CN108933447B - Multi-inverter system parameter self-adaptive control method based on mode switching under weak network - Google Patents
Multi-inverter system parameter self-adaptive control method based on mode switching under weak network Download PDFInfo
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- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- 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
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Abstract
The invention discloses a multi-inverter system parameter self-adaptive control method based on mode switching in a weak network. The invention provides a multi-inverter system parameter self-adaptive control method based on mode switching under a weak network, aiming at the problem that the system stability is generally improved by reducing the gain of a grid-connected inverter current regulator under the weak network, but the dynamic performance is simultaneously worsened.
Description
Technical Field
The invention relates to a control method for grid connection of a multi-inverter system, in particular to a mode switching-based multi-inverter system parameter self-adaptive control method under a weak network.
Background
With the rapid development of distributed power generation systems, grid-connected inverters are widely used. Due to the existence of long-distance transmission lines and a large number of voltage transformation devices in a remote distributed power generation system, the power grid presents a non-negligible equivalent impedance, so that the power grid presents weak power grid characteristics. At this time, a dynamic interconnection system is formed between a multi-inverter system composed of a plurality of grid-connected inverters and a power grid during grid-connected operation, and grid impedance of the system at a Point of Common Coupling (PCC) of the system causes grid-connected stability of the multi-inverter system to be reduced, so that resonance of output current of the grid-connected inverters is caused.
For a multi-inverter system composed of a plurality of grid-connected inverters under the condition of a weak power grid, the stability control method of the multi-inverter system also has deep theoretical analysis on the stability control method by academic papers and engineering methods applied in practice, for example:
1) huwei et al, published in "analysis of resonance characteristics of multi-inverter grid-connected system" on 7 th volume 34 of "power automation equipment" in 7 th month 2014. The method establishes an equivalent model of the multi-inverter grid-connected system, analyzes the influence of the number, the composition and the system control parameters of the grid-connected inverters on the system resonance characteristics, but only considers that the grid-connected inverters operate in a single current source mode, does not consider the situation that part of the grid-connected inverters operate in a voltage source mode, and indicates that the system stability can be improved by reducing the parameters of a current regulator on the premise of meeting the performance of the grid-connected inverters. However, this scheme reduces the control bandwidth of the grid-connected inverter and deteriorates the dynamic performance thereof.
2) Tang Zhendong et al, published in 2016 (11 th month of the year) (grid technology), 40 th volume, and a text of analysis of interaction influence among grid-connected control channels of multiple inverters under a weak grid. Aiming at the stability problem of a weak grid multi-inverter system, the change characteristic of interaction influence when the number of grid-connected inverters, control parameters and the equivalent impedance of a power grid are changed is analyzed. However, in the multi-inverter system analyzed by the article, only the grid-connected inverter is considered to operate in a single current source mode, and the occasion that part of the grid-connected inverter operates in a voltage source mode is not considered, and the article indicates that on the premise of meeting the performance of the grid-connected inverter, the interaction influence among control channels is weakened by reducing the parameters of a current regulator, so that the stability of the system is improved; meanwhile, the scheme can reduce the control bandwidth of the grid-connected inverter and deteriorate the dynamic performance of the grid-connected inverter in the current source mode.
3) The chinese patent document CN 105356507B entitled "dual-mode control method for LC type grid-connected inverter based on grid impedance adaptation" announced in 2017, 8, 29, is to realize switching between two grid-connected modes of a current source and a voltage source of a grid-connected inverter by grid impedance identification, so as to realize stable operation of the grid-connected inverter in a weak grid. However, the mode switching methods described above are based on a single grid-connected inverter, and a multi-inverter system including a plurality of grid-connected inverters is not designed.
In summary, the prior art has the following problems:
(1) the existing switching between the current source mode and the voltage source mode is based on a single grid-connected inverter, and a multi-inverter system formed by a plurality of grid-connected inverters is not designed.
(2) For a multi-inverter system under the condition of a weak power grid, the existing documents do not relate to the problem that the current regulator gain of a grid-connected inverter which still operates in a current source mode is improved in a self-adaptive manner by switching part of grid-connected inverters in the multi-inverter system into the voltage source grid-connected mode.
Disclosure of Invention
In order to overcome the limitations of the various technical schemes, the invention provides a multi-inverter system parameter self-adaptive control method based on mode switching under a weak network aiming at a multi-inverter system under a full current source mode under the weak network, which improves the system stability by generally reducing the gain of a grid-connected inverter current regulator, but simultaneously deteriorates the dynamic performance.
The object of the invention is thus achieved. The invention provides a multi-inverter system parameter self-adaptive control method based on mode switching under a weak network, wherein a multi-inverter system related to the control method comprises n grid-connected inverters, n is a positive integer and is greater than 1;
the control method comprises the following steps:
step 1, setting n grid-connected inverters to operate in a current source mode;
step 2, randomly selecting 1 grid-connected inverter from n grid-connected inverters, marking as a grid-connected inverter A, and setting the proportional coefficient of a current regulator of the grid-connected inverter A as Kp_n', and Kp_n' determined according to the following formula:
in the above formula, ρ is a proportionality coefficient and is 0<ρ<1; l is an equivalent inductance of an output filter of the grid-connected inverter A; f. ofsThe switching frequency of the grid-connected inverter A; kPWMBridge circuit PWM equivalent gain of the grid-connected inverter A;
step 3, obtaining the equivalent grid impedance of the grid-connected inverter A public coupling point through a grid impedance identification algorithm, and recording as Zg_est；
Step 4, setting the number of the rest n-1 grid-connected inverters needing to be adaptively switched to the voltage source mode as k, wherein k is 0,1,2, … and n-1, setting the equivalent grid impedance boundary values of the public coupling points of the rest n-1 grid-connected inverters, and obtaining the equivalent grid impedance Z of the public coupling point of the grid-connected inverter A according to the step 2g_estThe following judgment and operation are carried out:
when Z is satisfiedg_estWhen the current source mode is not more than the preset current source mode, the rest n-1 grid-connected inverters are kept in the current source mode;
when Z is satisfiedg_estWhen the voltage is higher than the preset value, the number k of the other n-1 grid-connected inverters which are self-adaptively switched to the voltage source mode is increased from 0 one by one until Z is metg_est≤；
Step 5, resetting current regulator parameters of the grid-connected inverter A, wherein the current regulator proportionality coefficient of the reset grid-connected inverter A is Kp_nAnd K isp_nDetermined according to the following formula:
and 6, ending the control flow.
