Cascade high-frequency link medium-voltage direct-hanging multi-terminal charging control system and method

Abstract

The application discloses a kind of cascade high-frequency chain medium voltage direct hanging multi-terminal charging control system and method, it is related to charging control field, system includes: three-phase cascade power unit, by net side ac filter, respectively access medium voltage ac bus A phase, B phase and C phase;Common low-voltage DC charging bus is parallel with single-phase high-frequency chain AC / DC power module;DC charging terminal is connected with common low-voltage DC charging bus.Can be directly accessed in the case where no power transformer is set medium voltage ac bus, and form unified low-voltage DC power supply interface, it is convenient for multiple DC charging terminal access;Without setting module level intermediate DC link and its large-capacity energy storage capacitor, reduce independent power conversion stage, it is favorable to improve system power density and reliability;Avoid sudden interruption of leakage inductance current path in commutation process, it is favorable to reduce leakage inductance caused by commutation overvoltage and improve the commutation reliability of matrix converter.

Description

Technical Field

[0001] This application relates to the field of charging control, specifically to a cascaded high-frequency chain medium-voltage direct-connection multi-terminal charging control system and method. Background Technology

[0002] With the electrification of electric vehicles, heavy trucks, and engineering vehicles, megawatt-level multi-terminal charging stations are facing higher requirements in terms of power supply capacity, equipment size, operating efficiency, and continuous power supply capability. Traditional charging stations typically use power frequency transformers to step down the 10kV or 35kV medium-voltage distribution network to the low-voltage AC bus, and then the low-voltage charging modules rectify the voltage to power the vehicle batteries. This solution suffers from problems such as the large size and weight of the power frequency transformers, high losses, large footprint, and high investment costs, making it difficult to meet the high power density and low-cost high-power charging demands.

[0003] To eliminate the need for power frequency transformers, existing technologies have proposed modular medium-voltage direct-connect charging systems. Some of these systems employ a two-stage power conversion structure (AC / DC and DC / DC), with an intermediate DC bus and energy storage capacitor housed within the power module. While this structure achieves voltage division on the medium-voltage side and high-frequency electrical isolation, it involves numerous power conversion stages and requires a large number of power devices and energy storage components within the module. Summary of the Invention

[0004] To address the aforementioned issues, this application proposes a cascaded high-frequency chain medium-voltage direct-connected multi-terminal charging control system, comprising: a three-phase cascaded power unit, a common low-voltage DC charging bus, and one or more DC charging terminals. The three-phase cascaded power unit is connected to phases A, B, and C of the medium-voltage AC bus via a grid-side AC filter; and each phase's cascaded branch includes multiple series-connected single-phase high-frequency AC / DC power modules; wherein the medium-voltage AC bus is a medium-voltage three-phase AC bus. The common low-voltage DC charging bus is connected in parallel with the single-phase high-frequency AC / DC power module through an output filtering stage; The DC charging terminal is connected to the common low-voltage DC charging bus.

[0005] In one example, the AC sides of each single-phase high-frequency chain AC / DC power module are connected in series to share the AC voltage of the corresponding phase.

[0006] In one example, the common low-voltage DC charging bus connects to one or more of the DC charging terminals, and charges the DC charging terminals through the common low-voltage DC charging bus. Each of the DC charging terminals is equipped with corresponding connection, pre-charge, and protection devices to enable the connection and disconnection between the DC charging terminal and the common low-voltage DC charging bus.

[0007] In one example, a single-phase high-frequency chain AC / DC power module includes a matrix converter, a high-frequency isolation transformer, and a secondary rectifier. The AC input terminal of the matrix converter is connected to the cascaded branch where it is located, and the output terminal is connected to the primary side of the high-frequency isolation transformer, which is used to directly modulate the AC input voltage of the single-phase high-frequency chain AC / DC power module into a high-frequency AC pulse voltage. No module-level intermediate DC link or its energy storage capacitor is provided between the matrix converter and the high-frequency isolation transformer. The high-frequency AC pulse voltage output from the secondary side of the high-frequency isolation transformer is rectified into DC output by the secondary side rectifier and connected to the common low-voltage DC charging bus.

