Time domain distance protection method and system combining control strategy and distributed capacitance current

Abstract

The invention discloses a time domain distance protection method and system combined with a control strategy and distributed capacitance current, and the method comprises the steps: employing negative sequence current to suppress the adaptability of a control strategy based on converters at two ends of a line in a double-end weak feed AC system, and recognizing the problems that negative sequence networks at two sides in a composite sequence network are open-circuited and a non-fault phase generates overvoltage; aiming at the identification problem, an MMC negative-sequence current suppression control strategy is improved into a negative-sequence voltage suppression and voltage reduction V / F cooperative control strategy; based on a cooperative control strategy, a transition resistance item in the distance protection equation is split into an equivalent transition resistance and equivalent transition inductance complex model, and a line pi-type equivalent model is adopted to deduce a fault differential equation; and solving a differential equation containing a fault distance through a least square method to realize high-precision fault positioning.

Description

Technical Field

[0001] This invention belongs to the field of protection of power line collection in new energy power stations, and specifically relates to a time-domain distance protection method and system that combines control strategies and distributed capacitance current. Background Technology

[0002] Flexible DC transmission technology is currently the main method for long-distance transmission of large-scale photovoltaic and other renewable energy sources. However, the typical double-ended weak-feed system is characterized by 100% power electronics, weak support, and low immunity. This means that when a double-ended weak-feed AC system fails, both ends of the line are weakly fed, fundamentally altering the fault characteristics and protection requirements. Furthermore, when the flexible DC side employs a negative-sequence current suppression control strategy, overvoltage problems arise in the non-faulty phases. To suppress overvoltage, a negative-sequence voltage suppression control strategy is used on the flexible DC side, resulting in a very large fault current in the faulty phase, leading to severe overcurrent. Existing systems generally employ low-voltage ride-through control and negative-sequence current suppression strategies for renewable energy systems, and V / f control and negative voltage current suppression strategies for flexible DC transmission systems. After the fault control strategy is improved, the positive and negative sequence control strategies in a double-ended weak-feed AC system cause a phase difference in the fault current flowing to both sides of the fault point, increasing the fault distance error calculated by traditional time-domain distance protection. Summary of the Invention

[0003] The purpose of this invention is to provide a time-domain distance protection method and system that combines control strategies and distributed capacitance current to solve the above-mentioned problems.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a time-domain distance protection method combining a control strategy and distributed capacitance current, comprising: Based on the adaptability of the negative sequence current suppression control strategy adopted by the converters at both ends of the line in the double-ended weak feeder AC system, the problem of open circuit of the negative sequence network on both sides and overvoltage of non-faulty phases in the composite sequence network is identified. To address the identification issue, the MMC negative sequence current suppression control strategy is improved into a coordinated control strategy of negative sequence voltage suppression and buck V / F. Based on the collaborative control strategy, the transition resistance term in the distance protection equation is decomposed into a complex model of equivalent transition resistance and equivalent transition inductance. The fault differential equation is derived using the π-type equivalent model of the line. The differential equation containing the fault distance is solved by the least squares method to achieve high-precision fault location.

[0005] Furthermore, the adaptability of the negative sequence current suppression control strategy adopted by the converters at both ends of the line in the dual-ended weak-feed AC system includes: The adaptability of the negative sequence current suppression control strategy is obtained by considering the same fault location with different transition resistances and the same transition resistance with different fault locations: With different transition resistances, when a single-phase ground fault occurs in a double-ended weakly fed AC system, the fault control strategy adopted by the converters on both sides of the line is the negative sequence current suppression control strategy. As the transition resistance increases, the error of the fault location calculation result gradually increases. When a single-phase ground fault occurs in a double-ended weak-feed AC system, the fault control strategy adopted by the converters on both sides of the line is the negative sequence current suppression control strategy. When the transition resistance remains unchanged, the error of the fault location calculation result will not differ due to the different fault locations. Similarly, the error of the fault location calculation result is significantly different under different transition resistances.

[0006] When both converters on both sides of a double-ended weak-feed AC system adopt a negative sequence current suppression control strategy, the time-domain distance protection under metallic faults performs well. However, both will experience a significant decrease in protection performance as the transition resistance increases.

[0007] Furthermore, identifying the problems that cause open circuits in the negative sequence networks on both sides of the composite sequence network and overvoltage in the non-faulty phases includes: When both converters at both ends of a double-ended weak-feed AC system adopt a negative sequence current suppression control strategy, the negative sequence components at both ends are affected by the negative sequence control, causing the negative sequence networks on both sides of the composite sequence network to open, resulting in overvoltage problems in the non-faulty phases of the double-ended weak-feed AC system. After a fault, the magnitude of the positive and negative sequence components is determined by the positive and negative sequence control strategy of the double-ended weak-feed AC system, while the zero sequence component is determined by the fault type and the transformer connection form. The control effect of the positive and negative sequence components indirectly affects the zero sequence component, resulting in the phase inconsistency of the positive and negative sequence currents on both sides of the fault point after the fault, which in turn leads to the phase inconsistency of the zero sequence component currents on both sides of the fault point.

[0008] Furthermore, regarding the identification problem, the MMC negative sequence current suppression control strategy is improved into a negative sequence voltage suppression and buck V / F coordinated control strategy, including: The converter on the new energy side adopts a negative sequence current suppression control strategy. The MMC adopts a buck V / F control and negative sequence voltage suppression combined control strategy. Under this control strategy, the system stability requirements and the protection requirements for fault characteristics are met.

[0009] Furthermore, based on the cooperative control strategy, the transition resistance term in the distance protection equation is decomposed into a complex model of equivalent transition resistance and equivalent transition inductance. The fault differential equation is derived using a line π-type equivalent model, including: When a single-phase ground fault occurs in a double-ended weakly fed AC system, the fault differential equation based on the π-type equivalent model of the line is derived; the fault differential equation considering the distributed capacitance line model after a single-phase ground fault is written as follows:

[0010] In the formula: , These represent the time-domain electrical quantity information of the voltage and current of phase A at the protection installation point on the new energy side during a fault. , These are the positive sequence resistance and inductance values ​​per unit length of the line, respectively. The voltage across the transition resistor; in for:

[0011] The transition resistance is represented by a complex equivalent, and the specific analysis is as follows: Introduce complex coefficients to the transition resistance

[0012] Therefore, the above formula can be expressed as:

[0013] Combining the transition resistance into a complex number, we get:

[0014] In the formula: , The coefficients are real. and These are the equivalent transition resistance and the equivalent transition inductance, respectively. Transition resistance Decomposed into equivalent transition resistance Add equivalent transition inductance In form, After the fault is obtained, the transition resistance is transformed into a complex equation, which is the single-phase ground fault differential equation of equivalent transition resistance and equivalent transition inductance: ; While considering the transition resistance, the influence of the distributed capacitive current of long-distance transmission lines should also be taken into account:

[0015] In the formula: ;

[0016] Parameters in the formula , , All data are obtained from line parameters and data acquisition equipment at the new energy side protection installation site. The fault point voltage is obtained by analyzing the fault loop between the local system and the fault point after the fault, and writing the KVL equation for the fault loop. Represented as:

[0017] In the formula: This refers to the zero-sequence current flowing into the fault point in the fault circuit; The relationship between the zero-sequence voltage at the protection installation point and the zero-sequence voltage at the fault point is obtained using the symmetrical component analysis method:

[0018] The zero-sequence current at the fault point is:

[0019] Based on the above derivation, the fault differential equation is: .

