A three-level dc / dc device open-circuit fault determination method

Abstract

The application discloses a three-level DC / DC device open-circuit fault judgment method, comprising the following steps: selecting a DC / DC unit corresponding to one photovoltaic string as a DC / DC unit to be tested, and driving and locking the DC / DC units corresponding to the remaining photovoltaic strings; collecting the inductor current and the 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 the bus voltage deviation in a single switching cycle; or, 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 in a single switching cycle; reselecting the DC / DC unit to be tested until the open-circuit fault judgment of all DC / DC units is completed. The application has the beneficial effects that the open-circuit fault of the three-level DC / DC unit can be monitored by means of software, and the accurate positioning of the open-circuit fault can also be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of inverters, in particular to a three-level DC / DC device open-circuit fault judgment method. BACKGROUND

[0002] There are various types of architectures of inverter systems, such as the typical string-type photovoltaic inverter system architecture shown in the figure. Figure 1 As an example of the typical string-type photovoltaic inverter system architecture, the front stage includes N photovoltaic strings PV, each of which is connected with a corresponding DC / DC unit, and then all the DC / DC units corresponding to the photovoltaic strings PV are connected in parallel at the bus to the DC side of the DC / AC unit, and the AC side of the DC / AC unit is connected to the grid. Among them, the common topology of the DC / DC unit is Boost or three-level Boost.

[0003] When a DC / DC fault occurs, the system needs to detect it in time, and the fault will not only affect the faulty module, but also cause the fault to spread, so that other modules cannot work normally, thereby causing the output voltage to be unable to be maintained and the entire system to collapse. Therefore, fault detection is a very important link. SUMMARY

[0004] One of the purposes of the present application is to provide a three-level DC / DC device open-circuit fault judgment method that can solve at least one of the defects in the background art.

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

[0006] Preferably, the DC / DC unit comprises two switch tubes, the open circuit fault scenarios of the DC / DC unit include an open circuit fault scenario of one of the switch tubes and an open circuit fault scenario of both of the switch tubes, and the DC / DC unit further comprises a normal working scenario; the single switch cycle comprises a switch tube on phase and a switch tube off phase in sequence; the inductance current change values corresponding to the three scenarios are different in the switch tube on 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 according to the inductance current change value calculated from the collected inductance current.

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

[0008] Preferably, for the open circuit fault scenario of one of the switch tubes, the calculation formula of the corresponding inductance current change value is: [(V PV -V dcx )×D×T s ] / L; for the open circuit fault scenario of both of the switch tubes, if the output voltage of the photovoltaic string is lower than the bus voltage, there is no inductance current; for the normal working scenario of the DC / DC unit, the calculation formula of the corresponding inductance current change value is: V PV ×(D×T s ) / L; wherein, V PV represents the output voltage of the photovoltaic string corresponding to the DC / DC unit, V dcx represents the half bus voltage, T s represents the switch cycle, D represents the duty ratio, and L represents the total inductance value of the DC / DC unit.

[0009] Preferably, the inductance change rate corresponding to the inductance current calculation comprises the following process: the peak value and the valley value of the inductance current are collected in the single switch cycle, and then the inductance current change value is calculated according to the difference between the peak value and the valley value; the peak value of the inductance current is collected at a time Δt before the end of the switch tube on phase; wherein, Δt represents the sampling delay time.

[0010] Preferably, the DC / DC unit includes two switch tubes, the open circuit fault scenarios of the DC / DC unit include a scenario in which one of the switch tubes has an open circuit fault and a scenario in which both of the switch tubes have open circuit faults; and the process of judging the three scenarios by using the bus voltage deviation includes the following steps: controlling the bus voltage by using the DC / DC unit to be tested, and monitoring the bus voltage; the bus voltage includes a total bus voltage and two half bus voltages; if the total bus voltage always fails to reach a set voltage V dc , and is close to the output voltage of the photovoltaic string, it is judged that the DC / DC unit to be tested is in the scenario in which both of the switch tubes have open circuit faults; otherwise, the difference between the two half bus voltages is calculated, and if the difference meets the threshold value requirement, it is judged that the DC / DC unit to be tested is in the scenario in which one of the switch tubes has an open circuit fault.

