Method for determining control period of modular cascaded battery energy storage system and modulation method

Abstract

The disclosure provides a control cycle determination method and a modulation method of a modular cascaded battery energy storage system, comprising: determining the sum of the minimum voltage of each corresponding module DC side, the sum of the maximum voltage of each corresponding module DC side, and the maximum value of the grid-connected phase voltage effective value; determining the preset maximum modulation ratio of the modular cascaded battery energy storage system according to the sum of the minimum voltage of each corresponding module DC side and the maximum value of the grid-connected phase voltage effective value; determining the control cycle according to the maximum modulation ratio, the maximum value of the sum of the maximum voltage of each corresponding module DC side, the preset modulation wave amplitude, and the preset modulation wave period; and determining the number of modules put into operation according to the instantaneous value of the modulation wave and the operation result of the series of the DC side voltage of each module according to the module charging and discharging priority. Through the disclosure, the utilization rate of the modular cascaded battery energy storage system is improved, and the accuracy of the output voltage and the output power quality of the modular cascaded battery energy storage system are improved.

Description

Technical Field

[0001] This disclosure relates to the field of power electronics and energy storage control technology, specifically to a method for determining the control cycle and a modulation method for a modular cascaded battery energy storage system. Background Technology

[0002] Modular converter technology enables efficient high-voltage, high-capacity power conversion and is widely used in motor drives, grid reactive power compensation, DC transmission, and medium-voltage direct-connected energy storage. Cascaded H-bridges and modular multilevel converters are two typical modular converter technologies. Modulation technology converts the modulated wave output by the control algorithm into switching commands for the converter's power devices, directly affecting the converter's efficiency and response speed, and is one of the core technologies of modular converters.

[0003] With the large-scale application of battery energy storage in grid-connected energy storage and new energy power plants, the demands for indicators such as efficiency, single-unit capacity, and dynamic response speed are increasing. Therefore, battery energy storage systems based on modular converter technology are receiving increasing attention and application. To obtain better output voltage and current harmonic characteristics, modular battery energy storage systems require a large number of modules per phase; typically no fewer than 8 for 6kV systems, no fewer than 12 for 10kV systems, and no fewer than 40 for 35kV systems.

[0004] Battery balancing control is an essential function of battery energy storage systems. The modulation technology of modular converters directly affects the implementation method of balancing control in modular battery energy storage systems. Modulation technologies for modular converters mainly include stacked carrier PWM modulation (LS-PWM), phase-shifted carrier PWM modulation (PS-PWM), and nearest-level modulation. Stacked carrier PWM modulation can cause uneven power distribution between modules, requiring carrier rotation between different modules, which is relatively complex. Nearest-level modulation is mainly used in applications where the number of modules per phase ranges from tens to hundreds, such as high-voltage flexible DC transmission. In high-voltage flexible DC transmission, capacitor voltage balance control maintains the capacitor voltage of each module at approximately equal levels. The number of modules to be put into operation is determined based on the ratio of the modulation voltage to the capacitor voltage.

[0005] Both carrier-stacked PWM modulation and carrier phase-shifted PWM modulation share the advantage of a high equivalent switching frequency and good output voltage and current waveforms. They can achieve excellent output voltage and current waveforms even with a relatively large number of submodules, while requiring less output filtering and easily meeting the relevant requirements of grid connection standards.

[0006] Basic carrier-layered PWM modulation compares a triangular carrier wave with a modulating wave, but with different average amplitudes but identical waveforms, to generate switching signals for the corresponding module power devices. This results in inconsistent conduction times for each module's power devices within a single power frequency cycle, leading to uneven power distribution between modules and inconsistent heat generation. To overcome these problems, carrier rotation between different modules is required, which is quite complex. Furthermore, to achieve module battery balancing, the differences between the batteries in each module must be considered during carrier rotation, resulting in numerous influencing factors that are interconnected and further complicate control.

[0007] The number of triangular carriers required for carrier-stacking PWM modulation is consistent with the number of modules. Basic carrier-stacking PWM modulation uses triangular carriers of equal amplitude and phase to compare with the modulating wave, generating switching signals for the module power devices. Therefore, the conduction time of each module's power device is exactly the same during operation, and the power and heat generation of each module's power devices are consistent. To achieve module battery balancing, adjusting the carrier magnitude for each module changes the conduction time of each module's power devices, thus altering the module's power. Battery balancing control has few influencing factors and is simple to implement.

