Method for analyzing low-frequency oscillation interaction between parallel direct-driven wind turbine generators

Abstract

The invention discloses a method for analyzing low-frequency oscillation interaction between parallel direct-driven wind turbine generators. The method comprises the following steps: establishing a parallel grid-connected simulation model of two direct-driven wind turbine generators; accessing to a simulation model of an alternating current system, linearizing at a balance point to obtain a system state space equation, calculating a characteristic value and participating in factor analysis, and analyzing low-frequency oscillation characteristics of the system; according to an analysis result, aiming at a low-frequency oscillation mode related to the direct-current voltage, establishing a damping torque model of a direct-current voltage loop mode of the direct-driven wind turbine generator; forming a two-machine parallel system damping torque model by using the damping torque model, and constructing an expression of interaction between units by using a damping transfer function; quantitatively evaluating the interaction between the direct-driven wind turbine generators; after evaluation, the system is expanded to a multi-machine parallel system and the interaction is analyzed. The mutual change trend is quantitatively analyzed in the form of the mutual damping coefficient, the dynamic coupling mechanism between the parallel direct-driven wind turbine generators is clearly and quantitatively elaborated, and the problem that the interaction mechanism between the generators cannot be quantitatively analyzed in the past is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of power transmission, and in particular to a method for analyzing low-frequency oscillation interaction between direct-drive wind turbines. Background Art

[0002] With the rapid development of renewable energy, as of June 2023, my country's cumulative installed capacity of wind power has reached 389 million kilowatts, ranking first in the world. It is estimated that by 2050, the cumulative installed capacity of wind power will reach 2.4 billion kilowatts, showing a rapid development trend. The local AC system where the wind turbine is located becomes a weak AC system. Under the condition of a weak AC system, the voltage at the public connection point where the wind turbine is connected to the grid is easily affected by changes in wind turbine power and grid strength, which destroys the ideal grid connection point voltage assumption and affects the operating stability of the wind turbine.

[0003] At present, domestic and foreign scholars have carried out outstanding and effective research on the low-frequency oscillation problem of direct-drive wind turbines under weak AC systems. Some researchers pointed out that there is a coupling relationship between the output power of direct-drive wind turbines and the grid-connected voltage under weak power grids. When the direct-drive wind turbines use terminal voltage vector control, this coupling relationship will destroy the stability of the system. The research results show that as the power grid strength weakens and the working conditions increase, the stability of the direct-drive wind turbines weakens. Some researchers further reveal the low-frequency oscillation mechanism by establishing a dynamic model of the direct-drive wind turbine from the perspective of "negative damping" or by establishing a transfer function model from the perspective of the "positive feedback" control loop, and propose physical concepts such as wind turbine inertia, damping component and synchronous component. Based on the understanding of the concepts of inertia, damping torque and synchronous torque of synchronous generators, it is revealed that the low-frequency oscillation of direct-drive wind turbines connected to the grid under weak AC system conditions is caused by insufficient damping components. Summary of the invention

[0004] Aiming at the dynamic coupling problem between parallel direct-drive wind turbines in a weak AC system, the present invention proposes a low-frequency oscillation interaction analysis method between parallel direct-drive wind turbines in order to achieve the purpose of quantitatively analyzing the interaction mechanism between the turbines.

[0005] The technical solutions adopted to implement the present invention are as follows:

[0006] The method for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines includes the following steps:

[0007] Establish a simulation model for two direct-drive wind turbines connected in parallel and connected to the grid;

[0008] The simulation model of the two direct-drive wind turbines connected in parallel to the grid is connected to the simulation model of the AC system, the system state space equation is linearized at the equilibrium point, the eigenvalue and participation factor analysis are calculated, and the low-frequency oscillation characteristics of the system are analyzed;

[0009] According to the analysis results of the low-frequency oscillation characteristics of the system, a damping torque model of the DC voltage loop mode of each direct-drive wind turbine is established for the low-frequency oscillation mode related to the DC voltage;

[0010] Using the damping torque model of the direct-drive wind turbine set, a damping torque model of a two-machine parallel system is formed, and an analytical expression of the interaction between the units is constructed using the damping transfer function;

[0011] Based on the initial operating parameters of the parallel system damping torque model, the perturbed grid strength, system operating conditions and control parameters are obtained, and the interaction between direct-drive wind turbines is quantitatively evaluated on this basis.

[0012] After the evaluation, the interaction operations between direct-drive wind turbines will be quantitatively evaluated and extended to multi-machine parallel systems to analyze the interaction analysis of multi-machine parallel systems.

[0013] Furthermore, the system state space equation is expressed as follows:

[0014]

[0015] In the formula: Δx is the state variable of the system; Δu is the input variable of the system; A and B are the state matrix and input matrix of the system respectively.

[0016] Furthermore, the input of the damping torque model is the electromagnetic power increment ΔP of the wind turbine ei The output of the model is the voltage increment Δi of the DC capacitor of the wind turbine di (i=1, 2).

[0017] Furthermore, the analytical expression is based on the first direct-drive wind turbine as the research object, with the mutual damping coefficient D dc12 The influence of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of:

[0018] D dc12 =-Re[G 12 ×G 121dc ×G 21 ] (2)

[0019] Where: R e Indicates the meaning of taking the real part, G 12 G represents the damping transfer function of wind turbine 1 on wind turbine 2; 22 G represents the damping transfer function of wind turbine 2 acting on itself; 21 G represents the damping transfer function of wind turbine 2 on wind turbine 1; 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop.

[0020] Furthermore, the establishment of a simulation model of two direct-drive wind turbines connected in parallel and connected to the grid includes a primary system of the two direct-drive wind turbines, a control system and a power grid.

[0021] Furthermore, the establishment of a parallel grid-connected simulation model of two direct-drive wind turbines comprises the following steps:

[0022] Direct-drive wind turbine modeling:

[0023] The direct-drive wind turbine group includes: a wind turbine, a permanent magnet synchronous generator, a four-quadrant converter, and a filter circuit; the wind turbine, permanent magnet synchronous generator, and machine-side converter of the direct-drive wind turbine group are all equivalent to controlled current sources, and the main circuit expression of the grid-side converter is as follows: (i=1,2)

[0024]

[0025] Where: i = 1 represents direct-drive wind turbine group 1, i = 2 represents direct-drive wind turbine group 2; E di 、E qi Indicates the converter output voltage dq axis voltage; L fi Indicates the converter filter inductance; U tdi , U tqi 、i di 、i qi are the dq axis components of the voltage at the grid-side converter terminal of the direct-drive wind turbine and the dq axis components of the output current of the grid-side converter respectively; C dci , U dci , P outi , P ini are the capacitance, voltage, output active power and input active power of the DC capacitor of the direct-drive wind turbine respectively; ω is the rated angular velocity.

