Impedance modeling method of grid-connected direct-drive wind turbine considering frequency coupling effect

Abstract

The application discloses a kind of network type direct drive wind turbine impedance modeling methods considering frequency coupling effect, comprising: the main circuit small signal model of network type direct drive wind turbine grid-connected system including grid impedance in frequency domain is constructed;Based on the analytical expression between modulation coefficient disturbance and grid-connected point voltage, current and inductance current disturbance signal is obtained by multi-harmonic linearization theory;The overall impedance matrix of network type direct drive wind turbine grid-connected system is obtained by simultaneously solving main circuit small signal model and analytical expression;Overall impedance under disturbance frequency ω p Is obtained by using overall impedance matrix, and the equivalent impedance of network type direct drive wind turbine is obtained by subtracting the impedance component corresponding to ω p Frequency in grid impedance matrix from overall impedance.The application can effectively reduce the modeling difficulty, obtain accurate and reliable equivalent impedance, and provide technical support for accurately analyzing grid-connected system stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power generation system stability, in particular to a grid-connected direct-drive wind turbine impedance modeling method considering frequency coupling effect. BACKGROUND

[0002] Direct-drive wind turbine has the advantages of mature technology, cost-effective, etc. and is widely used in wind power generation system as the mainstream model. However, with a large number of wind turbines connected to the grid, the system inertia and damping will be reduced, which is not conducive to system stability. Therefore, grid-connected control technology is widely valued because it can simulate the inertia and damping characteristics of traditional synchronous generators. In order to better analyze the small signal stability of grid-connected system of grid-connected direct-drive wind turbine, it is particularly important to establish a correct impedance model.

[0003] At present, in the existing research, when establishing the impedance model of grid-connected direct-drive wind turbine, only the single-frequency disturbance voltage input and double-frequency disturbance current output characteristics formed by the asymmetric control structure are considered, and the mutual coupling between the output double-frequency component and the grid is not involved, which will lead to inaccurate results when the system is analyzed for stability. Secondly, a few studies have established an equivalent impedance model considering frequency coupling by finding the coupling relationship based on the obtained VSG output impedance, but the coupling process involved in this method is relatively complex, which undoubtedly increases the difficulty of system modeling.

[0004] Therefore, a new grid-connected direct-drive wind turbine impedance modeling method is needed, which can effectively reduce the modeling difficulty and obtain accurate and reliable equivalent impedance to provide technical support for accurate analysis of grid-connected system stability. SUMMARY

[0005] Therefore, the purpose of the present application is to overcome the defects in the prior art and provide a grid-connected direct-drive wind turbine impedance modeling method considering frequency coupling effect, which can effectively reduce the modeling difficulty and obtain accurate and reliable equivalent impedance to provide technical support for accurate analysis of grid-connected system stability.

[0006] The grid-connected direct-drive wind turbine impedance modeling method considering frequency coupling effect of the present application comprises:

[0007] Applying voltage disturbance to the grid, constructing a main circuit small signal model of the grid-connected direct-drive wind turbine system in the frequency domain including the grid impedance;

[0008] Based on the multi-harmonic linearization theory, obtaining an analytical expression between the modulation coefficient disturbance and the voltage, current and inductance current disturbance signals at the point of common coupling (PCC);

[0009] Solving the main circuit small signal model and the analytical expression to obtain the overall impedance matrix Z of the grid-connected direct-drive wind turbine system acg ;

[0010] The overall impedance matrix Z is utilized acg , and the overall impedance Z p (ω acg ) under the disturbance frequency ω p is obtained, the overall impedance Z acg (ω p ) is subtracted from the impedance component Z g (ω p ) corresponding to the frequency ω g in the grid impedance matrix Z p , and the equivalent impedance of the grid-connected direct-drive wind turbine is obtained.

[0011] Further, the main circuit small signal model is as follows:

[0012]

[0013] wherein, and represent grid voltage, PCC point voltage, PCC point current and inductance current disturbance signals respectively, represents a modulation coefficient disturbance; Z g , Z L and Z C represent a grid impedance matrix, a filter inductance impedance matrix and a filter capacitance impedance matrix respectively, U dc represents a DC voltage steady-state matrix.

