A battery pack grid-connected balance control device and method based on a series power converter

Abstract

This invention discloses a battery pack grid-connected balancing control device and method based on a series power converter. On the basis of parallel droop control, a series I-P droop control is proposed to achieve stable power distribution. At the same time, SOC is introduced into the series droop control to achieve SOC balance, realizing the balance of battery module SOC, power and current. The balance factor is dynamically adjusted according to the range of SOC to adapt to the dynamically changing battery state and improve the response speed.

Description

Technical Field

[0001] This invention mainly relates to the field of energy storage and grid connection, specifically to a battery pack grid connection balance control device and method based on a series power converter. Background Technology

[0002] With the development of new energy power generation technologies, energy storage technology has become the most effective technical solution to address the problems of wind and solar power curtailment. In the large-scale application of energy storage systems, lithium-ion battery energy storage has the advantages of long service life and high flexibility, and is widely used in high-voltage DC energy storage systems such as electric vehicles, hybrid vehicles, and microgrids.

[0003] For high-power energy storage systems, multiple battery cells are typically connected in series and parallel to form a battery module in order to achieve high voltage levels. A battery pack contains multiple battery modules. Due to mismatches in battery parameters, series- and parallel battery modules suffer from poor consistency, state-of-charge (SOC) imbalance, and power mismatch losses. When the SOC of series-connected battery modules is unbalanced, if only one battery module is fully charged or discharged, the entire system will stop operating, reducing the utilization efficiency of the battery cells. On the other hand, continuing to charge or discharge when a battery module is fully charged or discharged can lead to overcharging or over-discharging, causing internal damage to some battery cells. Therefore, balancing the lithium-ion battery energy storage system is crucial for improving the utilization rate of battery cells and ensuring the safe and stable operation of the energy storage system.

[0004] The new generation of lithium-ion battery energy storage systems focuses on power electronic battery pack technology. Battery modules are cascaded with power electronic converters to control the charging or discharging current of the battery modules. Research has found that when a two-phase interleaved parallel bidirectional DC-DC converter is used for the system power converter, and the battery modules are connected to the low-voltage side, the current and bus voltage are self-stabilizing in discharge mode when the module controller independently controls the current. However, in charging mode, if the module controller independently controls the current, it will lead to unstable current and voltage divergence. Summary of the Invention

[0005] To address the aforementioned issues, this paper proposes a grid-connected balance control device and method for battery packs based on a series power converter, incorporating the concept of parallel droop control.

[0006] A battery pack grid-connected balance control method based on a series power converter is provided, including:

[0007] S1. Obtain the SOC value of the i-th battery module in the battery pack according to a predetermined cycle. i The battery pack comprises N battery modules, i=1,2...N;

[0008] S2. Based on the SOC value of the battery module. i And battery pack parameters, determine the duty cycle of multiple switching elements in each battery module;

[0009] S3. Based on the duty cycle of multiple switching elements in each battery module, control the corresponding switching elements to achieve grid-connected balance of the battery pack.

[0010] Further, in step S1, the SOC i The calculation formula is:

[0011] ,

[0012] Where SOC0 is the initial state of charge of the i-th battery module; Q is the rated capacity of the battery module; and d i I is the duty cycle of the switching element of the i-th battery module. o It is the bus-side cluster current.

[0013] Further, step S2 includes:

[0014] S21. Based on the battery pack parameters and the SOC value of the i-th battery module... i Calculate the reference output voltage V of the power converter for the i-th battery module. busi_ref ;

[0015] S22. Based on the reference output voltage V busi_ref and cluster current compensation coefficient k dp The droop coefficient g of the i-th battery module is obtained. di ;

[0016] S23. According to the droop coefficient g di Given the battery pack parameters, calculate the bus-side current I of the i-th battery module. oi_droop ;

[0017] S24. Calculate the battery-side reference current I of the i-th battery module. Bi_ref ,

[0018]

[0019] Among them, V busi V represents the bus-side voltage of the i-th battery module. Bi It is the actual battery-side voltage of the i-th battery module;

[0020] S25. Based on the actual battery-side current I of the i-th battery module Bi The duty cycle d is determined by the measured currents of the two inductors. 1_i and d 2_i .

[0021] Further, in step S21, the battery pack parameters include the DC-side voltage V of the battery pack. dc The average SOC value of the N battery modules avg The balance factor k of the i-th battery module pi The reference output voltage V busi_ref for

[0022] Furthermore, the balance factor k of the i-th battery module pi With SOC i The deviation value is positively correlated with the SOC value. i With SOC avg The difference.

[0023] Furthermore, in step S22, the cluster current compensation coefficient k dp Satisfying the relation:

[0024] ,

[0025] I o_ref I is the reference value for cluster current. set Preset cluster current reference value for each battery module.

