A single-phase five-level photovoltaic grid-connected inverter circuit

Abstract

The utility model discloses a single -phase five -level photovoltaic grid -connected inverter circuit, include: with the positive terminal of photovoltaic power supply PV connects the former stage DC DC conversion circuit, the latter stage five -level inverter circuit, by the first capacitor C1 and the second capacitor C2 series connection constitute direct current bus, it is used for connecting the former stage DC DC conversion circuit with the latter stage five -level inverter circuit. Utilize the scheme of the utility model, adopt the former stage DC DC conversion circuit to realize wide range maximum power point tracking, and charge by the direct current bus of two series capacitor constitution, has guaranteed the efficient energy capture and transmission. The latter stage five -level inverter circuit can generate the five -level alternating voltage waveform of sine wave closer through the flexible selection of different voltage source combination in the output end, greatly reduced total harmonic distortion, thereby reduced the requirement of filter volume and cost, and reduced the voltage stress of switch tube, improved the electric energy quality, reliability and life of whole system.

Description

Technical Field

[0001] This utility model generally relates to the field of power electronics technology. More specifically, this utility model relates to a single-phase five-level photovoltaic grid-connected inverter circuit. Background Technology

[0002] Against the backdrop of the current energy transition, distributed renewable energy (especially photovoltaics) is being integrated into the grid on a large scale. In this process, single-phase non-isolated (transformerless) photovoltaic grid-connected inverters have become the mainstream technical solution due to their advantages such as small size and low cost.

[0003] However, existing technologies have two major problems: First, safety and efficiency issues: due to the lack of transformer isolation, there is an electrical connection between the photovoltaic side and the grid side, which will generate leakage current. This leakage current will not only reduce system efficiency and cause electromagnetic interference, but more seriously, it may threaten the safety of equipment and personnel. Second, power quality issues: traditional two-level inverters generate a large amount of current harmonics during operation, polluting the power grid and failing to deliver high-quality clean power to the grid.

[0004] In view of this, there is an urgent need to provide a single-phase five-level photovoltaic grid-connected inverter circuit, which can suppress leakage current and improve power quality, thus realizing the safe and efficient utilization of solar energy. Utility Model Content

[0005] In order to solve at least one or more of the technical problems mentioned above, this utility model proposes a single-phase five-level photovoltaic grid-connected inverter circuit in several aspects.

[0006] This utility model provides a single-phase five-level photovoltaic grid-connected inverter circuit, comprising: a front-stage DC-DC converter circuit connected to the positive terminal of a photovoltaic power source PV; a rear-stage five-level inverter circuit; and a DC bus consisting of a first capacitor C1 and a second capacitor C2 connected in series, which is used to connect the front-stage DC-DC converter circuit and the rear-stage five-level inverter circuit; wherein, the negative terminal of the photovoltaic power source PV is configured to be connected to the neutral point of the power grid; the front-stage DC-DC converter circuit is used to transfer the energy of the photovoltaic power source PV to the DC bus to charge the first capacitor C1 and the second capacitor C2; the rear-stage five-level inverter circuit selects different voltages from the first capacitor C1, the second capacitor C2, and the series combination of the first capacitor C1 and the second capacitor C2 on the DC bus by controlling the selective conduction of multiple switching transistors, so as to generate a five-level AC voltage waveform at the output terminal.

[0007] In some embodiments, the front-stage DC-DC converter circuit employs a buck-boost circuit. The front-stage DC-DC converter circuit includes a first switch S11, a second switch S12, a first inductor L1, a first diode D1, and a second diode D2. The first terminal of the first switch S11 is connected to the positive terminal of the photovoltaic power supply PV. The first terminal of the second switch S12 is connected to the second terminal of the first switch S11 through the first inductor L1. The second terminal of the second switch S12 is connected to the neutral point of the power grid. The cathode of the first diode D1 is connected to the second terminal of the first switch S11, and the anode of the first diode D1 is connected to the first terminal of the DC bus. The anode of the second diode D2 is connected to the first terminal of the second switch S12, and the cathode of the second diode D2 is connected to the second terminal of the DC bus.

