Flexible grounding system high resistance fault line selection method based on low frequency transient zero sequence current distortion rate

Abstract

The application discloses a flexible grounding system high-resistance fault line selection method based on low-frequency transient zero sequence current distortion rate and belongs to the field of distribution network fault line selection. In the flexible grounding system, the single-phase grounding protection formed based on the steady-state fault characteristics generally has the problem of low transition resistance capacity, and the problem is further highlighted after the parallel small resistance is put into operation. Therefore, the application provides the flexible grounding system high-resistance fault line selection method based on the low-frequency transient zero sequence current distortion rate. The method calculates the low-frequency transient zero sequence current distortion rate ratio of each feeder in a specific time window, firstly realizes accurate identification of bus faults and outgoing line faults, and then identifies the fault line. The method is not affected by the disconnection of the zero sequence voltage transformer and the polarity reverse connection of the zero sequence current transformer, is almost not affected by the fault distance, and has low requirements on the accuracy of the transformer.

Description

Technical Field

[0001] This invention relates to the field of fault location in distribution networks, specifically to a high-resistivity fault location method for flexible grounding systems based on low-frequency transient zero-sequence current distortion rate. Background Technology

[0002] Flexible grounding is a new type of grounding method that combines the advantages of resonant grounding and low-resistance grounding. It overcomes the problems of high overvoltage levels and difficult fault location after a resonant grounding fault, while also addressing the issues of low-resistance grounding's inability to distinguish between transient and permanent grounding faults, resulting in low power supply reliability. The basic idea of ​​flexible grounding is to activate an arc suppression coil during normal operation and transient grounding faults, and then activate a parallel low-resistance resistor after determining the fault type to be a permanent grounding fault, thus achieving flexible switching of the neutral point grounding method. Currently, flexible grounding has been promoted and used in some regions of my country, and the State Grid Corporation of China and China Southern Power Grid Company have successively issued relevant regulations and technical demonstration results.

[0003] Flexible grounding systems generally use zero-sequence overcurrent protection, similar to that of low-resistance grounding systems. The setting value of this protection is typically set to avoid the maximum ground capacitance current of each feeder, usually between 40A and 60A, with a maximum withstand resistance of no more than 140Ω. However, single-phase high-resistance ground faults exceeding 140Ω frequently occur in distribution networks. After a fault occurs, the protection fails to operate, causing the fault to persist for a long time, easily escalating and even threatening personal safety. Furthermore, current line selection technology implemented in distribution networks has an accuracy rate of less than 20% when dealing with single-phase high-resistance ground faults. Therefore, research on high-resistance ground faults in flexible grounding systems is urgently needed to address the problems of existing protection systems failing to operate and difficulty in line selection under single-phase high-resistance ground fault conditions. Summary of the Invention

[0004] To address the challenges mentioned in the background section, this invention proposes a high-resistivity fault location method for flexible grounding systems based on low-frequency transient zero-sequence current distortion rate.

[0005] To achieve the above-mentioned technical objectives and effects, this invention provides the following technical solution: a high-resistivity fault location method for flexible grounding systems based on low-frequency transient zero-sequence current distortion rate, comprising the following steps:

[0006] Step 1: Monitor the neutral point current in real time and calculate the sudden change in the neutral point current amplitude. If satisfied If so, the fault detection system will be activated;

[0007] Step 2: After a short delay, if the fault disappears, return to Step 1, that is, the neutral point current amplitude drops below the threshold; otherwise, connect a small parallel resistor and proceed to Step 3.

[0008] Step 3: Filter the original waveforms of the zero-sequence current of each outgoing line to obtain the low-frequency transient zero-sequence current waveforms of each outgoing line, and calculate the distortion rate and distortion rate range ratio of the low-frequency transient zero-sequence current of each outgoing line within a specific time window. ;

[0009] Step 4: Based on Determine the type of fault. The fault was determined to be a busbar fault. The line is identified as having an outgoing fault, and the line with the lowest low-frequency transient zero-sequence current distortion rate is considered the faulty line.

[0010] Furthermore, the method for adjusting the sudden change in the neutral point current amplitude in step 1 is as follows:

[0011]

[0012] in, and These are the amplitude of the neutral point current and the amplitude of the maximum unbalanced current at the neutral point, respectively. The maximum unbalanced voltage amplitude of the system is 144.337V. Power frequency angular frequency, reliability factor Take 1.5, For the arc suppression coil inductance, the amplitude of the neutral point current is adjusted on-site based on the installed arc suppression coil inductance.

