Battery energy storage converter and fault tolerant control method

Abstract

This invention relates to the field of charge / discharge power conversion control for electrochemical energy storage batteries, and provides a battery energy storage converter and fault-tolerant control method. The converter includes multiple cascaded sub-units, each comprising a battery module, a PWM rectifier stage, a DC boost stage, and a switching switch. The PWM rectifier stage contains a series capacitor on its DC side, and the DC boost stage is a resonant soft-switching circuit. By switching the state of the switching switch, the battery module can be directly connected to the PWM rectifier stage or connected to the DC boost stage. This invention eliminates boost losses during normal operation and utilizes the resonant soft-switching circuit to efficiently boost and compensate for voltage during faults, significantly reducing switching losses and improving system operating efficiency and reliability.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage battery charge and discharge power conversion technology, and relates to a battery energy storage converter and a fault-tolerant control method. Background Technology

[0002] With the development of new energy power generation technologies, energy storage systems play a crucial role in addressing issues such as wind and solar power curtailment and the spatiotemporal distribution of energy. In high-power energy storage systems, multiple battery modules are typically connected in series to form a high voltage level. However, if the state of charge (SOC) of the series-connected battery modules is uneven, shutting down the entire battery module cluster only when one module is fully charged or fully discharged will severely reduce battery utilization efficiency. Continuing to charge or discharge will lead to overcharging or over-discharging, potentially causing battery overheating or even combustion and explosion. Furthermore, if the monitoring components integrated into the battery module malfunction, the module's operation must be stopped immediately to ensure system safety.

[0003] In existing technologies, configuring a corresponding sub-unit converter for each battery module and cascading them to form a module-cascaded multilevel converter is an effective solution. This solution facilitates balanced control of module states and allows for temporary disconnection of faulty units through bypass functionality. However, in low-voltage systems, when a sub-unit fails and is bypassed, maintaining the full AC voltage output of the entire system typically requires increasing the DC bus voltage of other sub-units. Traditional solutions often use a single-stage boost converter to handle the power of all battery modules, but this boost converter suffers from high switching device losses and high heat generation during operation, increasing the system's size, weight, and cost, and failing to meet the requirements of efficient and compact design. Therefore, how to reduce losses in the boost stage and improve the overall system efficiency while achieving fault tolerance and voltage compensation has become an urgent technical problem to be solved. Summary of the Invention

[0004] The purpose of this invention is to solve the technical problems of high loss and low efficiency of boost converters after fault bypass in the prior art, and to provide a battery energy storage converter and a fault-tolerant control method.

[0005] To achieve the above objectives, the present invention employs the following technical solution: The present invention provides a battery energy storage converter, comprising several phases connected in a star or delta configuration; each phase includes multiple cascaded sub-units, and each sub-unit includes: a battery module, a PWM rectifier stage, a DC boost stage, a first switching switch, and a second switching switch; The AC side of the PWM rectifier stage is cascaded with the PWM rectifier stages of other sub-units. The DC side of the PWM rectifier stage includes a first capacitor and a second capacitor connected in series. The connection point of the first capacitor and the second capacitor forms the midpoint of the DC bus. The positive terminal of the first capacitor forms the positive bus, and the negative terminal of the second capacitor forms the negative bus. The DC boost stage is a resonant soft-switching DC boost circuit. The input terminal of the DC boost stage is selectively connected to the battery module, and the output terminal of the DC boost stage is connected to both ends of the first capacitor. The first switching switch is connected between the positive terminal of the battery module and the positive terminal of the first capacitor; the second switching switch is connected between the positive terminal of the battery module and the midpoint of the DC bus; the negative terminal of the battery module is connected to the negative terminal of the second capacitor; when the first switching switch is closed and the second switching switch is open, the battery module is directly connected to the PWM rectifier stage, and the DC boost stage does not work; when the first switching switch is open and the second switching switch is closed, the battery module is connected to the input terminal of the DC boost stage, and the DC boost stage works to boost the voltage between the positive and negative terminals of the first capacitor.

[0006] The above solution, by setting a first switching switch and a second switching switch in conjunction with a resonant soft-switching DC boost circuit, achieves flexible switching between the direct-connection low-loss mode during normal operation and the boost compensation mode after fault bypass, thus solving the problem of high losses caused by the continuous operation of the Boost circuit in the traditional solution.

