AC micro-grid frequency control method and system based on hybrid energy storage system

Abstract

The invention provides an AC micro-grid frequency control method and system based on a hybrid energy storage system, and the method comprises the steps: constructing a virtual synchronous machine based on droop control, and carrying out the calculation of a relation between the load disturbance quantity delta Pac of an AC micro-grid and the frequency variation delta f, namely, an equivalent inertia model of the virtual synchronous machine; establishing a hybrid energy storage system consistent with the equivalent inertia model in response, and enabling the hybrid energy storage system to access the network in a virtual synchronous machine mode; the hybrid energy storage system is adjusted through a control loop of the BIC, the hybrid energy storage system is merged into the alternating-current micro-grid, and interconnection and intercommunication between the hybrid energy storage system and the alternating-current micro-grid are completed. Through the design of the control loop of the BIC, interconnection and intercommunication between the hybrid energy storage system and the AC micro-grid are realized, the purpose of friendly grid connection is achieved, and a basis is provided for the hybrid energy storage system to participate in primary frequency modulation of the AC micro-grid.

Description

Technical Field

[0001] The present invention relates to the field of electrical engineering technology, and in particular to a frequency control method and system for an AC microgrid based on a hybrid energy storage system, and in particular to control design and analysis of a hybrid energy storage system based on an equivalent inertia model in an AC microgrid. Background Art

[0002] Vigorously developing renewable energy is an inevitable choice to improve my country's energy structure, ensure national energy security, and achieve the "dual carbon" goal. The large-scale development and utilization of renewable energy can effectively reduce dependence on traditional fossil energy and imported energy, and reduce environmental pollution and greenhouse gas emissions. However, with the large-scale application of renewable energy such as photovoltaics and wind power, the inertia of the power grid system is greatly reduced because renewable energy is usually coupled to the power grid through inertia-free fast-response power converters. When the inertia is insufficient, changes in load will cause the grid frequency to fluctuate beyond the threshold, leading to unplanned load reduction, cascading failures, and even power system blackouts.

[0003] In order to increase the system inertia, enhance the frequency regulation capability of the power grid, and improve the stability and reliability of the power grid, energy storage system frequency regulation has been widely used. Traditional energy storage devices can be divided into two categories: energy type and power type. Energy type devices such as battery energy storage have high energy density, but weak transient response capability, which is suitable for smoothing the fluctuation of low-frequency power in the system. Power type devices such as supercapacitor (SC) energy storage have the advantage of high power density, so they are suitable for responding to high-frequency power in the system, but their capacity is generally smaller than that of batteries, which is not suitable for overcharging and discharging.

[0004] By integrating multiple energy storage units, hybrid energy storage systems can solve the problems of traditional energy storage devices to a certain extent, but the interconnection with AC microgrids still needs to be considered. Since the power response of energy storage systems is generally related to DC voltage, it is necessary to design a bidirectional interlinking converter (BIC) control loop to achieve friendly grid connection and give full play to the support role of hybrid energy storage systems for the frequency of AC microgrids.

[0005] The document H.Ali, "A Hybrid Energy Storage System Based on Supercapacitor and Electric Vehicle Batteries for Frequency Stability Improvement of Islanded Microgrids," 2022 23rd International Middle East Power Systems Conference (MEPCON), Cairo, Egypt, 2022, pp. 1-6, discloses a strategy for enhancing the frequency stability of islanded microgrids by using a hybrid energy storage system consisting of supercapacitors (SC) and electric vehicle (EV) batteries. Among them, SC is used with high power density to provide virtual inertia characteristics, and EV batteries are used with high energy density to provide virtual damping characteristics, thereby improving virtual inertia. However, the power distribution method designed in this document is too simple and is not friendly to the overall inertia analysis of the system after grid connection. This paper designs a power distribution method for a hybrid energy storage system based on the equivalent inertia model of a virtual synchronous machine, which caters to the characteristics of the existence of multiple composite energy sources in the power grid that operate in a virtual synchronous machine mode, and is more convenient for grid connection and system inertia analysis.

