CN116087678A - Online positioning method and system for high-voltage transmission cable sheath grounding fault - Google Patents

Abstract

The invention discloses a high-voltage transmission cable sheath ground fault on-line positioning method and a system, wherein the method collects the loop current of a head sheath and the loop current of a tail sheath of each loop of each section of a cable; calculating the circulation amplitude mutation quantity of the head end sheath and the circulation amplitude mutation quantity of the tail end sheath of each loop in each section, and the circulation difference value of the head end sheath and the tail end sheath; judging whether a fault section and a fault loop exist or not, outputting a fault section k and a fault loop j, and finally obtaining the specific position of the fault on the fault section k according to the current change rate of the first end and the tail end of the fault loop j after the fault and the current of the first end of the fault loop j before the fault. According to the invention, the cable line is segmented, the loop current is monitored on line only at the head end and the tail end of the segment, the loop current abrupt change is taken as the fault judgment basis of the protective layer, the transient identification fault can be generated in the fault of the protective layer, the influence of the transition resistance is avoided in the fault ranging formula, the real-time induced voltage is not required to be calculated, the ranging formula is simplified to the maximum extent, and the operability is high.

Description

A method and system for online positioning of high-voltage transmission cable sheath grounding fault

Technical Field

The present invention relates to the technical field of high-voltage transmission cable fault detection, and in particular to an online positioning method and system for a high-voltage transmission cable sheath grounding fault.

Background Art

Cables have the advantages of low maintenance workload and no space in the corridor, but they also have disadvantages such as slow fault detection and recovery and long maintenance cycle. When the cable is in operation, the cable sheath insulation fails, resulting in multiple grounding points, increased sheath circulation, and the current carrying capacity of the cable is affected. In severe cases, the cable will heat up and burn.

At present, the sheath fault location and distance measurement methods of power cables can be divided into online location and offline location methods. The traditional offline location method comprehensively applies current analysis method, low-voltage pulse method, step voltage method, etc. according to the line model, application principle and measurement equipment. The above fault location methods are basically to locate the cable offline fault through special equipment after the power cable is powered off, which increases the planned power outage time of the line and makes fault finding time-consuming and labor-intensive, and the efficiency needs to be improved.

Online positioning is achieved by online monitoring of cable line sheath related parameters, such as transient and sudden changes in sheath over-limit current, capturing sudden changes in sheath current, analyzing fault causes, and locating fault sections, providing technical conditions for line repair and early warning. A Chinese patent application with application number 202111443092.7 proposes a sheath grounding fault location method. The theoretical basis of this method is that the induced voltage in the equivalent circuit before and after the sheath single-phase grounding fault remains basically unchanged, and the non-fault phase circulating current remains basically unchanged. In actual situations, the parameters in the equivalent circuit after the fault will change to a certain extent. The degree of change is related to the fault location, transition resistance, etc., which will have a certain impact on the accuracy of fault distance measurement.

The online positioning method has problems such as requiring external signal generating equipment, only being able to achieve segment positioning but not precise positioning, complicated calculation methods, high costs and not being conducive to promotion.

Summary of the invention

The purpose of the present invention is to address the deficiencies of the above-mentioned background technology and provide a method and system for online positioning of sheath grounding faults in high-voltage transmission cables, so as to achieve real-time discovery and precise positioning of sheath single-phase grounding faults.

The present invention adopts the following technical solutions to achieve the above-mentioned invention object:

In a first aspect, the present application provides a method for online locating a sheath grounding fault of a high-voltage transmission cable, comprising the following steps:

Step 1: Collect the sheath circulation current at the head end and the sheath circulation current at the end of each loop of each section of the high-voltage transmission cable, wherein the head end is located in the incoming power side of the loop, and the end end is located in the receiving power side of the loop. The section is divided by the direct grounding point and the protective grounding point, and each section contains three loops;

末端护层环流幅值突变量

首末端护层环流差值

所述首末端护层环流差值

为所述首端护层环流与所述末端护层环流的差值；Step 2: Calculate the sudden change in the amplitude of the sheath current at the head end of each loop in each section

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

The difference between the first and the last protective layer circulation

is the difference between the head end sheath circulation and the end sheath circulation;

Where: k represents the kth segment (1≤k≤n), j represents the jth loop (j＝1, 2, 3), 1 represents the beginning of the segment, and 2 represents the end of the segment;

所述末端护层环流幅值突变量

与突变阈值的关系，以及所述首末端护层环流差值

的大小，对是否存在故障区段及故障回路进行判断，若存在所述故障区段及所述故障回路，则输出故障区段k以及故障回路j，若不存在所述故障区段或所述故障回路，则返回步骤1；Step 3: Determine the fault section and fault circuit; according to the sudden change of the amplitude of the head end sheath circulation current of each circuit in each section

The end sheath circulation amplitude mutation amount

The relationship between the mutation threshold and the difference in the sheath circulation at the first and the end

The size of is used to determine whether there is a fault section and a fault loop. If the fault section and the fault loop exist, the fault section k and the fault loop j are output. If the fault section or the fault loop does not exist, the process returns to step 1.

