Photovoltaic grid-tied power generation system and grid-tied control method for inverter

Abstract

A photovoltaic grid-tied power generation system and a grid-tied control method for an inverter. The method comprises: determining whether voltages to ground of positive and negative electrodes of an inverter at a photovoltaic connection side satisfy a preset balance condition (S101); if the voltages to ground of the positive and negative electrodes at the photovoltaic connection side does not satisfy the preset balance condition, determining the electrode of which the voltage to ground has a greater absolute value (S102); then controlling the equivalent impedance to ground of the corresponding electrode of a direct-current bus in the inverter to decrease (S103), thus causing the midpoint voltage of the direct-current bus to approach zero, and reducing the voltage difference between two ends of a contact of an alternating-current grid-tied relay while implementing grid-tied connection of the inverter; and then performing grid-tied control (S104), i.e., reducing a surge current when the alternating-current grid-tied relay is pulled in, and reducing the probability of ablation of the contact of the relay.

Description

Photovoltaic grid-connected power generation system and grid-connected control method of inverter

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on September 14, 2023, with application number 202311189448.8 and invention name “A photovoltaic grid-connected power generation system and grid-connected control method of inverter”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of photovoltaic grid-connected power generation, and in particular to a photovoltaic grid-connected power generation system and a grid-connected control method of an inverter. Background Art

[0003] In a grid-connected photovoltaic power generation system, the inverter typically receives DC power from multiple photovoltaic strings through its DC side, aggregates the power, and then inverts it uniformly. The resulting AC power is then connected to the grid via an AC grid-connected relay. In actual applications, two AC grid-connected relays are usually connected in series due to safety regulations and closed sequentially to complete the inverter's grid connection.

[0004] With the increase in photovoltaic application scenarios, changes in construction quality and aging of products, the insulation of the DC cable between the photovoltaic string and the inverter is easily damaged. When the negative or positive pole of the photovoltaic string decreases in resistance to ground due to weather or insulation failure, the DC side voltage of the inverter to ground will change, and a voltage difference will be formed at both ends of the AC grid-connected relay. When the AC grid-connected relay is energized, a large surge current will be generated due to this voltage difference, burning the relay contacts.

[0005] Therefore, there is an urgent need for a solution to protect the AC grid-connected relay when the inverter is connected to the grid.

[0006] Summary of the Invention

[0007] The present application provides a photovoltaic grid-connected power generation system and a grid-connected control method for an inverter, so as to reduce the voltage difference across the contacts of an AC grid-connected relay in the prior art when the inverter is connected to the grid, thereby reducing the probability of relay contact erosion.

[0008] To achieve the above objectives, this application provides the following technical solutions:

[0009] In a first aspect, the present application provides a grid-connected control method for an inverter, wherein the inverter is connected to a grid-connected point via a corresponding grid-connected relay branch, and the grid-connected control method includes:

[0010] Determining whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition;

[0011] If the voltage between the positive and negative electrodes of the photovoltaic access side and the ground does not meet the preset balance condition, determining the electrode with the larger absolute value of the voltage between the positive and negative electrodes and the ground;

[0012] Controlling the corresponding pole equivalent impedance to ground of the DC bus in the inverter to decrease;

[0013] Conduct grid-connected control.

[0014] Optionally, controlling the corresponding pole equivalent impedance to ground of the DC bus in the inverter to decrease includes:

[0015] A grounded resistor is connected to the corresponding pole of the DC bus.

[0016] Optionally, when the voltages of the positive and negative electrodes of the photovoltaic access side to the ground do not meet the preset balance condition, determining the electrode with a larger absolute value of the voltage to the ground includes:

[0017] Comparing the absolute value of the ground voltage of any pole on the photovoltaic access side of the inverter with the half-bus voltage of the DC bus;

[0018] If the absolute value of the voltage to ground of the pole on the photovoltaic access side of the inverter is greater than the half-bus voltage, the pole with the larger absolute value of the voltage to ground is determined; otherwise, the other pole with the larger absolute value of the voltage to ground is determined.

[0019] Optionally, comparing the absolute value of the ground voltage of any pole of the photovoltaic access side of the inverter with the half-bus voltage of the DC bus includes:

[0020] Comparing the absolute value of the voltage between the negative electrode and ground of the photovoltaic access side of the inverter with the half bus voltage; or,

[0021] The absolute value of the voltage between the positive electrode and ground of the photovoltaic access side of the inverter is compared with the half bus voltage.

[0022] Optionally, determining whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition includes:

[0023] Determine whether a deviation between an absolute value of a voltage to ground of any pole on the photovoltaic access side of the inverter and a half-bus voltage of the DC bus is greater than a preset deviation;

[0024] If the deviation is greater than the preset deviation, it is determined that the voltages of the positive and negative electrodes to the ground on the photovoltaic access side do not meet the preset balance condition; otherwise, it is determined that the voltages of the positive and negative electrodes to the ground on the photovoltaic access side meet the preset balance condition.

[0025] Optionally, determining whether a deviation between an absolute value of a voltage to ground at any pole of the photovoltaic access side of the inverter and a half-bus voltage of the DC bus is greater than a preset deviation includes:

[0026] Determine whether the deviation between the absolute value of the voltage to ground of the negative electrode of the photovoltaic access side of the inverter and the half-bus voltage is greater than a preset deviation; or

[0027] It is determined whether a deviation between an absolute value of a positive pole-to-ground voltage on the photovoltaic access side of the inverter and the half-bus voltage is greater than a preset deviation.

[0028] Optionally, determining whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition includes:

[0029] Determining whether a common-mode voltage difference between two sides of the grid-connected relay branch of the inverter connected to the grid-connected point is greater than a preset voltage difference;

[0030] If the common-mode voltage difference is greater than the preset voltage difference, it is determined that the voltage between the positive and negative electrodes of the photovoltaic access side and the ground does not meet the preset balance condition; otherwise, it is determined that the voltage between the positive and negative electrodes of the photovoltaic access side and the ground meets the preset balance condition.

