CN110912235B - Energy storage system and current equalizing method thereof - Google Patents
Energy storage system and current equalizing method thereof Download PDFInfo
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- CN110912235B CN110912235B CN201911284186.7A CN201911284186A CN110912235B CN 110912235 B CN110912235 B CN 110912235B CN 201911284186 A CN201911284186 A CN 201911284186A CN 110912235 B CN110912235 B CN 110912235B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
- H02J7/0013—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries acting upon several batteries simultaneously or sequentially
- H02J7/0014—Circuits for equalisation of charge between batteries
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
- H01M10/44—Methods for charging or discharging
- H01M10/441—Methods for charging or discharging for several batteries or cells simultaneously or sequentially
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Abstract
The invention provides an energy storage system and a current equalizing method thereof, wherein the energy storage system comprises: x current-sharing unit and X battery cluster, wherein: in each current equalizing unit, the first end of the uppermost DC/DC converter is connected with the direct current bus of the corresponding battery cluster, the second ends of the Y lowermost DC/DC converters are respectively connected with the corresponding battery pack of the corresponding battery cluster, and the second end of the uppermost DC/DC converter is directly connected with the first end of each lowermost DC/DC converter or is connected with the first end of each lowermost DC/DC converter through other DC/DC converters; therefore, the charging/discharging is carried out on the corresponding battery clusters through the DC/DC converters at all levels, so that the SOC balance of each battery cluster is realized, the repeated switching of the relay is avoided, the SOC change rates of each battery cluster are kept consistent in the whole process, the current equalization of each battery cluster can be realized, and the safety of the energy storage system is improved.
Description
Technical Field
The invention belongs to the technical field of energy storage batteries, and particularly relates to an energy storage system and a current equalizing method thereof.
Background
In a conventional energy storage system, each battery cluster is connected in parallel. Because the internal resistances of the battery clusters are different, and the currents in the charging and discharging processes are different, the SOC change rates of different battery clusters are different, so that the battery clusters in the energy storage system have fixed SOC differences and cannot be fully charged to exert the maximum performance of the system.
In the prior art, current sharing is realized by controlling the switching of each battery cluster. The dynamic method comprises the following steps: in the charging process of the energy storage system, if the charging current of the energy storage system is smaller than a preset value, cutting off each battery cluster from high to low according to the SOC; and in the discharging process of the energy storage system, if the discharging current of the energy storage system is smaller than a preset value, cutting off each battery cluster from low to high according to the SOC. The static and dynamic combination method comprises the following steps: in the charging process of the energy storage system, after a certain battery cluster is fully charged, stopping charging and cutting off all the battery clusters, then performing power supplementing operation once again, putting each battery cluster from low to high according to the SOC until at least one of the remaining battery clusters is fully charged, and repeating the operation until all the battery clusters are fully charged. The control process during the charging of the energy storage system is identical to the discharging process of the dynamic method.
However, in the two methods, the relay needs to be switched for many times, and each time, the relay is in on-load operation, so that the service life of the relay is greatly influenced, and meanwhile, safety accidents are easily caused by the adhesion of the relay.
Disclosure of Invention
In view of this, the present invention provides an energy storage system and a current equalizing method thereof, so as to keep SOC of each battery cluster dynamically consistent in the whole process when each battery cluster is charged and discharged.
The first aspect of the present invention discloses an energy storage system, including: x current-sharing units and X battery clusters, wherein X is an integer greater than 1; wherein:
each of the battery clusters is connected in parallel;
each current equalizing unit is connected with each battery cluster in parallel in a one-to-one correspondence manner;
the current equalizing unit comprises at least two stages of DC/DC converters;
in each current equalizing unit, the first end of the uppermost DC/DC converter is connected with the direct current bus of the corresponding battery cluster; the second ends of the Y lowest-level DC/DC converters are respectively connected with corresponding battery packs in corresponding battery clusters; the second end of the uppermost DC/DC converter is directly connected with the first end of each lowermost DC/DC converter or is connected with the first end of each lowermost DC/DC converter through other DC/DC converters; y is the number of the battery packs in the battery cluster, and Y is larger than 1.
Optionally, the current equalizing unit includes: two-stage DC/DC converters, which are respectively an uppermost DC/DC converter and Y lowermost DC/DC converters; wherein:
first terminals of the respective lowermost DC/DC converters are connected to second terminals of the uppermost DC/DC converters, respectively.
Optionally, each stage of DC/DC converter in the current equalizing unit is a unidirectional converter;
the first end of each DC/DC converter is the input end of the DC/DC converter, and the second end of each DC/DC converter is the output end of the DC/DC converter;
the uppermost DC/DC converter is a fixed voltage output, and the lowermost DC/DC converter is a fixed voltage input.
Optionally, each stage of DC/DC converter in the current equalizing unit is a bidirectional converter.
Optionally, the uppermost DC/DC converter in each current equalizing unit is disposed in the control box of the corresponding battery cluster, and each lowermost DC/DC converter in each current equalizing unit is externally hung on the corresponding battery pack.
