CN109525135B - Second-order repetitive control method of LCL type grid-connected inverter and grid-connected inverter - Google Patents

Second-order repetitive control method of LCL type grid-connected inverter and grid-connected inverter Download PDF

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CN109525135B
CN109525135B CN201811419317.3A CN201811419317A CN109525135B CN 109525135 B CN109525135 B CN 109525135B CN 201811419317 A CN201811419317 A CN 201811419317A CN 109525135 B CN109525135 B CN 109525135B
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current
control method
connected inverter
voltage
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CN109525135A (en
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杨兴武
李豪
徐依明
黄玉辉
符杨
赵剑飞
向大为
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Shanghai University of Electric Power
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters

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Abstract

The invention discloses a second-order repetitive control method of an LCL type grid-connected inverter, which comprises the following steps of detecting required voltage and current components by using voltage and current transformers; converting the detected voltage and current components under the three-phase static coordinate system into an alpha beta two-phase static coordinate system; acquiring a grid-connected current instruction signal synchronous with the PCC voltage through dq/alpha beta coordinate conversion; subtracting the obtained grid-connected current command signal synchronous with the PCC voltage and the detected current to obtain an error; and after passing through the second-order repetitive controller, the error is input to PWM modulation for modulation calculation, and the switching tube is driven to act. According to the system control method, when the power grid frequency changes, the system control method with high steady-state precision can be realized without adjusting the parameters of the controller, and the grid-connected inverter has excellent grid-connected current output when the rated power grid frequency and the power grid frequency change by adopting a composite structure of series connection of second-order repetitive control and proportional control.

Description

Second-order repetitive control method of LCL type grid-connected inverter and grid-connected inverter
Technical Field
The invention relates to the technical field of power electronics, in particular to a second-order repetitive control method of an LCL type grid-connected inverter and the grid-connected inverter.
Background
Distributed power generation is receiving more and more attention to deal with energy crisis and reduce environmental pollution. The grid-connected inverter is used as a main power interface unit for connecting a distributed power generation system and a power grid, has very important position, and has important significance for researching the stability and the grid-connected current quality.
In order to improve the quality of grid-connected current, an LCL type filter is often used in engineering to inhibit high-frequency switching frequency subharmonics generated by a grid-connected inverter, and the resonance problem caused by the subharmonics can be solved by an active damping strategy such as a capacitor branch series resistor, a capacitor current feedback, a capacitor voltage feedback or a state feedback and the like. The damping method of capacitance current feedback is more applied due to simplicity and excellent performance.
Due to the fact that various nonlinear devices and unbalanced loads are contained in the distributed power grid, various low-order harmonics are contained at the PCC voltage. The traditional hysteresis control, Proportional Integral (PI) control and Proportional Resonance (PR) control strategies have weak inhibition capacity on voltage harmonics, and the repetitive control based on the internal model principle utilizes the periodicity of interference signals, so that the interference of periodic harmonics in the PCC voltage can be effectively inhibited, and the design is simple and convenient. However, the internal model is easily affected by the frequency fluctuation of the power grid, so that the resonance frequency of the internal model deviates from the fundamental wave and the harmonic frequency of the power grid, and the control performance is reduced.
To solve this problem, many studies have been made by domestic and foreign scholars. An Improved regenerative control scheme for grid-connected inverter with frequency adaptation published by ZHao Q et al in IEEE Transactions on Power Electronics provides frequency adaptive repetitive control on the basis of adopting capacitance current feedback active damping, so that the resonance frequency of the repetitive control approaches the actual values of fundamental wave and harmonic wave frequency of a Power grid, thereby achieving ideal control performance. MAARTEN s, reicitive control for systems with uncertain period-time published on Automatica, proposes a high-order Repetitive control strategy from the idea of enhancing the periodic robustness of Repetitive control to disturbance signals, so that the strategy has stronger adaptability.
Disclosure of Invention
This section is for the purpose of summarizing some aspects of embodiments of the invention and to briefly introduce some preferred embodiments. In this section, as well as in the abstract and the title of the invention of this application, simplifications or omissions may be made to avoid obscuring the purpose of the section, the abstract and the title, and such simplifications or omissions are not intended to limit the scope of the invention.