[0008] In some embodiments, by controlling the first switch S11 and the corresponding switch in the subsequent five-level inverter circuit to turn on, a charging path is formed for the first capacitor C1 and / or the second capacitor C2; by controlling the second switch S12 and the corresponding switch in the subsequent five-level inverter circuit to turn on, a discharging path is formed for the first capacitor C1 and / or the second capacitor C2.

[0009] In some embodiments, the subsequent five-level inverter circuit includes a third switch S21, a fourth switch S22, a fifth switch S23, a sixth switch S24, a seventh switch S25, a second inductor L2, and a third capacitor C3; the first terminal of the third switch S21 is connected to the first terminal of the DC bus, the first terminal of the fourth switch S22 is connected to the connection point of the first capacitor C1 and the second capacitor C2, the second terminal of the fourth switch S22 is connected to the second terminal of the third switch S21, and the first terminal of the fifth switch S23 is connected to the second terminal of the DC bus. The second terminal of the fifth switch S23 is connected to the second terminal of the fourth switch S22. The first terminal of the sixth switch S24 is connected to the connection point of the first capacitor C1 and the second capacitor C2. The second terminal of the sixth switch S24 is connected to the neutral point of the power grid. The first terminal of the seventh switch S25 is connected to the second terminal of the fifth switch S23. The second terminal of the seventh switch S25 is connected to the neutral point of the power grid. The first terminal of the second inductor L2 is connected to the first terminal of the seventh switch S25. The second terminal of the second inductor L2 is connected to the neutral point of the power grid through the third capacitor C3.

[0010] In some embodiments, the five voltage levels include a first positive voltage level, a second positive voltage level, a first negative voltage level, a second negative voltage level, and a zero level.

[0011] In some embodiments, when the subsequent five-level inverter circuit outputs a first positive voltage level or a first negative voltage level, it simultaneously turns on the third switch S21, the fourth switch S22, and the seventh switch S25, or simultaneously turns on the fourth switch S22, the fifth switch S23, and the seventh switch S25; when the third switch S21, the fourth switch S22, and the seventh switch S25 are simultaneously turned on, the first capacitor C1 supplies power to the output terminal; when the fourth switch S22, the fifth switch S23, and the seventh switch S25 are simultaneously turned on, the second capacitor C2 supplies power to the output terminal.

[0012] In some embodiments, when the subsequent five-level inverter circuit outputs a second positive voltage level or a second negative voltage level, it simultaneously turns on the third switch S21, the fifth switch S23, and the seventh switch S25, so that the first capacitor C1 and the second capacitor C2 are connected in series to supply power to the output terminal.

[0013] In some embodiments, when the output of the subsequent five-level inverter circuit is at zero level, the seventh switch S25 is turned on.

[0014] By directly connecting the negative terminal of the photovoltaic (PV) power supply to the grid neutral point through the single-phase five-level photovoltaic grid-connected inverter circuit provided above, this embodiment of the invention effectively eliminates common-mode leakage current and significantly improves system safety and grid-connected stability. Simultaneously, it employs a front-stage DC-DC converter circuit to achieve wide-range maximum power point tracking and charges the DC bus composed of two series capacitors, ensuring efficient energy capture and transfer. The subsequent five-level inverter circuit, through flexible selection of different voltage source combinations, generates a five-level AC voltage waveform at the output that is closer to a sine wave, greatly reducing total harmonic distortion (THD). This reduces the requirements for filter size and cost, lowers the voltage stress on the switching transistors, and improves the overall system's power quality, reliability, and lifespan.

[0015] Furthermore, in some embodiments, the PV output voltage is flexibly adjusted through a front-stage DC-DC converter circuit. This efficiently converts photovoltaic energy into a stable DC power supply, adapting to a wide range of photovoltaic input voltage variations. This allows for more precise maximum power point tracking under different lighting conditions, maximizing energy capture. Simultaneously, it effectively charges the first and second capacitors in the DC bus, providing a stable and voltage-balanced DC bus for the subsequent five-level inverter circuit, thus ensuring the efficient and stable operation of the entire inverter circuit and providing high-quality AC output.