[0007] In one embodiment, the DC boost stage is a resonant soft-switching DC boost circuit with partial power processing, comprising: a first half-bridge, the DC side of which is connected between the positive bus and the negative bus; a second half-bridge, the DC side of which is connected between the midpoint of the DC bus and the negative bus; and an LC resonant branch, which is connected between the AC output terminal of the first half-bridge and the AC output terminal of the second half-bridge.

[0008] The above scheme further reduces the switching losses during the boost conversion process by using a resonant soft-switching circuit structure that handles part of the power.

[0009] In one implementation, both the first half-bridge and the second half-bridge include a number of power switching transistors connected in series.

[0010] In one implementation, the first half-bridge and the second half-bridge are alternately turned on, and phase-shift control is used between the first half-bridge and the second half-bridge to control the switching frequency of the power switch to operate in the inductive region above the LC resonant frequency, so as to realize the fully soft switching of the power switch.

[0011] The above scheme ensures that the power switching devices achieve full soft switching through phase-shift control and inductive region operation mode, which significantly improves the conversion efficiency.

[0012] In one implementation, each subunit also includes a bypass switch connected in parallel between two AC terminals of the PWM rectifier stage; when the subunit fails, the bypass switch closes, disconnecting the subunit from the cascaded subunits.

[0013] The above solution enables rapid disconnection of faulty sub-units through a bypass switch, ensuring the continuous operation capability of the system.

[0014] In one implementation, each subunit further includes a discharge resistor connected in parallel between the positive and negative terminals of the first capacitor, for discharging the voltage of the first capacitor to zero when the DC boost stage is not operating.

[0015] The above solution ensures that the voltage of the first capacitor is zero in the direct connection mode by using a discharge resistor, thus avoiding the impact of voltage floating on the system.

[0016] Furthermore, the present invention also provides a fault-tolerant control method, comprising the following steps: Step 1: When the battery energy storage converter is running normally, control all sub-units to work in the first mode, that is, control the first switching switch of each sub-unit to be closed and the second switching switch to be open, and block the DC boost stage, so that the battery module is directly connected to the PWM rectifier stage; Step 2: When a fault is detected in a sub-unit of a phase, the faulty sub-unit is controlled to enter the bypass state, and the phase is disconnected from the power grid. At the same time, the PWM rectifier stage of all sub-units in the phase is blocked. Step 3: For each normal sub-unit in this phase that has not experienced a fault, execute the second mode: first disconnect the first switching switch, and after confirming that it is disconnected, close the second switching switch; Step 4: Activate the DC boost stage of the normal sub-unit to increase the voltage across the first capacitor to compensate for the voltage loss caused by the bypass of the faulty sub-unit, so that the total DC bus voltage of the phase is restored to the level before the fault.

[0017] As one implementation method, Step 2, controlling the faulty subunit to enter the bypass state, specifically includes: controlling the power switching devices in at least two PWM rectifier stages of the faulty subunit to conduct, forming a current path; then closing the bypass switch of the faulty subunit; and after the bypass switch is closed by feedback, turning off the power switching devices in all PWM rectifier stages of the faulty subunit.

[0018] In one implementation, in Step 3, after disconnecting the first switching switch and before closing the second switching switch, a waiting delay is performed to ensure that the first switching switch is completely disconnected.

[0019] The above solution ensures reliable execution of the switching action by delaying the time, thus preventing shoot-through failure.

[0020] In one implementation, in Step 4, the DC boost stage increases the voltage across the first capacitor by adjusting the phase shift angle of the resonant soft-switching DC boost circuit, so as to stabilize the voltage of the first capacitor at a preset target value.

[0021] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a battery energy storage converter, which adopts a multi-stage cascaded sub-unit structure, allowing for flexible expansion of voltage levels to adapt to medium- and high-voltage energy storage grid-connected scenarios, thus improving system adaptability and modularity. The DC side of the PWM rectifier stage uses a series connection of a first capacitor and a second capacitor to divide the voltage, forming a positive bus, a negative bus, and a bus midpoint, providing a stable voltage division basis for voltage compensation. Two operating modes are switched via a first and a second switching switch. Under normal operating conditions, the battery module is directly connected to the PWM rectifier stage, completely eliminating DC boost stage losses and significantly improving system efficiency. The DC boost stage output is only connected to the first capacitor, achieving partial power processing, reducing the power rating and component stress of the boost circuit, and decreasing size and cost.

