A method for DC bus voltage stabilization in photovoltaic and energy storage microgrid based on feedforward variable structure PI control

Abstract

The present invention provides a method for stabilizing the DC bus voltage of a photovoltaic storage microgrid based on feedforward variable structure PI control. In a photovoltaic storage DC microgrid, in order to ensure smooth and stable energy flow between the battery and the DC bus, and to solve the problems of large DC bus voltage fluctuation and poor anti-interference performance caused by photovoltaic output uncertainty and frequent load disturbances, the present invention designs a bidirectional DC-DC converter control strategy based on variable structure PI control on the basis of establishing a small signal mathematical model of a bidirectional DC-DC converter. The strategy uses variable structure control to compensate for the real-time information of the interference signal, and combines feedforward control to enhance the robustness of the system.

Description

Technical Field

[0001] The present invention relates to the field of new energy technology, and in particular to a method for stabilizing direct current bus voltage of a photovoltaic energy storage microgrid based on feedforward variable structure PI control. Background Art

[0002] In recent years, people have paid increasing attention to energy shortages and environmental pollution, which has promoted the development of renewable energy such as photovoltaic power generation, wind power generation and hydropower generation. Among them, photovoltaic power generation has become one of the most promising power generation technologies because of its abundant resources, safety, reliability and pollution-free. However, the randomness and intermittency of photovoltaic power generation make it susceptible to the external environment, resulting in frequent fluctuations in the DC bus voltage in the photovoltaic storage DC microgrid. In the photovoltaic storage DC microgrid, the energy storage unit plays a vital role, which can not only alleviate the impact of unstable photovoltaic output on power quality, but also improve the power supply reliability and power utilization of the system. As the interface circuit between the DC bus and the energy storage unit, the bidirectional DC-DC converter plays an important role in stabilizing the DC bus voltage. However, the traditional PI-controlled bidirectional DC-DC converter is difficult to effectively cope with the situation of large voltage range and drastic load changes.

[0003] Traditional PI control has been widely used in the field of modern industrial control due to its advantages such as simplicity and fewer parameter settings. However, under multiple disturbances and high complexity, PI control is prone to cause system overshoot and oscillation. In the case of drastic load changes or large fluctuations in photovoltaic output, traditional PI controllers are difficult to respond quickly, resulting in a decrease in the dynamic performance of the system. In addition, PI control has limited ability to suppress external interference to the system, which can easily cause overshoot, oscillation and other phenomena in the system, affecting the dynamic performance and steady-state accuracy of the system. In addition, PI control usually does not take into account the nonlinear characteristics and uncertainties of the system, which makes it show great limitations in practical applications.

[0004] In order to deal with the problem of DC bus voltage fluctuation caused by external disturbances, researchers have proposed some improved control strategies. For example, the improved current compensation model predictive control strategy based on the residual generator not only effectively solves the problem, but also significantly improves the dynamic response performance of the system. In addition, a non-singular terminal sliding mode control strategy based on a nonlinear disturbance observer is designed, which can effectively deal with the voltage fluctuation problem caused by photovoltaic output power and load disturbances in the photovoltaic storage DC microgrid. The self-disturbance rejection control strategy based on fuzzy neural network realizes the total disturbance estimation and online optimization of controller parameters of the bidirectional DC-DC converter through the linear extended state observer and fuzzy neural network, thereby solving the problem of deterioration of control effect caused by operating condition uncertainty. Although these methods have shown significant effects in improving system performance, their high computational complexity and large demand for processor resources make these methods difficult to implement in single-chip microcomputers with limited resources.

[0005] Although some existing improved control strategies have achieved remarkable results in improving the performance of photovoltaic-storage DC microgrid systems, their high computational complexity and large demand for processor resources limit the promotion and implementation of these methods in practical applications. Therefore, it is particularly important to design a simple and effective control method. A simple and efficient control method can not only improve the dynamic response performance while ensuring system stability, but also reduce the requirements for hardware resources, enhance the feasibility of the practical application of the control strategy, and provide reliable guarantee for the stable operation of photovoltaic-storage DC microgrids. Summary of the invention

[0006] The purpose of the present invention is to provide a method for stabilizing the DC bus voltage of a photovoltaic storage microgrid based on feedforward variable structure PI control. In a photovoltaic storage DC microgrid, in order to ensure smooth and stable energy flow between the battery and the DC bus, and to solve the problem of large DC bus voltage fluctuation and poor anti-interference performance caused by photovoltaic output uncertainty and frequent load disturbances, the present invention designs a bidirectional DC-DC converter control strategy based on variable structure PI control on the basis of establishing a small signal mathematical model of a bidirectional DC-DC converter. The strategy uses variable structure control to compensate for the real-time information of the interference signal, and combines feedforward control to enhance the robustness of the system.

