A method and system for evaluating safety margin of power distribution system

Abstract

The present invention discloses a method and system for evaluating the safety margin of a distribution system, and relates to the technical field of electric power systems. At present, the safety margin evaluation ignores the voltage boundary and cannot truly reflect the safe operating space of the distribution network. The present invention includes the steps of: constructing a nonlinear power flow model and a voltage source converter model of an AC / DC distribution network based on the Distflow branch power flow method, and combining voltage constraints and capacity constraints to construct a distribution system safety domain that comprehensively considers power boundaries and voltage boundaries; linearizing the constructed nonlinear power flow model and various operating safety constraints to generate a linearized model, and calculating the static safety distance of each effective boundary of the distribution system based on the linearized model; calculating and evaluating the static safety margin of the distribution system based on the minimum static safety distance. This technical solution improves the existing distribution system safety margin evaluation method to more accurately evaluate the safety margin of the distribution system under different operating conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method and system for evaluating safety margin of a power distribution system. Background Art

[0002] With the accelerated advancement of the construction of new power systems, traditional distribution networks are facing profound changes. The access of a high proportion of distributed power sources leads to a bidirectional flow of power in the distribution network, and some lines face the risk of power flow exceeding the limit and node voltage exceeding the limit; at the same time, the significant increase in the proportion of flexible loads such as user-side energy storage equipment and electric vehicles has gradually transformed traditional energy consumers into energy producers and sellers, and the load characteristics have shown greater diversity, and the randomness and uncertainty of load changes have also increased significantly. These factors have further increased the difficulty of measuring the overall safety of the new distribution system and evaluating the safety margin of operation, bringing greater challenges to the safety analysis and preventive control of the distribution network.

[0003] The safety of the distribution network is a basic requirement for its operation. Safety assessment of the distribution network is the key to achieving operation optimization and risk prevention and control. Traditional distribution network safety analysis mostly adopts the "point-by-point method", which can only verify the operating safety of the distribution system point by point, and cannot comprehensively evaluate the overall safety margin of the system. Some scholars have proposed the concept and method of the Distribution System Security Region (DSSR) for AC distribution networks. The relationship between the system operating point and the operating safety boundary is characterized by the security region, so as to further calculate the safety margin of the current operating point of the distribution system. Some studies have proposed a distribution system safety assessment method that can calculate the power supply margin of the distribution network in real time and analyze the maximum power supply capacity from different levels such as the complete distribution network, region and node. However, the above studies mostly focus on the construction of the security domain of the AC distribution network, and fail to fully consider the impact of the access of distributed generators (DG) on the security domain and power supply capacity of the distribution system.

[0004] Most of the existing safety domain research focuses on AC distribution networks, and there are relatively few studies on the safety domain model of AC and DC distribution networks. Some researchers have proposed the concepts of static safety domain and static safety distance based on the DC power flow model, and analyzed their physical meanings. However, since the DC power flow model only considers the power boundary of the system and ignores the impact of the voltage boundary, when the reactive power compensation in the distribution network is insufficient, the line is long, or the network loss is large, the system will face low voltage risks. The safety domain constructed by simply considering the power boundary cannot truly reflect the safe operation space of the distribution network.

[0005] In summary, existing studies on the safety margin assessment of distribution systems mostly use the "point-by-point method" or the "domain method" to perform safety margin analysis. The distribution system safety analysis method based on the "domain method" usually uses a linear model of DC power flow. This method mainly focuses on the capacity boundary and ignores the voltage boundary, resulting in an incomplete characterization of the safe operation space of the distribution system. Therefore, there is an urgent need for a new safety domain model that can comprehensively consider the power boundary and the voltage boundary to more accurately evaluate the safety margin of the AC / DC distribution system under different operating conditions, thereby ensuring the safe and stable operation of the distribution system. Summary of the invention

[0006] The technical problem to be solved and the technical task proposed by the present invention are to improve and perfect the existing technical solutions, and provide a method for evaluating the safety margin of a power distribution system, so as to improve the existing method for evaluating the safety margin of a power distribution system by constructing a safety domain of an AC / DC power distribution system that comprehensively considers power boundaries and voltage boundaries, so as to more accurately evaluate the safety margin of a power distribution system under different operating conditions. To this end, the present invention adopts the following technical solutions.

[0007] A method for evaluating the safety margin of a power distribution system comprises the following steps:

[0008] 1) Based on the Distflow branch power flow method, a nonlinear power flow model and a voltage source converter model of the AC / DC distribution network are constructed. In combination with voltage constraints and capacity constraints, a distribution system safety domain that comprehensively considers power boundaries and voltage boundaries is constructed to obtain a safe operating range under different operating conditions;

[0009] 2) Linearize the constructed nonlinear power flow model and various operating safety constraints to generate a linearized model, and calculate the static safety distance of each effective boundary of the distribution system based on the linearized model; based on the minimum static safety distance, calculate and evaluate the static safety margin of the distribution system, so as to quantify the safety margin of the system under the current operating state.

[0010] This technical solution constructs a more comprehensive distribution system safety domain by considering both the power boundary and the voltage boundary at the same time, which enables the evaluation method to not only reflect the power limit of the system, but also accurately capture the system voltage limit, thereby more comprehensively reflecting the safe operation space of the distribution system. Based on the Distflow branch flow method, this technical solution can effectively handle the complexity of AC / DC hybrid distribution networks, and is suitable for better evaluation of system safety margins when a high proportion of distributed power sources are connected and the proportion of flexible loads increases. Through the linearization of nonlinear power flow models and operational safety constraints, this method can accurately calculate the static safety distance of each effective boundary, and evaluate the static safety margin of the distribution system based on this. In this way, the safety margin of the system under the current operating state can be quantified, which improves the accuracy of the evaluation. This technical solution can evaluate the safe operation range of the system under different operating conditions (such as load fluctuations, changes in the output of distributed power sources, etc.), has strong adaptability and practicality, and helps operators better understand the safety status of the system. Through a more comprehensive and accurate safety margin evaluation, system operators can identify potential safety risks earlier and take measures in advance, thereby enhancing the preventive control capabilities of the distribution system and ensuring the stable operation of the system.

[0011] As a preferred technical means: Step 1) includes: 1.1) Establishment of power flow model: Based on the Distflow branch power flow method, a nonlinear power flow model describing the power flow of the AC / DC distribution network is constructed, specifically including the power balance equation of the AC branch, the power transmission equation of the DC branch, and the relationship equation between the node voltage and current;

[0012] 1.2) Integration of converter model: The voltage source converter model is introduced to describe the role of the converter in the AC / DC distribution network through the power exchange equation, modulation strategy and its capacity limitation;

[0013] 1.3) Construction of safety domain: Based on the voltage constraints, capacity constraints and power boundary conditions of system operation, the safety domain of the distribution system is defined and constructed through the power flow model and converter model to define the safe operation range of the system under different operating conditions.

[0014] This technical solution can accurately describe the flow of power in AC / DC distribution networks through a nonlinear power flow model constructed based on the Distflow branch power flow method, better reflect the actual operating status of the system, and improve the accuracy of system analysis, especially in complex AC / DC hybrid networks. The introduction of the voltage source converter (VSC) model and its combination of power exchange equations, modulation strategies and capacity constraints enable the evaluation method to fully consider the role of the converter in the AC / DC distribution network, which is particularly important for modern distribution networks with a large number of distributed power sources connected, and can more accurately evaluate the operating safety of the system. By combining voltage constraints, capacity constraints and power boundary conditions of system operation, a comprehensive safety domain is constructed. The safety domain can accurately define the safe operating range of the system under different operating conditions, providing a comprehensive safety assessment framework, so that the safe operating space of the system can be more accurately characterized. This technical solution can accurately define the safe operating range of the system under different operating conditions. This means that it can effectively respond to changes in operating status caused by factors such as distributed power source access and load changes, and improve the adaptability and stability of the system. By adopting a safety domain construction method that comprehensively considers power boundaries and voltage boundaries, system operators can better understand the operating boundaries of the distribution network, thereby identifying potential safety risks in advance and taking preventive measures, significantly improving the safety and reliability of the distribution system.

