Control method for improving stability of direct-drive fan grid-connected system

Abstract

The invention discloses a control method for improving the stability of a direct-drive fan grid-connected system. The control method comprises the following steps: 1, equivalence of a wind turbine, a permanent magnet synchronous motor and a machine-side converter; 2, establishing a phase-locked loop control model; 3, establishing an adaptive virtual impedance control model; 4, constructing an all-pass filter; and 5, constructing a direct-drive fan current regulator control model by combining the power grid voltage feed-forward control of the all-pass filter and a parallel virtual impedance method. The output impedance phase of the direct-drive fan grid-connected inverter at the cut-off frequency is effectively improved, the phase margin of the system is further improved, the stability of the system is enhanced, and the method has good adaptability to a power grid.

Description

Technical Field

[0001] The present invention belongs to the field of automatic control of power systems, and particularly relates to a control method for improving the stability of a direct-drive wind turbine grid-connected system. Background Art

[0002] With the advancement of the energy transformation strategy, the proportion of new energy power generation represented by photovoltaic and wind power in the power system has been increasing continuously, significantly reducing the use of traditional fossil fuels and greenhouse gas emissions. However, new energy sources such as wind power are generally far from the main grid in terms of electrical distance, and the equivalent impedance of the grid is large. Resonance is likely to occur under the interaction between the new energy power generation system and the grid.

[0003] In the field of power system stability optimization, existing research mainly focuses on three directions: impedance reshaping, advanced algorithm optimization, and PLL improvement. However, the technical solutions in each direction have deficiencies. In the impedance reshaping direction, although some control strategies can improve the phase margin problem of the inverter output impedance at the crossover frequency, the improvement effect of the phase margin drops sharply under high grid impedance, making it difficult to meet the requirements of complex grid environments. In the advanced algorithm optimization direction, the adaptive feedforward control strategy can effectively enhance the system robustness by introducing a resonance link in the positive feedback loop and combining with grid impedance detection. However, this requires real-time observation of a large number of state variables, with extremely high requirements for the configuration accuracy and quantity of sensors, greatly increasing the hardware cost and system complexity. For the algorithm based on adding a lead compensation network to the capacitor current feedforward branch, since the parameter design highly depends on the online measurement technology of grid impedance, the parameter adjustment process is cumbersome in practical applications and it is difficult to quickly adapt to different grid scenarios. In the PLL improvement direction, the traditional PLL parameter optimization design mostly focuses on single parameter adjustment, making it difficult to simultaneously improve oscillation suppression and system stability in a wide frequency range. Although the adaptive PLL control strategy can dynamically adjust the PLL damping according to the disturbance amount and enhance the adaptability to weak grids, the additional impedance reshaping control link introduced by this strategy will inevitably weaken the system's attenuation ability for high-frequency harmonics, resulting in a decrease in the output power quality.

[0004] It is difficult for existing technical directions to achieve a balance in multiple aspects such as stability improvement, cost control, environmental adaptability, and power quality guarantee. There is an urgent need for a new control strategy to solve the above technical pain points and meet the strict requirements for the stable operation of the direct-drive wind turbine grid-connected system. Summary of the Invention

[0005] To solve the deficiencies of the above-mentioned existing technologies, the present invention proposes a control method for improving the stability of a direct-drive wind turbine grid-connected system, aiming to combine the grid voltage feedforward control of an all-pass filter and the parallel virtual impedance method to effectively improve the output impedance phase of the direct-drive wind turbine grid-connected inverter at the cut-off frequency, thereby expanding the system phase margin and enhancing the system stability to further enhance its adaptability to complex grid environments.

