Second harmonic current suppression method based on fractional order filter

Abstract

The invention relates to a secondary harmonic current suppression method based on a fractional filter, which can realize secondary harmonic current suppression by designing a control loop of a preceding-stage DC / DC converter, does not need to use additional devices, and greatly reduces hardware cost and system complexity. The fractional order band-pass filter with the adjustable order is used for replacing a traditional integer order replacement filter, and the dynamic response speed of the system is effectively improved. The fractional order notch filter is introduced into the voltage control loop, the stability margin of the system can be guaranteed by adjusting the order, and in addition, the second harmonic current suppression effect is further improved by adjusting the gain coefficient of the fractional order notch filter. The problem that the system stability is reduced and the second harmonic current suppression performance improvement effect is limited due to a traditional integer order notch filter is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of harmonic current control in a new energy distributed power generation system, and in particular to a second harmonic current suppression method based on a fractional-order filter. Background Art

[0002] In recent years, with the increasing depletion of traditional fossil energy and the intensification of environmental pollution, new energy distributed power generation systems have developed rapidly and have been widely used in agriculture, industry, and urban livelihood fields. As the core power electronic device for the efficient conversion of DC power to AC power, the two-stage single-phase inverter plays a key role in new energy distributed power generation systems. When the single-phase inverter is connected to the grid or working off the grid, its output power exhibits a periodic pulsation characteristic with a pulsation frequency of twice the AC output voltage frequency, resulting in the generation of second harmonic current in the DC input current. Second harmonic current can lead to problems such as reduced efficiency of the two-stage single-phase inverter, increased system cost, shortened life of energy storage batteries and fuel cells, and increased size of magnetic devices. Therefore, suppressing the second harmonic current in the input current of the two-stage single-phase inverter to improve system efficiency and reduce system cost has important engineering practical significance.

[0003] Second harmonic current suppression methods are mainly divided into two categories:

[0004] One is to absorb the second harmonic current by adding a passive or active power decoupling circuit;

[0005] The other is to achieve second harmonic current suppression through active control strategy.

[0006] The first method requires adding additional electronic components (such as inductors, capacitors, switches, etc.), which increases system costs and reduces efficiency.

[0007] The second method relies on the system's inherent front-stage DC / DC converter to achieve second harmonic current suppression without adding additional devices, thus not increasing the system's cost and energy loss, and has the advantages of low cost and high efficiency.

[0008] To suppress the second harmonic current in two-stage single-phase inverters, relevant active control strategies at home and abroad include:

[0009] The paper titled “Low Frequency Current Ripple Reduction Technique With Active Control in a Fuel Cell Power System With Inverter Load” proposes a second harmonic current suppression strategy based on dual-loop control. However, in order to suppress the second harmonic current, the cutoff frequency of the voltage outer loop is set very low, resulting in poor system dynamic performance.

[0010] The paper titled “A Bandpass Filter Incorporated Into the Inductor Current Feedback Path for Improving Dynamic Performance of the Front-End DC–DC Converter in Two-Stage Inverter” proposes a second harmonic current suppression strategy based on virtual resistance. The system response speed is accelerated by using an integer-order bandpass filter, but the harmonic suppression effect is poor.

[0011] The papers titled "On the Reduction of Second Harmonic Current and Improvement of Dynamic Response for Two-Stage Single-Phase Inverter" and "An Integrated Power Decoupling Method for Single-Phase EV Onboard Charger in V2G Application" improve second harmonic current suppression by adding integer-order notch filters to the voltage loop. However, the introduction of integer-order notch filters results in a negative phase shift approaching 90 degrees, reducing the system stability margin. Furthermore, these papers design the integer-order notch filters to have a gain of zero at the resonant frequency, thereby improving harmonic suppression. However, the improvement in harmonic suppression is limited.

[0012] In summary, in the prior art, when the second harmonic current is suppressed by introducing the inductor current through an integer-order bandpass filter for feedforward, the response speed of the integer-order bandpass filter is slow, resulting in a decrease in the dynamic performance of the system; when the second harmonic current suppression effect is improved by introducing an integer-order notch filter in the voltage control loop, the improvement effect is limited and will reduce the system stability margin. Summary of the Invention

[0013] The present invention aims at solving the problems in the prior art and provides a method for suppressing second harmonic current based on a fractional-order filter.