[0022] Furthermore, the DC boost stage employs partial power handling and a resonant soft-switching structure, handling only the power required for compensation, further reducing switching and conduction losses. The first and second half-bridges are connected across the positive-negative bus and the midpoint-negative bus, respectively, resulting in a symmetrical circuit structure, balanced voltage division, and stronger bus voltage stability. The LC resonant branch connects the AC terminals of the two half-bridges, forming a resonant power transmission path, providing the necessary circuit conditions for achieving full soft switching. The overall topology is simple, with fewer components, reducing control complexity and failure probability, and improving converter reliability and power density.

[0023] Furthermore, the alternating conduction and phase-shifting control of the first and second half-bridges enable continuous and linear adjustment of the output voltage, resulting in high control precision and fast dynamic response. The switching frequency operates in the inductive region above the LC resonant frequency, ensuring that all power switching devices achieve zero-voltage switching (ZVS) soft switching.

[0024] Furthermore, the sub-unit is equipped with a bypass switch, which can quickly short-circuit and disconnect the faulty sub-unit in case of a fault, without affecting the normal operation of other sub-units; the bypass switch is connected in parallel to the AC terminal of the PWM rectifier stage, and the disconnection method is direct and reliable, realizing fault isolation and uninterrupted system operation. Attached Figure Description

[0025] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the battery energy storage converter system structure according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the sub-unit main power circuit topology according to an embodiment of the present invention; Figure 3 This is a flowchart of the fault-tolerant control method according to an embodiment of the present invention. Detailed Implementation

[0027] 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, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0028] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0029] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0030] All terms used in this invention are explained herein and will not be repeated hereafter: PWM stands for Pulse Width Modulation. MOSFET: The full English name is Metal-Oxide-Semiconductor Field-Effect Transistor. IGBT: The full English name is Insulated Gate Bipolar Transistor. ZVS: The full English name is Zero Voltage Switching. LC: The full English name is Inductance-Capacitance, and the full Chinese name is Inductance-Capacitance. SOC stands for State of Charge.

[0031] The present invention will now be described in further detail with reference to the accompanying drawings: like Figure 1 and Figure 2 As shown, this embodiment provides a battery energy storage converter. The converter includes multiple cascaded sub-units, each sub-unit including a battery module, a PWM rectifier stage, a DC boost stage, a first switching switch S1, and a second switching switch S2.

[0032] Specifically, the AC sides of multiple sub-units are connected in series to form one phase of a cascaded multilevel converter. The three-phase converter can be connected in a star or delta configuration through a grid-connected filter network to form a complete energy storage system. Within each sub-unit, the AC side of the PWM rectifier stage is used for cascading with the PWM rectifier stages of other sub-units. The DC side of the PWM rectifier stage includes a first capacitor C1 and a second capacitor C2 connected in series. The connection point of the first capacitor C1 and the second capacitor C2 forms the midpoint of the DC bus. The positive terminal of the first capacitor C1 forms the positive bus, and the negative terminal of the second capacitor C2 forms the negative bus. This capacitor voltage divider structure decomposes the DC bus voltage into two parts, providing a circuit basis for subsequent voltage boosting.

[0033] The DC-DC boost stage is a resonant soft-switching DC-DC boost circuit. Its input is selectively connected to the battery module, and its output is connected to both ends of the first capacitor C1. A first switching switch S1 is connected between the positive terminal of the battery module and the positive terminal of the first capacitor C1; a second switching switch S2 is connected between the positive terminal of the battery module and the midpoint of the DC bus; the negative terminal of the battery module is connected to the negative terminal of the second capacitor C2. It should be understood that the first switching switch S1 and the second switching switch S2 can be electromagnetic contactors, relays, or solid-state switches with mechanical holding functions to achieve reliable physical switching of the circuit path.