[0026] Control system modeling:

[0027] The grid-side converter active power control adopts constant DC voltage control. The instantaneous value of the DC capacitor voltage and the reference value are subtracted and passed through the PI link to generate the d-axis current reference value. The reactive power control adopts constant terminal voltage amplitude control. The instantaneous value of the terminal voltage and the reference value are subtracted and passed through the PI link to generate the q-axis current reference value. The expression is as follows:

[0028]

[0029] Where: U dcrefi , U dci are the reference value and actual value of the DC capacitor voltage of the direct-drive wind turbine group; U tref , U t are the reference value and actual value of the voltage at the public grid connection point of the direct-drive wind turbines respectively; x 1i 、x2i 、x 3i 、x 5i is the state variable; k p1i , k p2i , k p3i , k p5i are the proportional coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively; k i1i , k i2i , k i3i , k i5i are the integral coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively;

[0030] Grid modeling: Without considering the dynamic process of the grid, the grid algebraic equation is established, and the expression is as follows:

[0031]

[0032] Where: X g is the reactance value of the AC system; U tx , U ty are the x-axis and y-axis components of the converter terminal voltage; U gx , U gy are the x-axis and y-axis components of the grid voltage; i xi 、i yi are the x-axis and y-axis components of the output current of the direct-drive wind turbine.

[0033] Furthermore, the mutual damping coefficient D dc12 The influence of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of G. 11 represents the damping function of wind turbine 1 on itself, G 12 The damping function of wind turbine 1 on wind turbine 2, G 22 represents the damping function of wind turbine 2 on itself, G 21 The damping function of wind turbine 2 on wind turbine 1, G 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop;

[0034] G 11 , G 12 , G 22 , G 21 , G 121dc The expression is as follows:

[0035]

[0036]

[0037]

[0038]

[0039]

[0040] Where: ΔU td1 , ΔU tq1 , Δi d1 , Δi q1 , ΔU td2 , ΔU tq2 , Δi d2 , Δi q2 is the d-axis and q-axis component increments of the grid connection point voltage and output current of wind turbine generator set 1 and wind turbine generator set 2; U td01 , U tq01 、i d01 、i q01 , U td02 , U tq02 、i d02 、i q02 I is the grid connection point voltage of wind turbine 1 and wind turbine 2 and the stable operating value of the d-axis and q-axis components of the output current; dc02 , U dc02 is the equivalent current source current value of wind turbine 2 and the stable operation value of DC capacitor voltage; PI 12 For the controller;

[0041] Therefore, the influence of wind turbine 2 on the DC voltage loop stability of wind turbine 1 is G dc (s) can be obtained by G 12 , G 121dc and G 21 Represented by the series connection.

[0042] Furthermore, the above is further extended to a multi-machine parallel system, and the interaction analysis of the multi-machine parallel system includes the following steps:

[0043] Establish a damping torque model for a multi-machine parallel system, take the first direct-drive wind turbine as the research object, and study the interaction with other direct-drive wind turbines;

[0044] The damping torque model of the DC voltage loop mode of each direct-drive wind turbine is established, where: the input quantity of the first direct-drive wind turbine is the electromagnetic power increment ΔP e1 , the output is the voltage increment ΔU of the DC capacitor dc1 , the damping transfer function is G dc1 , the expression is as follows:

[0045]

[0046] The input quantity of other direct-drive wind turbines is the voltage increment of the DC capacitor ΔU dci, the output is the electromagnetic power increment ΔP ei , the damping transfer function is G dci , the expression is as follows: (i=2,…n)

[0047]

[0048] Where: i = 1 represents direct-drive wind turbine group 1, i = 2 represents direct-drive wind turbine group 1, and is extended to n direct-drive wind turbine groups; D dci and K dci represents the DC voltage loop damping coefficient and synchronization coefficient of the i-th wind turbine;

[0049] Calculation of mutual damping coefficient D in multi-machine parallel system dcm , the expression is as follows:

[0050] D dcm =D dc2 +...+D dci (14)

[0051] Using the calculated mutual damping coefficient D dcm ,After perturbing the grid strength, system operating conditions and direct-drive wind turbine control parameters, the interaction among the parallel systems of multiple direct-drive wind turbines is quantitatively evaluated.

[0052] The low-frequency oscillation interaction analysis device between parallel direct-drive wind turbines includes:

[0053] A simulation model building module is used to build a simulation model of two direct-drive wind turbines connected in parallel and connected to the grid;

[0054] A calculation and analysis module, for using the simulation model of the parallel grid connection of the two direct-drive wind turbines to access the simulation model of the AC system, linearizing the system state space equation at the equilibrium point, calculating the eigenvalue and participation factor analysis, and analyzing the low-frequency oscillation characteristics of the system;

[0055] A damping torque model establishment module is used to establish a damping torque model of a DC voltage loop mode of each direct-drive wind turbine set according to the analysis results of the low-frequency oscillation characteristics of the system and for the low-frequency oscillation mode related to the DC voltage;

[0056] An analytical module, used to form a damping torque model of a two-machine parallel system using the damping torque model of the direct-drive wind turbine set, and to construct an analytical expression of the interaction between the units using a damping transfer function;

[0057] A quantification module is used to derive the perturbed grid strength, system operating conditions and control parameters based on the initial operating parameters of the parallel system damping torque model, and to quantitatively evaluate the interaction between direct-drive wind turbines on this basis;

[0058] The extended analysis module is used to evaluate the interaction operations between direct-drive wind turbines, expand the system to multiple parallel machines, and analyze the interaction between multiple parallel machines.

[0059] Furthermore, the system state space equation is expressed as follows:

[0060]

[0061] In the formula: Δx is the state variable of the system; Δu is the input variable of the system; A and B are the state matrix and input matrix of the system respectively;

[0062] The input of the damping torque model is the electromagnetic power increment ΔP of the wind turbine ei The output of the model is the voltage increment Δi of the DC capacitor of the wind turbine di (i=1, 2);

[0063] The analytical expression is based on the first direct-drive wind turbine as the research object, with the mutual damping coefficient D dc12 The influence of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of:

[0064] D dc12 =-Re[G 12 ×G 121dc ×G 21 ] (2)

[0065] Where: R e Indicates the meaning of taking the real part, G 12 G represents the damping transfer function of wind turbine 1 on wind turbine 2; 22 G represents the damping transfer function of wind turbine 2 acting on itself; 21 G represents the damping transfer function of wind turbine 2 on wind turbine 1; 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop;

[0066] The establishment of a parallel grid-connected simulation model for two direct-drive wind turbines includes a primary system, a control system and a power grid for the two direct-drive wind turbines;

