Composite control method for improving current control performance of three-phase LCL type grid-connected inverter

Abstract

The invention discloses a compound control method for improving the current control performance of a three-phase LCL type grid-connected inverter. The method comprises the following steps: establishing a system loop gain based on digital control delay; composite control is realized by adopting a mode of combining a capacitance current sampling modification mode and a delay compensation link, and the capacitance current sampling modification mode is used for shortening capacitance current sampling delay and comprises the steps of sampling grid-connected current at a wave trough of a triangular carrier wave, sampling capacitance current at a wave crest of the triangular carrier wave, and carrying out delay compensation on the grid-connected current. A modulation signal is loaded at the next grid-connected current sampling moment, and capacitance current sampling delay is shortened by half beat; the delay compensation link is used for compensating the influence of digital control delay, the influence of digital control delay is reduced by embedding a lead phase compensator in a capacitive current feedback loop, and the stability of the system and the quality of grid-connected current are improved. According to the invention, composite control is realized by adopting a mode of combining a capacitor current sampling modification mode and a delay compensation link addition mode.

Description

Technical Field

[0001] This invention relates to the field of LCL-type grid-connected inverters, and more specifically to a composite control method for improving the current control performance of three-phase LCL-type grid-connected inverters. Background Technology

[0002] With the development of clean and renewable energy, more and more distributed energy sources are replacing traditional fossil fuels and being integrated into the power grid. As the interface device for connecting distributed generation systems to the grid, the grid-connected inverter bears the important responsibility of integrating distributed energy into the grid. To reduce the harmonic content in the grid-connected current and improve its quality, filtering devices are required. Commonly used filters in engineering include L-type filters and LCL-type filters. Compared to L-type filters, LCL-type filters have better filtering performance, and require less inductance for the same filtering effect. However, an LCL-type filter is a third-order system with inherent resonant spikes, which, if not suppressed, can cause instability in the inverter system.

[0003] There are two existing methods for suppressing resonant spikes: active damping and passive damping. Compared to passive damping, active damping uses state variable feedback to virtually create a resistor to suppress resonant spikes, without adding additional passive components and thus avoiding additional power loss. Under digital control, the characteristics of the virtual resistance are altered by the digital control delay, and the equivalent virtual resistance becomes a frequency-dependent virtual impedance. The positive damping range of this virtual impedance is... , The sampling frequency is the resonant frequency of the system. Exceeded In the equivalent virtual impedance, positive damping becomes negative damping, which affects the robustness and stability of the system, especially at the resonant frequency. At times, the system cannot be stable; under weak grid conditions, the line impedance between the inverter and the grid will affect the resonant frequency of the system, making the system susceptible to instability.

[0004] There are three existing solutions: 1) Predictive control: using a state observer to predict the next state variable of the system in advance. However, in actual engineering, changes in circuit parameters and line impedance can cause deviations between the prediction model and the actual system, thus affecting the actual control performance; 2) Modifying the sampling method: reducing computational delay by increasing the number of samples. However, using only this method to solve the control delay problem will greatly increase the computational burden on the processor and may also introduce aliasing and switching noise; 3) Indirect delay compensation method: reducing the impact of delay on the active damping characteristics by introducing a delay compensation element in the active damping loop. However, using only this method to compensate for the system delay will make the control algorithm very complex.

[0005] Existing technical solutions typically employ a single approach to address the resonance problem of digitally controlled LCL grid-connected inverters and the impact of digital control delay on the system, which increases the system's control complexity. In weak grid conditions, the system's control becomes even more complex, and its robustness is insufficient.

