Anti-frequency interference V2G converter low harmonic combination control method

Abstract

The invention discloses an anti-frequency interference V2G converter low harmonic combination control method, the V2G converter is a voltage type PWM rectifier, and the method is characterized in that a voltage outer loop controller in the control method adopts a proportional integral controller, a moving average filter is directly connected in series to the output of the voltage outer loop controller; interference of periodic signals in the direct current bus voltage on the current reference signals is suppressed, so that the amplitude of the current reference signals is pure direct current quantity; in the current loop controller, a repetitive controller is used as a feedforward controller and is combined with a weighted average deadbeat controller, an insertion type composite controller is constructed, coefficients are rapidly updated by adopting a Lagrange interpolation method, and a delay value is continuously controlled, so that the repetitive controller and a moving average filter have frequency self-adaptive characteristics; the problem of performance degradation of a traditional fixed frequency controller during frequency fluctuation is solved, and the anti-frequency interference characteristic of the converter is enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of V2G converters, and particularly to a low-harmonic combined control method for a V2G converter with anti-frequency interference. Background Art

[0002] Under the background of the transformation of the automotive industry towards energy-saving and new energy vehicles, the large-scale development of electric vehicles has formed a considerable-capacity energy storage device, which can be used as a mobile energy storage device in the smart grid to improve the grid system, making the V2G (Vehicle-to-Grid) technology a research hotspot. However, when connecting electric vehicles to the grid for charging and discharging control, some new problems will occur, such as harmonic interference problems, frequency fluctuation problems, etc. The reason for harmonic generation lies in the non-linear power electronic devices inside the V2G system, and the main link for generating harmonics is the PWM rectification link (AC / DC unit). At the same time, load changes or other factors will cause fluctuations in the grid frequency, resulting in a decline in the harmonic suppression ability of the controller and affecting the stable operation of the grid.

[0003] The control strategy directly affects the harmonic performance of the grid-connected current. To reduce the harmonic content, the existing technology usually adopts a double-loop control scheme of a voltage outer loop plus a current inner loop, and the suppression of harmonics is mainly aimed at the current control strategy of the inner loop. The commonly used control methods for the current loop are feedback control based on the object model and feedback control based on the signal model.

[0004] Among them, the feedback control based on the object model can ensure stability and rapidity, but in practice, due to uncertain factors such as frequency fluctuations, parameter changes, and load disturbances, there is a deviation between the object model and the actual situation; while the feedback control based on the signal model can ensure the accuracy of the system and can achieve the error-free tracking of any waveform periodic signal, but it is limited by the internal model delay link of the signal and has a slow dynamic response. Summary of the Invention

[0005] The present invention proposes a low-harmonic combined control method for a V2G converter with anti-frequency interference, which solves the problem that the existing control methods are difficult to simultaneously meet the requirements of fast dynamic response and high-precision harmonic suppression.

[0006] To solve the above technical problems, the present invention provides a low-harmonic combined control method for a V2G converter with anti-frequency interference. The V2G converter is a voltage-type PWM rectifier, and is characterized in that the control method includes the following steps: Step S1: Obtain the voltage error signal by subtracting the DC-side voltage signal of the V2G converter from the DC voltage reference signal, and input the voltage error signal into the voltage outer loop controller, and the voltage outer loop controller is a proportional-integral controller; Step S2: Directly connect the moving average filter in series to the output of the voltage outer-loop controller to suppress the interference of the periodic signal in the DC bus voltage on the current reference signal, so that the amplitude iacpeak of the current reference signal is a pure DC value; Step S3: The output signal of the moving average filter is combined with the output signal of the phase-locked loop to generate a current reference signal, obtain the AC-side current signal, and subtract it from the current reference signal to obtain a current error signal, which is used as the input of the current-loop controller; Step S4: For the current-loop controller, use the repetitive controller as a feedforward controller, combine it with the weighted average deadbeat controller to construct a plug-in composite controller, and use the Lagrange interpolation method to quickly update the coefficients and continuously control the delay value, so that the repetitive controller and the moving average filter have frequency adaptive characteristics; Step S5: Generate the PWM control signal of the V2G converter according to the output signal of the composite controller.

