Cascade double active bridge open-circuit fault diagnosis method based on inductance impulse voltage

Abstract

The invention discloses a cascaded dual-active bridge open-circuit fault diagnosis method based on inductance impulse voltage. The method comprises the following steps: acquiring inductance current and inductance voltage of each subunit in a cascaded dual-active bridge in real time; extracting the waveform characteristics and detecting whether inductance impulse voltage exceeding a preset threshold exists in the inductance voltage or not; if the inductance impulse voltage is detected, it is judged that an IGBT overall open-circuit fault occurs; according to the corresponding relation between the occurrence time of the inductance impulse voltage and the system switch control time sequence, the group to which the faulted IGBT belongs is positioned; and comparing the inductance voltage waveform characteristics of each subunit, determining the subunit with a fault, and completing diagnosis. According to the invention, by monitoring and analyzing the inductance impulse voltage generated when the inductance current path is forcibly cut off, and by using the corresponding relation between the occurrence time of the inductance impulse voltage and the switch control time sequence, the technical problem that the fault subunit and the fault IGBT group are difficult to quickly and accurately locate when the whole open-circuit fault of the IGBT occurs in the cascaded dual-active bridge system is solved.

Description

Technical Field

[0001] This invention relates to the field of AC / DC distribution network relay protection, and in particular to a method for diagnosing open-circuit faults in cascaded dual active bridges based on inductor impulse voltage. Background Technology

[0002] With the widespread application of power electronic transformers in distribution networks connected to distributed photovoltaic systems, the reliability of their core power modules—cascaded dual active bridges (DABs)—has become increasingly prominent. DAB device failures are mainly classified into two categories: short-circuit faults and open-circuit faults. Currently, research on open-circuit faults, especially overall open-circuit faults (i.e., complete IGBT failure and disconnection), is still incomplete. Existing protection schemes mostly focus on short-circuit faults or overcurrent protection based on voltage and current amplitudes. For open-circuit faults, which are "hidden" faults that do not directly cause large-scale inrush currents but disrupt the normal operating mode of the circuit, the characteristics are not clearly understood, and effective real-time identification and location methods are lacking.

[0003] This problem is particularly complex in cascaded structures. Due to the power support and voltage equalization of normal sub-units, an open-circuit fault in a single IGBT does not immediately lead to system collapse. Its fault characteristics (such as output DC voltage sag) are partially masked by the cascaded system, and the evolution process is slow and insignificant. This makes traditional protection methods based on a single electrical parameter threshold (such as voltage sag) suffer from low sensitivity and long delays, making it difficult to pinpoint the specific sub-unit or even the specific IGBT in the faulty bridge arm in a timely and accurate manner. Furthermore, in pursuit of precise location, some existing research schemes tend to add additional voltage or current sampling points or rely on complex models and algorithms for state estimation. This undoubtedly increases system cost and computational burden, and reduces the practicality and engineering feasibility of the protection scheme.

[0004] Therefore, existing technologies urgently need a new technical solution to address the above problems. Summary of the Invention

[0005] To address the aforementioned shortcomings of existing technologies, this invention provides a method for diagnosing open-circuit faults in cascaded dual active bridge systems based on inductor impulse voltage. By monitoring and analyzing the inductor impulse voltage generated when the inductor current path is forcibly interrupted, and utilizing the correspondence between its occurrence time and the switching control timing, this method solves the technical problem of difficulty in quickly and accurately locating the faulty sub-unit and faulty IGBT group when an overall IGBT open-circuit fault occurs in a cascaded dual active bridge system.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A method for diagnosing open-circuit faults in a cascaded dual active bridge based on inductor impulse voltage includes the following steps:

[0008] S1. Real-time acquisition of inductor current and inductor voltage of each sub-unit in the cascaded dual active bridge;

[0009] S2. Extract the waveform characteristics of the inductor voltage and detect whether there is an inductor surge voltage exceeding a preset threshold; when the inductor surge voltage is detected, it is determined that an IGBT overall open circuit fault has occurred.

[0010] S3. Based on the correspondence between the occurrence time of the inductor surge voltage and the switching control timing of the cascaded dual active bridge, locate the faulty IGBT group.

[0011] S4. Compare the inductor voltage waveform characteristics of each sub-unit in the cascaded dual active bridge to determine the faulty sub-unit where the IGBT overall open circuit fault occurs.

[0012] S5. Output the fault diagnosis results, including the location information of the faulty IGBT group and the faulty sub-unit.

