A physical reduction method and terminal for calculating synchronous oscillation frequency of direct-drive wind turbine

Abstract

The present invention discloses a method and terminal for calculating the synchronous oscillation frequency of a direct-drive fan with physical order reduction. First, a first state space model of a direct-drive fan including a phase-locked loop, a current loop, and a voltage loop is established; then, a second state space model with reduced order after a limiting link is continuously saturated is obtained by using a physical order reduction method; finally, the sub / super synchronous oscillation frequency of the direct-drive fan caused by the limiting link is determined based on the second state space model by using a characteristic root analysis method. Therefore, the sub / super synchronous oscillation phenomenon of a direct-drive fan with continuous saturation of the limiting link after a large disturbance can be determined by using a state space model of a direct-drive fan with physical order reduction, thereby accurately calculating the frequency of the sub / super synchronous oscillation generated after the limiting link is saturated due to a large disturbance.

Description

Technical Field

[0001] The present invention relates to the technical field of stable operation analysis of power systems, and in particular to a physical order reduction method and terminal for calculating synchronous oscillation frequency of direct-drive wind turbines. Background Art

[0002] With the diversification of the application of power electronic devices on the source-grid-load side, the operation mode and dynamic characteristics of modern power systems have changed significantly, and are developing towards a "double-high" system with a high proportion of renewable energy generation and a high proportion of power electronic equipment. However, the stability problem of the "double-high" system cannot be ignored. In particular, the sub- / super-synchronous oscillation problem caused by power electronic converters has attracted widespread attention from scholars.

[0003] According to the mathematical mechanism of sub / super synchronous oscillations involving power electronic devices, they can be divided into negatively damped oscillations, forced oscillations and switching oscillations. Most existing studies focus on negatively damped oscillations under small disturbances, that is, when the parameters change, the damping of the corresponding sub / super synchronous oscillation mode will become negative, causing local oscillations with single frequency divergence. However, the probability of oscillations caused by nonlinear switching links is not small, and there are relatively few studies on oscillations caused by nonlinear switching links.

[0004] At present, there are two main aspects in the research on nonlinear switching links participating in / inducing oscillations. One is the oscillation approximate analysis method taking into account nonlinear switching links such as limiting, and the other is the mechanism by which nonlinear switching links such as limiting cause system oscillation instability.

[0005] In terms of the oscillation approximation analysis method taking into account nonlinear switching links such as limiting, the existing literature mainly uses the describing function method to analyze the impact of nonlinear links in converter control on the oscillation dynamics. The basic idea is to replace the nonlinear effect with the fundamental response, so that it is approximated as a linear system in the frequency domain and as a smooth system in the time domain.

[0006] In the study of the mechanism of system oscillation and instability caused by nonlinear switching links such as limiting, one is to consider that the limiting / switching effect causes the system to lose its equilibrium point, thereby causing small disturbance instability or transient instability. The other is to directly analyze the switching oscillation caused by limiting.

[0007] On the other hand, if some nonlinear conditions act during the oscillation process so that the system is restricted to a certain manifold (such as continuous saturation due to amplitude limiting), the oscillation analysis can be obtained in the mathematical model of the reduced-dimensional manifold according to the differential dynamics theory. However, the reduced-dimensional model is only meaningful mathematically and has no corresponding system in physics. Therefore, the above studies all analyzed the system that has undergone switching oscillations, and did not consider the changes in the dynamic behavior of the system caused by the nonlinear links after the stable system was disturbed. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a physical reduction-order direct-drive wind turbine synchronous oscillation frequency calculation method and terminal, which can calculate the oscillation frequency of the switching oscillation after the stable system is disturbed.

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0010] A method for calculating synchronous oscillation frequency of a direct-drive fan by physical reduction, comprising the steps of:

[0011] The first multi-order state space model of the direct-drive wind turbine is established by combining the state variables corresponding to the phase-locked loop, current loop and voltage loop inside the direct-drive wind turbine.

[0012] Physically reducing the first multi-order state space model by limiting and saturating to obtain a second multi-order state space model;

[0013] The second multi-order state space model is solved by using the characteristic root analysis method to calculate the sub-synchronous oscillation frequency or super-synchronous oscillation frequency of the direct-drive wind turbine caused by the limiting link.

[0014] In order to solve the above technical problems, another technical solution adopted by the present invention is:

[0015] A physical reduction order direct drive wind turbine synchronous oscillation frequency calculation terminal comprises a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the processor implements the following steps when executing the computer program:

[0016] The first multi-order state space model of the direct-drive wind turbine is established by combining the state variables corresponding to the phase-locked loop, current loop and voltage loop inside the direct-drive wind turbine.