[0007] Preferably, the transfer function of the repetitive controller is as follows:

[0008] where, , is the compensation gain; is a zero-phase low-pass filter, F is the fractional part of N, Ni is the integer part of N, is the system compensator, z is the complex variable of the z-transform, and N is the ratio of the sampling frequency to the frequency corresponding to the sampling sample period.

[0009] Preferably, the transfer function of the moving average filter is as follows:

[0010] where, z is the complex variable of the z-transform, and N is the ratio of the sampling frequency to the frequency corresponding to the sampling sample period.

[0011] Meanwhile, the present invention also proposes a V2G converter low-harmonic combined controller device with anti-frequency interference, including: a memory, a processor, and a computer program, characterized in that: the computer program is stored in the memory and is configured to be executed by the processor to implement the above-mentioned V2G converter low-harmonic combined control method with anti-frequency interference.

[0012] Meanwhile, the present invention also proposes a computer-readable storage medium, in which a computer program is stored, and the computer program is executed by a processor to implement the above-mentioned V2G converter low-harmonic combined control method with anti-frequency interference.

[0013] The advantages of the present invention at least include: 1. The controller can achieve zero-error tracking and eliminate periodic signal interference, that is, the current controller of the PWM rectifier achieves infinite gain at the harmonic frequency, making the output of the voltage controller of the PWM rectifier a stable DC quantity.

[0014] 2. By using the repetitive controller as a feedforward controller and combining it with the deadbeat controller, a plug-in composite controller structure can be constructed, which has the characteristics of precise control and fast response speed. To reduce the influence of frequency fluctuations on the performance of the converter, the Lagrange interpolation method is used to quickly update the coefficients and continuously control the delay value, enabling the repetitive controller and the moving average filter to have frequency adaptive characteristics, solving the problem of performance degradation of traditional fixed-frequency controllers during frequency fluctuations, and enhancing the anti-frequency interference characteristics of the converter. 3. Combining the frequency adaptive moving average filter and the frequency adaptive repetitive controller can adjust the parameters of the filter and the controller in real time according to the dynamic changes of the grid frequency, effectively solving the problem of performance degradation of traditional control methods during grid frequency fluctuations, enhancing the robustness and stability of the system, and being particularly suitable for the situation where the load changes frequently and the grid frequency is unstable in the V2G system. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a schematic flowchart of the method according to an embodiment of the present invention; Figure 2 is a schematic diagram of the low harmonic combined control structure of the anti-frequency interference V2G converter according to an embodiment of the present invention; Figure 3 is a schematic diagram of the main circuit and its controller of the single-phase full-bridge voltage-type PWM rectifier according to an embodiment of the present invention; Figure 4 is the amplitude-frequency response and phase-frequency response diagram of the moving average filter; Figure 5 is a schematic diagram comparing the simulation effects of the frequency adaptive moving average filter and the moving average filter according to an embodiment of the present invention; Figure 6 is a schematic diagram of the frequency adaptive repetitive controller according to an embodiment of the present invention; Figure 7 is the waveform diagram of the AC side voltage and current and the FFT analysis result diagram of the current when using conventional control at 50Hz frequency; Figure 8 is the waveform diagram of the AC side voltage and current and the FFT analysis result diagram of the current when using conventional control and repetitive control at 50Hz frequency; Figure 9 is the waveform diagram of the AC side voltage and current and the FFT analysis result diagram of the current when using conventional control, notch filter and repetitive controller at 50Hz frequency; Figure 10Waveform diagrams of the AC-side voltage and current and the FFT analysis results of the current when using conventional control, moving average filter, and repetitive controller at a frequency of 50 Hz; Figure 11 Waveform diagrams of the AC-side voltage and current and the FFT analysis results of the current when using moving average and repetitive control at a frequency of 50.2 Hz; Figure 12 Waveform diagrams of the AC-side voltage and current and the FFT analysis results of the current when using frequency-adaptive moving average and frequency-adaptive repetitive control at a frequency of 50.2 Hz; Figure 13 Dynamic switching waveform diagram of the tracking error of the AC-side current during the switching of multiple control strategies when the frequency is 50.2 Hz; Figure 14 Dynamic voltage and current waveform diagram when the load jumps when the AC-side voltage is 110 V; Figure 15 Histogram of the total harmonic of the AC-side current when different control strategies are adopted during different degrees of frequency fluctuations; Figure 16 Variation curve diagram of the THD of the AC-side current when different control strategies are adopted under normal frequency and frequency fluctuation conditions. Detailed implementation manners

