Three-level converter and control method therefor

Abstract

The present invention relates to the field of photovoltaic power generation, and provides a three-level converter and a control method therefor. The three-level converter comprises a first direct-current input end, a second direct-current input end, a neutral end, and a first power switch to a seventh power switch; a first end of the first power switch is respectively connected to the first direct-current input end and a first end of the third power switch; a second end of the second power switch is respectively connected to the second direct-current input end and a second end of the fourth power switch; a connecting point of a second end of the first power switch and a first end of the second power switch serves as a first alternating-current output end; a connecting point of a second end of the third power switch and a first end of the fourth power switch serves as a second alternating-current output end; the fifth power switch and the sixth power switch are connected in series between the first alternating-current output end and the neutral end; and the seventh power switch is connected between a connecting point of the fifth power switch and the sixth power switch and the second alternating-current output end. The three-level converter of the present invention can improve the grid connection and off-grid efficiency.

Description

A three-level converter and a control method thereof Technical Field

[0001] The present invention relates to the field of photovoltaic power generation, and in particular to a three-level converter and a control method thereof. Background Art

[0002] In photovoltaic power generation systems, inverters are an indispensable part that can convert the variable DC voltage generated by photovoltaic solar panels into AC voltage, which can be fed back to the commercial power transmission system or provided to various power loads.

[0003] North American residential photovoltaic inverters require phase-splitting during operation, meaning they can output two independent AC voltages with a 180° phase shift. As shown in Figure 1, the photovoltaic inverter 11 includes three output ports, L1, L2, and N. Output ports L1 and L2 are live wire ports, and output port N is a neutral wire port. The voltage between output ports L1 and L2 is twice the voltage between output ports L1 and N, or the voltage between output ports L1 and L2 is twice the voltage between output ports L2 and N. Load 1 is connected between output ports L1 and N, and power is supplied to load 1 via the voltage between output ports L1 and N; load 2 is connected between output ports N and L2, and power is supplied to load 2 via the voltage between output ports L2 and N; and load 3 is connected between output ports L1 and L2, and power is supplied to load 3 via the voltage between output ports L1 and L2.

[0004] The following explanation assumes that the voltage between output ports L1 and L2 is 240V. When the photovoltaic inverter 11 is operating off-grid, it can output two-phase 120V power. Specifically, the voltage between output ports L1 and N is 120V, which can power a 120V load 1. Furthermore, the voltage between output ports L2 and N is 120V, which can power a 120V load 2 or a 240V load 3. When the photovoltaic inverter 11 is operating off-grid, the two-phase 120V voltages of load 1 and load 2 may not be identical. Therefore, the photovoltaic inverter 11 must independently control the voltages of these two phases. When the photovoltaic inverter 11 is grid-connected, its output voltage is 240V. Load balancing requires the grid to balance the load, and the inverter itself cannot perform balancing adjustments. Therefore, there is no need to connect output port N of the photovoltaic inverter 11 to the grid's N line. In this case, the photovoltaic inverter 11 is considered a standard single-phase inverter.

[0005] Existing on-grid and off-grid photovoltaic inverters have the following problems: they cannot guarantee high efficiency both when connected and when off-grid. Some photovoltaic inverters are highly efficient when connected to the grid, but less efficient when off-grid, resulting in excessive system heat generation. Other photovoltaic inverters are inefficient and costly when connected to the grid, but highly efficient when off-grid. Therefore, ensuring efficient photovoltaic inverters both when connected and when off-grid is a pressing technical challenge for the industry.

[0006] Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a three-level converter that can improve grid-connected and off-grid efficiency and reduce system losses.

[0008] The present invention provides a three-level converter, comprising: a first DC input terminal, a second DC input terminal, a neutral terminal, a first power switch, a second power switch, a third power switch, a fourth power switch, a fifth power switch, a sixth power switch, and a seventh power switch; the first end of the first power switch is respectively connected to the first DC input terminal and the first end of the third power switch; the second end of the second power switch is respectively connected to the second DC input terminal and the second end of the fourth power switch; the connection point between the second end of the first power switch and the first end of the second power switch serves as a first AC output terminal; the connection point between the second end of the third power switch and the first end of the fourth power switch serves as a second AC output terminal; the fifth power switch and the sixth power switch are connected in series between the first AC output terminal and the neutral terminal; and the seventh power switch is connected between the connection point between the fifth power switch and the sixth power switch and the second AC output terminal.