[0006] The present invention adopts the following technical solutions to solve the technical problems:

[0007] A control method for improving the stability of a direct-drive wind turbine grid-connected system according to the present invention, wherein the direct-drive wind turbine grid-connected system is composed of a wind turbine, a permanent magnet synchronous motor, a machine-side converter, a grid-side converter, a DC capacitor, and an AC grid; characterized in that the control method comprises the following steps:

[0008] Step 1: Decouple the machine side and the grid side of the direct-drive wind turbine grid-connected system by using a DC capacitor, so as to equivalently regard the wind turbine, the permanent magnet synchronous motor, and the machine-side converter as a constant DC current source;

[0009] Step 2: After collecting the a-phase voltage u pcc_a , b-phase voltage u pcc_b , c-phase voltage u pcc_c , a-phase current i pcc_a , b-phase current i pcc_b and c-phase current i pcc_c at the grid connection point, perform coordinate transformation to obtain the d-axis voltage component u d , q-axis voltage component u q , d-axis current component i d and q-axis current component i q ;

[0010] Step 3: Use Equation (1) to construct a phase-locked loop control model for use in the control of the direct-drive wind turbine grid-connected system;

[0011] (1)

[0012] In Equation (1), K p_ll is the proportional coefficient of the PI regulator under the phase-locked loop; K i_pll is the integral coefficient of the PI regulator under the phase-locked loop; s is the Laplace operator; is the phase-locked angle;

[0013] Step 4: Establish an adaptive virtual impedance control model for the direct-drive wind turbine grid-connected system for use in the control of the direct-drive wind turbine grid-connected system;

[0014] Step 5: Use Equation (5) to construct an expression for the all-pass filter G i (s);

[0015] (5)

[0016] In formula (5), Q2 is the second quality factor, is the center frequency;

[0017] Step 6: Based on the all-pass filter G i (s), establish a current regulator control model for the grid-connected system of the direct-drive wind turbine, which is used in the control of the grid-connected system of the direct-drive wind turbine.

[0018] The feature of a control method for improving the stability of the grid-connected system of a direct-drive wind turbine according to the present invention also lies in that the step 4 includes:

[0019] Step 4.1: Use formula (2) to construct the expression of the notch filter G f (s);

[0020] (2)

[0021] In formula (2), Q1 is the first quality factor; is the fundamental angular frequency of the power grid;

[0022] Step 4.2: After the a-phase voltage u pcc_a , b-phase voltage u pcc_b , and c-phase voltage u pcc_c are processed by the notch filter G f (s), the a-phase harmonic component u pa , b-phase harmonic component u pb and c-phase harmonic component u pc are obtained;

[0023] Step 4.3: After coordinate transformation of the a-phase harmonic component u pa , b-phase harmonic component u pb and c-phase harmonic component u pc , the axis voltage harmonic component and axis voltage harmonic component are obtained;

[0024] Step 4.4: Use formula (3) to construct the expression of the low-pass filter ;

[0025] (3)

[0026] In formula (3), f1 is the fundamental frequency of the power grid;

[0027] Step 4.5: Use formula (4) to establish an adaptive virtual impedance control model;

[0028] (4)

[0029] In formula (4), Z is the virtual impedance, and K pR is the proportional coefficient of the PI regulator under the adaptive virtual impedance control; K iR is the integral coefficient of the PI regulator under the adaptive virtual impedance control; U pref is the reference value of the harmonic component.

[0030] Furthermore, the said step 6 includes:

[0031] Step 6.1, construct the PI regulator expression of the current regulator by using formula (6);

[0032] (6)

[0033] In formula (6), K pi is the proportional coefficient of the PI regulator under the current regulator; K ii is the integral coefficient of the PI regulator under the current regulator;

[0034] Step 6.2, establish the current regulator control model by using formula (7) according to the virtual impedance Z and the all-pass filter G i (s).

[0035] (7)

[0036] In formula (7), u id is the d-axis voltage modulation signal under the current regulator control; u iq is the q-axis voltage modulation signal under the current regulator control; i dref is the d-axis current reference value of the grid connection point; i qref is the q-axis current reference value of the grid connection point; L1 is the inductance of the LCL filter of the direct-drive wind turbine grid connection system close to the inverter output end.