[0008] In one example, the system further includes: a system controller; The system controller is used to sample the three-phase input current on the grid side, the voltage of the common low-voltage DC charging bus, and to acquire the phase information of the medium-voltage AC bus voltage and the polarity information of the AC input voltage of each single-phase high-frequency chain AC / DC power module. Based on the deviation between the voltage reference value and the voltage sampling value of the common low-voltage DC charging bus, a grid-side active current reference is generated, and the three-phase current reference is obtained by combining the phase information of the medium-voltage AC bus voltage; based on the deviation between the three-phase current reference and the grid-side three-phase input current, current closed-loop control is performed to obtain the modulation wave corresponding to each phase. For each single-phase high-frequency chain AC / DC power module in each phase, the corresponding modulation wave is shared, and each single-phase high-frequency chain AC / DC power module is allocated a high-frequency triangular carrier with the same frequency and amplitude, and whose phases are staggered according to a preset angle. The modulation wave corresponding to each single phase is compared with the high-frequency triangular carrier wave corresponding to each single-phase high-frequency chain AC / DC power module to obtain the PWM logic signal corresponding to each single-phase high-frequency chain AC / DC power module, and the matrix converter is determined to be in a non-zero level state or a zero level state based on the PWM logic signal. The switching state of the matrix converter is determined based on the polarity information of the AC input voltage of each single-phase high-frequency AC / DC power module, the polarity information of the primary side expected voltage of the high-frequency isolation transformer, and the PWM logic signal. For each single-phase high-frequency chain AC / DC power module in each phase, PWM modulation is performed according to its corresponding high-frequency triangular carrier wave, so that the high-frequency switching pulses of each single-phase high-frequency chain AC / DC power module are staggered in time; the cascaded branches of each phase operate according to the corresponding modulation wave, and after passing through the matrix converter, high-frequency isolation transformer and secondary rectifier in each single-phase high-frequency chain AC / DC power module, the corresponding DC output is obtained and connected in parallel to the common low-voltage DC charging bus.

[0009] In one example, the system controller is further configured to, when the matrix converter is in a non-zero level state, select the corresponding diagonal switch path in the matrix converter based on the combination of the polarity information of the AC input voltage of the single-phase high-frequency chain AC / DC power module and the polarity information of the desired voltage on the primary side of the high-frequency isolation transformer, and apply the AC input voltage of the single-phase high-frequency chain AC / DC power module to the primary side of the high-frequency isolation transformer with the corresponding polarity, so that the primary side of the high-frequency isolation transformer forms a high-frequency AC pulse voltage. When the matrix converter is in a zero-level state, the corresponding switch of the matrix converter is turned on, so that the two terminals of the primary side of the high-frequency isolation transformer are connected to the same terminal of the AC input side of the single-phase high-frequency chain AC / DC power module, so that the two ends of the primary side of the high-frequency isolation transformer are in an equipotential state and the primary voltage is zero, and a freewheeling path for the leakage inductance current of the high-frequency isolation transformer is formed.