[0034] The core of this embodiment lies in achieving two distinct operating modes through the combination of the states of two switching switches. When the first switching switch S1 is closed and the second switching switch S2 is open, the battery module is directly connected to the PWM rectifier stage, and the DC boost stage is not working. At this time, the positive terminal of the battery module is directly connected to the positive bus, and the negative terminal is connected to the negative bus, and the current path does not pass through the DC boost stage. Since the DC boost stage is bypassed, the system eliminates the conduction and switching losses caused by the boost circuit during normal operation, significantly improving the overall efficiency of the system. At the same time, since the voltage across the first capacitor C1 is zero or close to zero, the second capacitor C2 bears the entire battery voltage, and the system is in a low-loss direct-connection mode.

[0035] When the first switch S1 is open and the second switch S2 is closed, the battery module is connected to the input of the DC boost stage, which then operates to increase the voltage across the first capacitor C1. At this time, the positive terminal of the battery module is connected to the midpoint of the DC bus via the second switch S2, and the DC boost stage intervenes, boosting the battery voltage and outputting it to the first capacitor C1. In this mode, the sub-unit can output a bus voltage higher than the battery voltage, thus maintaining the total system output voltage by increasing the bus voltage of the remaining normal sub-units when some sub-units are bypassed due to a fault. Through the above hardware architecture, this embodiment achieves flexible switching between zero-loss bypass during normal operation and efficient boost compensation under fault conditions, solving the problem of low efficiency caused by continuous operation of the boost circuit in traditional solutions.

[0036] Specifically, in this embodiment, the DC boost stage is a resonant soft-switching DC boost circuit with partial power processing. This circuit specifically includes a first half-bridge, a second half-bridge, and an LC resonant branch.

[0037] Specifically, the DC side of the first half-bridge is connected between the positive and negative bus. The first half-bridge consists of power switches Q1 and Q2 connected in series. The drain of Q1 is connected to the positive bus (i.e., the positive terminal of the first capacitor C1), and the source of Q2 is connected to the negative bus (i.e., the negative terminal of the second capacitor C2). The source of Q1 and the drain of Q2 are connected and led out as the AC output terminal of the first half-bridge. The DC side of the second half-bridge is connected between the midpoint of the DC bus and the negative bus. The second half-bridge consists of power switches Q3 and Q4 connected in series. The drain of Q3 is connected to the midpoint of the DC bus (i.e., the connection point of the first capacitor C1 and the second capacitor C2), and the source of Q4 is connected to the negative bus. The source of Q3 and the drain of Q4 are connected and led out as the AC output terminal of the second half-bridge. An LC resonant branch is connected between the AC output terminals of the first and second half-bridges. This LC resonant branch includes a resonant inductor Lr and a resonant capacitor Cr connected in series. It should be understood that the power switching transistors Q1 to Q4 mentioned above can be selected from MOSFETs, IGBTs or other fully controllable power semiconductor devices, and this embodiment does not impose specific restrictions on them.

[0038] The partial power processing mentioned in this embodiment refers to the fact that, during operation, the DC boost stage does not process all the power output from the battery module like a traditional Boost circuit, but only processes a portion of the power flowing through the branch of the first capacitor C1. Since the DC side of the PWM rectifier stage is divided by a series connection of the first capacitor C1 and the second capacitor C2, in boost mode, the DC boost stage indirectly adjusts the voltage distribution of the entire DC bus by controlling the voltage across the first capacitor C1. This structural design allows the DC boost stage to achieve voltage boosting at a relatively small power level, thereby significantly reducing the current stress requirements on the power devices of the boost circuit, and reducing size and cost.

[0039] In terms of control strategy, the first and second half-bridges conduct alternately, and phase-shift control is used between them. Specifically, Q1 and Q2 in the first half-bridge conduct complementaryly, and Q3 and Q4 in the second half-bridge conduct complementaryly. By adjusting the phase shift angle between the two half-bridge drive signals, the direction and magnitude of energy transfer in the LC resonant branch can be controlled, thereby regulating the voltage across the first capacitor C1. More importantly, this embodiment controls the switching frequency to operate in the inductive region above the LC resonant frequency to achieve fully soft switching of the power switching device.