[0006] Therefore, while ensuring system stability, improving the system's robustness to weak power grids and optimizing the system control algorithm are issues that need to be considered and addressed. Summary of the Invention

[0007] The main objective of this invention is to provide a composite control method to improve the current control performance of a three-phase LCL grid-connected inverter. Under the premise of ensuring system stability, this invention optimizes the system's control algorithm and improves the system's robustness and stability under weak grid conditions by adopting an innovative control strategy that combines modifying the capacitor current sampling method with embedding a leading phase compensator in the capacitor current feedback loop.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a composite control method for improving the current control performance of a three-phase LCL grid-connected inverter, the method comprising: A transfer function from output current to reference current is established for an LCL-type grid-connected inverter with capacitor current as the damping source under weak grid conditions. A digital control delay is introduced, and a system loop gain based on the digital control delay is established on the basis of the transfer function. Composite control is achieved by combining modified capacitor current sampling method with the addition of delay compensation stage: Modifying the capacitor current sampling method to shorten the capacitor current sampling delay includes: sampling the grid-connected current at the trough of the triangular carrier wave, sampling the capacitor current at the peak of the triangular carrier wave, and loading the modulation signal at the next grid-connected current sampling time, thereby shortening the capacitor current sampling delay by half a beat of sampling delay. The delay compensation stage is used to compensate for the impact of digital control delay. By embedding a leading phase compensator in the capacitor current feedback loop, the impact of digital control delay is reduced, and the stability of the system and the quality of grid-connected current are improved. The output signal of the current controller and the output signal of the capacitor current feedback loop are calculated, and the output adjustment signal is transformed into the input modulation signal of the space vector pulse width modulation module to drive the LCL grid-connected inverter.

[0009] Preferably, for an LCL-type grid-connected inverter using capacitor current as the damping source under weak grid conditions, the transfer function from output current to reference current does not consider digital control delay, and the specific expression is as follows: ; in, Represents the transfer function. Represents a complex variable. Indicates the inverter-side inductance. Indicates the filter capacitor. Indicates the grid-side inductance. This indicates the transmission line impedance between the inverter and the power grid. For a leading phase compensator, Indicates current controller, This represents the gain of the PWM stage that controls the on / off state of the switching devices. The resonant angular frequency of the system is expressed as follows: .

[0010] Preferably, the digital control delay includes a 1-step sampling and calculation delay and a 0.5-step PWM delay, expressed as: ; in, Indicates digital control delay. Indicates the digital sampling period.

[0011] Preferably, the expression for the system loop gain based on digital control delay is: ; in, This represents the system loop gain.

[0012] Preferably, a current controller The expression is: ; in, This is the proportionality coefficient. The resonance coefficient, The angular frequency of the power grid. It is the resonant angular frequency.

[0013] Preferably, the resonant angular frequency ,in This represents the maximum allowable deviation for power grid frequency fluctuations.

[0014] Preferred, leading phase compensator The expression is: ; in, This represents the gain of the lead phase compensator. This represents the compensation coefficient of the leading phase compensator. It is used to modulate the magnitude of the compensation phase.

[0015] Preferably, the compensation coefficient .

[0016] Preferably, the parameter design of the leading phase compensator and current controller includes: Determine the cutoff frequency According to the cutoff frequency Determine the proportionality coefficient ; Regarding transmission line impedance Fluctuations, design system gain margin and Combined with the determined proportional coefficient Design the gain of the leading phase compensator The range of values ​​for ; From gain Choose a suitable value from the range of possible values; if no suitable value exists, increase the cutoff frequency. Re-determine the proportionality coefficient Or adjust the amplitude margin and ; According to gain The values ​​of and the range of phase margin of the system are used to design the resonance coefficient. .

[0017] Secondly, the present invention provides a three-phase LCL type inverter, which adopts the composite control method for improving the current control performance of the three-phase LCL type grid-connected inverter as described in any one of the first aspects.

[0018] The beneficial effects of this invention are: This invention provides a composite control method to improve the current control performance of a three-phase LCL grid-connected inverter. While ensuring system stability, it significantly optimizes the control complexity of a single control strategy by employing an innovative control strategy that combines modified capacitor current sampling with an embedded lead-phase compensator in the capacitor current feedback loop. The positive damping region of the equivalent virtual resistance of the capacitor current active damping obtained by this method is expanded to the entire Nyquist frequency range, making the system more adaptable to a wide range of line impedance variations under weak grid conditions, thus greatly improving the system's robustness and stability.