[0017] Physically reducing the first multi-order state space model by limiting and saturating to obtain a second multi-order state space model;

[0018] The second multi-order state space model is solved by using the characteristic root analysis method to calculate the sub-synchronous oscillation frequency or super-synchronous oscillation frequency of the direct-drive wind turbine caused by the limiting link.

[0019] The beneficial effects of the present invention are as follows: firstly, a first state space model of a direct-drive fan including state variables corresponding to a phase-locked loop, a current loop, and a voltage loop is established; then, a second state space model after continuous saturation of the limiting link is obtained by using a physical order reduction method; finally, the sub- / super-synchronous oscillation frequency of the direct-drive fan caused by the limiting link is determined based on the second state space model by using a characteristic root analysis method. Therefore, the sub- / super-synchronous oscillation phenomenon of a direct-drive fan with continuous saturation of the limiting link after a large disturbance can be determined by using a physical order reduction state space model of a direct-drive fan, thereby accurately calculating the frequency of the sub- / super-synchronous oscillation generated after the limiting link is saturated due to a large disturbance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a flow chart of a method for calculating synchronous oscillation frequency of a direct-drive wind turbine by physical reduction in an embodiment of the present invention;

[0021] Figure 2 A schematic diagram of a physical-order-reduced direct-drive wind turbine synchronous oscillation frequency calculation terminal according to an embodiment of the present invention;

[0022] Figure 3 It is a flow chart of a method for analyzing sub- / super-synchronous oscillation frequency of a physical reduced-order system switching type according to an embodiment of the present invention;

[0023] Figure 4 A structural diagram of a grid-connected direct-drive wind turbine system according to an embodiment of the present invention;

[0024] Figure 5 is a control block diagram of a machine-side converter according to an embodiment of the present invention;

[0025] Figure 6 is a control block diagram of a grid-side converter according to an embodiment of the present invention;

[0026] Figure 7 A phase-locked loop control block diagram of an embodiment of the present invention;

[0027] Figure 8 A time domain waveform comparison diagram before and after adding a limiting link in an embodiment of the present invention;

[0028] Fig. 9 This is a schematic diagram of the current limiting saturation condition after being subjected to a large disturbance according to an embodiment of the present invention;

[0029] Fig.10 Schematic diagram of root loci of characteristic roots of an embodiment of the present invention;

[0030] Description of labels:

[0031] 1. A physical-order reduced direct-drive wind turbine synchronous oscillation frequency calculation terminal; 2. A memory; 3. A processor. DETAILED DESCRIPTION

[0032] In order to explain the technical content, achieved objectives and effects of the present invention in detail, the following is an explanation in combination with the implementation modes and the accompanying drawings.

[0033] Please refer to Figure 1 The embodiment of the present invention provides a method for calculating the synchronous oscillation frequency of a direct-drive wind turbine by physical reduction, comprising the steps of:

[0034] The first multi-order state space model of the direct-drive wind turbine is established by combining the state variables corresponding to the phase-locked loop, current loop and voltage loop inside the direct-drive wind turbine.

[0035] Physically reducing the first multi-order state space model by limiting and saturating to obtain a second multi-order state space model;

[0036] The second multi-order state space model is solved by using the characteristic root analysis method to calculate the sub-synchronous oscillation frequency or super-synchronous oscillation frequency of the direct-drive wind turbine caused by the limiting link.

[0037] From the above description, it can be seen that the beneficial effects of the present invention are: first, a first state space model of a direct-drive fan including state variables corresponding to a phase-locked loop, a current loop, and a voltage loop is established; then, a second state space model after continuous saturation of the limiting link is obtained by using the physical order reduction method; finally, the sub- / super-synchronous oscillation frequency of the direct-drive fan caused by the limiting link is determined based on the second state space model by using the characteristic root analysis method. Therefore, the sub- / super-synchronous oscillation phenomenon of a direct-drive fan whose limiting link is continuously saturated after being subjected to a large disturbance can be determined by using the state space model of the direct-drive fan with physical order reduction, thereby accurately calculating the frequency of the sub- / super-synchronous oscillation generated after the limiting link is saturated due to a large disturbance.

