the power data includes the three-phase grid voltage, three-phase grid current and DC bus voltage of the AC collection bus of the offshore wind farm. The calculation module is used to calculate the three-phase output voltage reference value of the voltage regulation device based on the power data, including a voltage instantaneous amplitude calculation unit, a reactive component reference value calculation unit, an active component reference value calculation unit, a current phase calculation unit, and an output voltage reference value calculation unit. The instantaneous voltage amplitude calculation unit is used to calculate the instantaneous voltage amplitude of the AC busbar based on the three-phase grid voltage of the AC busbar. The specific formula is as follows: Among them, u wA u wB u wC Let u be the voltage of the three-phase power grid. wm The instantaneous amplitude of the voltage at the AC busbar; The reactive power component reference value calculation unit is used to generate the reactive power component reference value of the output voltage of the voltage regulation device through closed-loop control based on the instantaneous voltage amplitude. The specific formula is as follows: Among them, v wq * Let s be the reference value for the reactive component of the output voltage of the voltage regulating device, s be the Laplace operator, and k be the reference value. p1 and k i1 These are the proportional and integral control coefficients of the voltage controller for the AC bus, respectively. wm * This is a reference value for the voltage amplitude of the AC busbar; The active component reference value calculation unit is used to generate the active component reference value of the output voltage of the voltage regulation device through closed-loop control based on the voltage of the DC bus. The specific formula is as follows: Among them, v wd * k is the reference value for the active component of the output voltage of the voltage regulating device. p2 and k i2 These are the proportional and integral control coefficients of the voltage controller for the DC bus, respectively; U dcw * u is the reference value for the voltage of the DC bus. dcw The voltage of the DC bus; The current phase calculation unit is used to calculate the corresponding current phase based on the three-phase power grid current using a three-phase phase-locked loop algorithm. The output voltage reference value calculation unit is used to calculate the three-phase output voltage reference value of the voltage regulating device based on the output voltage reactive component reference value, the output voltage active component reference value, and the current phase, and through a coordinate transformation from a rotating coordinate system to a three-phase stationary coordinate system. The specific formula is as follows: Among them, v wA * v wA * v wA * This is the reference value for the three-phase output voltage of the voltage regulating device. The current phase corresponding to the three-phase power grid current; The voltage regulation module is used to input the three-phase output voltage reference value to the voltage regulation device, so that the voltage regulation device maintains the voltage stability of the AC bus and the DC bus according to the three-phase output voltage reference value. The frequency generation module is used to transmit the voltage information of the DC bus to the onshore converter station, so that the onshore converter station generates a first output frequency based on the voltage information of the DC bus and the onshore power data; The control module is used to adjust the actual output frequency of the onshore converter station according to the first output frequency.

7. The frequency control system for low-frequency power transmission in offshore wind power as described in claim 6, characterized in that, The onshore converter station generates a first output frequency based on the voltage information of the DC bus and the onshore power data, including: Acquire onshore power data, which includes three-phase onshore grid voltage and three-phase onshore grid current; The active power flowing into the onshore power grid is calculated based on the onshore power data. Based on the active power flowing into the onshore power grid and the voltage information of the DC bus, a first output frequency is generated using a preset optimized target transfer function and frequency optimization adjustment algorithm.

8. The frequency control system for low-frequency power transmission in offshore wind power as described in claim 7, characterized in that, The active power flowing into the onshore power grid is calculated based on the onshore power data using the following formula: Among them, u gA u gB u gC Let i be the voltage of the three-phase onshore power grid. gA i gB i gC The three-phase onshore power grid current; The specific formula for the optimized objective transfer function is as follows: Where, k pg and k dc These are the weighting factors for active power and DC bus voltage, respectively, U dcw,normal This refers to the rated DC bus voltage of the voltage regulating device.

9. The frequency control system for low-frequency power transmission in offshore wind power as described in claim 7, characterized in that, The frequency optimization adjustment algorithm includes: Initialize the output frequency, basic step size, step size factor, correction direction, and exit threshold; The output frequency is corrected based on the basic step size, step size factor, and correction direction, and the difference and absolute value of the optimized objective transfer function before and after the output frequency correction are calculated. If the absolute value is less than the exit threshold, the output frequency is determined as the average of the output frequencies before and after correction, and the algorithm exits. If the absolute value is greater than the exit threshold, the correction direction is updated according to the sign of the difference. Specifically, if the sign of the difference is negative, the correction direction remains unchanged; if the sign of the difference is positive, the correction direction is reversed. The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and the preset step size factor update formula, and the output frequency is further corrected until the algorithm exits.

10. The frequency control system for low-frequency power transmission in offshore wind power as described in claim 9, characterized in that, The specific formula for correcting the output frequency based on the basic step size, step size factor, and correction direction is as follows: Among them, f ko+1 with f ok These are the output frequencies before and after correction, respectively; k is the step size factor; d is the correction direction; and Δf0 is the basic step size. The step size factor is updated based on the optimized target transfer function before and after the output frequency correction and a preset step size factor update formula. The specific formula is as follows: Among them, T c h is the calculation cycle for the frequency optimization adjustment algorithm. fk+1 with h fk These are the optimized target transfer functions before and after output frequency correction, respectively.

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