A three-level SOC balancing method for MMC-BESS considering power device safety

Abstract

A MMC-BESS three-level SOC balancing method considering the safety of power devices belongs to the field of energy storage management. It includes: using the DC current distribution method to calculate the DC interphase balancing current superimposed on each phase; using the base frequency current injection method to calculate the base frequency balancing current between bridge arms superimposed on each phase; using the DC voltage distribution method or the AC voltage distribution method to achieve submodule-level SOC balancing; according to the performance parameters of the selected power device, setting its effective value monitoring threshold; monitoring the real-time effective value of each bridge arm current of the MMC-BESS; if the maximum value of the bridge arm current effective value exceeds the set threshold, recalculate the DC interphase balancing current superimposed on each phase to obtain the adjusted interphase balancing current superimposed on phase A; the adjusted interphase balancing currents of the other two phases are synchronously reduced in proportion. It dynamically adjusts the interphase balancing current and adaptively distributes it, which can prevent power devices from being damaged due to excessive balancing current.

Description

Technical Field

[0001] The present invention belongs to the field of operation control and management of electric energy storage systems, and in particular relates to an MMC-BESS three-level SOC balancing method considering the safety of power devices. Background Art

[0002] Using battery energy storage systems to smooth wind and photovoltaic power generation is an effective way to solve the problems of difficulty in connecting new energy to the grid and wind power “abandonment”.

[0003] The application of large-capacity battery energy storage in the power grid has changed the history of electricity being able to be transmitted but not stored, bringing revolutionary impacts to the production and operation of the power grid and greatly promoting the development of my country's smart grid.

[0004] With the promotion of energy storage technology in the power grid, the efficiency of renewable energy power generation and the quality of grid connection have been greatly improved, and the focus of public attention has shifted from the original "abandonment of solar and wind power" problems to the safety of energy storage systems.

[0005] The inconsistency and "short board effect" of batteries in the modular multilevel converter based BESS (MMC-BESS) will lead to overcharging, over-discharging and overcurrent in the battery pack, which will not only affect the life of the battery pack, but also may cause safety problems such as fire. Therefore, the balanced control of battery SOC (State of Charge, which refers to the ratio of the remaining capacity of a battery after a period of use to its capacity in a fully charged state) is the key.

[0006] The battery SOC balancing control of MMC-BESS can be divided into three levels, namely, inter-phase SOC balancing, inter-bridge arm SOC balancing, and inter-submodule SOC balancing.

[0007] The purpose of SOC balancing is to make the SOC of the batteries in the MMC-BESS approach consistent, in order to maximize the battery capacity utilization.

[0008] The more commonly used methods for inter-phase SOC balancing are the DC current distribution method and the zero-sequence voltage injection method. These two methods use the system's DC power and AC power respectively to achieve SOC balancing; inter-bridge arm SOC balancing is a first-level balancing unique to MMC-BESS. The fundamental frequency current injection method can be used. By taking advantage of the opposite phase characteristics of the upper and lower bridge arm voltages in the same phase, a fundamental frequency current with adjustable amplitude and the same or opposite phase to the AC output voltage of the phase is injected into the phase to achieve SOC balancing between bridge arms; the methods for sub-module SOC balancing include the DC voltage distribution method, the AC voltage distribution method, etc., which are similar to the inter-phase SOC balancing. DC deviation power or AC deviation power is generated according to the different SOCs of each sub-module to achieve SOC balancing.

[0009] The Chinese invention patent application with the application publication date of July 29, 2022 and the application publication number of CN 114825558 A discloses an active SOC balancing method for a cascaded energy storage system, which is implemented by the following steps: a) judging whether balancing is required; b) judging the charge and discharge mode; c) bypassing the battery cluster; d) SOC balancing control. The active SOC balancing method for a cascaded energy storage system disclosed in the technical scheme uses the difference between the standard deviation of the minimum voltage of the battery cluster and the preset value to judge whether SOC balancing needs to be performed, selects the SOC balancing mode according to the number of minimum cell voltages greater than and less than the minimum cell voltage mean, and finally performs SOC balancing control on the non-bypassed battery cluster. The active SOC balancing method for a cascaded energy storage system described in the technical scheme can achieve effective balancing of the state of charge SOC of the battery cluster in the cascaded energy storage system, effectively ensure the balancing of the remaining power of the battery cluster, and ensure the capacity of the entire cascaded energy storage system.

