Multi-target control method for permanent magnet direct drive fan under asymmetric fault of power grid

Abstract

The invention provides a multi-target control method for a permanent magnet direct drive fan under an asymmetric fault of a power grid, and solves the technical problems of power fluctuation suppression and current unbalance degree reduction control target coordination optimization in fault ride-through. Comprising the following steps: S1, establishing a positive and negative sequence coordinate system model of a permanent magnet direct-driven wind turbine generator grid-side converter, and carrying out positive and negative sequence separation on voltage and current of the grid-side converter; s2, grid-side converter reference current instructions of the three control targets are calculated; s3, constructing a uniform expression form of the reference current of the grid-side converter; s4, active power fluctuation suppression and negative sequence current suppression are selected as coordinated optimization control targets for control; and S5, constructing a multi-objective optimization function, solving to obtain a current reference value of the converter, and performing current closed-loop control on the grid-side converter. The method has the advantages that multi-target coordination optimization control over the permanent magnet direct-driven wind turbine generator set under the power grid voltage fault is achieved, and the low-voltage ride-through capacity and the grid connection performance of the permanent magnet direct-driven wind turbine generator set are enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of permanent magnet direct-drive wind turbines, and in particular to a multi-objective control method for permanent magnet direct-drive wind turbines under asymmetric power grid faults. Background Art

[0002] Permanent magnet direct-drive wind turbine systems, known for their exceptional reliability and low maintenance costs, have garnered widespread attention in the wind power sector. As wind power system transmission capacity continues to increase, asymmetric grid faults can cause stator current distortion and DC bus voltage fluctuations in permanent magnet direct-drive wind turbines, posing a serious threat to equipment safety and grid stability. Traditional control strategies often focus on a single objective, making it difficult to simultaneously coordinate multiple requirements during faults, including current over-limit suppression, power fluctuation suppression, and negative-sequence current compensation. Consequently, experts and scholars have conducted research on multi-objective coordinated optimization control methods.

[0003] Chinese patent publication number: CN118889526A, publication date: November 1, 2024, discloses a method for transient stability control of DC voltage of a doubly fed wind power grid-connected system under asymmetric grid fault. When an asymmetric short circuit fault occurs in the grid, the grid-side converter active current reference value is generated by the proportional link; the positive and negative sequence reactive current output by the doubly fed wind power grid-connected system during the asymmetric grid fault is obtained; the positive and negative sequence active current output by the doubly fed wind power grid-connected system during the asymmetric grid fault is calculated and obtained; finally, the positive and negative sequence active and reactive current command values ​​output by the rotor-side converter and the grid-side converter of the doubly fed wind power grid-connected system during the asymmetric grid fault are calculated; by coordinating the positive and negative sequence active and reactive current commands output by the rotor-side converter and the grid-side converter, the power fluctuation on the DC capacitor is eliminated, and the transient stability operation capability of the DC bus voltage of the doubly fed wind power grid-connected system is improved.

[0004] Chinese patent publication number: CN119448316A, publication date: February 14, 2025, discloses a method and system for fault ride-through of a new energy grid-connected converter under asymmetric faults. The method comprises: calculating the grid positive-sequence voltage amplitude and grid voltage imbalance based on the grid's positive and negative sequence parameters; and determining whether the grid is in a normal or faulty state. When the grid is in an asymmetric fault state, current limiting is performed using a calculation formula for the positive and negative sequence current commands under the asymmetric fault state based on the grid fault-related parameters. The limiting values ​​of the positive and negative sequence current commands of the new energy grid-connected converter under the asymmetric fault state are calculated, and feedback control is performed based on the limiting values ​​of the positive and negative sequence current commands to achieve fault ride-through. The present invention limits the current in the calculation formula for the positive and negative sequence current commands, thereby maximally meeting the positive and negative sequence active and reactive current control targets while ensuring that the converter does not overcurrent, thereby improving the grid's support capacity during the fault. This method has a single control target and does not systematically analyze each control target.

