Power grid current feedback and PCC voltage feedforward combined control strategy of LCL grid-connected inverter

Abstract

The invention relates to the field of grid-connected inverters, and particularly discloses a power grid current feedback and PCC voltage feedforward combined control strategy of an LCL grid-connected inverter, which comprises the following steps: strategy combination: the control strategy adopts a mode of combining a single power grid current feedback control loop (GCF) and a PCC unit voltage feedforward loop to realize active damping control; an LCL type grid-connected inverter structure; according to the invention, the structure and algorithm of the control system are remarkably simplified and the implementation complexity is reduced by reducing the capacitance current feedback control loop and adopting the combined action of the single-grid current feedback loop and the PCC unit voltage feed-forward loop; the upper limit of the positive damping region of the obtained equivalent virtual resistance of the dynamic system exceeds the Nyquist frequency, so that the system can better cope with the large-range change of the impedance of the power grid, and the robustness and stability of the system are improved; and a control strategy of combining power grid current feedback and PCC unit voltage feedforward is adopted, so that medium-high frequency oscillation of a current control loop of the three-phase LCL type grid-connected inverter is effectively suppressed.

Description

Technical Field

[0001] The invention belongs to the field of grid-connected inverters, and in particular to a grid current feedback and PCC voltage feedforward joint control strategy of an LCL grid-connected inverter. Background Art

[0002] The three-phase LCL grid-connected inverter is a widely used device in grid-connected inverters. It consists of a filter capacitor C, an inverter-side inductor L1, and a grid-side inductor L2 to form an LCL filter, which is used to efficiently transmit the electric energy generated by the inverter to the grid.

[0003] Existing technical solutions usually adopt a control strategy that combines dual closed-loop capacitor current and grid current feedback with PCC point voltage feedforward. This method requires processing multiple control loops at the same time, which increases the complexity of the system and the difficulty of implementation. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a joint control strategy of grid current feedback and PCC voltage feedforward of LCL grid-connected inverter to solve the control strategy of combining double closed loop of capacitor current and grid current feedback and PCC point voltage feedforward in the prior art. This method needs to process multiple control loops at the same time, which increases the complexity of the system and the difficulty of implementation.

[0005] The grid current feedback and PCC voltage feedforward joint control strategy of LCL grid-connected inverter includes:

[0006] Strategy combination: The control strategy adopts a single grid current feedback control loop (GCF) combined with a PCC unit voltage feedforward loop to achieve active damping control;

[0007] LCL type grid-connected inverter structure: The control strategy is applied to an LCL type grid-connected inverter including an LCL filter composed of a filter capacitor C, an inverter-side inductor L1 and a grid-side inductor L2, and an equivalent connection is achieved between the inverter and the grid through a common coupling point (PCC);

[0008] Single grid current feedback control loop: responsible for ensuring that the output grid current closely tracks the given reference current. A negative high-pass filter (NH) is embedded in the single grid current feedback control loop and operates as a damping controller (GH);

[0009] PCC unit voltage feed-forward loop: used to reduce background harmonic interference introduced by grid voltage, thereby improving current quality;

[0010] Modulation reference signal: The output signals of the damping controller GH, the current controller GC and the PCC unit voltage feedforward are combined and used as the input modulation reference signal of the space vector pulse width modulation (SVPWM) module to drive the operation of the LCL type grid-connected inverter.

[0011] Preferably, the damping controller G H The expression is:

[0012]

[0013] Where K C is the high-pass filter gain, ω d It represents the high-pass filter cutoff frequency, which is used to determine the frequency response characteristics of the damping controller.

[0014] Preferably, the current controller GC adopts a quasi-proportional resonance (PR) controller, and its expression is:

[0015]

[0016] Among them, K p is the proportionality coefficient, K r is the resonance coefficient, ωo is the resonance frequency, ω i is the resonant term bandwidth, which is used to achieve a smaller steady-state error at the fundamental frequency and improve the accuracy of current control.

