A method and system for frequency support control of offshore wind power to land power grid based on low-frequency power transmission and a storage medium

Abstract

This invention discloses a method, system, and storage medium for frequency support control of offshore wind power to onshore power grids based on low-frequency transmission, solving the problem that conventional LFAC offshore wind power systems cannot achieve frequency support without communication. The method includes: a receiving-end converter acquiring the frequency of the onshore AC power grid and obtaining the frequency deviation; mapping the frequency deviation to a low-frequency side voltage amplitude change through droop control and transmitting it to the offshore AC power grid; the wind turbine grid side detecting the voltage amplitude obtained from the offshore AC mapping, generating corresponding additional power, and combining it with the base power for load shedding operation, generating a total power command through power superposition, used to regulate the wind turbine output to achieve full-time frequency support of the onshore power grid under signal-free conditions. This invention transmits frequency information through voltage amplitude changes, relying on the stability of the receiving-end converter's dual closed-loop control to achieve accurate frequency support under communication-free conditions. It has a simple structure, reliable operation, and is suitable for long-distance offshore wind power transmission scenarios.

Description

Technical Field

[0001] This invention relates to the field of offshore wind power grid connection and low-frequency AC power transmission technology, and in particular to a method, system and storage medium for offshore wind power frequency support control of onshore power grid based on low-frequency power transmission. Background Technology

[0002] With the continuous advancement of global energy transition and the construction of my country's low-carbon energy system, offshore wind power has become an important component of the new power system, thanks to its advantages such as abundant resources, no land occupation, and minimal environmental impact, and its installed capacity has been growing rapidly year by year.

[0003] However, the large-scale development of deep-sea wind power faces technical challenges in long-distance and efficient power transmission: traditional power frequency AC transmission is difficult to meet the transmission needs of deep-sea wind power due to its large line inductance and high loss. Low-frequency AC transmission (LFAC) technology, with its advantages of low line inductance, long transmission distance, low loss and relatively controllable equipment cost, has gradually become the preferred technology for deep-sea wind power transmission, and its engineering application potential is becoming increasingly prominent.

[0004] Meanwhile, the power system is rapidly evolving towards high penetration of power electronics. A large number of offshore wind turbines are being connected to the grid via power electronic converters, replacing the rotational inertia support and frequency regulation capabilities of traditional synchronous generators. This has led to insufficient system frequency regulation resources and severe challenges to frequency stability. Against this backdrop, fully exploring the rotational inertia and frequency regulation potential of offshore wind turbines and achieving frequency support for the terrestrial power grid through LFAC links has become a key approach to enhancing the frequency stability of high-penetration power electronic systems, and is of great significance for building a new type of power system.

[0005] Existing offshore wind power transmission systems based on LFAC (such as...) Figure 1 As shown in the diagram, the receiving-end converter typically uses a phase-locked loop (PLL) to lock the onshore grid voltage vector. The power frequency side operates in grid-following mode, while the low-frequency side uses a voltage amplitude-frequency (VF) grid control mode, ensuring that the frequency and voltage amplitude of the LFAC link remain constant. Offshore wind turbines are connected to the LFAC low-frequency link via step-up transformers to achieve power transmission. However, the sending and receiving ends are essentially in a frequency decoupled state. Offshore wind turbines cannot effectively sense frequency changes in the onshore grid and cannot proactively provide inertia support and primary frequency regulation services.

[0006] If additional wired or wireless communication systems are built to achieve frequency information transmission, it will not only significantly increase the cost of engineering construction and subsequent maintenance, but may also face risks such as signal interference and link interruption in the complex marine environment of the deep sea, affecting the reliability and timeliness of frequency support.

[0007] Therefore, existing technologies cannot achieve communication-free frequency support for the land power grid from offshore wind power without increasing additional costs or reducing system stability. This has become a core technical bottleneck restricting the large-scale application of LFAC technology in the deep-sea wind power field and the frequency stability improvement of high-penetration power electronic systems. Summary of the Invention

[0008] The purpose of this invention is to solve the above-mentioned problems existing in the prior art by providing a method, system and storage medium for frequency support control of offshore wind power to onshore power grid based on low-frequency power transmission. By optimizing the control method, reliable transmission of frequency information of onshore power grid under the condition of no communication is achieved. This not only eliminates the need to add an additional communication system and increase hardware costs, but also takes into account both economic efficiency and operational stability. Furthermore, it fully taps the inertia and frequency regulation potential of offshore wind turbines, enhances the frequency stability of high-penetration power electronic systems, and adapts to the needs of large-scale transmission of deep-sea wind power.