[0037] An electronic device according to the present invention, comprising a memory and a processor, is characterized in that the memory is used to store a program for supporting the processor to execute the said control method, and the processor is configured to execute the program stored in the memory.

[0038] A computer-readable storage medium according to the present invention, on which a computer program is stored, is characterized in that the computer program executes the steps of the said control method when being run by a processor.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] 1. In view of the characteristic that the output impedance of the grid-connected inverter of a direct-drive wind turbine under traditional control shows capacitive impedance, by connecting a virtual impedance in parallel, the anti-interference ability against grid background harmonic voltage is improved, and the stability of the system is improved at the same time.

[0041] 2. The adaptive regulation based on virtual impedance proposed in the present invention can adaptively adjust the virtual impedance parameters according to the magnitude of the harmonic components contained in the grid-connected point voltage, which can effectively balance the relationship between power loss and system stability, and ensure the superiority of the direct-drive wind turbine grid-connected system under different operating conditions.

[0042] 3. The present invention combines the grid voltage feed-forward control and the method of connecting a virtual impedance in parallel to construct a control model of the current regulator of the direct-drive wind turbine, thereby increasing the phase of the output impedance of the grid-connected inverter of the direct-drive wind turbine, further increasing the phase margin of the system, enhancing the stability of the system, and having good adaptability to the grid. It can provide effective guidance for the design of the control strategy of the grid-connected inverter of the direct-drive wind turbine, and thus lay a solid foundation for improving the system stability. Brief Description of the Drawings

[0043] Figure 1 is the structure diagram of the direct-drive wind turbine grid-connected system;

[0044] Figure 2 is the control block diagram of the phase-locked loop;

[0045] Figure 3 is the structure diagram of the adaptive virtual impedance control;

[0046] Figure 4 is the frequency response curve diagram of the all-pass filter;

[0047] Figure 5 is the control block diagram of the current regulator;

[0048] Figure 6a is the grid-connected current diagram when the grid impedance Zg = 0.42 pu under the control of the traditional current regulator;

[0049] Figure 6b is the FFT result diagram of the grid-connected current when the grid impedance Zg = 0.42 pu under the control of the traditional current regulator;

[0050] Figure 6c is the grid-connected current diagram when the grid impedance Zg = 0.53 pu under the control of the traditional current regulator;

[0051] Figure 6d is the FFT result diagram of the grid-connected current when the grid impedance Zg = 0.53 pu under the control of the traditional current regulator;

[0052] Figure 7aGrid-connected current diagram when the grid impedance \(Z_g = 0.42\) pu under the control of an improved current regulator;

[0053] Figure 7b FFT result diagram of the grid-connected current when the grid impedance \(Z_g = 0.42\) pu under the control of an improved current regulator;

[0054] Figure 7c Grid-connected current diagram when the grid impedance \(Z_g = 0.53\) pu under the control of an improved current regulator;

[0055] Figure 7d FFT result diagram of the grid-connected current when the grid impedance \(Z_g = 0.53\) pu under the control of an improved current regulator. Specific implementation manner

[0056] The technical solution of the present invention will be further specifically described below in combination with the specific implementation manner and the drawings

[0057] In this embodiment, an improved current control strategy for a direct-drive wind turbine based on virtual impedance is carried out according to the following steps:

[0058] Step 1. The stability problem of the direct-drive wind turbine grid-connected system mainly arises from the interaction between the grid-side converter and the grid-connected system, and the DC capacitor decouples the machine side and the grid side of the direct-drive wind turbine grid-connected system, so that the wind turbine, the permanent magnet synchronous motor, and the machine-side converter are equivalent to a constant DC current source. The structural diagram of the direct-drive wind turbine grid-connected system is as shown in Figure 1 as follows;

[0059] Figure 1 where \(I\) dc is the input current of the constant current source; \(C\) dc is the DC capacitor; \(S\) a1 ~ \(S\) c2 are the bridge arm switching tubes; \(u\) ad , \(u\) bd and \(u\) cd are the three-phase bridge arm midpoint voltages; \(L1\), \(C\) f and \(L2\) form an LCL filter; \(R\) d is the damping resistor in series with the filter capacitor, \(u\) pcc_a , \(u\) pcc_b , \(u\) pcc_c and \(i\) 2a , \(i\) 2b , \(i\) 2c are the three-phase voltages and three-phase currents at the grid connection point respectively; \(Z\) g is the grid impedance; is the phase angle of the phase-locked loop.

