Maximum tidal energy extraction method based on fixed time extended state observer

Abstract

The invention discloses a maximum tidal energy extraction method based on a fixed time extended state observer. The method comprises the following steps: building a tidal energy capture model; linearizing the tidal energy capture model by adopting a feedback linearization theory to obtain a tidal energy capture linear control model; performing disturbance term estimation in a real-time state by using a fixed time extended state observer so as to compensate the negative influence of actual disturbance on the tidal energy capture linear control model; an optimal rotating speed tracking control strategy based on a maximum power point tracking control method is adopted, a pulse width modulation converter is utilized, the actual rotating speed of the permanent magnet synchronous generator is controlled to track the optimal rotating speed, and the optimal rotating speed is determined based on the actual flow speed and the optimal tip speed ratio formula; and controlling a stator d-axis actual current of the permanent magnet synchronous generator to track a stator current reference value, and realizing extraction of maximum tidal energy.

Description

Technical Field

[0001] The present invention relates to the technical field of tidal energy extraction, and in particular to a maximum tidal energy extraction method based on a fixed-time extended state observer. Background Art

[0002] Compared with vertical-axis tidal generators, horizontal-axis tidal generators have a stronger ability to capture tidal energy and are more suitable for large-scale megawatt-level tidal generators.

[0003] Tidal power generation systems and wind power generation systems (WPGS) are similar in structure, principle and technology. Similarly, when selecting a horizontal axis tidal generator, a permanent magnet synchronous generator (PMSG) or a doubly fed induction generator (DFIGs) can be used. Both generators are controlled by an inverter, have the characteristics of variable speed operation and high efficiency, and can be connected to the grid through the inverter. Tidal power generation systems based on DFIGs require a gearbox to match the speed of the tidal turbine and the generator. Previous studies have shown that gearboxes account for a high proportion of system failures. PMSG can be directly connected to the tidal turbine. Considering the harsh marine environment and subsequent maintenance costs, PMSG is more suitable for horizontal axis tidal power generation systems.

[0004] The MPPT control strategy used in tidal power generation systems is similar to the traditional MPPT control strategy used in WPGS. The main control strategies include the ramp method, the optimal power method, and the optimal blade tip speed ratio (OBTSR) method.

[0005] Traditional PI (TPI) controllers are widely used in industry due to their simple structure and high reliability. However, due to the time-varying operating conditions and high nonlinearity of tidal power generation systems, TPI controllers are only effective near the equilibrium point, making it difficult to achieve the maximum power point. Therefore, this type of controller cannot fully address the characteristics of tidal energy, such as turbulence and expansion effects, which may degrade the performance of tidal power generation systems. At the same time, maintaining the consistency of tidal power generation system parameters is challenging, and parameter variations may further degrade controller performance. Summary of the Invention

[0006] Purpose of the Invention: To address the problem that existing controllers cannot fully account for the characteristics of tidal energy, which can degrade the performance of tidal power generation systems, this invention proposes a maximum tidal energy extraction method based on a fixed-time extended state observer. This method, focusing on a horizontal-axis tidal power generation system based on a permanent magnet synchronous generator, employs an OBTSR control strategy based on the MPPT control method to maximize the conversion of tidal energy into electrical energy while significantly reducing the operating and maintenance costs of the tidal power generation system.

[0007] Technical solution: A maximum tidal energy extraction method based on a fixed-time extended state observer, comprising the following steps:

[0008] Step 1: Build a tidal energy capture model; the tidal energy capture model includes: a tidal turbine, a permanent magnet synchronous generator, a machine-side converter, a grid-side converter, a filter device, a transformer, and a power grid; the tidal turbine extracts kinetic energy from the tide and converts it into mechanical energy, which is transmitted to the permanent magnet synchronous generator, thereby driving the permanent magnet synchronous generator rotor to rotate and convert the mechanical energy into electrical energy; the permanent magnet synchronous generator cooperates with the machine-side converter to convert the mechanical energy output by the tidal turbine into electrical energy, which is then connected to the power grid through the grid-side converter, filter device, and transformer in sequence;

[0009] Step 2: Linearize the tidal energy capture model using feedback linearization theory to obtain a tidal energy capture linear control model;

[0010] Step 3: Use the fixed-time extended state observer to estimate the disturbance term in real time to compensate for the negative impact of the actual disturbance on the tidal energy capture linear control model;

[0011] Step 4: Adopting the optimal speed tracking control strategy based on the maximum power point tracking control method, using the pulse width modulation converter, control the actual speed of the permanent magnet synchronous generator to track the optimal speed, the optimal speed is determined based on the actual flow velocity and the optimal tip speed ratio formula; control the actual stator d-axis current of the permanent magnet synchronous generator to track the stator current reference value to ensure the extraction of maximum tidal energy.