[0009] The above-mentioned technical objective of this invention is mainly achieved through the following technical solution: a method for frequency support control of onshore power grids by offshore wind power based on low-frequency transmission, characterized in that its steps include: The receiving-end converter collects the frequency of the onshore AC power grid and obtains the frequency deviation; The frequency deviation is mapped to a voltage amplitude reference value on the low-frequency side by droop control, and a low-frequency voltage on the low-frequency side with constant frequency and amplitude that fluctuates synchronously with the frequency of the onshore power grid is output. The amplitude change of the low-frequency voltage is a non-communication physical carrier of the frequency information of the onshore power grid. The low-frequency voltage is transmitted to the offshore AC power grid via a low-frequency AC power transmission link and a fixed-ratio step-up transformer, so that the voltage amplitude of the offshore AC power grid synchronously reflects the frequency changes of the onshore AC power grid. The wind turbine grid-side converter detects the voltage amplitude transmitted to the offshore AC grid, generates primary frequency regulation additional power based on the deviation of the voltage amplitude from its rated operating point, and generates inertia support additional power based on the rate of change of the voltage amplitude. The additional power of the primary frequency regulation, the additional power of the inertia support, and the basic power of the wind turbine under reduced load are superimposed to obtain the reference value of the total power of the wind turbine, and a total power command for the wind turbine is formed. Based on the total power command, the output power of the wind turbine grid-side converter is adjusted to quickly respond to the frequency fluctuations of the onshore power grid, realize the all-time, high-precision frequency support of offshore wind power to the onshore power grid under the condition of no communication, and effectively ensure the stability of the power grid frequency.

[0010] This technical solution does not rely on an additional communication system. It transmits the frequency information of the onshore power grid through changes in low-frequency voltage amplitude, enabling offshore wind power to support the frequency of the onshore power grid. This solves the technical pain point of traditional frequency support relying on communication and being unable to respond when there is no communication. At the same time, by superimposing the additional power of primary frequency regulation and the additional power of inertia support with the basic power of wind turbine load reduction operation, the timeliness and stability of frequency support are ensured, achieving all-time frequency and high-precision frequency support.

[0011] As a further improvement and supplement to the above technical solution, the present invention adopts the following technical measures: Preferably, the frequency deviation is the difference between the actual frequency and the rated frequency of the onshore AC power grid. When the actual frequency of the onshore power grid is lower than the rated frequency, the receiving-end converter raises the voltage amplitude on the low-frequency side through droop control, so that the voltage of the offshore AC power grid rises synchronously, thereby transmitting the information that the frequency of the onshore power grid has decreased. When the actual frequency of the onshore power grid is higher than the rated frequency, the receiving-end converter lowers the voltage amplitude on the low-frequency side through droop control, so that the voltage of the offshore AC power grid decreases synchronously, thereby transmitting the information that the frequency of the onshore power grid has increased.

[0012] By defining the specific meaning of frequency deviation and the response logic of the receiving-end converter, the specific rules for adjusting the low-frequency side voltage amplitude of the receiving-end converter through droop control when the onshore power grid frequency fluctuates are clarified. This enables the voltage amplitude of the offshore AC power grid to accurately and synchronously reflect the frequency changes of the onshore power grid, ensuring the accuracy of frequency information transmission, providing accurate signal basis for subsequent wind turbine power regulation, and improving the accuracy of frequency support.

[0013] Preferably, the receiving-end converter is a matrix modular multilevel converter used to realize the power conversion between the low-frequency AC transmission link and the onshore power frequency grid; the control of the receiving-end converter includes power frequency side dual closed-loop control and low-frequency side dual closed-loop control; the power frequency side dual closed-loop control is used to stabilize the converter arm capacitor voltage and track the onshore grid voltage; the low-frequency side dual closed-loop control is used to track the voltage amplitude reference value to maintain the frequency of the low-frequency voltage on the low-frequency side constant.

[0014] By limiting the receiving-end converter to a matrix modular multilevel converter (M3C), it adapts to the power conversion requirements of low-frequency transmission (LFAC) links and onshore power frequency grids, ensuring the stability of power conversion. Through dual closed-loop control (division of labor between power frequency side and low-frequency side dual closed-loop control), it not only ensures the stability of the converter's own bridge arm capacitor voltage and synchronization with the onshore power grid, but also accurately tracks the voltage amplitude reference value, which is conducive to maintaining a constant low-frequency voltage frequency.

