Inverters and their self-testing methods

Abstract

This invention discloses an inverter and its self-testing method. Before the inverter is connected to the grid, the duty cycle of the switching transistor is adjusted from a set state to adjust the flying voltage across the flying capacitor. Based on whether the flying voltage can be stabilized at the set voltage, it can be determined whether at least one switching transistor is working properly. This allows it to determine whether the charging and discharging switching transistors are working properly, thereby timely detection of inverter hardware faults before grid connection, preventing the fault from escalating, and improving the safety and reliability of the inverter.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to an inverter and its self-testing method. Background Technology

[0002] Flying capacitor three-level boost circuits are increasingly being used in inverters due to their advantages such as increasing the equivalent switching frequency, reducing inductor size and cost, and reducing the complexity of inverter insulation impedance detection circuits.

[0003] In existing technologies, photovoltaic power plants are typically connected to the grid via multiple string inverters, each of which includes a flying capacitor three-level boost circuit. During self-testing of photovoltaic inverters in existing technologies, the success of the flying capacitor three-level boost circuit in the multi-string inverter is usually determined by analyzing the relationship between the bus voltage connected to the bus and the AC peak value.

[0004] However, the above judgment method cannot detect in time whether there is an abnormality in the three-level boost circuit of the flyover capacitor, which leads to the inverter being connected to the grid with hidden dangers, reducing the safety and reliability of the inverter. Summary of the Invention

[0005] This invention provides an inverter and its self-testing method, which can detect abnormal hardware states of the inverter in a timely manner before grid connection, thereby improving the safety and reliability of the inverter.

[0006] According to one aspect of the present invention, an inverter self-test method is provided. The inverter includes at least one flying capacitor three-level boost circuit. The flying capacitor three-level boost circuit includes a switching transistor and a flying capacitor. The flying capacitor is electrically connected to the switching transistor, and the output terminal of the flying capacitor three-level boost circuit is electrically connected to a bus. The inverter self-test method includes:

[0007] Before the inverter is connected to the grid, the duty cycle of the switching transistor is adjusted from the set state to adjust the flying voltage across the flying capacitor. In the set state, the flying voltage is the first voltage and the voltage on the bus is the second voltage. The first voltage is equal to half of the second voltage.

[0008] Whether at least one switching transistor is working properly can be determined by whether the cross voltage can be stabilized at the set voltage.

[0009] The voltage can be set to be greater than half of the second voltage, or the voltage can be set to be less than half of the second voltage.

[0010] Optionally, the switching transistors include a charging switch and a discharging switch, which are connected in series, and a flying capacitor is connected to the common connection terminal of the charging switch and the discharging switch.

[0011] Before the inverter is connected to the grid, the duty cycle of the switching transistor is adjusted from the set state to adjust the flying voltage across the flying capacitor. This includes: before the inverter is connected to the grid, in the first time period, controlling the first duty cycle of the charging switching transistor to be greater than the second duty cycle of the discharging switching transistor; in the second time period, controlling the first duty cycle to be equal to the second duty cycle, and the second time period is after the first time period.

[0012] Determining whether the switching transistor is working properly based on whether the flyover voltage can be stabilized at the set voltage includes: determining whether the charging switching transistor is working properly based on whether the flyover voltage is stabilized at the first set voltage during the second time period; the first set voltage is greater than half of the second voltage.

[0013] Optionally, during the first time period, controlling the first duty cycle corresponding to the charging switch to be greater than the second duty cycle corresponding to the discharging switch includes: during the first time period, controlling the first duty cycle corresponding to the charging switch to increase from 0 with a first slope, and controlling the second duty cycle of the discharging switch to increase from 0 with a second slope, wherein the first slope is greater than the second slope.

[0014] Optionally, before the inverter is connected to the grid, the duty cycle of the switching transistor is adjusted from the set state to adjust the flying voltage across the flying capacitor. This also includes: during the third time period, controlling the first duty cycle of the charging switching transistor to be less than the second duty cycle of the discharging switching transistor; during the fourth time period, controlling the first duty cycle to be equal to the second duty cycle, with the fourth time period following the third time period.

[0015] Determining whether the switching transistor is working properly based on whether the flyover voltage can be stabilized at the set voltage also includes: determining whether the discharge switching transistor is working properly based on whether the flyover voltage is stabilized at the second set voltage during the fourth time period; the second set voltage is less than half of the second voltage.

[0016] In this process, the fourth time period begins before the first time period, and the flying capacitor three-level boost circuit is in the set state at the start of the third time period.

[0017] Alternatively, the second time period may occur before the third time period, with the flying capacitor three-level boost circuit in the set state at the start of the first time period.

[0018] Optionally, during the third time period, controlling the first duty cycle corresponding to the charging switch to be less than the second duty cycle corresponding to the discharging switch includes: during the third time period, controlling the first duty cycle corresponding to the charging switch to increase from 0 with a third slope, and controlling the second duty cycle of the discharging switch to increase from 0 with a fourth slope, wherein the third slope is less than the fourth slope.

