Impedance remodeling method for doubly-fed fan

Abstract

The invention provides an impedance remodeling method for a doubly-fed fan, a machine-side converter and a grid-side converter of the doubly-fed fan both adopt a VSG control strategy, the doubly-fed fan comprises an active power error path, and the impedance remodeling method comprises the following steps: adding an impedance remodeling controller in the active power error path, the impedance remodeling controller comprises a wave trap and a filtering assembly, and the wave trap and the filtering assembly are connected in series; carrying out primary remodeling on the impedance through a wave trap to obtain first impedance; the first impedance is remodeled by the filtering assembly to obtain a second impedance. The technical problem that grid-connected broadband oscillation exists in a doubly-fed fan with a VSG control strategy adopted by a machine-side converter and a grid-side converter is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to an impedance reshaping method for a doubly-fed wind turbine. Background Art

[0002] A large number of renewable energy power generation equipment are connected to the power grid. Due to the differences in the geographical distribution of natural resources in China and the imbalance in economic development between regions, renewable energy power generation often needs to be transmitted over long distances. In actual projects, transmission lines use series compensation capacitors to compensate for reactance, improve system voltage, and reduce power loss. However, series compensation transmission lines and renewable energy power generation stations often experience interactive oscillation, causing grid failures.

[0003] Both the machine-side and grid-side converters adopt the dual virtual synchronous machine control structure of the grid-connected doubly-fed wind turbine with virtual synchronous machines. It can not only provide inertia and frequency support when wind power is connected to the grid, but also avoid the influence of the phase-locked loop on the grid-connected stability of the system, solving the problem of low system inertia and negative resistance effect easily caused by the phase-locked loop controller.

[0004] However, in the actual implementation process, there is such a problem: the doubly-fed wind turbine that adopts VSG control strategy for both the machine-side and grid-side converters has a broadband oscillation problem when connected to the grid. Summary of the invention

[0005] The present invention solves the technical problem that a double-fed wind turbine in which both the machine-side and the grid-side converters adopt a VSG control strategy has a wide-frequency oscillation when connected to the grid.

[0006] To solve the above problems, the present invention provides an impedance reshaping method for a doubly fed wind turbine. Both the machine-side converter and the grid-side converter of the doubly fed wind turbine adopt the VSG control strategy. The doubly fed wind turbine includes an active power error path. The impedance reshaping method includes: adding an impedance reshaping controller in the active power error path, wherein the impedance reshaping controller includes a notch filter and a filter component, and the notch filter and the filter component are connected in series; the impedance is reshaped once by the notch filter to obtain a first impedance; and the first impedance is reshaped by the filter component to obtain a second impedance.

[0007] Compared with the existing technology, the technical effect achieved by adopting this technical solution is as follows: the phase change amplitude of the 50Hz frequency point on the impedance Bode diagram of the doubly fed wind turbine using the VSG control strategy is large, so the negative damping frequency band around 50Hz is reshaped by adding a notch filter with a frequency of 50Hz; after adding the notch filter, the resonance at the 50Hz frequency point is suppressed, but the negative damping interval still exists, and the phase in some intervals is still greater than 90°, and the impedance of the doubly fed wind turbine under the control of the dual VSG needs to be reshaped. The negative damping frequency band of the doubly fed wind turbine impedance that needs to be reshaped is about 8Hz-50Hz. By adding a filter component in the active power error path, the frequency band that still has negative damping characteristics after adding the notch filter is further reshaped. In view of the broadband oscillation problem of the grid-connected doubly fed wind turbine controlled by the dual VSG, the impedance is reshaped to make it present positive damping characteristics in the full frequency band to avoid broadband oscillation caused by its interaction with the grid impedance. In terms of the selection of the reshaping path, a notch filter is first added to the VSG control structure to reshape the impedance of the doubly fed wind turbine. Since the impedance characteristics of the grid-side converter have little effect on the impedance of the doubly fed wind turbine, only the machine-side part is considered. Since the impedance reshaping only considers the impedance characteristics of the filter and does not consider the filtering characteristics of the filter, in the active power error path, the active power error signal during stable operation is close to 0, and will not affect the regulation process in the initial grid-connected stage.

