A power grid-oriented energy storage system cooperative control method

Abstract

The application discloses a power grid-oriented energy storage system cooperative control method and relates to the technical field of power grid storage control. The application comprises the following steps: inputting fixed parameters of a module; collecting parameters of each module in each cycle, obtaining an automatic power generation control power instruction and a direct-current bus voltage; calculating a total direct-current current reference value and limiting the total direct-current current reference value according to the automatic power generation control power instruction and the direct-current bus voltage in combination with the efficiency of a converter; calculating an absolute value of a vertical temperature difference as a characteristic temperature difference, obtaining liquid metal physical property parameters, and calculating a Rayleigh number; calculating a Nusselt number by using an empirical correlation and the Rayleigh number, combining a liquid metal thermal conductivity coefficient to calculate an equivalent convective heat transfer coefficient; distributing the total current based on the equivalent convective heat transfer coefficient to obtain reference currents of each module; limiting the reference currents and matching charge and discharge directions to form current instructions and issuing the current instructions to corresponding DC / DC converters; each DC / DC converter executes current closed-loop control, and an energy storage converter maintains the direct-current bus voltage and realizes power exchange.

Description

Technical Field

[0001] This invention belongs to the field of power grid storage control technology, and in particular relates to a collaborative control method for power grid-oriented energy storage systems. Background Technology

[0002] Liquid metal batteries, as an electrochemical energy storage technology designed for grid-scale energy storage, operate at temperatures maintained at several hundred degrees Celsius, with their internal electrode materials and electrolytes in a molten liquid state. In large-capacity energy storage power stations, multiple liquid metal battery modules are typically connected in parallel to the DC bus and then to the AC grid via an energy storage converter, undertaking secondary frequency regulation tasks for Automatic Generation Control (AGC). To respond to grid frequency fluctuations, AGC commands require the energy storage system to frequently switch between charging and discharging states and adjust power output within a second-scale timescale, resulting in rapid and significant changes in the current of each parallel module.

[0003] During the aforementioned operation, due to differences in Joule heat generation rate and heat dissipation conditions, the liquid metal inside the module exhibits significant uneven temperature distribution in the vertical direction, a phenomenon known as thermal stratification. Once thermal stratification forms and continues to accumulate, it will cause non-uniform corrosion of electrode materials, localized solidification or overheating of the electrolyte, seriously threatening the battery's cycle life and operational safety. Existing collaborative control methods generally rely on limited temperature information obtained from surface temperature sensors located on the module casing, combined with the principle of state-of-charge balance, for power allocation. However, liquid metal battery modules have a closed cavity structure, and internal heat transfer relies not only on thermal conduction but also on natural convection driven by temperature differences; surface temperature cannot reflect the real-time intensity of internal natural convection, and surface temperature-based equalization control is insufficient to suppress the formation and accumulation of thermal stratification from the perspective of thermal response synchronization.

[0004] Specifically, the intensity of natural convection is characterized by the Rayleigh number of the liquid metal, which depends on the vertical temperature difference within the module and the physical properties of the liquid metal at the current average temperature, such as the coefficient of thermal expansion, kinematic viscosity, and thermal diffusivity. Different modules exhibit varying natural convection intensities due to differences in manufacturing processes, aging processes, and uneven current distribution at the operating midpoint. This results in inconsistent temperature rise rates among modules under the same current. Existing surface temperature-based balancing strategies cannot detect these differences in internal convection states, and current allocation lacks a physical basis at the heat transfer level. Consequently, some modules continuously bear current loads that are mismatched with their heat dissipation capacity, leading to increasingly severe thermal stratification and ultimately causing significant deviations in the temperature fields between modules, affecting the overall performance and safety of the energy storage system. Therefore, the following solutions are proposed to address these issues. Summary of the Invention