The present invention has been made keeping in mind the above and/or other problems occurring in the prior art.
Therefore, one of the objects of the present invention is to provide a second order repetitive control method of an LCL type grid-connected inverter.
In order to solve the technical problems, the invention provides the following technical scheme: a second-order repetitive control method of an LCL type grid-connected inverter comprises the following steps of detecting required voltage and current components by using voltage and current transformers; converting the detected voltage and current components under the three-phase static coordinate system into an alpha beta two-phase static coordinate system; acquiring a grid-connected current instruction signal synchronous with the PCC voltage through dq/alpha beta coordinate conversion; subtracting the obtained grid-connected current command signal synchronous with the PCC voltage and the detected current to obtain an error; and after passing through the second-order repetitive controller, the error is input to PWM modulation for modulation calculation, and the switching tube is driven to act.
As a preferable scheme of the second-order repetitive control method of the LCL type grid-connected inverter of the present invention, wherein: the detecting the required voltage and current components comprises (1) detecting the voltage, grid-connected current and capacitance current at the PCC; (2) and acquiring a grid-connected current reference component under the dq coordinate.
As a preferable scheme of the second-order repetitive control method of the LCL type grid-connected inverter of the present invention, wherein: under the alpha beta two-phase static coordinate system, the method comprises the steps that (1) the position angle theta of the PCC voltage is obtained by the detected PCC voltage through a phase-locked loopPLL(ii) a (2) And carrying out abc/alpha beta coordinate conversion on the detected grid-connected current and the detected capacitance current.
As a preferable scheme of the second-order repetitive control method of the LCL type grid-connected inverter of the present invention, wherein: an equivalent continuous domain transfer function of the LCL filter:
Figure BDA0001880209220000021
wherein L is1Is an inverter side inductor, L2For the grid-side filter inductance, KcThe active damping coefficient is fed back for the capacitance current, and C is the capacitance.
As a preferable scheme of the second-order repetitive control method of the LCL type grid-connected inverter of the present invention, wherein: l is1＝4mH，L2＝1mH，C＝10μF，Kc＝63。
As a preferable scheme of the second-order repetitive control method of the LCL type grid-connected inverter of the present invention, wherein: the natural frequency and the damping ratio in the second-order repetitive control method are as follows:
Figure BDA0001880209220000022
if ε is 0.707, then Kc＝63。
As a preferable scheme of the second-order repetitive control method of the LCL type grid-connected inverter of the present invention, wherein: g in the LCL filterLCL_icThe(s) sampling period T is 100 mus.
Another object of the present invention is to provide a grid-connected inverter using a second order repetitive control method.
As a preferable scheme of the second-order repetitive control method of the LCL type grid-connected inverter of the present invention, wherein: the system comprises a processing component and a control component, wherein the processing component sends an output command to the control component to enable the control component to act; the control assembly comprises a first capacitor, an IGBT and a flat cable, the IGBT is connected with the first capacitor through the flat cable, and a processing method in the processing assembly adopts a second-order repetitive control method.
As a preferable scheme of the second-order repetitive control method of the LCL type grid-connected inverter of the present invention, wherein: the control assembly further comprises a second capacitor and an IGBT control circuit board, the second capacitor is connected with the first capacitor in parallel, and the capacitance of the second capacitor is larger than that of the first capacitor; one end of the IGBT control circuit board receives a command sent by the processing assembly, and the other end of the IGBT control circuit board controls the on-off state of the IGBT.
As a preferable scheme of the second-order repetitive control method of the LCL type grid-connected inverter of the present invention, wherein: the flat cable comprises a first bus bar, a second bus bar and a third bus bar; the IGBT comprises a first main terminal, a second main terminal, a third main terminal, a first auxiliary terminal and a second auxiliary terminal; the first main terminal is connected with the first busbar, the second main terminal is connected with the second busbar, and the third main terminal is connected with the third busbar; wherein the first main terminal and the second main terminal are connected to form direct current, and the third main terminal outputs alternating current.
