A multi-level energy storage system and power balancing method thereof

Abstract

The present invention relates to the technical field of energy storage systems, and discloses a multi-level energy storage system and a method for balancing electric quantity thereof. In the system, each phase unit includes at least two battery clusters arranged in parallel; the battery cluster includes a plurality of battery PACKs connected in series, and a PACK controller collects the single cell voltage and current of the battery PACK to determine the SOC of the battery PACK; the system controller determines the electric quantity balancing control instruction based on the single cell voltage, current and SOC of each battery PACK in each battery cluster, as well as the set imbalance threshold and the set balance threshold, and sends it to the corresponding cluster controller, which transmits it to the corresponding PACK controller, and the PACK controller controls the charge and discharge power of the corresponding battery PACK. The present invention can realize automatic balancing of electric quantity between and within clusters of a multi-level energy storage system under static working conditions of charging and discharging, power outage or standby, avoid the loss of battery stack capacity caused by electric quantity imbalance, and realize electric quantity balancing under all working conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of energy storage systems, and in particular to a multi-level energy storage system and a power balancing method thereof. Background Art

[0002] In the energy storage system, single cells are connected in series and parallel to form a battery box, and battery boxes are connected in series and parallel to form a battery cluster. Multiple battery clusters are then connected in series and parallel to form an energy storage battery stack. Multiple battery clusters are directly connected in parallel to the same DC busbar. In the energy storage system, the reasons for the loss of available capacity due to battery inconsistency include inconsistent series connection and inconsistent parallel connection.

[0003] When unbalanced battery clusters are directly connected in parallel, a circulation phenomenon will occur after the charge and discharge are completed. The voltage of each battery cluster is forced to be balanced. When the battery cluster with a smaller internal resistance is fully charged or discharged, the other battery clusters must stop charging and discharging, resulting in the battery clusters not being fully charged or fully discharged, causing battery capacity loss and temperature rise, accelerating battery degradation, and reducing the available capacity of the battery system. In addition, due to the small internal resistance of the battery, even if the voltage difference of the unbalanced battery cluster is only a few volts, the circulation between clusters will be very large. Uncontrolled circulation will cause some battery clusters to be overcharged or over-discharged, greatly affecting the charge and discharge efficiency and battery life, and even leading to serious safety accidents.

[0004] The current multi-level energy storage system is in its infancy and lacks a complete power balancing strategy. The balancing between clusters and the balancing between PACKs within a cluster must be completed during the charging and discharging process. If there is a lack of charging and discharging circuits under standby or non-grid-connected conditions, balancing cannot be performed. Moreover, balancing during the charging and discharging process may cause the current of a cluster to be higher or lower than that of other clusters. Generally, the balancing current is not very large. This is because the excessive balancing current plus the working current will cause the cluster to work in an overloaded state, which has a certain impact on the thermal control and stability of the product. The balancing function under standby or non-grid-connected conditions can increase the flexibility of the product operation mode and improve the initial state of the energy storage system during operation, thereby further enhancing the operating stability of the energy storage system. Therefore, the balancing strategy under standby or non-grid-connected conditions is an important research topic in this field. In addition, the inter-cluster balancing strategy of the traditional energy storage system with a common DC bus is not applicable to the multi-level energy storage system. Therefore, it is urgent to further improve the power balancing strategy of the multi-level energy storage system. Summary of the invention

[0005] The purpose of the present invention is to provide a multi-level energy storage system and a method for balancing the charge and discharge power of each battery PACK, so as to realize automatic balancing of the charge between and within clusters of a multi-level topology energy storage system, avoid the loss of battery stack capacity caused by imbalanced charge, and solve the problem that the energy storage system cannot balance the charge under static conditions, so as to realize the balance of charge under all conditions.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] A multi-level energy storage system comprises three phase units, the three phase units are respectively connected to a three-phase AC power grid, each phase unit comprises at least two battery clusters arranged in parallel, and each battery cluster is connected in series with a reactor;

[0008] The battery cluster includes a plurality of battery PACKs arranged in series, and the output end of each battery PACK is connected to a PACK controller; each battery cluster is provided with a cluster controller, and the PACK controller is communicatively connected to the cluster controller of the battery cluster in which it is located; the cluster controller is communicatively connected to the system controller;

[0009] The PACK controller is provided with a voltage detection module and a current detection module, which are respectively used to collect the single cell voltage and current of the battery PACK. The PACK controller determines the SOC of the battery PACK based on the single cell voltage and current of the battery PACK, and uploads it to the cluster controller;

[0010] The cluster controller is used to upload the single cell voltage, current and SOC of each battery PACK in the corresponding battery cluster to the system controller;

[0011] The system controller is used to determine the power balancing control instruction based on the single cell voltage, current and SOC of each battery PACK in each battery cluster, as well as the set imbalance threshold and the set balance threshold, and send it to the corresponding cluster controller, which transmits it to the corresponding PACK controller. The PACK controller controls the charging and discharging power of the corresponding battery PACK.