[0005] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution: This invention relates to a collaborative control method for energy storage systems oriented towards power grids, comprising the following steps: Step S1: Input the fixed parameters of the liquid metal battery module, the coefficients of the empirical correlation of natural convection heat transfer, the lookup function of liquid metal properties changing with temperature, and the limit of the safe operating range of the state of charge into the collaborative controller. Step S2: In each control cycle, collect the upper temperature, lower temperature, state of charge and current of each parallel module, and at the same time obtain the automatic generation control power command and DC bus voltage; Step S3: Calculate the total DC current reference value based on the automatic power generation control command and DC bus voltage, combined with the energy storage converter efficiency, and limit the total DC current reference value. Step S4: Calculate the vertical temperature difference based on the upper and lower temperatures of each module and take the absolute value as the characteristic temperature difference. Obtain the liquid metal physical property parameters by looking up a table function based on the average temperature of each module, and calculate the Rayleigh number that characterizes the intensity of internal natural convection. Step S5: Calculate the Nusselt number of each module using the empirical correlation and the Rayleigh number, and then calculate the equivalent convective heat transfer coefficient by combining the thermal conductivity of the liquid metal of each module at the current average temperature. Step S6: Based on the equivalent convective heat transfer coefficient of each module, the reference current of each module is proportional to the square root of the equivalent convective heat transfer coefficient according to the distribution relationship. The total DC current reference value is then distributed to obtain the reference current of each module. Step S7: Limit the reference current of each module to ensure that the current does not exceed the maximum allowable charging and discharging current and the direction matches the charging and discharging mode, generate a current command and send it to the corresponding DC / DC converter. Step S8: The DC / DC converter of each module performs current closed-loop control according to the current command, and the energy storage converter maintains the DC bus voltage constant and realizes the exchange of active power and reactive power. Step S9: Return to step S2 and enter the next control cycle, repeating steps S2 to S8.

[0006] Further, step S1 includes the following steps: Step S11: Input the fixed parameters of each liquid metal battery module, including the module feature height. Effective heat exchange area Module equivalent internal resistance Effective thermal mass of electrodes Specific heat capacity of liquid metal and the function of thermal conductivity with temperature ; Step S12: Input the coefficients in the empirical correlation of natural convection heat transfer calibrated by multiphysics simulation and thermal testing. Sum of Indices And a lookup table function storing the properties of liquid metal as a function of temperature, wherein the lookup table function includes a function of the coefficient of thermal expansion as a function of temperature. kinematic viscosity as a function of temperature and the function of thermal diffusivity with temperature .

[0007] Furthermore, step S2 includes the following steps: Step S21: Obtain the upper temperature of each parallel module from the battery management system via the communication bus. Lower temperature Current state of charge and module output current subscript Indicates the module number. , This refers to the number of modules connected in parallel. Step S22: Obtain DC bus voltage from the energy storage converter controller and the automatic generation control power commands issued by the dispatch master station .

[0008] Furthermore, step S3 includes the following steps: Step S31: Convert the automatic power generation control command into a total DC current reference value on the energy storage side. The total DC current reference value is used for subsequent inter-module current distribution. The calculation formula is as follows:

[0009] In the formula, The efficiency of the energy storage converter in rectification or inversion mode. A positive value indicates discharging, and a negative value indicates charging. The symbol indicates the direction of the current; Step S32: Calculate the... Limit the amplitude to ensure It shall not exceed the upper limit of the rated total current and dynamic capability of the energy storage system.

[0010] Furthermore, step S4 includes the following steps: Step S41: Calculate the vertical temperature difference of each module. The vertical temperature difference is used to characterize the degree of unevenness in the temperature distribution inside the module. The formula is as follows:

[0011] The absolute value is then taken to obtain the characteristic temperature difference, which serves as the temperature difference driving natural convection, as shown in the following formula:

[0012] when When the temperature difference is less than the preset threshold of 0.1K, the characteristic temperature difference will be maintained at the effective value of the previous control cycle or the preset minimum value of 0.1K will be directly assigned. Step S42: Based on the average temperature of each module The physical properties of the liquid metal at this average temperature, including the coefficient of thermal expansion, are obtained by using a lookup table function. Kinematic viscosity and thermal diffusivity ; Step S43: Calculate the instantaneous Rayleigh number of the liquid metal inside each module. The Rayleigh number is used to quantify the relative intensity of natural convection driven by the temperature gradient and is a core parameter for subsequent thermal co-distribution. The Rayleigh number formula is as follows:

[0013] In the formula, It is the acceleration due to gravity. This represents the module feature height.

[0014] Furthermore, step S5 includes the following steps: Step S51: Calculate the Nusselt number of each module using the empirical correlation of natural convection heat transfer. The Nusselt number characterizes the degree of enhancement of heat transfer by natural convection. The formula is as follows:

[0015] In the formula, and The coefficients in the empirical correlation; Step S52: Calculate the equivalent convective heat transfer coefficient of each module. The equivalent convective heat transfer coefficient comprehensively reflects the heat transfer capacity achieved by natural convection inside the module. The formula is as follows:

[0016] In the formula, The thermal conductivity of the liquid metal at the current average temperature of each module. This represents the module feature height.