[0009] Furthermore, the first power switch, the second power switch, the third power switch and the fourth power switch are high-frequency switch tubes; the fifth power switch, the sixth power switch and the seventh power switch are power-frequency freewheeling tubes.

[0010] Furthermore, when the three-level converter operates in an off-grid mode, a first load is connected between the first AC output terminal and the neutral terminal, and a second load is connected between the second AC output terminal and the neutral terminal.

[0011] Furthermore, when the three-level converter operates in a grid-connected mode, a grid is connected between the first AC output terminal and the second AC output terminal.

[0012] Furthermore, the first end of the fifth power switch is connected to the first end of the sixth power switch and the first end of the seventh power switch respectively.

[0013] Furthermore, the second end of the fifth power switch is connected to the second end of the sixth power switch and the second end of the seventh power switch respectively.

[0014] Furthermore, when the three-level converter operates in a first half cycle, the second power switch and the third power switch are normally off and the fifth power switch is normally on, the first power switch and the fourth power switch are turned on or off at the same time, and the sixth power switch and the seventh power switch are turned on or off at the same time, wherein the first power switch and the sixth power switch are turned on complementarily.

[0015] Furthermore, when the three-level converter operates in the second half cycle, the first power switch and the fourth power switch are normally off and the seventh power switch is normally on, the second power switch and the third power switch are turned on or off at the same time, and the fifth power switch and the sixth power switch are turned on or off at the same time, wherein the second power switch and the fifth power switch are turned on complementarily.

[0016] Furthermore, it also includes a first freewheeling branch and a second freewheeling branch; the first freewheeling branch is connected between the first AC output terminal and the neutral terminal, and the second freewheeling branch is connected between the neutral terminal and the second AC output terminal.

[0017] Furthermore, the conducting direction of the first freewheeling branch is opposite to the conducting direction of the second freewheeling branch.

[0018] Furthermore, the first freewheeling branch and the second freewheeling branch both include a diode and a power switch connected in series.

[0019] The present application also provides a control method applied to the above-mentioned three-level converter, comprising: when the three-level converter operates in a first half cycle, controlling the second power switch and the third power switch to be normally off and the fifth power switch to be normally on, the first power switch and the fourth power switch to be turned on or off simultaneously, and the sixth power switch and the seventh power switch to be turned on or off simultaneously, wherein the first power switch and the sixth power switch are turned on in a complementary manner;

[0020] When the three-level converter operates in the second half cycle, the first power switch and the fourth power switch are controlled to be normally off and the seventh power switch is controlled to be normally on, the second power switch and the third power switch are simultaneously turned on or off, and the fifth power switch and the sixth power switch are simultaneously turned on or off, wherein the second power switch and the fifth power switch are complementary to each other.

[0021] A three-level converter according to an embodiment of the present invention includes a first DC input terminal, a second DC input terminal, a neutral terminal, a first power switch, a second power switch, a third power switch, a fourth power switch, a fifth power switch, a sixth power switch, and a seventh power switch. By connecting and controlling these power switches, the three-level converter can operate in either a grid-connected or off-grid state, thereby improving the efficiency of the three-level converter in both the grid-connected and off-grid states and reducing system losses. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG1 is a schematic structural diagram of a photovoltaic inverter and its connection with a load in the prior art.

[0023] FIG2 is a schematic structural diagram of a single-phase inverter in the prior art operating in a grid-connected mode.

[0024] FIG3 is a schematic structural diagram of the inverter in FIG2 operating in an off-grid mode.

[0025] FIG4 is a schematic structural diagram of a T-shaped three-level inverter in the prior art operating in a grid-connected mode.

[0026] FIG5 is a schematic structural diagram of the inverter in FIG4 operating in an off-grid mode.

[0027] FIG6 is a structural diagram of a three-level converter according to an embodiment of the present application operating in an off-grid mode.

[0028] FIG7 is a schematic structural diagram of the three-level converter in FIG6 operating in a grid-connected mode.

[0029] FIG8 is a driving waveform diagram of the power switch of the three-level converter in FIG6 and FIG7.

[0030] FIG9 is a schematic structural diagram of a three-level converter operating in an off-grid mode according to another embodiment of the present application.

