Extreme temperature environment power grid safety assessment method based on heat-force-sag tide coupling

Abstract

The invention relates to the technical field of power system current-carrying level and line thermal stability analysis, in particular to an extreme temperature environment power grid safety assessment method based on thermal-force-sag tide coupling. According to the scheme, the thermal-force-sag coupling characteristic equation of the overhead line is fused into traditional load flow calculation, and the operation state of the power grid under the high-temperature condition is accurately predicted and evaluated. According to the method, the change of physical parameters such as the temperature, the stress and the sag of the line is considered, comprehensive estimation is carried out, and more accurate power system operation state evaluation is provided by integrating a thermal-force-sag coupling characteristic equation of the overhead line into traditional load flow calculation. According to the method, an electric power system operator can accurately predict the load change of a power grid and the physical response of a line in an extreme high-temperature environment, so that a scientific basis is provided for stable operation and timely regulation and control of the power grid.

Description

Technical Field

[0001] The present invention relates to the technical field of power system current carrying level and line thermal stability analysis, and in particular to a power grid safety assessment method in an extreme temperature environment based on thermal-mechanical-sag power flow coupling. Background Art

[0002] In recent years, many parts of the world have experienced continuous high temperature weather in summer. For example, some areas in Sichuan and Chongqing have experienced extreme high temperature weather with temperatures exceeding 40°C and lasting for more than 30 days. The emergence of continuous extreme high temperatures has brought great challenges to the supply and demand of electricity. On the one hand, continuous high temperatures drive the rapid increase in electricity load, and the grid lines or transmission channels of the power grid will be overloaded or even overloaded; on the other hand, in extremely hot weather, a large range of calm wind environment is very unfavorable for the heat dissipation of overhead lines, and overloaded lines are very likely to cause safety accidents such as heat damage and fusing. Therefore, how to accurately evaluate the power flow distribution and safety status of transmission lines is of great significance to ensure the safe and stable operation of the power grid in extremely hot weather.

[0003] In the traditional power system safety analysis, it is crucial to ensure the safe operation of overhead transmission lines, and the thermal rating of transmission lines is the key indicator for evaluating their safety status. The thermal rating includes static thermal rating (STR) and dynamic thermal rating (DTR). STR is the maximum current value that the line can safely carry under extreme meteorological conditions such as low wind speed, high ambient temperature and strong sunshine. This setting is usually conservative because it assumes the most unfavorable meteorological conditions, which may underestimate the actual transmission capacity of the line under other conditions. Relatively speaking, DTR is more flexible and can adjust the current carrying capacity of the line according to real-time changes in meteorological conditions. By considering factors such as wind speed, ambient temperature, and sunshine intensity, DTR can more effectively utilize the transmission capacity of the line while ensuring safety. But whether it is STR or DTR, they define the safe operation range of the transmission line in the form of transmission power or allowable current.

[0004] The operating status of transmission lines is affected by many factors, including external environmental conditions, current size, conductor temperature, sag and clearance, etc. There is a complex coupling relationship between these factors, which affect each other and jointly determine the safety status of the line. For example, an increase in current will cause the conductor temperature to rise, which in turn affects the sag and clearance, which may reduce the mechanical strength of the line and increase the risk of failure. In order to more comprehensively evaluate the safety status of transmission lines, these key parameters need to be monitored and analyzed in real time. Although there are some online monitoring devices with stress or sag measurement functions on the market, which can provide real-time data on the operating status of the line, these data are often isolated and lack prediction and analysis of changing trends. This means that although we can understand the status of the line at a certain moment, it is difficult to predict future changes and it is impossible to make a comprehensive assessment of the line safety of the entire system.

[0005] In summary, under extremely high temperature weather, the power grid lines are affected by the environment and high load, and the temperature, thermal stress and sag all change, which in turn causes the resistance and reactance parameter values ​​of the conductors to change. The changes in the resistance and reactance of the lines will affect the power flow distribution of the entire system. Therefore, the power flow distribution of the power system under high temperature climate is affected by the heat, force and sag of the line body, and there is a coupling relationship. However, this coupling characteristic has been ignored in the existing system power flow calculation and line safety assessment research, which will easily cause deviations in the results of safety assessment and optimization control. Summary of the invention

[0006] The technical problem solved by the present invention is to provide a method for evaluating power grid safety in extreme temperature environments based on thermal-mechanical-sag flow coupling, so as to solve the technical problem that the existing system flow calculation and line safety assessment ignore the influence of the thermal, mechanical and sag states of the line body on the power system flow distribution in high temperature climates, thereby causing deviations in the results of safety assessment and optimization control.

