A DC-transmitted offshore wind and solar energy collection system and its control method

Abstract

The present invention discloses a direct current (DC)-transmitted offshore wind and solar energy collection system and its control method, which are applied to the field of power grid technology. The system includes a DC bus, a DC-DC converter, a DC line, an inverter, an offshore converter station, an AC grid, a plurality of DC power transmission units, and a plurality of new energy power generation units; the output end of each of the new energy power generation units is connected in series with each of the DC power transmission units; the DC power transmission unit transmits electric energy to the DC-DC converter via the DC bus, wherein the DC bus is connected in parallel with a plurality of the DC power transmission units; the output end of the DC-DC converter transmits electric energy to the receiving end of the inverter via the DC line; the inverter transmits electric energy to the AC grid via the offshore converter station. The system and method provided by the embodiments of the present invention can realize the control of grid power and ensure the stability of the system.

Description

Technical Field

[0001] The present invention relates to the field of power grid technology, and in particular to a direct current (DC)-transmitted offshore wind and solar energy collection system and a control method thereof. Background Art

[0002] With the transformation of the global energy structure and the continuous advancement of renewable energy technology, offshore wind power, as a clean and renewable form of energy, is gradually becoming an important part of the new energy system architecture.

[0003] In the existing technology, the cable impedance of offshore wind and solar new energy platforms is complex. When multiple DC power transmission units are connected in parallel directly to the DC bus, the line impedance differences and inconsistent parameters between lines will lead to unstable power grid power based on offshore wind and solar new energy.

[0004] It can be seen that how to control the power of wind and solar energy power grids to ensure the stability of the system has become a technical problem that needs to be urgently solved by those skilled in the art. Summary of the Invention

[0005] The present invention provides a direct current transmitted offshore wind and solar new energy collection system and a control method thereof, so as to solve the power instability of the offshore wind and solar new energy power grid, thereby realizing power control of the offshore wind and solar new energy power grid and ensuring the stability of the system.

[0006] To solve the above technical problems, an embodiment of the present invention provides an offshore wind and solar energy collection system for DC transmission, comprising:

[0007] DC bus, DC-DC converter, DC line, inverter, offshore converter station, AC grid, several DC power transmission units and several renewable energy power generation units;

[0008] The output end of each of the new energy power generation units is connected in series with each of the direct current transmission units;

[0009] The DC power transmission unit transmits electric energy to the DC-DC converter via the DC bus, wherein the DC bus is connected in parallel with a plurality of the DC power transmission units;

[0010] The output end of the DC-DC converter transmits electric energy to the receiving end of the inverter through the DC line;

[0011] The inverter transmits electric energy to the AC power grid through the offshore converter station.

[0012] Another embodiment of the present invention provides a power control method for an offshore wind-solar energy collection system based on direct current transmission, which is applied to the above-mentioned offshore wind-solar energy collection system based on direct current transmission. The power control method for an offshore wind-solar energy collection system based on direct current transmission includes:

[0013] Obtain the voltage and current values of the circuits on both sides of each DC power transmission unit in the target offshore wind and solar energy collection system;

[0014] constructing a plurality of impedance models based on the voltage values and the current values of the circuits on both sides of each DC power transmission unit;

[0015] Recursively updating each of the impedance models using a recursive least squares method to obtain an impedance estimate, wherein the impedance estimate includes at least an estimated value of a resistance component and an estimated value of an inductance component;

[0016] Obtaining a virtual impedance value of the target offshore wind-solar new energy collection system based on the estimated value of the resistance component and the estimated value of the inductance component;

[0017] Smoothing the virtual impedance value to obtain a stable virtual impedance value;

[0018] Introducing a circulating current suppression coefficient, inputting all the circulating current suppression coefficients and all the stable virtual impedance values into the constructed power control model to obtain a power control signal, wherein each circulating current suppression coefficient is obtained by processing the current value of each circuit;

[0019] A power control strategy generated by the power control signal is executed.

[0020] As one preferred solution, the step of constructing several impedance models based on the voltage and current values of the circuits on both sides of each DC power transmission unit includes:

[0021] applying a step voltage to the voltage value to obtain a current response; applying a current pulse to the current value to obtain a voltage response;

[0022] performing convolution processing on the voltage response and the current response to obtain a time domain impulse response;

[0023] The time domain impulse response is processed using Fourier transform to obtain the impedance model.

