Frequency-voltage coordinated support control method, device and equipment for offshore wind power flexible direct current grid-connected system and medium

Abstract

The present application relates to the technical field of power system control, and particularly relates to a frequency-voltage coordinated support control method, device, equipment and medium for an offshore wind power flexible direct current grid-connected system, comprising detecting the voltage and frequency state of a grid system grid-connected point, judging whether a disturbance occurs, and determining whether the disturbance is a single voltage disturbance, a single frequency disturbance or a coupled disturbance of frequency and voltage; when the system simultaneously experiences a frequency and voltage disturbance, a frequency-voltage coordinated support control strategy is adopted, and zoning control is performed according to the voltage situation of the grid-connected point, specifically including three scenarios, on the basis of being easy to implement in engineering, the frequency-voltage coordinated support capability of the offshore wind power flexible direct current grid-connected system is significantly improved, the safe operation level of the grid is improved, the frequency and voltage of the offshore wind farm flexible direct current grid-connected system are coordinately controlled, the frequency-voltage coordinated active support capability of the system is improved, and support is provided for the operation analysis and planning of the offshore wind farm flexible direct current grid-connected system and the grid.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power system control, in particular to a frequency-voltage coordinated support control method, device and equipment of offshore wind power flexible direct current grid-connected system and a medium. BACKGROUND

[0002] Renewable energy has become an important development direction of national energy transformation, among which wind energy is one of the most widely used and fastest growing energies in the world. Offshore wind power has become an important direction of wind power development in recent years due to its stable resources, high utilization rate, short distance to load center and small occupation area. Flexible direct current transmission technology has become the preferred solution for offshore wind power grid connection due to its flexible control, power decoupling and island operation. However, with the increasing penetration rate of offshore wind power, the power system is facing increasing challenges in frequency and voltage support.

[0003] At present, the frequency-voltage coordinated support method of offshore wind power flexible direct current grid-connected system mainly focuses on the single field of frequency support or voltage support, and lacks comprehensive control means for frequency and voltage coupling disturbance. The existing method fails to fully consider the coupling effect between frequency and voltage when dealing with frequency and voltage disturbance, resulting in that the system cannot provide effective coordinated support when frequency and voltage fluctuate at the same time. In addition, the existing control strategy has high complexity and is difficult to implement in engineering, which cannot meet the stability requirements in actual operation. Therefore, there is an urgent need for a simple and efficient control strategy that can simultaneously improve the frequency and voltage support capability of offshore wind power flexible direct current grid-connected system and ensure the safety and stability of power grid operation.

[0004] The information disclosed in this BACKGROUND section is only intended to enhance the understanding of the general background of the application, and should not be construed as recognition or any form of suggestion that this information constitutes prior art that is known to those skilled in the art. SUMMARY

[0005] The present application provides a frequency-voltage coordinated support control method, device, equipment and medium of offshore wind power flexible direct current grid-connected system, thereby effectively solving the problems in the background art.

[0006] In order to achieve the above purpose, the technical solution adopted by the present application is as follows: a frequency-voltage coordinated support control method of offshore wind power flexible direct current grid-connected system, comprising the following steps:

[0007] Detecting the voltage and frequency state of the grid system grid-connected point, judging whether a disturbance occurs, and determining whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupling disturbance of frequency and voltage;

[0008] When the system only occurs single voltage disturbance, the reactive power output is adjusted based on the voltage deviation amplitude, the voltage support is carried out by adopting the constant voltage control strategy, and the active power output is kept unchanged;

[0009] When the system only occurs single frequency disturbance, the frequency support is carried out on the grid-connected system by adopting the virtual inertia control and the droop control based on the frequency deviation, and the reactive power output is ensured to maintain the system stability;

[0010] When the system simultaneously occurs frequency and voltage disturbance, the frequency-voltage collaborative support control strategy is adopted, the partition control is carried out according to the voltage situation of the grid-connected point, the active power priority or the reactive power priority mode is adopted by the grid-side modular multilevel converter station according to the voltage situation deviation to carry out the voltage support, and the frequency support is realized on the system by adopting the collaborative frequency modulation or the remote communication mode.

[0011] Further, when the system simultaneously occurs frequency and voltage disturbance, the frequency-voltage collaborative support control strategy is adopted, the partition control is carried out according to the voltage situation of the grid-connected point, the active power priority or the reactive power priority mode is adopted by the grid-side modular multilevel converter station according to the voltage situation deviation to carry out the voltage support, and the frequency support is realized on the system by adopting the collaborative frequency modulation or the remote communication mode, specifically including the following scenarios:

[0012] Scenario one: when the voltage of the grid-connected point is in the interval of 0.95p.u.-1.05p.u., the voltage belongs to normal fluctuation, no reactive power compensation is carried out, the active power priority mode is adopted by the grid-side modular multilevel converter station, and the collaborative frequency modulation control strategy is adopted by the offshore wind farm and the MMC-HVDC to support the frequency of the system;

[0013] Scenario two: when the voltage of the grid-connected point is in the interval of 1.05p.u.-1.1p.u. or 0.9p.u.-0.95p.u., the active power priority mode is continued to be adopted by the grid-side modular multilevel converter station, the reactive power output is adjusted to actively support the voltage of the grid-connected point, and the remote communication mode is adopted by the offshore wind farm and the MMC-HVDC to support the frequency of the system;

[0014] Scenario three: when the voltage of the grid-connected point is lower than 0.9p.u. or exceeds 1.1p.u., the reactive power priority mode is adopted by the grid-side modular multilevel converter station, the active power output is reduced according to the actual situation, the voltage support is carried out on the grid-connected point mainly by the reactive power output, and the remote communication mode is adopted by the offshore wind farm and the MMC-HVDC to support the frequency of the system.

