A control method for improving the stability of inverters in weak power grids

Abstract

The present invention discloses a control method for improving the stability of an inverter in a weak power grid, relates to the technical field of inverter control strategies, and solves the technical problem of system instability and insufficient power transmission capacity after the inverter is incorporated into a series compensation weak power grid. The key point of the technical solution is to adopt a q-axis voltage feedback control method based on voltage observation, place a voltage observer in the q-axis feedback path, and effectively estimate the actual q-axis voltage through the voltage observation link. The feedback method of the observed voltage is as follows: after the q-axis output voltage of the phase-locked loop passes through the voltage observation feedback link, it is fed back into the inverter current control system as the q-axis current command compensation value and the q-axis voltage feedforward compensation value, and participates in the control of the inverter. Because the observed voltage is relatively close to the actual q-axis voltage, when the observed voltage feedback control is adopted, the system instability can be effectively suppressed and the system stability can be improved.

Description

Technical Field

[0001] The present application relates to the field of inverter control strategies, and in particular to a control method for improving the stability of an inverter in a weak power grid. Background Art

[0002] Because series compensation can significantly increase the transmission capacity of power lines at a relatively low cost, it is widely used in modern power grids to enhance the grid's ability to transmit large-scale renewable energy generation. With the widespread use of grid-connected inverters in renewable energy generation systems, the interaction between grid-connected inverters and series-compensated lines can lead to subsynchronous resonance in the power system, potentially causing system failures and disrupting normal operation. Effectively suppressing this interaction between grid-connected inverters and series-compensated lines is a key technology for ensuring system stability.

[0003] It is currently found that the phase-locked loop is the main factor causing the interactive instability of the grid-connected inverter and the series compensation line. Therefore, suppressing the negative effect of the phase-locked loop on the grid stability can effectively improve the stability of the system. Traditionally, the q-axis voltage feedback output of the phase-locked loop is often used to suppress the negative effect of the phase-locked loop. The specific control principle diagram is as follows Figure 1 However, due to the dynamic effects of the phase-locked loop (PLL), there is a significant error between the q-axis voltage output by the PLL and the actual q-axis voltage. This means that the q-axis voltage output by the PLL is not the actual q-axis voltage. Since the actual q-axis voltage must be fed back to effectively suppress the negative effects of the PLL, and the error between the q-axis voltage output by the PLL and the actual q-axis voltage is large, the traditional method of using the q-axis voltage feedback output by the PLL cannot effectively suppress system instability caused by the interaction between the grid-connected inverter and the series compensation line. Summary of the Invention

[0004] The present application provides a control method for improving the stability of the inverter in a weak power grid. Its technical purpose is to effectively improve the system stability after the inverter is incorporated into the series compensation weak power grid and ensure the power transmission capacity of the series compensation power grid.

[0005] The above technical objectives of this application are achieved through the following technical solutions:

[0006] A control method for improving the stability of an inverter in a weak power grid, comprising:

[0007] The output voltage v of the phase-locked loop q axis abc Transformed into the component v in the two-phase rotating coordinate system d 、v q , v q Input to an observer; wherein the observer includes a first low-pass filter, a second low-pass filter, a first multiplier and a second multiplier;

[0008] Vq As the first signal, v q The second signal is sequentially passed through the proportional coefficient k1 and the first low-pass filter to obtain the second signal, and the second signal is sequentially passed through the proportional coefficient k2 and the second low-pass filter to obtain the third signal;

[0009] The first signal, the second signal and the third signal are superimposed and then input into the first multiplier after the proportional coefficient k3 and the proportional coefficient k d Get the q-axis current command compensation value;

[0010] The first signal, the second signal and the third signal are superimposed and then input into the second multiplier to obtain the q-axis voltage feedforward compensation value through the proportional coefficient k4;

[0011] The q-axis current command compensation value and the q-axis voltage feedforward compensation value are fed back to an inverter current control system to control the inverter.