[0010] However, the current three-level SOC balancing method for MMC-BESS only considers the SOC of the battery, and the calculation of the balancing current does not take into account the tolerance of the devices in the main circuit, so there are certain safety hazards.

[0011] When the battery SOC distribution of each submodule in the MMC-BESS is very dispersed, the deviation power used for SOC balancing at each level is large, which will lead to a large balancing current. At this time, the current flowing through the system power device may be too large, causing the device to heat up and even risk damaging the power device, which will affect the safe and stable operation of the entire system.

[0012] How to achieve SOC balance at each level while limiting the effective value of the current flowing through the power device is the key to ensuring the safe and reliable operation of the system. Summary of the invention

[0013] The technical problem to be solved by the present invention is to provide a MMC-BESS three-level SOC balancing method that takes into account the safety of power devices. On the basis of realizing the inter-phase balancing, inter-bridge arm balancing and inter-submodule balancing of the battery SOC in the modular multi-level battery energy storage system, it determines the distribution strategy of the SOC balancing current by detecting the real-time effective value of the bridge arm current, ensures that the power devices in the system are in a safe working area, and provides a calculation method for the balancing current. On the premise of realizing the three-level balancing of the battery SOC in the modular multi-level battery energy storage system, it can avoid overheating and damage of the device due to excessive balancing current, ensure safe and stable operation, and improve the working reliability of the system.

[0014] The technical solution of the present invention is to provide a MMC-BESS three-level SOC balancing method considering the safety of power devices, which is characterized by comprising the following steps:

[0015] (1) The DC current distribution method is used to calculate the DC phase-to-phase balanced current superimposed on each phase;

[0016] (2) The fundamental frequency current injection method is used to calculate the corresponding inter-arm fundamental frequency balanced current superimposed on each phase;

[0017] (3) Submodule-level SOC balancing achieved by using DC voltage distribution method or AC voltage distribution method;

[0018] (4) According to the performance parameters of the selected power device, considering a certain safety margin, set the effective value monitoring threshold;

[0019] (5) monitoring the real-time effective value of each bridge arm current of the MMC-BESS. If the maximum value among the six bridge arm current effective values ​​does not exceed the threshold, performing SOC balancing according to the balancing method described in steps (1) to (3);

[0020] (6) If the maximum value among the effective values ​​of the six bridge arm currents exceeds the set threshold, the balancing method is adjusted and the balancing current is recalculated;

[0021] (7) At this time, the submodule-level SOC balancing method remains unchanged, and the DC interphase balancing current superimposed on each phase is recalculated to obtain the adjusted interphase balancing current superimposed on phase A;

[0022] (8) After the adjustment, the interphase balanced current of phases B and C decreases synchronously according to the ratio k;

[0023] (9) A dynamic real-time adjustment mode is adopted to adaptively distribute the inter-phase balancing current to ensure that the heating of the device is within the safe working area during system operation, thereby preventing power devices from being damaged due to excessive balancing current.

[0024] Specifically, the MMC-BESS three-level SOC balancing method, on the basis of realizing inter-phase balancing, inter-bridge arm balancing and inter-submodule balancing of battery SOC in the modular multi-level battery energy storage system, determines the allocation strategy of SOC balancing current by detecting the real-time effective value of the bridge arm current, and on the premise of realizing the three-level balancing of battery SOC in the modular multi-level battery energy storage system, avoids overheating and damage of devices due to excessive balancing current, ensures safe and stable operation, and improves system working reliability.

[0025] Furthermore, the MMC-BESS three-level SOC balancing method adopts a DC current distribution method and a base frequency current injection method to achieve phase-to-phase balancing and bridge-arm balancing of the battery SOC in the MMC-BESS, and a DC voltage distribution method or an AC voltage distribution method is adopted for balancing between submodules; by detecting the real-time effective value of the bridge arm current, the distribution strategy of the SOC balancing current is determined to ensure that the power devices in the system are in a safe working area.

[0026] Specifically, the DC current distribution method is used to calculate the DC phase-to-phase balanced current superimposed on each phase:

[0027]

[0028] Where i phasej is the DC phase-to-phase balancing current superimposed on each phase; K phase is the phase balance coefficient; SOC BESS is the system average SOC; SOC j is the phase SOC of each phase; U dc is the DC bus voltage.

[0029] Specifically, the fundamental frequency current injection method is used to calculate the corresponding inter-bridge arm fundamental frequency balanced current superimposed on each phase:

[0030]

[0031] Where i armj K is the base frequency balanced current between bridge arms superimposed on each phase; arm is the bridge arm balance coefficient; SOC pj is the average SOC of the upper bridge arm of this phase; SOC nj is the average SOC of the lower bridge arm of this phase; U g is the voltage amplitude at the AC side of MMC-BESS; is the phase angle of each phase voltage on the AC side of MMC-BESS.

