An active SOC balancing method for cascaded energy storage systems

Abstract

The active SOC balancing method for the cascaded energy storage system of the present invention is implemented through the following steps: a). Determine whether balancing is required; b). Judge the charge and discharge mode; c). Bypass the battery cluster; d). SOC balancing control. The active SOC balancing method for the cascaded energy storage system of the present invention uses the difference between the standard deviation of the lowest voltage of the battery cluster and a preset value to determine whether to perform SOC balancing, selects the SOC balancing mode according to the number of the lowest cell voltages greater than and less than the average value of the lowest cell voltages, and finally performs SOC balancing control on the non-bypassed battery clusters. The active SOC balancing method for the cascaded energy storage system of the present invention can effectively balance the state of charge SOC of the battery clusters in the cascaded energy storage system, effectively ensure the balance of the remaining power of the battery clusters, and ensure the capacity of the entire cascaded energy storage system.

Description

Technical Field

[0001] The present invention relates to a method for SOC balancing, and more specifically, to an active SOC balancing method for a cascaded energy storage system. Background Art

[0002] After entering the 21st century, what restricts the survival and development of mankind is no longer war and disease, but the gradually exhausted fossil energy. Against this background, a new type of power grid composed of new energy power generation and energy storage devices has emerged. At present, the number of energy storage devices using electrochemical batteries as energy storage media has increased sharply, and it has gradually become the mainstream method of energy storage. However, due to the limitations of production processes and technologies, there are high inconsistencies in battery cells, especially the currently mainstream lithium iron phosphate battery cells. Traditional low-voltage energy storage, that is, energy storage with an AC grid-connected voltage less than 1 kV, has a small single-machine capacity and a large number of parallel connections, and does not have a battery balancing function, which seriously amplifies the barrel effect brought by the inconsistency of battery cells.

[0003] As a new type of energy storage converter, the high-voltage cascaded energy storage combines multiple battery clusters into a large energy storage system through a cascading method different from parallel connection. Due to the large number of cascaded battery clusters, the cascaded energy storage system will also have the problem of unbalanced SOC (state of charge) caused by the inconsistency of battery cells, which will affect the capacity of the entire system. Summary of the Invention

[0004] In order to overcome the above technical problems, the present invention provides an active SOC balancing method for a cascaded energy storage system.

[0005] The active SOC balancing method for the cascaded energy storage system of the present invention is such that the cascaded energy storage system consists of a power grid, several power units, an on / off grid switch, and a battery management system BMS. Several power units are connected in series to form three phases and are connected to the power grid through the on / off grid switch; each power unit is composed of an inverter circuit and a battery cluster connected to both ends thereof. The battery cluster is composed of M battery cells connected in series, and the battery management system BMS is used to collect battery cluster information including the voltage of the battery cells. Its characteristics are that the active SOC balancing method for the cascaded energy storage system is realized through the following steps:

[0006] a). Determine whether balancing is required. When the cascaded energy storage system receives a state of charge SOC balancing instruction, the battery management system BMS collects the data of the battery cells in the battery cluster of each power unit, and determines whether to execute the state of charge SOC balancing instruction according to the collected battery cell data. If it is necessary to execute the SOC balancing instruction, then step b) is executed; if it is not necessary to execute the SOC balancing instruction, then wait for the SOC balancing instruction to be issued again;

[0007] b). Charge and discharge mode determination. After entering the SOC balancing state, determine whether the cascaded energy storage system enters the charging SOC balancing mode or the discharging SOC balancing mode; after determining the balancing mode, execute step c);

[0008] c). Battery cluster bypass. After selecting the SOC balancing mode, obtain the M battery clusters with the largest cell voltage deviation by comparing and processing the cell data, and then bypass the K battery clusters with relatively large cell voltage deviation;

[0009] d). SOC balancing control. Finally, according to the selected SOC balancing mode, perform preset charging or discharging control on the non-bypassed battery clusters to achieve the balancing of the state of charge SOC of the entire cascaded energy storage system.