[0015] As a preferred technical means: the power flow model expression of step 1.1) includes:

[0016] (1)

[0017] (2)

[0018] (3)

[0019] (4)

[0020] (5)

[0021] (6)

[0022] (7)

[0023] (8)

[0024] (9)

[0025] (10)

[0026] Where: P AC,ij,tand Q AC,ij,t AC branch ij exist t The active power and reactive power of the first end at the moment; P AC,jk,t and Q AC,jk,t AC branch jk exist t The active power and reactive power of the first end at the moment; P AC,j,t and Q AC,j,t Communication nodes j exist t Active power and reactive power flowing out at all times; P AC,i,t and Q AC,i,t Communication nodes i exist t Active power and reactive power flowing out at all times; r AC,ij and x AC,ij AC branch ij resistance and reactance; V AC,i,t and V AC,j,t Communication nodes i and j exist t The voltage amplitude at the moment; I AC,ij,t For AC branch ij exist t The current at the moment; P AC,i,L,t and Q AC,L,t Communication nodes i exist t Active load and reactive load at the moment; P AC,j,DG,t and Q AC,j,DG,t They are AC DG nodes j exist t Active and reactive power output at all times; P AC,j,VSC,t and Q AC,j,VSC,t They are the nodes on the AC side of the converter j exist t The equivalent outflow active power and reactive power at the moment; P DC,ij,t For DC branch ij exist tThe active power at the first end at the moment; P DC,jk,t For DC branch jk exist t The active power at the first end at the moment; P DC,j,t DC node j exist t The outflow power at the moment; r DC,ij For DC branch ij Resistance; I DC,ij,t For DC branch ij exist t The current at the moment; V DC,i,t DC node i exist t The voltage amplitude at the moment; V DC,j,t DC node j exist t The voltage amplitude at the moment; P DC,j,L,t DC node j Active load; P DC,j,DG,t DC node j Active output of distributed generation; Equations (1), (2) and (4) represent the equation constraints of AC branch power flow; Equation (3) is the equation constraint of node voltage with phase angle relaxation; Equations (5) and (6) represent the AC node outflow power, including node load, node distributed generation output and converter equivalent injection power; Equations (7) and (9) represent the DC branch power flow equation constraints; Equation (8) represents the DC branch voltage equation constraint; Equation (10) represents the DC node outflow power.

[0027] The power flow model of this technical solution not only covers the power balance and node voltage relationship of the AC distribution network, but also includes the power transmission equation of the DC distribution network, which more comprehensively reflects the operating characteristics of the modern AC / DC hybrid distribution network and improves the applicability and accuracy of the model. By introducing the active power and reactive power outflow items on the AC side of the converter, the model accurately describes the role of the converter in the AC / DC distribution network, which can reflect the impact of the converter on the system power flow distribution, thereby improving the accurate evaluation of the system operation status. The model expression takes into account the load, power output and voltage conditions of the AC node and the DC node, covering the main influencing factors in the system, and can better capture the dynamic changes in the system caused by the access of distributed power sources, load changes, etc., thereby improving the reliability of the analysis results. By modeling factors such as AC and DC branch resistance, reactance and current, the power loss and power transmission of the line can be accurately calculated, which helps to more accurately evaluate the operating status and voltage distribution of the line, thereby supporting more accurate operation optimization and risk prevention and control. The model can adapt to the analysis needs of the distribution network under different operating conditions. Whether it is high load, large-scale access of distributed power sources, or dynamic operation of converters, it can be accurately evaluated through this model, providing a solid foundation for the safe assessment and optimized operation of the distribution network.

[0028] As a preferred technical means: in step 1.2), the converter model expression includes:

[0029] (11)

[0030] (12)

[0031] (13)

[0032] (14)

[0033] (15)

[0034] (16)

[0035] (17)

[0036] (18)

[0037] (19)

[0038] (20)

[0039] (twenty one)

[0040] Where: and AC side system t Inject the first k Active power and reactive power of each VSC converter; and Respectively k The equivalent resistance and reactance of each VSC converter; and Respectively with k AC side nodes connected to the VSC converters t Voltage and converter at the moment t The fundamental phase voltage at the moment; is the VSC equivalent AC branch current; is the phase active power input to the VSC; is the DC active power output by the VSC; For the k Inverter t The modulation degree at the moment, that is, the ratio of the modulation wave to the carrier amplitude; For the k Inverter t DC voltage utilization at the moment; For the k The DC side nodes connected to the VSC converters t The voltage amplitude at the moment; For the k Maximum capacity of a VSC converter; The upper limit of the DC power output of the VSC; and They are the upper and lower limits of the VSC DC side voltage respectively; and They are the upper and lower limits of the VSC AC side voltage respectively.

[0041] This technical solution describes in detail the key parameters of the converter in the AC and DC distribution network, such as power injection, loss, modulation ratio, and voltage utilization, through model expressions. It can accurately simulate the dynamic behavior and working characteristics of the converter, and help improve the accurate prediction and analysis of the actual operating status of the converter. The converter model expression not only takes into account the injection of active power and reactive power of the converter, but also includes the voltage, current and capacity limitations related to the converter. The operation constraints ensure that the model can reflect the physical limitations of the converter in actual operation, and improve the reality and reliability of the model. The introduction of parameters such as modulation, DC voltage utilization, and voltage upper and lower limits in the model enables the converter model to flexibly adapt to different operating conditions and working conditions, so that the model can adapt to various complex operating environments and is suitable for different application scenarios and operation strategy analysis. By strictly constraining the converter voltage modulation ratio and voltage utilization, it is ensured that the converter operates within a safe range, avoiding system instability caused by excessively high or low voltage, and helping to improve the stability and safety of the entire distribution system. The expression of the converter model can well adapt to the complex characteristics of the AC / DC hybrid distribution system. By considering the power transmission and voltage control on the AC and DC sides, the model is more effective in dealing with the coupling and mutual influence of the AC / DC distribution system.

[0042] As a preferred technical means: In step 1.3), the safety constraint condition expression includes:

[0043] (twenty two)

[0044] (twenty three)

[0045] (twenty four)

[0046] (25)

[0047] (26)

[0048] (27)

[0049] (28)

[0050] (29)

[0051] Where: S AC,ij,max For AC branch ij transmission capacity; S AC,i,VSC,max For Node i Converter capacity; P AC,i,DG,t，max For Nodei Upper limit of distributed power generation output; is the power factor angle of the distributed power source; V AC,i, max and V AC,i, min Node i The voltage upper and lower limits of the system; Formula (22) is the line capacity constraint, Formula (23) is the VSC capacity constraint, Formula (26) is the node voltage constraint, and Formulas (24) and (25) are the distributed generation output constraints; For DC distribution network lines i - j transmission capacity; DC distribution network node i The upper limit of the output of distributed power generation; and They are DC distribution network nodes i The voltage lower limit and voltage upper limit of the DC distribution network are as follows; Formula (27) is the line capacity constraint of the DC distribution network, Formula (28) is the output constraint of the distributed generation, and Formula (29) is the node voltage constraint of the DC distribution network.