[0032] Furthermore, the bridge arm balance coefficient K is selected arm The amplitude of the injected fundamental frequency current should be much smaller than the amplitude of the AC component of the bridge arm current.

[0033] Specifically, when recalculating the DC phase-to-phase balancing current superimposed on each phase, proceed as follows:

[0034] Let max(I RMSj ) exceeds the threshold, that is:

[0035] max(I RMSj )>I RMS_MAX (j = a) b, c)

[0036] The interphase balancing current superimposed on this phase before adjustment is i phasej The adjusted interphase balancing current superimposed on this phase is i phasej ′, then the interphase balancing current i superimposed on this phase phasej ′ should satisfy the following relationship:

[0037]

[0038] Where I dcj is the DC current of phase j when there is no SOC balancing method; I acj is the j-phase AC side current amplitude when there is no SOC balancing method;

[0039] The phase-to-phase balancing current superimposed on the phase after adjustment can be obtained:

[0040]

[0041] The interphase balanced currents of the other two phases after adjustment are synchronously reduced according to the ratio k.

[0042] Furthermore, the ratio k is obtained by the following formula:

[0043]

[0044] Furthermore, the MMC-BESS three-level SOC balancing method decouples the power on the AC side and the DC side and controls them respectively.

[0045] Furthermore, the MMC-BESS three-level SOC balancing method adopts carrier phase shift modulation to realize switch control of submodules.

[0046] Compared with the prior art, the advantages of the present invention are:

[0047] 1. The technical solution of the present invention, on the basis of realizing the inter-phase balancing, inter-bridge arm balancing and inter-submodule balancing of the battery SOC in the modular multi-level battery energy storage system, determines the allocation strategy of the SOC balancing current by detecting the real-time effective value of the bridge arm current, thereby ensuring that the power devices in the system are in a safe working area;

[0048] 2. The technical solution of the present invention provides reliable support for battery SOC balancing in MMC-BESS, adaptively distributes balancing current, ensures the safety and stability of system power devices, and ensures the normal functioning of the system;

[0049] 3. The technical solution of the present invention can avoid overheating and damage of devices due to excessive balancing current while achieving three-level SOC balancing of the modular multi-level battery energy storage system, thereby ensuring safe and stable operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 Schematic diagram of the topological structure of the main circuit of the MMC-BESS of the present invention;

[0051] Figure 2 is a system power control block diagram of the present invention;

[0052] Figure 3This is a control block diagram of the system SOC balancing method of the present invention. DETAILED DESCRIPTION

[0053] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0054] Figure 1 As shown in , it is a schematic diagram of the topological structure of the MMC-BESS main circuit of the present invention.

[0055] The MMC-BESS main circuit includes three phase clusters. Each phase cluster is divided into an upper bridge arm and a lower bridge arm. The bridge arm contains several sub-modules, and the number of sub-modules contained in the upper and lower bridge arms is the same.

[0056] In each phase cluster, the positive pole of the DC bus is connected to the output positive terminal of the first submodule of the upper bridge arm, and the output negative terminal is connected to the output positive terminal of the next submodule, and so on; the output negative terminal of the last submodule of the upper bridge arm is connected to the bridge arm inductor; the connection method of the lower bridge arm is the same as that of the upper bridge arm: after the bridge arm inductor and each submodule are connected in sequence, the output negative terminal of the last submodule is connected to the negative pole of the DC bus.

[0057] The midpoints of the upper and lower bridge arms are connected together to form an outlet for the AC grid voltage, and are led out through an AC inductor to form the AC side of the system, which is connected to a three-phase load. The entire system is a three-phase three-wire system.

[0058] The system can be connected to AC and DC grids. The submodules are mainly composed of switch devices, capacitors and batteries, and each submodule has two terminals for cascading with other submodules.

[0059] Obviously, in Figure 1 The submodule shown in the figure is a half-bridge structure, and the half-bridge circuit forms a parallel structure with the capacitor and the battery.

[0060] right Figure 1 The meanings of the various physical quantities appearing in the table are shown in the following table.

