High-resistance fault detection method based on waveform distortion rate

Abstract

The invention provides a high-resistance fault detection method based on a waveform harmonic distortion rate, and belongs to the field of power system power distribution network relay protection. Bus zero-sequence voltage is monitored through a mutual inductor, when bus zero-sequence voltage u0 is detected to be larger than Ku times of rated phase voltage UN, zero-sequence current signals of all feeder lines are detected, and the zero-sequence current signals of all the feeder lines are obtained through fast Fourier transform; the zero-sequence current signal of the line is filtered, and finally, the zero-sequence current distortion rate of each feeder line in the first half period and the second period after the small resistor is put is calculated, and the zero-sequence current distortion rate is used as a fault criterion. According to the method, only the zero-sequence current needs to be collected, data collection is convenient, weak fault signals during high-resistance faults can be effectively highlighted, and reliable detection of the high-resistance faults of the flexible grounding system is achieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of power system distribution network relay protection, and particularly relates to a high-resistance fault detection method based on waveform distortion rate. BACKGROUND

[0002] At present, single-phase grounding fault is still the fault with the highest occurrence probability in distribution network. In a flexible grounding system, when a single-phase grounding fault occurs, the fault process can be generally divided into three stages: a resonant grounding system stage, a parallel small resistance input stage and a small resistance exit stage. The existing researches still have deficiencies in the flexible grounding fault feature analysis and protection research methods, especially the transient feature analysis when the parallel small resistance is input. Therefore, it is urgent to research the transient features before and after the small resistance input, and then propose a high-resistance fault detection scheme adaptable to various fault conditions.

[0003] Therefore, we propose a high-resistance fault detection method based on waveform distortion rate to solve the problems proposed in the above background.

[0004] The above information disclosed in the background of the application is only used to increase the understanding of the background of the application, and therefore, it can include prior art known by those skilled in the art. SUMMARY

[0005] The purpose of the application is to provide a high-resistance fault detection method based on waveform distortion rate to solve the problem that the transient feature analysis of the parallel small resistance input stage in the current market is insufficient, and then the high-resistance fault detection is unreliable. Through the high-resistance fault detection method based on waveform distortion rate of the application, the fault can be accurately detected when a single-phase high-resistance grounding occurs in the distribution network.

[0006] To achieve the above purpose, the application provides a high-resistance fault detection method based on waveform harmonic distortion rate, which comprises the following steps:

[0007] Step 1: monitoring the bus zero sequence voltage, when the zero sequence voltage exceeds the set threshold and lasts for a predetermined time, inputting the small resistance;

[0008] Step 2: collecting the zero sequence current signals of each feeder, and using fast Fourier transform to decompose them into fundamental wave and multiple harmonics;

[0009] Step 3: based on the decomposed harmonic amplitude, calculating the zero sequence current waveform distortion rate of each feeder;

[0010] Step 4: comparing the waveform distortion rates of each feeder, and determining the fault line as the line with the smallest waveform distortion rate.

[0011] Preferably, the specific steps of step 1 are:

[0012] Step 1.1: Obtain the bus zero-sequence voltage u0 and the system rated phase voltage U N ;

[0013] Step 1.2: When u0≥K u U N Record the duration t. If t > 1s, then the small resistor is switched on; if t < 1s, the small resistor is not switched on; where K is the small resistor. u K is the tuning coefficient. u =15%.

[0014] Preferably, the specific steps of step 2 are as follows:

[0015] Step 2: Convert the zero-sequence current i of each feeder using the Fast Fourier Transform. k0 Decomposed into the fundamental frequency, second harmonic, third harmonic, and fourth harmonic:

[0016]

[0017] In the formula, i k0 Here, k represents the zero-sequence current of each feeder, and A represents the line number. ki φ represents the amplitude of each harmonic of each feeder. ki Let i be the initial phase angle of each harmonic of each feeder, and i be the harmonic order.