[0007] The basic solution provided by the present invention is a method for evaluating power grid security in extreme temperature environments based on thermal-mechanical-sag power flow coupling, comprising the following steps:

[0008] S1: Analyze the thermal-mechanical-sag change characteristics of the overhead transmission line under extreme high temperature environment, and obtain the thermal-mechanical-sag mapping relationship of the overhead transmission line, wherein the thermal-mechanical-sag mapping relationship of the overhead transmission line includes a thermal balance equation, a stress balance equation and a sag change equation;

[0009] S2: The Newton-Raphson method is used to establish the power flow equation according to the power balance of each node in the system, and then the line resistance and reactance characteristics are analyzed according to the thermal-mechanical-sag coupling characteristics of the overhead transmission line;

[0010] S3: Couple the system power flow equation and the thermal-mechanical-sag mapping relationship of the overhead transmission line according to the line impedance parameters;

[0011] S4: Alternately solve the system's power flow equation and the overhead transmission line thermal-mechanical-sag mapping relationship to determine whether the temperature, stress and sag changes converge;

[0012] S5: If converged, output the temperature, stress, sag and power flow results; otherwise, modify the line impedance parameters and repeat S3-S5 until convergence, so as to simultaneously estimate the system power flow and line temperature, stress and sag changes.

[0013] Further, the heat balance equation in S1 is as follows:

[0014]

[0015] Where i and j are the two end nodes of any sag in the system network line; T ij,t represents the temperature value of line ij at time t; m ij and c ij are the mass and material specific heat capacity of unit circuit ij respectively; and They are the Joule heat, solar radiation heat, convection heat and thermal radiation heat of unit line ij respectively.

[0016] Further, the stress balance equation in S1 is as follows:

[0017]

[0018] In the formula, σ ij,t and represents the horizontal stress of the unit line ij at time t and in the base state; e ij and E ij They represent the elastic coefficient and thermal expansion coefficient of the unit line ij respectively; is the line temperature at the base state; s ij is the unit line ij representing the authority; β ij The unit line ij represents the height difference angle.

[0019] Furthermore, the sag variation equation in S1 is as follows:

[0020]

[0021] In the formula, H ij,t Indicates the sag at the lowest span of the unit line ij at time t; γ ij represents the vertical load ratio of unit line ij; p ijThe unit line ij represents the height difference of the suspension points at both ends of the span; the safety characteristic variable of the unit line ij under extreme high temperature environment can be determined by the thermal-mechanical-sag mapping relationship of the overhead line [T ij,t ,σ ij,t ,H ij,t ] to comprehensively characterize the thermal safety and sag safety distance of the operating transmission lines.

[0022] Further, in S2, the power flow equation is established by using the Newton-Raphson method according to the power balance of each node in the system, including:

[0023]

[0024] Where P Gi,t , P Li,t are the active power values ​​of the generator set and load at node i at time t; Q Gi,t , Q Li,t are the reactive power values ​​of the generator set and load at node i at time t; U i,t is the voltage amplitude of node i at time t; θ ij ,t is the phase angle difference between node i and node j at time t.

[0025] Further, in S2, analyzing the line resistance and reactance characteristics according to the thermal-mechanical-sag coupling characteristics of the overhead transmission line includes:

[0026]

[0027] In the formula, G ij and B ij are the conductance and susceptance of line ij respectively; R ij and X ij are the AC resistance and reactance of the conductor respectively; α and are the temperature variation coefficients of resistance and reactance respectively; D is the geometric mean distance of the three-phase conductors; r is the conductor radius; ω is the synchronous angular velocity; μ and ρ are the line magnetic field and electric field constants respectively; is the reference temperature of the conductor; R reg is the DC resistance value at the reference temperature.

[0028] Further, the S4 includes the steps of:

[0029] S401: Calculate the power flow according to the current temperature;

[0030] S402: updating the temperature, stress and sag according to the calculated tidal flow results and recalculating the tidal flow;

[0031] S403: Determine whether the temperature, stress and sag changes have converged.

