Single-phase tree-grounding fault detection method and device

Abstract

The application provides a single-phase tree grounding fault detection method, comprising the following steps: collecting initial zero sequence voltage data in a power distribution network; screening the waveform amplitude of the initial zero sequence voltage data to obtain target zero sequence voltage with sudden change in waveform amplitude; judging whether the target zero sequence voltage has beat frequency characteristics; if yes, it is determined that the target zero sequence voltage has a first fault; otherwise, the next step is entered; separating the target zero sequence voltage by using a fixed point algorithm, and judging whether the separated zero sequence voltage has beat frequency characteristics; if yes, it is determined that the target zero sequence voltage has a second fault. The application also provides a single-phase tree grounding fault detection device. The single-phase tree grounding fault detection is based on the fixed point algorithm FastICA, so that the recognition speed is improved while the recognition ability is ensured, and the detection efficiency is effectively improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of signal processing and high-voltage power grid, and more particularly, the present application relates to a single-phase tree grounding fault detection method and device. BACKGROUND

[0002] Single-phase tree grounding fault is a common fault in distribution network, which is easy to cause forest fire and other hazards, so it is of great significance to design an algorithm that can monitor the single-phase tree grounding fault in real time. At present, the design of this kind of detection algorithm faces some challenges. First, single-phase tree grounding fault is a single-phase high-resistance grounding fault, and the neutral point of the distribution network is usually grounded through an arc suppression coil. However, due to the over-compensation of the size of the arc suppression coil, the amplitude of the commonly used detection quantity is low (the zero sequence voltage may be less than 15% of the phase voltage, and the fault current may be less than 1A) when a single-phase high-resistance grounding fault occurs, and the fault characteristic quantity is not obvious, which is easily affected by three-phase imbalance, noise, arc distortion and other factors. For single-phase tree grounding fault, the transition resistance is relatively large in single-phase high-resistance grounding fault, which makes the amplitude of the commonly used detection quantity more weak than other types of single-phase high-resistance grounding fault. Therefore, the traditional fault detection based on amplitude and polarity is difficult to apply, and the traditional protection device is almost unable to act effectively. Therefore, it is necessary to design a single-phase tree grounding fault detection method. SUMMARY

[0003] (I) Technical problems to be solved

[0004] In view of the above problems, the main purpose of the present application is to provide a single-phase tree grounding fault detection method and device.

[0005] (II) Technical solutions

[0006] In order to achieve the above purpose, the present application provides a single-phase tree grounding fault detection method, comprising: step S1, collecting initial zero sequence voltage data in the distribution network; step S2, screening the waveform amplitude of the initial zero sequence voltage data to obtain the target zero sequence voltage with sudden change of waveform amplitude; step S3, judging whether the target zero sequence voltage has beat frequency characteristics, if yes, determining that the target zero sequence voltage has the first fault; otherwise, entering the following step S4; step S4, separating the target zero sequence voltage by using fixed point algorithm, and judging whether the separated zero sequence voltage has beat frequency characteristics, if yes, determining that the target zero sequence voltage has the second fault.

[0007] In the above scheme, the initial zero sequence voltage data includes normal data and fault data, and step S1 includes: reading the parameters of three-phase power in the distribution network through voltage transformer in parallel to obtain the initial zero sequence voltage data; or reading the parameters of three-phase neutral point through voltage transformer to obtain the initial zero sequence voltage data.

[0008] In the above scheme, step S3 comprises: step S31, performing fast Fourier transform on the target zero sequence voltage to obtain a first frequency domain graph of the target zero sequence voltage; and step S32, determining that the target zero sequence voltage has a beat frequency characteristic in a case where a plurality of signal peaks exist within a preset neighborhood range of the power frequency in the first frequency domain graph, and a difference between the plurality of signal peaks and a waveform amplitude of the target zero sequence voltage is less than a preset difference range.

[0009] In the above scheme, step S4 comprises: step S41, reading a power frequency ω0 by performing fast Fourier transform on the target zero sequence voltage; step S42, constructing a signal observation matrix according to the power frequency ω0, and solving the observation matrix by using a fixed point algorithm to obtain a mixing matrix A; step S43, separating a power frequency signal in the target zero sequence voltage by using the mixing matrix A to obtain a separated zero sequence voltage; step S44, performing fast Fourier transform on the separated zero sequence voltage to obtain a second frequency domain graph of the separated zero sequence voltage; and step S45, determining whether the separated zero sequence voltage has a beat frequency characteristic according to the second frequency domain graph.

