A method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion

Abstract

This invention discloses a method for screening N-2 faults in overhead transmission lines that takes into account dynamic capacity expansion. Step 1: Obtain an evaluation index for the probability of occurrence of each N-2 fault scenario in the N-2 fault set of the overhead transmission line, taking into account dynamic capacity expansion factors. Step 2: After screening and sorting the N-2 fault set of the overhead transmission line based on the evaluation index value of each N-2 fault scenario, a set of N-2 fault scenarios to be processed is obtained. The method for screening N-2 faults in overhead transmission lines that takes into account dynamic capacity expansion provided by this invention can correctly screen out N-2 faults in overhead transmission lines with a high probability of occurrence, and at the same time accurately reflect the impact of dynamic capacity expansion technology on the screening of N-2 faults in overhead transmission lines.

Description

Technical Field

[0001] This invention relates to a method for screening N-2 faults in overhead transmission lines that takes into account dynamic capacity expansion, and belongs to the field of power system transmission line operation and maintenance technology. Background Technology

[0002] Power system reliability assessment must meet the "N-1" basic principle, and N-2 fault detection based on this principle is one of the key issues in reliability assessment. As described in Reference 1, "A Rapid Screening Method for N-2 Faults Based on Fuzzy Theory and Considering Component Hazard Factors" (Power System Technology, April 2017, Vol. 41, No. 4, pp. 1212-1217), the analytical method for enumerating N-2 fault states has the highest accuracy. However, large-scale power systems have numerous components, resulting in a large number of N-2 fault combinations. The computational workload for N-2 fault detection using the state enumeration method increases exponentially, easily leading to the "curse of dimensionality." For large-scale complex power systems, detailed calculations for each N-2 fault are unnecessary and impractical.

[0003] Reference 1 proposes an N-2 fault screening method based on fuzzy theory and component hazard factors. This method mainly screens N-2 faults based on the magnitude of the impact of the faulty component on the reliability of the entire system. However, it does not consider the probability of occurrence of the screened N-2 faults. This may lead to the failure work wasting valuable computing time and control resources on preventing N-2 faults with low probability of occurrence, while some N-2 faults with high probability of occurrence and certain hazards may be missed.

[0004] Reference 2, "An N-2 Fault Screening Method" (Patent Application Publication No. CN109167356A, Publication Date 2019.01.08), proposes an N-2 fault screening method based on branch overload factor. The branch overload factor in the paper defines the linear relationship between the change in active power of the disconnected branch and the target branch. The advantage of the proposed method is its fast calculation speed. However, since it ignores the influence of reactive power on fault screening, it cannot guarantee the accuracy of fault screening when applied to large power grids.

[0005] Therefore, in order to overcome the shortcomings of the prior art, those skilled in the art urgently need to propose an improved N-2 fault screening method. Summary of the Invention

[0006] Objective: In order to overcome the shortcomings of existing N-2 fault screening methods that do not accurately reflect dynamic capacity expansion technology, this invention provides an N-2 fault screening method for overhead transmission lines that takes into account dynamic capacity expansion.

[0007] Technical solution: To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0008] A method for screening N-2 faults in overhead transmission lines that takes into account dynamic capacity expansion includes the following steps:

[0009] Step 1: Obtain the evaluation index of the probability of occurrence of each N-2 fault scenario in the N-2 fault set of overhead transmission lines, taking into account dynamic capacity expansion factors.

[0010] Step 2: After filtering and sorting the N-2 fault sets of overhead transmission lines according to the evaluation index values ​​of each N-2 fault scenario, the set of N-2 fault scenarios that need to be processed is obtained.

[0011] As a preferred embodiment, the method for obtaining the evaluation indicators specifically includes the following steps:

[0012] Step 1.1: Calculate the outage probability for each overhead transmission line k, k∈1,2,…,N, based on the line outage probability model based on line power flow. L The probability of shutdown F [k] (S k ), where N L S represents the total number of overhead transmission lines. k Let be the apparent power transmitted by line k.

