SOC coordination method based on sliding mode control for photovoltaic multi-battery energy storage unit

Abstract

The present invention relates to microgrid technology, and specifically to a SOC coordination method based on sliding mode control of a photovoltaic multi-battery energy storage unit. The microgrid includes a photovoltaic system and a battery energy storage system; the battery energy storage system includes a sensor, a sliding mode controller, a DC / DC bidirectional rectifier and a battery. The sensor collects local current, voltage and SOC signals of the battery and sends them to the sliding mode controller and sends SOC information to adjacent nodes; a first threshold and a second threshold are set, and the battery is divided into three working modes according to the relationship between the SOC of the battery, the first threshold and the second threshold; the sliding mode controller determines the working mode of the battery according to the collected local SOC information, and sends a control signal to control the output voltage of the energy storage system to adjust the output power of the battery. This method can avoid the risk of deep discharge and overcharging of the battery energy storage system at a certain node in the running microgrid, and adapt to the real-time changing reference output voltage.

Description

Technical Field

[0001] The present invention belongs to the technical field of microgrids, and in particular relates to a SOC coordination method based on sliding mode control of a photovoltaic multi-battery energy storage unit. Background Art

[0002] As people pay more attention to environmental protection, microgrids powered by solar energy, wind energy, and tidal energy have been widely used. However, this type of power supply is affected by the external environment, and its output power is unstable, making it difficult to provide stable power to the load in the microgrid. Taking photovoltaic power generation as an example, its output power is determined by the light intensity and the external ambient temperature. Therefore, direct connection to the grid will affect the quality of power and hinder the operation of the grid. In response to the above problems, a parallel battery energy storage system is usually used as a buffer between the photovoltaic system and the load to offset the problem of photovoltaic output power fluctuations through battery charging / discharging. The above-mentioned microgrid composed of photovoltaics and batteries is usually called a photovoltaic energy storage microgrid. However, as the construction and use of photovoltaic energy storage microgrids continues to expand, their investment costs are increasing year by year. Therefore, the maintenance cost of the energy storage system will be an expense that cannot be ignored.

[0003] In order to extend the service life of the energy storage system in the microgrid and avoid the battery at a certain node in the energy storage system from facing deep discharge and overcharging, it is usually hoped that the SOC of each node in the microgrid is at the same level to extend the service life of the energy storage system.

[0004] When a microgrid based on a photovoltaic system is in operation, it is difficult to continuously supply power to the load due to the volatility of photovoltaic system power generation. Usually, a battery energy storage system (BESSs) is connected in parallel to a photovoltaic microgrid to solve the impact of the volatility of photovoltaic system power generation on the load. When the output power of the photovoltaic system is greater than the power required by the load, the battery energy storage system absorbs the excess power. Conversely, when the output power of the photovoltaic system is less than the power required by the load, the battery energy storage system discharges to compensate for the power difference between the photovoltaic system and the load.

[0005] Due to repeated charging / discharging of the battery energy storage system, some BESS systems may fall into a state of overcharge / deep discharge. On the one hand, long-term overcharge / deep discharge will affect the service life of the battery; on the other hand, when some battery energy storage systems in the microgrid stop working, the number of normally working energy storage systems will decrease. At this time, the remaining normally working energy storage systems will be overloaded, causing local overheating of the system and damaging the service life of the energy storage system. In serious cases, it may also cause a series of safety accidents. This is bound to affect the normal operation of the microgrid.

[0006] Therefore, in order to extend the service life of the energy storage system, when controlling the battery energy storage system, it is hoped that the SOC (state of charge) of each battery energy storage system is in a coordinated state during operation. Summary of the invention

[0007] In view of the problems existing in the background technology, the present invention provides a SOC collaborative operation method of a photovoltaic microgrid battery energy storage system based on a sliding mode controller, which can make the SOC of each node of the battery energy storage system tend to a coordinated and consistent state during operation, and at the same time have a suppressive effect on the uncertainty fluctuations of photovoltaic power generation.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions: a SOC coordination method based on sliding mode control of a photovoltaic multi-battery energy storage unit, the coordination method is implemented based on a microgrid system, the microgrid system includes a battery energy storage system and a photovoltaic system, the battery energy storage system includes: a sensor, a sliding mode controller, a DC / DC bidirectional rectifier and a battery; the photovoltaic system is connected to a boost rectifier to control the terminal voltage of the photovoltaic panel; the coordination method includes the following steps:

