CN112532096A - LCL inverter grid-connected device and method suitable for weak power grid - Google Patents

LCL inverter grid-connected device and method suitable for weak power grid Download PDF

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CN112532096A
CN112532096A CN202011397031.7A CN202011397031A CN112532096A CN 112532096 A CN112532096 A CN 112532096A CN 202011397031 A CN202011397031 A CN 202011397031A CN 112532096 A CN112532096 A CN 112532096A
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grid
inverter
lcl
phase
network side
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CN112532096B (en
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吕建国
徐炜基
孙状
史兆成
方斌
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Nanjing University of Science and Technology
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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
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/487—Neutral point clamped inverters
    • 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
    • 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
    • H02M1/08—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters
    • H02M1/084—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters using a control circuit common to several phases of a multi-phase system
    • H02M1/0845—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters using a control circuit common to several phases of a multi-phase system digitally controlled (or with digital control)
    • 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
    • H02M1/08—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters
    • H02M1/088—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters for the simultaneous control of series or parallel connected semiconductor devices
    • 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
    • H02M1/126—Arrangements for reducing harmonics from ac input or output using passive filters
    • 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
    • H02M7/493—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 the static converters being arranged for operation in parallel
    • 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
    • H02M7/53873—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 with digital control

Abstract

The invention discloses an LCL inverter grid-connected device and method suitable for a weak power grid. The device comprises a three-level inverter, a digital processing control module and a driving circuit, wherein the digital processing control module comprises a sampling unit, a closed-loop control unit, an active damping unit and a sine pulse width modulation unit. The method comprises the following steps: selecting capacitance values of capacitors of the LCL filter, network side inductors and inverter side inductor values; the optimal combination of proportional parameters of the PR controller and an active damping feedback coefficient is determined by the aid of the Laus stability criterion and by combination of resonance suppression conditions, amplitude margin constraint conditions and phase margin constraint conditions, so that the grid-connected inverter can stably run in the impedance change range of a weak power grid, has larger system bandwidth and good dynamic performance. The invention has the characteristics of low hardware cost, accurate control and wide application range, can stably run under the condition of weak power grid, effectively inhibits the harmonic component of the resonant frequency of the LCL filter and reduces the distortion rate of the network access current.

Description

LCL inverter grid-connected device and method suitable for weak power grid
Technical Field
The invention belongs to the technical field of power electronic conversion, and particularly relates to a grid-connected device and method of an LCL inverter suitable for a weak power grid.
Background
The LCL filter has the advantages of simple structure, good high-frequency filtering performance, low output harmonic content and the like, and is widely applied to new energy distributed grid-connected power generation occasions. However, due to the inherent characteristics of the LCL filter, the resonance characteristics of the LCL filter can significantly degrade the power quality at the output side. At present, for the problem of the resonant frequency of the LCL filter, two solutions are mainly provided: (1) an additional hardware damping circuit is adopted to inhibit LCL resonance; (2) and a software control method is adopted to inhibit LCL resonance. The latter method is generally used because the first method increases hardware costs. Under ideal power grid conditions, the existing active damping control method is relatively mature, such as an active damping control method based on a wave trap, an active damping control method based on filter capacitor current feedback, an active damping control method based on multi-state quantity hybrid feedback, and the like. In practical situations, however, the power grid is not an ideal fundamental wave power grid, weak grid impedance exists in the power grid, and under the condition of the weak grid, the stability of the control system is affected by the existence of weak grid inductance, which brings difficulty to the resonance suppression control of the active damping LCL filter.
Disclosure of Invention
The invention aims to provide an LCL inverter grid-connected device and method suitable for a weak power grid, so as to realize resonance suppression of an LCL filter under the condition of the weak power grid.