[0010] On the other hand, this application also proposes a cascaded high-frequency chain medium-voltage direct-connection multi-terminal charging control method, including: The sampling network side three-phase input current and the voltage of the common low-voltage DC charging bus are sampled, and the phase information of the medium-voltage AC bus voltage and the polarity information of the AC input voltage of each single-phase high-frequency chain AC / DC power module are obtained. Based on the deviation between the voltage reference value and the voltage sampling value of the common low-voltage DC charging bus, a grid-side active current reference is generated, and the three-phase current reference is obtained by combining the phase information of the medium-voltage AC bus voltage; based on the deviation between the three-phase current reference and the grid-side three-phase input current, current closed-loop control is performed to obtain the modulation wave corresponding to each phase. For each single-phase high-frequency chain AC / DC power module in each phase, the corresponding modulation wave is shared, and each single-phase high-frequency chain AC / DC power module is allocated a high-frequency triangular carrier with the same frequency and amplitude, and whose phases are staggered according to a preset angle. The modulation wave corresponding to each single phase is compared with the high-frequency triangular carrier wave corresponding to each single-phase high-frequency chain AC / DC power module to obtain the PWM logic signal corresponding to each single-phase high-frequency chain AC / DC power module, and the matrix converter is determined to be in a non-zero level state or a zero level state based on the PWM logic signal. The switching state of the matrix converter is determined based on the polarity information of the AC input voltage of each single-phase high-frequency AC / DC power module, the polarity information of the primary side expected voltage of the high-frequency isolation transformer, and the PWM logic signal. For each single-phase high-frequency chain AC / DC power module in each phase, PWM modulation is performed according to its corresponding high-frequency triangular carrier wave, so that the high-frequency switching pulses of each single-phase high-frequency chain AC / DC power module are staggered in time; the cascaded branches of each phase operate according to the corresponding modulation wave, and after passing through the matrix converter, high-frequency isolation transformer and secondary rectifier in each single-phase high-frequency chain AC / DC power module, the corresponding DC output is obtained and connected in parallel to the common low-voltage DC charging bus.

[0011] In one example, the method further includes: When the matrix converter is in a non-zero level state, based on the combination of the polarity information of the AC input voltage of the single-phase high-frequency chain AC / DC power module and the polarity information of the desired voltage on the primary side of the high-frequency isolation transformer, the corresponding diagonal switch path in the matrix converter is selected, and the AC input voltage of the single-phase high-frequency chain AC / DC power module is applied to the primary side of the high-frequency isolation transformer with the corresponding polarity, so that the primary side of the high-frequency isolation transformer forms a high-frequency AC pulse voltage. When the matrix converter is in a zero-level state, the corresponding switch of the matrix converter is turned on, so that the two terminals of the primary side of the high-frequency isolation transformer are connected to the same terminal of the AC input side of the single-phase high-frequency chain AC / DC power module, so that the two ends of the primary side of the high-frequency isolation transformer are in an equipotential state and the primary voltage is zero, and a freewheeling path for the leakage inductance current of the high-frequency isolation transformer is formed.

[0012] The cascaded high-frequency chain medium-voltage direct-connection multi-terminal charging control system proposed in this application can bring the following beneficial effects: 1. This application adopts an input series-output parallel structure with the medium-voltage AC side connected in series and the low-voltage DC side connected in parallel. Multiple single-phase high-frequency chain AC / DC power modules in each phase share the medium-voltage AC side voltage. The DC output of each single-phase high-frequency chain AC / DC power module is connected in parallel to the common low-voltage DC charging bus. It can be directly connected to the medium-voltage AC bus without setting up a power frequency transformer, and forms a unified low-voltage DC power supply interface, which is convenient for multiple DC charging terminals to be connected.

[0013] 2. The single-phase high-frequency AC / DC power module of this application consists of a matrix converter, a high-frequency isolation transformer and a secondary rectifier. The matrix converter directly converts the module's power frequency AC input into high-frequency AC, and after high-frequency isolation and secondary rectification, it forms a DC output. There is no need to set up a module-level intermediate DC link and its large-capacity energy storage capacitor, which reduces the number of independent power conversion stages and helps to improve the system power density and reliability.

[0014] 3. This application sets the matrix converter to a zero-level state, so that the two ends of the primary side of the high-frequency isolation transformer are connected to the same end of the module's AC input side during the zero-level period. While making the primary voltage zero, it provides a continuous path for the leakage inductance current of the high-frequency isolation transformer, avoiding sudden interruption of the leakage inductance current path during the commutation process. This helps to reduce the commutation overvoltage caused by leakage inductance and improve the commutation reliability of the matrix converter.

