Model-free active damping control method for lcl grid-connected inverter based on notch filter

Abstract

The application discloses a kind of LCL grid-connected inverter model-free active damping control methods based on wave filter, belong to power electronics technical field.The application is by current sensor acquisition net side and inverter side current, center frequency is fundamental wave by wave filter setting, obtain harmonic current, two are subtracted to obtain capacitor branch harmonic current, increase virtual resistance and inhibit resonance, its voltage drop is combined with inverter output voltage reference value after processing.At the same time, inverter side current differential equation is rewritten as super-local model form, unknown quantity is estimated using super-local extended state observer, and damping resonance compensation voltage is calculated, and finally the control voltage removed harmonic influence is obtained.The application does not need LCL model parameter, and is high in model parameter robustness, only with inverter side and net side current, reduce the number of sensors, reduce cost, effectively inhibit resonance sharp peak, improve power quality, and has important significance in the grid-connected inverter control in the field of power electronics.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power electronics, in particular to a model-free active damping control method for LCL grid-connected inverter based on notch filter. BACKGROUND

[0002] LCL grid-connected inverter is widely concerned due to its superior high-frequency harmonic suppression capability. Three-phase grid-connected inverter adopts LCL filter, which has better high-frequency harmonic attenuation and better filtering effect, but needs to consider the resonance suppression problem. Since LCL filter is a third-order system, there are problems such as high-frequency resonance and stable control difficulty.

[0003] The prior art weakens the resonance peak by increasing the damping. According to the source of the damping, it is mainly divided into two categories of passive damping method and active damping method. The method of increasing damping by connecting series and parallel resistors is called passive damping method. Although the passive damping method is simple and easy to implement, the additional damping resistor will produce additional power loss and reduce the high-frequency harmonic suppression capability of the LCL filter.

[0004] The active damping method is to feed back the voltage or current of the filter to the system as a state variable, which is equivalent to introducing a virtual damping resistor, so as to achieve the purpose of suppressing harmonic peak. Some documents propose an active damping scheme based on input grid current feedback. However, since the LCL model parameters are needed in the process of calculating the voltage compensation value, when the filter parameters change due to environmental changes, the active damping effect will be poor. SUMMARY

[0005] The purpose of the present application is to provide a model-free active damping control method for LCL grid-connected inverter based on notch filter, which can observe the current harmonics flowing into the capacitor branch for subsequent calculation, and can reduce the sensors of the capacitor branch. And without using LCL model parameters and capacitor voltage sampling value, the robustness of the active damping strategy to inductance parameters is greatly improved.

[0006] To achieve the above purpose, the present application provides a model-free active damping control method for LCL grid-connected inverter based on notch filter, comprising the following steps:

[0007] S1, the current sensor is used to collect the grid-side current and the inverter-side current in the LCL circuit, and the loop voltage equation is derived from the capacitor branch to the inverter side according to the normal circuit structure;

[0008] S2, using a notch filter to the collected grid-side current and inverter-side current, the center frequency of the notch filter is set to the fundamental wave, get all the harmonic frequencies except the fundamental frequency, and then get the grid-side filter current and inverter-side filter current except the fundamental current, which is considered as harmonic current; the center frequency of the notch filter affects the notch depth and bandwidth of the notch filter;

[0009] S3, the inverter-side filter current minus the grid-side filter current, get the harmonic current of the capacitor branch, and increase the virtual resistance on the capacitor branch to suppress resonance, calculate the voltage drop generated by the virtual resistance in the capacitor branch; assuming that the fundamental current is hindered by the notch filter, only the harmonic current flows through the capacitor loop, rewrite the loop voltage equation, and combine the voltage compensation with the inverter output voltage reference value to achieve the harmonic suppression effect of active damping;

[0010] S4, rewrite the inverter-side current differential equation in step S1 into a hyperlocal model form, use a hyperlocal extended state observer to estimate the unknown quantity in the hyperlocal model, and get an equation containing the current observation value and the hyperlocal model adjustment parameter;

[0011] S5, according to the equation obtained in step S4, calculate the damping resonance compensation voltage, subtract the obtained inverter reference voltage from the damping resonance compensation voltage, and get the final input control voltage without harmonic influence.

[0012] Preferably, in step S1, the current sensor is used to collect the grid-side current i g and the inverter-side current i conv , according to the normal circuit structure, the capacitor branch is derived to the inverter side, and the loop voltage equation is:

[0013]

[0014] Preferably, in step S2, the collected grid-side current i g and the inverter-side current i conv are used to use a notch filter, the center frequency of the notch filter is set to the fundamental wave, and all the harmonic frequencies except the fundamental frequency are obtained as follows:

[0015]

[0016] Where, ω ref is the center frequency of the notch filter, x1 x2 affects the notch depth and bandwidth of the notch filter, and the grid-side filter current i g NF and the inverter-side filter current i conv NF are obtained, which are considered as harmonic currents.