[0012] Furthermore, the tidal energy capture model is expressed as:

[0013] For a tidal turbine, the mechanical power captured is expressed as:

[0014]

[0015] Where, P t represents the mechanical power captured by the tidal turbine, ρ represents the density of seawater, A represents the area of the tidal turbine blades rotating one circle, R is the impeller radius of the tidal turbine, V represents the water velocity, C p (β,λ) represents the power coefficient of the tidal turbine, β represents the blade pitch angle, and λ represents the tip speed ratio;

[0016] Among them, the power coefficient of the tidal turbine is expressed as:

[0017]

[0018] Where, Indicates intermediate parameters:

[0019]

[0020] λ is expressed as:

[0021]

[0022] Where, ω t Indicates the speed of the permanent magnet synchronous generator;

[0023] The dynamic model of the permanent magnet synchronous generator is described by Park transformation in the dq coordinate system, which is expressed as:

[0024]

[0025] T g =p[(L d -L q )i sd i sq +i sq Φ]

[0026] Where V sd 、V sq are the d-axis stator voltage and the q-axis stator voltage, respectively, i sd 、i sq are the d-axis stator current and the q-axis stator current, R s is the stator resistance, L d is the stator d-axis inductance, p is the number of pole pairs, Φ is the flux linkage, T g is the electromagnetic torque, L q represents the stator q-axis inductance;

[0027] The motion equation of the permanent magnet synchronous generator is expressed as:

[0028]

[0029] Where J is the total moment of inertia, T t is the mechanical torque, expressed as:

[0030]

[0031] Furthermore, in step 2, the tidal energy capture model is linearized using the feedback linearization theory to obtain a tidal energy capture linear control model, which is expressed as:

[0032] Select V sd and V sq As the input of the linear control model for tidal energy capture, i sd and ω t As the output of the permanent magnet synchronous generator, the dynamic model of the permanent magnet synchronous generator is rewritten as:

[0033]

[0034] Where, represents the stator d-axis current i sd The first derivative of represents the stator q-axis current i sq The first derivative of

[0035] By linearizing the above input / output, we get:

[0036]

[0037] Where, represents the stator d-axis current i sd The first derivative of Represents the second derivative of the permanent magnet synchronous generator speed;

[0038]

[0039] Furthermore, in step 3, the use of a fixed-time extended state observer to perform disturbance term estimation in real time to compensate for the negative impact of actual disturbances on the tidal energy capture linear control model specifically includes:

[0040] Define the disturbance terms Ψ1(x) and Ψ2(x), which are expressed as:

[0041]

[0042] in, They are J, Φ, D1(x), D2(x), and L d , L q The nominal value of

[0043] Using the perturbation terms Ψ1(x) and ψ2(x), we can Rewritten as:

[0044]

[0045] Divide Ψ1(x) and ψ2(x) into differentiable and non-differentiable parts:

[0046]

[0047] Where, and represents the differentiable part of Ψ1(x) and Ψ2(x), and represents the non-differentiable part;

[0048] represents the first derivative of Ψ1(x), Denote the first-order derivative of Ψ2(x), let The following assumptions are made:

[0049] When the new state vector z 11 =i sd , Define z 21 =ω t , Indicates the speed of the permanent magnet synchronous generator ω t The first derivative of Export the following two subsystems:

[0050]

[0051] Where, Indicates the newly defined state quantity;

[0052] Design two FESOs to estimate z 12 and z 23 , expressed as:

[0053]

[0054] Where, Both represent the corresponding estimated values, which are estimated by the observers designed by FESO1 and FESO2, x 11 、X 12 、X 21 、X 22 、X 23 is a positive constant;

[0055]

[0056] In the formula, the symbol ○ represents the combination of functions, and the function and Expressed as:

[0057]

[0058] The estimated disturbance is used to compensate the actual disturbance. The control law of subsystems S1 and S2 based on FESO is expressed as:

[0059]

