Method for identifying inductance parameters in cable armoring current blocking equipment

Abstract

The invention provides a method for identifying inductance parameters in cable armoring current blocking equipment, which comprises the following steps of: calculating the action time of a switching device; according to a preset initial inductance value and the null set, determining the voltage at the two ends of the capacitor and the current flowing through the inductor after the switching device of the inductor branch is closed; taking the maximum value of the absolute values of all the elements as the peak value of the surge current; taking the maximum value of absolute values of all elements as the peak value of the impulse voltage; when the peak value of the surge current is not smaller than the set current safety limit value, updating the inductance value of the inductor and re-determining the voltage and the current at the two ends of the capacitor; when the peak value of the surge current is smaller than a set current safety limit value, determining whether the peak value of the surge voltage is smaller than voltages at two ends of the capacitor; and if yes, outputting the current inductance value as the inductance value of the inductor of the current blocking equipment. According to the method, the safety and the reliability of the cable armoring current blocking equipment under the overvoltage condition are effectively improved, and the parameter determination efficiency and the parameter determination accuracy are improved.

Description

Technical Field

[0001] This invention relates to the field of power system automation technology, and in particular to a method for identifying inductance parameters in cable armored current blocking devices. Background Technology

[0002] This invention relates to the field of stray current control and power equipment protection in urban subways, specifically to a method for determining inductance parameters in cable armored current blocking devices.

[0003] Most urban subway power supply systems adopt DC traction power supply mode. DC current returns through the rails. Due to the transition resistance of the rails, complete insulation from the ground is not achieved, resulting in some traction current leaking to the ground, forming stray currents. These stray currents from the subway intrude into the neutral point of the main transformer in the urban power grid through various metal paths in the ground and the urban power grid. This causes DC bias in the main transformer, leading to increased transformer vibration and noise, reduced service life, and even malfunctions in relay protection, affecting the safe and stable operation of the urban AC power grid. Therefore, suppressing stray currents at the transformer neutral point is of great significance. Research shows that cable armor is a critical path for stray current propagation. Blocking this critical path can significantly reduce stray currents at the neutral point of all main transformers in the power grid. Based on the characteristic of capacitors to "pass AC and block DC," inserting a capacitor in series on the common grounding wire of the cable armor can effectively block stray currents propagating in the cable armor. Based on this idea, "cable armor current blocking devices" have become a promising solution.

[0004] This device uses capacitors as the main circuit element. When the power grid experiences a short circuit, lightning strike, or operational overvoltage, transient currents of up to tens of kiloamperes flow through or are induced in the cable armor, causing dangerous overvoltages across the capacitors. To prevent this overvoltage from damaging the series capacitors, a bypass protection circuit consisting of inductors and switching devices is required. When the capacitor voltage exceeds a set threshold, the switch closes, introducing the inductor branch to shunt and clamp the voltage across the capacitor. At the moment the bypass circuit is activated, a strong electromagnetic transient process occurs in the capacitor and inductor circuits, triggering surge voltages and currents. The inductance parameter directly determines the severity of this transient process: an excessively large inductance value, while suppressing surge current, leads to excessively high surge voltages on the capacitor; an excessively small inductance value fails to effectively limit current surges, similarly threatening equipment safety. Therefore, determining the parameters of the bypass inductor becomes a key challenge in moving this novel blocking device from theoretical conception to engineering application.

[0005] The closest existing technology is a method for determining the inductor parameters of grid-connected converters in new energy power generation. This method establishes a transfer function relationship between the DC voltage and steady-state disturbances such as AC side frequency, voltage, and power, aiming to suppress DC bus voltage fluctuations and optimize steady-state operating performance. It determines the optimal range of AC side inductor parameters by calculating safety boundaries. However, the aforementioned existing technology aims to optimize inductor parameters under steady-state operation using a linear system approach. Since the linear models and boundary calculation methods relied upon by these existing technologies cannot be used to evaluate nonlinear, strongly coupled transient processes and the resulting surge currents and impulse voltages, their parameter determination methods are not portable and cannot achieve the dual safety objectives during transient processes. Summary of the Invention

[0006] The purpose of this invention is to propose a method for identifying inductance parameters in cable armored current blocking devices, thereby solving the technical problems of safety, reliability, and efficiency of the traditional empirical value method.

