A battery cluster series-parallel equalization energy storage system and its control method

Abstract

The present invention discloses a battery cluster series-parallel equalization energy storage system and its control method. The system includes: two groups of battery clusters, a low-voltage DC bus capacitor, multiple power electronic half-bridge units, multiple branch inductors, and multiple DC switches; each power electronic half-bridge unit is connected in parallel with the low-voltage DC bus capacitor, and after a branch inductor and a DC switch are connected in series at the output end, it serves as a current-sharing branch; the negative electrodes of the first group of multiple battery clusters are respectively connected to the first group of multiple current-sharing branches, and the corresponding positive electrodes are connected together to form the positive electrode of the medium-voltage DC bus. The positive electrodes of the second group of multiple battery clusters are respectively connected to the second group of multiple current-sharing branches, and the corresponding negative electrodes are connected together to serve as the negative electrode of the medium-voltage DC bus. The present invention simplifies the circuit by adopting a low-voltage and low-power half-bridge circuit with a common low-voltage DC bus, and at the same time eliminates the external power supply, further reducing the system cost and improving the efficiency.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and particularly to an energy storage system for series-parallel balancing of battery clusters and a control method thereof. Background Art

[0002] In the field of large-scale battery energy storage, centralized energy storage has the advantages of large single-machine capacity, low cost, and high PCS efficiency. However, it usually requires a large number of battery clusters to be directly paralleled. Due to the differences in internal resistance and voltage among the battery clusters, a large circulating current is easily generated, resulting in a decrease in the available capacity of the system and additional loss heating. To solve the problems of SOC imbalance and parallel circulating current among the battery clusters, the traditional method is to boost the voltage of each battery cluster through a first-stage full-power DC / DC converter and then parallel them. As Figure 1 shown, on the one hand, it can ensure the stability of the output voltage, and on the other hand, it can adapt to the voltage changes of different energy storage branches and control the output current of each energy storage branch. However, this DC / DC converter needs to work under high voltage and large current conditions simultaneously, which not only has large losses but also high costs. How to improve the performance and efficiency of large-capacity energy storage and reduce costs is a difficult problem that the current energy storage industry urgently needs to solve. Another idea is to use a partial power converter, and a low-voltage DC voltage source with adjustable voltage is connected in series at the output of each battery cluster to compensate the output voltage. Its schematic diagram is as Figure 2 shown, but this method requires one or more isolated DC power supplies powered externally, increasing the system cost and reducing the efficiency. Summary of the Invention

[0003] The present invention aims to solve at least one of the technical problems in the related art to some extent.

[0004] To avoid the circulating current problem caused by directly paralleling a large number of battery clusters in a centralized energy storage system, and at the same time reduce the power and cost of the DC / DC converter in a two-stage energy storage system, the present invention proposes a solution for a series-parallel balanced energy storage system of battery clusters, and its advantages include: (1) By adopting a low-voltage and low-power half-bridge circuit with a common low-voltage DC bus, the circuit is simplified and the cost is reduced; (2) Through the indirect series connection of two groups of battery clusters, on the one hand, the output voltage is higher and the capacity is larger, and on the other hand, the power of the low-voltage DC bus can be balanced, eliminating the external power supply and further reducing the system cost and improving the efficiency.

[0005] Another object of the present invention is to propose a control method for a series-parallel balanced energy storage system of battery clusters.

[0006] To achieve the above object, on the one hand, the present invention proposes a series-parallel balanced energy storage system of battery clusters, including:

[0007] Two groups of battery clusters, a low-voltage DC bus capacitor, multiple power electronic half-bridge units, multiple branch inductors, and multiple DC switches; each power electronic half-bridge unit is connected in parallel with the low-voltage DC bus capacitor, and after a branch inductor and a DC switch are connected in series at the output end, it serves as a current-sharing branch; the negative electrodes of the multiple battery clusters in the first group are respectively connected to the multiple current-sharing branches in the first group, and the corresponding positive electrodes are connected together to form the positive electrode of the medium-voltage DC bus. The positive electrodes of the multiple battery clusters in the second group are respectively connected to the multiple current-sharing branches in the second group, and the corresponding negative electrodes are connected together to serve as the negative electrode of the medium-voltage DC bus.

