MMC energy storage system standby operating mode zero sequence circulating current injection energy balancing method

Abstract

This invention discloses a method for zero-sequence circulating current injection energy equalization in standby conditions of an MMC energy storage system. Based on a three-phase, six-bridge-arm MMC energy storage system topology, a mathematical model of the zero-sequence circulating current is established, and a multi-objective optimization function is constructed with the objectives of maximizing energy equalization speed, minimizing system losses, and minimizing submodule voltage fluctuations. The optimal amplitude and phase of the zero-sequence circulating current are solved to generate a zero-sequence voltage command, which is superimposed onto a modulation wave via PI control to drive the switching devices to actively inject the circulating current. The injected zero-sequence circulating current is used to establish an energy transfer channel between the upper and lower bridge arms, realizing the transfer of energy from the high-SOC bridge arm to the low-SOC bridge arm. Submodule-level balancing is achieved through hierarchical coordinated control, and injection is terminated when the SOC variance reaches the target or the system exits standby. This invention achieves active energy equalization in standby conditions, transforming the harmful suppression of the zero-sequence circulating current into active utilization, balancing equalization speed and system safety, without increasing hardware costs.

Description

Technical Field

[0001] This invention relates to the field of modular multilevel converter energy storage system technology, and in particular to a method for zero-sequence circulating current injection energy equalization in standby conditions of an MMC energy storage system. Background Technology

[0002] Modular multilevel converters (MMCs) are widely used in high-voltage, high-capacity energy storage systems due to their modular design, high output power quality, and ease of expansion. In MMC energy storage systems, each submodule is connected to a battery energy storage unit on its DC side. Due to differences in battery manufacturing processes, uneven operating temperatures, and varying degrees of aging, the state of charge (SOC) of batteries in different submodules can be inconsistent, leading to overcharging or over-discharging of some submodules, severely impacting battery life and system safety.

[0003] In standby mode, the MMC output voltage tracks the grid voltage to maintain grid connection, but the system active power is close to zero, with no charging or discharging current. The SOC imbalance problem persists or even worsens during standby, directly affecting the available capacity and system performance during the next charge and discharge cycle.

[0004] Zero-sequence circulating current is an inherent physical phenomenon in MMC systems, mainly caused by factors such as different switching states of upper and lower bridge arm submodules, parameter deviations, and differences in state of charge (SOC). Traditional research generally regards zero-sequence circulating current as a harmful factor and tries to suppress it to a minimum through hardware (increasing bridge arm inductance) or software (PI / PR controller) methods. This approach ignores the potential value of zero-sequence circulating current as an energy exchange channel between bridge arms.

[0005] However, zero-sequence circulating current is essentially an energy transfer channel between bridge arms. If zero-sequence circulating current can be designed and actively injected, an energy transfer mechanism can be established under standby conditions. By controlling the direction and magnitude of the circulating current, energy from high-SOC submodules can be transferred to low-SOC submodules, significantly shortening the equalization time.

[0006] Currently, there is limited research on energy balance in MMC energy storage systems under standby conditions, particularly regarding the lack of systematic research on methods for energy redistribution using active zero-sequence circulating current injection. Therefore, a control method that can actively achieve rapid, efficient, and low-loss energy balance under standby conditions is urgently needed. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for zero-sequence circulating current injection energy balancing in standby conditions of MMC energy storage systems. By actively injecting and optimizing the zero-sequence circulating current, an energy transfer channel is established between bridge arms, enabling rapid SOC balancing of submodules under standby conditions. No additional hardware is required, and the method balances balancing speed, system losses, voltage fluctuations, and current stress, thereby improving the capacity utilization and service life of the energy storage system.

[0008] To achieve the above objectives, the present invention provides the following solution: A method for zero-sequence circulating current injection energy equalization in standby conditions of an MMC energy storage system, applied to an MMC energy storage system, includes: S1, System Modeling and State Identification: Based on the topology of a three-phase six-bridge-arm MMC energy storage system, a mathematical model of zero-sequence circulating current is established. By collecting the state of charge (SOC) of each sub-module, the system SOC variance and the SOC difference between the upper and lower bridge arms of each phase are calculated to identify whether the system is in standby mode. S2, Optimal Circulating Current Calculation: When the system is in standby mode and the SOC variance exceeds the preset threshold, a multi-objective optimization function is constructed with the objectives of maximizing energy balance speed, minimizing system loss, and minimizing submodule voltage fluctuation, and the amplitude and phase of the optimal zero-sequence circulating current are solved. S3, Circulating Current Command Generation and Injection: Based on the calculated optimal circulating current amplitude and phase, a zero-sequence voltage command is generated, and the actual circulating current is tracked by a PI controller to generate a modulation wave. The zero-sequence voltage component is superimposed on the modulation wave of the upper and lower bridge arms to drive the power switching devices and realize the active injection of zero-sequence circulating current. S4, Energy Balance Control: An energy transfer channel is established between the upper and lower bridge arms by injecting zero-sequence circulating current, so that energy flows from the bridge arm with high SOC to the bridge arm with low SOC until the system SOC variance drops below the target value. S5, Coordination Control and Termination: During the recirculation injection process, a hierarchical coordination control strategy is used to coordinate energy balance at the sub-module level, and the recirculation injection is stopped when the termination condition is met.

[0009] Furthermore, the mathematical model of the zero-sequence circulation described in step S1 is described by the following differential equation:

[0010] in, For bridge arm inductance, The equivalent resistance of the bridge arm. and These are the output voltages of the upper and lower bridge arms, respectively. This is the DC bus voltage. It is a zero-sequence circulation; The zero-sequence circulation is:

[0011] In the formula, and These are the currents of the upper and lower bridge arms, respectively.

[0012] Furthermore, the multi-objective optimization function in step S2 is:

[0013] In the formula, This indicates the expected equilibrium time at the current circulation amplitude, and... Inversely proportional; This represents the system loss at the current circulating current amplitude, and... Proportional; This indicates the voltage fluctuation amplitude under the current circulating current amplitude; These are the baseline value for equalization time, the baseline value for rated power loss, and the upper limit for voltage fluctuation, respectively, used to normalize various indicators; These are the weighting coefficients for balancing speed, loss, and voltage fluctuation, respectively, and satisfy the following conditions: .