[0031] FIG10 is a schematic structural diagram of a three-level converter operating in an off-grid mode according to another embodiment of the present application.

[0032] FIG11 is a structural diagram of a three-level converter operating in an off-grid mode according to another embodiment of the present application. DETAILED DESCRIPTION

[0033] The following is a clear and complete description of the technical solutions of the present invention in conjunction with the accompanying drawings. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0034] It should be understood that the present invention can be implemented in different forms and should not be interpreted as being limited to the embodiments proposed herein. On the contrary, providing these embodiments will make disclosure thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. In the accompanying drawings, for clarity, the sizes and relative sizes of layers and regions may be exaggerated, and the same reference numerals represent the same elements throughout. It should be understood that when an element or layer is referred to as "on ... ", "adjacent to ... ", "connected to " or "coupled to" other elements or layers, it can be directly on other elements or layers, adjacent to them, connected to or coupled to other elements or layers, or there can be intervening elements or layers. On the contrary, when an element is referred to as "directly on ... ", "directly adjacent to ... ", "directly connected to " or "directly coupled to" other elements or layers, there are no intervening elements or layers. It should be understood that although the terms first, second, third, etc. can be used to describe various elements, components, regions, layers and / or parts, these elements, components, regions, layers and / or parts should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Therefore, without departing from the teachings of the present invention, a first element, component, region, layer or section discussed below may be represented as a second element, component, region, layer or section.

[0035] Spatially relative terms such as "under," "beneath," "below," "under," "above," "above," etc., may be used herein for convenience of description to describe the relationship of one element or feature shown in the figures to other elements or features. It should be understood that the spatially relative terms are intended to include different orientations of the device in use and operation in addition to the orientations shown in the figures. For example, if the device in the drawings is flipped, then the elements or features described as "under" or "beneath" or "beneath" the other elements will be oriented as "over" the other elements or features. Thus, the exemplary terms "under" and "under" may include both the upper and lower orientations. The device may be oriented otherwise (rotated 90 degrees or in other orientations) and the spatial descriptors used herein are interpreted accordingly.

[0036] The purpose of the terms used herein is only to describe specific embodiments and is not intended to limit the present invention. When used herein, 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 terms "comprising" and / or "comprising", when used in this specification, determine the presence of the features, integers, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups. When used herein, the term "and / or" includes any and all combinations of the relevant listed items.

[0037] Referring to Figures 2 and 3 , Figure 2 is a schematic diagram of the structure of a conventional single-phase inverter operating in grid-connected mode, and Figure 3 is a schematic diagram of the structure of the inverter in Figure 2 operating in off-grid mode. The inverter includes a first input terminal, a neutral terminal N, a second input terminal, and power switches Q11-Q16. The voltage between the first input terminal and the neutral terminal N is the upper half bus voltage V1, and the voltage between the neutral terminal N and the second input terminal is the lower half bus voltage V2. The first end of the power switch Q11 is connected to the power switch Q13 and the first input end, respectively. The connection point between the first end of the power switch Q12 and the second end of the power switch Q11 serves as the first AC output end A. The connection point between the second end of the power switch Q13 and the first end of the power switch Q14 serves as the second AC output end B. The second end of the power switch Q12 is connected to the second end of the power switch Q14 and the second input end, respectively. Power switches Q15 and Q16 are connected in series between the first AC output end A and the second AC output end B. Power switches Q15 and Q16 are bidirectional switches, i.e., the first end of the power switch Q15 is connected to the first end of the power switch Q16, the second end of the power switch Q15 is connected to the second end of the power switch Q11, and the second end of the power switch Q16 is connected to the second end of the power switch Q13. In this embodiment, the power switches Q11-Q16 may be controllable switches such as IGBTs and MOSFETs. For example, the power switches Q11-Q16 are IGBTs, the first terminals of the power switches Q11-Q16 are drains, and the second terminals of the power switches Q11-Q16 are drains. Each of the power switches Q11-Q16 includes a controllable switch and a diode connected in antiparallel with the controllable switch.

[0038] 2 , when the inverter operates in grid-connected mode, the grid G ​​is connected between the first AC output terminal A and the second AC output terminal B. The inverter also includes inductors L1 and L2. One end of the grid G ​​is connected to the first AC output terminal A via inductor L1, and the other end of the grid G ​​is connected to the second AC output terminal B via inductor L2.

