A multi-objective control method for permanent magnet direct-driven wind turbine under grid asymmetric fault

Abstract

The application provides a kind of permanent magnet direct drive wind turbine multi-objective control method under power grid asymmetrical fault, solve the technical problem of power fluctuation suppression and current unbalance degree reduction control target coordination optimization in fault ride-through. Including the following steps: S1: establish the positive and negative sequence coordinate system model of permanent magnet direct drive wind turbine grid-side converter, separate the positive and negative sequence of grid-side converter voltage and current;S2: calculate the grid-side converter reference current instruction of three control targets;S3: construct the unified expression form of grid-side converter reference current;S4: select active power fluctuation suppression and negative sequence current suppression as the coordinated optimization control target for control;S5: construct multi-objective optimization function, obtain the converter current reference value, and carry out current closed-loop control on grid-side converter. The beneficial effects of the application are: the multi-objective coordinated optimization control of permanent magnet direct drive wind turbine under power grid voltage fault is realized, and the low voltage ride-through capability and grid-connected performance are enhanced.

Description

Technical Field

[0001] This invention relates to the field of permanent magnet direct drive wind turbine technology, and in particular to a multi-objective control method for permanent magnet direct drive wind turbines under grid asymmetric faults. Background Technology

[0002] Permanent magnet direct-drive wind power systems have attracted widespread attention in the wind power field due to their superior reliability and low maintenance costs. With the continuous increase in the transmission capacity of wind power systems, asymmetrical grid faults can cause stator current distortion and DC bus voltage fluctuations in permanent magnet direct-drive wind turbines, seriously threatening equipment safety and grid stability. Traditional control strategies often focus on a single objective, making it difficult to simultaneously coordinate multiple requirements such as current over-limit suppression, power fluctuation suppression, and negative sequence current compensation during fault periods. Experts and scholars have conducted relevant research on multi-objective coordinated optimization control methods.

[0003] Chinese Patent Publication No. CN118889526A, Publication Date: November 1, 2024, discloses a method for transient stability control of DC voltage in a doubly-fed induction generator (DFIG) wind power grid-connected system under asymmetrical grid faults. When an asymmetrical short-circuit fault occurs in the grid, the method obtains the reference value of the active current of the grid-side converter through a proportional element; obtains the positive and negative sequence reactive current output of the DFIG wind power grid-connected system during the asymmetrical grid fault; calculates and obtains the positive and negative sequence active current output of the DFIG wind power grid-connected system during the asymmetrical grid fault; finally, it calculates and obtains the positive and negative sequence active and reactive current command values ​​output by the rotor-side converter and the grid-side converter of the DFIG wind power grid-connected system during the asymmetrical grid fault; by coordinating the positive and negative sequence active and reactive current commands output by the rotor-side converter and the grid-side converter, it eliminates power fluctuations on the DC capacitor and improves the transient stability operation capability of the DC bus voltage of the DFIG wind power grid-connected system.

[0004] Chinese Patent Publication No. CN119448316A, published on February 14, 2025, discloses a fault ride-through method and system for a renewable energy grid-connected converter under asymmetrical fault conditions. The method includes: calculating the positive-sequence voltage amplitude and voltage imbalance of the power grid based on positive and negative sequence parameters; determining whether the power grid is in a normal or fault state; when the power grid is in an asymmetrical fault state, using the calculation formula of the positive and negative sequence current command under the asymmetrical fault state based on relevant grid fault parameters, calculating the limit value of the positive and negative sequence current command of the renewable energy grid-connected converter under the asymmetrical fault state, and performing feedback control based on the limit value of the positive and negative sequence current command to complete the fault ride-through. This invention limits the current in the calculation formula of the positive and negative sequence current command, thereby maximizing the satisfaction of the positive and negative sequence active and reactive current control objectives while ensuring that converter overcurrent does not occur, thus improving the grid support capability during fault periods. However, this method has a single control objective and cannot perform systematic analysis of various control objectives.

[0005] Chinese Patent Publication No. CN114825395B, published on December 17, 2024, discloses a control strategy for a flywheel energy storage grid-side converter under asymmetrical grid faults. First, a mathematical model of the grid-side converter based on a two-phase rotating dq coordinate system is constructed. Combining the control objectives of the grid-side converter, a direct current control strategy based on grid voltage orientation is adopted. The control system consists of an outer loop for DC voltage and inner loops for active and reactive current. Reference values ​​for positive and negative sequence currents are obtained through the matrix relationship between power and appropriate amounts of positive and negative sequence voltage and current. A dual-closed-loop current control strategy with positive and negative sequence separation is adopted to control the positive and negative sequence currents, thereby effectively suppressing the second harmonic fluctuation of active power. The maximum value of the three-phase current amplitude output by the grid-side converter in the abc coordinate system is calculated in real time and compared with the current limit value. If the current exceeds the limit, the current is adjusted by adjusting the power reference value to prevent it from exceeding the limit. However, this method only considers the current limit exceeding problem and does not comprehensively consider other problems such as power limit exceeding.