[0067] The establishment of a parallel grid-connected simulation model for two direct-drive wind turbines comprises the following steps:

[0068] Direct-drive wind turbine modeling:

[0069] The direct-drive wind turbine group includes: a wind turbine, a permanent magnet synchronous generator, a four-quadrant converter, and a filter circuit; the wind turbine, permanent magnet synchronous generator, and machine-side converter of the direct-drive wind turbine group are all equivalent to controlled current sources, and the main circuit expression of the grid-side converter is as follows: (i=1,2)

[0070]

[0071] Where: i = 1 represents direct-drive wind turbine group 1, i = 2 represents direct-drive wind turbine group 2; E di 、E qi Indicates the converter output voltage dq axis voltage; L fi Indicates the converter filter inductance; U tdi , U tqi 、i di 、i qi are the dq axis components of the voltage at the grid-side converter terminal of the direct-drive wind turbine and the dq axis components of the output current of the grid-side converter respectively; C dci , U dci , P outi , P ini are the capacitance value, voltage value, output active power and input active power of the DC capacitor of the direct-drive wind turbine generator set respectively; ω is the rated angular velocity;

[0072] Control system modeling:

[0073] The grid-side converter active power control adopts constant DC voltage control. The instantaneous value of the DC capacitor voltage and the reference value are subtracted and passed through the PI link to generate the d-axis current reference value. The reactive power control adopts constant terminal voltage amplitude control. The instantaneous value of the terminal voltage and the reference value are subtracted and passed through the PI link to generate the q-axis current reference value. The expression is as follows:

[0074]

[0075] Where: U dcrefi , U dci are the reference value and actual value of the DC capacitor voltage of the direct-drive wind turbine group; U tref , U t are the reference value and actual value of the voltage at the public grid connection point of the direct-drive wind turbines respectively; x 1i 、x 2i 、x 3i 、x 5i is the state variable; k p1i , k p2i , k p3i , k p5i are the proportional coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively; k i1i , k i2i , k i3i , k i5i are the integral coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively;

[0076] Grid modeling: Without considering the dynamic process of the grid, the grid algebraic equation is established, and the expression is as follows:

[0077]

[0078] Where: X g is the reactance value of the AC system; U tx , U ty are the x-axis and y-axis components of the converter terminal voltage; U gx , U gy are the x-axis and y-axis components of the grid voltage; i xi 、i yi are the x-axis and y-axis components of the output current of the direct-drive wind turbine;

[0079] The mutual damping coefficient D dc12 The influence of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of G. 11 represents the damping function of wind turbine 1 on itself, G 12 The damping function of wind turbine 1 on wind turbine 2, G 22 represents the damping function of wind turbine 2 on itself, G 21 The damping function of wind turbine 2 on wind turbine 1, G 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop;

[0080] G 11 , G 12 , G 22 , G 21 , G 121dc The expression is as follows:

[0081]

[0082]

[0083]

[0084]

[0085]

[0086] Where: ΔU td1 , ΔU tq1 , Δi d1 , Δi q1 , ΔU td2 , ΔU tq2 , Δi d2 , Δi q2 is the d-axis and q-axis component increments of the grid connection point voltage and output current of wind turbine generator set 1 and wind turbine generator set 2; U td01 , U tq01、i d01 、i q01 , U td02 , U tq02 、i d02 、i q02 I is the grid connection point voltage of wind turbine 1 and wind turbine 2 and the stable operating value of the d-axis and q-axis components of the output current; dc02 , U dc02 is the equivalent current source current value of wind turbine 2 and the stable operation value of DC capacitor voltage; PI 12 For the controller;

[0087] Therefore, the influence of wind turbine 2 on the DC voltage loop stability of wind turbine 1 is G dc (s) by G 12 , G 121dc and G 21 The series connection of

[0088] The above further extends to a multi-machine parallel system and analyzes the interaction of the multi-machine parallel system, including the following steps:

[0089] Establish a damping torque model for a multi-machine parallel system, take the first direct-drive wind turbine as the research object, and study the interaction with other direct-drive wind turbines;

[0090] The damping torque model of the DC voltage loop mode of each direct-drive wind turbine is established, where: the input quantity of the first direct-drive wind turbine is the electromagnetic power increment ΔP e1 , the output is the voltage increment ΔU of the DC capacitor dc1 , the damping transfer function is G dc1 , the expression is as follows:

[0091]

[0092] The input quantity of other direct-drive wind turbines is the voltage increment of the DC capacitor ΔU dci , the output is the electromagnetic power increment ΔP ei , the damping transfer function is G dci , the expression is as follows: (i=2,…n)

[0093]

[0094] Where: i = 1 represents direct-drive wind turbine group 1, i = 2 represents direct-drive wind turbine group 1, and is extended to n direct-drive wind turbine groups; D dci and K dci represents the DC voltage loop damping coefficient and synchronization coefficient of the i-th wind turbine;

[0095] Calculation of mutual damping coefficient D in multi-machine parallel system dcm , the expression is as follows:

[0096] D dcm =D dc2 +...+D dci (14)

[0097] Using the calculated mutual damping coefficient D dcm ,After perturbing the grid strength, system operating conditions and direct-drive wind turbine control parameters, the interaction among the parallel systems of multiple direct-drive wind turbines is quantitatively evaluated.

[0098] A computer device comprises a storage medium, a processor and a computer program stored on the storage medium and executable on the processor, wherein when the processor executes the computer program, the steps of any of the methods for analyzing low-frequency oscillation interactions between parallel direct-drive wind turbines are implemented.

[0099] A storage medium, characterized in that the storage medium stores a computer program, and the computer program is executed by a processor to perform the steps of any of the methods for analyzing low-frequency oscillation interactions between parallel direct-drive wind turbines.

[0100] Compared with the prior art, the beneficial effects are:

[0101] The damping torque model between parallel wind turbines is established, and the analytical expression of the interaction between turbines is constructed using the damping transfer function. The variation trend of the interaction with the grid strength, operating conditions and control parameters is quantitatively analyzed in the form of mutual damping coefficient. The dynamic coupling mechanism between parallel direct-drive wind turbines is clearly and quantitatively explained, which effectively solves the problem that previous research methods cannot quantitatively analyze the interaction mechanism between turbines. It has the advantages of scientific rationality, good applicability and clear mechanism. BRIEF DESCRIPTION OF THE DRAWINGS

[0102] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0103] Figure 1 It is a schematic diagram of the parallel topology structure and control system of direct-drive wind turbines;

[0104] Figure 2 It is the damping torque model diagram of two direct-drive wind turbines;

[0105] Figure 3 It is the block diagram of the damping torque model of multi-machine parallel system;

[0106] Figure 4 It is a schematic diagram of the change curve of the mutual damping coefficient under the change of the grid strength in the DC voltage loop mode after the parallel direct-drive wind turbines are connected to the AC system;

[0107] Figure 5It is a schematic diagram of the change curve of the mutual damping coefficient under the DC voltage loop mode when the parallel direct-drive wind turbines are connected to the AC system;

[0108] Figure 6 It is a schematic diagram of the change curve of the mutual damping coefficient under the change of control parameters in the DC voltage loop mode after the parallel direct-drive wind turbines are connected to the AC system. DETAILED DESCRIPTION

[0109] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0110] Example 1

[0111] See attached Figure 1 To Attachment Figure 3 The present invention provides a method for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines, which comprises the following steps:

[0112] S1. First, a simulation model of two direct-drive wind turbines connected in parallel and connected to the grid is established. The simulation model consists of the primary system, control system and power grid of the two direct-drive wind turbines.