[0008] In one example of the present invention, the filter component includes: a first band-stop filter, a second band-stop filter and a third band-stop filter connected in series in sequence.

[0009] Compared with the existing technology, the technical effect achieved by adopting this technical solution is: the negative damping frequency band of the doubly fed wind turbine impedance that needs to be reshaped is approximately 8Hz-50Hz, and the added impedance reshaping controller is required to provide damping for a specific frequency band while having little impact on the transmission signals in other frequency bands. Therefore, it is considered to add a filtering component as a series structure of three band-stop filters.

[0010] In one embodiment of the present invention, the transfer function of the notch filter is: ;in, is the notch filter notch frequency, is the notch filter damping coefficient.

[0011] In one embodiment of the present invention, the notch frequency of the notch filter is 50 Hz; and the damping coefficient of the notch filter is 0.5.

[0012] In one embodiment of the present invention, the first filter transfer function of the first band-stop filter is: ; The second filter transfer function of the second band-stop filter is: The third filter transfer function of the third band-stop filter is: ;in, is the first filter coefficient, is the second filter coefficient, is the third filter coefficient, ; is the first damping coefficient, is the second damping coefficient, is the third damping coefficient, ; is the first center frequency, is the second center frequency, is the third center frequency, .

[0013] In one example of the present invention, the first filter coefficient is 0.3, the second filter coefficient is 0.3, and the third filter coefficient is 0.5; the first damping coefficient is 1.2, the second damping coefficient is 1, and the third damping coefficient is 1; the first center frequency is 2×π×25, the second center frequency is 2×π×50, and the third center frequency is 2×π×100.

[0014] In one example of the present invention, the impedance reshaping method further includes: verifying the effectiveness of the impedance reshaping controller by using simulation software.

[0015] In one example of the present invention, the machine-side converter collects a first voltage and a first current at an asynchronous motor port; calculates a first output active power and a first reactive power of a computer-side part through the first voltage and the first current; uses the first output active power and the first reactive power as a first control signal of the machine-side converter; and calculates a reference angle and a rotation angle difference between a stator and a rotor according to a VSG control strategy to obtain a reference angle of a PWM modulation part at a grid-side converter.

[0016] In one example of the present invention, the grid-side converter collects the second voltage and the second current of the asynchronous motor port; calculates the second output active power and the second reactive power of the grid-side part through the second voltage and the second current; and uses the second output active power and the second reactive power as the second control signal of the grid-side converter.

[0017] After adopting the technical solution of the present invention, the following technical effects can be achieved: (1) To address the broadband oscillation problem of the grid-connected doubly-fed wind turbine controlled by dual VSGs, the impedance is reshaped to make it present positive damping characteristics in the full frequency band, thereby avoiding broadband oscillation caused by its interaction with the grid impedance. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A flowchart of an impedance reshaping method for a doubly-fed wind turbine provided in Embodiment 1 of the present invention; Figure 2 This is the grid-connected system structure diagram of the doubly-fed wind turbine; Figure 3The equivalent impedance model of the doubly-fed wind turbine series-compensated system connected to the power grid; Figure 4 This is the impedance Bode diagram of the doubly-fed wind turbine in the traditional case; Figure 5 This is the VSG control block diagram after adding the notch filter; Figure 6 The impedance Bode diagram of the doubly-fed wind turbine after adding the notch filter; Figure 7 Bode diagram of the impedance of the added filter component; Figure 8 The impedance Bode diagram of the doubly-fed wind turbine after adding the impedance reshaping controller; Fig. 9 This is the simulation waveform. DETAILED DESCRIPTION

[0019] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] [Example 1] See also Figure 1 The present invention provides an impedance reshaping method for a doubly-fed wind turbine. The machine-side converter and the grid-side converter of the doubly-fed wind turbine both adopt a VSG control strategy. The doubly-fed wind turbine includes an active power error path. The impedance reshaping method includes: Step S100: adding an impedance reshaping controller in the active power error path, wherein the impedance reshaping controller includes a notch filter and a filter component, and the notch filter and the filter component are connected in series; Step S200: reshape the impedance once by using a notch filter to obtain a first impedance; Step S300: reshape the first impedance by a filtering component to obtain a second impedance.