[0031] Optionally, determining whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition includes:

[0032] Determine whether the difference between the absolute values ​​of the voltages to ground of the positive and negative electrodes of the photovoltaic access side of the inverter is greater than a preset difference;

[0033] If the difference is greater than the preset difference, it is determined that the voltages of the positive and negative poles to the ground on the photovoltaic access side do not meet the preset balance condition; otherwise, it is determined that the voltages of the positive and negative poles to the ground on the photovoltaic access side meet the preset balance condition.

[0034] Optionally, after determining whether the voltages of the positive and negative electrodes of the photovoltaic access side of the inverter to the ground meet a preset balance condition, if the voltages of the positive and negative electrodes of the photovoltaic access side to the ground do not meet the preset balance condition, the method further includes: recording an evaluation parameter of whether the voltages of the positive and negative electrodes of the photovoltaic access side to the ground meet the preset balance condition;

[0035] After controlling the reduction of the equivalent impedance to ground of the corresponding pole of the DC bus in the inverter, the method further includes:

[0036] Determining whether the evaluation parameter changes in a direction that satisfies the preset balance condition;

[0037] If so, the step of performing grid-connected control is executed; otherwise, the equivalent impedance to ground of the corresponding pole of the DC bus is restored first, and then the step of performing grid-connected control is executed.

[0038] Optionally, after determining whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition, the method further includes:

[0039] If the voltage between the positive and negative electrodes of the photovoltaic access side and the ground meets the preset balance condition, the step of performing grid connection control is executed.

[0040] The second aspect of the present application provides a photovoltaic grid-connected power generation system, comprising: at least one inverter and its grid-connected relay branch; wherein,

[0041] The photovoltaic access side of the inverter is connected to at least one photovoltaic string;

[0042] The inverter is connected to the grid connection point via the corresponding grid connection relay branch;

[0043] The positive and negative poles of the DC bus in the inverter are each grounded through a set of switches and resistors connected in series;

[0044] The controller in the inverter performs grid-connected control using the grid-connected control method for the inverter as described in any one of the first aspects above;

[0045] The switch is controlled by the controller.

[0046] Optionally, the resistance of the resistor satisfies the requirement that, when the resistor is connected, the difference in equivalent impedance to ground between the two poles of the DC bus is less than a preset threshold.

[0047] Optionally, when the number of the inverters is greater than 1, when each of the controllers executes the grid-connected control method, it determines whether the voltage between the positive and negative poles of the photovoltaic access side of the corresponding inverter and the ground meets the preset balance condition by judging whether the common-mode voltage difference between the two sides of the corresponding grid-connected relay branch is greater than the preset voltage difference.

[0048] Optionally, the switch and the resistor are potential induced decay PID power supply components in the inverter.

[0049] Optionally, the grid-connected relay branch includes: at least two AC relays connected in series.

[0050] Optionally, the main circuit of the inverter includes an inverter unit;

[0051] The DC side of the inverter unit is connected to the photovoltaic access side of the inverter through the DC bus;

[0052] A bus capacitor is connected between the positive and negative poles of the DC bus;

[0053] The AC side of the inverter unit is connected to the AC side of the inverter.

[0054] Optionally, the main circuit of the inverter further includes: at least one boost unit;

[0055] One end of the boost unit serves as a corresponding end on the photovoltaic access side of the inverter, and is used to connect to a corresponding photovoltaic string;

[0056] The other end of the boost unit is connected to the DC side of the inverter unit through the DC bus.

[0057] The grid-connected control method for the inverter provided in the present application determines whether the voltages of the positive and negative poles to the ground on the photovoltaic access side of the inverter meet a preset balance condition. If the voltages of the positive and negative poles to the ground on the photovoltaic access side do not meet the preset balance condition, the method first determines the pole with the larger absolute value of the voltage to the ground. Then, the method controls the corresponding pole of the DC bus in the inverter to reduce the equivalent impedance to the ground, thereby causing the midpoint voltage of the DC bus to approach zero, thereby reducing the voltage difference across the contacts of the AC grid-connected relay when the inverter is connected to the grid. Grid-connected control is then performed to reduce the inrush current when the AC grid-connected relay is energized, thereby reducing the probability of relay contact erosion. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings to be used in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0059] FIG1 is a schematic structural diagram of a photovoltaic grid-connected power generation system provided by the prior art;

[0060] FIG2 is a schematic diagram of one phase of a grid-connected relay branch of an inverter provided by the prior art;

[0061] FIG3 is a flow chart of a grid-connected control method for an inverter provided in an embodiment of the present application;

[0062] FIG4 is a schematic diagram of the equivalent structure of a photovoltaic grid-connected power generation system provided in an embodiment of the present application;

[0063] FIG5 is another equivalent structural diagram of a photovoltaic grid-connected power generation system provided in an embodiment of the present application;

[0064] FIG6 is another flow chart of the grid-connected control method of the inverter provided in an embodiment of the present application;

[0065] FIG7 is a schematic diagram of a complete flow chart of a grid-connected control method for an inverter provided in an embodiment of the present application;

[0066] FIG8 is another complete flow chart of the grid-connected control method for an inverter provided in an embodiment of the present application;

[0067] FIG9 is a schematic structural diagram of a photovoltaic grid-connected power generation system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0068] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0069] In this application, the terms "comprises," "comprising," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not preclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0070] An inverter typically includes an inverter unit and at least one boost unit (Figure 1 illustrates a boost unit as an example). Due to safety regulations, as shown in Figure 1, the AC side of the inverter in the prior art utilizes two AC grid-connected relays connected in series, which are closed sequentially to connect the inverter to the grid. In a three-phase grid-connected inverter system, both AC grid-connected relays are three-phase relays, one of which includes relays K1 to K3, respectively, located on each phase line, and the other includes relays K4 to K6, respectively, located on each phase line. When the ground impedance of the photovoltaic strings connected to the photovoltaic access side of the inverter is abnormal, the AC grid-connected relays, which close after the inverter is connected to the grid, will generate a large common-mode voltage difference across their main contacts, generating a large inrush current at the moment of closure.