The invention has the beneficial effects that: according to the system control method, when the power grid frequency changes, the system control method with high steady-state precision can be realized without adjusting the parameters of the controller, and the grid-connected inverter has excellent grid-connected current output when the rated power grid frequency and the power grid frequency change without changing the parameters of the controller by adopting a composite structure of series connection of second-order repetitive control and proportional control; under the condition of keeping the steady-state precision of the system, the internal resources of the digital controller can be saved, and the calculation burden is reduced.
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In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without inventive exercise. Wherein:
fig. 1 is a three-phase LCL type grid-connected inverter topology and control structure diagram provided by the second order repetitive control method of the LCL type grid-connected inverter of the present invention;
FIG. 2 is a root trace diagram of an open-loop system provided by the second-order repetitive control method of the LCL type grid-connected inverter of the present invention;
FIG. 3 is a structure diagram of an SORC closed-loop system in the second-order repetitive control method of the LCL grid-connected inverter of the present invention;
fig. 4 is a p (z) amplitude-frequency characteristic diagram in the second-order repetitive control method of the LCL grid-connected inverter according to the present invention;
FIG. 5 is a SORC internal model structure diagram in the second-order repetitive control method of the LCL type grid-connected inverter of the present invention;
FIG. 6 is a comparison of CRC and SORC internal model amplitude-frequency characteristics in the second-order repetitive control method of the LCL grid-connected inverter according to the present invention;
FIG. 7 shows the error output response of the closed loop system when Q (z) is 0.95 in the second order repetitive control method of the LCL grid-connected inverter of the present invention;
FIG. 8 shows the error output response of the closed loop system when Q (z) is a low pass filter in the second order repetitive control method of the LCL grid-connected inverter according to the present invention;
FIG. 9 shows the error output response of the closed loop system using CRC and SORC in the second order repetitive control method of the LCL grid-connected inverter of the present invention;
FIG. 10 shows the gain of the internal models of CRC and SORC at the fundamental wave and the second harmonic along with the delta T in the second-order repetitive control method of the LCL grid-connected inverter0(%) graph of variation;
fig. 11(a) is a steady-state simulation waveform of PCC voltage and grid-connected current and a frequency spectrum thereof under a rated grid frequency in the second-order repetitive control method of the LCL type grid-connected inverter according to the present invention;
fig. 11(b) is a steady-state simulation waveform of CRC voltage and grid-connected current and a frequency spectrum thereof under a rated grid frequency in the second-order repetitive control method of the LCL type grid-connected inverter according to the present invention;
fig. 11(c) is steady state simulation waveforms of the SORC voltage and the grid-connected current and a frequency spectrum thereof under the rated grid frequency in the second-order repetitive control method of the LCL type grid-connected inverter of the present invention;
fig. 12(a) is a dynamic simulation waveform of CRC voltage and grid-connected current under a rated grid frequency in the second-order repetitive control method of the LCL type grid-connected inverter according to the present invention;
fig. 12(b) is a dynamic simulation waveform of the SORC voltage and the grid-connected current under the rated grid frequency in the second-order repetitive control method of the LCL type grid-connected inverter of the present invention;
fig. 13(a) is a simulated waveform of PCC voltage and grid-connected current and a spectrogram thereof when the grid frequency is 49.75Hz in the second-order repetitive control method of the LCL type grid-connected inverter according to the present invention;
fig. 13(b) is a simulated waveform of CRC voltage and grid-connected current and a spectrogram thereof when the grid frequency is 49.75Hz in the second-order repetitive control method of the LCL type grid-connected inverter according to the present invention;
fig. 13(c) is a graph showing simulated waveforms of the SORC voltage and the grid-connected current and frequency spectrums thereof when the grid frequency is 49.75Hz in the second-order repetitive control method of the LCL type grid-connected inverter according to the present invention;
fig. 14 is a schematic view of the internal overall structure of the grid-connected inverter using the second-order repetitive control method according to the present invention;
fig. 15 is a schematic view of the overall structure of the IGBT in the grid-connected inverter using the second-order repetitive control method according to the present invention.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways than those specifically described and will be readily apparent to those of ordinary skill in the art without departing from the spirit of the present invention, and therefore the present invention is not limited to the specific embodiments disclosed below.