[0039] In this embodiment, power switches Q11-Q14 are high-frequency switching transistors, and power switches Q15 and Q16 are power-frequency freewheeling transistors. During the positive half-cycle of the power grid, a controller (not shown) is connected to the control terminals of power switches Q11-Q16, respectively, to control power switches Q12 and Q13 to be normally off and power switch Q15 to be normally on. The controller turns power switches Q11 and Q14 on or off simultaneously, and controls power switches Q11 and Q16 to be complementary. During the negative half-cycle of the power grid, the controller turns power switches Q11 and Q14 off, and controls power switch Q16 to be normally on. The controller turns power switches Q12 and Q13 on or off simultaneously, and controls power switches Q12 and Q15 to be complementary. When the inverter's grid-connected current and voltage are in phase, taking the positive half-cycle of the grid as an example, when power switches Q11 and Q14 are on, the inverter's output voltage is the bus voltage, or V1 + V2. When power switches Q11 and Q14 are off, the current in inductors L1 and L2 flows through power switches Q15 and Q16, and the inverter's output voltage is zero. This indicates that current always flows through two power switches in the entire loop from the inverter input to the grid. This inverter cannot operate in split-phase output mode and cannot accommodate unbalanced load conditions between the two phases.

[0040] 3 , when the inverter operates in off-grid mode, the inductor L1 and the load Z1 are connected in series between the first AC output terminal A and the neutral terminal N, and the inductor L2 and the load Z2 are connected in series between the second AC output terminal B and the neutral terminal N. At this time, only power switches Q11-Q14 can be used for bipolar modulation. This modulation method will significantly increase system losses, reduce efficiency, and increase system heat generation.

[0041] Referring to Figures 4 and 5 , Figure 4 is a schematic diagram of the structure of a conventional T-shaped three-level inverter operating in grid-connected mode, and Figure 5 is a schematic diagram of the structure of the inverter in Figure 4 operating in off-grid mode. The inverter includes a first input terminal, a neutral terminal N, a second input terminal, and power switches Q21-Q27. The voltage between the first input terminal and the neutral terminal N is the upper half bus voltage V1, and the voltage between the neutral terminal N and the second input terminal is the lower half bus voltage V2. The first end of power switch Q21 is connected to the first input terminal and the first end of power switch Q23, respectively. The connection point between the second end of power switch Q21 and the first end of power switch Q22 serves as the first AC output terminal A. The connection point between the second end of power switch Q23 and the first end of power switch Q24 serves as the second AC output terminal B. The second end of power switch Q22 is connected to the second end of power switch Q24 and the second input terminal, respectively. Power switches Q25 and Q26 are connected in series between the first AC output terminal A and the neutral terminal N. Power switches Q27 and Q28 are connected in series between the second AC output terminal B and the neutral terminal N. In this embodiment, power switches Q25 and Q26 are bidirectional switches, while power switches Q27 and Q28 are bidirectional switches. In this embodiment, power switches Q21-Q24 are high-frequency switching transistors, while power switches Q25-Q28 are power-frequency freewheeling transistors. Power switches Q21-Q28 include controllable switches and diodes connected in antiparallel thereto.

[0042] 4 , when the inverter operates in grid-connected mode, the grid G ​​is connected between the first AC output terminal A and the second AC output terminal B. The inverter also includes inductors L1 and L2. One end of the grid G ​​is connected to the first AC output terminal A via inductor L1, and the other end of the grid G ​​is connected to the second AC output terminal B via inductor L2.

[0043] 5 , when the inverter operates in off-grid mode, the inductor L1 and the load Z1 are connected in series between the first AC output terminal A and the neutral terminal N, and the inductor L2 and the load Z2 are connected in series between the second AC output terminal B and the neutral terminal N.

[0044] In this embodiment, the inverter further includes a controller (not shown), which is connected to the control terminals of power switches Q21-Q28, respectively. During the positive half-cycle of the power grid, the controller controls power switches Q25 and Q28 to be normally on, and power switches Q22 and Q23 to be normally off. The controller also controls power switches Q21 and Q24 to be simultaneously on or off, and controls power switches Q26 and Q27 to be simultaneously on or off. The power switches Q21 and Q24 are turned on in a complementary manner to the power switches Q27 and Q26. When power switches Q21 and Q24 are turned on, current flows through two power switches in the entire circuit from the inverter input to the power grid. When power switches Q26 and Q27 are turned on for freewheeling, current flows through four power switches in the entire circuit. This inverter has four power switches participating in freewheeling during freewheeling, while the inverter in FIG2 has two power switches participating in freewheeling during freewheeling. It can be seen that the total consumption of this inverter during freewheeling is greater than the total loss of the inverter in FIG2 during freewheeling.

