Control strategy for extending the service life of energy storage batteries under constant active power and safe grid connection of photovoltaic storage systems

Abstract

The present invention discloses a control strategy for extending the service life of energy storage batteries under the condition of safe grid connection of a photovoltaic storage system at constant active power. Specifically, the strategy includes: when the photovoltaic storage system is grid-connected at constant active power, first setting the lower limit of the charge and discharge power of a single converter of the energy storage system; when there is a power difference between the active power generated by the photovoltaic system and the grid-connected constant active power reference value, designing the AC-side dq-axis component reference values of multiple converters by considering the variation pattern of lithium-ion battery life, the grid-connected harmonic content, and the upper and lower charge and discharge limits of the battery cluster SOC; then, after voltage and current dual-loop control, generating a modulation signal to control the converter, thereby achieving safe grid connection of the photovoltaic storage system at constant active power and extending the service life of the energy storage batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of multi-converter grid-connected energy storage power stations, and in particular to a control strategy for extending the service life of energy storage batteries under the condition of safe grid connection of a photovoltaic storage system with constant active power. Background Art

[0002] In recent years, renewable energy sources such as photovoltaics have experienced rapid development. However, due to the uncertainty inherent in photovoltaic power generation, its output power fluctuates. To mitigate this volatility, energy storage is required. When a photovoltaic storage system is connected to the grid with constant active power, a power difference will occur between the photovoltaic output power and the constant grid-connected active power command value. This power difference requires the coordination of an energy storage system. Therefore, how to rationally distribute this power difference using multiple power storage converters (PCSs) is a problem addressed by this invention.

[0003] Domestic and foreign scholars have proposed two main methods for the constant active power grid connection of photovoltaic storage systems. One is that the difference power is first participated by a battery cluster. When the power of the battery cluster reaches the rated power, a new battery cluster is put into power conversion to reduce the battery cluster participation in operation; the other is the idea of ​​sharing, in which the shortfall power is directly shared by multiple battery clusters to maintain the balance between the multiple battery clusters.

[0004] Since lithium-ion batteries age faster with increasing charge and discharge current, the charge and discharge currents should be minimized. This means that the power differential should be shared as evenly as possible across multiple PCSs to extend their lifespan. Furthermore, since AC-side harmonic filters are designed based on rated power, the closer the PCS output power is to rated power, the lower the grid-connected harmonic content. When the PCS output power is low, the amplitude of the fundamental component is also low. The definition of total harmonic distortion (THD) indicates that the grid-connected harmonic content can easily exceed 5%, failing to meet national standards for grid-connected harmonic content. To ensure that the grid-connected harmonic content of the energy storage system is less than 5%, a lower limit for the charge and discharge power of each PCS can be set. This lower limit can be provided by the manufacturer or determined empirically. The higher the power value, the lower the harmonic content at the grid-connected energy storage system. Summary of the Invention

[0005] The purpose of the present invention is to provide a control strategy for extending the service life of energy storage batteries under the condition of safe grid connection of a photovoltaic storage system with constant active power. When there is a difference between the grid-connected active power reference value of the photovoltaic storage system and the generated power of the photovoltaic system, the control between multiple PCSs is optimized based on the grid-connected harmonic content requirements of the energy storage system and the change law of the life characteristics of the lithium-ion battery, thereby extending the service life of the energy storage battery.

[0006] To achieve the above technical effects, the technical solution adopted by the present invention is a control strategy for extending the service life of energy storage batteries under the condition of constant active power and safe grid connection of the photovoltaic storage system, which specifically includes the following steps:

[0007] Step S1: Set the reference value of the constant active power of the solar-storage system connected to the grid P ref And the lower limit of charge and discharge power P of a single PCS min ;

[0008] Step S2: Collect the power generated by the photovoltaic system P v ;

[0009] Step S3: Determine the photovoltaic system power generation power P v Is it greater than or equal to the grid-connected constant active power reference value P ref If it is greater than, then the power participation of each PCS is set as follows:

[0010] If the difference power (P v -P ref ) is less than P min In order to ensure that the harmonic content of energy storage grid connection does not exceed 5%, all energy storage battery clusters stop absorbing the difference power. Since the difference power is generally small, it will basically not affect the power fluctuation of the grid.

[0011] Furthermore, if P min <P v -P ref ≤2P min When , the difference in power is completely absorbed by energy storage battery cluster 1, and the remaining battery clusters do not participate in the grid connection.

[0012] Furthermore, if 2P min <P v -P ref ≤3P min When , the energy storage battery cluster 1 and battery cluster 2 absorb the difference power equally, and the other battery clusters do not participate in the grid connection.