Preferably, the current source mode control steps are as follows:
step 1.1, collecting and outputting grid-connected current iga、igb、igcCollecting voltage u of point of common couplingpcca、upccb、upccc；
Step 1.2, according to the voltage u of the point of common coupling collected in step 1.1pcca、upccb、upcccObtaining the voltage dq axis component u of the point of common coupling through a transformation equation from a three-phase static coordinate system to a two-phase rotating coordinate systempccd、upccq(ii) a The voltage u of the point of common couplingpcca、upccb、upcccObtaining a voltage phase angle theta of a public coupling point through phase locking of a phase-locked loop (PLL);
the transformation equation from the three-phase stationary coordinate system to the two-phase rotating coordinate system of the voltage of the point of common coupling is as follows:
the formula for calculating the voltage phase angle theta of the point of common coupling is as follows:
wherein, ω is0Rated angular frequency, K, of voltage at point of common couplingp_PLLProportional adjustment factor, K, for phase-locked loop PI regulatorsi_PLLAn integral adjustment coefficient of a phase-locked loop PI adjuster is obtained, and s is a Laplace operator;
step 1.3, converting the output grid-connected current i collected in step 1.1 into a two-phase rotating coordinate system through a three-phase static coordinate system according to the voltage phase angle theta of the point of common coupling obtained in step 1.2ga、igb、igcConverting the output grid-connected current dq component i under a two-phase rotating coordinate systemgdAnd igq；
The transformation equation of the output grid-connected current from the three-phase static coordinate system to the two-phase rotating coordinate system is as follows:
step 1.4, setting and outputting a grid-connected current instruction signal igdref、igqrefAnd according to the output grid-connected current dq component i obtained in the step 1.3gdAnd igqObtaining a control signal u through a power grid current closed-loop control equationdAnd uq；
The closed-loop control equation of the power grid current is as follows:
wherein, KpFor the proportionality coefficient, K, of current regulators in the current closed-loop control equation of the networkiThe integral coefficient of a current regulator in a power grid current closed-loop control equation is obtained;
step 1.5, according to the voltage phase angle theta of the public coupling point obtained in the step 1.2, the control signal u obtained in the step 1.4 is useddAnd uqConverting the control signal component u into a control signal component u under a three-phase static coordinate system through a transformation equation from a two-phase rotating coordinate system to the three-phase static coordinate systema、ub、uc；
The transformation equation of the control signal from the two-phase rotating coordinate system to the three-phase static coordinate system is as follows:
ua＝udcosθ-uqsinθ
step 1.6, obtaining a control signal component u under the three-phase static coordinate system according to the step 1.5a、ub、ucRespectively with the pcc voltage u obtained in step 1.1pcca、upccb、upcccAdding to obtain three-phase full-bridge grid-connected inverter bridge arm voltage control signals, wherein the three-phase full-bridge grid-connected inverter bridge arm voltage control signals are respectively as follows: u. ofa+upcca、ub+upccb、uc+upcccAnd generating a switching signal of the power device of the grid-connected inverter through SVPWM modulation, and controlling the on-off of the power device of the three-phase full-bridge grid-connected inverter through a driving circuit.
Preferably, the grid impedance identification algorithm of step 3 includes the following steps:
step 3.1, injecting non-characteristic subharmonic current with the frequency of 75Hz at a PCC (point of common coupling);
step 3.2, sampling harmonic response voltage u at PCCpcchAnd harmonic response current igh；
Step 3.3, respectively responding the harmonic wave response voltage u through fast Fourier algorithm FFTpcchAnd harmonic response current ighPerforming spectrum analysis to obtain the amplitude value | U of harmonic response voltage component at 75Hz frequencypcch_75HzPhase ∠ U of harmonic response voltage component at | 75Hz frequencypcch_75HzAmplitude I of harmonic response current component at 75Hz frequencypcch_75HzPhase ∠ I of harmonic response current component at | 75Hz frequencypcch_75Hz(ii) a Obtaining the amplitude value | Z of the network impedance at the frequency of 75Hz according to the following formulagPhase ∠ Z of the grid impedance at | and 75Hz frequenciesg：
∠Zg＝∠Upcch_75Hz-∠Ipcch_75Hz；
Step (ii) of3.4 amplitude | Z of the grid impedance at a frequency of 75Hz obtained according to step 3.3gPhase ∠ Z of the grid impedance at | and 75Hz frequenciesgCalculating to obtain the power grid impedance identification value Z according to the following formulag_est：
Preferably, the voltage source mode control step of step 4 is as follows:
step 4.1, collecting and outputting grid-connected current iga、igb、igcCollecting voltage u of point of common couplingpcca、upccb、upccc；
Step 4.2, according to the output grid-connected current i collected in step 4.1ga、igb、igcObtaining an output grid-connected current αβ axis component i through a transformation equation from a three-phase static coordinate system to a two-phase static coordinate systemgα、igβ(ii) a Voltage u of point of common coupling collected according to step 4.1pcca、upccb、upcccObtaining a common coupling point voltage αβ axis component u through a transformation equation from a three-phase static coordinate system to a two-phase static coordinate systempccα、upccβ；
The transformation equation of the output grid-connected current from the three-phase static coordinate system to the two-phase static coordinate system is as follows:
the transformation equation of the voltage of the common coupling point from the three-phase static coordinate system to the two-phase static coordinate system is as follows:
step 4.3, according to the output grid-connected current αβ axis component i obtained in the step 4.2gα、igβAnd a common coupling point voltage αβ axis component upccα、upccβFirstly, the average active power is obtained through the average active power calculation equation