[0032] The principle and advantage of the present invention are as follows: the present scheme first analyzes the electrical coupling process of the overhead transmission line under an extremely high temperature environment, establishes a current-temperature rise process model based on the thermal balance theory, and then determines the relationship between the line temperature and the horizontal stress by the representative span method and uses the parabolic equation to infer the line sag change trend. By comprehensively analyzing the thermal-mechanical-sag change characteristics of the overhead transmission line under an extremely high temperature environment, and coupling and solving it with the power grid flow equation, the operation state of the power grid under high temperature conditions can be accurately predicted and evaluated. This method not only takes into account the changes in physical parameters such as temperature, stress and sag of the line, and makes a comprehensive estimate, but also provides a more accurate evaluation of the operation state of the power system by integrating the thermal-mechanical-sag coupling characteristic equation of the overhead line into the traditional flow calculation. It enables power system operators to accurately predict the load changes and physical responses of the power grid under extremely high temperature environments, thereby providing a scientific basis for the stable operation and timely regulation of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The present invention is a flowchart of the steps of an embodiment of a method for evaluating power grid security in an extreme temperature environment based on thermal-mechanical-sag power flow coupling.

[0034] Figure 2 This is a diagram of the electrical, thermal and mechanical coupling process of overhead lines in an extreme high temperature environment in an embodiment of an extreme temperature environment power grid security assessment method based on thermal-mechanical-sag power flow coupling of the present invention.

[0035] Figure 3 It is a diagram of the mutual coupling relationship between system flow and thermal-mechanical-sag of grid operating lines in an embodiment of an extreme temperature environment power grid security assessment method based on thermal-mechanical-sag flow coupling of the present invention.

[0036] Figure 4 A schematic diagram of an improved IEEE-39 node system for performing thermal-mechanical-sag coupling calculation and analysis of overhead transmission lines in an embodiment of an extreme temperature environment power grid security assessment method based on thermal-mechanical-sag power flow coupling of the present invention.

[0037] Figure 5 The thermal-mechanical-sag state change diagram of the overhead line of an embodiment of the extreme temperature environment power grid security assessment method based on thermal-mechanical-sag power flow coupling of the present invention.

[0038] Figure 6 This is a diagram of thermal-mechanical-sag safety evaluation results of overhead lines in an embodiment of a method for evaluating power grid safety in an extreme temperature environment based on thermal-mechanical-sag power flow coupling of the present invention. DETAILED DESCRIPTION

[0039] The following is further described in detail through specific implementation methods:

[0040] The specific implementation process is as follows:

[0041] Embodiment 1

[0042] Embodiment 1 is basically as shown in the attached Figure 1 As shown, a method for evaluating power grid security in extreme temperature environments based on thermal-mechanical-sag power flow coupling includes the following steps:

[0043] S1: Analyze the thermal-mechanical-sag change characteristics of the overhead transmission line under extreme high temperature environment, and obtain the thermal-mechanical-sag mapping relationship of the overhead transmission line, wherein the thermal-mechanical-sag mapping relationship of the overhead transmission line includes a thermal balance equation, a stress balance equation and a sag change equation;

[0044] S2: The Newton-Raphson method is used to establish the power flow equation according to the power balance of each node in the system, and then the line resistance and reactance characteristics are analyzed according to the thermal-mechanical-sag coupling characteristics of the overhead transmission line;

[0045] S3: Couple the system power flow equation and the thermal-mechanical-sag mapping relationship of the overhead transmission line according to the line impedance parameters;

[0046] S4: Alternately solve the system's power flow equation and the overhead transmission line thermal-mechanical-sag mapping relationship to determine whether the temperature, stress and sag changes converge;

[0047] S5: If converged, output the temperature, stress, sag and power flow results; otherwise, modify the line impedance parameters and repeat S3-S5 until convergence, so as to simultaneously estimate the system power flow and line temperature, stress and sag changes.

[0048] This scheme first analyzes the electrical coupling process of overhead transmission lines under extreme high temperature environments, establishes a current-temperature rise process model based on thermal balance theory, and then determines the relationship between line temperature and horizontal stress through the representative span method and uses the parabolic equation to infer the line sag change trend. By comprehensively analyzing the thermal-mechanical-sag change characteristics of overhead transmission lines under extreme high temperature environments, and coupling them with the power grid flow equation to solve them, the operation status of the power grid under high temperature conditions can be accurately predicted and evaluated. This method not only takes into account the changes in physical parameters such as temperature, stress and sag of the line, and makes a comprehensive estimate, but also provides a more accurate evaluation of the operation status of the power system by integrating the thermal-mechanical-sag coupling characteristic equation of the overhead line into the traditional power flow calculation. It enables power system operators to accurately predict the load changes and physical responses of the power grid under extreme high temperature environments, thereby providing a scientific basis for the stable operation and timely regulation of the power grid.

