PWM charging and discharging control strategy based on dynamic reconfigurable battery energy storage system

Abstract

According to the dynamic reconfigurable battery energy storage technology based on energy digitization, the application normal form of fixed series-parallel connection in the battery invention is changed, and the physical connection between batteries is changed from the traditional rigid connection of fixed series-parallel connection to flexible connection of program control; a good balance control effect is achieved by controlling the time of each battery connected into the charging and discharging loop. According to the dynamic reconfigurable battery energy storage system, a PWM charging and discharging control strategy under the condition is provided, a duty ratio signal is applied to a non-conducted battery monomer, so that the non-conducted battery monomer is not completely conducted in a period, and an output power instruction and an equalization effect are well considered. Meanwhile, compared with a traditional equalization control strategy, the equalization speed of the improved strategy is also improved, the safety and the energy efficiency of the battery energy storage system are greatly improved, and a new equalization control thought is provided for constructing a large-scale, long-life and low-cost battery energy storage system.

Description

Technical Field

[0001] The present invention relates to the field of balanced control of battery energy storage systems, and in particular to a balanced control strategy under high overload operating conditions. Background Art

[0002] With the rapid development of electrochemical energy storage, the inherent safety of energy storage systems has become increasingly prominent. Energy storage battery systems are often composed of several battery modules connected in series and parallel. The same specification and model of battery cells have natural differences in parameters such as voltage, state of charge, capacity, internal resistance, life, and self-discharge rate. These subtle differences will be amplified after forming the battery module, which makes the entire battery system highly discrete. In an energy storage system, the modules connected in series receive the same charge and discharge current. However, due to the existence of discrete phenomena, there are differences in the capacity of each battery module, and the charge and discharge capacity of the entire battery system depends on the smallest single cell, so a "barrel effect" will be formed.

[0003] During the charging and discharging process, the battery module with small capacity reaches the charge and discharge depth threshold first, restricting the charge and discharge capacity of other batteries in the battery system. If the battery system cannot be fully charged or fully discharged for a long time, part of the capacity will be lost, causing irreversible damage to the battery system capacity, the SOC of the entire battery system will be inaccurate, the discharge depth will decrease, and the operating efficiency will be reduced. Under long-term operation, the polarization of the battery will become more and more serious, and the available capacity of the battery system will further decrease, which will cause problems in the safety and economy of the entire battery system. Therefore, battery balancing technology is particularly important.

[0004] In recent years, digital energy storage systems based on dynamically reconfigurable battery networks have received widespread attention. Dynamically reconfigurable battery networks deeply couple batteries with low-power semiconductor devices, transforming batteries from electrochemical reaction devices into a new type of digital device. Through millisecond-level battery physical connection topology reconstruction, the heat accumulation and thermal runaway problems caused by overcharging and over-discharging are eliminated in principle, achieving intrinsic safety at the battery system level. In addition, dynamically reconfigurable battery networks can quickly diagnose and remove faulty batteries while maintaining the normal operation of electrochemical energy storage, greatly improving the availability and reliability of the system.

[0005] For the traditional digital energy storage balancing strategy, when the power demand does not exceed the capacity of the N-1 column battery, the N-selected N-1 (column) and M-selected M-1 (row) charging and discharging control is executed; when the power instruction exceeds the upper limit of the N-1 column battery capacity but does not reach the N column battery capacity, all N columns of batteries are put into use. At this time, the balancing strategy fails, and long-term operation has an adverse effect on the life of the battery cells. Summary of the invention

[0006] In order to solve the above problems, the present invention improves the traditional method and adopts PWM charge and discharge control. The so-called PWM charge and discharge control is to divide each delay interval into smaller time scales. When the power instruction exceeds the upper limit of the N-1 column battery capacity but does not reach the N column battery capacity, the ratio of the excess power to the single string capacity is calculated to obtain the duty cycle of each battery cell in the non-conducting column, and then the power electronic switch of the battery cell in the non-conducting column is opened and closed by PWM control. Compared with the conventional digital energy storage two-dimensional balancing strategy, the improved strategy is more suitable for high overload conditions, improves battery utilization, correspondingly extends battery life, and reduces energy storage costs.

[0007] The present invention proposes a PWM charge and discharge control strategy based on a dynamically reconfigurable battery energy storage system, which is characterized by quickly achieving energy storage system power balance while providing high overload power for the system, and mainly includes the following steps:

[0008] 1) Establish a circuit topology model of a dynamically reconfigurable battery energy storage system, which mainly includes establishing an N-parallel and M-series battery network topology based on lithium iron phosphate battery cells, and connecting a high-frequency power switch in series to each lithium iron phosphate battery cell, and connecting each N parallel cells in parallel with a bypass high-frequency power switch. The high-frequency power electronic switches here are all MOSFET devices.