[0008] The battery cluster series-parallel equalization energy storage system according to the embodiment of the present invention may further have the following additional technical features:

[0009] In an embodiment of the present invention, the power electronic half-bridge unit is composed of two switching devices connected in series. Among them, the emitter of the upper transistor is connected to the collector of the lower transistor and serves as the output end of the power electronic half-bridge unit.

[0010] In an embodiment of the present invention, the collectors of the upper transistors of all the power electronic half-bridge units are connected to the positive electrode of the low-voltage DC bus capacitor to form the positive electrode of the low-voltage DC bus, and the emitters of the lower transistors of all the power electronic half-bridge units are connected to the negative electrode of the low-voltage DC bus capacitor to form the negative electrode of the low-voltage DC bus.

[0011] In an embodiment of the present invention, the positive electrode N+ or the negative electrode N- of the low-voltage DC bus is used as the neutral point of the medium-voltage DC bus.

[0012] In an embodiment of the present invention, it is also used for: when used in a large-scale battery energy storage system, a grid-connected inverter is connected between the positive electrode of the medium-voltage DC bus and the neutral point N+ or N-, and another grid-connected inverter is connected between the same neutral point and the negative electrode. The two inverters are connected to the grid through a double-split transformer to obtain a first circuit connection method;

[0013] Or, a grid-connected inverter with a DC bus neutral point and a neutral point led out is connected between the positive electrode, the neutral point N+ or N-, and the negative electrode of the medium-voltage DC bus, and then connected to the grid through a step-up transformer to obtain a second circuit connection method;

[0014] Or, only a grid-connected inverter is connected between the positive electrode and the negative electrode of the medium-voltage DC bus as a load, the neutral point is not connected, and then connected to the grid through a step-up transformer to obtain a third circuit connection method.

[0015] In an embodiment of the present invention, the switching devices of the power electronic half-bridge unit include multiple types of power electronic devices such as IGBT, MOSFET, and HEMT.

[0016] In one embodiment of the present invention, it is assumed that the highest voltage in the first group of n battery clusters is U Lmax , and the lowest voltage is U Lmin ; the highest voltage in the second group of n battery clusters is U Rmax , and the lowest voltage is U Rmin ; the low-voltage DC bus voltage is U b . When the first circuit connection method or the second circuit connection method is adopted,

[0017] If the neutral point connected to the output end is N+, the output voltage U d1 of the first group of battery clusters in parallel satisfies:

[0018] U Lmax -U b <U d1 <U Lmin (1)

[0019] The output voltage U d2 of the second group of battery clusters in parallel satisfies:

[0020] U Rmax <U d2 <U b +U Rmin (2)

[0021] If the neutral point connected to the output end is N-, the output voltage U d1 of the first group of battery clusters in parallel satisfies:

[0022] U Lmax <U d1 <U b +U Lmin (3)

[0023] The output voltage U d2 of the second group of battery clusters in parallel satisfies:

[0024] U Rmax -U b <U d2 <U Rmin (4)

[0025] When the third circuit connection method is adopted,

[0026] the output voltage U d of the medium-voltage DC bus after the first group and the second group of battery clusters are connected in series satisfies:

[0027] U Lmax +U Rmax -U b <U d <U Lmin +U Rmin +Ub (5)

[0028] According to formulas (1)-(5), the magnitude of the low-voltage DC bus voltage U b satisfies:

[0029]

[0030] To achieve the above object, on the other hand, the present invention proposes a control method for a battery cluster series-parallel equalization energy storage system, which adopts voltage-current double closed-loop control. The method includes:

[0031] S1. Obtain the operating voltages of all battery clusters, and based on different circuit connection modes, obtain the set values of the output voltages of two groups of battery clusters and the low-voltage DC bus voltage according to multiple formulas;

[0032] S2. Set the allowable operating voltage range of the battery clusters. If the preset operating voltage range is exceeded, disconnect the DC switch of the corresponding current-sharing branch;

[0033] S3. Based on the set value of the low-voltage DC bus voltage in S1, perform closed-loop control on the low-voltage DC bus voltage, and adjust the set values of the output voltages of two groups of battery clusters or the set value of the medium-voltage DC bus voltage;

[0034] S4. Based on the adjusted set values of the output voltages of two groups of battery clusters in S3, perform closed-loop control on the output voltages of two groups of battery clusters to obtain the set value of the total current of all branches;

[0035] S5. Distribute the current set value of each branch according to the voltage or SOC level of the battery clusters in each branch;

[0036] S6. Perform closed-loop control on the inductor current of each branch according to the distributed current set value of each branch to obtain the switching control signal of the power electronic half-bridge unit of each branch.