[0061] Table 1 Meaning of symbols in MMC-BESS topology

[0062]

[0063]

[0064] It should be noted here that the AC side output voltage u sj The reference point is the neutral point O on the AC side, and the AC side grid voltage u gjThe reference point is the neutral point of the DC side, which is also the zero potential point O' of the system. The reference direction of the DC bus current is specified as flowing from the positive end of the DC bus to the bridge arm; the reference direction of the upper bridge arm current is flowing from the positive end of the DC bus to the bridge arm inductor; the reference direction of the lower bridge arm current is flowing from the bridge arm inductor to the negative end of the DC bus; the reference direction of the AC output phase current is flowing from the MMC AC outlet to the grid-connected inductor; the reference direction of the voltage in the system is consistent with the reference direction.

[0065] On this basis, the technical solution of the present invention provides a MMC-BESS three-level SOC balancing method considering the safety of power devices, which respectively adopts a DC current distribution method and a base frequency current injection method to achieve inter-phase balancing and inter-bridge arm balancing of the battery SOC in the MMC-BESS, and the inter-submodule balancing can adopt a DC voltage distribution method or an AC voltage distribution method; by detecting the real-time effective value of the bridge arm current, the distribution strategy of the SOC balancing current is determined to ensure that the power devices in the system are in a safe working area. The specific implementation steps are as follows:

[0066] (1) The DC current distribution method is used to calculate the DC phase-to-phase balanced current superimposed on each phase:

[0067]

[0068] Where i phasej ——DC phase-to-phase balancing current superimposed on each phase;

[0069] K phase ——phase balance coefficient;

[0070] SOC BESS ——System average SOC;

[0071] SOC j ——phase SOC of each phase;

[0072] U dc ——DC bus voltage;

[0073] (2) The fundamental frequency current injection method is used to calculate the corresponding bridge arm fundamental frequency balanced current superimposed on each phase:

[0074]

[0075] Where i armj ——Basic frequency balanced current between bridge arms superimposed on each phase;

[0076] K arm ——Balance coefficient between bridge arms;

[0077] SOC pj ——The average SOC of the upper bridge arm of this phase;

[0078] SOC nj ——The average SOC of the lower bridge arm of this phase;

[0079] U g ——Amplitude of voltage on the AC side of MMC-BESS;

[0080] ——The phase angle of each phase voltage on the AC side of MMC-BESS;

[0081] The selected bridge arm balance coefficient K arm The amplitude of the injected fundamental frequency current should be much smaller than the amplitude of the AC component of the bridge arm current;

[0082] (3) Submodule-level SOC balancing achieved by using DC voltage distribution method or AC voltage distribution method;

[0083] (4) According to the performance parameters of the selected power device, consider a certain safety margin and set the effective value monitoring threshold I RMS_MAX ;

[0084] (5) Monitor the real-time effective value of each bridge arm current of the MMC-BESS. If the maximum value of the six bridge arm current effective values ​​does not exceed the threshold value I RMS_MAX , then perform SOC balancing according to the balancing method described in steps (1) to (3);

[0085] (6) If the maximum value of the six bridge arm current effective values ​​max(I RMSj ) exceeds the set threshold I RMS_MAX , then adjust the balancing method and recalculate the balancing current: the submodule level SOC balancing generates a deviation voltage instead of a current. At this time, the submodule level SOC balancing method remains unchanged; the base frequency current used for balancing between bridge arms passes through the bridge arm balancing coefficient K arm The selection of is much smaller than the AC component in the bridge arm current, and its influence on the effective value of the bridge arm current can be ignored; the DC phase-to-phase balancing current superimposed on each phase is recalculated as follows:

[0086] Let max(I RMSj ) exceeds the threshold, that is:

[0087] max(I RMSj )>I RMS_MAX (j = a, b, c)

[0088] The interphase balancing current superimposed on this phase before adjustment is i phasej The adjusted interphase balancing current superimposed on this phase is i phasej ′,i phasej ′ should satisfy the following relationship:

[0089]

[0090] Where I dcj ——J-phase DC current when there is no SOC balancing method;

[0091] I acj ——The current amplitude of the j-phase AC side when there is no SOC balancing method;

[0092] The phase-to-phase balancing current superimposed on the phase after adjustment can be obtained:

[0093]

[0094] The interphase balanced current of the other two phases after adjustment is reduced synchronously according to the ratio k, which can be obtained by the following formula:

[0095]

[0096] In summary, the technical solution of the present invention, on the basis of realizing the inter-phase balance, inter-bridge arm balance and inter-submodule balance of the battery SOC in the modular multi-level battery energy storage system, determines the distribution strategy of the SOC balancing current by detecting the real-time effective value of the bridge arm current, thereby ensuring that the power devices in the system are in a safe working area.

