Method for restoring speed and MPPT mode of offshore wind turbine to suppress secondary frequency drop

Abstract

The present application relates to offshore wind turbine frequency modulation control technical field, especially offshore wind turbine speed and MPPT mode recovery method for restraining secondary frequency drop, the present application releases the stored rotational kinetic energy in the fan to compensate for the active loss of the system by adding droop control to the fan rotor side control, the fuzzy controller is used to monitor the speed and frequency change of the system in real time, the appropriate parameters are selected according to the rules, the problem of weak adaptability of fixed parameters in different scenes is solved, the fuzzy control rules meeting the target are designed, the active output is smoothed, the control mode is smoothly switched, the speed is smoothly recovered, and the secondary frequency drop depth is relieved.

Description

Technical Field

[0001] This invention relates to the field of frequency regulation control technology for offshore wind turbines, and in particular to a method for restoring the speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops. Background Technology

[0002] Offshore wind power boasts advantages such as cleanliness, high efficiency, and no land-based resource consumption, making its vigorous development crucial for achieving China's "dual-carbon" strategic goals. However, the speed recovery of wind turbines after frequency response is accompanied by sudden changes in active power, easily causing secondary frequency impacts on the power grid. While improved traditional torque limit control has reduced the minimum point of the secondary frequency drop under traditional recovery control, the speed recovery time remains relatively long. Using traditional P-control for speed recovery strategies, constant parameters cannot simultaneously address both speed recovery performance and the depth of the secondary frequency drop in different scenarios. Therefore, there is an urgent need to propose a speed recovery method that suppresses secondary frequency drops and is applicable to various scenarios. Summary of the Invention

[0003] This invention addresses the shortcomings of existing technologies by proposing a method for restoring the speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops. This invention provides frequency support to the power grid through droop control while generating speed recovery power using fuzzy P control, causing the control mode to automatically and smoothly switch back to Maximum Power Point Tracking (MPPT) mode. This achieves smooth recovery of wind turbine speed while suppressing secondary frequency drops.

[0004] To achieve the aforementioned objectives, the present invention employs the following technical solution: a method for suppressing secondary frequency drops in offshore wind turbine rotational speed and MPPT mode recovery, comprising the following steps:

[0005] S1. Collect the grid frequency f, calculate the system frequency deviation ∆f and the system frequency change rate df / dt; collect the rotor speed ω of the offshore wind turbine. r Calculate the rotor speed deviation ∆ω and the rotor speed change rate dω / dt; the system frequency deviation ∆f and rotor speed deviation ∆ω are specifically expressed as follows:

[0006] (1)

[0007] (2)

[0008] In formula (1), ∆f is the system frequency deviation, f is the real-time frequency of the system, and f0 is the rated frequency of the system;

[0009] In formula (2), ∆ω is the rotor speed deviation, ω r ω represents the real-time rotor speed of an offshore wind turbine. off For t off The rotor speed at time toff This is the moment when the engine speed recovers and starts;

[0010] S2. Determine whether the system frequency is stable. If the system meets the requirements for safe and stable operation of the power grid, the offshore wind turbine will operate in maximum power point tracking (MPPT) mode. If the requirements are not met, droop control will be used to participate in system frequency regulation.

[0011] S3. To suppress secondary frequency drops and shorten speed recovery time, fuzzy rules are formulated. The system frequency deviation ∆f and rotor speed deviation ∆ω are used as inputs to the fuzzy controller, and the output is the proportional control coefficient k of the P controller. P ;

[0012] S4, convert the k output by the fuzzy controller P As the gain of the P controller, the input is the difference between the rotor speed of the offshore wind turbine at the steady-state moment and the current rotor speed of the offshore wind turbine, and the output is the speed recovery power ∆P, which is calculated as follows:

[0013] (3)

[0014] In formula (3), ∆P is the speed recovery power of the offshore wind turbine, ω0 is the rotor speed of the offshore wind turbine at the steady state of the system, and ω r k is the real-time rotor speed of an offshore wind turbine. P For the proportional control coefficient of the P controller;

[0015] S5. Determine whether the system meets the speed recovery condition. If it does, the offshore wind turbine will obtain additional speed recovery power ∆P and enter the speed recovery stage until the speed is recovered.

