Pre-charging circuit of cascaded power electronic transformer suspension capacitor and control method of pre-charging circuit

Abstract

The invention discloses a pre-charging circuit of a cascaded power electronic transformer suspension capacitor and a control method of the pre-charging circuit, and belongs to the technical field of power electronics. The pre-charging circuit comprises a plurality of isolated DCDC booster circuits, a plurality of anti-reverse diodes and a pre-charging controller. And the pre-charging circuit provides independent and controllable charging voltage for each floating capacitor through the isolated DCDC booster circuit. And the pre-charging controller adopts a multi-channel sub-control framework, and is used for monitoring and regulating the charging process of each level of floating capacitor respectively, so that voltage balance and level-to-level management are realized. The isolated DCDC boost circuit provides electrical isolation and soft start boost functions, and the anti-reverse diode provides reverse blocking after pre-charging is completed, so that reverse discharge of the capacitor is prevented. The pre-charging circuit is simple in structure, flexible in control and low in cost, and the problem of high impact current of the cascaded middle-high voltage power electronic equipment during grid-connected starting operation is effectively avoided through pre-charging.

Description

Technical Field

[0001] This invention belongs to the field of power electronic devices and their control technology, specifically relating to a pre-charging circuit and control method for the floating capacitor of a cascaded power electronic transformer. Background Technology

[0002] Cascaded power electronic transformers are widely used in medium- and high-voltage power electronic conversion and are key equipment for achieving efficient power conversion and flexible control. Through the cascading of multiple modules, they can effectively improve voltage levels and power quality. However, the high inrush current during grid-connected startup of cascaded medium- and high-voltage power electronic conversion equipment remains a pressing technical challenge. Currently, traditional pre-charging of suspended capacitors involves directly drawing power from the grid and gradually increasing the modulation index of the cascaded modules. This method establishes a direct electrical connection with the grid, resulting in poor control flexibility and dependence on grid voltage. Furthermore, when charging multiple modules of suspended capacitors, it is difficult to precisely control the charging process of each module, easily leading to voltage imbalances. When cascaded power electronic equipment is connected to the grid, the uncertainty and imbalance of the initial voltage of the suspended capacitors generate high inrush currents. This not only damages power electronic devices, reducing equipment reliability and lifespan, but may also affect the stable operation of the grid. Therefore, how to achieve efficient and flexible pre-charging of the suspended capacitors in cascaded medium- and high-voltage power electronic converter modules, avoiding the high inrush current problem when connected to the grid, has become a pressing technical problem that needs to be solved by those skilled in the art. Summary of the Invention

[0003] To address the problems in the prior art, this invention provides a pre-charging circuit and control method for the floating capacitor of a cascaded power electronic transformer.

[0004] The technical solution of the present invention is as follows: In a first aspect, the present invention discloses a pre-charging circuit for the floating capacitors of a cascaded power electronic transformer, wherein the cascaded power electronic transformer includes multiple floating capacitors, and the pre-charging circuit includes a pre-charging controller and an isolated DC-DC boost circuit and a reverse protection diode equal in number to the floating capacitors. The isolated DC-DC boost circuit includes a power supply terminal, an output terminal, and a control terminal. An isolated DC-DC boost circuit, a reverse protection diode, and a floating capacitor constitute a pre-charge sub-circuit. In each pre-charge sub-circuit, the positive terminal of the output of the isolated DC-DC boost circuit is connected to the positive terminal of the reverse protection diode, the negative terminal of the reverse protection diode is connected to one end of the floating capacitor, and the other end of the floating capacitor is connected to the negative terminal of the output of the isolated DC-DC boost circuit. The power supply terminal of each isolated DC-DC boost circuit is connected to an external power supply, and the control terminal is connected to a pre-charge controller. The precharge controller includes a working sequence control unit and multiple precharge sub-circuit controllers, each controlling one precharge sub-circuit. The working sequence control unit receives an external precharge start command and then sends start instructions to the precharge sub-circuit controllers in a preset order. The precharge sub-circuit controllers control the isolated DC-DC boost circuit in the corresponding precharge sub-circuit to start working according to the received start instructions. At the same time, the precharge sub-circuit controllers collect the voltage of the floating capacitor in the corresponding precharge sub-circuit in real time. When the voltage reaches the preset range, the isolated DC-DC boost circuit stops working.

