A three-level inverter, photovoltaic system and control method

Abstract

The present application discloses a three-level inverter, a photovoltaic system, and a control method. The three-level inverter is a midpoint clamped three-level inverter, comprising: a bridge arm comprising a first switch module, a second switch module, a third switch module, and a fourth switch module connected in series, a clamping switch comprising a fifth switch module and a sixth switch module, and a controller; when the output voltage of the three-level inverter passes through zero, the controller controls the three-level inverter to output a first level, then controls the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off, and determines whether the three-level inverter outputs a second level, the second level being opposite to the first level. If not, it is determined that the three-level inverter has a fault. In the first half cycle of the level switching, a power frequency tube short circuits, and fault protection is triggered before a half-bus short circuit occurs to prevent a half-bus short circuit. For example, when a fault is determined, an alarm is issued in a timely manner and protective measures are taken, such as controlling the three-level inverter to shut down in a timely manner.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to a three-level inverter, a photovoltaic system, and a control method. Background Art

[0002] Existing 1100V or 1500V photovoltaic systems generally use three-level inverters. Common three-level inverters include I-type three-level, T-type three-level, and active-pole-clamp (ANPC) three-level.

[0003] When a three-level inverter is operating, if the switch tube cannot be turned off continuously, it will cause a half-bus short circuit of the inverter; if the short-circuit withstand capability of the switch tube in the inverter is poor, the switch tube will be damaged quickly, which may cause the entire three-level inverter to fail.

[0004] Therefore, it is crucial to accurately determine whether the switch tube of the three-level inverter has a fault that cannot be turned off. Summary of the Invention

[0005] In order to solve the above technical problems, the present application provides a three-level inverter, a photovoltaic system and a control method, which can accurately determine whether there is a switch failure in the three-level inverter.

[0006] The present application provides a three-level inverter, comprising: a first switch module, a second switch module, a third switch module, a fourth switch module, a fifth switch module, a sixth switch module and a controller;

[0007] The first end and the second end of the first switch module are connected to the positive bus and the first node respectively; the first end and the second end of the second switch module are connected to the first node and the output terminal respectively; the first end and the second end of the third switch module are connected to the output terminal and the second node respectively; the first end and the second end of the fourth switch module are connected to the second node and the negative bus; the first end and the second end of the fifth switch module are connected to the first node and the neutral line respectively; the first end and the second end of the sixth switch module are connected to the neutral line and the second node respectively;

[0008] The controller is configured to control the three-level inverter to output a first level when the output voltage of the three-level inverter crosses a zero point, and then control the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off, and determine whether the three-level inverter outputs a second level, the second level being opposite to the first level. If not, it is determined that the three-level inverter has a fault.

[0009] Preferably, the controller is specifically configured to control the three-level inverter to output a +1 level when the output voltage of the three-level inverter switches from the positive half cycle to the zero crossing point; then control the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off, and determine whether the three-level inverter outputs a -1 level. If not, it is determined that the three-level inverter has a fault.

[0010] Preferably, the controller is specifically configured to control the three-level inverter to output a -1 level when the output voltage of the three-level inverter switches from the negative half cycle to the zero crossing point; then control the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off, and determine whether the three-level inverter outputs a +1 level. If not, it is determined that the three-level inverter has a fault.

[0011] Preferably, the controller is specifically configured to control the three-level inverter to output the first level when the output voltage of the three-level inverter passes through a zero point and the output current of the three-level inverter is within a preset interval.

[0012] Preferably, the controller is specifically configured to control the three-level inverter to output a first level, and then control the first switch module, the second switch module, the third switch module and the fourth switch module to be turned off after a preset time period.

[0013] Preferably, it further comprises: a filter inductor connected to the output end of the three-level inverter;

[0014] The preset time period ton and the maximum current value Iref in the preset interval satisfy the following relationship:

[0015] Unbus×ton / 2L>Iref;

[0016] Among them, Unbus is the half bus voltage, and L is the inductance of the filter inductor.

[0017] Preferably, the fifth switch module includes at least a fifth switch tube, and the sixth switch module includes at least a sixth switch tube;

[0018] or,

[0019] The fifth switch module includes a fifth diode, and the sixth switch module includes a sixth diode.