[0005] Chinese patent publication number: CN114825395B, publication date: December 17, 2024, discloses a control strategy for a flywheel energy storage grid-side converter under asymmetric grid faults. First, a mathematical model of the grid-side converter based on a two-phase rotating dq coordinate system is constructed. In conjunction with the grid-side converter's control objectives, the grid-side converter adopts a direct current control strategy based on grid voltage orientation. The control system consists of an outer DC voltage loop and inner active and reactive current loops. Positive and negative sequence current reference values ​​are derived from the matrix relationship between power and the positive and negative sequence voltage and current quantities. A dual closed-loop current control strategy with positive and negative sequence separation is used to control the positive and negative sequence currents, effectively suppressing active power double frequency fluctuations. The maximum 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. If the current exceeds the limit, the power reference value is adjusted to keep the current within the limit. This method only considers the current limit exceeding issue and does not fully address other issues such as power limit exceeding.

[0006] How to solve the above technical problems is the subject faced by the present invention. Summary of the Invention

[0007] The purpose of the present invention is to provide a multi-objective control method for permanent magnet direct-drive wind turbines under asymmetric grid faults. To address the technical issues of traditional control methods under asymmetric grid faults, such as single control targets, current over-limit, and neglect of the voltage support effect of the control targets, the present invention proposes a multi-objective control method for permanent magnet direct-drive wind turbines under asymmetric grid faults. The method aims to achieve coordinated optimization of the control targets of power fluctuation suppression and current imbalance reduction, thereby improving the low voltage ride-through capability of the wind turbine and the grid-connected performance of the system.

[0008] The inventive concept of the present invention is as follows: the present invention provides a multi-objective control method for a permanent magnet direct-drive wind turbine under asymmetric fault of the power grid, 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 double second-order generalized integrator to separate the positive and negative sequence of the voltage and current of the grid-side converter; deriving the power matrix equation of the grid-side converter based on the instantaneous power theory, and respectively calculating the grid-side converter reference current instruction with the control objectives of suppressing the grid-side active power fluctuation, reactive power fluctuation and minimizing the grid-connected current imbalance; according to the reference current under the three control objectives obtained in step S2, The current regulation coefficient K is introduced to construct a unified expression of the grid-side converter reference current that takes into account the above three control objectives; the grid connection point voltage support equation is derived according to Kirchhoff's voltage law, and according to the influence of the three control objectives on the negative sequence voltage at the grid connection point, the suppression of active power fluctuation and the suppression of negative sequence current are selected as the coordinated optimization control objectives for control; the evaluation index of the coordinated optimization control objective in step S4 is defined, and a multi-objective optimization function is constructed based on this, and the optimal current regulation coefficient k is solved to obtain the converter current reference value, and the current closed-loop control of the grid-side converter is performed. The multi-objective control method of permanent magnet direct-drive wind turbines under asymmetric grid faults proposed in the present invention realizes the coordinated optimization of the control objectives of power fluctuation suppression and current imbalance reduction, improves the low voltage ride-through capability of the wind turbine, and improves the grid connection performance of the system.

[0009] In order to achieve the above-mentioned invention object, the present invention adopts a technical solution specifically as follows: a multi-objective control method for a permanent magnet direct-drive wind turbine under an asymmetric power grid fault, comprising the following steps:

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

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

[0012] S3: Based on the reference current instructions under the three control objectives obtained in step S2, a current adjustment coefficient K is introduced to construct a unified expression of the grid-side converter reference current that takes into account the above three control objectives;

[0013] S4: The grid connection point voltage support equation is derived based on Kirchhoff's voltage law. Based on 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 for control;

[0014] S5: Define the evaluation index of the coordinated optimization control target in step S4, and construct a multi-objective optimization function based on it to solve 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] The 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 double second-order generalized integrator to separate the positive and negative sequence of the grid-side converter voltage and current, as follows:

[0016] According to 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, and uses a dual second-order generalized integrator to separate and orient the positive and negative sequence of the grid-side converter voltage components, and to separate the positive and negative sequence of the grid-side converter current components.