[0005] Furthermore, the preset sequence is the startup sequence of the isolated DC-DC boost circuit; the startup sequence of the isolated DC-DC boost circuit is set by the following method: Before precharging the floating capacitor, the precharge sub-circuit controller collects the voltage of the floating capacitor in the precharge sub-circuit it controls and feeds it back to the working sequence control unit. The working sequence control unit sorts the received voltage values ​​from largest to smallest. The position of the floating capacitor in this sort corresponds to the start position of the isolated DC-DC boost circuit in its precharge sub-circuit, thereby determining the start sequence of the isolated DC-DC boost circuit.

[0006] Furthermore, when the pre-charge sub-circuit controller starts the isolated DC-DC boost circuit, it controls the output power of the isolated DC-DC boost circuit, thereby controlling the voltage change rate of the corresponding floating capacitor to a preset value.

[0007] Furthermore, each precharge sub-circuit controller receives an externally input capacitor start-up voltage and sets the preset range based on the capacitor start-up voltage; the preset range is from 0.99 times the capacitor start-up voltage to 1.01 times the capacitor start-up voltage.

[0008] Furthermore, the preset value is equal to the result of dividing the capacitor start-up voltage by the preset pre-charging time of the floating capacitor.

[0009] Secondly, the present invention also discloses a pre-charging control method for the floating capacitor of a cascaded power electronic transformer utilizing the pre-charging circuit, comprising: Step 1: Each pre-charge sub-circuit controller collects the voltage of the floating capacitor in the pre-charge sub-circuit it controls and feeds it back to the working sequence control unit. The working sequence control unit sorts the received voltage values ​​from largest to smallest. The position of the floating capacitor in this sort corresponds to the start position of the isolated DC-DC boost circuit in its respective pre-charge sub-circuit, thereby determining the start sequence of the isolated DC-DC boost circuit. Each pre-charge sub-circuit controller also receives the externally input capacitor start voltage, and then divides the capacitor start voltage by the preset floating capacitor pre-charge time as the preset voltage change rate of the corresponding floating capacitor. Meanwhile, the working sequence control unit detects in real time whether it has received a pre-charge start command; when the working sequence control unit receives a pre-charge start command, the working sequence control unit sends start commands to the pre-charge sub-circuit controller in sequence according to the start order of the isolated DC-DC boost circuit; Step 2: Each pre-charge sub-circuit controller controls the isolated DC-DC boost circuit in the corresponding pre-charge sub-circuit to start working according to the received instructions. That is, by adjusting the output power of the corresponding isolated DC-DC boost circuit, the voltage of the floating capacitor changes according to the preset voltage change rate. At the same time, each pre-charge sub-circuit controller collects the voltage of the floating capacitor in the corresponding pre-charge sub-circuit in real time. When the voltage reaches the preset range, it controls the isolated DC-DC boost circuit to stop working. Step 3: When all isolated DC-DC boost circuits stop working, the pre-charging of the floating capacitor is complete. Based on the above technical solution, the beneficial effects of the present invention compared with the prior art are: (1) The pre-charging circuit of the present invention has a simple structure, flexible control and strong isolation.

[0010] This invention achieves independent and controllable charging of multi-stage floating capacitors by configuring independent isolated DC-DC boost circuits and anti-reverse diodes in each stage of the floating capacitor. Each boost circuit employs an electrical isolation structure, preventing interference and avoiding the problem of cascaded voltage imbalance, thus significantly improving the electrical safety and operational stability of the system. Simultaneously, the circuit structure is compact, the hardware implementation is simple, and it facilitates modular design and maintenance.

[0011] (2) The pre-charge control method of the present invention realizes voltage balancing and hierarchical management of the floating capacitors, and the charging process is efficient and reliable. The pre-charge controller adopts a multi-channel sub-control architecture. Through the coordinated work of the working sequence control unit and each pre-charge sub-circuit controller, the voltage of each floating capacitor is sampled and dynamically regulated in real time. Each isolated DC-DC boost circuit starts up sequentially according to the capacitor voltage. Combined with the preset voltage change rate, the voltage is steadily increased and balanced charging is achieved, which effectively avoids the overcharging or undercharging phenomenon caused by uneven voltage distribution in the traditional scheme, thereby reducing the inrush current in the start-up stage and improving the start-up reliability of the system.