[0037] For the battery cluster series-parallel equalization energy storage system and control method according to the embodiments of the present invention, the circuit is simplified by adopting a low-voltage and low-power half-bridge circuit with a common low-voltage DC bus, and at the same time, the external power supply is omitted, further reducing the system cost and improving the efficiency.

[0038] The additional aspects and advantages of the present invention will be partially given in the following description, partially will become obvious from the following description, or will be understood through the practice of the present invention. Brief Description of the Drawings

[0039] The above-mentioned and / or additional aspects and advantages of the present invention will become obvious and easy to understand from the following description of the embodiments in conjunction with the drawings, where:

[0040] Figure 1 is a two-stage battery energy storage circuit of the prior art;

[0041] Figure 2 is a voltage source series compensated battery energy storage circuit of the prior art;

[0042] Figure 3 is a schematic structural diagram of a battery cluster series - parallel equalizing energy storage system according to an embodiment of the present invention;

[0043] Figures 4(a), 4(b) and 4(c) are respectively schematic diagrams of three connection modes of the battery cluster series - parallel equalizing energy storage system according to an embodiment of the present invention connected to a grid - connected inverter;

[0044] Figure 5 is a flowchart of a control method for a battery cluster series - parallel equalizing energy storage system according to an embodiment of the present invention;

[0045] Figure 6 is a schematic structural diagram of a photovoltaic power generation system according to an embodiment of the present invention. Detailed implementation manners

[0046] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the drawings and in combination with the embodiments.

[0047] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0048] The battery cluster series - parallel equalizing energy storage system and control method according to an embodiment of the present invention will be described below with reference to the drawings.

[0049] Figure 3 is a schematic structural diagram of a battery cluster series - parallel equalizing energy storage system according to an embodiment of the present invention. As Figure 3 shown,

[0050] The battery cluster series - parallel equalizing energy storage system proposed by the present invention includes two groups of n battery clusters each, 1 low - voltage DC bus capacitor, 2n power electronic half - bridge units, 2n branch inductors and 2n DC switches, where n is a positive integer greater than or equal to 2;

[0051] Exemplarily, the power electronic half - bridge unit is formed by two switching devices connected in series, and the emitter of the upper transistor is connected to the collector of the lower transistor and serves as the output terminal of the power electronic half - bridge unit;

[0052] Exemplarily, the upper tube collectors of all the power electronic half-bridge units are connected to the low voltage DC bus capacitor C b The positive electrode of the low-voltage DC bus is connected to form the positive electrode N+ of the low-voltage DC bus. The lower tube emitters of all the power electronic half-bridge units are connected to the low-voltage DC bus capacitor C b The negative pole is connected to form the negative pole N- of the low voltage DC bus;

[0053] Exemplarily, the output end of each power electronic half-bridge unit is connected in series with a branch inductor and a DC switch to serve as a current sharing branch;

[0054] Exemplarily, the negative electrodes of the first group of n battery clusters are respectively connected to the first group of n current sharing branches, and their positive electrodes are connected together to form the positive electrode DC+ of the medium voltage DC bus; the positive electrodes of the second group of n battery clusters are respectively connected to the second group of n current sharing branches, and their negative electrodes are connected together to serve as the negative electrode DC- of the medium voltage DC bus; the positive electrode N+ or negative electrode N- of the low voltage DC bus is used as the neutral point of the medium voltage DC bus;

[0055] Exemplarily, when used in a large-scale battery energy storage system, a grid-connected inverter can be connected between the positive pole DC+ and the neutral point N+ or N- of the medium voltage DC bus, and another grid-connected inverter can be connected between the same neutral point and the negative pole DC-. The two inverters are connected to the grid through a double-split transformer to obtain a first circuit connection mode, as shown in Figure 4(a).

