Single-phase transformerless photovoltaic inverter

Abstract

The application relates to a single-phase transformerless photovoltaic inverter. The photovoltaic inverter comprises an H-bridge inverter circuit, a first bridge arm and a second bridge arm connected in parallel, an input end of the H-bridge inverter circuit being connected with a photovoltaic array and being used for converting direct current output by the photovoltaic array into alternating current; a first branch being connected with a midpoint of the second bridge arm and a first end, the first end being a connection position of the first bridge arm and the second bridge arm with a positive electrode of the photovoltaic array; and a second branch being connected with a midpoint of the first bridge arm and a first end. In four modes, a common-mode voltage formed by the photovoltaic array is maintained at a preset target voltage. The application can reduce leakage current, thereby reducing the safety hazard of the leakage current to an operator.

Description

Technical Field

[0001] This application relates to the field of inverter technology, and in particular to a single-phase transformerless photovoltaic inverter. Background Technology

[0002] Grid-connected photovoltaic (PV) inverter systems are mainly divided into isolated and non-isolated types. From a safety perspective, PV grid-connected inverters typically require a transformer to achieve electrical isolation between the PV system and the AC load. However, isolation transformers reduce system efficiency and increase size, weight, and cost. Non-isolated PV grid-connected inverter systems eliminate the need for a power frequency isolation transformer, thus offering higher conversion efficiency and lower cost, and are widely used in small-to-medium power and string PV inverter systems.

[0003] However, without a transformer, the negative electrode of the photovoltaic array cannot be directly grounded, and there is a certain parasitic capacitance between the positive and negative electrodes of the photovoltaic array and the outside world. A loop is formed between the parasitic capacitance of the photovoltaic array and the ground, which will lead to a large leakage current and thus pose a safety hazard to the operators. Utility Model Content

[0004] Therefore, it is necessary to provide a single-phase transformerless photovoltaic inverter. Using this photovoltaic inverter can reduce leakage current, thereby reducing the safety hazards caused by leakage current to operators.

[0005] This application provides a single-phase transformerless photovoltaic inverter, comprising:

[0006] The H-bridge inverter circuit includes a first bridge arm and a second bridge arm connected in parallel. The input terminal of the H-bridge inverter circuit is connected to the photovoltaic array and is used to convert the DC power output by the photovoltaic array into AC power.

[0007] The first branch connects the midpoint of the second bridge arm to the first end, where both the first and second bridge arms are connected to the positive terminal of the photovoltaic array. In the positive half-cycle charging mode of the photovoltaic inverter, the first branch is used for charging and energy storage. In the positive half-cycle freewheeling mode of the photovoltaic inverter, the first branch is also used to form a discharge current loop with the H-bridge inverter circuit to achieve freewheeling, and to achieve electrical isolation between AC load and DC power.

[0008] The second branch connects the midpoint and the first end of the first bridge arm. In the negative half-cycle charging mode of the photovoltaic inverter, the second branch is used for charging and energy storage. In the negative half-cycle freewheeling mode of the photovoltaic inverter, the second branch is also used to form a discharge current loop with the H-bridge inverter circuit to achieve freewheeling, and to achieve electrical isolation between AC load and DC power.

[0009] In all four modes, the common-mode voltage generated by the photovoltaic array is maintained at the preset target voltage.

[0010] In one embodiment, the first bridge arm includes a first upper switch and a first lower switch connected in series, and the second bridge arm includes a second upper switch and a second lower switch connected in series.

[0011] The first branch includes a first branch switch and a first branch inductor disposed on the second bridge arm; the second branch includes a second branch switch and a second branch inductor disposed on the first bridge arm.

[0012] The first and second ends of the first branch inductor are connected to the second upper switch and the second lower switch, respectively. The output end of the first branch switch is connected to the first end and the second end of the first branch inductor. The first end of the first branch inductor is the midpoint of the second bridge arm for connecting the AC load.

[0013] The first and second ends of the second branch inductor are connected to the first upper switch and the first lower switch, respectively. The output end of the second branch switch is connected to the first end and the second end of the second branch inductor. The first end of the second branch inductor is the midpoint of the first bridge arm for connecting the AC load.

[0014] In one embodiment, the photovoltaic inverter further includes a first filter circuit, which includes a filter capacitor, a first filter inductor, and a second filter inductor.

[0015] The first end of the filter capacitor is connected to the midpoint of the first bridge arm, and the second end of the filter capacitor is connected to the midpoint of the second bridge arm.

[0016] The first terminal of the first filter inductor is connected to the first terminal of the filter capacitor, and the second terminal of the first filter inductor is connected to the AC load.

[0017] The first terminal of the second filter inductor is connected to the second terminal of the filter capacitor, and the second terminal of the second filter inductor is connected to the AC load.

[0018] In one embodiment, the photovoltaic inverter further includes a second filter circuit, which includes a common-mode inductor, a first capacitor, a second capacitor, a third capacitor, and a fourth capacitor.

[0019] The first terminal of the common-mode inductor is connected to the second terminal of the first filter inductor, the second terminal of the common-mode inductor is connected to the positive bus of the AC load, the third terminal of the common-mode inductor is connected to the second terminal of the second filter inductor, and the fourth terminal of the common-mode inductor is connected to the negative bus of the AC load.

[0020] The two ends of the first capacitor are connected to the first and third ends of the common-mode inductor, respectively.

[0021] The two ends of the second capacitor are connected to the second and fourth ends of the common-mode inductor, respectively.

[0022] The two ends of the third capacitor are connected to the second end of the common-mode inductor and the ground terminal, respectively.

[0023] The two ends of the fourth capacitor are connected to the fourth terminal of the common-mode inductor and the negative busbar of the AC load, respectively.

[0024] In one embodiment, the photovoltaic inverter further includes a first diode and a second diode; the cathode of the first diode is connected to a first end, the anode of the first diode is used to connect to the positive electrode of the photovoltaic array, the anode of the second diode is connected to a second end, and the cathode of the second diode is used to connect to the negative electrode of the photovoltaic array, wherein the first end is the connection point of the first upper switch and the second upper switch, and the second end is the connection point of the first lower switch and the second lower switch.

