CT disconnection discrimination method and system based on current track

Abstract

The invention discloses a CT disconnection discrimination method and system based on a current track, and the method comprises the steps: obtaining a historical current instantaneous value of a CT secondary circuit under a normal working condition, and carrying out the alignment and translation preprocessing, and obtaining an aligned and translated current; drawing the track of the aligned and translated current relative to the reference current and calculating the upper and lower bounds of the track; calculating the differential current of the CT secondary circuit according to the current instantaneous current value of the CT secondary circuit; if it is judged that the dynamic trajectory corresponding to the current instantaneous value and the upper and lower bounds of the trajectory do not meet the ellipse criterion and the differential current meets the differential criterion, a CT disconnection alarm is given after a set time is delayed. According to the invention, the current sensed by the protection device during high-resistance grounding and CT disconnection can be effectively distinguished, and the protection maloperation probability is reduced.

Description

Technical Field

[0001] This invention belongs to the field of power system protection and control technology, and relates to a method and system for CT line disconnection detection based on current trajectory. Background Technology

[0002] Relay protection devices are key equipment in substations. They perform various protection functions by sensing characteristic quantities such as current and voltage. Current, as the most important electrical characteristic quantity, directly affects the correctness of protection operation. For a long time, the integrity of the CT secondary circuit has been a key focus of the operation and maintenance of protection devices.

[0003] A current transformer (CT) open circuit, or secondary open circuit of a current transformer, is a typical current loop anomaly. Conventional criteria for CT open circuit detection generally use electrical quantities such as differential current and zero-sequence current. When these electrical quantities are below a certain threshold, a CT open circuit is considered to have occurred. However, due to the complexity and variety of actual power grid faults, such as high-resistance grounding faults where the amplitudes of differential current and zero-sequence current are also relatively small, protection devices risk misinterpreting high-resistance grounding faults as CT open circuits, potentially leading to widespread power outages.

[0004] Different manufacturers and different types of protection devices have different criteria for determining CT disconnection. Taking busbar protection as an example, for instance... Figure 1 The differential current criterion shown is the simplest and most commonly used criterion. Protection systems such as GE-B90, SEL 487B, and REB500 all use this criterion, which compares the differential current sensed by the protection with the open-circuit lockout value. For example, if the secondary current of a phase CT on a certain line is open, the current in that phase is almost zero. If the differential current calculated from the currents of each phase on each line is greater than the lockout value (this setting is much smaller than the operating setting, typically 0.15In), then a time-delay alarm and blockout of the bus protection will occur. Although the differential current criterion is simple and easy to operate, because it only judges the differential current, its ability to distinguish between CT open circuit and high-resistance grounding is extremely weak. In the case of high-resistance grounding, the differential current may increase to exceed the lockout value and remain for a period of time (exceeding the Ts delay). This can easily lead to misjudging such a high-resistance grounding fault as a CT open circuit, thereby blocking the protection device and delaying timely fault clearing. In response, some studies have proposed new criteria based on the cosine of the included angle and the Euclidean distance. By calculating the cosine of the included angle and the Euclidean distance, it is possible to determine whether the current is sinusoidal. This method can improve the differentiation between CT disconnection and high-resistance grounding. However, this method is not sensitive enough and has limited differentiation ability. When the CT senses the clipped periodic waveform, the probability of misjudgment is relatively high. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method and system for detecting CT line breaks based on current trajectories.

[0006] The present invention adopts the following technical solution.

[0007] The first aspect of this invention proposes a CT line breakage detection method based on current trajectory, comprising: The historical instantaneous current values ​​of the CT secondary circuit under normal operating conditions are obtained and preprocessed by alignment and translation to obtain the aligned and translated current. Plot the trajectory of the aligned and translated current with respect to the reference current and calculate the upper and lower bounds of the trajectory; Calculate the differential current of the CT secondary circuit based on the current instantaneous value of the CT secondary circuit. If it is determined that the dynamic trajectory corresponding to the current instantaneous current value and the upper and lower boundaries of the trajectory do not satisfy the ellipse criterion, and the differential current satisfies the differential criterion, then after a set delay time, a CT disconnection alarm is issued.

[0008] Preferably, the alignment and translation preprocessing includes: The instantaneous current value measured by the CT secondary circuit is fitted to the corresponding fundamental wave. The zero-crossing point of the current during the rising phase of the fundamental wave is shifted to the origin of the coordinate system to obtain the aligned current data. The aligned current data is then shifted by 90 degrees to obtain the aligned and shifted current.