[0045] In summary, when the inverter operates in grid-connected mode, the inverter in Figure 2 is more efficient than the inverter in Figure 4, and it also reduces the cost of two freewheeling diodes. However, when the inverter operates in off-grid mode, the efficiency of the inverter in Figure 2 is lower than that of Figure 4. Clearly, the two inverters mentioned above each have their own advantages and disadvantages, and it is difficult to guarantee that the inverter is highly efficient in both grid-connected and off-grid modes.

[0046] 6 and 7, FIG6 is a schematic structural diagram of a three-level converter according to an embodiment of the present application operating in an off-grid mode, and FIG7 is a schematic structural diagram of the three-level converter in FIG6 operating in a grid-connected mode. The three-level converter includes a first DC input terminal, a second DC input terminal, a neutral terminal N, a first power switch Q31, a second power switch Q32, a third power switch Q33, a fourth power switch Q34, a fifth power switch Q35, a sixth power switch Q36, and a seventh power switch Q37; a first end of the first power switch Q31 is connected to the first DC input terminal and the first end of the third power switch Q33 respectively; a second end of the second power switch Q32 is connected to the second DC input terminal and the second end of the fourth power switch Q34 respectively; a second end of the first power switch Q31 is connected to the first DC input terminal and the second end of the fourth power switch Q34 respectively; a second end of the first power switch Q31 ... power switch Q31 and the second end of the fourth power switch Q34 respectively; a second end of The connection point of the first end of the second power switch Q32 serves as the first AC output terminal A; the connection point of the second end of the third power switch Q33 and the first end of the fourth power switch Q34 serves as the second AC output terminal B; the fifth power switch Q35 and the sixth power switch Q36 are connected in series between the first AC output terminal A and the neutral terminal N, wherein the fifth power switch Q35 and the sixth power switch Q36 are bidirectional switches; and the seventh power switch Q37 is connected between the connection point of the fifth power switch Q35 and the sixth power switch Q36 and the second AC output terminal B. In this embodiment, the first end of the fifth power switch Q35 is connected to the first end of the sixth power switch Q36 and the first end of the seventh power switch Q37, respectively; the second end of the fifth power switch Q35 is connected to the first AC output terminal A; the second end of the sixth power switch Q36 is connected to the neutral terminal N; and the second end of the seventh power switch Q37 is connected to the second AC output terminal B. In this embodiment, the first to seventh power switches Q31 to Q37 each include a controllable switch and diodes D31 to D37 connected in anti-parallel thereto. The controllable switch may be an IGBT, a MOSFET, an IGCT, or the like. For example, when the controllable switch is an IGBT, the first end of the first to seventh power switches Q31 to Q37 is the drain of the IGBT, and the second end of the first to seventh power switches Q31 to Q37 is the source of the IGBT.

[0047] In this embodiment, the first to fourth power switches Q31 - Q34 are high-frequency switching tubes, and the fifth to seventh power switches Q35 - Q37 are power-frequency freewheeling tubes.

[0048] When the three-level converter operates in an off-grid mode, a first load Z1 is connected between the first AC output terminal A and the neutral terminal N. The first load Z1 can be connected in series with an inductor L1 and then connected between the first AC output terminal A and the neutral terminal N. A second load Z2 is connected between the second AC output terminal B and the neutral terminal N. The second load Z2 can be connected in series with an inductor L2 and then connected between the second AC output terminal B and the neutral terminal N. The first load Z1 and the second load Z2 are respectively split-phase output loads.

[0049] When the three-level inverter operates in grid-connected mode, the grid G ​​is connected between the first AC output terminal A and the second AC output terminal B, wherein an inductor L1 is connected between one end of the grid G ​​and the first AC output terminal A, and an inductor L2 is connected between the other end of the grid G ​​and the second AC output terminal B.