[0024] As one preferred solution, the recursive least squares method is used to recursively update each impedance model to obtain an impedance estimate, including:

[0025] Obtaining a preset impedance estimate and a preset covariance matrix;

[0026] The preset impedance estimation value, the preset covariance matrix and the current value obtained by real-time detection are input into the impedance model to obtain a gain vector;

[0027] The gain vector is recursively updated using a recursive least squares method to obtain the impedance estimation value.

[0028] As one preferred solution, obtaining the virtual impedance value of the target offshore wind-solar new energy collection system based on the estimated value of the resistance component and the estimated value of the inductance component includes:

[0029] obtaining a resistance component voltage based on the resistance component estimated value and the current value;

[0030] performing a differential process on the current value to obtain a differential current value;

[0031] Obtaining an inductance component voltage based on the inductance component estimated value and the differential current value;

[0032] The resistance component voltage and the inductance component voltage are processed using a Kalman gain function to obtain a virtual impedance value of the target offshore wind-solar new energy collection system.

[0033] As one preferred solution, the step of smoothing the virtual impedance value to obtain a stable virtual impedance value includes:

[0034] Determining step parameters based on the current value of the target offshore wind and solar energy new energy collection system;

[0035] The step parameter is introduced, and the virtual impedance value is smoothed using a step approximation method to obtain a stable virtual impedance value.

[0036] As one preferred solution, each of the circulating current suppression coefficients is obtained by processing the current value of each circuit, including:

[0037] Obtaining a circulating current value based on current values of circuits on both sides of each converter in the target offshore wind-solar new energy integration system;

[0038] Performing harmonic analysis on the circulating current value to obtain a circulating current signal to be suppressed;

[0039] Fuzzy logic control is performed on the circulation signal to obtain a circulation suppression coefficient.

[0040] As one preferred solution, the process of constructing the power control model includes:

[0041] obtaining an output power of the converter based on the voltage value and the current value;

[0042] Processing the output power of the converter using a droop control algorithm to obtain a power reference value;

[0043] obtaining a power error based on the output power of the converter and the power reference value;

[0044] Correcting the voltage value using the stable virtual impedance value, and obtaining a voltage reference value based on the correction result;

[0045] The circulating current suppression coefficient, the power reference value, and the voltage reference value are trained using a sequential Monte Carlo algorithm to obtain a power control model.

[0046] As one of the preferred solutions, after obtaining the power control model, the power control method of the offshore wind-solar new energy collection system based on DC transmission further includes:

[0047] The power control model is solved by Bayesian Nash equilibrium using simulated game theory to obtain a voltage-power equilibrium point, which reflects that the network flow is stable and meets the voltage constraints of the target offshore wind-solar new energy collection system.

[0048] As one preferred solution, after executing the power control strategy generated by the power control signal, the power control method of the offshore wind-solar new energy collection system based on DC transmission further includes:

[0049] Taking the average power of the target offshore wind-solar new energy collection system within a preset range of time as a benchmark, the Nyquist criterion is used to verify the stability of the power obtained after executing the power control strategy.

[0050] Compared with the prior art, the embodiments of the present invention have the following advantages:

[0051] The system of the present invention includes a DC bus, a DC-DC converter, a DC line, an inverter, an offshore converter station, an AC power grid, a plurality of DC power transmission units, and a plurality of new energy power generation units; the output end of each of the new energy power generation units is connected in series with each of the DC power transmission units; the DC power transmission unit transmits electric energy to the DC-DC converter via the DC bus, wherein the DC bus is connected in parallel with the plurality of DC power transmission units; the output end of the DC-DC converter transmits electric energy to the receiving end of the inverter via the DC line; and the inverter transmits electric energy to the AC power grid via the offshore converter station. Compared with the prior art, the present invention adds a DC transmission unit to the DC bus and the new energy power generation unit, and processes the electric energy when the electric energy enters the DC bus through the DC transmission unit. The processing process includes: obtaining the voltage value and current value of the circuit on both sides of each DC transmission unit in the target offshore wind-solar new energy collection system; constructing a number of impedance models based on the voltage value and the current value of the circuit on both sides of each DC transmission unit; using the recursive least squares method to recursively update each of the impedance models to obtain an impedance estimation value, wherein the impedance estimation value at least includes a resistance component estimation value and an inductance component estimation value; obtaining the virtual impedance value of the target offshore wind-solar new energy collection system based on the resistance component estimation value and the inductance component estimation value; smoothing the virtual impedance value to obtain a stable virtual impedance value; introducing a circulating current suppression coefficient, and inputting all the circulating current suppression coefficients and all the stable virtual impedance values into the constructed power control model to obtain a power control signal, wherein each circulating current suppression coefficient is obtained by processing the current value of each circuit; and executing the power control strategy generated by the power control signal. During this processing process, the DC power transmission unit builds a dynamic control basis by estimating the impedance and generating virtual impedance based on the real-time voltage and current values of the circuit. It then controls the power of the offshore wind and solar power grid through smoothing and circulating current suppression, ultimately achieving closed-loop stable control of the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is a structural diagram of an offshore wind and solar energy collection system based on DC transmission in one embodiment of the present invention;

[0053] Figure 2 It is a flow chart of a power control method of an offshore wind-solar new energy collection system based on DC transmission in one embodiment of the present invention.