[0016] The following clearly and completely describes the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the protection scope of the present invention.

[0017] As Figure 1 and Figure 2 shown, the embodiments of the present invention provide a low-harmonic combined control method for a V2G converter against frequency interference, including the following steps: Step S1: Use the difference between the DC bus voltage signal and the DC voltage reference signal of the V2G converter as the voltage error signal, and input the voltage error signal into a proportional-integral controller to generate the amplitude of the current reference signal.

[0018] Step S2: Input the amplitude of the current reference signal into a frequency-adaptive moving average filter, and the frequency-adaptive moving average filter dynamically adjusts the filter parameters according to the grid frequency.

[0019] Specifically, as Figure 3As shown in the figure, according to the main circuit topology of the single-phase full-bridge voltage-type PWM rectifier, the main circuit of the single-phase voltage-type PWM rectifier can be divided into three parts. The left side is the AC part, which consists of the AC voltage source vac and the filter inductor L; the middle is the rectifier bridge composed of the power switch tubes S1 - S4; the right side is the DC part, which consists of the energy storage capacitor C and the load resistor R, where the capacitor C maintains the stability of the DC-side output voltage.

[0020] The difference between the reference signal of the DC bus voltage and the feedback signal of the DC bus voltage is used as the voltage error signal. After passing through the PI controller, it is input to the frequency adaptive sliding average filter to generate the amplitude of the current reference signal.

[0021] Figure 4 It is the amplitude-frequency response and phase-frequency response diagram of the sliding average filter. The sliding average filter reduces random noise by calculating the average value of a group of continuous samples. Let the input signal be , and the output signal be , then the expression of the output signal of the sliding average filter is: ; In the formula, is the number of samples in one period of the sliding average filter.

[0022] For discrete signals, the z transformation can be used, and the expression of the obtained output signal is: ; In the formula, means delaying the sampling signal by sampling periods.

[0023] Thus, the expression of the transfer function of the sliding average filter can be obtained as: .

[0024] In the embodiment of the present invention, the sampling frequency is set to f s = 20KHz, and the corresponding frequency of the sampling period is f = 100Hz. Then, N = f s / f = 200. The amplitude-frequency response and phase-frequency response of the sliding average filter under this parameter are as shown in Figure 4 . From Figure 4It can be seen that at the double frequency of 100 Hz and its multiple frequencies, the gain of the filter is relatively large, and it can suppress the periodic signals of 100 Hz and its multiple frequencies. Therefore, directly connecting the moving average filter in series to the output of the voltage outer-loop controller can suppress the interference of the periodic signals in the DC bus voltage on the current reference signal, making the amplitude of the current reference signal a pure DC value. However, when the power grid frequency fluctuates, the sample period corresponds to a frequency f that is not an integer, resulting in N = f s / f also not being an integer, and it cannot be implemented with a fractional value N = f s / f , causing an error between the gain of the controller and the actual value. The delay at this time can be expressed as: .

[0025] Where N i = N , which is the integer part of N , and F = N - N i (1 ≥ F ≥ 0), which is the fractional part of N .

