Grid-connected current THD optimization method and device considering bus midpoint voltage fluctuation

Abstract

The embodiment of the invention provides a grid-connected current THD optimization method and device considering bus midpoint voltage fluctuation. The method comprises the steps that firstly, bus midpoint voltage data and three-phase operation data of an inverter are obtained; wherein the bus neutral-point voltage data comprises neutral-point voltage deviation and neutral-point voltage pulsation frequency; determining a first control signal according to the neutral-point voltage amplitude and the three-phase operation data; optimizing the first control signal according to the midpoint voltage deviation and the midpoint voltage pulsation frequency by taking the minimum grid-connected current THD as a target to obtain a second control signal; and finally, the inverter is controlled according to the second control signal. On the basis of a conventional control loop, consideration of voltage deviation and pulsation frequency of a bus midpoint is introduced, so that the influence of zero crossing point distortion of an inverter on grid-connected current THD is reduced, and network access current THD is effectively reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of inverter control, and in particular relates to a method and device for optimizing grid-connected current THD taking bus midpoint voltage fluctuation into account. Background Art

[0002] In response to the challenge of insufficient global energy supply, new energy technologies have played a vital role in alleviating the problem. In particular, solar photovoltaic grid-connected power generation technology has experienced rapid development in recent years due to its environmentally friendly and efficient power generation characteristics. However, with the large-scale deployment of photovoltaic grid-connected power generation systems, a series of problems have gradually emerged. The most significant problem is that the power electronic devices used in a large number of photovoltaic grid-connected power generation systems increase the total harmonic distortion (THD) of the grid current, which seriously reduces the quality of power.

[0003] At present, in order to reduce the grid current THD, while optimizing the inverter topology and improving the hardware performance, its control algorithm also needs to be improved. Summary of the invention

[0004] In view of this, the present invention provides a method and device for optimizing the grid-connected current THD taking into account bus midpoint voltage fluctuations, aiming to solve the problem that the control algorithm of the inverter in the prior art needs to be improved.

[0005] A first aspect of an embodiment of the present invention provides a method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuations, comprising:

[0006] Obtaining bus midpoint voltage data and three-phase operation data of the inverter; wherein the bus midpoint voltage data includes midpoint voltage deviation and midpoint voltage pulsation frequency;

[0007] Determining a first control signal according to the midpoint voltage amplitude and the three-phase operation data;

[0008] Taking the minimum THD of the grid-connected current as the goal, the first control signal is optimized according to the midpoint voltage deviation and the midpoint voltage pulsation frequency to obtain the second control signal;

[0009] The inverter is controlled according to the second control signal.

[0010] In a possible implementation, the three-phase operation data includes three-phase voltage and three-phase current; and determining the first control signal according to the midpoint voltage deviation and the three-phase operation data includes:

[0011] Determine the current reference value according to the midpoint voltage deviation, the three-phase voltage and the voltage outer loop;

[0012] A first control signal is determined according to the three-phase current, the current control amount and the current inner loop.

[0013] In a possible implementation, determining a current reference value according to a midpoint voltage deviation, three-phase voltages, and a voltage outer loop includes:

[0014] The three-phase voltages are transformed into coordinates to obtain the d-axis voltage and the q-axis voltage;

[0015] Determine a d-axis current reference value according to the d-axis voltage, the d-axis voltage reference value and the PI controller;

[0016] Determine a current reference value of the q-axis according to the q-axis voltage, the q-axis voltage reference value and the PI controller;

[0017] According to the midpoint voltage deviation and PI controller, the current reference value of the 0 axis is determined.

[0018] In a possible implementation, determining the first control signal according to the three-phase current, the current reference value and the current inner loop includes:

[0019] The three-phase currents are transformed into coordinates to obtain the d-axis current, q-axis current and 0-axis current;

[0020] Determine the d-axis control amount according to the d-axis current reference value, the d-axis current and the PI controller;

[0021] Determine the q-axis control amount according to the q-axis current reference value, the q-axis current and the PI controller;

[0022] Determine the 0-axis control amount according to the 0-axis current reference value, the 0-axis current and the PI controller;

[0023] The d-axis control amount, the q-axis control amount and the 0-axis control amount are used as the first control signal.

