A so c equalization control method for multiple energy storage units operating in parallel

Abstract

The application discloses a SoC equalization control method for multiple energy storage units in parallel operation, which comprises the following steps: estimating the SoC of the i-th energy storage unit by using the ampere-hour integration method; obtaining the estimated SoC of other energy storage units through a low-bandwidth communication line, and calculating the average SoC of the n energy storage units in parallel operation; adaptively adjusting the droop coefficient by using the SoC equalization control algorithm to obtain the dynamic correction droop coefficient of the i-th energy storage unit; obtaining the reference output voltage of the converter of the i-th energy storage unit, and obtaining the reference output current of the converter of the i-th energy storage unit by using the voltage loop PI control method; obtaining the switch duty cycle of the converter of the i-th energy storage unit by using the current loop PI control method; and controlling the actions of the switches of the converters of the energy storage units through complementary pulse signals to realize the SoC equalization of the energy storage units in parallel operation. The SoC equalization control method can accurately equalize the SoCs of the energy storage units in parallel operation.

Description

Technical Field

[0001] This invention belongs to the field of DC microgrid operation control and relates to a SoC equalization control method for parallel operation of multiple energy storage units. Background Technology

[0002] In recent years, the "dual carbon" target has driven the global power system towards a green and low-carbon transformation, bringing new opportunities for the development of microgrid technology. Compared to AC microgrids, DC microgrids do not require consideration of frequency control, reactive power compensation, harmonic suppression, and other issues, and have been widely used in distributed renewable energy generation scenarios. Because distributed power sources such as solar and wind power are intermittent, random, and fluctuating, multiple distributed energy storage units must be equipped to ensure the efficient absorption of renewable energy.

[0003] Considering safety and redundancy, multiple distributed energy storage units are usually connected to a DC microgrid in parallel. However, the state of charge (SoC) balancing control of multiple parallel energy storage units has always been a difficult point in the field of DC microgrid operation control. The main reasons are: (1) the initial SoC of energy storage units with different capacities is often inconsistent, making it difficult to achieve SoC balancing control during operation; (2) the initial SoC of energy storage units with the same capacity is inconsistent due to differences in manufacturing process, installation method, measurement results, etc. Even if the initial SoC is consistent, the SoC will gradually diverge due to the different line impedance between the output terminal of each energy storage converter and the DC bus; (3) the droop control method is often used as the current distribution method for multiple parallel energy storage units, but the traditional droop control method does not consider SoC information and cannot achieve SoC balancing during the operation of parallel energy storage units. The imbalance of SoC among parallel energy storage units can easily lead to the following two problems: (1) If no SoC critical threshold is set, some energy storage units will be overcharged or deeply discharged, thereby shortening the service life of the energy storage units; (2) If a SoC critical threshold is set, the energy storage units that reach the SoC critical threshold will be forced to exit the DC microgrid, thereby increasing the load burden on the remaining energy storage units, accelerating their performance degradation, and potentially causing system instability.

[0004] To achieve dynamic SoC balancing of parallel energy storage units during operation, the literature... [1]The paper "Double-quadrant state-of-charge-based droop control method for distributed energy storage systems in autonomous DC microgrids" designs the droop coefficient to be directly / inversely proportional to the nth power of the SoC during discharge / charge to achieve dynamic SoC balancing, and adjusts the SoC balancing speed by changing the value of n. However, this method leads to a significant bus voltage drop. (Reference: [link to literature]) [2] The paper "Distributed Secondary Level Control for Energy Storage Management in DC Microgrids" correlates System-on-Chips (SoC) with an exponential function, utilizing the fast convergence characteristic of the exponential function to adjust the droop coefficient and achieve rapid SoC balancing. However, this method does not consider the case of inconsistent energy storage cell capacities. (Reference...) [3] The paper "Improved Load Current Allocation Method for Distributed Energy Storage Systems with SOC Droop Control" combines exponential and power functions to further accelerate the SoC convergence speed of each energy storage unit. However, this method requires adjusting a large number of equalization parameters, increasing the complexity of the control algorithm. (Reference: ...) [4] The paper "Dynamic Equalization Strategy for Multiple Energy Storage SoCs in DC Microgrids Based on Adaptive Droop Control" adaptively adjusts the droop coefficient by combining the SoC with the arctangent function. It utilizes the limiting characteristic of the arctangent function to avoid excessive droop coefficient variation, but this reduces the SoC equalization speed. (Reference: ...) [5] The paper "N. Zhi, K. Ding, L. Du, and H. Zhang. An SOC-based virtual DC machine control for distributed storage systems in DC microgrids" uses the SoC of each energy storage unit as input to a fuzzy controller to adaptively adjust the droop coefficient to achieve dynamic SoC balancing. However, the complexity of the fuzzy rules increases significantly with the number of parallel energy storage units, making it difficult to apply in practice. (Reference: ...) [6] The SoC imbalance function is defined and used as the power exponent of the average SoC. SoC balancing is achieved by adaptively adjusting the droop coefficient, but this method results in a slow SoC balancing speed.