[0007] On the one hand, a method for identifying inductance parameters in cable armored current blocking devices is provided, including: The equivalent resistance of the inductor branch, the operating voltage of the switching device, and the capacitance of the capacitor branch are obtained. The operating time of the switching device is calculated based on the analytical expression of the short-circuit current when a single-phase ground fault occurs in the target cable circuit. The equivalent resistance of the inductor branch is the sum of the equivalent resistance of the switching device and the resistance of the inductor coil. Based on the preset initial inductance value and empty set, determine the voltage across the capacitor and the current flowing through the inductor after the switching device of the inductor branch is closed; In the inductor current sequence, the maximum absolute value of all elements is taken as the peak value of the surge current; in the capacitor voltage sequence, the maximum absolute value of all elements is taken as the peak value of the impulse voltage. When the peak value of the surge current is not less than the set current safety limit, update the inductor inductance value and redetermine the voltage and current across the capacitor; when the peak value of the surge current is less than the set current safety limit, determine whether the peak value of the impulse voltage is less than the voltage across the capacitor; if so, output the current inductance value as the inductance value of the current blocking device inductor.

[0008] Preferably, it also includes, If the peak value of the impulse voltage is not less than the voltage across the capacitor, it is determined that the capacitor capacity of the current blocking device is too small. The capacitor capacity is then increased, and the inductance value of the current blocking device inductor is re-determined based on the increased capacitor capacity. The current inductance value is output as the inductance value of the current blocking device until the peak value of the impulse voltage is less than the voltage across the capacitor.

[0009] Preferably, the empty set includes at least a first empty set and a second empty set; the first empty set is used to store the state variable value sequence of the current flowing through the inductor during the transient process; the second empty set is used to store the state variable value sequence of the voltage flowing across the capacitor during the transient process.

[0010] Preferably, the method further includes determining an analytical expression for the short-circuit current when a single-phase ground fault occurs in the target cable loop, based on the following steps. Obtain the steady-state system parameters of the target cable loop and the total fault loop resistance and total fault loop reactance under the most severe short-circuit condition of the target cable loop; Based on the time-domain analytical expression of the short-circuit current when a single-phase ground fault occurs in a cable circuit, the expressions for key parameters are determined.

[0011] Preferably, the key parameters include at least the current amplitude during normal system operation, the steady-state amplitude of the short-circuit current, the impedance angle during normal system operation, the short-circuit impedance angle, and the decay time constant.

[0012] Preferably, the determination of the key parameter expression includes, Current amplitude during normal system operation:

[0013] in, This represents the current amplitude during normal system operation. This refers to the phase voltage amplitude of the busbar to which the cable is connected. This represents the magnitude of the system's equivalent impedance. Amplitude of the steady-state component of the short-circuit current:

[0014] in, This represents the amplitude of the steady-state component of the short-circuit current. This is the magnitude of the total fault loop impedance; Impedance angle during normal system operation:

[0015] in, The impedance angle when the system is operating normally. The equivalent resistance of the load connected to the end of the target cable line. The equivalent reactance of the load connected to the end of the target cable line. The resistance of the target cable line is given by its total length. The reactance of the target cable line along its entire length; Short-circuit impedance angle:

[0016] in, It is the short-circuit impedance angle. The total fault loop resistance from the faulty phase outlet of the transformer to the neutral point. The total fault circuit reactance from the transformer fault phase outlet to the neutral point; decay time constant:

[0017] in, The decay time constant, This represents the decay time coefficient.