[0020] Furthermore, when a two-phase-to-ground fault occurs in a double-ended weakly fed AC system line, the fault differential equation based on the line's π-type equivalent model is derived:

[0021] In the formula: , , , These represent the time-domain electrical quantity information of voltage and current of phase A and phase B at the new energy side protection installation point during a fault, respectively. , The voltages at the fault points of phase A and phase B lines; in Represented as:

[0022] Similarly, the process of deriving the fault differential equation when a single-phase ground fault occurs on a line is as follows:

[0023] Separating the unknown parameters and known quantities, the final expression of the differential equation for the interphase fault is:

[0024] In the formula: ; ; ; ; ; ; ; .

[0025] Furthermore, the method of solving the differential equation containing the fault distance using the least squares method to achieve high-precision fault location includes: The differential terms in the fault differential equation are solved using the difference substitution method. Assuming the sampling interval is Ts, the differential terms are calculated using the following formula:

[0026] The fault differential matrix equation under multiple sets of sampled data is:

[0027] In the formula: ; ; ; ; ; ; ; ; The left side of the equation represents the voltage data collected at the protection installation point, while the right side represents the voltage obtained by fitting the fault circuit KVL equation using collected local voltage and current data. The equation contains three unknown variables: d, ... , The difference between both sides of the equation is the error between the actual measured value and the fitted value of the unknown quantity; where These represent information matrices composed of sampled voltage and current values ​​collected at the local protection installation location. These represent the first and second order differential matrices of the voltage and current collected at the protection installation point, respectively. When the cumulative error calculated from multiple sets of sampled data is minimized, the result obtained from calculating the unknown variables is considered equal to the actually collected data. The objective function of the fault differential equation calculation result is:

[0028] For the objective function of the fault differential equation calculation results, the unknown variable d is adjusted through cumulative error. , The method to find its minimum value is that all partial derivatives are 0:

[0029] When the difference between the fault locations obtained from three consecutive calculations all satisfy If the calculation is successful, the fault location is considered reliable. The fault location is then compared with the protection action criteria to determine whether the fault is within the designated area and whether the protection should activate. If any of the three consecutive calculation results show a fault location within the designated area, the protection will activate. If any one of these conditions is not met, the calculation result is considered unreliable, and more data points are calculated to continue the determination.

[0030] Secondly, the present invention provides a negative sequence component control system for a dual-ended weak feeder system during line faults, comprising: The problem identification module is used to identify problems caused by the negative sequence current suppression control strategy adopted by the converters at both ends of the line in a double-ended weak feeder AC system, which leads to open circuits in the negative sequence networks on both sides of the composite sequence network and overvoltage in the non-faulty phases. The collaborative control module is used to improve the MMC negative sequence current suppression control strategy into a collaborative control strategy of negative sequence voltage suppression and buck V / F to address the identification problem. The output module is used to decompose the transition resistance term in the distance protection equation into a complex model of equivalent transition resistance and equivalent transition inductance based on the cooperative control strategy, derive the fault differential equation using the line π-type equivalent model, and solve the differential equation containing the fault distance using the least squares method to achieve high-precision fault location.

[0031] Thirdly, the present invention provides a computer device including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor, when executing the computer program, implements the steps of the time-domain distance protection method combining control strategy and distributed capacitance current.

[0032] Fourthly, the present invention provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the steps of the time-domain distance protection method combining control strategy and distributed capacitance current.

[0033] Compared with the prior art, the present invention has the following technical effects: To improve system stability and enhance the fault characteristics of the double-ended weak-feed AC system after a fault, a negative-sequence current suppression control strategy is adopted on the converter on the renewable energy side. The MMC employs a combined step-down V / F control and negative-sequence voltage suppression control strategy, which meets the system stability requirements. Therefore, improving the control strategy is necessary, and this improvement can lead to improvements in the time-domain distance protection. In a double-ended weak-feed AC system, fast-acting protection can minimize the damage to the power electronic converter and other electronic equipment caused by rapidly rising overvoltages and overcurrents after a fault. Fast-acting also has a smaller impact on system stability and power quality, making it more suitable for double-ended weak-feed AC systems. Therefore, based on the improved control strategy, this invention selects an improved time-domain distance protection method. In long-distance transmission lines, the time-domain distance protection performance based on the RL line model cannot meet the requirements of long-distance transmission lines. The fundamental reason is that the influence of distributed capacitance current on the fault current in long-distance transmission lines cannot be ignored. Therefore, the second improvement in this application, considering the influence of distributed capacitance, proposes a time-domain distance protection based on the π-type equivalent model of the line, deriving the fault differential equation considering the distributed capacitance of the line. Furthermore, in time-domain distance protection, the algorithm for solving the differential equations and the required data window length both affect the computational accuracy of the time-domain distance protection. The proposed time-domain distance protection method can withstand a 300Ω transition resistance and 30dB noise interference, improving the performance of time-domain distance protection in a double-ended weak-feed AC system. Attached Figure Description Figure 1 This is the equivalent sequence network diagram for a single-phase ground fault.

[0034] Figure 2 This section compares the calculation results of the fault location of time-domain distance protection under single-phase ground fault with the change of transition resistance.

[0035] Figure 3 This section compares the time-domain distance protection ranging results under a single-phase ground fault with the changes in fault location.

[0036] Figure 4 The overall framework for improving time-domain distance protection performance.

[0037] Figure 5 This is the equivalent circuit for a single-phase (AG) ground fault based on the π-type line equivalent model.

[0038] Figure 6 Equivalent circuit diagram before and after the transition resistor is split Figure 7 This is the zero-sequence equivalent circuit under a single-phase ground fault.

[0039] Figure 8 This is the equivalent circuit for a two-phase (AB) fault based on the equivalent model of a π-type line.

[0040] Figure 9 This is a flowchart for the improved time-domain distance protection.

[0041] Figure 10 Improved time-domain distance protection performance under different transition resistances.

[0042] Figure 11 Simulation results for different fault locations.

[0043] Figure 12 Simulation results for different fault types.

[0044] Figure 13 To consider the time-domain distance protection performance before and after distributed capacitance.