[0011] Preferably, if the current DC / DC unit to be tested has a single switch tube open circuit fault, the bus needs to be controlled to a balanced state before judging the open circuit fault of the next DC / DC unit to be tested; and the bus balancing control is adapted to be completed by using the DC / AC unit.

[0012] Preferably, the DC / DC unit includes two switch tubes, the open circuit fault scenarios of the DC / DC unit include a scenario in which one of the switch tubes has an open circuit fault and a scenario in which both of the switch tubes have open circuit faults; and the process of judging the three scenarios by using the bus voltage deviation includes the following steps: controlling the bus voltage by using the DC / DC unit to be tested, and monitoring the bus voltage; the bus voltage includes a total bus voltage and two half bus voltages; if the total bus voltage always fails to reach a set voltage V dc , and is close to the output voltage of the photovoltaic string, it is judged that the DC / DC unit to be tested is in the scenario in which both of the switch tubes have open circuit faults; otherwise, the difference between the two half bus voltages is calculated, and if the difference meets the threshold value requirement, it is judged that the DC / DC unit to be tested is in the scenario in which one of the switch tubes has an open circuit fault.

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

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

[0015] Compared with the prior art, the beneficial effects of this application are as follows:

[0016] Software can be used to monitor open-circuit faults in three-level DC / DC units and accurately locate them. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the architecture of the string photovoltaic inverter system in this application.

[0018] Figure 2 For this application Figure 1 The diagram shows an architecture where the upper switch S11 of DC / DC#1 experiences an open-circuit fault.

[0019] Figure 3 For this application Figure 2 The diagram shows the state of the device when it is turned on.

[0020] Figure 4 For this application Figure 2 The diagram shows the state of the architecture when the device is turned off.

[0021] Figure 5 For this application Figure 2 A simplified schematic diagram of an open-circuit fault in the switch S11 of the architecture shown.

[0022] Figure 6 This is a schematic diagram of the overall workflow of this application.

[0023] Figure 7 For this application Figure 1 The diagram shows the architecture in which the lower switch S12 of DC / DC#1 experiences an open-circuit fault.

[0024] Figure 8 For this application Figure 1 The diagram shows an architecture where both switches of DC / DC#1 experience open-circuit faults.

[0025] Figure 9 This is a timing diagram illustrating the acquisition of the inductor current peak value in this application.

[0026] Figure 10 This is a schematic diagram of the workflow for determining open-circuit faults based on bus voltage deviation in this application. Detailed Implementation

[0027] The present application will now be further described in conjunction with specific embodiments. It should be noted that, in the description of this specification, the use of terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicates that the specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms should not be construed as necessarily referring to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0028] In the description of this application, it should be noted that the terms "center", "lateral", "longitudinal", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., which indicate the orientation and positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and should not be construed as limiting the specific protection scope of this application.

[0029] It should be noted that the terms "first," "second," etc., in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0030] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0031] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature being directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0032] The terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion, for example, a process, method, system, product, or device that includes a series of steps or units is not necessarily limited to those steps or units that are explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such process, method, product, or device.

[0033] To facilitate understanding of the technical solution of this application, we can first analyze the open-circuit fault of the DC / DC unit.

[0034] like Figure 1 and Figure 2 As shown, for a string photovoltaic inverter system, the number of photovoltaic strings is N, which can be labeled sequentially as PV#1 to PV#N; then the output voltage of each photovoltaic string is V. PV1 To V PVN Each photovoltaic string corresponds to a DC / DC unit, so N DC / DC units can be sequentially labeled as DC / DC#1 to DC / DC#N.

[0035] Each DC / DC unit includes a pair of inductors, a pair of switching transistors, and a pair of capacitors. The two inductors are respectively located at the positive and negative output terminals of the photovoltaic string, with inductance values ​​of Li1 and Li2. The two switching transistors are located between the positive and negative output terminals of the photovoltaic string, and are the upper switching transistor S. i1 and the lower switch S i2 Two capacitors are also respectively placed between the positive and negative output terminals of the photovoltaic string, and the capacitance values ​​of the two capacitors are C. oi1 and C oi2 Where i represents the serial number of the DC / DC unit.