[0012] (3) This invention features high efficiency and extremely low power consumption. During the charging process, the isolated DC-DC boost circuit achieves efficient energy transfer through soft-start boost control, and the anti-reverse diode provides reverse blocking after pre-charging to prevent reverse discharge of the capacitor. After pre-charging is completed, each sub-circuit automatically stops working, and the static power consumption of the circuit is extremely low, which can significantly improve the overall energy efficiency of the system. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the pre-charging circuit for the floating capacitor of the cascaded power electronic transformer of the present invention. Figure 2 This is a structural diagram of the pre-charge controller of the pre-charge circuit for the floating capacitor of the cascaded power electronic transformer of the present invention. Figure 3 This is a flowchart of the control method for the pre-charging circuit of the floating capacitor of the cascaded power electronic transformer of the present invention. Detailed Implementation

[0014] The present invention will be further described and illustrated below with reference to specific embodiments. The embodiments described are merely examples of the content of this disclosure and do not limit the scope of the invention. The technical features of each embodiment in the present invention can be combined accordingly, provided that there is no mutual conflict.

[0015] To address the issues of complex configuration and poor performance of existing pre-charging technologies, this invention proposes a pre-charging circuit and its control method for the floating capacitor of a cascaded power electronic transformer. This pre-charging circuit features a simple structure, convenient control, flexible configuration, and wider applicability. The pre-charging process consumes minimal power, and the circuit stops operating after charging is complete, exhibiting no static power consumption. It is suitable for soft-start applications involving pre-charging of medium- and high-voltage cascaded power electronic transformer modules.

[0016] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0017] The pre-charging target of the pre-charging circuit of this invention is the floating capacitor in a cascaded power electronic transformer, such as... Figure 1As shown, this cascaded power electronic transformer includes a single-phase / three-phase AC power grid, a circuit breaker, a downstream circuit (load), and n cascaded modules. Each cascaded module includes an AC / DC unit and a floating capacitor, meaning the cascaded power electronic transformer has n floating capacitors. The circuit topology of the AC / DC unit can be selected from AC / DC power electronic converters containing output capacitors, such as half-bridge converters, full-bridge converters, or ANPC converters.

[0018] A specific embodiment of the present invention provides a detailed description of the pre-charging circuit for the floating capacitor of a cascaded power electronic transformer.

[0019] like Figure 1 and Figure 2 As shown, the pre-charging circuit for the floating capacitors of a cascaded power electronic transformer provided by this invention includes: n isolated DC-DC boost circuits, n anti-reverse diodes, and a pre-charging controller. Each isolated DC-DC boost circuit can operate independently, thereby achieving distributed independent charging of multiple floating capacitors and avoiding voltage imbalance between stages. This pre-charging circuit is used to pre-charge each stage of the floating capacitors before the cascaded power electronic transformer starts up, in order to achieve voltage balance, avoid inrush current, and ensure safe system startup. The pre-charging controller is responsible for controlling the isolated DC-DC boost circuits to pre-charge each floating capacitor of the cascaded power electronic transformer.

[0020] An isolated DC-DC boost circuit includes a power supply terminal, an output terminal, and a control terminal. One isolated DC-DC boost circuit, one anti-reverse diode, and one floating capacitor constitute a pre-charge sub-circuit; therefore, there are n pre-charge sub-circuits in the pre-charge circuit. In each pre-charge sub-circuit, the positive terminal of the output of the isolated DC-DC boost circuit is connected to the positive terminal of the anti-reverse diode, the negative terminal of the anti-reverse diode is connected to one end of the floating capacitor, and the other end of the floating capacitor is connected to the negative terminal of the output of the isolated DC-DC boost circuit. The anti-reverse diode provides reverse blocking capability after the pre-charge circuit stops working. The power supply terminal of each isolated DC-DC boost circuit is connected to an external power supply, and the control terminal is connected to the pre-charge controller. The power supply is a multi-channel isolated DC output auxiliary power supply with split transformer isolation or current source isolation. Each DC current in the multi-channel isolated DC output auxiliary power supply is output to one isolated DC-DC boost circuit. Simultaneously, the power supply is also connected to the pre-charge controller, supplying power to the pre-charge controller.

