Load optimization control method for voltage source wind turbines

Abstract

To address the problem that existing technologies, which do not consider torque mismatch under fault ride-through and low-frequency oscillation conditions, cannot avoid system oscillations and shutdowns caused by torque mismatch during fault conditions, this invention provides a load optimization control method for voltage source wind turbines. This method optimizes the power coordination characteristics between the wind turbine's main control system and the converter under voltage source control mode, avoiding system oscillations and shutdowns caused by torque mismatch. Simultaneously, it reduces the load impact on the drive train shaft system of voltage source wind turbines under different grid conditions such as voltage surges, voltage dips, and low-frequency oscillations, thereby improving the mechanical lifespan of the wind turbine throughout its entire lifecycle. By calculating and superimposing electromagnetic torque disturbances based on different control objectives before and after a fault in the voltage source wind turbine, it avoids shutdowns caused by torque mismatch during fault ride-through. Furthermore, by calculating and superimposing electromagnetic torque disturbance components based on the expressions for stator and rotor current disturbance components caused by voltage disturbance components, it avoids load oscillations caused by system oscillations.

Description

Technical Field

[0001] This invention relates to the field of wind power control technology, and specifically to a load optimization method for voltage source type wind turbine generators. Background Technology

[0002] my country's wind power development has been rapid, becoming the country's third largest energy source. With the rapid increase in installed wind power capacity, its proportion of total installed power capacity is also rising, with some power grids now exceeding 30% of their total capacity. This annual increase in total installed wind power capacity has led to a decline in the proportion of synchronous generators in the power system. Simultaneously, the large-scale grid connection of power electronic devices has caused wind power to exhibit characteristics such as low disturbance rejection, weak inertia, and weak damping in the power system, further manifesting as weak frequency and voltage support. This is driving the transformation of the traditional power system, dominated by synchronous generators, into a new power system dominated by renewable energy sources.

[0003] To strengthen the coupling between wind power systems and the main power grid, address local grid weakening, and improve power system stability margins, the control methods for doubly-fed induction generator (DFIG) wind turbines have gradually differentiated into current-source active support control strategies and voltage-source active grid connection control strategies. The current-source active support control strategy primarily uses the generator's output active and reactive currents as control targets, utilizing collected system frequency and voltage data to modify the output active and reactive currents to achieve system frequency and voltage support. The voltage-source active grid connection control strategy primarily uses the generator's output voltage amplitude and phase as control targets, simulating the active-frequency and reactive-voltage regulation characteristics of a synchronous generator based on droop control technology or virtual synchronization technology, thus possessing frequency response and voltage support capabilities.

[0004] Doubly fed wind turbine generators are rigidly connected to the gearbox and achieve machine-grid interaction coupling through electromagnetic torque. At the same time, the increasing size of the unit and the complexity of the mechanical component structure lead to a decrease in the damping of key components such as the transmission chain of the doubly fed wind turbine and a decrease in the natural frequency. When the grid is disturbed (such as fault ride-through, frequency drop, low frequency oscillation), it will cause a sudden change in electromagnetic torque, resulting in a load impact on the transmission chain. In severe cases, it will induce mechanical torsional vibration, causing tooth breakage and shaft breakage.

[0005] To address the load impact caused by the mismatch between electromagnetic torque and drive shaft torque during grid disturbances, current-source wind turbines typically utilize observers to extract relevant variables and employ specific control laws to compensate for and superimpose the electromagnetic torque difference, thereby reducing load impact. Wind turbines under voltage-source control mode exhibit different external and load characteristics compared to those under current-source control mode. Furthermore, during normal operation, their output power is dependent on grid frequency and voltage, and they do not fully comply with the master control torque command, exhibiting weak torque following characteristics. Without effective torque compensation strategies and load optimization measures, they are more prone to abnormal speed control and load impact.

