CN113517720A - Fractional phase compensation control method and device for LCL grid-connected inverter under weak grid - Google Patents

Fractional phase compensation control method and device for LCL grid-connected inverter under weak grid Download PDF

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CN113517720A
CN113517720A CN202111061593.9A CN202111061593A CN113517720A CN 113517720 A CN113517720 A CN 113517720A CN 202111061593 A CN202111061593 A CN 202111061593A CN 113517720 A CN113517720 A CN 113517720A
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grid
signal
current
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fractional
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CN113517720B (en
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曾成碧
李苏丹
苗虹
王翰文
段述江
童广
朱胤杰
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Sichuan University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H02J3/241—The oscillation concerning frequency
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/40—Arrangements for reducing harmonics

Abstract

The invention relates to the technical field of grid-connected inverter control, in particular to a fractional phase compensation control method and device for an LCL grid-connected inverter under a weak power grid, wherein the fractional phase compensation control method comprises the following steps: firstly, respectively acquiring a voltage signal and a grid-connected current signal; secondly, extracting voltage phase angle information; generating a grid-connected current reference signal in a control system; dividing the obtained grid-connected current signal into two paths, wherein one path of the grid-connected current signal is fed back through grid-connected current to inhibit the inherent resonance peak of the LCL grid-connected inverter through active damping, and the other path of the grid-connected current signal is differed with the grid-connected current reference signal to obtain an error signal which is transmitted to a current controller for modulation; fourthly, the error signal is subjected to modulation action of the current controller and then is subjected to difference with the grid-connected current feedback active damping signal to obtain a modulation wave signal; and fifthly, comparing to obtain a control signal. The current controller structure is a proportional resonant parallel repetitive control structure with a fractional phase compensation function, is simple in design and can provide flexible fractional phase compensation under a weak power grid.

Description

Fractional phase compensation control method and device for LCL grid-connected inverter under weak grid
Technical Field
The invention relates to the technical field of grid-connected inverter control, in particular to a fractional phase compensation control method and device for an LCL grid-connected inverter under a weak power grid.
Background
The distributed energy is mainly installed in remote areas with small power loads, long-distance power transmission lines are needed for transmitting electric energy to a power load center, at the moment, the impedance of a power grid line is not negligible, and the power grid is in a weak power grid characteristic. Under a weak grid, grid-connected current harmonics are further amplified, and the inverter system is unstable under a more serious condition.
As a key device of distributed energy grid connection, a control strategy of a grid-connected inverter is widely researched. Various controllers are currently used for controlling the grid-connected inverter, such as a proportional integral controller (PI), a proportional resonant controller (PR), a dead-beat controller (DB), a Repetitive Controller (RC), and the like. Among them, RC has a simpler structure and easier digital implementation and parameter design due to its zero steady-state error tracking characteristic for periodic signals, compared with other controllers with similar control effects, and is gradually used in the control of grid-connected inverters under weak power grids.
The sampling frequency of the conventional RC is usually equal to the switching frequency of the inverter power electronic device (generally, 10kHz or higher frequency), which also determines the number of sampling points of the RC in one fundamental wave period
Figure 581021DEST_PATH_IMAGE001
Large (f s In order to be able to sample the frequency,fat the grid fundamental frequency). To reduce RCNAnd reduced occupation of memory cells, multi-rate repetitive control (MRC) is proposed, which can greatly reduce each base while ensuring a higher switching frequencyThe sampling frequency of the wave period reduces the operation load of the calculator.
In order to ensure stable operation of the system, the phase lag of the entire system must be taken into account. A suitable linear phase lead compensator is typically introducedz m k To compensate for the phase of the entire inverter system. But is limited to a digital implementation and,kbut only an integer. In the conventional RC, a proper integer is easy to findkTo compensate the phase of the whole system, but due to the low sampling frequency of the MRC, the integer phase compensator is liable to over-compensate or under-compensate the phase of the whole system, resulting in deteriorated current quality and even system instability. The prior document proposes that fractional phase compensation is realized by adopting a fractional filter based on Lagrange interpolation, but the greatest defect of the fractional filter based on Lagrange interpolation is that the filter coefficient needs to be changed along with the change of the filter coefficientkReal-time adjustments, greatly increasing the computational burden and memory consumption of the processor. However, the grid impedance under weak grid will vary with the grid operating mode and transmission distance, which also makes the phase compensation termkVariations of (c) are unavoidable. Therefore, it is desirable to provide a fractional phase compensation scheme that is simple in structure and flexible for weak grid applications.