[0050] Referring to FIG8 , FIG8 is a driving waveform diagram of the power switches of the three-level converter shown in FIG6 and FIG7 . The three-level converter further includes a controller (not shown) connected to control terminals of the first through seventh power switches Q31 through Q37 and configured to control the on / off switching of the first through seventh power switches Q31 through Q37. When the three-level converter operates in a first half-cycle (e.g., a positive half-cycle of a power grid), the controller controls the second and third power switches Q32 and Q33 to be normally off and the fifth power switch Q35 to be normally on. The first and fourth power switches Q31 and Q34 are simultaneously turned on or off, and the sixth and seventh power switches Q36 and Q37 are simultaneously turned on or off, with the first and sixth power switches Q31 and Q36 being complementary to each other. When the three-level converter operates in the second half cycle (for example, the negative half cycle of the power grid), the controller controls the first power switch Q31 and the fourth power switch Q34 to be normally off and the seventh power switch Q27 to be normally on, the second power switch Q32 and the third power switch Q33 to be turned on or off at the same time, and the fifth power switch Q35 and the sixth power switch Q36 to be turned on or off at the same time, wherein the second power switch Q32 and the fifth power switch Q35 are turned on complementarily.

[0051] When the three-level converter operates in grid-connected or off-grid mode and the two-phase loads Z1 and Z2 are matched, the line current at the neutral terminal N is zero. During power-frequency freewheeling, current flows through the fifth power switch Q35 and the seventh power switch Q37, while no current flows through the sixth power switch Q36. Therefore, the freewheeling circuit has only two power switches, achieving the same efficiency as the inverter operating in grid-connected mode as shown in Figure 2. When the three-level converter operates in off-grid mode and the first load Z1 and the second load Z2 are asymmetrical, current flows through the sixth power switch Q36. Assuming that the second load Z2 connected between the second AC output terminal B and the neutral point is unloaded, power is output only between the first AC output terminal A and the neutral terminal N, which is the same current path as the inverter in Figure 5.

[0052] As can be seen, the efficiency of the three-level converter in Figure 7 when operating in grid-connected mode is the same as the efficiency of the inverter in Figure 2 when operating in grid-connected mode. The worst-case efficiency of the three-level converter in Figure 6 when operating in off-grid mode is the same as the efficiency of the inverter in Figure 4 when operating in off-grid mode, and the best-case efficiency is the same as the efficiency of the inverter in Figure 2 when operating in grid-connected mode. The three-level converters in Figures 6 and 7 incorporate the advantages of the inverters in Figures 2 and 4, improving the efficiency of the three-level converters in both off-grid and grid-connected modes.

[0053] Referring to FIG9 , FIG9 is a schematic diagram of the structure of a three-level converter operating in an off-grid mode according to an embodiment of the present application. The similarities in structure and connection between the three-level converter in FIG9 and the three-level converter in FIG6 are not further described here. The following primarily describes the differences in structure and connection between the three-level converter in FIG9 and the three-level converter in FIG6 . The second end of the fifth power switch Q35 is connected to the second end of the sixth power switch Q36 and the second end of the seventh power switch Q37, respectively. The first end of the fifth power switch Q35 is connected to the first AC output terminal A, and the first end of the seventh power switch Q37 is connected to the second AC output terminal B.

[0054] Referring to Figures 10 and 11, Figure 10 is a schematic diagram of the structure of a three-level converter operating in off-grid mode according to one embodiment of the present application, and Figure 11 is a schematic diagram of the structure of a three-level converter operating in off-grid mode according to a fourth embodiment of the present invention. The three-level converter further includes a first freewheeling branch and a second freewheeling branch. The first freewheeling branch is connected between the first AC output terminal A and the neutral terminal N, and the second freewheeling branch is connected between the neutral terminal N and the second AC output terminal B. As shown in Figure 10, the first freewheeling branch includes a diode D38 and a power switch Q39 connected in series, wherein the anode of diode D38 is connected to the first AC output terminal A, the cathode of diode D38 is connected to the first terminal of power switch Q39, and the second terminal of power switch Q39 is connected to the neutral terminal N. The second freewheeling branch includes a power switch Q40 and a diode D41 connected in series, wherein the second terminal of power switch Q40 is connected to the neutral terminal N, the first terminal of power switch Q40 is connected to the cathode of diode D41, and the anode of diode D41 is connected to the second AC output terminal B. It should be noted that diodes D38 and D41 can be replaced with controllable switches. As shown in Figure 11, the first freewheeling branch includes a diode D39 and a power switch Q38 connected in series. The second end of the power switch Q38 is connected to the first AC output terminal A, the first end of the power switch Q38 is connected to the cathode of the diode D39, and the anode of the diode D39 is connected to the neutral terminal N. The second freewheeling branch includes a power switch Q41 and a diode D40 connected in series. The anode of the diode D40 is connected to the neutral terminal N, the cathode of the diode D40 is connected to the first end of the power switch Q41, and the second end of the power switch Q41 is connected to the second AC output terminal B. As can be seen, the conduction directions of the first and second freewheeling branches are opposite. The first and second freewheeling branches can solve the dead time of the three-level converter between the shutdown of the second power switch Q32 and the fourth power switch Q34 and the conduction of the fifth power switch Q35 to the power switch Q37. By providing a temporary freewheeling path through the first or second freewheeling branch, the three-level converter is more reliable and safe.