[0019] Optionally, during the first, second, third, and fourth time periods, both the first and second duty cycles are less than the preset duty cycle threshold.

[0020] Optionally, the absolute value of the difference between the first duty cycle and the second duty cycle is less than or equal to half of a preset duty cycle threshold.

[0021] Optionally, the inverter self-test method may also include: performing the inverter's start-up condition judgment or the closed-loop boost process of the flying capacitor three-level boost circuit;

[0022] The power-on condition judgment is performed before adjusting the duty cycle of the switching transistor to adjust the flying voltage across the flying capacitor from the set state, or after confirming that the switching transistor is working normally.

[0023] Determining the inverter's start-up conditions includes: determining whether the inverter's start-up conditions are met based on the photovoltaic output voltage and output power of the photovoltaic panel connected to the input terminal of the flying capacitor three-level boost circuit.

[0024] The closed-loop boost process is performed before adjusting the duty cycle of the switching transistor and the voltage across the flying capacitor from the set state, or after confirming that the switching transistor is working normally. The closed-loop boost process includes controlling the three-level boost circuit of the flying capacitor to boost the output voltage to the third voltage.

[0025] Optionally, the inverter self-test method also includes: if the overpass voltage can be stabilized at the set voltage to determine that each switching transistor is working normally and meets the start-up conditions, and after the closed-loop boost process, the inverter performs open-loop self-test and relay self-test; after the inverter passes the open-loop self-test and relay self-test, the inverter is controlled to operate in grid-connected mode.

[0026] According to another aspect of the present invention, an inverter is provided, including at least one flying capacitor three-level boost circuit, the flying capacitor three-level boost circuit including a switching transistor and a flying capacitor, the flying capacitor being electrically connected to the switching transistor; the inverter performs a self-test based on the inverter self-test method of any embodiment of the present invention.

[0027] The inverter and its self-testing method according to the present invention adjust the duty cycle of the switching transistors from a set state before the inverter is connected to the grid, thereby adjusting the flying voltage across the flying capacitor. Based on whether the flying voltage can be stabilized at the set voltage, it can be determined whether at least one switching transistor is working properly. This allows it to determine whether the charging and discharging switching transistors are working properly, thus timely detecting hardware faults in the inverter before grid connection, preventing the fault from escalating, and improving the safety and reliability of the inverter.

[0028] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of the present invention, nor is it intended to limit the scope of the invention. Other features of the invention will become readily apparent from the following description. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of a three-level boost circuit with a flying capacitor.

[0031] Figure 2 This is a flowchart of an inverter self-test method provided in an embodiment of the present invention;

[0032] Figure 3 This is a flowchart of another inverter self-test method provided in an embodiment of the present invention;

[0033] Figure 4 It is a graph of the first duty cycle and the second duty cycle;

[0034] Figure 5 It is a curve of the voltage across the capacitor;

[0035] Figure 6 It is another graph of the first duty cycle and the second duty cycle;

[0036] Figure 7 This is another curve showing the voltage across the capacitor;

[0037] Figure 8 This is a flowchart of another inverter self-test method provided in an embodiment of the present invention;

[0038] Figure 9 This is a flowchart of another inverter self-testing method provided in an embodiment of the present invention;

[0039] Figure 10 This is a flowchart of another inverter self-testing method provided in an embodiment of the present invention. Detailed Implementation

[0040] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0041] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0042] This invention provides a self-test method for an inverter, wherein the inverter includes at least one flying capacitor three-level boost circuit. Figure 1 This is a schematic diagram of a flying capacitor three-level boost circuit, for reference. Figure 1 The flying capacitor three-level boost circuit includes a switching transistor and a flying capacitor Cfly. The flying capacitor Cfly is electrically connected to the switching transistor, and the output terminal of the flying capacitor three-level boost circuit is electrically connected to the bus. In some optional embodiments, the inverter includes at least two flying capacitor three-level boost circuits. Figure 1 The example illustrates an inverter including two flying capacitor three-level boost circuits. The photovoltaic output voltages input to the corresponding flying capacitor three-level boost circuits from the photovoltaic panels connected to their input terminals can be the same or different. The output terminals of the flying capacitor three-level boost circuits can be connected in parallel to the bus. Specifically, the flying capacitor three-level boost circuit can include a charging switch VT12 and a discharging switch VT11. When the charging switch VT12 is on and the discharging switch VT11 is off, the flying capacitor Cfly can be charged. When the discharging switch VT11 is on and the charging switch VT12 is off, the flying capacitor Cfly can be discharged. Figure 2 This is a flowchart of an inverter self-test method provided in an embodiment of the present invention, see reference. Figure 2 The inverter's self-test method includes:

[0043] S110. Before the inverter is connected to the grid, the duty cycle of the switching transistor is adjusted from the set state to adjust the flying voltage across the flying capacitor.

[0044] In the set state, the flyover voltage is the first voltage, and the voltage on the bus is the second voltage. The first voltage is equal to half of the second voltage.