[0016] S6, Active power output P of offshore wind turbine with additional frequency regulation power and speed recovery power. W The maximum power P of the offshore wind turbine operating in maximum power point tracking mode MPPT In comparison, the maximum value is taken as the reference value P for the active power of offshore wind turbines. ref ;

[0017] S7. After the offshore wind turbine completes speed recovery, the reference value of active power P ref From active power output P W Maximum power P when smoothly switching back to MPPT mode MPPT This enables MPPT mode recovery.

[0018] Furthermore, as a preferred embodiment of the present invention, before the system provides frequency support and after the offshore wind turbine completes speed recovery, the wind turbine operates in maximum power point tracking mode, and its output electromagnetic power expression is:

[0019] (4)

[0020] In formula (4), P MPPT The maximum power of the offshore wind turbine operating in maximum power point tracking mode, ρ is the air density, R is the radius of the offshore wind turbine blade, and C is the maximum power of the offshore wind turbine operating in maximum power point tracking mode. p Maximum wind energy utilization coefficient, where v is wind speed and ω is the wind speed. r is the real-time rotor speed of the offshore wind turbine, and k0 is the coefficient of the maximum power point tracking curve.

[0021] Furthermore, as a preferred embodiment of the present invention, in step S2, the method by which the offshore wind turbine participates in system frequency regulation using droop control is as follows:

[0022] Additional active power ∆P generated by offshore wind turbines when droop control is used d for:

[0023] (5)

[0024] In formula (5), ΔP d K is the active power generated by the droop control of offshore wind turbines. d Here, Δf is the droop control coefficient, and Δf is the system frequency deviation.

[0025] Active power output P of offshore wind turbine W for:

[0026] (6)

[0027] At this time, the active power reference value P of the offshore wind turbine is... ref for:

[0028] (7)

[0029] In formula (7), the active power reference value P ref Active power output P for adding frequency regulation power to offshore wind turbines W The maximum power P of the offshore wind turbine operating in maximum power point tracking mode MPPT Compare and take the maximum value.

[0030] As a further preferred technical solution of the present invention, in step S3, the system frequency deviation ∆f and rotor speed deviation ∆ω are normalized by the speed deviation quantization factor K1 and the frequency deviation quantization factor K2, and then input into the fuzzy control after being subjected to amplitude limiting processing according to the fuzzy domain.

[0031] Furthermore, as a preferred embodiment of the present invention, the system frequency deviation ∆f and rotor speed deviation ∆ω are used as inputs to fuzzy control, and the proportional control coefficient k of the P controller is output. P Specifically:

[0032] Based on the variation law of power system frequency deviation ∆f and rotor speed deviation ∆ω, the fuzzy sets of frequency deviation and rotor speed deviation are set as {Z, S, MS, MB, B}, representing {zero, small, slightly small, slightly large, large}.

[0033] Establish fuzzy sets of frequency deviation and rotor speed deviation and proportional control coefficient k of P controller. P Fuzzy rules for fuzzy sets;

[0034] The frequency deviation and rotor speed deviation are fuzzified using a triangular membership function. Based on fuzzy rules, the fuzzification result is then defuzzified using the centroid method to obtain the proportional control coefficient k of the P controller. P .

[0035] Furthermore, as a preferred embodiment of the present invention, the fuzzy control rule of the fuzzy logic controller is: when ∆f is large, then k P When ∆ω is relatively small, then k P Larger; when ∆f is smaller, then k P Larger; when ∆ω is smaller, then k P Smaller; where ∆f is the system frequency deviation, ∆ω is the rotor speed deviation, and k P This is the proportional control coefficient for the P controller.

[0036] As a further preferred embodiment of the present invention, in step S5, when the speed recovery start condition is met, the selector switch is switched to channel 1, and the speed recovery control start condition is:

[0037] (8)

[0038] In formula (8), df / dt is the system frequency change rate and dω / dt is the rotor speed change rate.

