Island microgrid load flow calculation method, system and device and storage medium

Abstract

The invention discloses an island micro-grid load flow calculation method, system and device and a storage medium, and the method comprises the steps: obtaining the resistance, reactance and susceptance of a power transmission network line and a transformer at a rated frequency, and building a node power model considering the frequency characteristics; obtaining an active droop coefficient and a reactive droop coefficient of the distributed power supply, and further establishing a droop control model for load flow calculation; obtaining a dependency relationship of power of a load element on voltage and frequency, and further establishing a load model considering frequency characteristics; simultaneously establishing the three models to obtain an overall model for island micro-grid load flow calculation, and taking the system frequency as a variable to be solved; and determining the initial value of the system frequency, solving the overall model through iteration until the load flow calculation is converged, and obtaining the steady-state values of the voltage, the phase angle and the system frequency of each node. And the characteristic description of the distributed power supply and the load flow calculation result are more accurate.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of micro-grid, and relates to a method, system and device for power flow calculation of an island micro-grid and a storage medium. BACKGROUND

[0002] As an independent autonomous system mainly taking distributed power generation equipment as a power supply form, a micro-grid can fully utilize renewable energy generation of wind and light and reduce the impact of fluctuation of wind and light generation on a power grid. The micro-grid system usually has the capability of grid-connected operation. When a large power grid fails, the micro-grid is disconnected from the large power grid and is converted into an island operation mode, so as to ensure uninterrupted power supply for important loads in the system. In the island operation mode, the control mode of each distributed power supply in the micro-grid can be divided into master-slave control mode and peer-to-peer control mode. When the island micro-grid adopts the master-slave control mode, one distributed power supply adopts constant voltage and constant frequency control, and other distributed power supplies adopt constant power control. This control mode has a high degree of dependence on communication and has the risk of single point failure. In comparison, in the peer-to-peer control mode, each distributed power supply adopts droop control mode, which can realize plug-and-play function and has high system reliability.

[0003] In order to analyze the case of stable operation of the micro-grid and provide a basis for subsequent stability analysis and short-circuit analysis, it is necessary to perform power flow calculation, so as to determine key state quantities such as power and voltage of power supplies, networks and loads. In the traditional power flow calculation method, the generator node is usually treated as a PV node (mainly used for voltage regulation) or a PQ node (representing a load). This way does not consider the power regulation characteristics presented by the droop control of each distributed power supply. In addition, there is often a lack of sufficient large-capacity distributed power supply in the micro-grid to balance the power shortage. For the distributed power supply adopting droop control, the system power is actually balanced by each distributed power supply. Therefore, the method of setting a balance node used in the traditional power flow calculation method is difficult to be directly used in the power flow calculation process of the island micro-grid.

[0004] When the load changes, the large power grid basically stabilizes at the power frequency state by adopting primary frequency regulation and secondary frequency regulation control. Therefore, the influence of frequency change is usually not considered in the traditional power flow calculation method. For the island micro-grid adopting decentralized droop control, it does not adopt secondary frequency regulation control, which will cause deviation of the system frequency. In addition, due to the small capacity of the distributed power supply in the micro-grid and the lack of strong inertial support, the generated frequency deviation will be large. This large frequency deviation further affects the power transmission network and the load model in the power flow calculation. SUMMARY

[0005] The present application aims to overcome the above-mentioned prior art defects, and provides an island micro-grid power flow calculation method, system, device and storage medium, which models the droop characteristics of the distributed power supply in the power flow calculation process, and considers the frequency deviation caused by the droop characteristics and its influence on the power transmission network and the load model, so that the characteristic description of the distributed power supply and the result of the power flow calculation are more accurate.

[0006] To achieve the above-mentioned purpose, the present application adopts the following technical solutions: An island micro-grid power flow calculation method, comprising the following processes: Obtaining the resistance, reactance and susceptance of the power transmission network lines and transformers in the island micro-grid at the rated frequency, and then establishing a node power model considering the frequency characteristics; Obtaining the active droop coefficient and the reactive droop coefficient of the distributed power supply in the island micro-grid, and then establishing a droop control model for power flow calculation; Obtaining the power-voltage and frequency dependence relationship of the load element in the island micro-grid, and then establishing a load model considering the frequency characteristics; Simultaneously solving the node power model, the droop control model and the load model to obtain an overall model for island micro-grid power flow calculation, taking the system frequency as a variable to be solved; Determining the initial value of the system frequency, and solving the overall model by iteration until the power flow calculation converges to obtain the steady-state values of the node voltage, phase angle and system frequency.

