Photovoltaic power station large-capacity inverter parallel system grid-connected control method and system

Abstract

The application provides a photovoltaic station large-capacity inverter parallel system grid-connected control method and system, acquires a real-time value of a common-mode current; compares the real-time value of the common-mode current with a given reference value, and sends a difference value of the comparison into a common-mode resonant current controller to obtain a real-time control variable; controls common-mode voltage and resonant peaks by adjusting redundant small vector action time, and combines the obtained control variable to perform common-mode resonance and leakage current suppression; the application can effectively reduce resonant current and leakage current, and improves power quality and stability of the system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of inverter control, in particular to a grid-connected control method and system for large-capacity inverter parallel system of photovoltaic station. BACKGROUND

[0002] The statements in this section merely provide background technology related to the present application and do not necessarily constitute prior art.

[0003] In the application of photovoltaic power generation, three-level inverters are widely used due to their high output quality, low switching loss, and small device voltage stress. At the same time, with the increase of large-capacity occasions, the capacity of a single inverter often cannot meet the requirements, and the modular parallel form of distributed inverters with independent DC bus is an ideal solution.

[0004] In order to ensure the safe and stable operation of the photovoltaic system, leakage current suppression is an inherent problem that needs to be carefully considered. Leakage current is caused by high-frequency common-mode voltage on the parasitic capacitor, which can reduce system performance, such as increased electromagnetic interference, decreased power quality, and increased safety hazards.

[0005] To achieve leakage current suppression, the zero impedance shunt method is an efficient solution, i.e. changing the traditional LCL filter to an improved LCL filter. Compared with the traditional modulation scheme and the way of changing the hardware topology, it has low cost and is easy to implement, and can greatly reduce leakage current without increasing harmonics and losses. However, the zero impedance shunt method changes the common-mode loop of the system, which can cause common-mode resonance, resulting in current oscillation and reducing the stability of the system.

[0006] The inventors found that the traditional common-mode resonance suppression method is mainly for common DC bus parallel systems or single inverters, and cannot be applied to non-common DC bus modular parallel systems. First, the common DC bus parallel system often does not consider the parasitic capacitor, and compared with the non-common DC bus system, its system structure is relatively simple and does not consider the leakage current suppression problem. Second, although there are documents researching leakage current suppression and common-mode resonance suppression methods for single inverters, the single inverter model cannot describe the complex common-mode resonance between parallel inverters, especially considering the grid impedance, the common-mode resonance problem of non-common DC bus parallel inverter system is more serious, therefore, the existing method no longer has the ability to suppress leakage current and common-mode resonance in the non-common DC bus parallel inverter system, which seriously affects the performance, stability and safety of the system. SUMMARY

[0007] In order to solve the problems of the prior art, the present application provides a grid-connected control method and system for large-capacity inverter parallel system of photovoltaic station, which can effectively reduce the resonance current and leakage current, and improve the power quality and stability of the system.

[0008] To achieve the above object, the present application adopts the following technical solutions.

[0009] The first aspect of the present application provides a photovoltaic power station large-capacity inverter parallel system grid-connected control method.

[0010] A photovoltaic power station large-capacity inverter parallel system grid-connected control method comprises the following processes:

[0011] Obtaining a real-time value of common-mode current;

[0012] Comparing the real-time value of common-mode current with a given reference value, and feeding a difference value of comparison into a common-mode resonant current controller to obtain a real-time control variable; wherein, a common-mode voltage and a resonant peak are controlled by adjusting a redundant small vector action time, and the obtained control variable is combined to carry out common-mode resonance and leakage current suppression.

[0013] As an optional implementation manner, in the common-mode resonant current controller, a transfer function G iscm (s) from the common-mode excitation source to the inverter side common-mode current is:

[0014]

[0015] Wherein, G cm11 is an admittance transfer function of an inductive current loop, L2 is a second inductance in ILCL, C f is a capacitance in ILCL, Z t1 is a total equivalent impedance except the first inverter, Z g (s) is a grid impedance, and Z PV (s) is an equivalent impedance of a parasitic capacitance.