[0055] 6, 8 and 10, the working principle of the three-level converter is as follows:

[0056] When the three-level converter operates in the first cycle, the first power switch Q31 and the fourth power switch Q34 are turned on. The first DC input terminal, the first power switch Q31, the first load Z1, and the neutral terminal N form a current path. The second DC input terminal, the neutral terminal N, the fourth power switch Q34, and the second load Z2 form a current path. When the first power switch Q31 and the fourth power switch Q34 are turned off and before the sixth power switch Q36 and the seventh power switch Q37 are turned on, the neutral terminal N, the second load Z2, the power switch Q40, and the diode D41 form a freewheeling path. The diode D36, the power switch Q35, the neutral terminal N, and the first load Z1 form a freewheeling path. After the sixth power switch Q36 and the seventh power switch Q37 are turned on, the seventh power switch Q37, the sixth power switch Q36, the neutral terminal N, and the second load Z2 form a freewheeling path. The sixth power switch Q36, the fifth power switch Q35, the first load Z1, and the neutral terminal N form a freewheeling path.

[0057] When the three-level converter operates in the second cycle, the second power switch Q32 and the third power switch Q33 are turned on. The second power switch Q32, the second DC input terminal, the neutral terminal N, and the first load Z1 form a current path. The first DC input terminal, the third power switch Q33, the second load Z2, and the neutral terminal N form a current path. When the second power switch Q32 and the third power switch Q33 are turned off and before the sixth power switch Q36 and the fifth power switch Q35 are turned on, the neutral terminal N, the second load Z2, the power switch Q37, and the diode D36 form a freewheeling path. The diode D38, the power switch Q39, the neutral terminal N, and the first load Z1 form a freewheeling path. After the sixth power switch Q36 and the fifth power switch Q35 are turned on, the sixth power switch Q36, the seventh power switch Q37, the second load Z2, and the neutral terminal N form a freewheeling path. The fifth power switch Q35, the sixth power switch Q36, the neutral terminal N, and the first load Z1 form a freewheeling path.

[0058] It can be seen that the above-mentioned first freewheeling branch and the second freewheeling branch can solve the problem that a temporary freewheeling path can be provided by the first freewheeling branch or the second freewheeling branch during the dead time from the second power switch Q32 and the fourth power switch Q34 being turned off to the fifth power switch Q35 to the power switch Q37 being turned on in the three-level converter, thereby making the three-level converter more reliable and safe.

[0059] Figure 11 is a schematic diagram of the structure of a three-level converter in another embodiment of the present invention operating in an off-grid mode. The operating principle of the three-level converter in Figure 11 is similar to that of Figure 10 and will not be repeated here.

[0060] In one embodiment of the present invention, a control method for the above-mentioned three-level converter is provided, the control method comprising:

[0061] When the three-level converter operates in the first half cycle, the second power switch Q32 and the third power switch Q33 are controlled to be normally off and the fifth power switch Q35 is controlled to be normally on, the first power switch Q31 and the fourth power switch Q34 are simultaneously turned on or off, and the sixth power switch Q36 and the seventh power switch Q37 are simultaneously turned on or off, wherein the first power switch Q31 and the sixth power switch Q36 are complementary to each other;

[0062] When the three-level converter operates in the second half cycle, the first power switch Q31 and the fourth power switch Q34 are controlled to be normally off and the seventh power switch Q37 is controlled to be normally on, the second power switch Q32 and the third power switch Q33 are simultaneously turned on or off, and the fifth power switch Q35 and the sixth power switch Q36 are simultaneously turned on or off, wherein the second power switch Q32 and the fifth power switch Q35 are complementarily turned on.