[0014] This paper proposes a second harmonic current suppression method based on a fractional-order filter. This control method uses a fractional-order bandpass filter to suppress the second harmonic current while accelerating the system's dynamic performance. By adjusting the gain and order of the fractional-order notch filter, the second harmonic current suppression capability can be greatly improved while maintaining the system's phase margin. This achieves a comprehensive improvement in system dynamic performance, system stability, and second harmonic current suppression capability, solving the problems of poor dynamic performance, reduced system stability, and poor harmonic suppression in traditional control methods.

[0015] The technical solutions adopted by the present invention are as follows:

[0016] The second harmonic current suppression method based on fractional-order filter is applicable to a two-stage single-phase inverter system consisting of a front-stage DC / DC converter and a rear-stage DC / AC single-phase inverter, and also includes a fractional-order bandpass filter G FO-NF (s) and fractional-order notch filter G FO-BPF (s), the fractional-order bandpass filter G FO-NF (s) is used to speed up the system response speed, the fractional order notch filter G FO-BPF (s) is used to improve the second harmonic suppression effect and ensure the system phase margin, the method includes the following steps:

[0017] Step 1: In a single sampling period, the DC / DC converter output filter inductor current i L And the output voltage v o Sampling was performed separately.

[0018] Step 2: Set the reference voltage v of the output voltage of the pre-stage DC / DC converter ref The actual voltage v output by the previous DC / DC converter o Subtract and get the error e v .

[0019] Step 3: Convert the error e v With the fractional-order notch filter G FO-NF (s) is multiplied by the transfer function and filtered to obtain e v G FO-NF (s); where G FO-NF The expression of (s) is

[0020]

[0021] In the above formula, ξ1 represents the bandwidth of the fractional-order notch filter, ω o is the angular frequency of the AC output voltage, α represents the order of the fractional-order notch filter, s is the Laplace operator, and K represents the adjustable gain coefficient of the fractional-order notch filter.

[0022] Step 4: Setting up the PI Controller G v (s), and e obtained in step 3 v G FO-NF (s) and PI controller G v Multiply the transfer function of (s) to get e v G FO-NF (s)G v (s); G v The expression of (s) is

[0023]

[0024] In the above formula, k p is the proportionality coefficient, k i is the integration coefficient.

[0025] Step 5: Set up the PWM wave modulator to convert the inductor current i L Multiply by the virtual resistance coefficient R v G FO-BPF (s) / K PWM H(s), then passes through the fractional-order bandpass filter G FO-BPF After multiplying the transfer function of (s), it is then multiplied by the result e in step 4. v G FO-NF (s)G v (s) are subtracted, and the result of the subtraction is multiplied by the gain K of the PWM wave modulator PWM Generate the duty cycle D of the preceding DC / DC converter to achieve second harmonic current suppression.

[0026] The above R v represents the resistance of the introduced virtual resistor, H(s) is the characteristic voltage gain coefficient of the previous DC / DC converter, G FO-BPF (s) is a fractional-order bandpass filter, and its transfer function is

[0027]

[0028] In the above formula, β represents the order of the fractional-order bandpass filter, and ξ2 represents the bandwidth of the fractional-order bandpass filter;

[0029] Step 6: After steps 2 to 5, the DC port impedance Z of the front-stage DC / DC converter is finally obtained. dc The expression of (s) is

[0030]

[0031] By adjusting the coefficient R v Change the DC port impedance Z with K dc (s) at frequency 2ωo The amplitude at 0 is set to achieve the suppression of the second harmonic current.

[0032] Furthermore, in step 3, by adjusting the fractional-order notch filter G FO-NF The order α of (s) is used to improve the system phase margin, and the order value range is 0<α<1.

[0033] Furthermore, in step 5, the dynamic response speed is accelerated by adjusting the order β of the fractional-order bandpass filter, and the value range of the order β is 1<β<2.

[0034] Furthermore, by adjusting the gain coefficient K of the fractional-order notch filter, the DC port impedance Z can be increased. dc (s) at frequency 2ω o The amplitude at .

[0035] Furthermore, in step 6, the DC port impedance Z dc The denominator of the expression for (s) is at frequency 2ω o The amplitude range at is:

[0036]

[0037] Furthermore, the value range of the gain coefficient K is:

[0038]

[0039] Furthermore, by increasing the bandwidth of the voltage loop gain of the front-stage DC / DC converter, the second harmonic current suppression effect will not be affected, and the system dynamic performance can be improved while ensuring the second harmonic current suppression effect.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The second harmonic current suppression method based on the fractional-order filter provided by the present invention can achieve second harmonic current suppression by designing the control loop of the preceding DC / DC converter without using additional devices, greatly reducing hardware cost and system complexity.