[0017] The step S2 derives the grid-side converter power matrix equation based on the instantaneous power theory, and calculates the grid-side converter reference current command 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, are the d-axis and q-axis components of the grid voltage at the converter outlet in the positive sequence coordinate system respectively; are the d-axis and q-axis components of the grid voltage at the converter outlet in the negative sequence coordinate system respectively; are the d-axis and q-axis components of the grid-side current in the positive sequence coordinate system respectively; are the d-axis and q-axis components of the grid-side current in the negative-sequence coordinate system; P0 and Q0 are the average active power and 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 with the minimum grid current imbalance:

[0024]

[0025] The step S3 introduces the current adjustment coefficient K to construct a unified expression of the grid-side converter reference current, which is as follows:

[0026]

[0027] Among them, when K=1, the fluctuation of active power on the grid side can be suppressed; when K=0, the grid current imbalance can be minimized; when K=-1, the fluctuation of reactive power on the grid side can be suppressed; λ is the voltage imbalance,

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

[0029] The voltage support equation for the grid connection point is derived based on Kirchhoff's voltage law. Based on the symmetrical component method, the voltage support equation for an asymmetrical fault is obtained as shown below:

[0030]

[0031] Where, are the positive and negative sequence voltage amplitudes at the grid connection point, respectively; are the positive and negative sequence voltage amplitudes of the grid voltage respectively.

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

[0033]

[0034] It can be seen from the above formula that 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, and the magnitude and direction of the current components are affected by the control strategy.

[0035] Combining the above three control objectives on the voltage support effect of the grid connection point, the two control objectives of suppressing the active power fluctuation on the grid side and controlling the minimum imbalance of the grid current are both helpful to reduce the imbalance of the grid connection point voltage. The direction of the voltage generated on the equivalent impedance of the grid is the same as the negative sequence voltage of the grid On the contrary, the negative sequence voltage at the grid connection point decreases; while the reactive power fluctuation suppression control target is The direction of the voltage generated on the equivalent impedance of the grid is the same as the negative sequence voltage of the grid Similarly, the negative sequence voltage at the grid connection point increases, while the positive and negative sequence voltages are raised, 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 coordinated optimization control objectives.

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

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

[0038]

[0039] in, is the fluctuation of active power on the grid side; P sin is the sinusoidal component of active power fluctuation on the grid side; P cos is the cosine component of the active power fluctuation on the grid side; I′ is the grid current imbalance.

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

[0041]

[0042] S5.3: Construct a multi-objective function based on the evaluation indicators 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], achieving 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 target of suppressing the grid-side active power fluctuation. When the grid has high requirements for active power fluctuation, suppressing active power fluctuation is given priority, and the range of x1 is [0,1]. The coefficient x2 is the weight of reducing the grid-connected current imbalance. When the grid has high requirements for the grid-connected current imbalance, reducing the grid-connected current imbalance is given priority, and the range of x2 is [0,1]; and x1+x2=1.

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

[0046] After obtaining the positive and negative sequence dq axis current reference command, in order 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] Where: I maxis the maximum phase current amplitude of the grid-side converter; I lim The maximum output current allowed by the inverter. Usually, I lim The value is 1.2 to 1.5 times the rated output current of the grid-side converter; I + , I - are the positive and negative sequence current amplitudes, respectively.

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

[0050]

[0051] Where: are the reference values ​​of the d-axis and q-axis components of the grid-side current in the positive sequence coordinate system respectively; are the reference values ​​of 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, and closed-loop control is performed on the positive and negative sequence currents of the d and q axes of the network-measured converter. The differences between the positive and negative sequence current reference instructions of the d and q axes and the actual positive and negative sequence currents of the d and q axes are respectively input into the proportional-integral regulator for decoupling control. The resulting synthetic modulation instruction voltage is modulated by space vector pulse width modulation to generate the switching signals of the power devices of the network-measured converter.

[0053] Compared with the existing technology, the present invention has the following beneficial effects: It proposes a multi-objective control method for permanent magnet direct-drive wind turbines. This method selects optimized control objectives based on enhancing voltage support capability, constructs a multi-objective optimization function, solves the positive and negative sequence current reference instructions, and achieves coordinated optimization among various control objectives. Furthermore, by comprehensively considering the transformer phase shift characteristics, it further derives current peak constraints and limits over-limit currents, ensuring safe and stable system operation. This method achieves coordinated optimization of the control objectives of power fluctuation suppression and current imbalance reduction, enhancing the low voltage ride-through capability of the wind turbine and improving the system's grid-connected performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[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 voltage support principle of the grid connection point under positive and negative sequence conditions of the present invention.