[0060] Furthermore, in step 4, the optimal speed tracking control strategy based on the maximum power point tracking control method is adopted, and a pulse width modulation converter is used to control the actual speed of the permanent magnet synchronous generator to track the optimal speed, wherein the optimal speed is determined based on the actual flow velocity and the optimal tip speed ratio formula; the actual stator d-axis current of the permanent magnet synchronous generator is controlled to track the stator current reference value to ensure the extraction of maximum tidal energy. The specific operations include:

[0061] The optimal speed is expressed as:

[0062]

[0063] Where λ opt Indicates the power coefficient C of the tidal turbine p At maximum C pmax The corresponding tip speed ratio is: V represents the actual flow velocity, and R represents the impeller radius;

[0064] Control y2 = ω t Track its optimal reference y 2r =ω ref =vλ opt / R, at the same time, control y1=i sd Tracking 1r =i sdr =0;

[0065] To achieve GUUB trajectory error, v1 and v2 are defined as:

[0066]

[0067] Among them, k 11 、k 21 and k 22 is the positive control gain;

[0068] Physical variables including flux, inductance, current, mechanical speed, and total inertia are used to represent the FESOC law. The dynamic model of the PMSG in the dq coordinate system is represented by the FESOC law:

[0069]

[0070] Where, L d0 represents the nominal value of the stator d-axis inductance, Represents the first derivative of the stator d-axis current reference value;

[0071]

[0072] Where, L q0 Indicates the nominal value of the stator q-axis inductance;

[0073] The pulse width modulation converter obtains the stator voltage V sd and V sq , control the actual speed of the permanent magnet synchronous generator to track the optimal speed, control the actual current of the stator d axis of the permanent magnet synchronous generator to track the stator current reference value, and ensure the extraction of maximum tidal energy.

[0074] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0075] To reduce the operation and maintenance costs of tidal power generation systems, it is necessary to maximize the conversion of tidal energy into electricity. Therefore, it is necessary to adopt a maximum power point tracking (MPPT) control strategy similar to WPGS. MPPT control is key to improving the efficiency and reducing the cost of tidal power generation systems.

[0076] The MPPT control strategy adopted by the present invention is mainly based on the OBTSR method, which is characterized by high efficiency and fast speed response. However, since the density of seawater is much greater than that of air, the WPGS MPPT control strategy based on the OBTSR method cannot be directly applied to tidal power generation systems. It is necessary to add a filtering link to the MPPT control strategy to suppress the violent fluctuations of torque and power. In the OBTSR control method, the tip speed ratio (BTSR) of the ocean turbine is always near the optimal value. This requires the controller to accurately track the optimal speed of the tidal turbine. Therefore, efficient speed control is the key to realizing the MPPT strategy.

[0077] This invention improves the ESO in FESOC by utilizing a timed differentiator that can simultaneously estimate the system state at a fixed time and external disturbances. For tidal power generation systems, this invention eliminates the requirement for differentiability of the total disturbance, enhancing its practicality and relevance to actual conditions. Furthermore, the feasibility and effectiveness of FESOC are verified through experimental test results in various scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 Schematic diagram of the MPPT control method based on the OBTSR control strategy;

[0079] Figure 2 Schematic diagram of a method for extracting maximum tidal energy from a permanent magnet synchronous tidal power generation system based on a timed extended state observer;

[0080] Figure 3 It is a test platform for hardware experiments;

[0081] Figure 4 Schematic diagram of the response of the mechanical speed ωt under time-varying tidal flow speed;

[0082] Figure 5Schematic diagram of the response of the mechanical speed ωt to the uncertainty of the magnetic flux Φ;

[0083] Figure 6 Schematic diagram of the response of mechanical speed ωt to current sensor failure;

[0084] Figure 7 Schematic diagram of the power factor response to current sensor failure. DETAILED DESCRIPTION

[0085] The technical solution of this embodiment will now be further described with reference to the accompanying drawings and embodiments.

[0086] In order to ensure the maximum extraction efficiency of tidal energy, this embodiment proposes a maximum tidal energy extraction method for a permanent magnet synchronous tidal power generation system based on a timed extended state observer, which mainly includes the following steps:

[0087] Step 1: Build a tidal energy capture model. This tidal energy capture model includes a tidal turbine (MCT), a permanent magnet synchronous generator (PMSG), a machine-side converter, a grid-side converter, a filter, a transformer, and a power grid. The tidal turbine captures tides, converting the kinetic energy of the tidal flow into mechanical energy. The PMSG, in conjunction with the machine-side converter, converts the mechanical energy output by the tidal turbine into electrical energy. This electrical energy then passes through the grid-side converter, filter, and transformer before being connected to the power grid. The PMSG is controlled by a pulse-width modulation (PWM) converter.

