CN113489291B - Control method for positive feedback virtual impedance of LCL type grid-connected converter - Google Patents

Control method for positive feedback virtual impedance of LCL type grid-connected converter Download PDF

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CN113489291B
CN113489291B CN202110783438.1A CN202110783438A CN113489291B CN 113489291 B CN113489291 B CN 113489291B CN 202110783438 A CN202110783438 A CN 202110783438A CN 113489291 B CN113489291 B CN 113489291B
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lcl type
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positive feedback
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type grid
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CN113489291A (en
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杜贵平
朱天生
杨子江
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South China University of Technology SCUT
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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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • 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
    • H02J3/38—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
    • 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • 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
    • H02M7/5387—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 in a bridge configuration
    • H02M7/53871—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 in a bridge configuration with automatic control of output voltage or current
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/22—The renewable source being solar energy
    • H02J2300/24—The renewable source being solar energy of photovoltaic origin
    • 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

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Abstract

The invention discloses a control method of positive feedback virtual impedance of an LCL type grid-connected converter, which comprises the steps of sampling the current of a power grid side of the LCL type grid-connected converter, comparing the current of the power grid side with a given value to obtain a comparison signal, and sending the comparison signal into a current regulator to obtain a first error signal; sampling the terminal voltage of a filter capacitor of an LCL type filter in the LCL type grid-connected converter, and sending the terminal voltage to a voltage loop positive feedback control link to obtain a second error signal; and comparing the first error signal with the second error signal to obtain a pulse width modulation signal for driving the switching device, and driving the switching device by using the pulse width modulation signal to enable the effect of the virtual impedance to act on the LCL type grid-connected converter, thereby realizing the control of the positive feedback virtual impedance of the LCL type grid-connected converter. The method reduces the system loss, increases the low-frequency gain, reduces the total cost and achieves the effect of better inhibiting the resonance peak.

Description

Control method for positive feedback virtual impedance of LCL type grid-connected converter
Technical Field
The invention relates to the technical field of grid-connected converter network access current control, in particular to a method for controlling LCL type grid-connected converter positive feedback virtual impedance.
Background
Solar energy is one of the most important renewable energy sources, and it is a direction of attention and research in all countries of the world to convert it into electric energy and utilize it. The photovoltaic inversion control technology is a 'bridge' for ensuring that electric energy converted from light energy is smoothly merged into a power grid, and a filter is an indispensable part in the 'bridge'.
At low frequency, the amplitude-frequency curve of the LCL type filter is attenuated by-20 dB/ten times frequency, the phase angle is-90 degrees, along with the increase of frequency, the amplitude-frequency curve can generate a resonance peak at the resonance frequency, the phase angle jumps to-270 degrees, and then the amplitude-frequency curve is attenuated by-60 dB/ten times frequency. Therefore, the LCL type filter has strong inhibition capability on high-frequency harmonics, but a certain method must be adopted to damp a resonance peak, otherwise, the system is easy to be unstable. The article of the single-phase LCL inverter grid-connected technology research published by Liu Shang Wei shows that the resistor is connected in parallel at the two ends of the filter capacitor to effectively damp the resonance peak, but the resistor causes unnecessary loss. Therefore, the article adopts a method of sampling the capacitance current and feeding the capacitance current back to the control end, which is equivalent to the effect of connecting resistors in parallel at two ends of the filter capacitor, but in practice, the capacitance current is difficult to obtain, and extra cost is increased.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a control method of the positive feedback virtual impedance of an LCL type grid-connected converter, which carries out positive feedback control on the capacitor voltage of an LCL type filter through an inertia link, increases the damping of the system, realizes the inhibition of the resonance peak existing in the traditional LCL type filter, and achieves the damping effect of connecting resistors at two ends of a filter capacitor in parallel, namely realizes the virtual impedance.