[0026] When the frequency changes, a digital controller with a fixed sampling frequency f s is difficult to achieve the expected effect. For example, when the frequency fluctuates, at this time the gain of the filter is at the red line of Figure 4 , and the gain of the controller becomes significantly smaller, making it difficult to achieve the expected suppression effect. To make the moving average filter frequency adaptive, the traditional method is to ignore the fractional delay , and use the nearest integer delay or to replace the fractional delay . The harmonic suppression effect of this moving average filter based on integer delay may decay due to ignoring the fractional delay , especially when the sampling frequency f s is relatively low; while the frequency adaptive moving average filter based on the Lagrange interpolation method can accurately simulate the fractional delay , the problem of performance degradation of the traditional frequency adaptive moving average filter during frequency fluctuations is solved. Therefore, in the embodiments of the present invention, the Lagrange interpolation method is adopted, and the fractional digital delay in the control process is approximately expressed in the following form: ; .

[0027] As the number of interpolation times n increases, the true value can be approximated more accurately.

[0028] Figure 5 FIG. is a schematic diagram for comparing the simulation effects of the moving average filter and the frequency adaptive moving average filter. When the frequency fluctuates, for example, jumps from 50 Hz to 49.5 Hz, the filtering effects of the moving average filter corresponding to 50 Hz and the frequency adaptive moving average filter are as Figure 5 shown. The black line in the figure is the filtering effect diagram of the frequency adaptive moving average filter when the frequency jumps, the blue line is the filtering effect diagram of the moving average filter when the frequency jumps, and the red line is the comparison difference between the two. It can be seen that the frequency adaptive moving average filter has a good filtering effect when the frequency fluctuates, while there are obvious errors in the moving average filter with a fixed frequency.

[0029] Step S3: Multiply the amplitude of the filtered current reference signal by the AC side voltage signal of the V2G converter to generate an AC side current reference signal, and use the error between the AC side current reference signal and the AC side current signal as the current error signal.

[0030] Step S4: Input the current error signal into the frequency adaptive repetitive controller, and the frequency adaptive repetitive controller performs fractional delay compensation on the current error signal through the Lagrange interpolation method.

[0031] As Figure 6 shown is a schematic diagram of the frequency adaptive repetitive controller. When the frequency fluctuates, the sample period corresponding to the frequency f is not an integer, N = f s / f is also not an integer. The Lagrange interpolation method is also used to simulate the fractional delay. Finally, the expression of the repetitive control internal model of the frequency adaptive repetitive controller is obtained as: ; In the formula, is the repetitive control internal model of the frequency adaptive repetitive controller; is the delay link of the repetitive control internal model; is the period of the periodic signal; is the complex frequency domain operator; is the interpolation times of the frequency adaptive repetitive controller; is the resonant bandwidth; The transfer function of the frequency adaptive repetitive controller is: ; ; ; In the formula, is the transfer function of the frequency adaptive repetitive controller; is the compensation gain; is the integer part of; is the number of samples in one period of the frequency adaptive repetitive controller; is the fractional part of.

[0032] Figures 7 to 10 are respectively the waveform diagrams of the AC side voltage v ac and current i ac and the current FFT analysis result diagrams under normal frequency, that is, 50 Hz, when using conventional control, adding repetitive control, adding notch filter + repetitive controller, and adding moving average filter + repetitive controller. It can be seen that there are obvious distortions in the AC side current under conventional control, and the current THD is 12.77%; after adding repetitive control, the distortion of the AC side current decreases, and the current THD is 7.62%; after adding the notch filter, the distortion of the AC side current decreases again, and the current THD is 3.61%; while using the combined control strategy of moving average filter + repetitive controller, the distortion of the AC side current is significantly reduced, and according to the analysis, the current THD is only 0.63%, and the effect of suppressing periodic interference is significant. The experimental parameters are respectively: the switching frequency f s is 20 KHz, the filter inductor L is 3.4 mH, the capacitor C is 880 uF, the output voltage v dc is 300 V, and the output power P o is .

