Direct-drive fan system small interference stability index and sensitivity index broadband oscillation evaluation method based on mode damping energy function

Abstract

The invention discloses a broadband oscillation evaluation method for a small-interference stability index and a sensitivity index of a direct-drive fan system based on a mode damping energy function. The broadband oscillation evaluation method comprises the following steps: constructing the mode damping energy function containing the direct-drive fan system; based on the mode damping energy function, analyzing the small-interference stable energy of the system containing the direct-drive fan; verifying the direct-driven fan system according to an analysis result to obtain an evaluation model; and evaluating the direct-driven fan system to be evaluated according to the evaluation model to obtain an evaluation result. According to the method, complex modeling is avoided, and meanwhile, the change condition of each grid-connected unit on the overall damping level of the system and the influence of direct-drive fan grid connection on oscillation can be quantified through mode damping energy.

Description

Technical Field

[0001] The present invention belongs to the technical field of small disturbance stability of power system caused by grid-connected direct-drive wind turbines, and in particular to a broadband oscillation evaluation method of small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function. Background Art

[0002] As wind power grid connection is becoming more common in new power systems, the inertia level of the system is decreasing. At the same time, the volatility and randomness of wind power further complicate the analysis of low-frequency oscillations of the system. Therefore, the safe and stable operation of new power systems has also ushered in new challenges. Most of the existing studies have studied the impact of direct-drive wind turbines on low-frequency oscillations from a single perspective such as wind turbine parameters, converter control strategy or overall system impedance model, but lack complex modeling of direct-drive wind turbines or systems. Summary of the invention

[0003] In order to solve the above technical problems, the present invention proposes a wide-band oscillation evaluation method for the small disturbance stability index and sensitivity index of a direct-drive wind fan system based on the mode damping energy function, which avoids complex modeling and can also quantify the changes in the overall damping level of the system by each grid-connected unit and the impact of the direct-drive wind fan grid connection on the oscillation through the mode damping energy. Furthermore, by constructing the small disturbance stability evaluation index and sensitivity index of the direct-drive wind fan system based on the mode damping energy, a reasonable direct-drive wind fan grid-connected small disturbance stability control strategy is given.

[0004] To achieve the above object, the present invention provides a broadband oscillation evaluation method for small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function, comprising:

[0005] Construct the damping energy function of the direct-drive fan system mode;

[0006] Based on the mode damping energy function, the small disturbance stability energy of the system containing the direct-drive fan is analyzed;

[0007] The direct-drive fan system is verified based on the analytical results to obtain an evaluation model;

[0008] The direct-drive wind turbine system to be evaluated is evaluated according to the evaluation model to obtain an evaluation result.

[0009] Optionally, before constructing the damping energy function of the direct-drive wind turbine system mode, it is necessary to construct a new energy power system model that takes the damping coefficient into consideration.

[0010] Optionally, constructing a new energy power system model taking into account the damping coefficient includes:

[0011]

[0012] Among them, U d , U q are the voltages of the d-axis and q-axis, I d ,I q are the voltages of the d-axis and q-axis respectively, X d , X d ' are the synchronous reactance and transient reactance of the d-axis respectively, X q is the synchronous reactance of the q axis, E f is the stator electromagnetic electromotive force, E q 、E q ' are the no-load electromotive force and transient electromotive force of the q axis respectively, T d0 ' is the transient time constant of the d-axis, M is the inertia time constant, P m , P e are the mechanical power and electromagnetic power of the generator respectively, D is the winding damping coefficient, δ is the generator power angle, ω and ω0 are the rotor angular velocity of the generator and the initial value of the rotor angular velocity of the generator respectively.

[0013] Optionally, constructing a damping energy function for a system with a direct-drive fan includes:

[0014] Construct the linear equation of state of the system containing direct-drive wind turbine under small disturbance;

[0015] Solving the state linear equation;

[0016] The voltage angle increment at the node is obtained based on the solution of the state linear equation; the position and the number of small disturbance characteristic roots in all state variables are obtained based on the voltage angle increment, and a mode damping energy function of the direct-drive fan system is constructed.