[0045] Figure 14 This is a flowchart of the present invention. Detailed Implementation

[0046] The present invention will be further described below with reference to the accompanying drawings: Please see Figure 14 This invention provides a time-domain distance protection method combining control strategies and distributed capacitance current, comprising the following steps: Based on the adaptability of the negative sequence current suppression control strategy adopted by the converters at both ends of the line in the double-ended weak feeder AC system, the problem of open circuit of the negative sequence network on both sides and overvoltage of non-faulty phases in the composite sequence network is identified. To address the identification issue, the MMC negative sequence current suppression control strategy is improved into a coordinated control strategy of negative sequence voltage suppression and buck V / F. Based on the collaborative control strategy, the transition resistance term in the distance protection equation is decomposed into a complex model of equivalent transition resistance and equivalent transition inductance. The fault differential equation is derived using the π-type equivalent model of the line. The differential equation containing the fault distance is solved by the least squares method to achieve high-precision fault location.

[0047] Example 2: This invention provides a time-domain distance protection method combining control strategies and distributed capacitance current, comprising the following steps: First, the adaptability issues of distance protection in a two-terminal weakly fed AC system are analyzed. Second, based on the improved system control strategy, a time-domain distance protection method for a two-terminal weakly fed AC system considering the influence of control strategy and distributed capacitance current is proposed. This method proposes corresponding improvement methods based on the problems encountered in the time-domain distance protection of a two-terminal weakly fed AC system.

[0048] In analyzing the adaptive problems of distance protection in a two-terminal weak-feed AC system, the specific steps are as follows: Step 1: Analyze the basic principles of time-domain distance protection.

[0049] Step 2: Conduct a theoretical analysis of the adaptability of time-domain distance protection.

[0050] Step 3: Analyze the basic principles of time-domain distance protection.

[0051] Step 1 includes: Taking a three-phase metallic fault in the system as an example, the time-domain fault loop equation on the M side can be expressed as: (1) In the formula: R k and L k These are the equivalent resistance and equivalent inductance from the installation point to the fault point, respectively.

[0052] Equation (1) contains only two unknowns, Rk and Lk, and Rk is the largest unknown. k and L k All of these can be represented by the fault distance d after the fault, by writing the fault differential equation and solving R. k and L k The distance to the fault can then be obtained.

[0053] When both converters at both ends of a double-ended weak-feed AC system employ a negative-sequence current suppression control strategy, taking a single-phase ground fault as an example, the negative-sequence components at both ends are affected by the negative-sequence control, causing the negative-sequence networks on both sides of the composite sequence network to open, resulting in overvoltage problems in the non-faulty phases of the double-ended weak-feed AC system. Therefore, after a fault, this invention changes the MMC negative-sequence current suppression control strategy to a coordinated control of negative-sequence voltage suppression and step-down V / F, to avoid overvoltage damage to power electronic equipment in the non-faulty phases.

[0054] After the fault control strategy is improved, the fault current flowing to both sides of the fault point in the double-ended weakly fed AC system experiences a phase difference due to the positive and negative sequence control strategy, resulting in an increased fault distance error calculated by traditional time-domain distance protection. Therefore, it is necessary to consider the impact of the control strategy on the fault current phase and improve the time-domain distance protection principle.

[0055] Step 2 includes: As can be seen from equation (1), the protection principle of the time-domain distance element is line-oriented and does not require the use of Fourier algorithm to extract power frequency quantities. Therefore, theoretically, this protection is not affected by the frequency offset characteristics of power electronic systems such as new energy sources, and the protection action speed is fast.

[0056] For time-domain distance protection, current research mainly focuses on equivalent models of RL lines, neglecting the influence of line distributed capacitance, which leads to increased errors. Furthermore, the algorithm errors used in calculating fault distance are also significant. Differential algebra is the most commonly used algorithm for solving the differential terms in fault differential equations, and it generates computational errors, particularly affecting the transient process in the initial stage of a fault. In addition, the choice of data window length also affects computational stability and convergence speed. Generally, a longer data window results in smaller fluctuations and greater stability, but also reduces convergence speed. The sampling rate also affects the algorithm's accuracy.

[0057] In power system fault analysis, lines are typically simplified to a resistance-inductance (RL) model for calculation, ignoring the influence of distributed capacitance by default. From a circuit theory perspective, this model is valid for fundamental, harmonic, and aperiodic components, but it is essentially a lumped approximation of the actual line's distributed parameters. This simplification is only effective within a specific frequency band; when analyzing high-frequency components, the (RL) model deviates significantly from the actual physical characteristics. This limitation leads to reduced model accuracy, consequently affecting the accurate extraction and identification of fault characteristics by relay protection devices.

[0058] Furthermore, time-domain distance protection does not possess good resistance to transition resistance. When a non-metallic fault occurs in the system, taking a three-phase fault as an example, the time-domain equation on the M side can be expressed as: (2) In the above formula, due to the weak feed characteristic of the converter equipment, the additional term... Generally large, which makes R... k The calculated and actual values ​​of Lk may differ significantly.

[0059] When a single-phase ground fault with high resistance occurs in the system (taking phase A as an example), the time-domain fault loop equation corresponding to phase A on side M is: (3) In the formula: , These represent the compensation coefficients for zero-sequence resistance and inductance, respectively. The additional terms have relatively large values, which may lead to misjudgment of time-domain distance protection in single-phase high-resistance fault scenarios.

[0060] like Figure 1 As shown, both the positive and negative order components are controlled. Therefore: (4) From equation (4), the fault current can be obtained as: (5) Depend on Figure 1From equation (4), we can see that the magnitudes of the positive and negative sequence components after a fault are determined by the positive and negative sequence control strategy of the dual-terminal weak-feed AC system, while the zero-sequence component is determined by the fault type and the transformer wiring configuration. However, in the fault sequence network, there is a coupling relationship between the positive, negative, and zero-sequence components. Therefore, the control effect of the positive and negative sequence components indirectly affects the zero-sequence component, leading to an inconsistency in the phases of the positive and negative sequence currents on both sides of the fault point after the fault, which in turn leads to an inconsistency in the phases of the zero-sequence component currents on both sides of the fault point, resulting in a significant error in the fault distance calculation for the time-domain distance protection.

[0061] Furthermore, step 3 includes: To verify the adaptability of traditional time-domain distance protection based on the RL model in a two-terminal weak-feed AC system, the following analysis examines the adaptability of traditional time-domain distance protection with different transition resistors at the same fault location and with different fault locations using the same transition resistor.

[0062] 1) Different transition resistances Taking a single-phase ground fault (phase A) occurring at the midpoint of the line as an example, the performance of traditional distance protection is compared by calculating the fault location as the transition resistance increases using the least squares method.

