A fault diagnosis and fault-tolerant method for solid-state transformer

Abstract

The application discloses a kind of solid-state transformer fault diagnosis and fault-tolerant method, belong to the field of power electronics.The method real-time sampling net side voltage, current, inductance current, bridge arm midpoint voltage, DC capacitor voltage and power flow direction, based on cascaded H bridge switch state and DC capacitor voltage estimation net side voltage deviation, fast detection preceding stage open circuit fault;With inductance current bias and bridge arm midpoint voltage waveform distortion, accurately locate the open circuit fault of later stage;Switching modulation mode when preceding stage fault, combine current direction positioning fault tube, and realize fault-tolerant by reconstructing switch state, using healthy device and anti-parallel diode;When primary side fault of later stage, limit the maximum of phase-shifted ratio to suppress overcurrent, when secondary side fault, bypass fault bridge arm and utilize diode freewheeling to eliminate current bias.The application does not need to increase additional hardware, can fast diagnosis and non-shutdown fault-tolerant control of preceding stage and later stage open circuit fault, effectively improve system reliability and power transmission continuity.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a method for fault diagnosis and fault tolerance of solid-state transformers. Background Technology

[0002] Solid-state transformers (SSTs), as a new type of power electronic equipment, are modularly composed of numerous power switching devices, high-frequency components, and control systems. They offer advantages such as small size, flexible control, and power quality management, enabling them to adapt to the flexible and intelligent development needs of medium- and low-voltage distribution networks. They have become crucial equipment for new distribution networks and the integration of new energy sources. However, SSTs operate under complex and harsh conditions for extended periods, making their internal power switching transistors highly susceptible to open-circuit faults. In severe cases, this can lead to system shutdowns or even cascading grid failures, directly threatening power supply reliability.

[0003] Regarding the fault diagnosis and fault tolerance of solid-state transformers, scholars both domestically and internationally have conducted a series of studies. NagaBrahmendra Yadav Gorla et al. published "A Novel Open-Circuit Fault Detection and Localization Scheme for Cascaded H-Bridge Stage of a Three-Stage Solid-State Transformer" in *IEEE Transactions on Power Electronics* in 2021, proposing a switching modulation strategy to locate the fault in the switching transistor during an open-circuit fault in the cascaded H-bridge (CHB) stage of a solid-state transformer. Pei Xuejun et al. published "Open-Circuit Fault Diagnosis and Fault-Tolerant Strategies for Full-Bridge DC–DC Converters" in *IEEE Transactions on Power Electronics* in 2012, proposing to improve the reliability of the converter after a fault by adding redundant switches, bridge arms, or circuits. In 2025, Chen Min et al. proposed an optimized fault-tolerant control method for cascaded solid-state transformers in Chinese invention patent No. 202411655384.0. This method bypasses the faulty module after a fault, switches the redundant module to operation, and forwards the drive of the faulty module to the redundant module to achieve fault-tolerant operation.

[0004] However, the aforementioned prior art has the following drawbacks:

[0005] (1) Gorla et al.’s solution only focuses on fault location of the front-end CHB and does not discuss fault-tolerant control after the fault, which limits its engineering application. (2) The redundancy addition method proposed by Pei Xuejun et al. is more suitable for circuits with fewer device requirements, but it will greatly increase the system cost for solid transformers containing many sub-modules. (3) Although the method of Chen Min et al. achieved fault-tolerant operation, it did not accurately locate the fault switch tube in the solid-state transformer and directly switched the redundant module, resulting in a large waste of components.

[0006] Therefore, how to achieve rapid diagnosis and reliable fault-tolerant control of solid-state transformer faults without the need for additional sensors and a large number of redundant devices, and without causing significant waste of components, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] In view of this, the present invention provides a method for fault diagnosis and fault tolerance of solid-state transformers, which achieves rapid fault diagnosis and location and reliable fault-tolerant control without the need for additional sensors and redundant devices, and ensures safe and stable operation of the system under fault conditions without causing significant waste of components.

[0008] Therefore, the present invention provides the following technical solution: A fault diagnosis and fault tolerance method for a solid-state transformer, wherein the topology of the solid-state transformer includes n modules consisting of a front-stage cascaded H-bridge rectifier circuit and a rear-stage dual active bridge circuit; the method includes the following steps: S1. Sampling: Collect the grid-side voltage, grid-side current, DC coupling capacitor voltage of the cascaded H-bridge, as well as the transmission inductor current, primary-side bridge arm midpoint voltage, secondary-side bridge arm midpoint voltage, and power flow direction of the dual active bridge. S2. Front-end fault detection: Based on the switching state of the cascaded H-bridge and the DC coupling capacitor voltage, the grid-side voltage is estimated in real time, and the deviation between the grid-side voltage and the actual measured grid-side voltage is calculated. If the deviation exceeds the preset threshold, it is determined that the cascaded H-bridge has an open circuit fault, and the process proceeds to step S4; otherwise, the process proceeds to step S3. S3. Post-stage fault detection: Determine whether the bias type of the transmission inductor current matches the fault characteristics. If it does, determine that the dual active bridge has an open circuit fault and proceed to step S5; otherwise, determine that the system is operating normally and return to step S1. S4. Front-end fault-tolerant control: The modulation mode of the cascaded H-bridge is switched from phase-shifted unipolar SPWM to phase-shifted bipolar SPWM. The specific faulty switch is located by combining the grid current direction and output voltage characteristics. Then, the fault-tolerant operation is achieved by reconstructing the switch state and using the anti-parallel diodes of the healthy switch and the faulty switch to replace the fault path. S5. Post-fault diagnosis and fault tolerance: Based on the bias characteristics of the transmission inductor current and the waveform distortion of the midpoint voltage of the primary and secondary bridge arms, locate the position of the switch in the dual active bridge that has an open-circuit fault; if the fault is located on the primary side, limit the maximum shift ratio to 0.25 to suppress overcurrent; if the fault is located on the secondary side, block the drive pulse of the entire bridge arm containing the faulty switch, and only allow the bridge arm diodes to continue working to eliminate current bias.

