Three-level DC / DC device open-circuit fault judgment method

Abstract

The invention discloses a three-level DC / DC device open-circuit fault judgment method, and the method comprises the following steps: selecting a DC / DC unit corresponding to a photovoltaic string as a to-be-tested DC / DC unit, and carrying out the driving blocking of the DC / DC units corresponding to other photovoltaic strings; acquiring an inductive current and a bus voltage of the DC / DC unit to be tested; judging an open-circuit fault of the DC / DC unit to be tested based on the inductive current change value or the bus voltage deviation in the single-switch period; or the open-circuit fault of the DC / DC unit to be tested is judged based on the combination of the inductive current change value and the bus voltage deviation in the single-switch period; and reselecting the DC / DC unit to be tested until all the DC / DC units finish open-circuit fault judgment. The beneficial effects of the invention are that the open-circuit fault of the three-level DC / DC unit can be monitored through software, and the open-circuit fault can be accurately positioned.

Description

Technical Field

[0001] The present application relates to the technical field of inverters, and in particular to a method for determining an open-circuit fault of a three-level DC / DC device. Background Art

[0002] There are many types of inverter system architectures, including Figure 1 Taking the typical string-type PV inverter system architecture shown in the figure as an example, the front stage includes N PV strings, each of which is connected to a corresponding DC / DC unit. The DC / DC units corresponding to all PV strings are then connected in parallel to the DC side of the DC / AC unit at the busbar, and the AC side of the DC / AC unit is connected to the grid. Common DC / DC unit topologies are boost or three-level boost.

[0003] When a DC / DC circuit fails, it must be detected immediately by the system. This fault not only affects the faulty module but can also spread, causing other modules to malfunction, leading to an inability to maintain output voltage and the collapse of the entire system. Therefore, fault detection is a crucial step. Summary of the Invention

[0004] One of the objectives of the present application is to provide a method for determining an open circuit fault of a three-level DC / DC device that can solve at least one of the drawbacks of the above-mentioned background technology.

[0005] To achieve at least one of the above-mentioned purposes, the technical solution adopted in the present application is: a method for judging an open-circuit fault of a three-level DC / DC device, applied to a string-type photovoltaic inverter system, comprising the following steps: selecting a DC / DC unit corresponding to one photovoltaic string as the DC / DC unit to be tested, and driving and blocking the DC / DC units corresponding to the remaining photovoltaic strings; collecting the inductor current and bus voltage of the DC / DC unit to be tested; judging an open-circuit fault of the DC / DC unit to be tested based on a change in the inductor current or a bus voltage deviation within a single switching cycle; or judging an open-circuit fault of the DC / DC unit to be tested based on a combination of a change in the inductor current and a bus voltage deviation within a single switching cycle; after determining the open-circuit fault judgment result of the DC / DC unit to be tested, reselecting a new DC / DC unit to be tested from the DC / DC units corresponding to the remaining photovoltaic strings for open-circuit fault judgment, until the open-circuit fault judgment is completed for all DC / DC units.

[0006] Preferably, the DC / DC unit includes two switching tubes, and the open circuit fault scenarios of the DC / DC unit include the scenario where one of the switching tubes has an open circuit fault, and the scenario where both switching tubes have an open circuit fault; the DC / DC unit also includes a normal working scenario; a single switching cycle includes a switch tube conduction phase and a switch tube turn-off phase performed in sequence; the three scenarios have different corresponding inductor current change values ​​in the switch tube conduction phase, and thus, when judging the open circuit fault of the DC / DC unit to be tested, the scenario of the DC / DC unit to be tested is judged based on the inductor current change value calculated based on the collected inductor current.

[0007] Preferably, the inductor current change value of the DC / DC unit is suitable for calculation based on the bus voltage and the output voltage of the photovoltaic string, and the calculation formulas for the inductor current change value under different open circuit fault scenarios are different; when judging the open circuit fault of the DC / DC unit to be tested, the collected bus voltage is substituted into all the calculation formulas for calculation, and the calculation result is matched with the inductor current change value calculated by the collected inductor current, and the scenario in which the DC / DC unit to be tested is judged based on the matching result.

[0008] Preferably, for a scenario where an open circuit fault occurs in one of the switch tubes, the corresponding calculation formula for the inductor current change value is: [(V PV -V dcx )×D×T s ] / L; For the scenario where both switches have an open circuit fault, if the PV string output voltage is lower than the bus voltage, there is no inductor current; For the scenario where the DC / DC unit operates normally, the corresponding inductor current change value is calculated as follows: V PV ×(D×T s ) / L; where V PV Indicates the output voltage of the photovoltaic string corresponding to the DC / DC unit, V dcx Represents half bus voltage, T s represents the switching period, D represents the duty cycle, and L represents the total inductance of the DC / DC unit.