[0012] The beneficial effects of the present application are as follows: the present application adopts a q-axis voltage feedback control method based on voltage observation, placing a voltage observer in the q-axis feedback path, and effectively estimating the actual q-axis voltage through the voltage observation link. The feedback method of the observed voltage is as follows: after the q-axis output voltage of the phase-locked loop passes through the voltage observation feedback link, it is fed back into the inverter current control system as the q-axis current command compensation value and the q-axis voltage feedforward compensation value, participating in the control of the inverter. Because the observed voltage is relatively close to the actual q-axis voltage, when the observed voltage feedback control is adopted, the system instability can be effectively suppressed and the system stability can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a control structure diagram of the voltage feedback method of the prior art;

[0014] Figure 2 A control structure block diagram for a specific implementation of the control method of the present invention;

[0015] Figure 3 Schematic diagram of the simulation results of the output q-axis voltage disturbance and the actual q-axis voltage disturbance in the prior art;

[0016] Figure 4 Schematic diagram of the output voltage and current experimental results of an inverter using the prior art in a series compensation power grid;

[0017] Figure 5 Schematic diagram of the simulation results of the q-axis voltage disturbance output and the actual q-axis voltage disturbance using the technology of the present invention;

[0018] Figure 6 The figure is a schematic diagram of the output voltage and current experimental results of the inverter using the technology of the present invention in a series compensation power grid. DETAILED DESCRIPTION

[0019] The technical solution of this application will be described in detail below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0020] like Figure 2 As shown, the focus of implementing the control method described in this application is on the observer for voltage observation. The entire control system includes a current control system, a phase-locked loop, a PWM unit and an observer, specifically including inverter d-axis and q-axis current feedback and feedforward, d-axis and q-axis current controllers, d-axis and q-axis voltage feedforward and feedforward coefficients, a phase-locked loop, a PWM unit, observer-based voltage feedforward, current abc / dq conversion and voltage abc / dq conversion.

[0021] The phase-locked loop includes the abc / dq transformation of the grid voltage, the PI controller and the integral link. The specific execution steps are: the grid voltage is transformed by abc / dq to obtain the q-axis voltage v q , v q As the input of the phase-locked loop PI controller, the output of the PI controller is superimposed on the rated angular frequency ω0 of the grid voltage, and then passes through the integral link 1 / s to generate the angle θ required for the abc / dq conversion.

[0022] The calculation formula of the voltage abc / dq conversion is as follows:

[0023]

[0024] Where θ is the rotation angle of the dq coordinate system, which is obtained from the phase-locked loop output. The q-axis voltage v output by the phase-locked loop is q as the input of the observer.

[0025] The control method for improving the stability of the inverter in the weak power grid specifically includes:

[0026] The output voltage v of the phase-locked loop q axis abc Transformed into the component v in the two-phase rotating coordinate system d 、v q , v q Input to an observer; wherein the observer includes a first low-pass filter, a second low-pass filter, a first multiplier and a second multiplier;

[0027] V q As the first signal, v q The second signal is sequentially passed through the proportional coefficient k1 and the first low-pass filter to obtain the second signal, and the second signal is sequentially passed through the proportional coefficient k2 and the second low-pass filter to obtain the third signal;

[0028] The first signal, the second signal and the third signal are superimposed and then input into the first multiplier after the proportional coefficients k3 and k d Get the q-axis current command compensation value;

[0029] The first signal, the second signal and the third signal are superimposed and then input into the second multiplier to obtain the q-axis voltage feedforward compensation value through the proportional coefficient k4;

[0030] The q-axis current command compensation value and the q-axis voltage feedforward compensation value are fed back to an inverter current control system to control the inverter.

[0031] As a specific embodiment, the q-axis current command compensation value is fed back to the inverter current control system and superimposed with the q-axis current command for closed-loop feedback of the inverter current control system.

[0032] As a specific embodiment, the q-axis voltage feedforward compensation value is fed back to the inverter current control system and superimposed with the q-axis voltage modulation instruction for closed-loop feedback of the inverter current control system.

[0033] Figure 2 In the example, the first low-pass filter (low-pass filter 1) and the second low-pass filter (low-pass filter 2) are expressed as follows:

[0034]

[0035] Wherein, s represents the differential operator, ω1 represents the cutoff frequency of the first low-pass filter, ω2 represents the cutoff frequency of the second low-pass filter, and the value range of ω1 and ω2 is 5-20 rad / s.

[0036] As a specific embodiment, the low-pass filter in the present application can be a first-order filter or a second-order filter.

[0037] The calculation method of the proportional coefficients k1 and k2 is:

[0038]

[0039] Among them, v' d Indicates the grid voltage amplitude, k ppll and k ipll They represent the proportional coefficient and integral coefficient of the phase-locked loop PI controller respectively.