[0014] Furthermore, the amplitude of the optimal zero-sequence circulation described in step S2 is calculated according to the following formula:

[0015] in, This is the equalization gain coefficient; For SOC differences; The average battery capacity of the submodule is expressed in Ah. The desired equilibrium time constant is in seconds; 3600 is the unit conversion factor, converting seconds to hours. The phase of the optimal zero-sequence circulation is determined by the sign of the SOC difference. When the SOC of the upper arm is higher than that of the lower arm, the circulation direction is from the upper arm to the lower arm, and vice versa.

[0016] Furthermore, the active injection of the zero-sequence circulating current in step S3 is: By adjusting the modulation waves of the upper and lower bridge arms, the number of sub-modules put into operation in the upper and lower bridge arms is adjusted, and the sum of the output voltages of the upper and lower bridge arms, i.e. the common-mode voltage, is changed. When a voltage difference is generated between the common-mode voltage and the DC bus voltage, the voltage difference acts on both ends of the bridge arm inductor, driving the zero-sequence circulating current to flow in the closed loop between the bridge arms and reach the target amplitude.

[0017] Furthermore, the hierarchical coordination control strategy described in step S5 includes: The first layer is the circulating injection control, which calculates and updates the optimal circulating amplitude and phase based on the SOC difference in the first control cycle. The second layer is submodule balancing control. During the submodule switching process executed in the second control cycle, based on the direction of the bridge arm current and the submodule SOC, the low SOC submodule is prioritized to be switched on or the high SOC submodule is prioritized to be switched off. The second control cycle is shorter than the first control cycle.

[0018] Furthermore, the termination condition in step S5 includes at least one of the following: The SOC variance was reduced to the preset target value; The system exits standby mode and receives a charge / discharge command; The balancing time exceeds the preset maximum allowable time.

[0019] Furthermore, step S3 also includes limiting the amplitude of the calculated optimal zero-sequence circulating current. Under standby conditions, the limiting process restricts the amplitude of the optimal zero-sequence circulating current to within 6%-10% of the system rated current.

[0020] Furthermore, the zero-sequence circulating current injection in step S4 includes a continuous injection mode and a pulse injection mode; In continuous injection mode, a zero-sequence circulating current of constant amplitude is continuously injected until the SOC equilibrium is reached. The pulse injection mode alternates between circulating injection and stopping injection with a 50% duty cycle, where the injection time and the stopping time are equal.

[0021] Furthermore, in step S1, the identification conditions for the standby operating condition are: the system active power is less than 5% of the rated power, and the SOC variance exceeds the start-up threshold of 2%-3%.

[0022] According to specific embodiments of the present invention, the energy balancing method for zero-sequence circulating current injection in the standby condition of an MMC energy storage system provided by the present invention changes the traditional approach of completely suppressing zero-sequence circulating current, and actively injects optimized zero-sequence circulating current components, disclosing the following technical effects: Standby Energy Balancing: This solves the problem that traditional methods cannot achieve energy balancing in standby mode. It establishes an energy transfer channel between bridge arms through zero-sequence circulating current injection, realizing energy redistribution between sub-modules, with SOC balancing as the main implementation method.

[0023] Rapid equalization: Achieves active energy equalization that is impossible with traditional methods in standby mode; when used in conjunction with charging and discharging mode, the equalization time can be shortened by 40%-60% compared with traditional passive equalization methods.

[0024] No additional hardware required: Circulating current injection is achieved using MMC's existing power circuitry, eliminating the need for additional equalization circuitry or large-capacity bridge arm inductors, thus reducing system cost and size.

[0025] Multi-objective collaborative optimization: Taking into account performance indicators such as equalization speed, system loss, and voltage fluctuation, the optimal circulating current injection amount is calculated through optimization algorithms.

[0026] Improve system performance: Avoid reduced usable capacity and shortened battery life caused by severe SOC imbalance, and improve the overall performance of the energy storage system.

[0027] In summary, this invention solves the technical problem that existing technologies cannot perform energy balancing under standby conditions by adopting a complete technical path of standby state identification, multi-objective optimization, active injection of circulating current, energy balance control, hierarchical coordination and closed-loop termination. It realizes a paradigm shift in zero-sequence circulating current, constructs a control framework for multi-objective collaborative optimization, and achieves significant beneficial effects in filling technical gaps, improving balancing efficiency, reducing hardware costs, and ensuring system security. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the topology of a three-phase MMC energy storage system according to an embodiment of the present invention; Figure 2 This is a block diagram of the zero-sequence circulating current optimized injection control system according to an embodiment of the present invention; Figure 3 This is a flowchart illustrating the standby circulating current injection control process according to an embodiment of the present invention. Figure 4 The following is a standby power balance process curve of an embodiment of the present invention, wherein (a) is the zero-sequence circulating current injection waveform of phase A, (b) is the average SOC change of the upper and lower bridge arms, (c) is the SOC variance convergence curve of the whole system, and (d) is the additional loss of the circulating current. Figure 5 This is a diagram showing the relationship between the circulating current optimization parameters under standby conditions in an embodiment of the present invention. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] The purpose of this invention is to provide a method for zero-sequence circulating current injection energy balancing in standby conditions of an MMC energy storage system. By actively injecting and optimizing the zero-sequence circulating current, an energy transfer channel is established between bridge arms, enabling rapid SOC balancing of submodules under standby conditions. This method requires no additional hardware and takes into account balancing speed, system losses, voltage fluctuations, and current stress, thereby improving the capacity utilization and service life of the energy storage system.

[0032] The relevant terms in this invention are explained as follows: Modular multilevel converter (MMC): A power electronic converter topology consisting of multiple power submodules connected in series to form a bridge arm. In energy storage applications, each submodule typically integrates an independent battery energy storage unit, and they are cascaded to achieve direct high-voltage grid connection.

[0033] Zero-sequence circulating current: In an MMC system, the circulating current component that flows through the upper and lower bridge arms but not to the AC load. The zero-sequence circulating current circulates between the upper and lower bridge arms of a three-phase system, and its frequency is mainly an even multiple of the fundamental frequency (second harmonic, fourth harmonic, etc.).

[0034] Submodule: The basic building block of MMC, usually using a half-bridge or full-bridge topology, containing power switching devices and battery energy storage units.

[0035] SOC (State of Charge): The percentage of the battery's current remaining charge relative to its rated capacity.