[0015] Preferably, the low-frequency side dual closed-loop control adopts a d / q axis decoupling architecture, which forces the q-axis component of the low-frequency side voltage to be 0, aligns the voltage vector with the d-axis, and controls the low-frequency side voltage amplitude only through the d-axis; the low-frequency side dual closed-loop control introduces an equivalent reactance to achieve d / q axis current loop decoupling and eliminate inter-axis control interference.

[0016] The specific architecture of the low-frequency side dual closed-loop control is defined (adopting a d / q axis decoupling architecture and forcing the q axis voltage component to be 0), which enables precise control of the low-frequency side voltage amplitude only through the d axis, ensuring that the voltage amplitude change can accurately match the frequency deviation; the equivalent reactance is introduced to achieve d / q axis current loop decoupling, eliminate inter-axis control interference, improve the accuracy and stability of low-frequency side voltage amplitude control, avoid frequency information transmission distortion caused by control interference, and further ensure the reliability of frequency support.

[0017] Preferably, the receiving-end converter is further provided with a bridge arm equalization control stage and a coordinate transformation modulation stage; the bridge arm equalization control stage is used to suppress the bridge arm circulating current and equalize the voltage of the sub-module capacitors in the receiving-end converter; the coordinate transformation modulation stage is used to convert the power frequency side modulation amount and the low frequency side modulation amount into a three-phase modulation signal to generate a converter trigger pulse.

[0018] By limiting the additional control loop of the receiving-end converter (the arm balancing control loop effectively suppresses the arm circulating current), the voltage of the submodule capacitors is balanced, avoiding overload of the converter arm and damage to the submodule, thus ensuring the long-term stable operation of the receiving-end converter; the coordinate transformation modulation loop converts the power frequency side modulation and low frequency side modulation into three-phase modulation signals and generates trigger pulses, ensuring that the receiving-end converter can accurately execute control commands and stably output the required low frequency voltage, providing reliable control for frequency information transmission and avoiding the impact of frequency support due to abnormal converter operation.

[0019] Preferably, the primary frequency modulation additional power is obtained by multiplying the voltage amplitude deviation by the primary frequency modulation droop coefficient after low-pass filtering, and the inertia support additional power is obtained by multiplying the voltage amplitude change rate by the inertia droop coefficient after low-pass filtering.

[0020] By defining the generation method of additional power, the specific calculation logic of primary frequency regulation additional power and inertia support additional power is clarified. By multiplying the additional power with the droop coefficient through low-pass filtering, the stability and accuracy of additional power generation are ensured, avoiding sudden changes in additional power caused by voltage amplitude deviation and rate of change fluctuation, thereby ensuring the stability of the total power command of the wind turbine and improving the smoothness of frequency support.

[0021] Preferably, the base power for the wind turbine under reduced load operation is obtained by multiplying the maximum power output of the wind turbine at the maximum power point tracking by a preset reduced load coefficient. This is used to reserve power margin and ensure the stable implementation of primary frequency regulation and inertia support.

[0022] By limiting the generation method of the basic power of the wind turbine under load (obtained by multiplying the maximum power output of the wind turbine's maximum power point tracking MPPT by the load reduction factor), sufficient power margin is reserved for the wind turbine. This ensures that the wind turbine can output the additional power required for primary frequency regulation and inertia support when needed, avoiding the inability to achieve frequency support or insufficient support strength due to lack of power margin, and ensuring the stable execution of frequency support.

[0023] The technical solution of the second technical subject matter involved in this invention: A frequency support system for offshore wind power to onshore power grid based on low-frequency power transmission, used to implement the aforementioned control method, includes: The receiving-end converter is used to acquire the frequency of the onshore AC power grid, obtain the frequency deviation, map the frequency deviation to the voltage amplitude reference value on the low-frequency side through droop control, and output the low-frequency voltage on the low-frequency side. Low-frequency AC transmission links are used to connect receiving-end converters to offshore AC power grids to transmit electrical energy and voltage amplitude change signals. A fixed-ratio step-up transformer connects the offshore AC power grid and the LFAC link to transmit voltage amplitude changes on the low-frequency side to the offshore AC power grid, so that the voltage amplitude of the offshore AC power grid synchronously reflects the frequency changes of the onshore AC power grid. Offshore wind turbines have grid-side converters that detect the voltage amplitude of the offshore AC grid and generate additional power for primary frequency regulation and additional power for inertia support. Their control modules are used to superimpose the additional power for primary frequency regulation and additional power for inertia support with the basic power for wind turbine load reduction to generate a total power command for the wind turbine and regulate the wind turbine output.