[0009] A further improvement of the technical solution of the present invention is that: the front-stage cascaded H-bridge rectifier circuit includes: a grid-side AC voltage source connected in series with a grid-side inductor to the AC input terminal of the full-bridge rectifier circuit; the full-bridge rectifier circuit is composed of a first switch, a second switch, a third switch, a fourth switch, and a first diode, a second diode, a third diode, and a fourth diode connected in anti-parallel to it respectively; the full-bridge rectifier circuit converts the AC voltage into a DC voltage as the front-stage output, and the front-stage output terminal is connected in parallel with a bus capacitor and a subsequent dual active bridge circuit; The subsequent dual active bridge circuit includes: a primary-side full-bridge circuit, an intermediate-stage transformer, a transmission inductor, a secondary-side full-bridge circuit, and an output capacitor. The primary-side full-bridge circuit consists of a fifth switch, a sixth switch, a seventh switch, an eighth switch, and a fifth diode, a sixth diode, a seventh diode, and an eighth diode connected in anti-parallel to each switch. Its DC input terminal is connected in parallel with the bus capacitor, and its AC output terminal is connected to the primary winding of the intermediate stage transformer via the transmission inductor. The secondary-side full-bridge circuit consists of a ninth switch, a tenth switch, an eleventh switch, a twelfth switch, and a ninth diode, a tenth diode, an eleventh diode, and a twelfth diode connected in anti-parallel to the switch. Its AC input terminal is connected to the secondary winding of the intermediate stage transformer, and its DC output terminal is connected in parallel to the output capacitor.

[0010] A further improvement to the technical solution of the present invention is that step S2 specifically includes: 2.1 Calculate the equivalent logic signals of the bridge arms of the cascaded H-bridge S A and S B : Define the gate drive signals for the first, second, third, and fourth switches in the cascaded H-bridge as follows: s 1. s 2. s 3. s 4. The complementary signals are respectively , , , 1 indicates conduction, and 0 indicates deactivation; when When the current flows into the H-bridge, the direction of the current is the same as the grid current; when At this time, the current direction is that the grid current flows out of the H-bridge; complementary signal ; Let the bridge arm containing the first and second switching transistors be the bridge arm. S A The bridge arm containing the third and fourth switching transistors is the bridge arm. S B , When, it indicates the bridge arm S A In the on state, similarly. When, it indicates the bridge arm S B It is in the on state; Based on the conduction state of the switches in the cascaded H-bridge, the equivalent logic signals of the bridge arms of the H-bridge are... S A and S B The bridge arm logic formula is: ; S A and S B The value can only be 0 or 1, corresponding to the on and off states of the bridge arm; 2.2 Calculate the grid-side estimated voltage of the cascaded H-bridge V ane Compared with the measured grid-side voltage V in deviation e f : Grid-side estimated voltage V ane The calculation formula is: ; in, Vc This is the voltage of the DC coupling capacitor; Grid-side estimated voltage V ane Compared with the measured grid-side voltage V in deviation e f The calculation formula is: ; 2.3 Determining open-circuit faults in cascaded H-bridge devices: Set threshold: ; in, V dFor the forward voltage drop of the anti-parallel diode; like The fault flag is triggered, indicating that there is an open circuit fault in the cascaded H-bridge; like This is normal operating condition.

[0011] A further improvement to the technical solution of the present invention is that step S3 specifically includes: analyzing the sampled dual active bridge transmission inductor current, and if the peak value of the dual active bridge transmission inductor current is... If the voltage at the midpoint of the primary arm and the midpoint of the secondary arm of the dual active bridge exhibit significant waveform distortion, and the inductor current of the dual active bridge shows a fixed-direction bias, then it is determined that there is an open-circuit fault in the dual active bridge; otherwise, it is a normal operating condition.

[0012] A further improvement to the technical solution of this invention lies in: in step S4, the modulation mode of the cascaded H-bridge is switched from phase-shifted unipolar SPWM to phase-shifted bipolar SPWM, and the specific fault switch is located by combining the grid current direction and output voltage characteristics, specifically including: After determining that an open-circuit fault has occurred in the cascaded H-bridge, the modulation mode of the cascaded H-bridge is switched from phase-shifted unipolar SPWM to phase-shifted bipolar SPWM; after switching the modulation mode, the operating data of a complete power grid frequency cycle is collected. According to the direction of the grid current The periodic data is divided into positive and negative half-cycles for analysis; when At this time, the current direction is that the grid current flows into the H-bridge, and the period data is the positive half-cycle; when At that time, the current direction is the grid current flowing out of the H-bridge, and the period data is the negative half-cycle; When the switch state is [ s 1, s When 3]=[1,0], under bipolar SPWM modulation, the output voltage of the fault-free cascaded H-bridge satisfies: ; When the switch state is [ s 1, s When 3]=[0,1], under bipolar SPWM modulation, the output voltage of the fault-free cascaded H-bridge satisfies: ; in, V 0 indicates the measured output voltage of the cascaded H-bridge; V c This is the voltage of the DC coupling capacitor; When the direction of the detected grid current And the switch state is [ s 1, sWhen 3]=[1,0], if the measured output voltage of the cascaded H-bridge does not meet the requirements... If so, the faulty switch is determined to be the first switch. When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[1,0], if the measured output voltage of the cascaded H-bridge does not meet the requirements... If so, the faulty switch is determined to be the second switch. When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[0,1], if the measured output voltage of the cascaded H-bridge does not meet the requirements... If so, the faulty switch is determined to be the third switch. When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[0,1], if the measured output voltage of the cascaded H-bridge does not meet the requirements... If so, the faulty switch is determined to be the fourth switch.

[0013] A further improvement to the technical solution of this invention lies in: in step S4, fault-tolerant operation is achieved by reconstructing the switch state and replacing the fault path with an anti-parallel diode of the healthy switch and the faulty switch; specifically, this includes: The cascaded H-bridge is a fully controlled bridge topology. The first, second, third, and fourth switches, together with the first, second, third, and fourth diodes connected in anti-parallel, achieve bidirectional current transmission. An open circuit in a single switch will cause the bridge arm / diagonal where the faulty switch is located to lose its active switching capability, and the current path will be blocked. This will cause the output voltage of the cascaded H-bridge to lose the positive / negative half-cycle, introducing DC bias into the system. Based on the power flow direction in step S1 and the specific location of the faulty switch in the cascaded H-bridge in step S4, fault-tolerant control is performed: When the power flow direction When the faulty switch is in the same bridge arm, the other normal switch will continue to conduct, while the faulty switch will continue to be turned off, forming a stable current path through the anti-parallel diode. When the power flow direction When the faulty switch and the other switch in the same bridge arm are turned off, a stable current path is formed only through the anti-parallel diode.