[0025] In one embodiment, the photovoltaic inverter further includes a Z-source network, which includes a first network inductor, a second network inductor, a first network capacitor, and a second network capacitor.

[0026] The first end of the first network inductor is connected to the positive terminal of the photovoltaic array, and the second end of the first network inductor is connected to the first terminal.

[0027] The first terminal of the second network inductor is connected to the negative terminal of the photovoltaic array, and the second terminal of the second network inductor L7 is connected to the second end.

[0028] The first terminal of the first network capacitor is connected to the first terminal of the first network inductor, and the second terminal of the first network capacitor is connected to the second terminal of the second network inductor.

[0029] The first end of the second network capacitor is connected to the second end of the first network inductor, and the second end of the first network capacitor is connected to the first end of the second network inductor.

[0030] In one embodiment, when the photovoltaic inverter is in the positive half-cycle charging mode, the common-mode voltage formed by the photovoltaic array is half the sum of the midpoint voltage of the first arm and the midpoint voltage of the second arm.

[0031] In the positive half-cycle charging mode of the photovoltaic inverter, the first upper switch is turned on, the first lower switch is turned off, the second upper switch is turned off, the second lower switch is switched on and off at a preset frequency, the first branch switch is turned off, and the second branch switch is turned off. When the second lower switch is turned on, the current loop formed includes: photovoltaic array - first diode - first network inductor - first upper switch - first filter inductor - common mode inductor - AC load - common mode inductor - second filter inductor - first branch inductor - second lower switch - second network inductor - second diode photovoltaic array.

[0032] In one embodiment, when the photovoltaic inverter is in the negative half-cycle charging mode, the common-mode voltage formed by the photovoltaic array is half the sum of the midpoint voltage of the first bridge arm and the midpoint voltage of the second bridge arm.

[0033] In the negative half-cycle charging mode of the photovoltaic inverter, the first upper switch is turned off, the first lower switch is switched on and off at a preset frequency, the second upper switch is turned on, the second lower switch is turned off, the first branch switch is turned off, and the second branch switch is turned off, forming a current loop including: photovoltaic array - first diode - first network inductor - second upper switch - second filter inductor - common mode inductor - AC load - common mode inductor - first filter inductor - second branch inductor - first lower switch - second network inductor - second diode.

[0034] In one embodiment, when the photovoltaic inverter is in negative half-cycle freewheeling mode, the common-mode voltage formed by the photovoltaic array is half the sum of the midpoint voltage of the first arm and the midpoint voltage of the second arm.

[0035] In the positive half-cycle freewheeling mode of the photovoltaic inverter, the first upper switch is turned on, the first lower switch is turned off, the second upper switch is turned off, the second lower switch is turned off, the first branch switch is turned on, and the second branch switch is turned off, forming a current loop including: first upper switch - first filter inductor - common mode inductor - AC load - common mode inductor - second filter inductor - first branch inductor - first branch switch - first upper switch.

[0036] In one embodiment, when the photovoltaic inverter is in negative half-cycle freewheeling mode, the common-mode voltage formed by the photovoltaic array is half the sum of the midpoint voltage of the first arm and the midpoint voltage of the second arm.

[0037] In the negative half-cycle freewheeling mode of the photovoltaic inverter, the first upper switch is turned off, the first lower switch is turned off, the second upper switch is turned on, the second lower switch is turned off, the first branch switch is turned off, and the second branch switch is turned on, forming a current loop: second upper switch - second filter inductor - common mode inductor - AC load - common mode inductor - first filter inductor - second branch inductor - second branch switch - second upper switch - photovoltaic array.

[0038] The aforementioned single-phase transformerless photovoltaic inverter includes an H-bridge inverter circuit, a first branch, and a second branch. The H-bridge inverter circuit includes a first bridge arm and a second bridge arm connected in parallel. The input terminal of the H-bridge inverter circuit is connected to the photovoltaic array and is used to convert the DC power output from the photovoltaic array into AC power. The first branch connects the midpoint of the second bridge arm and the first end, where both the first and second bridge arms are connected to the positive terminal of the photovoltaic array. In the positive half-cycle charging mode of the photovoltaic inverter, the first branch is used for charging and energy storage. In the positive half-cycle freewheeling mode of the photovoltaic inverter, the first branch... One branch is also used to form a discharge current loop with the H-bridge inverter circuit to achieve freewheeling and to achieve electrical isolation between the AC load and the DC power. The second branch connects the midpoint and the first end of the first bridge arm. In the negative half-cycle charging mode of the photovoltaic inverter, the second branch is used for charging and energy storage. In the negative half-cycle freewheeling mode of the photovoltaic inverter, the second branch is also used to form a discharge current loop with the H-bridge inverter circuit to achieve freewheeling and to achieve electrical isolation between the AC load and the DC power. In all four modes, the common-mode voltage formed by the photovoltaic array is maintained at a preset target voltage. Through the embodiments of this application, in the positive half-cycle charging mode, negative half-cycle charging mode, positive half-cycle freewheeling mode, and negative half-cycle freewheeling mode of the photovoltaic inverter, the H-bridge inverter circuit, the first branch, and the second branch cooperate with each other to maintain the common-mode voltage formed by the photovoltaic array at a preset target voltage. A constant common-mode voltage can reduce leakage current, thereby reducing the safety hazards caused by leakage current to operators. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of this application or the conventional technology, the drawings used in the description of the embodiments or the conventional technology will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is one of the structural schematic diagrams of a photovoltaic inverter in one embodiment;

[0041] Figure 2 This is a second schematic diagram of the structure of a photovoltaic inverter in one embodiment;

[0042] Figure 3a This is one of the schematic diagrams of the drive signal in one embodiment;

[0043] Figure 3b This is a second schematic diagram of the drive signal in one embodiment;