[0017] Optionally, analyzing the small disturbance stability energy of the system containing the direct-drive fan based on the mode damping energy function includes:

[0018] Construct a small disturbance stability evaluation index for a direct-driven wind system based on the mode damping energy function;

[0019] Construct a direct-drive wind turbine grid-connected sensitivity index based on the damping energy of the key oscillation link mode;

[0020] Based on the evaluation index and the sensitivity index, the small disturbance stability energy of the system containing the direct-drive fan is analyzed.

[0021] Optionally, constructing a small disturbance stability evaluation index of a direct-driven wind system based on a mode damping energy function includes:

[0022]

[0023] Among them, λ KGDESSSIis the small disturbance stability evaluation index of the direct-drive wind system, n is the number of key participating units in the oscillation, V DiMAX1 、V DiMAX2 They are the first and second peaks of the i-mode damping energy of the key unit respectively.

[0024] Optionally, constructing a direct-drive wind turbine grid-connected sensitivity index based on the damping energy of the oscillation key link mode includes:

[0025]

[0026] Among them, λ DMDESI is the direct-drive wind turbine grid-connected sensitivity index, V bk 、V ak are the mode damping energy of the kth key oscillation participating unit before and after the intervention, n is the number of key oscillation participating units, and ΔPG is the change in the output active power of the direct-drive wind turbine.

[0027] Technical effect of the invention: The present invention discloses a wide-band oscillation evaluation method for small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function. Based on the energy method of transient problems and combined with the small disturbance stability analysis theory, the connection between energy and state variables of each generator set is established, and a mode damping energy function of a direct-drive wind turbine system is constructed. While avoiding complex modeling, the change in the overall damping level of the system and the impact of direct-drive wind turbine grid connection on oscillation can be quantified through the mode damping energy theory. Furthermore, through the constructed small disturbance stability evaluation index and sensitivity index of the direct-drive wind turbine system based on the mode damping energy, a reasonable small disturbance stability control strategy for direct-drive wind turbine grid connection is given, and the reliability of the proposed method is determined through damping verification. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0029] Figure 1 This is a wiring diagram of a 3-machine 9-node system according to an embodiment of the present invention;

[0030] Figure 2 Schematic diagram of mode damping energy distribution in mode 1 of an embodiment of the present invention;

[0031] Figure 3 Schematic diagram of mode damping energy distribution in mode 2 of an embodiment of the present invention;

[0032] Figure 4 It is a schematic diagram of the damping energy distribution of the embodiment of the present invention;

[0033] Figure 5 This is a schematic diagram of the damping energy amplitude of the key unit mode after the output of DDSG1 is increased in an embodiment of the present invention;

[0034] Figure 6 This is a schematic diagram of system mode damping energy distribution under the dominant oscillation mode of an embodiment of the present invention;

[0035] Figure 7 Schematic diagram of the change of damping energy of DDSG2 mode before and after the output is increased according to an embodiment of the present invention, wherein (a) is the change of damping energy of Ddsg1, (b) is the change of damping energy of Ddsg2, (c) is the change of damping energy of Ddsg3, and (d) is the change of damping energy of Ddsg7;

[0036] Figure 8 The present invention is a flowchart of a method for evaluating a wide-band oscillation of a small-disturbance stability index and a sensitivity index of a direct-drive wind turbine system based on a mode damping energy function according to an embodiment of the present invention. DETAILED DESCRIPTION

[0037] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0038] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0039] like Figure 8 As shown, in this embodiment, a method for evaluating the wide-band oscillation of the small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function is provided, comprising:

[0040] Construct the damping energy function of the direct-drive fan system mode;

[0041] The small disturbance stability energy of the system containing direct-drive wind turbines is analyzed based on the mode damping energy function;

[0042] The direct-drive fan system is verified based on the analytical results to obtain an evaluation model;

[0043] The direct-drive wind turbine system to be evaluated is evaluated according to the evaluation model to obtain an evaluation result.