[0045] Specifically, the charging switch VT12 and the discharging switch VT11 are driven by PWM pulses (Vge1 and Vge2) with a 180° phase shift between each other. By adjusting the pulse length, the flying voltage of the flying capacitor Cfly is adjusted to be equal to half of the bus voltage. (Reference) Figure 1 The flying capacitor three-level boost circuit also includes a first diode D12, a second diode D13, a third diode D11, and a fourth diode D14. The anode of the first diode D12 is connected to the collector of the discharge switch VT11 and the inductor L11, respectively. The cathode of the first diode D12 is connected to the anode of the second diode D13, and the cathode of the second diode D13 is connected to the first output terminal OUT1 of the flying capacitor three-level boost circuit. The emitter of the discharge switch VT11 is connected to the collector of the charging switch VT12, and the emitter of the charging switch VT12 is connected to the second output terminal OUT2 of the flying capacitor three-level boost circuit. The voltage between the first output terminal OUT1 and the second output terminal OUT2 is the output voltage of the flying capacitor three-level boost circuit. A first output capacitor Cout+ and a second output capacitor Cout- are connected in series between the first output terminal OUT1 and the second output terminal OUT2. A fourth diode D14 is connected between the common connection terminal of the first output capacitor Cout+ and the second output capacitor Cout- and the common connection terminal of the discharge switch VT11 and the charging switch VT12. The cathode of the fourth diode D14 is electrically connected to the anode of the third diode D11, and the cathode of the third diode D11 is connected to the common connection terminal of the first diode D12 and the second diode D13. One end of the flying capacitor Cfly is connected to the common connection terminal of the discharge switch VT11 and the charging switch VT12, and the other end of the flying capacitor Cfly is connected to the common connection terminal of the first diode D12 and the second diode D13. The input terminal of the flying capacitor three-level boost circuit can be connected to a photovoltaic panel, that is, the input voltage Vin of the flying capacitor three-level boost circuit is equal to the photovoltaic output voltage of the photovoltaic panel. The charging path of the flying capacitor Cfly is from the positive input terminal through inductor L11, first diode D12, flying capacitor Cfly, and charging switch VT12 to the negative input terminal. The discharging path of the flying capacitor Cfly is through the positive input terminal, inductor L11, discharging switch VT11, flying capacitor Cfly, second diode D13, first output capacitor Cout+, and second output capacitor Cout- back to the negative input terminal.

[0046] Due to the presence of the third diode D11 and the fourth diode D14, the flying capacitor three-level boost circuit can be in a set state when it is first powered on, that is, the voltage of the flying capacitor Cfly is equal to half of the output voltage of the flying capacitor three-level boost circuit. Figure 1As shown, the discharge switch VT11 and the charging switch VT12 use the same wafer specification, and their allowable voltage stress requirements are consistent. During operation of the flying capacitor three-level boost circuit, the fourth diode D14 is used to charge the flying capacitor Cfly when the discharge switch VT11 and the charging switch VT12 are not working (e.g., when the flying capacitor three-level boost circuit is first powered on); the third diode D11 is used for regenerative energy charging, that is, charging the flying capacitor Cfly when the bus voltage is higher than the output voltage of the flying capacitor three-level boost circuit (this operating condition almost never occurs in the inverter; if the flying voltage Vcly is greater than the voltage on the second output capacitor Cout-, grid energy backflow will occur, which is not allowed in inverters connected to generator equipment).

[0047] During operation, the voltage stress plateau V borne by the internal transistors such as the discharge switch VT11 and the first diode D12 ds1 It should be:

[0048] V ds1 =V cfly ;

[0049] That is, the voltage stress plateau V of the inner tube ds1 For the flying voltage V cfly .

[0050] During operation, the voltage stress plateau borne by external transistors such as charging switch VT12 and second diode D13 should be as follows:

[0051] V ds2 =V bus -V cfly ;

[0052] That is, the voltage stress plateau V of the outer tube ds2 Bus voltage V bus Subtract the flying voltage V cfly .

[0053] To ensure that both the discharge switch VT11 and the charging switch VT12 have sufficient voltage stress and sufficient rated margin during stable circuit operation, the following requirements must be met:

[0054] V ds1 =V ds2 ;

[0055] Thus, That is, the flying voltage V cfly Bus voltage V bus Half of it.

[0056] When charging and / or discharging the flying capacitor Cfly in subsequent control operations, the set state can also be achieved by adjusting the duty cycle of the drive signal of the discharge switch VT11 and / or the duty cycle of the drive signal of the charging switch VT12. This means the voltage of the flying capacitor Cfly is equal to half the output voltage of the three-level boost circuit. In this step, starting from the set state of the three-level boost circuit, the duty cycle of the switching transistors is adjusted to regulate the flying voltage across the flying capacitor Cfly. When the switching transistors are operating normally in the three-level boost circuit, the magnitude of the flying voltage will change. Because in the set state, the first voltage across the flying capacitor Cfly is equal to half the second voltage output by the three-level boost circuit, correspondingly, after the flying voltage changes, it is no longer equal to half the second voltage.

[0057] S120, Obtain the flying voltage.