[0009] 2) Monitor and collect various electrical parameters of the energy storage system, including the state of charge (SOC) of each battery cell, the output voltage and current of the energy storage system, and external power instructions.

[0010] 3) Establish a traditional digital energy storage balancing control model for a dynamically reconfigurable battery energy storage system, input the SOC data of each battery cell and the external power command into the control module, and control the on-off of the MOSFET through the control module based on the SOC data of each battery cell, thereby controlling the input and output of each battery cell, dynamically reconstructing the battery network, and finally achieving SOC balance of the entire energy storage system.

[0011] 4) Establish a bidirectional DC / DC converter control model and construct a current loop based on PID control.

[0012] 5) Constant power control is adopted for the bidirectional DC / DC converter. The external power command collected by monitoring is input into the bidirectional DC / DC converter control model. Different current commands are obtained based on different power commands. The output current of the energy storage system is made to track the current command through the PID current loop, thereby controlling the charging and discharging of the energy storage system.

[0013] 6) Improve the traditional SOC balancing control model, perform PWM control on the high-frequency power electronic switch according to the external power command, realize PWM charging and discharging control, and operate in full power mode when the energy storage system is fully loaded to ensure output power matching. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0015] Figure 1 The general flow chart for implementing the method of the present invention is as follows;

[0016] Figure 2 It is a schematic diagram of the energy storage system structure;

[0017] Figure 3 This is a comparison chart between traditional series-parallel battery packs and digital energy storage battery packs;

[0018] Figure 4 Reconstruct the structure diagram for digital energy storage battery;

[0019] Figure 5 It is a flow chart of power monitoring and topology reconstruction of reconfigurable battery pack;

[0020] Figure 6 This is a diagram of the traditional digital energy storage balancing strategy;

[0021] Figure 7 This is the PWM charge and discharge control balancing strategy diagram. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present invention are described in conjunction with the drawings of the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0023] See also Figure 1 The present invention proposes a PWM charge and discharge control strategy based on a dynamically reconfigurable battery energy storage system, comprising the following steps:

[0024] Step S10, establishing a circuit topology model of a dynamically reconfigurable battery energy storage system; Step S20, monitoring and collecting various electrical parameters of the energy storage system, including the state of charge SOC of each battery cell, the system's external output voltage and current, and external power instructions; Step S30, establishing a traditional SOC balancing control model of a dynamically reconfigurable battery energy storage system; Step S40, establishing a bidirectional DC / DC converter control model; Step S50, adopting constant power control for the bidirectional DC / DC converter; Step S60, improving the traditional SOC balancing control model to achieve PWM charging and discharging control.

[0025] The step S10 establishes a circuit topology model of a dynamically reconfigurable battery energy storage system, such as Figure 2 As shown, including:

[0026] Step S101, establish an N-parallel and M-series battery network based on lithium iron phosphate battery cells; Step S102, connect a high-frequency power switch in series to each lithium iron phosphate battery cell, and at the same time, each N parallel cells are connected in parallel with a bypass high-frequency power switch, and the high-frequency power electronic switches here are all MOSFET devices.

[0027] By introducing high-frequency power electronic switching devices, the "digitalization" of energy storage battery information and energy is realized, such as Figure 3 As shown, due to the limitations of the traditional series-parallel battery pack structure, when the power of a single battery exceeds the limit, the entire series battery pack will exit, and the power supply reliability of the energy storage battery will be seriously reduced. In addition, the "short board effect" of the battery is not conducive to the efficient use and economic recovery of energy and resources, which in turn causes a series of problems such as efficiency, safety, reliability, life and economy. The present invention proposes a new energy storage battery topology, which realizes the "digitalization" of the control signal through a high-frequency switch, adds a control switch to each battery cell, realizes the refinement of the control unit, and achieves the purpose of precise control of the battery cell power and independent charging and discharging through the precise capture, transmission and execution of digital signals.

[0028] In addition, if Figure 4 As shown, the physical connection between cells can also be dynamically reconstructed through software and energy exchange backplane according to the battery's own conditions and external working conditions, enhancing the adaptability and flexibility of the energy storage system to complex working conditions.

[0029] The step S20 monitors and collects various electrical parameters of the energy storage system, including the state of charge (SOC) of each battery cell, the system's external output voltage and current, and external power instructions.

[0030] The power monitoring and topology reconstruction of the energy storage battery network are the prerequisites for two-dimensional balancing. The imbalance of battery cell power makes the operation structure of large-scale battery networks complicated. Therefore, the dynamic optimization of complex battery network topology is essentially the battery power monitoring and input and exit control of complex networks. To achieve fine-grained and efficient control of battery cell power, it is necessary to grasp the characteristics and status of network nodes and the characteristics and performance of different network topologies composed of network nodes in real time, and then realize the topology reconstruction and performance optimization of battery energy storage systems through battery power monitoring and input and exit logic.