[0008] The number of triangular carriers required for carrier phase-shift PWM modulation is also consistent with the number of modules. Therefore, in battery energy storage systems with high voltage levels and a large number of modules, the requirements for control resources are higher. In cases with a large number of modules, the output voltage and current waveform quality can be easily satisfied through nearest-nearest-level modulation, although its equivalent switching frequency is higher and its losses are higher than those of nearest-nearest-level modulation.

[0009] Recently, level modulation has advantages in output waveform quality and low switching losses in applications with tens to hundreds of modules per phase. It is widely used in applications requiring a large number of modules to achieve high-voltage flexible DC transmission and in high-voltage dynamic reactive power generators. Given the differences in parameters and losses between different modules, a capacitor voltage sorting algorithm is generally used to maintain capacitor voltage balance across modules. This algorithm selectively switches modules based on the direction of current, ensuring that the total number of modules in operation remains constant. Capacitor voltage balancing can be completed within a 10ms timescale; therefore, at the power control level, the capacitor voltage of each module can be considered to be always the same, i.e., equal to the average voltage. The number of modules in operation can then be calculated based on the ratio of the modulation voltage to the average capacitor voltage.

[0010] However, even with a large number of modules, the aforementioned nearest-level modulation cannot be directly applied to modular battery energy storage systems. The reason is:

[0011] 1) The energy of the DC-side batteries in a modular battery energy storage system is far greater than the DC-side capacitors in flexible DC transmission systems and dynamic reactive power generator modules. Battery voltage does not change significantly over timescales of several minutes or even tens of minutes. When there is an imbalance among the modules in a modular battery energy storage system, the voltages of each module's batteries will differ considerably. Directly using the average voltage of the modules to calculate the required number of modules will cause significant errors, affecting the output voltage and current quality.

[0012] 2) The selection of the control frequency during near-level modulation is related to the DC-side voltage of the module. For modular battery energy storage systems, the DC-side voltage of each module is different. Selecting the control frequency based on the average voltage of the module batteries cannot fully utilize the advantages of the multi-level topology. Summary of the Invention

[0013] To address the shortcomings of existing technologies, the purpose of this disclosure is to provide a method for determining the control cycle and a modulation method for a modular cascaded battery energy storage system.

[0014] To achieve the above objectives, according to one aspect of this disclosure, a method for determining the control cycle and a modulation method for a modular cascaded battery energy storage system are provided, comprising:

[0015] Based on the pre-set design parameters of the modular cascaded battery energy storage system, determine the sum of the minimum DC-side voltages of each corresponding module and the sum of the maximum DC-side voltages of each corresponding module;

[0016] Based on the preset design parameters of the modular cascaded battery energy storage system, determine the maximum effective value of the grid-connected phase voltage;

[0017] The maximum modulation ratio of the preset modular cascaded battery energy storage system is determined based on the sum of the minimum DC side voltages of each corresponding module and the maximum effective value of the grid-connected phase voltage.

[0018] The control period is determined based on the preset maximum modulation ratio of the modular cascaded battery energy storage system, the maximum sum of the maximum DC side voltages of each corresponding module, the preset modulation wave amplitude, and the preset modulation wave period.

[0019] The instantaneous value of the modulated wave and the preset module charging and discharging priority of the modular cascaded battery energy storage system are obtained.

[0020] Based on the instantaneous value of the modulation wave and the result of the series calculation of the DC side voltage of each module according to the module charging and discharging priority, the number of modules to be put into operation according to the module charging and discharging priority is determined.

[0021] Based on the preset maximum modulation ratio of the modular cascaded battery energy storage system and the number of modules put into operation according to the module charging and discharging priority, the preset modular cascaded battery energy storage system is controlled to perform near-level modulation.

[0022] Optionally, determining the sum of the minimum DC-side voltages of each corresponding module and the sum of the maximum DC-side voltages of each corresponding module based on the preset design parameters of the modular cascaded battery energy storage system includes:

[0023] Based on the preset design parameters of the modular cascaded battery energy storage system, determine the DC-side voltage operating range of each module of the preset modular cascaded battery energy storage system.

[0024] Based on the DC-side voltage operating range of each module, determine the maximum DC-side voltage of all modules;

[0025] Based on the DC-side voltage operating range of each module, the minimum DC voltage of each corresponding module is summed, and the maximum DC voltage of each corresponding module is summed to determine the sum of the minimum DC voltage and the sum of the maximum DC voltage of each corresponding module.