[0009] Step 1: The sensor sends the collected voltage signal, current signal and SOC information of the battery energy storage system to the sliding mode controller;

[0010] Step 2: The sliding mode controller determines the working mode of the battery energy storage system based on the SOC information and calculates the reference voltage at the output end of the DC / DC bidirectional rectifier.

[0011] Step 3: The sliding mode controller generates a control signal through a set current droop control calculation formula to control the output voltage of the DC / DC bidirectional rectifier, thereby controlling the output power of the battery energy storage system.

[0012] In the above-mentioned SOC coordination method based on sliding mode control of a photovoltaic multi-battery energy storage unit, the implementation of step 2 includes the following steps:

[0013] Step 2.1: Set the first threshold SOC t1 and the second threshold SOC t2 , the battery energy storage system is divided into low-power operation mode, SOC coordinated operation mode and high-power operation mode;

[0014] Step 2.2: Determine whether the collected SOC value of the battery energy storage system is higher than the first threshold SOC t1 If yes, proceed to step 2.3; if no, determine that the battery energy storage system is running in the low power operation mode and SOC coordinated operation mode state;

[0015] Step 2.3: Determine whether the collected SOC value of the battery energy storage system is lower than the second threshold SOC t2 , if yes, it is determined that the battery energy storage system operates in the SOC coordinated operation mode; if no, it is determined that the battery energy storage system operates in the SOC coordinated operation mode and the high power operation mode state;

[0016] Step 2.4: Calculate the reference voltage at the output of the DC / DC bidirectional rectifier

[0017]

[0018] Among them, V nom Indicates the system setting value. represents the battery output current of the ith node, r i represents the initial droop coefficient of the i-th node, M represents the microgrid has M energy storage system units, sign(·) represents the sign function; let the SOC boundary limit of all batteries satisfy 0<SoC min <SoC max <1, then SoC i The piecewise function ΔR(SoC i ) is expressed as:

[0019]

[0020] is the first droop coefficient generated by the low power operation mode, is the second droop coefficient generated by the SOC cooperative operation mode, is the third droop coefficient generated by the high power operation mode;

[0021] Step 2.5: Calculate the first droop coefficient Second droop coefficient and the third droop coefficient

[0022] Step 2.5.1: First droop coefficient of low power operation mode The calculation formula is:

[0023]

[0024] in, Always a positive number, indicating the first droop coefficient Gain, SOC i Represents the SOC value of the i-th node, SOC t1 It is the first threshold set by the system;

[0025] Step 2.5.2: Second droop coefficient of SOC cooperative operation mode The calculation formula is:

[0026]

[0027] in, Indicates the second droop coefficient of the i-th node The gain coefficient, superscript j indicates the jth node, N i represents the set of neighbor nodes that have a communication relationship with the i-th node;

[0028] Step 2.5.3: The third droop coefficient of high power operation mode The calculation formula is:

[0029]

[0030] in, Indicates the third droop coefficient Gain, SoC t2 is the second threshold of the system;

[0031] In the above-mentioned SOC coordination method based on sliding mode control of photovoltaic multi-battery energy storage units, the implementation of step 3 includes:

[0032] Step 3.1: The control signal generated by the sliding mode controller is:

[0033]

[0034] Among them, S i is the sliding surface of the i-th node sliding mode controller, The reference voltage for the DC / DC bidirectional rectifier output, u i is the control signal of the first switch K1. i =1, the first switch K1 is turned on and the second switch K2 is turned off; when u i =0, the first switch K1 is turned off and the second switch K2 is turned on; and are the two gains of the sliding mode controller; sign(·) represents the sign function, is the output voltage of the battery energy storage system rectifier;

[0035] Step 3.2: The sliding mode controller is based on the reference voltage at the output of the DC / DC bidirectional rectifier. And the output voltage of the DC / DC bidirectional rectifier Generate a control signal to control the output voltage of the DC / DC bidirectional rectifier, thereby controlling the output power of the battery energy storage system.