[0013] Furthermore, in step 2, the brief delay duration is 2 seconds.

[0014] Furthermore, in step 2, the parallel small resistor is connected at the instant the next neutral point current crosses zero after a 2-second delay.

[0015] Furthermore, in step 3, the low-frequency transient zero-sequence current of the non-faulty line... for:

[0016]

[0017]

[0018]

[0019] in, , These represent the voltage amplitude at the fault point and the current amplitude of the arc suppression coil before the parallel small resistor is connected. , , These are the capacitance to ground of the non-faulty lines, the total capacitance to ground of the system, and the capacitance to ground of the faulty lines, respectively. , These represent the fault point voltage and the initial phase angle of the arc suppression coil current at the instant the parallel small resistor is connected. v For system detuning, For parallel small resistor values, For transition resistance;

[0020] Furthermore, in step 3, the low-frequency transient zero-sequence current of the faulty line... for:

[0021]

[0022] in, This refers to the capacitance to ground of the faulty line.

[0023] Furthermore, in step 3, the specific time window is 0s-0.02s after the parallel small resistor is put into operation.

[0024] Furthermore, in step 3, the filter cutoff frequency is 360Hz.

[0025] Furthermore, in step 3, the distortion rate range ratio is:

[0026]

[0027] in, , These represent the maximum and minimum values ​​of the low-frequency transient zero-sequence current distortion rate among all outgoing lines.

[0028] Furthermore, in step 4, the line selection steps are as follows:

[0029] Step 4.1, if the distortion rate range ratio If the fault is found, the fault is determined to be a busbar fault, and the fault selection process ends.

[0030] Step 4.2, if the distortion rate range ratio If the line is faulty, the line with the lowest low-frequency transient zero-sequence current distortion rate is considered the faulty line and is promptly disconnected.

[0031] The high-resistance fault location method for flexible grounding systems based on low-frequency transient zero-sequence current distortion rate disclosed in this invention is simple in principle and easy to implement. It does not require changes to the original protection devices of the line, is easy to operate, and has strong practicality. It can also avoid the influence of zero-sequence voltage transformer disconnection and zero-sequence current transformer polarity reversal. At the same time, this method also overcomes the problem of the sharp drop in steady-state fault components after the parallel small resistor is put into operation, which greatly reduces the accuracy requirements of the transformer and is almost unaffected by the fault distance. Attached Figure Description

[0032] Figure 1 This is a flowchart of the invention;

[0033] Figure 2It is a simulation topology diagram;

[0034] Figure 3 This is a waveform diagram showing the changes in the original zero-sequence current of each feeder;

[0035] Figure 4 This is the zero-sequence current waveform after filtering. Detailed Implementation

[0036] The invention will now be described in more detail with reference to the accompanying drawings and simulation examples.

[0037] A flexible grounding system high-resistivity fault location method based on low-frequency transient zero-sequence current distortion rate, such as... Figure 1 As shown, it includes the following steps:

[0038] Step 1: Monitor the neutral point current in real time and calculate the sudden change in the neutral point current amplitude. If satisfied If so, the fault detection system will be activated;

[0039] Step 2: After a short delay, if the fault disappears, return to Step 1, that is, the neutral point current amplitude drops below the threshold; otherwise, connect a small parallel resistor and proceed to Step 3.

[0040] Step 3: Filter the original waveforms of the zero-sequence current of each outgoing line to obtain the low-frequency transient zero-sequence current waveforms of each outgoing line, and calculate the distortion rate and distortion rate range ratio of the low-frequency transient zero-sequence current of each outgoing line within a specific time window. ;

[0041] Step 4: Based on Determine the type of fault. The fault was determined to be a busbar fault. The line is identified as having an outgoing fault, and the line with the lowest low-frequency transient zero-sequence current distortion rate is considered the faulty line.

[0042] Furthermore, the method for adjusting the sudden change in the neutral point current amplitude in step 1 is as follows:

[0043]

[0044] in, and These are the amplitude of the neutral point current and the amplitude of the maximum unbalanced current at the neutral point, respectively. The maximum unbalanced voltage amplitude of the system is 144.337V. Power frequency angular frequency, reliability factor Take 1.5, For the arc suppression coil inductance, the amplitude of the neutral point current is adjusted on-site based on the installed arc suppression coil inductance.

[0045] Furthermore, in step 2, the brief delay duration is 2 seconds.

