Offshore Wind Power Control System Based on Disturbance Observation and Global Fast Integral Sliding Mode

Abstract

The present invention provides an offshore wind power control system based on disturbance observation and global fast integral sliding mode, including an offshore wind turbine, a doubly-fed induction generator, an optimal angular velocity tracking module, and a global fast integral sliding mode control module; the offshore wind turbine is used to convert the captured offshore wind energy into mechanical energy; the doubly-fed induction generator is used to convert mechanical energy into electrical energy; the maximum power point tracking module is used to calculate the variant power coefficient curve at the current moment considering unknown errors; based on the variant power coefficient curve, a disturbance tracking strategy is adopted to calculate the optimal angular velocity; the global fast integral sliding mode control module is used to control the current by adopting a PI control strategy. In the outer loop controller, a sliding mode surface function is set and a global fast control law is adopted for control to quickly adjust to the optimal angular velocity.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore wind power, and particularly relates to an offshore wind power control system based on disturbance observation and global fast integral sliding mode. Background Art

[0002] New energy sources that replace fossil energy are attracting more and more attention. Offshore wind energy is currently favored by various countries and is expected to account for 20% of the global energy demand by 2030.

[0003] Maximum power point tracking (MPPT) has always been one of the research hotspots in offshore wind power generation systems. Among many methods, the perturbation observation method does not require obtaining the offshore wind speed value in advance. By simply applying a perturbation to the angular velocity of the turbine rotor, the turbine output power can move along the power-speed curve to the maximum power point. However, this method also has disadvantages. If a larger step size LSPO is used, a large steady-state oscillation will form around the maximum power point MPP. If a smaller step size SSPO is used, the tracking speed will be slower.

[0004] To solve the deficiencies of the perturbation observation method in the prior art, a method is usually used to roughly calculate the optimal offshore wind speed using the estimated offshore wind speed, and then track the true maximum power point with a small step. The value calculated using the offshore wind speed estimation divides the power-speed curve into several regions, and different step sizes are used in each region or the optimal constant is updated using the value calculated by the offshore wind speed estimation, and then the maximum power point is calculated. The combination of the perturbation observation method and the polynomial offshore wind speed estimation method can reduce the steady-state oscillation while increasing the tracking speed, but a multi-solution problem will occur during the process, resulting in a large error. Summary of the Invention

[0005] The present invention proposes an offshore wind power control system based on disturbance observation and global fast integral sliding mode to solve the problem of large errors when the perturbation observation method is combined with the polynomial offshore wind speed estimation method.

[0006] To solve the above technical problems, the present invention provides an offshore wind power control system based on disturbance observation and global fast integral sliding mode, including an offshore wind turbine, a doubly-fed induction generator, an optimal angular velocity tracking module, and a global fast integral sliding mode control module;

[0007] The offshore wind turbine is used to convert the captured offshore wind energy into mechanical energy;

[0008] The doubly-fed induction generator is used to convert the mechanical energy into electrical energy;

[0009] The maximum power point tracking module is used to calculate the variant power coefficient curve at the current moment considering unknown errors; based on the variant power coefficient curve, a perturbation tracking strategy is adopted to calculate the optimal angular velocity.

[0010] The global fast integral sliding mode control module is used to control the current by adopting a PI control strategy. In the outer loop controller, a sliding mode surface function is set and a global fast control law is adopted for control to quickly adjust to the optimal angular velocity.

[0011] Preferably, the steps for the maximum power point tracking module to calculate the variant power coefficient curve at the current moment include:

[0012] Step S11: Based on the unknown error, correct the offshore wind energy captured by the offshore wind turbine, and the expression is:

[0013]

[0014]

[0015] In the formula, ρ represents the air density, R represents the length of the moving blade, C p represents the power coefficient, W wt represents the angular velocity of the turbomachine, λ represents the tip speed ratio, e represents the error coefficient, P wto (k - 1) represents the actual optimal power of the offshore wind speed at the previous moment, W wto (k - 1) represents the actual optimal rotational speed of the offshore wind speed at the previous moment, λ0 = 8.1;

[0016] Step S12: Calculate the variant power coefficient based on the offshore wind energy:

[0017]

[0018] In the formula, Y(k) represents the variant power coefficient at the current moment.