[0042] By using a fractional-order bandpass filter with adjustable order instead of the traditional integer-order filter, the dynamic response speed of the system is effectively improved.

[0043] The present invention introduces a fractional-order notch filter into the voltage control loop, which can ensure the system stability margin by adjusting the order. In addition, the second harmonic current suppression effect is further improved by adjusting its gain coefficient, solving the problem of system stability degradation caused by traditional integer-order notch filters and the limited effect of improving the second harmonic current suppression performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is the structural block diagram of two-stage single-phase inverter;

[0045] Figure 2 This is the closed-loop control block diagram of the DC / DC converter;

[0046] Figure 3 for Figure 1 Schematic diagram of the structure of the controller;

[0047] Figure 4 is the Bode diagram of the fractional-order notch filter when the order α changes;

[0048] Figure 5 is the Bode diagram of the fractional-order bandpass filter when the order β changes;

[0049] Figure 6 is the dynamic response curve of integer-order bandpass filter and fractional-order bandpass filter;

[0050] Figure 7 The DC / DC converter input current waveform and FFT analysis results when the second harmonic current is not suppressed;

[0051] Figure 8 The input current waveform and FFT analysis results of the DC / DC converter based on the integer-order filter control strategy;

[0052] Figure 9 The input current waveform and FFT analysis results of the DC / DC converter under the proposed fractional-order filter-based control strategy. DETAILED DESCRIPTION

[0053] The present invention will be described below with reference to specific embodiments, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar components or components with the same or similar functions.

[0054] Directional terms used in the present invention, such as up, down, left, right, front, back, inside, outside, front, back, and side, are merely referenced to the accompanying drawings. The embodiments described below with reference to the accompanying drawings and the directional terms used are exemplary and intended only to explain the present invention and are not to be construed as limiting the present invention. Furthermore, the various specific process and material examples provided herein are examples of other processes and / or materials that one of ordinary skill in the art would recognize.

[0055] Please also refer to Figure 1 Figure 2 and Figure 3 ,in Figure 1 It is the structural block diagram of two-stage single-phase inverter;

[0056] Figure 2 This is the closed-loop control block diagram of the DC / DC converter; Figure 3 for Figure 1 Schematic diagram of the controller structure.

[0057] The second harmonic current suppression method based on fractional-order filter is applicable to a two-stage single-phase inverter system consisting of a front-stage DC / DC converter and a rear-stage DC / AC single-phase inverter, and also includes a fractional-order bandpass filter G FO-NF (s) and fractional-order notch filter G FO-BPF (s), the fractional-order bandpass filter G FO-NF (s) is used to speed up the system response speed, the fractional order notch filter G FO-BPF (s) is used to improve the second harmonic suppression effect and ensure the system phase margin, the method includes the following steps:

[0058] Step 1: In a single sampling period, the DC / DC converter output filter inductor current i L And the output voltage v o Sampling was performed separately.

[0059] Step 2: Set the reference voltage v of the output voltage of the pre-stage DC / DC converter ref The actual voltage v output by the previous DC / DC converter o Subtract and get the error e v .

[0060] Step 3: Convert the error e v With the fractional-order notch filter G FO-NF (s) is multiplied by the transfer function and filtered to obtain e v G FO-NF (s); where G FO-NF The expression of (s) is

[0061]

[0062] In the above formula, ξ1 represents the bandwidth of the fractional-order notch filter, ω o is the angular frequency of the AC output voltage, α represents the order of the fractional-order notch filter, s is the Laplace operator, and K represents the adjustable gain coefficient of the fractional-order notch filter.

[0063] Step 4: Setting up the PI Controller G v (s), and e obtained in step 3 v G FO-NF (s) and PI controller G v Multiply the transfer function of (s) to get e v GFO-NF (s)G v (s); G v The expression of (s) is

[0064]

[0065] In the above formula, k p is the proportionality coefficient, k i is the integration coefficient.

[0066] Step 5: Set up the PWM wave modulator to convert the inductor current i L Multiply by the virtual resistance coefficient R v G FO-BPF (s) / K PWM H(s), then passes through the fractional-order bandpass filter G FO-BPF After multiplying the transfer function of (s), it is then multiplied by the result e in step 4. v G FO-NF (s)G v (s) are subtracted, and the result of the subtraction is multiplied by the gain K of the PWM wave modulator PWM Generate the duty cycle D of the preceding DC / DC converter to achieve second harmonic current suppression.