[0039] Furthermore, as a preferred embodiment of the present invention, a speed recovery method based on fuzzy P control is used for speed recovery, specifically expressed as follows:

[0040] During the speed recovery period, the speed recovery power ∆P is:

[0041] (3)

[0042] Active power output P of offshore wind turbine W for:

[0043] (9)

[0044] At this time, the active power reference value P of the offshore wind turbine is... ref for:

[0045] (10)

[0046] In formula (10), the active power reference value P ref The active power output P of offshore wind turbines, which adds frequency regulation power and speed recovery power. W The maximum power P of the offshore wind turbine operating in maximum power point tracking mode MPPT The maximum value is obtained by comparison.

[0047] As a further preferred embodiment of the present invention, after the speed recovery is complete, the wind turbine switches to maximum power point tracking (MPPT) mode to control the output power of the unit, and the active power reference value P of the offshore wind turbine is... ref Smoothly switch to P MPPT The specific expression is:

[0048] (11);

[0049] The gain coefficient of the P controller is dynamically adjusted by fuzzy control based on the real-time detected frequency and speed deviations. Specifically, at the beginning of each control cycle, the P controller clears the k value used in the previous control cycle. P The new k obtained by the fuzzy controller P The updated P parameters are then stored in the P controller and maintained throughout the current cycle. The P controller uses the updated P parameters to control the controlled variables of the system.

[0050] As a further preferred technical solution of the present invention, the recovery power ∆P is small at the moment of starting the speed recovery control to avoid sudden changes in active power output and alleviate the secondary frequency drop; the recovery power ∆P gradually increases in the early stage of starting the speed recovery control to increase the rotor speed recovery speed and reduce the rotor speed recovery time; the recovery power ∆P gradually decreases to 0 in the later stage of starting the speed recovery control, so that the offshore wind turbine can realize the recovery of MPPT control mode.

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0052] This invention compensates for the system's active power loss by adding droop control to the fan rotor side control and releasing the rotational kinetic energy stored in the fan; it uses a fuzzy controller to monitor the changes in the system's speed and frequency in real time, and selects appropriate parameters according to the established rules, thus solving the problem of weak adaptability of set parameters under different scenarios; it designs fuzzy control rules that meet the objectives, so as to achieve smooth active power output, smooth switching of control modes, smooth recovery of speed and alleviate the depth of secondary frequency drop. Attached Figure Description

[0053] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0054] Figure 1 This is a schematic diagram of the method flow proposed in this invention;

[0055] Figure 2 This is a schematic diagram of the membership function of the fuzzy control input and output variables proposed in this invention;

[0056] Figure 3 This is a schematic diagram of the simulation system according to an embodiment of the present invention;

[0057] Figure 4(a) is a frequency curve of the power system when the wind speed is 9 m / s and the load disturbance of 0.1 pu is applied for 45 s in an example of the present invention.

[0058] Figure 4(b) is a rotor speed curve of an offshore wind turbine under a wind speed of 9 m / s and a load disturbance of 0.1 pu for 45 s.

[0059] Figure 4(c) is the active power output curve of the offshore wind turbine when the wind speed is 9 m / s and the load disturbance is 0.1 pu for 45 s in the example of the present invention.

[0060] Figure 5(a) is a frequency curve of the power system when the wind speed is 10 m / s and the load disturbance of 0.1 pu is applied for 45 s in an example of the present invention.

[0061] Figure 5(b) is a rotor speed curve of an offshore wind turbine under a wind speed of 10 m / s and a load disturbance of 0.1 pu for 45 s.

[0062] Figure 6(a) is a frequency curve of the power system when the wind speed is 9 m / s and the load disturbance of 0.15 pu is applied for 45 s in an example of the present invention.

[0063] Figure 6(b) is a rotor speed curve of an offshore wind turbine under a wind speed of 9 m / s and a load disturbance of 0.15 pu for 45 s. Detailed Implementation

[0064] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. Of course, the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0065] like Figure 1 As shown, the present invention proposes a method for restoring the rotational speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops, comprising the following steps:

[0066] S1: Collect the grid frequency f, calculate the system frequency deviation ∆f and the system frequency change rate df / dt; collect the rotor speed ω of the offshore wind turbine. r Calculate the system frequency deviation ∆ω and the rotor speed change rate dω / dt. The specific expressions for the system frequency deviation ∆f and rotor speed deviation ∆ω are as follows:

[0067] (1)

[0068] (2)

[0069] In formula (1), ∆f is the system frequency deviation, f is the real-time frequency of the system, and f0 is the rated frequency of the system; in formula (2), ∆ω is the rotor speed deviation, ω r ω represents the real-time rotor speed of an offshore wind turbine. off For t off The rotor speed at time t off This is the moment when the engine speed recovers and starts.