[0071] Figure 2 shows one phase of the inverter's AC side as an example, where K1 and K4 are the relays on the phase line of the two AC grid-connected relays, a11 and a12 are the main contact terminals of relay K1, b41 and b42 are the main contact terminals of relay K4, R5 is the ground impedance of the phase on the inverter's AC side, R1 is the ground impedance of the PV+ positive pole of the PV string, and R2 is the ground impedance of the PV- negative pole of the PV string. The following formula can be used to calculate the common-mode voltage u at the DC bus midpoint busn on the DC side of the inverter unit: busn :

[0072] Among them, u pv is the voltage of the photovoltaic string PV, that is, the voltage difference between the positive pole PV+ and the negative pole PV- of the photovoltaic string; u busIt is the bus voltage, that is, the voltage difference between the DC bus positive pole bus+ and the DC bus negative pole bus-.

[0073] After the inverter starts and relay K4 is closed and before relay K1 is closed, the common mode voltage u11 to ground is a Approximately equal to the bus midpoint voltage u busn The inverter voltage u of the inverter unit ac逆 The sum of the inverter voltage u before grid connection ac逆 It is approximately equal to the preset grid voltage u on the side of relay K4 away from the inverter ac网 , that is: u ac网 ≈u ac逆

[0074] Therefore, the common-mode voltage difference ΔV between the main contacts a11 and a12 of relay K1 is:

[0075] Because the inverter has: u before being connected to the grid pv ≈u bus , when the impedance of the photovoltaic string PV to ground is normal, that is, R1=R2, ΔV≈0; when the photovoltaic string PV is grounded, resulting in abnormal impedance of its positive pole PV+ or its negative pole PV- to ground, that is, R1<<R2, or R2<<R1, ΔV≈±0.5u bus At this time, there is a large voltage difference between the two ends of the main contacts of relay K1. When relay K1 is closed, the surge current will cause erosion of the main contacts.

[0076] Therefore, the present application provides a grid-connected control method for an inverter to reduce the voltage difference across the contacts of an AC grid-connected relay in the prior art when the inverter is connected to the grid, thereby reducing the probability of relay contact erosion.

[0077] Referring to FIG3 , the grid-connected control method of the inverter includes:

[0078] S101: Determine whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition.

[0079] The photovoltaic access side refers to the side of the inverter connected to the photovoltaic string, such as the input end of the boost unit shown in FIG1 .

[0080] This preset balance condition mainly characterizes the condition for the voltage balance between the positive and negative poles of the photovoltaic access side (i.e., PV+ and PV- shown in Figures 1 and 2) to the ground. When the photovoltaic string PV is grounded, resulting in abnormal impedance to the ground of its positive pole PV+ or its negative pole PV-, the voltage between the positive and negative poles of the photovoltaic access side and the ground will not meet the preset balance condition.

[0081] It is worth noting that Figures 1 and 2 are both shown as an example in which the photovoltaic access side includes only one positive and negative port. In actual applications, when the inverter includes multiple boost units, the input ends of each boost unit serve as the positive and negative ports of the photovoltaic input side respectively. If any of the positive and negative ports is grounded, the impedance to ground of the photovoltaic input side will be abnormal and the preset balance condition cannot be met.

[0082] If the voltage between the positive and negative electrodes of the photovoltaic access side and the ground does not meet the preset balance condition, S102 is executed.

[0083] S102: Determine the electrode with the larger absolute value of the voltage to ground.

[0084] Because the inverter has: u before being connected to the grid pv ≈u bus , when the impedance of the photovoltaic string PV to ground shown in Figure 1 or Figure 2 is normal, that is, R1=R2, |PV+voltage to ground|=|PV-voltage to ground|≈0.5u bus When the impedance of the photovoltaic string PV to ground is abnormal, for example, the positive pole PV+ of the photovoltaic string is grounded and R1<<R2, there is |PV+ voltage to ground|=0<0.5u bus , |PV-ground voltage|≈u bus >0.5u bus For example, when the negative pole PV- of the PV string is grounded and R2<<R1, |PV+ voltage to ground|≈u bus >0.5u bus , |PV-ground voltage|=0<0.5u bus .

[0085] Therefore, the process of S102 may specifically include:

[0086] (1) Compare the absolute value of the ground voltage of any pole on the photovoltaic access side of the inverter with the half-bus voltage of the DC bus.

[0087] For example, the specific method is to compare the absolute value of the negative pole-to-ground voltage of the photovoltaic access side with the half bus voltage, that is, to compare the |PV-to-ground voltage| shown in Figure 1 or Figure 2 with 0.5u bus .

[0088] (2) If the absolute value of the voltage to ground of the pole on the photovoltaic access side is greater than the half-bus voltage, the pole with the larger absolute value of the voltage to ground is determined; otherwise, the other pole with the larger absolute value of the voltage to ground is determined.

[0089] For example, if |PV-to-ground voltage|>0.5u bus , then the negative pole of the photovoltaic access side is determined to be the pole with the larger absolute value of the ground voltage; otherwise, that is, |PV-ground voltage|<0.5u bus, then the positive pole of the photovoltaic access side is determined to be the pole with the larger absolute value of the ground voltage.

[0090] In practical applications, considering the convenience of voltage acquisition and calculation, the above method is preferred, that is, comparing the |PV-to-ground voltage| shown in Figure 1 or Figure 2 with 0.5u bus To achieve S102, but it does not exclude the possibility of comparing |PV+ voltage to ground| shown in FIG1 or FIG2 with 0.5u bus To implement S102, this is only an example and is within the scope of protection of this application.

[0091] S103, controlling the equivalent impedance of the corresponding pole of the DC bus in the inverter to decrease.