[0015] 4. This application employs a common modulation wave and carrier phase-shift modulation for each single-phase high-frequency chain AC / DC power module within the same phase, so that the high-frequency switching pulses of each module are staggered in time, reducing the high-frequency pulsation on the AC side of the cascaded branch; under the condition that the parameters of each power module are basically the same, the single-phase high-frequency chain AC / DC power modules in the same phase carry the same series current and have similar average modulation effects, so that the average transmission power of each single-phase high-frequency chain AC / DC power module tends to be consistent. Attached Figure Description

[0016] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a schematic diagram of the cascaded single-phase high-frequency AC / DC power module and the three-phase cascaded access topology in the embodiments of this application; Figure 2 This is a schematic diagram of the architecture of a medium-voltage AC bus access type multi-DC charging terminal system in an embodiment of this application; Figure 3 This is a schematic diagram illustrating the decoupling principle of the matrix transformer in the embodiments of this application; Figure 4 This is a schematic diagram of multi-module carrier phase shift modulation in an embodiment of this application; Figure 5 This is a flowchart illustrating the cascaded high-frequency chain medium-voltage direct-connection multi-terminal charging control method in this application embodiment. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] The technical solutions provided by the various embodiments of this application are described in detail below with reference to the accompanying drawings.

[0019] like Figure 1As shown, this application provides a cascaded high-frequency chain medium-voltage direct-connected multi-terminal charging control system, including: a three-phase cascaded power unit, a common low-voltage DC charging bus, and one or more DC charging terminals.

[0020] like Figure 2 As shown, the three-phase cascaded power unit is connected to phases A, B, and C of the medium-voltage AC bus via a grid-side AC filter. Each phase includes multiple series-connected single-phase high-frequency chain AC / DC power modules (hereinafter referred to as power modules in some descriptions), with the number denoted as N, where N is an integer greater than or equal to 2. The AC sides of each single-phase high-frequency chain AC / DC power module are connected in series sequentially, sharing the AC voltage of the corresponding phase. The medium-voltage AC bus is a three-phase AC bus of medium-voltage level, which may include 10kV, 35kV, or other medium-voltage levels.

[0021] like Figure 2 As shown, the common low-voltage DC charging bus is connected in parallel with a single-phase high-frequency AC / DC power module through an output filter. The common low-voltage DC charging bus connects to one or more DC charging terminals, which are charged via the common low-voltage DC charging bus. Each DC charging terminal includes a DC / DC converter and a corresponding power battery. Each DC charging terminal is equipped with corresponding connection, pre-charge, and protection devices to enable connection and disconnection between the DC charging terminal and the common low-voltage DC charging bus.

[0022] like Figure 3 As shown, a single-phase high-frequency chain AC / DC power module includes a matrix converter, a high-frequency isolation transformer, and a secondary-side rectifier. The AC input terminal of the matrix converter is connected to the cascaded branch, and the output terminal is connected to the primary side of the high-frequency isolation transformer, used to directly modulate the AC input voltage of the single-phase high-frequency chain AC / DC power module into a high-frequency AC pulse voltage. The high-frequency isolation transformer performs electrical isolation and voltage transformation. The high-frequency AC pulse voltage output from the secondary side of the high-frequency isolation transformer is rectified into a DC output by the secondary-side rectifier and connected to a common low-voltage DC charging bus. No module-level intermediate DC link or its energy storage capacitor is set between the AC-side matrix converter and the high-frequency isolation transformer, thus forming a direct high-frequency chain AC / DC conversion structure of power frequency AC—high-frequency AC—DC.

[0023] In one embodiment, the system further includes: a system controller; The system controller is used to sample the three-phase input current on the grid side, the voltage of the common low-voltage DC charging bus, and to acquire the phase information of the medium-voltage AC bus voltage and the polarity information of the AC input voltage of each single-phase high-frequency AC / DC power module.

[0024] Based on the deviation between the voltage reference value and the voltage sampling value of the common low-voltage DC charging bus, a grid-side active current reference is generated, and the three-phase current reference is obtained by combining the phase information of the medium-voltage AC bus voltage. Based on the deviation between the three-phase current reference and the grid-side three-phase input current, current closed-loop control is performed to obtain the modulation wave corresponding to each phase (A, B, and C phases).