Optionally, the method further includes: PCS (Power Control System, energy storage converter) and two confluence units, wherein:
the positive pole of the direct current bus of each battery cluster is correspondingly connected with each input end of one confluence unit, and the negative pole of the direct current bus of each battery cluster is correspondingly connected with each input end of the other confluence unit;
and the output ends of the two confluence units are respectively connected with the positive electrode and the negative electrode of the direct current side of the PCS.
The second aspect of the present invention discloses a current sharing method for an energy storage System, which is characterized in that a BMS (battery management System) applied to the energy storage System according to any one of the first aspects of the present invention includes:
periodically determining the SOC (State of Charge) change rate of each battery cluster in the energy storage system until the energy storage system is in a full charge/full discharge State;
judging whether the energy storage system needs current sharing adjustment or not according to the SOC change rate of each battery cluster;
if the energy storage system needs current sharing adjustment, determining a balancing current value of each battery pack in the energy storage system, and sending each balancing current value to a corresponding current sharing unit, so that the corresponding current sharing unit provides corresponding balancing current for the corresponding battery pack by adjusting a Pulse Width Modulation (PWM) signal.
Optionally, if the energy storage system is in a charging process, the periodically determining the SOC change rate of each battery cluster in the energy storage system includes:
periodically taking the maximum value in the SOC of each battery pack of each battery cluster as the current SOC of each battery cluster, and calculating the ratio of the corresponding difference value of the current SOC and the previous SOC of each battery cluster to time;
if the energy storage system is in a discharging process, the periodically determining the SOC change rate of each battery cluster in the energy storage system includes:
periodically taking the minimum value in the SOC of each battery pack of each battery cluster as the current SOC of each battery cluster, and calculating the ratio of the corresponding difference value of the current SOC of each battery cluster and the previous SOC to time.
Optionally, the calculation formula of the SOC of each battery pack is:
SOCijt＝SOCij0+Δt*(Ii1+…+Iin)+Δt*(Iij1+…+Iijm)，n*Δt＝t，m≤n；
the current sampling method comprises the steps that SOCijt is the SOC of a jth battery pack in an ith battery cluster at the time t, SOCij0 is the SOC of the jth battery pack in the ith battery cluster at the initial time, Ii1 is the 1 st sampling current value of the ith battery cluster, Iin is the nth sampling current value of the ith battery cluster, delta t is a sampling interval, n is the current sampling frequency of the battery cluster within the time 0-t, and m is the current sampling frequency of the battery pack within the time 0-t.
Optionally, if the energy storage system is in a charging process, the SOC of each battery pack at the initial time is the maximum value among the SOCs of each battery core in each battery pack;
if the energy storage system is in a discharging process, the SOC of each battery pack at the initial moment is the minimum value of the SOC of each battery core in each battery pack;
the calculation formula of the SOC of each battery cell is as follows: SOC/(SOH × Qn), where SOH is SOH (State of health) of the cell, Q is available capacity of the cell, and Qn is rated capacity of the cell.
Optionally, the determining whether the energy storage system needs to perform current sharing adjustment according to the SOC change rate of each battery cluster includes:
taking the maximum value of the SOC change rates of each battery cluster as a first SOC change rate, taking the minimum value of the SOC change rates of each battery cluster as a second SOC change rate, and judging whether the difference value between the first SOC change rate and the second SOC change rate is smaller than or equal to a threshold value or not;
if the difference value between the first SOC change rate and the second SOC change rate is smaller than or equal to a threshold value, judging that each energy storage system does not need current sharing adjustment;
and if the difference value between the first SOC change rate and the second SOC change rate is larger than a threshold value, judging that each energy storage system needs current sharing adjustment.
Optionally, each of the equalizing current values is sent to a corresponding current equalizing unit, so that the corresponding current equalizing unit provides a corresponding equalizing current for the corresponding battery pack by adjusting the PWM signal, including:
and sending each balance current value to each lowest level DC/DC converter of each current equalizing unit so that each lowest level DC/DC converter of each current equalizing unit provides corresponding balance current for the corresponding battery pack by adjusting PWM signals.
Optionally, after sending each of the equalization current values to the corresponding current equalizing unit, the method further includes:
and if the input voltage and the output voltage of the DC/DC converter at the lowest stage in the current equalizing unit are the same, controlling each battery pack to execute corresponding switching according to a preset switching strategy.
Optionally, the preset switching strategy includes:
when the energy storage system is in a charging process, cutting off each battery cluster from high to low according to the SOC until each battery cluster is cut off;
and when the energy storage system is in a discharging process, cutting off each battery cluster from low to high according to the SOC until each battery cluster is cut off.