Disclosure of Invention
The method aims to solve the problems that the dynamic performance of the grid-connected inverter is poor under a weak power grid, integral phase compensation is easy to cause over-compensation or under-compensation of the phase of the whole system, grid-connected current quality is further reduced, and even system instability is caused, and meanwhile, the current fractional phase compensation strategy is complex in structure, large in memory consumption and the like. The invention provides a fractional phase compensation control method and device for an LCL grid-connected inverter under a weak power grid, which can overcome the defects.
The fractional phase compensation control method for the LCL grid-connected inverter under the weak grid is characterized by comprising the following steps of:
firstly, respectively acquiring a voltage signal and a grid-connected current signal at a common coupling point in an inverter system through a voltage sampling plate and a current sampling plate;
secondly, voltage signals acquired through the voltage sampling plate pass through a phase-locked loop to extract voltage phase angle information; generating a grid-connected current reference signal in a control system according to the voltage phase angle information and a grid-connected current amplitude value required by the inverter system;
dividing a grid-connected current signal acquired by a current sampling plate into two paths, wherein one path of the grid-connected current signal is subjected to grid-connected current feedback active damping to suppress an inherent resonance peak of the LCL grid-connected inverter, and the other path of the grid-connected current signal is subjected to difference with the grid-connected current reference signal to obtain an error signal which is transmitted to a current controller for modulation;
fourthly, the error signal is subjected to modulation action of a current controller and then is subjected to difference with a grid-connected current feedback active damping signal to obtain a modulation wave signal;
comparing the modulated wave signal with a triangular carrier wave generated in the SPWM generator to obtain a control signal; and controlling the on-off of the electronic devices in the grid-connected inverter through the control signal.
Preferably, the current controller is a composite controller of a proportional resonant controller with a fractional phase compensation function and a multi-rate repetitive controller connected in parallel, the specific value of the fractional phase compensation is determined according to weak grid impedance, and the grid impedance value is measured and calculated according to an impedance detection unit.
Preferably, the current controller comprises a PR controller and a MRC controller, the PR controller employing a high sampling rate TsThe MRC controller uses a low sampling rate T m PR controlled transfer function G pr And MRC controlled transfer function G mrc The following were used:
Figure 685112DEST_PATH_IMAGE003
；
wherein the content of the first and second substances,
Figure 136953DEST_PATH_IMAGE004
；Ts=1/f s ，f s is the sampling frequency; t is m =1/f s /m，mIs an integer, also called the sampling ratio;
Figure 719113DEST_PATH_IMAGE005
is the order of repetitive control;Q(z m ) Is an item added for improving the stability margin of the repeated control;S(z m ) Is a term introduced to accelerate the rate of decay at high frequencies of the control system;k r the gain compensator is MRC and is used for compensating the amplitude of an equivalent controlled object of the MRC;
Figure 471168DEST_PATH_IMAGE006
is a phase lead compensator of the MRC,kis an integer and is used for compensating the phase of an equivalent controlled object of the MRC;k p andk i proportional gain and integral gain of PR control respectively;
Figure 890036DEST_PATH_IMAGE007
a bandwidth factor for PR control;
Figure 344019DEST_PATH_IMAGE008
is the grid angular frequency.