[0018] Preferably, the specific steps of step 3 are as follows:

[0019] Step 3: Calculate the transient zero-sequence current waveform distortion rate (THD) of each feeder based on the amplitude values ​​of each harmonic extracted by the Fast Fourier Transform. k :

[0020]

[0021] In the formula, THD k Let A be the harmonic distortion rate of each feeder. k1 A k2 A k3 A k4 These represent the fundamental amplitude, second harmonic amplitude, third harmonic amplitude, and fourth harmonic amplitude of each feeder, respectively, with k being the line number.

[0022] Preferably, step 4 consists of the following steps:

[0023] Step 4: Final criterion, by comparing the zero-sequence current waveform distortion rate (THD) of each feeder. k The magnitude of the zero-sequence current waveform distortion rate (THD) is selected. k The smallest faulty line is the one with the shortest length.

[0024]

[0025] In the formula, l is the faulty line number, min{THD k} represents the minimum zero-sequence current waveform distortion rate in each feeder.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] In this invention, a Fast Fourier Transform is performed on the zero-sequence current signal of each feeder. The zero-sequence current distortion rate of each feeder is calculated using the decomposed fundamental, second, third, and fourth harmonic components to detect faulty lines. Furthermore, after connecting a small parallel resistor, only the zero-sequence current of each feeder needs to be acquired, making data acquisition convenient. The zero-sequence current waveform distortion rate calculated using the fundamental and three harmonic components can effectively highlight weak fault information, enabling effective detection of high-resistance ground faults.

[0028] The above overview is for illustrative purposes only and is not intended to be limiting in any way. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features of the invention will become readily apparent from the accompanying drawings and the following detailed description. Attached Figure Description

[0029] Figure 1 This is a flowchart of the high-resistivity fault detection method based on waveform distortion rate of the present invention;

[0030] Figure 2 This is a 10kV flexible grounding system model in Embodiment 2 of the present invention;

[0031] Figure 3 This refers to the zero-sequence current of each feeder in this invention;

[0032] Figure 4 This is the distribution of the zero-sequence current spectrum characteristics of each feeder in Embodiment 2 of the present invention; Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. It should be noted that the drawings are schematic and not illustrated to scale. For clarity and convenience, the relative sizes and proportions of the parts shown in the drawings have been exaggerated or reduced in size. Any size is only illustrative and not limiting.

[0034] Example 1

[0035] The high-resistivity fault detection method based on waveform distortion rate includes the following steps:

[0036] Step 1: Obtain the zero-sequence voltage u0 of each feeder bus and the rated phase voltage U of the system. N When u0≥K u U N Record the duration t. If t > 1s, then the small resistor is switched on; if t < 1s, then the small resistor is not switched on. Where K... u K is the tuning coefficient. u =15%. The zero-sequence current of each feeder is shown in the attached figure. Figure 3 As shown.

[0037] Step 2: Convert the zero-sequence current i of each feeder using the Fast Fourier Transform. k0 Decomposed into the fundamental frequency, second harmonic, third harmonic, and fourth harmonic:

[0038]

[0039] In the formula, i k0 The zero-sequence current of each feeder, k=1,2,3… represents the line number, A ki φ represents the amplitude of each harmonic of each feeder. ki Let i be the initial phase angle of each harmonic of each feeder, where i = 1, 2, 3... represents the harmonic order.

[0040] Step 3: Calculate the transient zero-sequence current waveform distortion rate (THD) of each feeder based on the amplitude values ​​of each harmonic extracted by the Fast Fourier Transform. k :

[0041]

[0042] In the formula, THD k Let A be the harmonic distortion rate of each feeder. k1 A k2 A k3 A k4 These represent the fundamental amplitude, second harmonic amplitude, third harmonic amplitude, and fourth harmonic amplitude of each feeder, respectively, with k=1,2,3… representing the line number.

[0043] Step 4: Final criterion, by comparing the zero-sequence current waveform distortion rate (THD) of each feeder. k The magnitude of the zero-sequence current waveform distortion rate (THD) is selected. k The smallest faulty line is the one with the shortest length.

[0044]

[0045] In the formula, l is the faulty line number, min{THD k} represents the minimum zero-sequence current waveform distortion rate in each feeder.