[0040] The so-called inductive region refers to the inductive impedance of the LC resonant branch at the switching frequency. When the circuit operates in the inductive region, the resonant current lags behind the voltage, allowing the current to naturally freewheel through the anti-parallel diode at the moment the switching transistor operates, thus achieving zero-voltage turn-on (ZVS) for the switching transistor. In contrast, traditional Boost converter circuits typically operate in hard-switching mode, resulting in high switching losses and severe electromagnetic interference. This embodiment, through a strategy combining phase-shift control with inductive region operation, achieves fully soft-switching operation of all power switching devices (Q1 to Q4), significantly reducing switching losses and electromagnetic interference, and improving the overall system efficiency. This low-loss characteristic is particularly important for boost operation after fault bypass, ensuring that the system maintains high energy conversion efficiency under abnormal operating conditions and avoiding a significant drop in overall system efficiency due to low efficiency in the boost stage.

[0041] Specifically, in one embodiment of the present invention, each subunit further includes a bypass switch and a discharge resistor, which play a key role in ensuring the system's fault tolerance and state maintenance.

[0042] Specifically, the bypass switch is connected in parallel between the two AC terminals of the PWM rectifier stage. Figure 2 In the illustrated embodiment, the bypass switch can be a mechanical contactor S3. When a sub-unit fails, the bypass switch closes, disconnecting the sub-unit from the cascaded system. It should be understood that, since the sub-units are AC-side cascaded structures, the bypass switch, located on the AC side, can directly short-circuit the output port of the faulty unit, ensuring that the unit's shutdown does not affect the current path of other normal sub-units in the same phase. In terms of hardware selection, the bypass switch is preferably a mechanical switch with arc-extinguishing capability. Its operating logic typically works in conjunction with the power switching devices of the PWM rectifier stage: before closing the bypass switch, a specific switch in the PWM rectifier stage can be turned on to establish a temporary current path; after the mechanical switch contacts close, the power devices are turned off, thus achieving a smooth, arc-free, and impact-free cut-off, ensuring the safety and reliability of the fault disconnection process.

[0043] In addition, each subunit also includes a discharge resistor connected in parallel across the first capacitor C1 to discharge the voltage of the first capacitor C1 to zero when the DC boost stage is not operating. In the direct-connect mode described in Embodiment 1, the first switching switch S1 is closed, and the battery module is directly connected to the PWM rectifier stage. At this time, the DC boost stage is in a non-operating state (blocked state). Without a discharge circuit, an uncertain floating voltage may remain across the first capacitor C1 due to parasitic parameters or previous operating history. This floating voltage may cause the first capacitor C1 to bear unnecessary voltage stress, and when the system needs to switch modes (from direct-connect mode to boost mode), the residual voltage may conflict with the expected control voltage, affecting the voltage stability at the moment of switching. Therefore, this embodiment uses a discharge resistor to slowly release the voltage of the first capacitor C1 to zero in the non-operating state by utilizing the energy dissipation characteristics of the resistor. In practical applications, high-resistance resistors are typically chosen for discharge. The resistance value must be set to balance discharge time and power loss: too high a resistance results in a slow discharge rate, potentially failing to completely discharge the voltage within the required time; too low a resistance leads to significant static losses during normal operation (boost mode). Preferably, the discharge resistor's resistance value should ensure that its losses during rated power operation constitute a minimal proportion of the total losses, thereby eliminating the risk of voltage floating without substantially impacting the overall system efficiency.

[0044] By combining the bypass switch and discharge resistor, this embodiment not only achieves rapid physical removal of the faulty sub-unit, but also ensures the determinism of the circuit state under normal operating conditions, eliminates the potential risks caused by voltage floating, and further improves the reliability and safety of the battery energy storage converter system.

[0045] This embodiment provides a fault-tolerant control method, which is implemented based on the battery energy storage converter hardware architecture described in the previous embodiment. For example... Figure 3 As shown, the method specifically includes the following steps: Step 1: During normal system operation, control all sub-units to operate in the first mode. Specifically, the controller closes the first switching switch and opens the second switching switch of each sub-unit, and blocks the drive signal of the DC boost stage, making the battery module directly connected to the PWM rectifier stage. In this state, the voltage of the battery module is directly applied to the DC bus formed by the first and second capacitors connected in series, the DC boost stage is in a non-operating state, and the system operates at maximum efficiency.