[0036] Compared with the prior art, the present invention has the following beneficial effects: the present invention can effectively avoid the risk of deep discharge and overcharge of the battery energy storage system at a node in the running microgrid. At the same time, the interference of the photovoltaic system to the operation of the power grid is fully considered, and the strong robustness of the sliding mode controller is used to make the output voltage of the rectifier in the battery energy storage system For the reference output voltage The response speed is improved, and it can adapt to the reference output voltage that changes in real time due to the fluctuation of photovoltaic system power generation; since SOC coordination only needs to collect SOC information of adjacent nodes, and the SOC changes slowly over time, communication delay and packet loss problems can be ignored during the communication process of adjacent nodes. The sliding mode controller of the present invention can effectively deal with problems such as interference caused by fluctuations in photovoltaic system power generation. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 A microgrid topology structure according to an embodiment of the present invention;

[0038] Figure 2 A flowchart of switching a battery operating mode according to an embodiment of the present invention;

[0039] Figure 3 The figure is a schematic diagram of the structure of a rectifier according to an embodiment of the present invention. DETAILED DESCRIPTION

[0040] The technical solutions in the embodiments of the present invention will be described clearly and completely below in combination with 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 creative work are within the scope of protection of the present invention.

[0041] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0042] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited thereto.

[0043] The formula symbol variable x in this embodiment i The superscript i in represents the i-th node.

[0044] This embodiment is implemented by the following technical solutions: Figure 1As shown in the figure, the microgrid system includes: a battery energy storage system and a photovoltaic system. The battery energy storage system includes: a sensor, a sliding mode controller, a DC / DC bidirectional rectifier and a battery. The boost rectifier connected to the photovoltaic system controls the terminal voltage of the photovoltaic panel so that it always runs in the maximum power output (MPPT) mode to more effectively utilize solar power generation.

[0045] In order to achieve SOC coordination, this embodiment proposes a SOC coordination method based on sliding mode control for a photovoltaic multi-battery energy storage unit. The battery energy storage system sets two SOC thresholds: the first threshold SOC t1 and the second threshold SOC t2 , the battery energy storage system is divided into three different working modes, namely: low power operation mode, SOC coordinated operation mode and high power operation mode. This control method is based on the current droop control. On the basis of traditional droop control, three different droop coefficients are added: the first droop coefficient Second droop coefficient and the third droop coefficient The bias droop coefficients correspond to the three different working modes of the battery energy storage system: low power operation mode, SOC coordinated operation mode and high power operation mode.

[0046] The sensor sends the collected local voltage signal, current signal, and SOC signals of local and adjacent nodes to the sliding mode controller;

[0047] The sliding mode controller determines the operating mode of the battery by analyzing the SOC value of the local node. Different operating modes will affect the output reference voltage of the battery energy storage system.

[0048] The sliding mode controller generates a control signal through the set droop control calculation formula to control the output voltage of the DC / DC bidirectional rectifier of the battery energy storage system, thereby controlling the output power of the battery energy storage system.

[0049] The SOC coordination method based on sliding mode control of a photovoltaic multi-battery energy storage unit includes the following steps:

[0050] The sensor sends the collected voltage, current and SOC related information to the sliding mode controller, which determines the working mode of the battery energy storage system based on the SOC information and calculates the reference voltage at the output end of the DC / DC bidirectional rectifier. The sliding mode controller is based on the reference voltage at the output of the DC / DC bidirectional rectifier. And the output voltage of the DC / DC bidirectional rectifier Generate a control signal to control the output voltage of the DC / DC bidirectional rectifier, thereby controlling the output power of the battery energy storage system.