[0005] The commonality among the above-mentioned SoC equalization control methods lies in their relatively fast SoC equalization speed in the early stages, but very slow speed in the later stages. This is because the SoC differences among the energy storage units are significant in the early stages of SoC equalization. The aforementioned SoC equalization control methods can adjust the droop coefficient of each energy storage unit by amplifying this difference. Different droop coefficients result in different output currents of the converters for each energy storage unit, thus affecting the rate of SoC change for each unit. However, in the later stages of SoC equalization, the SoCs of each energy storage unit gradually converge. The aforementioned SoC equalization control methods can no longer adjust the droop coefficient by amplifying the SoC equalization differences, causing the droop coefficients and converter output currents of each energy storage unit to become more uniform. This leads to a bottleneck in the SoC equalization process, ultimately making it difficult to achieve accurate SoC equalization results. Therefore, designing a simple and effective control method to ensure fast SoC equalization speeds in both the early and later stages of equalization has become crucial for the research on SoC equalization control of multi-parallel energy storage units. Summary of the Invention

[0006] To address the aforementioned problems, the present invention provides a SoC equalization control method for multiple energy storage units operating in parallel, comprising the following steps:

[0007] S1: Measure and collect the battery output current of the i-th energy storage unit, and estimate the SoC of the i-th energy storage unit using the ampere-hour integration method;

[0008] S2: Each energy storage unit transmits the estimated SoC to other energy storage units through a low-bandwidth communication line, and obtains the estimated SoC of other energy storage units through the low-bandwidth communication line, and calculates the average SoC of n parallel-running energy storage units locally.

[0009] S3: Based on the SoC and average SoC of the i-th energy storage unit, the droop coefficient is adaptively adjusted using the SoC equalization control algorithm to obtain the dynamic correction droop coefficient of the i-th energy storage unit.

[0010] S4: Combining droop control technology, the converter output current of the i-th energy storage unit is multiplied by the dynamically corrected droop coefficient to obtain the bus voltage drop of the i-th energy storage unit, and the difference between the rated bus voltage and the bus voltage drop of the i-th energy storage unit is obtained to obtain the converter reference output voltage of the i-th energy storage unit.

[0011] S5: After subtracting the converter reference output voltage of the i-th energy storage unit from the converter output voltage of the i-th energy storage unit, the voltage loop PI control method is used to obtain the converter reference output current of the i-th energy storage unit.

[0012] S6: After subtracting the converter reference output current of the i-th energy storage unit from the converter inductor current of the i-th energy storage unit, the duty cycle of the converter switch of the i-th energy storage unit is obtained by using the current loop PI control method.

[0013] S7: The duty cycle of the i-th energy storage unit is generated using pulse width modulation technology to obtain a pair of complementary pulse signals. Each energy storage unit converter controls the operation of its own switching transistor through the complementary pulse signals to achieve SoC equalization of each parallel-operating energy storage unit.

[0014] Furthermore, the estimation of the SoC of the i-th energy storage unit using the ampere-hour integration method employs the following formula:

[0015]

[0016] In the formula, SoC i For the current SoC of the i-th energy storage unit, SoC i0 For the initial SoC of the i-th energy storage unit, C bati Let I be the rated capacity of the i-th energy storage unit. bati Let be the output current of the i-th energy storage unit.

[0017] Furthermore: the formula for calculating the average SoC of n parallel-operated energy storage units is as follows:

[0018]

[0019] In the formula, SoC avg denoted as the average SoC of the parallel energy storage units, and n is the number of parallel energy storage units.

[0020] Furthermore: the SoC and average SoC input SoC equalization control module based on the i-th energy storage unit adaptively adjusts the droop coefficient using the SoC equalization control algorithm of the present invention to obtain the dynamically corrected droop coefficient of the i-th energy storage unit; the specific process is as follows:

[0021] The droop coefficient of the i-th energy storage unit is designed as an adaptive variable incorporating the SoC equalization control algorithm:

[0022]

[0023] In the formula, R vdi R is the adaptive droop coefficient for the i-th energy storage unit. vi ΔSoC represents the initial droop factor for each energy storage unit. i ΔSoC represents the SoC equalization difference for the i-th energy storage unit. i =SoC avg -SoC i m is the equalization speed regulation factor less than 1, Idci >0 indicates a discharge process, I dci <0 indicates the charging process.

[0024] When the energy storage unit discharges, taking two energy storage units connected in parallel as an example, the relationship between the converter output current and the droop coefficient is as follows:

[0025]

[0026] In the formula, I dc1 I is the output current of the first energy storage unit converter. dc2 R is the output current of the second energy storage unit converter. vd1 R is the adaptive droop factor for the first energy storage unit. vd2 R is the adaptive droop factor for the second energy storage unit. v1 R is the initial droop factor for the first energy storage unit. v2 ΔSoC1 is the initial droop coefficient of the second energy storage unit, ΔSoC2 is the SoC balance difference of the first energy storage unit, and ΔSoC2 is the SoC balance difference of the second energy storage unit.

[0027] When the energy storage unit is charging, taking two energy storage units connected in parallel as an example, the relationship between the converter output current and the droop coefficient is as follows:

[0028]

[0029] In the formula, I dc1 I is the output current of the first energy storage unit converter. dc2 R is the output current of the second energy storage unit converter. vd1 R is the adaptive droop factor for the first energy storage unit. vd2 R is the adaptive droop factor for the second energy storage unit. v1 R is the initial droop factor for the first energy storage unit. v2 ΔSoC1 represents the initial droop coefficient of the second energy storage unit, ΔSoC2 represents the SoC balance difference of the first energy storage unit, and ΔSoC2 represents the SoC balance difference of the second energy storage unit.

[0030] Furthermore: The converter reference output voltage for the i-th energy storage unit is calculated using the droop control technology.

[0031] U refi =U nom -R vdi I dci (6)

[0032] In the formula, U refi U is the converter reference output voltage for the i-th energy storage unit. nom This is the rated bus voltage.

[0033] Further: The converter reference output current for calculating the i-th energy storage unit is:

[0034]

[0035] In the formula, I refi U is the converter reference output current for the i-th energy storage unit. dci Let k be the output voltage of the i-th energy storage unit converter. PU k is the proportional gain of the voltage loop PI controller. IU 1 / s represents the integral coefficient of the voltage loop PI controller.