[0009] Preferably, calculating the corresponding inductor change rate based on the inductor current includes the following process: collecting the peak and valley values ​​of the inductor current within a single switching cycle, and then calculating the inductor current change value based on the difference between the peak and valley values; collecting the peak value of the inductor current Δt before the end of the switch tube conduction phase; wherein Δt represents the sampling delay time.

[0010] Preferably, the DC / DC unit includes two switching tubes, and the open circuit fault scenarios of the DC / DC unit include the open circuit fault scenario of one of the switching tubes and the open circuit fault scenario of both switching tubes; the judgment of the three scenarios based on the bus voltage deviation includes the following process: controlling the bus voltage through the DC / DC unit to be tested and monitoring the bus voltage; the bus voltage includes the total bus voltage and two half bus voltages; if the total bus voltage still fails to reach the set voltage V dc , and is close to the output voltage of the PV string, it is determined that the DC / DC unit to be tested is in a scenario where both switching tubes have open circuit faults. Otherwise, the difference between the two half-bus voltages is calculated. If the difference between the two meets the threshold value requirement, it is determined that the DC / DC unit to be tested is in a scenario where one of the switching tubes has open circuit faults.

[0011] Preferably, if a single switch tube open circuit fault exists in the current DC / DC unit to be tested, the bus needs to be re-controlled to a balanced state before determining the open circuit fault of the next DC / DC unit to be tested; bus balance control is suitable to be completed by the DC / AC unit.

[0012] Preferably, the DC / DC unit includes two switching tubes, and the open circuit fault scenarios of the DC / DC unit include a scenario in which one of the switching tubes has an open circuit fault, and a scenario in which both switching tubes have an open circuit fault. The process of judging the open circuit fault of the DC / DC unit by combining the inductor current change value and the bus voltage deviation is as follows: before selecting the DC / DC unit to be tested, the inductor current change values ​​of the DC / DC units corresponding to all photovoltaic strings in each switching cycle are monitored, and a first threshold value is set based on the average value of the obtained inductor current change values; the inductor current of the DC / DC unit to be tested is collected, and if there is no inductor current, it is judged that the DC / DC unit to be tested is in a scenario in which both switching tubes have an open circuit fault; otherwise, the inductor current change value is compared with the first threshold value, and if the inductor current change value is less than the first threshold value, the two half-bus voltages are collected and the difference is calculated; if the difference between the two half-bus voltages meets the second threshold value requirement, it is judged that the DC / DC unit to be tested is in a scenario in which one of the switching tubes has an open circuit fault.

[0013] Preferably, the two switching tubes of the DC / DC unit are an upper switching tube and a lower switching tube, respectively. The half-bus voltage corresponding to the upper switching tube is the upper half-bus voltage, and the half-bus voltage corresponding to the lower switching tube is the lower half-bus voltage. When the DC / DC unit to be tested is in a scenario where one of the switching tubes has an open circuit fault, if the upper half-bus voltage is greater than the lower half-bus voltage, it is judged that an open circuit fault has occurred in the upper switching tube; if the lower half-bus voltage is greater than the upper half-bus voltage, it is judged that an open circuit fault has occurred in the lower switching tube.

[0014] Preferably, the first threshold value is 70% to 80% of the average value of the inductor current variation value.

[0015] Compared with the prior art, the present invention has the following advantages: The open circuit fault of the three-level DC / DC unit can be monitored by software, and the open circuit fault can also be accurately located. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic diagram of the architecture of the string photovoltaic inverter system in this application.

[0017] Figure 2 For this application Figure 1 Schematic diagram of an architecture in which the upper switch S11 of DC / DC#1 in the architecture shown has an open circuit fault.

[0018] Figure 3 For this application Figure 2 Schematic diagram of the state of the device when it is turned on.

[0019] Figure 4 For this application Figure 2 Schematic diagram of the state of the shown architecture when the device is shut down.

[0020] Figure 5 For this application Figure 2 Schematic diagram of a simplified model of an open circuit fault in the switch tube S11 in the shown architecture.

[0021] Figure 6 This is a schematic diagram of the overall workflow of this application.

[0022] Figure 7 For this application Figure 1 Schematic diagram of an architecture in which the lower switch S12 of DC / DC#1 in the architecture shown has an open circuit fault.

[0023] Figure 8 For this application Figure 1 Schematic diagram of the architecture in which both switches of DC / DC #1 fail open circuit.

[0024] Figure 9 This is a timing diagram for collecting the peak value of the inductor current in this application.