[0063] When a single-phase ground fault occurs in a double-ended weakly fed AC system, the fault control strategy employed by the converters on both sides of the line is a negative sequence current suppression control strategy. Figure 4 It can be seen that under the traditional control strategy and the traditional time-domain distance protection, the error of the fault location calculation result gradually increases with the increase of the transition resistance. When the transition resistance is 50Ω, the fault location calculation result deviates from the actual fault location by more than 10km, and the error exceeds 10%. Therefore, the time-domain distance protection under the traditional control strategy can no longer meet the needs of the dual-end weak-feed AC system.

[0064] 2) Different fault locations like Figure 3 As shown, taking a single-phase ground fault (phase A) occurring at the midpoint of the line as an example, the results of calculating the fault location and the actual fault location using the least squares method under different fault locations are compared when the transition resistance remains unchanged.

[0065] When a single-phase ground fault occurs in a double-ended weakly fed AC system, the fault control strategy employed by the converters on both sides of the line is a negative sequence current suppression control strategy. Figure 3 Therefore, under traditional negative sequence control strategies and traditional time-domain distance protection, when the transition resistance remains constant, the error in fault location calculation does not differ significantly due to different fault locations. Similarly, the error in fault location calculation varies significantly under different transition resistances.

[0066] In summary, time-domain distance protection suffers from adaptability issues in double-ended weakly fed AC systems. When both converters on both sides of the line in a double-ended weakly fed AC system employ negative-sequence current suppression control strategies, the performance of time-domain distance protection under metallic faults is good. However, both methods experience a significant decrease in protection performance as the transition resistance increases. Therefore, it is necessary to improve the fault control strategy for double-ended weakly fed AC systems. Based on this, the influence of converter fault control strategies should be considered to improve the distance protection principle, and a time-domain distance protection method suitable for double-ended weakly fed AC systems should be proposed.

[0067] As the above analysis shows, time-domain distance protection, compared to power frequency distance protection, does not require the use of Fourier algorithms to extract the phase and amplitude information of voltage and current signals. Therefore, it is not affected by the system's frequency offset characteristics. Furthermore, time-domain distance protection operates faster than power frequency distance protection. In a double-ended weak-feed AC system, rapid operation can minimize the damage to power electronic equipment caused by overvoltage and overcurrent after a fault, and its impact on system stability and power quality is also smaller. Therefore, this invention chooses to improve the time-domain distance protection method.

[0068] Besides transition resistance, the most significant factor affecting the time-domain distance protection performance is the impact of distributed capacitance current on long-distance transmission lines. The influence of distributed capacitance on time-domain distance protection performance increases with the length of the transmission line. Therefore, in the context of long-distance AC collection lines, i.e., double-ended weak-feed AC systems, where large areas are primarily desert or Gobi areas, the impact of distributed capacitance current on the fault location calculation results of time-domain distance protection cannot be ignored. Therefore, considering the above factors, this invention proposes a time-domain distance protection method for double-ended weak-feed AC systems that considers both the control strategy of the converter and the influence of distributed capacitance current.

[0069] In the improved time-domain distance protection method that considers the control strategy and the influence of distributed capacitance current, the specific steps are as follows: Step 1: Improve the fault control strategy of the dual-terminal weak feeder AC system. This invention selects a negative sequence current suppression control strategy for the converter on the new energy side, and selects a buck V / F control and negative sequence voltage suppression coordinated control strategy for the MMC. Under this control strategy, the system stability requirements and the protection requirements for fault characteristics can be met.

[0070] Step 2: Considering the phase-controlled characteristics of the dual-terminal weak-feed AC system, the fault current in the transition resistance term of the distance protection fault differential equation is changed from the real part to a complex term, that is, it is split into two terms: transition resistance and transition inductance. This can fully avoid the influence of system characteristics on distance protection.

[0071] Step 3: Based on the π-type equivalent model of the line, derive the time-domain distance protection fault differential equation. On this basis, the derived and analyzed time-domain distance protection equation can meet the protection requirements for fault location calculation accuracy in a double-ended weak-feed AC system.

[0072] Step 4: Select a suitable algorithm for solving the differential equation, and comprehensively consider factors that affect the performance of the time-domain distance protection, such as the system sampling rate, the accuracy of the fault location under time-domain distance protection, and the selection of the data window length.

[0073] The above four points consider the impact of factors such as the diversity of converter control strategies in double-ended weak-feed AC systems, the special characteristics of double-ended weak-feed AC systems, the differences in line models, and the different algorithms for solving differential equations on time-domain distance protection. Based on these four points, time-domain distance protection is improved to enhance the performance of single-ended distance protection.

[0074] (1) Derivation of the differential equation for a single-phase ground fault when Figure 5 When a single-phase ground fault occurs in a medium-duty double-ended weak-feed AC transmission line (taking phase A fault as an example), the fault current from the new energy side converter and MMC flows into the fault point after the fault. To improve the performance of distance protection, the influence of the distributed capacitance current of long-distance transmission lines on the fault current is considered. Taking the π-type line equivalent model as an example, the fault differential equation based on the π-type line equivalent model is derived. The fault equivalent circuit after the fault is simplified as follows: Figure 5 As shown.

[0075] Depend on Figure 5 The fault differential equation for a line model considering distributed capacitance after a single-phase ground fault can be written, as shown in equation (6).

[0076] (6) In the formula: , These represent the time-domain electrical quantity information of the voltage and current of phase A at the protection installation point on the new energy side during a fault. , These are the positive sequence resistance and inductance values ​​per unit length of the line, respectively. This is the voltage across the transition resistor.

[0077] in for: (7) Because the sequence voltage and current phases change with time after a fault in a double-ended weakly fed AC system, this invention performs a complex equivalent transformation of the transition resistance to address the phase difference issue. The specific analysis is as follows: Introduce a complex coefficient to the transition resistance.

[0078] (8) Therefore, the above formula can be expressed as: (9) Combining the transition resistance into a complex number, we get: (10) In the formula: , The coefficients are real. and These are the equivalent transition resistance and equivalent transition inductance, respectively.

[0079] As can be seen from equation (10), the transition resistance can be... Decomposed into equivalent transition resistance Add equivalent transition inductance In the form of, such as Figure 6 As shown, Figure 6 (a) and Figure 6 (b) shows the equivalent circuit diagrams before and after the transition resistor is split, where... , Both are unknown quantities.

[0080] Substituting equation (10) into equation (9), we can obtain the differential equation for a single-phase ground fault, which transforms the transition resistance after the fault into an equivalent transition resistance and an equivalent transition inductance, as shown in equation (11).

[0081] (11) While considering the transition resistance, the influence of the distributed capacitance current of the long-distance transmission line is also considered, as shown in Equation (12).

[0082] (12) In the formula: ; .

[0083] The parameters in equation (12) , , All of these can be obtained from the line parameters and the data acquisition equipment at the protection installation point on the new energy side. However, the fault point voltage in equation (12) cannot be directly obtained from the acquisition equipment at the protection installation point. It is necessary to analyze the fault loop between the local system and the fault point after the fault and write the fault loop KVL equation to obtain the fault point voltage. It can be represented as: (13) In the formula: This refers to the zero-sequence current flowing into the fault point in the fault circuit.