[0020] The present application also provides a photovoltaic system, including the three-level inverter described above; the input end of the three-level inverter is used to connect to the photovoltaic array, or the input end of the three-level inverter is connected to the photovoltaic array through a DC-DC converter.

[0021] The present application also provides a control method for a three-level inverter, the three-level inverter including a first switch module, a second switch module, a third switch module, a fourth switch module, a fifth switch module, a sixth switch module, and a controller; a first end and a second end of the first switch module are respectively connected to a positive bus and a first node, a first end and a second end of the second switch module are respectively connected to the first node and an output terminal, a first end and a second end of the third switch module are respectively connected to the output terminal and a second node, a first end and a second end of the fourth switch module are respectively connected to the second node and a negative bus, a first end and a second end of the fifth switch module are respectively connected to the first node and an N line, and a first end and a second end of the sixth switch module are respectively connected to the N line and a second node.

[0022] When the output voltage of the three-level inverter passes through a zero point, controlling the three-level inverter to output a first level;

[0023] Controlling the first switch module, the second switch module, the third switch module and the fourth switch module to be turned off;

[0024] It is determined whether the three-level inverter outputs a second level, where the second level is opposite to the first level. If not, it is determined that a fault occurs in the three-level inverter.

[0025] Preferably, when the output voltage of the three-level inverter passes through zero, controlling the three-level inverter to output the first level specifically includes:

[0026] When the output voltage of the three-level inverter passes through a zero point and the output current of the three-level inverter is within a preset interval, the three-level inverter is controlled to output a first level.

[0027] Preferably, controlling the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off specifically includes:

[0028] After controlling the three-level inverter to output the first level for a preset time period, the first switch module, the second switch module, the third switch module and the fourth switch module are all controlled to be turned off.

[0029] Preferably, the preset time period ton and the maximum current value Iref in the preset interval satisfy the following relationship:

[0030] Unbus×ton / 2L>Iref;

[0031] Wherein, Unbus is the half bus voltage, and L is the inductance of the filter inductor connected to the output end of the three-level inverter.

[0032] It can be seen that this application has the following beneficial effects:

[0033] The three-level inverter provided by the present application mainly determines whether the second switch module and the third switch module can be correctly shut down when they need to be shut down. The present application controls the three-level inverter to output the first level when the output voltage of the three-level inverter passes through zero, and then controls the first switch module, the second switch module, the third switch module and the fourth switch module to be shut down, and determines whether the three-level inverter outputs the second level, which is opposite to the first level. If not, it is determined that the three-level inverter has a fault, that is, the power frequency tube is short-circuited within the first half cycle of the level switching, and the fault protection is triggered before the half-bus short-circuit to prevent the half-bus short-circuit. For example, when it is determined that a fault occurs, an alarm is issued in time and protective measures are taken, such as controlling the three-level inverter to shut down in time. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A schematic diagram of a three-level inverter provided in an embodiment of the present application;

[0035] Figure 2 This is a schematic diagram of T2 not being turned off;

[0036] Figure 3 A schematic diagram of another three-level inverter provided in this application;

[0037] Figure 4 A timing diagram of a switch tube provided in an embodiment of the present application;

[0038] Figure 5 Schematic diagram of a three-level inverter outputting +1 level;

[0039] Figure 6 Schematic diagram of the three-level inverter output -1 level;

[0040] Figure 7 A schematic diagram of a photovoltaic system provided in an embodiment of the present application;

[0041] Figure 8 A flow chart of a control method for a three-level inverter provided in an embodiment of the present application;

[0042] Figure 9 A flowchart of another method for controlling a three-level inverter provided in an embodiment of the present application;

[0043] Figure 10 This is a flow chart of another control method for a three-level inverter provided in an embodiment of the present application. DETAILED DESCRIPTION

[0044] In order to enable those skilled in the art to better understand the technical solution provided by this application, a specific application scenario is first introduced below.

[0045] The embodiments of the present application do not specifically limit the application scenarios of the three-level inverter. For example, it can be used in photovoltaic systems to convert the direct current (DC) power of the photovoltaic array into alternating current (AC) power for grid connection; it can also be used in wind power generation systems; and it can also be used in other situations where DC power needs to be inverted into AC power, such as energy storage systems.