[0010] In the above scheme, step S45 comprises determining that the separated zero sequence voltage has a beat frequency characteristic in a case where the following three conditions are met: a plurality of signal peaks exist within a preset neighborhood range of the power frequency in the second frequency domain graph; a difference between the plurality of signal peaks and a waveform amplitude of the target zero sequence voltage is less than a preset difference range; and an amplitude of the separated zero sequence voltage is not less than 5% of the waveform amplitude of the target zero sequence voltage.

[0011] In the above scheme, the reading of the power frequency ω0 in step S41 comprises: adding 0 to the back of the target zero sequence voltage data, and then performing fast Fourier transform on the target zero sequence voltage data with the added 0 to read the power frequency ω0.

[0012] In the above scheme, the signal observation matrix in step S42 is represented as:

[0013] X={U o (t),sin(ω0t),cos(ω0t)}

[0014] wherein, U o (t) represents the target zero sequence voltage; sin(ω0t) and cos(ω0t) represent a sine signal of the power frequency ω0 and a cosine signal of the power frequency ω0, respectively; and X represents the signal observation matrix.

[0015] The application further provides a single-phase contact tree grounding fault detection device, comprising: an initial zero sequence voltage reading module configured to read initial zero sequence voltage data in a power distribution network; a target zero sequence voltage screening module configured to screen zero sequence voltage with waveform amplitude mutation in the initial zero sequence voltage data; a first fault judgment module configured to judge whether the target zero sequence voltage has beat frequency characteristics, and if yes, determine that the target zero sequence voltage has a first fault; otherwise, enter the following second fault judgment module; and the second fault judgment module configured to separate the target zero sequence voltage by using a fixed point algorithm, and judge whether the separated zero sequence voltage has beat frequency characteristics, and if yes, determine that the target zero sequence voltage has a second fault.

[0016] Further, the first fault judgment module further comprises: a first frequency domain graph determination unit configured to perform fast Fourier transform on the target zero sequence voltage to obtain a first frequency domain graph of the target zero sequence voltage; and a first fault judgment unit configured to determine that the target zero sequence voltage has beat frequency characteristics in a case where it is determined that there are multiple signal peaks in a preset neighborhood range of the power frequency in the first frequency domain graph, and the difference between the multiple signal peaks and the waveform amplitude of the target zero sequence voltage is less than a preset difference range.

[0017] Further, the second fault judgment module comprises: a power frequency reading unit configured to read the power frequency ω0 by performing fast Fourier transform on the target zero sequence voltage; a mixed matrix determination unit configured to construct a power frequency ω0 signal observation matrix, solve the observation matrix by using a fixed point algorithm, and obtain a mixed matrix A; a signal separation unit configured to separate the power frequency signal in the target zero sequence voltage by using the mixed matrix A to obtain a separated zero sequence voltage; a second frequency domain graph determination unit configured to perform fast Fourier transform on the separated zero sequence voltage to obtain a second frequency domain graph of the separated zero sequence voltage; and a second fault judgment unit configured to judge whether the separated zero sequence voltage has beat frequency characteristics according to the second frequency domain graph, and if yes, determine that the target zero sequence voltage has a second fault.

[0018] (Three) beneficial effects

[0019] Compared with the prior art, the beneficial effects of the application include:

[0020] (1) The detection method can accurately distinguish the normal state from the fault state.

[0021] (2) The detection method effectively improves the detection efficiency, can accurately distinguish the normal state from the fault state while ensuring the recognition ability, and improves the detection speed of the algorithm. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 A flow chart of a single-phase contact tree grounding fault detection method according to an embodiment of the application is schematically shown.

[0023] Figure 2 A flow chart of the first fault judgment according to an embodiment of the present application is shown schematically.

[0024] Figure 3 A flow chart of the second fault judgment according to an embodiment of the present application is shown schematically.

[0025] Figure 4 A block diagram of the single-phase tree grounding fault detection device according to an embodiment of the present application is shown schematically.