[0013] Step 1.2: Calculate the N-1 fault scenario [k] caused by power flow transfer. The outage probability F(l) of each transmission line l in the set. [k] Wherein, N-1 fault scenario [k] represents a fault in overhead transmission line k. This represents the set of lines that are in normal operation under fault scenario [k] of N-1.

[0014] Step 1.3: Calculate the evaluation index PI for each N-2 fault scenario [k,l] according to formula (1). [k,l] Wherein, the N-2 fault scenario [k,l] represents a fault where the first fault is on overhead transmission line k, and the second fault is on overhead transmission line l. The calculation formula of formula (1) is as follows:

[0015]

[0016] As a preferred embodiment, the calculation formula for the line outage probability model based on line power flow is as follows:

[0017]

[0018] Among them, a k 、b k For parameters.

[0019] As a preferred embodiment, the a k 、b k The solution method includes the following steps:

[0020] When the line's ground-state power flow is within the normal range, obtain the statistical value of the line k outage probability. The calculation formula is as follows:

[0021]

[0022] In the formula: S max,normal It is the threshold value for the line's ground-state power flow exceeding the limit.

[0023] When the line's basic power flow reaches or exceeds the line's basic power flow over-limit threshold S max At that time, obtain the statistical value F of the probability of line k being out of service. [k] (S max The formula for calculating () is as follows:

[0024]

[0025] Substituting the statistical values ​​of formulas (2) and (3) into the line outage probability model based on line power flow, the parameter a is solved. k and b k .

[0026] As a preferred embodiment, the outage probability F(l) of the transmission line l is... [k] The acquisition method specifically includes the following steps:

[0027] Step 1.2.1: Set the effective time of the correction control measures The constraints are as shown in formula (4):

[0028]

[0029] in, The effective time of the corrective control measures after the occurrence of fault scenario [k] N-1 is given. This is a preset threshold.

[0030] Step 1.2.2: According to The probability of the constraint condition in formula (4) being true is derived from the superposition of exponential distributions.

[0031]

[0032] The probability The calculation formula is as follows:

[0033]

[0034] Where, α [k] β [k] For parameters.

[0035] Step 1.2.3: Based on probability Calculate the probability F(l) of transmission line l being out of service due to power flow shift in the N-1 fault scenario [k]. [k] The calculation formula is as follows:

[0036]

[0037] As a preferred embodiment, the The acquisition method specifically includes the following steps:

[0038] when When formula (5) is satisfied, calculate according to formula (6).

[0039] The calculation formula for formula (5) is as follows:

[0040]

[0041] Among them, T cinit,l Let be the initial temperature of line l. For the N-1 fault scenario [k], the steady-state final temperature of conductor l of the transmission line is given. τ represents the time when fault scenario [k] occurs (N-1). l Let l be the time constant of the thermal process of the transmission line conductor l. The acceptable short-term maximum temperature of the conductor in a transmission line.

[0042] The calculation formula for formula (6) is as follows:

[0043]

[0044] As a preferred embodiment, the α [k] β [k] The acquisition method specifically includes the following steps:

[0045] Based on historical data of the effective time of corrective control measures after the occurrence of fault scenario N-1[k], the expected effective time of the corrective control measures is calculated. and variance

[0046] According to expectations Calculation formula (7) and variance Calculate α using formula (8). [k] β [k] .

[0047] The calculation formula for formula (7) is as follows:

[0048]

[0049] The calculation formula for formula (8) is as follows:

[0050]

[0051] As a preferred embodiment, the The acquisition method specifically includes the following steps:

[0052] Construct the steady-state form of the heat balance equation.

[0053] Solving the steady-state form of the heat balance equation for T c .

[0054] make

[0055] The steady-state form of the heat balance equation is calculated using the following formula:

[0056] In the formula:

[0057] Let A be the fault-state current amplitude of line l under fault scenario [k] of N-1.

[0058] T c Let K be the steady-state temperature of the conductor.

[0059] R(T c ) is T c Resistance per unit length of conductor at temperature, Ω / m .

[0060] q s The solar radiation power absorbed per unit length of line, W / m .