[0026] Optionally, the maximum DC-side voltage of all modules is the same as the preset modulation wave amplitude.

[0027] Optionally, acquiring the instantaneous value of the modulated wave and the preset module charging and discharging priority of the modular cascaded battery energy storage system includes:

[0028] The instantaneous value of the modulated wave is obtained from the control strategy based on a preset first time interval through the modulation function;

[0029] The modulation function obtains the preset module charging and discharging priority of the modular cascaded battery energy storage system from the equalization algorithm based on a preset second time interval.

[0030] Optionally, the method further includes:

[0031] Based on the DC-side voltage of each priority module and the module's charging / discharging priority, the sequence calculation results are determined sequentially according to the module's charging / discharging priority, up to the DC-side voltage of each priority module.

[0032] Optionally, determining the number of modules to be put into operation according to the module charging and discharging priority based on the instantaneous value of the modulation wave and the result of a series of calculations of the DC-side voltage of each module according to the module charging and discharging priority includes:

[0033] If the result of the sequence calculation of the DC side voltage of the current priority module according to the module charging and discharging priority is greater than the instantaneous value of the modulation wave, the number of modules put into operation is the priority module from the first priority module to the previous priority module according to the module charging and discharging priority.

[0034] Optionally, determining the number of modules to be put into operation according to the module charging and discharging priority based on the instantaneous value of the modulation wave and the result of a series of calculations of the DC-side voltage of each module according to the module charging and discharging priority further includes:

[0035] If the result of the sequence calculation up to the DC-side voltage of the current priority module according to the module charging and discharging priority is not greater than the instantaneous value of the modulation wave, then the next priority module of the current priority module is taken as the new current priority module. The result of the sequence calculation up to the DC-side voltage of the new current priority module according to the module charging and discharging priority is compared with the instantaneous value of the modulation wave, until the result of the sequence calculation up to the DC-side voltage of the new current priority module is greater than the instantaneous value of the modulation wave. The number of modules put into operation is determined to be the priority module above the first priority module to the new current priority module according to the module charging and discharging priority.

[0036] Compared with the prior art, the embodiments disclosed herein have at least one of the following beneficial effects:

[0037] Through the above technical solution, based on the preset design parameters of the modular cascaded battery energy storage system, the control cycle is determined, thereby obtaining the preset optimal control cycle of the modular cascaded battery energy storage system. This provides design indicators for the design of the controller of the modular cascaded battery energy storage system. Under this control cycle, the modular cascaded battery energy storage system generates the most output voltage levels and has the best output voltage waveform quality. The nearest-level modulation method is used to modulate the modular cascaded battery energy storage system, improving the utilization rate of the modules, reducing the impact of module voltage inconsistency on the output voltage waveform, improving the accuracy of the output voltage and the quality of the output power, and improving the power output efficiency. Attached Figure Description

[0038] Other features, objects, and advantages of this disclosure will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0039] Figure 1 This is a flowchart illustrating a control cycle determination method and modulation method for a modular cascaded battery energy storage system according to an exemplary embodiment. Detailed Implementation

[0040] The present disclosure will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present disclosure, but do not limit the present disclosure in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present disclosure. These all fall within the protection scope of the present disclosure.

[0041] Figure 1 This is a flowchart illustrating a control cycle determination method and modulation method for a modular cascaded battery energy storage system according to an exemplary embodiment.

[0042] like Figure 1 As shown, this disclosure provides a method for determining the control cycle and a modulation method for a modular cascaded battery energy storage system, including steps S11 to S17.

[0043] S11, based on the preset design parameters of the modular cascaded battery energy storage system, determine the sum of the minimum DC side voltages of each corresponding module and the sum of the maximum DC side voltages of each corresponding module.

[0044] The design parameters of the preset modular cascaded battery energy storage system can refer to the design parameters of the DC-side voltage of each module of the preset modular cascaded battery energy storage system, such as the number of DC-side batteries in the module, the highest operating voltage of a single battery, and the lowest operating voltage of a single battery.

[0045] Maximum DC voltage of the module = highest operating voltage of a single cell * number of cells on the DC side of the module;

[0046] Minimum DC voltage of the module = minimum operating voltage of a single cell * number of cells on the DC side of the module.