[0036] Further: The calculation of the duty cycle of the converter switch of the i-th energy storage unit:

[0037]

[0038] In the formula, D i Let I be the duty cycle of the switch transistor in the i-th energy storage unit converter. Li Let k be the inductor current of the i-th energy storage unit converter. PI k is the proportional gain of the current loop PI controller. II represents the integral coefficient of the current loop PI controller.

[0039] Furthermore: the duty cycle input of the i-th energy storage unit is based on pulse width modulation generator technology to obtain a pair of complementary pulse signals. Each energy storage unit converter controls the operation of its respective switching transistor through the complementary pulse signals to achieve SoC equalization of each parallel-operating energy storage unit. Specifically, this includes:

[0040] Each energy storage unit converter controls the operation of the switching transistors through complementary pulse signals, so that the energy storage unit with a higher SoC has a larger discharge current during the discharge process and a smaller charging current during the charging process; the energy storage unit with a lower SoC has a smaller discharge current during the discharge process and a larger charging current during the charging process, until the output current of each parallel-operating energy storage unit converter is distributed proportionally to its rated capacity, ultimately achieving SoC balance among the parallel-operating energy storage units.

[0041] A SoC equalization control device for multiple energy storage units operating in parallel includes the following steps:

[0042] Acquisition module: used to measure and acquire the battery output current of the i-th energy storage unit, and to estimate the SoC of the i-th energy storage unit using the ampere-hour integration method;

[0043] Average SoC Calculation Module: This module is used to transmit the estimated SoC of each energy storage unit to other energy storage units through low-bandwidth communication lines, and to obtain the estimated SoC of other energy storage units through low-bandwidth communication lines, and to calculate the average SoC of n parallel-running energy storage units locally.

[0044] SoC equalization control module: Based on the SoC and average SoC of the i-th energy storage unit, the SoC equalization control algorithm is used to adaptively adjust the droop coefficient to obtain the dynamic correction droop coefficient of the i-th energy storage unit.

[0045] Droop control module: It is used to combine droop control technology to multiply the converter output current of the i-th energy storage unit by the dynamically corrected droop coefficient to obtain the bus voltage drop of the i-th energy storage unit, and to obtain the converter reference output voltage of the i-th energy storage unit by subtracting the rated bus voltage from the bus voltage drop of the i-th energy storage unit.

[0046] Voltage loop PI controller module: It is used to obtain the converter reference output current of the i-th energy storage unit by taking the difference between the converter reference output voltage of the i-th energy storage unit and the converter output voltage of the i-th energy storage unit, and then using the voltage loop PI control method.

[0047] Kongming's current loop PI controller: It is used to obtain the duty cycle of the converter switching transistor of the i-th energy storage unit by taking the difference between the converter reference output current of the i-th energy storage unit and the converter inductor current of the i-th energy storage unit and then using the current loop PI control method.

[0048] Pulse Width Modulation Generator Module: This module generates a pair of complementary pulse signals based on pulse width modulation technology, using the duty cycle of the i-th energy storage unit. Each energy storage unit converter controls the operation of its respective switching transistor through the complementary pulse signals, thereby achieving SoC equalization of the parallel-operating energy storage units.

[0049] The present invention provides a SoC equalization control method for parallel operation of multiple energy storage units, which has the following advantages:

[0050] (1) The SoC equalization control method of the present invention can achieve rapid SoC equalization of multiple parallel energy storage units in both the early and late stages of SoC equalization.

[0051] (2) The equalization speed regulation factor m introduced in the SoC equalization control method of the present invention only plays a major role in the later stage of SoC equalization, which avoids the coupling effect of the difference in SoC equalization state on the SoC equalization speed in the early stage of SoC equalization.

[0052] (3) The SoC equalization control method of the present invention can realize that the output current of each parallel energy storage unit converter is proportionally distributed according to the rated capacity of each energy storage unit, and can realize the precise SoC equalization of each parallel energy storage unit. Attached Figure Description

[0053] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0054] Figure 1 This is a flowchart of the implementation steps of the present invention;

[0055] Figure 2 This is a diagram of the DC microgrid structure of the present invention;

[0056] Figure 3 This is a control block diagram of the i-th energy storage unit converter of the present invention;

[0057] Figure 4 (a) is the energy storage unit SoC of Embodiment 1 of the present invention, and (b) is the output current simulation waveform of Embodiment 1 of the present invention;

[0058] Figure 5 (a) is the energy storage unit SoC of Embodiment 2 of the present invention, and (b) is the simulated waveform of the output current of Embodiment 2 of the present invention;

[0059] Figure 6 (a) is the energy storage unit SoC of Embodiment 3 of the present invention, and (b) is the output current simulation waveform of Embodiment 3 of the present invention. Detailed Implementation

[0060] It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0061] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0062] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0063] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps described in these embodiments do not limit the scope of the invention. It should also be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following figures denote similar items; therefore, once an item is defined in one figure, it need not be further discussed in subsequent figures.

[0064] In the description of this invention, it should be understood that the orientation or positional relationship indicated by directional terms such as "front, back, up, down, left, right", "horizontal, vertical, horizontal" and "top, bottom" is generally based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing this invention and simplifying the description. Unless otherwise stated, these directional terms do not 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 on the scope of protection of this invention. The directional terms "inner" and "outer" refer to the inner and outer contours relative to the outline of each component itself.