Furthermore, reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation of the invention. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments.
The invention relates to a second-order repetitive control method of an LCL type grid-connected inverter, which comprises the following steps:
firstly, detecting a required voltage and current component by using a voltage and current transformer, and acquiring a grid-connected current reference component under dq coordinates after detecting the voltage, grid-connected current and capacitance current at a PCC;
secondly, converting the detected voltage and current components under the abc three-phase static coordinate system into an alpha beta two-phase static coordinate system, and obtaining the position angle theta of the PCC voltage by the detected PCC voltage through a phase-locked loopPLLThen, carrying out abc/alpha beta coordinate transformation on the detected grid-connected current and the detected capacitance current;
a third step of combining the position angle theta in the second stepPLLAnd a grid-connected current reference component under the abc/alpha beta coordinate is obtained through dq/alpha beta coordinateStandard conversion is carried out, and a grid-connected current command signal synchronous with the PCC voltage is obtained;
fourthly, subtracting the grid-connected current command signal which is synchronous with the PCC voltage and is obtained in the third step from the grid-connected current in the second step to obtain an error;
the fifth step: and (4) after passing through a second-order repetitive controller, inputting the error obtained in the previous step into a PWM (pulse-width modulation) modulator for modulation calculation, thereby driving the switching tube to act.
Specifically, the topology and control structure of the three-phase LCL grid-connected inverter is shown in fig. 1, in which V isdcIs a DC side input voltage, L1Is an inverter side inductor, L2For the grid-side filter inductance, R1、R2Respectively, its parasitic resistance; c is a capacitor; u shapegAnd ZgRespectively the grid voltage and the grid impedance. The control system obtains a grid-connected current command signal i synchronous with the PCC voltage through a phase-locked loop2αβ *The inverter adopts compound control of series connection of SORC and P, and the LCL filter adopts a capacitance current feedback active damping method.
The P controller parameter Kp is first designed. Neglecting the network impedance Zg and the parasitic resistance, the equivalent continuous domain transfer function of the LCL filter can be obtained:
Figure BDA0001880209220000061
and Kc is a capacitance current feedback active damping coefficient. The parameters of the LCL filter are: l1 ═ 4mH, L2 ═ 1mH, and C ═ 10 μ F. The transfer function can be regarded as being formed by connecting an integral element and a second-order oscillation element in series. The natural frequency and damping of the second order element are as follows:
Figure BDA0001880209220000062
to obtain the best damping effect, let ε be 0.707, and Kc be 63. The sampling period T is taken as 100 μ s, and a zero-order keeper discretization method is adopted for GLCL _ ic(s). The root locus with KP as the open loop gain is plotted as shown in fig. 2. The figure shows that the setting range of KP is 0-0.23 when the system is stable.
The second is to design the SORC controller. Taking the α -axis current control as an example, the structure of the SORC closed loop system is shown in fig. 3. In the figure, P (z) is the equivalent control object of the SORC, i.e. the closed-loop transfer function in the case of only P control; w (z) is the forward channel of the SORC inner die; q (z) is an internal model improvement link; kr is the gain of SORC; s (z) is a low-pass filtering link; zm is a phase lead element that compensates for the phase lag introduced by P (z) and S (z).
The principle of setting Kr is to maintain the unity gain of the amplitude-frequency characteristic of krp (z) in the mid-low frequency band, and as can be seen from fig. 4, p (z) is already the unity gain in the low frequency band, so Kr takes 1. The order m of the lead element zm is adapted to be 9 to better compensate for the phase lag of p (z) and s (z).
The core of the repetitive controller is an internal model link. The designed SORC internal mold structure is shown in FIG. 5. The SORC inner-die becomes the traditional repetitive control (CRC) inner-die when the coefficients on the inner-die forward path become 0 and 1.