[0014] A further improvement to the technical solution of this invention lies in: in step S5, based on the bias characteristics of the transmission inductor current and the waveform distortion of the voltage at the midpoint of the primary and secondary bridge arms, the location of the switch in the dual active bridge that has an open-circuit fault is determined; specifically, this includes: 5.1 Calculate the inductor current under normal operating conditions of the dual active bridge: When the dual active bridge is operating normally, both the primary and secondary sides are driven by 50% duty cycle pulses, and the power is controlled by the shift ratio D. Under normal operating conditions, the transmission power of the dual active bridge is... P N The expression is: ; When the shift ratio D=0.5, the maximum transmission power of the dual active bridge is P Nmax and maximum inductor current i Lmax The expression is: ; ; When the shift ratio D=0.5, the maximum inductor current of the dual active bridge is i Lmax The expression is: ; in, n For transformer turns ratio, L r To transmit inductance, f s For switching frequency, u 1 represents the primary DC voltage. u 2 is the secondary DC voltage. i Lr To transmit inductor current; 5.2 Calculate the inductor current under primary-side fault conditions in a dual active bridge: The primary side full-bridge circuit includes four switching transistors: the fifth, sixth, seventh, and eighth. The fault characteristics of the fifth and eighth switching transistors are completely identical, and the fault characteristics of the sixth and seventh switching transistors are completely identical. The following derivation is based on an open-circuit fault in the fifth switching transistor: When the fifth switch is open, the forward direction i Lr Unable to pass through the fifth switching transistor, the freewheeling second diode is forced to conduct, resulting in four new operating states. i Lr The slope of the change is: ; in, m1, m 2, m 3, m 4 represents the inductor current under four different conditions. i Lr The slope of the change; The following is the expression for the inductor current at critical time points after a fault occurs in the original side full-bridge circuit: ; The fifth switching transistor in the primary-side full-bridge circuit. T 1. After the fault i Lr All values ​​are negative, indicating a clear negative bias; When the shift is compared At that time, the inductor current is at its maximum, and Substitution The expression is: ; At this point, the maximum value of the inductor current after the fifth switch in the original side full-bridge circuit fails. ; The open-circuit fault of the sixth switch is similar to the open-circuit fault of the fifth switch, but after the sixth switch fails... i Lr All values ​​are positive, indicating a clear positive bias; the maximum inductor current after the sixth switch on the primary side malfunctions is also 2. i Lmax ; 5.3 Calculation of inductor current under secondary-side fault in dual active bridge: The primary-side full-bridge circuit includes four switches: the ninth, tenth, eleventh, and twelfth. The fault characteristics of the ninth and twelfth switches are completely identical, as are the fault characteristics of the tenth and eleventh switches. The following derivation uses an open-circuit fault in the ninth switch as an example: When the ninth switch is open, the forward direction i Lr Unable to pass through the ninth switching transistor, the tenth freewheeling diode is forced to conduct, resulting in five new operating states. i Lr The slope of the change is consistent with the slope of the change after the original side fault occurs; The following is the expression for the inductor current at critical time points after a fault occurs in the secondary-side full-bridge circuit: ; After a fault occurs in the ninth switch of the secondary-side full-bridge circuit i Lr All values ​​are positive, indicating a clear positive bias; When the shift is compared DWhen the value is 0.5, the inductor current is at its maximum. D =0.5 Substitute The expression is: ; At this point, the maximum value of the inductor current after a fault occurs in the secondary-side full-bridge circuit. Numerically, it is not exactly equal to i Lmax It is twice that of the second-side full-bridge circuit, and has a negative component; moreover, the midpoint voltage of the bridge arm in the secondary-side full-bridge circuit will show a significant plateau segment in each switching cycle. 5.4 Locating faults on the primary and secondary sides of the dual active bridge: The bias direction, peak value, and midpoint voltage waveform of the secondary bridge arm of the dual active bridge transmission inductor in real time; If detected i Lr If there is a negative bias and no positive component, a voltage drop will occur at the midpoint of the primary side bridge arm, while the midpoint voltage of the secondary side bridge arm is normal. This corresponds to a fault in the fifth / eighth switch. If detected i Lr If the primary side bridge arm is positively biased and has no negative component, a voltage drop occurs at the midpoint of the primary side bridge arm. If the midpoint voltage of the secondary side bridge arm is normal, then the sixth / seventh switch is faulty. If detected i Lr If the voltage at the midpoint of the primary arm is normal when the voltage waveform at the midpoint of the secondary arm is positively biased and has a negative component, then the voltage waveform at the midpoint of the secondary arm shows a plateau segment, which corresponds to a fault in the ninth / twelfth switch. If detected i Lr If there is a negative bias and a positive component, the voltage at the midpoint of the primary arm is normal, but the voltage waveform at the midpoint of the secondary arm shows a plateau segment, then the fault corresponds to the 10th / 11th switch.

[0015] A further improvement to the technical solution of this invention is that: in step S5, if the fault is located on the primary side, the maximum value of the shift ratio is limited to 0.25 to suppress overcurrent; if the fault is located on the secondary side, the drive pulse of the faulty switch transistor in the entire bridge arm is blocked, and only the freewheeling operation of the bridge arm diode is retained to eliminate current bias; specifically including: If the open-circuit fault of the positioning switch occurs on the primary side of the dual active bridge converter, the limiting shift ratio will be... D The maximum value is 0.25. By reducing the transmission power, the inductor current is limited to the normal maximum value. If the open circuit fault of the positioning switch occurs on the secondary side of the dual active bridge converter, the drive pulse of the entire secondary bridge arm where the faulty switch is located is blocked, and only the bridge arm diode is kept working. The current bias is eliminated by bypassing the faulty bridge arm, thereby limiting the maximum current. When an open-circuit fault occurs on either the primary or secondary side of a dual active bridge, the dual active bridge can still maintain the same power transmission capability as under normal operating conditions, but the inductor current... i Lr The peak value can reach about twice that of normal operating conditions, and a one-way bias problem will occur; When an open-circuit fault occurs on the primary side of the dual active bridge, the maximum value of the shift ratio D is immediately limited from 0.5 to 0.25, and the peak inductor current is limited to the normal rated value. i Lmax Within this range, the maximum inductor current flowing at this time The expression for maximum transmission power is: ; When an open-circuit fault occurs on the secondary side of the dual active bridge, the entire bridge arm containing the faulty switch is directly bypassed. By blocking the drive pulses of the two switches within the bridge arm, current freewheeling is achieved using anti-parallel freewheeling diodes, eliminating current bias and limiting overcurrent. The maximum transmission power expression at this time is: ; Maximum inductor current flowing through i LrSBAmax The expression is: ; As can be seen, after an open-circuit fault occurs on the secondary side of the dual active bridge, the maximum current flowing through the inductor is limited to the normal rated value. i Lmax Within this range, the circuit can operate normally.