[0097] The technical solution of the present invention can avoid overheating and damage of devices due to excessive balancing current while achieving three-level balancing of battery SOC in a modular multi-level battery energy storage system, thereby ensuring safe and stable operation and improving system working reliability.

[0098] Example:

[0099] use Figure 2 In the power control strategy shown, the power on the AC side and the DC side are decoupled and controlled.

[0100] The MMC-BESS shown in this embodiment has a rated capacity of 50kW, a DC side voltage of 750V, an AC side voltage of 380V, includes 120 submodules, each bridge arm includes 20 submodules, the bridge arm inductance is 0.5mH, and the AC side grid-connected inductance is 0.5mH.

[0101] The power device used in this embodiment is MOSFET, specifically SFG180N10PF, and its current resistance is 180A at an ambient temperature of 25°C.

[0102] In this embodiment, the system average SOC is:

[0103] SOC BESS =50.8%

[0104] The average SOC of each phase is:

[0105] SOC a=49.4%

[0106] SOC b =49.7%

[0107] SOC c =53.3%

[0108] The average SOC of each bridge arm is:

[0109] SOC pa =51.4%

[0110] SOC na =47.5%

[0111] SOC pb =52.5%

[0112] SOC nb =46.8%

[0113] SOC pc =52.1%

[0114] SOC nc =54.5%

[0115] In this embodiment, carrier phase shift modulation is used to implement switch control of the submodules.

[0116] Set the DC bus current setpoint to 90A and the AC bus current amplitude to 110A.

[0117] The MMC-BESS three-level SOC balancing method considering the safety of power devices includes the following steps:

[0118] (1) The DC current distribution method is used to calculate the DC phase-to-phase balanced current superimposed on each phase:

[0119]

[0120]

[0121]

[0122] In this embodiment, the phase balance coefficient K phase Take 1000, U dc It is 750V.

[0123] (2) The fundamental frequency current injection method is used to calculate the corresponding bridge arm fundamental frequency balanced current superimposed on each phase:

[0124]

[0125]

[0126]

[0127] In this embodiment, the bridge arm balance coefficient K arm Take 10,U g It is 311V.

[0128] (3) Submodule-level SOC balancing achieved by using DC voltage distribution method or AC voltage distribution method;

[0129] (4) According to the selected SFG180N10PF MOSFET, considering a certain safety margin and the actual operating junction temperature, set its effective value monitoring threshold I RMS_MAX =50A;

[0130] (5) Monitor the real-time effective value of each bridge arm current of the MMC-BESS. If the maximum value of the six bridge arm current effective values ​​does not exceed the threshold value I RMS_MAX , then perform SOC balancing according to the balancing method described in steps (1) to (3);

[0131] (6) If the maximum value of the six bridge arm current effective values ​​max(I RMSj ) exceeds the set threshold I RMS_MAX , then adjust the balancing method: the submodule-level SOC balancing method remains unchanged, the bridge arm balancing method remains unchanged, and the DC phase-to-phase balancing current superimposed on each phase is recalculated as follows:

[0132] Assume that the effective value of the upper and lower bridge arm current of phase A exceeds the threshold, that is:

[0133] I RMSa >50A

[0134] The interphase balancing current superimposed on this phase before adjustment is i phasea =1.86667A, the adjusted interphase balancing current superimposed on this phase is i phasej ',have:

[0135]

[0136] The interphase balancing current of the other two phases after adjustment is reduced synchronously according to the ratio k, which can be obtained by the following formula:

[0137]

[0138] The interphase balancing current values ​​superimposed on phases B and C become:

[0139]

[0140]

[0141] The present invention can be widely used in the field of operation control and management of electric energy storage systems.

Claims

1. A MMC-BESS three-level SOC balancing method considering power device safety, characterized by The following steps are included: (1) The DC current distribution method is used to calculate the DC phase-to-phase balanced current superimposed on each phase; (2) The fundamental frequency current injection method is used to calculate the corresponding inter-arm fundamental frequency balanced current superimposed on each phase; (3) Submodule-level SOC balancing achieved by using DC voltage distribution method or AC voltage distribution method; (4) According to the performance parameters of the selected power device, considering a certain safety margin, set the effective value monitoring threshold; (5) monitoring the real-time effective value of each bridge arm current of the MMC-BESS. If the maximum value among the six bridge arm current effective values ​​does not exceed the threshold, performing SOC balancing according to the balancing method described in steps (1) to (3); (6) If the maximum value among the effective values ​​of the six bridge arm currents exceeds the set threshold, the balancing method is adjusted and the balancing current is recalculated; (7) At this time, the submodule-level SOC balancing method remains unchanged, and the DC interphase balancing current superimposed on each phase is recalculated to obtain the adjusted interphase balancing current superimposed on phase A; (8) After the adjustment, the interphase balanced current of phases B and C decreases synchronously according to the ratio k; (9) A dynamic real-time adjustment mode is adopted to adaptively distribute the inter-phase balancing current to ensure that the heating of the device is within the safe working area during system operation, thereby preventing power devices from being damaged due to excessive balancing current.