[0088] In tidal energy capture models, a permanent magnet synchronous generator (PMSG) is used as the core component, and its dynamic model is used to describe and analyze the system's dynamic behavior. To improve the operating efficiency of tidal power generation systems, energy must be extracted efficiently, which can be achieved through MPPT control on the machine side (MSC). DC capacitors not only reduce high-frequency ripple in the DC voltage but also enable decoupling control between the machine side (MSC) and the grid side (GSC), keeping the DC voltage near its rated value. For the grid side (GSC), its primary goal is to maintain the DC link voltage at its rated value and deliver power to the grid.

[0089] For a tidal turbine, the mechanical power that can be captured is expressed as:

[0090]

[0091] Where, P t represents the mechanical power captured by the tidal turbine, ρ represents the density of seawater, A represents the area of the tidal turbine blades rotating one circle, R is the impeller radius of the tidal turbine, V represents the water velocity, C p(β,λ) represents the power coefficient of the tidal turbine, β represents the blade pitch angle, and λ represents the blade tip speed ratio (BTSR).

[0092] The power coefficient of a tidal turbine is expressed as:

[0093]

[0094] Where, Indicates intermediate parameters;

[0095]

[0096] λ is expressed as:

[0097]

[0098] The dynamic model of the permanent magnet synchronous generator is usually described by Park transformation in the dq coordinate system, which is expressed as:

[0099]

[0100] T g =p[(L d -L q )i sd i sq +i sq Φ]

[0101] Where V sd 、V sq are the d-axis stator voltage and the q-axis stator voltage, respectively, i sd 、i sq are the d-axis stator current and the q-axis stator current, R s is the stator resistance, L d is the stator d-axis inductance, p is the number of pole pairs, Φ is the flux linkage, T g is the electromagnetic torque, L q represents the stator q-axis inductance;

[0102] The motion equation of the permanent magnet synchronous generator is expressed as:

[0103]

[0104] Where J is the total moment of inertia, T t is the mechanical torque, expressed as:

[0105]

[0106] In order to ensure that the tidal power generation system can extract the maximum efficiency of tidal energy, this embodiment adopts the OBTSR (Optimal Speed Ratio Tracking) control strategy based on the MPPT (Maximum Power Point Tracking) control method, such as Figure 1 As shown, this control strategy aims to ensure that the MCT system is always within the MCT power coefficient C p The maximum value C pmax For a certain β, there is only one λ corresponding to the maximum value C pmax , the corresponding tip speed ratio is called OBTSR(λ opt ), the corresponding optimal speed is expressed as:

[0107]

[0108] Where R represents the impeller radius.

[0109] Step 2: Use feedback linearization theory to linearize the tidal energy capture model constructed in step 1 to obtain a tidal energy capture linear control model. The specific operations include:

[0110] like Figure 2 As shown, select V sd and V sq As the input of the linear control model for tidal energy capture, i sd and ω t As the output of the permanent magnet synchronous generator, the dynamic model of PMSG is rewritten as:

[0111]

[0112] Where, represents the stator d-axis current i sd The first derivative of represents the stator q-axis current i sq The first derivative of

[0113] By linearizing the input / output of the above system, we can obtain:

[0114]

[0115]

[0116] For actual tidal power generation system, the parameters p, Φ, J, L d and L q The values of should all be non-zero. Therefore, D(x) is non-singular.

[0117] Define the disturbance terms Ψ1(x) and Ψ2(x), which include uncertainty, interference and nonlinear terms, and are expressed as:

[0118]

[0119] in, They are J, Φ, D1(x), D2(x), and Ld , L q Nominal value.