From the above technical solution, the present invention provides an energy storage system, including: x current-sharing units and X battery clusters, wherein X is a positive integer greater than 1; wherein: each battery cluster is connected in parallel, and each current equalizing unit is connected in parallel with each battery cluster in a one-to-one correspondence manner; the current equalizing units comprise at least two levels of DC/DC converters, and in each current equalizing unit, the first end of the uppermost DC/DC converter is connected with the direct current bus of the corresponding battery cluster; the second ends of the Y lowest-level DC/DC converters are respectively connected with corresponding battery packs in corresponding battery clusters, and the second end of the highest-level DC/DC converter is directly connected with the first end of each lowest-level DC/DC converter or is connected with other-level DC/DC converters; therefore, the charging/discharging is carried out on the corresponding battery clusters through the DC/DC converters at each level so as to realize the SOC balance of each battery cluster, the repeated switching of the relay is avoided, and the problem of limitation of the SOC range of the battery cluster when the current is adjusted due to the fact that the voltage difference between each battery cluster is not large can be avoided by adopting the DC/DC converters at least at two levels; in addition, the SOC change rate of each battery cluster is kept consistent in the whole process, current equalization of each battery cluster can be achieved, and the safety of the energy storage system is improved.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
Fig. 1 is a schematic diagram of an energy storage system according to an embodiment of the present invention;
fig. 2 is a flowchart of a current sharing method of an energy storage system according to an embodiment of the present invention;
fig. 3 is a flowchart of another current sharing method for an energy storage system according to an embodiment of the present invention;
fig. 4 is a flowchart of another current sharing method for an energy storage system according to an embodiment of the present invention;
fig. 5 is a flowchart of another current sharing method for an energy storage system according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In this application, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.
The embodiment of the invention provides an energy storage system, which aims to solve the problems that in the prior art, multiple switching of a relay is needed, and each time, the relay is in on-load operation, the service life of the relay is greatly influenced, and meanwhile, safety accidents are easily caused by relay adhesion.
This energy storage system includes: x current equalizing units and X battery clusters; wherein:
each battery cluster is connected in parallel; in each battery cluster, Y battery packs are sequentially connected in series, and X, Y are integers greater than 1.
It should be noted that the difference of internal resistances among the battery clusters is dynamically changed, and in order to realize current sharing among the battery clusters, a charge-discharge loop decoupled from the battery cluster needs to be connected in parallel to each battery cluster; and the current difference of each battery cluster reaches 40A, and the power required for current sharing is very large.
Each current equalizing unit is respectively connected with each battery cluster in parallel in a one-to-one correspondence mode, namely, each battery cluster is connected with an independent charging and discharging loop in parallel. The two current equalizing units and the two battery clusters are taken as an example for explanation, one current equalizing unit is connected with one battery cluster in parallel, and the current equalizing unit and the battery cluster form an independent charge-discharge loop; and the other current equalizing unit is connected with the other battery cluster in parallel, and the current equalizing unit and the battery cluster also form an independent charge-discharge loop.
For the decoupling zero of realizing two charge-discharge return circuits, can adopt the effect object of charge-discharge return circuit to be the battery cluster, the effect object of the return circuit that flow equalizes is the battery package, consequently, this current equalizing unit includes: at least two stages of DC/DC converters. In each current equalizing unit, the first end of the uppermost DC/DC converter is connected with the direct current bus of the corresponding battery cluster, and at the moment, each uppermost DC/DC converter and the corresponding battery cluster form a charge-discharge loop, so that the uppermost DC/DC converter can take electricity from the direct current bus or transmit electric energy to the direct current bus, and the requirement of high-power current equalization of each battery cluster is met; the second ends of the Y lowest-level DC/DC converters are respectively connected with corresponding battery packs in corresponding battery clusters, and at the moment, each lowest-level DC/DC converter and the corresponding battery pack form a current equalizing loop; the second end of the uppermost DC/DC converter is connected with the first end of each lowermost DC/DC converter directly or through other DC/DC converters.
Specifically, when the current equalizing unit includes two stages of DC/DC converters, in the current equalizing unit, the second terminals of the uppermost DC/DC converter are respectively connected to the first terminals of the Y lowermost DC/DC converters. When the current equalizing unit includes a plurality of stages of DC/DC converters, in the current equalizing unit, the second terminal of the uppermost DC/DC converter is directly connected to the first terminal of each of the lowermost DC/DC converters, or the second terminal of the uppermost DC/DC converter is connected to the first terminal of each of the lowermost DC/DC converters through another stage of DC/DC converter.
Referring to fig. 1 (which is shown by taking a two-stage DC/DC conversion as an example), specifically, the battery clusters C1 to CX are connected in parallel, the battery packs P11 to P1Y in the battery cluster C1 are sequentially connected in series, and by analogy, the battery packs PX1 to PXY in the battery cluster CX are sequentially connected in series, and it should be noted that each battery pack includes N battery cells sequentially connected in series.