[0058] Specifically, after adjusting the duty cycle of the charging switch and / or discharging switch, the flying voltage across the flying capacitor changes under the condition that the charging switch and discharging switch are working normally. In this step, the flying voltage across the flying capacitor can be obtained in real time or at regular intervals.

[0059] S130. Determine whether at least one switching transistor is working properly based on whether the transient voltage can be stabilized at the set voltage.

[0060] Specifically, the set voltage can be greater than half of the second voltage, or the set voltage can be less than half of the second voltage. Alternatively, the set voltage can be greater than 0 and less than the second voltage.

[0061] Specifically, through analysis of production line and field failure cases, the following typical failure modes are currently observed in the application of flying capacitor three-level boost circuits:

[0062] (1) The drive circuit of the discharge switch is damaged, and there is only a low level (optocoupler or drive circuit short circuit) or the gate of the discharge switch is short-circuited (such as internal bonding wire short circuit), which causes the discharge switch to be in an open circuit state.

[0063] (2) The drive circuit of the discharge switch is damaged, and there is only a high level (the drive optocoupler is damaged) or the collector and emitter of the discharge switch are short-circuited (such as the wafer is short-circuited), which causes the discharge switch to be in a short-circuit state.

[0064] (3) The driving circuit of the charging switch is damaged, and there is only a low level or the gate of the charging switch is short-circuited (such as the internal bonding line is short-circuited), which causes the charging switch to be in an open circuit state.

[0065] (4) The driving circuits of the charging switch and the discharging switch are damaged, resulting in only a high level or a short circuit between the collector and emitter of the charging switch (such as a short circuit damage to the wafer), causing the charging switch to be in a short circuit state.

[0066] (5) The drive power supply is damaged, causing both the charging switch and the discharging switch to be in an open circuit state.

[0067] (6) The internal wafers of the charging switch and the discharging switch are damaged. Both the charging switch and the discharging switch are in a short-circuit state.

[0068] Based on the above failure modes, the main issues lie in the proper functioning of two critical components in the flying capacitor three-level boost circuit: the charging switch and the discharging switch. Each of these switches has two failure modes: open circuit or short circuit. Therefore, determining whether the charging and discharging switches are functioning correctly before grid connection is crucial for identifying any hardware malfunctions in the circuit. In the flying capacitor three-level boost circuit, the discharging switch discharges the flying capacitor when it is on, while the charging switch charges it when it is on. By utilizing the charging and discharging characteristics of the flying capacitor by these switches, it is possible to determine if they are malfunctioning. This allows for the detection of hardware defects in the inverter boost circuit itself before grid connection, preventing the fault from escalating.

[0069] In this step, the normal operation of the charging switch and / or discharging switch can be determined by whether the flyback voltage can stabilize at the set voltage. Optionally, if the set voltage is greater than half of the second voltage, and the flyback voltage can stabilize at the set voltage, it indicates that the charging switch can conduct normally for charging as the flyback voltage gradually increases from the first voltage to the set voltage. Furthermore, since the flyback voltage does not continuously increase but stabilizes at the set voltage, it indicates that the charging switch can normally turn off, meaning the charging switch is working normally. If the flyback voltage cannot stabilize at the set voltage, it indicates that the charging switch is malfunctioning. Optionally, if the set voltage is less than half of the second voltage, and the flyback voltage can stabilize at the set voltage, it indicates that the flyback voltage gradually decreases from the first voltage to the set voltage, and the discharging switch can conduct normally for discharging. Furthermore, since the flyback voltage does not continuously decrease but stabilizes at the set voltage, it indicates that the discharging switch can normally turn off, meaning the discharging switch is working normally. If the flyback voltage cannot stabilize at the set voltage, it indicates that the discharging switch is malfunctioning. The inverter self-test method in this embodiment can perform self-tests on each flying capacitor three-level boost circuit in the inverter, and promptly determine whether the switching transistors in each flying capacitor three-level boost circuit are abnormal.

[0070] The inverter self-test method in this embodiment adjusts the duty cycle of the switching transistors from a set state before the inverter is connected to the grid, thereby adjusting the flying voltage across the flying capacitor. Based on whether the flying voltage can be stabilized at the set voltage, it can be determined whether at least one switching transistor is working properly. This allows it to determine whether the charging and discharging switching transistors are working properly, thus timely detecting inverter hardware faults before grid connection, preventing the faults from escalating, and improving the safety and reliability of the inverter.

[0071] As described above, the switching transistor includes a charging switch and a discharging switch, which are connected in series, and a flying capacitor is connected to the common connection terminal of the charging switch and the discharging switch.

[0072] Figure 3 This is a flowchart of another inverter self-test method provided in an embodiment of the present invention, see reference. Figure 3 Optionally, the inverter self-test method includes:

[0073] S210. Before the inverter is connected to the grid, during the first time period, the first duty cycle corresponding to the charging switch is greater than the second duty cycle corresponding to the discharging switch.