[0006] To address the issues of active power coordination and torque mismatch between the main control system and converter of voltage source wind turbines, some scholars have used main control torque commands and speed optimization control strategies to correct torque deviations in wind turbines during normal operation. At the same time, they have added a shaft load optimization control loop based on grid frequency and used grid frequency disturbances for additional torque compensation.

[0007] Existing methods only consider the torque mismatch problem of voltage source wind turbines under normal operating conditions and grid frequency disturbance conditions, without considering the torque mismatch problem under fault ride-through conditions and low-frequency oscillation conditions.

[0008] For example, the Chinese patent document "Control Method and System for Voltage Source Wind Turbine Considering Load and Speed ​​Constraints" (application number CN202010157813.7) discloses a control method and system for voltage source wind turbine considering load and speed constraints. This system includes a voltage source converter, a shaft load optimization control loop, a speed optimization control loop, and variable speed and pitch control. The voltage source converter outputs f to the shaft load optimization control loop and outputs ΔTf, Pg, and ωg to the speed optimization control loop. The variable speed and pitch control outputs Tgmd to the speed optimization control loop, which outputs Tgrd to the adder. Simultaneously, it outputs ωg to both the shaft load optimization control loop and the variable speed and pitch control loop. The shaft load optimization control loop outputs TBd to the adder, and the adder outputs Tgd to the voltage source converter. However, this system only considers torque mismatch under normal operating conditions and frequency disturbance conditions, without addressing torque mismatch under fault ride-through and low-frequency oscillation conditions. Summary of the Invention

[0009] To address the problem that existing technologies, which do not consider torque mismatch under fault ride-through and low-frequency oscillation conditions, cannot avoid system oscillations and shutdowns caused by torque mismatch under fault conditions, this invention optimizes the power coordination characteristics between the wind turbine main control system and the converter in voltage source control mode. This avoids system oscillations and shutdowns caused by torque mismatch, while also reducing the load impact on the transmission chain shaft system of the voltage source wind turbine under different grid conditions such as voltage surges, voltage dips, and low-frequency oscillations, thereby improving the mechanical life of the wind turbine throughout its entire life cycle.

[0010] The present invention provides a load optimization control method for voltage source wind turbines, comprising: utilizing negative feedback to eliminate the influence of disturbance components generated during voltage faults or system oscillations on shaft loads; using the actual generator speed as a filtered feedback quantity; calculating the offset between the given speed and the feedback quantity; converting the offset into a torque setpoint through a torque loop; and adjusting the output torque in conjunction with the actual operating conditions of the wind turbine, including low-voltage ride-through, high-voltage ride-through, and oscillation conditions. Thus, under fault ride-through conditions, electromagnetic torque disturbances are calculated and superimposed based on different control objectives before and after a fault in the voltage source wind turbine, avoiding fault shutdowns caused by torque mismatch during fault ride-through; under system oscillation conditions, electromagnetic torque disturbances are calculated and superimposed based on the expression for stator and rotor current disturbances caused by voltage disturbances, avoiding load oscillations caused by system oscillations; and calculating grid weight gain based on the actual operating frequency and voltage of the power grid to compensate for torque mismatches between the main control system and the converter caused by changes in the control strategy.

[0011] Preferably, under both low-voltage ride-through and high-voltage ride-through conditions, the offset between the given speed and the feedback quantity is output to the input of the torque control loop. The torque control loop outputs a torque setpoint, and the additional torque is calculated based on the actual operating conditions of the wind turbine. The sum of the torque setpoint and the additional torque is then output to the wind turbine to adjust its torque and eliminate disturbance components. Under the oscillation condition, after obtaining the sum of the torque setpoint and the additional torque, the sum needs to be multiplied by the grid weight gain function before being output to the wind turbine to adjust its torque and eliminate disturbance components.