[0036] It's important to know that an open-circuit fault in a DC / DC unit means that both switching transistors are open-circuited; for example... Figure 2 As shown, the example is the open-circuit fault of the upper switch S11 of DC / DC#1.

[0037] First, assume ideal conditions, the output voltage V of each photovoltaic string. PV1 = V PV2 =……=V PVN =V PV L11=L12=……= LN1=LN2=L / 2, where L represents the total inductance of the DC / DC unit; the duty cycle of each DC / DC unit is the same, D.

[0038] like Figure 3 As shown, in one switching cycle T s Inside, 0~D×T s During a certain period of time, the switching transistor of the DC / DC unit is turned on, which is the switching transistor conduction stage; Figure 3 The thick solid line represents the inductor current flow path. At this time, the normally operating DC / DC unit dominates, maintaining the total bus voltage; therefore, the rise in inductor current corresponding to the normally operating DC / DC unit is ΔI. x This is the change in inductor current; ΔI x The calculation formula is:

[0039] V PV =L×ΔI x / (D×T s (1)。

[0040] Similarly, such as Figure 4 As shown, in one switching cycle T s Inside, D×T s ~T s During a certain period of time, the switching transistors of the DC / DC unit are turned off, which is the switching transistor turn-off phase. Figure 4 The thick solid line represents the flow path of the inductor current. This shows the rise in inductor current ΔI corresponding to a DC / DC unit in normal operating condition. x The calculation formula is:

[0041] V PV -V dc =-L×ΔI x / (T s -D×T s (2)。

[0042] Among them, V dc This indicates the voltage in the busbar.

[0043] By combining the above formulas (1) and (2), we can obtain V. dc =V dc1 +V dc2 =V PV / (1-D). Wherein, Vdc1 This represents the voltage of the upper bus, V. dc2 This indicates the voltage of the lower half of the bus.

[0044] For DC / DC#1 with an open circuit fault, such as Figure 3 As shown, during the conduction phase of the switching transistor, due to the switching transistor S... 11 An open circuit fault occurred, and the switching transistor S... 12 While maintaining normal operation, this will draw current from the midpoint of the bus, thus affecting the bus voltage balance. Assume that during the switching transistor's conduction phase, the average inductor current is i. o Therefore, the model of DC / DC#1 can be simplified as follows: Figure 5 As shown.

[0045] We can assume that the capacitance C of the two capacitors in DC / DC#1 is... o11 =C o12 =C o ,like Figure 5 As shown, the bus voltage deviation ΔV during the conduction phase of the switching transistor can be obtained as: ΔV = V dc1 -V dc2 =i o ×D×T s / C o .

[0046] For DC / DC#1 with an open circuit fault, such as Figure 3 As shown, during 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 ΔI between the faulty DC / DC unit and the normal DC / DC unit is... x It is consistent; please refer to formula (2) above for details.

[0047] As the analysis above shows, in scenarios where the DC / DC unit has an open-circuit fault, without additional control, the system will experience bus voltage imbalance during the switching transistor conduction phase. In practice, when the bus voltage imbalance reaches a certain level, it will trigger bus overvoltage protection or bus imbalance protection.

[0048] Based on the above analysis results, this application provides a method for judging open-circuit faults in three-level DC / DC devices, applicable to string photovoltaic inverter systems, 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 blocking the drive of 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 a combination of the inductor current change value and bus voltage deviation within a single switching cycle. After determining the open-circuit fault judgment result of the DC / DC unit to be tested, selecting a new DC / DC unit to be tested from the remaining photovoltaic strings for open-circuit fault judgment, until all DC / DC units have completed the open-circuit fault judgment.

[0049] As can be understood from the aforementioned analysis, when an open-circuit fault occurs in the DC / DC unit, the inductor current generated by the inductor in the DC / DC unit will change, and this change in inductor current leads to an imbalance in the bus voltage. Therefore, when judging an open-circuit fault in a DC / DC unit, the judgment can be made based on the change in inductor current or the deviation in bus voltage; alternatively, a combination of both can be used.