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

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

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

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

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

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

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

[0064] Figure 10 It is a schematic diagram of the simulation results of the current limiting multi-objective coordinated control strategy under working condition b of the present invention. DETAILED DESCRIPTION

[0065] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. Of course, the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0066] Example 1

[0067] See also Figure 1 , the technical solution provided by this embodiment is a permanent magnet direct drive wind turbine system structure diagram based on Figure 1 As shown in the figure, it includes: wind turbine, permanent magnet synchronous generator, machine-side converter, DC bus capacitor and grid-side converter consisting of back-to-back converter, filter, transformer and grid connection line. The wind turbine is directly connected to the permanent magnet synchronous generator rotor, and the permanent magnet synchronous generator stator is connected to the grid through the back-to-back converter and transformer. The voltage support principle diagram of the grid connection point under positive and negative sequence is shown in the figure. Figure 2 The three control target positive and negative sequence voltage support principles are as follows: Figure 3 The block diagram of the multi-objective coordinated optimization control strategy of the permanent magnet direct-drive wind power system under asymmetric grid fault is shown in Figure 4 The method of the present invention specifically comprises 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, and use a double second-order generalized integrator to separate the positive and negative sequence of the grid-side converter voltage and current. According to 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, and uses a double second-order generalized integrator to separate and orient the positive and negative sequence of the grid-side converter voltage components, and to separate the positive and negative sequence of the grid-side converter current components.

[0069] S2: Based on the instantaneous power theory, the grid-side converter power matrix equation is derived. The grid-side converter reference current instructions are calculated with the control objectives of suppressing grid-side active power fluctuations, suppressing grid-side reactive power fluctuations, and minimizing grid current imbalance. The details are as follows:

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

[0071]

[0072] in, are the d-axis and q-axis components of the grid voltage at the converter outlet in the positive sequence coordinate system respectively; are the d-axis and q-axis components of the grid voltage at the converter outlet in the negative sequence coordinate system respectively; are the d-axis and q-axis components of the grid-side current in the positive sequence coordinate system respectively; are the d-axis and q-axis components of the grid-side current in the negative-sequence coordinate system; P0 and Q0 are the average active power and 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 with the minimum grid current imbalance:

[0076]

[0077] S3: Introduce the current regulation coefficient K to construct a unified expression for the grid-side converter reference current, as shown in the following formula:

[0078]

[0079] Among them, when K=1, the fluctuation of active power on the grid side can be suppressed; when K=0, the grid current imbalance can be minimized; when K=-1, the fluctuation of reactive power on the grid side can be suppressed; λ is the voltage imbalance,

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

[0081]

[0082] Where, are the positive and negative sequence voltage amplitudes at the grid connection point, respectively; The voltage support principle vector diagram of the grid connection point under positive and negative sequence is shown in the figure below. Figure 2 shown.

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

[0084]

[0085] It can be seen from the above formula that 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, and the magnitude and direction of the current components are affected by the control strategy.

[0086] Combining the above three control objectives on the voltage support effect of the grid connection point, the two control objectives of suppressing the active power fluctuation on the grid side and controlling the minimum imbalance of the grid current are both helpful to reduce the imbalance of the grid connection point voltage. The direction of the voltage generated on the equivalent impedance of the grid is the same as the negative sequence voltage of the grid On the contrary, the negative sequence voltage at the grid connection point decreases; while the reactive power fluctuation suppression control target is The direction of the voltage generated on the equivalent impedance of the grid is the same as the negative sequence voltage of the grid Similarly, the negative sequence voltage at the grid connection point increases, and at the same time, the positive and negative sequence voltages are raised, 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 control objectives for coordinated optimization. The positive and negative sequence voltage support principles under the traditional three control objectives are as follows: Figure 3 shown.

[0087] S5: Define the evaluation indicators of the coordinated optimization control objectives and construct a multi-objective optimization function based on them, as follows:

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

[0089]

[0090] in, is the fluctuation of active power on the grid side; P sin is the sinusoidal component of active power fluctuation on the grid side; P cos is the cosine component of the active power fluctuation on the grid side; I′ is the grid current imbalance.