[0013] As a preferred embodiment, in step S1, the cascaded dual active bridge adopts single-phase shift control; under the single-phase shift control, the upper and lower IGBT switches of the same bridge arm are 180° complementary conduction, and the conduction signals of the diagonal IGBTs are the same.

[0014] As a preferred embodiment, in step S2, the criteria for determining that an IGBT overall open-circuit fault has occurred include:

[0015] When an IGBT experiences an overall open-circuit fault, the inductor current path is forcibly cut off during its predetermined conduction period, resulting in an increase in the rate of change of the inductor current. According to the voltage balance relationship on the primary side of the cascaded dual active bridge, a significant inductor surge voltage will be generated on the current inductor at this time.

[0016] As a preferred embodiment, the voltage balance relationship on the primary side of the cascaded dual active bridge is expressed as follows:

[0017] ;

[0018] In the formula, This is the primary inverter output voltage; Inductor voltage; Voltage across the resistor; This refers to the primary voltage of the high-frequency transformer. Voltage across the resistor; This is the inductance value; This refers to the primary side current; This is the equivalent resistance of the primary side;

[0019] Ignoring the voltage across the resistor, the voltage balance relationship simplifies to:

[0020] .

[0021] As a preferred option, in step S3, locating the group to which the faulty IGBT belongs specifically includes:

[0022] A piecewise mathematical model of inductor current and inductor voltage within one switching cycle after a fault is constructed; wherein, the occurrence of the inductor surge voltage corresponds to the discontinuity point where the inductor current undergoes a forced abrupt change.

[0023] By matching the actual detected inductor surge voltage occurrence time with the theoretical switching time of each IGBT determined based on the switching control timing of the cascaded dual active bridge, the IGBT group experiencing an open-circuit fault can be located.

[0024] As a preferred option, the piecewise mathematical model of the inductor current within one switching cycle after a fault is expressed as follows:

[0025] ;

[0026] In the formula, These are the key time points in the switching control sequence;

[0027] The piecewise mathematical model of the inductor voltage within one switching cycle after a fault is expressed as follows:

[0028] ;

[0029] In the formula, This is the inductor voltage.

[0030] As a preferred embodiment, in step S4, the inductor voltage waveform characteristics of each sub-unit in the cascaded dual active bridge specifically include:

[0031] After detecting the inductor impulse voltage, the inductor voltage waveforms of each subunit in all cascaded dual active bridges are compared within the same time period; the subunits with periodic inductor impulse voltages in the inductor voltage waveforms are identified as faulty phases; the subunits with smooth inductor voltage waveforms that conform to normal modulation rules are identified as normal phases; wherein, the faulty phase is the faulty subunit.

[0032] As a preferred embodiment, after step S5, the method further includes: generating fault alarm information and triggering corresponding protection control commands based on the fault diagnosis results.

[0033] Compared with the prior art, the present invention has the following technical effects:

[0034] 1. This invention monitors the inductor voltage of each sub-unit in a cascaded dual active bridge in real time and identifies the characteristics of the inductor impulse voltage. Based on the operating characteristics of the inductor, it utilizes the inductor voltage and inductor current characteristics after an open-circuit fault occurs, compares and analyzes the inductor voltage and inductor current characteristics under normal conditions and during a fault, and determines the faulty sub-unit and the faulty IGBT group. This achieves rapid, accurate, and cost-free fault detection and location.

[0035] 2. The method of the present invention utilizes the phenomenon that an open circuit fault causes the current path to be forcibly cut off, and directly captures the characteristic inductance surge voltage generated on the inductor as a result. This characteristic is significant and is not masked by the voltage equalization effect of the cascaded system. Therefore, it has high sensitivity and fast response speed, and overcomes the shortcomings of traditional methods based on voltage amplitude drop, which have long delays.

[0036] 3. This invention achieves precise location of the faulty IGBT group by matching the occurrence time of the inductor surge voltage with the inherent switching control timing of the system, without the need for additional sensors or sampling points, thus reducing system complexity and cost and improving the engineering practicality of the solution. At the same time, in the cascaded system, by comparing the inductor voltage waveforms of each sub-unit, the faulty phase and the normal phase can be clearly distinguished, effectively suppressing fault propagation misjudgment and enhancing the reliability of the system. Attached Figure Description

[0037] To make the objectives, technical solutions, and advantages of the invention clearer, the invention will now be described in further detail with reference to the accompanying drawings, wherein:

[0038] Figure 1 This is a flowchart of a cascaded dual active bridge open-circuit fault diagnosis method based on inductor impulse voltage disclosed in this invention.