[0024] In a possible implementation, the first control signal is optimized according to the midpoint voltage deviation and the midpoint voltage pulsation frequency with the goal of minimizing the grid-connected current THD to obtain the second control signal, including:

[0025] With the goal of minimizing the grid-connected current THD, the first control signal is optimized according to the midpoint voltage deviation, the midpoint voltage pulsation frequency and the particle swarm algorithm to obtain the second control signal.

[0026] In one possible implementation, the objective function is:

[0027]

[0028]

[0029] Among them, minTHD is the objective function, In is the current component of the nth harmonic, I1 is the fundamental current component, N is the harmonic number considered, V pm is the midpoint voltage deviation, f p is the midpoint voltage pulsation frequency, Z n is the impedance of the system at the nth harmonic frequency. n (f p ) is a transfer function that describes the effect of the midpoint voltage pulsation frequency on the nth harmonic current component, which depends on the inverter type, modulation strategy, load characteristics, and filtering characteristics of the system.

[0030] In a possible implementation, controlling the inverter according to the second control signal includes:

[0031] The second control signal is input into the pulse width modulator after coordinate transformation to obtain the control signal of the inverter switch tube;

[0032] The inverter is controlled according to the control signal of the inverter switch tube.

[0033] In a possible implementation, the first control signal is optimized according to the midpoint voltage deviation and the midpoint voltage pulsation frequency with the goal of minimizing the grid-connected current THD to obtain the second control signal, including:

[0034] The midpoint voltage deviation, the midpoint voltage pulsation frequency, and the first control signal are input into the neural network model to obtain a grid-connected current THD prediction value;

[0035] Taking the minimization of the grid-connected current THD prediction value as the goal, the particle swarm algorithm is used for optimization to determine the optimal midpoint voltage deviation and midpoint voltage pulsation frequency;

[0036] The second control signal is determined according to the optimal mid-point voltage deviation and the optimal mid-point voltage pulsation frequency.

[0037] In a possible implementation, determining the second control signal according to the optimal midpoint voltage deviation and the optimal midpoint voltage pulsation frequency includes:

[0038] Determine the optimal 0-axis control amount according to the 0-axis control amount and the optimal midpoint voltage deviation;

[0039] The second control signal is determined according to the d-axis control amount, the q-axis control amount, the optimal 0-axis control amount and the optimal mid-point voltage pulsation frequency.

[0040] A second aspect of an embodiment of the present invention provides a grid-connected current THD optimization device taking into account bus midpoint voltage fluctuations, comprising:

[0041] An acquisition module is used to acquire bus midpoint voltage data and three-phase operation data of the inverter; wherein the bus midpoint voltage data includes midpoint voltage deviation and midpoint voltage pulsation frequency;

[0042] A determination module, used to determine a first control signal according to a midpoint voltage amplitude and three-phase operation data;

[0043] An optimization module, for optimizing the first control signal according to the midpoint voltage deviation and the midpoint voltage pulsation frequency with the goal of minimizing the grid-connected current THD, to obtain a second control signal;

[0044] The control module is used to control the inverter according to the second control signal.