2. The MMC-BESS three-level SOC balancing method considering power device safety according to claim 1, characterized in that The MMC-BESS three-level SOC balancing method, on the basis of realizing inter-phase balancing, inter-bridge arm balancing and inter-submodule balancing of battery SOC in the modular multi-level battery energy storage system, determines the distribution strategy of SOC balancing current by detecting the real-time effective value of the bridge arm current, and on the premise of realizing the three-level balancing of battery SOC in the modular multi-level battery energy storage system, avoids overheating and damage of devices due to excessive balancing current, ensures safe and stable operation, and improves the working reliability of the system.

3. The MMC-BESS three-level SOC balancing method considering power device safety according to claim 1, characterized in that The MMC-BESS three-level SOC balancing method adopts a DC current distribution method and a base frequency current injection method to achieve phase-to-phase balancing and bridge-arm balancing of the battery SOC in the MMC-BESS, and a DC voltage distribution method or an AC voltage distribution method is adopted for balancing between submodules. By detecting the real-time effective value of the bridge arm current, the distribution strategy of the SOC balancing current is determined to ensure that the power devices in the system are in a safe working area.

4. The MMC-BESS three-level SOC balancing method considering power device safety according to claim 1, characterized in that The DC current distribution method is used to calculate the corresponding DC phase-to-phase balanced current superimposed on each phase: Where i phasej is the DC phase-to-phase balancing current superimposed on each phase; K phase is the phase balance coefficient; SOC BESS is the system average SOC; SOC j is the phase SOC of each phase; U dc is the DC bus voltage.

5. The MMC-BESS three-level SOC balancing method considering power device safety according to claim 1, characterized in that The fundamental frequency current injection method is used to calculate the corresponding fundamental frequency balanced current between the bridge arms superimposed on each phase: Where i armj K is the base frequency balanced current between bridge arms superimposed on each phase; arm is the bridge arm balance coefficient; SOC pj is the average SOC of the upper bridge arm of this phase; SOC nj is the average SOC of the lower bridge arm of this phase; U g is the voltage amplitude at the AC side of MMC-BESS; is the phase angle of each phase voltage on the AC side of MMC-BESS.

6. The MMC-BESS three-level SOC balancing method considering power device safety according to claim 5, characterized in that The selected bridge arm balance coefficient K arm The amplitude of the injected fundamental frequency current should be much smaller than the amplitude of the AC component of the bridge arm current.

7. The MMC-BESS three-level SOC balancing method considering power device safety according to claim 1, characterized in that When recalculating the DC phase-to-phase balancing current superimposed on each phase, proceed as follows: Let max(I RMSj ) exceeds the threshold, that is: max(I RMSj )>I RMS_MAX (j=a,b,c) The interphase balancing current superimposed on this phase before adjustment is i phasej The adjusted interphase balancing current superimposed on this phase is i phasej ′, then the interphase balancing current i superimposed on this phase phasej ′ should satisfy the following relationship: Where I dcj is the DC current of phase j when there is no SOC balancing method; I acj is the j-phase AC side current amplitude when there is no SOC balancing method; The phase-to-phase balancing current superimposed on the phase after adjustment can be obtained: The interphase balanced currents of the other two phases after adjustment are synchronously reduced according to the ratio k.

8. The MMC-BESS three-level SOC balancing method considering power device safety according to claim 7, characterized in that The ratio k is obtained by the following formula:

9. The MMC-BESS three-level SOC balancing method considering power device safety according to claim 1, characterized in that The MMC-BESS three-level SOC balancing method decouples the power of the AC side and the DC side and then controls them.

10. The MMC-BESS three-level SOC balancing method considering power device safety according to claim 1, characterized in that The MMC-BESS three-level SOC balancing method adopts carrier phase shift modulation to realize switch control of submodules.

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