[0006] How to solve the above-mentioned technical problems is the challenge facing this invention. Summary of the Invention

[0007] The purpose of this invention is to provide a multi-objective control method for permanent magnet direct-drive wind turbines under grid asymmetric faults. Addressing the technical problems of traditional control methods under grid asymmetric faults, such as single control objective, current exceeding limits, and neglecting the voltage support effect of control objectives, this invention proposes a multi-objective control method for permanent magnet direct-drive wind turbines under grid asymmetric faults. Its purpose is to achieve coordinated optimization of control objectives for power fluctuation suppression and current imbalance reduction, improve the low-voltage ride-through capability of wind turbines, and enhance the grid connection performance of the system.

[0008] The inventive concept of this invention is as follows: This invention provides a multi-objective control method for permanent magnet direct-drive wind turbines under grid asymmetric faults, including: establishing a positive and negative sequence coordinate system model of the grid-side converter of the permanent magnet direct-drive wind turbine, and using a dual second-order generalized integrator to separate the positive and negative sequences of the voltage and current of the grid-side converter; deriving the power matrix equation of the grid-side converter based on instantaneous power theory, and calculating the reference current command of the grid-side converter with the control objectives of suppressing grid-side active power fluctuations, reactive power fluctuations, and minimizing grid-connected current imbalance; and calculating the reference current command of the grid-side converter under the three control objectives obtained in step S2. The invention proposes a multi-objective control method for permanent magnet direct-drive wind turbines under grid asymmetric faults. This method achieves coordinated optimization of power fluctuation suppression and current imbalance reduction control objectives, improves the low-voltage ride-through capability of wind turbines, and enhances the grid-side converter's grid connection performance. Based on Kirchhoff's voltage law, the grid connection point voltage support equation is derived. Considering the impact of the three control objectives on the negative-sequence voltage at the grid connection point, suppressing active power fluctuations and suppressing negative-sequence current are selected as coordinated optimization control objectives. An evaluation index for the coordinated optimization control objectives in step S4 is defined, and a multi-objective optimization function is constructed accordingly. The optimal current regulation coefficient k is then solved to obtain the converter current reference value, enabling closed-loop current control of the grid-side converter.

[0009] To achieve the aforementioned objectives, the present invention employs the following technical solution: a multi-objective control method for permanent magnet direct-drive wind turbines under grid asymmetric faults, comprising the following steps:

[0010] S1: Establish a positive and negative sequence coordinate system model of the grid-side converter of the permanent magnet direct-drive wind turbine, and use a double second-order generalized integrator to separate the positive and negative sequence of the voltage and current of the grid-side converter.

[0011] S2: Based on instantaneous power theory, derive the power matrix equation of the grid-side converter, and calculate the reference current command of the grid-side converter with the control objectives of suppressing grid-side active power fluctuations, reactive power fluctuations and minimizing grid-connected current imbalance.

[0012] S3: Based on the reference current commands under the three control objectives obtained in step S2, introduce the current regulation coefficient K to construct a unified expression form of grid-side converter reference current that takes into account the above three control objectives.

[0013] S4: Based on Kirchhoff's voltage law, the voltage support equation at the grid connection point is derived. Based on the influence of three control objectives on the negative sequence voltage at the grid connection point, suppressing active power fluctuations and suppressing negative sequence current are selected as coordinated optimization control objectives for control.

[0014] S5: Define the evaluation index of the coordinated optimization control objective in step S4, and construct a multi-objective optimization function accordingly. Solve for the optimal current regulation coefficient k, and then obtain the converter current reference value to perform current closed-loop control on the grid-side converter.

[0015] Step S1 establishes a positive and negative sequence coordinate system model of the grid-side converter of the permanent magnet direct-drive wind turbine, and uses a dual second-order generalized integrator to separate the positive and negative sequence of the voltage and current of the grid-side converter, as detailed below:

[0016] Based on the positive and negative sequence coordinate system model of the grid-side converter of the permanent magnet direct-drive wind turbine, the grid-side converter adopts a positive and negative sequence dual current vector control structure. It uses dual second-order generalized integrators to separate and orient the positive and negative sequence of the voltage components of the grid-side converter, and to separate the positive and negative sequence of the current components of the grid-side converter.

[0017] Step S2 derives the power matrix equation of the grid-side converter based on instantaneous power theory, and calculates the reference current command of the grid-side converter with the control objectives of suppressing grid-side active power fluctuations, suppressing grid-side reactive power fluctuations, and minimizing grid-connected current imbalance, as follows:

[0018] S2.1: Calculate the grid-side converter reference current expression for suppressing active power fluctuations:

[0019]

[0020] in, These are the d-axis and q-axis components of the converter outlet grid voltage in the positive sequence coordinate system, respectively. These are the d-axis and q-axis components of the converter outlet grid voltage in the negative sequence coordinate system, respectively. These are the d-axis and q-axis components of the grid-side current in the positive sequence coordinate system, respectively. Let d and q be the grid-side current components in the negative sequence coordinate system; P0 and Q0 are the average active power and average reactive power of the grid side, respectively.