[0017] Preferably, the control strategy also includes current and damping controller parameter design steps, specifically including:

[0018] a. Determine the current loop controller parameters, including the proportional coefficient K p and the resonance coefficient K r , to meet the system's control frequency ω c And the requirements of phase margin PM;

[0019] b. According to the specified amplitude margins GM1, GM2 and the selected K p , determine the grid impedance L g Changes in the high-pass filter gain K C The possible value range of , in order to ensure the robustness of the system;

[0020] c. From K C Select a suitable value from the range of possible values ​​to ensure that the system can operate stably under different grid impedance conditions. If the requirements of GM1, GM2 and PM are strict, K C If the robust value range of does not exist, then adjust the control frequency ω c and phase margin PM, and re-execute the design steps;

[0021] d. Verify the validity of the selected parameters and verify whether the dynamic performance and steady-state performance of the system meet the design requirements through simulation or experiments.

[0022] Preferably, the ω d The value of is preferably within the resonant frequency of the LCL filter r nearby.

[0023] Preferably, the control strategy realizes active damping by means of a joint virtual impedance, wherein the joint virtual impedance is formed by connecting in parallel a virtual impedance generated by a PCC unit voltage feedforward and a virtual impedance generated by a damping controller.

[0024] Preferably, the control strategy can generate a positive damping zone of the dynamic system equivalent virtual resistance that varies with the grid impedance and the damping controller parameters, and its upper limit exceeds the Nyquist frequency, thereby achieving strong robustness of the system in coping with changes in grid impedance.

[0025] Preferably, the control strategy adopts a dual closed-loop control of capacitor current and grid current plus a PCC unit voltage feed-forward loop.

[0026] Compared with the prior art, the present invention has the following beneficial effects: by reducing the capacitor current feedback control loop and adopting the combined effect of a single grid current feedback loop and a PCC unit voltage feedforward loop, the structure and algorithm of the control system are significantly simplified, and the complexity of implementation is reduced;

[0027] The upper limit of the positive damping region of the equivalent virtual resistance of the obtained dynamic system exceeds the Nyquist frequency, which enables the system to better cope with the large-scale changes in the grid impedance and improves the robustness and stability of the system.

[0028] The adopted control strategy combining grid current feedback with PCC unit voltage feedforward effectively suppresses the medium and high frequency oscillations of the current control loop of the three-phase LCL grid-connected inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The block diagram of the control system of the three-phase LCL type grid-connected inverter adopting the combined damping strategy of the present invention;

[0030] Figure 2 This is a control block diagram of a grid-connected inverter system using GCF active damping and PCC unit voltage feedforward strategy according to the present invention;

[0031] Figure 3 is the equivalent circuit of the combined damping method of the present invention;

[0032] Figure 4 For the present invention, K C >0: R eq Frequency characteristics diagram of

[0033] Figure 5 For the present invention, K C <0 time R eq Frequency characteristics diagram of

[0034] Figure 6 Flowchart of the design procedure for the present invention. DETAILED DESCRIPTION

[0035] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] like Figures 1 to 6 As shown:

[0037] Embodiment 1: The present invention provides a grid current feedback and PCC voltage feedforward joint control strategy for an LCL grid-connected inverter, including:

[0038] Strategy combination: The control strategy adopts a single grid current feedback control loop (GCF) combined with a PCC unit voltage feedforward loop to achieve active damping control;

[0039] LCL grid-connected inverter structure: The control strategy is applied to the LCL grid-connected inverter including the LCL filter composed of the filter capacitor C, the inverter side inductor L1 and the grid side inductor L2. The inverter and the grid are equivalently connected through the common coupling point (PCC);

[0040] Single grid current feedback control loop: responsible for ensuring that the output grid-connected current closely tracks the given reference current. A negative high-pass filter (NH) is embedded in the single grid current feedback control loop and operates as a damping controller (GH), mainly used to suppress possible resonance phenomena in the system.

[0041] PCC unit voltage feed-forward loop: used to reduce background harmonic interference introduced by grid voltage, thereby improving current quality;

[0042] Modulation reference signal: The output signals of the damping controller GH, the current controller GC and the PCC unit voltage feedforward are combined and used as the input modulation reference signal of the space vector pulse width modulation (SVPWM) module to drive the operation of the LCL type grid-connected inverter.