[0063] The above-mentioned embodiments improve the efficiency of the three-level converter when operating in the off-grid mode and the grid-connected mode.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A three-level converter, characterized in that: include: A first DC input terminal, a second DC input terminal, a neutral terminal, a first power switch, a second power switch, a third power switch, a fourth power switch, a fifth power switch, a sixth power switch and a seventh power switch; the first end of the first power switch is respectively connected to the first DC input terminal and the first end of the third power switch; the second end of the second power switch is respectively connected to the second DC input terminal and the second end of the fourth power switch; the connection point of the second end of the first power switch and the first end of the second power switch serves as a first AC output terminal; the connection point of the second end of the third power switch and the first end of the fourth power switch serves as a second AC output terminal; the fifth power switch and the sixth power switch are connected in series between the first AC output terminal and the neutral terminal; the seventh power switch is connected between the connection point of the fifth power switch and the sixth power switch and the second AC output terminal.

2. The three-level converter according to claim 1, characterized in that: The first power switch, the second power switch, the third power switch and the fourth power switch are high-frequency switch tubes; the fifth power switch, the sixth power switch and the seventh power switch are industrial frequency freewheeling tubes.

3. The three-level converter according to claim 1, characterized in that: When the three-level converter operates in an off-grid mode, a first load is connected between the first AC output terminal and the neutral terminal, and a second load is connected between the second AC output terminal and the neutral terminal.

4. The three-level converter according to claim 1, characterized in that: When the three-level converter operates in a grid-connected mode, a grid is connected between the first AC output terminal and the second AC output terminal.

5. The three-level converter according to claim 1, characterized in that: The first end of the fifth power switch is connected to the first end of the sixth power switch and the first end of the seventh power switch respectively.

6. The three-level converter according to claim 1, characterized in that: The second end of the fifth power switch is connected to the second end of the sixth power switch and the second end of the seventh power switch respectively.

7. The three-level converter according to claim 1, characterized in that: When the three-level converter operates in the first half cycle, the second power switch and the third power switch are normally off and the fifth power switch is normally on, the first power switch and the fourth power switch are turned on or off at the same time, the sixth power switch and the seventh power switch are turned on or off at the same time, and the first power switch and the sixth power switch are turned on complementarily.

8. The three-level converter according to claim 7, characterized in that: When the three-level converter operates in the second half cycle, the first power switch and the fourth power switch are normally off and the seventh power switch is normally on, the second power switch and the third power switch are turned on or off at the same time, the fifth power switch and the sixth power switch are turned on or off at the same time, and the second power switch and the fifth power switch are turned on complementarily.

9. The three-level converter according to claim 1, characterized in that: It also includes a first freewheeling branch and a second freewheeling branch; the first freewheeling branch is connected between the first AC output terminal and the neutral terminal, and the second freewheeling branch is connected between the neutral terminal and the second AC output terminal.

10. The three-level converter according to claim 9, characterized in that: The conduction direction of the first freewheeling branch is opposite to the conduction direction of the second freewheeling branch.

11. The three-level converter according to claim 9, characterized in that: The first freewheeling branch and the second freewheeling branch both include a diode and a power switch connected in series.

12. A control method for a three-level converter according to any one of claims 1 to 6 and 9 to 11, characterized in that: The control method comprises: When the three-level converter operates in the first half cycle, the second power switch and the third power switch are controlled to be normally off and the fifth power switch is controlled to be normally on, the first power switch and the fourth power switch are turned on or off at the same time, the sixth power switch and the seventh power switch are turned on or off at the same time, and the first power switch and the sixth power switch are turned on complementarily; When the three-level converter operates in the second half cycle, the first power switch and the fourth power switch are controlled to be normally off and the seventh power switch is controlled to be normally on, the second power switch and the third power switch are turned on or off at the same time, the fifth power switch and the sixth power switch are turned on or off at the same time, and the second power switch and the fifth power switch are turned on complementarily.

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