[0070] S2: Determine whether the system frequency is stable. If the system meets the requirements for safe and stable operation of the power grid, the offshore wind turbine will operate in maximum power point tracking (MPPT) mode. If the requirements are not met, droop control will be used to participate in system frequency regulation.

[0071] Before the system provides frequency support and after the offshore wind turbine completes speed recovery, the turbine operates in maximum power point tracking mode, and its output electromagnetic power expression is:

[0072] (4)

[0073] In formula (4), P MPPT The maximum power of the offshore wind turbine operating in maximum power point tracking mode, ρ is the air density, R is the radius of the offshore wind turbine blade, and C is the maximum power of the offshore wind turbine operating in maximum power point tracking mode. p Maximum wind energy utilization coefficient, where v is wind speed and ω is the wind speed. r is the real-time rotor speed of the offshore wind turbine, and k0 is the coefficient of the maximum power point tracking curve.

[0074] The method for offshore wind turbines to participate in system frequency regulation using droop control is as follows:

[0075] Additional active power ∆P generated by offshore wind turbines when droop control is used d for:

[0076] (5)

[0077] In formula (5), ΔP d K is the active power generated by the droop control of offshore wind turbines. dΔf is the droop control coefficient, and Δf is the system frequency deviation.

[0078] Active power output P of offshore wind turbine W for:

[0079] (6)

[0080] At this time, the active power reference value P of the offshore wind turbine is... ref for:

[0081] (7)

[0082] In formula (7), the active power reference value P ref Active power output P for adding frequency regulation power to offshore wind turbines W The maximum power P of the offshore wind turbine operating in maximum power point tracking mode MPPT Compare and take the maximum value.

[0083] S3: Fuzzy rules are formulated to suppress secondary frequency drops and shorten speed recovery time. The system frequency deviation ∆f and rotor speed deviation ∆ω are used as inputs to the fuzzy controller, and the output is the proportional control coefficient k of the P controller. P ;

[0084] The system frequency deviation ∆f and rotor speed deviation ∆ω are normalized and limited by the speed deviation quantization factor K1 and frequency deviation quantization factor K2 before being input into the fuzzy control.

[0085] The system frequency deviation ∆f and rotor speed deviation ∆ω are used as inputs to fuzzy control, and the output is the proportional control coefficient k of the P controller. P Specifically:

[0086] Based on the variation patterns of power system frequency deviation ∆f and rotor speed deviation ∆ω, the fuzzy sets of frequency deviation and rotor speed deviation are denoted as {Z, S, MS, MB, B}, representing {zero, small, slightly small, slightly large, large}, as follows. Figure 2 As shown, the universe of discourse for the input variable is [0, 1], and the universe of discourse for the output variable is [0, 8].

[0087] Establish fuzzy sets of frequency deviation and rotor speed deviation and proportional control coefficient k of P controller. P The fuzzy rules for fuzzy sets, and the fuzzy logic inference table are shown below:

[0088] Table 1 Fuzzy Logic Inference Table

[0089]

[0090] The frequency deviation and rotor speed deviation are fuzzified using a triangular membership function. Based on fuzzy rules, the fuzzification result is then defuzzified using the centroid method to obtain the proportional control coefficient k of the P controller. P。

[0091] The fuzzy control rules of the fuzzy logic controller are as follows:

[0092] When ∆f is large, then k P Smaller;

[0093] When ∆ω is large, then k P Larger;

[0094] When ∆f is small, then k P Larger;

[0095] When ∆ω is small, then k P Smaller.

[0096] Where ∆f is the system frequency deviation, ∆ω is the rotor speed deviation, and k P This is the proportional control coefficient for the P controller.