[0016] Advantages and positive effects of the present invention: 1. This invention utilizes only existing voltage and current sensors in the system (including grid-side voltage and current, DC bus voltage, transmission inductor current, bridge arm midpoint voltage, etc.) and software algorithms to estimate and judge in real time, which can realize the rapid detection and accurate location of open circuit faults of the front and rear stage switching transistors. There is no need to add additional hardware detection circuits or redundant devices, which significantly reduces the system hardware cost and design complexity.

[0017] 2. This invention estimates the grid-side voltage by combining the cascaded H-bridge switch state and the DC-coupled capacitor voltage, and compares this estimate with the measured value, enabling the determination of an open-circuit fault in the preceding stage in a very short time. Upon fault detection, the modulation mode is automatically switched from phase-shifted unipolar SPWM to phase-shifted bipolar SPWM, and the faulty switch is accurately located by combining the grid current direction and output voltage characteristics. Furthermore, by reconstructing the switch state, the fault path is replaced by the anti-parallel diodes of the healthy switch and the faulty switch. Fault-tolerant operation can be achieved simply by adjusting the control signal, without the need to replace components or shut down the system, ensuring the continuity of power transmission in the preceding stage.

[0018] 3. This invention targets dual active bridge (DAB) converters. By utilizing the bias characteristics of the transmission inductor current and the waveform distortion of the voltage at the midpoint of the primary and secondary bridge arms, it can accurately distinguish between open-circuit faults in the primary and secondary side switching transistors. For primary-side faults, by limiting the maximum shift ratio to 0.25, the transmission power is actively reduced, keeping the inductor current within a safe range. For secondary-side faults, the drive pulses of the faulty bridge arm switching transistor in the entire bridge arm are blocked, leaving only the freewheeling path of the bridge arm diodes, effectively eliminating current bias and limiting the maximum current. This hierarchical fault-tolerant strategy effectively prevents overcurrent damage to devices and ensures safe system operation.

[0019] 4. After detecting an open circuit fault in the preceding or following stage, this invention can enable the solid-state transformer to continue operating with the fault without interrupting power transmission by switching modulation strategies, reconstructing switch states, limiting the shift ratio, or bypassing the bridge arm. This significantly improves the power supply reliability and power transmission continuity of the system, and is particularly suitable for applications with high requirements for uninterrupted operation, such as data centers, medical power supplies, and rail transit.

[0020] 5. The fault diagnosis and fault tolerance method provided by this invention is based on common cascaded H-bridge and dual active bridge topologies. The fault characteristics are clear, the diagnostic logic is clear, and the fault tolerance control only involves the modification of the control algorithm without modifying the main circuit. It is easy to implement in digital controllers (such as DSP and FPGA) and has good engineering promotion value and application prospects. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a system-level overall diagram of the solid-state transformer in an embodiment of the present invention; Figure 2 This is a circuit topology diagram of a solid-state transformer with n=1 as an example in this embodiment of the invention; Figure 3 This is a flowchart of a solid-state transformer fault diagnosis and fault tolerance method provided in an embodiment of the present invention. Detailed Implementation

[0023] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0024] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0025] The topology used in this invention consists of a front-stage cascaded H-bridge rectifier circuit (CHB) and a rear-stage dual active bridge circuit (DAB). n It consists of several modules, such as Figure 1 As shown.

[0026] Now n For example, =1 Figure 2 As shown, the front-end cascaded H-bridge rectifier circuit includes: a grid-side AC voltage source. V in Network-side inductance L The AC input terminal of the full-bridge rectifier circuit is connected in series; the full-bridge rectifier circuit consists of the first switching transistor. S 1. Second switching transistor S 2. Third switching transistor S 3. Fourth switching transistor S 4 and the first diode connected in antiparallel to it. D 1. Second diode D 2. Third diode D 3. Fourth diode D 4. Composition: The full-bridge rectifier circuit converts AC voltage into DC voltage as the output of the pre-amplifier stage. A bus capacitor is connected in parallel to the output of the pre-amplifier stage. C 1 and subsequent dual active bridge circuits; The subsequent dual active bridge circuit includes: a primary-side full-bridge circuit, an intermediate stage transformer, and a transmission inductor. L rSecondary-side full-bridge circuit and output capacitor C 2; The original side full-bridge circuit is controlled by the fifth switching transistor. T 1. Sixth switching transistor T 2. Seventh switching transistor T 3. Eighth switching transistor T 4 and the fifth diode connected in antiparallel to it. M 1. Sixth diode M 2. Seventh Diode M 3. Eighth diode M It consists of 4 components, with its DC input terminal connected to the bus capacitor. C 1. Connected in parallel, its AC output terminal is connected via a transmission inductor. L r Connected to the primary winding of the intermediate stage transformer; The secondary-side full-bridge circuit consists of the ninth switching transistor. T 5. Tenth switching transistor T 6. Eleventh switching transistor T 7. Twelfth Switch T 8 and the ninth diode connected in antiparallel to it. M 5. Tenth diode M 6. Eleventh diode M 7. Twelfth Diode M It consists of 8 components, with its AC input terminal connected to the secondary winding of the intermediate stage transformer, and its DC output terminal connected to the output capacitor. C 2. Parallel connection.

[0027] like Figure 3 As shown, a fault diagnosis and fault tolerance method for solid-state transformers specifically includes the following steps: S1. Sampling: The grid-side voltage of the preceding cascaded H-bridge rectifier circuit (hereinafter referred to as CHB) V in Grid-side current of CHB I in The transfer inductor current flowing through the subsequent dual active bridge circuit (hereinafter referred to as DAB) i Lr The voltage at the midpoint of the primary arm of the DAB bridge u AB and the voltage at the midpoint of the secondary bridge arm u ab DC coupling capacitor voltage between CHB and DAB V C and power flow direction P ( P >0 indicates that the AC voltage and current are in the same direction, and P<0 indicates that the AC voltage and current are in opposite directions. The switching state of CHB is also sampled.

[0028] S2. Front-end fault detection: based on the switching state of the cascaded H-bridge and the DC-coupled capacitor voltage. V C Real-time estimation of CHB grid-side voltage V ane Calculate the deviation between it and the measured grid-side voltage. e f If the deviation exceeds the preset threshold, it is determined that the cascaded H-bridge has an open circuit fault, and the process proceeds to step S4; otherwise, the process proceeds to step S3. Specifically, it includes the following steps: 2.1 Calculate the equivalent logic signals of the bridge arms of the cascaded H-bridge S A and S B : First, define the following: the first switch of a cascaded H-bridge. S 1. Second switching transistor S 2. Third switching transistor S 3. Fourth switching transistor S The gate drive signals for 4 are respectively s 1. s 2. s 3. s 4. Complementary signal 0 is , , , , where 1 indicates conduction and 0 indicates deactivation.

