Impedance modeling method for optical storage DC micro-grid under energy management and regulation

Abstract

The invention discloses an impedance modeling method for an optical storage DC micro-grid under energy management regulation and control, and belongs to the technical field of optical storage DC micro-grid system modeling and stability analysis, and the method comprises the following steps: carrying out the impedance modeling of a photovoltaic unit based on MPPT control; performing impedance modeling on the energy storage unit based on power proportional distribution and cooperative control; dC side impedance modeling is carried out on the optical storage inverter based on power proportional distribution and cooperative control; and establishing a stability analysis model of the system based on photovoltaic, energy storage and inversion DC port impedance models. According to the invention, through research on the optical storage system, the impedance model of the optical storage DC micro-grid system considering energy management regulation is established, and the stability analysis criterion of the system is deduced on this basis, so that the stability of the optical storage DC micro-grid system under the energy management proportional regulation strategy can be effectively represented and analyzed; the method is of great significance to stable operation and power supply reliability of an optical storage DC micro-grid system under energy management.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic-storage DC microgrid system modeling and stability analysis technology, specifically to an impedance modeling method for photovoltaic-storage DC microgrids under energy management and control. Background Technology

[0002] With the development of DC distributed power generation and energy storage technologies, and the increasing prevalence of modern DC electrical equipment, DC microgrids have received widespread attention and research due to their advantages such as high efficiency, flexibility, and strong controllability. This has led to the significant development of photovoltaic-storage DC microgrids. However, many technical challenges have gradually emerged, such as the planning and design, energy management, and stability analysis of photovoltaic-storage DC microgrids. These challenges are also research hotspots in the field of DC microgrids. To ensure the stability and reliability of new energy system operation, it is necessary to conduct stability analysis research on photovoltaic-storage DC microgrids, providing a reference for further research and development in this field.

[0003] Existing research largely focuses on the stability of individual components, such as inverter control, DC-side photovoltaic and energy storage charging / discharging, lacking a global system-wide stability analysis. This leads to prominent coupling problems between components under dynamic operating conditions. During load surges, improper power distribution between the energy storage system and the grid can cause voltage oscillations or overloads. However, for impedance modeling analysis of photovoltaic-storage DC microgrids, most existing methods model the system from the AC side, rarely considering impedance modeling from the DC side. Furthermore, there is a lack of equivalent impedance analysis models for photovoltaic-storage DC microgrids that take into account energy management and control strategies. Therefore, this invention proposes an impedance modeling method for photovoltaic-storage DC microgrids under energy management and control. Summary of the Invention

[0004] The technical problem to be solved by this invention is: how to address the issue that existing research focuses on the stability of a single component and lacks stability analysis from a system-wide perspective, and provides an impedance modeling method for photovoltaic-storage DC microgrids under energy management and regulation.

[0005] The present invention solves the above-mentioned technical problems through the following technical solution, and the present invention includes the following steps:

[0006] S1: Based on MPPT control, impedance modeling is performed on the photovoltaic unit in the photovoltaic-storage DC microgrid system to obtain the equivalent output impedance model of the photovoltaic unit;

[0007] S2: Based on power ratio allocation and coordinated control, impedance modeling is performed on the energy storage unit in the photovoltaic-storage DC microgrid system to obtain the equivalent output impedance model of the energy storage unit;

[0008] S3: Based on power ratio allocation and coordinated control, DC-side impedance modeling of inverters in photovoltaic-storage DC microgrid systems is performed to obtain the equivalent output impedance model of inverters.

[0009] S4: Based on the equivalent output impedance model of photovoltaic unit, energy storage unit and inverter, establish a stability analysis model for photovoltaic-storage DC microgrid system.

[0010] Furthermore, in the photovoltaic-storage DC microgrid system, the photovoltaic array in the photovoltaic unit is connected to the DC bus via a Boost converter, and the energy storage battery in the energy storage unit is connected to the DC bus via a bidirectional DC-DC converter, and then connected to the grid via an LCL grid-connected inverter.