[0044] Figure 3c This is the third schematic diagram of the drive signal in one embodiment;

[0045] Figure 3dThis is a fourth schematic diagram of the drive signal in one embodiment;

[0046] Figure 4 This is the third schematic diagram of the structure of a photovoltaic inverter in one embodiment;

[0047] Figure 5 This is the fourth schematic diagram of the structure of a photovoltaic inverter in one embodiment;

[0048] Figure 6 This is the fifth schematic diagram of the structure of a photovoltaic inverter in one embodiment;

[0049] Figure 7 This is the sixth schematic diagram of the structure of a photovoltaic inverter in one embodiment;

[0050] Figure 8a This is one of the schematic diagrams of a current loop in one embodiment;

[0051] Figure 8b This is a second schematic diagram of the current loop in one embodiment;

[0052] Figure 8c This is a schematic diagram of the current loop in one embodiment;

[0053] Figure 8d This is a fourth schematic diagram of the current loop in one embodiment;

[0054] Figure 9a This is a schematic diagram of the simulation results of a conventional inverter in one embodiment;

[0055] Figure 9b This is a schematic diagram of the simulation results of a photovoltaic inverter in one embodiment.

[0056] Explanation of reference numerals in the attached figures:

[0057] 10. Photovoltaic inverter; 11. H-bridge inverter circuit; 12. First branch; 13. Second branch;

[0058] 111. First bridge arm; 112. Second bridge arm; PV, photovoltaic array; Grid, AC load;

[0059] S1, first upper switch transistor; S2, first lower switch transistor; S3, second upper switch transistor;

[0060] S4, second lower switch transistor; Q1, first branch switch transistor; L1, first branch inductor;

[0061] Q2, the second branch switching transistor; L2, the second branch inductor; C1, the filter capacitor;

[0062] L3, first filter inductor; L4, second filter inductor; L5, common-mode inductor;

[0063] Cx1, the first capacitor; Cx2, the second capacitor; Cy1, the third capacitor;

[0064] Cy2, fourth capacitor; D1, first diode; D2, second diode;

[0065] L6, first network inductor; L7, second network inductor; C2, first network capacitor;

[0066] C3, the second network capacitor. Detailed Implementation

[0067] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings, which illustrate embodiments of the present application. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that the disclosure of this application will be thorough and complete.

[0068] 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. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0069] It is understood that the terms "first," "second," etc., used herein may be used to describe various elements, but these elements are not limited by these terms. These terms are only used to distinguish one element from another. For example, without departing from the scope of this application, a first resistor may be referred to as a second resistor, and similarly, a second resistor may be referred to as a first resistor. Both the first resistor and the second resistor are resistors, but they are not the same resistor.

[0070] It is understood that the term "connection" in the following embodiments should be understood as "electrical connection," "communication connection," etc., if the connected circuits, modules, units, etc., have electrical signal or data transmission with each other.

[0071] It is understandable that "at least one" refers to one or more, and "multiple" refers to two or more. "At least a part of an element" refers to part or all of an element.

[0072] When used herein, the singular forms of “a,” “an,” and “the” may also include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprising / including” or “having,” etc., specify the presence of the stated features, wholes, steps, operations, components, parts, or combinations thereof, but do not preclude the possibility of the presence or addition of one or more other features, wholes, steps, operations, components, parts, or combinations thereof. Meanwhile, the term “and / or” as used in this specification includes any and all combinations of the associated listed items.

[0073] Grid-connected photovoltaic (PV) inverter systems are mainly divided into isolated and non-isolated types. From a safety perspective, PV grid-connected inverters typically require a transformer to achieve electrical isolation between the PV system and the AC load. However, isolation transformers reduce system efficiency and increase size, weight, and cost. Non-isolated PV grid-connected inverter systems eliminate the need for a power frequency isolation transformer, thus offering higher conversion efficiency and lower cost, and are widely used in small-to-medium power and string PV inverter systems.

[0074] However, without a transformer, the negative electrode of the photovoltaic array cannot be directly grounded, and there is a certain parasitic capacitance between the positive and negative electrodes of the photovoltaic array and the outside world. A loop is formed between the parasitic capacitance of the photovoltaic array and the ground, which will lead to a large leakage current and thus pose a safety hazard to the operators.

[0075] Research into the above issues revealed that the leakage current to ground is the common-mode current icm, and the common-mode current icm has the following relationship with the common-mode voltage Ucm:

[0076]

[0077] Among them, C PV Let t be the parasitic capacitance of the photovoltaic array. From the above relationship, it can be seen that the leakage current is directly proportional to the rate of change of the common-mode voltage Ucm. Therefore, the constantly changing common-mode voltage is the fundamental cause of the leakage current.

[0078] Based on the above findings, this application provides a single-phase transformerless photovoltaic inverter, which includes an H-bridge inverter circuit, a first branch, and a second branch. In the positive half-cycle charging mode, negative half-cycle charging mode, positive half-cycle freewheeling mode, and negative half-cycle freewheeling mode of the photovoltaic inverter, the H-bridge inverter circuit, the first branch, and the second branch cooperate with each other to maintain the common-mode voltage formed by the photovoltaic array at a preset target voltage. The constant common-mode voltage can reduce leakage current, thereby reducing the safety hazards caused by leakage current to operators.