[0021] In a specific embodiment, both the machine-side converter and the grid-side converter of the doubly-fed wind turbine adopt the VSG control strategy, that is, the doubly-fed wind turbine in this embodiment is a double-VSG-controlled doubly-fed wind turbine. Figure 2 This is the grid-connected system structure diagram of the double-fed wind turbine controlled by dual VSG. Figure 3 In , and R are the reactance, series compensation capacitor reactance and resistance of the transmission line respectively, and , It is connected in series with R to form a series compensation circuit.

[0022] The impedance Bode diagram characteristics of the doubly fed wind turbine using the VSG control strategy are as follows: Figure 4 As shown, it can be observed that the phase change amplitude at the 50Hz frequency point is large, so a notch filter with a frequency of 50Hz is added to reshape the negative damping frequency band around 50Hz; see Figure 6 As shown by the green curve in , after adding the notch filter, the resonance at the 50Hz frequency point is suppressed, but the negative damping range still exists, and the phase in some ranges is still greater than 90°. It is also necessary to reshape the impedance of the doubly fed wind turbine under dual VSG control. The negative damping frequency band of the doubly fed wind turbine impedance that needs to be reshaped is about 8Hz-50Hz. By adding a filter component in the active power error path, the frequency band that still has negative damping characteristics after adding the notch filter can be further reshaped. In view of the broadband oscillation problem of the grid-connected doubly fed wind turbine controlled by dual VSG, the impedance is reshaped to make it present positive damping characteristics in the full frequency band to avoid broadband oscillation caused by its interaction with the grid impedance.

[0023] Specifically, in the selection of the reshaping path, a notch filter is first added to the VSG control structure to reshape the impedance of the doubly fed wind turbine. Since the impedance characteristics of the grid-side converter have little effect on the impedance of the doubly fed wind turbine, only the machine-side part is considered. Since the impedance reshaping only considers the impedance characteristics of the filter and does not consider the filtering characteristics of the filter, in the active power error path, the active power error signal during stable operation is close to 0, and will not affect the regulation process in the initial grid-connected stage. Therefore, if Figure 5 As shown, when selecting the paths of the filter components in the generator-side and grid-side converter VSG control structures, the filter components are added to the active power error path.

[0024] Furthermore, the filter component includes: a first band-stop filter, a second band-stop filter and a third band-stop filter which are connected in series in sequence.

[0025] Specifically, the negative damping frequency band of the doubly-fed wind turbine impedance that needs to be reshaped is approximately 8Hz-50Hz. The added impedance reshaping controller is required to provide damping for a specific frequency band while having little impact on the transmission signals in other frequency bands. Therefore, it is considered to add a filtering component as a series structure of three band-stop filters.

[0026] Furthermore, the transfer function of the notch filter is: ;in, is the notch filter notch frequency, is the notch filter damping coefficient.

[0027] Specifically, after adding the notch filter, the control angle for: ; After adding small signal disturbance, VSG control angle small signal The expression is: ; Doubly fed wind turbine impedance The expression is: ; In the formula, , , , , , ; make and , then the impedance of the doubly fed wind turbine is In the expression , .

[0028] in, is the control angle, is the machine-side VSG control transfer function, is the fundamental frequency; is the VSG parameter damping coefficient, is the VSG parameter damping coefficient on the machine side, is the VSG parameter damping coefficient on the grid side; is the notch filter transfer function; is the given value of active power, is the given value of the active power on the machine side, is the grid-side active power given value; is the actual value of active power, is the actual value of the machine side active power, is the actual value of grid-side active power; is the control angle after adding small signal disturbance; is the small signal disturbance of active power.