[0046] As the name suggests, a three-level inverter outputs three voltage levels during operation: +1, 0, and -1. However, in practice, the switching transistors of a three-level inverter may fail to shut off during switching, resulting in a fault and short-circuiting half of the busbar. This is explained in detail below with reference to the accompanying figures.

[0047] It should be understood that the embodiments of the present application do not specifically limit the specific number of bridge arms of the three-level inverter, which is determined according to the number of phases in the inverter application scenario, such as a single-phase inverter or a three-phase inverter.

[0048] See also Figure 1 , which is a schematic diagram of a three-level inverter provided in an embodiment of the present application.

[0049] The three-level inverter provided in this embodiment includes: a first switch module, a second switch module, a third switch module, a fourth switch module, a fifth switch module, a sixth switch module and a controller;

[0050] The embodiments of this application do not specifically limit the number of switching transistors included in each switch module; generally, at least one controllable switching transistor is included. To increase voltage resistance, a switch module can include multiple controllable switching transistors connected in series. For ease of description, the following embodiments are described as each switch module including one switching transistor.

[0051] The embodiments of the present application do not specifically limit the type of controllable switch. The type of controllable switch can be any of the following: relay, insulated gate bipolar transistor (IGBT) or metal oxide semiconductor field effect transistor (MOSFET, hereinafter referred to as MOS tube), SiC MOSFET (Silicon Carbide Metal Oxide Semiconductor Field Effect Transistor, silicon carbide field effect transistor), etc. When the switch is a MOS tube, it can be a PMOS tube or an NMOS tube, which is not specifically limited in the embodiments of the present application.

[0052] The first end and the second end of the first switch module T1 are connected to the positive bus BSU+ and the first node A respectively. The first end and the second end of the second switch module T2 are connected to the first node A and the output terminal respectively. The first end and the second end of the third switch module T3 are connected to the output terminal and the second node B respectively. The first end and the second end of the fourth switch module T4 are connected to the second node B and the negative bus BUS- respectively. This embodiment does not specifically limit the specific forms of the fifth switch module and the sixth switch module, which can be diodes or controllable switch tubes. Figure 1 Taking a diode as an example, the first end and the second end of the fifth switch module D5 are connected to the first node A and the N line NBUS respectively; the first end and the second end of the sixth switch module D6 are connected to the N line and the second node B respectively.

[0053] Since the output side of the inverter is connected to the filter inductor, there is ripple in the output current, and the ripple coefficient is different for different designs. In practical applications, in order to prevent false triggering, a certain blind spot must be left when the current is near 0 for the three-level inverter. For the inverter in the photovoltaic system, the current is very small near the modulation zero point in the active state. When switching between the positive and negative half cycles, if the power frequency tube fails in the first half cycle (because the inverter works normally after the failure, it cannot be detected), it will inevitably cause a half-bus short circuit after switching. Therefore, in order to prevent a half-bus short circuit, the present application provides a solution for detecting whether the switching tube is faulty.

[0054] The following is combined with Figure 2 The following describes a situation where the second switch module T2 fails but does not shut down.

[0055] See also Figure 2 , this figure is a schematic diagram of T2 not being turned off.

[0056] When T1 and T2 of the three-level inverter are turned on and need to be switched off, T2 cannot be turned off due to a fault. At this time, T3 and T4 are still in the off state. Then, the current of the N line NBUS passes through D5 and T2 to reach the output end of the three-level inverter. The output end is connected to the filter inductor L. At this time, the output voltage Uac of the three-level inverter is the voltage of NBUS, that is, 0 level.

[0057] See also Figure 3 , which is a schematic diagram of another three-level inverter provided by this application.

[0058] Figure 3 and Figure 1 The difference is that the fifth switch module T5 and the sixth switch module T6 are both controllable switch tubes, and T5 and T6 both include anti-parallel diodes. Figure 1 The role of the diode is enough.

[0059] The three-level inverter provided in the embodiments of the present application primarily determines whether T2 and T3 can be properly shut down when required. Therefore, the present application determines whether the output voltage of the three-level inverter meets the requirements at the zero-crossing point of the output voltage to determine whether the switch tube has failed. Specifically, it determines whether the power-frequency tube has shorted in the first half cycle of the level switching. Fault protection is triggered before a half-bus short circuit occurs to prevent a half-bus short circuit. For example, when a fault is determined, a timely alarm is issued and protective measures are taken, such as controlling the three-level inverter to shut down in a timely manner.

