A four-port self-voltage balancer fault-tolerant circuit for bipolar dc microgrid and a control method thereof

Abstract

The application discloses a four-port self-voltage balancer fault-tolerant circuit for a bipolar direct-current microgrid and a control method thereof. The circuit comprises a first DAB branch, a second DAB branch, an LC fault-tolerant module and a controller. The first DAB branch is connected between a positive bus and a neutral line, the second DAB branch is connected between the neutral line and a negative bus, and the LC fault-tolerant module is connected between the two DAB branches. The controller judges the system state according to bus voltage, port current and fault current, and controls the circuit to switch between normal voltage balancing, port fault isolation and bus fault current limiting modes. In normal state, the LC fault-tolerant module transmits mismatched power to realize voltage balancing. In low-voltage short-circuit state, the fault branch is locked, and the LC module maintains voltage balancing of the non-fault branch. In medium-voltage short-circuit state, all switches are locked, a freewheeling diode is used to form a freewheeling channel, the fault current flows reversely into a sub-module capacitor, a fault current limiting inductor bears reverse voltage, and the fault current rapidly decreases. The application has voltage balancing and fault tolerance capabilities.

Description

Technical Field

[0001] This invention belongs to the field of DC power distribution, power electronic conversion and bipolar DC microgrid fault protection technology, and particularly relates to a fault tolerance method that uses an LC fault tolerance module to achieve bipolar bus voltage balance, low-voltage port fault isolation and DC bus short-circuit fault current suppression. Specifically, it is a fault tolerance circuit and control method for a four-port self-voltage balancer for bipolar DC microgrids. Background Technology

[0002] Bipolar DC microgrids typically include a positive bus, a neutral bus, and a negative bus. They can provide multiple voltage levels between the positive and neutral lines, between the negative and neutral lines, and between the positive and negative lines. They have advantages such as flexible power supply, high reliability, and high power quality, and are suitable for scenarios such as data centers, charging stations, distributed photovoltaics, energy storage systems, and DC load access.

[0003] In actual operation, distributed power sources, energy storage devices, and DC loads in bipolar DC microgrids are often connected to the positive and negative buses in an asymmetrical manner. When the loads on the positive and negative sides are inconsistent, or when a load on one side is dynamically switched, a power imbalance will occur between the positive and negative buses, leading to an increase in neutral current and a voltage shift between the positive and negative poles. This phenomenon will reduce the power supply quality on the load side and affect the safe and stable operation of the system.

[0004] Existing technologies typically employ voltage balancers to transfer unbalanced power between the positive and negative buses to maintain bipolar bus voltage balance. However, existing voltage balancers primarily focus on voltage equalization under normal operating conditions and lack the ability to withstand fault conditions. Especially when a port short-circuit fault or bus short-circuit fault occurs in a bipolar DC microgrid, a fault discharge loop may form between the bus capacitor, line inductance, and converter ports, causing the fault current to rise rapidly and endangering the safety of power switching devices and load equipment. Summary of the Invention

[0005] To address the problems of insufficient fault tolerance of voltage balancers, difficulty in isolating local port short-circuit faults, and excessively rapid rise of DC bus short-circuit fault current in existing bipolar DC microgrids, this invention provides a four-port self-voltage balancer fault tolerance circuit and its control method for bipolar DC microgrids. By setting up an LC fault tolerance module and controller, the system achieves voltage balance between the positive and negative buses during normal operation, achieves fault isolation during local port faults, and achieves rapid suppression of fault current during DC bus short-circuit faults.