[0016] Furthermore, in some embodiments, by explicitly controlling the first and second switches in the front-end DC-DC converter circuit to work in coordination with the switches in the rear-end inverter circuit, specific paths for charging and discharging the DC bus capacitor are defined, thereby endowing the circuit with the core capability of bidirectional power flow. This not only realizes the standard charging function from the photovoltaic side to the DC side, but also establishes a reverse discharge path, enabling the inverter to perform advanced grid support functions. This bidirectional control flexibility greatly enhances the inverter's versatility and grid friendliness, transforming it from a simple power generation unit into a smart grid device capable of actively participating in grid voltage and frequency regulation, significantly improving the system's applicability and added value.

[0017] Furthermore, in some embodiments, a specific and efficient implementation scheme for achieving five-level output is provided by precisely defining the topology of the five switches and filter elements in the subsequent inverter circuit. Compared to traditional five-level topologies that require more switches, this design significantly reduces the number of power devices, directly resulting in a comprehensive benefit of reduced cost, increased power density, and optimized efficiency. Moreover, it is this ingenious combination of switches that enables high-quality, low-harmonic five-level voltage output. This not only significantly reduces the size and cost of the output filter but also allows for the use of higher-performance low-voltage devices by distributing the voltage stress on the switches, thereby further improving the overall reliability and lifespan of the inverter. Attached Figure Description

[0018] The above and other objects, features, and advantages of the present invention will become readily understood by reading the following detailed description of exemplary embodiments with reference to the accompanying drawings. In the drawings, several embodiments of the present invention are shown by way of example and not limitation, and like or corresponding reference numerals denote like or corresponding parts, wherein:

[0019] Figure 1 An exemplary structural block diagram of a single-phase five-level photovoltaic grid-connected inverter circuit according to an embodiment of the present invention is shown;

[0020] Figure 2 A circuit diagram of a single-phase five-level photovoltaic grid-connected inverter circuit according to an embodiment of the present invention is shown;

[0021] Figure 3 A circuit diagram illustrating the charging of the first and second capacitors according to an embodiment of the present invention is shown.

[0022] Figure 4 A circuit diagram illustrating the discharge of the first and second capacitors according to an embodiment of the present invention is shown.

[0023] Figure 5A circuit diagram showing the output of a first positive voltage level or a first negative voltage level in one embodiment of the present invention is shown;

[0024] Figure 6 A circuit diagram showing the output of a first positive voltage level or a first negative voltage level is shown in another embodiment of the present invention;

[0025] Figure 7 A circuit diagram showing the output of a second positive voltage level or a second negative voltage level according to an embodiment of the present invention is shown;

[0026] Figure 8 A circuit diagram showing the zero-level output of an embodiment of this utility model is provided. Detailed Implementation

[0027] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present utility model.

[0028] It should be understood that the terms "comprising" and "including" used in the specification and claims of this utility model indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0029] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the scope of the invention. As used in this specification and claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used in this specification and claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes such combinations.

[0030] Figure 1 An exemplary structural block diagram of a single-phase five-level photovoltaic grid-connected inverter circuit according to an embodiment of the present invention is shown.

[0031] like Figure 1As shown, the single-phase five-level photovoltaic grid-connected inverter circuit 100 includes: a front-stage DC-DC converter circuit 110 connected to the positive terminal of the photovoltaic power source PV, a rear-stage five-level inverter circuit 120, and a DC bus 130. Specifically, the DC bus 130 is composed of a first capacitor C1 and a second capacitor C2 connected in series, and is used to connect the front-stage DC-DC converter circuit 110 and the rear-stage five-level inverter circuit 120.

[0032] In embodiments of this application, the negative terminal of the photovoltaic power source PV is configured to be connected to the neutral point of the power grid.