Let q (z) be 1, and plot the amplitude-frequency characteristics of CRC and SORC internal models as shown in fig. 6. It can be known that the gain and bandwidth of the inner model of the SORC are large at the fundamental wave and each harmonic frequency, and when the grid frequency fluctuates, the inner model can still maintain high control gain, which indicates that the SORC can improve the robustness of the system to the grid frequency change. Whereas q (z) in practical applications typically takes a constant less than 1 (e.g. 0.95) or a low pass filter. Wherein, the expression of the low-pass filter is:
Figure BDA0001880209220000071
in order to ensure that the amplitude-frequency characteristic of the SORC internal model has a faster attenuation speed at a middle-high frequency so as to improve the stability of a system, a low-pass filter with a cut-off frequency of 1.06kHz is designed, and the expression is as follows:
Figure BDA0001880209220000072
FIG. 7 shows a graph of the error output response of the closed loop system when the command signal is sinusoidal, with Q (z) taken as 0.95 and the low pass filter, respectively. It can be seen that q (z) is 0.95 or a low pass filter, which can make the system steady state error converge to zero.
As can be seen from fig. 5, the delay link (z-N) on the forward channel of the SORC internal model is one more than the CRC, and qualitative analysis shows that: the dynamic performance of the SORC system is somewhat degraded compared to the CRC system. Fig. 8 shows the error output response of the system using CRC and SORC when the command signal is sin (2 tt 50). It can be seen that the error response at steady state is 0, which indicates that both strategies can make the system have excellent steady state performance. From the aspect of error convergence speed, the error can be converged to zero by adopting the method that SORC is 2 power frequency periods more than CRC.
Assuming that the grid fundamental frequency is changed from 1/T0 to 1/[ T0(1+ δ T0) ], the gain of the CRC and SORC internal models at the fundamental and second harmonics is plotted as a function of δ T0 (%), as shown in FIGS. 9 and 10. Analysis from the graph: the gains in the CRC and SORC modes at the fundamental and second harmonic frequencies both decrease with increasing δ T0 (%), but the gains in the SORC mode decrease more slowly. Taking δ T0 (%) as 0.5 as an example, when the grid frequency becomes 49.75Hz or 50.25Hz, the gain of the CRC internal model at the fundamental wave is only 30dB, while the gain of the SORC internal model can reach 65 dB. Therefore, the designed SORC has stronger robustness to the change of the power grid frequency, and the aim of realizing higher steady-state precision without adjusting the parameters of the controller when the power grid frequency is changed is fulfilled.
In order to verify the correctness of the above theoretical analysis and the effectiveness of the designed SORC strategy, a simulation model of the three-phase LCL type grid-connected inverter is built under the Matlab/Simulink simulation environment. In order to simulate nonlinear devices and local loads, harmonics with harmonic numbers of 5, 7, 11, 13, 17 and 19 are injected into the power grid, and the content of the harmonics is 2.85%, 2.52%, 2.36%, 2.05%, 1.89% and 1.57%, respectively.
Firstly, testing the steady-state performance of the grid-connected inverter under the rated grid frequency. Fig. 11(a) - (c) show steady state simulation waveforms and frequency spectrums of PCC voltage and grid-connected current by applying CRC and SORC. As can be seen from the figure, the harmonic waves can be effectively suppressed by the two strategies, and the grid-connected current quality is improved.
And secondly, testing the dynamic performance of the grid-connected inverter. Fig. 12(a) and (b) show dynamic simulation waveforms of grid-connected current using CRC and SORC. It can be seen from fig. 12 that using the SORC, the error convergence time of the system is 2 power frequency cycles more than using the CRC, which is consistent with the theoretical analysis above.
Fig. 13(a) - (c) show simulated waveforms and frequency spectrums of CRC and SORC, PCC voltage and grid-connected current, respectively, when the grid frequency is 49.75 Hz. As can be seen from the figure: the grid frequency offset has a large influence on the CRC performance: firstly, the effective value (4.914A) of grid-connected current is more deviated from the specified value (5A) than the SORC (5.008A); and the second harmonic content of 11, 13, 17 and 19 in the current is higher. Therefore, the robustness of the SORC to the grid frequency is strong, i.e. the SORC can suppress the harmonics in the PCC voltage more effectively when the frequency changes. Simulation results under two working conditions of rated power grid frequency and power grid frequency change are integrated, and the designed SORC control strategy is proved to have remarkable superiority.