[0033] Figure 11 and Figure 12 are the AC side voltage v ac and current i ac when frequency fluctuation occurs, that is, when the frequency is 50.2 Hz, using the control strategy of moving average + repetitive control and the control strategy of frequency adaptive moving average + frequency adaptive repetitive control.Waveform diagram and current FFT analysis result diagram. It can be seen that when the control strategy of sliding average + repetitive control is adopted, there is obvious distortion in the AC side current, and the current THD is 5.60%; while when the control strategy of frequency adaptive sliding average + frequency adaptive repetitive control is adopted, the distortion of the AC side current is significantly reduced. According to the analysis, the current THD is only 0.79%, and the anti-frequency interference characteristic of the converter is significantly enhanced.

[0034] Figure 13 It is the dynamic switching waveform diagram of the AC side current tracking error when frequency fluctuation occurs, that is, when the frequency is 50.2Hz, switching from the conventional PI control to the combined control strategy without frequency adaptation and the combined control strategy with frequency adaptation. It can be intuitively seen from the figure that the steady-state tracking errors of the current under the three control strategies decrease in turn. After applying the combined control strategy with frequency adaptation, through the error correction of the frequency adaptive repetitive controller, the steady-state tracking error of the current quickly drops to about ±0.38A, which can not only ensure stability but also improve the rapidity of the control system.

[0035] Figure 14 It is the dynamic waveform of voltage and current when the load jumps when the AC side voltage is 110V. It can be seen that whether the output power jumps from 600W to 400W or from 400W to 600W, the control system can reach a new steady state again after about 2 power frequency cycles of transition time, verifying that the proposed combined control strategy can meet the requirement of rapidity.

[0036] Figure 15 It is the bar chart of the total harmonic of the AC side current when different degrees of frequency fluctuation occur, that is, when the frequency is 49.5Hz - 50.5Hz, and different control strategies are adopted. It can be seen that when the frequency fluctuates, there is obvious distortion in the AC side current under the conventional control strategy; when the combined control strategy without frequency adaptation is adopted, only when the frequency is 50Hz, the current THD is relatively low, and when the frequency fluctuates, the current THD increases significantly; when the combined control strategy proposed in this paper is adopted, whether the frequency fluctuates or under normal conditions, the current THD is at an extremely low level, and it has excellent anti-frequency interference characteristics.

[0037] Figure 16 It is the change curve diagram of the AC side current THD when the input voltage changes under normal frequency and when frequency fluctuation occurs, and different control strategies are adopted. It can be seen that as the input voltage increases, the current THD shows an increasing trend. After applying the proposed combined control strategy, even when frequency fluctuation occurs, the THD is lower than 1%, which has a very good effect on reducing the harmonic of the AC side current and has excellent anti-frequency interference characteristics.

[0038] From the analysis of the above experimental waveforms, it can be seen that the method of the embodiment of the present invention enhances the frequency interference resistance characteristic of the converter and eliminates the harmonic distortion of the grid-connected reference current by connecting a frequency adaptive sliding average filter in series at the output of the voltage loop; by inserting a frequency adaptive repetitive controller in front of the conventional deadbeat current controller in the current inner loop, the grid-connected current accurately tracks the reference current, further enhancing the frequency interference resistance characteristic of the converter and eliminating the harmonic distortion of the grid-connected current, while retaining the fast dynamic characteristic. Compared with the conventional control strategy, it can enhance the frequency interference resistance characteristic of the converter, reduce the current tracking error and achieve an ultra-low total harmonic distortion rate of the rectifier current, having significant advantages.

[0039] Step S5: Generate a PWM signal according to the output signal of the frequency adaptive repetitive controller, and adjust the current waveform on the AC side of the V2G converter according to the PWM signal.