[0013] Furthermore, if 3P min <P v -P ref ≤4P min When , the energy storage battery cluster 1, battery cluster 2 and battery cluster 3 absorb the difference power together, and the remaining battery clusters do not participate in the grid connection.

[0014] Furthermore, by analogy, if n·P min <P v -P ref ≤(n+1)P min Energy storage battery clusters 1, 2, 3, ..., and n absorb the difference in power equally, and the remaining battery clusters do not participate in the grid connection.

[0015] Step S4: Determine the photovoltaic system power generation power P v Is it less than the grid-connected constant active power reference value P ref If it is less than, then the power participation of each PCS is set as follows:

[0016] If the difference power (P ref -P v ) is less than P min In order to ensure that the harmonic content of energy storage grid connection does not exceed 5% and meet the grid connection active power demand, energy storage battery cluster 1 sends P min ,Since this power is generally small, it will not affect the power fluctuation of the grid, and the remaining battery clusters do not participate in the grid connection.

[0017] Furthermore, if P min <P ref -P v ≤2P min When , energy storage battery cluster 1 all generates the difference power, and the other battery clusters do not participate in the grid connection.

[0018] Furthermore, if 2P min <P ref -P v ≤3P min When , the energy storage battery cluster 1 and battery cluster 2 share the difference in power, and the remaining battery clusters do not participate in the grid connection.

[0019] Furthermore, if 3P min <P ref -P v ≤4P min When , the energy storage battery cluster 1, battery cluster 2 and battery cluster 3 share the difference power, and the remaining battery clusters do not participate in the grid connection.

[0020] Furthermore, by analogy, if n·P min <P ref -P v ≤(n+1)P min When , energy storage battery cluster 1, battery cluster 2, battery cluster 3, ..., battery cluster n all share the difference in power, and the remaining battery clusters do not participate in the grid connection.

[0021] Furthermore, when the PV-storage system is connected to the grid with constant active power and different power differentials occur, the power size of each PCS designed in steps S3 and S4 is as follows:

[0022]

[0023]

[0024]

[0025] Where, P ref P is the reference value of the constant active power of the photovoltaic storage system connected to the grid; v The power generated by the photovoltaic system; P min is the lower limit of charge and discharge power of a single PCS; P batt1 is the power size of the first PCS involved; n is the power conversion of the nth PCS involved; P batt n The power level of the nth PCS.

[0026] Step S5: The reference value of the AC current dq axis of the entire energy storage system is obtained by weighting the power of each PCS designed in step S3 and step S4, and obtaining the reference value i of the AC side current dq axis of each PCS. d_ref and i q_ref ;

[0027] Step S7: The reference value i of the dq axis current on the AC side of each PCS d_ref and i q_ref After performing voltage and current dual-loop control, SVPWM is generated to control the PCS. At the same time, the lower discharge limit of the energy storage battery cluster is set to 20%, and the upper charge limit is set to 90% to prevent the battery cluster from over-discharge and over-charge, thereby realizing coordinated control of the constant active power grid connection of the photovoltaic storage system.

[0028] The beneficial effect of the present invention is that when the photovoltaic storage system is connected to the grid with constant active power, the power distribution between multiple PCSs is designed based on the life variation characteristics of the lithium-ion battery and the grid-connected harmonic content requirements to support the constant active power grid-connection of the photovoltaic storage system. This invention can extend the service life of the lithium-ion battery of the energy storage system under the condition of safe grid-connection of the energy storage system, and will not have a negative impact on the constant active power grid-connection of the photovoltaic storage system. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0030] Figure 1 This is a topological diagram of an embodiment of the present invention;

[0031] Figure 2 Curve of the number of cycles of aging of lithium iron phosphate battery under different charge and discharge currents;

[0032] Figure 3Control block diagram of a single PCS participating in the grid connection of a PV-storage system with constant active power;