In order to achieve the purpose, the technical scheme provided by the invention is as follows: a control method for LCL type grid-connected converter positive feedback virtual impedance comprises the following steps:
grid side current I of sampling LCL type grid-connected converter2For grid side current I2With a given value IrefComparing to obtain a comparison signal, and sending the comparison signal into a current regulator to obtain a first error signal; sampling terminal voltage U of filter capacitor of LCL type filter in LCL type grid-connected converterCSending the second error signal to a voltage loop positive feedback control link to obtain a second error signal, and realizing the suppression of resonance peak through the voltage loop positive feedback control link to achieve the aim of filteringThe control effect of the resistor connected in parallel at the two ends of the filter capacitor of the device, namely a positive feedback control link of a voltage loop becomes virtual impedance; and comparing the first error signal with the second error signal to obtain a pulse width modulation signal for driving the switching device, and driving the switching device by using the pulse width modulation signal to enable the effect of the virtual impedance to act on the LCL type grid-connected converter, thereby realizing the control of the positive feedback virtual impedance of the LCL type grid-connected converter.
Further, the current regulator is selected from a PI regulator Gi(s) is represented by KP+KIS, where s is a complex frequency domain variable, KPAs a proportional parameter of the current regulator, KIIs an integral parameter of the current regulator.
Furthermore, the LCL type filter in the LCL type grid-connected converter consists of a converter side inductor, a filter capacitor and a network side inductor, and Z isL1(s)、ZL2(s)、ZC(s) respectively corresponding to the impedances of the converter side inductor, the network side inductor and the filter capacitor, neglecting parasitic parameters, and obtaining:
ZL1(s)＝sL1
ZL2(s)＝sL2
Figure BDA0003157855410000021
deriving the transfer function of the LCL-type filter as G1(s)：
Figure BDA0003157855410000022
In the formula: s is a complex frequency domain variable, L1Is the inductance value of the inverter side inductor, C is the capacitance value of the filter capacitor, L2Is the inductance, omega, of the network side inductornIs the natural resonant frequency, the expression is:
Figure BDA0003157855410000023
the LCL type filter is corrected by introducing damping, i.e. the first order term of s is added to the denominator of the transfer function of the LCL type filter, and the transfer function of the LCL type filter is G2(s)：
Figure BDA0003157855410000031
According to the transfer function G of the LCL type filter at that time2(s) to obtain the grid side current I of the LCL type grid-connected converter2The transfer function for the control signal is Gd0(s)：
Figure BDA0003157855410000032
In the formula: xi is the damping coefficient of the resonant pole, KuIs a coefficient, and the terminal voltage u of the bridge arm formed by switching tubes and the connecting end of the filterdcDetermining;
after a voltage loop positive feedback control link to be corrected is introduced, the transfer function G of the LCL type grid-connected converter at the moment is obtained through derivationd(s)：
Figure BDA0003157855410000033
In the formula: gF(s) is a voltage loop positive feedback control link to be corrected;
comparative Gd(s) and Gd0(s) to obtain GF(s)：
Figure BDA0003157855410000034
GF(s) is a first order differential element, with a negative sign in the expression, thus for GF(s) performing a calibration;
the frequency characteristic curves of the first-order differential element and the inertia element are symmetrical about a frequency axis, and G is obtainedF(s) correction as an inertial element to remove GF(s) watchNegative sign in the expression, corrected GF(s) to GVF(s)：
Figure BDA0003157855410000035
In the formula: gVF(s) is a positive feedback control link of the corrected voltage loop, A1、A2Two undetermined coefficients;
and (3) calculation of undetermined coefficients:
according to the equivalent relation between the differential link and the inertia link, the following steps are obtained:
Figure BDA0003157855410000041
further, the LCL type grid-connected converter is an LCL type grid-connected inverter or an LCL type grid-connected rectifier.
Compared with the prior art, the invention has the following advantages and beneficial effects:
1. the resonance peak of the LCL type filter is effectively inhibited, the damping effect which is the same as that of the resistor connected in parallel at the two ends of the capacitor is achieved, the virtual impedance is realized, the low-frequency harmonic wave is better inhibited, and the cost is not required to be additionally increased.
2. The invention adopts a positive feedback control mode of a voltage loop, reduces the loss, lowers the cost and simplifies the circuit structure.
3. The invention is suitable for photovoltaic grid-connected converters, including grid-connected inverters and grid-connected rectifiers.
Drawings
FIG. 1 is a flow chart of the method of the present invention.
Fig. 2 is a circuit diagram of the LCL type grid-connected inverter.
Fig. 3 is a control block diagram of an LCL type filter.
Fig. 4 is a graph showing the frequency characteristics of the LCL type filter.