[0039] Figure 2 This is a schematic diagram of the topology of the cascaded dual active bridge in an embodiment of the present invention;

[0040] Figure 3 This is a schematic diagram of the equivalent circuit of the primary side of the dual active bridge in an embodiment of the present invention;

[0041] Figure 4 This is a circuit conduction mode diagram after an open-circuit fault in the IGBT1 device in an embodiment of the present invention;

[0042] Figure 5 This is a voltage change diagram of the IGBT1 device after an open-circuit fault in an embodiment of the present invention;

[0043] Figure 6 This is a diagram showing the change in inductor voltage between the faulty phase and the non-faulty phase after an IGBT open-circuit fault in an embodiment of the present invention. Detailed Implementation

[0044] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0045] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to represent selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0046] It should be noted that similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the figures, or the orientation or positional relationship commonly used when the product is in use. They are only for the convenience of describing the invention 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. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance. In addition, the terms "horizontal," "vertical," etc., do not indicate that the component is required to be absolutely horizontal or suspended, but can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted. In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; 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; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0047] Example:

[0048] Addressing the critical role of cascaded dual active bridges (DABs) in power electronic transformers and the challenge of rapidly and accurately locating open-circuit faults, existing technologies suffer from the following shortcomings: Firstly, traditional protection methods primarily focus on short-circuit faults or rely on voltage and current amplitude threshold detection. They lack effective feature extraction methods for latent faults such as IGBT overall open circuits, which do not generate large current surges but disrupt circuit operating modes, resulting in low fault identification sensitivity and significant delays. Secondly, in cascaded structures, due to the power support and voltage balancing effects of normal sub-units, the fault characteristics of a single sub-unit (such as DC output voltage drops) are partially masked by the system, evolving slowly and weakening features, making fault location difficult. Existing solutions often require additional sensors or rely on complex algorithms, increasing system cost and complexity and limiting engineering practicality. To address the aforementioned technical problems, this invention proposes a cascaded dual active bridge open-circuit fault diagnosis method based on inductor impulse voltage. This method utilizes the transient event of the current path being forcibly interrupted due to an open-circuit fault. By leveraging the electromagnetic characteristics of the inductor itself, it directly extracts the inductor impulse voltage characteristics that inevitably occur when the fault occurs. By analyzing the timing of the inductor impulse voltage occurrence in conjunction with the control timing of the cascaded dual active bridge switches, it is possible to achieve rapid and accurate diagnosis of faulty sub-unit identification and specific faulty IGBT group location. Thus, it provides a low-cost and real-time reliable cascaded dual active bridge open-circuit fault diagnosis method in engineering applications.

[0049] Specifically, the open-circuit fault diagnosis method for cascaded dual active bridges based on inductor impulse voltage proposed in this invention has the following process: Figure 1 As shown. The fault diagnosis method provided in this embodiment of the invention is based on the analysis of the inductor current and inductor voltage characteristics of a cascaded dual active bridge, the topology of which is as follows. Figure 2 As shown, the object of study is as follows Figure 3 The equivalent circuit of the primary side of the cascaded dual active bridge is shown.

[0050] Specifically, the method in this embodiment includes the following steps:

[0051] S1. Real-time acquisition of inductor current and inductor voltage of each sub-unit in the cascaded dual active bridge;

[0052] In this embodiment, the DAB utilizes IGBT switching to achieve signal conversion. The signal first passes through an inverter circuit composed of four IGBTs, where the switching of different IGBT switches and freewheeling diodes transforms it into high-frequency AC power. Then, a high-frequency transformer converts the voltage level. In the secondary stage, different switching of the IGBTs in the rectifier circuit rectifies the signal into a DC signal, which is then output to the next stage. DAB control often employs single-phase-shift control. In this control mode, within the same bridge arm of the inverter or rectifier circuit, the upper and lower switches are 180° complementary, and the conduction signals of the diagonal IGBTs are identical. The conduction signals of the IGBTs on the high and low voltage sides differ by a phase shift angle. By changing the sign and magnitude of the phase shift angle, the direction of the current and the output DC voltage level can be altered.

[0053] S2. Extract the waveform characteristics of the inductor voltage and detect whether there is an inductor surge voltage exceeding a preset threshold; when the inductor surge voltage is detected, it is determined that an IGBT overall open circuit fault has occurred.