Preferably, the fractional phase compensation is realized by a general Newton structure fractional filter based on spline interpolation, and the specific implementation steps are as follows:
1) deducing a mathematical form of the general Newton structure fractional filter through a basic Farrow structure, wherein the mathematical form comprises the determination of a Farrow structure filter coefficient matrix and the determination of a conversion matrix from the Farrow structure to the Newton structure;
2) designing the order of the general Newton structure fractional filter, wherein the design needs to consider the characteristics of both the complexity of the filter structure and the accuracy of the approximation effect;
3) determination of a fractional phase compensation term depending on the grid impedance value and the phase behavior of the entire control systemkWherein the grid impedance is measured by an impedance detector.
Preferably, the order of the general Newton fractional filter based on spline interpolation is designed to be third order.
The invention also provides LCL grid-connected inverter fractional phase compensation control equipment under the weak power grid, which adopts the LCL grid-connected inverter fractional phase compensation control method under the weak power grid and comprises the following steps:
the signal acquisition unit is used for acquiring a voltage signal and a grid-connected current signal at a common coupling point in the inverter system;
a coordinate transformation unit comprising
Figure 480603DEST_PATH_IMAGE009
Coordinate transformation unit and
Figure 336432DEST_PATH_IMAGE010
a coordinate transformation unit for converting the collected abc three-phase signals into
Figure 64217DEST_PATH_IMAGE011
Two phase signal and
Figure 5497DEST_PATH_IMAGE011
converting the two-phase signals into abc three-phase signals;
the pre-processing unit extracts voltage phase angle information at a common coupling point through a phase-locked loop PLL (phase-locked loop), and obtains a grid-connected reference current signal in the control system according to the voltage phase angle information and a grid-connected current value amplitude required by the inversion system;
the control unit is used for subtracting the grid-connected reference current signal from the collected grid-connected current signal to obtain an error signal; inputting the error signal into a current controller for modulation to obtain a current modulation signal; subtracting the current modulation signal from the grid-connected current feedback active damping signal to obtain a modulation wave signal;
the impedance detection unit is used for monitoring and calculating the power grid impedance value of the weak power grid and sending the impedance value to the current controller so as to supply design parameters to the current controller;
the control signal generating unit is used for comparing the modulation wave signal with a carrier signal generated in the SPWM modulator to obtain a control signal; inputting the control signal to a grid-connected inverter to control the on-off of an electronic device in the grid-connected inverter;
and the control output unit is used for outputting the control signal and controlling the on-off of the electronic devices in the grid-connected inverter.
Compared with the prior art, the invention has the following advantages and beneficial effects:
1. the PR parallel MRC composite control structure adopted by the invention can effectively improve the dynamic response speed of the whole system, can realize the implementation of a control strategy with higher switching frequency and less storage consumption, and can effectively inhibit power grid harmonic waves and periodic disturbance with a relatively simple control structure;
2. the active damping mode adopted by the invention is grid-connected current feedback active damping, and compared with other active damping modes, the use of one current sampling is reduced;
3. in consideration of the fact that an integer phase compensation strategy easily causes over-compensation or under-compensation on the phase of the whole system under a weak power grid environment, and further causes serious influence on grid-connected current quality, the invention provides a fractional phase compensation control strategy, namely a fractional filter based on a general Newton structure is adopted to approximate a fractional phase compensation item, so that the stable range of a PRMRC strategy under the weak power grid is improved, and the grid-connected current quality is improved;
4. the fractional filter based on the Newton structure has the greatest advantages of simple structure and fixed coefficient, and is very suitable for the working condition that the impedance of a power grid fluctuates under a weak power grid.
Drawings
Fig. 1 is a flowchart of a fractional phase compensation control method for an LCL grid-connected inverter in a weak grid in embodiment 1;
fig. 2 is a circuit structure and control scheme diagram of an LCL grid-connected inverter system in the weak grid in embodiment 1;
FIG. 3 shows the control strategy of PR parallel MRC in example 1zA control block diagram of the domain;
FIG. 4 is a diagram of a third order Newton fractional filter in example 1;
FIG. 5 shows the grid inductance of example 1L g =3mH，kA grid-connected current waveform diagram when = 6;
FIG. 6 shows the grid inductance of example 1L g =3mH，kA grid-connected current waveform diagram when = 5;
FIG. 7 shows the grid inductance of example 1L g =3mH，kAnd (5.3) grid-connected current waveform diagram.