Fig. 5 is a control block diagram of the LCL type grid-connected inverter when no feedback control is applied.
Fig. 6 is a control block diagram of the LCL type grid-connected converter with voltage loop positive feedback control before correction.
Fig. 7 is a control block diagram of the LCL grid-connected inverter in the specific case.
Fig. 8 is a graph showing an open-loop frequency characteristic of the LCL grid-connected inverter in the specific case.
Fig. 9 is a closed loop step response diagram of the LCL grid-connected inverter in the specific case.
Detailed Description
The present invention will be described in further detail with reference to examples and drawings, but the embodiments of the present invention are not limited thereto.
As shown in fig. 1, the method for controlling the positive feedback virtual impedance of the LCL type grid-connected converter provided in this embodiment specifically includes the following steps:
sampling power grid side current I of LCL type grid-connected converter2For grid side current I2With a given value IrefComparing to obtain a comparison signal, and sending the comparison signal into a current regulator to obtain a first error signal; sampling terminal voltage U of filter capacitor of LCL type filter in LCL type grid-connected converterCSending the second error signal to a voltage loop positive feedback control link to obtain a second error signal, and realizing the suppression of resonance peaks through the voltage loop positive feedback control link, thereby achieving the control effect of connecting resistors in parallel at two ends of a filter capacitor of the filter, namely the voltage loop positive feedback control link becomes virtual impedance; and comparing the first error signal with the second error signal to obtain a pulse width modulation signal for driving the switching device, and driving the switching device by using the pulse width modulation signal to enable the effect of the virtual impedance to act on the LCL type grid-connected converter, thereby realizing the control of the positive feedback virtual impedance of the LCL type grid-connected converter.
The specific calculation process of the voltage loop positive feedback control link is as follows:
step 1: the transfer function of the LCL-type filter in the converter is derived from the circuit diagram.
Fig. 2 is a circuit diagram of the LCL type grid-connected inverter. In the figure VinIs used for inputting a direct current power supply; cinIs an input capacitance; q1, Q2, Q3 and Q4 form a full-bridge power supplyFour switching tubes of the circuit; u. ofdcThe terminal voltage of the connecting end of the full bridge circuit and the filter; vgIs an ac voltage source, which represents the power grid. L is1Is the inductance value, L, of the inverter-side inductor2The inductance value of the grid side inductor and C the capacitance value of the filter capacitor. ZL1、ZL2、ZCAnd the impedances respectively correspond to the inductance at the side of the converter, the inductance at the side of the power grid and the filter capacitor, and the influence of parasitic parameters is ignored. The expression of the impedance in the complex frequency domain can be obtained as follows:
ZL1(s)＝sL1
ZL2(s)＝sL2
Figure BDA0003157855410000061
in the formula, s is a complex frequency domain variable.
Fig. 3 is a control block diagram of an LCL type filter, in which the meaning of the parameters is mentioned above. Due to VgIs an AC source with 50Hz frequency and 10 sampling frequency6Hz, much greater than VgTherefore, it can be considered that within one sampling period, VgIs substantially unchanged so that VgTo the side current I of the power grid2Does not change with the change of the complex frequency domain variable s, so V is ignored in the subsequent analysisg. The transfer function of the LCL filter, i.e. the grid-side current I, can be derived from FIG. 32To bridge arm voltage udcTransfer function G of1(s) is:
Figure BDA0003157855410000062
in the formula:
Figure BDA0003157855410000063
is the natural resonant frequency.
It can be seen that the damping coefficient ξ of the LCL type filter is 0, and therefore a resonance spike is generated at the resonance frequency, as shown in fig. 4, fig. 4 is a frequency characteristic curve of the LCL type filter. As can be seen from fig. 4, the amplitude-frequency curve has a resonant peak at the resonant frequency, and at this frequency a-180 ° jump in phase occurs, which can destabilize the system.
Step 2: damping is introduced to suppress the resonance spikes.