[0046] Step 2: When a fault is detected in a sub-unit of a phase, the faulty sub-unit is controlled to enter bypass mode, and the phase is disconnected from the power grid. At the same time, the PWM rectifier stage of all sub-units in that phase is blocked. The core of this step is to achieve safe and shock-free disconnection of the faulty sub-unit.

[0047] Specifically, the operation of controlling the faulty subunit to enter the bypass state includes: first, controlling at least two power switching devices in the PWM rectifier stage of the faulty subunit to turn on, forming a current path. For example, specific switching combinations of different bridge arms of the PWM rectifier stage can be controlled to turn on, such as keeping the upper transistors of both bridge arms simultaneously on ( Figure 2 Q5 and Q7 are simultaneously on, while Q6 and Q8 are simultaneously off; or Q6 and Q8 are simultaneously on, while Q5 and Q7 are simultaneously off, thus establishing a low-impedance path between the AC side terminals. Subsequently, the bypass switch of the faulty subunit is closed. Since the current path has already been established by the power switching devices, the voltage between the contacts of the bypass switch (usually a mechanical contactor) is extremely low, even zero, at the moment of closure, thus achieving arc-free closure and avoiding contact erosion. After the bypass switch returns a closed state, all power switching devices in the PWM rectifier stages of the faulty subunit are turned off, completing the fault clearance. This logic of "establishing the path first, then closing the switch, and then disconnecting the devices" ensures a smooth transition in the bypass process, greatly improving the system's reliability.

[0048] Step 3: For each normal sub-unit in this phase that has not experienced a fault, perform a mode switch. Specifically, first disconnect the first switching switch, and after confirming the disconnection, close the second switching switch. This step follows the safety principle of "disconnect before connecting" to prevent a short circuit between the battery module's positive terminal and the DC bus midpoint caused by the simultaneous closing of the first and second switching switches.

[0049] Furthermore, after disconnecting the first switching switch and before closing the second switching switch, a waiting delay is included to ensure that the first switching switch is completely disconnected. It should be understood that the first switching switch is typically a mechanical contactor, and its contact separation process involves arcing time and mechanical action time. If the second switching switch is closed before the contacts are completely separated and the arc is extinguished, a short circuit risk may still occur. Therefore, by setting a reasonable delay (e.g., tens to hundreds of milliseconds), the controller only issues the command to close the second switching switch after receiving the disconnection feedback signal from the first switching switch or after the preset delay time has been reached, thereby completely eliminating the risk of shoot-through at the physical level.

[0050] Step 4: Activate the DC boost stage of the normal sub-unit to increase the voltage across the first capacitor, compensating for the voltage loss caused by the bypass of the faulty sub-unit, thus restoring the total DC bus voltage of that phase to the pre-fault level. After the mode switch is completed, the battery module is connected to the input of the DC boost stage, and the controller releases the blockade on the DC boost stage, allowing it to start working.

[0051] Specifically, the process of the DC-DC boost stage increasing the voltage across the first capacitor is achieved by adjusting the phase shift angle of the resonant soft-switching DC-DC boost circuit. The controller calculates the target phase shift angle based on the required compensation voltage of the current system and controls the energy transfer in the LC resonant branch by adjusting the phase difference between the drive signals of the first and second half-bridges, thereby stabilizing the voltage of the first capacitor at the preset target value. For example, if each sub-unit carried the rated voltage before the fault, and the remaining sub-units need to share the missing voltage after the fault, the controller increases the phase shift angle to increase the voltage output of the first capacitor, thus keeping the total voltage of that phase constant. Because the DC-DC boost stage uses resonant soft-switching technology, it can maintain high conversion efficiency during voltage boosting, ensuring the system's economy under fault-tolerant operating conditions.