[0051] The DC / DC bidirectional rectifier output reference voltage calculation formula is:

[0052]

[0053] Among them, V nom Indicates the system setting value. represents the battery output current of the ith node, r i represents the initial droop coefficient of the i-th node, M represents the microgrid has M energy storage system units, sign(·) represents the sign function; let the SOC boundary limit of all batteries satisfy 0<SoC min <SoC max <1, then SoC i The piecewise function ΔR(SoC i ) can be expressed as:

[0054]

[0055] For the additional bias droop factor and Only when the battery enters the corresponding working mode, its corresponding droop coefficient will take effect. For example, the reference voltage of the battery energy storage system in the SOC coordinated operation mode is: The reference voltage of the battery energy storage unit in the SOC cooperative operation mode in the low power operation mode is: The reference voltage of the battery energy storage unit in the SOC cooperative operation mode in the high power operation mode is:

[0056] The calculation formulas for the offset droop coefficients generated by the three different working modes are:

[0057] (1) Low power operation mode generates corresponding offset droop coefficient First droop coefficient The determination formula is:

[0058]

[0059] in, Always a positive number, indicating Gain, SOC i Represents the SOC value of the i-th node, SOC t1 It is the first threshold set by the system;

[0060] (2) The SOC cooperative operation mode generates the corresponding offset droop coefficient and the second droop coefficient The determination formula is:

[0061]

[0062] in Represents the i-th node The gain coefficient of the jth node, N i represents the set of networks that have a communication relationship with the i-th node;

[0063] (3) The high power operation mode generates the corresponding offset droop coefficient and the third droop coefficient The determination formula is:

[0064]

[0065] in express Gain, SoC t2 It is the second threshold of the system.

[0066] The SOC of the local node battery energy storage system is lower than the first threshold SOC t1 When the battery energy storage system enters the low power operation mode, due to the first droop coefficient The effect is to reduce the output voltage of the battery energy storage system, thereby reducing the output power of the local node battery.

[0067] The SOC of the local node battery energy storage system is lower than the second threshold SOC t2 When the SOC of the local node battery energy storage system is higher than that of the adjacent node battery energy storage system, due to the second droop coefficient The effect of the second droop coefficient is to increase the output voltage of the battery energy storage system, thereby increasing the output power of the local node battery energy storage system; when the SOC of the local node battery energy storage system is lower than the SOC of the adjacent node battery energy storage system, The function is to reduce the output voltage of the battery energy storage system, thereby reducing the output power of the local node battery energy storage system;

[0068] The SOC of the local node battery energy storage system is higher than the second threshold SOC t2 When the battery energy storage system enters the high power operation mode, due to the third droop coefficient The function is to increase the output voltage of the battery energy storage system, thereby increasing the output power of the local node battery energy storage system.

[0069] The sliding mode controller of this embodiment is a sliding mode controller for voltage tracking, which has strong robustness and can resist interference fluctuations generated by photovoltaics. The sliding mode controller generates a control signal to control the output voltage of the DC / DC bidirectional rectifier. Only when the battery energy storage system enters the corresponding working mode, the corresponding droop coefficient will take effect in the droop control process.

[0070] The method of generating control signal by sliding mode controller is:

[0071]

[0072] Among them, S i is the sliding surface of the i-th node sliding mode controller, The reference voltage for the DC / DC bidirectional rectifier output, u i is the control signal of the first switch K1. i =1, the first switch K1 is turned on and the second switch K2 is turned off; when u i =0, the first switch K1 is turned off and the second switch K2 is turned on; and are the two gains of the sliding mode controller; sign(·) represents the sign function, is the output voltage of the rectifier of the battery energy storage system. The sliding mode controller generates a control signal according to the sliding mode surface, so it can effectively suppress external interference.

[0073] In the specific implementation, the signals collected by the sensors of the battery energy storage system are: the output current i of the battery of the battery energy storage system b , the output voltage of the DC / DC bidirectional rectifier V dc The sensor sends the collected signal to the sliding mode controller, which determines the working mode of the battery energy storage system based on the collected SOC value.

[0074] like Figure 2 As shown, determining the working mode of the battery energy storage system includes:

[0075] 1) Determine whether the collected battery energy storage system SOC value is higher than the first threshold SOC set by the system t1 If yes, proceed to step 2); if not, determine that the battery energy storage system operates in low-power operation mode and SOC coordinated operation mode.