[0009] Preferably, the reference current is a 10-cycle sine wave of 0.15A.

[0010] Preferably, drawing the trajectory of the aligned and translated current with respect to the reference current includes: For each phase, the reference current of that phase is matched with the current after alignment and translation, and the trajectory of that phase is plotted.

[0011] Preferably, the calculation formulas for the upper and lower bounds of the trajectory are as follows: The upper bound of the trajectory is: ; The lower bound of the trajectory is: ; in, , for The reference current of the phase and the current after alignment and translation; , Let be the lengths of the major and minor axes of the trajectory.

[0012] Preferably, the ellipse criterion is: The dynamic trajectory of each phase lies between the upper and lower bounds of the trajectory of the corresponding phase.

[0013] Preferably, the differential criterion is: The effective value of the differential current in any phase is greater than the CT disconnection blocking value for the corresponding phase.

[0014] A second aspect of this invention proposes a CT line breakage detection system based on current trajectory, comprising: The data acquisition and processing module is used to acquire the historical instantaneous current value of the CT secondary circuit under normal operating conditions and perform alignment and translation preprocessing to obtain the aligned and translated current. The trajectory boundary calculation module is used to draw the trajectory of the aligned and translated current with respect to the reference current and calculate the upper and lower bounds of the trajectory; The differential current calculation module is used to calculate the differential current of the CT secondary circuit based on the current instantaneous value of the CT secondary circuit. The CT disconnection detection and alarm module is used to issue a CT disconnection alarm after a set delay time if it is determined that the dynamic trajectory corresponding to the current instantaneous value and the upper and lower boundaries of the trajectory do not satisfy the elliptic criterion and the differential current satisfies the differential criterion.

[0015] A third aspect of the present invention provides a terminal, including a processor and a storage medium; the storage medium is used to store instructions; the processor is used to perform operations according to the instructions to execute the steps of the method.

[0016] A fourth aspect of the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method.

[0017] Compared with the prior art, the beneficial effects of the present invention include at least the following: This invention pre-calculates the upper and lower bounds of the trajectory based on historical current data of the CT secondary circuit under normal operating conditions, determines whether the real-time dynamic trajectory and the upper and lower bounds of the trajectory satisfy the elliptic criterion, and combines the differential criterion to identify CT disconnection. This can effectively distinguish the current sensed by the protection device when there is high resistance grounding and CT disconnection, and reduce the probability of protection maloperation. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the differential current criterion; Figure 2 This is a schematic diagram of the reference current; Figure 3 The measured current is a sinusoidal current and the angle of translation at the zero point is... A schematic diagram of the trajectory when the angle is greater than or equal to 30 degrees; Figure 4 The measured current is a sinusoidal current and the angle of translation at the zero point is... Schematic diagrams of the trajectories at 45 degrees, 60 degrees, and 90 degrees; Figure 5 This is a schematic diagram of the trajectory when the measured current is a sinusoidal current but there is some interference. Figure 6 This is a schematic diagram of the trajectory when the measured current is a random disturbance current; Figure 7 This is a schematic diagram of the trajectory when the measured current is the clipping waveform; Figure 8 This is the CT line breakage criterion of the present invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0020] Embodiment 1 of this invention provides a CT disconnection detection method based on current trajectory. Practical results show that this method can reliably distinguish between CT disconnection and high-resistance grounding. Specifically, it includes: Step 1: Obtain the historical instantaneous current value of the CT secondary circuit under normal operating conditions and perform alignment and translation preprocessing to obtain the aligned and translated current; Step 2: Plot the trajectory of the aligned and translated current relative to the reference current and calculate the upper and lower bounds of the trajectory; Step 3: Calculate the differential current of the CT secondary circuit based on the current instantaneous value of the CT secondary circuit; Step 4: If it is determined that the dynamic trajectory corresponding to the current instantaneous current value and the upper and lower boundaries of the trajectory do not satisfy the ellipse criterion and the differential current satisfies the differential criterion, then after a set delay time, a CT disconnection alarm is issued.

[0021] The overall idea of ​​this invention is to compare the characteristic quantities of the sampled current from the CT scanner with the standard fundamental current to determine whether the sampled current truly indicates a CT disconnection.

[0022] The difference in characteristic quantities between CT disconnection and single-phase high-resistivity grounding current: The phase current of the branch where the CT disconnection occurs is sometimes almost zero, and the actual sampled current is zero or a disturbance with a very small amplitude. Sometimes there is a waveform in half of the cycle (the lower half of the cycle), while the other half of the cycle has only a very small current. After the transient process ends, the phase current of single-phase high-resistivity grounding current is a sine wave.