Then obtaining the average reactive power through an average reactive power calculation equation
The average active power calculation equation is:
the average reactive power calculation equation is:
wherein τ is a first-order low-pass filter time constant, and s is a laplacian operator;
step 4.4, obtaining the average active power according to the step 4.3
Obtaining the output angular frequency omega of the grid-connected inverter through an active power-frequency droop control equation; the active power-frequency droop control equation is as follows:
wherein, PnGiven an active power command, ω, for the grid-connected inverternGiven active power command P for grid-connected inverternNominal angular frequency, D, to which time correspondspThe active droop coefficient;
integrating the output angular frequency omega of the grid-connected inverter to obtain the output phase angle theta of the grid-connected inverter0Namely:
step 4.5, according to the voltage u of the public coupling point collected in step 4.1pcca、upccb、upcccAnd the output phase angle theta of the grid-connected inverter obtained according to the step 4.40Obtaining the voltage dq axis component u of the point of common coupling through a transformation equation from a three-phase static coordinate system to a two-phase rotating coordinate systempccd、upccq；
The transformation equation of the voltage of the common coupling point from a three-phase static coordinate system to a two-phase rotating coordinate system is as follows:
step 4.6, output grid-connected current i acquired according to step 4.1ga、igb、igcAnd the output phase angle theta of the grid-connected inverter obtained according to the step 4.40Obtaining output grid-connected current dq component i through a transformation equation from a three-phase static coordinate system to a two-phase rotating coordinate systemgdAnd igq；
The transformation equation of the output grid-connected current from the three-phase static coordinate system to the two-phase rotating coordinate system is as follows:
step 4.7, outputting the average reactive power of the grid-connected inverter obtained according to the step 4.3
Obtaining a voltage dq component reference value u of a point of common coupling of the grid-connected inverter through a reactive power-amplitude droop control equationpccdref、upccqrefThe reactive power-amplitude droop control equation is as follows:
upccqref＝0
wherein, UnGiven reactive power command Q for grid-connected inverternRated output voltage, D, corresponding to timeqIs the reactive droop coefficient;
step 4.8, firstly obtaining the voltage dq axis component u of the point of common coupling according to the step 4.5pccd、upccqAnd the value of the reference u of the voltage dq component of the pcc obtained in step 4.7pccdref、upccqrefAnd obtaining an output grid-connected current instruction signal i through an voltage loop control equationgdref、igqref；
The voltage loop control equation is:
wherein, Kp1Proportional control coefficient, K, for a PI regulator in a voltage loop control equationi1The integral control coefficient of a PI regulator in a voltage loop control equation;
step 4.9, firstly, according to the output grid-connected current instruction signal i obtained in step 4.8gdref、igqrefAnd according to the output grid-connected current dq component i obtained in the step 4.6gdAnd igqObtaining the control signal u by a current loop control equationdAnd uq；
The current loop control equation is:
wherein, Kp2Proportional control coefficient, K, of a PI regulator in a current loop control equationi2The integral control coefficient of a PI regulator in a current loop control equation;
step 4.10, obtaining the output phase angle theta of the grid-connected inverter according to the step 4.40The control signal u obtained in step 4.9dAnd uqConverting the control signal component u into a control signal component u under a three-phase static coordinate system through a transformation equation from a two-phase rotating coordinate system to the three-phase static coordinate systema、ub、uc；
The transformation equation of the control signal from the two-phase rotating coordinate system to the three-phase static coordinate system is as follows:
ua＝udcosθ0-uqsinθ0
step 4.11, obtaining the component u under the three-phase static coordinate system according to the step 4.10a、ub、ucRespectively with the pcc voltage u obtained in step 4.1pcca、upccb、upcccAdding to obtain three-phase full-bridge grid-connected inverter bridge arm voltage control signals, wherein the three-phase full-bridge grid-connected inverter bridge arm voltage control signals are respectively as follows: u. ofa+upcca、ub+upccb、uc+upcccAnd generating a switching signal of the power device of the grid-connected inverter through SVPWM modulation, and controlling the on-off of the power device of the three-phase full-bridge grid-connected inverter through a driving circuit.
Compared with the prior art, the invention has the beneficial effects that:
1. the method is simple to implement, and compared with a current source mode grid-connected inverter in a traditional multi-inverter system, the dynamic performance of the system is greatly improved while the system is ensured to operate stably;
2. the invention overcomes the problem that the stability of a multi-inverter system in a full current source mode under a weak grid is generally improved by reducing the gain of a current regulator of a grid-connected inverter, but the dynamic performance is deteriorated;
3. according to the invention, only part of grid-connected inverters in the multi-inverter system are switched to be in a voltage source grid-connected mode, and then the gain of the grid-connected inverter current regulator still operating in the current source mode is improved in a self-adaptive manner.
Drawings
Fig. 1 is a multi-inverter system topology structure under a weak grid adopted by the invention.
FIG. 2 is a flow chart of the present invention.
Fig. 3 is a schematic diagram of a control strategy when a single grid-connected inverter in a multi-inverter system operates in a current source mode under a weak grid.
Fig. 4 is a schematic diagram of a control strategy when a single grid-connected inverter in a multi-inverter system operates in a voltage source mode under a weak grid.