[0006] According to one aspect of the present invention, a four-port self-voltage balancer fault tolerance circuit for a bipolar DC microgrid is provided, comprising a first DAB branch, a second DAB branch, an LC fault tolerance module, and a controller; wherein, the first DAB branch is connected between the positive bus and the neutral line of the bipolar DC microgrid, and the second DAB branch is connected between the neutral line and the negative bus of the bipolar DC microgrid; the LC fault tolerance module is connected between the first DAB branch and the second DAB branch; the controller is connected to the first DAB branch, the second DAB branch, and the LC fault tolerance module respectively; the controller is used to determine the system operating state based on the bipolar bus voltage, port current, and bus fault current, and to control the first DAB branch, the second DAB branch, and the LC fault tolerance module to switch between normal voltage equalization mode, port fault isolation mode, and bus fault current limiting mode.

[0007] As a further technical solution, the LC fault tolerance module includes a first inductor, a first capacitor, at least one freewheeling diode, and a switching branch connected in series.

[0008] As a further technical solution, the first DAB branch and the second DAB branch each include a DAB sub-module, and each DAB sub-module includes a balancing inductor, a balancing capacitor and a freewheeling device.

[0009] As a further technical solution, a positive bus capacitor is connected between the positive bus and the neutral line, and a negative bus capacitor is connected between the neutral line and the negative bus.

[0010] According to one aspect of the present invention, a control method for a four-port self-voltage balancer fault tolerance circuit for a bipolar DC microgrid is provided, comprising: detecting bipolar bus voltage, port current and bus fault current;

[0011] The system's operating status is determined based on the detection results. Under normal operating conditions, the first and second DAB branches are controlled to operate, and mismatch power is transmitted between the first and second DAB branches through the LC fault tolerance module to maintain a balance between the output voltages of the first and second DAB branches. When a short-circuit fault is detected at the low-voltage input port, the switching devices in the DAB branch with the short-circuit fault are blocked, and the mismatch power continues to be transmitted between the DAB branches without faults through the LC fault tolerance module. When a short-circuit fault is detected at the medium-voltage DC output terminal, all switching devices are blocked, and a fault freewheeling channel is formed using the freewheeling diode in the LC fault tolerance module.

[0012] As a further technical solution, the detection of the short circuit fault at the low-voltage input port includes: determining that the input terminal voltage of the corresponding DAB branch is lower than a preset voltage threshold, or determining that the input current of the corresponding DAB branch has an abnormal decrease or reverse surge.

[0013] As a further technical solution, the detection of the short circuit fault at the medium-voltage DC output terminal includes: determining that the fault current at the medium-voltage DC output terminal exceeds a preset current threshold.

[0014] As a further technical solution, after all switching devices are blocked in case of a short circuit fault at the medium-voltage DC output terminal, the freewheeling diode in the LC fault tolerance module, together with the output capacitor of each DAB branch and the current-limiting inductor on the medium-voltage side, constitutes the fault energy transfer path.

[0015] As a further technical solution, in the fault energy transfer path, the fault current flows in reverse into the output capacitor of each DAB branch.

[0016] As a further technical solution, in the fault energy transfer path, the medium-voltage side current-limiting inductor is subjected to reverse voltage, causing the fault current to decrease until the fault circuit is cut off.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: First, the present invention can realize the unbalanced power transmission between the positive and negative buses of the bipolar DC microgrid by using LC fault-tolerant branches, which reduces the number of devices and control complexity caused by the multi-branch structure.

[0018] Secondly, this invention can not only achieve bipolar bus voltage balance under normal operating conditions, but also participate in fault protection under port short-circuit faults and bus short-circuit faults.

[0019] Third, when a local port short-circuit fault occurs, the present invention can isolate the faulty port and maintain the continuous power supply of the non-faulty side bus, thereby improving the power supply continuity of the bipolar DC microgrid.

[0020] Fourth, when a DC bus short-circuit fault occurs, the present invention can utilize the equalizing capacitor in the LC fault withstand branch to be connected in reverse to the fault circuit, so that the line inductor is subjected to reverse voltage, thereby achieving rapid attenuation of the fault current.

