Direct-driven fan system impedance modeling method considering frequency coupling

Abstract

The invention discloses a direct-driven fan system impedance modeling method considering frequency coupling, and the method comprises the steps: S1, obtaining a mathematical expression of a blade tip speed ratio and output power according to a wind turbine airflow model of a direct-driven fan; s2, according to equivalent circuit topology modeling under a dq coordinate system of the permanent magnet synchronous motor, obtaining an expression of electromagnetic torque and electromagnetic power; s3, according to the control structures of the machine-side converter and the grid-side converter of the direct-drive fan, modeling is carried out respectively, and an equivalent admittance mathematical expression of the machine-side converter and six disturbance admittance expressions in the direct-drive fan are obtained; and S4, based on the mathematical expressions of the output power, the electromagnetic power, the equivalent admittance and the disturbance admittance obtained in S1, S2 and S3, establishing the equivalent admittance / impedance of the direct-drive fan system. According to the method, multi-time-scale control and full-working-condition frequency domain port characteristics of the direct-driven fan system can be effectively represented, and the equivalent impedance model can accurately analyze the stability of the system.

Description

Technical Field

[0001] The present application relates to the field of new energy direct current power transmission, and in particular to a direct-drive wind turbine system impedance modeling method considering frequency coupling. Background Art

[0002] After the “dual carbon” goals were proposed, the power system dominated by generators gradually transformed into a power system dominated by new energy converters. In the past decade, the cumulative installed capacity of wind power has shown a rapid growth trend.

[0003] Since wind farms contain a large number of power electronic equipment, with the widespread application of these power electronic equipment, the problem of broadband oscillation in new energy bases has become increasingly prominent, seriously threatening the safe and stable operation of the power system and causing significant economic losses. Therefore, in order to reduce the oscillation risk of future high wind power penetration power systems, it is of great significance to reveal the broadband oscillation mechanism of wind turbines and conduct theoretical research on the oscillation law of direct-drive wind turbines connected to the grid.

[0004] Among the existing analysis methods for broadband oscillation problems in power electronic systems, the impedance method is an effective method. In summary, in order to more effectively and fully utilize energy, improve the level of wind power consumption, ensure the continuous and high-quality transmission of wind power to high-load areas, and transmit electricity at lower costs and line losses, an impedance modeling method for direct-drive wind turbine systems based on impedance analysis is urgently needed. Summary of the invention

[0005] The purpose of this application is to overcome the defects of the prior art and to disclose a method for impedance modeling of a direct-drive wind turbine system taking into account frequency coupling in order to overcome the time-varying characteristics of the converter and the problem that direct linearization is impossible in order to perform small signal modeling of the converter in a direct-drive wind turbine power generation system.

[0006] The purpose of this application is achieved through the following technical solutions:

[0007] A direct-drive fan system impedance modeling method considering frequency coupling, the direct-drive fan system impedance modeling method comprising:

[0008] S1: Based on the wind turbine airflow model of the direct-drive wind turbine, the mathematical expressions of the tip speed ratio and the output power are obtained;

[0009] S2: Based on the equivalent circuit topology modeling of the permanent magnet synchronous motor in the dq coordinate system, the expressions of electromagnetic torque and electromagnetic power are obtained;

[0010] S3: According to the control structure of the machine-side converter and the grid-side converter of the direct-drive wind turbine, the models are respectively constructed, and the equivalent admittance mathematical expression of the machine-side converter and the six disturbance admittance expressions in the direct-drive wind turbine are obtained;

[0011] S4: Based on the mathematical expressions of output power, electromagnetic power, equivalent admittance and disturbance admittance obtained by S1, S2 and S3, the equivalent admittance / impedance of the direct-drive wind turbine system is established.

[0012] According to a preferred implementation, in step S2, an equivalent circuit topology of the permanent magnet synchronous motor is established in the dq coordinate system, and the electromagnetic torque T is obtained based on the established equivalent circuit topology. e And the electromagnetic power P e The expression is as follows:

[0013]

[0014] P e =ω e T e

[0015] Among them, L sd and L sq Respectively represent the dq axis stator inductance, i sd 、i sq They represent the dq axis stator current, ψ f is the magnetic flux, ω e is the rotor electrical angular velocity, n p is the number of pole pairs of permanent magnet synchronous motor.