FIG. 5 is a block diagram of a grid impedance identification algorithm based on non-characteristic harmonic injection according to the present invention.
Fig. 6 shows a dynamic waveform of the output grid-connected current amplitude from half load to full load when the grid-connected inverter a in the multi-inverter system composed of 2 grid-connected inverters does not adopt the control strategy proposed by the present invention.
Fig. 7 shows a dynamic waveform of the output grid-connected current amplitude from half load to full load when the grid-connected inverter a in the multi-inverter system composed of 2 grid-connected inverters adopts the control strategy proposed by the present invention.
Detailed Description
The embodiment of the invention provides a mode switching-based multi-inverter system parameter self-adaptive control method under a weak network, which aims to solve the problems that in the prior art, the stability of a multi-inverter system under a full current source mode under the weak network is generally improved by reducing the gain of a current regulator of a grid-connected inverter, but the dynamic performance is deteriorated at the same time.
The technical scheme of the invention is clearly and completely described below with reference to the accompanying drawings.
The topological structure of the multi-inverter system under the weak grid adopted by the invention is shown in figure 1. The topological structure of the multi-inverter system under the weak grid consists of a plurality of identical grid-connected inverters, the number of the grid-connected inverters in the multi-inverter system is n, n is a positive integer, and n is>1; each grid-connected inverter topological structure comprises a direct current side filter capacitor CdcThree-phase bridge type inversion topology and grid-connected inverter side inductor L1Filter capacitor C and damping resistor RdGrid side inductor L2LCL type filter passes through PCC and has grid impedance ZgIs connected to the three-phase network ofgTo the network impedance ZgResistive component of LgTo the network impedance ZgOf the inductive component rgAnd LgForming the network impedance ZgGrid impedance ZgThe expression is as follows:
Zg＝rg+s·Lg
s in the formula is a laplace operator. In this example, n is 2, Cdc＝600μF，L1＝0.9mH，C＝40μF，Rd＝0.15Ω，L2＝0.1mH，rg＝0，Lg＝0.5mH。
FIG. 2 is a flow chart of the present invention. As can be seen from fig. 2, the present invention consists of the following steps:
step 1, setting n grid-connected inverters to operate in a current source mode.
Step 2, randomly selecting 1 grid-connected inverter from n grid-connected inverters, marking as a grid-connected inverter A, and setting the proportional coefficient of a current regulator of the grid-connected inverter A as Kp_n', and Kp_n' determined according to the following formula:
in the above formula, ρ is a proportionality coefficient and is 0<ρ<1; l is an equivalent inductance of an output filter of the grid-connected inverter A; f. ofsThe switching frequency of the grid-connected inverter A; kPWMBridge PWM equivalent gain of the grid-connected inverter A. In this embodiment, the 1 st grid-connected inverter is selected as the grid-connected inverter a, where ρ is 0.1, L is 1mH, and K isPWM＝1，fs16kHz, so Kp_n’≈0.533。
Step 3, obtaining the equivalent grid impedance of the grid-connected inverter A public coupling point through a grid impedance identification algorithm, and recording as Zg_est。
Step 4, setting the number of the rest n-1 grid-connected inverters needing to be adaptively switched to the voltage source mode as k, wherein k is 0,1,2, … and n-1, setting the equivalent grid impedance boundary values of the public coupling points of the rest n-1 grid-connected inverters, and obtaining the equivalent grid impedance Z of the public coupling point of the grid-connected inverter A according to the step 2g_estThe following judgment and operation are carried out:
when Z is satisfiedg_estWhen the current source mode is not more than the preset current source mode, the rest n-1 grid-connected inverters are kept in the current source mode;
when Z is satisfiedg_estWhen the voltage is higher than the preset value, the number k of the other n-1 grid-connected inverters which are self-adaptively switched to the voltage source mode is increased from 0 one by one until Z is metg_est≤。
In this example, ═ 1.2 mH.
Step 5, resetting current regulator parameters of the grid-connected inverter A, wherein the reset current of the grid-connected inverter AFlow regulator proportionality coefficient of Kp_nAnd K isp_nDetermined according to the following formula:
in the above formula, L is the equivalent inductance of the output filter of the grid-connected inverter A; f. ofsThe switching frequency of the grid-connected inverter A; kPWMBridge circuit PWM equivalent gain of the grid-connected inverter A; in this example, L ═ 1mH, KPWM＝1，fs16kHz, so Kp_n≈5.33。
And 6, ending the control flow.
Fig. 3 is a schematic diagram of a control strategy when a single grid-connected inverter in a multi-inverter system operates in a current source mode under a weak grid. As can be seen from fig. 3, the grid-connected inverter control strategy operating in the current source mode includes the following steps:
step 1.1, collecting and outputting grid-connected current iga、igb、igcCollecting voltage u of point of common couplingpcca、upccb、upccc。
Step 1.2, according to the voltage u of the point of common coupling collected in step 1.1pcca、upccb、upcccObtaining the voltage dq axis component u of the point of common coupling through a transformation equation from a three-phase static coordinate system to a two-phase rotating coordinate systempccd、upccq(ii) a The voltage u of the point of common couplingpcca、upccb、upcccAnd obtaining a voltage phase angle theta of the common coupling point through phase locking of a phase-locked loop (PLL).