[0021] S2.2: Calculate the grid-side converter reference current expression for suppressing reactive power fluctuations:

[0022]

[0023] S2.3: Calculate the grid-side converter reference current expression for minimizing grid-connected current imbalance:

[0024]

[0025] Step S3 introduces a current regulation coefficient K to construct a unified expression for the reference current of the grid-side converter, as follows:

[0026]

[0027] Where K=1, it can suppress grid-side active power fluctuations; K=0, it can minimize grid-connected current imbalance; K=-1, it can suppress grid-side reactive power fluctuations; λ is the voltage imbalance.

[0028] In step S4, the grid connection point voltage support equation is derived based on Kirchhoff's voltage law, and the control target is selected based on the principle of reducing the grid connection point voltage imbalance, as follows:

[0029] Based on Kirchhoff's voltage law, the voltage support equation at the grid connection point is derived. Using the symmetrical component method, the voltage support equation under asymmetrical faults is obtained, as shown below:

[0030]

[0031] In the formula, These are the positive and negative sequence voltage amplitudes at the grid connection point, respectively. These represent the positive and negative sequence voltage amplitudes of the power grid voltage, respectively.

[0032] Since the transverse component mainly affects the phase angle between the grid connection point and the grid voltage, and has little impact on the voltage amplitude at the grid connection point, the active current I is ignored. d The voltage drop across the reactance ωL and the reactive current I q The simplified PCC voltage support equation for the voltage drop across resistor R is as follows:

[0033]

[0034] As can be seen from the above equation, the positive sequence voltage rise and negative sequence voltage suppression depend on the direction and magnitude of the positive and negative sequence active and reactive currents, while the magnitude and direction of the current components are affected by the control strategy.

[0035] Combining the effects of the above three control objectives on grid connection point voltage support, both suppressing grid-side active power fluctuations and minimizing grid-connected current imbalance contribute to reducing grid connection point voltage imbalance. The active power fluctuation suppression control objective injects... The voltage generated across the equivalent impedance of the power grid is in the same direction as the negative sequence voltage of the power grid. Conversely, the negative sequence voltage at the grid connection point decreases; while the reactive power fluctuation suppression control target is met by the injected... The voltage generated across the equivalent impedance of the power grid is in the same direction as the negative sequence voltage of the power grid. Similarly, the increase in negative sequence voltage at the grid connection point raises both positive and negative sequence voltages, which is not conducive to improving the voltage imbalance at the grid connection point. Therefore, suppressing active power fluctuations and suppressing negative sequence current are selected as the coordinated optimization control objectives.

[0036] In step S5, an evaluation index for the coordinated optimization control objective is defined, and a multi-objective optimization function is constructed accordingly, as follows:

[0037] S5.1: Calculate the magnitude of active power fluctuation and current imbalance on the grid side:

[0038]

[0039] in, The magnitude of active power fluctuation on the grid side; P sin P represents the sinusoidal component of the active power fluctuation on the grid side. cos I' represents the cosine component of the active power fluctuation on the grid side; I' represents the grid-connected current imbalance.

[0040] S5.2: Define the evaluation index γ for the active power fluctuation of the grid-side converter. P and grid-connected current imbalance evaluation index γ i :

[0041]

[0042] S5.3: Construct a multi-objective function based on the evaluation metrics defined in step S5.2:

[0043] minF(K)=x1(γ P ) 2 +x2(γ i ) 2

[0044] Among them, the current regulation coefficient K ranges from [0,1] to achieve coordinated optimization between the grid-side active power fluctuation suppression and the grid-connected current balance target; the coefficient x1 is the weight of the grid-side active power fluctuation suppression target. When the grid has high requirements for active power fluctuation, the suppression of active power fluctuation is given priority, and the range of x1 is [0,1]; the coefficient x2 is the weight of the grid-connected current imbalance reduction. When the grid has high requirements for grid-connected current imbalance, the reduction of grid-connected current imbalance is given priority, and the range of x2 is [0,1]; and x1+x2=1.

[0045] S5.4: Solve for the optimal current regulation coefficient k based on the multi-objective function to obtain the reference value of the grid-side converter current, and perform closed-loop current control on the grid-side converter:

[0046] After obtaining the positive and negative sequence dq-axis current reference command, to ensure that the current amplitude does not exceed the maximum safety threshold during the low-voltage ride-through of the wind turbine, the reference current command is substituted into the following formula to determine whether the current amplitude exceeds the limit, as follows:

[0047]

[0048] In the formula: I maxThis represents the maximum phase current amplitude of the grid-side converter; I lim The maximum allowable output current of the inverter is, under normal circumstances, I. lim The value is taken as 1.2 to 1.5 times the rated output current of the grid-side converter; I + I - These represent the positive and negative sequence current amplitudes, respectively.

