Resonant grounding system fault section positioning method and device based on neutral point injection

Abstract

The application provides a neutral point injection-based fault section positioning method and device for a resonant grounding system, which comprises the following steps: selecting an injection current frequency according to a transformer winding connection mode, and then determining a current amplitude to configure an injection power supply; subsequently, designing a power frequency wave trap composed of an inductance L and a capacitance C, connecting the wave trap in series with the injection power supply, and then connecting the wave trap in parallel with a neutral point grounding loop, so that the injection line presents a circuit breaking characteristic to a power frequency current and a low resistance characteristic to an injection current; the innovation does not need to change a traditional grounding mode, and has high reliability; then, a fault zero sequence network is established based on a system topology, a distribution law of the injection current in a fault / non-fault section is analyzed, and a first-end and last-end current amplitude difference characteristic is extracted; finally, a positioning criterion is set according to the characteristic, and fault section identification is realized. The application effectively solves the technical problem that, when a single-phase grounding fault occurs in the resonant grounding system, a current transformer has insufficient precision and positioning accuracy is limited due to weak zero sequence current.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power distribution network, and particularly relates to a fault section positioning method for a resonance grounding system based on neutral point injection. BACKGROUND

[0002] Most of the existing urban distribution network lines use cables, which have the problem of excessive capacitive current. In order to avoid the phenomenon of arc grounding, resonance grounding is generally used at the neutral point of the Y-type winding of the transformer. When a single-phase grounding fault occurs in the line, due to the compensation effect of the arc suppression coil, the fault characteristics of the zero sequence current of the fault line are not obvious, which leads to inaccurate measurement or even failure to measure data by the zero sequence current transformer. Therefore, fault positioning becomes very difficult in the above-mentioned scenario. In the field of power system relay protection, different frequency harmonic currents are often injected into the system through the neutral point or Z-type grounding transformer. However, for the neutral point ungrounded system, the injection through the neutral point will cause secondary grounding of the neutral point, leading to protection misoperation or equipment damage. Therefore, it is necessary to consider the problem of how to select the topology structure without affecting the power frequency component during injection. The traditional injection method often uses 220Hz frequency, which is a non-three times and integer multiple harmonic. When the transformer winding is Dyn11 connection, the injected current will be induced to the high-voltage side through the low-voltage winding, affecting the power quality of the grid side. In addition, in the tree-shaped distribution network of multi-branch lines, the injected current will be shunted by the line distribution capacitance, and the current at the fault point may be small. Therefore, when the neutral point injection method is used in the resonance grounding system, the selection of the frequency of the injected current and how to reduce the shunt of the injected current at the non-fault point need to be studied.

[0003] The existing injection method selects a frequency of 60Hz (Chengxiao, Jiaoyanjun, Wang Lizong. Fault location of distribution network based on characteristic frequency signal injection method [J]. Shaanxi Electric Power, 2012, 40(01): 33-36.) characteristic current, but this frequency is close to the power frequency, and the extraction effect is limited. When extracting and processing the injected characteristic current, some studies extract and reconstruct specific components after decomposition (Single-phase grounding fault joint line selection considering distributed power injection harmonics [J]. Power Engineering and Technology, 2023, 42(04): 113-121.), and calculate the energy value to realize fault section positioning, but the results of this method are greatly affected by the human selection strategy. In the field of fault location of resonance grounding system, it can be considered to further optimize the frequency of the injected current based on the injection method, and use the power frequency wave trap to realize more accurate fault section positioning. SUMMARY

[0004] In order to improve the correctness of fault positioning when single-phase grounding fault occurs in a neutral point resonant grounding system and reduce the loss caused by line tripping, the application provides a neutral point injection-based fault section positioning method and device for a resonant grounding system, which mainly aims at the problem that the zero sequence current characteristics are not obvious when single-phase grounding fault occurs due to the resonant grounding compensation capacitor current, and proposes a new fault section positioning technology based on neutral point injection current frequency selection and power frequency wave trap design.

[0005] The technical scheme adopted by the application is:

[0006] A neutral point injection-based fault section positioning method for a resonant grounding system, comprising the following steps:

[0007] Step 1: According to the transformer winding connection mode, the flow rule of the injection current in the winding is analyzed, and the neutral point injection current frequency f is selected;

[0008] Step 2: The injection current distribution rule in the line is analyzed, and the neutral point injection current amplitude is determined. The injection power source is determined by the neutral point injection current frequency f and the neutral point injection current amplitude .