[0113] S2. Using the simulation model of two direct-drive wind turbines connected to the AC system constructed in step S1, linearize the system state space equation at the equilibrium point as shown in equation (15), calculate the eigenvalue and participation factor analysis, and analyze the low-frequency oscillation characteristics of the system.

[0114]

[0115] In the formula: Δx is the state variable of the system; Δu is the input variable of the system; A and B are the state matrix and input matrix of the system respectively.

[0116] S3. For the low-frequency oscillation mode related to the DC voltage in step S2, a damping torque model of the DC voltage loop mode of each direct-drive wind turbine is established. The input of the model is the electromagnetic power increment ΔP of the wind turbine. ei The output of the model is the voltage increment Δi of the DC capacitor of the wind turbine di (i=1, 2).

[0117] S4. Use the damping torque model of each direct-drive wind turbine in step S3 to form a damping torque model of the two-machine parallel system. Use the damping transfer function to construct an analytical expression for the interaction between the units. Take the first direct-drive wind turbine as the research object and use the mutual damping coefficient D dc12 The impact of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of

[0118] D dc12=-Re[G 12 ×G 121dc ×G 21 ] (16)

[0119] Where: G 12 G represents the damping transfer function of wind turbine 1 on wind turbine 2; 22 G represents the damping transfer function of wind turbine 2 acting on itself; 21 G represents the damping transfer function of wind turbine 2 on wind turbine 1; 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop.

[0120] S5. Based on the initial operating parameters in step S4, the grid strength, system operating conditions and control parameters are perturbed to quantitatively evaluate the interaction between the direct-drive wind turbines.

[0121] S6. Further expand to multi-machine parallel system and analyze the interaction analysis of multi-machine parallel system.

[0122] Existing research work mainly focuses on single-machine systems, while for the study of multi-machine parallel systems, the eigenvalue method is still used to analyze the impact of various influencing factors (operating conditions, control parameters, number of units, etc.) on oscillation, lacking a quantitative description of the interaction between units. To this end, the patent of this invention focuses on the dynamic coupling problem between parallel direct-drive wind turbines under weak AC systems, and uses damping transfer functions to construct an analytical expression for the interaction between units for the low-frequency oscillation mode of the parallel system of multiple direct-drive wind turbines. The interaction is quantitatively analyzed in the form of mutual damping coefficients as the intensity of the grid, operating conditions, and control parameters change.

[0123] Example 2

[0124] Step S1 includes:

[0125] S1-1. Modeling of direct-drive wind turbines:

[0126] The direct-drive wind turbine group includes: a wind turbine, a permanent magnet synchronous generator, a four-quadrant converter, and a filter circuit; the wind turbine, permanent magnet synchronous generator, and machine-side converter of the direct-drive wind turbine group are equivalent to a controlled current source, and the mathematical model of the main circuit of the grid-side converter is shown in formula (17) (i=1,2).

[0127]

[0128] Where: E di 、E qi Indicates the converter output voltage dq axis voltage; L fi Indicates the converter filter inductance; U tdi , U tqi 、i di 、iqi are the dq axis components of the voltage at the grid-side converter terminal of the direct-drive wind turbine and the dq axis components of the output current of the grid-side converter respectively; C dci , U dci , P outi , P ini are the capacitance, voltage, output active power and input active power of the DC capacitor of the direct-drive wind turbine respectively; ω is the rated angular velocity.

[0129] Control system modeling: The active power control of the grid-side converter adopts constant DC voltage control. The instantaneous value of the DC capacitor voltage and the reference value are subtracted and passed through the PI link to generate the d-axis current reference value; the reactive power control adopts constant terminal voltage amplitude control. The instantaneous value of the terminal voltage and the reference value are subtracted and passed through the PI link to generate the q-axis current reference value; the mathematical model is shown in formula (18).

[0130]

[0131] Where: U dcrefi , U dci are the reference value and actual value of the DC capacitor voltage of the direct-drive wind turbine group; U tref , U t are the reference value and actual value of the voltage at the public grid connection point of the direct-drive wind turbines respectively; x 1i 、x 2i 、x 3i 、x 5i is the state variable; k p1i , k p2i , k p3i , k p5i are the proportional coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively; k i1i , k i2i , k i3i , k i5i They are the integral coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively.

[0132] Grid modeling: Without considering the dynamic process of the grid, the grid algebraic equation is established, as shown in formula (19).

[0133]

[0134] Where: X g is the reactance value of the AC system; U tx , U ty are the x-axis and y-axis components of the converter terminal voltage; U gx , U gy are the x-axis and y-axis components of the grid voltage; i xi 、i yi are the x-axis and y-axis components of the output current of the direct-drive wind turbine.

[0135] Example 3

[0136] Step S4 includes:

[0137] S4-1.G 11 represents the damping function of wind turbine 1 on itself, G 12 The damping function of wind turbine 1 on wind turbine 2, G 22 represents the damping function of wind turbine 2 on itself, G 21 The damping function of wind turbine 2 on wind turbine 1, G 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop, G 11 , G 12 , G 22 , G 21 , G 121dc The expressions of are shown in equations (20) to (24).

[0138]

[0139]

[0140]

[0141]

[0142]

[0143] Where: ΔU td1 , ΔU tq1 , Δi d1 , Δi q1 , ΔU td2 , ΔU tq2 , Δi d2 , Δi q2 is the d-axis and q-axis component increments of the grid connection point voltage and output current of wind turbine generator set 1 and wind turbine generator set 2; U td01 , U tq01 、i d01 、i q01 , U td02 , U tq02 、i d02 、i q02 I is the grid connection point voltage of wind turbine 1 and wind turbine 2 and the stable operating value of the d-axis and q-axis components of the output current; dc02 , U dc02 is the equivalent current source current value of wind turbine 2 and the stable operation value of DC capacitor voltage; PI 12 For the controller.