[0050] The entire process of this application utilizes software to monitor open-circuit faults in a three-level DC / DC unit, thus avoiding increased hardware costs and making it easy to implement. Furthermore, when an open-circuit fault is detected in the DC / DC unit, it can accurately pinpoint the fault location. For ease of understanding, a detailed explanation will be provided below using specific examples.

[0051] Example 1: Determining open circuit faults by analyzing changes in inductor current.

[0052] In this example, the DC / DC unit includes two switching transistors. Open-circuit fault scenarios for the DC / DC unit include a scenario where one switching transistor fails, and a scenario where both switching transistors fail. The inductor current changes corresponding to these two open-circuit fault scenarios are different. Therefore, when determining the open-circuit fault of the DC / DC unit under test, the specific open-circuit fault scenario is determined based on the inductor current change value calculated from the collected inductor current; that is, the open-circuit fault is located based on the open-circuit fault scenario in which the DC / DC unit under test is located.

[0053] It is important to note that the DC / DC unit also includes a normal operating scenario. The inductor current change value in the normal operating scenario differs from the inductor current change value 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 fall within the normal operating scenario or within one of the two open-circuit fault scenarios. The specific determination requires based on the inductor current change value of the DC / DC unit.

[0054] Specifically, as the analysis above shows, the inductor current change of the DC / DC unit can be calculated based on the bus voltage and the output voltage of the photovoltaic string, and the calculation formula for the inductor current change differs under different open-circuit fault scenarios. Therefore, when judging the open-circuit fault of the DC / DC unit under test, the collected bus voltage is substituted into all the calculation formulas for calculation, and the calculation result is matched with the inductor current change calculated from the collected inductor current. Based on the matching result, the specific open-circuit fault scenario of the DC / DC unit under test is judged.

[0055] Understandably, when troubleshooting open-circuit faults in DC / DC units, it's necessary to collect not only the output voltage of the photovoltaic string corresponding to the DC / DC unit under test, but also the bus voltage and inductor current. The single-switching cycle T can be obtained from the collected inductor current. s The change in inductor current within the DC / DC converter is the increase in inductor current. Simultaneously, the change in inductor current of the DC / DC converter can also be calculated by combining the bus voltage and the output voltage of the photovoltaic string. Since it is unknown whether the DC / DC converter has malfunctioned, simply substituting the collected bus voltage and photovoltaic string output voltage into the formulas for calculating the change in inductor current under various open-circuit faults yields different calculation results. The scenario corresponding to the result that is the same as or substantially similar to the change in inductor current obtained directly through inductor current acquisition is used as the judgment result.

[0056] It should be noted that, as shown in the aforementioned analysis, the rate of change of inductor current in a faulty DC / DC unit and a normal DC / DC unit during the switching transistor conduction phase is different. Therefore, it is sufficient to simply substitute the collected data into the calculation formula for the inductor current rise value of the DC / DC unit during the switching transistor conduction phase.

[0057] Specifically, as shown in the aforementioned analysis, when the DC / DC unit is operating normally, the change in its corresponding inductor current ΔI during the switching transistor's conduction phase is... x The calculation formula is: ΔI x =V PV ×(D×T s ) / L.

[0058] As can be seen from the above analysis results, the upper switching transistor S of DC / DC#1 11 When an open-circuit fault occurs, the formula for calculating the change in inductor current ΔI1 during the switching transistor's conduction phase is: ΔI1=[(V PV -V dc1 )×D×T s ] / L.

[0059] Similarly, such as Figure 7 As shown, the lower switching transistor S of DC / DC#1 12 When an open-circuit fault occurs, the formula for calculating the change in inductor current ΔI1 during the switching transistor's conduction phase is: ΔI1=[(V PV -V dc2 )×D×T s ] / L.