The technical solution for realizing the purpose of the invention is as follows: an LCL type inverter grid-connected device suitable for a weak power grid comprises a three-level inverter, a digital processing control module and a driving circuit, wherein the three-level inverter is an LCL type NPC three-level inverter, and the digital processing control module comprises a sampling unit, a closed-loop control unit, an active damping unit and a sine pulse width modulation unit;
the sampling unit respectively collects three-phase voltage signals at the network side of the LCL filter and three-phase current signals at the network side of the LCL filter and transmits the three-phase voltage signals and the three-phase current signals to the closed-loop control unit;
the closed-loop control unit converts the network side voltage and the network side current under the static abc coordinate system into a static alpha beta coordinate system through Clarke conversion according to the acquired signals; the alpha and beta axis components i of the grid side current under an alpha and beta coordinate systemα、iβAn input active damping unit;
and the output end of the sinusoidal pulse width modulation unit is connected to each switching tube of each phase bridge arm of the three-level inverter through a driving circuit.
Further, the digital processing control modules are TMS320F28377D and EPM1270T chips.
A grid-connected control method of an LCL inverter adapting to a weak power grid comprises the following steps:
step 1, in each switching period, a sampling unit of a digital control module respectively collects a network side voltage signal u of an LCL filtera、ub、ucAnd net side current signal ia、ib、ic；
Step 2, the closed-loop control unit transforms the network side voltage and the network side current under the static abc coordinate system to the static alpha beta coordinate system through Clarke transformation according to the signals collected in the step 1;
step 3, calculating a closed loop transfer function of the system after the active damping loop and the PR controller are added under the s domain, and performing stability analysis on the system by using a Laus criterion;
step 4, analyzing the harmonic suppression condition of the resonant frequency of the LCL filter;
step 5, analyzing the amplitude margin requirement required to be met by the system;
step 6, analyzing the phase margin requirement required to be met by the system;
step 7, satisfying the stability stripSelecting an active damping feedback coefficient k which enables the system bandwidth to be maximum within the range of the resonance suppression condition, the amplitude margin requirement and the phase margin requirementadProportional link coefficient K of PR controllerpSo as to obtain a closed-loop control system with good dynamic performance;
step 8, calculating current setting by taking current sine as a target, subtracting the obtained current setting amount by taking the network side current as a feedback amount, adding the obtained current setting amount to the output of an active damping ring through a proportional resonance regulator, and outputting a three-phase modulation wave signal through Clarke inverse transformation;
and 9, generating a pulse width modulation signal by the three-phase modulation signal obtained in the step 8 through a sine pulse width modulation unit, wherein the pulse width modulation signal controls the working state of a switching tube of the inverter through a driving circuit.
Further, the stability analysis of the system in step 3 is specifically as follows:
the system stability analysis results were as follows:
Figure BSA0000226309960000021
in the formula, KpIs the proportional link coefficient, k, of the PR controlleradFor active damping feedback coefficient, L1Is inductance value, L, of inverter side of LCL filter2Is inductance value of LCL filter network side, C1Is the capacitance value, L, of the LCL filtergThe inductance value is weak grid inductance value.
Further, the analysis of the resonance suppression condition of the LCL filter in step 4 is specifically as follows:
system open loop transfer function Gop(s) is:
Figure BSA0000226309960000022
due to the presence of the zero-order keeper in the digital control, the actual resonance frequency ω of the LCL filterresOffset after using an active damping control loopResonant frequency of omegares', the resonance suppression analysis results are as follows:
Figure BSA0000226309960000023
in the formula, KPWMIs an inverter transfer function, TsFor sample time, A isres' substitution of the modulus of the real part on the denominator after the open-loop transfer function, B isres' substituting the modulus of the imaginary part on the denominator after the open loop transfer function.
A＝0.5C1L1(L2+Lg)Tsωres′4-(0.5(L1+L2+Lg)Ts+kadKPWM)ωres′2
B＝(L1+L2+Lg)ωres′-C1L1(L2+Lg)ωres′3
Further, the step 5 analyzes the amplitude margin requirement to be met by the system, and the specific result is as follows:
Figure BSA0000226309960000031
where GM is the amplitude margin of the system.
