Submodule control method for MMC retired power battery energy storage system

Abstract

The submodule control method for the MMC retired power battery energy storage system first sets the state of charge working range according to the remaining capacity of the retired power battery, and then divides the system submodule operation strategy into two types, the first type: the operation strategy when the state of charge of the system submodule battery does not exceed the limit; the second type: the operation strategy when the state of charge of the system submodule battery exceeds the limit; comprising the following steps: step 1: setting the upper and lower limits of the state of charge according to the remaining capacity of the energy storage module battery; step 2: normalizing the state of charge; step 3: judging whether the state of charge of the retired power battery exceeds the limit; step 4: when the state of charge does not exceed the limit, adopting the first submodule operation strategy; when exceeding the limit, adopting the second submodule operation strategy.

Description

Technical Field

[0001] The present invention relates to MMC battery energy storage system control, and in particular to an MMC submodule control technology capable of slowing down the capacity decay rate of retired power batteries and improving the capacity utilization rate of energy storage systems. Background Art

[0002] At present, there are about 6.78 million new energy vehicles in China. Data shows that by 2020, the cumulative retired power battery volume in my country has reached 200,000 tons, and it is expected that by 2025, the retired power battery volume will reach 800,000 tons. After screening and reorganization, the performance of retired power batteries can fully meet the requirements of low-speed electric vehicles, backup power supplies, power storage and other fields. In order to make full use of the residual value of retired power batteries, they can be introduced into grid energy storage power stations.

[0003] After a long period of use, the characteristics of power batteries such as voltage and capacity vary greatly, and the rate of capacity decay is further accelerated. This feature is more prominent in retired power batteries with relatively low remaining capacity. When designing sub-module control strategies for traditional MMC battery energy storage systems, generally only the problem of charge state balance is considered. If this strategy is applied to MMC retired power battery energy storage systems, it may cause the capacity of retired power batteries to decay too quickly, and the sub-module terminal voltages to differ greatly and be unstable. In response to this situation, the present invention proposes a sub-module control method for an MMC battery energy storage system that can effectively slow down the capacity decay rate of retired power batteries, stabilize the sub-module terminal voltage, and reduce the voltage differences between sub-modules. Summary of the invention

[0004] In order to alleviate the problems of too fast capacity attenuation, large differences in voltage characteristics and inconsistent state of charge when retired power batteries are used in MMC energy storage systems, a submodule control method for an MMC retired power battery energy storage system is provided.

[0005] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0006] First, the state of charge working range is set according to the remaining capacity of the retired power battery. Then, considering that the energy storage system is usually designed according to the maximum energy fluctuation of the power grid, the battery capacity is generally not fully utilized during operation. Therefore, the present invention divides the energy storage submodule operation strategy into two types according to whether the state of charge of the battery in the system exceeds the limit;

[0007] The steps include:

[0008] Step 1: setting a state of charge working range for each energy storage module according to the remaining capacity of the energy storage module battery;

[0009] Step 2, normalizing the state of charge;

[0010] Step 3, determining whether the state of charge of the retired power battery exceeds the limit;

[0011] Step 4: When the state of charge does not exceed the limit, the system adopts the first submodule operation strategy; when it exceeds the limit, the system adopts the second submodule operation strategy.

[0012] The benefits of the present invention are:

[0013] (1) The present invention sets a state of charge working range for each retired power battery energy storage unit according to the remaining capacity of the battery, so as to stabilize the battery voltage, slow down the battery capacity decay rate, and prevent the low remaining capacity power battery from retiring too quickly.

[0014] (2) The present invention designs two submodule operation strategies according to the battery state of charge. The first operation strategy reduces the average working time and average working power of retired power batteries with low remaining capacity, thereby slowing down the capacity decay rate of retired power batteries. The second operation strategy takes into account the balance of the submodule state of charge, thereby improving the capacity utilization of the energy storage power station. The combination of the two strategies takes into account both battery life and system capacity utilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the topological structure of the energy storage system; Figure 2 This is a schematic diagram of the switching sequence of the MMC bridge arm submodules. Figure 3 This is the first operation strategy flow chart; Figure 4 This is the second operation strategy flow chart; Figure 5 It is the overall control strategy flow chart. DETAILED DESCRIPTION

[0016] The specific embodiments of the present invention are further described below in conjunction with the accompanying drawings:

[0017] like Figure 1 As shown in the figure, the MMC retired power battery energy storage system has six bridge arms, each of which is composed of n retired power battery energy storage sub-modules (RSM) and a reactor in series. Unlike the common MMC, this system has no common DC bus voltage support. Figure 1 The capacitor in the circuit does not exist in the actual system. Point O′ is the imaginary neutral point. L0 is the bridge arm reactor. L is the equivalent reactance of the power grid. u pj is the three-phase upper bridge arm voltage; u nj is the three-phase lower bridge arm voltage; i pj is the three-phase upper bridge arm current; i nj is the three-phase lower bridge arm current; i j is the three-phase output current; e jis the grid side voltage. (j=a,b,c)

[0018] Starting from the MMC battery energy storage system and combining the characteristics of retired power batteries, the present invention designs a control method that can slow down the capacity decay rate of retired power batteries and improve the capacity utilization rate of energy storage power stations.

[0019] The specific implementation steps are as follows:

[0020] Step 1: Detect the remaining capacity of each battery and set a separate state of charge working range and balancing center for each energy storage module according to formulas (1) and (2):

[0021] State of charge working lower limit:

[0022]

[0023] State of Charge Working Upper Limit:

[0024] SOC p =85% (2)

[0025] The above principle is that the lower the remaining capacity of the battery, the narrower the charge state working range, so as to stabilize the MMC sub-module terminal voltage and slow down the capacity attenuation rate of retired power batteries.