[0014] Further, the expression of the DC voltage steady-state matrix U dc is as follows:

[0015]

[0016] wherein, U dc represents the intermediate DC side voltage of the direct-drive wind turbine system.

[0017] Further, the analytical expression is as follows:

[0018]

[0019] wherein, and represent PCC point voltage, PCC point current and inductance current disturbance signals respectively, represents a modulation coefficient disturbance; Q, P and R are coefficient matrices.

[0020] Further, the overall impedance matrix Z acg is as follows:

[0021]

[0022] wherein E is a unit matrix, Y L is a filter inductance admittance matrix, i.e. L -1 = Z L .

[0023] Further, the Toeplitz steady-state harmonic matrix is substituted into the main circuit small signal model of the main circuit, and the convolution operation is eliminated.

[0024] The beneficial effects of the present application are: the impedance modeling method of the grid-connected direct-driven wind turbine considering the frequency coupling effect disclosed in the present application establishes the overall impedance model of the grid-connected system of the grid-connected direct-driven wind turbine based on the multi-harmonic linearization theory, and then obtains the equivalent impedance model of the grid-connected direct-driven wind turbine containing the frequency coupling effect. The overall modeling method adopted in the present application only needs to calculate the grid impedance and the system output impedance under the disturbance frequency, and does not need to consider the coupling relationship of the positive and negative sequence impedances in the calculation process, the physical meaning is more clear, and the system modeling difficulty is effectively reduced; at the same time, the modeling based on the multi-harmonic linearization theory will make the impedance model only need to adjust the coefficient matrix when the system control structure changes, which is more conducive to the damping characteristic and stability analysis of the grid-connected direct-driven wind turbine under the variable control structure compared with the existing method. BRIEF DESCRIPTION OF DRAWINGS

[0025] The present application will be further described below in combination with the drawings and embodiments:

[0026] Figure 1 Fig. 1 is a schematic diagram of the overall structure of the grid-connected system of the grid-connected direct-driven wind turbine of the present application;

[0027] Figure 2 Fig. 2 is a schematic diagram of the virtual synchronous control structure of the grid-side converter in the grid-connected direct-driven wind turbine of the present application;

[0028] Figure 3 Fig. 3 is a schematic diagram of the equivalent impedance sweep result of the grid-connected direct-driven wind turbine of the present application;

[0029] Figure 4 Fig. 4 is a schematic diagram of the impedance modeling method flow of the grid-connected direct-driven wind turbine of the present application. DETAILED DESCRIPTION

[0030] The present application will be further described below in combination with the drawings and embodiments:

[0031] The present application discloses an impedance modeling method of a grid-connected direct-driven wind turbine considering the frequency coupling effect, comprising the following steps:

[0032] S1. Apply a voltage disturbance to the grid, and construct a main circuit small signal model of the grid-connected system of the grid-connected direct-driven wind turbine containing the grid impedance in the frequency domain;

[0033] S2. Based on the multiharmonic linearization theory, an analytical expression is obtained between the modulation coefficient disturbance and the disturbance signals of grid connection point voltage, current and inductor current;

[0034] S3. By combining the small-signal model and analytical expression of the main circuit, the overall impedance matrix Z of the grid-connected system of the grid-connected direct-drive wind turbine is obtained. acg ;

[0035] S4. Utilizing the overall impedance matrix Z acg The perturbation frequency ω is obtained. p The overall impedance Z acg (ω p ), the overall impedance Z acg (ω p Subtract the grid impedance matrix Z g ω p The impedance component Z corresponding to the frequency g (ω p ), thus obtaining the equivalent impedance of the grid-type direct-drive wind turbine.

[0036] This invention first establishes a small-signal model of the main circuit in the frequency domain; then, based on the control structure of the wind turbine grid-connected system, it obtains analytical expressions between the modulation coefficient and the disturbance signals of the grid connection point voltage, current, and inductor current, and establishes a small-signal model of the control circuit; finally, it combines the main circuit and the control circuit to obtain the overall impedance matrix Z of the wind turbine grid-connected system. acg Treating the grid impedance as part of a grid-connected direct-drive wind turbine, using Z... acg The impedance component Z corresponding to the disturbance frequency acg (ω p Subtract the grid impedance matrix Z g The impedance component Z corresponding to the disturbance frequency g (ω p Thus, the equivalent impedance of the grid-type direct-drive fan is obtained.