Further, the step 6 of analyzing the phase margin requirement to be met by the system has the following specific results:
Figure BSA0000226309960000032
where PM is the phase margin of the system, ωcIs the cut-off frequency of the system, ωresThe resonant frequency of the LCL filter.
Figure BSA0000226309960000033
K in step 7adAnd KpThe selection is as follows:
Figure BSA0000226309960000034
in the formula, ωc_maxThe maximum cut-off frequency of the system is expressed as follows:
Figure BSA0000226309960000035
a＝4C1 2L1 2(L2+Lg_max)2(1+(tan PM)2)
b＝-10-0.1GMTs 2(L1+L2+Lg_max)2(tan PM)2
c＝4(tan PM)Ts(L1+L2+Lg_max)210-0.1GM
d＝-4(L1+L2+Lg_max)210-0.1GM
Δ1＝b2+12ad
Δ2＝2b3+27ac2-72abd
Figure BSA0000226309960000036
in the formula, a is a cubic coefficient of a unitary cubic equation obtained by a constraint condition, b is a quadratic coefficient of the unitary cubic equation, c is a first order coefficient of the unitary cubic equation, d is a constant term coefficient of the unitary cubic equation, Δ 1 is a 1 discriminant of the unitary cubic equation, Δ 2 is a 2 discriminant of the unitary cubic equation, and Δ 3 is a 3 discriminant of the unitary cubic equation.
Compared with the prior art, the invention has the remarkable advantages that: (1) active damping is fed back through network side current, hardware cost is not increased, and LCL resonance suppression control is achieved; (2) and selecting control parameters which enable the system bandwidth to be larger, so that the system can stably run under the condition of a weak power grid and has good dynamic performance.
Drawings
Fig. 1 is a schematic structural diagram of an LCL type inverter grid-connected device adapted to impedance changes of a weak power grid according to the present invention.
Fig. 2 is a control block diagram of an LCL type NPC three-level inverter grid-connected system.
Fig. 3 is a topology diagram of an NPC three-level grid-connected inverter.
Fig. 4 is a simulated waveform diagram after adding active damping at ideal grid conditions.
FIG. 5 shows the inductance L in the weak gridgThe simulated waveform after adding active damping at 5 mH.
Detailed Description
The invention is described in further detail below with reference to the figures and the embodiments.
With reference to fig. 1, the LCL type inverter grid-connected device adapted to the weak grid of the present invention includes a three-level inverter, a digital processing control module and a driving circuit, wherein the three-level inverter is an LCL type NPC three-level inverter, and the digital processing control module includes a sampling unit, a closed-loop control unit, an active damping unit and a sinusoidal pulse width modulation unit; the sampling unit respectively collects three-phase voltage signals at the network side of the LCL filter and three-phase current signals at the network side of the LCL filter and transmits the three-phase voltage signals and the three-phase current signals to the closed-loop control unit; the closed-loop control unit converts the network side voltage and the network side current under the static abc coordinate system into a static alpha beta coordinate system through Clarke conversion according to the acquired signals; the alpha and beta axis components i of the grid side current under an alpha and beta coordinate systemα、iβAn input active damping unit; and the output end of the sine pulse width modulation unit is connected to each switching tube of each phase bridge arm of the three-level inverter through a driving circuit.
As a specific example, the digital processing control modules are TMS320F28377D and EPM1270T chips.
The invention relates to a control method of an LCL inverter grid-connected device suitable for a weak power grid, which comprises the following steps:
step 1, in each switching period, a sampling unit of a digital control module respectively collects a network side voltage signal u of an LCL filtera、ub、ucNet side current signal ia、ib、ic；
Step 2, the closed-loop control unit transforms the network side voltage and the network side current under the static abc coordinate system to the static alpha beta coordinate system through Clarke transformation according to the signals collected in the step 1;
clarke transforms the transform matrix into Tabc/αβ：
Figure BSA0000226309960000051
Through the steps, alpha and beta axis components u of the network side voltage under the static alpha and beta coordinate system are obtainedα、uβAnd alpha and beta axis components i of net side currentα、iβ；
And 3, calculating a closed loop transfer function of the system after the active damping loop and the PR controller are added under the s domain, and analyzing the stability of the system by using a Laus stability criterion.