[0056] Alternatively, a grid-connected inverter with a neutral point is connected between the positive pole DC+, the neutral point N+ or N-, and the negative pole DC- of the medium voltage DC bus, such as a diode-clamped three-level, an active neutral point-clamped three-level, a T-type three-level active neutral point-clamped five-level inverter with a DC bus neutral point, and then connected to the grid through a step-up transformer to obtain a second circuit connection mode, as shown in FIG4(b);

[0057] Alternatively, only one grid-connected inverter is connected between the positive pole DC+ and the negative pole DC- of the medium voltage DC bus as a load, the neutral point is not connected, and then connected to the grid through a step-up transformer, to obtain a third circuit connection mode, as shown in FIG4(c).

[0058] It can be understood that, compared with the existing patents, the negative electrode of the first battery cluster of the present invention is connected to the current sharing branch, and the positive electrode of the second battery cluster is connected to the current sharing branch. The advantages are: first, the indirect series connection of the two battery clusters is realized, and the output voltage is higher; second, by adopting a certain control method, the voltage of the low-voltage DC bus capacitor can be kept stable, and no external power supply is required.

[0059] Furthermore, assuming that the highest voltage in the first group of n battery clusters is U Lmax , the minimum voltage is ULmin ; The highest voltage in the second group of n battery clusters is U Rmax , and the lowest voltage is U Rmin ; The low-voltage DC bus voltage is U b .

[0060] When the connection method shown in Fig. 4(a) or Fig. 4(b) is adopted,

[0061] (1) If the neutral point connected to the output end is N+, the output voltage U of the first group of battery clusters in parallel d1 shall satisfy:

[0062] U Lmax -U b <U d1 <U Lmin (1)

[0063] The output voltage U of the second group of battery clusters in parallel d2 shall satisfy:

[0064] U Rmax <U d2 <U b +U Rmin (2)

[0065] (2) If the neutral point connected to the output end is N-, the output voltage U of the first group of battery clusters in parallel d1 shall satisfy:

[0066] U Lmax <U d1 <U b +U Lmin (3)

[0067] The output voltage U of the second group of battery clusters in parallel d2 shall satisfy:

[0068] U Rmax -U b <U d2 <U Rmin (4)

[0069] When the connection method shown in Fig. 4(c) is adopted,

[0070] The output voltage U of the medium-voltage DC bus after the first group and the second group of battery clusters are connected in series d shall satisfy:

[0071] U Lmax +U Rmax -U b <U d <U Lmin +U Rmin +U b (5)

[0072] According to formulas (1) to (5), the low-voltage DC bus voltage U b needs to satisfy:

[0073]

[0074] To minimize the loss, the set value of the low-voltage DC bus voltage is minimized on the premise of satisfying formula (6).

[0075] The battery cluster series-parallel equalizing energy storage system of the embodiment of the present invention simplifies the circuit by adopting a low-voltage and low-power half-bridge circuit with a common low-voltage DC bus, and at the same time eliminates the external power supply, further reducing the system cost and improving the efficiency.

[0076] To implement the above embodiment, as Figure 5 shown, a control method for the battery cluster series-parallel equalizing energy storage system is also provided in this embodiment. Adopting voltage-current double closed-loop control, the method includes:

[0077] S1. Obtain the operating voltage of the battery cluster, and based on different circuit connection modes, obtain the output voltages U d1 、U d2 or the total output voltage U d of two groups of battery clusters according to formulas (1) to (6), as well as the set value of the low-voltage DC bus voltage U b ;

[0078] S2. Set the allowable operating voltage range of the battery cluster. If it exceeds the preset operating voltage range, disconnect the DC switch of the corresponding current-sharing branch;

[0079] S3. Based on the set value of the low-voltage DC bus voltage in S1 and different circuit connection modes, perform closed-loop control on the low-voltage DC bus voltage, and adjust the set values U d1 、U d2 of the output voltages of two groups of battery clusters to achieve balanced control of the low-voltage DC bus voltage;

[0080] S4. Based on the adjusted set values of the output voltages of two groups of battery clusters in S3, perform closed-loop control on the output DC voltages of two groups of battery clusters to obtain the total current given value of all branches;

[0081] S5. Allocate the current given value of each branch according to the voltage or SOC level of the battery clusters in each branch;

[0082] S6. Perform closed-loop control on the inductor current of each branch according to the allocated current given value of each branch to obtain the switching control signal of the power electronic half-bridge unit of each branch.