[0007] The present invention provides a method for stabilizing direct current bus voltage of a photovoltaic energy storage microgrid based on feedforward variable structure PI control, the method comprising the following steps:

[0008] Step 1: Establish a photovoltaic-storage DC microgrid. The photovoltaic-storage DC microgrid structure mainly includes photovoltaic power generation units, loads, power electronic converters and energy storage units. In this structure, the discharge power of the energy storage unit is defined as a positive value, the charging power is defined as a negative value, and the grid absorption power is defined as a positive value; the relationship between the variables is as shown in the formula As shown, where: P ES is the output power of the battery; P Grid The power delivered by the DC bus to the grid; P load It is the power delivered by the DC bus to the load side; P PV is the output power of photovoltaic;

[0009] Step 2: Modeling of the energy storage bidirectional DC-DC converter, wherein the converter modeling is divided into a mathematical model in Boost mode and a mathematical model in Buck mode;

[0010] Step 3: Design of feedforward variable structure PI controller. The voltage outer loop control strategy based on feedforward variable structure PI control includes two parts: variable structure PI controller and feedforward controller. The variable structure PI controller is responsible for processing the main error of the system, reducing the steady-state error and dynamic error of the system by adjusting the control input, and dynamically adjusting the integral gain. k i To cope with the nonlinearity and dynamic changes of the system; the feedforward controller improves the robustness of the system by compensating for interference signals in real time. The feedforward controller quickly adjusts the control input according to the size and change trend of the interference signal without waiting for the system error to appear.

[0011] Further improvement is that: the present invention mainly studies the photovoltaic microgrid in the island operation mode, and realizes voltage stability by adjusting the power balance between the energy storage unit and the DC bus. Therefore, PGrid is set to 0. Based on the load power and the maximum output power of the photovoltaic array, the switching state of the bidirectional DC-DC converter is reasonably controlled to realize the bidirectional energy flow between the energy storage device and the DC bus, thereby ensuring the balance of the DC bus power and effectively suppressing voltage fluctuations.

[0012] The further improvement is that: in the mathematical model of the Boost mode in step 2, when the switches S1 and S2 are turned off, the diode D2 is cut off, and the diode D1 is turned on, the state space equation of the system is expressed as , V dc is the bus voltage; C b is the battery side capacitance; C d is the bus side capacitance; L is the inductance on the battery side; when the switch tube S2 is turned on, S1 is turned off, and the diodes D1 and D2 are turned off, the state space equation of the system is expressed as: ; Through the state space averaging method, the state space average equation of the circuit in Boost mode is obtained: , where: d Boost is the duty cycle in Boost working mode; I bat Represents the inductor current on the energy storage side; R dc Indicates the equivalent load on the DC bus side; L Represents the inductance on the energy storage side; V dc Represents the DC bus voltage; through small signal modeling and Laplace transform, the transfer function in Boost mode is obtained: .

[0013] Further improvement is that: the working state of the mathematical model in the Buck mode in step 2 is similar to that in the Boost mode, and the state space average equation and transfer function are: and , where: d Buck is the duty cycle in Buck working mode; R bat is the equivalent resistance on the energy storage side.

[0014] Further improvement is that in actual systems, saturation characteristics may significantly affect system performance. When debugging a bidirectional DC-DC converter, small signals are usually used for debugging, which may cause designers to underestimate the nonlinear effects of saturation characteristics and thus ignore their actual existence. However, in actual operation, when the system is subjected to large signal input or other factors that cause the control system to enter a saturated state, the system performance will drop significantly and may not meet the expected requirements. Therefore, when saturation occurs, it is necessary to introduce appropriate compensation links in order to maintain better performance indicators. Variable structure design technology has become a basic method in the design of control systems that deal with saturation characteristics. The core idea is to calculate the saturation error of the control input. V 1 -V 2 The integral is performed and adjusted by the adaptive coefficient, and then the adjustment result is added to the integral term of the PI controller. Adaptive adjustment rule β as follows: , where: α To adaptively adjust the gain coefficient, generally take 1; V 3 is the average gain coefficient; e 1 is the system feedback error; k i is the controller integral gain coefficient; V 3 The calculation method is as follows: , where: V max is the maximum output of the controller; V min is the minimum output of the controller.