[0038] Furthermore, the first multi-order state space model of the direct-drive fan is established by combining the state variables corresponding to the phase-locked loop, the current loop and the voltage loop inside the direct-drive fan, including:

[0039] In combination with the state variables corresponding to the direct-drive wind turbine and its internal phase-locked loop, current loop and voltage loop, a first multi-order state-space model of the direct-drive wind turbine is established. The first multi-order state-space model also includes the state variables corresponding to the wind turbine, permanent magnet synchronous generator, machine-side converter, DC capacitor and grid-side converter.

[0040] From the above description, it can be seen that the first multi-order state-space model takes the phase-locked loop into consideration compared with other PMSG models. When sub / super synchronous oscillation occurs, the grid-side converter voltage fluctuates, which has a certain impact on the phase angle of the phase-locked loop, and further has a certain impact on the Park transformation and inverse transformation of the control system. Therefore, considering the phase-locked loop can make the direct-drive wind turbine model more accurate; compared with other VSC models, the machine-side control is considered. When the machine-side control is ignored, the machine side of the direct-drive wind turbine is equivalent to a voltage-controlled current source, so that when sub / super synchronous oscillation occurs, the dynamic process of the machine-side VSC, synchronous generator, and wind rotor shaft system is ignored. The first multi-order state-space model considers the machine-side control, synchronous generator, and wind rotor shaft system, which can make the model more complete and the analysis process more accurate.

[0041] Furthermore, the physically reducing the first multi-order state space model by limiting saturation to obtain a second multi-order state space model includes:

[0042] When the first current output by the DC voltage of the DC capacitor through the regulation of the voltage outer loop reaches saturation and the first current is maintained in a saturated state, the DC capacitor and the voltage outer loop of the first multi-order state space model are omitted to obtain a second multi-order state space model after physical reduction.

[0043] From the above description, it can be seen that the stability of a high-order system can be judged by the characteristic roots of a low-order physical system, thereby reducing the computational complexity while ensuring the computational accuracy of the oscillation frequency.

[0044] Further, when the DC voltage of the DC capacitor reaches saturation through the first current output by the regulation of the voltage outer loop, and the first current is maintained in a saturated state, omitting the DC capacitor and the voltage outer loop of the first multi-order state space model includes:

[0045] The first current is a d-axis current reference value;

[0046] When the first current reaches saturation and remains in a saturated state, deleting a DC capacitance equation in the first multi-order state-space model, wherein the DC capacitance equation includes a DC capacitance voltage;

[0047] The voltage outer loop formula corresponding to the DC capacitor voltage in the first multi-order state-space model is deleted.

[0048] From the above description, it can be seen that when the d-axis current reference value reaches the limit, the DC capacitance equation no longer works, and because the DC capacitance equation includes the DC capacitance voltage, the mathematical expression of the equilibrium point cannot solve the DC capacitance voltage after deleting the DC capacitance equation. Therefore, when the d-axis current reference value reaches the limit, it can be considered that the voltage outer loop that generates the d-axis current reference value no longer works, which is equivalent to decoupling the DC capacitance and voltage loop from other parts of the system. The first multi-order state space model can be simplified through physical order reduction.

[0049] Further, the method of using the characteristic root analysis method to solve the second multi-order state space model and calculate the subsynchronous oscillation frequency and supersynchronous oscillation frequency of the direct-drive wind turbine caused by the limiting link includes:

[0050] respectively calculating the characteristic roots of the first multi-order state-space model and the second multi-order state-space model under different oscillation modes;

[0051] The unstable oscillation mode of the second multi-order state-space model is determined by comparing the calculation results of the characteristic roots, and the subsynchronous oscillation frequency or supersynchronous oscillation frequency of the direct-drive wind turbine is calculated according to the unstable oscillation mode.

[0052] From the above description, it can be seen that using the characteristic root analysis method to solve has the advantages of convenience and accuracy, and based on the characteristic root analysis method, the stability of a high-order system can be judged by the characteristic roots of a low-order physical system.

[0053] Please refer to Figure 2 Another embodiment of the present invention provides a physical reduction-order direct-drive wind turbine synchronous oscillation frequency calculation terminal, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the following steps when executing the computer program:

[0054] The first multi-order state space model of the direct-drive wind turbine is established by combining the state variables corresponding to the phase-locked loop, current loop and voltage loop inside the direct-drive wind turbine.

[0055] Physically reducing the first multi-order state space model by limiting and saturating to obtain a second multi-order state space model;

[0056] The second multi-order state space model is solved by using the characteristic root analysis method to calculate the sub-synchronous oscillation frequency or super-synchronous oscillation frequency of the direct-drive wind turbine caused by the limiting link.