[0015] Further, the voltage and frequency states of the grid-connected point of the power grid system are detected, whether the disturbance occurs is judged, and the disturbance is determined to be single voltage disturbance, single frequency disturbance, or coupled disturbance of frequency and voltage, and the step includes:

[0016] Real-time acquisition of voltage and frequency data of the grid system and grid connection point;

[0017] Comparing the voltage and frequency data with historical operation data to analyze fluctuation trend of the data;

[0018] Based on preset voltage and frequency threshold, setting multi-level alarm mechanism to determine whether the voltage or frequency exceeds the set threshold range;

[0019] According to the feedback result of the multi-level alarm mechanism, it is determined whether a disturbance occurs, and it is determined whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupled disturbance of frequency and voltage.

[0020] Further, when the system simultaneously occurs frequency and voltage disturbance, frequency-voltage collaborative support control strategy is adopted, which is used to cope with simultaneous voltage disturbance and load disturbance, and at least includes the following working conditions: voltage drop accompanied by load surge, voltage drop accompanied by load reduction, voltage rise accompanied by load surge, voltage rise accompanied by load reduction.

[0021] Further, the offshore wind farm and MMC-HVDC adopt collaborative frequency regulation control strategy to support the frequency of the system, and the collaborative frequency regulation control strategy to support the frequency of the system includes:

[0022] By adding virtual inertia control in offshore wind turbine, the inertia response of conventional synchronous generator is simulated, and the specific model includes:

[0023]

[0024] In the formula, ΔP in is the inertia support frequency provided by the wind turbine by releasing the rotor kinetic energy, k in is the inertia control coefficient, and f is usually the measured frequency of the onshore AC system;

[0025] The offshore wind turbine can also simulate the governor response of the generator through droop control, and the specific model includes:

[0026] △P dr = -k dr (f-f0);

[0027] In the formula, ΔP dr represents the frequency support power provided by the wind turbine through droop control, k dr represents the droop control coefficient, f0 and f represent the rated value and measured value of the onshore AC system frequency respectively;

[0028] To ensure power reserve, the offshore wind farm ensures power reserve through overspeed load shedding control, and for different load shedding rates d%, the model of wind turbine load shedding power includes:

[0029] P del = P MPPT (1-d%);

[0030] In the formula, P MPPT is the mechanical power output of the fan in the MPPT state.

[0031] Further, in the first scenario, the offshore wind farm and the MMC-HVDC adopt a coordinated frequency regulation control strategy as follows:

[0032] When the grid-connected point frequency changes, the DC voltage of the MMC-HVDC is adjusted according to the frequency change, and the change of the DC voltage enables the wind farm to receive frequency information from the land system; by adjusting the relationship between the DC voltage and the frequency, active support for the system frequency is achieved.

[0033] The relationship model between the DC voltage and the frequency includes:

[0034] f W = K W K DC △f+f W0 ;

[0035] In the formula, K DC represents the DC voltage control coefficient, K W represents the WFMMC frequency control coefficient; f W0 represents the steady-state frequency of the WFMMC, and △f represents the frequency deviation of the land system, i.e., the difference between the rated value and the actual value of the frequency.

[0036] Further, in the first scenario and the second scenario, the grid-side modular multilevel converter station adopts an active priority mode, i.e., a d-axis current priority mode, to preferentially guarantee active output, and the maximum reactive output is the maximum value remaining in the system capacity, which is realized by controlling the PI limiter in the current inner loop control.

[0037] The model of the actual values of the d-axis and q-axis currents in the active priority mode includes:

[0038]

[0039] In the formula, i d represents the actual value of the d-axis current, i q represents the actual value of the q-axis current, i qref and i dref represent the q-axis and d-axis current reference values, respectively, and i max represents the maximum current that the converter can withstand.

[0040] Further, in the scenario two, scenario three, the voltage support control strategy of the grid-side modular multilevel converter station on the grid point comprises:

[0041] When the system is normally running, the grid-side modular multilevel converter station adopts U dc control, and works in a rated reactive power state; when a voltage disturbance occurs, the grid-side modular multilevel converter station is switched to a fixed AC voltage mode to provide voltage support for the system;

[0042] The model of the voltage support control comprises:

[0043]

[0044] In the formula, i qref represents a q-axis current reference value, k v represents a reactive droop coefficient, V acref and V ac respectively represent an onshore AC system grid point AC voltage rated value and a measured value, K p and K i respectively represent reactive PI control coefficients, and s represents a commonly used representation of an integral element.

[0045] Further, in the scenario two, scenario three, the offshore wind farm and the MMC-HVDC provide frequency support to the system through a remote communication mode, the remote communication mode uses communication technology to directly transmit the GSMMC grid point frequency information to the offshore wind farm, so that the offshore wind farm participates in active frequency support of the system.

[0046] Further, in the scenario three, the grid-side modular multilevel converter station adopts a reactive power priority mode, reduces active power output according to actual conditions, and mainly uses reactive power output to provide voltage support for the grid point;

[0047] The reactive power priority mode, i.e., the q-axis current priority mode, prioritizes reactive power output, and the maximum active power output is the maximum residual value of system capacity, which is realized by PI limiting in current inner loop control;

[0048] The model of the d-axis and q-axis current actual values under the reactive power priority mode comprises:

[0049]

[0050] In the formula, i d represents a d-axis current actual value, i q represents a q-axis current actual value, i qref and i dref respectively represent q-axis and d-axis current reference values, i max represents the maximum current that the converter can withstand.