[0036] Zero-sequence circulating current optimization injection: Unlike traditional circulating current suppression strategies, this invention proposes to actively design and inject zero-sequence circulating current components with specific amplitude and phase to achieve energy redistribution and SOC equalization among sub-modules.

[0037] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] like Figure 1 As shown, the three-phase MMC energy storage system of this invention includes a three-phase six-bridge-arm structure, with each phase comprising an upper bridge arm and a lower bridge arm. Each bridge arm consists of N sub-modules connected in series (N is typically from a dozen to hundreds), and each sub-module includes power switching devices, a battery cell, and a BMS monitoring unit. The series inductor in the bridge arm is used for current limiting and buffering, with a typical value of 3-6 mH. The sub-modules adopt a half-bridge or full-bridge topology, with the DC side connected to the battery energy storage unit. For the specific circuit structure of the three-phase MMC energy storage system involved in this invention, please refer to the MMC battery energy storage system structure described in "CN202510222982.7 - An MMC Battery Energy Storage System and Its State of Charge Estimation Method".

[0039] like Figure 3 As shown, the judgment logic and control strategy for standby operation include: standby state detection, SOC variance judgment, parameter selection, mode selection (continuous / pulse), circulating current calculation, circulating current injection, SOC monitoring, and termination condition judgment. Specifically, the zero-sequence circulating current injection energy equalization method for standby operation of an MMC energy storage system provided by this embodiment of the invention is applied to an MMC energy storage system and includes: S1, System Modeling and State Recognition: Based on the topology of a three-phase six-bridge-arm MMC energy storage system, a mathematical model of zero-sequence circulating current is established. By collecting the state of charge (SOC) of each submodule, the system SOC variance and the SOC difference between the upper and lower bridge arms of each phase are calculated to identify whether the system is in standby mode.

[0040] Zero-sequence circulation is defined as follows: Zero-sequence circulating current is the current component that flows through the bridge arm but not to the AC side. Taking phase A as an example, the upper bridge arm current is... The current in the lower bridge arm is The output current is:

[0041] The zero-sequence circulation is:

[0042] The zero-sequence circulation flows between the upper and lower bridge arms and does not flow towards the AC side, which is its key characteristic.

[0043] Zero-sequence circulation mathematical model: The dynamic characteristics of the zero-sequence circulating current are determined by the inductance and resistance of the bridge arms, and its differential equation is:

[0044] in, For bridge arm inductance, The equivalent resistance of the bridge arm. and These are the output voltages of the upper and lower bridge arms, respectively. This is the DC bus voltage.

[0045] The equation is derived based on Kirchhoff's voltage law: it can be obtained by adding the voltage equations of the upper and lower bridge arm loops and substituting them into the definition of circulating current.

[0046] Fourier analysis reveals that the zero-sequence circulating current of the MMC mainly comprises a DC component, a second harmonic component, and higher harmonics. Under grid-connected operation, the MMC zero-sequence circulating current primarily exhibits a second harmonic component (100Hz for a 50Hz system). This invention achieves energy balancing by actively injecting a controllable low-frequency or DC circulating current component during standby. Specifically, in standby mode, a quasi-DC circulating current (frequency 0-10Hz) is injected, and energy transfer between the upper and lower bridge arms is achieved by controlling the direction and amplitude of the circulating current. When the system switches to grid-connected operation, a second harmonic circulating current component synchronized with the bridge arm voltage can be injected.

[0047] Circulation amplitude limit description: In this invention, the circulating current amplitude is limited to 8%-15% of the rated current. This limit was determined through simulation and testing of MMC energy storage systems of different capacities, and it can achieve rapid SOC equalization while avoiding excessive arm current and excessive circulating current losses. Under standby conditions, the circulating current amplitude is limited to 6%-10% of the rated current to reduce losses.

[0048] S2, Optimal circulation calculation: When the system is in standby mode and the SOC variance exceeds a preset threshold, a multi-objective optimization function is constructed with the objectives of maximizing energy balance speed, minimizing system loss, and minimizing submodule voltage fluctuation, and the amplitude and phase of the optimal zero-sequence circulating current are solved.

[0049] 1. Construction of multi-objective optimization function This invention proposes a multi-objective optimized zero-sequence circulating injection control, taking into account the following performance indicators: Control objective 1: Maximize energy equilibrium speed The SOC variance is defined as an indicator of the degree of energy balance; the smaller the SOC variance, the more consistent the energy states of each submodule.

[0050] Here, 6N indicates that the three-phase six-bridge arm contains a total of 6N sub-modules. The control objective is to maximize the rate of decrease of the SOC variance.

[0051] : indicates the first in the MMC energy storage system Real-time state of charge of each sub-module battery energy storage unit ( The range of values ​​is to ); : Represents all in the system The arithmetic mean of the real-time state of charge of each submodule battery energy storage unit is calculated using the following formula: .

[0052] Control objective 2: Minimize system losses It should be noted that the formula for the instantaneous value of the zero-sequence circulation defined above... Applicable to various operating conditions, its specific form depends on the control strategy's adjustment of the bridge arm current. The differences in numerical relationships and circuit states between the two circulating current forms involved in this invention are as follows: (1) Numerical relationships and waveforms: DC circulating current (quasi-DC state): When the controller adjusts the bridge arm voltage so that a constant current flows through the upper and lower bridge arms in the same direction (i.e., Substituting into the above definition formula, the result is... This is a DC constant. At this point, the effective value of the circulating current... Numerically equal to that DC amplitude .

[0053] Alternating current circulation (oscillating state): When the current in the upper and lower bridge arms contains AC components of the same phase (i.e., ), and calculate by substituting into the definition formula. The waveform is an alternating current waveform (usually a second harmonic or low frequency). At this time, the effective value of the circulating current is... For its peak ,Right now .

[0054] (2) Circuit conduction state: The DC circulating current mode corresponds to a "unidirectional through" energy flow state. In this mode, the current starts from the positive terminal of the DC bus, passes through the upper bridge arm submodule, the bridge arm inductor, and the lower bridge arm submodule in sequence, and finally flows back to the negative terminal of the DC bus, forming a stable energy transfer channel.

[0055] The AC circulating current mode corresponds to the "internal oscillation" of current between the upper and lower bridge arms and the submodule capacitors, with the current direction changing alternately with the period.

[0056] Based on the above definition, the circulating current loss of a three-phase six-arm bridge system is calculated as follows:

[0057] in, This is the RMS value of the circulating current. For a DC circulating current, the RMS value equals the DC value; for an AC circulating current, the RMS value is the peak value divided by [the value is missing from the original text]. .

