Three-level converter and its control method

Abstract

To provide a three-level converter and a control method thereof.SOLUTION: In a field of photovoltaic power generation, a three-level converter includes a first DC input end, a second DC input end, a neutral end, and first to seventh power switches. A first end of the first power switch is connected to the first DC input end and to a first end of the third power switch. A second end of the second power switch is connected to the second DC input end and to a second end of the fourth power switch. A connection point between a second end of the first power switch and a first end of the second power switch is a first AC output end. A connection point between a second end of the third power switch and a first end of the fourth power switch is a second AC output end. The fifth and sixth power switches are connected in series between the first AC output end and the neutral end. The seventh power switch is connected between the second AC output and a connection point between the fifth and sixth power switches. The three-level converter can increase efficiency in grid-connected and off-grid states.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to the field of solar power generation, and particularly to a three-level converter and its control method.

[0002] This application is based on a Chinese patent application with application number 202310704136.X and a filing date of June 14, 2023, and claims the priority of the Chinese patent application. The entire content of the Chinese patent application is incorporated herein by reference.

Background Art

[0003] In a solar power generation system, an inverter is an essential part. It can convert the variable DC voltage generated by a solar power generation panel into an AC voltage, feedback to a commercial power transmission system, or supply various electrical consumption loads.

[0004] North American residential photovoltaic inverters need to be phase-separated and stable during operation, that is, they can output two independent AC voltages and the phases can be shifted by 180°. As shown in Figure 1, the photovoltaic inverter 11 has three output ports L1, L2, and N. The output ports L1 and L2 are live wire ports, and the output port N is a zero line port. Here, the voltage between the output ports L1 and L2 is twice the voltage between the output port L1 and N, or the voltage between the output ports L1 and L2 is twice the voltage between the output port L2 and N. A load 1 is connected between the output port L1 and N, and the load 1 is supplied with power by the voltage between the output port L1 and N. A load 2 is connected between the output port N and L2, and the load 2 is supplied with power by the voltage between the output port L2 and N. A load 3 is connected between the output ports L1 and L2, and the load 3 is supplied with power by the voltage between the output ports L1 and L2.

[0005] Hereinafter, the voltage between output ports L1 and L2 will be described as 240V. However, this photovoltaic inverter 11 can output two-phase 120V during grid-off output. That is, the voltage between output port L1 and N is 120V, and it can supply power to a 120V load 1. The voltage between output port L2 and N is 120V, and it can supply power to a 120V load 2, and it can also supply power to a 240V load 3. When the photovoltaic inverter 11 outputs grid-off, it is not necessary for the loads 1 and 2 with two-phase 120V voltages to be exactly the same. The photovoltaic inverter 11 needs to independently control these two-phase voltages. When the photovoltaic inverter 11 is connected to the grid, the output voltage of the photovoltaic inverter 11 is 240V. The problem of load balance needs to be balanced by the grid, and the inverter itself cannot adjust the balance. Therefore, it is not necessary to connect the output port N of the photovoltaic inverter 11 to the N wire of the grid. In this case, the photovoltaic inverter 11 can be considered as a normal single-phase inverter.

[0006] Conventional grid-off photovoltaic inverters have the problem that they cannot guarantee high efficiency both when the photovoltaic inverter is connected to the grid and when it is grid-off. Currently, some photovoltaic inverters have high efficiency when connected to the grid and low efficiency when grid-off, and the heat generation of the system is large. However, some photovoltaic inverters have low efficiency and high cost when connected to the grid and high efficiency when grid-off. Therefore, ensuring high efficiency of the photovoltaic inverter both when connected to the grid and when grid-off is a technical problem that the industry needs to solve urgently.

Summary of the Invention

Problems to be Solved by the Invention

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

Means for Solving the Problems

[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 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 terminal of the first power switch is connected to the first DC input terminal and the first terminal of the third power switch respectively. The second terminal of the second power switch is connected to the second DC input terminal and the second terminal of the fourth power switch respectively. The connection point between the second terminal of the first power switch and the first terminal of the second power switch serves as a first AC output terminal. The connection point between the second terminal of the third power switch and the first terminal 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.

