CN112532096B - 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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CN112532096B
CN112532096B CN202011397031.7A CN202011397031A CN112532096B CN 112532096 B CN112532096 B CN 112532096B CN 202011397031 A CN202011397031 A CN 202011397031A CN 112532096 B CN112532096 B CN 112532096B
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inverter
grid
lcl
phase
active damping
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CN112532096A (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

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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 the proportional parameters of the PR controller and the active damping feedback coefficient is determined by the Laus stability criterion in combination with the resonance suppression condition, the amplitude margin constraint condition and the phase margin constraint condition, so that the grid-connected inverter can stably run in the impedance change range of the weak 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 inhibiting the LCL resonance by adopting a software control method. 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 system α 、i β An 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 filter a 、u b 、u c And net side current signal i a 、i b 、i c ；
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 requirement ad Proportional link coefficient K of PR controller p So 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, K p Is proportional link coefficient, k, of PR controller ad For active damping feedback coefficient, L 1 Is inductance value, L, of inverter side of LCL filter 2 Is inductance value of LCL filter network side, C 1 Is the capacitance value, L, of the LCL filter g The 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 G op (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 filter res The shift can occur after the active damping control loop is used, and the resonance frequency after the shift is omega res ', the resonance suppression analysis results are as follows:
Figure BSA0000226309960000023
in the formula, K PWM Is an inverter transfer function, T s For sample time, A is res ' substitution of the modulus of the real part on the denominator after the open-loop transfer function, B is res ' substituting the modulus of the imaginary part on the denominator after the open loop transfer function.
A＝0.5C 1 L 1 (L 2 +L g )T s ω res ′ 4 -(0.5(L 1 +L 2 +L g )T s +k ad K PWM )ω res ′ 2
B＝(L 1 +L 2 +L g )ω res ′-C 1 L 1 (L 2 +L g )ω res ′ 3
Further, the step 5 analyzes the requirement of the amplitude margin that the system needs to meet, and the specific result is as follows:
Figure BSA0000226309960000031
where GM is the amplitude margin of the system.
Further, the analysis on the phase margin requirement that the system needs to meet in step 6 is performed, and the specific result is as follows:
Figure BSA0000226309960000032
where PM is the phase margin of the system, ω c As the cut-off frequency of the system, ω res The resonant frequency of the LCL filter.
Figure BSA0000226309960000033
K in step 7 ad And K p The selection of (A) is as follows:
Figure BSA0000226309960000034
in the formula, ω c_max The maximum cut-off frequency of the system is expressed as follows:
Figure BSA0000226309960000035
a＝4C 1 2 L 1 2 (L 2 +L g_max ) 2 (1+(tan PM) 2 )
b＝-10 -0.1GM T s 2 (L 1 +L 2 +L g_max ) 2 (tan PM) 2
c＝4(tan PM)T s (L 1 +L 2 +L g_max ) 2 10 -0.1GM
d＝-4(L 1 +L 2 +L g_max ) 2 10 -0.1GM
Δ 1 ＝b 2 +12ad
Δ 2 ＝2b 3 +27ac 2 -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 variation of a weak 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 grid g The simulated waveform after adding active damping at 5 mH.
Detailed Description
The invention is further described in detail below with reference to the drawings and specific 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 the alpha and beta coordinate system α 、i β An 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 filter a 、u b 、u c Net side current signal i a 、i b 、i c ；
Step 2, the closed-loop control unit converts the network side voltage and the network side current under the static abc coordinate system to a static alpha beta coordinate system through Clarke conversion according to the signals collected in the step 1;
clarke transforms the transform matrix into T abc/αβ ：
Figure BSA0000226309960000051
Through the steps, the material is obtainedAlpha and beta axis components u of grid side voltage under alpha and beta stopping coordinate system α 、u β And alpha and beta axis components i of net side current α 、i β ；
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:
G c (s) is a current controller whose transfer function is as follows:
Figure BSA0000226309960000052
in the formula, K p For proportional controller gain, K r Is the fundamental resonant controller gain, omega i For fundamental harmonic resonance control of angular frequency, omega o For grid fundamental voltage angular frequency, the PR controller can realize non-static control on fundamental current.
G ad (s) is an active damping link, and the transfer function is as follows:
G ad (s)＝k ad s 2
wherein k is ad Is the active damping coefficient.
G ZOH (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 criterion p And k ad The following conditions need to be satisfied:
Figure BSA0000226309960000054
in the formula, K p Is the proportional link coefficient, k, of the PR controller ad For active damping feedback coefficient, L 1 Is inductance value, L, of inverter side of LCL filter 2 Is inductance value of LCL filter network side, C 1 Is the capacitance value, L, of the LCL filter g The 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 G op (s) is:
Figure BSA0000226309960000055
the harmonic suppression condition of the resonant frequency of the LCL filter is 20lgA ωres′ ＜0dB，ω res ' is the resonant frequency of the LCL filter after adding active damping, A ωres′ The expression of (a) is:
Figure BSA0000226309960000061
namely K p And k ad The following conditions need to be satisfied:
Figure BSA0000226309960000062
in the formula, K PWM Is an inverter transfer function, T s For sample time, A is res ' substitution of the modulus of the real part on the denominator after the open-loop transfer function, B is res ' substituting the modulus of the imaginary part on the denominator after the open loop transfer function.