[0015] A further improvement is that the expression of the variable structure PI control is as follows: , where: k pis the proportional gain coefficient of the controller. The feedforward controller compensates according to the size and change trend of the interference signal. The feedforward controller responds immediately when the interference occurs, without waiting for the occurrence of system deviation. Feedforward control is more timely and effective than traditional feedback control. The feedforward controller does not rely on the output error of the system, and compensates when the interference just occurs, thereby avoiding the influence of system lag. Its mathematical expression is as follows: , where: K ffc is the feedforward control coefficient; I batref It is the reference value of current loop control; U dcref It is the reference value of voltage loop control.

[0016] The beneficial effects of the present invention are as follows: in view of the uncertainty and load fluctuation problems of photovoltaic power generation output, a bidirectional DC-DC converter control strategy based on feedforward variable structure PI control is proposed. By introducing a variable structure PI controller, the control parameters are dynamically adjusted to effectively cope with the nonlinearity and dynamic changes of the system, thereby improving the control accuracy and response speed. The feedforward controller is used to compensate for external interference in real time, effectively improving the robustness and stability of the system. This method can better ensure the stability of the DC bus voltage and ensure the reliable operation of the photovoltaic storage microgrid system under various working conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of the structure of a photovoltaic energy storage DC microgrid.

[0018] Figure 2 It is the topology diagram of the energy storage bidirectional DC-DC converter.

[0019] Figure 3 It is the feedforward variable structure PI control block diagram.

[0020] Figure 4 It is a variable structure PI control structure diagram.

[0021] Figure 5 It is the bus voltage waveform during load disturbance.

[0022] Figure 6 It is the bus voltage waveform when the photovoltaic output is disturbed.

[0023] Figure 7 This is a diagram of the energy storage bidirectional DC-DC converter experimental platform.

[0024] Figure 8 This is the waveform of the output voltage when the input voltage suddenly increases.

[0025] Fig. 9 This is the output voltage waveform when the input voltage suddenly drops.

[0026] Fig.10 It is the output voltage waveform when the load power suddenly decreases.

[0027] Fig.11 It is the output voltage waveform when the load power suddenly increases. DETAILED DESCRIPTION

[0028] In order to deepen the understanding of the present invention, the present invention will be further described in detail below in conjunction with examples. The examples are only used to explain the present invention and do not constitute a limitation on the protection scope of the present invention.

[0029] In a photovoltaic DC microgrid, in order to ensure smooth and stable energy flow between the battery and the DC bus, and to solve the problem of large DC bus voltage fluctuations and poor anti-interference performance caused by photovoltaic output uncertainty and frequent load disturbances, the present invention designs a bidirectional DC-DC converter control strategy based on variable structure PI control on the basis of establishing a small signal mathematical model of a bidirectional DC-DC converter. This strategy uses variable structure control to compensate for the real-time information of the interference signal, and combines feedforward control to enhance the robustness of the system. Through simulation and STM32F4 experimental platform, the control strategy proposed by the present invention and the DC bus side voltage waveform of proportional integral control under photovoltaic and load disturbances are analyzed, proving the effectiveness of feedforward variable structure PI control in suppressing DC bus voltage fluctuations.

[0030] Figure 1 A typical photovoltaic-storage DC microgrid structure is shown, which mainly includes photovoltaic power generation unit, load, power electronic converter and energy storage unit. Among them, the discharge power of the energy storage unit is positive, the charging power is negative, and the grid absorption power is positive. Figure 1 Shows the relationship between variables: , where: P ES is the output power of the battery; P Grid The power delivered by the DC bus to the grid; P load It is the power delivered by the DC bus to the load side; P PV is the photovoltaic output power.

[0031] This example mainly discusses how to achieve voltage stability by controlling the power balance between the energy storage unit and the DC bus in the isolated operation mode of the photovoltaic microgrid. P Grid =0 According to the load power and the maximum power output by the photovoltaic array, the switching state of the bidirectional DC-DC converter is reasonably controlled to achieve bidirectional energy flow between the energy storage device and the DC bus, thereby ensuring power balance on the DC bus and suppressing voltage fluctuations.