[0024] This technical solution clearly defines the functions and collaborative relationships of each component of the system, enabling the engineering implementation of a frequency support control method for onshore power grids based on low-frequency power transmission for offshore wind power. Each component has a clear division of labor and works collaboratively, ensuring smooth execution of the entire process of frequency information transmission, power regulation, and frequency support under conditions without communication. This provides reliable hardware support for the control method and facilitates engineering application and promotion.

[0025] Preferably, the offshore wind turbine on the offshore wind turbine adopts a back-to-back converter structure, and its generator-side converter adopts constant DC voltage control to stabilize the DC bus voltage and ensure the power regulation capability of the grid-side converter.

[0026] Among them, the back-to-back converter structure is adapted to the power regulation requirements of offshore wind power. The turbine-side converter adopts constant DC voltage control, which can stabilize the DC bus voltage and provide a stable working environment for the power regulation of the grid-side converter. This ensures that the grid-side converter can accurately detect the voltage amplitude, generate additional power and execute the total power command, thus ensuring the reliability of wind turbine power regulation and indirectly improving the stability of frequency support.

[0027] The technical solution of the third technical subject matter involved in this invention: A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the aforementioned method for frequency support control of offshore wind power on land power grids based on low-frequency power transmission.

[0028] The beneficial effects of this invention are as follows: It eliminates the need for additional communication links, transmitting frequency information through low-frequency voltage amplitude changes to achieve frequency support under communication-free conditions, thus overcoming the drawbacks of traditional communication-dependent systems; the dual closed-loop control of the receiving-end converter (M3C) ensures constant low-frequency voltage and precise controllable amplitude, balancing power generation efficiency and frequency support accuracy; through power superposition with clear superposition logic, it retains the basic power generation function of the wind turbine while rapidly responding to frequency changes in the terrestrial power grid, achieving all-time frequency support; the system structure is simple, with clearly defined functions for each component, and it is compatible with existing LFAC and wind turbine topologies, featuring no redundant design and facilitating engineering implementation and promotion. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the prior art LFAC system involved in this invention.

[0030] Figure 2 This is a schematic block diagram of the control of the receiving-end converter (M3C) involved in this invention.

[0031] Figure 3 This is a schematic block diagram of the control of the grid-side converter of the wind turbine involved in the present invention.

[0032] Figure 4 This is a schematic diagram of the simulation response results of the power frequency side voltage waveform involved in the present invention.

[0033] Figure 5 This is a schematic diagram of the simulation response results of the power frequency side frequency waveform involved in the present invention.

[0034] Figure 6 This is a schematic diagram of the active power response curve of the grid-side converter of the wind turbine, which is involved in this invention.

[0035] Figure 7 This is a schematic diagram of the voltage response result curve of the offshore low-frequency power grid involved in the present invention. Detailed Implementation

[0036] The technical solution of the present invention will be further described in detail below through embodiments and in conjunction with the accompanying drawings.

[0037] Example 1: A method for frequency support control of onshore power grids by offshore wind power based on low-frequency transmission, comprising the following steps: The receiving-end converter collects the frequency of the onshore AC power grid and obtains the frequency deviation; The frequency deviation is mapped to a voltage amplitude reference value on the low-frequency side by droop control, and a low-frequency voltage on the low-frequency side with constant frequency and amplitude that fluctuates synchronously with the frequency of the onshore power grid is output. The amplitude change of the low-frequency voltage is a non-communication physical carrier of the frequency information of the onshore power grid. The low-frequency voltage is transmitted to the offshore AC power grid via a low-frequency AC power transmission link and a fixed-ratio step-up transformer, so that the voltage amplitude of the offshore AC power grid synchronously reflects the frequency changes of the onshore AC power grid. The wind turbine grid-side converter detects the voltage amplitude transmitted to the offshore AC grid, generates primary frequency regulation additional power based on the deviation of the voltage amplitude from its rated operating point, and generates inertia support additional power based on the rate of change of the voltage amplitude. The additional power of the primary frequency regulation, the additional power of the inertia support, and the basic power of the wind turbine under reduced load are superimposed to obtain the reference value of the total power of the wind turbine, and a total power command for the wind turbine is formed. Based on the total power command, the output power of the wind turbine grid-side converter is adjusted to quickly respond to the frequency fluctuations of the onshore power grid, realize the all-time, high-precision frequency support of offshore wind power to the onshore power grid under the condition of no communication, and effectively ensure the stability of the power grid frequency.