[0019] In addition, this method can greatly improve the control accuracy of the current of a three-phase LCL grid-connected inverter and reduce the harmonic components in the grid-connected current. Attached Figure Description

[0020] Figure 1 This is a system structure diagram of a composite control method for improving the current control performance of a three-phase LCL grid-connected inverter according to an embodiment of the present invention. Figure 2 This is a block diagram of a traditional capacitor current active damping control system. Figure 3 This is an equivalent control block diagram of a composite control method for improving the current control performance of a three-phase LCL grid-connected inverter according to an embodiment of the present invention. Figure 4 This is a block diagram of the virtual resistance equivalent control of active damping according to an embodiment of the present invention; Figure 5 This is a schematic diagram of a modified capacitor current sampling method according to an embodiment of the present invention; Figure 6 Different parameters of the leading phase compensator according to an embodiment of the present invention The corresponding frequency response curve; Figure 7 A current controller according to an embodiment of the present invention and advanced phase compensator Parameter design flowchart; Figure 8 The amplitude-frequency response curve of the equivalent virtual impedance of a traditional capacitor with active damping is shown. Figure 9 This is the amplitude-frequency response curve of the active damping equivalent virtual impedance when only the capacitor current sampling method is modified in this invention; Figure 10 The diagram shows the amplitude-frequency response curve of the active damping equivalent virtual impedance when the capacitor current sampling method and the advanced phase compensator are modified in this invention. Figure 11 The gain of a traditional capacitor current active damping loop under weak grid conditions is related to the line impedance. Frequency response curve under varying conditions; Figure 12 The loop gain of the control method of this invention is related to the power grid line impedance under weak power grid conditions. Frequency response curve under varying conditions; Figure 13 For traditional capacitor current active damping methods, the impedance of power lines in weak power grids The waveform of the grid-connected current under changing conditions; among which, Figure 13 (a) in the figure represents the line impedance. equal Frequency response curve at time, Figure 13 (b) in the figure represents the line impedance. equal Frequency response curve at time, Figure 13 (c) in the figure represents the line impedance. equal Frequency response curve at time; Figure 14 The control strategy proposed in this invention addresses the impedance of power grid lines in weak power grids. The waveform of the grid-connected current under changing conditions; among which, Figure 14 (a) in the figure represents the line impedance. equal The waveform of the grid-connected current at that time. Figure 14 (b) in the figure represents the line impedance. equal The waveform of the grid-connected current at that time. Figure 14 (c) in the figure represents the line impedance. equal The waveform of the grid-connected current at that time; Figure 15 The control strategy proposed in this invention addresses the impedance of power grid lines in weak power grids. A graph showing the total harmonic distortion rate of the grid-connected current under varying conditions; among which... Figure 15 (a) in the figure represents the line impedance. equal The graph of total harmonic distortion of grid-connected current at that time. Figure 15 (b) in the figure represents the line impedance. equal The graph of total harmonic distortion of grid-connected current at that time. Figure 15 (c) in the figure represents the line impedance. equal The graph shows the total harmonic distortion rate of the grid-connected current at that time. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments provided by this invention without inventive effort are within the scope of protection of this invention.

[0022] Obviously, the accompanying drawings described below are merely some examples or embodiments of the present invention. Those skilled in the art can apply the present invention to other similar scenarios based on these drawings without any inventive effort. Furthermore, it is understood that although the efforts made in this development process may be complex and lengthy, for those skilled in the art related to the content disclosed in this invention, modifications to design, manufacturing, or production based on the technical content disclosed in this invention are merely conventional technical means and should not be construed as insufficient disclosure of the present invention.

[0023] In this invention, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention may be combined with other embodiments without conflict.

[0024] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "a," "an," "an," "the," and similar words used in this invention do not indicate quantity limitation and may indicate singular or plural. The terms "comprising," "including," "having," and any variations thereof used in this invention are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or device that includes a series of steps or modules (units) is not limited to the listed steps or units, but may also include steps or units not listed, or may include other steps or units inherent to these processes, methods, products, or devices. The terms "connected," "linked," "coupled," and similar words used in this invention are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "A plurality" used in this invention refers to two or more. "And / or" describes the relationship between related objects, indicating that three relationships may exist; for example, "A and / or B" can represent: A alone, A and B simultaneously, and B alone. The character " / " generally indicates that the preceding and following objects have an "or" relationship. The terms "first," "second," and "third" used in this invention are merely to distinguish similar objects and do not represent a specific ordering of the objects.