[0011] Furthermore, in step S1, the specific process of impedance modeling for the photovoltaic unit is as follows:

[0012] S11: The mathematical model for the photovoltaic unit operating in MPPT control mode is established as follows:

[0013]

[0014] Among them, G pv The transfer function for the MPPT voltage loop control of the photovoltaic unit. This is the output voltage of the photovoltaic array. This is the DC bus reference voltage. The duty cycle of the photovoltaic unit Boost converter;

[0015] S12: Establish the equivalent output impedance model of the photovoltaic unit when operating in MPPT control mode. pv as follows:

[0016]

[0017] Among them, Z pvou Output current for photovoltaic units DC bus voltage to photovoltaic unit side The transfer function between them, G pvui Output current for photovoltaic units to photovoltaic array output voltage The transfer function between them, G pvud Duty cycle DC bus voltage to photovoltaic unit side The transfer function between them, H pvud Duty cycle to photovoltaic array output voltage The transfer function between them.

[0018] Furthermore, in step S2, the specific process of impedance modeling for the energy storage unit is as follows:

[0019] S21: The mathematical model for the energy storage unit operating under power proportional distribution and coordinated control is as follows:

[0020]

[0021] Among them, G cu G is the voltage loop transfer function under power proportional distribution and coordinated control. b Let K1 be the current loop transfer function of the energy storage unit, and K1 be the power allocation coefficient of the energy storage unit. The duty cycle of the bidirectional DC-DC converter for the energy storage unit. This refers to the DC bus voltage on the energy storage unit side. For the inductor current of the bidirectional DC-DC converter of the energy storage unit;

[0022] S22: Establish the equivalent output impedance model of the energy storage unit under power proportional allocation and coordinated control Z batt as follows:

[0023]

[0024] Among them, G bii Output current for energy storage unit Current to the bidirectional DC-DC converter inductor of the energy storage unit The transfer function between them, G bid Duty cycle Current to the bidirectional DC-DC converter inductor of the energy storage unit The transfer function between them, Z bou Output current for energy storage unit DC bus voltage to energy storage unit side The transfer function between them, G bud Duty cycle DC bus voltage to energy storage unit side The transfer function between them.

[0025] Furthermore, in step S3, the specific process of modeling the DC-side impedance of the inverter is as follows:

[0026] S31: The d-axis voltage and current dual-loop mathematical model of the inverter operating under power proportional distribution and coordinated control is established as follows:

[0027]

[0028] Among them, G ci Let K1 be the inverter current inner loop control transfer function, and K2 be the inverter power proportional distribution coefficient. This refers to the output current of the d-axis inverter. U is the inverter duty cycle. dc This is the steady-state value of the DC bus voltage. This is the DC bus voltage. Since the photovoltaic unit, energy storage unit, and inverter are connected in parallel, the DC bus voltage at the parallel connection point is... same;

[0029] S32: Establish the equivalent output impedance model of the inverter Z dc as follows:

[0030]

[0031] in, I 1d D represents the steady-state value of the output current of the d-axis inverter. d L is the steady-state value of the d-axis component of the inverter duty cycle, C is the LCL filter capacitor, L1 is the LCL filter inductor on the inverter side, L2 is the LCL filter inductor on the grid side, and L... g R is the power grid impedance, and R is the passive damping series resistance.

[0032] Furthermore, in step S4, the specific process of establishing the stability analysis model of the photovoltaic-storage DC microgrid system is as follows:

[0033] S41: The equivalent output impedance model of the photovoltaic unit, energy storage unit and inverter is equivalent to the form of voltage source and impedance in series, thus obtaining the equivalent model of photovoltaic-storage DC microgrid system.