[0033] In the embodiments of this application, the front-end DC-DC converter circuit 110 adopts a buck-boost circuit, and the front-end DC-DC converter circuit 120 is used to transfer the energy of the photovoltaic power source PV to the DC bus 130 to charge the first capacitor C1 and the second capacitor C2.

[0034] In the embodiments of this application, the subsequent five-level inverter circuit selects different voltages from the first capacitor C1, the second capacitor C2 and the series combination of the first capacitor C1 and the second capacitor C2 of the DC bus by controlling the selective conduction of multiple switching transistors, so as to generate a five-level AC voltage waveform at the output terminal.

[0035] In the embodiments of this application, the negative terminal of the first capacitor C1 serves as the first end of the DC bus, the negative terminal of the DC bus of the second capacitor C2 is connected to the positive terminal of the second capacitor C1, and the positive terminal of the DC bus of the second capacitor C2 serves as the second end of the DC bus.

[0036] The following is combined Figure 2 The specific composition and connection relationship of the front-stage DC-DC converter circuit 110, the rear-stage five-level inverter circuit 120 and the DC bus 130 are described in detail.

[0037] like Figure 2 As shown, the front-end DC-DC converter circuit 110 includes a first switch S11, a second switch S12, a first inductor L1, a first diode D1, and a second diode D2. The first terminal of the first switch S11 is connected to the positive terminal of the photovoltaic power supply PV. The first terminal of the second switch S12 is connected to the second terminal of the first switch S11 through the first inductor L1. The second terminal of the second switch S12 is connected to the neutral point of the power grid. The cathode of the first diode D1 is connected to the second terminal of the first switch S11, and the anode of the first diode D1 is connected to the first terminal of the DC bus. The anode of the second diode D2 is connected to the first terminal of the second switch S12, and the cathode of the second diode D2 is connected to the second terminal of the DC bus.

[0038] By connecting the first diode D1 in parallel with the first switch S11 and then in reverse, a good reverse current conduction capability can be achieved, resulting in a better waveform spectrum output.

[0039] The subsequent five-level inverter circuit 120 includes a third switch S21, a fourth switch S22, a fifth switch S23, a sixth switch S24, a seventh switch S25, a second inductor L2, and a third capacitor C3. The first terminal of the third switch S21 is connected to the first terminal of the DC bus. The first terminal of the fourth switch S22 is connected to the connection point of the first capacitor C1 and the second capacitor C2. The second terminal of the fourth switch S22 is connected to the second terminal of the third switch S21. The first terminal of the fifth switch S23 is connected to the second terminal of the DC bus. The second terminal of the fifth switch S23 is connected to the second terminal of the fourth switch S22. The first terminal of the sixth switch S24 is connected to the connection point of the first capacitor C1 and the second capacitor C2. The second terminal of the sixth switch S24 is connected to the neutral point of the power grid. The first terminal of the seventh switch S25 is connected to the second terminal of the fifth switch S23. The second terminal of the seventh switch S25 is connected to the neutral point of the power grid. The first terminal of the second inductor L2 is connected to the first terminal of the seventh switch S25. The second terminal of the second inductor L2 is connected to the neutral point of the power grid through the third capacitor C3.

[0040] Specifically, the third switch S21 and the fifth switch S23 constitute the basic inverter bridge arm, the fourth switch S22 acts as a clamping switch, and the sixth switch S24 and the seventh switch S25 are used to provide a path to the grid neutral point.

[0041] Specifically, the second terminal of the second inductor L2 is used as the output terminal.

[0042] In the embodiments of this application, the aforementioned first switch S11, second switch S12, third switch S21, fourth switch S22, fifth switch S23, sixth switch S24 and seventh switch S25 are all insulated gate bipolar transistors (IGBTs).

[0043] In the embodiments of this application, by controlling the first switch S11 and the corresponding switch in the subsequent five-level inverter circuit 120 to be turned on, a charging path is formed for the first capacitor C1 and / or the second capacitor C2.

