A SOC balancing control method for battery energy storage system based on droop control

Abstract

The present invention discloses a SOC balancing control method for a battery energy storage system based on droop control. The present invention first constructs an AC microgrid system with multiple energy storage converters connected in parallel; secondly, the three-phase output voltage and three-phase output current of each energy storage converter are obtained to obtain the output active power and output reactive power of the corresponding energy storage converter; then, the adjustment frequency and adjustment voltage value are calculated by improving the droop control; finally, the AC side inductor current of each energy storage converter is obtained, and according to the inductor current, three-phase output voltage and adjustment voltage value, the voltage and current dual-loop control is adopted to obtain the modulation wave of each energy storage converter through pulse width modulation. In the present invention, each energy storage converter only needs to consider the energy storage unit information of the corresponding DC side to meet the characteristics of droop control; and the balance of the energy storage battery in the charging state of the energy storage converter is considered to ensure the consistency of the energy storage battery in the grid-connected and off-grid states.

Description

Technical Field

[0001] The present invention relates to the technical field of microgrid battery energy storage system control, and in particular to a SOC balancing control method for a battery energy storage system based on droop control. Background Art

[0002] With the increasing depletion of traditional energy sources and their continued environmental pollution, governments around the world have increased their focus on sustainable economic development. Renewable energy sources (RES), such as photovoltaics and wind power, have garnered widespread attention and rapid development in recent years due to their minimal environmental pollution, low cost, and flexible installation and configuration. However, due to the intermittent and fluctuating nature of RES power generation, energy storage systems are typically installed within microgrids to ensure the stability and reliability of load power supply. However, within energy storage systems, energy storage cells are constrained by their manufacturing processes, leading to inconsistencies between individual cells. These differences can be amplified during use due to factors such as ambient temperature, discharge efficiency, and the impact of protection circuits on the battery pack. In particular, existing battery energy storage systems primarily utilize second-life batteries, and these batteries are numerous. This inconsistency results in circulating current losses and a weak link effect, significantly compromising system safety and reliability.

[0003] In the prior art, battery energy storage systems often use improved droop control to achieve battery consistency control. For example, a patent (grant number CN 111244931 B) proposes a self-balancing state of charge (SOC) control method for multiple energy storage modules operating in parallel. By introducing SOC into droop control, it achieves output power balance among parallel energy storage modules and SOC balance among energy storage batteries. However, each energy storage module needs to obtain the SOC of all energy storage batteries, which loses the communication-free feature of droop control. Another example is a patent (publication number CN 113507151 A) that proposes a SOC collaborative control method for multiple energy storage units. It uses a dynamic consistency algorithm with sparse communication to achieve information exchange among multiple energy storage units on a large scale. Although the communication range is reduced, only adjacent energy storage units need to obtain information from each other, but communication still occurs. At the same time, this patent achieves SOC balancing control of energy storage batteries by incorporating SOC and battery capacity into droop control. However, in the case of different battery capacities, SOC balancing can only be achieved between two energy storage batteries. Moreover, the above technologies only consider the SOC balance of the energy storage battery in the discharge state, and do not consider the battery charging state. Summary of the Invention

[0004] In response to the shortcomings of the above technologies, the present invention provides a SOC balancing control method for a battery energy storage system based on droop control, which not only has the advantage of droop control without communication, but also can achieve SOC balancing of battery energy storage systems with different battery capacities and in rectification states.

[0005] The technical solution adopted by the present invention to solve its technical problem is:

[0006] The present invention comprises the following steps:

[0007] Step 1: Build an AC microgrid system with multiple energy storage converters in parallel:

[0008] The AC microgrid system includes n energy storage converters, the AC sides of each energy storage converter are arranged in parallel, connected to the AC grid through the grid-connecting switch SS1, and the DC sides of each energy storage converter are connected to a battery cluster;

[0009] Step 2: Obtain the three-phase output voltage and three-phase output current of each energy storage converter, and obtain the output active power and output reactive power of the corresponding energy storage converter through power calculation;

[0010] Step 3: Obtain the SOC, maximum available capacity, and output voltage of each DC-side battery cluster, and obtain the regulated frequency and regulated voltage values through improved droop control based on the battery cluster SOC, maximum available capacity, output voltage, output active power, and output reactive power;

[0011] Step 4: Obtain the AC side inductor current of each energy storage converter. According to the inductor current, three-phase output voltage and adjustment voltage value, adopt voltage and current dual loop control, the inner loop is the inductor current, and the outer loop is the load voltage control, and obtain the modulation wave of each energy storage converter through pulse width modulation.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] The present invention obtains the SOC, maximum available capacity, and voltage of each battery cluster, adopts an improved droop control, determines the adjustment frequency and adjustment voltage value of each energy storage converter, controls the output active power of each energy storage converter, and realizes reasonable power distribution. The energy storage converter with a high SOC of the battery cluster has a high output power, and the energy storage converter with a low SOC has a low output power. In other words, the battery cluster with a high SOC discharges more, and the battery cluster with a low SOC discharges less. Ultimately, at a certain moment, SOC balance is achieved, eliminating circulating current and inconsistency. In previous technologies, based on SOC balance, each energy storage converter needs to obtain information from all energy storage units, and there is information exchange between energy storage converters, which loses the advantage of droop control without communication. In the present invention, each energy storage converter only needs to consider the information of the energy storage unit on the corresponding DC side, meeting the characteristics of droop control. It also considers the balance of energy storage batteries in the charging state of the energy storage converter, ensuring the consistency of energy storage batteries in grid-connected and off-grid states. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic structural diagram of the energy storage system of the present invention;