[0019] Preferably, the method for calculating the optimal angular velocity includes the following steps:

[0020] Step S21: Calculate the corresponding tip speed ratio based on the variant power coefficient curve;

[0021] Step S22: When there are multiple solutions for the tip speed ratio, a perturbation tracking strategy is adopted for selection;

[0022] Step S23: Based on the tip speed ratio obtained in Step S22, use a small step size △w wt to perform perturbation to track the optimal angular velocity.

[0023] Preferably, the expression for stopping tracking in the perturbation tracking strategy in Step S22 is:

[0024] |P wtoi (k)-P wtkoi (k)|≤η

[0025]

[0026]

[0027] Wherein, P wtoi (k) represents the i-th maximum power corresponding to the current power coefficient variant value, P wtoki (k) represents the i-th maximum power calculated from the optimal power curve, η represents the stop tracking threshold; A0(k) represents the optimal constant, W wtoi represents the i-th maximum rotational speed corresponding to the current power coefficient variant value.

[0028] Preferably, a small step size △w wt is used for perturbation in step S23, and the expression for stopping the perturbation is:

[0029]

[0030] Wherein, γ represents the stop perturbation threshold.

[0031] Preferably, the expression of the sliding mode surface function s is:

[0032]

[0033] Wherein, p > 0, q > 0, α1 > 1, 0 < α2 < 1, x represents the state variable, and t represents time.

[0034] Preferably, the method of controlling by using the global fast control law includes the following steps:

[0035] Step S31: Calculate the equivalent control term without considering the disturbance term;

[0036] Step S32: Set the switching control term considering the disturbance term;

[0037] Step S33: Obtain the control law of the global integral sliding mode based on the equivalent control term and the switching control term.

[0038] Preferably, the expression of the equivalent control term is:

[0039]

[0040] Wherein, α1 > 1, 0 < α2 < 1, b > 1 and is odd, p > 0, q > 0, x represents the state variable.

[0041] Preferably, the expression of the switching control term is:

[0042]

[0043] Wherein, D represents the upper bound of interference, s represents the sliding mode surface function, r1 > 0, r2 > 0, ε > 1 and is a positive odd number.

[0044] Preferably, the expression of the control law of the global integral sliding mode is:

[0045] u = u eq + u sw

[0046] Wherein, u represents the control law of the global fast integral sliding mode.

[0047] The beneficial effects of the present invention at least include: using the polynomial offshore wind speed estimation principle to roughly calculate the optimal angular velocity of the offshore wind turbine under the current offshore wind speed, and then using small steps to track to the precise maximum power point. At the same time, in order to improve the instantaneous tracking performance to the optimal power point, a global fast integral sliding mode controller is proposed by using the principle of non-singular fast sliding mode at the terminal, so that the offshore wind power generation system can quickly reach the maximum power point, and the overshoot and steady-state jitter are small. Description of the Drawings

[0048] Figure 1 Schematic diagram of the offshore wind power control system provided by the embodiment of the present invention;

[0049] Figure 2 Overall schematic diagram of the generator-side control system based on the doubly-fed motor in the embodiment of the present invention;

[0050] Figure 3 Schematic diagram of the variant power coefficient curve in the embodiment of the present invention;

[0051] Figure 4 Schematic diagram of the power and angular velocity values satisfying the current tip speed ratio in the embodiment of the present invention;

[0052] Figure 5 Schematic diagram of the disturbance tracking strategy process in the embodiment of the present invention;

[0053] Figure 6 Schematic diagram of the disturbance tracking strategy principle in the embodiment of the present invention;

[0054] Figure 7 Schematic diagram of the global fast integral sliding mode control in the embodiment of the present invention;

[0055] Figure 8 Schematic diagram of the simulation analysis of the offshore wind speed profile in the embodiment of the present invention;

[0056] Figure 9Schematic diagram of simulating and analyzing the angular velocity response of an offshore wind turbine in an embodiment of the present invention;

[0057] Figure 10 Schematic diagram of simulating and analyzing the power coefficient response of an offshore wind turbine in an embodiment of the present invention;