[0079] In one exemplary embodiment, such as Figure 1As shown, a single-phase transformerless photovoltaic inverter 10 is provided. The single-phase transformerless photovoltaic inverter 10 includes an H-bridge inverter circuit 11, a first branch 12, and a second branch 13. The H-bridge inverter circuit 11 includes a first bridge arm 111 and a second bridge arm 112 connected in parallel. The input terminal of the H-bridge inverter circuit is connected to a photovoltaic array (PV) and is used to convert the DC power output from the PV array into AC power. The first branch 12 connects the midpoint of the second bridge arm 112 and a first end, where both the first bridge arm 111 and the second bridge arm 112 are connected to the positive terminal of the PV array. In the positive half-cycle charging mode of the photovoltaic inverter 10, the first branch 12 is used for charging and energy storage. In the positive half-cycle freewheeling mode of the photovoltaic inverter 10, the first branch 12 is also used to connect with the H-bridge inverter circuit. 11 forms a discharge current loop to achieve freewheeling and is used to achieve electrical isolation between the AC load Grid and the DC power; the second branch 13 connects the midpoint and the first end of the first bridge arm 111. In the negative half-cycle charging mode of the photovoltaic inverter 10, the second branch 13 is used for charging and energy storage; in the negative half-cycle freewheeling mode of the photovoltaic inverter 10, the second branch 13 is also used to form a discharge current loop with the H-bridge inverter circuit 11 to achieve freewheeling and to achieve electrical isolation between the AC load Grid and the DC power; in the four modes, the common-mode voltage formed by the photovoltaic array PV is maintained at the preset target voltage.

[0080] In this embodiment, the single-phase transformerless photovoltaic inverter includes an H-bridge inverter circuit, a first branch, and a second branch. In the positive half-cycle charging mode, negative half-cycle charging mode, positive half-cycle freewheeling mode, and negative half-cycle freewheeling mode of the photovoltaic inverter, the H-bridge inverter circuit, the first branch, and the second branch cooperate with each other to maintain the common-mode voltage formed by the photovoltaic array at a preset target voltage. The constant common-mode voltage can reduce leakage current, thereby reducing the safety hazards caused by leakage current to operators.

[0081] like Figure 2As shown, the first bridge arm 111 includes a first upper switch S1 and a first lower switch S2 connected in series, and the second bridge arm 112 includes a second upper switch S3 and a second lower switch S4 connected in series; the first branch 12 includes a first branch switch Q1 and a first branch inductor L1 disposed on the second bridge arm 112, and the second branch 13 includes a second branch switch Q2 and a second branch inductor L2 disposed on the first bridge arm 111; the first end and the second end of the first branch inductor L1 are respectively connected to the second upper switch S3 and the second lower switch S4. The output terminal of the first branch switch Q1 is connected to the first end and the second end of the first branch inductor L1; the first end of the first branch inductor L1 is the midpoint of the second bridge arm 112 for connecting the AC load Grid; the first and second ends of the second branch inductor L2 are respectively connected to the first upper switch S1 and the first lower switch S2, and the output terminal of the second branch switch Q2 is connected to the first end and the second end of the second branch inductor L2; the first end of the second branch inductor L2 is the midpoint of the first bridge arm 111 for connecting the AC load Grid.

[0082] In this embodiment, a first branch switch Q1 and a second branch switch Q2 are added, allowing the first branch inductor L1 and the second branch inductor L2 to move between the bridge arms, thereby maintaining a constant common-mode voltage during the switching period of each switch and effectively suppressing leakage current.

[0083] In this embodiment of the application, combined with Figure 3a and Figure 8a In the positive half-cycle charging mode of the photovoltaic inverter 10, the operating modes and drive signals of the first upper switch S1, the first lower switch S2, the second upper switch S3, the second lower switch S4, the first branch switch Q1, and the second branch switch Q2 are as follows: Figure 3a As shown. In this mode, the first upper switch S1 is on, the first lower switch S2 is off, the second upper switch S3 is off, and the second lower switch S4 is switched on and off at a preset frequency. The first branch switch Q1 is off, and the second branch switch Q2 is off. When the second lower switch S4 is on, a current loop is formed (e.g., Figure 8a (As shown by the dashed line): The current loop from the photovoltaic array PV - first upper switch S1 - AC load Grid - first branch inductor L1 - second lower switch S4 - photovoltaic array PV.

[0084] Combination Figure 3b and Figure 8b In the positive half-cycle freewheeling mode of the photovoltaic inverter 10, the operating modes and drive signals of the first upper switch S1, the first lower switch S2, the second upper switch S3, the second lower switch S4, the first branch switch Q1, and the second branch switch Q2 are as follows: Figure 3bAs shown. In this mode, the first upper switch S1 is turned on, the first lower switch S2 is turned off, the second upper switch S3 is turned off, the second lower switch S4 is turned off, the first branch switch Q1 is turned on, and the second branch switch Q2 is turned off, forming a current loop (as shown). Figure 8b (As shown by the dashed line): First upper switch S1 - AC load Grid - First branch inductor L1 - First branch switch Q1 - First upper switch S1. That is, in the positive half-cycle freewheeling mode, freewheeling can be achieved by controlling the first branch switch Q1, and the photovoltaic array PV and AC load Grid can be isolated.

[0085] Combination Figure 3c and Figure 8c In the negative half-cycle charging mode of the photovoltaic inverter 10, the operating modes and drive signals of the first upper switch S1, the first lower switch S2, the second upper switch S3, the second lower switch S4, the first branch switch Q1, and the second branch switch Q2 are as follows: Figure 3c As shown. In this mode, the first upper switch S1 is off, the first lower switch S2 is switched on and off at a preset frequency, the second upper switch S3 is on, the second lower switch S4 is off, the first branch switch Q1 is off, and the second branch switch Q2 is off. When the first lower switch S2 is on, a current loop is formed (e.g., Figure 8c (As shown by the dashed line): Photovoltaic array PV - Second upper switch S3 - AC load Grid - Second branch inductor L2 - First lower switch S2 - Photovoltaic array PV.

[0086] Combination Figure 3d and Figure 8d In the negative half-cycle freewheeling mode of the photovoltaic inverter 10, the operating modes and drive signals of the first upper switch S1, the first lower switch S2, the second upper switch S3, the second lower switch S4, the first branch switch Q1, and the second branch switch Q2 are as follows: Figure 3d As shown. In this mode, the first upper switch S1 is off, the first lower switch S2 is off, the second upper switch S3 is on, the second lower switch S4 is off, the first branch switch Q1 is off, and the second branch switch Q2 is on, forming a current loop (as shown). Figure 8d (As shown by the dashed line): Second upper switch S3 - AC load Grid - Second branch inductor L2 - Second branch switch Q2 - Second upper switch S3. That is, in the negative half-cycle freewheeling mode, freewheeling can be achieved by controlling the second branch switch Q2, and the photovoltaic array PV and AC load Grid can be isolated.

