Single-phase parallel inverter control method based on power grid current feedback

Abstract

The invention relates to a single-phase parallel inverter control method based on power grid current feedback, and the method specifically comprises the steps: building a mathematical model of a multi-parallel inverter system, and building a simulation model; a grid-connected current is selected as a controlled object, and a system transfer function of the inverter adopting virtual impedance as active damping is obtained; analyzing parallel resonance; active damping is introduced, dynamic response is actively intervened through a grid-connected current feedback mechanism, and potential resonance peaks are inhibited; deducing an equivalent transfer function from the inverter output voltage to the grid-connected current after the active damping is introduced; virtual impedance is connected to the two sides of a capacitor of the LCL filter in parallel to obtain a transfer function from the output voltage of the inverter to the grid-connected current at the moment; a similar item having similar frequency response with the second-order differential item is used for replacement; substituting the transfer function to obtain an overall open-loop transfer function of the inverter system; the method has the advantages that a mathematical model is established, a grid-connected current feedback control strategy is introduced, and the resonance problem is effectively solved.

Description

Technical Field

[0001] The invention belongs to the technical field of photovoltaic inverters, and in particular relates to a single-phase parallel inverter control method based on grid current feedback. Background Art

[0002] The rapid development of clean energy technology today has made the method of parallel operation of multiple grid-connected inverters more and more common in microgrid systems. This system configuration requires that multiple inverters be connected to the grid through a common coupling point to achieve efficient parallel operation. However, with the increase in the number of inverters, the resonance mechanism of the system becomes more complicated. In a system where multiple grid-connected inverters operate in parallel, the grid impedance at the common coupling point becomes a key factor that cannot be ignored. Especially in a strong grid environment, the design parameters of traditional grid-connected inverters (with active damping characteristics) can no longer meet the requirements for stable operation of the system; in a weak grid environment, With the parallel connection of multiple inverters, the system faces complex multiple resonance challenges, which not only include the inherent resonance of the LCL filter itself, which is common to grid-connected inverters, but also involve the resonance phenomenon between the inverter and the grid and between inverters. At the same time, the increase in the number of inverters leads to an increase in the equivalent impedance of the grid and a shift of the resonant frequency point to a low frequency, which further increases the risk of system resonance and may have a negative impact on the stability of the system. Therefore, it is very necessary to provide a single-phase parallel inverter control method based on grid current feedback, which establishes a mathematical model, introduces a grid-connected current feedback control strategy, and effectively solves the resonance problem. Summary of the invention

[0003] The purpose of the present invention is to overcome the deficiencies of the prior art and to provide a single-phase parallel inverter control method based on grid current feedback, which establishes a mathematical model, introduces a grid-connected current feedback control strategy, and effectively solves the resonance problem.

[0004] The object of the present invention is achieved by: a single-phase parallel inverter control method based on grid current feedback, the method comprising the following steps:

[0005] Step 1: Establish a mathematical model of multiple parallel inverter systems, including circuit model and control model, and use Simulink to establish a LCL type single-phase parallel inverter simulation model;

[0006] Step 2: By selecting the grid-connected current i2 as the controlled object, the system transfer function of the inverter using virtual impedance as active damping can be obtained;

[0007] Step 3: The resonance of a single inverter is mainly due to its own characteristics, while the resonance of multiple inverters is caused by the mutual interference between the inverters, and the equivalent impedance between the inverter and the main grid will also cause series resonance. This is analyzed;

[0008] Step 4: In traditional LCL-type grid-connected inverters, the system is prone to resonance due to the existence of grid impedance. Especially when the number of inverters increases, the resonance problem becomes more significant. In order to meet this challenge, the concept of active damping is introduced, and the dynamic response of the system is actively intervened through the feedback mechanism of the grid-connected current to suppress potential resonance spikes.

[0009] Step 5: Derive the inverter output voltage U after introducing active damping inv The equivalent transfer function G to the grid-connected current i2 ieq ;

[0010] Step 6: Connect a virtual impedance R in parallel on both sides of the capacitor of the LCL filter v It can be obtained that the output voltage U of the inverter is inv The transfer function to the grid-connected current i2;

[0011] Step 7: At this time G ad There is a second-order differential term in (s), which may amplify the noise. Then this term may not be able to directly achieve the damping performance. Therefore, it is necessary to use a second-order differential term s 2 A similar term with a similar frequency response can be used to replace s 2 item;

[0012] Step 8: Substitute the transfer function of the current controller to obtain the overall open-loop transfer function of the inverter system.