[0052] To verify the effectiveness of the battery energy storage converter and its fault-tolerant control method provided by this invention, this embodiment uses a specific low-voltage module cascaded energy storage system as an example for detailed explanation. Figure 1 As shown, the battery energy storage converter has a three-phase star connection structure, with each phase consisting of N cascaded sub-units. Under normal operating conditions, the system line voltage is 380V, corresponding to an effective phase voltage of approximately 220V. Assuming the rated voltage of the battery module in each sub-unit is 100V, when the system is operating without faults, the controller controls all sub-units to operate in the first mode, i.e., the first switching switch is closed and the second switching switch is open, with the battery module directly connected to the PWM rectifier stage. In this mode, the DC bus voltage output by each sub-unit is the battery module voltage of 100V. After five sub-units are connected in series, the peak phase voltage is approximately 707V, which meets the 220V phase voltage output requirement. In this mode, the DC boost stage is in a locked state, resulting in no switching losses and optimal system efficiency.

[0053] Suppose that a fault is detected in the third sub-unit of phase A during operation. The system immediately initiates the fault-tolerant control process. First, the controller executes step 2, controlling the faulty third sub-unit to enter bypass mode. Specifically, it first controls the power switches Q5 and Q8 in the PWM rectifier stage of this sub-unit to conduct, forming a current path. Then, it closes the bypass switch S3. After S3 feedback closes, it turns off all power switches in the PWM rectifier stage. At the same time, phase A is disconnected from the grid, and the PWM rectifier stage drive signals of all sub-units are blocked.

[0054] Next, the controller executes Step 3, switching the modes of the remaining four normal sub-units in Phase A. Taking the first sub-unit as an example, the controller first disconnects the first switching switch S1 and waits for a preset delay time (e.g., 100ms) to ensure that the S1 contact is completely extinguished and disconnected. After confirming that S1 is disconnected, the controller closes the second switching switch S2. At this time, the positive terminal of the battery module is connected to the midpoint of the DC bus, the negative terminal is connected to the negative bus, and the input terminal of the DC boost stage is connected to the circuit.

[0055] After the mode switch is complete, proceed to Step 4 to start the DC boost stage. Since the third sub-unit is bypassed, only four sub-units remain in phase A. To maintain the original total voltage output capacity, each normal sub-unit needs to bear a higher bus voltage. The specific calculation is as follows: the original total DC link voltage borne by the five units was approximately 500V (100V). 5) Now, this needs to be handled by four units, so the target DC bus voltage for each unit needs to be increased to 125V. Since the battery module voltage is still 100V, the DC boost stage needs to increase the voltage across the first capacitor C1 to 25V, so that the positive bus voltage (the sum of the voltages of C1 and C2) reaches 125V. The controller controls the energy transfer from the battery module to the first capacitor C1 by adjusting the phase shift angle of the resonant soft-switching DC boost circuit, thus stabilizing the voltage of C1 at 25V.

[0056] During this boost process, the DC boost stage employs partial power processing, handling only the power flowing through the C1 branch, rather than the full power of the battery module. Simultaneously, because the switching frequency operates in the inductive region above the LC resonant frequency, power switching devices Q1 to Q4 all achieve zero-voltage turn-on (ZVS), significantly reducing switching losses. Experimental data shows that in boost mode, the conversion efficiency of this DC boost stage can reach over 98%, an improvement of approximately 2-3 percentage points compared to traditional hard-switching Boost circuits, effectively ensuring the system's economic efficiency during fault-tolerant operation. Ultimately, the total DC bus voltage of phase A recovers to 500V (4... (125V), the system was reconnected to the grid and put back into operation, achieving uninterrupted power supply after the fault.

[0057] It should be understood that the voltage values, number of sub-units, and delay times in the above embodiments are merely illustrative and do not constitute a limitation on the scope of protection of this invention. In practical applications, the relevant parameters can be adaptively adjusted according to different grid voltage levels, battery module configurations, and system redundancy. For example, if the system design has a higher voltage margin, the voltage rise of the sub-unit after a fault can be reduced accordingly to further reduce device stress.