[0087] In the above embodiments, the conversion from DC to AC can be achieved through the first upper switch and the first lower switch of the first bridge arm, and the second upper switch and the second lower switch of the second bridge arm. The freewheeling current can be achieved through the first branch inductor and the first branch switch in the first branch, and the second branch inductor and the second branch switch in the second branch. This also decouples the DC side of the photovoltaic array from the AC side of the AC load, effectively suppressing common-mode current.

[0088] In one exemplary embodiment, such as Figure 4 As shown, the photovoltaic inverter 10 also includes a first filter circuit, which includes a filter capacitor C1, a first filter inductor L3, and a second filter inductor L4. The first end of the filter capacitor C1 is connected to the midpoint of the first bridge arm 111, and the second end of the filter capacitor C1 is connected to the midpoint of the second bridge arm 112. The first end of the first filter inductor L3 is connected to the first end of the filter capacitor C1, and the second end of the first filter inductor L3 is connected to the AC load Grid. The first end of the second filter inductor L4 is connected to the second end of the filter capacitor C1, and the second end of the second filter inductor L4 is connected to the AC load Grid.

[0089] In this embodiment, the first branch inductor L1, the second branch inductor L2, the filter capacitor C1, the first filter inductor L3, and the second filter inductor L4 in the first filter circuit constitute an LCL filter.

[0090] Combination Figures 3a to 8d In the positive half-cycle charging mode of the photovoltaic inverter 10, a current loop is formed: photovoltaic array PV - first upper switch S1 - first filter inductor L3 - AC load Grid - second filter inductor L4 - first branch inductor L1 - second lower switch S4 - photovoltaic array PV.

[0091] In the positive half-cycle freewheeling mode of the photovoltaic inverter 10, a current loop is formed: first upper switch S1 - first filter inductor L3 - AC load Grid - second filter inductor L4 - first branch inductor L1 - first branch switch Q1 - first upper switch S1.

[0092] In the negative half-cycle charging mode of the photovoltaic inverter 10, a current loop is formed: photovoltaic array PV - second upper switch S3 - second filter inductor L4 - AC load Grid - first filter inductor L3 - second branch inductor L2 - first lower switch S2 - photovoltaic array PV.

[0093] In the negative half-cycle freewheeling mode of the photovoltaic inverter 10, a current loop is formed: second upper switch S3 - second filter inductor L4 - AC load Grid - first filter inductor L3 - second branch inductor L2 - second branch switch Q2 - second upper switch S3.

[0094] In the above embodiments, in the four modes, the first filter inductor and the second filter inductor are connected in series in the current loop and work together with the filter capacitor to filter the AC power converted by the H-bridge inverter circuit and reduce signal interference.

[0095] In one exemplary embodiment, such as Figure 4 As shown, the photovoltaic inverter 10 also includes a second filter circuit, which includes a common-mode inductor L5, a first capacitor Cx1, a second capacitor Cx2, a third capacitor Cy1, and a fourth capacitor Cy2. The first terminal of the common-mode inductor L5 is connected to the second terminal of the first filter inductor L3, the second terminal of the common-mode inductor L5 is connected to the positive bus of the AC load Grid, the third terminal of the common-mode inductor L5 is connected to the second terminal of the second filter inductor L4, and the fourth terminal of the common-mode inductor L5 is connected to the negative bus of the AC load Grid. The two terminals of the first capacitor Cx1 are connected to the first and third terminals of the common-mode inductor L5, respectively. The two terminals of the second capacitor Cx2 are connected to the second and fourth terminals of the common-mode inductor L5, respectively. The two terminals of the third capacitor Cy1 are connected to the second terminal of the common-mode inductor L5 and the ground terminal, respectively. The two terminals of the fourth capacitor Cy2 are connected to the fourth terminal of the common-mode inductor L5 and the negative bus of the AC load Grid, respectively.

[0096] In this embodiment, the common-mode inductor L5, the first capacitor Cx1, the second capacitor Cx2, the third capacitor Cy1, and the fourth capacitor Cy2 constitute an EMI (Electromagnetic Interference) filter.

[0097] Combination Figures 3a to 8d In the positive half-cycle charging mode of the photovoltaic inverter 10, a current loop is formed: photovoltaic array PV - first upper switch S1 - first filter inductor L3 - common mode inductor L5 - AC load Grid - common mode inductor L5 - second filter inductor L4 - first branch inductor L1 - second lower switch S4 - photovoltaic array PV.

[0098] In the positive half-cycle freewheeling mode of the photovoltaic inverter 10, a current loop is formed: first upper switch S1 - first filter inductor L3 - common mode inductor L5 - AC load Grid - common mode inductor L5 - second filter inductor L4 - first branch inductor L1 - first branch switch Q1 - first upper switch S1.

[0099] In the negative half-cycle charging mode of the photovoltaic inverter 10, a current loop is formed: photovoltaic array PV - second upper switch S3 - second filter inductor L4 - common mode inductor L5 - AC load Grid - common mode inductor L5 - first filter inductor L3 - second branch inductor L2 - first lower switch S2 - photovoltaic array PV.

[0100] In the negative half-cycle freewheeling mode of the photovoltaic inverter 10, the following current loop is formed: second upper switch S3 - second filter inductor L4 - common mode inductor L5 - AC load Grid - common mode inductor L5 - first filter inductor L3 - second branch inductor L2 - second branch switch Q2 - second upper switch S3.

[0101] In the above embodiments, in the four modes, the common-mode inductor inserted into the current loop can suppress common-mode interference in the current loop. Together with the first capacitor, the second capacitor, the third capacitor, and the fourth capacitor, it can effectively improve electromagnetic interference noise.