[0012] Furthermore, each phase unit includes two battery clusters arranged in parallel, and one ends of three battery clusters located on the same side of the three phase units are connected together to form a neutral point, and the other ends are respectively connected to the three-phase AC power grid through reactors.

[0013] The present invention also provides a method for balancing a multi-level energy storage system, which is applied to the multi-level energy storage system, and comprises:

[0014] Under shutdown or standby conditions, the output voltage of two or more battery clusters connected in parallel is controlled to form a charge-discharge loop, and the current between clusters in each phase unit is controlled to form a power transfer to achieve static DC balancing, which includes inter-cluster static DC balancing and intra-cluster static DC balancing;

[0015] The inter-cluster static DC balancing is as follows: if the SOC imbalance of the battery cluster exceeds the set imbalance threshold, the inter-cluster static DC balancing strategy is activated; if the SOC of the battery cluster is lower than the average SOC of the cluster, the inter-cluster current is controlled to control the charging of the battery cluster; if the SOC of the battery cluster is higher than the average SOC of the cluster, the inter-cluster current is controlled to control the discharge of the battery cluster to achieve inter-cluster power balancing;

[0016] The intra-cluster static DC balancing is as follows: if the SOC imbalance of the battery PACK exceeds the set imbalance threshold, the intra-cluster static DC balancing strategy is started; if the SOC of the battery PACK is lower than the battery cluster SOC, the output voltage of the battery PACK is increased by controlling the PACK controller to increase the power outflow; if the SOC of the battery PACK is higher than the battery cluster SOC, the output voltage of the battery PACK is reduced by controlling the PACK controller to reduce the power outflow, thereby achieving intra-cluster power balancing.

[0017] Furthermore, it also includes: inter-cluster balancing under discharge conditions:

[0018] When the average value of the single cell voltage is greater than or equal to 3.1V and the SOC imbalance of the battery cluster exceeds the set imbalance threshold, the inter-cluster balancing strategy is activated. If the SOC of the battery cluster is lower than the average SOC of the cluster, the discharge current of the battery cluster is reduced by controlling the PACK controller in the battery cluster; if the SOC of the battery cluster is higher than the average SOC of the cluster, the discharge current of the battery cluster is increased by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster power balance;

[0019] When the average value of the cell voltage is less than 3.1V, if the minimum cell voltage in the battery cluster is lower than the average cluster cell voltage minimum value, the discharge current of the battery cluster is reduced by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster charge balance.

[0020] Furthermore, it also includes: inter-cluster balancing under charging conditions:

[0021] When the average value of the single cell voltage is less than or equal to 3.4V and the SOC imbalance of the battery cluster exceeds the set imbalance threshold, the inter-cluster balancing strategy is activated. If the SOC of the battery cluster is lower than the average SOC of the cluster, the charging current of the battery cluster is increased by controlling the PACK controller in the battery cluster; if the SOC of the battery cluster is higher than the average SOC of the cluster, the charging current of the battery cluster is reduced by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster power balance;

[0022] When the average value of the single cell voltage is greater than 3.4V and the maximum value of the battery cluster single cell voltage is higher than the average cluster single cell voltage maximum value, the charging power of the battery cluster is reduced by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster power balance.

[0023] Further, the battery cluster SOC is the average SOC of all battery PACKs in the corresponding battery cluster;

[0024] The battery cluster SOC imbalance is the ratio of the absolute value of the difference between the battery cluster SOC and the cluster average SOC to the cluster average SOC;

[0025] If the SOC imbalance of the battery cluster is less than the set balance threshold, the SOC balance between clusters is determined;

[0026] The set imbalance threshold is greater than the set balance threshold.