[0006] Literature Chen Chongde. Research on hybrid energy storage participating in primary frequency regulation of photovoltaic power stations [D]. North China Electric Power University (Beijing), 2024, discloses a hybrid energy storage participating in photovoltaic primary frequency regulation capacity optimization configuration method based on improved adaptive noise complete set empirical mode decomposition, which decomposes and reconstructs the photovoltaic primary frequency regulation demand power into high-frequency and low-frequency components, respectively, as the frequency regulation instructions of flywheel and lithium battery energy storage, thereby improving the frequency stability of photovoltaic power stations. However, the calculation method of this literature is complicated. The hybrid energy storage system in this paper models the equivalent inertia of the virtual synchronous machine, which makes the mathematical derivation and transfer function simpler, and also effectively realizes the rapid support of the frequency of the AC microgrid.

[0007] To address the above problems, this paper proposes an AC microgrid frequency control method and system based on a hybrid energy storage system based on the equivalent inertia model of a virtual synchronous machine, which helps to increase the system inertia in the AC microgrid and provides a new idea for the control of the hybrid energy storage system. Summary of the invention

[0008] In view of the defects in the prior art, the object of the present invention is to provide an AC microgrid frequency control method and system based on a hybrid energy storage system.

[0009] According to the present invention, an AC microgrid frequency control method based on a hybrid energy storage system includes:

[0010] Step S1: Construct a virtual synchronous machine based on droop control and calculate the AC microgrid load disturbance ΔP ac The relationship between the frequency change Δf, that is, the equivalent inertia model of the virtual synchronous machine;

[0011] Step S2: establishing a hybrid energy storage system that is consistent with the response of the equivalent inertia model, so that the hybrid energy storage system is connected to the grid in the form of a virtual synchronous machine;

[0012] Step S3: The hybrid energy storage system is adjusted through the control loop of the BIC to be incorporated into the AC microgrid, thereby completing the interconnection between the hybrid energy storage system and the AC microgrid.

[0013] Preferably, the virtual synchronous machine based on droop control is a virtual inertia link added to the droop control loop;

[0014] The AC microgrid load disturbance ΔP ac The relationship with the frequency change Δf is as follows:

[0015]

[0016] In the formula, R ac represents the frequency droop coefficient; H represents the inertia coefficient of the AC network, and D represents the load damping constant of the AC network.

[0017] Preferably, the hybrid energy storage system includes two energy storage units: an SC and a battery.

[0018] Preferably, the method of connecting the hybrid energy storage system to the grid in the form of a virtual synchronous machine includes: performing reasonable power distribution on the two energy storage units based on an equivalent inertia model of the virtual synchronous machine;

[0019] Load power change ΔP of hybrid energy storage system d The power distribution of the two energy storage units, SC and battery, in the Laplace domain is as follows:

[0020]

[0021] Where P H Indicates the output power of SC, P L represents the output power of the battery, s represents the Laplace operator, D represents the equivalent damping factor, and m and n represent the characteristic parameters of the energy storage unit;

[0022] According to the above formula, the power output ΔP is obtained d The DC voltage change ΔV ds The relationship is as follows:

[0023]

[0024] The two energy storage units, battery and SC, are suitable for low-frequency power response and high-frequency power response respectively.

[0025] Preferably, step S3 includes providing a power reference value P of the BIC after the DC voltage variation per unit value and the frequency variation per unit value are passed through a proportional integral controller. ref , as shown below:

[0026]

[0027] Where ΔV dS Indicates the DC voltage change, V ref is the DC voltage reference value of the hybrid energy storage system, f ref is the AC microgrid frequency reference value, k p represents the proportional coefficient of the proportional-integral controller, k i represents the integral coefficient of the proportional-integral controller, and s represents the Laplace operator.

[0028] Then, the difference between the normalized DC voltage of the hybrid energy storage system and the AC microgrid frequency is used to provide a power reference.

[0029] According to the present invention, an AC microgrid frequency control system based on a hybrid energy storage system includes:

[0030] Module M1: Construct a virtual synchronous machine based on droop control and calculate the AC microgrid load disturbance ΔP ac The relationship between the frequency change Δf, that is, the equivalent inertia model of the virtual synchronous machine;

[0031] Module M2: Establishing a hybrid energy storage system that is consistent with the response of the equivalent inertia model, so that the hybrid energy storage system is connected to the grid in the form of a virtual synchronous machine;

[0032] Module M3: The hybrid energy storage system is regulated by the control loop of BIC to be incorporated into the AC microgrid, thereby completing the interconnection between the hybrid energy storage system and the AC microgrid.