[0018] Preferably, the calculation of the operating time of the switching device includes, The analytical expression for short-circuit current is approximated to determine the second-order approximate expression for short-circuit current; the second-order approximate expression for capacitor voltage is calculated based on the integral relationship between capacitor voltage and current and the second-order approximate expression for short-circuit current. Based on the operating voltage of the switching device, the operating time of the switching device is determined. The second-order approximate expression of the capacitor voltage is then set to be equal to the operating voltage of the switching device, thus obtaining the operating time of the switching device.

[0019] Preferably, after the switching device that determines the inductor branch is closed, the voltage across the capacitor and the current flowing through the inductor include, Define state variables and set system parameters and time parameters. The system parameters include at least the equivalent resistance, switching device operating voltage, capacitor capacitance, and current iteration inductance value. The time parameters include at least the start time (switching action time), end time, and time step. Set the initial value of the state variable at the instant the switching device closes; set the current time as the switching action time, and record the initial state variable value as the first element of the sequence and store it in the corresponding sequence.

[0020] Preferably, after the switching device that determines the inductor branch is closed, the voltage across the capacitor and the current flowing through the inductor also include, The Runge-Kutta coefficients are calculated based on the current state variables, and the state variables at the next moment are calculated using the Runge-Kutta coefficients based on the preset state equations describing the transient process of the system. The calculated state variable value is recorded as the a-th element in the sequence, and the state variable is updated.

[0021] Preferably, after the switching device that determines the inductor branch is closed, the voltage across the capacitor and the current flowing through the inductor also include, Based on the updated state variables, the current time value is compared with the end time. If the current time value is not greater than the end time, the current time value is updated according to the time step and the sequence index a is increased by 1. The Runge-Kutta coefficient is recalculated based on the current state variables until the current time value is greater than the end time. The voltage across the capacitor and the current flowing through the inductor are then output.

[0022] In summary, implementing the embodiments of the present invention has the following beneficial effects: The method for identifying inductance parameters in cable armored current blocking devices provided by this invention utilizes the Runge-Kutta method for numerical calculation. This method accurately reproduces and evaluates the complex electromagnetic transient processes generated when a bypass circuit is activated, thereby ensuring that the determined inductance parameters simultaneously limit surge current and impulse voltage within safe ranges. This effectively improves the safety and reliability of cable armored current blocking devices under overvoltage conditions. The iterative optimization method automates and optimizes the determination of inductance parameters, avoiding the shortcomings of traditional designs that rely on experience or repeated trials, and significantly improving the efficiency and accuracy of parameter determination. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, obtaining other drawings based on these drawings without creative effort still falls within the scope of the present invention.

[0024] Figure 1 This is a schematic diagram of the main process of a method for identifying inductance parameters in a cable armor current blocking device according to an embodiment of the present invention.

[0025] Figure 2 This is a schematic diagram of a cable armor current blocking device according to an embodiment of the present invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0027] like Figure 1 The diagram shown is a schematic representation of an embodiment of a method for identifying inductance parameters in a cable armored current blocking device provided by the present invention. In this embodiment, the cable armored current blocking device is as follows: Figure 2 As shown, the method includes the following steps: Step S1: Obtain the equivalent resistance of the inductor branch, the operating voltage of the switching device, and the capacitor capacity of the capacitor branch; and calculate the operating time of the switching device according to the analytical expression of the short-circuit current when a single-phase ground fault occurs in the target cable circuit; wherein, the equivalent resistance of the inductor branch is the sum of the equivalent resistance of the switching device and the resistance of the inductor coil. Step S2: Based on the preset initial inductance value and empty set, determine the voltage across the capacitor and the current flowing through the inductor after the switching device of the inductor branch is closed. Step S3: In the inductor current sequence, take the maximum absolute value of all elements as the peak value of the surge current; in the capacitor voltage sequence, take the maximum absolute value of all elements as the peak value of the impulse voltage. Step S4: When the peak value of the surge current is not less than the set current safety limit, update the inductor value and redetermine the voltage and current across the capacitor; when the peak value of the surge current is less than the set current safety limit, determine whether the peak value of the impulse voltage is less than the voltage across the capacitor; if so, output the current inductance value as the inductance value of the current blocking device inductor.