[0052] This technical solution covers various key safety constraints involved in AC and DC power distribution systems, including line transmission capacity, converter capacity, distributed power output, voltage upper and lower limits, etc., ensuring that the safety of the system is fully guaranteed under different operating conditions. By strictly constraining the capacity of AC and DC lines, voltage upper and lower limits, converter capacity, and distributed power output, the actual physical limitations of the system can be accurately reflected, which helps to avoid safety problems caused by exceeding physical limits in actual operation. Safety constraints are not only applicable to traditional AC distribution networks, but also to DC distribution networks and AC / DC hybrid distribution systems. In particular, the consideration of voltage and power constraints of DC distribution networks enables this solution to adapt to the complex structure of modern distribution systems. Since the constraints involve all aspects of system operation and take into account multiple boundaries of voltage and power, this provides accurate basic data for subsequent safety margin assessment, thereby improving the accuracy and reliability of the assessment. Through strict capacity and voltage constraints, it is ensured that the system will not fall into an unstable state due to overload or voltage anomaly, thereby ensuring the stability of the distribution system under various operating conditions. This technical solution takes into account the output limit and power factor angle of distributed power sources, and can effectively handle the impact of distributed power sources of different types and sizes on system security, and support the energy diversification needs in modern power distribution systems. The safety constraints of this technical solution provide specific boundary conditions for the construction of the system security domain, which can accurately define the safe operating range of the system and help to monitor and warn the operating status of the system in real time.

[0053] As a preferred technical means: In step 2), the linearized Distflow branch power flow model is used to replace the nonlinear power flow model:

[0054] (30)

[0055] (31)

[0056] (32)

[0057] (33)

[0058] (34)

[0059] Where: V AC,N is the rated voltage amplitude of the AC distribution network; V DC,N is the rated voltage of the DC distribution network.

[0060] The linearized model simplifies the complex nonlinear power flow calculation, reduces the difficulty of calculation, and enables the safety margin assessment of the distribution system to be completed in a shorter time; the linearized power flow model structure is simpler and easy to implement and apply in the actual system. Although the linearized model is an approximation of the actual system, it can still provide sufficiently accurate results under most operating conditions, is suitable for different loads and operating states, has strong adaptability, and can meet a variety of practical application needs. The linearized model is easier to integrate with other optimization algorithms, control strategies or analysis tools. Especially in the optimization and dispatching of large-scale power systems, the application of linear models is more extensive and can be seamlessly connected with existing systems. Since the linearized model no longer contains complex nonlinear equations, the state estimation, optimization and margin assessment of the distribution system can be quickly solved through linear programming or other linear solution methods, which not only simplifies the algorithm design, but also improves the stability and reliability of the solution. The linearized model helps to simplify the stability analysis of the distribution system. Through the linear model, researchers and engineers can more easily identify the possible instability factors in the system and take corresponding preventive measures. In the analysis of multi-level power systems, the linearized model can reduce the cumulative effect of errors. Since the linear model calculation process is relatively stable, it can maintain high accuracy in multiple calculation steps, thereby improving the reliability of the overall analysis. After linearization, the safety domain of the system can be quantified and evaluated more accurately, especially when evaluating the static safety margin. The linearized model provides a more direct calculation method, which helps to more accurately quantify the safety margin of the system under the current operating state.

[0061] As a preferred technical means: linearize the capacity constraints of equations (22) and (23), and approximate the circular constraints as an inscribed regular dodecagon, that is, replace the capacity constraints of equations (22) and (23) with linearized capacity constraint equations:

[0062] (35)

[0063] Where: , , is the coefficient of the linearized capacity constraint.

[0064] This technical solution approximates the circular constraint as a linear constraint of an inscribed regular dodecagon, which simplifies the complex nonlinear calculation, so that the linear programming method can be used to solve it during the optimization process, thereby greatly reducing the complexity of the calculation. The linearized constraint conditions make the calculation process more efficient. Linear programming problems are usually easier to solve than nonlinear programming problems, so results can be obtained in a shorter time, which improves the real-time and efficiency of the safety margin assessment of the distribution system. Linear constraints can be better compatible with existing optimization algorithms and solvers, especially in large-scale distribution networks. The linearized model can be more effectively combined with commercial optimization software and algorithms to support larger-scale system analysis. The linearization process simplifies the implementation process of the model, making it easier to implement and operate in actual systems, reducing the complexity of system implementation, and helping engineers to quickly apply the model to the analysis and optimization of actual distribution systems. By reasonably selecting an inscribed regular dodecagon to approximate the circular constraint, a certain accuracy can be maintained while ensuring the simplification of the model, which is suitable for most actual operating conditions. Converting complex circular constraints into linear constraints can reduce calculation errors.

[0065] As a preferred technical means: the static safety margin of the power distribution system is characterized by the static safety distance (Steady-State Security Distance, SSD), which is defined as the shortest Euclidean distance from the basic operating point of the system to the boundary of each safety domain, that is,

[0066] (36)

[0067] Where: M argin,t for t The safety margin of the power distribution system at all times; N is the number of effective boundaries of the safety domain of the distribution system; ω 0 is the current operating point expressed in terms of node load; F i The first in the static security region (Steady-State Security Region, SSR)i The set of operating points contained in the boundary; λ i Indicates the operating point to SSR i The shortest distance to the border.

[0068] As a quantitative indicator, SSD can accurately describe the safety margin of the system under the current operating state. By calculating the shortest distance from the current point to the safety domain boundary, it provides an accurate safety margin quantification value, which helps to evaluate the stability and reliability of the system. SSD can be used to easily compare the safety margin under different operating conditions. By comparing the SSD values ​​at different times or in different situations, the safety changes of the system under different conditions can be analyzed, supporting effective safety risk assessment and management. SSD can adapt to the dynamic changes of the distribution system. Since it is based on the distance between the current operating point and the safety domain boundary, it does not require too many assumptions or simplifications on the system, so it can reflect the actual safety status of the system in real time. SSD can be used as the basis for optimization and decision-making. By evaluating SSD, the most vulnerable areas or the most urgent safety risks in the system can be identified, thereby providing a basis for formulating corresponding improvement measures or optimization strategies. SSD takes into account the various safety domain boundaries of the system and calculates the shortest distance to each boundary, thereby comprehensively considering multiple safety factors, which helps to comprehensively evaluate the safety margin of the system, rather than just being limited to a specific constraint. Through simple distance calculation, the SSD method is easy to implement and compatible with existing distribution system monitoring and management systems, making it relatively easy to implement in practical applications and effectively integrated into the safety assessment process of the distribution system.

[0069] Another object of the present invention is to provide a power distribution system safety margin assessment system, the system is used to perform the above-mentioned power distribution system safety margin assessment method, the system comprising:

[0070] The power flow modeling module is used to build a nonlinear power flow model and a voltage source converter model of the AC / DC distribution network based on the Distflow branch power flow method, and to build a distribution system safety domain that comprehensively considers the power boundary and the voltage boundary by combining the voltage constraint and the capacity constraint, so as to obtain a safe operating range under different operating conditions;

[0071] A linearization processing module, used for performing linearization processing on the nonlinear power flow model and various operation safety constraints to generate a linearization model;

[0072] The safety margin calculation module is used to calculate the static safety distance of each effective boundary of the distribution system based on the linearized model, and calculate and evaluate the static safety margin of the distribution system based on the minimum static safety distance, so as to quantify the safety margin of the system under the current operating state.