[0046] Example 2

[0047] As shown in the attached diagram. Figure 1 The simulation model of a 10kV flexible grounding system is shown. Four feeders, L1 to L4, are set up, including overhead lines, cable lines, and a hybrid overhead-cable line. Zero-sequence current transformers are installed at the beginning of each line segment, and zero-sequence voltage transformers are installed on the busbars. The fault is assumed to occur at 6km along cable line L3, with a high-resistance A-phase grounding fault. The initial phase angle β = 60°, and the grounding resistance R... g =100Ω, line parameters are shown in the table:

[0048] Table 1 Parameters of Cables and Overhead Lines

[0049]

[0050] After performing a Fast Fourier Transform on the zero-sequence current waveform following a fault, the spectral characteristic distribution diagrams of each feeder are obtained. Figure 4 It can be seen that after the fault occurs, the fundamental frequency amplitude of the faulty feeder is much larger than its second, third, and fourth harmonic amplitudes, showing a significant difference from that of a healthy line. The amplitudes of the fundamental, second, third, and fourth harmonics are extracted. To further verify the adaptability of the high-resistivity fault detection algorithm based on waveform harmonic distortion rate, the zero-sequence current waveform harmonic distortion rate of each feeder is calculated using the proposed method. Simulation verification is performed under various typical fault conditions, and the distortion rate of each feeder under various conditions is calculated. The results are shown in Table 2.

[0051] Table 2 Simulation verification results

[0052]

[0053] As shown in Table 2, the waveform distortion rate of the faulty line is the smallest among the four feeders, thus enabling the detection of the line where the high-resistance grounding fault is located.

[0054] The accompanying drawings of the embodiments disclosed in this invention only involve structures relevant to the embodiments disclosed in this invention. Other structures can be referred to with common designs. Unless otherwise specified, the same embodiment and different embodiments of this invention can be combined with each other.

[0055] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high impedance fault detection method based on waveform harmonic distortion rate, characterized by, The method comprises the following steps: Step 1: monitoring the bus zero sequence voltage, and when the zero sequence voltage exceeds a set threshold and lasts for a predetermined time, a small resistance is put into operation; Step 2: collecting zero sequence current signals of each feeder, and decomposing the signals into fundamental waves and multiple harmonics by using fast Fourier transform; Step 3: calculating the zero sequence current waveform distortion rate of each feeder based on the decomposed harmonic amplitudes; Step 4: comparing the waveform distortion rates of each feeder, and determining the fault line as the line with the minimum waveform distortion rate.

2. The waveform harmonic distortion rate based high resistance fault detection method of claim 1, wherein, The specific steps of step 1 are as follows: Step 1.1: Obtain bus zero sequence voltage u0and system rated phase voltage U N ; Step 1.2: When u0≥K u U N , record its duration t, if t>1s, put in small resistance; if t<1s, do not put in small resistance; in the formula K u is the setting coefficient, K u =15%.

3. The waveform harmonic distortion rate based high resistance fault detection method of claim 1, wherein, The specific steps of step 2 are as follows: Step 2: The feeder zero sequence currents i k0 are decomposed into fundamental, 2nd harmonic, 3rd harmonic and 4th harmonic: where i k0 is the zero sequence current of each feeder, k is the line number, A ki is the amplitude of each harmonic of each feeder, φ ki is the initial phase angle of each harmonic of each feeder, i is the harmonic number.

4. The waveform harmonic distortion rate based high impedance fault detection method of claim 1, wherein, The specific steps of step 3 are as follows: Step 3: Calculate the waveform distortion rate THD of the transient zero sequence current of each feeder according to the amplitude of each harmonic extracted by the fast Fourier transform k : where THD k is the harmonic distortion of each feeder, A k1 , A k2 , A k3 , A k4 is the fundamental amplitude, 2nd harmonic amplitude, 3rd harmonic amplitude, 4th harmonic amplitude, respectively, of each feeder, and k is the line number.

5. The waveform harmonic distortion rate based high impedance fault detection method of claim 1, wherein, The specific steps of step 4 are as follows: Step 4: Final criterion, selection of the line with the lowest zero sequence current waveform distortion rate THD k k The line with the lowest THD is the faulty line:​ where l is the fault line number, min{THD k} is the minimum value of zero sequence current waveform distortion factor in each feeder.

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