[0016] According to a preferred implementation, in step S3, based on the rotor flux orientation speed-current dual closed-loop control, in the motor control, the d-axis output current is 0, that is, I sd =0, so that the q-axis output current is linearly related to the electromagnetic torque, so that the electromagnetic torque can be controlled by controlling the output value of the q-axis current;

[0017] The steady-state value of the modulation coefficient in the dq coordinate system is:

[0018]

[0019] Among them, M sd 、M sq is the steady-state value of the modulation coefficient, I sq ,I sd is the steady-state value of the dq-axis stator current, V dc is the DC side steady-state voltage, ω opt is the angular velocity of the wind turbine at rated operation, n p is the number of pole pairs of the permanent magnet synchronous motor, R s is the stator resistance;

[0020] After the frequency domain signal is linearized, the outer loop speed is a constant, and only the influence of the inner current loop on the impedance output characteristics is considered. At this time, the expressions of the d-axis and q-axis modulation coefficients are as follows:

[0021]

[0022] Among them, m sd (ω p )、m sq (ω p ) is the modulation coefficient in the dq axis coordinate system, V dc0 is the steady-state value of DC voltage, H sid (ω p ), H siq (ω p ) are the d-axis and q-axis current loop transfer functions, K sid , K siq is the current loop dq decoupling coefficient, ω p is the disturbance angular frequency.

[0023] According to a preferred implementation, the synchronous motor and the converter are taken as a front-stage whole, and the output of the corresponding front-stage whole is the voltage and current on the DC side. Then, the equivalent admittance is the ratio of the DC current disturbance to the DC voltage disturbance output on the DC side, so that the equivalent admittance Y of the machine-side converter can be obtained. ms The specific mathematical expression is:

[0024]

[0025] Among them, M sd 、M sq is the steady-state value of the modulation coefficient, R s is the stator winding resistance, L sd and L sq They represent the dq axis stator inductance respectively.

[0026] According to a preferred embodiment, when the AC side voltage is ω p When the voltage disturbance of frequency is caused, the grid connection point will have a frequency of ω p and 2ω0-ω p The disturbance current response is ω. p -ω0 disturbance current;

[0027] When there is ω on the DC side p When the voltage disturbance of the frequency is caused, it will also cause current disturbance of two frequencies on the AC side.

[0028] Therefore, the six disturbance admittance expressions in the direct-drive wind turbine are:

[0029]

[0030] Among them, Y ac1_PMSG (ω p ), Y ac3_PMSG(ω p ) respectively represent the introduction of ω at the grid connection point p When the voltage is disturbed by frequency, ω p ,ω p The response current on the AC side at -2ω0 frequency and the corresponding system admittance under the response current on the DC side; Y dc1_PMSG (ω p ), Y dc3_PMSG (ω p ) respectively represent the introduction of ω on the DC side p When the voltage is disturbed by frequency, ω p +ω0、ω p -ω0 frequency AC side response current and DC side response current corresponding to the system admittance; M dq0 ,I dq0 is the steady-state modulation coefficient and steady-state output current value under the steady-state dq axis, V dc0 is the steady-state voltage value of the DC side, H v is the GSC DC voltage controller, H gi is the GSC current controller;

[0031] T pll is the closed-loop transfer function of the phase-locked loop, Y C (ω p )=jω p C dc is the admittance of the DC bus capacitor, Z mdc (ω p ) is the equivalent impedance of the machine-side converter and DC capacitor, Y ms It is the admittance corresponding to the bus capacitance on the DC side of the direct-drive wind turbine converter.

[0032] According to a preferred implementation, in step S4, the self-admittance Y of the direct-drive wind turbine generator system PMSG_sef (ω p ) and the adjoint admittance Y PMSG_aef (ω p )for:

[0033]

[0034] Among them, k wt is the transformer ratio, Y sef_1 (ω p ), Y aef_1 (ω p ) are the self-admittance and adjoint admittance of the direct-drive fan, Y hf (ω p ) is the admittance of the filter;

[0035] According to the transfer relationship between equivalent impedance and self-admittance, adjoint admittance and grid impedance, the equivalent admittance Y of the direct-drive wind turbine can be obtained: PMSG_inv / Impedance Z PMSG_inv as follows:

[0036]

[0037] Among them, Y PMSG_sef (ω p ) is the self-admittance of the direct-drive wind turbine, Y PMSG_aef (ω p ) is the accompanying admittance of the direct-drive wind turbine, Represents Y PMSG_aef The conjugate of Y PMSG_sef The conjugate of Z s is the grid impedance, Z s The conjugation of.