[0017] Furthermore, step S6 includes the following steps: Step S61: Module The average temperature dynamics are described by a lumped heat capacity model, which expresses the relationship between the module's average temperature change rate and the Joule heat generation rate and convective heat dissipation. The model expression is as follows:

[0018] in The equivalent temperature of the external environment of the module. For flow through the module The current, Joule heat generation rate; thermal mass of each module Heat exchange area Under consistent conditions, if the average temperature change rate of each module is required... If they tend to be consistent, then the square of the current in each module will be... With convective heat transfer coefficient They are roughly proportional, thus yielding the module reference current and the equivalent convective heat transfer coefficient. The distribution is proportional to the square root; Step S62: Calculate the reference current for each module according to the allocation relationship. The allocation relationship incorporates the real-time state of natural convection inside the module into the current allocation, so that the module with strong convection heat transfer capability can bear a larger current, thereby achieving synchronous thermal response of each module. The formula for calculating the reference current is as follows:

[0019] In the formula, This is the reference value for the total DC current. For the number of parallel modules, For module indexing.

[0020] Furthermore, step S7 includes the following steps: Step S71: Set the reference current for each module The maximum allowable charge / discharge current of the corresponding module under the current temperature and state of charge. The comparison is performed, the current is limited, and the current direction is ensured to match the charging and discharging mode to obtain the module current command. ; Step S72: The cooperative controller transmits data via the high-speed communication bus. Send to the local control unit of the DC / DC converter corresponding to each module.

[0021] Furthermore, after obtaining the reference current of each module in step S6 and before limiting in step S7, the following steps are also included: Read the current state of charge of each module and the preset safe working area Compare; If a module's state of charge exceeds the safe operating range, the system switches to the state of charge balancing control mode. Modules with a state of charge exceeding the upper limit are current-limited during charging, and modules with a state of charge below the lower limit are current-limited during discharging. The power ratio of the corresponding module during reverse power is also increased to promote the recovery of the state of charge. Once all modules return to the safe operating range in terms of charge state, the system automatically switches back to the allocation mode based on the equivalent convective heat transfer coefficient in step S6.

[0022] Furthermore, the collaborative control cycle entered in step S1 The temperature is less than the thermal time constant of the liquid metal inside the module, so that the change in the average temperature of each module is small enough during the cyclic execution process from step S9 back to step S2, to ensure the effectiveness of the coordinated distribution based on the equivalent convective heat transfer coefficient.

[0023] The present invention has the following beneficial effects: 1. This invention calculates the Rayleigh number of the liquid metal inside each parallel module in real time to obtain the instantaneous intensity of natural convection, and derives the equivalent convective heat transfer coefficient accordingly, thus incorporating the internal heat transfer capacity into the current distribution decision. Compared with traditional balancing strategies that rely solely on surface temperature or state of charge, this method can directly sense the changes in the convective state driven by temperature difference inside the module, guiding the thermal response of each module to tend towards synchronization from the heat transfer mechanism level. This effectively alleviates the accumulation of heat stratification caused by frequent charging and discharging, reduces the risk of local overheating, and improves the consistency and long-term stability of battery module operation.

[0024] 2. This invention utilizes the existing upper and lower temperature measurement points of each module and the preset liquid metal property lookup table function to achieve online evaluation of the natural convection state inside the closed module cavity without adding a direct internal flow field measurement sensor. By calculating the Nusselt number through empirical correlation, the convective heat transfer capacity, which is difficult to measure directly, is transformed into an equivalent convective heat transfer coefficient that can be calculated online. The hardware modification is small and the implementation cost is low, while ensuring the reliable acquisition of the internal state information required for collaborative control.

[0025] 3. The current distribution strategy of this invention is derived from the lumped heat capacity model, which establishes a physical relationship between the square of the module current and the convective heat transfer coefficient, so that the distribution relationship has a clear heat transfer basis and there is no need to manually set dynamic weighting factors or empirical coefficients. The distribution ratio is automatically adjusted with the changes of Rayleigh number and convective heat transfer coefficient, and has good adaptive ability to the changes in internal physical properties caused by operating condition fluctuations and battery aging, avoiding the problem of performance degradation of fixed parameter distribution under complex operating environment.

[0026] 4. This invention organically integrates thermal synergy distribution with the tracking of automatic power generation control commands from the power grid. Under the premise of meeting the accuracy of total power response, the current of each module is distributed according to the square root ratio of the convective heat transfer coefficient, so that the system can continuously suppress the temperature field difference between modules while completing the frequency regulation task. This thermoelectric synergy architecture does not sacrifice the energy storage system's support performance for the power grid, and can take into account both the quality of frequency regulation service and the thermal safety of the battery itself. It is suitable for grid-connected energy storage scenarios that require rapid power response.