[0038] Next, the above technical solution will be further elaborated as follows: In practical applications, the frequency deviation is the difference between the actual frequency and the rated frequency of the onshore AC power grid. When the actual frequency of the onshore power grid is lower than the rated frequency, the receiving-end converter raises the voltage amplitude on the low-frequency side through droop control, so that the voltage of the offshore AC power grid rises synchronously, in order to transmit the information that the frequency of the onshore power grid has decreased. When the actual frequency of the onshore power grid is higher than the rated frequency, the receiving-end converter reduces the voltage amplitude on the low-frequency side through droop control, so that the voltage of the offshore AC power grid decreases synchronously, thereby transmitting the information that the frequency of the onshore power grid has increased.

[0039] The receiving-end converter adopts a matrix modular multilevel converter to realize the power conversion between the low-frequency AC transmission link and the onshore power frequency grid.

[0040] The control of the receiving-end converter includes dual closed-loop control on the power frequency side and dual closed-loop control on the low frequency side: The power frequency side dual closed-loop control is used to stabilize the converter arm capacitor voltage and track the onshore grid voltage; the low frequency side dual closed-loop control is used to track the voltage amplitude reference value to maintain the low frequency voltage frequency constant.

[0041] The low-frequency side dual closed-loop control adopts a d / q axis decoupling architecture, which forces the q-axis component of the low-frequency side voltage to be 0, aligns the voltage vector with the d-axis, and controls the low-frequency side voltage amplitude only through the d-axis. At the same time, the low-frequency side dual closed-loop control introduces an equivalent reactance to achieve d / q axis current loop decoupling and eliminate inter-axis control interference.

[0042] The receiving-end converter also includes a bridge arm equalization control stage and a coordinate transformation modulation stage. The arm equalization control circuit is used to suppress the arm circulating current and equalize the submodule capacitor voltage in the receiving-end converter. The coordinate transformation modulation stage is used to convert the power frequency side modulation and the low frequency side modulation into a three-phase modulation signal to generate a converter trigger pulse.

[0043] The primary frequency modulation additional power is obtained by multiplying the voltage amplitude deviation by the primary frequency modulation droop coefficient after low-pass filtering.

[0044] The additional power for inertia support is obtained by multiplying the voltage amplitude change rate by the inertia droop coefficient after low-pass filtering.

[0045] The base power for the wind turbine under reduced load operation is obtained by multiplying the maximum power output of the wind turbine at the maximum power point tracking by a preset reduced load coefficient. This is used to reserve power margin and ensure the stable implementation of primary frequency regulation and inertia support.

[0046] Next, the control flow of the frequency support control method for offshore wind power based on low-frequency transmission to onshore power grids will be described in detail with reference to the accompanying drawings: Step 1, as follows Figure 2 As shown, the receiving-end converter detects the frequency of the onshore power grid and obtains the voltage amplitude reference value on the low-frequency side of the receiving end through the following receiving-end converter droop control equation. :

[0047] in, It is the droop control coefficient between the power frequency of the receiving-end converter and the amplitude of the low-frequency voltage; and These are the nominal and actual values ​​of the onshore power grid frequency, respectively. It is the static operating point of the low-frequency side voltage amplitude of the receiving-end converter.

[0048] When the frequency of the onshore power grid decreases or increases, this equation will cause the voltage amplitude on the low-frequency side of the receiving-end converter to increase or decrease.

[0049] Step 2, as follows Figure 2 As shown, the voltage amplitude reference value and the frequency reference value of the low-frequency side of the receiving end Send to the low-frequency side dual closed-loop control, The rotation phase is obtained after integration. Meanwhile, because the target voltage vector on the low-frequency side is aligned with the d-axis of the dq0 coordinate system, it makes... , .

[0050] exist Figure 2 middle, (Right now , ), (Right now , These are the d-axis and q-axis components of the voltage and current on the low-frequency side, respectively. This is the equivalent reactance of the converter, used for decoupling the dq channel of the current loop. Then, after a coordinate transformation from dq0 to αβ0, the low-frequency modulation quantity is obtained. .

[0051] The current inner loop structure on the power frequency side of the receiving end is the same as the current inner loop structure on the low frequency side, wherein, (Right now , ), (Right now , These are the d-axis and q-axis components of the voltage and current on the power frequency side, respectively. In the outer loop on the power frequency side of the receiving end, and These are the reference and actual values ​​of the average voltage of the converter bridge arm modules, respectively, and are typically set... .against The d-axis outer loop control is used to ensure that the voltage and energy of the converter arm module remain stable, so that the converter can operate normally. and These are the reference value and the actual value of the reactive power on the power frequency side of the receiving end, respectively.