[0046] Furthermore, in step 2, the parallel small resistor is connected at the instant the next neutral point current crosses zero after a 2-second delay.

[0047] Furthermore, in step 3, the low-frequency transient zero-sequence current of the non-faulty line... for:

[0048]

[0049]

[0050]

[0051] in, , These represent the voltage amplitude at the fault point and the current amplitude of the arc suppression coil before the parallel small resistor is connected. , , These are the capacitance to ground of the non-faulty lines, the total capacitance to ground of the system, and the capacitance to ground of the faulty lines, respectively. , These represent the fault point voltage and the initial phase angle of the arc suppression coil current at the instant the parallel small resistor is connected. v For system detuning, For parallel small resistor values, For transition resistance;

[0052] Furthermore, in step 3, the low-frequency transient zero-sequence current of the faulty line... for:

[0053]

[0054] in, This refers to the capacitance to ground of the faulty line.

[0055] Furthermore, in step 3, the specific time window is 0s-0.02s after the parallel small resistor is put into operation.

[0056] Furthermore, in step 3, the filter cutoff frequency is 360Hz.

[0057] Furthermore, in step 3, the distortion rate range ratio is:

[0058]

[0059] in, , These represent the maximum and minimum values ​​of the low-frequency transient zero-sequence current distortion rate among all outgoing lines.

[0060] Furthermore, in step 4, the line selection steps are as follows:

[0061] Step 4.1, if the distortion rate range ratio If the fault is found, the fault is determined to be a busbar fault, and the fault selection process ends.

[0062] Step 4.2, if the distortion rate range ratio If the line is faulty, the line with the lowest low-frequency transient zero-sequence current distortion rate is considered the faulty line and is promptly disconnected.

[0063] Simulation and experimental verification

[0064] To verify the reliability and effectiveness of this invention, it is constructed based on a Real-Time Digital Simulation System (RTDS) as follows: Figure 2 The simulation model shown is a 10kV radial flexible grounding system. This simulation system includes four outgoing lines L1-L4, where L1-L3 are mixed lines of overhead and cable lines, L4 is an overhead line, and G is an infinite power source. R N A small resistor is connected in parallel at the neutral point, set to a commonly used field value of 10Ω. S A switch for switching on and off with a small parallel resistor. L p It is a neutral-point grounded arc suppression coil.

[0065] Table 1. Busbar grounding fault selection effect under different transition resistances.

[0066] 0.01 9.60 9.78 9.97 8.50 1.17 busbar correct 10 84.85 86.27 87.82 76.11 1.15 busbar correct 100 219.28 220.62 221.96 209.66 1.06 busbar correct 500 343.31 348.75 354.58 309.59 1.15 busbar correct 1000 433.51 441.56 449.23 385.77 1.16 busbar correct 2000 354.66 360.24 366.30 320.12 1.14 busbar correct 3000 353.35 358.91 364.91 318.58 1.15 busbar correct 4000 356.27 361.87 367.92 321.34 1.14 busbar correct

[0067] Table 2 shows the line selection effect under different transition resistance grounding faults on line L4.

[0068] 0.01 140.28 143.06 146.04 32.30 4.52 L4 correct 10 173.42 176.96 180.76 42.20 4.28 L4 correct 100 296.05 300.87 306.06 46.96 6.52 L4 correct 500 342.40 347.76 353.66 45.76 7.73 L4 correct 1000 351.15 356.69 362.65 42.73 8.49 L4 correct 2000 355.21 360.72 366.79 37.70 9.73 L4 correct 3000 356.24 361.83 367.89 33.80 10.88 L4 correct 4000 355.07 360.63 366.18 31.21 11.73 L4 correct

[0069] Table 3. Waveform distortion rate of feeder current at different fault distances

[0070] 2000 3km 355.10 360.65 366.64 37.77 L4 2000 6km 352.87 358.37 364.44 37.85 L4 2000 9km 358.58 364.01 369.87 37.12 L4 3000 3km 353.87 359.44 365.59 33.81 L4 3000 6km 355.30 360.92 366.92 33.89 L4 3000 9km 356.44 361.99 368.01 33.87 L4

[0071] To further verify the effectiveness of the proposed method, an RTDS was connected to a PA30B digital simulation power amplifier. The output current of the PA30B power amplifier was connected to an MG-LJK100J zero-sequence current transformer. The secondary output current of the transformer was connected to a circuit board. The output waveform was recorded using an oscilloscope. Finally, the proposed method was verified. The experimental results are shown in Table 4.