[0060] Step 2. Collect the a-phase voltage \(u\) pcc_a of the grid connection point, the b-phase voltage \(u\)pcc_b and the c-phase voltage u pcc_c and the a-phase current i pcc_a and the b-phase current i pcc_b and the c-phase current i pcc_c , and then perform a coordinate transformation to obtain the d-axis voltage component u d of the grid connection point, the q-axis voltage component u q , the d-axis current component i d and the q-axis current component i q .

[0061] Step 3: Use Equation (1) to construct a phase-locked loop control model for the control of the direct-drive wind turbine grid-connected system;

[0062] (1)

[0063] In Equation (1), K p_ll is the proportional coefficient of the PI regulator under the phase-locked loop; K i_pll is the integral coefficient of the PI regulator under the phase-locked loop; s is the Laplace operator; the phase-locked loop control structure is as shown in Figure 2 .

[0064] Step 4: Establish an adaptive virtual impedance control model for the direct-drive wind turbine grid-connected system for the control of the direct-drive wind turbine grid-connected system. The adaptive virtual impedance control structure is as shown in Figure 3 ; Figure 3 in is the PI regulator under the adaptive virtual impedance control.

[0065] Step 4.1: Use Equation (2) to construct the expression of the notch filter G f (s);

[0066] (2)

[0067] In Equation (2), Q1 is the first quality factor; is the fundamental angular frequency of the power grid;

[0068] Step 4.2: After the a-phase voltage u pcc_a , the b-phase voltage u pcc_b , and the c-phase voltage u pcc_c are processed by the notch filter G f (s), the a-phase harmonic component u pa , the b-phase harmonic component u pb and the c-phase harmonic component u pc are obtained.

[0069] Step 4.3: For the a-phase harmonic component u pa , the b-phase harmonic component u pb and the c-phase harmonic component u pcAfter coordinate transformation, we obtain the harmonic component of the shaft voltage and the harmonic component of the shaft voltage ;

[0070] Step 4.4: Construct the expression of the low-pass filter using Equation (3) ;

[0071] (3)

[0072] In Equation (3), f1 is the fundamental frequency of the power grid.

[0073] Step 4.5: Establish the adaptive virtual impedance control model using Equation (4);

[0074] (4)

[0075] In Equation (4), Z is the virtual impedance, K pR is the proportional coefficient of the PI regulator under adaptive virtual impedance control; K iR is the integral coefficient of the PI regulator under adaptive virtual impedance control; U pref is the reference value of the harmonic component, set to 5% of the fundamental component of the grid-connected point voltage;

[0076] The principle of adaptive virtual impedance control is: when the grid-connected system becomes unstable, , 1 / Z is adjusted according to the PI regulator until , and the determined 1 / Z at this time is the appropriate virtual impedance value for this system.

[0077] Step 5: Construct the expression of the all-pass filter G i (s) using Equation (5), and its frequency response curve of the all-pass filter is as Figure 4 shown.

[0078] (5)

[0079] In Equation (5), Q2 is the second quality factor, is the center frequency.

[0080] Step 6: Establish the current regulator control model for the direct-drive wind turbine grid-connected system and use it in the control of the direct-drive wind turbine grid-connected system;

[0081] Step 6.1: Construct the expression of the PI regulator under the current regulator using Equation (6);

[0082] (6)

[0083] In Equation (6), K piis the proportional coefficient of the PI regulator under the current regulator; K ii is the integral coefficient of the PI regulator under the current regulator.