[0102] In one exemplary embodiment, such as Figure 5 As shown, the photovoltaic inverter 10 also includes a first diode D1 and a second diode D2; the cathode of the first diode D1 is connected to the first end, and the anode of the first diode D1 is used to connect to the positive terminal of the photovoltaic array PV; the anode of the second diode D2 is connected to the second end, and the cathode of the second diode D2 is used to connect to the negative terminal of the photovoltaic array PV; wherein, the first end is the connection point of the first upper switch S1 and the second upper switch S3, and the second end is the connection point of the first lower switch S2 and the second lower switch S4.

[0103] In this embodiment, adding a first diode D1 and a second diode D2 to the DC side of the photovoltaic array (PV) forms a current loop from the positive terminal of the PV array to the H-bridge inverter circuit 11, the AC load Grid, the H-bridge inverter circuit 11, and back to the negative terminal of the PV array. However, it does not form a current loop in the opposite direction. In other words, adding the first diode D1 and the second diode D2 can achieve single-phase circuit clamping and effectively isolate the photovoltaic array from the AC load.

[0104] In one exemplary embodiment, such as Figure 6 As shown, the Z-source network includes a first network inductor L6, a second network inductor L7, a first network capacitor C2, and a second network capacitor C3. The first end of the first network inductor L6 is connected to the positive terminal of the photovoltaic array PV, and the second end of the first network inductor L6 is connected to the first end terminal. The first end of the second network inductor L7 is connected to the negative terminal of the photovoltaic array PV, and the second end of the second network inductor L7 is connected to the second end terminal. The first end of the first network capacitor C2 is connected to the first end of the first network inductor L6, and the second end of the first network capacitor C2 is connected to the second end of the second network inductor L7. The first end of the second network capacitor C3 is connected to the second end of the first network inductor L6, and the second end of the first network capacitor C2 is connected to the first end of the second network inductor L7.

[0105] In this embodiment, when the H-bridge inverter circuit 11 is operating, a bridge arm shoot-through situation may occur. For example, if the first upper switch S1 and the first lower switch S2 are both turned on simultaneously, the first bridge arm 111 will shoot through; if the second upper switch S3 and the second lower switch S4 are both turned on simultaneously, the second bridge arm 112 will shoot through. Since bridge arm shoot-through will affect the normal operation of the photovoltaic inverter 10, a dead time is set. During the dead time, both switches on the same bridge arm are turned off. However, this method is prone to causing output waveform distortion. To address this problem, a Z-source network is constructed using the first network inductor L6, the second network inductor L7, the first network capacitor C2, and the second network capacitor C3.

[0106] During the operation of the Z-source network, especially when a shoot-through occurs in the arm of the H-bridge inverter circuit 11, the energy flow changes drastically. Network capacitors have the ability to store charge; in the shoot-through state, the energy stored in the network inductor changes, and the network capacitors can absorb or release energy to buffer this fluctuation.

[0107] For example, when the first bridge arm 111 in the H-bridge inverter circuit 11 is directly connected, the current change of the first network inductor L6 and the second network inductor L7 causes voltage fluctuations. The first network capacitor C2 and the second network capacitor C3 can release or absorb electrical energy in time to avoid large voltage fluctuations, so that the common-mode voltage of the clamping point can be kept at a relatively stable level.

[0108] In the above embodiments, the use of Z-source network can effectively prevent false triggering and avoid shoot-through of the upper and lower switches on the same bridge arm. Since there is no need to set dead time, it can effectively reduce harmonic pollution of the output sine wave and improve the current waveform quality.

[0109] In an exemplary embodiment, when the photovoltaic inverter is in the positive half-cycle charging mode, the common-mode voltage formed by the photovoltaic array PV is half the sum of the midpoint voltage of the first bridge arm 111 and the midpoint voltage of the second bridge arm 112.

[0110] Combination Figures 3a to 8d In the positive half-cycle charging mode of the photovoltaic inverter 10, the first upper switch S1 is turned on, the first lower switch S2 is turned off, the second upper switch S3 is turned off, the second lower switch S4 is switched on and off at a preset frequency, the first branch switch Q1 is turned off, and the second branch switch Q2 is turned off. When the second lower switch S4 is turned on, a current loop is formed: photovoltaic array PV - first diode D1 - first network inductor L6 - first upper switch S1 - first filter inductor L3 - common mode inductor L5 - AC load Grid - common mode inductor L5 - second filter inductor L4 - first branch inductor L1 - second lower switch S4 - second network inductor L7 - second diode D2 - photovoltaic array PV.

[0111] Due to the alternating on and off of the switching transistor, such as Figure 8a As shown, high-frequency alternating voltages Uao1 and Ubo1 are formed between points a and b and point o of the DC bus.

[0112]

[0113]

[0114] U PV Ugrid is the DC voltage output by the photovoltaic array PV, Ugrid is the voltage of the AC load Grid, and the common-mode voltage Ucm1 is calculated according to the following formula:

[0115]

[0116] In an exemplary embodiment, when the photovoltaic inverter is in the negative half-cycle charging mode, the common-mode voltage formed by the photovoltaic array PV is half the sum of the midpoint voltage of the first bridge arm 111 and the midpoint voltage of the second bridge arm 112.

[0117] In the positive half-cycle freewheeling mode of the photovoltaic inverter 10, the first upper switch S1 is turned on, the first lower switch S2 is turned off, the second upper switch S3 is turned off, the second lower switch S4 is turned off, the first branch switch Q1 is turned on, and the second branch switch Q2 is turned off, forming a current loop: first upper switch S1 - first filter inductor L3 - common mode inductor L5 - AC load Grid - common mode inductor L5 - second filter inductor L4 - first branch inductor L1 - first branch switch Q1 - first upper switch S1.

[0118] like Figure 8b As shown, high-frequency alternating voltages Uao2 and Ubo2 are formed between points a and b and point o of the DC bus.

