Charge and Discharge Control Method for Batteries in Energy Storage Inverters

Abstract

The present invention discloses a charge and discharge control method for a storage battery in an energy storage converter, including: obtaining the state quantity of the current cycle; if the input power of the current cycle is less than or equal to the load power, the storage battery discharges or the load is reduced; if the input power of the current cycle is greater than the load power, the storage battery is charged, and it is further determined whether the terminal voltage of the storage battery reaches the full amount. If the terminal voltage of the storage battery does not reach the full amount, constant current mode charging is adopted, current prediction control is used, and the switching tube control signal is output according to the evaluation function; if the terminal voltage of the storage battery reaches the full amount, constant voltage mode charging is adopted. When the input power is less than the load power, the storage battery is controlled whether to discharge in combination with the SOC of the storage battery to avoid over-discharge. When the input power is greater than the load power, charging is completed by using power optimization configuration and current prediction control in the constant current mode, so as to improve the rapidity of tracking the current expectation value fluctuating with the charging power and effectively improve the charging efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of energy storage converters, and in particular to a method for controlling the charging and discharging of a storage battery in an energy storage converter. Background Art

[0002] The input and output power of a distributed energy storage converter changes at any time, with great uncertainty. The storage battery in the energy storage converter switches between the charging and discharging states. The existing charging and discharging control strategies for the storage battery in the energy storage converter are generally two-stage charging control, that is, the switching between constant current charging and constant voltage charging. When the battery terminal voltage does not reach the full voltage, the constant current mode is used for charging, and when it reaches the full voltage, it switches to the constant voltage mode for floating charging. Among them, the PI coefficient needs to be calibrated multiple times, and it needs to be recalibrated when the energy storage battery with different parameters is replaced in the system, and the portability of the controller is poor. And it may affect the service life of the battery due to insufficient charging power or the switching between charging and discharging. Summary of the Invention

[0003] To solve some or all of the technical problems existing in the above-mentioned prior art, the present invention provides a method for controlling the charging and discharging of a storage battery in an energy storage converter. The technical solution is as follows:

[0004] A method for controlling the charging and discharging of a storage battery in an energy storage converter is provided. The energy storage converter adopts a cascaded BOOST-BUCK circuit. The front-end interleaved parallel BOOST circuit adopts a constant voltage control method to obtain a stable DC bus voltage, and the back-end charging and discharging part selects a single-loop circuit. The method includes: obtaining the state quantity of the current cycle; if the input power of the current cycle is less than or equal to the load power, the storage battery discharges or the load is reduced; if the input power of the current cycle is greater than the load power, the storage battery is charged, and it is further determined whether the terminal voltage of the storage battery reaches the full amount. If the terminal voltage of the storage battery does not reach the full amount, the constant current mode is used for charging, and current prediction control is used to output a switching tube control signal according to the evaluation function; if the terminal voltage of the storage battery reaches the full amount, the constant voltage mode is used for charging.

[0005] In some optional implementation manners, the evaluation function is obtained through the following formula:

[0006] When the switching tube is turned on, that is, when S 21 = 1, the current equation flowing through the inductor L 3 is:

[0007] When the switching tube is turned off, that is, when S 21 = 0, the current equation flowing through the inductor L 3 is:

[0008] According to the mathematical model of the BUCK circuit, first-order discretization is performed to obtain:

[0009]

[0010]

[0011] Wherein, U dc (k), U b (k) and I b (k) are the bus voltage, the battery terminal voltage, and the battery charging current in the k-th cycle respectively. T s is the system sampling period. I 1b (k + 1) and I 2b (k + 1) are the battery charging current values corresponding to two switch states in the (k + 1)-th cycle respectively;

[0012] Finally, the evaluation function described by the following formula (5) is adopted to select the minimum J i value as the switch control signal, and cooperate with the calculated I b (k + 1) to achieve fast current tracking:

[0013] J i = |I ib (k + 1) - I bref (k + 1)| i = 1, 2 (5)

[0014] Wherein, if the evaluation function is smaller when i = 1, the switch is controlled to turn on; if the evaluation function is smaller when i = 2, the switch is controlled to turn off.