[0033] Preferably, the virtual synchronous machine based on droop control is a virtual inertia link added to the droop control loop;

[0034] The AC microgrid load disturbance ΔP ac The relationship with the frequency change Δf is as follows:

[0035]

[0036] In the formula, R ac represents the frequency droop coefficient; H represents the inertia coefficient of the AC network, and D represents the load damping constant of the AC network.

[0037] Preferably, the hybrid energy storage system includes two energy storage units: an SC and a battery.

[0038] Preferably, the method of connecting the hybrid energy storage system to the grid in the form of a virtual synchronous machine includes: performing reasonable power distribution on the two energy storage units based on an equivalent inertia model of the virtual synchronous machine;

[0039] Load power change ΔP of hybrid energy storage system d The power distribution of the two energy storage units, SC and battery, is as follows:

[0040]

[0041] Where P H Indicates the output power of SC, P L represents the output power of the battery, s represents the Laplace operator, D represents the equivalent damping factor, and m and n represent the characteristic parameters of the energy storage unit;

[0042] According to the above formula, the power output ΔP is obtained d The DC voltage change ΔV ds The relationship is as follows:

[0043]

[0044] The two energy storage units, battery and SC, are suitable for low-frequency power response and high-frequency power response respectively.

[0045] Preferably, the module M3 includes a proportional integral controller that provides a power reference value P of the BIC after the DC voltage variation per unit value and the frequency variation per unit value are passed through the controller. ref , as shown below:

[0046]

[0047] Where ΔV dS Indicates the DC voltage change, V ref is the DC voltage reference value of the hybrid energy storage system, f ref is the AC microgrid frequency reference value, k p represents the proportional coefficient of the proportional-integral controller, k i represents the integral coefficient of the proportional-integral controller, and s represents the Laplace operator;

[0048] Then, the difference between the normalized DC voltage of the hybrid energy storage system and the AC microgrid frequency is used to provide a power reference.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] 1. The present invention realizes the interconnection between the hybrid energy storage system and the AC microgrid through the design of the BIC control loop, achieves the purpose of friendly grid connection, and provides a basis for the hybrid energy storage system to participate in the primary frequency regulation of the AC microgrid.

[0051] 2. The present invention designs two energy storage units, SC and battery, for the hybrid energy storage system. The SC responds to the high-frequency power of the system, and the battery responds to the low-frequency power of the system, thereby achieving the effect of smoothing the frequency fluctuation of the AC microgrid.

[0052] 3. The present invention performs reasonable power distribution for the hybrid energy storage system based on the equivalent inertia model of the virtual synchronous machine. After being connected to the grid, it is convenient to analyze and adjust the overall inertia of the system while effectively increasing the inertia of the AC microgrid system. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0054] Figure 1 It is the frequency regulation inertia control block diagram of the virtual synchronous machine;

[0055] Figure 2 It is the BIC control block diagram of the present invention;

[0056] Figure 3 This is a block diagram of the frequency control of the AC microgrid by the hybrid energy storage system in the present invention;

[0057] Figure 4 This is a comparison diagram of frequency fluctuations in the present invention;

[0058] Figure 5 It is the output power diagram of the battery and SC in the present invention;

[0059] Figure 6 It is a schematic diagram of the working method of the present invention.

[0060] in, Figure 4 The horizontal and vertical axes represent the simulation time and grid frequency respectively. Figure 5 The horizontal and vertical axes represent the simulation time and the output power of the battery or SC, respectively. DETAILED DESCRIPTION

[0061] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0062] Embodiment 1

[0063] According to the present invention, a frequency control method of an AC microgrid based on a hybrid energy storage system is provided. Figure 6 As shown, including:

[0064] Step S1: Construct a virtual synchronous machine based on droop control and calculate the AC microgrid load disturbance ΔP ac The relationship between the frequency change Δf and the equivalent inertia model of the virtual synchronous machine is shown in Figure 2. A virtual inertia link is added to the droop control loop to obtain a virtual synchronous machine based on droop control. The AC microgrid load disturbance ΔP ac The relationship with the frequency change Δf is as follows:

[0065]

[0066] In the formula, R ac represents the frequency droop coefficient; H represents the inertia coefficient of the AC network, and D represents the load damping constant of the AC network.