[0025] Figure 10 This is a schematic diagram of the workflow for open circuit fault judgment based on bus voltage deviation in this application. DETAILED DESCRIPTION

[0026] Below, the present application is further described in conjunction with specific implementation methods. It should be noted that, in the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" and the like are intended to mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms should not be understood as necessarily referring to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification.

[0027] In the description of this application, it should be noted that for directional words, such as the terms "center", "horizontal", "longitudinal", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and so on, indicating the orientation and position relationship are based on the orientation or position relationship shown in the accompanying drawings, which is only for the convenience of describing this application and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and cannot be understood as limiting the specific scope of protection of this application.

[0028] It should be noted that the terms "first", "second", etc. in the description and claims of this application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence.

[0029] In this application, unless otherwise expressly specified or limited, terms such as "mounted," "connected," "connect," and "fixed" should be understood in a broad sense. For example, they may refer to connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; and internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on specific circumstances.

[0030] In this application, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Moreover, a first feature being "above," "above," and "above" a second feature may include the first feature being directly above or obliquely above the second feature, or may simply mean that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may include the first feature being directly below or obliquely below the second feature, or may simply mean that the first feature is lower in level than the second feature.

[0031] The terms "comprises" and "having" and any variations thereof in the specification and claims of this application are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus that includes a series of steps or units is not necessarily limited to those steps or units expressly listed, but may include other steps or units not expressly listed or inherent to such process, method, product or apparatus.

[0032] In order to facilitate the understanding of the technical solution of the present application, the open circuit fault of the DC / DC unit may be analyzed first.

[0033] like Figure 1 and Figure 2 As shown, for a string-type photovoltaic inverter system, the number of photovoltaic strings is N, and the N photovoltaic strings can be marked in order: PV#1 to PV#N; then the output voltage of each photovoltaic string corresponds to V PV1 To V PVN Each PV string corresponds to a DC / DC unit, so the N DC / DC units can also be marked in sequence as DC / DC#1 to DC / DC#N.

[0034] Each DC / DC unit includes a pair of inductors, a pair of switching tubes and a pair of capacitors. The two inductors are respectively set at the positive and negative output terminals of the photovoltaic string. The inductance values ​​of the two inductors are Li1 and Li2 respectively. The two switching tubes are set between the positive and negative output terminals of the photovoltaic string. The two switching tubes are upper switching tube S i1 and the lower switch tube S i2 ; Two capacitors are also set between the positive and negative output terminals of the photovoltaic string, and the capacitance of the two capacitors is C oi1 and C oi2 Wherein, i represents the serial number of the DC / DC unit.

[0035] It should be noted that the open circuit fault of the DC / DC unit is the open circuit fault of the two switching tubes; Figure 2 As shown, an open circuit fault occurs in the upper switch tube S11 of DC / DC#1 as an example.

[0036] Assume that under ideal conditions, the output voltage of each photovoltaic string is V PV1 = V PV2 =……=V PVN =V PV ; L11=L12=……= LN1=LN2=L / 2, L represents the total inductance value of the DC / DC unit; the duty cycle of each DC / DC unit is the same as D.

[0037] like Figure 3 As shown, in one switching cycle T s Internal, 0~D×T s During this time period, the switch tube of the DC / DC unit is turned on, which is the switch tube conduction stage; Figure 3 The thick solid line in the middle represents the flow path of the inductor current. At this time, the normally working DC / DC unit takes the lead and maintains the total bus voltage; then the rise value of the inductor current corresponding to the DC / DC unit in normal working state is ΔI x , which is the change in inductor current; ΔI x The calculation formula is: V PV =L×ΔI x / (D×T s ) (1).

[0038] Similarly, if Figure 4 As shown, in one switching cycle T s Inside, D×T s ~T s During this time period, the switch tube of the DC / DC unit is turned off, which is the switch tube off phase; Figure 4 The thick solid line in the middle represents the flow path of the inductor current. At this time, the rise value of the inductor current corresponding to the DC / DC unit in normal working state is ΔI x The calculation formula is: V PV -V dc = -L × ΔI x / (T s -D×T s ) (2).

[0039] Among them, V dc Indicates the voltage in the bus.

[0040] By combining the above formula (1) and formula (2), we can get V dc =V dc1 +V dc2 =V PV / (1-D). Where V dc1 Represents the upper half bus voltage, Vdc2 Indicates the lower half bus voltage.

[0041] For DC / DC#1 with open circuit fault, if Figure 3 As shown, during the conduction phase of the switch tube, due to the switch tube S 11 An open circuit fault occurs, and the switch tube S 12 Maintaining normal operation, current will be drawn from the bus midpoint, thus affecting the bus voltage balance. Assume that during the on-state phase of the switch tube, the average value of the inductor current is i o , then the model of DC / DC#1 can be simplified as follows Figure 5 shown.