[0031] Therefore, it is necessary to establish a battery information management system to measure the power, voltage, temperature and other information of each battery cell in real time through the battery energy exchange backplane, capture the battery cell power information, and store it on the information management platform. In addition, the battery network controller of the information management platform can accurately estimate and predict the health status and power information of the battery online, providing a platform foundation for the optimization of the battery network topology and subsequent two-dimensional balancing.

[0032] Step S30, establishing a traditional SOC balancing control model of a dynamically reconfigurable battery energy storage system.

[0033] The battery management system performs power monitoring and topology reconstruction. The energy storage battery network power monitoring flow chart is as follows: Figure 5 As shown. The battery power information management system captures the battery power information of the battery cells in real time and stores it on the information management platform. The upper and lower limits of the battery power threshold are set to 20% and 80% respectively. When the battery power is lower than 20% or higher than 80%, the energy storage system is exited to avoid the safety and reliability problems of the battery network due to the "short board effect". The battery with a power between 20% and 80% is an ideal cell, which provides charging and discharging guarantee for the subsequent load requirements under different working conditions.

[0034] When the battery power is lower than the threshold, that is, the battery power of a single battery is lower than 20%, the number of single batteries with power exceeding the limit is fed back through the information management platform. If only one battery single battery is lower than the threshold, the single battery is controlled to exit operation through the power electronic switch digital logic to form a new energy storage battery topology. When multiple battery single batteries are lower than the threshold, the power data of the information management platform is collected at this time, and the topology is exited one by one in the order of low to high power, so as to avoid system crash caused by the simultaneous exit of multiple single batteries. When the battery power is higher than the threshold, that is, the battery power of a single battery is higher than 80%, the number of single batteries with power exceeding the limit is fed back through the information management platform. If only one battery single battery is higher than the threshold, the single battery is controlled to exit operation through the power electronic switch digital logic to form a new energy storage battery topology. When multiple battery single batteries are higher than the threshold, the power data of the information management platform is collected at this time, and the topology is exited one by one in the order of high to low power.

[0035] When all batteries outside the threshold are removed, the battery information management platform starts power monitoring under the latest battery topology, and performs short-term prediction of battery power through historical power data and current power information to determine short-term changes in the energy storage topology. When the battery power is all within the threshold, a 2s delay is set. If the power information monitored within 2s is still within the ideal range, the topology reconstruction is completed and the two-dimensional balance control of the battery power begins. If the battery power exceeds the limit again within 2s, the single cell exit instruction will continue to be executed to form a new reconstructed topology.

[0036] For traditional digital energy storage balancing strategies, such as Figure 6 As shown, when the power instruction does not exceed the total power of N columns of batteries, the charge and discharge control of N selects N-1 (columns) and M selects M-1 (rows) is executed. That is, when the normal operation is in the discharge state, the single cell with the smallest SOC value among each N parallel battery cells is calculated and selected according to the collected SOC data of each battery cell, and the high-frequency power electronic switch of the single cell is disconnected to isolate it from the network, thereby achieving the purpose of column balance in the discharge state; then, a group of single cells with the smallest total SOC value among each N parallel single cells is calculated and selected, and the bypass power electronic switch of the group of battery cells is closed to isolate it from the network, thereby achieving the purpose of row balance in the discharge state. When the system is in the charging state during normal operation, the single cell with the largest SOC value among every N parallel-connected single cells is calculated and selected based on the collected SOC data of each battery cell, and the single cell is isolated from the network by disconnecting its high-frequency power electronic switch, thereby achieving the purpose of column balancing of the charging state; then a group of single cells with the largest total SOC value among every N parallel-connected single cells is calculated and selected, and the bypass power electronic switch of this group of battery cells is closed to isolate them from the network, thereby achieving the purpose of row balancing.

[0037] When the power command exceeds the upper limit of the N-1 battery column power but does not reach the N battery column power, all N battery columns will be put into use. At this time, the traditional digital energy storage balancing strategy fails, resulting in a waste of energy storage system capacity and an adverse effect on the life of the battery cells.

[0038] In the step S40, a bidirectional DC / DC converter control model is established, which is mainly a Buck-Boost bidirectional DC converter. In the step S50, a constant power control is adopted for the bidirectional DC / DC converter, and the external power command collected by monitoring is input into the bidirectional DC / DC converter control model. Different charge and discharge current commands are obtained based on different power commands, thereby controlling the energy storage system to be in a charging state or a discharging state.

[0039] The step S60 improves the traditional SOC balance control model to implement PWM charge and discharge control, such as Figure 7 As shown, it mainly includes:

[0040] When the power command exceeds the upper limit of the N-1 battery column power but does not reach the N battery column power, all N battery columns are put into use. At this time, the traditional digital energy storage balancing strategy has failed. In order to enable the energy storage system to retain the balancing function under such conditions, the existing control strategy needs to be improved.