[0027] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Attached Figure Description

[0028] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of 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 flowchart illustrating a collaborative control method for a power grid-oriented energy storage system according to 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] Please see Figure 1 As shown, this invention is a collaborative control method for energy storage systems oriented towards power grids, comprising the following steps: Step S1: Input the fixed parameters of the liquid metal battery module, the coefficients of the empirical correlation of natural convection heat transfer, the lookup function of liquid metal properties changing with temperature, and the limit of the safe operating range of the state of charge into the collaborative controller. Step S2: In each control cycle, collect the upper temperature, lower temperature, state of charge and current of each parallel module, and at the same time obtain the automatic generation control power command and DC bus voltage; Step S3: Calculate the total DC current reference value based on the automatic generation control power command and DC bus voltage, combined with the energy storage converter efficiency, and limit the total DC current reference value. Step S4: Calculate the vertical temperature difference based on the upper and lower temperatures of each module and take the absolute value as the characteristic temperature difference. Obtain the liquid metal physical property parameters by looking up a table function based on the average temperature of each module, and calculate the Rayleigh number that characterizes the intensity of internal natural convection. Step S5: Calculate the Nusselt number of each module using empirical correlations and Rayleigh numbers, and then calculate the equivalent convective heat transfer coefficient by combining the thermal conductivity of the liquid metal of each module at the current average temperature. Step S6: Based on the equivalent convective heat transfer coefficient of each module, the reference current of each module is proportional to the square root of the equivalent convective heat transfer coefficient according to the distribution relationship. The total DC current reference value is then distributed to obtain the reference current of each module. Step S7: Limit the reference current of each module to ensure that the current does not exceed the maximum allowable charging and discharging current and the direction matches the charging and discharging mode, generate a current command and send it to the corresponding DC / DC converter. Step S8: The DC / DC converter of each module performs current closed-loop control according to the current command, and the energy storage converter maintains the DC bus voltage constant and realizes the exchange of active power and reactive power. Step S9: Return to step S2 and enter the next control cycle, repeating steps S2 to S8.

[0032] Step S1 includes the following steps: Step S11: Enter the fixed parameters for each liquid metal battery module. The fixed parameters include the module feature height. Effective heat exchange area Module equivalent internal resistance Effective thermal mass of electrodes Specific heat capacity of liquid metal and the function of thermal conductivity with temperature ; Step S12: Input the coefficients in the empirical correlation of natural convection heat transfer calibrated by multiphysics simulation and thermal testing. Sum of Indices And a lookup table function storing the properties of liquid metal as a function of temperature, including a function of the coefficient of thermal expansion as a function of temperature. kinematic viscosity as a function of temperature and the function of thermal diffusivity with temperature .

[0033] Step S2 includes the following steps: Step S21: Obtain the upper temperature of each parallel module from the battery management system via the communication bus. Lower temperature Current state of charge and module output current subscript Indicates the module number. , This refers to the number of modules connected in parallel. Step S22: Obtain DC bus voltage from the energy storage converter controller and the automatic generation control power commands issued by the dispatch master station .

[0034] Step S3 includes the following steps: Step S31: Convert the automatic generation control power command into a total DC current reference value on the energy storage side. The total DC current reference value is used for subsequent inter-module current distribution. The calculation formula is as follows:

[0035] In the formula, The efficiency of the energy storage converter in rectification or inversion mode. A positive value indicates discharging, and a negative value indicates charging. The symbol indicates the direction of the current; Step S32: Calculate the... Limit the amplitude to ensure It shall not exceed the upper limit of the rated total current and dynamic capability of the energy storage system.

[0036] Step S4 includes the following steps: Step S41: Calculate the vertical temperature difference of each module. The vertical temperature difference is used to characterize the degree of unevenness in the temperature distribution inside the module. The formula is as follows:

[0037] The absolute value is then taken to obtain the characteristic temperature difference, which serves as the temperature difference driving natural convection. The formula is as follows:

[0038] when When the temperature difference is less than the preset threshold of 0.1K, the characteristic temperature difference will be maintained at the effective value of the previous control cycle or the preset minimum value of 0.1K will be directly assigned. Step S42: Based on the average temperature of each module The physical properties of the liquid metal at this average temperature, including the coefficient of thermal expansion, are obtained by using a lookup table function. Kinematic viscosity and thermal diffusivity ; Step S43: Calculate the instantaneous Rayleigh number of the liquid metal inside each module. The Rayleigh number is used to quantify the relative intensity of natural convection driven by the temperature gradient and is a core parameter for subsequent thermal co-distribution. The Rayleigh number formula is as follows:

[0039] In the formula, It is the acceleration due to gravity. This represents the module feature height.