[0021] Fifth, this invention can be deployed at load access nodes, distributed power access nodes, or energy storage access nodes of bipolar DC microgrids, and is applicable to data centers, charging stations, photovoltaic-storage DC-flexible systems, and low-voltage DC power distribution systems. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying 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.

[0023] Figure 1 This is a schematic diagram of a bipolar DC microgrid system structure provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of a four-port voltage balancer topology provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the voltage balancer current path under low-voltage fault mode provided in an embodiment of the present invention; Figure 4 These are key waveforms illustrating the operation of the voltage balancer under low-voltage fault mode, as provided in this embodiment of the invention. Figure 5 The voltage balancer transmission current versus phase shift angle curve provided in this embodiment of the invention; Figure 6 The following is a simulation result diagram of a low-voltage side short-circuit fault provided in an embodiment of the present invention; Figure 7 This is an equivalent circuit diagram of the balancer operating under medium-voltage fault mode provided in an embodiment of the present invention; Figure 8 This is a schematic diagram of the voltage balancer current path under medium-voltage fault mode provided in an embodiment of the present invention; Figure 9 The simulation results of the medium-voltage side short-circuit fault are shown in the embodiment of the present invention. Detailed Implementation

[0024] The terms “comprising” and “having”, and any variations thereof, in the specification, claims, and accompanying drawings of this invention are intended to cover a non-exclusive inclusion, such as a process, method, system, product, or apparatus that includes a series of steps or units, not necessarily limited to those explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0025] The block diagrams shown in the accompanying drawings are merely functional entities and do not necessarily correspond to physically independent entities. That is, these functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices. The flowcharts shown in the accompanying drawings are merely illustrative and do not necessarily include all content and operations / steps, nor do they necessarily have to be performed in the described order. For example, some operations / steps can be decomposed, while others can be combined or partially combined; therefore, the actual execution order may change depending on the specific circumstances.

[0026] 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 some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. In addition, the technical features of the various embodiments or individual embodiments provided by the present invention can be arbitrarily combined to form new technical solutions. Such combinations are not bound by the order of steps and / or structural composition patterns, but must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0027] like Figure 1 As shown, the bipolar DC microgrid includes a positive bus P, a neutral bus Z, and a negative bus N. A positive power supply loop is formed between the positive bus P and the neutral bus Z, and a negative power supply loop is formed between the negative bus N and the neutral bus Z. A bipolar power supply loop is formed between the positive bus P and the negative bus N. Distributed photovoltaic systems, energy storage systems, charging loads, and DC loads can be connected to different buses according to voltage level requirements.

[0028] like Figure 2 As shown, this embodiment provides a four-port self-voltage balancer fault tolerance circuit for bipolar DC microgrids, including a first DAB branch, a second DAB branch, an LC fault tolerance module, and a controller.

[0029] The first DAB branch is connected between the positive bus P and the neutral bus Z of the bipolar DC microgrid, and the second DAB branch is connected between the neutral bus Z and the negative bus N of the bipolar DC microgrid. A positive bus capacitor is connected between the positive bus P and the neutral bus Z, and a negative bus capacitor is connected between the neutral bus Z and the negative bus N.

[0030] The first DAB branch and the second DAB branch each include a DAB submodule (i.e., a power submodule), and each DAB submodule includes a balancing inductor, a balancing capacitor, and a freewheeling device. Specifically, each DAB submodule is a dual active bridge converter structure, with its primary side connected to the low-voltage input port and its secondary side connected to the LC fault tolerance module through a half-bridge circuit.

[0031] The LC fault tolerance module is connected between the first DAB branch and the second DAB branch. The LC fault tolerance module includes a first inductor, a first capacitor, at least one freewheeling diode, and a switching branch connected in series. This switching branch works with the bridge arm of the DAB submodule to enable or disable the LC fault tolerance module under controller commands. During normal operation, the LC fault tolerance module is used to transfer mismatched power between adjacent power submodules, achieving output voltage balancing of the power submodules.