[0047] The sum of the minimum DC-side voltages of each corresponding module represents the total minimum DC-side voltages of all corresponding modules, expressed as: Among them, U dcmin_j,x U represents the minimum DC-side voltage of the x-th module corresponding to j. dc_min_sum_j This represents the sum of the minimum DC side voltages of the corresponding modules in phase j, N represents the number of modules in each corresponding phase, a represents phase a, b represents phase b, and c represents phase c.

[0048] The sum of the maximum DC-side voltages of each corresponding module represents the total maximum DC-side voltages of all corresponding modules, expressed as: Among them, U dcmax_j,x U represents the maximum DC-side voltage of the x-th module corresponding to j. dc_max_sum_j This represents the sum of the maximum DC voltages of the corresponding module j, N represents the number of modules in each corresponding module, a represents phase a, b represents phase b, and c represents phase c.

[0049] S12, determine the maximum effective value of the grid-connected phase voltage based on the preset design parameters of the modular cascaded battery energy storage system.

[0050] The pre-set design parameters for the modular cascaded battery energy storage system may include the grid rated voltage and its fluctuation range, grid-connected inductance, and maximum grid-connected current.

[0051] The maximum effective value of the grid-connected phase voltage of a modular cascaded battery energy storage system is expressed as U. acmax .

[0052] S13. Determine the maximum modulation ratio of the pre-set modular cascaded battery energy storage system based on the sum of the minimum DC side voltages of each corresponding module and the maximum effective value of the grid-connected phase voltage.

[0053] Among them, the sum of the minimum DC side voltages of each corresponding module is as follows: the sum of the minimum DC side voltages of phase a module U dc_min_sum_a The sum of the minimum DC side voltages of phase b modules, U dc_min_sum_b The sum of the minimum DC side voltages of the C-phase module, U dc_min_sum_c .

[0054] The preset maximum modulation ratio of the modular cascaded battery energy storage system is M. max .

[0055] S14. Determine the control cycle based on the preset maximum modulation ratio of the modular cascaded battery energy storage system, the maximum sum of the maximum DC voltages of each corresponding module, the preset modulation wave amplitude, and the preset modulation wave period.

[0056] The maximum modulation ratio of the pre-set modular cascaded battery energy storage system is M. max The maximum sum of the maximum DC-side voltages of each corresponding module is: max[U dc_max_sum_a U dc_max_sum_b U dc_max_sum_c The preset modulation amplitude is U. dcmax The preset modulation wave period is T. m In grid-connected systems, T m It also indicates the power frequency cycle.

[0057] The preset modulation wave is a sinusoidal modulation wave.

[0058] The control period is T c .

[0059] According to a control cycle T c Within, the amplitude of the sinusoidal modulated wave is U dcmax To design control cycle T c .

[0060] S15, obtain the instantaneous value of the modulated wave and the preset module charging and discharging priority of the modular cascaded battery energy storage system.

[0061] The preset modular cascaded battery energy storage system's module charging and discharging priorities are named as the first priority module, the second priority module, ..., the nth priority module, from high to low, and their corresponding DC side voltages are U(1), U(2), U(3), ..., U(n).

[0062] S16. Based on the instantaneous value of the modulation wave and the result of the series calculation of the DC side voltage of each module according to the module charging and discharging priority, determine the number of modules to be put into operation according to the module charging and discharging priority.

[0063] S17, based on the preset maximum modulation ratio of the modular cascaded battery energy storage system and the number of modules put into operation according to the module charging and discharging priority, control the preset modular cascaded battery energy storage system to perform the nearest level modulation.

[0064] Through the above technical solution, based on the preset design parameters of the modular cascaded battery energy storage system, the control cycle is determined, thereby obtaining the preset optimal control cycle of the modular cascaded battery energy storage system. This provides design indicators for the design of the controller of the modular cascaded battery energy storage system. Under this control cycle, the modular cascaded battery energy storage system generates the most output voltage levels and has the best output voltage waveform quality. The nearest-level modulation method is used to modulate the modular cascaded battery energy storage system, improving the utilization rate of the modules, reducing the impact of module voltage inconsistency on the output voltage waveform, improving the accuracy of the output voltage and the quality of the output power, and improving the power output efficiency.

[0065] In one possible embodiment, S11 may include S21 to S23.