[0051] The application also comprises a frequency-voltage coordinated support control device for offshore wind power flexible direct current grid-connected system, which uses the method as described above, comprising:

[0052] a detection module for detecting the voltage and frequency state of the grid-connected point of the grid system, judging whether a disturbance occurs, and determining whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupled disturbance of frequency and voltage;

[0053] a voltage support module for adjusting reactive power output based on voltage deviation amplitude when the system only has a single voltage disturbance, adopting a constant voltage control strategy for voltage support, and keeping the active power output unchanged;

[0054] a frequency regulation support module for, when the system only has a single frequency disturbance, based on the frequency deviation, supporting the grid-connected system in frequency through virtual inertia control and droop control, and ensuring that the reactive power output maintains system stability;

[0055] a coordinated support module for, when the system has both frequency and voltage disturbances, adopting a frequency-voltage coordinated support control strategy, performing partition control according to the voltage situation of the grid-connected point, and adopting active power priority or reactive power priority mode for voltage support according to the voltage situation deviation of the grid-side modular multilevel converter station, and supporting the system in frequency through coordinated frequency regulation or remote communication mode.

[0056] The application also comprises a computer device comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, wherein the processor implements the method as described above when executing the computer program.

[0057] The application also comprises a storage medium having a computer program stored thereon, wherein the computer program is executable on a processor to implement the method as described above.

[0058] The application has the following beneficial effects:

[0059] For the frequency-voltage coupled disturbance of offshore wind power through flexible direct current grid-connected system, a frequency-voltage coordinated support strategy for offshore wind power flexible direct current grid-connected system based on voltage situation partition is proposed, which partitions according to the GSMMC AC voltage base value, and then adopts three different control strategies through the partition results; on the basis of being easy to implement in engineering, the strategy significantly improves the frequency-voltage coordinated support capability of offshore wind power through flexible direct current grid-connected system, improves the safe operation level of the grid, and the coordinated control idea provided by the method has strong universality, can be applied to multi-terminal offshore flexible direct current grid-connected system and land flexible direct current grid-connected system, and is not limited to wind farm and other new energy stations. BRIEF DESCRIPTION OF DRAWINGS

[0060] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings described below only constitute some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0061] Figure 1 Flowchart of a frequency-voltage coordinated support control method for an offshore wind power flexible HVDC grid-connected system;

[0062] Figure 2 Flowchart of a frequency-voltage coordinated support control method for an offshore wind power flexible HVDC grid-connected system;

[0063] Figure 3 Topological structure diagram of an offshore wind power flexible HVDC grid-connected system;

[0064] Figure 4 Schematic diagram of an additional frequency modulation control for an offshore wind farm;

[0065] Figure 5 Schematic diagram of a coordinated frequency modulation control for an offshore wind farm and an MMC-HVDC;

[0066] Figure 6 Schematic diagram of a voltage support control for a GSMMC;

[0067] Figure 7 Schematic diagram of a frequency-voltage coordinated support control device for an offshore wind power flexible HVDC grid-connected system;

[0068] Figure 8 Schematic diagram of a computer device. DETAILED DESCRIPTION

[0069] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments only constitute some embodiments of the present application, rather than all embodiments.

[0070] As shown in Figures 1 to 6 A frequency-voltage coordinated support control method for an offshore wind power flexible HVDC grid-connected system comprises the following steps:

[0071] S10: detecting the voltage and frequency state of a grid system grid-connected point, judging whether a disturbance occurs, and determining whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupled disturbance of frequency and voltage;

[0072] S20: when the system only has a single voltage disturbance, adjusting reactive power output based on the voltage deviation amplitude, adopting a constant voltage control strategy for voltage support, and keeping the active power output unchanged;

[0073] S30: When only a single frequency disturbance occurs in the system, frequency support is provided to the grid-connected system through virtual inertia control and droop control based on the frequency deviation condition, and reactive power output is ensured to maintain system stability;

[0074] S40: When frequency and voltage disturbances occur simultaneously in the system, a frequency-voltage coordinated support control strategy is adopted, and zoned control is performed according to the voltage situation at the grid connection point. The grid-side modular multilevel converter station adopts active power priority or reactive power priority mode for voltage support according to the voltage situation deviation, and through coordinated frequency regulation or remote communication mode, frequency support is provided to the system.

[0075] As a preferred embodiment of the above embodiment, when frequency and voltage disturbances occur simultaneously in the system, a frequency-voltage coordinated support control strategy is adopted, and zoned control is performed according to the voltage situation at the grid connection point. The grid-side modular multilevel converter station adopts active power priority or reactive power priority mode for voltage support according to the voltage situation deviation, and through coordinated frequency regulation or remote communication mode, frequency support is provided to the system, which specifically includes the following scenarios:

[0076] Scenario one: When the voltage at the grid connection point is in the range of 0.95 p.u.-1.05 p.u., the voltage belongs to normal fluctuation, no reactive power compensation is performed, the grid-side modular multilevel converter station (GSMMC) adopts active power priority mode, and the offshore wind farm and the MMC-HVDC (modular multilevel converter-high voltage direct current transmission system) adopt coordinated frequency regulation control strategy to provide frequency support to the system;

[0077] Scenario two: When the voltage at the grid connection point is in the range of 1.05 p.u.-1.1 p.u. or 0.9 p.u.-0.95 p.u., the grid-side modular multilevel converter station (GSMMC) continues to adopt active power priority mode and adjusts reactive power output to actively support the voltage at the grid connection point, and the offshore wind farm and the MMC-HVDC (modular multilevel converter-high voltage direct current transmission system) adopt remote communication mode to provide frequency support to the system;

[0078] Scenario three: When the voltage at the grid connection point is lower than 0.9 p.u. or higher than 1.1 p.u., the grid-side modular multilevel converter station (GSMMC) adopts reactive power priority mode and reduces active power output according to the actual situation to support the voltage at the grid connection point mainly through reactive power output, and the offshore wind farm and the MMC-HVDC (modular multilevel converter-high voltage direct current transmission system) adopt remote communication mode to provide frequency support to the system.