[0015] Figure 2 It is the energy storage converter operation control strategy of the present invention;

[0016] Figure 3 It is the improved droop control block diagram of the present invention. DETAILED DESCRIPTION

[0017] The present invention will be described in detail below through specific examples with reference to the accompanying drawings.

[0018] Take the parallel energy storage converter as an example, Figure 1 As shown, the energy storage system includes n energy storage converters. The AC sides of the energy storage converters are arranged in parallel and connected to the AC grid through the grid-connected switch SS1. The DC sides of the energy storage converters are each connected to an energy storage cluster; the battery cluster is composed of multiple battery cells connected in series and in parallel.

[0019] Based on the above energy storage system, the present invention provides a battery energy storage system SOC balancing control method based on droop control as follows: Figure 2 As shown:

[0020] Step 1: Obtain the three-phase output voltage u of each energy storage converter abc And the three-phase output current i abc After power calculation, the corresponding energy storage converter output active power P is obtained i and output reactive power Q i ;

[0021] Step 2: Obtain the SOC of each DC side battery cluster i , Maximum available capacity C Ni and the output voltage v bati , according to the battery cluster SOC i , Maximum available capacity C Ni , output voltage v bati And output active power P i and output reactive power Q i By improving the droop control, the regulation frequency f is obtained i And adjust the voltage value U i ;

[0022] According to the adjustment frequency and adjustment voltage of each energy storage converter, the output power of the energy storage converter is adjusted accordingly, so that the energy storage converter with a high SOC of the battery cluster has a higher output power, and the energy storage converter with a low SOC of the battery cluster has a lower output power. The SOC consistency is achieved by controlling the size of the output current.

[0023] Preferably, the improved droop control block diagram is as follows: Figure 3 As shown, the principle expression is as follows:

[0024]

[0025] Where i represents the number of energy storage converters, i = 1, 2, 3...n; f i represents the regulation frequency of the i-th energy storage converter; f n Indicates rated frequency; P i represents the active power of the i-th energy storage converter; v bati represents the DC side voltage of the i-th energy storage converter; G i represents the equalization factor, U i represents the regulated voltage value of the i-th energy storage converter; U n Indicates the rated voltage value; Q i represents the reactive power output by the i-th energy storage converter; K P and K Q Represents the droop coefficient of the improved droop control method.

[0026] In the equalization factor G i In the expression, SOC i Indicates the state of charge of the battery cluster corresponding to the i-th energy storage converter, SOC min Indicates the discharge limit of the battery cluster, which is 20%; C Ni represents the maximum available capacity of the battery cluster corresponding to the i-th energy storage converter; T represents the time of maximum rate charge and discharge within a safe range.

[0027] Preferably, step 2 of the method specifically includes the following steps:

[0028] Step S1: Obtain the SOC of each battery cluster i , Maximum available capacity C Ni When the energy storage converter works in the inverter state (the battery cluster supplies power to the load), according to the SOC of each battery cluster i , Maximum available capacity C Ni , battery cluster discharge limit SOC min The discharge balance factor G is calculated by the maximum rate charge and discharge time T within the safety range. i ;

[0029] When the energy storage converter works in the rectification state (AC grid charges the battery cluster), according to the SOC of each battery cluster i , Maximum available capacity C Ni The charge balancing factor G is calculated by the maximum rate charge and discharge time T within the safety range. i ;

[0030] Step S2: Obtain the battery cluster output voltage v bati , according to the battery cluster output voltage v bati , Equalization factor G i , active power P iand reactive power Q i Calculate the adjustment frequency f i and regulating voltage U i ;

[0031] Step S3: According to the adjustment frequency f i and regulating voltage U i After voltage synthesis and transformation, the d-axis given voltage component v is obtained dref and q-axis given voltage component v qref .

[0032] Step 3: Obtain the AC side inductor current i of each energy storage converter Labci , according to the inductor current i Labci , three-phase output voltage u abci And the given voltage component v dref 、v qref , adopts voltage and current dual loop control, the inner loop is the inductor current, the outer loop is the load voltage control, and the modulation wave of each energy storage converter is obtained through pulse width modulation;

[0033] Preferably, step three of the method specifically includes the following steps:

[0034] Step S1: Obtain the three-phase output voltage u of each energy storage converter abci and the inductor current i Labci , adjust the frequency f according to the droop control i The converted phase angle θ i , the three-phase output voltage u abci and the inductor current i Labci After dq transformation, the d-axis output voltage component u is obtained d , q-axis output voltage component u q and the d-axis inductor current component i Ld , q-axis inductor current component i Lq ;

[0035] Step S2: Set the d-axis given voltage component v dref The d-axis output voltage component u d Do the difference, the q-axis given voltage component v qref The q-axis output voltage component u q Make the difference and obtain the d-axis regulating current reference value and the q-axis regulating current reference through proportional-integral regulation;

[0036] Step S3: Subtracting the d-axis regulating current reference value from the d-axis inductor current component, and subtracting the q-axis regulating current reference value from the q-axis inductor current component, and obtaining a d-axis first regulating component and a q-axis first regulating component through proportional-integral regulation and decoupling.