[0029] when When the current flows into the H-bridge, the direction of the current is the same as the grid current; when At this time, the current direction is that the grid current flows out of the H-bridge; complementary signal .

[0030] First switching transistor S 1 and second switching transistors S The bridge arm where 2 is located is the bridge arm S A Third switching transistor S 3 and the fourth switching transistor S The bridge arm where 4 is located is the bridge arm. S B , When, it indicates the bridge arm S A In the on state, similarly. When, it indicates the bridge arm S B It is in the conductive state.

[0031] Based on the conduction state of the switches in the cascaded H-bridge, the equivalent logic signals of the bridge arms of the H-bridge are... S A andS B The bridge arm logic formula is: ; S A and S B The value can only be 0 or 1, corresponding to the on and off states of the bridge arm.

[0032] 2.2 Calculate the grid-side estimated voltage of the cascaded H-bridge V ane Compared with measured values V in deviation e f : Grid-side estimated voltage V ane The calculation formula is as follows: ; Grid-side estimated voltage V ane Compared with measured values V in deviation e f The calculation formula is as follows: ; 2.3 Determining open-circuit faults in cascaded H-bridge devices: Set threshold: ; in, V d This is the forward voltage drop of the anti-parallel diode.

[0033] like The fault flag is triggered, indicating an open-circuit fault in the cascaded H-bridge; if This is normal operating condition.

[0034] S3. Post-stage fault detection: Determine whether the bias type of the transmission inductor current matches the fault characteristics. If it does, determine that the dual active bridge has an open circuit fault and proceed to step S5; otherwise, determine that the system is operating normally and return to step S1. Specifically, this involves: determining open-circuit faults in the primary and secondary components of a dual active bridge; and sampling... i Lr If an analysis is performed, i Lr peak , u AB and u ab There is obvious waveform distortion, and iLr If a fixed-direction offset is observed, it indicates an open-circuit fault in the dual active bridge; otherwise, it is considered normal operation.

[0035] S4. Front-end fault-tolerant control: The modulation mode of the cascaded H-bridge is switched from phase-shifted unipolar SPWM to phase-shifted bipolar SPWM. The specific faulty switch is located by combining the grid current direction and output voltage characteristics. Then, the fault-tolerant operation is achieved by reconstructing the switch state and using the anti-parallel diodes of the healthy switch and the faulty switch to replace the fault path. Specifically, it includes: After determining that an open-circuit fault has occurred in the cascaded H-bridge, the modulation mode of the cascaded H-bridge is switched from phase-shifted unipolar SPWM to phase-shifted bipolar SPWM. During fault location, the main advantage of bipolar SPWM compared to unipolar SPWM is that the number of switch combinations in bipolar SPWM is only […]. s 1, s 3]=[1,0] and [ s 1, s 3]=[0,1], while the number of switch combinations in unipolar SPWM is four. Therefore, compared with the corresponding SPWM, only half of the switch combinations need to be verified in bipolar SPWM to locate the fault. After switching the modulation mode, the operating data of a complete power grid frequency cycle is collected; According to the direction of the grid current The periodic data is divided into positive and negative half-cycles for analysis; when At this time, the current direction is that the grid current flows into the H-bridge, and the period data is the positive half-cycle; when At that time, the current direction is the grid current flowing out of the H-bridge, and the period data is the negative half-cycle; When the switch state is [ s 1, s When 3]=[1,0], under bipolar SPWM modulation, the output voltage of the fault-free cascaded H-bridge satisfies: ; When the switch state is [ s 1, s When 3]=[0,1], under bipolar SPWM modulation, the output voltage of the fault-free cascaded H-bridge satisfies: ; in, V 0 represents the measured output voltage of the cascaded H-bridge.

[0036] When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[1,0], if the measured output voltage of the cascaded H-bridge does not meet the requirements... Then the faulty switch is determined to be the first switch. S 1.

[0037] When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[1,0], if the measured output voltage of the cascaded H-bridge does not meet the requirements... Then the faulty switch is determined to be the second switch. S 2.

[0038] When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[0,1], if the measured output voltage of the cascaded H-bridge does not meet the requirements... Then the faulty switch is determined to be the third switch. S 3.

[0039] When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[0,1], if the measured output voltage of the cascaded H-bridge does not meet the requirements... Then the faulty switch is determined to be the fourth switch. S 4.

[0040] Cascaded H-bridge fault-tolerant control: The cascaded H-bridge is a fully controlled bridge topology, with the first switch transistor... S 1. Second switching transistor S 2. Third switching transistor S 3. Fourth switching transistor S 4. In conjunction with the first anti-parallel diode D 1. Second diode D 2. Third diode D 3. Fourth diode D 4. Achieve bidirectional current transmission; an open circuit in a single switch will cause the bridge arm / diagonal where the faulty switch is located to lose its active switching capability, the current path will be blocked, the H-bridge output voltage will lack positive / negative half-cycles, and DC bias will be introduced into the system.

[0041] Based on the power flow direction in step S1 P And the specific location of the fault switch transistor in the cascaded H-bridge, to perform fault-tolerant control: When the power flow direction When the faulty switch is in the same bridge arm, the other normal switch will continue to conduct, while the faulty switch will continue to be turned off, forming a stable current path through the anti-parallel diode. When the power flow direction When the faulty switch and the other switch in the same bridge arm are turned off, a stable current path is formed only through the anti-parallel diode.

[0042] S5. Post-fault diagnosis and fault tolerance: Based on the bias characteristics of the transmission inductor current and the waveform distortion of the midpoint voltage of the primary and secondary bridge arms, locate the position of the switch in the dual active bridge that has an open-circuit fault; if the fault is located on the primary side, limit the maximum shift ratio to 0.25 to suppress overcurrent; if the fault is located on the secondary side, block the drive pulse of the entire bridge arm containing the faulty switch, and only allow the bridge arm diodes to continue working to eliminate current bias.

[0043] Specifically: Locating the open circuit fault on the primary and secondary sides of the DAB involves the following steps: 5.1 Calculate the inductor current under normal operating conditions of the dual active bridge: When the dual active bridge is operating normally, both the primary and secondary sides are driven by 50% duty cycle pulses, and the power is controlled by the shift ratio D. Under normal operating conditions, the transmission power of the dual active bridge is... P N The expression is: ; When the shift ratio D=0.5, the maximum transmission power of the dual active bridge is P Nmax and maximum inductor current i Lmax The expression is: ; ; When the shift ratio D=0.5, the maximum inductor current of the dual active bridge is i Lmax The expression is: ; in, n For transformer turns ratio, L r To transmit inductance, f s For switching frequency, u 1 represents the primary DC voltage. u 2 is the secondary DC voltage. i Lr This represents the inductor current.