所述末端护层环流幅值突变量

所述首末端护层环流差值

的二次计算，若所述二次计算的结果与所述步骤2的结果一致，则进行下一步，若所述二次计算的结果与所述步骤2的结果不一致，则返回步骤1；Step 4: Secondary calculation and judgment of the fault loop j: The sudden change of the amplitude of the sheath circulation current at the head end of the fault loop j

The end sheath circulation amplitude mutation amount

The difference between the first and the last protective layer circulation

If the result of the secondary calculation is consistent with the result of step 2, proceed to the next step; if the result of the secondary calculation is inconsistent with the result of step 2, return to step 1;

Step 5: Calculate the fault time t when the fault section k fails using a wavelet algorithm;

Step 6: Collect the instantaneous current at the beginning of the fault loop and the instantaneous current at the end of the fault loop after the fault of the fault loop j at t and t+2Δt; collect the instantaneous current at the beginning of the fault loop before the fault of the fault loop j at t-T and t+2Δt-T, where Δt is the unit sampling interval and T is the sheath current cycle;

Step 7: Calculate the current change rate at the beginning of loop j after the fault, the current change rate at the end of loop j after the fault, and the current change rate at the beginning of loop j before the fault:

The post-fault current change rate of the first end of loop j is obtained by calculation based on the instantaneous current of the first end of the post-fault loop;

The post-fault current change rate at the end of loop j is obtained by calculation based on the instantaneous current at the end of the post-fault loop;

The current change rate of the first end of the loop j before the fault is obtained by calculation based on the instantaneous current of the first end of the loop before the fault;

Step 8: Calculate the specific location where the fault occurs on the fault section k; calculate the ratio α of the fault distance to the total length of the fault section k based on the current change rate at the beginning of loop j after the fault, the current change rate at the end of loop j after the fault, and the current change rate at the beginning of loop j before the fault. The fault distance is the distance between the fault location and the beginning of the fault section k.

所述末端护层环流幅值突变量

所述首末端护层环流差值

Furthermore, the Fourier algorithm is used to calculate the sudden change in the amplitude of the head end sheath circulation current of each loop in each section.

The end sheath circulation amplitude mutation amount

The difference between the first and the last protective layer circulation

Furthermore, the mutation threshold is a set value set according to the circulating current value under normal operating conditions of the high-voltage transmission cable.

Furthermore, the step of determining whether the fault section and the fault circuit exist includes:

所述

且所述

则第k区段为所述故障区段，第j回路为所述故障回路，判断为存在所述故障区段及所述故障回路；When the

Said

And the

Then the kth section is the fault section, the jth loop is the fault loop, and it is determined that the fault section and the fault loop exist;

判断为不存在所述故障区段或所述故障回路。When the

It is determined that the faulty section or the faulty circuit does not exist.

Furthermore, the unit sampling interval Δt≤0.05ms; the sheath current period T＝0.02s.

Furthermore, the calculation formula of the current change rate at the first end of loop j after the fault is:

The calculation formula of the current change rate at the end of loop j after the fault is:

The calculation formula of the current change rate of the first end of the loop j before the fault is:

Wherein, _i1(t) and _i1(t+2Δt) are the instantaneous currents at the beginning of the loop after the fault; _i2(t) and _i2(t+2Δt) are the instantaneous currents at the end of the loop after the fault; _i1(tT) and _i1(t+2Δt-T) are the instantaneous currents at the beginning of the loop before the fault.

Furthermore, the calculation formula of the ratio α is:

Wherein: _R1 and _R2 are the head-end and terminal grounding resistances respectively, which can be obtained by zero-sequence current test method or grounding resistance test method; Re is the earth leakage resistance; R is the total sheath resistance of the fault loop j; L is the total sheath inductance of the fault loop j; i _sj is the head-end current of the fault loop j before the fault.

Furthermore, the calculation formula of the ratio α is applicable when the fault section k is a cross-connection section or a double-terminal section.

On the other hand, the present application provides an online precise positioning system for high-voltage transmission cable sheath grounding faults, including a foreground recording and analysis system and a background fault identification system;

The front-end recording and analysis system includes: a section management module, a section circulation calculation module, and a fault section and fault circuit judgment module;

The background fault identification system includes: a fault circuit secondary calculation and judgment module, a fault time t calculation module, a fault circuit circulation information collection module before and after the fault, a current change rate calculation module, and a fault circuit fault location positioning module;

The section management module is used to collect the head end sheath circulation current and the end sheath circulation current of each loop in each section of the high voltage transmission cable;

末端护层环流幅值突变量

首末端护层环流差值

所述首末端护层环流差值

为所述首端护层环流与所述末端护层环流的差值；The section circulation calculation module is used to calculate the sudden change in the amplitude of the sheath circulation at the head end of each loop in each section.

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

The difference of the circulating current of the first and the last protective layers

is the difference between the head end sheath circulation and the end sheath circulation;

Where: k represents the kth segment (1≤k≤n), j represents the jth loop (j＝1, 2, 3), 1 represents the beginning of the segment, and 2 represents the end of the segment;

所述末端护层环流幅值突变量

与突变阈值的关系，以及所述首末端护层环流差值

的大小，对是否存在故障区段及故障回路进行判断，若存在所述故障区段及所述故障回路，则输出故障区段k以及故障回路j，若不存在所述故障区段或所述故障回路，则返回所述区段管理模块；The fault section and fault circuit judgment module is based on the sudden change of the amplitude of the head end sheath circulation current of each circuit in each section.