[0061] q c The heat lost per unit length of line through thermal conduction, W / m .

[0062] q r The heat lost per unit length of line through thermal radiation, W / m .

[0063] Among them, R(T) c The formula for calculating ) is:

[0064] R(T c )=β·R 20 [1+α(T c -20)]

[0065] In the formula:

[0066] β is the AC / DC resistance ratio.

[0067] α is the temperature coefficient, 1 / ℃.

[0068] R 20 The DC resistance of the conductor at 20°C.

[0069] As the preferred option, q s The calculation formula is:

[0070] q s =γDS i

[0071] In the formula:

[0072] γ is the absorption coefficient of the conductor.

[0073] D is the diameter of the conductor, in meters (m).

[0074] S i Solar radiation intensity, W / m 2 .

[0075] As the preferred option, q c The calculation formula is:

[0076] q c =λ·E u ·π(T c -T a )

[0077] In the formula:

[0078] T a The ambient temperature is in K.

[0079] λ is the thermal conductivity of the air film in contact with the conductor, which is assumed to be constant and equal to 0.02585 W / m·K.

[0080] π is the mathematical constant for a circle.

[0081] E u It is the Euler number.

[0082] As the preferred option, q r The calculation formula is:

[0083]

[0084] In the formula:

[0085] σ is the Stefan-Boltzmann constant.

[0086] k e denoted as the surface emissivity of the conductor.

[0087] π is the mathematical constant for a circle.

[0088] D is the diameter of the conductor, in meters (m).

[0089] As a preferred option, E u The calculation formula is:

[0090] E u =0.65R e 0.2 +0.23R e 0.61

[0091] In the formula, R e It is the Reynolds number.

[0092] R e The calculation formula is:

[0093] R e =1.644×10 9 vD[T a +0.5(T c -T a )] -1.78

[0094] In the formula:

[0095] v represents wind speed, in m / s.

[0096] D is the diameter of the conductor, in meters (m).

[0097] As a preferred embodiment, step 2 specifically includes the following steps:

[0098] Step 2.1: Check the PI of each N-2 fault scenario [k,l] one by one. [k,l] If the judgment condition defined in formula (9) is met, then the N-2 fault scenario [k,l] is selected into the set of N-2 fault scenarios that need to be processed. C .

[0099] The calculation formula of formula (9) is as follows:

[0100] PI [k,l] >PI th (9)

[0101] Where: PI th This is the filtering threshold.

[0102] Step 2.2: Assess Π in descending order of evaluation index values. C The N-2 fault scenarios in the dataset are sorted to obtain the sorted set of N-2 fault scenarios that need to be processed.

[0103] As a preferred option, PIth The calculation formula is:

[0104]

[0105] In the formula: K is the minimum probability of outage for all transmission lines. d These are preset coefficients.

[0106] As a preferred option, S max,normal =0.8×S N S max =1.2×S N .

[0107] As the preferred option, K d =0.5.

[0108] Beneficial effects: The present invention provides a method for screening N-2 faults in overhead transmission lines that takes into account dynamic capacity expansion. This method can correctly screen out N-2 faults in overhead transmission lines with a high probability of occurrence, and at the same time, it accurately reflects the impact of dynamic capacity expansion technology on the screening of N-2 faults in overhead transmission lines. Attached Figure Description

[0109] Figure 1 A flowchart for fault screening of overhead transmission line n-2 that takes into account dynamic capacity expansion.

[0110] Figure 2 To overlay the exponential probability distribution curve (dashed line) and the probability density curve (solid line). Detailed Implementation

[0111] The present invention will be further described below with reference to specific embodiments.

[0112] like Figure 1 As shown, the N-2 fault screening method for overhead transmission lines considering dynamic capacity expansion proposed in this invention includes the following steps:

[0113] Step 1: Compile and organize the input data required for the N-2 fault screening method for overhead transmission lines.

[0114] Step 2: Calculate the evaluation index of the probability of occurrence of each N-2 fault scenario in the N-2 fault set of overhead transmission lines, taking into account dynamic capacity expansion factors, based on the input data.