[0044] 5.2 Calculate the inductor current under primary-side fault conditions in a dual active bridge: The primary side includes the fifth switching transistor. T 1. Sixth switching transistor T 2. Seventh switching transistor T 3. Eighth switching transistor T4. Four switching transistors, and a fifth switching transistor. T 1 and the eighth switch T 4. The fault characteristics are completely identical; the sixth switching transistor... T 2. The fault characteristics are completely consistent with those of the seventh switch transistor T3. The following discussion focuses on the fifth switch transistor. T Let's take an open-circuit fault as an example to derive the result: Fifth switching transistor T 1. When opening the road, in the forward direction i Lr Unable to pass the fifth switch transistor T 1. Freewheeling second diode D 2 is forcibly activated, forming 4 new working states. i Lr The slope of the change is: ; in, m 1, m 2, m 3, m 4 represents the inductor current under four different conditions. i Lr The slope of the change.

[0045] The following is the expression for the inductor current at critical time points after a fault occurs on the primary side: ; As can be seen from the above formula, the fifth switch on the original side occurs. T 1. After the fault i Lr All values ​​are negative, indicating a clear negative bias.

[0046] When the shift is compared At that time, the inductor current is at its maximum, and Substitution The expression can be obtained as follows: ; It can be seen that at this time, the fifth switch on the original side... T 1. Maximum value of inductor current after a fault occurs .

[0047] Sixth switch T 2. Open circuit fault and fifth switching transistor T 1. Similar to an open-circuit fault, but the sixth switch transistor T 2. After the fault i Lr All values ​​are positive, showing a clear positive bias; the original side sixth switch transistor T 2. The maximum value of the inductor current after a fault is also 2. i Lmax .

[0048] 5.3 Calculation of inductor current under secondary-side fault in dual active bridge: The secondary side includes the ninth switch. T 5. Tenth switching transistor T 6. Eleventh switching transistor T 7. Twelfth Switch T 8. Four switching transistors, and a ninth switching transistor. T 5 and the twelfth switch T 8. The fault characteristics are completely identical; the tenth switching transistor... T The fault characteristics of switch 6 and the eleventh switch T7 are completely consistent. The following discussion focuses on the ninth switch. T Let's take an open-circuit fault as an example to derive the following: Ninth switch T 5. When opening the road, in the forward direction i Lr Unable to pass the ninth switch T 5. Freewheeling diode (10th diode) D 6 is forcibly activated, forming 5 new working states. i Lr The slope of the change is consistent with the slope of the change after the original side fault occurs.

[0049] The following is the expression for the inductor current at critical time points after a fault occurs on the secondary side: ; As can be seen from the above formula, when the ninth switch on the secondary side occurs... T 5. After the malfunction i Lr All values ​​are positive, indicating a clear positive bias.

[0050] When the shift is compared D When the value is 0.5, the inductor current is at its maximum. D =0.5 Substitute The expression can be obtained as follows: ; It can be seen that the maximum value of the inductor current after a fault occurs on the secondary side is at this time. Numerically, it is not exactly equal to i Lmax It is twice that of the secondary bridge arm, and has a negative component. Furthermore, the voltage at the midpoint of the secondary bridge arm... u ab A distinct plateau segment appears in each switching cycle.

[0051] 5.4 Locating faults on the primary and secondary sides of the dual active bridge: Real-time monitoring of inductor current i Lr Bias direction, peak value and uab Waveform; If detected i Lr Negative bias with no positive component, voltage at the midpoint of the primary arm u AB This generates a voltage drop, and the voltage at the midpoint of the secondary bridge arm... u ab If normal, it corresponds to the fifth switching transistor. T 1 / Eighth Switch T 4. Fault.

[0052] If detected i Lr Positive bias with no negative component, voltage at the midpoint of the primary arm u AB This generates a voltage drop, and the voltage at the midpoint of the secondary bridge arm... u ab If normal, it corresponds to the sixth switching transistor. T 2 / Seventh Switch T 3. Fault.

[0053] If detected i Lr Positive bias with a negative component, voltage at the midpoint of the primary arm u AB Normal, voltage at the midpoint of the secondary arm. u ab If the waveform shows a plateau segment, it corresponds to the ninth switch. T 5 / Twelfth Switch T 8. Fault.

[0054] If detected i Lr Negative bias with a positive component, the voltage at the midpoint of the primary arm. u AB Normal, voltage at the midpoint of the secondary arm. u ab If the waveform shows a plateau segment, it corresponds to the tenth switching transistor. T 6 / Eleventh Switch T 7. Fault.

[0055] Dual active bridge fault-tolerant control: When an open-circuit fault occurs on either the primary or secondary side of a dual active bridge, the dual active bridge can still maintain the same power transmission capability as under normal operating conditions, but the inductor current... i Lr The peak value can reach about twice that of normal operating conditions, and a one-way bias problem will occur.

[0056] When an open-circuit fault occurs on the primary side of the dual active bridge, the maximum value of the shift ratio D is immediately limited from 0.5 to 0.25, and the peak inductor current is limited to the normal rated value. iLmax Within this range, the maximum inductor current flowing at this time The expression for maximum transmission power is: ; When an open-circuit fault occurs on the secondary side of the dual active bridge, the entire bridge arm containing the faulty switch is directly bypassed. By blocking the drive pulses of the two switches within the bridge arm, current freewheeling is achieved using anti-parallel freewheeling diodes, eliminating current bias and limiting overcurrent. The maximum transmission power expression at this time is: ; Maximum inductor current flowing through i LrSBAmax The expression is: ; As can be seen, after an open-circuit fault occurs on the secondary side of the dual active bridge, the maximum current flowing through the inductor is limited to the normal rated value. i Lmax Within this range, the circuit can operate normally.

[0057] In summary, this invention achieves rapid diagnosis, precise location, and non-stop fault-tolerant control of open-circuit faults in the front and rear stage switching transistors of solid-state transformers without increasing hardware costs, significantly improving the robustness and operational reliability of the system.