[0015] In some alternative implementation manners, to obtain the optimal I bref (k + 1) according to the real-time charging power, it is necessary to first calculate the optimally configured battery charging power P b (k + 1), which has an energy conservation relationship with the input power P in (k + 1) and the load power P load (k + 1):

[0016] P b (k + 1) = P load (k + 1) - P in (k + 1) (6)

[0017] P b is negative when the battery is in the charging state, and vice versa for the discharging state; A low-pass filter is introduced to optimize the configuration of the charging power, and the low-pass filter H(s) is as shown in formula (7):

[0018]

[0019] Wherein, T crepresents the filtering time constant, s is the differential operator, and the relationships between the above three powers and H(s) are as follows:

[0020]

[0021] Expressing Equation (8) in the time domain gives:

[0022]

[0023] In some alternative implementation manners, after obtaining P b (k + 1), I bref (k + 1) is obtained through the following formula:

[0024] I bref (k + 1) = P b (k + 1) / U b (k + 1)

[0025] where, U b (k + 1) and U b (k) are approximately the same numerically, and the value of U b (k) is used as the value of U b (k + 1).

[0026] In some alternative implementation manners, during constant - current mode charging, the filtering time constant is adjusted according to the change of the battery SOC: when SOC < 20%, Tc = Tc + dTc; when 20% ≤ SOC ≤ 80%, the charging power configuration is performed with a constant Tc value; when SOC > 80%, normal charge or discharge control is performed according to the positive and negative of Pb.

[0027] In some alternative implementation manners, when the input power of the current cycle is less than or equal to the load power, the battery SOC is further judged. If the battery SOC is greater than 20%, the battery discharges. If the battery SOC is less than or equal to 20%, the load is appropriately reduced or turned off.

[0028] The main advantages of the technical solution of the present invention are as follows:

[0029] The charge - discharge control method for the battery in the energy storage converter provided by the present invention monitors the current - cycle state. When the input power is less than the load power, it combines the battery SOC to control whether the battery discharges, avoiding over - discharge. When the input power is greater than the load power, charging is completed by using power optimization configuration combined with current prediction control in the constant - current mode, thereby improving the rapidity of tracking the current expectation value fluctuating with the charging power. When the system cannot reach the charging rated power or the energy storage device has insufficient power, the energy storage device is timely replenished with electric energy, effectively improving the charging efficiency, preventing the battery from being discharged due to long - term over - discharge, and avoiding shortening the battery service life. Brief Description of the Drawings

[0030] The drawings described herein are provided to further understand the embodiments of the present invention and form a part of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0031] Figure 1 is the topological structure diagram of the energy storage converter provided by an embodiment of the present invention;

[0032] Figure 2 is the charging control strategy diagram of the rear-end BUCK circuit in the energy storage converter provided by an embodiment of the present invention;

[0033] Figure 3 is the charge and discharge control flow chart of the storage battery in the energy storage converter provided by an embodiment of the present invention;

[0034] Figure 4 is the voltage of the front-end BOOST circuit in the energy storage converter provided by an embodiment of the present invention from startup to stable output to the DC bus;

[0035] Figure 5a is the voltage control adjustment diagram when the load changes suddenly in the energy storage converter provided by an embodiment of the present invention;

[0036] Figure 5b is the constant current charging current prediction control diagram in the energy storage converter provided by an embodiment of the present invention;

[0037] Figure 6 is the dynamic change curve diagram of the SOC of the storage battery in the energy storage converter provided by an embodiment of the present invention;

[0038] Figure 7 is the schematic diagram of the two-stage charging switching of the selected battery in the energy storage converter provided by an embodiment of the present invention. Detailed Embodiments

[0039] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention and the corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0040] The following will describe in detail the technical solutions provided by the embodiments of the present invention with reference to the drawings.

[0041] The topological structure diagram of the energy storage converter can be seen in the appendix Figure 1It adopts a cascaded BOOST-BUCK circuit to divide the DC side of the energy storage converter into two parts. The front-end staggered parallel BOOST circuit adopts a constant voltage control method to obtain a stable DC bus voltage. The control signal drives the two-phase IGBT switch tube with a duty ratio of 180°. This can prevent the voltage difference between the input terminal voltage and the rated bus voltage from being too large, and the duty ratio of a single IGBT is too large, causing the temperature of the switch tube to rise too quickly, resulting in a decrease in durability.