[0042] It can be assumed that the capacitance of the two capacitors of DC / DC#1 is C o11 =C o12 =C o ,like Figure 5 As shown, the bus voltage deviation ΔV during the switch-on phase can be obtained as: ΔV=V dc1 -V dc2 =i o ×D×T s / C o .

[0043] For DC / DC#1 with open circuit fault, if Figure 3 As shown, in the switch-off phase, since the switch is in the off state, the inductor current flows back through the corresponding capacitor. At this time, the inductor current rise value ΔI of the faulty DC / DC unit is different from that of the normal DC / DC unit. x are consistent, please refer to the above formula (2) for details.

[0044] The above analysis shows that if an open-circuit fault occurs in a DC / DC unit, without additional control, the system will experience bus voltage imbalance during the on-state phase of the switch. In practice, when the bus voltage imbalance reaches a certain level, bus overvoltage protection or bus imbalance protection will be triggered.

[0045] Based on the above analysis results, this application provides a three-level DC / DC device open circuit fault judgment method, which is applied to a string photovoltaic inverter system, such as Figure 6As shown, one preferred embodiment includes the following steps: selecting a DC / DC unit corresponding to one photovoltaic string as the DC / DC unit to be tested, and driving and blocking the DC / DC units corresponding to the remaining photovoltaic strings. Collecting the inductor current and bus voltage of the DC / DC unit to be tested. Judging the open circuit fault of the DC / DC unit to be tested based on the inductor current change value or bus voltage deviation within a single switching cycle. Alternatively, judging the open circuit fault of the DC / DC unit to be tested based on the combination of the inductor current change value and the bus voltage deviation within a single switching cycle. After determining the open circuit fault judgment result of the DC / DC unit to be tested, reselect a new DC / DC unit to be tested from the DC / DC units corresponding to the remaining photovoltaic strings for open circuit fault judgment, until all DC / DC units complete the open circuit fault judgment.

[0046] It is understandable that, as can be seen from the preceding analysis, when a DC / DC unit experiences an open-circuit switch failure, the inductor current generated by the unit's inductor will change, and this change in inductor current leads to an imbalance in the bus voltage. Therefore, when determining an open-circuit fault in a DC / DC unit, the fault can be determined based on either the change in inductor current or the deviation in bus voltage. Alternatively, a combination of the two can be used.

[0047] The entire process of this application uses software to monitor open-circuit faults in three-level DC / DC units, thus minimizing hardware costs and simplifying implementation. Furthermore, when an open-circuit fault in a DC / DC unit is detected, the fault can be accurately located. For ease of understanding, this process is described in detail below using specific examples.

[0048] Example 1: Determine an open circuit fault based on the change in inductor current.

[0049] In this example, the DC / DC unit includes two switching transistors. The open-circuit fault scenarios for the DC / DC unit include one in which one of the switching transistors fails, and both in which the two switching transistors fail. The two open-circuit fault scenarios correspond to different inductor current change values. Therefore, when determining an open-circuit fault in the DC / DC unit under test, the specific open-circuit fault scenario of the DC / DC unit under test is determined based on the inductor current change value calculated from the collected inductor current. In other words, the open-circuit fault is located based on the open-circuit fault scenario of the DC / DC unit under test.

[0050] It's important to note that the DC / DC unit also includes normal operating scenarios, and the inductor current change value in normal operating scenarios differs from the inductor current change values ​​in the two open-circuit fault scenarios. Therefore, when determining an open-circuit fault in the DC / DC unit, the corresponding inductor current change value may be in the normal operating scenario or in the two open-circuit fault scenarios. The specific judgment needs to be based on the inductor current change value of the DC / DC unit.

[0051] Specifically, the above analysis shows that the change in the inductor current of a DC / DC unit can be calculated based on the bus voltage and the output voltage of the PV string, and that the calculation formulas for the change in inductor current vary under different open-circuit fault scenarios. Therefore, when determining an open-circuit fault in the DC / DC unit under test, the collected bus voltage is substituted into all calculation formulas. The calculated result is then matched with the change in inductor current calculated from the collected inductor current. Based on this matching result, the specific open-circuit fault scenario of the DC / DC unit under test is determined.