[0115] Step 3: After filtering and sorting the N-2 fault set of overhead transmission lines according to the evaluation index value of each N-2 fault scenario, the set of N-2 fault scenarios that need to be processed is obtained.

[0116] Furthermore, the input data for step 1 mainly includes the following:

[0117] 1.1: Obtain ground-state power flow data through the power grid dispatching system.

[0118] 1.2: Calculate the power flow data of each overhead transmission line after an N-1 fault based on the ground state power flow data, and obtain the power flow data of each line in normal operation after each N-1 fault.

[0119] 1.3: Dynamic capacity expansion technology is used to acquire relevant data on the transmission line environment and conductor status of overhead transmission lines. This dynamic capacity expansion technology involves installing various condition monitoring devices on the transmission lines to collect real-time information on the transmission line environment and conductor status, and then transmitting the collected data to a data center for processing and display via wireless communication technology.

[0120] 1.4: Obtain relevant parameters for the line outage probability model based on line power flow.

[0121] 1.5: Obtain relevant parameters for the superposition exponential distribution of the effective time of corrective control measures after an N-1 fault in an overhead transmission line.

[0122] Furthermore, the calculation method for the evaluation indicators in step 2 includes the following steps:

[0123] Step 2.1: Calculate the outage probability for each overhead transmission line k, k∈1,2,…,N, based on the line outage probability model based on line power flow. L The probability of shutdown F [k] (S k ), where N L S represents the total number of overhead transmission lines. k This represents the apparent power transmitted by line k. It should be noted that each circuit in a double-circuit transmission line should be numbered differently.

[0124] Furthermore, the hyperbolic tangent function is used to fit and calculate the outage probability of transmission line k, resulting in a line outage probability model based on line power flow. The calculation formula is as follows:

[0125]

[0126] In the formula: S k Let a be the apparent power transmitted by line k. k 、b k For parameters.

[0127] Furthermore, among them, parameter a k and b k The solution method is as follows:

[0128] When the line's ground-state power flow is within the normal range, the line outage probability F [k] (S kIn the interval [0, S] max,normal The average value on [the data] should be equal to the statistical value of the outage probability of line k. The calculation formula is as follows:

[0129]

[0130] In the formula: S max,normal It is the threshold value for the line's ground-state power flow exceeding the limit.

[0131] When the line's basic power flow reaches or exceeds the line's basic power flow over-limit threshold S max When the line itself overheats and melts or the self-protection device activates, the probability of line outage is close to 1. The calculation formula is as follows:

[0132]

[0133] The parameter a can be solved using equations (2) and (3) and formula (1). k and b k Substituting this back into equation (1) yields the line outage probability model based on line power flow.

[0134] Step 2.2: Calculate the N-1 fault scenario [k] and the resulting faults due to power flow shift. The outage probability F(l) of each transmission line l in the set. [k] Wherein, fault scenario N-1[k] represents a fault in overhead transmission line k (the overhead transmission line number is k). This represents the set of lines that are in normal operation under fault scenario [k] of N-1.

[0135] Step 2.2.1: After the N-1 fault scenario [k], to avoid power flow transfer causing the transmission line l to shut down, the effective time of the correction control measures is determined. The following constraints must be met:

[0136]

[0137] In the formula: This is a preset threshold.

[0138] Furthermore, the present invention employs the following method to set...

[0139] Since the redistribution time of the power flow after the N-1 fault scenario [k] is very short, the temperature change of the conductor can be regarded as a step response to the change in power flow in the line. Therefore, the temperature change process is monotonic. As long as the control... If the conductor temperature at any given time remains within an acceptable short-term range, the safety of line operation during dynamic capacity expansion following an N-1 fault [k] can be guaranteed; therefore, The temperature of conductor l in the transmission line at any given time must satisfy the following constraint:

[0140]

[0141] In the formula:

[0142] Let L be the temperature of the transmission line conductor l at the moment when the correction control measures take effect under the N-1 fault scenario [k].