[0040] Step S5 includes the following steps: Step S51: Calculate the Nusselt number of each module using the empirical correlation of natural convection heat transfer. The Nusselt number characterizes the degree of enhancement of heat transfer by natural convection, and the formula is as follows:

[0041] In the formula, and These are the coefficients in the empirical correlation formula; Step S52: Calculate the equivalent convective heat transfer coefficient of each module. The equivalent convective heat transfer coefficient comprehensively reflects the heat transfer capacity achieved by natural convection inside the module. The formula is as follows:

[0042] In the formula, The thermal conductivity of the liquid metal at the current average temperature of each module. This represents the module feature height.

[0043] Step S6 includes the following steps: Step S61: Module The average temperature dynamics are described by a lumped heat capacity model, which expresses the relationship between the module's average temperature change rate and the Joule heat generation rate and convective heat dissipation. The model expression is as follows:

[0044] in The equivalent temperature of the external environment of the module. For flow through the module The current, Joule heat generation rate; thermal mass of each module Heat exchange area Under consistent conditions, if the average temperature change rate of each module is required... If they tend to be consistent, then the square of the current in each module will be... With convective heat transfer coefficient They are roughly proportional, thus yielding the module reference current and the equivalent convective heat transfer coefficient. The distribution is proportional to the square root; Step S62: Calculate the reference current for each module based on the allocation relationship. The allocation relationship incorporates the real-time state of natural convection within the module into the current allocation, allowing modules with strong convection heat transfer capabilities to bear a larger current, thus achieving synchronous thermal response of each module. The formula for calculating the reference current is as follows:

[0045] In the formula, This is the reference value for the total DC current. For the number of parallel modules, For module indexing.

[0046] Step S7 includes the following steps: Step S71: Set the reference current for each module The maximum allowable charge / discharge current of the corresponding module under the current temperature and state of charge. The comparison is performed, the current is limited, and the current direction is ensured to match the charging and discharging mode to obtain the module current command. ; Step S72: The cooperative controller transmits data via the high-speed communication bus. Send to the local control unit of the DC / DC converter corresponding to each module.

[0047] After obtaining the reference current of each module in step S6 and before limiting in step S7, the following steps are also included: Read the current state of charge of each module and the preset safe working area Compare; If a module's state of charge exceeds the safe operating range, the system switches to the state of charge balancing control mode. Modules with a state of charge exceeding the upper limit are current-limited during charging, and modules with a state of charge below the lower limit are current-limited during discharging. The power ratio of the corresponding module during reverse power is also increased to promote the recovery of the state of charge. Once all modules return to a safe operating range of charge, the system automatically switches back to the allocation mode based on the equivalent convective heat transfer coefficient in step S6.

[0048] The collaborative control cycle entered in step S1 The temperature is less than the thermal time constant of the liquid metal inside the module, so that the change in the average temperature of each module is small enough during the cyclic execution process from step S9 back to step S2, to ensure the effectiveness of the coordinated distribution based on the equivalent convective heat transfer coefficient.

[0049] One specific application of this embodiment is: This embodiment addresses a grid-side frequency regulation energy storage scenario. The energy storage system consists of four parallel liquid metal battery modules, connected to the 10kV AC grid via an energy storage converter, and responds to Automatic Generation Control (AGC) power commands issued by the dispatch master station. The control cycle of the collaborative controller is set to 1 second, which is less than the thermal time constant of the liquid metal within the modules, ensuring the effectiveness of thermal state estimation and current distribution. The specific collaborative control process is as follows: Step S1: System parameter preset Pre-enter the module's fixed parameters, heat transfer empirical correlation coefficients, physical property lookup functions, and SOC safety limits into the collaborative controller. The specific details are as follows: Step S11: Input of module fixed parameters Enter the unified structure and thermal parameters of 4 liquid metal battery modules: Module feature height Effective heat exchange area Module equivalent internal resistance Equivalent thermal mass of electrode and liquid metal The thermal conductivity of liquid metal as a function of temperature Preset in the form of a lookup table.

[0050] Step S12: Input of heat transfer correlation equation and physical property parameters Enter the empirical correlation coefficients for natural convection heat transfer, calibrated through multiphysics simulation and thermal testing: , ; A lookup table function for the temperature-dependent properties of a pre-set liquid metal, including the coefficient of thermal expansion. Kinematic viscosity Thermal diffusivity ; Enter the safe operating range for the charged state: Maximum allowable charging and discharging current of a single module Rated total current of energy storage system .