A＝0.5C 1 L 1 (L 2 +L g )T s ω res ′ 4 -(0.5(L 1 +L 2 +L g )T s +k ad K PWM )ω res ′ 2
B＝(L 1 +L 2 +L g )ω res ′-C 1 L 1 (L 2 +L g )ω res ′ 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 K p And k ad The 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 K p And k ad The following conditions need to be satisfied:
Figure BSA0000226309960000064
where PM is the phase margin of the system, ω c As the cut-off frequency of the system, ω res The 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 requirement p And active damping feedback coefficient k ad To obtain a closed loop control system with good dynamic performance, i.e. to find K satisfying the following equation p And k ad ：
Figure BSA0000226309960000066
In the formula, ω c_max The maximum cut-off frequency of the system is expressed as follows:
Figure BSA0000226309960000071
a＝4C 1 2 L 1 2 (L 2 +L g_max ) 2 (1+(tan PM) 2 )
b＝-10 -0.1GM T s 2 (L 1 +L 2 +L g_max ) 2 (tan PM) 2
c＝4(tan PM)T s (L 1 +L 2 +L g_max ) 2 10 -0.1GM
d＝-4(L 1 +L 2 +L g_max ) 2 10 -0.1GM
Δ 1 ＝b 2 +12ad
Δ 2 ＝2b 3 +27ac 2 -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 waves under a static alpha and beta coordinate system through a closed-loop control unit and an active damping unitSignal v αh 、v αpr 、v βh 、v βpr Two modulated wave signals v under the alpha axis under the static alpha and beta coordinate system αh 、v αpr Adding to obtain:
v α ＝v αh -v αpr
two modulated wave signals v under beta axis βh 、v βpr Adding to obtain:
v β ＝v βh -v βpr
through the steps, a modulation wave signal v under a static alpha beta coordinate system is obtained α 、v β ；
Step 8.2, putting the v under the stationary alpha beta coordinate system α 、v β Converting the matrix into T under the three-phase static coordinate system αβ/abc ：
Figure BSA0000226309960000073
Through the steps, three-phase modulation wave signals v under a three-phase static coordinate system are obtained a 、v b 、v c ；
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 system a 、v b 、v c And 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 v a Positive half cycle of (d), when v a When greater than the carrier, order S a1 、S a2 When the a-phase bridge arm outputs high level when the v is on a When smaller than the carrier, order S a2 、S a3 Conducting, and outputting zero level by the a-phase bridge arm; in thatv a Negative half cycle of (d), when v a When smaller than the carrier, order S a3 、S a4 When the a-phase bridge arm outputs low level when the v is turned on a When greater than the carrier, order S a2 、S a3 Conducting, 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 K p And active damping feedback coefficient k ad The stability condition, the resonance suppression condition, the system amplitude margin requirement and the system phase margin requirement need to be met, the system bandwidth is maximized, and a closed-loop control system with good dynamic performance is obtained, namely, K meeting the following equation is obtained p And k ad ：
Figure BSA0000226309960000082
In the formula, ω c_max The maximum cut-off frequency of the system is expressed as follows:
Figure BSA0000226309960000091
a＝4C 1 2 L 1 2 (L 2 +L g_max ) 2 (1+(tan PM) 2 )
b＝-10 -0.1GM T s 2 (L 1 +L 2 +L g_max ) 2 (tan PM) 2
c＝4(tan PM)T s (L 1 +L 2 +L g_max ) 2 10 -0.1GM
d＝-4(L 1 +L 2 +L g_max ) 2 10 -0.1GM
Δ 1 ＝b 2 +12ad
Δ 2 ＝2b 3 +27ac 2 -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 1 p And k ad The values of (A) are as follows:
Figure BSA0000226309960000093
FIG. 4 is a simulated waveform diagram after adding active damping at ideal grid conditions, and FIG. 5 is a diagram of weak grid inductance L g The device and the method for the LCL type inverter grid-connected system which are suitable for the weak grid can effectively realize stable operation under the condition of the weak grid, have good dynamic performance, inhibit the harmonic frequency subharmonic in the grid-side current 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 inhibited through the current feedback active damping at the network side, 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 a grid-connected inverter is facilitated, and the method has a great engineering application value.