[0143] This is the set of lines that are in normal operation under the N-1 fault scenario [k].

[0144] T cinit,l The initial temperature of line l is provided by the transmission line condition monitoring device.

[0145] The steady-state final temperature of conductor l of the transmission line after fault scenario [k] in N-1 fault scenario.

[0146] The time when fault scenario [k] occurs is N-1.

[0147] The effective time of the corrective control measures after the occurrence of fault scenario [k] is N-1.

[0148] The acceptable short-term maximum temperature of the conductor in a transmission line.

[0149] τ l Let be the time constant for the thermal process of conductor l in the transmission line.

[0150] Furthermore, in order to solve The steady-state form of the heat balance equation should be solved using Newton's iteration method, and the following result should be obtained. Value:

[0151]

[0152] In the formula:

[0153] Let A be the fault-state current amplitude of line l under fault scenario [k] of N-1.

[0154] T c Let K be the steady-state temperature of the conductor.

[0155] R(T c ) is T cResistance per unit length of conductor at temperature, Ω / m.

[0156] q s The solar radiation power absorbed per unit length of the line, in W / m.

[0157] q c This represents the amount of heat lost per unit length of the circuit through thermal conduction, expressed in W / m.

[0158] q r This represents the amount of heat lost per unit length of the line through thermal radiation, expressed in W / m.

[0159] Furthermore, among them, R(T) c The formula for calculating ) is:

[0160] R(T c )=β·R 20 [1+α(T c -20)] (7)

[0161] In the formula:

[0162] β is the AC / DC resistance ratio.

[0163] α is the temperature coefficient, 1 / ℃.

[0164] R 20 The DC resistance of the conductor at 20°C.

[0165] Furthermore, among which, q s The calculation formula is:

[0166] q s =γDS i (8)

[0167] In the formula:

[0168] γ is the absorption coefficient of the conductor.

[0169] D is the diameter of the conductor, in meters (m).

[0170] S i Solar radiation intensity, W / m 2 .

[0171] Furthermore, among which, q c The calculation formula is:

[0172] q c =λ·E u ·π(T c -T a (9)

[0173] In the formula:

[0174] T aThe ambient temperature is in K.

[0175] λ is the thermal conductivity of the air film in contact with the conductor, which is assumed to be constant and equal to 0.02585 W / m·K.

[0176] π is the mathematical constant for a circle.

[0177] E u For Euler numbers, the formula is:

[0178] E u =0.65R e 0.2 +0.23R e 0.61 (10)

[0179] R e Let be the Reynolds number, and its formula is:

[0180] R e =1.644×10 9 vD[T a +0.5(T c -T a )] -1.78 (11)

[0181] In the formula:

[0182] v represents wind speed, in m / s.

[0183] D is the diameter of the conductor, in meters (m).

[0184] Furthermore, among which, q r The calculation formula is:

[0185]

[0186] In the formula:

[0187] σ is the Stefan-Boltzmann constant, σ = 5.67 × 10 -8 W / m 2 ·K 4 .

[0188] k e denoted as the surface emissivity of the conductor.

[0189] π is the mathematical constant for a circle.

[0190] D is the diameter of the conductor, in meters (m).

[0191] when When formula (13) is satisfied, it can be set based on formula (14).

[0192]

[0193]

[0194] Step 2.2.2: According to The probability of the constraint condition defined in equation (4) being true is derived from the superimposed exponential distribution. The specific calculation process is as follows:

[0195] This invention uses a superimposed exponential distribution to describe the effective time of corrective control measures after an N-1 fault scenario [k]. Equations (15) and (16) respectively give the probability distribution functions of the superimposed exponential distribution. and probability density function

[0196]

[0197]

[0198] In the formula: α [k] β [k] For parameters, and It is a value at the effective time of the corrective control measures after the N-1 fault scenario [k].

[0199] Figure 2 The figure shows the probability density function curve of the superimposed exponential distribution, whose shape and size are determined by the parameter α. [k] β [k] Decide.