[0032] The equivalent circuit of energy flow on the energy storage side is as follows: Figure 2 As shown. Since the switching frequency of the bidirectional DC-DC converter is much higher than the speed of the microgrid state change, the power on the DC bus side can be approximately constant in a short period of time, which is equivalent to a constant power load. At the same time, the terminal voltage on the energy storage side remains unchanged in a short period of time and can be regarded as an ideal constant voltage source. Figure 2 middle: I load Indicates the load current on the DC bus side; I bat Represents the inductor current on the energy storage side; I o Indicates the output current; L Represents the inductance on the energy storage side; V bat Indicates the voltage on the energy storage side; R dc Indicates the equivalent load on the DC bus side.

[0033] When the switch S1 and S2 Shutdown, diode D2 Cut-off, diode D1 When conducting, the state space equation of the system is expressed as: , where: V dc is the bus voltage; C b is the battery side capacitance; C d is the bus side capacitance; L is the battery side inductance.

[0034] When the switch tube S2 is turned on, S1 is turned off, and the diodes D1 and D2 are turned off, the state space equation of the system is expressed as: .

[0035] Through the state space averaging method, the state space average equation of the circuit in Boost mode is obtained: , where: d Boost is the duty cycle in Boost working mode; I bat Represents the inductor current on the energy storage side; R dc Indicates the equivalent load on the DC bus side; L Represents the inductance on the energy storage side; V dc Indicates the DC bus voltage.

[0036] Through small signal modeling and Laplace transform, the transfer function in Boost mode is obtained: .

[0037] The working state of Buck mode is similar to that of Boost mode. In Buck mode, the state space average equation and transfer function are: , , where: d Buck is the duty cycle in Buck working mode; R bat is the equivalent resistance on the energy storage side.

[0038] The voltage outer loop control block diagram based on feedforward variable structure PI control is as follows: Figure 3 As shown in the figure, it includes two parts: variable structure PI controller and feedforward controller. The variable structure PI controller is responsible for processing the main error of the system, reducing the steady-state error and dynamic error of the system by adjusting the control input, and dynamically adjusting the integral gain. k i , to cope with the nonlinearity and dynamic changes of the system. The feedforward controller compensates for the interference signal based on the real-time information of the interference signal, and can immediately adjust the control input based on the size and change trend of the interference signal without waiting for the occurrence of system errors.

[0039] In actual systems, saturation characteristics can significantly affect performance. When debugging a bidirectional DC-DC converter, a small signal is usually used as the debugging signal, which may cause the designer to underestimate the nonlinear effect of the saturation characteristic and ignore its existence. However, in actual operation, when the system is subjected to a large signal input or other conditions that cause the control system to enter a saturated state, the system performance will drop significantly and may not meet the expected requirements. Therefore, in order to maintain good performance indicators when saturation occurs, it is necessary to introduce appropriate compensation links. Variable structure design technology has become a basic method in the design of control systems that deal with saturation characteristics. The core idea is to compensate for the saturation error of the control input. V1−V2 The integral is performed and adjusted through the adaptive coefficient, and the adjusted result is added to the integral term of the PI controller, thereby effectively compensating for the error caused by saturation and improving the performance of the control system.

[0040] Adaptive adjustment rules β as follows: , where: α To adaptively adjust the gain coefficient, generally take 1; V 3 is the average gain coefficient; e 1 is the system feedback error; k i is the controller integral gain coefficient.

[0041] V 3 The calculation method is as follows: , where: V max is the maximum output of the controller; V min is the minimum output of the controller.

[0042] In summary, the expression of variable structure PI control is as follows: .

[0043] The feedforward controller operates on the principle of compensation based on the amplitude and change trend of the interference signal. When interference occurs, the feedforward controller can respond immediately without waiting for the system deviation to appear. Therefore, feedforward control has higher timeliness and effectiveness than traditional feedback control. Since the feedforward controller does not rely on the system output error, it can compensate for the interference as soon as it occurs, thus avoiding the system lag effect. The mathematical expression is as follows: , where: K ffc is the feedforward control coefficient, generally taken as 2; I batref It is the reference value of current loop control; U dcref It is the reference value of voltage loop control.