[0042] The back-end charging and discharging part uses a bidirectional BOOST-BUCK circuit to realize forward buck charging and reverse boost discharging. Considering that the bus voltage is stably controlled by the front section and the power quality is high, a single-loop circuit is selected to save costs and reduce system size. The schematic diagram of the single-loop circuit can be found in the attached Figure 2 .

[0043] In the energy storage converter circuit of the present invention, the rated bus voltage is 600V, the system rated power is 15KW, the energy storage device adopts a nominal 512V / 18Ah high-voltage lithium-ion battery, the maximum charging current is 10A, and the maximum charging voltage (full-rated voltage) is 572V. The circuit is controlled by the GD32F407ZET6 chip, the switch device (IGBT) adopts the GD450HFTY120C6S of Jiaxing Star Company, and the maximum switching frequency can reach 20kHz.

[0044] The embodiment of the present invention provides a method for controlling the charging and discharging of a battery in an energy storage converter, which is used to control the charging and discharging of a battery in the energy storage converter of the above structure, as shown in the attached Figure 3 As shown, the method includes:

[0045] The state quantity of the current cycle is obtained. The state quantity of the current cycle may include the input power P of the current cycle (ie, the kth cycle) in (k), load power P load (k), bus voltage U dc (k) Battery terminal voltage U b (k) and battery charging current I b (k), battery charging current I b And other data.

[0046] If the input power of the current cycle is less than or equal to the load power, the battery is discharged or the load is reduced. At this time, if the battery has a lot of power, the battery is discharged to meet the load requirements. If the battery has less power, the battery cannot be discharged, otherwise it will cause the battery to be over-discharged and affect the battery life. At this time, the load power is reduced so that the input power can meet the load requirements.

[0047] If the input power in the current cycle is greater than the load power, the battery is charged. Further, it is determined whether the terminal voltage of the battery reaches the full value. If the terminal voltage of the battery does not reach the full value, constant current mode charging is adopted, and current prediction control is used to output the switching tube control signal according to the evaluation function. If the terminal voltage of the battery reaches the full value, constant voltage mode charging is adopted. During the constant current mode charging process, the switching tube signal is controlled through current tracking to optimize the charging process, make full use of the redundant power at the input end for charging, and improve the energy storage efficiency.

[0048] In summary, for the charge and discharge control method of the battery in the energy storage converter provided by the embodiment of the present invention, by monitoring the current cycle state, when the input power is less than the load power, whether the battery discharges is controlled in combination with the battery SOC to avoid over-discharge. When the input power is greater than the load power, charging is completed by using power optimization configuration combined with current prediction control in the constant current mode, so as to improve the rapidity of tracking the current expectation value fluctuating with the charging power, and realize timely replenishment of electric energy to the energy storage device when the system cannot reach the charging rated power or the energy storage device has insufficient power, effectively improve the charging efficiency, prevent the battery from being discharged due to long-term over-discharge, and avoid shortening the battery service life.

[0049] The charging control strategy of the BUCK circuit is as Figure 2 shown, U b and I b are the battery terminal voltage and the battery charging current respectively, where the current is taken as the positive direction from left to right. Constant current control uses current prediction control to output the switching tube control signal through the evaluation function. Constant voltage control is traditional PI control. The two are judged according to U b . Before U b reaches the full voltage, constant current charging is adopted, and after reaching, it is switched to constant voltage charging and added to the previous constant current charging control signal for output.

[0050] To implement the current prediction control for charging, the current value I b (k + 1) of the (k + 1)-th cycle needs to be calculated in advance in the k-th cycle, and then current tracking is performed according to the real-time I bref of the (k + 1)-th cycle to achieve the control target. Here, the cycle is the sampling cycle. When the battery is charged, the BUCK circuit is used for voltage and current control, and a mathematical model is built according to the different working states of the switching tube S 21 (S 21 = 1 indicates that the switching tube is turned on, and S 21 = 0 indicates that the switching tube is turned off).