[0120] Using the perturbation terms Ψ1(x) and Ψ2(x), we can can be rewritten as:

[0121]

[0122]

[0123] Step 3: Use the Fixed-Time Extended State Observer (FTESO) to estimate the disturbance term in real time. Figure 2 As shown, the specific operations include:

[0124] Considering the complex and ever-changing operating environment of permanent magnet synchronous tidal power generation systems, factors such as variable ocean currents, disturbances, and model parameter uncertainties must be considered. These challenges complicate the design of the control system, which must efficiently capture maximum tidal energy (MTE) under normal operating conditions while also having strong resistance to disturbances and parameter uncertainty. To mitigate the adverse effects of unknown disturbances and model parameter uncertainties on the tidal energy capture linear control model obtained in step 2, a fixed-time extended state observer (FTESO) is used to estimate the disturbance term in real time to compensate for the negative impact of actual disturbances on the tidal energy capture linear control model obtained in step 2.

[0125] Assume that the power coefficient C of the MCT provided in this embodiment is p The maximum value C pmax =0.411,λ opt =7.95. In the actual operating environment of a tidal power generation system, it is necessary to address not only changes in ocean current velocity but also known and unknown disturbances, such as changes in tidal power generation system parameters, torque disturbances, and equipment failures. These factors may affect the performance of the MPPT control. Therefore, the fixed-time extended state observer (FTESO) designed in this embodiment fully considers the actual operating environment of the tidal power generation system to ensure effective tidal energy capture. The specific operation is as follows:

[0126] During system operation, the parameters in the disturbance terms Ψ1(x) and Ψ2(x) may change abruptly. This may cause them to become non-differentiable over time. Therefore, ESOs cannot be directly used to estimate Ψ1(x) and Ψ2(x).

[0127] In this embodiment, Ψ1(x) and Ψ2(x) are divided into differentiable and non-differentiable parts:

[0128]

[0129] here, and represents the differentiable part of Ψ1(x) and Ψ2(x), and and represents the non-differentiable part.

[0130] represents the first derivative of Ψ1(x), Denote the first-order derivative of Ψ2(x), let The following assumptions are made:

[0131] When the new state vector z 11 =i sd , Define z 21 =ω t , Indicates the speed of the permanent magnet synchronous generator ω t The first derivative of The following two subsystems can be exported:

[0132]

[0133] Where, Indicates the newly defined state quantity;

[0134] Now we design two FESO to estimate z 12 and z 23 :

[0135]

[0136] Where, Both represent the corresponding estimated values, which are estimated by the observers designed by FESO1 and FESO2, χ 11 , χ 12 , χ 21 , χ 22 , χ 23 is a positive constant;

[0137]

[0138] In the formula, the symbol It is a combination of functions. Function and Expressed as:

[0139]

[0140]

[0141] The estimated disturbance is used to compensate the actual disturbance. The control law of subsystems S1 and S2 based on FESO is expressed as:

[0142]

[0143] To achieve MPPT, control y2 = ω t Track its optimal reference y 2r =ω ref =vλ opt / R. At the same time, y1=i sd Should track y 1r =i sdr = 0. Therefore, to achieve GUUB (global uniform ultimately bounded) trajectory error, v1 and v2 are defined as:

[0144]

[0145] Among them, k 11 、k 21 and k 22 is the positive control gain.

[0146] The FESOC control law can be expressed by physical variables such as flux, inductance, current, mechanical speed, and total inertia:

[0147]

[0148] Where, L d0 represents the nominal value of the stator d-axis inductance, Represents the first derivative of the stator d-axis current reference value.

[0149]

[0150] Where, L q0 Indicates the nominal value of the stator q-axis inductance.

[0151] The FESO proposed in this embodiment is designed based on the NCSF scheme. The standard nonlinear controller based on the NCSF scheme is expressed as:

[0152]

[0153] Its form based on physical variables can be summarized as follows:

[0154]

[0155] When designing NCSF, it is necessary to know V, ω t 、i sd , q and The measured values of , and the actual TPGS parameter values. In contrast, this embodiment only needs to know V, ω t The measured value of L d , Lq, J, nominal value of φ L d0 , L q0 , J0 and φ, and i sd Can be set to 0, i sd The results show that the controller proposed in this embodiment has the advantages of strong robustness and easy implementation.

[0156] In order to verify that the method of this embodiment can ensure the efficient power generation of the tidal energy power generation system, a hardware experimental test platform was built based on the real-time simulator OPAL-RT. Figure 3 As shown in the figure, the tidal power generation system model is compiled and loaded onto the hardware (OP4510) using RT-LAB software on a computer. Network-based communication is established via signal lines. An oscilloscope can visually display the system's status response.

[0157] The TPI controller, NCSF and adaptive control based on high-gain observer (ACHO) are now used for comparison with this embodiment.