[0026] Further, in step S23, the bus-side current I oi_droop for

[0027]

[0028] Among them, P Bi P is the actual battery-side power of the i-th battery module. Bi =V Bi I Bi V Bi I is the actual battery-side voltage of the i-th battery module. Bi Let I be the actual battery-side current of the i-th battery module. Bi .

[0029] Further, in step S25, the duty cycle d is determined. 1_i and d 2_i Specifically, this involves: setting the battery-side reference current I... Bi_ref Multiply by 0.5 to obtain the inductance reference current I flowing through the two inductors in the i-th battery module. Li_ref The inductor reference current I Li_ref The measured current I of the first inductor L1 L1_i After the difference is calculated, the duty cycle d is obtained through a PI controller. 1_i The inductor reference current I Li_refThe measured current I of the second inductor L2 L2_i After the difference is calculated, the duty cycle d is obtained through a PI controller. 2_i .

[0030] A control device is also provided, which employs a battery pack grid-connected balancing control method based on a series power converter. The device includes multiple sub-balancing control modules, each of which is cascaded with a low-voltage DC-DC power converter to form a battery power module. N is the number of battery power modules connected in series. The multiple sub-balancing control modules correspond one-to-one with the multiple battery modules of the battery pack. The sub-balancing control module includes a voltage balancing control module, an IP droop control module, and a current control module.

[0031] The voltage balance control module is used to obtain the converter's reference output voltage.

[0032] The IP droop control module is used to obtain the battery-side reference current.

[0033] The current control module is used to obtain the duty cycle.

[0034] Furthermore, the energy storage system of each battery module has two sets of switching units, each set of switching units including two pulse complementary switching elements. The current control module calculates the duty cycle of multiple switching elements, and the control device controls the corresponding multiple switching elements in each battery module according to the duty cycle of the multiple switching elements.

[0035] Compared with the prior art, the beneficial effects of the present invention are:

[0036] This invention combines the concept of parallel droop control with the introduction of series droop control to weaken the influence of circuit parameters, and introduces the battery module SOC into the droop control. The bus side voltage is distributed proportionally according to the SOC, thereby controlling the charging current. Thus, after the voltage distribution ratio is stable, the module power balance and SOC balance are achieved.

[0037] Furthermore, the coefficient kp is adaptively adjusted according to the SOC range to maintain a significant difference in the bus-side voltage of the battery modules, further reducing the required balancing time and improving the balancing speed. Under the same balancing parameters, the improved strategy requires less time, and the effect is more significant when the SOC range is small. Attached Figure Description

[0038] Figure 1 This is a circuit connection diagram of the battery pack grid-connected energy storage system based on a series power converter according to the present invention.

[0039] Figure 2 This is the IP droop characteristic curve of the cascaded modular energy storage system of this invention.

[0040] Figure 3 This is a control block diagram of the battery pack grid-connected balance control device of the present invention. Detailed Implementation

[0041] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0042] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0043] In the following description, terms such as “inner,” “outer,” “upper,” “lower,” “left,” and “right” are used only to indicate orientation or positional relationship for the convenience of describing the embodiments and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0044] Figure 1 This is a circuit connection diagram of the battery pack grid-connected energy storage system based on a series power converter according to the present invention. Multiple battery cells are connected in series to form battery module B. i (i=1,2,3……N). Each battery module is cascaded with a low-voltage DC-DC power converter to perform full power management, forming a battery power module, where N is the number of battery power modules connected in series. Each module integrates an independent balance control device for controlling the charging or discharging current, and the output side of each battery module is connected in series and integrated into the DC grid.

[0045] Ignoring power loss, the actual battery-side current I of the i-th battery module Bi It can be represented as

[0046] (1)

[0047] In formula (1), V busi V represents the bus-side voltage of the i-th battery module. Bi I is the actual battery-side voltage of the i-th battery module. busi This is the bus-side current of the i-th battery module. Since the output sides of each battery module are connected in series, the output current is equal to the bus-side cluster current I. o .

[0048] In discharge mode, the DC-DC power converter operates in boost mode. At this time, the relationship between the bus-side voltage of the i-th battery module and the battery module voltage is:

[0049] (2)

[0050] Where d i It is the duty cycle of the switching element of the i-th battery module.

[0051] If the battery current is positive in the specified discharge mode, then the battery module's SOC (State of Charge) is... i The expression is:

[0052] (3)

[0053] SOC o Q is the initial state of charge of the i-th battery module; Q is the rated capacity of the battery module, and the rated capacity of each battery module in the battery pack is the same.

[0054] Substituting formulas (1) and (2) into formula (3), we get:

[0055] (4)

[0056] As can be seen from the above formula (4), when determining the power, the SOC of each battery module is determined by the duty cycle of the battery module. In discharge mode, for battery modules with higher SOC, the bus-side voltage should be larger to increase the duty cycle; conversely, in charging mode, for battery modules with higher SOC, the bus-side voltage should be smaller, thereby shortening the duty cycle. Therefore, by controlling the duty cycle of the battery power module, the charging or discharging current can be adjusted to achieve SOC balance of the battery module.