[0066] S21, based on the preset design parameters of the modular cascaded battery energy storage system, determine the DC-side voltage operating range of each module of the preset modular cascaded battery energy storage system.

[0067] The DC-side voltage operating range for each module is represented as [U dcmin_j,x U dcmax_j,x In a three-phase system, j = a, b, c, x = 1, 2, ..., N, where N represents the number of modules in each corresponding phase. dcmin_j,x U represents the minimum DC-side operating voltage of the x-th module in phase j. dcmax_j,x This represents the maximum DC-side operating voltage of the x-th module in phase j.

[0068] S22, determine the maximum DC-side voltage of all modules based on the DC-side voltage operating range of each module.

[0069] Among them, the maximum DC-side voltage U of all modules dcmax :U dcmax =max[U dcmax_j,x ].

[0070] S23. Based on the DC-side voltage operating range of each module, sum the minimum DC voltage of each corresponding module and sum the maximum DC voltage of each corresponding module to determine the sum of the minimum DC voltage and the sum of the maximum DC voltage of each corresponding module.

[0071] Among them, the minimum DC voltage of each corresponding module is summed:

[0072]

[0073] Among them, U dc_min_sum_j U represents the sum of the minimum DC-side voltages of the corresponding module j. dcmin_j,x This represents the sum of the minimum DC-side voltages of the x-th module in phase j.

[0074] Sum the maximum DC-side voltage for each corresponding module:

[0075]

[0076] Among them, U dc_max_sum_j U represents the sum of the maximum DC-side voltages of the module corresponding to j. dcmax_j,x This represents the sum of the maximum DC-side voltages of the x-th module in phase j.

[0077] In one possible embodiment, in this disclosure, all the maximum DC-side voltage values ​​are the same as the preset modulation wave amplitude, i.e., all are U. dcmax .

[0078] In one possible embodiment, S13 may include:

[0079]

[0080] Among them, M max U represents the preset maximum modulation ratio of the modular cascaded battery energy storage system. acmax U represents the maximum effective value of the grid-connected phase voltage. dc_min_sum_a U represents the sum of the minimum DC side voltages of phase a module. dc_min_sum_b U represents the sum of the minimum DC-side voltages of the b-phase module. dc_min_sum_c This represents the sum of the minimum DC voltages of the c-phase module, and min[] represents the minimum value operation.

[0081] In one possible embodiment, S14 may include:

[0082]

[0083] Among them, T c U represents the control period. dcmax M represents the preset modulation amplitude. max U represents the preset maximum modulation ratio of the modular cascaded battery energy storage system. dc_max_sum_a U represents the sum of the maximum DC side voltages of phase a module. dc_max_sum_b U represents the sum of the maximum DC-side voltages of the b-phase module. dc_max_sum_c This represents the sum of the maximum DC-side voltages of the c-phase module, and max[] indicates the operation of taking the maximum value. dc_max_sum_a U dc_max_sum_b U dc_max_sum_c [] indicates the maximum value of the sum of the maximum DC side voltages of each corresponding module.

[0084] Based on the control cycle, the optimal control frequency can be determined, providing design parameters for the controller of the preset modular cascaded battery energy storage system and improving the output voltage and current quality of the preset modular cascaded battery energy storage system.

[0085] The pre-designed modular cascaded battery energy storage system disclosed herein employs a nearest-level modulation method. The controller of the pre-designed modular cascaded battery energy storage system uses a timing control method, with a power control period of Tp.

[0086] In one possible embodiment, S15 may include S31 to S32.

[0087] S31, the instantaneous value of the modulated wave is obtained from the control strategy based on a preset first time interval through the modulation function.

[0088] The modulation function can be implemented using software algorithms or hardware modules. It obtains the instantaneous value U of the modulated wave in the control strategy based on a preset first time interval through recent-level modulation. x .

[0089] S32, based on a preset second time interval, obtains the preset module charging and discharging priority of the modular cascaded battery energy storage system from the equalization algorithm through the modulation function.

[0090] The module charging and discharging priority ranking is obtained from the equalization algorithm by using the most recent level modulation based on a preset second time interval.

[0091] In one possible embodiment, a near-level control method for a modular cascaded battery energy storage system may further include S18.

[0092] S18, based on the DC side voltage of each priority module and the module charging and discharging priority, sequentially determine the sequence calculation results according to the module charging and discharging priority cutoff up to the DC side voltage of each priority module.