[0044] Furthermore, the construction of a new energy power system model considering the damping coefficient includes:

[0045] A single-machine infinite system is established. The generator adopts a third-order model and takes damping into account. The excitation system adopts a static excitation system and the mechanical power is constant. The distributed capacitance and loss of the line are not considered. The reactance is represented by X. The voltage of the single-machine infinite system is Where U is a constant, and the generator dq coordinate unit value mathematical model is shown in the following formula (1):

[0046]

[0047] In formula (1), U d , U q Refers to the voltage of the d-axis and q-axis respectively, I d ,I q Refers to the voltage of d-axis and q-axis respectively, X d , X d ' refers to the synchronous reactance and transient reactance of the d-axis respectively, X q Refers to the synchronous reactance of the q axis, E f Refers to the stator electromagnetic electromotive force, E q 、E q ' refers to the no-load electromotive force and transient electromotive force of the q axis respectively, T d0 ' refers to the d-axis transient time constant, M refers to the inertia time constant, P m , P e They refer to the mechanical power and electromagnetic power of the generator respectively, D refers to the winding damping coefficient, δ refers to the generator power angle, ω and ω0 refer to the rotor angular velocity of the generator and its initial value respectively.

[0048] Linearize equation (1) to get equation (2):

[0049]

[0050] In formula (2), ΔP e and ΔE q is the number of generations:

[0051]

[0052] When a single-machine infinite system is in steady-state operation and Δω approaches 0, then ΔP e ≈ΔT e ;

[0053] Single machine infinite system (not considering virtual excitation system adjustment ΔE f ) In steady-state conditions (ω0=1) and when Δω is very small, ΔP e ≈ΔT e Written as:

[0054]

[0055] When only the damping component of the mode energy is considered, the damping component in the generator output power increment is as follows:

[0056] ΔP de =T e Δω=D e Δω 2 ω0(5),

[0057] Furthermore, the damping energy function of the direct-drive fan system mode is constructed including:

[0058] Under small disturbances, the system containing the direct-drive fan can be expressed by a linear state equation, as shown in the following equation (6):

[0059]

[0060] In formula (6), ΔX = [Δx G , Δx Ddsg ] T refers to the n-dimensional state variable; Δx G =[Δδ i ,Δω i ,Δe iq ] T Refers to the state variable of the synchronous generator; Δx Ddsg =[Δv ωi ,Δω i ,,Δθ i ,Δi qsi ,Δi qci ] T Refers to the state variable of the fan; ΔU N =[Δu1,Δθ1,Δu2,Δθ2,…,Δu i ,Δθ i ] T , where Δu n ,Δθ n They refer to the node voltage amplitude increment and angle increment respectively, and A, B, C, and D refer to the coefficient matrix.

[0061] The general form of the solution to the state equation is:

[0062]

[0063] In formula (7), ψ j ,φ j refers to the left and right eigenvectors; λ j (j=1,...,n) refers to the eigenvalue; x0 refers to the initial value of the state variable. The specific matrix form is shown in formula (8):

[0064]

[0065] The voltage angle increment at node i is shown in equation (9):

[0066]

[0067] In formula (9), p refers to the position of the state variable in all state variables, q refers to the number of small disturbance characteristic roots, and the constructed mode damping energy function is shown in the following formula (10):

[0068]

[0069] The damping-related expression of the generator outlet power increment in the system is shown in the following equation (11):

[0070] ΔP de =T e Δω=D e Δω 2 ω0 (11),

[0071] The voltage angle increment at node i is shown in equation (12):

[0072]

[0073] Where: F = -D -1 C;

[0074] The active power increment and voltage angle increment at the generator output are simplified, where A1-A n Refers to the power increment factor, B1-B n It refers to the voltage angle increment coefficient. The final simplified result is expressed as follows:

[0075]

[0076] Therefore, the modal damping energy expression of the generator can be expressed by the following formula (14):

[0077]

[0078] In this case, the modal damping energy of the system consists of two parts: the autocorrelation mode and the cross-correlation mode:

[0079]

[0080] At present, we should accurately identify the dominant oscillation mode and then conduct further analysis, so the cross-correlation mode is ignored. At this time, the mode damping energy expression of the system is shown in the following formula (16):

[0081]

[0082] Furthermore, the small disturbance stability energy of the direct-drive fan system is analyzed based on the mode damping energy function, including:

[0083] Construct a small disturbance stability evaluation index for a direct-driven wind system based on the mode damping energy function;

[0084] Construct a direct-drive wind turbine grid-connected sensitivity index based on the damping energy of the key oscillation link mode;

[0085] Based on the evaluation index and the sensitivity index, the small disturbance stability energy of the system containing the direct-drive fan is analyzed.