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

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

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

[0013] Furthermore, the second terminal of the fifth power switch is connected to the second terminals of the sixth power switch and the seventh power switch respectively. Furthermore, when the three-level converter operates in the first half cycle, the second power switch and the third power switch are always off, the fifth power switch is always on, the first power switch and the fourth power switch are turned on or off simultaneously, the sixth power switch and the seventh power switch are turned on or off simultaneously, and here, the first power switch and the sixth power switch are conducting complementarily.

[0014] Furthermore, when the three-level converter operates in the second half cycle, the first power switch and the fourth power switch are always off, the seventh power switch is always on, the second power switch and the third power switch are turned on or off simultaneously, the fifth power switch and the sixth power switch are turned on or off simultaneously, and here, the second power switch and the fifth power switch are conducting complementarily.

[0015] Furthermore, it further includes a first freewheel branch and a second freewheel branch. The first freewheel branch is connected between the first AC output terminal and the neutral terminal, and the second freewheel branch is connected between the neutral terminal and the second AC output terminal.

[0016] Furthermore, the conduction direction of the first freewheel branch is opposite to that of the second freewheel branch.

[0017] Furthermore, both the first freewheel branch and the second freewheel branch include a diode and a power switch connected in series.

[0018] The present application provides a control method applied to the three-level converter described above. When the three-level converter operates in the first half cycle, the second power switch and the third power switch are always off, the fifth power switch is always on, the first power switch and the fourth power switch are turned on or off simultaneously, and the sixth power switch and the seventh power switch are turned on or off simultaneously. Here, the first power switch and the sixth power switch are conducting complementarily. When the three-level converter operates in the second half cycle, the first power switch and the fourth power switch are always off, the seventh power switch is always on, the second power switch and the third power switch are turned on or off simultaneously, and the fifth power switch and the sixth power switch are turned on or off simultaneously. Here, the second power switch and the fifth power switch are conducting complementarily.

Advantages of the Invention

[0019] The 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 the connection relationship of the above power switches and the corresponding control of the power switches, the three-level converter can be operated in a grid-connected state or a grid-off state, and the efficiency of the three-level converter in the grid-connected state and the grid-off state can be improved to reduce the loss of the system.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, the technical solution in the present invention will be clearly and detailedly described. However, it is obvious that the described embodiments are only a part of the present invention and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without additional work belong to the protection scope of the present invention.

[0022] Of course, the present invention may be implemented in various forms and should not be construed as limited to the embodiments described herein. Rather, the provision of these embodiments is to make the disclosure thorough and complete and to fully convey the scope of the present invention to those skilled in the art. In the drawings, the dimensions and relative dimensions of layers and regions may be exaggerated for clarity, and the same reference numerals are used throughout to indicate the same elements. Obviously, when an element or layer is referred to as being "on", "adjacent to", "connected to", or "coupled to" another element or layer, it can be directly on, adjacent to, connected to, or coupled to the other element or layer, or intervening elements or layers may be present. Conversely, when an element or layer is referred to as being "directly on", "directly adjacent to", "directly connected to", or "directly coupled to" another element or layer, there are no intervening elements or layers. Terms such as first, second, third, etc. can be used to describe various elements, components, regions, layers, and / or portions, but these elements, components, regions, layers, and / or portions should not be limited by these terms. These terms are merely used to distinguish one element, component, region, layer, or portion from another element, component, region, layer, or portion. Thus, the first element, component, region, layer, or portion discussed below can refer to the second element, component, region, layer, or portion without departing from the teachings of the present invention.

[0023] Spatial relationship terms, such as "below", "beneath", "underneath", "under", "above", "upper", etc., may be used herein for the sake of convenience in explanation to describe the relationship between one element or feature shown in the drawings and another element or feature.

[0024] Terms such as "upper" and "above" can be used in this specification for the sake of convenience in description to explain the relationship between one element or feature shown in the drawings and another element or feature. In addition to the orientation shown in the figures, it should be understood that the terms of spatial relationship are intended to include different orientations of the device during use and operation. For example, when the device in the drawings is inverted, an element or feature described as "below another element" or "beneath it" or "under" will be oriented "above" the other element or feature. Thus, the exemplary terms "below ~" and "under ~" can encompass both upper and lower orientations. The device may additionally be oriented (rotated 90 degrees or other orientations), and the spatial descriptors used in this specification will be interpreted accordingly.