[0028] In one specific embodiment of the present invention, step S5 is further included: If the peak value of the impulse voltage is not less than the voltage across the capacitor, it is determined that the capacitor capacity of the current blocking device is too small. The capacitor capacity is then increased, and the inductance value of the current blocking device inductor is re-determined based on the increased capacitor capacity. The current inductance value is output as the inductance value of the current blocking device until the peak value of the impulse voltage is less than the voltage across the capacitor.

[0029] In a specific embodiment, in step S1, the analytical expression of the short-circuit current when a single-phase ground fault occurs in the target cable circuit is determined according to the following steps, and the steady-state system parameters of the target cable circuit and the total fault circuit resistance and total fault circuit reactance under the most severe short-circuit condition of the target cable circuit are obtained; and the expression of key parameters is determined according to the time-domain analytical expression of the short-circuit current when a single-phase ground fault occurs in the cable circuit.

[0030] The target cable loop refers to a specific cable with the armored common grounding wire equipped with the cable armor current blocking device. Obtain the following steady-state parameters of the target cable loop: the phase voltage amplitude of the busbar to which the cable is connected. U m The system's power frequency f and the total resistance of the target cable line. R WL With reactance X WL The equivalent resistance of the load connected to the end of the target cable line R L and equivalent reactanceX L .

[0031] Obtain the total fault loop resistance and total fault loop reactance of the target cable loop under the most severe short-circuit condition. The most severe short-circuit condition refers to a short-circuit condition where the single-phase ground fault point is located at the beginning of the target cable loop. Obtain the total fault loop resistance from the transformer fault phase outlet to the neutral point. R ∑ and total fault circuit reactance X ∑ .in, R ∑ = R T + R C + R S + R G , X ∑ = X T + X C + X S + X G , R T and X T These are the equivalent phase resistance and equivalent phase reactance of the transformer, respectively. R C and X C These represent the resistance and reactance of the faulty phase conductor from the transformer outlet to the fault point. R S and X S These are the armor resistance and reactance from the fault point to the armor grounding point, respectively. R G and X G These are the grounding resistance and reactance of the armor grounding point, respectively. The armor grounding point refers to the grounding point of the common grounding wire of the armor where the armored current blocking equipment will be installed.

[0032] Determine the parameters in the analytical expression for short-circuit current and establish a complete analytical expression. When a single-phase ground fault occurs in a cable circuit, the time-domain analytical expression for its short-circuit current is as follows:

[0033] Calculate the key parameters in the above expression based on the system parameters obtained above (including the current amplitude during normal system operation). Im Steady-state amplitude of short-circuit current I m.k Impedance angle during normal system operation φ Short-circuit impedance angle φ k and decay time constant τ ).

[0034] The expression for determining the key parameters includes, Current amplitude during normal system operation:

[0035] in, This represents the current amplitude during normal system operation. This refers to the phase voltage amplitude of the busbar to which the cable is connected. This represents the magnitude of the system's equivalent impedance. ; Amplitude of the steady-state component of the short-circuit current:

[0036] in, This represents the amplitude of the steady-state component of the short-circuit current. This is the magnitude of the total fault loop impedance; Impedance angle during normal system operation: ;

[0037] in, The impedance angle when the system is operating normally. The equivalent resistance of the load connected to the end of the target cable line. The equivalent reactance of the load connected to the end of the target cable line. The resistance of the target cable line is given by its total length. The reactance of the target cable line along its entire length; Short-circuit impedance angle:

[0038] in, It is the short-circuit impedance angle. The total fault loop resistance from the faulty phase outlet of the transformer to the neutral point. The total fault circuit reactance from the transformer fault phase outlet to the neutral point; decay time constant:

[0039] in, The decay time constant, This represents the decay time coefficient.

[0040] In a specific embodiment, calculating the operating time of the switching device includes: approximating the analytical expression of the short-circuit current to determine a second-order approximate expression for the short-circuit current; calculating a second-order approximate expression for the capacitor voltage based on the integral relationship between the capacitor voltage and current and the second-order approximate expression for the short-circuit current; determining the operating time of the switching device based on its operating voltage; and setting the second-order approximate expression for the capacitor voltage equal to the operating voltage of the switching device to obtain the operating time of the switching device.