[0044] Specifically, by controlling the first switch S11, the second diode D2, and the third switch S21 and the seventh switch S25 in the subsequent five-level inverter circuit 120 to be turned on, while the remaining switches are in the off state, the first capacitor C1 and the second capacitor C2 are charged. For details, please refer to [reference needed]. Figure 3 .

[0045] Specifically, by controlling the first switch S11, the second diode D2, and the fourth switch S22 and the seventh switch S25 in the subsequent five-level inverter circuit 120 to be turned on, while the other switches are turned off, the second capacitor C2 is charged separately, thus achieving flexible selection of charging.

[0046] In the embodiments of this application, by controlling the second switch S12 and the corresponding switch in the subsequent five-level inverter circuit 120 to be turned on, a path for the discharge of the first capacitor C1 and / or the second capacitor C2 is formed.

[0047] Specifically, by controlling the second switch S12, the first diode D1, and the fifth switch S23 and the seventh switch S25 in the subsequent five-level inverter circuit 120 to be turned on, while the remaining switches are in the off state, the first capacitor C1 and the second capacitor C2 are discharged. For details, please refer to [reference needed]. Figure 4 .

[0048] Specifically, by controlling the second switch S12, the first diode D1, and the fourth switch S22 and the seventh switch S25 in the subsequent five-level inverter circuit 120 to be turned on, while the other switches are in the off state, the first capacitor C1 is discharged separately, thus achieving flexible selection of discharge.

[0049] In the embodiments of this application, the aforementioned five voltage levels include a first positive voltage level, a second positive voltage level, a first negative voltage level, a second negative voltage level, and a zero level. Specifically, the first positive voltage level is lower than the second positive voltage level, and the first negative voltage level is higher than the second negative voltage level.

[0050] In one embodiment of this application, when the subsequent five-level inverter circuit 120 outputs a first positive voltage level or a first negative voltage level, it simultaneously turns on the third switch S21, the fourth switch S22, and the seventh switch S25, so that the first capacitor C1 supplies power to the output terminal. For details, please refer to [reference needed]. Figure 5 .

[0051] In another embodiment of this application, when the subsequent five-level inverter circuit 120 outputs a first positive voltage level or a first negative voltage level, it simultaneously turns on the fourth switch S22, the fifth switch S23, and the seventh switch S25, so that the second capacitor C2 supplies power to the output terminal. For details, please refer to [reference needed]. Figure 6 .

[0052] In the embodiments of this application, when the subsequent five-level inverter circuit 120 outputs a second positive voltage level or a second negative voltage level, it simultaneously turns on the third switch S21, the fifth switch S23, and the seventh switch S25, so that the first capacitor C1 and the second capacitor C2 are connected in series to supply power to the output terminal. For details, please refer to [reference needed]. Figure 7.

[0053] In the embodiments of this application, when the output level of the subsequent five-level inverter circuit 120 is zero, the seventh switch S25 is turned on. For details, please refer to [reference needed]. Figure 8 .

[0054] In the process of forming the five-level circuit described above, by providing two different switching combinations for the first positive voltage level and the first negative voltage level, the circuit gains the unique ability to dynamically select either the first capacitor C1 or the second capacitor C2 for power supply during operation. This flexibility is its most critical advantage, enabling the control system to proactively and in real-time correct any potential voltage imbalance between the first capacitor C1 and the second capacitor C2, thereby ensuring the symmetry and accuracy of the output voltage step and ultimately generating high-quality AC power with extremely low harmonic content. Furthermore, by combining the use of the first capacitor C1 and the second capacitor C2 in series to generate either a second positive voltage level or a second negative voltage level, and achieving a clear path to zero level through a single switch, this entire switching strategy not only completely constructs the five-level waveform, but more importantly, through its built-in voltage balancing mechanism, it greatly improves the stability and reliability of the inverter operation, ensuring long-term high-quality power output.