[0027] Furthermore, it also includes: intra-cluster balancing under discharge conditions:

[0028] When the average value of the single cell voltage is greater than or equal to 3.1V and the SOC imbalance of the battery PACK exceeds the set imbalance threshold, the PACK balancing strategy is activated. If the SOC of the battery PACK is lower than the SOC of the battery cluster, the output voltage of the battery PACK is reduced by controlling the PACK controller to reduce the outflow of electricity; if the SOC of the battery PACK is higher than the SOC of the battery cluster, the output voltage of the battery PACK is increased by controlling the PACK controller to increase the outflow of electricity, thereby achieving intra-cluster electricity balance;

[0029] When the average value of the cell voltage is less than 3.1V, if the minimum value of the PACK cell voltage is lower than the minimum value of the average cluster cell voltage, the output voltage of the battery PACK is reduced by controlling the PACK controller, thereby achieving power balance between PACKs in the cluster.

[0030] Furthermore, it also includes: intra-cluster balancing under charging conditions:

[0031] When the average value of the single cell voltage is less than or equal to 3.4V, if the SOC imbalance of the battery PACK exceeds the set imbalance threshold, the PACK balancing strategy is activated. If the SOC of the battery PACK is lower than the SOC of the battery cluster, the output voltage of the battery PACK is increased by controlling the PACK controller to increase the power outflow; if the SOC of the battery PACK is higher than the SOC of the battery cluster, the output voltage of the battery PACK is reduced by controlling the PACK controller to reduce the power outflow, thereby achieving power balance within the cluster;

[0032] When the average value of the cell voltage is greater than 3.4V, if the maximum value of the PACK cell voltage is higher than the average maximum value of the cluster cell voltage, the output voltage of the battery PACK is reduced by controlling the PACK controller, thereby achieving power balance between PACKs in the cluster.

[0033] Further, the SOC imbalance of the battery PACK is the ratio of the absolute value of the difference between the battery cluster SOC and the battery PACK SOC to the battery cluster SOC;

[0034] The battery cluster SOC is the average SOC of all battery PACKs in the corresponding battery cluster;

[0035] If the SOC imbalance of the battery PACK is less than the set balance threshold, the SOC balance of the PACK in the cluster is determined;

[0036] The set imbalance threshold is greater than the set balance threshold.

[0037] According to the specific embodiments provided by the present invention, the multi-level energy storage system and the power balancing method thereof provided by the present invention disclose the following technical effects:

[0038] By adding a PACK controller to each battery PACK output, the power of each battery PACK can be controlled, and the power of the entire battery cluster can be controlled at the same time. Not only can inter-cluster balance be achieved, but also intra-cluster balance can be achieved by controlling the power of each battery PACK, which improves the efficiency and accuracy of power balancing and realizes automatic power balancing between clusters of multi-level topology energy storage systems.

[0039] 2. Under shutdown or standby conditions, the output voltage of two or more (3 or more) clusters connected in parallel is controlled to form a charge-discharge loop, and the current between clusters in each phase unit is controlled to form a power transfer, so as to achieve static DC balance, static DC balance between clusters and static DC balance within clusters, and solve the problem that the energy storage system cannot balance power under static conditions;

[0040] 3. Different power balancing strategies are formulated for different working conditions such as charging and discharging, shutdown or standby. By setting the balancing threshold and balance threshold as the standard for starting or stopping the power balancing strategy, if the SOC of the battery cluster or battery PACK is higher than the corresponding average value, the discharge current is increased or the charging current is reduced; if the SOC of the battery cluster or battery PACK is lower than the average value, the charging current is increased or the discharge current is reduced. Flexible balancing strategies can be used for safer and more intelligent control to achieve power balancing under all working conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] 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.

[0042] Figure 1 A hardware topology diagram of a multi-level energy storage system according to an embodiment of the present invention;

[0043] Figure 2A schematic diagram of a control architecture of a multi-level energy storage system according to an embodiment of the present invention;

[0044] Figure 3 This is a schematic diagram of inter-cluster static DC balancing control of a multi-level energy storage system according to an embodiment of the present invention;

[0045] Figure 4 Schematic diagram of static DC balancing control within a cluster of a multi-level energy storage system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0047] The purpose of the present invention is to provide a multi-level energy storage system and a method for balancing the charge and discharge power of each battery PACK, so as to realize automatic balancing of the charge between and within clusters of a multi-level topology energy storage system, avoid the loss of battery stack capacity caused by imbalanced charge, and solve the problem that the energy storage system cannot balance the charge under static conditions, so as to realize the balance of charge under all conditions.