[0025] The terms used in this specification are for the purpose of describing particular embodiments only and are not intended to limit the invention. As used herein, the singular forms "a", "one", and "the" shall include the plural forms as well, unless the context clearly dictates otherwise. The terms "comprising" and / or "including", as used herein, specify the presence of the described features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. As used herein, the term "and / or" includes any and all combinations of the associated listed items.

[0026] Referring to FIGS. 2 and 3, FIG. 2 shows a configuration in which a single-phase inverter in the prior art operates in the grid-connected mode, and FIG. 3 shows a configuration in which the inverter in FIG. 2 operates in the grid-off mode. The inverter includes a first input terminal, a neutral terminal N, a second input terminal, and power switches Q11-Q16. Here, the voltage between the first input terminal and the neutral terminal N is the upper bus voltage V1, and the voltage between the neutral terminal N and the second input terminal is the lower bus voltage V2. The first terminal of power switch Q11 is connected to power switch Q13 and the first input terminal respectively. The connection point between the first terminal of power switch Q12 and the second terminal of power switch Q11 is used as the first AC output terminal A. The connection point between the second terminal of power switch Q13 and the first terminal of power switch Q14 is used as the second AC output terminal B. The second terminal of power switch Q12 is connected to the second terminal of power switch Q14 and the second input terminal respectively. Power switches Q15, Q16 are connected in series between the first AC output terminal A and the second AC output terminal B. Here, power switches Q15, Q16 are bidirectional switches. That is, the first terminal of power switch Q15 is connected to the first terminal of power switch Q16, the second terminal of power switch Q15 is connected to the second terminal of power switch Q11, and the second terminal of power switch Q16 is connected to the second terminal of power switch Q13. In this embodiment, power switches Q11-Q16 may be controllable switches such as IGBTs, MOSFETs, etc. For example, power switches Q11-Q16 are IGBTs, the first terminals of power switches Q11-Q16 are drains, and the second terminals of power switches Q11-16 are drains. Each of power switches Q11-Q16 includes a controllable switch and a diode connected in anti-parallel to this controllable switch.

[0027] Referring to FIG. 2, when the inverter is operating in the grid-connected mode, a grid G is connected between the first AC output terminal A and the second AC output terminal B. The inverter further includes inductors L1 and L2. One end of the grid G is connected to the first AC output terminal A by inductor L1, and the other end of the grid is connected to the second AC output terminal B by inductor L2.

[0028] In this embodiment, power switches Q11 - Q14 are high - frequency switch tubes, and power switches Q15 and Q16 are commercial free - wheel tubes. During the positive half - cycle of the grid, a controller (not shown) is connected to the control terminals of power switches Q11 - Q16 respectively, controls power switches Q12 and Q13 to be always off, power switch Q15 to be always on, the controller controls power switches Q11 and Q14 to be on or off simultaneously, and also, the controller controls the complementary conduction of power switches Q11 and Q16. During the negative half - cycle of the grid, the controller controls power switches Q11 and Q14 to be always off, power switch Q16 to be always on, the controller controls power switches Q12 and Q13 to be on or off simultaneously, and also, the controller controls the complementary conduction of power switches Q12 and Q15. When the grid - connected current and voltage of the inverter are in the same phase, taking the positive half - cycle of the grid as an example, when power switches Q11 and Q14 are on, the output voltage of the inverter is the bus voltage, that is, 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 output voltage of the inverter is 0. It can be seen that through the entire circuit from the input end of the inverter to the grid, current is always flowing through two power switches. This inverter cannot be used in the split - phase output mode and cannot adjust the unbalanced operation mode of the split - phase two - phase load.

[0029] Referring to FIG. 3, when this inverter operates in the grid - off mode, inductor L1 and load Z1 are connected in series between the first AC output terminal A and the neutral terminal N, and inductor L2 and load Z2 are connected in series between the second AC output terminal B and the neutral terminal N. In this case, only bipolar modulation can be performed using power switches Q11 - Q14. Such a modulation method significantly increases the system loss, reduces the efficiency, and causes a large amount of heat generation in the system.