[0074] In the first time period, the first duty cycle corresponding to the charging switch is greater than the second duty cycle corresponding to the discharging switch, causing the flying capacitor to charge and its voltage to rise. In this embodiment, controlling the first duty cycle corresponding to the charging switch and the second duty cycle corresponding to the discharging switch, and thus controlling the flying voltage of the flying capacitor, can be achieved using either open-loop or closed-loop control. Figure 4 It is a graph of the first duty cycle and the second duty cycle. Figure 5 This is a graph showing the voltage across a capacitor, see reference. Figure 4 and Figure 5 Optionally, S210 includes: within the first time period t1, controlling the first duty cycle D12 corresponding to the charging switch to increase from 0 with a first slope, and controlling the second duty cycle D11 of the discharging switch to increase from 0 with a second slope, wherein the first slope is greater than the second slope. This ensures that at any time within the first time period t1, the first duty cycle D12 corresponding to the charging switch is greater than the second duty cycle D11 corresponding to the discharging switch, allowing the flying capacitor to be charged when the charging switch is functioning normally.

[0075] S220. During the second time period, the first duty cycle is controlled to be equal to the second duty cycle, and the second time period is after the first time period.

[0076] Combination Figure 4 and Figure 5In the case of using open-loop control to control the flying voltage Vfly of the flying capacitor, the first duty cycle D12 and the second duty cycle D11 can eventually be equal. In the case of closed-loop control, the first duty cycle D12 can also be made equal to the second duty cycle D11 in the second time period t2 by controlling the first duty cycle D12 and the second duty cycle D11.

[0077] S230. Determine whether the charging switch is working properly based on whether the crossover voltage is stable at the first set voltage during the second time period.

[0078] Wherein, the first set voltage is greater than half of the second voltage (that is...) Figure 5 (Vbus / 2).

[0079] As described above, if the charging switch is working normally, the flying voltage of the flying capacitor can stabilize during the second time period. Since the flying capacitor is charged during the first time period, its flying voltage will be higher than the first voltage under the set condition, i.e., greater than half of the second voltage on the bus. Therefore, in this step, by determining whether the charging switch is working normally based on whether the flying voltage can stabilize at a first set voltage greater than half of the second voltage during the second time period, it can be determined whether the charging switch is working normally. In some optional embodiments, the first set voltage can be set to be equal to the sum of half of the second voltage and a reference voltage, where the reference voltage value can be set relatively small, for example, less than or equal to 2% of the second voltage. Specifically, if the flying voltage can stabilize at the first set voltage during the second time period, it can be determined that the charging switch is working normally; if the flying voltage cannot stabilize at the first set voltage during the second time period, it indicates that the charging switch may have a short circuit or open circuit, indicating that the charging switch is not working normally.

[0080] S240. During the third time period, the first duty cycle corresponding to the charging switch is controlled to be less than the second duty cycle corresponding to the discharging switch.

[0081] During the third time period, the first duty cycle corresponding to the charging switch is controlled to be less than the second duty cycle corresponding to the discharging switch, causing the flying capacitor to discharge and its voltage to decrease. In this embodiment, controlling the first duty cycle corresponding to the charging switch and the second duty cycle corresponding to the discharging switch, and thus controlling the flying voltage of the flying capacitor, can be achieved using either open-loop or closed-loop control. Figure 6 It is another graph showing the first and second duty cycles. Figure 7 This is another graph showing the voltage across the capacitor, see reference. Figure 6 and Figure 7Optionally, S240 includes, within the third time period t3, controlling the first duty cycle D12 corresponding to the charging switch to increase from 0 with a third slope, and controlling the second duty cycle D11 of the discharging switch to increase from 0 with a fourth slope, where the third slope is less than the fourth slope. This ensures that at any time within the third time period t3, the first duty cycle D12 corresponding to the charging switch is less than the second duty cycle D11 corresponding to the discharging switch, allowing the flying capacitor to discharge under normal conditions when the discharging switch is operating normally.

[0082] S250. During the fourth time period, the first duty cycle is controlled to be equal to the second duty cycle. The fourth time period is after the third time period.

[0083] Combination Figure 6 and Figure 7 In the case of open-loop control of the flying capacitor's flying voltage Vfly, the first duty cycle D12 and the second duty cycle D11 can eventually be equal. In the case of closed-loop control, the first duty cycle D12 can also be made equal to the second duty cycle D11 during the fourth time period t4 by controlling the first duty cycle D12 and the second duty cycle D11.

[0084] S260. Determine whether the discharge switch is working properly based on whether the crossover voltage is stable at the second set voltage during the fourth time period.

[0085] Wherein, the second set voltage is less than half of the second voltage (that is...) Figure 7 (Vbus / 2).

[0086] As described above, if the discharge switch is working normally, the flying voltage of the flying capacitor can stabilize during the fourth time period. Since the flying capacitor is discharged during the third time period, its flying voltage will be lower than the first voltage under the set condition, i.e., less than half of the second voltage on the bus. Therefore, in this step, by determining whether the flying voltage can stabilize at a second set voltage less than half of the second voltage during the second time period, it can be determined whether the discharge switch is working normally. In some optional embodiments, the second set voltage can be set to be equal to the difference between half of the second voltage and the reference voltage. Specifically, if the flying voltage can stabilize at the second set voltage during the fourth time period, it can be determined that the discharge switch is working normally; if the flying voltage cannot stabilize at the second set voltage during the fourth time period, it indicates that the discharge switch may have a short circuit or open circuit, meaning the discharge switch is not working normally.