[0048] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0049] like Figure 1-Figure 2 As shown, the multi-level energy storage system provided by the present invention comprises: three phase units, the three phase units are respectively connected to the three-phase AC power grid, each phase unit comprises at least two battery clusters arranged in parallel, and each battery cluster is connected in series with a reactor;

[0050] The battery cluster includes a plurality of battery PACKs arranged in series, and the output end of each battery PACK is connected to a PACK controller; each battery cluster is provided with a cluster controller, and the PACK controller is communicatively connected to the cluster controller of the battery cluster in which it is located; the cluster controller is communicatively connected to the system controller;

[0051] The PACK controller is provided with a voltage detection module and a current detection module, which are respectively used to collect the single cell voltage and current of the battery PACK. The PACK controller determines the SOC of the battery PACK based on the single cell voltage and current of the battery PACK, and uploads it to the cluster controller; wherein the PACK controller calculates the SOC of each battery cell by ampere-hour integration, and calibrates the SOC of each battery cell by referring to the open circuit voltage method, and finally obtains the SOC of the single battery PACK by averaging. The internal battery cell voltage of the battery PACK can be collected by a dedicated AFE sampling chip, and the current can be collected by a shunt meter or a Hall sensor;

[0052] The cluster controller is used to upload the single cell voltage, current and SOC of each battery PACK in the corresponding battery cluster to the system controller;

[0053] The system controller is used to determine the power balancing control instruction based on the single cell voltage, current and SOC of each battery PACK in each battery cluster, as well as the set imbalance threshold and the set balance threshold, and send it to the corresponding cluster controller, which transmits it to the corresponding PACK controller. The PACK controller controls the charging and discharging power of the corresponding battery PACK.

[0054] The system controller can achieve the following by controlling the PACK controller: under charging conditions, the charging degree of the battery PACK with less power can be increased, and the charging degree of the battery PACK with more power can be reduced; under discharging conditions, the discharging degree of the battery PACK with less power can be reduced, and the discharging degree of the battery PACK with more power can be increased. The inter-cluster current is controlled according to the average SOC of the cluster. If the SOC of cluster A1 is too high, the output voltage of the battery PACK in cluster A1 can be uniformly fine-tuned, so that the overall voltage of cluster A1 is higher than that of cluster A2, thereby realizing inter-cluster energy transfer.

[0055] Taking the structure of 6 battery clusters as an example, each phase unit includes two battery clusters arranged in parallel, and one end of the three battery clusters on the same side of the three phase units are connected together to form a neutral point, and the other end is connected to the three-phase AC power grid through a reactor. Each battery cluster contains several PACK controllers and battery PACKs.

[0056] The multi-level topology energy storage system does not require PCS and can directly realize the AC grid-connected function of PCS. Figure 1 A, B and C are connected to the three-phase AC power grid to achieve grid-connected function.

[0057] The embodiment of the present invention provides a method for balancing electric power of a multi-level energy storage system, which is executed by a system controller and applied to the above-mentioned multi-level energy storage system, including two methods: inter-cluster balancing and intra-cluster PACK balancing:

[0058] 1. Inter-cluster balance

[0059] The inter-cluster balancing method provided by the present invention uses the battery cluster SOC or voltage as a control object to achieve dynamic balancing of the battery cluster power. The battery cluster SOC is the average SOC of all battery PACKs in the cluster.

[0060] The multi-level energy storage system has three working states: discharge, charge, standby or shutdown. During the charge and discharge process, the battery can be balanced by adjusting the cluster current. In standby or shutdown conditions, since there are two or more battery clusters in parallel in each phase unit, intra-phase energy transfer can be achieved through each cluster in the phase unit.

[0061] The inter-cluster balancing is achieved by controlling the current of the battery cluster. The control principle is to control the PACK controller in each cluster and adjust the total series voltage of the cascaded PACK controllers in the cluster, thereby controlling the cluster current and power.

[0062] The battery cluster power imbalance is judged by the battery cluster SOC and the single cell voltage within the cluster. Under the discharge condition, when the average single cell voltage is less than 3.1V, the imbalance is judged by the minimum single cell voltage of each cluster; when it is greater than or equal to 3.1V, the imbalance is judged by the cluster SOC. Under the charging condition, when the average single cell voltage is greater than 3.4V, the imbalance is judged by the maximum single cell voltage of each cluster; when it is less than or equal to 3.4V, the imbalance is judged by the cluster SOC.