[0144] Therefore, the influence of wind turbine 2 on the DC voltage loop stability of wind turbine 1 is G dc (s) can be obtained by G 12 , G 121dc and G 21 The mutual damping coefficient D dc12 The expression of is shown in formula (25).

[0145] D dc12 =-Re[G 12 ×G 121dc ×G 21 ] (25)

[0146] Example 4

[0147] Step S6 includes:

[0148] S6-1. Establish a damping torque model for a multi-machine parallel system, take the first direct-drive wind turbine as the research object, and study the interaction with other direct-drive wind turbines.

[0149] S6-2. Establish the damping torque model of the DC voltage loop mode of each direct-drive wind turbine, where: the input quantity of the first direct-drive wind turbine is the electromagnetic power increment ΔP e1 , the output is the voltage increment ΔU of the DC capacitor dc1 , the damping transfer function is G dc1 , the mathematical relationship is shown in formula (26); the input of other direct-drive wind turbines is the voltage increment ΔU of the DC capacitor dci , the output is the electromagnetic power increment ΔP ei , the damping transfer function is G dci , the mathematical relationship is shown in formula (27) (i=2,…n).

[0150]

[0151]

[0152] Where: D dci and K dci It represents the damping coefficient and synchronization coefficient of the DC voltage loop acting on the i-th wind turbine.

[0153] S6-3. Calculation of the mutual damping coefficient D in a multi-machine parallel system dcm , as shown in formula (28):

[0154] D dcm =D dc2 +...+D dci (28)

[0155] S6-4. Using the mutual damping coefficient D in step S6-3dcm ,After perturbing the grid strength, system operating conditions and direct-drive wind turbine control parameters, the interaction among the parallel systems of multiple direct-drive wind turbines is quantitatively evaluated.

[0156] Example 5

[0157] like Figures 4 to 6 As shown, the present invention focuses on the dynamic coupling problem between parallel direct-drive wind turbines in a weak AC system, and for the low-frequency oscillation mode of a parallel system of multiple direct-drive wind turbines, an analytical expression for the interaction between the units is constructed using the damping transfer function, and the changing trend of the interaction with the grid strength, operating conditions and control parameters is quantitatively analyzed in the form of mutual damping coefficients.

[0158] like Figure 4 As shown in the figure, the perturbation grid strength: with the increase of grid strength (SCR = 0.8 → 2), the mutual damping coefficient of the direct-drive wind turbine increases from -2 to -0.001. With the increase of grid strength, the mutual damping coefficient increases, which is beneficial to the stability of the two-machine grid-connected system.

[0159] like Figure 5 As shown in Figure 1, the operating conditions of the two units are perturbed: as the output power of unit 1 and unit 2 increases, the mutual damping coefficient between the two gradually increases, which is beneficial to the stability of the two-machine grid-connected system.

[0160] Depend on Figure 5 As shown, the control parameters of the two direct-drive wind turbines are perturbed: the proportional control parameter of the direct-drive wind turbine 1 remains unchanged, and as the proportional control parameter of the direct-drive wind turbine 2 increases, the mutual damping coefficient first increases and then decreases.

[0161] The above embodiments verify the effectiveness and feasibility of the low-frequency oscillation interaction analysis method between direct-drive wind turbines.

[0162] Example 6

[0163] The present invention further provides a low-point embodiment, which is a low-frequency oscillation interaction analysis device between parallel direct-drive wind turbines, comprising:

[0164] A simulation model building module is used to build a simulation model of two direct-drive wind turbines connected in parallel and connected to the grid;

[0165] A calculation and analysis module, for using the simulation model of the parallel grid connection of the two direct-drive wind turbines to access the simulation model of the AC system, linearizing the system state space equation at the equilibrium point, calculating the eigenvalue and participation factor analysis, and analyzing the low-frequency oscillation characteristics of the system;

[0166] A damping torque model establishment module is used to establish a damping torque model of a DC voltage loop mode of each direct-drive wind turbine set according to the analysis results of the low-frequency oscillation characteristics of the system and for the low-frequency oscillation mode related to the DC voltage;

[0167] An analytical module, used to form a damping torque model of a two-machine parallel system using the damping torque model of the direct-drive wind turbine set, and to construct an analytical expression of the interaction between the units using a damping transfer function;

[0168] A quantification module is used to derive the perturbed grid strength, system operating conditions and control parameters based on the initial operating parameters of the parallel system damping torque model, and to quantitatively evaluate the interaction between direct-drive wind turbines on this basis;

[0169] The extended analysis module is used to evaluate the interaction operations between direct-drive wind turbines, expand the system to multiple parallel machines, and analyze the interaction between multiple parallel machines.

[0170] The system state space equation is expressed as follows:

[0171]

[0172] In the formula: Δx is the state variable of the system; Δu is the input variable of the system; A and B are the state matrix and input matrix of the system respectively;

[0173] The input of the damping torque model is the electromagnetic power increment ΔP of the wind turbine ei The output of the model is the voltage increment Δi of the DC capacitor of the wind turbine di (i=1, 2);

[0174] The analytical expression is based on the first direct-drive wind turbine as the research object, with the mutual damping coefficient D dc12 The influence of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of:

[0175] D dc12 =-Re[G 12 ×G 121dc ×G 21 ] (2)

[0176] Where: R e Indicates the meaning of taking the real part, G 12 G represents the damping transfer function of wind turbine 1 on wind turbine 2; 22 G represents the damping transfer function of wind turbine 2 acting on itself; 21 G represents the damping transfer function of wind turbine 2 on wind turbine 1; 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop;

[0177] The establishment of a parallel grid-connected simulation model for two direct-drive wind turbines includes a primary system, a control system and a power grid for the two direct-drive wind turbines;

[0178] The establishment of a parallel grid-connected simulation model for two direct-drive wind turbines comprises the following steps:

[0179] Direct-drive wind turbine modeling:

[0180] The direct-drive wind turbine group includes: a wind turbine, a permanent magnet synchronous generator, a four-quadrant converter, and a filter circuit; the wind turbine, permanent magnet synchronous generator, and machine-side converter of the direct-drive wind turbine group are all equivalent to controlled current sources, and the main circuit expression of the grid-side converter is as follows: (i=1,2)

[0181]

[0182] Where: i = 1 represents direct-drive wind turbine group 1, i = 2 represents direct-drive wind turbine group 2; E di 、E qi Indicates the converter output voltage dq axis voltage; L fi Indicates the converter filter inductance; U tdi , U tqi 、i di 、i qi are the dq axis components of the voltage at the grid-side converter terminal of the direct-drive wind turbine and the dq axis components of the output current of the grid-side converter respectively; C dci , U dci , P outi , P ini are the capacitance value, voltage value, output active power and input active power of the DC capacitor of the direct-drive wind turbine generator set respectively; ω is the rated angular velocity;