[0092] In practical applications, S103 may specifically include: connecting a grounded resistor to a corresponding pole of the DC bus.

[0093] Still taking one phase of the inverter's AC side as an example, assuming that the relay that closes last is K1, the difference from FIG2 is that, to reduce the large voltage difference across the main contacts of relay K1, in this embodiment, when a ground fault occurs on the PV access side of the inverter, a corresponding grounding resistor is connected to one pole of the DC bus, such as R4 shown in FIG4 or R3 shown in FIG5.

[0094] Refer to Figure 4. Assume that the positive pole PV+ of the photovoltaic access side is grounded. At this time, R1<<R2, and the negative pole bus- of the DC bus is connected to a grounding resistor R4. At this time, recalculate the common mode voltage difference ΔV between the main contacts a11 and a12 of relay K1 R ,have:

[0095] Among them, R 24 The negative electrode equivalent impedance of the DC bus to ground when other resistances such as the sampling resistor are ignored, and Therefore, facing the situation of R1<<R2 at this time, compared with the situation in Figure 2, the ΔV in the case of Figure 4 R in Closer to 0.5, making ΔV R <ΔV.

[0096] It can be seen that the voltage difference across the main contacts of the relay K1 is reduced, thereby reducing the surge current when the relay K1 is closed and reducing the probability of burning of the main contacts of the relay K1.

[0097] In practical applications, the voltage difference ΔV between the main contacts a11 and a12 of relay K1 can be accurately adjusted by reasonably selecting the resistance value of resistor R4. R, so that it can be effectively reduced, so as to more effectively suppress the surge current when the relay K1 is closed; preferably, the resistance value of the resistor R4 can be selected to make the equivalent impedance of the positive and negative poles of the DC bus to the ground close, for example, the difference between the two can be less than a preset threshold, especially, R 24 =R1, then ΔV R ≈ 0. The situation of the other two phases on the AC side of the inverter is the same and will not be described in detail.

[0098] Refer to Figure 5. Assume that the negative pole PV- of the photovoltaic access side is grounded. At this time, R2<<R1, and the positive pole bus+ of the DC bus is connected to a grounding resistor R3. At this time, recalculate the voltage ΔV between the main contacts a11 and a12 of relay K1. R ,have:

[0099] Among them, R 13 The positive equivalent impedance of the DC bus to ground when other resistances such as the sampling resistor are ignored, and Therefore, facing the situation of R2<<R1, compared with the situation in Figure 2, the ΔV in the case of Figure 4 R in Closer to 0.5, making ΔV R <ΔV.

[0100] It can be seen that the voltage difference across the main contacts of the relay K1 is reduced, thereby reducing the surge current when the relay K1 is closed and reducing the probability of burning of the main contacts of the relay K1.

[0101] In practical applications, the voltage difference ΔV between the main contacts a11 and a12 of relay K1 can be accurately adjusted by reasonably selecting the resistance value of resistor R4. R , so that it can be effectively reduced, so as to more effectively suppress the surge current when the relay K1 is closed; preferably, the resistance value of the resistor R3 can be selected to make the equivalent impedance of the positive and negative poles of the DC bus to the ground close, for example, the difference between the two can be less than a preset threshold, especially, R 13 =R2, then ΔV R ≈ 0. The situation of the other two phases on the AC side of the inverter is the same and will not be described in detail.

[0102] S104: Perform grid connection control.

[0103] The grid-connected control process may include: issuing a start command, and closing two AC grid-connected relays in sequence; the details of this process can be found in the prior art and will not be described in detail here.

[0104] The grid-connected control method for the inverter provided in this embodiment, before performing grid-connected control, uses the above-mentioned principle to make the equivalent impedance of the positive and negative poles of the DC bus to ground close, that is, to cause the midpoint voltage of the DC bus to approach zero, thereby reducing the voltage difference across the contacts of the AC grid-connected relay when the inverter is connected to the grid. Grid-connected control is then performed to reduce the inrush current when the AC grid-connected relay is energized, thereby reducing the probability of relay contact erosion.

[0105] Based on the previous embodiment, this embodiment provides some optional examples for the specific implementation process of S101 in the grid connection control method, such as:

[0106] The S101 may include: determining whether the deviation between the absolute value of the ground voltage of any pole of the photovoltaic access side of the inverter and the half bus voltage of the DC bus is greater than a preset deviation; for example, it may be determined whether the difference between |PV-to-ground voltage| and 0.5u bus The deviation between |PV+ voltage to ground| and 0.5u bus Is the deviation between them greater than the preset deviation U dev If the deviation is greater than the preset deviation U dev , it is determined that the voltage between the positive and negative poles of the photovoltaic access side and the ground does not meet the preset balance condition; otherwise, it is determined that the voltage between the positive and negative poles of the photovoltaic access side and the ground meets the preset balance condition.

[0107] Alternatively, the step S101 may also include: determining whether the common mode voltage difference between the two sides of the grid-connected relay branch of the inverter connected to the grid-connected point is greater than a preset voltage difference; for example, determining whether the above ΔV is greater than the preset voltage difference U base If the common mode pressure difference is greater than the preset pressure difference U base , it is determined that the voltage between the positive and negative poles of the photovoltaic access side and the ground does not meet the preset balance condition; otherwise, it is determined that the voltage between the positive and negative poles of the photovoltaic access side and the ground meets the preset balance condition.

[0108] Alternatively, S101 may further include determining whether the difference between the absolute values ​​of the positive and negative ground voltages on the photovoltaic access side of the inverter is greater than a preset difference. For example, the difference between |PV+ voltage to ground| and |PV- voltage to ground| may be determined to be greater than a preset difference. If the difference is greater than the preset difference, it is determined that the positive and negative ground voltages on the photovoltaic access side do not meet a preset balance condition. Otherwise, it is determined that the positive and negative ground voltages on the photovoltaic access side meet the preset balance condition.

