Method for correcting positive virtual impedance of LCL type inverter within Nyquist frequency

Abstract

The invention relates to the technical field of grid-connected inverters, in particular to a method for correcting positive virtual impedance of an LCL type inverter in a Nyquist frequency, which comprises the following steps of: accurately determining parameters of capacitor voltage feedback branches in an LCL filter, establishing an equivalent formula of virtual impedance, and adjusting parameters in primary differential feedback so as to correct the positive virtual impedance of the LCL type inverter in the Nyquist frequency. The method successfully widens the positive damping frequency range, optimizes the impedance characteristics of the inverter through the virtual impedance of the parallel proportional term, the primary differential term and the secondary differential term and the calculation of the total parallel impedance formula, and obtains the impedance characteristics of the inverter by drawing various impedance-frequency curves. Comprising a virtual impedance-frequency curve, a parallel impedance-frequency curve and a primary differential impedance-frequency curve, and an intuitive impedance characteristic analysis tool is provided for a system designer and an engineer; and finally, by drawing a total impedance-frequency curve and verifying whether the system maintains the positive resistance characteristic in the Nyquist frequency range, the stability of the system is ensured.

Description

Technical Field

[0001] The invention relates to the technical field of grid-connected inverters, and in particular to a method for correcting positive virtual impedance of an LCL type inverter within the Nyquist frequency. Background Art

[0002] With the increasing shortage of traditional fossil energy, new energy power generation technology, especially wind power and photovoltaic power generation, has developed rapidly. As a key interface device between the new energy distributed power generation system and the large power grid, the performance of the grid-connected inverter has a great impact on the quality of grid-connected power.

[0003] In recent years, with the widespread application of power electronic equipment and the large-scale access of distributed power sources, the penetration rate of the power grid has continued to increase. This high penetration rate causes the grid impedance to vary over a wide range, making the grid present the characteristics of a weak or even extremely weak grid. At the same time, the grid voltage contains a large number of background harmonics, which not only reduce the power quality, but also may pose a threat to the stable operation of the system.

[0004] In order to reduce the impact of grid voltage background harmonics on the output current of grid-connected inverters, the industry has widely adopted grid voltage feedforward control technology. In grid-connected inverter systems, the point of common coupling (PCC) voltage is usually used as a substitute for the grid voltage. Since the inductance value of the LCL filter is inversely proportional to the rated capacity of the grid-connected inverter, the filter capacitor voltage can be approximately equal to the PCC voltage. In actual engineering applications, the grid-connected inverter usually needs to be connected to the grid through a step-up transformer. At this time, the leakage inductance of the step-up transformer can be used as the grid-side inductance, and the PCC voltage is equal to the filter capacitor voltage.

[0005] Based on the above principle, existing scholars have proposed a capacitor voltage full feedforward control strategy. This strategy effectively eliminates the influence of grid voltage on grid-connected current by using capacitor voltage instead of PCC point voltage for feedforward control. At the same time, the capacitor voltage full feedforward control strategy can also provide active damping for the grid-connected inverter, suppress the occurrence of harmonic resonance, and thus improve the operating stability of the LCL grid-connected inverter.

[0006] However, the digital control delay problem in digital control technology poses a new challenge to the stability of the grid-connected inverter system. The control delay makes the active damping no longer equivalent to a constant resistor in parallel with the filter capacitor, but becomes a virtual impedance that changes with the frequency. This change may cause the system to lose stability at certain frequencies. Therefore, in order to solve the delay problem in the full feedforward control of capacitor voltage, it is urgent to develop a correction method that can expand the frequency range of positive damping to improve the stability and reliability of the grid-connected inverter system. Summary of the invention

[0007] The invention provides a carbon fiber prepreg tape laying device, which can effectively solve the problems in the background technology.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is:

[0009] A method for correcting positive virtual impedance of an LCL inverter within the Nyquist frequency comprises the following steps:

[0010] Determine the parameters of each branch of capacitor voltage feedback in the LCL filter, establish the equivalent formula of virtual impedance, and draw the virtual impedance-frequency curve;

[0011] Based on the parallel effect of proportional term, first differential term and second differential term, the equivalent formula of virtual impedance after parallel connection is established, and the impedance-frequency curve after parallel connection is drawn;

[0012] By adjusting the parameters in the first differential feedback, the frequency range of positive damping can be broadened;

[0013] Based on the value range of the first differential item parameter, determine the drawing value and draw the first differential impedance-frequency curve;

[0014] Based on the impedance equivalent formula of proportional term, first differential term and second differential term, calculate the total parallel impedance formula;

[0015] A total impedance-frequency curve is drawn according to the total parallel impedance formula to verify whether the system maintains a positive resistance characteristic within the Nyquist frequency range.