[0054] Reference numerals:

[0055] Among them, 11. New energy power generation unit; 12. DC power transmission unit; 13. DC bus; 14. DC-DC converter; 15. DC line; 16. Inverter; 17. Offshore converter station; 18. AC power grid. DETAILED DESCRIPTION

[0056] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. The purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0057] In the description of the present invention, the terms "first," "second," "third," etc. are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, a feature specified as "first," "second," "third," etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0058] In the description of the present invention, it should be noted that, unless otherwise expressly specified and limited, the terms "installed", "connected" and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be a communication between the two components. The terms "vertical", "horizontal", "left", "right", "up", "down" and similar expressions used herein are for illustrative purposes only, and do not indicate or imply that the device or component referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0059] In describing the present invention, it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. Those skilled in the art will understand the specific meanings of the above terms in the present invention in specific circumstances.

[0060] With the transformation of the global energy mix and the rapid development of renewable energy, offshore wind power, as a clean and efficient form of energy, is gradually becoming a key area of energy development. In recent years, the scale of offshore wind power development has continued to expand, with wind farms located farther and farther offshore, and both the capacity of individual turbines and the total installed capacity of wind farms have also been increasing. In this context of large-scale offshore wind power development, wind farms are typically located tens or even hundreds of kilometers offshore, and the capacity of individual wind farms is relatively large, often requiring the power from multiple wind farms to be aggregated and transmitted using flexible direct current transmission technology.

[0061] In the existing technology, the cable impedance of offshore wind and solar new energy platforms is complex. When multiple DC power transmission units are connected in parallel directly to the DC bus, the line impedance differences and inconsistent parameters between lines will lead to unstable power grid power based on offshore wind and solar new energy.

[0062] To this end, the power of the wind and solar energy grid is controlled to ensure the stability of the system. An embodiment of the present invention provides an offshore wind and solar energy collection system with DC transmission. For details, please refer to Figure 1 , Figure 1 The figure shows a schematic structural diagram of an offshore wind and solar energy collection system for DC transmission in one embodiment of the present invention. The system includes:

[0063] A DC bus 13 , a DC-DC converter 14 , a DC line 15 , an inverter 16 , an offshore converter station 17 , an AC power grid 18 , several DC power transmission units 12 and several new energy power generation units 11 .

[0064] The connection relationship between the various components in the system includes: the output end of each of the new energy power generation units 11 is connected in series with each of the DC power transmission units 12; the DC power transmission unit transmits electric energy to the DC-DC converter 14 through the DC bus 13, wherein the DC bus 13 is connected in parallel with several of the DC power transmission units 12; the output end of the DC-DC converter 14 transmits electric energy to the receiving end of the inverter 16 through the DC line 15; the inverter 16 transmits electric energy to the AC power grid 18 through the offshore converter station 17.

[0065] Specifically, the present invention transmits the current obtained by the new energy power generation unit 11 such as wind power and photovoltaic power to the DC bus 13 through the DC transmission unit 12. In the prior art, since the line impedance of each new energy power generation unit 11 is different, unstable DC power is generated, resulting in unstable power input into the DC bus 13. Therefore, the present invention processes the current and voltage of the DC transmission unit 12. Specifically, the DC transmission unit constructs a dynamic control basis by performing impedance estimation and virtual impedance generation on the real-time voltage value and real-time current value of the circuit, and then controls the power of the offshore wind and solar new energy power grid through smoothing processing and circulating current suppression, thereby finally achieving closed-loop stable control of the power grid.

[0066] The DC-DC converter 14 is used to increase the DC voltage level and efficiently transmit it to the centralized converter station through the DC line.

[0067] The function of the inverter 15 is to convert high-voltage direct current into high-frequency alternating current and input it into the AC grid.