[0025] like Figure 4 As shown, for each single-phase high-frequency chain AC / DC power module in each phase, the corresponding modulation wave is shared, and each single-phase high-frequency chain AC / DC power module is allocated a high-frequency triangular carrier (also called carrier phase shift or carrier phase) with the same frequency and amplitude but with phases staggered according to a preset angle. The carrier phase corresponding to the i-th power module is: Where N is the number of power modules in the cascaded branch. When the device parameters, high-frequency isolation transformer parameters, and operating conditions of each power module are basically the same, carrier phase shift mainly changes the temporal distribution of the high-frequency switching pulses of each module, without changing the average modulation effect of each power module within the same statistical period. Therefore, the average transmission power of each power module tends to be consistent.

[0026] The modulation wave corresponding to each single phase is compared with the high-frequency triangular carrier wave corresponding to each single-phase high-frequency chain AC / DC power module to obtain the PWM logic signal corresponding to each single-phase high-frequency chain AC / DC power module, and the matrix converter is determined to be in a non-zero level state or a zero level state based on the PWM logic signal.

[0027] The switching state of the matrix converter is determined based on the polarity information of the AC input voltage of each single-phase high-frequency AC / DC power module, the polarity information of the primary side expected voltage of the high-frequency isolation transformer, and the PWM logic signal.

[0028] When the matrix converter is in a non-zero level state, based on the combination of the polarity information of the AC input voltage of the single-phase high-frequency chain AC / DC power module and the desired polarity information of the primary side voltage of the high-frequency isolation transformer, the corresponding diagonal switch path in the matrix converter is selected. This applies the AC input voltage of the single-phase high-frequency chain AC / DC power module to the primary side of the high-frequency isolation transformer with the corresponding polarity, thus generating a high-frequency AC pulse voltage on the primary side of the high-frequency isolation transformer. When the matrix converter is in a zero-level state, the corresponding switch in the matrix converter is turned on, connecting the two terminals of the primary side of the high-frequency isolation transformer to the same terminal of the AC input side of the single-phase high-frequency chain AC / DC power module. This ensures that the two ends of the primary side of the high-frequency isolation transformer are at the same potential, and the primary voltage is zero, forming a freewheeling path for the leakage inductance current of the high-frequency isolation transformer.

[0029] For each single-phase high-frequency chain AC / DC power module in each phase, PWM modulation is performed according to its corresponding high-frequency triangular carrier wave. This causes the high-frequency switching pulses of each single-phase high-frequency chain AC / DC power module to be staggered in time, thereby increasing the equivalent high-frequency ripple frequency on the AC side of the cascaded branch and reducing the high-frequency ripple component. Each phase's cascaded branch operates according to the corresponding modulation wave. After passing through the matrix converter, high-frequency isolation transformer, and secondary rectifier in each single-phase high-frequency chain AC / DC power module, the corresponding DC output is obtained and connected in parallel to the common low-voltage DC charging bus.

[0030] like Figure 5 As shown, this application also provides a cascaded high-frequency chain medium-voltage direct-connection multi-terminal charging control method, including: The sampling network side three-phase input current and the voltage of the common low-voltage DC charging bus are sampled, and the phase information of the medium-voltage AC bus voltage and the polarity information of the AC input voltage of each single-phase high-frequency chain AC / DC power module are obtained. Based on the deviation between the voltage reference value and the voltage sampling value of the common low-voltage DC charging bus, a grid-side active current reference is generated, and the three-phase current reference is obtained by combining the phase information of the medium-voltage AC bus voltage; based on the deviation between the three-phase current reference and the grid-side three-phase input current, current closed-loop control is performed to obtain the modulation wave corresponding to each phase. For each single-phase high-frequency chain AC / DC power module in each phase, the corresponding modulation wave is shared, and each single-phase high-frequency chain AC / DC power module is allocated a high-frequency triangular carrier with the same frequency and amplitude, and whose phases are staggered according to a preset angle. The modulation wave corresponding to each single phase is compared with the high-frequency triangular carrier wave corresponding to each single-phase high-frequency chain AC / DC power module to obtain the PWM logic signal corresponding to each single-phase high-frequency chain AC / DC power module, and the matrix converter is determined to be in a non-zero level state or a zero level state based on the PWM logic signal. The switching state of the matrix converter is determined based on the polarity information of the AC input voltage of each single-phase high-frequency AC / DC power module, the polarity information of the primary side expected voltage of the high-frequency isolation transformer, and the PWM logic signal. For each single-phase high-frequency chain AC / DC power module in each phase, PWM modulation is performed according to its corresponding high-frequency triangular carrier wave, so that the high-frequency switching pulses of each single-phase high-frequency chain AC / DC power module are staggered in time; the cascaded branches of each phase operate according to the corresponding modulation wave, and after passing through the matrix converter, high-frequency isolation transformer and secondary rectifier in each single-phase high-frequency chain AC / DC power module, the corresponding DC output is obtained and connected in parallel to the common low-voltage DC charging bus.