[0058] in, This represents the circulating current amplitude (RMS value). The control objective is to minimize circulating current loss while ensuring the balancing effect.

[0059] Control objective 3: Minimize voltage fluctuations Circulating current injection can cause voltage fluctuations in submodules. The control objective is to keep the voltage fluctuation amplitude within the allowable range (typically 5%-8% of the rated voltage).

[0060] fluctuation amplitude of submodule capacitor voltage This is mainly caused by the charging and discharging of capacitors by the current flowing through the submodule. When a circulating current is injected during standby operation, its estimation formula is:

[0061] in, The amplitude of the injected circulation. This is the switching cycle of the submodule (i.e., the reciprocal of the switching frequency). This refers to the capacitance value of the submodule.

[0062] This formula shows that voltage fluctuations are directly proportional to the injected circulating current amplitude and inversely proportional to the capacitance value and switching frequency. The control objective is to ensure... ,in, The rated voltage of the submodule. Allowable volatility coefficient (e.g., 0.05).

[0063] Control objective 4: Current stress limitation The peak value of the bridge arm current should not exceed the rated value, and it must be ensured that the sum of the output current component and the circulating current component does not exceed the limit.

[0064] Bridge arm current peak Real-time data is acquired using Hall current sensors installed on each arm of the MMC bridge. Internally, the real-time value of the arm current is calculated using the following formula:

[0065] in, This is the AC side output current (approximately 0 in standby mode). This is the injected zero-sequence circulation.

[0066] The logic for determining current stress limitation is as follows:

[0067] That is, the maximum absolute value of the bridge arm current collected in real time should not exceed the rated current of the power device. .

[0068] Multi-objective optimization function: Based on the above objectives, a weighted multi-objective function is constructed. Different weighting coefficients are set according to actual application requirements. For frequency regulation energy storage applications requiring rapid response, the equalization speed weight can be set to 0.6-0.7; for long-term energy storage applications, the loss weight can be set to 0.3-0.4.

[0069] To comprehensively balance equilibrium speed, system losses, and voltage fluctuations, the following weighted multi-objective optimization function is constructed. This function aims to find the optimal circulation amplitude. This minimizes the objective function value.

[0070] The meanings of the symbols in the formula are as follows: The estimated equilibrium time at the current circulation amplitude, and Inversely proportional; The system loss at the current circulating current amplitude, and Proportional; : Voltage fluctuation amplitude under the current circulating current amplitude; These are the baseline value for equalization time, the baseline value for rated power loss, and the upper limit for voltage fluctuation, respectively, used to normalize various indicators (eliminate the influence of dimensions). : These are the weighting coefficients for balancing speed, loss, and voltage fluctuation, respectively, and satisfy the following conditions: .

[0071] 2. Solving for the amplitude and phase of the optimal zero-sequence circulation. 2.1 SOC Difference Calculation The SOC difference between the upper and lower arms of each phase is defined as the difference between the average SOC of all submodules in the upper arm and the average SOC of all submodules in the lower arm:

[0072] A positive value indicates that the average SOC of the upper arm is higher than that of the lower arm, and energy needs to be transferred from the upper arm to the lower arm; a negative value indicates that the energy transfer direction is opposite.

[0073] 2.2 Calculation of Circulation Amplitude To achieve energy balance, the optimal circulating current amplitude is directly proportional to the SOC difference, directly proportional to the battery capacity, and inversely proportional to the desired balance time.

[0074] in: The equalization gain coefficient (dimensionless) typically ranges from 0.8 to 1.2, with a standard value of 1.0 indicating injection at the theoretically optimal current. SOC difference (represented as a decimal, e.g., 5% is represented as 0.05); The average battery capacity of the submodule (Ah); 3600 is the expected equilibrium time constant (seconds); 3600 is the unit conversion factor (seconds to hours).

[0075] The physical meaning of this formula is as follows: Considering that the SOC of the upper and lower bridge arms changes in opposite directions simultaneously, eliminating the difference in ΔSOC only requires transferring ΔSOC / 2 of the charge. The greater the difference in SOC and the larger the battery capacity, the more energy needs to be transferred, and the larger the injected circulating current; the shorter the target equalization time, the greater the circulating current required.

[0076] like Figure 5 As shown, the difference between the circulating current amplitude and the SOC, as well as the equalization time, exhibit a three-dimensional relationship under standby conditions.

[0077] Under typical standby operating conditions ( , This display uses a three-dimensional surface to show how the circulation amplitude varies with SOC differences (1%-10%) and equalization times (300-1800 seconds). The surface has hyperboloid characteristics and is superimposed with contour projections for accurate readings. Engineers can directly read the required circulation amplitude from the surface based on the actual SOC difference and the desired equalization time. Figure 5 core formula Presented in 3D visualization, it enables rapid mapping from theory to practice, providing an intuitive basis for optimizing standby operating parameters.

[0078] In typical parameters , Under these conditions, the surface shows that when the SOC difference increases from 1% to 10%, the required circulation amplitude increases significantly; when the desired equilibrium time increases from 300 seconds to 1800 seconds, the required circulation amplitude decreases accordingly, exhibiting hyperboloid characteristics. Engineers can directly read the required circulation amplitude from the surface based on the actual SOC difference and the desired equilibrium time.

[0079] 2.3 Amplitude Limiting Considering loss constraints and current stress limits, the calculated circulating current amplitude is limited to ensure that the circulating current amplitude does not exceed the set maximum value (usually 6%-10% of the rated current in standby mode).

[0080] 2.4 Circulation Phase Calculation Energy transfer mechanism explanation: In standby mode, a voltage difference is established between the upper and lower bridge arms. In conjunction with the injected DC circulating current To achieve power transfer .

[0081] Specific implementation method: By adjusting the difference in the number of upper and lower bridge arm sub-modules, a small voltage difference (typically 1%-3% of the rated voltage) is established between the upper and lower bridge arms.

[0082] Energy flow direction control: When the SOC of the upper bridge arm is higher than that of the lower bridge arm, increase the number of sub-modules connected to the upper bridge arm (or decrease the number of modules connected to the lower bridge arm) so that the voltage of the upper bridge arm is slightly higher than that of the lower bridge arm. With the help of the positive circulating current, energy flows from the upper bridge arm to the lower bridge arm. When the SOC of the lower bridge arm is higher than that of the upper bridge arm, the adjustment strategy is reversed and the energy flow direction is reversed.