[0037] When performing impedance modeling for grid-type direct-drive wind turbines, this invention only requires calculating the grid impedance and system output impedance at the disturbance frequency, and does not involve complex coupling relationships. This effectively reduces the difficulty of modeling grid-type direct-drive wind turbine systems and enables more accurate analysis of the damping characteristics and stability of grid-type direct-drive wind turbines under variable control structures.

[0038] In this embodiment, in step S1, the frequency coupling effect refers to the effect of injecting a frequency of ω at the point of common coupling (PCC). p The disturbance voltage will appear in the grid-connected current with a positive sequence ω. p Frequency and negative order ω pThe disturbance current is dominated by the coupling frequency component -2ω0, where ω0 is the system reference frequency. The system exhibits single-input multiple-output characteristics. Meanwhile, ω... p The disturbance current at the -2ω0 coupling frequency will generate a voltage disturbance at the coupling frequency at the PCC point through the grid impedance. This voltage disturbance will further couple into ω at the converter. p Frequency-disturbing current.

[0039] Apply voltage disturbances to the power grid, according to Figure 1 The system structure shown is used to establish a mathematical model of the main circuit of a grid-connected direct-drive wind turbine system in the frequency domain, including grid impedance. A small-signal model of the main circuit in the frequency domain is obtained through small-signal linearization. The small-signal model of the main circuit is as follows:

[0040]

[0041] in, and These represent disturbance signals for grid voltage, PCC point voltage, PCC point current, and inductor current, respectively. Indicates modulation coefficient perturbation; Z g Z L and Z C U represents the mains impedance matrix, the filter inductor impedance matrix, and the filter capacitor impedance matrix, respectively. dc This represents the steady-state matrix of DC voltage.

[0042] The DC voltage steady-state matrix U dc The expression is:

[0043]

[0044] Among them, U dc This indicates the intermediate DC side voltage of the direct-drive fan system.

[0045] It should be noted that substituting the Toeplitz steady-state harmonic matrix into the main circuit small-signal model can eliminate convolution operations, thus obtaining a simplified frequency domain small-signal model of the main circuit. The Toeplitz steady-state harmonic matrix is ​​a matrix of an existing type where all elements on each diagonal are identical.

[0046] In this embodiment, in step S2, according to Figure 2 The virtual synchronous control structure of the grid-side converter in the grid-connected system of the grid-type direct-drive wind turbine, as shown, yields an analytical expression for the relationship between the modulation coefficient and the disturbance signals of the grid connection point voltage, current, and inductor current: The analytical expression is:

[0047]

[0048] in, and represent PCC point voltage, PCC point current and inductance current disturbance signal respectively, represents modulation coefficient disturbance; Q, P and R are coefficient matrices.

[0049] The coefficient matrix Q can be solved according to the following method: the inductance current disturbance is subjected to current loop to obtain the modulation coefficient disturbance signal in dq coordinate system, and then subjected to dq-abc transformation to finally obtain A phase modulation coefficient disturbance signal Thus, the mathematical relationship between the inductance current disturbance and the modulation coefficient disturbance is obtained.

[0050] The coefficient matrix P can be solved according to the following method: the PCC voltage disturbance signal is subjected to active loop to form output phase disturbance, thereby affecting dq-abc transformation and abc-dq transformation, at this time, the steady-state modulation coefficient will be affected by the phase disturbance in dq-abc transformation; secondly, the steady-state current and the steady-state voltage of the grid-connected point will affect the modulation coefficient by acting on the phase disturbance signal in abc-dq transformation; finally, the disturbance voltage of the PCC point will affect the modulation coefficient by being subjected to voltage loop, and also cause the reference voltage to change by being subjected to reactive loop, thereby affecting the modulation coefficient.