[0045] The method and device for optimizing the grid-connected current THD taking into account the bus midpoint voltage fluctuation provided by the embodiment of the present invention first obtains the bus midpoint voltage data and three-phase operation data of the inverter; wherein the bus midpoint voltage data includes the midpoint voltage deviation and the midpoint voltage pulsation frequency; then determines the first control signal according to the midpoint voltage amplitude and the three-phase operation data; then optimizes the first control signal according to the midpoint voltage deviation and the midpoint voltage pulsation frequency with the goal of minimizing the grid-connected current THD to obtain the second control signal; finally controls the inverter according to the second control signal. By introducing the consideration of the voltage deviation and pulsation frequency of the bus midpoint on the basis of the conventional control loop, the influence of the inverter's zero-crossing distortion on the grid-connected current THD is reduced, and the grid-connected current THD is effectively reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0047] Figure 1 is a flow chart of an implementation method of a grid-connected current THD optimization method taking into account bus midpoint voltage fluctuations provided in an embodiment of the present invention;

[0048] Figure 2 It is a structural schematic diagram of a grid-connected current THD optimization device taking into account bus midpoint voltage fluctuations provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0049] In the following description, specific details such as specific system structures, technologies, etc. are provided for the purpose of illustration rather than limitation, so as to provide a thorough understanding of the embodiments of the present invention. However, it should be clear to those skilled in the art that the present invention may be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to prevent unnecessary details from obstructing the description of the present invention.

[0050] Figure 1 1 is a flow chart of the implementation of the method for optimizing the grid-connected current THD taking into account the bus midpoint voltage fluctuation provided by the embodiment of the present invention. Figure 1 As shown, in some embodiments, a method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuations includes:

[0051] S110, obtaining bus midpoint voltage data and three-phase operation data of the inverter; wherein the bus midpoint voltage data includes a midpoint voltage deviation and a midpoint voltage pulsation frequency;

[0052] In the embodiment of the present invention, the three-phase operation data is specifically the three-phase current and three-phase voltage at the output end of the inverter. The midpoint voltage deviation is the voltage amplitude of the bus midpoint. In an ideal state, the voltage of the bus midpoint is 0, so its amplitude can be defined as the deviation. After collecting the voltage signal of the bus midpoint, the conventional algorithm is used to perform frequency analysis to obtain the midpoint voltage pulsation frequency.

[0053] S120, determining a first control signal according to the midpoint voltage amplitude and the three-phase operation data;

[0054] In some embodiments, the three-phase operation data includes three-phase voltage and three-phase current; S120 includes: determining the current reference value based on the midpoint voltage deviation, the three-phase voltage and the voltage outer loop; determining the first control signal based on the three-phase current, the current control amount and the current inner loop.

[0055] In the embodiment of the present invention, the inverter adopts voltage and current dual-loop control, and its specific control method is as follows:

[0056] In some embodiments, a current reference value is determined based on a midpoint voltage deviation, three-phase voltages and a voltage outer loop, including: performing coordinate transformation on the three-phase voltages to obtain a d-axis voltage and a q-axis voltage; determining a d-axis current reference value based on the d-axis voltage, the d-axis voltage reference value and a PI controller; determining a q-axis current reference value based on the q-axis voltage, the q-axis voltage reference value and a PI controller; and determining a 0-axis current reference value based on the midpoint voltage deviation and the PI controller.

[0057] In the embodiment of the present invention, the d-axis voltage and the d-axis voltage reference value are subtracted, and the obtained d-axis voltage deviation is input into the PI controller to obtain the d-axis current reference value. Similarly, the q-axis voltage and the q-axis voltage reference value are subtracted, and the obtained q-axis voltage deviation is input into the PI controller to obtain the q-axis current reference value. In an ideal state, the voltage at the midpoint of the bus is 0, and the voltage reference value of the 0-axis is set to 0. Therefore, the midpoint voltage deviation is input into the PI controller to obtain the 0-axis current reference value.

[0058] In some embodiments, a first control signal is determined based on the three-phase current, the current reference value and the current inner loop, including: performing coordinate transformation on the three-phase current to obtain a d-axis current, a q-axis current and a 0-axis current; determining the d-axis control quantity based on the d-axis current reference value, the d-axis current and the PI controller; determining the q-axis control quantity based on the q-axis current reference value, the q-axis current and the PI controller; determining the 0-axis control quantity based on the 0-axis current reference value, the 0-axis current and the PI controller; and using the d-axis control quantity, the q-axis control quantity and the 0-axis control quantity as the first control signal.