Claims (6)

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 LCL filter network side and three-phase current signals at the LCL filter network side 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 three-phase voltage signals and three-phase current signals in a static abc coordinate system into a static alpha beta coordinate system through Clarke conversion according to the collected signals;
the alpha and beta axis components i of the three-phase current signal under the alpha and beta coordinate system are converted into α 、i β An input active damping unit;
the three-phase voltage signal and the three-phase current signal are transmitted to the alpha and beta axis components i through the modulated wave signal generated by the PR controller in the closed-loop control unit α 、i β The modulation wave signals obtained by the active damping unit are subjected to subtraction and sent to the sine pulse width modulation 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;
the method comprises the following steps:
step 1, in each switching period, a sampling unit of a digital processing control module respectively collects three-phase voltage signals u on the network side of an LCL filter a 、u b 、u c Three-phase current signal i on the side of the summing network a 、i b 、i c ；
Step 2, the closed-loop control unit converts the three-phase voltage signals and the three-phase current signals under the static abc coordinate system into a static alpha beta coordinate system through Clarke conversion according to the signals collected in the step 1;
step 3, calculating a closed-loop transfer function of the system after the active damping unit and a PR controller in the closed-loop control unit are added under the s domain, and analyzing the stability of the system by using the 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 requirement ad Proportional link coefficient K of PR controller p So 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 a three-phase current signal on the network side as a feedback amount, performing difference on the current setting amount and the output of the active damping unit after passing through a PR (pulse-width modulation) controller, and performing Clarke inverse transformation to output a three-phase modulation wave signal;
9, generating a pulse width modulation signal by the three-phase modulation wave 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;
the stability analysis of the system was carried out with the following results:
Figure FSB0000199695190000021
wherein, K p Is the proportional link coefficient, k, of the PR controller ad Is an active damping feedback coefficient, L 1 Is inductance value, L, of inverter side of LCL filter 2 Is inductance value of LCL filter network side, C 1 Is the capacitance value, L, of the LCL filter g The inductance value is weak grid inductance value.
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. The LCL inverter grid-connected device suitable for the weak grid according to claim 1, wherein the analysis of the harmonic suppression condition of the resonant frequency of the LCL filter is as follows:
system open loop transfer function G op (s) is:
Figure FSB0000199695190000022
wherein, T s To sample time, K PWM Is the inverter transfer function;
due to the presence of the zero-order keeper in the digital control, the actual resonance frequency ω of the LCL filter res The resonance frequency after the shift is omega res ', the resonance suppression analysis results are as follows:
Figure FSB0000199695190000023
wherein A is represented by the formula res ' substitution of the modulus of the real part on the denominator after the open-loop transfer function, B is res ' the modulus of the imaginary part on the denominator after the substitution of the open-loop transfer function:
Figure FSB0000199695190000024
Figure FSB0000199695190000025
4. the LCL inverter grid-connected device suitable for the weak grid according to claim 1, wherein the amplitude margin requirement which needs to be met by a system is analyzed, and the specific result is as follows:
Figure FSB0000199695190000026
where GM is the amplitude margin of the system.
5. The LCL inverter grid-connected device suitable for the weak grid according to claim 1, wherein the phase margin requirement required to be met by the system is analyzed, and the specific results are as follows:
Figure FSB0000199695190000027
in the formula, T s To sample time, K PWM For the inverter transfer function, PM is the phase margin of the system, ω c Is the cut-off frequency of the system, ω res For the resonant frequency of the LCL filter:
Figure FSB0000199695190000031
6. the LCL inverter grid-connected device suitable for the weak grid according to claim 1, wherein the step 7 is performedK is ad And K p The selection is as follows:
Figure FSB0000199695190000032
wherein, T s To sample time, K PWM For the inverter transfer function, GM is the amplitude margin of the system, PM is the phase margin of the system, L g_max A weak grid inductance value corresponding to a Short Circuit Ratio (SCR) equal to 10;
in the formula, ω c_max The maximum cut-off frequency of the system is expressed as follows:
Figure FSB0000199695190000033
a＝4C 1 2 L 1 2 (L 2 +L g_max ) 2 (1+(tan PM) 2 )
b＝-10 -0.1GM T s 2 (L 1 +L 2 +L g_max ) 2 (tan PM) 2
c＝4(tan PM)T s (L 1 +L 2 +L g_max ) 2 10 -0.1GM
d＝-4(L 1 +L 2 +L g_max ) 2 10 -0.1GM
Δ 1 ＝b 2 +12ad
Δ 2 ＝2b 3 +27ac 2 -72abd
Figure FSB0000199695190000034
wherein a is the cubic coefficient of the unitary cubic equation obtained from the constraint condition, b is the quadratic coefficient of the unitary cubic equation, c is the first order coefficient of the unitary cubic equation, d is the constant term coefficient of the unitary cubic equation, and delta 1 1 discriminant being a unitary cubic equation，Δ 2 2 discriminant, Δ, of a cubic equation of unity 3 Is a 3-degree discriminant of a unitary cubic equation.
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