[0052] It is understandable that when judging the open circuit fault of the DC / DC unit, it is necessary not only to collect the output voltage of the photovoltaic string corresponding to the DC / DC unit to be tested, but also to collect the bus voltage and inductor current. Based on the collected inductor current, the single switching cycle T s The inductor current change value within the circuit is the inductor current rise value. The inductor current change value of the DC / DC unit can also be calculated by combining the bus voltage and the output voltage of the PV string. Since it is unknown whether the DC / DC unit has failed at this time, it is only necessary to substitute the collected bus voltage and the output voltage of the PV string into the inductor current change value calculation formula for various open-circuit faults. Different calculation results can be obtained. The scenario corresponding to the inductor current change value obtained by directly collecting the inductor current is used as the judgment result.

[0053] It should be noted that, based on the aforementioned analysis results, the rate of change of the inductor current in the faulty DC / DC unit and the normal DC / DC unit during the switch-on phase is different. Therefore, it is sufficient to simply substitute the collected data into the calculation formula for the inductor current rise value corresponding to the DC / DC unit during the switch-on phase.

[0054] Specifically, from the above analysis results, it can be seen that when the DC / DC unit is working normally, the corresponding change value of the inductor current during the switch tube conduction phase ΔI x The calculation formula is: ΔI x =V PV ×(D×T s ) / L.

[0055] From the above analysis results, we can see that the upper switch S of DC / DC#1 11 When an open circuit fault occurs, the corresponding change in the inductor current during the on-state phase of the switch tube, ΔI1, is calculated as follows: ΔI1=[(V PV -V dc1 )×D×T s ] / L.

[0056] Similarly, if Figure 7 As shown, the lower switch tube S of DC / DC#1 12 When an open circuit fault occurs, the corresponding change in the inductor current during the on-state phase of the switch tube, ΔI1, is calculated as follows: ΔI1=[(V PV -V dc2 )×D×T s ] / L.

[0057] Based on the upper switch tube S of DC / DC#1 11 and the lower switch tube S 12 The calculation formulas for the inductor current change value when an open circuit fault occurs can be obtained. When the DC / DC unit is in a scenario where one of the switch tubes has an open circuit fault, the calculation formula for the corresponding inductor current change value is: [(V PV -V dcx )×D×T s ] / L; where V dcx = represents half the bus voltage. This means there are two corresponding calculation formulas for the scenario where one of the switches has an open circuit fault. Substituting the collected data into the two formulas, the results determine whether the DC / DC unit is experiencing an open circuit fault in the upper or lower switch.

[0058] Similarly, if Figure 8 As shown, if the upper switch tube S of DC / DC#1 11 and the lower switch tube S 12 Both switches of DC / DC#1 are open-circuit faults. If the output voltage of the PV string is lower than the bus voltage, the diode corresponding to DC / DC#1 will not conduct, the inductor will not be magnetized, and no current will flow through the inductor.

[0059] For ease of understanding, the following table uses DC / DC#1 as an example to calculate the inductor current change during the switch-on phase under different fault scenarios.

[0060] In this example, to obtain the rising value of the inductor current, it is necessary to collect the peak and valley values ​​of the inductor current and then calculate the change value of the inductor current based on the difference between the peak and valley values. To ensure the accuracy of the peak and valley current collection, the sampling time needs to be advanced to take into account the sampling delay Δt. Since the inductor current is discontinuous, its valley current value is zero, so only the peak current needs to be sampled. Taking DC / DC#1 as an example, the peak current collection process includes the following: Based on Figure 9 As shown in the timing diagram, the peak value of the inductor current is collected at Δt time before the end of the switch tube conduction phase, and the peak current I is obtained. 1_p .

[0061] It should be noted that, from the above analysis process, the upper switch S of DC / DC#1 11 When an open circuit fault occurs, current will be drawn from the bus midpoint, thus affecting the bus voltage balance; specifically, the upper bus voltage V dc1 Gradually increases, the lower bus voltage V dc2 Similarly, the lower switch S of DC / DC#1 12 When an open circuit fault occurs, the upper bus voltage V dc1 Gradually decreases, the lower bus voltage V dc2 Gradually increases. When determining an open-circuit fault in a string-type photovoltaic inverter system, if an upper switch tube open-circuit fault occurs in one DC / DC unit and a lower switch tube open-circuit fault occurs in another DC / DC unit, the bus voltage on the DC side of the DC / AC unit may remain balanced when the two factors affect it. This may cause a failure in fault identification and, in turn, the spread of the open-circuit fault in the DC / DC unit. Therefore, in the technical solution of this application, the open-circuit fault determination of the DC / DC unit is performed by selecting the DC / DC unit to be tested individually, while the remaining DC / DC units are in a drive-blocked state.

[0062] Example 2: Determine open circuit faults based on bus voltage deviation.

[0063] As can be seen from the above content, the DC / DC unit includes two switching tubes. The open circuit fault scenarios of the DC / DC unit include the open circuit fault scenario of one of the switching tubes and the open circuit fault scenario of both switching tubes. At the same time, the DC / DC unit also includes normal operation scenarios.