[0059] In an embodiment of the present invention, the d-axis current is subtracted from the current reference value of the d-axis, and the obtained d-axis current deviation is input into the PI controller to obtain the d-axis control amount. Similarly, the q-axis current is subtracted from the current reference value of the q-axis, and the obtained q-axis current deviation is input into the PI controller to obtain the q-axis control amount. The 0-axis current is subtracted from the current reference value of the 0-axis, and the obtained 0-axis current deviation is input into the PI controller to obtain the 0-axis control amount. Among them, the current reference value of the 0-axis depends on the current from the midpoint of the line to the ground, which will only be a DC component. If the midpoint of the bus is connected to the ground through an ideal passive filter (such as a capacitor), then under ideal conditions, the current from the midpoint of the bus to the ground will only be a DC component, without pulsation. This is because the ideal filter can completely filter out the AC pulsation and only allow the DC component to pass.

[0060] Since the above reference value is set based on an ideal state, the control of the inverter cannot well follow the actual change of the busbar, and the grid-connected current THD still has room for optimization. Therefore, the present invention is optimized through the following steps.

[0061] S130, taking the minimum THD of the grid-connected current as the goal, optimizing the first control signal according to the midpoint voltage deviation and the midpoint voltage pulsation frequency to obtain a second control signal;

[0062] In some embodiments, S130 includes: taking the minimum grid-connected current THD as a goal, optimizing the first control signal according to the midpoint voltage deviation, the midpoint voltage pulsation frequency and the particle swarm algorithm to obtain the second control signal.

[0063] In some embodiments, the objective function is:

[0064]

[0065]

[0066] Among them, minTHD is the objective function, I n is the current component of the nth harmonic, I1 is the fundamental current component, N is the harmonic number considered, V pm is the midpoint voltage deviation, f p is the midpoint voltage pulsation frequency, Z n is the impedance of the system at the nth harmonic frequency. n (f p ) is a transfer function that describes the effect of the midpoint voltage pulsation frequency on the nth harmonic current component, which depends on the inverter type, modulation strategy, load characteristics, and filtering characteristics of the system.

[0067] In an embodiment of the present invention, in the PSO algorithm, each particle represents a potential solution, including the amplitude and frequency of the midpoint voltage pulsation. When initializing the particle swarm, it is necessary to randomly assign an initial position and speed to each particle. Subsequently, the fitness of each particle is evaluated, that is, the corresponding objective function value (THD) is calculated. An optimization strategy that minimizes THD is selected. Then, according to the individual optimal solution (pbest) and the global optimal solution (gbest) of each particle, the speed and position of the particle are updated. The process of evaluating fitness and updating particle speed and position is repeated until the stop condition is met (for example, a preset number of iterations is reached or THD is less than a certain threshold). Finally, after the algorithm ends, the particle corresponding to the global optimal solution (gbest) is the optimal solution that minimizes THD. In this way, the amplitude and frequency of the midpoint voltage pulsation that minimizes THD can be determined.

[0068] In some embodiments, S130 includes: inputting the midpoint voltage deviation, the midpoint voltage pulsation frequency, and the first control signal into the neural network model to obtain a grid-connected current THD prediction value; minimizing the grid-connected current THD prediction value as the goal, optimizing using a particle swarm algorithm, and determining the optimal midpoint voltage deviation and midpoint voltage pulsation frequency; determining the second control signal based on the optimal midpoint voltage deviation and the optimal midpoint voltage pulsation frequency.

[0069] In the embodiment of the present invention, in addition to the particle swarm, the optimal midpoint voltage deviation and midpoint voltage pulsation frequency can also be calculated by a neural network, and its loss function is used to measure the difference between the THD value predicted by the network and the actual THD value.