[0087] like Figure 3As shown, in some optional embodiments of the present invention, the self-test of the charging switch (including S210-S230) can be performed before the self-test of the discharging switch (including S240-S260), and the corresponding second time period precedes the third time period. At the start of the first time period, the flying capacitor three-level boost circuit is in a set state. In this case, the self-test of the charging switch starts from the set state, and then the self-test of the discharging switch is performed.

[0088] In another optional embodiment of the present invention, the self-test of the charging switch (including S210-S230) can be performed after the self-test of the discharging switch (including S240-S260), with the corresponding fourth time period preceding the first time period, and the flying capacitor three-level boost circuit in a set state at the start of the third time period. In this case, the self-test of the discharging switch begins from the set state, and then the self-test of the charging switch is performed.

[0089] Combination Figure 4 and Figure 6 Optionally, during the first, second, third, and fourth time periods, both the first and second duty cycles are less than the preset duty cycle threshold Dmin, thereby keeping the voltage fluctuation on the bus within a preset range.

[0090] The preset duty cycle threshold can be set according to the allowable fluctuation of the bus voltage. Optionally, the preset duty cycle threshold is less than or equal to 5%. Since the magnitude of the first duty cycle and the second duty cycle affects the output voltage of the flying capacitor three-level boost circuit, it also affects the bus voltage. In this embodiment, setting the first duty cycle and the second duty cycle to be less than the preset duty cycle threshold during the first, second, third, and fourth time periods can reduce the fluctuation of the bus voltage.

[0091] In some optional embodiments of the present invention, the absolute value of the difference between the first duty cycle and the second duty cycle is less than or equal to half of a preset duty cycle threshold. This further ensures that the impact of adjusting the first and second duty cycles on the bus voltage is reduced, and further reduces bus voltage fluctuations.

[0092] Figure 8 This is a flowchart of another inverter self-test method provided in an embodiment of the present invention. Figure 9 This is a flowchart of another inverter self-test method provided in an embodiment of the present invention. Figure 10 This is a flowchart of another inverter self-test method provided in an embodiment of the present invention, see reference. Figures 8-10Optionally, the inverter self-test method further includes: determining the inverter's power-on conditions or performing a closed-loop boost process for the flying capacitor three-level boost circuit. In some optional embodiments of the present invention, the inverter self-test method may include determining the inverter's power-on conditions and performing a closed-loop boost process for the flying capacitor three-level boost circuit.

[0093] refer to Figures 8-10 Optionally, the power-on condition determination is performed before adjusting the duty cycle of the switching transistor to regulate the flying voltage across the flying capacitor from the set state, or after confirming that the switching transistors (including charging and discharging switches) are working normally. The closed-loop boost process is performed before adjusting the duty cycle of the switching transistor to regulate the flying voltage across the flying capacitor from the set state, or after confirming that the switching transistors (including charging and discharging switches) are working normally; the closed-loop boost process is performed after the power-on condition determination.

[0094] The inverter start-up condition determination includes: determining whether the inverter start-up conditions are met based on the photovoltaic output voltage and output power of the photovoltaic panel connected to the input terminal of the flying capacitor three-level boost circuit. Optionally, the inverter start-up conditions are determined to be met if the photovoltaic output voltage of the photovoltaic panel reaches a set voltage threshold and the output power reaches a set output power threshold.

[0095] The closed-loop boost process includes controlling the flying capacitor three-level boost circuit to boost the output voltage to the third voltage. This third voltage can be set according to the requirements of the inverter circuit connected to the bus of the flying capacitor three-level boost circuit, ensuring that the bus voltage meets the inverter circuit's input voltage requirements and guarantees power quality after grid connection. This closed-loop boost process can be achieved by controlling the duty cycle of the charging and discharging switches.

[0096] Optionally, if the normal operation of each switching transistor is determined based on whether the crossover voltage can be stabilized at the set voltage and the start-up conditions are met, and after the closed-loop boost process, the inverter performs open-loop self-test and relay self-test, and after the inverter's open-loop self-test and relay self-test pass, the inverter is controlled to operate in grid-connected mode.

[0097] Specifically, in addition to the flying capacitor three-level boost circuit, the inverter also includes an inverter circuit and relays. In this embodiment, after confirming that each flying capacitor three-level boost circuit is fault-free, before controlling the inverter to operate in grid-connected mode, an open-loop self-test of the inverter and a self-test of the relays are performed to detect faults in other structures of the inverter in a timely manner, further prevent the fault from escalating, and ensure the safety and reliability of the inverter.