[0016] As an optional implementation manner, the differential-mode current adopts current closed-loop control, the three-phase output current of the inverter is changed through abc / dq, and then a difference is made with a reference value, and a modulation wave is obtained through a PI controller and dq / abc transformation.

[0017] As an optional implementation manner, the DC side voltage adopts a proportional controller, the upper and lower capacitor voltages are sampled, a difference is made, compared with a reference value, fed into a DC voltage controller, and injected into a modulation wave to realize midpoint voltage balance control.

[0018] As an optional implementation manner, the common-mode current output by each inverter is determined by its own common-mode excitation source and the common-mode excitation sources of other inverters in the parallel system, wherein:

[0019] The admittance transfer function of the inductive current loop excitation is:

[0020]

[0021] The admittance transfer function of the common-mode loop leakage current external excitation is:

[0022]

[0023] Wherein, Z t1 is the total equivalent impedance except the first inverter, Z g (s) is the grid impedance, Z PV (s) is the equivalent impedance of the parasitic capacitance, G op (s) is the open-loop gain of the common-mode excitation source, Z eq (s) is the open-loop equivalent output impedance, Z t2 is the total equivalent impedance except the i-th and the first inverters.

[0024] As an optional implementation, the redundant vector is adjusted by a proportional controller, and the DC side midpoint voltage balance control is performed on each inverter.

[0025] As an optional implementation, the admittance transfer functions of the internal excitation and the external excitation contain resonance peaks f1, f2 and f3:

[0026]

[0027] Wherein, L1 is the first inductance in the ILCL, L2 is the second inductance in the ILCL, C f is the capacitance in the ILCL, m is the number of inverters, C PV is the parasitic capacitance.

[0028] The second aspect of the present application provides a photovoltaic power station large-capacity inverter parallel system grid-connected control system.

[0029] A photovoltaic power station large-capacity inverter parallel system grid-connected control system comprises:

[0030] The data acquisition module is configured to acquire the real-time value of the common-mode current.

[0031] The common-mode resonance and leakage current control module is configured to compare the real-time value of the common-mode current with a given reference value, and send the difference of the comparison to a common-mode resonance current controller to obtain a real-time control variable; wherein the common-mode voltage and the resonance peak are controlled by adjusting the action time of the redundant small vector, and the common-mode resonance and leakage current suppression are performed in combination with the obtained control variable.

[0032] The third aspect of the present application provides a computer readable storage medium, which stores a program, and the program is executed by a processor to realize the steps of the photovoltaic power station large-capacity inverter parallel system grid-connected control method according to the first aspect of the present application.

[0033] The fourth aspect of the present application provides an electronic device, comprising a memory, a processor and a program stored in the memory and executable on the processor, wherein the processor implements the steps in the grid-connected control method of the photovoltaic station large-capacity inverter parallel system according to the first aspect of the present application when executing the program.

[0034] Compared with the prior art, the present application has the following beneficial effects:

[0035] 1. The photovoltaic station large-capacity inverter parallel system grid-connected control method and system according to the present application first establishes a common-mode equivalent model of the parallel inverter system without common DC bus, finds the generation mechanism of the common-mode current and the coupling relationship between different inverters, and lays a foundation for realizing leakage current and common-mode resonance current suppression.

[0036] 2. The photovoltaic station large-capacity inverter parallel system grid-connected control method and system according to the present application adopts an inverter-side common-mode current feedback method, without the need for additional current sensors, thereby reducing the suppression cost.

[0037] 3. The photovoltaic station large-capacity inverter parallel system grid-connected control method and system according to the present application can realize the suppression of leakage current and common-mode resonance current at the same time through the zero-impedance shunt method and the proposed common-mode resonance control method, without the need for additional complex modulation and additional hardware circuits, and the stability and power quality of the system are better.

[0038] 4. The photovoltaic station large-capacity inverter parallel system grid-connected control method and system according to the present application are not only suitable for parallel three-level T-type inverters, but also suitable for two-level, neutral-point-clamped, multi-level and other different converter topologies, and suitable for complex multi-inverter high-power application occasions, and have strong practicability and wide application scenarios.