[0070] In some embodiments, a second control signal is determined based on an optimal midpoint voltage deviation and an optimal midpoint voltage pulsation frequency, including: determining an optimal 0-axis control amount based on a 0-axis control amount and an optimal midpoint voltage deviation; and determining a second control signal based on a d-axis control amount, a q-axis control amount, an optimal 0-axis control amount, and an optimal midpoint voltage pulsation frequency.

[0071] The control of the 0-axis current will affect the balance of the three-phase current. Therefore, in the embodiment of the present invention, after calculating the optimal midpoint voltage deviation, a new 0-axis control amount is calculated to update the first control signal to the second control signal. The specific calculation formula is as follows:

[0072]

[0073] Among them, V dc is the bus DC voltage, i0 is the 0-axis control quantity, V offset It is a voltage deviation caused by circuit imbalance or other factors.

[0074] After calculating the second control signal and controlling it, it is necessary to monitor the pulsation frequency according to the optimal midpoint voltage pulsation frequency to ensure system stability. High switching frequency will bring greater switching losses. When reducing switching losses, the switching frequency must not be lower than the product of the optimal midpoint voltage pulsation frequency and the preset coefficient to avoid THD increase.

[0075] S140: Control the inverter according to the second control signal.

[0076] In some embodiments, the inverter is controlled according to the second control signal, including: the second control signal is coordinate transformed and input into a pulse width modulator to obtain a control signal of the inverter switch tube; and the inverter is controlled according to the control signal of the inverter switch tube.

[0077] It should be understood that the order of execution of the steps in the above embodiment does not necessarily mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiment of the present invention.

[0078] Figure 2 Schematic diagram of the structure of the grid-connected current THD optimization device taking into account bus midpoint voltage fluctuations provided by an embodiment of the present invention. Figure 2 As shown, in some embodiments, a grid-connected current THD optimization device 2 taking into account bus midpoint voltage fluctuations comprises:

[0079] The acquisition module 210 is used to acquire the bus midpoint voltage data and three-phase operation data of the inverter; wherein the bus midpoint voltage data includes the midpoint voltage deviation and the midpoint voltage pulsation frequency;

[0080] A determination module 220, configured to determine a first control signal according to a midpoint voltage amplitude and three-phase operation data;

[0081] An optimization module 230 is used to optimize the first control signal according to the midpoint voltage deviation and the midpoint voltage pulsation frequency with the goal of minimizing the grid-connected current THD, so as to obtain a second control signal;

[0082] The control module 240 is used to control the inverter according to the second control signal.

[0083] Optionally, the determination module 220 is used to: determine the current reference value according to the midpoint voltage deviation, the three-phase voltage and the voltage outer loop; determine the first control signal according to the three-phase current, the current control amount and the current inner loop.

[0084] Optionally, the determination module 220 is used to: perform coordinate transformation on the three-phase voltage to obtain the d-axis voltage and the q-axis voltage; determine the d-axis current reference value based on the d-axis voltage, the d-axis voltage reference value and the PI controller; determine the q-axis current reference value based on the q-axis voltage, the q-axis voltage reference value and the PI controller; determine the 0-axis current reference value based on the midpoint voltage deviation and the PI controller.

[0085] Optionally, the determination module 220 is used to: perform coordinate transformation on the three-phase current to obtain a d-axis current, a q-axis current and a 0-axis current; determine the d-axis control quantity according to the d-axis current reference value, the d-axis current and the PI controller; determine the q-axis control quantity according to the q-axis current reference value, the q-axis current and the PI controller; determine the 0-axis control quantity according to the 0-axis current reference value, the 0-axis current and the PI controller; and use the d-axis control quantity, the q-axis control quantity and the 0-axis control quantity as the first control signal.

[0086] Optionally, the optimization module 230 is used to optimize the first control signal according to the midpoint voltage deviation, the midpoint voltage pulsation frequency and the particle swarm algorithm with the goal of minimizing the grid-connected current THD to obtain the second control signal.