[0109] In practical applications, the present invention is not limited to the above three implementation forms. Any method that can determine whether the voltage between the positive and negative poles of the photovoltaic access side and the ground is balanced can be adopted, depending on the specific application environment, and is within the scope of protection of this application.

[0110] In addition, after S101, the grid-connected control method further includes: if the voltage between the positive and negative electrodes of the photovoltaic access side and the ground meets the preset balance condition, there is no need to adjust the common-mode voltage difference between the two sides of the grid-connected relay branch, and S104 can be directly executed.

[0111] On the basis of the above embodiment, the grid connection control method is further provided as shown in FIG6 . After S101 , if the voltage between the positive and negative electrodes of the photovoltaic access side and the ground does not meet the preset balance condition, the method further includes:

[0112] S201. Recording evaluation parameters for determining whether the voltages between the positive and negative electrodes of the photovoltaic access side and the ground meet preset balance conditions.

[0113] The evaluation parameter may refer to any of the following used in S101: the deviation between the absolute value of the voltage to ground of any pole on the photovoltaic access side and the half-bus voltage, the common-mode voltage difference between the two sides of the grid-connected relay branch, and the difference between the absolute values ​​of the voltage to ground of the positive and negative poles on the photovoltaic access side; it may be determined according to the specific judgment process used in S101 and is not limited here.

[0114] Moreover, after S103, it also includes:

[0115] S202: Determine whether the evaluation parameter changes in a direction that satisfies a preset balance condition.

[0116] If the evaluation parameter changes in a direction that satisfies the preset balance condition, for example, the deviation between the absolute value of the voltage to ground of any pole on the photovoltaic access side and the half-bus voltage becomes smaller, the common-mode voltage difference between the two sides of the grid-connected relay branch becomes smaller, or the difference between the absolute values ​​of the voltage to ground of the positive and negative poles on the photovoltaic access side becomes smaller, then execute S104; otherwise, execute S203 first, and then execute S104.

[0117] S203. Restore the equivalent impedance of the corresponding pole of the DC bus to the ground.

[0118] Specifically, S203 may be to disconnect the resistor connected in S103.

[0119] R4 shown in Figure 4 and R3 shown in Figure 5 are devices for adjusting the voltage between the corresponding poles and the ground on the photovoltaic access side. Whether they are connected is independent of the grid-connected procedure. From the above embodiment, it can be seen that connecting the corresponding pole grounding resistor will make the positive and negative poles of the photovoltaic access side tend to the half-bus voltage. At this time, the common-mode voltage difference between the two sides of the grid-connected relay branch is detected. If it is reduced, the grid can be directly connected. However, there are differences in the on-site environment. It is possible that the deviation between the positive and negative poles of the photovoltaic access side and the half-bus voltage after adjustment is greater than before adjustment. In this case, it is necessary to first cut out the connected resistor and then perform grid connection in the scenario before adjustment.

[0120] It is worth noting that the two AC relays in the grid-connected relay branch can be controlled to energize sequentially when S104 is executed. Alternatively, after S101, if the voltage between the positive and negative electrodes of the photovoltaic access side and the ground does not meet the preset balance condition, one of the AC relays in the grid-connected relay branch can be energized first, and only the other AC relay can be energized when S104 is executed. The specific application environment can be determined and all are within the scope of protection of this application.

[0121] Two complete examples of the grid-connected control method are given below:

[0122] (1) A solution to adjust the voltage between the negative pole and the ground on the photovoltaic access side.

[0123] This solution reduces the common-mode voltage difference across the main contacts of the AC relay, which is closed last before the inverter is connected to the grid, by adjusting the voltage between the negative electrode of the photovoltaic access side and the ground. As shown in Figure 7, the specific process of software-controlled inverter startup is as follows:

[0124] First, detect the voltage between the negative pole of the photovoltaic access side and the ground |PV-voltage to ground| and the half bus voltage 0.5u bus Whether the deviation between exceeds the preset deviation U dev .

[0125] a. Does not exceed the preset deviation U dev , then the start command is issued and the inverter is connected to the grid.

[0126] b. Exceeding the preset deviation U dev , then an AC relay in the grid relay branch is attracted, and the voltage between the negative pole of the photovoltaic access side and the ground|PV-voltage between the ground|and the half bus voltage 0.5u are recorded. bus The deviation between pv-bus .

[0127] Then, if |PV-to-ground voltage| < 0.5u bus ,but:

[0128] a. Connect resistor R3, delay 1s to read the voltage between the negative pole of the photovoltaic access side and the ground | PV- to ground | and the half bus voltage 0.5u bus The deviation between npv-bus .

[0129] b.If U npv-bus pv-bus , then grid connection control is carried out.

[0130] c. If U npv-bus >U pv-bus , then disconnect the connected resistor R3 and then perform grid-connected control.

[0131] If |PV-to-ground voltage|>0.5u​bus ,but:

[0132] a. Connect resistor R4, delay 1s to read the voltage between the negative pole of the photovoltaic access side and the ground | PV- to ground | and the half bus voltage 0.5u bus The deviation between ppv-bus .

[0133] b.If U ppv-bus pv-bus , then grid connection control is carried out.

[0134] c. If U ppv-bus >U pv-bus , then disconnect the connected resistor R4 and then perform grid-connected control.

[0135] (2) Scheme for adjusting common-mode voltage difference.

[0136] Since solution (1) is only applicable to the grid-connected scenario of a single inverter, when multiple inverters are connected to the grid at the same time, there is a situation where the grid-connected time is inconsistent. In this scenario, some inverters are connected to the grid, and some inverters are not connected to the grid. Then the grid neutral point to ground voltage u grid To change, the grid neutral point to ground voltage u grid Taking this into consideration, the voltage ΔV′ between the two ends of the main contacts a11 and a12 of the relay K1 that closes later is:

[0137] Since there is a voltage u between the neutral point of the grid and the ground grid The original evaluation parameter used to determine the connection of the PV grid was the difference between the negative pole-to-ground voltage and the half-bus voltage on the PV connection side. This would result in errors and could increase the common-mode voltage difference after adjustment. Therefore, this solution directly uses the common-mode voltage difference on both sides of the grid-connected relay branch before the inverter is connected to the grid, which is more accurate. As shown in Figure 8, the conditions for software-controlled inverter startup are as follows:

[0138] First, check whether the common mode voltage difference on both sides of the grid-connected relay branch exceeds the preset voltage difference U base .