[0049] In this embodiment, S1 mainly analyzes the thermal-mechanical-sag change characteristics of overhead transmission lines under extreme high temperature environments, including: establishing an overhead line current-temperature rise process model based on thermal balance theory, determining the relationship between line temperature and horizontal stress by the representative span method, and further calculating the sag change trend of the lowest point within the representative span of the line through the parabola equation, thereby obtaining the thermal-mechanical-sag mapping relationship of the overhead line. Specifically, the thermal-mechanical-sag mapping relationship of the overhead transmission line includes:

[0050] Heat balance equation:

[0051]

[0052] Where i and j are the two end nodes of any sag in the system network line; T ij,t represents the temperature value of line ij at time t; m ij and c ij are the mass and material specific heat capacity of unit circuit ij respectively; and They are the Joule heat, solar radiation heat, convection heat and thermal radiation heat of unit line ij respectively.

[0053] Stress balance equation:

[0054]

[0055] In the formula, σ ij,t and represents the horizontal stress of the unit line ij at time t and in the base state; e ij and E ij They represent the elastic coefficient and thermal expansion coefficient of the unit line ij respectively; is the line temperature at the base state; s ij is the unit line ij representing the authority; β ij The unit line ij represents the height difference angle.

[0056] Sag change equation:

[0057]

[0058] In the formula, H ij,t Indicates the sag at the lowest span of the unit line ij at time t; γ ij represents the vertical load ratio of unit line ij; p ij The unit line ij represents the height difference of the suspension points at both ends of the span; the safety characteristic variable of the unit line ij under extreme high temperature environment can be determined by the thermal-mechanical-sag mapping relationship of the overhead line [T ij,t ,σ ij,t ,H ij,t] to comprehensively characterize the thermal safety and sag safety distance of the operating transmission lines.

[0059] As shown in Figure 2, the thermal-mechanical-sag characteristics of overhead transmission lines under extreme high temperature environments are analyzed. Under extreme high temperature environments, overhead lines have an electro-thermal-mechanical coupling process of "external environment increases → load demand increases → current increases → conductor temperature increases → sag increases → conductor clearance across objects decreases".

[0060] In S2, this scheme uses the Newton-Raphson method to establish the system power flow equation based on the power balance of each node in the system, and the line resistance R is obtained from the thermal-mechanical-sag coupling characteristics of the transmission line. ij (t) and reactance X ij (t) has time-varying characteristics, so the system admittance matrix is ​​also subject to the constraints of thermal-mechanical-sag. The details are as follows:

[0061] According to the power balance of each node in the system, the Newton-Raphson method is used to establish the power flow equation, including:

[0062]

[0063] Where P Gi,t , P Li,t are the active power values ​​of the generator set and load at node i at time t; Q Gi,t , Q Li,t are the reactive power values ​​of the generator set and load at node i at time t; U i,t is the voltage amplitude of node i at time t; θ ij ,t is the phase angle difference between node i and node j at time t.

[0064] Analysis of line resistance and reactance characteristics based on thermal-mechanical-sag coupling characteristics of overhead transmission lines includes:

[0065]

[0066] In the formula, G ij and B ij are the conductance and susceptance of line ij respectively; R ij and X ij are the AC resistance and reactance of the conductor respectively; α and are the temperature variation coefficients of resistance and reactance respectively; D is the geometric mean distance of the three-phase conductors; r is the conductor radius; ω is the synchronous angular velocity; μ and ρ are the line magnetic field and electric field constants respectively; is the reference temperature of the conductor; R ref is the DC resistance value at the reference temperature.

[0067] like Figure 3As shown in the figure, the analysis of the thermal-stress-vertical coupling characteristics of overhead transmission lines shows that the line resistance R ij (t) and reactance X ij (t) has time-varying characteristics. Therefore, the power system admittance matrix is ​​also restricted by the thermal stress sag, and the line transmission current will also be affected. There is a coupling relationship between the system flow and the thermal stress sag of the power grid operation line.