[0083] The physical essence of this method is: by asymmetrically switching sub-modules, a power transmission channel is established between bridge arms while maintaining zero AC output current. Specifically, the differential-mode component of the bridge arm voltage ( ) is used to track the mains voltage and maintain the output current at zero; common-mode component ( Used to generate DC circulating current The two are decoupled and controlled, and do not interfere with each other.

[0084] 2.5 Three-phase circulating flow coordination The circulating current in this invention is mainly used for energy balancing between the upper and lower bridge arms within each phase. The three phases can operate independently using the same control strategy. The amplitude of the circulating current in each phase is calculated separately based on the SOC difference between the upper and lower bridge arms of that phase. In standby mode, since no current is output from the AC side, the circulating current in each phase bridge arm only circulates within that phase, preventing three-phase imbalance or neutral point potential shift. SOC balancing between three-phase submodules (i.e., energy transfer between different phases) can be handled separately as a supplementary balancing method and is not within the main scope of this invention.

[0085] S3, Circulation Command Generation and Injection: Based on the calculated optimal circulating current amplitude and phase, a zero-sequence voltage command is generated, and the actual circulating current is tracked by a PI controller to generate a modulation wave. The zero-sequence voltage component is superimposed on the modulation wave of the upper and lower bridge arms to drive the power switching devices and realize the active injection of zero-sequence circulating current.

[0086] Step S3 specifically includes: 1. Zero-sequence voltage control Zero-sequence circulating current is achieved by adjusting the sum of the voltages of the upper and lower bridge arms. The zero-sequence voltage is defined as:

[0087] The zero-sequence circulating current can be adjusted by controlling the zero-sequence voltage without affecting the output voltage (the output voltage depends on the difference in bridge arm voltages).

[0088] 2. PI Controller Design A PI controller is used to track the desired circulating current. The input of the PI controller is the deviation between the desired circulating current and the actual circulating current, and the output is the zero-sequence voltage command.

[0089] The PI controller parameters are designed based on the bridge arm inductance and the desired control bandwidth. For systems with a bridge arm inductance of 3-6mH and a desired control bandwidth of 200-500rad / s, the typical proportional gain is 5-10 and the integral gain is 100-500.

[0090] 3. Modulation Strategy Implementation Zero-sequence voltage is achieved through a PWM modulation strategy. For commonly used carrier phase-shift modulation, a zero-sequence component is superimposed on the original modulation wave. The modulation waves of the upper and lower bridge arms are simultaneously superimposed with the same zero-sequence component, thereby changing the sum of the bridge arm voltages to achieve circulating current control without affecting the output voltage.

[0091] Specifically, the active injection of the zero-sequence circulating current is achieved by the main controller (such as a DSP or FPGA) by controlling the on and off of the power semiconductor devices (such as IGBTs or MOSFETs) inside the submodule.

[0092] The physical mechanism is as follows: The controller adjusts the modulation wave signal of the upper and lower bridge arms according to the calculated circulating current command, changes the number of sub-modules put into operation in the upper and lower bridge arms, and thus adjusts the sum of the output voltages of the upper and lower bridge arms (i.e., common mode voltage). When a voltage difference is generated between the common mode voltage and the DC bus voltage, the voltage difference acts on the two ends of the bridge arm inductor, thereby driving the zero-sequence circulating current to flow in the closed loop and reach the target amplitude.

[0093] 4. Explanation of the impact on the power grid side Zero-sequence circulating current flows between the three-phase bridge arms and does not flow to the AC side, therefore it has no direct impact on the grid-side output current and power quality. However, when injecting circulating current, it is necessary to ensure that the system can still meet the grid dispatching commands. When a power command is received, the circulating current control should respond quickly (within 5 seconds) and adjust the amplitude.

[0094] S4, Energy Balance Control: An energy transfer channel is established between the upper and lower bridge arms by injecting zero-sequence circulation, allowing energy to flow from the bridge arm with high SOC to the bridge arm with low SOC, until the system SOC variance drops below the target value.

[0095] Step S4 specifically includes: Standby mode is the optimal application scenario for the zero-sequence circulating current optimization injection method. Traditional circulating current suppression methods cannot achieve energy balance under this condition, while the method of this invention can actively inject circulating current to achieve rapid balance.

[0096] 1. Startup conditions The standby zero-sequence circulating current injection equalization is initiated when the following conditions are met: The system is in standby mode (active power is less than 5% of rated power); SOC variance exceeds the threshold (typical value 2%-3%). The standby time is expected to be longer than the minimum equalization time (e.g., 10 minutes).

[0097] 2. Control Parameter Selection In standby mode, since there is no output current constraint, a relatively large circulating current can be injected to accelerate equalization. The equalization gain coefficient can be set to 1.2-1.5, with an expected equalization time of 600-900 seconds (10-15 minutes). The upper limit of the injected circulating current amplitude is determined by loss constraints and voltage fluctuation constraints, typically 6%-10% of the rated current, ensuring that the circulating current loss does not exceed the allowable value (typically 0.5%-1% of the rated power).

[0098] 3. Circulation Injection Mode (1) Continuous injection mode A constant-amplitude zero-sequence circulating current is continuously injected until the State of Charge (SOC) is balanced to the target. This is suitable for scenarios with large SOC differences that require rapid balancing. The control flow is as follows: calculate the optimal circulating current amplitude and phase → PI controller tracks the desired circulating current → monitor SOC changes in real time → stop injection after the target is reached.

[0099] (2) Pulse injection mode To further reduce losses, a pulsed circulating current injection strategy can be adopted. Specifically, the injection time is 45 seconds (injecting a relatively large value of circulating current), the stop time is 45 seconds (the circulating current drops to zero, reducing losses), and this cycle is repeated until the SOC is balanced.

[0100] A 90-second pulse period (50% duty cycle) represents a trade-off between balancing equalization effectiveness and control complexity. Too short a period increases the controller load, while too long a period slows down the equalization speed. Compared to continuous injection mode, pulse injection mode, while maintaining the same injection current amplitude, doubles the equalization time but reduces the average thermal load on power devices by approximately 50%. This is beneficial for controlling device junction temperature and extending device lifespan, making it suitable for applications sensitive to device thermal stress or with poor environmental heat dissipation.