[0021] The pre-charge controller includes a working sequence control unit and multiple pre-charge sub-circuit controllers. Each pre-charge sub-circuit controller controls one pre-charge sub-circuit, enabling independent charging control of each stage of the floating capacitors. Before pre-charging the floating capacitors, the pre-charge sub-circuit controller collects the voltage of the floating capacitors in its controlled pre-charge sub-circuit and feeds it back to the working sequence control unit. The working sequence control unit sorts the received voltage values ​​from largest to smallest. The position of the floating capacitor in this sort corresponds to the start-up position of the isolated DC-DC boost circuit in its respective pre-charge sub-circuit, thus determining the start-up sequence of the isolated DC-DC boost circuit.

[0022] The working sequence control unit is used to receive the pre-charge start command from the external input, and then send the start command to the pre-charge sub-circuit controller in sequence according to the start sequence of the isolated DC-DC boost circuit.

[0023] Before receiving the start command, each precharge sub-circuit controller also receives an externally input capacitor start voltage. U* Then, the capacitor-start voltage is used. U* The result of dividing by the preset pre-charging time of the floating capacitor is used as the preset voltage change rate of the corresponding floating capacitor.

[0024] After receiving the start command, each pre-charge sub-circuit controller will control the isolated DC-DC boost circuit in its corresponding pre-charge sub-circuit to start working. This means controlling the output power of the isolated DC-DC boost circuit to ensure the voltage change rate of the corresponding floating capacitor is a preset voltage change rate. Simultaneously, the pre-charge sub-circuit controller will also collect the voltage of the floating capacitor in the corresponding pre-charge sub-circuit in real time. When the voltage reaches a preset range, the isolated DC-DC boost circuit will stop working. The preset range is 0.99 times the capacitor start voltage. U* Up to 1.01 times the capacitor startup voltage U* .

[0025] In a specific embodiment of the present invention, the isolated DC-DC boost circuit is a DC / DC converter with isolation and boost functions. The DC / DC converter may adopt a flyback circuit structure, a forward boost circuit structure, or a push-pull boost circuit structure.

[0026] In a preferred embodiment of the present invention, the anti-reverse diode is equipped with a withstand voltage greater than the capacitor's start-up voltage. U* Schottky diodes or fast recovery diodes are used to ensure high-frequency characteristics and safety.

[0027] In this invention, the pre-charge sub-circuit controller includes a voltage sampling module and a control signal output module. The voltage sampling module is used to sample the voltage values ​​of each stage of the floating capacitor in real time; the control signal output module is used to provide PWM drive signals for each isolated DC-DC boost circuit.

[0028] like Figure 3 As shown, the present invention also provides a control method for the pre-charging circuit of the floating capacitor in a cascaded power electronic transformer. The method includes the following steps: 1) After the pre-charging circuit is powered on, the pre-charging controller is in standby mode. The working sequence control unit in the pre-charging controller continuously detects whether it receives a pre-charging start command from the external host computer. The pre-charge sub-circuit controller collects the voltage of the floating capacitor in the pre-charge sub-circuit it controls and feeds it back to the working sequence control unit. The working sequence control unit sorts the received voltage values ​​from largest to smallest. The position of the floating capacitor in this sort corresponds to the start position of the isolated DC-DC boost circuit in its pre-charge sub-circuit, thereby determining the start sequence of the isolated DC-DC boost circuit. Simultaneously, each precharge subcircuit controller receives an externally input capacitor start-up voltage. U* Then, the capacitor-start voltage is used. U* The result of dividing by the preset pre-charging time of the floating capacitor is used as the preset voltage change rate of the corresponding floating capacitor.

[0029] 2) When the working sequence control unit receives the precharge start command, the working sequence control unit sends start commands to the precharge sub-circuit controller in sequence according to the start order of the isolated DC-DC boost circuit.

[0030] 3) Each pre-charge sub-circuit controller controls the isolated DC-DC boost circuit in the corresponding pre-charge sub-circuit to start working according to the received instructions. That is, by adjusting the output power of the corresponding isolated DC-DC boost circuit, the voltage of the floating capacitor is changed at a preset voltage change rate.

[0031] 4) The controller of each pre-charge sub-circuit collects the voltage of the floating capacitor in the corresponding pre-charge sub-circuit in real time. When the voltage is greater than 0.99... U* And less than 1.01 U* At this time, the isolated DC-DC boost circuit stops working.

[0032] 5) When all isolated DC-DC boost circuits stop working, the pre-charging of the floating capacitor is completed.