[0054] In this embodiment, the criteria for determining that an IGBT overall open-circuit fault has occurred include:

[0055] When an IGBT experiences an overall open-circuit fault, the inductor current path is forcibly cut off during its predetermined conduction period, resulting in an increase in the inductor current change rate. Since the DAB operating mode under single-phase-shift control is symmetrical within one conduction cycle, a significant inductor surge voltage will be generated on the current inductor at this time, according to the voltage balance relationship of the primary side of the cascaded dual active bridge.

[0056] The voltage balance relationship on the primary side of this cascaded dual active bridge is expressed as follows:

[0057] ;

[0058] In the formula, This is the primary inverter output voltage; Inductor voltage; Voltage across the resistor; This refers to the primary voltage of the high-frequency transformer. Voltage across the resistor; This is the inductance value; This refers to the primary side current; This is the equivalent resistance of the primary side;

[0059] In practice, the equivalent resistance on the primary side is small and can be ignored in calculations. Therefore, the voltage balance relationship simplifies to:

[0060] .

[0061] To further clarify the fault mechanism, in the specific application of this embodiment, taking the open-circuit fault of the IGBT before t1 as an example, the dynamic process after the bridge arm where it is located is broken is analyzed. Before t1, since the bridge arm where IGBT1 is located is originally in the open state, the DAB is operating normally at this time. Analyzing one conduction cycle, it can be divided into three stages according to the current loop, such as... Figure 4 As shown.

[0062] Stage 1: [t1, t2), S2 and S3 are off. The current should have flowed through D1 and D4, but since the bridge arm containing D1 is open-circuited at this time, there is no other path for it to continue flowing, and the current is forced to drop to 0, resulting in an inrush voltage on the inductor. At the same time, the gate signal of S4 is triggered normally, and S4 conducts after bearing a positive voltage, forming a new path with D2. The current drops sharply to zero and then increases again.

[0063] Phase 2: [t2, t4), at this time S2 and S3 are still off, S4 continues to conduct, and the current still flows through D2 and S4 to form a circuit.

[0064] Phase 3: [t4,t7], at this time, the gate signals of S2 and S3 are triggered, the two devices are turned on under positive voltage, S4 is turned off, and DAB works normally.

[0065] S3. Based on the correspondence between the occurrence time of the inductor surge voltage and the switching control timing of the cascaded dual active bridge, locate the faulty IGBT group.

[0066] In this embodiment, a piecewise mathematical model of inductor current and inductor voltage within one switching cycle after a fault is constructed; wherein, the occurrence of the inductor surge voltage corresponds to the discontinuity point where the inductor current undergoes a forced abrupt change;

[0067] By matching the actual detected inductor surge voltage occurrence time with the theoretical switching time of each IGBT determined based on the switching control timing of the cascaded dual active bridge, the IGBT group experiencing an open-circuit fault can be located.

[0068] In a specific application of this embodiment, after a primary IGBT open-circuit fault occurs, the circuit can work normally during the half-cycle in which the bridge arm containing the faulty IGBT is not conducting. However, during the half-cycle in which it should be conducting, the open-circuit fault causes the bridge arm containing the faulty IGBT to be unable to freewheel, and the current freewheels through the freewheeling diode of another IGBT on the bridge arm.

[0069] Because the conduction status of the IGBT changes, the UAB will change during the first half of the original operating cycle of the faulty IGBT:

[0070] In stages 1 and 2: [t1, t4), after the inductor voltage change, a new path is formed by D2, S4, the inductor, and the primary side of the transformer. At this time, U AB It is zero.

[0071] In stage 3: [t4, t7], at this time S2 and S3 are turned on, U AB The value remains within the normal range.

[0072] In the very short time following the fault, the voltage across the transformer changes very little. Therefore, during the analysis phase, it can be temporarily assumed that the voltage amplitude across the transformer remains unchanged during this process.

[0073] At the end of each cycle, an open-circuit fault interrupts the current path, forcing the current to zero and generating a huge voltage across the inductor. Ignoring the extremely short time following the forced current interruption, the inductor current and voltage after the fault can be expressed as follows:

[0074] ;

[0075] In the formula, These are the key time points in the switching control sequence;

[0076] The piecewise mathematical model of the inductor voltage within one switching cycle after a fault is expressed as follows:

[0077] ;

[0078] In the formula, This is the inductor voltage.