[0025] This invention provides a composite control method to improve the current control performance of a three-phase LCL grid-connected inverter, comprising: establishing a model of a three-phase LCL grid-connected inverter under weak grid conditions; establishing a transfer function from output current to reference current for the LCL grid-connected inverter with capacitor current as the damping source; establishing a loop gain considering digital control delay based on the transfer function; analyzing the impact of digital control delay on the system; proposing to first reduce part of the delay impact by modifying the capacitor current sampling method, and then adding a delay compensator in the feedback channel to reduce another part of the impact; strategy composition: the control strategy adopts a combination of modifying the capacitor current sampling method and delay compensation. This composite control strategy reduces the complexity of a single method and expands the positive damping of the equivalent virtual resistance of the dynamic system to the entire Nexter frequency, thereby improving the robustness and stability of the system.

[0026] Example 1: This invention provides a composite control method for improving the current control performance of a three-phase LCL grid-connected inverter, comprising: The model of a three-phase LCL grid-connected inverter under a weak power grid is as follows: Figure 1 As shown, the control strategy of this invention is applied to an inverter containing an LCL filter composed of a filter capacitor C, an inverter-side inductor L1, and a grid-side inductor L2. In a weak grid environment, there is a line impedance between the inverter and the grid. Perform equivalent joins; A transfer function from output current to reference current is established for an LCL-type grid-connected inverter with capacitor current as the damping source under weak grid conditions. Based on this transfer function, a system loop gain considering digital control delay is established. Composite control strategy: The control strategy achieves composite control by combining modification of capacitor current sampling method with the addition of delay compensation stage; Modify the capacitor current sampling method: to shorten the capacitor current sampling delay, sample the grid current at the trough of the triangular carrier wave, sample the capacitor current at the peak of the triangular carrier wave, and load the modulation signal at the next grid current sampling time, thereby shortening the capacitor current sampling delay by half a beat of sampling delay. Delay compensation: Used to compensate for the effects of digital control delays, a leading phase compensator is embedded in the capacitor current feedback loop. This reduces the impact of digital control delay on the system and improves system stability and grid-connected current quality. Modulation reference signal: Current controller The output signal and the capacitor current feedback loop output signal are calculated to produce the output adjustment signal. After coordinate transformation, the signal becomes the input modulation signal for the Space Vector Pulse Width Modulation (SVPWM) module, which is used to drive the LCL grid-connected inverter.

[0027] As can be seen from the above, the model of a three-phase LCL grid-connected inverter under weak grid conditions is based on the inverter-side inductance. , grid-side inductor Filter capacitor The inverter filter section is composed of line impedance. Achieving grid connection; establishing a transfer function from grid-connected current to reference current for an LCL-type grid-connected inverter using capacitor current as a damping source under weak grid conditions, for analyzing the system's frequency characteristics; strategy combination: the control strategy achieves composite control of the inverter system by combining modification of the capacitor current sampling method with the addition of a delay compensation loop; modification of capacitor current sampling method: compensating for part of the impact of digital control delay on the system by modifying the capacitor current sampling method; delay compensation: compensating for another part of the impact of digital control delay on the system by embedding a leading phase compensator in the capacitor current feedback loop; modulating the reference signal: current controller. The output signal and the capacitor current feedback loop output signal are calculated to produce the output adjustment signal. The coordinate transformation serves as the input modulation signal for the Space Vector Pulse Width Modulation (SVPWM) module, driving the LCL-type grid-connected inverter. The control strategy proposed in this invention optimizes the control complexity of a single control strategy, combining the advantages of two control strategies to improve system stability and adaptability to a wide range of grid impedance variations under weak grid conditions, which is of great significance for practical engineering applications.

[0028] For LCL-type grid-connected inverters operating in weak grids with capacitor current as the damping source, the transfer function from output current to reference current, without considering digital control delay, is specifically expressed as follows: ; in, Represents the transfer function. Represents a complex variable. Indicates the inverter-side inductance. Indicates the filter capacitor. Indicates the grid-side inductance. This indicates the transmission line impedance between the inverter and the power grid. For the proposed leading phase compensator, Indicates current controller, This represents the gain of the PWM stage that controls the on / off state of the switching devices. The resonant angular frequency of the system is expressed as follows: .