[0033] Figure 4 Flowchart of overall thinking; DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0035] Figure 1 The topological structure diagram of the photovoltaic storage system grid-connected in this embodiment uses three battery clusters connected to the photovoltaic system. The energy storage battery is a 12.8V-40Ah lithium iron phosphate battery. The battery is discharged and charged with different currents and then its aging is measured (SOC is discharged from 100% to 20% and then charged to 100%). The battery aging cycle number curves under different charge and discharge currents are shown as follows: Figure 2 shown. Figure 2 It can be seen that as the charge and discharge current increases, the aging cycle of the energy storage battery increases faster and shows a nonlinear growth. Because the larger the charge and discharge current, the more serious the polarization of the battery internal material, which will make the irreversible cycle of battery charge and discharge more serious, so the battery capacity will decay sharply. For this reason, the charge and discharge current of the energy storage battery should be reduced as much as possible to extend the service life of the energy storage battery. The charge and discharge power of the energy storage battery cluster is related to the charge and discharge current of the battery. Therefore, based on Figure 2 A principle for extending the grid connection time of energy storage batteries when the PV-storage system is connected to the grid at constant active power can be derived: the charge and discharge current of the lithium-ion battery should be minimized. Specifically, multiple PCSs should be assigned the power differential to extend the service life of the energy storage battery. However, since the AC-side filter parameters are designed based on rated power, if the grid-connected active power of the energy storage is low, the harmonic content on the AC side will be high, potentially exceeding the 5% grid-connected harmonic content requirement specified by the national standard. Therefore, it is necessary to set a lower limit for the charge and discharge power of a single PCS to meet grid connection requirements. In this embodiment, based on experience, when the power contribution of a single PCS reaches above 40% of the rated power, the grid-connected harmonic content can be guaranteed to be less than 5%. Therefore, in this embodiment, the lower limit for the charge and discharge power of a single PCS is set at 40% of the rated power.

[0036] Based on the above-mentioned life cycle variation law of lithium-ion batteries and the grid-connected harmonic content requirements, the charge and discharge power of each battery cluster at different differential powers under constant active power of the solar-storage system can be obtained as follows:

[0037]

[0038]

[0039]

[0040] Where, P ref P is the reference value of the constant active power of the photovoltaic storage system connected to the grid; v The power generated by the photovoltaic system; P N_batt is the rated power of a single PCS; n is the nth PCS participating in power conversion; P batt1 is the power size of the first PCS; P batt n The power level of the nth PCS.

[0041] During the process of grid connection with a constant active power of the PV-storage system, the control of the power participation of each PCS is reflected in controlling the reference value of the d-axis component of the AC side current of each PCS. Since the energy storage system only outputs active power, the reference value of the q-axis component is set to 0. The reference value of the d-axis component is as follows:

[0042]

[0043]

[0044]

[0045] Where i d_ref is the reference value of the AC side current of the energy storage system under the d-axis component; i d_ref 1 is the reference value of the first PCS AC side under the d-axis component; i d_ref n It is the reference value of the AC side of the nth PCS under the d-axis component.

[0046] Further, such as Figure 3 As shown in the control block diagram of the first PCS, after obtaining the dq-axis component reference values for this PCS, SVPWM is generated through dual-loop voltage and current control to control the PCS. Furthermore, to prevent over-discharge and over-charge of the energy storage battery, a lower discharge limit of 20% and an upper charge limit of 90% are set, respectively. Exceeding these limits will halt the operation of the battery cluster, ultimately achieving the coordinated control of the present invention.

[0047] In summary, the overall idea flow chart of the present invention is as follows Figure 4 shown.

[0048] Furthermore, in order to demonstrate the superiority of the control of the present invention, corresponding simulation verification is carried out, and the simulation parameters are shown in Table 1:

[0049] Table 1 Simulation parameters of the constant active power grid-connected PV storage system

[0050]

[0051] When setting the grid-connected constant active power P ref When the active power is 50 kW, during the process of constant active power grid connection, when the light intensity changes, the photovoltaic system will generate different amounts of active power. The simulation results of the energy storage system participation are shown in Table 2:

[0052] Table 2 Grid connection of the photovoltaic storage system with constant active power at different photovoltaic powers

[0053]

[0054]

[0055] As shown in Table 2, the control strategy designed by the present invention consistently keeps the harmonic content of the grid-connected energy storage system within 5%. However, using a traditional control strategy can result in harmonic content exceeding 5%. For example, in the case of conventional control No. 2, a large amount of harmonics is injected into the grid, impacting other devices on the grid. Furthermore, when comparing the number of aging cycles at 160% of the battery's rated capacity (discharging the battery's SOC from 100% to 20% and then charging it to 100%, a total of 160%), the control strategy designed by the present invention has a clear advantage. For example, when the number is 5, the degree of aging at 160% of the rated capacity is 1.25 times (0.8625 / 0.692≈1.25) greater under the conventional control strategy than under the control strategy designed by the present invention. The same applies to Nos. 6 and 7. Therefore, the control strategy designed by the present invention can be effectively applied to the grid-connected operation of a photovoltaic storage system with constant active power, effectively extending the service life of lithium-ion batteries when the grid-connected energy storage system's harmonic content meets the requirements.