[0009] Step 3: A power frequency wave trap is designed, the power frequency wave trap comprises a wave trap inductance L and a capacitance C, and the power frequency wave trap and the injection power source are connected in series and then connected in parallel with the neutral point grounding loop to form an injection line, so that the injection line shows a broken circuit to the power frequency current flowing from the fault point to the ground through the neutral point, and shows a low resistance characteristic to the injection current emitted by the injection power source;

[0010] Step 4: A neutral point resonant grounding distribution network fault zero sequence network is established based on the system topology and the injection line, the distribution rule of the injection current in the fault / non-fault section is analyzed, and then the current amplitude difference characteristics of the first and last ends are obtained;

[0011] Step 5: Based on the current amplitude difference characteristics obtained in step 4, the fault section positioning criterion is set, and the fault section identification is completed.

[0012] Further, the determination process of the injection frequency f in step 1 is:

[0013] The suppression characteristics of the transformer Dyn11 winding to the 3nth harmonic are analyzed, when f=9th harmonic, the three-phase currents are in the same phase to form a zero sequence component, after circulating through the delta winding, the current flowing into the power grid side is ≤0.1A, so the 9th harmonic is selected as the injection frequency.

[0014] Further, the determination process of the injection current amplitude in step 2 is:

[0015] Based on the f=9th harmonic of the output of step 1, the following constraints are met:

[0016] (1) The maximum zero sequence current of the line is less than or equal to the residual current limit value of the system grounding fault ;

[0017] (2) The zero sequence current injected at the first end of the line farthest from the neutral point is greater than or equal to the minimum measurable current of the transformer ;

[0018] The final determination of the injected current amplitude selection strategy is:

[0019]

[0020] In the formula: is the zero sequence current injected at the first end of the farthest line, and the minimum measurable current of the transformer is the zero sequence current injected at the secondary side of the transformer corresponding to the time.

[0021] Further, the design process of the wave trap parameters in step 3 is:

[0022] Based on the f=9th harmonic of the output of step 1, the capacitance and inductance parallel structure is designed to make:

[0023] (1) The equivalent impedance at power frequency to block the power frequency current;

[0024] (2) The equivalent impedance at 9th harmonic to realize low resistance path;

[0025] Where the equivalent impedance at 9th harmonic is , and the requirement is , L is the inductance of the wave trap, and C is the capacitance of the wave trap.

[0026] Further, the analysis process of the current amplitude difference characteristics in step 4 is:

[0027] Based on the injected current amplitude obtained in step 2 and the power frequency wave trap parameters obtained in step 3, a zero sequence network model is constructed, and the following is simulated:

[0028] (1) The current change percentage at the first and last ends of the non-fault section is less than or equal to 25%;

[0029] (2) The current at the first end of the fault section is more than 4 times that at the last end;

[0030] (3) The current deviation rate at the first end of the last fault line is more than 3 times that of the non-fault line.

[0031] Further, the setting process of the positioning criterion in step 5 is: based on the current amplitude difference characteristics output by step 4, set:

[0032] (1) Non-terminal line criterion: if the current variation percentage of the first and last ends of a line is > 25%, the fault section is determined;

[0033] (2) Terminal line criterion: if the current offset rate of the first end of a line is the maximum value, the fault section is determined.

[0034] Further, the zero sequence network model in step 4 includes distributed capacitance, line impedance and the power frequency wave trap designed in step 3, wherein the wave trap presents low impedance to the 9th harmonic, ensuring that the injected current preferentially flows through the fault section.

[0035] Further, the current variation percentage calculation formula in step 5 is , and the current offset rate calculation formula is , wherein represents the first end injected current of the non-terminal feeder i, represents the last end injected current of the non-terminal feeder i, is the inherent injected current of the terminal feeder n, and the size is the injected current amplitude generated at the first end of the line by the neutral point injected current during normal operation of the system, is the injected current extracted at the first end of the terminal feeder n during the fault.

[0036] A neutral point injection-based fault section positioning device for a resonant grounding system, comprising:

[0037] An injected current frequency determination module for analyzing the flow-through rule of the injected current in the winding according to the transformer winding connection mode, and selecting the neutral point injected current frequency f;

[0038] An injected current amplitude determination module for analyzing the distribution rule of the injected current in the line, and determining the neutral point injected current amplitude , from the neutral point injected current frequency f and the neutral point injected current amplitude An injected power source determination module for determining the injected power source;

[0039] An injected line determination module for designing a power frequency wave trap, wherein the power frequency wave trap includes a wave trap inductance L and a capacitance C, and the power frequency wave trap and the injected power source are connected in series and then connected in parallel with the neutral point grounding loop to form an injected line, so that the injected line presents a circuit break to the power frequency current flowing from the fault point to the ground through the neutral point, and presents a low resistance characteristic to the injected current emitted by the injected power source;

[0040] A fault zero sequence network establishment module for establishing a neutral point resonant grounding distribution network fault zero sequence network based on the system topology and the injected line, analyzing the distribution rule of the injected current in the fault / non-fault section, and then obtaining the first and last end current amplitude difference characteristics;

[0041] A fault section identification module is configured to set a fault section positioning criterion based on the obtained current amplitude difference feature, and complete fault section identification.