[0043] From the above, we can see that the strategy combination: a single grid current feedback control loop is combined with a PCC unit voltage feedforward loop to achieve active damping control; LCL type grid-connected inverter structure: composed of filter capacitor C, inverter side inductor L1 and grid side inductor L2, the inverter and the grid are connected through a common coupling point (PCC); single grid current feedback control loop: ensure that the output grid-connected current closely tracks the given reference current, and embed a negative high-pass filter as a damping controller to suppress system resonance; PCC unit voltage feedforward loop: reduce the background harmonic interference introduced by the grid voltage and improve the current quality; modulation reference signal: the output signal combination of the damping controller, current controller and PCC unit voltage feedforward is used as the input of the space vector pulse width modulation module to drive the inverter to work; this strategy simplifies the control algorithm structure, reduces the number of hardware devices, reduces the size of the equipment, and realizes the strong robustness of the system to changes in grid impedance.

[0044] Embodiment 2: This embodiment is basically the same as the previous embodiment, except that the damping controller G H The expression is:

[0045]

[0046] Where K C is the high-pass filter gain, ω d It represents the high-pass filter cutoff frequency, which is used to determine the frequency response characteristics of the damping controller.

[0047] Specifically, the control system parameters include the current loop controller parameters and the control parameters K in the damping loop. C and ω d , where K C and ω d Determines the upper and lower limit frequencies of the positive damping zone. In order to obtain the best performance of the control system, ω d The value of is preferably within the resonant frequency of the LCL filter r Therefore, choose ω d =ω r .

[0048] The use of a quasi-proportional resonant PR controller can achieve a smaller steady-state error at the fundamental frequency, so a PR controller is used. The current controller GC uses a quasi-proportional resonant (PR) controller, and its expression is:

[0049]

[0050] Among them, K p is the proportionality coefficient, K r is the resonance coefficient, ω0 is the resonance frequency, ω i To consider the resonant bandwidth required by -3dB, that is, at ω0±ω iThe gain of the resonance term is 0.707K r In order to ensure that the PR regulator has sufficient gain when the grid frequency fluctuates, ω i =2πΔf=πrad / s.

[0051] like Figure 6 :

[0052] Specifically, the control strategy also includes current and damping controller parameter design steps, including:

[0053] a. Determine the current loop controller parameters, including the proportional coefficient K p and the resonance coefficient K r , to meet the system's control frequency ω c And the requirements of phase margin PM;

[0054] b. According to the specified amplitude margin GM1, GM2 and the selected K p , determine the grid impedance L g Changes in the high-pass filter gain K C The possible value range of , in order to ensure the robustness of the system;

[0055] c. From K C Select a suitable value from the range of possible values ​​to ensure that the system can operate stably under different grid impedance conditions. If the requirements of GM1, GM2 and PM are strict, K C If the robust value range of does not exist, then adjust the control frequency ω c and phase margin PM, and re-execute the design steps;

[0056] d. Verify the validity of the selected parameters and verify whether the dynamic performance and steady-state performance of the system meet the design requirements through simulation or experiments.

[0057] Specifically, d The value of is preferably within the resonant frequency of the LCL filter r nearby.

[0058] Among them, according to Figure 1 , and applying the superposition theorem, vPCC can be expressed as

[0059]

[0060] Figure 2 for Figure 1 The equivalent control block diagram of the system shown in the continuous domain, where two virtual impedances Z are connected in parallel across the capacitor PCC and Z NHGCF The expression is as follows:

[0061]

[0062]

[0063] Two virtual impedances Z PCC and Z NHGCF can be regarded as the parallel connection of virtual resistance and virtual reactance respectively, so the combined virtual resistance and reactance can be expressed as:

[0064]

[0065] The combined active damping method is equivalent to connecting two virtual impedances Z in parallel at both ends of the capacitor. PCC and Z NHGCF ,like Figure 3 shown.

[0066] According to the above analysis, the grid voltage will introduce harmonics into the grid-connected current, thus causing the current waveform to be distorted. Therefore, the introduction of PCC unit voltage feedforward in the grid current feedback control can achieve the suppression of harmonics in the grid-connected current. eq =R NHGCF / / R PCC , Figure 4 and Figure 5 Draw R eq The frequency characteristic diagram of r and ω′ r are the frequency and angular frequency of the system respectively; ω s is the sampling angular frequency.

[0067] Figure 4 In, when L g = 0, according to the above formula, the voltage at the PCC point is equal to the grid voltage. Since there is no capacitor voltage component, the voltage feedforward has no damping effect. NHGCF Provides damping. RU and ω RL R eq (f r ′) are negative upper and lower limit frequencies, for K C >0, when ω r ′>ω RL , R NHGCF <0; and when ω r ′<ω RL , R NHGCF >0. When L g Not 0, R PCC participate in the joint damping effect, because R NHGCF and R PCC In (ω s / 3,ω s / 2) is negative, so R eq It is also negative in this frequency region. L gIncrease from 0 to L gmax , R eq The lower limit of the positive region is always 0, while the upper limit changes with L g increases with the increase of ω, but will not exceed ω s / 3.