[0200] The effective time of correction control measures that satisfy the superimposed exponential distribution The expectations are as follows:

[0201]

[0202] and The variance is as follows:

[0203]

[0204] Based on the statistical results of historical data, the expected value and variance of the correction control time are given, and the parameter α can be obtained from equations (17) and (18). [k] β [k] The value of .

[0205] Based on the definition of the probability distribution function, the probability that constraint (4) holds can be calculated. As shown in the following formula:

[0206]

[0207] in, It is given by equation (15).

[0208] Step 2.2.3: Calculate the probability F(l) of transmission line l being out of service due to power flow shift after N-1 fault scenario [k]. [k] :

[0209]

[0210] Step 2.3: Calculate the evaluation index PI for each N-2 fault scenario [k,l] using the following formula. [k,l] , The N-2 fault scenario [k,l] represents a fault where the first fault is on overhead transmission line k (the overhead transmission line number is k), and the second fault is on overhead transmission line l (the overhead transmission line number is l).

[0211]

[0212] PI [k The larger the value of [k,l], the greater the probability of the corresponding N-2 fault scenario [k,l] occurring.

[0213] Furthermore, the method in step 3 includes the following steps:

[0214] Step 3.1: Check the PI of each N-2 fault scenario [k,l] one by one. [k,l] k∈1,2,…,N L If it satisfies the judgment condition defined in equation (22), then the N-2 fault scenario [k,l] is selected into the "set of N-2 fault scenarios to be processed" Π C .

[0215]

[0216] In the above formula, the screening threshold PI th It is given by the following formula:

[0217]

[0218] In the above formula: Let K be the minimum probability of outage for all transmission lines, where 0 < K. d ≤1 is the preset coefficient.

[0219] Step 3.2: Rank Π according to the evaluation index values ​​from largest to smallest. C The N-2 fault scenarios in the dataset are sorted to obtain the sorted "set of N-2 fault scenarios that need to be processed".

[0220] The values ​​of each parameter in this invention should be set according to the actual requirements of the project. Suggested values ​​for some parameters are as follows: S max,normal =0.8×SN (S N (Rated capacity of the line), S max =1.2×S N (S N (Rated capacity of the line), K d =0.5.

[0221] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for screening N-2 faults in overhead transmission lines that takes into account dynamic capacity expansion, characterized in that: Includes the following steps: Step 1: Obtain the evaluation index of the probability of occurrence of each N-2 fault scenario in the N-2 fault set of overhead transmission lines, taking into account dynamic capacity expansion factors; Step 2: After filtering and sorting the N-2 fault sets of overhead transmission lines according to the evaluation index values ​​of each N-2 fault scenario, the set of N-2 fault scenarios that need to be processed is obtained. The method for obtaining the evaluation indicators specifically includes the following steps: Step 1.1: Calculate the probability of line outage for each overhead transmission line based on the line power flow-based line outage probability model. , probability of shutdown ,in, This represents the total number of overhead transmission lines. For the line The apparent power transmitted; Step 1.2: Calculate N-1 fault scenarios Later, due to the shift in the tide, The collection of transmission lines probability of shutdown Among them, the N-1 fault scenario Indicates overhead transmission lines The fault, Represents the N-1 fault scenario The following is a set of lines that are currently in normal operating condition. ; Step 1.3: Calculate each N-2 fault scenario according to formula (1). Evaluation indicators Among them, the N-2 fault scenario The initial fault was reported to be an overhead transmission line. The first fault was a fault in the overhead transmission line, and the second fault was a fault in the overhead transmission line. ; The calculation formula for formula (1) is as follows: （1）； The calculation formula for the line outage probability model based on line power flow is as follows: ； in, , For parameters.

2. The method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion as described in claim 1, characterized in that: The , The solution method includes the following steps: When the line's ground-state power flow is within the normal range, obtain the line... Operational Disruption Probability Statistics The calculation formula is as follows: （2）； Where: It is the threshold value for the line's ground-state power flow exceeding the limit; When the line's basic power flow reaches or exceeds the line's basic power flow over-limit threshold At that time, obtain the line Operational Disruption Probability Statistics The calculation formula is as follows: （3）； Substituting the statistical values ​​from formulas (2) and (3) into the line outage probability model based on line power flow, the parameters are solved. and .