[0039] The advantages of the additional aspects of the present application will be partially given in the following description, partially become obvious from the following description, or be understood through the practice of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0040] The accompanying drawings, which form a part of the specification, are included to provide a further understanding of the application and are incorporated herein for explanation by referring to the exemplary embodiments thereof.

[0041] Figures 1(a)-1(b) Fig. 1 and Fig. 2 are respectively a parallel three-level inverter topology structure diagram and a common-mode resonance current equivalent circuit diagram of zero-impedance shunt;

[0042] Figures 2(a)-2(b) Fig. 3 and Fig. 4 are respectively a simplified equivalent circuit diagram of internal excitation and external excitation of the common-mode loop;

[0043] Figure 3are the amplitude-frequency response curves of the internal excitation and the external excitation of the parallel system respectively;

[0044] Figures 4(a)-4(b) are the amplitude-frequency response curves of the admittance transfer function of the internal excitation and the external excitation of the parallel system respectively with the number of parallel inverters m;

[0045] Figure 5 is the control block diagram of the proposed common-mode current control strategy;

[0046] Figure 6 is the overall control block diagram of the three-level inverter with the ILCL filter (improved LCL filter);

[0047] Figures 7(a)-7(b) are the current waveforms without the control method of the application and with the proposed method respectively;

[0048] Figures 8(a)-8(c) are the current spectrums of a single inverter without the method, parallel inverters without the method and parallel inverters with the method respectively. DETAILED DESCRIPTION

[0049] The application will be further described below in conjunction with the accompanying drawings and embodiments.

[0050] It should be noted that the following detailed description is exemplary in nature and is intended to provide further description of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0051] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments in accordance with the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups thereof.

[0052] The embodiments in the application and the features in the embodiments can be combined with each other without conflict.

[0053] Embodiment 1:

[0054] As described in the background, to achieve leakage current suppression, the zero impedance shunt method is used in the parallel inverters without common DC bus, the traditional LCL filter is changed to the improved LCL filter, which can greatly reduce the leakage current without increasing the harmonics and losses, has low cost and is simple to implement; however, the zero impedance shunt method changes the common-mode loop of the system, thereby causing common-mode resonance, which can cause current oscillation and reduce the stability of the system.

[0055] Therefore, the embodiment 1 of the present application provides a leakage current and common-mode resonance suppression method for a non-common DC bus parallel inverter system, including the following processes:

[0056] S1: An equivalent model of the non-common DC bus parallel inverter system with an improved LCL filter is established, and the internal excitation and external excitation of the common-mode resonance current of the inverter itself and other inverters are considered, and the action mechanism of the internal excitation and external excitation on the common-mode current in different frequency ranges is found.

[0057] S2: Based on the above model and analysis, the coupling relationship between multiple inverters is further considered, and it is found that, unlike a single inverter system, the common-mode resonance peak of the system is affected by the number of inverters, and with the increase of the number of inverters, the leakage current resonance peak gradually shifts to low frequency, and the common-mode resonance characteristics also change. In addition, when the zero impedance shunt method is used to suppress the leakage current, the inherent resonance peak will be inevitably introduced, and all common-mode loops of the parallel system contain the inherent resonance peak, and all common-mode resonance currents flow through the common-mode loop of the improved LCL filter on the inverter side.

[0058] S3: The inverter-side common-mode current feedback method is adopted, the three-phase output currents of each inverter are added to obtain the operating state data of the common-mode resonance current, and then compared with the given reference value, and the difference is sent to the common-mode resonance current controller to obtain the real-time control variable, and the common-mode voltage and resonance peak are reduced by adjusting the action time of the redundant small vector, so as to realize the common-mode resonance and leakage current suppression.

[0059] S4: The control gain of the common-mode control loop is calculated, and the control parameter constraint condition of the common-mode resonance current controller is derived and calculated, which provides a reference for the controller design.

[0060] The non-common DC bus parallel inverter system includes a main circuit and an improved LCL filter, the main circuit is a T-type three-level modular parallel circuit, the DC sides of each inverter are independent of each other, the AC output sides of the inverters are directly connected in parallel after being filtered by the improved LCL filter, and are connected to the power grid through the grid impedance.