[0052] In addition, the phase-locked loop (PLL) locks the voltage vector at the grid connection point of the onshore AC power grid and outputs a rotating phase. This is used to realize the coordinate system transformation from dq0 to αβ0 and obtain the modulation amount on the power frequency side. .

[0053] Step 3: The bridge arm equalization control circuit outputs the bridge arm equalization modulation amount used for circulating current suppression. (include , , , (Four components), the power frequency side modulation generated by the power frequency side dual closed-loop control, the low frequency side modulation generated by the low frequency side dual closed-loop control, and the zero-sequence component. and (where zero-order - zero-order components) Together, they form the following modulation matrix in the dual αβ0 coordinate system. :

[0054] Then, the modulation matrix in the abc coordinate system is obtained through the following double αβ0 inverse transformation steps. :

[0055] in, It is the Clarke constant-amplitude transformation matrix; yes The transpose of . Next, The trigger pulses are sent to the modulation system to obtain the trigger pulses of each capacitor module in the nine arms of the receiving-end converter.

[0056] Step 4, as follows Figure 3 As shown, offshore wind turbines use fixed-ratio step-up transformers ( Therefore, the voltage of the offshore power grid With the low-frequency link voltage at the sending end The relationship between them can be represented as:

[0057] At this point, the frequency changes of the onshore power grid will be reflected in the voltage amplitude of the receiving end low-frequency side, and then this information will be transmitted to the offshore low-frequency AC power grid.

[0058] Step 5: The offshore wind turbine adopts a back-to-back converter structure, where the turbine-side converter uses a conventional constant DC voltage control method. The grid-side converter first detects the amplitude of the low-frequency AC grid voltage vector at the offshore site. :

[0059] in, (Right now , ) are respectively The d-axis and q-axis components.

[0060] Then, after passing through the low-pass filter (LPF), the power corresponding to the primary frequency modulation response (i.e., the primary frequency modulation additional power) is determined according to the following droop control equation. :

[0061] in, It is the droop control coefficient between the primary frequency regulation response power of the wind turbine and the voltage amplitude of the offshore low-frequency power grid; It is the transfer function of LPF; This is the static operating point of the offshore low-frequency power grid voltage amplitude. Furthermore, the power corresponding to the inertia support of the wind turbine system (i.e., the additional power of the inertia support) is calculated using the following equation. :

[0062] in, This is the droop control coefficient between the voltage amplitude and inertial power of the offshore low-frequency power grid. Additionally, based on the wind turbine speed... To obtain the maximum output power Then, multiply by the load reduction factor. calculate Therefore, the reference value for the total output power of the wind turbine. It can be represented as:

[0063] The wind turbine's machine-side converter uses a current control method, meaning it is controlled based on the actual output power of the converter. The d-axis current reference value is obtained through a proportional-integral circuit. Meanwhile, the q-axis current reference value The trigger pulses for each arm of the converter are then obtained through closed-loop current control and modulation system.

[0064] At this point, the offshore wind power system based on LFAC has inertial support without communication and primary frequency regulation capability. That is, the frequency information of the onshore power grid is mapped to the voltage amplitude of the offshore low-frequency AC power grid. Then, the offshore wind turbine dynamically utilizes the standby power of the unit by detecting the voltage amplitude information of the offshore low-frequency AC power grid to achieve frequency support for the onshore power grid.

[0065] Experimental verification: like Figure 4 and Figure 5 As shown, the experimental conditions were that the frequency of the onshore power grid decreased from 50Hz to 49.8Hz in 2 seconds. It is easy to see that the receiving-end converter maintained stable operation, and in 2 seconds, according to the receiving-end droop control equation, the voltage amplitude was reduced on the low-frequency side while keeping the frequency constant at 20Hz.

[0066] The experimental results show that the offshore wind power frequency support control method for onshore power grid based on low-frequency power transmission involved in this invention can transmit the frequency information of the onshore power grid to the offshore power grid in a non-communication manner through the low-frequency power transmission link, providing a necessary foundation for offshore wind turbines to achieve inertia and primary frequency response.

[0067] like Figure 6 and Figure 7As shown, the wind turbine maintains MPPT operation mode, and its output power does not generate a frequency response from the onshore power grid before 2 seconds. However, at 2 seconds, the voltage amplitude of the offshore power grid increases with the frequency change of the onshore power grid, and the output power of the wind turbine increases instantaneously, providing inertia for the onshore power grid. At the same time, the wind turbine also performs a frequency response, that is, after 2 seconds, it injects more active power into the power system through the wind turbine's reserve power to suppress the frequency change of the onshore power grid.