[0097] The gain coefficient of the P controller is dynamically adjusted by fuzzy control based on the real-time detected frequency and speed deviations. The specific steps are as follows:

[0098] At the beginning of each control cycle, the P controller clears the k used in the previous control cycle. P ;

[0099] The new k obtained by the fuzzy controller P Update the P controller and maintain this control parameter throughout the current cycle;

[0100] The P controller uses the updated P parameters to control the controlled variables of the system.

[0101] S4: Convert the k output by the fuzzy controller P As the gain of the P controller, the input is the difference between the rotor speed of the offshore wind turbine at the steady-state moment and the current rotor speed of the offshore wind turbine, and the output is the speed recovery power ∆P, which is calculated as follows:

[0102] (3)

[0103] In formula (3), ∆P is the speed recovery power of the offshore wind turbine, ω0 is the rotor speed of the offshore wind turbine at the steady state of the system, and ω r k is the real-time rotor speed of an offshore wind turbine. P This is the proportional control coefficient for the P controller.

[0104] S5: Determine whether the system meets the speed recovery condition. If it does, the offshore wind turbine will obtain additional speed recovery power ∆P and enter the speed recovery stage until the speed is recovered.

[0105] When the speed recovery start condition is met, the selector switch is switched to channel 1. The speed recovery control start condition is as follows:

[0106] (8)

[0107] In formula (8), df / dt is the system frequency change rate and dω / dt is the rotor speed change rate.

[0108] During the speed recovery period, the speed recovery power ∆P is:

[0109] (3)

[0110] Active power output P of offshore wind turbine W for:

[0111] (9)

[0112] At this time, the active power reference value P of the offshore wind turbine is... ref for:

[0113] (10)

[0114] In formula (10), the active power reference value P ref The active power output P of offshore wind turbines, which adds frequency regulation power and speed recovery power. W The maximum power P of the offshore wind turbine operating in maximum power point tracking mode MPPT The maximum value is obtained by comparison.

[0115] S6: Active power output P of offshore wind turbine with additional frequency regulation power and speed recovery power W The maximum power P of the offshore wind turbine operating in maximum power point tracking mode MPPT The maximum value is used as a reference value P for the active power of offshore wind turbines. ref ;

[0116] S7: Reference value P of active power after the offshore wind turbine completes speed recovery. ref From active power output P W Smoothly switch back to maximum power P MPPT This enables MPPT mode recovery.

[0117] Once the rotational speed is restored, the wind turbine switches to Maximum Power Point Tracking (MPPT) mode to control the unit's output power. The active power reference value P for offshore wind turbines... ref Smoothly switch to PMPPT The specific expression is:

[0118] (11)

[0119] In step S4, the recovery power ∆P is relatively small at the moment of starting the speed recovery control to avoid sudden changes in active power output and alleviate the secondary frequency drop. In the early stage of starting the speed recovery control, the recovery power ∆P gradually increases to increase the rotor speed recovery speed and reduce the rotor speed recovery time. In the later stage of starting the speed recovery control, the recovery power ∆P gradually decreases to 0, and the offshore wind turbine realizes the recovery of MPPT control mode.

[0120] To verify the effectiveness of the control proposed in this invention in restoring speed performance and suppressing secondary frequency drops under different operating conditions, the technical effects of this invention will be explained in detail below with simulation examples.

[0121] To verify the effectiveness of the proposed method for suppressing secondary frequency drops and restoring the rotational speed and MPPT mode of offshore wind turbines under different wind speeds and disturbance scenarios, a simulation system including an offshore wind farm and an onshore synchronous machine was built on the MATLAB / Simulink platform. Figure 3 As shown in the figure; in addition, the parameters of offshore wind farms and onshore synchronous turbines are shown in Table 2;

[0122] Table 2 Simulation Parameters

[0123]

[0124] The following analysis compares the frequency support and speed recovery degree of offshore wind turbines under two scenarios: maximum power point tracking control, speed recovery control based on traditional torque limit control, speed recovery control based on constant P control, and speed recovery control based on fuzzy P control, when a load disturbance of 0.1 pu occurs at 45 s and the wind speed is 9 m / s and 10 m / s.