[0058] Figure 11 Schematic diagram of simulating and analyzing the angular velocity response of an offshore wind turbine under an offshore wind power generation system in an embodiment of the present invention;

[0059] Figure 12 Diagram of the angular velocity response of an offshore wind turbine under a complete offshore wind power generation system in an embodiment of the present invention through simulation analysis. Detailed implementation manners

[0060] Next, in conjunction with the accompanying drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0061] As Figure 1 shown, an embodiment of the present invention provides an offshore wind power control system based on disturbance observation and global fast integral sliding mode, including an offshore wind turbine, a doubly-fed induction generator, an optimal angular velocity tracking module, and a global fast integral sliding mode control module.

[0062] The offshore wind turbine is used to convert the captured offshore wind energy into mechanical energy; the doubly-fed induction generator is used to convert the mechanical energy into electrical energy.

[0063] Specifically, as Figure 2 shown, the overall configuration of the machine-side control scheme of an offshore wind power generator WECS based on a doubly-fed motor DFIG is described. The offshore wind power generator converts the captured offshore wind energy into mechanical energy, and then the doubly-fed motor converts the mechanical energy into electrical energy.

[0064] The mechanical power captured by the offshore wind power generator can be expressed as:

[0065]

[0066] In the formula, ρ represents the air density, R represents the length of the moving blade, C p represents the power coefficient, λ represents the tip speed ratio, v represents the wind speed, and β represents the pitch angle.

[0067] The power coefficient can be expressed in terms of the tip speed ratio and the pitch angle as:

[0068]

[0069] The tip speed ratio is expressed as:

[0070]

[0071] where ω wt represents the angular velocity of the turbomachine.

[0072] The aerodynamic torque of the turbine can be expressed as:

[0073]

[0074] The dynamic equation of the turbine can be expressed as follows:

[0075]

[0076] where J a represents the total moment of inertia of the drive system, B represents the total damping coefficient, n g represents the transmission ratio of the gearbox, T em represents the electromagnetic torque.

[0077] After the Park transformation, the three-phase stationary coordinate system becomes a two-phase synchronous rotating coordinate system. The dynamic model of the doubly-fed induction motor is further decoupled by adopting the stator magnetic field oriented vector control strategy. The decoupled rotor voltage equation can be expressed as:

[0078]

[0079] where u rd and u rq respectively represent the d-axis component and q-axis component of the rotor voltage, i rd and i rq respectively represent the d-axis component and q-axis component of the rotor current, σ represents the leakage magnetic coefficient, ω d represents the slip angular velocity, L s represents the stator inductance, L r represents the rotor inductance, L m represents the rotor mutual inductance, R r represents the three-phase winding, Ψ s represents the total magnetic flux linkage of the phase winding;

[0080]

[0081] where ω s represents the angular velocity of the stator magnetic field, ω r represents the electrical angular velocity of the rotor, n p represents the number of pole pairs of the generator, ω rm represents the mechanical angular velocity of the generator rotor, n g represents the transmission ratio of the gearbox.

[0082] The converted electromagnetic torque T em is expressed as:

[0083]

[0084] The expression of the instantaneous active and reactive power on the stator side can be simplified to:

[0085]

[0086] In the formula, P s represents the active power, Q s represents the reactive power, and u s represents the stator voltage.

[0087] The maximum power point tracking module is used to calculate the variant power coefficient curve at the current moment considering the unknown error; based on the variant power coefficient curve, a perturbation tracking strategy is adopted to calculate the optimal angular velocity;

[0088] Specifically, considering the unknown error e, combining equations (1) and (3), it can be known that at the current moment, the offshore wind energy captured by the turbine can be expressed as:

[0089]

[0090] Since e will not change in a short time, it can be expressed as:

[0091]

[0092] In the formula, e represents the error coefficient, P wto (k - 1) represents the actual optimal power of the offshore wind speed at the previous moment, ω wto (k - 1) represents the actual optimal rotational speed of the offshore wind speed at the previous moment, and λ0 = 8.1.