[0012] Preferably, the additional torque is used to eliminate torque disturbance components. The additional torque is numerically equal to the torque disturbance component but in the opposite direction. The torque disturbance component is the difference between the motor electromagnetic torque after a voltage fault or system oscillation and the motor electromagnetic torque without considering disturbances. Without considering disturbances, the doubly-fed motor electromagnetic torque can be expressed as:

[0013] T e =1.5n p L m (i sq i rd -i sd i rq );

[0014] In the formula, T e The electromagnetic torque of the doubly-fed generator unit; n p L is the number of pole pairs of the generator; m The mutual inductance between the stator and rotor in the dq-axis coordinate system; i sd i sq ird i rq These are the d-axis component of the stator current, the q-axis component of the stator current, the d-axis component of the rotor current, and the q-axis component of the rotor current, respectively. When a voltage fault or system oscillation occurs, disturbance components are generated in the stator and rotor currents. These disturbance components affect the unit load through the application of electromagnetic torque. The current disturbance component is represented as...

[0015]

[0016] In the formula, Δi sdq , Δi rdq These are the disturbance values ​​of the stator dq-axis component and the rotor dq-axis component, respectively. The superscript t indicates the current value at the moment before the disturbance, and t+1 indicates the current value after the disturbance occurs.

[0017] Substituting the current disturbance component into the expression for the electromagnetic torque of the doubly-fed motor, we can obtain:

[0018]

[0019] Therefore, when a disturbance occurs in the power grid, a torque disturbance component ΔT will appear in the electromagnetic torque. e , can be represented as

[0020] Preferably, under low-voltage ride-through conditions, the expressions for the stator and rotor current disturbance components of the wind turbine are obtained based on the stator and rotor current control laws before and after low-voltage ride-through faults and the principle of non-abrupt flux linkage of voltage source wind turbines. Substituting these expressions for the stator and rotor current disturbance components under low-voltage ride-through conditions into the expression for the torque disturbance components when grid disturbances occur yields the torque disturbance components under low-voltage ride-through conditions. The disturbance components of the stator and rotor currents under low-voltage ride-through conditions are expressed as follows:

[0021] In the formula, This is the active power setpoint of the wind turbine after the fault; V g t+1 V g t These represent the voltage amplitudes before and after a fault at the wind turbine terminals, respectively; I max K represents the maximum output current of the converter. l L is the dynamic reactive current proportional coefficient of the wind turbine generator; s In the dq-axis coordinate system, P represents the self-inductance of the equivalent two-phase stator winding; s Q s These represent the active and reactive power of the unit before the low-voltage ride-through fault; Δi sq , Δi rq, Δi sd , Δi rd These are the current disturbance values ​​of the stator q-axis component, rotor q-axis component, stator d-axis component, and rotor d-axis component, respectively. By substituting the expressions for the disturbance components of the stator and rotor currents of the wind turbine under low voltage ride-through conditions into the expressions for the torque disturbance components when the power grid experiences disturbances, the torque disturbance components under low voltage ride-through conditions can be obtained.

[0022] Preferably, under the high-voltage ride-through condition, based on the stator and rotor current control laws of the voltage source wind turbine before and after a high-voltage fault, expressions for the stator and rotor current disturbance components can be derived. Substituting these expressions for the stator and rotor current disturbance components under the high-voltage ride-through condition into the expression for the torque disturbance components when a disturbance occurs in the power grid, the torque disturbance components under the high-voltage ride-through condition can be obtained. The stator and rotor current disturbance components under the high-voltage ride-through condition are:

[0023] In the formula, K h The dynamic reactive current proportional coefficient of the wind turbine is given by substituting the expressions for the stator and rotor current disturbance components under high voltage ride-through conditions into the expression for the torque disturbance components when the grid experiences disturbances.