[0083] Specifically, the steps for performing closed-loop control on the low-voltage DC bus voltage based on different circuit connection methods in step S3 are as follows:

[0084] To achieve the goal of eliminating the external power supply, it is necessary to ensure that the voltage of the low-voltage DC bus always remains stable. Taking the connection method shown in Fig. 4(a) or Fig. 4(b) and connecting to the neutral point N+ as an example, when the energy storage system discharges, the first battery cluster will cause the low-voltage DC bus capacitor to charge, and the second battery cluster will cause the low-voltage DC bus capacitor to discharge. Therefore, the voltage of the low-voltage DC bus capacitor can be controlled by adjusting the output DC bus voltage.

[0085] Specifically: when the energy storage system discharges, if the low-voltage DC bus voltage is higher than the rated value, the output voltage U of the first battery cluster can be increased to reduce the charging amount of the low-voltage DC bus capacitor; and / or the output voltage U of the second battery cluster can be increased to increase the discharging amount of the low-voltage DC bus capacitor. d1 And / or increase the output voltage U of the second battery cluster to increase the discharging amount of the low-voltage DC bus capacitor. d2 If the low-voltage DC bus voltage is lower than the rated value, the output voltage U of the first battery cluster can be decreased to increase the charging amount of the low-voltage DC bus capacitor; and / or the output voltage U of the second battery cluster can be decreased to reduce the discharging amount of the low-voltage DC bus capacitor.

[0086] If the low-voltage DC bus voltage is lower than the rated value, the output voltage U of the first battery cluster can be decreased to increase the charging amount of the low-voltage DC bus capacitor; and / or the output voltage U of the second battery cluster can be decreased to reduce the discharging amount of the low-voltage DC bus capacitor. d1 And / or decrease the output voltage U of the second battery cluster to reduce the discharging amount of the low-voltage DC bus capacitor. d2 If the low-voltage DC bus voltage is lower than the rated value, the output voltage U of the first battery cluster can be decreased to increase the charging amount of the low-voltage DC bus capacitor; and / or the output voltage U of the second battery cluster can be decreased to reduce the discharging amount of the low-voltage DC bus capacitor.

[0087] Furthermore, the present invention can also be used for the series and parallel connection of photovoltaic strings. Since photovoltaic power generation only requires unidirectional power output, the lower transistor of the first power electronic half-bridge unit and the upper transistor of the second power electronic half-bridge unit can be simplified to diodes, as Figure 6 shown.

[0088] According to the control method of the battery cluster series and parallel connection equalization energy storage system of the embodiment of the present invention, it avoids the uneven current problem caused by direct parallel connection of battery clusters, reduces the power and cost of the DC / DC converter, eliminates the external power supply, and realizes effective control of the output current of each battery cluster.

[0089] In the description of this specification, the descriptions referring to terms such as "one embodiment", "some embodiments", "examples", "specific examples", or "some examples", etc., mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, without contradiction, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0090] In addition, the terms "first" and "second" are used only for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise specifically and clearly defined.

Claims

1. A battery cluster series-parallel balanced energy storage system, characterized in that: include: Two groups of battery clusters, a low-voltage DC bus capacitor, multiple power electronic half-bridge units, multiple branch inductors and multiple DC switches; the power electronic half-bridge unit is connected in series with two switch devices, wherein the emitter of the upper tube is connected to the collector of the lower tube and serves as the output end of the power electronic half-bridge unit, the upper tube collectors of all the power electronic half-bridge units are connected to the positive electrode of the low-voltage DC bus capacitor to form the positive electrode N+ of the low-voltage DC bus, and the lower tube emitters of all the power electronic half-bridge units are connected to the negative electrode of the low-voltage DC bus capacitor to form the negative electrode N- of the low-voltage DC bus; the output end of each power electronic half-bridge unit is connected in series with a branch inductor and a DC switch to form a current sharing branch; the negative electrodes of the first group of multiple battery clusters are respectively connected to the first group of multiple current sharing branches, and the corresponding positive electrodes are connected together to form the positive electrode of the medium-voltage DC bus, and the positive electrodes of the second group of multiple battery clusters are respectively connected to the second group of multiple current sharing branches, and the corresponding negative electrodes are connected together to serve as the negative electrode of the medium-voltage DC bus.