[0093] Based on formula (10) and formula (11), the variant power coefficient Y(k) at the current moment can be obtained:

[0094]

[0095] Based on formulas (5) and (8), the offshore wind energy P captured by the wind turbine wt can be expressed as:

[0096]

[0097] In the formula, B represents the total damping coefficient.

[0098] It can be known from formula (13) that given i at the current moment rq and ω wt , the P at the current moment can be obtained wt, thus obtaining Y(k) at the current moment. Y(k) is determined by the blade characteristics of the fan itself and is not affected by the offshore wind speed. In the embodiment of the present invention, its curve is as shown in Figure 3 . It can be found that the overall trend of the variant power coefficient curve is decreasing. When Y ∈ [1.665×10 -3 , 2.09×10 -3 , the variant power coefficient curve is in a fluctuating state, and Y(k) at the current moment corresponds to at least two λ solutions.

[0099] In order to obtain a faster solution speed and solve the multi-solution problem, the embodiment of the present invention divides the curve into 3 parts longitudinally and 5 parts horizontally. Longitudinally, it is bounded by Y a = 1.45×10 -3 , Y b = 2.45×10 -3 . Horizontally, it is bounded by λ a = 1.6839, λ b = 2.4896, λ c = 4.2804, λ d = 6.6785.

[0100] When Y ≥ 2.45×10 -3 or Y ≤ 1.45×10 -3 , there is only one λ solution, and λ at each offshore wind speed can satisfy the current Y(k). However, knowing the current turbine angular velocity and power, it can be known that there is a unique λ of the offshore wind speed that satisfies the current Y(k). The optimal angular velocity of the current offshore wind speed can be deduced through formula (3):

[0101]

[0102] When 1.45×10 -3 ≤ Y ≤ 2.45×10 -3 , it corresponds to three solutions of λ1(k), λ2(k), and λ3(k). Moreover, λ1(k), λ2(k), and λ3(k) at each offshore wind speed satisfy the current Y(k). As shown in Figure 4 , the corresponding optimal angular velocities ω o1 < ω o2 < ω o3 . Therefore, the embodiment of the present invention adopts a perturbation tracking strategy for selection, and its flowchart is as shown in Figure 5 . The judgment condition for stopping tracking is:

[0103]

[0104]

[0105] Since the unknown error cannot be changed in a short time, the expression of the optimal constant is as follows:

[0106]

[0107] In the formula, P wtoi (k) represents the i-th maximum power corresponding to the current power coefficient variant value, and P wtoki (k) represents the i-th maximum power calculated by the optimal power curve, η represents the stop tracking threshold; A0(k) represents the optimal constant, and ω wtoi represents the i-th maximum rotational speed corresponding to the current power coefficient variant value.

[0108] Considering reasons such as system errors, a small step size △w wt perturbation is adopted to approximate the most real optimal angular velocity. As Figure 6 shown, through the combination of the above methods, the rapidity, low oscillation, and accuracy of tracking are guaranteed.

[0109] The global fast integral sliding mode control module is used to control the current by adopting a PI control strategy. In the outer loop controller, a sliding mode surface function is set and a global fast control law is adopted for control to quickly adjust to the optimal angular velocity.

[0110] In the embodiment of the present invention, the current inner loop controller on the machine side of the offshore wind power conversion system adopts a PI control strategy to control the current. In the outer loop controller, in order to improve the transient tracking speed, a new sliding mode surface and reaching law are adopted, which can quickly track the optimal angular velocity. The control block diagram is as Figure 7 shown.

[0111] Specifically, the state variable x = ω o -ω wt is defined and substituted into Equation (5) to obtain the state equation as follows:

[0112]

[0113] In the formula, is the control input, d0 represents the total disturbance term composed of uncertain parameters and external disturbances in the dynamic equation, d represents the total disturbance term under the state equation, and D represents the disturbance upper bound.

[0114] In order to ensure the rapid convergence of the state variable, a global fast integral sliding mode surface function is adopted:

[0115]

[0116] In the formula, p > 0, q > 0, α1 > 1, 0 < α2 < 1, x represents the state variable, and t represents time.