[0007] Optionally, the process of establishing the node power model considering the frequency characteristics is: According to the resistance, reactance and susceptance of the lines and transformers at the rated frequency, the admittance considering the frequency characteristics is calculated to form a node admittance matrix considering the frequency characteristics; The node power model is established based on the node admittance matrix, and the node power model includes a node active power error equation and a reactive power error equation.

[0008] Optionally, the process of establishing the droop control model for power flow calculation is: According to the active droop coefficient of the distributed power supply, an equation of the relationship between the system frequency and the active output is established; According to the reactive droop coefficient of the distributed power supply, an equation of the relationship between the node voltage and the reactive output is established.

[0009] Optionally, the process of establishing the load model considering the frequency characteristics is: According to the dependence relationship of the active power of the load element on the voltage and frequency, an active load model equation is established; According to the dependence relationship of the reactive power of the load element on the voltage and frequency, a reactive load model equation is established.

[0010] Optionally, the Newton-Raphson method is used to iteratively solve the overall model, including: a Jacobian matrix is constructed, which contains the partial derivatives of the active and reactive power errors of the nodes with respect to the voltage amplitudes, phase angles of the nodes, and the system frequency; the correction values of the variables are calculated according to the Jacobian matrix and the power errors of the current iteration point; and the values of the voltage amplitudes, phase angles of the nodes, and the system frequency are updated.

[0011] Optionally, the process of determining the initial value of the system frequency is as follows: the system power imbalance is estimated based on the active power reference values of the distributed power sources and the active power reference values of the loads; the initial value of the system frequency is calculated in combination with the active droop coefficients of the distributed power sources.

[0012] Optionally, the convergence condition of the power flow calculation is that the maximum value of the absolute values of the active power errors and the absolute values of the reactive power errors of all the nodes is less than a preset convergence tolerance.

[0013] A power flow calculation system for an islanded microgrid, comprising: a node power model construction module, configured to obtain the resistance, reactance, and susceptance of the transmission network lines and transformers in the islanded microgrid at the rated frequency, and then establish a node power model considering the frequency characteristics; a droop control model construction module, configured to obtain the active droop coefficients and the reactive droop coefficients of the distributed power sources in the islanded microgrid, and then establish a droop control model for power flow calculation; a load model construction module, configured to obtain the power-voltage and power-frequency dependence relationship of the load elements in the islanded microgrid, and then establish a load model considering the frequency characteristics; an overall model construction module, configured to combine the node power model, the droop control model, and the load model to obtain an overall model for power flow calculation of the islanded microgrid, with the system frequency as a variable to be solved; an iteration module, configured to determine the initial value of the system frequency, iteratively solve the overall model until the power flow calculation converges, and obtain the steady-state values of the node voltages, phase angles, and the system frequency.

[0014] A computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the power flow calculation method for the islanded microgrid when executing the computer program.

[0015] A computer readable storage medium, which stores a computer program, wherein the computer program is executable by a processor to implement the steps of the power flow calculation method for the islanded microgrid.

[0016] Compared with the prior art, the present application has the following beneficial effects: The present application builds a droop control model for power flow calculation and incorporates it into the overall model. The method follows the basic operating principle that distributed power sources in an island microgrid participate in power balance and frequency and voltage regulation through droop characteristics, avoids the balance node assumption in traditional methods which is not applicable to such scenarios, and thus more realistically simulates the behavior of distributed power sources. Secondly, the method explicitly takes the system frequency as a variable to be solved and takes into account the frequency characteristics in the node power model and the load model, which is in line with the physical principle that the lack of secondary frequency modulation in an island microgrid may result in significant frequency deviation, and this deviation may affect network parameters and load response, so that the calculation result can more accurately reflect the actual operating state. Finally, by estimating the system frequency initial value based on the total droop characteristics of the system and the initial power imbalance, the method uses the basic principle of droop control to optimize the starting point of iterative solution, improving the convergence performance of the numerical calculation method. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 A flow chart of power flow calculation for the island microgrid of the embodiment of the present application; Figure 2 A framework diagram of the microgrid system used for testing the embodiment of the present application; Figure 3 A convergence curve diagram of power flow calculation of the embodiment of the present application; Figure 4 A simulation curve diagram of the system of the embodiment of the present application. DETAILED DESCRIPTION

[0018] In order for those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should be within the scope of protection of the present application.