[0079] Aiming at the frequency-voltage coupling disturbance of offshore wind power through flexible direct current grid-connected system, a frequency-voltage collaborative support strategy of offshore wind power through flexible direct current grid-connected system based on voltage situation zoning is provided, the system is divided into zones according to the GSMMC alternating current voltage standard value, and then three different control strategies are adopted according to the zoning results; the strategy significantly improves the frequency-voltage collaborative support capability of the offshore wind power through flexible direct current grid-connected system, improves the safe operation level of the power grid, and the collaborative control idea provided by the method has strong universality, can be applied to multi-terminal offshore flexible grid-connected system and land flexible grid-connected system, and is not limited to wind power station and other new energy stations.

[0080] The prior art focuses on single frequency or voltage support control, and lacks comprehensive processing of coupled disturbances of both, and the application introduces a frequency-voltage collaborative support control strategy in S40, which can control the system according to the voltage situation when frequency and voltage disturbances occur at the same time. This control strategy can select different priorities (such as active power priority or reactive power priority) according to different voltage states (such as normal fluctuation, slight deviation and serious deviation), optimize the collaborative support effect of frequency and voltage in different cases, and realize more stable power grid operation.

[0081] The application divides the voltage situation of the grid-connected point into three scenarios (scenario one, scenario two and scenario three), and adopts different strategies in each scenario. For normal fluctuation, no reactive power compensation is performed, for slight deviation, reactive power is adjusted for support, and for serious deviation, reactive power output is prioritized and active power output is reduced. Through this voltage situation-based zoning control, the system can flexibly cope with disturbances of different degrees.

[0082] In the control strategy, the priority of active and reactive power is flexibly adjusted according to different voltage disturbance conditions. When the voltage is in normal fluctuation, active power output is prioritized, while in the case of serious voltage deviation, reactive power support is prioritized, and active power output is reduced if necessary. This dynamic adjustment improves the tolerance of the system to voltage fluctuations in the power grid and ensures more efficient frequency-voltage support.

[0083] It should be noted that, Figure 3 The offshore wind farm through flexible direct current grid-connected system topology structure diagram shown in the application is only an example, and the corresponding topology structure diagram should be established according to the actual situation of the offshore wind farm through flexible direct current grid-connected system in actual application. The power transmission between the offshore wind farm and the onshore power grid is realized through the direct current transmission line between WFMMC and GSMMC. The system has the ability of frequency-voltage collaborative support control, uses R chop to stabilize the direct current voltage, and ensures that voltage support and frequency regulation can be effectively performed when voltage or frequency disturbance occurs.

[0084] As a preferred embodiment of the above, in step S10, the voltage and frequency states of the grid system at the grid connection point are detected, it is determined whether a disturbance occurs, and it is determined whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupled frequency and voltage disturbance, and the step includes:

[0085] S11: A high-precision sensor is installed at the grid connection point to collect voltage and frequency data of the grid system at the grid connection point in real time;

[0086] S12: The voltage and frequency data are compared with historical operation data to analyze the fluctuation trend of the data;

[0087] S13: Based on preset voltage and frequency thresholds, a multi-level alarm mechanism is set to determine whether the voltage or frequency exceeds the set threshold range;

[0088] S14: According to the feedback result of the multi-level alarm mechanism, it is determined whether a disturbance occurs, and it is determined whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupled frequency and voltage disturbance.

[0089] The multi-level alarm mechanism can trigger different levels of alarms according to different degrees of voltage or frequency deviation, thereby improving the flexibility of the system. According to the severity of the disturbance, the system can make a graded response, avoiding excessive intervention in the case of small disturbances, ensuring the accuracy of the control strategy, and at the same time, quickly taking action when a large disturbance occurs, improving the emergency handling capability of the power grid.

[0090] In this embodiment, in step S40, when the system simultaneously occurs frequency and voltage disturbance, a frequency-voltage coordinated support control strategy is adopted, and the control strategy is mainly used to cope with simultaneous voltage disturbance and load disturbance, and at least includes the following working conditions: voltage drop accompanied by load surge, voltage drop accompanied by load sudden decrease, voltage rise accompanied by load surge, and voltage rise accompanied by load sudden decrease.

[0091] By providing corresponding control strategies for simultaneous disturbances of various complex working conditions (such as voltage drop and load surge / sudden decrease), the system can dynamically adjust the frequency and voltage support, ensuring that the power grid can still maintain stability even in complex disturbance conditions; By considering the joint disturbance of voltage and load, the system can respond more flexibly and quickly, avoiding problems such as frequency instability, overvoltage or undervoltage when the load suddenly changes or the voltage changes, thereby improving the operation safety of the entire power grid; Considering multiple complex disturbance combination scenarios, the system can also quickly adjust when responding to single voltage, frequency disturbance and load disturbance, thereby improving the adaptability and resilience of the system to various working conditions.

[0092] For example, Figure 4As shown, the control block diagram shown is a schematic diagram of the offshore wind farm frequency support control strategy, involving virtual inertia control (S41), droop control (S42) and over-speed load shedding control (S43) described in claim 9, by combining these control mechanisms, the wind farm can quickly respond to frequency disturbances and provide continuous frequency support, while providing power backup, ensuring that the grid frequency is effectively adjusted and restored when fluctuating, the inertia and droop frequency support control is as follows:

[0093] As a preferred embodiment of the above, in step S40, the offshore wind farm cooperates with the MMC-HVDC (Modular Multilevel Converter-High Voltage Direct Current Transmission System) to adopt a coordinated frequency regulation control strategy to support the system, the coordinated frequency regulation control strategy to support the system includes:

[0094] S41: By adding virtual inertia control in the offshore wind turbine, the inertia response of a conventional synchronous generator is simulated, and the specific model includes:

[0095]

[0096] In the formula, ΔP in is the inertia support frequency provided by the wind turbine by releasing the rotor kinetic energy, k in is the inertia control coefficient, f is usually the measured frequency of the onshore AC system;

[0097] S42: The offshore wind turbine can also simulate the governor response of the generator through droop control, and the specific model includes:

[0098] △P dr = -k dr (f-f0);

[0099] In the formula, ΔP dr represents the frequency support power provided by the wind turbine through droop control, k dr represents the droop control coefficient, f0 and f represent the rated value and measured value of the onshore AC system frequency respectively;

[0100] S43: To ensure power backup, the offshore wind farm ensures power backup through over-speed load shedding control, for different load shedding rates d%, the model of wind turbine load shedding power includes:

[0101] P del = P MPPT (1-d%);

[0102] In the formula, P MPPT is the mechanical power output of the wind turbine operating in the MPPT state.