[0073] The power flow modeling module of this technical solution uses the Distflow branch power flow method to accurately construct the nonlinear power flow model and voltage source converter model of the AC / DC distribution network, ensuring comprehensive consideration of the system power and voltage, thereby more accurately defining the safety domain of the distribution system. By combining the constraints of the power boundary and the voltage boundary, the power flow modeling module can construct a distribution system safety domain that comprehensively considers all key factors. The comprehensive safety domain construction helps to more accurately define the safe operating range of the system under different operating conditions. The linearization processing module linearizes the nonlinear model and the safety constraints to generate a linearized model. The linearization processing simplifies the calculation process, improves the calculation efficiency, and makes it easier to perform safety margin calculation and optimization analysis in practical applications. The safety margin calculation module calculates the static safety distance of each effective boundary based on the linearization model, and evaluates the safety margin of the system through the minimum static safety distance, dynamically reflects the current safety status of the system, and provides real-time safety margin quantification values ​​for managers. This technical solution can quantify the complex safety margin evaluation process into specific numerical indicators (static safety distance and safety margin), making the evaluation of the system safety status more intuitive, operational and easy to understand. By quantifying the safety margin, the system can help managers identify potential risks in the system and support optimal decision-making, providing detailed information on the system's safety status and helping to formulate effective improvement measures and response strategies. By comprehensively considering power and voltage constraints and performing linear processing, the accuracy of the distribution system's safety margin assessment is improved, thereby enhancing the overall reliability and stability of the system.

[0074] As a preferred technical means: the power flow modeling module includes:

[0075] A power flow model building unit is used to build a nonlinear power flow model describing the power flow of the AC / DC distribution network based on the Distflow branch power flow method, including the power balance equation of the AC branch, the power transmission equation of the DC branch, and the relationship equation between the node voltage and current;

[0076] The converter model integration unit is used to introduce the voltage source converter model to describe the role of the converter in the AC and DC distribution network through power exchange equations, modulation strategies and capacity limitations;

[0077] The safety domain construction unit is used to define and construct the safety domain of the distribution system by combining voltage constraints, capacity constraints and power boundary conditions of system operation through power flow models and converter models to define the safe operation range of the system under different operating conditions;

[0078] The linearization processing module comprises:

[0079] A nonlinear power flow model linearization unit is used to linearize the nonlinear power flow model in the distribution network, including converting the AC and DC power flow equations in the distribution network into linear equations by approximate methods for subsequent calculations;

[0080] Safety constraint linearization unit, used to linearize various operational safety constraint conditions;

[0081] A linearization coefficient calculation unit is used to calculate and generate the coefficients required in the linearization process; the coefficients are calculated based on the current operating point or reference point of the system and used to generate an approximate linear equation;

[0082] The model adjustment unit is used to adjust and optimize the linearized model during the model linearization process to ensure the accuracy and effectiveness of the linear model;

[0083] The error analysis unit is used to evaluate the error introduced in the linearization process and to calibrate or optimize the linearization model based on the error analysis results to ensure the accuracy of the model in practical applications;

[0084] The safety margin calculation module comprises:

[0085] The static safety distance calculation unit is used to calculate the static safety distance of each effective boundary of the power distribution system. By calculating the Euclidean distance between the current system operation point and each safety domain boundary, the distance between the system and the safety boundary in the current state is determined;

[0086] The minimum safety distance identification unit is used to identify the minimum static safety distance among all calculated static safety distances. The minimum safety distance indicates the situation where the system is closest to the safety boundary in a certain direction and is a key parameter for evaluating the overall safety margin of the system.

[0087] A safety margin evaluation unit is used to calculate and evaluate the static safety margin of the system based on the minimum static safety distance, convert the minimum static safety distance into the safety margin value of the system, and quantify the safety margin of the system under the current operating state;

[0088] Boundary condition analysis unit, used to analyze different operating boundary conditions of the power distribution system to identify which conditions have the greatest impact on the safety margin of the system and determine the operating conditions that are most susceptible to the system, thereby providing a basis for further optimization;

[0089] A margin result display unit is used to visualize or output the calculated safety margin results to the user, including displaying the safety margin value in the form of a graph or table to help the user intuitively understand the safety status of the system;

[0090] The dynamic adjustment and correction unit is used to dynamically adjust the calculation model or system parameters to improve the safety margin and adjust the system operating parameters in real time when it is found that the safety margin is insufficient or close to the boundary.

[0091] This technical solution accurately describes the power flow of AC and DC distribution networks and the impact of converters through the power flow model construction unit and the converter model integration unit, ensuring the accuracy of the model and providing a solid foundation for subsequent safety margin calculations. The safety domain construction unit combines voltage constraints, capacity constraints, and power boundary conditions to comprehensively construct the safety domain of the distribution system, which can provide the system's safe operating range under different operating conditions and ensure the comprehensiveness of the evaluation results.

[0092] The linearization processing module significantly reduces the computational complexity by linearizing complex nonlinear models and constraints, enabling efficient and rapid calculation of safety margins in practical applications, thus meeting the requirements of the power distribution system for real-time performance and efficiency.

[0093] The error analysis unit is used to evaluate the errors that may be introduced during the linearization process, and the linear model is optimized and corrected through the model adjustment unit to ensure the accuracy and reliability of the linearization model.

[0094] The safety margin calculation module can accurately evaluate the safety margin of the system under the current operating state and identify the minimum safety distance through the calculation of static safety distance, so as to accurately quantify the safety margin of the system and provide accurate safety information for operators.

[0095] The boundary condition analysis unit can identify the operating conditions that have the greatest impact on the system safety margin, provide a basis for further optimization of the system, and help operators take preventive measures to improve the overall safety of the system.

[0096] The margin result display unit can intuitively display the calculated safety margin results to the user in the form of graphics or tables, helping the user to quickly understand the safety status of the system and provide support for decision-making.

[0097] The system has dynamic adjustment and correction functions. When it detects that the safety margin is close to or insufficient, it can adjust the system parameters in real time and provide a dynamic safety margin improvement plan, thereby enhancing the adaptability and safety of the system.

[0098] This technical solution can adapt to AC and DC distribution networks of different sizes and types, and has the ability to comprehensively consider converters and distributed power sources. It is suitable for complex and changeable modern distribution network systems and has a wide range of application scenarios.

[0099] Beneficial effects:

[0100] This paper proposes a method for evaluating the safety margin of a distribution system, conducts in-depth research on the safety domain of the distribution system, constructs the static safety domain of the distribution system by combining voltage and capacity constraints, and proposes a method for screening effective boundaries from the perspective of the safety domain and a linear solution method for the static safety margin. By calculating the static safety distance of each effective boundary, the full-dimensional information of the safety domain of the distribution system is fully reflected, which has the advantages of high efficiency and high precision. The specific summary is as follows:

[0101] 1. Based on the Distflow branch power flow method, a nonlinear power flow model and a voltage source converter (VSC) model of the AC / DC distribution network were constructed. Combined with voltage and capacity constraints, the power and voltage boundaries were comprehensively considered to construct the safety domain of the distribution system, providing an accurate model basis for system safety margin assessment.

[0102] 2. A linear processing method for each operational safety constraint condition is proposed. The static safety distance of each effective boundary of the system is solved by linearization. Based on the minimum static safety distance, the static safety margin of the system is accurately calculated and evaluated.

[0103] 3. A method for screening effective boundaries from the perspective of safety domain is proposed, and combined with the linearization solution method of static safety margin, the efficiency of system safety margin assessment is greatly improved, so that the safe operation status of the system can be evaluated more comprehensively and accurately. BRIEF DESCRIPTION OF THE DRAWINGS

[0104] Figure 1 This is the VSC converter station model of the present invention.