[0023] The bus voltage remains unchanged for the branch where the CT is disconnected, but the bus voltage changes for the branch with a single-phase high-resistance grounding. (This invention does not use voltage characteristics).

[0024] The sampling frequencies of the method and apparatus of this invention are closely related, and the calculation basis is as follows: The sampling frequency of mainstream protection devices in China is usually 1200Hz, that is, the current and voltage are calculated at 24 points (20ms) per cycle. The reference voltage is a 10-cycle sine wave that should normally be 57.7V, the reference current is a 10-cycle sine wave of 0.15In, the secondary current rating is taken as 1A, and the current is a 10-cycle sine wave of reference current of 0.15A.

[0025] Reference current ,Right now ,like Figure 2 As shown. This invention compares the current sampled from the CT circuit with the reference current.

[0026] More preferably, regarding the preprocessing of current data, the present invention allows the instantaneous current value measured by the CT circuit to... for

[0027] This invention primarily relies on current identification to determine its correlation with sinusoidal current. First, the instantaneous current value measured by the CT circuit is fitted to the corresponding fundamental frequency. Then, the zero-crossing point of the current during the rising phase of the fundamental frequency is shifted to the origin of the coordinate system. The current data after the zero-crossing point shift is then obtained. for And record the angle of translation, such as Then Translate it by 90 degrees to obtain for .

[0028] The specific analysis regarding trajectory calculation is as follows: (1) The measured current is a sinusoidal current and When the temperature is greater than or equal to 30 degrees: Reference current Aligned with and shifted to the instantaneous current value For comparison, taking two phase A currents as an example, the approach is as follows: Figure 3 As shown, if it is a high-resistance ground, after the transient process ends, the reference current... and instantaneous current value Given two sinusoidal currents, their trajectories will be elliptical, in reality... The calculation of the elliptical trajectory at 30 degrees, 45 degrees, 60 degrees, and 90 degrees is as follows: Figure 4 As shown (45 degrees, 60 degrees, and 90 degrees correspond to the blue, orange, and green tracks respectively), in the graph, the vertical axis is... The x-axis is All units are in amperes (A). The reference current is the amplitude. A. Without loss of generality, the measured current amplitude of the CT is 0.5A.

[0029] (2) The measured current is a sinusoidal current and When the temperature is below 30 degrees Celsius: Reference current Aligned with and shifted to the instantaneous current value In comparison, the rules for both are the same. Figure 4 Green oval.

[0030] (3) When the measured current is a sinusoidal current but there is some interference: Based on the preceding phase angle analysis, regardless of the measured current phase angle, the reference current will be used. Aligned with and shifted to the instantaneous current value Both comparisons yield clear elliptic curves. Below, we present aligned and translated elliptic curves. For example, an interference analysis is performed, assuming that random interference reaches 5%, 10%, and 20% of the amplitude (positive direction), and the reference current is compared. and the instantaneous value of the current is translated and aligned Then there is such Figure 5 The trajectories shown (5%, 10%, and 20% correspond to the blue, orange, and green trajectories respectively) are plotted in the graph, with the vertical axis representing... The x-axis is All units are in Å. In this trajectory, the length of the major axis of the observed ellipse (in the positive y-direction) is at... (Note that due to coordinate axis compression, what appears to be a short axis is actually a long axis.)

[0031] (4) The measured current is a random disturbance current: Assuming the CT is disconnected, and the actual disconnection is a random small current, the reference current will be used. Aligned with and shifted to the instantaneous current value In comparison, the following analysis examines random disturbances of different amplitudes: 0.05A, 0.1A, and 0.15A, comparing them with the reference current. and the instantaneous value of the current is translated and aligned Then there is such Figure 6 The trajectories shown (0.05A, 0.1A, and 0.15 correspond to the blue, orange, and green trajectories respectively) are plotted in the graph, with the vertical axis representing the coordinates. The x-axis is All values ​​are in amperes (A). The red curve represents the contrasting sine waves, showing a clear difference in trajectory characteristics between the sine waves generated by a small current after a wire break and those generated by a high-resistance grounding.