[0084] Step 6.2. Based on the virtual impedance Z and the all-pass filter G i (s), establish the current regulator control model using Equation (7);

[0085] (7)

[0086] In Equation (7), u id is the d-axis voltage modulation signal under the current regulator control; u iq is the q-axis voltage modulation signal under the current regulator control; i dref is the d-axis current reference value at the grid connection point; i qref is the q-axis current reference value at the grid connection point; L1 is the inductor of the LCL filter of the direct-drive wind turbine grid-connected system close to the inverter output.

[0087] The current regulator control block diagram is as Figure 5 shown. The control of this current regulator combines the grid voltage feedforward control of the all-pass filter and the parallel virtual impedance method. The grid voltage feedforward control realizes the leading compensation of the phase without changing the signal amplitude, improves the phase margin of the direct-drive wind turbine grid-connected system at the crossover frequency, and further improves the stability of the system. The parallel virtual impedance improves the anti-interference ability of the grid voltage background harmonics while further ensuring the phase margin of the system.

[0088] Step 7. Apply the control strategies in Steps 3 - 6 to the control of the direct-drive wind turbine grid-connected system, so as to achieve the effects of enhancing the system stability and improving the adaptability to the grid.

[0089] In this embodiment, an electronic device includes a memory and a processor. The memory is used to store a program that supports the processor to execute the above method, and the processor is configured to execute the program stored in the memory.

[0090] In this embodiment, a computer-readable storage medium stores a computer program on the computer-readable storage medium. When the computer program is run by a processor, it executes the steps of the above method.

[0091] Embodiment:

[0092] Taking the simulink simulation of the direct-drive wind turbine grid-connected system model as an example:

[0093] 1. In simulink, build a direct-drive wind turbine grid-connected system model as Figure 1 shown, and the model parameters are shown in Table 1.

[0094] Table 1

[0095]

[0096] 2. Under the control of the traditional current regulator, the grid impedance is adjusted to operate at 0.42 pu and 0.53 pu respectively for simulation. At this time, the grid-connected point current and the FFT analysis results are as follows Figure 6a 、 Figure 6b 、 Figure 6c 、 Figure 6d shown. It can be seen from the figure that when the grid impedance is 0.42 pu, the system is in a stable state, and the total harmonic distortion (THD) is 0.33%. When the grid impedance is 0.53 pu, the grid-connected inverter is coupled with the grid impedance at this time, the inverter generates resonance, the grid-connected current waveform is severely distorted, and the THD is 33.26%. The simulation results show that under the control of the traditional current regulator, there are stability problems in the grid-connected system of direct-drive wind turbines when the grid impedance is large.

[0097] 3. Under the control of the improved current regulator, the grid impedance is adjusted to operate at 0.42 pu and 0.53 pu respectively for simulation. At this time, the grid-connected point current and the FFT analysis results are as follows Figure 7a 、 Figure 7b 、 Figure 7c 、 Figure 7d shown. Combining Figure 6b 、 Figure 6d 、 Figure 7b and Figure 7d indicates that with the addition of the improved current regulator, when the grid impedance is 0.42 pu, the THD is reduced from 0.33% to 0.27%, and the system stability is further improved. When the grid impedance is 0.53 pu, the THD is reduced from 33.26% to 3.86%, the harmonic content is significantly reduced, and the system changes from unstable to stable operation. Therefore, the embodiments prove the effectiveness of the method proposed in the present invention.