[0105] Figure 2 It is a schematic diagram of the static safety distance of the present invention.

[0106] Figure 3 This is the modified IEEE33 AC / DC power distribution system of the present invention.

[0107] Figure 4 It is the distance from the operating point of the present invention to each effective boundary.

[0108] Figure 5 It is the daily safety margin of the power distribution system of the present invention. DETAILED DESCRIPTION

[0109] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings.

[0110] Embodiment 1:

[0111] This embodiment is a method for evaluating the safety margin of a power distribution system, which includes the following steps:

[0112] S1: Based on the Distflow branch power flow method, a nonlinear power flow model and a voltage source converter model of the AC / DC distribution network are constructed. In combination with voltage constraints and capacity constraints, a distribution system safety domain that comprehensively considers power boundaries and voltage boundaries is constructed to obtain a safe operating range under different operating conditions.

[0113] S2: Linearize the constructed nonlinear power flow model and various operating safety constraints to generate a linearized model, and calculate the static safety distance of each effective boundary of the distribution system based on the linearized model; based on the minimum static safety distance, calculate and evaluate the static safety margin of the distribution system, so as to quantify the safety margin of the system under the current operating state.

[0114] In the above technical solution, step S1 performs security domain modeling on the power distribution system according to the power flow constraints and operation safety constraints of the power system operation. The specific method is as follows:

[0115] The distribution system safety domain model mainly includes the AC / DC distribution network power flow model, converter station model and common distribution network safety constraints. The nonlinear power flow of the AC / DC distribution network is described based on the Distflow branch power flow model:

[0116] (1)

[0117] (2)

[0118] (3)

[0119] (4)

[0120] (5)

[0121] (6)

[0122] (7)

[0123] (8)

[0124] (9)

[0125] (10)

[0126] Where: P AC,ij,t and Q AC,ij,t AC branch ij exist t The active power and reactive power of the first end at the moment; P AC,jk,t andQ AC,jk,t AC branch jk exist t The active power and reactive power of the first end at the moment; P AC,j,t and Q AC,j,t Communication nodes j exist t Active power and reactive power flowing out at all times; P AC,i,t and Q AC,i,t Communication nodes i exist t Active power and reactive power flowing out at all times; r AC,ij and x AC,ij AC branch ij resistance and reactance; V AC,i,t and V AC,j,t Communication nodes i and j exist t The voltage amplitude at the moment; I AC,ij,t For AC branch ij exist t The current at the moment; P AC,i,L,t and Q AC,L,t Communication nodes i exist t Active load and reactive load at the moment; P AC,j,DG,t and Q AC,j,DG,t They are AC DG nodes j exist t Active and reactive power output at all times; P AC,j,VSC,t and Q AC,j,VSC,t They are the nodes on the AC side of the converter j exist t The equivalent outflow active power and reactive power at the moment; P DC,ij,t For DC branch ij exist t The active power at the first end at the moment; P DC,jk,t For DC branch jk exist t The active power at the first end at the moment; P DC,j,tDC node j exist t The outflow power at the moment; r DC,ij For DC branch ij Resistance; I DC,ij,t For DC branch ij exist t The current at the moment; V DC,i,t DC node i exist t The voltage amplitude at the moment; V DC,j,t DC node j exist t The voltage amplitude at the moment; P DC,j,L,t DC node j Active load; P DC,j,DG,t DC node j Active output of distributed power sources.

[0127] In addition to the above power flow constraints, the operation of the distribution system must also meet the following safety constraints:

[0128] (twenty two)

[0129] (twenty three)

[0130] (twenty four)

[0131] (25)

[0132] (26)

[0133] (27)

[0134] (28)

[0135] (29)

[0136] Where: S AC,ij,max For AC branch ij transmission capacity; S AC,i,VSC,max For Node i Converter capacity; P AC,i,DG,t，max For Node i Upper limit of distributed power generation output; is the power factor angle of the distributed power source;V AC,i, max and V AC,i, min Node i The voltage upper and lower limits.

[0137] In the flexible AC / DC power distribution system, the DC distribution network and the AC distribution network are often interconnected through a voltage source converter, with VSC as the energy conversion interface between the DC side and the AC side. On the one hand, it can realize the bidirectional flow of active power in the AC / DC distribution network, and on the other hand, it can realize a certain degree of reactive power regulation. The VSC converter station model is shown in the attached figure. Figure 1 shown.

[0138] The VSC converter station model is:

[0139] (11)

[0140] (12)

[0141] (13)

[0142] (14)

[0143] (15)

[0144] (16)

[0145] (17)

[0146] (18)

[0147] (19)

[0148] (20)

[0149] (twenty one)

[0150] Where: and AC side system t Inject the first k Active power and reactive power of each VSC converter; and Respectively k The equivalent resistance and reactance of each VSC converter; and Respectively with k AC side nodes connected to VSC converters t Voltage and converter at the moment tThe fundamental phase voltage at the moment; is the VSC equivalent AC branch current; is the phase active power input to the VSC; is the DC active power output by the VSC; For the k Inverter t The modulation degree at the moment, that is, the ratio of the modulation wave to the carrier amplitude; For the k Inverter t DC voltage utilization at the moment; For the k The DC side nodes connected to the VSC converters t The voltage amplitude at the moment; For the k Maximum capacity of a VSC converter; The upper limit of the DC power output of the VSC; and They are the upper and lower limits of the VSC DC side voltage respectively; and They are the upper and lower limits of the VSC AC side voltage respectively.

[0151] The static safety region of the distribution system refers to the set of load operating points that meet the equality constraints and inequality constraints of the distribution system operation. The static safety region of the distribution system consists of all operating points that meet all the above constraints, and each operating point is represented by the net outflow power of each node.

[0152] Furthermore, step S2 linearizes each operational safety constraint condition, linearizes and solves the static safety distance of each effective boundary of the system, and obtains the static safety margin of the system according to the static safety distance. The specific method is as follows:

[0153] For the AC / DC distribution network power flow model, the linearized Distflow branch power flow model is used as a replacement:

[0154] (30)

[0155] (31)

[0156] (32)

[0157] (33)

[0158] (34)

[0159] Where: V AC,N is the rated voltage amplitude of the AC distribution network; VDC,N is the rated voltage of the DC distribution network.

[0160] The capacity constraint is mathematically represented as the interior of a circle, which is approximated by a regular dodecagon inscribed in the circle. Therefore, the capacity constraint can be replaced by the following linear constraint:

[0161] (35)

[0162] Where: , , is the coefficient of the linearized capacity constraint.

[0163] Since the boundary of the static safety domain is represented by a group of mutually coupled nonlinear implicit functions, it is difficult to solve analytically. The steady-state security distance (SSD) is used to characterize the operating safety margin of the distribution system, thereby reflecting the global information of the distribution system and serving as an evaluation indicator for safety control. The SSD of the distribution system is defined as the shortest Euclidean distance from the basic operating point of the system to each boundary of the SSR. ω 0 represents a safe operating point of the system. λ i Indicates the operating point to SSR i The shortest distance to the boundary, that is

[0164] (36)

[0165] Where: M argin,t for t The safety margin of the power distribution system at all times; N is the number of effective boundaries of the safety domain of the distribution system; ω 0 is the current operating point expressed in terms of node load; F i For SSR i The set of operating points contained in the boundary; λ i Indicates the operating point to SSR i The shortest distance to the border.