[0041] The specific process includes calculating the operating time t1 of the switching device using an analytical approximation method.

[0042] Step 221: Approximate the analytical expression for the short-circuit current to obtain an expression that is easy to integrate. i sc ( t )exist t Performing a Taylor expansion near 0 and retaining the second-order terms, we obtain the second-order approximate expression for the short-circuit current. i sc.second ( t ):

[0043] Step 222: Calculate the second-order approximate expression for the capacitor voltage based on the integral relationship between capacitor voltage and current.

[0044] voltage across the capacitor v C ( t ) and the current flowing through it i sc ( t Satisfies the integral relation:

[0045] The second-order approximate current expression for the short-circuit current obtained in step 221 is used. i sc.second ( t Substituting into the above equation and integrating, and similarly retaining the second-order terms, we obtain a second-order approximate expression for the capacitor voltage. v C.second ( t ):

[0046] Step 223: Determine the operating time of the switching device based on its operating voltage. Let the second-order approximate expression for the capacitor voltage obtained in step 222 be used. v C.second (t) equals V 0. Solving this equation yields the operating time of the switching device. t 1:

[0047] in: .

[0048] In a specific embodiment, in step S2, the empty set includes at least a first empty set and a second empty set; the first empty set is used to store the state variable value sequence of the current flowing through the inductor during the transient process; the second empty set is used to store the state variable value sequence of the voltage flowing across the capacitor during the transient process; two empty sets are defined. Y 1 and Y 2 are used to store the current flowing through the inductor during transient processes. i L ( t ) and the voltage across the capacitor v C ( t The sequence of state variable values. The first value in this sequence is... a Each element is denoted as . y 1_{ a}and y 2_{ a}.in, Y 1 represents the inductor current sequence. Y 2 represents the capacitor voltage sequence. Let... n =1, set the initial inductance value L n ,in n This represents the number of iterations.

[0049] In this embodiment, in step S2, after the switching device of the determined inductor branch is closed, the voltage across the capacitor and the current flowing through the inductor include, Define state variables and set system parameters and time parameters. The system parameters include at least the equivalent resistance, switching device operating voltage, capacitor capacitance, and current iteration inductance value. The time parameters include at least the start time (switching action time), end time, and time step. Set the initial value of the state variable at the instant the switching device closes; set the current time as the switching action time, and record the initial state variable value as the first element of the sequence and store it in the corresponding sequence.

[0050] The Runge-Kutta coefficients are calculated based on the current state variables, and the state variables at the next moment are calculated using the Runge-Kutta coefficients based on the preset state equations describing the transient process of the system. The calculated state variable value is recorded as the a-th element in the sequence, and the state variable is updated.

[0051] Based on the updated state variables, the current time value is compared with the end time. If the current time value is not greater than the end time, the current time value is updated according to the time step and the sequence index a is increased by 1. The Runge-Kutta coefficient is recalculated based on the current state variables until the current time value is greater than the end time. The voltage across the capacitor and the current flowing through the inductor are then output.

[0052] The whole process can be understood as, substitution L n Solving using the Runge-Kutta method iterative formula L = L n When the switching device in the inductor branch is closed, the voltage across the capacitor... v C ( t and the current flowing through the inductor i L ( t ).

[0053] Step 251: Define state variables:

[0054] Step 252: Set system parameters, including equivalent resistance. R Switching device operating voltage V0, capacitor capacitance C Current iteration inductance value L n Set the time parameter; the start time is the switch action time. t 1. The end time is t end The time step is ∆ t .in t end = t 1 + 0.002s. ∆t The value is selected based on the required accuracy; a typical value is 1e-6s.