[0086] Furthermore, the small disturbance stability evaluation index of the direct-driven wind system based on the mode damping energy function is constructed, including:

[0087] In order to quantify the small disturbance stability from the energy perspective, the present invention maps the small disturbance degree of the system to the stability degree of the damping energy of the key units involved in the oscillation, and quantifies the stability degree of different oscillation links under the corresponding oscillation mode from the damping energy, amplitude and frequency consumed by each generator under different oscillation modes, detects the dominant oscillation mode of the system and the distribution of its mode damping energy, and then finds the damping energy under the key unit mode with the highest energy distribution level, and then quantitatively analyzes the small disturbance stability of the system, and proposes a key unit mode damping energy stability index λ KGDESSSI (Key Generator damping energy small signal stability index), which is shown in the following formula (17):

[0088]

[0089] Among them, λ KGDESSSI is the small disturbance stability evaluation index of the direct-drive wind system, n is the number of key participating units in the oscillation, V DiMAX1 、V DiMAX2 They are respectively the first and second peaks of the i-mode damping energy of the key unit. The index value is proportional to the attenuation of the mode damping energy at this time, that is, the smaller the index value, the worse the small disturbance stability of the system, and vice versa.

[0090] Furthermore, the direct-drive wind turbine grid-connected sensitivity index based on the damping energy of the key oscillation mode is constructed, including:

[0091] Existing studies have shown that in a direct-drive wind turbine grid-connected power system, the amplitude and attenuation rate of the damping energy of the key participating unit mode can reflect the degree of influence of the output change of a specific direct-drive wind turbine on the oscillation. In order to intuitively reflect the influence of direct-drive wind turbines at different positions in the direct-drive wind turbine grid-connected system on the small disturbance stability of the system, the present invention proposes a direct-drive wind turbine mode damping energy sensitivity index λ DMDESI (Direct drive synchronousgeneratorMode Dampling Energy SensitivityIndex):

[0092]

[0093] Among them, λ DMDESI is the direct-drive wind turbine grid-connected sensitivity index, V bk 、V ak are the mode damping energy of the kth key oscillation participating unit before and after the intervention, n is the number of key oscillation participating units, ΔPG is the change in the output active power of the direct-drive wind turbine. This indicator can be used to quantitatively analyze the influence of direct-drive wind turbines in different positions on oscillation. In addition, the optimal position for the direct-drive wind turbine to be connected to the grid can be determined according to the sensitivity index, which can not only ensure the installed capacity of the direct-drive wind turbine, but also maximize the small disturbance stability of the system.

[0094] A specific application example of the present invention is as follows:

[0095] (1) 3-machine 9-node system

[0096] The 3-machine 9-node system with direct-drive wind turbines is considered as the research scenario, and nodes 4, 7, and 9 are used as candidate nodes for direct-drive wind turbines to be connected to the grid. Each node is connected to a 20MW direct-drive wind turbine with an upper limit of 50MW. In order to explore how to use λ KGDESSSI and λ DMDESI Determine the influence of the increase of the output of each direct-drive fan unit on the system stability and sensitivity. The system wiring diagram is as follows: Figure 1 As shown,

[0097] First, for Figure 2 , Figure 3 The modal damping energy distribution under the two oscillation modes shown in FIG. 2 is taken as the dominant oscillation mode. The modal damping energy distribution of each unit under this mode is shown in FIG. Figure 4 As shown, the key units participating in the oscillation at this time are selected: No. 3 synchronous generator, No. 2 wind turbine and No. 3 wind turbine. The output of No. 4, No. 7 and No. 9 direct-drive wind turbines is increased respectively. At this time, the wind power penetration rate in the system increases, and the change of the key unit mode damping energy caused by the increase of the direct-drive wind turbine output in the dominant oscillation mode is calculated as follows Figure 5shown.

[0098] from Figure 5 It can be found that in the system of 3 machines and 9 nodes, after the output of the direct-drive wind turbines at each node increases, the damping energy of each key unit mode changes, but this does not change the system's oscillation key units and the system's dominant oscillation mode.

[0099] To verify the above viewpoint, the small disturbance stability index of the mode damping energy of each key unit is calculated, and the λ under the dominant oscillation mode after the output of the direct-drive wind turbine increases is obtained. KGDESSSI As shown in Table 1, the changes in the damping energy of the key unit modes before and after the wind turbine output of each node is increased are observed, and the influence of the oscillation of a certain direct-drive wind turbine is deeply analyzed.