[0063] (1) Inter-cluster balance under discharge conditions

[0064] When the average value of the single cell voltage is greater than or equal to 3.1V, and the SOC imbalance of the battery cluster exceeds the set imbalance threshold, the inter-cluster balancing strategy is activated. If the SOC of the battery cluster is lower than the average SOC of the cluster, the discharge current of the battery cluster is reduced by controlling the PACK controller in the battery cluster; if the SOC of the battery cluster is higher than the average SOC of the cluster, the discharge current of the battery cluster is increased by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster charge balance; when the average value of the single cell voltage is less than 3.1V, if the minimum value of the single cell voltage in the battery cluster is lower than the minimum value of the average cluster single cell voltage, the discharge current of the battery cluster is reduced by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster charge balance.

[0065] (2) Inter-cluster balance under charging conditions

[0066] When the average value of the single cell voltage is less than or equal to 3.4V, and the SOC imbalance of the battery cluster exceeds the set imbalance threshold, the inter-cluster balancing strategy is activated. If the SOC of the battery cluster is lower than the average SOC of the cluster, the charging current of the battery cluster is increased by controlling the PACK controller in the battery cluster; if the SOC of the battery cluster is higher than the average SOC of the cluster, the charging current of the battery cluster is reduced by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster power balance; when the average value of the single cell voltage is greater than 3.4V, and the maximum value of the single cell voltage of the battery cluster is higher than the maximum value of the average cluster single cell voltage, the charging power of the battery cluster is reduced by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster power balance.

[0067] (3) Inter-cluster static DC balancing

[0068] In shutdown or standby conditions, the output voltage of two or more clusters connected in parallel is controlled to form a charge-discharge loop, and the current between clusters in the phase is controlled to form a power transfer, thereby achieving static DC balance between clusters. If the SOC imbalance of the battery cluster exceeds the set imbalance threshold, the static DC balance strategy between clusters is activated. If the SOC of the battery cluster is lower than the average SOC of the cluster, the charging of the battery cluster is controlled by controlling the current between clusters; if the SOC of the battery cluster is higher than the average SOC of the cluster, the discharge of the battery cluster is controlled by controlling the current between clusters to achieve power balance between clusters.

[0069] The basic principle of static DC balancing between clusters is that the battery cluster with the largest power transfers energy to the battery cluster with the smallest power, and the rest do not participate in energy conversion.

[0070] For example, Figure 3 As shown, if the SOC of cluster A1 is too high, the system controller will slightly increase the output voltage amplitude of each module in cluster A1, causing the voltage of cluster A1 to be slightly greater than the voltage of cluster A2, resulting in part of the energy of A1 being transferred to cluster A2, thereby achieving power balancing.

[0071] The inter-cluster SOC balance judgment is to compare the cluster average SOC with the single cluster SOC to obtain the battery cluster SOC imbalance. If the battery cluster SOC imbalance is greater than the imbalance threshold, the inter-cluster SOC is judged to be imbalanced. If the battery cluster SOC imbalance is less than the set balance threshold, the inter-cluster SOC is judged to be balanced. Generally, the imbalance threshold is greater than the balance threshold. The battery cluster SOC imbalance = the ratio of the absolute value of the difference between the average SOC value of the cluster in the same phase and the SOC value of the single cluster to the cluster average SOC, and the obtained data unit is %.

[0072] The imbalance threshold is generally set to 3%, and the balance threshold is generally set to 2%.

[0073] If the SOC imbalance of the battery cluster is between the set imbalance threshold and the set balance threshold, balancing will not be started. The purpose of this is to reduce the use of balancing, reduce unnecessary system losses, and avoid frequent switching of the system between balancing and not balancing.

[0074] 2. Intra-cluster PACK balancing

[0075] The intra-cluster PACK balancing method provided by the present invention uses the SOC of the battery PACK or the voltage of the battery cell as the control object to achieve dynamic balancing of the PACK power. The SOC of the battery PACK is the average SOC of all the cells in the PACK.

[0076] The multi-level energy storage system has three working states: discharging, charging, standby or shutdown.

[0077] The intra-cluster PACK power balancing controls the PACK controller in each cluster so that the PACK controller in the cluster controls the average output voltage of the battery PACK, thereby controlling the charge and discharge current of each PACK.

[0078] The power imbalance of the battery PACK within the cluster is judged by the SOC of the battery PACK and the single cell voltage between the PACKs within the cluster. Under the discharge condition, when the average single cell voltage is less than 3.1V, the imbalance is judged by the minimum single cell voltage of each battery PACK; when it is greater than or equal to 3.1V, the imbalance is judged by the SOC of the battery PACK; under the charging condition, when the average single cell voltage is greater than 3.4V, the imbalance is judged by the maximum single cell voltage of each PACK; when it is less than or equal to 3.4V, the imbalance is judged by the SOC of the battery PACK.