As can be known from step 1, the reason why the LCL type filter generates the resonance peak is that the damping coefficient ξ is 0, so that the damping is introduced to correct the system, namely, the damping is the first order term of s is added to the denominator of the transfer function of the LCL type filter. The transfer function of the LCL filter is G2(s)：
Figure BDA0003157855410000064
Fig. 5 is a control block diagram of the LCL type grid-connected inverter when no feedback control is applied, and d in the figure is a control signal of the switching device. According to G2(s) and the control block diagram in FIG. 5, the grid side current I of the LCL type grid-connected converter can be obtained2The transfer function for the control signal d is Gd0(s)：
Figure BDA0003157855410000071
In the formula: xi is the damping coefficient of the resonant pole, KuIs a coefficient, and the terminal voltage u of the bridge arm formed by switching tubes and the connecting end of the filterdcThe decision is 400 in this design.
And step 3: and introducing filter capacitor voltage positive feedback of the LCL type filter.
After introducing a positive feedback control link of a voltage loop to be corrected, a control block diagram of the LCL type grid-connected converter system is shown in fig. 6. The voltage feedback point of the filter capacitor is KuPreviously, it was represented that the capacitor voltage was fed back into the control signal. According to the graph 6, the transfer function G of the LCL type grid-connected converter can be obtained through derivationd(s)：
Figure BDA0003157855410000072
In the formula: gFAnd(s) is a positive feedback control link of the voltage loop to be corrected.
Comparative Gd(s) and Gd0(s) to obtain GF(s)：
Figure BDA0003157855410000073
GF(s) is a negative first-order differential link, the expression has a negative sign, and the actual feedback mode is negative feedback. Therefore, it is to be directed to GF(s) correction is made so that the feedback mode is positive feedback.
And 4, step 4: and correcting the first-order differential element into an inertial element.
The relationship between the negative first-order differential element and the positive first-order differential element on the frequency characteristic curve is symmetrical about the frequency axis. The relationship between the inertia element and the positive first-order differential element on the frequency characteristic curve is also symmetrical about the frequency axis. Therefore, the inertia differential element can be used to replace the negative first order differential element. G is to beFAnd(s) correcting the inertial element. Corrected GF(s) to GVF(s)：
Figure BDA0003157855410000074
In the formula: a. the1、A2Are two undetermined coefficients.
And 5: and calculating the undetermined coefficient.
According to the relation between the differential element and the inertia element coefficient, the following can be obtained:
Figure BDA0003157855410000081
in the formula: xi is in the range of 0.4-0.8, where xi is 0.7.
The above analysis is based on the LCL type grid-connected inverter, but the same is also applicable to the LCL type grid-connected rectifier. This is because the LCL grid-connected rectifier and the LCL grid-connected inverter are different only in the direction of current flow, and therefore the same control strategy can be adopted, and therefore the grid-connected inverter only needs to be analyzed.
In the following, we take an LCL type grid-connected inverter with a rated power of 5KW as an example. The switching frequency is 40kHz, and the network voltage is 220V/50 Hz. L is1The value of 320uH, C3 uF, L2Values 2600 uH. The undetermined coefficients are found with reference to the formula given above and are adjusted appropriately. Final A1Take 0.0005, A240000.
Fig. 7 is a control block diagram of a control method employed by the present invention. In the figure IrefIs the grid side current I2Reference value of Gi(s) is the transfer function of the PI regulator. The selection of parameters for the PI regulator is described below.
The PI regulator expression is:
Figure BDA0003157855410000082
in the present invention, the ratio parameter KPTake 0.295, integral parameter KI294 is taken.
And at this point, the parameter setting is finished.
Fig. 8 shows a frequency characteristic curve of the LCL grid-connected inverter. As is obvious from comparison with FIG. 3, the resonance peak almost disappears, the phase margin is 54.1 degrees, and the system stability is greatly improved. Because the open-loop transfer function has no pole on the right half plane and the frequency of the phase-frequency characteristic curve crossing the (2k +1) pi line is zero in the range that the amplitude-frequency characteristic curve of the transfer function is greater than 0, the closed-loop system is stable.
The step response of the closed loop system is shown in fig. 9. As can be seen from the figure, the grid-connected current I2The response time of (2) is 0.01s, and the response speed is high.
In summary, conventional LCL filters have resonant spikes. The passive damping method of connecting the capacitors in parallel at two ends of the filter capacitor can effectively damp the resonance peak, but can generate larger loss. The active damping method based on capacitive current feedback requires a plurality of sensors to cooperate, which increases the cost of the system. The invention provides a novel virtual impedance feedback control method. Compared with the prior art, the invention has the innovation points that the capacitance voltage feedback link is adjusted into a first-order inertia link and a proportion link, and is positively fed back to the input end of the PWM modulator, namely fed back to the control signal, so that the low-frequency gain of the system is improved, the stability of the system is also improved, and the system can respond quickly and is worthy of popularization.