3. The method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion as described in claim 1, characterized in that: The power transmission line probability of service interruption The acquisition method specifically includes the following steps: Step 1.2.1: Set the effective time of the correction control measures The constraints are as shown in formula (4): （4）； in, For the N-1 fault scenario The effective time of corrective control measures after an event occurs. The preset threshold; Step 1.2.2: According to The probability of the constraint condition in formula (4) being true is derived from the superposition of exponential distributions. ; The probability The calculation formula is as follows: ； in, , For parameters; Step 1.2.3: Based on probability Calculate N-1 fault scenarios Later, due to power flow shifts, the transmission lines... probability of shutdown The calculation formula is as follows: 。 4. The method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion as described in claim 3, characterized in that: The The acquisition method specifically includes the following steps: when When formula (5) is satisfied, calculate according to formula (6). ; The calculation formula for formula (5) is as follows: (5）； in, For the line The initial temperature, For the N-1 fault scenario Rear transmission lines The final steady-state temperature of the conductor, For the N-1 fault scenario The moment of occurrence, For power transmission lines The time constant of the thermal process of a conductor For acceptable transmission lines The short-term highest temperature of a conductor; The calculation formula for formula (6) is as follows: (6）。 5. The method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion according to claim 3, characterized in that: The , The acquisition method specifically includes the following steps: Based on the N-1 fault scenario Historical data on the effective time of corrective control measures after an event are used to calculate the expected effective time of the corrective control measures. and variance ; According to expectations Calculation formula (7) and variance Calculate using formula (8) and solve for the results. , ; The calculation formula for formula (7) is as follows: （7）； The calculation formula for formula (8) is as follows: （8）。 6. The method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion according to claim 4, characterized in that: The The acquisition method specifically includes the following steps: Construct the steady-state form of the heat balance equation; Solving the steady-state form of the heat balance equation ; make ; The steady-state form of the heat balance equation is calculated using the following formula: ； Where: For the N-1 fault scenario Downline The magnitude of the accidental current. ; The steady-state temperature of the conductor. ; for Resistance per unit length of conductor at temperature ; The solar radiation power absorbed per unit length of the line. ; This refers to the amount of heat lost per unit length of circuit through thermal conduction. ; This refers to the amount of heat lost per unit length of the circuit through thermal radiation. ; in, The calculation formula is: ； Where: The AC / DC resistance ratio; For temperature coefficient, ; For conductor DC resistance.

7. The method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion as described in claim 6, characterized in that: The calculation formula is: ； Where: The conductor absorption coefficient; The diameter of the wire. ; For solar radiation intensity, .

8. The method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion as described in claim 6, characterized in that: The calculation formula is: ； Where: For ambient temperature, ; The thermal conductivity of the air film in contact with the conductor is assumed to be constant and equal to... ; Pi; It is the Euler number.

9. The method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion as described in claim 6, characterized in that: The calculation formula is: ； Where: It is the Stefan-Boltzmann constant; The surface emissivity of the conductor; Pi; The diameter of the wire. ; For ambient temperature, .

10. The method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion according to claim 1, characterized in that: Step 2 specifically includes the following steps: Step 2.1: Check each N-2 fault scenario one by one. of If the judgment condition defined in formula (9) is met, then the N-2 fault scenario will be... Select the set of N-2 fault scenarios to be processed. ; The calculation formula of formula (9) is as follows: （9）； In the formula: The filtering threshold; Step 2.2: Assess the evaluation indicators in descending order of their values. The N-2 fault scenarios in the dataset are sorted to obtain the sorted set of N-2 fault scenarios that need to be processed. .

11. The method for screening N-2 faults in overhead transmission lines considering dynamic capacity expansion as described in claim 10, characterized in that: The calculation formula is: ； In the formula: This represents the minimum probability of outage for all transmission lines. These are preset coefficients.

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