[0030] Referring to FIGS. 4 and 5, FIG. 4 is a diagram showing the configuration in which a T-shaped three-level inverter in the prior art operates in the grid connection mode, and FIG. 5 is a diagram showing the configuration in which the inverter of FIG. 4 operates in the grid-off mode. The inverter includes a first input terminal, a neutral terminal N, a second input terminal, and power switches Q21-Q27. Here, the voltage between the first input terminal and the neutral terminal N is the upper bus voltage V1, and the voltage between the neutral terminal N and the second input terminal is the lower bus voltage V2. The first terminal of power switch Q21 is connected to the first input terminal and the first terminal of power switch Q23 respectively. The connection point between the second terminal of power switch Q21 and the first terminal of power switch Q22 is taken as the first AC output terminal A, and the connection point between the second terminal of power switch Q23 and the first terminal of power switch Q24 is taken as the second AC output terminal B. The second terminal of power switch Q22 is connected to the second terminal 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, and 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, and power switches Q27 and Q28 are bidirectional switches. In this embodiment, power switches Q21-Q24 are high-frequency switch tubes, and power switches Q25-Q28 are commercial free-wheel tubes. Power switches Q21-Q28 include a controllable switch and a diode connected in anti-parallel thereto.

[0031] Referring to FIG. 4, when the inverter operates in the grid connection mode, a grid G is connected between the first AC output terminal A and the second AC output terminal B. The inverter further includes inductors L1 and L2. One end of the grid G is connected to the first AC output terminal A by the inductor L1, and the other end of the grid is connected to the second AC output terminal B by the inductor L2. Referring to FIG. 5, when the inverter operates in the grid-off 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.

[0032] In this embodiment, the inverter further includes a controller (not shown) connected to the control terminals of power switches Q21-Q28 respectively. In the positive half cycle of the grid, the controller controls power switches Q25 and Q28 to be always on, and power switches Q22 and Q23 to be always off. The controller controls power switches Q21 and Q24 to be turned on or off simultaneously, and controls power switches Q26 and Q27 to be turned on or off simultaneously. Here, power switches Q21 and Q24 conduct complementarily with power switches Q27 and Q26. When power switches Q21 and Q24 are conducting, current flows through two power switches in the entire circuit from the inverter input terminal to the grid. When power switches Q26 and Q27 are conducting and freewheeling, current flows through four power switches in the entire circuit. The inverter has four power switches involved in freewheeling during freewheeling, while the inverter in Figure 2 has two power switches involved in freewheeling during freewheeling. As can be seen, the total loss of the inverter during freewheeling is greater than the total loss of the inverter in Figure 2 during freewheeling.

[0033] As described above, when the inverter operates in the grid-connected mode, the efficiency of the inverter in Figure 2 is higher than that of the inverter in Figure 4, and moreover, the cost of two freewheel diodes is reduced. However, when the inverter operates in the grid-off mode, the inverter in Figure 2 is less efficient than Figure 4. It can be seen that the above two types of inverters each have advantages and disadvantages, and it is difficult to ensure that the inverter operates efficiently in both the grid-connected mode and the grid-off mode.

[0034] Referring to FIGS. 6 and 7, FIG. 6 is a diagram showing the configuration in which the three-level converter of an embodiment of the present application operates in the grid-off mode, and FIG. 7 is a diagram showing the configuration in which the three-level converter in FIG. 6 operates in the 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. The first terminal of the first power switch Q31 is connected to the first DC input terminal and the first terminal of the third power switch Q33, respectively. The second terminal of the second power switch Q32 is connected to the second DC input terminal and the second terminal of the fourth power switch Q34, respectively. The connection point between the second terminal of the first power switch Q31 and the first terminal of the second power switch Q32 is used as a first AC output terminal A, and the connection point between the second terminal of the third power switch Q33 and the first terminal of the fourth power switch Q34 is used as a 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. Here, the fifth power switch Q35 and the sixth power switch Q36 are bidirectional switches. 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 terminal of the fifth power switch Q35 is connected to the first terminal of the sixth power switch Q36 and the first terminal of the seventh power switch Q37, respectively. The second terminal of the fifth power switch Q35 is connected to the first AC output terminal A. The second terminal of the sixth power switch Q36 is connected to the neutral terminal N. The second terminal of the seventh power switch Q37 is connected to the second AC output terminal B. In this embodiment, the first power switch Q31 to the seventh power switch Q37 all include a controllable switch and a diode D31-D37 connected in anti-parallel thereto. Here, the controllable switch may be an IGBT, a MOSFET, an IGCT, etc. When the controllable switch is an IGBT, the first terminal of the first power switch Q31 to the seventh power switch Q37 is the drain of the IGBT, and the second terminal of the first power switch Q31 to the seventh power switch Q37 is the source of the IGBT.