The end sheath circulation amplitude mutation amount

The relationship between the mutation threshold and the difference in the sheath circulation at the first and the end

The size of , determines whether there is a fault section and a fault loop, if the fault section and the fault loop exist, outputs the fault section k and the fault loop j, if the fault section or the fault loop does not exist, returns to the section management module;

所述末端护层环流幅值突变量

所述首末端护层环流差值

的二次计算，若所述二次计算的结果与所述区段环流计算模块的结果一致，则进行故障时刻t的计算，若所述二次计算的结果与所述区段环流计算模块的结果不一致，则返回所述区段管理模块；The fault circuit secondary calculation and judgment module is used for the sudden change of the amplitude of the head end sheath circulation current of the fault circuit j

The end sheath circulation amplitude mutation amount

The difference between the first and the last protective layer circulation

If the result of the secondary calculation is consistent with the result of the section circulation calculation module, the fault time t is calculated; if the result of the secondary calculation is inconsistent with the result of the section circulation calculation module, the fault time t is returned to the section management module;

The fault time t calculation module uses a wavelet algorithm to calculate the fault time t when the fault occurs in the fault section k;

The fault loop current information collection module before and after the fault collects the instantaneous current at the first end of the fault loop of the fault loop j at t and t+2Δt after the fault, the instantaneous current at the end of the fault loop after the fault, and the instantaneous current at the first end of the fault loop of the fault loop j at t-T and t+2Δt-T before the fault;

The current change rate calculation module is used to calculate the current change rate at the beginning of loop j after a fault, the current change rate at the end of loop j after a fault, and the current change rate at the beginning of loop j before a fault;

The fault position locating module of the fault circuit calculates the ratio α of the fault distance to the total length of the fault section k according to the current change rate of the beginning end of the circuit j after the fault, the current change rate of the end end of the circuit j after the fault, and the current change rate of the beginning end of the circuit j before the fault. The fault distance is the distance between the fault position and the beginning end of the fault section k.

Furthermore, the foreground recording and analysis system communicates with the background fault identification system through a communication module.

The beneficial effects of the present invention are:

(1) The method for accurately locating sheath faults online proposed by the present invention divides the transmission cable line into sections, performs online monitoring of circulating current only at the beginning and end of each section, and uses the sudden change in circulating current as the basis for judging sheath faults, thereby being able to instantly identify sheath faults when they occur.

(2) The transient current calculation method is innovatively applied to accurately locate the single-point grounding fault of the protective layer. The principle of instantaneous voltage equality is applied to minimize the error. The influence of transition resistance is avoided in the fault distance measurement formula, and there is no need to calculate the real-time induced voltage. The distance measurement formula is simplified to the maximum extent, and the engineering application is highly operational.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of a method for online locating a sheath grounding fault of a high-voltage transmission cable.

Figure 2 is a schematic diagram of the cross-interconnection section of the sheath of a high-voltage transmission cable.

Figure 3 is a schematic diagram of the double-terminal section of the sheath of a high-voltage transmission cable.

Figure 4 is a schematic diagram of a single-terminal section of a high-voltage transmission cable sheath.

FIG5 is a transient current waveform diagram of the front and rear sheath circuits of a high-voltage transmission cable sheath grounding fault.

FIG6 is an equivalent circuit diagram of an online positioning method for high-voltage transmission cable sheath grounding fault before a single-point grounding fault occurs.

FIG7 is an equivalent circuit diagram of a method for online positioning of a high-voltage transmission cable sheath grounding fault after a single-point grounding fault occurs.

FIG8 is a block diagram of a high-voltage transmission cable sheath grounding fault online location system.

Among them: 1-three-phase cable sheath; 2-direct grounding box at the beginning and end of the section; 3-circulating current monitoring device at the beginning; 4-circulating current monitoring device at the end; 5-direct grounding box at the beginning of the section; 6-end protection grounding box.

DETAILED DESCRIPTION

It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The method of the present invention is further described in detail below with reference to the accompanying drawings.

This embodiment discloses a method for online locating a sheath grounding fault of a high-voltage transmission cable as shown in FIG1 , which specifically includes the following steps:

Step 1: Collect the sheath circulation current at the head end and the sheath circulation current at the end of each loop of each section of the high-voltage transmission cable. The head end is located on the incoming power side of the loop, and the end end is located on the receiving power side of the loop. The section is divided by the direct grounding point and the protective grounding point. Each section contains three loops.

末端护层环流幅值突变量

首末端护层环流差值

首末端护层环流差值

为首端护层环流与末端护层环流的差值；Step 2: Calculate the sudden change in the amplitude of the sheath current at the head end of each loop in each section

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

Difference of the sheath circulation at the beginning and end

is the difference between the sheath circulation at the head end and the sheath circulation at the end;

Where: k represents the kth segment (1≤k≤n), j represents the jth loop (j＝1, 2, 3), 1 represents the beginning of the segment, and 2 represents the end of the segment;

末端护层环流幅值突变量

与突变阈值的关系，以及首末端护层环流差值

的大小，对是否存在故障区段及故障回路进行判断：Step 3: Determine the fault section and fault circuit; according to the sudden change of the sheath current amplitude at the head end of each circuit in each section

Sudden change of the end sheath current amplitude

Relationship with mutation threshold and difference in sheath circulation at the beginning and end

The size of the fault section and fault circuit is determined:

If there is a fault section and a fault loop, then output the fault section k and the fault loop j;

If there is no fault section or fault circuit, return to step 1;

末端护层环流幅值突变量

首末端护层环流差值

的二次计算，Step 4: Secondary calculation and judgment of fault loop j; sudden change of the amplitude of the sheath circulating current at the head end of fault loop j

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

The secondary calculation of

If the result of the second calculation is consistent with the result of step 2, proceed to the next step;

If the result of the second calculation is inconsistent with the result of step 2, return to step 1;

Step 5: Use wavelet algorithm to calculate the fault time t when the fault occurs in the fault section k;

Step 6: Collect the instantaneous current at the beginning of the fault loop and the instantaneous current at the end of the fault loop after the fault at t and t+2Δt; collect the instantaneous current at the beginning of the fault loop before the fault at t-T and t+2Δt-T, where Δt is the unit sampling interval and T is the sheath current cycle;