A control block diagram of the LCL NPC three-level inverter grid-connected system is shown in fig. 2, wherein:
Gc(s) is a current controller whose transfer function is as follows:
Figure BSA0000226309960000052
in the formula, KpFor proportional controller gain, KrIs the fundamental resonant controller gain, omegaiFor fundamental harmonic controller angular frequency, omegaoFor grid fundamental voltage angular frequency, the PR controller can realize non-static control on fundamental current.
Gad(s) isThe transfer function of the active damping link is as follows:
Gad(s)＝kads2
wherein k isadIs the active damping coefficient.
GZOH(s) is a zero order keeper with a transfer function of:
Figure BSA0000226309960000053
the system stability analysis results were as follows:
to stabilize the system, all coefficients in the first column of the Laus table need to be made positive, i.e., K, according to the Laus stability criterionpAnd kadThe following conditions need to be satisfied:
Figure BSA0000226309960000054
in the formula, KpIs the proportional link coefficient, k, of the PR controlleradFor active damping feedback coefficient, L1Is inductance value, L, of inverter side of LCL filter2Is inductance value of LCL filter network side, C1Is the capacitance value, L, of the LCL filtergThe inductance value is weak grid inductance value.
Step 4, analyzing the harmonic suppression condition of the resonant frequency of the LCL filter;
system open loop transfer function Gop(s) is:
Figure BSA0000226309960000055
the harmonic suppression condition of the resonant frequency of the LCL filter is 20lgAωres′＜0dB，ωres' is the resonant frequency of the LCL filter after adding active damping, Aωres′The expression of (a) is:
Figure BSA0000226309960000061
namely KpAnd kadThe following conditions need to be satisfied:
Figure BSA0000226309960000062
in the formula, KPWMIs an inverter transfer function, TsFor sample time, A isres' substitution of the modulus of the real part on the denominator after the open-loop transfer function, B isres' substituting the modulus of the imaginary part on the denominator after the open loop transfer function.
A＝0.5C1L1(L2+Lg)Tsωres′4-(0.5(L1+L2+Lg)Ts+kadKPWM)ωres′2
B＝(L1+L2+Lg)ωres′-C1L1(L2+Lg)ωres′3
Step 5, analyzing the amplitude margin requirement required to be met by the system;
in order to ensure that the relative stability of the system is good, the amplitude margin GM of the system should meet the condition that GM is more than or equal to 3dB, namely KpAnd kadThe following conditions need to be satisfied:
Figure BSA0000226309960000063
where GM is the amplitude margin of the system.
Step 6, analyzing the phase margin requirement required to be met by the system;
in order to ensure that the relative stability of the system is good, the phase margin PM of the system should meet the condition that PM is more than or equal to 45deg, namely KpAnd kadThe following conditions need to be satisfied:
Figure BSA0000226309960000064
where PM is the phase margin of the system, ωcIs the cut-off frequency of the system, ωresThe resonant frequency of the LCL filter.
Figure BSA0000226309960000065
And 7, selecting a proportional link coefficient K of the PR controller which enables the system bandwidth to be maximum within the range of meeting the stability condition, the resonance suppression condition, the system amplitude margin requirement and the system phase margin requirementpAnd active damping feedback coefficient kadTo obtain a closed loop control system with good dynamic performance, i.e. to find K satisfying the following equationpAnd kad：
Figure BSA0000226309960000066
In the formula, ωc_maxThe maximum cut-off frequency of the system is expressed as follows:
Figure BSA0000226309960000071
a＝4C1 2L1 2(L2+Lg_max)2(1+(tan PM)2)
b＝-10-0.1GMTs 2(L1+L2+Lg_max)2(tan PM)2
c＝4(tan PM)Ts(L1+L2+Lg_max)210-0.1GM
d＝-4(L1+L2+Lg_max)210-0.1GM
Δ1＝b2+12ad
Δ2＝2b3+27ac2-72abd
Figure BSA0000226309960000072
in the formula, a is a cubic coefficient of a unitary cubic equation obtained by a constraint condition, b is a quadratic coefficient of the unitary cubic equation, c is a first order coefficient of the unitary cubic equation, d is a constant term coefficient of the unitary cubic equation, Δ 1 is a 1 discriminant of the unitary cubic equation, Δ 2 is a 2 discriminant of the unitary cubic equation, and Δ 3 is a 3 discriminant of the unitary cubic equation.