[0034] S42: When the energy storage unit operates in the discharge state, its impedance can be integrated with that of the photovoltaic unit and connected to the power supply terminal. The equivalent model of the photovoltaic-storage DC microgrid system is further simplified into a model, thus obtaining the DC bus-side port voltage u of the photovoltaic-storage DC microgrid system. dc The expression for the stability analysis model of the photovoltaic-storage DC microgrid system is as follows:

[0035]

[0036] Among them, Z d Z represents the total equivalent output impedance of the photovoltaic unit and the energy storage unit. dc u is the total equivalent output impedance of the inverter. d u is the total equivalent output voltage of the photovoltaic unit and the energy storage unit. ref This is the equivalent output voltage of the inverter;

[0037] S43: Based on the derived u dc Its stability is derived from the expression based on the impedance ratio Z. d / Z dcThe decision, based on the Nyquist stability criterion, is that when the system impedance is greater than Z... d / Z dc When the Nyquist curve does not include the point (-1, j0), it indicates that the photovoltaic-storage DC microgrid system is stable, and when the system impedance ratio Z d / Z dc When the Nyquist curve includes the point (-1, j0), it indicates that the photovoltaic-storage DC microgrid system is unstable, thus enabling stability analysis.

[0038] Compared with existing technologies, this invention has the following advantages: The energy management-controlled photovoltaic-storage DC microgrid impedance modeling method integrates energy management strategies into impedance modeling, achieving a comprehensive analysis of the system's global stability and providing a new approach to the stability analysis of photovoltaic-storage DC microgrid systems. In the stability analysis of photovoltaic-storage DC microgrid systems, existing research often focuses on the stability of single components, such as the inverter control component, the DC-side photovoltaic and energy storage charging / discharging components. However, the photovoltaic-storage DC microgrid system impedance model proposed in this invention starts from the global system perspective, incorporating the power regulation relationship in energy management into impedance modeling by introducing a power ratio allocation coefficient. This establishes an accurate photovoltaic-storage DC microgrid system model, which more accurately reflects the actual operating characteristics of the photovoltaic-storage DC microgrid compared to traditional technologies. Attached Figure Description

[0039] Figure 1 This is a main circuit diagram of the photovoltaic-storage DC microgrid system in an embodiment of the present invention;

[0040] Figure 2 This is a schematic diagram of the control of a photovoltaic-storage DC microgrid system in an embodiment of the present invention;

[0041] Figure 3 This is a schematic diagram of photovoltaic MPPT control in an embodiment of the present invention;

[0042] Figure 4 This is a diagram illustrating the intelligent control of the energy storage and LCL grid-connected inverter in an embodiment of the present invention.

[0043] Figure 5 This is a diagram of the equivalent model of the system in an embodiment of the present invention;

[0044] Figure 6 This is a simplified model diagram of the system in an embodiment of the present invention;

[0045] Figure 7 The impedance ratio Nyquist plot is shown in the embodiment of the present invention.

[0046] Figure 8 This is a simulation diagram of three-phase voltage and current in an embodiment of the present invention. Detailed Implementation

[0047] The embodiments of the present invention are described in detail below. These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0048] The main circuit of the photovoltaic-storage DC microgrid system consists of Figure 1 This means that the photovoltaic array in the photovoltaic unit is connected to the DC bus through a Boost converter, and the energy storage battery in the energy storage unit is connected to the DC bus through a bidirectional DC-DC converter, and then connected to the grid through an LCL grid-connected inverter.

[0049] Step S1: The stability of the photovoltaic-storage DC microgrid system is analyzed using impedance analysis. First, based on the state-space averaging method, equivalent impedance models of the Boost converter, bidirectional DC-DC converter, and inverter under the control strategy are established, such as... Figures 2 to 4 The photovoltaic unit shown adopts MPPT control for its operation. The energy storage unit and inverter adopt a power proportional allocation and collaborative control strategy. This strategy can stabilize the bus voltage by having the bidirectional DC-DC converter and inverter in the energy storage unit work together to adjust the power through collaborative control. Furthermore, it can realize the power allocation between the two converters based on proportional changes. This not only enables the tracking of dispatch commands, but also greatly reduces the impact of transmission power limits.