[0068] Figure 5 In, when L g = 0, similar to the above analysis, only R NHGCF Provides damping effect, for K C <0, when ω RU >ω r ′>ω RL , R NHGCF >0; and when ω r ′<ω RU , R NHGCF <0. When L g When it is not 0, R PCC Participate in the joint damping effect, as L g Increase from 0 to L gmax , R eq The upper and lower limits of the frequency in the positive region move to the left at the same time, and when K C As R increases gradually, eq The upper and lower frequency limits of the positive region of move to the right at the same time, thus forming a dynamic positive damping region whose width is close to ω s / 3. In addition, the upper limit of this dynamic damping region can exceed the Nyquist frequency ω s / 2, and the lower limit can reach 0Hz.

[0069] According to the above analysis results, it can be concluded that the PCC unit voltage feedforward and NHGCF (K C The combined damping method combined with active damping can produce a dynamic positive damping zone, the upper and lower limit frequencies of which can be adjusted with the grid impedance L g The size of the NHGCF can also be adjusted by adjusting the parameter K C To control. In order to deal with the grid impedance L g A wide range of changes, choose a suitable K C Robust control of the system can be achieved.

[0070] From the above, it can be seen that this strategy combines a single grid current feedback control loop with a PCC unit voltage feedforward loop, and realizes active damping by means of a joint virtual impedance. The joint virtual impedance is formed by connecting the virtual impedance generated by the PCC unit voltage feedforward and the virtual impedance generated by the damping controller in parallel, which can generate a positive damping zone of the equivalent virtual resistance of the dynamic system that changes with the grid impedance and the damping controller parameters. Its upper limit exceeds the Nyquist frequency, achieving strong robustness of the system to cope with changes in grid impedance.

[0071] In addition, the expression of the damping controller includes the high-pass filter gain and the cutoff angular frequency, which are used to determine the frequency response characteristics of the damping controller and obtain the best performance by reasonably designing the control system parameters. Compared with the existing technology, this strategy has the advantages of simple algorithm, small equipment size, and wide robust control range.

[0072] Embodiment 3: This embodiment is basically the same as the previous embodiment, except that the control strategy implements active damping by means of a joint virtual impedance, wherein the joint virtual impedance is formed by connecting in parallel the virtual impedance generated by the PCC unit voltage feedforward and the virtual impedance generated by the damping controller.

[0073] Specifically, the control strategy can generate a positive damping zone of the dynamic system equivalent virtual resistance that varies with the grid impedance and the damping controller parameters, and its upper limit exceeds the Nyquist frequency, thus achieving strong robustness of the system to changes in grid impedance.

[0074] Specifically, the control strategy adopts a dual closed-loop control of capacitor current and grid current plus a PCC unit voltage feed-forward loop.

[0075] As can be seen from the above, this control strategy adopts a single grid current feedback control loop (GCF) combined with a PCC unit voltage feedforward loop to achieve active damping through a joint virtual impedance. The joint virtual impedance is formed by the virtual impedance generated by the PCC unit voltage feedforward and the virtual impedance generated by the damping controller in parallel, which can generate a positive damping zone of the equivalent virtual resistance of the dynamic system that changes with the grid impedance and the damping controller parameters. Its upper limit exceeds the Nyquist frequency, achieving strong robustness of the system to changes in grid impedance;

[0076] At the same time, the use of a single grid current feedback control loop simplifies the algorithm structure, reduces the number of current transformers, reduces the size of the equipment, and improves the robustness and performance of the system.

[0077] The standard parts used in the present invention can all be purchased from the market, and the special-shaped parts can be customized according to the description and the drawings. The specific connection methods of each part adopt conventional means such as mature bolts, rivets, welding, etc. in the prior art. The machinery, parts and equipment all adopt conventional models in the prior art, and the circuit connection adopts the conventional connection method in the prior art, which will not be described in detail here. The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field.

[0078] In the description of the present invention, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. "Multiple" means two or more, unless otherwise clearly and specifically defined.

[0079] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0080] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0081] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, unless they are contradictory.