[0068] Another embodiment of the present invention provides a power control method for an offshore wind and solar energy collection system based on DC transmission. For details, see Figure 2 , Figure 2 The figure shows a flow chart of a power control method for an offshore wind-solar energy collection system based on DC transmission in one embodiment of the present invention, the method comprising:

[0069] S1: Obtain the voltage and current values of the circuits on both sides of each DC power transmission unit in the target offshore wind and solar energy collection system;

[0070] S2: constructing a plurality of impedance models based on the voltage values and the current values of the circuits on both sides of each DC power transmission unit;

[0071] S3: recursively updating each of the impedance models using a recursive least squares method to obtain an impedance estimate, wherein the impedance estimate includes at least an estimated value of a resistance component and an estimated value of an inductance component;

[0072] S4: Obtaining a virtual impedance value of the target offshore wind-solar new energy integration system based on the estimated value of the resistance component and the estimated value of the inductance component;

[0073] S5: Smoothing the virtual impedance value to obtain a stable virtual impedance value;

[0074] S6: Introducing a circulating current suppression coefficient, inputting all the circulating current suppression coefficients and all the stable virtual impedance values into the constructed power control model to obtain a power control signal, wherein each circulating current suppression coefficient is obtained by processing the current value of each circuit;

[0075] S7: Execute the power control strategy generated by the power control signal.

[0076] The above method controls the power of the offshore wind and solar energy collection system that transmits DC power based on the DC power transmission unit as the execution entity.

[0077] In step S1, the voltage and current values of the circuits on both sides of each DC power transmission unit in the target offshore wind and solar energy collection system are monitored in real time. The voltage and current values can be used to calculate the power and impedance characteristics, analyze the circulating current, load changes and dynamic responses, and reflect the real-time operating status of the power grid.

[0078] In step S2, based on the voltage value and the current value of the circuits on both sides of each DC power transmission unit, a plurality of impedance models are constructed, including:

[0079] A step voltage is applied to the voltage value to obtain a voltage response; a current pulse is applied to the current value to obtain a current response; a convolution process is performed on the voltage response and the current response to obtain a time domain impulse response; and the time domain impulse response is processed using Fourier transform to obtain the impedance model.

[0080] Typically, step voltage or current pulses are artificially applied to stimulate the dynamic response of the power grid, making it easier to extract the impedance characteristics of the system.

[0081] Specifically, the voltage amplitude is instantaneously changed on the DC bus or AC side, and the current response after the voltage change is recorded. The step response reflects the low-frequency impedance characteristics of the system. A short high-frequency current pulse (such as a rectangular wave) is injected into the circuit, and the transient response of the voltage is recorded. The current pulse response reflects the high-frequency impedance characteristics.

[0082] The convolution operation establishes the time-domain relationship between voltage and current to characterize the system's dynamic impedance characteristics. Specifically, the time axis of the step / pulse excitation is aligned to eliminate measurement delays.

[0083] The convolution operation combines the responses to two stimuli in the time domain, eliminating the limitations of a single test and improving model accuracy. By combining the stimulus signal (step or pulse) with the response signal through convolution, the system's time-domain impulse response is derived and characterized. The impulse response represents the system's dynamic behavior to a transient perturbation and directly reflects the time-domain correlation between voltage and current.

[0084] Converting the time-domain impulse response into a frequency-domain impedance model reveals the impedance behavior of the system at different frequencies, while locating the resonant frequency or high-frequency noise-sensitive areas, providing a basis for stability analysis and control design.

[0085] There are several circuits in the target offshore wind and solar energy collection system. An impedance model is constructed based on the voltage and current values of each circuit.

[0086] In step S3, each of the impedance models is recursively updated using the recursive least squares method to obtain an impedance estimate, wherein the impedance estimate includes at least a resistance component estimate and an inductance component estimate, specifically including: obtaining a preset impedance estimate and a preset covariance matrix; inputting the preset impedance estimate, the preset covariance matrix and the current value obtained by real-time detection into the impedance model to obtain a gain vector; and recursively updating the gain vector using the recursive least squares method to obtain the impedance estimate.

[0087] First, the impedance estimation value is initialized, such as the resistance component estimation value and the inductance component estimation value are both zero, and the preset covariance matrix represents the credibility of the initial parameter estimation. Generally, the covariance matrix is initialized as a diagonal matrix, and the diagonal elements reflect the confidence of the initial parameter error.

[0088] The real-time detected current value is input into the impedance model, and the voltage response is predicted in combination with the current impedance estimate. The gain vector is calculated based on the deviation between the predicted voltage and the actual measured voltage, the covariance matrix, and the current data. The larger the gain, the more significant the impact of the current data on the parameter correction.