[0084] The relationship between the zero-sequence voltage at the protection installation point and the zero-sequence voltage at the fault point can be obtained using the symmetrical component analysis method: (14) Based on equation (14), the zero-sequence fault circuit can be obtained as follows: Figure 7 As shown.

[0085] Depend on Figure 7 The zero-sequence current at the fault point can be obtained as follows: (15) Based on the above derivation, the fault differential equation can be obtained as follows: (16) Given that the distributed capacitance of cable lines is typically at the μF / km level, while that of overhead lines is typically at the nF / km level, and that the impact of distributed capacitance on the line is closely related to the line's environment, line design parameters, operating conditions, voltage level, etc., and that high-voltage lines have even lower distributed capacitance due to their thicker insulation layer, and considering that the research object of this invention is the development of new energy in the Gobi Desert region, and the lines are generally overhead lines, the square term of distributed capacitance in equation (16) is very small and has little impact on the calculation result of the final fault location. Therefore, the square term of distributed capacitance in equation (16) can be omitted, and equation (16) can be simplified as follows: (17) By separating the unknown parameters and known quantities in equation (17), the final expression of the differential equation for a single-phase ground fault can be obtained as follows: (18) In the formula: ; ; ; ; ; ; ; .

[0086] In equation (18), the unknowns are the fault distance d and the equivalent transition resistance. Equivalent Transition Inductance After a fault, the three unknown variables remain unchanged. At this time, the unknown variables can be obtained by using the voltage and current data sampled at the protection installation point to solve the fault differential equation through an algorithm.

[0087] (2) Derivation of differential equations for interphase faults The differential equation for a two-phase interphase fault is derived using the new energy side as an example. When Figure 8When a two-phase phase-to-phase fault occurs in a medium-duty double-ended weak-feed AC system (taking the AB phase-to-phase fault as an example), considering the influence of the distributed capacitance of the long-distance line, and taking the π-type line equivalent model as an example, the fault differential equation based on the π-type line equivalent model is derived. The simplified fault equivalent circuit after the fault is shown in the figure below.

[0088] according to Figure 8 The differential equations for two-phase faults based on the π-type equivalent model of the line can be written as shown in equation (19).

[0089] (19) In the formula: , , , These represent the time-domain electrical quantity information of voltage and current of phase A and phase B at the new energy side protection installation point during a fault, respectively. , These are the voltages at the fault points of phase A and phase B lines.

[0090] in It can be represented as: (20) Similarly, the process of deriving the fault differential equation when a single-phase ground fault occurs on a line can be followed to obtain: (twenty one) By separating the unknown parameters and known quantities in equation (17), the final expression of the differential equation for the interphase fault in the two phases can be obtained as follows: (twenty two) In the formula: ; ; ; ; ; ; ; .

[0091] Equation (22) and Equation (16) both have three unknowns d, , Furthermore, since these three factors remain essentially unchanged after a fault occurs, the fault distance, equivalent transition resistance, and equivalent transition inductance can be solved using a system of equations composed of multiple sets of sampled values ​​calculated by the least squares method. Similarly, the derivation for two-phase ground faults, three-phase inter-phase faults, and three-phase ground faults is similar to the above.

[0092] Step 4 includes: Based on the theoretical derivation of the fault differential equations for single-phase grounding faults and two-phase interphase faults in time-domain distance protection, it can be seen that the differential equations after the fault contain three unknowns to be solved: d, ... , The voltage and current data in the differential equation can be collected from the protection installation point on the new energy side. The fault differential equation can be solved by collecting multiple sets of sampled data. Since the voltage and current data obtained by sampling are all discrete values, the differential term in the equation can be solved by the difference substitution method. Let the sampling interval be Ts, then the differential term can be calculated according to equation (23).

[0093] (twenty three) Taking a phase-A ground fault as an example, according to equations (23) and (18), the fault differential matrix equation under multiple sets of sampled data is as follows: (twenty four) In the formula: ; ; ; ; ; ; ; .

[0094] In equation (24), the left side of the equation represents the voltage data collected at the protection installation point, and the right side represents the voltage obtained by fitting the fault circuit KVL equation through the collection of local voltage and current data. The equation contains three unknown variables d, , The difference between both sides of the equation is the error between the actual measured value and the fitted value of the unknown quantity. In equation (24) These represent information matrices composed of sampled voltage and current values ​​collected at the local protection installation location. These represent the first and second order differential matrices of the voltage and current collected at the protection installation point, respectively. Based on the least squares method described above, when the cumulative error obtained from multiple sets of sampled data is minimized, the result obtained from calculating the unknown variables can be considered equal to the actually collected data. In summary, the objective function for the fault differential equation calculation result can be written as: (25) The objective function of the fault differential equation calculation results can be adjusted by accumulating the error to control the unknown variable d. , The minimum value can be found by the method that all partial derivatives are 0, as shown in equation (26).

[0095] (26) The above analysis shows that the voltage and current data collected at the protection installation point can be used to solve the unknown variables in the fault differential equation constructed based on the distributed capacitance and the phase difference of the current on both sides of the fault point, thus realizing the requirement of the dual-end weak-feed AC system for fast protection and fault location.

[0096] In summary, the time-domain distance protection fault differential equation was derived based on the equivalent model of a π-type line, taking into account both distributed capacitance and the phase difference between the currents on both sides. The protection distance flowchart is shown below. Figure 4-9 As shown in the figure. In the figure: the parameter represents the difference in fault distance calculated from the interval sampling, i.e. Where k represents the kth sampling point, and represents the time of the kth sampling point. This invention sets the fault to occur at time zero. The specific operation steps of the time-domain distance protection are as follows: After a fault occurs, the protection installation point collects voltage and current signals. The converter then uses the detected voltage signal to determine if a fault has occurred. Based on the degree of voltage amplitude drop, the converter switches from normal steady-state operation control to a collaborative fault control strategy for both converters. On the new energy side, the protection installation point collects local voltage and current data and extracts voltage and current components in specific frequency bands using a bandpass filter. The presence of a zero-sequence component in the current signal after filtering is detected; if present, it indicates a ground fault; otherwise, it indicates a phase-to-phase fault. The number of phases affected by the voltage drop then determines whether it is a single-phase / two-phase ground fault, or a two-phase / three-phase phase-to-phase fault. Different fault differential equations are applied for different fault types. The least squares method is used to solve these fault differential equations.