[0038] The aforementioned main scheme of the present application and its further options can be freely combined to form multiple schemes, all of which are schemes that can be adopted and claimed for protection in the present application. After understanding the scheme of the present application, those skilled in the art can understand that there are multiple combinations based on the prior art and common knowledge, all of which are technical schemes to be protected by the present application, and they are not exhaustively listed here.

[0039] Beneficial effects of this application:

[0040] The impedance modeling method of the direct-drive wind turbine system in the present application is an impedance modeling method of the direct-drive wind turbine that takes into account the mathematical models of the wind turbine, permanent magnet synchronous motor, machine-side converter and grid-side converter. It can analyze the factors affecting the safety and stability of the direct-drive wind turbine grid-connected system, determine the safe and stable operating range of the system, and based on this, can put forward effective operating suggestions for ensuring the safety and stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a flow chart of the impedance modeling method of the present invention;

[0042] Figure 2 It is a basic structural diagram of the direct-drive wind turbine generator set of the present invention;

[0043] Figure 3 is a dq axis equivalent circuit diagram of the permanent magnet synchronous generator of the present invention;

[0044] Figure 4 This is a control structure diagram of the direct-drive wind turbine side converter of the present invention;

[0045] Figure 5 is a diagram showing the influence of the transformer and filter of the present invention on the output admittance;

[0046] Figure 6 It is the relationship between the voltage disturbance of the present invention and the current response of AC and DC at different frequencies. DETAILED DESCRIPTION

[0047] The following describes the embodiments of the present application through specific examples, and those skilled in the art can easily understand other advantages and effects of the present application from the contents disclosed in this specification. The present application can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present application. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0048] refer to Figure 1 and Figure 2 As shown, the present application discloses a direct-drive wind turbine system impedance modeling method considering frequency coupling, comprising the following steps:

[0049] Step S1. According to an ideal wind turbine airflow model of a direct-drive wind turbine, mathematical expressions of the tip speed ratio and the output power are obtained.

[0050] Step S2. Based on the equivalent circuit topology modeling of the permanent magnet synchronous motor in the dq coordinate system, the expressions of electromagnetic torque and electromagnetic power are obtained.

[0051] Step S3. Model the direct-drive wind turbine machine-side rectifier rotor flux oriented speed-current dual closed-loop control control structure and the grid-side converter control structure respectively to obtain the equivalent admittance mathematical expression of the machine-side rectifier and the six disturbance admittance expressions in the direct-drive wind turbine.

[0052] Step S4. Use the mathematical expressions of output power, equivalent admittance, and disturbance admittance obtained by S1, S2, and S3 to finally establish the equivalent admittance / impedance of the direct-drive wind turbine system.

[0053] Preferably, in step S2, an equivalent circuit topology of the permanent magnet synchronous motor is established in the dq coordinate system, such as Figure 3 , so the expressions of electromagnetic torque and electromagnetic power are as follows:

[0054]

[0055] P e =ω e T e

[0056] Among them, L sd and L sq Respectively represent the dq axis stator inductance, i sd 、i sq They represent the dq axis stator current, ψf is the magnetic flux, ω e is the rotor electrical angular velocity, n p is the number of pole pairs of the permanent magnet synchronous motor, T e , P e are the electromagnetic torque and electromagnetic power of the permanent magnet synchronous motor respectively. The power loss of the mechanical part in the transmission process is ignored here, and it is approximately assumed that the kinetic energy captured by the wind turbine is equal to the electromagnetic power output by the motor.