[0057] From the above description, we can know that firstly, we establish the first state space model of the direct-drive fan including the state variables corresponding to the phase-locked loop, current loop and voltage loop; then, we use the physical order reduction method to obtain the second state space model after the limiting link is continuously saturated; finally, we use the characteristic root analysis method to determine the sub / supersynchronous oscillation frequency of the direct-drive fan caused by the limiting link based on the second state space model. Therefore, we can use the physical order reduction direct-drive fan state space model to determine the oscillation frequency for the sub / supersynchronous oscillation phenomenon of the direct-drive fan with continuous saturation of the limiting link after a large disturbance, so as to accurately calculate the frequency of the sub / supersynchronous oscillation generated after the limiting link is saturated due to a large disturbance.

[0058] Furthermore, the first multi-order state space model of the direct-drive fan is established by combining the state variables corresponding to the phase-locked loop, the current loop and the voltage loop inside the direct-drive fan, including:

[0059] In combination with the state variables corresponding to the direct-drive wind turbine and its internal phase-locked loop, current loop and voltage loop, a first multi-order state-space model of the direct-drive wind turbine is established. The first multi-order state-space model also includes the state variables corresponding to the wind turbine, permanent magnet synchronous generator, machine-side converter, DC capacitor and grid-side converter.

[0060] From the above description, it can be seen that the first multi-order state-space model takes the phase-locked loop into consideration compared with other PMSG models. When sub / super synchronous oscillation occurs, the grid-side converter voltage fluctuates, which has a certain impact on the phase angle of the phase-locked loop, and further has a certain impact on the Park transformation and inverse transformation of the control system. Therefore, considering the phase-locked loop can make the direct-drive wind turbine model more accurate; compared with other VSC models, the machine-side control is considered. When the machine-side control is ignored, the machine side of the direct-drive wind turbine is equivalent to a voltage-controlled current source, so that when sub / super synchronous oscillation occurs, the dynamic process of the machine-side VSC, synchronous generator, and wind rotor shaft system is ignored. The first multi-order state-space model considers the machine-side control, synchronous generator, and wind rotor shaft system, which can make the model more complete and the analysis process more accurate.

[0061] Furthermore, the physically reducing the first multi-order state space model by limiting saturation to obtain a second multi-order state space model includes:

[0062] When the first current output by the DC voltage of the DC capacitor through the regulation of the voltage outer loop reaches saturation and the first current is maintained in a saturated state, the DC capacitor and the voltage outer loop of the first multi-order state space model are omitted to obtain a second multi-order state space model after physical reduction.

[0063] From the above description, it can be seen that the stability of a high-order system can be judged by the characteristic roots of a low-order physical system, thereby reducing the computational complexity while ensuring the computational accuracy of the oscillation frequency.

[0064] Further, when the DC voltage of the DC capacitor reaches saturation through the first current output by the regulation of the voltage outer loop, and the first current is maintained in a saturated state, omitting the DC capacitor and the voltage outer loop of the first multi-order state space model includes:

[0065] The first current is a d-axis current reference value;

[0066] When the first current reaches saturation and remains in a saturated state, deleting a DC capacitance equation in the first multi-order state-space model, wherein the DC capacitance equation includes a DC capacitance voltage;

[0067] The voltage outer loop formula corresponding to the DC capacitor voltage in the first multi-order state-space model is deleted.

[0068] From the above description, it can be seen that when the d-axis current reference value reaches the limit, the DC capacitance equation no longer works, and because the DC capacitance equation includes the DC capacitance voltage, the mathematical expression of the equilibrium point cannot solve the DC capacitance voltage after deleting the DC capacitance equation. Therefore, when the d-axis current reference value reaches the limit, it can be considered that the voltage outer loop that generates the d-axis current reference value no longer works, which is equivalent to decoupling the DC capacitance and voltage loop from other parts of the system. The first multi-order state space model can be simplified through physical order reduction.

[0069] Further, the method of using the characteristic root analysis method to solve the second multi-order state space model and calculate the subsynchronous oscillation frequency and supersynchronous oscillation frequency of the direct-drive wind turbine caused by the limiting link includes:

[0070] respectively calculating the characteristic roots of the first multi-order state-space model and the second multi-order state-space model under different oscillation modes;

[0071] The unstable oscillation mode of the second multi-order state-space model is determined by comparing the calculation results of the characteristic roots, and the subsynchronous oscillation frequency or supersynchronous oscillation frequency of the direct-drive wind turbine is calculated according to the unstable oscillation mode.