[0019] It is to be understood that the terminology "first", "second", and the like used in the specification and the claims of the application as well as the foregoing drawings is merely used to distinguish similar objects and does not necessarily imply a specific order or chronology. It is to be understood that the use of the term "including" and "comprising" and variations thereof, is intended to cover the various steps, procedures, components, objects, and the like, without limiting them to the steps, procedures, components, objects, and the like, which are recited clearly. It is to be understood that the use of the term "including" and "comprising" and variations thereof, is intended to cover the various steps, procedures, components, objects, and the like, without limiting them to the steps, procedures, components, objects, and the like, which are recited clearly.

[0020] As shown in the figure, the island micro-grid power flow calculation method considering the droop characteristics of the distributed power supply comprises the following steps: Figure 1 1) Obtain the resistance, reactance and susceptance of the transmission network lines and transformers in the island micro-grid at the rated frequency, and then establish a node power model considering the frequency characteristics.

[0021] 2) Obtain the active droop coefficient and reactive droop coefficient of the distributed power supply in the island micro-grid, and then establish a droop control model for power flow calculation.

[0022] 3) Obtain the power-voltage and frequency dependence of the load element in the island micro-grid, and then establish a load model considering the frequency characteristics.

[0023] 4) Combine the node power model, the droop control model and the load model to obtain the overall model for island micro-grid power flow calculation, and determine the variables to be solved in the model.

[0024] 5) Determine the initial value of the power flow calculation, then give the specific iteration format of the power flow calculation using the Newton-Raphson method, and finally solve by iteration until the power flow converges.

[0025] In the step 1), the network equation of the island micro-grid expressed by the node admittance matrix is specifically as follows: (1) In the formula, Y is the self-admittance, which is the sum of the branch admittance connected to the node; Y is the mutual admittance, which is equal to the negative value of the branch admittance connected between the node and the node i Y is the mutual admittance, which is equal to the negative value of the branch admittance connected between the node and the node Y is the mutual admittance, which is equal to the negative value of the branch admittance connected between the node and the node Y is the mutual admittance, which is equal to the negative value of the branch admittance connected between the node and the node i Y is the mutual admittance, which is equal to the negative value of the branch admittance connected between the node and the node j Y is the mutual admittance, which is equal to the negative value of the branch admittance connected between the node and the node Y is the mutual admittance, which is equal to the negative value of the branch admittance connected between the node and the node Y is the mutual admittance, which is equal to the negative value of the branch admittance connected between the node and the node Y is the mutual admittance, which is equal to the negative value of the branch admittance connected between the node and the node are peak values. Admittance which can be formed according to the line and transformer parameters, wherein the admittance of the line or transformer considering the frequency characteristics is: (2) wherein: , and are the resistance, inductive reactance and capacitive reactance of the line or transformer at the rated frequency, respectively; and are the system frequency and its reference value, respectively; and respectively represent the admittance formed by the impedance and capacitive reactance of the line or transformer. Accordingly, the node power model considering the frequency characteristics is: (3) wherein: and respectively represent the active and reactive power errors of the node i ; and respectively represent the active and reactive power of the distributed power on the node i , and if there is no power source, and ; and respectively represent the active and reactive power of the load on the node i , and if there is no load, and ; and are the real part and imaginary part of the element in the node admittance matrix, and they are functions of the frequency f ; .

[0026] In the step 2), the specific form of the droop control adopted by the distributed power in the island micro-grid is: (4) wherein: is the reference value of the voltage of the node i , and its value is a peak value; and are the active and reactive droop coefficients, respectively; and are the reference values of the active and reactive power of the distributed power on the node i . The specific form of the droop control model used for power flow calculation is: (5) In step 3), the specific form of the load model considering frequency characteristics in the islanded microgrid is as follows: (6) In the formula: and They are nodes i Reference values ​​for the active and reactive power output of distributed power sources; and Indicates the corresponding coefficient; , , and , , Let represent the corresponding coefficients, and satisfy the following relationship: (7) In step 4), the node power model (3), droop control model (5), and load model (6) are combined to obtain the specific form of the overall model for power flow calculation of the islanded microgrid: (8) For those N In a system with 2 nodes, equation (8) contains 2... N The equations are as follows. Taking the voltage phase angle at node 1 as the reference angle, that is, let... The variable to be solved in equation (8) is and and system frequency f A total of 2 N There are several unknowns. The model is based on nodes. i This holds true when both distributed power sources and loads exist simultaneously; when nodes i When there are no distributed power sources and / or loads, it is necessary to make , and / or , .