Detailed Description
For a further understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings and examples. It is to be understood that the examples are illustrative of the invention and not limiting.
Example 1
As shown in fig. 1, the present embodiment provides a fractional phase compensation control method for an LCL grid-connected inverter in a weak grid, which includes the following steps:
firstly, respectively acquiring a voltage signal and a grid-connected current signal at a common coupling point in an inverter system through a voltage sampling plate and a current sampling plate;
secondly, voltage signals acquired through the voltage sampling plate pass through a phase-locked loop to extract voltage phase angle information; generating a grid-connected current reference signal in a control system according to the voltage phase angle information and a grid-connected current amplitude value required by the inverter system;
dividing a grid-connected current signal acquired by a current sampling plate into two paths, wherein one path of the grid-connected current signal is subjected to grid-connected current feedback active damping to suppress an inherent resonance peak of the LCL grid-connected inverter, and the other path of the grid-connected current signal is subjected to difference with the grid-connected current reference signal to obtain an error signal which is transmitted to a current controller for modulation;
fourthly, the error signal is subjected to modulation action of a current controller and then is subjected to difference with a grid-connected current feedback active damping signal to obtain a modulation wave signal;
comparing the modulated wave signal with a triangular carrier wave generated in the SPWM generator to obtain a control signal; and controlling the on-off of the electronic devices in the grid-connected inverter through the control signal.
The current controller is a composite controller with a proportional resonant controller with a fractional phase compensation function and a multi-rate repetitive controller connected in parallel, the specific value of the fractional phase compensation is determined according to weak grid impedance, and the grid impedance value is measured and calculated according to an impedance detection unit.
The current controller comprises a PR controller and an MRC controller, wherein the PR controller adopts a high sampling rate TsThe MRC controller uses a low sampling rate T m PR controlled transfer function G pr And MRC controlled transfer function G mrc The following were used:
Figure 414613DEST_PATH_IMAGE013
；
wherein the content of the first and second substances,
Figure 534403DEST_PATH_IMAGE004
；Ts=1/f s ，f s is the sampling frequency; t is m =1/f s /m，mIs an integer, also called the sampling ratio;
Figure 151198DEST_PATH_IMAGE005
is the order of repetitive control;Q(z m ) Is an item added for improving the stability margin of the repeated control;S(z m ) Is a term introduced to accelerate the rate of decay at high frequencies of the control system;k r the gain compensator is MRC and is used for compensating the amplitude of an equivalent controlled object of the MRC;
Figure 127244DEST_PATH_IMAGE006
is a phase lead compensator of the MRC,kis an integer and is used for compensating the phase of an equivalent controlled object of the MRC;k p andk i proportional gain and integral gain of PR control respectively;
Figure 323739DEST_PATH_IMAGE007
a bandwidth factor for PR control;
Figure 826265DEST_PATH_IMAGE008
is the grid angular frequency.
The fractional phase compensation is realized by a general Newton structure fractional filter based on spline interpolation, and the specific realization steps are as follows:
1) deducing a mathematical form of the general Newton structure fractional filter through a basic Farrow structure, wherein the mathematical form comprises the determination of a Farrow structure filter coefficient matrix and the determination of a conversion matrix from the Farrow structure to the Newton structure;
2) designing the order of the general Newton structure fractional filter, wherein the design needs to consider the characteristics of both the complexity of the filter structure and the accuracy of the approximation effect;
3) determination of a fractional phase compensation term depending on the grid impedance value and the phase behavior of the entire control systemkWherein the grid impedance is measured by an impedance detector.
The order of a general Newton fractional filter based on spline interpolation is designed to be third order.