[0049] If I max >I lim This indicates that the current amplitude exceeds the limit, and the reference current command needs to be limited, as shown in the following formula.

[0050]

[0051] In the formula: These are the reference values ​​for the d-axis and q-axis components of the grid-side current in the positive sequence coordinate system, respectively. These are the reference values ​​for the d-axis and q-axis components of the grid-side current in the negative sequence coordinate system.

[0052] A positive and negative sequence dual current loop structure is adopted for decoupling control. The positive and negative sequence currents of the grid-tested converter along the d and q axes are controlled in a closed loop. The difference between the reference commands for the positive and negative sequence currents of the d and q axes and the actual positive and negative sequence currents of the d and q axes are input to the proportional-integral regulator for decoupling control. The resulting synthesized modulation command voltage is used to generate the switching signals of the power devices of the grid-tested converter through space vector pulse width modulation.

[0053] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention proposes a multi-objective control method for permanent magnet direct-drive wind turbines. It selects optimized control objectives with the goal of enhancing voltage support capability, constructs a multi-objective optimization function, solves for positive and negative sequence current reference commands, and achieves coordinated optimization among various control objectives. Simultaneously, considering the transformer phase shift characteristics, it further derives current peak constraint conditions, limits over-limit currents, and ensures safe and stable system operation. This achieves coordinated optimization of power fluctuation suppression and current imbalance control objectives, improves the low-voltage ride-through capability of wind turbines, and enhances the grid connection performance of the system. Attached Figure Description

[0054] 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.

[0055] Figure 1 This is a structural diagram of the permanent magnet direct drive wind turbine system of the present invention.

[0056] Figure 2 This is a schematic diagram of the grid connection point voltage support principle under positive and negative sequence of the present invention.

[0057] Figure 3This invention relates to the principle of positive and negative sequence voltage support for three control targets.

[0058] Figure 4 This invention presents a block diagram of a multi-objective coordinated optimization control strategy for a permanent magnet direct-drive wind power system under grid asymmetric faults.

[0059] Figure 5 This is a schematic diagram of the simulation results under the active power fluctuation suppression strategy in operating condition a of the present invention.

[0060] Figure 6 This is a schematic diagram of the simulation results under the negative sequence current suppression strategy in operating condition a of the present invention.

[0061] Figure 7 This is a schematic diagram of the simulation results of the current limiting multi-objective coordinated control strategy under operating condition a of the present invention.

[0062] Figure 8 This is a schematic diagram of the simulation results under the active power fluctuation suppression strategy in operating condition b of the present invention.

[0063] Figure 9 This is a schematic diagram of the simulation results under the negative sequence current suppression strategy in operating condition b of the present invention.

[0064] Figure 10 This is a schematic diagram of the simulation results of the current limiting multi-objective coordinated control strategy under operating condition b of the present invention. Detailed Implementation

[0065] 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.

[0066] Example 1

[0067] See Figure 1 The technical solution provided in this embodiment is as follows: a structural block diagram of a permanent magnet direct-drive fan system is shown in the figure. Figure 1 As shown, the system includes: a wind turbine, a permanent magnet synchronous generator (PMSG), a turbine-side converter, a DC bus capacitor, a back-to-back converter consisting of a grid-side converter, a filter, a transformer, and grid connection lines. The wind turbine is directly connected to the rotor of the PMSG, and the stator of the PMSG is connected to the grid via the back-to-back converter and the transformer. The schematic diagram of the grid connection point voltage support under positive and negative sequence conditions is shown below. Figure 2 As shown. The three control target positive and negative sequence voltage support principles are as follows: Figure 3 As shown in the figure. The block diagram of the multi-objective coordinated optimization control strategy for a permanent magnet direct-drive wind power system under grid asymmetric fault conditions is as follows. Figure 4 As shown. The method of the present invention specifically includes the following steps:

[0068] S1: Establish a positive and negative sequence coordinate system model of the grid-side converter of the permanent magnet direct-drive wind turbine. Use a dual second-order generalized integrator to separate the positive and negative sequence of the voltage and current of the grid-side converter. Based on the positive and negative sequence coordinate system model of the grid-side converter of the permanent magnet direct-drive wind turbine, the grid-side converter adopts a positive and negative sequence dual current vector control structure. Use a dual second-order generalized integrator to separate and orient the positive and negative sequence of the voltage components of the grid-side converter, and separate the positive and negative sequence of the current components of the grid-side converter.