[0117] To ensure that the system is robust throughout the dynamic process, it is necessary to ensure that the initial state reaches the sliding surface s = 0. Therefore, a first-order low-pass filter can be used to process the initial target angular velocity ω o (0). After processing, the initial value x(0) of the sliding surface function is 0, thus ensuring that the initial state reaches the sliding surface.

[0118] When the system enters the sliding mode, there is That is:

[0119]

[0120] As can be seen from formula (20), when the error state |x| > 1, plays a major role; when the error state |x| < 1 and approaches the equilibrium point, plays a major role. Therefore, during the sliding mode motion stage, the proposed sliding surface can enable the error state variable x to achieve global rapid convergence.

[0121] Due to external disturbances and parameter uncertainties, the actual system trajectory always moves in a zigzag shape along the sliding surface. To make the system trajectory always move towards the sliding surface, the embodiment of the present invention uses an equivalent sliding mode control method to design a global rapid control law.

[0122] Without considering the disturbance, assuming that the entire system reaches the sliding surface, combining formula (18) and formula (20) gives the equivalent control term u eq :

[0123]

[0124] When considering the disturbance term, to ensure that the system can reach the sliding surface, a new switching control term u sw is designed to compensate for the disturbance term, and the expression is as follows:

[0125]

[0126] In the formula, D represents the upper bound of the disturbance, s represents the sliding surface function, r1 > 0, r2 > 0, ε > 1 and is a positive odd number.

[0127] It can be seen that when |s| > 1, -r2s ε plays a major role. Therefore, during the approaching motion stage, the proposed switching control term can enable the system to globally and rapidly reach the sliding surface. As can be seen from formula (21), the equivalent control term is essentially continuous. As can be seen from formula (22), during the approaching process, the sign function of the compensation term is not continuous, which may lead to high-frequency chattering. To reduce chattering, the embodiment of the present invention uses the softsign function to replace the sign function, and the expression is:

[0128]

[0129] In the formula, when γ is closer to 0, the softsign function is closer to the sign function. Combining Equation (21) and Equation (23), the control law of the global fast integral sliding mode can be obtained as follows:

[0130] u = u eq + u sw (24).

[0131] To verify the stability condition of the system proposed in the present invention, the Lyapunov function is taken as:

[0132]

[0133] Taking the derivative of V gives:

[0134]

[0135] When D > |d| and γ is close to 0, the proposed method can ensure the asymptotic stability of the system in the global sense.

[0136] To verify the effectiveness and rapidity of the embodiments of the present invention, a DFIG-based offshore wind power generation system model was established using MATLAB / Simulink. The proposed method was simulated under a step offshore wind speed, and the specific changes in the offshore wind speed are as Figure 8 .

[0137] First, the proposed disturbance tracking strategy HPO of the offshore wind turbine was verified separately. The following simulations were all carried out under ideal instantaneous tracking, as Figure 9 shown. It can be found that the steady-state oscillation caused by LSPO is high and the convergence time is fast, the steady-state oscillation caused by SSPO is low and the convergence time is slow, while the convergence time caused by HPO is not only fast, but also the steady-state oscillation is maintained at a low level. And as Figure 10 shown, HPO demonstrates good performance. It can not only make the response fluctuation of the power coefficient generally small, but also quickly converge to the optimal value C pmax = 0.48 under the sudden change of the offshore wind speed.

[0138] To further verify the feasibility of the system proposed in the present invention, the following simulations were all carried out under a complete offshore wind power generation system, and the optimal reference angular velocity was obtained through HPO. Figure 11 For the angular velocity response of the offshore wind turbine under a complete offshore wind power generation system, it can be seen that the angular velocity response is fast, the overshoot is small, and the chattering is small under the sudden change of the offshore wind speed; in Figure 12 , although the response fluctuations of the power coefficients under the global fast integral sliding mode control GFSMC and the traditional integral sliding mode control SMC are the same in the face of the sudden change of the offshore wind speed, the tracking speed of GFSMC is faster.

[0139] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. Only the preferred embodiments of the present invention are expressed. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent of the present invention. As long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0140] It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention should be subject to the appended claims.