[0035] In this embodiment, the first to fourth power switches Q31 - Q34 are high - frequency switch tubes, and the fifth to seventh power switches Q35 - Q37 are commercial - frequency free - wheeling tubes.

[0036] When the three - level converter operates in the grid - off mode, a first load Z1 is connected between the first AC output terminal A and the neutral terminal N. Here, the first load Z1 can be connected between the first AC output terminal A and the neutral terminal N after being connected in series with an inductor L1. A second load Z2 is connected between the second AC output terminal B and the neutral terminal N. Here, the second load Z2 can be connected between the second AC output terminal B and the neutral terminal N after being connected in series with an inductor L2. The first load Z1 and the second load Z2 are respectively loads of split - phase output.

[0037] When the three - level inverter operates in the grid - connected mode, a grid G is connected between the first AC output terminal A and the second AC output terminal B. Here, 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.

[0038] Referring to FIG. 8, FIG. 8 is a driving waveform diagram of the power switches of the three-level converter in FIGS. 6 and 7. The three-level converter further includes a controller (not shown) connected to the control ends of the first power switch Q31 to the seventh power switch Q37, and is used to control the on or off of the first power switch Q31 to the seventh power switch Q37. When the three-level converter operates in the first half cycle (for example, the positive half cycle of the grid), the controller keeps the second power switch Q32 and the third power switch Q33 always off, keeps the fifth power switch Q35 always on, controls the first power switch Q31 and the fourth power switch Q34 to be turned on or off simultaneously, and controls the sixth power switch Q36 and the seventh power switch Q37 to be turned on or off simultaneously. Here, the first power switch Q31 and the sixth power switch Q36 conduct complementarily. When the three-level converter operates in the second half cycle (for example, the negative half cycle of the grid), the controller keeps the first power switch Q31 and the fourth power switch Q34 always off, keeps the seventh power switch Q27 always on, controls the second power switch Q32 and the third power switch Q33 to be turned on or off simultaneously, and controls the fifth power switch Q35 and the sixth power switch Q36 to be turned on or off simultaneously. Here, the second power switch Q32 and the fifth power switch Q35 conduct complementarily.

[0039] When the three-level converter operates in the grid-connected mode or the grid-off mode and the two-phase loads Z1 and Z2 are matched, the line current at the neutral terminal N is 0. During commercial-frequency freewheeling, the current flows through the fifth power switch Q35 and the seventh power switch Q37, but no current flows through the sixth power switch Q36. Therefore, the freewheel loop has only two power switches, and the efficiency of the inverter operating in the grid-connected mode as shown in Figure 2 can be achieved. When the three-level converter operates in the grid-off mode and the first load Z1 and the second load Z2 are asymmetric, current flows through the sixth power switch Q36. If the second load Z2 connected between the second AC output terminal B and the neutral point is idle, power is output only between the first AC output terminal A and the neutral terminal N, which is the same as the current path of the inverter in Figure 5.

[0040] The efficiency of the three-level converter operating in the grid-connected mode in Figure 7 is the same as that of the inverter operating in the grid-connected mode in Figure 2. The worst-case efficiency of the three-level converter operating in the grid-off mode in Figure 6 is the same as that of the inverter operating in the grid-off mode in Figure 4. In the optimal situation, it can be seen that the efficiency is the same as that of the inverter operating in the grid-connected mode in Figure 2. The three-level converters in Figures 6 and 7 make use of the advantages of the inverters in Figures 2 and 4 to improve the operating efficiency of the three-level converters in the grid-off mode and the grid-connected mode.

[0041] Referring to FIG. 9, FIG. 9 is a diagram showing a configuration in which a three-level converter operates in a grid-off mode according to an embodiment of the present application. Regarding the parts where the configuration and connection relationship of the three-level converter in FIG. 9 are the same as those of the three-level converter in FIG. 6, the description will be omitted. Hereinafter, the description will focus on the differences between the configuration and connection relationship of the three-level converter in FIG. 9 and the three-level converter in FIG. 6. The second terminal of the fifth power switch Q35 is connected to the second terminal of the sixth power switch Q36 and the second terminal of the seventh power switch Q37 respectively. The first terminal of the fifth power switch Q35 is connected to the first AC output terminal A. The first terminal of the seventh power switch Q37 is connected to the second AC output terminal B.