[0024] Preferably, under the oscillation condition, the flux linkage disturbance component caused by the grid voltage disturbance component in the xy-axis coordinate system is transformed into a flux linkage disturbance component in the dq-axis coordinate system. The stator and rotor disturbance currents induced by the flux linkage disturbance component in the dq-axis coordinate system are calculated to obtain the expressions for the stator and rotor current disturbance components. These expressions are then substituted into the expression for the torque disturbance component when the grid experiences disturbances to obtain the torque disturbance component under the oscillation condition. The stator and rotor current disturbance components under the oscillation condition are:

[0025]

[0026] In the formula, θ is the angle difference when the initial rotation angles are both 0; by substituting the expression for the stator and rotor current disturbance components under oscillation conditions into the expression for the torque disturbance components when the power grid experiences disturbances, the torque disturbance components under oscillation conditions can be obtained.

[0027] Preferably, the power grid weighting gain function G(V) introduced under the oscillation condition is... g The expression for f is:

[0028] In the formula, α and β are the torque compensation frequency component coefficients, with β ranging from [0.7, 1); λ is the torque compensation voltage component coefficient, ranging from (0, 0.15). During fault ride-through, due to the change in the controlled object of the voltage source wind turbine, only the superposition of the torque disturbance component command is needed to solve the power matching problem between the main control system and the converter. Under oscillating conditions, since the controlled object of the voltage source wind turbine remains consistent with normal operation, the superposition of the torque disturbance command can only eliminate the oscillating component in the torque command, and cannot solve the torque mismatch problem between the wind turbine main control system and the converter. Therefore, the grid weight gain function G(V) is introduced. g f) is used to solve the compatibility problem between the two.

[0029] Beneficial effects: 1. Under fault ride-through conditions, electromagnetic torque disturbances are calculated and superimposed based on different control targets before and after a fault in voltage source wind turbine units, avoiding the problem of fault shutdown caused by torque mismatch during fault ride-through.

[0030] 2. Under system oscillation conditions, the electromagnetic torque disturbance component can be calculated and superimposed based on the expression of the stator and rotor current disturbance component caused by the voltage disturbance component, thereby avoiding load oscillation caused by system oscillation.

[0031] 3. Calculate the grid weight gain based on the actual operating frequency and voltage of the grid to compensate for the torque mismatch between the main control system and the converter caused by changes in the control strategy. Attached Figure Description

[0032] Figure 1 This is a control principle diagram of the load optimization control method for voltage source type wind turbine generators of the present invention;

[0033] Figure 2 This is a schematic diagram of the active current segmentation threshold under different fault voltages in the load optimization control method for voltage source type wind turbines of the present invention. Detailed Implementation

[0034] like Figure 1 As shown, a specific embodiment of the load optimization control method for voltage source wind turbines of the present invention is as follows: Without considering disturbances, the electromagnetic torque of the doubly-fed induction generator can be expressed as: T e =1.5n p L m (i sq i rd -i sd i rq In the formula, T e The electromagnetic torque of the doubly-fed generator unit; n p L is the number of pole pairs of the generator; m The mutual inductance between the stator and rotor in the dq-axis coordinate system; i sd isq i rd i rq These are the dq-axis components of the stator and rotor currents, respectively.

[0035] When a voltage fault or system oscillation occurs, a disturbance component will be generated in the stator and rotor currents. This disturbance component affects the unit load by exerting electromagnetic torque. The current disturbance component is expressed as:

[0036]

[0037] In the formula, Δi sdq , Δi rdq These are the disturbance values ​​of the stator and rotor dq axis components, respectively. The superscript t indicates the current value at the moment before the disturbance, and t+1 indicates the current value after the disturbance occurs.

[0038] Substituting the disturbance component into the expression for the electromagnetic torque of the doubly-fed motor, we can obtain...

[0039]

[0040] Therefore, when a disturbance occurs in the power grid, a torque disturbance component ΔT will appear in the electromagnetic torque. e , can be represented as:

[0041]

[0042] The influence of disturbance components on shaft loads can be eliminated through negative feedback; the principle is as follows: Figure 1 As shown, W d To give the controller the speed, W m The generator feedback speed is T, where err is the difference between the given speed and the feedback speed. set P is the torque setpoint for the torque loop output. s Q s These represent the active and reactive power of the unit before the low-voltage ride-through fault, T. ref G(V) is the sum of the given torque and the additional torque. g f) is the grid weight gain function, and the additional torque is selected according to the low voltage ride-through condition, high voltage ride-through condition, and oscillation condition.