[0139] a. Does not exceed the preset pressure difference U base , then the start command is issued and the inverter is connected to the grid.

[0140] b. Exceeding the preset pressure difference U base , then an AC relay in the grid-connected relay branch is attracted, and the common mode voltage difference U on both sides of the grid-connected relay branch is recorded at this time commode1 .

[0141] Then, if |PV-to-ground voltage| < 0.5u bus ,but:

[0142] ​a. Connect resistor R3 and read the common mode voltage difference U on both sides of the grid-connected relay branch. commode2 , wait for data collection to stabilize.

[0143] b.If U commode2 commode1 , then grid connection control is carried out.

[0144] c. If U commode2 >U commode1 , then disconnect the connected resistor R3 and then perform grid-connected control.

[0145] If |PV-to-ground voltage|>0.5u bus ,but:

[0146] a. Connect resistor R4 and read the common mode voltage difference U on both sides of the grid-connected relay branch. commode2 , wait for data collection to stabilize.

[0147] b.If U commode2 commode1 , then grid connection control is carried out.

[0148] c. If U commode2 >U commode1 , then disconnect the connected resistor R4 and then perform grid-connected control.

[0149] The above two solutions are only optional examples and are not limited thereto. When executing the grid-connected control method, appropriate evaluation parameters can be selected according to the actual application environment, all of which are within the scope of protection of this application.

[0150] Due to the diversity of on-site power grid environments, the ground impedances (such as R1 and R2 shown in Figures 2, 4, and 5) vary in size. Before grid connection, this grid-connected control method detects any evaluation parameters, such as the negative pole-to-ground voltage on the photovoltaic access side, to determine whether it is necessary to connect a resistor to the corresponding pole of the DC bus or not to connect any resistor, and then proceed with grid connection. This method can adapt to complex power grid environments.

[0151] Another embodiment of the present application further provides a photovoltaic grid-connected power generation system, as shown in FIG9 , comprising: at least one (one is shown as an example) inverter 100 and its grid-connected relay branch 200; wherein:

[0152] The photovoltaic access side of the inverter 100 is connected to at least one photovoltaic string; the photovoltaic access side has at least one positive and negative ports, and any of the positive and negative ports can be connected to at least one photovoltaic string.

[0153] ​​As shown in Figure 9, the main circuit of the inverter 100 includes an inverter unit 101; the DC side of the inverter unit 101 is connected to the photovoltaic access side of the inverter 100 through a DC bus; a bus capacitor 102 is connected between the positive and negative poles of the DC bus; and the AC side of the inverter unit 101 is connected to the AC side of the inverter 100.

[0154] In actual applications, the main circuit of the inverter 100 may further include: at least one (one is shown as an example in the figure) boost unit 103; one end of the boost unit 103 serves as the corresponding end of the photovoltaic access side of the inverter 100, that is, the positive and negative ports of the above-mentioned photovoltaic access side, used to connect to the corresponding photovoltaic string; the other end of the boost unit 103 is connected to the DC side of the inverter unit 101 through a DC bus.

[0155] The inverter 100 is connected to the grid connection point through the corresponding grid connection relay branch 200; in addition, the photovoltaic grid-connected power generation system usually also includes: a transformer; the transformer is arranged between the grid connection relay branch 200 and the grid connection point. For details, please refer to the existing technology and will not be repeated here.

[0156] The grid-connected relay branch 200 may include at least two AC relays connected in series, such as the two AC relays 201 and 202 shown in FIG9 , and FIG9 is taken as an example where both AC relays are three-phase relays, wherein the AC relay 201 includes relays K1 to K3 respectively arranged on each phase line, and the AC relay 202 includes relays K4 to K6 respectively arranged on each phase line; for a single-phase photovoltaic grid-connected power generation system, these two AC relays will both be single-phase relays and are not illustrated here.

[0157] In inverter 100, the positive and negative poles of the DC bus are each grounded via a series-connected set of switches and resistors. Referring to Figure 9 , the positive pole of the DC bus, bus+, is grounded via a series-connected switch K7 and resistor R3; the negative pole of the DC bus, bus-, is grounded via a series-connected switch K8 and resistor R4. The order in which the switches and resistors are connected in series within each set is not limited. Figure 9 illustrates a resistor connected between a switch and ground, but this is not a limitation.

[0158] The controller in inverter 100 performs grid-connected control using the grid-connected control method described in any of the above embodiments. The specific process and principles of this grid-connected control method can be found in the above embodiments and will not be repeated here. Switches K7 and K8 are controlled by the controller. Specifically, when switch K7 is controlled closed, resistor R3 is connected; when switch K7 is controlled open, resistor R3 is disconnected; when switch K8 is controlled closed, resistor R4 is connected; when switch K8 is controlled open, resistor R4 is disconnected. In practical applications, the switch can be a relay or an electronic switch, etc., which is not limited here.

[0159] Referring to Figure 9 , when the negative-to-ground impedance R2 on the photovoltaic access side is much greater than the positive-to-ground impedance R1, the negative-to-ground potential of the DC bus increases dramatically. When the inverter 100 is connected to the grid, an AC relay in the grid-connected relay branch 200 is first attracted, such as the AC relay 202 on the grid side including relays K4, K5, and K6. At this time, a large common-mode voltage difference is generated across the main contacts of the other AC relay, namely the AC relay 201 on the inverter side including relays K1, K2, and K3. By attracting the switch K8, the voltage difference across the main contacts of the AC relay 201 is reduced, thereby reducing the surge current generated when the relays K1, K2, and K3 are closed.