[0058] 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 them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for fault diagnosis and fault tolerance of solid-state transformers, characterized in that, The topology of the solid-state transformer includes n modules consisting of a front-stage cascaded H-bridge rectifier circuit and a rear-stage dual active bridge circuit; the method includes: S1. Sampling: Collect the grid-side voltage, grid-side current, DC coupling capacitor voltage of the cascaded H-bridge, as well as the transmission inductor current, primary-side bridge arm midpoint voltage, secondary-side bridge arm midpoint voltage, and power flow direction of the dual active bridge. S2. Front-end fault detection: Based on the switching state of the cascaded H-bridge and the DC coupling capacitor voltage, the grid-side voltage is estimated in real time, and the deviation between the grid-side voltage and the actual measured grid-side voltage is calculated. If the deviation exceeds the preset threshold, it is determined that the cascaded H-bridge has an open circuit fault, and the process proceeds to step S4; otherwise, the process proceeds to step S3. S3. Post-stage fault detection: Determine whether the bias type of the transmission inductor current matches the fault characteristics. If it does, determine that the dual active bridge has an open circuit fault and proceed to step S5; otherwise, determine that the system is operating normally and return to step S1. S4. Front-end fault-tolerant control: The modulation mode of the cascaded H-bridge is switched from phase-shifted unipolar SPWM to phase-shifted bipolar SPWM. The specific faulty switch is located by combining the grid current direction and output voltage characteristics. Then, the fault-tolerant operation is achieved by reconstructing the switch state and using the anti-parallel diodes of the healthy switch and the faulty switch to replace the fault path. S5. Post-fault diagnosis: Based on the bias characteristics of the transmission inductor current and the waveform distortion of the voltage at the midpoint of the primary and secondary bridge arms, locate the position of the switch in the dual active bridge that has an open circuit fault.

2. The solid-state transformer fault diagnosis and fault-tolerance method according to claim 1, characterized in that, The pre-stage cascaded H-bridge rectifier circuit includes: a grid-side AC voltage source connected in series with a grid-side inductor to the AC input terminal of the full-bridge rectifier circuit; the full-bridge rectifier circuit consists of a first switch, a second switch, a third switch, a fourth switch, and a first diode, a second diode, a third diode, and a fourth diode connected in anti-parallel to them respectively; the full-bridge rectifier circuit converts AC voltage into DC voltage as the pre-stage output, and a bus capacitor and a subsequent dual active bridge circuit are connected in parallel at the pre-stage output terminal; The subsequent dual active bridge circuit includes: a primary-side full-bridge circuit, an intermediate-stage transformer, a transmission inductor, a secondary-side full-bridge circuit, and an output capacitor. The primary-side full-bridge circuit consists of a fifth switch, a sixth switch, a seventh switch, an eighth switch, and a fifth diode, a sixth diode, a seventh diode, and an eighth diode connected in anti-parallel to each switch. Its DC input terminal is connected in parallel with the bus capacitor, and its AC output terminal is connected to the primary winding of the intermediate stage transformer via the transmission inductor. The secondary-side full-bridge circuit consists of a ninth switch, a tenth switch, an eleventh switch, a twelfth switch, and a ninth diode, a tenth diode, an eleventh diode, and a twelfth diode connected in anti-parallel to the switch. Its AC input terminal is connected to the secondary winding of the intermediate stage transformer, and its DC output terminal is connected in parallel to the output capacitor.

3. The solid-state transformer fault diagnosis and fault-tolerance method according to claim 2, characterized in that, The aforementioned front-end fault detection includes: S21. Calculate the equivalent logic signals of the bridge arms of the cascaded H-bridge. S A and S B : Define the gate drive signals for the first, second, third, and fourth switches in the cascaded H-bridge as follows: s 1. s 2. s 3. s 4. The complementary signals are respectively , , , 1 indicates conduction, and 0 indicates deactivation; when When the current flows into the H-bridge, the direction of the current is the same as the grid current; when At this time, the current direction is that the grid current flows out of the H-bridge; complementary signal ; Let the bridge arm containing the first and second switching transistors be the bridge arm. S A The bridge arm containing the third and fourth switching transistors is the bridge arm. S B , When, it indicates the bridge arm S A In the on state, similarly. When, it indicates the bridge arm S B It is in the on state; Based on the conduction state of the switches in the cascaded H-bridge, the equivalent logic signals of the bridge arms of the H-bridge are... S A and S B The bridge arm logic formula is: ； S A and S B The value can only be 0 or 1, corresponding to the on and off states of the bridge arm; S22. Calculate the estimated grid-side voltage of the cascaded H-bridge. V ane Compared with the measured grid-side voltage V in deviation e f : Grid-side estimated voltage V ane The calculation formula is: ； in, Vc This is the voltage of the DC coupling capacitor; Grid-side estimated voltage V ane Compared with the measured grid-side voltage V in deviation e f The calculation formula is: ； S23. Determine if the cascaded H-bridge device has an open circuit fault: Set threshold: ; in, V d For the forward voltage drop of the anti-parallel diode; like The fault flag is triggered, indicating that there is an open circuit fault in the cascaded H-bridge; like This is normal operating condition.

4. The solid-state transformer fault diagnosis and fault-tolerance method according to claim 1, characterized in that, The subsequent fault detection includes: analyzing the sampled dual active bridge transmission inductor current; if the peak value of the dual active bridge transmission inductor current is... If the voltage at the midpoint of the primary arm and the midpoint of the secondary arm of the dual active bridge exhibit significant waveform distortion, and the inductor current of the dual active bridge shows a fixed-direction bias, then it is determined that there is an open-circuit fault in the dual active bridge; otherwise, it is a normal operating condition.

5. The solid-state transformer fault diagnosis and fault-tolerance method according to claim 1, characterized in that, The modulation mode of the cascaded H-bridge is switched from phase-shifted unipolar SPWM to phase-shifted bipolar SPWM. The specific faulty switching transistor is located by combining the grid current direction and output voltage characteristics, including: After determining that an open-circuit fault has occurred in the cascaded H-bridge, the modulation mode of the cascaded H-bridge is switched from phase-shifted unipolar SPWM to phase-shifted bipolar SPWM; after switching the modulation mode, the operating data of a complete power grid frequency cycle is collected. According to the direction of the grid current The periodic data is divided into positive and negative half-cycles for analysis; when At this time, the current direction is that the grid current flows into the H-bridge, and the period data is the positive half-cycle; when At that time, the current direction is the grid current flowing out of the H-bridge, and the period data is the negative half-cycle; When the switch state is [ s 1, s When 3]=[1,0], under bipolar SPWM modulation, the output voltage of the fault-free cascaded H-bridge satisfies: ； When the switch state is [ s 1, s When 3]=[0,1], under bipolar SPWM modulation, the output voltage of the fault-free cascaded H-bridge satisfies: ； in, V 0 indicates the measured output voltage of the cascaded H-bridge; V c This is the voltage of the DC coupling capacitor; When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[1,0], if the measured output voltage of the cascaded H-bridge does not meet the requirements... If so, the faulty switch is determined to be the first switch. When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[1,0], if the measured output voltage of the cascaded H-bridge does not meet the requirements... If so, the faulty switch is determined to be the second switch. When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[0,1], if the measured output voltage of the cascaded H-bridge does not meet the requirements... If so, the faulty switch is determined to be the third switch. When the direction of the detected grid current And the switch state is [ s 1, s When 3]=[0,1], if the measured output voltage of the cascaded H-bridge does not meet the requirements... If so, the faulty switch is determined to be the fourth switch.