[0042] Referring to FIGS. 10 and 11, FIG. 10 is a diagram showing the configuration in which the three-level converter of an embodiment of the present application operates in the grid-off mode, and FIG. 11 is a diagram showing the configuration in which the three-level converter according to the fourth embodiment of the present invention operates in the grid-off mode. The three-level converter further includes a first freewheel branch and a second freewheel branch. The first freewheel branch is connected between the first AC output terminal A and the neutral terminal N, and the second freewheel branch is connected between the neutral terminal N and the second AC output terminal B. As shown in FIG. 10, the first freewheel branch includes a diode D38 and a power switch Q39 connected in series. Here, the anode of the diode D38 is connected to the first AC output terminal A, the cathode of the diode D38 is connected to the first terminal of the power switch Q39, and the second terminal of the power switch Q39 is connected to the neutral terminal N. The second freewheel branch includes a power switch Q40 and a diode D41 connected in series. The second terminal of the power switch Q40 is connected to the neutral terminal N, the first terminal of the power switch Q40 is connected to the cathode of the diode D41, and the anode of the diode D41 is connected to the second AC output terminal B. Note that the diodes D38 and D41 can be replaced with controllable switches. As shown in FIG. 11, the first freewheel branch includes a diode D39 and a power switch Q38 connected in series. Here, the second terminal of the power switch Q38 is connected to the first AC output terminal A, the first terminal 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 freewheel 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 terminal of the power switch Q41, and the second terminal of the power switch Q41 is connected to the second AC output terminal B. It can be seen that the conduction directions of the first freewheel branch and the second freewheel branch are opposite.The first freewheel branch and the second freewheel branch can provide a temporary freewheel path by the first freewheel branch or the second freewheel branch during the dead time from when the second power switch Q32 and the fourth power switch Q34 turn off until the power switches Q35 to Q37 conduct, making the three-level converter more reliable and safe.

[0043] Referring to FIGS. 6, 8, and 10, the operating principle of the three-level converter is as follows.

[0044] When the three-level converter operates in the first cycle, the first power switch Q31 and the fourth power switch Q34 are turned on, and 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. Before 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 not yet turned on, the neutral terminal N, the second load Z2, the power switch Q40, and the diode D41 form a freewheel path. The diode D36, the power switch Q35, the neutral terminal N, and the first load Z1 form a freewheel 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 freewheel path. The sixth power switch Q36, the fifth power switch Q35, the first load Z1, and the neutral terminal N form a freewheel path.

[0045] When the three-level converter operates in the second period, turn on the second power switch Q32 and the third power switch Q33. The second power switch Q32, the second DC input terminal, the neutral terminal N, and the first load Z1 form a current path, and the first DC input terminal, the third power switch Q33, the second load Z2, and the neutral terminal N form a current path. Before turning off the second power switch Q32 and the third power switch Q33 and without turning on the sixth power switch Q36 and the fifth power switch Q35 yet, the neutral terminal N, the second load Z2, the power switch Q37, and the diode D36 form a freewheel path, the diode D38, the power switch Q39, the neutral terminal N, and the first load Z1 form a freewheel path. After turning on the sixth power switch Q36 and the fifth power switch Q35, the sixth power switch Q36, the seventh power switch Q37, the second load Z2, and the neutral terminal N form a freewheel path, and the fifth power switch Q35, the sixth power switch Q36, the neutral terminal N, and the first load Z1 form a freewheel path.

[0046] Obviously, the first freewheel branch and the second freewheel branch can provide a temporary freewheel path by the first freewheel branch or the second freewheel branch during the dead time from when the second power switch Q32 and the fourth power switch Q34 are turned off until the power switch Q37 conducts from the fifth power switch Q35, making the three-level converter more reliable and safe.

[0047] FIG. 11 is a diagram showing the structure of the three-level converter according to another embodiment of the present application operating in the grid-off mode. Since the operating principle of the three-level converter in FIG. 11 is similar to the operating principle in FIG. 10, it will not be described further here.