[0076] 2) Determine whether the collected battery energy storage system SOC value is lower than the second threshold SOC set by the system t2 If so, it is determined that the battery energy storage system operates in the SOC coordinated operation mode; if not, it is determined that the battery energy storage system operates in the SOC coordinated operation mode and the high power operation mode.

[0077] The sliding mode controller calculates the three bias droop coefficients according to the formula: Second droop coefficient The third droop coefficient The sliding mode controller selects the corresponding droop bias coefficient according to the working mode of the battery energy storage system and calculates the output reference voltage of the DC / DC bidirectional rectifier.

[0078] The DC / DC bidirectional rectifier output reference voltage calculation formula is:

[0079]

[0080] Among them, V nom Indicates the system setting value. represents the battery output current of the ith node, r i represents the initial droop coefficient of the i-th node, M represents the microgrid has M energy storage system units, sign(·) represents the sign function; let the SOC boundary limit of all batteries satisfy 0<SoC min <SoC max <1, then SoC i The piecewise function ΔR(SoC i ) can be expressed as:

[0081]

[0082] For the additional bias droop factor and Only when the battery enters the corresponding working mode, its corresponding droop coefficient will take effect. For example, the reference voltage of the battery energy storage system in the SOC coordinated operation mode is: The reference voltage of the battery energy storage unit in the SOC cooperative operation mode in the low power operation mode is: The reference voltage of the battery energy storage unit in the SOC cooperative operation mode in the high power operation mode is:

[0083] The calculation formulas for the offset droop coefficients generated by the three different working modes are:

[0084] a. The first droop coefficient formula for low power operation mode is:

[0085]

[0086] in, Always a positive number, indicating the first droop coefficient Gain, SOC i Represents the SOC value of the i-th node, SOC t1 It is the working mode threshold set by the system.

[0087] b. The second droop coefficient formula of SOC cooperative operation mode is:

[0088]

[0089] in Indicates the second droop coefficient of the i-th node The gain coefficient of the jth node, N i represents the set of networks that have a communication relationship with the i-th node;

[0090] c. The third droop coefficient formula for high power operation mode is:

[0091]

[0092] in Indicates the third droop coefficient Gain, SoC t2 The threshold set for the system.

[0093] The voltage control method of this embodiment enables the battery energy storage system to achieve SOC coordination of batteries at each node during repeated charging and discharging, preventing the battery energy storage system at a certain node from being overcharged or deeply discharged due to unreasonable load sharing. The battery energy storage system enters a low-power operation mode, due to the first droop coefficient The battery energy storage system enters the SOC cooperative operation mode. When the SOC of the local node battery energy storage system is higher than that of the adjacent node battery energy storage system, due to the second droop coefficient The effect of the second droop coefficient is to increase the output voltage of the battery energy storage system, thereby increasing the output power of the local node battery energy storage system; when the SOC of the local node battery energy storage system is lower than the SOC of the adjacent node battery energy storage system, The effect of the third droop factor is to reduce the output voltage of the battery energy storage system, thereby reducing the output power of the local node battery energy storage system. The battery energy storage system will enter a high power operation mode. The function is to increase the output voltage of the battery energy storage system, thereby increasing the output power of the local node battery energy storage system.

[0094] The formula for the sliding mode controller to generate the control signal is:

[0095]

[0096] Among them, S i is the sliding surface of the i-th node sliding mode controller, The reference voltage for the DC / DC bidirectional rectifier output, u i is the control signal of the first switch K1. i =1, the first switch K1 is turned on and the second switch K2 is turned off; when u i =0, the first switch K1 is turned off and the second switch K2 is turned on; and are the two gains of the sliding mode controller; sign(·) represents the sign function, is the output voltage of the battery energy storage system rectifier.

[0097] like Figure 3 As shown, the control signal of the sliding mode controller controls the first switch K1 of the DC / DC bidirectional rectifier, while the control signal of the sliding mode controller controls the control state of the second switch K2 of the DC / DC bidirectional rectifier, which is opposite to the control state of the first switch K1. The mutual conduction and cutoff of the two switches make the inductance in the DC / DC bidirectional rectifier switch repeatedly between energy storage and discharge, thereby realizing the control of the output voltage of the DC / DC bidirectional rectifier.