[0055] Step 253: Set the instant the switching device closes, i.e., at t = t Initial values ​​of state variables at time 1:

[0056] Set the current time t current = t 1. and initialize a =1. Record the initial state variable value as the first element of the sequence and store it in the corresponding sequence:

[0057] Step 254: Define the state equations describing the transient process of the system:

[0058] Step 255: Based on the current state variable y 1,curent and y 2,current Calculate the Runge-Kutta coefficient:

[0059] Step 256: Based on the state equation of the transient process of the system, calculate the state variables at the next moment using the Runge-Kutta coefficients. y 1 _new and y 2 _new :

[0060] Step 257: Calculate the state variable values y 1 _new and y 2 _new , respectively as the first in the sequence a Each element is recorded:

[0061] Step 258: Update state variables:

[0062] Step 259: Determine: t current Is it greater than t end If yes, proceed to step S3; otherwise, follow the formula. t current = t current +∆ t Update the current time and simultaneously update the sequence index. a Increase by 1, then jump to step 254.

[0063] In step S3, in the inductor current sequence Y In step 1, retrieve all elements. y 1_{ a The maximum absolute value is the peak value of the surge current. I L.Peak In capacitor voltage sequence Y In step 2, retrieve all elements. y 2_{ a The maximum absolute value is the peak value of the impulse voltage. V C.Peak .

[0064] In step S4, determine I L.Peak Is it less than I set If yes, proceed to step S5; otherwise, follow the formula. L n+1 = L n +∆ L Update the inductor inductance value and proceed to step S2. ∆L This represents the increment of the inductance value of the inductor. I set This refers to the safety limit for surge current.

[0065] In step S5, determine V C.Pea k Is it less than V set If so, output the current inductance value L. n The inductance value is used as the inductor value of the current blocking device; otherwise, it indicates that the capacitor capacitance of the current blocking device is too small and needs to be increased. C And return to step S1. Wherein V set This refers to the safety limit for impulse voltage.

[0066] In summary, implementing the embodiments of the present invention has the following beneficial effects: The method for identifying inductance parameters in cable armored current blocking devices provided by this invention utilizes the Runge-Kutta method for numerical calculation. This method accurately reproduces and evaluates the complex electromagnetic transient processes generated when a bypass circuit is activated, thereby ensuring that the determined inductance parameters simultaneously limit surge current and impulse voltage within safe ranges. This effectively improves the safety and reliability of cable armored current blocking devices under overvoltage conditions. The iterative optimization method automates and optimizes the determination of inductance parameters, avoiding the shortcomings of traditional designs that rely on experience or repeated trials, and significantly improving the efficiency and accuracy of parameter determination.

[0067] The above description discloses only preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Therefore, equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for identifying inductance parameters in cable armored current blocking equipment, characterized in that, include: The equivalent resistance of the inductor branch, the operating voltage of the switching device, and the capacitance of the capacitor branch are obtained. The operating time of the switching device is calculated based on the analytical expression of the short-circuit current when a single-phase ground fault occurs in the target cable circuit. The equivalent resistance of the inductor branch is the sum of the equivalent resistance of the switching device and the resistance of the inductor coil. Based on the preset initial inductance value and empty set, determine the voltage across the capacitor and the current flowing through the inductor after the switching device of the inductor branch is closed; In the inductor current sequence, the maximum absolute value of all elements is taken as the peak value of the surge current; in the capacitor voltage sequence, the maximum absolute value of all elements is taken as the peak value of the impulse voltage. When the peak value of the surge current is not less than the set current safety limit, update the inductor inductance value and redetermine the voltage and current across the capacitor; when the peak value of the surge current is less than the set current safety limit, determine whether the peak value of the impulse voltage is less than the voltage across the capacitor; if so, output the current inductance value as the inductance value of the current blocking device inductor.

2. The method as described in claim 1, characterized in that, It also includes, If the peak value of the impulse voltage is not less than the voltage across the capacitor, it is determined that the capacitor capacity of the current blocking device is too small. The capacitor capacity is then increased, and the inductance value of the current blocking device inductor is re-determined based on the increased capacitor capacity. The current inductance value is output as the inductance value of the current blocking device until the peak value of the impulse voltage is less than the voltage across the capacitor.