[0166] The larger the static safety distance, the greater the safe operation margin of the distribution system at this operating point. The output of distributed power sources and random loads in the distribution system can fluctuate within a larger range. The voltage amplitude, line flow and AC / DC system control quantity have a larger adjustment range. In the scenario of large-scale distributed renewable energy access, the system is less likely to exceed the steady-state limit, and the system operation is safer.

[0167] Attached Figure 2The SSD from the current operating point to each effective boundary of the SSR is shown. It is worth noting that the SSD of some boundaries is equal to the distance from the current operating point to the boundary (i.e., the length of the vertical segment from the operating point to the boundary), while the SSD of some boundaries is the distance from the endpoint of the boundary of the current operating point. Because the SSR of the distribution system is the result of the mutual coupling of various constraints, the SSR boundary obtained based on formula (36) must be a line segment of finite length rather than a straight line. Therefore, the SSD of some safety domain boundaries is the distance from the current operating point to the endpoint of the safety boundary. According to the short board effect, the system safety margin is the shortest SSD from the current operating point to each effective boundary of the SSR.

[0168] In order to verify the effectiveness of the proposed method, the safety margin evaluation of the distribution system is carried out by taking the modified IEEE 33-node example. Figure 3 As shown in the figure, the safety domain boundary and static safety distance are obtained by using the Gurobi solver in Matlab software. The hardware environment of the test system is an AMD Ryzen 7 5800H 3.20 GHz processor with a memory capacity of 16 GB.

[0169] The rated voltage of the AC distribution network is 12.66 kV, the rated voltage of the DC distribution network is ±10 kV, the capacity of the converter station is 2 MVA, and the other calculation parameters are shown in Table 1.

[0170] Table 1 Example parameters

[0171]

[0172] Note: Nodes 51-54 are VSC equivalent AC side nodes; 55-58 are VSC equivalent DC side nodes; VSC converter station equivalent R VSC =0.5Ω, X VSC =1.5Ω.

[0173] The static safety domain state variables select the net active outflow and reactive outflow of each node, so the safety domain dimension of this example is 100 dimensions. The boundary of the static safety domain is determined by all the constraints set above, but not all constraints can constitute the effective boundary of the static safety domain. This embodiment uses the static safety distance solution method to determine whether the constraint is a valid boundary. The system steady-state operation point takes the system standard data and uses the AC / DC power flow algorithm to calculate the branch power flow and node voltage. The static safety distance from the operating point to each boundary is obtained by formula (36). If the optimization solution has no solution, it means that the boundary is an ineffective boundary and the boundary corresponding to the constraint condition falls outside the SSR; if there is a solution, it means that the boundary corresponding to the constraint condition is a valid boundary. Through the above method, 33 effective boundaries are finally obtained, including line capacity constraint boundaries and node voltage constraint boundaries. The static safety distance from the steady-state operation point of the distribution system to each boundary is shown in the attached figure. Figure 4 shown.

[0174] The static safety distances from each operating point to different effective boundaries obtained by formula (36) convey the global information of the current operating point in the static safety domain of the system. The operating safety margin of the current operating point of the system can be known from the short board effect. The traditional steady-state flow verification can only give the margin of the flow calculation value of each electrical quantity (active, reactive, and voltage, etc.) compared with the constraint limit, but cannot give the global operation margin calculation value. The method of this embodiment can give global information feedback on the static safety domain when the AC / DC system is in steady-state operation.

[0175] When the AC / DC distribution system is at the current operating point and under the constraints mentioned above, the 10 boundaries with the shortest effective boundary distances of the operating point and their distances are shown in Table 2.

[0176] Table 2 The most dangerous effective boundary and static safety distance

[0177]

[0178] Under the current operating point and the constraints mentioned above, the most dangerous boundary of the system is reflected in Table 2. As can be seen from Table 2, the current dangerous boundary is mainly the voltage boundary, indicating that the safety margin of the node voltage is relatively small, and the risk of crossing the lower limit of the voltage is relatively high. Since reactive compensation equipment is not installed in the current distribution system, when the load at the operating point is large, the risk of low voltage in the distribution system will increase further as the line increases and the network loss increases. If the load of each dimension of the current example operating point is reduced, the static safety distance of each voltage effective boundary of the system will increase, and the voltage margin will increase. The static safety distances of the above boundaries are shown in Table 3. Compared with the original operating point, the static safety distance of the voltage effective boundary increases, and the safety margin increases.

[0179] Table 3 Effective boundary and static safety distance

[0180]

[0181] Based on the distributed photovoltaic and load forecasting method, the daily load curve of each node can be obtained. From the short board effect, it can be known that the current operating safety margin of the system is the minimum static safety distance from the current operating point to each effective boundary. According to the short-term deterministic prediction results of load and distributed photovoltaic output, the daily safety margin change curve of the AC / DC distribution system can be predicted as shown in the attached figure. Figure 5 shown.

[0182] Embodiment 2:

[0183] This embodiment provides a power distribution system safety margin assessment system, which is used to execute the above-mentioned power distribution system safety margin assessment method, and the system includes:

[0184] The power flow modeling module is used to build a nonlinear power flow model and a voltage source converter model of the AC / DC distribution network based on the Distflow branch power flow method, and to build a distribution system safety domain that comprehensively considers the power boundary and the voltage boundary by combining the voltage constraint and the capacity constraint, so as to obtain a safe operating range under different operating conditions;

[0185] The linearization processing module is used to perform linearization processing on the nonlinear power flow model and various operation safety constraints to generate a linearized model;

[0186] The safety margin calculation module is used to calculate the static safety distance of each effective boundary of the distribution system based on the linearization model, and calculate and evaluate the static safety margin of the distribution system based on the minimum static safety distance, so as to quantify the safety margin of the system under the current operating state.

[0187] The power flow modeling module of this embodiment uses the Distflow branch power flow method, which can accurately construct the nonlinear power flow model and voltage source converter model of the AC / DC distribution network, ensure comprehensive consideration of the system power and voltage, and thus more accurately define the safety domain of the distribution system. By combining the constraints of the power boundary and the voltage boundary, the power flow modeling module can construct a distribution system safety domain that comprehensively considers all key factors. The comprehensive safety domain construction helps to more accurately define the safe operation range of the system under different operating conditions. The linearization processing module linearizes the nonlinear model and the safety constraints to generate a linearized model. The linearization processing simplifies the calculation process, improves the calculation efficiency, and makes it easier to perform safety margin calculation and optimization analysis in practical applications. The safety margin calculation module calculates the static safety distance of each effective boundary based on the linearization model, and evaluates the safety margin of the system through the minimum static safety distance, dynamically reflects the current safety state of the system, and provides real-time safety margin quantification values ​​for managers. This technical solution can quantify the complex safety margin evaluation process into specific numerical indicators (static safety distance and safety margin), making the evaluation of the system safety state more intuitive, operational and easy to understand. By quantifying the safety margin, the system can help managers identify potential risks in the system and support optimal decision-making, providing detailed information on the system's safety status and helping to formulate effective improvement measures and response strategies. By comprehensively considering power and voltage constraints and performing linear processing, the accuracy of the distribution system's safety margin assessment is improved, thereby enhancing the overall reliability and stability of the system.