[0042] Further, the setting process of the positioning criterion in the fault section identification module is as follows: based on the output current amplitude difference feature, setting:

[0043] (1) Non-end section line criterion: if the current variation percentage of a line at the two ends is greater than 25%, the fault section is determined;

[0044] (2) End section line criterion: if the current offset rate of the line at the first end is the maximum value, the fault section is determined;

[0045] The current variation percentage calculation formula is The current offset rate calculation formula is , wherein represents the first end injection current of the non-end section feeder i, represents the end injection current of the non-end section feeder i, is the inherent injection current of the end section feeder n, and the size is the injection current amplitude of the neutral point injection current at the first end of the line during normal operation of the system, is the injection current extracted at the first end of the end section feeder n during the fault.

[0046] The beneficial effects of the present application are as follows:

[0047] (1) The present application is aimed at a resonant grounding system, and solves the problem of unobvious fault characteristics without affecting the compensation effect of the arc suppression coil on the fault point;

[0048] (2) The power frequency wave choke proposed by the present application allows power frequency current to pass through, avoiding the occurrence of secondary neutral grounding after the fault;

[0049] (3) The principle of the present application is simple, the wave choke is designed with reference to the pilot protection, only common electrical elements are used, the technology is mature, and the grounding mode is not changed, the system has high reliability, and is conducive to the application and popularization of the project. BRIEF DESCRIPTION OF DRAWINGS

[0050] Figure 1 is a distributed photovoltaic distribution network topology structure diagram according to an embodiment of the present application;

[0051] Figure 2 is a neutral point resonant grounding distribution network fault zero sequence network structure diagram in an embodiment of the present application;

[0052] Figure 3 is a flowchart of a neutral point injection-based resonant grounding system fault section positioning method according to an embodiment of the present application;

[0053] Figure 4 A simulation model schematic diagram of the embodiment of the present application;

[0054] Figure 5 A current injection waveform diagram of the embodiment of the present application;

[0055] Figure 6 A 9th harmonic voltage and current amplitude diagram of each section line head when the embodiment of the present application is fault-free;

[0056] Figure 7 A 9th harmonic current amplitude diagram of each section line head when the embodiment of the present application is line 2 metallic single-phase grounding;

[0057] Figure 8 A current amplitude diagram of the first and last ends of a non-end section line when the embodiment of the present application is line 2 metallic single-phase grounding;

[0058] Figure 9 A 9th harmonic current amplitude diagram of each section line head when the embodiment of the present application is line 3 metallic single-phase grounding;

[0059] Figure 10 A current amplitude diagram of the first and last ends of a non-end section line when the embodiment of the present application is line 3 metallic single-phase grounding;

[0060] Figure 11 A current amplitude diagram of each end section line head when the embodiment of the present application is line 7 metallic single-phase grounding. DETAILED DESCRIPTION

[0061] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described below in a clear and complete manner with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0062] Please refer to Figure 3 The embodiment of the present application provides a neutral point injection-based resonant grounding system fault section positioning method, which comprises the following steps:

[0063] Step 1: Since the distribution network voltage reduction is Dyn11 connection, its closed loop can provide circulation path for 3rd harmonic and its integer multiple harmonic currents. When the injected current is 3rd harmonic, the phase relationship of three-phase currents is:

[0064] (1);

[0065] Since the three-phase currents are in phase, the 3rd harmonic current is zero sequence current, which satisfies the relationship as formula (2).

[0066] (2);

[0067] The 3rd harmonic magnetic motive force in the delta loop is superimposed:

[0068] (3);

[0069] The circulating current generated by the magnetic motive force in the winding is:

[0070] (4);

[0071] In the formula, The 3rd harmonic impedance is very small, so a large 3rd harmonic circulating current will be generated in the delta winding. The magnetic motive force generated by the circulating current is:

[0072] (5);

[0073] In the formula, k is the winding coefficient, and N is the number of turns. The 3rd harmonic magnetic motive force balance equation in the delta winding is:

[0074] (6);

[0075] In the formula, is the original 3rd harmonic magnetic motive force in the transformer, is the compensation magnetic motive force. The compensation magnetic motive force generates a 3rd harmonic voltage on the grid side , so the current flowing into the grid side through the transformer delta winding is:

[0076] (7);

[0077] In the formula, is the equivalent impedance on the grid side under the 3rd harmonic, which is large, so the 3rd harmonic current flowing into the grid side through the transformer delta winding is almost 0, which can be treated as open circuit. Although the 6th harmonic and the 9th harmonic are both 3n harmonics, the 6th harmonic is a negative sequence component, and its three-phase phase relationship is:

[0078] (8);

[0079] As can be seen from formula (8), the 6th harmonic current is three-phase symmetrical, and thus the three-phase winding cancels each other out, so that the 6th harmonic magnetic potential and voltage cannot be generated on the winding and the grid side. However, the 6th harmonic is greatly affected by the system unbalance degree, when a single-phase grounding fault occurs in the system, the injected 6th harmonic is affected by the system asymmetry at the time of fault, and the three-phase current is no longer strictly symmetrical, and the negative sequence current component that cannot be symmetrically canceled out is generated in the transformer winding. Moreover, due to the negative sequence characteristic of the 6th harmonic, it cannot generate circulating current in the delta winding to suppress the circulating current, so that the injected 6th harmonic current will flow into the grid side when the system is unbalanced, thereby expanding the influence range.

[0080] The 9th harmonic three-phase phase relationship is shown in formula (9):

[0081] (9);

[0082] As can be seen from formula (9), the 9th harmonic and the 3rd harmonic are both zero sequence components, so the suppression effect of the Dyn11 transformer on the 9th harmonic is the same as that on the 3rd harmonic, which is better than that on the 6th harmonic, so when selecting the injected current frequency, the 3rd harmonic and the 9th harmonic current are preferentially selected. Considering the accuracy of the characteristic current extraction after injection, the 9th harmonic is injected at the neutral point.

[0083] Step 2: Since there are a large number of capacitive and inductive elements in the zero sequence network, the impedance size is affected by the frequency of the zero sequence current flowing through. As can be seen from the inductive reactance calculation formula , the higher the current frequency, the greater the inductive reactance. As can be seen from the capacitive reactance calculation formula , the size decreases with the increase of the frequency. The capacitive current of the 10kV distribution network is generally tens to hundreds of amperes, and according to the 5% over-compensation, the size of the neutral point arc suppression coil is generally 0.1-2H. When the inductance is the minimum value 0.1H, the equivalent impedance of the arc suppression coil to the injected 9th harmonic current is still as high as 282.6Ω, and the larger the arc suppression coil, the equivalent impedance increases by several times, so before the size of the transition resistance exceeds the equivalent impedance of the arc suppression coil, the current flowing into the ground through the arc suppression coil is very small. When the injected current is the 9th harmonic, the line inductance increases to 9 times the power frequency, and the capacitive reactance decreases to 1 / 9 of the power frequency, at this time the line inductance cannot be ignored and it should be equivalent to a capacitive branch.

[0084] According to the GB / T50064-2014 "Design Specification for Overvoltage Protection and Insulation Coordination of AC Electrical Devices", when an automatic tracking compensation arc suppression device is used, the system grounding fault residual current Should not be greater than 10A. To reduce the impact of injection current on the equipment and protection in the system, the size of the traditional low-frequency (f<50Hz) injection current is generally limited to below 10A, and the injection duration is generally less than 0.5s. Considering that it is difficult for CT to measure when the current is less than 0.5A, the injection current flowing into the farthest line head should be ensured to be not less than 0.5A, that is , at this time the injection current corresponding to the low-voltage side of the transformer is , wherein is a proportional coefficient representing the amplitude multiple relationship of the injection current at two positions, and at this time the injection current corresponding to the neutral point is According to the above analysis, the constraint condition of the injection current amplitude can be obtained as formula (10):

[0085] (10);

[0086] Considering that the present application adopts 9th harmonic current, the above analysis shows that its ground capacitance reactance is very small, so the ground capacitance current diversion is greater than the power frequency, and therefore the present application takes the maximum value within the range specified in the specification as the injection current size. The injection current amplitude is determined as , to further reduce its impact on the equipment, the injection duration is shortened on the basis of the traditional low-frequency injection method, and the injection duration is required to be not more than 0.2s.

[0087] Step 3: design a power frequency wave trap, which is a parallel structure composed of inductance L and capacitance C, which utilizes the parallel resonance of the power frequency current flowing through the wave trap to realize the conduction of high-frequency injection current and hinder the power frequency current. When the inductance and the capacitance are connected in parallel, the equivalent impedance is:

[0088] (11);

[0089] When the frequency meets , the parallel impedance is infinite, so it plays a role in cutting off the power frequency and the frequency current near it. For the 9th harmonic, the equivalent impedance is:

[0090] (12);

[0091] When formula (12) meets the constraint condition , there is Z9=4.95<5Ω, at this time the corresponding inductance L=0.14H and the capacitance C=72.6uf. After the power frequency wave trap is connected in series with the injection power supply, it is connected in parallel with the neutral point grounding circuit to form an injection line, so that the injection line shows a broken circuit to the power frequency current flowing from the fault point to the ground through the neutral point, and shows a low resistance characteristic to the injection current emitted by the injection power supply.