[0058] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A battery energy storage converter, characterized in that, It includes several phases connected in a star or delta configuration; each phase includes multiple cascaded sub-units, each of which includes: Battery module, PWM rectifier stage, DC boost stage, first switching switch and second switching switch; The AC side of the PWM rectifier stage is cascaded with the PWM rectifier stages of other sub-units. The DC side of the PWM rectifier stage includes a first capacitor and a second capacitor connected in series. The connection point of the first capacitor and the second capacitor forms the midpoint of the DC bus. The positive terminal of the first capacitor forms the positive bus, and the negative terminal of the second capacitor forms the negative bus. The DC boost stage is a resonant soft-switching DC boost circuit. The input terminal of the DC boost stage is selectively connected to the battery module, and the output terminal of the DC boost stage is connected to both ends of the first capacitor. The first switching switch is connected between the positive terminal of the battery module and the positive terminal of the first capacitor; The second switching switch is connected between the positive terminal of the battery module and the midpoint of the DC bus; The negative terminal of the battery module is connected to the negative terminal of the second capacitor; When the first switching switch is closed and the second switching switch is open, the battery module is directly connected to the PWM rectifier stage, and the DC boost stage is not working; when the first switching switch is open and the second switching switch is closed, the battery module is connected to the input terminal of the DC boost stage, and the DC boost stage works to boost the voltage between the positive and negative terminals of the first capacitor.

2. The battery energy storage converter according to claim 1, characterized in that, The DC boost stage is a resonant soft-switching DC boost circuit, including: The first half-bridge, wherein the DC side of the first half-bridge is connected between the positive bus and the negative bus; The second half-bridge has its DC side connected between the midpoint of the DC bus and the negative bus. An LC resonant branch is connected between the AC output terminal of the first half-bridge and the AC output terminal of the second half-bridge.

3. The battery energy storage converter according to claim 2, characterized in that, Both the first half-bridge and the second half-bridge include several power switching transistors connected in series.

4. The battery energy storage converter according to claim 3, characterized in that, The first half-bridge and the second half-bridge are alternately turned on, and phase-shift control is used between the first half-bridge and the second half-bridge to control the switching frequency of the power switch to operate in the inductive region above the LC resonant frequency.

5. The battery energy storage converter according to claim 1, characterized in that, Each of the sub-units also includes a bypass switch connected in parallel between two AC terminals of the PWM rectifier stage; when the sub-unit fails, the bypass switch closes, disconnecting the sub-unit from the cascaded sub-units.

6. The battery energy storage converter according to claim 1, characterized in that, Each of the sub-units further includes a discharge resistor connected in parallel between the positive and negative terminals of the first capacitor, used to discharge the voltage of the first capacitor to zero when the DC boost stage is not operating.

7. A fault-tolerant control method, applied to the battery energy storage converter according to any one of claims 1 to 6, characterized in that, Includes the following steps: Step 1: When the battery energy storage converter is running normally, control all sub-units to work in the first mode, that is, control the first switching switch of each sub-unit to be closed and the second switching switch to be open, and block the DC boost stage, so that the battery module is directly connected to the PWM rectifier stage; Step 2: When a fault is detected in a sub-unit of a phase, the faulty sub-unit is controlled to enter the bypass state, and the phase is disconnected from the power grid. At the same time, the PWM rectifier stage of all sub-units in the phase is blocked. Step 3: For each normal sub-unit in this phase that has not experienced a fault, execute the second mode: first disconnect the first switching switch, and after confirming that it is disconnected, close the second switching switch; Step 4: Activate the DC boost stage of the normal sub-unit to increase the voltage across the first capacitor to compensate for the voltage loss caused by the bypass of the faulty sub-unit, so that the total DC bus voltage of the phase is restored to the level before the fault.

8. The fault-tolerant control method according to claim 7, characterized in that, Step 2, which involves controlling the faulty subunit to enter the bypass state, specifically includes: The power switching devices in at least two PWM rectifier stages of the faulty subunit are turned on to form a current path; Then close the bypass switch of the faulty subunit; After the bypass switch feedback is closed, the power switching devices in all PWM rectifier stages of the faulty subunit are turned off.

9. The fault-tolerant control method according to claim 7, characterized in that, In Step 3, a waiting delay is performed after the first switching switch is turned off and before the second switching switch is turned on.

10. The fault-tolerant control method according to claim 7, characterized in that, In Step 4, the DC boost stage increases the voltage across the first capacitor by adjusting the phase shift angle of the resonant soft-switching DC boost circuit.

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