[0032] (5) The measured current is a clipped waveform: Assuming the CT is disconnected, and the measured disconnection waveform is a half-cycle clipping waveform (assuming it's the lower half-cycle clipping), the reference current is... Aligned with and shifted to the instantaneous current value By comparison, since the upper half of the waveform is more complete, an upper half-elliptic curve can be obtained, while the lower half-curve will differ from the original lower half-elliptic curve. The following section uses aligned and translated waveforms... Taking a comparative analysis as an example, the characteristics of the clipping waveform trajectory are analyzed. Assuming the amplitude is reduced to 50%, 20%, 10%, and 0% in the second half of the cycle, the reference current is compared. and the instantaneous value of the current is translated and aligned Then there is such Figure 7 The trajectories shown (50%, 20%, 10%, 0% correspond to blue, orange, green, and red trajectories respectively) are plotted in the graph, with the vertical axis representing the trajectories. The x-axis is All units are in amperes (A). It is evident that when a clipped waveform appears, the difference in trajectory characteristics between a small current after a wire break and a high-resistance grounded sine wave is significant. In the portion of the trajectory below the x-axis, the waveform differs greatly from the sine wave trajectory.

[0033] Based on the above research, the two waveforms of CT line breakage and the trajectory of high-resistance grounding are quite different, and can be distinguished by trajectory criteria (ellipse criteria).

[0034] Compare the reference current with the aligned and translated current, and plot the trajectory of the translated current relative to the reference current. That is: ; Considering a certain degree of interference tolerance, and taking into account the actual range of axis length variation in the embodiments, we have: Upper bound of the trajectory: Lower bound of the trajectory: .

[0035] If the real-time trajectory ellipse of the reference current and the current after alignment and translation of a certain phase is between the upper and lower bounds of the trajectory, then the ellipse criterion of a certain phase is considered to be satisfied, that is, the sampled current of a certain phase is considered to be a sine wave.

[0036] In specific implementation, the elliptic criterion is that the dynamic trajectory of each phase is between the upper and lower bounds (including the boundary) of the corresponding phase trajectory. If the dynamic trajectory of each phase is between the upper and lower bounds (including the boundary) of the corresponding phase trajectory (i.e., the elliptic criterion is satisfied), then it is not a CT disconnection current. If the dynamic trajectory of any phase is outside the upper and lower bounds of the corresponding phase trajectory (i.e., the elliptic criterion is not satisfied), and the differential criterion (the effective value of the differential current of any phase is greater than the CT disconnection blocking value of the corresponding phase) is satisfied, then it is a CT disconnection current.

[0037] The ellipse criterion, combined with the differential criterion, forms the final CT disconnection criterion. At this point, the CT disconnection criterion for a certain branch is as follows: Figure 8 As shown.

[0038] Taking line protection as an example, the instantaneous current value measured by the CT circuit on this side (M side) is The instantaneous current measured in the CT circuit on the line side (N side) is ; At this time, the difference current between the two ends of a cycle wave for

[0039] The effective value of differential current is ; It is easy to see that if the CT scan of a certain phase on this side (such as phase A) is broken, then Extremely small If it remains unchanged, It will increase.

[0040] A data window is formed using 24 points, and calculations are performed using a scrolling method. ;once , , If any one of them is true, then it satisfies Figure 8 The logic block diagram shows the condition "a certain phase difference current is greater than the CT disconnection blocking value". If the elliptic criterion is not met at this time, a CT disconnection is reported after a delay of Ts seconds (typically 200ms).

[0041] Embodiment 2 of the present invention provides a CT line breakage detection system based on current trajectory, comprising: The data acquisition and processing module is used to acquire the historical instantaneous current value of the CT secondary circuit under normal operating conditions and perform alignment and translation preprocessing to obtain the aligned and translated current. The trajectory boundary calculation module is used to draw the trajectory of the aligned and translated current with respect to the reference current and calculate the upper and lower bounds of the trajectory; The differential current calculation module is used to calculate the differential current of the CT secondary circuit based on the current instantaneous value of the CT secondary circuit. The CT disconnection detection and alarm module is used to issue a CT disconnection alarm after a set delay time if it is determined that the dynamic trajectory corresponding to the current instantaneous value and the upper and lower boundaries of the trajectory do not satisfy the elliptic criterion and the differential current satisfies the differential criterion.

[0042] Embodiment 3 of the present invention provides a terminal, including a processor and a storage medium; the storage medium is used to store instructions; the processor is used to perform operations according to the instructions to execute the steps of the method.

[0043] Embodiment 4 of the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method.

[0044] Compared with the prior art, the beneficial effects of the present invention include at least the following: This invention pre-calculates the upper and lower bounds of the trajectory based on historical current data of the CT secondary circuit under normal operating conditions, determines whether the real-time dynamic trajectory and the upper and lower bounds of the trajectory satisfy the elliptic criterion, and combines the differential criterion to identify CT disconnection. This can effectively distinguish the current sensed by the protection device when there is high resistance grounding and CT disconnection, and reduce the probability of protection maloperation.