[0103] By adding virtual inertia control in wind turbines, the system can quickly respond when frequency disturbance occurs. Virtual inertia control uses the rotor kinetic energy of wind turbines to simulate the inertia response of conventional synchronous generators, providing short-term frequency support for the system. This rapid response can effectively suppress frequency fluctuations and enhance the frequency stability of the power grid. Based on virtual inertia control, the system continuously adjusts the frequency through droop control, simulating the governor response of conventional generators. Droop control can gradually adjust the power output of wind turbines according to the frequency deviation, ensuring that the frequency returns to the rated value. This gradual recovery mechanism ensures frequency stability and prevents long-term frequency deviation in the system. Through overspeed load shedding control, offshore wind farms can provide power backup for the system. Through the load shedding model, wind turbines can reserve a certain amount of power backup to ensure a quick response when load surges or frequency drops sharply, further enhancing the stability and emergency handling capacity of the system.

[0104] In step S40, as shown in Figure 5 The cooperative frequency control diagram of offshore wind farms and MMC-HVDC (Modular Multilevel Converter-High Voltage Direct Current Transmission System) is shown, covering wind farm active power control, WFMMC frequency control, and GSMMC direct current voltage control. In scenario one, offshore wind farms and MMC-HVDC (Modular Multilevel Converter-High Voltage Direct Current Transmission System) adopt the following cooperative frequency control strategy:

[0105] When the grid-connected point frequency changes, the direct current voltage of MMC-HVDC (Modular Multilevel Converter-High Voltage Direct Current Transmission System) adjusts with the frequency change, and the change of direct current voltage makes the wind farm receive frequency information from the land system. By adjusting the relationship between direct current voltage and frequency, active support for system frequency is achieved.

[0106] The relationship model between direct current voltage and frequency includes:

[0107] f W =K W K DC △f+f W0 ;

[0108] In the formula, K DC represents the direct current voltage control coefficient, K W represents the WFMMC frequency control coefficient; f W0 represents the steady-state frequency of WFMMC, and △f represents the frequency deviation of the land system, i.e. the difference between the rated value and the actual value.

[0109] By adjusting the DC voltage of the MMC-HVDC (Modular Multilevel Converter-High Voltage Direct Current Transmission System) system, the frequency information can be accurately transmitted to the wind farm, so that the wind farm can obtain the frequency change information from the land system in real time and quickly respond according to the information, ensuring the accuracy and timeliness of frequency regulation, realizing the coordinated support of frequency and voltage, and being particularly suitable for coping with large frequency fluctuations; the offshore wind farm can actively adjust its output power based on frequency changes to support the grid frequency, which enhances the frequency regulation effect of the wind farm in the grid, effectively reduces the frequency regulation burden of the land grid, and improves the stability and safety of the system.

[0110] As a preferred embodiment of the above, in step S40, in scenario one and scenario two, the grid-side modular multilevel converter station (GSMMC) adopts an active priority mode, i.e., a d-axis current priority mode, to preferentially guarantee active output and to maximize the remaining maximum value of system capacity as reactive output, and to achieve this by PI limiting in the current inner loop control;

[0111] The model of the actual values of the d-axis and q-axis currents in the active priority mode includes:

[0112]

[0113] In the formula, i d represents the actual value of the d-axis current, i q represents the actual value of the q-axis current, i qref and i dref respectively represent the reference values of the q-axis and d-axis currents, i max represents the maximum current that the converter can withstand.

[0114] In the active priority mode, the system first guarantees the output of active power, especially when the grid-connected point voltage is within the normal fluctuation range (such as scenario one and scenario two), by preferentially adjusting the d-axis current, the active output of the wind farm can be maximized, the power generation efficiency of the wind power system can be improved, and the stable supply of power can be ensured; by dynamically adjusting the reactive output according to the system capacity remaining value, the system can quickly respond when the grid needs additional voltage support, and the limitation of the maximum output of the q-axis current is realized by the remaining capacity of the system, so that the reactive power compensation can flexibly adapt to different voltage fluctuation conditions, and excessive or insufficient reactive output is avoided.

[0115] In step S40, as shown in Figure 6 , a GSMMC voltage support control schematic diagram is shown, and in scenario two and scenario three, the voltage support control strategy of the grid-side modular multilevel converter station (GSMMC) for the grid-connected point includes:

[0116] When the system is in normal operation, the grid-side modular multilevel converter station (GSMMC) adopts U dc control and works in the rated reactive power state; when a voltage disturbance occurs, the grid-side modular multilevel converter station (GSMMC) switches to the fixed AC voltage mode to provide voltage support for the system.

[0117] The model of the voltage support control comprises:

[0118]

[0119] In the formula, i qref represents the q-axis current reference value, k v represents the reactive droop coefficient, V acref and V ac respectively represent the rated value and the measured value of the AC voltage at the land AC system grid connection point, K p and K i respectively represent the reactive PI control coefficients, and s represents the commonly used representation of the integral element.