[0093] Based on the module charging and discharging priorities, the sequence calculation results of the DC side voltage cutoff to each priority module are determined:

[0094] ...

[0096]

[0097] Where U(1) represents the DC side voltage of the first priority module, U(2) represents the result of a series of calculations based on the module charging and discharging priority cutoff until the DC-side voltage of the first priority module is reached, and U(2) represents the DC-side voltage of the second priority module. This represents the result of a sequence calculation based on the DC-side voltage of the module up to the second priority module, according to the module's charging and discharging priority. U(k-1) represents the DC-side voltage of the (k-1)th priority module, and U(k) represents the DC-side voltage of the kth priority module. This represents the result of a series of calculations based on the DC-side voltage of the kth module, according to the module's charging and discharging priority.

[0098] In one possible embodiment, S16 may include S41.

[0099] S41, if the result of the sequence calculation of the DC side voltage of the current priority module according to the module charging and discharging priority is greater than the instantaneous value of the modulation wave, the number of modules put into operation is the priority module from the first priority module to the previous priority module according to the module charging and discharging priority.

[0100] In one possible embodiment, S16 may also include S42.

[0101] If the result of the sequence calculation of the DC side voltage of the current priority module according to the module charging and discharging priority is not greater than the instantaneous value of the modulation wave, then the next priority module of the current priority module is taken as the new current priority module. The result of the sequence calculation of the DC side voltage of the new current priority module according to the module charging and discharging priority is compared with the instantaneous value of the modulation wave, until the result of the sequence calculation of the DC side voltage of the new current priority module is greater than the instantaneous value of the modulation wave. The number of modules put into operation is determined according to the module charging and discharging priority from the first priority module to the previous priority module of the new current priority module.

[0102] For example, if Then the first priority module will be deployed;

[0103] like Then the first priority module and the second priority module will be deployed;

[0104] If so Then the first priority module, the second priority module, and the third priority module will be deployed;

[0105] And so on, if Then the first priority module, the second priority module, ..., the (k-1)th priority module and the kth priority module are deployed.

[0106] Until Stop comparing the sequence calculation result of the DC side voltage of the current priority module up to the new current priority module with the instantaneous value of the modulation wave. The (k+1)th priority module, the (k+2)th priority module, ..., the nth priority module are not put into operation. Then the modules that should be put into operation now are the first priority module to the kth priority module according to the module charging and discharging priority. The number of modules put into operation according to the module charging and discharging priority is k.

[0107] By determining the modules to be put into operation and the number of modules to be put into operation according to the charging and discharging priority of the modules and the actual DC side voltage of each priority module, the impact of voltage differences between modules is reduced and the module utilization rate is improved. By adopting the nearest level modulation method, the output voltage of the preset modular cascaded battery energy storage system is made as close as possible to the modulated wave.

[0108] The specific embodiments of this disclosure have been described above. It should be understood that this disclosure is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the substantive content of this disclosure. The above-described preferred features can be used in any combination without conflict.

Claims

1. A method for determining the control period and a method for modulating a modular cascaded battery energy storage system, characterized in that, The method comprises the following steps: determining the sum of minimum voltage of each module DC side and the sum of maximum voltage of each module DC side according to the design parameters of the preset modular cascaded battery energy storage system; determining the maximum value of grid-connected phase voltage effective value according to the design parameters of the preset modular cascaded battery energy storage system; determining the maximum modulation ratio of the preset modular cascaded battery energy storage system according to the sum of minimum voltage of each module DC side and the maximum value of grid-connected phase voltage effective value; determining the control period according to the maximum modulation ratio of the preset modular cascaded battery energy storage system, the maximum value of the sum of maximum voltage of each module DC side, the preset modulation wave amplitude, and the preset modulation wave period; obtaining the instantaneous value of the modulation wave and the module charge-discharge priority of the preset modular cascaded battery energy storage system; determining the number of modules put into operation according to the instantaneous value of the modulation wave and the operation result of the sequence of DC side voltage of each module according to the module charge-discharge priority; controlling the preset modular cascaded battery energy storage system to perform nearest level modulation according to the maximum modulation ratio of the preset modular cascaded battery energy storage system and the number of modules put into operation according to the module charge-discharge priority.