[0089] Among them, the gain vector is mainly used to balance the weights of "new measurement data" and "historical estimated values", determine the influence of current data on parameter update, and at the same time reduce the interference of measurement noise on parameter estimation through the gain vector.

[0090] The gain vector and prediction error (the difference between the actual voltage and the model-predicted voltage) are recursively updated using the recursive least squares method to correct the resistance and inductance components. If the current data indicates an increase in system inductance (such as a sudden load change), the algorithm automatically increases the impedance estimate while reducing the value of the covariance matrix, indicating that the confidence in the parameter estimate has increased. If the data noise is large, the covariance matrix retains a high value to maintain the flexibility of parameter updates.

[0091] Feed the updated resistance and inductance back into the impedance model for virtual impedance generation, circulating current suppression, or power distribution optimization.

[0092] Through this process, the recursive least squares method provides real-time and adaptive impedance parameter estimation capabilities for offshore wind and solar power grids, which is the core technical support for ensuring system stability and power control.

[0093] In step S4, the virtual impedance value of the target offshore wind-solar new energy collection system is obtained based on the estimated value of the resistance component and the estimated value of the inductance component, specifically including obtaining the resistance component voltage based on the estimated value of the resistance component and the current value; differentiating the current value to obtain the differential current value; obtaining the inductance component voltage based on the estimated value of the inductance component and the differential current value; and processing the resistance component voltage and the inductance component voltage using the Kalman gain function to obtain the virtual impedance value of the target offshore wind-solar new energy collection system.

[0094] In offshore wind and solar renewable energy power grids, the process of generating virtual impedance values based on the estimated values of resistance and inductance components aims to dynamically simulate the additional impedance characteristics to optimize system stability and control performance.

[0095] First, the real-time detected current value is multiplied by the resistance estimate updated by the recursive least squares method to obtain the resistance component voltage. The resistance component voltage reflects the ohmic loss (such as cable heating) generated when the current flows through the line or equipment. It is directly related to the real-time current value and the resistance parameter. It provides a static (low-frequency) voltage component for the construction of virtual impedance, which is used to offset the negative impact of the actual resistance.

[0096] The real-time current signal is then digitally differentiated to calculate the rate of change of the current difference between the current at the current moment and the previous moment, yielding the differential current value. Preferably, the digital differentiation is performed using the backward differentiation method. In this process, the current differential value reflects the instantaneous rate of change of the current, such as current fluctuations caused by sudden load changes or switching operations, and is used to characterize the dynamic characteristics of the system. Simultaneously, the differential operation amplifies high-frequency noise and fast transient processes, providing input for the inductive component voltage.

[0097] The differential current value is multiplied by the estimated inductance value to obtain the inductance component voltage. The inductance component voltage reflects the system's inertial response to current changes, that is, it suppresses current mutations. By simulating the inductance voltage drop, it offsets the voltage fluctuation caused by the actual inductance and improves the dynamic stability of the system.

[0098] The resistance component voltage and the inductance component voltage are processed using a Kalman gain function to obtain a virtual impedance value for the target offshore wind-solar renewable energy integration system. The Kalman gain function dynamically adjusts the contribution weights of the resistance component voltage and the inductance component voltage based on the system noise level and measurement reliability. The Kalman gain function then eliminates measurement noise (such as sensor error) and model error through filtering to output a more accurate virtual impedance value.

[0099] However, this process places high demands on computing power, and the algorithm needs to complete calculations within milliseconds to meet the rapid response requirements of power electronic control.

[0100] Through this process, the virtual impedance value becomes the core bridge connecting parameter estimation and closed-loop control, significantly improving the dynamic performance and robustness of offshore wind and solar power grids.

[0101] In step S5, the virtual impedance value is smoothed to obtain a stable virtual impedance value, specifically including determining a step parameter based on the current value of the target offshore wind-solar new energy collection system; introducing the step parameter, and using the step approximation method to smooth the virtual impedance value to obtain a stable virtual impedance value.

[0102] The absolute value or rate of change of the current is calculated in real time, and the current characteristics are mapped to step parameters according to preset rules. The rule is: the higher the current rate of change, the smaller the step parameter. When the current changes dramatically, the step parameter is small, allowing for more precise step adjustments and quickly tracking changes in virtual impedance. When the current is stable, the step parameter is large, reducing the adjustment frequency and preventing frequent fluctuations from interfering with the control signal.

[0103] According to the size or change rate of the real-time current value, the step size of the step parameter is dynamically adjusted to balance the smoothing effect and response speed. Based on the difference between the virtual impedance target value to be tracked in the upstream control and the current virtual impedance value, the number of steps required is determined and the impedance is updated step by step.