[0065] For ease of description, spatial relative terms such as "above," "over," "on the upper surface of," "above," etc., are used herein to describe the spatial positional relationship of a device or feature as shown in the figures to other devices or features. It should be understood that spatial relative terms are intended to encompass different orientations in use or operation besides the orientation of the device as described in the figures. For example, if the device in the figures is inverted, a device described as "above" or "above" other devices or structures would subsequently be positioned as "below" or "under" other devices or structures. Thus, the exemplary term "above" can include both "above" and "below." The device may also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatial relative descriptions used herein will be interpreted accordingly.

[0066] Furthermore, it should be noted that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0067] Figure 1 This is a flowchart illustrating the implementation steps of the present invention, providing a SoC equalization control method for multiple energy storage units operating in parallel, comprising the following steps:

[0068] S1: Measure and collect the battery output current of the i-th energy storage unit, and estimate the SoC of the i-th energy storage unit using the ampere-hour integration method;

[0069] S2: Each energy storage unit transmits the estimated SoC to other energy storage units through a low-bandwidth communication line, and obtains the estimated SoC of other energy storage units through the low-bandwidth communication line, and calculates the average SoC of n parallel-running energy storage units locally.

[0070] S3: Based on the SoC and average SoC of the i-th energy storage unit, the droop coefficient is adaptively adjusted using the SoC equalization control algorithm to obtain the dynamic correction droop coefficient of the i-th energy storage unit.

[0071] S4: Combining droop control technology, the converter output current of the i-th energy storage unit is multiplied by the dynamically corrected droop coefficient to obtain the bus voltage drop of the i-th energy storage unit, and the difference between the rated bus voltage and the bus voltage drop of the i-th energy storage unit is obtained to obtain the converter reference output voltage of the i-th energy storage unit.

[0072] S5: After subtracting the converter reference output voltage of the i-th energy storage unit from the converter output voltage of the i-th energy storage unit, the voltage loop PI control method is used to obtain the converter reference output current of the i-th energy storage unit.

[0073] S6: After subtracting the converter reference output current of the i-th energy storage unit from the converter inductor current of the i-th energy storage unit, the duty cycle of the converter switch of the i-th energy storage unit is obtained by using the current loop PI control method.

[0074] S7: The duty cycle of the i-th energy storage unit is generated using pulse width modulation technology to obtain a pair of complementary pulse signals. Each energy storage unit converter controls the operation of its own switching transistor through the complementary pulse signals to achieve SoC equalization of each parallel-operating energy storage unit.

[0075] The above-described embodiments of the present invention provide a SoC equalization control method for parallel operation of multiple energy storage units. Each energy storage unit locally collects its battery output current, estimates its current SoC value using the ampere-hour integral method, and obtains the SoC values ​​of other energy storage units through a low-bandwidth communication line. The average SoC value of all energy storage units is calculated locally, and the SoC equalization control algorithm of the present invention adaptively adjusts the droop coefficient. Combined with droop control technology and voltage-current dual closed-loop control technology, the operation of the converter switching transistors is controlled by PWM to achieve SoC equalization of each parallel energy storage unit. This SoC equalization control method for parallel operation of multiple energy storage units can achieve rapid SoC equalization in both the early and late stages of equalization, regardless of whether the rated capacities of the parallel energy storage units are the same. When the rated capacities of the parallel energy storage units are the same, the output current of the converters for each parallel energy storage unit can be evenly distributed; when the rated capacities of the parallel energy storage units are different, the output current of the converters for each parallel energy storage unit can be reasonably distributed according to the ratio of the rated capacities of the energy storage units.

[0076] Wherein, the estimation of the SoC of the i-th energy storage unit using the ampere-hour integration method adopts the following formula:

[0077] In the formula, SoC i For the current SoC of the i-th energy storage unit, SoC i0 For the initial SoC of the i-th energy storage unit, C bati Let I be the rated capacity of the i-th energy storage unit. bati Let be the output current of the i-th energy storage unit.

[0078] The formula for calculating the average SoC of n parallel-operated energy storage units is as follows:

[0079]

[0080] In the formula, SoC avg denoted as the average SoC of the parallel energy storage units, and n is the number of parallel energy storage units.

[0081] Among them, the specific steps of step S3 include: designing the droop coefficient of the i-th energy storage unit as an adaptive change amount including the SoC balancing control algorithm:

[0082]

[0083] In the formula, R vdi is the adaptive droop coefficient of the i-th energy storage unit, R vi is the initial droop coefficient of each energy storage unit, ΔSoC i is the SoC balancing difference of the i-th energy storage unit, ΔSoC i = SoC avg - SoC i , m is a balancing speed regulation factor less than 1, I dci > 0 indicates the discharging process, I dci <0 indicates the charging process;

[0084] When the energy storage unit discharges, taking two parallel energy storage units as an example, the relationship between the converter output current and the droop coefficient is:

[0085]

[0086] In the formula, I dc1 is the output current of the converter of the first energy storage unit, I dc2 is the output current of the converter of the second energy storage unit, R vd1 is the adaptive droop coefficient of the first energy storage unit, R vd2 is the adaptive droop coefficient of the second energy storage unit, R v1 is the initial droop coefficient of the first energy storage unit, R v2 is the initial droop coefficient of the second energy storage unit, ΔSoC1 is the SoC balancing difference of the first energy storage unit, and ΔSoC2 is the SoC balancing difference of the second energy storage unit.