[0160] When the negative-pole-to-ground impedance R2 on the photovoltaic access side is much smaller than the positive-pole-to-ground impedance R1, the negative-to-ground potential of the DC bus decreases sharply. When the inverter 100 is connected to the grid, an AC relay in the grid-connected relay branch 200 is first attracted, such as AC relay 202 on the grid side, which includes relays K4, K5, and K6. At this time, a large common-mode voltage difference is generated across the main contacts of the other AC relay, namely AC relay 201 on the inverter side, which includes relays K1, K2, and K3. By attracting switch K7, the voltage difference across the main contacts of AC relay 201 is reduced, thereby reducing the inrush current generated when relays K1, K2, and K3 are closed.

[0161] When the negative electrode-to-ground impedance R2 of the photovoltaic access side is close to the positive electrode-to-ground impedance R1, the voltage difference across the main contacts of relays K1, K2, and K3 in the AC relay 201 is small, and the above-mentioned adjustment is not required. The surge current generated when relays K1, K2, and K3 are closed is small, meeting the device specifications.

[0162] In the prior art, when the impedance to ground of the negative or positive pole of the photovoltaic access side decreases, a large common-mode voltage difference will be generated across the main contacts of the last AC relay closed before the inverter is connected to the grid, resulting in a large inrush current at the moment of closing. This embodiment connects the positive and negative poles of the DC bus (bus+ and bus- as shown in FIG9 ) in series with the ground using resistors and switches. This allows an AC relay (such as 202) in the grid-connected relay branch 200 to be closed when or before it is closed, by closing the corresponding switch. This causes the voltage to the ground of the negative pole of the photovoltaic access side / DC bus to approach the negative half of the bus voltage, and the midpoint voltage of the DC bus to approach zero. This reduces the voltage difference across the main contacts of the last AC relay (such as 201) closed before the inverter 100 is connected to the grid when the impedance to ground of the negative or positive pole of the photovoltaic access side decreases, thereby reducing the erosion of the main contacts of each relay by the inrush current.

[0163] It should be noted that the above description is based on the example that the AC relay 202 in the grid-connected relay branch 200 is attracted first and the AC relay 201 is attracted later. In actual applications, the order of attraction of the two is not limited and can be determined according to the specific application environment, which is within the protection scope of this application.

[0164] It should also be noted that in this photovoltaic grid-connected power generation system, when the number of inverters 100 is greater than 1, when each controller thereof executes the grid-connected control method, it specifically determines whether the voltage between the positive and negative poles of the photovoltaic access side of the corresponding inverter 100 and the ground meets the preset balance condition by judging whether the common-mode voltage difference between the two sides of the corresponding grid-connected relay branch 200 is greater than the preset voltage difference.

[0165] In practical applications, the resistance values ​​of resistors R3 and R4 are not limited and can range from 0 to any impedance value. Preferably, the resistance values ​​of resistors R3 and R4 satisfy the following requirement: when connected, the difference in the equivalent impedance to ground between the two poles of the DC bus is less than a preset threshold. For a description of the resistance values ​​of these two resistors, please refer to the above embodiment and will not be repeated here.

[0166] In addition, the above-mentioned switches and resistors can be PID (Potential Induced Degradation) power supply components in the inverter 100. That is, the PID power supply can be directly used to achieve equivalent impedance adjustment for the corresponding poles of the DC bus without modifying the hardware circuit or adding additional hardware facilities. Only through software control logic and detection judgment, the erosion of the contacts by the instantaneous surge current when the AC relay is connected to the grid can be reduced.

[0167] Switches K7 and K8 regulate the voltage to ground on the PV access side. Their on / off control is independent of grid connection control. After closing either switch and connecting the connected resistor to the corresponding DC bus pole, if the voltage to ground on the PV access side / DC bus negative pole deviates significantly from the negative half bus voltage, the switch must be opened and grid connection must be performed as before regulation.

[0168] The same or similar parts between the various embodiments in this specification can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the system or system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment. The system and system embodiments described above are merely schematic, wherein the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or they may be distributed on multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of this embodiment. Ordinary technicians in this field can understand and implement it without making any creative efforts.

[0169] Professionals may further appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the above description has generally described the components and steps of each example according to their functions. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professionals and technicians may use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of the present invention.

[0170] With respect to the above description of the disclosed embodiments, the features described in the various embodiments in this specification may be interchanged or combined to enable those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein, but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A grid-connected control method for an inverter, characterized in that: The inverter is connected to the grid connection point through a corresponding grid connection relay branch, and the grid connection control method includes: Determine whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition; If the voltage between the positive and negative electrodes of the photovoltaic access side and the ground does not meet the preset balance condition, then determine the electrode with a larger absolute value of the voltage between the positive and negative electrodes and the ground; Controlling the corresponding pole equivalent impedance to ground of the DC bus in the inverter to decrease; Conduct grid-connected control.

2. The grid-connected control method of the inverter according to claim 1, characterized in that: Controlling the reduction of the equivalent impedance to ground of the corresponding pole of the DC bus in the inverter includes: A grounded resistor is connected to the corresponding pole of the DC bus.

3. The grid-connected control method of the inverter according to claim 1, characterized in that: When the voltage between the positive and negative electrodes of the photovoltaic access side and the ground does not meet the preset balance condition, determining a electrode with a larger absolute value of the voltage between the positive and negative electrodes and the ground includes: Comparing the absolute value of the voltage to ground of any pole of the photovoltaic access side of the inverter with the half bus voltage of the DC bus; If the absolute value of the voltage to ground of the pole at the photovoltaic access side of the inverter is greater than the half-bus voltage, the pole is determined to be the pole with the larger absolute value of the voltage to ground; otherwise, the other pole is determined to be the pole with the larger absolute value of the voltage to ground.