6. The solid-state transformer fault diagnosis and fault-tolerance method according to claim 5, characterized in that, Fault-tolerant operation is achieved by reconfiguring switch states and using anti-parallel diodes of healthy and faulty switches to replace faulty paths, including: The cascaded H-bridge is a fully controlled bridge topology. The first, second, third, and fourth switches, together with the first, second, third, and fourth diodes connected in anti-parallel, achieve bidirectional current transmission. An open circuit in a single switch will cause the bridge arm / diagonal where the faulty switch is located to lose its active switching capability, and the current path will be blocked. This will cause the output voltage of the cascaded H-bridge to lose the positive / negative half-cycle, introducing DC bias into the system. Fault-tolerant control is performed based on the power flow direction and the specific location of the faulty switch in the cascaded H-bridge: When the power flow direction When the faulty switch is in the same bridge arm, the other normal switch will continue to conduct, while the faulty switch will continue to be turned off, forming a stable current path through the anti-parallel diode. When the power flow direction When the faulty switch and the other switch in the same bridge arm are turned off, a stable current path is formed only through the anti-parallel diode.

7. The solid-state transformer fault diagnosis and fault-tolerance method according to claim 1, characterized in that, Based on the bias characteristics of the transmission inductor current and the waveform distortion of the midpoint voltage of the primary and secondary bridge arms, the location of the switch in the dual active bridge with an open-circuit fault is determined, including: S51. Calculate the inductor current under normal operating conditions of the dual active bridge: When the dual active bridge is operating normally, both the primary and secondary sides are driven by 50% duty cycle pulses, and the power is controlled by the shift ratio D. Under normal operating conditions, the transmission power of the dual active bridge is... P N The expression is: ； When the shift ratio D=0.5, the maximum transmission power of the dual active bridge is P Nmax and maximum inductor current i Lmax The expression is: ； ； When the shift ratio D=0.5, the maximum inductor current of the dual active bridge is i Lmax The expression is: ； in, n For transformer turns ratio, L r To transmit inductance, f s For switching frequency, u 1 represents the primary DC voltage. u 2 is the secondary DC voltage. i Lr To transmit inductor current; S52. Calculate the inductor current under a fault on the primary side of a dual active bridge: The primary side full-bridge circuit includes four switching transistors: the fifth, sixth, seventh, and eighth. The fault characteristics of the fifth and eighth switching transistors are completely identical, and the fault characteristics of the sixth and seventh switching transistors are completely identical. When the fifth switch is open, the forward direction i Lr Since the fifth switching transistor cannot pass through, the freewheeling second diode is forced to conduct, forming four new operating states. After a fault occurs in the primary-side full-bridge circuit, the current expression for the inductor current at critical time points is as follows: ； The fifth switching transistor in the primary-side full-bridge circuit. T 1. After the fault i Lr All values ​​are negative, indicating a clear negative bias, and the maximum inductor current is 2. i Lmax ; After the sixth switch in the primary-side full-bridge circuit fails... i Lr All values ​​are positive, indicating a clear positive bias; the maximum inductor current after the sixth switch on the primary side malfunctions is also 2. i Lmax ; S53. Calculate the inductor current under a fault on the secondary side of a dual active bridge: The primary-side full-bridge circuit includes four switches: the ninth, tenth, eleventh, and twelfth. The fault characteristics of the ninth and twelfth switches are completely identical, as are the fault characteristics of the tenth and eleventh switches. The following derivation uses an open-circuit fault in the ninth switch as an example: When the ninth switch is open, the forward direction i Lr Since the ninth switching transistor cannot pass through, the tenth freewheeling diode is forced to conduct, resulting in five new operating states. i Lr The slope of the change is consistent with the slope of the change after the original side fault occurs; After a fault occurs in the secondary-side full-bridge circuit, the current expression for the inductor current at critical time points is as follows: ； After a fault occurs in the ninth switch of the secondary-side full-bridge circuit i Lr All values ​​are positive, indicating a clear positive bias; the maximum value of the inductor current. Numerically, it is not exactly equal to i Lmax It is twice that of the second-side full-bridge circuit, and has a negative component; moreover, the midpoint voltage of the bridge arm in the secondary-side full-bridge circuit will show a significant plateau segment in each switching cycle. S54. Locate the fault on the primary and secondary sides of the dual active bridge: The bias direction, peak value, and midpoint voltage waveform of the secondary bridge arm of the dual active bridge transmission inductor in real time; If detected i Lr If there is a negative bias and no positive component, a voltage drop will occur at the midpoint of the primary side bridge arm, while the midpoint voltage of the secondary side bridge arm is normal. This corresponds to a fault in the fifth / eighth switch. If detected i Lr If the primary side bridge arm is positively biased and has no negative component, a voltage drop occurs at the midpoint of the primary side bridge arm. If the midpoint voltage of the secondary side bridge arm is normal, then the sixth / seventh switch is faulty. If detected i Lr If the voltage at the midpoint of the primary arm is normal when the voltage waveform at the midpoint of the secondary arm is positively biased and has a negative component, then the voltage waveform at the midpoint of the secondary arm shows a plateau segment, which corresponds to a fault in the ninth / twelfth switch. If detected i Lr If there is a negative bias and a positive component, the voltage at the midpoint of the primary arm is normal, but the voltage waveform at the midpoint of the secondary arm shows a plateau segment, then the fault corresponds to the 10th / 11th switch.

8. The solid-state transformer fault diagnosis and fault-tolerance method according to claim 7, characterized in that, After fault diagnosis, the following steps are also taken: if the fault is located on the primary side, the maximum value of the shift ratio is limited to 0.25 to suppress overcurrent; if the fault is located on the secondary side, the drive pulse of the fault switch transistor in the entire bridge arm is blocked, and only the bridge arm diode is allowed to continue to operate to eliminate current bias.

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