[0060] Based on the upper switching transistor S of DC / DC#1 11 and the lower switch S 12 The formulas for calculating the inductor current change when an open-circuit fault occurs can be used to obtain the formula for calculating the inductor current change when one of the switching transistors in the DC / DC unit experiences an open-circuit fault: [(V PV -V dcx )×D×T s ] / L; where V dcx This represents the half-bus voltage. There are two calculation formulas for a scenario where one of the switching transistors experiences an open-circuit fault. The collected data is then substituted into these two formulas for calculation. Based on the specific calculation results, it is determined whether the DC / DC unit is experiencing an open-circuit fault in the upper or lower switching transistor.

[0061] Similarly, such as Figure 8 As shown, if the upper switching transistor S of DC / DC#1 11 and the lower switch S 12 Both open-circuit faults occur, meaning that DC / DC#1 is in a scenario where both switching transistors are open-circuit faulted. If the output voltage of the photovoltaic string is lower than the bus voltage at this time, the diode corresponding to DC / DC#1 will not conduct, the inductor will not be magnetized, and there will be no current in the inductor.

[0062] For ease of understanding, the following table shows the calculation formulas for the inductor current change during the switching transistor conduction stage under different fault scenarios, using DC / DC#1 as an example:

[0063]

[0064] 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 in inductor current based on the difference between the peak and valley values. To ensure the accuracy of the collected peak and valley currents, the sampling time needs to be advanced considering the sampling delay Δt. Since the inductor current is discontinuous, its valley current value is zero, so only the sampling of the peak current needs to be considered. Taking DC / DC#1 as an example, the peak current acquisition of the inductor current includes the following process: Based on Figure 9 The timing diagram shown acquires the peak inductor current at a time Δt before the end of the switching transistor's conduction phase, thus obtaining the peak current I. 1_p .

[0065] It should be noted that, as can be seen from the aforementioned analysis, the switching transistor S on DC / DC#1... 11 When an open-circuit fault occurs, current will be drawn from the midpoint of the bus, thus affecting the balance of the bus voltage; specifically, the upper bus voltage V dc1 The voltage V of the lower bus gradually increases. dc2 Gradually decrease. Similarly, under DC / DC#1, the switching transistor S... 12 When an open circuit fault occurs, the voltage V on the upper bus will decrease. dc1 The voltage gradually decreases, and the lower bus voltage V dc2 The fault gradually increases. Therefore, when diagnosing open-circuit faults in a string photovoltaic inverter system, if one DC / DC unit experiences an open-circuit fault in its upper switch and another in its lower switch, the combined effect on the DC-side bus voltage of the DC / AC unit may result in a balanced bus voltage. This could lead to fault identification failure and the propagation of open-circuit faults in the DC / DC units. Therefore, in the technical solution of this application, open-circuit fault diagnosis of DC / DC units is performed by selecting only the DC / DC unit under test, while the remaining DC / DC units are in drive lockout.

[0066] Example 2: Using bus voltage deviation to determine open circuit faults.

[0067] As can be seen from the foregoing, the DC / DC unit includes two switching transistors. The open-circuit fault scenarios of the DC / DC unit include the scenario where one of the switching transistors fails to open, and the scenario where both switching transistors fail to open; at the same time, the DC / DC unit also includes the normal operation scenario.

[0068] The difference between this example and Example 1 is that the judgment of three scenarios based on bus voltage deviation includes the following process: controlling the bus voltage through the DC / DC unit under test and monitoring the bus voltage; the bus voltage includes the total bus voltage and the two half-bus voltages; if the total bus voltage consistently fails to reach the set voltage V... dcIf the voltage is close to the output voltage of the photovoltaic string, it is determined that the DC / DC unit under test is in a scenario where both switching transistors have open circuit faults; otherwise, the difference between the voltages of the two half-buses is calculated. If the difference between the two half-buses meets the threshold value requirement, it is determined that the DC / DC unit under test is in a scenario where one of the switching transistors has an open circuit fault.