[0101] 4. Adaptive Adjustment Strategy During the balancing process, the control parameters are dynamically adjusted based on the real-time changes in SOC differences: As the SOC difference decreases, gradually reduce the circulation amplitude to avoid over-adjustment; When the SOC convergence speed is lower than expected, the equalization gain coefficient should be increased appropriately. When the circulating current loss exceeds the set threshold, switch to pulse injection mode.

[0102] 5. Termination Conditions Circulation injection shall be stopped when any of the following conditions are met: SOC variance reduced to the target value (typically 1%-1.5%). The system exits standby mode (receives a charge / discharge command). The balancing time exceeds the maximum allowable time (e.g., 2 hours).

[0103] 6. Promotion and application of charging and discharging conditions The method of this invention can also be extended to charging and discharging conditions. During charging or discharging, a small-amplitude circulating current (with an upper limit of 8%-12% of the rated current) can be injected to coordinate with the output current to achieve energy balance. The control strategy is similar to that in standby mode, but it is necessary to coordinate the circulating current injection and the output current to avoid exceeding the bridge arm current limit. Specifically, when the bridge arm current is detected to be close to 90% of the rated value, the charging and discharging power is appropriately reduced or the circulating current injection amplitude is limited. However, standby mode remains the best application scenario for this method because there is no output current constraint, and a larger circulating current can be injected to achieve faster balancing.

[0104] like Figure 4 As shown, the standby power balancing process curves of the embodiment are illustrated, including four sub-plots: Sub-figure (a): Phase A zero-sequence circulating current waveform, showing the quasi-DC circulating current injected under standby conditions, with an amplitude of 8A, containing slowly changing low-frequency components (0-10Hz) for energy transfer direction control; Subgraph (b): Average SOC change curves of the upper and lower arms of phase A. The upper arm decreases from 78.5% to 77.3%, while the lower arm increases from 74.8% to 76.1%. During the equilibration process, the two curves gradually become consistent. Subplot (c): The SOC variance change curve of the entire system, decreasing from 2.6% to 1.0%, reaching the target in approximately 1665 seconds; Subgraph (d): Circulating current loss curve, which is stable at about 38.4W, accounting for 0.384% of the rated power.

[0105] S5, Coordination Control and Termination: During the recirculation injection process, a hierarchical coordination control strategy is used to coordinate energy balance at the submodule level, and the recirculation injection is stopped when the termination condition is met.

[0106] The injection of zero-sequence circulating current will cause changes in the energy state of the submodule, which needs to be coordinated with the energy balance control of the submodule.

[0107] 1. Traditional submodule balance control Traditional submodule balancing control is achieved through a sorting method: submodules are selectively switched based on their State of Charge (SOC) and the direction of the bridge arm current. When the bridge arm current is positive (submodule charging), submodules with low SOC are switched on first; when the current is negative (discharging), submodules with high SOC are switched on first.

[0108] 2. Hierarchical Coordination and Control Strategy This invention proposes a hierarchical coordination control: The first layer is the circulating current injection control, which calculates the optimal circulating current amplitude and phase based on the SOC difference and generates a zero-sequence voltage command. The control cycle is 100-500ms. This is the slow control layer, responsible for the overall energy balance strategy.

[0109] The second layer is the submodule balancing control. Under the premise of meeting the circulating current injection requirements, it achieves balancing between submodules through an improved sorting method, with a control cycle of 1-10ms. This is the fast control layer, responsible for the selection of submodules for each switching cycle.

[0110] 3. Improved sorting method The improved sequencing method considers both the output current component and the circulating current component. The bridge arm current equals the output current component plus or minus the circulating current component.

[0111] The submodule switching criterion has been modified as follows: when the bridge arm current is positive, submodules with lower SOC are switched on first; when the bridge arm current is negative, submodules with higher SOC are switched on first. Since the circulating current is designed according to the balancing requirements, this improved sorting method can work synergistically with circulating current injection to further accelerate energy balancing.

[0112] 4. Voltage fluctuation suppression The following measures are taken to suppress submodule voltage fluctuations caused by circulating current: Select a sufficiently large submodule capacitor or battery capacity to keep voltage fluctuations within an acceptable range (5%-8% of rated voltage). Real-time monitoring of submodule voltage fluctuations; when the voltage exceeds 80% of the allowable value, dynamically reducing the circulating current injection amplitude.

[0113] like Figure 2 As shown, the zero-sequence circulating current injection energy equalization method for standby conditions of the MMC energy storage system described in this invention can be implemented through a zero-sequence circulating current optimization injection control system, which includes the following modules: SOC Acquisition and Calculation Module: Acquires SOC from each submodule in real time (via the Battery Management System (BMS)) and calculates the average SOC, SOC variance, and SOC difference. The sampling period is 100-500ms.

[0114] Operating condition identification module: Determines the current operating condition based on power commands and actual power, with a focus on identifying whether it is in standby mode and determining whether equalization needs to be started based on the SOC status.

[0115] Optimal Circulation Calculation Module: Based on the current State of Charge (SOC), this module executes a multi-objective optimization algorithm to calculate the optimal circulation amplitude and phase. This module is the core of the entire control system.

[0116] The circulating current closed-loop control module acquires the actual bridge arm current, calculates the actual circulating current, and the PI controller generates a zero-sequence voltage command based on the deviation. The control cycle is 100μs-1ms, ensuring fast and accurate tracking of the circulating current.

[0117] Modulation and Submodule Control Module: This module superimposes the zero-sequence voltage onto the modulation wave, executes an improved sorting method, and generates switching signals for each submodule. The PWM carrier frequency is 1-3kHz.

[0118] Anomaly protection module: BMS communication fault protection: When BMS communication is interrupted for more than 100ms, the circulating current injection is immediately stopped and the system switches to the traditional circulating current suppression mode. Temperature protection: When the power device temperature exceeds 80°C, the circulating current amplitude is automatically reduced by 50%. Overcurrent protection: When the arm current exceeds 95% of the rated value, the circulating current amplitude is immediately limited or the injection is stopped. The key control parameters and tuning methods in the solution described in this invention include: (1) Equalization gain coefficient: The typical value for standby operation is 1.2-1.5, with a recommended standard value of 1.3. This can be fine-tuned according to actual needs: 1.2 for conservative mode and 1.4-1.5 for fast mode. This parameter directly affects the circulating current amplitude. Figure 5 As shown, under the same SOC difference and equilibrium time conditions, The larger the gain, the greater the required circulating current amplitude and the faster the balancing speed, but the losses also increase accordingly. If applied to charging and discharging conditions, the gain needs to be determined based on the battery's allowable charge and discharge rate: if the battery allows 1C charging and discharging, the balancing gain can be 1.0-1.5; if only 0.5C is allowed, the balancing gain should be 0.5-0.8.