[0100] Table 1

[0101]

[0102] The changes in the modal damping energy amplitude and sensitivity index before and after the output of No. 1, No. 2 and No. 3 direct-drive fans increases are shown in Table 2. By calculating the changes in the modal damping energy amplitude and sensitivity index, it can be seen that the sensitivity of No. 2 direct-drive fan at node 4 is the smallest and its impact on oscillation is the weakest, while the sensitivity of No. 3 direct-drive fan at node 9 is the largest and its impact on oscillation is the strongest.

[0103] Table 2

[0104]

[0105] In addition, by combining the analysis of small disturbance stability evaluation indicators, it can be seen that the connection of direct-drive wind turbines at nodes 7 and 9 can enhance the stability of the system, among which the degree of enhancement at node 9 is better than that at node 7. The connection of No. 2 direct-drive wind turbine at node 4 will deteriorate the stability of the system, but the impact is not significant. Therefore, it can be concluded that the grid-connected node order of the three direct-drive wind turbines should be node 9, node 7 and node 4.

[0106] Afterwards, the change of damping ratio before and after the output of direct-drive fans at different positions is verified, as shown in Table 3. As can be seen from Table 3, when the output of direct-drive fans at different positions increases, the damping ratio of the system is also constantly changing. The damping ratio of direct-drive fan No. 2 decreases after the output increases, and the direct-drive fan No. 3 has the most significant impact on the system damping ratio after the output increases. It has the best improvement effect on the system damping ratio and the most absorbent effect on oscillation, which is basically consistent with the conclusion drawn by the proposed method, proving the feasibility of the proposed method.

[0107] Table 3

[0108]

[0109] (2) New England 10-machine 39-node system

[0110] In order to further verify the universality and objectivity of the proposed method, a 10-machine 39-bus system was selected, and the incorporation of multiple direct-drive wind turbines was used as the operating condition.

[0111] First, direct-drive wind turbines are added to nodes 7, 20, 22, 23, 25, 29, and 31 respectively, and the output of direct-drive wind turbines at different locations is increased equally. The changes in the mode damping energy of each unit are as follows Figure 6 As shown in the figure, the key oscillation links are direct-drive wind turbines No. 1, No. 2, No. 3 and No. 7. Figure 7 Schematic diagram of the change of damping energy in DDSG2 mode before and after the output is increased in an embodiment of the present invention, wherein Figure 7 (a) is the change of Ddsg1 damping energy, Figure 7 (b) is the change of Ddsg2 damping energy. Figure 7 (c) is the change of Ddsg3 damping energy, Figure 7 (d) shows the change of damping energy of Ddsg7.

[0112] Increase the output of the direct-drive wind turbine at each direct-drive wind turbine grid-connected node and calculate the small disturbance stability index λ of the key unit at this time KGDESSSI The results are shown in Table 4. In the 10-machine 39-node system, as the output of the direct-drive wind turbines at each node increases, the key units of the system remain unchanged. At this time, the change amplitude and sensitivity index of the key unit mode damping energy when the output of the direct-drive wind turbines at different nodes is increased are shown in Table 5. ΔV kb It refers to the change in the damping energy of the key unit mode after the output of the same direct-drive wind turbine is increased equally. The impact of the increase in the output of the direct-drive wind turbines at different nodes on the oscillation is also different, and the degree of change in the damping energy of the key unit mode of the system is also different.

[0113] By comparing the λ in Table 4 and Table 5 KGDESSSI , DMDESI From the analysis of the two indicators, it can be seen that after the output of direct-drive wind turbines No. 1, 2, 3, 5 and 6 was increased, the system stability deteriorated, among which No. 1 and 2 were affected to a greater extent, and No. 3, 5 and 6 were affected to a lesser extent; after the addition of direct-drive wind turbines No. 4 and 7, the system small disturbance stability index value increased, among which the sensitivity index value of direct-drive wind turbine No. 7 was larger, and the degree of influence on system oscillation was more significant. Obviously, the small disturbance stability of the system was enhanced at this time. Therefore, under the premise that the maximum installed capacity of each access position remains unchanged, the grid-connected order of each direct-drive wind turbine is 7-4-6-3-5-2-1.