[0068] Experimental results show that the frequency support control method for offshore wind power based on low-frequency transmission for onshore power grids involved in this embodiment can effectively realize the inertia support and primary frequency regulation of offshore wind power for onshore power grids under conditions without communication, and has good dynamic response performance and operational stability.

[0069] Example 2: Technical solution of the second technical subject matter involved in this invention: A frequency support system for offshore wind power to onshore power grid based on low-frequency power transmission, used to implement the control method described in Example 1, includes: The receiving-end converter is used to acquire the frequency of the onshore AC power grid, obtain the frequency deviation, map the frequency deviation to the voltage amplitude reference value on the low-frequency side through droop control, and output the low-frequency voltage on the low-frequency side. Low-frequency AC transmission links are used to connect receiving-end converters to offshore AC power grids to transmit electrical energy and voltage amplitude change signals. A fixed-ratio step-up transformer connects the offshore AC power grid and the LFAC link to transmit voltage amplitude changes on the low-frequency side to the offshore AC power grid, so that the voltage amplitude of the offshore AC power grid synchronously reflects the frequency changes of the onshore AC power grid. Offshore wind turbines have grid-side converters that detect the voltage amplitude of the offshore AC grid and generate additional power for primary frequency regulation and additional power for inertia support. Their control modules are used to superimpose the additional power for primary frequency regulation and additional power for inertia support with the basic power for wind turbine load reduction to generate a total power command for the wind turbine and regulate the wind turbine output.

[0070] The offshore wind turbines on the offshore wind turbines adopt a back-to-back converter structure. Their generator-side converters use constant DC voltage control to stabilize the DC bus voltage and ensure the power regulation capability of the grid-side converters.

[0071] Among them, the back-to-back converter structure is adapted to the power regulation requirements of offshore wind power. The turbine-side converter adopts constant DC voltage control, which can stabilize the DC bus voltage and provide a stable working environment for the power regulation of the grid-side converter. This ensures that the grid-side converter can accurately detect the voltage amplitude, generate additional power and execute the total power command, thus ensuring the reliability of wind turbine power regulation and indirectly improving the stability of frequency support.

[0072] Example 3: The technical solution of the third technical subject matter involved in this invention: A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the frequency support control method for offshore wind power to onshore power grid based on low-frequency power transmission as described in Embodiment 1.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Various modifications and variations can be made to the above embodiments. 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.

Claims

1. A method for frequency support control of onshore power grids by offshore wind power based on low-frequency transmission, characterized in that, The steps include: The receiving-end converter collects the frequency of the onshore AC power grid and obtains the frequency deviation; The frequency deviation is mapped to a voltage amplitude reference value on the low-frequency side by droop control, and a low-frequency voltage on the low-frequency side with constant frequency and amplitude that fluctuates synchronously with the frequency of the onshore power grid is output. The amplitude change of the low-frequency voltage is a non-communication physical carrier of the frequency information of the onshore power grid. The low-frequency voltage is transmitted to the offshore AC power grid via a low-frequency AC power transmission link and a fixed-ratio step-up transformer, so that the voltage amplitude of the offshore AC power grid synchronously reflects the frequency changes of the onshore AC power grid. The wind turbine grid-side converter detects the voltage amplitude transmitted to the offshore AC grid, generates primary frequency regulation additional power based on the deviation of the voltage amplitude from its rated operating point, and generates inertia support additional power based on the rate of change of the voltage amplitude. The additional power of the primary frequency regulation, the additional power of the inertia support, and the basic power of the wind turbine under reduced load are superimposed to obtain the reference value of the total power of the wind turbine, and a total power command for the wind turbine is formed. Based on the total power command, the output power of the wind turbine grid-side converter is adjusted to quickly respond to the frequency fluctuations of the onshore power grid, realize the all-time, high-precision frequency support of offshore wind power to the onshore power grid under the condition of no communication, and effectively ensure the stability of the power grid frequency.