[0183] Control system modeling:

[0184] The grid-side converter active power control adopts constant DC voltage control. The instantaneous value of the DC capacitor voltage and the reference value are subtracted and passed through the PI link to generate the d-axis current reference value. The reactive power control adopts constant terminal voltage amplitude control. The instantaneous value of the terminal voltage and the reference value are subtracted and passed through the PI link to generate the q-axis current reference value. The expression is as follows:

[0185]

[0186] Where: U dcrefi , U dci are the reference value and actual value of the DC capacitor voltage of the direct-drive wind turbine group; U tref , U t are the reference value and actual value of the voltage at the public grid connection point of the direct-drive wind turbines respectively; x 1i 、x 2i、x 3i 、x 5i is the state variable; k p1i , k p2i , k p3i , k p5i are the proportional coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively; k i1i , k i2i , k i3i , k i5i are the integral coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively;

[0187] Grid modeling: Without considering the dynamic process of the grid, the grid algebraic equation is established, and the expression is as follows:

[0188]

[0189] Where: X g is the reactance value of the AC system; U tx , U ty are the x-axis and y-axis components of the converter terminal voltage; U gx , U gy are the x-axis and y-axis components of the grid voltage; i xi 、i yi are the x-axis and y-axis components of the output current of the direct-drive wind turbine;

[0190] The mutual damping coefficient D dc12 The influence of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of G. 11 represents the damping function of wind turbine 1 on itself, G 12 The damping function of wind turbine 1 on wind turbine 2, G 22 represents the damping function of wind turbine 2 on itself, G 21 The damping function of wind turbine 2 on wind turbine 1, G 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop;

[0191] G 11 , G 12 , G 22 , G 21 , G 121dc The expression is as follows:

[0192]

[0193]

[0194]

[0195]

[0196]

[0197] Where: ΔU td1 , ΔU tq1 , Δi d1 , Δi q1 , ΔU td2 , ΔU tq2 , Δi d2 , Δi q2 is the d-axis and q-axis component increments of the grid connection point voltage and output current of wind turbine generator set 1 and wind turbine generator set 2; U td01 , U tq01 、i d01 、i q01 , U td02 , U tq02 、i d02 、i q02 I is the grid connection point voltage of wind turbine 1 and wind turbine 2 and the stable operating value of the d-axis and q-axis components of the output current; dc02 , U dc02 is the equivalent current source current value of wind turbine 2 and the stable operation value of DC capacitor voltage; PI 12 For the controller;

[0198] Therefore, the influence of wind turbine 2 on the DC voltage loop stability of wind turbine 1 is G dc (s) by G 12 , G 121dc and G 21 The series connection of

[0199] The above further extends to a multi-machine parallel system and analyzes the interaction of the multi-machine parallel system, including the following steps:

[0200] Establish a damping torque model for a multi-machine parallel system, take the first direct-drive wind turbine as the research object, and study the interaction with other direct-drive wind turbines;

[0201] The damping torque model of the DC voltage loop mode of each direct-drive wind turbine is established, where: the input quantity of the first direct-drive wind turbine is the electromagnetic power increment ΔP e1 , the output is the voltage increment ΔU of the DC capacitor dc1 , the damping transfer function is G dc1 , the expression is as follows:

[0202]

[0203] The input quantity of other direct-drive wind turbines is the voltage increment of the DC capacitor ΔU dci , the output is the electromagnetic power increment ΔPei , the damping transfer function is G dci , the expression is as follows: (i=2,…n)

[0204]

[0205] Where: i = 1 represents direct-drive wind turbine group 1, i = 2 represents direct-drive wind turbine group 1, and is extended to n direct-drive wind turbine groups; D dci and K dci represents the DC voltage loop damping coefficient and synchronization coefficient of the i-th wind turbine;

[0206] Calculation of mutual damping coefficient D in multi-machine parallel system dcm , the expression is as follows:

[0207] D dcm =D dc2 +...+D dci (14)

[0208] Using the calculated mutual damping coefficient D dcm ,After perturbing the grid strength, system operating conditions and direct-drive wind turbine control parameters, the interaction among the parallel systems of multiple direct-drive wind turbines is quantitatively evaluated.

[0209] Example 7

[0210] The present invention further provides an embodiment, which is a computer device, including a storage medium, a processor, and a computer program stored on the storage medium and executable on the processor, wherein when the processor executes the computer program, the steps of the method for analyzing low-frequency oscillation interactions between parallel direct-drive wind turbines described in any one of Embodiments 1-2 are implemented.

[0211] Example 8

[0212] The present invention further provides an embodiment, which is a storage medium, wherein the storage medium stores a computer program, and the computer program is executed by a processor to perform the steps of the method for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines as described in any of Embodiments 1-2.

[0213] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0214] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0215] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0216] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0217] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for analyzing the interaction between low-frequency oscillations of parallel direct-drive wind turbines, characterized in that: The steps include: Establish a simulation model for two direct-drive wind turbines connected in parallel and connected to the grid; The simulation model of the two direct-drive wind turbines connected in parallel to the grid is connected to the simulation model of the AC system, the system state space equation is linearized at the equilibrium point, the eigenvalue and participation factor analysis are calculated, and the low-frequency oscillation characteristics of the system are analyzed; According to the analysis results of the low-frequency oscillation characteristics of the system, a damping torque model of the DC voltage loop mode of each direct-drive wind turbine is established for the low-frequency oscillation mode related to the DC voltage; Using the damping torque model of the direct-drive wind turbine set, a damping torque model of a two-machine parallel system is formed, and an analytical expression of the interaction between the units is constructed using the damping transfer function; Based on the initial operating parameters of the parallel system damping torque model, the perturbed grid strength, system operating conditions and control parameters are obtained, and the interaction between direct-drive wind turbines is quantitatively evaluated on this basis. After the evaluation, the interaction operations between direct-drive wind turbines will be quantitatively evaluated, expanded to multi-machine parallel systems, and the interactions between multi-machine parallel systems will be analyzed.

2. The method for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines according to claim 1, characterized in that: The system state space equation is expressed as follows: In the formula: Δx is the state variable of the system; Δu is the input variable of the system; A and B are the state matrix and input matrix of the system respectively; The input of the damping torque model is the electromagnetic power increment ΔP of the wind turbine ei The output of the model is the voltage increment Δi of the DC capacitor of the wind turbine di (i=1, 2); The analytical expression is based on the first direct-drive wind turbine as the research object, with the mutual damping coefficient D dc12 The influence of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of: D dc12 =-Re[G 12 ×G 121dc ×G 21 ] (2) Where: R e Indicates the meaning of taking the real part, G 12 G represents the damping transfer function of wind turbine 1 on wind turbine 2; 22 G represents the damping transfer function of wind turbine 2 acting on itself; 21 G represents the damping transfer function of wind turbine 2 on wind turbine 1; 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop.