[0072] From the above description, it can be seen that using the characteristic root analysis method to solve has the advantages of convenience and accuracy, and based on the characteristic root analysis method, the stability of a high-order system can be judged by the characteristic roots of a low-order physical system.

[0073] The above-mentioned physical reduction order direct-drive wind turbine synchronous oscillation frequency calculation method and terminal of the present invention are suitable for determining the sub-synchronous oscillation frequency of the grid-connected direct-drive wind turbine with continuous saturation of amplitude limiting after being subjected to a large disturbance by using the state space model of the grid-connected direct-drive wind turbine with physical reduction order. The following is an explanation through a specific implementation method:

[0074] Embodiment 1

[0075] Please refer to Figure 1 and Figure 3 , a physical reduction method for calculating the synchronous oscillation frequency of a direct-drive wind turbine, comprising the steps of:

[0076] S1. Combine the state variables corresponding to the phase-locked loop, current loop and voltage loop inside the direct-drive wind turbine to establish the first multi-order state space model of the direct-drive wind turbine.

[0077] Wherein, step S1 specifically includes:

[0078] In combination with the state variables corresponding to the direct-drive wind turbine and its internal phase-locked loop, current loop and voltage loop, a first multi-order state-space model of the direct-drive wind turbine is established. The first multi-order state-space model also includes the state variables corresponding to the wind turbine, permanent magnet synchronous generator, machine-side converter, DC capacitor and grid-side converter.

[0079] Please refer to Figure 4 The main circuit of the grid-connected direct-drive wind turbine includes the wind turbine, permanent magnet synchronous generator, machine-side converter, grid-side converter and its control system, phase-locked loop and other parts.

[0080] S11. Calculate the mechanical torque equation obtained by the wind turbine from wind energy:

[0081] T m =0.5ρC p A r V w 3 / Ω W ;

[0082] Where V w Indicates wind speed, Ω w represents the wind turbine speed, ρ represents the air density, A r Indicates the rotor area, C p represents the wind energy conversion coefficient, T m Indicates the mechanical torque of the fan.

[0083] S12, direct-drive permanent magnet synchronous generator The wind turbine rotor and the generator are directly connected, and its motion equation is:

[0084] pΩ w =(T m -T e ) / J;

[0085] Where p represents the differential operator, J represents the moment of inertia of the wind turbine blade, and T e Represents the electromagnetic torque of the generator.

[0086] The stator winding of the permanent magnet generator adopts the motor convention and only considers the positive sequence component. The dynamic model of the permanent magnet synchronous generator in the dq synchronous rotating coordinate system is:

[0087]

[0088] Where, L s , R s Respectively represent the stator inductance and resistance, u ds 、u qs and i ds 、i qs Represent the dq axis components of stator voltage and current respectively, n p represents the number of permanent magnet motor pole pairs, ψ f represents the flux of the permanent magnet passing through the rotor, ω s represents the electrical angular velocity of the permanent magnet generator, Ω represents the mechanical angular velocity of the generator, Ω=Ω w .

[0089] S13, please refer to Figure 5 , q-axis stator current i qs The control goal is to achieve maximum wind energy capture, and the d-axis stator current i ds The control value of is 0. Under this control strategy, the control equation of the machine-side converter can be obtained:

[0090]

[0091] In the formula, i dsref Indicates the d-axis stator current reference value of the machine-side converter, i qsref Indicates the q-axis stator current reference value of the machine-side converter, k p1 , k i1 , k p2 , k i2 represents the control coefficient of the PI controller of the machine-side converter, C pmax represents the maximum wind energy utilization coefficient, Indicates the reference value of electromagnetic torque under maximum wind energy control.

[0092] S14, DC capacitance equation is:

[0093] pU dc =(1.5u ds i ds +1.5u qs i qs -1.5e d i d -1.5e q i q ) / CU dc ;

[0094] In the formula, e d 、e q and i d 、i qThey represent the dq axis components of the grid-side converter voltage and current, respectively; C represents the DC capacitance, and U dc Indicates the DC capacitor voltage.

[0095] S15, please refer to Figure 6 , the grid-side converter adopts constant DC voltage control and reactive power control. Under this control strategy, the dynamic equation of the grid-side converter when the limit is not saturated is:

[0096]

[0097] In the formula, i gdref 、i gqref They represent the dq axis current reference value components of the grid-side converter, i gd 、i gq They represent the dq axis current components of the grid-side converter after Parker transformation, k pc , k ic They respectively represent the proportional coefficient and integral coefficient of the inner loop PI link of the grid-side converter.