The present embodiment further provides a fractional phase compensation control device for an LCL grid-connected inverter in a weak grid, which adopts the fractional phase compensation control method for an LCL grid-connected inverter in a weak grid, and includes:
the signal acquisition unit is used for acquiring a voltage signal and a grid-connected current signal at a common coupling point in the inverter system;
a coordinate transformation unit comprising
Figure 99114DEST_PATH_IMAGE009
Coordinate transformation unit and
Figure 740618DEST_PATH_IMAGE010
a coordinate transformation unit for converting the collected abc three-phase signals into
Figure 757116DEST_PATH_IMAGE011
Two phase signal and
Figure 442044DEST_PATH_IMAGE011
converting the two-phase signals into abc three-phase signals;
the pre-processing unit extracts voltage phase angle information at a common coupling point through a phase-locked loop PLL (phase-locked loop), and obtains a grid-connected reference current signal in the control system according to the voltage phase angle information and a grid-connected current value amplitude required by the inversion system;
the control unit is used for subtracting the grid-connected reference current signal from the collected grid-connected current signal to obtain an error signal; inputting the error signal into a current controller for modulation to obtain a current modulation signal; subtracting the current modulation signal from the grid-connected current feedback active damping signal to obtain a modulation wave signal;
the impedance detection unit is used for monitoring and calculating the power grid impedance value of the weak power grid and sending the impedance value to the current controller so as to supply design parameters to the current controller;
the control signal generating unit is used for comparing the modulation wave signal with a carrier signal generated in the SPWM modulator to obtain a control signal; inputting the control signal to a grid-connected inverter to control the on-off of an electronic device in the grid-connected inverter;
and the control output unit is used for outputting the control signal and controlling the on-off of the electronic devices in the grid-connected inverter.
The proposed fractional phase compensation item can expand the stable range of PR parallel MRC control under a weak grid and improve grid-connected current quality.
The fractional phase compensation item is realized by a general Newton structure fractional filter based on spline interpolation, and the filter can be obtained by Farrow structure derivation;
when phase compensation term
Figure 948111DEST_PATH_IMAGE006
In (1)kWhen fractional, the phase compensation term can be expressed as
Figure 85701DEST_PATH_IMAGE014
Wherein, in the step (A),k i is composed ofkThe integer part of (a) is,dis composed ofkThe fractional part of (a) is,
Figure 640310DEST_PATH_IMAGE015
the fractional part is realized by a general Newton structure fractional filter based on spline interpolation.
The order of the filter is determined before the specific structure of the filter is determinedMIn other words, when the Newton fractional filter is third order, the filter has both the characteristics of simple structure and better approximation effect, and the third order Newton fractional filter based on spline interpolation is adopted in this embodiment.
Example 2
The embodiment provides a fractional phase compensation control method for an LCL grid-connected inverter under a weak power grid, which comprises the following steps:
1. respectively acquiring a voltage signal and a grid-connected current signal at a common coupling point in an inverter system through a voltage sampling unit 101 and a current sampling unit 102, as shown in fig. 2;
2. the voltage signal and the grid-connected current signal at the common coupling point respectively pass through the first
Figure 851848DEST_PATH_IMAGE009
Coordinate system transformer 103 and second
Figure 718698DEST_PATH_IMAGE009
The coordinate system converter 104 converts the three-phase signal into a two-phase signal;
3. through the second
Figure 891053DEST_PATH_IMAGE009
The voltage signal transformed by the coordinate system transformer 104 passes through a phase-locked loop PLL105 to extract voltage phase angle information; according to the phase angle information of the voltage signal
Figure 498621DEST_PATH_IMAGE016
And the grid-connected current amplitude required by the inverter systemI ref Generating a grid-connected current reference signal in the whole control systemi ref ；
4. Through the first
Figure 564666DEST_PATH_IMAGE009