[0029] is the positive sequence disturbance voltage amplitude, is the positive sequence disturbance current amplitude; is the frequency domain voltage ratio, It represents a simplified expression and has no physical meaning; is the terminal voltage amplitude, is the output current amplitude; s is the basic circuit parameter, j is the basic circuit parameter, is the fundamental frequency; Control PWM modulation wave amplitude for the machine-side VSG; for In exponential form, It is a term generated in the calculation and has no physical meaning. is the VSG control power angle, is the output current angle; is the rotor angular frequency; and is the frequency domain expression of the converter terminal voltage and the grid connection point voltage, is the frequency domain expression of the excitation inductance, and are the frequency domain expressions of stator and rotor winding leakage inductance, respectively. , , , and is the imputed value; PWM modulation wave amplitude; To control the delay frequency domain expression, To control signal delay; and are the cutoff angular frequencies of the low-pass filters for voltage and current signals respectively; is the rotation angle difference between the stator and rotor; J is the VSG parameter moment of inertia, is the VSG parameter moment of inertia on the machine side, is the VSG parameter moment of inertia on the grid side; is the VSG parameter damping coefficient, is the VSG parameter damping coefficient on the machine side, is the VSG parameter damping coefficient on the grid side. The impedance Bode diagram of the double-fed wind turbine controlled by dual VSG after the VSG control transfer function is reshaped is shown in Figure 6 As shown in the figure, it can be seen that the resonance characteristics at the 50Hz frequency point are suppressed.

[0030] Preferably, the notch frequency of the notch filter is 50 Hz; and the damping coefficient of the notch filter is 0.5.

[0031] Furthermore, the first filter transfer function of the first band-stop filter is: ; The second filter transfer function of the second band-stop filter is: The third filter transfer function of the third band-stop filter is: ;in, is the first filter coefficient, is the second filter coefficient, is the third filter coefficient, ; is the first damping coefficient, is the second damping coefficient, is the third damping coefficient, ; is the first center frequency, is the second center frequency, is the third center frequency, .

[0032] Specifically, Figure 6As shown in the figure, after adding the notch filter, the resonance at the 50Hz frequency point is suppressed, but the negative damping range still exists, and the phase in some ranges is still greater than 90°. It is also necessary to reshape the impedance of the doubly fed wind turbine under dual VSG control. The negative damping frequency band of the doubly fed wind turbine impedance that needs to be reshaped is approximately 8Hz-50Hz. The added impedance reshaping controller is required to provide damping for a specific frequency band while having little effect on the transmission signals in other frequency bands. Therefore, it is considered to add a filter component as a series structure of three band-stop filters, and the expressions of the band-stop filters are respectively , and .

[0033] The impedance Bode diagram of the filter components added in stage 2 is as follows: Figure 7 The notch filter only has an attenuation effect on the transmission signal of a certain frequency band. The impedance of the doubly fed wind turbine after adding the notch filter and filtering components is as follows: Figure 8 As shown, the blue curve is the impedance of the dual-VSG controlled doubly fed wind turbine after reshaping in the first stage (that is, the Bode diagram of the impedance of the dual-VSG controlled doubly fed wind turbine after only adding a notch filter), and the green curve is the impedance of the dual-VSG controlled doubly fed wind turbine after reshaping in the second stage (that is, the Bode diagram of the impedance of the dual-VSG controlled doubly fed wind turbine after adding a notch filter and a filter component). After two stages of reshaping, the impedance of the dual-VSG controlled doubly fed wind turbine presents a positive damping characteristic in the entire frequency band.

[0034] Preferably, the first filter coefficient is 0.3, the second filter coefficient is 0.3, and the third filter coefficient is 0.5; the first damping coefficient is 1.2, the second damping coefficient is 1, and the third damping coefficient is 1; the first center frequency is 2×π×25, the second center frequency is 2×π×50, and the third center frequency is 2×π×100.

[0035] Furthermore, the impedance reshaping method also includes: verifying the effectiveness of the impedance reshaping controller through simulation software.

[0036] Specifically, the effectiveness of the impedance reshaping strategy was verified through MATLAB software simulation. Fig. 9 As shown, the double-fed wind turbine is connected to the grid at 0 seconds of simulation, and the series compensation device with a series compensation degree of 10 is put into use at 1 second of simulation (that is, Figure 3 For the series compensation line in the simulation), the impedance reshaping controller is activated at 1.4 seconds, and the grid-connected current returns to a stable state after adjustment, verifying the effectiveness of the impedance reshaping strategy.