[0098] S16, please refer to Figure 7 , the dynamic equation of the phase-locked loop is:

[0099]

[0100] In the formula, v td 、v tq Respectively represent the voltage at the phase-locked loop measurement point, v ta 、v tb 、v tc represents the measured voltage dq axis component obtained after Park transformation, k pp , k ip Represents the proportional integral coefficient of the phase-locked loop PI link, where ω=ω pll -ω 0 .

[0101] S17. The current differential equation of the main circuit is:

[0102]

[0103] In the formula, v d 、v q They represent the d and q axis components of the grid side voltage, L g Indicates the inductance of lines, transformers, power grids, etc.

[0104] v gdq With the grid voltage amplitude U g The relationship is:

[0105]

[0106] S18, combining the equations and formulas in step S11 to step S17, to obtain the first multi-order state space model of the direct-drive wind turbine, that is, the simplified 13th-order grid-connected model of the direct-drive wind turbine:

[0107]

[0108] S2. Physically reduce the order of the first multi-order state space model by limiting and saturation to obtain a second multi-order state space model.

[0109] Among them, when the first current output by the DC voltage of the DC capacitor through the regulation of the voltage outer loop reaches saturation and the first current is maintained in a saturated state, the DC capacitor and the voltage outer loop of the first multi-order state space model are omitted to obtain a second multi-order state space model after physical reduction.

[0110] Specifically, the sub / supersynchronous oscillation phenomenon of the PMSG (permanent magnet synchronous generator) system with limited participation after a large disturbance is as follows Figure 8 As shown in the figure, after adding the limiting link, the system becomes a non-smooth system, and is a local positive damping system near the equilibrium point. Under the same disturbance, the system oscillates with an oscillation frequency of 139.5Hz. Please refer to Fig. 9 ,i dref After adding the limiting link, the waveform is disturbed by the large disturbance. dref Continues to saturate after reaching the upper limit.

[0111] i dref Reach i dmax This is because after the initial deviation is given, the power at both ends of the DC capacitor is unbalanced, causing the DC voltage deviation. The DC voltage is adjusted through the voltage outer loop to output i dref , thereby maintaining the power balance of the DC capacitor (assuming that the current ideally tracks the current reference value at this time). However, the initial value is large, resulting in a large power imbalance, which makes the voltage loop output i dref Reaching the limit means that the voltage loop regulation capability is limited by the overcurrent capability of the device, and it is difficult to maintain power balance, which causes the DC voltage to fluctuate within a higher range and continue to increase. dref Location in i dmax The voltage loop control fails, which is equivalent to decoupling the DC capacitor and the voltage loop from other parts of the system. dref =i dmax In this case, the original model can be equivalent to a physical reduced-order model that ignores the DC capacitance and the voltage outer loop dynamics.

[0112] S21. The first current is a d-axis current reference value. When the first current reaches saturation and remains in a saturated state, a DC capacitance equation in the first multi-order state-space model is deleted, wherein the DC capacitance equation includes a DC capacitance voltage.

[0113] Specifically, when the d-axis current reference value reaches the limit, it is considered that i dref =i dmax , so that in the original system of equations, the equation px=U dc -U dref No longer works, after deleting this equation, the original 13th-order state space model is mathematically reduced. At this time, the state space model becomes a 12th-order model, with a total of 12 state variables (including U dc ).

[0114] S22. Delete the voltage outer loop formula corresponding to the DC capacitor voltage in the first multi-order state-space model.

[0115] Specifically, since there are a total of 12 state variables in the 12th-order model (including U dc ), the mathematical expression of its equilibrium point cannot be used to describe the state variable U dc The solution is obtained, that is, the mathematical reduced order system (MROS) has no equilibrium point, and the MROS state space model has no corresponding system in physics.

[0116] Therefore, consider the 11th-order physical reduced-order system (PROS) after removing the voltage outer loop. PROS has physical meaning. When the d-axis current reference value reaches the limit, it can be considered that the voltage outer loop that generates the d-axis current reference value no longer works, which is equivalent to decoupling the DC capacitor and the voltage loop from other parts of the system. At this time, px=U can be deleted. dc -U dref and pU dc =(1.5u ds i ds +1.5u qs i qs -1.5e d i d -1.5e q i q ) / CU dc These two equations.