Claims

1. A marine wind power control system based on disturbance observation and global fast integral sliding mode, characterized in that: Including an offshore wind turbine, a doubly-fed induction generator, an optimal angular velocity tracking module, and a global fast integral sliding mode control module; The offshore wind turbine is used to convert the captured offshore wind energy into mechanical energy; The doubly-fed induction generator is used to convert the mechanical energy into electrical energy; The maximum power point tracking module is used to calculate the variant power coefficient curve at the current moment considering unknown errors; based on the variant power coefficient curve, a perturbation tracking strategy is adopted to calculate the optimal angular velocity; The global fast integral sliding mode control module is used to control the current using a PI control strategy. In the outer loop controller, a sliding mode surface function is set and a global fast control law is adopted for control to quickly adjust to the optimal angular velocity; The steps for the maximum power point tracking module to calculate the variant power coefficient curve at the current moment include: Step S11: Based on the unknown error, correct the offshore wind energy captured by the offshore wind turbine, and the expression is: where ρ represents the air density, R represents the length of the moving blade, C p represents the power coefficient, W wt represents the angular velocity of the turbomachine, λ represents the tip speed ratio, e represents the error coefficient, P wto (k - 1) represents the actual optimal power of the offshore wind speed at the previous moment, W wto (k - 1) represents the actual optimal rotational speed of the offshore wind speed at the previous moment, λ0 = 8.1; Step S12: Calculate the variant power coefficient based on the offshore wind energy: In the formula, Y(k) represents the variant power coefficient at the current moment.

2. The offshore wind power control system based on disturbance observation and global fast integral sliding mode according to claim 1, characterized in that: The method for calculating the optimal angular velocity includes the following steps: Step S21: Calculate the corresponding tip speed ratio based on the variant power coefficient curve; Step S22: When there are multiple solutions for the tip speed ratio, a perturbation tracking strategy is adopted for selection; Step S23: Based on the tip speed ratio obtained in Step S22, use a small step size △w wt to perform perturbations to track the optimal angular velocity.

3. The offshore wind power control system based on disturbance observation and global fast integral sliding mode according to claim 2, wherein: The expression for stopping tracking using the perturbation tracking strategy in Step S22 is: |P wtoi (k)-P wtkoi (k)|≤η Wherein, P wtoi (k) represents the i-th maximum power corresponding to the current power coefficient variant value, P wtoki (k) represents the i-th maximum power calculated from the optimal power curve, η represents the stop tracking threshold; A0(k) represents the optimal constant, W wtoi represents the i-th maximum rotational speed corresponding to the current power coefficient variant value.

4. A marine wind power control system based on disturbance observation and global fast integral sliding mode according to claim 2, characterized in that: In step S23, a small step size △w is adopted wt for perturbation. The expression for stopping perturbation is as follows: In the formula, γ represents the stopping perturbation threshold.

5. A marine wind power control system based on disturbance observation and global fast integral sliding mode according to claim 1, characterized in that: The expression for the sliding mode surface function s is: In the formula, p > 0, q > 0, α1 > 1, 0 < α2 < 1, x represents the state variable, and t represents time.

6. The offshore wind power control system based on disturbance observation and global fast integral sliding mode according to claim 1, wherein: The method for controlling using the global fast control law includes the following steps: Step S31: Calculate the equivalent control term without considering the disturbance term; Step S32: Set the switching control term considering the disturbance term; Step S33: Based on the equivalent control term and the switching control term, obtain the control law of the global integral sliding mode.

7. The offshore wind power control system based on disturbance observation and global fast integral sliding mode according to claim 6, characterized in that: The expression for the equivalent control term is: In the formula, α1 > 1, 0 < α2 < 1, b > 1 and is an odd number, p > 0, q > 0, and x represents the state variable.

8. A marine wind power control system based on disturbance observation and global fast integral sliding mode according to claim 7, characterized in that: The expression for the switching control term is: In the formula, D represents the upper bound of the disturbance, s represents the sliding mode surface function, r1 > 0, r2 > 0, and ε > 1 and is a positive odd number.

9. The offshore wind power control system based on disturbance observation and global fast integral sliding mode according to claim 8, characterized in that: The expression for the control law of the global integral sliding mode is: u = u eq + u sw In the formula, u represents the control law of the global fast integral sliding mode.

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