[0125] When the wind speed is 9 m / s, as shown in Figures 4(a) to 4(c), when the doubly-fed induction generator (DFIG) wind turbine uses maximum power point tracking (MPPT) control, the wind turbine cannot provide frequency response service to the grid. At 45 s, no speed recovery strategy is initiated, the turbine's active power output and speed remain unchanged, and the maximum grid frequency deviation is 0.710 Hz. Traditional torque limit control instantaneously reduces the electromagnetic power output by 0.05 pu at the speed recovery initiation moment, providing transient support for the system frequency. The secondary frequency drop depth is reduced to 0.045 Hz, maintaining the DFIG output unchanged until it matches the maximum power point tracking curve P. MPPTEqually, the DFIG, under maximum power point tracking control, completes the speed recovery process, extending the speed recovery time; the rotor speed recovers to ω0 in 76 s. The constant P coefficient control wind turbine speed recovery strategy, at the moment of strategy startup, the turbine's frequency regulation output reaches the set limit of 0.2 pu, and the frequency experiences a second drop of 0.200 Hz compared to the startup moment of the recovery strategy, with the drop depth exceeding the lowest point of the first drop; the speed recovery time is 64.4 s. The wind turbine speed recovery strategy using fuzzy P control proposed in this invention achieves a smaller frequency regulation output at startup, gradually increasing smoothly, and decreasing later according to the decrease in frequency deviation and speed frequency difference, with only a 0.036 Hz frequency drop, significantly reducing the second frequency drop, while simultaneously increasing the rotor speed recovery speed; the rotor speed recovery time is within 60 s. In summary, the improved strategy proposed in this invention effectively alleviates the second frequency drop phenomenon, shortens the speed recovery time, and improves system stability.

[0126] When the wind speed is increased to 10 m / s, the speed recovery strategy is initiated at 45 s. As shown in Figure 5(a), all three recovery strategies experience a secondary frequency drop due to the release of rotor kinetic energy by the fan. The system frequency decreases by 0.022 Hz, 0.038 Hz, and 0.011 Hz respectively compared to the start-up time. It is evident that the improved strategy proposed in this invention effectively alleviates the problem of secondary frequency drop caused by speed recovery. As shown in Figure 5(b), the rotor speed recovery times for the four strategies are 70.7 s, 73.1 s, 65.3 s, and 61.4 s respectively. The improved strategy increases the rotor speed recovery time.

[0127] By comparing the method of the present invention, the improved strategy for speed recovery under different wind speeds can smoothly achieve the desired result of the wind turbine output first increasing and then decreasing, alleviate the secondary frequency drop, and accelerate the speed recovery time.

[0128] The following analysis and comparison will be conducted on the frequency support and speed recovery degree of offshore wind turbines under the following load disturbances: maximum power point tracking control, speed recovery control based on traditional torque limit control, speed recovery control based on constant P control, and speed recovery control based on fuzzy P control, when the system experiences load disturbances of 0.1 pu and 0.15 pu at 45 s with a wind speed of 9 m / s.

[0129] Figures 6(a) and 6(b) show the simulation results with a load disturbance of 0.15 pu. When the offshore wind turbine adopts maximum power point tracking control, it does not participate in system frequency regulation, and the active power output and speed of the turbine remain unchanged. The maximum deviation of the grid frequency is 0.710 Hz. The proposed offshore wind turbine speed recovery control based on fuzzy P control achieves a 0.045% increase in the minimum point of the second frequency drop compared to the speed recovery control based on traditional torque limit control, and a 17.461% increase compared to the speed recovery control based on constant P control. The rotor speed recovery time is between 64 s and 72 s, and the speed recovery time of the proposed improved strategy is better than that of the traditional recovery strategy.

[0130] The system is subjected to a large power load disturbance, resulting in a severe secondary frequency drop. The system needs to provide more frequency modulation power to maintain the safety and stability of the system frequency. This paper proposes an improved strategy. Although the output changes are more volatile due to instability, it still meets the characteristics of gradually increasing and then decreasing, which alleviates the secondary frequency drop phenomenon and ensures a short speed recovery time.