[0158] from Figure 4 It can be seen that the TPI control strategy cannot provide satisfactory tracking performance when facing rapidly changing ocean currents. However, the method of this embodiment achieves tracking performance similar to that of ACHO and NCSF. It should be noted that ACHO achieves similar control performance to the method of this embodiment based on high gain. The magnetic flux linkage drops from the nominal value of 1 p.u. to 0.99 pu at 9.4 seconds. Figure 5 It can be seen that the method of this embodiment and ACHO are almost unaffected by the change of magnetic flux, and still maintain efficient speed tracking performance. Before the magnetic flux changes, the NCSF control strategy achieves tracking performance similar to that of ACHO and the method of this embodiment. However, after the magnetic flux changes, its speed tracking performance is significantly lower than that of the method of this embodiment and ACHO, and even a large steady-state error occurs. This is mainly because NCSF has high requirements for the accuracy of system parameters and state variables during the controller design process. Once the values of certain parameters or state variables deviate, it may lead to a decline in control performance or even controller failure. Since the current sensor failed at 7.4 seconds, the measured value was only 90% of the actual value. At 19.4 seconds, the measured value was only 70% of the actual value. The method of this embodiment and ACHO can perform real-time estimation and compensation for unknown interference caused by current sensor failure. Therefore, Figure 6The experimental results show that the two controllers are almost unaffected by the current sensor failure and always maintain efficient speed tracking performance. The results show that the method of this embodiment is insensitive to the current sensor failure. However, when the sensor fails, NCSF does not have the ability to track the speed and even has a steady-state error. This is mainly because it requires the understanding of known and unknown interference during the controller design process. Known and unknown interference in the controller design process. From Figure 7 The experimental results show that the tidal power coefficient Cp under NCSF control is far from the maximum value when the current sensor fails, while the other two control strategies are close to the maximum value of Cp. The above verifies the effectiveness and rationality of the method of this embodiment.

Claims

1. A maximum tidal energy extraction method based on a fixed-time extended state observer, characterized by: The following steps are involved: Step 1: Build a tidal energy capture model; The tidal energy capture model includes: a tidal turbine, a permanent magnet synchronous generator, a machine-side converter, a grid-side converter, a filter device, a transformer, and a grid. The tidal turbine extracts kinetic energy from the tide and converts it into mechanical energy. This mechanical energy is transmitted to the permanent magnet synchronous generator, thereby driving the permanent magnet synchronous generator rotor to rotate and converting the mechanical energy into electrical energy. The permanent magnet synchronous generator cooperates with the machine-side converter to convert the mechanical energy output by the tidal turbine into electrical energy. The electrical energy is sequentially connected to the grid through the grid-side converter, the filter device, and the transformer. Step 2: Linearize the tidal energy capture model using feedback linearization theory to obtain a tidal energy capture linear control model; Step 3: Use the fixed-time extended state observer to estimate the disturbance term in real time to compensate for the negative impact of the actual disturbance on the tidal energy capture linear control model; Step 4: Adopting the optimal speed tracking control strategy based on the maximum power point tracking control method, using the pulse width modulation converter, control the actual speed of the permanent magnet synchronous generator to track the optimal speed, the optimal speed is determined based on the actual flow velocity and the optimal tip speed ratio formula; control the actual stator d-axis current of the permanent magnet synchronous generator to track the stator current reference value to ensure the extraction of maximum tidal energy.

2. The maximum tidal energy extraction method based on a fixed-time extended state observer according to claim 1 is characterized in that: The tidal energy capture model is expressed as: For a tidal turbine, the mechanical power captured is expressed as: Where, P t represents the mechanical power captured by the tidal turbine, ρ represents the density of seawater, A represents the area of the tidal turbine blades rotating one circle, R is the impeller radius of the tidal turbine, V represents the water velocity, C p (β,λ) represents the power coefficient of the tidal turbine, β represents the blade pitch angle, and λ represents the tip speed ratio; Among them, the power coefficient of the tidal turbine is expressed as: Where, Indicates intermediate parameters: λ is expressed as: Where, ω t Indicates the speed of the permanent magnet synchronous generator; The dynamic model of the permanent magnet synchronous generator is described by Park transformation in the dq coordinate system, which is expressed as: T g =p[(L d -L q )i sd i sq +i sq Φ] Where V sd 、V sq are the d-axis stator voltage and the q-axis stator voltage, respectively, i sd 、i sq are the d-axis stator current and the q-axis stator current, R s is the stator resistance, L d is the stator d-axis inductance, p is the number of pole pairs, Φ is the flux linkage, T g is the electromagnetic torque, L q represents the stator q-axis inductance; The motion equation of the permanent magnet synchronous generator is expressed as: Where J is the total moment of inertia, T t is the mechanical torque, expressed as:

3. The maximum tidal energy extraction method based on a fixed-time extended state observer according to claim 2 is characterized in that: In step 2, the tidal energy capture model is linearized using the feedback linearization theory to obtain a tidal energy capture linear control model, which is expressed as: Select V sd and V sq As the input of the linear control model for tidal energy capture, i sd and ω t As the output of the permanent magnet synchronous generator, the dynamic model of the permanent magnet synchronous generator is rewritten as: Where, represents the stator d-axis current i sd The first derivative of represents the stator q-axis current i sq The first derivative of By linearizing the above input / output, we get: Where, represents the stator d-axis current i sd The first derivative of Represents the second derivative of the permanent magnet synchronous generator speed; 4. The maximum tidal energy extraction method based on a fixed-time extended state observer according to claim 3 is characterized in that: In step 3, the use of a fixed-time extended state observer to perform disturbance term estimation in real time to compensate for the negative impact of actual disturbances on the tidal energy capture linear control model specifically includes: Define the disturbance terms Ψ1(x) and Ψ2(x), which are expressed as: Among them, J0, v0, L d0 、L q0 They are J, Φ, D1(x), D2(x), and L d 、L q The nominal value of Using the perturbation terms Ψ1(x) and Ψ2(x), we can Rewritten as: Divide Ψ1(x) and Ψ2(x) into differentiable and non-differentiable parts: Where, and represents the differentiable part of Ψ1(x) and Ψ2(x), and represents the non-differentiable part; represents the first derivative of Ψ1(x), Denote the first-order derivative of Ψ2(x), let The following assumptions are made: When the new state vector z 11 =i sd , Define z 21 =ω t , Indicates the speed of the permanent magnet synchronous generator ω t The first derivative of Export the following two subsystems: Where, Indicates the newly defined state quantity; Design two FESOs to estimate z 12 and z 23 , expressed as: Where, Both represent the corresponding estimated values, which are estimated by the observers designed by FESO1 and FESO2, χ 11 、X 12 、X 21 、X 22 、X 23 is a positive constant; In the formula, the symbol Represents a combination of functions, function and Expressed as: The estimated disturbance is used to compensate the actual disturbance. The control law of subsystems S1 and S2 based on FESO is expressed as:

5. The maximum tidal energy extraction method based on a fixed-time extended state observer according to claim 4 is characterized in that: In step 4, the optimal speed tracking control strategy based on the maximum power point tracking control method is adopted, and a pulse width modulation converter is used to control the actual speed of the permanent magnet synchronous generator to track the optimal speed, and the optimal speed is determined based on the actual flow rate and the optimal tip speed ratio formula; The actual stator D-axis current of the permanent magnet synchronous generator is controlled to track the stator current reference value to ensure the maximum tidal energy extraction. The specific operations include: The optimal speed is expressed as: Where λ opt Indicates the power coefficient C of the tidal turbine p At maximum C pmax The corresponding tip speed ratio is: V represents the actual flow velocity, and R represents the impeller radius; Control y2 = ω t Track its optimal reference y 2r =ω ref =vλ opt / R, at the same time, control y1=i sd Tracking 1r =i sdr =0; To achieve GUUB trajectory error, v1 and v2 are defined as: Among them, k 11 、k 21 and k 22 is the positive control gain; Physical variables including flux, inductance, current, mechanical speed, and total inertia are used to represent the FESOC law. The dynamic model of the PMSG in the dq coordinate system is represented by the FESOC law: Where, L d0 represents the nominal value of the stator d-axis inductance, Represents the first derivative of the stator d-axis current reference value; Where, L q0 Indicates the nominal value of the stator q-axis inductance; The pulse width modulation converter obtains the stator voltage V sd and V sq , control the actual speed of the permanent magnet synchronous generator to track the optimal speed, control the actual current of the stator d axis of the permanent magnet synchronous generator to track the stator current reference value, and ensure the extraction of maximum tidal energy.

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