[0040] A low-harmonic combined control method for a V2G converter with frequency interference resistance provided by an embodiment of the present invention can enhance the frequency interference resistance characteristic of the converter, eliminate the harmonic distortion of the grid-connected reference current and make the grid-connected current accurately track the reference current by connecting a filter and a controller, thereby realizing an ultra-low harmonic rectifier. The control unit samples the AC side voltage signal and the AC side current signal, obtains an error signal by comparing the reference signal and the DC bus voltage feedback signal, inputs it to a frequency adaptive sliding average filter after a PI controller, multiplies the signal generated by the frequency adaptive sliding average filter by the AC side voltage signal as the reference signal of the AC side current, compares this signal with the AC side current signal to obtain the current error signal, and inputs the current error signal to the corresponding controller to generate a PWM control signal after a plug-in composite controller. The PWM control signal generated by the controller acts on the switching tubes of the rectifier bridge of the PWM rectifier. Among them, the DC bus voltage reference signal is used to control the output voltage to be constant; the frequency adaptive sliding average filter is used to enhance the frequency interference resistance characteristic of the converter and eliminate the influence of even multiple power frequency ripples in the DC voltage feedback signal, thereby eliminating the harmonic distortion of the grid-connected reference current; the composite controller can realize the accurate tracking of the grid-connected current to the reference current, enhance the frequency interference resistance characteristic of the converter and eliminate the harmonic distortion and tracking error of the grid-connected current, while retaining the fast dynamic characteristic.

[0041] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. Only the preferred embodiments of the present invention are expressed. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the present invention. As long as the combination of these technical features does not conflict, it should be considered as the scope recorded in this specification.

[0042] It should be noted that, for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the appended claims.

Claims

1. A low-harmonic combined control method for a V2G converter against frequency interference, where the V2G converter is a voltage-source PWM rectifier, characterized in that, The control method includes the following steps: Step S1: Obtain the voltage error signal by subtracting the DC-side voltage signal of the V2G converter from the DC voltage reference signal, and input the voltage error signal into the voltage outer-loop controller, which is a proportional-integral controller; Step S2: Connect the moving average filter directly in series with the output of the voltage outer-loop controller to suppress the interference of the periodic signal in the DC bus voltage on the current reference signal, so that the amplitude of the current reference signal is a pure DC value; Step S3: The output signal of the moving average filter is combined with the output signal of the phase-locked loop to generate the current reference signal, obtain the AC-side current signal, and subtract it from the current reference signal to obtain the current error signal, which is used as the input of the current loop controller; Step S4: For the current loop controller, use the repetitive controller as the feedforward controller, combine it with the weighted average deadbeat controller to construct an inserted composite controller, and use the Lagrange interpolation method to quickly update the coefficients and continuously control the delay value, so that the repetitive controller and the moving average filter have frequency adaptive characteristics; Step S5: Generate the PWM control signal of the V2G converter according to the output signal of the composite controller.

2. A low-harmonic combined control method for a V2G converter with anti-frequency interference according to claim 1, characterized in that: The transfer function of the repetitive controller is as follows: Among them, , is the compensation gain; is a zero-phase low-pass filter, F is the fractional part of N, Ni is the integer part of N, is the system compensator, z is the complex variable of the z-transform, and N is the ratio of the sampling frequency to the frequency corresponding to the sampling sample period.

3. A low-harmonic combined control method for a V2G converter with anti-frequency interference according to claim 1, characterized in that: The transfer function of the moving average filter is as follows: Where z is the complex variable of the z-transform, and N is the ratio of the sampling frequency to the corresponding frequency of the sampling sample period.

4. A low-harmonic combined controller device for a V2G converter with anti-frequency interference, comprising: A memory, a processor, and a computer program, characterized in that: the computer program is stored in the memory and is configured to be executed by the processor to implement a low-harmonic combined control method for a V2G converter against frequency interference according to any one of claims 1 to 3.

5. A computer-readable storage medium, characterized in that: A computer program is stored in the computer-readable storage medium, and the computer program is executed by the processor to implement a low-harmonic combined control method for a V2G converter against frequency interference according to any one of claims 1 to 3.

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