[0119] (2) Equilibrium time constant: For frequency modulation applications requiring fast response, the equalization time should be 600-900 seconds; for energy-intensive applications, it can be 1200-1800 seconds.

[0120] (3) PI controller parameters: The design is based on the bridge arm inductance and the desired control bandwidth. For example, when the bridge arm inductance is 5mH and the desired bandwidth is 300rad / s, the proportional gain is 6-7 and the integral gain is 200-300.

[0121] (4) SOC imbalance threshold: The typical value for standby conditions is 2%-3%. If the threshold is too small, the equalization process will be initiated frequently, increasing losses; if the threshold is too large, the equalization process will not be timely.

[0122] (5) Maximum circulation amplitude: The typical value of standby current is 6%-10% of the rated current, and it is necessary to balance the speed and system losses.

[0123] Control system hardware configuration requirements: The controller is recommended to use a DSP or FPGA with a main frequency of ≥200MHz, an AD sampling accuracy of ≥12 bits, and SOC data to be obtained from BMS via CAN bus (baud rate ≥500kbps). The circulating current control cycle is 100μs and the interrupt response time is required to be <10μs.

[0124] In a specific application of this invention, taking a 10kV / 1MW MMC energy storage system as an example, standby energy balancing is performed, specifically including: 1. System Parameters A 10kV / 1MW three-phase MMC energy storage system is established, with the following system parameters: Rated power: 1MW; DC bus voltage: 10kV; Number of sub-modules per phase arm: N = 18; Submodule rated voltage: 550V (18 × 550V = 9.9kV); Battery type: Lithium iron phosphate; Single submodule battery capacity: 200Ah; Bridge arm inductance: 5mH; Bridge arm resistance: 0.1Ω; Rated current: 100A; Power grid frequency: 50Hz; 2. Initial State and Problem Description The system is in standby mode (active power is 0). Data collected via BMS shows: Average SOC of the upper arm of phase A: 78.5%; Average SOC of the lower arm in phase A: 74.8%; SOC difference: 3.7%; System-wide SOC variance: 2.6% (exceeding the 2% threshold for standby startup). If no measures are taken, the SOC difference will persist, causing some submodules to reach their limits prematurely during the next charge / discharge cycle, resulting in wasted capacity. Traditional circulating current suppression methods cannot achieve equalization under standby conditions.

[0125] In this embodiment, the control process of the zero-sequence circulating current injection energy equalization method for the standby condition of the MMC energy storage system is as follows: Step 1: Balanced Startup Judgment The control system detected that the system is in standby mode, the SOC variance is 2.6% exceeding the 2% start-up threshold, and the SOC difference of phase A is 3.7%, requiring equalization. The standby operating condition zero-sequence circulating current optimization injection equalization start-up conditions are met.

[0126] Step 2: Optimal circulation calculation Based on the standby mode control strategy, set the equalization gain coefficient. Target equilibrium time Seconds, SOC difference Battery capacity .

[0127] According to the formula for calculating the circulation amplitude:

[0128] The upper limit of the circulating current (8% of the rated current of 100A, i.e., 8A) was checked. The theoretical value of 22.2A exceeded the upper limit, so amplitude limiting was performed. Finally, the amplitude of the injected circulating current in phase A was determined to be 8A.

[0129] Since the SOC of the upper bridge arm is higher than that of the lower bridge arm, the circulating current direction is set to positive (flowing from the positive terminal of the DC bus through the upper bridge arm to the AC side and then back to the negative terminal through the lower bridge arm). At the same time, the number of sub-modules connected to the upper bridge arm is increased so that the voltage of the upper bridge arm is slightly higher than that of the lower bridge arm, so that energy can be transferred from the upper bridge arm to the lower bridge arm.

[0130] Step 3: Circulation Closed-Loop Control and Injection The desired circulating current is a quasi-DC circulating current with an amplitude of 8A. A PI controller is used to track the desired circulating current, with the PI parameters set to a proportional coefficient of 6.5 and an integral coefficient of 300.

[0131] After the recirculation injection is started, the changes in the system's operating status are shown in Table 1. Table 1 ; Data Explanation: Circulation loss: ; The submodule voltage fluctuation is approximately 13V (2.4% of the rated voltage), which is within the allowable range.

[0132] It should be noted that the formula The calculation is for the "ideal circulation value under unlimited amplitude constraint". When the theoretical value exceeds the amplitude limit, the actual equilibrium time will be extended accordingly. In this embodiment, the estimated actual equilibrium time after amplitude constraint is as follows: Seconds; In actual testing, due to the synergistic effect of the improved sorting method (contributing approximately 30%-50% of the additional balancing effect), according to According to the physical laws, an 8A current eliminates a 3.7% SOC difference (requiring the transfer of 3.7Ah of charge), with a theoretical equalization time of approximately 1665 seconds (28 minutes). Actual testing closely matches the theoretical calculations.

[0133] Step 4: Equilibrium Termination At t=1665s (approximately 28 minutes), the controller detected that the SOC difference between the upper and lower arms of phase A had dropped to 0, and the SOC variance had dropped to 1.0%, meeting the equilibrium termination condition. The circulating current injection amplitude was gradually reduced (ramp-down over 5 seconds), and eventually the circulating current dropped to zero, and the system returned to normal standby state.

[0134] Equalization effect and comparison: Equilibrium time: Approximately 1665 seconds (28 minutes); SOC variance decreased from 2.6% to 1.0%, a reduction of 61%; Total energy loss: 38.4W × 1665s = 63.9kJ (0.018kWh), accounting for a very small portion of the total system capacity; The circulating current loss accounts for 0.384% of the rated power: 38.4W / 1MW = 0.384%.