2. The method for frequency support control of offshore wind power on onshore power grid based on low-frequency power transmission according to claim 1, characterized in that, The frequency deviation is the difference between the actual frequency and the rated frequency of the onshore AC power grid. When the actual frequency of the onshore power grid is lower than the rated frequency, the receiving-end converter raises the voltage amplitude on the low-frequency side through droop control, so that the voltage of the offshore AC power grid rises synchronously, thereby transmitting the information that the frequency of the onshore power grid has decreased. When the actual frequency of the onshore power grid is higher than the rated frequency, the receiving-end converter lowers the voltage amplitude on the low-frequency side through droop control, so that the voltage of the offshore AC power grid decreases synchronously, thereby transmitting the information that the frequency of the onshore power grid has increased.

3. The method for frequency support control of offshore wind power on onshore power grid based on low-frequency power transmission according to claim 1, characterized in that, The receiving-end converter is a matrix modular multilevel converter used to realize the power conversion between the low-frequency AC transmission link and the onshore power frequency grid. The control of the receiving-end converter includes power frequency side dual closed-loop control and low-frequency side dual closed-loop control. The power frequency side dual closed-loop control is used to stabilize the converter arm capacitor voltage and track the onshore grid voltage. The low-frequency side dual closed-loop control is used to track the voltage amplitude reference value to maintain the frequency of the low-frequency voltage on the low-frequency side constant.

4. The method for frequency support control of offshore wind power on onshore power grid based on low-frequency power transmission according to claim 3, characterized in that, The low-frequency side dual closed-loop control adopts a d / q axis decoupling architecture, which forces the q-axis component of the low-frequency side voltage to be 0, aligns the voltage vector with the d-axis, and controls the low-frequency side voltage amplitude only through the d-axis. The low-frequency side dual closed-loop control introduces an equivalent reactance to achieve d / q axis current loop decoupling and eliminate inter-axis control interference.

5. The method for frequency support control of offshore wind power on onshore power grid based on low-frequency power transmission according to claim 3, characterized in that, The receiving-end converter is further provided with a bridge arm equalization control stage and a coordinate transformation modulation stage; the bridge arm equalization control stage is used to suppress the bridge arm circulating current and equalize the voltage of the sub-module capacitors in the receiving-end converter; the coordinate transformation modulation stage is used to convert the power frequency side modulation amount and the low frequency side modulation amount into a three-phase modulation signal to generate a converter trigger pulse.

6. The method for frequency support control of offshore wind power on onshore power grid based on low-frequency power transmission according to claim 1, characterized in that, The primary frequency modulation additional power is obtained by multiplying the voltage amplitude deviation by the primary frequency modulation droop coefficient after low-pass filtering, and the inertia support additional power is obtained by multiplying the voltage amplitude change rate by the inertia droop coefficient after low-pass filtering.

7. The method for frequency support control of offshore wind power on onshore power grid based on low-frequency power transmission according to claim 1, characterized in that, The base power for the wind turbine under reduced load operation is obtained by multiplying the maximum power output of the wind turbine at the maximum power point tracking by a preset reduced load coefficient. This is used to reserve power margin and ensure the stable implementation of primary frequency regulation and inertia support.

8. A frequency support system for onshore power grids based on offshore wind power transmission with low-frequency transmission, characterized in that, To implement the control method according to any one of claims 1-7, comprising: The receiving-end converter is used to acquire the frequency of the onshore AC power grid, obtain the frequency deviation, map the frequency deviation to the voltage amplitude reference value on the low-frequency side through droop control, and output the low-frequency voltage on the low-frequency side. Low-frequency AC transmission links are used to connect receiving-end converters to offshore AC power grids to transmit electrical energy and voltage amplitude change signals. A fixed-ratio step-up transformer connects the offshore AC power grid and the LFAC link to transmit voltage amplitude changes on the low-frequency side to the offshore AC power grid, so that the voltage amplitude of the offshore AC power grid synchronously reflects the frequency changes of the onshore AC power grid. Offshore wind turbines have grid-side converters that detect the voltage amplitude of the offshore AC grid and generate additional power for primary frequency regulation and additional power for inertia support. Their control modules are used to superimpose the additional power for primary frequency regulation and additional power for inertia support with the basic power for wind turbine load reduction to generate a total power command for the wind turbine and regulate the wind turbine output.

9. The offshore wind power frequency support system for onshore power grids based on low-frequency power transmission according to claim 8, characterized in that, The offshore wind turbines on the offshore wind turbines adopt a back-to-back converter structure. Their generator-side converters use constant DC voltage control to stabilize the DC bus voltage and ensure the power regulation capability of the grid-side converters.

10. A computer-readable storage medium, characterized in that, The storage medium stores a computer program, which, when executed by a processor, implements the method for frequency support control of offshore wind power on land power grid based on low-frequency power transmission as described in any one of claims 1-7.

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