[0029] Digital control delay includes a 1-step delay introduced by the digital processing chip calculation and a 0.5-step delay introduced by the zero-order holder (ZOH) in PWM modulation. Digital control delay uses... The specific expression is: ; in, Indicates the digital sampling period.

[0030] After introducing digital control delay, the control block diagram of a traditional capacitor current active damping LCL type grid-connected inverter is as follows: Figure 2 As shown, with grid-connected current To achieve the control objective, the open-loop transfer function of the system is: .

[0031] It can be converted into an impedance equivalent form, such as Figure 3 As shown, the virtual impedance The specific expression is: ; in, This is the feedback coefficient.

[0032] Virtual impedance can be decomposed into virtual resistance. and virtual reactance The parallel form, specifically expressed as: ; in, The virtual impedance equivalent to the active damping of the capacitor current when the digital control delay is not considered is expressed as follows: .

[0033] Virtual impedance The amplitude-frequency response curve is as follows Figure 8 As shown, the virtual impedance is composed of the resistor and reactance The system is composed of parallel components, in which the resistor providing damping for the system is... It exhibits positive damping within the frequency range, which can suppress the system's resonant peaks. Exhibiting negative resistance over a frequency range can cause right-half-plane poles in the system, making system stability more demanding. When the system's resonant frequency... The system will become completely unstable at that time. The reactance component of the impedance... It exhibits inductive characteristics within a certain range. It exhibits capacitive characteristics within a certain range.

[0034] This invention modifies the capacitor current sampling method as follows: Figure 5 As shown, the grid-connected current is sampled at the trough of the triangular carrier wave, the capacitor current is sampled at the peak of the triangular carrier wave, and the modulation signal is loaded at the next grid-connected current sampling time. Therefore, the control delay of the grid-connected current loop is... The sampling delay is one beat, and the control delay of the capacitor current loop is... Half-beat sampling delay. The specific mathematical expression is: ; in, Indicates the first Beat modulation signal; Indicates the first Pulse current controller Output value; Indicates the first Tap the capacitor current feedback signal.

[0035] By advancing the capacitor current feedback sampling time by half a beat, the digital control delay of the capacitor current feedback loop by half a beat can be reduced, and the open-loop transfer function of the system can be improved. The expression is: ; Compared to the unmodified capacitor current sampling method, it reduces the digital control delay by half a beat.

[0036] Using the same method as described above, it is converted into an equivalent impedance form, at which point the virtual impedance... The expression is: .

[0037] Decompose it into virtual resistors using the same method as described above. and virtual reactance The parallel form, specifically, is expressed as follows: .

[0038] Virtual impedance The amplitude-frequency response curve is as follows Figure 9 As shown, the virtual impedance is composed of the resistor and reactance The system is composed of parallel components, in which the resistor providing damping for the system is... It exhibits positive damping within the frequency range, which can suppress the system's resonant peaks. Exhibiting negative resistance over a frequency range can cause right-half-plane poles in the system, making system stability more demanding. When the system's resonant frequency... The system will become completely unstable at that time. The reactance component of the impedance... It exhibits inductive characteristics within its range.

[0039] Therefore, this invention embeds a leading phase compensator in the capacitor current feedback loop, the specific expression of which is as follows: ; in, The gain of the lead phase compensator is the same as the feedback coefficient. , The compensation coefficient of the leading phase compensator is given, and its range is given. It is used to modulate the magnitude of the compensation phase.

[0040] Different compensation coefficients of the leading phase compensator Frequency response curves as shown Figure 6 As shown, with The angle of the compensation phase gradually increases as the compensation phase increases. Time can be compensated The phase lag is exactly the phase lag caused by the half-beat sampling delay.

[0041] like Figure 4 As shown, after embedding a leading phase compensator in the capacitor current feedback loop, the equivalent virtual impedance is... The expression is: .

[0042] To make the analysis more intuitive, the impedance form is converted into the admittance form, and... , Substitute and simplify: .