Referring to fig. 14 to 15, the present invention further provides a grid-connected inverter using a second-order repetitive control method, the inverter including a processing module 100 and a control module 200, and the processing module 100 sends a command output from the processing module 100 to the control module 200 to operate the control module 200.
It should be noted that the processing method within the processing assembly 100 employs a second order repetitive control method.
The control assembly 200 includes a first capacitor 201, an IGBT202, an IGBT control circuit board 205, and a flat cable 203, and the IGBT202 is connected to the first capacitor 201 through the flat cable 203 and to the IGBT control circuit board 205. The IGBT control board 205 has one end connected to the processing unit 100 and the other end connected to the IGBT202 to control the on/off state of the IGBT 202.
In this chain of relationships, the processing component 100 is the brain, the IGBT control board 205 is the hand, the IGBT202 is the light, the hand is controlled by the brain, the hand operates the light, the processing component 100 controls the circuit on the IGBT control board 205, and the IGBT control board 205 operates the on/off state of the IGBT 202.
The control component 200 further includes a second capacitor 204, the second capacitor 204 is connected in parallel with the first capacitor 201, and a capacitance of the second capacitor 204 is smaller than a capacitance of the first capacitor 201.
It should be noted that, in this embodiment, preferably 3 IGBTs are connected to one control IGBT control circuit board 205, and each control IGBT control circuit board 205 is provided with one second capacitor 204, so the number of the second capacitors 204 is also preferably 3.
It should be noted that the second capacitor 204 is located differently from the first capacitor 201 and functions differently. The first capacitor 201 has a larger capacitance but operates at a slower speed, and the second capacitor 204 has a smaller capacitance but operates at a faster speed, both of which complement the control element 200. Since the first capacitor 201 has a large capacitance, it is sufficient to have a large capacitance, and the second capacitor 204 has a small capacitance and a high operation speed, it is preferable to be a non-inductive capacitor, and the effect can be achieved regardless of the type.
In the present embodiment, the bus bar 203 includes a first bus bar 203a, a second bus bar 203b and a third bus bar 203c, and the IGBT202 includes a first main terminal 202a, a second main terminal 202b, a third main terminal 202c, a first auxiliary terminal 202d and a second auxiliary terminal 202 e. The first main terminal 202a is connected to the first bus bar 203a, the second main terminal 202b is connected to the second bus bar 203b, and the third bus bar 203c is connected to the third main terminal 202 c.
The first main terminal 202a and the second main terminal 202b are connected to form a direct current, and the third main terminal 202c outputs an alternating current. Therefore, the first bus bar 203a and the second bus bar 203b are respectively connected with the positive electrode and the negative electrode of the external power supply and output through the third bus bar 203 c.
Preferably, the control assembly 200 further includes an inductor 206, and the inductor 206 is disposed at the front ends of the first busbar 203a and the second busbar 203b to filter input power.
It should be noted that the above-mentioned embodiments are only for illustrating the technical solutions of the present invention and not for limiting, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, which should be covered by the claims of the present invention.