[0104] If the current changes suddenly midway, such as the step parameters are dynamically reduced due to current fluctuations, the remaining number of steps and step length are recalculated to ensure smoothness.

[0105] In step S5, the continuous virtual impedance target value is decomposed into multiple discrete step segments. The impedance value in each segment is kept constant and gradually approaches the target value. Through the step transition, the virtual impedance is prevented from directly jumping to the target value, thereby preventing instantaneous impact on the power controller or modulation signal.

[0106] In step S6, a circulating current suppression coefficient is introduced, and all the circulating current suppression coefficients and all the stable virtual impedance values are input into the constructed power control model to obtain a power control signal, wherein each circulating current suppression coefficient is obtained by processing the current value of each circuit.

[0107] Each of the circulating current suppression coefficients is obtained by processing the current value of each circuit, specifically including:

[0108] Based on the current values of the circuits on both sides of each converter in the target offshore wind-solar new energy collection system, a circulating current value is obtained; harmonic analysis is performed on the circulating current value to obtain a circulating current signal to be suppressed; fuzzy logic control is performed on the circulating current signal to obtain a circulating current suppression coefficient.

[0109] Specifically, the current values on both sides of the converter (such as the upper and lower bridge arms or different converter station exits) are monitored in real time. By measuring the current difference between the circuits on both sides of the converter, that is, the circulating current value, which directly reflects the size of the circulating current, the circulating current value is subjected to fast Fourier transform (FFT) or bandpass filtering to extract specific frequency components, and the amplitude and phase of the target harmonic are recorded as input for fuzzy logic control. The input quantity is converted into fuzzy linguistic variables, and the membership function is defined. If the circulating current amplitude is "large" and the rate of change is "fast", the suppression coefficient is greatly increased. If the circulating current amplitude is "small", the current suppression coefficient is maintained.

[0110] Preferably, the fuzzy output is converted into an accurate circulation suppression coefficient by a center of gravity method or a maximum membership method.

[0111] Harmonic analysis can accurately separate the circulating current components that need to be suppressed, avoiding "one-size-fits-all" suppression, and fuzzy logic automatically adapts to different operating conditions (such as sudden changes in wind speed and load fluctuations) without the need for manual parameter readjustment. Through this process, the circulating current suppression coefficient achieves closed-loop control from "passive detection" to "active suppression", significantly improving the reliability and power quality of offshore wind and solar power grids.

[0112] Specifically, the process of building the power control model includes:

[0113] The output power of the converter is obtained based on the voltage value and the current value; the output power of the converter is processed using a droop control algorithm to obtain a power reference value; a power error is obtained based on the output power of the converter and the power reference value; the voltage value is corrected using the stable virtual impedance value, and a voltage reference value is obtained based on the correction result; the circulating current suppression coefficient, the power reference value and the voltage reference value are trained using a sequential Monte Carlo algorithm to obtain a power control model.

[0114] Specifically, the converter's output power is determined from its voltage and current values. The voltage droop slope is then defined as the voltage changes with power. Based on the current power and the droop curve, a target power reference value is generated. During this process, the power load of each converter is dynamically distributed within the grid based on the preset droop characteristics, preventing single-point overload.

[0115] The circulating current suppression coefficient, the power reference value, and the voltage reference value are trained using the Sequential Monte Carlo algorithm to obtain a power control model. Sequential Monte Carlo (SMC) is a probabilistic algorithm based on random sampling and dynamic updates. It is primarily used for state estimation and parameter optimization in dynamic system modeling. Its core concept is to gradually approximate the true state or optimal solution of a complex system through an iterative "sampling-evaluation-resampling" process. Therefore, the power control model constructed using the Sequential Monte Carlo algorithm can obtain the optimal power control signal.

[0116] After obtaining the power control model, the Bayesian Nash equilibrium of the power control model is solved by using simulation game to obtain the voltage-power equilibrium point, which reflects that the network flow is stable and meets the voltage constraints of the target offshore wind-solar new energy collection system.

[0117] This process replaces centralized control with game equilibrium to adapt to the decentralized decision-making needs of offshore power grids. In addition, the Bayesian framework effectively handles environmental interference and incomplete information problems. The equilibrium point naturally integrates voltage constraints, avoiding the limitations of manual weighting of constraints in traditional optimization.