[0188] The power flow modeling module includes:

[0189] A power flow model building unit is used to build a nonlinear power flow model describing the power flow of the AC / DC distribution network based on the Distflow branch power flow method, including the power balance equation of the AC branch, the power transmission equation of the DC branch, and the relationship equation between the node voltage and current;

[0190] The converter model integration unit is used to introduce the voltage source converter model to describe the role of the converter in the AC and DC distribution network through power exchange equations, modulation strategies and capacity limitations;

[0191] The safety domain construction unit is used to define and construct the safety domain of the distribution system by combining voltage constraints, capacity constraints and power boundary conditions of system operation through power flow models and converter models to define the safe operation range of the system under different operating conditions;

[0192] The linearization processing module includes:

[0193] A nonlinear power flow model linearization unit is used to linearize the nonlinear power flow model in the distribution network, including converting the AC and DC power flow equations in the distribution network into linear equations by approximate methods for subsequent calculations;

[0194] Safety constraint linearization unit, used to linearize various operational safety constraint conditions;

[0195] A linearization coefficient calculation unit is used to calculate and generate the coefficients required in the linearization process; the coefficients are calculated based on the current operating point or reference point of the system and used to generate an approximate linear equation;

[0196] The model adjustment unit is used to adjust and optimize the linearized model during the model linearization process to ensure the accuracy and effectiveness of the linear model;

[0197] The error analysis unit is used to evaluate the error introduced in the linearization process and to calibrate or optimize the linearization model based on the error analysis results to ensure the accuracy of the model in practical applications;

[0198] The safety margin calculation module includes:

[0199] The static safety distance calculation unit is used to calculate the static safety distance of each effective boundary of the power distribution system. By calculating the Euclidean distance between the current system operation point and each safety domain boundary, the distance between the system and the safety boundary in the current state is determined;

[0200] The minimum safety distance identification unit is used to identify the minimum static safety distance among all calculated static safety distances. The minimum safety distance indicates the situation where the system is closest to the safety boundary in a certain direction and is a key parameter for evaluating the overall safety margin of the system.

[0201] A safety margin evaluation unit is used to calculate and evaluate the static safety margin of the system based on the minimum static safety distance, convert the minimum static safety distance into the safety margin value of the system, and quantify the safety margin of the system under the current operating state;

[0202] Boundary condition analysis unit, used to analyze different operating boundary conditions of the power distribution system to identify which conditions have the greatest impact on the safety margin of the system and determine the operating conditions that are most susceptible to the system, thereby providing a basis for further optimization;

[0203] A margin result display unit is used to visualize or output the calculated safety margin results to the user, including displaying the safety margin value in the form of a graph or table to help the user intuitively understand the safety status of the system;

[0204] The dynamic adjustment and correction unit is used to dynamically adjust the calculation model or system parameters to improve the safety margin and adjust the system operating parameters in real time when it is found that the safety margin is insufficient or close to the boundary.

[0205] This technical solution accurately describes the power flow of AC and DC distribution networks and the impact of converters through the power flow model construction unit and the converter model integration unit, ensuring the accuracy of the model and providing a solid foundation for subsequent safety margin calculations. The safety domain construction unit combines voltage constraints, capacity constraints, and power boundary conditions to comprehensively construct the safety domain of the distribution system, which can provide the system's safe operating range under different operating conditions and ensure the comprehensiveness of the evaluation results.

[0206] The linearization processing module significantly reduces the computational complexity by linearizing complex nonlinear models and constraints, enabling efficient and rapid calculation of safety margins in practical applications, thus meeting the requirements of the power distribution system for real-time performance and efficiency.

[0207] The error analysis unit is used to evaluate the errors that may be introduced during the linearization process, and the linear model is optimized and corrected through the model adjustment unit to ensure the accuracy and reliability of the linearization model.

[0208] The safety margin calculation module can accurately evaluate the safety margin of the system under the current operating state and identify the minimum safety distance through the calculation of static safety distance, so as to accurately quantify the safety margin of the system and provide accurate safety information for operators.

[0209] The boundary condition analysis unit can identify the operating conditions that have the greatest impact on the system safety margin, provide a basis for further optimization of the system, and help operators take preventive measures to improve the overall safety of the system.

[0210] The margin result display unit can intuitively display the calculated safety margin results to the user in the form of graphics or tables, helping the user to quickly understand the safety status of the system and provide support for decision-making.

[0211] The system has dynamic adjustment and correction functions. When it detects that the safety margin is close to or insufficient, it can adjust the system parameters in real time and provide a dynamic safety margin improvement plan, thereby enhancing the adaptability and safety of the system.

[0212] This technical solution can adapt to AC and DC distribution networks of different sizes and types, and has the ability to comprehensively consider converters and distributed power sources. It is suitable for complex and changeable modern distribution network systems and has a wide range of application scenarios.

[0213] Through the above-mentioned detailed description of a distribution system safety margin assessment method in this specification, those skilled in the art can clearly understand a distribution system safety margin assessment system in this embodiment. For the system disclosed in the second embodiment, since it corresponds to the method disclosed in the first embodiment and has corresponding functional modules and beneficial effects, the relevant parts can be referred to the method part description.

[0214] The above-mentioned method for evaluating the safety margin of a power distribution system is a specific embodiment of the present invention, which has embodied the substantial characteristics and progress of the present invention. It can be modified equivalently according to actual use needs under the guidance of the present invention, and all of them are within the protection scope of this scheme.

Claims

1. A method for evaluating the safety margin of a power distribution system, characterized in that The following steps are involved: 1) Based on the Distflow branch power flow method, a nonlinear power flow model and a voltage source converter model of the AC / DC distribution network are constructed. In combination with voltage constraints and capacity constraints, a distribution system safety domain that comprehensively considers power boundaries and voltage boundaries is constructed to obtain a safe operating range under different operating conditions; 2) Linearize the constructed nonlinear power flow model and various operating safety constraints to generate a linearized model, and calculate the static safety distance of each effective boundary of the distribution system based on the linearized model; Based on the minimum static safety distance, calculate and evaluate the static safety margin of the power distribution system, so as to quantify the safety margin of the system under the current operating state; Step 1) includes: 1.1) Establishment of power flow model: Based on the Distflow branch power flow method, a nonlinear power flow model describing the power flow of AC and DC distribution networks is constructed, including the power balance equation of the AC branch, the power transmission equation of the DC branch, and the relationship equation between node voltage and current; 1.2) Integration of converter model: The voltage source converter model is introduced to describe the role of the converter in the AC / DC distribution network through the power exchange equation, modulation strategy and its capacity limitation; 1.3) Construction of safety domain: Based on voltage constraints, capacity constraints and power boundary conditions of system operation, the safety domain of the distribution system is defined and constructed through power flow model and converter model to define the safe operation range of the system under different operating conditions; Safety constraint expressions include: (22) (23) (24) (25) (26) (27) (28) (29) Where: P AC,ij,t and Q AC,ij,t AC branch ij exist t The active power and reactive power of the first end at the moment; S AC,ij,max For AC branch ij transmission capacity; S AC,i,VSC,max For Node i Converter capacity; P AC,i,DG,t，max For Node i Upper limit of distributed power generation output; is the power factor angle of the distributed power source; V AC,i, max and V AC,i, min Node i The voltage upper and lower limits; For DC distribution network lines i - j transmission capacity; DC distribution network node i The upper limit of the output of distributed power generation; and They are DC distribution network nodes i The lower and upper voltage limits; V AC,i,t For communication nodes i exist t The voltage amplitude at the moment; P DC,ij,t For DC branch ij exist t The active power at the first end at the moment; V DC,i,t DC node i exist t The voltage amplitude at the moment; In step 2), the linearized Distflow branch power flow model is used to replace the nonlinear power flow model; The capacity constraint condition is linearized by approximating the circular constraint condition as an inscribed regular dodecagon, that is, replacing the capacity constraint condition with a linearized capacity constraint equation: (35) Where: , , is the coefficient of the linearized capacity constraint.