[0069] Understandably, during the switching transistor's conduction phase, taking DC / DC #1 as an example, when the switching transistor 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, a midpoint current flows out of the bus, causing the upper bus voltage V to increase. dc1 It will gradually exceed the lower bus voltage V dc2 Currently, the switching transistor 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, a midpoint current flows into the bus, causing the lower bus voltage V to increase. dc2 It will gradually exceed the upper bus voltage V dc1 When the switching transistor S is on 11 and the lower switch 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, making voltage boosting impossible. When the switching transistor S... 11 and the lower switch S 12 When all are operating normally, the upper bus voltage V dc1 and lower bus voltage V dc2 Maintain voltage balance.

[0070] It should be noted that, due to fluctuations in bus voltage, a threshold value ΔV can be set for the bus voltage deviation ΔV to ensure the accuracy of diagnosing open-circuit faults in switching transistors. limit Only when the bus voltage deviation exceeds this threshold value ΔV limit Only then can it be confirmed that a deviation in the bus voltage has occurred, indicating that one of the switching transistors has experienced an open-circuit fault. For ease of understanding, the specific working process of this example will be described in detail below.

[0071] Specifically, such as Figure 10As shown, firstly, DC / DC #1 corresponding to PV #1 can be selected as the test circuit DC / DC unit, i.e., the DC / DC unit to be tested; then, the drive of the DC / DC units corresponding to other PV sides is blocked, and the total bus voltage is controlled to V through the DC / DC unit on the test circuit side. dc .

[0072] After operating for a period of time, the total voltage of the bus can be monitored; the specific operating time can be selected according to actual needs, such as 2 seconds.

[0073] If the total bus voltage consistently fails to reach V dc And it has been near the output voltage of PV#1; determine the two switching transistors S of DC / DC#1. 11 and S 12 All are in an open-circuit fault state. Otherwise, the voltage V of the upper bus will be affected. dc1 and lower half bus voltage V dc2 Monitoring is performed to determine the bus voltage deviation |V dc1 -V dc2 Is it greater than the threshold value ΔV? limit If the line voltage deviation is |V dc1 -V dc2 | Greater than the threshold value ΔV limit Then proceed to the next step of analysis.

[0074] If the upper bus voltage V dc1 If it is too high, it indicates that the upper switching transistor S 11 An open circuit fault occurs; if the lower half bus voltage V dc2 If it's too high, it indicates that the switching transistor S... 12 An open circuit fault has occurred.

[0075] After completing the open-circuit fault detection for DC / DC#1, the open-circuit fault detection test is performed on the next PV-side DC / DC unit. This involves using PV#2 as the DC / DC unit to be tested and repeating the open-circuit fault detection process. After each DC / DC unit's open-circuit fault detection process is completed, the sequence number of the next DC / DC unit to be tested can be determined. If the sequence number is N+1, it indicates that all N PV-side DC / DC units have completed the open-circuit fault detection test, and the test results can then be statistically analyzed.

[0076] It is important to note that if a single switch in the DC / DC unit under test has an open-circuit fault, the bus voltage will be unbalanced after the open-circuit fault detection for that unit ends. Therefore, to ensure the accuracy of the next test, the bus needs to be restored to a balanced state before performing the open-circuit fault detection test on the next DC / DC unit. There are various methods for bus balance control; in this example, it is preferred to use a DC / AC unit to achieve bus balance.

[0077] Example 3: Open circuit faults are determined by combining the changes in inductor current and the deviation of bus voltage.

[0078] As can be seen from the foregoing, the DC / DC unit includes two switching transistors. The open-circuit fault scenarios of the DC / DC unit include the scenario where one of the switching transistors fails to open, and the scenario where both switching transistors fail to open; at the same time, the DC / DC unit also includes the normal operation scenario.

[0079] The difference between this example and Examples 1 and 2 lies in that, before selecting the DC / DC unit to be tested, the inductor current change value of the corresponding DC / DC unit of all photovoltaic strings is monitored in each switching cycle, and a first threshold value is set based on the average value of the obtained inductor current change value. 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 switches 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 voltages of the two half-buses 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 switches has an open circuit fault. Compared with Examples 1 and 2, this example can simultaneously consider the changes in inductor current and bus voltage, making the open circuit fault judgment result of the DC / DC unit more reliable and accurate.