The transformation equation from the three-phase stationary coordinate system to the two-phase rotating coordinate system of the voltage of the point of common coupling is as follows:
the formula for calculating the voltage phase angle theta of the point of common coupling is as follows:
wherein ω is0Rated angular frequency, K, of voltage at point of common couplingp_PLLProportional adjustment factor, K, for phase-locked loop PI regulatorsi_PLLAnd s is a Laplace operator, and is an integral regulation coefficient of the phase-locked loop PI regulator. In the embodiment of the present invention, ω0＝314rad/s，Kp_PLL＝2000，Ki_PLL＝1。
Step 1.3, converting the output grid-connected current i collected in step 1.1 into a two-phase rotating coordinate system through a three-phase static coordinate system according to the voltage phase angle theta of the point of common coupling obtained in step 1.2ga、igb、igcConverting the output grid-connected current dq component i under a two-phase rotating coordinate systemgdAnd igq。
The transformation equation of the output grid-connected current from the three-phase static coordinate system to the two-phase rotating coordinate system is as follows:
step 1.4, setting and outputting a grid-connected current instruction signal igdref、igqrefAnd according to the output grid-connected current dq component i obtained in the step 1.3gdAnd igqObtaining a control signal u through a power grid current closed-loop control equationdAnd uq；
The closed-loop control equation of the power grid current is as follows:
wherein, KpProportional control coefficient, K, for current regulator in current closed-loop control equation of power networkiThe integral control coefficient of a current regulator in a power grid current closed-loop control equation is obtained;
step 1.5, according to the voltage phase angle theta of the public coupling point obtained in the step 1.2, the control signal u obtained in the step 1.4 is useddAnd uqConverting the control signal component u into a control signal component u under a three-phase static coordinate system through a transformation equation from a two-phase rotating coordinate system to the three-phase static coordinate systema、ub、uc。
The transformation equation of the control signal from the two-phase rotating coordinate system to the three-phase static coordinate system is as follows:
ua＝udcosθ-uqsinθ
step 1.6, obtaining a control signal component u under the three-phase static coordinate system according to the step 1.5a、ub、ucRespectively with the pcc voltage u obtained in step 1.1pcca、upccb、upcccAdding to obtain three-phase full-bridge grid-connected inverter bridge arm voltage control signals, wherein the three-phase full-bridge grid-connected inverter bridge arm voltage control signals are respectively as follows: u. ofa+upcca、ub+upccb、uc+upcccAnd generating a switching signal of the power device of the grid-connected inverter through SVPWM modulation, and controlling the on-off of the power device of the three-phase full-bridge grid-connected inverter through a driving circuit.
Fig. 4 is a schematic diagram of a control strategy when a single grid-connected inverter in a multi-inverter system operates in a voltage source mode under a weak grid. As can be seen from fig. 4, the control strategy of the grid-connected inverter operating in the voltage source mode in step 4 of the present invention includes the following steps:
step 4.1, collecting and outputting grid-connected current iga、igb、igcCollecting voltage u of point of common couplingpcca、upccb、upccc。
Step 4.2, according to the output grid-connected current i collected in step 4.1ga、igb、igcObtaining an output grid-connected current αβ axis component i through a transformation equation from a three-phase static coordinate system to a two-phase static coordinate systemgα、igβ(ii) a Voltage u of point of common coupling collected according to step 4.1pcca、upccb、upcccObtaining a common coupling point voltage αβ axis component u through a transformation equation from a three-phase static coordinate system to a two-phase static coordinate systempccα、upccβ。
The transformation equation of the output grid-connected current from the three-phase static coordinate system to the two-phase static coordinate system is as follows:
the transformation equation of the voltage of the common coupling point from the three-phase static coordinate system to the two-phase static coordinate system is as follows:
step 4.3, according to the output grid-connected current αβ axis component i obtained in the step 4.2gα、igβAnd a common coupling point voltage αβ axis component upccα、upccβFirstly, the average active power is obtained through the average active power calculation equation
Then obtaining the average reactive power through an average reactive power calculation equation
The average active power calculation equation is:
the average reactive power calculation equation is:
where τ is the first order low pass filter time constant and s is the laplacian operator. In the present embodiment, τ is 0.00667 s.
Step 4.4, obtaining the average active power according to the step 4.3
Obtaining the output angular frequency omega of the grid-connected inverter through an active power-frequency droop control equation; the active power-frequency droop control equation is as follows:
wherein, PnGiven an active power command, ω, for the grid-connected inverternGiven active power command P for grid-connected inverternNominal angular frequency, D, to which time correspondspThe active droop coefficient. In the embodiment of the present invention, ωn＝314rad/s，Pn＝20kW，Dp＝0.0001。
Integrating the output angular frequency omega of the grid-connected inverter to obtain the output phase angle theta of the grid-connected inverter0Namely:
step 4.5, according to the voltage u of the public coupling point collected in step 4.1pcca、upccb、upcccAnd the output phase angle theta of the grid-connected inverter obtained according to the step 4.40Obtaining the voltage dq axis component u of the point of common coupling through a transformation equation from a three-phase static coordinate system to a two-phase rotating coordinate systempccd、upccq。
The transformation equation of the voltage of the common coupling point from a three-phase static coordinate system to a two-phase rotating coordinate system is as follows:
step 4.6, output grid-connected current i acquired according to step 4.1ga、igb、igcAnd the output phase angle theta of the grid-connected inverter obtained according to the step 4.40Obtaining output grid-connected current dq component i through a transformation equation from a three-phase static coordinate system to a two-phase rotating coordinate systemgdAnd igq。