[0044] The sudden change of load power and photovoltaic output power is the main reason for the DC bus voltage fluctuation. Therefore, this paper constructs a Figure 1 The photovoltaic storage DC microgrid system shown in Figure 1 is used to simulate the sudden changes in the output power of the photovoltaic array and the load power. To simplify the analysis, a pure resistive load is used on the load side, and the change in load power is simulated by adjusting the resistance value. In order to verify the effectiveness and advantages of the proposed control strategy in the microgrid system, the feedforward variable structure PI control is compared with the classical PI control. To ensure the fairness of the comparison, the PI controller has been adjusted to the optimal working state to evaluate the performance of the two control strategies in dealing with voltage fluctuations.

[0045] Under the condition that the light intensity is kept at 800W / m², the load power suddenly decreases from 8kW to 4kW at t=4 seconds, and then increases from 4kW to 8kW at t=7 seconds. Figure 5The waveform comparison of the DC bus voltage of the bidirectional DC-DC converter under two different control strategies is shown. Since the photovoltaic output power remains unchanged, the load power decreases at t=4 seconds, resulting in excess power in the DC bus, which increases the bus voltage. At this time, the bidirectional DC-DC converter switches to Buck mode and transfers the excess power to the energy storage device, allowing the battery to enter a charging state. On the contrary, the increase in load power at t=4 seconds requires the energy storage device to provide additional energy to the DC bus to maintain power balance. In this case, the battery is in a discharged state, and the bidirectional DC-DC converter operates in Boost mode, transferring energy from the battery to the DC bus. During load disturbances, the bus voltage fluctuates as shown in Figure 5 shown.

[0046] Under the condition that the load power remains unchanged, the light intensity decreases from 800W / m² to 200W / m² at t=4 seconds, and increases again from 200W / m² to 800W / m² at t=6 seconds. At t=4 seconds, due to the decrease in photovoltaic output power, the battery needs to provide additional energy to the DC bus to maintain the dynamic balance of power. In this case, the bidirectional DC-DC converter operates in Boost mode, the battery is in a discharge state, and energy is transferred from the battery to the DC bus. At t=6 seconds, the increase in light intensity causes the output power of the photovoltaic array to increase, thereby increasing the DC bus voltage. At this time, the bidirectional DC-DC converter switches to Buck mode, the battery enters the charging state, and absorbs excess energy from the DC bus side to stabilize the bus voltage. During load disturbances, the bus voltage fluctuates as follows: Figure 6 shown.

[0047] To further verify the effectiveness of feedforward variable structure PI control, Figure 7 As shown in the figure, a small voltage prototype experimental platform based on the STM32F407 chip is built. The technical parameters of the system are as follows: input voltage is 12V, output voltage is 30V, and switching frequency is 30kHz. The experimental platform includes a host computer, an energy storage unit, a programmable electronic load, and a bidirectional DC-DC converter, in which the energy storage unit is replaced by a programmable DC power supply.

[0048] During the experiment, a programmable DC power supply and a programmable electronic load were used to simulate various conditions where the bus voltage was subject to real-time interference, including a sudden increase or decrease in bus voltage, and a sudden increase or decrease in load. To verify the performance of the proposed feedforward variable structure PI control, the classical PI control and the feedforward variable structure PI control were used as a comparative experiment for the voltage outer loop control. Figure 8 , Fig. 9 , Fig.10 , Fig.11The output voltage waveform under different working conditions is displayed, and the anti-interference performance of the system is evaluated by the maximum voltage change Δu and the adjustment time Δt.

[0049] This embodiment proposes a bidirectional DC-DC converter control strategy based on feedforward variable structure PI control to solve the DC bus voltage fluctuation problem caused by photovoltaic power generation and load disturbance in the photovoltaic storage DC microgrid. By introducing the feedforward variable structure PI control strategy in the voltage loop, the response speed and robustness of the system to external disturbances are improved. Experimental results show that the control strategy can effectively reduce voltage fluctuations and response time under different working conditions. Future research will further optimize the control algorithm and explore its application under more complex working conditions to improve the overall performance and reliability of the photovoltaic storage DC microgrid.