[0092] Step 4: for example Figure 1The typical neutral point resonance grounding distribution network is shown in Figure 1. Figure 2 The neutral point resonance grounding distribution network fault zero sequence network is shown in Figure 2. Since the transformers all adopt Dyn11 connection mode, the zero sequence network does not contain loads and distributed photovoltaic power sources. Since the distribution network is a multi-branch structure, it is processed in sections, and the line with complete nodes without branches is equivalent to a section, Figure 1 The distribution network is divided into 8 sections of lines, and transformers are installed at the beginning and end of each section of line to collect voltage and current information at the point. As shown in Figure 3, Figure 2 As shown in Figure 4, the injected current flows through the wave trap and generates a partial 9th harmonic voltage drop at the neutral point. A small part of the current flows into the ground through the arc suppression coil, and the remaining current flows into the bus and flows to the lower level line from the bus. Since the line has a large inductive reactance and the end of the line in the zero sequence network is equivalent to an open circuit, the zero sequence voltage generated by the 9th harmonic of the line is mainly affected by the capacitive voltage rise effect, and the zero sequence voltage gradually increases with the direction of the current.

[0093] The line before the fault point is defined as the upstream line of the fault point, and the line after the fault point is defined as the downstream line. Since a ground short circuit occurs at the fault point, the current measured at the beginning of the fault section is the remaining part of the injected current after being shunted by the ground capacitance of the upstream branch line. Therefore, the size of the injected current decreases with the flow direction upstream of the fault point. Since the fault resistance at the fault point is smaller than the equivalent impedance of the ground capacitance, the current mainly flows into the ground through the point, so only a small part of the injected current remains downstream of the fault point. As the fault resistance increases, the current shunted at the fault point becomes smaller, but in general scenarios (0~300Ω), the shunt at the fault point is the main part, and the injected current measured at the end of the fault section is significantly lower than the current measured at the beginning. For the non-fault section upstream of the fault, since the length of each section of the line does not exceed 10 km, the difference between the currents measured at both ends of the section is only the capacitance current of the line to the ground, which does not change significantly.

[0094] Step 5: Based on the analysis of the change of the injected current of the fault feeder in step 4, the difference in the amplitude of the injected current of the fault line and the healthy feeder is further quantified, and the current change percentage of the non-end section line at both ends is defined:

[0095] (13);

[0096] wherein, is the current change percentage of the line i, is the 9th harmonic zero sequence current amplitude at the beginning of the line i, is the 9th harmonic zero sequence current amplitude at the end of the line i. Through simulation verification, the difference between the injected current amplitudes at both ends of the non-fault section is generally not more than 25%, so the threshold value b is taken as 25% in the embodiment of the present application.

[0097] When the fault section is the end section, i.e. directly connected with the load or photovoltaic power supply, the line end and the load cannot form a zero sequence passage, and the injected 9th harmonic current cannot flow through, so no matter whether the section line is faulty, the injected current measured by the end transformer is 0, and the fault section cannot be identified. When the fault occurs in the end section, the 9th harmonic currents measured at the beginning and end of the non-end section line have little difference, and since there are many line branches and multiple end sections, it is still necessary to identify the fault section in all end sections.

[0098] In the present application, the size of the injected current, the system topology and the line parameters are constant, so when no ground fault occurs, the 9th harmonic current flowing into the beginning of each end section is a constant value. As shown in Figure 1 , the section numbers n are 1-8 in turn, and the inherent amplitude of the injected current measured at the beginning of the line n when no fault occurs is . When the line i is faulty, the amplitude of the 9th harmonic current measured at the beginning of the line i is , where the line i is any end section. At this time, the amplitude of the 9th harmonic current measured at the beginning of other end sections is . Since the fault line i provides a ground passage, it provides more injected current, and the amplitude is obviously larger than the inherent current amplitude . The non-fault end section line has no structural change in the section line, so the current change is smaller than that of the fault end section line. Based on the above analysis, the injected current offset rate of the beginning of the end section line is proposed:

[0099] (14);

[0100] The injected current offset rate of the beginning of the end section line is the largest line, which is the fault line. Therefore, no matter where the fault point is located, the fault section can be accurately identified.

[0101] Based on the above analysis: the difference between the current change rate at the beginning and end of the non-end section line when the non-end section line is faulty and normal, and the difference between the injected current offset rate at the beginning of the end section line when the end section line is faulty and normal, the fault section positioning criterion is set: for the non-end section line, if the amplitude of the 9th harmonic current at the beginning is reduced by more than b compared with the 9th harmonic current at the end, it is the fault section. If all non-end section lines do not meet the condition, it is determined that the end section line is faulty, and the injected current offset rate of the beginning of the end section line is the largest line, which is the fault line.

[0102] The fault section positioning process of the present application is shown in Figure 3 .