[0032] The controller is connected to the first DAB branch, the second DAB branch, and the LC fault tolerance module, respectively. The controller is used to collect the bipolar bus voltage, the port current of each DAB branch, and the bus fault current at the medium-voltage DC output terminal. Based on these electrical quantities, it determines the system operating status and controls the first DAB branch, the second DAB branch, and the LC fault tolerance module to switch between normal voltage equalization mode, port fault isolation mode, and bus fault current limiting mode.

[0033] This embodiment provides a control method for the fault tolerance circuit of the above-mentioned four-port self-voltage balancer, including the following steps.

[0034] (a) System status detection and mode determination

[0035] The controller monitors the bipolar bus voltage (positive-to-neutral voltage, negative-to-neutral voltage), the voltage and current at the low-voltage input ports of each DAB branch, and the bus fault current at the medium-voltage DC output terminal in real time. Based on the monitoring results, the controller determines the system's operating status.

[0036] When the bipolar bus voltage deviation is within the allowable range and the current at each port is normal, it is judged to be in normal operating condition;

[0037] When the voltage at the low-voltage input port of a certain DAB branch is lower than the preset voltage threshold (e.g., 0.3 to 0.5 times the rated voltage), or when the input current drops abnormally or reverses, it is determined that a short circuit fault has occurred at the low-voltage input port of that DAB branch.

[0038] When the fault current at the medium-voltage DC output terminal exceeds the preset current threshold (e.g., 2 to 3 times the rated current), it is determined that a short circuit fault has occurred at the medium-voltage DC output terminal.

[0039] (II) Normal Operating Mode

[0040] Under normal operating conditions, the controller controls the first and second DAB branches to operate, enabling the two DAB submodules to connect to the system. Simultaneously, the controller keeps the LC fault tolerance module in a conducting state, transmitting mismatch power between the first and second DAB branches through the LC fault tolerance module.

[0041] Specifically, under normal operating conditions, the control system collects the output voltage of each power submodule and controls the operation of the LC fault tolerance module based on the deviation between each output voltage and the target equalization voltage. When the output voltage of a power submodule is higher than the target equalization voltage, the LC fault tolerance module transfers the excess energy of that power submodule to an adjacent low-voltage power submodule; when the output voltage of a power submodule is lower than the target equalization voltage, the LC fault tolerance module compensates for the energy of that power submodule. Thus, the output voltages of each power submodule remain balanced, thereby achieving output voltage balance between the first DAB branch and the second DAB branch.

[0042] (III) Short-circuit fault handling mode for low-voltage input port

[0043] When the controller detects a short circuit fault at the low-voltage input port of the first DAB branch or the second DAB branch (e.g., the input voltage is lower than 0.3 times the rated voltage, or the input current experiences a reverse surge), the controller immediately blocks all switching devices in the DAB branch where the short circuit fault occurred, preventing the medium-voltage side from transmitting power to the low-voltage fault point.

[0044] When a short-circuit fault occurs at the low-voltage port, the input voltage of the faulty power submodule drops rapidly, and the input current exhibits abnormal changes. Upon detecting this anomaly, the control system determines that a low-voltage port short-circuit fault has occurred in the corresponding power submodule. During the fault, all switches in the submodule except Q1 and Q2 need to be latched. Ignoring the DAB submodule circuit, the operation of the proposed four-port balancer can be divided into four modes (a, b, c, and d) based on the state of the switches, as follows: Figure 3 As shown, its working waveform is as follows Figure 4 As shown, the blue arrow represents the positive direction of the current.

[0045] According to the state-space averaging method, we can deduce i B1 and i B2 The average value is:

[0046]

[0047] in, and The inductor current is the current flowing through the LC branch between the upper and lower capacitors. For output voltage, For switching frequency, The inductance value of the LC branch. This is the phase shift angle.