3. The method as described in claim 2, characterized in that, The empty set includes at least a first empty set and a second empty set; the first empty set is used to store the state variable value sequence of the current flowing through the inductor during the transient process; the second empty set is used to store the state variable value sequence of the voltage flowing across the capacitor during the transient process.

4. The method as described in claim 3, characterized in that, It also includes determining an analytical expression for the short-circuit current when a single-phase-to-ground short circuit occurs in the target cable loop, based on the following steps. Obtain the steady-state system parameters of the target cable loop and the total fault loop resistance and total fault loop reactance under the most severe short-circuit condition of the target cable loop; Based on the time-domain analytical expression of the short-circuit current when a single-phase ground fault occurs in a cable circuit, the expressions for key parameters are determined.

5. The method as described in claim 4, characterized in that, The key parameters include at least the current amplitude during normal system operation, the steady-state amplitude of the short-circuit current, the impedance angle during normal system operation, the short-circuit impedance angle, and the decay time constant.

6. The method as described in claim 5, characterized in that, The expression for determining the key parameters includes, Current amplitude during normal system operation: in, This represents the current amplitude during normal system operation. This refers to the phase voltage amplitude of the busbar to which the cable is connected. This represents the magnitude of the system's equivalent impedance. Amplitude of the steady-state component of the short-circuit current: in, This represents the amplitude of the steady-state component of the short-circuit current. This is the magnitude of the total fault loop impedance; Impedance angle during normal system operation: in, The impedance angle when the system is operating normally. The equivalent resistance of the load connected to the end of the target cable line. The equivalent reactance of the load connected to the end of the target cable line. The resistance of the target cable line is given by its total length. The reactance of the target cable line along its entire length; Short-circuit impedance angle: in, It is the short-circuit impedance angle. The total fault loop resistance from the faulty phase outlet of the transformer to the neutral point. The total fault circuit reactance from the transformer fault phase outlet to the neutral point; decay time constant: in, The decay time constant, This represents the decay time coefficient.

7. The method as described in claim 6, characterized in that, The calculation of the operating time of the switching device includes By approximating the analytical expression of the short-circuit current, a second-order approximate expression for the short-circuit current is determined. The second-order approximate expression for the capacitor voltage is calculated based on the integral relationship between capacitor voltage and current and the second-order approximate expression for short-circuit current. Based on the operating voltage of the switching device, the operating time of the switching device is determined. The second-order approximate expression of the capacitor voltage is then set to be equal to the operating voltage of the switching device, thus obtaining the operating time of the switching device.

8. The method as described in claim 7, characterized in that, After the switching device that determines the inductor branch is closed, the voltage across the capacitor and the current flowing through the inductor include, Define state variables and set system parameters and time parameters. The system parameters include at least the equivalent resistance, switching device operating voltage, capacitor capacitance, and current iteration inductance value. The time parameters include at least the start time (switching action time), end time, and time step. Set the initial value of the state variable at the instant the switching device closes; set the current time as the switching action time, and record the initial state variable value as the first element of the sequence and store it in the corresponding sequence.

9. The method as described in claim 8, characterized in that, After the switching device that determines the inductor branch is closed, the voltage across the capacitor and the current flowing through the inductor also include, The Runge-Kutta coefficients are calculated based on the current state variables, and the state variables at the next moment are calculated using the Runge-Kutta coefficients based on the preset state equations describing the transient process of the system. The calculated state variable value is recorded as the a-th element in the sequence, and the state variable is updated.

10. The method as described in claim 9, characterized in that, After the switching device that determines the inductor branch is closed, the voltage across the capacitor and the current flowing through the inductor also include, Based on the updated state variables, compare the current time value with the end time; If the current time value is not greater than the end time, then update the current time value according to the time step and increment the sequence index a by 1, and recalculate the Runge-Kutta coefficient according to the current state variable until the current time value is greater than the end time, and output the voltage across the capacitor and the current flowing through the inductor.

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