[0060] Next, continue to combine Figure 1 The three-level inverter shown is used to introduce the working principle of the technical solution provided in the embodiment of the present application.

[0061] The controller (not shown in the figure) is used to control the three-level inverter to output a first level when the output voltage of the three-level inverter passes through zero, and then control the first switch module T1, the second switch module T2, the third switch module T3 and the fourth switch module T4 to be turned off, and determine whether the three-level inverter outputs a second level. The second level is opposite to the first level. If not, it is determined that the three-level inverter has a fault.

[0062] It should be understood that the first level is not limited to +1 or -1. Similarly, the second level is not limited to +1 or -1. When the first level is +1, the second level is -1; when the first level is -1, the second level is +1, and the level states of the two are opposite.

[0063] The output voltage of the three-level inverter crosses zero, and whether the zero crossing occurs can be determined based on the phase of the output voltage. For example, a phase-locked loop can be used to obtain the phase of the output voltage, and whether the zero crossing occurs can be determined based on the phase.

[0064] In addition, in the three-level inverter provided in the embodiment of the present application, the controller modulates the output voltage at the zero crossing point. Specifically, the level state after zero crossing can be determined based on the level state before zero crossing. For example, when the output voltage of the inverter switches from the positive half cycle to the zero crossing point, the three-level inverter is controlled to output a +1 level. Then, the first switch module T1, the second switch module T2, the third switch module T3, and the fourth switch module T4 are all controlled to be turned off to determine whether the three-level inverter outputs a -1 level. If not, it is determined that the three-level inverter has a fault. That is, the level states of the three-level inverter output before and after the four switch modules are turned off need to be opposite, that is, zero-crossing switching is achieved; if the level states are not opposite, the four switch modules are at a +1 level before turning off and at a +1 level or a 0 level after turning off, which is considered to be a fault of the switch tube and cannot be turned off, which easily causes a half-bus short circuit.

[0065] It should be understood that the controller can control the three-level inverter to output the +1 level by controlling the timing of the four switch modules. Determining whether the three-level inverter outputs the -1 level is primarily determined by sampling the inverter's output voltage. Specifically, if the output voltage is less than a first preset voltage value, the inverter is considered to output the -1 level. Similarly, determining whether the three-level inverter outputs the +1 level is also determined by sampling the inverter's output voltage. For example, if the output voltage is greater than a second preset voltage value, the inverter is considered to output the +1 level. The second preset voltage value is greater than the first preset voltage value.

[0066] Similarly, the controller is specifically configured to control the three-level inverter to output a -1 level when the output voltage of the three-level inverter switches from the negative half cycle to the zero crossing point; and then control the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off, to determine whether the three-level inverter outputs a +1 level. If not, that is, the output is a -1 level or a 0 level, then it is determined that the three-level inverter has a fault.

[0067] In addition, a preset current range is set to accurately determine whether the switch is faulty. This is to prevent the impact of protection malfunction caused by positive and negative fluctuations in current zero crossing. The controller is specifically configured to control the three-level inverter to output the first level when the output voltage of the three-level inverter crosses zero and the output current of the three-level inverter is within a preset range, for example, [-Iref, Iref].

[0068] The controller is specifically configured to control the three-level inverter to output a first level and, after a preset period of time, to control the first, second, third, and fourth switch modules to shut down. The preset time is set to allow sufficient time for the output current to commutate, ensuring that the output current flows out during the positive half-cycle and flows in during the negative half-cycle.

[0069] Generally, a three-level inverter includes a filter inductor connected to the output terminal of the three-level inverter;

[0070] The preset time period ton and the maximum current value Iref in the preset interval satisfy the following relationship:

[0071] Unbus×ton / 2L>Iref;

[0072] Among them, Unbus is the half bus voltage, and L is the inductance of the filter inductor.

[0073] The following is a detailed introduction combined with the timing diagram and current path diagram.

[0074] See also Figure 4 , this figure is a timing diagram of a switching tube provided in an embodiment of the present application.

[0075] For example, the output voltage of a three-level inverter switches from a positive half-cycle to a negative half-cycle and outputs a 0 level before crossing the zero point. Furthermore, the output current of the three-level inverter is within a preset interval [-Iref, Iref].