In the current equalizing unit 1, the second end of the DC/DC converter 11 is connected to a battery pack P11 in a battery cluster C1, the second end of the DC/DC converter 12 is connected to a battery pack P12 in a battery cluster C1, and so on, the second end of the DC/DC converter 1Y is connected to a battery pack P1Y in a battery cluster C1, the first ends of the DC/DC converters 11 to 1Y are connected to the second end of the DC/DC converter 1, and the first end of the DC/DC converter 1 is connected to a DC bus of a battery cluster C1. In the same way, in the xth current equalizing unit, the second end of the DC/DC converter X1 is connected with the battery pack PX1 in the battery cluster CX, the second end of the DC/DC converter X2 is connected with the battery pack PX2 in the battery cluster CX, in the same way, the second end of the DC/DC converter XY is connected with the battery pack PXY in the battery cluster CX, the first ends of the DC/DC converters X1 to XY are connected with the second end of the DC/DC converter X, and the first end of the DC/DC converter X is connected with the DC bus of the battery cluster CX.
In the 1 st current equalizing unit, the DC/DC converters 11 to 1Y are the lowest level DC/DC converters, the DC/DC converter 1 is the highest level DC/DC converter, and the other current equalizing units are similar, and are not described herein again and are within the protection scope of the present application. Each uppermost DC/DC converter is preferably arranged in the control box of the corresponding battery cluster, and each lowermost DC/DC converter is preferably hung on the corresponding battery pack, and of course, the positions of each lowermost DC/DC converter, the uppermost DC/DC converter, and the other DC/DC converters may be other positions, as the case may be, and are all within the protection scope of the present application.
It should be noted that, in the above connection relations, the connection relation of the polarities of the devices is satisfied, that is, the positive electrode of one end of one device is connected to the positive electrode of the corresponding end of the other device, and the negative electrode of one end of one device is connected to the negative electrode of the corresponding end of the other device.
In addition, each battery cluster is provided with a current sampling device so as to sample the current of the corresponding battery cluster through each current sampling device, and each battery pack in the battery cluster can also be provided with a current sampling device so as to sample the current of the corresponding battery pack through each current sampling device.
In this embodiment, charging/discharging is performed on the corresponding battery clusters through each level of DC/DC converter to realize SOC balance of each battery cluster, so that the risk of insufficient service life and adhesion caused by multiple switching of the relay is avoided, the charging/discharging time of the energy storage system is shortened, and the efficiency of the energy storage system is improved on the basis of not increasing the system cost.
The description of the value is that if one DC/DC converter is configured for each battery cluster, the parallel connection of the battery clusters is changed into the parallel connection of the DC/DC converters, so that the decoupling between different battery clusters is realized, and the SOC change rates of the battery clusters are balanced. However, since the operating power of the battery cluster is large, a DC/DC converter with a large power-tolerant value needs to be used, which results in high circuit cost; in addition, for an LFP (lithium iron phosphate, whose molecular formula is LiFePO) system, when a battery cluster is in a range of 30-80% SOC, the SOC difference of each battery cluster is not large even if the SOC difference is large, and therefore, the DC/DC converter cannot greatly adjust the current between different battery clusters, and is limited by the SOC range of the battery cluster when adjusting the current, that is, the scheme cannot achieve the uniformity of the SOC change rate in the whole process.
In the embodiment, the problem that the SOC range of each battery cluster is limited when the current is adjusted due to the fact that the voltage difference between the battery clusters is small can be solved by adopting at least two stages of DC/DC converters, the SOC change rates of the battery clusters are kept consistent in the whole process, current equalization of the battery clusters can be achieved, the safety of the energy storage system is improved, the power resisting value of the battery clusters can be reduced by adopting at least two stages of DC/DC converters, the circuit cost is reduced, and the control difficulty of the energy storage system is reduced.
In practical applications, each stage of the DC/DC converter in the current equalizing unit is a unidirectional converter or a bidirectional converter. Specifically, when each of the DC/DC converters is a unidirectional converter, at this time, charging of each battery pack can be realized, a first end of each of the DC/DC converters is an input end of the DC/DC converter, and a second end thereof is an output end of the DC/DC converter. The uppermost DC/DC converter is a fixed voltage output, and the lowermost DC/DC converter is a fixed voltage input; when all the DC/DC converters are bidirectional converters, the charging and discharging of the battery pack can be realized. Considering the complexity of the control, a unidirectional converter is preferably used, but of course, depending on the actual application, it is within the scope of the present application.
Optionally, the energy storage system further comprises: PCS and two bus units.
Referring to fig. 1, the positive electrode of the dc bus of each battery cluster is correspondingly connected to each input end of the confluence unit 1, and the negative electrode of the dc bus of each battery cluster is correspondingly connected to each input end of the confluence unit 2; the output ends of the two confluence units are respectively connected with the positive electrode and the negative electrode of the direct current side of the PCS.
It should be noted that, in actual operation of the energy storage system, a difference between a maximum value and a minimum value in the current of each battery cluster may reach 40A, and the SOC difference between corresponding battery clusters may reach more than 10%. If the SOC is not adjusted, the SOC difference among the battery clusters in the multiple charging and discharging processes can be accumulated, and the performance of the energy storage system is influenced.
Therefore, an embodiment of the present invention provides a current sharing method for an energy storage system, which is applied to the BMS of the energy storage system provided in the above embodiment, and the specific structure of the energy storage system is referred to the above embodiment and is not described herein again. Referring to fig. 2, the current sharing method of the energy storage system includes:
s101, periodically determining the SOC change rate of each battery cluster in the energy storage system until the energy storage system is in a full charge/full discharge state.