[0010] For the active SOC balancing method of the cascaded energy storage system of the present invention, the determination of whether balancing is required in step a) is specifically implemented through the following steps:

[0011] After the cascaded energy storage system receives the SOC balancing instruction, in the cell data of the battery clusters collected by the battery management system BMS, obtain the lowest cell voltage V in each battery cluster minj , j = 1, 2,..., N = 1, N is the number of battery clusters; then calculate the average value V of the lowest cell voltages of the battery clusters through formula (1) AVG :

[0012] V AVG =(V min1 +V min2 +......+V minj +......+V minN ) / N (1)

[0013] In formula (1), 1 ≤ j ≤ N;

[0014] Then calculate the standard deviation S of the lowest voltages of the battery clusters through formula (2) N :

[0015]

[0016] Then determine whether the standard deviation S of the lowest voltages of the battery clusters is greater than or equal to the preset value S of the voltage standard deviation through formula (3) N ;

[0017] S N ≥S (3)

[0018] If formula (3) holds, it indicates that the SOC balancing instruction needs to be executed; if formula (3) does not hold, it indicates that the SOC balancing instruction does not need to be executed.

[0019] The active SOC balancing method for the cascaded energy storage system of the present invention, the charge and discharge mode judgment in step b) is realized through the following steps:

[0020] b-1). Set variables and initial values, set integer variables a, b, and j, and set the initial values a = 0, b = 0, j = 1; execute step b-2);

[0021] b-2). Compare the cell voltage with the average value, and judge whether formula (4) holds:

[0022] V minj -V AVG >0 (4)

[0023] If formula (4) holds, then execute a = a + 1; if formula (4) does not hold, then judge whether formula (5) holds:

[0024] V minj -V AVG <0 (5)

[0025] If formula (5) holds, then execute b = b + 1; execute step b-3);

[0026] b-3). Judgment of termination condition, then judge whether j = N holds, if it holds, then execute step b-4); if it does not hold, then execute j = j + 1, and then jump to execute step b-2);

[0027] b-4). Judge the charge and discharge mode, and then compare the magnitudes of a and b. If a is greater than b, it indicates that the number of battery clusters with the lowest cell voltage greater than V AVG is more than the number of battery clusters with the lowest cell voltage less than V AVG , then execute the discharge SOC balancing mode; if a is less than or equal to b, then execute the charge SOC balancing mode.

[0028] The battery cluster bypass in step c) of the active SOC balancing method for the cascaded energy storage system of the present invention is realized through the following steps:

[0029] Obtain the battery clusters that meet the conditions through formula (6):

[0030] |V minj -V AVG |>V0 (6)

[0031] V0 is a preset voltage value greater than 0, j = 1, 2,..., N; arrange the battery clusters that meet formula (6) in descending order of the difference between the lowest cell voltage and V AVG , and select the first M battery clusters for battery cluster bypass, M≤3.

[0032] The active SOC equalization method for the cascaded energy storage system of the present invention, during the charging or discharging control process with a preset value for the non-bypassed battery clusters in step d), with power P N and energy W N perform charging or discharging control on the non-bypassed battery clusters; where 0.1W ≥ W N > 0, 0.2P ≥ P N > 0, W is the rated capacity of a single battery cluster, and P is the rated charge-discharge power of a single battery cluster.

[0033] For the active SOC equalization method of the cascaded energy storage system of the present invention, the preset value S of the voltage difference standard deviation in step a) satisfies: 0.4 ≥ S ≥ 0.01; the preset voltage value V0 in step c) satisfies: 1.0 ≥ V0 ≥ 0.05.