[0026] Figure 5 A component block diagram of the first fault judgment module according to an embodiment of the present application is shown schematically.

[0027] Figure 6 A component block diagram of the second fault judgment module according to an embodiment of the present application is shown schematically. DETAILED DESCRIPTION

[0028] In order to make the objects, technical solutions and advantages of the present application clearer, further detailed description will be made to the present application with reference to the embodiments and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0029] For a more complete understanding of the present application, reference is made to the following Figure 1 , Figure 1 A flow chart of the single-phase tree grounding fault detection method according to an embodiment of the present application is shown schematically.

[0030] As shown in Figure 1 , the embodiment of the present application provides a single-phase tree grounding fault detection method, which comprises:

[0031] Step S1, collecting initial zero-sequence voltage data in a power distribution network;

[0032] Step S2, screening the waveform amplitude of the initial zero-sequence voltage data to obtain target zero-sequence voltage with sudden change of waveform amplitude;

[0033] Step S3, judging whether the target zero-sequence voltage has beat frequency characteristics, if yes, determining that the target zero-sequence voltage has the first fault; otherwise, entering the following step S4;

[0034] Step S4, separating the target zero-sequence voltage by using fixed-point algorithm, and judging whether the separated zero-sequence voltage has beat frequency characteristics, if yes, determining that the target zero-sequence voltage has the second fault.

[0035] Wherein, the initial zero-sequence voltage data comprises normal data and fault data, and step S1 comprises:

[0036] The voltage transformer is installed along the power transmission line, and the parameters of three-phase power in the power distribution network are read in parallel through the voltage transformer to obtain initial zero sequence voltage data; or the parameters of the three-phase power neutral point are read through the voltage transformer to obtain initial zero sequence voltage data.

[0037] In the embodiment, please refer to Figure 2 , Figure 2 The flow chart of the first fault judgment according to the embodiment of the application is schematically shown.

[0038] The first fault judgment step S3 comprises:

[0039] Step S31, the target zero sequence voltage is subjected to fast Fourier transform to obtain a first frequency domain graph of the target zero sequence voltage;

[0040] Step S32, in the case that it is determined that there are multiple signal peaks in the preset neighborhood range of the power frequency in the first frequency domain graph, and the difference between the multiple signal peaks and the amplitude of the target zero sequence voltage waveform is less than the preset difference range, it is determined that the target zero sequence voltage has beat frequency characteristics.

[0041] In the embodiment, please refer to Figure 3 , Figure 3 The flow chart of the second fault judgment according to the embodiment of the application is schematically shown.

[0042] The second fault judgment step S4 comprises:

[0043] Step S41, the target zero sequence voltage is subjected to fast Fourier transform to read the power frequency ω0;

[0044] Step S42, according to the power frequency ω0, a signal observation matrix is constructed, and a fixed point algorithm is used to solve the observation matrix to obtain a mixing matrix A;

[0045] Step S43, the power frequency signal in the target zero sequence voltage is separated using the mixing matrix A to obtain a separated zero sequence voltage;

[0046] Step S44, the separated zero sequence voltage is subjected to fast Fourier transform to obtain a second frequency domain graph of the separated zero sequence voltage;

[0047] Step S45, whether the separated zero sequence voltage has beat frequency characteristics is judged according to the second frequency domain graph.

[0048] Specifically, step S45 comprises determining that the separated zero sequence voltage has beat frequency characteristics in the case that the following three conditions are met:

[0049] In the second frequency domain diagram, there are multiple signal peaks within the preset neighborhood of the power frequency; the difference between the multiple signal peaks and the amplitude of the target zero-sequence voltage waveform is less than the preset difference range; the amplitude of the separated zero-sequence voltage is not less than 5% of the amplitude of the target zero-sequence voltage waveform.

[0050] Specifically, step S41, which involves performing a fast Fourier transform on the target zero-sequence voltage and reading the power frequency ω0, includes:

[0051] The target zero-sequence voltage data is appended with zeros, and then a fast Fourier transform is performed on the zero-added target zero-sequence voltage data to read the power frequency ω0.