[0114] Table 4

[0115]

[0116] Table 5

[0117]

[0118] To verify the reliability of the proposed strategy, the changes in the system damping ratio after the output of direct-drive fans at different positions is observed, and the output of direct-drive fans No. 1 to No. 7 is increased respectively. The changes in the system damping ratio under the dominant oscillation mode are observed. It can be seen that the system damping ratio of direct-drive fans No. 1, No. 2, No. 3, No. 5 and No. 6 decreases after the output increases. Obviously, the system stability deteriorates at this time, while the system damping ratio of direct-drive fans No. 4 and No. 7 increases after the output increases, among which the change degree of direct-drive fan No. 7 is larger, so the system stability is improved at this time. Among them, the system stability reaches the best state after the output of fan No. 7 is increased, and the system damping ratio changes less after the output of direct-drive fan No. 4 is increased. Therefore, the degree of change in system stability is small, which is consistent with the conclusion of the proposed method and proves that the proposed method is feasible.

[0119] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A broadband oscillation evaluation method for small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function, characterized in that: include: Construct the damping energy function of the direct-drive fan system mode; Based on the mode damping energy function, the small disturbance stability energy of the system containing the direct-drive fan is analyzed; The direct-drive fan system is verified based on the analytical results to obtain an evaluation model; The direct-drive wind turbine system to be evaluated is evaluated according to the evaluation model to obtain an evaluation result.

2. The broadband oscillation evaluation method of small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function according to claim 1, characterized in that: Before constructing the damping energy function of the direct-drive wind turbine system mode, it is necessary to build a new energy power system model that takes the damping coefficient into consideration.

3. The broadband oscillation evaluation method of small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function as claimed in claim 2, characterized in that: The construction of a new energy power system model considering the damping coefficient includes: Among them, U d , U q are the voltages of the d-axis and q-axis, I d ,I q are the voltages of the d-axis and q-axis respectively, X d , X d ' are the synchronous reactance and transient reactance of the d-axis respectively, X q is the synchronous reactance of the q axis, E f is the stator electromagnetic electromotive force, E q 、E q ' are the no-load electromotive force and transient electromotive force of the q axis respectively, T d0 ' is the transient time constant of the d-axis, M is the inertia time constant, P m , P e are the mechanical power and electromagnetic power of the generator respectively, D is the winding damping coefficient, δ is the generator power angle, ω and ω0 are the rotor angular velocity of the generator and the initial value of the rotor angular velocity of the generator respectively.

4. The broadband oscillation evaluation method of small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function according to claim 1, characterized in that: The construction of the damping energy function of the direct-drive fan system mode includes: Construct the linear equation of state of the system containing direct-drive wind turbine under small disturbance; Solving the state linear equation; The voltage angle increment at the node is obtained based on the solution of the state linear equation; the position and the number of small disturbance characteristic roots in all state variables are obtained based on the voltage angle increment, and a mode damping energy function of the direct-drive fan system is constructed.

5. The broadband oscillation evaluation method of small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function according to claim 1, characterized in that: The analysis of the small disturbance stability energy of the direct-drive fan system based on the mode damping energy function includes: Construct a small disturbance stability evaluation index for a direct-driven wind system based on the mode damping energy function; Construct a direct-drive wind turbine grid-connected sensitivity index based on the damping energy of the key oscillation link mode; Based on the evaluation index and the sensitivity index, the small disturbance stability energy of the system containing the direct-drive fan is analyzed.

6. The broadband oscillation evaluation method of small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function according to claim 1, characterized in that: The small disturbance stability evaluation index of the direct-driven wind system based on the mode damping energy function includes: Among them, λ KGDESSSI is the small disturbance stability evaluation index of the direct-drive wind system, n is the number of key participating units in the oscillation, V DiMAX1 、V DiMAX2 They are the first and second peaks of the i-mode damping energy of the key unit respectively.

7. The broadband oscillation evaluation method of small disturbance stability index and sensitivity index of a direct-drive wind turbine system based on a mode damping energy function according to claim 1, characterized in that: The direct-drive wind turbine grid-connected sensitivity index based on the damping energy of the key oscillation mode includes: Among them, λ DMDESI is the direct-drive wind turbine grid-connected sensitivity index, V bk 、V ak are the mode damping energy of the kth key oscillation participating unit before and after the intervention, n is the number of key oscillation participating units, and ΔPG is the change in the output active power of the direct-drive wind turbine.

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