[0103] a. After the fault occurs, the neutral point zero sequence voltage rises, and the line selection starts.

[0104] b. Injecting a 9th harmonic current with an amplitude of from the neutral point into the system for 0.2 s.

[0105] c. Extracting the steady-state amplitude of the 9th harmonic current at the two ends of each section of the line and recording.

[0106] d. Calculating the percentage of current change at the two ends of the non-fault section If it exists , it indicates that the fault section is the line n, and if it does not exist, it indicates that the fault point is located in the end section of the line.

[0107] e. Comparing the amplitude of the 9th harmonic current at the end of each section of the line with the inherent 9th harmonic current amplitude of the line when no fault occurs, and calculating the current deviation rate of each section of the line, and the largest deviation rate is the fault line.

[0108] The embodiment of the present application also provides a neutral point injection-based fault section positioning device for a resonant grounding system, comprising:

[0109] an injection current frequency determination module, configured to select a neutral point injection current frequency f according to the transformer winding connection mode and analyze the flow rule of the injection current in the winding;

[0110] an injection current amplitude determination module, configured to analyze the distribution rule of the injection current in the line and determine a neutral point injection current amplitude , wherein the neutral point injection current frequency f and the neutral point injection current amplitude are used to determine an injection power supply;

[0111] an injection line determination module, configured to design a power frequency wave trap, the power frequency wave trap comprising a wave trap inductance L and a wave trap capacitance C, and the power frequency wave trap and the injection power supply are connected in series and then connected in parallel with a neutral point grounding loop to form an injection line, so that the injection line appears as a circuit breaker for the power frequency current flowing from the fault point to the ground through the neutral point, and appears as a low resistance for the injection current emitted by the injection power supply;

[0112] a fault zero sequence network establishment module, configured to establish a neutral point resonant grounding distribution network fault zero sequence network based on the system topology and the injection line, analyze the distribution rule of the injection current in the fault / non-fault section, and then obtain the current amplitude difference characteristics of the two ends;

[0113] a fault section identification module, configured to set the fault section positioning criterion based on the obtained current amplitude difference characteristics and complete the fault section identification.

[0114] In the fault section identification module, the setting process of the positioning criterion is: based on the output current amplitude difference characteristics, setting:

[0115] (1) Non-terminal line criterion: if the current variation percentage of a line at both ends is > 25%, the fault section is determined;

[0116] (2) Terminal line criterion: if the current offset rate at the head of a line is the maximum value, the fault section is determined;

[0117] The current variation percentage calculation formula is , and the current offset rate calculation formula is , wherein represents the head injection current of the non-terminal feeder i, represents the end injection current of the non-terminal feeder i, is the inherent injection current of the terminal feeder n, and the size is the injection current amplitude of the neutral point injection current at the head of the line during normal operation of the system, is the injection current extracted at the head of the terminal feeder n during the fault.

[0118] Example model: based on the PSCAD / EMTDC platform, a neutral point resonance grounding 10kV distribution network model as shown in Figure 4 is established.

[0119] Example 1: simulation verification of different fault resistance sizes

[0120] In order to study the transition resistance tolerance of the invention after a single-phase grounding fault occurs in the resonance grounding system, a fault occurs at the head of line section 2 at 0.5km at 0.5s, a 9th harmonic current with an amplitude of is injected into the system at 0.8s, and lasts for 0.2s, as shown in Figure 5 . When no fault occurs, the 9th harmonic zero sequence voltage and current amplitudes measured at the head of each section line are as shown in Figure 6 (a) and (b). It can be seen that due to the influence of the line capacity effect, the farther the line is from the injection power source, the higher the 9th harmonic zero sequence voltage.

[0121] When a metallic grounding occurs, the 9th harmonic zero sequence current amplitudes measured at the head of each section line are as shown in Figure 7 . It can be seen that the 9th harmonic zero sequence current of the upstream line through the shortest path between the fault section and the injection power source is relatively large, while the 9th harmonic zero sequence current of the downstream line of the fault section and the branch of the non-shortest path of the upstream line is relatively small. As shown in Figure 8 (b), the current measured at the head of the fault section is 10.06A, the end current is 2.43A, and the current variation percentage is:

[0122] (15);

[0123] The value is much greater than the threshold of 25%, so it is determined as a fault section. And the non-end section line 1 and line 3 similar to line 2 are not directly connected to the load, Figure 8 The current amplitude graph of the first and last ends of the non-end section line when the line 2 is metallic single-phase grounding in the embodiment of the application; Figure 9 The 9th harmonic current amplitude graph of the first end of each section line when the line 3 is metallic single-phase grounding in the embodiment of the application; Figure 10 The current amplitude graph of the first and last ends of the non-end section line when the line 3 is metallic single-phase grounding in the embodiment of the application; the current amplitude and the current change percentage of the first and last ends of each section line are shown in Table 1:

[0124] Table 1: Current change percentage of each non-end section line under different fault resistances

[0125]

[0126] As shown in Table 1, under different transition resistances, the current change percentage of the first and last ends of the fault section is always the largest, and exceeds the threshold of 25%, so the fault section can be identified by this feature.