[0069] S2: Based on instantaneous power theory, derive the power matrix equation of the grid-side converter, and calculate the reference current command of the grid-side converter with the control objectives of suppressing grid-side active power fluctuations, suppressing grid-side reactive power fluctuations, and minimizing grid-connected current imbalance, as follows:

[0070] S2.1: Calculate the grid-side converter reference current expression for suppressing active power fluctuations:

[0071]

[0072] in, These are the d-axis and q-axis components of the converter outlet grid voltage in the positive sequence coordinate system, respectively. These are the d-axis and q-axis components of the converter outlet grid voltage in the negative sequence coordinate system, respectively. These are the d-axis and q-axis components of the grid-side current in the positive sequence coordinate system, respectively. Let d and q be the grid-side current components in the negative sequence coordinate system; P0 and Q0 are the average active power and average reactive power of the grid side, respectively.

[0073] S2.2: Calculate the grid-side converter reference current expression for suppressing reactive power fluctuations:

[0074]

[0075] S2.3: Calculate the grid-side converter reference current expression for minimizing grid-connected current imbalance:

[0076]

[0077] S3: Introducing the current regulation coefficient K, a unified expression for the reference current of the grid-side converter is constructed, as shown in the following formula:

[0078]

[0079] Where K=1, it can suppress grid-side active power fluctuations; K=0, it can minimize grid-connected current imbalance; K=-1, it can suppress grid-side reactive power fluctuations; λ is the voltage imbalance.

[0080] S4: Based on Kirchhoff's voltage law, the voltage support equation at the grid connection point is derived. The control target is selected based on the principle of reducing the voltage imbalance at the grid connection point. Using the symmetrical component method, the voltage support equation under asymmetrical faults is obtained, as shown in the following equation:

[0081]

[0082] In the formula, These are the positive and negative sequence voltage amplitudes at the grid connection point, respectively. These represent the positive and negative sequence voltage amplitudes of the grid voltage, respectively. The voltage support principle vector diagram at the grid connection point under positive and negative sequence voltages is shown below. Figure 2 As shown.

[0083] Since the transverse component mainly affects the phase angle between the grid connection point and the grid voltage, and has little impact on the voltage amplitude at the grid connection point, the active current I is ignored. d The voltage drop across the reactance ωL and the reactive current I q The simplified PCC voltage support equation for the voltage drop across resistor R is as follows:

[0084]

[0085] As can be seen from the above equation, the positive sequence voltage rise and negative sequence voltage suppression depend on the direction and magnitude of the positive and negative sequence active and reactive currents, while the magnitude and direction of the current components are affected by the control strategy.

[0086] Combining the effects of the above three control objectives on grid connection point voltage support, both suppressing grid-side active power fluctuations and minimizing grid-connected current imbalance contribute to reducing grid connection point voltage imbalance. The active power fluctuation suppression control objective injects... The voltage generated across the equivalent impedance of the power grid is in the same direction as the negative sequence voltage of the power grid. Conversely, the negative sequence voltage at the grid connection point decreases; while the reactive power fluctuation suppression control target is met by the injected... The voltage generated across the equivalent impedance of the power grid is in the same direction as the negative sequence voltage of the power grid. Similarly, an increase in the negative-sequence voltage at the grid connection point simultaneously raises both the positive-sequence and negative-sequence voltages, which is detrimental to improving the voltage imbalance at the grid connection point. Therefore, suppressing active power fluctuations and suppressing negative-sequence current are selected as the control objectives for coordinated optimization. The supporting principles for positive and negative-sequence voltages under the three traditional control objectives are as follows: Figure 3 As shown.

[0087] S5: Define the evaluation index for the coordinated optimization control objective, and construct a multi-objective optimization function based on it, as follows:

[0088] S5.1: Calculate the magnitude of active power fluctuation and current imbalance on the grid side:

[0089]

[0090] in, The magnitude of active power fluctuation on the grid side; P sin P represents the sinusoidal component of the active power fluctuation on the grid side. cos I' represents the cosine component of the active power fluctuation on the grid side; I' represents the grid-connected current imbalance.

[0091] S5.2: Define the evaluation index γ for the magnitude of active power fluctuation in grid-side converters. P and grid-connected current imbalance evaluation index γ i :

[0092]

[0093] S5.3: Construct a multi-objective function based on the evaluation metrics defined in step S5.2:

[0094] minF(K)=x1(γ P ) 2 +x2(γ i ) 2

[0095] Among them, the current regulation coefficient K ranges from [0,1] to achieve coordinated optimization between the grid-side active power fluctuation suppression and the grid-connected current balance target; the coefficient x1 is the weight of the grid-side active power fluctuation suppression target. When the grid has high requirements for active power fluctuation, the suppression of active power fluctuation is given priority, and the range of x1 is [0,1]; the coefficient x2 is the weight of the grid-connected current imbalance reduction. When the grid has high requirements for grid-connected current imbalance, the reduction of grid-connected current imbalance is given priority, and the range of x2 is [0,1]; and x1+x2=1.