The grid-connected current signal converted by the coordinate system converter 103 is divided into two paths, one path inhibits the inherent property of the LCL inverter through the grid-connected current feedback active damping 106The other path of the resonance peak is connected with the grid-connected current reference signali ref Making a difference to obtain an error signal and transmitting the error signal to a current controller for modulation;
5. the error signal is subjected to the adjustment action of the current controller and then is subjected to difference with a grid-connected current feedback active damping signal to obtain a modulation wave signal; wherein: the current controller is a composite controller with a fractional phase compensation proportional resonant controller connected with a multi-rate repetitive controller (PRMRC) in parallel, and the specific value of the fractional phase compensation is determined according to the impedance of a weak power grid;
6. FIG. 3 shows a specific structure of the current controller of FIG. 2, the transfer function of PR controlG pr And MRC controlled transfer functionG mrc The following were used:
Figure 615798DEST_PATH_IMAGE018
；
wherein the content of the first and second substances,
Figure 259138DEST_PATH_IMAGE004
；Ts=1/f s ，f s is the sampling frequency; t is m =1/f s /m，mIs an integer, also called the sampling ratio;
Figure 421129DEST_PATH_IMAGE005
is the order of repetitive control;Q(z m ) Is an item added for improving the stability margin of the repeated control;S(z m ) Is a term introduced to accelerate the rate of decay at high frequencies of the control system;k r the gain compensator is MRC and is used for compensating the amplitude of an equivalent controlled object of the MRC;
Figure 79031DEST_PATH_IMAGE006
is a phase lead compensator of the MRC,kis an integer and is used for compensating the phase of an equivalent controlled object of the MRC;k p andk i are respectively PR controlProportional gain and integral gain of;
Figure 284753DEST_PATH_IMAGE007
a bandwidth factor for PR control;
Figure 166122DEST_PATH_IMAGE008
is the grid angular frequency.
In order to realize fractional phase compensation, a fractional filter is introduced into MRC control, and the stable range of PRMRC under a weak power grid is improved;
Figure 381071DEST_PATH_IMAGE006
phase lead compensator for MRC and limited to digital implementationkThe integer phase compensation PRMRC can only be an integer, but the integer phase compensation PRMRC is easy to destabilize the system under the weak power gridkWhen it is a fraction, the following holds:
Figure 156129DEST_PATH_IMAGE019
；
wherein the content of the first and second substances,k i is thatkThe integer part of (a) is,dis composed ofkA fraction part of (a); fractional part phase lead compensation term
Figure 80223DEST_PATH_IMAGE020
The general Newton fractional filter can be approximated by a fractional filter approximation of the general Newton structure
Figure 576451DEST_PATH_IMAGE020
In designing the approximation thereof
Figure 329512DEST_PATH_IMAGE020
When needed to use
Figure 100022DEST_PATH_IMAGE021
Substitution
Figure 381968DEST_PATH_IMAGE022
(ii) a And simultaneously, the order of the general Newton structure fractional filter is determined before the specific structure of the general Newton structure fractional filter is determined。
7. The power grid impedance value required by the current controller is detected and calculated by the impedance detection unit 109;
8. the modulated wave signal passes through
Figure 706770DEST_PATH_IMAGE010
The coordinate system converter 107 performs conversion, and the converted signal is compared with a triangular carrier wave generated in the SPWM generator 108 to obtain a control signal; and the on-off of the electronic devices in the inverter is controlled by the control signal.
After analysis and verification, whenMIn other words, when the Newton fractional structure filter is third order, the filter has both the characteristics of simple structure and better approximation effect, so that the general Newton fractional structure filter in this embodiment is designed to be third order and simultaneously based on spline interpolation, and therefore, a coefficient of the third-order spline interpolation filter of the general Newton structure based on linear interpolation can be obtained:
Figure 935626DEST_PATH_IMAGE024
；
C spline for the coefficient matrix of the fractional filter based on the third-order spline interpolation,
Figure 762241DEST_PATH_IMAGE025
and
Figure 496979DEST_PATH_IMAGE026
the inverse of the Farrow to Newton structure conversion matrix and the transpose of the inverse, respectively.
The general Newton architecture fractional filter expression is as follows:
Figure 89503DEST_PATH_IMAGE028
；
Figure 935100DEST_PATH_IMAGE030
；
Figure DEST_PATH_IMAGE032
；
Figure DEST_PATH_IMAGE033
；
Figure DEST_PATH_IMAGE034
and N' -1 is the filter order.