[0118] Among them, simulated game is a process of simulating the strategic interaction of multiple intelligent agents (such as multiple converters in the power grid) through computer modeling and simulation technology. Participants (converters) dynamically adjust their own strategies (such as power output, voltage reference value) according to preset rules and real-time data. The goal is to achieve a global optimal or equilibrium state.

[0119] Bayesian Nash equilibrium is an equilibrium concept in game theory, which describes the state in which all participants choose the optimal strategy based on probabilistic beliefs in a scenario with multiple participants and incomplete information (such as unknown strategies of other participants or the status of the power grid).

[0120] In summary, simulated games are tools for strategy verification and multi-agent coordination, revealing the complex interaction effects under decentralized control through simulation; Bayesian Nash equilibrium is the target state in game theory, ensuring that the strategies of each converter are both stable and compliant under incomplete information; the combination of the two provides an intelligent autonomous coordination mechanism for offshore wind and solar power grids, taking into account efficiency, stability and security.

[0121] A power control strategy generated by the power control signal is executed.

[0122] After executing the power control strategy generated by the power control signal, the stability of the power obtained after executing the power control strategy is verified using the Nyquist criterion based on the average power of the target offshore wind-solar new energy collection system within a preset range of time.

[0123] After executing the power control strategy, the process of verifying the system stability through the Nyquist criterion aims to ensure that the offshore wind and solar renewable energy power grid maintains stable operation during dynamic regulation and avoid power oscillations or voltage collapse.

[0124] Among them, the Nyquist Criterion is a classic frequency-domain method used in control engineering to determine the stability of closed-loop systems. It predicts whether instability will occur after the loop is closed by analyzing the system's open-loop frequency response (i.e., the characteristics when feedback is not closed).

[0125] In offshore wind and solar power grids, it helps engineers quickly identify potential oscillation risks in power control strategies and ensure system robustness through parameter tuning.

[0126] Compared with the prior art, the embodiments of the present invention have the following advantages:

[0127] The system of the present invention includes a DC bus, a DC-DC converter, a DC line, an inverter, an offshore converter station, an AC power grid, a plurality of DC power transmission units, and a plurality of new energy power generation units; the output end of each of the new energy power generation units is connected in series with each of the DC power transmission units; the DC power transmission unit transmits electric energy to the DC-DC converter via the DC bus, wherein the DC bus is connected in parallel with the plurality of DC power transmission units; the output end of the DC-DC converter transmits electric energy to the receiving end of the inverter via the DC line; and the inverter transmits electric energy to the AC power grid via the offshore converter station. Compared with the prior art, the present invention adds a DC transmission unit to the DC bus and the new energy power generation unit, and processes the electric energy when the electric energy enters the DC bus through the DC transmission unit. The processing process includes: obtaining the voltage value and current value of the circuit on both sides of each DC transmission unit in the target offshore wind-solar new energy collection system; constructing a number of impedance models based on the voltage value and the current value of the circuit on both sides of each DC transmission unit; using the recursive least squares method to recursively update each of the impedance models to obtain an impedance estimation value, wherein the impedance estimation value at least includes a resistance component estimation value and an inductance component estimation value; obtaining the virtual impedance value of the target offshore wind-solar new energy collection system based on the resistance component estimation value and the inductance component estimation value; smoothing the virtual impedance value to obtain a stable virtual impedance value; introducing a circulating current suppression coefficient, and inputting all the circulating current suppression coefficients and all the stable virtual impedance values into the constructed power control model to obtain a power control signal, wherein each circulating current suppression coefficient is obtained by processing the current value of each circuit; and executing the power control strategy generated by the power control signal. During this processing process, the DC power transmission unit builds a dynamic control basis by estimating the impedance and generating virtual impedance based on the real-time voltage and current values of the circuit. It then controls the power of the offshore wind and solar power grid through smoothing and circulating current suppression, ultimately achieving closed-loop stable control of the grid.