[0117] The second multi-order state space model is:

[0118]

[0119] S3. Use characteristic root analysis to solve the second multi-order state space model, and calculate the sub-synchronous oscillation frequency or super-synchronous oscillation frequency of the direct-drive wind turbine caused by the limiting link.

[0120] S31 . Calculate the characteristic roots of the first multi-order state-space model and the second multi-order state-space model respectively under different oscillation modes.

[0121] Specifically, the stability of the equilibrium point in the grid-connected direct-drive wind turbine system is determined by the eigenvalue of the corresponding Jacobian matrix J. The typical parameters of the first multi-order state space model and the second multi-order state space model are shown in Table 1.

[0122] Table 1 System parameters

[0123]

[0124] Under the same control parameters as the first multi-order state-space model, when the i of the physical reduced-order model dref =i dmax When , its characteristic roots can be compared with the characteristic roots of the first multi-order state space model, as shown in Table 2.

[0125] Table 2 Characteristic roots and equilibrium point properties of physical reduced-order systems

[0126]

[0127] It can be seen that the state space model of the direct-drive wind turbine after physical order reduction has a pair of unstable oscillation modes 3. At this time, the first multi-order state space model originally has a stable operating point, but the large disturbance causes the current limit of the system to be continuously saturated. The limit saturation effect can physically reduce the first multi-order state space model. The reduced model has an unstable oscillation mode 3, resulting in oscillation.

[0128] S32. Determine the unstable oscillation mode of the second multi-order state-space model by comparing the calculation results of the characteristic roots, and calculate the subsynchronous oscillation frequency or supersynchronous oscillation frequency of the direct-drive wind turbine according to the unstable oscillation mode.

[0129] Analyze the root loci of physical reduced-order models with i dref Changes such as Fig.10 As shown, with i dref from i dmax When the current gradually increases from 600A, the oscillation mode 3 gradually shifts to the right, the damping decreases, and at i dmax = 975A and reaches the right half plane, the oscillation mode becomes unstable, indicating that the physical reduced-order model is dref As the limit saturates, the system stability becomes weaker. dmax When =1050A, the model exhibits negative damping characteristics.

[0130] The oscillation frequency analyzed by physical order reduction method is 141.94Hz, while the oscillation frequency of the first multi-order state space model obtained by detailed PSCAD electromagnetic transient simulation is 139.5Hz, and the relative error between the two is 1.74%, which is basically consistent. Therefore, the consistency of oscillation frequency verifies the accuracy of determining sub / supersynchronous oscillation frequency by physical order reduction.

[0131] In summary, the switching oscillation phenomenon of the system under positive damping under large disturbance in this embodiment can be explained as follows: the large disturbance causes the system current limit with a stable operating point to be continuously saturated, which is equivalent to being reduced to a PROS with a negative damping equilibrium point, thereby generating divergent oscillations. And by calculating the oscillation frequency through physical reduction, based on the characteristic root analysis method, the characteristic roots of the low-order physical system determine the stability of the high-order system, which can ensure the accuracy of the oscillation frequency calculation.

[0132] Embodiment 2

[0133] Please refer to Figure 2 A physically reduced-order direct-drive wind turbine synchronous oscillation frequency calculation terminal 1 includes a memory 2, a processor 3, and a computer program stored in the memory 2 and executable on the processor 3. When the processor 3 executes the computer program, each step of a physically reduced-order direct-drive wind turbine synchronous oscillation frequency calculation method of embodiment 1 is implemented.

[0134] In summary, the present invention provides a method and terminal for calculating the synchronous oscillation frequency of a direct-drive wind turbine with physical order reduction. Aiming at the sub-synchronous oscillation phenomenon of a grid-connected direct-drive wind turbine with continuous saturation of the amplitude limit after a large disturbance, the sub-synchronous oscillation frequency is determined by using a state space model of a grid-connected direct-drive wind turbine with physical order reduction. Specifically, firstly, a 13th-order simplified state space model of a grid-connected direct-drive wind turbine system including a phase-locked loop, a current loop, a voltage loop and other links is established; secondly, by using the physical order reduction method, an 11th-order state space model of a grid-connected direct-drive wind turbine ignoring the dynamics of the DC capacitor and the voltage outer loop after the amplitude limit link is continuously saturated is obtained; finally, based on the established state space model of the direct-drive wind turbine with physical order reduction, the sub-synchronous oscillation frequency of the direct-drive wind turbine caused by the amplitude limit link is determined by the characteristic root analysis method. The results show that the switching oscillation phenomenon of the system under positive damping under large disturbance can be explained as follows: the large disturbance causes the current amplitude limit of the system with a stable operating point to be continuously saturated, which is equivalently reduced to a physical order reduction model with a negative damping equilibrium point, thereby generating divergent oscillations.