[0120] When the system is in normal operation, the GSMMC is in the fixed reactive power state to ensure the stability of the reactive output of the system, and when a voltage disturbance occurs, the GSMMC can automatically switch to the fixed AC voltage mode to provide voltage support. This dynamic working mode switching improves the adaptability of the system and enables the system to respond in time under voltage fluctuation to ensure the stability of the grid voltage. Through the combination of the reactive droop control and the PI control, the system can flexibly adjust the reactive power output. The reactive droop coefficient k v ensures the rapid response of the voltage deviation, and the PI controller accurately adjusts the reactive power output through the proportion K p and the integral K i to prevent the voltage fluctuation from having a long-term impact on the system. Such a double control mechanism effectively improves the accuracy and dynamic response capability of the voltage support.

[0121] In the present embodiment, in step S40, in scenario two and scenario three, the offshore wind farm and the MMC-HVDC (modular multilevel converter-high voltage direct current transmission system) provide frequency support to the system through a remote communication mode. The remote communication mode directly transmits the frequency information at the GSMMC grid connection point to the offshore wind farm by using communication technology, so that the offshore wind farm participates in the active frequency support of the system.

[0122] Through the remote communication technology, the frequency information of the grid-side MMC point can be transmitted to the offshore wind farm in real time, so that the wind farm can quickly respond to the change of the frequency, and the information transmission in a fast manner greatly reduces the information lag, enhances the frequency regulation response capability of the wind farm, and thus improves the frequency stability of the power grid; through receiving the frequency information of the grid-side point, the offshore wind farm can work cooperatively with the MMC-HVDC (modular multilevel converter-high voltage direct current transmission system) system for frequency regulation, and through the cooperative mechanism, the linkage mechanism between the wind farm and the onshore power grid can be formed to effectively share the load of frequency regulation when the frequency disturbance occurs, and the frequency regulation efficiency of the whole system is improved.

[0123] As a preferred embodiment of the above, in step S40, in scenario three, the grid-side modular multilevel converter station (GSMMC) adopts a reactive power priority mode, and according to the actual situation, the active power output is reduced, and the voltage support at the grid-side point is mainly realized by the reactive power output.

[0124] The reactive power priority mode is a q-axis current priority mode, which preferentially guarantees the reactive power output, and the maximum active power output is the maximum residual value of the system capacity, and the PI limiting is realized through the control of the current inner loop control.

[0125] The model of the actual values of the d-axis and q-axis currents in the reactive power priority mode comprises:

[0126]

[0127]

[0128] In the formula, i d represents the actual value of the d-axis current, i q represents the actual value of the q-axis current, i qref and i dref respectively represent the reference values of the q-axis and d-axis currents, i max represents the maximum current that can be borne by the converter.

[0129] When a serious voltage disturbance occurs in the power grid (such as scenario three), through the reactive power priority mode, the system can preferentially guarantee the reactive power output, thereby effectively supporting the voltage of the power grid, and the voltage stability is crucial for the safe operation of the power system, and through preferentially providing the reactive power support, the system can quickly respond to the voltage fluctuation and restore stability; in the reactive power priority mode, the system reduces the active power output according to the actual situation, so as to ensure that the power system can provide more capacity for the reactive power, and thus the system can flexibly adjust the balance between the active power and the reactive power in the voltage support, avoid the operation of the system in an overload state, and guarantee the stability of the equipment and the system.

[0130] The application also comprises a frequency-voltage cooperative support control device for an offshore wind power flexible direct current grid-connected system, which uses the method as described above, such as Figure 7As shown, comprising:

[0131] A detection module is configured to detect the voltage and frequency state of the grid system and determine whether a disturbance occurs and whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupled disturbance of frequency and voltage;

[0132] A voltage support module is configured to adjust the reactive power output based on the voltage deviation amplitude when the system only has a single voltage disturbance, adopt a constant voltage control strategy to support the voltage, and keep the active power output unchanged;

[0133] A frequency support module is configured to, when the system only has a single frequency disturbance, support the frequency of the grid system through virtual inertia control and droop control based on the frequency deviation, and ensure that the reactive power output maintains the stability of the system;

[0134] A cooperative support module is configured to, when the system has a coupled disturbance of frequency and voltage, adopt a frequency-voltage cooperative support control strategy, perform partition control according to the voltage situation of the grid point, and adopt an active power priority mode or a reactive power priority mode to support the voltage according to the voltage situation deviation of the grid-side modular multilevel converter station, and support the frequency of the system through cooperative frequency regulation or a remote communication mode.

[0135] Please refer to Figure 8 The computer device provided in the embodiment of the application has the structure shown in the structural schematic diagram of the computer device. The computer device 400 provided in the embodiment of the application comprises a processor 410 and a memory 420. The memory 420 stores a computer program executable by the processor 410. The computer program is executed by the processor 410 to perform the method as above.

[0136] The embodiment of the application further provides a storage medium 430. The storage medium 430 stores a computer program. The computer program is run by the processor 410 to perform the method as above.

[0137] The storage medium 430 can be implemented by any type of volatile or nonvolatile storage devices or a combination thereof, such as static random access memory (SRAM), electrically erasable programmable read-only memory (EEPROM), erasable programmable read only memory (EPROM), programmable read only memory (PROM), read-only memory (ROM), magnetic storage, flash memory, magnetic or optical disk.

[0138] In the description of the present application, the terms "first", "second", "third", etc. are used only to describe purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second", etc. can explicitly or implicitly include one or more of the features. The meaning of "plurality" is two or more, unless otherwise explicitly specified and limited.

[0139] In the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connecting", "fixing" and the like should be understood in a broad sense, for example, can be fixed connection, can also be detachable connection, or integral; can be mechanical connection, can also be electrical connection; can be directly connected, can also be indirectly connected through an intermediate medium, can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0140] In the description of the present application, the description of the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present application, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine the different embodiments or examples described in the present application and the features of the different embodiments or examples without contradiction.