[0013] In step 2, by selecting the grid-connected current i2 as the controlled object, the system transfer function of the inverter using virtual impedance as active damping can be obtained, specifically: From this, the relationship between each inverter branch current and the output voltage and the grid voltage can be obtained, that is: in, Among them, G 1i =1 / Z 1i , G 2i =1 / Z 2i , G Ci =1 / Z Ci , G g =1 / Z g , In the formula, G 11 To G nm is the inductance in the inverter; G g1 To G gnand Z g is the grid impedance; u g is the grid voltage; G Ci is the capacitance in the i-th inverter; G eqi is the equivalent impedance in the i-th inverter; G ij is an inductor in the multi-inverter system; Z 1i is the inverter-side inductance in the ith inverter; Z 2i is the grid-side inductance in the ith inverter; Z Ci is the capacitance in the ith inverter.

[0014] The resonance of a single inverter in step 3 is mainly due to its own characteristics, while the resonance of multiple inverters is caused by the mutual interference between the inverters. In addition, due to the equivalent impedance of the connection between the inverter and the main grid, series resonance will also occur. This is analyzed as follows: in the context of a weak grid, the series impedance cannot be ignored. In order to simplify the analysis process, it is assumed that the inverter parameters are consistent and the structure is symmetrical, then G 11 =G 22 =... =G nn ,G ij (i≠j) values ​​are equal, G g1 =G g2 =... =G gn ,i1=i2=...=i n ,u inv1 =u inv2 =...=u invn , then i can be expressed as: i=[G 11 +(n-1)G 12 ]u inv +G g U g , so the open-loop transfer function G from grid current to inverter voltage is: Wherein, L1 and L2 are filter inductors on the inverter side and the grid side of the grid-connected inverter respectively; s is a complex variable representing the frequency in the Laplace transform domain.

[0015] The equivalent transfer function G in step 5 ieq for:

[0016] The transfer function in step 6 is: Among them, R v is the resistance of the virtual impedance connected in parallel at both ends of the filter capacitor. Combining the formulas in step 5 and step 6, we can get the G of the active damping controller. ad The transfer function is:

[0017] At this time, Gad There is a second-order differential term in (s), which may amplify the noise. Then this term may not be able to directly achieve the damping performance. Therefore, it is necessary to use a second-order differential term s 2 A similar term with a similar frequency response can be used to replace s 2 Term, specifically: introduce a new second-order transfer function G1(s), whose expression is: Among them, ω c is the critical frequency of G1(s) with infinite gain; ξ is the damping coefficient; According to steps 6 and 7, the new AD controller expression can be obtained;

[0018] The overall open-loop transfer function of the inverter system in step 8 is:

[0019] Beneficial effects of the present invention: The present invention is a single-phase parallel inverter control method based on grid current feedback. During use, the present invention pays in-depth attention to multiple parallel photovoltaic inverters, especially the resonance risk problem that may be faced when using LCL filters in a weak grid environment, and proposes a new solution. Not only a mathematical model of the system is established, but also an innovative grid-connected current feedback control strategy is introduced through deep resonance analysis. This strategy not only focuses on the optimal design of system parameters, but also pays more attention to improving the inverter phase margin and amplitude margin, thereby fundamentally solving the resonance problem of multi-inverter parallel systems; the present invention has the advantages of establishing a mathematical model, introducing a grid-connected current feedback control strategy, and effectively solving the resonance problem. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a structural diagram of a system of multiple parallel inverters of the present invention.

[0021] Figure 2 This is a block diagram of the improved grid-connected current feedback control strategy of the present invention.

[0022] Figure 3 The schematic diagram of the grid-connected inverter network current counter-active damping control system of the present invention. DETAILED DESCRIPTION

[0023] The present invention will be further described below in conjunction with the accompanying drawings.