[0076] The specific working process is as follows:

[0077] Phase a: At this stage, it works at the zero level of the first half of the cycle, the current is relatively small, and the current outflow is uncertain;

[0078] Phase b: Since the zero-crossing switching requires the output of a +1 level, the switching needs to increase the dead time;

[0079] Phase C: Maintain the preset time period ton and output +1 level. Maintaining ton is to ensure that the output current flows out, that is, +1 level, and the output current is not lower than Iref; for details, please refer to Figure 5 , the figure shows a three-level inverter outputting +1 level, T1 and T2 are both turned on, T3 and T4 are both turned off, and the output current is greater than or equal to Iref.

[0080] Phase d: According to the shutdown sequence of the I-type three-level inverter, turn off T1 and T2. At this time, all four switches T1-T4 are turned off. After T1 and T2 are completely turned off, it is determined whether the output voltage is -1 level. If so, it indicates that the three-level inverter is working normally. For example, Figure 6 As shown, the output is -1 level, BUS- passes through the anti-parallel diodes of T3 and T4 to the filter inductor L. If the three-level inverter works normally, the operation of each switch tube can be controlled according to the normal working sequence. If not, that is, the three-level inverter outputs +1 level or 0 level, it means that T2 cannot be turned off, and continue as shown. Figure 2 As shown in the figure, T2 cannot be turned off and the output is 0 level. There is a fault in T2 or the drive circuit of T2. In order to protect the switch, fault feedback is required, such as an alarm, to promptly control the inverter to shut down.

[0081] The above embodiment is introduced by taking the zero-crossing point when switching from the positive half cycle to the negative half cycle as an example. Similarly, when the zero-crossing point when switching from the negative half cycle to the positive half cycle occurs, it can be determined whether T3 can be turned off normally.

[0082] The photovoltaic system provided by the embodiment of the present application is described below with reference to the accompanying drawings, taking the application of a three-level inverter in a photovoltaic system as an example.

[0083] See also Figure 7 , which is a schematic diagram of a photovoltaic system provided in an embodiment of the present application.

[0084] The photovoltaic system provided in this embodiment includes the three-level inverter described in the above embodiment; the photovoltaic system can be a single-phase system for home use or a three-phase photovoltaic system connected to the grid. It should be understood that Figure 1 The figure shows only the three bridge arms of one phase.

[0085] The input of the three-level inverter 1000 is connected to the photovoltaic array PV, or the input of the three-level inverter 1000 is connected to the photovoltaic array PV via a DC-DC converter. It should be understood that a DC-DC converter may also be included between the input of the three-level inverter and the photovoltaic array PV. The embodiments of the present application do not specifically limit the type of the three-level inverter; for example, it may be a string inverter or a centralized inverter.

[0086] Since the photovoltaic system provided in the embodiment of the present application includes a three-level inverter, it is possible to promptly determine whether a switch module has a fault that cannot be shut down, and then promptly issue an alarm or shut down the system, thereby avoiding causing a larger safety accident and causing the entire photovoltaic system to shut down.

[0087] Based on the three-level inverter and photovoltaic system provided in the above embodiments, the present application further provides a control method for the three-level inverter, which is described in detail below with reference to the accompanying drawings.

[0088] See also Figure 8 , which is a flow chart of a control method for a three-level inverter provided in an embodiment of the present application.

[0089] This embodiment provides a control method for a three-level inverter, wherein the three-level inverter includes a first switch module, a second switch module, a third switch module, a fourth switch module, a fifth switch module, a sixth switch module, and a controller; a first end and a second end of the first switch module are respectively connected to a positive bus and a first node, a first end and a second end of the second switch module are respectively connected to the first node and an output terminal, a first end and a second end of the third switch module are respectively connected to the output terminal and a second node, a first end and a second end of the fourth switch module are respectively connected to the second node and a negative bus, a first end and a second end of the fifth switch module are respectively connected to the first node and an N line, and a first end and a second end of the sixth switch module are respectively connected to the N line and a second node.