[0067] Synchronous generators have inertia and damping support, and can easily perform primary frequency modulation with the support of control links such as excitation and speed regulator. Virtual synchronous machine refers to the use of control technology to allow the inverter to simulate the damping and rotational inertia of the synchronous generator. The hybrid energy storage system is connected to the grid in the form of a virtual synchronous machine, which can effectively increase the inertia of the system and provide support for the system frequency.

[0068] A virtual inertia link is added to the droop control loop, and a virtual synchronous machine based on droop control is obtained. Figure 1 This is the frequency regulation control block diagram of the virtual synchronous machine. HP 、T CH and T RH is the thermal turbine coefficient, T G is the time constant of the speed regulator. The electromechanical characteristics of the virtual synchronous machine are defined as:

[0069]

[0070] Where: H and D represent the load inertia coefficient and damping factor related to frequency; P m and P l Represent mechanical input power and output power respectively; f g is the AC grid frequency.

[0071] In steady state, the mechanical input power P m Equal to load power P l and damping power D(ff g), so the frequency remains unchanged and f is a constant. When a small signal load power disturbance occurs, the frequency shifts by Δf. Differentiating equation (1) yields the swing equation (2):

[0072]

[0073] According to formula (2), the load power fluctuation ΔP l The change of inertial power and input power ΔP m The large power difference will cause a large frequency deviation Δf in the AC microgrid under load transfer events. The introduction of load inertia coefficient H and damping factor D effectively reduces the frequency fluctuation caused by the power difference and maintains the stability of the system frequency.

[0074] According to the inertia control block diagram of the virtual synchronous machine, the AC microgrid load disturbance ΔP can be obtained: ac Relationship with frequency change Δf:

[0075]

[0076] Where: R ac is the frequency droop coefficient; H is the AC network inertia coefficient; D is the AC network load damping constant.

[0077] The present invention refers to equation (3) as the equivalent inertia model of the virtual synchronous machine.

[0078] Step S2: Establish a hybrid energy storage system that is consistent with the response of the equivalent inertia model, and connect the hybrid energy storage system to the grid in the form of a virtual synchronous machine. The hybrid energy storage system includes two energy storage units, SC and battery. Based on the equivalent inertia model of the virtual synchronous machine, the two energy storage units are reasonably allocated power. Through the design of the BIC control loop, the interconnection between the hybrid energy storage system and the AC microgrid is achieved, achieving the effect of smoothing the frequency fluctuation of the AC microgrid and enhancing the anti-interference ability of the system.

[0079] In order to connect the hybrid energy storage system to the grid as a virtual synchronous machine, it is necessary to make the response of the hybrid energy storage system consistent with the equivalent inertia model in step S2. The hybrid energy storage system consists of two energy storage units, a battery and an SC, which are respectively suitable for low-frequency power response and high-frequency power response. The response functions of the two energy storage units are designed as follows:

[0080]

[0081] Where V represents the actual voltage; V n Indicates rated voltage; P H Indicates the output power of SC, P Lrepresents the output power of the battery; D is the equivalent damping factor; m and n are the characteristic parameters of the designed energy storage unit.

[0082] In order to meet the power balance condition, the sum of the power outputs of the two energy storage units needs to be equal to the load power change ΔP of the system. d Equal, that is:

[0083] P H +P L =ΔP d (5)

[0084] According to equations (4) and (5), the system load power change ΔP can be derived d The power distribution of the two energy storage units is as follows:

[0085]

[0086] Load disturbance ΔP d It is also the power output of the hybrid energy storage system. According to equations (6-1) and (6-2), the power output ΔP can be obtained. d The DC voltage change ΔV ds Relationship:

[0087]

[0088] Comparing equation (3) and equation (7), it can be seen that the hybrid energy storage system is consistent with the equivalent inertia model.