[0087] Assume that in the above formula R v1 = R v2 and SoC1 < SoC2. Then, during the discharging process of the energy storage unit, the SoC balancing difference ΔSoC2 of the second energy storage unit with a higher SoC < 0, and its droop coefficient R vd2 is smaller, so its converter output current I dc2 is larger; the SoC balancing difference ΔSoC1 of the first energy storage unit with a lower SoC > 0, and its droop coefficient R vd1 is larger, so its converter output current I dc1 is smaller;

[0088] When the energy storage unit charges, taking two parallel energy storage units as an example, the relationship between the converter output current and the droop coefficient is:

[0089]

[0090] Where, I dc1 is the output current of the first energy storage unit converter, I dc2 is the output current of the second energy storage unit converter, R vd1 is the adaptive droop coefficient of the first energy storage unit, R vd2 is the adaptive droop coefficient of the second energy storage unit, R v1 is the initial droop coefficient of the first energy storage unit, R v2 is the initial droop coefficient of the second energy storage unit, ΔSoC1 is the SoC balance difference of the first energy storage unit, and ΔSoC2 is the SoC balance difference of the second energy storage unit.

[0091] Assume that in the above formula R v1 = R v2 and SoC1 < SoC2. Then, during the charging process of the energy storage unit, the SoC balance difference ΔSoC2 of the second energy storage unit with a higher SoC is less than 0, and its droop coefficient R vd2 is larger, so the output current I dc2 of its converter is smaller; the SoC balance difference ΔSoC1 of the first energy storage unit with a lower SoC is greater than 0, and its droop coefficient R vd1 is smaller, so the output current I dc1 of its converter is larger;

[0092] Formulas (4) and (5) also show that in the early stage of SoC balance, |ΔSoC i | is much greater than zero, and the influence of m on the SoC balance speed is small. The SoC balance speed in this stage is mainly determined by |ΔSoC i |; in the later stage of SoC balance, |ΔSoC i | tends to zero, and the influence of m on the SoC balance speed is large. The SoC balance speed in this stage is mainly determined by m.

[0093] Among them, the step S4 specifically includes: combining the droop control technology to calculate the reference output voltage of the converter of the i-th energy storage unit:

[0094] U refi = U nom - R vdi I<00​​​​​​​​​​Specifically, step S5 includes: calculating the converter reference output current of the i-th energy storage unit:

[0097]

[0098] In the formula, I refi U is the converter reference output current for the i-th energy storage unit. dci Let k be the output voltage of the i-th energy storage unit converter. PU k is the proportional gain of the voltage loop PI controller. IU 1 / s represents the integral coefficient of the voltage loop PI controller.

[0099] Specifically, step S6 includes: calculating the duty cycle of the converter switch of the i-th energy storage unit.

[0100]

[0101] In the formula, D i Let I be the duty cycle of the switch transistor in the i-th energy storage unit converter. Li Let k be the inductor current of the i-th energy storage unit converter. PI k is the proportional gain of the current loop PI controller. II represents the integral coefficient of the current loop PI controller.

[0102] The duty cycle input of the i-th energy storage unit is based on pulse width modulation generator technology to obtain a pair of complementary pulse signals. Each energy storage unit converter controls the operation of its respective switching transistor through the complementary pulse signals to achieve SoC equalization of the parallel-operating energy storage units. Specifically, this includes:

[0103] Each energy storage unit converter controls the operation of the switching transistors through complementary pulse signals, so that the energy storage unit with a higher SoC has a larger discharge current during the discharge process and a smaller charging current during the charging process; the energy storage unit with a lower SoC has a smaller discharge current during the discharge process and a larger charging current during the charging process, until the output current of each parallel-operating energy storage unit converter is distributed proportionally to its rated capacity, ultimately achieving SoC balance among the parallel-operating energy storage units.

[0104] A SoC equalization control device for multiple energy storage units operating in parallel includes the following steps:

[0105] Acquisition module: used to measure and acquire the battery output current of the i-th energy storage unit, and to estimate the SoC of the i-th energy storage unit using the ampere-hour integration method;

[0106] Average SoC Calculation Module: This module is used to transmit the estimated SoC of each energy storage unit to other energy storage units through low-bandwidth communication lines, and to obtain the estimated SoC of other energy storage units through low-bandwidth communication lines, and to calculate the average SoC of n parallel-running energy storage units locally.

[0107] SoC equalization control module: Based on the SoC and average SoC of the i-th energy storage unit, the SoC equalization control algorithm is used to adaptively adjust the droop coefficient to obtain the dynamic correction droop coefficient of the i-th energy storage unit.

[0108] Droop control module: It is used to combine droop control technology to multiply the converter output current of the i-th energy storage unit by the dynamically corrected droop coefficient to obtain the bus voltage drop of the i-th energy storage unit, and to obtain the converter reference output voltage of the i-th energy storage unit by subtracting the rated bus voltage from the bus voltage drop of the i-th energy storage unit.

[0109] Voltage loop PI controller module: It is used to obtain the converter reference output current of the i-th energy storage unit by taking the difference between the converter reference output voltage of the i-th energy storage unit and the converter output voltage of the i-th energy storage unit, and then using the voltage loop PI control method.

[0110] Kongming's current loop PI controller: It is used to obtain the duty cycle of the converter switching transistor of the i-th energy storage unit by taking the difference between the converter reference output current of the i-th energy storage unit and the converter inductor current of the i-th energy storage unit and then using the current loop PI control method.

[0111] Pulse Width Modulation Generator Module: This module generates a pair of complementary pulse signals based on pulse width modulation technology, using the duty cycle of the i-th energy storage unit. Each energy storage unit converter controls the operation of its respective switching transistor through the complementary pulse signals, thereby achieving SoC equalization of the parallel-operating energy storage units.