Claims (6)

1. A second-order repetitive control method of an LCL type grid-connected inverter is characterized by comprising the following steps: the second order repetitive control method comprises the following steps,
detecting required voltage and current components by using a voltage and current transformer, and acquiring a grid-connected current reference component under dq coordinates after detecting the voltage, grid-connected current and capacitance current at the PCC;
converting the detected voltage and current components under the three-phase static coordinate system into an alpha beta two-phase static coordinate system, and obtaining the position angle theta of the PCC voltage by the detected PCC voltage through a phase-locked loopPLLThen, carrying out abc/alpha beta coordinate transformation on the detected grid-connected current and the detected capacitance current;
coupling position angle thetaPLLAnd grid-connected current reference components under abc/alpha beta coordinates, and acquiring a grid-connected current command signal synchronous with the PCC voltage through dq/alpha beta coordinate conversion;
subtracting the obtained grid-connected current command signal synchronous with the PCC voltage and the detected current to obtain an error;
after passing through the second-order repetitive controller, the error is input to PWM modulation for modulation calculation, and a switching tube is driven to act;
the second-order repetitive controller adopts compound control of series connection of SORC and P, the LCL filter adopts a capacitance current feedback active damping method, a parameter Kp of the P controller is designed, and an equivalent continuous domain transfer function of the LCL filter can be obtained by neglecting the impedance Zg and parasitic resistance of a power grid:
Figure FDA0002787352560000011
where Kc is the inverse of the capacitance currentFeeding active damping coefficient, L1Is an inverter side inductor, L2The transfer function is a filter inductor at the side of a power grid, C is a capacitor, and the transfer function can be regarded as being formed by connecting an integral link and a second-order oscillation link in series, wherein the natural frequency and the damping of the second-order link are as follows:
Figure FDA0002787352560000012
to obtain the optimal damping effect, if epsilon is 0.707, Kc is 63;
and further comprising the following steps of designing the SORC controller:
in the SORC closed-loop system structure, P (z) is an equivalent control object of the SORC, namely a closed-loop transfer function only under the control of P;
w (z) is used as a forward channel of an SORC internal model, Q (z) is used as an internal model improvement link, Kr is used as a gain of the SORC, S (z) is used as a low-pass filtering link, zm is used as a phase advance link and is used for compensating phase lag brought by P (z) and S (z), Kr is 1, and the order m of the advance link zm is 9;
when the coefficients on the inner model forward channel become 0 and 1, the SORC inner model becomes the traditional repetitive control (CRC) inner model;
in order to ensure that the amplitude-frequency characteristic of the SORC internal model has a faster attenuation speed at a middle-high frequency so as to improve the stability of a system, a low-pass filter with a cut-off frequency of 1.06kHz is defined, and the expression is as follows:
Figure FDA0002787352560000021
when q (z) takes 0.95, the system is stable and the low pass filter converges the system steady state error to zero.
2. The second order repetitive control method of the LCL type grid-connected inverter according to claim 1, characterized in that: l is1＝4mH，L2＝1mH，C＝10μF，Kc＝63。
3. The second order repetitive control method of the LCL type grid-connected inverter according to claim 2, characterized in that: g in the LCL filterLCL_icThe(s) sampling period T is 100 mus.
4. The grid-connected inverter of the second order repetitive control method of the LCL type grid-connected inverter according to any one of claims 1 to 3, characterized in that: the device comprises a processing component (100) and a control component (200), wherein the processing component (100) sends the output command to the control component (200) to make the control component (200) act;
wherein the control component (200) comprises a first capacitor (201), an IGBT (202) and a flat cable (203), and the IGBT (202) is connected with the first capacitor (201) through the flat cable (203);
wherein the processing method in the processing assembly (100) adopts a second-order repetitive control method.
5. The grid-connected inverter of the second order repetitive control method of the LCL type grid-connected inverter according to claim 4, wherein: the control assembly (200) further comprises a second capacitor (204) and an IGBT control circuit board (205),
the second capacitor (204) is connected with the first capacitor (201) in parallel, and the capacitance of the second capacitor (204) is larger than that of the first capacitor (201);
one end of the IGBT control circuit board (205) receives a command sent by the processing component (100), and the other end controls the on-off state of the IGBT (202).
6. The grid-connected inverter of the second order repetitive control method of the LCL type grid-connected inverter according to claim 5, wherein: the flat cable (203) comprises a first bus bar (203a), a second bus bar (203b) and a third bus bar (203 c);
the IGBT (202) comprises a first main terminal (202a), a second main terminal (202b), a third main terminal (202c), a first auxiliary terminal (202d) and a second auxiliary terminal (202 e);
the first main terminal (202a) is connected with the first busbar (203a), the second main terminal (202b) is connected with the second busbar (203b), and the third main terminal (202c) is connected with the third busbar (203 c);
wherein the first main terminal (202a) and the second main terminal (202b) are connected to form a direct current, and the third main terminal (202c) outputs an alternating current.
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