The transformation equation of the output grid-connected current from the three-phase static coordinate system to the two-phase rotating coordinate system is as follows:
step 4.7, outputting the average reactive power of the grid-connected inverter obtained according to the step 4.3
Obtaining a voltage dq component reference value u of a point of common coupling of the grid-connected inverter through a reactive power-amplitude droop control equationpccdref、upccqrefThe reactive power-amplitude droop control equation is as follows:
upccqref＝0
wherein, UnGiven reactive power command Q for grid-connected inverternRated output voltage, D, corresponding to timeqIs the reactive droop coefficient. In the embodiment of the invention, Un＝220V，Qn＝0，Dq＝0.0001。
Step 4.8, firstly obtaining the voltage dq axis component u of the point of common coupling according to the step 4.5pccd、upccqAnd the value of the reference u of the voltage dq component of the pcc obtained in step 4.7pccdref、upccqrefAnd obtaining an output grid-connected current instruction signal i through an voltage loop control equationgdref、igqref。
The voltage loop control equation is:
wherein, Kp1Proportional control coefficient, K, for a PI regulator in a voltage loop control equationi1Is the integral control coefficient of the PI regulator in the voltage loop control equation. In the examples of the present invention, Kp1＝1，Ki1＝1000。
Step 4.9, firstly, according to the output grid-connected current instruction signal i obtained in step 4.8gdref、igqrefAnd according to the output grid-connected current dq component i obtained in the step 4.6gdAnd igqObtaining the control signal u by a current loop control equationdAnd uq。
The current loop control equation is:
wherein, Kp2Proportional control coefficient, K, of a PI regulator in a current loop control equationi2Is the integral control coefficient of the PI regulator in the current loop control equation. In the examples of the present invention, Kp2＝100，Ki2＝0。
Step 4.10, obtaining the output phase angle theta of the grid-connected inverter according to the step 4.40The control signal u obtained in step 4.9dAnd uqConverting the control signal component u into a control signal component u under a three-phase static coordinate system through a transformation equation from a two-phase rotating coordinate system to the three-phase static coordinate systema、ub、uc。
The transformation equation of the control signal from the two-phase rotating coordinate system to the three-phase static coordinate system is as follows:
ua＝udcosθ0-uqsinθ0
step 4.11, obtaining the component u under the three-phase static coordinate system according to the step 4.10a、ub、ucRespectively with the pcc voltage u obtained in step 4.1pcca、upccb、upcccAdding to obtain three-phase full-bridge grid-connected inverter bridge arm voltage control signals, wherein the three-phase full-bridge grid-connected inverter bridge arm voltage control signals are respectively as follows: u. ofa+upcca、ub+upccb、uc+upcccAnd generating a switching signal of the power device of the grid-connected inverter through SVPWM modulation, and controlling the on-off of the power device of the three-phase full-bridge grid-connected inverter through a driving circuit.
FIG. 5 is a block diagram of a grid impedance identification method based on non-characteristic harmonic injection according to the present invention. According to fig. 5, the power grid impedance identification algorithm of step 3 of the present invention comprises the following steps:
step 3.1, injecting a non-characteristic subharmonic current with the frequency of 75Hz at the PCC. In the present example, the injection frequency of 75Hz of the non-characteristic subharmonic current amplitude is 2A;
step 3.2, sampling harmonic response voltage u at PCCpcchAnd harmonic response current igh；
Step 3.3, respectively responding the harmonic wave response voltage u through fast Fourier algorithm FFTpcchAnd harmonic response current ighPerforming spectrum analysis to obtain the amplitude value | U of harmonic response voltage component at 75Hz frequencypcch_75HzPhase ∠ U of harmonic response voltage component at | 75Hz frequencypcch_75HzAmplitude I of harmonic response current component at 75Hz frequencypcch_75HzPhase ∠ I of harmonic response current component at | 75Hz frequencypcch_75Hz(ii) a Obtaining the amplitude value | Z of the network impedance at the frequency of 75Hz according to the following formulagPhase ∠ Z of the grid impedance at | and 75Hz frequenciesg：
∠Zg＝∠Upcch_75Hz-∠Ipcch_75Hz；
Step 3.4, obtaining the amplitude value | Z of the power grid impedance at the frequency of 75Hz according to the step 3.3gPhase ∠ Z of the grid impedance at | and 75Hz frequenciesgCalculating to obtain the power grid impedance identification value Z according to the following formulag_est：
In the embodiment of the present invention, fig. 6 is a dynamic waveform of a grid-connected current amplitude from half load to full load output when a grid-connected inverter a in a multi-inverter system composed of 2 grid-connected inverters does not adopt the control strategy proposed by the present invention. At this time, 2 inverters of the multi-inverter system are all operated in the current source mode and the current regulator scaling factor of the grid-connected inverter a is set to K according to the procedure shown in fig. 2p_n' 0.533. As can be seen from fig. 6, the output current waveform of the grid-connected inverter a has obvious low-order harmonics, and the current dynamic regulation transition time is also longer when the command current is stepped from half load to full load. Fig. 7 shows a dynamic waveform of the output grid-connected current amplitude from half load to full load when the grid-connected inverter a in the multi-inverter system composed of 2 grid-connected inverters adopts the control strategy proposed by the present invention. At this time, the grid-connected inverter a operates in the current source mode, and the other grid-connected inverter operates in the voltage source mode. Setting the current regulator scaling factor of the grid-connected inverter a to K according to the steps shown in fig. 2p_n'. 5.33, as can be seen by comparing fig. 6 and 7, the output current waveform of the grid-connected inverter a at this time has the low order harmonics disappeared and the dynamic adjustment transition time from half load to full load becomes smaller. As can be seen from fig. 6 and 7, compared with the current source mode grid-connected inverter in the conventional multi-inverter system, the mode switching-based multi-inverter system parameter adaptive control method in the weak grid provided by the invention has the advantages that the stable operation of the system is ensured, and the dynamic performance of the system is greatly improved.