[0024] Example 1

[0025] like Figure 1-3 As shown, a single-phase parallel inverter control method based on grid current feedback comprises the following steps:

[0026] Step 1: Establish a mathematical model of multiple parallel inverter systems, including circuit model and control model, and use Simulink to establish a LCL type single-phase parallel inverter simulation model;

[0027] Step 2: By selecting the grid-connected current i2 as the controlled object, the system transfer function of the inverter using virtual impedance as active damping can be obtained;

[0028] Step 3: The resonance of a single inverter is mainly due to its own characteristics, while the resonance of multiple inverters is caused by the mutual interference between the inverters, and the equivalent impedance between the inverter and the main grid will also cause series resonance. This is analyzed;

[0029] Step 4: In traditional LCL-type grid-connected inverters, the system is prone to resonance due to the existence of grid impedance. Especially when the number of inverters increases, the resonance problem becomes more significant. In order to meet this challenge, the concept of active damping is introduced, and the dynamic response of the system is actively intervened through the feedback mechanism of the grid-connected current to suppress potential resonance spikes.

[0030] Step 5: Derive the inverter output voltage U after introducing active damping inv The equivalent transfer function G to the grid-connected current i2 ieq ;

[0031] Step 6: Connect a virtual impedance R in parallel on both sides of the capacitor of the LCL filter v It can be obtained that the inverter output voltage U inv The transfer function to the grid-connected current i2;

[0032] Step 7: At this time G ad There is a second-order differential term in (s), which may amplify the noise. Then this term may not be able to directly achieve the damping performance. Therefore, it is necessary to use a second-order differential term s 2 A similar term with a similar frequency response can be used to replace s 2 item;

[0033] Step 8: Substitute the transfer function of the current controller to obtain the overall open-loop transfer function of the inverter system.

[0034] The present invention is a single-phase parallel inverter control method based on grid current feedback. During use, the present invention pays in-depth attention to multiple parallel photovoltaic inverters, especially the resonance risk problem that may be faced when using LCL filters in a weak grid environment, and proposes a new solution. Not only a mathematical model of the system is established, but also an innovative grid-connected current feedback control strategy is introduced through deep resonance analysis. This strategy not only focuses on the optimal design of system parameters, but also pays more attention to improving the inverter phase margin and amplitude margin, thereby fundamentally solving the resonance problem of the multi-inverter parallel system; the present invention has the advantages of establishing a mathematical model, introducing a grid-connected current feedback control strategy, and effectively solving the resonance problem.

[0035] Example 2

[0036] like Figure 1-3 As shown, a single-phase parallel inverter control method based on grid current feedback comprises the following steps:

[0037] Step 1: Establish a mathematical model of multiple parallel inverter systems, including circuit model and control model, and use Simulink to establish a LCL type single-phase parallel inverter simulation model, such as Figure 1 As shown;

[0038] In this embodiment, in the figure: i n Indicates the grid-side output current of the nth inverter after LCL filtering; u invn represents the output voltage of the nth inverter; Z1 and Z2 represent the inductive reactance values ​​of the filter inductors L1 and L2 of each inverter; Z3 represents the capacitive reactance value of the filter capacitor C; Z g Represents the equivalent impedance value of the power grid, that is, Z1=L1s, Z2=L1s, Z3=1 / C3s.

[0039] Step 2: By selecting the grid-connected current i2 as the controlled object, the system transfer function of the inverter using virtual impedance as active damping can be obtained, such as Figure 2 As shown;

[0040] In this embodiment, V pcc is the voltage at the common coupling point; G c is the current controller; k PWM is the equivalent model of the full-bridge inverter circuit. Here, k PWM Approximately Among them U tri is the amplitude of the triangular carrier; G ad_1 It is an active damping controller;

[0041] pass Figure 1 , we can get the relationship between each inverter branch current and output voltage and grid voltage, namely: in, Among them, G 1i =1 / Z 1i , G 2i =1 / Z 2i , G Ci =1 / Z Ci , G g =1 / Z g , In the formula, G 11 To G nm is the inductance in the inverter; G g1 To G gn and Z g is the grid impedance; u g is the grid voltage; G Ci is the capacitance in the i-th inverter; G eqi is the equivalent impedance in the i-th inverter; G ij is an inductor in the multi-inverter system; Z 1i is the inverter-side inductance in the ith inverter; Z 2i is the grid-side inductance in the ith inverter; Z Ci is the capacitance in the ith inverter.