[0048] Therefore, it can be seen that within one switching cycle, power is transferred from the upper capacitor to the lower capacitor or vice versa, while the capacitor energy on the LC fault-tolerant module remains unchanged. Draw... about The schematic diagram is as follows Figure 5 As shown. When When power is transferred from the upper capacitor to the lower capacitor; when At this time, power is transferred from the lower capacitor to the upper capacitor. For ease of controller design, the capacitor shown in the image should be selected. The voltage is adjusted accordingly. Based on the above analysis, the LC power equalizer type fault tolerance module also has voltage equalization functionality, ensuring that its output voltage does not drop when a low-voltage side fault occurs, and the operation of other sub-modules is not affected. A ±2 kV bipolar DC microgrid was built in MATLAB / Simlink. Short-circuit faults at the medium and low voltage ports were simulated using a series connection of a switch and a resistor. The simulation parameters are shown in Table 1.

[0049] Table 1. Parameters of a four-port voltage balancer with LC fault tolerance topology

[0050]

[0051] Simulation results of low-voltage side short-circuit fault are as follows Figure 6 As shown, the short-circuit switches on the input ports of the two submodules are turned on at t = 50 ms. Assuming a fault is detected 5 ms later, the faulty submodule stops operating. It can be seen that... Figure 6 In (a), the voltage of the two faulty submodules drops rapidly to 0. Figure 6 (b) shows the input current of the submodule. When a fault occurs, the current of both submodules also drops rapidly and there is a certain reverse surge current. The reason is that the input capacitor discharges rapidly to the fault point. Figure 6 (c) shows the output voltage of the submodule. Under the action of the LC power equalizer, although the output voltage of the submodule fluctuates to some extent, it can eventually remain stable and balanced, with the fluctuation amplitude not exceeding 170 V. This verifies that the LC fault-tolerant connector can continue to maintain the operation of non-faulty modules when there is a fault at the low-voltage port, thus improving system reliability.

[0052] (iv) Short circuit fault handling mode at medium voltage DC output terminal

[0053] When a short-circuit fault occurs on the medium-voltage bus side, since the fault occurrence time is relatively short, the electrical quantities of the sub-module and the medium-voltage DC bus side can be approximated as constant. The equivalent circuit diagram is as follows: Figure 7 As shown, where R f For short-circuit resistance, its current i f This represents the fault current. The main purpose of the fault tolerance module is to protect the converter itself from damage. Therefore, in the following analysis of medium-voltage port short-circuit faults, it is assumed that the medium-voltage DC bus is disconnected from the circuit after the short-circuit fault occurs.

[0054] The process of LC fault tolerance topology disconnecting a medium-voltage side fault includes four stages, such as... Figure 8 As shown. The fault occurrence stages are as follows: Figure 8 As shown in (a), before the LC power equalizer type fault tolerance module takes short-circuit fault response measures, the capacitors of the sub-modules connected in series will discharge through the short-circuit point, the capacitor voltage will drop, and the short-circuit fault current i f Continued to rise;

[0055] Short-circuit fault current freewheeling stage 1 Figure 8 As shown in (b), when a short-circuit fault is detected, all switching transistors are blocked and the submodule stops operating. Since the inductor current cannot change abruptly, the anti-parallel diode of Q2 and diode D in the submodule conduct, and the submodule capacitor and bus inductance L... bus Short-circuit resistance R f Together, they form a freewheeling path. During this stage, the bus inductor experiences a large reverse voltage, the fault current drops rapidly, and fault energy is transferred to the sub-module capacitor, causing its voltage to rise. Simultaneously, the LC fault withstand module capacitor forms a discharge circuit under diode conduction conditions, releasing stored energy. However, since this discharge circuit relies on diode conduction, the LC fault withstand module capacitor cannot discharge to the short-circuit point independently. Therefore, when the fault current drops to 0, the LC fault withstand module discharge circuit is simultaneously cut off, without affecting the fault protection function.