It should be noted that the SOC of the battery clusters is related to the current of the battery clusters, so that the current of each battery cluster is determined by determining the SOC change rate of each battery cluster, and the SOC change rates of the battery clusters are kept consistent by combining the subsequent steps, that is, the current of each battery cluster can be kept consistent, thereby realizing current equalization and full charge/full discharge state of each battery cluster.
It should be noted that the granularity of the time period is adjustable, that is, the time duration of one period is adjustable, the granularity of the time period depends on the task period and the system requirement, and the specific value is determined according to the actual application environment, and all of them are within the protection scope of the present application. After determining the SOC change rate of each battery cluster in the energy storage system, executing the following steps:
and S102, judging whether the energy storage system needs current sharing adjustment or not according to the SOC change rate of each battery cluster.
Specifically, it may be determined whether the SOC change rates of the battery clusters are consistent, or the difference between the SOC change rates of the battery clusters is within a preset range. Of course, step S102 may also be other manners capable of determining whether the energy storage system needs to be adjusted for current sharing, which are not described herein again and are all within the protection scope of the present application.
It should be noted that, if each battery cluster in the energy storage system does not have a current sharing, each battery cluster cannot be in a full charge/full discharge state, and the maximum performance of the energy storage system cannot be exerted.
Therefore, when the battery clusters in the energy storage system are not current-sharing, namely the SOC of each battery cluster is inconsistent, the energy storage system is judged to need current-sharing adjustment, and when the battery clusters in the energy storage system are current-sharing, namely the SOC of each battery cluster is consistent, the energy storage system is judged not to need current-sharing adjustment. In addition, whether the voltages of the battery clusters are balanced or not can also be used as an auxiliary judgment condition according to the actual situation, and the description is omitted here, and the auxiliary judgment condition is within the protection scope of the application.
If the energy storage system needs current sharing adjustment, step S103 is executed.
S103, determining the balance current value of each battery pack in the energy storage system, and sending each balance current value to the corresponding current sharing unit, so that the corresponding current sharing unit provides corresponding balance current for the corresponding battery pack through the PWM signal.
It should be noted that, when each stage of the DC/DC converters in the current equalizing unit is a unidirectional converter, each equalizing current value is sent to the corresponding current equalizing unit, so that the corresponding current equalizing unit provides the corresponding charging current for the corresponding battery pack through the PWM signal. When all levels of DC/DC converters in the current equalizing units are bidirectional converters, all equalizing current values are sent to the corresponding current equalizing units, so that the corresponding current equalizing units provide corresponding charging current or discharging current for the corresponding battery packs through PWM signals.
Specifically, each equalization current value is sent to each lowest level DC/DC converter of each current equalizing unit, so that each lowest level DC/DC converter of each current equalizing unit provides a corresponding equalization current for the corresponding battery pack by adjusting the PWM signal.
It should be noted that, calculating the SOH of each battery pack, counting the current of each battery pack, and keeping the current of each battery pack/the SOH of the battery pack the same, that is, keeping the SOC of each battery pack the same and the SOC change rate of each battery pack the same, at this time, it is necessary to charge the battery pack with slow charging in the charging process, and charge the battery pack with fast discharging in the discharging process; and the current equalizing loops corresponding to the battery packs in the battery clusters work simultaneously.
In this embodiment, whether the energy storage system needs current-sharing adjustment is judged according to the SOC change rate of each battery cluster, when the energy storage system needs current-sharing adjustment, the equalizing current value of each battery pack in the energy storage system is determined, and each equalizing current value is sent to the corresponding current-sharing unit, so that the corresponding current-sharing unit provides corresponding equalizing current for the corresponding battery pack through the PWM signal, thereby realizing current sharing of each battery cluster in the energy storage system and exerting the maximum performance of the energy storage system.
Further, after step S103, the method further includes: and if the input voltage and the output voltage of the DC/DC converter at the lowest level in the current equalizing unit are the same, controlling each battery pack to execute corresponding switching according to a preset switching strategy.
Presetting a switching strategy comprises the following steps: when the energy storage system is in a charging process, cutting off each battery cluster from high to low according to the SOC until each battery cluster is cut off; when the energy storage system is in a discharging process, each battery cluster is cut off from low to high according to the SOC until each battery cluster is cut off.
Specifically, when the energy storage system is in a charging process, if the input voltage and the output voltage of a lowest-level DC/DC converter in the current equalizing unit are the same, each battery cluster is controlled to be cut off from high to low according to the SOC until each battery cluster is cut off; when the energy storage system is in a discharging process, if the input voltage and the output voltage of the DC/DC converter at the lowest level in the current equalizing unit are the same, each battery cluster is controlled to be cut off from low to high according to the SOC until each battery cluster is cut off.