[0042] Step 3: The resonance of a single inverter is mainly due to its own characteristics, while the resonance of multiple inverters is caused by the mutual interference between the inverters, and the equivalent impedance between the inverter and the main grid will also cause series resonance. This is analyzed;

[0043] In this embodiment, in the context of a weak power grid, the series impedance cannot be ignored. To simplify the analysis process, it is assumed that the inverter parameters are consistent and the structure is symmetrical, then G 11 =G 22 =... =G nn ,G ij (i≠j) values ​​are equal, G g1 =G g2 =... =G gn ,i1=i2=...=i n ,u inv1 =u inv2 =...=u invn , then i can be expressed as: i=[G 11 +(n-1)G 12 ]u inv +G g U g , so the open-loop transfer function G from grid current to inverter voltage is: Wherein, L1 and L2 are filter inductors on the inverter side and the grid side of the grid-connected inverter respectively; s is a complex variable representing the frequency in the Laplace transform domain.

[0044] Step 4: In traditional LCL-type grid-connected inverters, the system is prone to resonance due to the existence of grid impedance. Especially when the number of inverters increases, the resonance problem becomes more significant. In order to meet this challenge, the concept of active damping is introduced, and the dynamic response of the system is actively intervened through the feedback mechanism of the grid-connected current to suppress potential resonance spikes.

[0045] In this embodiment, the principle of active damping is to adjust the output of the inverter by introducing appropriate current feedback control to inject active current into the system, thereby realizing active regulation of the resonant frequency. This method can not only suppress the generation of resonant peaks, but also improve the stability and dynamic performance of the system. The specific principle is as follows: Figure 3 As shown;

[0046] In the figure: ref is the reference current injected into the grid; G c is the transfer function of the outer loop; G c (s) is a current regulator implemented using a proportional resonant (PR) controller, which is expressed as: Among them, k p is the proportional gain; k r is the resonant gain; ω0 is the basic angular frequency of the power grid, ω0=314rad / s; k PWM is the gain of the full-bridge inverter circuit.

[0047] Step 5: From Figure 2 It can be deduced that after the introduction of active damping, the inverter output voltage U inv The equivalent transfer function G to the grid-connected current i2 ieq for:

[0048] Step 6: Connect a virtual impedance R in parallel on both sides of the capacitor of the LCL filter v It can be obtained that the inverter output voltage U inv The transfer function to the grid-connected current i2;

[0049] In this embodiment, the transfer function is: Among them, R v is the resistance of the virtual impedance connected in parallel at both ends of the filter capacitor. Combining the formulas in step 5 and step 6, we can get the G of the active damping controller. ad The transfer function is:

[0050] Step 7: At this time G adThere is a second-order differential term in (s), which may amplify the noise. Then this term may not be able to directly achieve the damping performance. Therefore, it is necessary to use a second-order differential term s 2 A similar term with a similar frequency response can be used to replace s 2 item;

[0051] In this embodiment, a new second-order transfer function G1(s) is introduced, and its expression is: Among them, ω c is the critical frequency at which G1(s) has infinite gain, where ω c =6rad / s; ξ is the damping coefficient, which is 0.707; According to step 6 and step 7, the new AD controller expression can be obtained;

[0052] Step 8: Substitute the transfer function of the current controller into Figure 2 The block diagram in the figure can be used to obtain the overall open-loop transfer function of the inverter system.

[0053] In this embodiment, the overall open-loop transfer function of the inverter system is:

[0054] The present invention is a single-phase parallel inverter control method based on grid current feedback. During use, the present invention pays in-depth attention to multiple parallel photovoltaic inverters, especially the resonance risk problem that may be faced when using LCL filters in a weak grid environment, and proposes a new solution. Not only a mathematical model of the system is established, but also an innovative grid-connected current feedback control strategy is introduced through deep resonance analysis. This strategy not only focuses on the optimal design of system parameters, but also pays more attention to improving the inverter phase margin and amplitude margin, thereby fundamentally solving the resonance problem of the multi-inverter parallel system; the present invention has the advantages of establishing a mathematical model, introducing a grid-connected current feedback control strategy, and effectively solving the resonance problem.