Claims

1. A method for stabilizing DC bus voltage of a photovoltaic energy storage microgrid based on feedforward variable structure PI control, characterized in that: The method comprises the following steps: Step 1: Establish a photovoltaic-storage DC microgrid. The photovoltaic-storage DC microgrid structure mainly includes photovoltaic power generation units, loads, power electronic converters and energy storage units. In this structure, the discharge power of the energy storage unit is defined as a positive value, the charging power is defined as a negative value, and the grid absorption power is defined as a positive value; the relationship between the variables is as shown in the formula P ES =P Gird +P Load -P PV As shown, where: P ES is the output power of the battery; P Grid The power transmitted by the DC bus to the grid; P load P is the power delivered by the DC bus to the load side; PV is the output power of photovoltaic; Step 2: Modeling of the energy storage bidirectional DC-DC converter, wherein the converter modeling is divided into a mathematical model in Boost mode and a mathematical model in Buck mode; Step 3: Design of feedforward variable structure PI controller. The voltage outer loop control strategy based on feedforward variable structure PI control includes two parts: variable structure PI controller and feedforward controller. The variable structure PI controller is responsible for processing the main error of the system, reducing the steady-state error and dynamic error of the system by adjusting the control input, and dynamically adjusting the integral gain k. i To cope with the nonlinearity and dynamic changes of the system; the feedforward controller improves the robustness of the system by compensating for the interference signal in real time. The feedforward controller quickly adjusts the control input according to the size and change trend of the interference signal without waiting for the system error to appear; In the step 3, the feedforward variable structure PI controller is designed to integrate the saturation error V1-V2 of the control input, and adjust it through the adaptive coefficient, and then add the adjustment result to the integral term of the PI controller. The adaptive adjustment rule β is as follows: Where: α is the adaptive adjustment gain coefficient, which is generally 1; V3 is the average gain coefficient; e1 is the system feedback error; k i is the controller integral gain coefficient; V3 is calculated as follows: V3 = (V max +V min ) / 2, where: V max is the maximum output of the controller; V min is the minimum output of the controller; the expression of the variable structure PI control is as follows: u(t)=K p e(t)+k i β, where: k p is the proportional gain coefficient of the controller. The feedforward controller compensates according to the size and change trend of the interference signal. The feedforward controller responds immediately when the interference occurs, without waiting for the occurrence of system deviation. The feedforward controller does not rely on the output error of the system and compensates when the interference just occurs, thereby avoiding the influence of system lag. Its mathematical expression is as follows: batref (t) = K ffc ×[V dcref -V dc (t)]+V2(t), where: K ffc is the feedforward control coefficient; I batref is the reference value of current loop control; U dcref It is the reference value of voltage loop control.

2. A method for stabilizing DC bus voltage of a photovoltaic energy storage microgrid based on feedforward variable structure PI control as claimed in claim 1, characterized in that: In the step 1, the photovoltaic storage microgrid achieves voltage stability by adjusting the power balance between the energy storage unit and the DC bus in the island operation mode; therefore, PGrid=0 is set; based on the load power and the maximum output power of the photovoltaic array, the switching state of the bidirectional DC-DC converter is reasonably controlled to achieve bidirectional energy flow between the energy storage device and the DC bus, thereby ensuring the balance of the DC bus power and effectively suppressing voltage fluctuations.

3. The method for stabilizing DC bus voltage of a photovoltaic energy storage microgrid based on feedforward variable structure PI control according to claim 1, characterized in that: In the mathematical model of the Boost mode in step 2, when the switches S1 and S2 are turned off, the diode D2 is cut off, and the diode D1 is turned on, the state space equation of the system is expressed as follows: V dc is the bus voltage; C b is the battery side capacitance; C d is the bus side capacitance; L is the battery side inductance; when the switch tube S2 is turned on, S1 is turned off, and the diodes D1 and D2 are turned off, the state space equation of the system is expressed as: Through the state space averaging method, the state space average equation of the circuit in Boost mode is obtained: Where: d Boost is the duty cycle in Boost working mode; I bat Represents the inductor current on the energy storage side; R dc Indicates the equivalent load on the DC bus side; L indicates the inductance on the energy storage side; V dc Represents the DC bus voltage; through small signal modeling and Laplace transform, the transfer function in Boost mode is obtained:

4. A method for stabilizing DC bus voltage of a photovoltaic energy storage microgrid based on feedforward variable structure PI control as claimed in claim 3, characterized in that: The working state of the mathematical model in the Buck mode in step 2 is similar to that in the Boost mode, and the state space average equation and transfer function are: and Where: d Buck is the duty cycle in Buck working mode; R bat is the equivalent resistance on the energy storage side.

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