[0055] By directly connecting the negative terminal of the photovoltaic (PV) power supply to the grid neutral point through the single-phase five-level photovoltaic grid-connected inverter circuit provided above, this embodiment of the invention effectively eliminates common-mode leakage current and significantly improves system safety and grid-connected stability. Simultaneously, it employs a front-stage DC-DC converter circuit to achieve wide-range maximum power point tracking and charges the DC bus composed of two series capacitors, ensuring efficient energy capture and transfer. The subsequent five-level inverter circuit, through flexible selection of different voltage source combinations, generates a five-level AC voltage waveform at the output that is closer to a sine wave, greatly reducing total harmonic distortion (THD). This reduces the requirements for filter size and cost, lowers the voltage stress on the switching transistors, and improves the overall system's power quality, reliability, and lifespan.

[0056] Furthermore, in some embodiments, the PV output voltage is flexibly adjusted through a front-stage DC-DC converter circuit. This efficiently converts photovoltaic energy into a stable DC power supply, adapting to a wide range of photovoltaic input voltage variations. This allows for more precise maximum power point tracking under different lighting conditions, maximizing energy capture. Simultaneously, it effectively charges the first and second capacitors in the DC bus, providing a stable and voltage-balanced DC bus for the subsequent five-level inverter circuit, thus ensuring the efficient and stable operation of the entire inverter circuit and providing high-quality AC output.

[0057] Furthermore, in some embodiments, by explicitly controlling the first and second switches in the front-end DC-DC converter circuit to work in coordination with the switches in the rear-end inverter circuit, specific paths for charging and discharging the DC bus capacitor are defined, thereby endowing the circuit with the core capability of bidirectional power flow. This not only realizes the standard charging function from the photovoltaic side to the DC side, but also establishes a reverse discharge path, enabling the inverter to perform advanced grid support functions. This bidirectional control flexibility greatly enhances the inverter's versatility and grid friendliness, transforming it from a simple power generation unit into a smart grid device capable of actively participating in grid voltage and frequency regulation, significantly improving the system's applicability and added value.

[0058] Furthermore, in some embodiments, a specific and efficient implementation scheme for achieving five-level output is provided by precisely defining the topology of the five switches and filter elements in the subsequent inverter circuit. Compared to traditional five-level topologies that require more switches, this design significantly reduces the number of power devices, directly resulting in a comprehensive benefit of reduced cost, increased power density, and optimized efficiency. Moreover, it is this ingenious combination of switches that enables high-quality, low-harmonic five-level voltage output. This not only significantly reduces the size and cost of the output filter but also allows for the use of higher-performance low-voltage devices by distributing the voltage stress on the switches, thereby further improving the overall reliability and lifespan of the inverter.

[0059] While various embodiments of the present invention have been shown and described herein, it will be apparent to those skilled in the art that such embodiments are provided by way of example only. Many modifications, alterations, and alternatives will occur to those skilled in the art without departing from the spirit and intent of the present invention. It should be understood that various alternatives to the embodiments of the present invention described herein may be employed in the practice of the present invention. The appended claims are intended to define the scope of protection of the present invention and therefore cover equivalents or alternatives within the scope of these claims.

Claims

1. A single-phase five-level photovoltaic grid-connected inverter circuit, comprising: A front-end DC-DC converter circuit connected to the positive terminal of a photovoltaic power source (PV). The subsequent five-level inverter circuit; A DC bus consisting of a first capacitor C1 and a second capacitor C2 connected in series is used to connect the front-stage DC-DC converter circuit and the rear-stage five-level inverter circuit. The negative terminal of the photovoltaic power source PV is configured to be connected to the neutral point of the power grid; The front-end DC-DC converter circuit is used to transfer the energy of the photovoltaic power source PV to the DC bus to charge the first capacitor C1 and the second capacitor C2. The subsequent five-level inverter circuit selects different voltages from the first capacitor C1, the second capacitor C2, and the series combination of the first capacitor C1 and the second capacitor C2 on the DC bus by controlling the selective conduction of multiple switching transistors, so as to generate a five-level AC voltage waveform at the output terminal.