It should be noted that, when the energy storage system is in a discharging process and a charging process, the specific process of step S101 in fig. 2 in the embodiment of the present invention is different, and two cases are respectively described herein as follows:
when the energy storage system is in a charging process, the SOC change rate of each battery cluster in the energy storage system is periodically determined in step S101 until the energy storage system is in a full charge/full discharge state, which is shown in fig. 3 and includes:
s201, periodically taking the maximum value in the SOC of each battery pack of each battery cluster as the current SOC of each battery cluster, and calculating the ratio of the corresponding difference value of the current SOC and the previous SOC of each battery cluster to time.
Specifically, taking the SOC of one battery cluster as an example, the SOC of each battery pack is first obtained as SOCi1, …, SOCij, …, SOCiX, and the previous SOCin-1 of the battery cluster, the current SOCin of the battery cluster is determined according to the formula SOCin ═ SOCi1, …, SOCij, …, SOCiX } max, and then the SOC change rate of the battery cluster is obtained according to the formula (SOCin-SOCn-1)/T. It should be noted that the above steps are repeatedly executed at specific time intervals until the energy storage system is in a full charge/full discharge state.
Wherein, the SOCi 1-SOCiX are SOC of the battery packs i 1-iX, T is the time interval between SOCin and SOCn-1, and the time of the previous SOCin-1 is 13: 00, if the current SOCin time is 14:00, T is 1 hour, and the unit of T may be minutes and seconds, which are also within the scope of the present application.
When the energy storage system is in a discharging process, determining the SOC change rate of each battery cluster in the energy storage system periodically in step S101 until the energy storage system is in a full charging/full discharging state; referring to fig. 4, including:
s301, periodically taking the minimum value in the SOC of each battery pack of each battery cluster as the current SOC of each battery cluster, and calculating the ratio of the corresponding difference value of the current SOC and the previous SOC of each battery cluster to time.
Specifically, taking the SOC of one battery cluster as an example, the SOC of each battery pack is first obtained as SOCi1, …, SOCij, …, SOCiX, and the previous SOCin-1 of the battery cluster, the current SOCin of the battery cluster is determined according to the formula SOCi ═ { SOCi1, …, SOCij, …, SOCiX } min, and then the SOC change rate of the battery cluster is obtained according to the formula (SOCin-SOCn-1)/T. It should be noted that the above steps are repeatedly executed at specific time intervals until the energy storage system is in a full charge/full discharge state.
In practical application, the calculation formula of the SOC of each battery pack is as follows:
SOCijt＝SOCij0+Δt*(Ii1+…+Iin)+Δt*(Iij1+…+Iijm)，n*Δt＝t，m≤n；
the current sampling method comprises the steps that SOCijt is the SOC of a jth battery pack in an ith battery cluster at the time t, SOCij0 is the SOC of the jth battery pack in the ith battery cluster at the initial time, Ii1 is the 1 st sampling current value of the ith battery cluster, Iin is the nth sampling current value of the ith battery cluster, delta t is a sampling interval, n is the current sampling frequency of the battery cluster within the time 0-t, and m is the current sampling frequency of the battery pack within the time 0-t. The reason the above equation uses a summation equation rather than integration is that the current sampling in the energy storage system is discrete and not continuous.
It should be noted that, if the energy storage system is in a charging process, the SOC of each battery pack at the initial time is the maximum value among the SOCs of each battery cell in each battery pack. If the energy storage system is in the discharging process, the SOC of each battery pack at the initial moment is the minimum value of the SOC of each battery core in each battery pack.
In addition, the calculation formula of the SOC of each cell is as follows: SOC/(SOH × Qn), where SOH is SOH of the cell, Q is available capacity of the cell, and Qn is rated capacity of the cell.
SOHi is { SOHi1, …, SOHij, …, SOHiX } min, where SOHi is the SOH of the ith battery cluster, and SOHi1 to SOHiX are the SOH of each battery pack in the ith battery cluster, that is, the SOH of each battery cluster is the minimum value of the SOH of each battery cell in the battery cluster.
Optionally, in the embodiment of the present invention, in step S102 in fig. 2 to 4, it is determined whether the energy storage system needs current sharing adjustment according to the SOC change rate of each battery cluster, referring to fig. 5 (shown on the basis of fig. 2), including:
s401, taking the maximum value of the SOC change rates of the battery clusters as a first SOC change rate, and taking the minimum value of the SOC change rates of the battery clusters as a second SOC change rate.
S402, judging whether the difference value of the first SOC change rate and the second SOC change rate is smaller than or equal to a threshold value.
If the difference value between the first SOC change rate and the second SOC change rate is smaller than or equal to a threshold value, namely { SOC11, …, SOCXY } max- { SOC11, …, SOCXY } min is smaller than or equal to the threshold value, judging that each energy storage system does not need current sharing adjustment;
and if the difference value between the first SOC change rate and the second SOC change rate is larger than a threshold value, namely the threshold value of { SOC11, …, SOCXY } max- { SOC11, …, SOCXY } min >, it is determined that each energy storage system needs current sharing adjustment.