[0097] After implementing the least squares method, the impact of factors such as the required data window length and sliding window length on the accuracy of the calculated results needs to be considered. For the differential terms in the fault differential equation, this invention uses difference substitution, thus requiring sliding window calculation. When the sliding window has two data points, the calculation accuracy cannot meet the requirements of the time-domain distance protection proposed in this invention. When the sliding window is longer than one power frequency cycle (20ms), the calculation error no longer decreases significantly. Considering the speed of the time-domain distance protection algorithm, the sliding window should not be too long. Therefore, taking into account both calculation accuracy and the speed of the protection algorithm, this invention selects a sliding window length of half a power frequency cycle. That is, the protection algorithm proposed in this invention begins to output the calculated unknown results half a power frequency cycle after the fault.

[0098] When the difference between the fault locations obtained from three consecutive calculations all satisfy If the calculated fault location is found to be within the designated area, and the protection mechanism is checked against the calculated fault location, it can be determined whether the fault is within the designated area and whether the protection should operate. If any of the three consecutive calculation results show a fault location within the designated area, the calculation is considered reliable. If any one of these conditions is not met, the calculation result is considered unreliable, and more data points need to be calculated for continued evaluation. If the fault calculation result within 100ms after protection activation indicates that the above conditions have been met, the entire protection group will reset.

[0099] Example 1: Simulation Analysis of Protection Performance under Different Transition Resistances Taking a single-phase ground fault (AG) at the midpoint f2 of a double-ended weakly fed AC system as an example, the performance of the proposed improved time-domain distance protection method for double-ended weakly fed AC systems, considering the influence of control strategy and distributed capacitance current, is verified under different transition resistances (0.01Ω, 100Ω, 200Ω, 300Ω). The least squares method requires a sliding window length of 10ms. The performance of the improved time-domain distance protection method is affected by different transition resistances as follows: Figure 10 As shown.

[0100] Depend on Figure 10 It can be seen that, during faults within the zone, the improved time-domain distance protection method proposed in this invention, with the increase of the transition resistance, calculates fault distances that fluctuate around the actual fault location. When the transition resistance is less than 100Ω, the fault location calculated by the improved time-domain distance protection method proposed in this invention can quickly calculate the vicinity of the fault point. When the transition resistance is 200Ω and 300Ω, the improved time-domain distance protection method proposed in this invention can quickly calculate the fault point, and the average values ​​of the calculated fault locations when the transition resistance is 0.01Ω, 100Ω, 200Ω, and 300Ω are 48.92km, 46.50km, 45.48km, and 46.78km, respectively. Therefore, the improved time-domain distance protection proposed in this invention can reliably identify faults within the zone, and the error is within the allowable range of time-domain distance protection. Therefore, when faults occur at different locations, the improved distance protection method proposed in this invention shows improved performance compared to the traditional time-domain distance protection method in a two-terminal weak-feed AC system.

[0101] Example 2: Simulation Analysis of Protection Performance under Different Fault Locations The improved time-domain distance protection method proposed in this invention improves the fault differential equation of the time-domain distance protection by considering the influence of the control strategy and distributed capacitance current based on the improved converter control strategy. Based on this, the fault location is obtained using the least squares method. To study the performance of the improved time-domain distance protection method proposed in this invention under different fault locations (20km, 50km, and 80km from the renewable energy side), a single-phase ground fault (AG) is used as an example to verify the performance of the improved time-domain distance protection proposed in this invention under the same transition resistance. The results are as follows: Figure 11 As shown.

[0102] like Figure 11As shown, this invention improves the fault differential equation of time-domain distance protection by considering the influence of control strategy and distributed capacitance current based on the improved converter fault control strategy. When a single-phase ground fault occurs at different fault locations, the improved time-domain distance protection is used to calculate the fault distance. Simulation results show that when the transition resistance is 300Ω, the improved time-domain distance protection method proposed in this invention can quickly calculate the fault point at different fault locations, and the error is within the allowable range of time-domain distance protection. Therefore, the improved distance protection method proposed in this invention has good characteristics at different fault locations.

[0103] Example 3: Simulation Analysis of Protection Performance under Different Fault Types To verify the performance of the proposed time-domain distance protection during symmetrical and asymmetrical faults in a dual-ended weak-feed AC system, an asymmetrical fault was set at the midpoint of the line within the zone, taking a single-phase-to-ground fault (AG) as an example. A symmetrical fault was set at the midpoint of the line within the zone, taking a three-phase-to-phase fault (ABC) as an example. The simulation results of the time-domain distance protection under different fault types are as follows: Figure 4-12 As shown.

[0104] like Figure 12 As shown, under different types of faults within the area, the average fault distance calculated for a single-phase ground fault is 48.92 km, with an absolute error of 1.08%. For a three-phase inter-phase fault, the calculated fault distance for phase A is 49.74 km, with an absolute error of 0.26%, and the fault distance can be calculated quickly after the fault occurs. Furthermore, the protection range of distance protection stage I is 80% of the total line length. The fault distance calculated by the proposed time-domain distance protection under different fault types can reliably identify faults within the area, and the results meet the requirements of distance protection stage I.

[0105] Example 4: Comparative Analysis of Time-Domain Distance Protection Performance Before and After Considering Distributed Capacitance The above analysis demonstrates that this invention employs a negative-sequence current suppression control strategy on the new energy side converter of a double-ended weak-feed AC system, and a combined step-down V / F control and negative-sequence voltage suppression control strategy for the MMC. Compared to when both converters on both sides of the line in a double-ended weak-feed AC system employ a negative-sequence current suppression control strategy, the fault location error calculated by the time-domain distance protection is smaller. This section verifies the performance difference of the time-domain distance protection criterion derived from the fault differential equation based on the line RL equivalent model and the line π-type equivalent model considering the line distributed capacitance when the new energy side converter of the double-ended weak-feed AC system adopts a negative-sequence current suppression control strategy and the MMC adopts step-down V / F control. Taking a single-phase ground fault (AG) at the line midpoint f2 as an example, the simulation results are as follows: Figure 13 As shown.

[0106] Depend on Figure 13Comparing the fault distances calculated by time-domain distance protection based on different line equivalent models, the average fault distance at 20ms is 57.56km based on the line RL equivalent model and 49.75km based on the line π-type equivalent model. With a transition resistance of 100Ω, the fault distance at 20ms is 68.97km based on the line RL equivalent model and 46.54km based on the line π-type equivalent model. Therefore, the fault location calculated using the fault differential equation derived from the line π-type equivalent model has a smaller error.

[0107] Table 1 shows the average value and error of the fault differential equations derived for different fault types at the same fault location with and without considering the influence of distributed capacitance under different transition resistances, calculated using the least squares method with the same time window length.

[0108] Table 1. Impact of distributed capacitance on the results of the time-domain distance protection algorithm.