[0089] Step S3: The hybrid energy storage system is adjusted through the control loop of the BIC to be incorporated into the AC microgrid, thereby completing the interconnection between the hybrid energy storage system and the AC microgrid.

[0090] The hybrid energy storage system needs to be integrated into the AC microgrid through BIC to play a frequency regulation role. The control block diagram of BIC is as follows Figure 2 shown. Determined by formula (7), Provided by the control of the AC microgrid. The superscript “-” indicates the per-unit value of the corresponding physical quantity, that is, the actual value divided by the reference value. and After a proportional-integral controller, the power reference value P of BIC is provided. ref , which can be expressed by formula (8). ref After being mapped into current at a certain ratio, it passes through a proportional-integral controller and finally obtains the switching signal of the inverter through PWM modulation.

[0091]

[0092] Where Vref is the DC voltage reference value of the hybrid energy storage system; f ref is the AC microgrid frequency reference value.

[0093] According to formula (8), the difference between the DC voltage of the hybrid energy storage system and the AC microgrid frequency is used to provide a power reference to support frequency stability. The BIC control loop makes the dynamic response between the DC side voltage and the AC side frequency consistent. Assuming that the proportional integrator adjustment time is much smaller than the subgrid control time, the following constraints apply:

[0094]

[0095] By controlling the power flow through the BIC, the hybrid energy storage system supports the frequency of the AC microgrid and completes the transfer of virtual inertia between the hybrid energy storage system and the AC microgrid. As the inertia increases, the AC microgrid will gain better frequency stability.

[0096] The present invention aims to increase the inertia of the AC microgrid and improve the frequency regulation capability of the system. It provides a solution for the energy storage system to regulate the frequency of the AC microgrid, effectively reduces the peak value of the frequency fluctuation of the AC microgrid, and enhances the stability of the power grid. The equivalent inertia model of the virtual synchronous machine of the present invention clarifies the mechanism of the virtual synchronous machine to smooth the frequency fluctuation of the AC microgrid, and provides a new strategy for the control design of the hybrid energy storage system. The interconnection between the hybrid energy storage system and the AC microgrid is controlled and designed using BIC, making the grid connection more friendly. The hybrid energy storage system adopted is composed of two energy storage units, SC and battery, which have the advantages of both and realizes rapid support for the frequency of the AC microgrid.

[0097] Further, in conjunction with the accompanying drawings, the AC microgrid frequency control method based on the hybrid energy storage system of the present invention is specifically described as follows:

[0098] like Figure 3 The hybrid energy storage system shown is connected to the grid through BIC to provide frequency support for the AC microgrid, which adopts droop control. When load power fluctuations occur in the grid, the hybrid energy storage system can reduce the peak value of frequency deviation, smooth frequency fluctuations, and enhance the anti-interference ability of the grid. The parameter settings of the overall system are shown in Table 1.

[0099] Table 1 System parameters

[0100]

[0101]

[0102] when Figure 3 When load power fluctuation occurs in the AC microgrid shown in the figure, the corresponding frequency response is as follows Figure 4As shown, the fluctuation peak value is large when there is no hybrid energy storage connected to the grid, and the fluctuation peak value is small when the energy storage system participates in frequency modulation. It can be seen that the hybrid energy storage system plays an effective supporting role in the frequency of the AC microgrid.

[0103] When the hybrid energy storage system is frequency modulating, the power distribution between the battery and the SC is as follows: Figure 5 As shown, pol represents the output power of the battery and poh represents the output power of the SC, which is in line with the design expectations.

[0104] Embodiment 2

[0105] The present invention also provides an AC microgrid frequency control system based on a hybrid energy storage system. The AC microgrid frequency control system based on the hybrid energy storage system can be implemented by executing the process steps of the AC microgrid frequency control method based on the hybrid energy storage system, that is, those skilled in the art can understand the AC microgrid frequency control method based on the hybrid energy storage system as a preferred implementation of the AC microgrid frequency control system based on the hybrid energy storage system.