[0072] Table 4 Experimental Data Results

[0073] busbar 2000 333.43 332.86 338.29 359.05 1.08 busbar correct busbar 3000 330.83 326.15 329.36 306.51 1.08 busbar correct busbar 4000 321.04 305.07 333.26 332.90 1.09 busbar correct L4 2000 323.55 320.61 338.2 39.98 8.46 L4 correct L4 3000 326.99 341.93 344.39 38.21 9.01 L4 correct L4 4000 330.62 345.19 344.35 28.04 12.31 L4 correct L3 2000 328.43 344.41 147.64 320.69 2.33 L3 correct L3 3000 326.42 327.84 178.03 326.10 1.84 L3 correct L3 4000 329.03 327.56 169.57 326.71 1.94 L3 correct

[0074] Figure 3 and Figure 4The figures show the original waveform changes of the zero-sequence current of each feeder before and after the parallel small resistor is connected when a 3000Ω single-phase high-resistance ground fault occurs at the end of line L4, and the current waveform after filtering. The results show that the fault handling strategy of the present invention can work correctly under different ground fault resistances in flexible grounding systems, and has high practicality.

[0075] The above description and embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for selecting high-resistivity faults in a flexible grounding system based on low-frequency transient zero-sequence current distortion rate, characterized in that, Includes the following steps: Step 1: Monitor the neutral point current in real time and calculate the sudden change in the neutral point current amplitude. If satisfied If so, the fault detection system will be activated; Step 2: After a short delay, if the fault disappears, return to Step 1, that is, the neutral point current amplitude drops below the threshold; otherwise, connect a small parallel resistor and proceed to Step 3. Step 3: Filter the original waveforms of the zero-sequence current of each outgoing line to obtain the low-frequency transient zero-sequence current waveforms of each outgoing line, and calculate the distortion rate and distortion rate range ratio of the low-frequency transient zero-sequence current of each outgoing line within a specific time window. ; Low-frequency transient zero-sequence current in non-faulty lines for: Low-frequency transient zero-sequence current of faulty circuit for: in, , These are the voltage amplitude at the fault point and the current amplitude of the arc suppression coil before the parallel small resistor is connected, respectively. , , These are the capacitance to ground of the non-faulty lines, the total capacitance to ground of the system, and the capacitance to ground of the faulty lines, respectively. , These represent the fault point voltage and the initial phase angle of the arc suppression coil current at the instant the parallel small resistor is connected, respectively; v is the system detuning degree. For parallel small resistor values, For transition resistance; Distortion range ratio used to distinguish between bus faults and outgoing line faults for: in, , These represent the maximum and minimum values ​​of the low-frequency transient zero-sequence current distortion rate among all outgoing lines; Step 4: Based on Determine the type of fault. The fault was determined to be a busbar fault. The line is identified as having an outgoing fault, and the line with the lowest low-frequency transient zero-sequence current distortion rate is considered the faulty line.

2. The high-resistivity fault location method for flexible grounding systems based on low-frequency transient zero-sequence current distortion rate as described in claim 1, characterized in that, The abrupt change in the neutral point current amplitude in step 1 is: in, and These are the amplitude of the neutral point current and the amplitude of the maximum unbalanced current at the neutral point, respectively. The maximum unbalanced voltage amplitude of the system is 144.337V. The power frequency angular frequency, reliability factor Take 1.5, This is the inductance of the arc suppression coil.

3. The high-resistivity fault location method for flexible grounding systems based on low-frequency transient zero-sequence current distortion rate as described in claim 1, characterized in that, The specific time window in step 3 is 0s-0.02s after the parallel small resistor is put into operation.

4. The high-resistivity fault location method for flexible grounding systems based on low-frequency transient zero-sequence current distortion rate as described in claim 1, characterized in that, The filter cutoff frequency in step 3 is 360Hz.

5. The high-resistivity fault location method for flexible grounding systems based on low-frequency transient zero-sequence current distortion rate as described in claim 1, characterized in that, The method for distinguishing between busbar faults and outgoing line faults in step 4 is as follows: The fault was determined to be a busbar fault. The fault was determined to be an outgoing line fault.

6. The high-resistivity fault location method for flexible grounding systems based on low-frequency transient zero-sequence current distortion rate as described in claim 1, characterized in that, The fault selection method in step 4 is as follows: the line with the lowest low-frequency transient zero-sequence current distortion rate within a specific time window is regarded as the faulty line.

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