[0057] Preferably, in step S3, the mathematical expressions of the circuit voltage and current under different shaft systems are as follows:

[0058]

[0059] Among them, R s is the stator winding resistance, u sd 、u sq Represent the dq axis stator voltage, L sd and L sq Respectively represent the dq axis stator inductance, i sd 、i sq Respectively represent the dq axis stator current, ω e and ω m They are electrical angular velocity and mechanical angular velocity respectively, and the relationship between them is ω e =n p ω m , where n p is the number of pole pairs of the permanent magnet synchronous motor, ψ f For magnetic link.

[0060] Furthermore, based on the rotor flux orientation speed-current dual closed-loop control, since in the motor control, the d-axis output current is 0, that is, I sd =0, so the q-axis output current is linearly related to the electromagnetic torque. The electromagnetic torque can be controlled by controlling the output value of the q-axis current. The specific control structure diagram is as follows: Figure 4 The steady-state value of the modulation coefficient in the dq coordinate system is:

[0061]

[0062] Among them, M sd 、M sq is the steady-state value of the modulation coefficient, I sq ,I sd is the steady-state value of the dq-axis stator current, V dc is the DC side steady-state voltage, ω opt is the angular velocity of the wind turbine at rated operation, n p is the number of pole pairs of the permanent magnet synchronous motor, R s is the stator resistance.

[0063] After the frequency domain small signal is linearized, the outer loop speed can be approximately considered as a constant, and only the influence of the inner current loop on the impedance output characteristics is considered. At this time, the expressions of the d-axis and q-axis modulation coefficients are as follows:

[0064]

[0065] Among them, m sd (ω p )、m sq (ω p ) is the modulation coefficient in the dq axis coordinate system; V dc0 is the steady-state value of DC voltage, H sid (ω p ), H siq (ω p ) are the d-axis and q-axis current loop transfer functions, K sid , K siq is the current loop dq decoupling coefficient, ω p is the disturbance angular frequency.

[0066] The synchronous motor and the rectifier are regarded as a front-stage whole, the output of which is the voltage and current on the DC side. Therefore, the equivalent admittance is the ratio of the DC current disturbance to the DC voltage disturbance output on the DC side. Combining the above formula, the specific mathematical expression of the equivalent admittance can be obtained as follows:

[0067]

[0068] Among them, M sd 、M sq is the steady-state value of the modulation coefficient, K sid , K siq is the current loop dq decoupling coefficient, R s is the stator winding resistance, L sd and L sq Respectively represent the dq axis stator inductance, I sq ,I sd is the steady-state value of the dq-axis stator current, H sid (ω p ), H siq (ω p ) are the d-axis and q-axis current loop transfer functions, ω p is the disturbance angular frequency.

[0069] Furthermore, when the grid voltage has ω p When the voltage disturbance of frequency is caused, the grid connection point will have a frequency of ω p and 2ω0-ω p The disturbance current response is ω. p -ω0 disturbance current; when there is ω on the DC side pWhen the voltage disturbance of the frequency is high, it will also bring two-frequency current disturbance to the AC side. Therefore, the six disturbance admittance expressions in the direct-drive wind turbine are:

[0070]

[0071] Among them, Y ac1_PMSG (ω p ), Y ac2_PMSG (ω p ), Y ac3_PMSG (ω p ) are introduced at the grid connection point p When the voltage is disturbed by frequency, ω p ,ω p The response current on the AC side at -2ω0 frequency and the corresponding system admittance under the response current on the DC side; Y ac1_PMSG (ω p ), Y dc2_PMSG (ω p ), Y ac3_PMSG (ω p ) respectively represent the introduction of ω on the DC side p When the voltage is disturbed by frequency, ω p +ω0、ω p -ω0 frequency AC side response current and DC side response current corresponding to the system admittance; M dq0 ,I dq0 is the steady-state modulation coefficient and steady-state output current value under the steady-state dq axis, V dc0 is the steady-state voltage value of the DC side, H v is the GSC DC voltage controller, H gi is the GSC current controller; T pll is the closed-loop transfer function of the phase-locked loop, Y C (ω p )=jω p C dc is the admittance of the DC bus capacitor, ω p is the disturbance frequency, ω0 is the fundamental frequency, and j is the imaginary unit. mdc (ω p ) is the equivalent impedance of the machine-side converter and DC capacitor, and its expression is:

[0072]

[0073] where Y C (ω p ) is the admittance of the DC bus capacitor, Y ms It is the admittance corresponding to the bus capacitance on the DC side of the direct-drive wind turbine converter.