[0043] Under low voltage ride-through conditions, the control law of a voltage source wind turbine before and after a fault can be expressed as follows:

[0044]

[0045] In the formula, ω s K represents the angular velocity of the two-phase rotating coordinate system of the stator. P K QThese are the active power droop coefficient and the reactive power droop coefficient, respectively; V s ref The stator voltage setpoint; These represent the reference values ​​for active current and reactive current after a fault, respectively. These are the active power setpoint and reactive power setpoint of the unit before the fault, respectively; ω base For the angular velocity of a two-phase synchronous rotating coordinate system, generally ω base =2πf;V base This is a voltage reference value, typically V. base =1.0pu; V g This represents the voltage amplitude after the fault.

[0046] Regarding the stator and rotor current control laws and the principle of non-abrupt flux linkage before and after low-voltage ride-through faults in voltage-source wind turbines, the specific expressions for the stator and rotor current disturbance components can be written as follows:

[0047]

[0048] In the formula This refers to the active power setpoint of the wind turbine after the fault. When the unit was in normal operation before the fault, it generally had the following values: Vg t+1 , These represent the voltage amplitudes before and after a fault at the wind turbine terminals, respectively; I max This is the maximum output current of the converter, typically 1.5 times the rated current of the generator set; K l The dynamic reactive current proportional coefficient of the wind turbine generator is defined by the national standard, and its value range is 1.5 ≤ K. l ≤3; L s In the dq-axis coordinate system, represents the self-inductance of the equivalent two-phase stator winding.

[0049] Based on the different active power outputs and the varying values ​​of the dynamic reactive current proportional coefficient of the unit before fault ride-through, a graph can be drawn as follows: Figure 2 The active current piecewise threshold curves shown are for different fault voltages. From the curves, it can be seen that during a fault, the active current first increases monotonically and then decreases monotonically. The voltage threshold V of the piecewise function... th Unique within the low-voltage ride-through range, by substituting the expressions for the disturbance components of the stator and rotor currents of the wind turbine under low-voltage ride-through conditions into the expression for the torque disturbance component when the grid experiences disturbances, the torque disturbance component ΔT under low-voltage ride-through conditions can be obtained. e :

[0050]

[0051] Under high-voltage ride-through conditions, considering the stator and rotor current control laws and the principle of non-abrupt flux linkage before and after a high-voltage fault in voltage-source wind turbines, the stator and rotor current disturbance components can be specifically expressed as follows:

[0052]

[0053] In the formula, K h The dynamic reactive current proportional coefficient of the wind turbine generator is K, and its value range is K according to national standards. h ≥1.5.

[0054] Substituting the expressions for the stator and rotor current disturbance components under high voltage ride-through conditions into the expression for the torque disturbance components when the power grid experiences disturbances, we can obtain the expression for the torque disturbance components under high voltage ride-through conditions:

[0055]

[0056] Under low-frequency oscillation conditions, the grid voltage has a frequency of f. d The disturbance component u sx and u sy Time (i.e., the frequency of the xy coordinate axis rotation of the disturbance component is ω) d =2πf d Furthermore, the frequency of the voltage disturbance component is greater than the rated frequency, i.e., ω d >ω base This will induce a disturbance component Ψ in the magnetic flux. sx Ψ sy Specifically, it can be expressed as The xy magnetic flux disturbance component Ψ sx Ψ sy The disturbance component caused by transforming to the dq-axis coordinate system is ΔΨ. sdq Specifically, it can be expressed as:

[0057]

[0058] In the formula, θ d =θ d0 +ω d t is the angle of rotation along the x and y axes, θ d0 θ is the initial angle of rotation along the x and y axes. base =θ b0 +ω base t is the angle θ during rotation along the dq axis. b0 Let dq be the initial rotation angle; when both initial rotation angles are 0, the angle difference θ can be expressed as θ = θ d -θ base =(ω d -ω base )t.