[0098] The above are only preferred embodiments of the present invention, and are not intended to limit the implementation methods and protection scope of the present invention. Those skilled in the art should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the contents of the specification of the present invention should be included in the protection scope of the present invention.

Claims

1. A SOC coordination method based on sliding mode control for a photovoltaic multi-battery energy storage unit, characterized by: The collaborative method is implemented based on a microgrid system, which includes a battery energy storage system and a photovoltaic system. The battery energy storage system includes: a sensor, a sliding mode controller, a DC / DC bidirectional rectifier and a battery; the photovoltaic system is connected to a boost rectifier to control the terminal voltage of the photovoltaic panel; the collaborative method includes the following steps: Step 1: The sensor sends the collected voltage signal, current signal and SOC information of the battery energy storage system to the sliding mode controller; Step 2: The sliding mode controller determines the working mode of the battery energy storage system based on the SOC information and calculates the reference voltage at the output end of the DC / DC bidirectional rectifier. The implementation of step 2 includes the following steps: Step 2.1: Set the first threshold SOC t1 and the second threshold SOC t2 , the battery energy storage system is divided into low-power operation mode, SOC coordinated operation mode and high-power operation mode; Step 2.2: Determine whether the collected SOC value of the battery energy storage system is higher than the first threshold SOC t1 If yes, proceed to step 2.3; if no, determine that the battery energy storage system is running in the low power operation mode and SOC coordinated operation mode state; Step 2.3: Determine whether the collected SOC value of the battery energy storage system is lower than the second threshold SOC t2 , if yes, it is determined that the battery energy storage system operates in the SOC coordinated operation mode; if no, it is determined that the battery energy storage system operates in the SOC coordinated operation mode and the high power operation mode state; Step 2.4: Calculate the reference voltage at the output of the DC / DC bidirectional rectifier Among them, V nom Indicates the system setting value. represents the battery output current of the ith node, r i represents the initial droop coefficient of the i-th node, M represents the microgrid has M energy storage system units, sign(·) represents the sign function; let the SOC boundary limit of all batteries satisfy 0 <SoC min <SoC max <1, then SoC i The piecewise function ΔR(SoC i ) is expressed as: is the first droop coefficient generated by the low power operation mode, is the second droop coefficient generated by the SOC cooperative operation mode, is the third droop coefficient generated by the high power operation mode; Step 2.5: Calculate the first droop coefficient separately Second droop coefficient and the third droop coefficient Step 2.5.1: First droop coefficient of low power operation mode The calculation formula is: in, Always a positive number, indicating the first droop coefficient Gain, SOC i represents the SOC value of the i-th node, SOC t1 It is the first threshold set by the system; Step 2.5.2: Second droop coefficient of SOC cooperative operation mode The calculation formula is: in, Indicates the second droop coefficient of the i-th node The gain coefficient, superscript j indicates the jth node, N i represents the set of neighbor nodes that have a communication relationship with the i-th node; Step 2.5.3: The third droop coefficient of high power operation mode The calculation formula is: in, Indicates the third droop coefficient Gain, SoC t2 is the second threshold of the system; Step 3: The sliding mode controller generates a control signal through a set current droop control calculation formula to control the output voltage of the DC / DC bidirectional rectifier, thereby controlling the output power of the battery energy storage system; The implementation of step 3 includes: Step 3.1: The control signal generated by the sliding mode controller is: Among them, S i is the sliding surface of the i-th node sliding mode controller, The reference voltage for the DC / DC bidirectional rectifier output, u i is the control signal of the first switch K1. i =1, the first switch K1 is turned on and the second switch K2 is turned off; when u i =0, the first switch K1 is turned off and the second switch K2 is turned on; and are the two gains of the sliding mode controller; sign(·) represents the sign function, is the output voltage of the battery energy storage system rectifier; Step 3.2: The sliding mode controller is based on the reference voltage at the output of the DC / DC bidirectional rectifier. And the output voltage of the DC / DC bidirectional rectifier Generate a control signal to control the output voltage of the DC / DC bidirectional rectifier, thereby controlling the output power of the battery energy storage system.

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