[0033] In one specific embodiment of the present invention, the cascaded power electronic transformer includes six cascaded modules, each containing one floating capacitor. Therefore, the cascaded power electronic transformer has six floating capacitors with a rated capacitance of 470µF and a rated withstand voltage of 1200V. The pre-charge circuit parameters are shown in Table 1.

[0034] Table 1 The pre-charging circuit control method for the floating capacitor of the cascaded power electronic transformer in this embodiment is as follows: Step 1: After the pre-charging circuit is powered on, the pre-charging controller is in standby mode. The working sequence control unit in the pre-charging controller continuously detects whether it receives a pre-charging start command from the external host computer. The pre-charge sub-circuit controller collects the voltage of the floating capacitor in the pre-charge sub-circuit it controls and feeds it back to the working sequence control unit. The working sequence control unit sorts the received voltage values ​​from largest to smallest. The position of the floating capacitor in this sort corresponds to the start position of the isolated DC-DC boost circuit in its pre-charge sub-circuit, thereby determining the start sequence of the isolated DC-DC boost circuit. Simultaneously, each precharge subcircuit controller receives an externally input capacitor start-up voltage. U* Then, the capacitor-start voltage is used. U* The result of dividing by the preset pre-charging time of the floating capacitor is used as the preset voltage change rate of the corresponding floating capacitor. The preset voltage change rate is 600V / 2s, or 300.

[0035] Step 2: When the working sequence control unit receives the pre-charge start command, it sends start commands to the pre-charge sub-circuit controllers sequentially according to the start order of the isolated DC-DC boost circuits. Step 3: Each pre-charge sub-circuit controller controls the corresponding isolated DC-DC boost circuit in its pre-charge sub-circuit to start working according to the received command. Specifically, it adjusts the output power of the corresponding isolated DC-DC boost circuit to change the voltage of the floating capacitor at a preset voltage change rate.

[0036] Step 4: The controller of each pre-charge sub-circuit collects the voltage of the floating capacitor in the corresponding pre-charge sub-circuit in real time. When the voltage is greater than 0.99×600 and less than 1.01×600, the isolated DC-DC boost circuit is stopped.

[0037] Step 5: When all isolated DC-DC boost circuits stop working, the pre-charging of the floating capacitor is completed.

[0038] The pre-charging circuit for the floating capacitors of the cascaded power electronic transformer of this invention consists of multiple isolated DC-DC boost circuits, anti-reverse diodes, and a pre-charging controller. The pre-charging circuit uses the power supply as its input and provides independently controllable charging voltages to each stage of the floating capacitors through the isolated DC-DC boost circuits. The pre-charging controller employs a multi-channel sub-control architecture to monitor and regulate the charging process of each stage of the floating capacitors, achieving voltage balancing and hierarchical management. The isolated DC-DC boost circuits provide electrical isolation and soft-start boost functions, while the anti-reverse diodes provide reverse blocking after pre-charging to prevent reverse discharge of the capacitors. The proposed pre-charging circuit is compact, flexible in control, and low in cost, effectively avoiding the generation of high inrush currents during system startup. Multi-channel independent boost control achieves voltage balancing of the floating capacitors, significantly improving the reliability and operational safety of the cascaded system. The circuit automatically exits the operating state after pre-charging, exhibiting extremely low static power consumption, making it suitable for safe pre-charging applications in medium- and high-voltage cascaded power electronic transformers.

[0039] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. Those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A pre-charging circuit for the floating capacitors of a cascaded power electronic transformer, wherein the cascaded power electronic transformer comprises multiple floating capacitors, characterized in that, The pre-charge circuit includes a pre-charge controller, an isolated DC-DC boost circuit equal in number to the floating capacitors, and a reverse protection diode. The isolated DC-DC boost circuit includes a power supply terminal, an output terminal, and a control terminal. An isolated DC-DC boost circuit, a reverse protection diode, and a floating capacitor constitute a pre-charge sub-circuit. In each pre-charge sub-circuit, the positive terminal of the output of the isolated DC-DC boost circuit is connected to the positive terminal of the reverse protection diode, the negative terminal of the reverse protection diode is connected to one end of the floating capacitor, and the other end of the floating capacitor is connected to the negative terminal of the output of the isolated DC-DC boost circuit. The power supply terminal of each isolated DC-DC boost circuit is connected to an external power supply, and the control terminal is connected to a pre-charge controller. The precharge controller includes a working sequence control unit and multiple precharge sub-circuit controllers, each controlling one precharge sub-circuit. The working sequence control unit receives an external precharge start command and then sends start instructions to the precharge sub-circuit controllers in a preset order. The precharge sub-circuit controllers control the isolated DC-DC boost circuit in the corresponding precharge sub-circuit to start working according to the received start instructions. At the same time, the precharge sub-circuit controllers collect the voltage of the floating capacitor in the corresponding precharge sub-circuit in real time. When the voltage reaches the preset range, the isolated DC-DC boost circuit stops working.