[0090] S801: When the output voltage of the three-level inverter passes through a zero point, control the three-level inverter to output a first level;

[0091] When the output voltage of the three-level inverter passes through a zero point, controlling the three-level inverter to output a first level specifically includes:

[0092] When the output voltage of the three-level inverter passes through a zero point and the output current of the three-level inverter is within a preset interval, the three-level inverter is controlled to output a first level.

[0093] S802: Controlling the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off;

[0094] Controlling the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off specifically includes:

[0095] After controlling the three-level inverter to output the first level for a preset time period, the first switch module, the second switch module, the third switch module, and the fourth switch module are all controlled to be turned off. The preset time period ton and the maximum current value Iref in the preset interval satisfy the following relationship:

[0096] Unbus×ton / 2L>Iref;

[0097] Wherein, Unbus is the half bus voltage, and L is the inductance of the filter inductor connected to the output end of the three-level inverter.

[0098] S803: Determine whether the three-level inverter outputs a second level, where the second level is opposite to the first level. If not, determine that the three-level inverter has a fault.

[0099] The control method of the three-level inverter provided by the present application mainly determines whether the second switch module and the third switch module can be correctly shut down when they need to be shut down. The present application controls the three-level inverter to output the first level when the output voltage of the three-level inverter passes through zero, and then controls the first switch module, the second switch module, the third switch module and the fourth switch module to be turned off, and determines whether the three-level inverter outputs the second level, which is opposite to the first level. If not, it is determined that the three-level inverter has a fault, that is, the power frequency tube is short-circuited within the first half cycle of the level switching, and the fault protection is triggered before the half-bus short-circuit to prevent the half-bus short-circuit. For example, when it is determined that a fault occurs, an alarm is issued in time and protective measures are taken, such as controlling the three-level inverter to shut down in time.

[0100] In order to make the control method provided in the embodiment of the present application clearer, it is introduced in two cases below. The first case is the control when the output voltage switches from the positive half cycle to the zero-crossing point of the negative half cycle, and the second case is the control when the output voltage switches from the negative half cycle to the zero-crossing point of the positive half cycle.

[0101] See also Figure 9 , which is a flow chart of another control method for a three-level inverter provided in an embodiment of the present application.

[0102] S901: Determine whether the output voltage of the three-level inverter is at a zero-crossing point from a positive half-cycle to a negative half-cycle. If yes, execute S902;

[0103] S902: Determine whether the output current of the three-level inverter is within a preset range. If yes, execute S903;

[0104] S903: Control the three-level inverter to output the +1 level for a preset period of time;

[0105] S904: Turn off all switches of the three-level inverter according to the timing sequence;

[0106] S905: Determine whether the three-level inverter outputs a -1 level. If yes, execute S906; otherwise, execute S907.

[0107] S906: Turn on the switch tube corresponding to the zero level of the negative half cycle after switching according to the timing to complete the zero-crossing switching.

[0108] S907: Report a fault and shut down the three-level inverter.

[0109] This embodiment can detect whether the second switch tube has a fault that cannot be turned off when crossing zero from the positive half-cycle to the negative half-cycle. When a fault occurs, the three-level inverter is promptly controlled to shut down, thereby avoiding the problem of half-bus short circuit, protecting the safety of each switch tube, and improving the reliability of the three-level inverter.

[0110] See also Figure 10 , which is a flow chart of another control method for a three-level inverter provided in an embodiment of the present application.

[0111] S1001: Determine whether the output voltage of the three-level inverter is at a zero-crossing point from the negative half cycle to the positive half cycle. If yes, execute S1002;

[0112] S1002: Determine whether the output current of the three-level inverter is within a preset range. If yes, execute S1003;

[0113] S1003: Control the three-level inverter to output the -1 level for a preset period of time;

[0114] S1004: Turn off all switches of the three-level inverter according to the timing sequence;

[0115] S1005: Determine whether the three-level inverter outputs a +1 level. If yes, execute S1006; otherwise, execute S1007.

[0116] S1006: Turn on the switch tube corresponding to the zero level of the positive half cycle after switching according to the timing to complete the zero-crossing switching.

[0117] S1007: Report a fault and control the three-level inverter to shut down.

[0118] This embodiment can detect whether the third switch tube has a fault that cannot be turned off when crossing zero from the negative half-cycle to the positive half-cycle. When a fault occurs, the three-level inverter is promptly controlled to shut down, thereby avoiding the problem of half-bus short circuit, protecting the safety of each switch tube, and improving the reliability of the three-level inverter.