[0109] As shown in Table 1, compared to the time-domain distance protection algorithm considering distributed capacitance proposed in this invention, the fault location error calculated by the time-domain distance protection algorithm without considering distributed capacitance is significant, reaching 6.02% for two-phase inter-phase faults. This makes incorrect distance protection operation more likely. Both time-domain distance protection methods show increasing errors with increasing transition resistance. However, the maximum error of the time-domain distance protection algorithm considering distributed capacitance proposed in this invention is only 1.66% when the transition resistance is 300Ω. The fault location errors calculated by the time-domain distance protection algorithm proposed in this invention are all less than 1.66%, and the time-domain distance protection can operate correctly. This indicates that the time-domain distance protection method proposed in this invention reduces the fault distance error caused by phase and distributed capacitance, demonstrating an improvement in the ability to withstand transition resistance.

[0110] Example 5: Analysis of the impact of noise on the performance of improved time-domain distance protection To test the noise immunity of the proposed improved time-domain distance protection method, noise levels of 40dB, 35dB, and 30dB were added to the fault data. Taking the fault at location f2 (50km) as an example, with a transition resistance of 0.01Ω, the least squares method with the same time window length was used for calculation. The average value and error of the results obtained by the proposed time-domain distance protection method are shown in Table 2.

[0111] Table 2. Impact of different noise levels on the performance of the improved time-domain distance protection system.

[0112] As shown in Table 2, with the increase of noise, the calculated fault location and error under different fault types change little. Comparing the fault location and error calculated by the time-domain distance protection proposed in this invention under different fault locations, the changes are also small. It can be concluded that the time-domain protection proposed in this invention can still reliably identify faults under 30dB noise interference and has good noise resistance.

[0113] This invention proposes a time-domain distance protection method for a dual-terminal weakly fed AC system that considers the influence of control strategies and distributed capacitance current: (1) First, the adaptability of time-domain distance protection in a double-ended weak-feed AC system was analyzed. Simulation results show that when the transition resistance is 50Ω, the fault location error calculated by the fault differential equation derived from the RL model is greater than 10%. Therefore, the traditional time-domain distance protection has an adaptability problem in a double-ended weak-feed AC system.

[0114] (2) Secondly, the factors affecting the time-domain distance protection performance in a double-ended weak-feed AC system were analyzed. The results show that the control strategy leading to the phase inconsistency of the currents on both sides and the distributed capacitance current both affect the performance of the time-domain distance protection.

[0115] (3) Finally, based on the equivalent model of the π-type line, the present invention derives the fault differential equation and converts the transition resistance into real and imaginary parts, and uses the least squares method to calculate the unknown variables. Simulation results show that the proposed time-domain distance protection method can withstand a 300Ω transition resistance and 30dB noise interference, thus improving the performance of time-domain distance protection in a double-ended weak-feed AC system.

[0116] In another embodiment of the present invention, a negative sequence component control system for line faults in a dual-ended weak feeder system is provided, which can be used to implement the above-mentioned time-domain distance protection method combining control strategy and distributed capacitance current. Specifically, the system includes: The problem identification module is used to identify problems caused by the negative sequence current suppression control strategy adopted by the converters at both ends of the line in a double-ended weak feeder AC system, which leads to open circuits in the negative sequence networks on both sides of the composite sequence network and overvoltage in the non-faulty phases. The collaborative control module is used to improve the MMC negative sequence current suppression control strategy into a collaborative control strategy of negative sequence voltage suppression and buck V / F to address the identification problem. The output module is used to decompose the transition resistance term in the distance protection equation into a complex model of equivalent transition resistance and equivalent transition inductance based on the cooperative control strategy, derive the fault differential equation using the line π-type equivalent model, and solve the differential equation containing the fault distance using the least squares method to achieve high-precision fault location.

[0117] The module division in this embodiment of the invention is illustrative and represents only one logical functional division. In actual implementation, other division methods may be used. Furthermore, the functional modules in the various embodiments of the invention can be integrated into a single processor, exist as separate physical entities, or be integrated into a single module. The integrated modules described above can be implemented in hardware or as software functional modules.

[0118] In another embodiment of the present invention, a computer device is provided, comprising a processor and a memory. The memory stores a computer program, which includes program instructions. The processor executes the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing and control core of the terminal, suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions in the computer storage medium to achieve a corresponding method flow or corresponding function. The processor described in this embodiment of the present invention can be used to operate a time-domain distance protection method combining control strategies and distributed capacitance current.

[0119] In another embodiment of the present invention, a storage medium is provided, specifically a computer-readable storage medium (Memory), which is a memory device in a computer device used to store programs and data. It is understood that the computer-readable storage medium here can include both the built-in storage medium in the computer device and extended storage media supported by the computer device. The computer-readable storage medium provides storage space that stores the terminal's operating system. Furthermore, the storage space also stores one or more instructions suitable for loading and execution by a processor. These instructions can be one or more computer programs (including program code). It should be noted that the computer-readable storage medium here can be a high-speed RAM memory or a non-volatile memory, such as at least one disk storage device. The processor can load and execute one or more instructions stored in the computer-readable storage medium to implement the corresponding steps of the time-domain distance protection method combining control strategies and distributed capacitance current in the above embodiments.

[0120] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0121] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0122] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0123] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0124] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A time-domain distance protection method combining control strategies and distributed capacitance current, characterized in that, include: Based on the adaptability of the negative sequence current suppression control strategy adopted by the converters at both ends of the line in the double-ended weak feeder AC system, the problem of open circuit of the negative sequence network on both sides and overvoltage of non-faulty phases in the composite sequence network is identified. To address the identification issue, the MMC negative sequence current suppression control strategy is improved into a coordinated control strategy of negative sequence voltage suppression and buck V / F. Based on the collaborative control strategy, the transition resistance term in the distance protection equation is decomposed into a complex model of equivalent transition resistance and equivalent transition inductance. The fault differential equation is derived using the π-type equivalent model of the line. The differential equation containing the fault distance is solved by the least squares method to achieve high-precision fault location.

2. The time-domain distance protection method combining control strategy and distributed capacitance current according to claim 1, characterized in that, The adaptability of the negative sequence current suppression control strategy adopted by the converters at both ends of the line in the dual-ended weak-feed AC system includes: The adaptability of the negative sequence current suppression control strategy is obtained by considering the same fault location with different transition resistances and the same transition resistance with different fault locations: With different transition resistances, when a single-phase ground fault occurs in a double-ended weakly fed AC system, the fault control strategy adopted by the converters on both sides of the line is the negative sequence current suppression control strategy. As the transition resistance increases, the error of the fault location calculation result gradually increases. When a single-phase ground fault occurs in a double-ended weakly fed AC system, the fault control strategy adopted by the converters on both sides of the line is the negative sequence current suppression control strategy. When the transition resistance remains unchanged, the error of the fault location calculation result will not differ due to the different fault locations. Similarly, the error of the fault location calculation result under different transition resistances is obvious. When both converters on both sides of a double-ended weak-feed AC system adopt a negative sequence current suppression control strategy, the time-domain distance protection under metallic faults performs well. However, both will experience a significant decrease in protection performance as the transition resistance increases.