[0045] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0046] Computer-readable storage media can be tangible devices capable of holding and storing instructions for use by an instruction execution device. Computer-readable storage media can be, for example—but not limited to—electrical storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital multifunction disc (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or recessed protrusions storing instructions thereon, and any suitable combination of the foregoing. The computer-readable storage media used herein are not to be construed as transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses through fiber optic cables), or electrical signals transmitted through wires.

[0047] The computer-readable program instructions described herein can be downloaded from computer-readable storage media to various computing / processing devices, or downloaded via a network, such as the Internet, local area network, wide area network, and / or wireless network, to an external computer or external storage device. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards them to the computer-readable storage media in the respective computing / processing device.

[0048] Computer program instructions used to perform the operations of this disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, status setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The computer-readable program instructions may execute entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer may be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN)—or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, electronic circuitry, such as programmable logic circuitry, field-programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), is personalized by utilizing the status information of the computer-readable program instructions to implement various aspects of this disclosure.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A CT line breakage detection method based on current trajectory, characterized in that, The method includes: The historical instantaneous current value of the CT secondary circuit under normal operating conditions is obtained and preprocessed by alignment and translation to obtain the current after alignment and translation. Plot the trajectory of the aligned and translated current with respect to the reference current and calculate the upper and lower bounds of the trajectory; Calculate the differential current of the CT secondary circuit based on the current instantaneous value of the CT secondary circuit. If it is determined that the dynamic trajectory corresponding to the current instantaneous current value and the upper and lower boundaries of the trajectory do not satisfy the ellipse criterion, and the differential current satisfies the differential criterion, then after a set delay time, a CT disconnection alarm is issued.

2. The CT line breakage detection method based on current trajectory according to claim 1, characterized in that: The alignment and translation preprocessing includes: The instantaneous current value measured by the CT secondary circuit is fitted to the corresponding fundamental wave. The zero-crossing point of the current during the rising phase of the fundamental wave is shifted to the origin of the coordinate system to obtain the aligned current data. The aligned current data is then shifted by 90 degrees to obtain the aligned and shifted current.

3. The CT line breakage detection method based on current trajectory according to claim 1, characterized in that: The reference current is a 10-cycle sine wave of 0.15A.

4. The CT line breakage detection method based on current trajectory according to claim 1, characterized in that: The process of plotting the aligned and translated current trajectory with respect to the reference current includes: For each phase, the reference current of that phase is matched with the current after alignment and translation, and the trajectory of that phase is plotted.

5. The CT line breakage detection method based on current trajectory according to claim 1, characterized in that: The specific formulas for calculating the upper and lower bounds of the trajectory are as follows: The upper bound of the trajectory is: ; The lower bound of the trajectory is: ; in, , for The reference current of the phase and the current after alignment and translation; , Let be the lengths of the major and minor axes of the trajectory.

6. The CT line breakage detection method based on current trajectory according to claim 1, characterized in that: The elliptic criterion is: The dynamic trajectory of each phase lies between the upper and lower bounds of the trajectory of the corresponding phase.

7. The CT line breakage detection method based on current trajectory according to claim 1, characterized in that: The differential criterion is: The effective value of the differential current in any phase is greater than the CT disconnection blocking value for the corresponding phase.

8. A CT line breakage detection system based on current trajectory, comprising the method described in any one of claims 1-7, characterized in that, The system includes: The data acquisition and processing module is used to acquire the historical instantaneous current value of the CT secondary circuit under normal operating conditions and perform alignment and translation preprocessing to obtain the aligned and translated current. The trajectory boundary calculation module is used to draw the trajectory of the aligned and translated current with respect to the reference current and calculate the upper and lower bounds of the trajectory; The differential current calculation module is used to calculate the differential current of the CT secondary circuit based on the current instantaneous value of the CT secondary circuit. The CT disconnection detection and alarm module is used to issue a CT disconnection alarm after a set delay time if it is determined that the dynamic trajectory corresponding to the current instantaneous value and the upper and lower boundaries of the trajectory do not satisfy the elliptic criterion and the differential current satisfies the differential criterion.

9. A terminal, comprising a processor and a storage medium; characterized in that: The storage medium is used to store instructions; The processor is configured to operate according to the instructions to perform the steps of the method according to any one of claims 1-7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the program implements the steps of the method according to any one of claims 1-7.

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