[0043] At this point, the resistor can be... and reactance The specific expression is as follows: .

[0044] By modifying the capacitor current sampling method and embedding a leading phase compensator in the capacitor current feedback loop as proposed in this invention, the frequency response curve of the equivalent virtual impedance is as follows: Figure 10 As shown, the proposed control strategy can expand the positive damping component of the equivalent virtual impedance to [a larger value]. The entire Nyquist frequency range significantly improves the system's robustness, and the reactance component is... It exhibits inductive properties within a certain range.

[0045] Example 2: This invention discloses a composite control method for improving the current control performance of a three-phase LCL grid-connected inverter, comprising: a current controller. Parameter design and lead phase compensator gain The design.

[0046] Current controller A quasi-proportional resonant (QPR) controller is used, and the specific expression is as follows: ; in, This is the proportionality coefficient. The resonance coefficient, Here, the grid frequency is taken as... , This is the resonant bandwidth, used to design a suitable bandwidth to improve the accuracy of grid-connected current control.

[0047] resonant frequency The design follows the principle that the power grid frequency fluctuation of small PV power plants is Maximum allowable deviation To ensure that the current controller has sufficient gain when the grid frequency fluctuates, therefore .

[0048] like Figure 7 As shown, the current controller and advanced phase compensator Gain The design steps specifically include: (1) First, the amplitude-frequency characteristic curve is plotted based on the open-loop transfer function of the LCL grid-connected inverter. Second, the open-loop cutoff frequency is designed. (To ensure the dynamic performance of the system, the open-loop cutoff frequency) The design should not be too small; the invention selects... for Next, find the cutoff frequency. Gain margin at The value, finally, according to the formula Calculate The value of .

[0049] (2) Regarding the impedance of the power grid Two gain margins in the wave design system and In addition to the above design The values ​​are selected to design the lead phase compensator. The range of values ​​for .

[0050] (3) In The value range should be selected to ensure that the LCL grid-connected inverter system has sufficient stability margin. The range of values ​​for is too narrow, resulting in no suitable value to ensure sufficient stability margin for the system. It is advisable to try increasing the open-loop cutoff frequency of the system. Redesign and adjustment and To redesign The range of values ​​for .

[0051] (4) Based on the parameters Parameter selection and system phase margin The range of values ​​(usually to ensure that the system has sufficient stability margin) The value range is generally within Design resonance coefficient .

[0052] (5) Verify the effectiveness of the designed parameters, draw the Bode plot of the system based on the loop gain of the system and verify the stability of the system by simulation to see if it meets the design requirements.

[0053] Example 3: Based on the composite control method for improving the current control performance of a three-phase LCL grid-connected inverter proposed in this invention, simulation verification was performed. The specific simulation parameters are as follows: DC power supply for Grid voltage for Grid frequency for Filter capacitor for Inverter-side inductor for Grid-side inductor for The modulation method uses bipolar PWM modulation, and the modulation carrier amplitude is... =1, switching frequency for sampling frequency for According to the calculations performed using the method steps in Example 2, the advanced phase compensator... Gain The compensation coefficient is 0.04. =1, current controller proportionality coefficient The resonance coefficient is 0.0525. 30, resonant broadband for power grid line impedance The fluctuation range is .

[0054] Based on the control strategy proposed in this invention and the parameters of the aforementioned three-phase LCL grid-connected inverter, the system impedance under weak grid conditions can be plotted as a function of the grid line impedance. exist To highlight the superiority of the control strategy proposed in this invention, the amplitude-frequency response curve of the open-loop transfer function varying within a certain range is compared with that of the traditional capacitor current active damping open-loop transfer function. The proportional coefficient in the traditional capacitor current feedback loop is compared with the gain coefficient of the lead phase compensator in this paper. The values ​​are the same, and the other parameters are also the same.