[0040] The calculation method of the proportional coefficient k3 is:

[0041]

[0042] in, Indicates the d-axis current command; proportional coefficient k d The value range is 1 to 2.

[0043] The calculation method of the proportional coefficient k4 is: k4 = 1-k vf ; where k vf Represents the feedforward coefficient.

[0044] The current control system consists of d-axis current control and q-axis current control. The specific execution steps are as follows:

[0045] d-axis current command and d-axis current feedback i d After the difference is calculated, it is used as the input of the controller, and then the output of the controller, the q-axis current i q and ω0L product, d-axis voltage v d The product of the three and the feedforward coefficient is added to obtain a part of the PWM unit modulation instruction v id * , where L is the filter inductance value;

[0046] q-axis current command After the q-axis current command compensation value in the voltage observer feedback is superimposed, it is combined with the q-axis current feedback i q The difference is obtained as the input of the controller, and then the output of the controller, the d-axis current i d and ω0L product, q-axis voltage v q The other part of the PWM unit modulation instruction v is obtained by adding the product of the feedforward coefficient and the q-axis voltage feedforward compensation value in the voltage observer feedback. iq * ;

[0047] d-axis current feedback i d and q-axis current feedback i q By the output current i abc The current abc / dq conversion is obtained. The specific calculation formula of abc / dq conversion is as follows:

[0048]

[0049] Where θ is the rotation angle of the dq coordinate system, which is obtained from the phase-locked loop output.

[0050] The PWM unit is v id * and v iq * is the input;

[0051] The output of the PWM unit is used to control the three-phase inverter.

[0052] The controller used in the current control system and the phase-locked loop of the present invention is basically the same as the controller in the traditional control method, and still only uses one PI regulator, which maintains the advantages of simple system structure and good robustness. Figures 3 to 6 The experimental results in

[15] demonstrate that the present invention can more accurately capture the true q-axis voltage disturbance compared to existing technologies. When the more realistic q-axis voltage disturbance captured by the present invention is used for feedback control, the stability of the converter in a series-compensated weak grid is effectively improved.

[0053] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A control method for improving the stability of an inverter in a weak power grid, characterized in that: include: The output voltage v of the phase-locked loop q axis abc Transformed into the component v in the two-phase rotating coordinate system d 、v q , v q Input to an observer; wherein the observer includes a first low-pass filter, a second low-pass filter, a first multiplier and a second multiplier; V q As the first signal, v q The second signal is sequentially passed through the proportional coefficient k1 and the first low-pass filter to obtain the second signal, and the second signal is sequentially passed through the proportional coefficient k2 and the second low-pass filter to obtain the third signal; The first signal, the second signal and the third signal are superimposed and then input into the first multiplier after the proportional coefficients k3 and k d Get the q-axis current command compensation value; The first signal, the second signal and the third signal are superimposed and then input into the second multiplier to obtain the q-axis voltage feedforward compensation value through the proportional coefficient k4; The q-axis current command compensation value and the q-axis voltage feedforward compensation value are fed back to an inverter current control system to control the inverter.

2. The control method according to claim 1, wherein: The q-axis current command compensation value is fed back to the inverter current control system and superimposed with the q-axis current command for closed-loop feedback of the inverter current control system.

3. The control method according to claim 1 or 2, characterized in that: The q-axis voltage feedforward compensation value is fed back to the inverter current control system and superimposed with the q-axis voltage modulation instruction for closed-loop feedback of the inverter current control system.

4. The control method according to claim 3, wherein: The first low-pass filter and the second low-pass filter are respectively expressed as: Where s represents the differential operator, ω1 represents the cutoff frequency of the first low-pass filter, ω2 represents the cutoff frequency of the second low-pass filter, and the value range of ω1 and ω2 is 5-20 rad / s; The calculation method of the proportional coefficients k1 and k2 is: Among them, v' d Indicates the grid voltage amplitude, k ppll and k ipll They represent the proportional coefficient and integral coefficient of the phase-locked loop PI controller respectively.

5. The control method according to claim 4, wherein: The calculation method of the proportional coefficient k3 is: in, Indicates the d-axis current command; Proportional coefficient k d The value range is 1 to 2.

6. The control method according to claim 5, wherein: The calculation method of the proportional coefficient k4 is: k4=1-k vf ; Among them, k vf Represents the feedforward coefficient.

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