[0050] Step S2: The impedance model established from each element is equivalent to, as shown below. Figure 5 The system model shown is such that the photovoltaic unit, energy storage unit, and inverter unit are all equivalent to a voltage source system with impedance and constant voltage source connected in series.

[0051] Step S3: Based on the impedance models of each DC converter, study the small-signal stability of the photovoltaic-storage DC microgrid system, and determine the small-signal stability of the photovoltaic-storage DC microgrid system based on the frequency domain analysis method and the Nyquist criterion.

[0052] In step S1, establishing the impedance model of the photovoltaic-storage DC microgrid includes the following steps.

[0053] Step S11: Applying the state-space averaging method, the small-signal state-space equation of the photovoltaic Boost converter can be listed as follows:

[0054]

[0055] Among them, L pv C is the inductor of the photovoltaic unit Boost converter. pv C is the input capacitor of the photovoltaic unit Boost converter. dcpv For the output capacitor of the photovoltaic unit Boost converter, I is the inductor current of the photovoltaic unit Boost converter.Lpv D represents the steady-state value of the inductor current in the photovoltaic unit Boost converter. pv This represents the steady-state duty cycle value of the photovoltaic unit Boost converter. The duty cycle of the photovoltaic unit Boost converter. For the photovoltaic array output current, This is the output voltage of the photovoltaic array. U is the DC bus voltage on the photovoltaic unit side. dcpv This represents the steady-state value of the DC bus voltage on the photovoltaic unit side. It provides the output current for the photovoltaic unit.

[0056] Based on the above equation, the transfer function relationship equation between the variables of the Boost converter can be derived as follows when closed-loop control is not considered.

[0057]

[0058] Among them, Z pvou Output current for photovoltaic units DC bus voltage to photovoltaic unit side The transfer function between them, G pvui Output current for photovoltaic units to photovoltaic array output voltage The transfer function between them, G pvud Duty cycle DC bus voltage to photovoltaic unit side The transfer function between them, H pvud Duty cycle to photovoltaic array output voltage The transfer function between them.

[0059] Step S12: As Figure 3 As shown, the mathematical model for the outer voltage loop is derived when the photovoltaic unit adopts MPPT control mode:

[0060]

[0061] Among them, G pv The transfer function for the MPPT voltage loop control of the photovoltaic unit. This is the reference voltage for the DC bus.

[0062] The equivalent output impedance model for a photovoltaic module operating in MPPT mode is as follows:

[0063]

[0064] Step S13: Applying the state-space averaging method, the small-signal state-space equation of the bidirectional DC-DC converter of the energy storage unit can be listed as follows:

[0065]

[0066] Among them, L Lb C is the inductor for the bidirectional DC-DC converter of the energy storage unit. b For the output capacitor of the bidirectional DC-DC converter of the energy storage unit, I is the inductor current of the bidirectional DC-DC converter for the energy storage unit. Lb D represents the steady-state value of the inductor current in the bidirectional DC-DC converter of the energy storage unit. b This represents the steady-state duty cycle value of the bidirectional DC-DC converter in the energy storage unit. The duty cycle of the bidirectional DC-DC converter in the energy storage unit. This is the output voltage of the energy storage battery. U is the DC bus voltage on the energy storage unit side. dcb This represents the steady-state value of the DC bus voltage on the energy storage unit side. This provides the output current for the energy storage unit.

[0067] Based on the above equation, the transfer function relationship equations between the variables of the bidirectional DC-DC converter of the energy storage unit without considering closed-loop control can be derived as follows:

[0068]

[0069] Among them, G bii Output current for energy storage unit Current to the bidirectional DC-DC converter inductor of the energy storage unit The transfer function between them, G bid Duty cycle Current to the bidirectional DC-DC converter inductor of the energy storage unit The transfer function between them, Z bou Output current for energy storage unit DC bus voltage to energy storage unit side The transfer function between them, G bud Duty cycle DC bus voltage to energy storage unit side The transfer function between them.