[0079] (1) Intra-cluster balance under discharge conditions:

[0080] Under discharge conditions, when the average value of the single cell voltage is greater than or equal to 3.1V and the SOC imbalance of the battery PACK exceeds the set imbalance threshold, the PACK balancing strategy is activated. If the SOC of the battery PACK is lower than the SOC of the battery cluster, the output voltage of the battery PACK is reduced by controlling the PACK controller to reduce the outflow of electricity; if the SOC of the battery PACK is higher than the SOC of the battery cluster, the output voltage of the battery PACK is increased by controlling the PACK controller to increase the outflow of electricity, thereby achieving intra-cluster electricity balance;

[0081] When the average value of the cell voltage is less than 3.1V, if the minimum value of the PACK cell voltage is lower than the minimum value of the average cluster cell voltage, the output voltage of the battery PACK is reduced by controlling the PACK controller, thereby achieving power balance between PACKs in the cluster.

[0082] (2) Intra-cluster balancing under charging conditions:

[0083] When the average value of the single cell voltage is less than or equal to 3.4V, if the SOC imbalance of the battery PACK exceeds the set imbalance threshold, the PACK balancing strategy is activated. If the SOC of the battery PACK is lower than the SOC of the battery cluster, the output voltage of the battery PACK is increased by controlling the PACK controller to increase the power outflow; if the SOC of the battery PACK is higher than the SOC of the battery cluster, the output voltage of the battery PACK is reduced by controlling the PACK controller to reduce the power outflow, thereby achieving power balance within the cluster;

[0084] When the average value of the cell voltage is greater than 3.4V, if the maximum value of the PACK cell voltage is higher than the average maximum value of the cluster cell voltage, the output voltage of the battery PACK is reduced by controlling the PACK controller, thereby achieving power balance between PACKs in the cluster.

[0085] like Figure 4 As shown in the figure, taking the energy storage system of 6 clusters as an example, if it is in the charging state, the power of PACKn inside cluster A1 is too high and exceeds the battery cluster SOC. At this time, the system controller detects that the power of PACKn inside cluster A1 is too high, and controls the output voltage amplitude of PACK controller n inside cluster A1 to reduce the charging power of the PACK because the cluster current of a battery cluster is consistent. The SOC of PACKn will gradually approach the cluster SOC.

[0086] (3) Static DC balancing within a cluster:

[0087] In shutdown or standby conditions, there is no cluster current under shutdown conditions, and balancing cannot be completed. Therefore, it is necessary to generate an inter-cluster current as a condition for balancing the PACK power within the cluster. If the battery PACK power imbalance exceeds the set imbalance threshold, the PACK balancing strategy is started. If the battery clusters within the phase are balanced at this time, an inter-cluster current is generated through a certain charging and discharging sequence, and this inter-cluster current is used to achieve PACK balancing within the cluster under discharge or charging conditions. Therefore, the cluster current is the same as the current flowing through the PACK. Controlling the cluster current is achieved by controlling the PACK output voltage. The cluster with a high PACK voltage is controlled as a discharge cluster, and the cluster with a low PACK voltage is controlled as a charging cluster.

[0088] For example, each phase unit includes two battery clusters. If the estimated balancing time is T, first increase the voltage of cluster A1 so that cluster A1 discharges to cluster A2. Cluster A1 works in the discharging condition and cluster A2 works in the charging condition. After maintaining T / 2, reduce the voltage of cluster A1 so that cluster A2 discharges to cluster A1. Cluster A1 works in the charging condition and cluster A2 works in the discharging condition. After balancing for T time, clusters A1 and A2 still maintain balance, and the PACK balance within the cluster also maintains balance. If the battery clusters within the phase are not balanced at this time, the battery cluster with more power will provide the battery cluster with less power, so that the PACK within the cluster can achieve PACK balance under the charging and discharging conditions.

[0089] Under different working conditions, the SOC balance of the PACK in the cluster is judged by comparing the SOC of the battery cluster with the SOC of the battery PACK to obtain the SOC imbalance of the battery PACK. If the SOC imbalance of the battery PACK is greater than the set imbalance threshold, the SOC imbalance of the battery PACK in the cluster is judged. If the SOC imbalance of the battery PACK is less than the set balance threshold, the SOC of the PACK in the battery cluster is judged to be balanced. Generally, the imbalance threshold is greater than the balance threshold. The SOC imbalance of the battery PACK = the ratio of the absolute value of the difference between the SOC of the battery cluster and the SOC of the battery PACK to the SOC of the battery cluster, and the obtained data unit is %. Among them, the battery cluster SOC is the average SOC of all battery PACKs in the cluster.