[0048] An embodiment of the present invention provides a control method applied to the three-level converter. When the three-level converter operates in the first half cycle, the second power switch Q32 and the third power switch Q33 are always off, the fifth power switch Q35 is always on, the first power switch Q31 and the fourth power switch Q34 are turned on or off simultaneously, and the sixth power switch Q36 and the seventh power switch Q37 are controlled to be turned on or off simultaneously. Here, the first power switch Q31 and the sixth power switch Q36 conduct complementarily. When the three-level converter operates in the second half cycle, the first power switch Q31 and the fourth power switch Q34 are always off, the seventh power switch Q37 is always on, the second power switch Q32 and the third power switch Q33 are turned on or off simultaneously, and the fifth power switch Q35 and the sixth power switch Q36 are controlled to be turned on or off simultaneously. Here, the second power switch Q32 and the fifth power switch Q35 conduct complementarily.

[0049] The above embodiments improve the operating efficiency of the three-level converter in the grid-off mode and the grid-connected mode.

[0050] Finally, each of the above embodiments does not limit the technical solution of the present invention, but is only for explaining the technical solution of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art will understand that the technical solutions described in the foregoing embodiments can be modified, or some or all of them can be equivalently replaced. These modifications and replacements do not deviate from the essence of the corresponding technical solutions from the scope of the technical solutions of each embodiment of the present invention.

Claims

1. 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 terminal of the first power switch is connected to the first DC input terminal and the first terminal of the third power switch respectively. The second terminal of the second power switch is connected to the second DC input terminal and the second terminal of the fourth power switch respectively. The connection point of the second terminal of the first power switch and the first terminal of the second power switch is used as a first AC output terminal. The connection point of the second terminal of the third power switch and the first terminal of the fourth power switch is used 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. A three-level converter characterized by the above.

2. When the three-level converter operates in the grid-off 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. The three-level converter according to claim 1, characterized by the above.

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

4. The first terminal of the fifth power switch is connected to the first terminal of the sixth power switch and the first terminal of the seventh power switch respectively. The three-level converter according to claim 1, characterized by the above.

5. The second terminal of the fifth power switch is connected to the second terminal of the sixth power switch and the second terminal of the seventh power switch respectively. The three-level converter according to claim 1, characterized by the above.

6. When the three-level converter operates in the first half cycle, the second power switch and the third power switch are always off, the fifth power switch is always on, the first power switch and the fourth power switch are turned on or off simultaneously, and the sixth power switch and the seventh power switch are turned on or off simultaneously. Here, the first power switch and the sixth power switch are conducting complementarily. The three-level converter according to claim 1, characterized by the above.

7. When the three-level converter operates in the second half cycle, the first power switch and the fourth power switch are always off, the seventh power switch is always on, the second power switch and the third power switch are turned on or off simultaneously, the fifth power switch and the sixth power switch are turned on or off simultaneously. Here, the three-level converter according to claim 6, wherein the second power switch and the fifth power switch conduct complementarily.

8. The three-level converter further includes a first freewheel branch and a second freewheel branch. The first freewheel branch is connected between the first AC output terminal and the neutral terminal, and the second freewheel branch is connected between the neutral terminal and the second AC output terminal. The three-level converter according to claim 1, characterized in that.

9. The three-level converter according to claim 8, characterized in that the conduction direction of the first freewheel branch is opposite to the conduction direction of the second freewheel branch.

10. The three-level converter according to claim 8, characterized in that both the first freewheel branch and the second freewheel branch include a diode and a power switch connected in series.

11. A control method applied to the three-level converter according to any one of claims 1 to 5 and 8 to 10, wherein When the three-level converter operates in the first half cycle, the second power switch and the third power switch are always off, the fifth power switch is always on, the first power switch and the fourth power switch are turned on or off simultaneously, and the sixth power switch and the seventh power switch are turned on or off simultaneously. Here, the first power switch and the sixth power switch conduct complementarily, and When the three-level converter operates in the second half cycle, the first power switch and the fourth power switch are always off, the seventh power switch is always on, the second power switch and the third power switch are turned on or off simultaneously, and the fifth power switch and the sixth power switch are turned on or off simultaneously. Here, the control method is characterized by including that the second power switch and the fifth power switch conduct complementarily.

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