[0064] The difference between this example and Example 1 is that the judgment of the three scenarios based on bus voltage deviation includes the following process: the bus voltage is controlled and monitored by the DC / DC unit to be tested; the bus voltage includes the total bus voltage and two half bus voltages; if the total bus voltage still cannot reach the set voltage V dc, and is close to the output voltage of the PV string, it is determined that the DC / DC unit to be tested is in a scenario where both switching tubes have open circuit faults. Otherwise, the difference between the two half-bus voltages is calculated. If the difference between the two meets the threshold value requirement, it is determined that the DC / DC unit to be tested is in a scenario where one of the switching tubes has open circuit faults.

[0065] It is understandable that, in the on-state of the switch, taking DC / DC#1 as an example, when the upper switch S 11 When an open circuit fault occurs, the bus voltage deviation ΔV=V dc1 -V dc2 =i o ×D×T s / C o ; At this time, the midpoint current flows out of the bus, making the upper bus voltage V dc1 Will gradually become higher than the lower bus voltage V dc2 . The current switch tube S 12 When an open circuit fault occurs, the bus voltage deviation ΔV=V dc1 -V dc2 =-i o ×D×T s / C o ; At this time, the midpoint current flows into the bus, making the lower bus voltage V dc2 Will gradually become higher than the upper bus voltage V dc1 When the switch S 11 and the lower switch tube S 12 When an open circuit fault occurs, the output voltage of the photovoltaic string corresponding to DC / DC#1 is basically equal to the bus voltage and cannot be boosted. 11 and the lower switch tube S 12 When both are working normally, the upper bus voltage V dc1 and the lower bus voltage V dc2 Maintain voltage balance.

[0066] It should be noted that due to the fluctuation of bus voltage, in order to ensure the accuracy of judging the open circuit fault of the switch tube, a threshold value ΔV can be set for the bus voltage deviation ΔV limit , only when the bus voltage deviation exceeds this threshold value ΔV limit Only when the bus voltage is deviated can it be confirmed that one of the switches has an open circuit fault. For ease of understanding, the specific working process of this example can be described in detail below.

[0067] Specifically, such as Figure 10As shown, first, the DC / DC#1 corresponding to PV#1 can be selected as the test DC / DC unit, that is, the DC / DC unit to be tested; then the DC / DC units corresponding to the PV sides of other roads are driven and blocked, and the total bus voltage is controlled to V by the DC / DC units on the test road side. dc .

[0068] After a period of action, the total bus voltage can be monitored; the specific action time can be selected according to actual needs, such as 2s.

[0069] If the total bus voltage cannot reach V dc , and is always near the output voltage of PV#1; determine the two switching tubes S of DC / DC#1 11 and S 12 Otherwise, the upper half bus voltage V dc1 and the lower half bus voltage V dc2 Monitor and judge the bus voltage deviation |V dc1 -V dc2 Is it greater than the threshold value ΔV? limit ; If the line voltage deviation |V dc1 -V dc2 |Greater than the threshold value ΔV limit , then proceed to the next step of analysis.

[0070] If the upper half bus voltage V dc1 If it is too high, it means that the upper switch tube S 11 An open circuit fault occurs; if the lower half bus voltage V dc2 If it is too high, it means that the lower switch tube S 12 An open circuit fault has occurred.

[0071] After completing the open-circuit fault determination for DC / DC#1, the next PV-side DC / DC unit is tested for open-circuit faults. This means that the PV#2-side DC / DC unit is used as the DC / DC unit to be tested and the above open-circuit fault determination process is repeated. After completing the open-circuit fault determination process for each DC / DC unit, the sequence number of the next DC / DC unit to be tested can be determined. If the sequence number is N+1, it means that all N PV-side DC / DC units have completed the open-circuit fault determination test. At this point, the test results can be statistically analyzed.

[0072] It should be noted that if a single switch tube in the DC / DC unit under test has an open-circuit fault, then after the open-circuit fault determination for the current DC / DC unit under test is completed, the bus voltage will be in an unbalanced state. Therefore, to ensure the accuracy of the next test process, the bus needs to be re-balanced before the open-circuit fault determination test for the next DC / DC unit under test is performed. There are many specific methods for bus balancing control. In this example, bus balancing is preferably achieved through DC / AC units.

[0073] Example 3: Determine an open circuit fault by combining the inductor current change value and the bus voltage deviation.

[0074] As can be seen from the above content, the DC / DC unit includes two switching tubes. The open circuit fault scenarios of the DC / DC unit include the open circuit fault scenario of one of the switching tubes and the open circuit fault scenario of both switching tubes. At the same time, the DC / DC unit also includes normal operation scenarios.