[0080] Specifically, in this example, a larger amount of inductor current can be collected during the switching cycle T. s The average of all the rising values ​​within the range can reflect the basic level of the inductor current when the DC / DC unit is operating normally. Therefore, the average value of the collected inductor current rises can be used as the first threshold value according to a set ratio. The specific ratio can be selected according to the actual needs of those skilled in the art. For example, 70% to 80% of the average inductor current change value can be chosen as the first threshold value, preferably 75% of the average inductor current change value.

[0081] After obtaining the first threshold value, the inductor current of the DC / DC unit under test can be collected and the rise value can be calculated. If the inductor current of the DC / DC unit under test is not collected, as can be seen from Example 1, the DC / DC unit under test is in a scenario where both switching transistors are open-circuit faulted.

[0082] If the inductor current is collected and the calculated rise in inductor current is less than the first threshold value, it indicates that the DC / DC unit under test may have an open-circuit fault. It can be marked as a suspected open-circuit fault, and the bus voltage can be monitored. If an imbalance in the bus voltage is detected, i.e., a bus voltage deviation of |V is observed... 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 switching transistors. Subsequently, the faulty switching transistor can 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.

[0083] The basic principles, main features, and advantages of this application have been described above. Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely the principles of this application. Various changes and modifications can be made to this application without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claims. The scope of protection claimed by this application is defined by the appended claims and their equivalents.

Claims

1. A method for determining open-circuit faults in three-level DC / DC devices, applied to string photovoltaic inverter systems, characterized in that, Includes the following steps: Select the DC / DC unit corresponding to one photovoltaic string as the DC / DC unit to be tested, and disable the drive of the DC / DC units corresponding to the other photovoltaic strings. Collect the inductor current and bus voltage of the DC / DC unit under test; The open-circuit fault of the DC / DC unit under test is determined based on the change in inductor current during 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 all DC / DC units have completed the open-circuit fault judgment. The DC / DC unit includes two switching transistors. The open-circuit fault scenarios of the DC / DC unit include a scenario where one of the switching transistors fails to open, and a scenario where both switching transistors fail to open; the DC / DC unit also includes a normal operation scenario. 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 formula for the inductor current change value is different in different scenarios. When determining the open-circuit fault of the DC / DC unit under test, the collected bus voltage is substituted into all the calculation formulas for calculation. The calculation result is matched with the inductor current change value calculated by the collected inductor current. Based on the matching result, the scenario of the DC / DC unit under test is determined.

2. The method for determining open-circuit faults in a three-level DC / DC device as described in claim 1, characterized in that, A single switching cycle includes a sequential turn-on phase and a turn-off phase. The inductor current change values ​​corresponding to the turn-on phase differ in the three scenarios. Therefore, when judging the open circuit fault of the DC / DC unit under test, the scenario of the DC / DC unit under test is determined based on the inductor current change value calculated from the collected inductor current.

3. The method for determining open-circuit faults in a three-level DC / DC device as described in claim 1, characterized in that, For a scenario where one of the switching transistors experiences an open-circuit fault, the formula for calculating the corresponding change in inductor current is: [(V PV -V dcx )×D×T s ] / L; In the scenario where both switching transistors experience open-circuit faults, if the output voltage of the photovoltaic string is lower than the bus voltage, there is no inductor current. For the normal operation of a DC / DC unit, the formula for calculating the corresponding inductor current change is: V PV ×(D×T s ) / L; Among them, V PV V represents the output voltage of the photovoltaic string corresponding to the DC / DC unit. dcx T represents the half-bus voltage. s The switching cycle is represented by D, the duty cycle by L, and the total inductance of the DC / DC unit by L.

4. The method for determining open-circuit faults in a three-level DC / DC device as described in claim 1, characterized in that, The process for calculating the corresponding change in inductor current through inductor current includes the following steps: collecting the peak and valley values ​​of the inductor current within a single switching cycle, and then calculating the change in inductor current based on the difference between the peak and valley values. Before the end of the switching transistor's conduction phase The peak value of the inductor current is collected at time t. in, t represents the sampling delay time.

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