Claims (4)
1. A multi-inverter system parameter self-adaptive control method based on mode switching under a weak network is characterized in that a multi-inverter system related to the control method comprises n grid-connected inverters, n is a positive integer and is greater than 1;
the control method comprises the following steps:
step 1, setting n grid-connected inverters to operate in a current source mode;
step 2, randomly selecting 1 grid-connected inverter from n grid-connected inverters, marking as a grid-connected inverter A, and setting the proportional coefficient of a current regulator of the grid-connected inverter A as Kp_n', and Kp_n' determined according to the following formula:
in the above formula, ρ is a proportionality coefficient and is 0<ρ<1; l is an equivalent inductance of an output filter of the grid-connected inverter A; f. ofsFor grid-connected invertersThe switching frequency of A; kPWMBridge circuit PWM equivalent gain of the grid-connected inverter A;
step 3, obtaining the equivalent grid impedance of the grid-connected inverter A public coupling point through a grid impedance identification algorithm, and recording as Zg_est；
Step 4, setting the number of the rest n-1 grid-connected inverters needing to be adaptively switched to the voltage source mode as k, wherein k is 0,1,2, … and n-1, setting the equivalent grid impedance boundary values of the public coupling points of the rest n-1 grid-connected inverters, and obtaining the equivalent grid impedance Z of the public coupling point of the grid-connected inverter A according to the step 2g_estThe following judgment and operation are carried out:
when Z is satisfiedg_estWhen the current source mode is not more than the preset current source mode, the rest n-1 grid-connected inverters are kept in the current source mode;
when Z is satisfiedg_estWhen the voltage is higher than the preset value, the number k of the other n-1 grid-connected inverters which are self-adaptively switched to the voltage source mode is increased from 0 one by one until Z is metg_est≤；
Step 5, resetting current regulator parameters of the grid-connected inverter A, wherein the current regulator proportionality coefficient of the reset grid-connected inverter A is Kp_nAnd K isp_nDetermined according to the following formula:
and 6, ending the control flow.
2. The method for adaptively controlling parameters of a multi-inverter system based on mode switching in a weak network according to claim 1, wherein the current source mode in step 1 is controlled by the following steps:
step 1.1, collecting and outputting grid-connected current iga、igb、igcCollecting voltage u of point of common couplingpcca、upccb、upccc；
Step 1.2, according to the voltage u of the point of common coupling collected in step 1.1pcca、upccb、upcccThree-phase stationary coordinateObtaining the voltage dq axis component u of the point of common coupling by a transformation equation from a system to a two-phase rotating coordinate systempccd、upccq(ii) a The voltage u of the point of common couplingpcca、upccb、upcccObtaining a voltage phase angle theta of a public coupling point through phase locking of a phase-locked loop (PLL);
the transformation equation from the three-phase stationary coordinate system to the two-phase rotating coordinate system of the voltage of the point of common coupling is as follows:
the formula for calculating the voltage phase angle theta of the point of common coupling is as follows:
wherein, ω is0Rated angular frequency, K, of voltage at point of common couplingp_PLLProportional adjustment factor, K, for phase-locked loop PI regulatorsi_PLLAn integral adjustment coefficient of a phase-locked loop PI adjuster is obtained, and s is a Laplace operator;
step 1.3, converting the output grid-connected current i collected in step 1.1 into a two-phase rotating coordinate system through a three-phase static coordinate system according to the voltage phase angle theta of the point of common coupling obtained in step 1.2ga、igb、igcConverting the output grid-connected current dq component i under a two-phase rotating coordinate systemgdAnd igq；
The transformation equation of the output grid-connected current from the three-phase static coordinate system to the two-phase rotating coordinate system is as follows:
step 1.4, setting and outputting a grid-connected current instruction signal igdref、igqrefAnd according to the output grid-connected current dq component i obtained in the step 1.3gdAnd igqObtaining a control signal u through a power grid current closed-loop control equationdAnd uq；
The closed-loop control equation of the power grid current is as follows:
wherein, KpFor the proportionality coefficient, K, of current regulators in the current closed-loop control equation of the networkiThe integral coefficient of a current regulator in a power grid current closed-loop control equation is obtained;
step 1.5, according to the voltage phase angle theta of the public coupling point obtained in the step 1.2, the control signal u obtained in the step 1.4 is useddAnd uqConverting the control signal component u into a control signal component u under a three-phase static coordinate system through a transformation equation from a two-phase rotating coordinate system to the three-phase static coordinate systema、ub、uc；
The transformation equation of the control signal from the two-phase rotating coordinate system to the three-phase static coordinate system is as follows:
ua＝udcosθ-uqsinθ
step 1.6, obtaining a control signal component u under the three-phase static coordinate system according to the step 1.5a、ub、ucRespectively obtained by the step 1.1Voltage u of the point of common couplingpcca、upccb、upcccAdding to obtain three-phase full-bridge grid-connected inverter bridge arm voltage control signals, wherein the three-phase full-bridge grid-connected inverter bridge arm voltage control signals are respectively as follows: u. ofa+upcca、ub+upccb、uc+upcccAnd generating a switching signal of the power device of the grid-connected inverter through SVPWM modulation, and controlling the on-off of the power device of the three-phase full-bridge grid-connected inverter through a driving circuit.