[0051] Step S2: Real-time status acquisition At the start of the k-th control cycle, the collaborative controller collects the overall system operating status via the communication bus: Step S21: Module-side status acquisition Operating data for the four parallel modules were obtained from the battery management system: Module 1 has an upper temperature of 723.0K, a lower temperature of 718.0K, a current state of charge (SOC) of 0.55%, and a real-time output current of 140A; Module 2 has an upper temperature of 725.0K, a lower temperature of 719.0K, a current SOC of 0.54%, and a real-time output current of 142A; Module 3 has an upper temperature of 722.0K, a lower temperature of 717.0K, a current SOC of 0.56%, and a real-time output current of 138A; Module 4 has an upper temperature of 724.0K, a lower temperature of 718.0K, a current SOC of 0.55%, and a real-time output current of 141A.

[0052] Step S22: Grid-side command acquisition Obtain DC bus voltage from energy storage converter controller And the AGC power commands issued by the main dispatch station. (A positive value indicates the discharge condition).

[0053] Step S3: Calculation and Limiting of Total DC Current Reference Value Step S31: Total DC current conversion Combined with the discharge efficiency of the energy storage converter Convert the AGC power command into a DC-side total current reference value: The current is positive, corresponding to the discharge direction, which is consistent with the AGC command operating condition.

[0054] Step S32: Total Current Limiting The system's rated total current is 800A, calculated as follows: Since the limit was not exceeded, the reference value of the total DC current remained unchanged at 657.89A after the limit was set.

[0055] Step S4: Calculation of Characteristic Temperature Difference and Rayleigh Number Step S41: Calculation of vertical temperature difference and characteristic temperature difference Calculate the vertical temperature difference and characteristic temperature difference for each module, taking module 1 as an example: Vertical temperature difference: Characteristic temperature difference (absolute value): Since this value is greater than the preset threshold of 0.1K, the calculated value is used directly. Similarly, the following calculation is obtained: , , .

[0056] Step S42: Look up the average temperature and physical property parameters in the table. Calculate the average temperature of each module, taking module 1 as an example: Similarly, we can conclude that: , , .

[0057] Based on the average temperature of each module, the corresponding liquid metal physical property parameters are obtained through a preset lookup table function: The coefficient of thermal expansion of module 1 is ; kinematic viscosity is ; Thermal diffusivity is ; Thermal conductivity is ; The coefficient of thermal expansion of module 2 is ; kinematic viscosity is ; Thermal diffusivity is ; Thermal conductivity is ; The coefficient of thermal expansion of module 3 is ; kinematic viscosity is ; Thermal diffusivity is ; Thermal conductivity is ; The coefficient of thermal expansion of module 4 is ; kinematic viscosity is ; Thermal diffusivity is ; Thermal conductivity is .

[0058] Step S43: Instantaneous Rayleigh number calculation Rayleigh quantifies the natural convection intensity inside the module, and takes the gravitational acceleration as... Taking module 1 as an example: Substitute the values: Similarly, the calculation yields: , , .

[0059] Step S5: Calculation of equivalent convective heat transfer coefficient Step S51: Nusselt number calculation The Nusselt number of each module is calculated using the empirical correlation of natural convection, taking module 1 as an example: Similarly, we can conclude that: , , .

[0060] Step S52: Calculation of equivalent convective heat transfer coefficient Combining thermal conductivity and characteristic height, the equivalent convective heat transfer coefficient is calculated, taking module 1 as an example: Similarly, we can conclude that: , , .

[0061] Step S6: Current distribution based on thermal coordination Step S61: Derivation of Assignment Relations The module's average temperature dynamically satisfies the lumped heat capacity model: Among them, the equivalent temperature of the environment , The Joule heating rate. Due to the thermal mass of the four modules. Heat exchange area Internal resistance To ensure consistency, if the average temperature change rate of each module is made to be uniform, then the square of the current... With convective heat transfer coefficient It is directly proportional, meaning that the module reference current is directly proportional to the square root of the equivalent convective heat transfer coefficient.

[0062] Step S62: Calculation of reference current for each module The total weight is the sum of the square roots of the heat transfer coefficients of each module: The calculation yields: Taking module 1 as an example, the reference current is: Similarly, the calculation yields: , , .

[0063] This allocation result allows modules 2 and 4, which have stronger convective heat transfer capabilities, to bear a larger current, thereby synchronizing the temperature rise rate of each module and suppressing the accumulation of heat stratification.

[0064] Step S7: Reference current limiting and command issuance Step S71: Current Limiting Process The maximum allowable charge / discharge current for a single module is 200A. The calculated reference currents for each module are all less than 200A, and the current direction is discharge, which matches the AGC command condition. Therefore, the current command after limiting is consistent with the reference current. , , , .

[0065] Step S72: Issuance of Instructions The collaborative controller sends current commands from each module to the corresponding local control unit of the DC / DC converter via a high-speed CAN communication bus.