[0017] Preferably, in step S3, the voltage drop generated by the virtual resistance in the capacitor branch is:

[0018]

[0019] Assuming that the fundamental current is hindered by the trap filter, only the harmonic current flows through the capacitor loop, formula (1) is changed to the following form:

[0020]

[0021] As the voltage compensation quantity is combined with the inverter output voltage reference value, the harmonic suppression effect of active damping is realized.

[0022] Preferably, in step S4, the inverter side current differential equation of formula (9) is rewritten into the form of hyperlocal model, and the expression is as follows:

[0023]

[0024] Where F contains the disturbance quantity and the unknown quantity, and α is the coefficient of model input;

[0025] The unknown quantity in formula (11) is estimated by using the hyperlocal extended state observer, and formula (11) is rewritten in the following form:

[0026]

[0027] Where,

[0028] i erro =i eso -i conv NF (13);

[0029] i eso is the current observation value, y1 and y2 are the adjustment parameters of the hyperlocal model.

[0030] Preferably, in step S5, the damping harmonic compensation voltage v r comp is calculated according to formula (12), the obtained inverter reference voltage v c is subtracted by the damping harmonic compensation voltage v r comp , and the final input inverter reference voltage v comp is obtained, which is the control voltage removing the harmonic effect, and the specific expression is:

[0031]

[0032] v comp= v c -v r comp (15)。

[0033] Therefore, the application adopts the above-mentioned structure, and the LCL grid-connected inverter model-free active damping control method based on a notch filter has the following beneficial effects:

[0034] (1) In the process of calculating the damping resonance compensation voltage v r comp , the LCL model parameters are not needed, so the model parameters have high robustness.

[0035] (2) Only the inverter side current and the grid side current are needed, and the capacitor voltage is not needed, so the number of sensors is reduced, and the cost is reduced.

[0036] The technical solutions of the application will be further described in detail below through the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0037] Fig. 1 It is an equivalent model diagram of the LCL grid-connected inverter of the LCL grid-connected inverter model-free active damping control method based on a notch filter.

[0038] Fig. 2 It is an experimental data diagram of the LCL grid-connected inverter model-free active damping control method based on a notch filter. DETAILED DESCRIPTION

[0039] The technical solutions of the application will be further described in detail below through the drawings and examples.

[0040] Unless otherwise defined, the technical terms or scientific terms used in the application should be understood as the usual meanings understood by those skilled in the art to which the application belongs. The "first", "second" and similar words used in the application do not represent any order, number or importance, but are only used to distinguish different components. "Include" or "contain" and similar words mean that the elements or objects before the words cover the elements or objects listed after the words and their equivalents, and do not exclude other elements or objects. "Connected" or "connected" and similar words are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to represent relative positional relationships, and when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0041] EMBODIMENT

[0042] As Figs. 1-2As shown, the application provides a trap filter-based LCL grid-connected inverter model-free active damping control method, including the following steps

[0043] Step S1: applying a current sensor to collect the grid-side current i g and the inverter-side current i conv , according to the normal circuit structure, deriving from the capacitor branch to the inverter side, at this time the loop voltage equation is:

[0044]

[0045] Step S2: using a trap filter on the collected grid-side current i g and the inverter-side current i conv The trap filter can be regarded as a filter that rapidly attenuates the input signal at a certain frequency point to achieve the filtering effect of hindering the passage of signals at this frequency. The trap filter belongs to a kind of band-stop filter, and the center frequency of the trap filter is set to the fundamental wave, and the purpose is to obtain all harmonic frequencies except the fundamental wave frequency:

[0046]

[0047] Where ω ref is the center frequency of the trap filter, x1 and x2 affect the trap depth and bandwidth of the trap filter, and the grid-side filtered current i g NF and the inverter-side filtered current i conv NF are obtained, which are regarded as harmonic currents.

[0048] Step S3: subtracting the grid-side filtered current i conv NF from the inverter-side filtered current i g NF , the harmonic current of the capacitor branch can be obtained, and a virtual resistor is added to the harmonic current of the capacitor branch to suppress resonance (a resistor is directly connected in series in the capacitor branch to become passive damping, but this method has large power loss, therefore we replace the passive damping with this virtual resistor to achieve the effect of resonance suppression), the voltage drop generated by the virtual resistor in the capacitor branch is:

[0049]

[0050] Assuming that the fundamental wave current is hindered by the trap filter and only the harmonic current flows through the capacitor loop, formula (1) can be changed to the following form:

[0051]

[0052] The voltage compensation quantity can be combined with the inverter output voltage reference value to achieve the harmonic suppression effect of active damping.

[0053] Step 4: The inverter-side current differential equation of formula (9) is rewritten into the form of a hyperlocal model, which is to estimate the actual parameters of the line and the possible changing quantities as disturbance terms, so as to ensure that even if the actual parameters of the line suddenly change, the control effect can still be kept within the normal range as follows:

[0054]

[0055] Where F contains the disturbance quantity and the unknown quantity, and a is the coefficient of the model input.