[0051] When the switching tube is turned on, that is, when S 21 = 1, the current equation flowing through the inductor L 3 is:

[0052] The switching transistor is turned off, i.e., when S 21 = 0, the current equation flowing through the inductor L 3 is:

[0053] According to the mathematical model of the BUCK circuit, first-order discretization is performed to obtain:

[0054]

[0055]

[0056] In the formula, U dc (k), U b (k) and I b (k) are the bus voltage, battery terminal voltage, and battery charging current in the k-th cycle respectively, T s is the system sampling period, I 1b (k + 1) and I 2b (k + 1) are the battery charging current values corresponding to the two switching states in the (k + 1)-th cycle respectively. Finally, the evaluation function described by the following formula (5) is used to select the smallest J i value as the switching transistor control signal, and cooperate with the calculated I b (k + 1) to achieve fast current tracking:

[0057] J i = |I ib (k + 1) - I bref (k + 1)| i = 1, 2 (5)

[0058] Among them, if the evaluation function is smaller when i = 1, the switching transistor is controlled to turn on; if the evaluation function is smaller when i = 2, the switching transistor is controlled to turn off.

[0059] To obtain the optimal I bref (k + 1) according to the real-time charging power, the optimal configured battery charging power P b (k + 1) needs to be calculated first. It has an energy conservation relationship with the input power P in (k + 1) and the load power P load (k + 1):

[0060] P b (k + 1) = P load (k + 1) - P in (k + 1) (6)

[0061] P b When P is negative, the battery is in the charging state; otherwise, it is in the discharging state. A low-pass filter is introduced to optimize the configuration of the charging power. The low-pass filter H(s) is shown in formula (7):

[0062]

[0063] In the formula, T c represents the filtering time constant, s is the differential operator, and the relationships between the above three powers and H(s) are as follows:

[0064]

[0065] Expressing Equation (8) in the time domain gives:

[0066]

[0067] From Equation (9), P b (k + 1) can be obtained in relation to the input power and the low-pass filtering constant T c in the k-th cycle. The larger T c is, the larger the charging power; conversely, the smaller it is. According to the change in the SOC of the battery, T c also changes accordingly. The SOC of the battery is divided into three working regions. When SOC < 20%, T c = T c + dT c , and the battery is charged quickly with a relatively large power. When 20% ≤ SOC ≤ 80%, the charging power is configured with a constant T c value. When SOC > 80%, normal charging or discharging control is performed according to the positive or negative of P b . Finally, based on the P b (k + 1) calculated in real time and the battery terminal voltage, the current to be tracked is converted. The current set value I bref (k + 1) = P b (k + 1) / U b (k + 1). Among them, U b (k + 1) is approximately the same as U b (k) in value, and the value of U b (k) is used as the value of U b (k + 1).

[0068] In summary, the overall flowchart of the battery charge and discharge control can be seen in Appendix Figure 3 .

[0069] In one embodiment, Figure 4 it shows the voltage of the front-end BOOST circuit from startup to stable output to the DC bus. To prevent excessive circuit shock caused by too large a bus voltage difference at startup, a pre-charge resistor is added for pre-charging, and the ramp setting is 135 V / s. After the bus voltage stabilizes to the expected value of 600 V, charging is started. Initially, charging is carried out with a maximum current of 10 A, and the current ripple is about 0.5 A.

[0070] Figure 5a The rated power output of the middle input terminal is 10KW. When the load suddenly increases from 0KW to 9KW, the bus voltage is instantaneously pulled down. After about 0.5s of adjustment, it stabilizes to the expected value of 600V, and the charging voltage decreases accordingly as the charging power drops.

[0071] Figure 5b It is shown in the middle that during the above change process, the current predictive control realizes the current tracking of the charging current from the stable maximum value to the current after the charging power decreases.

[0072] Figure 6 When charging the battery under the influence of the above power conversion, the SOC change curve of the battery after adding power configuration is compared with the SOC change curve of the battery without adding power optimization configuration under the same conditions. Adding power optimization configuration greatly improves the charging efficiency of the battery in the energy storage converter.