[0079] At the extremely brief moment when the current is cut off, based on the inductor's own voltage-current relationship and the above formula, the inductor voltage value at this time can be obtained as follows:

[0080] .

[0081] To verify the correctness of the above analysis and the effectiveness of the proposed method, simulation verification was performed. Figure 5 The simulation waveform and theoretical calculation waveform of the inductor voltage of the fault phase after an IGBT open-circuit fault are shown. As can be seen from the figure, the inductor voltage undergoes a drastic change at the moment of IGBT switching, generating a significant inductor impulse voltage. The simulation value is in high agreement with the theoretical calculation value, which proves the accuracy of the segmented mathematical model and the impulse voltage generation mechanism.

[0082] S4. Compare the inductor voltage waveform characteristics of each sub-unit in the cascaded dual active bridge to determine the faulty sub-unit where the IGBT overall open circuit fault occurs.

[0083] The specific features of comparing the inductor voltage waveform characteristics of each subunit in the cascaded dual active bridge include: after detecting inductor impulse voltage, comparing the inductor voltage waveforms of each subunit in all cascaded dual active bridges within the same time period; identifying the subunit with periodic inductor impulse voltage in the inductor voltage waveform as a fault phase; and identifying the subunit with a smooth inductor voltage waveform that conforms to the normal modulation rule as a normal phase; wherein, the fault phase is the faulty subunit.

[0084] In this specific application of the embodiment, when a single IGBT device experiences an overall open-circuit fault in a cascaded configuration, the normal phase provides power support relative to the faulty phase. Therefore, the DC voltage output of the faulty phase tends to decrease after the fault occurs, while the output of the normal phase increases. Consequently, the rate of decrease in the final output DC voltage is slower compared to a single phase. The more cascaded devices there are, the slower the voltage decreases. Simultaneously, with an increase in the number of cascaded devices, the final output voltage after the fault also increases.

[0085] Within the sub-unit, as time increases, the output of each port of the normal phase will gradually change due to the decrease in DC voltage. However, in the very short time after a fault occurs, although the inductor voltage of the faulty phase will experience periodic surges, the inductance change of the normal phase is almost negligible. Figure 6 As shown, within a very short time after the fault occurs, the circuit enters a steady state after a very short transient process. The faulty phase experiences an impulse voltage at the moment of IGBT switching, while the voltage of the normal phase remains at a normal value.

[0086] Because the current interruption time is extremely short, the induced voltage across the inductor is extremely high. The above analysis shows that the timing of the inductor spike voltage after the current interruption is related to the switching time of the faulty IGBT. Since the conduction status of the IGBTs on opposite sides of each circuit in a dual active bridge is the same, the inductor spike voltage appears at the same time after a fault occurs in the same group of IGBTs. The group of faulty IGBTs can be determined based on the timing of the voltage appearance.

[0087] S5. Output the fault diagnosis results, including the location information of the faulty IGBT group and the faulty sub-unit.

[0088] In this embodiment, after the fault diagnosis is completed, isolation, alarm or fault-tolerant control commands are triggered according to the located faulty sub-unit and faulty IGBT group information. Finally, a diagnostic report containing the fault location, type and timestamp is output.

[0089] In summary, the open-circuit fault diagnosis method for cascaded dual active bridges based on inductor impulse voltage proposed in this embodiment has the following technical advantages compared with the prior art:

[0090] 1. This invention monitors the inductor voltage of each sub-unit in a cascaded dual active bridge in real time and identifies the characteristics of the inductor impulse voltage. Based on the operating characteristics of the inductor, it utilizes the inductor voltage and inductor current characteristics after an open-circuit fault occurs, compares and analyzes the inductor voltage and inductor current characteristics under normal conditions and during a fault, and determines the faulty sub-unit and the faulty IGBT group. This achieves rapid, accurate, and cost-free fault detection and location.

[0091] 2. The method of the present invention utilizes the phenomenon that an open circuit fault causes the current path to be forcibly cut off, and directly captures the characteristic inductance surge voltage generated on the inductor as a result. This characteristic is significant and is not masked by the voltage equalization effect of the cascaded system. Therefore, it has high sensitivity and fast response speed, and overcomes the shortcomings of traditional methods based on voltage amplitude drop, which have long delays.