[0056] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention are included in the scope of protection of the present invention.

Claims

1. A control strategy for extending the service life of energy storage batteries under the condition of constant active power and safe grid connection of photovoltaic and energy storage systems, characterized by: The specific steps include: Step S1: Set the reference value of the constant active power of the solar-storage system connected to the grid P ref And the lower limit of charge and discharge power P of a single PCS min ; Step S2: Collect the power generated by the photovoltaic system P v ; Step S3: Determine the photovoltaic system power generation power P v Is it greater than or equal to the grid-connected constant active power reference value P ref , if it is greater than, set the power participation of each PCS; Step S4: Determine the photovoltaic system power generation power P v Is it less than the grid-connected constant active power reference value P ref , if it is less than, set the power participation of each PCS; Step S5: The reference value of the AC current dq axis of the entire energy storage system is obtained by weighting the power of each PCS designed in step S3 and step S4, and obtaining the reference value i of the AC side current dq axis of each PCS. d_ref and i q_ref ; Step S6: The reference value i of the dq axis current on the AC side of each PCS d_ref and i q_ref After performing dual-loop voltage and current control, SVPWM is generated to control the PCS. At the same time, the lower discharge limit of the energy storage battery cluster is set to 20%, and the upper charge limit is set to 90% to prevent the battery cluster from over-discharging and over-charging, thereby achieving coordinated control of the constant active power grid connection of the photovoltaic storage system. The specific content of step S3 is: If the difference power (P v -P ref ) is less than P min To ensure that the harmonic content of energy storage grid connection does not exceed 5%, all energy storage battery clusters stop absorbing the difference power; Furthermore, if P min <P v -P ref ≤2P min When , energy storage battery cluster 1 absorbs all the difference power, and the other battery clusters do not participate in the grid connection; Furthermore, if 2P min <P v -P ref ≤3P min When , the energy storage battery cluster 1 and battery cluster 2 absorb the difference power equally, and the other battery clusters do not participate in the grid connection; Furthermore, if 3P min <P v -P ref ≤4P min When , the energy storage battery cluster 1, battery cluster 2 and battery cluster 3 absorb the difference power equally, and the remaining battery clusters do not participate in the grid connection; Furthermore, by analogy, if n·P min <P v -P ref ≤(n+1)P min When , energy storage battery cluster 1, battery cluster 2, battery cluster 3, ..., battery cluster n absorb the difference power evenly, and the remaining battery clusters do not participate in the grid connection; The specific content of step S4 is: If the difference power (P ref -P v ) is less than P min In order to ensure that the harmonic content of energy storage grid connection does not exceed 5% and meet the grid connection power demand, energy storage battery cluster 1 emits P min ,The remaining battery clusters do not participate in grid connection; Furthermore, if P min <P ref -P v ≤2P min When , the energy storage battery cluster 1 will all generate the difference power, and the other battery clusters will not participate in the grid connection; Furthermore, if 2P min <P ref -P v ≤3P min When , the energy storage battery cluster 1 and battery cluster 2 share the difference power, and the remaining battery clusters do not participate in the grid connection; Furthermore, if 3P min <P ref -P v ≤4P min When , the energy storage battery cluster 1, battery cluster 2 and battery cluster 3 will share the difference power, and the remaining battery clusters will not participate in the grid connection; Furthermore, by analogy, if n·P min <P ref -P v ≤(n+1)P min When , energy storage battery cluster 1, battery cluster 2, battery cluster 3, ..., battery cluster n all share the difference in power, and the remaining battery clusters do not participate in the grid connection.

2. The control strategy for extending the service life of energy storage batteries under the condition of safe grid connection of a photovoltaic energy storage system with constant active power according to claim 1, wherein step S1 is characterized by: The lower limit of charge and discharge power of a single PCS is P min The goal is to reduce the charging and discharging power of the PCS as much as possible while keeping the harmonic content below 5% when a single PCS is connected to the grid, thereby reducing the output current of the energy storage battery and extending the battery life.

3. According to the control strategy for extending the service life of energy storage batteries under the condition of safe grid connection of a photovoltaic energy storage system with constant active power according to claim 1, the general characteristics of steps S3 and S4 are as follows: When the PV-storage system is connected to the grid with constant active power and different power differentials occur, the designed power participation of each PCS is as follows: Where, P ref P is the reference value of the constant active power of the photovoltaic storage system connected to the grid; v The power generated by the photovoltaic system; P min is the lower limit of the charge and discharge power of a single PCS; n is the nth PCS participating in power conversion; P batt1 is the power size of the first PCS; P battn The power level of the nth PCS.

Images