[0090] The imbalance threshold is generally set to 3%, and the balance threshold is generally set to 2%.

[0091] If the SOC imbalance of the battery PACK is between the set imbalance threshold and the set balance threshold, balancing will not be started. The purpose of this is to reduce the use of balancing, reduce unnecessary system losses, and avoid frequent switching of the system between balancing and not balancing.

[0092] The hardware topology of the multi-level energy storage system provided by the present invention is that each battery PACK has a PACK controller, which can control the charge and discharge power of each battery PACK, and can not only achieve inter-cluster balance, but also achieve intra-cluster balance by controlling the power of each battery PACK. The power balancing method provided by the present invention can achieve safer and smarter control through a flexible balancing strategy; the balancing strategy is to control the cluster SOC and the battery cell voltage, and start or stop the inter-cluster balancing strategy by setting the imbalance threshold to the balance threshold. That is, if the SOC is higher than the average value, the discharge current is increased or the charging current is reduced, and if the SOC is lower than the average value, the charging current is increased or the discharge current is reduced.

[0093] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for balancing electric power in a multi-level energy storage system, applied to a multi-level energy storage system, characterized in that: The multi-level energy storage system comprises three phase units, which are respectively connected to a three-phase AC power grid, each phase unit comprises at least two battery clusters arranged in parallel, and each battery cluster is connected in series with a reactor, and is characterized in that: The battery cluster includes a plurality of battery PACKs arranged in series, and the output end of each battery PACK is connected to a PACK controller; each battery cluster is provided with a cluster controller, and the PACK controller is communicatively connected to the cluster controller of the battery cluster in which it is located; the cluster controller is communicatively connected to the system controller; The PACK controller is provided with a voltage detection module and a current detection module, which are respectively used to collect the single cell voltage and current of the battery PACK. The PACK controller determines the SOC of the battery PACK based on the single cell voltage and current of the battery PACK, and uploads it to the cluster controller; The cluster controller is used to upload the single cell voltage, current and SOC of each battery PACK in the corresponding battery cluster to the system controller; The system controller is used to determine the power balancing control instruction based on the single cell voltage, current and SOC of each battery PACK in each battery cluster, as well as the set imbalance threshold and the set balance threshold, and send it to the corresponding cluster controller, which transmits it to the corresponding PACK controller, and the PACK controller controls the charge and discharge power of the corresponding battery PACK; The power balancing method comprises: Under shutdown or standby conditions, the output voltage of two or more battery clusters connected in parallel is controlled to form a charge-discharge loop, and the current between clusters in each phase unit is controlled to form a power transfer to achieve static DC balancing, which includes inter-cluster static DC balancing and intra-cluster static DC balancing; The inter-cluster static DC balancing is as follows: if the SOC imbalance of the battery cluster exceeds the set imbalance threshold, the inter-cluster static DC balancing strategy is activated; if the SOC of the battery cluster is lower than the average SOC of the cluster, the inter-cluster current is controlled to control the charging of the battery cluster; if the SOC of the battery cluster is higher than the average SOC of the cluster, the inter-cluster current is controlled to control the discharge of the battery cluster to achieve inter-cluster power balancing; The intra-cluster static DC balancing is as follows: if the SOC imbalance of the battery PACK exceeds the set imbalance threshold, the intra-cluster static DC balancing strategy is started. When the battery cluster where the battery PACK is located is a discharge cluster, if the SOC of the battery PACK is higher than the SOC of the battery cluster, the output voltage of the battery PACK is increased by controlling the PACK controller to increase the power outflow; if the SOC of the battery PACK is lower than the SOC of the battery cluster, the output voltage of the battery PACK is reduced by controlling the PACK controller to reduce the power outflow, thereby achieving intra-cluster power balancing; The battery cluster SOC is the average SOC of all battery packs in the corresponding battery cluster; The battery cluster SOC imbalance is the ratio of the absolute value of the difference between the battery cluster SOC and the cluster average SOC to the cluster average SOC; If the SOC imbalance of the battery cluster is less than the set balance threshold, the SOC balance between clusters is determined; The SOC imbalance of the battery PACK is the ratio of the absolute value of the difference between the battery cluster SOC and the battery PACK SOC to the battery cluster SOC; The battery cluster SOC is the average SOC of all battery PACKs in the corresponding battery cluster; If the SOC imbalance of the battery PACK is less than the set balance threshold, the SOC balance of the PACK in the cluster is determined; The set imbalance threshold is greater than the set balance threshold. If the SOC imbalance of the battery cluster or the SOC imbalance of the battery PACK is between the set imbalance threshold and the set balance threshold, balancing is not started.