[0128] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A power control method for an offshore wind and solar energy collection system based on direct current transmission, characterized in that: Applicable to an offshore wind and solar energy collection system for DC transmission, the offshore wind and solar energy collection system for DC transmission includes: DC bus, DC-DC converter, DC line, inverter, offshore converter station, AC grid, several DC power transmission units and several renewable energy power generation units; The output end of each of the new energy power generation units is connected in series with each of the direct current transmission units; The DC power transmission unit transmits electric energy to the DC-DC converter via the DC bus, wherein the DC bus is connected in parallel with a plurality of the DC power transmission units; The output end of the DC-DC converter transmits electric energy to the receiving end of the inverter through the DC line; The inverter transmits electric energy to the AC power grid through the offshore converter station; The power control method of the offshore wind and solar energy collection system based on DC transmission includes: Obtain the voltage and current values of the circuits on both sides of each DC power transmission unit in the target offshore wind and solar energy collection system; constructing a plurality of impedance models based on the voltage values and the current values of the circuits on both sides of each DC power transmission unit; Recursively updating each of the impedance models using a recursive least squares method to obtain an impedance estimate, wherein the impedance estimate includes at least an estimated value of a resistance component and an estimated value of an inductance component; Obtaining a virtual impedance value of the target offshore wind-solar new energy collection system based on the estimated value of the resistance component and the estimated value of the inductance component; Smoothing the virtual impedance value to obtain a stable virtual impedance value; Introducing a circulating current suppression coefficient, inputting all the circulating current suppression coefficients and all the stable virtual impedance values into the constructed power control model to obtain a power control signal, wherein each circulating current suppression coefficient is obtained by processing the current value of each circuit, including: Obtaining a circulating current value based on current values of circuits on both sides of each converter in the target offshore wind-solar new energy integration system; Performing harmonic analysis on the circulating current value to obtain a circulating current signal to be suppressed; Performing fuzzy logic control on the circulation signal to obtain a circulation suppression coefficient; A power control strategy generated by the power control signal is executed.

2. The power control method of the offshore wind and solar energy collection system based on DC transmission according to claim 1, characterized in that: The step of constructing a plurality of impedance models based on the voltage and current values of the circuits on both sides of each DC power transmission unit includes: applying a step voltage to the voltage value to obtain a current response; applying a current pulse to the current value to obtain a voltage response; performing convolution processing on the voltage response and the current response to obtain a time domain impulse response; The time domain impulse response is processed using Fourier transform to obtain the impedance model.

3. The power control method of the offshore wind and solar energy collection system based on DC transmission according to claim 1, characterized in that: The recursive least square method is used to recursively update each impedance model to obtain an impedance estimate, including: Obtaining a preset impedance estimate and a preset covariance matrix; The preset impedance estimation value, the preset covariance matrix and the current value obtained by real-time detection are input into the impedance model to obtain a gain vector; The gain vector is recursively updated using a recursive least squares method to obtain the impedance estimation value.

4. The power control method of the offshore wind and solar energy collection system based on DC transmission according to claim 1, characterized in that: Obtaining the virtual impedance value of the target offshore wind-solar new energy collection system based on the estimated value of the resistance component and the estimated value of the inductance component includes: obtaining a resistance component voltage based on the resistance component estimated value and the current value; performing a differential process on the current value to obtain a differential current value; Obtaining an inductance component voltage based on the inductance component estimated value and the differential current value; The resistance component voltage and the inductance component voltage are processed using a Kalman gain function to obtain a virtual impedance value of the target offshore wind-solar new energy collection system.

5. The power control method of the offshore wind and solar energy collection system based on DC transmission according to claim 1, characterized in that: The step of smoothing the virtual impedance value to obtain a stable virtual impedance value includes: Determining step parameters based on the current value of the target offshore wind and solar energy new energy collection system; The step parameter is introduced, and the virtual impedance value is smoothed using a step approximation method to obtain a stable virtual impedance value.

6. The power control method of the offshore wind and solar energy collection system based on DC transmission according to claim 1, characterized in that: The process of constructing the power control model includes: obtaining an output power of the converter based on the voltage value and the current value; Processing the output power of the converter using a droop control algorithm to obtain a power reference value; obtaining a power error based on the output power of the converter and the power reference value; Correcting the voltage value using the stable virtual impedance value, and obtaining a voltage reference value based on the correction result; The circulating current suppression coefficient, the power reference value, and the voltage reference value are trained using a sequential Monte Carlo algorithm to obtain a power control model.

7. The power control method of the offshore wind and solar energy collection system based on DC transmission according to claim 1, characterized in that: After obtaining the power control model, the power control method of the offshore wind-solar new energy collection system based on DC transmission further includes: The power control model is solved by Bayesian Nash equilibrium using simulated game theory to obtain a voltage-power equilibrium point, which reflects that the network flow is stable and meets the voltage constraints of the target offshore wind-solar new energy collection system.

8. The power control method of the offshore wind and solar energy collection system based on DC transmission according to claim 1, characterized in that: After executing the power control strategy generated by the power control signal, the power control method of the offshore wind-solar new energy collection system based on direct current transmission further includes: Taking the average power of the target offshore wind-solar new energy collection system within a preset range of time as a benchmark, the Nyquist criterion is used to verify the stability of the power obtained after executing the power control strategy.

Images