Claims

1. A single-phase parallel inverter control method based on grid current feedback, characterized in that: The method comprises the following steps: Step 1: Establish a mathematical model of multiple parallel inverter systems, including circuit model and control model, and use Simulink to establish a LCL type single-phase parallel inverter simulation model; Step 2: By selecting the grid-connected current i2 as the controlled object, the system transfer function of the inverter using virtual impedance as active damping can be obtained; Step 3: The resonance of a single inverter is mainly due to its own characteristics, while the resonance of multiple inverters is caused by the mutual interference between the inverters, and the equivalent impedance between the inverter and the main grid will also cause series resonance. This is analyzed; Step 4: In traditional LCL-type grid-connected inverters, the system is prone to resonance due to the existence of grid impedance. Especially when the number of inverters increases, the resonance problem becomes more significant. In order to meet this challenge, the concept of active damping is introduced, and the dynamic response of the system is actively intervened through the feedback mechanism of the grid-connected current to suppress potential resonance spikes. Step 5: Derive the inverter output voltage U after introducing active damping inv The equivalent transfer function G to the grid-connected current i2 ieq ; Step 6: Connect a virtual impedance R in parallel on both sides of the capacitor of the LCL filter v It can be obtained that the output voltage U of the inverter is inv The transfer function to the grid-connected current i2; Step 7: At this time G ad There is a second-order differential term in (s), which may amplify the noise. Then this term may not be able to directly achieve the damping performance. Therefore, it is necessary to use a second-order differential term s 2 A similar term with a similar frequency response can be used to replace s 2 item; Step 8: Substitute the transfer function of the current controller to obtain the overall open-loop transfer function of the inverter system.

2. A single-phase parallel inverter control method based on grid current feedback as claimed in claim 1, characterized in that: In step 2, by selecting the grid-connected current i2 as the controlled object, the system transfer function of the inverter using virtual impedance as active damping can be obtained, specifically: From this, the relationship between each inverter branch current and the output voltage and the grid voltage can be obtained, that is: in, Among them, G 1i =1 / Z 1i , G 2i =1 / Z 2i , G Ci =1 / Z Ci , G g =1 / Z g , In the formula, G 11 To G nm is the inductance in the inverter; G g1 To G gn and Z g is the grid impedance; u g is the grid voltage; G Ci is the capacitance in the i-th inverter; G eqi is the equivalent impedance in the i-th inverter; G ij is an inductor in the multi-inverter system; Z 1i is the inverter-side inductance in the ith inverter; Z 2i is the grid-side inductance in the ith inverter; Z Ci is the capacitance in the ith inverter.

3. The single-phase parallel inverter control method based on grid current feedback according to claim 1, characterized in that: The resonance of a single inverter in step 3 is mainly due to its own characteristics, while the resonance of multiple inverters is caused by the mutual interference between the inverters. In addition, due to the equivalent impedance of the connection between the inverter and the main grid, series resonance will also occur. This is analyzed as follows: in the context of a weak grid, the series impedance cannot be ignored. In order to simplify the analysis process, it is assumed that the inverter parameters are consistent and the structure is symmetrical, then G 11 =G 22 =... =G nn ,G ij (i≠j) values ​​are equal, G g1 =G g2 =... =G gn ,i1=i2=...=i n ,u inv1 =u inv2 =...=u invn , then i can be expressed as: i=[G 11 +(n-1)G 12 ]u inv +G g U g , so the open-loop transfer function G from grid current to inverter voltage is: Wherein, L1 and L2 are filter inductors on the inverter side and the grid side of the grid-connected inverter respectively; s is a complex variable representing the frequency in the Laplace transform domain.

4. The single-phase parallel inverter control method based on grid current feedback according to claim 1, characterized in that: The equivalent transfer function G in step 5 ieq for:

5. A single-phase parallel inverter control method based on grid current feedback as claimed in claim 4, characterized in that: The transfer function in step 6 is: Among them, R v is the resistance of the virtual impedance connected in parallel at both ends of the filter capacitor. Combining the formulas in step 5 and step 6, we can get the G of the active damping controller. ad The transfer function is:

6. A single-phase parallel inverter control method based on grid current feedback as claimed in claim 5, characterized in that: At this time, G ad There is a second-order differential term in (s), which may amplify the noise. Then this term may not be able to directly achieve the damping performance. Therefore, it is necessary to use a second-order differential term s 2 A similar term with a similar frequency response can be used to replace s 2 Term, specifically: introduce a new second-order transfer function G1(s), whose expression is: Among them, ω c is the critical frequency of G1(s) with infinite gain; ξ is the damping coefficient; According to steps 6 and 7, the new AD controller expression can be obtained; 7. The single-phase parallel inverter control method based on grid current feedback according to claim 1, characterized in that: The overall open-loop transfer function of the inverter system in step 8 is:

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