[0082] In the drawings of the embodiments disclosed in the present invention, only the structures related to the embodiments disclosed in the present invention are involved, and other structures can refer to the general design. In the absence of conflict, the same embodiment and different embodiments of the present invention can be combined with each other.

[0083] Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. The grid current feedback and PCC voltage feedforward joint control strategy of LCL grid-connected inverter is characterized by: include: Strategy combination: The control strategy adopts a single grid current feedback control loop (GCF) combined with a PCC unit voltage feedforward loop to achieve active damping control; LCL type grid-connected inverter structure: The control strategy is applied to an LCL type grid-connected inverter including an LCL filter composed of a filter capacitor C, an inverter-side inductor L1 and a grid-side inductor L2, and an equivalent connection is achieved between the inverter and the grid through a common coupling point (PCC); Single grid current feedback control loop: responsible for ensuring that the output grid current closely tracks the given reference current. A negative high-pass filter (NH) is embedded in the single grid current feedback control loop and operates as a damping controller (GH); PCC unit voltage feed-forward loop: used to reduce background harmonic interference introduced by grid voltage, thereby improving current quality; Modulation reference signal: The output signals of the damping controller GH, the current controller GC and the PCC unit voltage feedforward are combined and used as the input modulation reference signal of the space vector pulse width modulation (SVPWM) module to drive the operation of the LCL type grid-connected inverter.

2. The grid current feedback and PCC voltage feedforward joint control strategy of the LCL grid-connected inverter as claimed in claim 1, characterized in that: The damping controller G H The expression is: Where K C is the high-pass filter gain, ω d It represents the high-pass filter cutoff frequency, which is used to determine the frequency response characteristics of the damping controller.

3. The grid current feedback and PCC voltage feedforward joint control strategy of the LCL grid-connected inverter as claimed in claim 2, characterized in that: The current controller GC adopts a quasi-proportional resonant (PR) controller, and its expression is: Among them, K p is the proportionality coefficient, K r is the resonance coefficient, ωo is the resonance frequency, ω i is the resonant term bandwidth, which is used to achieve a smaller steady-state error at the fundamental frequency and improve the accuracy of current control.

4. The grid current feedback and PCC voltage feedforward joint control strategy of the LCL grid-connected inverter as claimed in claim 3, characterized in that: The control strategy also includes current and damping controller parameter design steps, specifically including: a. Determine the current loop controller parameters, including the proportional coefficient K p and the resonance coefficient K r , to meet the system's control frequency ω c And the requirements of phase margin PM; b. According to the specified amplitude margins GM1, GM2 and the selected K p , determine the grid impedance L g Changes in the high-pass filter gain K C The possible value range of , in order to ensure the robustness of the system; c. From K C Select a suitable value from the range of possible values ​​to ensure that the system can operate stably under different grid impedance conditions. If the requirements of GM1, GM2 and PM are strict, K C If the robust value range of does not exist, then adjust the control frequency ω c and phase margin PM, and re-execute the design steps; d. Verify the validity of the selected parameters and verify whether the dynamic performance and steady-state performance of the system meet the design requirements through simulation or experiments.

5. The grid current feedback and PCC voltage feedforward joint control strategy of the LCL grid-connected inverter as claimed in claim 4, characterized in that: The ω d The value of is preferably within the resonant frequency of the LCL filter r nearby.

6. The grid current feedback and PCC voltage feedforward joint control strategy of the LCL grid-connected inverter as claimed in claim 5, characterized in that: The control strategy realizes active damping by means of a joint virtual impedance, wherein the joint virtual impedance is formed by connecting in parallel a virtual impedance generated by a PCC unit voltage feedforward and a virtual impedance generated by a damping controller.

7. The grid current feedback and PCC voltage feedforward joint control strategy of the LCL grid-connected inverter as claimed in claim 6, characterized in that: The control strategy can generate a positive damping zone of the dynamic system equivalent virtual resistance that varies with the grid impedance and the damping controller parameters, and its upper limit exceeds the Nyquist frequency, thereby achieving strong robustness of the system in coping with grid impedance changes.

8. The grid current feedback and PCC voltage feedforward joint control strategy of the LCL grid-connected inverter as claimed in claim 1, characterized in that: The control strategy adopts a dual closed-loop control of capacitor current and grid current plus a PCC unit voltage feed-forward loop.

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