[0096] S5.4: Solve for the optimal current regulation coefficient k based on the multi-objective function to obtain the reference value of the grid-side converter current, and perform closed-loop current control on the grid-side converter:

[0097] After obtaining the positive and negative sequence dq-axis current reference command, to ensure that the current amplitude does not exceed the maximum safety threshold during the low-voltage ride-through of the wind turbine, the reference current command is substituted into the following formula to determine whether the current amplitude exceeds the limit, as follows:

[0098]

[0099] In the formula: I max This represents the maximum phase current amplitude of the grid-side converter; I lim The maximum allowable output current of the inverter is, under normal circumstances, I. lim The value is taken as 1.2 to 1.5 times the rated output current of the grid-side converter; I + I - These represent the positive and negative sequence current amplitudes, respectively.

[0100] If I max >I lim This indicates that the current amplitude exceeds the limit, and the reference current command needs to be limited, as shown in the following formula.

[0101]

[0102] In the formula: These are the reference values ​​for the d-axis and q-axis components of the grid-side current in the positive sequence coordinate system, respectively. These are the reference values ​​for the d-axis and q-axis components of the grid-side current in the negative sequence coordinate system.

[0103] A block diagram of a multi-objective coordinated optimization control strategy for a permanent magnet direct-drive wind power system under grid asymmetric fault conditions, such as... Figure 4 As shown, a dual-current-loop structure with positive and negative sequences is used for decoupling control. Closed-loop control is performed on the positive and negative sequence currents of the grid-tested converter along the d and q axes. The differences between the reference commands for the positive and negative sequence currents of the d and q axes and the actual currents of the positive and negative sequences of the d and q axes are input to the proportional-integral regulator for decoupling control. The resulting synthesized modulation command voltage is then used to generate the switching signals for the power devices of the grid-tested converter through space vector pulse width modulation.

[0104] Through the above steps S1-S5, multi-objective control of permanent magnet direct-drive wind turbines under grid voltage faults can be achieved. With supporting the grid connection point voltage as the core, the coordinated control objective is selected. By introducing the current regulation coefficient, the active power fluctuation on the grid side and the grid connection current imbalance can be reduced at the same time. This realizes multi-objective control of permanent magnet direct-drive wind turbines under grid voltage faults and enhances their low voltage ride-through capability and grid connection performance.

[0105] Example 2

[0106] To verify the feasibility of the multi-objective control strategy proposed in this invention, we will verify the traditional active power suppression and negative sequence current suppression strategies, as well as the multi-objective control strategy under current limiting constraints proposed in this invention.

[0107] a) The simulation duration is 0.7s. From 0 to 0.4s, the system is in normal operation. From 0.4 to 0.6s, a single-phase ground fault occurs in the grid voltage. The voltage amplitude of phase A drops to 0.5pu, the voltage imbalance δ=0.2, P0=1.8MW, and Q0=0.8Mvar.

[0108] Figure 5The simulation results are for the active power fluctuation suppression strategy. This strategy eliminates active power fluctuations, but there is a significant imbalance in the grid-side output current. The current amplitudes of the three phases A, B, and C are 1.34 pu, 1.31 pu, and 0.95 pu, respectively, and the current imbalance reaches 20%. Although the positive sequence voltage increases by 1.92% and the negative sequence voltage decreases by 3.66%, its peak current exceeds the safe current limit threshold by 11.67%, which can easily trigger the unit's overcurrent protection, causing the unit to trip and the low voltage ride-through to fail.

[0109] Figure 6 The simulation results under the negative sequence current suppression strategy show that the three-phase current is balanced, with a current amplitude of 1.15 pu and high current quality; however, the active power fluctuation amplitude reaches 0.2 pu, resulting in obvious second harmonic fluctuation of DC bus voltage; secondly, the positive sequence voltage at the grid connection point increases by 1.92%, while the negative sequence voltage remains at 0.164 pu.

[0110] Under this operating condition, the voltage imbalance δ = 0.2. To improve the converter current quality, the current weighting coefficient x1 is given as 0.7, and the power weighting coefficient x2 is given as 0.3. The optimal value of k is 0.299. Figure 7 To account for the simulation results of the current-limiting multi-objective coordinated control strategy, the current imbalance was reduced to 5.98%. The positive-sequence voltage increased from 0.833 pu to 0.848 pu, the negative-sequence voltage decreased from 0.164 pu to 0.162 pu, and the active power fluctuation amplitude was 0.13 pu. The DC bus voltage was limited to 1.1 pu by the unloading circuit, exhibiting slight fluctuations. This strategy increased the positive-sequence voltage (by 1.92%) while suppressing the negative-sequence voltage (by 1.22%), and significantly reduced the current imbalance, achieving coordinated optimization between the control objectives of negative-sequence current suppression and active power fluctuation elimination.