[0131] This invention aims to provide a method for restoring the speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops. Addressing the problems of weak parameter adaptability of traditional P controllers in different scenarios and the significant secondary frequency drop caused by the speed recovery process, this invention proposes a strategy for restoring the speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops. The wind turbine provides frequency support to the grid through droop control, and generates speed recovery power using fuzzy P control, causing the control mode to automatically and smoothly switch back to MPPT control mode. This achieves smooth turbine output and speed recovery while mitigating the depth of the secondary frequency drop. Simulation results show that the proposed method can effectively suppress secondary frequency drops while ensuring the speed recovery time of offshore wind turbines.

[0132] The specific implementation schemes described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific implementation schemes of the present invention and are not intended to limit the scope of the present invention. Any equivalent changes and modifications made by those skilled in the art without departing from the concept and principles of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for restoring the rotational speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops, characterized in that, Includes the following steps: S1. Collect the real-time frequency f of the power grid system, and calculate the system frequency deviation ∆f and the system frequency change rate df / dt; collect the real-time rotor speed ω of the offshore wind turbine. r Calculate the rotor speed deviation ∆ω and the rotor speed change rate dω / dt; the system frequency deviation ∆f and rotor speed deviation ∆ω are specifically expressed as follows: (1) (2) In formula (1), ∆f is the system frequency deviation, f is the real-time frequency of the system, and f0 is the rated frequency of the system; In formula (2), ∆ω is the rotor speed deviation, ω r ω represents the real-time rotor speed of an offshore wind turbine. off For t off The rotor speed at time t off This is the moment when the engine speed recovers and starts; S2. Determine whether the system frequency is stable. If the system meets the requirements for safe and stable operation of the power grid, the offshore wind turbine will operate in maximum power point tracking (MPPT) mode. If the requirements are not met, droop control will be used to participate in system frequency regulation. In step S2, the method for offshore wind turbines to participate in system frequency regulation using droop control is as follows: Additional active power ∆P generated by offshore wind turbines when droop control is used d : (5) In formula (5), ΔP d The additional active power generated for droop control of offshore wind turbines, K d Here, Δf is the droop control coefficient, and Δf is the system frequency deviation. Active power output P of offshore wind turbine W for: (6) At this time, the reference value of the active power P of the offshore wind turbine is... ref for: (7) In formula (7), the active power reference value P ref Active power output P for adding frequency regulation power to offshore wind turbines W The maximum power P of offshore wind turbines operating in maximum power point tracking (MPPT) mode MPPT Compare and take the maximum value; S3. To suppress secondary frequency drops and shorten speed recovery time, fuzzy rules are formulated. The system frequency deviation ∆f and rotor speed deviation ∆ω are used as inputs to the fuzzy controller, and the output is the proportional control coefficient k of the P controller. P ; S4, convert the k output by the fuzzy controller P As the proportional control coefficient of the P controller, the input is the difference between the rotor speed of the offshore wind turbine at the steady state and the real-time rotor speed of the offshore wind turbine, and the output is the speed recovery power ∆P, which is calculated as follows: (3) In formula (3), ∆P is the speed recovery power of the offshore wind turbine, ω0 is the rotor speed of the offshore wind turbine at the steady state of the system, and ω r k is the real-time rotor speed of an offshore wind turbine. P For the proportional control coefficient of the P controller; S5. Determine whether the system meets the startup conditions for speed recovery control. If it does, the offshore wind turbine will obtain speed recovery power ∆P and enter the speed recovery stage until the speed is recovered. S6, Active power output P of offshore wind turbine with additional frequency regulation power and speed recovery power. W The maximum power P of offshore wind turbines operating in maximum power point tracking (MPPT) mode MPPT The maximum value is taken as the reference value P for the active power of offshore wind turbines. ref ; A speed recovery method based on fuzzy P control is adopted for speed recovery, as detailed below: During the speed recovery period, the speed recovery power ∆P is: (3) Active power output P of offshore wind turbine W for: (9) At this time, the active power reference value P of the offshore wind turbine is... ref for: (10) In formula (10), the active power reference value P ref The active power output P of offshore wind turbines, which adds frequency regulation power and speed recovery power. W The maximum power P of offshore wind turbines operating in maximum power point tracking (MPPT) mode MPPT The comparison yields the maximum value; S7. After the offshore wind turbine completes speed recovery, the active power reference value P ref From active power output P W The maximum power P that smoothly switches back to maximum power point tracking (MPPT) mode. MPPT This enables recovery of the Maximum Power Point Tracking (MPPT) mode.