Step 8, calculating current set by taking current sine as a target, subtracting the obtained current set quantity by taking the network side current as a feedback quantity, performing difference on the obtained current set quantity and the output of an active damping ring after passing through a proportional resonance regulator, and outputting a three-phase modulation wave signal through Clarke inverse transformation;
step 8.1, obtaining 4 paths of modulation wave signals v under a static alpha beta coordinate system through a closed-loop control unit and an active damping unitαh、vαpr、vβh、vβprTwo modulated wave signals v under the alpha axis under the static alpha and beta coordinate systemαh、vαprAdding to obtain:
vα＝vαh-vαpr
two modulated wave signals v under beta axisβh、vβprAdding to obtain:
vβ＝vβh-vβpr
through the steps, a modulation wave signal v under a static alpha beta coordinate system is obtainedα、vβ；
Step 8.2, putting the v under the stationary alpha beta coordinate systemα、vβConverting the matrix into T under the three-phase static coordinate systemαβ/abc：
Figure BSA0000226309960000073
Through the steps, the three-phase modulation wave under the three-phase static coordinate system is obtainedSignal va、vb、vc；
And 9, generating a pulse width modulation signal by the three-phase modulation signal obtained in the step 8 through a sine pulse width modulation unit, wherein the pulse width modulation signal controls the working state of a switching tube of the inverter through a driving circuit, and specifically comprises the following steps:
three-phase modulation wave signal v under three-phase static coordinate systema、vb、vcAnd the pulse width modulation signal is sent to a sine pulse width modulation unit to generate a pulse width modulation signal, and the pulse width modulation signal controls the working state of a switching tube of the three-level inverter through a driving circuit to realize the control of the resonance suppression of the LCL filter.
The modulation rule of the NPC three-phase three-level inverter is shown in FIG. 3, taking a-phase bridge arm as an example, at vaPositive half cycle of (d), when vaWhen greater than the carrier, order Sa1、Sa2When the a-phase bridge arm outputs high level when the v is onaWhen smaller than the carrier, order Sa2、Sa3Conducting, and outputting zero level by the a-phase bridge arm; at vaNegative half cycle of (d), when vaWhen smaller than the carrier, order Sa3、Sa4When the a-phase bridge arm outputs low level when the v is onaWhen greater than the carrier, order Sa2、Sa3Conducting, and outputting zero level by the a-phase bridge arm; b. the modulation rules of the c-phase bridge arms are the same.
Example 1
In the embodiment, a three-level inverter circuit is built by using a Simulink tool in MATLAB, the direct current is inverted by the three-level inverter circuit to output three-phase voltage after passing through a direct current bus capacitor, and stable three-phase sinusoidal voltage is output through an LCL filter circuit.
The electrical parameter settings during the simulation are as in table 1:
TABLE 1
Figure BSA0000226309960000081
Selected PR controller proportional link coefficient KpAnd active damping feedback coefficient kadThe stability condition, the resonance suppression condition and the system amplitude are required to be metMargin requirement, system phase margin requirement and maximizing system bandwidth to obtain a closed-loop control system with good dynamic performance, i.e. finding K satisfying the following equationpAnd kad：
Figure BSA0000226309960000082
In the formula, ωc_maxThe maximum cut-off frequency of the system is expressed as follows:
Figure BSA0000226309960000091
a＝4C1 2L1 2(L2+Lg_max)2(1+(tan PM)2)
b＝-10-0.1GMTs 2(L1+L2+Lg_max)2(tan PM)2
c＝4(tan PM)Ts(L1+L2+Lg_max)210-0.1GM
d＝-4(L1+L2+Lg_max)210-0.1GM
Δ1＝b2+12ad
Δ2＝2b3+27ac2-72abd
Figure BSA0000226309960000092
in the formula, a is a cubic coefficient of a unitary cubic equation obtained by a constraint condition, b is a quadratic coefficient of the unitary cubic equation, c is a first order coefficient of the unitary cubic equation, d is a constant term coefficient of the unitary cubic equation, Δ 1 is a 1 discriminant of the unitary cubic equation, Δ 2 is a 2 discriminant of the unitary cubic equation, and Δ 3 is a 3 discriminant of the unitary cubic equation.