[0067] The above R v represents the resistance of the introduced virtual resistor, H(s) is the characteristic voltage gain coefficient of the previous DC / DC converter, G FO-BPF (s) is a fractional-order bandpass filter, and its transfer function is

[0068]

[0069] In the above formula, β represents the order of the fractional-order bandpass filter, and ξ2 represents the bandwidth of the fractional-order bandpass filter;

[0070] Step 6: After steps 2 to 5, the DC port impedance Z of the front-stage DC / DC converter is finally obtained. dc The expression of (s) is

[0071]

[0072] By adjusting the coefficient R v Change the DC port impedance Z with K dc (s) at frequency 2ω o The amplitude at 0 is set to achieve the suppression of the second harmonic current.

[0073] The DC port impedance Z dc The denominator of the expression for (s) is at frequency 2ω o The amplitude range at is:

[0074]

[0075] The value range of the gain coefficient K is:

[0076]

[0077] like Figure 4 As shown, Figure 4 is the Bode diagram of the fractional-order notch filter when the order α changes. In step 3, by adjusting the fractional-order notch filter G FO-NF The order α of (s) is used to improve the system phase margin. The order value range is 0<α<1, which improves the suppression effect of the second harmonic current. The larger the impedance amplitude, the better the second harmonic current suppression effect.

[0078] When the order α = 1, it is an integer-order notch filter with a maximum negative phase shift of 88.1°, significantly reducing the system stability margin. As the order α decreases, the maximum negative phase shift also decreases. When α = 0.6, the maximum negative phase shift of the fractional-order notch filter is 52.6°. Therefore, compared with integer-order notch filters, fractional-order notch filters can improve system stability.

[0079] like Figure 5 As shown, Figure 5 is the Bode diagram of the fractional-order bandpass filter when the order β changes. In step 5, the dynamic response speed is accelerated by adjusting the order β of the fractional-order bandpass filter. The value range of the order β is 1<β<2.

[0080] As the order β increases, the maximum negative phase shift of the fractional-order bandpass filter decreases.

[0081] By adjusting the gain coefficient K of the fractional-order notch filter, the DC port impedance Z can be increased. dc (s) at frequency 2ω o The amplitude at .

[0082] By increasing the bandwidth of the voltage loop gain of the preceding DC / DC converter, the second harmonic current suppression effect is not affected, thereby improving the system's dynamic performance while maintaining the second harmonic current suppression effect. Conventional methods require setting the voltage loop bandwidth very low to suppress the second harmonic current. This method improves the second harmonic current suppression effect by adjusting the gain K of the fractional-order notch filter, eliminating the need to reduce the voltage loop bandwidth, thereby improving the system's dynamic performance.

[0083] like Figure 6 As shown, Figure 6The dynamic response curves of integer-order bandpass filter and fractional-order bandpass filter are shown in Figure 2. It can be found that compared with the integer-order bandpass filter, the fractional-order bandpass filter has a faster response speed, which proves that the fractional-order bandpass filter can speed up the system response speed.

[0084] like Figure 7 As shown, Figure 7 The DC / DC converter input current waveform and FFT analysis results are shown when the second harmonic current is not suppressed. As can be seen from the figure, the input current contains a significant second harmonic component. The FFT analysis results show that the second harmonic current accounts for 35.67%.

[0085] like Figure 8 As shown, Figure 8 The input current waveform and FFT analysis results of the DC / DC converter based on the integer-order filter control strategy are shown in the figure;

[0086] The input current waveform and FFT analysis results of the DC / DC converter based on the integer-order filter control strategy (α=1, β=1, K=0) are shown. The second harmonic component in the input current is reduced, and the FFT analysis results show that the second harmonic current accounts for 2.25%.

[0087] like Figure 9 As shown, Figure 9 The figure shows the input current waveform and FFT analysis results of the DC / DC converter under the proposed fractional-order filter-based control strategy.

[0088] The input current waveform and FFT analysis results of the DC / DC converter using a fractional-order filter (α = 1.2, β = 0.7, K = -0.5) control strategy are shown. The second harmonic component in the input current is significantly reduced, with FFT analysis results showing that the second harmonic current accounts for only 0.51%.

[0089] The results show that compared with integer-order filters, the proposed control method can further improve the second harmonic current suppression capability, dynamic response speed and system stability margin.

[0090] Compared with the prior art, the present invention has the following beneficial effects:

[0091] The second harmonic current suppression method based on the fractional-order filter provided by the present invention can achieve second harmonic current suppression by designing the control loop of the preceding DC / DC converter without using additional devices, greatly reducing hardware cost and system complexity.

[0092] By using a fractional-order bandpass filter with adjustable order instead of the traditional integer-order filter, the dynamic response speed of the system is effectively improved.