[0031] In one embodiment, the method further includes: When the matrix converter is in a non-zero level state, based on the combination of the polarity information of the AC input voltage of the single-phase high-frequency chain AC / DC power module and the polarity information of the desired voltage on the primary side of the high-frequency isolation transformer, the corresponding diagonal switch path in the matrix converter is selected, and the AC input voltage of the single-phase high-frequency chain AC / DC power module is applied to the primary side of the high-frequency isolation transformer with the corresponding polarity, so that the primary side of the high-frequency isolation transformer forms a high-frequency AC pulse voltage. When the matrix converter is in a zero-level state, the corresponding switch of the matrix converter is turned on, so that the two terminals of the primary side of the high-frequency isolation transformer are connected to the same terminal of the AC input side of the single-phase high-frequency chain AC / DC power module, so that the two ends of the primary side of the high-frequency isolation transformer are in an equipotential state and the primary voltage is zero, and a freewheeling path for the leakage inductance current of the high-frequency isolation transformer is formed.

[0032] 1. This application adopts an input series-output parallel structure with the medium-voltage AC side connected in series and the low-voltage DC side connected in parallel. Multiple single-phase high-frequency chain AC / DC power modules in each phase share the medium-voltage AC side voltage. The DC output of each single-phase high-frequency chain AC / DC power module is connected in parallel to the common low-voltage DC charging bus. It can be directly connected to the medium-voltage AC bus without setting up a power frequency transformer, and forms a unified low-voltage DC power supply interface, which is convenient for multiple DC charging terminals to be connected.

[0033] 2. The single-phase high-frequency AC / DC power module of this application consists of a matrix converter, a high-frequency isolation transformer and a secondary rectifier. The matrix converter directly converts the module's power frequency AC input into high-frequency AC, and after high-frequency isolation and secondary rectification, it forms a DC output. There is no need to set up a module-level intermediate DC link and its large-capacity energy storage capacitor, which reduces the number of independent power conversion stages and helps to improve the system power density and reliability.

[0034] 3. This application sets the matrix converter to a zero-level state, so that the two ends of the primary side of the high-frequency isolation transformer are connected to the same end of the module's AC input side during the zero-level period. While making the primary voltage zero, it provides a continuous path for the leakage inductance current of the high-frequency isolation transformer, avoiding sudden interruption of the leakage inductance current path during the commutation process. This helps to reduce the commutation overvoltage caused by leakage inductance and improve the commutation reliability of the matrix converter.

[0035] 4. This application employs a common modulation wave and carrier phase-shift modulation for each single-phase high-frequency chain AC / DC power module within the same phase, so that the high-frequency switching pulses of each module are staggered in time, reducing the high-frequency pulsation on the AC side of the cascaded branch; under the condition that the parameters of each power module are basically the same, the single-phase high-frequency chain AC / DC power modules in the same phase carry the same series current and have similar average modulation effects, so that the average transmission power of each single-phase high-frequency chain AC / DC power module tends to be consistent.