Step 7: Calculate the current change rate at the beginning of loop j after the fault, the current change rate at the end of loop j after the fault, and the current change rate at the beginning of loop j before the fault:

The rate of change of the current at the first end of loop j after the fault is calculated based on the instantaneous current at the first end of the loop after the fault;

The rate of change of the current at the end of loop j after the fault is calculated based on the instantaneous current at the end of the loop after the fault;

The current change rate at the first end of loop j before the fault is calculated based on the instantaneous current at the first end of the loop before the fault;

Step 8: Calculate the specific location of the fault on the fault section k; calculate the ratio α of the fault distance to the total length of the fault section k according to the current change rate at the beginning of loop j after the fault, the current change rate at the end of loop j after the fault, and the current change rate at the beginning of loop j before the fault. The fault distance is the distance between the fault location and the beginning of the fault section k.

In this embodiment, the basic quantities such as the amplitude mutation amount and phase of the sheath circulation current are calculated within a cycle, and then it is determined whether there is a fault section or a fault circuit. If there is a fault section or a fault circuit, the fault time is calculated. I represents the amplitude or effective value within a cycle, and i represents the instantaneous value.

Specifically, step 1 realizes the collection of circulating current information of each loop of each section of the high-voltage cable. Each area is divided into sections based on the direct grounding point and the protective grounding point, and is divided into cross-connection sections, double-terminal sections, and single-terminal sections, as shown in Figures 2, 3, and 4. Each section includes three loops.

For a two-end grounding system or a single-end grounding system, they are A-phase loop, B-phase loop, and C-phase loop; for a cross-interconnected grounding system, they are A-B-C loop, B-C-A loop, C-A-B loop or A-C-B loop, B-A-C loop, C-B-A loop.

Figure 2 is a schematic diagram of the cross-interconnected section of the sheath of a high-voltage transmission cable. In the figure, 1 is the sheath A, B, and C of the three-phase cable, which are transposed twice and directly grounded at both ends to form a sheath loop. In this embodiment, the sheath current is cross-interconnected to form three loops A-B-C, B-C-A, and C-A-B. A-B-C is the first loop, B-C-A is the second loop, and C-A-B is the third loop.

In this embodiment, the incoming call direction is set as the line head end, and the receiving power direction is set as the line end. For different section types, the circulating current monitoring device is installed at the head and end of the section, specifically: 2 is the direct grounding box at the head and end of the section; 3 is the head end circulating current monitoring device, which monitors the A, B, C three-phase circulating current and the total grounding current respectively; 4 is the end circulating current monitoring device, which monitors the A, B, C three-phase circulating current and the total grounding current respectively.

Figure 3 is a schematic diagram of a double-terminal section of a high-voltage transmission cable sheath. In the figure, 1 is a three-phase cable sheath, the ends of which are grounded through a direct grounding box; 2 is a direct grounding box at the beginning and end of the section; 3 is a head-end circulating current monitoring device, which monitors the A, B, and C three-phase circulating currents and the total grounding current respectively; 4 is a terminal circulating current monitoring device, which monitors the A, B, and C three-phase circulating currents and the total grounding current respectively. The cable segment length of the double-end grounding system is usually around 600m. In recent years, single-segment long cables have been gradually used and double-end direct grounding methods are mostly used. The cable segment length can reach more than 1000m. As the segment length increases, it is more difficult to find sheath defects. The sheath fault ranging method proposed in this embodiment can effectively solve this problem and help the development of single-segment long cables. The three loops are A-phase loop, B-phase loop, and C-phase loop.

Figure 4 is a schematic diagram of a single-terminal connection section of a high-voltage transmission cable sheath. In the figure, 1 is a three-phase cable sheath, whose ends are grounded through a direct grounding box; 5 is a direct grounding box at the beginning of the section; 6 is a terminal protection grounding box; 3 is a head-end circulating current monitoring device, which monitors the A, B, C three-phase circulating current and the total grounding current; 4 is a terminal circulating current monitoring device, which monitors the A, B, C three-phase circulating current and the total grounding current.

Step 1: The sheath circulating current at the head end and the sheath circulating current at the end of each loop of each section of the high-voltage transmission cable are collected through the circulating current monitoring device located at the head and end of the section.

末端护层环流幅值突变量

首末端护层环流差值

Step 2: Use Fourier algorithm to calculate the sudden change of the sheath current amplitude at the head end of each loop in each section.

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

In this embodiment, the circulation mutation data of the three loops in the kth section are calculated by step 2:

末端护层环流幅值突变量

首末端护层环流差值

ABC loop: Sudden change in the amplitude of the current circulating in the first end sheath

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

末端护层环流幅值突变量

首末端护层环流差值

BCA loop: Sudden change in the amplitude of the current circulating in the first end sheath

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

末端护层环流幅值突变量

首末端护层环流差值

CAB loop: Sudden change in the amplitude of the sheath current at the head end

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

末端护层环流幅值突变量

与突变阈值的关系，以及首末端护层环流差值

的大小，对是否存在故障区段及故障回路进行判断。Step 3: According to the sudden change of the amplitude of the sheath circulation at the head end of each loop in each section

Sudden change of the end sheath current amplitude

Relationship with mutation threshold and difference in sheath circulation at the beginning and end

The size of the fault section and fault circuit can be determined.