[0070] Step S14: As Figure 4 As shown, the mathematical model for the energy storage unit operating under proportional distribution is as follows:

[0071]

[0072] Among them, G cu G is the voltage loop control transfer function under power proportional distribution and coordinated control. bK1 is the current loop control transfer function of the energy storage unit, and K1 is the power proportional allocation coefficient of the energy storage unit.

[0073] The equivalent output impedance model of the energy storage unit under energy management and regulation is as follows:

[0074]

[0075] Step S15: Select the synchronous rotating dq coordinate system as the basis for analysis, considering active power transmission on the DC side with the d-axis as the active axis and the q-axis as the reactive power control axis. Based on the orthogonal decoupling principle, neglecting grid asymmetry and harmonic components, the q-axis and cross-coupling are ignored. The grid-connected inverter is connected to the grid via an LCL filter, and passive damping with a series resistor on the capacitor is used to ensure system stability. Based on this, DC-side impedance modeling is performed on the grid and the grid-connected inverter, and the small-signal state-space equations under the inverter's d-axis are listed as follows:

[0076]

[0077] in, This refers to the output current of the d-axis inverter. The current is the d-axis grid-side inductor current. The current is the d-axis capacitor current. For the DC-side inverter input current, For DC bus input current, This is the DC bus voltage. This refers to the output voltage of the d-axis inverter. The inverter's duty cycle. U is the grid voltage along the d-axis. dc I is the steady-state value of the DC bus voltage. 1d D represents the steady-state value of the output current of the d-axis inverter. d Here, C is the steady-state value of the d-axis component of the duty cycle, and C is the LCL filter capacitor. dc L1 is the DC-side capacitor of the inverter, L2 is the LCL filter inductor on the inverter side, and L3 is the LCL filter inductor on the grid side. g R is the power grid impedance, and R is the passive damping series resistance.

[0078] From this, we can deduce that:

[0079]

[0080] Step S16: Establish a dual-loop mathematical model of the inverter's d-axis voltage and current under energy management control:

[0081]

[0082] Among them, G ciLet K1 be the inverter current inner loop control transfer function, and K2 be the inverter power proportional distribution coefficient. Since the photovoltaic unit, energy storage unit, and inverter are connected in parallel, their DC bus voltages at the parallel connection points are the same. All three are equal.

[0083] The equivalent output impedance model of the inverter is derived as follows:

[0084]

[0085] In step S2, the stability analysis model of the system is established based on the photovoltaic, energy storage, and inverter DC port impedance models, including the following steps:

[0086] Step S21: The impedance model obtained through the above steps is equivalent to a voltage source and an impedance connected in series, as shown below. Figure 5 As shown;

[0087] Step S22: When the energy storage unit is operating in a discharge state, its impedance can be integrated with that of the photovoltaic unit and connected to the power supply terminal. The system can then be further equivalent to... Figure 6 The simplified model shown leads to the following expression for the DC bus side port voltage of the system:

[0088]

[0089] Among them, Z d Z represents the total equivalent output impedance of the photovoltaic unit and the energy storage unit. dc u is the total equivalent output impedance of the inverter. d u is the total equivalent output voltage of the photovoltaic unit and the energy storage unit. ref This is the equivalent output voltage of the inverter;

[0090] In step S3, the small-signal stability of the system is determined based on the Nyquist criterion using frequency domain analysis.

[0091] Step S31: Define Z d / Z dc The system impedance ratio, also known as the secondary loop gain, can be seen from equation (15) to determine its stability by Z. d / Z dc According to the Nyquist stability criterion, when the system impedance ratio Z... d / Z dc When the Nyquist curve does not include the point (-1,j0), the photovoltaic-storage DC microgrid system is stable.