[0037] Step S4: performing inverse transformation on the d-axis first regulation component and the q-axis first regulation component to obtain a three-phase regulation voltage of the energy storage converter, and controlling the operation of the energy storage converter through pulse width modulation.

Claims

1. A battery energy storage system SOC balancing control method based on droop control, applied to a microgrid with multiple energy storage converters running in parallel, characterized by The method comprises the following steps: Step 1: Build an AC microgrid system with multiple energy storage converters in parallel: The AC microgrid system includes n energy storage converters, the AC sides of each energy storage converter are arranged in parallel, connected to the AC grid through the grid-connecting switch SS1, and the DC sides of each energy storage converter are connected to a battery cluster; Step 2: Obtain the three-phase output voltage and three-phase output current of each energy storage converter, and obtain the output active power and output reactive power of the corresponding energy storage converter through power calculation; Step 3: Obtain the SOC, maximum available capacity, and output voltage of each DC-side battery cluster, and obtain the regulated frequency and regulated voltage values through improved droop control based on the battery cluster SOC, maximum available capacity, output voltage, output active power, and output reactive power; Step 4: Obtain the AC side inductor current of each energy storage converter, and adopt voltage and current dual loop control based on the inductor current, three-phase output voltage and adjustment voltage value, with the inner loop being the inductor current and the outer loop being the load voltage control, to obtain the modulation wave of each energy storage converter through pulse width modulation; Step three specifically includes the following steps: Step S1: Obtain the SOC and maximum available capacity of each battery cluster. When the energy storage converter works in the inverter state, the SOC, maximum available capacity and discharge limit SOC of each battery cluster are obtained. min The discharge balance factor is calculated based on the maximum rate charge and discharge time within the safety range; When the energy storage converter is working in the rectification state, the charge balancing factor is calculated based on the SOC of each battery cluster, the maximum available capacity and the maximum rate charge and discharge time within the safe range; Step S2: obtaining the battery cluster output voltage, and calculating the regulated frequency and regulated voltage according to the battery cluster output voltage, balancing factor, active power, and reactive power; Step S3: According to the adjustment frequency and the adjustment voltage, a d-axis given voltage component and a q-axis given voltage component are obtained through voltage synthesis and transformation processing; The expression of the improved droop control is as follows: When the energy storage converter operates in the rectification state: f i =f n -K P (P i -G i v bati ) U i =U n -K Q Q i When the energy storage converter works in the inverter state: f i =f n -K P (P i -G i v bati ) U i =U n -K Q Q i Where i represents the number of energy storage converters, f i represents the regulation frequency of the i-th energy storage converter, f n Indicates rated frequency, P i represents the active power of the i-th energy storage converter, v bati represents the DC side voltage of the i-th energy storage converter, G i represents the equalization factor, U i represents the regulated voltage value of the i-th energy storage converter, U n Indicates the rated voltage value, Q i represents the reactive power output by the i-th energy storage converter, K P and K Q Indicates the droop coefficient in the improved droop control, SOC i Indicates the state of charge of the battery cluster corresponding to the i-th energy storage converter, SOC min Indicates the discharge limit of the battery cluster, C Ni represents the maximum available capacity of the battery cluster corresponding to the i-th energy storage converter, and T represents the time of maximum rate charge and discharge within a safe range.

2. The SOC balancing control method for a battery energy storage system based on droop control according to claim 1, characterized in that: Step 4 specifically includes the following steps: Step S1: Obtain the AC side output voltage and inductor current of each energy storage converter, and according to the angular frequency obtained by droop control, perform dq transformation on the three-phase output voltage and inductor current to obtain a d-axis output voltage component, a q-axis output voltage component, and a d-axis inductor current component, and a q-axis inductor current component; Step S2: Subtracting the d-axis given voltage component from the d-axis voltage component, and subtracting the q-axis given voltage component from the q-axis output voltage component, and obtaining a d-axis regulating current reference value and a q-axis regulating current reference value through proportional-integral regulation; Step S3: Subtracting the d-axis regulating current reference value from the d-axis inductor current component, and subtracting the q-axis regulating current reference value from the q-axis inductor current component, and obtaining a d-axis first regulating component and a q-axis first regulating component through proportional-integral regulation and decoupling. Step S4: performing inverse transformation on the d-axis first regulation component and the q-axis first regulation component to obtain a three-phase regulation voltage of the energy storage converter, and controlling the operation of the energy storage converter through pulse width modulation.

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