[0059] Therefore, the frequency is ω d The disturbance component u sxy A frequency of (ω) will be induced in the dq rotating coordinate system rotating at synchronous speed. d -ω base The disturbance component ΔΨ sdq The flux disturbance component can be induced to have a frequency of (ω) through the unit flux equation. d -ω base The stator and rotor disturbance current Δi sdq and Δi rdq Based on the above analysis and the principle of non-abrupt flux linkage, the specific expressions for the stator and rotor current disturbance components can be written as follows:

[0060]

[0061] Substituting the expressions for the stator and rotor current disturbance components under oscillating conditions into the expressions for the torque disturbance components when the power grid experiences disturbances, we can obtain the expression for the torque disturbance components under oscillating conditions:

[0062]

[0063] When a fault occurs, due to the change in the control object of a voltage-source wind turbine, simply adding a torque disturbance component command can resolve the power matching issue between the main control system and the converter. However, under oscillating conditions, since the control object of a voltage-source wind turbine remains consistent with that under normal operation, adding a torque disturbance command can only eliminate the oscillating component in the torque command and cannot resolve the torque mismatch issue between the wind turbine's main control system and the converter. Therefore, a grid weighting gain function G(V) is introduced. g To address the compatibility issue between the two, the power grid weight gain function can be expressed as:

[0064]

[0065] In the formula, α and β are the frequency component coefficients of torque compensation, and there is a relationship between them: α = 2(1-β). The specific value needs to be determined according to the strength of the voltage source wind turbine connected to the grid. The value of β is in the range of [0.7, 1). λ is the voltage component coefficient of torque compensation. The specific value needs to be determined according to the strength of the voltage source wind turbine connected to the grid. The value of λ is in the range of (0, 0.15).

Claims

1. A load optimization control method for voltage source type wind turbine generators, characterized in that, Negative feedback is used to eliminate the impact of disturbance components generated during voltage faults or system oscillations on the shaft load. The actual generator speed is filtered and used as the feedback quantity. The offset between the given speed and the feedback quantity is calculated and converted into a torque setpoint through a torque loop. The output torque is then adjusted in conjunction with the actual operating conditions of the wind turbine. The actual operating conditions of the wind turbine include low voltage ride-through, high voltage ride-through, and oscillation conditions. The torque disturbance component is calculated based on the actual operating conditions. The torque disturbance component is the difference between the electromagnetic torque of the motor after a voltage fault or system oscillation and the electromagnetic torque of the motor without considering the disturbance. An additional torque with the same value but opposite direction as the torque disturbance component is set. Under low voltage ride-through and high voltage ride-through conditions, the sum of the torque setpoint and the additional torque is output to the wind turbine to adjust the torque of the wind turbine and thus eliminate the disturbance component. Under oscillation conditions, the sum of the torque setpoint and the additional torque is multiplied by the grid weight gain function and then output to the wind turbine to adjust the torque and eliminate the disturbance component.

2. The load optimization control method for voltage source type wind turbine generators according to claim 1, characterized in that, Based on the stator current and rotor current control laws and the principle of non-abrupt flux linkage before and after the low voltage ride-through fault of the voltage source wind turbine, the expressions of the stator current and rotor current disturbance components are obtained under the low voltage ride-through condition. By substituting the expressions for the disturbance components of the stator and rotor currents of the wind turbine under low voltage ride-through conditions into the expression for the torque disturbance components when the power grid experiences disturbances, the torque disturbance components under low voltage ride-through conditions can be obtained.