The mutation threshold is a set value set according to the circulating current value under normal operation of the line. The steps for judging whether there is a fault section and a fault circuit include:

则第k区段为故障区段，第j回路为故障回路，判断为存在故障区段及故障回路；when

Then the kth section is the faulty section, the jth loop is the faulty loop, and it is determined that there are faulty sections and faulty loops;

判断为不存在故障区段或故障回路。when

It is determined that there is no faulty section or faulty circuit.

则第3回路为故障回路，判断结果为：存在故障区段及故障回路，输出故障区段k以及故障回路3。In this embodiment, if the sheath current amplitude mutations at the beginning and end of the second and third loops of the k section are greater than the mutation threshold, the k section is a faulty section; further verify which loop of the k section is faulty, and after judging the third loop

Then the third loop is a faulty loop, and the judgment result is: there is a faulty section and a faulty loop, and the faulty section k and the faulty loop 3 are output.

为k区段第三回路(C-A-B回路)首端电流，

为k区段第三回路(C-A-B回路)末端电流。可见在故障时间t＝0.15s，第三回路首末端电流在故障时刻发生突变，而后到达新的稳态；非故障回路电流连续变化至新的稳态，故障前后电流幅值仅发生小幅度变化。When a single-point grounding fault occurs in the sheath, the transient change process of the sheath circulating current in this embodiment is shown in FIG5 , where I _s1 is the current of the first loop (ABC loop) of the k section in this embodiment, i.e., the non-fault loop current;

is the current at the first end of the third loop (CAB loop) in the k section,

is the current at the end of the third circuit (CAB circuit) in section k. It can be seen that at the fault time t = 0.15s, the current at the end of the third circuit changes suddenly at the moment of the fault, and then reaches a new steady state; the current in the non-fault circuit changes continuously to a new steady state, and the current amplitude changes only slightly before and after the fault.

In another embodiment, if there is no sudden change in the amplitude of the circulating current at the head and end of the sheath in the k section that is not less than the sudden change threshold, it means that the effective value of the circulating current has not changed in adjacent cycles, there is no fault section, and the process returns to step 1.

In another embodiment, if there are head and end sheath circulation amplitude mutations greater than or equal to the mutation threshold in the k section, but there is no loop with a head and end sheath circulation difference greater than 2A, then there is no faulty loop and the process returns to step 1.

In step 3, the faulty section is first judged based on the sudden change in the amplitude of the sheath circulating current at the head and the end, and then the specific faulty circuit in the faulty section is judged based on the difference in the sheath circulating current at the head and the end. By judging the difference between the head and the end of the circuit, the change in the line load will cause the synchronous change in the sudden change in the amplitude of the sheath circulating current at the head and the end. If the difference in the sheath circulating current at the head and the end is not judged, the change in load will be judged as a fault. This step effectively avoids misjudgment of the fault.

末端护层环流幅值突变量

首末端护层环流差值

的二次计算，若二次计算的结果与步骤2的结果一致，则进行下一步，若二次计算的结果与步骤2的结果不一致，则返回步骤1。Step 4: The sudden change of the sheath current amplitude at the head end of the fault circuit j in the fault section k

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

If the result of the secondary calculation is consistent with the result of step 2, proceed to the next step. If the result of the secondary calculation is inconsistent with the result of step 2, return to step 1.

The fault judgment accuracy is further improved through secondary calculation, and the misjudgment rate of this method is reduced.

The fault section k and the fault loop 3 are determined in step 3. In step 4, the mutation of the fault loop 3 is further calculated. In step 5, the wavelet algorithm is used to calculate the fault time t when the fault occurs in the fault section k.

Step 6: Collect the instantaneous current at the beginning of the fault loop and the instantaneous current at the end of the fault loop after the fault of the fault loop j at t and t+2Δt; collect the instantaneous current at the beginning of the fault loop before the fault of the fault loop j at t-T and t+2Δt-T, where Δt is the unit sampling interval and T is the sheath current cycle;

Collect the instantaneous current at the beginning of the fault circuit 3 after the fault at time t and t+2Δt, including: i _1(t) and i _1(t+2Δt) , and the instantaneous current at the end of the fault circuit after the fault, including: i _2(t) and i _2(t+2Δt) .

The unit sampling interval Δt≤0.05m. The unit sampling interval can be selected according to the detection requirements. If the sampling interval is too large, the detection accuracy will be reduced. In this embodiment, the unit sampling interval is 0.05s.

The instantaneous current at the first end of the fault circuit 3 before the fault at the time tT and t+2Δt-T is collected, including: i _1(tT) , i _1(t+2Δt-T) . In this embodiment, the sheath current period T=0.02s.

Step 7: Calculate the current change rate at the beginning of loop j after the fault based on the instantaneous current at the beginning of the loop after the fault, calculate the current change rate at the end of loop j after the fault based on the instantaneous current at the end of the loop after the fault, and calculate the current change rate at the beginning of loop j before the fault based on the instantaneous current at the beginning of the loop before the fault.

According to the data collected at multiple times, at least two data at different times can be selected to calculate the conversion rate. In this embodiment, the current change rate at the head end of loop 3 after the fault, the current change rate at the end of loop 3 after the fault, and the current change rate at the head end of loop 3 before the fault are calculated. The specific calculation formula is:

The calculation formula for the current change rate at the first end of loop j after a fault is:

The calculation formula for the current change rate at the end of the rear loop j is:

The calculation formula for the current change rate at the first end of loop j before the fault is:

Step 8: Calculate the ratio α of the fault distance to the total length of the fault section k.