Claims

1. A control method for improving the stability of a direct-drive wind turbine grid-connected system, where the direct-drive wind turbine grid-connected system is composed of a wind turbine, a permanent magnet synchronous motor, a machine-side converter, a grid-side converter, a DC capacitor, and an AC power grid; characterized in that, The control method comprises the following steps: Step 1: Decouple the machine side and the grid side of the direct-drive wind turbine grid-connected system by using a DC capacitor, so as to equivalently transform the wind turbine, the permanent magnet synchronous motor and the machine-side converter into a constant DC current source; Step 2: After collecting the a-phase voltage u pcc_a of the grid connection point, the b-phase voltage u pcc_b , the c-phase voltage u pcc_c , the a-phase current i pcc_a , the b-phase current i pcc_b and the c-phase current i pcc_c , perform a coordinate transformation to obtain the d-axis voltage component u d of the grid connection point, the q-axis voltage component u q , the d-axis current component i d and the q-axis current component i q ; Step 3: Construct a phase-locked loop control model by using Equation (1) for the control of the direct-drive wind turbine grid-connected system; (1) In Equation (1), K p_ll is the proportional coefficient of the PI regulator under the phase-locked loop; K i_pll is the integral coefficient of the PI regulator under the phase-locked loop; s is the Laplace operator; is the phase-locked angle; Step 4: Establish an adaptive virtual impedance control model for the direct-drive wind turbine grid-connected system for the control of the direct-drive wind turbine grid-connected system; Step 5. Construct an all-pass filter G using Equation (5) i (s) expression (5) In formula (5), Q2 is the second quality factor, is the center frequency; Step 6: Based on the all-pass filter G i (s), establish the current regulator control model of the direct-drive wind turbine grid-connected system for use in the control of the direct-drive wind turbine grid-connected system.

2. The control method for improving the stability of the grid-connected system of a direct-drive wind turbine according to claim 1, wherein, The said Step 4 comprises: Step 4.

1. Construct the notch filter G using Equation (2) f (s) expression (2) In formula (2), Q1 is the first quality factor; is the fundamental angular frequency of the power grid; Step 4.2, the a-phase voltage u pcc_a , the b-phase voltage u pcc_b , and the c-phase voltage u pcc_c After being processed by the limiter G f (s), the a-phase harmonic component u pa , the b-phase harmonic component u pb and the c-phase harmonic component u pc are obtained; Step 4.3: After performing coordinate transformation on the a-phase harmonic component u pa , the b-phase harmonic component u pb , and the c-phase harmonic component u pc , the -axis voltage harmonic component and -axis voltage harmonic component are obtained; Step 4.

4. Construct a low-pass filter using Equation (3) for the expression; (3) In Equation (3), f1 is the fundamental grid frequency; Step 4.5: Establish an adaptive virtual impedance control model by using Equation (4); (4) In Equation (4), Z is the virtual impedance, and K pR is the proportional coefficient of the PI regulator under adaptive virtual impedance control; K iR is the integral coefficient of the PI regulator under adaptive virtual impedance control; U pref is the reference value of the harmonic component.

3. A control method for improving the stability of a grid-connected system of a direct-drive wind turbine according to claim 2, characterized in that, The said Step 6 comprises: Step 6.1: Construct a PI regulator expression of the current regulator by using Equation (6); (6) In Equation (6), K pi is the proportionality coefficient of the PI regulator under the current regulator; K ii is the integral coefficient of the PI regulator under the current regulator; Step 6.

2. Based on the virtual impedance Z and the all-pass filter G i (s), establish the current regulator control model using Equation (7); (7) In Equation (7), u id is the d-axis voltage modulation signal controlled by the current regulator; u iq is the q-axis voltage modulation signal controlled by the current regulator; i dref is the reference value of the d-axis current at the grid connection point; i qref is the reference value of the q-axis current at the grid connection point; L1 is the inductor of the LCL filter of the direct-drive wind turbine grid-connected system close to the inverter output end.

4. An electronic device, comprising a memory and a processor, characterized in that, The memory is used for storing a program for supporting the processor to execute the control method described in any one of Claims 1-3, and the processor is configured to execute the program stored in the memory.

5. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is run by the processor, it executes the steps of the control method described in any one of Claims 1-3.

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