[0091] S5.2: Define the grid-side converter active power fluctuation evaluation index γ P and grid current imbalance evaluation index γ i :

[0092]

[0093] S5.3: Construct a multi-objective function based on the evaluation indicators 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], achieving 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 target of suppressing the grid-side active power fluctuation. When the grid has high requirements for active power fluctuation, suppressing active power fluctuation is given priority, and the range of x1 is [0,1]. The coefficient x2 is the weight of reducing the grid-connected current imbalance. When the grid has high requirements for the grid-connected current imbalance, reducing the grid-connected current imbalance is given priority, and the range of x2 is [0,1]; and x1+x2=1.

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

[0097] After obtaining the positive and negative sequence dq axis current reference command, in order 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] Where: I max is the maximum phase current amplitude of the grid-side converter; I lim The maximum output current allowed by the inverter. Usually, I lim The value is 1.2 to 1.5 times the rated output current of the grid-side converter; I + , I - are the positive and negative sequence current amplitudes, respectively.

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

[0101]

[0102] Where: are the reference values ​​of the d-axis and q-axis components of the grid-side current in the positive sequence coordinate system respectively; are the reference values ​​of the d-axis and q-axis components of the grid-side current in the negative-sequence coordinate system.

[0103] The block diagram of the multi-objective coordinated optimization control strategy of the permanent magnet direct-drive wind power system under the asymmetric fault of the power grid is as follows: Figure 4 As shown in the figure, a dual-current loop structure with positive and negative sequences is used for decoupling control. Closed-loop control is performed on the d- and q-axis positive and negative sequence currents of the network-measured converter. The difference between the d- and q-axis positive and negative sequence current reference commands and the d- and q-axis positive and negative sequence current actual values ​​is input into the proportional-integral regulator for decoupling control. The resulting synthetic modulation command voltage is modulated by space vector pulse width modulation to generate the switching signals for the power devices of the network-measured converter.

[0104] Through the above steps S1-S5, multi-objective control of the permanent magnet direct-drive wind turbine under grid voltage fault can be achieved. The coordinated control target is selected with the grid connection point voltage as the core. By introducing the current regulation coefficient, the grid-side active power fluctuation and grid current imbalance can be reduced at the same time, thereby achieving multi-objective control of the permanent magnet direct-drive wind turbine under grid voltage fault and enhancing its low voltage ride-through capability and grid connection performance.

[0105] Example 2

[0106] In order to verify the feasibility of the multi-objective control strategy proposed in the present invention, the traditional active power suppression, negative sequence current suppression strategy and the multi-objective control strategy under current limit constraint proposed in the present invention are verified.

[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 grounding 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 following are simulation results using 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-phase A, B, and C phases are 1.34 pu, 1.31 pu, and 0.95 pu, respectively, with a current imbalance of 20%. Although the positive-sequence voltage increases by 1.92% and the negative-sequence voltage decreases by 3.66%, the peak current exceeds the safe current limit threshold by 11.67%, easily triggering 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 currents are balanced, with a current amplitude of 1.15 pu, indicating high current quality. However, the active power fluctuation amplitude reaches 0.2 pu, resulting in obvious double-frequency fluctuations in the 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 is 0.2. To improve the converter current quality, the current weight coefficient x1 is set to 0.7, and the power weight coefficient x2 is set to 0.3. The optimal k value is 0.299. Figure 7 Simulation results from a multi-objective coordinated control strategy based on current limiting show that current imbalance is reduced to 5.98%. The positive-sequence voltage is increased from 0.833 pu to 0.848 pu, while the negative-sequence voltage is reduced from 0.164 pu to 0.162 pu, resulting in an active power fluctuation of 0.13 pu. The DC bus voltage is limited to 1.1 pu by the unloading circuit, with slight fluctuations. This strategy increases the positive-sequence voltage by 1.92% while suppressing the negative-sequence voltage by 1.22%, significantly reducing current imbalance and 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 grounding fault occurs in the grid voltage, and the voltage amplitude of phase A drops to 0.2pu. The grid imbalance δ = 0.36, P0 = 1.6MW, and Q0 = 1.2Mvar.