[0034] The beneficial effects of the present invention are as follows: For the active SOC equalization method of the cascaded energy storage system of the present invention, first, the battery management system BMS collects the lowest cell voltage of each battery cluster and calculates the average value of the lowest cell voltages, and then uses the difference between the standard deviation of the lowest battery cluster voltage and the preset value to determine whether to perform SOC equalization; then, according to the number of battery clusters with the lowest cell voltage greater than and less than the average value of the lowest cell voltages, select whether to perform charging or discharging SOC equalization mode, bypass the first M battery clusters with a relatively large difference between the lowest cell voltage and the average value, and finally perform SOC equalization control on the non-bypassed battery clusters. It can be seen that the active SOC equalization method of the cascaded energy storage system of the present invention can effectively equalize the state of charge SOC of the battery clusters in the cascaded energy storage system, effectively ensure the balance of the remaining power of the battery clusters, and ensure the capacity of the entire cascaded energy storage system. Description of the Drawings

[0035] Figure 1 is the circuit schematic diagram of the cascaded energy storage system of the present invention;

[0036] Figure 2 is the flow chart of the active SOC equalization method of the cascaded energy storage system of the present invention.

[0037] In the figure: 1 power grid, 2 power unit, 3 grid-connected and off-grid switch, 4 battery cluster, 5 converter circuit. Detailed Embodiments

[0038] The present invention will be further described below in conjunction with the drawings and embodiments.

[0039] As Figure 1As shown in the figure, the circuit schematic diagram of the cascaded energy storage system of the present invention is given. It is composed of a power grid 1, a number of power units 2, and a grid-connected / off-grid switch 3. The power grid 1 is a three-phase AC power grid connected to the cascaded energy storage system. A number of power units 2 are connected in series to form a three-phase and are connected to the power grid 1 through the grid-connected / off-grid switch 3. Each of the shown power units 2 is composed of an inverter circuit 5 and a battery cluster 4. The battery cluster 4 is composed of M battery cells connected in series. The inverter circuit 5 can selectively operate in the rectification and inversion states. The battery management system BMS is used to collect battery cluster information including the voltage of the battery cells. The power unit 2 converts the alternating current on the power grid 1 into direct current through rectification and stores it in the battery cluster 4 to achieve electrical energy storage; the power unit 2 converts the electrical energy stored in the battery cluster 4 into alternating current through inversion and inputs it to the power grid 1 to achieve peak shaving and valley filling of the power grid 1.

[0040] As Figure 2 shown in the figure, the flowchart of the active SOC balancing method for the cascaded energy storage system of the present invention is given, which is implemented through the following steps:

[0041] a). Determine whether balancing is required. When the cascaded energy storage system receives the state of charge SOC balancing instruction, the battery management system BMS collects the cell data in the battery cluster of each power unit, and determines whether to execute the state of charge SOC balancing instruction according to the collected cell data. If the SOC balancing instruction needs to be executed, go to step b); if the SOC balancing instruction does not need to be executed, wait for the resending of the SOC balancing instruction;

[0042] In this step, the determination of whether balancing is required is specifically implemented through the following steps:

[0043] When the cascaded energy storage system receives the state of charge SOC balancing instruction, in the cell data of the battery cluster collected by the battery management system BMS, obtain the lowest cell voltage V in each battery cluster minj , j = 1, 2,..., N = 1, N is the number of battery clusters; then calculate the average value V of the lowest cell voltage of the battery cluster through formula (1) AVG :

[0044] V AVG =(V min1 +V min2 +......+V minj +......+V minN ) / N (1)

[0045] In formula (1), 1 ≤ j ≤ N;

[0046] Then calculate the standard deviation S of the lowest voltage of the battery cluster through formula (2) N :

[0047]

[0048] Then, the standard deviation S of the lowest voltage of the battery cluster is judged through formula (3) N whether it is greater than or equal to the preset value S of the voltage standard deviation;

[0049] S N ≥ S (3)

[0050] If formula (3) holds, it indicates that the SOC equalization instruction needs to be executed; if formula (3) does not hold, it indicates that the SOC equalization instruction does not need to be executed.