[0066] Step S8: Low-level closed-loop execution After receiving the current command, each module DC / DC converter uses a PI control algorithm to perform current closed-loop tracking, stabilizing the module output current near the command value; the energy storage converter maintains the DC bus voltage at a constant 800V, and completes the active power exchange with the AC grid according to the AGC command, while providing reactive power support according to the grid demand.

[0067] Step S9: Periodic Cycle After the current control cycle is completed, return to step S2 and enter the next 1-second control cycle to continuously execute the status acquisition, calculation and command issuance process.

[0068] SOC Equalization Mode Switching Instructions If the state of charge of module 3 is collected in a certain control cycle If the value falls below the lower limit of the safety range by 0.1, the system will automatically switch from the thermal collaborative distribution mode to the state-of-charge equalization control mode. Under discharge conditions, the current of module 3 is limited to 30% of its rated value. At the same time, the current distribution ratio of module 3 under charging conditions is increased, which promotes its SOC to recover quickly. When all modules SOC return After the interval, the system automatically switches back to the heat-coordinated distribution mode based on the equivalent convective heat transfer coefficient.

[0069] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0070] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A collaborative control method for energy storage systems oriented towards power grids, characterized in that, Includes the following steps: Step S1: Input the fixed parameters of the liquid metal battery module, the coefficients of the empirical correlation of natural convection heat transfer, the lookup function of liquid metal properties changing with temperature, and the limit of the safe operating range of the state of charge into the collaborative controller. Step S2: In each control cycle, collect the upper temperature, lower temperature, state of charge and current of each parallel module, and at the same time obtain the automatic generation control power command and DC bus voltage; Step S3: Calculate the total DC current reference value based on the automatic power generation control command and DC bus voltage, combined with the energy storage converter efficiency, and limit the total DC current reference value. Step S4: Calculate the vertical temperature difference based on the upper and lower temperatures of each module and take the absolute value as the characteristic temperature difference. Obtain the liquid metal physical property parameters by looking up a table function based on the average temperature of each module, and calculate the Rayleigh number characterizing the intensity of internal natural convection. Step S5: Calculate the Nusselt number of each module using the empirical correlation and the Rayleigh number, and then calculate the equivalent convective heat transfer coefficient by combining the thermal conductivity of the liquid metal of each module at the current average temperature. Step S6: Based on the equivalent convective heat transfer coefficient of each module, the reference current of each module is proportional to the square root of the equivalent convective heat transfer coefficient according to the distribution relationship. The total DC current reference value is then distributed to obtain the reference current of each module. Step S7: Limit the reference current of each module to ensure that the current does not exceed the maximum allowable charging and discharging current and that the direction matches the charging and discharging mode, generate a current command and send it to the corresponding DC / DC converter. Step S8: The DC / DC converter of each module performs current closed-loop control according to the current command, and the energy storage converter maintains the DC bus voltage constant and realizes the exchange of active power and reactive power. Step S9: Return to step S2 and enter the next control cycle, repeating steps S2 to S8.

2. The collaborative control method for a grid-oriented energy storage system according to claim 1, characterized in that, Step S1 includes the following steps: Step S11: Input the fixed parameters of each liquid metal battery module, including the module feature height. Effective heat exchange area Module equivalent internal resistance Effective thermal mass of electrodes Specific heat capacity of liquid metal and the function of thermal conductivity with temperature ; Step S12: Input the coefficients in the empirical correlation of natural convection heat transfer calibrated by multiphysics simulation and thermal testing. Sum of Indices And a lookup table function storing the properties of liquid metal as a function of temperature, wherein the lookup table function includes a function of the coefficient of thermal expansion as a function of temperature. kinematic viscosity as a function of temperature and the function of thermal diffusivity with temperature .

3. The collaborative control method for a grid-oriented energy storage system according to claim 1, characterized in that, Step S2 includes the following steps: Step S21: Obtain the upper temperature of each parallel module from the battery management system via the communication bus. Lower temperature Current state of charge and module output current subscript Indicates the module number. , This refers to the number of modules connected in parallel. Step S22: Obtain DC bus voltage from the energy storage converter controller And the automatic generation control power commands issued by the dispatch master station .

4. The collaborative control method for a grid-oriented energy storage system according to claim 1, characterized in that, Step S3 includes the following steps: Step S31: Convert the automatic power generation control command into a total DC current reference value on the energy storage side. The total DC current reference value is used for subsequent inter-module current distribution. The calculation formula is as follows: In the formula, The efficiency of the energy storage converter in rectification or inversion mode. A positive value indicates discharging, and a negative value indicates charging. The symbol indicates the direction of the current; Step S32: Calculate the... Limit the amplitude to ensure It shall not exceed the upper limit of the rated total current and dynamic capability of the energy storage system.