[0098] in, Figure 8The diagram illustrates a scenario where the power-on judgment condition and the closed-loop boost process are performed before the charging switch self-test (including S210-S230 in the above embodiments) and the discharging switch self-test (including S240-S260 in the above embodiments). Figure 9 The diagram shows that the power-on judgment condition is performed before the charging switch self-test (including S210-S230 in the above embodiment) and the discharging switch self-test (including S240-S260 in the above embodiment), and the closed-loop boost process is performed after the charging switch self-test (including S210-S230 in the above embodiment) and the discharging switch self-test (including S240-S260 in the above embodiment). Figure 10 The diagram shows that the power-on judgment condition and the closed-loop boost process are performed after the charging switch self-test (including S210-S230 in the above embodiment) and the discharging switch self-test (including S240-S260 in the above embodiment).

[0099] refer to Figure 8 The inverter's self-test method includes:

[0100] Initially, the inverter is in standby mode.

[0101] S310. Determine if the power-on conditions are met. That is, perform a power-on condition check.

[0102] If yes, then execute S320; otherwise, return to standby mode.

[0103] S320 performs the closed-loop boost process of the flying capacitor three-level boost circuit.

[0104] S330, Determine whether the charging switch transistor has passed the self-test.

[0105] If yes, execute S340; otherwise, execute S350.

[0106] S340. Determine whether the self-test of the discharge switch tube has passed.

[0107] If yes, then execute S360; otherwise, execute S350.

[0108] S350 controls the Luffy cross-capacitor three-level boost circuit and the corresponding maximum power point tracking controller to standby mode and issues a fault warning.

[0109] S360 controls the inverter to perform open-loop self-test and relay self-test, and determines whether the inverter open-loop self-test and relay self-test are successful.

[0110] If yes, then execute S370; otherwise, execute S380.

[0111] S370 controls the inverter to operate in grid-connected mode.

[0112] S380: The inverter is confirmed to be in a fault state, and a warning is issued.

[0113] refer to Figure 9 The inverter's self-test method includes:

[0114] Initially, the inverter is in standby mode.

[0115] S410. Determine if the power-on conditions are met. That is, perform a power-on condition check.

[0116] If yes, then execute S420; otherwise, return to standby mode.

[0117] S420: Determine whether the charging switch transistor has passed the self-test.

[0118] If yes, execute S430; otherwise, execute S440.

[0119] S430. Determine whether the self-test of the discharge switch tube has passed.

[0120] If yes, then execute S450; otherwise, execute S440.

[0121] S440 controls the Luffy cross-capacitor three-level boost circuit and the corresponding maximum power point tracking controller to standby mode and issues a fault warning.

[0122] S450 performs the closed-loop boost process of the flying capacitor three-level boost circuit.

[0123] S460: Control the inverter to perform open-loop self-test and relay self-test, and determine whether the inverter open-loop self-test and relay self-test are successful.

[0124] If yes, then execute S470; otherwise, execute S480.

[0125] S470 controls the inverter to operate in grid-connected mode.

[0126] S480: The inverter is confirmed to be in a fault state, and a warning is issued.

[0127] refer to Figure 10 The inverter's self-test method includes:

[0128] Initially, the inverter is in standby mode.

[0129] S510, Determine whether the charging switch transistor has passed the self-test.

[0130] If yes, execute 520; otherwise, execute S530.

[0131] S520. Determine whether the self-test of the discharge switch tube has passed.

[0132] If yes, then execute S540; otherwise, execute S530.

[0133] S530 controls the Luffy cross-capacitor three-level boost circuit and the corresponding maximum power point tracking controller to standby mode and issues a fault warning.

[0134] S540: Determine if the power-on conditions are met. This means performing a power-on condition check.

[0135] If yes, then execute S550; otherwise, return to standby mode.

[0136] S550 performs the closed-loop boost process of the flying capacitor three-level boost circuit.

[0137] S560 controls the inverter to perform open-loop self-test and relay self-test, and determines whether the inverter open-loop self-test and relay self-test are successful.

[0138] If yes, then execute S570; otherwise, execute S580.

[0139] S570 controls the grid-connected operation of the inverter.

[0140] S580: Determines that the inverter is in a fault state and issues a warning.

[0141] This invention also provides an inverter, which includes at least one flying capacitor three-level boost circuit. The flying capacitor three-level boost circuit includes a switching transistor and a flying capacitor, and the flying capacitor is electrically connected to the switching transistor. The inverter performs a self-test based on the inverter self-test method of any of the above embodiments of this invention, and has the beneficial effects of the inverter self-test method of any of the above embodiments of this invention, which will not be described in detail here.

[0142] It should be understood that the various forms of processes shown above can be used, with steps reordered, added, or deleted. For example, the steps described in this invention can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution of this invention can be achieved, and this is not limited herein.