[0141] Any processes or methods described in the flowcharts or otherwise described herein can be understood as representing modules, segments, or portions of code that include one or more executable instructions for implementing specific logic functions or steps, and alternative implementations are within the scope of the preferred embodiments of the application that are directed towards the functions performed by the steps, in which the order of performance can be changed, functions can be performed in parallel, or in reverse order, as appropriate, and appropriate combinations thereof, as would be understood by one skilled in the art.

[0142] The logic and / or steps represented in the flowcharts or otherwise described herein, for example, can be considered as a list of executable instructions for implementing the logic function, and can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor- containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions, or a combination thereof. For the purposes of this specification, a "computer-readable medium" can be any apparatus that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-readable medium can specifically include the following, which are non-exhaustive listings: electrical connections (electrical apparatus), portable computer disks (magnetic apparatus), random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or Flash memory), optical fiber devices, and portable compact disk read-only memory (CDROM). Additionally, the computer-readable medium can even be paper or other suitable medium upon which the program can be printed, because the program can be electronically obtained, for example, by

[0143] It should be understood that portions of the application can be implemented in hardware, software, firmware, or combinations thereof. In the above embodiments, the various steps or methods can be implemented in software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, any of the following technologies, or combinations thereof, can be used: discrete logic circuitry having logic gates for implementing logic functions upon data signals, application specific integrated circuits having appropriate combinational logic gates, programmable gate arrays (PGA), field programmable gate arrays (FPGA), and so on.

[0144] Those skilled in the art can understand that all or part of the steps of the method carried out by the above-mentioned embodiments can be instructed by a program to complete the relevant hardware, and the program can be stored in a computer readable storage medium. When the program is executed, it includes one of the steps of the method embodiment or a combination thereof.

[0145] The storage medium mentioned above can be a read-only memory, a magnetic disk or an optical disk, etc. Although the embodiments of the present application have been shown and described above, it should be understood that the above-mentioned embodiments are exemplary and cannot be understood as limiting the present application. Those skilled in the art can make changes, modifications, replacements and variations to the above-mentioned embodiments within the scope of the present application.

Claims

1. A method for frequency-voltage coordinated support control of offshore wind power flexible direct grid connection system, characterized in that, The method comprises the following steps: detecting the voltage and frequency state of the grid system and the grid connection point, judging whether a disturbance occurs, and determining whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupled frequency and voltage disturbance; when the system only has a single voltage disturbance, adjusting the reactive power output based on the voltage deviation amplitude, adopting a constant voltage control strategy to support the voltage, and keeping the active power output unchanged; when the system only has a single frequency disturbance, supporting the frequency of the grid connection system through virtual inertia control and droop control based on the frequency deviation, and ensuring that the reactive power output maintains the stability of the system; when the system has both a frequency and a voltage disturbance, adopting a frequency-voltage collaborative support control strategy, performing zoned control according to the voltage situation of the grid connection point, and supporting the voltage through active power priority or reactive power priority mode according to the voltage situation deviation of the grid connection point, and supporting the frequency of the system through collaborative frequency regulation or remote communication mode.

2. The offshore wind power flexible direct grid connection system frequency- voltage collaborative support control method according to claim 1, characterized in that, when the system has both a frequency and a voltage disturbance, adopting a frequency-voltage collaborative support control strategy, performing zoned control according to the voltage situation of the grid connection point, and supporting the voltage through active power priority or reactive power priority mode according to the voltage situation deviation of the grid connection point, and supporting the frequency of the system through collaborative frequency regulation or remote communication mode, specifically including the following scenarios: Scenario one: when the voltage of the grid connection point is in the range of 0.95p.u.-1.05p.u., the voltage belongs to normal fluctuation, no reactive power compensation is performed, the grid-side modular multilevel converter station adopts an active power priority mode, and the offshore wind farm and the MMC-HVDC adopt a collaborative frequency regulation control strategy to support the frequency of the system; Scenario two: when the voltage of the grid connection point is in the range of 1.05p.u.-1.1p.u. or 0.9p.u.-0.95p.u., the grid-side modular multilevel converter station continues to adopt the active power priority mode and adjusts the reactive power output to actively support the voltage of the grid connection point, and the offshore wind farm and the MMC-HVDC adopt a remote communication mode to support the frequency of the system; Scenario three: when the voltage of the grid connection point is lower than 0.9p.u. or higher than 1.1p.u., the grid-side modular multilevel converter station adopts a reactive power priority mode and reduces the active power output according to the actual situation to support the voltage of the grid connection point mainly through the reactive power output, and the offshore wind farm and the MMC-HVDC adopt a remote communication mode to support the frequency of the system.

3. The offshore wind power flexible direct grid connection system frequency-voltage collaborative support control method according to claim 2, characterized in that, The step of detecting the voltage and frequency state of the grid system and the grid connection point, judging whether a disturbance occurs, and determining whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupled frequency and voltage disturbance comprises: real-time acquisition of voltage and frequency data of the grid system and the grid connection point; comparison of the voltage and frequency data with historical operation data to analyze the fluctuation trend of the data; based on preset voltage and frequency thresholds, setting a multi-level alarm mechanism to judge whether the voltage or frequency exceeds the set threshold range; According to the feedback result of the multi-level alarm mechanism, it is determined whether a disturbance occurs, and it is judged whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupled disturbance of frequency and voltage.

4. The offshore wind power flexible direct grid connection system frequency-voltage collaborative support control method according to claim 2, characterized in that, When the system simultaneously occurs frequency and voltage disturbance, a frequency-voltage collaborative support control strategy is adopted, which is used to cope with simultaneous voltage disturbance and load disturbance, and at least includes the following working conditions: voltage drop accompanied by load surge, voltage drop accompanied by load reduction, voltage rise accompanied by load surge, voltage rise accompanied by load reduction.