2. The system according to claim 1, characterized in that The positive pole N+ or negative pole N- of the low voltage DC bus is used as the neutral point of the medium voltage DC bus.

3. The system according to claim 2, characterized in that Also used for: when used in a large-scale battery energy storage system, a grid-connected inverter is connected between the positive pole and the neutral point N+ or N- of the medium voltage DC bus, and another grid-connected inverter is connected between the same neutral point and the negative pole, and the two inverters are connected to the grid through a double-split transformer to obtain a first circuit connection mode; Or, a grid-connected inverter having a neutral point of the DC bus and leading out the neutral point is connected between the positive pole, the neutral point N+ or N-, and the negative pole of the medium voltage DC bus, and then connected to the grid through a step-up transformer to obtain a second circuit connection mode; Alternatively, only one grid-connected inverter is connected between the positive and negative poles of the medium voltage DC bus as a load, the neutral point is not connected, and then connected to the grid through a step-up transformer to obtain a third circuit connection mode.

4. The system according to claim 1, characterized in that The switching devices of the power electronic half-bridge unit include multiple types of IGBT, MOSFET, and HEMT power electronic devices.

5. The system according to claim 3, characterized in that Assume that the highest voltage in the first group of n battery clusters is U Lmax , the minimum voltage is U Lmin ; The highest voltage in the second group of n battery clusters is U Rmax , the minimum voltage is U Rmin ; The low voltage DC bus voltage is U b , when the first circuit connection mode or the second circuit connection mode is adopted, If the neutral point connected to the output end is N+, the output voltage U d1 satisfy: IN Lmax -IN b d1 Lmin (1)​​ The output voltage U of the second battery cluster after parallel connection d2 satisfy: IN Rmax d2 b +U Rmin (2)​​ If the neutral point connected to the output end is N-, the output voltage U of the first battery cluster after parallel connection d1 satisfy: IN Lmax d1 b +U Lmin (3)​​ The output voltage U of the second battery cluster after parallel connection d2 satisfy: When the third circuit connection method is adopted, The output voltage U of the medium voltage DC bus after the first and second battery clusters are connected in series d satisfy: IN Lmax +U Rmax -IN b d Lmin +U Rmin +U b (5)​​ According to equations (1) to (5), the low-voltage DC bus voltage U b The size meets:

6. A control method for a battery cluster series-parallel balanced energy storage system according to any one of claims 1 to 5, characterized in that: Adopt voltage and current dual closed-loop control, the method includes: S1, obtaining the operating voltages of all battery clusters, and obtaining the set values ​​of the output voltages of two battery clusters and the low-voltage DC bus voltage according to multiple formulas based on different circuit connection methods; S2, setting the operating voltage range allowed by the battery cluster. If the operating voltage exceeds the preset range, disconnecting the DC switch of the corresponding current sharing branch; S3, according to the low-voltage DC bus voltage setting value of S1, close-loop control is performed on the low-voltage DC bus voltage to adjust the output voltage of the two battery clusters or the setting value of the medium-voltage DC bus voltage; S4, according to the output voltage setting values ​​of the two battery clusters adjusted in S3, performing closed-loop control on the output voltages of the two battery clusters to obtain a total current given value of all branches; S5, allocating a current given value to each branch according to the voltage or SOC of the battery cluster of each branch; S6, performing closed-loop control on the inductor current of each branch according to the assigned current given value of each branch, and obtaining a switch control signal of the power electronic half-bridge unit of each branch.

7. The method according to claim 6, characterized in that Based on different circuit connection methods, the low-voltage DC bus voltage is closed-loop controlled, including: When the first circuit connection mode or the second circuit connection mode is adopted and the neutral point N+ is connected, when the energy storage system is discharged, if the low-voltage DC bus voltage is higher than the rated value, the output voltage of the first battery cluster is increased, so that the charging amount of the low-voltage DC bus capacitor is reduced; and / or the output voltage of the second battery cluster is increased, so that the discharge amount of the low-voltage DC bus capacitor is increased; If the low-voltage DC bus voltage is lower than the rated value, the output voltage of the first battery cluster is reduced, thereby increasing the charge amount of the low-voltage DC bus capacitor; and / or the output voltage of the second battery cluster is reduced, thereby reducing the discharge amount of the low-voltage DC bus capacitor.

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