[0087] Optionally, the objective function is:

[0088]

[0089] Among them, minTHD is the objective function, I n is the current component of the nth harmonic, I1 is the fundamental current component, N is the harmonic number considered, V pm is the midpoint voltage deviation, f p is the midpoint voltage pulsation frequency, Z n is the impedance of the system at the nth harmonic frequency. n (f p) is a transfer function that describes the effect of the midpoint voltage pulsation frequency on the nth harmonic current component, which depends on the inverter type, modulation strategy, load characteristics, and filtering characteristics of the system.

[0090] Optionally, the control module 240 is used to: perform coordinate transformation on the second control signal and input the signal into a pulse width modulator to obtain a control signal of the inverter switch tube; and control the inverter according to the control signal of the inverter switch tube.

[0091] Optionally, the optimization module 230 is used to: input the midpoint voltage deviation, the midpoint voltage pulsation frequency, and the first control signal into the neural network model to obtain the grid-connected current THD prediction value; minimize the grid-connected current THD prediction value as the goal, use the particle swarm algorithm for optimization, and determine the optimal midpoint voltage deviation and midpoint voltage pulsation frequency; determine the second control signal based on the optimal midpoint voltage deviation and the optimal midpoint voltage pulsation frequency.

[0092] Optionally, the optimization module 230 is used to: determine the optimal 0-axis control amount based on the 0-axis control amount and the optimal mid-point voltage deviation; determine the second control signal based on the d-axis control amount, the q-axis control amount, the optimal 0-axis control amount and the optimal mid-point voltage pulsation frequency.

[0093] The high-speed packaging machine control device based on cigarette box appearance defect detection provided in this embodiment can be used to execute the above method embodiments. Its implementation principle and technical effects are similar, and this embodiment will not be repeated here.

[0094] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0095] Those skilled in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

[0096] In the embodiments provided by the present invention, it should be understood that the disclosed devices / controllers and methods can be implemented in other ways. For example, the device / controller embodiments described above are only schematic, for example, the division of modules or units is only a logical function division, and there may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0097] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0098] In addition, each functional unit in each embodiment of the present invention may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0099] If the integrated module / unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the present invention implements all or part of the processes in the above-mentioned embodiment method, and can also be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a computer-readable storage medium, and the computer program can implement the steps of the above-mentioned various method embodiments when executed by the processor. Among them, the computer program includes computer program code, and the computer program code can be in source code form, object code form, executable file or some intermediate form. Computer-readable media may include: any entity or device capable of carrying computer program code, recording medium, U disk, mobile hard disk, disk, optical disk, computer memory, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), electrical carrier signal, telecommunication signal and software distribution medium, etc.

[0100] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention.

Claims

1. A method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuations, characterized in that: include: Obtaining bus midpoint voltage data and three-phase operation data of the inverter; wherein the bus midpoint voltage data includes midpoint voltage deviation and midpoint voltage pulsation frequency; Determining a first control signal according to the midpoint voltage amplitude and the three-phase operation data; Taking the minimum THD of the grid-connected current as the goal, the first control signal is optimized according to the midpoint voltage deviation and the midpoint voltage pulsation frequency to obtain the second control signal; The inverter is controlled according to the second control signal.

2. The method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuation according to claim 1, characterized in that: The three-phase operation data includes three-phase voltage and three-phase current; determining a first control signal according to the midpoint voltage deviation and the three-phase operation data includes: Determining a current reference value according to the midpoint voltage deviation, the three-phase voltage and the voltage outer loop; The first control signal is determined according to the three-phase current, the current control variable and the current inner loop.

3. The method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuation according to claim 2, characterized in that: Determining a current reference value according to the midpoint voltage deviation, the three-phase voltage and the voltage outer loop includes: Performing coordinate transformation on the three-phase voltage to obtain a d-axis voltage and a q-axis voltage; Determining a d-axis current reference value according to the d-axis voltage, the d-axis voltage reference value and the PI controller; Determine a q-axis current reference value according to the q-axis voltage, the q-axis voltage reference value and the PI controller; According to the midpoint voltage deviation and the PI controller, the current reference value of the 0 axis is determined.