[0075] The difference between this example and Examples 1 and 2 is that before selecting the DC / DC unit to be tested, the inductor current change values ​​of the DC / DC units corresponding to all photovoltaic strings are monitored during each switching cycle, and a first threshold value is set based on the average value of the obtained inductor current change values. The inductor current of the DC / DC unit to be tested is collected. If there is no inductor current, it is determined that the DC / DC unit to be tested is in a scenario where both switching tubes have open circuit faults; otherwise, the inductor current change value is compared with the first threshold value. If the inductor current change value is less than the first threshold value, the two half-bus voltages are collected and the difference is calculated; if the difference between the two half-bus voltages meets the second threshold value requirement, it is determined that the DC / DC unit to be tested is in a scenario where one of the switching tubes has an open circuit fault. Compared with Examples 1 and 2, this example can take into account both the change in inductor current and the change in bus voltage, making the open circuit fault judgment result of the DC / DC unit more reliable and accurate.

[0076] Specifically, in this example, firstly, a large amount of inductor current can be collected during the switching period T s The average of all rising values ​​within the inductor current can reflect the basic level of the inductor current during normal operation of the DC / DC unit. Therefore, the average of the collected inductor current rising values ​​can be used as the first threshold value according to a set ratio. The specific ratio can be selected based on the actual needs of those skilled in the art. For example, 70% to 80% of the average value of the inductor current change can be selected as the first threshold value, and 75% of the average value of the inductor current change is preferred.

[0077] After obtaining the first threshold value, the inductor current of the DC / DC unit to be tested can be collected and the rising value can be calculated. If the inductor current of the DC / DC unit to be tested is not collected, it can be seen from Example 1 that the DC / DC unit to be tested is in a scenario where both switching tubes of the DC / DC unit to be tested have open circuit faults.

[0078] If the inductor current is collected and the calculated inductor current rise value is less than the first threshold value, it indicates that the DC / DC unit to be tested may have an open circuit fault. It can be marked as a suspected open circuit fault and the bus voltage can be monitored. If the bus voltage is monitored to be unbalanced, that is, the bus voltage deviation |V dc1 -V dc2 |Greater than the second threshold value ΔV limit This indicates that the DC / DC unit under test is experiencing an open-circuit fault in one of its switches. The switch with the open-circuit fault can then be located based on the specific deviation of the half-bus voltage. The specific process can be found in Example 2 and will not be repeated here.

[0079] The above describes the basic principles, main features, and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above-described embodiments. The above-described embodiments and the specification merely illustrate the principles of the present application. Various changes and improvements may be made to the present application without departing from the spirit and scope of the present application. These changes and improvements fall within the scope of the present application for which protection is sought. The scope of protection claimed by the present application is defined by the appended claims and their equivalents.

Claims

1. A method for determining an open-circuit fault in a three-level DC / DC device, applied to a string photovoltaic inverter system, characterized in that: The steps include: Select the DC / DC unit corresponding to one photovoltaic string as the DC / DC unit to be tested, and block the drive of the DC / DC units corresponding to the remaining photovoltaic strings; Collect the inductor current and bus voltage of the DC / DC unit to be tested; Determine the open circuit fault of the DC / DC unit to be tested based on the inductor current change value or bus voltage deviation within a single switching cycle; Alternatively, an open circuit fault of the DC / DC unit to be tested is determined based on a combination of a change in the inductor current and a bus voltage deviation within a single switching cycle; After determining the open circuit fault judgment result of the DC / DC unit to be tested, a new DC / DC unit to be tested is selected from the DC / DC units corresponding to the remaining photovoltaic strings for open circuit fault judgment until the open circuit fault judgment of all DC / DC units is completed.

2. The method for determining an open circuit fault of a three-level DC / DC device according to claim 1, wherein: The DC / DC unit includes two switching tubes. The open circuit fault scenarios of the DC / DC unit include the scenario where one of the switching tubes has an open circuit fault, and the scenario where both switching tubes have an open circuit fault. The DC / DC unit also includes the normal operation scenario. A single switching cycle includes a switch-on phase and a switch-off phase, which are performed sequentially. The three scenarios correspond to different changes in the inductor current during the switch-on phase. Therefore, when determining an open-circuit fault in the DC / DC unit to be tested, the scenario of the DC / DC unit to be tested is determined based on the inductor current change value calculated from the collected inductor current.