3. The adaptive control method for the parameters of the multi-inverter system based on mode switching under the weak grid according to claim 1, wherein the grid impedance identification algorithm in step 3 comprises the following steps:
step 3.1, injecting non-characteristic subharmonic current with the frequency of 75Hz at a PCC (point of common coupling);
step 3.2, sampling harmonic response voltage u at PCCpcchAnd harmonic response current igh；
Step 3.3, respectively responding the harmonic wave response voltage u through fast Fourier algorithm FFTpcchAnd harmonic response current ighPerforming spectrum analysis to obtain the amplitude value | U of harmonic response voltage component at 75Hz frequencypcch_75HzPhase ∠ U of harmonic response voltage component at | 75Hz frequencypcch_75HzAmplitude I of harmonic response current component at 75Hz frequencypcch_75HzPhase ∠ I of harmonic response current component at | 75Hz frequencypcch_75Hz(ii) a Obtaining the amplitude value | Z of the network impedance at the frequency of 75Hz according to the following formulagPhase ∠ Z of the grid impedance at | and 75Hz frequenciesg：
∠Zg＝∠Upcch_75Hz-∠Ipcch_75Hz；
Step 3.4, obtaining the amplitude value | Z of the power grid impedance at the frequency of 75Hz according to the step 3.3gPhase ∠ Z of the grid impedance at | and 75Hz frequenciesgThe network impedance is calculated according to the following formulaIdentification value Zg_est：
4. The weak grid mode-switching-based multi-inverter system parameter adaptive control method according to claim 1, wherein the voltage source mode control step of step 4 is as follows:
step 4.1, collecting and outputting grid-connected current iga、igb、igcCollecting voltage u of point of common couplingpcca、upccb、upccc；
Step 4.2, according to the output grid-connected current i collected in step 4.1ga、igb、igcObtaining an output grid-connected current αβ axis component i through a transformation equation from a three-phase static coordinate system to a two-phase static coordinate systemgα、igβ(ii) a Voltage u of point of common coupling collected according to step 4.1pcca、upccb、upcccObtaining a common coupling point voltage αβ axis component u through a transformation equation from a three-phase static coordinate system to a two-phase static coordinate systempccα、upccβ；
The transformation equation of the output grid-connected current from the three-phase static coordinate system to the two-phase static coordinate system is as follows:
the transformation equation of the voltage of the common coupling point from the three-phase static coordinate system to the two-phase static coordinate system is as follows:
step 4.3, according to the output grid-connected current αβ axis component i obtained in the step 4.2gα、igβAnd a common coupling point voltage αβ axis component upccα、upccβFirstly, the average active power is obtained through the average active power calculation equation
Then obtaining the average reactive power through an average reactive power calculation equation
The average active power calculation equation is:
the average reactive power calculation equation is:
wherein τ is a first-order low-pass filter time constant, and s is a laplacian operator;
step 4.4, obtaining the average active power according to the step 4.3
Obtaining the output angular frequency omega of the grid-connected inverter through an active power-frequency droop control equation; the active power-frequency droop control equation is as follows:
wherein, PnGiven an active power command, ω, for the grid-connected inverternGiven active power command for grid-connected inverterPnNominal angular frequency, D, to which time correspondspThe active droop coefficient;
integrating the output angular frequency omega of the grid-connected inverter to obtain the output phase angle theta of the grid-connected inverter0Namely:
step 4.5, according to the voltage u of the public coupling point collected in step 4.1pcca、upccb、upcccAnd the output phase angle theta of the grid-connected inverter obtained according to the step 4.40Obtaining the voltage dq axis component u of the point of common coupling through a transformation equation from a three-phase static coordinate system to a two-phase rotating coordinate systempccd、upccq；
The transformation equation of the voltage of the common coupling point from a three-phase static coordinate system to a two-phase rotating coordinate system is as follows:
step 4.6, output grid-connected current i acquired according to step 4.1ga、igb、igcAnd the output phase angle theta of the grid-connected inverter obtained according to the step 4.40Obtaining output grid-connected current dq component i through a transformation equation from a three-phase static coordinate system to a two-phase rotating coordinate systemgdAnd igq；
The transformation equation of the output grid-connected current from the three-phase static coordinate system to the two-phase rotating coordinate system is as follows:
step 4.7, outputting the average reactive power of the grid-connected inverter obtained according to the step 4.3
Obtaining a voltage dq component reference value u of a point of common coupling of the grid-connected inverter through a reactive power-amplitude droop control equationpccdref、upccqrefThe reactive power-amplitude droop control equation is as follows:
upccqref＝0
wherein, UnGiven reactive power command Q for grid-connected inverternRated output voltage, D, corresponding to timeqIs the reactive droop coefficient;
step 4.8, firstly obtaining the voltage dq axis component u of the point of common coupling according to the step 4.5pccd、upccqAnd the value of the reference u of the voltage dq component of the pcc obtained in step 4.7pccdref、upccqrefAnd obtaining an output grid-connected current instruction signal i through an voltage loop control equationgdref、igqref；
The voltage loop control equation is:
wherein, Kp1Proportional control coefficient, K, for a PI regulator in a voltage loop control equationi1The integral control coefficient of a PI regulator in a voltage loop control equation;
step 4.9, firstly, according to the output grid-connected current instruction signal i obtained in step 4.8gdref、igqrefAnd according to the output grid-connected current dq component i obtained in the step 4.6gdAnd igqObtaining the control signal u by a current loop control equationdAnd uq；
The current loop control equation is:
wherein, Kp2Proportional control coefficient, K, of a PI regulator in a current loop control equationi2The integral control coefficient of a PI regulator in a current loop control equation;
step 4.10, obtaining the output phase angle theta of the grid-connected inverter according to the step 4.40The control signal u obtained in step 4.9dAnd uqConverting the control signal component u into a control signal component u under a three-phase static coordinate system through a transformation equation from a two-phase rotating coordinate system to the three-phase static coordinate systema、ub、uc；
The transformation equation of the control signal from the two-phase rotating coordinate system to the three-phase static coordinate system is as follows:
ua＝udcosθ0-uqsinθ0
step 4.11, obtaining the component u under the three-phase static coordinate system according to the step 4.10a、ub、ucRespectively with the pcc voltage u obtained in step 4.1pcca、upccb、upcccAdding to obtain three-phase full-bridge grid-connected inverter bridge arm voltage control signals, wherein the three-phase full-bridge grid-connected inverter bridge arm voltage control signals are respectively as follows: u. ofa+upcca、ub+upccb、uc+upcccAnd generating a switching signal of the power device of the grid-connected inverter through SVPWM modulation, and controlling the on-off of the power device of the three-phase full-bridge grid-connected inverter through a driving circuit.
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