[0135] Compared to traditional methods: Energy balance cannot be achieved in standby mode; Under 0.2C charging conditions, it takes about 25 minutes to reduce the SOC variance to 1.4% using passive methods (while the present invention reduces it to 1.0% within 28 minutes, showing better performance). The method of this invention completes standby power equalization within 28 minutes (traditional methods cannot achieve any equalization in standby mode). Compared to the purely passive equalization method under 0.2C charging conditions (approximately 40 minutes), this method reduces the equalization time by about 30%, fully demonstrating the advantage of active circulating current injection for energy equalization under standby conditions.

[0136] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for zero-sequence circulating current injection energy equalization in standby mode of an MMC energy storage system, applied to an MMC energy storage system, characterized in that, include: S1, System Modeling and State Identification: Based on the topology of a three-phase six-bridge-arm MMC energy storage system, a mathematical model of zero-sequence circulating current is established. By collecting the state of charge (SOC) of each sub-module, the system SOC variance and the SOC difference between the upper and lower bridge arms of each phase are calculated to identify whether the system is in standby mode. S2, Optimal Circulating Current Calculation: When the system is in standby mode and the SOC variance exceeds the preset threshold, a multi-objective optimization function is constructed with the objectives of maximizing energy balance speed, minimizing system loss, and minimizing submodule voltage fluctuation, and the amplitude and phase of the optimal zero-sequence circulating current are solved. S3, Circulating Current Command Generation and Injection: Based on the calculated optimal circulating current amplitude and phase, a zero-sequence voltage command is generated, and the actual circulating current is tracked by a PI controller to generate a modulation wave. The zero-sequence voltage component is superimposed on the modulation wave of the upper and lower bridge arms to drive the power switching devices and realize the active injection of zero-sequence circulating current. S4, Energy Balance Control: An energy transfer channel is established between the upper and lower bridge arms by injecting zero-sequence circulating current, so that energy flows from the bridge arm with high SOC to the bridge arm with low SOC until the system SOC variance drops below the target value. S5, Coordination Control and Termination: During the recirculation injection process, a hierarchical coordination control strategy is used to coordinate energy balance at the sub-module level, and the recirculation injection is stopped when the termination condition is met.

2. The method for zero-sequence circulating current injection energy equalization in standby mode of an MMC energy storage system according to claim 1, characterized in that, The mathematical model of the zero-sequence circulation described in step S1 is described by the following differential equation: in, For bridge arm inductance, The equivalent resistance of the bridge arm. and These are the output voltages of the upper and lower bridge arms, respectively. This is the DC bus voltage. It is a zero-sequence circulation; The zero-sequence circulation is: In the formula, and These are the currents of the upper and lower bridge arms, respectively.

3. The method for zero-sequence circulating current injection energy equalization in standby mode of an MMC energy storage system according to claim 2, characterized in that, The multi-objective optimization function mentioned in step S2 is: In the formula, This indicates the expected equilibrium time at the current circulation amplitude, and... Inversely proportional; This represents the system loss at the current circulating current amplitude, and... Proportional; This indicates the voltage fluctuation amplitude under the current circulating current amplitude; These are the baseline value for equalization time, the baseline value for rated power loss, and the upper limit for voltage fluctuation, respectively, used to normalize various indicators; These are the weighting coefficients for balancing speed, loss, and voltage fluctuation, respectively, and satisfy the following conditions: .

4. The method for zero-sequence circulating current injection energy equalization in standby mode of an MMC energy storage system according to claim 2, characterized in that, The amplitude of the optimal zero-sequence circulation described in step S2 is calculated according to the following formula: in, This is the equalization gain coefficient; For SOC differences; The average battery capacity of the submodule is expressed in Ah. The desired equilibrium time constant is in seconds; 3600 is the unit conversion factor, converting seconds to hours. The phase of the optimal zero-sequence circulation is determined by the sign of the SOC difference. When the SOC of the upper arm is higher than that of the lower arm, the circulation direction is from the upper arm to the lower arm, and vice versa.

5. The method for zero-sequence circulating current injection energy equalization in standby mode of an MMC energy storage system according to claim 2, characterized in that, The active injection of the zero-sequence circulating current in step S3 is: By adjusting the modulation waves of the upper and lower bridge arms, the number of sub-modules put into operation in the upper and lower bridge arms is adjusted, and the sum of the output voltages of the upper and lower bridge arms, i.e. the common-mode voltage, is changed. When a voltage difference is generated between the common-mode voltage and the DC bus voltage, the voltage difference acts on both ends of the bridge arm inductor, driving the zero-sequence circulating current to flow in the closed loop between the bridge arms and reach the target amplitude.

6. The method for zero-sequence circulating current injection energy equalization in standby mode of an MMC energy storage system according to claim 1, characterized in that, The hierarchical coordination control strategy described in step S5 includes: The first layer is the circulating injection control, which calculates and updates the optimal circulating amplitude and phase based on the SOC difference in the first control cycle. The second layer is submodule balancing control. During the submodule switching process executed in the second control cycle, based on the direction of the bridge arm current and the submodule SOC, the low SOC submodule is prioritized to be switched on or the high SOC submodule is prioritized to be switched off. The second control cycle is shorter than the first control cycle.

7. The method for zero-sequence circulating current injection energy equalization in standby mode of an MMC energy storage system according to claim 1, characterized in that, The termination condition in step S5 includes at least one of the following: The SOC variance was reduced to the preset target value; The system exits standby mode and receives a charge / discharge command; The balancing time exceeds the preset maximum allowable time.

8. The method for zero-sequence circulating current injection energy equalization in standby mode of an MMC energy storage system according to claim 1, characterized in that, Step S3 also includes limiting the amplitude of the calculated optimal zero-sequence circulating current. Under standby conditions, the limiting process restricts the amplitude of the optimal zero-sequence circulating current to within 6%-10% of the system rated current.

9. The method for zero-sequence circulating current injection energy equalization in standby mode of an MMC energy storage system according to claim 1, characterized in that, The zero-sequence circulating current injection in step S4 includes continuous injection mode and pulse injection mode; In continuous injection mode, a zero-sequence circulating current of constant amplitude is continuously injected until the SOC equilibrium is reached. The pulse injection mode alternates between circulating injection and stopping injection with a 50% duty cycle, where the injection time and the stopping time are equal.

10. The method for zero-sequence circulating current injection energy equalization in standby mode of an MMC energy storage system according to claim 1, characterized in that, In step S1, the conditions for identifying the standby operating condition are: the system active power is less than 5% of the rated power, and the SOC variance exceeds the start-up threshold of 2%-3%.

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