[0119]

[0120]

[0121] In the continuous flow phase, point b and point o are not directly connected, which can be equivalently represented as follows:

[0122]

[0123] U PV Ucm2 is the DC voltage output by the photovoltaic array PV, Ugrid is the voltage of the AC load Grid, and the common-mode voltage Ucm2 is calculated according to the following formula:

[0124]

[0125] In an exemplary embodiment, when the inverter is in negative half-cycle freewheeling mode, the common-mode voltage generated by the photovoltaic array PV is half the sum of the midpoint voltage of the first bridge arm 111 and the midpoint voltage of the second bridge arm 112.

[0126] In the negative half-cycle charging mode of the photovoltaic inverter 10, the first upper switch S1 is off, the first lower switch S2 is switched on and off at a preset frequency, the second upper switch S3 is on, the second lower switch S4 is off, the first branch switch Q1 is off, and the second branch switch Q2 is off. When the first lower switch S2 is on, a current loop is formed: photovoltaic array PV - first diode D1 - first network inductor L6 - second upper switch S3 - second filter inductor L4 - common mode inductor L5 - AC load Grid - common mode inductor L5 - first filter inductor L3 - second branch inductor L2 - first lower switch S2 - second network inductor L7 - second diode D2 - photovoltaic array PV.

[0127] like Figure 8c As shown, high-frequency alternating voltages Uao3 and Ubo3 are formed between points a and b and point o of the DC bus.

[0128]

[0129]

[0130] U PV Ucm3 is the DC voltage output by the photovoltaic array PV, Ugrid is the voltage of the AC load Grid, and the common-mode voltage Ucm3 is calculated according to the following formula:

[0131]

[0132] In an exemplary embodiment, when the inverter is in negative half-cycle freewheeling mode, the common-mode voltage generated by the photovoltaic array PV is half the sum of the midpoint voltage of the first bridge arm 111 and the midpoint voltage of the second bridge arm 112.

[0133] In the negative half-cycle freewheeling mode of the photovoltaic inverter 10, the first upper switch S1 is turned off, the first lower switch S2 is turned off, the second upper switch S3 is turned on, the second lower switch S4 is turned off, the first branch switch Q1 is turned off, and the second branch switch Q2 is turned on, forming a current loop: second upper switch S3 - second filter inductor L4 - common mode inductor L5 - AC load Grid - common mode inductor L5 - first filter inductor L3 - second branch inductor L2 - second branch switch Q2 - second upper switch S3.

[0134] like Figure 8d As shown, high-frequency alternating voltages Uao4 and Ubo4 are formed between points a and b and point o of the DC bus.

[0135]

[0136]

[0137] U PV Ugrid is the DC voltage output by the photovoltaic array PV, Ugrid is the voltage of the AC load Grid, and the common-mode voltage Ucm4 is calculated according to the following formula:

[0138]

[0139] Simulation was performed using simulation software under the following conditions: the photovoltaic array PV was set to 10kW, its parasitic capacitance to ground was 500nF, the DC high-voltage bus voltage was 400V, unipolar control was used, the switching frequency was 20kHz, the rated output AC voltage RMS value was 220V, and the output power was 6kW. (Refer to...) Figure 9a Simulation results of the traditional structure shown and Figure 9b The simulation results of the embodiments of this application show that the peak leakage current of the traditional H4 full-bridge inverter is as high as 600mA, while the leakage current of the embodiments of this application is less than 30mA, which has a significant suppression effect and can greatly reduce the system's leakage current to ground.

[0140] In the above embodiments, in the positive half-cycle charging mode, negative half-cycle charging mode, positive half-cycle freewheeling mode and negative half-cycle freewheeling mode of the photovoltaic inverter, the common-mode voltage formed by the photovoltaic array can be maintained at the preset target voltage, that is, the common-mode voltage can be kept constant, thus effectively suppressing leakage current.

[0141] In the description of this specification, references to terms such as "some embodiments," "other embodiments," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative descriptions of the above terms do not necessarily refer to the same embodiments or examples.

[0142] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0143] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these modifications and improvements all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A single-phase transformerless photovoltaic inverter, characterized in that, include: The H-bridge inverter circuit includes a first bridge arm and a second bridge arm connected in parallel. The input terminal of the H-bridge inverter circuit is connected to a photovoltaic array and is used to convert the direct current output by the photovoltaic array into alternating current. The first branch connects the midpoint of the second bridge arm and the first end, where both the first and second bridge arms are connected to the positive terminal of the photovoltaic array. In the positive half-cycle charging mode of the photovoltaic inverter, the first branch is used for charging and energy storage. In the positive half-cycle freewheeling mode of the photovoltaic inverter, the first branch is also used to form a discharge current loop with the H-bridge inverter circuit to achieve freewheeling, and to achieve electrical isolation between the AC load and the DC power. The second branch connects the midpoint and the first end of the first bridge arm. In the negative half-cycle charging mode of the photovoltaic inverter, the second branch is used for charging and energy storage. In the negative half-cycle freewheeling mode of the photovoltaic inverter, the second branch is also used to form a discharge current loop with the H-bridge inverter circuit to achieve freewheeling, and to achieve electrical isolation between the AC load and the DC power. In all four modes, the common-mode voltage generated by the photovoltaic array is maintained at a preset target voltage.

2. The single-phase transformerless photovoltaic inverter according to claim 1, characterized in that, The first bridge arm includes a first upper switch and a first lower switch connected in series, and the second bridge arm includes a second upper switch and a second lower switch connected in series. The first branch includes a first branch switch and a first branch inductor disposed on the second bridge arm, and the second branch includes a second branch switch and a second branch inductor disposed on the first bridge arm. The first end and the second end of the first branch inductor are respectively connected to the second upper switch and the second lower switch, and the output end of the first branch switch is connected to the first end and the second end of the first branch inductor; the first end of the first branch inductor is the midpoint of the second bridge arm for connecting the AC load; The first and second ends of the second branch inductor are respectively connected to the first upper switch and the first lower switch, and the output end of the second branch switch is connected to the first end and the second end of the second branch inductor; the first end of the second branch inductor is the midpoint of the first bridge arm for connecting the AC load.