[0051] The coefficient matrix R can be solved according to the following method: the PCC current disturbance signal is subjected to active loop to form output phase disturbance, thereby affecting dq-abc transformation and abc-dq transformation, at this time, the steady-state modulation coefficient will be affected by the phase disturbance in dq-abc transformation; secondly, the steady-state current and the steady-state voltage of the PCC point will affect the modulation coefficient by acting on the phase disturbance signal in abc-dq transformation; finally, the disturbance current of the PCC point will also affect the modulation coefficient by being subjected to reactive loop, thereby causing the reference voltage to change.

[0052] In this embodiment, in step S3, the main circuit small signal model is combined with the analytical expression to obtain the overall impedance matrix Z of the grid-connected system of the grid-connected direct-driven wind turbine acg ; the overall impedance matrix Z acg is:

[0053]

[0054] Wherein, E is a unit matrix, Y L is a filter inductance admittance matrix, that is, Y L -1 = Z L .

[0055] In this embodiment, in step S4, the overall impedance matrix Z of the system is used to obtain the overall impedance Z of the system at the disturbance frequency ω acg p acg ​​(ω p ), subtracting the grid impedance matrix Z g p corresponding to the impedance component Z g (ω p ), thereby obtaining the equivalent impedance Z ac_eq (ω p ) of the grid-connected direct-drive wind turbine, and the expression is:

[0056] Z ac_eq (ω p ) = Z acg (ω p ) - Z g (ω p ).

[0057] To verify the correctness of the modeling method in the present application, a simulation model of the grid-connected direct-drive wind turbine is built in Matlab / Simulink, and the frequency scanning result is compared with the impedance model for verification. Figure 3 The equivalent impedance scanning result of the grid-connected direct-drive wind turbine considering frequency coupling when the wind speed is 12 m / s is shown in the figure, the solid line is the equivalent impedance, and the "O" mark is the simulation scanning result. Figure 3 It can be seen that the equivalent impedance established according to the modeling method of the present application has high consistency with the simulation scanning result, which verifies the effectiveness and correctness of the modeling method of the present application.

[0058] Finally, it should be pointed out that the above embodiments are only used to illustrate the technical solutions of the present application and not to limit it, although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the purpose and scope of the technical solutions of the present application, and they should be covered in the scope of the claims of the present application.​

Claims

1. A method for impedance modeling of grid-connected direct-drive wind generators considering frequency coupling effects, characterized in that: The application relates to a method for analyzing the small-signal stability of a grid-connected direct-drive wind turbine system. The method comprises the following steps: ； wherein, , , and represent grid voltage, PCC point voltage, PCC point current and inductance current disturbance signal respectively, represents modulation coefficient disturbance; , and represent grid impedance matrix, filter inductance impedance matrix and filter capacitance impedance matrix respectively, represents DC voltage steady-state matrix; the expression of the DC voltage steady-state matrix is: ； wherein U dc represents the intermediate DC side voltage of the direct drive fan system; Applying a voltage disturbance to the power grid, constructing a main circuit small-signal model of a grid-connected direct-drive wind turbine system containing grid impedance in the frequency domain; the main circuit small-signal model is: The small signal model of the main circuit and the analytical expression are combined to obtain the overall impedance matrix of the grid-connected system of the grid-connected direct-driven wind turbine ; the overall impedance matrix is ； wherein is the identity matrix, is the filter inductance admittance matrix, i.e. ; , and are coefficient matrices; Using the global impedance matrix The perturbation frequency is obtained. Overall impedance , to the overall impedance Subtract the grid impedance matrix middle impedance components corresponding to frequency The equivalent impedance of the grid-type direct-drive wind turbine is obtained.

2. The impedance modeling method of networked direct-drive wind generators considering frequency coupling effects according to claim 1, characterized in that: Based on the multi-harmonic linearization theory, an analytical expression is obtained between the modulation coefficient disturbance and the voltage, current and inductance current disturbance signals at the grid-connected point; ； wherein, , and represent the PCC point voltage, PCC point current and inductance current disturbance signal, respectively, denotes the modulation coefficient disturbance; , and are coefficient matrices.

3. The impedance modeling method of networked direct-drive wind generators considering frequency coupling effects according to claim 1, characterized in that: The analytical expression is: The Toeplitz steady-state harmonic matrix is substituted into the main circuit small-signal model to eliminate the convolution operation.

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