2. A method for evaluating safety margin of a power distribution system according to claim 1, characterized in that: The power flow model expression in step 1.1) includes: (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) Where: P AC,ij,t and Q AC,ij,t AC branch ij exist t The active power and reactive power of the first end at the moment; P AC,jk,t and Q AC,jk,t AC branch jk exist t The active power and reactive power of the first end at the moment; P AC,j,t and Q AC,j,t Communication nodes j exist t Active power and reactive power flowing out at all times; P AC,i,t and Q AC,i,t Communication nodes i exist t Active power and reactive power flowing out at all times; r AC,ij and x AC,ij AC branch ij resistance and reactance; V AC,j,t For communication nodes j exist t The voltage amplitude at the moment; I AC,ij,t For AC branch ij exist t The current at the moment; P AC,i,L,t and Q AC,L,t Communication nodes i exist t Active load and reactive load at the moment; P AC,j,DG,t and Q AC,j,DG,t They are AC DG nodes j exist t Active and reactive power output at all times; P AC,j,VSC,t and Q AC,j,VSC,t They are the nodes on the AC side of the converter j exist t The equivalent outflow active power and reactive power at the moment; P DC,jk,t For DC branch jk exist t The active power at the first end at the moment; P DC,j,t DC node j exist t The outflow power at the moment; r DC,ij For DC branch ij Resistance; I DC,ij,t For DC branch ij exist t The current at the moment; V DC,j,t DC node j exist t The voltage amplitude at the moment; P DC,j,L,t DC node j Active load; P DC,j,DG,t DC node j Active output of distributed generation; Equations (1), (2) and (4) represent the equation constraints of AC branch power flow; Equation (3) is the equation constraint of node voltage with phase angle relaxation; Equations (5) and (6) represent the AC node outflow power, including node load, node distributed generation output and converter equivalent injection power; Equations (7) and (9) represent the DC branch power flow equation constraints; Equation (8) represents the DC branch voltage equation constraint; Equation (10) represents the DC node outflow power.

3. A method for evaluating safety margin of a power distribution system according to claim 2, characterized in that: In step 1.2), the converter model expression includes: (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) Where: and AC side system t Inject the first k Active power and reactive power of each VSC converter; and Respectively k The equivalent resistance and reactance of each VSC converter; and Respectively with k AC side nodes connected to the VSC converters t Voltage and converter at the moment t The fundamental phase voltage at the moment; is the VSC equivalent AC branch current; is the phase active power of the output VSC; is the DC active power output by the VSC; For the k Inverter t The modulation degree at the moment, that is, the ratio of the modulation wave to the carrier amplitude; For the k Inverter t DC voltage utilization at the moment; For the k The DC side nodes connected to the VSC converters t The voltage amplitude at the moment; For the k Maximum capacity of a VSC converter; The upper limit of the DC power output of the VSC; and They are the upper and lower limits of the VSC DC side voltage respectively; and They are the upper and lower limits of the VSC AC side voltage respectively.

4. A method for evaluating the safety margin of a power distribution system according to claim 3, characterized in that: In step 2), the linearized Distflow branch power flow model is used to replace the nonlinear power flow model: (30) (31) (32) (33) (34) Where: V AC,N is the rated voltage amplitude of the AC distribution network; V DC,N is the rated voltage of the DC distribution network.

5. A method for evaluating safety margin of a power distribution system according to claim 4, characterized in that: The static safety margin of the power distribution system is characterized by the static safety distance (Steady-State Security Distance, SSD), which is defined as the shortest Euclidean distance from the basic operating point of the system to the boundary of each safety domain, that is, (36) Where: M argin,t for t The safety margin of the power distribution system at all times; N is the number of effective boundaries of the safety domain of the distribution system; ω 0 is the current operating point expressed in terms of node load; F i The first in the static security region (Steady-State Security Region, SSR) i The set of operating points contained in the boundary; λ i Indicates the operating point to SSR i The shortest distance to the border.

6. A power distribution system safety margin assessment system, characterized in that: The system is used to perform the method according to any one of claims 1 to 5, and the system comprises: The power flow modeling module is used to build a nonlinear power flow model and a voltage source converter model of the AC / DC distribution network based on the Distflow branch power flow method, and to build a distribution system safety domain that comprehensively considers the power boundary and the voltage boundary by combining the voltage constraint and the capacity constraint, so as to obtain a safe operating range under different operating conditions; A linearization processing module, used for performing linearization processing on the nonlinear power flow model and various operation safety constraints to generate a linearization model; A safety margin calculation module, used to calculate the static safety distance of each effective boundary of the power distribution system based on the linearized model, and calculate and evaluate the static safety margin of the power distribution system based on the minimum static safety distance, so as to quantify the safety margin of the system under the current operating state; The power flow modeling module includes: A power flow model building unit is used to build a nonlinear power flow model describing the power flow of the AC / DC distribution network based on the Distflow branch power flow method, including the power balance equation of the AC branch, the power transmission equation of the DC branch, and the relationship equation between the node voltage and current; The converter model integration unit is used to introduce the voltage source converter model to describe the role of the converter in the AC and DC distribution network through power exchange equations, modulation strategies and capacity limitations; The safety domain construction unit is used to define and construct the safety domain of the distribution system by combining voltage constraints, capacity constraints and power boundary conditions of system operation through power flow models and converter models to define the safe operation range of the system under different operating conditions; The linearization processing module comprises: A nonlinear power flow model linearization unit is used to perform linearization processing on the nonlinear power flow model in the distribution network, including converting the AC and DC power flow equations in the distribution network into linear equations by an approximate method; The safety constraint linearization unit is used to linearize various operational safety constraint conditions.

7. A power distribution system safety margin assessment system according to claim 6, characterized in that: The linearization processing module also includes: A linearization coefficient calculation unit is used to calculate and generate the coefficients required in the linearization process; the coefficients are calculated based on the current operating point or reference point of the system and are used to generate an approximate linear equation; A model adjustment unit is used to adjust and optimize the linearized model during the model linearization process; The error analysis unit is used to evaluate the error introduced in the linearization process and to correct or optimize the linearization model based on the error analysis results to ensure the accuracy of the model in practical applications.

8. A power distribution system safety margin assessment system according to claim 7, characterized in that: The safety margin calculation module includes: The static safety distance calculation unit is used to calculate the static safety distance of each effective boundary of the power distribution system. By calculating the Euclidean distance between the current system operation point and each safety domain boundary, the distance between the system and the safety boundary in the current state is determined; The minimum safety distance identification unit is used to identify the minimum static safety distance among all calculated static safety distances. The minimum safety distance indicates the situation where the system is closest to the safety boundary in a certain direction and is a key parameter for evaluating the overall safety margin of the system. A safety margin evaluation unit is used to calculate and evaluate the static safety margin of the system based on the minimum static safety distance, convert the minimum static safety distance into the safety margin value of the system, and quantify the safety margin of the system under the current operating state; Boundary condition analysis unit, used to analyze different operating boundary conditions of the power distribution system to identify which conditions have the greatest impact on the safety margin of the system and determine the operating conditions that are most susceptible to the system, thereby providing a basis for further optimization; A margin result display unit is used to visualize or output the calculated safety margin results to the user, including displaying the safety margin value in the form of a graph or table to help the user intuitively understand the safety status of the system; The dynamic adjustment and correction unit is used to dynamically adjust the calculation model or system parameters to improve the safety margin and adjust the system operation parameters in real time when it is found that the safety margin is insufficient or close to the boundary.

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