[0055] Figure 11The amplitude-frequency response curve of the open-loop transfer function of the traditional capacitor current active damping system shows that, with the impedance of the power grid line... As the value increases, the system's resonance peak gradually shifts to the left, and the system's resonance peak exhibits a trend of first increasing and then decreasing, indicating that it is in a state of flux. The sampling frequency reaches its maximum value, and the system phase occurs twice at that point. The jump in impedance is due to the fact that the portion of the virtual impedance in the equivalent active damping of a traditional capacitor current abruptly changes from providing positive damping to providing negative damping at this point. The system changes with the line impedance... The increase in current also gradually led to instability, confirming the shortcomings of traditional active capacitor current damping in terms of insufficient robustness to wide range of line impedance variations under weak grid conditions. Figure 12 The amplitude-frequency response curve of the open-loop transfer function of the proposed scheme is shown in the figure. It can be seen that as the line impedance increases... As the amplitude increases, the system's resonant peak shows a decreasing trend, and the overall peak value is significantly lower than that of the traditional scheme. Furthermore, the system's phase frequency curve does not exhibit a second jump. This is because the proposed scheme extends the equivalent virtual impedance of active damping to the entire Nyquist frequency. The resistive part of the equivalent virtual impedance continuously provides a positive damping effect for the system, which greatly improves the system's stability and robustness to a wide range of line impedance variations under weak power grid conditions.

[0056] Figure 14 The proposed solution in this invention varies with the impedance of the power grid lines under weak power grid conditions. exist To highlight the superiority of the proposed scheme, the simulated waveform of the grid-connected current varying within the range will be compared with the simulated waveform of the grid-connected current with active damping of the capacitor current.

[0057] Figure 13 In the diagram, (a), (b), and (c) represent the line impedances, respectively. equal , and The grid-connected current waveform at that time shows that in a strong power grid Traditional active damping control strategies for capacitor current have good grid-connected current control performance, but they gradually lose stability as the system line impedance increases. The system had already become unstable when The system completely diverged, once again confirming the shortcomings of traditional active capacitor current damping in weak power grids, which is insufficient robustness to a wide range of line impedance variations and poor system stability. Figure 14 The grid-connected current waveform using the scheme proposed in this invention is shown below. Figure 14 (a) in the figure represents the line impedance. equal The waveform of the grid-connected current at that time. Figure 14 (b) in the figure represents the line impedance. equal The waveform of the grid-connected current at that time. Figure 14 (c) in the figure represents the line impedance. equal The grid-connected current waveform at that time shows that as the line impedance... exist The grid-connected current remained under very good control despite variations within the specified range, further demonstrating the superiority of the proposed scheme.

[0058] Figure 15 The proposed solution in this invention varies with the impedance of the power grid lines under weak power grid conditions. exist Analysis of the total harmonic distortion (THD) of the grid-connected current when varying within a certain range, including Figure 15 (a) in the figure represents the line impedance. equal The graph of total harmonic distortion of grid-connected current at that time. Figure 15 (b) in the figure represents the line impedance. equal The graph of total harmonic distortion of grid-connected current at that time. Figure 15 (c) in the figure represents the line impedance. equal The graph of total harmonic distortion of grid-connected current shows that the control strategy described in this invention effectively suppresses harmonics and improves the quality of grid-connected current.

[0059] Based on the above technical solutions and simulation analysis, it can be seen that the present invention discloses a composite control method for improving the current control performance of a three-phase LCL grid-connected inverter, which has the following advantages: (1) Without increasing costs, this invention improves the stability and robustness of the three-phase grid-connected inverter system by combining modified capacitor current sampling method with delay compensation in an innovative design. (2) By effectively combining the two strategies, the advantages of the two control strategies are combined, the control difficulty of using a single control strategy is optimized, and the complexity of the control system is simplified. (3) By adopting the control strategy proposed in this invention, the frequency range of the resistor that provides positive damping for the three-phase LCL inverter system in the virtual impedance of the active damping equivalent can be expanded to the entire Nyquist frequency range, thereby improving the range of selectable parameters for the three-phase LCL system. (4) The control strategy of the present invention can effectively suppress harmonic components in the grid-connected current, whether it is used in a strong grid or a weak grid, and greatly improve the quality of the grid-connected current.

[0060] It should be noted that, depending on the implementation needs, the various steps / components described in this invention can be broken down into more steps / components, or two or more steps / components or parts of the operation of steps / components can be combined into new steps / components to achieve the purpose of this invention.