[0061] Fig. 1(a) is a non-common DC bus parallel T-type inverter topology, which includes a main circuit and an improved LCL filter, the main circuit is a T-type three-level modular parallel circuit, each phase contains four IGBTs, each IGBT is directly controlled by a controller in the control system through a conditioning circuit and a driving circuit; the DC sides of each inverter are independent of each other, the AC output sides of the inverters are directly connected in parallel after being filtered by the improved LCL filter, and are connected to the power grid through the grid impedance; the improved LCL filter is connected to the neutral point of the DC side capacitor at the common point of the LCL filter, forming a low-impedance high-frequency common-mode current circuit, thereby realizing the shunt and suppression of the common-mode current.

[0062] The control system comprises a protection circuit, a driving circuit, a sampling conditioning circuit and a control circuit, signals of the three-level inverter are connected to the controller through the sampling conditioning circuit, the controller communicates with the protection circuit to realize overcurrent and overvoltage protection, the controller is connected to the driving circuit, and PWM signals output by the controller are sent to control electrodes of the switching tubes through isolation amplification of the driving circuit to control turn-on and turn-off of the switching tubes.

[0063] The sampling conditioning circuit collects and conditions inverter-side output currents of the parallel inverters, DC-side capacitor voltages and three-phase grid voltages.

[0064] Fig. 1(b) is a common-mode equivalent circuit diagram, G op is an open-loop gain of the common-mode excitation source, Z eq , Z g , Z PV are respectively an open-loop equivalent output impedance, a grid impedance and a parasitic capacitance equivalent impedance.

[0065] The above parameters are represented as:

[0066]

[0067]

[0068] Z g (s) = L g s / 3 (3)

[0069] Z PV (s) = 1 / sC PV (4)

[0070] Due to the existence of the grid impedance, the coupling relationship between the parallel inverters changes the characteristics of the common-mode loop. According to Fig. 1(b) and the superposition theorem, a parallel common-mode model and an admittance transfer function of each inverter can be obtained as follows:

[0071]

[0072] In the formula, G cmii is an admittance transfer function of the common-mode excitation source u cmi of the i-th inverter to the common-mode current i cmi of the i-th inverter; G cmij (i≠j) is an admittance transfer function of the common-mode excitation source u cmj of the j-th inverter to the common-mode current i cmi of the i-th inverter.

[0073] The leakage current i cmi of the i-th inverter is represented as:

[0074]

[0075] Since the parameters of each inverter in a parallel system are usually the same, the admittance transfer function in equation (5) has the following mathematical relationship:

[0076]

[0077] Therefore, equation (5) can be further simplified to:

[0078]

[0079] As shown in equation (8), in a multi-parallel inverter system, the common-mode current of each inverter is coupled to all common-mode excitation sources. That is, the common-mode current output by each inverter is determined by its own common-mode excitation source and the common-mode excitation sources of other inverters in the parallel system, which are defined as internal excitations (G0, G1, G2, G3, G4, G5, G6, G7, G8, G9, G1, G2 ...3, G4, G5, G6, G7cmii ) and external incentives (G cmij ).

[0080] Figures 2(a) and 2(b) are simplified equivalent circuit diagrams of the internal and external excitations of the common-mode circuit. In Figure 2(a), the total equivalent impedance excluding the first inverter is Z. t1 , can be represented as:

[0081]

[0082] Figure 2(b) shows the equivalent model of the external excitation of the i-th inverter to the 1-th inverter. Zt2 is the total equivalent impedance excluding the i-th and 1-th inverters, expressed as...

[0083]

[0084] Therefore, the admittance transfer function of the internal and external excitations of the common-mode leakage current circuit is obtained as follows:

[0085]

[0086]

[0087] Figure 3 The amplitude-frequency response curves for internal and external excitations are given, with the number of parallel inverters set to m = 2. From equations (11) and (12), it can be seen that the characteristic equations for internal and external excitations are the same. Therefore, from... Figure 3 As can be seen, the amplitude-frequency response curves of the internal and external excitations have the same common-mode resonance peak. However, the amplitudes of the internal and external excitations differ across different frequency ranges. In the low-frequency range, the amplitude of the internal excitation is higher than that of the external excitation, indicating that the common-mode current is more significantly affected by the internal excitation in this frequency range. In the high-frequency range, the admittances of the two curves are essentially the same. At this point, the common-mode current is more easily affected by other inverters.