2. The pre-charging circuit for the floating capacitor of the cascaded power electronic transformer according to claim 1, characterized in that, The preset sequence is the startup sequence of the isolated DC-DC boost circuit; the startup sequence of the isolated DC-DC boost circuit is set by the following method: Before precharging the floating capacitor, the precharge sub-circuit controller collects the voltage of the floating capacitor in the precharge sub-circuit it controls and feeds it back to the working sequence control unit. The working sequence control unit sorts the received voltage values ​​from largest to smallest. The position of the floating capacitor in this sort corresponds to the start position of the isolated DC-DC boost circuit in its precharge sub-circuit, thereby determining the start sequence of the isolated DC-DC boost circuit.

3. The pre-charging circuit for the floating capacitor of the cascaded power electronic transformer according to claim 1, characterized in that, When the pre-charge sub-circuit controller starts the isolated DC-DC boost circuit, it controls the output power of the isolated DC-DC boost circuit. By controlling the output power of the isolated DC-DC boost circuit, the voltage change rate of the corresponding floating capacitor is made to a preset value.

4. The pre-charging circuit for the floating capacitor of the cascaded power electronic transformer according to claim 3, characterized in that, Each precharge sub-circuit controller receives an externally input capacitor start-up voltage and sets a preset range based on the capacitor start-up voltage; the preset range is from 0.99 times the capacitor start-up voltage to 1.01 times the capacitor start-up voltage.

5. The pre-charging circuit for the floating capacitor of the cascaded power electronic transformer according to claim 4, characterized in that, The preset value is equal to the result of dividing the capacitor start-up voltage by the preset pre-charging time of the floating capacitor.

6. The pre-charging circuit for the floating capacitor of the cascaded power electronic transformer according to claim 1, characterized in that, The power supply is a multi-channel isolated DC output auxiliary power supply with split transformer isolation or current source isolation.

7. The pre-charging circuit for the floating capacitor of the cascaded power electronic transformer according to claim 1, characterized in that, The isolated DC-DC boost circuit is a DC / DC converter with isolation and boost functions. The DC / DC converter adopts a flyback circuit structure, a forward boost circuit structure, or a push-pull boost circuit structure.

8. A method for controlling the pre-charging of the floating capacitor in a cascaded power electronic transformer using the pre-charging circuit described in any one of claims 1-7, characterized in that, include: Step 1: Each pre-charge sub-circuit controller collects the voltage of the floating capacitor in the pre-charge sub-circuit it controls and feeds it back to the working sequence control unit. The working sequence control unit sorts the received voltage values ​​from largest to smallest. The position of the floating capacitor in this sort corresponds to the start position of the isolated DC-DC boost circuit in its respective pre-charge sub-circuit, thereby determining the start sequence of the isolated DC-DC boost circuit. Each pre-charge sub-circuit controller also receives the externally input capacitor start voltage, and then divides the capacitor start voltage by the preset floating capacitor pre-charge time as the preset voltage change rate of the corresponding floating capacitor. Meanwhile, the working sequence control unit detects in real time whether it has received a pre-charge start command; when the working sequence control unit receives a pre-charge start command, the working sequence control unit sends start commands to the pre-charge sub-circuit controller in sequence according to the start order of the isolated DC-DC boost circuit; Step 2: Each pre-charge sub-circuit controller controls the isolated DC-DC boost circuit in the corresponding pre-charge sub-circuit to start working according to the received instructions. That is, by adjusting the output power of the corresponding isolated DC-DC boost circuit, the voltage of the floating capacitor changes according to the preset voltage change rate. At the same time, each pre-charge sub-circuit controller collects the voltage of the floating capacitor in the corresponding pre-charge sub-circuit in real time. When the voltage reaches the preset range, it controls the isolated DC-DC boost circuit to stop working. Step 3: When all isolated DC-DC boost circuits stop working, the pre-charging of the floating capacitor is completed.

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