5. The offshore wind power flexible direct grid connection system frequency- voltage collaborative support control method according to claim 2, characterized in that, The offshore wind farm and the MMC-HVDC adopt a collaborative frequency regulation control strategy to support the frequency of the system, and the collaborative frequency regulation control strategy to support the frequency of the system includes: By adding virtual inertia control in the offshore wind turbine, the inertia response of the conventional synchronous generator is simulated, and the specific model includes: where ΔP in is the frequency of the inertial support provided by the wind turbine through the release of the rotor kinetic energy, k in is the inertial control coefficient, and f is the frequency of the land-based AC system, typically 50 or 60 Hz. The offshore wind turbine can also simulate the governor response of the generator through droop control, and the specific model includes: ΔP dr = -k dr (f-f0); where ΔP dr represents the frequency support power provided by the wind turbine through droop control, k dr represents the droop control coefficient, f0and f represent the rated value and measured value of the onshore AC system frequency, respectively; To ensure power reserve, the offshore wind farm ensures power reserve through overspeed load shedding control, and the model of wind turbine load shedding power for different load shedding rates d% includes: P del = P MPPT (1 - d%) ; In the formula, P MPPT is the mechanical power output of the fan when operating in the MPPT state.

6. The offshore wind power flexible direct grid connection system frequency-voltage collaborative support control method according to claim 2, characterized in that, In the scenario one, the offshore wind farm and the MMC-HVDC adopt a collaborative frequency regulation control strategy as follows: When the grid-connected point frequency changes, the DC voltage of the MMC-HVDC adjusts with the frequency change, and the change of the DC voltage makes the wind farm receive frequency information from the onshore system; By adjusting the relationship between the DC voltage and the frequency, active support is realized for the system frequency; The relationship model between the DC voltage and the frequency includes: f W = K W K DC Δf + f W0 ; where K DC represents the DC voltage control coefficient, K W represents the WFMMC frequency control coefficient; f W0 represents the steady state frequency of the WFMMC, and Δf represents the terrestrial system frequency deviation, i.e. the difference between the frequency rating and the actual value.

7. The offshore wind power flexible direct grid connection system frequency-voltage co-support control method according to claim 2, characterized in that, In the scenario one and the scenario two, the grid-side modular multilevel converter station adopts an active power priority mode, i.e., a d-axis current priority mode, to preferentially guarantee active power output, and the maximum reactive power output is the maximum remaining value of the system capacity, which is realized by PI limiting in the current inner loop control; The model of the actual values of the d-axis and q-axis currents in the active power priority mode includes: where i d represents the d-axis current actual value, i q represents the q-axis current actual value, i qref and i dref respectively represent the q-axis d-axis current reference values, i max represents the maximum current that the inverter can withstand.

8. The offshore wind power flexible direct grid connection system frequency- voltage collaborative support control method according to claim 2, characterized in that, In the scenario two and the scenario three, the voltage support control strategy of the grid-side modular multilevel converter station on the grid-connected point includes: When the system is in normal operation, the grid-side modular multilevel converter station adopts U dc / Q control and works in the rated reactive power state; when a voltage disturbance occurs, the grid-side modular multilevel converter station switches to the constant AC voltage mode to provide voltage support for the system; The model of the voltage support control includes: In the formula, i qref represents the q-axis current reference value, k v represents the reactive droop coefficient, V acref and V ac respectively represent the rated value and the measured value of the AC voltage at the land AC system grid connection point, K p and K i respectively represent the reactive PI control coefficient, and s represents the commonly used representation of the integral element.

9. The offshore wind power flexible direct grid connection system frequency-voltage collaborative support control method according to claim 2, characterized in that, In the scenario two and the scenario three, the offshore wind farm and the MMC-HVDC support the frequency of the system through a remote communication mode, which directly transmits the frequency information of the GSMMC grid-connected point to the offshore wind farm by using communication technology, so that the offshore wind farm participates in active frequency support of the system.

10. The offshore wind power flexible direct grid connection system frequency- voltage collaborative support control method according to claim 2, characterized in that, In the scenario three, the grid-side modular multilevel converter station adopts a reactive power priority mode, i.e., a q-axis current priority mode, to preferentially guarantee reactive power output, and the maximum active power output is the maximum remaining value of the system capacity, which is realized by PI limiting in the current inner loop control; The model of the actual values of the d-axis and q-axis currents in the reactive power priority mode includes: The method according to any one of claims 1 to 10 is used, including: where i d represents the d-axis current actual value, i q represents the q-axis current actual value, i qref and i dref respectively represent the q-axis d-axis current reference values, i max represents the maximum current that the inverter can withstand.

11. A frequency-voltage coordinated support control device for offshore wind power flexible direct grid connection system, characterized in that, ​ a detection module configured to detect voltage and frequency states of a grid system and determine whether a disturbance occurs, and determine whether the disturbance is a single voltage disturbance, a single frequency disturbance, or a coupled frequency and voltage disturbance; a voltage support module configured to adjust reactive power output based on a voltage deviation amplitude when the system only experiences a single voltage disturbance, adopt a constant voltage control strategy to support voltage, and keep active power output unchanged; a frequency support module configured to, when the system only experiences a single frequency disturbance, support frequency of the grid system through virtual inertia control and droop control based on a frequency deviation, and ensure that reactive power output maintains system stability; a coordinated support module configured to, when the system experiences both frequency and voltage disturbances, adopt a frequency-voltage coordinated support control strategy, perform partition control according to voltage states of a grid point, and adopt active power priority or reactive power priority mode to support voltage according to the voltage state deviation of a grid-side modular multilevel converter station, and support frequency of the system through coordinated frequency regulation or a remote communication mode.

12. A computer device comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, The processor executes the computer program to implement the method of any one of claims 1-10.

13. A storage medium having stored thereon a computer program, characterized in that The computer program is executed by the processor to implement the method of any one of claims 1-10.

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