[0112] Figure 8The following are waveforms related to 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 double frequency oscillation. The current imbalance reached 36%, and the current amplitudes of the three phases ABC were 1.77 pu, 1.71 pu, and 0.9 pu, respectively. The current was seriously out of limit, endangering the safe operation of the system. The positive sequence voltage increased from 0.73 pu to 0.759 pu (an increase of 3.97%), and the negative sequence voltage decreased from 0.263 pu to 0.254 pu (a decrease of 3.54%). This strategy suppressed active power fluctuations, raised the positive sequence voltage, and suppressed the negative sequence voltage, but the peak current exceeded the safe current limit threshold by 47.5%.

[0113] Figure 9 The following waveforms are simulated using the negative-sequence current suppression strategy. Under this strategy, the three-phase currents are balanced, with a current amplitude of 1.33 pu. The output current exceeds the safe operating threshold, potentially causing converter overcurrent risks. The active power fluctuation amplitude is 0.36 pu, and the DC bus voltage exhibits significant frequency-doubled oscillations. The positive-sequence voltage increases from 0.73 pu to 0.759 pu (a 3.97% increase), while the negative-sequence voltage is 0.263 pu, showing no significant effect. This strategy only raises the positive-sequence voltage while achieving three-phase current balance. However, 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 large, so suppressing active power fluctuations is prioritized. The current weight coefficient x1 is set to 0.4, and the power weight coefficient x2 is set to 0.6. The optimal k value is 0.608. Figure 10 This is a multi-objective coordinated control simulation waveform. The current imbalance is 21.8%, and the current of each phase is effectively limited to a safe range; due to the current limit constraint, the active output is reduced to 0.65pu, but the active fluctuation amplitude is only 0.13pu; at the same time, the positive sequence voltage is raised to 0.751pu (increased by 2.88%), and the negative sequence voltage is reduced to 0.257pu (reduced by 2.28%). Under the multi-objective coordinated control strategy proposed in the present invention, the three-phase currents can be limited to a safe current range, achieving coordinated optimization between reducing current imbalance and eliminating active fluctuations while supporting the grid-connected point voltage, improving the grid-connected performance of the grid-side converter, 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 in the scope of protection of the present invention.

Claims

1. A multi-objective control method for a permanent magnet direct-drive wind turbine under asymmetric grid fault, characterized in that: The following steps are involved: Step S1: 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 double second-order generalized integrator to perform positive and negative sequence separation on the voltage and current of the grid-side converter; Step S2: deriving the power matrix equation of the grid-side converter based on the instantaneous power theory, and calculating the grid-side converter reference current command with the three control objectives of suppressing grid-side active power fluctuation, reactive power fluctuation, and minimizing grid current imbalance; Step S3: Based on the reference current instructions under the three control objectives obtained in step S2, the three control objectives are suppressing grid-side active power fluctuations, reactive power fluctuations, and minimizing grid-connected current imbalance. A current adjustment coefficient K is introduced to construct a unified expression of the grid-side converter reference current that takes into account the above three control objectives. Step S4: deriving the grid connection point voltage support equation based on Kirchhoff's voltage law, and selecting active power fluctuation suppression and negative sequence current suppression as coordinated optimization control objectives based on the impact of the three control objectives on the grid connection point negative sequence voltage; Step S5: Define the evaluation index of the coordinated optimization control target in step S4, and construct a multi-objective optimization function based on it, solve 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 a permanent magnet direct-drive wind turbine under asymmetric grid fault according to claim 1 is characterized in that: The 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 double second-order generalized integrator to separate the positive and negative sequence of the grid-side converter voltage and current, as follows: According to 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, and uses a dual second-order generalized integrator to separate and orient the positive and negative sequence of the grid-side converter voltage components, and to separate the positive and negative sequence of the grid-side converter current components.