3. The time-domain distance protection method combining control strategy and distributed capacitance current according to claim 2, characterized in that, Identify the problems that cause open circuits in the negative sequence networks on both sides of the composite sequence network and overvoltage in the non-faulty phases, including: When both converters at both ends of a double-ended weak-feed AC system adopt a negative sequence current suppression control strategy, the negative sequence components at both ends are affected by the negative sequence control, causing the negative sequence networks on both sides of the composite sequence network to open, resulting in overvoltage problems in the non-faulty phases of the double-ended weak-feed AC system. After a fault, the magnitude of the positive and negative sequence components is determined by the positive and negative sequence control strategy of the double-ended weak-feed AC system, while the zero sequence component is determined by the fault type and the transformer connection form. The control effect of the positive and negative sequence components indirectly affects the zero sequence component, resulting in the phase inconsistency of the positive and negative sequence currents on both sides of the fault point after the fault, which in turn leads to the phase inconsistency of the zero sequence component currents on both sides of the fault point.

4. The time-domain distance protection method combining control strategy and distributed capacitance current according to claim 1, characterized in that, To address the identification problem, the MMC negative sequence current suppression control strategy is improved into a coordinated control strategy of negative sequence voltage suppression and buck V / F, including: The converter on the new energy side adopts a negative sequence current suppression control strategy. The MMC adopts a buck V / F control and negative sequence voltage suppression combined control strategy. Under this control strategy, the system stability requirements and the protection requirements for fault characteristics are met.

5. The time-domain distance protection method combining control strategy and distributed capacitance current according to claim 1, characterized in that, The aforementioned collaborative control strategy decomposes the transition resistance term in the distance protection equation into a complex model of equivalent transition resistance and equivalent transition inductance, and derives the fault differential equation using a line π-type equivalent model, including: When a single-phase ground fault occurs in a double-ended weakly fed AC system, the fault differential equation based on the π-type equivalent model of the line is derived; the fault differential equation considering the distributed capacitance line model after a single-phase ground fault is written as follows: In the formula: , These represent the time-domain electrical quantity information of the voltage and current of phase A at the protection installation point on the new energy side during a fault. , These are the positive sequence resistance and inductance values ​​per unit length of the line, respectively. The voltage across the transition resistor; in for: The transition resistance is represented by a complex equivalent, and the specific analysis is as follows: Introduce complex coefficients to the transition resistance Therefore, the above formula can be expressed as: Combining the transition resistance into a complex number, we get: In the formula: , The coefficients are real. and These are the equivalent transition resistance and the equivalent transition inductance, respectively. Transition resistance Decomposed into equivalent transition resistance Add equivalent transition inductance In form, After the fault is obtained, the transition resistance is transformed into a complex equation, which is the single-phase ground fault differential equation of equivalent transition resistance and equivalent transition inductance: ； While considering the transition resistance, the influence of the distributed capacitive current of long-distance transmission lines should also be taken into account: In the formula: ; Parameters in the formula , , All data are obtained from line parameters and data acquisition equipment at the new energy side protection installation site. The fault point voltage is obtained by analyzing the fault loop between the local system and the fault point after the fault, and writing the KVL equation for the fault loop. Represented as: In the formula: This refers to the zero-sequence current flowing into the fault point in the fault circuit; The relationship between the zero-sequence voltage at the protection installation point and the zero-sequence voltage at the fault point is obtained using the symmetrical component analysis method: The zero-sequence current at the fault point is: Based on the above derivation, the fault differential equation is: 。 6. The time-domain distance protection method combining control strategy and distributed capacitance current according to claim 5, characterized in that, When a two-phase-to-ground fault occurs in a double-ended weakly fed AC system line, the fault differential equation based on the line's π-type equivalent model is derived: In the formula: , , , These represent the time-domain electrical quantity information of voltage and current of phase A and phase B at the new energy side protection installation point during a fault, respectively. , The voltages at the fault points of phase A and phase B lines; in Represented as: Similarly, the process of deriving the fault differential equation when a single-phase ground fault occurs on a line is as follows: Separating the unknown parameters and known quantities, the final expression of the differential equation for the interphase fault in the two phases is: In the formula: ； ； ； ； ； ； ； 。 7. The time-domain distance protection method combining control strategy and distributed capacitance current according to claim 1, characterized in that, The method of solving the differential equation containing the fault distance using the least squares method to achieve high-precision fault location includes: The differential terms in the fault differential equation are solved using the difference substitution method. Assuming the sampling interval is Ts, the differential terms are calculated using the following formula: The fault differential matrix equation under multiple sets of sampled data is: In the formula: ； ； ； ； ； ； ； ； The left side of the equation represents the voltage data collected at the protection installation point, while the right side represents the voltage obtained by fitting the fault circuit KVL equation using collected local voltage and current data. The equation contains three unknown variables: d, ... , The difference between both sides of the equation is the error between the actual measured value and the fitted value of the unknown quantity; where These represent information matrices composed of sampled voltage and current values ​​collected at the local protection installation location. These represent the first and second order differential matrices of the voltage and current collected at the protection installation point, respectively. When the cumulative error calculated from multiple sets of sampled data is minimized, the result obtained from calculating the unknown variables is considered equal to the actually collected data. The objective function of the fault differential equation calculation result is: For the objective function of the fault differential equation calculation results, the unknown variable d is adjusted through cumulative error. , The method to find its minimum value is that all partial derivatives are 0: When the difference between the fault locations obtained from three consecutive calculations all satisfy If the calculation is successful, the fault location is considered reliable. The fault location is then compared with the protection action criteria to determine whether the fault is within the designated area and whether the protection should activate. If any of the three consecutive calculation results show a fault location within the designated area, the protection will activate. If any one of these conditions is not met, the calculation result is considered unreliable, and more data points are calculated to continue the determination.

8. A negative sequence component control system for a dual-ended weak feeder system during line faults, characterized in that, include: The problem identification module is used to identify problems caused by the negative sequence current suppression control strategy adopted by the converters at both ends of the line in a double-ended weak feeder AC system, which leads to open circuits in the negative sequence networks on both sides of the composite sequence network and overvoltage in the non-faulty phases. The collaborative control module is used to improve the MMC negative sequence current suppression control strategy into a collaborative control strategy of negative sequence voltage suppression and buck V / F to address the identification problem. The output module is used to decompose the transition resistance term in the distance protection equation into a complex model of equivalent transition resistance and equivalent transition inductance based on the cooperative control strategy, derive the fault differential equation using the line π-type equivalent model, and solve the differential equation containing the fault distance using the least squares method to achieve high-precision fault location.

9. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the steps of the time-domain distance protection method combining control strategy and distributed capacitance current as described in any one of claims 1 to 7.

10. A computer-readable storage medium storing a computer program, characterized in that, When the computer program is executed by the processor, it implements the steps of the time-domain distance protection method combining control strategy and distributed capacitance current as described in any one of claims 1 to 7.

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