[0092] Step S32: Based on the impedance models of each DC converter, study the small-signal stability of the photovoltaic DC microgrid system, and determine the small-signal stability of the system based on the Nyquist criterion of frequency domain analysis.

[0093] A photovoltaic-storage DC microgrid system can be built using Matlab / Simulink. In this embodiment, to verify the effectiveness of the proposed method, the voltage outer loop control transfer function G under power ratio allocation and cooperative control is analyzed. cu Different proportional control parameters K pu The dynamic characteristics of the system are verified through simulation studies.

[0094]

[0095] Figure 7 The red curve shown is K. pu When Z equals 2 d / Z dc The Nyquist plot shows that the curve does not include the point (-1, j0), indicating that the system is stable. The blue curve represents K. pu When Z equals 20 d / Z dc The Nyquist plot shows that the curve contains the point (-1, j0), indicating that the system is unstable. Therefore, by... Figure 7 Analysis shows that as K pu The system gradually becomes unstable as the number of cells increases. Figure 8 The display shows different K values. pu The simulated waveforms of the three-phase voltage and current of the system are shown in the figure. It can be seen that the system is stable before t = 1.28s. At t = 1.28s, K... pu Increasing the value from 2 to 20 causes the entire system to become unstable, and Figure 7 Nyquist Figure 1 Therefore, it is evident that the impedance model of the photovoltaic-storage DC microgrid based on energy management regulation proposed in this invention is correct.

[0096] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for impedance modeling of photovoltaic-storage DC microgrids under energy management and control, characterized in that, Includes the following steps: S1: Based on MPPT control, impedance modeling is performed on the photovoltaic unit in the photovoltaic-storage DC microgrid system to obtain the equivalent output impedance model of the photovoltaic unit; S2: Based on power ratio allocation and coordinated control, impedance modeling is performed on the energy storage unit in the photovoltaic-storage DC microgrid system to obtain the equivalent output impedance model of the energy storage unit; S3: Based on power ratio allocation and coordinated control, DC-side impedance modeling of inverters in photovoltaic-storage DC microgrid systems is performed to obtain the equivalent output impedance model of inverters. S4: Based on the equivalent output impedance model of photovoltaic unit, energy storage unit and inverter, establish a stability analysis model for photovoltaic-storage DC microgrid system.

2. The impedance modeling method for a photovoltaic-storage DC microgrid under energy management and control according to claim 1, characterized in that, In the photovoltaic-storage DC microgrid system, the photovoltaic array in the photovoltaic unit is connected to the DC bus through a Boost converter, and the energy storage battery in the energy storage unit is connected to the DC bus through a bidirectional DC-DC converter, and then connected to the grid through an LCL grid-connected inverter.

3. The impedance modeling method for a photovoltaic-storage DC microgrid under energy management and control according to claim 2, characterized in that, In step S1, the specific process of impedance modeling for the photovoltaic unit is as follows: S11: The mathematical model for the photovoltaic unit operating in MPPT control mode is established as follows: Among them, G pv The transfer function for the MPPT voltage loop control of the photovoltaic unit. This is the output voltage of the photovoltaic array. This is the DC bus reference voltage. The duty cycle of the photovoltaic unit Boost converter; S12: Establish the equivalent output impedance model of the photovoltaic unit when operating in MPPT control mode. pv as follows: Among them, Z pvou Output current for photovoltaic units DC bus voltage to photovoltaic unit side The transfer function between them, G pvui Output current for photovoltaic units to photovoltaic array output voltage The transfer function between them, G pvud Duty cycle DC bus voltage to photovoltaic unit side The transfer function between them, H pvud Duty cycle to photovoltaic array output voltage The transfer function between them.