The derivation process of the ratio α calculation formula is:

In this embodiment, the sheath current is cross-connected and has three loops, ABC, BCA, and CAB. ABC is the first loop, BCA is the second loop, and CAB is the third loop. The fault occurs in the third loop (CAB). The equivalent circuit diagram before the single-point grounding fault occurs is shown in FIG6 . In the figure, _is1 , _is2 , and _is3 are the loop currents of each loop; _isz is the total grounding loop current; _Zai , _Zbi , and _Zci (1≤i≤3) are the impedances of the sheath of each section of the line; _uai , _ubi , and _uci (1≤i≤3) are the induced voltages generated by the core current on the metal sheath, and _uai ′, _ubi ′, and _uci ′ (1≤i≤3) are the induced voltages of the sheath current; _R1 and _R2 are grounding resistances, and _Re is the earth leakage resistance. According to Kirchhoff's law, the equation for loop CAB (the third loop) is:

Where R is the total resistance of the third circuit sheath; L is the total inductance of the third circuit sheath. In the induced voltage, the main core current induced voltage part plays a leading role.

A single-point grounding fault occurs in the third loop CAB loop. Figure 7 is the equivalent circuit diagram after the single-point grounding fault occurs. The fault occurs in the first section of the CAB loop. In the figure, _i1 is the head-end current of the CAB loop after the fault, _i2 is the end-end current of the CAB loop after the fault, the fault distance l is the distance between the fault location and the head of the fault section k, and _R3 is the grounding resistance of the fault point, that is, the transition resistance. The instantaneous voltage of the CAB loop satisfies the formula:

α is the ratio of the fault distance l to the total length of the fault section k. Assume that t is the first sampling time point of the sheath current acquisition device after the fault occurs, the unit sampling interval is △t, t+△t is the first sampling time point after the fault occurs, and t+2△t is the second sampling time point after the fault occurs. In the microsecond time interval after the fault occurs, the transient change of the sheath current has little effect on the total induced voltage on the sheath. It can be concluded that at time t, the instantaneous voltage of the circuit under the condition of ground fault and no fault is equal, and the circuit current of the non-fault phase is equal. That is, at time t, the right sides of equations (4) and (5) are equal, and simplified to obtain equation (6):

Formula (3) is the derived formula for measuring the distance of a sheath single-point grounding fault using transient current, where _is3 is the fault circuit current, i.e., the third circuit (CAB) current.

In summary, the calculation formula for the ratio α is:

Wherein: _R1 and _R2 are the head-end and terminal grounding resistances respectively, which can be obtained by zero-sequence current test method or grounding resistance test method; Re is the earth leakage resistance, which takes the classic value of 0.0493Ω/km; R is the total sheath resistance of fault loop j; L is the total sheath inductance of fault loop j; i _sj is the head-end current of fault loop j before the fault.

L is the total inductance of the cross-connected unit sheath, which can be obtained by calculation or measurement. The calculation method is calculated according to formula (8).

In formula (4), De is the equivalent depth of the earth; r is the average geometric radius of the metal sheath; _Lz is the total length of the cross-connected unit.

Each parameter in the fault distance measurement formula derived by this technology can be obtained by calculation or measurement. The calculation method is simple, highly operational, and convenient for practical engineering application.

The calculation formula of the ratio α is applicable when the fault section k is a cross-connection section or a double-terminal section.

For the A-C-B loop, B-A-C loop, and C-B-A loop of the cross-interconnected section, it is only necessary to replace the A phase data at the end with C phase data, the B phase data with A phase data, and the C phase data with B phase data.

For sections with direct grounding at both ends, during the calculation process, it is only necessary to replace the A-phase data at the end in the ranging formula derived using the cross-connected system C-A-B loop as an example with B-phase data, the B-phase data with C-phase data, and the C-phase data with A-phase data.

For a single-ended grounding system, as shown in FIG4 , the technical solution of this embodiment can determine the fault section but cannot accurately locate it.

The online accurate positioning method of sheath fault proposed in this embodiment divides the transmission cable line into sections, performs online monitoring of circulating current only at the beginning and end of the section, and uses the sudden change of circulating current as the basis for judging sheath fault, which can instantly identify the fault when sheath fault occurs. The transient current calculation method is innovatively applied to accurately locate the sheath single-point grounding fault, and the principle of instantaneous voltage equality is applied to minimize the error. The influence of transition resistance is avoided in the fault distance formula, and there is no need to calculate the real-time induced voltage, which simplifies the distance formula to the maximum extent, and has high operability in engineering applications.

On the other hand, the present application provides an online precise positioning system for high-voltage transmission cable sheath grounding faults. The system composition block diagram is shown in Figure 8. The system includes a front-end recording and analysis system and a back-end fault identification system. The front-end recording and analysis system and the back-end fault identification system are shown in the dotted box in the figure.

The front-end recording and analysis system includes: section management module, section circulation calculation module, fault section and fault circuit judgment module;

The background fault identification system includes: a fault circuit secondary calculation and judgment module, a fault time t calculation module, a fault circuit circulation information collection module before and after the fault, a current change rate calculation module, and a fault circuit fault location positioning module;

The section management module is used to collect the sheath circulation current at the head end and the sheath circulation current at the end of each loop in each section of the high-voltage transmission cable, and maintain the information and grounding method of the head and end of each section;

末端护层环流幅值突变量

首末端护层环流差值

首末端护层环流差值

为首端护层环流与末端护层环流的差值；The section circulation calculation module is used to calculate the sudden change of the sheath circulation amplitude at the head end of each loop in each section.