[0041] At this time, the balancing strategy is that when the energy storage system is in the discharging state, if the discharge power is less than the upper limit of the power of the N-1 battery string clusters, the N-1 battery string cluster with larger power is discharged; if the discharge power is greater than the upper limit of the power of the N battery string clusters, all the N battery strings are discharged; if the discharge power is greater than the upper limit of the power of the N-1 battery string clusters and less than the upper limit of the power of the N battery string clusters, the battery string with the smallest power is PWM discharged. At this time, the PWM signal duty cycle α of the power electronic switch is expressed by the ratio of the excess power to the power of the single battery string cluster, that is,

[0042]

[0043] Similarly, when the energy storage system is in the charging state, if the charging power is less than the upper power limit of the N-1 battery string clusters, the N-1 battery string clusters with smaller power are charged; if the discharge power is greater than the upper power limit of the N battery string clusters, all the N battery strings are charged; if the charging power is greater than the upper power limit of the N-1 battery string clusters and less than the upper power limit of the N battery string clusters, the battery string with the largest power is PWM charged. At this time, the PWM signal duty cycle α of the power electronic switch is also expressed by the ratio of the excess power to the power of a single battery string cluster.

[0044] In summary, when the dynamic reconfigurable battery energy storage system adopts PWM charge and discharge control strategy, it is beneficial to improve battery utilization, better compensate for the differences between individual battery cells, achieve power balance, and effectively extend battery life. Since this strategy does not require the installation of additional devices, it is more advantageous in construction cost and can reduce the construction cost of the energy storage system.

[0045] The above-mentioned PWM charging and discharging control strategy based on the dynamically reconfigurable battery energy storage system can be realized and has broad application prospects.

[0046] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A PWM charge and discharge control strategy based on a dynamically reconfigurable battery energy storage system, characterized in that Rapidly achieve energy storage system power balancing while providing high overload power to the system, including the following steps: Step S10, establishing a circuit topology model of a dynamically reconfigurable battery energy storage system; Step S20, monitoring and collecting various electrical parameters of the energy storage system, including the state of charge (SOC) of each battery cell, the system's external output voltage and current, and external power instructions; Step S30, establishing a traditional SOC balancing control model of a dynamically reconfigurable battery energy storage system; Step S40, establishing a bidirectional DC / DC converter control model; Step S50, adopting constant power control for the bidirectional DC / DC converter; Step S60, improving the traditional SOC balancing control model to achieve PWM charging and discharging control.

2. The method according to claim 1, characterized in that: The step S10 comprises: Step S101, establishing a battery network based on N parallel and M series of lithium iron phosphate battery cells; Step S102, a high-frequency power switch is connected in series to each lithium iron phosphate battery cell, and each N parallel-connected cells are connected in parallel with a bypass high-frequency power switch, wherein the high-frequency power electronic switches are all MOSFET devices.

3. The method according to claim 1, characterized in that The step S20 includes: Step S201, monitoring and collecting the SOC of the lithium iron phosphate battery cell; Step S202, monitoring and collecting the output voltage and current of the dynamic reconfigurable battery energy storage system; Step S203: monitor and collect external power instructions.

4. An energy storage system detection unit, used to monitor and collect the battery cell SOC, energy storage system output voltage and current, and external power command data mentioned in claim 3.

5. The method according to claim 1, characterized in that The step S30 includes: Step S301, inputting the SOC data of each battery cell and the external power command obtained in step S20 into the control module; Step S302, based on the above data, the switching on and off of MOSFET is controlled to control the switching on and off of each battery cell, so as to realize dynamic reconstruction of the battery network.

6. The method according to claim 1, characterized in that The step S40 includes: Step S401, establishing a current loop based on PID control.

7. The method according to claim 1, characterized in that The step S50 includes: Step S501, inputting the external power command collected by monitoring into the bidirectional DC / DC converter control model, and obtaining different current commands based on different power commands; Step S502, controlling the charging and discharging of the energy storage system according to different current instructions.

8. The method according to claim 1, characterized in that The step S60 includes: Step S601, when the power instruction exceeds the output power of N-1, PWM control is performed on the high-frequency power electronic switch to increase the output power of the energy storage system; Step S602, running in full power mode after full load to ensure that the output power matches the power instruction.

9. According to claim 1, a PWM charge and discharge control strategy based on a dynamically reconfigurable battery energy storage system is characterized in that: The hardware structure of the device is mainly divided into a main circuit and a control program part, wherein the main circuit is composed of a dynamically reconfigurable battery energy storage system and a bidirectional DC / DC converter. In order to test the relevant control strategy mentioned in claim 1.

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