[0061] Those skilled in the art will readily understand that the above are merely preferred embodiments of the present invention and are 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 within the scope of protection of the present invention.

Claims

1. A composite control method for improving the current control performance of a three-phase LCL grid-connected inverter, characterized in that, include: A transfer function from output current to reference current is established for an LCL-type grid-connected inverter with capacitor current as the damping source under weak grid conditions. A digital control delay is introduced, and a system loop gain based on the digital control delay is established on the basis of the transfer function. Composite control is achieved by combining modified capacitor current sampling method with the addition of delay compensation stage: Modified capacitor current sampling method is used to shorten capacitor current sampling delay, including: sampling grid current at the trough of triangular carrier wave, sampling capacitor current at the peak of triangular carrier wave, and loading modulation signal at the next grid current sampling time, thus shortening capacitor current sampling delay by half a beat; Delay compensation stage is used to compensate for the impact of digital control delay, by embedding a leading phase compensator in capacitor current feedback loop to reduce the impact of digital control delay and improve system stability and grid current quality; The output signal of the current controller and the output signal of the capacitor current feedback loop are calculated, and the output adjustment signal is transformed into the input modulation signal of the space vector pulse width modulation module to drive the LCL grid-connected inverter.

2. The composite control method for improving the current control performance of a three-phase LCL grid-connected inverter according to claim 1, characterized in that, For an LCL-type grid-connected inverter using capacitor current as the damping source in a weak grid environment, the transfer function from output current to reference current, without considering digital control delay, is expressed as follows: ； in, Represents the transfer function. Represents a complex variable. Indicates the inverter-side inductance. Indicates the filter capacitor. Indicates the grid-side inductance. This indicates the transmission line impedance between the inverter and the power grid. For a leading phase compensator, Indicates a current controller. This represents the gain of the PWM stage that controls the on / off state of the switching devices. The resonant angular frequency of the system is expressed as follows: 。 3. The composite control method for improving the current control performance of a three-phase LCL grid-connected inverter according to claim 2, characterized in that, The digital control delay includes a 1-step sampling and calculation delay and a 0.5-step PWM delay, expressed as: ； in, Indicates digital control delay. Indicates the digital sampling period.

4. The composite control method for improving the current control performance of a three-phase LCL grid-connected inverter according to claim 3, characterized in that, The expression for the system loop gain based on digitally controlled delay is: ； in, This represents the system loop gain.

5. The composite control method for improving the current control performance of a three-phase LCL grid-connected inverter according to any one of claims 2 to 4, characterized in that, Current controller The expression is: ； in, This is the proportionality coefficient. The resonance coefficient, The angular frequency of the power grid. It is the resonant angular frequency.

6. The composite control method for improving the current control performance of a three-phase LCL grid-connected inverter according to claim 5, characterized in that, Resonant angular frequency ,in This represents the maximum allowable deviation for power grid frequency fluctuations.

7. The composite control method for improving the current control performance of a three-phase LCL grid-connected inverter according to claim 5, characterized in that, Leading phase compensator The expression is: ； in, This represents the gain of the lead phase compensator. This represents the compensation coefficient of the leading phase compensator. It is used to modulate the magnitude of the compensation phase.

8. The composite control method for improving the current control performance of a three-phase LCL grid-connected inverter according to claim 1, characterized in that, compensation coefficient .

9. The composite control method for improving the current control performance of a three-phase LCL grid-connected inverter according to claim 7, characterized in that, The parameter design of the leading phase compensator and current controller includes: Determine the cutoff frequency According to the cutoff frequency Determine the proportionality coefficient ; Regarding transmission line impedance Fluctuations, design system gain margin and Combined with the determined proportional coefficient Design the gain of the leading phase compensator The range of values ​​for ; From gain Choose a suitable value from the range of possible values; if no suitable value exists, increase the cutoff frequency. Re-determine the proportionality coefficient Or adjust the amplitude margin and ; According to gain The values ​​of and the range of phase margin of the system are used to design the resonance coefficient. .

10. A three-phase LCL inverter, characterized in that, The inverter employs the composite control method described in any one of claims 1 to 9 for improving the current control performance of a three-phase LCL grid-connected inverter.

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