[0112] Figure 2 This is a DC microgrid structure diagram of the present invention, which mainly includes: a first energy storage unit battery 1, a second energy storage unit battery 2, a third energy storage unit battery 3, a first energy storage unit converter 4, a second energy storage unit converter 5, a third energy storage unit converter 6, a first line impedance 7, a second line impedance 8, a third line impedance 9, a controllable load resistor 10, and a controllable DC power supply 11.

[0113] The first energy storage unit converter 4 includes: a first inductor L1, a first capacitor C1, a first switch S1, and a second switch S2;

[0114] The second energy storage unit converter 5 includes: a second inductor L2, a second capacitor C2, a third switch S3, and a fourth switch S4;

[0115] The third energy storage unit converter 6 includes: a third inductor L3, a third capacitor C3, a fifth switch S5, and a sixth switch S6;

[0116] Each energy storage unit is connected in parallel to the DC bus via an energy storage unit converter. The controllable load resistor 10 is used to simulate the resistance of electrical equipment in a DC microgrid.

[0117] The controllable DC power supply 11 is used to simulate distributed power sources in a DC microgrid.

[0118] The first energy storage unit battery 1 is connected to one end of the first energy storage unit converter 4, the other end of the first energy storage unit converter 4 is connected to one end of the first line impedance 7, and the other end of the first line impedance 7 is connected to the controllable load resistor 10 and the controllable DC power supply 11.

[0119] The first energy storage unit battery 2 is connected to one end of the first energy storage unit converter 5, the other end of the first energy storage unit converter 5 is connected to one end of the first line impedance 8, and the other end of the first line impedance 8 is connected to the controllable load resistor 10 and the controllable DC power supply 11.

[0120] The first energy storage unit battery 3 is connected to one end of the first energy storage unit converter 6, the other end of the first energy storage unit converter 6 is connected to one end of the first line impedance 9, and the other end of the first line impedance 9 is connected to the controllable load resistor 10 and the controllable DC power supply 11.

[0121] Figure 3 This is the control block diagram of the i-th energy storage unit converter of the present invention. Its working principle will be explained below: The measured energy storage unit battery output current I... bati The SoC estimation module is input, and the SoC is estimated using the ampere-hour integration method. i SoC i And the SoC calculation module calculates the SoC based on the SoC input average of other energy storage units obtained through low-bandwidth communication lines. avg SoC i and SoC avg Input the SoC equalization control loop and adaptively adjust the droop coefficient using formula (3) of this invention to obtain the dynamically corrected R. vdi Then, by combining droop control technology and voltage-current dual closed-loop control technology, a complementary PWM pulse signal S is obtained. i and S jBy controlling the switching action of the converter transistors through PWM, dynamic SoC equalization of each parallel-operating energy storage unit is ultimately achieved. The control principle of each energy storage unit converter is simple and easy to implement. The SoC equalization control algorithm can be easily embedded into traditional droop controllers and voltage-current dual closed-loop controllers, ensuring that each parallel-operating energy storage unit has a fast SoC equalization speed in both the early and late stages of equalization.

[0122] Example 1

[0123] To verify the feasibility and effectiveness of the SoC equalization control method for parallel operation of multiple energy storage units proposed in this invention, a system was built based on MATLAB / Simulink software as follows: Figure 2 The simulation model shown. Each parallel energy storage unit converter adopts... Figure 3 The control structure is shown below. System simulation parameters are shown in Table 1.

[0124] Table 1 System Simulation Parameters

[0125]

[0126] Figure 4 (a) is the energy storage unit SoC of Embodiment 1 of the present invention, and (b) is the output current simulation waveform of Embodiment 1 of the present invention;

[0127] The initial SoCs of the three energy storage units were 80%, 70%, and 60%, respectively, with a rated capacity of 10Ah and an initial droop factor of 3Ω. The load power demand was 20kW, and the output power of the controllable DC power supply was 0. Throughout the simulation, the load power demand was provided by all three energy storage units, each operating in discharge mode. Through the adjustment of the control method proposed in this invention, the SoCs of the three energy storage units gradually converged, achieving SoC equalization and equal distribution of output current around 17 seconds. The simulation results of Example 1 show that the control method proposed in this invention can ensure that the parallel energy storage units achieve SoC equalization during the discharge process, exhibiting a fast SoC equalization speed in both the early and late stages of equalization, and high SoC equalization accuracy.

[0128] Figure 5 (a) is the energy storage unit SoC of Embodiment 2 of the present invention, and (b) is the simulated waveform of the output current of Embodiment 2 of the present invention;

[0129] The initial SoCs of the three energy storage units were 50%, 40%, and 30%, respectively, with a rated capacity of 10Ah and an initial droop factor of 3Ω. The load power demand was 20kW, and the output power of the controllable DC power supply was 40kW. Throughout the simulation, the controllable DC power supply provided the load power demand but still had surplus power, which was absorbed by all three energy storage units, each operating in charging mode. Through the adjustment of the control method proposed in this invention, the SoCs of the three energy storage units gradually converged, achieving SoC equalization and equal distribution of output current around 19 seconds. The simulation results of Example 2 show that the control method proposed in this invention can ensure that the parallel energy storage units achieve SoC equalization during charging, exhibiting a fast SoC equalization speed in both the early and late stages of equalization, and high SoC equalization accuracy.

[0130] Figure 6 (a) is the energy storage unit SoC of Embodiment 3 of the present invention, and (b) is the output current simulation waveform of Embodiment 3 of the present invention.