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

Difference of the sheath circulation at the beginning and end

is the difference between the sheath circulation at the head end and the sheath circulation at the end;

Where: k represents the kth segment (1≤k≤n), j represents the jth loop (j＝1, 2, 3), 1 represents the beginning of the segment, and 2 represents the end of the segment;

末端护层环流幅值突变量

与突变阈值的关系，以及首末端护层环流差值

的大小，对是否存在故障区段及故障回路进行判断，若存在故障区段及故障回路，则输出故障区段k以及故障回路j，若不存在故障区段或故障回路，则返回区段管理模块；The fault section and fault circuit judgment module is based on the sudden change of the sheath circulation amplitude at the head end of each circuit in each section.

Sudden change of the end sheath current amplitude

Relationship with mutation threshold and difference in sheath circulation at the beginning and end

The size of is used to determine whether there is a faulty section and a faulty circuit. If there is a faulty section and a faulty circuit, the faulty section k and the faulty circuit j are output. If there is no faulty section or a faulty circuit, the section management module is returned.

末端护层环流幅值突变量

首末端护层环流差值

的二次计算，若二次计算的结果与区段环流计算模块的结果一致，则进行故障时刻t的计算，若区段环流计算模块的结果与区段环流计算模块的结果不一致，则返回区段管理模块。Fault circuit secondary calculation and judgment module, used for the sudden change of the sheath current amplitude at the head end of fault circuit j

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

If the result of the secondary calculation is consistent with the result of the section circulation calculation module, the fault time t is calculated. If the result of the section circulation calculation module is inconsistent with the result of the section circulation calculation module, it returns to the section management module.

The fault time t calculation module uses wavelet algorithm to calculate the fault time t when the fault occurs in the fault section k;

The fault circuit circulation information collection module before and after the fault collects the instantaneous current at the beginning of the fault circuit j at the time t and t+2Δt after the fault, the instantaneous current at the end of the circuit after the fault, and the instantaneous current at the beginning of the fault circuit j at the time t-T and t+2Δt-T before the fault;

The current change rate calculation module is used to calculate the current change rate at the beginning of loop j after a fault, the current change rate at the end of loop j after a fault, and the current change rate at the beginning of loop j before a fault.

The fault location positioning module of the fault circuit calculates the ratio α of the fault distance to the total length of the fault section k according to the current change rate of the first end of the circuit j after the fault, the current change rate of the last end of the circuit j after the fault, and the current change rate of the first end of the circuit j before the fault. The fault distance is the distance between the fault position and the first end of the fault section k.

The front-end recording and analysis system communicates with the back-end fault identification system through a communication module.

The system of this embodiment, relying on the derived fault distance measurement formula, realizes online identification and positioning of single-point grounding faults in the protective layer through functional design, and realizes automatic identification, accurate positioning, low cost, simplicity and convenience.

The above description is only a preferred example of the invention and is not intended to limit the invention. Although the invention is described in detail with reference to the above examples, those skilled in the art can still modify the technical solutions recorded in the above examples or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, etc. made within the spirit and principle of the invention shall be included in the protection scope of the invention.

Claims (10)

1. A method for online locating a high-voltage transmission cable sheath grounding fault, characterized in that it comprises the following steps: Step 1: Collect the sheath circulating current at the head end and the sheath circulating current at the end of each loop in each section of the high-voltage transmission cable; Step 2: Calculate the sudden change in the amplitude of the sheath current at the head end of each loop in each section

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

The difference of the circulating current of the first and the last protective layers

is the difference between the head end sheath circulation and the end sheath circulation, wherein: k represents the kth section (1≤k≤n), j represents the jth loop (j＝1, 2, 3), 1 represents the head end of the section, and 2 represents the end of the section; Step 3: Determine the fault section and fault circuit; according to the sudden change of the amplitude of the head end sheath circulation current of each circuit in each section

The end sheath circulation amplitude mutation amount

The relationship between the mutation threshold and the difference in the sheath circulation at the first and the end

The size of , determines whether the fault section and the fault loop exist, if the fault section and the fault loop exist, outputs the fault section k and the fault loop j, if the fault section or the fault loop does not exist, returns to step 1; Step 4: Secondary calculation and judgment of the fault loop j; the sudden change of the amplitude of the sheath current at the head end of the fault loop j

The end sheath circulation amplitude mutation amount

The difference of the circulating current of the first and the last protective layers

If the result of the secondary calculation is consistent with the result of step 2, proceed to the next step; if the result of the secondary calculation is inconsistent with the result of step 2, return to step 1; Step 5: Calculate the fault time t when the fault section k fails using a wavelet algorithm; Step 6: Collect the instantaneous current at the beginning of the fault loop and the instantaneous current at the end of the fault loop after the fault of the fault loop j at t and t+2△t; collect the instantaneous current at the beginning of the fault loop before the fault of the fault loop j at t-T and t+2△t-T, where △t is the unit sampling interval and T is the sheath current cycle; Step 7: Calculate the current change rate at the beginning of loop j after the fault, the current change rate at the end of loop j after the fault, and the current change rate at the beginning of loop j before the fault: The post-fault current change rate of the first end of loop j is obtained by calculation based on the instantaneous current of the first end of the post-fault loop; The post-fault current change rate at the end of loop j is obtained by calculation based on the instantaneous current at the end of the post-fault loop; The current change rate of the first end of the loop j before the fault is obtained by calculation based on the instantaneous current of the first end of the loop before the fault; Step 8: Calculate the specific location where the fault occurs on the fault section k; calculate the ratio α of the fault distance to the total length of the fault section k according to the current change rate at the beginning of loop j after the fault, the current change rate at the end of loop j after the fault, and the current change rate at the beginning of loop j before the fault. The fault distance is the distance between the fault location and the beginning of the fault section k. 2. A method for online positioning of high-voltage transmission cable sheath grounding faults according to claim 1, characterized in that the head end is located on the incoming side of the loop, the end end is located on the receiving side of the loop, the section is divided by the direct grounding point and the protective grounding point, and each section contains three loops. 3. A method for online positioning of a high-voltage transmission cable sheath grounding fault according to claim 1, characterized in that the mutation threshold is a set value set according to the circulating current value under normal operating conditions of the high-voltage transmission cable. 4. A method for online locating a sheath grounding fault of a high-voltage transmission cable according to claim 1, characterized in that the step of judging whether there is a fault section and a fault circuit comprises: When the amplitude of the circulating current in the head end sheath changes suddenly