[0036] The above description is merely an embodiment of this application and is not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. A cascaded high-frequency chain medium-voltage direct-connection multi-terminal charging control system, characterized in that, include: Three-phase cascaded power unit, common low-voltage DC charging bus, and one or more DC charging terminals; The three-phase cascaded power unit is connected to phases A, B, and C of the medium-voltage AC bus via a grid-side AC filter; and each phase's cascaded branch includes multiple series-connected single-phase high-frequency AC / DC power modules; wherein the medium-voltage AC bus is a medium-voltage three-phase AC bus. The common low-voltage DC charging bus is connected in parallel with the single-phase high-frequency AC / DC power module through an output filtering stage; The DC charging terminal is connected to the common low-voltage DC charging bus.

2. The system according to claim 1, characterized in that, The AC sides of each single-phase high-frequency AC / DC power module are connected in series to share the AC voltage of the corresponding phase.

3. The system according to claim 1, characterized in that, The common low-voltage DC charging bus connects to one or more of the DC charging terminals, and charges the DC charging terminals through the common low-voltage DC charging bus. Each of the DC charging terminals is equipped with corresponding connection, pre-charge, and protection devices to enable the connection and disconnection between the DC charging terminal and the common low-voltage DC charging bus.

4. The system according to claim 1, characterized in that, A single-phase high-frequency AC / DC power module includes a matrix converter, a high-frequency isolation transformer, and a secondary rectifier; The AC input terminal of the matrix converter is connected to the cascaded branch where it is located, and the output terminal is connected to the primary side of the high-frequency isolation transformer, which is used to directly modulate the AC input voltage of the single-phase high-frequency chain AC / DC power module into a high-frequency AC pulse voltage. No module-level intermediate DC link or its energy storage capacitor is provided between the matrix converter and the high-frequency isolation transformer. The high-frequency AC pulse voltage output from the secondary side of the high-frequency isolation transformer is rectified into DC output by the secondary side rectifier and connected to the common low-voltage DC charging bus.

5. The system according to claim 4, characterized in that, The system also includes: a system controller; The system controller is used to sample the three-phase input current on the grid side, the voltage of the common low-voltage DC charging bus, and to acquire the phase information of the medium-voltage AC bus voltage and the polarity information of the AC input voltage of each single-phase high-frequency chain AC / DC power module. Based on the deviation between the voltage reference value and the voltage sampling value of the common low-voltage DC charging bus, a grid-side active current reference is generated, and the three-phase current reference is obtained by combining the phase information of the medium-voltage AC bus voltage; based on the deviation between the three-phase current reference and the grid-side three-phase input current, current closed-loop control is performed to obtain the modulation wave corresponding to each phase. For each single-phase high-frequency chain AC / DC power module in each phase, the corresponding modulation wave is shared, and each single-phase high-frequency chain AC / DC power module is allocated a high-frequency triangular carrier with the same frequency and amplitude, and whose phases are staggered according to a preset angle. The modulation wave corresponding to each single phase is compared with the high-frequency triangular carrier wave corresponding to each single-phase high-frequency chain AC / DC power module to obtain the PWM logic signal corresponding to each single-phase high-frequency chain AC / DC power module, and the matrix converter is determined to be in a non-zero level state or a zero level state based on the PWM logic signal. The switching state of the matrix converter is determined based on the polarity information of the AC input voltage of each single-phase high-frequency AC / DC power module, the polarity information of the primary side expected voltage of the high-frequency isolation transformer, and the PWM logic signal. For each single-phase high-frequency chain AC / DC power module in each phase, PWM modulation is performed according to its corresponding high-frequency triangular carrier wave, so that the high-frequency switching pulses of each single-phase high-frequency chain AC / DC power module are staggered in time; the cascaded branches of each phase operate according to the corresponding modulation wave, and after passing through the matrix converter, high-frequency isolation transformer and secondary rectifier in each single-phase high-frequency chain AC / DC power module, the corresponding DC output is obtained and connected in parallel to the common low-voltage DC charging bus.