2. The method of claim 1, wherein, The step of determining the sum of minimum voltage of each module DC side and the sum of maximum voltage of each module DC side according to the design parameters of the preset modular cascaded battery energy storage system comprises the following steps: determining the DC side voltage operation interval of each module of the preset modular cascaded battery energy storage system according to the design parameters of the preset modular cascaded battery energy storage system; determining the maximum value of DC side voltage of all modules according to the DC side voltage operation interval of each module; determining the sum of minimum voltage of each module DC side and the sum of maximum voltage of each module DC side according to the DC side voltage operation interval of each module.

3. The method of claim 2, wherein, The maximum value of DC side voltage of all modules is the same as the preset modulation wave amplitude.

4. The method of claim 1, wherein, The step of determining the maximum modulation ratio of the preset modular cascaded battery energy storage system according to the sum of minimum voltage of each module DC side and the maximum value of grid-connected phase voltage effective value comprises the following steps: wherein M max represents the maximum modulation ratio of the preset modular cascaded battery energy storage system, U acmax represents the maximum value of the effective value of the grid-connected phase voltage, U dc_min_sum_a represents the sum of the minimum voltages on the DC side of the a-phase modules, U dc_min_sum_b represents the sum of the minimum voltages on the DC side of the b-phase modules, U dc_min_sum_c represents the sum of the minimum voltages on the DC side of the c-phase modules, min[] represents the minimum value operation.

5. The method of claim 1, wherein, The step of determining the control period according to the maximum modulation ratio of the preset modular cascaded battery energy storage system, the maximum value of the sum of maximum voltage of each module DC side, the preset modulation wave amplitude, and the preset modulation wave period comprises the following steps: wherein, T c represents the control period, U dcmax represents the preset modulation wave amplitude, M max represents the preset maximum modulation ratio of the modular cascaded battery energy storage system, U dc_max_sum_a represents the sum of maximum voltages on the DC side of phase a modules, U dc_max_sum_b represents the sum of maximum voltages on the DC side of phase b modules, U dc_max_sum_c represents the sum of maximum voltages on the DC side of phase c modules, max[] represents the maximum value operation, max[U dc_max_sum_a ,U dc_max_sum_b ,U dc_max_sum_c ] represents the maximum value of the sum of maximum voltages on the DC side of the corresponding modules of each phase, T m represents the preset modulation wave period.

6. The method of claim 1, wherein, The step of obtaining the instantaneous value of the modulation wave and the module charge-discharge priority of the preset modular cascaded battery energy storage system comprises the following steps: obtaining the instantaneous value of the modulation wave from the control strategy based on a preset first time interval through a modulation function; obtaining the module charge-discharge priority of the preset modular cascaded battery energy storage system from the equalization algorithm based on a preset second time interval through the modulation function.

7. The method of claim 1, wherein, The method further comprises the following steps: According to the DC side voltage of each priority module and the module charge-discharge priority, a series operation result of the DC side voltage of each priority module is determined in sequence according to the module charge-discharge priority.

8. The method of claim 7, wherein, According to the DC side voltage of each priority module and the module charge-discharge priority, a series operation result of the DC side voltage of each priority module is determined in sequence according to the module charge-discharge priority, comprising: ... wherein U(l) represents the DC side voltage of the first priority module, represents the series operation result up to the DC side voltage of the first priority module according to the module charge-discharge priority, U(2) represents the DC side voltage of the second priority module, represents the series operation result up to the DC side voltage of the second priority module according to the module charge-discharge priority, U(k-1) represents the DC side voltage of the k-1th priority module, and U(k) represents the DC side voltage of the kth priority module, represents the series operation result up to the DC side voltage of the kth module according to the module charge-discharge priority.

9. The method of claim 1, wherein, The determination of the module number according to the module charge-discharge priority comprises: If the series operation result of the DC side voltage of the current priority module is greater than the instantaneous value of the modulation wave, the module number is the last priority module from the first priority module to the current priority module according to the module charge-discharge priority.

10. The method of claim 9, wherein, The determination of the module number according to the module charge-discharge priority further comprises: If the series operation result of the DC side voltage of the current priority module is not greater than the instantaneous value of the modulation wave, the next priority module of the current priority module is taken as a new current priority module, and the series operation result of the DC side voltage of the new current priority module is compared with the instantaneous value of the modulation wave until the series operation result of the DC side voltage of the new current priority module is greater than the instantaneous value of the modulation wave, and the module number is determined as the last priority module from the first priority module to the last priority module of the new current priority module according to the module charge-discharge priority.

Images