[0052] For example, the sampled data is stored in an Excel file with sampling ranges of 1, 2, 3, 4, and 5. Zeros are added to the end of the floating-point data to obtain 1, 2, 3, 4, 5, 0, 0, 0, 0, which is then subjected to a Fourier transform.

[0053] For example, in the step of constructing the signal observation matrix based on the power frequency ω0, and solving the observation matrix using the fixed-point algorithm to obtain the hybrid matrix A, the signal observation matrix can be represented as:

[0054] X={U o (t), sin(ω0t), cos(ω0t)}

[0055] Among them, U o (t) represents the target zero-sequence voltage; sin(ω0t) and cos(ω0t) represent the sine signal and cosine signal of the power frequency ω0, respectively; X represents the signal observation matrix.

[0056] Once a zero-sequence voltage indicating a second fault is detected, the final result will be output for appropriate actions such as tripping and dispatching personnel for inspection.

[0057] The present invention also provides a single-phase tree grounding fault detection device. The device provided in the embodiments of the present invention will be described below. The device described below corresponds to the method described above.

[0058] Figure 4 A block diagram of a single-phase tree-touch grounding fault detection device according to an embodiment of the present invention is shown schematically.

[0059] like Figure 4 As shown, the single-phase tree grounding fault detection device 400 may include, for example:

[0060] The initial zero-sequence voltage reading module 401 is used to read the initial zero-sequence voltage data in the distribution network.

[0061] The target zero-sequence voltage screening module 402 is used to screen zero-sequence voltages with abrupt changes in waveform amplitude in the initial zero-sequence voltage data.

[0062] The first fault judgment module 403 is used to determine whether the target zero-sequence voltage has beat frequency characteristics. If it does, the target zero-sequence voltage is determined to have a first fault; otherwise, it proceeds to the second fault judgment module.

[0063] The second fault judgment module 404 is used to separate the target zero-sequence voltage using a fixed-point algorithm, and to determine whether the separated zero-sequence voltage has beat frequency characteristics. If so, it is determined that the target zero-sequence voltage has a second fault.

[0064] Figure 5 The diagram illustrates the components of the first fault determination module 403 according to an embodiment of the present invention.

[0065] like Figure 5 As shown, the first fault determination module 403 may include, for example, a first frequency domain diagram determination unit 501 and a first fault determination unit 502;

[0066] The first frequency domain diagram determination unit 501 is used to perform a fast Fourier transform on the target zero-sequence voltage to obtain the first frequency domain diagram of the target zero-sequence voltage.

[0067] The first fault judgment unit 502 is used to determine that the target zero-sequence voltage has beat frequency characteristics when there are multiple signal peaks in the preset neighborhood of the power frequency in the first frequency domain diagram, and the difference between the amplitude of the multiple signal peaks and the target zero-sequence voltage waveform is less than a preset difference range.

[0068] Figure 6 The diagram illustrates the components of the second fault determination module 404 according to an embodiment of the present invention.

[0069] like Figure 6 As shown, the second fault judgment module 404 may include: a power frequency reading unit 601, a hybrid matrix determination unit 602, a signal separation unit 603, a second frequency domain diagram determination unit 604, and a second fault judgment unit 605;

[0070] The power frequency reading unit 601 is used to perform a fast Fourier transform on the target zero-sequence voltage and read the power frequency ω0.

[0071] The hybrid matrix determination unit 602 is used to construct the power frequency ω0 signal observation matrix and solve the observation matrix using a fixed-point algorithm to obtain the hybrid matrix A.

[0072] The signal separation unit 603 uses a hybrid matrix A to separate the power frequency signal in the target zero-sequence voltage to obtain the separated zero-sequence voltage;

[0073] The second frequency domain graph determination unit 604 performs fast Fourier transform on the separated zero sequence voltage to obtain a second frequency domain graph of the separated zero sequence voltage.

[0074] The second fault determination unit 605 determines whether the separated zero sequence voltage has a beat frequency feature according to the second frequency domain graph, and if so, determines that the target zero sequence voltage has a second fault.

[0075] It should be noted that the embodiment modes of the device part are similar to the embodiment modes of the method part, and the technical effects achieved are also similar. For specific details, please refer to the above method embodiment mode part, which will not be repeated here.