6. The system according to claim 5, characterized in that, The system controller is also used to, when the matrix converter is in a non-zero level state, select the corresponding diagonal switch path in the matrix converter according to the combination of the polarity information of the AC input voltage of the single-phase high-frequency chain AC / DC power module and the polarity information of the desired voltage on the primary side of the high-frequency isolation transformer, and apply the AC input voltage of the single-phase high-frequency chain AC / DC power module to the primary side of the high-frequency isolation transformer with the corresponding polarity, so that the primary side of the high-frequency isolation transformer forms a high-frequency AC pulse voltage. When the matrix converter is in a zero-level state, the corresponding switch of the matrix converter is turned on, so that the two terminals of the primary side of the high-frequency isolation transformer are connected to the same terminal of the AC input side of the single-phase high-frequency chain AC / DC power module, so that the two ends of the primary side of the high-frequency isolation transformer are in an equipotential state and the primary voltage is zero, and a freewheeling path for the leakage inductance current of the high-frequency isolation transformer is formed.

7. A method for controlling multi-terminal charging via a cascaded high-frequency chain with direct voltage connection, characterized in that, include: The sampling network side three-phase input current and the voltage of the common low-voltage DC charging bus are sampled, and the phase information of the medium-voltage AC bus voltage and the polarity information of the AC input voltage of each single-phase high-frequency chain AC / DC power module are obtained. Based on the deviation between the voltage reference value and the voltage sampling value of the common low-voltage DC charging bus, a grid-side active current reference is generated, and the three-phase current reference is obtained by combining the phase information of the medium-voltage AC bus voltage; based on the deviation between the three-phase current reference and the grid-side three-phase input current, current closed-loop control is performed to obtain the modulation wave corresponding to each phase. For each single-phase high-frequency chain AC / DC power module in each phase, the corresponding modulation wave is shared, and each single-phase high-frequency chain AC / DC power module is allocated a high-frequency triangular carrier with the same frequency and amplitude, and whose phases are staggered according to a preset angle. The modulation wave corresponding to each single phase is compared with the high-frequency triangular carrier wave corresponding to each single-phase high-frequency chain AC / DC power module to obtain the PWM logic signal corresponding to each single-phase high-frequency chain AC / DC power module, and the matrix converter is determined to be in a non-zero level state or a zero level state based on the PWM logic signal. The switching state of the matrix converter is determined based on the polarity information of the AC input voltage of each single-phase high-frequency AC / DC power module, the polarity information of the primary side expected voltage of the high-frequency isolation transformer, and the PWM logic signal. For each single-phase high-frequency chain AC / DC power module in each phase, PWM modulation is performed according to its corresponding high-frequency triangular carrier wave, so that the high-frequency switching pulses of each single-phase high-frequency chain AC / DC power module are staggered in time; the cascaded branches of each phase operate according to the corresponding modulation wave, and after passing through the matrix converter, high-frequency isolation transformer and secondary rectifier in each single-phase high-frequency chain AC / DC power module, the corresponding DC output is obtained and connected in parallel to the common low-voltage DC charging bus.

8. The method according to claim 7, characterized in that, The method further includes: When the matrix converter is in a non-zero level state, based on the combination of the polarity information of the AC input voltage of the single-phase high-frequency chain AC / DC power module and the polarity information of the desired voltage on the primary side of the high-frequency isolation transformer, the corresponding diagonal switch path in the matrix converter is selected, and the AC input voltage of the single-phase high-frequency chain AC / DC power module is applied to the primary side of the high-frequency isolation transformer with the corresponding polarity, so that the primary side of the high-frequency isolation transformer forms a high-frequency AC pulse voltage. When the matrix converter is in a zero-level state, the corresponding switch of the matrix converter is turned on, so that the two terminals of the primary side of the high-frequency isolation transformer are connected to the same terminal of the AC input side of the single-phase high-frequency chain AC / DC power module, so that the two ends of the primary side of the high-frequency isolation transformer are in an equipotential state and the primary voltage is zero, and a freewheeling path for the leakage inductance current of the high-frequency isolation transformer is formed.

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