4. The grid-connected control method of the inverter according to claim 3, characterized in that: Comparing the absolute value of the voltage to ground of any pole of the photovoltaic access side of the inverter with the half bus voltage of the DC bus, including: Comparing the absolute value of the voltage between the negative pole of the photovoltaic access side of the inverter and the ground with the half bus voltage; or, The absolute value of the voltage between the positive pole of the photovoltaic access side of the inverter and the ground is compared with the half bus voltage.

5. The grid-connected control method of the inverter according to claim 1, characterized in that: Determining whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition includes: Determine whether the deviation between the absolute value of the voltage to ground of any pole of the photovoltaic access side of the inverter and the half-bus voltage of the DC bus is greater than a preset deviation; If the deviation is greater than the preset deviation, it is determined that the voltage between the positive and negative electrodes of the photovoltaic access side and the ground does not meet the preset balance condition; otherwise, it is determined that the voltage between the positive and negative electrodes of the photovoltaic access side and the ground meets the preset balance condition.

6. The grid-connected control method of the inverter according to claim 5, characterized in that: Determining whether the deviation between the absolute value of the voltage to ground of any pole of the photovoltaic access side of the inverter and the half-bus voltage of the DC bus is greater than a preset deviation includes: Determine whether the deviation between the absolute value of the voltage between the negative electrode of the photovoltaic access side of the inverter and the half-bus voltage is greater than a preset deviation; or, It is determined whether a deviation between an absolute value of a positive pole-to-ground voltage at a photovoltaic access side of the inverter and the half-bus voltage is greater than a preset deviation.

7. The grid-connected control method of the inverter according to claim 1, characterized in that: Determining whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition includes: For the grid-connected relay branch of the inverter connected to the grid-connected point, determining whether the common-mode voltage difference between the two sides thereof is greater than a preset voltage difference; If the common-mode voltage difference is greater than the preset voltage difference, it is determined that the voltage between the positive and negative electrodes of the photovoltaic access side and the ground does not meet the preset balance condition; otherwise, it is determined that the voltage between the positive and negative electrodes of the photovoltaic access side and the ground meets the preset balance condition.

8. The grid-connected control method of the inverter according to claim 1, characterized in that: Determining whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition includes: Determine whether the difference between the absolute values ​​of the voltages of the positive and negative electrodes to the ground on the photovoltaic access side of the inverter is greater than a preset difference; If the difference is greater than the preset difference, it is determined that the voltage between the positive and negative electrodes of the photovoltaic access side and the ground does not meet the preset balance condition; otherwise, it is determined that the voltage between the positive and negative electrodes of the photovoltaic access side and the ground meets the preset balance condition.

9. The grid-connected control method of the inverter according to any one of claims 1 to 8, characterized in that: After determining whether the voltages of the positive and negative electrodes of the photovoltaic access side of the inverter to the ground meet the preset balance condition, if the voltages of the positive and negative electrodes of the photovoltaic access side to the ground do not meet the preset balance condition, the method further includes: recording the judgment parameters of whether the voltages of the positive and negative electrodes of the photovoltaic access side to the ground meet the preset balance condition; After controlling the corresponding pole equivalent impedance to ground of the DC bus in the inverter to decrease, the method further includes: Determining whether the evaluation parameter changes in a direction that satisfies the preset balance condition; If so, the step of performing grid-connected control is executed; otherwise, the corresponding pole equivalent impedance to ground of the DC bus is restored first, and then the step of performing grid-connected control is executed.

10. The grid-connected control method of the inverter according to any one of claims 1 to 9, characterized in that: After determining whether the voltage between the positive and negative electrodes of the photovoltaic access side of the inverter and the ground meets a preset balance condition, the method further includes: If the voltage between the positive and negative electrodes of the photovoltaic access side and the ground meets the preset balance condition, the step of performing grid connection control is executed.

11. A photovoltaic grid-connected power generation system, characterized in that: include: At least one inverter and its grid-connected relay branch; wherein, The photovoltaic access side of the inverter is connected to at least one photovoltaic string; The inverter is connected to the grid connection point via the corresponding grid connection relay branch; The positive and negative poles of the DC bus in the inverter are each grounded through a set of switches and resistors connected in series; The controller in the inverter performs grid-connected control by using the grid-connected control method of the inverter according to any one of claims 1 to 10; The switch is controlled by the controller.

12. The photovoltaic grid-connected power generation system according to claim 11, characterized in that: The resistance value of the resistor satisfies that: when the resistor is connected, the difference between the two poles of the DC bus's equivalent impedance to ground is less than a preset threshold.

13. The photovoltaic grid-connected power generation system according to claim 11, characterized in that: When the number of the inverters is greater than 1, when each of the controllers executes the grid-connected control method, it determines whether the voltage between the positive and negative electrodes of the photovoltaic access side of the corresponding inverter meets the preset balance condition by judging whether the common-mode voltage difference between the two sides of the corresponding grid-connected relay branch is greater than the preset voltage difference.

14. The photovoltaic grid-connected power generation system according to claim 11, characterized in that: The switch and the resistor are potential induced decay PID power supply components in the inverter.

15. The photovoltaic grid-connected power generation system according to claim 11, characterized in that: The grid-connected relay branch includes at least two AC relays connected in series.

16. The photovoltaic grid-connected power generation system according to any one of claims 11 to 15, characterized in that: The main circuit of the inverter includes an inverter unit; The DC side of the inverter unit is connected to the photovoltaic access side of the inverter through the DC bus; A bus capacitor is connected between the positive and negative poles of the DC bus; The AC side of the inverter unit is connected to the AC side of the inverter.

17. The photovoltaic grid-connected power generation system according to claim 16, characterized in that: The main circuit of the inverter further includes: at least one boost unit; One end of the boost unit is used as a corresponding end of the photovoltaic access side of the inverter, and is used to connect the corresponding photovoltaic string; The other end of the boost unit is connected to the DC side of the inverter unit through the DC bus.

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