3. The method for determining an open circuit fault of a three-level DC / DC device according to claim 2, wherein: The inductor current change value of the DC / DC unit is suitable for calculation based on the bus voltage and the output voltage of the PV string, and the calculation formula of the inductor current change value is different in different scenarios; When judging an open-circuit fault of the DC / DC unit to be tested, the collected bus voltage is substituted into all calculation formulas for calculation, and the calculation result is matched with the inductor current change value calculated by the collected inductor current. Based on the matching result, the scenario in which the DC / DC unit to be tested is judged.

4. The method for determining an open circuit fault of a three-level DC / DC device according to claim 3, wherein: For a scenario where one of the switches has an open circuit fault, the corresponding inductor current change is calculated as follows: [(V PV -V dcx )×D×T s ] / L; In the scenario where both switches have an open circuit fault, if the PV string output voltage is lower than the bus voltage, there will be no inductive current. For the normal operation scenario of the DC / DC unit, the corresponding calculation formula for the inductor current change value is: V PV ×(D×T s ) / L; Among them, V PV Indicates the output voltage of the photovoltaic string corresponding to the DC / DC unit, V dcx Represents half bus voltage, T s represents the switching period, D represents the duty cycle, and L represents the total inductance of the DC / DC unit.

5. The method for determining an open circuit fault of a three-level DC / DC device according to claim 2, wherein: Calculating the corresponding inductor change rate based on the inductor current includes the following process: collecting the peak and valley values ​​of the inductor current within a single switching cycle, and then calculating the inductor current change value based on the difference between the peak and valley values; The peak value of the inductor current is collected at Δt time before the end of the switch tube conduction phase; Where Δt represents the sampling delay time.

6. The method for determining an open circuit fault of a three-level DC / DC device according to claim 1, wherein: The DC / DC unit includes two switching tubes. The open circuit fault scenarios of the DC / DC unit include the open circuit fault scenario of one of the switching tubes and the open circuit fault scenario of both switching tubes. The judgment of three scenarios based on bus voltage deviation includes the following process: The bus voltage is controlled and monitored by the DC / DC unit to be tested; the bus voltage includes the total bus voltage and two half bus voltages; If the total bus voltage does not reach the set voltage V dc , and is close to the output voltage of the PV string, it is determined that the DC / DC unit under test is in a scenario where both switch tubes have open circuit faults; Otherwise, the difference between the two half-bus voltages is calculated. If the difference between the two meets the threshold value requirement, it is determined that the DC / DC unit to be tested is in a scenario where one of the switch tubes has an open circuit fault.

7. The method for determining an open circuit fault of a three-level DC / DC device according to claim 6, wherein: If a single switch tube open circuit fault exists in the current DC / DC unit to be tested, the bus needs to be controlled to a balanced state before the open circuit fault judgment of the next DC / DC unit to be tested is performed; bus balance control is suitable for completion through the DC / AC unit.

8. The method for determining an open circuit fault of a three-level DC / DC device according to claim 1, wherein: The DC / DC unit includes two switching tubes. The open circuit fault scenarios of the DC / DC unit include the open circuit fault scenario of one of the switching tubes and the open circuit fault scenario of both switching tubes. The process of judging the open circuit fault of the DC / DC unit by combining the inductor current change value and the bus voltage deviation is as follows: Before selecting the DC / DC unit to be tested, monitoring the inductor current change values ​​of the DC / DC units corresponding to all photovoltaic strings in each switching cycle, and setting a first threshold value based on the average value of the obtained inductor current change values; The inductor current of the DC / DC unit to be tested is collected. If there is no inductor current, it is determined that the DC / DC unit to be tested is in a scenario where both switching tubes have open circuit faults; Otherwise, the inductor current change value is compared with the first threshold value. If the inductor current change value is less than the first threshold value, the two half-bus voltages are collected and the difference is calculated; If the difference between the two half-bus voltages meets the second threshold requirement, it is determined that the DC / DC unit to be tested is in a scenario where one of the switch tubes has an open circuit fault.

9. The method for determining an open circuit fault of a three-level DC / DC device according to claim 6 or 8, wherein: The two switching tubes of the DC / DC unit are an upper switching tube and a lower switching tube. The half bus voltage corresponding to the upper switching tube is the upper half bus voltage, and the half bus voltage corresponding to the lower switching tube is the lower half bus voltage. When the DC / DC unit to be tested is in a scenario where one of the switch tubes has an open circuit fault, if the upper bus voltage is greater than the lower bus voltage, it is determined that an open circuit fault has occurred in the upper switch tube; if the lower bus voltage is greater than the upper bus voltage, it is determined that an open circuit fault has occurred in the lower switch tube.

10. The method for determining an open circuit fault of a three-level DC / DC device according to claim 8, wherein: The first threshold value is 70% to 80% of the average value of the inductor current change.

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