In this embodiment, it is achieved that during any one time period, { SOC11, …, SOCXY } max- { SOC11, …, SOCXY } min ≦ threshold, i.e., the rates of change of SOC for the various battery clusters are approximately consistent, thereby allowing different battery clusters to be fully charged at approximately the same time.
For convenience of understanding, the energy storage system is described herein as including 2 battery clusters, each battery cluster including 2 battery packs, and each battery pack including 2 battery cells, as follows:
the energy storage system comprises battery clusters C1 and C2, the battery cluster C1 comprises battery packs P11 and P12, the battery cluster C2 comprises battery packs P21 and P22, and each battery pack comprises two battery cores which are marked as a battery core 1 and a battery core 2; the rated capacity of each cell is 100 Ah. Assuming constant main circuit current, other basic parameters are shown in table 1:
TABLE 1 various parameters of the energy storage system
According to the table, when the energy storage system is in a charging process, the BMS calculates the SOH and the SOC of the battery pack according to the SOH and the SOC of each battery cell, further calculates the SOC change rate of each battery cluster and each battery pack according to the main circuit current, further calculates the equalizing current value Iij of each battery pack, and transmits the equalizing current value Iij to the corresponding lowest level DC/DC converter.
And the DC/DC converter at the lowest level receives the balance current value transmitted by the BMS, and adjusts the PWM signal according to the balance current value to provide corresponding balance current for the corresponding battery pack.
The steps are iterated gradually until the energy storage system is full, namely the battery core in at least one battery cluster reaches the upper limit voltage. When the energy storage system is in the discharging process, similarly to the charging process, the details are not repeated here, and the difference is that the SOC calculations are different until the energy storage system is discharged, that is, the battery cell in at least one battery cluster reaches the lower limit voltage.
In this embodiment, the current equalizing size is calculated by using the SOC change rate of the battery packs, so that the SOC of the battery clusters is consistent, and the SOC of the battery packs is also consistent, that is, the SOC change rates of the battery packs are consistent, so that the energy storage system can adjust the current equalizing size of the battery packs; and calculating the balance current value of the next time period by using the parameter dynamic of the last state of the previous time period, and gradually iterating to realize the SOC dynamic consistency of each battery cluster and each battery pack.
The embodiments in the present specification are described in a progressive manner, and the same and similar parts among the embodiments are referred to each other, and each embodiment focuses on the differences from the other embodiments. In particular, the system or system embodiments are substantially similar to the method embodiments and therefore are described in a relatively simple manner, and reference may be made to some of the descriptions of the method embodiments for related points. The above-described system and system embodiments are only illustrative, wherein the units described as separate parts may or may not be physically separate, and the parts displayed as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of the present embodiment. One of ordinary skill in the art can understand and implement it without inventive effort.
Those of skill would further appreciate that the various illustrative elements and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both, and that the various illustrative components and steps have been described above generally in terms of their functionality in order to clearly illustrate this interchangeability of hardware and software. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the implementation. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (14)
1. An energy storage system, comprising: x current-sharing units and X battery clusters, wherein X is an integer greater than 1; wherein:
each of the battery clusters is connected in parallel;
each current equalizing unit is connected with each battery cluster in parallel in a one-to-one correspondence manner;
the current equalizing unit comprises at least two stages of DC/DC converters;
in each current equalizing unit, the first end of the uppermost DC/DC converter is connected with the direct current bus of the corresponding battery cluster; the second ends of the Y lowest-level DC/DC converters are respectively connected with corresponding battery packs in corresponding battery clusters; the second end of the uppermost DC/DC converter is directly connected with the first end of each lowermost DC/DC converter or is connected with the first end of each lowermost DC/DC converter through other DC/DC converters; y is the number of the battery packs in the battery cluster, and Y is larger than 1.
2. The energy storage system of claim 1, wherein the current share unit comprises: two-stage DC/DC converters, which are respectively an uppermost DC/DC converter and Y lowermost DC/DC converters; wherein:
first terminals of the respective lowermost DC/DC converters are connected to second terminals of the uppermost DC/DC converters, respectively.
3. The energy storage system according to claim 1 or 2, characterized in that each stage of DC/DC converter in the current equalizing unit is a unidirectional converter;
the first end of each DC/DC converter is the input end of the DC/DC converter, and the second end of each DC/DC converter is the output end of the DC/DC converter;
the uppermost DC/DC converter is a fixed voltage output, and the lowermost DC/DC converter is a fixed voltage input.
4. The energy storage system according to claim 1 or 2, wherein each stage of the DC/DC converter in the current equalizing unit is a bidirectional converter.
5. The energy storage system according to claim 1 or 2, wherein the uppermost DC/DC converter in each current equalizing unit is arranged in a control box of the corresponding battery cluster, and each lowermost DC/DC converter in each current equalizing unit is hung on the corresponding battery pack.