2. The method for restoring the speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops according to claim 1, characterized in that, Before the system provides frequency support and after the offshore wind turbine completes speed recovery, the turbine operates in Maximum Power Point Tracking (MPPT) mode, and its output electromagnetic power expression is: (4) In formula (4), P MPPT The maximum power of the offshore wind turbine operating in Maximum Power Point Tracking (MPPT) mode, ρ is the air density, R is the radius of the offshore wind turbine blade, and C is the maximum power of the offshore wind turbine operating in MPPT mode. p Maximum wind energy utilization coefficient, where v is wind speed and ω is the wind speed. r is the real-time rotor speed of the offshore wind turbine, and k0 is the coefficient of the maximum power point tracking curve.

3. The method for restoring the speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops according to claim 2, characterized in that, In step S3, the system frequency deviation ∆f and rotor speed deviation ∆ω are normalized by the speed deviation quantization factor K1 and the frequency deviation quantization factor K2, and then input into the fuzzy controller after being limited according to the fuzzy universe of discourse.

4. The method for restoring the speed and MPPT mode of offshore wind turbines by suppressing secondary frequency drops according to claim 3, characterized in that, The system frequency deviation ∆f and rotor speed deviation ∆ω are used as inputs to the fuzzy controller, and the output is the proportional control coefficient k of the P controller. P Specifically: Based on the variation law of system frequency deviation ∆f and rotor speed deviation ∆ω, the fuzzy sets of system frequency deviation and rotor speed deviation are set as {Z, S, MS, MB, B}, representing {zero, small, slightly small, slightly large, large}. Establish fuzzy sets of system frequency deviation and rotor speed deviation and proportional control coefficient k of P controller. P Fuzzy control rules for fuzzy sets; The system frequency deviation and rotor speed deviation are fuzzified using triangular membership functions. Based on fuzzy control rules, the fuzzification result is then defuzzified using the centroid method to obtain the proportional control coefficient k of the P controller. P .

5. The method for restoring the speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops according to claim 4, characterized in that, The fuzzy control rule of the fuzzy controller is: when ∆f is large, then k P When ∆ω is relatively small, then k P Larger; when ∆f is smaller, then k P Larger; when ∆ω is smaller, then k P Smaller; where ∆f is the system frequency deviation, ∆ω is the rotor speed deviation, and k P This is the proportional control coefficient for the P controller.

6. The method for restoring the speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops according to claim 5, characterized in that, Step S5 satisfies the start condition for speed recovery control. The selector switch is switched to channel 1. The start condition for speed recovery control is: (8) In formula (8), df / dt is the system frequency change rate and dω / dt is the rotor speed change rate.

7. The method for restoring the speed and MPPT mode of offshore wind turbines by suppressing secondary frequency drops according to claim 6, characterized in that, Once the rotational speed is restored, the wind turbine switches to Maximum Power Point Tracking (MPPT) mode to control the unit's output power. The active power reference value P for offshore wind turbines... ref Smoothly switch to P MPPT The specific expression is: (11)； The proportional control coefficient of the P controller is dynamically adjusted by the fuzzy controller based on the real-time detected system frequency deviation and rotor speed deviation. Specifically, at the beginning of each control cycle, the P controller clears the k value used in the previous control cycle. P The new k obtained by the fuzzy controller P The updated proportional control coefficient is then incorporated into the P controller and maintained throughout the current cycle. The P controller uses this updated proportional control coefficient to control the controlled variable of the system.

8. The method for restoring the speed and MPPT mode of offshore wind turbines to suppress secondary frequency drops according to claim 7, characterized in that, At the moment of startup of speed recovery control, the speed recovery power ∆P is relatively small, which avoids sudden changes in active power output and alleviates the secondary frequency drop. In the early stage of startup of speed recovery control, the speed recovery power ∆P gradually increases, which increases the rotor speed recovery speed and reduces the rotor speed recovery time. In the later stage of startup of speed recovery control, the speed recovery power ∆P gradually decreases to 0, and the offshore wind turbine achieves MPPT mode recovery.

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