In summary, a specific structure of the general third-order Newton fractional filter of the present embodiment can be obtained, as shown in fig. 4, it can be seen that the fractional filter coefficient provided in the present embodiment is fixed, and when the value of the fractional phase compensation term needs to change along with the change of the grid impedance, the calculation burden and the memory consumption can be greatly reduced.
In order to verify the effectiveness of the fractional phase compensation control method of the LCL grid-connected inverter under the weak power grid, a simulation model of the three-phase LCL type grid-connected inverter based on MATLAB/Simulink is built, and meanwhile, a corresponding experiment platform is designed and built, wherein specific parameters are shown in a table 1.
TABLE 1 grid-tied inverter parameters
Figure DEST_PATH_IMAGE035
In a simulation model of the three-phase LCL grid-connected inverter, the grid impedance is expressed by pure inductance (the resistive component in the grid impedance has an active effect on the stability of the whole system), and the grid inductance is setL g =3mH, phase compensation term
Figure 588191DEST_PATH_IMAGE006
In (1)kThe integers 6, 5 and the fraction 5.3 are respectively taken, and the simulation results are respectively shown in fig. 5, 6 and 7, wherein the graphs are grid-connected current waveforms. As can be seen from the simulation results shown in fig. 5, 6 and 7, the integer phase compensation cannot ensure that the system can obtain better grid-connected current quality, when the system is in operationk=6, total harmonic distortion of grid-connected current(THD) 3.24%; when in usekWhen =5, the THD of the grid-connected current is 2.41%; whether or notk=5 or 6, the grid-connected current waveform distortion is severe. FIG. 7 is a drawing showingkAccording to the simulation result of the system when the current is 5.3, the THD of the grid-connected current is 1.49%, and compared with integer phase compensation, the fractional phase compensation strategy greatly improves the grid-connected current quality of the system.
The present invention and its embodiments have been described above schematically, without limitation, and what is shown in the drawings is only one of the embodiments of the present invention, and the actual structure is not limited thereto. Therefore, if the person skilled in the art receives the teaching, without departing from the spirit of the invention, the person skilled in the art shall not inventively design the similar structural modes and embodiments to the technical solution, but shall fall within the scope of the invention.

Claims (6)

1. The fractional phase compensation control method of the LCL grid-connected inverter under the weak grid is characterized by comprising the following steps: the method comprises the following steps:
firstly, respectively acquiring a voltage signal and a grid-connected current signal at a common coupling point in an inverter system through a voltage sampling plate and a current sampling plate;
secondly, voltage signals acquired through the voltage sampling plate pass through a phase-locked loop to extract voltage phase angle information; generating a grid-connected current reference signal in a control system according to the voltage phase angle information and a grid-connected current amplitude value required by the inverter system;
dividing a grid-connected current signal acquired by a current sampling plate into two paths, wherein one path of the grid-connected current signal is subjected to grid-connected current feedback active damping to suppress an inherent resonance peak of the LCL grid-connected inverter, and the other path of the grid-connected current signal is subjected to difference with the grid-connected current reference signal to obtain an error signal which is transmitted to a current controller for modulation;
fourthly, the error signal is subjected to modulation action of a current controller and then is subjected to difference with a grid-connected current feedback active damping signal to obtain a modulation wave signal;
comparing the modulated wave signal with a triangular carrier wave generated in the SPWM generator to obtain a control signal; and controlling the on-off of the electronic devices in the grid-connected inverter through the control signal.
2. The fractional phase compensation control method for the LCL grid-connected inverter under the weak grid according to claim 1, characterized by comprising the following steps: the current controller is a Proportional Resonance (PR) controller with a fractional phase compensation function and is connected with a composite controller of a multi-rate repetitive control (MRC) in parallel, a specific value of the fractional phase compensation is determined according to weak grid impedance, and a grid impedance value is measured and calculated according to an impedance detection unit.