3. The single-phase transformerless photovoltaic inverter according to claim 2, characterized in that, The photovoltaic inverter also includes a first filter circuit, which includes a filter capacitor, a first filter inductor, and a second filter inductor. The first end of the filter capacitor is connected to the midpoint of the first bridge arm, and the second end of the filter capacitor is connected to the midpoint of the second bridge arm. The first end of the first filter inductor is connected to the first end of the filter capacitor, and the second end of the first filter inductor is connected to the AC load. The first end of the second filter inductor is connected to the second end of the filter capacitor, and the second end of the second filter inductor is connected to the AC load.

4. The single-phase transformerless photovoltaic inverter according to claim 3, characterized in that, The photovoltaic inverter also includes a second filter circuit, which includes a common-mode inductor, a first capacitor, a second capacitor, a third capacitor, and a fourth capacitor. The first terminal of the common-mode inductor is connected to the second terminal of the first filter inductor, the second terminal of the common-mode inductor is connected to the positive bus of the AC load, the third terminal of the common-mode inductor is connected to the second terminal of the second filter inductor, and the fourth terminal of the common-mode inductor is connected to the negative bus of the AC load. The two ends of the first capacitor are respectively connected to the first and third ends of the common-mode inductor; The two ends of the second capacitor are respectively connected to the second and fourth ends of the common-mode inductor; The two ends of the third capacitor are respectively connected to the second end of the common-mode inductor and the ground end; The two ends of the fourth capacitor are respectively connected to the fourth end of the common-mode inductor and the negative bus of the AC load.

5. The single-phase transformerless photovoltaic inverter according to claim 4, characterized in that, The photovoltaic inverter further includes a first diode and a second diode; the cathode of the first diode is connected to a first end, the anode of the first diode is used to connect to the positive electrode of the photovoltaic array, the anode of the second diode is connected to a second end, and the cathode of the second diode is used to connect to the negative electrode of the photovoltaic array, wherein the first end is the connection point of the first upper switch and the second upper switch, and the second end is the connection point of the first lower switch and the second lower switch.

6. The single-phase transformerless photovoltaic inverter according to claim 5, characterized in that, The photovoltaic inverter also includes a Z-source network, which includes a first network inductor, a second network inductor, a first network capacitor, and a second network capacitor. The first end of the first network inductor is connected to the positive terminal of the photovoltaic array, and the second end of the first network inductor is connected to the first end. The first end of the second network inductor is connected to the negative terminal of the photovoltaic array, and the second end of the second network inductor L7 is connected to the second end. The first end of the first network capacitor is connected to the first end of the first network inductor, and the second end of the first network capacitor is connected to the second end of the second network inductor. The first end of the second network capacitor is connected to the second end of the first network inductor, and the second end of the first network capacitor is connected to the first end of the second network inductor.

7. The single-phase transformerless photovoltaic inverter according to claim 6, characterized in that, When the photovoltaic inverter is in the positive half-cycle charging mode, the common-mode voltage formed by the photovoltaic array is half the sum of the midpoint voltage of the first bridge arm and the midpoint voltage of the second bridge arm. In the positive half-cycle charging mode of the photovoltaic inverter, the first upper switch is turned on, the first lower switch is turned off, the second upper switch is turned off, the second lower switch is turned on and off at a preset frequency, the first branch switch is turned off, and the second branch switch is turned off. When the second lower switch is turned on, the current loop formed includes: the photovoltaic array - the first diode - the first network inductor - the first upper switch - the first filter inductor - the common mode inductor - the AC load - the common mode inductor - the second filter inductor - the first branch inductor - the second lower switch - the second network inductor - the second diode photovoltaic array.

8. The single-phase transformerless photovoltaic inverter according to claim 6, characterized in that, When the photovoltaic inverter is in the negative half-cycle charging mode, the common-mode voltage formed by the photovoltaic array is half the sum of the midpoint voltage of the first bridge arm and the midpoint voltage of the second bridge arm. In the negative half-cycle charging mode of the photovoltaic inverter, the first upper switch is turned off, the first lower switch is switched on and off at a preset frequency, the second upper switch is turned on, the second lower switch is turned off, the first branch switch is turned off, and the second branch switch is turned off, forming a current loop including: the photovoltaic array - the first diode - the first network inductor - the second upper switch - the second filter inductor - the common mode inductor - the AC load - the common mode inductor - the first filter inductor - the second branch inductor - the first lower switch - the second network inductor - the second diode.

9. The single-phase transformerless photovoltaic inverter according to claim 6, characterized in that, When the photovoltaic inverter is in the positive half-cycle freewheeling mode, the common-mode voltage formed by the photovoltaic array is half the sum of the midpoint voltage of the first bridge arm and the midpoint voltage of the second bridge arm. In the positive half-cycle freewheeling mode of the photovoltaic inverter, the first upper switch is turned on, the first lower switch is turned off, the second upper switch is turned off, the second lower switch is turned off, the first branch switch is turned on, and the second branch switch is turned off, forming a current loop including: the first upper switch - the first filter inductor - the common mode inductor - the AC load - the common mode inductor - the second filter inductor - the first branch inductor - the first branch switch - the first upper switch.

10. The single-phase transformerless photovoltaic inverter according to claim 6, characterized in that, When the photovoltaic inverter is in the negative half-cycle freewheeling mode, the common-mode voltage formed by the photovoltaic array is half the sum of the midpoint voltage of the first bridge arm and the midpoint voltage of the second bridge arm. In the negative half-cycle freewheeling mode of the photovoltaic inverter, the first upper switch is off, the first lower switch is off, the second upper switch is on, the second lower switch is off, the first branch switch is off, and the second branch switch is on, forming a current loop: second upper switch - second filter inductor - common mode inductor - AC load - common mode inductor - first filter inductor - second branch inductor - second branch switch - second upper switch - photovoltaic array.

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