3. The method for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines according to claim 1, characterized in that: The simulation model of establishing two direct-drive wind turbines in parallel and connected to the grid includes the primary systems of the two direct-drive wind turbines, a control system and a power grid.

4. The method for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines according to claim 1, characterized in that: The establishment of a parallel grid-connected simulation model for two direct-drive wind turbines comprises the following steps: Direct-drive wind turbine modeling: The direct-drive wind turbine group includes: a wind turbine, a permanent magnet synchronous generator, a four-quadrant converter, and a filter circuit; the wind turbine, permanent magnet synchronous generator, and machine-side converter of the direct-drive wind turbine group are all equivalent to controlled current sources, and the main circuit expression of the grid-side converter is as follows: (i=1,2) Where: i = 1 represents direct-drive wind turbine group 1, i = 2 represents direct-drive wind turbine group 2; E di 、E qi Indicates the converter output voltage dq axis voltage; L fi Indicates the converter filter inductance; U tdi , U tqi 、i di 、i qi are the dq axis components of the voltage at the grid-side converter terminal of the direct-drive wind turbine and the dq axis components of the output current of the grid-side converter respectively; C dci , U dci , P outi , P ini are the capacitance value, voltage value, output active power and input active power of the DC capacitor of the direct-drive wind turbine generator set respectively; ω is the rated angular velocity; Control system modeling: The grid-side converter active power control adopts constant DC voltage control. The instantaneous value of the DC capacitor voltage and the reference value are subtracted and passed through the PI link to generate the d-axis current reference value. The reactive power control adopts constant terminal voltage amplitude control. The instantaneous value of the terminal voltage and the reference value are subtracted and passed through the PI link to generate the q-axis current reference value. The expression is as follows: Where: U dcrefi , U dci are the reference value and actual value of the DC capacitor voltage of the direct-drive wind turbine group; U tref , U t are the reference value and actual value of the voltage at the public grid connection point of the direct-drive wind turbines respectively; x 1i 、x 2i 、x 3i 、x 5i is the state variable; k p1i , k p2i , k p3i , k p5i are the proportional coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively; k i1i , k i2i , k i3i , k i5i are the integral coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively; Grid modeling: Without considering the dynamic process of the grid, the grid algebraic equation is established, and the expression is as follows: Where: X g is the reactance value of the AC system; U tx , U ty are the x-axis and y-axis components of the converter terminal voltage; U gx , U gy are the x-axis and y-axis components of the grid voltage; i xi 、i yi are the x-axis and y-axis components of the output current of the direct-drive wind turbine.

5. The method for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines according to claim 2, characterized in that: The mutual damping coefficient D dc12 The influence of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of G. 11 represents the damping function of wind turbine 1 on itself, G 12 The damping function of wind turbine 1 on wind turbine 2, G 22 represents the damping function of wind turbine 2 on itself, G 21 The damping function of wind turbine 2 on wind turbine 1, G 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop; G 11 , G 12 , G 22 , G 21 , G 121dc The expression is as follows: Where: ΔU td1 , ΔU tq1 , Δi d1 , Δi q1 , ΔU td2 , ΔU tq2 , Δi d2 , Δi q2 is the d-axis and q-axis component increments of the grid connection point voltage and output current of wind turbine generator set 1 and wind turbine generator set 2; U td01 , U tq01 、i d01 、i q01 , U td02 , U tq02 、i d02 、i q02 I is the grid connection point voltage of wind turbine 1 and wind turbine 2 and the stable operating value of the d-axis and q-axis components of the output current; dc02 , U dc02 is the equivalent current source current value of wind turbine 2 and the stable operation value of DC capacitor voltage; PI 12 For the controller; Therefore, the influence of wind turbine 2 on the DC voltage loop stability of wind turbine 1 is G dc (s) by G 12 , G 121dc and G 21 Represented by the series connection.

6. The method for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines according to claim 1, characterized in that: The above further extends to a multi-machine parallel system and analyzes the interaction of the multi-machine parallel system, including the following steps: Establish a damping torque model for a multi-machine parallel system, take the first direct-drive wind turbine as the research object, and study the interaction with other direct-drive wind turbines; The damping torque model of the DC voltage loop mode of each direct-drive wind turbine is established, where: the input quantity of the first direct-drive wind turbine is the electromagnetic power increment ΔP e1 , the output is the voltage increment ΔU of the DC capacitor dc1 , the damping transfer function is G dc1 , the expression is as follows: The input quantity of other direct-drive wind turbines is the voltage increment of the DC capacitor ΔU dci , the output is the electromagnetic power increment ΔP ei , the damping transfer function is G dci , the expression is as follows: (i=2,…n) Where: i = 1 represents direct-drive wind turbine group 1, i = 2 represents direct-drive wind turbine group 1, and is extended to n direct-drive wind turbine groups; D dci and K dci represents the DC voltage loop damping coefficient and synchronization coefficient of the i-th wind turbine; Calculation of mutual damping coefficient D in multi-machine parallel system dcm , the expression is as follows: D dcm =D dc2 +...+D dci (14) Using the calculated mutual damping coefficient D dcm ,After perturbing the grid strength, system operating conditions and direct-drive wind turbine control parameters, the interaction among the parallel systems of multiple direct-drive wind turbines is quantitatively evaluated.

7. A device for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines, characterized in that: include: A simulation model building module is used to build a simulation model of two direct-drive wind turbines connected in parallel and connected to the grid; A calculation and analysis module, for using the simulation model of the parallel grid connection of the two direct-drive wind turbines to access the simulation model of the AC system, linearizing the system state space equation at the equilibrium point, calculating the eigenvalue and participation factor analysis, and analyzing the low-frequency oscillation characteristics of the system; A damping torque model establishment module is used to establish a damping torque model of a DC voltage loop mode of each direct-drive wind turbine set according to the analysis results of the low-frequency oscillation characteristics of the system and for the low-frequency oscillation mode related to the DC voltage; An analytical module, used to form a damping torque model of a two-machine parallel system using the damping torque model of the direct-drive wind turbine set, and to construct an analytical expression of the interaction between the units using a damping transfer function; A quantification module is used to derive the perturbed grid strength, system operating conditions and control parameters based on the initial operating parameters of the parallel system damping torque model, and to quantitatively evaluate the interaction between direct-drive wind turbines on this basis; The extended analysis module is used to evaluate the interaction operations between direct-drive wind turbines, expand the system to multiple parallel machines, and analyze the interaction between multiple parallel machines.