[0068] Therefore, in S3, the overhead line thermal-mechanical-sag coupled power flow calculation model is used, and the line impedance parameters are used as the link to couple and solve the power flow equation of the power grid with the thermal balance equation, the stress balance equation and the sag change equation. The line impedance parameters in this embodiment include line resistance parameters and reactance parameters.

[0069] Preferably, S4 includes the steps of:

[0070] S401: Calculate the power flow according to the current temperature;

[0071] S402: updating the temperature, stress and sag according to the calculated tidal flow results and recalculating the tidal flow;

[0072] S403: Determine whether the temperature, stress and sag changes have converged.

[0073] In this embodiment, the system power flow equation and the overhead line thermal-stress-sag mapping relationship are solved alternately. Alternating the two relationships means that in the iteration process, the power flow is first calculated according to the current temperature, and then the temperature, stress and sag are updated according to the calculated power flow results, and then the power flow is recalculated. Determining whether these parameters converge, that is, whether a stable state is reached, is a key step to ensure the safe operation of the power system.

[0074] In S5, when the power flow equation of the power system and the thermal-mechanical-sag mapping relationship of the overhead line are alternately solved to reach a convergence state, the system will output the specific values ​​of the current voltage level, line power flow, temperature, stress and sag. If these parameters do not converge, the impedance parameters of the line need to be adjusted to reflect the actual physical changes. Then, the above iterative solution process is repeated until all parameters stabilize. The purpose of this process is to ensure that during the operation of the power grid, not only the flow of electricity can be accurately predicted, but also the physical state of the line can be monitored and predicted in real time, thereby improving the stability and reliability of the power grid.

[0075] In summary, this solution can achieve a comprehensive prediction of power grid flow and line temperature, stress, and sag changes. This model provides a more accurate evaluation of the power system operation status by integrating the thermal-mechanical-sag coupling characteristic equations of overhead lines into traditional power flow calculations. It enables power system operators to accurately predict the load changes of the power grid and the physical response of the lines in extremely high temperature environments, thus providing a scientific basis for the stable operation and timely regulation of the power grid.

[0076] In the specific implementation process, Figure 4 As shown in the figure, the improved IEEE-39 node system is analyzed for example. The reference voltage and capacity of the example system are selected as 230kV and 100MVA respectively. Without affecting the validity verification of the model in this paper, the capacity and load of all generator sets in the original system are expanded by 1.5 times, assuming that the conductor type of each line in the system is LGJ400 / 35.

[0077] Through the extreme temperature environment power grid security assessment method based on thermal-mechanical-sag power flow coupling in this scheme, the changes in the thermal-mechanical-sag state quantities of some transmission lines L3-18, L4-14, L16-21, L17-27 and L25-26 in the example system under extreme high temperature meteorological environment are calculated, as shown in the following figure: Figure 5 As shown. The state variables of the line are fluctuating, but they are still within the safe range. At the same time, it can be seen that the meteorological environment along the overhead line is an important factor affecting the conductor temperature, stress and sag, especially the external ambient temperature. L17-27 has a large thermal-mechanical-sag difference compared to L4-14. Therefore, it is particularly important to consider the meteorological conditions of the overhead line in the system safety assessment or calculation.

[0078] Figure 6 The corresponding thermal-mechanical-sag safety assessment is given for the same line (L17-27); by comparing the calculation results of the two methods, it can be found that the power flow calculation method proposed in this scheme more objectively reflects the thermal-mechanical-sag safety status of the line; in extremely high temperature weather conditions, power flow calculations that do not consider the thermal-mechanical-sag coupling characteristics are likely to reduce the safety risk assessment value to a low level, seriously affecting the subsequent control or scheduling decisions of the system.