[0016] Furthermore, an equivalent formula of virtual impedance is established, including:

[0017] The impedance equivalent formula of proportional feedback is specifically expressed as:

[0018]

[0019] Where, L1 represents the inverter machine-measured inductance value, in mH; G d (s) represents a delay of 1.5 beats, i.e. e -1.5sTs ; s represents the Laplace operator, Ts represents the sampling period;

[0020] The impedance equivalent formula of the secondary differential feedback is specifically expressed as:

[0021]

[0022] Where C represents the inverter capacitance value.

[0023] Furthermore, the equivalent formula of virtual impedance after parallel connection is established, including:

[0024]

[0025] Among them, R eq (f) represents the equivalent impedance at frequency f; f r1 Indicates the resonant frequency of the measured inductance L1 and capacitance C.

[0026] Furthermore, the parameter range in the first differential feedback is calculated through the curve and formula of the parallel virtual impedance changing with frequency, and the positive damping frequency range is widened. Specifically:

[0027] The equivalent impedance formula of the first differential term is specifically expressed as:

[0028]

[0029] The equivalent resistance formula of the first differential term is specifically expressed as:

[0030]

[0031] Among them, Q is the first differential term parameter, T sam Represents the switching cycle.

[0032] Furthermore, the total parallel impedance formula is calculated by combining the proportional term, the first differential term and the second differential term formula, including:

[0033] The total parallel resistance formula is specifically expressed as:

[0034]

[0035] Furthermore, in the impedance-frequency curve, the resistor equivalent formula of the proportional feedback is specifically expressed as:

[0036]

[0037] The resistor equivalent formula for the secondary differential term feedback is specifically expressed as:

[0038]

[0039] Furthermore, it is checked whether the total impedance-frequency curve is a positive resistance within the Nyquist frequency range to determine whether the correction method is effective.

[0040] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps of the above-mentioned method for correcting positive virtual impedance of an LCL inverter within the Nyquist frequency.

[0041] An electronic device comprises a memory, a processor and a computer program stored in the memory and running on the processor. When the processor executes the program, the steps of the positive virtual impedance correction method of the LCL inverter within the Nyquist frequency are implemented.

[0042] The beneficial effects of the present invention are:

[0043] The scheme of the present invention corrects the output impedance of the inverter by accurately determining the parameters of each branch of the capacitor voltage feedback in the LCL filter and establishing an equivalent formula of the virtual impedance; specifically, by adjusting the parameters in the first differential feedback, the positive damping frequency range is successfully widened, which helps the system maintain stable damping characteristics in a wider frequency range and reduces the risks of resonance and oscillation.

[0044] The impedance characteristics of the inverter are optimized by paralleling the virtual impedance of the proportional term, the first differential term and the second differential term and calculating the total parallel impedance formula. By drawing a variety of impedance-frequency curves, including virtual impedance-frequency curves, impedance-frequency curves after parallel connection and first differential impedance-frequency curves, an intuitive impedance characteristic analysis tool is provided for system designers and engineers. Finally, by drawing the total impedance-frequency curve and verifying whether the system maintains the positive resistance characteristics within the Nyquist frequency range, the stability of the system is ensured. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0046] Figure 1 This is a control flow chart for correcting the internal resistance of the Nyquist frequency to be all positive resistance in the present invention;

[0047] Figure 2 It is a graph of proportional resistance and secondary differential impedance-frequency in the present invention;

[0048] Figure 3 The impedance-frequency curve diagram after the proportional term resistor and the secondary differential term resistor are connected in parallel in the present invention;

[0049] Figure 4 It is a primary differential impedance-frequency curve diagram in the present invention;

[0050] Figure 5 This is a total impedance-frequency curve diagram in the present invention. DETAILED DESCRIPTION

[0051] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0052] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly on the other element or there may be a central element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be a central element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only and do not represent the only implementation method.