[0073] Figure 7 After charging in constant current mode for a period of time, the battery terminal voltage Ub reaches the full value of 572V, and then switches to constant voltage charging mode ( Figure 7 at the dotted line in the middle), the charging current continuously decreases. At this time, the battery is in floating charge state until the current decreases to 0 to complete charging.

[0074] It should be noted that in this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. In addition, in this article, "front", "rear", "left", "right", "up" and "down" are all referenced with the placement state shown in the drawings.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A charge and discharge control method for a storage battery in an energy storage converter, characterized in that, the energy storage converter adopts a cascaded BOOST - BUCK circuit, the front - end interleaved parallel BOOST circuit adopts a constant - voltage control method to obtain a stable DC bus voltage, and the back - end charge and discharge part selects a single - loop circuit. The method includes: Obtaining the state quantity of the current cycle; If the input power of the current cycle is less than or equal to the load power, the storage battery discharges or the load is reduced; If the input power of the current cycle is greater than the load power, the storage battery is charged. Further, it is judged whether the terminal voltage of the storage battery reaches the full value. If the terminal voltage of the storage battery does not reach the full value, a constant - current mode is used for charging, current prediction control is used, and the control signal of the switching tube is output according to the evaluation function; if the terminal voltage of the storage battery reaches the full value, a constant - voltage mode is used for charging; The evaluation function is obtained through the following formula: The switch is turned on, that is, when S 21 = 1, the current flowing through the inductor L 3 is given by the equation: The switching transistor is turned off, i.e., S 21 = 0, the current flowing through the inductor L 3 The current equation is: First - order discretization is performed according to the mathematical model of the BUCK circuit to obtain: where U dc (k), U b (k) and I b (k) are the bus voltage, the battery terminal voltage, and the battery charging current in the k-th cycle respectively, T s is the system sampling period, I 1b (k + 1) and I 2b (k + 1) are the battery charging current values corresponding to the two switching states in the (k + 1)-th cycle respectively; Finally, the evaluation function described by the following formula (5) is adopted to select the smallest J i value as the switching tube control signal, and cooperate with the calculated I b (k + 1) to achieve fast current tracking: J i = |I ib (k + 1)-I bref (k + 1)| where i = 1, 2 (5) wherein, if the evaluation function is smaller when i = 1, the switching tube is controlled to turn on; if the evaluation function is smaller when i = 2, the switching tube is controlled to turn off; To obtain the optimal I bref (k + 1) based on the real-time charging power, it is necessary to first calculate the charging power P b (k + 1) of the optimal configuration of the storage battery, which has an energy conservation relationship with the input power P in (k + 1) and the load power P load (k + 1): bref (k + 1), it is necessary to first calculate the charging power P b (k + 1) of the optimal configuration of the storage battery, which is related to the input power P in (k + 1) and the load power P load (k + 1) there is an energy conservation relationship: P b (k + 1) = P load (k + 1) - P in (k + 1)(6) P b When it is negative, the battery is in the charging state; otherwise, it is in the discharging state. A low-pass filter is introduced to optimize the configuration of the charging power. The low-pass filter H(s) is shown in Equation (7) as follows: where T c represents the filtering time constant, s is the differential operator, and the relationships between the above three powers and H(s) are as follows: Expressing Equation (8) in the time domain gives: Obtain P b After (k + 1), obtain I through the following formula bref (k + 1): I bref (k + 1) = P b (k + 1) / U b (k + 1) Among them, U b (k + 1) is approximately equal in value to U b (k), and the value of U b (k) is used as the value of U b (k + 1).

2. The charge and discharge control method for a storage battery in an energy storage converter according to claim 1, characterized in that, during constant - current mode charging, the filtering time constant is adjusted according to the change of the SOC of the storage battery: When SOC < 20%, T c = T c + dT c ; When 20% ≤ SOC ≤ 80%, the charging power is configured at a constant T c value; When SOC > 80%, normal charge or discharge control is performed according to P b in a positive or negative manner.

3. The charge and discharge control method for a storage battery in an energy storage converter according to claim 1, characterized in that, when the input power of the current cycle is less than or equal to the load power, the SOC of the storage battery is further judged. If the SOC of the storage battery is greater than 20%, the storage battery discharges; if the SOC of the storage battery is less than or equal to 20%, the load is appropriately reduced or turned off.

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