[0076] Any one or at least part of the functions of any one or more of the modules according to the embodiments of the present application can be implemented in one module. Any one or more of the modules according to the embodiments of the present application can be split into multiple modules for implementation. Any one or more of the modules according to the embodiments of the present application can be at least partially implemented as a hardware circuit, such as a field programmable gate array (FPGA), a programmable logic array (PLA), a system on chip, a system on board, a system on package, an application specific integrated circuit (ASIC), or any other reasonable way of hardware or firmware through integration or packaging of circuits, or in any one of software, hardware and firmware or in an appropriate combination of any one or more of them. Alternatively, one or more of the modules according to the embodiments of the present application can be at least partially implemented as a computer program module that can perform corresponding functions when running.

[0077] In summary, the single-phase tree grounding fault detection method and device provided by the present application first detects whether the zero sequence voltage amplitude changes to preliminarily select the fault signal, then performs the first fault determination through the beat frequency feature of the zero sequence voltage, and if no obvious fault can be determined, removes the unbalanced component of the power distribution network by combining FastICA (fixed point algorithm) to perform the second fault determination. In order to improve the recognition speed of the algorithm, the detection efficiency can be effectively improved, the normal state and the fault state can be effectively distinguished while ensuring the recognition ability, and the detection speed of the algorithm is improved.

[0078] The computer program product of the present application can be a computer program product comprising a computer readable storage medium and a computer program mechanism embedded in the computer readable storage medium. The computer readable storage medium is not to be construed as a transitory signal per se. The computer readable storage medium is a tangible (non-transitory) computer-readable medium having computer-readable program code embodied thereon. The computer readable storage medium is a tangible computer-readable medium that can cause a processor or computer to perform methods described herein. The computer readable storage medium can be, but is not limited to, one or more of the following: an electronic storage device, a magnetic storage device, an optical storage device, a tape, a floppy disk, a CD-ROM, CDRW, DVD, a DVD-ROM, a hard disk, a solid state drive, a memory card, a ROM, a PROM, an EPROM, a FLASH-EPROM, optical storage, a magnetic storage, or a tape. Specifically, the computer readable program code can comprise one or more instructions enabling a processor or computer to perform at least some of the methods described herein.

[0079] Those skilled in the art will appreciate that, although specific exemplary embodiments of the application have been described herein for illustrative purposes, various modifications and changes in light thereof will be obvious to those skilled in the art. Accordingly, the scope of the application is not intended to be limited to the above described embodiments but rather is intended to be limited only by the scope of the claims appended hereto.

[0080] Similarly, it is to be understood that the use of certain of the above terms are merely used as descriptive terms, and not as a limitation on the scope of the application as claimed. In this document, the terms "comprises", "comprising", "has", "having", "includes", "including", "contains", "containing" or variations thereof are used as equivalent to "consists essentially of", "consisting essentially of", "consists of", and "consisting of" unless otherwise specified. Where the term "comprises", "comprising", "has", "having", "includes", "including", "contains", "containing" or variations thereof is used in the specification, it is used in the sense of "consisting essentially of", "consisting essentially of", "consists essentially of", "consisting of", and "consists of", as those terms are interpreted in accordance with the dictionary.

[0081] The above described embodiments are further detailed to explain the purpose, technical solutions and beneficial effects of the present application. It should be understood that the above described embodiments are merely specific embodiments of the present application and are not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A single phase touch tree ground fault detection method, characterized by, The method comprises the following steps: Step S1, collecting initial zero sequence voltage data in a power distribution network; Step S2, screening the waveform amplitude of the initial zero sequence voltage data to obtain target zero sequence voltage with sudden change in waveform amplitude; Step S3, determining whether the target zero sequence voltage has beat frequency characteristics, and if so, determining that the target zero sequence voltage has a first fault; Otherwise, the following step S4 is entered; Step S4, separating the target zero sequence voltage by using a fixed point algorithm, and determining whether the separated zero sequence voltage has beat frequency characteristics, and if so, determining that the target zero sequence voltage has a second fault; The step S3 comprises: Step S31, performing fast Fourier transform on the target zero sequence voltage to obtain a first frequency domain graph of the target zero sequence voltage; Step S32, determining that the target zero sequence voltage has beat frequency characteristics in the case that a plurality of signal peaks exist in a preset neighborhood range of the power frequency in the first frequency domain graph, and the difference between the plurality of signal peaks and the waveform amplitude of the target zero sequence voltage is less than a preset difference range; The step S4 comprises: Step S41: performing fast Fourier transform on the target zero sequence voltage to read the power frequency ; Step S42: according to the power frequency , construct a signal observation matrix, solve the observation matrix by using the fixed point algorithm, and obtain a mixing matrix A; Step S43: separating the power frequency signal in the target zero sequence voltage by using the mixing matrix A to obtain the separated zero sequence voltage; Step S44: performing fast Fourier transform on the separated zero sequence voltage to obtain a second frequency domain graph of the separated zero sequence voltage; Step S45: determining whether the separated zero sequence voltage has beat frequency characteristics according to the second frequency domain graph.