[0053] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0054] like Figure 1 The control strategy flow chart of the positive virtual impedance correction method of the LCL inverter within the Nyquist frequency shown in the figure includes the following steps:

[0055] Determine the parameters of each branch of the capacitor voltage feedback in the LCL filter, establish an equivalent formula for virtual impedance, and draw a virtual impedance-frequency curve; establish an equivalent formula for virtual impedance after parallel connection based on the parallel effect of proportional terms, first differential terms, and second differential terms, and draw an impedance-frequency curve after parallel connection; broaden the positive damping frequency range by adjusting the parameters in the first differential feedback; determine the drawing value based on the first differential term parameter value range and draw a first differential impedance-frequency curve; calculate the total parallel impedance formula based on the impedance equivalent formula of the proportional terms, first differential terms, and second differential terms; draw a total impedance-frequency curve based on the total parallel impedance formula to verify whether the system maintains the positive resistance characteristic within the Nyquist frequency range.

[0056] By obtaining the parameters of each branch of capacitor voltage feedback (such as proportional term, first differential term, and second differential term), a basis is provided for subsequent steps. Specifically, an equivalent formula for virtual impedance is established, including:

[0057] The impedance equivalent formula of proportional feedback is specifically expressed as:

[0058]

[0059] Where, L1 represents the inverter machine-measured inductance value, in mH; G d (s) represents a delay of 1.5 beats, i.e. e -1.5sTs ; s represents the Laplace operator, Ts represents the sampling period;

[0060] The impedance equivalent formula of the secondary differential feedback is specifically expressed as:

[0061]

[0062] Where C represents the inverter capacitance value.

[0063] The impedance characteristics of different feedback branches are described by establishing an equivalent formula of virtual impedance.

[0064] By drawing the impedance-frequency curve, see Figure 2 As shown, the impedance characteristics of each feedback branch can be intuitively understood as the frequency changes, which facilitates analysis of whether the impedance of each branch is positive resistance or negative resistance at different frequencies.

[0065] In this embodiment, see the system parameters shown in the following table:

[0066] Machine side inductance 1mH Grid side inductor 2mH capacitance 2μF

[0067] Furthermore, the virtual impedance effect after parallel connection is considered to analyze its influence on system stability. Specifically, the virtual impedance equivalent formula after parallel connection is derived, which considers the parallel effect of proportional term, first differential term and second differential term to draw the virtual impedance-frequency curve after parallel connection. For details, see Figure 3 As shown, by selecting a series of frequency points and calculating the equivalent impedance value at each frequency point, a curve reflecting the change of impedance with frequency can be obtained, which is convenient for observing the change of impedance characteristics after parallel connection.

[0068] Specifically, the equivalent formula of virtual impedance after parallel connection is established, including:

[0069]

[0070] Among them, R eq (f) represents the equivalent impedance at frequency f; f r1 Indicates the resonant frequency of the measured inductance L1 and capacitance C, T sam Represents the switching cycle.

[0071] The introduction of virtual impedance is to improve the stability of LCL inverter, especially in the high frequency band. By adjusting the value of virtual impedance, the frequency response of the system can be affected, thereby optimizing the performance of the system; the equivalent formula of virtual impedance takes into account the change of frequency, so that at different frequencies, the virtual impedance can present different characteristics to adapt to the needs of the system.

[0072] Furthermore, the value range of the first differential term parameter Q is determined to optimize the impedance characteristics after parallel connection; the parameter range in the first differential feedback is calculated through the curve and formula of the change of the parallel virtual impedance with frequency, and the positive damping frequency range is widened. Specifically:

[0073] The equivalent impedance formula of the first differential term is specifically expressed as:

[0074]

[0075] The equivalent resistance formula of the first differential term is specifically expressed as:

[0076]

[0077] Among them, Q is the first differential term parameter, T sam The equivalent impedance formula of the first differential term and the equivalent resistance formula of the first differential term respectively describe the relationship between the equivalent impedance and equivalent resistance and the first differential feedback parameter, sampling frequency or switching period.

[0078] In this embodiment, according to the calculated parameter range, appropriate values ​​are selected to draw a differential impedance-frequency curve, see Figure 4 As shown, the Q range is finally determined to be (0.0357, 0.0735). By adjusting the parameter Q, the value of the equivalent impedance and equivalent resistance can be affected, thereby changing the frequency response of the system.

[0079] Furthermore, the total parallel impedance value formula is calculated based on the first differential term formula and the proportional term and second differential term formula; specifically, the total parallel impedance formula is calculated by combining the proportional term, the first differential term and the second differential term formula, including:

[0080] The total parallel resistance formula is specifically expressed as:

[0081]

[0082] Based on the equivalent formula of the determined virtual resistance, an impedance-frequency curve is drawn; in the impedance-frequency curve, the equivalent formula of the resistance of the proportional term feedback is specifically expressed as:

[0083]

[0084] The resistor equivalent formula for the secondary differential term feedback is specifically expressed as:

[0085]

[0086] In the proportional term feedback, the resistance is proportional to the frequency and inductance, and inversely proportional to the sine of the sampling time, which means that the resistance may increase with increasing frequency; in the quadratic differential term feedback, the resistance is inversely proportional to the sine of the frequency, capacitance, and sampling time, which means that the resistance may decrease with increasing frequency.