2. The method for balancing the electric quantity of a multi-level energy storage system according to claim 1, characterized in that: In the multi-level energy storage system, each phase unit includes two battery clusters arranged in parallel, one end of the three battery clusters located on the same side of the three phase units are connected together to form a neutral point, and the other ends are connected to the three-phase AC power grid through reactors.

3. The method for balancing the electric quantity of a multi-level energy storage system according to claim 1, characterized in that: Also includes: Inter-cluster balance under discharge conditions: When the average value of the single cell voltage is greater than or equal to 3.1V and the SOC imbalance of the battery cluster exceeds the set imbalance threshold, the inter-cluster balancing strategy is activated. If the SOC of the battery cluster is lower than the average SOC of the cluster, the discharge current of the battery cluster is reduced by controlling the PACK controller in the battery cluster; if the SOC of the battery cluster is higher than the average SOC of the cluster, the discharge current of the battery cluster is increased by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster power balance; When the average value of the cell voltage is less than 3.1V, if the minimum cell voltage in the battery cluster is lower than the average cluster cell voltage minimum value, the discharge current of the battery cluster is reduced by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster charge balance.

4. The method for balancing the electric quantity of a multi-level energy storage system according to claim 1, characterized in that: Also includes: Inter-cluster balancing under charging conditions: When the average value of the single cell voltage is less than or equal to 3.4V and the SOC imbalance of the battery cluster exceeds the set imbalance threshold, the inter-cluster balancing strategy is activated. If the SOC of the battery cluster is lower than the average SOC of the cluster, the charging current of the battery cluster is increased by controlling the PACK controller in the battery cluster; if the SOC of the battery cluster is higher than the average SOC of the cluster, the charging current of the battery cluster is reduced by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster power balance; When the average value of the single cell voltage is greater than 3.4V and the maximum value of the battery cluster single cell voltage is higher than the average cluster single cell voltage maximum value, the charging power of the battery cluster is reduced by controlling the PACK controller in the battery cluster, thereby achieving inter-cluster power balance.

5. The method for balancing the electric quantity of a multi-level energy storage system according to claim 1, characterized in that: Also includes: Intra-cluster balance under discharge conditions: When the average value of the single cell voltage is greater than or equal to 3.1V and the SOC imbalance of the battery PACK exceeds the set imbalance threshold, the PACK balancing strategy is activated. If the SOC of the battery PACK is lower than the SOC of the battery cluster, the output voltage of the battery PACK is reduced by controlling the PACK controller to reduce the outflow of electricity; if the SOC of the battery PACK is higher than the SOC of the battery cluster, the output voltage of the battery PACK is increased by controlling the PACK controller to increase the outflow of electricity, thereby achieving intra-cluster electricity balance; When the average value of the cell voltage is less than 3.1V, if the minimum value of the PACK cell voltage is lower than the minimum value of the average cluster cell voltage, the output voltage of the battery PACK is reduced by controlling the PACK controller, thereby achieving power balance between PACKs in the cluster.

6. The method for balancing the electric quantity of a multi-level energy storage system according to claim 1, characterized in that: Also includes: Intra-cluster balancing under charging conditions: When the average value of the single cell voltage is less than or equal to 3.4V, if the SOC imbalance of the battery PACK exceeds the set imbalance threshold, the PACK balancing strategy is activated. If the SOC of the battery PACK is lower than the SOC of the battery cluster, the output voltage of the battery PACK is increased by controlling the PACK controller to increase the power inflow; if the SOC of the battery PACK is higher than the SOC of the battery cluster, the output voltage of the battery PACK is reduced by controlling the PACK controller to reduce the power inflow, thereby achieving power balance within the cluster; When the average value of the cell voltage is greater than 3.4V, if the maximum value of the PACK cell voltage is higher than the average maximum value of the cluster cell voltage, the output voltage of the battery PACK is reduced by controlling the PACK controller, thereby achieving power balance between PACKs in the cluster.

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