[0051] b). Charge and discharge mode judgment. After entering the SOC equalization state, then judge whether the cascaded energy storage system enters the charging SOC equalization mode or the discharging SOC equalization mode; after judging the equalization mode, execute step c);

[0052] In this step, the charge and discharge mode judgment is realized through the following steps:

[0053] b-1). Set variables and initial values. Set integer variables a, b, and j, and set the initial values a = 0, b = 0, j = 1; execute step b-2);

[0054] b-2). Comparison of the cell voltage with the mean value. Judge whether formula (4) holds:

[0055] V minj -V AVG > 0 (4)

[0056] If formula (4) holds, execute a = a + 1; if formula (4) does not hold, judge whether formula (5) holds:

[0057] V minj -V AVG < 0 (5)

[0058] If formula (5) holds, execute b = b + 1; execute step b-3);

[0059] b-3). Judgment of termination condition. Then judge whether j = N holds. If it holds, execute step b-4); if it does not hold, execute j = j + 1, and then jump to execute step b-2);

[0060] b-4). Judge the charge and discharge mode. Then compare the magnitudes of a and b. If a is greater than b, it indicates that the number of battery clusters with the lowest cell voltage greater than V AVG is more than the number of battery clusters with the lowest cell voltage less than V AVG Then execute the discharging SOC equalization mode; if a is less than or equal to b, execute the charging SOC equalization mode.

[0061] c). Battery cluster bypass: After selecting the SOC equalization mode, M battery clusters with the largest cell voltage deviation are obtained by comparing and processing the cell data, and then K battery clusters with relatively large cell voltage deviation are bypassed;

[0062] In this step, the battery cluster bypass is realized through the following steps:

[0063] Obtain the battery clusters that meet the conditions through formula (6):

[0064] |V minj -V AVG |>V0 (6)

[0065] V0 is a preset voltage value greater than 0, j = 1, 2,..., N; Arrange the battery clusters that meet formula (6) in descending order according to the difference between the lowest cell voltage and V AVG , and select the first M battery clusters for battery cluster bypass, M≤3.

[0066] d). SOC equalization control: Finally, according to the selected SOC equalization mode, perform preset charging or discharging control on the non-bypassed battery clusters to achieve the equalization of the state of charge SOC of the entire cascaded energy storage system.

[0067] In step d), during the process of performing preset charging or discharging control on the non-bypassed battery clusters, charge or discharge the non-bypassed battery clusters with power P N and energy W N ; where 0.1W≥W N >0, 0.2P≥P N >0, W is the rated capacity of a single battery cluster, and P is the rated charge and discharge power of a single battery cluster.

[0068] The preset voltage difference value S mentioned in step a) satisfies: 0.4≥S≥0.01; The preset voltage value V0 in step c) satisfies: 1.0≥V0≥0.05.

[0069] It can be seen that the active SOC equalization method for the cascaded energy storage system disclosed in the present invention is characterized in that it starts by judging through the standard deviation of the cell voltages of the batteries, and then achieves the SOC equalization function of the battery clusters of the cascaded energy storage system through the unit automatic bypass technology. In short, the SOC equalization strategy for the battery clusters of the cascaded energy storage converter mentioned in the present invention has been proven to be able to achieve the active equalization of the SOC of the battery clusters in the case of impedance mismatch in the parallel device, and it is a simple, fast and effective SOC equalization method for the cascaded energy storage system.