8. The device for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines according to claim 7, characterized in that: The system state space equation is expressed as follows: In the formula: Δx is the state variable of the system; Δu is the input variable of the system; A and B are the state matrix and input matrix of the system respectively; The input of the damping torque model is the electromagnetic power increment ΔP of the wind turbine ei The output of the model is the voltage increment Δi of the DC capacitor of the wind turbine di (i=1, 2); The analytical expression is based on the first direct-drive wind turbine as the research object, with the mutual damping coefficient D dc12 The influence of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of: D dc12 =-Re[G 12 ×G 121dc ×G 21 ] (2) Where: R e Indicates the meaning of taking the real part, G 12 G represents the damping transfer function of wind turbine 1 on wind turbine 2; 22 G represents the damping transfer function of wind turbine 2 acting on itself; 21 G represents the damping transfer function of wind turbine 2 on wind turbine 1; 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop; The establishment of a parallel grid-connected simulation model for two direct-drive wind turbines includes a primary system, a control system and a power grid for the two direct-drive wind turbines; The establishment of a parallel grid-connected simulation model for two direct-drive wind turbines comprises the following steps: Direct-drive wind turbine modeling: The direct-drive wind turbine group includes: a wind turbine, a permanent magnet synchronous generator, a four-quadrant converter, and a filter circuit; the wind turbine, permanent magnet synchronous generator, and machine-side converter of the direct-drive wind turbine group are all equivalent to controlled current sources, and the main circuit expression of the grid-side converter is as follows: (i=1,2) Where: i = 1 represents direct-drive wind turbine group 1, i = 2 represents direct-drive wind turbine group 2; E di 、E qi Indicates the converter output voltage dq axis voltage; L fi Indicates the converter filter inductance; U tdi , U tqi 、i di 、i qi are the dq axis components of the voltage at the grid-side converter terminal of the direct-drive wind turbine and the dq axis components of the output current of the grid-side converter respectively; C dci , U dci , P outi , P ini are the capacitance value, voltage value, output active power and input active power of the DC capacitor of the direct-drive wind turbine generator set respectively; ω is the rated angular velocity; Control system modeling: The grid-side converter active power control adopts constant DC voltage control. The instantaneous value of the DC capacitor voltage and the reference value are subtracted and passed through the PI link to generate the d-axis current reference value. The reactive power control adopts constant terminal voltage amplitude control. The instantaneous value of the terminal voltage and the reference value are subtracted and passed through the PI link to generate the q-axis current reference value. The expression is as follows: Where: U dcrefi , U dci are the reference value and actual value of the DC capacitor voltage of the direct-drive wind turbine group; U tref , U t are the reference value and actual value of the voltage at the public grid connection point of the direct-drive wind turbines respectively; x 1i 、x 2i 、x 3i 、x 5i is the state variable; k p1i , k p2i , k p3i , k p5i are the proportional coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively; k i1i , k i2i , k i3i , k i5i are the integral coefficients of the DC voltage loop, the inner current loop and the terminal voltage control loop respectively; Grid modeling: Without considering the dynamic process of the grid, the grid algebraic equation is established, and the expression is as follows: Where: X g is the reactance value of the AC system; U tx , U ty are the x-axis and y-axis components of the converter terminal voltage; U gx , U gy are the x-axis and y-axis components of the grid voltage; i xi 、i yi are the x-axis and y-axis components of the output current of the direct-drive wind turbine; The mutual damping coefficient D dc12 The influence of the second direct-drive wind turbine on the first direct-drive wind turbine is quantitatively analyzed in the form of G. 11 represents the damping function of wind turbine 1 on itself, G 12 The damping function of wind turbine 1 on wind turbine 2, G 22 represents the damping function of wind turbine 2 on itself, G 21 The damping function of wind turbine 2 on wind turbine 1, G 121dc It represents the transfer function from the input to the output of wind turbine 2 when analyzing the DC voltage loop; G 11 , G 12 , G 22 , G 21 , G 121dc The expression is as follows: Where: ΔU td1 , ΔU tq1 , Δi d1 , Δi q1 , ΔU td2 , ΔU tq2 , Δi d2 , Δi q2 is the d-axis and q-axis component increments of the grid connection point voltage and output current of wind turbine generator set 1 and wind turbine generator set 2; U td01 , U tq01 、i d01 、i q01 , U td02 , U tq02 、i d02 、i q02 I is the grid connection point voltage of wind turbine 1 and wind turbine 2 and the stable operating value of the d-axis and q-axis components of the output current; dc02 , U dc02 is the equivalent current source current value of wind turbine 2 and the stable operation value of DC capacitor voltage; PI 12 For the controller; Therefore, the influence of wind turbine 2 on the DC voltage loop stability of wind turbine 1 is G dc (s) by G 12 , G 121dc and G 21 The series connection of The above further extends to a multi-machine parallel system and analyzes the interaction of the multi-machine parallel system, including the following steps: Establish a damping torque model for a multi-machine parallel system, take the first direct-drive wind turbine as the research object, and study the interaction with other direct-drive wind turbines; The damping torque model of the DC voltage loop mode of each direct-drive wind turbine is established, where: the input quantity of the first direct-drive wind turbine is the electromagnetic power increment ΔP e1 , the output is the voltage increment ΔU of the DC capacitor dc1 , the damping transfer function is G dc1 , the expression is as follows: The input quantity of other direct-drive wind turbines is the voltage increment of the DC capacitor ΔU dci , the output is the electromagnetic power increment ΔP ei , the damping transfer function is G dci , the expression is as follows: (i=2,…n) Where: i = 1 represents direct-drive wind turbine group 1, i = 2 represents direct-drive wind turbine group 1, and is extended to n direct-drive wind turbine groups; D dci and K dci represents the DC voltage loop damping coefficient and synchronization coefficient of the i-th wind turbine; Calculation of mutual damping coefficient D in multi-machine parallel system dcm , the expression is as follows: D dcm =D dc2 +...+D dci (14) Using the calculated mutual damping coefficient D dcm ,After perturbing the grid strength, system operating conditions and direct-drive wind turbine control parameters, the interaction among the parallel systems of multiple direct-drive wind turbines is quantitatively evaluated.

9. A computer device, characterized in that: The method comprises a storage medium, a processor and a computer program stored on the storage medium and executable on the processor, wherein when the processor executes the computer program, the steps of the method for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines as described in any one of claims 1 to 8 are implemented.

10. A storage medium, characterized in that: The storage medium stores a computer program, and the computer program is used by a processor to execute the steps of the method for analyzing low-frequency oscillation interaction between parallel direct-drive wind turbines as described in any one of claims 1-8.

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