[0131] The initial SoC of the three energy storage units is 50%, with rated capacities of 15Ah, 10Ah, and 5Ah, respectively, and initial droop coefficients of 1Ω, 1.5Ω, and 3Ω, respectively. The initial load demand power is 20kW, which rises to 50kW at 15s, and the controllable DC power supply output power is 40kW. From 0 to 5s, conventional droop control is used to control the converter. Since the droop coefficient of conventional droop control is fixed and does not consider the SoC information and rated capacity ratio of the energy storage units, the output current of each energy storage unit is the same, causing the SoC to gradually diverge. At 5s, the converter control method is switched from conventional droop control to the control method proposed in this invention. The SoC of each energy storage unit gradually converges, and around 12s, SoC balance is achieved and the output current is distributed according to the rated capacity of the energy storage units in a 3:2:1 ratio. At 15s, the load demand power increases, and each energy storage unit switches from charging mode to discharging mode, while still maintaining SoC balance and output current ratio distribution. Simulation results in Example 3 show that the control method proposed in this invention can ensure that energy storage units with inconsistent capacities achieve fast and accurate SoC balancing during charging / discharging, and distribute their output current according to the rated capacity ratio of each energy storage unit, further verifying the effectiveness and superiority of the control method proposed in this invention.

[0132] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

[0133] [1]

[0134] [2]TROliveira,WWAGSilva,and PFDGarcia.Distributed secondarylevel control for energy storage management in DC microgrids[J].IEEETransactions on Smart Grid,2017,8(6):2597-2607.

[0135] [3] Li Pengcheng, Zhang Chunjiang, Yuan Ranran, Kan Zhizhong, Chen Yufei. Improved load current distribution method for distributed energy storage system with SOC droop control [J]. Proceedings of the CSEE, 2017, 37(13):3746-3754.

[0136] [4] Zhang Yun, Wang Chen, Wang Yi, Han Bing. Dynamic balancing strategy of multiple energy storage SOCs in DC microgrid based on adaptive droop control [J]. Journal of North China Electric Power University (Natural Science Edition), 2020, 47(5):21-29.

[0137] [5] NLDiaz, T. Dragicevic, JCVasquez, and JMGuerrero. Intelligent distributed generation and storage units for DC microgrids—A new concept oncooperative control without communications beyond droop control [J]. IEEETransactions on Smart Grid, 2014, 5(5): 2476-2485.

[0138] [6]N.Zhi,K.Ding,L.Du,and H.Zhang.An SOC-based virtual DC machinecontrol for distributed storage systems in DC microgrids[J].IEEE Transactionson Energy Conversion,2020,35(3):1411-1420.

Claims

1. A SoC equalization control method for multiple energy storage units operating in parallel, characterized in that: Includes the following steps: S1: Measure and collect the battery output current of the i-th energy storage unit, and estimate the SoC of the i-th energy storage unit using the ampere-hour integration method; S2: Each energy storage unit transmits the estimated SoC to other energy storage units through a low-bandwidth communication line, and obtains the estimated SoC of other energy storage units through the low-bandwidth communication line, and calculates the average SoC of n parallel-running energy storage units locally. S3: Based on the SoC and average SoC of the i-th energy storage unit, the droop coefficient is adaptively adjusted using the SoC equalization control algorithm to obtain the dynamic correction droop coefficient of the i-th energy storage unit. The dynamic correction droop coefficient of the i-th energy storage unit is designed as an adaptive change factor incorporating the SoC equalization control algorithm: (3) where R vdi is the adaptive droop coefficient of the i-th energy storage unit, R vi is the initial droop coefficient of each energy storage unit, ΔSoC i is the SoC equalization difference of the i-th energy storage unit, ΔSoC i = SoC avg - SoC i , m is an equalization speed factor less than 1, I dci > 0 indicates a discharging process, I dci < 0 indicates a charging process; S4: Combining droop control technology, the converter output current of the i-th energy storage unit is multiplied by the dynamically corrected droop coefficient to obtain the bus voltage drop of the i-th energy storage unit, and the difference between the rated bus voltage and the bus voltage drop of the i-th energy storage unit is obtained to obtain the converter reference output voltage of the i-th energy storage unit. S5: After subtracting the converter reference output voltage of the i-th energy storage unit from the converter output voltage of the i-th energy storage unit, the voltage loop PI control method is used to obtain the converter reference output current of the i-th energy storage unit. S6: After subtracting the converter reference output current of the i-th energy storage unit from the converter inductor current of the i-th energy storage unit, the current loop PI control method is used to obtain the duty cycle of the converter switching transistor of the i-th energy storage unit. S7: The duty cycle of the i-th energy storage unit is generated using pulse width modulation technology to obtain a pair of complementary pulse signals. Each energy storage unit converter controls the operation of its own switching transistor through the complementary pulse signals to achieve SoC equalization of each parallel-operating energy storage unit.

2. The SoC equalization control method for multiple energy storage units operating in parallel according to claim 1, characterized in that The SoC estimation of the i-th energy storage unit using the ampere-hour integration method employs the following formula: (1) where SoC i is the current SoC of the i-th energy storage unit, SoC i0 is the initial SoC of the i-th energy storage unit, C bati is the rated capacity of the i-th energy storage unit, I bati is the output current of the i-th energy storage unit.

3. The SoC equalization control method for multiple energy storage units operating in parallel according to claim 2, characterized in that The formula for calculating the average SoC of n parallel-operating energy storage units is as follows: (2) where SoC avg is the average SoC of the parallel energy storage units, and n is the number of parallel energy storage units.