[0093] The present invention introduces a fractional-order notch filter into the voltage control loop, which can ensure the system stability margin by adjusting the order. In addition, the second harmonic current suppression effect is further improved by adjusting its gain coefficient, solving the problem of system stability degradation caused by traditional integer-order notch filters and the limited effect of improving the second harmonic current suppression performance.

[0094] 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 and improvements 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 second harmonic current suppression method based on a fractional-order filter is applicable to a two-stage single-phase inverter system consisting of a front-stage DC / DC converter and a back-stage DC / AC single-phase inverter. The characteristics are: Also includes a fractional-order bandpass filter G FO-NF (s) and fractional-order notch filter G FO-BPF (s), the fractional-order bandpass filter G FO-NF (s) is used to speed up the system response speed, the fractional order notch filter G FO-BPF (s) is used to improve the second harmonic suppression effect and ensure the system phase margin, the method includes the following steps: Step 1: In a single sampling period, the DC / DC converter output filter inductor current i L And the output voltage v o Sampling was carried out separately; Step 2: Set the reference voltage v of the output voltage of the pre-stage DC / DC converter ref The actual voltage v output by the previous DC / DC converter o Subtract and get the error e v ; Step 3: Convert the error e v With the fractional-order notch filter G FO-NF (s) is multiplied by the transfer function and filtered to obtain e v G FO-NF (s); where G FO-NF The expression of (s) is In the above formula, ξ1 represents the bandwidth of the fractional-order notch filter, ω o is the angular frequency of the AC output voltage, α represents the order of the fractional-order notch filter, s is the Laplace operator, and K represents the adjustable gain coefficient of the fractional-order notch filter; Step 4: Setting up the PI Controller G v (s), and e obtained in step 3 v G FO-NF (s) and PI controller G v Multiply the transfer function of (s) to get e v G FO-NF (s)G v (s); G v The expression of (s) is In the above formula, k p is the proportionality coefficient, k i is the integration coefficient; Step 5: Set up the PWM wave modulator to convert the inductor current i L Multiply by the virtual resistance coefficient R v G FO-BPF (s) / K PWM H(s), then passes through the fractional-order bandpass filter G FO-BPF After multiplying the transfer function of (s), it is then multiplied by the result e in step 4. v G FO-NF (s)G v (s) are subtracted, and the result of the subtraction is multiplied by the gain K of the PWM wave modulator PWM Generate the duty cycle D of the previous DC / DC converter to achieve second harmonic current suppression; The above R v represents the resistance of the introduced virtual resistor, H(s) is the characteristic voltage gain coefficient of the previous DC / DC converter, G FO-BPF (s) is a fractional-order bandpass filter, and its transfer function is In the above formula, β represents the order of the fractional-order bandpass filter, and ξ2 represents the bandwidth of the fractional-order bandpass filter; Step 6: After steps 2 to 5, the DC port impedance Z of the front-stage DC / DC converter is finally obtained. dc The expression of (s) is: By adjusting the coefficient R v Change the DC port impedance Z with K dc (s) at frequency 2ω o The amplitude at is set to achieve the suppression of the second harmonic current.

2. The method for suppressing second harmonic current based on fractional-order filter according to claim 1, characterized in that: In step 3, by adjusting the fractional-order notch filter G FO-NF The order α of (s) is used to improve the system phase margin, and the order value range is 0<α<1.

3. The method for suppressing second harmonic current based on fractional-order filter according to claim 1, characterized in that: In step 5, the dynamic response speed is accelerated by adjusting the order β of the fractional-order bandpass filter, and the value range of the order β is 1<β<2.

4. The method for suppressing second harmonic current based on fractional-order filter according to claim 1, characterized in that: By adjusting the gain coefficient K of the fractional-order notch filter, the DC port impedance Zdc(s) can be increased at a frequency of 2ω. o The amplitude at .

5. The method for suppressing second harmonic current based on fractional-order filter according to claim 4, characterized in that: In step 6, the DC port impedance Z dc The denominator of the expression for (s) is at frequency 2ω o The amplitude range at is:

6. The method for suppressing second harmonic current based on fractional-order filter according to claim 4, characterized in that: The value range of the gain coefficient K is:

7. The method for suppressing second harmonic current based on fractional-order filter according to claim 1, characterized in that: By increasing the bandwidth of the voltage loop gain of the front-stage DC / DC converter, the second harmonic current suppression effect will not be affected, and the system dynamic performance can be improved while ensuring the second harmonic current suppression effect.

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