[0088] In addition, as shown in Figure 3 the admittance transfer functions of the internal and external excitations contain resonance peaks f1, f2 and f3, respectively, which are expressed as:

[0089]

[0090]

[0091]

[0092] As can be seen from equation (15), f3 of the parallel system is determined by the number of inverters. At the same time, for each inverter in the m-parallel inverter system, the grid impedance is equivalent to m times of that of a single inverter system. While the single inverter system has only two resonance peaks f1 and f3, and f3 is a fixed value, the common-mode resonance characteristics of the parallel system and the single inverter system are different and the resonance peaks to be suppressed are different, so the existing method cannot be directly applied.

[0093] Figures 4(a) and 4(b) are amplitude-frequency response curves of the admittance transfer functions of the internal and external excitations of the parallel system with the number of parallel inverters m varying. As can be seen, the resonance peaks f1 and f2 of the internal and external excitations are not affected by the number of parallel inverters. With the increase of the number of inverters m, the resonance peak f3 gradually shifts to low frequency. This is consistent with the foregoing theoretical analysis.

[0094] Since f1 has a low and constant frequency, the common-mode current can be suppressed by adjusting the amplitude of the resonance peak. Compared with f1, f2 and f3 are generally higher than the switching frequency and uncontrollable. More importantly, all the common-mode resonance currents are excited by the common-mode excitation source and flow through the improved LCL common-mode loop on the inverter side. Therefore, by suppressing the common-mode current on the inverter side, the leakage current can be effectively alleviated.

[0095] Figure 5 The control block diagram of the proposed common-mode current control strategy is shown in Figure 3, where G c (s) is the transfer function of the PI controller; K PWM is the gain of the PWM; G d (s) is the control delay of the digital processor. The transfer function G iscm from the common-mode excitation source to the common-mode current on the inverter side can be obtained from Figure 4 as:

[0096]

[0097] In order to effectively suppress the three common-mode resonance peaks f1, f2 and f3, the common-mode current on the inverter side is fed back into the common-mode controller, and the output control signal is obtained through the delay link G d (s) and the transfer function of the parallel system to obtain the actual output current.

[0098] Figure 6 The overall control block diagram of the three-level inverter with the parallel ILCL filter (improved LCL filter) realizes accurate tracking of the inverter output current, suppression of the common-mode resonance and control of the midpoint voltage through current closed-loop control, common-mode current control and midpoint voltage control.

[0099] Specifically, first, the inverter side current iABCm of each inverter is sampled and calculated, and in the differential-mode current loop, a PI controller is used to realize accurate tracking of the differential-mode reference current of each inverter; second, since the differential-mode circuit and the common-mode circuit are independent of each other, the proposed common-mode current controller is used to improve power quality. By suppressing the common-mode excitation source and the resonance peak, the leakage current and the common-mode circulating current are effectively suppressed; finally, since each inverter has an independent DC bus, the control of each inverter needs to be balanced for the midpoint voltage on the DC side, and a proportional controller is used to adjust the redundant vector to realize the control.

[0100] Fig. 7 (a)-(b) shows the current waveforms under the condition of not using the control method of the present application and using the proposed method, from top to bottom, the first inverter output current, the second inverter output current, the leakage current and the common-mode resonance circulating current, the reference currents of the two inverters are 15A and 25A respectively; as can be seen from Fig. 7 (a), when the proposed method of the present application is not used, the common-mode resonance will cause the current to oscillate violently, and the system will become unstable, and the leakage current and the common-mode resonance circulating current will increase continuously; as shown in Fig. 7 (b), when the method of the present application is used, the common-mode resonance is effectively suppressed, the current ripple is greatly reduced, the amplitude of the leakage current is reduced to 20mA, and the common-mode resonance circulating current is significantly weakened.