4. The SoC equalization control method for multiple energy storage units operating in parallel according to claim 1, characterized in that The SoC and average SoC input SoC equalization control module based on the i-th energy storage unit adaptively adjusts the droop coefficient using the SoC equalization control algorithm of this invention to obtain the dynamically corrected droop coefficient of the i-th energy storage unit; the specific process is as follows: When the energy storage unit discharges, taking two energy storage units connected in parallel as an example, the relationship between the converter output current and the droop coefficient is as follows: (4) wherein I dc1 is the output current of the first energy storage unit converter, I dc2 is the output current of the second energy storage unit converter, R vd1 is the adaptive droop coefficient of the first energy storage unit, R vd2 is the adaptive droop coefficient of the second energy storage unit, R v1 is the initial droop coefficient of the first energy storage unit, R v2 is the initial droop coefficient of the second energy storage unit, ΔSoC1is the SoC equalization difference of the first energy storage unit, ΔSoC2is the SoC equalization difference of the second energy storage unit; When the energy storage unit is charging, taking two energy storage units connected in parallel as an example, the relationship between the converter output current and the droop coefficient is as follows: (5) where I dc1 is the output current of the first energy storage unit converter, I dc2 is the output current of the second energy storage unit converter, R vd1 is the adaptive droop coefficient of the first energy storage unit, R vd2 is the adaptive droop coefficient of the second energy storage unit, R v1 is the initial droop coefficient of the first energy storage unit, R v2 is the initial droop coefficient of the second energy storage unit, ΔSoC1is the SoC equalization difference of the first energy storage unit, and ΔSoC2is the SoC equalization difference of the second energy storage unit.

5. The SoC equalization control method for multiple energy storage units operating in parallel according to claim 1, characterized in that Using droop control technology, calculate the converter reference output voltage for the i-th energy storage unit: (6) wherein U refi is the converter reference output voltage of the i-th energy storage unit, U nom is the rated bus voltage.

6. The SoC equalization control method of claim 5, wherein Calculate the converter reference output current for the i-th energy storage unit: (7) where I refi is the reference output current of the converter of the i-th energy storage unit, U dci is the output voltage of the converter of the i-th energy storage unit, k PU is the proportional coefficient of the voltage loop PI controller, k IU is the integral coefficient of the voltage loop PI controller, 1 / s denotes integration.

7. The SoC equalization control method of claim 6, wherein Calculate the duty cycle of the converter switch for the i-th energy storage unit: (8) In the formula, D i is the duty ratio of the i-th energy storage unit converter switch tube, I Li is the inductor current of the i-th energy storage unit converter, k PI is the proportional coefficient of the current loop PI controller, k II is the integral coefficient of the current loop PI controller.

8. The SoC equalization control method of claim 1, wherein The duty cycle input of the i-th energy storage unit is based on pulse width modulation generator technology to obtain a pair of complementary pulse signals. Each energy storage unit converter controls the operation of its respective switching transistor through the complementary pulse signals to achieve SoC equalization of the parallel-operating energy storage units. Specifically, this includes: Each energy storage unit converter controls the operation of the switching transistors through complementary pulse signals, so that the energy storage unit with a higher SoC has a larger discharge current during the discharge process and a smaller charging current during the charging process; the energy storage unit with a lower SoC has a smaller discharge current during the discharge process and a larger charging current during the charging process, until the output current of each parallel-operating energy storage unit converter is distributed proportionally to its rated capacity, ultimately achieving SoC balance among the parallel-operating energy storage units.

9. A SoC equalization control device for multiple energy storage units operating in parallel, characterized in that: include: Acquisition module: used to measure and acquire the battery output current of the i-th energy storage unit, and to estimate the SoC of the i-th energy storage unit using the ampere-hour integration method; Average SoC Calculation Module: This module is used by each energy storage unit to transmit the estimated SoC to other energy storage units through low-bandwidth communication lines, and to obtain the estimated SoC of other energy storage units through low-bandwidth communication lines, and to calculate the average SoC of n parallel-running energy storage units locally. SoC equalization control module: Based on the SoC and average SoC of the i-th energy storage unit, the SoC equalization control algorithm is used to adaptively adjust the droop coefficient to obtain the dynamic correction droop coefficient of the i-th energy storage unit. The dynamic correction droop coefficient of the i-th energy storage unit is designed as an adaptive change factor incorporating the SoC equalization control algorithm: (3) where R vdi is the adaptive droop coefficient of the i-th energy storage unit, R vi is the initial droop coefficient of each energy storage unit, ΔSoC i is the SoC equalization difference of the i-th energy storage unit, ΔSoC i = SoC avg - SoC i , m is an equalization speed factor less than 1, I dci > 0 indicates a discharging process, I dci < 0 indicates a charging process; Droop control module: It is used to combine droop control technology to multiply the converter output current of the i-th energy storage unit by the dynamically corrected droop coefficient to obtain the bus voltage drop of the i-th energy storage unit, and to obtain the converter reference output voltage of the i-th energy storage unit by subtracting the rated bus voltage from the bus voltage drop of the i-th energy storage unit. Voltage loop PI controller module: It is used to obtain the converter reference output current of the i-th energy storage unit by taking the difference between the converter reference output voltage of the i-th energy storage unit and the converter output voltage of the i-th energy storage unit, and then using the voltage loop PI control method. Current loop PI controller module: It is used to obtain the duty cycle of the converter switching transistor of the i-th energy storage unit by taking the difference between the converter reference output current of the i-th energy storage unit and the converter inductor current of the i-th energy storage unit and then using the current loop PI control method. Pulse Width Modulation Generator Module: This module generates a pair of complementary pulse signals based on pulse width modulation technology, using the duty cycle of the i-th energy storage unit. Each energy storage unit converter controls the operation of its respective switching transistor through the complementary pulse signals, thereby achieving SoC equalization of the parallel-operating energy storage units.

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