K can be obtained from the simulation parameters in Table 1pAnd kadThe values of (A) are as follows:
Figure BSA0000226309960000093
FIG. 4 is a simulated waveform diagram after active damping is added under ideal grid conditions, and FIG. 5 is a diagram of the inductance L of a weak gridgThe LCL type inverter grid-connected device and the method which are suitable for the weak power grid can effectively realize stable operation under the condition of the weak power grid, have good dynamic performance, inhibit the harmonic frequency subharmonic in the current on the grid side and reduce the total harmonic distortion rate of the current.
In summary, the invention provides a grid-connected control method of an LCL inverter suitable for a weak power grid, which utilizes a Laus stability criterion to perform stability analysis on a system, calculates conditions required to be met by inhibiting resonance of an LCL filter, calculates amplitude margin requirements and phase margin requirements required to be met by the system, obtains a parameter which enables cut-off frequency to be maximum as a system parameter, adds outputs of an active damping unit and outputs of a closed-loop control unit, obtains a three-phase modulation wave after Clarke inverse transformation, generates a sine pulse width modulation signal through a sine pulse width modulation unit, and controls working states of all switching tubes of a three-level inverter through a driving circuit to realize control of resonance inhibition of the LCL filter. According to the invention, the LCL resonant frequency subharmonic is suppressed through the network side current feedback active damping, the dynamic response performance of the system is improved, the harmonic of the output current is reduced, the waveform quality is improved, the grid connection of the grid-connected inverter is facilitated, and the method has great engineering application value.

Claims (8)

1. The LCL type inverter grid-connected device suitable for the weak power grid is characterized by comprising a three-level inverter, a digital processing control module and a driving circuit, wherein the three-level inverter is an LCL type NPC three-level inverter, and the digital processing control module comprises a sampling unit, a closed-loop control unit, an active damping unit and a sine pulse width modulation unit;
the sampling unit respectively collects three-phase voltage signals at the network side of the LCL filter and three-phase current signals at the network side of the LCL filter and transmits the three-phase voltage signals and the three-phase current signals to the closed-loop control unit;
the closed-loop control unit converts the network side voltage and the network side current under the static abc coordinate system into a static alpha beta coordinate system through Clarke conversion according to the acquired signals;
the alpha and beta axis components i of the grid side current under an alpha and beta coordinate systemα、iβAn input active damping unit;
and the output end of the sinusoidal pulse width modulation unit is connected to each switching tube of each phase bridge arm of the three-level inverter through a driving circuit.
2. The LCL inverter grid-connected device suitable for the weak grid according to claim 1, wherein the digital processing control module is TMS320F28377D and EPM1270T chips.