3. The fractional phase compensation control method for the LCL grid-connected inverter under the weak grid according to claim 2, characterized in that: the current controller comprises a PR controller and an MRC controller, wherein the PR controller adopts a high sampling rate TsThe MRC controller uses a low sampling rate T m PR controlled transfer function G pr And MRC controlled transfer function G mrc The following were used:
Figure 341410DEST_PATH_IMAGE002
；
wherein the content of the first and second substances,
Figure 296597DEST_PATH_IMAGE003
；Ts=1/f s ，f s is the sampling frequency; t is m =1/f s /m，mIs an integer, also called the sampling ratio;
Figure 477042DEST_PATH_IMAGE004
is the order of repetitive control;Q(z m ) Is an item added for improving the stability margin of the repeated control;S(z m ) Is a term introduced to accelerate the rate of decay at high frequencies of the control system;k r the gain compensator is MRC and is used for compensating the amplitude of an equivalent controlled object of the MRC;
Figure 278250DEST_PATH_IMAGE005
is a phase lead compensator of the MRC,kis an integer and is used for compensating the phase of an equivalent controlled object of the MRC;k p andk i proportional gain and integral gain of PR control respectively;
Figure 354791DEST_PATH_IMAGE006
a bandwidth factor for PR control;
Figure 797273DEST_PATH_IMAGE007
is the grid angular frequency.
4. The fractional phase compensation control method for the LCL grid-connected inverter under the weak grid according to claim 3, characterized by comprising the following steps: the fractional phase compensation is realized by a general Newton structure fractional filter based on spline interpolation, and the specific realization steps are as follows:
1) deducing a mathematical form of the general Newton structure fractional filter through a basic Farrow structure, wherein the mathematical form comprises the determination of a Farrow structure filter coefficient matrix and the determination of a conversion matrix from the Farrow structure to the Newton structure;
2) designing the order of the general Newton structure fractional filter, wherein the design needs to consider the characteristics of both the complexity of the filter structure and the accuracy of the approximation effect;
3) determination of a fractional phase compensation term depending on the grid impedance value and the phase behavior of the entire control systemkWherein the grid impedance is measured by an impedance detector.
5. The fractional phase compensation control method for the LCL grid-connected inverter under the weak grid according to claim 4, characterized by comprising the following steps: the order of a general Newton fractional filter based on spline interpolation is designed to be third order.
6. A fractional phase compensation control device of an LCL grid-connected inverter under a weak power grid is characterized in that: the fractional phase compensation control method for the LCL grid-connected inverter under the weak grid as claimed in any one of claims 1 to 5 is adopted, and comprises the following steps:
the signal acquisition unit is used for acquiring a voltage signal and a grid-connected current signal at a common coupling point in the inverter system;
a coordinate transformation unit comprising
Figure 765098DEST_PATH_IMAGE008
Coordinate transformation unit and
Figure 422476DEST_PATH_IMAGE009
a coordinate transformation unit for converting the collected abc three-phase signals into
Figure 919185DEST_PATH_IMAGE010
Two phase signal and
Figure 851893DEST_PATH_IMAGE010
converting the two-phase signals into abc three-phase signals;
the pre-processing unit extracts voltage phase angle information at a common coupling point through a phase-locked loop PLL (phase-locked loop), and obtains a grid-connected reference current signal in the control system according to the voltage phase angle information and a grid-connected current value amplitude required by the inversion system;
the control unit is used for subtracting the grid-connected reference current signal from the collected grid-connected current signal to obtain an error signal; inputting the error signal into a current controller for modulation to obtain a current modulation signal; subtracting the current modulation signal from the grid-connected current feedback active damping signal to obtain a modulation wave signal;
the impedance detection unit is used for monitoring and calculating the power grid impedance value of the weak power grid and sending the impedance value to the current controller so as to supply design parameters to the current controller;
the control signal generating unit is used for comparing the modulation wave signal with a carrier signal generated in the SPWM modulator to obtain a control signal; inputting the control signal to a grid-connected inverter to control the on-off of an electronic device in the grid-connected inverter;
and the control output unit is used for outputting the control signal and controlling the on-off of the electronic devices in the grid-connected inverter.
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