[0027] In step 5), the initial voltage magnitude and phase angle used for power flow calculation are: (9) In the formula: and These are the initial values ​​of the voltage magnitude and phase angle used for power flow calculation. Ignoring power consumption on the transmission network and assuming the load voltage is at its rated value, we obtain an approximate power balance equation for the entire system: (10) Further consideration of the specific forms in equations (5) and (6) yields: (11) Therefore, the initial value of the system frequency used for power flow calculation can be: (12) In the formula: The initial system frequency is used for power flow calculation. The specific iterative format for solving the power flow using the Newton-Laurel method is as follows: (13) In the formula: , , For the first k Initial value for the next iteration; , , For the first k The correction value for the next iteration; , , For the first k The result of the next iteration; and The first k The active and reactive power deviation in the next iteration; For the first k The Jacobian matrix of the nth iteration; where when node i When distributed power sources and loads exist simultaneously, and for: (14) In the formula: When node i When there are no distributed power sources and / or loads, it is necessary to make , and / or , Furthermore, the specific form of the Jacobian matrix is: (15) When node i When distributed power sources and loads coexist, the elements in the Jacobian matrix take the following form: (16) and (17) The discriminant for trend convergence is: (18) In the formula: E k Indicates the first k The error of the next iteration; Indicates tolerance. When the node iWhen there are no distributed power sources and / or loads, it is necessary to make , and / or , .

[0028] To verify the power flow calculation method for islanded microgrids that takes into account the droop characteristics of distributed generation proposed in this embodiment, the effectiveness of the method was verified on an islanded microgrid system containing three distributed generation sources.

[0029] A framework diagram of a microgrid system is shown below. Figure 2 As shown. The corresponding model parameters are shown in Table 1, where... R 0, X L0 and X C0 These represent the resistance, inductive reactance, and capacitive reactance per unit length of line at power frequency, respectively. l ij Represents a node i and nodes j The line length between them. Furthermore, the parameters and power ratings of the three distributed power sources in the microgrid remain identical.

[0030] Table 1 Microgrid Model Parameters

[0031] Next, based on the line parameters, the node admittance matrix considering frequency variations in the islanded microgrid is obtained, and then a node power model considering frequency characteristics is formed. Additionally, based on the distributed generation and load models and parameters, droop control models for power flow calculations and load models considering frequency characteristics are established respectively. After obtaining the overall system model, the Newton-Laurel method is used to solve the system's power flow. (The tolerance is set to 10.) -8 The power flow calculation converged after 6 iterations. The convergence curve of the power flow calculation is shown below. Figure 3 As shown in the figure. Furthermore, the power flow calculation results are shown in Table 2, where the power generated by the distributed generation is a positive value; the power absorbed by the load is an amplitude value. Additionally, the system frequency is 49.824Hz. At this time, the impedance per unit length of the line is 0.2 + j0.4982Ω, and the capacitive reactance is 9.96 × 10⁻⁶. -4 Ω.

[0032] Table 2. Microgrid power flow calculation results

[0033] Finally, the simulation curves of the system are as follows: Figure 4 As shown. The simulation considered the RLC filter model of the distributed power source and the control equations for voltage and current. Furthermore, low-pass filtering of the power was considered in the droop control. (Comparison) Figure 4As shown in Table 2, the frequency and voltage amplitude of the simulated system are consistent with the power flow calculation results in Table 2.

[0034] The following are embodiments of the apparatus of the present invention, which can be used to execute embodiments of the method of the present invention. For details not omitted in the apparatus embodiments, please refer to the embodiments of the method of the present invention.

[0035] In another embodiment of the present invention, a power flow calculation system for an islanded microgrid is provided. This system can be used to implement the above-mentioned power flow calculation method for an islanded microgrid. Specifically, the system includes a node power model construction module, a droop control model construction module, a load model construction module, an overall model construction module, and an iteration module.

[0036] The node power model construction module is used to obtain the resistance, reactance, and susceptance of the transmission network lines and transformers in the islanded microgrid at the rated frequency, and then establish a node power model that takes into account the frequency characteristics.