5. The collaborative control method for a grid-oriented energy storage system according to claim 1, characterized in that, Step S4 includes the following steps: Step S41: Calculate the vertical temperature difference of each module. The vertical temperature difference is used to characterize the degree of unevenness in the temperature distribution inside the module. The formula is as follows: The absolute value is then taken to obtain the characteristic temperature difference, which serves as the temperature difference driving natural convection, as shown in the following formula: when When the temperature difference is less than the preset threshold of 0.1K, the characteristic temperature difference will be maintained at the effective value of the previous control cycle or the preset minimum value of 0.1K will be directly assigned. Step S42: Based on the average temperature of each module The physical properties of the liquid metal at this average temperature, including the coefficient of thermal expansion, are obtained by using a lookup table function. Kinematic viscosity and thermal diffusivity ; Step S43: Calculate the instantaneous Rayleigh number of the liquid metal inside each module. The Rayleigh number is used to quantify the relative intensity of natural convection driven by the temperature gradient and is a core parameter for subsequent thermal co-distribution. The Rayleigh number formula is as follows: In the formula, It is the acceleration due to gravity. This represents the module feature height.

6. The collaborative control method for a grid-oriented energy storage system according to claim 1, characterized in that, Step S5 includes the following steps: Step S51: Calculate the Nusselt number of each module using the empirical correlation of natural convection heat transfer. The Nusselt number characterizes the degree of enhancement of heat transfer by natural convection. The formula is as follows: In the formula, and The coefficients in the empirical correlation; Step S52: Calculate the equivalent convective heat transfer coefficient of each module. The equivalent convective heat transfer coefficient comprehensively reflects the heat transfer capacity achieved by natural convection inside the module. The formula is as follows: In the formula, The thermal conductivity of the liquid metal at the current average temperature of each module. This represents the module feature height.

7. The collaborative control method for a grid-oriented energy storage system according to claim 1, characterized in that, Step S6 includes the following steps: Step S61: Module The average temperature dynamics are described by a lumped heat capacity model, which expresses the relationship between the module's average temperature change rate and the Joule heat generation rate and convective heat dissipation. The model expression is as follows: in The equivalent temperature of the external environment of the module. For flow through the module The current, Joule heat generation rate; thermal mass of each module Heat exchange area Under consistent conditions, if the average temperature change rate of each module is required... If they tend to be consistent, then the square of the current in each module will be... With convective heat transfer coefficient They are roughly proportional, thus yielding the module reference current and the equivalent convective heat transfer coefficient. The distribution is proportional to the square root; Step S62: Calculate the reference current for each module according to the allocation relationship. The allocation relationship incorporates the real-time state of natural convection inside the module into the current allocation, so that the module with strong convection heat transfer capability can bear a larger current, thereby achieving synchronous thermal response of each module. The formula for calculating the reference current is as follows: In the formula, This is the reference value for the total DC current. For the number of parallel modules, For module indexing.

8. The collaborative control method for a grid-oriented energy storage system according to claim 1, characterized in that, Step S7 includes the following steps: Step S71: Set the reference current for each module The maximum allowable charge / discharge current of the corresponding module under the current temperature and state of charge. The comparison is performed, the current is limited, and the current direction is ensured to match the charging and discharging mode to obtain the module current command. ; Step S72: The cooperative controller transmits data via the high-speed communication bus. Send to the local control unit of the DC / DC converter corresponding to each module.

9. The collaborative control method for a grid-oriented energy storage system according to claim 1, characterized in that, After obtaining the reference current of each module in step S6 and before limiting in step S7, the following steps are also included: Read the current state of charge of each module and the preset safe working area Compare; If a module's state of charge exceeds the safe operating range, the system switches to the state of charge balancing control mode. Modules with a state of charge exceeding the upper limit are current-limited during charging, and modules with a state of charge below the lower limit are current-limited during discharging. The power ratio of the corresponding module during reverse power is also increased to promote the recovery of the state of charge. Once all modules return to the safe operating range under their state of charge, the system automatically switches back to the allocation mode based on the equivalent convective heat transfer coefficient in step S6.

10. The collaborative control method for a grid-oriented energy storage system according to claim 1, characterized in that, The collaborative control cycle entered in step S1 The temperature is less than the thermal time constant of the liquid metal inside the module, so that the change in the average temperature of each module is small enough during the cyclic execution process from step S9 back to step S2, to ensure the effectiveness of the coordinated distribution based on the equivalent convective heat transfer coefficient.

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