[0127] Example 2: Simulation verification of different fault positions

[0128] In order to study whether the application can identify different fault positions after single-phase grounding fault occurs in the resonant grounding system, metallic grounding faults are set at the first end of line section 3 at 1 km and the first end of line 7 at 5 km at 0.5 s, and a 9th harmonic current with an amplitude of 30 A is injected into the system at 0.8 s, lasting for 0.2 s.

[0129] When the fault occurs at the first end of section 3 at 1 km, the 9th harmonic current at the first and last ends of each non-end section line is as shown in Table 2. Figure 10

[0130] Table 2: Current change percentage of non-end section line when line 7 is faulty

[0131]

[0132] As shown in Table 2, the current change percentage of line 3 is greater than the threshold and significantly greater than that of the other two lines, so the fault line is determined as line 3.

[0133] When the fault occurs in line 7, the current at the first and last ends of each non-end section line, i.e. line 1, 2 and 3, is shown in Table 3:

[0134] Table 3: Current change percentage of non-end section line when line 7 is faulty

[0135] ​

[0136] It can be seen that when the end section line has a metallic fault, the main reason for the change of the non-end section line current is the coupling effect of the ground capacitance current and the injected current, resulting in the injected current and the capacitance current flowing to the fault point. But it can be seen that the change amplitude of the non-end section line current is still limited, which does not meet the threshold, so it can be judged that the fault is located in the end section line. The fault current amplitude of the end section line 4, 5, 6, 7, 8 head end is collected as shown in Figure 11 It can be seen that when the end section line has a metallic fault, the main reason for the change of the non-end section line current is the coupling effect of the ground capacitance current and the injected current, resulting in the injected current and the capacitance current flowing to the fault point. But it can be seen that the change amplitude of the non-end section line current is still limited, which does not meet the threshold, so it can be judged that the fault is located in the end section line. The fault current amplitude of the end section line 4, 5, 6, 7, 8 head end is collected as shown in The results are shown in Table 4:

[0137] Table 4: Current displacement rate of end section line when line 7 has a fault

[0138]

[0139] It can be seen from Table 4 that the fault section is line 7, and it can be seen from the above simulation verification that the present application is not affected by the fault location.

[0140] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any changes or replacements within the technical scope disclosed by the present application can be easily thought by any person skilled in the art, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for fault section location in a resonant grounded system based on neutral injection, characterized by, The method comprises the following steps: Step 1: According to the transformer winding connection mode, analyze the flow rule of the injected current in the winding, and select the neutral point injected current frequency f; Step 2: Analyze the distribution law of injected current in the line to determine the amplitude of the neutral point injected current The frequency f of the neutral point injected current and the amplitude of the neutral point injected current Determine the injected power supply; Step 3: Design a power frequency wave trap, the power frequency wave trap comprising a wave trap inductance L and a capacitance C, and the power frequency wave trap and the injected power source are connected in series and then connected in parallel with the neutral point grounding loop to form an injected line, so that the injected line presents a broken circuit to the power frequency current flowing from the fault point to the ground through the neutral point, and presents a low resistance characteristic to the injected current emitted by the injected power source; Step 4: Based on the system topology and the injected line, establish a neutral point resonant grounding distribution network fault zero sequence network, analyze the distribution rule of the injected current in the fault / non-fault section, and then obtain the current amplitude difference characteristics of the first and last ends; Step 5: Based on the current amplitude difference characteristics obtained in step 4, set the fault section positioning criterion, and complete the fault section identification; The injected current amplitude in step 2 The determination process is as follows: Based on the f=9th harmonic output in step 1, the following constraints are met: (1) The maximum zero sequence current of the line is less than or equal to the residual current limit value of the system grounding fault ; (2) The zero sequence current injected at the head of the line farthest from the neutral point is greater than or equal to the minimum measurable current of the transformer ; The final determination of the injected current amplitude selection strategy is: ； In the formula: The zero sequence current injected at the farthest line end is the minimum measurable current of the transformer The zero sequence current injected at the transformer secondary side at this time The design process of the wave trap parameters in step 3 is: Based on the f=9th harmonic output in step 1, a capacitance and inductance parallel structure is designed to make: (1) Equivalent impedance under power frequency to block the power frequency current; (2) Equivalent impedance under 9th harmonic to achieve a low resistance path; where Zeq is the equivalent impedance at the 9th harmonic , and requires , L is the choke inductance, and C is the choke capacitance. The setting process of the positioning criterion in step 5 is: based on the current amplitude difference characteristics output in step 4, set: (1) Non-end section line criterion: if the current change percentage of the first and last ends of a line is greater than 25%, it is determined as a fault section; (2) End section line criterion: if the current offset rate of the first end of a line is the maximum value, it is determined as a fault section.