[0037] The droop control model building module is used to obtain the active droop coefficient and reactive droop coefficient of distributed generation in islanded microgrids, and then establish a droop control model for power flow calculation.

[0038] The load model building module is used to obtain the power dependence of load elements on voltage and frequency in an islanded microgrid, and then establish a load model that takes into account frequency characteristics.

[0039] The overall model building module is used to combine the node power model, droop control model and load model to obtain an overall model for power flow calculation of islanded microgrids, with the system frequency as the variable to be solved.

[0040] The iterative module is used to determine the initial value of the system frequency. It iteratively solves the overall model until the power flow calculation converges, and obtains the steady-state values ​​of the voltage, phase angle and system frequency of each node.

[0041] The iteration module is used to solve the overall model iteratively until the power flow calculation converges, and obtain the steady-state values ​​of the voltage, phase angle and system frequency of each node.

[0042] In another embodiment of the present invention, a terminal device is provided, comprising a processor and a memory. The memory stores a computer program, the computer program including program instructions, and the processor executes the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or it may be other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), or field-programmable gate arrays (FPGAs). Gate Array (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc., are the computing and control core of the terminal. They are suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions to realize the corresponding method flow or corresponding function. The processor described in this embodiment of the invention can be used for the operation of the power flow calculation method for isolated microgrids, including: obtaining the resistance, reactance and susceptance of the transmission network lines and transformers in the isolated microgrid at the rated frequency, and then establishing a node power model considering frequency characteristics; obtaining the active droop coefficient and reactive droop coefficient of the distributed power sources in the isolated microgrid, and then establishing a droop control model for power flow calculation; obtaining the power dependence of the load elements on voltage and frequency in the isolated microgrid, and then establishing a load model considering frequency characteristics; combining the node power model, droop control model and load model to obtain the overall model for power flow calculation of the isolated microgrid, with the system frequency as the variable to be solved; determining the initial value of the system frequency, and iteratively solving the overall model until the power flow calculation converges to obtain the steady-state values ​​of the voltage, phase angle and system frequency of each node.

[0043] In another embodiment, the present invention also provides a computer-readable storage medium (Memory), which is a memory device in a terminal device for storing programs and data. It is understood that the computer-readable storage medium here may include both the built-in storage medium in the terminal device and extended storage media supported by the terminal device. The computer-readable storage medium provides storage space that stores the terminal's operating system. Furthermore, the storage space also stores one or more instructions suitable for loading and execution by a processor, which may be one or more computer programs (including program code). It should be noted that the computer-readable storage medium here may be high-speed RAM or non-volatile memory, such as at least one disk storage device.

[0044] One or more instructions stored in a computer-readable storage medium can be loaded and executed by the processor to implement the corresponding steps of the power flow calculation method for isolated microgrids in the above embodiments. One or more instructions in the computer-readable storage medium are loaded and executed by the processor in the following steps: obtaining the resistance, reactance, and susceptance of the transmission network lines and transformers in the isolated microgrid at the rated frequency, and then establishing a node power model considering frequency characteristics; obtaining the active droop coefficient and reactive droop coefficient of the distributed generation in the isolated microgrid, and then establishing a droop control model for power flow calculation; obtaining the power dependence of the load elements on voltage and frequency in the isolated microgrid, and then establishing a load model considering frequency characteristics; combining the node power model, droop control model, and load model to obtain the overall model for power flow calculation of the isolated microgrid, with the system frequency as the variable to be solved; determining the initial value of the system frequency, and iteratively solving the overall model until the power flow calculation converges to obtain the steady-state values ​​of the voltage, phase angle, and system frequency of each node.

[0045] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0046] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0047] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0048] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0049] The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0050] In the above embodiments of this application, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0051] In the several embodiments provided in this application, it should be understood that the disclosed technical content can be implemented in other ways. The device embodiments described above are merely illustrative; for example, the division of units can be a logical functional division, and in actual implementation, there may be other division methods. For instance, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the displayed or discussed mutual coupling, direct coupling, or communication connection may be through some interfaces; the indirect coupling or communication connection between units or modules may be electrical or other forms.

[0052] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0053] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

[0054] It should be understood that the above description is for illustrative purposes and not for limitation. Many embodiments and applications beyond the provided examples will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of this patent should not be determined by reference to the above description, but rather by reference to the foregoing claims and the full scope of their equivalents. For purposes of completeness, all articles and references, including patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended as a waiver of that subject matter, nor should it be construed as an indication that the applicant has not considered that subject matter as part of the disclosed inventive subject matter.