The end sheath circulation amplitude mutation amount

And the difference of the sheath circulation at the first and the end is

Then the kth section is the fault section, the jth loop is the fault loop, and it is determined that the fault section and the fault loop exist; When the amplitude of the circulating current in the head end sheath changes suddenly

Or the sudden change of the end sheath current amplitude

Or the difference of the sheath circulation at the beginning and end

It is determined that the faulty section or the faulty circuit does not exist. 5. A method for online positioning of a high-voltage transmission cable sheath grounding fault according to claim 1, characterized in that the unit sampling interval △t≤0.05ms; the sheath current period T＝0.02s. 6. A method for online positioning of a high-voltage transmission cable sheath grounding fault according to claim 1, characterized in that the calculation formula for the current change rate at the first end of the loop j after the fault is:

The calculation formula of the current change rate at the end of loop j after the fault is:

The calculation formula of the current change rate at the first end of the loop j before the fault is:

Wherein, _i1(t) and _i1(t+2△t) are the instantaneous currents at the beginning of the loop after the fault; _i2(t) and _i2(t+2△t) are the instantaneous currents at the end of the loop after the fault; _i1(tT) and _i1(t+2△tT) are the instantaneous currents at the beginning of the loop before the fault. 7. A method for online locating a high-voltage transmission cable sheath grounding fault according to claim 1, characterized in that the calculation formula of the ratio α is:

Wherein: _R1 and _R2 are the head-end and terminal grounding resistances respectively, which can be obtained by zero-sequence current test method or grounding resistance test method; Re is the earth leakage resistance; R is the total sheath resistance of the fault loop j; L is the total sheath inductance of the fault loop j; _isj is the head-end current of the fault loop j before the fault. 8. A method for online positioning of a high-voltage transmission cable sheath grounding fault according to claim 1, characterized in that the calculation formula of the ratio α is applicable when the fault section k is a cross-connected section or a double-terminal section. 9. An online precise positioning system for high-voltage transmission cable sheath grounding faults, including a front-end recording and analysis system and a back-end fault identification system; The front-end recording and analysis system includes: a section management module, a section circulation calculation module, and a fault section and fault circuit judgment module; The background fault identification system includes: a fault circuit secondary calculation and judgment module, a fault time t calculation module, a fault circuit circulation information collection module before and after the fault, a current change rate calculation module, and a fault circuit fault location positioning module; The section management module is used to collect the head end sheath circulation current and the end sheath circulation current of each loop in each section of the high voltage transmission cable; The section circulation calculation module is used to calculate the sudden change in the amplitude of the sheath circulation at the head end of each loop in each section.

Sudden change of the end sheath current amplitude

Difference of the sheath circulation at the beginning and end

The difference of the circulating current of the first and the last protective layers

is the difference between the head end sheath circulation and the end sheath circulation; Where: k represents the kth segment (1≤k≤n), j represents the jth loop (j＝1, 2, 3), 1 represents the beginning of the segment, and 2 represents the end of the segment; The fault section and fault circuit judgment module is based on the sudden change of the amplitude of the head end sheath circulation current of each circuit in each section.

The end sheath circulation amplitude mutation amount

The relationship between the mutation threshold and the difference in the sheath circulation at the first and the end

The size of , determines whether there is a fault section and a fault loop, if the fault section and the fault loop exist, outputs the fault section k and the fault loop j, if the fault section or the fault loop does not exist, returns to the section management module; The fault circuit secondary calculation and judgment module is used for the sudden change of the amplitude of the head end sheath circulation current of the fault circuit j

The end sheath circulation amplitude mutation amount

The difference between the first and the last protective layer circulation

If the result of the secondary calculation is consistent with the result of the section circulation calculation module, the fault time t is calculated; if the result of the secondary calculation is inconsistent with the result of the section circulation calculation module, the fault time t is returned to the section management module; The fault time t calculation module uses a wavelet algorithm to calculate the fault time t when the fault occurs in the fault section k; The fault loop current information collection module before and after the fault collects the instantaneous current at the beginning of the fault loop of the fault loop j at t and t+2△t, the instantaneous current at the end of the fault loop, and the instantaneous current at the beginning of the fault loop of the fault loop j at t-T and t+2△t-T before the fault; The current change rate calculation module is used to calculate the current change rate at the beginning of loop j after a fault, the current change rate at the end of loop j after a fault, and the current change rate at the beginning of loop j before a fault; The fault position locating module of the fault circuit calculates the ratio α of the fault distance to the total length of the fault section k according to the current change rate of the beginning end of the circuit j after the fault, the current change rate of the end end of the circuit j after the fault, and the current change rate of the beginning end of the circuit j before the fault. The fault distance is the distance between the fault position and the beginning end of the fault section k. 10. An online precise positioning system for high-voltage transmission cable sheath grounding faults according to claim 9, characterized in that the foreground recording and analysis system communicates with the background fault identification system through a communication module.

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