[0057] Figure 2 The IP droop characteristic curves of the cascaded modular energy storage system of this invention are shown. The vertical axis of the coordinate system represents the cluster current, and the horizontal axis represents the battery capacity. The current bias point of different droop characteristic curves at zero voltage is set to the same point I. set To distribute power, the droop coefficient g of different battery modules is... di Designed differently. Droop characteristics and current reference I. o_ref Horizontal lines intersect at different points. The larger the sag factor, the more negligible the original impedance becomes. If it is too large, the current variation range during power fluctuations will be too large, and the impedance difference will be too small, which is not conducive to power distribution; if it is too small, the allowable power range will be too large, making it difficult to achieve stable power distribution.

[0058] Droop control will cause the cluster current to deviate from the reference value I. o_ref In this balancing strategy, the current offset is pre-compensated to the cluster current command value I. set In this process, the bus-side current reference is made close to the cluster current reference value I. o_ref Therefore, the bus-side current I after droop control oi_droop The reference expression is:

[0059] (5)

[0060] Where k dp It is the cluster current compensation coefficient, P Bi P is the actual battery-side power of the i-th battery module. Bi =V Bi I Bi V Bi I is the actual battery-side voltage of the i-th battery module. Bi Let I be the actual battery-side current of the i-th battery module. Bi .

[0061] Figure 3 This is a control block diagram of the battery pack grid-connected balancing control device of the present invention. It includes N sub-balancing control modules, each corresponding one-to-one with one of the N battery modules. Each sub-balancing control module consists of three main modules: a voltage balancing control module, an IP droop control module, and a current control module.

[0062] The voltage balance control module is used to obtain the converter's reference output voltage.

[0063] The IP droop control module is used to obtain the battery-side reference current.

[0064] The current control module is used to obtain the duty cycle.

[0065] Taking the output control process of the i-th sub-balance control module as an example, the i-th self-balancing control module corresponds to the i-th battery module and includes the i-th voltage balance control module, the i-th IP droop control module, and the i-th current control module.

[0066] The reference output voltage V of the i-th voltage balance control module busi_ref for:

[0067] (6)

[0068] The average SOC value is obtained by summing the SOC values ​​of all battery modules and dividing by the number of battery modules N. avg The SOC value of the i-th module is SOC i .

[0069] k p As an introduced balance factor, K corresponds to each battery module. p They are different. The SOC of the i-th battery module i Value and SOC avg The larger the difference, the greater the balance factor K corresponding to the i-th battery module. piThe value is also correspondingly larger to accelerate the charging speed of the module. k is set according to the difference in SOC size. p This can accelerate the process of achieving SOC balance among various battery modules. The voltage balancing module incorporates SOC into voltage balancing control, allowing for differentiated settings of the reference output voltage V for each module. busi_ref It accelerates charging speed and achieves SOC balance.

[0070] The bus-side reference current I of the i-th IP droop control module oi_droop for

[0071] (7)

[0072] Among them, P Bi Let k be the actual battery-side power of the i-th battery module. dp For the current compensation coefficient, g di V is the droop coefficient of the i-th battery module. Bi I is the actual battery-side voltage of the i-th battery module. Bi Let P be the actual battery-side current of the i-th battery module. Bi =V Bi ×I Bi .

[0073] In the IP droop control module, the current offset is pre-compensated to the converter cluster current I. o In the middle, the converter cluster current I o Approximate reference output current I o_ref Set the reference current for each battery module to the same value I. set , then I o_ref with I set Existence Relationship:

[0074] (8)

[0075] By compensating the target reference current for droop control in advance to the output current during droop control, the control response speed can be accelerated.

[0076] The battery-side reference current I of the i-th battery module Bi_ref ,

[0077] (9)

[0078] Among them, V busi This represents the bus-side voltage of the i-th battery module.

[0079] In the current control module, from Figure 1As can be seen from the energy storage system circuit diagram, each battery pack is connected to a converter. The converter can be simply viewed as consisting of two BUCK-BOOST circuits: BUCK-BOOST1 is composed of capacitor C1, capacitor C2, inductor L1, and switching elements S1 and S2; BUCK-BOOST2 is composed of capacitor C1, capacitor C2, inductor L2, and switching elements S3 and S4. The battery pack output current I... B Half of this is the magnitude of the current flowing through the inductor, and the inductor reference current I flowing through the two inductors in the i-th battery module. Li_ref Represented as: I Li_ref =I Bi_ref ×0.5. Where, I Bi_ref This is the battery-side reference current of the i-th battery module. Each switching element consists of a transistor and a diode connected in anti-parallel.