Claims

1. A method for power flow calculation of islanded microgrid, characterized in that, The method comprises the following steps: obtaining the resistance, reactance and susceptance of the transmission network line and transformer in the island micro-grid at the rated frequency, and then establishing a node power model considering the frequency characteristics; obtaining the active and reactive droop coefficients of the distributed power supply in the island micro-grid, and then establishing a droop control model for power flow calculation; obtaining the power-voltage and frequency dependence of the load element in the island micro-grid, and then establishing a load model considering the frequency characteristics; simultaneously solving the node power model, the droop control model and the load model to obtain an overall model for power flow calculation of the island micro-grid, taking the system frequency as a variable to be solved; determining the initial value of the system frequency, and iteratively solving the overall model until the power flow calculation converges to obtain the steady-state values of the node voltage, phase angle and system frequency.

2. The islanded microgrid power flow calculation method of claim 1, wherein, The process of establishing the node power model considering the frequency characteristics is as follows: calculating the admittance considering the frequency characteristics according to the resistance, reactance and susceptance of the line and the transformer at the rated frequency, and forming a node admittance matrix considering the frequency characteristics; establishing a node power model based on the node admittance matrix, wherein the node power model comprises a node active power error equation and a node reactive power error equation.

3. The islanded microgrid power flow calculation method of claim 1, wherein, The process of establishing the droop control model for power flow calculation is as follows: establishing an equation of the relationship between the system frequency and the active output according to the active droop coefficient of the distributed power supply; establishing an equation of the relationship between the node voltage and the reactive output according to the reactive droop coefficient of the distributed power supply.

4. The islanded microgrid power flow calculation method of claim 1, wherein, The process of establishing the load model considering the frequency characteristics is as follows: establishing an active load model equation according to the dependence of the active power of the load element on the voltage and frequency; establishing a reactive load model equation according to the dependence of the reactive power of the load element on the voltage and frequency.

5. The islanded microgrid power flow calculation method of claim 1, wherein, The Newton-Raphson method is used to iteratively solve the overall model, including: constructing a Jacobian matrix, wherein the Jacobian matrix comprises the partial derivatives of the node active and reactive power errors with respect to the node voltage amplitude, the phase angle and the system frequency; calculating the correction value of each variable according to the Jacobian matrix and the power error at the current iteration point; and updating the values of the node voltage amplitude, the phase angle and the system frequency.

6. The islanded microgrid power flow calculation method of claim 1, wherein, The process of determining the initial value of the system frequency is as follows: estimating the system power imbalance based on the active output reference value of each distributed power supply and the active power reference value of each load; calculating the initial value of the system frequency in combination with the active droop coefficients of each distributed power supply.

7. The islanded microgrid power flow calculation method of claim 1, wherein, The convergence condition of the power flow calculation is that the maximum value of the absolute values of the active power error and the reactive power error at all nodes is less than a preset convergence tolerance.

8. An islanded microgrid power flow calculation system, characterized by, The method comprises the following steps: a node power model construction module, configured to obtain the resistance, reactance and susceptance of the transmission network line and transformer in the island micro-grid at the rated frequency, and then establish a node power model considering the frequency characteristics; a droop control model construction module, configured to obtain the active and reactive droop coefficients of the distributed power supply in the island micro-grid, and then establish a droop control model for power flow calculation; A load model construction module is configured to obtain power-voltage and frequency dependence of load elements in the islanded microgrid, and to establish a load model considering frequency characteristics; An overall model construction module is configured to jointly construct the node power model, the droop control model and the load model to obtain an overall model for islanded microgrid power flow calculation, with system frequency as a variable to be solved; An iteration module is configured to determine an initial value of the system frequency, and to solve the overall model by iteration until power flow calculation converges, to obtain steady-state values of node voltages, phase angles and the system frequency.

9. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, The processor executes the computer program to implement the steps of the islanded microgrid power flow calculation method according to any one of claims 1 to 7.

10. A computer-readable storage medium storing a computer program, the computer program comprising instructions that, when executed by a computer, cause the computer to perform the method of any one of claims 1 to 9. The computer program is executed by the processor to implement the steps of the islanded microgrid power flow calculation method according to any one of claims 1 to 7.

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