2. The method of claim 1, wherein, The determination process of the injected current frequency f in step 1 is: The suppression characteristics of the transformer Dyn11 winding to the 3nth harmonic are analyzed, when f=9th harmonic, the three-phase currents are in phase to form a zero sequence component, after circulating through the triangular winding, the current flowing into the power grid side is ≤0.1A, so the f=9th harmonic is selected as the injected frequency.

3. The method of claim 1, wherein, The analysis process of the current amplitude difference characteristics in step 4 is: The amplitude of the injected current obtained in step 2 And the power frequency wave trap parameters obtained in step 3, a zero sequence network model is constructed, and the simulation results are obtained: (1) The current change percentage of the first and last ends of the non-fault section is ≤25%; (2) The first end current of the fault section is 4 times or more than the last end; (3) The current offset rate of the first end of the end fault line is 3 times or more than that of the non-fault line.

4. The method of claim 3, wherein, The zero sequence network model in step 4 includes distributed capacitance, line impedance and the power frequency wave trap designed in step 3, wherein the wave trap presents low impedance to the 9th harmonic, ensuring that the injected current preferentially flows through the fault section.

5. The method of claim 1, wherein, The current variation percentage calculation formula in step 5 is The current offset rate calculation formula is Wherein represents the head end injection current of the non-terminal section feeder i, represents the end injection current of the non-terminal section feeder i, is the inherent injection current of the terminal section feeder n, and the size is the injection current amplitude generated by the neutral point injection current at the head end of the section line during normal operation of the system, is the injection current extracted at the head end of the terminal section feeder n during the fault.

6. A neutral injection based resonant ground system fault section location device characterized in that, It comprises: An injected current frequency determination module, configured to analyze the flow rule of the injected current in the winding according to the transformer winding connection mode, and select the neutral point injected current frequency f; The injection current amplitude determination module is configured to analyze the distribution law of the injection current in the line and determine the neutral point injection current amplitude The neutral point injection current frequency f and the neutral point injection current amplitude determine the injection power supply; An injected line determination module, configured to design a power frequency wave trap, the power frequency wave trap comprising a wave trap inductance L and a capacitance C, and the power frequency wave trap and the injected power source are connected in series and then connected in parallel with the neutral point grounding loop to form an injected line, so that the injected line presents a broken circuit to the power frequency current flowing from the fault point to the ground through the neutral point, and presents a low resistance characteristic to the injected current emitted by the injected power source; A fault zero sequence network establishment module, configured to establish a neutral point resonant grounding distribution network fault zero sequence network based on the system topology and the injected line, analyze the distribution rule of the injected current in the fault / non-fault section, and then obtain the current amplitude difference characteristics of the first and last ends; A fault section identification module is configured to set a fault section positioning criterion based on the obtained current amplitude difference feature, and complete fault section identification; The injection current amplitude The determination process is as follows: Based on the output f=9th harmonic, the following constraints are met: (1) The maximum zero sequence current of the line is less than or equal to the residual current limit value of the system grounding fault ; (2) The zero sequence current injected at the head of the line farthest from the neutral point is greater than or equal to the minimum measurable current of the transformer ; The final injection current amplitude selection strategy is determined as: ； In the formula: The zero sequence current injected at the farthest line head is the minimum measurable current of the transformer The zero sequence current injected at the transformer secondary side at this time; The design process of the wave trap parameters is as follows: Based on the output f=9th harmonic, a parallel structure of capacitance and inductance is designed to make: (1) Equivalent impedance under power frequency to block the power frequency current; (2) Equivalent impedance under 9th harmonic to achieve a low resistance path; where Z is the equivalent impedance at the 9th harmonic , and requires , L is the choke inductance, and C is the choke capacitance. The setting process of the positioning criterion is as follows: based on the current amplitude difference feature output in step 4, the following is set: (1) Non-end section line criterion: if the current change percentage of the first and last ends of a line is greater than 25%, the fault section is determined; (2) End section line criterion: if the current offset rate of the first end of a line is the maximum value, the fault section is determined; The current percentage change calculation formula is The current offset rate calculation formula is Wherein represents the head end injection current of the non-terminal section feeder i, represents the end injection current of the non-terminal section feeder i, is the inherent injection current of the terminal section feeder n, and the size is the injection current amplitude generated by the neutral point injection current at the head end of the section line during normal operation of the system, is the injection current extracted at the head end of the terminal section feeder n during the fault.

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