3. The load optimization control method for voltage source type wind turbine generators according to claim 2, characterized in that, The expression for the torque disturbance component under the low voltage ride-through condition is as follows: In the formula, This refers to the torque disturbance component under low voltage ride-through conditions. The number of pole pairs of the generator. The mutual inductance between the stator and rotor in the dq-axis coordinate system; This represents the d-axis component of the rotor current before the fault. In the dq-axis coordinate system, represents the self-inductance of the equivalent two-phase stator winding. This represents the d-axis component of the stator current before the fault. is the dynamic reactive current proportional coefficient of the wind turbine; Qs is the reactive power of the unit before the low-voltage ride-through fault occurs. , These are the voltage amplitudes before and after the fault, respectively; , These are the q-axis components of the rotor current and stator current before the fault, respectively. This is the active power setpoint for the wind turbine after a low-voltage ride-through fault. For piecewise function voltage threshold, This is the maximum current that the converter can output.

4. The load optimization control method for voltage source type wind turbine generators according to claim 2, characterized in that, Under the high-voltage ride-through condition, based on the stator current and rotor current control laws of the voltage source wind turbine before and after a high-voltage fault, expressions for the stator current and rotor current disturbance components can be derived. Substituting these expressions for the stator current and rotor current disturbance components under the high-voltage ride-through condition into the expression for the torque disturbance components when the grid experiences disturbances, the torque disturbance components under the high-voltage ride-through condition can be obtained.

5. The load optimization control method for voltage source type wind turbine generators according to claim 4, characterized in that, The expression for the torque disturbance component under the high voltage ride-through condition is as follows: In the formula, K h This is the dynamic reactive current proportional coefficient of the wind turbine generator. This refers to the torque disturbance component under high voltage ride-through conditions. The number of pole pairs of the generator. The mutual inductance between the stator and rotor in the dq-axis coordinate system; This represents the d-axis component of the rotor current before the fault. In the dq-axis coordinate system, represents the self-inductance of the equivalent two-phase stator winding. d is the stator current before the fault; Qs is the reactive power of the unit before the low-voltage ride-through fault. , These are the voltage amplitudes before and after the fault, respectively; , These are the q-axis components of the rotor current and stator current before the fault, respectively. This is the active power setpoint for the wind turbine after a low-voltage ride-through fault.

6. The load optimization control method for voltage source type wind turbine generators according to claim 2, characterized in that, Under the oscillation condition, the flux disturbance component caused by the grid voltage disturbance component in the xy-axis coordinate system is transformed into the flux disturbance component in the dq-axis coordinate system. The stator and rotor disturbance currents induced by the flux disturbance component in the dq-axis coordinate system are calculated to obtain the expressions for the stator and rotor current disturbance components. Then, these expressions are substituted into the expression for the torque disturbance component when the grid experiences disturbances to obtain the torque disturbance component under the oscillation condition.

7. The load optimization control method for voltage source type wind turbine generators according to claim 6, characterized in that, The expression for the torque disturbance component under the oscillation condition is as follows: Where θ is the angle θ when rotating along the xy-axis. d The angle θ when rotating with respect to the dq axis base angular difference, θ=θ d -θ base =(ω d -ω base )t,ω base Let ω be the angular velocity of the two-phase synchronous rotating coordinate system. d Let x be the frequency of the disturbance component's rotation along the xy-axis. This refers to the torque disturbance component under oscillating conditions. The number of pole pairs of the generator. The mutual inductance between the stator and rotor in the dq-axis coordinate system; This represents the d-axis component of the rotor current before the fault. In the dq-axis coordinate system, represents the self-inductance of the equivalent two-phase stator winding. d is the stator current before the fault; Ps and Qs are the active power and reactive power of the unit before the low voltage ride-through fault, respectively. , These are the q-axis components of the rotor current and stator current before the fault, respectively.

8. The load optimization control method for voltage source type wind turbine generators according to claim 6, characterized in that, The power grid weight gain function G(V) introduced under the oscillation condition g The expression for f is: ； In the formula, α , β For torque compensation frequency component coefficients, β The value range of is [0.7, 1); λ is the torque compensation voltage component coefficient, and the value range of λ is (0, 0.15], V g This represents the voltage amplitude.

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