2. The single-phase five-level photovoltaic grid-connected inverter circuit according to claim 1, characterized in that, The front-stage DC-DC converter circuit adopts a buck-boost circuit. The front-stage DC-DC converter circuit includes a first switch S11, a second switch S12, a first inductor L1, a first diode D1, and a second diode D2. The first terminal of the first switch S11 is connected to the positive terminal of the photovoltaic power supply PV. The first terminal of the second switch S12 is connected to the second terminal of the first switch S11 through the first inductor L1. The second terminal of the second switch S12 is connected to the neutral point of the power grid. The cathode of the first diode D1 is connected to the second terminal of the first switch S11, and the anode of the first diode D1 is connected to the first terminal of the DC bus. The anode of the second diode D2 is connected to the first terminal of the second switch S12, and the cathode of the second diode D2 is connected to the second terminal of the DC bus.

3. The single-phase five-level photovoltaic grid-connected inverter circuit according to claim 2, characterized in that, By controlling the first switch S11 and the corresponding switch in the subsequent five-level inverter circuit to be turned on, a charging path is formed for the first capacitor C1 and / or the second capacitor C2. By controlling the second switch S12 and the corresponding switch in the subsequent five-level inverter circuit to conduct, a path is formed for the first capacitor C1 and / or the second capacitor C2 to discharge.

4. The single-phase five-level photovoltaic grid-connected inverter circuit according to claim 1, characterized in that, The subsequent five-level inverter circuit includes a third switch S21, a fourth switch S22, a fifth switch S23, a sixth switch S24, a seventh switch S25, a second inductor L2, and a third capacitor C3. The first end of the third switch S21 is connected to the first end of the DC bus. The first end of the fourth switch S22 is connected to the connection point of the first capacitor C1 and the second capacitor C2. The second end of the fourth switch S22 is connected to the second end of the third switch S21. The first end of the fifth switch S23 is connected to the second end of the DC bus. The second end of the fifth switch S23 is connected to the second end of the fourth switch S22. The first end of the sixth switch S24 is connected to the connection point of the first capacitor C1 and the second capacitor C2. The second end of the sixth switch S24 is connected to the neutral point of the power grid. The first end of the seventh switch S25 is connected to the second end of the fifth switch S23. The second end of the seventh switch S25 is connected to the neutral point of the power grid. The first end of the second inductor L2 is connected to the first end of the seventh switch S25. The second end of the second inductor L2 is connected to the neutral point of the power grid through the third capacitor C3.

5. The single-phase five-level photovoltaic grid-connected inverter circuit according to claim 4, characterized in that, The five voltage levels include a first positive voltage level, a second positive voltage level, a first negative voltage level, a second negative voltage level, and a zero level.

6. The single-phase five-level photovoltaic grid-connected inverter circuit according to claim 5, characterized in that, When the subsequent five-level inverter circuit outputs the first positive voltage level or the first negative voltage level, it simultaneously turns on the third switch S21, the fourth switch S22 and the seventh switch S25, or simultaneously turns on the fourth switch S22, the fifth switch S23 and the seventh switch S25. When the third switch S21, the fourth switch S22 and the seventh switch S25 are turned on at the same time, the first capacitor C1 supplies power to the output terminal. When the fourth switch S22, the fifth switch S23, and the seventh switch S25 are turned on simultaneously, the second capacitor C2 supplies power to the output terminal.

7. The single-phase five-level photovoltaic grid-connected inverter circuit according to claim 5, characterized in that, When the subsequent five-level inverter circuit outputs the second positive voltage level or the second negative voltage level, it simultaneously turns on the third switch S21, the fifth switch S23 and the seventh switch S25, so that the first capacitor C1 and the second capacitor C2 are connected in series to supply power to the output terminal.

8. The single-phase five-level photovoltaic grid-connected inverter circuit according to claim 5, characterized in that, When the output of the subsequent five-level inverter circuit is at zero level, the seventh switch S25 is turned on.

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