[0056] The unknown quantity in formula (11) is estimated using a hyperlocal extended state observer, and formula (11) can be rewritten in the following form:

[0057]

[0058] Where

[0059] i erro =i eso -i conv NF (13);

[0060] i eso is the current observation value, and y1 and y2 are the adjustment parameters of the hyperlocal model.

[0061] Step S5: The damping harmonic compensation voltage v r comp is calculated according to formula (12). c Subtract the damping harmonic compensation voltage v r comp from the obtained inverter reference voltage v comp , to obtain the final reference voltage v comp of the inverter, which is the control voltage without harmonic influence.

[0062]

[0063] v c -v r comp (15).

[0064] Therefore, the application adopts the above-mentioned model-free active damping control method of the LCL grid-connected inverter based on the notch filter, filters the inverter measurement current and the grid-side current respectively using the notch filter, obtains the current harmonics after filtering the fundamental wave, subtracts the grid-side current harmonics from the inverter-side current harmonics, can accurately calculate the current harmonics flowing into the capacitor branch for subsequent calculation, and can reduce the sensors of the capacitor branch. The harmonic current equation of the capacitor branch is rewritten into a hyper-local model form, unknown parts in the model are estimated using the extended state method, without using the LCL model parameters and the capacitor voltage sampling value, and the robustness of the active damping strategy to the inductance parameters is greatly improved.

[0065] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit it, although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that: it can still modify or equivalently replace the technical solutions of the present application, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present application.

Claims

1. A method for model-free active damping control of LCL grid-connected inverter based on notch filter, characterized in that: The method comprises the following steps: S1, the current sensor is used to collect the grid-side current and the inverter-side current in the LCL circuit, and the loop voltage equation is derived from the capacitor branch to the inverter side; S2, the grid-side current and the inverter-side current collected are filtered by a notch filter, the center frequency of the notch filter is set to the fundamental wave, all harmonic frequencies except the fundamental wave frequency are obtained, and then the grid-side filter current and the inverter-side filter current except the fundamental wave current are obtained, which are regarded as harmonic currents; the center frequency of the notch filter affects the notch depth and bandwidth of the notch filter; S3, the inverter-side filter current is subtracted from the grid-side filter current to obtain the harmonic current of the capacitor branch, a virtual resistor is added to the harmonic current of the capacitor branch to suppress resonance, and the voltage drop generated by the virtual resistor in the capacitor branch is calculated; assuming that the fundamental wave current is hindered by the notch filter, only the harmonic current flows through the capacitor loop, the loop voltage equation is rewritten, the rewritten equation is combined with the inverter output voltage reference value to realize the harmonic suppression effect of active damping; S4, the inverter-side current differential equation in step S1 is rewritten into the form of a hyperlocal model, unknown quantities in the hyperlocal model are estimated by using a hyperlocal extended state observer, and an equation containing current observation values and hyperlocal model adjustment parameters is obtained; S5, according to the equation obtained in step S4, the damping resonance compensation voltage is calculated, the obtained inverter reference voltage is subtracted by the damping resonance compensation voltage, and the final input control voltage without harmonic influence is obtained; In step S1, the current sensor is applied to collect the grid-side current in the LCL circuit The inverter current is measured The inverter-side derivation from the capacitor branch, at which time the loop voltage equation: （1）； In step S3, the voltage drop generated by the virtual resistor in the capacitor branch is: （9）； Assuming that the fundamental wave current is hindered by the notch filter, only the harmonic current flows through the capacitor loop, formula (1) is changed into the following form: （10）； The active damping harmonic suppression effect is realized by combining the voltage compensation amount with the inverter output voltage reference value. In step S4, formula (9) is rewritten into the form of a hyperlocal model, and the expression is as follows: （11）； wherein comprising a disturbance quantity and an unknown quantity, a coefficient for the model input; Unknown quantities in formula (11) are estimated by using a hyperlocal extended state observer, and formula (11) is rewritten into the following form: （12）； wherein, （13）； is a current observation value, and is a tuning parameter of the super-local model.

2. The method of claim 1, wherein the method is based on a wave-trap filter based LCL grid-connected inverter model-free active damping control method. In step S2, the acquired grid-side current and the inverter-side current Using a notch filter, the center frequency of the notch filter is set to the fundamental wave, and all harmonic frequencies except the fundamental wave frequency are obtained as follows: （2）； （3）； （4）； （5）； （6）； （7）； （8）； wherein is the center frequency of the notch filter, affects the notch depth and bandwidth of the notch filter, resulting in a grid-side filter current , the inverter-side filter current , is considered a harmonic current.

3. The method of claim 1, wherein the method is characterized by: In step S5, the damping resonance compensation voltage is calculated according to formula (12) The obtained inverter reference voltage Subtract the damping resonance compensation voltage The final input inverter reference voltage This voltage is the control voltage without harmonic influence, and the specific expression is （14）； （15）。

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