[0079] The above are only embodiments of the present invention. Common knowledge such as the known specific structures and characteristics in the scheme is not described in detail here. Ordinary technicians in the relevant field know all the common technical knowledge in the technical field to which the invention belongs before the application date or priority date, can obtain all the existing technologies in the field, and have the ability to apply conventional experimental means before that date. Ordinary technicians in the relevant field can improve and implement this scheme in combination with their own abilities under the enlightenment given by this application. Some typical known structures or known methods should not become obstacles for ordinary technicians in the relevant field to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several deformations and improvements can be made, which should also be regarded as the scope of protection of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A method for evaluating power grid security in extreme temperature environments based on thermal-mechanical-sag power flow coupling, characterized in that: Includes steps: S1: Analyze the thermal-mechanical-sag change characteristics of the overhead transmission line under extreme high temperature environment, and obtain the thermal-mechanical-sag mapping relationship of the overhead transmission line, wherein the thermal-mechanical-sag mapping relationship of the overhead transmission line includes a thermal balance equation, a stress balance equation and a sag change equation; S2: The Newton-Raphson method is used to establish the power flow equation according to the power balance of each node in the system, and then the line resistance and reactance characteristics are analyzed according to the coupling characteristics of the overhead transmission line; S3: Couple the system power flow equation and the thermal-mechanical-sag mapping relationship of the overhead transmission line according to the line impedance parameters; S4: Alternately solve the system's power flow equation and the overhead transmission line thermal-mechanical-sag mapping relationship to determine whether the temperature, stress and sag changes converge; S5: If converged, output the temperature, stress, sag and power flow results; otherwise, modify the line impedance parameters and repeat S3-S5 until convergence, so as to simultaneously estimate the system power flow and line temperature, stress and sag changes.

2. The method for evaluating power grid security in extreme temperature environments based on thermal-mechanical-sag power flow coupling according to claim 1 is characterized in that: The heat balance equation in S1 is as follows: Where i and j are the two end nodes of any sag in the system network line; T ij,t represents the temperature value of line ij at time t; m ij and c ij are the mass and material specific heat capacity of unit circuit ij respectively; and They are the Joule heat, solar radiation heat, convection heat and thermal radiation heat of unit line ij respectively.

3. The method for evaluating power grid security in extreme temperature environments based on thermal-mechanical-sag power flow coupling according to claim 2 is characterized in that: The stress balance equation in S1 is as follows: In the formula, σ ij,t and represents the horizontal stress of the unit line ij at time t and in the base state; e ij and E ij They represent the elastic coefficient and thermal expansion coefficient of the unit line ij respectively; is the line temperature at the base state; s ij is the unit line ij representing the authority; β ij The unit line ij represents the height difference angle.

4. The method for evaluating power grid security in extreme temperature environments based on thermal-mechanical-sag power flow coupling according to claim 3 is characterized in that: The sag variation equation in S1 is as follows: In the formula, H ij,t Indicates the sag at the lowest span of the unit line ij at time t; γ ij represents the vertical load ratio of unit line ij; p ij The unit line ij represents the height difference of the suspension points at both ends of the span; the safety characteristic variable of the unit line ij under extreme high temperature environment can be determined by the thermal-mechanical-sag mapping relationship of the overhead line [T ij,t ,σ ij,t ,H ij,t ] to comprehensively characterize the thermal safety and sag safety distance of the operating transmission lines.

5. The method for evaluating power grid security in extreme temperature environments based on thermal-mechanical-sag power flow coupling according to claim 4 is characterized in that: In S2, the power flow equation is established by using the Newton-Raphson method according to the power balance of each node in the system, including: Where P Gi,t , P Li,t are the active power values ​​of the generator set and load at node i at time t; Q Gi,t , Q Li,t are the reactive power values ​​of the generator set and load at node i at time t; U i,t is the voltage amplitude of node i at time t; θ ij ,t is the phase angle difference between node i and node j at time t.

6. The method for evaluating power grid security in extreme temperature environments based on thermal-mechanical-sag power flow coupling according to claim 5 is characterized in that: In S2, analyzing the line resistance and reactance characteristics according to the thermal-mechanical-sag coupling characteristics of the overhead transmission line includes: In the formula, G ij and B ij are the conductance and susceptance of line ij respectively; R ij and X ij are the AC resistance and reactance of the conductor respectively; α and are the temperature variation coefficients of resistance and reactance respectively; D is the geometric mean distance of the three-phase conductors; r is the conductor radius; ω is the synchronous angular velocity; μ and ρ are the line magnetic field and electric field constants respectively; is the reference temperature of the conductor; R ref is the DC resistance value at the reference temperature.

7. The method for evaluating power grid security in extreme temperature environments based on thermal-mechanical-sag power flow coupling according to claim 6 is characterized by: The S4 comprises the steps of: S401: Calculate the power flow according to the current temperature; S402: updating the temperature, stress and sag according to the calculated tidal flow results and recalculating the tidal flow; S403: Determine whether the temperature, stress and sag changes have converged.

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