[0074] Furthermore, for the grid-side converter of the direct-drive wind turbine, there is a certain coupling relationship between the voltage disturbance and the current disturbance of different frequencies of AC and DC. Considering that there are high-pass filters and box transformers at the grid connection point in the actual direct-drive wind turbine, these two have a significant impact on the output impedance of the direct-drive wind turbine, so they cannot be ignored. Figure 5 , at this time the self-admittance and adjoint admittance of the direct-drive wind turbine are:

[0075]

[0076]

[0077]

[0078]

[0079] Among them, Y PMSG_sef (ω p ) is the self-admittance of the direct-drive wind turbine, Y PMSG_aef (ω p ) is the accompanying admittance of the direct-drive wind turbine, k wt is the transformer ratio, Y sef_1 (ω p ), Y aef_1 (ω p ) are the self-admittance and adjoint admittance of the direct-drive fan, Y hf (ω p ) is the filter admittance, ω p is the disturbance frequency.

[0080] According to the transfer relationship between equivalent impedance and self-admittance, adjoint admittance, and grid impedance, such as Figure 6 , the equivalent admittance / impedance of the direct-drive fan is as follows:

[0081]

[0082]

[0083] Among them, Y PMSG_inv (ω p ), Y PMSG_inv (ω p ) are the equivalent admittance and equivalent impedance of the direct-drive fan, Y PMSG_sef (ω p ) is the self-admittance of the direct-drive wind turbine, Y PMSG_aef (ω p ) is the adjoint admittance of the direct-drive wind turbine, * indicates conjugate, Z s is the grid impedance, ω p is the disturbance frequency.

[0084] The present invention comprehensively considers that the small signal modeling of the converter in the direct-drive wind turbine system must overcome the time-varying characteristics of the converter and the asymmetry of the converter dq axis, and proposes a direct-drive wind turbine system impedance modeling method considering frequency coupling. The impedance model provides a model basis for the stability analysis of the interconnection between the wind power system and the new energy system. It can analyze the safety and stability influencing factors of the direct-drive wind turbine grid-connected system, determine the safe and stable operation range of the system, and can propose effective operation suggestions for ensuring the safety and stability of the system.

[0085] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A direct-drive wind turbine system impedance modeling method considering frequency coupling, characterized in that: The direct-drive wind turbine system impedance modeling method comprises: S1: Based on the wind turbine airflow model of the direct-drive wind turbine, the mathematical expressions of the tip speed ratio and the output power are obtained; S2: Based on the equivalent circuit topology modeling of the permanent magnet synchronous motor in the dq coordinate system, the expressions of electromagnetic torque and electromagnetic power are obtained; S3: According to the control structure of the machine-side converter and the grid-side converter of the direct-drive wind turbine, the models are respectively constructed, and the equivalent admittance mathematical expression of the machine-side converter and the six disturbance admittance expressions in the direct-drive wind turbine are obtained; S4: Based on the mathematical expressions of output power, electromagnetic power, equivalent admittance and disturbance admittance obtained by S1, S2 and S3, the equivalent admittance / impedance of the direct-drive wind turbine system is established.

2. The direct-drive wind turbine system impedance modeling method according to claim 1, characterized in that: In step S2, an equivalent circuit topology of the permanent magnet synchronous motor is established in the dq coordinate system, and the electromagnetic torque T is obtained based on the established equivalent circuit topology. e And the electromagnetic power P e The expression is as follows: P e =ω e T e Among them, L sd and L sq Respectively represent the dq axis stator inductance, i sd 、i sq They represent the dq axis stator current, ψ f is the magnetic flux, ω e is the rotor electrical angular velocity, n p is the number of pole pairs of permanent magnet synchronous motor.