[0135] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent transformations made using the contents of the present invention's specification and drawings, or directly or indirectly applied in related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A physical reduction method for calculating the synchronous oscillation frequency of a direct-drive wind turbine. It is characterized in that Includes steps: Combined with the state variables corresponding to the phase-locked loop, current loop and voltage loop inside the direct-drive wind turbine, a first multi-order state-space model of the direct-drive wind turbine is established: Combined with the state variables corresponding to the phase-locked loop, current loop and voltage loop inside the direct-drive wind turbine, a first multi-order state-space model of the direct-drive wind turbine is established, wherein the first multi-order state-space model also includes the state variables corresponding to the wind turbine, the permanent magnet synchronous generator, the machine-side converter, the DC capacitor and the grid-side converter The first multi-order state space model is physically reduced in order by limiting saturation to obtain a second multi-order state space model: when the first current output by the DC voltage of the DC capacitor through the regulation of the voltage outer loop reaches saturation and the first current is maintained in a saturated state, the DC capacitor and the voltage outer loop of the first multi-order state space model are omitted to obtain a physically reduced second multi-order state space model; When the DC voltage of the DC capacitor reaches saturation through the first current output by the regulation of the voltage outer loop, and the first current is maintained in a saturated state, omitting the DC capacitor and the voltage outer loop of the first multi-order state space model includes: the first current is a d-axis current reference value; when the first current reaches saturation and is maintained in a saturated state, deleting the DC capacitor equation in the first multi-order state space model, the DC capacitor equation including the DC capacitor voltage; deleting the voltage outer loop formula corresponding to the DC capacitor voltage in the first multi-order state space model; The second multi-order state-space model is solved by the characteristic root analysis method, and the sub-synchronous oscillation frequency or super-synchronous oscillation frequency of the direct-drive fan caused by the limiting link is calculated: the characteristic roots of the first multi-order state-space model and the second multi-order state-space model are calculated respectively under different oscillation modes; the unstable oscillation mode of the second multi-order state-space model is determined by comparing the calculation results of the characteristic roots, and the sub-synchronous oscillation frequency or super-synchronous oscillation frequency of the direct-drive fan is calculated according to the unstable oscillation mode.

2. A physical reduction-order direct-drive wind turbine synchronous oscillation frequency calculation terminal, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, It is characterized in that When the processor executes the computer program, the following steps are implemented: In combination with the state variables corresponding to the phase-locked loop, current loop and voltage loop inside the direct-drive wind turbine, a first multi-order state-space model of the direct-drive wind turbine is established: In combination with the state variables corresponding to the phase-locked loop, current loop and voltage loop inside the direct-drive wind turbine, a first multi-order state-space model of the direct-drive wind turbine is established, wherein the first multi-order state-space model also includes the state variables corresponding to the wind turbine, the permanent magnet synchronous generator, the machine-side converter, the DC capacitor and the grid-side converter; The first multi-order state space model is physically reduced in order by limiting saturation to obtain a second multi-order state space model: when the first current output by the DC voltage of the DC capacitor through the regulation of the voltage outer loop reaches saturation and the first current is maintained in a saturated state, the DC capacitor and the voltage outer loop of the first multi-order state space model are omitted to obtain a physically reduced second multi-order state space model; When the DC voltage of the DC capacitor reaches saturation through the first current output by the regulation of the voltage outer loop, and the first current is maintained in a saturated state, omitting the DC capacitor and the voltage outer loop of the first multi-order state space model includes: the first current is a d-axis current reference value; when the first current reaches saturation and is maintained in a saturated state, deleting the DC capacitor equation in the first multi-order state space model, the DC capacitor equation including the DC capacitor voltage; deleting the voltage outer loop formula corresponding to the DC capacitor voltage in the first multi-order state space model; The second multi-order state-space model is solved by the characteristic root analysis method, and the sub-synchronous oscillation frequency or super-synchronous oscillation frequency of the direct-drive fan caused by the limiting link is calculated: the characteristic roots of the first multi-order state-space model and the second multi-order state-space model are calculated respectively under different oscillation modes; the unstable oscillation mode of the second multi-order state-space model is determined by comparing the calculation results of the characteristic roots, and the sub-synchronous oscillation frequency or super-synchronous oscillation frequency of the direct-drive fan is calculated according to the unstable oscillation mode.

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