[0119] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present application. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A three-level inverter, characterized in that: include: A first switch module, a second switch module, a third switch module, a fourth switch module, a fifth switch module, a sixth switch module and a controller; The first end and the second end of the first switch module are connected to the positive bus and the first node respectively, and the first end and the second end of the second switch module are connected to the first node and the output end respectively; The first end and the second end of the third switch module are connected to the output end and the second node respectively; The first end and the second end of the fourth switch module are connected to the second node and the negative bus respectively; the first end and the second end of the fifth switch module are connected to the first node and the N line respectively; The first end and the second end of the sixth switch module are connected to the N line and the second node respectively; The controller is configured to control the three-level inverter to output a first level when the output voltage of the three-level inverter crosses zero and the output current of the three-level inverter is within a preset range, and then control the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off, and determine whether the three-level inverter outputs a second level, the second level being opposite to the first level. If not, determine that the three-level inverter has a fault.

2. The three-level inverter according to claim 1, characterized in that: The controller is specifically configured to control the three-level inverter to output a +1 level when the output voltage of the three-level inverter switches from a positive half-cycle to a zero-crossing point; then control the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off, and determine whether the three-level inverter outputs a -1 level. If not, determine that the three-level inverter has a fault.

3. The three-level inverter according to claim 1, characterized in that: The controller is specifically configured to control the three-level inverter to output a -1 level when the output voltage of the three-level inverter switches from a negative half cycle to a zero crossing point; then control the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off, and determine whether the three-level inverter outputs a +1 level. If not, determine that the three-level inverter has a fault.

4. The three-level inverter according to claim 1, characterized in that: The controller is specifically configured to control the three-level inverter to output a first level, and then control the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off after a preset time period.

5. The three-level inverter according to claim 4, characterized in that: Also includes: a filter inductor connected to an output end of the three-level inverter; The preset time period ton and the maximum current value Iref of the preset interval satisfy the following relationship: Unbus×ton / 2L>Iref; Wherein, Unbus is the half bus voltage, and L is the inductance of the filter inductor.

6. The three-level inverter according to claim 1, characterized in that: The fifth switch module includes at least a fifth switch tube, and the sixth switch module includes at least a sixth switch tube; or, The fifth switch module includes a fifth diode, and the sixth switch module includes a sixth diode.

7. A photovoltaic system, characterized in that: A three-level inverter comprising any one of claims 1 to 6; The input end of the three-level inverter is used to connect to the photovoltaic array, or the input end of the three-level inverter is connected to the photovoltaic array through a DC-DC converter.

8. A control method for a three-level inverter, characterized in that: The three-level inverter includes a first switch module, a second switch module, a third switch module, a fourth switch module, a fifth switch module, a sixth switch module and a controller; a first end and a second end of the first switch module are respectively connected to the positive bus and the first node, and a first end and a second end of the second switch module are respectively connected to the first node and the output terminal; The first end and the second end of the third switch module are connected to the output end and the second node respectively; The first end and the second end of the fourth switch module are connected to the second node and the negative bus respectively; the first end and the second end of the fifth switch module are connected to the first node and the N line respectively; The first end and the second end of the sixth switch module are connected to the N line and the second node respectively; When the output voltage of the three-level inverter passes through a zero point and the output current of the three-level inverter is within a preset interval, controlling the three-level inverter to output a first level; Controlling the first switch module, the second switch module, the third switch module and the fourth switch module to be turned off; It is determined whether the three-level inverter outputs a second level, where the second level is opposite to the first level; if not, it is determined that a fault occurs in the three-level inverter.

9. The control method according to claim 8, characterized in that: The controlling the first switch module, the second switch module, the third switch module, and the fourth switch module to be turned off specifically includes: After controlling the three-level inverter to output the first level for a preset time period, the first switch module, the second switch module, the third switch module and the fourth switch module are controlled to be turned off.

10. The control method according to claim 9, characterized in that: The preset time period ton and the maximum current value Iref of the preset interval satisfy the following relationship: Unbus×ton / 2L>Iref; Wherein, Unbus is the half bus voltage, and L is the inductance of the filter inductor connected to the output end of the three-level inverter.

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