[0026] Step 2: By changing the number of upper and lower bridge arm submodules, the MMC output is as close to the modulated wave as possible. p With n n Indicates the number of submodules required for the upper and lower bridge arms, then n p With n n It can be obtained by the following formula:

[0027]

[0028] Where: U ref is the modulation wave reference voltage; U C is the submodule voltage.

[0029] Step 3: Determine the submodule operation strategy to be adopted based on whether the battery state of charge exceeds the limit. When the bridge arm current charges the energy storage submodule, detect whether any battery exceeds the upper limit of the state of charge, that is, SOC'≥1. When the bridge arm current discharges the energy storage submodule, detect whether any battery exceeds the lower limit of the state of charge, that is, SOC'≤0. If a battery exceeds the limit, switch from the first operation strategy to the second operation strategy. Among them, SOC' is the normalized battery state of charge, and the formula is shown in formula (4).

[0030] The two operation strategies are as follows:

[0031] The first operation strategy: Figure 2 This is an example diagram of the switching of a phase arm submodule in one and a half cycles of a 5-level MMC retired power battery energy storage system under the control of this strategy using the nearest level approximation modulation method. Figure 2 In the figure, the area above the dotted line represents the upper bridge arm, and the area below the dotted line represents the lower bridge arm; A, B, C, and D represent the four energy storage submodules of the upper bridge arm of one phase, and their remaining battery capacities are arranged in ascending order; white represents that the module is in the removed state, and the shaded area represents that the module is in the engaged state.

[0032] This strategy makes the submodules with relatively low remaining capacity work for a shorter time in each cycle. Since the system bridge arm submodules are connected in series, under this switching mode, the shorter the working time of the module, the smaller the average working current and average working power, thereby effectively slowing down the capacity decay rate of retired power batteries with low remaining capacity. The strategy process is as follows Figure 3 shown.

[0033] The second operating strategy: When the state of charge of retired power batteries in the system exceeds the limit, it is necessary to introduce balancing operations to maximize the utilization of system capacity.

[0034] In order to balance the state of charge within the set state of charge working range, the state of charge is first normalized according to the following formula:

[0035]

[0036] Similar to the submodule switching under the first strategy, when the bridge arm current is charging current, the n retired power battery energy storage submodules with smaller SOC' are put into operation; when the bridge arm current is discharging current, the n retired power battery energy storage submodules with larger SOC' are put into operation. By switching the submodules according to the above strategy, the energy storage submodules with smaller SOC' can work longer and have greater charging power during charging; the energy storage submodules with larger SOC' can work longer and have greater discharge power during discharge, thereby achieving balanced operation. The strategy process is as follows Figure 4 shown.

[0037] The overall process of the strategy is as follows Figure 5 As shown. First, measure the state of charge and remaining capacity of each battery, and calculate the number of sub-modules n that need to be put into use this time. Then set the state of charge working range of each battery, and calculate the normalized state of charge value. After that, combine the current direction to determine whether the state of charge of any battery in the system exceeds the limit. If it does not exceed the limit, the first operation strategy is adopted, that is, the sub-module with higher remaining capacity is put into use; if it exceeds the limit, the second operation strategy is adopted, that is, when the bridge arm current is the charging current, the sub-module with lower SOC′ of the bridge arm is put into use, and when discharging, the sub-module with larger SOC′ is put into use.

[0038] The above is a specific implementation method of the present invention, but it is not limited to the above method. Changes made by workers in this technical field without creative labor should also fall within the protection scope of the present invention.

Claims

1. A submodule control method for an MMC retired power battery energy storage system, characterized in that: First, the state of charge working range is set according to the remaining capacity of the retired power battery. Then, the system submodule operation strategy is divided into two types: the first type: the operation strategy when the state of charge of the system submodule battery does not exceed the limit; the second type: the operation strategy when the state of charge of the system submodule battery exceeds the limit; The steps include: S1. Set the upper and lower limits of the state of charge according to the remaining capacity of the energy storage module battery; The specific feature of S1 is that The state of charge working range is set according to the following formula. The state of charge setting formula is as follows: State of charge working lower limit: State of Charge Working Upper Limit: SOC p =85% (2) Where C is the remaining capacity of the battery; C g is the rated capacity of the battery; SOC n The lower limit of the state of charge setting; SOC p The upper limit of the working state of charge is set; S2, normalizing the state of charge; In S2, the state of charge normalization process is as follows: Wherein, SOC is the state of charge of the battery, and SOC' is the normalized state of charge value; S3, judging whether the state of charge of the retired power battery exceeds the limit; In S3, the method for judging whether the state of charge exceeds the limit is as follows: When discharging, SOC'≤0 or when charging, SOC'≥1, the state of charge is in an out-of-limit state; When the bridge arm current charges the energy storage submodule, the battery state of charge value exceeds the upper limit of the state of charge, SOC'≥1, and the battery state of charge is in an out-of-limit state; when the bridge arm current discharges the energy storage submodule, the battery state of charge value exceeds the lower limit of the state of charge, SOC'≤0, and the battery state of charge is in an out-of-limit state; S4: When the state of charge does not exceed the limit, the first submodule operation strategy is adopted; when the state of charge exceeds the limit, the second submodule operation strategy is adopted; In S4, the first operation strategy is as follows: n represents the number of sub-modules that need to be put into operation each time, and each time a sub-module is put into operation, n energy storage sub-modules with larger remaining battery capacity are put into operation; the second operation strategy is as follows: when the bridge arm current is a charging current, n retired power battery energy storage sub-modules with smaller SOC' are put into operation; when the bridge arm current is a discharging current, n sub-modules with larger SOC' are put into operation.

Images