[0056] Short-circuit fault current freewheeling stage 2 Figure 8 As shown in (c), during the freewheeling phase 1 of the short-circuit fault current, the inductor of the LC fault withstand module bears the voltages of the two sub-module capacitors plus its own capacitor voltage. The branch inductor current increases rapidly. When the current of the LC fault withstand module equals the fault current, diode D1 on sub-module 1 will be cut off, and the capacitor of sub-module 1 will exit the freewheeling circuit. Its freewheeling current is entirely transferred to the LC fault withstand module, so the current of diode D2 on sub-module 2 will also drop to 0 in a short time. During this phase, the capacitor voltage of sub-module 1 remains constant, the capacitor voltage of sub-module 2 increases, and the capacitor voltage of the LC fault withstand module decreases.

[0057] Short circuit fault disconnection Figure 8 As shown in (d), when the short-circuit fault current drops to 0, the freewheeling diode is turned off due to reverse voltage, and no current flows in the circuit, thus the short-circuit fault is cut off.

[0058] Simulation waveform of medium voltage side short circuit fault as follows Figure 9 As shown, the fault is simulated by turning on the medium-voltage side short-circuit switch at t = 5 ms. Figure 9 The voltage at the output terminal of the submodule in (a) drops, while Figure 9 The fault current in (b) rises rapidly because the submodule output capacitor discharges in series to the fault point. Assume the fault is detected and all switches are blocked after 0.5 ms. At this time, under the action of the fault-tolerant module, the fault current flows in reverse into the submodule capacitor, causing its voltage to rise, while the medium-voltage side inductor, due to the large reverse voltage, experiences a rapid drop in fault current to 0. The entire process from the start of the protection measure to the fault being cut off takes 0.8 ms. This demonstrates that the LC fault-tolerant topology can transfer fault energy to the submodule capacitor and quickly cut off the fault circuit when a medium-voltage side short-circuit fault occurs, protecting the equipment itself.

[0059] This invention, through the aforementioned method, enables the LC power equalizer to simultaneously possess voltage equalization functionality under normal operation, the ability to maintain operation of non-faulty modules under low-voltage port short-circuit faults, and the ability to rapidly cut off fault current under medium-voltage port short-circuit faults. This method can improve the fault tolerance and operational reliability of stand-alone photovoltaic medium-voltage DC-DC converters.

[0060] In summary, this invention discloses a fault tolerance topology and control method for an LC power equalizer for bipolar DC microgrid voltage balancers, belonging to the field of power electronic converter technology. Addressing the problem of insufficient short-circuit fault tolerance at the ports of existing voltage balancers, this invention, based on a device-multiplexed LC power equalizer, adds a diode D to the secondary side of each DAB submodule. k and switching transistor M k The newly formed bridge arm constructs an indirect series output structure for submodules; during normal operation, the switching transistor M... k Conducting diode D k The system shuts down while retaining the original submodule power transmission and LC fault-tolerant module voltage equalization functions. In the event of a low-voltage port short-circuit fault, the DAB switch of the faulty submodule is blocked, and power is transferred between submodules through the LC power equalizer to maintain output voltage balance across all submodules, ensuring continuous operation of non-faulty submodules. In the event of a medium-voltage port short-circuit fault, all switches are blocked, and a freewheeling circuit is formed using diodes and anti-parallel diodes of the switches to transfer fault energy to the submodule output capacitor, cutting off the fault current within microseconds. This invention eliminates the need for a large number of additional switching devices, simultaneously achieving uninterrupted operation under low-voltage port faults and rapid self-protection under medium-voltage port faults, significantly improving the converter's reliability and operating efficiency.

[0061] 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 therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the technical solutions of the embodiments of the present invention.