6. The energy storage system of claim 1 or 2, further comprising: energy storage converter PCS and two converge the unit, wherein:
the positive pole of the direct current bus of each battery cluster is correspondingly connected with each input end of one confluence unit, and the negative pole of the direct current bus of each battery cluster is correspondingly connected with each input end of the other confluence unit;
and the output ends of the two confluence units are respectively connected with the positive electrode and the negative electrode of the direct current side of the PCS.
7. A current sharing method for an energy storage system is characterized in that a battery management system BMS applied to the energy storage system according to any one of claims 1 to 6 comprises the following steps:
periodically determining the SOC change rate of each battery cluster in the energy storage system until the energy storage system is in a full charge/full discharge state;
judging whether the energy storage system needs current sharing adjustment or not according to the SOC change rate of each battery cluster;
if the energy storage system needs current sharing adjustment, the balance current value of each battery pack in the energy storage system is determined, and each balance current value is sent to the corresponding current sharing unit, so that the corresponding current sharing unit provides corresponding balance current for the corresponding battery pack by adjusting the Pulse Width Modulation (PWM) signal.
8. The current sharing method for the energy storage system according to claim 7, wherein if the energy storage system is in a charging process, the periodically determining the SOC change rate of each battery cluster in the energy storage system includes:
periodically taking the maximum value in the SOC of each battery pack of each battery cluster as the current SOC of each battery cluster, and calculating the ratio of the corresponding difference value of the current SOC and the previous SOC of each battery cluster to time;
if the energy storage system is in a discharging process, the periodically determining the SOC change rate of each battery cluster in the energy storage system includes:
periodically taking the minimum value in the SOC of each battery pack of each battery cluster as the current SOC of each battery cluster, and calculating the ratio of the corresponding difference value of the current SOC of each battery cluster and the previous SOC to time.
9. The current sharing method of the energy storage system according to claim 8, wherein the calculation formula of the SOC of each battery pack is as follows:
SOCijt＝SOCij0+Δt*(Ii1+…+Iin)+Δt*(Iij1+…+Iijm)，n*Δt＝t，m≤n；
the current sampling method comprises the steps of sampling current of a jth battery pack in an ith battery cluster, wherein SOCijt is the SOC of the jth battery pack in the ith battery cluster at a time t, SOCij0 is the SOC of the jth battery pack in the ith battery cluster at an initial time, Ii1 is a 1 st sampling current value of the ith battery cluster, Iin is an nth sampling current value of the ith battery cluster, Iij1 is a 1 st sampling current value of the jth battery pack in the ith battery cluster, Iijm is an mth sampling current value of the jth battery pack in the ith battery cluster, delta t is a sampling interval, n is the current sampling frequency of the battery cluster within a time 0-t, and m is the current sampling frequency of the battery pack within a time 0-t.
10. The current sharing method for the energy storage system according to claim 9, wherein if the energy storage system is in a charging process, the SOC of each battery pack at an initial time is a maximum value among the SOCs of each battery cell in each battery pack;
if the energy storage system is in a discharging process, the SOC of each battery pack at the initial moment is the minimum value of the SOC of each battery core in each battery pack;
the calculation formula of the SOC of each battery cell is as follows: and SOC/(SOH × Qn), where SOH is the battery health SOH of the battery cell, Q is the available capacity of the battery cell, and Qn is the rated capacity of the battery cell.
11. The method for current sharing in an energy storage system according to claim 7, wherein the determining whether the energy storage system needs current sharing adjustment according to the SOC change rate of each battery cluster includes:
taking the maximum value of the SOC change rates of each battery cluster as a first SOC change rate, taking the minimum value of the SOC change rates of each battery cluster as a second SOC change rate, and judging whether the difference value between the first SOC change rate and the second SOC change rate is smaller than or equal to a threshold value or not;
if the difference value between the first SOC change rate and the second SOC change rate is smaller than or equal to a threshold value, judging that each energy storage system does not need current sharing adjustment;
and if the difference value between the first SOC change rate and the second SOC change rate is larger than a threshold value, judging that each energy storage system needs current sharing adjustment.
12. The current sharing method of the energy storage system according to claim 7, wherein the sending each balancing current value to a corresponding current sharing unit so that the corresponding current sharing unit provides a corresponding balancing current for a corresponding battery pack by adjusting a PWM signal comprises:
and sending each balance current value to each lowest level DC/DC converter of each current equalizing unit so that each lowest level DC/DC converter of each current equalizing unit provides corresponding balance current for the corresponding battery pack by adjusting PWM signals.
13. The current sharing method for the energy storage system according to any one of claims 7 to 12, wherein after sending each of the balancing current values to the corresponding current sharing unit, the method further comprises:
and if the input voltage and the output voltage of the DC/DC converter at the lowest stage in the current equalizing unit are the same, controlling each battery pack to execute corresponding switching according to a preset switching strategy.
14. The current sharing method for the energy storage system according to claim 13, wherein the preset switching strategy includes:
when the energy storage system is in a charging process, cutting off each battery cluster from high to low according to the SOC until each battery cluster is cut off;
and when the energy storage system is in a discharging process, cutting off each battery cluster from low to high according to the SOC until each battery cluster is cut off.
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