The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such modifications are intended to be included in the scope of the present invention.

Claims (3)

1. A control method for LCL type grid-connected converter positive feedback virtual impedance is characterized by comprising the following steps:
sampling power grid side current I of LCL type grid-connected converter2For grid side current I2With a given value IrefComparing to obtain a comparison signal, and sending the comparison signal into a current regulator to obtain a first error signal; sampling terminal voltage U of filter capacitor of LCL type filter in LCL type grid-connected converterCSending the second error signal to a voltage loop positive feedback control link to obtain a second error signal, and realizing the suppression of resonance peaks through the voltage loop positive feedback control link, thereby achieving the control effect of connecting resistors in parallel at two ends of a filter capacitor of the filter, namely the voltage loop positive feedback control link becomes virtual impedance; comparing the first error signal with the second error signal to obtain a pulse width modulation signal for driving the switching device, and driving the switching device by using the pulse width modulation signal to enable the effect of the virtual impedance to act on the LCL type grid-connected converter, thereby realizing the control of the positive feedback virtual impedance of the LCL type grid-connected converter;
the LCL type filter in the LCL type grid-connected converter consists of a converter side inductor, a filter capacitor and a grid side inductor, and Z isL1(s)、ZL2(s)、ZC(s) respectively corresponding to the impedances of the converter side inductor, the network side inductor and the filter capacitor, neglecting parasitic parameters, and obtaining:
ZL1(s)＝sL1
ZL2(s)＝sL2
Figure FDA0003584213520000011
deriving the transfer function of the LCL-type filter as G1(s)：
Figure FDA0003584213520000012
In the formula: s is a complex frequency domain variable, L1Is the inductance value of the inverter side inductor, C is the capacitance value of the filter capacitor, L2Is the inductance, omega, of the network side inductornIs the natural resonant frequency, the expression is:
Figure FDA0003584213520000013
the LCL type filter is corrected by introducing damping, i.e. the first order term of s is added to the denominator of the transfer function of the LCL type filter, and the transfer function of the LCL type filter is G2(s)：
Figure FDA0003584213520000021
According to the transfer function G of the LCL type filter at the time2(s) to obtain the grid side current I of the LCL type grid-connected converter2A transfer function for the control signal of Gd0(s)：
Figure FDA0003584213520000022
In the formula: xi is the damping coefficient of the resonant pole, KuIs a coefficient, and the terminal voltage u of the bridge arm formed by switching tubes and the connecting end of the filterdcDetermining;
after a voltage loop positive feedback control link to be corrected is introduced, the transfer function G of the LCL type grid-connected converter at the moment is obtained through derivationd(s)：
Figure FDA0003584213520000023
In the formula: gF(s) is a voltage loop positive feedback control link to be corrected;
comparison Gd(s) and Gd0(s) to obtain GF(s)：
Figure FDA0003584213520000024
GF(s) is a first order differential element, with a negative sign in the expression, thus for GF(s) performing a calibration;
the frequency characteristic curves of the first-order differential element and the inertia element are symmetrical about a frequency axis, and G is obtainedF(s) correction as an inertial element to remove GF(s) minus sign in the expression, corrected GF(s) to GVF(s)：
Figure FDA0003584213520000025
In the formula: gVF(s) is a positive feedback control link of the corrected voltage loop, A1、A2Two undetermined coefficients;
and (3) calculation of undetermined coefficients:
according to the equivalent relation between the differential link and the inertia link, the following steps are obtained:
Figure FDA0003584213520000031
2. the method for controlling the LCL type grid-connected converter positive feedback virtual impedance according to claim 1, characterized in that: the current regulator is selected from PI regulator Gi(s) is represented by KP+KIS, where s is a complex frequency domain variable, KPAs a proportional parameter of the current regulator, KIIs an integral parameter of the current regulator.
3. The method for controlling the LCL type grid-connected converter positive feedback virtual impedance according to claim 1, characterized in that: the LCL type grid-connected converter is an LCL type grid-connected inverter or an LCL type grid-connected rectifier.
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