2. The single phase touch tree ground fault detection method of claim 1, wherein, The initial zero sequence voltage data comprises normal data and fault data, and the step S1 comprises: reading the parameters of three-phase power in the power distribution network through a voltage transformer in parallel to obtain the initial zero sequence voltage data; or reading the parameters of the three-phase power neutral point through the voltage transformer to obtain the initial zero sequence voltage data.

3. The single phase touch tree ground fault detection method of claim 1, wherein, The step S45 comprises determining that the separated zero sequence voltage has beat frequency characteristics in the case that the following three conditions are met: a plurality of signal peaks exist in a preset neighborhood range of the power frequency in the second frequency domain graph; the difference between the plurality of signal peaks and the waveform amplitude of the target zero sequence voltage is less than a preset difference range; the amplitude of the separated zero sequence voltage is not less than 5% of the waveform amplitude of the target zero sequence voltage.

4. The single phase touch tree ground fault detection method of claim 1, wherein, The reading of the power frequency frequency in the step S41 comprises: The target zero sequence voltage data is processed by appending 0 at the end, and fast Fourier transform is performed on the target zero sequence voltage data with 0 appended, to read the power frequency .

5. The single phase touch tree ground fault detection method of claim 1, wherein, The signal observation matrix in the step S42 is expressed as: wherein, represents the target zero sequence voltage; respectively represent a sine signal of the power frequency and a cosine signal of the power frequency ; X represents a signal observation matrix.

6. A single phase touch tree ground fault detection device, characterized by, The method comprises the following steps: An initial zero sequence voltage reading module for reading initial zero sequence voltage data in a power distribution network; A target zero sequence voltage screening module for screening zero sequence voltage with sudden change in waveform amplitude in the initial zero sequence voltage data; A first fault determination module for determining whether the target zero sequence voltage has beat frequency characteristics, and if so, determining that the target zero sequence voltage has a first fault; otherwise, the following second fault determination module is entered; A second fault determination module for separating the target zero sequence voltage by using a fixed point algorithm, and determining whether the separated zero sequence voltage has beat frequency characteristics, and if so, determining that the target zero sequence voltage has a second fault; The first fault determination module further comprises: The first frequency domain graph determination unit is configured to perform fast Fourier transform on the target zero sequence voltage to obtain a first frequency domain graph of the target zero sequence voltage. The first fault determination unit is configured to determine that the target zero sequence voltage has a beat frequency characteristic in a case where it is determined that a preset neighborhood range of the power frequency in the first frequency domain graph has a plurality of signal peaks, and a difference between the plurality of signal peaks and a waveform amplitude of the target zero sequence voltage is less than a preset difference range. The second fault determination module includes: A power frequency reading unit is configured to read the power frequency by performing a fast Fourier transform on the target zero sequence voltage. ; a mixing matrix determination unit configured to construct a power frequency mixing matrix a signal observation matrix, and solving the observation matrix by using a fixed point algorithm to obtain the mixing matrix A; The signal separation unit is configured to separate the power frequency signal in the target zero sequence voltage using the mixing matrix A to obtain a separated zero sequence voltage. The second frequency domain graph determination unit is configured to perform fast Fourier transform on the separated zero sequence voltage to obtain a second frequency domain graph of the separated zero sequence voltage. The second fault determination unit is configured to determine whether the separated zero sequence voltage has a beat frequency characteristic according to the second frequency domain graph, and determine that the target zero sequence voltage has a second fault if yes.

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