3. The multi-objective control method for a permanent magnet direct-drive wind turbine under asymmetric grid fault according to claim 1 is characterized in that: The step S2 derives the grid-side converter power matrix equation based on the instantaneous power theory, and calculates the grid-side converter reference current command with the control objectives of suppressing grid-side active power fluctuations, suppressing grid-side reactive power fluctuations, and minimizing grid-connected current imbalance, including the following steps: Step S2.1: Calculate the grid-side converter reference current expression for suppressing active power fluctuations: in, are the d-axis component and q-axis component of the grid voltage at the converter outlet in the positive sequence coordinate system respectively; are the d-axis component and q-axis component of the grid voltage at the converter outlet in the negative sequence coordinate system respectively; are the d-axis component and q-axis component of the grid-side current in the positive sequence coordinate system respectively; are the d-axis component and q-axis component of the grid-side current in the negative-sequence coordinate system; P0 and Q0 are the average active power and reactive power of 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 with the minimum grid current imbalance:

4. The multi-objective control method for a permanent magnet direct-drive wind turbine under asymmetric grid fault according to claim 1 is characterized in that: In step S3, the current adjustment coefficient K is introduced to construct a unified expression of the grid-side converter reference current as follows: Among them, when K=1, the active power fluctuation on the grid side is suppressed; when K=0, the grid current imbalance is minimized; when K=-1, the reactive power fluctuation on the grid side is suppressed; λ is the voltage imbalance, 5. The multi-objective control method for a permanent magnet direct-drive wind turbine under asymmetric grid fault according to claim 1 is characterized in that: In step S4, the grid connection point voltage support equation is derived according to Kirchhoff's voltage law, as follows: The voltage support equation for the grid connection point is derived based on Kirchhoff's voltage law. Based on the symmetrical component method, the voltage support equation for an asymmetrical fault is obtained as shown below: Where, are the positive and negative sequence voltage amplitudes at the grid connection point, respectively; are the positive and negative sequence voltage amplitudes of the grid voltage respectively; Active current I d The voltage drop across the reactance ωL and the reactive current I q The voltage drop across the resistor R and the PCC voltage support equation are shown below:

6. The multi-objective control method for a permanent magnet direct-drive wind turbine under asymmetric grid fault according to claim 1 is characterized in that: In step S5, the evaluation index of the coordinated optimization control objective is defined, and a multi-objective optimization function is constructed based on the evaluation index, which includes the following steps: Step S5.1: Calculate the grid-side active power fluctuation and current imbalance: in, is the fluctuation of active power on the grid side; P sin is the sinusoidal component of active power fluctuation on the grid side; P cos is the cosine component of the active power fluctuation on the grid side; I′ is the unbalance degree of the grid current; Step S5.2: Define the grid-side converter active power fluctuation evaluation index γ P and grid current imbalance evaluation index γ i : Step S5.3: Construct a multi-objective function based on the evaluation indicators defined in step S5.2: minF(K)=x1(γ P ) 2 +x2(γ i ) 2 The current regulation coefficient K ranges from [0, 1], achieving 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, and the x1 range is [0, 1]. The coefficient x2 is the weight of reducing the grid-connected current imbalance, and the x2 range is [0, 1]. And x1 + x2 = 1. Step S5.4: Solve the optimal current regulation coefficient k according to the multi-objective function, obtain the grid-side converter current reference value, and perform current closed-loop control on the grid-side converter: After solving for the positive and negative sequence dq axis current reference command, since 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: Where: I max is the maximum phase current amplitude of the grid-side converter; I lim is the maximum output current allowed by the inverter, I lim The value is 1.2 to 1.5 times the rated output current of the grid-side converter; I + , I - are the positive and negative sequence current amplitudes respectively; If I max >I lim , indicating that the current amplitude exceeds the limit, the reference current command is limited, as shown in the following formula: Where: are the reference values ​​of the d-axis component and q-axis component of the grid-side current in the positive sequence coordinate system respectively; are the reference values ​​of the d-axis component and q-axis component 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, and closed-loop control is performed on the positive and negative sequence currents of the d and q axes of the network-measured converter. The differences between the positive and negative sequence current reference instructions of the d and q axes and the actual positive and negative sequence currents of the d and q axes are respectively input into the proportional-integral regulator for decoupling control. The synthesized modulation instruction voltage is modulated by space vector pulse width modulation to generate the switching signal of the power device of the network-measured converter.

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