3. A grid-connected control method for an LCL inverter adapting to a weak power grid is characterized by comprising the following steps:
step 1, in each switching period, a sampling unit of a digital control module respectively collects a network side voltage signal u of an LCL filtera、ub、ucAnd net side current signal ia、ib、ic；
Step 2, the closed-loop control unit transforms the network side voltage and the network side current under the static abc coordinate system to the static alpha beta coordinate system through Clarke transformation according to the signals collected in the step 1;
step 3, calculating a closed loop transfer function of the system after the active damping loop and the PR controller are added under the s domain, and performing stability analysis on the system by using a Laus criterion;
step 4, analyzing the harmonic suppression condition of the resonant frequency of the LCL filter;
step 5, analyzing the amplitude margin requirement required to be met by the system;
step 6, analyzing the phase margin requirement required to be met by the system;
step 7, selecting an active damping feedback coefficient k which enables the system bandwidth to be maximum within the range of meeting the stability condition, the resonance suppression condition, the amplitude margin requirement and the phase margin requirementadProportional link coefficient K of PR controllerpSo as to obtain a closed-loop control system with good dynamic performance;
step 8, calculating current setting by taking current sine as a target, subtracting the obtained current setting quantity by taking the network side current as a feedback quantity, performing difference on the obtained current setting quantity and the output of an active damping ring after passing through a proportional resonance regulator, and outputting a three-phase modulation wave signal through Clarke inverse transformation;
and 9, generating a pulse width modulation signal by the three-phase modulation signal obtained in the step 8 through a sine pulse width modulation unit, wherein the pulse width modulation signal controls the working state of a switching tube of the inverter through a driving circuit.
4. The LCL inverter grid-connected control method suitable for the weak grid according to claim 3, wherein the stability analysis of the system in step 3 is specifically as follows:
the system stability analysis results were as follows:
Figure FSA0000226309950000021
wherein, KpIs the proportional link coefficient, k, of the PR controlleradFor active damping feedback coefficient, L1Is inductance value, L, of inverter side of LCL filter2Is inductance value of LCL filter network side, C1Is the capacitance value, L, of the LCL filtergThe inductance value is weak grid inductance value.
5. Analysis of the LCL filter resonant frequency harmonic rejection conditions according to claim 3, specifically as follows:
system open loop transfer function Gop(s) is:
Figure FSA0000226309950000022
due to the presence of the zero-order keeper in the digital control, the actual resonance frequency ω of the LCL filterresThe shift can occur after the active damping control loop is used, and the resonance frequency after the shift is omegares', the resonance suppression analysis results are as follows:
Figure FSA0000226309950000023
in the formula, KPWMIs an inverter transfer function, TsFor sample time, A isres' substitution of the modulus of the real part on the denominator after the open-loop transfer function, B isres' the modulus of the imaginary part on the denominator after the substitution of the open-loop transfer function:
Figure FSA0000226309950000027
Figure FSA0000226309950000028
6. the method of claim 3, wherein the amplitude margin requirement to be met by the system is analyzed, and the specific results are as follows:
Figure FSA0000226309950000024
where GM is the amplitude margin of the system.
7. The method for analyzing the phase margin requirement to be met by the system according to claim 3, wherein the specific results are as follows:
Figure FSA0000226309950000025
where PM is the phase margin of the system, ωcIs the cut-off frequency of the system, ωresFor the resonant frequency of the LCL filter:
Figure FSA0000226309950000026
8. the LCL inverter grid-connected control method suitable for the weak grid according to claim 3, wherein the k in the step 7 isadAnd KpThe selection is as follows:
Figure FSA0000226309950000031
in the formula, ωc_maxThe maximum cut-off frequency of the system is expressed as follows:
Figure FSA0000226309950000032
a＝4C1 2L1 2(L2+Lg_max)2(1+(tan PM)2)
b＝-10-0.1GMTs 2(L1+L2+Lg_max)2(tan PM)2
c＝4(tan PM)Ts(L1+L2+Lg_max)210-0.1GM
d＝-4(L1+L2+Lg_max)210-0.1GM
Δ1＝b2+12ad
Δ2＝2b3+27ac2-72abd
Figure FSA0000226309950000033
in the formula, a is a cubic coefficient of a unitary cubic equation obtained by a constraint condition, b is a quadratic coefficient of the unitary cubic equation, c is a first order coefficient of the unitary cubic equation, d is a constant term coefficient of the unitary cubic equation, Δ 1 is a 1 discriminant of the unitary cubic equation, Δ 2 is a 2 discriminant of the unitary cubic equation, and Δ 3 is a 3 discriminant of the unitary cubic equation.
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