[0080] In the i-th current control module, d 1_i and d 2_i These represent the duty cycles of switching elements S1 and S3 in the i-th battery module. The i-th battery module includes four switching elements S1-S4. The pulses of switching elements S1 and S2 are complementary, and the pulses of switching elements S3 and S4 are complementary. Li_ref The real-time inductor current is compared with the measured current I of the first inductor L1. L1_i The measured current I of the second inductor L2 L2_i After the difference is calculated, the values ​​are input to the PI controller G. pi(s) The duty cycle d of S1 is obtained by controlling the process. 1_i Duty cycle d of S3 2_i The duty cycle of the converter can be adjusted using a PI controller to achieve current control.

[0081] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A battery pack grid-connected balance control method based on a series power converter, characterized in that, include: S1. Obtain the SOC value of the i-th battery module in the battery pack according to a predetermined cycle. i The battery pack comprises N battery modules, i=1,2...N; S2. Based on the SOC value of the battery module. i The battery pack parameters are used to determine the duty cycle of multiple switching elements in each battery module; each battery module is cascaded with a low-voltage DC-DC power converter to form a battery power module; the energy storage system of each battery module has two sets of switching units, each set of switching units including two pulse complementary switching elements; S3. Based on the duty cycle of multiple switching elements in each battery module, control the corresponding switching elements to achieve grid-connected balance of the battery pack; In step S1, the SOC i The calculation formula is: ， Where SOC0 is the initial state of charge of the i-th battery module; Q is the rated capacity of the battery module; and d i I is the duty cycle of the switching element of the i-th battery module. o It is the bus-side cluster current; Step S2 includes: S21. Based on the battery pack parameters and the SOC value of the i-th battery module... i Calculate the reference output voltage V of the power converter for the i-th battery module. busi_ref ; S22. Based on the reference output voltage V busi_ref and cluster current compensation coefficient k dp The droop coefficient g of the i-th battery module is obtained. di The cluster current compensation coefficient kdp satisfies the following relationship: ， Io_ref is the cluster current reference value, and Iset is the preset cluster current reference value for each battery module; S23. According to the droop coefficient g di The actual power P on the battery side of the i-th battery module Bi and the preset cluster current reference value I for each battery module set Calculate the bus-side current I of the i-th battery module. oi_droop ; The bus-side current I oi_droop for ， Among them, P Bi P is the actual battery-side power of the i-th battery module. Bi =V Bi I Bi V Bi I is the actual battery-side voltage of the i-th battery module. Bi This represents the actual battery-side current of the i-th battery module. S24. Calculate the battery-side reference current I of the i-th battery module. Bi_ref , ， Among them, V busi V represents the bus-side voltage of the i-th battery module. Bi It is the actual battery-side voltage of the i-th battery module; S25. Based on the actual battery-side current I of the i-th battery module Bi The duty cycle d is determined by the measured currents of the two inductors. 1_i and d 2_i Specifically, this involves determining the duty cycle d. 1_i and d 2_i Specifically, this involves: setting the battery-side reference current I... Bi_ref Multiply by 0.5 to obtain the inductance reference current I flowing through the two inductors in the i-th battery module. Li_ref The inductor reference current I Li_ref The measured current I of the first inductor L1 L1_i After the difference is calculated, the duty cycle d is obtained through a PI controller. 1_i The inductor reference current I Li_ref The measured current I of the second inductor L2 L2_i After the difference is calculated, the duty cycle d is obtained through a PI controller. 2_i .

2. The control method according to claim 1, characterized in that: In step S21, the battery pack parameters include the DC-side voltage V of the battery pack. dc The average SOC value of the N battery modules avg The balance factor k of the i-th battery module pi The reference output voltage V busi_ref for 。 3. The control method according to claim 2, characterized in that: The balance factor k of the i-th battery module pi With SOC i The deviation value is positively correlated with the SOC value. i With SOC avg The difference.

4. A control device for implementing the battery pack grid-connected balance control method based on a series power converter as described in any one of claims 1-3, characterized in that, It includes multiple sub-balance control modules, each battery module cascaded with a low-voltage DC-DC power converter to form a battery power module, where N is the number of battery power modules connected in series. Each sub-balance control module corresponds one-to-one with a battery module in the battery pack. Each sub-balance control module includes a voltage balance control module, an IP droop control module, and a current control module. The voltage balance control module is used to obtain the converter's reference output voltage. The IP droop control module is used to obtain the battery-side reference current. The current control module is used to obtain the duty cycle.

5. The control device according to claim 4, characterized in that: The energy storage system of each battery module has two sets of switching units. Each set of switching units includes two pulse-complementary switching elements. The current control module calculates the duty cycle of multiple switching elements, and the control device controls the corresponding multiple switching elements in each battery module according to the duty cycle of the multiple switching elements.

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