[0106] According to the present invention, an AC microgrid frequency control system based on a hybrid energy storage system includes:

[0107] Module M1: Construct a virtual synchronous machine based on droop control and calculate the AC microgrid load disturbance ΔP ac The relationship between the frequency change Δf and the equivalent inertia model of the virtual synchronous machine. The virtual synchronous machine based on droop control adds a virtual inertia link in the droop control loop. The AC microgrid load disturbance ΔP ac The relationship with the frequency change Δf is as follows:

[0108]

[0109] In the formula, R ac represents the frequency droop coefficient; H represents the inertia coefficient of the AC network, and D represents the load damping constant of the AC network.

[0110] Module M2: Establish a hybrid energy storage system that is consistent with the response of the equivalent inertia model, so that the hybrid energy storage system is connected to the grid in the form of a virtual synchronous machine. The hybrid energy storage system includes two energy storage units, SC and battery. The method of connecting the hybrid energy storage system to the grid in the form of a virtual synchronous machine includes: based on the equivalent inertia model of the virtual synchronous machine, reasonably allocating power to the two energy storage units. Load power change ΔP of the hybrid energy storage system d The power distribution of the two energy storage units, SC and battery, is as follows:

[0111]

[0112] Where P H Indicates the output power of SC, P L represents the output power of the battery, s represents the Laplace operator, D represents the equivalent damping factor, and m and n represent the characteristic parameters of the energy storage unit. According to the above formula, the power output ΔP is obtained d The DC voltage change ΔV ds The relationship is as follows:

[0113]

[0114] The two energy storage units, battery and SC, are suitable for low-frequency power response and high-frequency power response respectively.

[0115] Module M3: The hybrid energy storage system is regulated by the control loop of BIC to be integrated into the AC microgrid, completing the interconnection between the hybrid energy storage system and the AC microgrid. Module M3 includes providing the BIC power reference value P after passing the DC voltage change per unit value and the frequency change per unit value through a proportional integral controller. ref , as shown below:

[0116]

[0117] Where ΔV dS Indicates the DC voltage change, V ref is the DC voltage reference value of the hybrid energy storage system, f ref is the AC microgrid frequency reference value, k p represents the proportional coefficient of the proportional-integral controller, k i represents the integral coefficient of the proportional-integral controller, and s represents the Laplace operator.

[0118] Then, the difference between the normalized DC voltage of the hybrid energy storage system and the AC microgrid frequency is used to provide a power reference.

[0119] Those skilled in the art know that, in addition to realizing the system and its various devices, modules, and units provided by the present invention in a purely computer-readable program code, it is entirely possible to realize the same functions in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system and its various devices, modules, and units provided by the present invention can be considered as a hardware component, and the devices, modules, and units included therein for realizing various functions can also be regarded as structures within the hardware component; the devices, modules, and units for realizing various functions can also be regarded as both software modules for realizing the method and structures within the hardware component.

[0120] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A frequency control method for an AC microgrid based on a hybrid energy storage system, characterized in that: include: Step S1: Construct a virtual synchronous machine based on droop control and calculate the AC microgrid load disturbance ΔP ac The relationship between the frequency change Δf, that is, the equivalent inertia model of the virtual synchronous machine; Step S2: establishing a hybrid energy storage system that is consistent with the response of the equivalent inertia model, so that the hybrid energy storage system is connected to the grid in the form of a virtual synchronous machine; Step S3: The hybrid energy storage system is adjusted through the control loop of the BIC to be incorporated into the AC microgrid, thereby completing the interconnection between the hybrid energy storage system and the AC microgrid.

2. The AC microgrid frequency control method based on a hybrid energy storage system according to claim 1, characterized in that: The virtual synchronous machine based on droop control adds a virtual inertia link in the droop control loop; The AC microgrid load disturbance ΔP ac The relationship with the frequency change Δf is as follows: In the formula, R ac represents the frequency droop coefficient; H represents the inertia coefficient of the AC network, and D represents the load damping constant of the AC network.

3. The AC microgrid frequency control method based on a hybrid energy storage system according to claim 1, characterized in that: The hybrid energy storage system includes two energy storage units: SC and battery.