[0111] b) The simulation duration is 0.7s. From 0 to 0.4s, the system is in normal operation. From 0.4 to 0.6s, a single-phase ground fault occurs in the grid voltage. The voltage amplitude of phase A drops to 0.2pu. The grid imbalance is δ=0.36, P0=1.6MW, and Q0=1.2Mvar.

[0112] Figure 8The diagram shows the relevant waveforms under the active power fluctuation suppression strategy. This strategy significantly reduces active power fluctuations, with the fluctuation amplitude approaching 0 p.u., effectively suppressing the DC bus voltage frequency doubling oscillation. The current imbalance reaches 36%, with the three-phase (A, B, and C) current amplitudes of 1.77 pu, 1.71 pu, and 0.9 pu, respectively, severely exceeding limits and jeopardizing system safety. The positive-sequence voltage increases from 0.73 pu to 0.759 pu (an increase of 3.97%), while the negative-sequence voltage decreases from 0.263 pu to 0.254 pu (a decrease of 3.54%). This strategy suppresses active power fluctuations, increases the positive-sequence voltage, and suppresses the negative-sequence voltage, but the peak current exceeds the safe current limit threshold by 47.5%.

[0113] Figure 9 The simulation waveforms are shown under the negative sequence current suppression strategy. Under this strategy, the three-phase current is balanced, with a current amplitude of 1.33 pu. The output current exceeds the safe current operating threshold, which may easily lead to overcurrent risk in the converter. The active power fluctuation amplitude is 0.36 pu, and the DC bus voltage exhibits significant frequency doubling oscillation. The positive sequence voltage is increased from 0.73 pu to 0.759 pu (an increase of 3.97%), while the negative sequence voltage is 0.263 pu, showing no significant effect. This strategy only increases the positive sequence voltage and achieves three-phase current balance, but its active power fluctuation amplitude is large, reaching 45%, and the peak current exceeds the safe current limit threshold by 10.83%.

[0114] The voltage imbalance is relatively large, so suppressing active power fluctuations is given priority. The current weighting coefficient x1 is given as 0.4, and the power weighting coefficient x2 is given as 0.6. The optimal value of k is 0.608. Figure 10 The simulation waveforms for multi-objective coordinated control are shown. The current imbalance is 21.8%, and the current in each phase is effectively limited within a safe range. Due to the current limiting constraint, the active power output drops to 0.65 pu, but the active power fluctuation amplitude is only 0.13 pu. Simultaneously, the positive sequence voltage rises to 0.751 pu (an increase of 2.88%), and the negative sequence voltage decreases to 0.257 pu (a decrease of 2.28%). Under the multi-objective coordinated control strategy proposed in this invention, all three-phase currents can be limited within a safe current range. This achieves coordinated optimization between reducing current imbalance and eliminating active power fluctuations, while simultaneously supporting the grid connection point voltage, improving the grid-side converter's grid connection performance, and enhancing the system's low-voltage ride-through capability.

[0115] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A multi-objective control method for permanent magnet direct-drive wind turbines under power grid asymmetric faults, characterized in that, Includes the following steps: Step S1: Establish a positive and negative sequence coordinate system model of the grid-side converter of the permanent magnet direct-drive wind turbine, and use a dual second-order generalized integrator to separate the positive and negative sequence of the voltage and current of the grid-side converter. Step S2: Based on instantaneous power theory, derive the power matrix equation of the grid-side converter, and calculate the grid-side converter reference current command with the three control objectives of suppressing grid-side active power fluctuations, reactive power fluctuations, and minimizing grid-connected current imbalance. Step S2 derives the power matrix equation of the grid-side converter based on instantaneous power theory, and calculates the reference current command of the grid-side converter with the control objectives of suppressing grid-side active power fluctuations, suppressing grid-side reactive power fluctuations, and minimizing grid-connected current imbalance. This includes the following steps: Step S2.1: Calculate the grid-side converter reference current expression for suppressing active power fluctuations: ； in, , These are the d-axis and q-axis components of the converter outlet grid voltage in the positive sequence coordinate system, respectively. , These are the d-axis and q-axis components of the converter outlet grid voltage in the negative sequence coordinate system, respectively. , These are the d-axis and q-axis components of the grid-side current in the positive sequence coordinate system, respectively. , These are the d-axis and q-axis components of the grid-side current in the negative sequence coordinate system. , These are the average active power and average reactive power on the grid side, respectively. Step S2.2: Calculate the grid-side converter reference current expression for suppressing reactive power fluctuations: ； Step S2.3: Calculate the grid-side converter reference current expression that minimizes grid-connected current imbalance: ； Step S3: Based on the reference current command under the three control objectives obtained in step S2, the three control objectives are to suppress the active power fluctuation, reactive power fluctuation and minimize the grid current imbalance. Introduce the current regulation coefficient K and construct a unified expression form of grid-side converter reference current that takes into account the above three control objectives. Step S4: Derive the grid connection point voltage support equation based on Kirchhoff's voltage law. Based on the influence of the three control objectives on the negative sequence voltage at the grid connection point, select suppressing active power fluctuations and suppressing negative sequence current as coordinated optimization control objectives for control. Step S5: Define the evaluation index of the coordinated optimization control objective in step S4, construct a multi-objective optimization function based on it, solve for the optimal current regulation coefficient K, obtain the converter current reference value, and perform current closed-loop control on the grid-side converter.