Claims

1. A four-port self-voltage balancer fault tolerance circuit for bipolar DC microgrids, characterized in that, The system includes a first DAB branch, a second DAB branch, an LC fault tolerance module, and a controller. The first DAB branch is connected between the positive bus and the neutral line of the bipolar DC microgrid, and the second DAB branch is connected between the neutral line and the negative bus of the bipolar DC microgrid. The LC fault tolerance module is connected between the first DAB branch and the second DAB branch. The controller is connected to the first DAB branch, the second DAB branch, and the LC fault tolerance module. The controller is used to determine the system operating status based on the bipolar bus voltage, port current, and bus fault current, and to control the first DAB branch, the second DAB branch, and the LC fault tolerance module to switch between normal voltage equalization mode, port fault isolation mode, and bus fault current limiting mode.

2. The four-port self-voltage balancer fault tolerance circuit for bipolar DC microgrids according to claim 1, characterized in that, The LC fault tolerance module includes a first inductor, a first capacitor, at least one freewheeling diode, and a switching branch connected in series.

3. The four-port self-voltage balancer fault tolerance circuit for bipolar DC microgrids according to claim 1, characterized in that, The first DAB branch and the second DAB branch each include a DAB submodule, and each DAB submodule includes a balancing inductor, a balancing capacitor and a freewheeling device.

4. The four-port self-voltage balancer fault tolerance circuit for bipolar DC microgrids according to claim 1, characterized in that, A positive bus capacitor is connected between the positive bus and the neutral line, and a negative bus capacitor is connected between the neutral line and the negative bus.

5. A control method for a four-port self-voltage balancer fault tolerance circuit in a bipolar DC microgrid, characterized in that, include: Detect bipolar bus voltage, port current, and bus fault current; Determine the system's operating status based on the test results; Under normal operating conditions, the first DAB branch and the second DAB branch are controlled to work, and the mismatch power is transmitted between the first DAB branch and the second DAB branch through the LC fault tolerance module, so that the output voltage of the first DAB branch and the output voltage of the second DAB branch are kept balanced. When a short-circuit fault is detected at the low-voltage input port, the switching devices in the DAB branch where the short-circuit fault occurred are blocked, and the mismatch power continues to be transmitted between the DAB branches where the fault did not occur through the LC fault tolerance module. When a short-circuit fault is detected at the medium-voltage DC output terminal, all switching devices are locked out, and the freewheeling diode in the LC fault tolerance module is used to form a fault freewheeling channel.

6. The control method for a four-port self-voltage balancer fault tolerance circuit for a bipolar DC microgrid according to claim 5, characterized in that, The detection of short-circuit faults at the low-voltage input port includes: determining that the input voltage of the corresponding DAB branch is lower than a preset voltage threshold, or determining that the input current of the corresponding DAB branch experiences an abnormal drop or reverse surge.

7. The control method for a four-port self-voltage balancer fault tolerance circuit for a bipolar DC microgrid according to claim 5, characterized in that, The detection of short-circuit faults at the medium-voltage DC output terminal includes: determining that the fault current at the medium-voltage DC output terminal exceeds a preset current threshold.

8. The control method for a four-port self-voltage balancer fault tolerance circuit for a bipolar DC microgrid according to claim 5, characterized in that, After all switching devices are blocked by a short-circuit fault at the medium-voltage DC output terminal, the freewheeling diode in the LC fault tolerance module, together with the output capacitor of each DAB branch and the current-limiting inductor on the medium-voltage side, constitutes the fault energy transfer path.

9. The control method for a four-port self-voltage balancer fault tolerance circuit for a bipolar DC microgrid according to claim 8, characterized in that, In the fault energy transfer path, the fault current flows in reverse into the output capacitor of each DAB branch.

10. The control method for a four-port self-voltage balancer fault tolerance circuit for a bipolar DC microgrid according to claim 9, characterized in that, In the fault energy transfer path, the medium-voltage side current-limiting inductor is subjected to a reverse voltage, causing the fault current to decrease until the fault circuit is disconnected.

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