[0143] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A self-testing method for an inverter, characterized in that, The inverter includes at least one flying capacitor three-level boost circuit, which includes a switching transistor and a flying capacitor. The flying capacitor is electrically connected to the switching transistor, and the output terminal of the flying capacitor three-level boost circuit is electrically connected to the bus. The inverter self-test method includes: Before the inverter is connected to the grid, the duty cycle of the switching transistor is adjusted from a set state to adjust the flying voltage across the flying capacitor; wherein, in the set state, the flying voltage is a first voltage, the voltage on the bus is a second voltage, and the first voltage is equal to half of the second voltage; Whether at least one of the switching transistors is working properly is determined based on whether the jump voltage can be stabilized at the set voltage. Wherein, the set voltage is greater than half of the second voltage, or the set voltage is less than half of the second voltage; the switching transistor includes a charging switching transistor and a discharging switching transistor, the charging switching transistor and the discharging switching transistor are connected in series, and the flying capacitor is connected to the common connection terminal of the charging switching transistor and the discharging switching transistor; Before the inverter is connected to the grid, the duty cycle corresponding to the switching transistor is adjusted from the set state to adjust the flying voltage across the flying capacitor. This includes: before the inverter is connected to the grid, during a first time period, controlling the first duty cycle corresponding to the charging switching transistor to be greater than the second duty cycle corresponding to the discharging switching transistor; during a second time period, controlling the first duty cycle to be equal to the second duty cycle, wherein the second time period is after the first time period. Determining whether the switching transistor is working properly based on whether the flyover voltage can be stabilized at a set voltage includes: determining whether the charging switching transistor is working properly based on whether the flyover voltage is stabilized at a first set voltage during the second time period; the first set voltage is greater than half of the second voltage.

2. The inverter self-test method according to claim 1, characterized in that, The step of controlling the first duty cycle corresponding to the charging switch to be greater than the second duty cycle corresponding to the discharging switch during the first time period includes: controlling the first duty cycle corresponding to the charging switch to increase from 0 with a first slope during the first time period, and controlling the second duty cycle of the discharging switch to increase from 0 with a second slope, wherein the first slope is greater than the second slope.

3. The inverter self-test method according to claim 1, characterized in that, Before the inverter is connected to the grid, the duty cycle corresponding to the switching transistor is adjusted from the set state to adjust the flying voltage across the flying capacitor. The method further includes: during the third time period, controlling the first duty cycle corresponding to the charging switching transistor to be less than the second duty cycle corresponding to the discharging switching transistor; during the fourth time period, controlling the first duty cycle to be equal to the second duty cycle, wherein the fourth time period is after the third time period. The step of determining whether the switching transistor is working properly based on whether the transient voltage can be stabilized at a set voltage further includes: determining whether the discharge switching transistor is working properly based on whether the transient voltage is stabilized at a second set voltage during the fourth time period; the second set voltage is less than half of the second voltage. Wherein, the fourth time period is before the first time period, and at the beginning of the third time period, the flying capacitor three-level boost circuit is in the set state; Alternatively, the second time period occurs before the third time period, and the flying capacitor three-level boost circuit is in the set state at the beginning of the first time period.

4. The inverter self-test method according to claim 3, characterized in that, The step of controlling the first duty cycle corresponding to the charging switch to be less than the second duty cycle corresponding to the discharging switch during the third time period includes: controlling the first duty cycle corresponding to the charging switch to increase from 0 with a third slope during the third time period, and controlling the second duty cycle of the discharging switch to increase from 0 with a fourth slope, wherein the third slope is less than the fourth slope.

5. The inverter self-test method according to claim 3, characterized in that, During the first time period, the second time period, the third time period, and the fourth time period, both the first duty cycle and the second duty cycle are less than the preset duty cycle threshold.

6. The inverter self-test method according to claim 5, characterized in that, The absolute value of the difference between the first duty cycle and the second duty cycle is less than or equal to half of the preset duty cycle threshold.

7. The inverter self-test method according to claim 1, characterized in that, Also includes: Perform the inverter start-up condition judgment or the closed-loop boost process of the flying capacitor three-level boost circuit; The power-on condition determination is performed before adjusting the duty cycle of the switching transistor from the set state to adjust the flying voltage across the flying capacitor, or after determining that the switching transistor is working normally. The determination of the inverter's start-up conditions includes: determining whether the inverter's start-up conditions are met based on the photovoltaic output voltage and output power of the photovoltaic panel connected to the input terminal of the flying capacitor three-level boost circuit. The closed-loop boost process is performed before adjusting the duty cycle of the switching transistor from the set state to adjust the flying voltage across the flying capacitor, or after confirming that the switching transistor is working normally; the closed-loop boost process includes controlling the flying capacitor three-level boost circuit to boost the voltage in a closed loop until the output voltage reaches the third voltage.

8. The inverter self-test method according to claim 7, characterized in that, Also includes: If the switching transistors are determined to be working normally based on whether the jump voltage can be stabilized at the set voltage and the start-up conditions are met, and after the closed-loop boost process, the inverter is controlled to perform open-loop self-test and relay self-test. After the inverter open-loop self-test and relay self-test pass, the inverter is controlled to operate in grid-connected mode.

9. An inverter, characterized in that, The inverter includes at least one flying capacitor three-level boost circuit, the flying capacitor three-level boost circuit includes a switching transistor and a flying capacitor, the flying capacitor being electrically connected to the switching transistor; the inverter performs a self-test based on the inverter self-test method according to any one of claims 1-8.

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