[0092] 3. This invention achieves precise location of the faulty IGBT group by matching the occurrence time of the inductor surge voltage with the inherent switching control timing of the system, without the need for additional sensors or sampling points, thus reducing system complexity and cost and improving the engineering practicality of the solution. At the same time, in the cascaded system, by comparing the inductor voltage waveforms of each sub-unit, the faulty phase and the normal phase can be clearly distinguished, effectively suppressing fault propagation misjudgment and enhancing the reliability of the system.

[0093] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A cascaded dual active bridge open-circuit fault diagnosis method based on inductive surge voltage, characterized in that, Includes the following steps: S1. Real-time acquisition of inductor current and inductor voltage of each sub-unit in the cascaded dual active bridge; S2. Extract the waveform characteristics of the inductor voltage and detect whether there is an inductor surge voltage exceeding a preset threshold; when the inductor surge voltage is detected, it is determined that an IGBT overall open circuit fault has occurred. S3. Based on the correspondence between the occurrence time of the inductor surge voltage and the switching control timing of the cascaded dual active bridge, locate the faulty IGBT group. S4. Compare the inductor voltage waveform characteristics of each sub-unit in the cascaded dual active bridge to determine the faulty sub-unit where the IGBT overall open circuit fault occurs. S5. Output the fault diagnosis results, including the location information of the faulty IGBT group and the faulty sub-unit.

2. The inductive surge voltage based cascaded dual active bridge open circuit fault diagnostic method of claim 1, wherein, In step S1, the cascaded dual active bridge adopts single-phase shift control; under the single-phase shift control, the upper and lower IGBT switches of the same bridge arm are 180° complementary conduction, and the conduction signals of the diagonal IGBTs are the same. 3.The inductive surge voltage based cascaded dual active bridge open-circuit fault diagnostic method of claim 1, wherein, In step S2, the criteria for determining that an IGBT overall open-circuit fault has occurred include: When an IGBT experiences an overall open-circuit fault, the inductor current path is forcibly cut off during its predetermined conduction period, resulting in an increase in the rate of change of the inductor current. According to the voltage balance relationship on the primary side of the cascaded dual active bridge, a significant inductor surge voltage will be generated on the current inductor at this time.

4. The open-circuit fault diagnosis method for cascaded dual active bridges based on inductive impulse voltage according to claim 3, characterized in that, The voltage balance relationship on the primary side of the cascaded dual active bridge is expressed as follows: ； In the formula, This is the primary inverter output voltage; Inductor voltage; Voltage across the resistor; This refers to the primary voltage of the high-frequency transformer. Voltage across the resistor; This is the inductance value; This refers to the primary side current; This is the equivalent resistance of the primary side; Ignoring the voltage across the resistor, the voltage balance relationship simplifies to: 。 5. The method for diagnosing open-circuit faults in cascaded dual active bridges based on inductive impulse voltage according to claim 1, characterized in that, In step S3, locating the group to which the faulty IGBT belongs specifically includes: A piecewise mathematical model of inductor current and inductor voltage within one switching cycle after a fault is constructed; wherein, the occurrence of the inductor surge voltage corresponds to the discontinuity point where the inductor current undergoes a forced abrupt change. By matching the actual detected inductor surge voltage occurrence time with the theoretical switching time of each IGBT determined based on the switching control timing of the cascaded dual active bridge, the IGBT group experiencing an open-circuit fault can be located.

6. The method for diagnosing open-circuit faults in cascaded dual active bridges based on inductive impulse voltage according to claim 5, characterized in that, The piecewise mathematical model of the inductor current within one switching cycle after a fault is expressed as follows: ； In the formula, These are the key time points in the switching control sequence; The piecewise mathematical model of the inductor voltage within one switching cycle after a fault is expressed as follows: ； In the formula, This is the inductor voltage.

7. The method for diagnosing open-circuit faults in cascaded dual active bridges based on inductive impulse voltage according to claim 1, characterized in that, In step S4, the inductor voltage waveform characteristics of each sub-unit in the cascaded dual active bridge specifically include: After detecting the inductor impulse voltage, the inductor voltage waveforms of each subunit in all cascaded dual active bridges are compared within the same time period; the subunits with periodic inductor impulse voltages in the inductor voltage waveforms are identified as faulty phases; the subunits with smooth inductor voltage waveforms that conform to normal modulation rules are identified as normal phases; wherein, the faulty phase is the faulty subunit.

8. The method for diagnosing open-circuit faults in cascaded dual active bridges based on inductive impulse voltage according to claim 1, characterized in that, After step S5, the method further includes: generating fault alarm information and triggering corresponding protection control commands based on the fault diagnosis results.

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