Claims

1. An active SOC balancing method for a cascaded energy storage system. The cascaded energy storage system is composed of a power grid (1), a number of power units (2), a grid-connected / off-grid switch (3), and a battery management system BMS. A number of power units are connected in series to form three phases and are connected to the power grid through the grid-connected / off-grid switch; each power unit is composed of an inverter circuit (5) and a battery cluster (4) connected to both ends thereof, and the battery management system BMS is used to collect battery cluster information including cell voltages; it is characterized in that, The active SOC equalization method for the cascaded energy storage system is realized through the following steps: a). Determine whether equalization is required. After the cascaded energy storage system receives the state of charge (SOC) equalization instruction, the battery management system (BMS) collects the cell data in the battery clusters of each power unit, and determines whether to execute the SOC equalization instruction according to the collected cell data. If the SOC equalization instruction needs to be executed, go to step b); if the SOC equalization instruction does not need to be executed, wait for the SOC equalization instruction to be issued again; b). Charge and discharge mode judgment. After entering the SOC equalization state, then judge whether the cascaded energy storage system enters the charging SOC equalization mode or the discharging SOC equalization mode; after judging the equalization mode, go to step c); c). Battery cluster bypass. After selecting the SOC equalization mode, M battery clusters with the largest cell voltage deviation are obtained by comparing and processing the cell data, and then K battery clusters with large cell voltage deviation are bypassed; d). SOC equalization control. Finally, according to the selected SOC equalization mode, charge or discharge control with a preset value is performed on the non-bypassed battery clusters to achieve the equalization of the state of charge (SOC) of the entire cascaded energy storage system; The charge and discharge mode judgment described in step b) is realized through the following steps: b-1). Set variables and initial values. Set integer variables a, b, and j, and set the initial values a = 0, b = 0, and j = 1; go to step b-2); b-2). Comparison of cell voltage with the mean value. Judge whether formula (4) holds: V minj V AVG > 0 (4) If formula (4) holds, then execute a = a + 1; if formula (4) does not hold, then judge whether formula (5) holds: V minj V AVG <0 (5) If formula (5) holds, then execute b = b + 1; go to step b-3); b-3). Termination condition judgment. Then judge whether j = N holds. If it holds, go to step b-4); if it does not hold, then execute j = j + 1, and then jump back to step b-2); b - 4). Determine the charge - discharge mode, then compare the magnitudes of a and b. If a is greater than b, it indicates that the number of battery clusters with the lowest cell voltage greater than V AVG is more than the number of battery clusters with the lowest cell voltage less than V AVG . Then execute the discharge SOC equalization mode; if a is less than or equal to b, execute the charge SOC equalization mode.

2. The active SOC equalization method for the cascaded energy storage system according to claim 1, wherein The determination of whether equalization is required described in step a) is specifically realized through the following steps: After the cascaded energy storage system receives the state of charge (SOC) equalization instruction, the minimum cell voltage V in each battery cluster is obtained from the cell data of the battery clusters collected by the battery management system (BMS). minj , where j = 1, 2,..., N, and N is the number of battery clusters. Then, the average value V of the minimum cell voltages of the battery clusters is calculated using formula (1). AVG : V AVG = (V min1 + V min2 +...... + V minj +...... + V minN ) / N(1) In formula (1), 1 ≤ j ≤ N; Then, the standard deviation S of the minimum voltage of the battery cluster is obtained through Equation (2). N : Then, the standard deviation S of the lowest voltage of the battery cluster is judged through formula (3). N Whether it is greater than or equal to the preset value S of the standard deviation of the voltage; S N ≥ S(3) If formula (3) holds, it indicates that the SOC equalization instruction needs to be executed; if formula (3) does not hold, it indicates that the SOC equalization instruction does not need to be executed.

3. The active SOC equalization method for the cascaded energy storage system according to claim 1, characterized in that The battery cluster bypass described in step c) is realized through the following steps: Obtain the battery clusters that meet the conditions through formula (6): |V minj -V AVG | > V0 (6) V0 is a preset voltage value greater than 0, j = 1, 2, …, N; the battery clusters that satisfy formula (6) are arranged in descending order according to the difference between the lowest cell voltage and V AVG and the first M battery clusters are selected for battery cluster bypass, where M ≤ 3.

4. The active SOC balancing method for the cascaded energy storage system according to claim 3, wherein During the charging or discharging control process of the non-bypass battery clusters with preset values in step d), with power P N and energy W N to perform charging or discharging control on the non-bypass battery clusters; where 0.1W ≥ W N > 0, 0.2P ≥ P N > 0, W is the rated capacity of a single battery cluster, and P is the rated charge-discharge power of a single battery cluster.

5. The active SOC equalization method for the cascaded energy storage system according to claim 2, characterized in that: The preset value S of the voltage standard deviation described in step a) satisfies: 0.4 ≥ S ≥ 0.01; the preset voltage value V0 described in step c) satisfies: 1.0 ≥ V0 ≥ 0.05.

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