[0037] Furthermore, the machine-side converter collects a first voltage and a first current of the asynchronous motor port; A first output active power and a first reactive power of the computer side portion are calculated by a first voltage and a first current; Using the first output active power and the first reactive power as a first control signal of the machine-side converter; According to the VSG control strategy, the reference angle and the rotation angle difference between the stator and the rotor are calculated to obtain the reference angle of the PWM modulation part at the grid-side converter end.

[0038] Furthermore, the grid-side converter collects a second voltage and a second current at the asynchronous motor port; Calculate the second output active power and the second reactive power of the grid side part by the second voltage and the second current; The second output active power and the second reactive power are used as second control signals of the grid-side converter.

[0039] Specifically, the machine-side converter collects the first voltage and the first current of the asynchronous motor port, and the first output active power and the first reactive power of the computer-side part are used as the first control signal, and the reference angle of the PWM modulation part is the reference angle calculated by the VSG control strategy. Subtract the rotation angle difference between the stator and rotor The grid-side converter collects the second output active power and the second reactive power of the grid-side part as the grid-side converter VSG control signal (ie, the second control signal) to modulate the converter drive signal.

[0040] 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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An impedance reshaping method for a doubly-fed wind turbine, characterized in that: The machine-side converter and the grid-side converter of the doubly-fed wind turbine both adopt the VSG control strategy, the doubly-fed wind turbine includes an active power error path, and the impedance reshaping method includes: adding an impedance reshaping controller in the active power error path, wherein the impedance reshaping controller comprises a notch filter and a filter component, and the notch filter and the filter component are connected in series; Reshaping the impedance once by the notch filter to obtain a first impedance; The first impedance is reshaped by the filtering component to obtain a second impedance.

2. The impedance reshaping method according to claim 1, characterized in that: The filter assembly comprises: a first band-stop filter, a second band-stop filter and a third band-stop filter which are connected in series in sequence.

3. The impedance reshaping method according to claim 1, characterized in that: The transfer function of the notch filter is: ; in, is the notch filter notch frequency, is the notch filter damping coefficient.

4. The impedance reshaping method according to claim 3, characterized in that: The notch frequency of the notch filter is 50 Hz; The damping coefficient of the wave trap is 0.

5.

5. The impedance reshaping method according to claim 2, characterized in that: The first filter transfer function of the first band-stop filter is: ; The second filter transfer function of the second band-stop filter is: The third filter transfer function of the third band-stop filter is: ; in, is the first filter coefficient, is the second filter coefficient, is the third filter coefficient, ; is the first damping coefficient, is the second damping coefficient, is the third damping coefficient, ; is the first center frequency, is the second center frequency, is the third center frequency, .

6. The impedance reshaping method according to claim 5, characterized in that: The first filter coefficient is 0.3, the second filter coefficient is 0.3, and the third filter coefficient is 0.5; The first damping coefficient is 1.2, the second damping coefficient is 1, and the third damping coefficient is 1; The first center frequency is 2×π×25, the second center frequency is 2×π×50, and the third center frequency is 2×π×100.

7. The impedance reshaping method according to claim 1, characterized in that: The impedance reshaping method further comprises: The effectiveness of the impedance reshaping controller is verified by simulation software.

8. The impedance reshaping method according to claim 1, characterized in that: The machine-side converter collects a first voltage and a first current at a port of the asynchronous motor; Calculate the first output active power and the first reactive power of the computer side portion through the first voltage and the first current; Using the first output active power and the first reactive power as a first control signal of the machine-side converter; The reference angle and the rotation angle difference between the stator and the rotor are calculated according to the VSG control strategy to obtain the reference angle of the PWM modulation part of the grid-side converter.

9. The impedance reshaping method according to claim 1, characterized in that: The grid-side converter collects a second voltage and a second current at a port of the asynchronous motor; Calculate the second output active power and the second reactive power of the grid-side part according to the second voltage and the second current; The second output active power and the second reactive power are used as second control signals of the grid-side converter.

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