[0101] Fig. 8 (a)-(c) are current spectra under the condition of not using the method for a single inverter, not using the method for parallel inverters and using the method for parallel inverters respectively; as can be seen from Fig. 8 (a), in a single inverter system, resonance occurs at frequencies f1 and f3; but when the number of inverters increases, the f2 common-mode resonance peak is added in the parallel system in Fig. 8 (b), and the frequency of the f3 resonance peak shifts to the low frequency band, which is consistent with the theoretical analysis result. When the proposed method of the present application is used, as can be seen from Fig. 8 (c), the common-mode resonance peaks are suppressed, and in particular, the resonance peak f1 is almost 0, and the stability of the system is effectively improved.

[0102] Embodiment 2:

[0103] The second aspect of the present application provides a photovoltaic station large-capacity inverter parallel system grid-connected control system, comprising:

[0104] The data acquisition module is configured to acquire a real-time value of the common-mode current;

[0105] The common mode resonance and leakage current control module is configured to compare the real-time value of the common mode current with a given reference value, and send the difference of the comparison to a common mode resonance current controller to obtain a real-time control variable; wherein the common mode voltage and resonance peak are controlled by adjusting the redundant small vector action time, and the obtained control variable is combined to suppress the common mode resonance and leakage current.

[0106] The working method of the system is the same as the grid-connected control method of the photovoltaic power station large-capacity inverter parallel system provided in Embodiment 1, and will not be described here.

[0107] Embodiment 3

[0108] Embodiment 3 of the present application provides a computer readable storage medium having a program stored thereon, and the program is executed by a processor to implement the steps in the grid-connected control method of the photovoltaic power station large-capacity inverter parallel system as described in Embodiment 1 of the present application.

[0109] Embodiment 4

[0110] Embodiment 4 of the present application provides an electronic device, which includes a memory, a processor, and a program stored in the memory and executable on the processor, and the processor implements the steps in the grid-connected control method of the photovoltaic power station large-capacity inverter parallel system as described in Embodiment 1 of the present application when executing the program.

[0111] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of a hardware embodiment, a software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer usable storage media (including but not limited to disk storage and optical storage, etc.) containing computer usable program code.

[0112] The present application is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of flows and / or blocks in the flowcharts and / or block diagrams can be implemented by computer program instructions. These computer program instructions can be provided to a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices produce a device for implementing the functions specified in one or more flows in the flowcharts and / or one or more blocks in the block diagrams.

[0113] These computer program instructions can also be stored in a computer readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instructions which implement the function specified in the flowchart or flowsheets and / or block or blocks of the block diagrams.

[0114] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart or flowsheets and / or block or blocks of the block diagrams.

[0115] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by computer programs instructing related hardware, and the programs can be stored in a computer readable storage medium. When the programs are executed, the processes of the above-mentioned embodiments can be included. The storage medium can be a magnetic disc, an optical disc, a read-only memory (ROM) or a random access memory (RAM), etc.

[0116] The above only describes the preferred embodiments of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A grid-connected control method for a photovoltaic plant large-capacity inverter parallel system, characterized by: The DC side of each inverter is independent of each other, the AC output side of the inverter is directly connected in parallel after being filtered by an improved LCL filter, and is connected to the power grid through the impedance of the power grid; the improved LCL filter is connected to the neutral point of the DC side capacitor through the common point of the LCL filter, forming a low-impedance high-frequency common-mode current circuit; The method comprises the following steps: The three-phase output currents of each inverter are added to obtain real-time operation state data of the common-mode current; The real-time value of the common-mode current is compared with a given reference value, and the difference is fed into a common-mode resonant current controller to obtain a real-time control variable; wherein the common-mode voltage and the resonant peak are controlled by adjusting the action time of the redundant small vector, and the common-mode resonance and leakage current suppression are performed in combination with the obtained control variable; the common-mode current output by each inverter is determined by its own common-mode excitation source and the common-mode excitation sources of other inverters in the parallel system, wherein: The admittance transfer function of the internal excitation of the leakage current common-mode loop is: The admittance transfer function of the external excitation of the leakage current common-mode loop is: ； 2. The grid-connected control method of the photovoltaic station large-capacity inverter parallel system according to claim 1, wherein: where Z t1 Ztotal is the total equivalent impedance excluding the i-th inverter, Zgrid is the grid impedance, Zparasitic is the parasitic capacitance equivalent impedance, G op G(s) is the open loop gain of the common mode excitation source, Z eq Zopen(s) is the open loop equivalent output impedance, Z t2 Ztotal is the total equivalent impedance excluding the i-th and the first inverters; the admittance transfer function of the internal and external excitations contains the resonance peaks f1, f2 and f3: ， ， ； wherein L1 is a first inductance in the ILCL, L2 is a second inductance in the ILCL, C f is a capacitance in the ILCL, m is a number of inverters, C PV is a parasitic capacitance; the ILCL is an improved LCL filter. In the common-mode resonant current controller, the transfer function from the common-mode excitation source to the common-mode current on the inverter side is:

3. The grid-connected control method of the photovoltaic station large-capacity inverter parallel system according to claim 1, wherein: wherein, Y is the admittance transfer function for common mode loop inrush excitation, L2 is a second inductance in the ILCL, C is a capacitance in the ILCL, t1 Ztot is the total equivalent impedance except for the first inverter, Zgrid is the grid impedance, Zpar is the parasitic capacitance equivalent impedance. The differential-mode current adopts current closed-loop control, the three-phase output currents of the inverter are changed through abc / dq, and the difference between the changed three-phase output currents and a reference value is obtained; the difference is input into a PI controller and a dq / abc converter to obtain a modulation wave.

4. The grid-connected control method of the photovoltaic station large-capacity inverter parallel system according to claim 1, wherein: The DC side voltage adopts a proportional controller, the difference between the upper and lower capacitor voltages is sampled, and the difference is compared with a reference value and fed into a DC voltage controller to be injected into a modulation wave, so as to realize midpoint voltage balance control.

5. The grid-connected control method of the photovoltaic station large-capacity inverter parallel system according to claim 1, wherein: The redundant vector is adjusted through a proportional controller to perform DC side midpoint voltage balance control on each inverter. The DC side of each inverter is independent of each other, the AC output side of the inverter is directly connected in parallel after being filtered by an improved LCL filter, and is connected to the power grid through the impedance of the power grid; the improved LCL filter is connected to the neutral point of the DC side capacitor through the common point of the LCL filter, forming a low-impedance high-frequency common-mode current circuit; 6. A grid-connected control system for a photovoltaic plant large capacity inverter parallel system, characterized in that: The method comprises the following steps: The three-phase output currents of each inverter are added to obtain real-time operation state data of the common-mode current; The real-time value of the common-mode current is compared with a given reference value, and the difference is fed into a common-mode resonant current controller to obtain a real-time control variable; wherein the common-mode voltage and the resonant peak are controlled by adjusting the action time of the redundant small vector, and the common-mode resonance and leakage current suppression are performed in combination with the obtained control variable; the common-mode current output by each inverter is determined by its own common-mode excitation source and the common-mode excitation sources of other inverters in the parallel system, wherein: The admittance transfer function of the internal excitation of the leakage current common-mode loop is: ； The admittance transfer function of the common-mode loop outside excitation of the leakage current is: where Z t1 Ztotal is the total equivalent impedance excluding the 1st inverter, Zgrid is the grid impedance, Zparasitic is the parasitic capacitance equivalent impedance, G op G(s) is the open loop gain of the common mode excitation source, Z eq Zopen is the open loop equivalent output impedance, Z t2 Ztotal is the total equivalent impedance excluding the i-th and 1st inverters; the admittance transfer function of the internal and external excitations contains the resonance peaks f1, f2 and f3: ， ， ； wherein, L 1 is a first inductance in the ILCL, L 2 is a second inductance in the ILCL, f C is a capacitance in the ILCL, m is a number of inverters, PV C is a parasitic capacitance; the ILCL is an improved LCL filter.

7. A computer-readable storage medium having stored thereon a program, characterized in that, The program, when executed by a processor, implements the steps in the grid-connected control method of the large-capacity inverter and grid-connected system of a photovoltaic power station according to any one of claims 1-5.

8. An electronic device comprising a memory, a processor, and a program stored in the memory and executable on the processor, characterized by The program, when executed by a processor, implements the steps in the grid-connected control method of the large-capacity inverter and grid-connected system of a photovoltaic power station according to any one of claims 1-5.

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