2. The multi-objective control method for permanent magnet direct-drive wind turbines under power grid asymmetric faults according to claim 1, characterized in that, Step S1 establishes a positive and negative sequence coordinate system model of the grid-side converter of the permanent magnet direct-drive wind turbine, and uses a dual second-order generalized integrator to separate the positive and negative sequence of the voltage and current of the grid-side converter, as detailed below: Based on the positive and negative sequence coordinate system model of the grid-side converter of the permanent magnet direct-drive wind turbine, the grid-side converter adopts a positive and negative sequence dual current vector control structure. It uses dual second-order generalized integrators to separate and orient the positive and negative sequence of the voltage components of the grid-side converter, and to separate the positive and negative sequence of the current components of the grid-side converter.

3. The multi-objective control method for a permanent magnet direct-drive wind turbine under asymmetrical grid faults according to claim 1, characterized in that, Step S3 introduces a current regulation coefficient K, and constructs a unified expression for the reference current of the grid-side converter as follows: ； When K=1, the grid-side active power fluctuation is suppressed; when K=0, the grid-connected current imbalance is minimized; when K=-1, the grid-side reactive power fluctuation is suppressed. For voltage imbalance, .

4. The multi-objective control method for permanent magnet direct-drive wind turbines under power grid asymmetric faults according to claim 1, characterized in that, In step S4, the voltage support equation at the grid connection point is derived based on Kirchhoff's voltage law, as follows: Based on Kirchhoff's voltage law, the voltage support equation at the grid connection point is derived. Using the symmetrical component method, the voltage support equation under asymmetrical faults is obtained, as shown below: ； In the formula, , These are the positive and negative sequence voltage amplitudes at the grid connection point, respectively. , These are the positive and negative sequence voltage amplitudes of the power grid voltage, respectively. active current The voltage drop and reactive current generated on the reactance ωL The voltage drop across resistor R is given by the following PCC voltage support equation: 。 5. The multi-objective control method for a permanent magnet direct-drive wind turbine under asymmetrical grid faults according to claim 3, characterized in that, Step S5 defines the evaluation index of the coordinated optimization control objective and constructs a multi-objective optimization function accordingly, including the following steps: Step S5.1: Calculate the magnitude of active power fluctuation and current imbalance on the grid side: ； ； in, The magnitude of active power fluctuation on the grid side; This represents the sinusoidal component of the active power fluctuation on the grid side. The cosine component of the active power fluctuation on the grid side; For grid-connected current imbalance; Step S5.2: Define the evaluation index for the magnitude of active power fluctuation of the grid-side converter. and grid-connected current imbalance evaluation indicators : ； ； Step S5.3: Construct a multi-objective function based on the evaluation metrics defined in step S5.2: ； The current regulation coefficient K ranges from [0,1], achieving coordinated optimization between grid-side active power fluctuation suppression and grid-connected current balance objectives; the coefficient The weighting of the target for suppressing grid-side active power fluctuations, The range is [0,1]; coefficients To reduce the weight of grid-connected current imbalance, The range is [0,1]; and ; Step S5.4: Solve for the optimal current regulation coefficient K according to the multi-objective function to obtain the reference value of the grid-side converter current, and perform closed-loop current control on the grid-side converter: After obtaining the positive and negative sequence dq axis current reference commands, since the current amplitude during the low-voltage ride-through of the wind turbine does not exceed the maximum safety threshold, the reference current command is substituted into the following formula to determine whether the current amplitude exceeds the limit, as follows: ； In the formula: This represents the maximum phase current amplitude of the grid-side converter. This is the maximum allowable output current of the inverter. The value is taken as 1.2 to 1.5 times the rated output current of the grid-side converter; , These are the positive and negative sequence current amplitudes, respectively; like This indicates that the current amplitude has exceeded the limit, and the reference current command is limited, as shown in the following formula: ； In the formula: , These are the reference values ​​for the d-axis component and q-axis component of the grid-side current in the positive sequence coordinate system, respectively. , These are the reference values ​​for the d-axis and q-axis components of the grid-side current in the negative sequence coordinate system. A positive and negative sequence dual current loop structure is adopted for decoupling control. The positive and negative sequence currents of the grid-tested converter along the d and q axes are controlled in a closed loop. The difference between the reference commands for the positive and negative sequence currents of the d and q axes and the actual positive and negative sequence currents of the d and q axes are input into the proportional-integral regulator for decoupling control. The resulting synthesized modulation command voltage is then used to generate the switching signals of the power devices of the grid-tested converter through space vector pulse width modulation.

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