[0087] Check whether the total impedance-frequency curve is positive resistance within the Nyquist frequency range to determine whether the correction method is effective. According to the total impedance formula, draw the total impedance-frequency curve to determine whether it is positive resistance within the Nyquist frequency range. Figure 5 shown.

[0088] The present invention also discloses a computer-readable storage medium on which a computer program is stored. When the program is executed by a processor, the steps of the above-mentioned method for correcting the positive virtual impedance of an LCL-type inverter within the Nyquist frequency are realized.

[0089] The present invention also discloses an electronic device, including a memory, a processor and a computer program stored in the memory and running on the processor. When the processor executes the program, the steps of the above-mentioned method for correcting the positive virtual impedance of the LCL inverter within the Nyquist frequency are implemented.

[0090] Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for correcting positive virtual impedance of an LCL inverter within the Nyquist frequency, characterized in that: The following steps are involved: Determine the parameters of each branch of capacitor voltage feedback in the LCL filter, establish the equivalent formula of virtual impedance, and draw the virtual impedance-frequency curve; Based on the parallel effect of proportional term, first differential term and second differential term, the equivalent formula of virtual impedance after parallel connection is established, and the impedance-frequency curve after parallel connection is drawn; By adjusting the parameters in the first differential feedback, the frequency range of positive damping can be broadened; Based on the value range of the first differential item parameter, determine the drawing value and draw the first differential impedance-frequency curve; Based on the impedance equivalent formula of proportional term, first differential term and second differential term, calculate the total parallel impedance formula; A total impedance-frequency curve is drawn according to the total parallel impedance formula to verify whether the system maintains a positive resistance characteristic within the Nyquist frequency range.

2. The method for correcting positive virtual impedance of LCL inverter within Nyquist frequency according to claim 1, characterized in that: Establish the equivalent formula of virtual impedance, including: The impedance equivalent formula of proportional feedback is specifically expressed as: Where, L1 represents the inverter machine-measured inductance value, in mH; G d (s) represents a delay of 1.5 beats, i.e. e -1.5sTs ; s represents the Laplace operator, Ts represents the sampling period; The impedance equivalent formula of the secondary differential feedback is specifically expressed as: Where C represents the inverter capacitance value.

3. The method for correcting positive virtual impedance of LCL inverter within Nyquist frequency according to claim 1, characterized in that: The equivalent formula of virtual impedance after parallel connection is established, including: Among them, R eq (f) represents the equivalent impedance at frequency f; f r1 Indicates the resonant frequency of the measured inductance L1 and capacitance C.

4. The method for correcting positive virtual impedance of LCL inverter within Nyquist frequency according to claim 1, characterized in that: Through the curve and formula of the parallel virtual impedance changing with frequency, the parameter range in the first differential feedback is calculated, and the positive damping frequency range is widened. Specifically: The equivalent impedance formula of the first differential term is specifically expressed as: The equivalent resistance formula of the first differential term is specifically expressed as: Among them, Q is the first differential term parameter, T sam Represents the switching cycle.

5. The method for correcting positive virtual impedance of LCL inverter within Nyquist frequency according to claim 1, characterized in that: Combining the proportional term, first differential term, and second differential term formulas, the total parallel impedance formula is calculated, including: The total parallel resistance formula is specifically expressed as:

6. The method for correcting positive virtual impedance of LCL inverter within Nyquist frequency according to claim 2, characterized in that: In the impedance-frequency curve, the resistor equivalent formula of proportional feedback is specifically expressed as: The resistor equivalent formula for the secondary differential term feedback is specifically expressed as:

7. The method for correcting positive virtual impedance of LCL inverter within Nyquist frequency according to claim 1, characterized in that: Check whether the total impedance-frequency curve is positive resistance within the Nyquist frequency range to determine whether the correction method is effective.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method for correcting positive virtual impedance of an LCL inverter within the Nyquist frequency as described in any one of claims 1 to 7 are implemented.

9. An electronic device comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that: When the processor executes the program, the steps of the method for correcting positive virtual impedance of an LCL inverter within the Nyquist frequency as described in any one of claims 1 to 7 are implemented.

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