4. The impedance modeling method for a photovoltaic-storage DC microgrid under energy management and control according to claim 3, characterized in that, In step S2, the specific process of impedance modeling for the energy storage unit is as follows: S21: The mathematical model for the energy storage unit operating under power proportional distribution and coordinated control is as follows: Among them, G cu G is the voltage loop transfer function under power proportional distribution and coordinated control. b Let K1 be the current loop transfer function of the energy storage unit, and K1 be the power allocation coefficient of the energy storage unit. The duty cycle of the bidirectional DC-DC converter for the energy storage unit. This refers to the DC bus voltage on the energy storage unit side. For the inductor current of the bidirectional DC-DC converter of the energy storage unit; S22: Establish the equivalent output impedance model of the energy storage unit under power proportional allocation and coordinated control Z batt as follows: Among them, G bii Output current for energy storage unit Current to the bidirectional DC-DC converter inductor of the energy storage unit The transfer function between them, G bid Duty cycle Current to the bidirectional DC-DC converter inductor of the energy storage unit The transfer function between them, Z bou Output current for energy storage unit DC bus voltage to energy storage unit side The transfer function between them, G bud Duty cycle DC bus voltage to energy storage unit side The transfer function between them.

5. The impedance modeling method for a photovoltaic-storage DC microgrid under energy management and control according to claim 4, characterized in that, In step S3, the specific process of modeling the DC-side impedance of the inverter is as follows: S31: The d-axis voltage and current dual-loop mathematical model of the inverter operating under power proportional distribution and coordinated control is established as follows: Among them, G ci Let K1 be the inverter current inner loop control transfer function, and K2 be the inverter power proportional distribution coefficient. This refers to the output current of the d-axis inverter. U is the inverter duty cycle. dc This is the steady-state value of the DC bus voltage. This is the DC bus voltage. Since the photovoltaic unit, energy storage unit, and inverter are connected in parallel, the DC bus voltage at the parallel connection point is... same; S32: Establish the equivalent output impedance model of the inverter Z dc as follows: in, I 1d D represents the steady-state value of the output current of the d-axis inverter. d L is the steady-state value of the d-axis component of the inverter duty cycle, C is the LCL filter capacitor, L1 is the LCL filter inductor on the inverter side, L2 is the LCL filter inductor on the grid side, and L... g R is the power grid impedance, and R is the passive damping series resistance.

6. The method for impedance modeling of a photovoltaic-storage DC microgrid under energy management and control according to claim 5, characterized in that, In step S4, the specific process of establishing the stability analysis model of the photovoltaic-storage DC microgrid system is as follows: S41: The equivalent output impedance model of the photovoltaic unit, energy storage unit and inverter is equivalent to the form of voltage source and impedance in series, thus obtaining the equivalent model of photovoltaic-storage DC microgrid system. S42: When the energy storage unit operates in the discharge state, its impedance can be integrated with that of the photovoltaic unit and connected to the power supply terminal. The equivalent model of the photovoltaic-storage DC microgrid system is further simplified into a model, thus obtaining the DC bus-side port voltage u of the photovoltaic-storage DC microgrid system. dc The expression for the stability analysis model of the photovoltaic-storage DC microgrid system is as follows: Among them, Z d Z represents the total equivalent output impedance of the photovoltaic unit and the energy storage unit. dc u is the total equivalent output impedance of the inverter. d u is the total equivalent output voltage of the photovoltaic unit and the energy storage unit. ref This is the equivalent output voltage of the inverter; S43: Based on the derived u dc Its stability is derived from the expression based on the impedance ratio Z. d / Z dc The decision, based on the Nyquist stability criterion, is that when the system impedance is greater than Z... d / Z dc When the Nyquist curve does not include the point (-1, j0), it indicates that the photovoltaic-storage DC microgrid system is stable, and when the system impedance ratio Z d / Z dc When the Nyquist curve includes the point (-1, j0), it indicates that the photovoltaic-storage DC microgrid system is unstable, thus enabling stability analysis.

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