4. The AC microgrid frequency control method based on a hybrid energy storage system according to claim 3 is characterized in that: The method of connecting the hybrid energy storage system to the grid in the form of a virtual synchronous machine includes: reasonably allocating power to the two energy storage units based on the equivalent inertia model of the virtual synchronous machine; Load power change ΔP of hybrid energy storage system d The power distribution of the two energy storage units, SC and battery, is as follows: Where P H Indicates the output power of SC, P L represents the output power of the battery, s represents the Laplace operator, D represents the equivalent damping factor, and m and n represent the characteristic parameters of the energy storage unit; According to the above formula, the power output ΔP is obtained d The DC voltage change ΔV ds The relationship is as follows: The two energy storage units, battery and SC, are suitable for low-frequency power response and high-frequency power response respectively.

5. The AC microgrid frequency control method based on a hybrid energy storage system according to claim 1, characterized in that: Step S3 includes providing a power reference value P of the BIC after passing the DC voltage variation per unit value and the frequency variation per unit value through a proportional integral controller. ref , as shown below: Where, ΔV dS Indicates the DC voltage change, V ref is the DC voltage reference value of the hybrid energy storage system, f ref is the AC microgrid frequency reference value, k p represents the proportional coefficient of the proportional-integral controller, k i represents the integral coefficient of the proportional-integral controller, and s represents the Laplace operator; Then, the difference between the normalized DC voltage of the hybrid energy storage system and the AC microgrid frequency is used to provide a power reference.

6. An AC microgrid frequency control system based on a hybrid energy storage system, characterized in that: include: Module M1: Construct a virtual synchronous machine based on droop control and calculate the AC microgrid load disturbance ΔP ac The relationship between the frequency change Δf, that is, the equivalent inertia model of the virtual synchronous machine; Module M2: Establishing a hybrid energy storage system that is consistent with the response of the equivalent inertia model, so that the hybrid energy storage system is connected to the grid in the form of a virtual synchronous machine; Module M3: The hybrid energy storage system is regulated by the control loop of BIC to be incorporated into the AC microgrid, thereby completing the interconnection between the hybrid energy storage system and the AC microgrid.

7. The AC microgrid frequency control system based on a hybrid energy storage system according to claim 6, characterized in that: The virtual synchronous machine based on droop control adds a virtual inertia link in the droop control loop; The AC microgrid load disturbance ΔP ac The relationship with the frequency change Δf is as follows: In the formula, R ac represents the frequency droop coefficient; H represents the inertia coefficient of the AC network, and D represents the load damping constant of the AC network.

8. The AC microgrid frequency control system based on a hybrid energy storage system according to claim 6, characterized in that: The hybrid energy storage system includes two energy storage units: SC and battery.

9. The AC microgrid frequency control system based on a hybrid energy storage system according to claim 8, characterized in that: The method of connecting the hybrid energy storage system to the grid in the form of a virtual synchronous machine includes: reasonably allocating power to the two energy storage units based on the equivalent inertia model of the virtual synchronous machine; Load power change ΔP of hybrid energy storage system d The power distribution of the two energy storage units, SC and battery, is as follows: Where P H Indicates the output power of SC, P L represents the output power of the battery, s represents the Laplace operator, D represents the equivalent damping factor, and m and n represent the characteristic parameters of the energy storage unit; According to the above formula, the power output ΔP is obtained d The DC voltage change ΔV ds The relationship is as follows: The two energy storage units, battery and SC, are suitable for low-frequency power response and high-frequency power response respectively.

10. The AC microgrid frequency control system based on a hybrid energy storage system according to claim 6, characterized in that: Module M3 includes a proportional integral controller that provides a power reference value P of the BIC after passing the DC voltage change per unit value and the frequency change per unit value through a proportional integral controller. ref , as shown below: Where ΔV dS Indicates the DC voltage change, V ref is the DC voltage reference value of the hybrid energy storage system, f ref is the AC microgrid frequency reference value, k p represents the proportional coefficient of the proportional-integral controller, k i represents the integral coefficient of the proportional-integral controller, and s represents the Laplace operator; Then, the difference between the normalized DC voltage of the hybrid energy storage system and the AC microgrid frequency is used to provide a power reference.

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