4. The method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuation according to claim 3, characterized in that: Determining the first control signal according to the three-phase current, the current reference value and the current inner loop includes: Performing coordinate transformation on the three-phase current to obtain a d-axis current, a q-axis current and a 0-axis current; Determining a d-axis control amount according to the d-axis current reference value, the d-axis current and a PI controller; Determining a q-axis control amount according to the q-axis current reference value, the q-axis current and a PI controller; Determine the 0-axis control amount according to the 0-axis current reference value, the 0-axis current and the PI controller; The d-axis control amount, the q-axis control amount, and the 0-axis control amount are used as a first control signal.

5. The method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuation according to claim 4, characterized in that: Taking the minimum THD of the grid-connected current as the goal, the first control signal is optimized according to the midpoint voltage deviation and the midpoint voltage pulsation frequency to obtain the second control signal, including: With the goal of minimizing the grid-connected current THD, the first control signal is optimized according to the midpoint voltage deviation, the midpoint voltage pulsation frequency and the particle swarm algorithm to obtain the second control signal.

6. The method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuation according to claim 5, characterized in that: The objective function is: Wherein, minTHD is the objective function, I n is the current component of the nth harmonic, I1 is the fundamental current component, N is the harmonic number considered, V pm is the midpoint voltage deviation, f p is the midpoint voltage pulsation frequency, Z n is the impedance of the system at the nth harmonic frequency. n (f p ) is a transfer function that describes the effect of the midpoint voltage pulsation frequency on the nth harmonic current component, which depends on the inverter type, modulation strategy, load characteristics, and filtering characteristics of the system.

7. The method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuation according to claim 1, characterized in that: The step of controlling the inverter according to the second control signal includes: The second control signal is input into a pulse width modulator after coordinate transformation to obtain a control signal of an inverter switch tube; The inverter is controlled according to the control signal of the inverter switch tube.

8. The method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuation according to claim 1, characterized in that: Taking the minimum THD of the grid-connected current as the goal, the first control signal is optimized according to the midpoint voltage deviation and the midpoint voltage pulsation frequency to obtain the second control signal, including: Inputting the midpoint voltage deviation, the midpoint voltage pulsation frequency, and the first control signal into a neural network model to obtain a grid-connected current THD prediction value; Taking the minimization of the grid-connected current THD prediction value as the goal, the particle swarm algorithm is used for optimization to determine the optimal midpoint voltage deviation and midpoint voltage pulsation frequency; The second control signal is determined according to the optimal mid-point voltage deviation and the optimal mid-point voltage pulsation frequency.

9. The method for optimizing grid-connected current THD taking into account bus midpoint voltage fluctuation according to claim 8, characterized in that: Determining a second control signal according to an optimal midpoint voltage deviation and an optimal midpoint voltage pulsation frequency includes: Determining an optimal 0-axis control amount according to the 0-axis control amount and the optimal midpoint voltage deviation; A second control signal is determined according to the d-axis control amount, the q-axis control amount, the optimal 0-axis control amount and the optimal mid-point voltage pulsation frequency.

10. A grid-connected current THD optimization device taking into account bus midpoint voltage fluctuations, characterized in that: include: An acquisition module is used to acquire bus midpoint voltage data and three-phase operation data of the inverter; wherein the bus midpoint voltage data includes midpoint voltage deviation and midpoint voltage pulsation frequency; A determination module, configured to determine a first control signal according to the midpoint voltage amplitude and the three-phase operation data; The optimization module is used to optimize the first control signal according to the midpoint voltage deviation and the midpoint voltage pulsation frequency with the goal of minimizing the grid-connected current THD to obtain a second control signal; the control module is used to control the inverter according to the second control signal.

Images