3. The direct-drive wind turbine system impedance modeling method according to claim 2, characterized in that: In step S3, the speed-current dual closed-loop control based on the rotor flux orientation is performed. In the motor control, the d-axis output current is 0, that is, I sd =0, so that the q-axis output current is linearly related to the electromagnetic torque, so that the electromagnetic torque can be controlled by controlling the output value of the q-axis current; The steady-state value of the modulation coefficient in the dq coordinate system is: Among them, M sd 、M sq is the steady-state value of the modulation coefficient, I sq ,I sd is the steady-state value of the dq-axis stator current, V dc is the DC side steady-state voltage, ω opt is the angular velocity of the wind turbine at rated operation, n p is the number of pole pairs of the permanent magnet synchronous motor, R s is the stator resistance; After the frequency domain signal is linearized, the outer loop speed is a constant, and only the influence of the inner current loop on the impedance output characteristics is considered. At this time, the expressions of the d-axis and q-axis modulation coefficients are as follows: Among them, m sd (ω p )、m sq (ω p ) is the modulation coefficient in the dq axis coordinate system, V dc0 is the steady-state value of DC voltage, H sid (ω p ), H siq (ω p ) are the d-axis and q-axis current loop transfer functions, K sid , K siq is the current loop dq decoupling coefficient, ω p is the disturbance angular frequency.

4. The impedance modeling method of a direct-drive wind turbine system according to claim 3, characterized in that: The synchronous motor and converter are regarded as the front-end whole, and the output of the corresponding front-end whole is the voltage and current on the DC side. The equivalent admittance is the ratio of the DC current disturbance to the DC voltage disturbance output on the DC side, so the equivalent admittance Y of the machine-side converter can be obtained: ms The specific mathematical expression is: Among them, M sd 、M sq is the steady-state value of the modulation coefficient, R s is the stator winding resistance, L sd and L sq They represent the dq axis stator inductance respectively.

5. The direct-drive wind turbine system impedance modeling method according to claim 4, characterized in that: When the AC side voltage exists p When the voltage disturbance of frequency is caused, the grid connection point will have a frequency of ω p and 2ω0-ω p The disturbance current response is ω. p -ω0 disturbance current; When there is ω on the DC side p When the voltage disturbance of the frequency is caused, it will also cause current disturbance of two frequencies on the AC side. Therefore, the six disturbance admittance expressions in the direct-drive wind turbine are: Among them, Y ac1_PMSG (ω o ), Y ac3_PMSG (ω p ) respectively represent the introduction of ω at the grid connection point p When the voltage is disturbed by frequency, ω p ,ω p The response current on the AC side at -2ω0 frequency and the corresponding system admittance under the response current on the DC side; Y dc1_PMSG (ω p ), Y dc3_PMSG (ω p ) respectively represent the introduction of ω on the DC side p When the voltage is disturbed by frequency, ω p +ω0、ω p -ω0 frequency AC side response current and DC side response current corresponding to the system admittance; M dq0 ,I dq0 is the steady-state modulation coefficient and steady-state output current value under the steady-state dq axis, V dc0 is the steady-state voltage value of the DC side, H v is the GSC DC voltage controller, H gi is the GSC current controller; T pll is the closed-loop transfer function of the phase-locked loop, Y C (ω p )=jω p C dc is the admittance of the DC bus capacitor, Z mdc (ω p ) is the equivalent impedance of the machine-side converter and DC capacitor, Y ms It is the admittance corresponding to the bus capacitance on the DC side of the direct-drive wind turbine converter.

6. The direct-drive wind turbine system impedance modeling method according to claim 5, characterized in that: In step S4, the self-admittance Y of the direct-drive wind turbine PMSG_sef (ω p ) and the adjoint admittance Y PMSG_aef (ω p )for: Among them, k wt is the transformer ratio, Y sef_1 (ω p ), Y aef_1 (ω p ) are the self-admittance and adjoint admittance of the direct-drive fan, Y hf (ω p ) is the admittance of the filter; According to the transfer relationship between equivalent impedance and self-admittance, adjoint admittance and grid impedance, the equivalent admittance Y of the direct-drive wind turbine can be obtained: PMSG_inv / Impedance Z PMSG_inv as follows: Among them, Y PMSG_sef (ω p ) is the self-admittance of the direct-drive wind turbine, Y PMSG_aef (ω p ) is the accompanying admittance of the direct-drive wind turbine, Represents Y PMSG_aef The conjugate of Y PMSG_sef The conjugate of Z s is the grid impedance, Z s The conjugation of.

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